Partial hydatidiform moles have impaired differentiated function (human chorionic gonadotropin and human placental lactogen secretion) in response to epidermal growth factor and 8-bromo-cyclic adenosine monophosphate

OBJECTIVE

The null hypothesis is that partial hydatidiform moles have normal differentiated function (human chorionic gonadotropin and human placental lactogen secretion) in response to epidermal growth factor and 8-bromo-cyclic adenosine monophosphate.

STUDY DESIGN

Two complete moles, 10 partial hydatidiform moles, and 19 normal first-trimester placentas in monolayer culture were exposed to 10 ng/ml epidermal growth factor, 1 mmol/L 8-bromo-cyclic adenosine monophosphate plus 1 mmol/L theophylline, or control. Human chorionic gonadotropin and human placental lactogen secretion was measured. Frequency of response to stimuli was compared by chi 2 analysis, and hormone secretion was compared by analysis of variance.

RESULTS

Partial moles demonstrated reduced frequencies of response of human chorionic gonadotropin and human placental lactogen to epidermal growth factor (partial moles 2/8 and 2/8, respectively; normal placentas 16/19 and 7/18, respectively; p less than 0.025) and of human chorionic gonadotropin to 8-bromo-cyclic adenosine monophosphate (partial moles 3/5, normal placentas 13/16; p less than 0.005).

CONCLUSION

Partial hydatidiform moles demonstrate impaired human chorionic gonadotropin and human placental lactogen secretory responsiveness to epidermal growth factor and cyclic nucleotides in comparison with normal first-trimester trophoblast.

Pituitary-placental interaction: hypophysectomy modulates the secretion of mouse placental lactogen-I

To determine whether the pituitary gland influences the concentration of mouse placental lactogen-I (mPL-I) in maternal serum, pregnant Swiss Webster mice were hypophysectomized or sham-operated on day 9 of gestation. Blood was collected on days 10-13 and 18, and the mPL-I concentration of the serum was measured by RIA. The serum mPL-I concentration of hypophysectomized mice was significantly higher than that of sham-operated and intact mice on days 10-13. There was no difference in mPL-I concentration on day 18 of pregnancy among the groups. Similar elevations in serum mPL-I concentration were observed when hypophysectomy was performed on day 10 of pregnancy. Steady state levels of placental mPL-I messenger RNA (mRNA) were analyzed by Northern hybridization. No differences were observed in the amount of mPL-I mRNA among hypophysectomized, sham-operated, and intact mice. These results demonstrate that the pituitary gland exerts inhibitory control over the maternal serum mPL-I concentration. This control does not appear to be affected at the level of steady state amounts of mPL-I mRNA.

Initiation of placental lactogen-I production by blastocysts growing on a two-dimensional surface and in hanging drops

The mouse blastocyst undergoes a program of protein secretion during the perimplantation period including the initial production of placental lactogen-I (mPL-I) on day 6 of pregnancy. Although blastocysts collected on day 5 also produce mPL-I when cultured to form outgrowths on plastic dishes, it was not known if embryos have an intrinsic ability to produce mPL-I during culture in vitro, or if a specific uterine influence is necessary. Earlier experiments also suggested that attachment of the trophoblast to a stable surface may be a prerequisite for synthesis of mPL-I. Both questions were addressed by examining mPL-I production by day 3 and day 4 embryos cultured either on a two-dimensional tissue culture dish surface or in hanging drops. The presence of intracellular mPL-I was assayed by immunohistochemistry, while the secreted hormone was detected by its known position in two-dimensional electrophoresis gels. These experiments demonstrated that these earlier stage embryos do have an intrinsic program for mPL-I production which proceeds in vitro under various culture conditions. Synthesis of mPL-I occurred in embryos suspended in hanging drops as well as in those spreading on a solid two-dimensional surface, thus showing that adhesion to a surface is not required for production of this hormone. Although some mPL-I synthesis was seen in embryos cultured in medium containing BSA as the sole macromolecule, the inclusion of fibronectin either in dishes, where it supports attachment, or in the hanging drop system stimulated mPL-I secretion. Serum supplementation in both culture systems further increased growth and differentiation of the embryo, as well as mPL-I secretion, compared to fibronectin supplementation.

