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