The Journal of Clinical Investigation | October 2001 | Volume 108 | Number 8 1105

The history of diabetes mellitus isreplete with many therapies, nearly all,including insulin, first given withoutany knowledge of a mechanism ofaction. Beginning with early Egyptianphysicians who, according to the EbersPapyrus of 1500 B.C.E., prescribed amixture of cakes, wheat grains, freshgrits, green lead, earth, and water, upto the more recent introduction of thethiazolidinediones, knowledge ofphysiologic effects preceded by yearsor centuries any knowledge as to whatthe medicament actually did.

In medieval times, a prescription ofGalega officinalis was said to relieve theintense urination accompanying thedisease that came to have the name ofdiabetes mellitus. G. officinalis, alsoknown as Goat’s rue, the French lilacor Italian fitch (Figure 1), was alsogiven during the plague epidemics topromote perspiration and has beenused as a galactogogue in cows. Theactive ingredient in the French lilacthat produced the lowering of bloodglucose was shown to be galegine orisoamylene guanidine (1). A curiouschapter in the history of guanidine-based hypoglycemic agents arose fromthe mistaken notion that the tetany ofhypoparathyroidism was due to theproduction of increased guanidine fol-lowing parathyroidectomy, leading tothe demonstration that an infusion ofguanidine produced lowering of bloodglucose (2). While guanidine itself andcertain derivatives are too toxic for thetreatment of diabetes mellitus, thebiguanides (two linked guanidinerings) have proved useful, and threebiguanides became available for dia-betes therapy in the 1950s. Phen-

formin and buformin, the formerbecoming quite popular in the 1960s,were withdrawn from the pharma-copoeia in the early 1970s due to theemergence of frequent lactic acidosisand increased cardiac mortality (1).Metformin, a less lipophilic biguanide,proved safer and, after 20 years of usein Europe, was approved for use in theUSA in 1995.

Metformin in the clinicMetformin is now a mainstay of thera-py in the treatment of type 2 diabetesand, as noted in the recently terminat-ed Diabetes Prevention Program, is

also an effective agent to decrease therisk of development of the disease (1,3, 4). In the United Kingdom Prospec-tive Diabetes Study, metformin alsoproved to be effective in decreasingdiabetes-related death, myocardialinfarction, and stroke (5). Metforminnot only lowers blood glucose but alsoinhibits adipose tissue lipolysis,reduces circulating free fatty acids, anddiminishes VLDL production (1, 3, 4).Among the known actions of met-formin are an improvement in insulinsensitivity in muscle and liver, adecrease in hepatic glucose productionvia gluconeogenesis, an increase in

The blooming of the French lilac

Lee A. WittersEndocrine-Metabolism Division, Departments of Medicine and Biochemistry, Dartmouth Medical School, Remsen 322, Hanover, New Hampshire 03755-3833, USA. Phone: (603) 650-1909; Fax: (603) 650-1727; E-mail: lee.a.witters@dartmouth.edu.

J. Clin. Invest. 108:1105–1107 (2001). DOI:10.1172/JCI200114178.

CommentarySee related article, pages 1167–1174.

Figure 1The bloom of the French lilac. This blooming G.officinalis (Goat’s rue; French lilac; Italian fitch) isrich in guanidine. The plant’s long-recognizedhypoglycemic properties led eventually to the syn-thesis of the biguanide compound metformin.

peripheral glucose utilization (pre-dominantly through a stimulation ofinsulin-mediated muscle glucoseuptake and glycogen synthesis), andpositive effects on insulin receptorexpression and tyrosine kinase activity(1). In addition, metformin appears tosuppress the gluconeogenic effects ofglucagon and to increase the translo-cation of glucose transporters to thecell surface (4). The exact molecularlocus of metformin’s effects, however,remain obscure. Two recent studiessuggested that its target, albeit indi-rect, is the respiratory chain complex I,leading to an inhibition of mitochon-drial respiration (6, 7).

Metformin and the AMP-activated protein kinaseIn this issue of the JCI, Zhou andcoworkers provide evidence that theelusive target of metformin’s actions isthe AMP-activated protein kinase(AMPK) (8). In studies performed inisolated hepatocytes and rat skeletalmuscles, they demonstrate that met-formin leads to AMPK activation,accompanied by an inhibition of lipo-genesis (due to inactivation of acetyl-CoA carboxylase and suppression oflipogenic enzyme expression), suppres-sion of the expression of SREBP-1 (acentral lipogenic transcription factor),and a modest stimulation of skeletalmuscle glucose uptake. Similar hepat-ic effects are seen in metformin-treatedrats. Based on the use of a newly dis-covered AMPK inhibitor, their datasuggest that the ability of metforminto suppress glucose production inhepatocytes requires AMPK activation.Many of these effects are similar to thatof 5-aminoimidazole-4-carboxamideribonucleoside (AICAR), a known,albeit nonspecific, activator of AMPK(9). However, metformin does not leadto AMPK activation in vitro, indicatingthat it activates the kinase indirectly.Thus, the tale of metformin’s proximalcellular target continues.