Molecular cloning and expression of rat placental lactogen-Iv, a variant of rPL-I present in late pregnant rat placenta

Late pregnant rat placenta was found to contain a messenger RNA (mRNA) that encodes an additional member of the prolactin-growth hormone family. This polypeptide was detected by hybridization of complementary DNA (cDNA) for rPL-I to a 1 kilobase mRNA transcript in late pregnant rat placenta. A cDNA clone for the new polypeptide was isolated from a phage lambda-gt10 library containing cDNA synthesized from day 18 rat placental mRNA. Sequencing of the day 18 cDNA clone revealed that it was most closely related structurally to rPL-I and at the amino acid terminal was identical to the rPL-I variant (rPL-Iv). The sequence of rPL-Iv (223 amino acids) is 85% homologous to rPL-I (230 amino acid). There are 71 base pair changes (85% nucleotide homology) between rPL-Iv and rPL-I which are not confined to any specific region of the molecule and which result in 36 amino acid changes. In vitro translation of rPL-Iv mRNA produced by transcription of the cDNA template yielded a 26 kilodalton polypeptide, the size of the expected precursor protein. In situ hybridization studies indicated that mRNA for rPL-Iv was present primarily in cytotrophoblasts of the basal zone as early as day 15 of gestation with some hybridization in a few giant cells as well.

Clinical counterpoint: the physiology of placental lactogen in human pregnancy

In summary, current evidence strongly suggests that PL may play a pivotal role during pregnancy, acting through distinct PL receptors to regulate and coordinate growth and metabolism in the mother and fetus. In early and midgestation, PL may be secreted preferentially into the fetal circulation, exerting growth-promoting effects at a time when the rate of linear growth of the fetus is maximal. Subsequently, during the latter half of pregnancy, the metabolic actions of PL in the mother and fetus may predominate, ensuring the optimal supply of nutrients to the fetus and utilization of the nutrients by fetal tissues. It therefore appears that PL affects fetal growth both by exerting effects on the fetus and the mother. Although hPL acts as "growth hormone of pregnancy," the regulation of the synthesis and secretion of hPL appears to be markedly different than that of GH.

Placental lactogen secretion during prolonged-pregnancy in the rat: the ovary plays a pivotal role in the control of placental function

The serum of rats at mid-pregnancy contains at least 2 distinct placental lactogen (PL)-like substances tentatively termed placental lactogen-alpha (PL-alpha) and placental lactogen-beta (PL-beta) (Endocrinol Japon 38: 533-540, 1991). We have investigated the secretory patterns of three placental lactogens (PL-alpha, PL-beta and placental lactogen-II) during normal pregnancy and in two prolonged-pregnancy models. Pregnancy was prolonged by the introduction of new corpora lutea by inducing ovulation on day 15 of pregnancy by successive treatments with PMSG (30 IU/rat, sc on day 12) and hCG (10 IU/rat, iv on day 14), and in the second model by progesterone implants on day 15 of pregnancy. During normal pregnancy, each of the 3 PLs exhibited only one secretory peak in the serum; PL-alpha and PL-beta on day 12 and placental lactogen II (PL-II) on day 20. Interestingly, in the rats with new sets of corpora lutea, serum PL-alpha and PL-beta levels began to increase again on day 18 and showed peaks on day 20 for PL-alpha and on day 22 for PL-beta. In this model, the initiation of PL-II secretion was not affected, but high levels were maintained until day 26, when parturition occurred. In rats receiving either PMSG or hCG, the secretory patterns of the PLs were similar to as those during normal pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of mouse decidual calcyclin: a novel stimulator of mouse placental lactogen-II secretion

