Secondary metabolites of mulberry leaves exert anti-lung cancer activity through regulating the PD-L1/PD-1 signaling pathway

Lung cancer ranks the top of malignancies that cause cancer-related deaths worldwide.The leaves of Morus alba L are traditional Chinese medicine widely applied in respiratory diseases.Our previous work has demonstrated the anti-lung cancer effect of secondary metabolites of mulberry leaf,but their mechanism of action has still not fully elucidated.We synthesized Moracin N(MAN)-Probe conjugated with alkyne to label lung cancer cells and identified protein targets by chemical proteomic analysis.MAN and its probe exerted similar growth-inhibitory effect on human lung cancer cells.Chemical proteomic results showed that MAN targeted the programmed death ligand 1(PD-L1)checkpoint pathway and T cell receptor(TCR)signaling pathway,indicating its immune-regulatory function.Cell-free surface plasmon resonance(SPR)results showed the direct interaction of MAN with PD-L1 protein.Molecular docking analysis demonstrated that MAN bound to E158 residue of PD-L1 protein.MAN downregulated the expression levels of PD-L1 in a time-and dose-dependent manner and disrupted the PD-L1/programmed death 1(PD-1)binding,including other secondary metabolites of mulberry leaves Guangsangon E(GSE)and Chalcomoracin(CMR).Human peripheral blood mononuclear cells(PBMCs)co-cultured with MAN-treated A549 cells,resulting in the increase of CD8^(+)GZMB^(+)T cells and the decrease of CD8^(+)PD-1^(+)T cells.It suggested that MAN exerts anti-cancer effect through blocking the PD-L1/PD-1 signaling.In vivo,MAN combined with anti-PD-1 antibody significantly inhibited lung cancer development and metastasis,indicating their synergistic effect.Taken together,secondary metabolites of mulberry leaves target the PD-L1/PD-1 signaling,enhance T cell-mediated immunity and inhibit the tumorigenesis of lung cancer.Their modulatory effect on tumor microenvironment makes them able to enhance the therapeutic efficacy of immune checkpoint inhibitors in lung cancer.