Characteristics of the Prolactin Stimulation of c-fos mRNA Levels in Mouse Mammary Gland Explants

Abstract

The signal transduction pathway(s) for the prolactin (PRL) regulation of lactogenic processes in the mammary are not fully known. Recent studies indicate that the PRL occupancy of its receptor activates a tyrosine kinase (JAK-2), which is constitutively associated with the receptor. In the present studies, we have characterized the PRL stimulation of c-fos mRNA accumulation in mouse mammary gland explants that were precultured with insulin and cortisol for 36 hr. In time course studies, an initial effect (2-fold increase) of PRL was apparent after 15 min, a 4-fold increase occurred after 60 min, and a 2-fold increase was apparent after 2 hr. Dose-response experiments indicated an initial effect with 5 ng/ml PRL and a maximum effect with 500 ng/ml PRL. The accumulation of c-fos; mRNA in the cultured mammary tissues was also increased by human GH, recombinant bovine PRL, cycloheximide, and phorbol 12-myristate 13-acetate (TPA). The cycloheximide and PRL responses were additive, thus indicating that cycloheximide did not "superinduce" the PRL stimulation of c-fos mRNA accumulation. Downregulation of protein kinase C (PKC) by pretreating mammary tissues for 1 day with TPA did not affect the magnitude of the PRL stimulation of c-fos mRNA accumulation. The findings from these studies are consistent with c-fos playing a role in the PRL stimulation of lactogenic processes in the mammary gland. PRL had no effect on c-jun mRNA accumulation under any of the experimental conditions employed in the c-fos studies.