Related Experiment Videos
Modulation of intracellular signal transduction pathways by the hepatitis B virus transactivator pX
G Natoli1, M L Avantaggiati, P Chirillo
1Istituto di I Clinica Medica, University of Rome, Italy.
Abstract:
The mechanisms by which pX, the transactivator of the hepatitis B virus (HBV), exerts its effects on transcription of viral and cellular genes and affects cell-growth regulation have not yet been fully defined. Previous reports suggested the possibility of a direct interaction of pX, which lacks intrinsic DNA-binding activity, with components of the cellular transcription machinery. More recent investigations support the hypothesis that pX might activate cellular kinases involved in transcriptional regulation and growth control. We characterized the mechanisms of AP-1 transcription factor activation by pX and, in particular, the role of cellular proteins involved in the intracellular signal transduction of growth-factor receptors. The observation that the overexpression of c-fos and c-jun in the cells results in a clear augmentation of the effects of pX on TRE-directed transcription and the induction of the DNA-binding activity of c-jun/c-fos heterodimers by AP1-depleted nuclear extracts from pX-expressing cells strongly supports the involvement of post-translational modifications. In both HeLa and undifferentiated F9 cells, pX was able to increase the activity of exogenous transfected c-jun but not of c-jun mutants bearing mutations in the serine residues located in the amino-terminal transcriptional activation domain. Moreover, by use of Ha-ras and Raf-1 dominant negative mutants, we show that both Ha-ras and Raf-1 are required for pX-induced activation of c-jun transcriptional activity.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Hepatitis B virus (HBV) pX protein activates transcription via the AP-1 pathway, involving c-jun and c-fos. This process requires post-translational modifications and the Ha-ras/Raf-1 signaling cascade.
Area of Science:
- Virology
- Molecular Biology
- Cellular Signaling
Background:
- The hepatitis B virus (HBV) pX protein's role in gene transcription and cell growth is not fully understood.
- pX lacks DNA-binding activity, suggesting indirect mechanisms for its transactivation effects.
Purpose of the Study:
- To elucidate the mechanisms of AP-1 transcription factor activation by HBV pX.
- To investigate the involvement of cellular signaling pathways, particularly growth-factor receptor signal transduction, in pX-mediated effects.
Main Methods:
- Overexpression of c-fos and c-jun to assess effects on TRE-directed transcription.
- Analysis of AP-1 DNA-binding activity in nuclear extracts from pX-expressing cells.
- Utilizing dominant-negative mutants of Ha-ras and Raf-1 to probe signaling pathways.
Main Results:
- Overexpression of c-fos and c-jun enhanced pX effects on TRE-directed transcription.
- pX induced DNA-binding activity of c-jun/c-fos heterodimers, indicating post-translational modifications.
- pX increased c-jun activity, dependent on serine residues in its activation domain, and required Ha-ras and Raf-1 signaling.
Conclusions:
- HBV pX activates transcription through the AP-1 pathway.
- Post-translational modifications and the Ha-ras/Raf-1 signaling cascade are crucial for pX-induced c-jun activation and transcriptional activity.