Interaction of hepatitis B virus with cellular defense mechanisms in relation to liver carcinogenesis

Hepatocellular carcinoma (HCC) is one of the major causes of cancer morbidity and mortality worldwide. However, its incidence is subject to variations: high incidence in sub-Saharan Africa and Asia, intermediate in Mediterranean countries and the Middle East, and low in Western European countries and North America. While different etiological factors have been implicated in the development of HCC, infection with the hepatotropic viruses hepatitis B virus (HBV) and hepatitis C virus (HCV), and exposure to liver carcinogens are the most frequently addressed as directly implicated in HCC development. Epidemiological studies indicate that infection with HBV and exposure to the cyclic mycotoxin Aflatoxin B1 constitute the major risk factors in some regions where the incidence of HCC is elevated. Strikingly, in areas were there is coexistent exposure to both these agents, the incidence of HCC is even higher. In order to elucidate the molecular mechanisms that may lead to a concomitant effect of both HBV and Aflatoxin B1 in HCC development, I hypothesized that HBV may decrease cellular defense mechanisms such as detoxification of active metabolites and/or DNA repair processes increasing the likelihood of mutations.; Detoxification of reactive compounds by phase II enzymes is an important cellular defense process involved in cell susceptibility to carcinogens. I have demonstrated that the activity of human Glutathione Transferase A1 (hGSTAl) enzyme is down-regulated in HBV infected cells. This data correlates with a decrease in protein and mRNA levels. I linked this inhibition---at least partially---to a transcriptional down-regulation of hGSTAl gene expression by the x protein of the HBV (HBx). Strikingly, Oltipraz (4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione), which has been found to have cancer chemoprotective properties, overcomes the effect of HBx on the hGSTAl promoter in in-vitro experiments. This result adds new evidence for the importance of the use of Oltipraz as an HCC chemoprotective agent.; I have demonstrated that the HBx is directly involved in the decrease of Nucleotide Excision Repair (NER) activity. Although HBx direct interaction with the tumor suppressor protein p53 was postulated to lead to decreased DNA repair mechanisms, I have shown that this process occurs in both p53-proficient and p53-deficient cells. The results of my work provide evidence that HBx-induced NER inhibition is associated with down-regulation of the expression of the TFIIH factors XPB and XPD. The decreased expression of both genes is regulated by HBx at the transcriptional level. These results were observed in both p53-proficient and p53-deficient cell lines. Interestingly, HBx was found to be capable of transcriptional down-regulation while maintaining its transactivation capacity. In addition, liver tissue from transgenic mice for HBx show decreased levels of XPB and XPD corroborating the results obtained in vitro.; Hence, the HBx protein seems to alter two cellular defense mechanisms that can increase susceptibility to liver carcinogenesis.