Cholesterol efflux-mediated signal transduction in mammalian sperm: Cholesterol release signals an increase in protein tyrosine phosphorylation during mouse sperm capacitation

We previously demonstrated that mouse sperm capacitation is accompanied by a time-dependent increase in protein tyrosine phosphorylation that is depen dent on the presence of BSA, Ca2+, and NaHCO3, all three of which are also required for this maturational event. We also demonstrated that activation of protein kinase A (PK-A) is upstream of this capacitation-associated incr ease in protein tyrosine phosphorylation. BSA is hypothesized to modulate c apacitation through the removal of cholesterol from the sperm plasma membra ne. In this report, we demonstrate that incubation of mouse sperm medium co ntaining BSA results in a release of cholesterol from the sperm plasma memb rane to the medium; release of this sterol does not occur in medium devoid of BSA. We next determined whether cholesterol release leads to changes in protein tyrosine phosphorylation. Blocking the action of BSA by adding exog enous cholesterol-SO4- to the BSA-containing medium inhibits the increase i n protein tyrosine phosphorylation as well as capacitation. This inhibitory effect is overcome by (1) the addition of increasing concentrations of BSA at a given concentration of cholesterol-SO4- and (2) the addition of dibut yryl cAMP plus IBMX. High-density lipoprotein (HDL), another cholesterol bi nding protein, also supports the capacitation-associated increase in protei n tyrosine phosphorylation through a cAMP-dependent pathway, whereas protei ns that do not interact with cholesterol have no effect. HDL also supports sperm capacitation, as assessed by fertilization in vitro. Finally, we prev iously demonstrated that HCO3- is necessary for the capacitation-associated increase in protein tyrosine phosphorylation and demonstrate here, by exam ining the effectiveness of HCO3- or BSA addition to sperm on protein tyrosi ne phosphorylation, that the HCO3- effect is downstream of the site of BSA action. Taken together, these data demonstrate that cholesterol release is associated with the activation of a transmembrane signal transduction pathw ay involving PK-A and protein tyrosine phosphorylation, leading to function al maturation of the sperm. (C) 1999 Academic Press.