Insulin resistance (IR) is common in chronic hepatitis C (CHC) and associates with reduced virological response to Pegylated-Interferon (PEG-IFN)/Ribavirin therapy, but the underlining mechanisms are still unclear. We have previously shown that, in CHC patients, insulin plasma levels are inversely related to antiviral effect induced by PEG-IFN. Therefore, we have here investigated "in vitro" the effect of insulin on IFN-alpha intracellular signaling as well as that of IFN-alpha on insulin signaling.

HepG2 cells, pre-incubated with or without insulin, were stimulated with IFN-alpha2b and mRNA and protein expression of IFN-stimulated genes (ISGs) were measured at different time points. The role of intracellular SOCS3 was evaluated by the siRNA strategy. To assess the effect of IFN-alpha on insulin signaling, HepG2 were pre-incubated with or without IFN before addition of insulin and cells were then analyzed for IRS-1 and for Akt/PKB Ser473 phosphorylation. Insulin (100 and 1000nM) significantly reduced in a dose-dependent fashion IFN-induced gene expression of PKR (p=0.017 and p=0.0017, respectively), MxA (p=0.0103 and p=0.00186) and OAS-1 (p=0.002 and p=0.006). Insulin also reduced IFN-alpha induced PKR protein expression. Although insulin was confirmed to increase SOCS3 expression, siRNASOCS3 did not restore ISGs expression after insulin treatment. IFN-alpha was found to reduce, in dose dependent fashion, IRS-1 gene expression as well as Akt/PKB Ser473 phosphorylation induced by insulin.

CONCLUSIONS: These results provide evidence of reciprocal interference between insulin and IFN-alpha signaling in liver cells. These findings may contribute to understand the role of insulin in CHC: IR might be favored by endogenous cytokines including IFN-alpha, and the resulting hyperinsulinemia then reduces the antiviral response to exogenous IFN in a vicious circle of clinical significance.