Inhibition of endothelial cell proliferation by platelet factor-4 involves a unique action on S phase progression

Beta-sheet is the bioactive conformation of the anti-angiogenic anginex peptide

Anginex is a designed peptide 33mer that functions as a cytokine-like agent to inhibit angiogenesis. Although this short linear peptide has been shown by NMR and CD to form a nascent beta-sheet conformation in solution, the actual bioactive structure formed upon binding to its receptor on the surface of endothelial cells could be quite different. By using a series of double-cysteine disulphide-bridged analogues, we provide evidence in the present study that the beta-sheet is in fact the bioactive conformation of anginex. CD and NMR spectral analysis of the analogues indicate formation of a beta-sheet conformation. Three functional assays, endothelial cell proliferation, apoptosis and in vitro angiogenesis, were performed on all analogues. As long as the placement of disulphide bonds preserved the beta-strand alignment, as in the proposed bioactive conformation, bioactivities were preserved. Knowledge of the bioactive conformation of anginex will aid in the design of smaller molecule mimetics of this potent anti-angiogenic peptide.

Glu-Leu-Arg-negative CXC chemokine interferon gamma inducible protein-9 as a mediator of epidermal-dermal communication during wound repair

Normal wound healing is a complex, highly regulated dynamic process that requires co-ordinate responses of both epidermal and dermal compartments. To accomplish the healing process several growth factors, chemokines, and matrix elements signal both cell proliferation and migration during the inflammatory and reparative phases and limit these responses during the remodeling phase. We have found that the Glu-Leu-Arg-negative CXC chemokines interferon gamma inducible protein 10, monokine induced by interferon gamma, and platelet factor 4, limit fibroblast responsiveness to growth factors, but the functioning of these factors in wound healing remains uncertain. We hypothesized that the keratinocyte-derived member of this Glu-Leu-Arg-negative CXC family, interferon gamma inducible protein 9 (IP-9) CXCL11 (also known as I-TAC, beta-R1, and H-174) signals to the dermal compartment to synchronize the re-epithelialization process. Interferon gamma inducible protein 9 was produced after mechanical wounding of a keratinocyte monolayer, suggesting for the first time that this could be a wound response factor. Interferon gamma inducible protein 9 limited epidermal growth factor (EGF)-induced fibroblast motility (57+/-7%) by the same protein kinase A (KA)-mediated inhibition of calpain activation and cell de-adhesion as described for interferon gamma inducible protein 10. Surprisingly, interferon gamma inducible protein 9 enhanced growth factor-induced motility in undifferentiated keratinocytes (137+/-19%) as determined in a two-dimensional in vitro wound healing assay, and interferon gamma inducible protein 9 alone promoted motility in undifferentiated keratinocytes (49+/-10% of epidermal growth factor-induced motility). A stimulated keratinocyte/target cell coculture system revealed that interferon gamma inducible protein 9 acts as a soluble keratinocyte-derived paracrine factor for both fibroblasts and keratinocytes. Further, we found that in both fibroblasts and undifferentiated keratinocytes, interferon gamma inducible protein 9 exerted its action through modulation of a cytosolic protease, calpain. Interestingly, interferon gamma inducible protein 9 increased calpain activity in undifferentiated keratinocytes, whereas the same chemokine inhibited the calpain activity in fibroblasts. This provides for a model whereby redifferentiated basal keratinocytes could limit fibroblast repopulation of the dermis underlying healed wounds while simultaneously promoting re-epithelialization of the remaining provisional wound.

Anti-tumor activity of the novel angiogenesis inhibitor anginex

Anginex is a novel cytokine-like peptide with potent anti-angiogenic activity, which operates specifically against angiogenically-activated endothelial cells via prevention of cell adhesion/migration on the extracellular matrix and subsequent induction of apoptosis. Here, we demonstrate that anginex inhibits tumor growth in vivo in mouse xenograft models. In the MA148 ovarian carcinoma model, tumor growth was inhibited dose-dependently by up to 80% when systemically administered via osmotic mini-pumps starting at the time of tumor cell inoculation. The optimal dose was found to be 10 mg/kg per day. When tested against established tumors, mini-pump-administered anginex demonstrated essentially the same effectivity at this optimal dose, whereas once or twice-daily injections were only half as effective. When anginex was conjugated to human serum albumin, effectivity was significantly improved, most likely due to increased bioavailability of the conjugate. Immunohistochemical analysis of microvessel density indicated that the anti-tumor activity of anginex is mediated by angiogenesis inhibition. This was confirmed in an in vitro angiogenesis assay based on tube formation in a collagen gel. Animals demonstrated no signs of toxicity as judged by unaltered behavior, normal weight gain, blood markers and macro- and microscopic morphology of internal organs upon autopsy. Overall, these in vivo studies indicate that anginex is an effective anti-tumor agent.

