Migratory and proliferative effect of platelet-derived growth factor in rabbit retinal endothelial cells: evidence of an autocrine pathway of platelet-derived growth factor

Research progress on the role of PDGF/PDGFR in type 2 diabetes

Platelet-derived growth factors (PDGFs) are basic proteins stored in the α granules of platelets. PDGFs and their receptors (PDGFRs) are widely expressed in platelets, fibroblasts, vascular endothelial cells, platelets, pericytes, smooth muscle cells and tumor cells. The activation of PDGFR plays a number of critical roles in physiological functions and diseases, including normal embryonic development, cellular differentiation, and responses to tissue damage. In recent years, emerging experimental evidence has shown that activation of the PDGF/PDGFR pathway is involved in the development of diabetes and its complications, such as atherosclerosis, diabetic foot ulcers, diabetic nephropathy, and retinopathy. Research on targeting PDGF/PDGFR as a treatment has also made great progress. In this mini-review, we summarized the role of PDGF in diabetes, as well as the research progress on targeted diabetes therapy, which provides a new strategy for the treatment of type 2 diabetes.

The platelet derived growth factor-B polymorphism is associated with risk of severe fever with thrombocytopenia syndrome in Chinese individuals

Background

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by a novel bunyavirus named SFTS virus (SFTSV). We hypothesize that host genetic variations may contribute to susceptibility to SFTS.

Results

Compared with the rs1800818 AA genotype, AG + GG genotypes were significantly associated with increased susceptibility to SFTS (odds ratio, 1.66, 95% confidence interval = 1.28-2.16; P < 0.001). By using the ELISA assay, we observed that PDGF-BB concentration was significantly reduced in acute phase of patients than in the controls (P < 0.001) and recovered patients at 6 month (P = 0.007) and 12 month (P = 0.003). A persistently reduced PDGF-BB was also revealed from the SFTSV-infected C57BL/6J mice (P < 0.001). The rs1800818 G allele was associated with decreased serum PDGF-BB levels in SFTS patients at their early infection (P = 0.015). In accordance, the relative mRNA levels of the at-risk G allele of 1800818 were lower than those of the A allele in heterozygous cell from acute phase of SFTS patients. PDGF-B rs1800818 conferred no susceptibility to severe or fatal outcome in SFTS patients.

Materials and Methods

An initially small-scale case-control association study guided the selection of platelet derived growth factor-B (PDGF-B) rs1800818 in 1020 SFTS patients and 1353 controls. Functional analyses were conducted to verify the biological significance of rs1800818 polymorphism.

Conclusions

Our findings suggest that the PDGF-B rs1800818 polymorphism might play a role in mediating the susceptibility to SFTS.

A Critical Analysis of the Available In Vitro and Ex Vivo Methods to Study Retinal Angiogenesis

Angiogenesis is a biological process with a central role in retinal diseases. The choice of the ideal method to study angiogenesis, particularly in the retina, remains a problem. Angiogenesis can be assessed through in vitro and in vivo studies. In spite of inherent limitations, in vitro studies are faster, easier to perform and quantify, and typically less expensive and allow the study of isolated angiogenesis steps. We performed a systematic review of PubMed searching for original articles that applied in vitro or ex vivo angiogenic retinal assays until May 2017, presenting the available assays and discussing their applicability, advantages, and disadvantages. Most of the studies evaluated migration, proliferation, and tube formation of endothelial cells in response to inhibitory or stimulatory compounds. Other aspects of angiogenesis were studied by assessing cell permeability, adhesion, or apoptosis, as well as by implementing organotypic models of the retina. Emphasis is placed on how the methods are applied and how they can contribute to retinal angiogenesis comprehension. We also discuss how to choose the best cell culture to implement these methods. When applied together, in vitro and ex vivo studies constitute a powerful tool to improve retinal angiogenesis knowledge. This review provides support for researchers to better select the most suitable protocols in this field.

Platelet derived growth factor inhibitors: A potential therapeutic approach for ocular neovascularization

Retinochoroidal vascular diseases are the leading causes of blindness in the developed world. They include diabetic retinopathy (DR), retinal vein occlusion, retinopathy of prematurity, age-related macular degeneration (AMD), and pathological myopia, among many others. Several different therapies are currently under consideration for the aforementioned disorders. In the following section, agents targeting platelet-derived growth factor (PDGF) are discussed as a potential therapeutic option for retinochoroidal vascular diseases. PDGF plays an important role in the angiogenesis cascade that is activated in retinochoroidal vascular diseases. The mechanism of action, side effects, efficacy, and the potential synergistic role of these agents in combination with other treatment options is discussed. The future of treatment of retinochoroidal vascular diseases, particularly AMD, has become more exciting due to agents such as PDGF antagonists.

