CA-2+-INDEPENDENT INDUCTION OF ACROSOME REACTION BY PROTEIN-KINASE-C IN HUMAN SPERM

We report that activated protein kinase C (PKC) can induce acrosome reaction independently of elevated Ca2+. Addition of 12-O-tetradecanoyl phorbol-13-acetate or the membrane-permeable diacylglycerol analog 1-oleoyl-2-acetylglycerol to ejaculated human sperm resulted in stimulation of acrosomal reaction (2- to 3-fold), provided the sperm underwent capacitation. Induction of acrosome reaction by 12-O-tetradecanoyl phorbol-13-acetate was blocked by the PKC inhibitor staurosporine or by down-regulation of endogenous PKC, but not by removal of extracellular Ca2+. Acrosome reaction was also enhanced by the Ca2+ ionophore ionomycin in a Ca2+-dependent, PKC-independent fashion. Immunohistochemical analysis with type-specific PKC antibodies revealed the presence of PKCalpha and PKCbetaII in the equatorial segment, whereas PKCbetaI and PKCepsilon staining was found in the principal piece of the tail. Acrosome reaction, thus far believed to be induced only by elevated Ca2+, can therefore be triggered by activated PKC in a Ca2+-independent fashion. The PKC subtypes potentially involved in acrosome reaction are most likely a and betaII, whereas the betaI- and epsilon-subspecies might be involved in regulation of flagellar motility of human sperm.