Distribution of selenoproteins in mouse mammary epithelial cells in vitro and in vivo.

Resolution of selenium-containing proteins synthesized by mouse mammary gland cells was achieved using the technique of two-dimensional gel electrophoresis. Radioactive selenium as H2[75Se]O3 was incorporated into relatively few proteins within mammary gland cells maintained in vitro and cells of mammary gland tissue in vivo. The pattern of selenoproteins obtained was identical qualitatively between a nontumorigenic differentiated cell line, COMMA-D, and a tumorigenic cell line, MOD. Eleven selenoproteins ranging in molecular weight from 12,000-78,000 were detected and a total of 25 spots were visible indicating charge heterogeneity of some of the proteins. A major selenoprotein (Mr 26,000) migrated identically with the subunit form of glutathione peroxidase, a well-characterized protein containing four selenocysteine residues. Other major selenoproteins had molecular weights of 58,000, 22,000, 18,000, and 14,000. Analysis of the total cellular protein extract and of each of the five major proteins indicated that selenium was incorporated as selenocysteine in the proteins. Incorporation of selenium as selenomethionine into cellular proteins was detected only when selenomethionine was provided in the culture medium. Cleavage of 75Se-labeled proteins with N-chlorosuccinimide produced polypeptides of different molecular weights indicating that the Mr 58,000, 26,000, and 22,000 selenoproteins were dissimilar in the amino acid sequences containing the selenoamino acid. The pattern of selenoproteins of mammary gland cells in vivo was similar to that obtained for cells in culture and most other tissues in vivo. These results provide evidence for the presence of multiple selenium-containing proteins in mammary epithelial cells. The possible significance of these proteins in selenium-mediated inhibition of cell growth awaits future clarification.