Understanding regulatory pathways of neovascularization in diabetes

Diabetes mellitus and its associated comorbidities represent a significant health burden worldwide. Vascular dysfunction is the major contributory factor in the development of these comorbidities, which include impaired wound healing, cardiovascular disease and proliferative diabetic retinopathy. While the etiology of abnormal neovascularization in diabetes is complex and paradoxical, the dysregulation of the varied processes contributing to the vascular response are due in large part to the effects of hyperglycemia. In this review, we explore the mechanisms by which hyperglycemia disrupts chemokine expression and function, including the critical hypoxia inducible factor-1 axis. We place particular emphasis on the therapeutic potential of strategies addressing these pathways; as such targeted approaches may one day help alleviate the healthcare burden of diabetic sequelae.

Yin and Yang of PDGF-mediated signaling pathway in the context of HIV infection and drug abuse

The control and eradication of neurological complications associated with AIDS continues to be an important goal in efforts toward improving the well being of HIV-infected patients. Although combined antiretroviral therapies have contributed significantly to increasing the longevity of patients by suppressing the virus burden in the systemic compartments, the prevalence of HIV-associated neurological disorders continues to be on the rise. This in turn, leads to an impaired quality of life of the infected individuals who continue to suffer from mild to moderate cognitive decline and memory loss. Developing therapeutic interventions that reverse neuronal injury in the context of HIV infection, is thus of paramount importance in the field. Our previous studies have demonstrated that platelet-derived growth factor (PDGF) has a neuroprotective potential against HIV envelope protein gp120 and Tat. Paradoxically, PDGF is also a cerebrovascular permeant with deleterious effects on the blood-brain barrier resulting in increased influx of monocytes in the CNS. Herein, we review the opposing roles of PDGF in the context of HIV-associated neurodegenerative disorder (HAND).

Cocaine-mediated induction of platelet-derived growth factor: implication for increased vascular permeability

Neuroinflammation associated with advanced HIV-1 infection is often exacerbated in cocaine-abusing, HIV-infected patients. The underlying mechanisms could, in part, be attributed to the increased impairment of blood brain barrier integrity in the presence of cocaine. Platelet-derived growth factor (PDGF) has been implicated in several pathologic conditions, specifically attributable to its potent mitogenic effects. Its modulation by drug abuse, however, has received very little attention. In the present study, we demonstrated cocaine-mediated induction of PDGF-BB in human brain microvascular endothelial cells through the binding to its cognate σ receptor. Furthermore, this effect was mediated, with subsequent activation of mitogen-activated protein kinases and Egr-1 pathways, culminating ultimately into increased expression of PDGF-BB. Cocaine exposure resulted in increased permeability of the endothelial barrier, and this effect was abrogated in mice exposed to PDGF-BB neutralizing antibody, thus underscoring its role as a vascular permeant. In vivo relevance of these findings was further corroborated in cocaine-treated mice that were administered neutralizing antibody specific for PDGF-BB as well as in Egr-1(-/-) mice. Understanding the regulation of PDGF-BB expression may provide insights into the development of potential therapeutic targets for neuroinflammation associated with HIV infection and drug abuse.

Vascular Wall Maturation and Prolonged Angiogenic Effect by Endothelial-Specific Platelet-Derived Growth Factor Expression

BACKGROUND

The implementation of angiogenic gene therapy at clinics is hindered by the transience of the therapeutic effect. Recruiting vascular wall smooth muscle cells, a process termed 'maturation', can stabilize newly formed vessels.

OBJECTIVE

To induce angiogenesis followed by vessel maturation in a murine ischemic limb model by endothelial cell-specific promoter regulated expression of vascular endothelial growth factor (VEGF) and platelet-derived growth factor-BB (PDGF-BB).

METHODS

We constructed adenoviral vectors containing angiogenic factors VEGF and PDGF-B regulated by a modified preproendothelin-1 (PPE-1-3x) promoter and investigated their angiogenic effect in a murine ischemic limb model.

RESULTS

VEGF gene therapy increased perfusion and the vessel density in the limb shortly after expression with PPE-1-3x promoter or cytomegalovirus (CMV) promoter vectors, but only PPE-1-3xVEGF treatment exhibited a sustained effect. Expression of PDGF-B by PPE-1-3x promoter resulted in morphological maturation of the vasculature and further increased the perfusion, while nonspecific expression of PDGF-B with CMV promoter had no therapeutic effect. Regulation of dual therapy with VEGF and PDGF-B by PPE-1-3x promoter resulted in an early-onset, sustained angiogenic effect accompanied by vessel maturation.

