Induction of ZEB Proteins by Inactivation of RB Protein Is Key Determinant of Mesenchymal Phenotype of Breast Cancer

We previously showed that depletion of the retinoblastoma protein (RB) induces down-regulation of the adhesion molecule E-cadherin and thereby triggers the epithelial-mesenchymal transition. To further characterize the effect of RB inactivation on the phenotype of cancer cells, we have now examined RB expression in human breast cancer cell lines and clinical specimens. We found that RB-inactive cells exhibit a mesenchymal-like morphology and are highly invasive. We also found that ZEB proteins, transcriptional repressors of the E-cadherin gene, are markedly up-regulated in these cells in a manner sensitive to the miR-200 family of microRNAs. Moreover, depletion of ZEB in RB-inactive cells suppressed cell invasiveness and proliferation and induced epithelial marker expression. These results implicate ZEB in induction of the epithelial-mesenchymal transition, as well as in maintenance of the mesenchymal phenotype in RB-inactive cells. We also developed a screening program for inhibitors of ZEB1 expression and thereby identified several cyclin-dependent kinase inhibitors that blocked both ZEB1 expression and RB phosphorylation. Together, our findings suggest that RB inactivation contributes to tumor progression not only through loss of cell cycle control but also through up-regulation of ZEB expression and induction of an invasive phenotype.