Direct repression of MYB by ZEB1 suppresses proliferation and epithelial gene expression during epithelial-to-mesenchymal transition of breast cancer cells.

INTRODUCTION: Epithelial-to-mesenchymal transition (EMT) promotes cell migration and is important in metastasis. Cellular proliferation is often downregulated during EMT, and the reverse transition (MET) in metastases appears to be required for restoration of proliferation in secondary tumors. We studied the interplay between EMT and proliferation control by MYB in breast cancer cells. METHODS: MYB, ZEB1, and CDH1 expression levels were manipulated by lentiviral small-hairpin RNA (shRNA)-mediated knockdown/overexpression, and verified with Western blotting, immunocytochemistry, and qRT-PCR. Proliferation was assessed with bromodeoxyuridine pulse labeling and flow cytometry, and sulforhodamine B assays. EMT was induced with epidermal growth factor for 9 days or by exposure to hypoxia (1% oxygen) for up to 5 days, and assessed with qRT-PCR, cell morphology, and colony morphology. Protein expression in human breast cancers was assessed with immunohistochemistry. ZEB1-MYB promoter binding and repression were determined with Chromatin Immunoprecipitation Assay and a luciferase reporter assay, respectively. Student paired t tests, Mann-Whitney, and repeated measures two-way ANOVA tests determined statistical significance (P < 0.05). RESULTS: Parental PMC42-ET cells displayed higher expression of ZEB1 and lower expression of MYB than did the PMC42-LA epithelial variant. Knockdown of ZEB1 in PMC42-ET and MDA-MB-231 cells caused increased expression of MYB and a transition to a more epithelial phenotype, which in PMC42-ET cells was coupled with increased proliferation. Indeed, we observed an inverse relation between MYB and ZEB1 expression in two in vitro EMT cell models, in matched human breast tumors and lymph node metastases, and in human breast cancer cell lines. Knockdown of MYB in PMC42-LA cells (MYBsh-LA) led to morphologic changes and protein expression consistent with an EMT. ZEB1 expression was raised in MYBsh-LA cells and significantly repressed in MYB-overexpressing MDA-MB-231 cells, which also showed reduced random migration and a shift from mesenchymal to epithelial colony morphology in two dimensional monolayer cultures. Finally, we detected binding of ZEB1 to MYB promoter in PMC42-ET cells, and ZEB1 overexpression repressed MYB promoter activity. CONCLUSIONS: This work identifies ZEB1 as a transcriptional repressor of MYB and suggests a reciprocal MYB-ZEB1 repressive relation, providing a mechanism through which proliferation and the epithelial phenotype may be coordinately modulated in breast cancer cells.

Remodeling of purinergic receptor-mediated Ca2+ signaling as a consequence of EGF-induced epithelial-mesenchymal transition in breast cancer cells.

BACKGROUND: The microenvironment plays a pivotal role in tumor cell proliferation, survival and migration. Invasive cancer cells face a new set of environmental challenges as they breach the basement membrane and colonize distant organs during the process of metastasis. Phenotypic switching, such as that which occurs during epithelial-mesenchymal transition (EMT), may be associated with a remodeling of cell surface receptors and thus altered responses to signals from the tumor microenvironment. METHODOLOGY/PRINCIPAL FINDINGS: We assessed changes in intracellular Ca(2+) in cells loaded with Fluo-4 AM using a fluorometric imaging plate reader (FLIPR(TETRA)) and observed significant changes in the potency of ATP (EC(50) 0.175 µM (-EGF) versus 1.731 µM (+EGF), P<0.05), and the nature of the ATP-induced Ca(2+) transient, corresponding with a 10-fold increase in the mesenchymal marker vimentin (P<0.05). We observed no change in the sensitivity to PAR2-mediated Ca(2+) signaling, indicating that these alterations are not simply a consequence of changes in global Ca(2+) homeostasis. To determine whether changes in ATP-mediated Ca(2+) signaling are preceded by alterations in the transcriptional profile of purinergic receptors, we analyzed the expression of a panel of P2X ionotropic and P2Y metabotropic purinergic receptors using real-time RT-PCR and found significant and specific alterations in the suite of ATP-activated purinergic receptors during EGF-induced EMT in breast cancer cells. Our studies are the first to show that P2X(5) ionotropic receptors are enriched in the mesenchymal phenotype and that silencing of P2X(5) leads to a significant reduction (25%, P<0.05) in EGF-induced vimentin protein expression. CONCLUSIONS: The acquisition of a new suite of cell surface purinergic receptors is a feature of EGF-mediated EMT in MDA-MB-468 breast cancer cells. Such changes may impart advantageous phenotypic traits and represent a novel mechanism for the targeting of cancer metastasis.

Heat shock proteins as novel therapeutic targets in cancer.

Heat shock proteins (HSPs) are evolutionarily conserved molecules synthesised by cells exposed to sub-lethal stresses. Acting as molecular chaperones, HSPs protect cells from environmental stress damage by assisting in proper folding and stabilisation of proteins. In addition, they help to sequester severely damaged proteins for degradation. Owing to the nature of their function, HSPs are often found to be overexpressed in a wide range of cancers. Members of the HSP family have been implicated in cancer growth as promoting tumour cell proliferation as well as inhibiting cellular death pathways. In recent years, several HSP90 client proteins have been validated as clinically important therapeutic targets for treatment of cancer, and inhibitors of HSP90 have emerged as potentially beneficial anticancer agents. This review explores the involvement of HSPs in cancer and the development of several anticancer agents with promising therapeutic applications.