Loss of Secreted Frizzled-Related Protein-1 Expression in Renal Cell Carcinoma Reveals a Critical Tumor Suppressor Function

EZH2 regulates sFRP4 expression without affecting the methylation of sFRP4 promoter DNA in colorectal cancer cell lines

Abnormal activation of the Wnt signaling pathway is found in 90% of colorectal cancers (CRCs). Secreted frizzled-related protein 4 (sFRP4) serves as an antagonist of the canonical Wnt signaling pathway. Epigenetic alterations, including changes in DNA methylation and histone methylation, may influence the expression of sFRP4. Polycomb group (PcG) proteins are epigenetic transcriptional repressors that selectively repress gene expression by forming polycomb repressive complexes (PRCs). Enhancer of zeste homolog 2 (EZH2), the core component of PRC2, is a histone-lysine N-methyltransferase that interacts with DNA methyltransferases. In the present study, the promoter DNA methylation status of sFRP4 in CRC cell lines was analyzed and the underlying mechanisms of action governing this modification was investigated. Firstly, the DNA methylation status of the sFRP4 promoter in CRC cell lines was assessed using methylation-specific PCR. Subsequently, the mRNA and protein levels of sFRP4 were measured using real-time qPCR and western blot analysis, respectively, to determine whether the DNA methylation status of the sFRP4 promoter is correlated with its transcriptional levels. To screen for important epigenetic modifiers that may regulate the promoter DNA methylation status of sFRP4, the expression levels of PcG proteins were examined by gene array analysis. ChIP-qPCR was performed to test whether the selected PcG proteins directly bind the promoter region of sFRP4. Finally, the downregulated PcG proteins EZH2, chromobox 7 (CBX7) and jumonji and AT-rich interaction domain containing 2 (JARID2) were identified and their association with sFRP4 expression levels and Wnt/β-catenin signaling pathway activity were investigated. The present study revealed that sFRP4 was hypermethylated in the promoter region and downregulated during the progression of the CRC cell lines from Dukes A to Dukes C. Expression levels of PcG proteins EZH2, CBX7 and JARID2 were upregulated and positively associated with the aberrantly activated Wnt signaling pathway in the CRC cell lines. EZH2, CBX7 and JARID2 were all enriched in the sFRP4 promoter region in CRC cells. EZH2 downregulation did not affect the promoter DNA methylation status of sFRP4 but increased its expression levels and decreased CRC cell proliferation. DNA methylation controls the expression of sFRP4. EZH2 regulates sFRP4 expression without affecting the DNA hypermethylation of the sFRP4 promoter and influences CRC cell proliferation and Wnt/β-catenin signaling pathway activities.

Wnt inhibitory factor-1 functions as a tumor suppressor through modulating Wnt/β-catenin signaling in neuroblastoma

Neuroblastoma is the most common extracranial solid tumor in childhood and is associated with serious morbidity and mortality. The effective treatment of neuroblastoma remains one of the major challenges in pediatric oncology. The Wnt signaling pathway has been shown to play a significant role in the pathogenesis of adult and pediatric tumors. WIF-1 has been identified as an important Wnt antagonist which inhibits Wnt/β-catenin signaling by directly binding to Wnt proteins. However, the expression and function of WIF-1 in neuroblastoma remains unknown. The present study showed that WIF-1 was downregulated with high level promoter methylation in neuroblastoma cells, and was significantly upregulated after exposure to demethylating agent. This finding suggests that downregulation of WIF-1 was associated with its promoter methylation in neuroblastoma. To further study the potential function of WIF-1 in neuroblastoma, we constructed a plasmid that over-expressed WIF-1 and transfected the plasmid into one neuroblastoma cell line SK-N-SH. We found that restoration of WIF-1 inhibited the growth and proliferation of neuroblastoma cells in vitro. Moreover, Wnt/β-catenin signaling activity and target genes expression were reduced by WIF-1 restoration. These results provide support that WIF-1 is downregulated and functions as a tumor suppressor by antagonizing Wnt/β-catenin signaling in neuroblastoma, suggesting a potential role as a therapeutic target in neuroblastoma.

Chronic cadmium exposure induces transcriptional activation of the Wnt pathway and upregulation of epithelial-to-mesenchymal transition markers in mouse kidney

The transition metal cadmium (Cd) is an environmental pollutant which damages the kidneys. Chronic Cd exposure may induce renal fibrosis and/or cancer, but the signaling pathways involved are not understood. The Wnt pathway is a key signaling cascade responsible for renal development, fibrosis and cancer. Hence the effect of chronic in vivo Cd exposure (100 mg/l drinking water for 12 weeks) on transcriptional activation of the Wnt pathway and markers of epithelial-to-mesenchymal transition (EMT) was investigated in mouse kidneys. Cd exposure increased kidney Cd content from 0.023+/-0.001 microg/g to 61+/-7 microg/g wet weight (means+/-S.D. of 6-7 animals). This was accompanied by increased expression of Wnt ligands (Wnt3a/6/7a/7b/9a/9b/10a/11), as determined by RT-PCR. The Wnt receptors Frizzled (Fz1/2/4,5,7-10) were also upregulated, as were the co-receptors low-density lipoprotein receptor-related proteins 5/6. Immunoblots with Wnt10a and Fz7 antibodies also revealed increased protein expression induced by Cd exposure. In contrast, Wnt antagonists were largely unaffected. Upregulation of Wnt signaling components induced by Cd was corroborated by increased expression of Wnt target genes, i.e. cell proliferation and survival genes c-Myc, cyclin D1 and the multidrug transporter P-glycoprotein Abcb1b, which promote malignancy. Lastly the EMT markers Twist, fibronectin and collagen I, but not alpha-smooth muscle actin, were also upregulated, suggesting that Cd-induced changes of renal epithelial tissue characteristics towards fibrosis and cancer may be mediated by Wnt signaling.

Functional significance of Wnt inhibitory factor-1 gene in kidney cancer

Wnt inhibitory factor-1 (WIF-1) has been identified as one of the secreted antagonists that bind Wnt protein. WIF-1 has been described as a tumor suppressor in various types of cancer. However, the molecular function of WIF-1 gene has never been examined in human renal cell carcinoma (RCC). Therefore, we hypothesized that WIF-1 functions as a tumor suppressor gene and overexpression of this gene may induce apoptosis and inhibit tumor growth in RCC cells. Immunohistochemistry and real-time reverse transcription-PCR revealed that WIF-1 was significantly downregulated in RCC samples and RCC cell lines, respectively. Bisulfite sequencing of the WIF-1 promoter region in RCC cell lines showed it to be densely methylated, whereas there was no methylation of WIF-1 promoter in normal kidney. Significant inhibition of cell growth and colony formation in WIF-1-transfected cells compared with controls were observed. WIF-1 transfection significantly induced apoptosis and suppressed in vivo tumor growth. Also, Wnt signaling activity and beta-catenin expression were reduced by WIF-1 transfection. In conclusion, this is the first report documenting that the WIF-1 is downregulated by promoter methylation and functions as a tumor suppressor gene by inducing apoptosis in RCC cells.