Epithelial to Mesenchymal Transition: A Mechanism that Fuels Cancer Radio/Chemoresistance

Abstract

Epithelial to mesenchymal transition (EMT) contributes to tumor progression, cancer cell invasion, and therapy resistance. EMT is regulated by transcription factors such as the protein products of the SNAI gene family, which inhibits the expression of epithelial genes. Several signaling pathways, such as TGF-beta1, IL-6, Akt, and Erk1/2, trigger EMT responses. Besides regulatory transcription factors, RNA molecules without protein translation, micro RNAs, and long non-coding RNAs also assist in the initialization of the EMT gene cluster. A challenging novel aspect of EMT research is the investigation of the interplay between tumor microenvironments and EMT. Several microenvironmental factors, including fibroblasts and myofibroblasts, as well as inflammatory, immune, and endothelial cells, induce EMT in tumor cells. EMT tumor cells change their adverse microenvironment into a tumor friendly neighborhood, loaded with stromal regulatory T cells, exhausted CD8⁺ T cells, and M2 (protumor) macrophages. Several EMT inhibitory mechanisms are instrumental in reversing EMT or targeting EMT cells. Currently, these mechanisms are also significant for clinical use.

Keywords: Krüppel-like factors (KLFs); MRX34; NRF2; PD-L1; neurotrophin; silibinin.

Figure 1

Main EMT regulatory mechanisms. EMT is a product of internal and external events in cells. It is mainly induced by external factors, such as TGF-β1, neurotrophins, or interleukin-6, which activate several signaling pathways such as WNT, Smads, STATs, Akt, or Erk1/2. The activated pathways drive transcription factors (Goosecoid, Snail, Slug, Twist, nuclear factor erythroid-2-related factor 2 (NRF2), and Krüppel-like factors (KLFs)) to bind to special responding sequences in DNA, and, consequently, these mainly suppress the expression of genes related to epithelial differentiation. The best-known event in this process is the down-regulation of E-cadherin by the SNAI transcription factors, Snail, and Slug. At the same time, mesenchymal gene products, such as vimentin, are induced during the transcriptional changes in the EMT process, where the Smad, HIF-1α, and KLF4 transcription factors are directly involved [43,44]. The transcription regulatory process during EMT does not only rely on transcription factors, but also on long noncoding RNAs (lncRNAs) and microRNAs. The external EMT-inducing factors, such as TGF-β1, neurotrophins, or interleukin-6, are available from the fibroblastic and inflammatory microenvironment, but tumor cells also actively participate in the completion of an EMT program [45]. Based on the experimental data of several research groups, including ours, it has been found that acidic extracellular conditions and, in a further step, a low oxygen tension (hypoxia) are necessary for EMT. Interestingly, EMT tumor cells produce immune checkpoint regulatory molecules and actively remodel their immune microenvironment into a more tumor-friendly one from a hostile neighborhood.

Figure 2

The TGF-β1 gradient causes heterogeneity in the tumor cell nests. There are higher TGF-β1 concentrations near the tumor vasculature, and the tumor cell nest has a decreasing gradient from its border towards the middle, which generates heterogeneity between the tumor–stroma interface and the tumor cell nest center [47]. The TGF-β1 gradient contributes to the heterogenic architecture of the tumor cell nest, where the invasive front has a completely different functional and differentiation profile than the core of the tumor cell nest. Additionally, at the invasive front, tumor cells interact with fibroblasts, leukocytes, and other stromal cells. Some scattered tumor cells undergo EMT at the tumor cell nest–stroma interface and contain both epithelial and mesenchymal markers. The figure is based on a figure published by Mayorca-Giuliani and Erler [48], which has been updated by the published data of Nieto et al. [29].

Figure 3

Vimentin represents an extended fibroblastic component in a primary oral squamous cell carcinoma (OSCC). Tissue section of human OSCC with the detection of vimentin (magenta) and pan-cytokeratin (green) and cell nuclei counterstained with DAPI (blue). Tumor cells form nests with a decreasing representation of the epithelial differentiation marker pan-cytokeratin from the middle of the tumor cell nests to their border. Among the tumor cell nests, the stroma contains large amounts of vimentin⁺ fibroblasts. This image was taken by the TissueFaxs^TM system (Tissuegnostics Vienna), and the bar represents 100 µm.

Figure 4

Multinuclear giant cells are therapy surviving intermediates. Hostile environmental conditions and hypoxia, in concert with EMT triggering factors, might induce EMT cells and polyploid giant cancer cells (PGCCs). These cells are also produced by serum-free culture conditions using conditioned media of oral squamous cell carcinoma cells and fibroblasts at pH = 6.7. These giant cells are also frequently the surviving cells of cultured OSCC cell lines during cisplatin or mitomycin C treatments. After the treatment cycles, the giant cells contribute to the expansion of the cell culture of normal epithelial tumor cell nests.