Expression of CtBP family protein isoforms in breast cancer and their role in chemoresistance

Abstract

Background information. CtBPs [C-terminal (of E1A) binding protein] have roles in the nucleus as transcriptional co-repressors, and in the cytoplasm in the maintenance of vesicular membranes. CtBPs are expressed from two genes, CTBP1 and CTBP2, mRNA products of which are alternatively spliced at their 5′-ends to generate distinct protein isoforms. Extensive molecular and cellular analyses have identified CtBPs as regulators of pathways critical for tumour initiation, progression and response to therapy. However, little is known of the expression or regulation of CtBP isoforms in human cancer, nor of the relative contributions of CTBP1 and CTBP2 to the tumour cell phenotype.Results. Expression of CtBP proteins and CTBP1 and CTBP2 mRNA splice forms in breast cancer cell lines and tumour tissue was examined. CtBP1 proteins are identifiable as a single band on Western blots and are ubiquitously detectable in breast tumour samples, by both Western blotting and immunohistochemistry. CtBP1 is present in six of six breast cancer cell lines, although it is barely detectable in SKBr3 cells due to reduced CTBP1 mRNA expression. In the cell lines, the predominant CTBP1 mRNA splice form encodes CtBP1-S protein; in tumours, both major CTBP1 mRNA splice forms are variably expressed. CtBP2 proteins are ubiquitously expressed in all lines and tumour samples. The predominant CTBP2 mRNA encodes CtBP2-L, although an alternatively spliced form that encodes CtBP2-S, previously unidentified in humans, is expressed at low abundance. Both CtBP2-L and CtBP2-S are readily detectable as two distinct bands on Western blots; here we show that the CTBP2-L mRNA is translated from two AUG codons to generate both CtBP2-L and CtBP2-S. We have also identified an autoregulatory feedback mechanism whereby CtBP protein abundance is maintained in proliferating breast cancer cells through the post-transcriptional regulation of CtBP2. This feedback is disrupted by UV-C radiation or exposure to cisplatin. Finally, we demonstrate that CtBP1 and CtBP2 both have p53-dependent and -independent roles in suppressing the sensitivity of breast cancer cells to mechanistically diverse cancer chemotherapeutic agents.Conclusions. These studies support recent evidence that CtBP family proteins represent potential targets for therapeutic strategies for the treatment of cancer in general, and breast cancer in particular.

C-terminus of Hsc70-interacting protein regulates C-terminal binding protein 2 and the expression of its target genes

“…CHIP protein is decreased in human breast cancer tissues and is inversely correlated with their malignancy [21]. The physiological relevance of CHIP and CtBP2 to the pathogenesis of human breast cancer has never been addressed, but our present study implies that the elevated CtBP2 observed in human breast cancer [12] may be due to inefficient regulation caused by low levels of CHIP protein and may contribute to the anti-apoptotic property of human breast cancer by repressing pro-apoptotic gene expression.…”

“…CtBP2 was recognized as an oncogenic protein that represses pro-apoptotic genes, and elevated CtBP2 is implicated in various human cancers including breast cancer [12][13][14], suggesting that maintaining a proper level of CtBP2 is crucial for preventing tumorization. Our study provides the first evidence that CHIP as an E3 ubiquitin ligase negatively regulates CtBP2 in a ubiquitin-proteasome-dependent manner.…”

“…Up-regulation of CtBPs has been reported. Elevated CtBP1 protein is implicated in initiation of human adenoma [11], and CtBP2 protein is highly expressed in breast [12] and ovarian cancers [13]. In addition, high expression of both proteins has been reported in colon cancer [14].…”

Quantitative proteomic analyses of mammary organoids reveals distinct signatures after exposure to environmental chemicals

“…A subset of the protein variants with potential cancer relevance is shown in Table 1. The isoforms of the NADH-binding transcriptional repressor CtBP1 are important regulators of tumor suppressors (61) that are required for high-fidelity replication of DNA during mitosis (38,62) and play a role in breast cancer chemoresistance (63). Isoform 2 of CtBP1 was down-regulated by PCB and, to a lesser extent, by Pht in organoids treated with these chemicals.…”

5-Fluorouracil derivatives: a patent review (2012 – 2014)

“…The CTBP1 gene encodes for a phosphoprotein that acts in the nucleus as a transcriptional repressor and in the cytoplasm in the maintenance of vesicular membranes [55]. CTBP1 degradation on colon cancer depends on the tumor suppressor adenomatous polyposis coli (APC); when this protein is mutated, CTBP1 increased resulting in the initiation of the adenoma formation [56].…”