In Vitro Effect of Melatonin on the PD-L1 Ligand in A549 and A549 Neuroendocrine Phenotype Cell Lines

Neuroendocrine tumors (NETs) are considered between the most aggresive cancers; NETs develop in several tissues particularly gastrointestinal tract, pancreas, thyroid and lungs; lung cancer is also the most mortal type globally. There are limited models for NETs, although the neuroendocrine phenotype of adenocarcinoma A549 cells (A549NED) has been studied, and has been shown to produce biogenic amines in a differential pattern. This neuroendocrine phenotype of lung cancer cells regulates the immune response in vitro and in vivo, although the complete molecular pathways to achieve the immunomodulation have not been studied. In this regard, the transmembrane protein death ligand 1 (PD-L1) is overexpressed in some types of cancer cells, giving them the ability to camouflage themselves against the immune system. Nowadays, therapeutic has taken advantage of this molecular target, with well documented safety and efficacy for PD-L1 inhibitors in neuroendocrine lung cancer tumors. On the other hand, melatonin can exert immunomodulatory effects and is considered a significant molecule for the neuro-immuno-endocrine axis. It has also been proposed as a promising agent to modulate the tumor microenvironment. In this work we evaluated the effect of melatonin on the neuroendocrine phenotype of A549 cells deprived from fetal bovine serum in the culture media or with 2% of the supplementation; the expression of PD-L1 was evaluated in these conditions. There was an inhibitory effect in proliferation exerted by 2.5 mM melatonin, in both neuroendocrine phenotype models of A549 cells at 24 and 48 hours. The expression of PD-L1, melatonin receptors and KRAS was assessed by rt-PCR and flow cytometry (PD-L1 only) demonstrating for the first time the presence of PD-L1 in the neuroendocrine phenotypes of A549. Our results suggest that in these conditions melatonin supplementation could not prevent PD-L1 expression, but it had an anti-proliferative effect on A549 and A549NED cells, therefore making it viable as an adyuvant therapy in lung cancer.