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

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

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

Pregnancy-Associated Plasma Protein A Induces Inflammatory Cytokine Expression by Activating IGF-I/PI3K/Akt Pathways

Pregnancy-associated plasma protein A (PAPP-A) was previously reported to be an inflammatory biomarker and a prognostic marker of acute coronary syndrome (ACS) and involved in the process of atherosclerosis and plaque rupture. However, the role of PAPP-A in inflammation is poorly understood. In this study, we aimed to investigate the role of PAPP-A in macrophage activation and inflammatory cytokine production. RAW264.7 macrophages were treated with or without PAPP-A. Reverse-transcriptase quantitative real-time PCR (RT-qPCR) and Western blot were performed to detect gene and protein expressions. The concentration of monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) in culture supernatants was determined by ELISA. Results showed that PAPP-A significantly stimulated the expression of MCP-1, TNF-α, and IL-6 at both transcriptional and translational levels in a dose-dependent and time-dependent manner. The secretion of these inflammatory cytokines by macrophages was also increased after PAPP-A treatment. Moreover, PAPP-A activated the IGF-I/PI3K/Akt signaling pathway in macrophages. The PAPP-A-mediated upregulation of MCP-1, TNF-α, and IL-6 mRNA and protein levels were strongly inhibited by PI3K inhibitors or IGF-IR siRNA, indicating that the upregulation of MCP-1, TNF-α, and IL-6 could involve the IGF-I/PI3K/Akt pathway. Together, this study demonstrates that PAPP-A activates the macrophage signaling pathway (IGF-I/PI3K/Akt), which drives the expression and production of inflammatory cytokines known to contribute to the initiation and progression of ACS. These findings indicate that PAPP-A may play a proinflammatory role in the pathophysiology of ACS and serve as a potential therapeutic target.

Insulin-like growth factor-binding protein-4 inhibits epithelial growth and proliferation in the rodent intestine

Insulin-like growth factor-binding protein-4 (IGFBP-4) is a binding protein that modulates the action of insulin-like growth factor-1 (IGF-1), a growth factor whose presence is required for the intestinotrophic effects of glucagon-like peptide-2 (GLP-2). GLP-2 is a gut hormone that uses both IGF-1 and epidermal growth factor (EGF) as intermediary factors to promote intestinal growth. Therefore, to elucidate the mechanism through which IGFBP-4 regulates IGF-1 activity in the intestine, proliferation assays were conducted using rat intestinal epithelial cells (IEC-6). IGF-1 and EGF synergistically enhanced proliferation, an effect that was dose-dependently decreased by IGFBP-4 ( P < 0.05-0.001) in an IGF-1 receptor (R)- and MEK1/2- but not a phosphatidylinositol 3-kinase-dependent manner ( P > 0.05 for IGFBP-4 effects with IGF-1R and MEK1/2 inhibitors). Intestinal organoids derived from IGFBP-4 knockout mice demonstrated significantly greater Ki-67 expression and an enhanced surface area increase in response to IGF-1 treatment, compared with organoids from control mice ( P < 0.05-0.01). GLP-2 is also known to increase the mucosal expression of IGFBP-4 mRNA. To investigate whether this occurs through the actions of its intermediaries, IGF-1 and EGF, inducible intestinal epithelial-IGF-1R knockout and control mice were treated for 10 days with and without the pan-ErbB inhibitor, CI-1033. However, no differences in mucosal IGFBP-4 mRNA expression were found for any of the treatment groups ( P > 0.05). Consistently, IEC-6 cells treated with IGF-1 and/or EGF displayed no alteration in IGFBP-4 mRNA or in cellular and secreted IGFBP-4 protein ( P > 0.05). Overall, this study establishes that endogenous IGFBP-4 plays an important role in inhibiting IGF-1-induced intestinal epithelial proliferation and that mucosal IGFBP-4 expression is independent of IGF-1 and EGF. NEW & NOTEWORTHY This study demonstrates, for the first time, the inhibitory role of locally expressed insulin-like growth factor-binding protein-4 (IGFBP-4) on the intestinal proliferative actions of IGF-1 and supports the notion of the synergistic roles of IGF-1 and EGF in promoting intestinal epithelial growth. In turn, intestinal IGFBP-4 expression was not found to be regulated by IGF-1 and/or EGF.

