in epithelial cancers has been associated with progression and resistant to
anti-cancer therapies. Several forms of plasticity have been documented,
including epithelial mesenchymal transition (EMT), endothelial-mesenchymal
transition and neuroendocrine-epithelial transition. Cellular plasticity plays
a major role in the progression of cancer and the acquisition of mesenchymal
cancer stem cell-like phenotypes has been correlated with poor prognosis.
The ability of cancer cells to undergo an EMT has been implicated as a major factor driving metastasis, through the acquisition of enhanced migratory and invasive properties. However it is also clear that by undergoing this process the cancer cells become resistant to a number of targeted therapies. Recent retrospective analysis of phase 3 clinical trial samples has revealed that a poorer response to erlotinib in the 2/3rd line setting in NSCLC was associated with a loss of E-cadherin (an epithelial tumor marker), suggesting that tumors that had undergone EMT were less responsive to EGFR-directed therapy. In addition an EMT phenotype has been reported in a number of EGFR-mutant NSCLC patients who have progressed while on erlotinib therapy.
In order to understand the full impact of these clinical observations and identify mechanisms of resistance in mesenchymal tumor cells we have modeled EMT in a number of different ways in vitro. We have used panels of NSCLC cell lines that are in a fixed epithelial or mesenchymal state, induced an EMT with TGFb, or driven an EMT through prolonged exposure to EGFR-TKi targeted therapy (erlotinib). Using large-scale phosphoproteomic and transcriptomic datasets we used a systems biology approach to uncover important observations relating to the role of EMT as a drug-resistance mechanism. Firstly, these models confirm the clinical observations and show that tumor cells that have undergone EMT are less responsive to a number of targeted agents including EGFR and IGF1R-IR directed agents. Secondly, they reveal the plasticity of the EMT process where three distinct stages of EMT: epithelial, ‘metastable’ mesenchymal and ‘epigenetically-fixed’ mesenchymal are observed. Thirdly, upon undergoing EMT tumor cells acquire novel mechanisms of cellular signaling not apparent in their epithelial counterparts. These include receptor tyrosine kinase (RTK) autocrine and paracrine loops, such as PDGFR, FGFR, AXL and integrin a5b1 and up regulation of IL-6 and IL-11 mediated JAK-STAT signaling. Reciprocal activation of PDGFR signaling through EGFR inhibition was observed in the mesenchymal state. Lastly, these models indicate that as part of the EMT process the tumor cells display a CD44high/CD24low cancer stem cell phenotype and show enhanced colony formation.
These observations reinforce the important role that EMT can have in driving drug resistance in tumor cells and highlight the wide diversity of mechanisms that can be used by tumor cells to evade targeted drug therapy. An understanding of these mechanisms and the contexts in which they are most likely to arise will have important implications in driving combinatorial drug therapy in cancer patients in the future.