protein kinase (AMPK) and the histone/protein deacetylase SIRT1 are fuel-sensing molecules that have coexisted in cells throughout evolution. When a cell's energy state is diminished, AMPK activation restores energy balance by stimulating catabolic processes that generate ATP and downregulating anabolic processes that consume ATP but are not acutely needed for survival. SIRT1 in turn is best known historically for producing genetic changes that mediate the increase in longevity caused by calorie restriction. Although the two molecules have been studied intensively for many years, only recently has it become apparent that they have similar effects on diverse processes such as cellular fuel metabolism, inflammation, and mitochondrial function. In this review we will examine the evidence that these similarities occur because AMPK and SIRT1 both regulate each other and share many common target molecules. In addition, we will discuss the clinical relevance of these interactions and in particular the possibility that their dysregulation predisposes to disorders such as type 2 diabetes and atherosclerotic cardiovascular disease and is a target for their therapy.adenosine 5=-monophosphate-activated protein kinase; sirtuin 1; metabolic syndrome; mitochondrial function; insulin resistance; peroxisome proliferator-activated receptor-␥ coactivator-1␣; type 2 diabetes; atherosclerosis AMP-ACTIVATED PROTEIN KINASE (AMPK) is a fuel-sensing enzyme that is activated by decreases in a cell's energy state as reflected by an increased AMP/ATP ratio. When activated, it initiates metabolic and genetic events that restore ATP levels by stimulating processes that generate ATP (e.g., fatty acid oxidation) and inhibiting others that consume ATP but are not acutely required for survival (e.g., triglyceride and protein synthesis, cell proliferation) (40). In addition, AMPK sets in motion changes in mitochondrial biogenesis and function (37) that could more chronically increase the ability of a cell to generate ATP and diminish oxidative stress and other potentially adverse cellular events (78). The sirtuins are a family of evolutionarily conserved NAD ϩ -dependent histone/protein deacetylases that are also widely regarded as fuel-sensing molecules. They have many actions (see below) but are perhaps best known for their role in mediating the increase in longevity caused by caloric restriction in various species, including yeast, worms, and possibly mammals (21, 85). Seven sirtuins have been identified in mammalian cells. Of these the most studied and the focus of this review is silent information regulator T1 (SIRT1).AMPK and the sirtuins are present in all eukaryotic cells and probably have coexisted throughout evolution (7, 29). Although both molecules have been studied intensively, the similarities in their regulation and in their actions on such diverse processes as cellular metabolism, inflammation, and mitochondrial function have only recently become apparent. In this review we will examine the evidence that these similarities occur, ...

SIRT1, a histone/protein deacetylase, and AMP-activated protein kinase (AMPK) are key enzymes responsible for longevity and energy homeostasis. We examined whether a mechanistic connection exists between these molecules that involves the major AMPK kinase LKB1. Initial studies demonstrated that LKB1 is acetylated in cultured (HEK293T) cells, mouse white adipose tissue, and rat liver. In the 293T cells, SIRT1 overexpression diminished lysine acetylation of LKB1 and concurrently increased its activity, cytoplasmic/nuclear ratio, and association with the LKB1 activator STRAD. In contrast, short hairpin RNA for SIRT1, where studied, had opposite effects on these parameters. Mass spectrometric analysis established that acetylation of LKB1 occurs on multiple, but specific, lysine residues; however, only mutation of lysine 48 to arginine, which mimics deacetylation, reproduced all of the effects of activated SIRT1. SIRT1 also affected downstream targets of LKB1. Thus its overexpression increased AMPK and acetyl-CoA carboxylase phosphorylation, and conversely, RNA interference-mediated SIRT1 knockdown reduced AMPK phosphorylation and that of another LKB1 target MARK1. Consistent with the results in cultured cells, total LKB1 lysine acetylation was decreased by 60% in the liver of 48-h starved rats compared with starved-refed rats, and this was associated with modest but significant increases in both LKB1 and AMPK activities. These results suggest that LKB1 deacetylation is regulated by SIRT1 and that this in turn influences its intracellular localization, association with STRAD, kinase activity, and ability to activate AMPK.LKB1 is a serine-threonine protein kinase that phosphorylates and activates 13 downstream kinases (1), one of which is AMP-activated protein kinase (AMPK), 2 a key enzyme that regulates cellular energy state, growth, inflammation, and mitochondrial function (2). LKB1, when not associated with other proteins, is located predominantly in the nucleus because of its N-terminal nuclear localization signal. However, LKB1 activation takes place predominantly in the cytoplasm, after it complexes with STRAD (STE-related adapter) and MO25 (mouse protein 25) (1, 3). Once activated, LKB1 has been demonstrated to phosphorylate AMPK on Thr-172, an event required for its activation (4). On the other hand, no specific mechanism for regulating the activation and inactivation of the kinase activity of LKB1 has been described. Indeed, it has been suggested that LKB1 may be constitutively active and that its effects on AMPK phosphorylation (e.g. in contracting muscle) may be governed by the action of phosphatases (1, 20). SIRT1, a class III NAD ϩ -dependent histone/protein deacetylase, has been implicated in the longevity induced by caloric restriction in species ranging from Caenorhabditis elegans to rodents (5). It has been suggested that it may work in part by activating AMPK (5). The expression and deacetylation activities of SIRT1 are enhanced by increases in NAD ϩ levels or the NAD ϩ /NADH ratio, such as occur dur...

