Purification and characterization of pregnancy associated plasma protein A (PAPP-A)

The role of pregnancy associated plasma protein-A in triple negative breast cancer: a promising target for achieving clinical benefits

Pregnancy associated plasma protein-A (PAPP-A) plays an integral role in breast cancer (BC), especially triple negative breast cancer (TNBC). This subtype accounts for the most aggressive BC, possesses high tumor heterogeneity, is least responsive to standard treatments and has the poorest clinical outcomes. There is a critical need to address the lack of effective targeted therapeutic options available. PAPP-A is a protein that is highly elevated during pregnancy. Frequently, higher PAPP-A expression is detected in tumors than in healthy tissues. The increase in expression coincides with increased rates of aggressive cancers. In BC, PAPP-A has been demonstrated to play a role in tumor initiation, progression, metastasis including epithelial-mesenchymal transition (EMT), as well as acting as a biomarker for predicting patient outcomes. In this review, we present the role of PAPP-A, with specific focus on TNBC. The structure and function of PAPP-A, belonging to the pappalysin subfamily, and its proteolytic activity are assessed. We highlight the link of BC and PAPP-A with respect to the IGFBP/IGF axis, EMT, the window of susceptibility and the impact of pregnancy. Importantly, the relevance of PAPP-A as a TNBC clinical marker is reviewed and its influence on immune-related pathways are explored. The relationship and mechanisms involving PAPP-A reveal the potential for more treatment options that can lead to successful immunotherapeutic targets and the ability to assist with better predicting clinical outcomes in TNBC.

The Pregnancy-Associated Plasma Protein-A (PAPP-A) Story

Pregnancy-associated plasma protein-A (PAPP-A) was first identified in the early 1970s as a placental protein of unknown function, present at high concentrations in the circulation of pregnant women. In the mid-to-late 1990s, PAPP-A was discovered to be a metzincin metalloproteinase, expressed by many nonplacental cells, that regulates local insulin-like growth factor (IGF) activity through cleavage of high-affinity IGF binding proteins (IGFBPs), in particular IGFBP-4. With PAPP-A as a cell surface-associated enzyme, the reduced affinity of the cleavage fragments results in increased IGF available to bind and activate IGF receptors in the pericellular environment. This proteolytic regulation of IGF activity is important, since the IGFs promote proliferation, differentiation, migration, and survival in various normal and cancer cells. Thus, there has been a steady growth in investigation of PAPP-A structure and function outside of pregnancy. This review provides historical perspective on the discovery of PAPP-A and its structure and cellular function, highlights key studies of the first 50 years in PAPP-A research, and introduces new findings from recent years.

PAPPA-mediated adipose tissue remodeling mitigates insulin resistance and protects against gestational diabetes in mice and humans

Pregnancy is a physiological state of continuous adaptation to changing maternal and fetal nutritional needs, including a reduction of maternal insulin sensitivity allowing for appropriately enhanced glucose availability to the fetus. However, excessive insulin resistance in conjunction with insufficient insulin secretion results in gestational diabetes mellitus (GDM), greatly increasing the risk for pregnancy complications and predisposing both mothers and offspring to future metabolic disease. Here, we report a signaling pathway connecting pregnancy-associated plasma protein A (PAPPA) with adipose tissue expansion in pregnancy. Adipose tissue plays a central role in the regulation of insulin sensitivity, and we show that, in both mice and humans, pregnancy caused remodeling of adipose tissue evidenced by altered adipocyte size, vascularization, and in vitro expansion capacity. PAPPA is known to be a metalloprotease secreted by human placenta that modulates insulin-like growth factor (IGF) bioavailability through prolteolysis of IGF binding proteins (IGFBPs) 2, 4, and 5. We demonstrate that recombinant PAPPA can stimulate ex vivo human adipose tissue expansion in an IGFBP-5- and IGF-1-dependent manner. Moreover, mice lacking PAPPA displayed impaired adipose tissue remodeling, pregnancy-induced insulin resistance, and hepatic steatosis, recapitulating multiple aspects of human GDM. In a cohort of 6361 pregnant women, concentrations of circulating PAPPA are inversely correlated with glycemia and odds of developing GDM. These data identify PAPPA and the IGF signaling pathway as necessary for the regulation of maternal adipose tissue physiology and systemic glucose homeostasis, with consequences for long-term metabolic risk and potential for therapeutic use.

Pregnancy-associated plasma protein-A proteolytic activity in rat vertebral cell cultures: Stimulation by dexamethasone-a potential mechanism for glucocorticoid regulation of osteoprogenitor proliferation and differentiation

Glucocorticoids (GCs) at physiological concentrations stimulate osteoprogenitor proliferation and differentiation in rat bone cell populations, and this is mediated in part by an increased response to insulin-like growth factors (IGFs). Since IGF binding proteins (IGFBPs) modulate IGF actions, we evaluated whether the increased IGF responsiveness might be associated with decreased inhibitory IGFBP-4 peptide levels. Rat vertebral cells were cultured for up to 20 days with or without dexamethasone (Dex). Cell layer proteins were extracted at day 6, 8, 14, and 20, conditioned media (CM) collected at day 8, 14, and 20, and total RNA isolated at day 14 and 20 of culture. Western blotting showed that cell layer IGFBP-4 levels were lower, while IGFBP-4 protease activity in CM was higher, in Dex-treated cultures. Addition of pregnancy-associated plasma protein-A (PAPP-A) antibody to CM abrogated IGFBP-4 proteolysis. PAPP-A mRNA levels were the same in control and Dex-treated cultures as evaluated by RT-PCR. Our data demonstrate that activity of the IGFBP-4 protease, PAPP-A, in rat bone cell cultures is increased by Dex via post-transcriptional mechanisms. Since IGFBP-4 mRNA levels in Dex-treated cultures were the same as in controls at day 8, slightly lower than in controls at day 14, and higher than in controls at day 20 as shown previously, the decreased IGFBP-4 peptide levels in Dex-treated cultures likely result from increased IGFBP-4 proteolysis by the elevated PAPP-A enzymatic activity. Our findings underscore a novel mechanism whereby GCs increase IGF responses in rat bone cells via PAPP-A-induced IGFBP-4 proteolysis.

