Disorders caused by genetic defects associated with GH-dependent genes: PAPPA2 defects

Growth hormone (GH) and its mediator, insulin-like growth factor-1 (IGF-1), have long been recognized as central to human growth physiology. IGF-1 is known to complex with IGF binding proteins as well as with the acid labile subunit (ALS) in order to prolong its half-life in circulation. Factors regulating the bioavailability of IGF-1 (i.e. the balance between free and bound IGF-1) were less well understood. Recently, pregnancy-associated plasma protein-A2 (PAPP-A2) was discovered as a protease which specifically cleaves IGF-binding protein (IGFBP)-3 and -5. PAPP-A2 deficient patients present with characteristic findings including growth failure, elevated total IGF-1 and -2, IGFBPs, and ALS, but decreased percentage of free to total IGF-1. Additionally, patients with PAPP-A2 deficiency have impairments in glucose metabolism and bone mineral density (BMD). Treatment with recombinant human IGF-1 (rhIGF-1) improved height SD scores, growth velocity, body composition, and dysglycemia. Mouse models recapitulate many of the human findings of PAPP-A2 deficiency. This review summarizes the function of PAPP-A2 and its contribution to the GH-IGF axis through an examination of PAPP-A2 deficient patients and mouse models, thereby emphasizing the importance of the regulation of IGF-1 bioavailability in human growth.

Characterization of mouse pericardial fat: regulation by PAPP-A

Although implicated in cardiovascular disease, little is known about the fat surrounding the heart. In humans, epicardial fat is the visceral fat depot of the heart, which directly contacts the myocardium. This strategically placed fat depot is thought to produce bioactive molecules that could affect cardiac function. A major limitation in understanding the biology of epicardial fat is its restricted access in humans and its seeming absence in commonly-used experimental animal models. Although laboratory mice do not have epicardial fat per se, they do have a fat depot around the heart. In this study, we found that mouse pericardial fat has the molecular signature, small adipocyte size, and resistance to differentiation consistent with visceral fat. In addition, we show that mouse pericardial fat is regulated by pregnancy-associated plasma protein-A (PAPP-A), a key modulator of local insulin-like growth factor bioavailability. PAPP-A is highly expressed in mouse pericardial fat at levels equivalent to those in mesenteric visceral fat and 10-fold higher than in subcutaneous inguinal fat (P = .0003). Cultured pre-adipocytes isolated from pericardial fat show 2-fold increased PAPP-A secretion compared to pre-adipocytes isolated from inguinal fat. Furthermore, PAPP-A knock-out mice fed a high fat diet for 20 weeks have significantly reduced pericardial fat (by 60%; P < .0001) compared to wild-type littermates. There was no significant difference in inguinal fat between wild-type and PAPP-A knock-out mice. These data characterize a new mouse model of visceral-like pericardial fat and lay a foundation for understanding its role in human heart disease.

PAPP-A in normal human mesangial cells: effect of inflammation and factors related to diabetic nephropathy

Insulin-like growth factors (IGFs) are implicated in the development of diabetic nephropathy (DN) and are shown to increase proliferation and extracellular matrix production in mesangial cells. The IGF system is complex and is composed of ligands, receptors, six binding proteins (IGF BPs) and a novel zinc metalloproteinase - pregnancy-associated plasma protein (PAPP)-A. PAPP-A increases the local bioavailability of IGF through the cleavage of IGF BP-4. Mesangial expansion is a major component of DN, and PAPP-A is shown to be increased in the glomeruli of patients with DN. Therefore, we determined the expression of PAPP-A and components of the IGF system in normal human mesangial cells (HMCs) and their regulation by factors known to be involved in DN. Under basal conditions, HMCs expressed PAPP-A, IGF1 receptor and all six IGF BPs. Interleukin (IL)-1β was the most potent stimulus for PAPP-A expression (5-fold) followed by tumor necrosis factor (TNF)-α (2.5-fold). This PAPP-A was secreted, cell associated and proteolytically active. IL1β also increased IGF BP-1expression (3-fold) with either reduction or no effect on other IGF BPs. Generally, TNF-α treatment decreased IGF BP expression. No treatment effect on PAPP-A or IGF BPs was seen with IL6, IGFs, advanced glycation end products or prolonged hyperglycemia. In addition, stimulation of HMCs with IGF1 alone or IGF1 complexed to wild-type, but not protease-resistant, IGF BP-4 led to increased [(3)H]-thymidine incorporation. In conclusion, these novel findings of PAPP-A and its regulation by proinflammatory cytokines, as well as the comprehensive analysis of the IGF system regulation in HMCs, suggest a mechanism by which inflammatory states such as DN can impact IGF activity in the kidney.

PAPP-A2 expression by osteoblasts is required for normal postnatal growth in mice

OBJECTIVE

Pregnancy associated plasma protein-A2 (PAPP-A2) is a protease that cleaves insulin-like growth factor binding protein-5 (IGFBP-5), the most abundant IGFBP in bone. Deletion of Pappa2 reduces postnatal growth and bone length in mice. The aim of this study was to determine whether locally produced PAPP-A2 is required for normal bone growth.

DESIGN

We deleted Pappa2 primarily in osteoblasts by crossing conditional Pappa2 deletion mice with mice expressing Cre recombinase under the control of the Sp7 (Osterix) promoter. Effects of disrupting Pappa2 in Sp7-expressing cells were examined by measuring body mass and tail length at 3, 6, 10 and 12 weeks of age and bone dimensions at 12 weeks.

RESULTS

Body mass, tail length, and linear bone dimensions were significantly reduced at all ages by osteoblast-specific Pappa2 deletion. Mice homozygous for the conditional Pappa2 deletion allele and carrying the Cre transgene were smaller than controls carrying the Cre transgene, whereas mice homozygous for the conditional Pappa2 deletion allele were not smaller than controls when comparing mice not carrying the transgene. This result unambiguously demonstrates that PAPP-A2 produced by Sp7 expressing cells is required for normal growth. However, constitutive Pappa2 deletion had greater effects than osteoblast-specific Pappa2 deletion for many traits, indicating that post-natal growth is also affected by other sources of PAPP-A2. Immunohistochemistry revealed that PAPP-A2 localized in the epiphysis and metaphysis as well as osteoblasts, consistent with a role in bone growth.

CONCLUSION

Locally-produced PAPP-A2 is required for normal bone growth.

