YAP1-Mediated CDK6 Activation Confers Radiation Resistance in Esophageal Cancer – Rationale for the Combination of YAP1 and CDK4/6 Inhibitors in Esophageal Cancer

Abstract

Purpose: Esophageal cancer is a lethal disease that is often resistant to therapy. Alterations of YAP1 and CDK6 are frequent in esophageal cancer. Deregulation of both molecules may be responsible for therapy resistance.Experimental Design: Expressions of YAP1 and CDK6 were examined in esophageal cancer cells and tissues using immunoblotting and immunohistochemistry. YAP1 expression was induced in esophageal cancer cells to examine YAP1-mediated CDK6 activation and its association with radiation resistance. Pharmacologic and genetic inhibitions of YAP1 and CDK6 were performed to dissect the mechanisms and assess the antitumor effects in vitro and in vivo.Results: YAP1 expression was positively associated with CDK6 expression in resistant esophageal cancer tissues and cell lines. YAP1 overexpression upregulated CDK6 expression and transcription, and promoted radiation resistance, whereas treatment with the YAP1 inhibitor, CA3, strongly suppressed YAP1 and CDK6 overexpression, reduced Rb phosphorylation, as well as sensitized radiation-resistant/ YAP1 high esophageal cancer cells to irradiation. CDK4/6 inhibitor, LEE011, and knock down of CDK6 dramatically inhibited expression of YAP1 and sensitized resistant esophageal cancer cells to irradiation indicating a positive feed-forward regulation of YAP1 by CDK6. In addition, suppression of both the YAP1 and CDK6 pathways by the combination of CA3 and LEE011 significantly reduced esophageal cancer cell growth and cancer stem cell population (ALDH1 þ and CD133 þ ), sensitized cells to irradiation, and showed a strong antitumor effect in vivo against radiation-resistant esophageal cancer cells.Conclusions: Our results document that a positive crosstalk between the YAP1 and CDK6 pathways plays an important role in conferring radiation resistance to esophageal cancer cells. Targeting both YAP1 and CDK6 pathways could be a novel therapeutic strategy to overcome resistance in esophageal cancer.

YAP1 amplification as a prognostic factor of definitive chemoradiotherapy in nonsurgical esophageal squamous cell carcinoma

“…YAP1 overexpression has been identified in non‐small cell lung cancer, urothelial carcinoma of the bladder, and pancreatic cancer, and is consistently correlated with unfavorable clinical outcomes . YAP1 amplification was also reported in 4%‐6% of ESCC patients, and was suggested by independent in vitro studies to mediate chemo‐ and radioresistance by downregulating PTEN, and upregulating EGFR and CDK6 expression . While local relapse is a major challenge for patients receiving dCRT, it is important to identify high‐risk patient groups for close monitoring.…”

“…[44][45][46] YAP1 amplification was also reported in 4%-6% of ESCC patients, 23,47 and was suggested by independent in vitro studies to mediate chemo-and radioresistance by downregulating PTEN, 48 and upregulating EGFR and CDK6 expression. 49,50 While local relapse is a major challenge for patients receiving dCRT, it is important to identify high-risk patient groups for close monitoring. Our study identified that a subgroup of patients with YAP1 CNV amplification tended to have earlier local relapse, and therefore, may require more intensive clinical interventions.…”

CDK4/6 inhibitors: a novel strategy for tumor radiosensitization

“…Moreover, the combination not only decreases colony numbers but also reduces the tumor sphere formation ability [45]. The tumor sphere formation ability is associated with cancer stem cell properties, which includes increased radiation resistance [50,51]. Huang et al [45] reported that the combination of 6 Gy radiation and 5 μM palbociclib dramatically reduced tumor sphere formation in Huh7 (hepatocellular carcinoma, HCC) cells compared to ionizing radiation (IR) alone.…”

“…Several in vivo studies demonstrated that palbociclib in combination with IR increased the median survival time by 1.2-to 3.3-fold in mice with brain malignancy xenografts [42,43,52]. The combined therapy also dramatically reduced the tumor weight and volume without no obvious systemic toxicity as assessed by mouse body weight [30,45,50]. The Ki-67 staining in xenografts was drastically reduced, revealing the reduction of tumor proliferation [30,44,50].…”

Inhibiting CBX4 efficiently suppress YAP nucleus translocation in hepatocellular carcinoma cells to protect against sorafenib resistance

“…The mounting evidence suggests that CSCs are particularly resistant to chemotherapy [44], and cells with sorafenib resistance maintain their CSC properties [27]. Our previous findings indicated that YAP1 plays a critical role in CSC self-renewal and tumor formation and that suppressing YAP1 could be an effective way to prevent the maintenance of CSCs [21]. In this study, we propose that targeting the CBX4-YAP1 axis could viable in treating CSCs and might be a novel strategy for SR HCC.…”