Glycosaminoglycans promote HARP/PTN dimerization

Pleiotrophin: Activity and mechanism

Pleiotrophin (PTN) is a potent mitogenic cytokine with a high affinity for the polysaccharide glycosaminoglycan (GAG). Although it is most strongly associated with neural development during embryogenesis and the neonatal period, its expression has also been linked to a plethora of other physiological events including cancer metastasis, angiogenesis, bone development, and inflammation. A considerable amount of research has been carried out to understand the mechanisms by which PTN regulates these events. In particular, PTN has now been shown to bind a diverse collection of receptors including many GAG-containing proteoglycans. These interactions lead to the activation of many intracellular kinases and, ultimately, activation and transformation of cells. Structural studies of PTN in complex with both GAG and domains from its non-proteoglycan receptors reveal a binding mechanism that relies on electrostatic interactions and points to PTN-induced receptor oligomerization as one of the possible ways PTN uses to control cellular functions.

The role of pleiotrophin in bone repair

Bone has an enormous capacity for growth, regeneration, and remodelling, largely due to induction of osteoblasts that are recruited to the site of bone formation. Although the pathways involved have not been fully elucidated, it is well accepted that the immediate environment of the cells is likely to play a role via cell–matrix interactions, mediated by several growth factors. Formation of new blood vessels is also significant and interdependent to bone formation, suggesting that enhancement of angiogenesis could be beneficial during the process of bone repair. Pleiotrophin (PTN), also called osteoblast-specific factor 1, is a heparin-binding angiogenic growth factor, with a well-defined and significant role in both physiological and pathological angiogenesis. In this review we summarise the existing evidence on the role of PTN in bone repair.

Pleiotrophin expression during odontogenesis

Pleiotrophin (PTN) is an extracellular matrix-associated growth factor and chemokine expressed in mesodermal and ectodermal cells. It plays an important role in osteoblast recruitment and differentiation. There is limited information currently available about PTN expression during odontoblast differentiation and tooth formation, and thus the authors aimed to establish the spatiotemporal expression pattern of PTN during mouse odontogenesis. Immortalized mouse dental pulp (MD10-D3, MD10-A11) and odontoblast-like (M06-G3) and ameloblast-like (EOE-3M) cell lines were grown and samples prepared for immunocytochemistry, Western blot, and conventional and quantitative PCR analysis. Effects of BMP2, BMP4, and BMP7 treatment on PTN expression in odontoblast-like M06-G3 cells were tested by quantitative PCR. Finally, immunohistochemistry of sectioned mice mandibles and maxillaries at developmental stages E16, E18, P1, P6, P10, and P28 was performed. The experiments showed that PTN, at both the mRNA and protein level, was expressed in all tested epithelial and mesenchymal dental cell lines and that the level of PTN mRNA was influenced differentially by the bone morphogenetic proteins. The authors observed initial expression of PTN in the inner enamel epithelium with prolonged expression in the ameloblasts and odontoblasts throughout their stages of maturation and strong expression in the terminally differentiated and enamel matrix-secreting ameloblasts and odontoblasts of the adult mouse incisors and molars.

16-kDa fragment of pleiotrophin acts on endothelial and breast tumor cells and inhibits tumor development

Pleiotrophin (PTN) is a 136-amino acid secreted heparin-binding protein that is considered as a rate-limiting growth and an angiogenic factor in the onset, invasion, and metastatic process of many tumors. Its mitogenic and tumorigenic activities are mediated by the COOH-terminal residues 111 to 136 of PTN, allowing it to bind to cell surface tyrosine kinase-linked receptors. We investigated a new strategy consisting in evaluating the antitumor effect of a truncated PTN, lacking the COOH-terminal 111 to 136 portion of the molecule (PTNDelta111-136), which may act as a dominant-negative effector for its mitogenic, angiogenic, and tumorigenic activities by heterodimerizing with the wild-type protein. In vitro studies showed that PTNDelta111-136 selectively inhibited a PTN-dependent MDA-MB-231 breast tumor and endothelial cell proliferation and that, in MDA-MB-231 cells expressing PTNDelta111-136, the vascular endothelial growth factor-A and hypoxia-inducible factor-1alpha mRNA levels were significantly decreased by 59% and 71%, respectively, compared with levels in wild-type cells. In vivo, intramuscular electrotransfer of a plasmid encoding a secretable form of PTNDelta111-136 was shown to inhibit MDA-MB-231 tumor growth by 81%. This antitumor effect was associated with the detection of the PTNDelta111-136 molecule in the muscle and tumor extracts, the suppression of neovascularization within the tumors, and a decline in the Ki-67 proliferative index. Because PTN is rarely found in normal tissue, our data show that targeted PTN may represent an attractive and new therapeutic approach to the fight against cancer.

