We reported that chronic activation of phosphatidylinositol 3-kinase (PI3K) by the overexpression of membrane-targeted p110CAAX induced proinflammatory gene expression in rat vascular smooth muscle cells (VSMCs) through the induction of CCAAT/enhancer binding protein- (C/EBP-) and C/EBP-. To examine the antiinflammatory effect of nitric oxide (NO) on the proinflammatory gene expression, we investigated the effects of sodium nitroprusside (SNP) on the monocyte chemoattractant protein-1 (MCP0-1) gene expression in VSMCs under chronic activation of PI3K. At low concentration (0.05 mM) of SNP, but not at high concentration (0.5-1.0 mM), it significantly reduced MCP-1 mRNA and protein expression as well as its transcriptional activity. We found that SNP induced C/EBP homologous protein (CHOP) expression, which reduced the C/EBP activity induced by chronic activation of PI3K in a dose-dependent manner up to 1.0 mM. Consistently, the increase in CHOP expression significantly reduced the MCP-1 promoter activity induced by PI3K. However, the overexpression of CHOP alone up-regulated MCP-1 promoter activity in a dose-dependent manner up to high concentration. Deletion analysis of MCP-1 promoter and electrophoretic mobility shift assay identified the CHOP-response element (CHOP-RE) at the region between -190 and -179 bp of MCP-1 promoter. By CHOP0-RE used as a decoy, the increase in promoter activity of MCP-1 induced by either CHOP or SNP was suppressed significantly. Thus, CHOP induced by an NO-donor has bidirectional effects on MCP-1 gene expression: it decreases the gene expression by inhibition of C/EBPs, and it increases the gene expression through the CHOP-RE.