Lipid balance in HepG2 cells: active synthesis and impaired mobilization.

The human hepatoma cell line HepG2 in culture medium synthesized fatty acids de novo (144 f 9 nmol fatty acid/mg protein per 24 h) at a rate similar to that observed in freshly prepared rat hepatocytes (192 f 8 nmol/mg per 24 h) and in primary cultures of rat hepatocytes (165.4 f 29.3 nmol/ mg per 24 h). In HepG2 cells, fatty acid synthesis was inhibited by extracellular oleate (0.75 mM), and, to a lesser extent, by glucagon M). Insulin (7.8 x M) had a mild stimula- tory effect. Fatty acid synthesis was not influenced by lipogenic precursors (lactate plus pyruvate), substances which, in rat hepatocytes, had pronounced stimulatory effects. Fatty acid syn- thesis rates were also unchanged in the presence of prostaglandin E2 (PGE2). In general, compared to rat hepatocytes, fatty acid synthesis in HepG2 cells was less sensitive to manipulation of the culture medium in vitro. HepG2 cells had a high capacity for triacylglycerol synthesis from extracellular oleate (469 f 43 nmol triacylglycerol/mg protein per 24 h) but phospholipid syn- thesis was relatively low (15.8 + 0.4% of total glycerolipids). Very little of the above newly synthesized triacylglycerol was secreted as lipoprotein (4.62 5 0.88 nmol triacylglycerol/mg protein per 24 h) resulting in a large intracellular accumulation of triacylglycerol. This was exacerbated by the absence of any detectable ketogenesis. The secretion of triacylglycerol produced from de novo synthesized fatty acids was also very low in HepG2 compared to that observed in primary cultures of rat hepato- cytes. In HepG2, the capacity for triacylglycerol + phospholipid synthesis from exogenous fatty acids was far higher than that from endogenous synthesized fatty acids. Lipoprotein triacyl- glycerol secretion was inhibited by insulin in HepG2. However, glucagon and PGEZ, which inhibit this process in rat hepato- cytes, were without effect. In contrast to rat hepatocytes, most of the lipoprotein triacylglycerol in HepGZ was secreted without prior lipolysis and re-esterification of intracellular triacyl- glycerol. This reflected a very low overall rate of intracellular