Folic acid inhibits homocysteine-induced proliferation of human arterial smooth muscle cells

Purpose: An elevated plasma homocysteine level has been identified as an independent risk factor for atherosclerosis. Whether this represents a marker for vascular disease or a direct effect on the vasculature remains unclear. Because vascular smooth muscle cells (VSMCs) play an integral role in the atherosclerotic process, we studied the effect of homocysteine on human infragenicular VSMC proliferation and the role of folic acid in reversing the homocysteine effect. Methods: Human infragenicular VSMCs harvested from amputation specimens were studied. Various cell groups were exposed to physiologic (6.25 mu mol/L and 12.5 mu mol/L) and pathologic (25 mu mol/L to 500 mu mol/L) concentrations of homocysteine. Similar groups were simultaneously exposed to 20 nmol/L of folic acid. Cell counts and DNA synthesis, as reflected by [methyl-H-3]-thymidine incorporation, were performed at 6 days and 24 hours, respectively. Additional groups were exposed to various combinations of folic acid (20 nmol/L), vitamin B-6 (145 nmol/L), and vitamin B-12 (0.45 nmol/L) in the presence of homocysteine (25, 50, and 250 mu mol/L). Results: Homocysteine resulted in a dose-dependent increase in DNA synthesis and cell proliferation. Cell counts increased significantly at homocysteine concentrations ranging from 25 mu mol/L to 500 mu mol/L (P < .05), with a maximal increase of 98% at 500 mu mol/L of homocysteine. The addition of 20 nmol/L folic acid resulted in significant inhibition of cell proliferation at all homocysteine concentrations studied (P < .001). Maximal inhibition of 70% occurred in the cells exposed to 50 mu mol/L of homocysteine. The increases in [methyl-H-3]-thymidine incorporation ranged from 36% at 6 mu mol/L homocysteine to a maximum of 247% at 500 mu mol/L homocysteine. All increases were statistically significant (P < .05). The addition of 20 nmol/L folic acid resulted in significant inhibition of DNA synthesis (P < .002). Vitamins B-6 and B-12 did not demonstrate significant antiproliferative properties. Conclusion: A possible role of homocysteine in the formation of atherosclerotic lesions is through a direct proliferative effect on VSMCs in a dose-dependent fashion. Folic acid intake at levels available in dietary supplements may prove protective in hyperhomocysteinemia-induced atherosclerosis. Vitamins B-6 and B-12 alone do not appear to exhibit a substantial inhibitory effect in the setting of elevated homocysteine levels.