Lipid metabolism during in vitro induction of the lipocyte phenotype in hepatic stellate cells.

Molecular mechanisms of lipid synthesis and their controls in hepatic stellate cells are not known. We have previously proposed that, in contrast to other fat storing cells, hepatic stellate cells are not involved in energy storage, but they represent a particular cell population specialized in storage of lipid-soluble substances, the major one being probably retinol. In agreement with this hypothesis, induction of the lipocyte phenotype in stellate cells is not under the control of insulin, but responds to retinoids and other molecules that modify the gene expression program in these cells. In the present study we have monitored the activity of the two major enzymes involved in lipid synthesis during the induction of the lipocyte phenotype in hepatic stellate cells: glycerol-3-phosphate dehydrogenase (GPDH) that mediates the de novo lipid synthesis, and lipoprotein lipase that mediates incorporation of plasma lipids. In early stages of lipocyte induction, both pathways of lipid synthesis are activated. When lipocytes have already constituted the lipid droplets, lipoprotein lipase pathway is downregulated, while GPDH activity remains high. Adult liver has been reported to lack lipoprotein lipase, but under stress, lipase activity was detected around and at the surface of the intrahepatic vasculature. We have now shown that the lipase activity can be induced in the hepatic stellate cells, located in the Disse's space. The high lipoprotein lipase activity under acute induction of lipocyte phenotype, followed by the low activity under conditions of metabolic equilibrium, are in compass with the increased activity of this enzyme under stress, and its low activity in adult liver parenchyma under normal conditions.