signaling in and out: long-noncoding rnas in tumor hypoxia...review open access signaling in and...

REVIEW Open Access

Signaling in and out: long-noncoding RNAsin tumor hypoxiaTse-Chun Kuo1, Hsing-Jien Kung1,2,3,4,5 and Jing-Wen Shih2,3,5,6*

Abstract

Over the past few years, long non-coding RNAs (lncRNAs) are recognized as key regulators of gene expression atchromatin, transcriptional and posttranscriptional level with pivotal roles in various biological and pathologicalprocesses, including cancer. Hypoxia, a common feature of the tumor microenvironment, profoundly affects geneexpression and is tightly associated with cancer progression. Upon tumor hypoxia, the central regulator HIF(hypoxia-inducible factor) is upregulated and orchestrates transcription reprogramming, contributing to aggressivephenotypes in numerous cancers. Not surprisingly, lncRNAs are also transcriptional targets of HIF and serve aseffectors of hypoxia response. Indeed, the number of hypoxia-associated lncRNAs (HALs) identified has risen sharply,illustrating the expanding roles of lncRNAs in hypoxia signaling cascade and responses. Moreover, through extra-cellular vesicles, lncRNAs could transmit hypoxia responses between cancer cells and the associatedmicroenvironment. Notably, the aberrantly expressed cellular or exosomal HALs can serve as potential prognosticmarkers and therapeutic targets. In this review, we provide an update of the current knowledge about theexpression, involvement and potential clinical impact of lncRNAs in tumor hypoxia, with special focus on theirunique molecular regulation of HIF cascade and hypoxia-induced malignant progression.

Keywords: Tumor hypoxia, Long non-coding RNA, lncRNA, HIF-1α, Hypoxia-associated lncRNAs, HAL,Extracellular vesicles

BackgroundHypoxia-associated lncRNAs (HALs) emerging as newlydriving factors in tumorigenesisIn rapidly growing solid tumors, hypoxia is a common,microenvironmental characteristics, caused by insufficientvascularization, and the high tumor metabolic demands[1]. Accumulating evidence has demonstrated that tumorhypoxia is involved in the initial oncogenic transform-ation, but is also tightly linked to aggressive cancer pheno-types, such as metastases, recurrences and resistance to

therapy [2–4]. Upon hypoxia, to survive, cancer cells co-opt the fundamental adaptive responses to this stressthrough modulating the central mediator of hypoxic re-sponse, the hypoxia-inducible factor-1 (HIF-1) complex.The HIF-1 complex is a heterodimeric assembly of

bHLH-PAS (basic helix-loop-helix DNA binding proteinsof the PER-ARNT-SIM family) transcriptional factors,comprised of a constitutively expressed, stable HIF-1βsubunit and an oxygen-sensitive HIF-1α subunit that de-termines HIF-1 activity [5, 6]. In mammals, two HIF-1αhomologs, HIF-2α and HIF-3α (also known as IPAS-1; in-hibitory PAS (Per/Arnt/Sim) domain protein), have beenidentified. Similar to HIF-1α, HIF-2α is also sensitive tooxygen concentration and can interact with HIF-1β toform the HIF-2 heterodimeric complex. Due to the struc-tural similarity in DNA binding and dimerization domainsas well as the difference in their transactivation domains,

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* Correspondence: [email protected] Institute of Cancer Biology and Drug Discovery, College of MedicalScience and Technology, Taipei Medical University, Taipei 11031, Taiwan,ROC3Ph.D. Program for Cancer Biology and Drug Discovery, College of MedicalScience and Technology, Taipei Medical University, Taipei 11031, Taiwan,ROCFull list of author information is available at the end of the article

Kuo et al. Journal of Biomedical Science (2020) 27:59 https://doi.org/10.1186/s12929-020-00654-x

http://crossmark.crossref.org/dialog/?doi=10.1186/s12929-020-00654-x&domain=pdfhttp://orcid.org/0000-0002-0417-9015http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/publicdomain/zero/1.0/mailto:[email protected]

HIF-1α and HIF-2α regulate both common as well as dis-tinct sets of target genes. Meanwhile, HIF-3α, an isoformlacking the transactivation domain, has a dominant nega-tive effect on HIF-dependent gene transcription [7, 8].In the presence of sufficient oxygen, HIF-1α subunits are

post-translationally modified by a family of dioxygenases(prolyl hydroxylase domain-containing dioxygenases PHD1,2 and 3, also known as EGLN1-3, Egl-9 family hypoxia in-ducible factor 1-3,). Upon hydroxylation, HIF-1α subunitsare recognized by the E3 ubiquitin ligase, VHL (vonHippel-Lindau tumor suppressor protein), leading to thepoly-ubiquitination and subsequent rapid degradationthrough the ubiquitin-proteasome pathway (Fig. 1a). Underhypoxic conditions, the PHD dioxygenase activity is inhib-ited, and the accumulated HIF-1α subunit translocates intothe nucleus, dimerizing with HIF-1β and binding to theHREs (hypoxia response elements; the consensus 5′-(A/G)CGTG-3′ nucleotide sequence) within the promoterregions of HIF target genes to stimulate downstream tran-scriptional activation of multiple hypoxia responsive genes(Fig. 1a), eliciting a wide spectrum of cellular adaptations,such as decreased apoptosis, enhanced angiogenesis, prolif-eration, migration and invasion [1, 9–11]. In addition toprotein coding genes, it has been widely acknowledged thatthe non-coding transcriptome is also responsive to hypoxiaand play critical roles in the hypoxic response and HIF-1associated cancer progression [12–16].With recent advances in high-throughput sequencing, it

is recognized that only a small fraction (< 2%) of the tran-scriptional output encodes proteins whereas the vast major-ity encode a variety of non-coding RNAs. Among thesenon-coding RNA species, long (> 200 bp) non-codingRNAs (lncRNAs) are a large class of regulatory transcripts[17], including lincRNAs (long intergenic RNAs), long in-tronic ncRNAs, pseudogenes, TCRs (transcribed ultra-conserved regions), asRNAs (antisense RNAs) and eRNAs(enhancer RNAs) [18]. According to the latest human gen-ome annotation (GRch38, GENCODE release 33, January2020; www.gencodegenes.org), 48,438 transcripts originat-ing from 17,952 loci were identified as lncRNAs. Althoughless than 1% has been functionally annotated, growingevidence suggested the vital roles of these lncRNAs in regu-lation of gene expression at various stages, such as imprint-ing, transcription, RNA interference, RNA splicing, andtranslation control [19–23]. It is now believed that the dis-tinctive RNA biochemical properties, such as base-pairingability, dynamic expression and flexible structure, endowthese lncRNAs with multi-functionality [24–28]. Collect-ively, it is now well appreciated that, through acting assignals, decoys, guides or scaffolds, lncRNA could act as acrucial player of biological regulation [23–25, 27, 29–33].Over the last few years, a large number of dysregulated

lncRNAs have been associated with numerous diseases,including cancer [34–37]. While a few cancer-associated

lncRNAs have been well characterized [27, 38], the func-tions of most remain largely unknown. Dysregulation ofmany cancer-associated lncRNAs is linked to both clini-copathological features and survival outcomes of pa-tients, suggesting that functional annotation of theselncRNAs will eventually identify new venues for earlydiagnosis and therapy of cancer [39]. Several studieshave shown that the modulation of lncRNAs in responseto hypoxia could play a regulatory role in HIF signalingcascade [14–16, 40, 41]. Here, we refer to these uniquetranscripts as “hypoxia-associated lncRNAs” (HALs).These RNA molecules are involved in multiple hypoxia-driven cancer progression pathways. In this review, weprovide an updated summary of the tumor HALs, with aspecific emphasis on the crosstalk between theselncRNA species and cellular hypoxia response (Table 1and Additional file 1: Table S1). We address currentmodels describing the functional involvement of thesenew players in cancer progression, highlighting their rele-vant clinical potential as cancer biomarkers or therapeutictargets. Our discussion is centered on tumor hypoxia. Forthe functional roles of lncRNAs in hypoxia-induced kid-ney/hepatic/myocardial injury and neuromuscular orcardiovascular diseases, interested readers are referred to anumber of comprehensive reviews published in recentyears [127–132].