CONCLUSIONS

Systemic gene therapy with the angiogenic factors VEGF and PDGF-B under angiogenic- endothelial cell-specific regulation was effective in inducing functionally and morphologically mature vasculature.

Endothelial survival factors and spatial completion, but not pericyte coverage of retinal capillaries determine vessel plasticity

Pericyte loss and capillary regression are characteristic for incipient diabetic retinopathy. Pericyte recruitment is involved in vessel maturation, and ligand-receptor systems contributing to pericyte recruitment are survival factors for endothelial cells in pericyte-free in vitro systems. We studied pericyte recruitment in relation to the susceptibility toward hyperoxia-induced vascular remodeling using the pericyte reporter X-LacZ mouse and the mouse model of retinopathy of prematurity (ROP). Pericytes were found in close proximity to vessels, both during formation of the superficial and the deep capillary layers. When exposure of mice to the ROP was delayed by 24 h, i.e., after the deep retinal layer had formed [at postnatal (p) day 8], preretinal neovascularizations were substantially diminished at p18. Mice with a delayed ROP exposure had 50% reduced avascular zones. Formation of the deep capillary layers at p8 was associated with a combined up-regulation of angiopoietin-1 and PDGF-B, while VEGF was almost unchanged during the transition from a susceptible to a resistant capillary network. Inhibition of Tie-2 function either by soluble Tie-2 or by a sulindac analog, an inhibitor of Tie-2 phosphorylation, resensitized retinal vessels to neovascularizations due to a reduction of the deep capillary network. Inhibition of Tie-2 function had no effect on pericyte recruitment. Our data indicate that the final maturation of the retinal vasculature and its resistance to regressive signals such as hyperoxia depend on the completion of the multilayer structure, in particular the deep capillary layers, and are independent of the coverage by pericytes.

Revascularization of ischemic tissues by PDGF-CC via effects on endothelial cells and their progenitors

The angiogenic mechanism and therapeutic potential of PDGF-CC, a recently discovered member of the VEGF/PDGF superfamily, remain incompletely characterized. Here we report that PDGF-CC mobilized endothelial progenitor cells in ischemic conditions; induced differentiation of bone marrow cells into ECs; and stimulated migration of ECs. Furthermore, PDGF-CC induced the differentiation of bone marrow cells into smooth muscle cells and stimulated their growth during vessel sprouting. Moreover, delivery of PDGF-CC enhanced postischemic revascularization of the heart and limb. Modulating the activity of PDGF-CC may provide novel opportunities for treating ischemic diseases.

Targeting the Platelet-Derived Growth Factor Receptor in Antivascular Therapy for Human Ovarian Carcinoma

PURPOSE

We sought to determine whether blockade of platelet-derived growth factor receptor (PDGF-R) activation by oral administration of a PDGF-R tyrosine kinase inhibitor (STI571) alone or in combination with i.p. paclitaxel can inhibit the progression of tumors caused by human ovarian carcinoma cells growing in the peritoneal cavity of female nude mice.

EXPERIMENTAL DESIGN

In several different experiments, paclitaxel-sensitive and paclitaxel-resistant metastatic human ovarian carcinoma cells were injected into the peritoneal cavity of nude mice. Seven days later, groups (n = 10) of mice began receiving a control treatment, STI571 alone, paclitaxel alone, or a combination of STI571 and paclitaxel. The mice were necropsied after 45 days of treatment.

RESULTS

Treatment with combination therapy significantly reduced tumor weight (relative to control or single-agent therapy) in all three human ovarian cancer cell lines. Immunohistochemical analyses revealed that PDGF-R activation was blocked by STI571 administered alone or in combination with paclitaxel. Tumor-associated endothelial cells expressed both PDGF-R and phosphorylated PDGF-R. In mice receiving combination therapy, tumor-associated endothelial cells underwent apoptosis, leading to decreases in microvessel density and tumor cell proliferation relative to control and single-agent therapy.

CONCLUSIONS

These results show that administration of a PDGF-R tyrosine kinase inhibitor in combination with paclitaxel impairs the progression of ovarian cancer in the peritoneal cavity of nude mice, in part, by blockade of PDGF, an endothelial cell survival factor, which results in the increased apoptosis of tumor-associated endothelial cells.

