A major part of platelet-derived growth factor purified from human platelets is a heterodimer of one A and one B chain

Release kinetics of growth factors loaded into β-TCP ceramics in an in vitro model

Introduction

β-TCP ceramics are bone replacement materials that have recently been tested as a drug delivery system that can potentially be applied to endogenous substances like growth factors found in blood platelets to facilitate positive attributes.

Methods

In this work, we used flow chamber loading to load β-TCP dowels with blood suspensions of platelet-rich plasma (PRP), platelet-poor plasma (PPP), or buffy coat (BC) character. PRP and BC platelet counts were adjusted to the same level by dilution. Concentrations of TGF-β1, PDGF-AB, and IGF-1 from dowel-surrounding culture medium were subsequently determined using ELISA over 5 days. The influence of alginate was additionally tested to modify the release.

Results

Concentrations of TGF-β1 and PDGF-AB increased and conclusively showed a release from platelets in PRP and BC compared to PPP. The alginate coating reduced the PDGF-AB release but did not reduce TGF-β1 and instead even increased TGF-β1 in the BC samples. IGF-1 concentrations were highest in PPP, suggesting circulating levels rather than platelet release as the driving factor. Alginate samples tended to have lower IGF-1 concentrations, but the difference was not shown to be significant.

Discussion

The release of growth factors from different blood suspensions was successfully demonstrated for β-TCP as a drug delivery system with release patterns that correspond to PRP activation after Ca2+-triggered activation. The release pattern was partially modified by alginate coating.

PDGF-BB is involved in phosphate regulation via the phosphate transporters in human neuroblastoma SH-SY5Y cells

Inorganic phosphate (Pi) is the second most abundant inorganic ion in the body. Since abnormalities in Pi metabolism are risk factors for various diseases, serum Pi levels are strictly controlled. Type-III sodium-dependent Pi transporters, PiT-1 (encoded by SLC20A1) and PiT-2 (encoded by SLC20A2), are distributed throughout the tissues of the body, including the central nervous system, and are known to be responsible for extracellular to intracellular Pi transport. Platelet-derived growth factor (PDGF) is a major growth factor of mesenchymal cells. PDGF-BB, a homodimer of PDGF-B, regulates intracellular Pi by increasing PiT-1 expression in vascular smooth muscle cells. However, the effects of PDGF-BB on Pi transporters in neurons have yet to be reported. Here, we investigated the effect of PDGF-BB on Pi transporters in human neuroblastoma SH-SY5Y cells. PDGF-BB did not induce SLC20A1 mRNA expression, but it increased the intracellular uptake of Pi via PiT-1 in SH-SY5Y cells. Among the signaling pathways associated with PDGF-BB, AKT signaling was shown to be involved in the increase in Pi transport. In addition, the PDGF-BB-induced increase in Pi mediated neuroprotective effects in SLC20A2-suppressed cells, in an in vitro model of the pathological condition found in idiopathic basal ganglia calcification. Moreover, the increase in Pi uptake was found to occur through promotion of intracellular PiT-1 translocation to the plasma membrane. Overall, these results indicate that PDGF-BB exerts neuroprotective effects via Pi transport, and they demonstrate the potential utility of PDGF-BB against abnormal Pi metabolism in neurons.

PDGFRβ+/c-kit+ pulp cells are odontoblastic progenitors capable of producing dentin-like structure in vitro and in vivo

Background

Successful pulp regeneration depends on identification of pulp stem cells capable of differentiation under odontoblastic lineage and producing pulp-dentinal like structure. Recent studies demonstrate that platelet-derived growth factor (PDGF) plays an important role in damage repair and tissue regeneration. The aim of this study was to identify a subpopulation of dental pulp cells responsive to PDGF and with dentin regeneration potential.

Methods

Pulp tissues were isolated from 12 freshly extracted human impacted third molars. Pulp cells were sorted by their expression of PDGFRβ and stem cell marker genes via flow cytometry. For the selected cells, proliferation was analyzed by a colorimetric cell proliferation assay, differentiation was assessed by real time PCR detection the expression of odontoblast marker genes, and mineralization was evaluated by Alizarin Red S staining. GFP marked PDGFRβ⁺/c-kit⁺ pulp cells were transplanted into emptied root canals of nude rat lower left incisors. Pulp-dentinal regeneration was examined by immunohistochemistry.

Results

PDGFRβ⁺/c-kit⁺ pulp cells proliferated significantly faster than whole pulp cells. In mineralization media, PDGFRβ⁺/c-kit⁺ pulp cells were able to develop under odontoblastic linage as demonstrated by a progressively increased expression of DMP1, DSPP, and osteocalcin. BMP2 seemed to enhance whereas PDGF-BB seemed to inhibit odontoblastic differentiation and mineralization of PDGFRβ⁺/c-kit⁺ pulp cells. In vivo root canal transplantation study revealed globular dentin and pulp-like tissue formation by PDGFRβ⁺/c-kit⁺ cells.

Conclusions

PDGFRβ⁺/c-kit⁺ pulp cells appear to have pulp stem cell potential capable of producing dentinal like structure in vitro and in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12903-016-0307-8) contains supplementary material, which is available to authorized users.

Commercially available topical platelet-derived growth factor as a novel agent to accelerate burn-related wound healing

The authors investigated whether the application of platelet-derived growth factor (PDGF) to donor site wounds would speed healing in a porcine model. In a red duroc pig model, three wounds that were 3 inches × 3 inches were created with a dermatome (0.06-inch depth) on one side of two different animals. These wounds were digitally and laser Doppler (LDI) imaged and biopsied immediately pre- and postwound creation and every 2 days for 2 weeks. A set of identical wounds were subsequently created on the opposite side of the same animals and treated with topical PDGF (becaplermin gel 0.01%, 4 g/wound) immediately on wounding. PDGF-treated wounds were imaged and biopsied as above. Digital images of wounds were assessed for epithelialization by clinicians using an ordinal scale. Perfusion units (PU) were evaluated by LDI. Wound healing was evaluated by hematoxylin and eosin histological visualization of an epithelium and intact basement membrane. First evidence of partial epithelialization was seen in control and PDGF-treated wounds within 7.7 ± 1.4 and 6.4 ± 1.1 days postwounding, respectively (P=.03). Completely epithelialized biopsies were seen in control and PDGF-treated wounds at 11.7 ± 2.6 and 9.6 ± 1.5 days, respectively (P=.02). Clinician evaluation of digital images showed that on day 9, control wounds were, on average, 48.3 ± 18.5% epithelialized vs 57.2 ± 20.2% epithelialized for PDGF-treated wounds. At day 16, control wounds showed an average of 72.9 ± 14.6% epithelialization and PDGF-treated wounds showed an average of 90 ± 11.8%epithelialization. Overall, PDGF-treated wounds had statistically significantly higher scores across all timepoints (P=.02). Average perfusion units as measured by LDI were similar for control and PDGF-treated wounds at time of excision (225 ± 81and 257 ± 100, respectively). On day 2 postwounding, average PU for control wounds were 803 and were 764 for PDGF-treated wounds. Control wounds maintained higher PU values compared with PDGF-treated wounds at all time points and returned to excision PU values by day 12.2 ± 1.1 postwounding. PDGF-treated wounds reached the same values by day 9.7 ± 2.3 (P=.03). The authors conclude that topical PDGF speeds time to epithelialization of partial-thickness wounds in a porcine model as evidenced by histology, clinical appearance, and time to return to prewounding vascularity.

