DIFFERENTIAL-EFFECTS OF SPHINGOMYELIN HYDROLYSIS AND CHOLESTEROL TRANSPORT ON OXYSTEROL-BINDING PROTEIN-PHOSPHORYLATION AND GOLGI LOCALIZATION

The deposition of de novo synthesized and lipoprotein-derived choleste rol at the plasma membrane and trans port to the endoplasmic reticulum is dependent on sphingomyelin (SM) content. Here we show that hydroly sis of plasma membrane SM in Chinese hamster ovary cells by exogenous bacterial sphingomyelinase resulted in enhanced cholesterol esterifica tion at the endoplasmic reticulum and rapid dephosphorylation of the o xysterol-binding protein (OSBP), a cytosolic/Golgi receptor for oxyste rols such as 25-hydroxycholesterol. After sphingomyelinase treatment, restoration of OSBP phosphorylation closely paralleled resynthesis of SM and down-regulation of cholesterol. ester synthesis. SM hydrolysis activated an okadaic acid-sensitive phosphatase that was not stimulate d in Chinese hamster ovary cells by short chain ceramides. Agents that specifically Mocked sphingomyelinase-mediated delivery of cholesterol to acyl-CoA:cholesterol acyltransferase (U18666A) or promoted cholest erol efflux to the medium (cyclodextrin) did not inhibit OSBP dephosph orylation. SM hydrolysis also promoted OSBP translocation from a vesic ular compartment to the Golgi apparatus. Cyclodextrin and U18666A also caused OSBP translocation to the Golgi apparatus, suggesting that OSB P movement is coupled to changes in the cholesterol content of the pla sma membrane or Golgi apparatus. These results identify OSBP as a pote ntial target of SM turnover and cholesterol mobilization at the plasma membrane and/or Gels apparatus.