ReviewLncRNAs as emerging driving forces in cancerprogression upon tumor hypoxiaGiven the pivotal roles of lncRNA in hypoxia-associatedtumorigenesis pathways, multiple approaches have beenapplied in the identification of hypoxia-regulated lncRNAs[87, 90]. A comprehensive analysis coupling RNA-seq withChIP-seq [12] revealed the extensive involvement of HIF-1αand HIF-2α in the transcriptional regulation of lncRNAsupon hypoxia. In recent years, the rapid expansion of re-search on lncRNAs has provided additional insights intothose associated with cellular hypoxia response. Table 1 pre-sents an updated list of these hypoxia-associated lncRNAs(HALs). Upon hypoxia, most HALs are up-regulated. HIFcould directly promote the expression of these hypoxia-inducible lncRNAs through binding to the HREs (hypoxiaresponse elements) located in their promoter (Table 1) [41].lncRNA-LET [93], CF129 [54] and CRPAT4 [56] are amongthe few which are down-regulated in hypoxic conditions.Notably, lncRNA-SARCC is able to respond to hypoxicstress differentially in a VHL-dependent manner [94].Most of the HALs identified have impacts on cancer

progression, although the mechanistic details are not allclear. Table 1 shows an overview of the tumor HALs.We summarize in the table, their potential moleculartarget related to hypoxic responses as well as their re-ported functions and signaling pathways. These HALs

Kuo et al. Journal of Biomedical Science (2020) 27:59 Page 2 of 25

http://www.gencodegenes.org

Fig. 1 Regulations of HIF-1 activity by HALs. a Regulation of HIF-1. Under normoxia (green arrows), HIF-1α subunit is hydroxylated by PHDs (prolyl hydroxylasedomain proteins). Hydroxylation residues within HIF-1α facilitates interaction of HIF-1α with the E3 ubiquitin ligase VHL protein, targeting HIF-1α forpolyubiquitination and subsequent proteasome-dependent degradation. Upon hypoxia (red arrows), the PHDs and other prolyl hydroxylases are inhibited,leading to HIF-1α stabilization and translocation into nucleus. After dimerization with its transcriptional partner HIF-1β and recruitment of co-activators (e.g. CBP/p300), the HIF-1 heterodimer binds the HRE (hypoxia response element) of target genes to regulate transcription. b Transcriptional co-activator. Hypoxia-induced LncHIFCAR could directly interact with HIF-1α and facilitate the recruitment of HIF-1α and p300 cofactor to the target loci, thereby upregulating HIF-1target genes. c Recruitment of transcription factor. HIF-1α-induced LncRNA-MTA2TR could recruit ATF3 to the promoter area of MTA2, thereby transcriptionallyupregulating the expression of oncogenic MTA2. MTA2 can subsequently enhance HIF-1α protein accumulation via deacetylation, forming a feedback loop toamplify HIF-1 signaling. dmRNA stability control. The expression of lncRNA-LET is repressed through hypoxia-induced HDAC3, which reduces the histone H3and H4 acetylation at the LncRNA-LET promoter. Decreased lncRNA-LET expression reduces the lncRNA-LET–mediated degradation of HIF-1α negative regulator,NF90, leading to HIF-1α accumulation. e ceRNA/miRNA sponge. Hypoxia-induced H19 could upregulate HIF-1α expression by absorbing miRNA let-7 andnullifying let-7-mediated HIF1AmRNA suppression. f Molecular decoy. lincRNA-p21 is able to disrupt the interaction between HIF-1α and its negative regulatorVHL via separate binding to both HIF-1α and VHL, thereby blocking VHL-dependent HIF-1α degradation. g Complex scaffold. LINK-A-mediated recruitment andenzymatic activation of BRK and LRRK2 kinases could facilitate phosphorylation of HIF-1α at specific residues. These phosphorylation modifications preventsubsequent HIF-1α degradation and enhance the association between HIF-1α and cofactor p300, thereby upregulating HIF-1 target genes. See text for a moredetailed discussion


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType

sClinicalassociation

Functio

nal

Impact

Interactor

Target/Effect

MechanisticClassificatio

nRefs

aHIF

(HIF1A-AS2)

Not

furthe

rindu

cedin

nonp

apillary

disease,bu

tcanbe

indu

cedin

lymph

ocytes

N.D.

(2Pu

tativeHREs)

Renalcarcino

ma

•Up-regu

latedin

non-papillary

clear-cellrenal

carcinom

a

N.D.

HIF1A

mRN

AHIF1A

mRN

Astability

mRN

Astab

ility

control

(Binding

ofHIF1A-AS2

totheHIF1A

mRN

A3′-UTR

couldpo

ssiblyexpo

seAU-richelem

entsand

thus

increase

the

degradationof

HIF1A

mRN

A)

[42,

43]

Up-regu

lated

N.D.

Hum

anum

bilicalvein

endo

thelialcells

(HUVECs)

•Up-regu

latedin

HUVECsin

hypo

xia

HUVECsviability

↑ Migratio

nability

↑ Tube

form

ation

↑

miR-153-3p

Theexpression

ofHIF-1α

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-153-3p-med

iated

repression

ofHIF-1α

expression

)

[44]

Up-regu

lated

N.D.

Bladde

rcancer

•Upreg

ulated

inbladde

rcancer

aftercisplatin

treatm

ent

Cisplatin

resistance

↑N.D.

Prom

oting

HMGA1

expression

Tran

scription

alregulation

(HIF1A

-AS2

prom

oting

theexpression

ofHMGA1,

which

physicallyinteracts

with

p53,p6

3,andp7

3,andthereforeinhibits

theirtranscrip

tional

activity

onBax)

[45]

Up-regu

lated

HIF-1αand/or

HIF-2α

depe

nden

t(2

HREs

iden

tified)

Mesen

chym

alGlioblastoma

Stem

-like

Cells(M

-GSC

s)

•Upreg

ulated

inM-GSC

sGrowth

ofM-

GSC

s↑

Neurosphe

re-

form

ing

capacity

ofM-

GSC

s↑

Glioblastoma

tumor

grow

th↑

IGF2BP2and

DHX9

Mainten

ance

ofexpression

ofHMGA1

Com

plexscaffold

(The

direct

interaction

amon

gHIF1A-AS2,

IGF2BP2andDHX9

isne

eded

forHMGA1

expression

)

[46,

47]

Up-regu

lated

N.D.

Epith

elialo

varian

cancer

(EOC)

•Up-regu

lated

inEO

CCellapo

ptosis↓

Cellp

roliferation

↑ Tumorigen

esis↑

Tumor

grow

th↑

N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

htheaH

IF-m

ediated

regu

latio

nof

certain

keymito

chon

drial

apop

tosispathway-

relatedge

nes,

includ

ingBcl-2,Bax,

Caspase-7,and

Caspase-9)

[48]

AGAP2-AS1

Up-regu

lated

N.D.

Hep

atocellular

carcinom

a(HCC)

•Up-regu

latedin

HCC

•Correlatedwith

adverseclinical

features

andpo

orprog

nosisof

HCC

Cellp

roliferation

↑ Migratio

nand

invasion

↑EM

Tprog

ression

↑

miR-16-5p

Theexpression

ofANXA

11Se

que

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-16-5p

-med

iated

repression

ofANXA

11)

[49]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

Apo

ptosis↓

ANRIL(CDKN

2B-

AS1)

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Osteo

sarcom

a•Up-regu

latedin

osteosarcoma

Hypoxicviability

↑ Hypoxia-

indu

ced

Invasion

↑Hypoxia-

indu

ced

apop

tosis↓

N.D.

N.D.

Unc

lear

mecha

nism

(Possiblythroug

hep

igen

eticmod

ificatio

n)

[50]

BC005927

Up-regu

lated

HIF-1α

depe

nden

t(2

HREs

iden

tified)

Gastriccancer

(GC)

•Up-regu

latedin

GC

•Correlatedwith

high

ertumor-nod

e-metastasisstages

andpo

orer

prog

noses

Metastasis↑

N.D.

N.D.

Tran

scription

alregulation

(The

neighb

oringge

ne,

EPHB4,a

metastasis-

relatedge

ne,is

regu

latedby

BC005927)

[51]

BX111887

(ZEBTR)

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Pancreaticcancer

(PC)

•Upreg

ulated

inPC

•Correlatedwith

late

TNM

stage,lymph

atic

invasion

anddistant

metastasis

Proliferatio

n↑

Migratio

n↑

Invasion

↑

YB1

ZEB1

prom

oter

Tran

scription

alregulation

(BX111

prom

otes

ZEB1

transcrip

tionby

recruitin

gYB1to

ZEB1

prom

oter)

[52]

CASC9

N.D.

N.D.

Nasop

haryng

eal

carcinom

a(NPC

)Up-regu

latedin

NPC

tissues

Glycolysisand

tumorigen

esis↑

Cellg

rowth

↑

HIF-1α

Thestability

ofHIF-1α

ProteinStab

ility

(CASC9

interactswith

HIF-1αanden

hances

the

stabilizatio

nof

HIF-1α)

[53]

CF129

(lncRNA-

CF129145.1)

Dow

n-regu

lated

Dow

nreg

ulated

bybind

ingof

HIF-1α/HDAC1

complex

toCF129

prom

oter

Pancreaticcancer

(PC)

•Dow

n-regu

latedin

PC•Low

CF129

expression

pred

ictedshortoverall

survival

Invasion

and

metastasis↓

p53andE3

ligase

MKRN1

FOXC

2transcrip

tion

Post-Translation

almod

ification

(CF129

directlybind

sto

p53andE3

ligaseMKRN1,

indu

cing

p53protein

ubiquitin

ationand

degradation,and

thereb

ysupp

ressing

FOXC

2transcrip

tion)

[54]

CPS1-IT1

Dow

n-regu

lated

(treatmen

tof

hypo

xiamim

etic,

CoC

l 2)

N.D.

Colorectalcancer

Dow

n-regu

latedin

colorectalcancer

EMTand

autoph

agy↓

N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

hsupp

ressingexpression

levelsof

HIF-1α,LC

3-I,

LC3-II,Beclin-1

andEM

Tassociated

proteins

unde

rhypo

xia)

[55]

CRPAT4

(RP11-225B17)

Dow

n-regu

lated

HIF-1α

depe

nden

t,HIF-

2αinde

pend

ent

Clear

cellrenalcell

carcinom

a(ccRCC)

•Up-regu

latedin

ccRC

C•Associatedwith

poor

overallsurvivaland

prog

ression-fre

e

Cellm

igratio

n↑

Proliferatio

n↑

N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

htheCR

PAT4-m

ediated

regu

latio

nof

migratio

n-

[56]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

survival

associated

gene

AVL9

expression

)

DAN

CRN.D.

