Cell Cycle Regulation and Adult T Cell Leukemia

描述（由申请方提供）：HTLV-I是成人T细胞白血病/淋巴瘤（ATL）的病原体，ATL是一种CD 4 + T细胞恶性肿瘤，其病因与病毒反式激活因子/癌蛋白Tax相关。我们已经表明，税收可以激活后期促进复合物/细胞周期体（APC/C），E3泛素连接酶，控制中期到后期的过渡和有丝分裂退出。Tax激活APC/C导致包括细胞周期蛋白A、细胞周期蛋白B、securin和Skp 2在内的有丝分裂调节因子的过早多聚泛素化和降解。Skp 2是另一种称为SCFSkp 2的E3泛素连接酶的底物靶向亚基，其介导细胞周期蛋白E/A-CDK 2抑制剂（CKI）p27的破坏。Tax诱导的APC/C对Skp 2的降解导致SCFSkp 2失活和p27稳定。另一种CKI p21的mRNA水平也由于Tax的启动子激活和mRNA稳定而急剧增加。p21和p27的激增反过来诱导细胞衰老，称为Tax-induced rapid senescence（Tax-IRS）。正如预期的那样，被HTLV-1感染的HeLa和SupT 1 T细胞也在衰老中停滞。相比之下，缺乏p21和p27的细胞，如HOS，在HTLV-1感染或Tax表达后逃避Tax-IRS并继续分裂。然而，它们以多核化和DNA非整倍体的形式发展出显著的核异常。重要的是，p27的下调和p21的错误定位在Tax表达HTLV-1转化的T（HTxT）细胞中是常见的，并且最有可能通过PI 3 K-Akt途径的激活而发生。我们的最新研究结果表明，税收引起的DNA复制许可因子，Cdt 1，在S/G2的显着积累。在一个最令人兴奋的最近的发展，我们发现，抑制NF-？通过税务激活B完全废除了Tax-IRS。我们的研究结果提出了几个重要的问题：如何持续NF-？B激活导致p21/p27上调和衰老？NF-？B激活位于APC/C激活和Cdt 1积累的上游。Tax诱导的Cdt 1的积累是否会导致DNA再复制/超复制，它能否解释Tax表达细胞中观察到的显著核异常？最后，在SupT 1和HeLa细胞中由HTLV-1诱导的衰老样停滞表明，由HTLV-1有效感染的原代T细胞也可能经历衰老。如果是这样，那么一种诱导衰老的病毒是如何导致白血病的呢？HTxT细胞系如何适应Tax并继续分裂？为了解决这些问题，并阐明HTLV-1感染导致T细胞转化和ATL的途径，提出了三个具体目标：（1）描绘从持续NF-？Tax诱导的B活化至衰老;（2）研究HTLV和Tax诱导的染色体不稳定性的原因和生物学效应;（3）阐明细胞适应Tax和HTLV-1（被Tax和HTLV-1转化）的机制。

DESCRIPTION (provided by applicant): HTLV-I is the causative agent of adult T-cell leukemia/lymphoma (ATL), a malignancy of CD4+ T cells whose etiology is associated with the viral transactivator/oncoprotein, Tax. We have shown that Tax can activate the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that controls metaphase to anaphase transition and mitotic exit. APC/C activation by Tax leads to the premature poly-ubiquitination and degradation of mitotic regulators including cyclin A, cyclin B, securin, and Skp2. Skp2 is the substrate-targeting subunit of another E3 ubiquitin ligase known as SCFSkp2, which mediates the destruction of cyclin E/A-CDK2 inhibitor (CKI), p27. The degradation of Skp2 by APC/C as induced by Tax leads to SCFSkp2 inactivation and p27 stabilization. The mRNA level of another CKI, p21, also increases sharply as a result of promoter activation and mRNA stabilization by Tax. The great surge in p21 and p27 in turn induces cellular senescence termed Tax-induced rapid senescence (Tax-IRS). As expected, HeLa and SupT1 T cells infected by HTLV-1 arrest in senescence as well. By contrast, cells deficient in p21 and p27, such as HOS, escape Tax-IRS and continue to divide after HTLV-1 infection or Tax expression. They, however, develop dramatic nuclear abnormalities in the form of multinucleation and DNA aneuploidy. Importantly, down-regulation of p27 and mis-localization of p21 are common in Tax-expressing HTLV-1-transformed T (HTxT) cells and most likely occur through activation of the PI3K- Akt pathway. Our latest results showed that Tax caused significant accumulation of the DNA replication licensing factor, Cdt1, during S/G2. In a most exciting recent development, we found that inhibition of NF-?B activation by Tax completely abrogated Tax-IRS. Our findings raise several important questions: How does persistent NF-?B activation lead to p21/p27 upregulation and senescence? Does NF-?B activation lie upstream of APC/C activation and Cdt1 accumulation? Does the accumulation of Cdt1 induced by Tax lead to DNA re/hyper-replication and can it explain the dramatic nuclear abnormalities seen in Tax-expressing cells? Finally, the senescence-like arrest induced by HTLV-1 in SupT1 and HeLa cells suggests that primary T cells productively infected by HTLV-1 likely also undergo senescence. If so, then how does a virus that induces senescence cause leukemia? How do HTxT cell lines manage to adapt to Tax and continue to divide? To address these questions and to elucidate the pathway by which HTLV-1 infection leads to T cell transformation and ATL, three specific aims are proposed: (1) to delineate the pathway leading from persistent NF-?B activation by Tax to senescence; (2) to investigate the cause and biological effects of HTLV- and Tax-induced chromosome instability; and (3) to elucidate the mechanisms by which cells become adapted to (transformed by) Tax and HTLV-1.