Currently, first-line treatment for patients with EGFR mutated advanced NSCLC could prescribe single targeted agents such as osimertinib, gefitinib and erlotinib. For lung cancer patients with T790M mutation, osimertinib as first-line treatment or as subsequent treatment after initial targeted therapy should be given priority. However, osimertinib may still face the problem of drug resistance after a period of therapeutic time. The mechanism of osimertinib resistance is very complex. The most important mechanism is C797S mutation of EGFR gene. C797S mutation is located in the EGFR tyrosine kinase region, which can inhibit the effect of osimertinib and mediate the drug resistance of lung cancer cells. For C797S mutation, EAI045 has been developed to overcome osimertinib resistance.
For patients without C797S mutation, it has been reported that HER2 or c-Met amplification pathways are involved in osimertinib resistance. Abnormal activation of HER2 can activate downstream pathways such as PI3K/Akt and MEK/MAPK through the formation of homologous or heterologous dimer, which leads to abnormal proliferation of tumor cells and the production of osimertinib resistance; c-met amplification bypasses EGFR activation of downstream PI3K/Akt mediated signal pathway, leading to resistance to osimertinib. The activation of RAS, a downstream signaling pathway, can also lead to osimertinib resistance. In addition to these reasons, there are still many unknown mechanisms of osimertinib resistance, especially in the front-line settings. Therefore, illustrating the resistance mechanism of osimertinib and overcoming the resistance problem of the third generation EGFR-TKIs have become one of the research focuses for NSCLC.
Till date, there were several resistance mechanisms in previous studies such as FLAURA and AURA3. However, it was still not clear whether other mechanisms of acquired resistance of osimertinib existed when used as front line treatment. As were showed in previous studies, EMT can be resistant to traditional anticancer drugs and EGFR-TKIs. EMT can promote the growth, migration and metastasis of tumor cells. It has been found that EMT tumor cells own the abilities of self-renewal, unlimited proliferation and anti-apoptosis, and highly express CD133, CD44 + and ABCG2, which is very similar to the characteristics of cancer stem cells, which may be the potential mechanism of osimertinib resistance.
In our study, we found that in osimertinib resistant NSCLC cells, the expression level of EMT related protein E-cadherin was lower than that of sensitive cells, while the expression level of ID1 and vimentin was higher than that of sensitive cells. In the study, ID1 expression level was closely related to E-cadherin and vimentin both in osimertinib sensitive and resistant cells. Alteration of ID1 expression in H1975/OR cells could change the expression of E-cadherin. Downregulating ID1 expression of H1975/OR cells could promote the apoptosis induced by osimertinib and block cell cycle at G1/G0 stage. Our study indicated that ID1 may induce EMT in T790M positive NSCLC, which mediates drug resistance of osimertinib. These results may compensate the pre-existing osimertinib resistance mechanisms.
In previous studies, we know that ID1 can participate in liver metastasis of lung cancer cells through EMT and knockout of ID1 may lead to decreased expression of vimentin, TGF-β, Snail. Down-regulation of ID1 expression can not only inhibit EMT formation, but also induce tumor apoptosis. It has been reported that ID1 knockout in ovarian cancer cells can significantly inhibit the growth and invasion of tumor cells, and promote the apoptosis of tumor cells. In colon cancer HCT116 cells, ID1 inhibited apoptosis induced by chemotherapy drugs and ultraviolet light. In small cell lung cancer, it has been reported that the high expression of ID1 can significantly inhibit the apoptosis of tumor cells. Our study is highly in line with results of these former studies.
In head and neck cancer cells, snail-induced EMT enables cancer cells to maintain their tumor stem cell-like properties, thus increasing resistance of chemotherapy and invasiveness. When tumor cells produce EMT, they often secrete more cytokines such as CXCL9 and CXCL10, which can inhibit NK cells' killing function on tumor stem cells, thus promoting tumor immune escape. Our study found that in T790M mutant NSCLC, increased ID1 expression can mediate EMT formation, inhibit cell apoptosis and promote cell proliferation.Down-regulation of ID1 expression can block cell cycle at G1/G0 stage. After ID1 expression level changes, does the proportion of tumor stem cells change? At present, there is no report that ID1-mediated EMT promotes the formation of tumor stem cells to regulate the immune escape of T790M mutated lung cancer. We will conduct more studies to investigated these mechanisms.
However, our study may also have several deficiencies. Firstly, the role of ID1 in osimertinib resistance was only explored in H1975 and H1975/OR cells. It is hard to confirm whether ID1 is definitely related to drug resistance of osimertinib in other cell lines. Therefore, further studies is warranted to include more NSCLC cell lines harboring T790M mutation. Secondly, due to shortage of time and funding, several experiments were not investigated in the current study such as the correlation of ID1, EMT downstream signaling pathway and immune escape after osimertinib resistance. Furthermore, our study was only conducted in vitro, but in vivo experiments such as nude mouse xenograft model were not performed. Hence, the role of ID1 need to be carefully verified in animal experiments. Last but not the least, our study did not involved in human specimens, which made the experiment results a little pale as compared to other similar researches. As a consequence, further exploration of ID1 in T790M positive lung cells is needed.
In conclusion, our study reveal the mechanism of ID1 mediated resistance to osimertinib in T790M positive non-small cell lung cancer through EMT, which may provide new ideas and methods for clinical treatment of EGFR mutated NSCLC after osimertinib resistance. Further studies are needed to illustrate the deep mechanism of osimertinib resistance as first-line targeted therapy.