The survival-promoting peptide Y-P30 enhances binding of pleiotrophin to syndecan-2 and -3 and supports its neuritogenic activity

Y-P30 is a polypeptide produced by peripheral blood mononuclear cells of the maternal immune system during pregnancy. The peptide passes the blood-placenta barrier and accumulates in neurons of the developing infant brain, where it enhances survival of thalamic neurons and displays neuritogenic activities. In this study, we identify pleiotrophin (PTN) and syndecan-2 and -3 as direct binding partners of Y-P30. PTN is known to promote neurite outgrowth of thalamic neurons due to its association with the proteoglycan syndecan-3. Via spontaneous oligomerization Y-P30 can capture large macromolecular complexes containing PTN and potentially syndecans. Accordingly, the neuritogenic activity of Y-P30 in thalamic primary cultures requires the presence of PTN in the media and binding to syndecans. Thus, we propose that the neurite outgrowth promoting actions of Y-P30 during brain development are essentially based on its association with the PTN/syndecan signaling complex. This identifies a new mechanism of communication between the nervous and the immune system that might directly affect the wiring of the brain during development.

Identification of candidate angiogenic inhibitors processed by matrix metalloproteinase 2 (MMP-2) in cell-based proteomic screens: disruption of vascular endothelial growth factor (VEGF)/heparin affin regulatory peptide (pleiotrophin) and VEGF/Connective tissue growth factor angiogenic inhibitory complexes by MMP-2 proteolysis

Matrix metalloproteinases (MMPs) exert both pro- and antiangiogenic functions by the release of cytokines or proteolytically generated angiogenic inhibitors from extracellular matrix and basement membrane remodeling. In the Mmp2-/- mouse neovascularization is greatly reduced, but the mechanistic aspects of this remain unclear. Using isotope-coded affinity tag labeling of proteins analyzed by multidimensional liquid chromatography and tandem mass spectrometry we explored proteome differences between Mmp2-/- cells and those rescued by MMP-2 transfection. Proteome signatures that are hallmarks of proteolysis revealed cleavage of many known MMP-2 substrates in the cellular context. Proteomic evidence of MMP-2 processing of novel substrates was found. Insulin-like growth factor binding protein 6, follistatin-like 1, and cystatin C protein cleavage by MMP-2 was biochemically confirmed, and the cleavage sites in heparin affin regulatory peptide (HARP; pleiotrophin) and connective tissue growth factor (CTGF) were sequenced by matrix-assisted laser desorption ionization-time of flight mass spectrometry. MMP-2 processing of HARP and CTGF released vascular endothelial growth factor (VEGF) from angiogenic inhibitory complexes. The cleaved HARP N-terminal domain increased HARP-induced cell proliferation, whereas the HARP C-terminal domain was antagonistic and decreased cell proliferation and migration. Hence the unmasking of cytokines, such as VEGF, by metalloproteinase processing of their binding proteins is a new mechanism in the control of cytokine activation and angiogenesis.

