Morphine suppresses the immune function of lung cancer by up-regulating MAEL expression

BACKGROUND: Patients with cancer rely on morphine for analgesia, while studies have indicated morphine can induce immunosuppression in cancer. Therefore, investigating the immunosuppressive roles and molecular mechanism of morphine on lung cancer progression is imperative. METHODS: Lactate dehydrogenase (LDH) release assay was used to determine the cytotoxicity of morphine to lung cancer cells. The percentage of CD4(+) and CD8(+) T cells was detected by flow cytometry. In addition, Maelstrom (MAEL), Nrf2, and PTEN were determined by western blot and RT-qPCR. Immune factors programmed death-ligand 1 (PD-L1), transforming growth factor (TGF-β), interleukin (IL)-10, and IL-2 were determined by western blot and ELISA assay. RESULTS: Morphine increased the levels of PD-L1, TGF-β, and IL-10, while decreased IL-2 level. Morphine enhanced MAEL expression in A549 cells and H460 cells. Morphine up-regulated Nrf2 and down-regulated PTEN, and morphine-induced MAEL up-regulation was reversed by PTEN. However, MAEL silencing inhibited the enhanced effects of morphine on cell viability and proliferation of A549 cells. Furthermore, morphine treatment reduced the LDH release and the percentage of CD8(+) T cells, and increased the ratio of CD4(+)/CD8(+) T cells and tumor weight. Meanwhile, MAEL silencing reversed the effects of morphine on immune factors (PD-L1, TGF-β, IL-10, and IL-2), the percentage of CD8(+) T cells, and the ratio of CD4(+)/CD8(+) T cells. CONCLUSION: Morphine activated MAEL in lung cancer cells by Nrf2/PTEN pathway and regulated the immune factors, thereby promoting tumor immune escape. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s40360-022-00632-z.