N.D.

Nasop

haryng

eal

carcinom

a(NPC

)•Up-regu

latedin

NPC

•Associatedwith

poor

prog

nosis

Metastasis↑

Invasion

↑NF90/NF45

complex

HIF-1αmRN

Astability

mRN

Astab

ility

control

(DAN

CRcouldincrease

HIF-1αmRN

Astability

throug

hinteractingwith

theNF90/NF45complex)

[57]

DAR

S-AS1

Up-regu

lated

HIF-1α

depe

nden

t,Bu

tHIF-2αinde

pend

-en

t(2

HREs

iden

tified)

Myeloma

•Up-regu

latedin

myeloma

•Correlatedwith

poor

prog

nosis

Survival↑

Tumorigen

esis↑

RBM39

RBM39

stability

Post-Translation

almod

ification

(The

interactionbe

tween

DAR

S-AS1and

RNA-binding

protein39

(RBM

39)im

pede

sthe

interactionbe

tween

RBM39

andits

E3ub

iquitin

ligaseRN

F147,

preven

tingRBM39

from

degradation)

[58]

EIF3J-AS1(EIF3J-

DT)

Up-regu

lated

N.D.

Hep

atocellular

carcinom

a(HCC)

•Up-regu

latedin

HCC

tissues

•Correlatedwith

tumor

size,vascularinvasion

,tumor

stageandpo

orprog

nosis

Cellp

roliferation

↑ Migratio

n↑

Invasion

↑

miR-122-5p

Theexpression

ofCTN

ND2

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-122-5p-med

iated

repression

ofCTN

ND2)

[59]

ENST00000480739

(RPL13AP23)

N.D.

N.D.

Pancreaticdu

ctal

aden

ocarcino

ma

(PDAC)

•Dow

n-regu

latedin

PDAC

•Associatedwith

tumor

node

metastasis(TNM)

stageandlymph

node

metastasis

•Inde

pend

entriskfactor

forPD

ACsurvival

followingsurgery

Invasion

↓OS-9mRN

A&

protein↑

N.D.

Transcrip

tionof

OS-9(Neg

ative

regu

latio

nof

HIF-1α)

Epigen

etican

dtran

scription

alregulation

(ENST00000480739

indu

cesOS-9expression

atthetranscrip

tional

level,po

ssiblythroug

hmod

ifyingtheH3K27

acetylationlevelo

fOS9

gene

prom

oter)

[60]

FALEC

Up-regu

lated

HIF-1αindu

cible

Prostate

cancer

(PCa)

•Up-regu

latedin

PCa

•Inde

pend

entprog

nostic

factor

Cellp

roliferation

↑ Migratio

nand

invasion

↑

N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

htheFALEC-med

iated

regu

latio

nof

p21andits

downstream

compo

nents

expression

)

[61]

FAM201A

N.D.

N.D.

Non

-smallcelllun

gcancer

(NSC

LC)

•Up-regu

latedin

tissues

obtained

from

NSC

LCpatientsresistantto

radiothe

rapy

Cellp

roliferation

↑ Apo

ptosis

(und

erX-rayir-

radiation)

↓

miR-370

Theexpression

ofEG

FRSe

que

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-370-m

ediated

repression

ofEG

FR)

[62]

FEZF1-AS1

N.D.

N.D.

Pancreaticcancer

•Upreg

ulated

inCellp

roliferation

miR-142

and

Theexpression

Seque

stration

of[63]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

pancreaticcancer

↑ Invasion

↑miR-133a

ofHIF-1αand

EGFR

miRNAs

(Dow

n-regu

latio

nof

miR-142-and

miR-133a-med

iated

repression

ofHIF-1αand

EGFR

expression

)

GAPLINC

Up-regu

lated

HIF-1α(2

HREs

iden

tified)

(2HREs)

Gastriccancer

•Upreg

ulated

inGC

•Highexpression

ofGAPLINCcorrelates

with

poorer

survival

•GAPLINCcorrelates

with

CD44

activation

Proliferatio

n↑

Apo

ptosis↓

Invasion

↑Migratio

n↑

miR-211-3p

Theexpression

ofCD44

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-211-3p-med

iated

repression

ofCD44)

[64,

65]

H19

Up-regu

lated

N.D.

Breastcancer

stem

cells

(BCSC

s)•H19

expression

strong

lycorrelates

with

PDK1

inprim

arybreast

carcinom

as

Glycolysis↑

BCSC

mainten

ance

↑

let-7

Theexpression

ofHIF-1α

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

let-7-med

iatedrepression

ofHIF-1αexpression

)

[66]

Up-regu

lated

N.D.

Multip

leMyeloma

(MM)

N.D.

Theexpression

ofthehypo

xia

indu

cedge

nes

↑ Adh

esionon

stromalcells

↑

N.D.

N.D.

HIF-1αnu

clea

rtran

sloc

ation

(H19

isrequ

iredfor

HIF-1αnu

clear

translocationandthe

expression

ofthe

hypo

xia-indu

cedge

nes,

such

asCXC

R4andSnail)

[67]

Up-regu

lated

HIF-1α

depe

nden

t(3

HREs

iden

tified)

Glioblastoma(GBM

)•Up-regu

latedin

GBM

•Correlatedwith

poor

prog

nosis

•TheHIF-1αlevelswere

positivelycorrelated

with

H19

levelsin

GBM

specim

ens

Migratio

nand

invasion

↑Tumor

grow

th↑

EMT↑

miR-181d

Theexpression

ofβ-catenin

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-181d-med

iated

repression

ofβ-catenin

expression

)

[68–

71]

Up-regu

lated

N.D.

Prostate

Cancer

•Upreg

ulated

byestrog

enor

hypo

xia

•Redu

cedup

oncombine

dtreatm

ent

Cellm

otility

↓Invasion

↓N.D.

Repression

ofbe

ta3and

beta4Integrins

Unc

lear

mecha

nism

(Com

bine

dEstrog

enandHypoxiatreatm

ent

couldcauseH19

down-regu

latio

n,followed

byup

-reg

ulationof

both

β3andβ4

Integrinsand

E-cadh

erin)

[72]

Up-regu

lated

N.D.

Breastcancer,N

on-

smallcelllun

gcarcin-

oma(NSC

LC)

•Up-regu

latedin

NSC

LCwith

chronicob

structive

pulm

onarydisease

(COPD

)•Up-regu

latedin

all

Migratio

nand

invasion

↑Tumor

grow

th↑

EMT↑

N.D.

Up-regu

latio

nof

miR-675-5p

Unc

lear

mecha

nism

(H19

couldindu

ceup

regu

latio

nof

miR-675-5p,

whe

reas

P53

isatarget

gene

of

[73,

74]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

common

metastatic

sitestested

miR-675-5pandP53

downstream

target

gene

sinvolved

inEM

T,survival

andtumorigen

esisare

thereb

yrepressed)

HAS2-AS1

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Oralsqu

amou

scell

carcinom

a(OSC

C)

•Up-regu

latedin

OSC

CEM

T↑

N.D.

N.D.

Unc

lear

mecha

nism

(HAS2-AS1-med

iated

hypo

xia-indu

cedEM

Tis

depe

nden

ton

cell-adhe

sion

molecule

CD44

andRH

AMM)

[75]

HIF2PUT

N.D.

N.D.

Osteo

sarcom

a•Expression

ofHIF2PUT

iscorrelated

with

HIF2A

mRN

A

Cellp

roliferation

andmigratio

n↓

Expression

ofCSC

marker

CD133↓

Sphe

re-fo

rming

ability

↓

N.D.

Transcrip

tionof

HIF2A

Tran

scription

alregulation

(HIF-2αwas

positively

regu

latedby

lncRNA

HIF2PUT)

[76]

N.D,

N.D.

Osteo

sarcom

acancer

stem

cell

•Dow

n-regu

latedin

osteosarcomacelllines

•Astrong

positive

correlationbe

tween

relativeHIF2PUTand

HIF-2αlevelin

osteosarcomacancer

tissues

Proliferatio

n↓

Migratio

nand

invasion

↓Sphe

re-

form

ation↓

N.D.

N.D.

Unc

lear

mecha

nism

(May

partlythroug

hHIF2PUT-med

iated

regu

latio

nof

HIF-2

expression

)

[77]

HINCU

T-1(uc.475)

Up-regu

lated

HIF-1α

depe

nden

t(3

HREs

iden

tified)

Colon

andbreast

cancer

celllines

N.D.

Hypoxiccell

proliferatio

n↑

N.D.

N.D.

Tran

scription

alregulation

(HINCU

T-1isrequ

iredfor

theexpression

ofOGT

mRN

Aexpression

and

glob

alO-GlcNAcylatio

nof

proteins)

[78]

HOTAIR

N.D.

N.D.

Renalcellcarcino

ma

•Upreg

ulated

and

correlated

with

tumor

prog

ression

RCC

proliferatio

n↑

Migratio

nand

EMT↑

Apo

ptosis↓

miR-217

Theexpression

ofHIF-1α/AXL

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-217-m

ediated

repression

ofHIF-1α/AXL

expression

)

[79]

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Non

-smallcelllun

gcarcinom

a(NSC

LC)

•Highlevelo

fHOTAIRis

associated

with

poor

clinicalou

tcom

ein

multip

lecancers

Cellp

roliferation

unde

rhypo

xia↑

Invasion

&migratio

nun

der

hypo

xia↑

Apo

ptosisun

der

hypo

xia↓

N.D.

N.D.

