THE FATE OF CHOLESTEROL EXITING LYSOSOMES

Cholesterol released from ingested low density lipoproteins in lysosom es moves both to the plasma membrane and to the endoplasmic reticulum (ER) where it is re-esterified, Whether cholesterol can move directly from lysosomes to ER or first must traverse the plasma membrane has no t been established, To examine this question, the endocytic pathway of rat hepatoma cells was loaded at 18 degrees C with low density lipopr oteins (LDL) labeled with [H-3]cholesteryl linoleate, and the label th en was chased at 37 degrees C, The hydrolysis of the accumulated ester proceeded linearly for several hours, Almost all of the released [H-3 ]cholesterol moved to the plasma membrane rapidly and without a discer nable lag, In contrast, the re-esterification in the ER of the release d [H-3]cholesterol showed a characteristic lag of 0.5-1 h, These data are inconsistent with direct cholesterol transfer from lysosomes to ER ; rather, they suggest movement through the plasma membrane. Furthermo re, we found that progesterone, imipramine and 3-beta-[2-(diethylamino )ethoxy]androst-5-en-17-one (U18666A) strongly inhibited the re-esteri fication of lysosomal cholesterol in tile ER, However, contrary to pre vious reports, they did not block transfer of [H-3]cholesterol from ly sosomes to the cell surface, Therefore, the site of action of these ag ents was not at the lysosomes. We suggest instead that their known abi lity to block cholesterol movement from the plasma membrane to the ER accounts for the inhibition of lysosomal cholesterol esterification. T hese findings are consistent with the hypothesis that cholesterol rele ased from lysosomes passes through the plasma membrane or, its way to the ER rather than proceeding there directly, As a result, ingested ch olesterol is subject to the same homeostatic regulation as the bulk of cell cholesterol, which is located in the plasma membrane.