The roles of PDGF in development and during neurogenesis in the normal and diseased nervous system

The four platelet-derived growth factor (PDGF) ligands and PDGF receptors (PDGFRs), α and β (PDGFRA, PDGFRB), are essential proteins that are expressed during embryonic and mature nervous systems, i.e., in neural progenitors, neurons, astrocytes, oligodendrocytes, and vascular cells. PDGF exerts essential roles from the gastrulation period to adult neuronal maintenance by contributing to the regulation of development of preplacodal progenitors, placodal ectoderm, and neural crest cells to adult neural progenitors, in coordinating with other factors. In adulthood, PDGF plays critical roles for maintenance of many specific cell types in the nervous system together with vascular cells through controlling the blood brain barrier homeostasis. At injury or various stresses, PDGF modulates neuronal excitability through adjusting various ion channels, and affecting synaptic plasticity and function. Furthermore, PDGF stimulates survival signals, majorly PI3-K/Akt pathway but also other ways, rescuing cells from apoptosis. Studies imply an involvement of PDGF in dendrite spine morphology, being critical for memory in the developing brain. Recent studies suggest association of PDGF genes with neuropsychiatric disorders. In this review, we will describe the roles of PDGF in the nervous system, from the discovery to recent findings, in order to understand the broad spectrum of PDGF in the nervous system. Recent development of pharmacological and replacement therapies targeting the PDGF system is discussed.

Platelet-derived growth factors and their receptors: structural and functional perspectives

The four types of platelet-derived growth factors (PDGFs) and the two types of PDGF receptors (PDGFRs, which belong to class III receptor tyrosine kinases) have important functions in the development of connective tissue cells. Recent structural studies have revealed novel mechanisms of PDGFs in propeptide loading and receptor recognition/activation. The detailed structural understanding of PDGF-PDGFR signaling has provided a template that can aid therapeutic intervention to counteract the aberrant signaling of this normally silent pathway, especially in proliferative diseases such as cancer. This review summarizes the advances in the PDGF system with a focus on relating the structural and functional understandings, and discusses the basic aspects of PDGFs and PDGFRs, the mechanisms of activation, and the insights into the therapeutic antagonism of PDGFRs. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Regenerative tendon and ligament healing: opportunities with recombinant human platelet-derived growth factor BB-homodimer

Intrinsic tendon healing in response to injury is a reparative process that often results in formation of scar tissue with functional and mechanical properties inferior to those of the native tendon. Development of therapies that can promote regenerative, rather than reparative, healing hold the promise of improving patient recovery from tendon and ligament injuries by producing tissue that is morphologically and functionally equivalent to the native tissue. One therapeutic approach that has been a frequent topic of investigation in the preclinical literature is the use of recombinant human platelet-derived growth factor-BB (rhPDGF-BB) to augment tendon and ligament repair. The chemotactic, mitogenic, and pro-angiogenic properties of rhPDGF-BB have been shown to result in recruitment and proliferation of tenogenic cells and a commensurate boost in extracellular matrix deposition and organization, improving the morphological and biomechanical properties of healing tendons and ligaments. The outcomes of the preclinical studies reviewed here strongly suggest that rhPDGF-BB will provide a new therapeutic opportunity to improve the treatment of injured tendons and ligaments.

Platelet lysate from whole blood-derived pooled platelet concentrates and apheresis-derived platelet concentrates for the isolation and expansion of human bone marrow mesenchymal stromal cells: production process, content and identification of active components

Background aims

The clinical use of human mesenchymal stromal cells (MSC) requires ex vivo expansion in media containing supplements such as fetal bovine serum or, alternatively, human platelet lysate (PL).

Methods

Platelet concentrates were frozen, quarantine stored, thawed and sterile filtered to obtain PL. PL content and its effect on fibroblast-colony-forming unit (CFU-F) formation, MSC proliferation and large-scale expansion were studied.

Results

PL contained high levels of basic fibroblast growth factor (bFGF), soluble CD40L (sCD40L), vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), platelet-derived growth factor AA (PDGF-AA), platelet-derived growth factor AB/BB (PDGF-AB/BB), chemokine (C-C) ligand 5 (CCL5; RANTES) transforming growth factor-β1 (TGF-β1) and chemokine (C-X-C) ligand 1/2/3 (GRO), with low batch-to-batch variability, and most were stable for up to 14 days. Inhibition of PDGF-BB and bFGF decreased MSC proliferation by about 20% and 50%, respectively. The strongest inhibition (about 75%) was observed with a combination of anti-bFGF + anti-PDGF-BB and anti-bFGF + anti-TGF-β1 + anti-PDGF-BB. Interestingly, various combinations of recombinant PDGF-BB, bFGF and TGF-β1 were not sufficient to promote cell proliferation. PL from whole blood-derived pooled platelet concentrates and apheresis platelet concentrates did not differ significantly in their growth-promoting activity on MSC.

Conclusions

PL enhances MSC proliferation and can be regarded as a safe tool for MSC expansion for clinical purposes. \in particular, PDGF-BB and bFGF are essential components for the growth-promoting effect of PL, but are not sufficient for MSC proliferation.

Significance of the platelet-derived growth factor in periodontal tissue regeneration

AIM

The aim was to review the significance of the platelet derived growth factor (PGDF) in periodontal tissue regeneration.

METHODS AND RESULTS

Databases were searched using the following terms in different combinations: "growth factors", "guided bone regeneration", "guided tissue regeneration", "periodontal", "platelet rich plasma" and "platelet derived growth factor". Titles and abstracts of articles obtained using the above-described criteria were then screened by the authors and checked for agreement. The next step was to hand-search the reference lists of original and review studies that were found to be relevant in the previous step. PDGF has a stimulatory effect on the DNA replication and chemotaxis of osteoblasts, fibroblasts, leukocytes, monocytes, neutrophils periodontal and alveolar bone cells. Proliferation of mesenchymal stem cells is also promoted by supplement treatment with PDGF. PDGF in combination with other growth factors enhances periodontal tissue repair.

CONCLUSIONS

The PDGF plays a significant role in periodontal bone and tissue regeneration.

