Abstract 2003: C-terminal binding protein (CtBP): An emerging oncogene and small molecule drug target in solid tumors
Authors: Sumner Evan T. , Korwar Sudha , Morris Benjamin L. , Dcona Martin M. , Hilbert Brendan J. , Royer William E. , Ellis Keith C. , Grossman Steven 
Published: July 2016

C-terminal binding proteins 1 and 2 (CtBP) are transcriptional coregulators whose overexpression has been linked to poor prognosis and/or chemoresistance in most common solid tumor types. CtBP exerts oncogenic activities through modulation of gene expression programs governing cell survival, epithelial/mesenchymal transitions, migration/invasion, and cell cycle, and is also required for colon cancer tumor initiating cell function. CtBP, however, has never been formally characterized as an oncogene. We now show that Ctbp2 exhibits transforming activity in immortalized NIH 3T3 as well as mouse and human primary cells. Ctbp2 alone, or in cooperation with SV40 large T antigen (LT) transformed NIH 3T3 cells and MEF’s, respectively, to anchorage independence with similar efficiency to mutant H-Ras. Human BJ foreskin fibroblasts were also transformed to anchorage independence by CtBP2 in cooperation with LT, SV40 small T antigen, and h-TERT with efficiency similar to H-Ras, indicating that CtBP overexpression, as found in the majority of human colon, ovary, breast, and prostate cancers, may be a key oncogenic driver gene. To confirm the physiologic role of Ctbp2 in a mouse tumor model with Ctbp overexpression, we bred Apcmin/+ mice to Ctbp2 hemizygote (Ctbp2+/-) mice, which are otherwise healthy. CtBP is a known target of the APC E3 ligase and is thus stabilized in APC mutated human colon cancers and is found in high levels in APCmin polyps. Remarkably, survival to humane endpoint at 37 weeks for Apcmin/+ vs. Apcmin/+-Ctbp2+/- mice was 0% vs. 100% (p &lt;0.001). As CtBP also encodes a targetable intrinsic dehydrogenase that forms an NADH-dependent oligomerization surface, our cell and mouse model data supports the idea that CtBP could be an attractive small molecule drug target in tumors where it is upregulated. We have therefore synthesized a series of dehydrogenase substrate competitive inhibitor compounds based on CtBP's natural substrate 4-methylthio-2-oxobutyric acid (MTOB) that utilize a phenylpyruvate backbone and demonstrate nanomolar enzymatic IC50's with micromolar cellular GI50’s, while exhibiting minimal off target inhibition of lactate dehydrogenase. Phenylpyruvate—based CtBP inhibitors biochemically caused dissolution of CtBP oligomers, which are the active transcriptional conformation, and exhibited on-target transcriptional effects in cells, with reversal of CtBP transcriptional repression of key cancer target genes, such as E-cadherin, Bik, and BRCA1. In summary, we have characterized CtBP2 as a cellular proto-oncogene that can transform primary mouse and human cells in a manner similar to activated H-Ras, genetically validated Ctbp2 as a therapeutic target in the Apcmin/+ mouse model, and identify promising small molecule lead inhibitors for future therapeutic development.

Citation Format: Evan T. Sumner, Sudha Korwar, Benjamin L. Morris, Martin M. Dcona, Brendan J. Hilbert, William E. Royer, Keith C. Ellis, Steven Grossman. C-terminal binding protein (CtBP): An emerging oncogene and small molecule drug target in solid tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2003.