Roles of Endogenously Synthesized Sterols in the Endocytic Pathway

The effect(s) of endogenously synthesized cholesterol (endo-CHOL) on the endosomal system in mammalian cells has not been examined. Here we treated Chinese hamster ovary cell lines with lovastatin (a hydroxymethylglutaryl-CoA reductase inhibitor) and mevalonate (a precursor for isoprenoids) to block endo-CHOL synthesis and then examined its effects on the fate of cholesterol liberated from low density lipoprotein (LDL-CHOL). The results showed that blocking endo-CHOL synthesis for 2 h or longer does not impair the hydrolysis of cholesteryl esters but partially impairs the transport of LDL-CHOL to the plasma membrane. Blocking endo-CHOL synthesis for 2 h or longer also alters the localization patterns of the late endosomes/lysosomes and retards their motility, as monitored by time-lapse microscopy. LDL-CHOL overcomes the effect of blocking endo-CHOL synthesis on endosomal localization patterns and on endosomal motility. Overexpressing Rab9, a key late endosomal small GTPase, relieves the endosomal cholesterol accumulation in Niemann-Pick type C1 cells but does not revert the reduced endosomal motility caused by blocking endo-CHOL synthesis. Our results suggested that endo-CHOL contributes to the cholesterol content of late endosomes and controls its motility, in a manner independent of NPC1. These results also supported the concept that endosomal motility plays an important role in controlling cholesterol trafficking activities.