Expression of pregnancy-associated plasma protein-A (PAPP-A) during human villous trophoblast differentiation in vitro

Pregnancy-associated placental protein-A (PAPP-A), first isolated from maternal serum, has been identified as a metalloprotease cleaving insulin-like growth factor binding protein-4 (IGFBP-4). The source of PAPP-A during pregnancy is unclear. We therefore investigated PAPP-A expression during in vitro human villous cytotrophoblast cell (CT) differentiation into syncytiotrophoblast (ST). CT were isolated from normal first trimester, second trimester and term placentae (n=10) and cultured to form ST. PAPP-A mRNA was quantified by real-time PCR, and PAPP-A protein expression was studied by immunocytochemistry and TRACE technology with specific monoclonal antibodies. PAPP-A mRNA expression in total placental extracts increased during the course of pregnancy. PAPP-A protein was detected in the cytoplasm of both CT and ST. ST formation in vitro was associated with a 19-fold increase in PAPP-A mRNA expression and an 8-fold increase in PAPP-A secretion into the culture medium. No significant difference in PAPP-A production was observed between cultured cells isolated from early and term placentae. In conclusion, PAPP-A production in vitro, is associated to the differentiation of villous cytotrophoblast cells into syncytiotrophoblast, independently of the age of gestation.

Localization and cellular distribution of pregnancy-associated plasma protein-a and major basic protein in human ovary and corpora lutea throughout the menstrual cycle

OBJECTIVE

To assess the expression and cellular distribution of pregnancy-associated plasma protein-A (PAPP-A) and major basic protein (MBP) in human ovarian tissue during the menstrual cycle.

DESIGN

Ovarian tissues (n = 50) and corpora lutea (n = 18) were obtained from patients undergoing hysterectomy/oophorectomy for benign conditions and tissue sections were immunostained for MBP and PAPP-A.

SETTING

University medical center.

INTERVENTION(S)

Immunostaining of tissue sections using antibodies to PAPP-A and MBP.

MAIN OUTCOME MEASURE(S)

Microscopic evaluation to assess the presence, distribution, and cellular co-localization of MBP and PAPP-A and to describe any variations in their expression during the menstrual cycle.

RESULT(S)

Major basic protein (MBP) is found in several ovarian cell types throughout the menstrual cycle. The MBP immunostaining of ovarian follicles varied depending on the size, with primordial follicles staining in the ooplasm with a lack of staining in the granulosa and theca cells. In the intermediate/mature follicles, MBP was immunolocalized in theca, but not in granulosa cells except in the mature follicles. Pregnancy-associated plasma protein-A (PAPP-A) was immunolocalized in primordial follicle ooplasm, theca externa of intermediate/mature follicles, and in granulosa cells with increased intensity as luteinization progressed. The luteal tissue is the major site of MBP and PAPP-A with highest intensity found during the midluteal phase associated with both small and large luteal cells.

CONCLUSION(S)

The expression and distinct pattern of MBP and PAPP-A cellular localization in human ovarian tissue during folliculogenesis and in luteal tissue suggest that their individual and combined actions in a cell specific fashion may play a role in growth and differentiation of theca, granulosa, and luteal cells.

Expression of recombinant human pregnancy-associated plasma protein-A and identification of the proform of eosinophil major basic protein as its physiological inhibitor

Pregnancy-associated plasma protein-A (PAPP-A), originally known from human pregnancy serum, has recently been demonstrated to be a metzincin superfamily metalloproteinase involved in normal and pathological insulin-like growth factor (IGF) physiology. PAPP-A specifically cleaves IGF-binding protein (IGFBP)-4, one of six antagonists of IGF action, which results in release of IGF bound to IGFBP-4. IGFBP-4 is the only known PAPP-A substrate. Its cleavage by PAPP-A uniquely depends on the presence of IGF. We here report mammalian expression and purification of recombinant 1547-residue PAPP-A (rPAPP-A). The recombinant protein is secreted as a homodimer of about 400 kDa composed of two 200-kDa disulfide-bound subunits. Antigenically and functionally, rPAPP-A behaves like the native protein. In human pregnancy, PAPP-A is known to circulate as a 500-kDa disulfide-bound 2:2 complex with the proform of eosinophil major basic protein (proMBP), PAPP-A/proMBP. A comparison between rPAPP-A and pregnancy serum PAPP-A/proMBP complex surprisingly reveals a difference greater than 100-fold in proteolytic activity, showing that proMBP functions as a proteinase inhibitor in vivo. We find that polyclonal antibodies against PAPP-A abrogate all detectable IGFBP-4 proteolytic activity in pregnancy serum, pointing at PAPP-A as the dominating, if not the only, IGFBP-4 proteinase present in the circulation. We further show that pregnancy serum and plasma contain traces (<1%) of uncomplexed PAPP-A with a much higher specific activity than the PAPP-A/proMBP complex. The measurable activity of the PAPP-A/proMBP complex probably results from the presence of a minor subpopulation of partly inhibited PAPP-A that exists in a 2:1 complex with proMBP. Inhibition of PAPP-A by proMBP represents a novel inhibitory mechanism with the enzyme irreversibly bound to its inhibitor by disulfide bonds.

Interaction of human pregnancy-associated plasma protein-A with serine proteinases

Human pregnancy-associated plasma protein A (PAPP-A) inhibited significantly the proteolytic activity of bovine trypsin and human plasmin. Trypsin or plasmin treatment of PAPP-A resulted in the generation of a major 85 kDa component and the rapid cleavage of internal thiol esters. The results indicated that both of these serine proteinases bound in a 1:1 stoichiometry to PAPP-A. The PAPP-A-bound enzymes were found to be enzymatically active towards small synthetic substrates and inaccessible to inactivation by soybean trypsin inhibitor and alpha 1-proteinase inhibitor. The mechanism of proteinase inhibition was likely to be entrapment, as described for alpha 2-macroglobulin.

Isolation and characterization of circulating complex between human pregnancy-associated plasma protein-A and proform of eosinophil major basic protein

The plasma protein previously known as pregnancy associated plasma protein-A (PAPP-A) and believed to contain only one kind of polypeptide chain has recently been shown to be a complex containing two different chains in equimolar amounts. One of the chains is now defined as the PAPP-A subunit, and the other has been identified as the proform of eosinophil major basic protein (proMBP) (Oxvig et al. (1993) J. Biol. Chem. 268, 12243-12246). A procedure for large scale preparation of the circulating complex (PAPP-A/proMBP) from pooled pregnancy serum is described. The amino acid and carbohydrate compositions of the isolated reduced and carboxymethylated PAPP-A (199 kDa) and proMBP subunits (38 kDa), and of the intact PAPP-A/proMBP have been determined. The PAPP-A and proMBP subunits contain 13.4% (w/w) and 38.6% (w/w) carbohydrate, respectively, and the intact complex contains 17.4% (w/w) carbohydrate. The PAPP-A subunit contains N-bound carbohydrate groups. In contrast, the proMBP subunit contains both N- and O-bound groups as well as glycosaminoglycan, previously found among plasma proteins only in inter-alpha-trypsin inhibitor and pre-alpha-trypsin inhibitor. It is shown that PAPP-A/proMBP can competitively inhibit human leucocyte elastase (KI = (5-10) x 10(-9) M) at an ionic strength of 0.075, but the inhibition is negligible at ionic strengths greater than 0.15. Human cathepsin G is also competitively inhibited (KI approx. 1 x 10(-6) M). The inhibition of both enzymes is most likely due to interactions with the glycosaminoglycan moiety of PAPP-A/proMBP. It is concluded that PAPP-A/proMBP is neither a potent nor a specific inhibitor of human leucocyte elastase.

Biochemical properties, activities, and presence in biologic fluids of eosinophil granule major basic protein

The existence of a proform of MBP is predicted from the sequence of MBP cDNA clones. ProMBP has been purified from the supernatants of CHO cells transfected with cDNA encoding prepro MBP. Purification involved heparin-Sepharose affinity purification followed by two sequential size fractionation steps over Sephadex G-100 and yielded proMBP with a molecular mass of 33 kd. Recombinant proMBP from the heparin-Sepharose column was subjected to isoelectric focusing followed by SDS-PAGE and Western blot analysis. The results indicated that most of the 33 kd form of proMBP focused predominantly between pI 4.2 and 5.1, with a major peak at a pI of approximately 4.9. Analyses of the carbohydrates associated with the purified 33 kd form of recombinant proMBP indicated the addition of 4856 to 5150 Da by carbohydrates characteristic of the complex type. Consistent with the hypothesis that the function of the propiece is to neutralize MBP toxicity during granule processing, proMBP lacked MBP cytostimulatory properties and actually blocked the effect of MBP in two different systems, basophil histamine release and neutrophil activation. In addition, as a measure of toxicity, proMBP did not inhibit protein synthesis, whereas MBP markedly reduced protein synthesis. The mechanisms by which MBP exerts its actions both as a cytostimulant and as a toxin are not known; however, it is known that cationic MBP readily reacts with acidic lipids. Using artificial liposomes as targets, MBP caused a disordering of the lipid bilayer membrane, resulting in fusion and lysis. Therefore, MBP may act both as a cytostimulant and as a toxin because of its marked cationicity and its ability to disorder lipid membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of pregnancy-associated plasma protein-A by electrophoresis

Gradient polyacrylamide gel electrophoresis, isoelectric focusing and multidimensional immunoelectrophoretic techniques have been applied in order to physico-chemically characterize pregnancy-associated plasma protein-A (PAPP-A). By lectin affinity immunoelectrophoresis, PAPP-A contained sialic acid, glucose/mannose and N-acetyl-alpha-D-galactosamine. Immunoelectrophoretic analyses after incubation with various glycolases confirmed these findings and demonstrated that PAPP-A contained glucuronic acid, perhaps in chondroitin sulphate moities, thus indicating that PAPP-A may be a proteoglycan rather than a glycoprotein. Analysis by metal chelate and dye ligand affinity immunoelectrophoresis demonstrated many similarities between PAPP-A and alpha 2-macroglobulin (alpha 2M). However, unlike alpha 2M, PAPP-A did not form immunologically reactive complexes when incubated with proteases. Furthermore, as demonstrated by autoradiographic studies, PAPP-A did not contain internal thiolester groups, thus indicating that PAPP-A cannot inhibit proteases by molecular entrapment and, despite the homotetrameric molecular conformation, PAPP-A and alpha 2M may not have evolved from a common ancestral protein.

Suppression of lymphocyte activation by a soluble factor released from the human placental chorionic membrane: chemical analysis and functional characterization

A heat-resistant factor that markedly and reversibly inhibits human lymphocyte activation was discovered in culture medium of human placental chorion. The chorionic factor inhibits lymphocyte proliferation in response to polyclonal mitogens and in the mixed leukocyte response. The inhibitory action is most effective if the factor is added during the first 24 h of lymphocyte culture and is reversible. The chorionic factor is sensitive to proteinase K, pepsin and bovine pancreatic protease; its activity is completely lost after papain digestion and following treatment with trichloracetic acid (TCA). The factor prevents the expression of IL-2 receptors and class II MHC glycoproteins (HLA-DR) on phytohemagglutinin-stimulated PBMC but does not affect the expression of MHC class I molecules. It inhibits the replication of IL-2-dependent CTLD cells but is without effect on the growth of various human or murine cell lines or acute leukemic cells. Human placental chorion is thus capable of releasing in vitro a nontoxic heat-resistant factor with protein characteristics that reversibly inhibits processes associated with the early stages of lymphoid cell triggering. This factor may play a role in an immunoregulatory mechanism that prevents maternally mediated immune rejection of the conceptus.

Placental-type plasminogen activator inhibitor in preeclampsia

The present cross-sectional study in patients with preeclampsia and gestational hypertension and in gestational age-matched controls was undertaken to investigate further the fibrinolytic system in these conditions. In preeclampsia we observed increased levels of total plasminogen activator inhibitor (p less than 0.001) but low levels of placental-type plasminogen activator inhibitor (p less than 0.05) compared with controls. The levels of placental-type plasminogen activator inhibitor were even more reduced (p less than 0.002) in pregnancies with a poor fetal outcome. It is concluded that placental-type plasminogen activator inhibitor does not contribute to the increased levels of total plasminogen activator inhibitor activity in preeclampsia. Placental-type plasminogen activator inhibitor levels correlated significantly with birth weight and placenta weight and may therefore reflect placental function.

Identification of a low molecular weight polypeptide of pregnant bovine uterine origin (LMW-UDF): influence on coagulation system in vitro. Part I

Acidified extracts of pregnant bovine uterus were found to contain a heparin-affinity polypeptide(s). The eluted peak was assayed in the Chandler loop method for whole blood and fibrin thrombus generation. Significant differences in the mean weight and size of whole blood thrombi (243.5 +/- 50 mg and 3.5 cm +/- 0.6 cm) occurred with 1 microgram of purified polypeptide when compared to control samples containing albumin (47.87 +/- .30 g and 0.8 cm +/- 0.3 cm). This activity was detected by the Chandler loop method with 100 ng to 10 micrograms of protein when analyzed in a dose-response fashion. When tested for fibrin thrombus formation this activity persisted. Experiments with 125I fibrinogen revealed no net increase or decrease of uptake into whole blood thrombi when purified polypeptide was added. The heparin-affinity polypeptide(s) possessed a molecular weight of approximately 6-4 kDa when examined by SDS-PAGE. We have therefore designated this polypeptide as low molecular weight-uterine derived factor (LMW-UDF).

New placental proteins: chemistry, physiology and clinical use

In the past decade, several new placental proteins have been isolated and studied. The 'pregnancy-specific' beta 1-glycoprotein (SP1) is a major placental product with unusual physicochemical properties that has been extensively investigated, but its biological function remains uncertain. Pregnancy-associated plasma protein A (PAPP-A), a glycoprotein of mol. wt 400,000, has effects in vitro on the coagulation and complement cascades, probably by its properties of protease inhibition. Placental protein 5 (PP5) may be involved in the coagulation and fibrinolytic systems, and in follicle maturation and semen liquefaction. 'Placental protein 12' (PP12) is not a product of the placenta at all; it appears to be produced in the female genital tract under the influence of progesterone and may also be produced by proliferating liver cells. Further study may reveal new roles for these placental proteins beyond their traditional roles as tumour markers.

Pulsatile secretion of pregnancy-associated plasma protein-A (PAPP-A) in non-pregnant women

No changes in PAPP-A levels could be detected when plasma samples were obtained every second day throughout the entire menstrual cycle in three healthy women. But when measured every 30 min throughout 12 consecutive hours, in four healthy women, PAPP-A followed a pulsatile pattern which may originate in the endometrium since no PAPP-A pulses were detected in a long-term hysterectomized woman, or in two men. The pulses were not related to levels of oestradiol, progesterone, FSH or LH.

In vitro release of pregnancy-associated plasma protein-A (PAPP-A) by human endometrial cells

Evidence has now accumulated to sustain the idea that pregnancy-associated plasma protein-A (PAPP-A) is not pregnancy specific: it can be localized immunohistochemically in nonpregnant human endometrium. Thus, it was of interest to see if a primary cell culture of human endometrial cells was capable of producing PAPP-A. Immunoreactive PAPP-A-like material was detected in the culture medium of glandular as well as stromal cells. The presence of PAPP-A in endometrial culture medium was specific and could not be attributed to a nonspecific protease effect on the tracer used in radioimmunoassay. The production rate of PAPP-A by stromal cells could be enhanced by the addition of medroxyprogesterone acetate (MPA). Estradiol (E2) alone or in combination with MPA did not modify the production rate of PAPP-A. The production rate of PAPP-A by glandular cells remained unchanged after treatment with MPA and/or E2. These results confirm that in nonpregnant women PAPP-A is a progesterone-dependent protein produced by the endometrium.

Binding of pregnancy-associated plasma protein-A (PAPP-A) to placental subfractions

Pregnancy-associated plasma protein-A (PAPP-A) has been shown to exert immunosuppressive effects both on complement and on lymphoblastogenesis. It was of interest to see whether this protein could bind to syncytiotrophoblast microvillous membranes since these represent the effective interface between fetal tissue and the immunocompetent mother. Placental subfractions were prepared according to established techniques. PAPP-A was purified from different sources (pregnancy serum and plasma, retroplacental serum, placental extracts) and labelled with radioactive iodine. It could be shown that radioactive PAPP-A, irrespective of its biological origin was primarily binding to brush border membrane preparations but that significant binding was also seen with plasma membrane preparations. The binding was specific since alpha 2-macroglobulin (a structurally related protein to PAPP-A) was unable to displace bound radioactive PAPP-A. Scatchard plot representation of the data indicated that the affinity of PAPP-A for its binding site was of the same order of magnitude as reported for the insulin or GnRH receptors of the trophoblast (Ka 2 10(7)-2.5 10(8) M-1). These results are in agreement with the immunohistochemical localization of PAPP-A on the trophoblast providing that the small binding to plasma membrane preparations is due to contamination.

Structural changes in alpha-2- and ovomacroglobulins studied by gel chromatography and electron microscopy

The structural change that occurs in alpha-2-macroglobulin upon its interaction with methylamine or chymotrypsin was studied by high-performance gel chromatography and electron microscopy. The result enabled us to estimate the Stokes radius of the protein as 8.8 nm and 7.9 nm before and after binding with the proteinase, respectively. The methylamine-treated protein also had the Stokes radius of 7.9 nm. Similar studies on the chicken and crocodilian ovomacroglobulins showed that these homologues of alpha 2-macroglobulin had Stokes radii of 9.2-9.3 nm and 8.5-8.7 nm before and after binding with chymotrypsin. Their Stokes radii did not change as a result of the methylamine treatment. Electron micrographs of the native and altered forms of the three proteins are presented. This study introduces a simple and quantitative method to study the structural change of alpha 2-macroglobulin and its homologues.

Placental plasminogen activator activity in pre-eclampsia

Plasminogen activator is one of the principal enzymes involved in the fibrinolytic system. The plasminogen activator activity in placental tissue was examined using a chromogenic substrate in 9 patients with pre-eclampsia and in 9 women with uncomplicated pregnancy, who were delivered between 37 and 42 weeks of gestation. In the patients with pre-eclampsia, the plasminogen activator activity was significantly (P less than 0.05) decreased compared with that in the control group of normal pregnant women. The present findings suggest a decreased fibrinolytic activity in the placental tissue of pre-eclamptic patients, which is responsible for fibrin deposition in the placental tissue and may contribute to placental ischemia and insufficiency in pre-eclampsia.

Seminal plasma levels of PAPP-A in normospermic and oligospermic men and tissue localization of PAPP-A in the male genital tract

Radioimmunoassay, gel filtration, isoelectric focusing, and immunoperoxidase methods were used to study the levels, properties, and localization of pregnancy-associated plasma protein A (PAPP-A) in the human seminal plasma and male genital tract. Seminal plasma specimens from 20 normospermic and 20 oligospermic men were studied. PAPP-A was found in 30 of 40 samples, the levels ranging from undetectable to 135 micrograms/liter (median 35 micrograms/liter) in the normospermic group and from undetectable to 111 micrograms/liter (median 21 micrograms/liter) in the oligospermic group. There was no significant difference in the PAPP-A levels between the two groups, and no correlation was observed between the seminal plasma PAPP-A levels and the volume of seminal plasma or the sperm count or the viability of spermatozoa. Serial dilutions of seminal plasma and PAPP-A standard yielded parallel dose-response curves, and incubation with seminal plasma did not result in any change in the elution pattern of [125I]PAPP-A in gel filtration. PAPP-A-immunoreactive material from seminal plasma eluted as two peaks, the major one corresponding to the elution volume of purified PAPP-A and the minor eluting more slowly. The isoelectric point of seminal plasma PAPP-A was 4.3-4.7 and that of term pregnancy serum was 4.2-4.6. In the immunoperoxidase staining, PAPP-A was seen in the epithelium of the prostate, seminal vesicle, and the ampullar part of the vas deferens but not in the testis, epididymis, proximal parts of vas deferens or urethra. The results confirm the occurrence of PAPP-A in the seminal plasma but do not suggest any clinical utility for seminal plasma PAPP-A measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

The development and validation of a radioimmunoassay for human pregnancy-associated plasma protein A (PAPP-A)

A specific and sensitive radioimmunoassay for human pregnancy-associated plasma protein A (PAPP-A) is described. This assay utilized tracer repurified by affinity chromatography on heparin-Sepharose. The most sensitive assay developed had a detection limit of 5 microliters PAPP-A in serum, and this protein could not be detected in sera from non-pregnant subjects. During pregnancy, mean circulating levels rose from 0.6 mg/l at 6 weeks gestation to 110 mg/l at term, and PAPP-A was detected in 44 of 101 umbilical blood samples. The use of the anticoagulants EDTA and acid citrate dextrose resulted in significantly different results in the plasma compared to the corresponding serum sample. PAPP-A was also measured in amniotic fluid.

The disappearance rate of pregnancy-associated plasma protein-A (PAPP-A) after the end of normal and abnormal pregnancies

PAPP-A is a macromolecular glycoprotein associated with human pregnancy. In vitro, PAPP-A is produced by explant cultures of trophoblast and decidua. The present work was undertaken to see if the presence of decidua had any effect on the disappearance rate of PAPP-A after removal of the placenta either by surgery or by spontaneous delivery. PAPP-A was measured before and at different times after a normal delivery (n = 6), after a termination of early pregnancy (n = 11) and after surgery for ectopic pregnancy (n = 8). The half life of PAPP-A after normal delivery (52.9 +/- 25.8 h, SD) was significantly (p less than 0.03) less than after a first trimester termination (93.9 +/- 41.6 h). After surgery for ectopic pregnancy in patients with curetted decidua, PAPP-A disappeared significantly faster (p less than 0.005) then in patients with intact decidua (84.1 +/- 17.8 vs 241.2 +/- 81.5 h). These results indicate that PAPP-A continues to be produced by the decidua after removal of the trophoblast in early pregnancy.

Enzyme immunometric assay for the determination of pregnancy associated plasma protein A (PAPP-A) with the antigen as solid phase (conjoint IEMA)

Purified pregnancy associated plasma protein A (PAPP-A) can be effectively bound to polystyrene microtitre plates. This immobilized antigen competes with the added serum PAPP-A of unknown concentration for the limited amount of peroxidase-labeled monospecific anti-PAPP-A antibody incubated simultaneously. The sensitivity is 0.1 WHO unit/ml and non-specific binding is 1.0%.

Pregnancy-associated plasma protein-A (PAPP-A): concentration in uterine fluid and immunohistochemical localization in the endometrium

Pregnancy-associated plasma protein-A (PAPP-A) was localized immunohistochemically in the endometrium and measured in uterine fluid of non-pregnant women. The variations of PAPP-A concentrations in uterine fluid during the menstrual cycle paralleled those found in the endometrium. In patients receiving hormone therapy there was a significant correlation between the uterine fluid PAPP-A concentration and the progestogen to oestrogen potency ratio of the hormonal treatment. The presence of PAPP-A in the uterine fluid cannot simply be explained by blood contamination or cell damage. These results are interpreted as indirect evidence for an exocrine as well as an endocrine secretion of PAPP-A by the endometrium which might be influenced by hormones.

Pregnancy-associated plasma protein A: purification under mild conditions, peptide mapping and tests for possible interactions with trypsin, plasmin and complement

Pregnancy-associated plasma protein A (PAPP-A) has been obtained in an enriched state under mild conditions of purification involving cibacron blue dye-ligand chromatography, negative antibody-affinity chromatography and gel filtration. The product was a mixture of PAPP-A and alpha-2-macroglobulin (alpha 2M). The ability of these proteins to bind 125I-trypsin and 125I-plasmin was studied. In contrast to previous studies by others, there was no evidence that PAPP-A could bind either protease. It is pointed out that protease-inhibitory activity in samples of PAPP-A can be due to the presence of very small amounts of contaminating alpha 2M in the preparations of PAPP-A. In further experiments PAPP-A did not inhibit the complement-induced lysis of sensitized red cells. Finally, cyanogen bromide peptide mapping experiments failed to yield evidence of structural homologies between PAPP-A and alpha 2M.

Immunohistochemical localization of pregnancy-associated plasma protein A in decidua and trophoblast: comparison with human chorionic gonadotrophin and fibrin

In an immunoperoxidase study, pregnancy-associated placental protein A (PAPP-A) was localized to the cytoplasm of decidual stromal cells, the villous syncytiotrophoblast and on the surface of placental trophoblast. A decidual and a trophoblastic origin is suggested for PAPP-A, and its presence on the surface of the syncytiotrophoblast is interpreted as a possible immunoprotective layer.

In vitro production of pregnancy-associated plasma protein A by human decidua and trophoblast

Immunohistochemical techniques and direct measurements of pregnancy-associated plasma protein A (PAPP-A) have demonstrated the presence of PAPP-A in trophoblast and decidua. The purpose of the present study was to investigate the possibility that these tissues are capable of producing PAPP-A in vitro. Trophoblast and decidua were obtained from term deliveries and from legal surgical terminations of pregnancy (7 to 12 weeks). In addition to trophoblast and decidua, myometrium was also obtained during two hysterectomies in the first trimester of pregnancy. Tissues were incubated in medium 199 at 37 degrees C under an oxygen/carbon dioxide atmosphere. Media containing either pregnancy-associated serum or non-pregnancy-associated serum were changed after 8 hours of incubation in medium 199 alone. In addition to PAPP-A, human placental lactogen (hPL) and prolactin (Prl) were measured in homogenates and media by radioimmunoassays in order to confirm the viability of the cultured tissues. Addition of pregnancy-associated serum to the media induced a significant release of PAPP-A from trophoblast and decidua when compared to that in control cultures. Non-pregnancy-associated serum had no effect. Myometrium did not release any measurable PAPP-A into the medium even in the presence of pregnancy-associated serum. Cycloheximide added to pregnancy-associated serum significantly inhibited the release of PAPP-A from trophoblast and decidua. These last tissues, irrespective of the culture condition, released significantly more PAPP-A as well as hPL and Prl than was initially present in the tissue. These data demonstrate that PAPP-A is released in vitro by trophoblast and decidua (but not by myometrium) and that this release can be magnified by a factor present only in pregnancy-associated serum. The release of PAPP-A, hPL, and Prl is considered as a de novo production since concentration of these proteins are higher in media and tissues after incubation compared to concentrations initially present in the tissue before culture and since cycloheximide significantly inhibits the release of PAPP-A, Prl, and hPL from the cultured tissues.

Pregnancy-associated plasma protein A (PAPP-A) specifically inhibits the third component of human complement (C3)

Pregnancy-associated plasma protein A (PAPP-A), a macromolecular glycoprotein associated with pregnancy, was shown to inhibit complement-induced haemolysis and to bind heparin reversibly. Because of the inhibitory effects of heparin on the complement cascade it was not clear if the inhibition of complement activity observed with PAPP-A (isolated from heparin plasma) was attributable to the heparin moiety bound to PAPP-A. This work demonstrates that heparin exerts an inhibitory effect on complement activity but that heparin-free PAPP-A is also inhibitory. PAPP-A specifically inhibits C3 by binding to this complement subcomponent and not by inhibiting C3 convertase as demonstrated for C3 inactivator.

Two-site enzyme immunoassay for pregnancy associated plasma protein A (PAPP-A)

Pregnancy associated plasma protein A (PAPP-A) is one of the newly isolated placental proteins whose concentration in the maternal circulation increases during pregnancy. The limited sensitivity of the immunoelectrophoretic techniques first developed to measure PAPP-A meant that assays could only be done in the second half of pregnancy but now radioimmunoassays have been set up in order to determine the concentration of this protein in early pregnancy as well. The two-site enzymometric immunoassay described here is characterised by a sensitivity and a precision which are comparable to the results obtained by radioimmunoassay. Neither the structurally similar alpha 2-macroglobulin, nor other pregnancy associated proteins interfered in the assay. The tracer, monospecific rabbit anti-PAPP-A coupled covalently to horseradish peroxidase, is stable for months when stored adequately.

Biochemical aspects of conceptus--endometrial interactions

Mammalian conceptuses must provide a chemical signal to the maternal system to insure maintenance of corpus luteum (CL) function and of progesterone production and continuation of uterine endometrial secretory activity. These events insure that the developing conceptus is provided with appropriate nutrients, regulatory enzymes and endocrine state to allow successful establishment and maintenance of pregnancy. Pig blastocysts begin to produce estrogens by Day 11 of pregnancy, which prevents secretion of the uterine luteolytic factor (PGF2 alpha) in an endocrine direction, but allows secretion in an exocrine direction, i.e., into the uterine lumen. Therefore, CL are "protected." Blastocyst estrogens also trigger secretion of a group of proteins, including uteroferrin, an iron transport protein, and a family of protease inhibitors whose biosynthesis within the uterine glandular epithelium is under the control of progesterone. Estrogen also appears to promote accumulation of glucose and fructose within the uterine lumen. A complex in vivo "culture medium" is thereby established to promote conceptus development. Pig blastocysts do not undergo invasive implantation within the uterine lumen although they produce the protease, plasminogen activator. Invasion may be prevented by endometrial secretion of progesterone-induced protease inhibitors which are produced in large amounts. In addition to estrogens of conceptus origin, calcium and prostaglandins PGF2 alpha and E2 may affect the uterine vasculature, water and electrolyte transport, capillary permeability, conceptus steroid production, and related events during pregnancy. The blastocysts of the large domestic animals also secrete proteins which include a large glycoprotein (Mr approximately 600,000) and a small acidic protein (Mr approximately 17,000). The latter has been purified from sheep and named ovine trophoblast protein I. These proteins may play unique roles in early pregnancy with respect to establishment and maintenance of pregnancy in the ewe, sow, mare, and cow.

Pregnancy-associated plasma protein-A (PAPP-A) inhibits thrombin-induced coagulation of plasma

Concentrations of immunoreactive PAPP-A have been found significantly lower in the serum as compared to heparin or EDTA plasma from the same patients. After coagulation significant amounts of PAPP-A remain associated with the clot. Purified PAPP-A inhibits thrombin induced coagulation of plasma. This inhibition cannot be attributed to a direct effect of PAPP-A on thrombin. It is exerted via an activation of endogenous antithrombin III since the inhibitory effect of PAPP-A on thrombin induced coagulation in a euglobulin system can be observed only if antithrombin III is added. The fact that protamine sulphate is capable of neutralizing the inhibitory effects of PAPP-A made us postulate that PAPP-A, like heparin, possesses strongly acidic residues which bind to protamine.

An ELISA for pregnancy-associated plasma protein A

An enzyme-linked immunoassay for pregnancy-associated plasma protein A is described. The assay is sensitive (7.6 IU/l) and precise (overall CV varied between 8.9% and 10.8%). Recovery was 100% over the working range. Parallel behaviour was observed between the International Reference Preparation 78/610 and pregnancy serum, thus allowing calibration of a laboratory standard pool against IRP 78/610.

Interaction between heparin and human pregnancy-associated plasma protein A (PAPP-A): a simple purification procedure

Human pregnancy-associated plasma protein A (PAPP-A) binds to heparin-Sepharose. This affinity chromatography preceded by molecular sieve chromatography provides a simple two-step purification procedure of PAPP-A from late pregnancy plasma. One hundred percent of the applied PAPP-A was recovered, with more than 40% being electrophoretically homogeneous after the two procedures. The remaining PAPP-A could be purified by negative affinity chromatography on anti-total human serum immobilized on agarose.

Prognostic significance of the new placental proteins in trophoblastic disease

Circulating levels of four specific placental proteins, human chorionic gonadotrophin (hCG), Schwangerschafts protein 1 (SP1), placental protein 5 (PP5) and pregnancy-associated plasma protein A (PAPP-A), were measured in 31 patients with hydatidiform mole before treatment. Seven patients subsequently developed clinical choriocarcinoma and three of them had pulmonary metastases. The estimations of hCG, PP5 and PAPP-A levels were found to be of no value in the prediction of malignant sequelae. Levels of SP1 greater than or equal to 16.5 i.u./l were associated with a higher incidence of subsequent malignant disease (P less than 0.05), the risk being at least nine times greater in these patients. The predictive value of high levels of SP1 was 35%, the specificity 45.8% and sensitivity 100%.

Circulating levels of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotrophin (hCG) in intrauterine and extrauterine pregnancies

Pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotrophin (hCG) have been measured in the plasma of pregnant and non-pregnant women attending the outpatient clinic for suspicion of pregnancy. The plasma concentrations obtained were grouped into intrauterine or extrauterine pregnancies and compared with values obtained in non-pregnant patients with similar periods of amenorrhoea. Patients with ectopic pregnancies had slightly lower PAPP-A levels and significantly lower hCG concentrations than those in women with normal intrauterine pregnancies. Non-pregnant women had very low hCG and PAPP-A levels compared with those in pregnant patients. These data suggest that in patients with extrauterine pregnancies the poorly sustained ectopic trophoblast is unable to produce normal concentrations of hCG and probably PAPP-A and that the slightly diminished levels of PAPP-A in ectopic pregnancies might be derived from a decidual production.

Is pregnancy-associated plasma protein A a tumor marker?

Pregnancy-associated plasma protein A (PAPP-A) was measured in 159 patients with various nontrophoblastic tumors. In three patients with trophoblastic tumors, PAPP-A and human chorionic gonadotropin (hCG) were measured before operation and weekly thereafter. Circulating levels of PAPP-A in patients with nontrophoblastic tumors were not significantly different from the levels in the matched control groups. Circulating levels of PAPP-A and hCG were significantly elevated preoperatively in patients with trophoblastic tumors, whereas the postoperative concentrations of both proteins gradually decreased. The conclusion is that, probably, PAPP-A cannot be considered to be a marker for nontrophoblastic disease, and that measurements of PAPP-A in patients with trophoblastic cancers do not provide any advantage over measurements of hCG.

Amniotic fluid and plasma concentrations of pregnancy-associated plasma protein-A (PAPP-A) throughout pregnancy: comparison with other fetoplacental products

The development of a sensitive radioimmunoassay has enabled measurements of pregnancy-associated plasma protein-A (PAPP-A) to be performed from early pregnancy. The present paper compares the plasma concentrations of PAPP-A with the levels of two trophoblastic proteins, human placental lactogen (hPL) and the beta-subunit of human chorionic gonadotrophin (beta-hCG), with a steroid of fetoplacental origin, total oestriol (total E3), and with a fetal protein, alpha-fetoprotein (AFP). PAPP-A was also measured in amniotic fluid and in maternal urine. In contrast with the secretion of the other substances studied, which either reach a plateau or even decrease during the last 4 weeks of pregnancy, PAPP-A steadily increased in the maternal circulation from 7 to 40 weeks gestation. It is proposed that PAPP-A production is either not related to placental mass or that PAPP-A is not of trophoblastic origin. The increase of PAPP-A in amniotic fluids parallels the increase in maternal blood; virtually no PAPP-A is excreted in urine.

Pregnancy-associated plasma protein-A (PAPP-A) and hCG in early pregnancy

Pregnancy-associated plasma protein-A (PAPP-A) is a recently described glycoprotein of unknown biological function. The development of a radioimmunoassay enabled us to measure plasma levels of PAPP-A and human chorionic gonadotrophin (hCG) in 12 non-pregnant volunteers and in 159 women in early pregnancy attending the outpatient clinic for legal abortion. PAPP-A but not hCG was measurable in all non-pregnant women. In pregnant patients (with 36 to 86 days of amenorrhea) hCG reached a peak value (163.1-197.6 ng/ml) between the 9th and the 13th week whereas PAPP-A steadily increased throughout this period of pregnancy. Between the 6th and the 13th week after the last menstrual period, levels of PAPP-A increased proportionally more than hCG. This work provides the first evidence of a PAPP-A production in non-pregnant subjects and the very early marked increase of PAPP-A secretion during pregnancy.

Pregnancy associated plasma protein-A: interaction with heparin in crossed affinity immunoelectrophoresis

A specific interaction between pregnancy associated plasma protein A- (PAPP A) and heparin has been demonstrated using heparin affinity crossed immunoelectrophoresis applied to late pregnancy serum. The presence of heparin in the first dimensional gel accelerated the anodic migration of six serum proteins with beta electrophoretic mobility and one protein in the alpha mobile region. Two of the six beta mobile proteins were identified as antithrombin III and beta lipoprotein, the alpha mobile protein being PAPP-A. The migration distance of other proteins originating in the placenta (pregnancy specific beta, glycoprotein), fetus (alpha-fetoprotein), leucocytes (pregnancy zone protein), or maternal liver (alpha, antitrypsin alpha 2 macroglobulin) was not altered. The interaction of PAPP-A with heparin was therefore independent of molecular size, charge, and site of origin indicating a specific high affinity interaction between PAPP-A and heparin. There results indicate that PAPP-A may be involved locally with the coagulation system in the maintenance of placental circulation.

Pregnancy-associated plasma protein A: circulating levels during normal pregnancy

The development of specific and sensitive electroimmunoassays for a recently identified high molecular weight alpha-2 mobile pregnancy-specific protein (pregnancy-associated plasma protein A, PAPP-A or SP4) is described. These assays have permitted the detection of circulating levels of PAPP-A (10 microgram/L) as early as the fifth week of pregnancy. In all 18 subjects studied, the levels of PAPP-A rose from first detection in the first trimester until delivery at term. The development of these assays now permit the evaluation of PAPP-A measurement as a diagnostic test of early pregnancy and as an index of fetal well-being throughout gestation.

Purification of pregnancy-associated plasma protein-A by a two step affinity chromatographic procedure

A high molecular weight pregnancy specific protein (PAPP-A) was purified by immunospecific affinity chromatography. The purification procedure was based on a new method for affinity chromatography using direct coupling of precipitable immune complexes to the gel matrix. The procedure was performed in two steps: a positive immunospecific affinity chromatography followed by a negative affinity chromatography in which balanced amounts of anti-contaminant antibodies were used as ligant. The purified material at a concentration of 300 microgram/ml was tested by immunoelectrophoretic methods and no contaminants were detectable. WHO beta-1 SP1 reference material 78610 contained 45 microgram PAPP-A/ml.

Pregnancy-associated plasma protein-A: an inhibitor of the complement system

Pregnancy-associated plasma protein-A (PAPP-A) was purified to homogenicity from third-trimester pregnancy plasma using ion exchange chromatography and affinity chromatography on con A-Sepharose and anti-human serum-Sepharose. The elution of PAPP-A, alpha 2-macroglobulin, Cl-inhibitor and alpha 1-antitrypsin was followed by immuno-diffusion. the eluates were tested for stimulation or inhibition of complement-induced haemolysis. PAPP-A markedly inhibits the haemolytic activity of complement.

Studies on human pregnancy-associated plasma protein A. Purification by affinity chromatography and structural comparisons with alpha 2-macroglobulin

Pregnancy-associated plasma protein-A (PAPP-A) has been purified by a combination of methods including antibody-affinity chromatography. The resultant protein, obtained in 16% yield from maternal serum, appeared as a single major component on non-denaturing polyacrylamide and SDS/polyacrylamide gel electrophoresis. The protein showed a single component when analysed by isoelectric focusing under denaturing conditions in the presence and absence of reduction and had a pI of 4.34 and 4.42 respectively. These pI values were indistinguishable from those of alpha 2-macroglobulin (alpha 2M). The molecular weight of the PAPP-A polypeptide as shown by SDS/polyacrylamide-gel electrophoresis was 187000, with a minor component of mol.wt. 82500 that was attributed to proteolysis. Since native PAPP-A had a molecular weight on gel chromatography very similar to that of alpha 2M (620000--820000), it was concluded that PAPP-A was a homotetramer. In the absence of reduction, a high-molecular-weight (420000) protomer of PAPP-A was found. It was deduced that PAPP-A, like alpha 2M, is a dinner, whose protomers are composed of disulphide-linked polypeptide chains. It was found that the molecular weight of the PAPP-A polypeptide exceeded that of alpha 2M by 3.3%, but that the total carbohydrate content of PAPP-A exceeded that of alpha 2M by 10% and that its neutral carbohydrate content exceeded that of alpha 2M by between 7.4 and 9.0%. The significance of the estimated molecular weights of alpha 2M (181000) and its major tryptic fragments is discussed in the light of published values. A tryptic fragment alpha 2M (82500 mol.wt.) was apparently the same size as the major tryptic fragment of PAPP-A.