mechanistic lung cancer

Despite well-documented health risks, a substantial proportion of men and women in the developed world continue to habitually smoke cigarettes1. Making mat-ters worse, smoking prevalence rates are on the rise in developing nations, all but ensuring an increased inci-dence of cigarette smoking-related diseases in the years to come. Given the paucity of disease-modifying thera-pies for most of these disorders, substantial increases in mortality are expected2. Of the numerous smoking-related maladies — coronary artery disease, periph-eral vascular disease, urinary bladder cancer, stroke and so on — the most common is chronic obstructive pulmonary disease (COPD), and the most deadly is lung cancer. Thus, it has been a concerning revelation that these two diseases are linked, with the presence of COPD increasing the incidence of lung cancer and lung cancer death3.

For decades, the occurrence of lung cancer in patients with COPD was thought to reflect the common aetiological agent, cigarettes. Observation of high lung cancer incidence rates in specialty COPD clinics chal-lenged this idea, and prompted controlled epidemio-logical studies to address this question. First reported by Skillrud and colleagues4 in 1986, the presence of COPD, as determined by a reduction in airflow, was associated with an increase in lung cancer incidence. Subsequently, this finding has been reproduced in numerous studies with large sample sizes that have carefully controlled for cigarette smoke dosage5–9 (TABLE 1). However, little progress has been made towards identifying common mechanistic links for these heterogeneous diseases, which can both be characterized by multiple disease sub-phenotypes.

COPD is loosely defined as the presence of airflow obstruction (BOX 1) in the setting of chronic exposure to noxious particulate matter (cigarette smoke)10, and it is comprised of two major components — the obstruction of the breathing tubes (airways disease) and the oblitera-tion of the tiny air sacs in the peripheral regions of the lung (emphysema)11. There is undoubtedly a substantial component of genetic susceptibility to the development of COPD, as only about 15–25% of smokers will develop one or both of the components of COPD. Currently, an estimated 20 million Americans are afflicted with COPD, which is the fourth leading cause of death in the United States12.

Lung cancer is considerably less common than COPD; however, it accounts for ~160,000 deaths annu-ally in the United States, with only 15% of patients still alive 5 years after diagnosis13. Approximately 20% of lung cancer cases are small-cell lung cancer (SCLC) by histology, with the other 80%, which includes adeno-carcinoma (ADCA), squamous cell carcinoma (SCCA), large-cell carcinoma and bronchoalveolar cell carci-noma, being lumped together as non-small-cell lung cancer (NSCLC)14. It seems that each of these subtypes of lung cancer is linked to COPD, although data are somewhat limited with respect to uncommon histo-logical subtypes. As with COPD, only a minority of cigarette smokers will develop lung cancer, highlight-ing the importance of genetic susceptibility to disease. Furthermore, about 10% of lung cancers arise in people who have never smoked (never smokers)15.

The link between these two diseases has garnered substantial attention over the past few years owing to a few insightful clinical studies and the looming burden on

Clinical Research Division, Fred Hutchinson Cancer Research Center and Division of Pulmonary and Critical Care, University of Washington, Seattle, Washington 98109, USA. e-mail: [email protected]:10.1038/nrc3477Published online 7 March 2013

Mechanistic links between COPD and lung cancerA. McGarry Houghton

Abstract | Numerous epidemiological studies have consistently linked the presence of chronic obstructive pulmonary disease (COPD) to the development of lung cancer, independently of cigarette smoking dosage. The mechanistic explanation for this remains poorly understood. Progress towards uncovering this link has been hampered by the heterogeneous nature of the two disorders: each is characterized by multiple sub-phenotypes of disease. In this Review, I discuss the nature of the link between the two diseases and consider specific mechanisms that operate in both COPD and lung cancer, some of which might represent either chemopreventive or chemotherapeutic targets.

REVIEWS

NATURE REVIEWS | CANCER ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Cancer | AOP, published online 7 March 2013; doi:10.1038/nrc3477

© 2013 Macmillan Publishers Limited. All rights reserved

mailto:[email protected]

Proximal lung cancersProximal refers to cancers arising in the large airways (bronchi) that are easily accessible by bronchoscopy. These lesions are most commonly squamous cell carcinoma or small-cell lung cancer by histology.

Distal lung cancersDistal cancers arising in the peripheral zones of the lung are more difficult to sample using bronchoscopy. These lesions are most commonly adenocarcinoma by histology.

healthcare systems worldwide16. Despite the increased attention, the nature of the link between COPD and lung cancer remains obscure. This is due, in large part, to the seemingly polar opposite nature of the two dis-eases. Emphysema is characterized by the destruction of matrix structures, epithelial cell death and a vanish-ing blood supply (alveolar capillary dropout). By stark contrast, cancers display the ability to avoid apopto-sis, proliferate uncontrollably and create new vascular networks. However, as discussed below, there are key shared mechanisms that may represent links between these two diseases, and potentially chemopreventive or chemotherapeutic targets.

Emphysema and airflow obstruction confer riskSeveral studies subsequent to Skillrud’s seminal report were able to confirm the original findings that COPD increased cancer risk, but were unable to pinpoint which aspects of COPD were involved. Fortunately, the increas-ing use of computed tomography (CT) imaging of the chest provided clinicians with a tool that could non- invasively identify the presence of one key aspect of COPD — emphysema. Subsequently, researchers began to use this methodology to sub-phenotype COPD subjects into those with emphysema or airflow obstruction, or those with both.

Wilson and colleagues17 used a patient cohort that was defined in this way to demonstrate that the presence of emphysema, even when controlled for airflow obstruc-tion, conferred an increased risk of lung cancer17. Notably, most of these studies also demonstrate a risk for declining forced expiratory volume in 1 second (FEV1), although it remains unclear whether this is a surrogate marker for emphysema, or, more likely, whether components of COPD other than emphysema confer a proportion of the risk. These findings have been reproduced in four independent studies18–21, and have not been refuted, with the caveat that the presence of radiographic emphysema was determined by semi-quantitative scoring by radiolo-gists, and not by automated software programs that fail to detect the presence of mild emphysema22,23. One of these studies also demonstrated an increase in lung cancer mortality, in addition to increased incidence21.

Unlike COPD, emphysema has a strict anatomical definition: the permanent enlargement of the periph-eral airspaces of the lung distal to the terminal bronchioles. The pathology of emphysema can be char-acterized by the presence of proteinases in excess of their inhibitors, increased apoptosis of lung epithelial and endothelial cells, excessive oxidative burden, and an inflammatory cell infiltrate composed of CD4+ and CD8+ lymphocytes, as well as macrophages and neutrophils24. The airways disease component of COPD consists of a similar inflammatory cell infiltrate, mucous hypersecretion, sub-epithelial cell fibrosis and airway wall thickening25.

It remains unclear whether airways disease and air-space disease confer a regional or a global risk of lung cancer. Typically, airways disease and smoking exposure would be associated with proximal lung cancers, such as SCCA and SCLC. Similarly, emphysema, being a periph-erally located disease, would assumedly nurture ADCA development (distal lung cancers). Surprisingly, the lim-ited data available on the subject suggest that emphy-sema confers a greater risk for SCLC and SCCA, than it does for ADCA20. Further studies with more carefully phenotyped subjects will be required to definitively answer this question.

Given the fact that COPD predisposes one to lung cancer, emerging studies are now examining how the key pathological features of the airways disease and airspace disease in COPD may lead to lung cancer development (FIG. 1). The leading candidates are considered below.

Mechanisms linking COPD and lung cancerGenetic predisposition. COPD and lung cancer are prime examples of the gene-by-environment theory of disease pathogenesis. The responsible environmental exposure, tobacco cigarette smoke, has been well documented for both. However, only a small proportion of smokers will develop either disease (or both), strongly implying a role for genetic susceptibility to disease (TABLE 2). There are only two clearly established genetic causes of COPD, and both of them cause an emphysema-predominant pheno-type. One is cutis laxa, an inherited defect that results in decreased elastic fibre content in the lungs and skin26, and is sufficiently rare not to be discussed here. The other is

At a glance

•Numerousepidemiologicalstudieshaveconsistentlydemonstratedanincreasedincidenceoflungcancerinpatientswhohavechronicobstructivepulmonarydisease(COPD).

•TheemphysemacomponentofCOPD,whichischaracterizedbyexcessiveinflammationandmatrixdestruction,issufficienttoconferanincreasedriskforlungcancer.Takentogether,theepidemiologicalliteraturesuggeststhatentitiesinvolvedintheairwaysandairspacecomponentsofCOPDarebothoperativeinincreasinglungcancer risk.

•BothCOPDandlungcancerinvolveasubstantialroleforgeneticsusceptibilitytodisease,asonlyaminorityofchroniccigarettesmokerswilldevelopone,orboth,ofthediseases.Severalsinglenucleotidepolymorphisms(SNPs)incandidategenefamilies(forexample,detoxifyingenzymes,proteinases,anti-proteinasesandcytokines)havebeenimplicatedindiseasepathogenesisforbothCOPDandlungcancer,andmayconferaproportionofthe risk.

•Theoxidantandnoxiousstressencounteredinthelungsofcigarettesmokersisoverwhelming.Thesespeciescausesufficientdamagetosomeepithelialcellssuchthattheyundergoapoptosis,resultinginemphysema.TheyadditionallyrepresentgenotoxicstresscapableofDNAadductformation,therebypromotingtheearlieststagesofcarcinogenesis.

•InflammatorycellinfiltratesarecommontobothCOPDandlungcancer.Theirquantityandqualitymustbetakenincontext.Inflammationencounteredinemphysemaistypicallycytotoxicanddestructivetomatrixstructures.Suchcellswouldnotbeexpecttopromotethegrowthofanexistingtumour,butwouldprovidethenecessarygenotoxicstressfortumourinitiation.Onceformed,smalltumourspolarizeimmunecellstoalternativelyactivatedphenotypes,whichpromotetumourgrowthandangiogenesis.

•Matrix-degradingenzymes,especiallythosecapableofdegradingelastin(elastases)areessentialforthedevelopmentofemphysema.Manyoftheseenzymeshavebeenshowntopromotelungtumourgrowthbyavarietyofmechanisms,includingenhancedcellularproliferationandincreasedangiogenesis,whichpermitsendovascularinvasion.Therefore,theseenzymesarelikelytorepresentaproportionofthelinkbetweenemphysemaandlung cancer.

•AsoperativemechanismslinkingCOPDtolungcancerarediscovered,theopportunityforchemopreventionwillarise.Ideally,newtherapieswillbedevelopedthathavetheabilitytoretardCOPDprogressionwhilereducinglungcancer risk.

