Anti-tumour effect of melatonin in pancreatic adenocarcinoma through neutrophil modulation

Abstract

Pancreatic adenocarcinoma (PAAD) is a highly lethal form of cancer worldwide. Unfortunately, the long-term survival rate remains alarmingly low, hovering around 13%. One of the main challenges is that PAAD often remains asymptomatic until reaching advanced stages, making early diagnosis difficult. Consequently, surgical removal, which is the only effective curative option, may no longer be viable by the time the disease is detected. As a result, systemic therapies become the primary approach. Despite significant progress in the development of anti-cancer drugs in recent decades, PAAD remains highly resistant to treatments, with only a few drugs showing satisfactory responses in patients. The prognosis of PAAD patients is significantly influenced by the tumour microenvironment. Proper regulation within this environment has the potential to improve patient survival. On the other hand, melatonin, an endogenous hormone produced by the pineal gland and various organs, plays a crucial role in regulating sleep-wake cycles. The duration and quality of sleep greatly affect melatonin production, and therefore melatonin is often regarded as a natural dietary supplement. Melatonin exhibits diverse biological activities and contributes significantly to immune modulation, antioxidative stress responses, and anti-tumour functions in the body. In our study, the impact of melatonin on PAAD was investigated using mice models. It was discovered that melatonin supplementation effectively inhibits tumour growth, while blocking the melatonin receptor pathway worsens tumour progression. Remarkably, this anti-tumour effect is independent of cytotoxicity but closely associated with tumour-associated neutrophils (TANs). Depletion of TANs using an anti-Ly6G antibody reverses the effects of melatonin. Melatonin induces infiltration and activation of TANs, leading to apoptosis of PAAD cells. Cytokine arrays reveal that this induction originates from melatonin-treated tumour cells secreting CXCL2, rather than directly affecting tumour-associated macrophages or TANs. Knockdown of CXCL2 in tumour cells eliminates melatonin-induced neutrophil migration and activation. The melatonin-induced neutrophils exhibit an N1-like anti-tumour phenotype, characterized by phenotypic changes and increased formation of neutrophil extracellular traps (NETs). These NETs cause tumour cell apoptosis through direct cell-to-cell contact. Next, proteomics analysis reveals that this reactive oxygen species-mediated inhibition is fuelled by fatty acid oxidation (FAO) in neutrophils, and inhibiting FAO abolishes the anti-tumour effect. Finally, analysis of PAAD patient specimens reveals lower melatonin levels in tumour tissues compared to adjacent tissues. Pancreatic melatonin levels are associated with patients' overall survival. Consistently, CXCL2 expression correlates with neutrophil infiltration in patients' tumours. Combining CXCL2 or TANs with NET markers predicts a better prognosis for patients. In summary, an anti-tumour mechanism of melatonin involving the recruitment of N1-neutrophils and the beneficial formation of NETs was revealed in this study.