Role of multidrug resistance P-glycoproteins in cholesterol metabolism

Cellular cholesterol homeostasis is maintained by regulated uptake of LDL-derived cholesterol, biosynthesis of cholesterol, and conversion of cholesterol to cholesteryl esters. Intracellular sterol transport between the plasma membrane and the endoplasmic reticulum (ER) is an important aspect of cholesterol esterification and biosynthesis. Progesterone inhibits these processes by blocking the transport of cholesterol and cholesterol precursors to the ER. Recent studies suggest that progesterone acts by inhibiting the activity of one or more of the multidrug resistance (MDR) P-glycoproteins. Structurally different inhibitors of MDR activity were shown to inhibit cholesterol esterification and biosynthesis in cell lines originating from multiple tissue types. A common mechanism requiring MDR activity is likely involved in both cholesterol biosynthesis and esterification. Using biochemical and genetic markers, the steps required for esterification of LDL-derived cholesterol were dissected. MDR inhibitors were shown to block a late step in delivery of cholesterol to the ER. Cells treated with MDR inhibitors also excluded a fluorescent cholesterol analog from the ER and accumulated novel vesicles. MDR inhibitors did not alter the general distribution of cholesterol in the plasma membrane as monitored by HDL-mediated cholesterol efflux. In addition, inhibition of MDR1 activity at the plasma membrane with MRK-16 antibody did not correlate with a decrease in cholesterol esterification or biosynthesis. MDR activity appears to be required for vesicular delivery of cholesterol to the ER after its departure from the plasma membrane. MDR P-glycoproteins catalyze translocation of phospholipids across the membrane bilayer; this activity may be required for vesicle generation or vesicle fusion events. The involvement of MDR P-glycoproteins in two cholesterol related diseases, coronary heart disease (CHD) and Smith-Lemli-Opitz syndrome (SLOS) was investigated. Coinheritance of the MDR genetic locus was analyzed in sibling pairs with CHD. Inherited abnormalities at the MDR genetic locus do not cause CHD, but quantitative trait analysis revealed that they do influence circulating triglyceride levels. Lymphoblasts from three SLOS patients with distinct phenotypes showed normal MDR mediated drug efflux, and normal subcellular localization of cholesterol and 7-dehydrocholesterol, a sterol that accumulates in SLOS patients. Differences in cholesterol biosynthesis that likely result from different mutations in 3?-hydroxysterol-?7-reductase are described.