R E V I E W S

2 | ADVANCE ONLINE PUBLICATION www.nature.com/reviews/cancer


A1ATThere are more than 75 known mutations in SERPINA1. The normal allele has been designated M. The most commonly encountered abnormal alleles are Z and S. A1AT deficiency is most commonly seen in ZZ or SZ subjects. Common carrier states include MZ and MS.

a hereditary deficiency of α1 antitrypsin (A1AT; encoded by SERPINA1). A1AT is the physiological inhibitor of neutrophil elastase (NE; also known as HLE (encoded by ELANE)), which is a potent neutrophil-derived proteinase that is capable of degrading elastin, the ‘rubber band’ protein that provides the lung its ability to recoil follow-ing inhalation27. Unfortunately, functional elastic fibres cannot be generated after adolescence, and so emphysema is thought to be an irreversible process28. Of the original five subjects identified as A1AT-deficient, three also had emphysema29. This observation formed the cornerstone of the proteinase–antiproteinase hypothesis of emphy-sema, which holds that when the proteinase burden in the lungs exceeds that of the antiproteinase shield (A1AT), elastin degradation and emphysema result30. Subsequent studies using the intratracheal administration of elasto-lytic enzymes into the lungs of rodents validated this hypothesis by reproducing key features of emphysema pathology31.

It remains unclear whether A1AT deficiency pro-motes lung tumorigenesis. Patients homozygous for the condition typically develop severe emphysema with reduced (or no) cigarette smoke exposure, and have shortened lifespans32. Therefore, these subjects benefit from a limited carcinogen exposure that places them at a low risk for cancer, and that possibly masks genetic susceptibility. However, there is evidence that carriers for A1AT deficiency are subject to a 70% increased risk for lung cancer33. Additionally, the imbalance of exces-sive NE activity coupled with reduced A1AT inhibitory capacity has been shown to correlate with lung cancer risk34. Mechanistically, A1AT deficiency could contrib-ute to lung tumorigenesis through increased NE activ-ity or due to the loss of the intracellular pro-apoptotic properties of A1AT35.

Genetic-mapping studies have identified several sin-gle nucleotide polymorphisms (SNPs) that are associated with both COPD and lung cancer. Many of these reside in

Table 1 | Studies linking COPD and lung cancer*

Study Number of participants

Outcome FEV1 (% predicted)‡ Emphysema

Skilrud et al.4 226 Incidence Cancers in 8.8% of cases (FEV1

<70%) versus 2.0% of controls (FEV

1 >85%); P = 0.024

NA

Tockman et al.5 4,395 Mortality •Cohort 1: RR 4.85 for FEV1 <60%

versus >60%; P = 0.002•Cohort 2: RR 2.72 for FEV

1

60–85% versus >85%; P = 0.043

NA

Speizer et al.7 8,427 Mortality Quartile-based FEV1 analysis

confers cancer risk (RR 2.0–8.27)NA

Lange et al.6 13,946 Mortality •RR 2.1 (95% CI 1.3–3.4) for FEV1

40–79% versus >80%•RR 3.9 (95% CI 2.2–7.2) for FEV

1

<40% versus >80%

NA

de Torres et al.19 1,166 Incidence RR 2.89 (95% CI 1.14–7.27) for FEV

1/FVC ratio <70% versus

>70%

Semi-quantitative radiographic emphysema, RR 3.13 (95% CI 1.32–7.44)

Wilson et al.17 3,638 Incidence OR 2.09 (95% CI 1.33–3.27) for any GOLD stage (FEV

1/FVC <70%)

Semi-quantitative radiographic emphysema, OR 3.56 (95% CI 2.21–5.73). After controlling for airflow obstruction, OR 3.14 (95% CI 1.91–5.15) for radiographic emphysema

Li et al.20 1,015 Incidence NA Semi-quantitative radiographic emphysema. Any = OR 2.79 (95% CI 2.05–3.81), >5% = 3.80 (95% CI 2.78–5.19), >10% = OR 3.33 (95% CI 2.30–4.82)

Zulueta et al.21 9,047 Mortality NA Semi-quantitative radiographic emphysema, HR 1.7 (95% CI 1.1–2.5); P = 0.013

Maldanado et al.23 1,520 Incidence Cancer risk conferred by decreasing FEV

1, OR 1.15 (95% CI

1.00–1.32; P = 0.046); and FEV1/

FVC <70%, OR 1.29 (95% CI 1.02–1.62; P = 0.0310)

Automated volumetric determination of radiographic emphysema was not associated with lung cancer risk, OR 1.042 (95% CI, 0.816–1.329; P = 0.743)

CI, confidence interval; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; GOLD, Global Initiative for Chronic

Obstructive Lung Disease; HR, hazard ratio; NA, not applicable; OR, odds ratio; RR, relative risk. *All studies controlled for age and cigarette consumption. ‡The FEV

1 is reported as the percentage that would be predicted for that individual based on parameters

that are known to influence the FEV1, such as gender, age, height and race.

R E V I E W S



Nature Reviews | Cancer

Normal alveoli

Alveoli with emphysema

Normal bronchi

Bronchitis

candidate gene families, such as proteinases, detoxifying enzymes and inflammatory cytokines (discussed below). Other genes implicated in COPD pathogenesis in recent linkage and genome-wide association studies (GWASs) include SERPINE2, Hedgehog interacting protein (HHIP) and family with sequence similarity 13, member A (FAM13A)36–38. The majority of GWASs carried out for COPD or lung cancer have identified the same locus on chromosome 15q25 that maps to CHRNA3 and CHRNA5, which encode nicotinic acetylcholine receptors38–41. Although there is some evidence that nicotine could be involved in lung cancer and COPD pathogenesis, it is dif-ficult to ignore the fact that CHRNA3 and CHRNA5 are also associated with cigarette smoke consumption and nicotine dependence. As all of the diseases that have been linked to CHRNA3 and CHRNA5 — COPD, lung cancer, peripheral vascular disease and nicotine dependence — are disorders that can arise as a result of cigarette smok-ing, it is quite possible that they are simply functioning as surrogates for tobacco smoke exposure.

Epigenetic changes may link COPD and lung can-cer through common methylation markings and sub-sequent changes in gene expression that are induced by cigarette smoking. DNA hypermethylation has been implicated in the altered expression of numer-ous oncogenes and tumour suppressors in lung can-cer, most notably for RAS association domain family member 1 (RASSF1A), O-6-methylguanine-DNA methyltransferase (MGMT), cyclin-dependent kinase inhibitor 2A (CDKN2A), retinoblastoma 1 (RB1), glutathione S-transferase (GSTP1) and transforming growth factor-β receptor II (TGFBRII)42–48. Of these, CDKN2A, which encodes the tumour suppressors p16 (also known as INK4A) and ARF, represents a common methylation mark between COPD and lung cancer42. The first genome-wide epigenetic study in COPD sub-jects was recently completed and identified 349 CpG sites that were significantly associated with COPD49. Many of these sites have previously been reported to confer lung cancer risk.

Box 1 | COPD sub-phenotypes

Chronicobstructivepulmonarydisease(COPD)encompassesanumberofdiseaseentitiesinthelungthatresultinchronicairflowobstruction.Classically,COPDwassubdividedintotwodiseases:chronicbronchitisandemphysema(seethefigure).PhysicianssoonrealizedthatmostpatientswithCOPDsufferedfromacombinationofthetwocomponents,withtherelativecontributionofthetwovaryingfromcasetocase.State-of-the-artdiseasephenotypingofCOPDsubjectsincludesanassessmentofbothcomponents,andtheintegrationofpulmonaryfunctiontests(PFTs),computedtomography(CT)chestassessmentforthepresenceofradiographicemphysema,andhistorytakingofsymptomsandexposures,mostnotablyforcigarettesmoking.DetailedassessmentscarriedoutinthiswayhavedemonstratedthatsomepatientswithemphysemahavenormalPFTs(technicallythesepatientsdonothaveCOPD)andthatsomepatientswithsevereCOPDdonothaveanyevidenceofemphysema.Ofcourse,patientsmostcommonlyexhibitacombinationofthetwodisorders.Keycomponentsofthisanalysisinclude:•Forcedvitalcapacity(FVC).Thetotalvolumeofairthatcanbeexhaledfollowingafullinspiration.Resultsexpressedinlitresandpercentagepredicted.Avalueof<80%predictedwouldbeconsideredabnormal.

•Forcedexpiratoryvolumein1 second(FEV1).Thetotalvolumeofairthatcanbeexhaledin1 secondfollowingafull

inspiration.Resultsexpressedinlitresandpercentagepredicted.Avalueof<80%predictedwouldbeconsideredabnormal.DiseaseseverityinCOPDisdeterminedbythisvalue,andisthebasisforGlobalInitiativeforChronicObstructiveLungDisease(GOLD)classification,whichisusedinclinicalstudies.

•FEV1/FVCratio.Thepercentageofthetotalvolumeofairexhaledinthefirstsecondofexhalation.Avalueof<70%

indicatesthepresenceofairflowobstructionandisthemajorcriterionforadiagnosisofCOPD.Mousemodelsdisplaythesamelevelofdiseaseheterogeneityashumans.Thestate-of-the-artmousemodeltostudy

emphysemapathogenesisiscigarettesmokeexposure.Thismodeldisplaysmanyofthekeyfeaturesofemphysema,includingpermanentenlargementoftheperipheralairspacesandacharacteristicinflammatorycellinfiltrate.However,themodeldoesnotrecapitulatetheairwaysdiseaseorbronchitisaspectofhumanCOPD.Thisismostlyduetoanatomicaldifferencesbetweenmiceandhumans,withmicehavingjustfivegenerationsofbranchingairwayscomparedwith>20inhumans.Additionally,micedonotdisplaymucoushypersecretioninresponsetocigarettesmoke.Onthebasisofrecentapproaches,itislikelythatnovelgeneticmodelswillbeabletoinducemucoussecretioninthemouse,which,whencoupledwithcigarettesmokeexposure,willmorecloselyresemblehumanCOPD.However,itisunlikelythatsuchmanipulationswilleverbeabletoreproducethecomplexityofairwaysdiseaseinhumanCOPD.

R E V I E W S




Genetics:• Process oxidant or noxious stress • EPHX, CYPs, MPO and NRF2

Cell cycle regulation:• Avoid apoptosis • Uncontrolled proliferation

Inflammation:• Field propagation • Cytotoxic versus growth promoting

Cytokines:• NF-κB activation • Regulate tumour microenvironment

Proteinases:• Matrix degradation • Release growth factors

?

