ABSTRACT

Palbociclib, a selective CDK4/6 kinase inhibitor, is approved in combination with endocrine therapies for the treatment of advanced estrogen receptor positive (ER+) breast cancer. In pre-clinical cancer models, CDK4/6 inhibitors act primarily as cytostatic agents. In two commonly studied ER+ breast cancer cell lines (MCF7 and T47D), CDK4/6 inhibition drives G1-phase arrest and the acquisition of a senescent-like phenotype, both of which are reversible upon palbociclib withdrawal (incomplete senescence). Here we identify an ER+ breast cancer cell line, CAMA1, in which palbociclib treatment induces irreversible cell cycle arrest and senescence (complete senescence). In stark contrast to T47D and MCF7 cells, mTORC1 activity is not stably suppressed in CAMA1 cells during palbociclib treatment. Importantly, inhibition of mTORC1 signaling either by the mTORC1 inhibitor rapamycin or by knockdown of Raptor, a unique component of mTORC1, during palbociclib treatment of CAMA1 cells blocks the induction of complete senescence. These results indicate that sustained mTORC1 activity promotes complete senescence in ER+ breast cancer cells during CDK4/6 inhibitor-induced cell cycle arrest. Consistent with this mechanism, genetic depletion of TSC2, a negative regulator of mTORC1, in MCF7 cells resulted in sustained mTORC1 activity during palbociclib treatment and evoked a complete senescence response. These findings demonstrate that persistent mTORC1 signaling during palbociclib-induced G1 arrest is a potential liability for ER+ breast cancer cells, and suggest a strategy for novel drug combinations with palbociclib.

摘要

Palbociclib 是一种选择性 CDK4/6 激酶抑制剂，被批准与内分泌疗法联合用于治疗晚期雌激素受体阳性 (ER+) 乳腺癌。在临床前癌症模型中，CDK4/6 抑制剂主要作为细胞抑制剂。在两种普遍研究的 ER+ 乳腺癌细胞系（MCF7 和 T47D）中，CDK4/6 抑制驱动 G1 期停滞和衰老样表型的获得，这两者在 palbociclib 戒断（不完全衰老）后都是可逆的。在这里，我们确定了一种 ER+ 乳腺癌细胞系 CAMA1，其中 palbociclib 治疗诱导不可逆的细胞周期停滞和衰老（完全衰老）。与 T47D 和 MCF7 细胞形成鲜明对比的是，在 palbociclib 治疗期间，CAMA1 细胞中的 mTORC1 活性并未得到稳定抑制。重要的，在 palbociclib 治疗 CAMA1 细胞期间，通过 mTORC1 抑制剂雷帕霉素或通过敲除 Raptor（mTORC1 的独特成分）抑制 mTORC1 信号会阻止完全衰老的诱导。这些结果表明，在 CDK4/6 抑制剂诱导的细胞周期停滞期间，持续的 mTORC1 活性促进了 ER+ 乳腺癌细胞的完全衰老。与此机制一致，MCF7 细胞中 TSC2（mTORC1 的负调节因子）的遗传耗竭导致在 palbociclib 治疗期间持续的 mTORC1 活性并引起完全的衰老反应。这些发现表明，palbociclib 诱导的 G1 期阻滞期间持续的 mTORC1 信号传导是 ER+ 乳腺癌细胞的潜在负担，并提出了一种与 palbociclib 组合的新药物策略。