Regulation of Scavenger Receptor Expression in Smooth Muscle Cells by Protein Kinase C

Abstract

Phorbol esters increase scavenger-receptor mRNA expression and receptor activity in smooth muscle cells (SMCs). Our present results demonstrate that activation of protein kinase C (PKC) mediates this increase in receptor expression. This conclusion is based on the findings that (1) phorbol esters induced translocation of PKC-α from the cytosol to the membrane fraction; (2) PKC inhibitors blocked the effect of phorbol esters on receptor expression; (3) diacylglycerol, a physiological PKC agonist, enhanced scavenger-receptor activity; and (4) in cotransfected human SMCs, constitutively active PKC-α stimulated the expression of a reporter gene under control of the scavenger-receptor promoter. Phorbol ester treatment of SMCs increased intracellular reactive oxygen, and the increase in receptor activity was reduced 30% by the antioxidant N-acetyl cysteine (NAC), suggesting a role for reactive oxygen in phorbol ester–mediated receptor regulation. Furthermore, direct treatment of SMCs with reactive oxygen species increased scavenger-receptor activity. In rabbit SMCs, 100 μmol/L H₂O₂ alone slightly increased scavenger-receptor mRNA and protein expression. In combination, 100 μmol/L H₂O₂ and 10 μmol/L vanadate, which promotes formation of OH and enhances the inhibition of protein tyrosine phosphatase by H₂O₂, increased scavenger-receptor mRNA expression 25-fold in rabbit SMCs and 8-fold in human SMCs. NAC reduced the effect of H₂O₂ and vanadate by 93%. The increase in SMC scavenger-receptor expression occurs at the level of gene transcription. Receptor mRNA half-life was unchanged after treatment with either phorbol esters or reactive oxygen (≈14.5 hours), and induction by phorbol esters increased SMC scavenger-receptor mRNA transcription, as determined by nuclear run-on assay. Multiple cytokines and growth factors that contribute to the generation of reactive oxygen species are present in atherosclerotic lesions. These factors may all contribute to the upregulation of SMC scavenger-receptor activity and therefore to the formation of smooth muscle foam cells.