The effects of secretagogue(s) from mouse decidual tissue on the release of mouse placental lactogen-II (mPL-II) were studied. Decidual tissue was obtained from 10- and 11-day-pregnant mice. The tissue was homogenized, extracted, and the tissue extract was made 50% saturated with ammonium sulfate. Both the precipitate and supernatant were tested for their ability to stimulate mPL-II release from cultured trophoblasts. The supernatant contained an activity to stimulate the release of mPL-II. This activity was further purified using column chromatography. The purification resulted in isolation of a protein with a mol wt of 20 K as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions and 6 K under reducing conditions. Further characterization of this protein showed that it binds calcium and has an amino acid sequence that is highly homologous with calcyclin expressed in mouse embryonic fibroblast cells and with calcyclin from other species. This protein was designated mouse decidual calcyclin. Antiserum was raised against the purified decidual calcyclin for development of an RIA and for immunoblots. Western blots of various mouse tissue extracts and mouse serum from different physiological stages showed that the concentration of calcyclin was highest in decidual tissue. Detectable levels were found in extracts from trophoblast, lung, and stomach, but the concentrations in these tissues were about 100 times lower than in decidua. Decidual calcyclin was not detectable in mouse serum. Cultured decidual cells released calcyclin into the medium. On average, this release was about 7.8 ng/micrograms DNA.24 h. The rate of release did not change significantly during 4 days of culture. The ratio of calcyclin in cells per calcyclin released during 24 h averaged 2.3 and did not change significantly during the culture period. The purified decidual calcyclin stimulated the release of mPL-II from cultured trophoblasts in a dose-dependent manner at concentrations from 0.01 to 1 microgram/ml. The maximum stimulation averaged about 1.5 times above control. It is concluded that decidual calcyclin may be of physiological importance for the regulation of mPL-II secretion.

An antigenic determinant common to human chorionic somatomammotropin and human growth hormone revealed by limited proteolysis of immune complexes

An epitope of human chorionic somatomammotropin for one of the monoclonal antibodies raised against the whole antigen has been identified. We compared the release of peptides from limited proteolysis of the antigen in the presence and absence of the related antibody. Using enzymes of different specificity, we could determine the amino acid sequence that can be considered at least inclusive of the epitope. The monoclonal antibody selected is 100% cross-reactive with human growth hormone, so the antigenic determinant identified is shared by the two protein hormones.

Characteristics of growth hormone binding to liver microsomes of pregnant women

Specific binding of hGH to liver microsomes of nonpregnant women and men is low, usually 10% of less in RRA. In pregnant women, however, we found that this binding is 10 times higher. The binding reaction shared the properties common to receptor systems: time, temperature and cation dependence, saturability and reversibility, hGH specific binding without cations was 10 times lower. The cross-reactions of hPRL and human placental lactogen with hGH were 0.49 +/- 0.16% and 0.10 +/- 0.05%, respectively. 125I-hPRL and 125I-human placental lactogen binding to microsomes of two controls and two pregnant women were very low and poorly reproducible. The Scatchard analysis revealed two hGH binding sites, one with an association constant (KA) of 2.7 +/- 0.1 x 10(10) M-1, and the other with a KA of 1.5 +/- 0.1 x 10(9) M-1. In one nonpregnant woman, we found a single hGH binding site with a KA of 1.5 x 10(9) M-1, confirming results previously reported in the literature. A hGH RRA was set up with microsomes of pregnant women. Acromegalic sera produced curves parallel to the hGH standard and pituitary dwarf serum had no 125I-hGH displacing activity. Sera of pregnant women produced curves divergent to the hGH standard and showed a 125I-hGH displacing activity 20 to 40 times higher than could be predicted by hGH levels determined by RIA. Cord sera and sera from puerperal women had similar hGH levels as determined by either RRA or RIA (r = 0.93, P less than 0.001, slope = 0.85, n = 25). Our results show the existence of specific GH receptors and serum factor(s) with high 125I-hGH displacing activity from these receptors in pregnancy. These findings must be related to several metabolic changes of pregnancy, such as glucose intolerance, hyperinsulinemia, increased lipolysis, and ketogenesis.

