Abstract 2026: CHK1 inhibition synergizes with cisplatin to promote mitotic cell death via activation of caspase 2 and downregulation of E2F1 in small cell lung cancer
Authors: Hsu Wei-Hsun , Zhao Xiaoliang , Kim In-Gyu , Rao Guanhua , McCutcheon Justine , Hsu Shuo-Tse , Dowlati Afshin , Zhang Yu-Wen , Giaccone Giuseppe 
Published: July 2017

Abstract
Platinum-based chemotherapy remains the standard treatment for patients with small cell lung cancer (SCLC), one of the deadliest cancers, but the benefit of such therapy is often hampered by rapid emergence of drug resistance. Thus far, there is no targeted therapy available for SCLC, mainly due to the lack of targetable oncogenic driver alterations; nevertheless, over 90% of SCLC tumors harbor inactivating mutations in tumor suppressor p53, an important checkpoint regulator for the control of cell cycle progression in the event of DNA damage. SCLC cells with p53-deficiency heavily rely on the checkpoint kinase 1 (CHK1), a key checkpoint regulator, to prevent the progression of cell cycle through G2/M in the presence of damaged DNA. Using CHK1 inhibitors and siRNA knockdown approaches, we explored the potential therapeutic benefit of CHK1 inhibition in SCLC models in combination with cisplatin, a DNA damaging agent. We demonstrated that CHK1 inhibition was able to release G2/M cell cycle arrest induced by cisplatin, and synergized with cisplatin to induce mitotic cell death especially in the p53-deficient SCLC cell lines. CHK1 inhibition activated caspase 2 and downregulated the expression of E2F1 transcription factor in these cells. We showed that activation of caspase 2 was required for the mitotic cell death induced by the combination of cisplatin and CHK1 inhibition. Interestingly, the upregulation of E2F1 induced by CHK1 inhibition was mediated in part by caspase 2, and E2F1 played a role in the DNA damage checkpoint control as knockdown of E2F1 enhanced cytotoxicity induced by cisplatin or CHK1 inhibitor in GLC4 cells. However, ectopic overexpression of E2F1 also led to increase of phospho-histone H3, γH2AX and cleaved PARP, and significantly enhanced cisplatin cytotoxicity, indicating that proper dosage of E2F1 is critical for DNA damage checkpoint response. These data provide a mechanistic insight of CHK1-mediated DNA damage checkpoint control and the anticancer effect of CHK1 inhibition against SCLC. In tumor samples derived from SCLC patients treated with platinum based chemotherapy, we found that high expression of CHK1 and E2F1 significantly correlated with poorer overall survival, further suggesting a link between CHK1 and E2F1 and potentially collaborative roles of these two factors in DNA damage checkpoint response. Moreover, we showed that CHK1 inhibition resensitized cisplatin-resistant cells to cisplatin treatment and induced mitotic cell death via caspase activation in these cells. These data indicate that CHK1 inhibition may enhance the benefit of cisplatin treatment for SCLC and overcome cisplatin resistance. We are presently confirming the in vitro results of CHK1 inhibition in combination with cisplatin treatment in xenograft tumor models in athymic nude mice.
Citation Format: Wei-Hsun Hsu, Xiaoliang Zhao, In-Gyu Kim, Guanhua Rao, Justine McCutcheon, Shuo-Tse Hsu, Afshin Dowlati, Yu-Wen Zhang, Giuseppe Giaccone. CHK1 inhibition synergizes with cisplatin to promote mitotic cell death via activation of caspase 2 and downregulation of E2F1 in small cell lung cancer [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 2026. doi:10.1158/1538-7445.AM2017-2026