Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles

Chandrahasa R. Yellaturu, Xiong Deng, Lauren M. Cagen, Henry G. Wilcox, Charles M. Mansbach, Shadab A. Siddiqi, Edwards Park, Rajendra Raghow, Marshall Elam

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Abstract

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding SREBP-1c(sterol regulatory element-binding protein-1c). Nascent SREBP-1c is synthesized and embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi in coatomer protein II (COPII) vesicles where two sequential cleavages generate the transcriptionally active NH2-terminal fragment, nSREBP-1c. There is limited indirect evidence to suggest that insulin may also regulate the post-translational processing of the nascent SREBP-1c protein. Therefore, we designed experiments to directly assess the action of insulin on the post-translational processing of epitope-tagged full-length SREBP-1c and SREBP-2 proteins expressed in cultured hepatocytes. We demonstrate that insulin treatment led to enhanced post-translational processing of SREBP-1c, which was associated with phosphorylation of ER-bound nascent SREBP-1c protein that increased affinity of the SREBP-1c cleavage-activating protein (SCAP)-SREBP-1c complex for the Sec23/24 proteins of the COPII vesicles. Furthermore, chemical and molecular inhibitors of the phosphoinositide 3-kinase pathway and its downstream kinase protein kinase B (PKB)/Akt prevented both insulin-mediated phosphorylation of nascent SREBP-1c protein and its post-translational processing. Insulin had no effect on the proteolysis of nascent SREBP-2 under identical conditions.Wealso show that in vitro incubation of an active PKB/Akt enzyme with recombinant full-length SREBP-1c led to its phosphorylation. Thus, insulin selectively stimulates the processing of SREBP-1c in rat hepatocytes by enhancing the association between the SCAP-SREBP-1c complex and COPII proteins and subsequent ER to Golgi transport and proteolytic cleavage. This effect of insulin is tightly linked to phosphoinositide 3-kinase and PKB/Akt-dependent serine phosphorylation of the precursor SREBP-1c protein.

Original languageEnglish (US)
Pages (from-to)7518-7532
Number of pages15
JournalJournal of Biological Chemistry
Volume284
Issue number12
DOIs
StatePublished - Mar 20 2009

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Coatomer Protein
Sterol Regulatory Element Binding Protein 1
Phosphorylation
Association reactions
Insulin
Processing
Proto-Oncogene Proteins c-akt
Endoplasmic Reticulum
Proteins
1-Phosphatidylinositol 4-Kinase
Phosphatidylinositols
Hepatocytes
Sterol Regulatory Element Binding Proteins
Proteolysis
Gene encoding

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology
  • Cell Biology

Cite this

Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles. / Yellaturu, Chandrahasa R.; Deng, Xiong; Cagen, Lauren M.; Wilcox, Henry G.; Mansbach, Charles M.; Siddiqi, Shadab A.; Park, Edwards; Raghow, Rajendra; Elam, Marshall.

In: Journal of Biological Chemistry, Vol. 284, No. 12, 20.03.2009, p. 7518-7532.

Research output: Contribution to journalArticle

Yellaturu, Chandrahasa R. ; Deng, Xiong ; Cagen, Lauren M. ; Wilcox, Henry G. ; Mansbach, Charles M. ; Siddiqi, Shadab A. ; Park, Edwards ; Raghow, Rajendra ; Elam, Marshall. / Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles. In: Journal of Biological Chemistry. 2009 ; Vol. 284, No. 12. pp. 7518-7532.
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AU - Wilcox, Henry G.

AU - Mansbach, Charles M.

AU - Siddiqi, Shadab A.

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AU - Raghow, Rajendra

AU - Elam, Marshall

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