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Accelerated proliferation of arterial smooth muscle cells (ASMC) plays an important role in the development of atherosclerosis, which preferentially affects the infragenicular vasculature in patients with diabetes mellitus. High insulin and glucose levels, which are present in patients with type II diabetes, have an additive effect in infragenicular ASMC proliferation in vitro. Thiamine is a coenzyme important in intracellular glucose metabolism. The objective of this study is to determine the effect of thiamine on human infragenicular ASMC proliferation induced by high glucose and insulin levels in vitro. Human infragenicular ASMC isolated from diabetic patients undergoing lower extremity amputation were used. Cells were cultured at 37 degrees C in 5% CO(2). Cells were identified as ASMC by immunohistochemical analysis. Cells from passages 3-5 were exposed to glucose concentrations of 0.1 and 0.2% with and without insulin concentrations of 100 ng/mL and 1000 ng/mL, in the presence or absence of 200 microM of thiamine. Standard hemocytometry and (3)H-thymidine incorporation quantified cell proliferation after incubation for 6 days and 24 hr, respectively. The data suggest that thiamine inhibits human infragenicular ASMC proliferation induced by high glucose and insulin. Vitamin B1 intake may prove important in delaying the atherosclerotic complications of diabetes.

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 micromol/L and 12.5 micromol/L) and pathologic (25 micromol/L to 500 micromol/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-(3)H]-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 micromol/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 micromol/L to 500 micromol/L (P <.05), with a maximal increase of 98% at 500 micromol/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 micromol/L of homocysteine. The increases in [methyl-(3)H]-thymidine incorporation ranged from 36% at 6 micromol/L homocysteine to a maximum of 247% at 500 micromol/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.