Activation of MAPK by modified low-density lipoproteins in vascular smoothmuscle cells

A high concentration of circulating low-density lipoproteins (LDL) is a maj or risk factor for atherosclerosis. Native LDL and LDL modified by glycatio n and/or oxidation are increased in diabetic individuals. LDL directly stim ulate vascular smooth muscle cell (VSMC) proliferation; however, the mechan isms remain undefined. The extracellular signal-regulated kinase (ERK) path way mediates changes in cell function and growth. Therefore, we examined th e cellular effects of native and modified LDL on ERK phosphorylation in VSM C. Addition of native, mildly modified (oxidized, glycated, glycoxidized) a nd highly modified (highly oxidized, highly glycoxidized) LDL at 25 mug/ml to rat VSMC for 5 min induced a fivefold increase in ERK phosphorylation. T o elucidate the signal transduction pathway by which LDL phosphorylate ERK, we examined the roles of the Ca2+/calmodulin pathway, protein kinase C (PK C), src kinase, and mitogen-activated protein kinase kinase (MEK). Treatmen t of VSMC with the intracellular Ca2+ chelator EGTA-AM (50 mmol/l) signific antly increased ERK phosphorylation induced by native and mildly modified L DL, whereas chelation of extracellular Ca2+ by EGTA (3 mmol/l) significantl y reduced LDL-induced ERK phosphorylation. The calmodulin inhibitor N-(6-am inohexyl)-1-naphthalenesulfonamide (40 mu mol/l) significantly decreased ER K phosphorylation induced by all types of LDL. Downregulation of PKC with p horbol myristate acetate (5 mmol/l) markedly reduced LDL-induced ERK phosph orylation. Pretreatment of VSMC with a cell-permeable MEK inhibitor (PD-980 59, 40 mu mol/l) significantly decreased ERK phosphorylation in response to native and modified LDL. These findings indicate that native and mildly an d highly modified LDL utilize similar signaling pathways to phosphorylate E RK and implicate a role for Ca2+/calmodulin, PKC, and MEK. These results su ggest a potential link between modified LDL, vascular function, and the dev elopment of atherosclerosis in diabetes.