Distinct placental lactogen and prolactin (lactogen) receptors in bovine endometrium

Specific binding sites for bovine placental lactogen (bPL) and the lactogenic hormone, prolactin, have been detected in endometrial membranes isolated from uteri of mid-pregnant heifers. The specific binding of human growth hormone (hGH) (used to monitor the presence of lactogenic binding sites) and of bPL was increased approximately 4-fold following treatment of the membranes with 4 M MgCl2. Binding was found to be ligand specific, membrane protein concentration-, time- and temperature-dependent and reversible. Scatchard analysis of bPL and hGH competition binding data revealed curvilinear plots with dissociation constants for the high affinity sites of 4.1 x 10(-11) M and 6.4 x 10(-11) M, respectively. The maximum capacity of binding of bPL at the high affinity site was 21 fmol/mg). membrane protein while approximately twice the level of binding was measured for hGH (39 fmol/mg). Both hGH and bGH, but not ovine prolactin, competed with [125I]bPL for binding. The concentrations of hGH and bGH needed to effectively compete were however 100-fold higher than those required for unlabeled bPL. No specific binding of radiolabeled bGH was detected in endometrial tissue suggesting the absence of bGH receptors. Preferential competition of [125I]hGH binding was observed by prolactin and bPL. From these data it may be inferred that hGH binding is indicative of the presence of both lactogenic (prolactin) and bPL binding sites in endometrial tissue. The presence of distinct bPL receptors in the endometrium from mid-pregnant cows suggests a possible role for bPL in the maintenance of pregnancy.

Thyroid hormone as a biological amplifier of differentiated trophoblast function in early pregnancy

Direct effects of T3 or T4 on the trophoblast function were investigated in vitro using an organ culture system of human placental tissues. Explants of trophoblastic tissues obtained from normal early and term placentas were cultured with or without graded doses of T3 or T4 for 5 days in a serum-free condition. Addition of T3 (10(-8) mol/l) resulted in the maximum increase in daily secretion of progesterone, estradiol-17 beta as well as hCG alpha, hCG beta, hCG and hPL by cultured early placental tissues. Increases in progesterone and estradiol-17 beta secretion caused by the addition of T3 were further augmented in response to concomitant addition of pregnenolone and testosterone, respectively, suggesting that T3 (10(-8) mol/l) enhances 3 beta-hydroxysteroid dehydrogenase and aromatase activity in the placenta. These stimulatory effects of T3 (10(-8) mol/l) on the trophoblast endocrine function were also found with the use of T4 (10(-7) mol/l). Addition of higher or lower concentrations of T3 or T4 gave attenuated effects. These results suggest that the optimal concentration of thyroid hormone is needed for it to exert its maximally stimulatory action on trophoblast endocrine function. Unlike early placental tissues, cultured term placental tissues did not respond to the addition of T3 or T4 with increased endocrine activity. Thus, the frequent occurrence of spontaneous abortion in early pregnancy during the state of hypothyroidism or hyperthyroidism may represent a direct consequence of inadequate thyroid hormone availability at the level of placental trophoblasts, followed by diminished expression of trophoblast endocrine function.

