Hepatitis B virus X protein induces RNA polymerase III-dependent gene transcription and increases cellular TATA-binding protein by activating the Ras signaling pathway.
Our previous studies have shown that the hepatitis B virus protein, X, activates all three classes of RNA polymerase III (pol III)-dependent promoters by increasing the cellular level of TATA-binding protein (TBP) (H.-D. Wang et al., Mol. Cell. Biol. 15:6720-6728, 1995), a limiting transcription component (A. Trivedi et al., Mol. Cell. Biol. 16:6909-6916, 1996). We have investigated whether these X-mediated events are dependent on the activation of the Ras/Raf-1 signaling pathway. Transient expression of a dominant-negative mutant Ras gene (Ras-ala15) in a Drosophila S-2 stable cell line expressing X (X-S2), or incubation of the cells with a Ras farnesylation inhibitor, specifically blocked both the X-dependent activation of a cotransfected tRNA gene and the increase in cellular TBP levels. Transient expression of a constitutively activated form of Ras (Ras-val12) in control S2 cells produced both an increase in tRNA gene transcription and an increase in cellular TBP levels. These events are not cell type specific since X-mediated gene induction was also shown to be dependent on Ras activation in a stable rat 1A cell line expressing X. Furthermore, increases in RNA pol III-dependent gene activity and TBP levels could be restored in X-S2 cells expressing Ras-ala15 by coexpressing a constitutively activated form of Raf-1. These events are serum dependent, and when the cells are serum deprived, the X-mediated effects are augmented. Together, these results demonstrate that the X-mediated induction of RNA pol III-dependent genes and increase in TBP are both dependent on the activation of the Ras/Raf-1 signaling cascade. In addition, these studies define two new and important consequences mediated by the activation of the Ras signal transduction pathway: an increase in the central transcription factor, TBP, and the induction of RNA pol III-dependent gene activity.
Wang
H D
HD
Department of Molecular Pharmacology, School of Pharmacy, University of Southern California, Los Angeles 90033, USA.
Trivedi
A
A
Johnson
D L
DL
D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D017931:DNA Primers / Q000235:genetics; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004330:Drosophila; D011905:Genes, ras; D006515:Hepatitis B virus / Q000235:genetics* / Q000378:metabolism* / Q000472:pathogenicity; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics; D012320:RNA Polymerase III / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D015398:Signal Transduction; D035181:TATA-Box Binding Protein; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D015533:Transcriptional Activation; D054334:Viral Regulatory and Accessory Proteins
D017931:DNA Primers; D004268:DNA-Binding Proteins; D035181:TATA-Box Binding Protein; D015534:Trans-Activators; D014157:Transcription Factors; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein; D019908:Proto-Oncogene Proteins c-raf; D012320:RNA Polymerase III