Abstract
Compared with other milk proteins, αS1-casein (encoded by CSN1S1) is associated with higher rates of allergies in humans. Although some studies have revealed that CSN1S1 affects a variety of cellular physiological processes such as immune response and proliferation in various cells, whether CSN1S1 regulates other milk proteins in ruminants is not known. In this study, we observed a negative Pearson correlation between the contents of αS1-casein and β-casein in goat milk. Thus, we used isolated primary goat mammary epithelial cells along with adenoviral infection or small interference RNA to alter abundance of CSN1S1 and examine its role in milk protein synthesis regulation. Overexpressing CSN1S1 through adenoviral transfection decreased β-casein mRNA (CSN2) and protein abundance, whereas interference of CSN1S1 resulted in a significant increase in β-casein abundance. CSN1S1 reduced phosphorylation level of Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5a (STAT5a). The transcription factor STAT5a activates CSN2 transcription by binding with its promoter region and then promotes β-casein synthesis. Furthermore, CSN1S1 inhibited CSN2 promoter activity and β-casein synthesis by modulating the JAK2/STAT5a signaling pathway. No changes in abundance were detected for αS2-casein (CSN1S2), κ-casein (CSN3) and major whey proteins (LALBA, BLG). Overall, our results underscored a mechanism whereby CSN1S1 inhibits β-casein synthesis via inhibition of STAT5a. The data suggested that knocking down CSN1S1 not only reduced the content of αS1-casein, but also increased the abundance of major milk proteins including β-casein. Thus, the present study provides a theoretical basis for manipulating αS1-casein mRNA to improve quality of ruminant milk for human consumption.