Transforming growth factor beta 1 inhibits placental differentiation and human chorionic gonadotropin and human placental lactogen secretion

Previously, no inhibitors of placental differentiation have been described. In this study, we determined the effect of transforming growth factor beta 1 (TGF beta 1) on cytotrophoblast differentiation. Monolayer cultures of pure cytotrophoblasts were exposed to 0.001-10 ng/ml TGF beta 1 with and without the presence of 10 ng/ml epidermal growth factor (EGF), an inducer of placental differentiation. Over 7 days of culture, in 11 separate experiments, phase contrast microscopy demonstrated marked inhibition of EGF-induced syncytial formation by TGF beta 1. Basal human (h)CG and h-placental lactogen (PL) release were reduced compared to control by fractions of 0.75 (TGF beta 1/control) and 0.54, respectively. EGF alone induced fractional (EGF/control) increases in hCG and hPL release of 2.46 and 2.68, respectively. However, this stimulation was significantly inhibited by 10 ng/ml TGF beta 1. Dose-response studies showed that maximal TGF beta 1 inhibition of EGF-stimulated hormone secretion occurred at 0.1 ng/ml or more TGF beta 1. Partial differentiation (syncytium formation) occurred despite the presence of TGF beta 1, suggesting a portion of cytotrophoblasts were committed to differentiation at the time of culture. We conclude that TGF beta 1 acts as a major inhibitor of trophoblast differentiation and concomitant peptide hormone secretion.

Mutational analysis and protein engineering of receptor-binding determinants in human placental lactogen

Human placental lactogen (hPL) shares 85% sequence identity to human growth hormone (hGH) yet has some very different receptor-binding properties. For example, hPL binds 2300-fold weaker than hGH to the hGH receptor, yet these two hormones have similar affinities for prolactin receptors. We have expressed hPL in Escherichia coli, and we show that, like hGH, hPL requires zinc for tight binding to the extracellular domain of the human prolactin receptor (hPRLbp). In fact, hPL contains virtually the same receptor-binding determinants and zinc ligands (His-18, His-21, and Glu-174) that hGH uses for coordinating zinc in the hGH.hPRLbp complex. As with hGH, mutation of Glu-174 to Ala in hPL reduces the affinity for the hPRLbp by 1400-fold. We can increase the affinity of hPL by over 200-fold for the hGHbp by installing four hGH receptor determinants that are not conserved in hPL. By simultaneously introducing E174A, we produced a pentamutant whose binding affinity for the hGHbp is only 1.6-fold weaker than hGH, but whose binding affinity for the hPRLbp is weaker by greater than 1000-fold relative to wild-type hPL. Thus, we have identified an hPRLbp epitope in hPL, "recruited" an hGHbp epitope into hPL, and produced receptor selective analogs of hPL that are designed to bind tightly to either, neither, or both receptors. Such variants should be important molecular probes to link specific receptor-binding, activation, and biological events.

[Immunocytochemical localization of human chorionic gonadotropin and placental lactogen in normal placentae]

Preliminary observations on immunocytochemical localization of human chorionic gonadotropin (hCG) and human placental lactogen (hPL) were made in 35 placentae of normal pregnancy at various stages of development. By analysing the processing regularities of the two hormones and comparing the characteristics of immunocytochemical localization and hematoxylin-eosin stain on various trophoblasts in the normal placentae, the findings showed: (1) presence of the three types of trophoblasts, namely, cytotrophoblast (CT), intermediate trophoblast (IT), and syncytiotrophoblast (ST) was confirmed in normal placentae of first-trimester pregnancy. (2) IT has distinctive immunocytochemical features that distinguishes itself from CT and ST. In the first-trimester, ST contains a large amount of hCG which sharply diminishes thereafter, but hPL in ST increases with the fetal age. IT contains hPL all through the pregnancy period and the peak-value occurs in the second trimester. CT is devoid of hCG and hPL. The results indicated: (1) IT is more like ST but different from CT. (2) IT contains chiefly hPL and hCG only locally in early pregnancy which demonstrates that the processing of hPL is in the more well differentiated cells whereas hCG is in the less differentiated cells.