Insulin- like Growth Factor-Binding Protein Action in Bone Tissue: A Key Role for Pregnancy- Associated Plasma Protein-A

The insulin-like growth factor (IGF) axis is required for the differentiation, development, and maintenance of bone tissue. Accordingly, dysregulation of this axis is associated with various skeletal pathologies including growth abnormalities and compromised bone structure. It is becoming increasingly apparent that the action of the IGF axis must be viewed holistically taking into account not just the actions of the growth factors and receptors, but also the influence of soluble high affinity IGF binding proteins (IGFBPs).There is a recognition that IGFBPs exert IGF-dependent and IGF-independent effects in bone and other tissues and that an understanding of the mechanisms of action of IGFBPs and their regulation in the pericellular environment impact critically on tissue physiology. In this respect, a group of IGFBP proteinases (which may be considered as ancillary members of the IGF axis) play a crucial role in regulating IGFBP function. In this model, cleavage of IGFBPs by specific proteinases into fragments with lower affinity for growth factor(s) regulates the partition of IGFs between IGFBPs and cell surface IGF receptors. In this review, we examine the importance of IGFBP function in bone tissue with special emphasis on the role of pregnancy associated plasma protein-A (PAPP-A). We examine the function of PAPP-A primarily as an IGFBP-4 proteinase and present evidence that PAPP-A induced cleavage of IGFBP-4 is potentially a key regulatory step in bone metabolism. We also highlight some recent findings with regard to IGFBP-2 and IGFBP-5 (also PAPP-A substrates) function in bone tissue and briefly discuss the actions of the other three IGFBPs (-1, -3, and -6) in this tissue. Although our main focus will be in bone we will allude to IGFBP activity in other cells and tissues where appropriate.

Immunolocalization and biochemical evidence of pregnancy-associated plasma protein A in the intervertebral disc

STUDY DESIGN

Studies were approved by the authors' Human Subjects Institutional Review Board and Institutional Animal Care and Use Committee. Anulus tissue was used in studies of the immunocytochemical localization of pregnancy-associated plasma protein A (PAPP-A) in disc tissue from the sand rat and from human disc surgical specimens and specimens from control donors. Cultured human disc cells were also tested for production of PAPP-A.

OBJECTIVES

(1) To determine the immunohistochemical localization of PAPP-A in human and sand rat discs; (2) To test for gene expression of PAPP-A in the human disc in vivo and in vitro production by cultured cells; and (3) To test for expression of insulin-like growth factor binding proteins (IGFBP)-2, -4, and -5 in vivo and in vitro by human disc cells.

SUMMARY OF BACKGROUND DATA

PAPP-A is a metalloproteinase expressed by several cell types, including fibroblasts, osteoblasts, and smooth muscle cells. PAPP-A has an extremely important role because it cleaves IGFBP-2, -4, and -5 in the extracellular matrix, thereby increasing the bioavailability of IGF to nearby cells. METHODS.: Specimens of human disc tissue and lumbar discs from sand rats were assessed for immunocytochemical localization of PAPP-A, and the percentage of positive cells determined. Human disc cells in three-dimensional culture were assessed for production of PAPP-A using an enzyme linked immunosorbent assay. Molecular gene expression studies were carried out using microarray analysis.

RESULTS

Positive cytoplasmic immunolocalization of PAPP-A was present in the majority of cells of the human and sand rat outer anulus (OA). In the human outer anulus, the percentage of cells positive for PAPP-A localization did not differ in Grades I-II discs vs. Grades III-V discs (OA: 77.4% +/- 10.5 vs. 75.1% +/- 7.4 [mean +/- SEM] respectively). In the inner anulus, however, the percentage of cells positive for PAPP-A localization in more degenerate discs was significantly greater than the percentage in healthier discs (60.7% +/- 10.1 vs. 15.6 +/- 5.4, P = 0.024). % positive cells in the inner anulus correlated significantly with disc grade (r = 0.579; P = 0.01). Over a 5-day three-dimensional culture period, human anulus cells produced and secreted abundant PAPP-A into the culture media. Molecular studies confirmed the expression of IGFBP-2, -4, and -5 both in vivo and in vitro.

CONCLUSIONS

Data provide important new insights into disc cell expression of PAPP-A at the translational level. The presence of a significantly greater proportion of cells positive for PAPP-A in the inner anulus of more degenerate Grade III-V discs compared with healthier Grade I-II discs supports our previous observation of increased gene expression of PAPP-A in more degenerated discs. Biochemical data shown here documented production of PAPP-A by disc cells in vitro. Production of PAPP-A by disc cells is important since PAPP-A cleaves IGF-binding proteins, and makes IGF-I, a potent mitogen and antiapoptotic agent, available to cells. Future studies are indicated to further investigate PAPP-A and IGF-BP function in the disc.

Impact of pregnancy-associated plasma protein-a deletion on the adult murine skeleton

INTRODUCTION

The metalloproteinase, pregnancy-associated plasma protein-A (PAPP-A) functions to enhance local insulin-like growth factor (IGF)-I bioavailability through cleavage of inhibitory IGF binding proteins. Because IGF-I is an important regulator of skeletal growth and remodeling and PAPP-A is highly expressed by osteoblastic cells, we hypothesized that, in the absence of PAPP-A, bone physiology would be compromised because of a blunting of local IGF-I action even in the presence of normal circulating IGF-I levels.

