Modulation of glutathione S-transferase alpha by hepatitis B virus and thechemopreventive drug oltipraz

Persistent infection by hepatitis B virus (HBV) and exposure to chemical ca rcinogens correlates with the prevalence of hepatocellular carcinoma in end emic areas. The precise nature of the interaction between these factors is not known, Glutathione S-transferases (GST) are responsible for the cellula r metabolism and detoxification of a variety of cytotoxic and carcinogenic compounds by catalysis of their conjugation with glutathione. Diminished GS T activity could enhance cellular sensitivity to chemical carcinogens. We h ave investigated GST isozyme expression in hepatocellular HepG2 cells and i n an HBV-transfected subline. Total GST activity and selenium-independent g lutathione peroxidase activity are significantly decreased in HBV transfect ed cells. On immunoblotting, HBV transfected cells demonstrate a significan t decrease in the level of GST Alpha class. Cytotoxicity assays reveal that the HBV transfected cells are more sensitive to a wide range of compounds known to be detoxified by GST Alpha conjugation, Although no significant di fference in protein half-life between the two cell lines was found, semiqua ntitative reverse transcription-polymerase chain reaction shows a reduced a mount of GST Alpha mRNA in the transfected cells, Because the HBV x protein (HBx) seems to play a role in HBV transfection, we also demonstrated that expression of the HBx gene into HepG2 cells decreased the amount of GST Alp ha protein, Transient transfection experiments using both rat and human GST Alpha (rGSTA5 and hGSTA1) promoters in HepG2 cells show a decreased CAT ac tivity upon HBx expression, supporting a transcriptional regulation of both genes by HBx. This effect is independent of HBx interaction with Sp1. Trea tment with oltipraz, an inducer of GST Alpha, partially overcomes the effec t of HBx on both promoters. Promoter deletion studies indicate that oltipra z works through responsive elements distinct from AP1 or NF-kappaB transcri ption factors. Thus, REV infection alters phase II metabolizing enzymes via different mechanisms than those modulated by treatment with oltipraz.