Sphingomyelin transport to the cell surface occurs independently of protein secretion in rat hepatocytes.

Abstract

The transport of sphingomyelin from its site of synthesis, in the cis/medial-Golgi, to the plasma membrane has been investigated in monolayer cultures of rat hepatocytes. Sphingomyelin was synthesized intracellularly from [methyl-3H]choline, and the arrival of sphingomyelin at the plasma membrane was monitored by treatment of the cells with sphingomyelinase, which hydrolyzed only sphingomyelin at the cell surface. When hepatocytes were maintained at temperatures from 5 to 37 degrees C, both protein secretion and sphingomyelin trafficking to the cell surface were inhibited at low temperatures, especially at temperatures below 25 degrees C. Treatment of cells with the energy poison potassium cyanide prevented protein secretion and reduced the incorporation of [3H]choline into sphingomyelin, but did not affect the percentage of newly synthesized sphingomyelin that was transported to the cell surface. Brefeldin A and monensin, both of which disrupt protein movement through the Golgi apparatus, were used to evaluate the mechanism of transport of sphingomyelin to the plasma membrane. Although brefeldin A and monensin each effectively blocked protein secretion, sphingomyelin movement to the cell surface was not inhibited, even though sphingomyelin synthesis normally occurs in the Golgi. We conclude that the most likely mechanism of sphingomyelin transport to the cell surface is a vesicle-mediated process (because of the known topology of sphingomyelin synthesis and distribution). However, the putative vesicles that are involved in the movement of sphingomyelin to the plasma membrane are distinct from those that transport proteins for secretion. In addition, the route of trafficking of sphingomyelin to the plasma membrane is not one that requires an obligatory passage through the Golgi apparatus.