Cross J C, Wen P, Rutter W J
Hormone Research Institute, University of California-San Francisco 94143-0534.
Proc Natl Acad Sci U S A. 1993 Sep 1;90(17):8078-82. doi: 10.1073/pnas.90.17.8078.
The X protein of hepatitis B virus (HBV-X) can act as a transactivator of transcription but its mechanism of action remains obscure. We have analyzed HBV-X transactivation in several cell types using 13 unrelated viral and cellular promoters and found that transactivation is more or less apparent in most cell types and is promiscuous and unrelated to specific sequence motifs within the target promoters. In general, though, HBV-X appears to act on enhancer elements since HBV-X had no effect on a minimal promoter, whereas HBV-X was able to transactivate after insertion of an AP-1 minienhancer. Several lines of evidence exclude the possibility that HBV-X interacts directly with the AP-1 enhancer or its binding proteins and suggest that the proximal target of HBV-X is peripheral to the transcription complex. This hypothesis is supported by the observation that inhibition of serine/threonine kinases, which regulate AP-1 activity (phorbol ester down-regulation or staurosporine inhibition of protein kinase C and a dominant negative mutant of Raf-1), blocked the ability of HBV-X to transactivate without affecting basal promoter activity. Furthermore, basal transcription from the AP-1-dependent promoter was increased by overexpression of protein kinase C and Raf-1 but HBV-X was unable to further stimulate, indicating that these kinases act subsequently to HBV-X. These data suggest that transactivation by HBV-X is an indirect result of the activation of cellular serine/threonine kinases including protein kinase C and Raf-1. This mode of action implies that HBV-X may affect other cellular processes, besides transcription, that are regulated by these kinases.
乙型肝炎病毒（HBV）的X蛋白（HBV-X）可作为转录反式激活因子，但其作用机制仍不清楚。我们使用13种不相关的病毒和细胞启动子，在几种细胞类型中分析了HBV-X的反式激活作用，发现反式激活在大多数细胞类型中或多或少都很明显，且具有混杂性，与靶启动子内的特定序列基序无关。不过一般而言，HBV-X似乎作用于增强子元件，因为HBV-X对最小启动子无影响，而在插入AP-1微型增强子后，HBV-X能够进行反式激活。几条证据排除了HBV-X直接与AP-1增强子或其结合蛋白相互作用的可能性，并表明HBV-X的近端靶标位于转录复合物的外周。这一假说得到以下观察结果的支持：抑制调节AP-1活性的丝氨酸/苏氨酸激酶（佛波酯下调或星形孢菌素抑制蛋白激酶C以及Raf-1的显性负突变体）可阻断HBV-X的反式激活能力，而不影响基础启动子活性。此外，蛋白激酶C和Raf-1的过表达增加了AP-1依赖性启动子的基础转录，但HBV-X无法进一步刺激，这表明这些激酶在HBV-X之后起作用。这些数据表明，HBV-X的反式激活是细胞丝氨酸/苏氨酸激酶（包括蛋白激酶C和Raf-1）激活的间接结果。这种作用模式意味着，除了转录之外，HBV-X可能还会影响受这些激酶调节的其他细胞过程。
