HEPATITIS-B VIRUS HBX PROTEIN ACTIVATES RAS-GTP COMPLEX-FORMATION ANDESTABLISHES A RAS, RAF, MAP KINASE SIGNALING CASCADE

Hepatitis B virus produces a small (154-amino acid) transcriptional tr ansactivating protein, HBx, which is required for viral infection and has been implicated in virus-mediated liver oncogenesis. However, the molecular mechanism for HBx activity and its possible influence on cel l proliferation have remained obscure. A number of studies suggest tha t HBx may stimulate transcription by indirectly activating transcripti on factors, possibly by influencing cell signaling pathways. We now pr esent biochemical evidence that HBx activates Ras and rapidly induces a cytoplasmic signaling cascade linking Ras, Raf, and mitogen-activate d protein kinase (MAP kinase), leading to transcriptional transactivat ion. HBx strongly elevates levels of GTP-bound Ras, activated and phos phorylated Raf, and tyrosine-phosphorylated and activated MAP kinase. Transactivation of transcription factor AP-1 by HBx is blocked by inhi bition of Ras or Raf activities but not by inhibition of Ca2+- and dia cylglycerol-dependent protein kinase C. HBx was also found to stimulat e DNA synthesis in serum-starved cells. The hepatitis B virus HBx prot ein therefore stimulates Ras-GTP complex formation and promotes downst ream signaling through Raf and MAP kinases, and may influence cell pro liferation.