There is no reliable model for the study of the many aspects of HBV infection, despite the use of the chimpanzee. Although several alternative methods have been previously developed for the in vitro study of HBV infection, there was still an urgent need for new in vitro infection models including for the ability of HBV to integrate into host cell genome. To date, mostly chimpanzees, but also recombinant mice, marmosets, woodchucks and ducks have been used as animal models for the study of the HBV infection (Schinazi, R. F., Ilan, E., Black, P. L. and Dagan, S. (1999). Cell-based animal models for hepatitis B and C viruses. Antivir. Chem. Chemother. 10, 99-114). Animals have been employed particularly for the evaluation of antiviral compounds.
In this scientific article, we have shown that following the early linearization of the viral genome, the integration of the viral DNA into the cellular genomic DNA did occur rapidly after 8 days of infection with the addition of two hormones, insulin and dexamethasone, in the cell culture medium. It is noticeable that in such an hepatoma infection system, no production of progeny virions in the cell culture supernatant was observed, whatever the cell culture conditions employed. By using this method we have shown that HepG2 and HuH-7 cell lines infected in vitro with HBV were still positive for HBV DNA, as revealed by polymerase chain reaction, two years after the onset of the infection. Some cell culture formulations enriched with hormones or oligoelements, increased the expression of the HBsAg (antigen) at the cell surface of the infected cells. Finally, the infection of the cells did result in dramatic cytopathic changes such as vacuolization, increase in the cell size and growth in soft agar, suggesting a potential transforming effect of HBV..
Figure. Involvement of the low density lipoproteins during the infection of hepatocytes with HBV in vitro. The LDL receptor and the annexin V binding protein have been proposed to mediate HBV adsorption and penetration into the hepatocytes. Both molecules do bind to low density lipoproteins (LDLs).
Since HBV does bind to the apolipoprotein H present in the LDLs. HBV might thus enter into the cells via the LDL receptor and/or the annexin V binding protein. The use of dextran sulfate prior to infection is removing the free lipoproteins from the cell surface and facilitating the subsequent binding and penetration of the LDL-HBV complexes into the hepatocytes.
As yet, despite some progresses, a major therapeutic breakthrough remains to be achieved. Hence new targets for antiviral therapy will have to be defined and obviously, detailed knowledge of the molecular mechanisms of hepatitis B virus (HBV) replication is required. To date, several types of regimen are currently available to reduce and/or inhibit the replication of the virus. However, a strong need does exist for the screening and the identification of new antioncogenic agents during the chronic, silent HBV infection when HBV DNA further integrates into the host genome, favouring cancerogenesis. The model system reported in this article may thus serve to look for new pharmacological targets to interfere with HBV DNA integration, and to screen in vitro for candidate inhibitors of therapeutic potential, without the use of animals. |
