Ca²⁺-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 l-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-tetra-decanoyl phorbol-13-acetate was blocked by the PKC inhibitor stau-rosporine 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 PKCα and PKCβII in the equatorial segment, whereas PKCβI and PKCϵ 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 α and βII, whereas the βI- and ϵ-subspecies might be involved in regulation of flagellar motility of human sperm.