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 +/- 9 nmol fatty acid/mg protein per 24 h) at a ra te similar to that observed in freshly prepared rat hepatocytes (192 /- 8 nmol/mg per 24 h) and in primary cultures of rat hepatocytes (165 .4 +/- 29.3 nmol/mg per 24 h). In HepG2 cells, fatty acid synthesis wa s inhibited by extracellular oleate (0.75 mM), and, to a lesser extent , by glucagon (10(-7) M). Insulin (7.8 x 10(-8) M) had a mild stimulat ory effect. Fatty acid synthesis was not influenced by lipogenic precu rsors (lactate plus pyruvate), substances which, in rat hepatocytes, h ad pronounced stimulatory effects. Fatty acid synthesis rates were als o unchanged in the presence of prostaglandin E(2) (PGE(2)). Tn 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 extracell ular oleate (469 +/- 43 nmol triacylglycerol/mg protein per 24 h) but phospholipid synthesis was relatively low (15.8 +/- 0.4% of total glyc erolipids). Very little of the above newly synthesized triacylglycerol was secreted as lipoprotein (4.62 +/- 0.88 nmol triacylglycerol/mg pr otein per 24 h) resulting in a large intracellular accumulation of tri acylglycerol. This was exacerbated by the absence of any detectable ke togenesis. The secretion of triacylglycerol produced from de novo synt hesized fatty acids was also very low in HepG2 compared to that observ ed in primary cultures of rat hepatocytes. In HepG2, the capacity for triacylglycerol + phospholipid synthesis from exogenous fatty acids wa s far higher than that from endogenous synthesized fatty acids. Lipopr otein triacylglycerol secretion was inhibited by insulin in HepG2. How ever, glucagon and PGE(2), which inhibit this process in rat hepatocyt es, were without effect. In contrast to rat hepatocytes, most of the l ipoprotein triacylglycerol in HepG2 was secreted without prior lipolys is and re-esterification of intracellular triacylglycerol. This reflec ted a very low overall rate of intracellular triacylglycerol lipolysis in HepG2. These results suggest that, although lipid synthesis is ver y active in HepG2, there is a defect in lipid mobilization (lipolysis, secretion, and oxidation) that results in excessive intracellular sto rage of triacylglycerol.