Unc

lear

mecha

nism

(Possiblythroug

hHOTAOR-med

iated

epigen

eticmod

ificatio

n)

[80,

81]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

HOTTIP

Up-regu

lated

HIF-1α

depe

nden

tGlioma

•Up-regu

latedin

glioma

•Associatedwith

metastasisandpo

orpatient

survival

EMT↑

Invasion

↑Migratio

n↑

miR-101

Theexpression

ofZEB1

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-101-m

ediated

repression

ofZEB1)

[82]

IDH1-AS1

N.D.

N.D.

(c-M

yc-m

ediated

repression

)

Multip

lecelllines

(HeLa,HCT116,H

1299,

P493

and293T)

N.D.

Glycolysis↓

IDH1

IDH1

dimerization

ProteinDim

erization

(IDH1-AS1interactswith

IDH1andprom

otes

ItsHom

o-dimerization)

[83]

LINC01436

Up-regu

lated

N.D.

Non

-smallcelllun

gcancer

(NSC

LC)

•Up-regu

latedin

NSC

LC•Associatedwith

poor

overallsurvival

Cellg

rowth

↑Migratio

nand

invasion

↑

miR-30a-3p

Theexpression

ofEPAS1

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-30a-3p-med

iated

repression

ofEPAS1)

[84]

lincRNA-p21

(TP53COR1)

Up-regu

lated

HIF-1α

depe

nden

t&

preferen

ce(2

HREs

iden

tified)

Cervical,lung

and

breastcancer

celllines

N.D.

Hypoxic

glycolysis↑

Tumor

grow

th↑

HIF-1αand

VHL

Thedisrup

tion

oftheVH

L-HIF-

1αinteraction

Protein-Protein

InteractionDecoy

(Stabilizationof

HIF-1αby

disrup

tingtheVH

L-HIF-1α

Interaction)

[85]

Up-regu

lated

N.D.

Hep

atom

a,glioma

N.D.

Apo

ptosis↓

Cellp

roliferation

andmotility

↑Autop

hagy

↑

N.D.

N.D.

Unc

lear

mecha

nism

(LincRNA-p21could

prom

oteautoph

agyof

hypo

xictumor

cells

byup

-reg

ulatingHIF-1α

proteinlevelsand

supp

ressing

Akt/m

TOR/P70S6K

sign

alingpathways)

[86]

linc-RO

RUp-regu

lated

N.D.

Hep

atocellularcancer

Up-regu

latedin

malignant

liver

cancer

cells

Cellviability

durin

ghypo

xia

↑ Tumor

grow

th↑

miR-145

Theexpression

ofp70S6K1

(RPS6KB1)

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR145-med

iated

repression

ofp7

0S6K1

expression

)

[87]

LINK-A

(LINC01139)

N.D.

N.D.

Triple-neg

ativebreast

cancer

•Upreg

ulated

inTN

BC•Highlevelsof

LINK-A

correlated

with

unfavorable

recurren

ce-free

survival

forbreastcancer

patients

Glycolysis↑

Tumor

grow

th↑

BRKand

LRRK2kinase

HIF-1α

phosph

orylation

Com

plexscaffold

(LINK-Afacilitates

the

recruitm

entof

BRKand

LRRK2kinase

activation,

thereb

ycausingHIF-1α

stabilizatio

n,HIF-1α/p3

00interaction,andactivation

ofHIF-1αtranscrip

tional

prog

ramsun

derno

rmoxic

cond

ition

s)

[88]

LncHIFCA

R(M

IR31HG)

Up-regu

lated

HIF-1α

depe

nden

tOralcancer

•Up-regu

latedin

oral

cancer

Hypoxic

glycolysis↑

HIF-1α

Activationof

HIF-1

sign

aling

Tran

scription

alregulation

[89]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

•Highlevelsof

LncHIFCA

Rpred

ictedworse

overall

survivalandrecurren

ce-

freesurvival

Tumor

metastasis↑

Invasion

and

migratio

n↑

Hypoxiccell

proliferatio

n↑

Sphe

re-fo

rming

ability

↑

(LncHIFCA

Ractsas

HIF-1α

coactivator)

lncRNA-AK058003

Up-regu

lated

N.D.

Gastriccancer

Up-regu

latedin

GC

Invasion

&migratio

n↑

Metastasis↑

N.D.

N.D.

Epigen

eticregulation

(AK058003expression

ispo

sitivelycorrelated

with

SNCG

expression

and

SNCG

prom

oter

demethylatio

n)

[90]

lncRNA-EFNA3

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Breastcancer

Astrong

correlation

betw

eenhigh

EFNA3

expression

andshorter

metastasis-fre

esurvival

inbreastcancer

patients

Cell

extravasation↑

Metastatic

dissem

ination↑

miR-210

Theexpression

ofEFNA3

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-210-m

ediated

repression

ofEFNA3)

[91]

lncRNA-HAL

(lnc-METTL16-2)

Up-regu

lated

HIF-1α

depe

nden

t(3

putativeHREs

foun

d)

Breastcancer

Up-regu

latedin

triple

negativebreastcancer

Migratio

n↑

Cancerstem

cell

phen

otype↑

Mam

mosph

eres

↑ Clono

genic

grow

th↑

Histone

sand

hnRN

Ps.

N.D.

Unc

lear

mecha

nism

(The

bind

ingof

lncRNA-HAL

tohiston

esandhn

RNPs

may

sugg

est

aparticipationat

the

chromatin

leveland

transcrip

tionalreg

ulation)

[92]

lncRNA-LET

(NPTN-IT1)

Dow

n-regu

lated

HIF-1α

depe

nden

t(Indirect:H

istone

deacetylation)

Lung

squamou

s-cell

cancer

(LSC

C),he

pato-

cellularcarcinom

a(HCC)a

ndcolorectal

cancer

(CRC

)

•Dow

n-regu

latedin

inLSCC

,HCCandCRC

•Correlatedwith

hypo

xia,

histon

eacetylation

disorder

andmetastasis

inHCC

Metastasis↓

Invasion

↓NF90(RNA-

bind

ing

protein)

HIF1A

mRN

Astability

mRN

Astab

ility

control

(The

associationbe

tween

lncRNA-LETandNF90

proteinen

hanced

the

degradationof

NF90,

thereb

yde

creasing

HIF1A

mRN

A)

[93]

lncRNA-SARC

C(lnc-P2RY1-1)

VHL-de

pend

ent

HIF-2α

depe

nden

t(1

HRE

iden

tified)

Renalcellcarcino

ma

Differen

tially

regu

lated

byhypo

xiain

avon

Hippe

l-Lindau(VHL)-

depe

nden

tmanne

rin

RCCclinicalspecim

ens

Hypoxiccell

cycle

prog

ression

(VHL-restored

RCCcells)↑

Hypoxiccell

cycle

prog

ression

(VHL-mutant

RCCcells)↓

AR

(and

roge

nreceptor)

AR

ubiquitin

ation

and

degradation

Post-Translation

almod

ification

(lncRNA-SARC

Ccould

prom

oteARde

gradation

viaub

iquitin

-med

iated

proteo

lysisto

supp

ress

AR/HIF-2α/C-M

YCsign

als)

[94]

lncTCF7(W

SPAR

)Up-regu

lated

N.D.

Glioma

•Up-regu

latedin

glioma

•Associatedwith

WHO

gradeandtumor

size

Cellm

igratio

n↑

Proliferatio

n↑

Tumorigen

icity

N.D.

N.D.

Unc

lear

mecha

nism

(LncTCF7

couldprom

ote

themigratio

nand

[95]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

↑proliferatio

nof

glioma

cellpartially

throug

hactivatingtheWnt

sign

allingpathway)

MALAT1

Up-regu

lated

HIF-2α

depe

nden

t&

preferen

ce(1HRE)

Hep

atocellular

carcinom

aN.D.

Cellg

rowth

↑Glycolysis↑

Migratio

n&

invasion

↑Vasculature

form

ation↑

Metastasis↑

N.D.

N.D.

Post-Translation

almod

ification

(MALAT1de

creases

hydroxylationof

HIF-1α

/HIF-2α,po

ssiblythroug

hdisassociatio

nof

theVH

Lproteinfro

mHIF-1α

/HIF-2α)

[96,

97]

Up-regu

lated

N.D.

Lung

aden

ocarcino

ma

N.D.

Proliferatio

n↑

Migratio

n↑

Invasion

↑

PTB-

associated

splicing

factor

(PSF)

GAG

E6prom

oter

Tran

scription

alregulation

(The

physicalinteraction

ofMALAT1andPSF

released

thebind

ingof

PSFto

GAG

E6prom

oter)

[98,

99]

Up-regu

lated

N.D.

Hep

atocellular

carcinom

aN.D.

Proliferatio

n↑

Migratio

nand

invasion

↑Apo

ptosis↓

miR-200a

N.D.

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-200a)

[100]

MEG

3Up-regu

lated

N.D.

Pheo

chromocytom

aN.D.

