Although mutation of APC or CTNNB1 (b-catenin) is rare in breast cancer, activation of Wnt signalling is nonetheless thought to play an important role in breast tumorigenesis, and epigenetic silencing of Wnt antagonist genes, including the secreted frizzled-related protein (SFRP) and Dickkopf (DKK) families, has been observed in various tumours. In breast cancer, frequent methylation and silencing of SFRP1 was recently documented; however, altered expression of other Wnt antagonist genes is largely unknown. In the present study, we found frequent methylation of SFRP family genes in breast cancer cell lines (SFRP1, 7 out of 11, 64%; SFRP2, 11 out of 11, 100%; SFRP5, 10 out of 11, 91%) and primary breast tumours (SFRP1, 31 out of 78, 40%; SFRP2, 60 out of 78, 77%; SFRP5, 55 out of 78, 71%). We also observed methylation of DKK1, although less frequently, in cell lines (3 out of 11, 27%) and primary tumours (15 out of 78, 19%). Breast cancer cell lines express various Wnt ligands, and overexpression of SFRPs inhibited cancer cell growth. In addition, overexpression of a b-catenin mutant and depletion of SFRP1 using small interfering RNA synergistically upregulated transcriptional activity of T-cell factor/lymphocyte enhancer factor. Our results confirm the frequent methylation and silencing of Wnt antagonist genes in breast cancer, and suggest that their loss of function contributes to activation of Wnt signalling in breast carcinogenesis. British Journal of Cancer (2008) Wnt ligands are secreted proteins that bind to transmembrane receptors in the Frizzled (Fz) family. During normal developmental processes, the resultant Wnt signalling plays essential roles in the regulation of cell proliferation, patterning and fate determination (Cadigan and Nusse, 1997). The binding of Wnt to Fz leads to dephosphorylation and stabilisation of b-catenin, enabling it to be translocated into the nucleus, where it interacts with members of the T-cell factor/lymphocyte enhancer factor (TCF/LEF) family of transcription factors to stimulate the expression of target genes. This signalling pathway is strongly implicated in tumorigenesis; indeed, the first mammalian Wnt isoform was identified based on its ability to promote mouse mammary tumorigenesis (Polakis, 2000). In addition, aberrant nuclear and cytoplasmic localisation of b-catenin is frequently observed in human breast cancer (Lin et al, 2000;Ryo et al, 2001;Chung et al, 2004). In contrast to colorectal cancer (CRC), however, mutation of APC, AXIN or CTNNB1 (b-catenin) is rare in breast cancer, indicating that other mechanisms are responsible for the activation of b-catenin. These mechanisms could include increased expression of Wnt ligand (Huguet et al, 1994;Dale et al, 1996;Bui et al, 1997) and/or the loss of Wnt antagonists.Several classes of secreted Wnt antagonists are known, including the Cerberus, Wnt inhibitory factor 1, secreted frizzled-related protein (SFRP) and the Dickkopf (DKK) families (Kawano and Kypta, 2003). The SFRP family is comprised of five secreted glycopr...

Activation of Wnt signaling has been implicated in tumorigenesis, and epigenetic silencing of Wnt antagonist genes has been detected in various cancers. In the present study, we examined the expression and methylation of DICKKOPF (DKK) family genes in gastrointestinal cancer cell lines. We found that all known DKK genes were frequently silenced in colorectal cancer (CRC) cells (DKK1, 3/9, 33%; DKK2, 8/9, 89%; DKK3, 5/9, 56% and DKK4, 5/9, 56%), but not in normal colon mucosa. DKK1, -2 and -3 have 5' CpG islands, and show an inverse relation between expression and methylation. DKK methylation also was frequently observed in gastric cancer (GC) cell lines (DKK1, 6/16, 38%; DKK2, 15/16, 94% and DKK3, 10/16, 63%), but was seen less frequently in hepatocellular carcinoma and pancreatic cancer cell lines. DKKs also were frequently methylated in primary CRCs (DKK1, 7/58, 12%; DKK2, 45/58, 78% and DKK3, 12/58, 21%) and GCs (DKK1, 15/31, 48%; DKK2, 26/31, 84% and DKK3, 12/31, 39%). Against a background of CTNNB1 or APC mutations, Dickkopfs (Dkks) were less effective inhibitors of Wnt signaling than secreted frizzled-related proteins, though over-expression of Dkks suppressed colony formation of CRC cells with such mutations. Our results demonstrate that DKKs are frequent targets of epigenetic silencing in gastrointestinal tumors, and that loss of DKKs may facilitate tumorigenesis through beta-catenin/T-cell factor-independent mechanisms.

