Changes of sphingolipid species in the phenotype conversion from myofibroblasts to lipocytes in hepatic stellate cells.

Sphingolipids play a relevant role in cell-cell interaction, communication, and migration. We studied the sphingolipid content in the murine hepatic stellate cell line GRX, which expresses the myofibroblast phenotype, and can be induced in vitro to display the fat-storing phenotype. Lipid modifications along this induction were investigated by labeling sphingolipids with [(14)C]galactose, [(14)C]serine, or [(14)C]choline, and determination of fatty acid composition of sphingomyelin. The total ganglioside content and the GM2 synthase activity were lower in myofibroblasts. Both phenotypes presented similar gangliosides of the a-pathway: GM2, GM1, and GD1a as well as their precursor GM3. Sphingomyelin and all the gangliosides were expressed as doublets; the upper/lower band ratio increased in lipocytes, containing more long-chain fatty acids in retinol-induced lipocytes as compared to the insulin/indomethacin induced ones. Time-course experiments indicated a transfer of metabolic precursors from phosphatidylcholine to sphingomyelin in the two phenotypes. Taken together, these results indicate that myofibroblast and lipocytes can use distinct ceramide pools for sphingolipid synthesis. Differential ganglioside expression and presence of the long-chain saturated fatty acids suggested that they may participate in formation of distinct membrane microdomains or rafts with specific functions on the two phenotypes of GRX-cells.