Hypoxia-

indu

cedPC

12cellinjury

↑

Methylatio

nproteins

(DNMT3a,

DNMT3b,

andMBD

1)

TIMP2

prom

oter

methylatio

nEp

igen

eticregulation

(MEG

3recruited

methylatio

nproteins

DNMT3a,DNMT3b,

and

MBD

1andaccelerated

TIMP2

prom

oter

methylatio

n,which

inturn

inhibitedits

expression

)

[101]

MTA2TR

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Pancreaticcancer

(PC)

Upreg

ulated

inPC

tissues

Cellp

roliferation

↑ Invasion

↑

Activating

transcrip

tion

factor

3(ATF3)

Theexpression

ofMTA

2(M

TA2stabilizes

theHIF-1αvia

deacetylation)

Tran

scription

alregulation

(MTA2TRtranscrip

tionally

upregu

latesMTA

2expression

byrecruitin

gATF3to

theprom

oter

area

ofMTA2)

[102]

NEAT1

Up-regu

lated

HIF-2α

depe

nden

tNon

-smallcelllun

gcancer

(NSC

LC)

•Up-regu

latedin

NSC

LC•Associatedwith

TNM

stageandmetastasis

Cellp

roliferation

↑ Migratio

nand

invasion

↑

miR-101-3p

SOX9

/Wnt/β-

catenin

sign

aling

pathway

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-101-3p-med

iated

repression

ofSO

X9/W

nt/β-caten

insign

alingpathway)

[103]

Up-regu

lated

HIF-2α

Breastcancer

Highexpression

ofNEAT1

Proliferatio

n↑

N.D.

N.D.

Com

plexscaffold

[12,


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

depe

nden

t&

preferen

ceisassociated

with

poor

survivalof

breastcancer

patients

Apo

ptosis↓

Clono

genic

survival↑

Paraspeckle

form

ation↑

(Indu

cesparaspeckle

form

ation,thereb

yen

hancingcancer

cell

survivalin

hypo

xia)

104–

106]

NDRG

-OT1

(lnc-NDRG

1-1)

Up-regu

lated

N.D.

Breastcancer

•N.D.

N.D.

NDRG

1NDRG

1de

gradation

Post-Translation

almod

ification

(NDRG

-OT1

could

prom

oteNDRG

1de

gradationvia

ubiquitin

-med

iated

proteo

lysis)

[107]

NORA

DUp-regu

lated

N.D.

Pancreaticcancer

(PC)

•Upreg

ulated

inPC

•Correlatedwith

shorter

overallsurvival

Migratio

n↑

Invasion

↑EM

T↑

Metastasis↑

miR-125a-3p

Theexpression

ofRh

oASe

que

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-125a-3p

-med

iated

repression

ofRh

oA)

[108]

NUTF2P3-001

(NUTF2P3)

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Pancreaticcancer

•Upreg

ulated

inpancreaticcancer

•Apo

sitivecorrelation

betw

eenNUTF2P3

andKRAS

•Associatedwith

tumor

stageandprog

nosis

Cellviability,

proliferatio

n↑

Invasion

↑KRASexpression

↑ Metastasis↑

miR-3923

Theexpression

ofKRAS

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-3923-med

iated

repression

ofKRAS)

[109]

PCGEM

1Up-regu

lated

N.D.

Gastriccancer

(GC)

Up-regu

latedin

GC

Invasion

and

metastasis↑

N.D.

N.D.

Unc

lear

mecha

nism

(Partially

throug

hregu

latin

gSN

AI1,a

key

transcrip

tionfactor

ofEM

T)

[110]

PVT1

N.D.

N.D.

Nasop

haryng

eal

carcinom

a(NPC

)•Up-regu

latedin

NPC

•Up-regu

latio

nis

associated

with

apo

orprog

nosisin

NPC

patients

NPC

cell

proliferatio

n↑

Colon

yform

ation↑

Invivo

tumorigen

esis↑

KAT2A

(chrom

atin

mod

ificatio

nfactor)

Transcrip

tionof

NF90(RNA-

bind

ing

protein)

Epigen

eticregulation

(PVT1serves

asascaffold

forKA

T2A,w

hich

med

iatesH3K9

acetylation,recruitin

gthe

nuclearreceptor

bind

ing

proteinTIF1βto

activate

NF90transcrip

tion,

thereb

yincreasing

HIF-1α

mRN

Astability)

[111]

N.D.

N.D.

Hep

atocellular

carcinom

a(HCC)

Up-regu

latedin

HCC

tissues

andcelllines

Cellp

roliferation

↑ Migratio

n↑

Invasion

and

ironup

take

↑Apo

ptosis↓

miR-150

Theexpression

ofHIG2

(Hypoxia-

indu

cible

protein2)

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-150-m

ediated

repression

ofHIG2)

[112]

N.D.

N.D.

Gastriccancer

•Upreg

ulated

inGC

GCcell

miR-186

Theexpression

Seque

stration

of[113]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

tissues

andcelllines

•Highexpression

levels

correlated

with

advanced

tumor

stage

andlymph

node

metastasis

proliferatio

n↑

GCcellinvasion

↑

ofHIF-1α

miRNAs

(Dow

n-regu

latio

nof

miR-186-m

ediated

repression

ofHIF-1α

expression

)

Up-regu

lated

N.D.

Non

-smallcelllun

gcancer

(NSC

LC)

•Up-regu

latedin

HIF-1α

high

grou

pcompared

with

HIF-1αlow

grou

p•Neg

ativelycorrelated

with

miR-199a-5p

expression

inNSC

LCtissues

Cellp

roliferation

↑miR-199a-5p

Theexpression

ofHIF-1α

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-199a-5p

-med

iated

repression

ofHIF-1α

expression

)

[114]

Up-regu

lated

treatm

entof

hypo

xiamim

etic

CoC

l 2)

N.D.

CervicalC

ancer

•Up-regu

latedin

Cervicalcancer

•Correlateswith

poorer

overallsurvival

Cellp

roliferation

↑ Migratio

nand

invasion

↑Apo

ptosis↓

Cisplatin

resistance

↑

N.D.

N.D.

Unc

lear

mecha

nism

(Possibleinvolvem

entof

theinteractionwith

nucleo

lin)

[115]

RERT-lncRNA

(RAB

4B-EGLN

2)N.D.

N.D.

Hep

atocellular

carcinom

a(HCC)

Theexpression

levelsof

RERT-lncRNAandEG

LN2

weresign

ificantly

correlated

inHCC

EGLN

2expression

↑N.D.

N.D.

Tran

scription

alregulation

(RERT-lncRNAindu

ces

EGLN

2/PH

D1expression

atthetranscrip

tional

level)

[116]

UBE2CP3

N.D.

N.D.

Hep

atocellular

carcinom

a(HCC)

•Up-regu

latedin

HCC,

espe

ciallyin

high

EV(end

othe

lialvessel)

density

tissues

•UBE2C

P3expression

combine

dwith

EVde

nsity

isassociated

with

HCCpatient

prog

nosis

Proliferatio

n↑

Migratio

n↑

Tube

form

ation

↑

N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

hUBE2C

P3-in

duced

increase

inthesecretion

ofVEGFA

into

the

supe

rnatantviaactivation

oftheERK/HIF-1α

sign

alingpathway)

[117]

UCA

1Up-regu

lated

HIF-1α-

depe

nden

tEstrog

enreceptor

(ER)-

positivebreastcancer

N.D.

Tamoxifen

resistance

↑miR-18a

Theexpression

ofHIF-1α

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-18a-m

ediated

repression

ofHIF-1α

expression

)

[118]

Up-regu

lated

N.D.

Hypoxia-resistant

gastric

cancer

(HRG

C)

Upreg

ulated

inHRG

Ccells

Migratio

n↑

miR-7-5p

Theexpression

ofEG

FRSe

que

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-7-5p-med

iated

repression

ofEG

FR)

[119]


Table

1|H

AL-med

iatedHIFsign

alingcontroland

cancer

prog

ression(Con

tinued)

lncRNA

Status

upon

hypo

xia

HIFinvolvem

ent

CancerType


Functio

nal

Impact

Interactor

Target/Effect


nRefs

Up-regu

lated

N.D.

Acute

myeloid

leukem

ia(AML)

Upreg

ulated

following

ADR(adriamycin)-b

ased

chem

othe

rapy

Cytotoxiceffect

ofADR↓

HIF-1α-

depe

nden

tglycolysis↑

miR-125a

Theexpression

ofHK2

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-125a-med

iated

repression

ofHK2)

[120]

Up-regu

lated

HIF-1α

depe

nden

t(2

HREs)

Bladde

rcancer

•Upreg

ulated

inbladde

rcancer

•UCA

1expression

associated

with

the

clinicalstageand

histolog

icgradeof

bladde

rcancer

Cellp

roliferation

unde

rhypo

xia↑

Invasion

&migratio

nun

der

hypo

xia↑

Apo

ptosisun

der

hypo

xia↓

N.D.

N.D.

Unc

lear

mecha

nism

(UCA

1couldmod

ulate

theexpression

ofseveral

gene

sinvolved

intumorigen

icpo

tential,

drug

resistance

and

embryonicde

velopm

ent)

[121,

122]

Up-regu

lated

HIF-1α

depe

nden

t(1

HRE

iden

tified)

Osteo

sarcom

aN.D.

Cellg

rowth

↑N.D.

N.D.

Unc

lear

mecha

nism

(May

partially

throug

hinactivatingthePTEN

/AKT

sign

alingpathway)

[123]

WT1-AS

Up-regu

lated

HIF-1

depe

nden

t(DNA

demethylatio

nof

theCpG

island

)

Myeloid

Leukem

ia•Upreg

ulated

inWilm

s’tumors

•Abe

rrantWT1-AS

splicingoftenfoun

din

acutemyeloid

leukem

ia

N.D.

N.D.

N.D.

