INHIBITION OF GROWTH AND P21(RAS) METHYLATION IN VASCULAR ENDOTHELIAL-CELLS BY HOMOCYSTEINE BUT NOT CYSTEINE

Although hyperhomocysteinemia has been recognized recently as a preval ent risk factor for myocardial infarction and stroke, the mechanisms b y which it accelerates arteriosclerosis have not been elucidated, most ly because the biological effects of homocysteine can only be demonstr ated at very high concentrations and can be mimicked by cysteine, whic h indicates a lack of specificity, We found that 10-50 mu m of homocys teine (a range that overlaps levels observed clinically) but not cyste ine inhibited DNA synthesis in vascular endothelial cells (VEC) and ar rested their growth at the G(1) phase of the cell cycle. Homocysteine in this same range had no effect on the growth of vascular smooth musc le cells (VSMC) or fibroblasts, Homocysteine decreased carboxyl methyl ation of p21(ras) (a G(1) regulator whose activity is regulated by pre nylation and methylation in addition to GTP-GDP exchange) by 50% in VE C but not VSMC, a difference that may be explained by the ability of h omocysteine to dramatically increase levels of S-adenosylhomocysteine, a potent inhibitor of methyltransferase, in VEC but not VSMC. Moreove r, homocysteine-induced hypomethylation in VEC was associated with a 6 6% reduction in membrane-associated p21(ras) and a 67% reduc- tion in extracellular signal-regulated kinase 1/2, which is a member of the mi togen-activated protein (MAP) kinase family, Because the MAP kinases h ave been implicated in cell growth, the p21(ras)-MAP kinase pathway ma y represent one of the mechanisms that mediates homocysteine's effect on VEC growth, VEC damage is a hallmark of arteriosclerosis. Homocyste ine-induced inhibition of VEC growth may play an important role in thi s disease process.