Cellular plasticity
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.