Biosynthesis of selenoproteins in cultured bovine mammary cells

The biosynthesis of selenoproteins was studied in relation to milk formation and mammary cell biology by incubating the bovine mammary cell line MAC-T with (75Se)selenite. Intracellular proteins and proteins secreted into the cell culture medium were separated by 2D electrophoresis, the selenoproteins were detected by autoradiography, and the proteins were identified by MALDI-TOF. Approximately 35 75Se-containing spots were found in the cell proteins from MAC-T cells. Among them, one-third showed high intensity. The strongest spot was identified as glutathione peroxidase 1. About 20 spots were observed in protein precipitated from cell culture medium, one-third of them being distinctly visible. In an attempt to study a perturbation of the system, the effect of retinoic acid (RA) on the formation of selenoproteins was investigated. The concentration of 75Se in total cell protein was reduced by about 35% in cells cultured with RA compared with control cells, while the opposite effect was observed in protein precipitated from cell culture medium, which contained 60% more 75Se in RA-treated samples than in controls. There were also indications that RA might affect different selenoproteins in different ways. The methods described provide a promising approach for further studies of the regulation of selenoprotein formation in the mammary gland.