Transactivation of proto-oncogene c-Myc by hepatitis B virus transactivator MHBs.

C-terminally truncated hepatitis B virus (HBV) middle size surface proteins (MHBs) has been shown to be a transcriptional activator and may be relevant to hepatocarcinogenesis by transactivating gene expression. In the present study, a pcDNA3.1(-)-MHBs vector coding for MHBs truncated at amino acid 167 (MHBs) was constructed and transfected into the HepG2 hepatoma cell line. mRNA and protein expression of MHBs in the cells was detected by reverse transcription-polymerase chain reaction (RT-PCR) and western blot analysis. A cDNA library of genes transactivated by the truncated protein in HepG2 cells was made in pGEM-T Easy using suppression subtractive hybridization. The cDNAs were sequenced and analyzed with BLAST searching against the sequences in GenBank. The results showed that certain sequences, such as that of human proto-oncogene c-Myc, may be involved in tumor development. An expression vector pCAT3/c-Myc containing the chloramphenicol acetyltransferase (CAT) gene under the control of a c-Myc promoter was generated, and the transcriptional transactivating effect of MHBs on the c-Myc promoter was investigated by RT-PCR and western blotting. MHBs was found to upregulate the transcriptional activity of the promoter, as well as transcription and translation of c-Myc. MHBs was also shown to transactivate SV40 immediate early promoter, and transcriptionally transactivate the expression of human c-Myc. These findings provide new directions for studying the biological functions of MHBs, and for a better understanding of the tumor development mechanisms of HBV infection.