Epigen

eticregulation

(WT1-ASmed

iateshypo

xia-

indu

cedWT-1mRN

Aup

regu

latio

nthroug

hmod

ulatinghiston

emethylatio

n)

[124,

125]

ZEB2-AS1

Up-regu

lated

HIF-1α

depe

nden

tGastriccancer

(GC)

•Upreg

ulated

inGC

•Correlatedwith

poor

differentiatio

n,lymph

node

metastasisand

distantmetastasis

Cellp

roliferation

andgrow

th↑

Invasion

↑In

vivo

tumor

grow

th↑

miR-143-5p

Theexpression

ofHIF-1α

Seque

stration

ofmiRNAs

(Dow

n-regu

latio

nof

miR-143-5p-med

iated

repression

ofHIF-1α

expression

)

[126]

Abb

reviation:

CRCcolorectal

cancer,C

SCcancer

stem

cell,EM

Tep

ithelial–mesen

chym

altran

sitio

n,GCGastriccancer,H

CChe

patocellularcancer,H

REhy

poxiarespon

seelem

ent,HUVECs

human

umbilical

vein

endo

thelialcells,ICC

Immun

ocytoche

mistry,LC

lung

cancer,M

-GSC

sMesen

chym

alglioblastomamultiformestem

-like

cells,N

.D.N

otde

term

ined

,NSC

LCno

n-sm

allcelllun

gcarcinom

a,OSC

COralsqu

amou

scell

carcinom

a,PD

ACpa

ncreaticdu

ctal

aden

ocarcino

ma,RC

CRe

nalC

ellC

arcino

ma,RN

Prib

onucleicprotein,

TNM

tumor,n

ode,

metastasis,VH

Lvo

nHippe

l-Linda

uprotein,

WHOWorld

Health

Organ

ization


may also have hypoxia-independent functions. For thesake of conciseness, those targets are not included in thetable. In addition, some of these lncRNAs can be capturedby exosomes and transmitted to tumor microenvironmentto exert their functions and further propagate the hypoxicresponses (Table 2). Notably, several HALs, such asUCA1, PVT1, H19 and MALAT1, might adapt more thanone action mode in different cancer types. In the discus-sion below, we highlight the selected few HALs to illus-trate their mechanisms of actions.

HAL-mediated epigenetic and transcriptional regulationA large number of lncRNAs are localized in the nucleus,participating in various biological processes, includingchromatin organization, nuclear structure, transcrip-tional and post-transcriptional regulation of gene expres-sion. With regard to chromatin organization, thepangenomic investigations of RNA–protein interactionshave shown that two hypoxia-inducible, oncogenic anti-sense RNAs ANRIL (also known as CDKN2B antisenseRNA 1) and HOTAIR (HOX transcript antisense RNA)[50, 80] could interact with different histone-modifyingcomplexes, and have thus been proposed to impact thechromatin modification and transcriptional state [138].However, whether these two antisense RNAs are in-volved in modulating gene expression in response tohypoxia via epigenetic modification or chromatin re-organization remains to be characterized. In addition,WT1-AS could mediate hypoxia-induced upregulation of

oncogenic transcription factor WT-1 in cis throughmodulating histone H3K4 and H3K9 methylationaround the transcription start site of WT1 mRNA, con-tributing to acute myeloid leukemia (AML) progression[124]. Similarly, in gastric cancer, lncRNA-AK058003,which could be profoundly induced by hypoxia, residesupstream of SNCG (synuclein gamma, a synuclein familymember, promotes migration, invasion and metastasis)and enhances SNCG expression in cis through demethyl-ation of SNCG promoter CpG islands, thereby drivinghypoxia-induced metastasis [90]. In the context of naso-pharyngeal carcinoma (NPC), up-regulated PVT1 couldserve as a scaffold for a transcriptional activator, the his-tone acetyltransferase KAT2A, to activate transcriptionof NF90. NF90, a RNA-binding protein, has been re-ported to stabilize many target mRNAs, including HIF1AmRNA. Indeed, the upregulated NF90 increased HIF1AmRNA stability and promoted malignant transformationof NPC cells [111]. In addition, in hypoxia-injured pheo-chromocytoma cells, up-regulated MEG3 (maternallyexpressed gene 3) could recruit methylation proteinsDNMT3a, DNMT3b and MBD1 to facilitate TIMP2 pro-moter methylation, which in turn inhibited the expres-sion of this cell cycle arrest inducer TIMP2. Moreover, aHIF-1α negative regulator, OS-9, is reported to facilitateHIF-1α hydroxylation and subsequent proteasomal deg-radation through tethering the interaction between HIF-1α and prolyl hydroxylases (PHDs) [139]. Interestingly,in pancreatic ductal adenocarcinoma (PDAC), another

Table 2 | HALs identified extracellularly

LncRNA Extracellular space identified Cell to Cell Transfer Functional Impact Mechanism Ref

aHIF(HIF1A-AS2)

Serum(aHIF level in serum correlates withits expression in matchedectopic endometria)

Endometriotic cyst stromal cells(ECSCs)-derived exosomes tohuman umbilical vein endothelialcells (HUVECs)

Elicits proangiogenicbehavior in HUVECs, thusfacilitating endometriosisangiogenesis.

Activates VEGF-A, VEGF-D, and b-FGF in HUVECs

[133]

CCAT2 Exosomes secreted from culturedglioma cells

U87-MG glioma cells to HUVECs Promotes HUVECangiogenesis and inhibitsapoptosis induced byhypoxia

Promotes VEGF-A, TGF-βand Bcl2 expression.Inhibits BAX and caspase3 expression

[134]

HISLA(LINC01146)

Extracellular vesicles secreted bytumor associated fibroblasts (TAMs)

TAMs to breast cancer cells Enhances aerobicglycolysis and apoptoticresistance of cancer cells

Stabilizes HIF-1α [135]

PVT1 Exosomes secreted from culturedcolon cancer cells. Cancer cells withmore aggressive phenotypes havemore extracellular PVT1

Not determined Promotes cellproliferation and inhibitsapoptosis.

[136]

linc-ROR Exosomes secreted from culturedhepatocellular carcinoma cells

HCC cancer cells to cancer cells Promotes cell survival ofrecipient cells

Through a miR-145–HIF-1α signaling module toincrease HIF-1αexpression

[87]

UCA1 Exosomes secreted from culturedbladder cancer cells & serum

Bladder cancer 5637 cells withhigh expression of UCA1 tobladder cancer UMUC2 cells withlow expression of UCA1

Promotes cellproliferation, migrationand invasion of recipientcellsPromotes xenograftgrowth

Through regulating theexpression of genesinvolved in EMT (E-cad,MMP9, vimentin)

[137]


lncRNA ENST00000480739 could inhibit HIF-1α by up-regulating OS9 (osteosarcoma amplified-9) expressionthrough enhancing the acetylation of H3K27 within OS9gene promoter [60]. Of note, in PDAC, the level ofENST00000480739 is markedly downregulated, andnegatively correlated with lymph node metastasis, inagreement with its negative regulatory role in HIF-1 sig-naling [60]. As ENST00000480739 resides upstream ofthe OS9 promoter region, this lncRNA also act in cis toinduce OS9 transcription.Apart from chromatin structure remodeling, a series

of HALs could modulate transcription and therebyfine-tune the HIF network. For instance, lncRNAHIF2PUT (HIF-2α promoter upstream transcript),RERT-lncRNA and hypoxia-inducible BC005927 are allfound to act in cis to up-regulate neighboringprotein-coding genes HIF2A (encodes HIF-2α),EGLN2 (encodes prolyl hydroxylase PHD1) andEPHB4 (encodes Ephrin type-B receptor 4, ametastasis-related gene), at the transcriptional level,respectively [51, 76, 116].Moreover, HALs could directly act on specific tran-

scription factors through physical interactions tomodulate their transactivation activities. We recentlyidentified a hypoxia-inducible lncRNA LncHIFCAR(long noncoding HIF-1α co-activating RNA, alsoknown as MIR31HG) acting as a HIF-1α co-activatorvia direct interaction with HIF-1α, thereby enhancingthe binding of HIF-1α and cofactor p300 to the targetloci (Fig. 1b). As the abundance of the HIF complexincreases, the hypoxia-induced HIF-1 signaling cas-cade is augmented to further promote subsequentcancer progression [89]. Meanwhile, in pancreaticcancer, HIF-1α-induced lncRNA-MTA2TR (MTA2transcriptional regulator RNA) transcriptionally up-regulates the expression of oncogenic MTA2 (metas-tasis associated protein 2) by recruiting ATF3 (acti-vating transcription factor 3) to the promoter area ofMTA2 [102]. Subsequently, MTA2 can enhance theaccumulation of HIF-1α protein via MTA2-mediatedHIF-1α deacetylation and stabilization, which furtheractivates HIF-1α transcriptional activity, forming feed-back loops to augment HIF-1 signaling [102] (Fig. 1c).In addition, through binding to PSF (PTB-associatedsplicing factor), hypoxia-induced lncRNA MALAT1released PSF from its downstream proto-oncogeneGAGE6 (proto-oncogene G antigen 6) and activatedits transcription, thereby promoting proliferation, mi-gration and invasion of lung adenocarcinoma cells[98, 99]. Given the extraordinary variety of transcrip-tional regulatory machinery discovered in the cell, itis anticipated that more lncRNAs-mediated regulationon hypoxia-induced transcriptional program will beunraveled in the imminent future.

