ROLE OF PHOSPHOLIPASE A(2) IN EXPRESSION OF THE SCAVENGER PATHWAY IN CULTURED AORTIC SMOOTH-MUSCLE CELLS STIMULATED WITH PHORBOL 12-MYRISTATE 13-ACETATE

We have demonstrated that cultured intimal smooth muscle cells (SMC) f rom thickened intima can metabolize acetylated low-density lipoprotein (LDL) by a scavenger pathway, but medial SMC from normal arteries can not. In this study we investigated the expression mechanism of the sca venger pathway in medial SMC using a phorbol ester. Medial SMC were in cubated with 10(-10)-10(-7) M phorbol 12-myristate 13-acetate (PMA) fo r 1-24 h and then their degradation of I-125-labelled acetylated LDL w as assayed. Unstimulated SMC degraded little acetylated LDL, but incub ation for 24 h with PMA dose-dependently stimulated its degradation by SMC, the optimal PMA concentration being 1 x 10(-8) M. Induction of e xpression of the scavenger pathway required more than 4 h of incubatio n with PMA and was completely inhibited by cycloheximide. In addition expression of the scavenger pathway was not transient but stable. Indu ction of expression of the scavenger pathway by PMA was not inhibited by protein kinase C inhibitors, but was inhibited about 50 % by phosph olipase A(2) inhibitors. The study, using various phorbol esters, indi cated that induction of the scavenger pathway was well correlated with their ability to stimulate phospholipase A(2) in medial SMC but not w ith their ability to activate protein kinase C. Moreover, incubation w ith exogenous phospholipase A(2) (0.1-10 units/ml) or its product, lys ophosphatidylcholine (0.01-100 mu g/ml) dose-dependently increased deg radation of I-125-labelled acetylated LDL in medial SMC. Lysophosphati dylcholine was most effective in various lysophospholipids. These resu lts suggest that PMA induced the scavenger pathway in part by stimulat ing phospholipase A(2) in medial SMC, and that a product, lysophosphat idylcholine, is a mediator of expression of the scavenger pathway.