The chronic hyperinsulinemia that accompanies obesity, metabolic syndrome, and type II diabetes mellitus sustains increased hepatic lipogenesis via transcriptional upregulation of lipogenic enzymes. This results in overproduction of VLDL by the liver with resulting accumulation of atherogenic triglyceride-rich lipoproteins in the plasma. Induction of lipogenesis by insulin is mediated by sterol regulatory element binding protein-1c (SREBP-1c) via transcriptional upregulation. SREBP-1c is synthesized as an integral membrane protein localized in the ER where it is associated with its chaperone (SCAP) and regulatory proteins (Insig2, Sec24). We have found that, in addition to promoting transcription of nascent SREBP-1c, insulin also stimulates transport of the SREBP-SCAP complex to the golgi where it undergoes proteolytic cleavage to release the transcriptionally active N-terminal fragment of SREBP-1c (SREBP-1c processing), thereby amplifying the transcriptional effects of insulin. Conversely, cAMP (a surrogate for glucagon) prevents processing of SREBP-1c. Thus, processing of SREBP-1c is a novel and potentially important additional level by which insulin and other factors regulate this pivotal transcription factor. Although the mechanisms by which processing of the cholesterol regulating isoform, SREBP-2, by sterol balance have been extensively studied, little is known about regulation of SREBP-1c processing by insulin and cAMP. We have also found that nascent SREBP-1c is phosphorylated in the presence of insulin via MAPK and PI-3K dependent pathways. Inhibitors of these pathways prevent both phosphorylation of SREBP-1c and insulin induced processing indicating that phosphorylation of SREBP-1c (and possibly of other participating proteins) may mediate the effect of insulin on processing. We therefore propose to delineate the mechanism(s) by which insulin and cAMP regulate proteolytic processing of SREBP-1c by identifying (1) the sites on SREBP-1c that are phosphorylated by insulin, (2) the signaling pathways that mediate the effect of insulin to stimulate and cAMP to inhibit processing, and (3) the functional significance of phosphorylation for regulation of SREBP-1c processing. We will also determine the additional roles of regulation of ER membrane cholesterol content and Insig2 expression by insulin in SREBP-1c processing. These studies will be carried out both in vitro in primary rat hepatocytes and in an in vivo model of hyperinsulinemia, the corpulent JCR:LA-cp rat. Relevance: in obesity, metabolic syndrome, and adult-onset diabetes mellitus, high insulin levels promote formation of fat (triglyceride) by the liver. The resulting elevated levels of triglyceride in the plasma increase the risk of heart attack and stroke. This research examines the role of an important regulatory protein, SREBP-1c in the response of the liver to high insulin levels. The goal of this research is to identify new treatments for hyperlipidemia by understanding the mechanisms by which insulin regulates this protein.

Agency
National Institute of Health (NIH)
Institute
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Type
Research Project (R01)
Project #
5R01DK075504-04
Application #
7880862
Study Section
Atherosclerosis and Inflammation of the Cardiovascular System Study Section (AICS)
Program Officer
Silva, Corinne M
Project Start
2007-07-01
Project End
2012-06-30
Budget Start
2010-07-01
Budget End
2012-06-30
Support Year
4
Fiscal Year
2010
Total Cost
$262,051
Indirect Cost
Name
University of Tennessee Health Science Center
Department
Pharmacology
Type
Schools of Medicine
DUNS #
941884009
City
Memphis
State
TN
Country
United States
Zip Code
38163
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Dong, Qingming; Giorgianni, Francesco; Deng, Xiong et al. (2014) Phosphorylation of sterol regulatory element binding protein-1a by protein kinase A (PKA) regulates transcriptional activity. Biochem Biophys Res Commun 449:449-54
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Sharma, Pragya; Thakran, Shalini; Deng, Xiong et al. (2013) Nuclear corepressors mediate the repression of phospholipase A2 group IIa gene transcription by thyroid hormone. J Biol Chem 288:16321-33
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Attia, Ramy R; Connnaughton, Sara; Boone, Lindsey R et al. (2010) Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by thyroid hormone: role of the peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha). J Biol Chem 285:2375-85
Song, Shulan; Attia, Ramy R; Connaughton, Sara et al. (2010) Peroxisome proliferator activated receptor alpha (PPARalpha) and PPAR gamma coactivator (PGC-1alpha) induce carnitine palmitoyltransferase IA (CPT-1A) via independent gene elements. Mol Cell Endocrinol 325:54-63
Elam, Marshall B; Yellaturu, Chandrahasa; Howell, George E et al. (2010) Dysregulation of sterol regulatory element binding protein-1c in livers of morbidly obese women is associated with altered suppressor of cytokine signaling-3 and signal transducer and activator of transcription-1 signaling. Metabolism 59:587-98

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