Role of epithelial-to-mesenchymal transition (EMT) in drug sensitivity and metastasis in bladder cancer

Abstract

Epithelial-to-mesenchymal transition (EMT) is a process that plays essential roles in development and wound healing that is characterized by loss of homotypic adhesion and cell polarity and increased invasion and migration. At the molecular level, EMT is characterized by loss of E-cadherin and increased expression of several transcriptional repressors of E-cadherin expression (Zeb-1, Zeb-2, Twist, Snail, and Slug). Early work established that loss of E-cadherin and increased expression of MMP-9 was associated with a poor clinical outcome in patients with urothelial tumors, suggesting that EMT might also be associated with bladder cancer progression and metastasis. More recently, we have used global gene expression profiling to characterize the molecular heterogeneity in human urothelial cancer cell lines (n = 20) and primary patient tumors, and unsupervised clustering analyses revealed that the cells naturally segregate into two discrete "epithelial" and "mesenchymal" subsets, the latter consisting entirely of muscle-invasive tumors. Importantly, sensitivity to inhibitors of the epidermal growth factor receptor (EGFR) or type-3 fibroblast growth factor receptor (FGFR3) was confined to the "epithelial" subset, and sensitivity to EGFR inhibitors could be reestablished by micro-RNA-mediated molecular reversal of EMT. The results suggest that EMT coordinately regulates drug resistance and muscle invasion/metastasis in urothelial cancer and is a dominant feature of overall cancer biology.

Fig. 1

Molecular markers of EMT. Epithelial markers are displayed in blue, mesenchymal in red. a Regulation of E-cadherin expression. Upstream signals, and in particular TGFβ, increase expression of transcriptional repressors of E-cadherin (Zeb-1, Zeb-2, Twist, Snail, and Slug) that function by binding to two so-called “E-box” elements that are located within the E-cadherin promoter. This results in recruitment of histone deacetylases and possibly histone and DNA methyltransferases to the E-cadherin promoter, resulting in transcriptional silencing. Members of the miR200 family of micro-RNAs directly antagonize this process by binding to multiple copies of specific “seed” sequences located within the mRNAs encoding Zeb-1 and Zeb-2, resulting in transcript degradation and inhibition of translation. b. Other representative markers of the “epithelial” and “mesenchymal” states. The lists provided are not comprehensive, and the potential mechanistic relationships between these markers and E-cadherin signaling has in most cases not been determined

Fig. 2

Relationship of EMT to the two tracks of bladder cancer progression. We compared the expression of four “mesenchymal” markers and E-cadherin in gene expression profiles obtained from superficial and muscle-invasive urothelial cancers using data from two publically available datasets. Fold changes and statistical signficance are indicated. Note that upregulation of the mesenchymal markers and downregulation of E-cadherin are significantly associated with muscle-invasive disease *Fold change: Invasive tumors/Superficial tumors.