LACK OF REQUIREMENT FOR STEROL CARRIER PROTEIN-2 IN THE INTRACELLULAR TRAFFICKING OF LYSOSOMAL CHOLESTEROL

Previous work has established that the absence of peroxisomes, as occurs in Zellweger syndrome, is accompanied by the absence of cellular sterol carrier protein-2 (SCP2). In the present study, Zellweger-syndrome fibroblasts and peroxisome-deficient CHO-ZR78 cells were used to study the role of SCP2 in the intracellular transport of low density lipoprotein (LDL)derived lysosomal cholesterol. By immunoblotting, peroxisome-deficient cells were confirmed to contain either no detectable SCP2 or far less SCP2 than corresponding normal cells. To monitor the transport of lysosomal cholesterol to the plasma membrane, we measured efflux of lysosomal cholesterol to HDL(3) or phospholipid vesicles. SCP2-deficient cells, in comparison to normal cells, demonstrated little or no impairment in this efflux, suggesting that SCP2 is not required for the efficient delivery of lysosomal cholesterol to the plasma membrane. To examine the role of SCP2 in the delivery of lysosomal cholesterol to acyl-CoA:cholesterol acyltransferase (ACAT) in the rough endoplasmic reticulum (RER), the lysosomal and whole-cell cholesterol pools were differentially labeled, and then the ACAT-mediated esterification of each pool was measured in response to an 8-h incubation with native LDL. For both cholesterol pools, esterification was stimulated by LDL, and the responses in normal and Zellweger cells were similar, demonstrating that SCP2 is required for neither the stimulation of ACAT that follows LDL uptake nor for the transport of lysosomal cholesterol to the RER. These findings suggest that some major aspects of lysosomal cholesterol trafficking in cells can occur by mechanisms not involving SCP2.