Effect of micellar p-sitosterol on cholesterol metabolism in CaCo-2 cells

CaCo-2 cells were used to address the effect of the plant sterol, psitosterol, on cholesterol trafficking, cholesterol metabolism, and apoB secretion. Compared to cells incubated with micelles (5 mM taurocholate and 250 p~ oleic acid) containing cholesterol, which caused an increase in the influx of plasma membrane cholesterol to the endoplasmic reticulum and increased the secretion of cholesteryl esters derived from the plasma membrane, 0-sitosterol did not alter cholesterol trafficking or cholesteryl ester secretion. Including psitosterol in the micelle together with cholesterol attenuated the influx of plasma membrane cholesterol and prevented the secretion of cholesteryl esters derived from the plasma membrane. Stigmasterol and campesterol had effects similar to psitosterol, although campesterol did promote a modest influx of plasma membrane cholesterol. Including psitosterol in the micelle with cholesterol decreased the uptake of cholesterol. Compared to cholesterol, 60% less p-sitosterol was taken up by CaCo-2 cells. No observable esterification of psitosterol was appreciated and the transport of the plant sterol to the basolateral medium was negligible. Cholesterol synthesis and HMGCoA reductase activities were decreased in cells incubated with p-sitosterol. This was associated with a decrease in reductase mass and mRNA levels. Cholesteryl ester synthesis and ACAT activities were unaltered by p-sitosterol. Both stigmasterol and campesterol decreased reductase activity, but only campesterol increased ACAT activity. psitosterol did not affect the secretion of apoB mass.Il The results suggest that p-sitosterol does not promote cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. psitosterol interferes with the uptake of micellar cholesterol causing less plasma membrane cholesterol to influx arid less cholesteryl ester to be secreted. Despite its lack of effect on cholesterol trafficking, psitosterol decreases cholesterol synthesis at the level of HMGCoA reductase genc expression.Field, F. J., E. Born, and S . N. Mathur. Effect of micellar psitosterol on cholesterol metabolism in CaCo-2 crlls. ,I. I>i{id RES. 1997. 38: 348-360. Supplementary key words cholesterol trafficking cholesterol nhsorption apoB HMGCoA redut taw A(',\T The human small intestine can differentiate between luminal cholesterol and the plant sterol 0-sitosterol. Despite differing from cholesterol by only an ethyl group on carbon number 24, the absorption of 0-sitosterol is approximately 1/10 that of cholesterol (1). Although a number of mechanisms have been addressed in an attempt to explain this observation, it remains unresolved (2-7). In individuals suffering from the rare disorder 0-sitosterolemia, the intestinal absorption and plasma levels of plant sterols are markedly increased. This disorder is also characterized by accelerated atherosclerosis resulting in cutaneous xanthomas, coronary heart disease, and death at a young age (8, 9, review). For reasons that are not clear, cholesterol metabolism in these patients is also abnormal. Absorption of cholesterol from the small intestine is not altered, but results from balance and isotopic turnover studies have suggested that total body cholesterol synthesis in individuals with psitosterolemia is decreased (10, 11). In support of these studies, the actiLity of' HMGCoA reductase in liver and circulating monocytes has been found to be significantly decreased in these patients, whereas LDL receptor activity is increased (12, 13). It has been suggested that there is an inherent defect in the HMGCoA reductase gene to explain these