Basis for rapid efflux of biosynthetic desmosterol from cells

Previous work shows that the efflux of biosyn- thetic desmosterol from cells is three times more efficient than that of cholesterol. To explain this difference, we la- beled CHO-K1 cells with ( 3 H)acetate precursor and mea- sured sterols in the whole cells, plasma membranes and ca- veolae, and those released to high density lipoprotein (HDL 3 ). The ( 3 H)desmosterol-to-( 3 H)cholesterol ratio was similar in the plasma membrane and whole cells but was greater in HDL 3 , suggesting that the more efficient efflux of desmosterol is due to more rapid desorption from the plasma membrane. The ratio in caveolae was similar to that in whole cells, arguing against selective delivery of desmos- terol to caveolae as an explanation for the more rapid ef- flux of this sterol. Additionally, to demonstrate that the enhanced release of desmosterol was not due to enhanced intracellular cycling, we made vesicles from CHO-cell plasma membranes labeled with ( 3 H)desmosterol or ( 14 C)choles- terol, and the rapid release of desmosterol was demon- strated in this system. To characterize sterol efflux from a simple lipid bilayer system, we measured the transfer of cholesterol and desmosterol between large unilamellar vesi- cles (LUV), and found that desmosterol transferred two to three times more rapidly than cholesterol. A similar differ- ential was seen when HDL 3 or low density lipoprotein (LDL) served as the acceptor. These results show that the greater efflux efficiency of biosynthetic desmosterol can be attributed to more efficient desorption from the plasma membrane, and that this difference is a property of the ste- rols' association with the lipid bilayer. In vivo, the rapid ef- flux of biosynthetic sterol intermediates, followed by effi- cient delivery to the liver, may constitute an important mechanism for preventing various types of pathology asso- ciated with these materials.— Phillips, J. E., W. V. Rodrigueza, and W. J. Johnson. Basis for rapid efflux of biosynthetic des- mosterol from cells. J. Lipid Res. 1998. 39: 2459-2470.