Benn J, Schneider R J
Department of Biochemistry, New York University School of Medicine, NY 10016.
Proc Natl Acad Sci U S A. 1994 Oct 25;91(22):10350-4. doi: 10.1073/pnas.91.22.10350.
Hepatitis B virus produces a small (154-amino acid) transcriptional transactivating 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 cell proliferation have remained obscure. A number of studies suggest that HBx may stimulate transcription by indirectly activating transcription factors, possibly by influencing cell signaling pathways. We now present biochemical evidence that HBx activates Ras and rapidly induces a cytoplasmic signaling cascade linking Ras, Raf, and mitogen-activated protein kinase (MAP kinase), leading to transcriptional transactivation. HBx strongly elevates levels of GTP-bound Ras, activated and phosphorylated Raf, and tyrosine-phosphorylated and activated MAP kinase. Transactivation of transcription factor AP-1 by HBx is blocked by inhibition of Ras or Raf activities but not by inhibition of Ca(2+)- and diacylglycerol-dependent protein kinase C. HBx was also found to stimulate DNA synthesis in serum-starved cells. The hepatitis B virus HBx protein therefore stimulates Ras-GTP complex formation and promotes downstream signaling through Raf and MAP kinases, and may influence cell proliferation.
乙型肝炎病毒产生一种小的（154个氨基酸）转录反式激活蛋白HBx，它是病毒感染所必需的，并且与病毒介导的肝脏肿瘤发生有关。然而，HBx活性的分子机制及其对细胞增殖的可能影响仍不清楚。许多研究表明，HBx可能通过间接激活转录因子来刺激转录，可能是通过影响细胞信号通路。我们现在提供生化证据表明，HBx激活Ras并迅速诱导一个连接Ras、Raf和丝裂原活化蛋白激酶（MAP激酶）的细胞质信号级联反应，导致转录反式激活。HBx强烈提高GTP结合型Ras、活化和磷酸化的Raf以及酪氨酸磷酸化和活化的MAP激酶的水平。HBx对转录因子AP-1的反式激活被Ras或Raf活性的抑制所阻断，但不被钙和二酰基甘油依赖性蛋白激酶C的抑制所阻断。还发现HBx能刺激血清饥饿细胞中的DNA合成。因此，乙型肝炎病毒HBx蛋白刺激Ras-GTP复合物的形成，并通过Raf和MAP激酶促进下游信号传导，可能影响细胞增殖。
