Impaired redox adaptation through SLC38A5-dependent glutamine metabolism contributes to melatonin-mediated anoikis sensitivity and metastasis suppression in bone cancer.

Malignant bone cancer, primarily osteosarcoma and chondrosarcoma, are highly aggressive neoplasms originating from bone tissue and characterized by a strong propensity for lung metastasis. Tumor cells evade anoikis through reactive oxygen species (ROS)-mediated redox homeostasis, which modulates signaling cascades that promote proliferation, survival, and metastatic invasion into distant sites. Melatonin, synthesized primarily by the pineal gland, has been implicated in cancer prevention and therapy due to its inhibitory effects on bone cancer growth and progression. Little is known about the mechanisms underlying anoikis resistance in bone cancer cells or whether melatonin can therapeutically modulate this process. We demonstrated that glutamine metabolism is essential for bone cancer cells to maintain anoikis resistance. Melatonin treatment disrupted glutamine metabolism and altered redox homeostasis, as evidenced by increased ROS accumulation and reduced NADPH/NADP⁺ ratios under anchorage-independent conditions. Notably, solute carrier family 38 member 5 (SLC38A5), a glutamine transporter, was identified as a critical regulator of bone cancer progression, with higher SLC38A5 expression correlating with poorer clinical outcomes. Melatonin suppressed SLC38A5 expression and attenuated anoikis resistance through inhibition of the PI3K-Akt signaling pathway. Consistently, reduced SLC38A5 expression was associated with decreased lung metastasis in melatonin-treated groups in an orthotopic mouse model. Collectively, our findings reveal a previously unrecognized role of melatonin in modulating glutamine-dependent redox balance and anoikis resistance in bone cancer. This study highlights SLC38A5-mediated glutamine metabolism as a critical determinant of metastatic potential and supports melatonin and SLC38A5 as promising therapeutic targets for osteosarcoma and chondrosarcoma.