A Sphingomyelin Transfer Protein in Rat Tumors and Fetal Liver

Abstract

Affinity resins containing covalently bound phospholipids were used to compare the affinity of the phosphatidylcholine transfer protein from beef liver and a low-specificity lipid transfer protein from rat hepatoma 27 to phosphatidylcholine and sphingomyelin. Binding experiments demonstrated that the beef liver protein associates specifically with immobilized phosphatidylcholine whereas the hepatoma protein showed a preference for sphingomyelin.Purified antiserum raised against the hepatoma sphingomyelin transfer protein was used to determine the presence of that protein in the cytosol of various experimental tumors as well as in those of normal, regenerating and fetal rat liver. The protein was found to be well expressed in all the tumors examined and in fetal liver as determined by immunodiffusion whereas only minute amounts could be detected in normal liver and in 30-h regenerated liver. The presence of the sphingomyelin transfer protein in cytosol was in parallel with the presence of sphingomyelin in the corresponding mitochondria. It is suggested that the occurrence of sphingomyelin in tumor and fetal liver mitochondria may be due to protein-catalyzed sphingomyelin transfer from the endoplasmic reticulum.A number of phospholipid transfer proteins have been purified from a variety of biological tissues (for reviews see [I, 21 and also [3-91). A common feature of all these intracellular transport lipoproteins is that they either do not use sphingomyelin as substrate or catalyze the transfer of sphingomyelin to a much smaller extent than that of phosphatidylcholine. Recently a new lipid transfer protein has been isolated from rat hepatoma 27 which is capable of transferring sphingomyelin at least as efficiently as phosphatidylcholine [lo]. Such a lipid transfer protein could not be detected in rat liver, and the liver postmicrosomal supernatant was not capable of transferring sphingomyelin from rat liver microsomes to mitochondria [lo].Since mitochondria from various tumors frequently show elevated sphingomyelin levels [I 1 -151 it seemed possible that the occurrence of sphingomyelin in tumor mitochondria may be related to the action of sphingomyelin transfer proteins that are absent or less expressed, in normal cells. In this connection it was thought of interest to study further the phospholipid specificity of the rat hepatoma lipid transfer protein and to find out whether the mitochondrial sphingomyelin content in different rat tissues correlates with the presence of sphingomyelin transfer protein in the corresponding cytosol. In this paper we compare the phosphatidylcholine and sphingomyelin binding capacities of the hepatoma lipid transfer protein to those of the specific phosphatidylcholine transfer protein from beef liver using affinity resins containing covalently bound sphingomyelin and phosphatidylcholine. We report further on an immunological examination of the presence of the hepatoma lipid transfer protein in the cytosol of several rat tumors, fetal, regenerating and normal liver. In addition dat...

The Organizing Potential of Sphingolipids in Intracellular Membrane Transport

“…In cells, transfer of sphingolipids may be stimulated by protein factors. One protein has been identified that in vitro stimulates transfer of glycosphingolipids between membranes (198), whereas proteins that can transfer SM have been identified as well (73,83). Whether these proteins actually function as transfer proteins in the cell remains to be demonstrated, as is the case for the phospholipid transfer proteins first identified over 30 years ago (388,417).…”

Sphingolipid transport in eukaryotic cells

“…Since no vesicular transport occurs to the mitochondria, the mechanism of transport of SM to the mitochondria was most likely monomeric. It will be interesting why SM is available for monomeric transfer under those conditions and whether there is a function for an SM transfer protein [298,299].…”