Regulation of sperm EGFR by ouabain leads to initiation of the acrosome reaction
Haim Breitbart, Limor Daniel-Dafna, Shirel Weiss, Debby Itzkovitz & Nir Etkovitz
It is well accepted that sperm acrosome reaction occurs after the binding of the capacitated sperm to the egg zona-pellucida. This study describes a novel idea regarding the regulation of the sperm epidermal growth factor receptor (EGFR) under physiological conditions and the relevance to the acrosome reaction. Ouabain, a known Na/K ATPase blocker is present in our blood and in the female reproductive tract. We showed here for the first time that physiological concentrations (nano-molar) of ouabain enhance phosphorylation of EGFR on tyr-845, stimulate Ca2+ influx and induce the occurrence of the acrosome reaction in sperm. These effects could be only seen if very low concentrations of EGF (0.1 ng/ml or 0.016 nM), were added together with ouabain. The three processes are inhibited by EGFR blocker suggesting that trans-activation of the EGFR is involved. Moreover, our data revealed that protein kinase A and the tyrosine kinase Src, two known kinases involved in EGFR activation in sperm, mediate the acrosome reaction induced by ouabain. Ouabain alone (without adding EGF) at relatively high concentration (10 μM) could enhance EGFR phosphorylation, Ca2+ influx and acrosome reaction, and the three processes were inhibited by EGFR blockers, further indicating that ouabain induce the acrosome reaction by a mechanism mediated by the trans-activation of EGFR. In conclusion, this study represents an interesting regulation of EGFR by physiological concentration of ouabain and EGF found in the female reproductive tract. None of these compounds can activate the EGFR when present at very low doses, however when both are present, the interaction of ouabain with the Na/K ATPase leads to the priming of the EGFR which undergoes its full activation by EGF.