Platelets are mitogenic for periosteum-derived cells

The early stages of bone regeneration are associated with a high mitogenic activity of periosteal cells. Here we addressed the question of whether platelets that accumulate within the developing haematoma can account for this tissue response. Addition of platelets, platelet-released supernatants, platelet membranes, and microparticles to bovine periosteum-derived cells resulted in an increase in 3H-thymidine incorporation; lipid extracts had no effect. Platelet-released supernatants retained their activity after incubation at 56 degrees C, but not at 100 degrees C. Gel chromatographic analysis revealed the highest mitogenic activity at approximately 35 kD. Of the factors released from activated platelets, basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) increased 3H-thymidine incorporation. The mitogenic activity of platelet-released supernatants was decreased by anti-PDGF, and anti-bFGF antibodies. Platelet-released supernatants increased the number of proliferating periosteum-derived cells as determined by the expression pattern of Ki67. Platelet-released supernatants also resulted in a stimulation of cell proliferation in periosteal explants. These results suggest that platelets have the potential to stimulate the mitogenic response of the periosteum during bone repair.

Rat cerebral endothelial cells express trk C and are regulated by neurotrophin-3

Cerebral endothelial cells (CEC) are critical for formation of the vascular system in the mammalian central nervous system (CNS). We focused on the neurotrophin (NT) for its possible involvement in signaling for the regulation of CEC to control formation and maintenance of the vascular system in CNS in comparison of rat cerebral endothelial cells (RCEC) with rat aortic endothelial cells (RAEC). We found that (1) trk C, a receptor for neurotrophin-3 (NT-3), is dominantly expressed in RCEC, but trk B, a receptor for brain-derived neurotrophic factor, is dominantly expressed in RAEC; (2) NT-3 inhibited the proliferation of RCEC; and (3) NT-3 stimulated the production of nitric oxide (NO) with increases in protein expression of endothelial NO synthase. These data indicated that NT may regulate and/or maintain the functions of the brain microvasculature through the regulation of CEC.

Photoreceptor-specific overexpression of platelet-derived growth factor induces proliferation of endothelial cells, pericytes, and glial cells and aberrant vascular development: an ultrastructural and immunocytochemical study

Platelet-derived growth factor (PDGF) is necessary for the normal development of the retinal vasculature and its overexpression is likely to contribute to proliferative retinal disorders, such as proliferative vitreoretinopathy. Transgenic mice that overexpress PDGF-B in the photoreceptors (rho/PDGF-B mice) develop traction retinal detachment. In the present study, a detailed histopathological analysis was performed in rho/PDGF-B mice. In these transgenic mice, endothelial cells, pericytes, and glial cells begin to proliferate at postnatal day 7 (P7). All three cell types increase in numbers, forming a highly vascularized cell mass, which reaches a maximum thickness at P14. Cords of endothelial cells and glia invade the retina and exert traction, generating retinal folds; however, the deep capillary bed never forms. Griffonia simplicifolia isolectin B4 (GSA)-positive endothelial cells form tubes and penetrate the retina to the level of the outer plexiform layer, but they never interconnect to form the deep capillary bed. The vessels within the cell mass are patent, but have a very immature morphology. They often are thin-walled with fenestrations. Pericytes and glial cells are usually found in clusters and are not associated with the abnormal vessels. The lack of this association may account for the failure to form a mature vasculature.

Angiogenesis stimulated by PDGF-CC, a novel member in the PDGF family, involves activation of PDGFR-alphaalpha and -alphabeta receptors

A newly discovered PDGF isoform, PDGF-CC, is expressed in actively angiogenic tissues such as placenta, some embryonic tissues, and tumors. We test the possibility that PDGF-CC promotes angiogenesis in vivo. The core domain (mature form) of human PDGF-CC is sufficiently potent to stimulate neovascularization in the mouse cornea. The corneal angiogenic response induced by PDGF-CC is robust although the area of neovascularization is smaller than those of FGF-2- and VEGF-stimulated angiogenesis. Similarly, PDGF-BB and PDGF-AB induce angiogenic responses virtually indistinguishable from PDGF-CC-stimulated vessels. In contrast, PDGF-AA displays only a weak angiogenic response in the mouse cornea. Although there was no significant difference in incorporation of mural cells to the newly formed blood vessels induced by PDGF-BB and -CC, the percentage of mural cell positive vessels induced by PDGF-AA was greater than those induced by FGF-2, PDGF-BB, and PDGF-CC. In the developing chick embryo, PDGF-CC induced branch sprouts from established blood vessels. In PDGF receptor-transfected endothelial cells, PDGF-CC activated the PDGF receptor alpha subunit (PDGFR-alpha). PDGF-CC, but not PDGF-AA, was able to activate PDGFR-beta receptor in endothelial cells that coexpress both alpha and beta forms of receptors. Thus, the PDGF-CC-mediated angiogenic response is most likely transduced by PDGF-alphaalpha and -alphabeta receptors. These data demonstrate that the PDGF family is a complex and important group of proangiogenic factors.

