TRANSPORT OF SPHINGOMYELIN TO THE CELL-SURFACE IS INHIBITED BY BREFELDIN-A AND IN MITOSIS, WHERE C-6-NBD-SPHINGOMYELIN IS TRANSLOCATED ACROSS THE PLASMA-MEMBRANE BY A MULTIDRUG TRANSPORTER ACTIVITY

Sphingomyelin is a major lipid of the mammalian cell surface, The view that sphingomyelin, after synthesis in the Golgi lumen, reaches the o uter leaflet of the plasma membrane on the inside of carrier vesicles has been challenged by inconsistencies in the results of transport stu dies, To investigate whether an alternative pathway to the cell surfac e exists for sphingomyelin, brefeldin A and mitotic cells were used to block vesicular traffic between the Golgi complex and the plasma memb rane, Exogenous sphingomyelinase was applied in the cold to assay for the presence of sphingomyelin on the surface of CHO cells, Newly synth esized radiolabeled sphingomyelin was found to equilibrate with cell s urface sphingomyelin within 1.5 hours at 37 degrees C, Brefeldin A and mitosis inhibited this transport but, surprisingly, not the surface a ppearance of the short-chain sphingomyelin analog xadiazol-4-yl]aminoh exanoyl(C-6-NBD)-sphingomyelin as assayed by depletion of this lipid i n the medium by the scavenger albumin, Transport of C-6-NBD-sphingomye lin in the presence of brefeldin A was blocked by cyclosporin A and PS C 833, inhibitors of the multidrug resistance P-glycoprotein, The same was observed in HepG2 and HeLa cells, and for short-chain glucosylcer amide, which demonstrates the general nature of the transporter-depend ent sphingolipid translocation across the plasma membrane.