Transcriptional Modulation of Hepatitis B Virus Through Multiple Regulatory Pathways
Ondracek, Caitlin R.
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Hepatitis B Virus (HBV) is a human pathogen that chronically infects approximately 400 million people worldwide. Although a vaccine exists to protect against HBV infection, there is no effective treatment for chronic carriers of this disease. The objective of this research is to understand the in vivo mechanisms through which HBV is regulated. We have established the capability of the transcriptional coactivator, peroxisome proliferator-activated receptor- γ coactivator 1α (PGC1α), to enhance viral biosynthesis by interacting with nuclear receptors required for recruitment of the transcriptional machinery necessary for viral RNA biosynthesis. While PGC1α has been demonstrated to be a powerful factor in enhancing viral biosynthesis in cell culture, modest results in vivo exemplify the significant challenges in demonstrating its specificity. The coactivators PGC1β, CBP, SRC1, and GRIP are also capable of enhancing viral biosynthesis, suggesting that compensation at the level of transcriptional coactivation exists, and that PGC1α is but one member in a multifactorial network influencing HBV biosynthesis. These specific coactivators can enhance nuclear receptor-mediated HBV gene transcription as well as gene activation by Fox transcription factors, presenting a previously unknown role for Fox transcription factors in HBV biosynthesis. It is likely that these transcription factors may be differentially expressed throughout the lifespan of a chronically infected patient, allowing the virus to persist through changing metabolic states. We also demonstrate that Akt/PKB (v-akt murine thymoma viral oncogene homolog/protein kinase B) is capable of regulating HBV biosynthesis through both transcriptional and post-transcriptional stages of the viral life cycle. This supports the hypothesis that HBV is metabolically regulated by signal transduction pathways in the liver. Akt activity is dependent on the insulin/PI3K/PDK1/Akt signal transduction pathway which is activated by growth/feeding conditions and inhibited during stress/starvation. This is consistent with previous work that has shown that caloric restriction has enhanced HBV biosynthesis, whereas loss of HBV has occurred during hepatocyte proliferation and tumor formation. Chronic HBV infection is associated with the development of liver disease and hepatocellular carcinoma. Developing the liver-specific PTEN-null HBV transgenic mouse, which produces constitutive Akt, allows the examination of the role of Akt in vivo. The aged PTEN-null mouse progressively develops hepatocellular carcinoma, which is often associated with Akt activation resulting from either growth factor stimulation or mutations within the Akt signal transduction pathway. HBV nucleocapsid protein expression and DNA replication are quite limited in tumors but relatively robust in adjacent nontumor liver tissue. Surprisingly, HBV 3.5kb RNA transcript levels are the same in both tumor and nontumor tissue. This suggests that secondary signaling events in the tumor post-transcriptionally inhibit viral biosynthesis. While viral biosynthesis has been previously observed to be limited in tumors, this study demonstrates that only distinct stages of the viral lifecycle are inhibited. Overall, this work provides insight into how HBV can adapt to various nutritional states and metabolic stresses. The role of metabolic signaling pathways, including the PI3K/Akt signaling pathway, bridges the metabolic state of the hepatocyte with HBV biosynthesis. Ultimately, our studies promote a more detailed mechanistic understanding of how chronic HBV persists in the liver and may identify novel targets for the future development of therapeutic agents.
Subjecthepatitis B virus