Raf plus TGFβ-dependent EMT is initiated by endocytosis and lysosomal degradation of E-cadherin

Abstract

Oncogenic Ras interferes with adhesive functions of epithelial cells, but requires tumor growth factor β (TGFβ) signaling to cause epithelial–mesenchymal transition (EMT) and tumor progression in model systems. To investigate the mechanisms by which Ras and TGFβ pathways cooperate in EMT induction, we introduced a tamoxifen-inducible version of Raf-1 (RafER) into fully polarized, mammary epithelial cells (EpH4). EMT characterized by loss of E-cadherin expression and upregulation of invasiveness-promoting genes was induced by TGFβ plus 4-hydroxytamoxifen (4HT) activation of RafER. Downregulation of E-cadherin by RafER plus TGFβ was detectable in total cell lysates after 48 h and much earlier in detergent-insoluble fractions of E-cadherin. Both pathways cooperated to strongly enhance endocytosis of E-cadherin, mainly via the clathrin-dependent route. Pulse-chase experiments showed decreased E-cadherin protein stability in cells stimulated with TGFβ and 4HT and increased E-cadherin half-life in the presence of monensin. Monensin and chloroquine prevented E-cadherin degradation to different extent, but only monensin effectively blocked the loss of E-cadherin from the junctional complexes. Both lysosome inhibitors caused accumulation of E-cadherin vesicles, some of which were positive for Cathepsin D and lysosome-associated membrane protein 1 (LAMP-1). In addition, TGFβ and mitogen-activated protein kinase hyperactivation synergistically induced E-cadherin ubiquitination, suggesting that the cooperation of Raf and TGFβ favors lysosomal degradation of E-cadherin instead of its recycling. Our data indicate that early stages of EMT involve cooperative, post-translational downregulation of E-cadherin, whereas loss of E-cadherin via transcriptional repression is a late event in EMT.