Abstract 5862: YAP1 mediated CDK6 activation confers radiation resistance in esophageal cancer: rationale for the combination of YAP1 and CDK4/6 inhibitors in EC
Authors: Li Fan , Dong Xiaoquan , Zhu Haitao , Scott Ailing W. , Ma Lang , Jin Jiankang , Estrella Jeannelyn Santiano , Skinner Heath , Johnson Randy L. , Song Shumei , Ajani Jaffer A. 
Published: July 2017

Abstract
Background: Esophageal carcinoma (EC) is a lethal disease with high incidence globally and often exist therapy resistance. Alterations (either overexpression or amplification) of YAP1 and CDK4/6 were found frequently in esophageal cancer. Deregulation of these pathways may represent key elements for resistance in esophageal cancer.
Methods: Expression of YAP1 and CDK4/6 were examined in esophageal tumor tissues as well as cell lines using immunohistochemistry and immunoblotting. Inducible YAP1 overexpression in EC cells by lentivirus system was performed to test YAP1 mediated CDK4/6 expression and activation and association with radiation resistance. YAP inhibitor CA3 and CDK4/6 inhibitor Lee001 were used to test their antitumor activities in vitro and in vivo. Cell proliferation assay (MTS), Flow cytometry and immunofluorecence and tumor sphere formation assay were performed to test cell cycle distribution, cancer stem cell (CSC) population maintenance and over growth and therapy resistance.
Results: We demonstrate that overexpressed YAP1 is positively associated with CDK4/6 expression in resistant tumor tissues and EC cell lines. Overexpression YAP1 by inducible lentivirus system in EC cells up-regulates expression of CDK4/6 at level of its transcription, while knock down YAP1 in JHESO EC cells dramatically decreased CDK6 expression as well as its transcription. CA3, a novel YAP1 inhibitor, was able to decrease activation of CDK4/6 and phosphorylation RB and cell cycle progression induced by YAP1. Interestingly, we found that inducible YAP1 high EC cells are resistant to radiation resistance and CDK6 knockout by Lenti-Crisp sensitize radiation resistant in EC cells induced by YAP1. Further, we demonstrated radiation resistant cells are enriched CSC population and demonstrated increased level of both YAP1 and CDK4/6, while blocking the YAP and CDK4/6 pathway by using their inhibitor CA3 or Lee001 reduced EC cell growth and CSCs properties especially in EC resistant cells. Most importantly, we found combination of Lee001 and CA3 exert a synergistic anticancer effect in radiation resistant EC cells with high YAP1 and CDK4/6 in Vitro and in Vivo.
Conclusions: Our data provide evidence that YAP1 mediated CDK4/6 up-regulation plays an important role in conferring radiation resistance in esophageal cancer cells. Targeting both YAP1 and CDK4/6 may provide novel therapeutic strategies in EC.
Corresponding Authors: Shumei Song, Tel:713-834-6144; Email: ssong@mdanderson.org; or Jaffer A. Ajani, Tel: 713-792-3685; Email: jajani@mdanderson.org
Citation Format: Fan Li, Xiaoquan Dong, Haitao Zhu, Ailing W. Scott, Lang Ma, Jiankang Jin, Jeannelyn Santiano Estrella, Heath Skinner, Randy L. Johnson, Shumei Song, Jaffer A. Ajani. YAP1 mediated CDK6 activation confers radiation resistance in esophageal cancer: rationale for the combination of YAP1 and CDK4/6 inhibitors in EC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5862. doi:10.1158/1538-7445.AM2017-5862