Inhibition of the mitogenic, angiogenic and tumorigenic activities of pleiotrophin by a synthetic peptide corresponding to its C-thrombospondin repeat-I domain

Pleiotrophin (PTN), is a heparin-dependent growth factor involved in angiogenesis and tumor growth. PTN contains a thrombospondin repeat-I (TSR-I) motif in its two beta-sheet domains that are involved in its binding to heparin and its neurite outgrowth activity. Based on the importance of the binding of PTN to heparin in its dimerization and biological activities, we have designed two synthetic peptides, P(13-39) and P(65-97) corresponding to a part of the N-terminal and C-terminal TSR-I motif of PTN, respectively. P(65-97) inhibited the mitogenic, tumorigenic and angiogenic activities of PTN, as well as the mitogenic and an angiogenic activity of fibroblast growth factor-2 (FGF-2). However, P(65-97) had no effect on the mitogenic activity of epidermal growth factor, which does not bind heparin. P(65-97) but not P(13-39) inhibited the binding of PTN and to a lesser extent of FGF-2 to heparin using an immunoassay and an optical biosensor assay and bound directly to heparin with a K(d) of 120 nM. These findings suggest that P(65-97), containing amino acids 65-97 of the TSR-I motif of the C-terminal domain of PTN, inhibits the activities of PTN and FGF-2 by virtue of its ability to bind heparin very effectively and so compete with the growth factors for their polysaccharide co-receptor.

The Binding of Chondroitin Sulfate to Pleiotrophin/Heparin-binding Growth-associated Molecule Is Regulated by Chain Length and Oversulfated Structures

Pleiotrophin is an 18-kDa heparin-binding growth factor, which uses chondroitin sulfate (CS) proteoglycan, PTPzeta as a receptor. It has been suggested that the D-type structure (GlcA(2S)beta1-3GalNAc(6S)) in CS contributes to the high affinity binding between PTPzeta and pleiotrophin. Here, we analyzed the interaction of shark cartilage CS-D with pleiotrophin using a surface plasmon resonance biosensor to reveal the importance of D-type structure. CS-D was partially digested with chondroitinase ABC, and fractionated using a Superdex 75pg column. The > or =18-mer CS fractions showed significant binding to pleiotrophin, and the longer fractions had stronger affinity for pleiotrophin than the shorter ones. The approximately 46-mer CS fraction bound to densely immobilized pleiotrophin with high affinity (K(D) = approximately 30 nM), and the binding reactions fitted the bivalent analyte model. However, when the density of the immobilized pleiotrophin was lowered, the strength of affinity remarkably decreased (K(D) = approximately 2.5 microM), and the reactions no longer fitted the model and were considered to be monovalent binding. The 20 approximately 24-mer fractions showed low affinity binding to densely immobilized pleiotrophin (K(D) = 3 approximately 20 microM), which seemed to be monovalent. When approximately 22-mer CS oligosaccharides were fractionated by strong anion exchange HPLC, each fraction differed in affinity for pleiotrophin (K(D) = 0.36 approximately >10 microM), and the affinity correlated with the amounts of D- and E- (GlcAbeta1-3GalNAc(4S,6S)) type oversulfated structures. These results suggest that the binding of pleiotrophin to CS is regulated by multivalency with CS approximately 20 mer as a unit and by the amounts of oversulfated structures.

Immunodetection of heparin-binding growth associated molecule (pleiotrophin) in striatal interneurons

Pleiotrophin (PTN), a developmentally-regulated trophic factor, is over-expressed in the striatum of parkinsonian rats. Because striatal PTN can provide trophic support to dopamine neurons, we identified the cellular types containing PTN in the striatum of adult rats. By means of fluorescent double-immunolabeling, we found PTN to co-localize with a neuronal nuclei marker but not with glial fibrillary acidic protein. The number, distribution, and morphology of the PTN-immunolabeled cells suggested that they were interneurons. Further double-immunolabeling studies ruled out PTN localization to calretinin- and parvalbumin-containing interneurons. Instead, approximately 40% of the PTN-immunolabeled neurons contained nitric oxide synthase or somatostatin and approximately 60% expressed the vesicular acetylcholine transporter, supporting that they were GABAergic nitric oxide synthase/somatostatin-containing and cholinergic interneurons. Further work is necessary to determine if PTN from striatal interneurons can provide trophic support to dopamine neurons.