IntroductionThe fuel-sensing enzyme AMP-activated protein kinase (AMPK) was first described as an enzyme activated by changes in the AMP/ATP ratio that could both increase cellular ATP generation (e.g., fatty acid oxidation) and diminish ATP use for less critical processes (e.g., fatty acid, triglyceride, and protein synthesis) (1). In addition to glucose transport, lipid and protein synthesis, and fuel metabolism, AMPK regulates a wide array of other physiological events, including cellular growth and proliferation, mitochondrial function and biogenesis, and factors that have been linked to insulin resistance (IR), including inflammation, oxidative and ER stress, and autophagy ( Figure 1A). Furthermore, AMPK does so by phosphorylating both key enzymes and transcriptional activators and coactivators.Here, we examine 2 hypotheses suggested by more recent studies: (a) dysregulation of AMPK plays an important role in the pathogenesis of IR and metabolic syndrome-associated diseases in humans and experimental animals; and (b) strategies that activate AMPK can be harnessed for the prevention and treatment of these abnormalities. These hypotheses emanated from associations between the metabolic syndrome and some downstream targets of AMPK, such as glucose transport and lipogenesis (2-8). In addition, exercise (9) and electrically induced contractions (10) were shown to activate AMPK. These observations, coupled with epidemiological evidence that diseases associated with the metabolic syndrome (e.g., type 2 diabetes, hypertension, atherosclerotic cardiovascular disease [ASCVD], and even certain cancers) are less prevalent in physically active people (11-13) and the demonstration that regular exercise improves whole-body insulin action (11, 12), suggest a central role for AMPK in regulating insulin sensitivity. Such studies also raise the possibility that pharmacological AMPK activators as well as exercise could be used for ameliorating IR in type 2 diabetes (4,8).In model systems, sustained decreases in AMPK activity accompany IR, whereas AMPK activation increases insulin sensitivity (5, 6, 13). In addition, decreases in AMPK activity accompanying IR were described in adipose tissue of humans with Cushing's syndrome, an effect attributable to high levels of glucocorticoids (14), and in a subgroup of very obese patients undergoing bariatric surgery who were insulin resistant (15, 16). The latter comprise approximately 75% of bariatric surgery patients and show a greater predisposition to metabolic syndrome-associated diseases than do the remaining 25% of such patients who are equally obese, but less hyperinsulinemic and more insulin sensitive (17-19). Insulin resistance in physiology and diseaseStudies with a perfused rat hindquarter preparation demonstrated that insulin-stimulated glucose uptake in skeletal muscle is reduced in fed versus fasted rats (20) and in sedentary versus recently exercised rats (21), suggesting that the fed and sedentary rats are essentially more insulin resistant. Such IR is physiological, rat...

AMP-activated protein kinase (AMPK) is activated in adipocytes during exercise and other states in which lipolysis is stimulated. However, the mechanism(s) responsible for this effect and its physiological relevance are unclear. To examine these questions, 3T3-L1 adipocytes were treated with cAMP-inducing agents (isoproterenol, forskolin, and isobutylmethylxanthine), which stimulate lipolysis and activate AMPK. When lipolysis was partially inhibited with the general lipase inhibitor orlistat, AMPK activation by these agents was also partially reduced, but the increases in cAMP levels and cAMP-dependent protein kinase (PKA) activity were unaffected. Likewise, small hairpin RNA-mediated silencing of adipose tissue triglyceride lipase inhibited both forskolin-stimulated lipolysis and AMPK activation but not that of PKA. Forskolin treatment increased the AMP:ATP ratio, and this too was reduced by orlistat. When acyl-CoA synthetase, which catalyzes the conversion of fatty acids to fatty acyl-CoA, was inhibited with triacsin C, the increases in both AMPK activity and AMP:ATP ratio were blunted. Isoproterenol-stimulated lipolysis was accompanied by an increase in oxidative stress, an effect that was quintupled in cells incubated with the AMPK inhibitor compound C. The isoproterenol-induced increase in the AMP:ATP ratio was also much greater in these cells. In conclusion, the results indicate that activation of AMPK in adipocytes by cAMP-inducing agents is a consequence of lipolysis and not of PKA activation. They suggest that AMPK activation in this setting is caused by an increase in the AMP:ATP ratio that appears to be due, at least in part, to the acylation of fatty acids. Finally, this AMPK activation appears to restrain the energy depletion and oxidative stress caused by lipolysis. AMP-activated protein kinase (AMPK)2 is a sensor of cellular energy state that responds to metabolic stresses and other regulatory signals. The mechanism of activation of AMPK is a complex phenomenon and is still not fully understood, although it is recognized that it involves phosphorylation of the critical Thr-172 residue of its ␣ catalytic subunit by upstream kinases such as LKB1, Ca 2ϩ -calmodulin-dependent protein kinase kinase, and possibly Tak1, a member of the mitogenactivated protein kinase kinase kinase family (1-5). AMPK is also regulated by AMP allosterically and the current view is that this both increases AMPK activity directly and makes it a poorer substrate for phosphatases (6).The role and regulation of AMPK in muscle, liver, and various cultured cells have been extensively studied. It is now well established that energy depletion because of starvation, hypoxia, and exercise increases the intracellular AMP:ATP ratio and secondarily AMPK activity (7). Upon its activation, a major role of AMPK is to replete cellular energy stores by stimulating processes that generate ATP, such as fatty acid oxidation, and inhibiting ATP-consuming pathways (e.g. lipogenesis, triglyceride synthesis, and gluconeogenesis) that are not acutel...