Immunohistological demonstration of intermediate trophoblast in the diagnosis of uterine versus ectopic pregnancy: a retrospective survey and results of a prospective trial

Immunohistological demonstration of human placental lactogen (hPL) in non-villous, mononuclear intermediate trophoblastic cells may be of routine diagnostic value, when chorionic villi are absent in endometrial curettings from patients suspected of miscarriage of an intrauterine pregnancy. The histological presence and distribution of hPL was investigated in endometrial curettings from 90 patients studied retrospectively (47 had ectopic pregnancies, 14 miscarriages, and 29 legal abortions), and a consecutive, prospective series of 50 patients (40 had miscarriages and 10 had ectopic pregnancies) without chorionic villi in their endometrial curettings. Non-specific background staining was not a problem. The retrospective survey disclosed that hPL was a highly sensitive marker of intrauterine pregnancy (sensitivity = 0.98). In the prospective series, the predictive value of positive staining for hPL for intrauterine pregnancy was 1.00, and the sensitivity of hPL, as an indicator of uterine gestation, was 0.62. In absence of specific hPL-staining, the risk of ectopic pregnancy was about 50%. The immunohistochemical demonstration of hPL is a useful tool for identifying patients who are suspected of having had a miscarriage, but for whom evidence in the form of chorionic villi in endometrial curettings is lacking.

Patterns of spontaneous pulsatile secretion of human chorionic gonadotropin and pregnancy specific beta 1 glycoprotein by superfused placental explants in first and last trimester. Lack of episodic human placental lactogen secretion

We have recently reported that during superfusion of placental explants, human chorionic gonadotropin secretion is episodic. In the present work we have examined, using the superfusion model, the pattern of secretion of other glycoprotein hormones, pregnancy specific beta 1 glycoprotein and human placental lactogen, in the first trimester and term placenta. This was done by evaluating the pulsatile pattern by two different computerized programmes. By using different sampling intervals (6-0.5 min) of the effluent, two distinct patterns of hCG secretion were detected in the first trimester: a short one, occurring every 8-9 min and the other every 18-25 min. In contrast, at term the only episodic pattern detected was every 40-50 min, with a low amplitude, 20-30% above baseline, and a declining baseline. In two out of seven placentas, no pulsatility was detected. The secretion of pregnancy specific beta 1 glycoprotein was pulsatile, occurring every 14-15 min in the first trimester and every 6-7 min at term. Finally, the secretion of human placental lactogen at term was not pulsatile. The levels of this glycoprotein in the first trimester placenta were below detection limits. In conclusion, the patterns of the three glycoproteins during superfusion are different, suggesting different paracrine/autocrine control mechanisms. The dynamic superfusion model also enables identification of thus far not reported gestational age-dependent differences in the secretion pattern of hCG and pregnancy specific beta 1 glycoprotein.

Localization of placental lactogen-I in trophoblast giant cells of the mouse placenta

The purpose of this investigation was to identify the cellular origin of placental lactogen-I (PL-I) expression in the mouse placenta and to cytologically define the transition from PL-I to PL-II expression during gestation. PL-I mRNA expression was assessed by in situ hybridization, and expression of PL-I and PL-II protein was determined by immunocytochemical analysis. PL-I mRNA and protein were localized to trophoblast giant cells. Trophoblast giant cells ceased producing PL-I at midgestation and began expressing PL-II. PL-I immunoreactivity was present in trophoblast giant cells on Days 9 and 10 of gestation but was not detectable in trophoblast giant cells on Day 11 of gestation. Immunoreactive PL-II-producing giant cells were detected first on Day 10 of gestation, continuing on Day 11 of gestation. Expression of PL-I and PL-II signals a significant functional transition in trophoblast giant cells of the developing mouse placenta.

