Phytosterols decrease prostaglandin release in cultured P388D ₁/MAB macrophages

Cardiovascular disease (CVD) remains the leading cause of death in Western societies. Atherosclerosis is a major cardiovascular related disorder that is responsible for 50% of all mortality in the United States. Several epidemiological studies suggest that consumption of a plant-based diet is associated with a decreased incidence of cardiovascular abnormalities. Phytosterols, especially β-sitosterol, are plant sterols that have been shown to exert protective effects against cardiovascular diseases as well as many types of cancer. Monocyte/macrophage cells are involved with the inflammatory process. Accumulation of these cells in arteries is one of the initial events leading to atherosclerosis. Macrophages are capable of supplying the atherosclerotic vessel with substantial amounts of prostaglandins. Prostaglandins have been shown by numerous studies to play a key role in the atherosclerosis process. They can affect platelet aggregation, vasodilation or constriction of blood vessels, and the adherence of monocytes to the vessel walls. The purpose of this study was to examine the effect of phytosterols on the release of PGE₂ and PGI₂ from lipopolysaccharide (LPS)-stimulated P388D₁/MAB macrophage cells. P388D₁/MAB cells were supplemented with 16 μM cholesterol, β-sitosterol or campesterol using cyclodextrin as a vehicle. Phytosterol supplementation led to a significant decrease in cellular growth at various time points throughout a 7-day treatment period, especially after 3 days of treatment. Macrophages incorporated the supplemented phytosterols into their membranes which accounted for 26% of total membrane sterols. Cholesterol supplementation at 16 μM however, had no effect on membrane sterols. Supplementation with 16 μM concentration of β-sitosterol or campesterol resulted in a significant inhibition of PGE₂ and PGI₂ release from macrophage cells as compared to the vehicle control. Of the two phytosterols, β-sitosterol supplementation exhibited a greater inhibitory effect. PGE₂ release was decreased 68% by β-sitosterol and 55% by campesterol, while cholesterol supplementation was not as effective, as it led to a 37% decrease. Similarly, release of PGI₂ from macrophages was inhibited 67% by β-sitosterol and 52% by campesterol treatment, while enrichment of the cells with cholesterol, led to a 35% decrease in PGI₂ release. The decrease in prostaglandin release was not due to alteration in the expression of cPLA₂ and COX-2 enzymes which suggests that alterations in the activities of these enzymes may be responsible for the observed changes in prostaglandin release. It was concluded that phytosterol incorporation into macrophages may offer protection from atherosclerosis by reducing their prostaglandin release and thus slowing down the atheroma development.