β-sitosterol inhibits HT-29 human colon cancer cell growth and alters membrane lipids

The purpose of the present study was to examine the effect of β-sitosterol, the main dietary phytosterol on the growth of HT-29 cells, a human colon cancer cell line. In addition, the incorporation of this phytosterol into cellular membranes and how this might influence the lipid composition of the membranes were investigated. Tumor cells were grown in DMEM containing 10% FBS and supplemented with sterols (cholesterol or β-sitosterol) at final concentrations up to 16 μM. The sterols were supplied to the media in the form of sterol cyclodextrin complexes. The cyclodextrin used was 2-hydroxypropyl-β-cyclodextrin. The sterol to cyclodextrin molar ratio was maintained at 1:300. The study indicated that 8 and 16 μM β-sitosterol were effective at cel growth inhibition as compared to cholesterol or to the control (no sterol supplementation). After supplementation with 16μM β-sitosterol for 9 days, cell growth was only one-third that of cells supplemented with equimolar concentration of cholesterol. No effect was observed on total membrane phospholipid concentration. At 16 μM β-sitosterol supplementation, membrane cholesterol was reduced by 26%. Cholesterol supplementation resulted in a significant increase in the cholesterol/phospholipid compared to either β-sitosterol supplemented cells or controls. There was a 50% reduction on membrane sphingomyelin (SM) of cells grown in 16 μM β-sitosterol. Additional changes were observed in the fatty acid composition of minor phospholipids of β-sitosterol supplemented cells, such as SM, phosphatidylserine (PS), and phosohatidylinositol (PI). Only in the case of PI, was there an effect of these fatty acid changes on the unsaturation index, β-sitosterol incorporation in an increase in the U.I. It is possible that the observed growth inhibition by β-sitosterol may be mediated through the influence of signal transduction pathways that involve membrane phospholipids.