Graduate School of Biomedical Sciences; Department of Cancer Biology
ADP-Ribosylation Factors; Apoptosis; Tumor Suppressor Protein p53; Phosphoproteins; DNA-Binding Proteins; Tumor Suppressor Protein p14ARF; Colorectal Neoplasms; Genes, Tumor Suppressor; Academic Dissertations; Dissertations, UMMS
ARF, a potent tumor suppressor, positively regulates p53 by antagonizing MDM2, a negative regulator of p53, which in turn, results in either apoptosis or cell cycle arrest. ARF also suppresses the proliferation of cells lacking p53, and loss of ARF in p53-null mice, compared with ARF-null or p53-null mice, results in a broadened tumor spectrum and decreased tumor latency. This evidence suggests that ARF exerts both p53-dependent and p53-independent tumor suppressor activity. However, the molecular pathway and mechanism of ARF’s p53-independent tumor suppressor activity is not understood.
The antiapoptotic, metabolically regulated, transcriptional corepressor C-terminal binding protein 2 (CtBP2) has been identified as a specific target of ARF’s p53-independent tumor suppression. CtBPs are phosphoproteins with PLDLS-binding motif and NADH-binding central dehydrogenase domains. ARF interacts with CtBP1 and CtBP2 both in vitro and in vivo, and induces their proteasome-mediated degradation, resulting in p53-independent apoptosis in colon cancer cells. ARF’s ability to target CtBP2 for degradation, and its induction of p53-independent apoptosis requires an intact interaction with CtBP2, and phosphorylation at S428 of CtBP2. As targets for inhibition by ARF, CtBPs are candidate oncogenes, and their expression is elevated in a majority of human colorectal adenocarcinomas specimens in comparison to normal adjacent tissue. Relevant to its targeting by ARF, there is an inverse correlation between ARF and CtBP expression, and CtBP2 is completely absent in a subset of colorectal adenocarcinomas that retains high levels of ARF protein.
CtBPs are activated under conditions of metabolic stress, such as hypoxia, and they repress epithelial and proapoptotic genes. BH3-only genes such as Bik, Bim and Bmf have been identified as mediators of ARF-induced, CtBP2-mediated p53-indpendent apoptosis. CtBP2 repressed BH3-only genes in a tissue specific manner through BKLF (Basic kruppel like factor)-binding elements. ARF regulation of BH3-only genes also required intact interaction with CtBP2. ARF antagonism of CtBP repression of Bik and other BH3-only genes may play a critical role in ARF-induced p53-independent apoptosis, and in turn, tumor suppression.
To study the physiologic effect of ARF/CtBP2 interaction at the organismal level, the p19ArfL46D knock-in mice, in which the Arf/CtBP2 interaction was abrogated, was generated. Analysis of the primary cells derived from these mice, revealed that the Arf/CtBP2 interaction contributes to regulation of cell growth and cell migration. Overexpression of CtBP in human tumors, and ARF antagonism of CtBP repression of BH3-only gene expression and CtBP-mediated cell migration may therefore play a critical role in the p53-independent tumor suppressor function/s of ARF.
Kovi, RC. Defining the Role of CtBP2 in p53-Independent Tumor Suppressor Function of ARF: A Dissertation. (2009). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 433. http://escholarship.umassmed.edu/gsbs_diss/433
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