Niemann-Pick type C1 (NPC1) overexpression alters cellular cholesterol homeostasis

The Niemann-Pick type C1 (NPC1) protein is a key participant in intracellul ar trafficking of low density lipoprotein cholesterol, but its role in regu lation of sterol homeostasis is not well understood. To characterize furthe r the function of NPC1, we generated stable Chinese hamster ovary (CHO) cel l lines overexpressing the human NPC1 protein (CHO/NPC1). NPC1 overexpressi on increases the rate of trafficking of low density lipoprotein cholesterol to the endoplasmic reticulum and the rate of delivery of endosomal cholest erol to the plasma membrane (PM). CHO/NPC1 cells exhibit a 1.5-fold increas e in total cellular cholesterol and up to a 2.9-fold increase in PM cholest erol. This increase in PM cholesterol is closely paralleled by a 3-fold inc rease in de novo cholesterol synthesis. Inhibition of cholesterol synthesis results in marked redistribution of PM cholesterol to intracellular sites, suggesting an unsuspected role for NPC1 in internalization of PM cholester ol, Despite elevated total cellular cholesterol, CHO/NPC1 cells exhibit inc reased cholesterol synthesis, which may be attributable to both resistance to oxysterol suppression of sterol-regulated gene expression and to reduced endoplasmic reticulum cholesterol levels under basal conditions. Taken tog ether, these studies provide important new insights into the role of NPC1 i n the determination of the levels and distribution of cellular cholesterol.