SIDE-CHAIN STRUCTURE IS CRITICAL FOR THE TRANSPORT OF STEROLS FROM LYSOSOMES TO CYTOPLASM

Macrophages take up and metabolize negatively charged liposomes contai ning free cholesterol efficiently, resulting in a massive accumulation of cholesteryl esters and triacylglycerols in their cytoplasm (Nishik awa, K., Arai, H, and Inoue, K. (1990) J. Biol. Chem. 265, 5226-5231). This system was used to assess the effects of structural variation of sterol on the intracellular transport and the metabolism of endocytos ed sterols by the cells. Liposomes containing phytosterols with an ext ra one (campesterol) or two (beta-sitosterol, stigmasterol, fucosterol ) carbons at the C-24 position of the cholesterol side-chain were endo cytosed as efficiently as those containing cholesterol without exhibit ing any apparent toxicity on the cells. Esterification of endocyotosed phytosterols was, however, extremely low; campesterol esterification was only 20% that of cholesterol and either beta-sitosterol or stigmas terol was not esterified appreciably. A morphological study showed tha t the endocytosed phytosterols were accumulated in the phagolysosomes of the cells. Blocking of esterification of endocytosed cholesterol by an acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor did not lead to cholesterol accumulation in the phagolysosomes. These data suggest that accumulation of endocytosed phytosterols in phagolysosomes is no t a consequence of the inability of the cell to esterify sterols in th e endoplasmic reticulum. In the light of these observations, we conclu de that cultured macrophages can discriminate between sterols that dif fer only by a methyl or ethyl group at the C-24 position at their lyso somal compartment.