MATERIALS AND METHODS

pQCT, muCT, histomorphometry, and mechanical strength testing were performed on bones from PAPP-A knockout (KO) mice and wildtype (WT) littermates at 2-12 mo of age. IGF-I levels and bone formation and resorption markers were determined in sera from these animals.

RESULTS

Volumetric BMD in PAPP-A KO mice measured by pQCT at the femoral midshaft, which is primarily cortical bone, was 10% less than WT at 2 mo. This difference was maintained at 4, 6, and 12 mo. Cortical thickness at this site was similarly decreased. On the other hand, trabecular bone at the distal femur (pQCT) and in the tibia (muCT) showed age-progressive decreases in bone volume fraction in PAPP-A KO compared with WT mice. Tibial muCT indicated a 46% relative decrease in trabecular bone volume/total volume (BV/TV) and a 28% relative decrease in trabecular thickness in PAPP-A KO compared with WT mice at 6 mo. These trabecular deficiencies in PAPP-A KO mice corresponded to a weakening of the bone. Serum markers and bone histomorphometry indicated that the primary impact of PAPP-A is on skeletal remodeling resulting in a state of low-turnover osteopenia in adult PAPP-A KO mice. Circulating IGF-I levels were not altered in PAPP-A KO mice.

CONCLUSIONS

PAPP-A is a bone growth regulatory factor in vivo and, in its absence, mice show skeletal insufficiency in mass, density, architecture, and strength. The data suggest a primary role for PAPP-A in modulating local IGF bioavailability for trabecular bone remodeling.

Insulin-like growth factor-binding proteins and bone metabolism

Insulin-like growth factor-binding proteins (IGFBPs) are important regulators of bone metabolism. However, their precise roles are not fully understood, since IGFBPs can have both enhancing and inhibiting effects on IGF action, depending on context and posttranslational modifications, as well as IGF-independent effects. This review focuses on recent findings from cell culture, rodent models, and clinical studies concerning local IGFBP-2, IGFBP-4, and IGFBP-5 action in bone.

Effects of insulin-like growth factor binding proteins in bone -- a matter of cell and site

The actions of the insulin-like growth factor (IGF)-system are controlled by six IGF-binding proteins (IGFBPs). The IGFBPs are thought to affect local effects of IGF-I and IGF-II due to higher affinity if compared to IGF-I receptors and due to cell-type specific IGFBP expression patterns. It was found in IGFBP knockout models that the IGFBP family is functionally redundant. Thus, functional analysis of potential effects of IGFBPs is dependent on descriptive studies and models of IGFBP overexposure in vitro and in vivo. In the literature, the role of the IGFBPs for bone growth is highly controversial and, to date, no systematic look has been taken at IGFBPs resolving functional aspects of IGFBPs at levels of cell types and specific locations within bones. Since IGFBPs are thought to represent local modulators of the IGF actions and also exert IGF-independent effects, this approach is particularly reasonable on a physiological level. By sorting the huge number of in part controversial results on IGFBP effects in bone present in the literature for distinct cell types and bone sites it is possible to generate a focused, more specific and a less controversial picture of IGFBP functions in bone.

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.

Real-time RT-PCR quantification of pregnancy-associated plasma protein-A mRNA abundance in bovine granulosa and theca cells: effects of hormones in vitro

Ovarian follicular growth and dominance are controlled by a series of hormonal and intraovarian events including a decrease in intrafollicular IGF-binding proteins -2, -4 and -5 levels. Proteolytic enzymes such as pregnancy-associated plasma protein-A (PAPP-A) degrade IGFBPs and increase bioavailability of IGF-I and -II during follicular development. The objective of this study was to determine the effect of IGF-I, IGF-II, insulin (INS), LH, FSH, estradiol (E2), leptin or cortisol on ovarian PAPP-A mRNA levels. Granulosa (GC) from small (SM) (1-5 mm) and large (LG) (8-22 mm) follicles as well as theca cells (TC) from LG follicles were collected from bovine ovaries and cultured for 48 h in medium containing 10% FCS and then treated with various hormones in serum-free medium for an additional 24 h. Cells were treated with various concentrations (3-500 ng/ml) and combinations of IGF-I, IGF-II, FSH, LH, E2, INS, leptin and (or) cortisol for 24 h (Experiments 1-10). PAPP-A mRNA levels were measured using quantitative real-time RT-PCR. In SM-GC and LG-GC, none of the treatments significantly affected (P>0.10) PAPP-A mRNA abundance. In LG-TC, IGF-I, LH or cortisol did not affect (P>0.10) PAPP-A mRNA levels, whereas INS with or without LH decreased (P<0.05) PAPP-A mRNA. E2 alone decreased PAPP-A mRNA levels in LG-TC, and E2 amplified the insulin-induced inhibition of PAPP-A mRNA abundance in LG-TC. We conclude that control of PAPP-A mRNA abundance in granulosa and theca cells differs, and that E2 may be part of an intraovarian negative feedback system which may reduce the bioavailable IGFs in the theca layer during growth and selection of follicles.