AMPK, first identified as a kinaseactive on HMG-CoA reductase, is oneimportant member of a largeserine/threonine protein kinase family,called the AMPK/SNF1 family (10–12).This family is found in all eukaryotes,including plants, yeast, Drosophila,Caenorhabditis elegans, and mammals.AMPK is a heterotrimeric enzyme, con-sisting of an α catalytic subunit andnoncatalytic, regulatory β and γ sub-

units. Each AMPK subunit is repre-sented in an isoform family, derivedfrom different genes. The binding ofthe β/γ subunits to the α subunit regu-lates enzyme activity, targeting, and αprotein turnover (13). Both the α (mul-tisite phosphorylation via upstreamAMPKKs) and β (myristoylation, mul-tisite phosphorylation) subunits areregulated by posttranslational modifi-cation (10–12, 14). The γ subunits,while not posttranslationally modified,are composed of repetitive CBSdomains (for cystathione β-synthasewhere they were first identified) andare critical for AMP regulation (15, 16).

AMPK subunits appear to beexpressed in all mammalian tissueswith a mixture of isoforms and specif-ic heterotrimers represented in a tissue-specific fashion. AMPK is subject tocomplex regulation via allosteric andcovalent mechanisms, which alter cat-alytic subunit activity, its heterotrimerstructure, and its cellular localization(10–12). Allosteric regulation by AMPand ATP was the first recognized regu-latory mechanism. The enzyme is verysensitive to AMP activation (Ka 20 µM)and is inhibited at high ATP. There isno consensus AMP binding site on anyof the subunits; AMP allostery appearsto be due to a higher-order structure,involving all the subunits, but especial-ly the γ subunit. This sensitivity toAMP and ATP creates a metabolic sen-sor that is able to monitor very smallchanges in cellular ATP content. Dur-ing early ATP depletion, as in limitedglucose availability, glycogen exhaus-tion, muscular exercise, or hypoxia,adenylate kinase catalyzes the resyn-thesis of ATP from accumulating ADP(2ADP→ATP+AMP). This means thatthe ratio of AMP to ATP actually variesas a function of (ADP/ATP)2, makingAMPK very sensitive to minimal ATPdepletion. While Zhou et al. report nochanges in cellular ATP levels in theirstudy, no measurements of AMP aregiven. Subtle but significant changes incellular adenine nucleotide pools, per-haps as a function of metformin’s abil-ity to inhibit cellular respiration, mightaccount for its AMPK-activatingeffects. Creatine phosphate, whichaccumulates in skeletal muscle onlywhen a surfeit of ATP exists, also servesas a negative allosteric regulator. ZMP(the monophosphorylated derivative ofAICAR) mimics the actions of AMP toactivate AMPK; AICAR, like met-

formin, has been shown to decreasehepatic glucose production and gluco-neogenesis, to inhibit lipolysis, lipoge-nesis, and lipogenic gene expression,and to increase skeletal muscle glucoseuptake (9, 17–19).

Targeting AMPKIs AMPK a promising target for thetreatment of diabetes mellitus? Itsknown protein and cellular targets cer-tainly suggest so (20). Several studies,with varying degrees of experimentalrigor, point to its important roles in thesuppression of gluconeogenesis in theliver, promotion of glucose uptake inskeletal muscle, inhibition of fatty acidand sterol synthesis, increases in fattyacid metabolism through oxidation,and inhibition of lipolysis (17–19, 21).AMPK also phosphorylates and acti-vates endothelial nitric oxide synthase(eNOS), which could have positiveeffects on capillary blood flow impor-tant for the diabetic (22). Some cautionis appropriate, however, for otheractions of an activated AMPK couldproduce deleterious effects. AMPK isknown to interact with and inhibit thecystic fibrosis transmembrane conduc-tance regulator (CFTR) (23). Under cer-tain conditions, AMPK activation maylead to an inhibition of insulin secre-tion (24). The promotion of fatty acidoxidation (through its ability to phos-phorylate and inhibit acetyl-CoA car-boxylase, lowering malonyl-CoA) coulddecrease cardiac function and efficien-cy after ischemia during reperfusion(25). Recently, a mutation in the geneencoding AMPK’s γ-2 subunit has beenpinpointed as a molecular defect under-lying ventricular pre-excitation in theWolff-Parkinson-White syndrome (26).This mutation, which maps to the firstCBS domain of the gamma subunit,leads to an AMP-independent activa-tion of AMPK activity (15); thus,heightened AMPK activity could lead todevelopmental and/or functional aber-rancy of cardiac conduction. AMPKactivation could also have undesirableeffects on cellular differentiation, notlimited to its known effects on adipo-genesis (27).

Nonetheless, the emerging centralrole of AMPK in the regulation of car-bohydrate and lipid metabolism pro-vides a compelling story, heightenedby the current report of Zhou andcoworkers on metformin (8). Thebloom of the French lilac may be real-

1106 The Journal of Clinical Investigation | October 2001 | Volume 108 | Number 8

ized in the flowering of a new directionin the therapy of diabetes mellitus.

Acknowledgments The author would like to acknowledgethe support of NIH grant DK-35712.

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The Journal of Clinical Investigation | October 2001 | Volume 108 | Number 8 1107