INDUCTION OF THE DNA-BINDING ACTIVITY OF C-JUN C-FOS HETERODIMERS BY THE HEPATITIS-B VIRUS TRANSACTIVATOR PX

The hepatitis B virus (HBV) X protein (pX) is capable of activating tr anscription regulated by viral and cellular promoters containing bindi ng sites for different transcription factors, including AP1. In this s tudy we have analyzed the mechanisms of AP1 induction by pX. The hepat itis B virus transactivator was able to activate TRE (12-O-tetradecano ylphorbol-13-acetate response element)-directed transcription in diffe rent cell lines, including HepG2, HeLa, CV1, and PLC/PRF/5 cells. pX-i nduced AP1 activation in HepG2 cells was associated with an increase i n the DNA-binding activity of c-Jun/c-Fos heterodimers, which was not dependent either on an increase in the overall amount of c-Fos and c-J un proteins in the cells or on formation of dimers between pX and the two proteins, thus suggesting the involvement of posttranslational mod ifications of the transcription factor. The observation that the overe xpression of c-Jun and c-Fos in the cells results in a strong augmenta tion of the effect of pX on TRE-directed transcription is additional e vidence indicating the involvement of posttranscriptional modification s of c-Jun/c-Fos heterodimers. The increased AP1 binding observed in t he presence of pX was unaffected by the protein kinase C inhibitors ca lphostin C and sphingosine and by the protein kinase A inhibitor HA100 4, while it was almost completely blocked by staurosporine, a potent a nd nonspecific protein kinase inhibitor, suggesting that protein kinas e C- and A-independent phosphorylation events might play a role in the phenomenon. The ability of pX also to increase TRE-directed transcrip tion in cell lines in which AP1-binding activity is not increased (i.e ., HeLa, CV1, and PLC/PRF/5 cells) suggests that pX can activate canon ical TRE sites by different mechanisms as well.