Lipids

, Volume 45, Issue 12, pp 1089–1100

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

  • Ghada A. Soliman
  • Hugo A. Acosta-Jaquez
  • Diane C. Fingar
Original Article

DOI: 10.1007/s11745-010-3488-y

Cite this article as:
Soliman, G.A., Acosta-Jaquez, H.A. & Fingar, D.C. Lipids (2010) 45: 1089. doi:10.1007/s11745-010-3488-y

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.

Keywords

mTORmTORC1RapamycinLipid metabolismLipolysisAdipocytes

Abbreviations

ATGL

Adipocyte triacylglycerol lipase

AMPK

AMP activated protein kinase

ATP

Adenosine triphosphate

cAMP

Cyclic adenosine monophosphate

eIF4E

Eukaryotic initiation factor-4E

4EBP1/PHAS-I

eIF-4E-binding protein or protein-1/heat and acid stable-activated by insulin

HEAT

Huntington elongation factor 3, the A subunit of protein phosphatase 2A, and TOR1

HSL

Hormone sensitive lipase

FBS

Fetal bovine serum

FRAP

FKBP-12 rapamycin associated protein

FRB

FKBP12-rapamycin binding domain

FFA

Free fatty acids

FKBP12

FK506 binding protein 12

GAP

GTPase activating protein

GβL

G protein β subunit-like protein also known as mLST8

GLUT 4

Glucose transporter 4

IRS

Insulin receptor substrate

LKB

Tumor suppressor protein

LPAAT

Lysophosphatidic acid acyl transferase

MEFs

Mouse embryonic fibroblasts

MGL

Monoacylglycerol lipase

mTOR

Mammalian target of rapamycin (TOR)

mTOR P-S2481

mTOR phosphorylated on serine 2481

mTORC1

Mammalian target of rapamycin complex 1

mTORC2

Mammalian target of rapamycin complex 2

NCS

Newborn calf serum

NEFA

Nonesterified fatty acids

RAPA

Rapamycin

Raptor

Regulatory associated protein of mammalian target of rapamycin

Rheb

Ras homolog enriched in brain

Rictor

Rapamycin-insensitive companion of mTOR

RII

Regulatory subunit II of PKA

TOS

TOR signaling motif

PA

Phosphatidic acid

PDE

Phosphodiesterase

PLD

Phospholipase D

PI3K

Phosphatidylinositol 3-OH kinase

Pol I

Polymerase I

PKA

Protein kinase A

PKB/AKT

Protein kinase B

PPAR γ

Peroxisome proliferator-activated receptor-γ

PTEN

Phosphatase and tensin homologue deleted on chromosome 10

S6K1

p70 ribosomal protein S6 kinase 1

S6K1 P-T389

S6K1 phosphorylated on threonine 389

TAG

Triacylglycerol

TSC

Tuberous sclerosis complex

VLDL

Very low density lipoprotein

Copyright information

© AOCS 2010

Authors and Affiliations

  • Ghada A. Soliman
    • 1
    • 3
  • Hugo A. Acosta-Jaquez
    • 2
  • Diane C. Fingar
    • 1
    • 2
  1. 1.Division of Metabolism, Endocrinology, and Diabetes, Department of MedicineUniversity of Michigan Medical SchoolAnn ArborUSA
  2. 2.Department of Cell and Developmental BiologyUniversity of Michigan Medical SchoolAnn ArborUSA
  3. 3.Department of Family and Consumer SciencesWestern Michigan UniversityKalamazooUSA