The activity of transcription factor Stat5 responds to prolactin, growth hormone, and IGF-I in rat and bovine mammary explant culture

Signal transducer and activator of transcription-5 (Stat5) is known to play a critical role in prolactin-induced p-casein gene transcription in rodent s. In nonmammary cells, Stat5 is activated by multiple hormones and cytokin es, including growth hormone. We hypothesized that Stat5 may serve as a com mon point in the signal transduction pathways of hormones that promote milk protein gene expression in bovine mammary cells, which are regulated by GH and IGF-I in addition to prolactin. Assays for Stat5 DNA binding activity and protein were validated in mammary explant culture. The Stat5 protein ab undance was not changed by any of the short-term hormonal treatments used i n our study, suggesting that short-term regulation of Stat5 is predominantl y at the level of protein activation. Both rat and bovine explant culture s howed a rapid stimulation of Stat5 DNA binding activity by prolactin, GH, a nd IGF-I at the high concentrations typically used in explant cultures as w ell as at levels within physiologic ranges. Growth hormone stimulated Stat5 activity at a lower concentration in bovine than in rat cultures, but in b oth species the presence of GH increased the response of Stat5 activity to prolactin. These results suggest that transcription factor Stat5 may repres ent part of a common route by which different extracellular signals converg e and are transduced intracellularly to coordinately regulate cell function in the mammary gland.