HAL-mediated post-transcriptional controlHALs also participate in post-transcriptional regulationincluding mRNA stability and miRNA-mediated genesilencing.

mRNA stability control Three HALs, lncRNA-LET(Long noncoding RNA Low Expression in Tumor),DANCR (Differentiation Antagonizing Non-ProteinCoding RNA) and HIF1A-AS2 (HIF1A Antisense RNA2; also known as aHIF), have all been reported to affectHIF1A mRNA stability. lncRNA-LET expression is gen-erally suppressed in various types of tumors, whereashypoxia-induced HDAC3 (histone deacetylase 3) couldrepress its expression by reducing the histone acetylationof the lncRNA-LET promoter region [93, 140]. Mechan-istically, lncRNA-LET is bound to NF90 (nuclear factor90), which increases NF90 degradation by the prote-asome. As RNA binding protein NF90 could stabilizeHIF1A mRNA [93, 141], the downregulation of lncRNA-LET upon hypoxia plays a key role in the stabilization ofNF90 protein, thereby increasing HIF-1A mRNA stabilityupon hypoxia and accordingly hypoxia-induced cancercell invasion [93] (Fig. 1d). Likewise, in nasopharyngealcarcinoma, another oncogenic lncRNA DANCR was up-regulated and associated with lymph lode metastasis andpoor survival [57]. Through interaction with the NF90/NF45 complex, DANCR could increase HIF1A mRNAstability, leading to metastasis and disease progression.In addition, another hypoxia-inducible antisense

lncRNA HIF1A-AS2, was shown to be up-regulated invarious tumors [42, 43, 46, 142, 143] and could differen-tially regulate HIF-1α and HIF-2α expression duringlong-term hypoxic conditions [43, 47]. Upon acute hyp-oxia, HIF-1α and HIF-2α were similarly induced. Inter-estingly, during prolonged hypoxia, these two proteinswere differentially regulated as HIF-1α protein levelgradually decreased due to a reduction in its mRNA sta-bility, whereas HIF-2α protein remained upregulated.Meanwhile, long-term hypoxia also induced an increasein HIF1A-AS2, whose gene promoter harbors functionalHREs. During prolonged hypoxia, HIF1A-AS2 couldbind to its sense counterpart, the HIF-1A mRNA 3′-UTR, and possibly expose the AU-rich elements in thisregion, thereby destabilizing HIF-1A mRNA to conveytarget gene specificity [43, 47]. Paradoxically, HIF1A-AS2was also shown to sequester miR153-3p (see next sec-tion) to enhance HIF-1A expression [44]. Thus, themode of action of HIF1A-AS2 is complex and likelycontext-dependent.

miRNA sponges A wealth of lncRNAs adapt a well-characterized, common mechanism, “ceRNA (competingendogenous RNA)” or “RNA sponges”, to repressmiRNA-mediated gene silencing. The ceRNAs compete


for shared miRNAs, sequester these miRNAs and dimin-ish their silencing effect on target mRNAs.Functional manipulations have demonstrated that sev-

eral HALs, such as lincRNA-ROR [87], PVT1 [113, 114],HIF1A-AS2 [44], UCA1 [118], HOTAIR [79], FEZF1-AS1[63], ZEB2-AS1 [126] and H19 [66], could act as a‘ceRNA’ to reduce individual specific miRNA-mediatedHIF1A mRNA destabilization and thereby restoring HIF-1α levels and consequently promote cancer progression(Table 1). Specifically, in breast cancer stem cells, by ab-sorbing endogenous miRNA let-7 and aborting let-7-mediated HIF1A mRNA suppression, hypoxia-inducedH19 could stimulate HIF-1α expression [66] (Fig. 1e). Inaddition, in glioblastoma, hypoxia-induced H19 up-regulation has been shown to confer an aggressivebehavior by sequestering miR-181d and nullifying itssuppression on an oncogenic EMT-associated factor, β-catenin [68].In a similar way, certain HALs could act as a ceRNA

to modulate other hypoxia-responsive regulators thanHIF-1α. In gastric cancer, GAPLINC (Gastric Adeno-carcinoma Associated, Positive CD44 Regulator, LongIntergenic Non-Coding RNA) is a HIF-1α direct, tran-scriptional downstream target, and could promote inva-sive tumor progression [64]. Mechanistically, GAPLINCcould serve as a decoy for miR-211-3p to restore thelevels of cancer stem cell marker CD44, enhancingtumor progression [65]. Aside from GAPLINC, NORAD[108], UCA1 [119, 120], HOTTIP [82], EIF3J-AS1 [59],MALAT1 [100], FAM201A [62], AGAP2-AS1 [49],LINC01436 [84], NEAT1 [103], NUTF2P3 [109]lncRNAs were shown to function in this way (Table 1).Collectively, in response to hypoxia, the crosstalkamong the lncRNA and miRNA transcriptomes build areciprocal repression feedback network, eliciting con-cordant shift to transcriptional reprogram. Further ex-ploration of this pertinent co-working group oflncRNAs and miRNAs under hypoxic conditions wouldhelp appreciate this emerging additional layer of post-transcriptional regulation governed by HALs.

HAL-mediated control of protein activity, stability and/orhigher-order complex formationIn addition to acting as ceRNAs to modulate gene ex-pression through interaction with miRNAs, HALs havemultiple molecular modes to act at the protein level tofurther modulate gene expression. One of the hypoxia-induced lncRNAs, PVT1 (plasmacytoma variant trans-location 1), was implicated in cervical cancer progres-sion, likely through its interaction with a multifunctionalshuttling protein, nucleolin [115]. In multiple cancer celllines, HIF-1-induced lincRNA-p21 provides another ex-ample as to how HALs modulate hypoxia response byprotein sequestration. Through separate binding to HIF-

1α and VHL, lincRNA-p21 could increase HIF-1α accu-mulation by disruption of the VHL/HIF-1α interactionand subsequent attenuation of VHL-mediated HIF-1αubiquitination and degradation [85] (Fig. 1f). AnotherHIF-1α binding lncRNA CASC9 (cancer susceptibilitycandidate 9) is highly expressed in nasopharyngeal car-cinoma (NPC) tissues. CASC9 could interact with andstabilize HIF-1α, promoting the glycolysis and tumori-genesis of NPC cells [53].Nevertheless, in addition to fine-tuning the activity of

one single protein, HALs can also dynamically modulatehigher-order protein organizations by serving as scaffoldsor molecular decoys. In mesenchymal glioblastoma stem-like cells, through direct binding to two RNA binding pro-teins, DHX9 (ATP-dependent RNA helicase A) andIGF2BP2 (insulin-like growth factor 2 mRNA-bindingprotein 2), lncRNA HIF1A-AS2 could facilitate the inter-action between this protein complex and their mRNA tar-get HMGA1 (high mobility group AT-hook 1), therebyenhancing HMGA1 expression as well as the downstreammolecular response to hypoxic stress [46, 47].In triple-negative breast cancer (TNBC), LINK-A (long

intergenic non-coding RNA for kinase activation) has acritical role in the growth factor-induced HIF-1α signal-ing under normoxic conditions [88]. LINK-A is requiredfor the recruitment of BRK (breast tumor kinase) andsubsequent enzymatic activation, which is stimulated byHB-EGF (Heparin-binding EGF-like growth factor) sig-nal. HB-EGF mediates the heterodimerization of EGFR(epidermal growth factor receptor) and GPNMB (trans-membrane glycoprotein NMB) to form ‘EGFR:GPNMB’complex. Due to its direct interaction with BRK andLRRK2 (leucine-rich repeat kinase 2), LINK-A could re-cruit these two kinases to EGFR:GPNMB heterodimer,thereby inducing their kinase activities, resulting in HIF-1α phosphorylation: BRK-mediated HIF-1α phosphoryl-ation at Tyr565, a phosphorylation preventing theadjacent Pro564 hydroxylation of HIF-1α and subsequentHIF-1α degradation under normoxic conditions; andLRRK2-mediated HIF-1α phosphorylation at Ser797,which facilitates the interaction of HIF-1α with the tran-scriptional cofactor p300 [88] (Fig. 1g). In TNBC sam-ples, both LINK-A abundance and HIF-1 signalingactivation are correlated with cancer progression andshorter survival, revealing potential therapeutic targetsfor TNBC [88].An additional novel function of lncRNAs is their

structural role in the assembly of nuclear domains. Forinstance, MALAT1 (metastasis-associated lung adenocar-cinoma transcript 1, also known as NEAT2) and NEAT1(nuclear enriched abundant transcript 1) are located intwo well-characterized nuclear bodies, nuclear specklesand paraspeckles, respectively. Also known as SC35 spli-cing domains, nuclear speckles are membrane-less


compartments and their formation involves “phase-sep-aration” mediated by aggregated lncRNAs and proteins.Being an abundant component of the nuclear speckles,MALAT1 associates with numerous splicing factors andother SR (serine/arginine-rich) proteins, and is requiredfor their correct localization to the nuclear speckles, al-though the overall nuclear speckle assembly is notdependent on the abundance of MALAT1 [144, 145]. Sofar, the functional involvement of MALAT1 in RNAsplicing in response to hypoxia remains to be deter-mined. In contrast, lncRNA NEAT1 is shown to be anessential architectural component of nuclear para-speckles [144, 145]. The precise function of paraspecklesremains largely elusive, but proposed to regulate geneexpression via the retention of hyper-edited RNA andother multifunctional factors in the nucleus [104]. Giventhe functional involvement of both MALAT1 andNEAT1 in nuclear structure, further investigation of theextent to which these nuclear structures and their asso-ciated transcription reprogramming respond to hypoxiawill deepen our understanding of the cellular dynamicresponse to hypoxia.

