Inhibition of polo like kinase 1 in sarcomas induces apoptosis that is dependent on Mcl-1 suppression

Sarcomas are rare cancers and the current treatments in inoperable or metastatic disease have not been shown to prolong survival. In order to develop novel targeted therapies, we tested the efficacy of polo-like kinase 1 (PLK-1) inhibitor (TAK-960) in sarcoma. All the sarcoma cell lines were sensitive to TAK-960 with IC50s in the low nanomolar range. We chose MPNST, CHP100 and LS141 for our studies and of which MPNST cells exclusively underwent polyploidy after a delay in mitosis for about 18 hours; CHP100 cells, after a 24h mitotic delay, died of apoptosis; LS141, after a delay in mitosis stayed at 4N with mild apoptosis. Apoptosis induced by TAK-960 in CHP100 was associated with down-regulation of Mcl-1 and the effect was recapitulated by down-regulating PLK1 by siRNA, confirming that the effect of TAK-960 on Mcl-1 expression is target specific. With suppression of Mcl-1 by siRNA, TAK-960 induced apoptosis in MPNST cells as well. These effects were confirmed <i>in vivo</i>, such that TAK-960 more effectively inhibited CHP100 than MPNST xenografts. In the setting of PLK-1 inhibition, Mcl-1 down regulation is shown to be an important determinant of apoptosis. Collectively, the net effect of this is to drive cells to apoptosis, resulting in a greater anti-tumor effect <i>in vivo</i>. Therefore, targeting PLK-1 should have a greater impact in treating sarcomas provided there is concomitant suppression of Mcl-1. These results further indicate that Mcl-1 could be an important biomarker to predict sensitivity to the induction of apoptosis by PLK-1 targeted therapy in sarcoma.

肉瘤是一类罕见恶性肿瘤，目前针对不可手术或转移性肉瘤的治疗方案尚未被证实可延长患者生存期。为开发新型靶向治疗手段，我们评估了polo样激酶1（polo-like kinase 1, PLK-1）抑制剂TAK-960在肉瘤中的抗肿瘤活性。所有肉瘤细胞系对TAK-960均表现出敏感性，其半数抑制浓度（IC50）处于低纳摩尔范围。我们选取MPNST、CHP100与LS141三种细胞系开展本研究：其中MPNST细胞在有丝分裂延迟约18小时后，仅发生多倍体化；CHP100细胞经24小时有丝分裂延迟后，通过凋亡途径死亡；LS141细胞则在有丝分裂延迟后维持4N倍体状态，仅伴随轻度凋亡。TAK-960在CHP100细胞中诱导的凋亡与Mcl-1的表达下调相关，而通过小干扰RNA（small interfering RNA, siRNA）下调PLK1可重现该效应，证实TAK-960对Mcl-1表达的调控作用具有靶点特异性。当通过siRNA抑制Mcl-1后，TAK-960同样可诱导MPNST细胞发生凋亡。上述效应在体内（in vivo）实验中得到验证：TAK-960对CHP100异种移植瘤的抑制效果优于MPNST异种移植瘤。研究表明，在PLK-1抑制的背景下，Mcl-1的下调是决定细胞凋亡发生的关键因素。综上，该通路的净效应是促使细胞发生凋亡，进而在体内产生更强的抗肿瘤效果。因此，若能同时抑制Mcl-1，靶向PLK-1的治疗方案在肉瘤治疗中可发挥更显著的作用。本研究进一步提示，Mcl-1可作为预测肉瘤患者对PLK-1靶向治疗诱导凋亡敏感性的重要生物标志物。