A comparison of bone generation capability in rabbits using tooth ash and plaster of Paris with platelet-rich plasma or fibrin sealant

OBJECTIVES

Increased attention has been focused on determining the most efficacious materials for generalized bone grafts. This article presents the results of a histomorphometric analysis of bone healing in the calvaria of rabbits. The study compared the use of a tooth ash and plaster of Paris mixture alone, in association with platelet-rich plasma (PRP), and in association with fibrin sealant.

STUDY DESIGN

Twelve rabbits were divided into control (group 1) and experimental groups (groups 2, 3, and 4). Group 1 was maintained as an unfilled control, and tooth ash and plaster of Paris were used in group 2, tooth ash and plaster of Paris with PRP were used in group 3, and tooth ash and plaster of Paris with fibrin sealant (Tissucol Duo Quick) were used in group 4. One-half of the animals were killed after 4 weeks, and the rest were killed after 8 weeks. Bone samples were taken from the defect areas, and newly formed bone was analyzed histomorphometrically.

RESULTS

The rate of new bone formation in groups 2, 3, and 4 was significantly higher than the rate in the control group. The rate of new bone formation in groups 3 and 4 was higher than the rate in group 2, but the difference was not statistically significant.

CONCLUSION

The concomitant use of PRP or fibrin sealant with tooth ash and plaster of Paris graft materials may have a positive effect on bone healing.

DNA functionalized gold nanoparticles for bioanalysis

Gold nanoparticles (Au NPs) have become one of the most interesting sensing materials because of their unique size- and shape-dependent optical properties, high extinction coefficients, and super-quenching capability. Au NPs that are bioconjugated with DNA (DNA-Au NPs) have been demonstrated for selective and sensitive detection of analytes such as mercury(ii) ions, platelet-derived growth factor (PDGF), and adenosine triphosphate (ATP). This review focuses on approaches using DNA-Au NPs for colorimetric, fluorescent, and scattering detection of biopolymers and small solutes. We highlight the important roles that the size and concentration of Au NPs, the length and sequence of DNA, the nature of the capping agents, and the ionic strength and pH of solution play in determining the specificity and sensitivity of the nanosensors for the analytes. The advantages and disadvantages of different detection methods for sensing of interesting analytes using DNA-Au NPs will be discussed.

Sustained platelet-derived growth factor receptor alpha signaling in osteoblasts results in craniosynostosis by overactivating the phospholipase C-gamma pathway

The development and growth of the skull is controlled by cranial sutures, which serve as growth centers for osteogenesis by providing a pool of osteoprogenitors. These osteoprogenitors undergo intramembranous ossification by direct differentiation into osteoblasts, which synthesize the components of the extracellular bone matrix. A dysregulation of osteoblast differentiation can lead to premature fusion of sutures, resulting in an abnormal skull shape, a disease called craniosynostosis. Although several genes could be linked to craniosynostosis, the mechanisms regulating cranial suture development remain largely elusive. We have established transgenic mice conditionally expressing an autoactivated platelet-derived growth factor receptor alpha (PDGFRalpha) in neural crest cells (NCCs) and their derivatives. In these mice, premature fusion of NCC-derived sutures occurred at early postnatal stages. In vivo and in vitro experiments demonstrated enhanced proliferation of osteoprogenitors and accelerated ossification of osteoblasts. Furthermore, in osteoblasts expressing the autoactivated receptor, we detected an upregulation of the phospholipase C-gamma (PLC-gamma) pathway. Treatment of differentiating osteoblasts with a PLC-gamma-specific inhibitor prevented the mineralization of synthesized bone matrix. Thus, we show for the first time that PDGFRalpha signaling stimulates osteogenesis of NCC-derived osteoblasts by activating the PLC-gamma pathway, suggesting an involvement of this pathway in the etiology of human craniosynostosis.

Recombinant human platelet-derived growth factor: biology and clinical applications

The abilities of bone to remodel, fractures to repair, and bone grafts to incorporate are all fundamental reflections of the bone remodeling cycle. This process is characterized by the recruitment and differentiation of osteoblastic and osteoclastic cell populations, whose cellular activities are coordinated and regulated by an elaborate system of growth factors and cytokines. One of the crucial biological factors responsible for reparative osseous activity is platelet-derived growth factor (PDGF). The potent stimulatory effects of PDGF as a chemoattractant and mitogen for mesenchymal cells (including osteogenic cells), along with its ability to promote angiogenesis, have been demonstrated in a variety of preclinical models predicting maxillofacial, spine and appendicular skeletal, and soft-tissue applications. The biological profile of PDGF, including its ability to recruit osteoprogenitor cells, makes it particularly suited to address the skeletal defects that are seen with comorbid conditions such as osteoporosis, diabetes, and the effects of smoking. The clinical success and safety that have been demonstrated with use of recombinant human PDGF (rhPDGF) in the repair of periodontal defects have led to U.S. Food and Drug Administration (FDA) approval of rhPDGF for this indication. Ongoing pilot and pivotal trials in the United States and internationally will continue to clarify the promising role of PDGF in the treatment of challenging skeletal disorders.

Structural and functional aspects of platelet-derived growth factor and its receptors

Platelet-derived growth factor (PDGF) is a dimeric molecule that exists as homodimers or heterodimers of related polypeptide chains (A and B). Two types of PDGF receptor have been identified. The PDGF alpha-receptor binds all three isoforms with high affinity whereas the beta-receptor binds only PDGF-BB with high affinity, PDGF-AB with low affinity and does not appear to bind PDGF-AA. The alpha- and beta-receptors are structurally related, each having an intracellular protein tyrosine kinase domain. Ligand-induced functional activation of the receptors appears to involve receptor dimerization. Binding of PDGF to its receptor is followed by internalization and degradation of the ligand-receptor complex. Experiments with mutant receptors have shown that ligand-induced internalization is not absolutely dependent on the kinase activity of the beta-receptor. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral version of the PDGF B chain gene and SSV-transformation is mediated by an autocrine PDGF-like growth factor. Formal evidence that the expression of the PDGF beta-receptor is sufficient to confer susceptibility to SSV-transformation has been obtained using porcine endothelial cells expressing a recombinant human beta-receptor. PDGF is a chemotactic agent for several cell types. Recent experiments have shown that the PDGF beta-receptor mediates a chemotactic response and that this effect requires an intact protein tyrosine kinase activity.

