Growth hormone (GH) deficiency in adults is associated with fatty liver disease and several features of the metabolic syndrome. Conversely, obesity is characterized by impaired GH function. Here, researchers aimed to clarify hepatic GH signaling and its metabolic consequences in nonalcoholic fatty liver disease. In humans, GHR and IGF-1 levels were measured in liver samples from 29 obese patients with nonalcoholic steatohepatitis (NASH) or simple steatosis. The cellular effects of GH on insulin signaling were investigated in GH receptor (GHR) knockdown HepG2 cells. Hepatic IGF-1 expression levels reflecting GH action were significantly lower and fasting glucose concentrations were higher in patients with NASH than in patients with simple steatosis. GHR knockdown in hepatocytes resulted in hyperglycemic output, manifested by decreased glycogen content, increased gluconeogenesis, and diminished insulin signaling. These data suggest that GH signaling is diminished in the liver of NASH patients and is associated with decreased hepatic insulin sensitivity and metabolic activity. Reduced hepatic GH activity may contribute to insulin resistance in obese NASH patients.
Insulin signaling activity in hepatocytes was assessed by quantifying Akt phosphorylation (pAKT). As shown in Figure 1, compared with non-silenced control HepG2 cells, Akt phosphorylation at threonine 308 was significantly reduced by 84 ± 9%, and phosphorylation at serine 473 was significantly reduced by 75 ± 7% in GHR knockdown cells. Incubation of GHR knockdown cells with 200 µg/ml IGF-1 for 24 hours did not eliminate the negative effects of Akt phosphorylation in GHR-inhibited cells. G6Pase M-RNA levels were significantly increased by 57 ± 20% in GHR-inhibited cells. Expression levels were also significantly higher in GHR-inhibited cells pre-incubated with IGF-1. Supplementation with IGF-1 significantly increased G6Pase expression in both GHR knockdown and control cells. PC and PCK1 M-RNA levels were higher in GHR knockdown cells than in control cells, but this did not reach statistical significance. The M-RNA expression levels of DGAT2, PPARγ, PPARα, SREBP1c, CPT1, FAS, and SCD-1 were similar between GHR knockdown cells and control cells.
Figure 1. pAKT/AKT in GHR-knockdown cells. (Rufinatscha K, et al., 2018)