Research On The Colorectal Cancer
CRC is a very heterogeneous type of cancer, subdividing the tumors into molecular subtypes aids in prognostic and therapeutic predictions in a more patient specific way. Coincidentally, all classification systems reported in literature, show that CRCs classified with a mesenchymal-like phenotype and high stromal influx have a worse overall and relapse free survival. In addition, this subset of cancers poorly respond to adjuvant chemo- or radiotherapy. It is of high importance to find a therapeutic strategy against the poor prognosis and highly resistant CMS4 subtype of CRC. The onesize-fits-all chemotherapy is long overdue and quite ineffective against CMS4. As described above, CMS4 CRCs have a mesenchymal-like phenotype (loss of E-cadherin, gain of Vimentin/N-cadherin etc.) as well as high stromal invasion. Several theories circulate upon the resistance mechanism of (CMS4) mesenchymal transitioned CRC cells. The question is whether this resistance against chemotherapy is due to EMT, so the mesenchymal-like phenotype of the cell and gained stem cell-like properties, or due to the TME properties.
Activation of the tumor-suppressor TGF-β (CMS4 marker) in early stage cancers inhibits proliferation16,20. This could contribute to the resistance toward chemotherapy (in early stage CMS4), because the basic mechanism of chemotherapy relies on inducing cytotoxicity in fast dividing cells51.
This feature of chemotherapy might also be the reason why dormant metastatic cells are unaffected by chemotherapy and result in relapse. The switch of transforming TGF-β from a tumor suppressor into an proto-oncogene is a result of an additional mutation in p5352. Which leads to increased proliferation, EMT, invasiveness and metastatic properties. TGF-β seems to be the key player in the autocrine loop and cross-talk between CAFs, endothelial-, immune-, and cancer cells. Not to mention its role as inducer of EMT, which is necessary for metastasis. TGF-β pathway activation is shown to be successfully inhibited and several therapeutic agents are now in phase I clinical trials including antiTGF-β or anti-TGF-β-receptor monoclonal antibodies, small molecules targeting the TGF-β-receptor, anti-sense oligonucleotides, chimeric proteins and vaccines. Inhibiting TGF-β itself, or the receptor would maybe stop the autocrine loop of the TGF-β signaling pathway. However, it may be associated
with the adverse side-effect of influencing normal stromal cells. In clinical trials, Galunisertib, a TGF-β receptor inhibitor results in cardiac toxicity. They resolved this issue by using Galunisertib in an intermittent pattern as a single agent or in combination with chemotherapy against glioblastomas, pancreatic and liver cancer53. The use of TGF-β inhibitors must be used with caution due to the dual role of TGF-β in cancer proliferation. As well as the fact that TGF-β is involved in wound healing and epithelial homeostasis suggest that long term use might not be feasible in the clinic. On top of that, the possibility exists that TGF-β inhibition may even be tumor promoting.
Yang et al. showed in an in vitro study that CRC cell-lines (HT29 and KM12L4) whom acquire resistance towards Oxaliplatin, undergo EMT and have an 8-15-fold increase in invasion and migration35. Cell-line based experiments lack the stromal factor, suggesting that intracellular triggered EMT could indeed contribute to chemo-resistance. In an EMT-lineage tracking breast cancer mouse model established by Fischer et al., EMT and non-EMT FACS-sorted lung metastasis of breast tumors were cultured in vitro. Cultured EMT sorted cells, with a confirmed EMT-status by RT-qPCR, showed higher resistance towards cyclophosphamide.
Notably, here the stromal component was missing in the in vitro setting, yet the cells maintained their mesenchymal-like phenotype. Suggesting that not only the stroma is responsible for sustaining the mesenchymal phenotype, but also that there must be an underlying intrinsic alteration sustaining the mesenchymal phenotype. Keeping in mind that this was a breast cancer model and not a CRC model, underlying genetic differences could be the reason for maintained EMT. Nonetheless, it still shows mesenchymal cell specific resistance towards chemotherapy. The mechanism of chemo-resistance in these mesenchymal transformed cell lines is due to decreased proliferation rate, apoptotic tolerance and elevated expression of chemo-resistant genes and drug metabolizing enzymes including drug transporters, aldehyde dehydrogenases (ALDH), cytochrome p450s, and glutathione metabolism related enzymes. Chemotherapy is toxic to cells with low ALDH levels, increase of ALDH expression contributes to the chemo resistance in these cells28. In addition, elevated ALDH activity is a cancer stem cell (CSC) marker, supporting the stem cell-like properties mesenchymal cells have, resulting in increased therapy resistance. A suggestion counteracting the acquired resistance would be a therapeutic combination of chemotherapy with an ALDH inhibitor.