Specificity of two-site immunometric assays

The assessment of specificity in two-site immunometric assays is complex. Conventional approaches based on standard substitution with cross-reactant may yield misleading results. An 'in-house' immunoradiometric assay (IRMA) for human growth hormone (hGH) appeared free from cross-reactivity with human placental lactogen (hPL) in a single point standard substitution experiment performed by the United Kingdom External Quality Assessment Scheme (UK EQAS). However, detailed studies based on the substitution of hGH standards with hPL cross-reactant, the addition of hPL cross-reactant to hGH standards and the recovery of added hGH from endogenous cross-reactant (sera from pregnant subjects) revealed gross cross-reactivity of hPL in the hGH assay. We recommend that the validation of assay specificity should require the demonstration of quantitative recovery of a range of analyte concentrations from human sera that contain varying endogenous concentrations of cross-reactant.

Melittin stimulates phosphoinositide hydrolysis and placental lactogen release: arachidonic acid as a link between phospholipase A2 and phospholipase C signal-transduction pathways

Previous investigations from this laboratory have implicated both phospholipase A2 and phospholipase C in the regulation of human placental lactogen release from human trophoblast. To study further the role of endogenous phospholipase A2 and the relationship between phospholipase A2 activation and phosphoinositide metabolism, we examined hPL and [3H]-inositol release from trophoblast cells in response to agents that stimulate or inhibit the endogenous enzyme. Melittin (0.5-2.0 micrograms/ml) stimulated rapid, dose-dependent, and reversible increases in the release of hPL, prostaglandin E, and [3H]-inositol. Mepacrine (0.1-0.25 mM) inhibited this stimulation. However, mepacrine had no effect on the stimulation of hPL and [3H]-inositol release by exogenous arachidonic acid (AA). These results indicate that the stimulation by melittin of phosphoinositide metabolism and hPL release is mediated by initial activation of phospholipase A2. Furthermore, the results support the possibility that AA, released as a consequence of phospholipase A2 activation, can act as a second messenger linking the two phospholipase pathways.

Adenosine 3':5'-cyclic monophosphate (cAMP) is not the mediator of kappa opiate effect on human placental lactogen release

We previously reported that kappa opiates stimulated the release of human placental lactogen (hPL) from human placental cells. In this study, we investigated the role of adenylate cyclase as a potential cellular mediator of such an effect. Incubations with ethylketocyclazocine (EKC) led to a time- and dose-dependent inhibition of adenylate cyclase activity. The maximal inhibition was 45 +/- 5% of control value after 15 min exposure to 10(-7)M EKC. This inhibition was reversed by opiate antagonist naloxone and was specific to kappa opiate type. Preincubation of human trophoblastic cells with 0.1 microgram/ml Islet-Activating-Protein (IAP; also called pertussis toxin) did not modify basal adenylate cyclase activity but abolished the inhibition of adenylate cyclase activity by EKC, indicating that the effect of opiates on cAMP production was mediated by an IAP-sensitive GTP binding protein. Also, IAP stimulated basal hPL release; the control levels were 22.4 ng/ml and 46.5 ng/ml without and with IAP respectively. However, the EKC-stimulated hPL levels were unchanged by preincubation with IAP. This difference in cAMP and hPL response in IAP-treated cells suggested that the opiate receptors are not directly coupled to adenylate cyclase. This hypothesis was confirmed by 1) experiments on placental membranes showing that in absence of the cytoplasmic elements (membranes only), EKC had no effect on membrane adenylate cyclase and 2) experiments on placental cells showing that dibutyryl-cAMP (dbcAMP) stimulated hPL release.

An islet-activating protein-sensitive G-protein is involved in dopamine inhibition of both angiotensin-stimulated inositol phosphate production and human placental lactogen release in human trophoblastic cells

