Ethacrynic acid suppresses B7-H4 expression involved in epithelial-mesenchymal transition of lung adenocarcinoma cells via inhibiting STAT3 pathway

Lung cancer is characterized by high incidence and mortality. 90% of cancer deaths are caused by metastases. The epithelial-mesenchymal transition (EMT) process in cancer cells is a prerequisite for the metastatic process. Ethacrynic acid (ECA) is a loop diuretic that inhibits the EMT process in lung cancer cells. EMT has been related to the tumour immunemicroenvironment. However, the effect of ECA on immune checkpoint molecules in the context of cancer has not been fully identified. In the present study, we found that sphingosylphosphorylcholine (SPC) and TGF-β1, awell-known EMT inducer, induced the expression of B7-H4 in lung cancer cells. We also investigated the involvement of B7-H4 in the SPC-induced EMT process. Knockdown of B7-H4 suppressed SPC-induced EMT, while B7-H4 overexpression enhanced EMT of lung cancer cells. ECA inhibited SPC/TGF-β1-induced B7-H4 expression via suppression of STAT3 activation. Moreover, ECA inhibits the colonization of mice lung by tail vein-injected LLC1 cells. ECA-treated mice increased the CD4-positive T cells in lung tumour tissues. In summary, these results suggested that ECA inhibits B7-H4 expression via STAT3 inhibition, leading to SPC/TGF-β1-induced EMT. Therefore, ECA might be an immune oncological drug for B7-H4-positive cancer, especially lung cancer.