PAPP-A affects tendon structure and mechanical properties

Pregnancy-associated plasma protein-A (PAPP-A) serves to increase local insulin-like growth factor (IGF) stimulation of proliferation and differentiation in many tissues through proteolysis of inhibitory IGF-binding proteins. The purpose of this study was to investigate the effects of PAPP-A on tendon structure and mechanical properties. A total of 30 tails from 6-month-old mice were tested with 10 tails in each of following groups: PAPP-A knockout (KO), skeletal-specific PAPP-A overexpressing transgenic (Tg) and wild type (WT). Morphologically, the total tail cross-sectional area (CSA), individual tissue CSAs of bone, muscle and tendon, and fascicle diameter were measured. A fascicle pullout test was performed to assess stiffness and strength of interfascicular structures. Fascicles were mechanically characterized through low and high displacement rate uniaxial tension tests providing modulus at each rate, hysteresis area and stress relaxation ratio. The KO mice had a smaller total tail CSA (p<0.05), fascicle diameter (p<0.05), absolute tendon CSA (p<0.05), fast and slow stiffness (p<0.05 for both) and larger hysteresis area (p<0.05) compared to WT and Tg mice. On the other hand, the Tg mice had a larger fascicle diameter (p<0.05), absolute tendon CSA (p<0.05), higher interfascicular strength and stiffness (p<0.05) and lower fascicular modulus at low displacement rates (p<0.05) compared to WT and KO mice. Tg mice also had larger total tail CSA area (p<0.05) and smaller hysteresis area (p<0.05) than KO mice, and larger normalized tendon CSA (p<0.05) than WT mice. Based on these data, we conclude that PAPP-A affects fascicle structure, thereby affecting tendon phenotype.

Causal Modeling of Cancer-Stromal Communication Identifies PAPPA as a Novel Stroma-Secreted Factor Activating NFκB Signaling in Hepatocellular Carcinoma

Inter-cellular communication with stromal cells is vital for cancer cells. Molecules involved in the communication are potential drug targets. To identify them systematically, we applied a systems level analysis that combined reverse network engineering with causal effect estimation. Using only observational transcriptome profiles we searched for paracrine factors sending messages from activated hepatic stellate cells (HSC) to hepatocellular carcinoma (HCC) cells. We condensed these messages to predict ten proteins that, acting in concert, cause the majority of the gene expression changes observed in HCC cells. Among the 10 paracrine factors were both known and unknown cancer promoting stromal factors, the former including Placental Growth Factor (PGF) and Periostin (POSTN), while Pregnancy-Associated Plasma Protein A (PAPPA) was among the latter. Further support for the predicted effect of PAPPA on HCC cells came from both in vitro studies that showed PAPPA to contribute to the activation of NFκB signaling, and clinical data, which linked higher expression levels of PAPPA to advanced stage HCC. In summary, this study demonstrates the potential of causal modeling in combination with a condensation step borrowed from gene set analysis [Model-based Gene Set Analysis (MGSA)] in the identification of stromal signaling molecules influencing the cancer phenotype.

All living cells rely on communication with other cells to ensure their function and survival. Molecular signals are sent among cells of the same cell type and from cells of one cell type to another. In cancer, not only the cancer cells themselves are responsible for the malignancy, but also stromal (non-cancerous) cells and the molecular signals they send to cancer cells are important factors that determine the severity and outcome of the disease. Therefore, the identification of stromal signals and their influence on cancer cells is important for the development of novel treatment strategies. With a computational systems biology model of stroma-cancer cell communication, we have compiled a set of ten proteins secreted by stromal cells that shape the cancer phenotype. Most importantly, our causal analysis uncovered Pregnancy-Associated Plasma Protein A (PAPPA) as a novel paracrine inducer of the pro-tumorigenic NFκB signaling pathway. In liver cancer patients, higher levels of PAPPA protein indicate a more progressed tumor stage, confirming its clinical relevance.

Motor and memory testing of long-lived pregnancy-associated plasma protein--a knock-out mice

Mice deficient in pregnancy-associated plasma protein-A (PAPP-A), an IGF binding protein protease, have been shown to be resistant to experimentally induced atherosclerosis and diabetic nephropathy, and, in the laboratory environment, live 30-40% longer than wild-type littermates in association with delayed incidence and occurrence of age-related neoplasms and degenerative diseases.

OBJECTIVE

PAPP-A is highly expressed in the cerebellum and hippocampus of the mouse brain. Therefore, the studies presented here were aimed at determining motor behavior, learning and retention in PAPP-A knock-out (KO) mice compared to wild-type (WT) littermates with age.

DESIGN

Balance and coordination were assessed using an accelerating rotarod; learning and memory were assessed in a Stone T-maze.

RESULTS

Time on the rotarod decreased with age but there was no significant difference between PAPP-A KO and WT mice at any of the testing ages. Latency to reach the goal box and number of errors committed in the Stone T-maze did not change with age and there were no significant differences between PAPP-A KO and WT mice.

CONCLUSION

Lack of PAPP-A in mice did not impact central regulation of coordination, learning or memory.

Pregnancy-associated plasma protein A (PAPP-A) and preeclampsia

Pregnancy-associated plasma protein A (PAPP-A) is a key regulator of insulin-like growth factor bioavailability essential for normal fetal development. In maternal blood, this protein increases with gestational age and then rapidly decreases after delivery. It is routinely used for Down syndrome screening in the first trimester of pregnancy, and its decrease compared to a normal pregnancy indicates an increased risk for both chromosomal anomalies and adverse pregnancy outcomes. It belongs to a group of biomarkers that predict later preeclampsia development, primarily early onset preeclampsia; however, it should be combined with a Doppler ultrasonography of the uterine artery (pulsatile index) and other biochemical and maternal factors to achieve a higher detection rate with an acceptable false positivity rate. Some studies have demonstrated an even more pronounced decrease of PAPP-A in the early second trimester of pregnancy in women who subsequently develop preeclampsia compared with women who do not develop preeclampsia. Conversely, during the last trimester of pregnancy, its concentration increases even more in patients with preeclampsia than in patients without. It is also detectable at very low levels in nonpregnant individuals, and a higher concentration indicates an adverse effect in patients with acute coronary syndromes or stable atherosclerotic disease and in patients with end-stage renal disease who are being treated with hemodialysis.

Inducible reduction in pregnancy-associated plasma protein-A gene expression inhibits established atherosclerotic plaque progression in mice

Pregnancy-associated plasma protein-A (PAPP-A) is a novel zinc metalloproteinase implicated in cardiovascular disease. The aim of this study was to determine whether a reduction in PAPP-A expression in the adult affects the progression of established atherosclerotic plaque. Apolipoprotein E-null mice were fed a high-fat diet for 5 weeks to initiate early-stage plaque development before tamoxifen-inducible, Cre recombinase-mediated excision of the floxed PAPP-A gene. High-fat feeding was continued, and after 10 weeks the aorta and brachiocephalic artery were harvested for atherosclerotic plaque analyses of overall burden and morphology, respectively. An inducible decrease in PAPP-A gene expression significantly inhibited atherosclerotic plaque progression as assessed by a 70% reduction in plaque burden in the aorta (P = .012) without an effect on the elevated circulating levels of cholesterol and triglycerides in this model. Furthermore, this reduction in PAPP-A prevented the development of advanced plaque with necrotic cores and buried fibrous caps in the brachiocephalic artery. These data indicate PAPP-A as a potential target to limit progression of established atherosclerotic plaque.