Platelet factor 4 inhibits FGF2-induced endothelial cell proliferation via the extracellular signal-regulated kinase pathway but not by the phosphatidylinositol 3-kinase pathway

Platelet factor 4 (PF-4) is a member of the chemokine family with powerful antiangiogenic properties. The mechanism by which PF-4 inhibits endothelial cell proliferation is unclear. We investigated the effects of PF-4 on the intracellular signal transduction induced by basic fibroblast growth factor (FGF2). We found that PF-4 (10 microg/mL) inhibited the FGF2-induced proliferation of adrenal cortex capillary endothelial (ACE) cells. The inhibition of MEK1/2 (mitogen-activated protein kinase kinase) by PD98059 or of PI3K (phosphatidylinositol 3-kinase) by Ly294002 abolished the proliferation induced by FGF2, suggesting that ACE cell proliferation required dual signaling through both the extracellular signal-regulated kinase (ERK) and PI3K pathways. Ly294002 had no significant effect on ERK phosphorylation, whereas PD98059 had a weak effect on the phosphorylation of Akt, suggesting that 2 separate cascades are required for ACE cell proliferation. The addition of PF-4 (10 microg/mL) significantly inhibited ERK phosphorylation (95%), showing that PF-4 acted directly on or upstream from this kinase. Surprisingly, PF-4 did not affect FGF2-induced Akt phosphorylation. This suggests that PF-4 disrupts FGF2 signaling via an intracellular mechanism of inhibition. To exclude the possibility that PF-4 inhibited the binding of FGF2 to only one FGF receptor, preferentially activating the ERK pathway, we investigated the effect of PF-4 on FGF2-induced ERK and Akt phosphorylation, using mutant heparan sulfate-deficient Chinese hamster ovary cells transfected with the FGF-R1 cDNA. The addition of PF-4 (1 microg/mL) significantly inhibited ERK phosphorylation (90%), with no effect on Akt phosphorylation, suggesting that PF-4 acts downstream from the FGF-R1 receptor. In conclusion, this is the first report showing that PF-4 inhibits FGF2 activity downstream from its receptor.

Inhibition of prostate tumor angiogenesis by the tumor suppressor CEACAM1

We have previously shown that CEACAM1, a cell-adhesion molecule, acts as a tumor suppressor in prostate carcinoma. Expression of CEACAM1 in prostate cancer cells suppresses their growth in vivo. However, CEACAM1 has no effect on the growth of prostate cancer cells in vitro. This difference suggests that the antitumor effect of CEACAM1 may be due to inhibition of tumor angiogenesis, perhaps by increased secretion of antiangiogenic molecules from the cells. In this study, we have demonstrated that expression of CEACAM1 in DU145 prostate cancer cells induced the production of a factor or factors that specifically blocked the growth of endothelial but not epithelial cells. Conditioned medium from the CEACAM1-expressing cells but not control luciferase-expressing cells inhibited endothelial cell migration up a gradient of stimulatory vascular endothelial growth factor in vitro and inhibited corneal neovascularization induced by basic fibroblast growth factor in vivo. Moreover, conditioned medium from CEACAM1-expressing cells induced endothelial cell apoptosis in vitro. Only medium conditioned by CEACAM1 mutants that were able to suppress tumor growth in vivo could cause endothelial cell apoptosis. These observations suggest that CEACAM1-mediated tumor suppression in vivo is, at least in part, due to the ability of CEACAM1 to inhibit tumor angiogenesis.

The CXC chemokine cCAF stimulates precocious deposition of ECM molecules by wound fibroblasts, accelerating development of granulation tissue

BACKGROUND

During wound repair, fibroblasts orchestrate replacement of the provisional matrix formed during clotting with tenascin, cellular fibronectin and collagen III. These, in turn, are critical for migration of endothelial cells, keratinocytes and additional fibroblasts into the wound site. Fibroblasts are also important in the deposition of collagen I during scar formation. The CXC chemokine chicken Chemotactic and Angiogenic Factor (cCAF), is highly expressed by fibroblasts after wounding and during development of the granulation tissue, especially in areas where extracellular matrix (ECM) is abundant. We hypothesized that cCAF stimulates fibroblasts to produce these matrix molecules.

