Transactivation by Hepatitis B Virus X Protein is Promiscuous and Dependent on Mitogen-Activated Cellular Serine/Threonine Kinases

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 90; no. 17; pp. 8078 - 8082
• Main Authors: Cross, James C., Wen, Ping, Rutter, William J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01.09.1993; National Acad Sciences; National Academy of Sciences
• Subjects: Animals; Base Sequence; Binding Sites; Biochemistry; Cell Line; Cellular biology; Chloramphenicol O-Acetyltransferase - metabolism; DNA; Down regulation; Enhancer Elements, Genetic; Enzyme Activation; HeLa Cells; Hep G2 cells; Hepatitis; Hepatitis B; Hepatitis B virus; Hepatitis B virus - genetics; Humans; L cells; Medical research; Molecular Sequence Data; Oligodeoxyribonucleotides - chemical synthesis; Oligodeoxyribonucleotides - metabolism; Plasmids; Protein Kinase C - metabolism; Protein Kinases - metabolism; Protein-Serine-Threonine Kinases - metabolism; Proteins; Proto-Oncogene Proteins - metabolism; Proto-Oncogene Proteins c-jun - metabolism; Proto-Oncogene Proteins c-raf; Restriction Mapping; T lymphocytes; Trans-Activators - biosynthesis; Trans-Activators - metabolism; Transactivation; Transcription, Genetic; Transcriptional Activation; Transfection; Tumor Cells, Cultured; Viruses
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.90.17.8078

The X protein of hepatitis B virus (HBV-X) can act as a transactivator of transcription but its mechanism of action remains obscure. We have analyzed HBV-X trans-activation in several cell types using 13 unrelated viral and cellular promoters and found that transactivation is more or less apparent in most cell types and is promiscuous and unrelated to specific sequence motifs within the target promoters. In general, though, HBV-X appears to act on enhancer elements since HBV-X had no effect on a minimal promoter, whereas HBV-X was able to transactivate after insertion of an AP-1 minienhancer. Several lines of evidence exclude the possibility that HBV-X interacts directly with the AP-1 enhancer or its binding proteins and suggest that the proximal target of HBV-X is peripheral to the transcription complex. This hypothesis is supported by the observation that inhibition of serine/threonine kinases, which regulate AP-1 activity (phorbol ester down-regulation or staurosporine inhibition of protein kinase C and a dominant negative mutant of Raf-1), blocked the ability of HBV-X to transactivate without affecting basal promoter activity. Furthermore, basal transcription from the AP-1-dependent promoter was increased by overexpression of protein kinase C and Raf-1 but HBV-X was unable to further stimulate, indicating that these kinases act subsequently to HBV-X. These data suggest that transactivation by HBV-X is an indirect result of the activation of cellular serine/threonine kinases including protein kinase C and Raf-1. This mode of action implies that HBV-X may affect other cellular processes, besides transcription, that are regulated by these kinases.