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Abbreviations: AMPK, AMP kinase; BMI, body mass index; HOMA-IR, homeostatic model assessment of insulin resistance.

Inflammation and infiltration of immune cells in white adipose tissue have been implicated in the development of obesity-associated insulin resistance. Likewise, dysregulation of the fuel-sensing enzyme AMP-activated protein kinase (AMPK) has been proposed as a pathogenetic factor for these abnormalities based on both its links to insulin action and its anti-inflammatory effects. In this study, we examined the relationships between AMPK activity, the expression of multiple inflammatory markers in visceral (mesenteric and omental) and abdominal subcutaneous adipose tissue, and whole-body insulin sensitivity in morbidly obese patients (BMI 48 ± 1.9 kg/m2) undergoing gastric bypass surgery. AMPK activity was assessed by western-blots (P-AMPK/T-AMPK) and mRNA levels of various markers of inflammation by qRT-PCR. Patients were stratified as insulin sensitive obese or insulin resistant obese according to their HOMA-IR values. The results indicate that AMPK activity is lower in visceral than in subcutaneous abdominal adipose tissue of these patients and that this is associated with an increased expression of multiple inflammatory genes. They also revealed that AMPK activity is lower in adipose tissue of obese patients who are insulin resistant (HOMA-IR > 2.3) than in BMI-matched insulin sensitive subjects. Furthermore, this difference was evident in all three fat depots. In conclusion, the data suggest that there are close links between reduced AMPK activity and inflammation in white adipose tissue, and whole-body insulin resistance in obese humans. Whether adipose tissue AMPK dysregulation is a causal factor for the development of the inflammation and insulin resistance remains to be determined.

Apoptosis of pericytes (PCs) is an early event in diabetic retinopathy. It is generally thought to be a consequence of sustained hyperglycemia. In keeping with this, long-term (>7 days) incubation of cultured PCs in a high-glucose media has been shown to increase apoptosis. We examine here whether the saturated free fatty acid palmitate, the concentration of which is often elevated in diabetes, has similar effects on cultured PCs. Incubation with 0.4 mmol/l palmitate for 24 h induced both oxidant stress and apoptosis, as evidenced by a sixfold increase in DCF fluorescence and a twofold increase in caspase-3 activation, respectively. NAD(P)H oxidase appeared to be involved in these responses, since overexpression of dominant-negative subunits of NAD(P)H oxidase, such as phox47(DN), diminished oxidant stress, and phox67(DN) and N-17 RAC1(DN) prevented the increase in caspase-3 activity. Likewise, overexpression of vRAC, a constitutively active RAC1, increased caspase-3 activity to the same extent as palmitate alone. The effects of vRAC and palmitate were not additive. In parallel with the increases in oxidative stress, the redox-sensitive transcription factor nuclear factor-B (NF-B) was activated in cells incubated with 0.4 mmol/l palmitate. Furthermore, inhibition of NF-B activation by various means inhibited caspase-3 activation. Finally, incubation with palmitate increased the cellular content of ceramide, a molecule linked to apoptosis and increases in oxidative stress and NF-B activation in other cells. In keeping with such a role, in PCs both coincubation with fumonisin B1 (a ceramide synthase inhibitor) and overexpression of ceramidase I reversed the proapoptotic effect of palmitate. On the other hand, they increased rather than decreased DCF fluorescence. In conclusion, the results suggest that palmitate-induced apoptosis in PCs is associated with activation of NAD(P)H oxidase and NF-B and an increase in ceramide. The precise interactions between these molecules in causing apoptosis and the importance of oxidant stress as a contributory factor remain to be determined.