Abstract 324: SIK2 inhibitors regulate DNA repair pathway and sensitize ovarian cancer to PARP1 inhibitors
Authors: Lu Zhen , Mao Wequn , Pang Lan , Santiago-O'Farrill Janice M. , Yang Haling , Ahmed Ahmed , Vankayalapati Hariprasad , Bast Robert C. 
Published: August 2018

Abstract
Genomic instability is a recognized hallmark of cancer. Germline mutations in critical DNA-repair and DNA-damage response genes, including BRCA1and BRCA2, predispose to cancer development, but also create vulnerabilities that can be exploited for cancer therapy. The successful development of PARP inhibitors for cancers with BRCA1/2 mutations and a deficiency in homologous recombination DNA repair has provided proof of concept. Inhibition of kinases that promote transcription of DNA repair genes might provide targets that enhance the activity of PARP-inhibitors. One novel candidate, salt-inducible kinase 2 (SIK2), is overexpressed in approximately 30% of high grade serous ovarian cancers. Inhibition of SIK2 induces tetraploidy, triggers apoptotic cell death, prevents metastasis, reduces AKT/survivin signaling and decreases phosphorylation of class-IIa histone deacetylases (HDACs) enhancing inhibition of gene expression. In contrast to other HDACs, class-IIa HDACs function as signal-responsive transcriptional corepressors for the myocyte enhancer factor-2 (MEF2) family of transcription factors. MEF2s play important roles in regulating growth factor responses, neuronal survival, T-cell apoptosis and, importantly, B-cell DNA double strand break (DSB) repair. Here we ask whether inhibition of SIK2 can enhance HDAC class-IIa inhibition of MEF activity, decrease DNA repair and enhance sensitivity to the PARP inhibitor Olaparib in ovarian cancer cells.
Treatment with SIK2 inhibitors (ARN3236 or ARN3261) enhanced sensitivity to Olaparib in each of 8 ovarian cancer cell lines without BRCA1/2 mutations (OC316, OVCAR8, IGROV1, A2780, HCC5030, HCC5032, SKOv3 and OVCAR5) and significantly reduced the IC50 of Olaparib in two PARP inhibitor-resistant ovarian cancer cell lines (A2780CP-R and UWB1-289-10R). Synergistic cytotoxicity, judged with a Chou-Talaylay combination index (CI), was observed in all 8 cell lines. Treatment with a SIK2 inhibitor decreased the phosphorylation of class-IIa HDAC4/5/7, abolished class-IIa HDAC4/5/7-associated transcriptional activity of MEF2 and decreased MEF2 binding to regulatory regions with high-chromatin accessibility in DNA repair genes, resulting in repression of critical gene expression in DNA repair pathways. DNA damage, judged by rH2AX expression was enhanced by SIK2 inhibition. These observations not only provide new insights into the transcriptional regulation of DNA repair gene expression, but also have important implications for enhancing sensitivity of high grade serous ovarian cancers to PARP inhibition, with or without BRCA1 or BRCA2 germline mutations.
Citation Format: Zhen Lu, Wequn Mao, Lan Pang, Janice M. Santiago-O'Farrill, Haling Yang, Ahmed Ahmed, Hariprasad Vankayalapati, Robert C. Bast. SIK2 inhibitors regulate DNA repair pathway and sensitize ovarian cancer to PARP1 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 324.