Insulin-induced de novo lipid synthesis occurs mainly via mTOR-dependent regulation of proteostasis of SREBP-1c

Qingming Dong, Gipsy Majumdar, Robert N. O’Meally, Robert N. Cole, Marshall B. Elam, Rajendra Raghow

Research output: Contribution to journalArticle

Abstract

Insulin stimulates de novo lipid synthesis in the liver and in cultured hepatocytes via its ability to activate sterol regulatory element-binding protein 1c (SREBP-1c). Although PI3K-AKT-mTORC1-p70S6K-signaling kinases are known to drive feed-forward expression of SREBP-1c, the identity of the phosphorylated amino acid residue(s) putatively involved in insulin-stimulated de novo lipogenesis remains elusive. We obtained in silico and mass spectrometry evidence, that was combined with siRNA strategies, to discover that insulin-induced phosphorylation of serine 418, serine 419, and serine 422 in rat SREBP-1c was most likely mediated by p70S6 kinase. Here, for the first time, we show that insulin-induced phosphorylation of these 3 serine residues mainly impinged on the mechanisms of proteostasis of both full-length and mature SREBP-1c in the McArdle-RH7777 hepatoma cells. Consistent with this conclusion, nascent SREBP-1c, substituted with phosphomimetic aspartic acid residues at these 3 sites, was resistant to proteasomal degradation. As a consequence, endoplasmic reticulum to Golgi migration and proteolytic maturation of pSREBP-1c was significantly enhanced which led to increased accumulation of mature nSREBP-1c, even in the absence of insulin. Remarkably, aspartic acid substitutions at S418, S419 and S422 also protected the nascent SREBP-1c from ubiquitin-mediated proteasome degradation thus increasing its steady-state levels and transactivation potential in the nucleus. These complementary effects of p70S6K-mediated phosphorylation on proteostasis of pSREBP-1c were necessary and sufficient to account for insulin’s ability to enhance transcription of genes controlling de novo lipogenesis in hepatocytes.

Original languageEnglish (US)
JournalMolecular and Cellular Biochemistry
DOIs
StateAccepted/In press - Jan 1 2019

Keywords

  • Mass spectrometry
  • Mechanistic target of rapamycin (mTOR)
  • Phosphorylation
  • Proteasome degradation
  • SREBP-1c
  • Ubiquitin

ASJC Scopus subject areas

  • Molecular Biology
  • Clinical Biochemistry
  • Cell Biology

Fingerprint Dive into the research topics of 'Insulin-induced de novo lipid synthesis occurs mainly via mTOR-dependent regulation of proteostasis of SREBP-1c'. Together they form a unique fingerprint.

  • Cite this