Abstract 1600: Keeping glioblastoma (GBM)in check by targeting the CHK1-STAT3-CIP2A axis

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Abstract Checkpoint kinase1 (CHK1) is a DNA damage kinase, which has been observed to be constitutively active in human cancers. Unfortunately a number of new generation CHK1 inhibitors, currently in clinical trials, have shown deleterious effects on normal cells, thus limiting their clinical utility. Therefore, identification and targeting of cancer specific effectors of CHK1 signaling has emerged as an attractive therapeutic alternative. Analysis of the REMBRANDT and TCGA datasets revealed a strong positive correlation for CHK1 and CIP2A expression in 422/522 and 356/454 glioma patients respectively. By contrast, there was negligible correlation observed in normal samples. Additionally, high mRNA expression of both CHK1 and CIP2A was associated with reduced overall survival in glioma patients and marked a more aggressive form of the disease. Notably, CIP2A amplification found in 14.72% cases in the Rembrandt study was associated with worse overall survival in GBM patients. Mechanistically, we identify STAT3 as a transcriptional mediator for CHK-dependent CIP2A expression in GBM cells. We demonstrate that CHK1 positively regulates CIP2A transcription by promoting phosphorylation of STAT3 at Tyrosine 705 both in vitro and in vivo. Further, depletion of STAT3 by siRNA or chemical inhibitors resulted in decreased CIP2A expression in GBM cells. Functionally, both CHK1 and CIP2A promoted viability, clonogenicity and anchorage-independent growth of GBM cells. Importantly, inhibition of cancer cell viability and clonogenicity by Chk1 inhibition can be partially rescued by exogenous overexpression of CIP2A. Higher CIP2A, p-S345-CHK1 and p-Ty705-STAT3 levels were observed in de novo and patient-derived GBM, and U251MG cells relative to levels in normal human astrocytes (NHA). Analogously, CHK1 and CIP2A mRNA expression is increased 10 to 15 fold in genetically engineered mouse(GEM) models of human GBM (Pdgf-Cre-driven Ptenf/f and Pdgf-Cre-driven Ptenf/f;/p53f/f double mutant mice compared with wild-type mice). Finally, using xenograft mouse models we show that both PF477736 (a small molecule inhibitor of CHK1) and CIP2A depletion inhibits tumor growth. These results highlight CHK1 and CIP2A expression as potential diagnostic and prognostic markers in human GBMs. Further, these findings identify CIP2A as a cancer specific therapeutic target downstream of constitutively active CHK1 and STAT3 signaling in GBM. Together, these data serves as a platform for using CIP2A expression in GBM, to either directly target CIP2A or to stratify patients for CHK1 and/or STAT3 inhibition. Note: This abstract was not presented at the meeting. Citation Format: Anchit Khanna, Brett Stringer, Bryan Day, Kathleen Ensbey, Han Shen, Andrew Boyd, Kerrie McDonald, John E. Pimanda. Keeping glioblastoma (GBM)in check by targeting the CHK1-STAT3-CIP2A axis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1600. doi:10.1158/1538-7445.AM2014-1600