Transcriptional Activation of Scavenger Receptor Expression in Human Smooth Muscle Cells Requires AP-1/c-Jun and C/EBPβ

Abstract

—Reactive oxygen species generated by treatment of smooth muscle cells (SMCs) with either phorbol 12-myristate 13-acetate or with the combination of H₂O₂ and vanadate strongly induce expression of the class A scavenger receptor (SR-A) gene. In the current studies, cis-acting elements in the proximal 245 bp of the SR-A promoter were shown to direct luciferase reporter expression in response to oxidative stress in both SMCs and macrophages. A composite activating protein-1 (AP-1)/ets binding element located between –67 and –50 bp relative to the transcriptional start site is critical for macrophage SR-A activity. Mutation of either the AP-1 or the ets component of this site also prevented promoter activity in SMCs. Mutation of a second site located between –44 and –21 bp, which we have identified as a CCAAT/enhancer binding protein (C/EBP) element, reduced the inducible activity of the promoter in SMCs by 50%, suggesting that combinatorial interactions between these sites are necessary for optimal gene induction. Interactions between SMC nuclear extracts and the SR-A promoter were analyzed by electrophoretic mobility shift assay. c-Jun/AP-1 binding activity, specific for the –67- to –50-bp site, was induced in SMCs by the same conditions that increased SR-A expression. Moreover, phorbol 12-myristate 13-acetate, H₂O₂, or the combination of H₂O₂ and sodium orthovanadate (vanadate) activated c-Jun–activating kinase. The binding activity within SMC extracts specific for the C/EBP site was shown to be C/EBPβ in SMCs. Taken together, these findings demonstrate that reactive oxygen species regulate the interactions between c-Jun/AP-1 and C/EBPβ in the SR-A promoter. Furthermore, induction of oxidative stress in THP-1 cells, with a combination of 10 μmol/L vanadate and 100 μmol/L H₂O₂, induced macrophage differentiation, adhesion, and SR activity. These data suggest that vascular oxidative stress may contribute to the induction of SR-A expression and thereby promote the uptake of oxidatively modified low density lipoprotein by both macrophage and SMCs to produce foam cells in atherosclerotic lesions.