901-122 Oxidized LDL Stimulates Mitogen-activated Protein Kinases in Smooth Muscle Cells and Macrophages

Abstract

Cellular proliferation and lipid infiltration in vessels are important events in atherogenesis. Recent data suggest an important role for LDL and oxidized LDL (ox-LDL) in the initiation and maintenance of atherosclerosis. The mechanisms by which LDL and ox-LDL affect cellular proliferation are not clear. A critical signal transduction event universally stimulated by growth factors is activation of mitogen-activated protein kinases (MAPK). We therefore examined the effect of LDL and ox-LDL on MAPK activity in cultured smooth muscle cells (SMC) and macrophages by using an in-gel-kinase assay with myelin basic protein as substrate. LDL and ox-LDL stimulated MAPK in a time-and dose-dependent manner in baboon, monkey, and rat SMC, but not in bovine endothelial cells. Ox-LDL also stimulated MAPK in macrophages but the effect was only present after freshly isolated cell as were cultured for 7 days. The maximal response to ox-LDL (340% increase over control) occurred at 50 μg/ml at 5 min in rat SMC, with half-maximal response at 10 μg/ml. Ox-LDL stimulated MAPK 2-fold more than native LDL. Stimulation of MAPK was mediated by protein kinase C (PKC) as phorbol ester pretreatment for 24 hr blocked MAPK activation. The active moiety was a lipid, based on the ability to extract the MAPK activating material from ox-LDL using chloroform, These data show that LDL stimulates MAPK in SMC via a PKC-dependent pathway, that oxidation of LDL potentiates the effect, and that a lipid is involved. These data establish a new growth related signal, MAPK activation, that is more powerfUlly stimulated by ox-LDL than LDL in SMC. In addition, there appears to be dynamic regulation of the cellular response to ox-LDL based on the differences between freshly isolated and cultured macrophages. This knowledge should be useful in understanding the mechanisms by which ox-LDL promotes SMC proliferation and macrophage dysfunction in atherogenesis.