STAT5 activation linked to B1 intergrin protein abundance in differentially milked bovine mammary glands

Prolactin (PRL) is important in the regulation of milk synthesis in mammary epithelial cells (MEC). In cattle the circulating levels of PRL are not limiting, suggesting the possible involvement of other factors that may control the response to PRL at the cellular level. The effects of milking frequency on milk synthesis are controlled locally within mammary glands and involve PRL signaling. To investigate this, udder halves of 10 dairy cows were milked either four-times-daily (4x) or once-daily (1x) for 14 days in early lactation. Mammary biopsies were obtained from both udder halves and changes in PRL and associated pathways were measured. The abundance of STAT5A mRNA was higher after 4x milking, whereas the mRNA abundance PRL receptor and STAT3 were lower. In 4x mammary tissues, the protein levels of STAT5, activated STAT5 and β1-integrin were higher whereas the protein levels of the long isoform of PRL receptor and activated STAT3 were lower. The activation of STAT5 correlated strongly with major milk protein gene mRNA abundance and β1-integrin protein levels. These results confirm that major milk protein gene expression is associated with STAT5 activation and also suggests that the STAT5 and β1-integrin signaling pathways are linked. Modulation of β1-integrin abundance in response to changes in milking frequency may be a mechanism that controls the MEC ability to respond to PRL and therefore its secretory activity.