Differential induction of glioblastoma migration and growth by two forms of pleiotrophin

Glioblastoma is the most common malignant brain tumor of adults and one of the most lethal cancers. The secreted growth factor pleiotrophin (PTN) promotes glioblastoma migration and proliferation, initiating its oncogenic activities through two cell surface receptors, the protein tyrosine phosphatase receptor zeta (PTPRZ1) and the anaplastic lymphoma kinase (ALK), respectively. Here, we report on the presence and purification of two naturally occurring forms of PTN (18 and 15 kDa) that differentially promote glioblastoma migration and proliferation. Using a panel of glioblastoma cell lines, including low passage patient-derived cultures, we demonstrate that PTN15 promotes glioblastoma proliferation in an ALK-dependent fashion, whereas immobilized PTN18 promotes haptotactic migration of glioblastoma cells in a PTPRZ1-dependent fashion. Mass spectrometric analysis indicated that PTN15 differs from PTN18 by processing of 12 C-terminal amino acids. To demonstrate clinical relevance, we show that PTN15, PTN18, and PTPRZ1 are significantly overexpressed in glioblastoma relative to normal brain at both mRNA and protein levels using microarray, Western blot, and tissue microarray analyses on human tumors. These results indicate that the PTN18-PTPRZ1 and the PTN15-ALK signaling pathways represent potentially important therapeutic targets for glioblastoma invasion and growth.

Identification of heparin affin regulatory peptide domains with potential role on angiogenesis

Heparin affin regulatory peptide (HARP) is a growth factor displaying high affinity for heparin. It is present in the extracellular matrix of many tissues, interacting with heparan sulfate and dermatan/chondroitin sulfate glycosaminoglycans. We have previously shown that HARP is implicated in the control of angiogenesis and its effects are mimicked, at least in part, by synthetic peptides that correspond to its N and C termini. In the present work, we show that HARP is cleaved by plasmin, leading to the production of five peptides that correspond to distinct domains of the molecule. Heparin, heparan sulfate and dermatan sulfate, at various HARP to glycosaminoglycan ratios, partially protect HARP from plasmin degradation. The molecules with higher affinity to HARP are the more protective, heparin being the most efficient. The peptides that are produced from cleavage of HARP by plasmin, affect in vivo and in vitro angiogenesis and modulate the angiogenic activity of vascular endothelial growth factor on human umbilical vein endothelial cells. Similar results were obtained in vitro with recombinant HARP peptides, identical to the peptides generated after treatment of HARP with plasmin. These results suggest that different regions of HARP may induce or inhibit angiogenesis.

Dominant negative effectors of heparin affin regulatory peptide (HARP) angiogenic and transforming activities

Heparin affin regulatory peptide (HARP) is an heparin-binding growth factor, highly expressed in several primary human tumors and considered as a rate-limiting angiogenic factor in tumor growth, invasion, and metastasis. Implication of this protein in carcinogenesis is linked to its mitogenic, angiogenic, and transforming activities. Recently, we have demonstrated that the C-terminal residues 111-136 of HARP are required for its mitogenic and transforming activities (Bernard-Pierrot, I., Delbe, J., Caruelle, D., Barritault, D., Courty, J., and Milhiet, P. E. (2001) J. Biol. Chem. 276, 12228-12234). In this paper, HARP deleted of its last 26 amino acids was shown to act as a dominant negative effector for its mitogenic, angiogenic, transforming, and tumor-formation activities by heterodimerizing with the wild type protein. Similarly, the synthetic corresponding peptide P111-136 displayed in vitro inhibition of wild type HARP activities, but in this case, the inhibition was mainly explained by the competition of the peptide with HARP for the binding to the extracellular domain of the high affinity ALK receptor.

Proteoglycans: pericellular and cell surface multireceptors that integrate external stimuli in the mammary gland

