Pregnancy-associated Plasma Proteins Research Articles

Pregnancy-associated plasma proteins and Stanniocalcin-2 – Novel players controlling IGF-I physiology

IGF-I was originally discovered as a GH-dependent growth factor stimulating longitudinal growth. Currently, however, it has become evident that the biological activities of IGF-I extend well beyond those of a simple growth factor and impact such processes as insulin sensitivity, aging, cancer and cardiovascular disease.The vast majority of IGF-I is tightly bound to IGF-binding proteins (IGFBPs), which renders IGF-I unable to stimulate the IGF-I receptor (IGF-IR) in vivo. This binding means that liberation of IGF-I from the IGFBPs is an important step controlling IGF-I action. In this context, IGFBP-cleaving enzymes appear to play a key role. Enzymatic cleavage of the IGFBPs markedly lowers their ligand affinity, and as a consequence, IGF-I becomes liberated and hence available for stimulation of the IGF-IR.Two of the best-characterized IGFBP-cleaving enzymes are pregnancy-associated plasma protein-A (PAPP-A) and its paralog PAPP-A2. The two enzymes (often referred to as pappalysins) regulate the liberation of IGF-I in a highly controlled manner. PAPP-A is believed to act predominantly in tissues, serving to liberate IGF-I at the cell surface in close proximity to the IGF-IR. In keeping with this notion, mice lacking PAPP-A exhibit reduced body size, despite having normal circulating IGF-I concentrations. In contrast, human findings indicate that altered PAPP-A2 activity changes circulating IGF-I concentrations, although PAPP-A2 is also present in high concentrations in tissues. Thus, PAPP-A2 appears to impact circulating, as well as tissue, IGF-I activity. The enzymatic activity of PAPP-A and PAPP-A2 was recently discovered to be regulated by the protein Stanniocalcin-2 (STC2). By binding to the enzymatic sites of PAPP-A and PAPP-A2, STC2 inhibits their activity.To date, the majority of findings demonstrating the ability of pappalysins and STC2 to regulate IGF-I action are from preclinical studies. However, clinical studies are now beginning to emerge. In this review, we will summarize our data on STC2, PAPP-A and PAPP-A2 in humans. These results indicate that pappalysins and STC2 constitute an important IGF-I activity-regulating system that warrants further investigation.

Effects of oestrogen and human growth hormone on pregnancy-associated plasma proteins in the rat.

The serum concentrations of pregnancy-associated murine protein-1 (PAMP-1), acute phase alpha 2-macroglobulin, albumin, transferrin, and complement factor 3(C3) were followed in male rats during continuous infusions of oestradiol-17 beta and human growth hormone. Three different patterns of protein response could be distinguished. A distinct acute phase response without any additive influence of the given hormones was recorded for alpha 2-macroglobulin, whereas the levels of albumin, transferrin and C3 were virtually unaffected throughout the experiment. Growth hormone gave a rapid and pronounced increase of PAMP-1 levels, whereas the response to oestradiol of this 'steroid-sensitive' protein was significantly weaker and delayed. It is suggested that the apparent oestrogenic influence on certain pregnancy-associated plasma proteins is mediated via growth hormone.

Radioimmunoassay for a progestagen-associated protein of the human endometrium.

We have previously demonstrated the presence of a progestagen-associated endometrial protein (PEP) in women. In this communication, we report the development of a RIA for PEP. We describe the use of this method for determining changes in PEP concentration that occur in the endometrium, amniotic fluid, and serum during the normal menstrual cycle and pregnancy. This assay used a partially purified PEP as the reference standard, purified radioiodinated PEP as the tracer, and a goat antibody to PEP as the specific antibody. The assay appears to be highly specific, since the tracer did not bind to antibodies to several known pregnancy-associated plasma, uterine, or placental proteins, including human transferrin, α1-anti-trypsin, ceruloplasmin, human PRL, placental protein SP1, pregnancy zone protein, α-fetoprotein, hCG, pregnancy-associated plasma proteins, and rabbit uteroglobin. Also, hCG, human PRL, and rabbit uteroglobin did not compete for the binding of the tracer to anti-PEP. The detection threshold of...

