Abstract
GIP (glucose-dependent insulinotropic polypeptide) is a gastrointestinal hormone that regulates pancreatic islet function. Additionally, emerging evidence suggests an important physiological role for GIP in the regulation of adipocyte metabolism. In previous studies on the lipogenic effects of GIP, it was shown to increase adipocyte lipoprotein lipase (LPL) activity in both differentiated 3T3-L1 cells and human adipocytes through a pathway involving activation of protein kinase B (PKB)/Akt. In the current study, we examined the effects of GIP on LPL gene expression. GIP in the presence of insulin increased LPL gene expression in human adipocytes and LPL promoter activity in GIP receptor-expressing HEK-293 cells, and both effects were greatly reduced by the transcription inhibitor actinomycin D. Subsequent studies established that GIP increased phosphorylation of Serine 133 in cAMP-response element binding protein (CREB) and the nuclear localization of cAMP-responsive CREB coactivator 2 (TORC2) through a pathway involving phosphatidylinositol 3-kinase (PI3-K), PKB, and AMP-activated protein kinase (AMPK). However, in the presence of insulin, GIP failed to activate the cAMP/PKA pathway. Knockdown of CREB and TORC2 using RNA interference reduced LPL expression, supporting a functional regulatory role. GIP-induced phospho-CREB and TORC2 were shown to bind to a cAMP-response element (-II) site in the human LPL promoter and GIP increased protein-protein interactions of these two factors. The lipogenic effects of GIP in the presence of insulin are therefore at least partially mediated by upregulation of adipocyte LPL gene transcription through a pathway involving PI3-K/PKB/AMPK-dependent CREB/TORC2 activation.