Cancer is caused by various environmental, genetic and epigenetic factors. Research in the Biochemistry Division focuses on elucidating the mechanisms of carcinogenesis and cancer progression using biochemical and molecular biological approaches. These studies are aimed at future application to the prevention and early diagnosis of human cancer.

Investigation of genetic susceptibility to colon carcinogenesis was conducted using aberrant crypt foci (ACF) as a surrogate biomarker in 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)-induced colon carcinogenesis. By genetic analysis, including haplotype mapping, using various rat strains, one of the candidate loci for the susceptibility genes (Sct) was narrowed down to a 0.8-Mb region around D16Rat129. One gene within this region was differentially expressed in the colon between F344 and ACI rats, regardless of PhIP treatment. Genetic polymorphisms in the 3'-UTR region of this gene were detected between the two strains and could be a candidate for Sct.

Dysplastic ACF detected by the differential staining method were demonstrated to contain various types of pre-neoplastic lesions of the colon. b-Catenin accumulation in colon epithelial cells was observed as early as at the stage of low-grade dysplastic ACF. Accumulation of Snd1, the function of which is discussed below, was detected in almost all dysplastic ACF, and a half of the cases of non-dysplastic ACF showed Snd1 accumulation ⁽²²⁾.

Array-based comparative genomic hybridization (aCGH) analysis indicated that all intestinal tumors induced in F344 rats by PhIP or combined administration of PhIP + azoxymethane (AOM) harbored an amplification of an approximately 500-kb region on chromosome 2, which is currently under validation by quantitative PCR. Amplification of ptprz1 was also detected in renal cancer induced by chronic oxidative stress, using aCGH analysis ⁽²³⁾.

The role of inflammation was analyzed in colon carcinogenesis. Mouse strain differences in inflammatory responses to dextran sulfate sodium (DSS) were correlated with enhanced colon tumorigenesis in the DSS-PhIP colon carcinogenesis model ⁽²⁴⁾. aCGH analysis in the colon tumors of mice suggested the existence of microdeletions throughout the genome. In relation to this study, genetic and pathological analysis of inflammation-associated regenerative mucosa was performed in the rat colitis model ⁽²⁵⁾ and the effect of 5-aminosalicylic acid was analyzed in the mouse colitis model ⁽²⁶⁾. Also, physical exercise modified the induction of ACF in rats ⁽²⁷⁾, and visceral fat obesity was significantly associated with the number of ACF in humans ⁽²⁸⁾. Related works that were published are listed ^{(29, 30)}.

Recently, post-transcriptional control of gene expression by the RNA-induced silencing complex (RISC) and small RNA species, including microRNA, was shown to have a major impact on human carcinogenesis. SND1, previously reported to be a component of RISC, is frequently upregulated in human colon cancers. Overexpression of SND1 in normal intestinal epithelial cells promoted cell proliferation mediated by downregulation of the tumor- suppressor APC without significant changes of its mRNA level, suggesting control of APC expression at the translational level. SND1 overexpression was detectable in colonic lesions even before b-catenin accumulation. It is thus suggested that overexpression of SND1 may contribute to the early stages of colon carcinogenesis through the translational regulation of target mRNA, including APC, acting to promote cell proliferation ⁽²²⁾.

Altered expression profile of miRNA is frequently found in human cancers. miR-34a was isolated as an upregulated gene by screening 157 human miRNAs after treatment of HCT 116 colon cancer cells with adriamycin. Expression of miR-34a was tightly regulated by the tumor suppressor p53. Forced expression of miR-34a induced senescence-like growth arrest in colon cancer cell lines. A member of the E2F family was coincidentally downregulated by miR-34a, indicating that senescence-like growth arrest could be due to the repression of positive regulators of the cell cycle. Furthermore, about 36% of human colon cancer specimens were found to show lower miR-34a expression as compared with matched normal counterparts. Therefore, miR-34a is suggested to be a novel type of tumor suppressor, and abrogation of miR-34a function could contribute to aberrant cell proliferation in normal cells, especially by allowing the cells to escape the premature senescence triggered by various environmental stresses ⁽³¹⁾.

G-rich short tandem repeats in the genome are hypermutable by various stimuli, and are frequently altered in various tumors. Formation of unusual DNA structures in these repeats is suspected to be responsible for their hypermutable feature. UP1/hnRNP A1 and hnRNP A3 have been identified as the proteins binding to the mouse hypervariable d(CAGGG)_n repeat. UP1 was demonstrated to unfold the intramolecular quadruplex structure of d(CAGGG)₅ and to abrogate the arrest of DNA synthesis at the d(GGG)_n site. UP1/hnRNP A1 also unfolded the intramolecular quadruplex structure of telomeric d(TTAGGG)₄ repeat and is suggested to be involved in telomere maintenance ⁽³²⁾. hnRNP A3 bound tightly to telomeric repeats and is suspected to protect the telomere ⁽³³⁾. Related work that was published is listed ⁽³⁴⁾. In relation to this study, analyses of translesion DNA synthesis at the PhIP-C8-dG adduct are ongoing.

The role of polyADP-ribosylation in cancer development is being studied using genetically modified mouse models. In vivo, accumulated poly(ADP-ribose) in cells harboring DNA damage might be released into the circulation upon cell destruction and further metabolized. Poly (ADP-ribose) was found to be metabolized to yield ribosyladenosine in mouse blood. The study on ribosyladenosine as a possible biomarker for cell death after DNA damage is ongoing.