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

Authors: GAUTHIER, MARIE-SOLEIL - BOSTON UNIVERSITY; MIYOSHI, HIDEAKI - HOKKAIDO UNIVERSITY; SOUZA, SANDRA - NOVARTIS; CACICEDO, JOSE - BOSTON UNIVERSITY; SAHA, ASISH - BOSTON UNIVERSITY; Greenberg, Andrew; RUDERMAN, NEIL - BOSTON UNIVERSITY

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2008
Publication Date: 6/13/2008
Citation: Gauthier, M., Miyoshi, H., Souza, S., Cacicedo, J.M., Saha, A., Greenberg, A.S., Ruderman, N.B. 2008. Amp-activated protein kinase is activated as a consequence of lipolysis in the adipocyte. Journal of Biological Chemistry. 283(24):16514-16524.

Interpretive Summary: The mechanisms by which hormones increase fat breakdown and the burning of fat in fat cells are unclear. In this paper we demonstrate that increased fat breakdown actually increases the burning of fat as well. We show that the process of fat breakdown increases the activity of a protein called AMPK. AMPK is a protein whose actions are increased with exercise and has been demonstrated to increase fat breakdown. Since certain types of exercise, such as jogging, increases the hormones investigated in this paper, these data suggest that hormones not only increase the breakdown of fat but also facilitate the burning of fat. Additionally, we demonstrate that increasing the actions of AMPK in fat cells also results in decreased levels of certain breakdown products of fats that increase the risk for development of diabetes. Future studies will examine how nutrition and exercise increase fat breakdown and fat burning.

Technical 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 agents that increase cAMP levels (isoproterenol, forskolin, and isobutylmethylxanthine), which are known to stimulate lipolysis and activate AMPK. When lipolysis was partially inhibited by co-incubating the cells with the general lipase inhibitor orlistat, AMPK activation by these agents was also partially diminished, but the increases in cAMP levels and cAMP-dependent protein kinase (PKA) activity were unaffected. Likewise, small hairpin RNAmediated 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 (4-fold), 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, both the increases in AMPK activity and in the AMP:ATP ratio were blunted. Isoproterenol-stimulated lipolysis was accompanied by an increase in reactive oxygen species production, an effect that was quintupled in adipocytes incubated with the AMPK inhibitor compound C. The isoproterenol-induced increase in the AMP:ATP ratio was also much greater in these cells, indicating they were more energy-stressed. In conclusion, the results indicate that activation of AMPK in adipocytes by agents that increase cAMP levels is a consequence of lipolysis and not the direct result of increases in cAMP levels or PKA activity. They also 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 AMPKactivation appears to restrain the energy depletion and oxidative stress caused by lipolysis.