LAUSR

Abstract Sperm cells can be damaged during the semen cryopreservation process, decreasing their fertilizing ability. Physical damage and oxidative stress may occur during the freeze–thawing process. Antioxidants such as the native antioxidant melatonin can potentially improve cryopreservation outcomes. In this study, we added melatonin to cryoprotectant to examine its effect on frozen–thawed human sperm. We found that adding 0.1 mM melatonin to cryoprotectant significantly increased sperm viability (24.80 ± 0.46% vs. 20.97 ± 1.27%, P < 0.05) and membrane integrity (P < 0.05), and decreased intracellular reactive oxygen species and lipid peroxidation damage. Furthermore, mRNA levels of the transcription factor NF‐E2‐related factor‐2 and its downstream genes were significantly increased. Resistance to oxidative stress was enhanced and expression of the antiapoptotic gene Bcl‐2 was increased by inclusion of 0.1 mM melatonin in the cryoprotectant. Moreover, 0.1 mM melatonin upregulated the expression of heat shock protein 90 (HSP90), which confers resistance to stressors in frozen–thawed sperm. Results obtained upon addition of inhibitors of melatonin receptors (luzindole and 4‐P‐PDOT) and an HSP90 inhibitor (geldanamycin) in the cryoprotectant demonstrated that melatonin promoted HSP90 translation via the melatonin receptor MT1 and increased adenosine triphosphate levels, thus increasing the viability of thawed sperm. HighlightsMelatonin increases motility and membrane integrity of frozen–thawed sperm.Melatonin decreases the ROS content of frozen–thawed human sperm.Melatonin promotes HSP90 expression in frozen–thawed human sperm via MT1.