Differential activation of STATs 3 and 5 during mammary gland development.

We have investigated the activity of STAT family members throughout a mammary developmental cycle. Transcripts for Stat 5 were upregulated during pregnancy whilst STAT1 and STAT3 mRNAs were expressed at constant levels. DNA binding complexes containing both STAT5a and 5b showed differing affinities for two naturally occurring STAT5 binding sites. In the involuting mammary gland STAT5 activity decreased whereas STAT3 was specifically activated. These observations reveal a complex pattern of activation of STAT factors during mammary growth, differentiation and remodelling and provide the first evidence for the involvement of STAT3 in development of the mammary gland.