Aptamer-modified gold nanoparticles for colorimetric determination of platelet-derived growth factors and their receptors

We have developed a highly specific sensing system for platelet-derived growth factors (PDGFs) and platelet-derived growth factor receptors (PDGFR) that uses gold nanoparticles (GNPs). We synthesized GNPs modified with an aptamer (Apt-GNPs) that is specific to PDGFs and used them to detect PDGFs by monitoring the changes in the color and extinction of the Apt-GNPs that occur as a result of aggregation. The color of the Apt-GNPs changes from red to purple at low concentrations (<400 nM), but changes only slightly at higher concentrations (>400 nM). We found that the sensitivity of the Apt-GNPs for the three PDGFs is highly salt-dependent, with an optimum condition of 200 mM NaCl. We obtained biphasic curves when plotting of the ratios of the extinction coefficients of the Apt-GNPs at 650 and 530 nm against the concentrations of PDGF-AA at various concentrations of Apt-GNPs. The linear ranges of the increases and decreases in this extinction ratio are 2.5-10 and 10-20 nM, respectively, for 0.42 nM Apt-GNPs and 25-75 and 75-200 nM, respectively, for 8.4 nM Apt-GNPs. When using 8.4 nM Apt-GNPs, the corresponding linear ranges of the increases and decreases in this extinction ratio are 15-100 and 100-400 nM, respectively, for PDGF-AB and 35-150 and 150-400 nM, respectively, for PDGF-BB. In addition, we have developed a homogeneous assay to detect the PDGF receptor-beta (PDGFR-beta) at concentrations as low as 3.2 nM, on the basis of the competition between the Apt-GNPs and PDGFR-beta for PDGF-BB. The results we present in this paper imply that there are practical applications of Apt-GNPs in protein analysis and cancer diagnosis.

Regulation of the stepwise proteolytic cleavage and secretion of PDGF-B by the proprotein convertases

Platelet-derived growth factor-B (PDGF-B) is important for normal tissue growth and maintenance and its overexpression has been linked to several diseases, including cancer, fibrotic disease and atherosclerosis. Here, we show that synthesized as a precursor, proPDGF-B is converted to a mature form by proteolytic cleavage at two sites and its N-terminal cleavage is a prerequisite for processing at its C-terminus. The first cleavage occurs at residues RGRR81/, and the second cleavage close to residues ARPVT190, just before the C-terminal amino-acid sequence crucial for PDGF-B retention to cell surface. Cotransfection of a Furin-deficient cell line LoVo-C5 with proPDGF-B and different PC members revealed that Furin, PACE4, PC5, and PC7 are candidate proPDGF-B convertases. This finding is consistent with the in vitro digestions of a synthetic peptide mimicking the cleavage site of proPDGF-B. The processing of proPDGF-B is blocked by site-directed mutagenesis of the RGRR81/ sequence and by various PC inhibitors. Mutation of the PDGF-A and/or PDGF-B convertase sites, revealed that processing of both A and B chains is required for the formation of mature PDGF-B dimers and that the processing of the B chain controls the level of secreted and matrix-bound PDGF-BB forms. Our findings emphasize the importance of the convertase-directed processing of proPDGF-B at the RGRR81/ sequence for PDGF-B maturation and secretion.

To go or not to go: Migration of human mesenchymal progenitor cells stimulated by isoforms of PDGF

The recruitment of mesenchymal progenitor cells (MPCs) and their subsequent differentiation to osteoblasts is mandatory for bone development, remodeling, and repair. To study the possible involvement of platelet-derived growth factor (PDGF) isoforms, primary human MPCs and osteogenic differentiated progenitor cells (dOB) were examined for chemotaxic response to homodimeric human platelet-derived growth factor AA, -BB, and heterodimeric PDGF-AB. The role of PDGF receptors was addressed by preincubation with PDGF receptor alpha and beta chain specific antibodies. Migration of MPCs, dOB, and primary osteoblasts (OB) was stimulated by the addition of rhPDGF-AA, rhPDGF-BB, and rhPDGF-AB. The effect was highest in MPCs and for rhPDGF-BB, and declining with osteogenic differentiation. Preincubation with the receptor alpha specific antibody decreased the CI to borderline values while pretreatment with the receptor beta specific antibody led to a complete loss of chemotactic response to PDGF isoforms. In control experiments, basal migration values and rhBMP-2 as well as rxBMP-4 induced chemotaxis of MPC were not influenced by the addition of receptor alpha or beta antibodies. Interestingly, without preincubation the parallel exposure of MPC to rhTGF-beta1 instantaneously leads to a selective loss of migratory stimulation by rhPDGF-AA. The chemotactic effect of PDGF isoforms for primary human MPCs and the influence of osteogenic differentiation suggest a functional role for recruitment of MPCs during bone development and remodeling. Moreover, these observations may be useful for novel approaches towards guided tissue regeneration or tissue engineering of bone.

alphaB-crystallin is phosphorylated during myocardial infarction: involvement of platelet-derived growth factor-BB

alphaB-crystallin is the most abundant low-molecular-weight heat shock protein in heart and recent studies have demonstrated that it plays a cardioprotective role during myocardial infarction both in vivo and in vitro. On the other hand, platelet-derived growth factor (PDGF), a potent serum mitogen, has been reported to improve cardiac function after myocardial infarction. In the present study, using a mouse myocardial infarction model, we investigated whether alphaB-crystallin is phosphorylated during myocardial infarction and the implication of PDGF-BB. Phosphorylation of alphaB-crystallin at Ser-59 was time dependently induced and plasma PDGF-BB levels were concomitantly increased. Moreover, PDGF-BB-stimulated phosphorylation of alphaB-crystallin was suppressed by SB203580, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, in primary cultured cardiac myocytes. Our results indicate that PDGF-BB induces phosphorylation of alphaB-crystallin via p38 MAP kinase during myocardial infarction.

Kinetics control preferential heterodimer formation of platelet-derived growth factor from unfolded A- and B-chains

The folding and assembly of platelet-derived growth factor (PDGF), a potent mitogen involved in wound-healing processes and member of the cystine knot growth factor family, was studied. The kinetics of the formation of disulfide-bonded dimers were investigated under redox reshuffling conditions starting either from unfolded and reduced PDGF-A- or B-chains or an equimolar mixture of both chains. It is shown that in all cases the formation of disulfide-bonded dimers is a very slow process occurring in the time scale of hours with a first-order rate-determining step. The formation of disulfide-bonded PDGF-AA or PDGF-BB homodimers displayed identical kinetics, indicating that both monomeric forms as well as the dimerized homodimer have similar folding and assembly pathways. In contrast, the formation of the heterodimer occurred three times more rapidly compared with the formation of the homodimers. As both monomeric forms revealed similar renaturation kinetics, it can be concluded that the first-order rate-determining folding step does not occur during monomer folding but must be attributed to conformational rearrangements of the dimerized, not yet disulfide-bonded protein. These structural rearrangements allow a more rapid formation of intermolecular disulfide bonds between the two different monomers of a heterodimer compared with the formation of the disulfide bonds between two identical monomers. The preferential formation of disulfide-bonded heterodimers from an equimolar mixture of unfolded A- and B-chains is thus a kinetically controlled process. Moreover, similar activation enthalpies for the formation of all different isoforms suggest that faster heterodimerization is controlled by entropic factors.