Pregnancy-associated plasma protein (PAPP)-A expressed in the mammary gland controls epithelial cell proliferation and differentiation

Lactation is an important event in all-mammalian species. To investigate the role of pregnancy-associated plasma protein (PAPP)-A in lactogenesis, we determined (i) PAPP-A expression in mouse mammary glands and (ii) the biological functions of PAPP-A in mammary epithelial cells. PAPP-A mRNA level was low during early mid pregnancy and increased during mid-late pregnancy, and then slightly decreased during lactation. Cell proliferation signals, but not differentiation, increased PAPP-A mRNA expression in HC11 mammary epithelial cells. Treatment of recombinant PAPP-A protein stimulated HC11 cell proliferation and suppressed the expression of β-casein mRNA, which is one of the milk proteins and cell differentiation marker. Surprisingly, in forcing expression experiment, PAPP-A increased β-casein mRNA expression. Our data suggest that PAPP-A has different roles on intracellular expressing and extracellular treatment to mammary epithelial cells. Taken together, in early pregnancy, circulating PAPP-A protein might be supplied from other organs and stimulates mammary gland growth. In contrast, during mid-late pregnancy, local PAPP-A expression begins and enhances cell differentiation within mammary epithelial cell.

Timing of prenatal maternal exposure to severe life events and adverse pregnancy outcomes: a population study of 2.6 million pregnancies

OBJECTIVE

To identify the impact of timing of prenatal stress exposure on offspring risk for shortened gestational age, preterm birth (PTB), low birth weight (LBW), and small for gestational age (SGA), using a population-based sample.

METHODS

Swedish longitudinal population registries were linked to study all individuals born in Sweden from 1973 to 2004. Prenatal maternal stress exposure was defined as death of the father of the child or first-degree relative of the mother. Using linear and logistic regression, timing of stress exposure was examined across pregnancy, by month, and by novel periods created based on month of stress exposure findings.

RESULTS

A total of 2,618,777 live-born, singleton infants without congenital anomalies were included; 32,286 were exposed to prenatal maternal stress. Examining associations between stress exposure and outcome by the month revealed that risk increases midgestation, particularly after months 5 and 6. Combining months 1 to 4, 5 and 6, and 7 to 9 as potential periods of differing vulnerability, it was found that stress during period 2 (months 5 and 6) was associated with the greatest risk for shortened gestational age (-0.52 days, standard error = 0.15, p = .0006), PTB (odds ratio [OR], 1.24; 99% confidence interval [CI], 1.08-1.42), LBW (OR, 1.38; 99% CI, 1.19-1.61), and SGA (OR, 1.25; 99% CI, 1.05-1.49).

CONCLUSIONS

Risk for shortened GA, PTB, LBW, and SGA are greater post stress exposure during the 5th and/or 6th month of pregnancy. It may be beneficial to refine future analyses to these months. Possible mechanisms include alterations in the hypothalamic-pituitary-adrenal axis and associated stress-responsive molecular regulators.

Inactivation of insulin-like-growth factors diminished the anabolic effects of pregnancy-associated plasma protein-A (PAPP-A) on bone in mice

In vivo studies have provided ubiquitous evidence that pregnancy-associated plasma protein-A (PAPP-A) functions as a potent anabolic factor. While some evidence supports the prediction that increasing IGF bioavailability contributes to the anabolic effects of PAPP-A, definitive evidence has been lacking. This important issue has been addressed in this study using a unique mouse model in which PAPP-A was overexpressed in bone either alone or together with a protease-resistant IGFBP-4 analog (PRBP-4) which serves as an IGF inhibitor. PAPP-A transgenic mice exhibited a 25% increase in skull bone mineral density (BMD) whereas PRBP-4 transgenic mice showed a 20-25% decrease in this parameter at an age of 3months. Femur/tibia size-related parameters were significantly increased in PAPP-A transgenic mice but decreased in PRBP-4 transgenic mice. This data clearly demonstrates that PAPP-A transgenic mice exhibit opposite phenotypes in both flat bone and long bone compared to PRBP-4 transgenic mice which have reduced IGF bioavailability in bone. Importantly, PRBP-4 and PRBP-4/PAPP-A double transgenic mice shared essentially identical phenotypes in both flat and long bones. Calvarial thickness, skull BMD and long bone parameters were reduced to similar degrees in PRBP-4 and PRBP-4/PAPP-A transgenic mice relative to wild-type littermates. Our findings provide compelling evidence that PAPP-A increases bone formation primarily by increasing IGF bioavailability and that other alternative pathways may play a negligible role in mediating the anabolic effect of PAPPA in bone. This clear definition of PAPP-A's mechanism of action is critical for future translational studies on the therapeutic application of PAPP-A.

Metabolic consequences of pregnancy-associated plasma protein-A deficiency in mice: exploring possible relationship to the longevity phenotype

Mice born with the deletion of the gene for pregnancy-associated plasma protein-A (PAPP-A), a model of reduced local IGF activity, live approximately 30% longer than their wild-type (WT) littermates. In this study, we investigated metabolic consequences of PAPP-A gene deletion and possible relationship to lifespan extension. Specifically, we determined whether 18-month-old PAPP-A knockout (KO) mice when compared with their WT littermates have reduced energy expenditure and/or altered glucose-insulin sensitivity. Food intake, and total energy expenditure and resting energy expenditure as measured by calorimetry were not different between PAPP-A KO and WT mice when subjected to the analysis of covariance with body weight as the covariate. However, there was an increase in spontaneous physical activity in PAPP-A KO mice. Both WT and PAPP-A KO mice exhibited mild insulin resistance with age, as assessed by fasting glucose/insulin ratios. Oral glucose tolerance and insulin sensitivity were not significantly different between the two groups of mice, although there appeared to be a decrease in the average size of the pancreatic islets in PAPP-A KO mice. Thus, neither reduced 'rate of living' nor altered glucose-insulin homeostasis can be considered key determinants of the enhanced longevity of PAPP-A KO mice. These findings are discussed in the context of those from other long-lived mouse models.

Impact of reduced insulin-like growth factor-1/insulin signaling on aging in mammals: novel findings

Growth hormone deficiency or resistance resulting from spontaneous or experimentally produced mutations in laboratory mice delay aging and increase lifespan. Alterations in insulin-like growth factor-1 (IGF-1) and insulin signaling emerged as likely mechanisms linking growth hormone and aging, and increased longevity was reported in mice with selective deletion of IGF-1 receptor in all tissues or insulin receptor in fat. Recent studies in mice with reduced IGF-1 levels or deletion of pregnancy-associated plasma protein-A, a protease that cleaves one of the IGF-1 binding proteins, strongly support the role of IGF-1 in the control of longevity. Reports of increased lifespan in mice with deletion of insulin receptor substrate (IRS) 1, reduced expression of IRS2, or selective deletion of IRS2 in the brain specifically implicate the IRS-PI3K-Akt-Foxo signaling pathway (which is shared by IGF-1 and insulin) in the control of aging. These important novel findings also strengthen the evidence for evolutionary conservation of mechanisms regulating lifespan in worms, insects and mammals.

