AMP-activated Protein Kinase Is Activated as a Consequence of Lipolysis in the Adipocyte

Abstract

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...

Integration of hormonal and nutrient signals that regulate leptin synthesis and secretion

“…Interplay between cAMP and AMPK signaling in adipocytes may also contribute to the ␤-AR-mediated decrease in leptin release (23,31,87). Koh et al (31) showed that ␤-AR agonists increase the AMP/ATP ratio and activate AMPK activity in rat adipocytes.…”

“…Koh et al (31) showed that ␤-AR agonists increase the AMP/ATP ratio and activate AMPK activity in rat adipocytes. A recent paper by Gauthier et al (23) showed that ␤-AR-stimulated lipolysis increases use of ATP for FA activation and that this activates AMPK activity. Thus it seems possible that the decline in serum leptin associated with activation of the sympathetic nervous system with fasting, cold, or exercise involves the cross-talk among the PKA, AMPK, and mTOR pathways, as illustrated in Fig.…”

“…Lee, S. Fried, unpublished observations). ␤-AR agonists also stimulate triglyceride (TG) breakdown, which may increase use of ATP for formation of fatty acyl (FA) CoA, leading to increased AMP and the activation of AMP-activated protein kinase (AMPK) (23,87). AMPK activation is known to inhibit the mTOR pathway, decreasing general protein synthesis and possibly also relative rates of leptin mRNA translation.…”

Lipid metabolism and exercise

“…Regarding adipocytes responses to acute exercise, Wohlers et al demonstrated that acute exercise elevated the expression of ATGL and HSL Ser660 phosphorylation 21) . Another study demonstrated that AMP-activated protein kinase (AMPK), a sensor of cellular energy state that responds to metabolic stress and other regulatory signals, was activated during lipolysis in adipocytes due to an increase in the AMP/ATP ratio 22) . As well, Ruderman et al demonstrated that AMPK activity was increased in adipose tissue 30 min following treadmill running exercise in normal rats 23) , and the administration of AMPK activator N 1 -(β-D-Ribofuranosyl)-5-aminoimidazole-4-carboxamide (AICAR) to rats diminished adiposity 24) .…”

Augmenting energy expenditure by mitochondrial uncoupling: a role of AMP-activated protein kinase

“…It seems counter-intuitive (Daval et al 2006) that AMPK inhibits lipolysis, although it is itself activated in situations of increased lipolysis. It could be speculated (Djouder et al 2010;Sponarova et al 2005;Daval et al 2006;Gauthier et al 2008) that AMPK-induced reduction of lipolysis could be a feed back mechanism limiting the cellular energy drain, because part of the fatty acids released can be re-activated to acyl-CoA and re-esterified which requires ATP and generates AMP. Also, accumulation of large amounts of fatty acids could be deleterious for the cells.…”