Close relationship between the platelet activation marker CD62 and the granular release of platelet-derived growth factor

The expression of CD62 on the surface of platelets is considered to be an indicator of platelet degranulation and secretion. We characterized the relationship between CD62 expression and platelet-derived growth factor (PDGF)(AB) and PDGF(BB) secretion in response to thrombin-receptor activating peptide (TRAP). The principal findings were 1) expression of CD62 as a constituent of platelet alpha-granule membrane and secretion of PDGF, an important ingredient of alpha-granules, can be stimulated by TRAP-induced activation in a dose-dependent fashion; 2) the activation marker and secretion product are closely correlated with each other; and 3) changes in the CD62 expression induced by a drug, namely clopidogrel, or by a disease, namely diabetes, are paralleled by changes in PDGF secretion. Although CD62 is perceived as an activation marker of platelets indicating enhanced aggregability and secretion of alpha-granular content, the proof that the CD62 status and its modifications reflect directly the actual secretion of the most important platelet mitogen, PDGF, has so far not been given. This ex vivo-in vitro study shows that at least for the activation pathway provided by the PAR-1 receptor for which TRAP is the selective agonist, CD62 expression on platelets could be a surrogate for their secretory activity.

Overexpression of PDGF-A in the lung epithelium of transgenic mice produces a lethal phenotype associated with hyperplasia of mesenchymal cells

Transgenic mice expressing platelet-derived growth factor A chain (PDGF-A) in the distal lung epithelium from the surfactant protein C (SPC) promoter were generated to investigate the role of this growth factor in lung development. Expression of the SPC-PDGFA transgene resulted in an enlarged, nonfunctional lung and perinatal lethality caused by failure to initiate ventilation. Histologic analysis of embryonic day (E) 16.5 lungs revealed increased mesenchymal cells and acinar buds and decreased bronchioles and dilated airspaces in SPC-PDGFA transgenic mice. At E18.5, nontransgenic lungs exhibited lung morphology typical of the saccular stage of lung development, including dilated airspaces, thin respiratory epithelium and mesenchyme, and elastin fiber deposition in primary septa. In contrast, E18.5 transgenic lungs retained many features of the canalicular stage of lung development, including undilated airspaces, cuboidal respiratory epithelium, thickened mesenchyme, and lack of parenchymal elastin deposition. These results indicate that PDGF-A is a potent growth factor for mesenchymal cells in the developing lung and that the downregulation of PDGF-A expression that normally occurs in the lung during late gestation is required for transition from the canalicular to the saccular stage of lung development.

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+ granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Retinoic acid stimulates immature lung fibroblast growth via a PDGF-mediated autocrine mechanism

All trans-retinoic acid (RA) enhances alveolarization in neonates and reinitiates alveolarization in emphysematous adult rat lungs, suggesting that RA may stimulate cell proliferation by upregulating growth factor ligand and/or receptor expression either indirectly or directly by acting on RA-responsive genes encoding growth factors. We report that RA and 1,25-dihydroxyvitamin D(3) (Vit D), alone and in combination, significantly increase [(3)H]thymidine incorporation in cultured fetal and postnatal rat lung fibroblasts (P < 0.05). The greatest increase (11-fold) was seen in 4-day cells treated with the two agents in combination (P < 0.0001). [(3)H]thymidine incorporation was age dependent. The greatest response to RA occurred in 4-day fibroblasts (P < 0.01), whereas the response to Vit D was greatest in embryonic day 20 fibroblasts (P < 0.001). Neutralizing antibody to platelet-derived growth factor (PDGF)-AB decreased [(3)H]thymidine incorporation in response to RA alone or in combination with Vit D, indicating a role for PDGF. Expression of mRNAs for PDGF-A and PDGF receptor (PDGFR)-alpha and -beta was upregulated at the transcriptional level in an age- and treatment-dependent manner. Thus exogenous RA may influence alveolarization by stimulating fibroblast proliferation through a PDGF-mediated autocrine mechanism, which is enhanced when RA and Vit D are administered in combination.

Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response

The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll-off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet-derived growth factor (PDGF)-BB can modulate the open channel probability of NMDA receptors by activating PDGF-beta receptors, its contribution to hypoxic ventilatory roll-off was examined. Administration of PDGF-BB, but not PDGF-AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll-off occurred in mice heterozygous for a mutation in the PDGF-beta receptor. Administration of a PDGF-beta receptor antagonist to wild-type littermates elicited similar declines in hypoxic ventilatory roll-off. The relative abundance of PDGF-beta receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF-beta receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll-off formation. Increased PDGF-B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF-B chain immunoreactivity colocalized with approximately 40% of nucleus of the solitary tract neurons, demonstrating hypoxia-induced c-Fos enhancements. Thus, PDGF-BB release and PDGF-beta receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll-off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

Renaturation of heterodimeric platelet-derived growth factor from inclusion bodies of recombinant Escherichia coli using size-exclusion chromatography

A procedure for renaturation of heterodimeric platelet-derived growth factor (PDGF-AB) from inclusion bodies of recombinant Escherichia coli using size-exclusion chromatography is described. Either prepurified or crude PDGF-AB inclusion bodies solubilized with guanidinium hydrochloride were subjected to buffer exchange from denaturing to renaturing conditions during chromatography. Renaturation of PDGF-AB involves folding of the solubilized and unfolded molecules into dimerization competent monomers during size-exclusion chromatography and subsequent dimerization of folded monomers into the biologically active heterodimeric growth factor. Optimized conditions result in an overall yield of 75% active PDGF-AB with respect to size-exclusion chromatography and subsequent dimerization. The described approach allows renaturation at high protein concentrations and circumvents aggregation which is observed when refolding is carried out by dilution.

Bone marrow effects of anagrelide therapy in patients with myelofibrosis with myeloid metaplasia

In a prospective study investigating the therapeutic role of anagrelide in myelofibrosis with myeloid metaplasia, 20 patients received anagrelide in daily oral doses of 0.5-3 mg. 17 patients were evaluable and received anagrelide for a median of 2 years (range 0.5-4 years). No patient had a clinically appreciable benefit. Bone marrow (BM) examinations at baseline and after 6 and 12 months of treatment were available for 17, 17 and 12 patients, respectively. In all evaluable cases, BM megakaryocyte number increased after 6 months of anagrelide treatment. Also, Ulex europaeus agglutinin-1 staining of megakaryocytes revealed a left-shifted maturation pattern in most patients with a platelet response to anagrelide. However, megakaryocyte staining intensity for transforming (TGF-beta) and platelet-derived (PDGF) growth factors was not affected consistently by treatment. No patient had a >/=2 grade change in either BM fibrosis or osteosclerosis. These in-vivo data support our previous in-vitro observations that anagrelide interferes with megakaryocyte maturation rather than proliferation. Lack of a positive treatment effect is consistent with the finding that anagrelide did not significantly alter megakaryocyte expression of TGF-beta and PDGF.

