MODULATION OF INTRACELLULAR SIGNAL-TRANSDUCTION PATHWAYS BY THE HEPATITIS-B VIRUS TRANSACTIVATOR PX

The mechanisms by which pX, the transactivator of the hepatitis B viru s (HBV), exerts its effects on transcription of viral and cellular gen es 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 th e cellular transcription machinery, More recent investigations support the hypothesis that pX might activate cellular kinases involved in tr anscriptional regulation and growth control. We characterized the mech anisms of AP-I transcription factor activation by pX and, in particula r, the role of cellular proteins involved in the intracellular signal transduction of growth-factor receptors. The observation that the over expression of c-fos and c-jun in the cells results in a clear augmenta tion of the effects of pX on TRE-directed transcription and the induct ion of the DNA-binding activity of c-jun/c-fos heterodimers by API-dep leted nuclear extracts from pX-expressing cells strongly supports the involvement of post-translational modifications. In both HeLa and undi fferentiated F9 cells, pX was able to increase the activity of exogeno us transfected c-jun but not of c-jun mutants bearing mutations in the serine residues located in the amino-terminal transcriptional activat ion domain. Moreover, by use of Ha-l as and Raf-l dominant negative mu tants, we show that both Ha-ras and Raf-l are required for pX-induced activation of c-jun transcriptional activity. In addition, we show tha t pX is able to cooperate with Raf-l, one of the components of the mit ogen-activated signaling pathway of the cells, and with the reactive o xygen intermediate H2O2 in c-jun activation. Our results are consisten t with the hypothesis that at least one site of action of pX is periph eral and is located upstream of the ras gene products. (C) Journal of Hepatology.