TGF-β and Hypoxia/Reoxygenation Promote Radioresistance of A549 Lung Cancer Cells through Activation of Nrf2 and EGFR

Although many studies have examined the roles of hypoxia and transforming growth factor-(TGF-) beta separately in the tumor microenvironment, the effects of simultaneous treatment with hypoxia/reoxygenation and TGF-beta on tumor malignancy are unclear. Here, we investigated the effects of redox signaling and oncogenes on cell proliferation and radioresistance in A549 human lung cancer cells in the presence of TGF-beta under hypoxia/reoxygenation conditions. Combined treatment with TGF-beta and hypoxia activated epidermal growth factor receptor (EGFR) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2), a redox-sensitive transcription factor. Interestingly, Nrf2 knockdown suppressed the effects of combined treatment on EGFR phosphorylation. In addition, blockade of EGFR signaling also suppressed induction of Nrf2 following combined treatment with hypoxia and TGF-beta, indicating that the combined treatment induced positive crosstalk between Nrf2 and EGFR. TGF-beta and hypoxia/reoxygenation increased the accumulation of reactive oxygen species (ROS), while treatment with N-acetyl-l-cysteine abolished the activation of Nrf2 and EGFR. Treatment with TGF-beta under hypoxic conditions increased the proliferation of A549 cells compared with that after vehicle treatment. Moreover, cells treated with the combined treatment exhibited resistance to ionizing radiation (IR), and knockdown of Nrf2 increased IR-induced cell death under these conditions. Thus, taken together, our findings suggested that TGF-beta and hypoxia/reoxygenation promoted tumor progression and radioresistance of A549 cells through ROS-mediated activation of Nrf2 and EGFR.