Evaluation of the somatogenic activity of bovine placental lactogen with cell lines transfected with the bovine somatotropin receptor

Studies have shown that bovine placental lactogen (bPL) has partial somatog enic activity in vivo even though binding results clearly indicate bPL does not cause homodimerization of the bovine somatotropin receptor (bST-R). To help understand the receptor binding versus biological activity of bovine somatotropin (bST) and bPL we have developed a homologous model system. Ful l length bSTR was stably transfected into a murine lymphoid cell line, Ba/F 3 and a hamster kidney cell line, BHK. From both transfected cell lines, cl ones were isolated (Ba/F3-C1 and BHK-24) which demonstrated specific bindin g of bST and, or bPL. Bovine ST stimulated proliferation of the Ba/F3-C1 cl onal line over a dose range of 10 to 3000 pM with an EC50 of 100 pM. A bST variant(des 1-4 bST) and porcine ST (pST) which both have approximately 10% of the binding affinity for bST-R as native bST were 1 and 10 % as potent as bST in this bioassay, respectively. This suggests that affinity and biol ogical activity are correlated for this system. Proliferation was initiated through the bST-R because addition of a monoclonal antibody which recogniz es the extracellular domain of bST-R and inhibits binding of bST to its rec eptor, inhibited bST-stimulated mitosis. However, even though the affinity of bPL for the bST-R is similar to that of bST, bPL antagonized the prolife rative action of bST with an IC50 of 1 nM. Components of the somatogenic si gnal transduction pathway were also evaluated in both cell lines. Addition of bST to the cell cultures increased phosphorylation of JAK2 in Ba/F3-C1 a nd BHK-24 cells in a dose-responsive manner but bPL failed to increase phos phorylation of JAK2 in either cell line. In summary, these data support the hypothesis that ST-R homodimerization is necessary for bioactivity in this model system but fail to explain apparent somatogenic activity of bPL in. vivo.