RESULTS

Here we show that this chemokine can stimulate precocious deposition of tenascin, fibronectin and collagen I, but not collagen III. Studies in culture and in vivo show that tenascin stimulation can also be achieved by the N-terminal 15 aas of the protein and occurs at the level of gene expression. In contrast, stimulation of fibronectin and collagen I both require the entire molecule and do not involve changes in gene expression. Fibronectin accumulation appears to be linked to tenascin production, and collagen I to decreased MMP-1 levels. In addition, cCAF is chemotactic for fibroblasts and accelerates their migration.

CONCLUSIONS

These previously unknown functions for chemokines suggest that cCAF, the chicken orthologue of human IL-8, enhances healing by rapidly chemoattracting fibroblasts into the wound site and stimulating them to produce ECM molecules, leading to precocious development of granulation tissue. This acceleration of the repair process may have important application to healing of impaired wounds.

Increased plasma thrombospondin-1 (TSP-1) levels are associated with the TNF alpha-308A allele in children with juvenile dermatomyositis

Vascular occlusion is more frequent in children with juvenile dermatomyositis (JDM) who have the TNF alpha-308A allele. One of the potent anti-angiogenic factors is thrombospondin-1 (TSP-1). This study investigated the association of the TNF alpha-308A allele with circulating levels of angiogenic mediators, TSP-1, and platelet factor 4 (PF4) using fresh, platelet-poor plasma (PPP). The TNF alpha-308A allele was characterized by PCR amplification and NcoI digestion. Concentrations of TSP-1 and PF4 in PPP from 31 JDM patients and 25 matched pediatric controls were determined by ELISA. The majority of the JDM children with the TNF alpha-308A allele (7/12) produced more TSP-1 than their TNF alpha-308G counterparts (P < 0.05), and their TSP-1 values were inversely related to those for PF4 (P < 0.0006). We conclude that the increased circulating concentrations of TSP-1 associated with the TNF alpha-308A allele suggest that this anti-angiogenic regulator may play a significant role in the augmented vascular occlusion observed in JDM children with this genetic marker.

Aging and survival of cutaneous microvasculature

The growth and turnover of blood vessels in the skin is fundamental in normal development, wound repair, hair follicle cycling, tumor cell metastasis, and in many different states of cutaneous pathology. Whereas many investigations are focused on mechanisms of angiogenesis in the skin, the influence of cellular aging and replicative senescence (i.e., the inability, after a critical number of population doublings, to replicate) on microvascular remodeling events has received relatively less attention. In this article, we review the clinical and pathologic relationships associated with cutaneous vascular aging and update current knowledge of endothelial cell survival characteristics. A hypothesis is presented in which endothelial cell aging and survival are linked to molecular mechanisms controlling cell proliferation, quiescence, apoptosis, and cellular senescence. We review recent results demonstrating how activation of telomerase in human dermal microvascular endothelial cells affects their durability both in vitro and in vivo and conclude by linking these studies with current concepts involving endothelial cell precursors, control of postnatal somatic cell telomerase activity, and murine model systems.

Signal transduction of chemokine platelet factor 4 in human erythroleukemia cells

Previous data have demonstrated that CXC-chemokine platelet factor 4 (PF4) inhibits the proliferation of the human erythroleukemia cell line (HEL). However, the mechanism of action is unclear at present. The signaling transduction induced by PF4 in the HEL was compared with that induced by transforming growth factor beta1 (TGF-beta1), which is also a potent inhibitor of HEL growth. It was found that PF4 had no inhibitory effect on intracellular calcium levels in resting HEL cells. When HEL cells were stimulated with interleukin-3 (IL-3), a rapid increase in the intracellular level of free calcium occurred within 15 to 20 seconds, and this increase was followed by a sustained increase that gradually declined until resting levels were reached 30 to 40 minutes later. PF4 dramatically decreased the transient rise of [Ca2+] and protein kinase C (PKC) activity of HEL cells induced by IL-3. However, PF4 had no inhibitory effect on PKC activation in resting HEL cells. Furthermore, PF4 was found to down-regulate significantly protein tyrosine kinase (PTK) activity. In contrast, TGF-beta1 induced an increase in intracellular free calcium concentration and PKC and PTK activity in HEL cells. Furthermore, PF4 significantly increased the messenger RNA (mRNA) level of p21waf1 in HEL cells. These data demonstrate that PF4 acts on HEL cells through a signaling transduction pathway, which is different from that of TGF-beta1 and is related to the up-regulatory mRNA level of p21waf1 in HEL cells.

