Identification and characterization of a novel functional estrogen receptor on human sperm membrane that interferes with progesterone effects

The presence of a novel functional estrogen receptor on the human sperm sur face has been demonstrated by using different experimental approaches. Liga nd blot analysis of sperm lysates, using peroxidase-conjugated estradiol as probe, identified a specific estradiol-binding protein of approximately 29 -kDa apparent molecular mass. The same protein band was also revealed by us ing alpha H222 antibody, which is directed against the steroid binding doma in of the genomic estrogen receptor. The biological effects of estrogen rec eptor were investigated by analyzing calcium fluxes, tyrosine phosphorylati on, and acrosome reaction (AR) in response to 17 beta-estradiol (17 beta E- 2) and by measuring the steroid influence on calcium and AR in responses to progesterone (P), a well-known physiological stimulus for human spermatozo a. Our results demonstrate that 17 beta E-2 induces a rapid and sustained i ncrease of intracellular calcium concentrations ([Ca2+](i)). This effect is totally dependent on the presence of extracellular calcium, because it is completely abolished in a calcium-depleted medium. The dose-response curve for calcium increase to 17 beta E-2 is biphasic with a first component in t he nanomolar range (effective concentration 50 = 0.60 +/- 0.12 nmol/L) and a second component in the micromolar range (EC50 = 3.80 +/- 0.26 mu mol/L). 17 beta E-2 stimulates tyrosine phosphorylation of several sperm proteins, including the 29-kDa protein band, and determines a reduction of calcium r esponse to P, finally resulting in inhibition of P-stimulated sperm AR. Con versely, no direct effect of 17 beta E-2 is observed on AR. 17 beta E-2 eff ects on calcium are clearly mediated by a membrane receptor, because they a re reproduced by the membrane-impermeable conjugate of the hormone BSA-E, a nd reduced by sperm preincubation with alpha H222 antibody. Taken together, our results clearly show the presence of a functional surface estrogen rec eptor, of 29 kDa, on human spermatozoa. This receptor may play a role in th e modulation of nongenomic action of P in these cells during the process of fertilization.