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

Previous work has established that the absence of peroxisomes, as occu rs in Zellweger syndrome, is accompanied by the absence of cellular st erol carrier protein-2 (SCP2). In the present study, Zellweger-syndrom e fibroblasts and peroxisome-deficient CHO-ZR78 cells were used to stu dy the role of SCP2 in the intracellular transport of low density lipo protein (LDL)derived lysosomal cholesterol. By immunoblotting, peroxis ome-deficient cells were confirmed to contain either no detectable SCP 2 or far less SCP2 than corresponding normal cells. To monitor the tra nsport of lysosomal cholesterol to the plasma membrane, we measured ef flux of lysosomal cholesterol to HDL(3) or phospholipid vesicles. SCP2 -deficient cells, in comparison to normal cells, demonstrated little o r no impairment in this efflux, suggesting that SCP2 is not required f or the efficient delivery of lysosomal cholesterol to the plasma membr ane. To examine the role of SCP2 in the delivery of lysosomal choleste rol to acyl-CoA:cholesterol acyltransferase (ACAT) in the rough endopl asmic 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 nativ e LDL. For both cholesterol pools, esterification was stimulated by LD L, and the responses in normal and Zellweger cells were similar, demon strating that SCP2 is required for neither the stimulation of ACAT tha t 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.