Mechanism of viral hepatitis-mediated liver carcinogenes

细胞内稳态是由许多细胞蛋白的活动严格调节的。这些蛋白的亚细胞定位和稳定性对其活性至关重要。例如，通过识别核定位信号（NLS）和/或核输出信号（NES），特定受体将蛋白质运入或运出细胞核。最近发现的一个NES是一个短的、疏水的、富含亮氨酸的基序，它是介导大型载体蛋白和mrna的核输出的必要和充分的基序。许多细胞转录因子、癌蛋白、细胞周期调节因子和肿瘤抑制蛋白的活性已被报道由它们的NES调节。有趣的是，许多病毒蛋白也利用Crm1/ ran介导的途径，尽管其中一些病毒蛋白被认为足够小，可以被动地通过核孔复合物（NPC）扩散。这些致癌病毒蛋白获得了NES活性并修饰了核输出，这一事实表明核输出可能是病毒介导的肿瘤发生的有效靶点。

Cellular homeostasis is regulated tightly by the activities of many cellular proteins. The subcellular localization and stability of these proteins are critical for their activities. Examples include proteins that are transported into or out of the nucleus by specific receptors through the recognition of nuclear-localization signals (NLS) and/or nuclear export signals (NES). One recently identified NES is a short, hydrophobic, leucine-rich motif that is necessary and sufficient to mediate nuclear export of large carrier proteins and mRNAs. The activity of many cellular transcription factors, oncoproteins, cell cycle regulators and tumor suppressor proteins has been reported to be regualted by their NES. Interestingly, many viral proteins also utilize the Crm1/Ran-mediated pathway, even though some of these viral proteins are thought to be small enough to passively diffuse through the nuclear pore complex (NPC). The fact that these oncogenic viral proteins have acquired NES activity and modified nuclear export implies that nuclear export may be an efficiet target for viral-mediated oncogenesis. In this study, we are examining the hypothesis that both the X protein (HBx) of hepatitis B virus (HBV) and core protein (HC-core) of hepatitis C virus (HCV), two major risk factors for hepatocellular carcinoma, may induce neoplastic transformation by disregulating the Crm1/Ran-mediated pathways. Recently, we discovered that the HBx and HC-core proteins contain functional NESs. Unlike other cellular NES-containing proteins, HBx binds to and sequesters Crm1 in the cytoplasm, thereby modulating Crm1-mediated nuclear export of other cellular proteins including the NFkB/IkBa complex. Similarly, HC-core-mediated activation of the NFkB/IkBa complex also depends on the presence of NESs. These findings suggest that multiple cellular functions associated with HBx or HC-core may be due, in part, to their influence on the Crm1/Ran-mediated pathway. Because Crm1 and its cofactor Ran GTPase also play a key role in mitosis initiation, the inactivation of Crm1 by HBx or HC-core also may induce genomic instability. Our initial results indicate that HBx induces centrosome amplification and aberrant mitosis. These results led us to generate a novel testable hypothesis that Crm1 may be a common target for viral hepatitis-mediated oncogenicity and provide a foundation for a possible involvement of Crm1 in human carcinogenesis.