mTORC1 Inhibition via Rapamycin Promotes Triacylglycerol Lipolysis and Release of Free Fatty Acids in 3T3‐L1 Adipocytes

Abstract

Signaling by mTOR complex 1 (mTORC1) promotes anabolic cellular processes in response to growth factors, nutrients, and hormonal cues. Numerous clinical trials employing the mTORC1 inhibitor rapamycin (aka sirolimus) to immuno-suppress patients following organ transplantation have documented the development of hypertriglyceridemia and elevated serum free fatty acids (FFA). We therefore investigated the cellular role of mTORC1 in control of triacylglycerol (TAG) metabolism using cultured murine 3T3-L1 adipocytes. We found that treatment of adipocytes with rapamycin reduced insulin-stimulated TAG storage ~50%. To determine whether rapamycin reduces TAG storage by upregulating lipolytic rate, we treated adipocytes in the absence and presence of rapamycin and isoproterenol, a β2-adrenergic agonist that activates the cAMP/protein kinase A (PKA) pathway to promote lipolysis. We found that rapamycin augmented isoproterenol-induced lipolysis without altering cAMP levels. Rapamycin enhanced the isoproterenol-stimulated phosphorylation of hormone sensitive lipase (HSL) on Ser-563 (a PKA site), but had no effect on the phosphorylation of HSL S565 (an AMPK site). Additionally, rapamycin did not affect the isoproterenol-mediated phosphorylation of perilipin, a protein that coats the lipid droplet to initiate lipolysis upon phosphorylation by PKA. These data demonstrate that inhibition of mTORC1 signaling synergizes with the β-adrenergic-cAMP/PKA pathway to augment phosphorylation of HSL to promote hormone-induced lipolysis. Moreover, they reveal a novel metabolic function for mTORC1; mTORC1 signaling suppresses lipolysis, thus augmenting TAG storage.

Mice with an adipocyte-specific lipin 1 separation-of-function allele reveal unexpected roles for phosphatidic acid in metabolic regulation

“…It should be noted that PA was recently found to suppress formation of the mTORC2 complex (26); thus, further work is needed to determine whether mTORC1 is specifically activated by this pool of PA. We also acknowledge that we cannot rule out the possibility that alterations in other lipid species upstream or downstream of PA in these pathways are mediating these signaling events. Our observation that Torin inhibits PDE activity is consistent with previous work showing that inhibition of mTOR promotes lipolysis in 3T3-L1 adipocytes (23). Protein kinase B/Akt regulates PDE3B activity via direct phosphorylation, and a parallel link between PDE4 and mTOR is possible.…”

“…5C). PA also has been mechanistically linked to activation of mammalian target of rapamycin (mTOR) signaling (22), which may regulate lipolytic activity as well (23). Adipose tissue of Adn-Lpin1 −/− mice or fld mice, which also exhibit adipose tissue PA accumulation (5), showed increased phosphorylation of mTOR (Ser2448) and its downstream target p70 S6 kinase (S6K) compared with WT littermates (Fig.…”

TORC1 Inhibition Induces Lipid Droplet Replenishment in Yeast

“…TORC1 inhibition by rapamycin has different outcomes in yeast and mammalian cells. Our results showing that TORC1 inhibition in yeast leads to LD synthesis do not agree with the role of the mTOR pathway in mammals, where it is well established that the activation of the TORC1 pathway is essential to promote lipogenesis (28,61).…”

Proline oxidase–adipose triglyceride lipase pathway restrains adipose cell death and tissue inflammation

“…12 Moreover, recent works have reported that drugs affecting longevity, such as rapamycin (mTOR inhibitor) and AICAR (AMPK agonist), are also able to induce ATGL in adipocytes. [42][43][44][45] Therefore, it is likely that in our system mTOR and/or AMPK could be the upstream inducers of POX activation.…”