Post-transcriptional control of the level of mRNA by hepatitis B virus X gene in the transient expression system using human hepatic cells

Background: Hepatitis B virus (HBV) infection is closely related to the development of not only acute or chronic hepatitis, but also hepatocellular carcinoma. Among the HBV genes, the X gene has been implicated in the carcinogenicity of this virus as a major causative factor by its ability to activate viral and cellular genes in trans via protein-protein interaction with cellular factors without binding to DNA. Results: To explore the possibility of other functions of the X gene, we examined the effect of X protein on the transient expression system of simian virus 40 (SV40) large T-antigen or chloramphenicol acetyltransferase (CAT) mRNA using SV40 promoter or EF-1 alpha (human elongation factor la) promoter, by co-transfecting an X gene expression plasmid to human hepatic cell lines, HepG2 and Huh7, In contrast to the SV40 promoter-mediated expression, the level of both T-antigen and CAT mRNAs expressed from the EF-1 alpha promoter was strikingly decreased by X protein in both hepatic cells. The nuclear run-on assay and the mRNA decay experiment using actinomycin D, indicated that the effect of X protein on the lowering of the level of chimeric mRNA was due to the degradation of mRNA, but not repression of transcriptional initiation. Moreover, this effect was dependent on the 22 bp sequence in the 5' untranslated region of mRNA derived from the EF-1 alpha promoter. Conclusion: The present data suggest a new function of the X gene to post-transcriptionally control the stability of mRNA through the 5' untranslated region derived from the EF-1 alpha promoter in human hepatic cells.