In isolated human trophoblastic cells, dopamine (DA) significantly inhibited the angiotensin-II (AII)-stimulated inositol phosphate (IP) accumulation by 44 +/- 8% (EC50, 0.5 +/- 0.2 microM) and human placental lactogen (hPL) release by 85 +/- 5% (EC50, 1.0 +/- 0.8 microM). These effects were blocked by sulpiride, a specific D2 antagonist. On the contrary, scherring 23390 (a specific D1 antagonist) and propranolol (a specific beta-adrenergic antagonist) were ineffective, suggesting that these DA effects are mediated through a DA receptor of the D2 subtype. The mechanism by which DA inhibited AII-stimulated inositol phosphate production implicates a GTP-binding protein sensitive to the islet-activating-protein (IAP), since DA's effects on IP accumulation and hPL release were blocked by this toxin. To further characterize this GTP-binding protein, particulate fractions of placental cells were incubated with [alpha-32P]NAD and IAP. Solubilized extracts were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two proteins of 40 and 41 kDa mol wt were specifically ADP ribosylated. They were probably involved in the DA inhibitory processes, since IAP treatment, known to suppress the effects of DA, also reduced the labeling of these two molecules by around 40%. The effects of AII and DA on hPL release appear to be insensitive to the external calcium concentration, since the results were not significantly different in normal (1.8 mM Ca2+) and low Ca2+ (10(-5) M Ca2+) concentrations. On the other hand, increasing the intracellular concentration of cAMP by adding forskolin did not modify the effect of DA on either IP accumulation or hPL release, suggesting that cAMP is not implicated in hPL release from freshly isolated human trophoblastic cells.

Characterization of glycosylated bovine placental lactogen and the effect of enzymatic deglycosylation on receptor binding and biological activity

Bovine placental lactogen (bPL) is a glycoprotein hormone that has both somatogenic and lactogenic properties. Purified preparations of the hormone contain many isoforms that are separated by isoelectric focusing. The sequence for bPL contains one consensus site for an N-linked oligosaccharide and many potential sites for O-linked sugars. To determine whether the isoforms are the result of differences in glycosylation, the oligosaccharide portion of bPL was partially characterized. In addition, a number of the isoforms were isolated and enzymatically deglycosylated to determine the effect of O- and N-linked glycosylation on biological activity. Biological activity was assessed in a somatotropin radioreceptor assay and also in the Nb2 lymphoma lactogenic bioassay. The structure of N-linked oligosaccharide was found to be sialylated and triantennary and appeared to be the same for all of the different charge isomers. Compositional analysis suggested that O-linked oligosaccharides were also present. Treatment of the intact hormone with neuraminidase resulted in the loss of some, but not all, of the isoforms, suggesting that a large degree of the charge heterogeneity is due to posttranslational modifications unrelated to glycosylation. Enzymatic removal of N-linked oligosaccharides from native bPL resulted in a 1.2-2.3-fold increase in binding to the somatotropin receptor, whereas receptor binding was unaffected by enzymatic removal of O-linked oligosaccharide. Lactogenic activity was affected very little by the removal of either type of oligosaccharide. The data suggests that glycosylation of bPL may have a small effect on receptor specificity, but that overall its presence does not dramatically affect receptor binding or biological activity.

Modification of arginine residues in human growth hormone by 1,2-cyclohexanedione: effects on the binding capacity to lactogenic and somatogenic receptors

Reactivity of arginine residues in human growth hormone was studied by reaction with 1,2-cyclohexanedione. Kinetic analysis of the data showed a good fit to a pseudo first order curve, with an apparent velocity constant k = 1.26 x 10(-2) min-1 and a maximum modification of 9.6 out of the 11 arginines of the molecule. Modification led to a decrease in binding capacity to both lactogenic and somatogenic rat liver receptors. In either case Tsou plots suggest that the modification of two arginine residues is responsible for this behavior, although it cannot be ascertained whether the two relevant residues are the same for both receptor types. Circular dichroism studies indicated no apparent changes in protein conformation in the modified hormone. Binding capacity was restored upon regeneration of arginines by incubation with Tris-HCl buffer. Only the carboxy-terminal peptide was isolated by HPLC from a tryptic digest of succinylated Arg-modified hGH, indicating that 183 is the nonreacting arginine residue.