Role of CBX7 in breast cancer stem cell regulation

Abstract

Chromoprotein homologue 7 (CBX7) is a member of the polycomb repressive complex (PRC)-1 which play an important role in several biological processes including stem cell regulation, differentiation, and cancer developments via epigenetic regulation of gene expression together with PRC-2. However, the role of CBX7 in cancer stem cell (CSC) regulation and tumorigenicity of human breast cancers remains unknown. Here, this study demonstrated that CBX7 has a tumor suppressive function by negatively regulating breast CSCs. I found that CBX7 expression was downregulated in population of breast CSCs compared with non-CSCs. Furthermore, small-hairpin RNA (shRNA)-mediated CBX7 knockdown in MCF10A immortalized breast cell lines and MDA-MB-231 breast cancer cell lines showed the increased percentage of CD44+/CD24-/ESA+ breast CSC population and reinforced self-renewal of breast CSCs as confirmed by flow cytometry, tumorsphere forming and anchorage independent growth assays. Similarly, tetracycline-inducible CBX7 overexpression in MCF7 breast cancer cell lines reduced percentage and self-renewal activity of breast CSCs. Consistently, xenograft with CBX7 knockdown MDA-MB-231 cells exhibited the enhanced in vivo tumor-initiating ability. I also identified a PcG-independent regulatory mechanism of breast CSCs by CBX7. Using a gene expression microarray analysis, I found that several identified CBX7-target genes including diminished expression of Dickkopf-1 (DKK-1) were involved in the Wnt signaling pathway. Moreover, I confirmed that CBX7 positively regulates the expression of DKK-1, a Wnt antagonist, by cooperating with p300 acetyltransferase to enhance the histone acetylation of DKK-1 promoter. Consistent with this observation, CBX7 inhibited GSK3-β phosphorylation, nuclear translocation of β-catenin and subsequent TCF/LEF promoter activity and its target c-Myc expression, indicating the negative regulation of the Wnt/-catenin/TCF pathway by CBX7. Furthermore, treatment with DKK-1 inhibitor, WAY-262611, in CBX7 overexpressing breast cancer cells showed recovery of the Wnt signaling pathway and consequent rescue of breast CSC activity. Taken together, these findings suggested that CBX7 acts as a novel epigenetic modifier of DKK-1 gene by cooperating with p300 and inhibits breast CSC activity through DKK-1-mediated suppression of the Wnt signaling pathway. Therefore, CBX7 could be a crucial tumor suppressor as a novel negative regulator of breast CSCs.