COPD COPD with cancer

Tumour

Fields of injury. Lung epithelial and endothelial cell apop tosis is a key feature of emphysema pathogene-sis50,51, and is seemingly the polar opposite of the uncon-trolled cellular proliferation that is observed in cancer. However, viewed another way, the inability of some lung cells to arrest or apoptose, owing to mutation or epigenetic changes in the cell cycle regulatory machin-ery (for example, TP53 or CDKN2A), might increase the population of lung epithelial cells from which tumours could arise. Debate persists as to whether apoptosis is a primary event in response to cigarette smoke, or whether it is secondary to cigarette smoke-induced inflammation and matrix destruction52. Regardless, it is universally accepted that lung epithelial cells are exposed to many pro-apoptotic stimuli in COPD, and that lung cancers arising in patients with COPD display a common cancer hallmark: increased resistance to pro-apoptotic stimuli53.

Whether apoptosis in emphysema is induced by the toxic and carcinogenic effects of cigarette smoke (a direct or primary effect) or the indirect (secondary) effects that are mediated by the host’s cytokine-rich inflammatory response to smoke, or both, is important to consider because injury to lung epithelial cells by the primary and secondary effects of cigarette smoke is associated with ‘field cancerization’, which is applicable to both COPD and lung cancer54. The concept of a field of cells at risk was first used to explain the presence of tissue that appeared histologically normal adjacent to oral cancers that shared some of the same molecular alterations as the cancer itself 55. With respect to the lung, there are in essence two fields at risk — airway epithelium and peripheral airspace (alveolar) epithelium (FIG. 2). A single clone with a compromising genetic mutation (in TP53, for example) could simply repopulate large portions of the airway epithelium independently of inflammation, and this has been described56. By contrast, synchronous primary lung cancers in humans typically demonstrate generally different mutation profiles, suggesting that the

cancers arose in the same field of injury, but from dif-ferent cells of origin57. Most commonly, the combined effects of cigarette smoke create large fields of injury in which unique clones featuring commonly identified genetic alterations (such as those in epidermal growth factor receptor (EGFR), KRAS and TP53) are thought to expand regionally58,59. Such expansion provides a larger number of cells in which additional mutations could occur, and increases the odds that a given cell could attain the required number of mutations to become malignant. Thus, airway epithelia in smokers share genetic similari-ties and differences with neighbouring regions of injury based on expanding clones in the same field.

Developing neoplastic lesions also have the capacity to expand and alter the field in which they originated. Tumours developing in the Lox–Stop–Lox (LSL)-KrasG12D mouse model of lung cancer synthesize and release numerous pro-inflammatory cytokines and chemokines, including CXC motif chemokine ligand 1 (CXCL1), CXCL2, CXCL5, TGFβ and CC motif chemokine ligand 2 (CCL2; also known as MCP1)60. This results in the recruit-ment of myeloid cells to sites of tumorigenesis where they promote lung tumour growth61. Additionally, these macrophages and neutrophils represent an added source of reactive oxygen species (ROS) that pose additional genotoxic stress on cells residing in the field.

Lung airway epithelial cells tend to follow a classical pattern of tumour development that progresses from metaplasia to dysplasia to carcinoma in situ and finally to invasive carcinoma (SCCA), with each of these events occurring in the field at risk. Genomic analysis of histo-logically normal airway epithelial cells in smokers at risk for lung cancer can distinguish between those who have lung cancer and those who do not62. Furthermore, signatures of PI3K hyperactivity can be found in both dysplastic precursor lesions and in mature lung cancers, highlighting a key early event63. However, this signature does not confer risk of COPD.

Figure 1 | Candidate mechanisms linking COPD to lung cancer. There are several mechanisms associated with the presence of chronic obstructive pulmonary disease (COPD) that might be associated with the development of lung cancer. These include inflammation and associated cytokines, smoking, alterations to cell cycle regulation and the presence of specific proteinases produced by immune cells and other stromal cells. Genetic and epigenetic changes are also likely to confer a risk of developing one or both diseases. CYP, cytochrome P450; EPHX, epoxide hydrolase; MPO, myeloperoxidase; NF-κB, nuclear factor-κB; NRF2, nuclear factor erythroid 2-related factor 2.

R E V I E W S



Bronchoalveolar lavage(BAL). To sample the contents of the distal lung, physicians insert a bronchoscope into the smallest airway in which it can fit. Once ‘wedged’ into position, saline is infused into the lung, followed by suctioning to return the fluid, which now contains cells and proteins.

A similar paradigm has been suggested for the devel-opment of lung ADCA. This model suggests that alveolar epithelial cells progress to atypical alveolar hyperplasias (AAHs), adenomas and finally to ADCA. This stepwise progression is clearly visible in mice with activating muta-tions in Kras64. In this case, the pathological features are attributable to the cell of origin, known as the broncho-alveolar stem cell (BASC)65. BASCs represent a potential link between emphysema and cancer because they would be under substantial pressure to replenish lung epithelial cells that have apoptosed as a result of emphysema. Such increased proliferation of stem cells is thought to increase the risk of them acquiring pro-tumorigenic mutations. Although the evidence for this hypothesis is strong in mouse models, it remains unclear whether it is operative in human lung ADCA in general. However, the identifica-tion of mutations in EGFR, KRAS and TP53 in both AAH and ADCA, supports the concept that these cancers progress in a stepwise manner from a common cell of origin66.

Inflammation. Essentially all smokers develop mac-rophage and neutrophil infiltration in their lungs, which is itself not sufficient to cause COPD67,68. Patients with COPD develop more pronounced inflammation when compared with smokers without COPD, the extent of which posi-tively correlates with disease severity69. It is noteworthy that the destructive inflammatory cell infiltrates that are operative in emphysema are found both in the airways and in the airspaces, and therefore could contribute to the development of both proximal and distal lung cancers.

Chronic inflammation is also a common feature in lung cancer70. At first glance, the inflammatory cell profiles seem to be similar in the two disorders, being comprised of macrophages, neutrophils, and CD4+ and CD8+ lymphocytes. Closer inspection, however, suggests

that the characteristics of the immune cells identified in COPD differ from those found in lung cancer. Analysis of bronchoalveolar lavage (BAL) fluid from COPD sub-jects suggests that these cells are polarized towards a T helper 1 (TH1) phenotype, as shown by cell surface markers and substantial interferon-γ (IFNγ) production71. This would suggest that alveolar macrophages in patients with COPD would be skewed towards an M1 phenotype, which has not been clearly demonstrated. Lung macro-phages are found in abundance in COPD, and display a mixed phenotype of both M1 and M2 markers72,73. However, all smoke-exposed lung macrophages damage the lung matrix in that proteinase expression is fairly uni-form, and seems to be independent of whether macro-phages are of an M1 or an M2 phenotype. A simplified view of the inflammatory infiltrate in COPD is that IFNγ that is produced by TH1 cells elicits the production of IFNγ-inducible cytokines from neighbouring CD8+ lym-phocytes74. These cytokines, which include CXCL10 (also known as IP10), have been shown to induce the expres-sion of macrophage elastase (also known as MMP12) from macrophages through interaction with CXCR4. Release of interleukin-8 (IL-8) from resident alveolar macrophages and lung epithelial cells is jointly responsible for the pres-ence of neutrophilic infiltrates in patients with COPD75. Recent studies also point to an important role for TH17 lymphocytes in the propagation and the maintenance of inflammation in COPD76,77.

Although a TH1 or cytotoxic profile would be desir-able in the tumour microenvironment, it is rarely encountered78. Most solid tumours have an immune cell infiltrate polarized towards a TH2 phenotype and the corresponding alternatively activated M2 macrophage79. Copious monocytes and neutrophils at various stages of development (often referred to as myeloid-derived sup-pressor cells (MDSCs)) are frequently encountered in lung cancer, and represent a commonality to COPD80–82. Whether from the activity of MDSCs, regulatory T cells (TRegs), co-regulatory molecules or other factors, the net result is the suppression of cytotoxic T lymphocyte func-tion and enhanced tumour viability83–85. As has been done for COPD, recent studies have explored the role of TH17 lympho cytes in solid cancers, although whether these cells promote or suppress tumour development is unclear86,87.

The nature of inflammatory cells surrounding lung cancers that reside in emphysematous lungs is the result of competing disease microenvironments (FIG. 3), and must be considered in context. The cytotoxic environ-ment identified in COPD would suppress existing cancers, but it is conducive to tumour initiation. ROS supplied by macrophages and neutrophils provide the necessary geno-toxic stress for DNA adduct formation and subsequent genetic mutation88. Once formed, early stage neoplasms might alter the surrounding microenvironment by releas-ing cytokines and chemokines (such as tumour necrosis factor-α (TNFα), IL-1β and IL-6) that skew the immune cell composition89. Nuclear factor-κB (NF-κB) activation in cancers and myeloid cells is a key step in this process90 that results in the polarization of existing immune cell populations to an alternative phenotype, as well as having direct effects on the tumour cells.

Table 2 | Genetic susceptibilities to COPD and lung cancer

Gene COPD Lung cancer

SERPINA1 MZ heterozygotes associated with COPD (P = 0.04)151

A1AT carrier rate (12.3%) exceeded expected control rate (P = 0.002)152

MMP1 Combined MMP1 and MMP12 SNPs associated with rapid decline in lung function115

MMP1 promoter SNP associated with lung cancer risk (OR 1.8; 95% CI 1.3–2.4)153

CYP1A1 Homozygous *2A allele significantly higher in severe COPD (P <0.01)154

M1 homozygous genotype found in 4.10% cancers versus 1.69% controls155

EPHX1 Increased COPD risk for exon 3 variant both as heterozygote (OR 3.0; 95% CI 1.2–7.1) and homozygote (OR 2.4; 95% CI 1.1–5.1)156,157

Lung cancer risk associated with high EPHX activity (P <0.02)158

CHRNA3 and CHRNA5

CHRNA3 and CHRNA5 locus significantly associated with both radiographic emphysema (P <0.0002) and airflow obstruction (P = 0.004)38

CHRNA3 and CHRNA5 locus strongly associated with lung cancer in three independent studies40

MPO NA Reduced risk (OR 0.5; 95% CI, 0.29–0.88) of lung cancer with A/G allele (reduced expression)93

CHRNA3, cholinergic receptor, neuronal nicotinic, α-polypeptide 3; CI, confidence interval; COPD, chronic obstructive pulmonary disease; CYP1A1, cytochrome P450 subfamily 1, polypeptide 1; EPHX1, epoxide hydrolase 1; MMP, matrix metalloproteinase; MPO, myeloperoxidase; MZ, individuals that have one normal allele of SERPINA1 and a commonly encountered abnormal allele designated Z; NA, not applicable; OR, odds ratio; SNP, single nucelotide polymorphism.

