HTLV-I Tax activates the anaphase promoting complex

描述（由申请人提供）：HTLV-I是成人T细胞白血病/淋巴瘤（ATL）的病原体。已知HTLV-1癌蛋白Tax可诱导DNA损伤、微核形成、多核化和DNA非整倍体。Tax诱导的基因组不稳定性被认为在ATLL的发展中起重要作用。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导致细胞周期调节因子（包括细胞周期蛋白A、细胞周期蛋白B、securin和Skp 2）的过早多聚泛素化和降解。Skp 2是另一种称为SCFSkp 2的E3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Tax诱导的APC/C激活引起的mRNA稳定而急剧增加。p21和p27的大量激增反过来诱导快速衰老（Tax-IRS）。重要的是，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，逃避Tax-IRS，并在HTLV-1感染或Tax表达后继续分裂。然而，它们发生了戏剧性的多核化和双链DNA断裂，最有可能是组成性APC/C激活的结果。正如所预期的，在大量表达Tax的HTLV-1转化的T（HTxT）细胞中，p27下调，p21错误定位。p21和p27缺陷显然允许HTxT细胞克服Tax诱导的衰老。与APC/C的过早激活对细胞有害的想法一致，我们最近发现，除了p21和p27缺陷之外，原型HTxT细胞系MT4表达低得多的APC/C的底物靶向亚基Cdh 1水平。 来自多个实验室的最新数据表明，通过耗尽APC/C的S/G2抑制剂Cdt 1而过早激活APC/C，破坏Cdt 1-双生蛋白失衡并促进DNA的重新复制。异常的DNA复制（再复制/超复制）反过来导致多核化、双链DNA断裂和DNA损伤检查点激活。Skp 1缺失还导致Skp 2被APC/C过早降解、p27稳定化和衰老。值得注意的是，Tc 1敲低的细胞周期表型与Tax相似。这些新发现验证了我们的观察结果，并提出了许多更有趣的问题，这些问题将在以下三个具体目标中得到解决：目标1，阐明Tax激活APC/C的机制;目标2，研究Tax介导的APC/C激活的生物学后果;目标3，确定转化的T细胞如何适应Tax表达。

DESCRIPTION (provided by applicant): HTLV-I is the causative agent of adult T-cell leukemia/lymphoma (ATL). HTLV-1 oncoprotein Tax is known to induce DNA damage, micronuclei formation, multinucleation, and DNA aneupoidy. The genomic instability induced by Tax is thought to play an important role in ATLL development. We have shown that the 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 premature poly- ubiquitination and degradation of cell cycle regulators including cyclin A, cyclin B, securin, and Skp2. Skp2 is the substrate-targeting subunit of another E3 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 by Tax and mRNA stabilization due to Tax-induced APC/C activation. The massive surge in p21 and p27 in turn induces rapid senescence (Tax-IRS). Importantly, 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, escaped Tax- IRS and continue to divide after HTLV-1 infection or Tax expression. They however developed dramatic multinucleation and double-stranded DNA breaks, most likely as a result of constitutive APC/C activation. As might be expected, p27 is down-regulated and p21, mis-localized, in HTLV-1-transformed T (HTxT) cells which express Tax abundantly. The p21 and p27 deficiencies apparently allow HTxT cells to overcome Tax-induced senescence. Consistent with the idea that precocious activation of APC/C is deleterious to the cell, we recently found that the prototypic HTxT cell line, MT4, in addition to p21 and p27 deficiencies, expressed a much lower level of Cdh1, the substrate-targeting subunit of APC/C. Recent data from multiple labs indicate that premature APC/C activation through the depletion of Emi1, the S/G2 inhibitor of APC/C, disrupts Cdt1-geminin imbalance and promotes re-replication of DNA. The aberrant DNA replication (re-/hyper-replication) in turn causes multinucleation, double-stranded DNA breaks and DNA damage checkpoint activation. Emi1 depletion also results in premature degradation of Skp2 by APC/C, p27 stabilization, and senescence. Remarkably, the cell cycle phenotypes of Emi1 knockdown resemble those of Tax. These new findings validate our observations and raise many more interesting questions that will be addressed in the following three specific aims: aim 1, to elucidate the mechanism by which Tax activates APC/C; aim 2, to investigate the biological consequences of Tax-mediated APC/C activation; and aim 3, to determine how transformed T cells adapt to Tax expression.