HAL-mediated control of hypoxia response via unclearmechanismAs listed in Table 1, most of the HALs identified withprofound impact on tumorigenesis have not yet been ex-amined in mechanistic detail. However, other reports re-garding the same lncRNA with functionalcharacterization might reveal clues about their biologicalroles in response to hypoxia. For instance, lncRNAPCGEM1 was found to be overexpressed in gastric can-cer, and could be induced by hypoxia [110]. In gastriccancer cells, PCGEM1 could promote the invasion andmetastasis through activating the expression of SNAI1, akey transcription factor of EMT, though the underlyingmechanism remains elusive [110]. Notably, in prostatecancer, our group previously reported that the oncogenicPCGEM1 could promote chromatin recruitment of c-Myc and enhances its transactivation activity throughdirect physical interaction [146]. As SNAI1 is a well-characterized downstream gene of c-Myc, the possiblefunctional role of the PCGEM1/c-Myc/SNAI1 signalingaxis in hypoxia-associated cancer progression warrantsfurther investigation.In summary, as noted in the above sections, given

the relatively large size and the structural flexibility oflncRNAs, it is to be expected that they interact withmultiple RNA or protein components and have multi-functions, perhaps in a context-dependent manner. Assuch, their roles in hypoxia responses and in tumorprogression may differ appreciably in different cancertypes.

LncRNAs as predictive biomarkers and therapeutic targetsfor hypoxic tumorExtracellular vesicles-containing HALs and their biologiceffects on tumorigenesisExtracellular vesicles are effective devices for transport-ing biomolecules among various cells types [147, 148].Based on the difference in size and biogenesis, cell-derived extracellular vesicles can be broadly divided intotwo main categories: exosomes (30–100 nm in diameter)and microvesicles. Together with proteins and othernon-coding RNAs, emerging evidence has shown thatlncRNAs are packaged into exosomes [149, 150], andthe abundance of lncRNAs in exosomes correlates withtheir expression level in the cell of origin [151]. Throughexosomal transfer, several lncRNAs are shown to po-tentiate cell responses to hypoxia between cancer cells[87], as well as between cancer cell and the associatedmicroenvironment [150]. Table 2 summarizes hypoxia-associated lncRNAs identified extracellularly. For ex-ample, linc-ROR was found abundant in tumor cells aswell as in exosomes derived from tumor cells [87]. It isincreased both in cells or exosomes during hypoxia, andit up-regulates HIF-1α expression by absorbing miR-145.By co-culture systems, linc-ROR-containing exosomesincrease HIF-1a transcription in recipient cells [87].Hypoxia can shape and fine tune specific macrophagephenotypes in the tumor milieu that are known to pro-mote tumor progression [152]. Chen et al found lncRNAHISLA (also known as LINC01146), secreted by tumor-associated macrophages, stabilized HIF-1α and enhancedaerobic glycolysis in cancer cells, leading to contagiousmetabolic reprogramming within tumor regions [150].PVT1, a lncRNA that often co-amplifies with c-myc andfunctions as miRNA sponge to upregulate HIF-1α ex-pression [153, 154], is another example of exosomaltransfer between TAMs (tumor associated macrophages)and cancer cells. PVT1 is detected in exosomes derivedfrom colon cancer cells, particularly in more aggressivecells [136]. In granulocytic myeloid-derived suppressorcells (G-MDSCs), PVT1 was up-regulated by HIF-1αunder hypoxia and contributed to immunosuppression,given its depletion reduced the suppression of these cellson T-cells and delayed tumor progression [155]. Otherexosomal-transferred lncRNAs that are implicated incancer cells during hypoxia include UCA1 in bladdercancer for promoting tumor growth and EMT [137], andCCAT2 for glioma’s resistance to apoptosis and angio-genesis [134].The functions of lncRNAs in exosomes for tumor pro-

gression await to be explored given a significant level ofnon-coding RNAs are revealed in exosomes (and ele-vated upon hypoxia) whereas only a small fraction hasbeen studied [149, 150, 156]. Accordingly, it is conceiv-able that multiple tumor phenotypes and signaling


pathways are affected upon exosomal loading. Indeed, bymicroarray analyses, Mao et al showed hundreds oflncRNAs, together with other transcripts, are changed inendothelial cell recipients of exosomes derived fromsquamous cancer cells [157]. Importantly, they foundexosomes obtained from hypoxic condition facilitateangiogenesis and metastasis better than those obtainedfrom normoxic condition in a xenograft model. Similareffects between normoxic exosomes and hypoxic exo-somes on angiogenesis were found in a mouse xenograftmodel of glioblastoma, with additional effect on acceler-ating tumor expansion at later stage [158]. The elevationin transcripts by exosomes could result from direct genetransfer, or sequential effects mediated by the trans-ferred genes. By which mechanism lncRNAs are selectedto be packaged in the exosomes upon stimuli is notknown; nevertheless, these studies revealed exosomes asa means by which hypoxia in the tumor microenviron-ment facilitates tumor cells to spread and progress.

Diagnostic potential of HALsSeveral HALs with known oncogenic functions havebeen detected in patient-derived exosomes, includingH19 in serum from patients with bladder cancer [159],HOTAIR in urinary exosomes from patients with urothe-lial bladder cancer [156], UCA1 in serum from bladdercancer patients [137], and HIF1A-AS2 in patients withendometriosis [133]. Future studies aimed at identifyinghypoxia-responsive transcripts in extracellular vesicleswould surely reveal more players in this aspect. Bearingdifferential expression patterns between normal and ma-lignant stages and/or tumor size, oncogenic lncRNAsthat can be detected extracellularly would potentiallyserve as non-invasive biomarkers for early detection,prognosis prediction, and disease surveillance. PCA3,up-regulated in > 90% of men with prostate cancer, is anexample of this [160]. A urine-based assay has been ap-proved by the United States Food and Drug Administra-tion (FDA) since 2012 as an alternative diagnostic testfor patients undergoing repeat prostate biopsy or withprevious negative prostate biopsy.As described above, there is considerable evidence in-

dicating hypoxia as a progression factor for tumor devel-opment [161]. Hypoxia promotes angiogenesis, tumormetastasis, immune evasion and therapy-resistance. Theoxygenation status of tumor was reported to influencelocal tumor response to radiation treatment, as well asoverall survival in a variety of tumors [162–164]. Che-motherapeutic drugs, such as Docetaxel and Sorafenib,also tend to be more effective in normoxic conditions[165, 166]. The hypoxic regions in tumors are infiltratedwith cells which promote tumor tolerance (regulatory T-cells, myeloid-derived suppressor cells, and macrophages),while antitumor T-cells are devoid and inhibited by HIF-

1α-mediated accumulation of extracellular adenosine[167–169]. PD-L1 (Programmed death-ligand 1), a ligandexpressed by tumor cells or myeloid-derived suppressorcells to suppress T-cell’s anti-tumor immunity, is up-regulated by and a direct target of HIF-1α during hypoxia[170]. It has become increasingly apparent that hypoxia intumors fosters immune suppression and prevents effectiveimmunotherapy. Considering the ill-effects of hypoxia, itis important to detect and to overcome tumor hypoxiaeven before therapy starts, for the best of patient care.By far, while there has been a great deal of interest in

methodologies to measure hypoxia in patients, an effi-cient, non-invasive, while sensitive method to detectsmall regions of hypoxia that frequently occur in the tu-mors is still lacking [163]. A few metabolic markers(HIF-1α, HIF-2, CA9 and GLUT1) have been used to as-sess low oxygen tensions by immunohistochemistry[171, 172]; however, the application of them in clinic islimited given that their expressions can be triggered byfactors other than hypoxia and that biopsies only repre-sent a small sampling of the tumor. As exosome com-position mirrors the hypoxia status of tumors [158], ahypoxia signature may be formulated based on the exo-somal hypoxia-responsive transcripts including HALs toevaluate oxygenation in the body for clinical exploit-ation, once our knowledge is advanced.

Therapeutic potential of HALs (targeting hypoxia in cancertherapy, a lncRNA perspective)Several approaches have been proposed to target hypoxiain tumor [161, 163]. These include drugs that induce celldeath selectively in hypoxic cells, e.g. hypoxia-activatedprodrugs, or drugs sensitizing hypoxic cells to radiation.Since the adaptive response to hypoxia mainly orientsfrom the transactivation of HIF signaling, some ap-proaches seek to block hypoxia-induced responses by tar-geting HIFs and the related signaling, or to targetpathways that also play pivotal roles in hypoxia adaptation,such as signaling involving mTOR, DNA damage re-sponse, and the unfolded protein response. In that regard,HALs that are elevated upon hypoxia and contribute totumor progression in pre-clinical studies could potentiallyserve as molecular targets, e.g. PVT, LncHIFCAR, etc. (seeTable 1) [41]. By contrast, HALs that are repressed inorder to magnify hypoxia response, such as lncRNA-LET,could be induced for therapeutic intervention.Various strategies have been developed to modulate

RNAs. Silencing lncRNAs by small interfering RNAs, anti-sense oligonucleotides (ASOs), or ribozymes and deoxy-nucleotides are well demonstrated in pre-clinical studies.Until now, three ASOs and one aptamer therapies havebeen approved by the FDA for diseases and a handful ofothers are in clinical trials. The development of short oli-gonucleotides that fold into three-dimensional structures,


aptamers, offers a greater specificity as they target sp

signaling in and out: long-noncoding rnas in tumor hypoxia...review open access signaling in and...

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