R E V I E W S




Cancer

Mixed populations withfields expanded byinflammation

Emphysemawith AAH

Clonalexpansion

Emphysema

Mixedpopulations

Normalalveoli

T cellMacrophage

Neutrophil

Inhalation ofcigarette smoke

AAH

Oxidative and noxious stress. Tobacco cigarette smoke delivers >4,000 distinct chemicals to the lungs with each breath. Several of these substances possess harmful or addictive properties, including nicotine, carbon mon-oxide, 4-methylnitrosamino-1-[3-pyridyl]-1-butanone (NNK), N-nitroso derivatives and polycyclic aromatic hydrocarbons (PAHs). Xenobiotic metabolism is the process by which host enzymes detoxify and eliminate these noxious agents from the system91. The first step is activation of the agent (by oxidation, reduction or hydrolysis) by phase I enzymes, which include haem oxygenase 1 (HO1), myeloperoxidase (MPO), members of the cytochrome P450 (CYP) family and microsomal epoxide hydrolase (EPHX) family. This is followed by conjugation by phase II enzymes (most notably GSTs), which allows excretion.

Although these systems exist to detoxify foreign sub-stances, they can occasionally transform relatively harm-less molecules into harmful carcinogens. For example, PAHs (such as benzo-a-pyrene) are not themselves toxic.

Unfortunately, members of the CYP family and EPHX1 can generate metabolites of PAHs that are carcinogenic92. As such, functional SNPs in some of these enzymes alter the risk of COPD and/or lung cancer development, and probably represent a component of genetic susceptibility to both diseases (TABLE 2).

MPO, found only in myeloid cells, is another phase I enzyme that is implicated in the inadvertent generation of toxic metabolites. Accordingly, a lower functioning SNP in MPO confers protection against lung cancer93. An additional function of MPO is the generation of the unique oxygen radical hypochlorous acid (HOCl−), which additionally contributes to the oxidative stress provided by cigarette smoke-induced inflammatory cell infiltrates.

The burden of ROS placed on lung epithelial cells in the lungs of smokers is overwhelming. Direct effects of cigarette smoke coupled with macrophage- and neutrophil-derived ROS combines to place high genotoxic and apoptotic stress on lung cells. The genes that encode enzymes that can reduce this stress (such as catalase) con-tain an antioxidant response element, which is activated by the transcription factor, nuclear factor erythroid 2-related factor 2 (NRF2)94. Nrf2−/− mice display increased epithelial cell apoptosis and emphysema compared with wild-type controls when exposed to cigarette smoke95. Therefore, augmentation of NRF2 activity would seem to be an ideal strategy to slow emphysema pathogenesis while reduc-ing the genotoxic stress and the tumour-initiating burden placed on lung epithelia in patients with COPD. There are studies in Nrf2-deficient mice using primary and metastatic tumours that support this strategy96. On the basis of such results, drugs that enhance NRF2 activity have been developed as novel chemopreventive agents97,98. Unfortunately, the role of NRF2 in cancer has proved any-thing but straightforward, with pro-tumour functions also reported99, and recently reviewed in depth100. Most con-cerning are the recent reports that activating mutations in NRF2 confer resistance to conventional chemotherapy and correlate with poor clinical outcomes, specifically in NSCLC101. In this regard, NRF2 function is similar to many of the key entities that potentially link COPD and lung cancer. It has a different role in tumour initiation, where its expression is associated with cancer prevention, from the role it has in established cancer, where its expres-sion can be growth promoting. Thus, future strategies to manipulate NRF2 function in lung disease will require application in a context-specific manner.

The extracellular matrix and proteinases. Numerous key pathogenic roles have been described for matrix-degrading enzymes in both emphysema and lung can-cer (TABLE 3). Members of the cysteine proteinase, serine proteinase and MMP families have been extensively studied with respect to emphysema pathogenesis, espe-cially those capable of degrading elastic fibres, an essen-tial event in the development of this disease. Important roles for non-elastolytic proteinases in emphysema have also been described, most notably for MMP1 (also known as collagenase I). Additionally, many of these enzymes degrade the inhibitors of one another (A1AT and the tissue inhibitors of metalloproteinases (TIMPs)),

Figure 2 | Fields at risk. A proposed model for lung adenocarcinoma development in the setting of emphysema is shown on the left-hand side of the figure. Emphysema develops in the setting of enhanced inflammation and excess proteinase burden. In this context, a mutation develops (such as in TP53), which expands to form atypical alveolar hyperplasia (AAH). The AAH eventually develops into an adenocarcinoma (shown by blue cells) with assistance from a tumour-promoting inflammatory cell infiltrate. Proposed models for airway epithelial cell-derived cancers are shown on the right-hand side of the figure. Each different coloured cell in the lower airway depicts a unique mutation (purple, green and red cells), all of which occurred in the same field. The middle airway depicts the expansion of a single clone (shown in yellow), which has been described for TP53. The upper airway depicts the most accepted model. In this case, there are single mutation expansions (shown in green, purple and red) and areas where those fields have developed additional mutations (dark purple and green). Each of these fields would be expanded by the presence of surrounding inflammatory cell infiltrates.

R E V I E W S




COPD:• Cytotoxic • Genotoxic• Matrix degrading

• MMP12 • ROS

• NE • ROS

• IL-4 • IL-13

TH1

TH2

CD8

CD8

M1

TH17

TH17

M2

M2

M2

CCL2

IFNγ CXCL10

VEGF

VEGF

Cancer:• Angiogenic • Myeloid suppressive• Growth promoting

IL-8

TGFβ

IL-1β

IL-6

a b

thereby augmenting the others potency102. Some of these proteinases may represent mechanistic links between emphysema and lung cancer by contributing to lung tis-sue destruction in emphysema and by promoting lung tumour growth and invasiveness.

MMP9 (also known as gelatinase B) is produced from numerous sources in the lung, most notably from alveo-lar macrophages. Mmp9−/− mice are not protected from cigarette smoke-induced emphysema103, which might be because mouse MMP9 has reduced elastolytic activity com-pared with the human enzyme. Indeed, MMP9 accounts for a substantial proportion of the elastolytic capacity of human alveolar macrophages104,105. Furthermore, MMP9 activity in BAL fluid correlates with the extent of emphy-sema in human subjects106. Macrophage- and neutrophil-derived MMP9 is essential for tumour angiogenesis in several tumour types107. The extracellular matrix (ECM) sequesters large amounts of vascular endothelial growth factor (VEGF), which becomes biologically available on MMP9 cleavage108. MMP9 has also been reported to gener-ate angiostatic peptides from the ECM, although the pre-dominant effect of the enzyme in mouse models has been tumour promoting109,110.

MMP1 is an interstitial collagenase that contributes to the growth of most solid tumours and promotes metas-tasis formation. Mechanistically, MMP1 increases the bioavailability of ligands for EGFR and degrades matrix structures111,112, enabling tumour invasion. In a study of metastatic lesions obtained from patients with breast

cancer, MMP1 was the most highly expressed gene in the metastatic lesion113. With respect to emphysema, overex-pression of MMP1 in transgenic mice creates the charac-teristic airspace enlargement that is seen in emphysema114. Furthermore, polymorphisms in the MMP1 promoter are predictive of disease severity in patients with COPD115.

MMP12 is a fairly macrophage-specific proteinase that is required for the development of cigarette smoke-induced emphysema in mice116. It is one of the most highly expressed genes in the alveolar macrophages of subjects with COPD, and promoter polymorphisms gen-erating increased MMP12 activity have been associated with disease severity in COPD117. Although proteinase-mediated destruction of lung matrix in emphysema is a concerted effort between the different elastases, the evidence for MMP12 as a pathological contributor is strongest. Unfortunately, MMP12 is one of the rare MMPs that is known to be tumour suppressive, essen-tially rendering it a useless target for the treatment of lung cancer118,119. MMP12 generates angiostatic peptides from precursor proteins, most notably angiostatin from plasminogen and endostatin from type XVIII collagen120.

NE probably represents part of the link between COPD and lung cancer. Its role in emphysema patho-genesis has been known for decades121, though it is now recognized that NE simply contributes to elastic fibre degradation, rather than being solely responsible for it122. NE has recently been shown to promote lung tumour growth in a Kras mouse model of lung adenocar-cinoma123. NE enters tumour endosomes and degrades a target substrate, insulin receptor substrate 1 (IRS1). Depletion of cellular IRS1 affects PI3K signalling, which is activated by a number of growth factors associated with tumour progression. Whether NE predicts poor outcomes in human lung cancer is not yet known. However, NE activity has been correlated with disease progression in invasive breast cancer124,125.

Proteinases can also affect disease pathogenesis in an indirect manner by generating unique matrix frag-ments that display novel biologically active properties on cleavage. Such matrix fragments have been termed matrikines. With respect to emphysema, matrix frag-ments for elastin, laminin and collagen have all been reported to possess chemotactic properties for inflam-matory cells. More specifically, elastin fragments recruit macrophages126, and both laminin 5 fragments127 and the collagen tripeptide repeat pro-gly-pro (PGP) recruit neutrophils128. Inhibition of these peptide fragments sig-nificantly alters inflammatory cell composition in vivo, suggesting that matrikines rival the more highly recog-nized cytokines and chemokines for the maintenance of chemotactic gradients in disease. Matrikines almost certainly influence the inflammatory cell composition surrounding tumours, although reports of direct effects of these matrix fragments on cancer cells are lacking.

Populations at riskScreening. The National Lung Screening Trial (NLST) was a large, multicentre, clinical trial that compared chest CT screening for lung cancer versus conventional care (no screening). The results, published in 2011,

Figure 3 | Immune cell profiles in COPD and lung cancer. a |The immune cell phenotype in chronic obstructive pulmonary disease (COPD) is T helper 1 (T

H1)

predominant, as shown by cell surface markers and interferon-γ (IFNγ) production. CD8+ T lymphocytes release IFNγ-inducible cytokines that affect macrophage function, which are of a mixed phenotype. Neutrophils release reactive oxygen species (ROS) and granular contents enhancing tissue damage. b | Tumours secrete biologically active molecules at an early stage, which polarize immune cells towards a T

H2 phenotype, which is characterized

by interleukin-4 (IL-4) production. Macrophages are typically of the alternatively activated M2 phenotype, which promote tumour growth and angiogenesis. Myeloid cells (neutrophils and monocytes) of varying stages of development contribute to lung cancer growth both directly (releasing growth-promoting substances) and indirectly by suppressing cytotoxic T lymphocyte function. CXCL10, C-X-C motif chemokine 10; MMP, matrix metalloproteinase; NE, neutrophil elastase; TGFβ, transforming growth factor-β; VEGF, vascular endothelial growth factor.

R E V I E W S



MedicareA national health insurance programme in the United States, and the largest payer of health care services in the United States.

