Stimulation of protein tyrosine phosphorylation by a progesterone receptor on the cell surface of human sperm

Mature human sperm initiate a rapid Ca2+ influx and the acrosomal exocytosis in response to progesterone. Recent evidence indicates that both events can be induced by antibody-mediated cross-linking of a sperm surface progesterone receptor. In many other systems in which signal is generated by receptor cross-linking, protein phosphorylation on tyrosine residues is involved in the signal transduction across the plasma membrane. In this study we examined whether tyrosine phosphorylation is implicated in the function of the sperm surface progesterone receptor, too. The effect of progesterone on the phosphorylation of proteins from a sperm membrane lysate was evaluated by in vitro kinase assay and by phosphoamino acid analysis using [gamma-32P]ATP as precursor. These experiments revealed a selective increase in the tyrosine phosphorylation of a 94-kilodalton phosphoprotein in the presence of progesterone. To decide whether the progesterone-induced increase in protein tyrosine phosphorylation is actually due to the hormone action on the cell surface, living sperm were treated with a cell-impermeant progesterone receptor agonist, and the resulting changes in the cellular level of phosphotyrosine proteins were examined. These experiments showed a clear relationship between the agonist binding and an increase in the phosphotyrosine concentration in the respective cells. This relationship was lost in the presence of genistein, which also efficiently inhibited the phosphorylation of the 94-kilodalton protein and the progesterone-induced acrosomal exocytosis. These results lead to the hypothesis that protein tyrosine phosphorylation is involved in signal transduction through the sperm surface progesterone receptor and may be implicated in nongenomic steroid effects in other cell types.