Although antineoplastic agents have an important role in the treatment of NSCLC, their treatment efficiency is commonly low due to the tumor cell resistance. The membrane transport proteins play a key role in drug metabolism. In particular, the drug efflux process mediated by ATP-binding cassette (ABC) transporter plays a major role in the chemotherapy drug resistance in cancer cells . A large body of work has been conducted on the drug resistance-related Multi-drug resistance 1 (MDR1), multidrug resistance-associated protein (MRP) and ATP-binding cassette transporter 2 (ABCG 2), which function through different mechanisms [12–16].
Molecular mechanisms of tumor cell resistance were another hot point. It was found that molecular pathological pathways of lung cancer were related with potential drug influence also. Structural of EGFR changes leading to the activating properties. Insertions in exon 19 are likely to respond to TKI therapy . Lung adenocarcinoma with predominant SMPC (stromal invasive micropapillary component) may be associated with a poor prognosis and have different phenotypic and genotypic characteristics .
ATPase ion pump is an ATP-dependent active transport carrier, which transports Na+, K+, H+, Ca2+ and Cu2+ out of the cells and organelles. V-ATPase is a macromolecular complex enzyme of ATPase and is expressed on the vacuolar membrane of cytoplasm (microsome), as well as the cell membrane. V-ATPase produces or maintains the transmembrane electrochemical ionic gradient that is related to the accumulation, intracellular distribution and sensitivity of the anticancer drugs . The overexpression of V-ATPase in tumor cells is of great significance to the maintenance of cytoplasmic alkaline environment, promotion of tumor cell growth, improvement of the extracellular acid environment, promotion of cell invasion and metastasis [8, 20]. Furthermore, it induces the invasive phenotype in the tumor cells [21, 22]. The transport capacity of ABCG2 for methotrexate, folic acid, mitoxantrone and topotecan can be enhanced under low pH conditions in tumor cell lines, such that at pH 5.5, the transport capacity of ABCG2 for drugs is five times higher than that of the normal conditions [23, 24]. The drug resistance could be possibly related to the changes in the pH gradient between the extracellular environment and the cytoplasm. It is believed that V-ATPase plays a major role in the regulation of intracellular pH value [25–27]. Available data on V-ATPase have mainly been reported based on the plant studies [28, 29] with the exception of a few reports on liver, breast, pancreatic, esophageal, gastric carcinomas as well as melanoma cells [8–11]. However, no reports are available on the correlation between the V-ATPase expression in NSCLC and the drug resistance in the relevant cancer tissues.
In this study, the results of the immunohistochemical and immunofluorescence assays showed that V-ATPase was overexpressed in NSCLC, while its expression rate was significantly lower in the adjacent normal tissue (data not shown). In terms of different histological types of NSCLC, the V-ATPase expression rate was 71.43% and 83.72% in squamous cell lung cancer and lung adenocarcinoma. The Mann–Whitney rank sum test indicated that there were significant differences in the V-ATPase expression rate between the two cancer tissues (P < 0.001). The rank sum test among different grades of squamous cell lung cancer and different differentiation degrees of adenocarcinoma further showed significant differences (P = 0.014 and 0.012, respectively). These results suggested that was overexpressed in NSCLC. Furthermore, it was demonstrated that the V-ATPase expression rate of lung adenocarcinoma was higher than that of the squamous cell lung cancer and higher in grade ΙΙΙ adenocarcinoma than that of the well-differentiated adenocarcinoma. In clinical practice, it is commonly found that lung adenocarcinoma is more resistant to chemotherapy drugs than squamous cell lung cancer, and that the resistance level is often related to the degree of malignancy. Together these findings indicated that the V-ATPase expression could be potentially related to the drug resistance in NSCLC.
In order to further examine the relationship between the V-ATPase expression and the drug resistance in NSCLC, we carried out the cancer tissue drug sensitivity test. Spearman rank correlation analysis was performed for the results of the drug sensitivity test of cyclophosphamide, gemcitabine, doxorubicin, paclitaxel and cisplatin in NSCLC tissues and for the V-ATPase expression in the corresponding tissues. All the obtained P-values were less than 0.05 and all the coefficients of rank correlation were less than −0.30. The correlation was found to be higher in lung adenocarcinoma as compared to that of the squamous cell lung cancer. However, this correlation was not significant for paclitaxel when tested separately in squamous cell lung cancer and lung adenocarcinoma. Overall, there was a negative correlation between the V-ATPase expression and drug sensitivity in the NSCLC tissues. Therefore, the V-ATPase expression had a strong positive correlation with the drug resistance in the NSCLC tissues. This finding suggested that the V-ATPase expression could be related to the drug resistance in the NSCLC tissues. The V-ATPase expression rate had a stronger correlation with the drug resistance in the lung adenocarcinoma tissue as compared to that of the squamous cell lung cancer tissue.
Although this study did not prove that the expression of V-ATPase was directly linked to the drug resistance of cancer tissues, other experiments have already shown that the activity of V-ATPase was increased in multidrug-resistant cell lines. Its subunit genes were upregulated under the action of antineoplastic agents [30, 31]. Therefore, we speculated that V-ATPase was related to the drug resistance in tumor tissues. However, further experiments are needed to unravel the exact mechanism.