Young adult bone marrow-derived endothelial precursor cells restore aging-impaired cardiac angiogenic function

Delivery of young bone marrow-derived stem cells offers a novel approach for restoring the impaired senescent cardiac angiogenic function that may underlie the increased morbidity and mortality associated with ischemic heart disease in older individuals. Recently, we reported that alterations in endothelial cells of the aging heart lead to a dysregulation in the cardiac myocyte platelet-derived growth factor (PDGF)-B-induced paracrine pathway, which contributes to impaired cardiac angiogenic function. Based on these results, we hypothesized that cellular restoration of the PDGF pathway by bone marrow-derived endothelial precursor cells (EPCs) could reverse the aging-associated decline in angiogenic activity. In vitro studies revealed that young murine (3-month-old) bone marrow-derived EPCs recapitulated the cardiac myocyte-induced expression of PDGF-B, whereas EPCs from the bone marrow of aging mice (18-month-old) did not express PDGF-B when cultured in the presence of cardiac myocytes. Transplantation of young, but not old, genetically marked syngeneic bone marrow cells into intact, unirradiated aging mice that populated the endogenous senescent murine bone marrow incorporated into the neovasculature of subsequently transplanted syngeneic neonatal myocardium. Moreover, the young bone marrow-derived EPCs restored the senescent host angiogenic PDGF-B induction pathway and cardiac angiogenesis, with graft survival and myocardial activity in the aging murine host (cardiac allograft viability: 3-month-old controls, 8/8; 18-month-old controls, 1/8; 18-month-old donors receiving bone marrow from 3-month-old mice, 15/16; or 18-month-old mice, 0/6; P<0.05). These results may offer a foundation for the development of novel therapies for the prevention and treatment of cardiovascular disease associated with aging.

The pathogenesis of diabetic retinopathy: old concepts and new questions

Hyperglycaemia appears to be a critical factor in the aetiology of diabetic retinopathy and initiates downstream events including: basement membrane thickening, pericyte drop out and retinal capillary non-perfusion. More recently, focus has been directed to the molecular basis of the disease process in diabetic retinopathy. Of particular importance in the development and progression of diabetic retinopathy is the role of growth factors (eg vascular endothelial growth factor, placenta growth factor and pigment epithelium-derived factor) together with specific receptors and obligate components of the signal transduction pathway needed to support them. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. How does hyperglycaemia regulate retinal vessels? Which growth factors are most important and at what stage of retinopathy do they operate? What is the preferred point in the growth factor signalling cascade for therapeutic intervention? Answers to these questions will provide the basis for new therapeutic interventions in a debilitating ocular condition.

Effects of angiogenic growth factor combinations on retinal endothelial cells

The aim of this paper was to determine if growth factors, known to be upregulated in proliferative diabetic retinopathy, exerted combined effects on retinal endothelial cells. The authors explored the individual and collective actions of insulin-like growth factor I (IGF-I), vascular endothelial growth factor (VEGF), platelet-derived growth factor-BB (PDGF-BB), fibroblast growth factor-2 (FGF-2) and placenta growth factor (PlGF) on several parameters that reflect the angiogenic potential of endothelial cells. The effect of growth factors on cell migration and survival/proliferation was examined using primary cultures of bovine retinal endothelial cells (BREC). The authors also determined the growth factor action on capillary-like tube formation on a reconstituted basement membrane matrix and on the newly described phenomenon of secondary sprouting, in which endothelial cell colonies spontaneously survive, proliferate, migrate and invade the matrix after the original capillary-like tubes have collapsed. Sprouting cells were positive for von Willebrand factor and could aggregate into larger tubes with lumens. Incubation with VEGF+IGF-I or PlGF+FGF-2 enhanced tube stability by 40-50%, more than each growth factor alone or other combinations (5-20%). The concurrent addition of four growth factors did not improve the response seen with growth factor pairs. Surprisingly, PDGF-BB induced tube collapse. IGF-I and FGF-2 mildly enhanced BREC proliferation/survival (5-15%). However, VEGF+IGF-I or PlGF+FGF-2 increased BREC proliferation/survival by 25% under low serum conditions, whereas combinations of all four growth factors exerted a clearly synergistic effect (250% increase). PDGF-BB or FGF-2 stimulated secondary sprouting and were the only factors capable of exerting this effect alone. Even though VEGF, IGF-I or PlGF were not effective, if administered in pairs, they demonstrated increased responses. PDGF-BB was also able to enhance the effect of FGF-2+IGF-I+VEGF on BREC secondary sprouting, but not of any of them individually. No other growth factor tested was able to significantly improve the action of combinations of three other growth factors. VEGF increased cell migration in a wounded monolayer assay two-fold and PDGF-BB, 2.5 times, but other individual growth factors were ineffective. PlGF+FGF-2 enhanced cell migration more than each factor alone. VEGF+IGF-I+PlGF+FGF-2, however, increased cell migration four-fold. In summary, this study indicates that growth factors, overexpressed in diabetic retinopathy eyes, enhance the angiogenic characteristics of cultured cells (tube formation, proliferation, secondary sprouting and migration). Their effects, however, can be greatly augmented by other growth factors that alone exert little or no action. Therefore, diabetic retinal neovascularization may result from the additive or synergistic action of several growth factors.