Proteoglycans consist of a core protein and an associated glycosaminoglycan (GAG) chain of heparan sulfate, chondroitin sulfate, dermatan sulfate or keratan sulfate, which are attached to a serine residue. The core proteins of cell surface proteoglycans may be transmembrane, e.g., syndecan, or GPI-anchored, e.g., glypican. Many different cell surface and matrix proteoglycan core proteins are expressed in the mammary gland and in mammary cells in culture. The level of expression of these core proteins, the structure of their GAG chains, and their degradation are regulated by many of the effectors that control the development and function of the mammary gland. Regulatory proteins of the mammary gland that bind GAG include many growth factors and morphogens (fibroblast growth factors, hepatocyte growth factor/scatter factor, members of the midkine family, wnts), matrix proteins (collagen, fibronectin, and laminin), enzymes (lipoprotein lipase) and microbial surface proteins. Structural diversity within GAG chains ensures that each protein-GAG interaction is as specific as necessary and a number of sequences of saccharides that recognize individual proteins have been elucidated. The GAG-protein interactions serve to regulate the signal output of growth factor receptor tyrosine kinase and hence cell fate as well as the storage and diffusion of extracellular protein effectors. In addition, GAGs clearly coordinate stromal and epithelial development, and they are active participants in mediating cell-cell and cell-matrix interactions. Since a single proteoglycan, even if it carries a single GAG chain, can bind multiple proteins, proteoglycans are also likely to act as multireceptors which promote the integration of cellular signals.

The lysine-rich C-terminal tail of heparin affin regulatory peptide is required for mitogenic and tumor formation activities

Heparin affin regulatory peptide (HARP) is a 18-kDa heparin-binding polypeptide that is highly expressed in developing tissues and in several primary human tumors. It seems to play a key role in cellular growth and differentiation. In vitro, HARP displays mitogenic, angiogenic, and neurite outgrowth activities. It is a secreted protein that is organized in two beta-sheet domains, each domain containing a cluster of basic residues. To assess determinants involved in the biological activities of HARP, C-terminally truncated proteins were produced in Chinese hamster ovary-K1 cells and tested for their mitogenic, tumor formation in nude mice and neurite outgrowth activities. Our data clearly indicate that the residues 111-136 of the lysine-rich C-terminal domain are involved in the mitogenic and tumor formation activities of HARP. Correlatively, no signal transduction was detected using the corresponding mutant, suggesting the absence of HARP binding to its high affinity receptor. However, this C-terminal domain of HARP is not involved in the neurite outgrowth activity. We also demonstrate that HARP signal peptide cleavage could led to two maturated forms that are both but differentially mitogenic.

HARP induces angiogenesis in vivo and in vitro: implication of N or C terminal peptides

HARP (heparin affin regulatory peptide) is a growth factor displaying high affinity for heparin. In the present work, we studied the ability of human recombinant HARP as well as its two terminal peptides (HARP residues 1-21 and residues 121-139) to promote angiogenesis. HARP stimulates endothelial cell tube formation on matrigel, collagen and fibrin gels, stimulates endothelial cell migration and induces angiogenesis in the in vivo chicken embryo chorioallantoic membrane assay. The two HARP peptides seem to be involved in most of the angiogenic effects of HARP. They both stimulate in vivo angiogenesis and in vitro endothelial cell migration and tube formation on matrigel. We conclude that HARP has an angiogenic activity when applied exogenously in several in vitro and in vivo models of angiogenesis and its NH(2) and COOH termini seem to play an important role.

Endothelial cell proliferation induced by HARP: implication of N or C terminal peptides

HARP (Heparin Affin Regulatory Peptide) is a 18-kDa secreted protein displaying high affinity for heparin. It has neurite outgrowth-promoting activity, while there are conflicting results regarding its mitogenic activity. In the present work, we studied the effect of human recombinant HARP expressed in bacterial cells as well as two peptides (HARP residues 1-21 and residues 121-139) on the proliferation of three endothelial cell types derived from human umbilical vein (HUVEC), rat adrenal medulla (RAME), and bovine brain capillaries (BBC) either added as a soluble form in the cell culture medium or coated onto the culture plate. HARP added in a soluble form in the culture medium had no effect on the proliferation of BBC, HUVEC, and RAME cells. However, when immobilized onto the cell culture plate, HARP had a concentration-dependent mitogenic effect on both BBC cells and HUVEC. The peptides presented as soluble factor induced a significant concentration-dependent mitogenic effect on BBC cells but only a small effect on HUVEC and RAME cells. When they were immobilized onto the cell culture plate, the mitogenic effect was much greater. The most responsive cells were BBC that expressed and secreted in the culture medium the higher amounts of HARP.