Transgenic overexpression of pregnancy-associated plasma protein-A increases the somatic growth and skeletal muscle mass in mice

Although IGFs are indispensable to skeletal muscle development, little information is available regarding the mechanisms regulating the local action of IGFs in skeletal muscle tissues. Here we tested the hypothesis that pregnancy-associated plasma protein-A (PAPP-A), a member of the metalloproteinase superfamily, promotes skeletal muscle formation in vivo through degrading IGF binding proteins (IGFBPs), which increases the bioavailability of IGFs. Expression of PAPP-A is significantly increased in muscle five days after muscle injury in mice. Targeted overexpression of PAPP-A using a muscle-specific promoter significantly increased the prenatal/postnatal growth, skeletal muscle weight, and muscle fiber area in mice. These anabolic effects were reproduced using F2/F3 progeny. Free IGF-I concentration was severalfold higher in the conditioned medium (CM) of ex vivo cultured muscle from the transgenic mice, compared with the wild-type littermate muscle. Accordingly, the proliferation of C2C12 myoblasts was significantly increased in the presence of CM from cultured skeletal muscle of the transgenic mice, compared with the controls. This observed increase in myoblast proliferation was abolished on addition of noncleavable IGFBP-4 peptide, which reduced free IGF-I concentration back to the basal level of the wild-type CM. Furthermore, proliferation and differentiation of C2C12 myoblasts was increased by transient overexpression of proteolytically active PAPP-A but not by inactive mutant PAPP-A (E483/A). Collectively, we identified PAPP-A as a novel regulator of prenatal/postnatal growth and skeletal muscle formation in vivo. Moreover, our studies provide the first experimental evidence that IGFBP degradation is a key determinant in modulating the local action of IGFs in muscle.

PAPP-A: a possible pathogenic link to the instability of atherosclerotic plaque

The rupture of coronary atherosclerotic plaque and subsequent thrombus formation are major events underlying acute coronary syndromes (ACS). Pregnancy associated plasma protein A (PAPP-A) is a member of the metzincin superfamily of metalloproteinases originally identified in the serum of pregnant women. Recent studies indicate that ACS is associated with elevated serum concentrations of PAPP-A. PAPP-A level is not only a marker of plaque instability favoring the progression to myocardial infarction, but is indicative of a poor prognosis even after the occurrence of an acute ischemic event caused by plaque instability. Why PAPP-A expression in unstable plaques is high is a puzzling problem. We hypothesized that PAPP-A is a possible cause of the instability of atherosclerotic plaque which plays a role in ACS. Studies found that PAPP-A was abundantly expressed in both eroded and ruptured plaques, but was only minimally expressed in stable plaques. Other studies have also demonstrated that patients with hyperechoic or isoechoic carotid plaques exhibit significantly higher PAPP-A levels than those with hypoechoic early carotid lesions. If the hypothesis is confirmed, administration of PAPP-A monoclonal antibodies may be used to eliminate the pathogen. It will be a new target point to treat ACS.

Genetic deletion of pregnancy-associated plasma protein-A is associated with resistance to atherosclerotic lesion development in apolipoprotein E-deficient mice challenged with a high-fat diet

Pregnancy-associated plasma protein-A (PAPP-A), a metalloproteinase in the insulin-like growth factor (IGF) system, is markedly upregulated in human atherosclerotic plaque. To determine whether PAPP-A plays an active role in the development of atherosclerosis, we crossed mice lacking apolipoprotein E (ApoE) with PAPP-A-deficient mice, generating ApoE knock-out (KO), PAPP-A KO, wild-type (WT/WT), and ApoE/PAPP-A double KO (KO/KO) mice. These mice were fed a high-fat diet starting at 7 weeks of age. Total serum cholesterol levels were elevated similarly in the ApoE KO and KO/KO mice and were 10-fold higher than in the WT/WT and PAPP-A KO mice. WT/WT and PAPP-A KO mice showed little or no lesion development even after 20 weeks of diet. ApoE KO mice had a progressive increase in aortic lesion area over 20 weeks of diet. In comparison, lesion area was reduced 60% to 80% in KO/KO mice. Lesions of ApoE KO aortas had 8- to 20-fold increases in PAPP-A, IGFBP-4, and IGF-I mRNA levels compared with nonlesional areas, whereas IGF-I receptor levels were equivalent--conditions for enhanced lesional IGF activity. Consistent with this, an in vivo marker of IGF-I receptor-mediated action was increased 10-fold in lesions from ApoE KO compared with KO/KO aortas. These data indicate that PAPP-A plays a critical role in lesion development in a mouse model of atherosclerosis, at least in part, through amplification of local IGF-I bioavailability.

Pregnancy associated plasma protein-A is necessary for expeditious fracture healing in mice

Pregnancy-associated plasma protein A (PAPP-A), a metalloproteinase that regulates IGF bioavailability in vitro through cleavage of inhibitory IGF-binding protein-4 (IGFBP-4), has been implicated in skeletal development and injury repair responses. However, direct in vivo data are lacking. In this study, we used PAPP-A knock-out (KO) mice to determine the role of PAPP-A in fracture repair. Stabilized mid-shaft fractures were produced in femurs of 3-month-old mice. At 14 days post-fracture, complete bony bridging of the fracture callus was seen radiographically in wild-type but not in PAPP-A KO mice. Histological examination 5 to 28 days post-fracture showed reductions in the amount of intramembranous bone formation, cartilage production, endochondral ossification and remodeling in PAPP-A KO compared with wild-type mice. However, fracture healing appeared similar in both groups at 42 days post-fracture when analyzed by histology. A similar degree of healing strength in wild-type and PAPP-A KO femurs was demonstrated by mechanical testing at 28 and 42 days post-fracture. Untreated cultures of day 5 fracture calluses from wild-type mice showed robust IGFBP-4 protease activity and IGF receptor phosphorylation, whereas fracture calluses from PAPP-A KO mice had no IGFBP-4 protease activity and reduced IGF receptor phosphorylation. These data demonstrate a marked delay in fracture healing in PAPP-A KO compared with wild-type mice, and suggest that PAPP-A is necessary in the early phases of the process for expeditious fracture repair. The ability of PAPP-A to enhance local IGF action may be an important mechanism for optimizing the fracture repair response.

Pregnancy-associated plasma protein-A (PAPP-A): a local regulator of IGF bioavailability through cleavage of IGFBPs

Pregnancy-associated plasma protein-A (PAPP-A) was originally isolated in 1974, as one of four proteins of placental origin found in high concentrations in the blood of pregnant women. In the early 1990s several laboratories reported novel protease activity against insulin-like growth factor binding protein-4 (IGFBP-4) in media conditioned by several cell types. This activity was unique, as it appeared to require the presence of IGF to cleave IGFBP-4. In 1999, this IGF-dependent IGFBP-4 protease activity was isolated from human fibroblast conditioned media and identified as PAPP-A. Subsequently, PAPP-A was shown to be expressed by a variety of cell types, and thus no longer could be considered to be just "pregnancy-associated". This review will describe what is currently known about the structure of PAPP-A and about its function as an IGFBP protease, with a focus on new insights obtained through study of a PAPP-A knock-out mouse model and on potential clinical applications.

