Nuclear localisation of the transcription factor Stat5b is associated withovine milk protein gene expression during lactation but not during late pregnancy or forced weaning

Localisation patterns of the transcription factor Stat5b in the udders from pregnant, lactating and involuting ewes were compared with the expression patterns of two major milk protein genes alpha-lactalbumin and alpha(S1) ca sein. Stat5b was detected in the cytoplasm and nuclei of epithelial cells a t all stages of mammary gland development. A consistent positive relationsh ip between the nuclear localisation of Stat5b in lactating mammary alveolar epithelial cells, and the presence of milk protein gene mRNA was apparent during lactation and early involution. Conversely, there was little evidenc e of nuclear localisation of Stat5b in non-lactating mammary alveolar epith elial cells during lactation and early involution. This supports the observ ation that during lactation, Stat5b may play a role in milk protein gene ex pression. However, during pregnancy and later involution, while Stat5b was observed to be present in mammary epithelial cell nuclei and cytoplasm, no relationship between this and the presence of milk protein gene mRNA was ap parent. This suggests that during late pregnancy and in later involution, S tat5b may be involved in processes other than initiation of milk protein ge ne transcription.