Glucocorticoid (GC) excess (Cushings syndrome) is characterized by central obesity, insulin resistance and in up to 20% of cases, non-alcoholic fatty liver disease (NAFLD). NAFLD is a progressive spectrum of disease ranging from hepatic steatosis to steatohepatitis, fibrosis and cirrhosis. Many processes contribute to lipid accumulation within heaptocytes including de novo lipogenesis which includes the rate-limiting carboxylation of acetyl CoA to malonyl-CoA by acetyl CoA carboxylase (ACC) and conversion to palmitate by fatty acid synthase (FAS). We have hypothesized that increased GC exposure drives insulin resistance and promotes the development of NAFLD.
C3A cells (human hepatoma derived), which express the GC receptor, were treated with cortisol (250 nM, 24 h) in the presence or absence of insulin (80 nM, 24 h). Lipogenic gene expression was determined by real-time PCR and lipogenesis measured by 1-[14C] acetate incorporation into triglyceride.
Cortisol increased mRNA expression of ACC2 (0.005±0.003 (control) vs 0.008±0.004 (cortisol), P<0.05) and FAS (0.071±0.007 (control) vs 0.110±0.019 (cortisol), P<0.05) but not ACC1, diacyly glycerol acyl transferase (DGAT) and glycerol phosphate acyl transferase (GPAT). However, cortisol decreased functional lipogenesis (78.68±4.5% vs control (100%), P<0.05) probably reflecting rate-limiting, post-transcriptional regulation. Interestingly, in the absence of cortisol, insulin only had a modest effect to increase lipogenesis and this did not reach statistical significance (114.9±12.4% vs control (100%), P=ns). In contrast, when C3A cells were pre-treated with cortisol for 24 h, the stimulatory effect of insulin upon lipogenesis was augmented (mean % increase in lipogenesis following insulin, 14.9±12.4% (control) vs 38.2±12.3% (cortisol), P<0.05).
In C3A cells, in the absence of insulin, GCs decreased lipogenesis despite increasing FAS and ACC2 expression. Consequent increased hepatic free fatty acid exposure may contribute to hepatic insulin resistance. Our data also suggest that GCs may potentially enhance the ability of insulin to promote lipid storage in hepatocytes.