Cell Surface Detachment of Pregnancy-associated Plasma Protein-A Requires the Formation of Intermolecular Proteinase-Inhibitor Disulfide Bonds and Glycosaminoglycan Covalently Bound to the Inhibitor

The metzincin metalloproteinase pregnancy-associated plasma protein-A (PAPP-A, pappalysin-1) promotes cell growth by proteolytic cleavage of insulin-like growth factor-binding proteins 4 and 5, causing the release of bound insulin-like growth factors. PAPP-A binds an unknown cell-surface heparan sulfate proteoglycan, suggesting that it controls insulin-like growth factor signaling spatially. In human pregnancy, the majority of PAPP-A circulates as a disulfide-bonded complex with its inhibitor, the proform of eosinophil major basic protein (proMBP). Interestingly, Ser-62 of proMBP is substituted with a glycosaminoglycan (GAG) chain, possibly a heparan sulfate type, and the PAPP-A.proMBP complex is unable to bind to the cell surface. We show here that proMBP detaches surface-bound PAPP-A in a process that depends on the proMBP GAG and also on the formation of intermolecular disulfide bonds between PAPP-A and proMBP. Unlike what was expected, we demonstrate that the GAG of proMBP is not required for PAPP-A.proMBP complex formation and that proMBP residues His-137, Ser-178, Arg-179, and Asn-181 are important for the recognition of PAPP-A. Using a mouse model, we find that the half-life of circulating PAPP-A and proMBP in complex is severalfold higher than both of the uncomplexed proteins, further suggesting that the PAPP-A.proMBP complex is formed at the cell surface in vivo rather than in the circulation. Further supporting this, we show that formation of the PAPP-A.proMBP complex at the cell surface proceeds rapidly compared with the slow rate of complex formation in solution. Because both PAPP-A and proMBP are expressed ubiquitously, this model may be applicable to many tissues in which insulin-like growth factor bioavailability is locally regulated.

Pregnancy-associated plasma protein-A increases osteoblast proliferation in vitro and bone formation in vivo

Pregnancy-associated plasma protein (PAPP)-A, a protease for IGF binding protein (IGFBP)-2, -4, and -5, may enhance IGF action by increasing its bioavailability. Here we have determined the role and mechanism of action of PAPP-A in the regulation of osteoblast proliferation in vitro and bone metabolism in vivo. Recombinant PAPP-A (100 ng/ml) significantly increased osteoblast proliferation and free IGF-I concentration. These effects were abolished by noncleavable IGFBP-4, suggesting that PAPP-A promotes osteoblast proliferation by increasing IGF bioavailability. To determine whether PAPP-A exerts an anabolic effect on bone in vivo, we developed transgenic mice that overexpress PAPP-A in osteoblasts using the 2.3-kb rat type I collagen promoter. Consistent with the increase in IGFBP-4 proteolysis, free IGF-I concentration was significantly increased in the conditioned medium of cultured osteoblasts derived from transgenic mice compared with the wild-type littermates. Calvarial bone thickness, bone marrow cavity, and skull bone mineral density were significantly increased in transgenic mice. Bone size-related parameters in femur and tibia such as total bone area and periosteal circumference as determined by peripheral quantitated computed tomography and histological analysis were significantly increased in transgenic mice. Bone formation rate and osteoid surface were increased by more than 2-fold, whereas bone resorbing surface was unaffected. These anabolic effects were sustained with aging. These findings provide strong evidence that PAPP-A acts as a potent anabolic factor in the regulation of bone formation. Thus, enhancing IGF bioavailability by PAPP-A can be a powerful strategy in the treatment of certain metabolic diseases such as osteoporosis.

The insulin-like growth factor system: IGFs, IGF-binding proteins and IGFBP-proteases

Insulin-like growth factors (IGF-I/-II) are not only the endocrine mediators of growth hormone-induced metabolic and anabolic actions but also polypeptides that act in a paracrine and autocrine manner to regulate cell growth, differentiation, apoptosis and transformation. The IGF system is a complex network comprised of two growth factors (IGF-I and -II), cell surface receptors (IGF-IR and -IIR), six specific high affinity binding proteins (IGFBP-I to IGFBP-6), IGFBP proteases as well as several other IGFBP-interacting molecules, which regulate and propagate IGF actions in several tissues. Besides their broad-spectrum physiological and pathophysiological functions, recent evidence suggests even a link between IGFs and different malignancies.

Pregnancy-associated plasma protein-a is involved in matrix mineralization of human adult mesenchymal stem cells and angiogenesis in the chick chorioallontoic membrane

Pregnancy-associated plasma protein A (PAPP-A) is an IGF binding protein 4 protease that can function to increase local IGF-I bioavailability. Aside from its assumed role during pregnancy, in vitro and in vivo studies have indicated roles for PAPP-A in IGF-I-mediated wound healing, vascular repair, and bone formation. Because bone morphogenetic protein 2 (BMP-2) is known to up-regulate Igf-I gene expression, we hypothesized that PAPP-A may be involved in BMP-2 mechanisms in bone formation. To test this hypothesis, we quantified gene expression of Papp-A in response to BMP-2 treatment and runt-related transcription factor 2, Osterix, and Igf-I in response to PAPP-A protein treatment in human adult mesenchymal stem cells. Our results demonstrate that BMP-2 directly up-regulated Papp-A gene and protein expression. Purified PAPP-A protein directly up-regulated runt-related transcription factor 2 and Igf-I gene expression but not Osterix. When added in combination with recombinant human BMP-2, PAPP-A increased matrix mineralization in the absence of dexamethasone. PAPP-A further demonstrated an angiogenic effect in the chick chorioallontoic membrane, which implicates a critical developmental role and possible therapeutic potential. Our findings suggest that PAPP-A functions in the formation of mineralized tissues through direct up-regulation of key genes. Furthermore, PAPP-A is involved in the formation of new blood vessels, which is essential for proper bone regeneration.

Functions for proteinases in the ovulatory process

The ovary is a unique and dynamic organ in respect to rapid and extensive degrees of tissue development and remodeling that are periodically repeated in the female reproductive activity. Ovulation is a directed and sequential process accompanied by broad-spectrum proteolysis and culminates in the follicular rupture to release the matured oocyte. This review will focus on the potential roles of six representative proteinases that are involved in various aspects of ovulatory processes: matrix metalloproteinases (MMPs), plasminogen activator (PA)/plasmin, a disintegrin and metalloproteinase domain with thrombospondin motif (ADAMTS), cathepsin-L, pregnancy-associated plasma protein-A (PAPP-A), and bone morphogenetic protein 1/mammalian Tolloid (BMP-1/mTld). Based on the studies of expression and function, these selected proteinases provide and share diverse functions ranging from cleaving components of the extracellular matrix (ECM) to modulating non-ECM molecules, such as various growth factors and their binding proteins. Consistently, the genetic deletion of each individual gene in mice shows their functional overlap in the reproductive activity.