ß-agonistInhaled drug that stimulates the ß-adrenergic receptors located on airway epithelial cells. Their stimulation results in the dilation of the airways. Commonly used for the treatment of asthma and chronic obstructive pulmonary disease.

showed a 20% reduction in lung cancer mortality for the screening group129. Despite this, CT screening for lung cancer has not become routine clinical prac-tice, and Medicare in the United States has not revealed whether they will pay for this screening. The three major limitations to this approach are the number of non-cancerous nodules that would require diagnostic work-up, the very large population at risk (smokers) and the costs for each. There is obvious interest in nar-rowing this population at risk, knowing that only one in nine smokers will develop lung cancer. Therein lies the interest in COPD, which defines a population that is at a particularly high risk for lung cancer. Ongoing studies are attempting to improve the current scoring systems for lung cancer risk130, by including factors such as airflow obstruction and radiographic emphysema to the already well-defined risk factors of age, smoking his-tory and asbestos exposure. Additionally, COPD sub-jects represent a population at a sufficiently high risk for use in chemoprevention studies.

Chemoprevention and chemotherapy. Identification of mechanisms linking COPD to lung cancer holds the promise that these can serve as therapeutic targets for COPD and chemopreventive measures for lung cancer. There was enthusiasm over the report that the chronic use of inhaled corticosteroids (ICS) in COPD reduced mortal-ity from lung cancer131. However, large prospective trials failed to demonstrate a survival benefit for the chronic use of ICS with or without a long-acting ß-agonist132.

There are several ongoing clinical trials for lung cancer chemoprevention that might also be relevant to COPD. Interest in cyclooxygenase (COX) signalling as a chemopreventive target is based on its success in other malignancies, data from mouse models133 and the asso-ciation of increased COX2 expression with poor out-comes in NSCLC134. COX2 generates prostaglandin E2 (PGE2) from the membrane phospholipid arachadonic acid. PGE2 promotes carcinogenesis in a variety of ways, including resistance to apoptosis, increased angio genesis and enhanced invasion135. A recent Phase IIb clinical trial using the COX2 inhibitor celecoxib in a high-risk smoking population demonstrated a reduction in Ki-67 (proliferating antigen) labelling index in the bronchial epithelium, which was used as a surrogate for reduced cancer risk136. Unfortunately, celecoxib confers a risk of cardiovascular disease, for which cigarette smokers are already at an increased risk of developing137.

An alternative method to affect arachadonic acid metabolism would be to enhance the production of pros-tacyclin (PGI2), the counterpart to PGE2. Whereas PGE2 is usually highly expressed in cancer, and less highly expressed in normal lung tissue, the opposite is true for PGI2 (REF. 138). As such, prostacyclin has tumour- suppressive functions, and displays anti-proliferative and anti-metastatic properties139. Oral prostacyclin (iloprost) was recently studied in a randomized Phase II study using smokers with endobronchial dysplasia as the study group. The iloprost group had a significant improvement in endobronchial pathology140, which functioned as a

Table 3 | Candidate proteinases in emphysema and lung cancer

Proteinase Source Matrix substrates Promotes emphysema?

Promotes cancer?

Refs

Neutrophil elastase

PMNs Elastin, CI, CIII, CIV, laminin, fibronectin and TIMPs

Yes Yes 122,123

Proteinase 3 PMNs Elastin, CIV, laminin and fibronectin

Yes ? 159

Cathepsin S Macrophages and other cell types

Elastin, CI, CIII, laminin and fibronectin

Yes Yes 160,161

Cathepsin L Macrophages and other cell types


? Yes 162

Cathepsin K Macrophages and other cell types


? ? 163

MMP1 Stromal cells CI, CIII and A1AT Yes Yes 112,114

MMP2 Stromal cells Elastin, CI, CIV, laminin, fibronectin and A1AT

? Yes 164

MMP3 Stromal cells Elastin, CIII, CIV, laminin, fibronectin and A1AT

No Yes 165

MMP8 PMNs CI, CIII and A1AT No No 166

MMP9 Macrophages, PMN and other cell types

Elastin, CI, CIV, laminin and A1AT

Yes Yes 106,107, 109

MMP12 Macrophages Elastin, CI, CIV, fibronectin, laminin and A1AT

Yes No 116,119

MMP13 Stromal cells CI, CIII and CIV No Yes 167

MMP14 Stromal cells and macrophages

CI, CIII, CIV, fibronectin and laminin

? Yes 168,169

CI, collagen type I; CIII, collagen type III; CIV, collagen type IV; MMP, matrix metalloproteinase; PMNs, polymorphonuclear leukocytes; TIMP, tissue inhibitors of metalloproteinase.

R E V I E W S



surrogate end point for lung cancer risk. Additional tri-als with iloprost will be required to demonstrate actual decreases in lung cancer incidence.

Treatment options of proven benefit are limited for patients with established COPD. For example, the only drug that has been shown to prolong survival for COPD subjects is supplemental oxygen, and this has been known for decades141. Recent approaches have focused on path-way hyperactivity, including direct and indirect inhibition of NF-κB. Theophylline indirectly suppresses NF-κB by activating histone deacetylase 2 (HDAC2), which restores sensitivity to ICS in patients with COPD142. Direct target-ing of NF-κB has been shown to reduce airway inflamma-tion and carcinogen-induced cancers in mice143. Clinical investigations in humans are underway.

Patients with NSCLC have benefited from the recent development of targeted therapies, most notably for EGFR144 and anaplastic lymphoma kinase (ALK)145. Although EGFR and ALK mutations are only identi-fied in small subsets of NSCLC, targeted therapies for these pathways are of proven benefit. EGFR inhibition has also been attempted in COPD subjects, as EGF and its related ligands stimulate mucous hypersecretion146. Unfortunately, the initial studies have been negative in this respect. The completion of the recent genome-sequencing studies for SCCA147 and ADCA148 has pro-vided researchers with an improved description of the mutations that occur in lung cancer. Once data emerge identifying which of these represent driving mutations, additional targeted therapies are likely to follow.

Future directionsThe identification of mechanisms linking COPD and lung cancer has long been hampered by the heterogene-ity of the disorders. The recently developed methods to sub-phenotype COPD subjects has allowed researchers to carry out enlightening clinical studies with respect to which aspects of COPD pathogenesis confer lung cancer risk. However, there are still several unanswered questions regarding the links between COPD and lung cancer. For example, does emphysema confer a local-regional risk for lung cancer, or a global one? Which histological sub-types of lung cancer are conferred by emphysema? And which ones are influenced by airways disease? What is the mutational profile of lung cancers arising on a back-ground of COPD? There are few to no data regarding the mutational status or pathway activation of tumours aris-ing in such subjects, and whether they differ with coex-isting COPD. Fortunately, studies using modern disease

sub-phenotyping to answer these questions are already underway, along with a concerted effort to identify novel biomarkers to narrow the population at risk. These studies and answers to the above questions should help to guide the direction of additional mechanistic studies.

A clear limitation of the field is simply that few studies have examined human subjects or used preclinical models in which both diseases are represented. Apart from the few clinical studies assessing lung cancer incidence in COPD subjects, essentially all of the studies reviewed here were carried out in subjects with either COPD or lung cancer, but not both. Additionally, the mechanistic possibilities discussed here were mostly generated in mouse models of cigarette smoke-induced emphysema or cancer, but not both. Development of the necessary preclinical models to simultaneously study both disorders will not be trivial. As discussed in BOX 1, mice are simply not a good model to study the airways disease component of COPD, as a result of the anatomical differences from human airways. Scientists will have to use translational approaches and expand current methodologies using bronchial epithelial cell cultures and primary human lung cancer specimens. Although mouse models of cigarette smoke-induced emphysema and lung cancer can be combined, the results will prove difficult to interpret. The dosages of cigarette smoke required to generate emphysema in mice produce a sufficiently cytotoxic inflammatory cell infiltrate, so that discontinuation of smoking actually accelerates lung tumour growth. Reported by Witschi and colleagues149 in 1997, this observation has been conveniently ignored, but must now be re-addressed if these models are to be used for the combined study of these diseases. Adjustment of cigarette smoke dosage and duration, as well as the care-ful choice of cancer models, should enable investigators to recapitulate the disease microenvironments that exist in human subjects.

Although the increased cancer risk conferred by COPD was the first lung disease to garner attention in this way, it will not be the last. Idiopathic pulmo-nary fibrosis (IPF) carries an approximately sevenfold increased lung cancer risk150. A better understanding of the mechanisms linking IPF and cancer should identify new therapeutic targets, as is the hope for COPD. Finally, there is one unequivocal preventive measure for COPD and lung cancer: smoking abstinence. Although smoking cessation will never lower COPD and lung cancer risk to zero in asymptomatic smokers, it is the only clearcut way to reduce their mortality, and should remain the top clinical priority.

1. Peto, R., Chen, Z. M. & Boreham, J. Tobacco-the growing epidemic. Nature Med. 5, 15–17 (1999).

2. Youlden, D. R., Cramb, S. M. & Baade, P. D. The International Epidemiology of Lung Cancer: geographical distribution and secular trends. J. Thorac. Oncol. 3, 819–831 (2008).

3. Wasswa-Kintu, S., Gan, W. Q., Man, S. F., Pare, P. D. & Sin, D. D. Relationship between reduced forced expiratory volume in one second and the risk of lung cancer: a systematic review and meta-analysis. Thorax 60, 570–575 (2005).

4. Skillrud, D. M., Offord, K. P. & Miller, R. D. Higher risk of lung cancer in chronic obstructive pulmonary disease. A prospective, matched, controlled study.

Ann. Intern. Med. 105, 503–507 (1986).The first study to show increased lung cancer incidence in patients with COPD.

5. Tockman, M. S., Anthonisen, N. R., Wright, E. C. & Donithan, M. G. Airways obstruction and the risk for lung cancer. Ann. Intern. Med. 106, 512–518 (1987).

6. Lange, P., Nyboe, J., Appleyard, M., Jensen, G. & Schnohr, P. Ventilatory function and chronic mucus hypersecretion as predictors of death from lung cancer. Am. Rev. Respir. Dis. 141, 613–617 (1990).

7. Speizer, F. E., Fay, M. E., Dockery, D. W. & Ferris, B. G. Jr. Chronic obstructive pulmonary disease mortality in six U.S. cities. Am. Rev. Respir. Dis. 140, S49–S55 (1989).

8. Sin, D. D., Anthonisen, N. R., Soriano, J. B. & Agusti, A. G. Mortality in COPD: role of comorbidities. Eur. Respir. J. 28, 1245–1257 (2006).

9. Young, R. P. et al. COPD prevalence is increased in lung cancer, independent of age, sex and smoking history. Eur. Respir. J. 34, 380–386 (2009).

