Possible Involvement of PI3K in Prolactin-Stimulated Milk Product Formation and Iodide Transport in Mouse Mammary Explants

Abstract

The in vitro effect of wortmannin, an inhibitor of PI3 kinase, on prolactin (PRL) stimulated P70^S6K, iodide transport, and milk product synthesis were investigated in cultured mouse mammary tissues. Mouse mammary gland explants were initially incubated for 24 hr in media M199 containing 1 μg/ml insulin and 10^-7 M Cortisol. A subsequent treatment with wortmannin impeded, in a dose-dependent fashion, the PRL stimulation of casein, lipid, and lactose synthesis as well as the PRL stimulation of iodide transport. Rapamycin (25 ng/ml), an inhibitor of P70^S6K, also inhibited the effect of PRL on iodide transport; this drug was earlier shown to inhibit PRL effects on milk product synthesis. These results suggest the possible involvement of p20^s6K and p13-kinase in PRL-stimulated milk product formation and iodide transport in mouse mammary explants. Since wortmannin caused a diminished cellular content of P70^S6K and a reduced extent of P70^S6K migration in polyacrylamide gels (likely due to dephosphorylation), PI3-kinase likely lies upstream in the PRL signaling pathway for P70^S6K activation.