Studies on regulation of IGF (insulin-like growth factor)-binding protein (IGFBP) 4 proteolysis by pregnancy-associated plasma protein-A (PAPP-A) in cells treated with phorbol ester

PAPP-A (pregnancy-associated plasma protein-A) is produced by hSFs (human skin fibroblasts) and hOBs (human osteoblasts) and enhances the mitogenic activity of IGFs (insulin-like growth factors) by degradation of IGFBP-4 (insulin-like growth factor-binding protein 4). PKC (protein kinase C) activation in these cells led to reduction in IGFBP-4 proteolysis. This study was undertaken to determine the mechanism by which activation of PKC suppresses IGFBP-4 proteolysis. Treatment of hSFs/hOBs with TPA (PMA; 100 nM) reduced IGFBP-4 proteolysis without significantly decreasing the PAPP-A level in the CM (conditioned medium). Immunodepletion of the proform of eosinophil major basic protein (proMBP), a known PAPP-A inhibitor, from CM of TPA-treated cells (TPA CM) failed to increase IGFBP-4 proteolytic activity. Transduction of hSFs with proMBP retrovirus increased the concentration of proMBP up to 30 ng/ml and led to a moderate reduction in IGFBP-4 proteolysis. In contrast, TPA treatment blocked IGFBP-4 proteolysis but failed to induce a detectable amount of proMBP in the CM. While proMBP overexpression led to the formation of a covalent proMBP-PAPP-A complex and reduced the migration of PAPP-A on SDS/PAGE, TPA treatment dose- and time-dependently increased the conversion of a approximately 470 kDa PAPP-A form (PAPP-A470) to a approximately 400 kDa PAPP-A form (PAPP-A400). Since unreduced PAPP-A400 co-migrated with the 400 kDa recombinant PAPP-A homodimer and since PAPP-A monomers from reduced PAPP-A470 and PAPP-A400 co-migrated on SDS/PAGE, conversion of PAPP-A470 to PAPP-A400 is unlikely to be caused by proteolytic cleavage of PAPP-A. Consistent with the data showing that the increase in the ratio of PAPP-A400/PAPP-A470 is correlated with the extent of reduction in IGFBP-4 proteolysis, partially purified PAPP-A400 exhibited a 4-fold reduction in IGFBP-4 proteolytic activity compared with PAPP-A470. These data suggest that a novel mechanism, namely conversion of PAPP-A470 to the less-active PAPP-A400, could account for the TPA-induced suppression of PAPP-A activity.

Follicular development: the role of the follicular microenvironment in selection of the dominant follicle

The importance of endocrine signals in the regulation of follicular development has long been recognized. However, the follicular microenvironment also plays a critical role in determining follicular fate. This review summarizes our studies on the role of the intrafollicular IGF system in selection of the dominant follicle (DF) in cattle. During the bovine estrous cycle, the largest antral follicles develop in two or three successive waves of follicular recruitment and selection of a DF. High concentrations of estradiol in the follicular fluid are the hallmark of dominant and preovulatory follicles and are associated with lower concentrations of low molecular weight (MW) insulin-like growth factor binding proteins (IGFBP-2, -4, and -5), which can prevent binding of IGF to its receptor. Our studies have shown that dominant and preovulatory follicles also have much higher levels of an IGFBP-4/-5 protease activity, which is the bovine equivalent of the human IGFBP-4 protease, pregnancy-associated plasma protein-A (PAPP-A). Studies of follicles isolated just after the emergence of the DF showed that PAPP-A is present in the follicular fluid of the DF as soon as it can be detected as morphologically dominant. To examine whether higher levels of PAPP-A in one follicle of the cohort (the future DF) precedes morphological dominance, the four largest follicles were isolated from pairs of bovine ovaries obtained before one follicle of the cohort was significantly larger the others, around the time that one follicle was first detected as morphologically dominant and after dominance was well established. Analysis of the temporal sequence of changes in estradiol, low MW IGFBPs, free IGF, and PAPP-A in the follicular fluid suggested that an increase in PAPP-A is the earliest biochemical difference yet detected in the future DF and that follicular selection is the result of a progressive series of changes beginning with the acquisition of PAPP-A, which leads to a decrease in IGFBP-4 and -5 and an increase in free IGF, which synergizes with FSH to increase estradiol production. Co-dominant follicles, induced by injection of small doses of recombinant bovine (rb) FSH, both had levels of PAPP-A similar to the single DF of control heifers, supporting the hypothesized role of FSH in the induction of PAPP-A in the DF. Taken together, these results suggest a critical role for FSH-induced PAPP-A, and thus for free IGF, in the selection of the DF. In contrast, other experiments provided evidence for a deleterious effect of IGF on the initiation of bovine follicular growth and the survival of primordial and primary follicles in vitro. These results underscore the importance of the follicular microenvironment in determining follicular fate and indicate that its effects can be stage-specific.

[Role of proteins of the macroglobulin family in the mechanisms of infection]

Information on the properties of proteins of the macroglobulin family taking part in the host protection from viral, fungal and bacterial pathogens is reviewed. High plasticity and polyfunctional character of these proteins makes it possible to realize different protective functions. They inhibit the lysis of the cell wall by binding the hydrolases of the pathogen thus blocking its penetration into the cell, directly participate in the presentation of antigens to immunocompetent cells, transport antibacterial substances (interferons, lysozyme) to the zone of infection. In addition, macroglobulins take part in the apoptosis regulation in infected cells, utilization of the lysosomal enzymes of annihilated pathogens. The complexes of macroglobulins with some proteins are powerful inductors of antibody production. Further studies of the properties of these proteins will result in a better understanding of the nature of infectious process. The possibility of artificial formation of macroglobulin complexes with pathogen components or with substances possessing protective or anti-inflammatory properties opens prospects for using these proteins in the fields of vaccinology, gene therapy and molecular biology.

Pregnancy-Associated Plasma Protein A and Its Endogenous Inhibitor, the Proform of Eosinophil Major Basic Protein (proMBP), Are Related to Complex Stenosis Morphology in Patients With Stable Angina Pectoris

Background— The metalloproteinase pregnancy-associated plasma protein-A (PAPP-A) has been implicated in coronary plaque disruption. Its endogenous inhibitor, the proform of eosinophil major basic protein (proMBP), may also play a role in this process. Atheromatous plaque disruption often presents as complex angiographic lesions. We sought to assess whether PAPP-A, proMBP, and PAPP-A/ProMBP ratio are markers of angiographic plaque complexity in patients with chronic stable angina.

Methods and Results— We studied 396 stable angina patients (age 63±10 years, 230 men) of whom 289 had angiographically documented coronary artery disease (≥75% stenosis). All coronary stenoses ≥30% diameter reduction (n =531 in 322 patients) were assessed and classified as complex (n =228) or smooth (n =303) by previously validated criteria. PAPP-A, proMBP, and C-reactive protein (hs-CRP) serum levels were measured by ELISA. Patients with complex coronary stenoses had a significantly ( P <0.001) higher PAPP-A/proMBP ratio (3.1±1.2 versus 2.7±0.8×10 −3 ) and PAPP-A levels (5.9±1.6 versus 5.1±1.4 mIU/L) than those without. On univariate analysis, male gender ( P <0.001), age ( P <0.001), previous history of myocardial infarction ( P =0.013), reduced ejection fraction ( P <0.001), severe coronary artery disease ( P <0.001), aspirin treatment ( P <0.001), PAPP-A levels ( P <0.001), and PAPP-A/proMBP ratio ( P <0.001) were correlated with the number of complex stenoses. Multiple regression analysis showed that male gender, age, severe coronary artery disease, and PAPP-A/proMBP ratio were independent predictors of the number of angiographically complex stenoses.