10. Pauwels, R. A., Buist, A. S., Calverley, P. M., Jenkins, C. R. & Hurd, S. S. Global strategy for the diagnosis, management, and prevention of chronic obstructive pulmonary disease. NHLBI/WHO Global Initiative for Chronic Obstructive Lung Disease (GOLD) Workshop summary. Am. J. Respir. Crit. Care Med. 163, 1256–1276 (2001).

R E V I E W S



11. Shapiro, S. D. & Ingenito, E. P. The pathogenesis of chronic obstructive pulmonary disease: advances in the past 100 years. Am. J. Respir. Cell. Mol. Biol. 32, 367–372 (2005).

12. Mannino, D. M. Epidemiology and global impact of chronic obstructive pulmonary disease. Semin. Respir. Crit. Care Med. 26, 204–210 (2005).

13. Jemal, A. et al. Global cancer statistics. CA Cancer J. Clin. 61, 69–90 (2011).

14. Travis, W. D. et al. International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society: international multidisciplinary classification of lung adenocarcinoma: executive summary. Proc. Am. Thorac. Soc. 8, 381–385 (2011).

15. Sun, S., Schiller, J. H. & Gazdar, A. F. Lung cancer in never smokers--a different disease. Nature Rev. Cancer 7, 778–790 (2007).

16. Punturieri, A., Szabo, E., Croxton, T. L., Shapiro, S. D. & Dubinett, S. M. Lung cancer and chronic obstructive pulmonary disease: needs and opportunities for integrated research. J. Natl Cancer Inst. 101, 554–559 (2009).

17. Wilson, D. O. et al. Association of radiographic emphysema and airflow obstruction with lung cancer. Am. J. Respir. Crit. Care Med. 178, 738–744 (2008).Definitive study showing that the presence of radiographic emphysema confers lung cancer risk.

18. Ueda, K. et al. Computed tomography-diagnosed emphysema, not airway obstruction, is associated with the prognostic outcome of early-stage lung cancer. Clin. Cancer Res. 12, 6730–6736 (2006).

19. de Torres, J. P. et al. Assessing the relationship between lung cancer risk and emphysema detected on low-dose CT of the chest. Chest 132, 1932–1938 (2007).

20. Li, Y. et al. Effect of emphysema on lung cancer risk in smokers: a computed tomography-based assessment. Cancer Prev. Res. 4, 43–50 (2011).

21. Zulueta, J. J. et al. Emphysema scores predict death from COPD and lung cancer. Chest 141, 1216–1223 (2012).

22. Wilson, D. O. et al. Quantitative computed tomography analysis, airflow obstruction, and lung cancer in the pittsburgh lung screening study. J. Thorac. Oncol. 6, 1200–1205 (2011).

23. Maldonado, F. et al. Are airflow obstruction and radiographic evidence of emphysema risk factors for lung cancer? A nested case-control study using quantitative emphysema analysis. Chest 138, 1295–1302 (2010).

24. Taraseviciene-Stewart, L. & Voelkel, N. F. Molecular pathogenesis of emphysema. J. Clin. Invest. 118, 394–402 (2008).

25. Barnes, P. J. Chronic obstructive pulmonary disease. N. Engl. J. Med. 343, 269–280 (2000).

26. Berk, D. R., Bentley, D. D., Bayliss, S. J., Lind, A. & Urban, Z. Cutis laxa: a review. J. Am. Acad. Dermatol. 66, 842.e1–842.e17 (2012).

27. Brantly, M., Nukiwa, T. & Crystal, R. G. Molecular basis of α-1-antitrypsin deficiency. Am. J. Med. 84, 13–31 (1988).

28. Shapiro, S. D., Endicott, S. K., Province, M. A., Pierce, J. A. & Campbell, E. J. Marked longevity of human lung parenchymal elastic fibers deduced from prevalence of D-aspartate and nuclear weapons-related radiocarbon. J. Clin. Invest. 87, 1828–1834 (1991).

29. Heidelberger, K. P. α-1-antitrypsin deficiency: a review; 1963–1975. Ann. Clin. Lab Sci. 6, 110–117 (1976).

30. Shapiro, S. D. The pathogenesis of emphysema: the elastase:antielastase hypothesis 30 years later. Proc. Assoc. Am. Physicians 107, 346–352 (1995).

31. Gross, P., Pfitzer, E. A., Tolker, E., Babyak, M. A. & Kaschak, M. Experimental emphysema: its production with papain in normal and silicotic rats. Arch. Environ. Health 11, 50–58 (1965).

32. Larsson, C. Natural history and life expectancy in severe α1-antitrypsin deficiency, Pi Z. Acta Med. Scand. 204, 345–351 (1978).

33. Yang, P. et al. α1-antitrypsin deficiency carriers, tobacco smoke, chronic obstructive pulmonary disease, and lung cancer risk. Arch. Intern. Med. 168, 1097–1103 (2008).

34. Yang, P. et al. α1-antitrypsin and neutrophil elastase imbalance and lung cancer risk. Chest 128, 445–452 (2005).

35. Petrache, I. et al. A novel antiapoptotic role for α1-antitrypsin in the prevention of pulmonary emphysema. Am. J. Respir. Crit. Care Med. 173, 1222–1228 (2006).

36. Demeo, D. L. et al. The SERPINE2 gene is associated with chronic obstructive pulmonary disease. Am. J. Hum. Genet. 78, 253–264 (2006).

37. Cho, M. H. et al. Variants in FAM13A are associated with chronic obstructive pulmonary disease. Nature Genet. 42, 200–202 (2010).

38. Pillai, S. G. et al. Loci identified by genome-wide association studies influence different disease-related phenotypes in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 182, 1498–1505 (2010).

39. Thorgeirsson, T. E. et al. A variant associated with nicotine dependence, lung cancer and peripheral arterial disease. Nature 452, 638–642 (2008).One of the original studies linking polymorphisms in CHRNA3 and CHRNA5 to cigarette consumption, nicotine dependence and lung cancer.

40. Hung, R. J. et al. A susceptibility locus for lung cancer maps to nicotinic acetylcholine receptor subunit genes on 15q25. Nature 452, 633–637 (2008).

41. Amos, C. I. et al. Genome-wide association scan of tag SNPs identifies a susceptibility locus for lung cancer at 15q25.1. Nature Genet. 40, 616–622 (2008).

42. Belinsky, S. A. et al. Aberrant promoter methylation in bronchial epithelium and sputum from current and former smokers. Cancer Res. 62, 2370–2377 (2002).

43. Belinsky, S. A. Silencing of genes by promoter hypermethylation: key event in rodent and human lung cancer. Carcinogenesis 26, 1481–1487 (2005).

44. Zhang, H. T. et al. Defective expression of transforming growth factor β receptor type II is associated with CpG methylated promoter in primary non-small cell lung cancer. Clin. Cancer Res. 10, 2359–2367 (2004).

45. Zochbauer-Muller, S. et al. Aberrant promoter methylation of multiple genes in non-small cell lung cancers. Cancer Res. 61, 249–255 (2001).

46. Ohtani-Fujita, N. et al. CpG methylation inactivates the promoter activity of the human retinoblastoma tumor-suppressor gene. Oncogene 8, 1063–1067 (1993).

47. Dammann, R., Takahashi, T. & Pfeifer, G. P. The CpG island of the novel tumor suppressor gene RASSF1A is intensely methylated in primary small cell lung carcinomas. Oncogene 20, 3563–3567 (2001).

48. Belinsky, S. A. et al. Aberrant methylation of p16(INK4a) is an early event in lung cancer and a potential biomarker for early diagnosis. Proc. Natl Acad. Sci. USA 95, 11891–11896 (1998).

49. Qiu, W. et al. Variable DNA methylation is associated with chronic obstructive pulmonary disease and lung function. Am. J. Respir. Crit. Care Med. 185, 373–381 (2012).

50. Kasahara, Y. et al. Inhibition of VEGF receptors causes lung cell apoptosis and emphysema. J. Clin. Invest. 106, 1311–1319 (2000).The semimal report highlighting the importance of cellular apoptosis in the pathogenesis of emphysema.

51. Aoshiba, K., Yokohori, N. & Nagai, A. Alveolar wall apoptosis causes lung destruction and emphysematous changes. Am. J. Respir. Cell. Mol. Biol. 28, 555–562 (2003).

52. Shapiro, S. D. Vascular atrophy and VEGFR-2 signaling: old theories of pulmonary emphysema meet new data. J. Clin. Invest. 106, 1309–1310 (2000).

53. Hanahan, D. & Weinberg, R. A. Hallmarks of cancer: the next generation. Cell 144, 646–674 (2011).The definitive review on the key features that define malignancy.

54. Steiling, K., Ryan, J., Brody, J. S. & Spira, A. The field of tissue injury in the lung and airway. Cancer Prev. Res. 1, 396–403 (2008).

55. Slaughter, D. P., Southwick, H. W. & Smejkal, W. Field cancerization in oral stratified squamous epithelium; clinical implications of multicentric origin. Cancer 6, 963–968 (1953).

56. Franklin, W. A. et al. Widely dispersed p53 mutation in respiratory epithelium. A novel mechanism for field carcinogenesis. J. Clin. Invest. 100, 2133–2137 (1997).

57. Chang, Y. L. et al. Clonality and prognostic implications of p53 and epidermal growth factor receptor somatic aberrations in multiple primary lung cancers. Clin. Cancer Res. 13, 52–58 (2007).

58. Wistuba, I. I. et al. Molecular damage in the bronchial epithelium of current and former smokers. J. Natl Cancer Inst. 89, 1366–1373 (1997).

59. Tang, X. et al. EGFR tyrosine kinase domain mutations are detected in histologically normal respiratory epithelium in lung cancer patients. Cancer Res. 65, 7568–7572 (2005).

60. Ji, H. et al. K-ras activation generates an inflammatory response in lung tumors. Oncogene 25, 2105–2112 (2006).

61. Sparmann, A. & Bar-Sagi, D. Ras-induced interleukin-8 expression plays a critical role in tumor growth and angiogenesis. Cancer Cell 6, 447–458 (2004).This is the seminal report of RAS-induced expression of IL-8. This work confirms that cancers have the ability to manipulate host immune cell function to their benefit.

62. Spira, A. et al. Airway epithelial gene expression in the diagnostic evaluation of smokers with suspect lung cancer. Nature Med. 13, 361–366 (2007).This study validates the concept that cells residing in the field of risk can predict the presence of distal cancers on the basis of their genetic profiles.

63. Gustafson, A. M. et al. Airway PI3K pathway activation is an early and reversible event in lung cancer development. Sci. Transl. Med. 2, 26ra25 (2010).

