Pamela Norton Associate Professor, Microbiology and Immunology Associate Director, Drexel Institute for Biotechnology and Virology Research Ph.D., 1986, Tufts University, Boston, Massachusetts 700 East Butler Ave. Doylestown, PA 18901 Tel: 215-489-4903 Fax: 215-489-4920 Email: Pamela.Norton@DrexelMed.edu

Keywords:

Hepatitis B virus, antiviral agents, extracellular matrix, liver disease, RNA processing

Research Interests

Chronic viral hepatitis is one of the major causes of liver disease, and represents a critical risk factor for liver cirrhosis and hepatocellular carcinoma. Infection with hepatitis B virus (HBV) can result in chronic infection, but the events that lead from infection to severe disease many years later are incompletely understood. An important theme of our research is to investigate virus-host cell molecular interactions and understand their role in virus pathology. Studies to identify changes that HBV infection causes within the infected host cell utilize cultured cell based models of HBV infection. These studies have identified a number of host cell genes that are either up- or down-regulated by the presence of HBV. A number of apolipoprotein genes are down-regulated; current investigation centers on determining the molecular mechanism by which the change in gene regulation occurs, and whether metabolic perturbations are a hallmark of viral infection. In contrast, the gene for the extracellular matrix protein fibronectin is up-regulated by the HBV transactivator, HBx. As fibronectin is also up-regulated early in models of fibrotic liver injury, it is possible that the virus contributes directly in the pathogenesis of liver fibrosis, the common prequel to cirrhosis.

Vaccination has proven effective in decreasing the incidence of new HBV infections, but has little therapeutic efficacy. The devastating consequences of chronic HBV infection compel our efforts to develop novel antiviral drugs. The drugs that are currently available target the viral polymerase (lamivudine and adefovir) or the host immune system (interferon). We have been characterizing members of a novel class of antiviral agents based on modifications of the prototype imino sugar deoxynojirimycin or DNJ (right). The imino sugars inhibit host glucosidases interfering with correct glycan processing; secretion of incompletely processed glycoproteins from the endoplasmic reticuluum is impaired. Certain viral glycoproteins such as the HBV MHBs and LHBs proteins have a strong requirement for correct glycoprocessing for secretion. Thus, blocking glycoprotein secretion selectively reduces virus secretion. DNJ derivatives with an N-alkyl side chain (for example, compound 169) are considerably more potent inhibitors of viral envelope glycoprotein secretion than DNJ itself. One member of this class of compounds will be tested in against the related woodchuck hepatitis virus in conjunction with therapeutic vaccination to test the hypothesis that antigen reduction will enhance vaccination efficacy. An exciting outcome of this study was the finding that drug treatment enhanced the cellular immune response to viral antigens. This drug-induced response was associated with reactivity to novel epitopes generated by de-N-glycosylation prior to proteasomal processing and presentation. The ability of certain imino sugars to augment epitope presentation by MHC class I in cultured cells suggests a possible role for these compounds in stimulating the production of virus-specific cytolytic CD8+ T cells.

We also have been investigating the ability of the host cell to recognize and defend itself against HBV infection. Interferon can reduce HBV replication in cell culture as well as in vivo, suggesting that non-immune factors contribute to anti-viral activity. We have found that the gene for a recently identified anti-viral host factor, APOBEC3G, is induced by interferon. The APOBEC3s belong to a family of cytidine deaminases thought to result in increased mutation of DNA (and possibly RNA). Studies are aimed at identifying the effects of APOBEC3 family members on HBV replication.

Selected Publications

1. Gorski, G. K., Aros, M. C., and P. A. Norton.  Characterization of fibronectin alternative splicing during mouse liver development reveals a novel isoform present transiently during late gestation.  Gene Expression, 6: 139-149, 1996.
   
   
2. Mirza, A., Liu, S. -L., Frizell, E., Zhu, J., Maddukuri, S., Martinez, J., Davies, P., Schwarting, R., Norton, P., and M. A. Zern.  A role for tissue transglutaminase in hepatic injury and fibrogenesis, and its regulation by nuclear factor-kappaB (NF-kappaB).  Am. J. Physiol., 35: G281-G288, 1997.
   