Conclusions— In patients with stable angina, PAPP-A and PAPP-A/proMBP ratio are associated with angiographic plaque complexity.

Up date on IGFBP-4: regulation of IGFBP-4 levels and functions, in vitro and in vivo

Of the six known high affinity insulin-like growth factor binding-proteins (IGFBPs), IGFBP-4 appears to be unique in that it is the only IGFBP that functions mostly like a traditional binding protein. In this regard, none of the IGF independent effects that have been ascribed for other IGFBPs have been described for IGFBP-4. However, recent in vitro and in vivo studies, in particular the recent identification of pregnancy-associated plasma protein-A as a major IGFBP-4 protease, are consistent with the idea that IGFBP-4 is an extremely important component of IGF system in several tissues including gonads and bone. In this review, we have provided an update on IGFBP-4 research and we have summarized our current understanding of the regulation of levels and actions of IGFBP-4 and proteolytic fragments both in vitro and in vivo.

Metalloproteinase pregnancy-associated plasma protein A is a critical growth regulatory factor during fetal development

Pregnancy-associated plasma protein A (PAPPA) is a metzincin superfamily metalloproteinase in the insulin-like growth factor (IGF) system. PAPPA increases IGF bioavailability and mitogenic effectiveness in vitro through regulated cleavage of IGF-binding protein 4 (IGFBP4). To determine its function in vivo, we generated PAPPA-null mice by gene targeting. Mice homozygous for targeted disruption of the PAPPA gene were viable but 60% the size of wild-type littermates at birth. The impact of the mutation was exerted during the early embryonic period prior to organogenesis, resulting in proportional dwarfism. PAPPA, IGF2 and IGFBP4 transcripts co-localized in wild-type embryos, and expression of IGF2 and IGFBP4 mRNA was not altered in PAPPA-deficient embryos. However,IGFBP4 proteolytic activity was completely lacking in fibroblasts derived from PAPPA-deficient embryos, and IGFBP4 effectively inhibited IGF-stimulated mitogenesis in these cells. These results provide the first direct evidence that PAPPA is an essential growth regulatory factor in vivo, and suggest a novel mechanism for regulated IGF bioavailability during early fetal development.

Review: immunomodulatory activity of pregnancy-associated plasma protein-A

This mini-review highlights the growing number of indications for the immunological importance of pregnancy-associated plasma protein-A (PAPP-A), which is a remote member of the alpha-macroglobulin plasma protein family. PAPP-A can bind a variety of cytokines and specifically cleave a binding protein for insulin-like growth factors, thereby serving as a modulator of cytokine activity. Important immune functions, such as lymphocyte proliferation response to alloantigens and lectins and expression of HLA-DR molecules are predominantly suppressed in vitro by PAPP-A. It is likely that the immunoregulatory properties of PAPP-A are very similar to that of alpha 2-macroglobulin. The experimental data allows us to suppose that PAPP-A serves to prevent the recognition of the fetus by the maternal immune system and to suppress locally the host's immune response to the tumour.

Pregnancy-associated plasma protein A (PAPP-A): theoretical and clinical aspects

Pregnancy-associated plasma protein A (PAPP-A) is an important pregnancy protein. PAPP-A exists in pregnancy serum as a heterotetrameric 2:2 complex with the proform of eosinophil major basic protein (proMBP), forming an approximately 500 kDa and called PAPP-A/proMBP. The gene of PAPP-A has been assigned to human chromosome 9q 33.1. PAPP-A belongs to the metzincin superfamily of metalloproteinases. It contains five short consensus repeats (SCR) and three the lin-notch repeats (LNR) and in addition a putative Zn binding site. The main site of both PAPP-A and proMBP synthesis during pregnancy is the placenta as shown by in situ hybridization. PAPP-A seems to be the predominating IGFBP-4 proteinase in pregnancy serum. In the women the levels of PAPP-A are highest during pregnancy, when plasma levels increase by a factor of about 150 as compared to the nonpregnant state. PAPP-A is the most abundant in the peripheral maternal circulation. Determination of PAPP-A in pregnancy serum has a limited value in some complication in gravidity such as a threatened abortion, ectopic gravidity, preeclampsia or diabetes mellitus. PAPP-A determination will gain increasing importance since this protein seems to be the major biochemical marker of Down syndrome in the first trimester of pregnancy. Maternal serum levels of PAPP-A in the first trimester are significantly reduced when a fetus affected by Down syndrome is present. Low first trimester maternal serum levels were found not only in trisomy 21 but also in non-Down syndrome fetal aneuploidies. Another contribution of PAPP-A determination may be in differentation of stable and unstable angina pectoris. (Tab. 3, Fig. 5, Ref. 78.)

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.

The physiology of folliculogenesis: the role of novel growth factors

OBJECTIVE

To assess major physiological events underlying folliculogenesis, including FSH-dependent dominant follicle (DF) formation, LH/hCG signaling, and the role of novel regulatory molecules in these developmental processes.

DESIGN

Review of some of the past and recent advances in ovarian biology, focusing attention on [1] two novel oocyte-derived growth factors, growth differentiation factor-9 (GDF-9) and bone morphogenetic protein (BMP-15); and [2] a recently discovered follicular insulin-like growth factor binding protein-4 (IGFBP-4) protease, pregnancy-associated plasma protein-A (PAPP-A), that can degrade the FSH antagonist IGFBP-4.

RESULT(S)

Oocyte-derived GDF-9 and BMP-15 are obligatory for folliculogenesis and female fertility in laboratory animals through their ability to stimulate granulosa cell proliferation and modulate FSH-dependent cytodifferentiation. The expression of these growth factors in human primary oocytes supports the hypothesis that GDF-9 and BMP-15 could be involved in ovary function in women. Pregnancy-associated plasma protein-A is a marker for the human dominant follicle and its product the corpus luteum, raising the possibility that this putative FSH antagonist might regulate FSH bioactivity during folliculogenesis and luteogenesis.

CONCLUSION(S)

Oocyte-derived and granulosa-derived regulatory proteins perform very important functions in FSH-dependent folliculogenesis. The current challenges are to understand the role of these novel proteins in ovary physiology and pathophysiology in women.

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.

