Abstract 5373: Intact regulation of G1/S transition renders esophageal squamous cell carcinomas sensitive to PI3Kα inhibitors by blocking SKP2-mediated p21 degradation
Authors: Wang Yuxiang , Zhang Xu , Zhang Xi , Shen Yanyan , Ma Qingyang , Qiao Yuemei , Wang Yi , Yang Biyu , Xu Lan , Ge Gaoxiang , Hu Landian , Kong Xiangyin , Yang Chunhao , Ding Jian , Chen Yi , Meng Linghua 
Published: June 2022

Abstract
Esophageal cancer is the seventh most common cancer and the sixth leading cause of cancer-related mortality in the world, of which about 80% are esophageal squamous cell carcinomas (ESCC). Mutation or amplification of PIK3CA frequently occurs in ESCC patients, indicating PI3Kα is a promising target for ESCC treatment. CYH33 is a novel PI3Kα-selective inhibitor, which is currently in clinical trials for the treatment of solid tumors including advanced ESCC. PI3Kα inhibitors including CYH33 and the marketed alpelisib displayed highly variable activity against the proliferation of a panel of ESCC cells, reflecting the heterogeneity of ESCC. We sought to discover predictive biomarkers for the efficacy of PI3Kα inhibitors in ESCC by evaluating its activity to against the growth of a panel of 16 clinically relevant patient-derived xenografts (PDXs). We found that PDXs with CCND1 amplification were more sensitive to CYH33, which was consistent with the positive correlation between CYH33 activity and CyclinD1 expression in PDXs detected by tissue microarray. As Cyclin D1 plays a critical role in G1/S transition in cell cycle progression, proteins regulating G1 phase were profiled in ESCC cells. Elevated expression of Cyclin D1, p21 and Rb was found in CYH33-sensitive cells compared to those in resistant cells, indicating intact G1/S regulation in sensitive cells. Accordingly, CYH33 significantly arrested sensitive cells at G1 phase, while it had little effect in resistant cells. The cellular level of p21 as well as p21 bound to the CDK4/6-Cyclin D1 or CDK2-Cyclin E complex increased upon CYH33 treatment in sensitive cells, which was accompanied with decreased phosphorylation of Rb. Though CYH33 had little effect on the mRNA level of p21, its treatment prolonged the half-life of p21 protein in sensitive cells. CYH33 was found to impede the interaction between the E3 ubiquitin ligase SKP2 and p21 in sensitive cells, resulting in decreased ubiquitination of p21. However, CYH33 failed to stabilize and accumulate p21 in resistant cells. In conclusion, we found that ESCC cells with intact G1 phase regulation were more sensitive to CYH33. CYH33 arrested sensitive ESCC cells at G1 phase via inhibiting the ubiquitination and degradation of p21 by SKP2, which further resulted in decreased activity of CDK4/6 and CDK2 as well as Rb phosphorylation. These findings revealed the insight mechanism of the differential activity CYH33 against ESCC and provided potential candidate biomarkers for the treatment of ESCC.
Citation Format: Yuxiang Wang, Xu Zhang, Xi Zhang, Yanyan Shen, Qingyang Ma, Yuemei Qiao, Yi Wang, Biyu Yang, Lan Xu, Gaoxiang Ge, Landian Hu, Xiangyin Kong, Chunhao Yang, Jian Ding, Yi Chen, Linghua Meng. Intact regulation of G1/S transition renders esophageal squamous cell carcinomas sensitive to PI3Kα inhibitors by blocking SKP2-mediated p21 degradation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5373.