Properties of the progestagen-dependent protein of the human endometrium.

Antigens A and B, shown to be associated with the progestagen-dominated human endometrium, were partly purified and their properties studied. The antigens were recovered in the crude nuclei, the heavy particulate fraction and cytosol of decidua-rich tissue from early pregnancy. The antigens in cytosol were enriched by a combination of Concanavalin A-Sepharose chromatography and polyacrylamide gel electrophoresis. The immunological reactivity of the antigens after partial purification by Concanavalin A-Sepharose chromatography was retained after 30 min exposure to 4-85 degrees C at pH 7.4, or after 2 h to pH 2-12 at 22 degrees C. Trypsin, but not pepsin, RNase, DNase or neuraminidase, completely destroyed immunological reactivity of both antigens. The apparent molecular weight of both antigens determined by filtration on Sephadex G100 was 48 000. The isoelectric point of both antigens was approximately 4.9. The antigens were not immunologically related to transferrin, ceruloplasmin, alpha-1-antitrypsin, ferritin, uteroglobin, alpha-fetoprotein, human chorionic gonadotrophin, pregnancy-associated plasma proteins or pregnancy zone protein. Furthermore, the antisera to Antigens A and B did not react with the decidual cytosol of pregnant baboons or of pseudopregnant rats.

Pregnancy-associated plasma proteins in pregnant and pseudopregnant rats.

As in the human, the rat pregnancy-associated plasma proteins consisted of four distinct entities as revealed by immunological methods. All showed gradual increases during gestation and rapid disappearance postpartum. They could be physicochemically, although not immunologically, tentatively related to the human pregnancy proteins. None of them was detectable in the serum of rats at various stages of pseudopregnancy, nor in nonpregnant rats.

Immunological Relationships of Human and Subhuman Primate Pregnancy-Associated Plasma Proteins

(1) Four pregnancy-associated plasma proteins cross-reactive with antibodies to the human pregnancy proteins were detected in several species of pregnant subhuman primates. In the case of the two apes studied (chimpanzee and orangutan), these appeared to be immunologically identical to the human PAPPs (PAPP-A, -B, -C, and HCS). In the old world monkeys analyzed, equivalent partially cross-reactive PAPPs were found; while in the new world squirrel monkey, only faint traces of cross-reactive PAPP-C and HCS were observed with the most sensitive methods used. (2) As in the human, these proteins appeared to be specific for pregnancy, not being detectable in nonpregnant animals, female or male. (3) The pregnant chimpanzee possessed significantly higher concentrations of PAPP-A and PAPP-C than did women at an equivalent stage of pregnancy, while the HCS and PAPP-B levels appeared to be approximately the same. (4) The primate PAPP-C analogues were more complex than human PAPP-C, often revealing multiple gel diffusion patterns with subfractions of differing electrophoretic mobilities. (5) Based on the changes of electrophoretic mobility upon exposure to neuraminidase, the subhuman primate as well as human PAPP-A and PAPP-C appeared to be glycoproteins containing sialic acid. (6) In the chimpanzee and rhesus monkey, as in the human, the levels of PAPP-A, PAPP-C, and HCS were appreciably higher during the third trimester of pregnancy than they were during the second trimester.

Placental localization of human pregnancy--associated plasma proteins.

Frozen sections of human placenta were examined for the presence of four human pregnancy proteins, pregnancy-associated plasma proteins A and C (PAPP-A and PAPP-C), human chorionic somatomammotropin (hCS), and pregnancy zone protein (PZP), by the indirect immunofluorescence technique. Monospecific rabbit antiserums to PAPP-A, PAPP-C and hCS all stained the trophoblast cytoplasm equivalently in a continuous layer, usggesting that the same trophoblast cells synthesize all three pregnancy proteins. In contrast, PZP was localized in blood vessel walls, parenchymal structures within the villous, as well as in the trophoblast cytoplasm. Its distribution in the latter was relatively inhomogeneous, tending to be more intense on the basement membrane side.

