Characterization of cyclin D1 as a Putative Kaiso Target Gene

Abstract

<p> Kaiso is a unique member of the BTB/POZ (Broad complex, Tramtrak, Bric à brac,/Pox virus and zinc finger) zinc finger family of transcription factors with established roles in development and tumourigenesis. Kaiso was originally identified as a novel binding partner of the Armadillo catenin p120^ctn, a cytosolic co-factor and regulator of the cell-cell adhesion molecule and tumor suppressor E-cadherin. In addition to their roles in cell adhesion, the multifunctional Armadillo catenins also regulate gene expression, thus providing at least two mechanisms for their contribution to tumourigenesis. The discovery of a novel interaction between p120^ctn and the transcription factor Kaiso was therefore consistent with gene regulatory roles for Armadillo catenins. Interestingly, Kaiso represses transcription via a sequence-specific DNA binding site (TCCTGCnA) as well as through methylated CpG di-nucleotides, and one role of nuclear p120^ctn is to inhibit Kaiso DNA-binding and transcriptional repression. We recently identified sequence-specific Kaiso binding sites in a subset of Wnt/β-catenin/TCF tumour-associated target genes, and here we present data characterizing cyclin D1 as a putative Kaiso target gene.</p> <p> Kaiso binds the cyclin D1 promoter in vitro and in vivo, and artificial promoter assays revealed that Kaiso overexpression results in the repression of a cyclin D1 promoter luciferase reporter. Since cyclin D1 is highly amplified in ~50% of human breast tumours, and a cancer profiling array demonstrated that Kaiso is misexpressed in ~40% of human breast tumours, we hypothesized that Kaiso represses and regulates cyclin D1 expression to inhibit breast tumourigenesis. In fact, examination of Kaiso expression in human breast cell lines demonstrated that cyclin D1 mRNA levels were upregulated in Kaiso-depleted cells. My studies further revealed that methylation-dependent Kaiso-DNA binding may contribute to Kaiso's transcriptional repression of the cyclin D1 promoter. We also determined that Kaiso inhibits, while p120^ctn activates, β-catenin-mediated activation of the cyclin D1 promoter. These findings further support a role for Kaiso and p120^ctn in breast tumourigenesis via their modulation of the canonical Wnt signaling pathway which is highly implicated in human tumourigenesis. Together these findings support our hypothesis that Kaiso regulates cyclin D1 expression. However, further studies are required to elucidate the mechanism employed by Kaiso to elicit cyclin D1 repression and to examine how this activity may contribute to breast tumourigenesis.</p>