Cardiovascular disease (CVD) is the leading cause of death in the USA and other industrialized countries. A large number of epidemiological studies have established a direct correlation between diet and the development and progression of atherosclerosis. Several studies have shown the incidence of CVD to be lower in populations consuming a predominantly plant-based diet, as compared to meat-based diets. Besides being low in fat and cholesterol, vegetarian and Asian diets contain a large variety of phytochemicals, which may function in the body. For example, phytosterols (PS) are plant sterols that interfere with the absorption of cholesterol from the intestine when present in adequate amounts. Although PS may also function at a cellular level in the body, there are few studies examining the action of PS on cells involved in atherosclerosis. The purpose of this study was to examine the effect of dietary PS on vascular smooth muscle cell (VSMC) growth and function, since VSMC play a central role in the development of atherosclerosis. VSMC were treated with 16 microM cholesterol, 25-hydroxycholesterol, campesterol and beta-sitosterol (SIT) using an ethanol as a vehicle. Cell growth was determined by cell counting and cell proliferation by DNA synthesis, which was measured by [(3)H]-thymidine incorporation. Cholesterol supplementation had no effect on cell growth and proliferation. 25-Hydroxycholesterol decreased cell growth by 68% and DNA synthesis by 99%. SIT was found to inhibit VSMC growth more effectively than campesterol. Of the two PS, campesterol decreased cell growth by 16% and SIT decreased cell growth by 30%. DNA synthesis was decreased 25% by SIT supplementation but was not influenced by campesterol or cholesterol supplementation. Cholesterol, campesterol and SIT were not cytotoxic to VSMC and did not significantly alter cell viability. 25-Hydroxycholesterol, however, was cytotoxic and decreased cell viability by 45% as determined by lactate dehydrogenase release and a trypan blue dye exclusion test. De novo cholesterol synthesis was decreased 28% by campesterol, 49% by SIT and 23% by cholesterol. Beta-sitosterol exhibited a greater effect on cholesterol synthesis than campesterol or cholesterol supplementation. Measurement of cell sterol content demonstrated incorporation of PS into VSMC at the expense of cholesterol. Campesterol decreased VSMC cholesterol content by 36%, representing 40% of the total sterol content following treatment. Beta-sitosterol decreased VSMC cholesterol by 41% following supplementation and represented 49% of the total sterol amount. Cholesterol treatment did not alter the cholesterol content of the cells. Prostacyclin production was significantly altered by PS treatment. Basal prostacyclin release was increased 43% by campesterol and 81% by SIT. A23187 stimulated prostacyclin release was increased 25% by campesterol and 54% by SIT. SIT supplementation induced a greater effect on prostacyclin release from VSMC than cholesterol or campesterol supplementation. The in vitro results presented here suggest that dietary PS, especially SIT, may offer protection from the VSMC hyperproliferation found in atherosclerosis. Further in vivo research is needed to support these observations.
Copyright 2001 Harcourt Publishers Ltd.