Relation of obstetric parameters to the concentrations of four pregnancy-associated plasma proteins at term in normal gestation

The relations between normal obstetric parameters and the maternal levels of four pregnancy-associated plasma proteins (PAPP) were studied, with the use of plasma samples taken from 187 normal pregnant women within seven days before delivery. PAPP-A levels were correlated with placental and newborn weights. The levels of this pregnancy protein was higher in primigravid women and in groups with higher diastolic blood pressure than in other groups. Women with extremely high PAPP-A concentrations were likely to have extremely large placental weights and to be primigravid. On the other hand, an extremely large placental weight was not necessarily associated with a high PAPP-A level. PAPP-C was not correlated with placental or newborn weight. The relationship between PAPP-C and maternal age, as well as maternal weight, was significant by one but not in the other three statistical analyses employed. The pregnancy zone protein was found to be correlated with parity. In primigravid women, this protein additionally showed an inverse correlation with the placental weight. Human chorionic somatomammotropin was significantly related to placental weight and inversely related to maternal weight. Its relationship with newborn weight was best seen in primigravid subjects. Other parameters (systolic blood pressure, one- and five-minute Apgar scores, weeks of gestation, days before delivery, newborn sex, and newborn bilirubin level) were not related to any of these pregnancy proteins.

Quantitative analysis of pregnancy-associated plasma proteins in human placenta.

By immunochemical methods and simultaneous measurements of several normal plasma proteins, human placenta was shown to contain elevated quantities of four pregnancy-associated plasma proteins (PAPP's). In the order of increasing amounts, PAPP-A, PAPP-C, PAPP-B, and human chorionic somatomammotropin (PAPP-D) all were present in placenta extracts in quantities greater than could be expected on the basis of their content in maternal blood. In sharp contrast, the placental content of pregnancy zone protein could be entirely accounted for by the maternal plasma present in the placenta. All of the PAPP's appeared to be readily extractible from placental tissue with buffered saline, the large bulk of them being solubilized in the first extraction procedure. However, absorption studies indicated that appreciable quantities of the PAPP's were still present in the insoluble placental residue after 12 sequential extractions with saline. The chorioamniotic membranes were not significantly enriched in any of the PAPP's. Immunochemical analysis of unwashed placental tissue extracts for the PAPP's IgA, and IgM (maternal blood derived), as well as albumin and transferrin (maternal and fetal blood derived), permitted calculations to be made of the amount of blood and PAPP's in placenta. On the basis of these data, it was roughly estimated that a 400-g placenta (wet weight) would occupy 312 ml in volume, and would contain 144 ml of blood. Of this blood, 36 ml would be derived from the mother.

Human pregnancy-associated plasma proteins during the postpartum period

The plasma levels of four pregnancy-associated plasma proteins (PAPP's) and pregnancy zone protein (PZP) were studied in women after delivery of a single viable infant by gel immunodiffussion methods. Data from 89 random samples and 85 serial specimens from five women revealed that both PAPP-B and HPL (PAPP-D) disappeared within a day after delivery. PAPP-A showed a rapid drop in the first 2 or 3 days post partum and became nondetectable in 4 to 6 weeks, with a half-life of 3 to 4 days. PAPP-C had a sharp decrease by the second postpartum day and was not detected 3 to 4 weeks later, with a half-life of 1 to 2 days. None of the PAPP's was detected again during the rest of the 14 postpartum weeks studied. In contrast, the PZP showed a much slower decrease or even a temporary increase during the first 2 weeks post partum; it remained readily detectable over the entire 14 weeks studied.

