Physiologic concentrations of glucose regulate fatty acid synthase activity in HepG2 cells by mediating fatty acid synthase mRNA stability.

Carbohydrate feeding of animals results in striking increases in hepatic fatty acid synthesis but much of this induction is presumed to be hormone mediated. To clarify the mechanisms responsible for the specific effect of carbohydrate on fatty acid synthesis, the effects of D-glucose on the expression of human fatty acid synthase (FAS) in HepG2 cells cultured in serum-free medium were studied. Northern blots of total RNA from these cells showed a single FAS mRNA band of 9.3 kilobases that was increased 2.7-5.4-fold in the presence of D-glucose. Lactate and citrate but not L-glucose mimicked this effect. Glucose induction of FAS mRNA was time- and concentration-dependent and most of the increase in FAS message was detected within the range of physiologic glucose concentrations. Glucose induction of FAS mRNA, protein synthetic rate, and enzyme activity were similar suggesting that glucose regulates FAS expression at a pretranslational level in this system. Transcription run-off experiments showed that glucose feeding was associated with no change in the FAS transcription initiation rate despite a 5-fold increase in FAS mRNA. FAS mRNA stability as determined by actinomycin D chase was 7-fold greater in the presence of glucose. Differences between FAS mRNA levels in cells grown in the presence versus the absence of glucose were inhibited by cycloheximide but not puromycin, suggesting that glucose regulation of FAS mRNA stability is not dependent on translation. Glucose, at physiologic concentrations and in the absence of hormones, appears to regulate FAS gene expression in HepG2 cells predominantly by mediating FAS mRNA stability.