Mechanistic Target of Rapamycin Complex 1 (mTORC1) and Adenosine Monophosphate Activated Protein Kinase (AMPK) are part of a signaling node in a network of opposing metabolic pathways. In conditions of nutritional sufficiency, mTORC1 is active and stimulates pathways that generate proteins, nucleic acids, and lipids necessary for cell growth and proliferation. In conditions of nutritional insufficiency, AMPK is activated to restore the energy charge of the cell by promoting catabolic processes and inhibiting anabolic processes. Viruses rely on cellular mechanisms for the generation of substrates needed for their replication and have evolved numerous interactions with cellular factors that regulate production of these substrates. We will focus on how hepatitis B virus (HBV) affects metabolic homeostasis in hepatocytes. This HBV effect likely establishes an environment suitable for its own replication while altering the metabolic set-point and rendering hepatocytes more or less sensitive to stressors. Chronic HBV infection is a cause of hepatocellular carcinoma (HCC); however, HBV-associated HCC develops after decades of infection, suggesting that HBV is weakly oncogenic and may only be a co-factor in HCC development. The HBV HBx protein is necessary for HBV replication and has a role in HBV-associated oncogenesis. We have shown that HBx activates mTORC1 and AMPK, that HBV replication is suppressed by mTORC1 and promoted by AMPK, and that expression of HBV proteins is elevated in the context of signals that inhibit global mRNA translation. A protein that was named HBx-interacting protein (HBXIP) was identified in a screen for cellular HBx-interacting proteins. HBXIP (renamed Late Endosomal/ Lysosomal Adaptor, MAPK and mTOR Activator 5 (Lamtor-5)) was recently identified as a component of the Ragulator complex, which regulates mTORC1 activity; this positions HBx at a metabolic signaling node. Our goals are to determine if HBV infection alters metabolic homeostasis established by opposing mTORC1 and AMPK pathways, whether this sensitizes or de-sensitizes cells to stresses that signal through these pathways, and whether a consequence of this is alteration of cellular protein synthesis. The results of these studies will lay the foundation for more comprehensive analyses of how HBV affects hepatocyte metabolism and how this influences expression of cellular and HBV proteins. We hypothesize that HBV interferes with cellular homeostasis to favor its own replication and that this alters the response of hepatocytes to other stresses. Over time, this leads to disruptions in hepatocyte survival and proliferation and contributes to HCC development. We will: 1. Examine whether HBx re-sets the homeostatic metabolic balance in hepatocytes, resulting in altered sensitivity of the cells to signaling through either mTORC1 or AMPK; and 2. Determine the effects of HBV- induced mTORC1 and AMPK activation on the efficiency of translation of viral and cellular mRNAs.
Although the link between a chronic hepatitis B virus (HBV) infection and the development of hepatocellular carcinoma (HCC) is established, the development of HBV-associated HCC typically requires decades of a chronic infection, suggesting that HBV may serve more as a co-factor in HCC development. We have shown that HBV alters glucose metabolism and energy-sensing signal-transduction pathways in hepatocytes, which could reset the metabolic baseline profile of hepatocytes to favor HBV replication. We will test the hypothesis that this metabolic reset sensitizes hepatocytes to cellular sensors, thereby affecting cell signal transduction pathways that could influence HCC development.