Immunological Comparison of Various Human Pregnancy-Associated Plasma Proteins

Direct immunodiffusion comparison with specific antisera demonstrated that all of the four pregnancy-associated plasma proteins (PAPPs) described in our laboratory are distinct from the pregnancy zone protein (von Schoultz); alpha2-pregnoglobulin (Berne); pregnancy-associated alpha2-glycoprotein (SP3) (Bohn); new serum alpha2-macroglobulin (Stimson); PAG (Horne); pregnancy-associated alpha2-globulin (Kasukawa); Pal (McLaren), and Xh protein (Dunston). All the latter proved to be immunologically idnetical to each other, and apparently represent the same protein described under different names. It was confirmed that none of the PAPPs is immunochemically related to placental alkaline phosphatase. PAPP-A, PAPP-C, and the pregnancy zone protein, but not HPL (PAPP-D), showed a decreased anodic mobility when treated with neuraminidase; their reactivities with antibody were essentially unaffected by the ezyme, however. Certain detergents had no effect on the immunological reactivities of the PAPPs and the pregnancy zone protein in whole plasma, while butanol and desoxycholate partially, and urea, SDS and cetylpyridinium largely inactivated them.

Measurement of pregnancy-associated plasma proteins during human gestation.

Studies of four pregnancy-associated plasma proteins (PAPP's) were made on 206 plasma samples obtained from 175 pregnant women between 12 and 44 wk of gestation. The concentration of three PAPP's (A, C, and D) were assayed by quantitative crossed immunoelectrophoresis. They showed a gradual but small rate of increase during the 2nd trimester, which became more rapid in the 3rd trimester. PAPP-A continued to rise steadily during this period, while PAPP-C and PAPP-D (recently identified as human placental lactogen) tended to reach a plateau. Although PAPP-B could not be quantitated because of technical problems, it was detected in over 50% of the samples from the last 2 mo of gestation, but was almost never seen in those obtained during the 12th-28th wk of gestation. Various parameters were analyzed to determine their possible correlation with the PAPP levels during the last month of gestation. The race and age of the mother showed no influence on any of the PAPP's, while parity, sex of fetus, and infant birth weights appeared to affect the plasma concentration of some of the PAPP's. In the two instances studied, mothers of twins showed abnormally high PAPP levels.

Three pregnancy-associated human plasma proteins: purification, monospecific antisera and immunological indentification.

Three human pregnancy-associated plasma proteins (PAPPs), which were detected only during pregnancy with appropriately absorbed antisera to pregnancy plasma, were purified from third trimester plasma. The authors employed combinations of sequential DEAE-cellulose and hydroxylapatite chromatography; Sephadex G-100, G-200 and Sepharose 6B gel filtration; isoelectric focusing; and/or ammonium sulfate salting out. Each final product was shown to contain only one PAPP, to be immunologically free of most normal plasma proteins, and to have specific activities 100- to 145-fold greater than in the original pregnancy plasma. Rabbits immunized with these preparations responded with antisera which became mono-specific to their respective PAPP after appropriate absorption. PAPP-D, the one with low molecular weight (20,000), was shown immunologically to represent human placental lactogen. Another, PAPP-C, has been found to be identical to the pregnancy-specific <i>β</i><sub>1</sub>-glycoprotein described by Bohn (designated by him as SP<sub>1</sub> protein). The other, PAPP-A, of molecular weight 750,000, is distinct from the other two pregnancy-associated proteins of Bohn (designated SP<sub>2</sub> and SP<sub>3</sub>), as well as the α<sub>2</sub>-pregnoglobulin of Berne. Since the latter and SP<sub>3</sub> proved to be identical immunologically, and since it has been reported that SP<sub>3</sub>, Xh factor, pregnancy-associated globulin (PAG or Pal), and the pregnancyzone proteins are identical to each other, PAPP-A may represent a new pregnancy-specific protein. None of the PAPPs were detected in sera from 102 patients with various malignancies.

