Binding of placental lactogen and growth hormone to fetal sheep fibroblasts

Growth hormone (GH) regulates growth and development in the postnatal period but lacks somatotropic activity in the fetus. In contrast, the placental hormone placental lactogen (PL) stimulates amino acid transport, DNA synthesis, and somatomedin production in isolated fetal tissues, suggesting that PL may function as a "fetal GH." To elucidate the mechanisms by which PL exerts GH-like effects in fetal tissues, we examined the binding of PL, GH, and prolactin to cultured skin fibroblasts obtained from midgestational fetal lambs. Ovine fetal fibroblasts bound radiolabeled ovine PL (oPL) specifically and with high affinity (EC50 0.20 nM). In competitive displacement assays using 125I-oPL as the radioligand, the potency of unlabeled oPL was eight to 12 times greater than that of ovine GH and congruent to 1000 times greater than that of ovine prolactin. Covalent cross-linking of 125I-oPL (22 kD) to ovine fetal fibroblasts revealed a specific hormone-receptor complex with an apparent M(r) of 130,000, suggesting that the high affinity oPL binding site has a molecular mass of approximately 108 kD. The specific bindings of radiolabeled ovine GH (0.6% per 250 micrograms protein) and ovine prolactin (0.04% per 250 micrograms protein) were only 1/15 and 1/230 that of radiolabeled oPL (9.1% per 250 micrograms protein), and no specific cross-linking of 125I-ovine GH or 125I-ovine prolactin to ovine fetal fibroblasts was detected. These findings demonstrate preferential binding of PL by isolated fetal sheep fibroblasts in culture, providing a cellular mechanism whereby PL may exert growth-promoting effects in the fetus.