| Summary: | 博士 === 國立臺灣大學 === 毒理學研究所 === 94 === Recently, it has been reported that the nuclei of colon carcinoma cell lines contain constitutively active Tcf4 / b-catenin complexes as a direct consequence of either loss of function of the tumor suppressor gene APC or gain of function mutations in b-catenin itself. This is believed to result in the uncontrolled transcription of TCF target genes, leading to transformation of colon epithelial cells and initiation of polyp formation. Regulation of the transcriptional activity of b-catenin / Tcf4 has important implications for embryonic development as well as for carcinogenesis in the intestinal epithelium. Prior to discuss the potential role for LEF / TCF transcription factors in cancer, it is important to outline the mechanism by which they have been proposed to operate. Although LEF / TCF transcription factors bind directly to DNA through their HMG domains, they are incapable of independently activating gene transcription. On the other hand, most experimental data support the view that TCF is a repressor when Wnt does not convert it into an activator. TCF can repress target genes as well as activate those same target genes in cells instructed to change developmental fate, there are several mechanisms by which this switch is achieved.
We are interested in the possibility of the bridging protein that interacts with TCF during the regulation of transcriptional activity. Because of its predominant expression in the human colonic epithelium, the full-length Tcf4 is used as bait in yeast two-hybridization, and Daxx is isolated. Daxx, a human cell death associated protein, has been reported to mediate the Fas / JNK-dependent signals in the cytoplasm. However, several lines of evidence have suggested that Daxx is mainly located in the nucleus and functions as a transcriptional regulator. Co-immunoprecipitation in HEK-293T cells and yeast two-hybrid screen in Y190 cells are performed to identify the interaction between Tcf4 and Daxx, and co-immunoprecipitation is also used to map the binding regions of Tcf4. In the nucleus, Daxx reduces DNA binding activity of Tcf4 and represses Tcf4 transcriptional activity. Overexpression of Daxx alteres expression of genes downstream of Tcf4, including cyclin D1 and Hath-1, and induces G1 phase arrest in colon cancer cells. Besides, a reduction in Daxx protein expression is observed in colon adenocarcinoma tissue when compared with normal colon tissue. These findings suggest a possible physiological function of Daxx, via interaction with Tcf4, to regulate cell cycle progression, and hence cell proliferation and differentiation. Furthermore, a small form of Tcf4 is observed specifically in SDS-PAGE when Daxx is co-expressed in a dose- and time-dependent manner. Moreover, transcriptional activity of Tcf4 is enhanced by SUMO-1 and PIAS3, but this transactivation still can be repressed by Daxx. Taken together, these findings not only outline the functional link between Tcf4 and Daxx, but also provide a new idea to develop novel cancer therapies.
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