Expression of biologically active rat prolactin in mammalian COS-1 cells

Rat prolactin (PRL) cDNA was constructed in mammalian expression vector, pSVL. Transient expression of rat PRL was performed in COS-1 cells by the DEAE-dextran method. The production of recombinant rat PRL started within 48 h from the cells and reached the level of 1.0-1.5 micrograms/ml/5 x 10(5) cells. The molecular size of recombinant rat PRL was the same as that of standard rat PRL (Mw: 23,000), suggesting successful removal of the signal peptide. The radioimmunoassay and isoelectric focusing analysis showed that recombinant rat PRL has almost the same immunological and biochemical characteristics as those of standard rat PRL. As biological tests, receptor-binding activity, Nb 2 node lymphoma cell growth activity, and mammary gland stimulating activity were examined. The radioreceptor assay showed that recombinant rat PRL has binding activity to mammary microsomal membrane similar to that of standard rat PRL. Recombinant rat PRL also stimulated the growth of Nb 2 lymphoma cells as standard rat PRL. Finally it was shown that recombinant rat PRL promotes the synthesis of the secretory materials in the lumen of mouse mammary gland with the same potency as that of standard rat PRL. In conclusion, recombinant rat PRL, which was produced in mammalian cells in the present experiment, has immunological, biochemical and biological characteristics similar to those of standard PRL, and has full bioactivity.