Identification of the polycomb group protein SU(Z)12 as a potential molecular target for human cancer therapy

Abstract

We have previously identified SU(Z)12 as an E2F target gene. Because many E2F target genes encode proteins that are critical for the control of cell proliferation, we have further characterized the regulation and expression of SU(Z)12. To understand the molecular mechanisms responsible for expression of SU(Z)12 mRNA, we have analyzed the promoter region. We found that the SU(Z)12 gene is controlled by dual promoters, one of which functions bidirectionally. In addition to the E2F binding site, we have identified two binding sites for T cell factor (TCF)/beta-catenin complexes. Using gel mobility shift assays, we demonstrated that both TCF sites can be bound by TCF4. TCF/beta-catenin complexes have been shown to be a critical regulator of gene expression in tumors of the colon, breast, and liver. Accordingly, we have used chromatin immunoprecipitation assays to confirm that TCF4/beta-catenin complexes are bound to the SU(Z)12 promoter in colon cancer cells but not in HeLa cells. We next adapted the chromatin immunoprecipitation assay for use with primary colon tumor samples, and, using matched pairs of normal and tumor tissue obtained from several different colon cancer patients, we demonstrate that levels of beta-catenin bound to the SU(Z)12 promoter are increased in colon tumors. Finally, we show that the SU(Z)12 mRNA is up-regulated in a number of different human tumors, including tumors of the colon, breast, and liver. Recent studies have found that SU(Z)12 is a component of the Drosophila ESC-E(Z) and the human EED-EZH2 Polycomb chromatin remodeling complexes. Therefore, we suggest that SU(Z)12, which may modulate the tumor phenotype by changing gene expression profiles, may be a logical target for the design of a new antitumor agent