Experimental models of growth factor-mediated angiogenesis and blood-retinal barrier breakdown

Following chronic ischemia, vascular endothelial growth factor (VEGF) is induced primarily in the ganglion cell layer of the retina. This often results in neovascularization (NV) that originates from the vascular bed closest to the ganglion cell layer. To study the effects of VEGF, independent lines of transgenic mice that express VEGF in the lens and in the retina have been generated. Expression in the lens results in excessive proliferation and accumulation of angioblasts and endothelial cells in proximity to the lens. However, VEGF expression is not sufficient to direct blood vessel organization or maturation in the prenatal mouse. Abnormal vessels do form on the retinal surface, but not until the second postnatal week. In transgenic mice expressing VEGF in the photoreceptors, NV originates from the deep capillary bed--the vascular bed closest to the photoreceptors. NV is accompanied by localized blood-retinal barrier breakdown. NV is also induced in PDGF-B transgenic mice. PDGF-B expression in the lens occurs prenatally and, during this time, mainly affects the perilenticular vessels. Postnatally, transgenic mice expressing PDGF-B in the lens or photoreceptors show a similar phenotype. In both models, a highly vascularized cell mass containing endothelial cells, pericytes, and glia forms in the superficial retina, and the formation of the deep capillary bed is inhibited. The phenotype suggests that an additional factor is necessary for the maturation and penetration of vascular endothelial cells into the retina to form the deep capillary bed.

New paradigms for the treatment of cancer: the role of anti-angiogenesis agents

Angiogenesis, the sprouting of new blood vessels, plays a role in diverse disease states including cancer, diabetic retinopathy, age-related macular degeneration, rheumatoid arthritis, psoriasis, atherosclerosis, and restenosis. With regard to cancer, the clinical association of tumor vascularity with tumor aggressiveness has been clearly demonstrated in numerous tumor types. The observation of increased microvessel density in tumors not only serves as an independent prognostic indicator, but also suggests that anti-angiogenic therapy may be an important component of treatment regimens for cancer patients. The complexity of the angiogenic process, which involves both positive and negative regulators, provides a number of targets for therapy. Many positive regulators, including growth factor receptors, matrix metalloproteinases, and integrins, have been correlated with increased vascularity of tumors and poor prognosis for patient survival. Thus, these serve as ideal targets for anti-angiogenesis therapy. Many inhibitors of these targets are currently undergoing clinical evaluation as potential anti-cancer agents. In this article, we discuss the role of positive regulators in angiogenesis and tumor growth and describe the anti-angiogenic agents under development.

Expression of hepatocyte growth factor/scatter factor and c-Met in human dental papilla and fibroblasts from dental papilla

Hepatocyte growth factor/scatter factor (HGF/SF), a broad-spectrum and multifunctional cytokine, is essential for the development of tissues including tooth. Here it was found that the HGF/SF content of human dental papillae obtained from 8 to 16-year-old individuals decreased significantly with age. Cultured fibroblasts prepared from the dental papillae of individuals of different ages produced HGF/SF at almost the same rate, but the sensitivities of the cells to interleukin-1alpha and tumour necrosis factor-alpha for the production of HGF/SF increased with age. Generally, mesenchymal cells such as fibroblasts produce HGF/SF but do not express c-Met, a receptor for HGF/SF, yet fibroblasts in dental papilla and cultured fibroblasts prepared from dental papilla did express c-Met, as determined by immunohistochemistry, in situ hybridization and reverse transcription-polymerase chain reaction. Recombinant human [125I]iodo-HGF/SF specifically bound to cell-surface macromolecules with a mol. wt of 146,000, which is the same as that of the beta-subunit of c-Met. The physiological role of c-Met on fibroblasts in dental papilla is unknown, but the addition of 2 ng of HGF/SF per ml to the culture medium significantly stimulated DNA synthesis in the cells, as determined by pulse labelling with [3H]thymidine. Exogenous HGF/SF also stimulated secretion by the cells of vascular endothelial growth factor, a cytokine that induces blood vessel-formation. These results suggest that HGF/SF may be involved in tooth development via autocrine mechanisms.

