Investigating the Role of CtBP in Colorectal Cancer

Abstract

Recent studies in breast cancer tissue have demonstrated that C-terminal binding protein (CtBP) over-expression induces stem cell-like features and genome instability; whereas CtBP depletion or caloric restriction reverses gene expression and increases DNA repair. C-terminal binding proteins (CtBP) are dimeric nuclear factors that function as transcriptional regulators in response to changes in the cellular level of NADH. Based on the binding affinity of CtBP for NADH, CtBP can function as a sensor of cellular metabolism and therefore has the potential to act as a metabolic marker in known obesity-related cancers. Thus, we decided to study CtBP expression and localization in colorectal cancer (CRC), a known obesity-related cancer. To study the potential role of CtBP as a metabolic marker in obesity-related cancer, we first subjected a panel of colorectal (HT-29, HCT-15, and SW480), lung (NCI-H1299) and breast (MCF-7) cancer cell lines to a number of metabolic perturbations including hypoxia and hyperglycemia and then observed the expression of known CtBP target genes. The results from these experiments suggested that CtBP does not function as a transcriptional regulator in the colorectal cancer cell lines studied, even while under conditions which are known to elevate cellular NADH levels. We then determined that simultaneous knockdown of CtBP 1 and 2 had no effect of the mRNA expression of known CtBP targets, suggesting again that it was not functioning as a transcription repressor in the colorectal cells. Furthermore we determined the subcellular localization of CtBP1 and CtBP2 in all the cell types interrogated. From these localization experiments we determined that CtBP 1 and CtBP2 were found in primarily in the cytoplasm of the CRC cell lines studied, in contrast to the breast and lung cancer cell lines, where they were found in the nucleus. Based on these findings, we conclude that CtBP does not act as a transcriptional regulator in the colorectal cancer cell lines tested. To test the therapeutic implications of this finding, we also treated the panel of cancer cell lines with purported inhibitors of CtBP and performed MTT assays at 24, 48, and 72 hours to determine the amount of cell death. The results of these experiments showed no significant differences in response to the inhibitors among the different cell types, thus suggesting that the inhibitors used potentially act through a mechanism other than one in which CtBP is responsible.