Stat5 phosphorylation status and DNA-binding activity in the bovine and murine mammary glands

The transcription factors Stat5a and Stat5b are mediators of prolactin sign alling in mammary epithelial cells, and are thought to play a role in lacto genesis, In cultured cells, activation of stat5 activity through phosphoryl ation results in Stat5 binding to the promoters of at least some of the mil k protein genes, thereby stimulating their transcription. However, the mamm ary biology of Stat5 differs between species, and the role of Stat5 in the bovine mammary gland is not fully understood. We have generated an antibody that specifically recognises the phosphorylated forms of Stat5a and Stat5b and used it to compare the levels of phosphorylated Stat5 with Stat5 DNA-b inding activity in bovine and murine mammary tissue. Both Stat5 DNA-binding activity and phosphorylation status in the bovine mammary gland were at ne ar-maximal levels at late pregnancy (27-35 days prior to calving), when at least three of the major milk proteins are not highly expressed. In additio n, these studies revealed significant animal-to-animal variation in the lev el of Stat5 activity in both species. The results are consistent with a rol e in terminal differentiation of mammary epithelial cells. They also sugges t that the stimulation of high-level expression of milk protein genes in th e bovine mammary gland is not through activation of the prolactin receptor- Jak2-Stat5 pathway. (C) 2001 Elsevier Science Ireland Ltd. All rights reser ved.