Insulin activates CCAAT/enhancer binding proteins and proinflammatory gene expression through the phosphatidylinositol 3-kinase pathway in vascular smooth muscle cells

Abstract

Phosphatidylinositol 3-kinase (PI3K) is a key molecule mediating signals of insulin in vascular smooth muscle cells (VSMCs). To examine the effect of chronic activation of PI3K on the gene expression of VSMCs, membrane-targeted p110CAAX, a catalytic subunit of PI3K, was overexpressed in rat VSMCs by adenovirus-mediated gene transfer. Similar to insulin's effects, cells overexpressing p110CAAX exhibited a 10- to 15-fold increase in monocyte chemoattractant protein-1 (MCP-1) mRNA expression as compared with the control cells. Electrophoretic mobility shift assay analysis showed that the overexpression of p110CAAX activated neither the NF-κB binding nor the activator protein (AP-1) binding activities. We found that two CCAAT/enhancer binding protein (C/EBP) binding sites located between 2.6 and 3.6 kb upstream of the MCP-1 gene were responsible for the induction by p110CAAX. The overexpression of C/EBP-β and C/EBP-δ but not C/EBP-α caused 6- to 8-fold induction of MCP-1 promoter activity. Consistently, the overexpression of p110CAAX as well as insulin induced mRNA expression and nuclear expression of C/EBP-β and C/EBP-δ in VSMCs. These results clearly indicate that the activation of PI3K induced proinflammatory gene expression through activating C/EBP-β and C/EBP-δ but not NF-κB, which may explain the proinflammatory effect of insulin in the insulin-resistant state.