Effects of human platelet-derived growth factor-AB on sarcoma growth in vitro and in vivo

Platelet-derived growth factor (PDGF) has been proposed to play an important role in the growth of tumors. In order to study the effects of PDGF-AB on tumor growth in vivo, sarcoma-bearing mice were treated with PDGF-AB. The tumors, a malignant fibrous histiocytoma and an osteosarcoma, had functional PDGF receptors in vitro, as demonstrated by stimulation of PDGF-AB using a [3H]thymidine incorporation assay. Immunohistochemistry also revealed that both sarcoma xenografts expressed PDGF receptors. The tumor-bearing mice were given human PDGF-AB for 14 days, either continuously by an intraperitoneally placed mini-osmotic pump, or by daily injections. No effects on tumor growth in vivo were observed, as measured by tumor volume, autoradiography or cell cycle distribution. The histological appearance and ploidy of the tumors remained unaltered. The results indicate that, although the tumor cells are stimulated by PDGF-AB in vitro, the in vivo milieu or tumor growth pattern may render the tumors less susceptible to exogenously administered PDGF-AB in vivo.

Emphysematous lesions, inflammation, and fibrosis in the lungs of transgenic mice overexpressing platelet-derived growth factor

Because of its expression pattern and its potent effects on mesenchymal cells, platelet-derived growth factor (PDGF) has been implicated as an important factor in epithelial-mesenchymal cell interactions during normal lung development and in the pathogenesis of fibrotic lung disease. To further explore the role of PDGF in these processes, we have developed transgenic mice that express the PDGF-B gene from the lung-specific surfactant protein C (SPC) promoter. Adult SPC-PDGFB transgenic mice exhibited lung pathology characterized by enlarged airspaces, inflammation, and fibrosis. Emphysematous changes frequently occurred throughout the lung, but inflammation and fibrotic lesions were usually confined to focal areas. The severity of this phenotype varied significantly among individual mice within the same SPC-PDGFB transgenic lineage. A pathology similar to that observed in adult mice was noted in lungs from transgenic mice as young as 1 week of age. Neonatal transgenic mice exhibited enlarged saccules and thickened primary septa. Results of these studies indicated that overexpression of PDGF-B induced distinct abnormalities in the developing and adult lung and led to a complex phenotype that encompassed aspects of both emphysema and fibrotic lung disease.

The role of alpha and beta platelet-derived growth factor receptor in the vascular response to injury in nonhuman primates

Restenosis remains a significant clinical problem associated with mechanical interventional procedures for arterial revascularization or repair, including coronary angioplasty and stenting. Studies with rodents have established that platelet-derived growth factor (PDGF), a potent chemotactic and mitogenic agent for vascular smooth muscle cells, is a key mediator of lesion formation after vascular injury. To further explore this hypothesis in a more clinically relevant model, neutralizing monoclonal antibodies (mAbs) were used to examine the effect of selective inhibition of alpha or beta PDGF receptor (PDGFR) on neointima formation in nonhuman primates. Carotid arteries were injured by surgical endarterectomy and femoral arteries by balloon catheter dilatation. Immunostaining revealed that both injuries induced cell proliferation and the upregulation of beta PDGFR but not alpha PDGFR. By 7 days after injury, beta PDGFR staining was limited to the luminal region of the media, the small areas of neointima, and the adventitia. Nearly all bromodeoxyuridine-positive cells were found in these regions as well. After 30 days, a concentric neointima that stained strongly for beta PDGFR had formed in the carotid and femoral arteries. Treatment of baboons with anti-beta PDGFR mAb 2A1E2 for 6 days after injury reduced the carotid artery and femoral artery lesion sizes by 37% (P<0.05) and 48% (P<0.005), respectively, when measured at 30 days. Under the same conditions, treatment with anti-alpha PDGFR mAb 2H7C5 had no effect. These findings suggest that PDGF mediates neointima formation through the beta PDGFR, and that antagonism of this pathway may be a promising therapeutic strategy for reducing clinical restenosis.

Processing of PDGF gene products determines interactions with glycosaminoglycans

The platelet derived growth factor (PDGF), a mitogen for mesenchymal cells, may be bound to and inhibited by heparin and other glycosaminoglycans. PDGF is a homo- or heterodimer of A- and B-chains. They occur as short (A109 and B110) and long (A125 and B160) isoforms. The latter contain basic carboxyl-terminal extensions. Dimeric A125 binds to heparin through its basic extension in a two-step reaction. The mechanism involves a conformational change and is consistent with a Monod-Wyman-Changeux allosteric model. Previous indirect experiments suggested that three critical amino acids (basic R111, K116 and polar T125) might be involved. Here, direct binding experiments using dimeric full-length mutants in surface plasmon resonanse analysis showed that all three critical amino acids in an R(X)4K(X)8T-motif contributed in a concerted manner to the high affinity binding. Mutations of these amino acids to alanine resulted in large thermodynamic changes, loss of the allosteric mechanism and order(s) of magnitude lower binding affinity. The binding mechanism and affinity of long dimeric rB were similar to the mutants. Short dimeric rA109 and rB110 showed 100 times lower binding affinity than rA125. Consequently, interactions with glycosaminoglycans in tissues varies between PDGF isoforms and may influence their local accumulation and activity.

Biological mediators for periodontal regeneration

A review of the literature on the use of growth-regulatory molecules in the oral cavity permits a model in which to consider approaches to oral tissue engineering. These concepts apply to periodontal regeneration and to regeneration of alveolar bone. In either case, the formation of tissues is complex but proceeds in a deliberate and orderly sequence. In these sequence of events resulting in either bone or cementum formation, periodontal ligament and bone can be stimulated at various points. Different signals can apparently be used to stimulate tissue formation including mitogenic signals and differentiation factors. Additionally, both hard and soft tissue stimulatory molecules appear to be permissive. Classic receptor-mediated peptides or extracellular matrix molecules for soft and hard tissues appear to allow stimulation of tissue formation cascades. Importantly, it also appears that the stimulatory event is transitory (that is, short-lived) and leads itself to a sequence of cellular events. These cellular events in turn stimulate a number of subsequent events (such as chemotaxis, proliferation, differentiation or angiogenesis), which lead to further progression of tissue formation. While a solid scientific rationale exists for the use of a variety of growth and attachment factors in regeneration of oral tissues, only a small number are being pursued clinically. Many therapeutic regimens have failed in preclinical testing or have resulted in limited regenerative capacity. The mitogenic polypeptides that stimulate soft tissue growth (such as platelet-derived growth factor) and both hard and soft tissue growth (such as transforming growth factor-beta) appear to have not led to successful enough outcomes to facilitate further work towards regulatory approval. The demonstrated ability of bone morphogenetic proteins to generate substantial quantities of bone suggest many applications in the oral cavity where this is the only tissue desired. Another therapeutic candidate is enamel matrix derivative, a set of matrix proteins. Enamel matrix derivative appears to stimulate first acellular cementum formation, which may allow for functional periodontal ligament formation. It will be of interest in the future to determine whether the protein matrix contains classic mitogenic or differentiation factors as well as the amelogenins. It is also evident that the bone morphogenetic proteins permit periodontal ligament formation. The conditions for stimulating predictable periodontal ligament tissues with bone morphogenetic proteins however are not known. It is clear that the bone morphogenetic proteins are excellent molecules for stimulating oral bone formation. The results of all these studies will determine the future therapeutic potential for these growth molecules such that they may be used to optimally stimulate and direct specific points along tissue formation cascades.

