EXPRESSION OF GROWTH HORMONE-BINDING PROTEIN WITH A HYDROPHILIC CARBOXYL-TERMINUS BY THE MOUSE PLACENTA - STUDIES IN-VIVO AND IN-VITRO

GH-binding protein (GHBP) or GH receptor is present in numerous extrah epatic tissues in the rodent. From mid- to late gestation in the mouse , the maternal serum concentration of GHBP increases 30- to 50-fold. W e have investigated whether the placenta might synthesize GHBP and pot entially contribute to this increase. RNA was isolated from placentas and subjected to Northern analysis using a cDNA probe to the shared re gion of GHBP and GH receptor-encoding mRNAs. From day 8 to day 18 of g estation, the GHBP-encoding mRNA (1.4 kb) increased 45-fold in quantit y. The GH receptor-encoding mRNA (4.2 kb) increased sixfold by day 14 and then remained steady until day 18. These changes which were not co -ordinated parallel reported changes in the steady-state concentration s of 1.4 and 4.2 kb mRNAs in maternal liver, suggesting shared regulat ory factors. Extracts of freshly isolated trophoblasts were assayed fo r GHBP with a radioimmunoassay specific for GHBP with a hydrophilic ca rboxyl terminus. The cytosolic content of immunoreactive GHBP increase d fourfold from mid- to late gestation. Trophoblasts were isolated fro m placentas and cultured for 2 days on collagen gels in defined medium . Cultured cells were at least 90% viable and secreted mouse placental lactogen-II (mPL-II). Immunocytochemistry was carried out simultaneou sly on cells cultured from day 7 to day 17 of gestation using a monocl onal antibody (MAb 4.3), which recognizes the hydrophilic C-terminus o f GHBP. Cell-localized GHBP was present in trophoblasts cultured for 2 days, but GHBP was not detectable by radioimmunoassay or by immunopre cipitation in concentrated culture media from cultures treated with 10 0 ng mouse GH/ml or 100 ng mPL-II/ml or from untreated cultures. RNA w as isolated from cells cultured in an identical manner to those analys ed by immunocytochemistry. Three GH receptor/GHBP mRNA species of 8, 4 .2 and 1.4 kb were observed. The quantity of 4.2 and 1.4 kb mRNAs did not change significantly in cultures from day 7 to day 15 of gestation but, in cultures from day 17 of gestation, the amount of 14 kb mRNA d ropped significantly, while that of the 42 kb mRNA remained unchanged. GHBP- and GH receptor-encoding mRNAs are not co-ordinately regulated in vivo or in vitro. Although mPL-II was secreted into the medium by c ultured trophoblasts, secretion of GHBP could not be detected. The cul ture medium may not contain the specific factors required for secretio n of placental GHBP, or placental GHBP may not be destined for secreti on. The results show that GHBP (as distinct from GH receptor) is expre ssed by the placenta in viva and trophoblasts in vitro. From mid-gesta tion onwards, GHBP mRNA increases dramatically in vivo and the cytosol ic content of GHBP in freshly isolated trophoblasts increases. This su ggests an important local regulatory role for placental GHBP during ge station.