CDK4/6 inhibition presents as a therapeutic option for pediatric and adult germ cell tumors and induces cell cycle arrest and apoptosis via canonical and non-canonical mechanisms

<b>Background: </b>Germ cell tumors (GCTs) are the most common solid malignancies in young men. Although high cure rates at early stages can be achieved, metastases, resistance to cisplatin-based therapy, and late toxicities still represent a lethal threat, arguing for the need of new therapeutic options. In this study, we analyzed the potential of CDK4/6 inhibitors palbociclib and ribociclib (PaRi) as molecular drugs to treat cisplatin-resistant and -sensitive pediatric and adult GCTs.<b>Results:</b> We demonstrate that adult GCTs<i> </i>highly express <i>CDK4</i>, while pediatric GCTs strongly express <i>CDK6</i>instead. Thus, both GCT types are potentially treatable by PaRi. GCTs presented as highly sensitive towards PaRi, which caused a decrease in viability, cell cycle arrest and apoptosis. Although GCTs mainly arrested in G1/G0 phase, some embryonal carcinoma (EC) cell lines were able to bypass the G1/S checkpoint and progressed to G2/M phase. We found that upregulation of <i>CDK3</i> and downregulation of many mitosis-regulation factors, like the <i>HAUS</i> genes, might be responsible for bypassing the G1/S checkpoint and termination of mitosis, respectively. We postulate that GCT cells do not tolerate these alterations in the cell cycle and eventually induce apoptosis. <b>Conclusion:</b> Our study highlights PaRi as therapeutic options for cisplatin-resistant and -sensitive pediatric and adult GCTs.

**背景：** 生殖细胞肿瘤（Germ cell tumors, GCTs）是青年男性最常见的实体恶性肿瘤。尽管早期阶段可获得较高的治愈率，但转移、顺铂为基础的治疗耐药以及治疗相关晚期毒性仍会构成致命威胁，因此亟需开发新的治疗方案。本研究分析了细胞周期蛋白依赖性激酶4/6（CDK4/6）抑制剂帕博西尼（palbociclib）与瑞博西尼（ribociclib，合称PaRi）作为分子药物，治疗顺铂耐药及顺铂敏感的儿童与成人生殖细胞肿瘤的潜力。
**结果：** 本研究证实，成人生殖细胞肿瘤高表达CDK4，而儿童生殖细胞肿瘤则显著高表达CDK6，因此两类生殖细胞肿瘤均存在PaRi治疗的潜在可能。生殖细胞肿瘤对PaRi呈现高度敏感性，PaRi可降低细胞活力、引发细胞周期阻滞并诱导细胞凋亡。尽管生殖细胞肿瘤大多阻滞于G1/G0期，但部分胚胎癌（Embryonal carcinoma, EC）细胞系可绕过G1/S检验点，进展至G2/M期。本研究发现，CDK3的上调以及诸多有丝分裂调控因子（如HAUS基因）的下调，可能分别与G1/S检验点的绕过和有丝分裂终止相关。本研究推测，生殖细胞肿瘤细胞无法耐受此类细胞周期异常，最终将诱发细胞凋亡。
**结论：** 本研究证实，PaRi可作为顺铂耐药及顺铂敏感的儿童与成人生殖细胞肿瘤的治疗选择。