Mast cells and their mediators in cutaneous wound healing--active participants or innocent bystanders?

Mast cells are traditionally viewed as effector cells of immediate type hypersensitivity reactions. There is, however, a growing body of evidence that the cells might play an important role in the maintenance of tissue homeostasis and repair. We here present our own data and those from the literature elucidating the possible role of mast cells during wound healing. Studies on the fate of mast cells in scars of varying ages suggest that these cells degranulate during wounding, with a marked decrease of chymase-positive cells, although the total number of cells does not decrease, based on SCF-receptor staining. Mast cells contain a plethora of preformed mediators like heparin, histamine, tryptase, chymase, VEGF and TNF-alpha which, on release during the initial stages of wound healing, affect bleeding and subsequent coagulation and acute inflammation. Various additional vasoactive and chemotactic, rapidly generated mediators (C3a, C5a, LTB4, LTC4, PAF) will contribute to these processes, whereas mast cell-derived proinflammatory and growth promoting peptide mediators (VEGF, FGF-2, PDGF, TGF-beta, NGF, IL-4, IL-8) contribute to neoangiogenesis, fibrinogenesis or re-epithelization during the repair process. The increasing number of tryptase-positive mast cells in older scars suggest that these cells continue to be exposed to specific chemotactic, growth- and differentiation-promoting factors throughout the process of tissue remodelling. All these data indicate that mast cells contribute in a major way to wound healing. their role as potential initiators of or as contributors to this process, compared to other cell types, will however have to be further elucidated.

Aggregating human platelets stimulate the expression of thrombin receptors in cultured vascular smooth muscle cells via the release of transforming growth factor-beta1 and platelet-derived growth factorAB

BACKGROUND

Thrombin and the thrombin receptor have been implicated in the proliferation of vascular smooth muscle cells (VSMCs) observed after angioplasty and in atherosclerosis. Because thrombin receptor activation is an irreversible proteolytic event, the marked upregulation of the smooth muscle cell thrombin receptor after vascular injury may account for the maintained mitogenic activity of thrombin. The present study was designed to determine whether aggregating platelets stimulate thrombin receptor expression in cultured VSMCs and, if so, to identify the mediators.

METHODS AND RESULTS

Thrombin receptor expression was assessed by Northern and Western blot analyses and functionally by measuring the release of 6-keto prostaglandin F1alpha. Platelet-derived products (PDPs) released by aggregating human platelets enhanced thrombin receptor mRNA levels in a time- and concentration-dependent manner, an effect that was potentiated by transient acidification of PDPs, which release bioactive transforming growth factor (TGF)-beta1, and that was slightly inhibited by ketanserin. Among several factors known to be released by aggregating platelets, only TGF-beta1, platelet-derived growth factorAB (PDGF(AB)), and serotonin mimicked the PDP effect. The level of membrane thrombin receptor protein was increased in TGF-beta1-treated VSMCs. Pretreatment of VSMCs with either acidified PDP, or TGF-beta1 increased the alpha-thrombin-stimulated release of 6-keto prostaglandin F1alpha. This effect was blunted by incubating acidified PDP with either a TGF-beta- or a PDGF-neutralizing antibody.

CONCLUSIONS

Aggregating human platelets stimulate the expression of thrombin receptors in VSMCs through the release of TGF-beta1, PDGF(AB), and, to a lesser extent, serotonin. The upregulation of the thrombin receptor by products released by aggregating platelets may sustain the mitogenic activity of thrombin in the vascular wall at sites of injury.

Effect of phenotype on the transcription of the genes for platelet-derived growth factor (PDGF) isoforms in human smooth muscle cells, monocyte-derived macrophages, and endothelial cells in vitro

Proliferation of arterial smooth muscle cells (ASMCs) contributes considerably to enlargement of the arterial wall during atherosclerosis. The platelet-derived growth factor (PDGF) is a well-known mitogen and chemoattractant for ASMCs. Quantitative reverse transcription-polymerase chain reaction showed that cells appearing in atherosclerotic lesions, such as ASMCs, endothelial cells, and monocytes/macrophages, expressed mRNAs for both PDGF A and B chains in vitro, with the highest expression in endothelial cells. On proliferation, ASMCs and endothelial cells upregulated PDGF A mRNA. Differentiation of macrophages increased the amount of both mRNAs. Thus, the regulation of PDGF A- and B-chain expression depends on cell types and phenotypic states of the cells, which have also been found in vivo in human atherosclerotic lesions. PDGF A can be produced as short and long isoforms. The latter binds with high affinity to glycosaminoglycans. Irrespective of phenotype, only the minor part of total PDGF A mRNA consisted of the long variant in ASMCs, while endothelial cells produced 40% of total PDGF A as the long form. The differentiation of macrophages increased the production of the long PDGF A mRNA from 10% to 40%. Thus, increasing numbers of stimulated cells in the atherosclerotic lesion may increase the transcription of PDGF isoforms, and particularly of the long PDGF A isoform. Together with increasing amounts of ASMC-derived proteoglycans in developing lesions, this may contribute to accumulation of PDGF in the arterial wall matrix, resulting in prolonged stimulation of ASMCs.

Rapid activation of PDGF-A and -B expression at sites of lung injury in asbestos-exposed rats

The development of interstitial pulmonary fibrosis is associated with a variety of inflammatory mediators, including peptide growth factors and cytokines. In the work presented here, we have asked whether or not platelet-derived growth factor (PDGF)-A and -B genes and proteins are expressed in anatomic and temporal patterns consistent with this factor playing a role in the disease process. Using an established rat model of asbestos-induced fibroproliferative lung disease, we demonstrate elevated levels of PDGF-A and -B mRNAs in total lung RNA immediately after a single 5-h exposure to approximately 1,000 fibers/ml of chrysotile asbestos. In situ hybridization revealed the PDGF-A and -B in RNAs primarily in macrophages and bronchiolar-alveolar epithelial cells at sites of initial fiber deposition and lung injury. There was clear evidence of PDGF-A and -B mRNAs in interstitial cells as well. The pattern of in situ hybridization was entirely consistent with the appearance (established by immunohistochemistry) of PDGF-A and -B proteins by 24 h post-exposure in the same cell types. Both mRNAs and proteins remained detectable at the fiber deposition sites for almost 2 wk post-exposures. These findings are consistent with our previous studies showing increased mesenchymal cell proliferation and fibroproliferative lesions that progress at the sites where PDGF-A and -B are expressed. Although it is clear that multiple growth factors are produced simultaneously at sites of initial injury, we suggest that the PDGF isoforms could be playing a central role in the disease process based upon their potent mitogenic effects upon mesenchymal cells.

