Hepatitis B virus pX activates NF-kappa B-dependent transcription through a Raf-independent pathway.
In this study, we characterized the molecular events involved in the activation of the ubiquitous transcription factor NF-kappa B by the viral transactivator pX. pX expression in HeLa cells determines a manyfold increase in NF-kappa B-dependent transcription, which is associated with an increase in p50/p65 heterodimer DNA-binding activity. Since the I kappa B-alpha inhibitory subunit proteolytic degradation, which follows its phosphorylation/modification, is a key event in NF-kappa B activation by different stimuli (such as growth factors, phorbol esters, tumor necrosis factor, UV irradiation, and oxygen radicals), we investigated pX effects on I kappa B-alpha, as well as the possible involvement of known signalling pathways in pX-induced NF-kappa B-dependent transcription. We observed that although pX had no direct effect on p50 or p65, it was able to restore the I kappa B-alpha-suppressed p50/p65 activity. More directly, the stable expression of pX in HeLa cells resulted in reduced levels of I kappa B-alpha in the cytoplasm. Pretreatment of the cells with H7, calphostin C, tyrphostin 25, or N-acetylcysteine did not impair the effects of pX on NF-kappa B, thus ruling out the involvement of protein kinase C, tyrosine kinases, and oxygen radicals. Finally, while most of the known NF-kappa B-activating agents converge on Raf-1 protein kinase, when Raf-1 activity is blocked by overexpression of a dominant negative mutant, the effects of pX on NF-kappa B are not impaired. Thus, we suggest that although pX is able to activate the Ras/Raf-1-signalling pathway, it triggers NF-kappa B activation by an as yet unidentified Raf-1-independent pathway.
Fondazione Andrea Cesalpino, Università degli Studi di Roma La Sapienza, Italy.
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D001483:Base Sequence; D004268:DNA-Binding Proteins / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D020672:I-kappa B Proteins; D008969:Molecular Sequence Data; D000072000:NF-KappaB Inhibitor alpha; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism*; D052002:NF-kappa B p50 Subunit; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf; D015398:Signal Transduction; D015534:Trans-Activators / Q000378:metabolism*; D051996:Transcription Factor RelA; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured; D054334:Viral Regulatory and Accessory Proteins
D004268:DNA-Binding Proteins; D020672:I-kappa B Proteins; D016328:NF-kappa B; D052002:NF-kappa B p50 Subunit; C000606204:NFKBIA protein, human; D011518:Proto-Oncogene Proteins; D015534:Trans-Activators; D051996:Transcription Factor RelA; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein; D000072000:NF-KappaB Inhibitor alpha; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf