Recombinant PAPP-A resistant insulin-like growth factor binding protein 4 (dBP4) inhibits angiogenesis and metastasis in a murine model of breast cancer

BACKGROUND

The Insulin-like growth factor (IGF) pathway plays a role in tumour development and progression. In vivo, IGF1 activity is regulated by the IGF binding proteins (IGFBPs). IGFBP4 inhibits the activity of IGF1 but proteolytic cleavage by pregnancy-associated plasma protein-A (PAPP-A) releases active IGF1. A modified IGFBP4, dBP4, which was resistant to PAPP-A cleavage but retained IGF1 binding capacity, was engineered, expressed in Human Embryonic Kidney (HEK) 293 cells and purified. This study examined the effects of dBP4 on IGF1-induced cell migration, invasion and angiogenesis in vitro. The effect of intra-tumour injections of dBP4 on tumour angiogenesis and metastasis was examined using the 4T1.2luc orthotopic model of breast cancer.

METHODS

PAPP-A resistance and IGF binding capacity of dBP4 were characterized by Western blot and surface plasmon resonance, respectively. 4T1.2luc are mouse mammary adenocarcinoma cells transfected with luciferase to allow in vivo imaging. The effect of dBP4 on IGF1-induced Akt activation in 4T1.2luc cells was assessed by Western blot. Cell migration and invasion assays were performed using 4T1.2luc cells. Angiokit™ assays and Matrigel® implants were used to assess the effects of dBP4 on angiogenesis in vitro and in vivo, respectively. An orthotopic breast cancer model - 4T1.2luc cells implanted in the mammary fat pad of BALB/c mice - was used to assess the effect of intra tumour injection of purified dBP4 on tumour angiogenesis and metastasis. Tumour growth and lung metastasis were examined by in vivo imaging and tumour angiogenesis was evaluated by CD31 immunohistochemistry.

RESULTS

Our engineered, PAPP-A resistant IGFBP4 (dBP4) retained IGF1 binding capacity and inhibited IGF1 activation of Akt as well as IGF1-induced migration and invasion by 4T1.2 mammary adenocarcinoma cells. dBP4 inhibited IGF1-induced angiogenesis in vitro and in Matrigel implants in vivo. Direct intra-tumour injection of soluble dBP4 reduced angiogenesis in 4T1.2 luc mammary tumours tumour and reduced lung metastasis.

CONCLUSION

A PAPP-A resistant IGFBP4, dBP4, inhibits angiogenesis and metastasis in 4T1.2 mammary fat pad tumours. This study highlights the therapeutic potential of dBP4 as an approach to block the tumour-promoting actions of IGF1.

A robust immunoassay for pregnancy-associated plasma protein-A2 based on analysis of circulating antigen: establishment of normal ranges in pregnancy

Pregnancy-associated plasma protein-A (PAPP-A) and PAPP-A2, two homologous metzincin metalloproteases, are both tightly linked to regulation within the insulin-like growth factor (IGF) system because of their specific cleavage of IGF binding proteins. Recent studies suggest that PAPP-A may be involved in clinical conditions related to unwanted cellular growth, and the circulating levels of PAPP-A is an established biomarker in prenatal screening for chromosomal abnormalities. Microarray data indicate that PAPP-A2 has potential as a biomarker for pre-eclampsia. However, well-characterized immunological methods of quantification are not available. We therefore developed monoclonal antibodies against recombinant PAPP-A2. The antibodies were epitope mapped against recombinantly expressed chimeras between PAPP-A2 and PAPP-A. Furthermore, circulating PAPP-A2 was immunoaffinity purified and characterized by sequence analysis and mass spectrometry. Unlike PAPP-A, PAPP-A2 is a noncovalent dimer in which each subunit of 1558 amino acids originates from all of the 22 predicted coding exons. A previously hypothesized variant (PAPP-E) does not exist, but low amounts of a C-terminally truncated PAPP-A2 variant was detected. A sensitive and robust ELISA for full-length PAPP-A2 was developed and used to establish normal ranges of PAPP-A2 through pregnancy. The functional sensitivity of this ELISA at 20% CV was 0.08 ng/ml, and the serum concentration of PAPP-A2 was found to increase during pregnancy in agreement with placental synthesis. The existence of this assay will enable an assessment of the biomarker potential of PAPP-A2 in pre-eclampsia as well as other clinical conditions.

Proteolytic degradation of IGF-binding protein (IGFBP)-2 in equine ovarian follicles: involvement of pregnancy-associated plasma protein-A (PAPP-A) and association with dominant but not subordinated follicles

In the ovary of mammalian species, terminal follicular growth is accompanied by a decrease in intrafollicular levels of IGF-binding protein-2 (IGFBP-2) and IGFBP-4. The decrease in IGFBP-4 levels is essentially due to an increase in proteolytic cleavage by intrafollicular pregnancy-associated plasma protein-A (PAPP-A) in growing healthy follicles. The decrease in IGFBP-2 levels is partly due to a decrease in mRNA expression by follicular cells. In addition, we have recently shown that IGFBP-2 is also proteolytically cleaved by PAPP-A in bovine and porcine growing follicles. In the present work, we showed that follicular fluid from late dominant equine follicles (35 mm diameter) contains a proteolytic activity against IGFBP-2. First follicular fluid from dominant follicles contained lower levels of native IGFBP-2 than the corresponding serum, as assessed by Western ligand blotting. In contrast, immunoblotting experiments showed much higher levels of a 12 kDa proteolytic fragment in dominant follicular fluid than in the serum. Moreover, equine dominant follicular fluid was able to induce proteolysis of exogenous recombinant bovine (rb)IGFBP-2, this degradation being dose-dependently enhanced by IGFs. The proteolytic activity against IGFBP-2 in equine follicles was partially immunoneutralized by a polyclonal antibody raised against human PAPP-A. Moreover, cleavage of rbIGFBP-2 by equine follicular fluid was dose-dependently inhibited by a peptide derived from the heparin-binding domain of IGFBP-5, as well as by peptides derived from other heparin-binding domain-containing proteins such as connective tissue growth factor, vitronectin and heparin-interacting protein, previously shown to inhibit PAPP-A. Finally, the proteolytic activity was very low in subordinate follicles, was high in both early (25 mm diameter) and late (35 mm diameter) dominant follicles, and was slightly lower in preovulatory follicles recovered 35 h after human chorionic gonadotropin (hCG) treatment.Overall, these data show that in the equine ovary, the selection of dominant follicles is associated with an increase of the proteolytic degradation of IGFBP-2 by PAPP-A, as for IGFBP-4, and potentially other protease(s), probably contributing to the increase in IGF bioavailability. In atretic subordinate follicles, the decrease in the proteolytic degradation of IGFBP-2, probably due in part to a direct inhibition by peptides containing heparin-binding domains, contributes to the increase in IGFBP-2 levels and the decrease in IGF bioavailability. The expression of PAPP-A and IGFBP-2 mRNA during folliculogenesis remain to be investigated in the mare.

Screening for Down's syndrome in early and late first and second trimester using six maternal serum markers

The efficiency of six maternal serum markers for Down's syndrome (DS), alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), free beta-hCG, pregnancy-associated plasma protein-A (PAPP-A), the proform of eosinophil major basic protein (ProMBP), pregnancy-specific-beta-1-glycoprotein (SP(1)), and combinations thereof, was examined. Discriminant analysis in 156 DS pregnancies and 546 controls defined three effective combinations of serum marker logMoMs (multiples of the median in control samples) in three gestational age windows, i.e. Index I (weeks 7-9) = 0.52 logMoM ProMBP + 0.28 logMoM PAPP-A - logMoM SP(1); Index II (weeks 10-12) = 1.94 logMoM free beta-hCG - logMoM SP(1), and Index III (weeks 15-19) = 0.78 logMoM free beta-hCG + 1.12 logMoM ProMBP - logMoM AFP. The estimated detection rates of indices and age for a false-positive rate (FPR) of 5% were 73% for Index I, 69% for Index II, and 60% for Index III. Including the ultrasound marker nuchal translucency, using a DS at term risk of 1 : 400 as cut-off, the detection rates of the indices increased to 86, 83, and 82% for FPRs of 4.3, 4.1, and 5.8%, respectively. The indices are promising markers for screening for DS.

Identification and regulation of the IGFBP-4 protease and its physiological inhibitor in human trophoblasts and endometrial stroma: evidence for paracrine regulation of IGF-II bioavailability in the placental bed during human implantation

The IGF family plays an important role in implantation and placental physiology. IGF-II is abundantly expressed by placental trophoblasts, and IGF binding protein (IGFBP)-4, a potent inhibitor of IGF actions, is the second most abundant IGFBP in the placental bed, expressed exclusively by the maternal decidua. Proteolysis of IGFBP-4 results in decreased affinity for IGF peptides, thereby enhancing IGF actions. In the current study, we have identified the IGFBP-4 protease and its inhibitor in human trophoblast and decidualized endometrial stromal cell cultures, and we have investigated their regulation in an effort to understand control of IGF-II bioavailability at the placental-decidual interface in human implantation. IGFBP-4 protease activity was detected in conditioned media (CM) from human trophoblasts and decidualized endometrial stromal cells using (125)I-IGFBP-4 substrate. Identification of the IGFBP-4 protease as pregnancy-associated plasma protein-A (PAPP-A) was confirmed by specific immunoinhibition and immunodepletion of the IGFBP-4 protease activity with specific PAPP-A antibodies. The IGFBP-4 protease activity was IGF-II-dependent in trophoblast CM. In decidualized stromal CM, PAPP-A/IGFBP-4 protease activity was also IGF-II-dependent, but was evident only when IGF-II was added in molar excess of the predominant IGFBP in decidualized stromal cell CM, IGFBP-1, supporting bioavailable IGF-II as a key cofactor of IGFBP-4 proteolysis by PAPP-A. Cultured first and second trimester human trophoblasts (n = 5) secreted PAPP-A into CM with mean +/- SEM levels of 172.4 +/- 32.8 mIU/liter.10(5) cells, determined by specific ELISA. PAPP-A in trophoblast CM (n = 3) and did not change in the presence of IGF-II (1-100 ng/ml). Cultured human endometrial stromal cells (n = 4) secreted low levels of PAPP-A (6.25 +/- 3.6 mIU/liter.10(5) cells). A physiological inhibitor of PAPP-A, the proform of eosinophil major basic protein (proMBP), was detected in trophoblast CM at levels of 1853 +/- 308 mIU/liter.10(5) cells, determined by specific ELISA, and was nearly undetectable in CM of human endometrial stromal cells. Upon in vitro decidualization of endometrial stromal cells with progesterone, PAPP-A levels in CM increased nearly 9-fold without a concomitant change in proMBP. In contrast to the experiments with trophoblasts, IGF-II and the IGF analogues, Leu(27) IGF-II, and Des (1-6) IGF-II, resulted in a dose-dependent decrease of PAPP-A levels in decidualized endometrial stromal CM by 70-90%, and a dose-dependent increase in proMBP of 14- to 41-fold. The data demonstrate conclusively that the IGF-II-dependent IGFBP-4 protease of human trophoblast and decidual origin is PAPP-A. Furthermore, the differential regulation of decidual PAPP-A and proMBP by insulin-like peptides supports a role for trophoblast-derived IGF-II as a paracrine regulator of these maternal decidual products that have the potential to regulate IGF-II bioavailability at the trophoblast-decidual interface. Overall, the data underscore potential roles for a complex family of enzyme (PAPP-A), substrate (IGFBP-4), inhibitor (proMBP), and cofactor (IGF-II) in the placental bed during human implantation.

Pregnancy-associated plasma protein-A (PAPP-A) in ovine, bovine, porcine, and equine ovarian follicles: involvement in IGF binding protein-4 proteolytic degradation and mRNA expression during follicular development

IGF binding protein-4 (IGFBP-4) proteolytic degradation is a common feature of preovulatory follicles from human, ovine, bovine, porcine, and equine ovary. In all these species, the protease is a zinc-dependent metalloprotease and its ability to degrade IGFBP-4 is IGF dependent. The human intrafollicular IGFBP-4-degrading protease has recently been identified as pregnancy-associated plasma protein-A (PAPP-A). The aim of this study was to investigate whether PAPP-A is also involved in IGFBP-4 degradation in ovine, bovine, porcine, and equine preovulatory follicles and to study the expression of PAPP-A mRNA in bovine and porcine granulosa cells from different classes of follicles. Immunoneutralization and immunoprecipitation with polyclonal antibodies raised against human PAPP-A inhibited IGFBP-4 proteolytic degradation in preovulatory follicular fluid from the four species studied. As previously reported for the intrafollicular proteolytic activity degrading IGFBP-4, recombinant human PAPP-A generated in vitro 17- and 10-kDa IGFBP-4-proteolytic fragments. Recombinant PAPP-A activity was also shown to be IGF dependent and was inhibited by heparin-binding domain-containing peptides. In all mammalian species studied, the PAPP-A sequences showed high degree of identity. Moreover, the PAPP-A gene was localized on porcine chromosome 1 (1q29-1q213), in agreement with the localization of human PAPP-A gene on human chromosome 9q33.1. In bovine and porcine ovaries, real-time quantitative RT-PCR showed that PAPP-A mRNA expression in granulosa cells was maximal in fully differentiated follicles and was positively correlated with expression of P450 aromatase and LH receptor mRNAs. Overall, these data show that PAPP-A is responsible for IGFBP-4 degradation in ovine, bovine, porcine, and equine preovulatory follicles. The high expression of PAPP-A mRNA in granulosa cells from large, differentiated follicles suggest that it is a new functional marker of follicular development.

Pregnancy-associated plasma protein A as a marker of acute coronary syndromes

BACKGROUND

Circulating markers indicating the instability of atherosclerotic plaques could have diagnostic value in unstable angina or acute myocardial infarction. We evaluated pregnancy-associated plasma protein A (PAPP-A), a potentially proatherosclerotic metalloproteinase, as a marker of acute coronary syndromes.

METHODS

We examined the level of expression of PAPP-A in eight culprit unstable coronary plaques and four stable plaques from eight patients who had died suddenly of cardiac causes. We also measured circulating levels of PAPP-A, C-reactive protein, and insulin-like growth factor I (IGF-I) in 17 patients with acute myocardial infarction, 20 with unstable angina, 19 with stable angina, and 13 controls without atherosclerosis.

RESULTS

PAPP-A was abundantly expressed in plaque cells and extracellular matrix of ruptured and eroded unstable plaques, but not in stable plaques. Circulating PAPP-A levels were significantly higher in patients with unstable angina or acute myocardial infarction than in patients with stable angina and controls (P<0.001). A PAPP-A threshold value of 10 mlU per liter identified patients who had acute coronary syndromes with a sensitivity of 89.2 percent and a specificity of 81.3 percent. PAPP-A levels correlated with levels of C-reactive protein and free IGF-I, but not with markers of myocardial injury (troponin I and the MB isoform of creatine kinase).

CONCLUSIONS

PAPP-A is present in unstable plaques, and circulating levels are elevated in acute coronary syndromes; these increased levels may reflect the instability of atherosclerotic plaques. PAPP-A is a new candidate marker of unstable angina and acute myocardial infarction.

Mutational analysis of the proteolytic domain of pregnancy-associated plasma protein-A (PAPP-A): classification as a metzincin

The bioavailability of insulin-like growth factor (IGF)-I and -II is controlled by six IGF-binding proteins (IGFBPs 1-6). Bound IGF is not active, but proteolytic cleavage of the binding protein causes release of IGF. Pregnancy-associated plasma protein-A (PAPP-A) has recently been found to cleave IGFBP-4 in an IGF-dependent manner. To experimentally support the hypothesis that PAPP-A belongs to the metzincin superfamily of metalloproteinases, all containing the elongated zinc-binding motif HEXXHXXGXXH (His-482-His-492 in PAPP-A), we expressed mutants of PAPP-A in mammalian cells. Substitution of Glu-483 with Ala causes a complete loss of activity, defining this motif as part of the active site of PAPP-A. Interestingly, a mutant with Glu-483 replaced by Gln shows residual activity. Known metzincin structures contain a so-called Met-turn, whose strictly conserved Met residue is thought to interact directly with residues of the active site. By further mutagenesis we provide experimental evidence that Met-556 of PAPP-A, 63 residues from the zinc-binding motif, is located in a Met-turn of PAPP-A. Our hypothesis is also supported by secondary-structure prediction, and the ability of a 55-residue deletion mutant (d[S498-Y552]) to express and retain antigenecity. However, because PAPP-A differs in the features defining the individual established metzincin families, we suggest that PAPP-A belongs to a separate family. We also found that PAPP-A can undergo autocleavage, and that autocleaved PAPP-A is inactive. A lack of unifying elements in the sequences around the found cleavage sites of PAPP-A and a variant suggests steric regulation of substrate specificity.

Pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growth factor binding protein (IGFBP)-5 independent of IGF: implications for the mechanism of IGFBP-4 proteolysis by PAPP-A

Pregnancy-associated plasma protein-A (PAPP-A) has recently been identified as the proteinase responsible for cleavage of insulin-like growth factor binding protein (IGFBP)-4, an inhibitor of IGF action, in several biological fluids. Cleavage of IGFBP-4 by PAPP-A is believed to occur only in the presence of IGF. We here report that in addition to IGFBP-4, PAPP-A also cleaves IGFBP-5. Cleavage occurs at one site, between Ser-143 and Lys-144 of IGFBP-5. In the presence of IGF, IGFBP-4 and -5 are cleaved with similar rates by PAPP-A. Interestingly, cleavage of IGFBP-5 by PAPP-A does not require the presence of IGF, but is slightly inhibited by IGF. These findings have implications for the mechanism of proteolysis of IGFBP-4 by PAPP-A, suggesting that IGFBP-4 binds IGF, which then becomes a PAPP-A substrate. Using highly purified, recombinant proteins, we establish that (1) PAPP-A cleavage of IGFBP-4 can occur in the absence of IGF, although the rate of hydrolysis is very slow, and (2) IGF is unable to bind to PAPP-A. We thus conclude that IGF enhances proteolysis by binding to IGFBP-4, not by interaction with PAPP-A, which could not previously be ruled out.

Pregnancy-associated plasma protein-A2 (PAPP-A2), a novel insulin-like growth factor-binding protein-5 proteinase

A novel metalloproteinase with similarity to pregnancy-associated plasma protein-A (PAPP-A), which we denoted PAPP-A2, has been identified. Through expression in mammalian cells we showed that recombinant PAPP-A2 polypeptide of 1558 residues resulted from processing of a 1791-residue prepro-protein. Unlike PAPP-A, PAPP-A2 migrated as a monomer (of 220 kDa) in non-reducing SDS-polyacrylamide gel electrophoresis. The prepro-parts of PAPP-A2 and PAPP-A are not homologous, but mature PAPP-A2 shares 45% of its residues with PAPP-A. Because PAPP-A specifically cleaves insulin-like growth factor-binding protein (IGFBP)-4, one of six known modulators of IGF-I and -II, we looked for a possible PAPP-A2 substrate among the members of this family. We showed that PAPP-A2 specifically cleaved IGFBP-5 at one site, between Ser-143 and Lys-144. In contrast to the cleavage of IGFBP-4 by PAPP-A that strictly requires the presence of IGF, the cleavage of IGFBP-5 by PAPP-A2 was IGF-independent. Recent data firmly establish PAPP-A and IGFBP-4 as an important functional pair in several systems. Because of its close relationship with PAPP-A, both structurally and functionally, PAPP-A2 is a likely candidate IGFBP-5 proteinase in many tissues and conditioned media where IGFBP-5 proteolysis has been reported.

Pregnancy-associated plasma protein-a is the insulin-like growth factor binding protein-4 protease secreted by human ovarian granulosa cells and is a marker of dominant follicle selection and the corpus luteum

Insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and IGFBP proteases are important in ovarian function. IGFs stimulate granulosa steroidogenesis, an effect that is inhibited by IGFBP-4 and augmented by IGFBP-4 proteolysis. We have recently identified the IGFBP-4 protease in human ovarian follicular fluid (FF) as pregnancy-associated plasma protein-A (PAPP-A). In the current study, we identify the IGFBP-4 protease secreted by cultured human ovarian granulosa cells as PAPP-A, based on specific immunoinhibition and immunodepletion of the IGFBP-4 protease activity with PAPP-A polyclonal antibodies and immunorecognition by PAPP-A monoclonal antibodies in ELISA. PAPP-A was barely detectable in conditioned media (CM) from granulosa derived from </=9 mm androgen-dominant follicles, but was secreted by cultured granulosa from estrogen-dominant follicles >/=9 mm, coincident with dominant follicle selection, and by luteinizing granulosa. PAPP-A levels in CM from the latter did not change in response to IGF-II or hCG (100 ng/mL). A naturally occurring inhibitor of PAPP-A, proform of eosinophil major basic protein (proMBP), was detected by ELISA in estrogen-dominant follicular fluid FF, but not in CM from granulosa or luteinizing granulosa cells treated with IGF-II (0-200 ng/mL), FSH (0-100 ng/mL) or hCG (0-100 ng/mL), suggesting an alternative source (other than granulosa) for proMBP, compared to PAPP-A. The data demonstrate granulosa cells as a source of PAPP-A in human ovary and suggest that PAPP-A is a marker of ovarian follicle selection and corpus luteum formation. In addition the data suggest complex regulation of this system in human ovary.

Insulin-like growth factor binding protein-4 protease produced by smooth muscle cells increases in the coronary artery after angioplasty

Insulin-like growth factor (IGF)-I stimulates vascular smooth muscle cell (VSMC) migration and proliferation, which are fundamental to neointimal hyperplasia in postangioplasty restenosis. IGF-I action is modulated by several high-affinity IGF binding proteins (IGFBPs). IGFBP-4 is the predominant IGFBP produced by VSMCs and is a potent inhibitor of IGF-I action. However, specific IGFBP-4 proteases can cleave IGFBP-4 and liberate active IGF-I. In this study, we document IGFBP-4 protease produced by human and porcine coronary artery VSMCs in culture as pregnancy-associated plasma protein-A (PAPP-A). This was shown by a distinctive IGFBP-4 cleavage pattern, specific inhibition of IGFBP-4 protease activity with PAPP-A polyclonal antibodies, and immunorecognition of PAPP-A by monoclonal antibodies. Furthermore, we found a 2-fold increase in IGFBP-4 protease activity in injured porcine VSMC cultures in vitro (P<0.05). We also evaluated IGFBP-4 protease/PAPP-A expression in vivo after coronary artery balloon injury. Twenty-five immature female pigs underwent coronary overstretch balloon injury, and vessels were examined at defined time points after the procedure. Abundant PAPP-A expression was observed in the cytoplasm of medial and neointimal cells 7, 14, and 28 days after angioplasty (P<0.01 vs control). The highest PAPP-A labeling indices were located in the neointima (36.1+/-2.1%) and the media (31.7+/-1.2%) 28 days after injury. Western blot analysis confirmed increased PAPP-A in injured vessels. PAPP-A, a regulator of IGF-I bioavailability through proteolysis of IGFBP-4, is thus expressed by VSMCs in vitro and in restenotic lesions in vivo. These results suggest a possible role for PAPP-A in neointimal hyperplasia.

An isolated, surface-expressed I domain of the integrin alphaLbeta2 is sufficient for strong adhesive function when locked in the open conformation with a disulfide bond

We introduced disulfide bonds to lock the integrin alphaLbeta2 I domain in predicted open, ligand binding or closed, nonbinding conformations. Transfectants expressing alphaLbeta2 heterodimers containing locked-open but not locked-closed or wild-type I domains constitutively adhered to intercellular adhesion molecule-1 (ICAM-1) substrates. Locking the I domain closed abolished constitutive and activatable adhesion. The isolated locked-open I domain bound as well as the activated alphaLbeta2 heterodimer, and binding was abolished by reduction of the disulfide. Lovastatin, which binds under the conformationally mobile C-terminal alpha-helix of the I domain, inhibited binding to ICAM-1 by alphaLbeta2 with wild-type, but not locked-open I domains. These data establish the importance of conformational change in the alphaL I domain for adhesive function and show that this domain is sufficient for full adhesive activity.

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.

Quantification and characterization of pregnancy-associated complexes of angiotensinogen and the proform of eosinophil major basic protein in serum and amniotic fluid

BACKGROUND

The proform of eosinophil major basic protein (ProMBP) exists in serum from pregnant women complexed with a variable fraction of angiotensinogen (Ang). A subfraction further binds complement C3dg in a 2:2:2 complex. The function, physiology, and clinical importance of ProMBP complexes are unknown, and the specific quantification of these complexes has not been possible.

METHODS

We developed an ELISA for the ProMBP/Ang complexes, using a monoclonal antibody against ProMBP for capture and a chicken anti-human Ang antiserum for detection. Calibrators were standardized with WHO IRP 78/610 for pregnancy proteins in the assay range 0.95-15.6 mIU/L.

RESULTS

The concentrations of ProMBP/Ang complexes in serum of nonpregnant blood donors (n = 79) were log-normally distributed with a central 95th interval of 985-3655 mIU/L. In pregnancy, mean serum concentrations were increased from week 7, and the concentrations reached term concentrations in week 18. ProMBP/Ang complexes eluted in gel filtration as a broad peak with a molecular mass of approximately 230 kDa. The concentration of ProMBP/Ang/C3dg increased during blood coagulation, suggesting that the ProMBP/Ang/C3dg complex may be a marker of complement activation.

CONCLUSIONS

ProMBP/Ang complexes are present in serum from nonpregnant persons as well as pregnant women, and the direct assays described here will make it possible to study the biochemistry and the clinical significance of different ProMBP complexes in pathological conditions and pregnancy.

Folding and function of I domain-deleted Mac-1 and lymphocyte function-associated antigen-1

In those integrins that contain it, the I domain is a major ligand recognition site. The I domain is inserted between beta-sheets 2 and 3 of the predicted beta-propeller domain of the integrin alpha subunit. We deleted the I domain from the integrin alpha(M) and alpha(L) subunits to give I-less Mac-1 and lymphocyte function-associated antigen-1 (LFA-1), respectively. The I-less alpha(M) and alpha(L) subunits were expressed in association with the wild-type beta(2) subunit on the surface of transfected cells and bound to all the monoclonal antibodies mapped to the putative beta-propeller and C-terminal regions of the alpha(M) and alpha(L) subunits, suggesting that the folding of these domains is independent of the I domain. I-less Mac-1 bound to the ligands iC3b and factor X, but this binding was reduced compared with wild-type Mac-1. In contrast, I-less Mac-1 did not bind to fibrinogen or denatured bovine serum albumin. Binding to iC3b and factor X by I-less Mac-1 was inhibited by the function-blocking antibody CBRM1/32, which binds to the beta-propeller domain of the alpha(M) subunit. I-less LFA-1 did not bind its ligands intercellular adhesion molecule-1 and -3. Thus, the I domain is not essential for the folding, heterodimer formation, and surface expression of Mac-1 and LFA-1 and is required for binding to some ligands, but not others.

The status of half-cystine residues and locations of N-glycosylated asparagine residues in human eosinophil peroxidase

The determination by protein chemistry methods of the half-cystine status in human eosinophil peroxidase (EPO) is reported. EPO is two-chained and has a total of 14 half-cystine residues. Cys141 and Cys152 form an intrachain bridge in the light chain of EPO. Disulfide bridges connect Cys253 and Cys263, Cys257 and Cys287, Cys359 and Cys370, Cys570 and Cys635, and Cys676 and Cys701, forming five intrachain disulfide bridges in the heavy chain of EPO. Cys291 and Cys455 are found to be unpaired, containing free sulfhydryl groups. The pattern of disulfide bridges is in agreement with that predicted from the X-ray crystallographic structure of canine myeloperoxidase (MPO) (Zeng, J., and Fenna, R. E. (1992) J. Mol. Biol. 226, 185-207) to be general for the class of mammalian peroxidases, including EPO, MPO, lactoperoxidase (LPO), and thyroid peroxidase (TPO). Of four candidate sites in EPO for attachment of glucosamine-based carbohydrate, Asn327 and Asn363 are occupied, whereas Asn700 and Asn708 are unsubstituted. Furthermore, a discrepancy in the literature regarding the sequence of residues 645-659 is resolved.

Evidence that the insulin-like growth factor binding protein-4 protease in human ovarian follicular fluid is pregnancy associated plasma protein-A

Ovarian insulin-like growth factor binding protein-4 (IGFBP-4) proteolysis is involved in the regulation of follicular development, but until now the identity of the responsible enzyme was unknown. In this study, we identify the IGFBP4 protease in human follicular fluid as pregnancy associated plasma protein-A (PAPP-A) based on distinctive IGFBP-4 cleavage pattern, the same protease inhibitor profile, specific inhibition and immunodepletion of IGFBP-4 protease activity with PAPP-A polyclonal antibodies, and immunorecognition by PAPP-A monoclonal antibodies in ELISA. Furthermore, PAPP-A levels in estrogen-dominant and androgen-dominant follicular fluids reflect their IGFBP-4 proteolytic activity. PAPP-A was also secreted by human granulosa cells, the reputed source of IGFBP-4 protease activity in follicular fluid. We have the molecular and biochemical tools to begin to delineate the regulation and biological function of PAPP-A in normal and dysregulated follicular development and atresia.

Messenger ribonucleic acid levels of pregnancy-associated plasma protein-A and the proform of eosinophil major basic protein: expression in human reproductive and nonreproductive tissues

PAPP-A/proMBP, the complex of pregnancy-associated plasma protein-A (PAPP-A) and the proform of eosinophil major basic protein (proMBP), circulates at increasing levels during pregnancy. The major site of synthesis is the placenta, in which PAPP-A mRNA has been localized to the syncytiotrophoblast and the placental X cells, whereas proMBP mRNA has been localized to the placental X cells only. The function of PAPP-A/proMBP and its components has remained speculative for years. Recently, however, it has been shown that PAPP-A specifically cleaves insulin-like growth factor (IGF) binding protein-4 in an IGF-dependent manner. Female reproductive and nonreproductive tissues have previously been reported to contain PAPP-A immunoreactivity, based on studies using preparations of anti(PAPP-A/proMBP), now known to recognize both PAPP-A and proMBP, and other irrelevant antigens. To analyze for the presence of PAPP-A and proMBP mRNA, a sensitive semiquantitative reverse transcription (RT) polymerase chain reaction (PCR) method was developed. Reverse-transcribed poly(A)(+) RNA was used as a template in a competitive PCR. PAPP-A and proMBP mRNA levels were normalized against the level of beta-actin mRNA. Both mRNA species were significantly more abundant in term placenta than in other tissues analyzed. All analyzed tissues, including endometrium, myometrium, colon, and kidney, contained both PAPP-A and proMBP mRNA.

The proform of eosinophil major basic protein: a new maternal serum marker for Down syndrome

The proform of eosinophil major basic protein (proMBP), the most abundant protein in the eosinophil specific granule, is synthesized by the placenta and secreted into the maternal circulation, where it is found complex-bound to pregnancy-associated plasma protein-A (PAPP-A) and other proteins. We examined the potential of proMBP as a maternal serum marker for fetal Down syndrome (DS) by determining its maternal serum concentration (MSpMBP) in 25 Down syndrome (DS) pregnancies and 152 control pregnancies in the first trimester, and in 105 DS pregnancies and 156 control pregnancies in the second trimester. The median (95 per cent confidence interval) MSpMBP MoM in DS pregnancies (n=15) was 0.66 (0.49-0.79) in gestational weeks 5-9; 1.06 (0.71-1.97) in weeks 10-12 (n=10) and 1.62 (1.18-1.98) in weeks 14-20 (n=105). Using parameterized receiver operator characteristics analysis for proMBP as a single marker for DS, detection rates (DRs) of 22 per cent and 38 per cent, for false-positive rates (FPRs) of 5 per cent, were found in weeks 5-9 (using MSpMBP</=cut-off) and weeks 14-20 (using MSpMBP>/=cut-off), respectively. When age and MSpMBP were used as markers in combination, a DR of 36.8 per cent for an FPR of 5.5 per cent was obtained in weeks 5-9 using a risk cut-off of 1:250. In weeks 14-20 the DR was 48.4 per cent for an FPR of 5.3 per cent using the same risk cut-off. This makes proMBP a marker comparable in diagnostic efficiency to human chorionic gonadotrophin (hCG), and exceeding that of alpha-fetoprotein (AFP) and unconjugated oestriol (uE3), in the second trimester.

Biochemical evidence for heme linkage through esters with Asp-93 and Glu-241 in human eosinophil peroxidase. The ester with Asp-93 is only partially formed in vivo

The covalent heme attachment has been extensively studied by spectroscopic methods in myeloperoxidase and lactoperoxidase (LPO) but not in eosinophil peroxidase (EPO). We show that heme linkage to the heavy chain is invariably present, whereas heme linkage to the light chain of EPO is present in less than one-third of EPO molecules. Mass analysis of isolated heme bispeptides supports the hypothesis of a heme b linked through two esters to the polypeptide. Mass analysis of heme monopeptides reveals that >90% have a nonderivatized methyl group at the position of the light chain linkage. Apparently, an ester had not been formed during biosynthesis. The light chain linkage could be formed by incubation with hydrogen peroxide, in accordance with a recent hypothesis of autocatalytic heme attachment based on studies with LPO (DePillis, G. D., Ozaki, S., Kuo, J. M., Maltby, D. A., and Ortiz de Montellano P. R. (1997) J. Biol. Chem. 272, 8857-8860). By sequence analysis of isolated heme peptides after aminolysis, we unambiguously identified the acidic residues, Asp-93 of the light chain and Glu-241 of the heavy chain, that form esters with the heme group. This is the first biochemical support for ester linkage to two specific residues in eosinophil peroxidase. From a parallel study with LPO, we show that Asp-125 and Glu-275 are engaged in ester linkage. The species with a nonderivatized methyl group was not found among LPO peptides.

Regulation of human ADAM 12 protease by the prodomain. Evidence for a functional cysteine switch

The ADAMs (a disintegrin and metalloprotease) are a family of multidomain proteins that are believed to play key roles in cell-cell and cell-matrix interactions. We have shown recently that human ADAM 12-S (meltrin alpha) is an active metalloprotease. It is synthesized as a zymogen, with the prodomain maintaining the protease in a latent form. We now provide evidence that the latency mechanism of ADAM 12 can be explained by the cysteine switch model, in which coordination of Zn2+ in the active site of the catalytic domain by a cysteine residue in the prodomain is critical for inhibition of the protease. Replacing Cys179 with other amino acids results in an ADAM 12 proform that is proteolytically active, but latency can be restored by placing cysteine at other positions in the propeptide. None of the amino acids adjacent to the crucial cysteine residue is essential for blocking activity of the protease domain. In addition to its latency function, the prodomain is required for exit of ADAM 12 protease from the endoplasmic reticulum. Tissue inhibitor of metalloprotease-1, -2, and -3 were not found to block proteolytic activity of ADAM 12, hence a physiological inhibitor of ADAM 12 protease in the extracellular environment remains to be identified.

The insulin-like growth factor (IGF)-dependent IGF binding protein-4 protease secreted by human fibroblasts is pregnancy-associated plasma protein-A

Proteolytic cleavage of the six known insulin-like growth factor binding proteins (IGFBPs) is a powerful means of rapid structure and function modification of these important growth-regulatory proteins. Intact IGFBP-4 is a potent inhibitor of IGF action in vitro, and cleavage of IGFBP-4 has been shown to abolish its ability to inhibit IGF stimulatory effects in a variety of systems, suggesting that IGFBP-4 proteolysis acts as a positive regulator of IGF bioavailability. Here we report the isolation of an IGF-dependent IGFBP-4-specific protease from human fibroblast-conditioned media and its identification by mass spectrometry microsequencing as pregnancy-associated plasma protein-A (PAPP-A), a protein of unknown function found in high concentrations in the maternal circulation during pregnancy. Antibodies raised against PAPP-A both inhibited and immunodepleted IGFBP-4 protease activity in human fibroblast-conditioned media. Moreover, PAPP-A purified from pregnancy sera had IGF-dependent IGFBP-4 protease activity. PAPP-A mRNA was expressed by the human fibroblasts and osteoblasts, and PAPP-A protein was secreted into the culture medium. In conclusion, we have identified an IGF-dependent IGFBP protease and at the same time assigned a function to PAPP-A. This represents an unanticipated union of two areas of research that were not linked in any way before this report.

Conformational changes in tertiary structure near the ligand binding site of an integrin I domain

For efficient ligand binding, integrins must be activated. Specifically, a conformational change has been proposed in a ligand binding domain present within some integrins, the inserted (I) domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340]. This proposal remains controversial, however, despite extensive crystal structure studies on the I domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340; Liddington, R. & Bankston, L. (1998) Structure (London) 6, 937-938; Qu, A. & Leahy, D. J. (1996) Structure (London) 4, 931-942; and Baldwin, E. T., Sarver, R. W., Bryant, G. L., Jr., Curry, K. A., Fairbanks, M. B., Finzel, B. C. , Garlick, R. L., Heinrikson, R. L., Horton, N. C. & Kelly, L. L. (1998) Structure (London) 6, 923-935]. By defining the residues present in the epitope of a mAb against the human Mac-1 integrin (alphaMbeta2, CD11b/CD18) that binds only the active receptor, we provide biochemical evidence that the I domain itself undergoes a conformational change with activation. This mAb, CBRM1/5, binds the I domain very close to the ligand binding site in a region that is widely exposed regardless of activation as judged by reactivity with other antibodies. The conformation of the epitope differs in two crystal forms of the I domain, previously suggested to represent active and inactive receptor. Our data suggests that conformational differences in the I domain are physiologically relevant and not merely a consequence of different crystal lattice interactions. We also demonstrate that the transition between the two conformational states depends on species-specific residues at the bottom of the I domain, which are proposed to be in an interface with another integrin domain, and that this transition correlates with functional activity.

The structure of the beta-propeller domain and C-terminal region of the integrin alphaM subunit. Dependence on beta subunit association and prediction of domains

The alphaM subunit of integrin Mac-1 contains several distinct regions in its extracellular segment. The N-terminal region has been predicted to fold into a beta-propeller domain composed of seven beta-sheets each about 60 amino acid residues long, with the I-domain inserted between beta-sheets 2 and 3. The structure of the C-terminal region is unknown. We have used monoclonal antibodies (mAbs) as probes to study the dependence of the structure of different regions of the alphaM subunit on association with the beta2 subunit in the alphaM/beta2 heterodimer. All of the mAbs to the I-domain immunoprecipitated the unassociated alphaM precursor and reacted with the alphaM subunit expressed alone on the surface of COS cells. By contrast, four mAbs to the beta-propeller domain did not react with the unassociated alphaM precursor nor with the uncomplexed alphaM subunit expressed on COS cell surface. The four mAbs were mapped to three subregions in three different beta-sheets, making it unlikely that each recognized an interface between the alpha and beta subunits. These results suggest that folding of different beta-propeller subregions is coordinate and is dependent on association with the beta2 subunit. The segment C-terminal to the beta-propeller domain, residues 599-1092, was studied with nine mAbs. A subset of four mAbs that reacted with the alphaM/beta2 complex but not with the unassociated alphaM subunit were mapped to one subregion, residues 718-759, and five other mAbs that recognized both the unassociated and the complexed alphaM subunit were localized to three other subregions, residues 599-679, 820-882, and 943-1047. This suggests that much of the region C-terminal to the beta-propeller domain folds independently of association with the beta2 subunit. Our data provide new insights into how different domains in the integrin alpha and beta subunits may interact.

Experimental support for a beta-propeller domain in integrin alpha-subunits and a calcium binding site on its lower surface

Integrins are large, heterodimeric surface molecules of wide importance in cell adhesion. The N-terminal half of all integrin alpha-subunits contains seven weak sequence repeats of approximately 60 amino acids that are important in ligand binding and have been predicted to fold cooperatively into a single beta-propeller domain with seven beta-sheets. We provide evidence supporting this model with a mouse mAb to human Mac-1 (alphaM beta2, CD11b/CD18). This antibody, CBRM1/20, binds to amino acid residues that are in different repeats and are 94 residues apart in the primary structure in the loop between strands 1 and 2 of beta-sheet 5 and in the loop between strands 3 and 4 of beta-sheet 6. The 1-2 loops of beta-sheets 5-7 in integrins have EF hand-like Ca2+-binding motifs. CBRM1/20 binds to Mac-1 in the presence of Ca2+ or Sr2+ with an EC50 of 0.2 mM. Mg2+ or Mn2+ cannot substitute. Antibodies to other epitopes on the Mac-1 beta-propeller domain bind in the absence of calcium. mAb CBRM1/20 does not block ligand binding. Thus, the region on the lower surface of the beta-propeller domain to which mAb CBRM1/20 binds does not bind ligand and, furthermore, cannot bind other integrin domains, such as those of the beta-subunit.

Double-monoclonal immunofluorometric assays for pregnancy-associated plasma protein A/proeosinophil major basic protein (PAPP-A/proMBP) complex in first-trimester maternal serum screening for Down syndrome

Four double-monoclonal time-resolved immunofluorometric assays (TrIFMAs) have been developed for the specific determination of pregnancy-associated plasma protein A/proeosinophil major basic protein (PAPP-A/ proMBP) complex in first-trimester maternal serum samples. The assays have a functional sensitivity of < 4 mIU/L and a working range from 4 to 1000 mIU/L. These 4 assays, together with a polyclonal sandwich TrIFMA, were compared for their ability to discriminate between normal pregnancies (n = 149) and pregnancies carrying a Down syndrome fetus (n = 36) in maternal serum screening samples from gestational weeks 4-13. In 26 Down syndrome pregnancies from gestational weeks 7-12, the median PAPP-A multiples of the median concentration in controls (MoMs) determined by monoclonal antibody combinations 234-3/234-2*, 234-4/234-2*, 234-4/234-5*, and 234-5/234-6* were 0.35, 0.37, 0.42, and 0.44, respectively, whereas the median MoM determined by the polyclonal assay was 0.56. ROC curve analysis also showed that better overall diagnostic accuracy and detection rates were achieved by the monoclonal TrIFMAs than by the polyclonal TrIFMA. This report is the first to describe assays that specifically measure PAPP-A/proMBP complex without possible interference from other proMBP-containing complexes.

Simple and reliable procedure for PCR amplification of genomic DNA from yeast cells using short sequencing primers

Yeast is widely used in molecular biology. Heterologous expression of recombinant proteins in yeast involves screening of a large number of recombinants. We present an easy and reliable procedure for amplifying genomic DNA from freshly grown cells of the methylotrophic yeast Pichia pastoris by means of PCR without any prior DNA purification steps. This method involves a simple boiling step of whole yeast cells in the presence of detergent, and subsequent amplification of genomic DNA using short sequencing primers in a polymerase chain reaction assay with a decreasing annealing temperature.

Complete cDNA sequence of the preproform of human pregnancy-associated plasma protein-A. Evidence for expression in the brain and induction by cAMP

A cDNA that encodes the prepropeptide of pregnancy-associated plasma protein-A (preproPAPP-A), a putative metalloproteinase, has been cloned and sequenced. PAPP-A is synthesized in the placenta as a 1627-residue precursor preproprotein with a putative 22-residue signal peptide and a highly basic propeptide of 58 residues. The prepro-PAPP-A-encoding transcript contains a region with an extremely high G+C content and has an unusually long 5' untranslated region with several upstream short ORF. No alternatively spliced products could be identified by means of Northern blotting experiments or with rapid amplification of 5' cDNA ends experiments. A stretch within the 5' untranslated region shows sequence identities to a partial cDNA isolated from brain and a to cAMP-inducible sequence from a choriocarcinoma cell line.