Abstract. Although mutations of APC, CTNNB1 (ß-catenin) and AXIN1 are rare in oral squamous cell carcinoma (OSCC), activation of the Wnt signaling pathway is thought to play an important role in oral carcinogenesis. In the present study, we examined the relationship between Wnt signaling and epigenetic alteration of the secreted frizzled-related protein (SFRP) genes in OSCC. We frequently detected loss of membrane localization of ß-catenin and its cytoplasmic or nuclear accumulation in OSCC cell lines, although these cell lines showed no APC or CTNNB1 (ß-catenin) mutations and no methylation of CDH1 (E-cadherin). By contrast, we frequently detected methylation of SFRP1 (7/17, 41%) SFRP2 (16/17, 94%) and SFRP5 (14/17, 82%) in a panel of OSCC cell lines, as well as in specimens of primary tumors collected from 44 OSCC patients (SFRP1, 10/42, 24%; SFRP2, 16/44, 36%; SFRP5, 7/43, 16%). We also observed that OSCC cell lines express various Wnt ligands, and that ectopic expression of SFRPs inhibited cancer cell proliferation. Our results confirm the frequent methylation and silencing of SFRP genes in OSCC, and suggest that their loss of function contributes to activation of Wnt signaling that leads to cell proliferation during oral carcinogenesis.

Genetic and epigenetic alterations in tumor-suppressor genes play important roles in human neoplasia. Ras signaling is often activated in oral squamous cell carcinoma (OSCC), although Ras mutations are rarely detected in Japanese OSCC patients, and the mechanisms underlying the gene's activation remain unclear. Here, we examined the expression of Ras association family (RASSF) genes in a panel of OSCC cell lines and found that RASSF2 is often downregulated by DNA methylation in OSCC cells. In addition, aberrant methylation of RASSF2 was detected in 12 of 46 (26%) primary OSCC, and 18 (39%) of those OSCC showed methylation of at least one RASSF gene. Ectopic expression of RASSF2 in OSCC cells suppressed cell growth and induced apoptosis. A RASSF2 deletion mutant lacking the Ras-association domain, which was therefore unable to interact with Ras, exhibited less pro-apoptotic activity than the full-length protein, indicating that the pro-apoptotic activity of RASSF2 is related to its association with Ras. Genomic screening of genes regulated by RASSF2 showed that genes involved in immune responses, angiogenesis, and metastasis are suppressed by RASSF2. Our results suggest that epigenetic inactivation of RASSF2 plays an important role in OSCC tumorigenesis, and that RASSF2 may be a useful molecular target for the diagnosis and treatment of OSCC. (Cancer Sci 2008; 99: 958-966) E pidemiological studies have shown that oral squamous cell carcinoma (OSCC) is one of the most commonly occurring human neoplasias (1) and it appears that alcohol and tobacco play key roles in its development. (2,3) Although it has been suggested that OSCC arises through the accumulation of multiple genetic changes (e.g. p53 mutations), the precise molecular mechanism underlying the development of OSCC remains unclear. In addition to genetic changes, recent studies indicate that epigenetic silencing of cancer-related genes by DNA methylation also plays an important role in tumorigenesis in OSCC. (5,6) Inactivation of tumor-suppressor and cancer-related genes occurs via such epigenetic changes as DNA methylation, histone deacetylation, and histone methylation in the 5′-CpG island of the genes.(7) DNA methylation is reversible by pharmacological inhibition of DNA methyltransferase, and can be a molecular target for cancer therapy. (8) In OSCC, genes involved in cell cycle regulation, apoptosis, DNA repair, and cellular differentiation are reportedly involved, (6,(9)(10)(11)(12) although the role of DNA methylation in OSCC tumorigenesis is not fully understood.The RAS signaling pathway is frequently activated in human tumors (13) and activating mutations of k-ras and Ha-ras have been detected in OSCC associated with chewing tobacco. (14,15) In contrast, Ras mutations are rare among Japanese OSCC patients. (16) Moreover, recent studies indicate that, in addition to Ras mutation, inactivation of negative effectors of Ras may play a significant role in the development of tumors. Among these negative effectors, the Ras association family (RASSF) is a...

The results of this study suggest that knockdown of GLUT-1 inhibits sensitization of oral squamous cells to CDDP during hypoxia in HSC-2, Ca9-22, and SAS cells.