Eotaxin/CCL11 suppresses IL-8/CXCL8 secretion from human dermal microvascular endothelial cells

The CC chemokine eotaxin/CCL11 is known to bind to the receptor CCR3 on eosinophils and Th2-type lymphocytes. In this study, we demonstrate that CCR3 is expressed on a subpopulation of primary human dermal microvascular endothelial cells and is up-regulated by TNF-alpha. We found that incubation of human dermal microvascular endothelial cells with recombinant eotaxin/CCL11 suppresses TNF-alpha-induced production of the neutrophil-specific chemokine IL-8/CXCL8. The eotaxin/CCL11-suppressive effect on endothelial cells was not seen on IL-1beta-induced IL-8/CXCL8 release. Eotaxin/CCL11 showed no effect on TNF-alpha-induced up-regulation of growth-related oncogene-alpha or IFN-gamma-inducible protein-10, two other CXC chemokines tested, and did not affect production of the CC chemokines monocyte chemoattractant protein-1/CCL2 and RANTES/CCL5, or the adhesion molecules ICAM-1 and E-selectin. These results suggest that eotaxin/CXCL11 is not effecting a general suppression of TNF-alphaR levels or signal transduction. Suppression of IL-8/CXCL8 was abrogated in the presence of anti-CCR3 mAb, pertussis toxin, and wortmannin, indicating it was mediated by the CCR3 receptor, G(i) proteins, and phosphatidylinositol 3-kinase signaling. Eotaxin/CCL11 decreased steady state levels of IL-8/CXCL8 mRNA in TNF-alpha-stimulated cells, an effect mediated in part by an acceleration of IL-8 mRNA decay. Eotaxin/CCL11 may down-regulate production of the neutrophil chemoattractant IL-8/CXCL8 by endothelial cells in vivo, acting as a negative regulator of neutrophil recruitment. This may play an important biological role in the prevention of overzealous inflammatory responses, aiding in the resolution of acute inflammation or transition from neutrophilic to mononuclear/eosinophilic inflammation.

The cxc chemokine cCAF stimulates differentiation of fibroblasts into myofibroblasts and accelerates wound closure

Chemokines are small cytokines primarily known for their roles in inflammation. More recently, however, they have been implicated in processes involved in development of the granulation tissue of wounds, but little is known about their functions during this process. Fibroblasts play key roles in this phase of healing: some fibroblasts differentiate into myofibroblasts, alpha-smooth muscle actin (SMA)-producing cells that are important in wound closure and contraction. Here we show that the CXC chemokine chicken chemotactic and angiogenic factor (cCAF) stimulates fibroblasts to produce high levels of alpha-SMA and to contract collagen gels more effectively than do normal fibroblasts, both characteristic properties of myofibroblasts. Specific inhibition of alpha-SMA expression resulted in abrogation of cCAF-induced contraction. Furthermore, application of cCAF to wounds in vivo increases the number of myofibroblasts present in the granulation tissue and accelerates wound closure and contraction. We also show that these effects in culture and in vivo can be achieved by a peptide containing the NH2-terminal 15 amino acids of the cCAF protein and that inhibition of alpha-SMA expression also results in inhibition of N-peptide-induced collagen gel contraction. We propose that chemokines are major contributors for the differentiation of fibroblasts into myofibroblasts during formation of the repair tissue. Because myofibroblasts are important in many pathological conditions, and because chemokines and their receptors are amenable to pharmacological manipulations, chemokine stimulation of myofibroblast differentiation may have implications for modulation of functions of these cells in vivo.

The chemokine CXCL13 (BCA-1) inhibits FGF-2 effects on endothelial cells

Several chemokines, belonging to both the CXC and CC classes, act as positive or negative regulators of angiogenesis. We sought to investigate the role of CXCL13, B cell-attracting chemokine 1 (BCA-1), also known as B-lymphocyte chemoattractant (BLC), on endothelial cell functions. We tested the effect of CXCL13 on HUVEC chemotaxis and proliferation in the presence of fibroblast growth factor (FGF)-2 and found that such chemokine inhibits FGF-2-induced functions, while is not active by itself. To test whether other FGF-2-mediated biological activities may be affected, we evaluated the ability of CXCL13 to rescue HUVEC from starvation-induced apoptosis, as FGF-2 is a survival factor for endothelial cells, and found that CXCL13 partially inhibits such rescue. Multiple mechanisms may be responsible for these biological activities as CXCL13 displaces FGF-2 binding to endothelial cells, inhibits FGF-2 homodimerization, and induces the formation of CXCL13-FGF-2 heterodimers. Our data suggest that CXCL13 may modulate angiogenesis by interfering with FGF-2 activity.