64. Jackson, E. L. et al. Analysis of lung tumor initiation and progression using conditional expression of oncogenic K-ras. Genes Dev. 15, 3243–3248 (2001).This is the initial report of the LSL-Kras mouse that develops lung adenoma and adenocarcinoma. It is the mouse model that is most commonly used to study lung cancer.

65. Kim, C. F. et al. Identification of bronchioalveolar stem cells in normal lung and lung cancer. Cell 121, 823–835 (2005).

66. Yatabe, Y., Borczuk, A. C. & Powell, C. A. Do all lung adenocarcinomas follow a stepwise progression? Lung Cancer 74, 7–11 (2011).

67. Karimi, R., Tornling, G., Grunewald, J., Eklund, A. & Skold, C. M. Cell recovery in bronchoalveolar lavage fluid in smokers is dependent on cumulative smoking history. PLoS ONE 7, e34232 (2012).

68. Merchant, R. K., Schwartz, D. A., Helmers, R. A., Dayton, C. S. & Hunninghake, G. W. Bronchoalveolar lavage cellularity. The distribution in normal volunteers. Am. Rev. Respir. Dis. 146, 448–453 (1992).

69. Hogg, J. C. et al. The nature of small-airway obstruction in chronic obstructive pulmonary disease. N. Engl. J. Med. 350, 2645–2653 (2004).Definitive study characterizing the quality and quantity of airway inflammation in patients with COPD.

70. Coussens, L. M. & Werb, Z. Inflammation and cancer. Nature 420, 860–867 (2002).

71. Grumelli, S. et al. An immune basis for lung parenchymal destruction in chronic obstructive pulmonary disease and emphysema. PLoS Med. 1, e8 (2004).Seminal study providing a mechanistic basis for the role of lymphocytes in COPD pathogenesis in humans.

72. Shaykhiev, R. et al. Smoking-dependent reprogramming of alveolar macrophage polarization: implication for pathogenesis of chronic obstructive pulmonary disease. J. Immunol. 183, 2867–2883 (2009).

73. Kunz, L. I. et al. Smoking status and anti-inflammatory macrophages in bronchoalveolar lavage and induced sputum in COPD. Respir. Res. 12, 34 (2011).

74. Maeno, T. et al. CD8+ T Cells are required for inflammation and destruction in cigarette smoke-induced emphysema in mice. J. Immunol. 178, 8090–8096 (2007).

75. Mio, T. et al. Cigarette smoke induces interleukin-8 release from human bronchial epithelial cells. Am. J. Respir. Crit. Care Med. 155, 1770–1776 (1997).

76. Shan, M. et al. Lung myeloid dendritic cells coordinately induce TH1 and TH17 responses in human emphysema. Sci. Transl. Med. 1, 4ra10 (2009).

77. Chen, K. et al. IL-17RA is required for CCL2 expression, macrophage recruitment, and emphysema in response to cigarette smoke. PLoS ONE 6, e20333 (2011).

78. de Visser, K. E., Eichten, A. & Coussens, L. M. Paradoxical roles of the immune system during cancer development. Nature Rev. Cancer 6, 24–37 (2006).

79. Mantovani, A., Sozzani, S., Locati, M., Allavena, P. & Sica, A. Macrophage polarization: tumor-associated macrophages as a paradigm for polarized M2 mononuclear phagocytes. Trends Immunol. 23, 549–555 (2002).

80. Gabrilovich, D. I. & Nagaraj, S. Myeloid-derived suppressor cells as regulators of the immune system. Nature Rev. Immunol. 9, 162–174 (2009).

R E V I E W S



81. Fridlender, Z. G. et al. Polarization of tumor-associated neutrophil phenotype by TGF-β: “N1” versus “N2” TAN. Cancer Cell 16, 183–194 (2009).

82. Moghaddam, S. J. et al. Promotion of lung carcinogenesis by chronic obstructive pulmonary disease-like airway inflammation in a K-ras-induced mouse model. Am. J. Respir. Cell. Mol. Biol. 40, 443–453 (2009).

83. Gallina, G. et al. Tumors induce a subset of inflammatory monocytes with immunosuppressive activity on CD8+ T cells. J. Clin. Invest. 116, 2777–2790 (2006).

84. Chen, M. L. et al. Regulatory T cells suppress tumor-specific CD8 T cell cytotoxicity through TGF-β signals in vivo. Proc. Natl Acad. Sci. USA 102, 419–424 (2005).

85. Whiteside, T. L. Immune suppression in cancer: effects on immune cells, mechanisms and future therapeutic intervention. Semin. Cancer Biol. 16, 3–15 (2006).

86. Wang, L. et al. IL-17 can promote tumor growth through an IL-6-Stat3 signaling pathway. J. Exp. Med. 206, 1457–1464 (2009).

87. Martin-Orozco, N. et al. T helper 17 cells promote cytotoxic T cell activation in tumor immunity. Immunity 31, 787–798 (2009).

88. Haqqani, A. S., Sandhu, J. K. & Birnboim, H. C. Expression of interleukin-8 promotes neutrophil infiltration and genetic instability in mutatect tumors. Neoplasia 2, 561–568 (2000).

89. Dinarello, C. A. The paradox of pro-inflammatory cytokines in cancer. Cancer Metastasis Rev. 25, 307–313 (2006).

90. Takahashi, H., Ogata, H., Nishigaki, R., Broide, D. H. & Karin, M. Tobacco smoke promotes lung tumorigenesis by triggering IKKβ- and JNK1-dependent inflammation. Cancer Cell 17, 89–97 (2010).

91. Zhang, J. Y., Wang, Y. & Prakash, C. Xenobiotic-metabolizing enzymes in human lung. Curr. Drug Metab. 7, 939–948 (2006).

92. Dix, T. A. & Marnett, L. J. Metabolism of polycyclic aromatic hydrocarbon derivatives to ultimate carcinogens during lipid peroxidation. Science 221, 77–79 (1983).

93. Feyler, A. et al. Point: myeloperoxidase -463G - a polymorphism and lung cancer risk. Cancer Epidemiol. Biomarkers Prev. 11, 1550–1554 (2002).

94. Nguyen, T., Nioi, P. & Pickett, C. B. The Nrf2-antioxidant response element signaling pathway and its activation by oxidative stress. J. Biol. Chem. 284, 13291–13295 (2009).

95. Rangasamy, T. et al. Genetic ablation of Nrf2 enhances susceptibility to cigarette smoke-induced emphysema in mice. J. Clin. Invest. 114, 1248–1259 (2004).

96. Satoh, H. et al. Nrf2-deficiency creates a responsive microenvironment for metastasis to the lung. Carcinogenesis 31, 1833–1843 (2010).

97. Kensler, T. W. et al. Modulation of the metabolism of airborne pollutants by glucoraphanin-rich and sulforaphane-rich broccoli sprout beverages in Qidong, China. Carcinogenesis 33, 101–107 (2012).

98. Shureiqi, I. & Baron, J. A. Curcumin chemoprevention: the long road to clinical translation. Cancer Prev. Res. 4, 296–298 (2011).

99. DeNicola, G. M. et al. Oncogene-induced Nrf2 transcription promotes ROS detoxification and tumorigenesis. Nature 475, 106–109 (2011).

100. Sporn, M. B. & Liby, K. T. NRF2 and cancer: the good, the bad and the importance of context. Nature Rev. Cancer 12, 564–571 (2012).

101. Solis, L. M. et al. Nrf2 and Keap1 abnormalities in non-small cell lung carcinoma and association with clinicopathologic features. Clin. Cancer Res. 16, 3743–3753 (2010).

102. Liu, Z. et al. The serpin α1-proteinase inhibitor is a critical substrate for gelatinase B/MMP-9 in vivo. Cell 102, 647–655 (2000).

103. Atkinson, J. J. et al. The role of matrix metalloproteinase-9 in cigarette smoke-induced emphysema. Am. J. Respir. Crit. Care Med. 183, 876–884 (2011).

104. Russell, R. E. et al. Release and activity of matrix metalloproteinase-9 and tissue inhibitor of metalloproteinase-1 by alveolar macrophages from patients with chronic obstructive pulmonary disease. Am. J. Respir. Cell. Mol. Biol. 26, 602–609 (2002).

105. Russell, R. E. et al. Alveolar macrophage-mediated elastolysis: roles of matrix metalloproteinases, cysteine, and serine proteases. Am. J. Physiol. Lung Cell. Mol. Physiol. 283, L867–L873 (2002).

106. Vignola, A. M. et al. Sputum metalloproteinase-9/tissue inhibitor of metalloproteinase-1 ratio correlates with airflow obstruction in asthma and chronic bronchitis. Am. J. Respir. Crit. Care Med. 158, 1945–1950 (1998).

107. Nozawa, H., Chiu, C. & Hanahan, D. Infiltrating neutrophils mediate the initial angiogenic switch in a mouse model of multistage carcinogenesis. Proc. Natl Acad. Sci. USA 103, 12493–12498 (2006).

108. Bergers, G. et al. Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis. Nature Cell Biol. 2, 737–744 (2000).

109. Coussens, L. M., Tinkle, C. L., Hanahan, D. & Werb, Z. MMP-9 supplied by bone marrow-derived cells contributes to skin carcinogenesis. Cell 103, 481–490 (2000).

110. Itoh, T. et al. Reduced angiogenesis and tumor progression in gelatinase A-deficient mice. Cancer Res. 58, 1048–1051 (1998).

111. Gupta, G. P. et al. Mediators of vascular remodelling co-opted for sequential steps in lung metastasis. Nature 446, 765–770 (2007).

112. Lu, X. et al. ADAMTS1 and MMP1 proteolytically engage EGF-like ligands in an osteolytic signaling cascade for bone metastasis. Genes Dev. 23, 1882–1894 (2009).

113. Minn, A. J. et al. Genes that mediate breast cancer metastasis to lung. Nature 436, 518–524 (2005).

114. D’Armiento, J., Dalal, S. S., Okada, Y., Berg, R. A. & Chada, K. Collagenase expression in the lungs of transgenic mice causes pulmonary emphysema. Cell 71, 955–961 (1992).

115. Joos, L. et al. The role of matrix metalloproteinase polymorphisms in the rate of decline in lung function. Hum. Mol. Genet. 11, 569–576 (2002).

116. Hautamaki, R. D., Kobayashi, D. K., Senior, R. M. & Shapiro, S. D. Requirement for macrophage elastase for cigarette smoke-induced emphysema in mice. Science 277, 2002–2004 (1997).This was the first study to combine cigarette smoke exposure and gene-targeted mice, which is now commonplace. It was also the first study to demonstrate a key role for MMP12 in cigarette smoke-induced emphysema.

117. Hunninghake, G. M. et al. MMP12, lung function, and COPD in high-risk populations. N. Engl. J. Med. 361, 2599–2608 (2009).

