Kaiso represses the cell cycle gene cyclin D1 via sequence-specific and methyl-CpG-dependent mechanisms

Kaiso is the first member of the POZ family of zinc finger transcription factors reported to bind DNA with dual-specificity in both a sequence- and methyl-CpG-specific manner. Here, we report that Kaiso associates with and regulates the cyclin D1 promoter via the consensus Kaiso binding site (KBS), and also via methylated CpG-dinucleotides. The methyl-CpG sites appear critical for Kaiso binding to the cyclin D1 promoter, while a core KBS in close proximity to the methyl-CpGs appears to stabilize Kaiso DNA binding. Kaiso's binding to both sites was demonstrated in vitro using electrophoretic mobility shift assays (EMSA) and in vivo using Chromatin immunoprecipitation (ChIP). To elucidate the functional relevance of Kaiso's binding to the cyclin D1 promoter, we assessed Kaiso overexpression effects on a minimal cyclin D1 promoter-reporter that contains both KBS and CpG sites. Kaiso repressed this minimal cyclin D1 promoter-reporter in a dose-dependent manner and transcriptional repression occurred in a KBS-specific and methyl-CpG-dependent manner. Collectively our data validates cyclin D1 as a Kaiso target gene and demonstrates a mechanism for Kaiso binding and regulation of the cyclin D1 promoter. Our data also provides a mechanistic basis for how Kaiso may regulate other target genes whose promoters possess both KBS and methyl-CpG sites.

Kaiso regulates Znf131-mediated transcriptional activation

Kaiso is a dual-specificity POZ-ZF transcription factor that regulates gene expression by binding to sequence-specific Kaiso binding sites (KBS) or methyl-CpG dinucleotide pairs. Kaiso was first identified as a binding partner for the epithelial cell adhesion regulator p120(ctn). The p120(ctn)/Kaiso interaction is reminiscent of the beta-catenin/TCF interaction and several studies have suggested that Kaiso is a negative regulator of the Wnt/beta-catenin TCF signaling pathway. To gain further insight into Kaiso's function, we performed a yeast two-hybrid screen using the Kaiso POZ domain as bait. This screen identified the POZ-ZF protein, Znf131, as a Kaiso-specific binding partner. GST pull-down assays confirmed that the interaction is mediated via the POZ domain of each protein, and co-immunoprecipitation experiments further supported an in vivo Kaiso-Znf131 interaction. Using a Cyclic Amplification and Selection of Targets (CAST) approach, we identified the 12-base pair DNA palindrome sequence GTCGCR-(X)(n)-YGCGAC as a potential Znf131 binding element (ZBE). In vitro studies using electrophoretic mobility shift assay (EMSA) demonstrated that Znf131 binds the ZBE via its zinc finger domain. Znf131 DNA-binding specificity was confirmed using competition assays and ZBE mutational analyses. An artificial promoter-reporter construct containing four tandem copies of the ZBE was constructed and used to assess Znf131 transcriptional properties. We observed dose-dependent transcriptional activation of this artificial promoter-reporter by Znf131 in both epithelial and fibroblast cells, suggesting that Znf131 is a transcriptional activator. Kaiso overexpression significantly decreased the Znf131-mediated transcriptional activation, and interestingly, co-expression of the Kaiso-specific interaction partner p120(ctn) relieved Kaiso's inhibition of Znf131-mediated transcriptional activation. These findings indicate that Znf131 is a transcriptional activator, a less common function of POZ-ZF proteins, that is negatively regulated by its heterodimerization partner Kaiso.

Nuclear trafficking of the POZ-ZF protein Znf131

Znf131 is a member of the BTB/POZ family of transcription factors with roles in development and carcinogenesis. Like many members of this protein family, Znf131 displays robust nuclear localization in cultured cells, but the mechanism(s) of Znf131 nuclear trafficking is unknown. Here, we report the mechanism of Znf131 nuclear localization. Visual inspection of the Znf131 amino acid sequence revealed three basic regions (BR-1, -2 and -3) with the potential to serve as nuclear localization signals (NLS). Of the three basic regions, only BR-1 functioned independently to efficiently target heterologous beta-gal-GFP fusion proteins to HeLa cell nuclei. However, a Znf131 truncation mutant containing BR-2 and BR-3 efficiently targeted heterologous beta-gal-GFP fusion proteins to HeLa cell nuclei. Mutational analysis of full-length GFP-tagged Znf131 revealed that loss of any one BR alone did not prevent Znf131 nuclear localization. This apparent redundancy in NLS activity was due to the fact that intact BR-1 or BR-2 alone could target full-length Znf131 to nuclei. Consequently, simultaneous mutation of BR-1 and BR-2 abolished full-length Znf131 nuclear localization. Therefore, BR-1 and BR-2 are functional NLSs for Znf131 and as such are designated NLS-1 and NLS-2. Finally, wild type Znf131, and not a Znf131 NLS-defective mutant (NLS-1m/NLS-2m) interacted preferentially with the nuclear import receptor Importin-alpha3 in vitro.