   
3. Kuo, B. A., and P. A. Norton.  Role of fibronectin exon B sequences in exon recognition.  Nucl. Acids Res., 27: 3945-3952, 1999.
   
   
4. Zhu, J., Wu, J., Frizell, E., Liu, S. -L., Bashey, R., Rubin, R., Norton, P., and M. A. Zern.  Rapamycin inhibits stellate cell proliferation in vitro and limits fibrogenesis in an in vivo model of liver fibrosis.  Gastroenterol., 117: 1198-1204, 1999.
   
   
5. Wu, J., Liu, S. -L. Zhu, J. L., Norton, P. A., Nojiri, S., Hoek, J. B., and M. A. Zern.  Roles of tissue transglutaminase in ethanol-induced inhibition of hepatocyte proliferation and beta 1-adrenergic signal transduction.  J. Biol. Chem., 275: 22213-22219, 2000.
   
   
6. Norton, P. A., and L. C. Steel, eds. "Gene Transfer Methods: Introducing DNA into Living Cells and Organisms", BioTechniques Press, Eaton Publishing, Natick, MA, 2000.
   
   
7. Kuo, B. A., Uporova, T. M., Liang, H., Bennett, V. D., Tuan, R. S., and P. A. Norton.  Alternative splicing during chondrogenesis: modulation of fibronectin exon EIIIA splicing by SR proteins.  J. Cell. Biochem., 86: 45-55, 2002.
   
   
8. Norton, P. A., Gong, Q., Mehta, A. S., Lu, X., and T. M. Block.  Hepatitis B virus-mediated changes in apolipoprotein mRNA abundance in cultured hepatoma cells.  J. Virol., 77: 5503-5506, 2003.
   
   
9. Norton, P. A., and C. J. Pachuk.  Methods for DNA introduction into mammalian cells. In "Gene Transfer and Expression in Mammalian Cells", S.C. Makrides, ed., Elsevier Science B.V. Amsterdam, 263-277, 2003.
   
   
10. Norton, P. A., Reis, H. M. G. P. V., Prince, S., Larkin, J., Pan, J., Liu, J., Gong, Q., Zhu, M., and M. A. Feitelson.  Activation of fibronectin gene expression by Hepatitis B virus X antigen.  J. Viral. Hep., 11: 332-341, 2004.
    
    
11. Norton, P. A., Conyers, B., Gong, Q., Steel, L. F., Block, T. M., and A. S. Mehta.  Assays for glucosidase inhibitors with potential antiviral activities: secreted alkaline phosphatase as a surrogate marker.  J. Virol. Methods, 124: 167-172, 2005.
    
    
12. Gu, B., Mason, P., Wang, L., Norton, P., Bourne, N., Moriarty, R., Mehta, A., Despande, M., Shah, R., and T. Block.  Antiviral profiles of novel iminocyclitol compounds against bovine viral diarrhea virus, West Nile virus, Dengue virus and hepatitis B virus.  Antiviral Chem. Chemother., 18: 49-59, 2006.
    
    
13. Liang, H., Tuan, R. S., and P. A. Norton.  Forced expression of a splicing factor promotes chondrogenesis.  Exp. Cell Res., 313: 1518-1532, 2007.
    
    
14. Liu, Y., Simsek, E., Norton, P., Sinnathamby, G., Philip, R., Block, T. M., Zhou, T., and A. S. Mehta.  The role of the downstream signal sequences in the maturation of the HBV Middle Surface glycoprotein: development of a novel therapeutic vaccine candidate.  Virology, 365: 10-19, 2007.
    
    
15. Norton, P. A., Comunale, M. A., Krakover, J., Rodemich, L., Pirog, N., D'Amelio, A., Philip, R., Mehta, A. S., and T. M. Block.  N-linked glycosylation of the liver cancer biomarker GP73.  J. Cell. Biochem., in press, 2007.