118. Acuff, H. B. et al. Analysis of host- and tumor-derived proteinases using a custom dual species microarray reveals a protective role for stromal matrix metalloproteinase-12 in non-small cell lung cancer. Cancer Res. 66, 7968–7975 (2006).

119. Houghton, A. M. et al. Macrophage elastase (matrix metalloproteinase-12) suppresses growth of lung metastases. Cancer Res. 66, 6149–6155 (2006).

120. Cornelius, L. A. et al. Matrix metalloproteinases generate angiostatin: effects on neovascularization. J. Immunol. 161, 6845–6852 (1998).

121. Kuhn, C., Yu, S. Y., Chraplyvy, M., Linder, H. E. & Senior, R. M. The induction of emphysema with elastase. II. Changes in connective tissue. Lab Invest. 34, 372–380 (1976).

122. Shapiro, S. D. et al. Neutrophil elastase contributes to cigarette smoke-induced emphysema in mice. Am. J. Pathol. 163, 2329–2335 (2003).

123. Houghton, A. M. et al. Neutrophil elastase-mediated degradation of IRS-1 accelerates lung tumor growth. Nature Med. 16, 219–223 (2010).

124. Foekens, J. A. et al. The prognostic value of polymorphonuclear leukocyte elastase in patients with primary breast cancer. Cancer Res. 63, 337–341 (2003).

125. Akizuki, M. et al. Prognostic significance of immunoreactive neutrophil elastase in human breast cancer: long-term follow-up results in 313 patients. Neoplasia 9, 260–264 (2007).

126. Houghton, A. M. et al. Elastin fragments drive disease progression in a murine model of emphysema. J. Clin. Invest. 116, 753–759 (2006).

127. Mydel, P. et al. Neutrophil elastase cleaves laminin-332 (laminin-5) generating peptides that are chemotactic for neutrophils. J. Biol. Chem. 283, 9513–9522 (2008).

128. Weathington, N. M. et al. A novel peptide CXCR ligand derived from extracellular matrix degradation during airway inflammation. Nature Med. 12, 317–323 (2006).Seminal study demonstrating the importance of matrix fragments (independently of cytokines and chemokines) on the recruitment and maintenance of inflammatory cell infiltrates in the setting of lung injury.

129. Aberle, D. R. et al. Reduced lung-cancer mortality with low-dose computed tomographic screening. N. Engl. J. Med. 365, 395–409 (2011).This study reports the results of a large, multi-centre, randomized, placebo-controlled clinical trial, which demonstrated a 20% reduction in lung cancer mortality from the use of chest CT screening.

130. Tammemagi, C. M. et al. Lung cancer risk prediction: prostate, lung, colorectal and ovarian cancer screening trial models and validation. J. Natl Cancer Inst. 103, 1058–1068 (2011).

131. Parimon, T. et al. Inhaled corticosteroids and risk of lung cancer among patients with chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 175, 712–719 (2007).

132. Calverley, P. M. et al. Salmeterol and fluticasone propionate and survival in chronic obstructive pulmonary disease. N. Engl. J. Med. 356, 775–789 (2007).

133. Stolina, M. et al. Specific inhibition of cyclooxygenase 2 restores antitumor reactivity by altering the balance of IL-10 and IL-12 synthesis. J. Immunol. 164, 361–370 (2000).

134. Khuri, F. R. et al. Cyclooxygenase-2 overexpression is a marker of poor prognosis in stage I non-small cell lung cancer. Clin. Cancer Res. 7, 861–867 (2001).

135. Greenhough, A. et al. The COX-2/PGE2 pathway: key roles in the hallmarks of cancer and adaptation to the tumour microenvironment. Carcinogenesis 30, 377–386 (2009).

136. Mao, J. T. et al. Lung cancer chemoprevention with celecoxib in former smokers. Cancer Prev. Res. 4, 984–993 (2011).

137. Solomon, S. D. et al. Cardiovascular risk associated with celecoxib in a clinical trial for colorectal adenoma prevention. N. Engl. J. Med. 352, 1071–1080 (2005).

138. Heasley, L. E. et al. Induction of cytosolic phospholipase A2 by oncogenic Ras in human non-small cell lung cancer. J. Biol. Chem. 272, 14501–14504 (1997).

139. Keith, R. L. et al. Pulmonary prostacyclin synthase overexpression chemoprevents tobacco smoke lung carcinogenesis in mice. Cancer Res. 64, 5897–5904 (2004).

140. Keith, R. L. et al. Oral iloprost improves endobronchial dysplasia in former smokers. Cancer Prev. Res. 4, 793–802 (2011).

141. Nocturnal Oxygen Therapy Trial Group. Continuous or nocturnal oxygen therapy in hypoxemic chronic obstructive lung disease: a clinical trial. Ann. Intern. Med. 93, 391–398 (1980).

142. Ito, K. et al. Decreased histone deacetylase activity in chronic obstructive pulmonary disease. N. Engl. J. Med. 352, 1967–1976 (2005).

143. Adcock, I. M., Chung, K. F., Caramori, G. & Ito, K. Kinase inhibitors and airway inflammation. Eur. J. Pharmacol. 533, 118–132 (2006).

144. Shepherd, F. A. et al. Erlotinib in previously treated non-small-cell lung cancer. N. Engl. J. Med. 353, 123–132 (2005).

145. Soda, M. et al. Identification of the transforming EML4-ALK fusion gene in non-small-cell lung cancer. Nature 448, 561–566 (2007).

146. Woodruff, P. G. et al. Safety and efficacy of an inhaled epidermal growth factor receptor inhibitor (BIBW 2948 BS) in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 181, 438–445 (2010).

147. Hammerman, P. S. et al. Comprehensive genomic characterization of squamous cell lung cancers. Nature 489, 519–525 (2012).

148. Imielinski, M. et al. Mapping the hallmarks of lung adenocarcinoma with massively parallel sequencing. Cell 150, 1107–1120 (2012).

149. Witschi, H. et al. The carcinogenicity of environmental tobacco smoke. Carcinogenesis 18, 575–586 (1997).

150. Hubbard, R., Venn, A., Lewis, S. & Britton, J. Lung cancer and cryptogenic fibrosing alveolitis. A population-based cohort study. Am. J. Respir. Crit. Care Med. 161, 5–8 (2000).

151. Sandford, A. J., Weir, T. D., Spinelli, J. J. & Pare, P. D. Z and S mutations of the α1-antitrypsin gene and the risk of chronic obstructive pulmonary disease. Am. J. Respir. Cell. Mol. Biol. 20, 287–291 (1999).

152. Yang, P. et al. α1-antitrypsin deficiency allele carriers among lung cancer patients. Cancer Epidemiol. Biomarkers Prev. 8, 461–465 (1999).

R E V I E W S



FURTHER INFORMATIONA. McGarry Houghton’s homepage: http://depts.washington.edu/pulmcc/directory/bio/houghton.html

ALL LINKS ARE ACTIVE IN THE ONLINE PDF

153. Zhu, Y., Spitz, M. R., Lei, L., Mills, G. B. & Wu, X. A single nucleotide polymorphism in the matrix metalloproteinase-1 promoter enhances lung cancer susceptibility. Cancer Res. 61, 7825–7829 (2001).

154. Cheng, S. L., Yu, C. J. & Yang, P. C. Genetic polymorphisms of cytochrome p450 and matrix metalloproteinase in chronic obstructive pulmonary disease. Biochem. Genet. 47, 591–601 (2009).

155. Dialyna, I. A., Miyakis, S., Georgatou, N. & Spandidos, D. A. Genetic polymorphisms of CYP1A1, GSTM1 and GSTT1 genes and lung cancer risk. Oncol. Rep. 10, 1829–1835 (2003).

156. Xiao, D. et al. Relationship between polymorphisms of genes encoding microsomal epoxide hydrolase and glutathione S-transferase P1 and chronic obstructive pulmonary disease. Chin. Med. J. 117, 661–667 (2004).

157. Park, J. Y., Chen, L., Wadhwa, N. & Tockman, M. S. Polymorphisms for microsomal epoxide hydrolase and genetic susceptibility to COPD. Int. J. Mol. Med. 15, 443–448 (2005).

158. Benhamou, S., Reinikainen, M., Bouchardy, C., Dayer, P. & Hirvonen, A. Association between lung cancer and microsomal epoxide hydrolase genotypes. Cancer Res. 58, 5291–5293 (1998).

159. Kao, R. C., Wehner, N. G., Skubitz, K. M., Gray, B. H. & Hoidal, J. R. Proteinase 3. A distinct human polymorphonuclear leukocyte proteinase that produces emphysema in hamsters. J. Clin. Invest. 82, 1963–1973 (1988).

160. Zheng, T. et al. Role of cathepsin S-dependent epithelial cell apoptosis in IFN-γ-induced alveolar remodeling and pulmonary emphysema. J. Immunol. 174, 8106–8115 (2005).

161. Yang, Y. et al. Cathepsin S mediates gastric cancer cell migration and invasion via a putative network of metastasis-associated proteins. J. Proteome Res. 9, 4767–4778 (2010).

162. Yang, Z. & Cox, J. L. Cathepsin L increases invasion and migration of B16 melanoma. Cancer Cell Int. 7, 8 (2007).

163. Golovatch, P. et al. Role for cathepsin K in emphysema in smoke-exposed guinea pigs. Exp. Lung Res. 35, 631–645 (2009).

164. Brooks, P. C. et al. Localization of matrix metalloproteinase MMP-2 to the surface of invasive cells by interaction with integrin α v β 3. Cell 85, 683–693 (1996).

165. Radisky, D. C. et al. Rac1b and reactive oxygen species mediate MMP-3-induced EMT and genomic instability. Nature 436, 123–127 (2005).

166. Balbin, M. et al. Loss of collagenase-2 confers increased skin tumor susceptibility to male mice. Nature Genet. 35, 252–257 (2003).

167. Kudo, Y. et al. Matrix Metalloproteinase-13 (MMP-13) Directly and indirectly promotes tumor angiogenesis. J. Biol. Chem. 287, 38716–38728 (2012).

168. Deshmukh, H. S. et al. Matrix metalloproteinase-14 mediates a phenotypic shift in the airways to increase mucin production. Am. J. Respir. Crit. Care Med. 180, 834–845 (2009).

169. Hotary, K., Li, X. Y., Allen, E., Stevens, S. L. & Weiss, S. J. A cancer cell metalloprotease triad regulates the basement membrane transmigration program. Genes Dev. 20, 2673–2686 (2006).

Competing interests statementThe author declares no competing financial interests.

R E V I E W S



http://depts.washington.edu/pulmcc/directory/bio/houghton.html

http://depts.washington.edu/pulmcc/directory/bio/houghton.html

mechanistic lung cancer

Documents