Effect of rapamycin on prolactin-stimulated S6 kinase activity and milk product formation in mouse mammary explants

Abstract

The in vitro effects of rapamycin on prolactin (PRL)-stimulated S6 kinase activity and milk product synthesis were investigated in cultured mouse mammary tissues. Mouse mammary gland explants were initially incubated for 24 h in M199 media containing 1 μg/ml insulin and 10-7 M cortisol. A subsequent treatment of the tissues with 1 μg/ml PRL for 12 h caused a 98% increase in S6 kinase activity in the cytosolic fraction; effects were not observed at earlier times. PRL at or above 500 ng/ml was needed to elicit a maximum stimulation of S6 kinase activity, and this response was specific for lactogenic hormones (PRL, hPL, hGH). Rapamycin, an inhibitor of 70 K S6 kinase, was employed to assess the possible physiological role of S6 kinase in the PRL stimulation of milk product formation. Rapamycin (25-100 ng/ml) impeded, in a dose-dependent fashion, the PRL stimulation of casein, lipid and lactose synthesis in concert with its inhibition of cytoplasmic S6 kinase activity. These results suggest a possible role for the activation of 70 K kinase in the signalling pathway for the PRL regulation of milk product synthesis in the mammary gland.