Thiram inhibits human sperm motility and capacitation by increasing mitochondrial O2- and reducing PTK and PAK4 activity at environmental concentrations

Background Environmental chemicals may suppress sperm motility (SM), hyperactivation, and capacitation, thus contributing to male infertility. This investigation was designed to examine the impacts of thiram on human SM and capacitation. Methods We incubated the fungicide thiram with normal human spermatozoa to investigate sperm metrics. Results Thiram markedly suppressed SM and hyperactivation, demonstrating half maximum effective concentration (EC ₅₀ ) values below 5 M. In addition, thiram inhibited reactive oxygen species and the formation of H ₂ O ₂ with EC ₅₀ values of 0.9 and 1.4 µM, respectively. It markedly increased mitochondrial O ²⁻ with an EC ₅₀ value of 0.9 µM. Sperm DNA fragmentation remained unaffected at concentrations up to 10 M. The thiram-induced reduction in SM and forward motility (FM) proved to be irreversible. Neither forskolin, 8Br-cAMP, pentoxifylline, progesterone, vitamin E, nor A23187 was capable of preventing the thiram-induced decline in SM and FM. Thiram suppressed spontaneous capacitation processes. Additional investigation revealed that thiram reduced tyrosine protein kinase levels with an EC ₅₀ value of around 0.27 M, decreased p21-activated kinase 4 levels with an EC ₅₀ value of around 0.69 M, and induced mitochondrial structural damage in normal sperm at concentrations as low as 2.5 M. Conclusion Thiram causes irreversible inhibition of human SM, FM, and capacitation through elevation of mitochondrial superoxide anion levels and reduction of tyrosine protein kinase levels and p21-activated kinase 4 activity, while compromising mitochondrial ultrastructural integrity.