The role of epithelial to mesenchymal transition in resistance to epidermal growth factor receptor tyrosine kinase inhibitors in non-small cell lung cancer

Abstract

Inhibition of the epidermal growth factor receptor (EGFR) is an important strategy when treating non-small cell lung cancer (NSCLC) patients. However, intrinsic resistance or development of resistance during the course of treatment constitutes a major challenge. The knowledge on EGFR-directed tyrosine kinase inhibitors (TKIs) and their biological effect keeps increasing. Within the group of patients with EGFR mutations some benefit to a much higher degree than others, and for patients lacking EGFR mutations a subset experience an effect. Up to 70% of patients with EGFR mutations and 10-20% of patients without EGFR mutations initially respond to the EGFR-TKI erlotinib, but there is a severe absence of good prognostic markers. Despite initial effect, all patients acquire resistance to EGFR-TKIs. Multiple mechanisms have implications in resistance development, but much is still to be explored. Epithelial to mesenchymal transition (EMT) is a transcriptionally regulated phenotypic shift rendering cells more invasive and migratory. Within the EMT process lays a need for external or internal stimuli to give rise to changes in central signaling pathways. Expression of mesenchymal markers correlates to a bad prognosis and an inferior response to EGFR-TKIs in NSCLC due to the contribution to a resistant phenotype. A deeper understanding of the role of EMT in NSCLC and especially in EGFR-TKI resistance-development constitute one opportunity to improve the benefit of TKI treatment for the individual patient. Many scientific studies have linked the EMT process to EGFR-TKI resistance in NSCLC and our aim is to review the role of EMT in both intrinsic and acquired resistance to EGFR-TKIs.

Keywords: Epidermal growth factor receptor (EGFR); epithelial to mesenchymal transition (EMT); non-small cell lung cancer (NSCLC); tyrosine kinase inhibitor (TKI).

Figure 1

EMT induces a cellular phenotypic shift. Epithelial cells are characterized by the presence of apico-basal polarity, intact cell-cell contacts, barrier function, and lack of movement. During the EMT process, cells with partial EMT appear characterized by the incipient presence of a front-rear polarity and have leaky cell-cell contacts. At the same time upregulation of the mesenchymal markers vimentin, N-cadherin, ZEB1 and 2, SNAIL, SLUG and matrix metalloproteinases begin. In addition, these partial EMT cells express lower amounts of E-cadherin and ZO-1 than epithelial cells. After finalization of EMT, the established mesenchymal cells have a clear front-rear polarity and no or few cell contacts. The expression of mesenchymal proteins is increased, and a cadherin gene-expression shift, most often from E-cadherin into N-cadherin expression, appears. EMT, epithelial to mesenchymal transition.

Figure 2

Models for induction of EMT during erlotinib resistance development. Above: erlotinib treatment induces increased IGF1R activation, which restores signaling through PI3K/AKT and MAPK pathways. Restoration of the signaling triggers transcription of mesenchymal transcription factors and leads to an IGF1R-dependent partial or full EMT. Eventually some cells might only depend on EMT signals in order to survive and hence no longer rely on IGF1R-signaling. Below: most cancer cells produce TGF-β, but TGF-β has both tumor-promoting and tumor-suppressing roles at early stages. When the cells are treated with erlotinib, the balance may shift so that cells primarily benefit from the tumor-promoting TGF-β signals. TGF-β initiates EMT through inhibition of miR-200 family and hence upregulation of mesenchymal transcription factors. In addition, TGF-β upregulates the EGFR repressor MIG6 and restores PI3K/AKT signaling. Secondly, PI3K/AKT signaling can activate a kinase shift, giving rise to the expression of another RTK. Hereby, MAPK signaling is restored and EMT solidified (40). EMT, epithelial to mesenchymal transition; TGF-β, transforming growth factor-β; EGFR, epidermal growth factor receptor; RTK, receptor tyrosine kinase.

Figure 3

Inhibition of EMT in NSCLC cells. (A) The HDAC inhibitor entinostat maintains an open chromatin structure and the transcription of E-cadherin. Entinostat hereby enforces the epithelial state and prevent EMT; (B) selumetinib inhibits MEK and thus the MAPK pathways. As the MAPK pathway is involved in receptor tyrosine kinase signaling as well as TGF-β signaling, selumetinib has potential both as a first line treatment and as a treatment upon resistance development. EMT, epithelial to mesenchymal transition; NSCLC, non-small cell lung cancer; HDAC, histone deacetylase; TGF-β, transforming growth factor-β.