News and Publications

Transcription and Replication of Hepatitis B Virus

H. Tang, K.E. Banks, A.L. Anderson, A. McLachlan

Infection with hepatitis B virus (HBV) is a worldwide health problem. The clinical consequences of HBV infection can be acute or chronic and range from the subclinical to the fatal forms of the disease. An estimated 200 million to 500 million persons in the world are chronic HBV carriers, and to date, no reliable treatment is available for them. Understanding the life cycle of HBV in detail may reveal potential targets for antiviral therapy. HBV replicates by reverse transcription of the viral pregenomic RNA encoded by the HBV DNA genome. Consequently, transcription of the viral genome and the regulation of pregenomic RNA synthesis, in particular, are essential steps in viral replication.

Chronic HBV infection is commonly treated with interferon. Interferon suppresses viral replication and can relieve the clinical signs and symptoms associated with the infection. However, interferon therapy does not efficiently resolve chronic HBV infections, and the molecular mechanisms of how interferon contributes to the reduction in HBV replication are not well understood. Interferon might modulate HBV transcription directly by activating interferon-stimulated gene factor 3 or members of the interferon regulatory factor (IRF) family of transcription factors. The presence of an interferon-stimulated response element (ISRE)--interferon regulatory element (IRE) sequence in the enhancer 1/X-gene promoter region of the HBV genome supports this contention.

We showed that the HBV enhancer 1/X-gene promoter ISRE-IRE sequence can respond to treatment with interferon. In addition, expression of interferon-stimulated gene factor 3 also supports transcription from a promoter containing the enhancer 1/X-gene promoter ISRE-IRE sequence, indicating that treatment with interferon probably directly activates this transcription factor. The IRF1 and IRF7 transcription factors also increase transcription from the enhancer 1/X-gene promoter ISRE-IRE sequence. These findings support the suggestion that members of the IRF family of transcription factors can directly bind to the enhancer 1/X-gene promoter ISRE-IRE sequence.

However, of note, IRFs activate expression of interferon genes, and interferon can activate expression of IRF genes. The complex interplay between interferon expression, activation of interferon-stimulated gene factor 3, and IRF gene expression suggests that several of these transcription factors may modulate expression of HBV genes in response to treatment with interferon by interacting with the enhancer 1/X-gene promoter ISRE-IRE sequence. Possibly, these interactions play a role in modulating HBV replication in vivo during interferon therapy.

A complication associated with HBV infection is the appearance of natural viral variants during the course of the disease. A natural HBV variant associated with induction of serum antibodies to hepatitis B e antigen contains 2 nucleotide substitutions in the proximal binding site for nuclear hormone receptors in the nucleocapsid promoter. These nucleotide substitutions prevent binding of the heterodimer comprising retinoid X receptor a and peroxisome proliferator activated receptor a without greatly altering the efficiency of binding of hepatocyte nuclear factor 4 to this recognition sequence.

Analysis of HBV transcription and replication indicates that the heterodimers support higher levels of pregenomic RNA transcription from the wild-type than from the variant nucleocapsid promoter, producing higher levels of wild-type replication intermediates. In contrast, hepatocyte nuclear factor 4 supports higher levels of pregenomic RNA transcription from the variant than from the wild-type nucleocapsid promoter, producing higher levels of variant replication intermediates. These observations indicate that replication of wild-type and variant viruses can be differentially regulated by the liver-specific transcription factors that bind to the proximal nuclear hormone receptor binding site of the nucleocapsid promoter. This finding may help explain the selection of specific viral variants that occurs during an antiviral immune response. To investigate this possibility in vivo, we are using HBV transgenic mice that synthesize this naturally occurring viral variant.

PUBLICATIONS

Raney, A.K., Eggers, C.M., Kline, E.F., Guidotti, L.G., Pontoglio, M., Yaniv, M., McLachlan, A. Nuclear covalently closed circular viral genomic DNA in the liver of hepatocyte nuclear factor 1 a-null hepatitis B virus transgenic mice. J. Virol. 75:2900, 2001.

Raney, A.K., Kline, E.F., Tang, H., McLachlan, A. Transcription and replication of a natural hepatitis B virus nucleocapsid promoter variant is regulated in vivo by peroxisome proliferators. Virology 289:239, 2001.

Tang, H., Banks, K.E., Anderson, A.L., McLachlan, A. Hepatitis B virus transcription and replication. Drug News Perspect., in press.

Tang, H., McLachlan, A. Transcriptional regulation of hepatitis B virus by nuclear hormone receptors is a critical determinant of viral tropism. Proc. Natl. Acad. Sci. U. S. A. 98:1841, 2001.

Tang, H., Raney, A.K., McLachlan, A. Replication of the wild type and a natural hepatitis B virus nucleocapsid promoter variant is differentially regulated by nuclear hormone receptors in cell culture. J. Virol. 75:8937, 2001.

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