Melatonin Alleviates the Toxicity of High Nicotinamide Concentrations in Oocytes: Potential Interaction with Nicotinamide Methylation Signaling.

Despite the numerous studies on melatonin and nicotinamide (NAM, the active form of vitamin B3), the linkage between these two biomolecules in the context of signaling pathways regulating preimplantation embryo development has not yet been investigated. In this study, we used bovine oocyte model to elucidate the effect of melatonin on the developmental competence of oocytes under the stress of high NAM concentrations. Results showed that NAM (20 mM) administration during in vitro maturation (IVM) significantly reduced oocyte maturation and actin distribution, while induced reactive oxygen species (ROS) accumulation and mitochondrial dysfunction, the multiple deleterious effects that were alleviated by melatonin (10-7 M). The RT-qPCR and/or immunofluorescence showed upregulation of the apoptosis (Caspase-3, Caspase-9, and BAX), autophagy (Beclin-1, LC3A, LC3B, ATG7, LAMP1, and LAMP2), cell cycle (P21, P27, and P53), and DNA damage (COX2 and 8-OxoG) specific markers in oocytes matured under NAM treatment, compared to NAM-melatonin dual-treated and the untreated ones. In addition, the total cleavage and blastocyst development rate, as well as the total number of cells and the inner cell mass (ICM) per blastocyst, were reduced, while DNA fragmentation was induced, in the group of NAM sole treatment than NAM-melatonin cotreatment and control. Inspecting the underlying mechanisms behind NAM-associated toxicity revealed an increase in transcription pattern of NAM methylation (NNMT and AHCY) genes in NAM-treated oocytes while the opposite profile was observed upon melatonin supplementation. In conclusion, to our knowledge, this is the first study reporting that melatonin can protect oocytes and embryos from NAM-induced injury through its ROS-scavenging activity together with potential interaction with NAM methylation signaling.