Identification of regulatory sequences in the promoter of the PDGF B-chain gene in malignant mesothelioma cell lines

Platelet-derived growth factor (PDGF) B-chain mRNA is readily detectable in malignant mesothelioma (MM) cell lines, but not in normal mesothelial (NM) cell lines. The high affinity receptor for PDGF B-chain dimers, the PDGF beta-receptor, is expressed in MM cell lines. NM cell lines predominantly express the PDGF alpha-receptor. Coexpression of the PDGF beta-receptor and its ligand may lead to an autocrine growth stimulating loop in the malignant cell type. In nuclear run off experiments, PDGF B-chain mRNA was detectable in MM cells only, indicating an increased level of transcription in this cell type. The proximal promoter of the PDGF B-chain gene contains DNaseI hypersensitive (DH) sites and mediates reporter gene activation in both normal and malignant cells. Nuclear proteins, extracted from both cell types, interact with DNA sequences within the proximal promoter around bp-64 to -61 relative to the transcription start site. Electrophoretic mobility shift assays (EMSAs) indicate that these factors are more abundantly present in the malignant than in the normal cell type. A DH site around -9.9 kb was found in both cell types. When tested in CAT assays, this region exerted a stimulatory effect on transcription in malignant cells. The elevated level of transcription of the PDGF B-chain gene in malignant cells may well be the result of interaction of regulatory sites in the proximal promoter and an enhancing element located at -9.9 kb from the transcription start site.

Production of cell-associated PDGF-AA by a human sarcoma cell line: evidence for a latent autocrine effect

The alternative splicing of platelet-derived growth factor A-chain is known to result in 2 different protein products. One variant is encoded by transcripts containing the 69 nts representing exon 6 (PDGF-AA(L)), and one variant is encoded by transcripts in which exon 6 is excluded (PDGF-AA(S). Transfection assays have suggested that the long splice variant of the A-chain is mainly associated with membrane- and matrix-associated heparan sulphate proteoglycans, whereas the shorter variant is soluble. We describe a human sarcoma cell line (U-2197) that expresses a high level of PDGF-A transcripts. Immunoprecipitations revealed cell-associated protein products of mainly 24, 28 and 33 kDa and less abundant forms of 40-45 kDa, while no PDGF was found in the medium. Analysis of extracellular medium in a radioreceptor assay confirmed that PDGF was not secreted by the U-2197 cells. The addition to U-2197 cultures of a carboxy terminal peptide that specifically competes with the binding of the long splice variant of PDGF-AA to extracellular matrix and cell membranes resulted in the release of 3 PDGF-AA-specific dimeric proteins with molecular masses of 33, 37 and 45 kDa. Furthermore, polymerase chain reaction studies discriminating between the long and the short splice variants of the PDGF-A transcripts revealed that U-2197 expressed relatively higher amounts of the long splice variant compared with U-343 MGa Cl 2:6, which is known to secrete PDGF-AA. These cell-associated forms of PDGF, released to the medium by adding carboxy terminal peptide, increased the tyrosine kinase activity of the endogenous PDGF alpha-receptor.

Site-directed mutagenesis of the N-linked glycosylation site in platelet-derived growth factor B-chain results in diminished intracellular retention

Pulse-chase analysis of human platelet-derived growth factor (PDGF) B-chain was conducted in stably transfected Chinese hamster ovary cells to determine precisely the kinetics of processing, intracellular trafficking and secretion. Newly synthesized 31 kDa monomers of the B-chain (p31) dimerized rapidly via disulfide bonds to a p54 species (t1/2 < 30 min). The p54 dimer was processed to a group of intracellular, cell surface (suramin-releasable) and secreted forms whose rates of appearance and disappearance from the cell were measured over a 48 h period. The newly synthesized p31 species was quantitatively converted to p27 by treatment with endoglycosidase H, consistent with efficient N-glycosylation at a site in the N-terminal propeptide region (Asn63-Met64-Thr65). Interruption of B-chain glycosylation by oligodeoxynucleotide-directed mutagenesis resulted in a significant increase in suramin-releasable forms at the cell surface (p34-38) and a concomitant decrease in accumulation of an intracellular p24 species. The glycosylation-defective mutant exhibited slight increases in receptor binding and mitogenic activity. Our results suggest that N-linked glycosylation of the B-chain is not important for formation of mitogenically active protein, but that it plays a role in early intracellular sorting and proteolytic processing events.

PDGF mediates a neuron-astrocyte interaction in the developing retina

Astrocytes invade the developing retina from the optic nerve head, over the axons of retinal ganglion cells (RGCs). RGCs express the platelet-derived growth factor A-chain (PDGF-A) and retinal astrocytes the PDGF alpha-receptor (PDGFR alpha), suggesting that PDGF mediates a paracrine interaction between these cells. To test this, we inhibited PDGF signaling in the eye with a neutralizing anti-PDGFR alpha antibody or a soluble extracellular fragment of PDGFR alpha. These treatments inhibited development of the astrocyte network. We also generated transgenic mice that overexpress PDGF-A in RGCs. This resulted in hyperproliferation of astrocytes, which in turn induced excessive vasculogenesis. Thus, PDGF appears to be a link in the chain of cell-cell interactions responsible for matching numbers of neurons, astrocytes, and blood vessels during retinal development.

Corneal endothelium and growth factors

Human corneal endothelium is characterized by a low regenerative capacity, mainly because of a low mitotic activity, and therefore complete regeneration of the endothelial layer after injury is precluded. A decrease in endothelial cell density and an abnormal cell mosaic, which may occur after many conditions, can compromise the integrity of the endothelial monolayer, resulting in corneal decompensation with reduced vision and eventual need for penetrating keratoplasty. It would be beneficial to have growth factors that can help to maintain or restore the integrity of this delicate endothelial monolayer by maintaining or increasing the endothelial cell density or to stimulate the regeneration during wound healing. Growth factors represent a group of signalling peptides which influence diverse cellular processes, including cell proliferation, differentiation, migration and survival. One of the areas that has received great interest is its enhancement of wound healing. In this review the effects of three most effective growth factors (EGF, PDGF, FGF) on corneal endothelium, especially on wound healing in human corneal buttons, will be discussed.