Deletion of C3G in hepatocytes impairs full liver maturation and alters glucose homeostasis.

Abstract

C3G (RapGEF1) regulates the biology of liver hepatic progenitor cells and hepatocarcinoma cells, but its role in hepatocytes remained unknown. Therefore, we generated a mouse model lacking C3G in hepatocytes (C3GKO^Alb), which showed liver damage as evidenced by increased fibrosis, liver macrophages and serum transaminases activity. Furthermore, impaired liver maturation was observed in C3GKO^Alb mice demonstrated by the low expression of hepatocyte specific proteins (i.e. HNF4α), but higher levels of Alpha-fetoprotein, and stemness markers (i.e. CD133). Glucose homeostasis was also altered in C3GKO^Alb mice, as well as insulin and glucagon effects on hepatocytes, which resulted in reduced serum glucose levels and an enhanced response to glucagon. In addition, the expression of several glycolytic and gluconeogenic enzymes, as well as the levels of the active form of Glycogen phosphorylase (PYGL), were upregulated in livers from C3GKO^Alb mice, being remarkable the increased Pyruvate kinase isoform 2 (PKM2) levels accompanied by higher serum lactate concentrations. An increased expression of the ketogenic enzyme 3-hydroxy 3-methylglutaryl-CoA (HMG) synthase (Hmgcs2) was also found in these livers in parallel to elevated blood levels of beta-hydroxy-butyrate. Moreover, the fasting response was enhanced in C3GKO^Alb mice as compared to wt animals. Hence, livers lacking C3G in hepatocytes showed a higher expression of gluconeogenic, lipogenic and ketogenic enzymes than livers from wt mice and enhanced ketogenesis. Mechanistically, data support a PTBP1-mediated upregulation of PKM2 expression in hepatocytes lacking C3G, which leads to enhanced glycolysis. Other metabolic alterations are likely due to the defective insulin signaling and the enhanced glucagon signaling through a cAMP-PKA-dependent mechanism. In summary, we have identified a novel role for C3G in the liver as a key mediator of hepatocyte differentiation and metabolic functions of hepatocytes. Hence, its absence leads to an immature phenotype and an altered response to insulin and glucagon, favoring glucagon actions.