Genomic and Proteomic Approaches to Colorectal Carcinogenesis

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Genomic and proteomic approaches to colorectal carcinogenesis (PDF:136KB)
β-Catenin is the downstream effector of the Wnt signaling pathway and is involved in the process of colorectal carcinogenesis. T-cell factor-4 (TCF4) regulates a certain set of genes related to growth and differentiation of intestinal epithelial cells, and aberrant transactivation of these TCF4-regulated genes by β-catenin protein plays a crucial role in early intestinal carcinogenesis.

Suppression of β-catenin-evoked gene transactivation of colorectal cancer cells by dominant-negative TCF4 switches off genes involved in cell proliferation and switches on genes involved in cell differentiation (1). Induction of dominant-negative TCF4 has been reported to restore the epithelial cell polarity of a colorectal cancer cell line and convert the cell line into a single layer of columnar epithelium, indicating that colorectal cancer cells still require accumulation of β-catenin protein, thereby transactivating the target genes of TCF4, to maintain cell proliferation, depolarization, and dedifferentiation (2).


Identification of the target genes/proteins of the β-catenin/TCF4 complex (PDF:66KB)
By using global gene (GeneChip oligonucleotide microarray) and protein (2D-DIGE and isotope-coded affinity tagging and mass spectrometry) expression analyses we succeeded in identifying several molecules whose expression is regulated by the β-catenin/TCF4 complex (3,4,9). The human multidrug resistance-1 (MDR1) (ABCB1) gene contains multiple TCF/LEF-binding elements in its promoter and is one of the immediate targets of the β-catenin/TCF4 complex (1).

Functional analyses of the target genes/proteins of the β-catenin/TCF4 complex (PDF:568KB)
Biological involvement of MDR1 in intestinal tumorigenesis has been verified in a mouse model. Mdr1-deficient Min (Apc^{Min/+Mdr1a/b-/-}) mice developed significantly fewer intestinal polyps than did control (Apc^{Min/+Mdr1a/b+/+}) mice (5). SF1 negatively regulates negatively -catenin-evoked gene transactivation and cell proliferation (9), and consistent with this Sf1^+/- mice exhibited greater susceptibility to colon tumorigenesis induced by azoxymethane than wild-type (Sf1^+/+) mice (10).

Protein composition of the β-catenin and T-cell factor-4 (TCF4) nuclear complex (PDF:39KB)
In our series of proteomic studies we identified fusion/translocated in liposarcoma (FUS/TLS), poly(ADP-ribose) polymerase-1 (PARP-1), Ku70, Ku80, DNA topoisomerase IIα (Topo IIα), and splicing factor-1 (SF1) as putative components of the β-catenin and TCF4 nuclear complex (6,7,8,9,12). Two of these component proteins, Topo IIα(11) and PARP-1(6,8), are enhancers of the β-catenin and TCF/LEF transcriptional complexes, and SF1 (9, 10), FUS/TLS (7), and Ku70 (8) are suppressors. Topo II is a known target of drugs that are currently being widely used for cancer chemotherapy. We have demonstrated that Topo IIα a functional component of the β-catenin and TCF4 complex (11) and a potential drug target. Although there is a correlation between nuclear localization of ƒÀ-catenin and neoplastic phonotypes of colorectal cancer, the mechanisms underlying nuclear import of ƒÀ-catenin, which lacks a nuclear localization signal, remain poorly understood. We have found that TCF-4 interacts with nuclear pore complex (NPC) including SUMO E3 ligase, Ran BP2, that sumoylates TCF4(PDF:122KB). This sumoylated TCF-4 reinforces the interaction between TCF-4 and ƒÀ-catenin and thereafter accelerates nuclear transport of ƒÀ-catenin/TCF-4 complex (12). We therefore believe that RanBP2 becomes a therapeutic target for colorectal cancer.


References
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4. Naishiro et al., Oncogene, 24: 3141-3153, 2005.
5. Yamada et al., Cancer Res, 63: 895-901, 2003.
6. Idogawa et al., Gastroenterology, 128: 1919-1936, 2005.
7. Sato et al., Gastroenterology, 129: 1226-11236, 2005.
8. Idogawa et al., Cancer Res, 67: 911-918, 2007.
9. Shitashige et al., Gastroenterology, 132: 1039-1054, 2007.
10. Shitashige et al., Cancer Sci, 98: 1862-1867, 2007.
11. Huang et al., Gastroenterology, 133: 1569-1578, 2007.
12. Shitashige et al., Gastroenterology, 134: 1961- 1971, 2008.