Identification of angiotensinogen and complement C3dg as novel proteins binding the proform of eosinophil major basic protein in human pregnancy serum and plasma

In sera from pregnant women, pregnancy-associated plasma protein-A (PAPP-A) circulates as a disulfide-bound complex (approximately 474 kDa) with the proform of eosinophil major basic protein (proMBP) (Oxvig, C., Sand, O., Kristensen, T., Gleich, G. J., and Sottrup-Jensen, L. (1993) J. Biol. Chem. 268, 12243-12246). We have produced monoclonal antibodies (mAbs) against the PAPP-A.proMBP complex and established a radioimmunoassay utilizing a mAb recognizing the PAPP-A subunit. Surprisingly, serum levels of proMBP exceed those of PAPP-A four to 10-fold on a molar basis throughout pregnancy. This result prompted an investigation of the status of proMBP in pregnancy. Using a proMBP-specific mAb two novel proMBP complexes have been isolated by chromatographic techniques. Based on sequence analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and reaction with specific antibodies, one is shown to be a 2:2 disulfide-bound complex (approximately 200 kDa) between proMBP and angiotensinogen. The other is a 2:2:2 complex (approximately 300 kDa) between proMBP, angiotensinogen, and complement C3dg. Circulating proMBP in pregnancy is thus present in three types of complexes. These results suggest that specific interactions between the complexed proteins occur in pregnancy, and the possibility is raised that their interactions are important in the pathophysiology of pregnancies associated with hypertension.

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)

Localization of pregnancy-associated plasma protein-A and colocalization of pregnancy-associated plasma protein-A messenger ribonucleic acid and eosinophil granule major basic protein messenger ribonucleic acid in placenta

BACKGROUND

The human eosinophil granule major basic protein (MBP), a 13.8 kilodalton cationic polypeptide constituting the core of the eosinophil granule, is cytotoxic to parasites and numerous mammalian cells. Concentrations of a molecule immunochemically similar to eosinophil granule MBP are present in maternal plasma, and MBP mRNA has been localized to placental X cells by in situ hybridization. Eosinophil granule MBP is initially translated as a nontoxic precursor (proMBP), containing a 9.9 kilodalton acidic pro-portion that is believed to neutralize MBP toxicity. Recent analyses of sera from pregnant women have revealed that pregnancy-associated plasma protein-A (PAPP-A), previously thought to be a homotetramer of PAPP-A subunits, is actually composed of PAPP-A subunits bound by disulfide bonds to equimolar amounts of proMBP molecules to form a complex, PAPP-A/proMBP. In addition, the PAPP-A subunit nucleotide and deduced amino acid sequence have been determined from cloned cDNA. The PAPP-A monomer found in plasma contains 1547 amino acid residues.

EXPERIMENTAL DESIGN

Because of the new evidence that PAPP-A is complexed with proMBP, previous studies on the localization of PAPP-A using antibodies to PAPP-A must be questioned. To determine the localization of the PAPP-A subunit, immunofluorescence was performed on normal placental tissues using proMBP absorbed anti-PAPP-A antibody. Furthermore, the expression of PAPP-A mRNA was investigated by in situ hybridization.

RESULTS

Immunofluorescence staining with proMBP absorbed anti-PAPP-A antibody showed that PAPP-A is localized to placental septa, anchoring villi, and the syncytia of chorionic villi, whereas MBP is localized only to septa and anchoring villi. By in situ hybridization, PAPP-A mRNA is detected in placental X cells and syncytiotrophoblasts, but MBP mRNA is localized only to placental X cells.

CONCLUSIONS

The presence of PAPP-A mRNA and PAPP-A subunit protein in placental X cells and syncytiotrophoblasts indicates that both X cells and syncytiotrophoblasts synthesize the PAPP-A subunit, whereas only X cells synthesize proMBP.

Location and nature of carbohydrate groups in proform of human major basic protein isolated from pregnancy serum

From human pregnancy serum we have isolated the proform of eosinophil major basic protein (proMBP), which forms a complex with pregnancy-associated plasma protein-A (PAPP-A), PAPP-A/proMBP. It is shown that proMBP contains O-linked glycan bound to Ser-24, Thr-25 (fully substituted), and to Thr-23 and Thr-34 (partially substituted). N-linked glycan is bound to Asn-86 and O-linked glycosaminoglycan is bound to Ser-62 (both fully substituted). From the RP-HPLC elution profile and mass spectra of tryptic peptides it is found that proMBP is extremely heterogeneous with respect to glycosylation.

Localization of disulfide bridges and free sulfhydryl groups in human eosinophil granule major basic protein

Eosinophil granule major basic protein (117 residues) is known to contain free sulfhydryl groups. Here we have located in the amino acid sequence the half-cystine residues present as cysteine, and identified those engaged in disulfide bridges. Of the 9 half-cystine residues, 5 are unpaired cysteines (Cys2, Cys23, Cys42, Cys64, and Cys96), while 4 form disulfides (Cys20-Cys115, and Cys92-Cys107).

Amino acid sequence of human pregnancy-associated plasma protein-A derived from cloned cDNA

The amino acid sequence of human pregnancy-associated plasma protein-A (PAPP-A), a component of the circulating complex with the proform of eosinophil major basic protein (proMBP), has been determined from partial protein sequencing and from sequencing of cloned cDNA. The PAPP-A monomer contains 1547 amino acid residues, but is derived from a larger precursor of placental origin. PAPP-A contains 82 Cys residues, which are all bridged, 14 putative sites for N-glycosylation, and 7 putative sites for attachment of glycosaminoglycan groups. The C-terminal part of PAPP-A contains 5 approximately 60-residue motifs related to the short consensus repeats of complement proteins and selectins. The SCRs presently known can be grouped into three classes: complement-type, class I; selectin-type, class II; PAPP-A-type, class III. PAPP-A further contains three approximately 26-residue motifs, related to the lin-notch motifs of proteins regulating early tissue differentiation, and, in addition, a putative Zn2+ binding site similar to that found in many metalloproteinases has been identified. Apart from these features, the PAPP-A sequence is not related to other known protein sequences.

Circulating human pregnancy-associated plasma protein-A is disulfide-bridged to the proform of eosinophil major basic protein

A previously unrecognized association between pregnancy-associated plasma protein-A (PAPP-A) and the proform of eosinophil major basic protein (proMBP) is demonstrated. PAPP-A isolated from pooled pregnancy serum is shown to be a disulfide-bridged complex with proMBP (PAPP-A/proMBP) in which the subunits of the constituents are present in a 1:1 molar ratio. The results are the outcome of analysis of tryptic and CNBr/tryptic peptides from PAPP-A/proMBP, sequence analysis of intact and reduced and carboxymethylated PAPP-A/proMBP, and reaction with monoclonal antibodies directed against MBP and its proform. In addition, it is shown that commercial polyclonal anti-PAPP-A is polyspecific, also reacting with MBP.