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.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

PDGF and TGF-beta contribute to the natural course of human IgA glomerulonephritis

PDGF and TGF-beta are known mediators of mesangial cell proliferation and matrix expansion. The presence of these regulatory factors was examined in 30 renal biopsies from patients with IgA glomerulonephritis (IgA-GN) at the mRNA and protein level. Normal renal tissue served as control. The mRNA expression of PDGF A/B chains, PDGF-beta R and TGF-beta 1 was evaluated by means of RT/PCR with subsequent Southern blot hybridization and/or non-radioactive in situ hybridization. In addition, PDGF-AB/BB, PDGF-beta R, TGF-beta isoforms (beta 1, beta 1 + 2, beta 2 + 3), the small TGF-beta 1 latency associated peptide (TGF-beta 1 LAP) and the extracellular matrix proteins tenascin and decorin were analyzed by immunocytochemistry. The expression of growth factors was correlated with light microscopic and clinical features. Compared to normal control kidneys, an increased expression of PDGF-BB/PDGF-beta R mRNAs and the corresponding proteins was observed in all biopsies with IgA-GN. Up-regulation was related to the degree of glomerular proliferation and the extent of fibrosing interstitial lesions. In contrast, there was a discordance between TGF-beta 1 mRNA and protein expression (evaluated by immunocytochemistry). In all biopsies, irrespective of the stage of the disease, abundant TGF-beta 1 transcripts were detected, whereas TGF-beta 1 immunoreactivity was expressed to a lesser degree and disclosed a more variable staining pattern. In patients with significant proliferative glomerular lesions and minor tubulointerstitial alterations, TGF-beta 1 positivity was confined to areas of glomerular proliferation, whereas in cases with more severe histology including sclerosing lesions TGF-beta 1 immunoreactivity was less prominent. The distribution and the intensity of TGF-beta 1 LAP staining commonly exceeded the positivity noted for TGF-beta 1, indicating only limited TGF-beta 1 activation. A decreased reactivity for tenascin accompanied the morphological features of glomerular sclerosis. The staining patterns and the fact that only very few inflammatory cells, particularly CD68 positive monocytes/macrophages, were detected in glomeruli confirm that predominantly resident glomerular cells (mesangial and endothelial cells) are the major source of up-regulated growth factor production in IgA-GN. Since the expression of PDGF-AB/BB paralleled the severity of proliferative glomerular changes, PDGF seems to represent a potential indicator of activity in this condition. It is suggested that an imbalance between PDGF and TGF-beta (by restricted translation and/or activation) production contribute to the progressive nature of IgA-GN.

The role of growth factors in the development of diabetic retinopathy

Proliferation of retinal blood vessels is one of the most striking features of advanced diabetic retinopathy. This feature has led to the conclusion that the normal balance of growth factors, which usually serves to keep angiogenesis in check, is disturbed in diabetic retinopathy. A considerable amount of work has been performed in the field of angiogenesis within the last decade. Much of this is applicable to diabetic eye disease, but due to the lack of an animal model, few studies have been performed directly on models of diabetic retinopathy. This review examines the literature as it relates to diabetic retinopathy.

Mechanism of angiogenic effects of saponin from ginseng Radix rubra in human umbilical vein endothelial cells

1. The effects of saponin from Ginseng Radix rubra on angiogenesis (tube formation) and its key steps (protease secretion, proliferation and migration) in human umbilical vein endothelial cells (HUVEC) were examined to elucidate the mechanism of the tissue repairing effects of Ginseng Radix rubra. The effect on a wound healing model was also studied. 2. Tube formation was measured by an in vitro system. The activity and immunoreactivity of tissue-type plasminogen activator (tPA) as a protease for angiogenesis and the immunoreactivity of its inhibitor, plasminogen activator inhibitor-1 (PAI-1), were measured in conditioned medium of HUVEC stimulated for 24 h with saponin. Cell proliferation was measured by counting the cell numbers at 2-7 days after seeding. Migration was measured by Boyden's chamber method. The effect on wound healing was studied in the skin of diabetic rats. 3. Saponin at 10-100 micrograms ml-1 significantly stimulated tube formation by HUVEC in a dose-dependent manner. Saponin in a similar concentration-range increased the secretion of tPA from HUVEC as estimated by immunoreactivity and enzyme activity. On the other hand, PAI-1 immunoreactivity was slightly increased at 10 micrograms ml-1 of saponin, but then was significantly decreased at 50 and 100 micrograms ml-1. Cell proliferation was only slightly enhanced by 1-100 micrograms ml-1 of saponin, but migration was significantly enhanced by 10-100 micrograms ml-1 in a dose-dependent manner. Moreover, saponin stimulated wound healing with enhanced angiogenesis in vivo. 4. These results indicate that saponin stimulates tube formation mainly by modifying the balance of protease/protease inhibitor secretion from HUVEC and enhancing the migration of HUVEC, and that it is effective in vivo.