Characterization of four human pregnancy-associated plasma proteins

Four proteins have been detected in third-trimester human pregnancy plasma by gel immunodiffusion methods, using antisera to pregnancy plasma that had been exhaustively absorbed with nonpregnancy plasma. These antisera failed to react with plasma from untreated normal nonpregnant females or from males. Two of the pregnancy-associated plasma proteins (PAPP-B and PAPP-C) migrated as β-globulins and the other two (PAPP-A and PAPP-D) as α2-globulins in immunoelectrophoresis. PAPP-B was regularly detected only by crossed-immunoelectrophoresis with the reagents available and it was not further studied here. Proteinases destroyed the immunologic reactivities of PAPP-A, C, and D, but none of them was inactivated by ribonuclease and deoxyribonuclease, and they did not appear to contain lipids. They were all stable to heat at 60° C. for 30 minutes, but were variably labile at higher temperatures. The three PAPP's also showed varying lability at pH extremes. They were well separated from each other in gel filtration on Sephadex G-200 and Sepharose 6B, in DEAE-cellulose column chromatography, and in sucrose density gradient ultracentrifugation, but not by step-wise salting out precipitation or gradient solubilization with ammonium sulfate. Molecular weights estimated by gel filtration were found to be 750,000 for PAPP-A, 110,000 for PAPP-C, and 20,000 for PAPP-D. Their isoelectric points were found by isoelectric focusing in ampholine-sucrose gradients: PAPP-A, pH 4.4; PAPP-C, pH 3.8; PAPP-D, pH 5.7. PAPP-C contained iron detectable by histochemical staining methods; it was heterogeneous and had minor components of different charge and isoelectric point (pH 6.0).

Pregnancy-associated serum antigens in the rat and mouse.

SummaryRabbits were hyperimmunized with late rat and mouse pregnancy serum. After thorough absorption of the antisera with nonpregnancy plasma of the same strains of animal, up to 4 pregnancy-associated serum antigens were revealed in both rat and mouse pregnancy sera by immunodiffusion methods. They were mostly species-specific, no cross-reactions being seen with human pregnancy-associated plasma proteins. Only a partial cross-reaction was found between one of the rat and mouse pregnancy antigens. The rat pregnancy-associated serum antigens were not detected 10 days after parturition, nor were they found in the serum of newborn rats.

Pregnancy-associated plasma proteins in trophoblastic disease

Pregnancy-associated plasma proteins in trophoblastic disease

Plasma acid phosphatase in normal and pre-eclamptic pregnancy

Plasma acid phosphatase activity was measured in nonpregnant, normal pregnant, and severely pre-eclamptic women. The third trimester of pregnancy was associated with a rise in plasma acid phosphatase and severe pre-eclampsia with a marked rise in this enzyme. Possible sources of this elevation in acid phosphatase activity are the placenta, kidneys, pregnancy-associated plasma proteins, and blood platelets—with platelets the most likely.

Observations on the origin of pregnancy-associated plasma proteins

1. The “alpha-2 globulin of pregnancy” has been found in the sera of nonpregnant females and in increased amounts in trophoblastic disease, during pregnancy, and during the administration of Enovid. The placenta, therefore, is not essential to the synthesis of this protein.2. The pregnancy-associated phosphatase and two cystine aminopeptidases, not being observed in patients with trophoblastic disease or during the administration of Enovid. seem to be specific for pregnancy. Since they are also absent from fetal blood, the placenta is the probable site of origin of these enzymes.3. Both Enovid administration and trophoblastic disease elicit the appearance of other aminopeptidase bands. one of which is similar in mobility to one of the pregnancy-associated cystine aminopeptidases. It can be differentiated from it by means of incubation with neuraminidase.4. The possibility of further development of the zymogram technique for the differentiation of trophoblastic disease and pregnancy is discussed.