Insulin Regulates Adipocyte Lipolysis via an Akt-Independent Signaling Pathway

Abstract

After a meal, insulin suppresses lipolysis through the activation of its downstream kinase, Akt, resulting in the inhibition of protein kinase A (PKA), the main positive effector of lipolysis. During insulin resistance, this process is ineffective, leading to a characteristic dyslipidemia and the worsening of impaired insulin action and obesity. Here, we describe a noncanonical Akt-independent, phosphoinositide-3 kinase (PI3K)-dependent pathway that regulates adipocyte lipolysis using restricted subcellular signaling. This pathway selectively alters the PKA phosphorylation of its major lipid droplet-associated substrate, perilipin. In contrast, the phosphorylation of another PKA substrate, hormone-sensitive lipase (HSL), remains Akt dependent. Furthermore, insulin regulates total PKA activity in an Akt-dependent manner. These findings indicate that localized changes in insulin action are responsible for the differential phosphorylation of PKA substrates. Thus, we identify a pathway by which insulin regulates lipolysis through the spatially compartmentalized modulation of PKA.

S.M.C. designed, performed, analyzed, and interpreted all experiments, except for those performed by D.F.T. and D.N.G., and wrote the manuscript. D.F.T. generated the Akt2 shRNA 3T3-L1 cells and designed, performed, and analyzed the glucose uptake assay using these cells. D.F.T. also performed side-by-side comparisons of Akt and PI3K inhibitor with Akt kinase assays and also helped write the manuscript. R.M.E. generated the Akt2^lox/lox mice. D.N.G. generated the Akt2^lox/lox immortalized cell line, performed the assays in primary adipocytes, and helped write the manuscript. B.R.M. generated the Akt2^lox/lox primary cell line. L.M.D. and A.S.D. performed lipid droplet isolations. M.J.B. designed, analyzed, and interpreted the experiments and wrote the manuscript.

We thank Michelle Bland, Russell Miller, and Min Wan for the critical reading of the manuscript and Harry Subramanian for assistance with the lipolysis assays. We also thank Constantine Londos (NIH, NIDDK) for the generous gift of perilipin antibody.

This work was supported by NIH grant R01-DK56886 to M.J.B. and the Penn DERC (NIH grant P30 DK19525), which provides partial support to the Vector Core at the University of Pennsylvania.