Pregnancy-associated plasma protein-A-induced inhibition of human leukocyte elastase: an artifact

Pregnancy-associated plasma protein A (PAPP-A), was reported to be an inhibitor in many in vitro systems. Since it was shown that the inhibition of coagulation and complement activity attributed to PAPP-A was in fact due to a contamination by heparin occurring during the purification process, we undertook the present study to see whether the reported PAPP-A-induced inhibition of human leukocyte elastase (HLE) could also be attributed to heparin contamination. PAPP-A was purified from maternal pregnancy EDTA plasma by a method which was previously shown to eliminate contaminating heparin: this preparation was inactive in the HLE assay. But PAPP-A isolated by heparin-Sepharose chromatography, or a PAPP-A-free washing of the heparin-Sepharose column were both inhibitors of HLE. Furthermore the inactive PAPP-A preparation, when incubated with the PAPP-A-free washing of the heparin-Sepharose column, yielded a high molecular weight preparation which inhibited HLE. It is concluded that PAPP-A is not an inhibitor of HLE and that the inhibition of HLE previously attributed to PAPP-A was due to contaminating heparin.

Three pregnancy proteins (PP12, PP14, and PAPP-A): their biological and clinical relevance

The three pregnancy proteins, PP12, PP14, and PAPP-A, reviewed here are all produced by the endometrium under the influence of progesterone. Their production is low during the secretory phase and increases dramatically after decidualization and pregnancy. PP12 and PP14 are synthesized by the epithelial cells and PAPP-A is synthesized by the stromal cells. Reflecting perhaps the relative abundance of stromal cells, PAPP-A concentrations increase progressively to term, whereas the levels of PP12 and PP14 level off and even decline after the 20th week of gestation. These proteins are also found in nonpregnant subjects in extrauterine sites: follicles, follicular fluid, luteal cells, and fallopian tubes, and in males, in seminal vesicles and seminal fluid. PP12 has been found in several forms of cancer, although not with sufficient frequency to make it a useful tumor marker. The biologic function of these proteins is still subject to speculation, but they do reflect the biosynthetic capacity of decidualized endometrium, and especially PP14 may find clinical application in the management of infertility patients.

In vitro effects of pregnancy-associated plasma protein-A: artifacts due to heparin

Pregnancy-associated plasma protein-A (PAPP-A) has been reported to inhibit elastase activity, lymphoblastogenesis, complement activity, and thrombin-induced coagulation of fibrinogen. Since some of these results are controversial, we reevaluate here the effects of PAPP-A in these last two systems. By molecular sieve chromatography, PAPP-A immunoreactivity and inhibitory activity on thrombin and complement were dissociated. A PAPP-A-free washing of the heparin-Sepharose column used during the purification of PAPP-A showed inhibitory activities similar to those of purified PAPP-A. Furthermore, a preparation of PAPP-A that had not been submitted to heparin-Sepharose chromatography during purification was not active in either assays. Thus, the anticoagulant and anti-complement effects previously attributed to PAPP-A were due to a contaminant of low molecular mass. We believe that this contaminant is probably heparin. A protocol to eliminate free and PAPP-A-bound heparin is presented herein, and implications for other previously reported in vitro effects of PAPP-A are discussed.

Potential role of pregnancy-associated plasma protein-A in human reproduction

By radioimmunoassay, pregnancy-associated plasma protein-A (PAPP-A) was undetectable in matched follicular and luteal phase serum samples (n = 17) or in the peripheral circulation of normal males (n = 17). However, seminal plasma (91.5%), cervical mucus (100%) and pre-ovulatory follicular fluid (99.6%) were consistently PaPP-A positive. In addition to PAPP-A, four circulating protease inhibitors (PIs) were detected in pooled seminal plasma whereas pooled follicular fluid contained an additional six. Follicular concentrations of serum PIs were inversely related to molecular size. By contrast, PAPP-A formed a positive concentration gradient across the blood-reproductive tract barrier suggesting PAPP-A production within the reproductive tract. A minor proportion (1.7%) of ejaculated spermatozoa were coated with PAPP-A, as demonstrated by direct immunofluorescence. Since PAPP-A specifically inhibits leucocyte elastase, it is suggested that PAPP-A coated spermatozoa were "selected" to overcome localized phagocytic-proteolytic degradation. The physiological significance of these findings are discussed in relation to human reproduction.

Pregnancy-associated plasma protein-A and placental protein 5 in human seminal plasma

Human seminal plasma contains two glycoproteins which are physiochemically and immunologically indistinguishable from pregnancy-associated plasma protein-A and placental protein 5. Seminal concentrations of both glycoproteins did not correlate with clinical assessment of semen quality. Furthermore, analysis of split ejaculates indicated a nontesticular origin for both proteins, which are possibly secreted into the distal portions of the tract by the accessory glands (prostate gland and seminal vesicles). The physiological significance of these findings has yet to be determined. However, it is suggested that pregnancy-associated plasma protein-A, a known potent inhibitor of leukocyte elastase, protects the deposited sperm against proteolytic attack originating from the localized leukocyte reaction within the female reproductive tract, thus contributing towards sperm survival within this immunologically hostile environment and enabling fertilization to occur.

Pregnancy-associated plasma protein-A and placental protein 5 in human ovarian follicular fluid

By sensitive and specific radioimmunoassays PAPP-A and PP5 were detected in follicular aspirates obtained from women undergoing ovarian hyperstimulation for oocyte harvest prior to in vitro fertilization and embryo transfer. Follicular and pregnancy-derived PAPP-A were immunologically and physicochemically indistinguishable. Similarly, pregnancy- and nonpregnancy-derived PP5 were immunologically indistinguishable. However, in addition to the 18- and 36-K species, a larger species having a molecular size greater than 140K was found in the follicular fluid. Mean follicular PAPP-A and PP5 concentrations were 727 mIU/L and 1376 mAU/L, respectively, with no significant correlation between follicular PAPP-A, PP5, and steroid concentrations. There was, however, a significant but negative relationship with follicular volume. Preliminary in vitro studies indicated that both proteins were synthesized by granulosa cells in preparation for follicular rupture. Follicular PP5, like antithrombin III, interacted reversibly with heparin and thrombin affinity matrices, suggesting a potential biological role as a follicular anticoagulant, whereas PAPP-A, a specific and potent inhibitor of leukocyte elastase, contributes to the maintenance of proteolytic homeostasis and the protection of spermatozoa and embryo against proteolytic attack originating from the maternal leukocytes.

Observations on the source of pregnancy-associated plasma protein A (PAPP-A)

Pregnancy-associated plasma protein A (PAPP-A) was measured in 26 women before and after hysterectomy. The PAPP-A concentration declined significantly in 24 of them. It is concluded that the uterus is a source of the protein.

Pregnancy-associated plasma protein A (PAPP-A) in non-pregnant subjects

A newly developed enzyme immunoassay technique was applied to the measurement of pregnancy-associated plasma protein A (PAPP-A) in the serum of women in the proliferative and luteal phases of the cycle, in hysterectomized and postmenopausal women and in the serum and seminal plasma of males. PAPP-A was detected in some individuals in all the categories of women and in seminal plasma but not in male serum. It is surmised that there must be a source of PAPP-A other than the placenta and that PAPP-A may be a maternal protein whose biosynthesis is stimulated by pregnancy rather than a product peculiar to the trophoblast.