Novel target genes of the Wnt pathway and statistical insights into Wnt target promoter regulation

The Wnt pathway controls biological processes via the regulation of target gene expression. The expression of direct Wnt target genes, e.g. cyclin D1 and MYC, is activated by the transcription factor TCF, which binds to specific sequence motifs in the promoter. Indirect target genes are regulated via transcription regulators, which are targets of the Wnt pathway. As an example, MYC regulates the MYC interacting zinc finger protein-1 (MIZ-1), which is able to inhibit the expression of the indirect target p21WAF1. We intended to identify new Wnt target genes and to get a deeper insight into the regulatory mechanisms of Wnt target gene expression. For this we analyzed the differential expression pattern of Wnt-1 activated cells by microarray analysis. We identified 43 sequences including eight expressed sequence tags (ESTs), which showed increased transcript levels, and 104 sequences including 19 ESTs with decreased RNA levels. Northern blot and real-time quantitative PCR analysis of the differential expression levels of 15 genes confirmed the differential expression trends of eight candidate genes. When the Wnt pathway was regulated at the lower level of glycogen synthase kinase-3 beta (GSK-3 beta) or adenomatous polyposis coli (APC), we detected discrepant expression trends. We compared the number of binding sites of transcription factors in the genomic regions of all candidate target genes with the number of sites in control genes. We found that the genomic regions of the down-regulated genes include an increased number of putative MIZ-1 binding sites. Our study introduces several new Wnt target genes and provides indications that the specific gene expression pattern depends on the type of the activation trigger or the level of interference with the Wnt pathway. Furthermore, our data indicate that a high proportion of Wnt target genes are regulated by indirect mechanisms.