Oxidized LDL Stimulates Mitogen-Activated Protein Kinases in Smooth Muscle Cells and Macrophages

Abstract

It has been proposed that oxidized LDL is more atherogenic than native LDL. However, the mechanisms by which native LDL and oxidized LDL alter function of cells in the vessel wall remain undefined. A signal transduction pathway that mediates many changes in cell function is the mitogen-activated protein (MAP) kinase cascade. We therefore examined the effect of native LDL and oxidized LDL on MAP kinase activity in cultured vascular smooth muscle cells (VSMC), endothelial cells, and macrophages by using an in-gel-kinase assay and anti-phosphotyrosine MAP kinase antibodies. Native LDL and LDL oxidized by the addition of Cu²⁺(Cu²⁺-oxidized LDL) stimulated MAP kinase in a time- and dose-dependent manner in baboon and rat VSMC but not in bovine endothelial cells. Cu²⁺-oxidized LDL stimulated MAP kinase in human monocyte-derived macrophages, but the effect was much greater in cells cultured for 7 days compared with 1 day, suggesting dynamic regulation of the cellular response to oxidized LDL. In rat VSMC, the maximal MAP kinase response to Cu²⁺-oxidized LDL was significantly greater than the response to native LDL. Cu²⁺-oxidized LDL was more potent, with half-maximal activation at 15 μg/mL versus 30 μg/mL for native LDL. Stimulation of MAP kinase appeared to involve protein kinase C, since phorbol ester pretreatment for 24 hours blocked MAP kinase activation. Oxidation of LDL by other methods showed that activation of MAP kinase was not well correlated with lipid peroxides or aldehydes, suggesting that other components present in oxidized LDL were responsible. The active moiety appeared to be lipid based on extraction of oxidized LDL with organic solvents. These data indicate that LDL stimulates MAP kinase in VSMC, oxidation of LDL potentiates the effect, a lipid moiety is involved, and Cu²⁺-oxidized LDL activation of MAP kinase is cell-type specific. These findings suggest a role for MAP kinase in the pathways by which oxidized LDL contributes to altered cellular function associated with atherogenesis.