Isoforms of platelet-derived growth factor and its receptors in epiretinal membranes: immunolocalization to retinal pigmented epithelial cells

Epiretinal membranes (ERMs) form on the inner surface of the retina in conjunction with various ocular disease processes, but the factors controlling their development are not understood. The predominant cell types involved are retinal pigmented epithelial (RPE) cells and retinal glia. Cultured RPE cells secrete platelet-derived growth factor (PDGF), which is chemotactic and mitogenic for both RPE cells and retinal glia and, therefore, could be involved in the development of ERMs. In the present study, we performed immunohistochemical staining for PDGF A chain (PDGF-A), PDGF B chain (PDGF-B), and both types of PDGF receptors (PDGFr alpha and PDGFr beta) on ERMs associated with various disease processes. PDGF-A is detected in most ERMs, regardless of the associated disease process, and it appears to be localized predominantly in RPE cells, recognized by the presence of pigment and the immunohistochemical demonstration of some or all of the following RPE-associated epitopes: class III beta-tubulin, keratin, the 65-kDa microsomal protein recognized by the RPE9 antibody, and cellular retinaldehyde-binding protein. PDGF-B is found only in minor subpopulations of cells in about half of the ERMs evaluated and, with only occasional exceptions, appears to be localized almost entirely in blood-borne cells found in and around vessels in vascularized ERMs. Both PDGFr alpha and PDGFr beta are demonstrated in most ERMs with neither isotype consistently predominating: they are found predominantly on RPE cells with many cells expressing both receptor types. ERMs with little or no RPE cell component contain little or no PDGF and PDGF receptor, whereas those in which the RPE cell represents the major cell type, have widespread PDGF and PDGF receptor positivity. These findings show that RPE cells in ERMs produce PDGF-A and PDGF alpha and PDGF beta receptors and suggest that autocrine and paracrine stimulation with PDGF may be involved in ERM pathogenesis.

Regulation of Fibronectin by Platelet-Derived Growth Factors in Cultured Rat Thoracic Aortic Smooth Muscle Cells

Induction of fibronectin (FN) gene expression by platelet-derived growth factor (PDGF) isoforms in rat thoracic aortic smooth muscle cells (SMC) was examined. PDGF-BB enhances FN levels in SMC cultures in a time- and concentration-response fashion. PDGF-AA and PDGF-AB show no effect on FN levels. The effects of insulin and insulin-like growth factor-I (IGF-I) on PDGF-BB-induced FN levels were examined. No additivity of FN levels is observed between PDGF-BB and insulin and/or IGF-I. Experiments also show that PDGF-BB enhances FN mRNA levels, implying that acquisition of additional FN mRNA units accounts for the increase in FN levels. Induction of FN and FN mRNA levels by PDGF-BB could be one of the initial events in vascular SMC proliferation and extracellular matrix expansion, leading to atherosclerosis and hypertension. Copyright 1995 S. Karger AG, Basel

Integrin alpha v beta 3 antagonists promote tumor regression by inducing apoptosis of angiogenic blood vessels

A single intravascular injection of a cyclic peptide or monoclonal antibody antagonist of integrin alpha v beta 3 disrupts ongoing angiogenesis on the chick chorioallantoic membrane (CAM). This leads to the rapid regression of histologically distinct human tumors transplanted onto the CAM. Induction of angiogenesis by a tumor or cytokine promotes vascular cell entry into the cell cycle and expression of integrin alpha v beta 3. After angiogenesis is initiated, antagonists of this integrin induce apoptosis of the proliferative angiogenic vascular cells, leaving preexisting quiescent blood vessels unaffected. We demonstrate therefore that ligation of integrin alpha v beta 3 is required for the survival and maturation of newly forming blood vessels, an event essential for the proliferation of tumors.