GSK-3β as a Potential Coordinator of Anabolic and Catabolic Pathways in Hepatitis C Virus Insulin Resistance

Abstract

<b><i>Introduction:</i></b> Chronic hepatitis C infection can result in insulin resistance (IR). We have previously shown that it occurs through the interaction of pathways for glucose homeostasis, insulin signaling, and autophagy. But it is not known how soon the pathways are activated and how IR is related to the signals generated by catabolic and anabolic conditions occurring in infected cells. We have extended our studies to a cell culture system mimicking acute infection and to downstream pathways involving energy-sensor AMPK and nutrient-sensor mTOR that are active in catabolic and anabolic processes within the infected cells. <b><i>Methods:</i></b> Huh7 liver cells in culture were infected with hepatitis C virus (HCV). We performed proteomics analysis of key proteins in infected cells by Western blotting and IP experiments, with or without IFNα exposure as a component of conventional therapeutic strategy. <b><i>Results:</i></b> We present evidence that (a) IRS-1 Ser312, Beclin-1, protein conjugate Atg12-Atg5 or GS Ser641 are up-regulated early in infection presumably by activating the same pathways as utilized for persistent infection; (b) Bcl-XL, an inhibitor of both autophagy and apoptosis, is present in a core complex with IRS-1 Ser312 and Beclin-1 during progression of IR; (c) AMPK level remains about the same in infected cells where it is activated by phosphorylation at Thr172 concomitant with increased autophagy, a hallmark of catabolic conditions; (d) an mTOR level that promotes anabolism is increased rather than decreased under an expanded autophagy; (e) hypophosphorylation of translational repressor 4E-BP1 downstream of mTOR is suggestive of reduced protein synthesis; and (f) β-catenin, is up-regulated but not phosphorylated suggesting indirectly our previous contention that its kinase, GSK-3β, is mostly in an inactive state. <b><i>Conclusion:</i></b> We report that in the development of IR following chronic infection, anabolic and catabolic pathways are activated early, and the metabolic interaction occurs possibly in a core complex with IRS-1 Ser312, Beclin-1, and autophagy inhibitor Bcl-XL. Induction of autophagy is usually controlled by a two-edged mechanism acting in opposition under anabolic and catabolic conditions by AMPK/mTOR/4E-BP1 pathway with GSK-3β-mediated feedback loops. However, we have observed an up-regulation of mTOR along with an up-regulation of AMPK caused by HCV infection is a deviation from the normal scenario described above which might be of therapeutic interest.