Plk1 in Chemo-resistance of Cancer

摘要：紫杉醇和阿霉素已被广泛用于治疗各种癌症。然而，化疗失败的一个主要问题是长时间暴露后对药物介导的细胞死亡逐渐产生耐药性。p21CIP1/WAF1最初被认为是导致细胞周期停滞的一个因素，它可以通过p53依赖性和非依赖性机制诱导。p21作为细胞增殖抑制剂，在药物诱导的肿瘤抑制中发挥重要作用。然而，现在人们认为p21在药物治疗后可以根据细胞环境发挥促凋亡和抗凋亡的功能。对于带有野生型p53的肿瘤，阿霉素治疗会激活p53/p21通路，导致细胞周期阻滞和凋亡。p53/p21通路失活是阿霉素诱导的细胞周期阻滞的后续恢复所必需的。相反，对紫杉醇介导的细胞死亡的抗性与p21水平升高有关，p21的敲除可恢复紫杉醇敏感性。因此，控制肿瘤细胞中的p21水平对于紫杉醇和阿霉素以及其他药物的疗效至关重要。由于p21是一种寿命短且高度非结构化的蛋白，因此对其降解率的调节对其胞内水平的调节有重要作用。p21与GTSE1/Hsp90/WISp39复合物在G2期和Sgt1/Hsp90复合物在G1期的结合保护其不被蛋白酶体降解。在正常细胞中，p21在有丝分裂时通过apccdc20依赖途径降解，在S期通过scfskp2介导途径降解。然而，p21在癌细胞的整个细胞周期中是稳定的。polo样激酶1 （Plk1）是许多细胞周期相关事件的关键调节因子，在几种类型的癌症中过度表达。由于我们已经确定GTSE1和Sgt1是两个Plk1底物，我们假设Plk1在癌症中的升高水平促进了p21的稳定性，从而促进了化疗耐药。我们的工作模型如下：Plk1磷酸化GTSE1阻止apccdc20介导的有丝分裂中p21的降解，Plk1磷酸化Sgt1抑制S期scfskp2相关的p21降解。这两种磷酸化事件都导致p21稳定，导致癌细胞抵抗紫杉醇介导的细胞死亡。对于携带WT p53的癌细胞，阿霉素治疗导致细胞周期阻滞。在随后的恢复过程中，Sgt1和GTSE1的Plk1磷酸化导致p53通路失活，从而允许细胞周期重新进入，从而有助于癌细胞抵抗阿霉素介导的凋亡。为了验证我们的假设，我们将首先验证Plk1在癌细胞对紫杉醇/阿霉素介导的细胞凋亡的抗性中起关键作用。在机制上，我们将确定Plk1磷酸化GTSE1和Sgt1如何调节培养细胞和异种移植肿瘤的化疗。如果成功，预测的结果将确定Plk1作为一个新的靶点，抑制它将防止化疗耐药，从而提高化疗的疗效。

DESCRIPTION (provided by applicant): Plk1 in chemoresistance of cancer Abstract: Paclitaxel and doxorubicin have been widely used to treat various cancers. However, one major problem for the failure of the chemotherapy is gradual development of resistance to drug-mediated cell death after prolonged exposure. Initially identified as a factor that leads to cell cycle arrest, p21CIP1/WAF1 can be induced by both p53-dependent and -independent mechanisms. As an inhibitor of cell proliferation, p21 plays an important role in drug-induced tumor suppression. However, it is now accepted that p21 can assume both pro- and anti- apoptotic functions after drug treatment depending on cellular context. For tumors with wild type p53, doxorubicin treatment causes activation of the p53/p21 pathway, resulting in cell cycle arrest and apoptosis. Inactivation of the p53/p21 pathway is required for subsequent recovery from doxorubicin-induced cell cycle arrest. In contrast, resistance to paclitaxel-mediated cell death has been correlated to elevated levels of p21 and knock-down of p21 restores paclitaxel sensitivity. Therefore, controlling p21 levels in tumor cells is critical for the efficacy of paclitaxel and doxorubicin, and possibly, other drugs. Because p21 is a short-lived and highly unstructured protein, modulation of its degradation rate significantly contributes to the regulation of its intracellular level. Association of p21 with the GTSE1/Hsp90/WISp39 complex in G2 phase and the Sgt1/Hsp90 complex in G1 phase protects it from degradation by the proteasome. In normal cells, p21 is degraded via the APCCdc20-dependent pathway in mitosis and the SCFSkp2-mediated pathway in S phase. However, p21 is stabilized in cancer cells throughout the cell cycle. Polo-like kinase 1 (Plk1), a critical regulator of many cell cycle-related events, is overexpressed in several types of cancers. Because we have identified GTSE1 and Sgt1 as two Plk1 substrates, we hypothesize that elevated levels of Plk1 in cancers promote p21 stability, thus contributing to chemoresistance. Our working model is as follows: Plk1 phosphorylation of GTSE1 prevents APCCdc20-mediated degradation of p21 in mitosis, and Plk1 phosphorylation of Sgt1 inhibits SCFSkp2-associated degradation of p21 in S phase. Both phosphorylation events lead to p21 stabilization, resulting in resistance of cancer cells to paclitaxel-mediated cell death. For cancer cells harboring WT p53, doxorubicin treatment results in cell cycle arrest. During the subsequent recovery, Plk1 phosphorylation of both Sgt1 and GTSE1 leads to inactivation of the p53 pathway to allow cell cycle re-entry, thus contributing to resistance of cancer cells to doxorubicin-mediated apoptosis. To test our hypothesis, we will first validate that Plk1 plays a critical role in resistance of cancer cells to paclitaxel/doxorubicin-mediated apoptosis. Mechanistically, we will determine how Plk1 phosphorylation of GTSE1 and Sgt1 regulates chemotherapy in both cultured cells and xenograft tumors. If successful, the predicted results will identify Plk1 as a novel target, inhibition of which will prevent chemoresistance, thus increasing the efficacy of chemotherapy.