Role of the mammalian Target of Rapamycin (mTOR) Complex 1 signaling in beta‐adrenergic‐stimulated triacylglycerol (TAG) lipolysis and free fatty acid (FFA) release in 3T3‐L1 adipocytes

Abstract

mTOR is a Ser/Thr kinase that integrates metabolic signals and is found in two complexes, a rapamycin‐sensitive complex (mTORC1) and a rapamycin‐insensitive complex (mTORC2). We have shown that rapamycin (RAPA) induces hypertriglyceridemia in transplant patients and increases FFA in guinea pigs. Therefore, we investigated a role for mTORC1 signaling in control of TAG metabolism.Using 3T3‐L1 adipocytes, we found that RAPA reduces cellular TAG content (2 fold p<0.05). To elucidate the mechanisms, we determined whether RAPA increases lipolytic rates. Thus we treated 3T3‐L1 adipocytes with Isoproterenol (Iso), a β‐adrenergic agonist that activates cAMP‐dependent Protein Kinase A (PKA), in the absence or presence of RAPA. We find that RAPA enhances Iso‐induced lipolysis, as measured by FFA release (p<0.01), but has no effect on cAMP levels. RAPA increases the phosphorylation of hormone sensitive lipase (HSL) on Ser 563, a site phosphorylated by PKA. RAPA also augments Iso‐mediated phosphorylation of perilipin (a protein that coats TAG droplet whose phosphorylation is critical for lipolysis,) which co‐immunoprecipitated with HSL and adipose triglyceride lipase. These data suggest that RAPA leads to increased lipolysis due to synergism with the β‐adrenergic‐PKA pathway. Moreover, these data may reveal a novel role for mTORC1 signaling in the regulation of FFA metabolism (Supported by AHA 0750060Z and K01 DK60654 to GS).