REGULATION OF SCAVENGER RECEPTOR EXPRESSION IN SMOOTH-MUSCLE CELLS BYPROTEIN-KINASE-C - A ROLE FOR OXIDATIVE STRESS

Phorbol esters increase scavenger-receptor mRNA expression and recepto r activity in smooth muscle cells (SMCs). Our present results demonstr ate that activation of protein kinase C (PKC) mediates this increase i n receptor expression. This conclusion is based on the findings that ( 1) phorbol esters induced translocation of PKC-alpha from the cytosol to the membrane fraction; (2) PKC inhibitors blocked the effect of pho rbol esters on receptor expression; (3) diacylglycerol, a physiologica l PKC agonist, enhanced scavenger-receptor activity; and (4) in cotran sfected human SMCs, constitutively active PKC-alpha stimulated the exp ression of a reporter gene under control of the scavenger-receptor pro moter. Phorbol ester treatment of SMCs increased intracellular reactiv e oxygen, and the increase in receptor activity was reduced 30% by the antioxidant N-acetyl cysteine (NAG), suggesting a role for reactive o xygen in phorbol ester-mediated receptor regulation. Furthermore, dire ct treatment of SMCs with reactive oxygen species increased scavenger- receptor activity. In rabbit SMCs, 100 mu mol/L H2O2 alone slightly in creased scavenger-receptor mRNA and protein expression. In combination , 100 mu mol/L H2O2 and 10 mu mol/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 S MCs and 8-fold in human SMCs. NAC reduced the effect of H2O2 and vanad ate by 93%. The increase in SMC scavenger-receptor expression occurs a t the level of gene transcription. Receptor mRNA half-life was unchang ed after treatment with either phorbol esters or reactive oxygen (appr oximate to 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 g eneration of reactive oxygen species are present in atherosclerotic le sions. These factors may all contribute to the upregulation of SMC sca venger-receptor activity and therefore to the formation of smooth musc le foam cells.