Solution structure and interaction with basic and acidic fibroblast growth factor of a 3-kDa human platelet factor-4 fragment with antiangiogenic activity

Platelet factor-4 is a protein belonging to the family of ELR-negative CXC chemokines which binds to fibroblast growth factor and inhibits its mitogenic activity. Platelet factor-4 also inhibits tumor growth by mechanisms involving antiangiogenesis. Antiangiogenic activity in vitro has also been shown for the 24-residue C-terminal fragment of the protein, which decreases the affinity between basic fibroblast growth factor and its cell-surface receptor. In this study, the preferential conformation of this fragment in solution has been determined and has been found to be composed of two helical subdomains. In addition, we show that the fragment forms a specific 1:1 complex with acidic and basic fibroblast growth factors and that both subdomains are probably required for inhibition of fibroblast growth factor-driven mitogenesis. Finally, we show that the binding of the fragment alters the structure of the fibroblast growth factors, although some of such alterations do not seem related with the inhibition of mitogenic activity. Since this fragment has recently been shown to inhibit fibroblast growth factor-induced angiogenesis in vivo when injected intraperitoneally, these results are relevant for developing new antiangiogenic treatments.

Identification of a blood-derived chemoattractant for neutrophils and lymphocytes as a novel CC chemokine, Regakine-1

Chemokines constitute a large family of chemotactic cytokines that selectively attract different blood cell types. Although most inflammatory chemoattractants are only induced and released in the circulation during acute infection, a restricted number of CXC and CC chemokines are constitutively present in normal plasma at high concentrations. Here, such a chemotactic protein was purified to homogeneity from serum and fully identified as a novel CC chemokine by mass spectrometry and amino acid sequence analysis. The protein, tentatively designated Regakine-1, shows less than 50% sequence identity with any known chemokine. This novel CC chemokine chemoattracts both neutrophils and lymphocytes but not monocytes or eosinophils. Its modest chemotactic potency but high blood concentration is similar to that of other chemokines present in the circulation, such as hemofiltrate CC chemokine-1, platelet factor-4, and beta-thromboglobulin. Regakine-1 did not induce neutrophil chemokinesis. However, it synergized with the CXC chemokines interleukin-8 and granulocyte chemotactic protein-2, and the CC chemokine monocyte chemotactic protein-3, resulting in an at least a 2-fold increase of the neutrophil and lymphocyte chemotactic response, respectively. The biologic effects of homogeneous natural Regakine-1 were confirmed with chemically synthesized chemokine. Like other plasma chemokines, it is expected that Regakine-1 plays a unique role in the circulation during normal or pathologic conditions.

Anginex, a designed peptide that inhibits angiogenesis

Novel beta-sheet-forming peptide 33-mers, betapep peptides, have been designed by using a combination approach employing basic folding principles and incorporating short sequences from the beta-sheet domains of anti-angiogenic proteins. One of these designed peptides (betapep-25), named anginex, was observed to be potently anti-angiogenic. Anginex specifically inhibits vascular endothelial cell proliferation and induces apoptosis in these cells, as shown by flow-cytometric detection of sub-diploid cells, TUNEL (terminal deoxyribonucleotidyl transferase-mediated dUTP-nick-end labelling) analysis and cell morphology. Anginex also inhibits endothelial cell adhesion to and migration on different extracellular matrix components. Inhibition of angiogenesis in vitro is demonstrated in the sprout-formation assay and in vivo in the chick embryo chorio-allantoic membrane angiogenesis assay. Comparison of active and inactive betapep sequences allows structure-function relationships to be deduced. Five hydrophobic residues and two lysines appear to be crucial to activity. This is the first report of a designed peptide having a well-defined biological function as a novel cytokine, which may be an effective anti-angiogenic agent for therapeutic use against various pathological disorders, such as neoplasia, rheumatoid arthritis, diabetic retinopathy and restenosis.

Cell cycle-dependent expression of CXC chemokine receptor 3 by endothelial cells mediates angiostatic activity

Endothelial cell receptors for the angiostatic chemokines IFN-gamma-inducible protein of 10 kDa (IP-10) and monokine induced by IFN-gamma (Mig) have not yet been identified, and the mechanisms responsible for the effects of these chemokines on angiogenesis are still unclear. IP-10 and Mig share a common functional receptor on activated T lymphocytes, named CXC chemokine receptor 3 (CXCR3). Using in situ hybridization and immunohistochemistry, we show that CXCR3 is expressed by a small percentage of microvascular endothelial cells in several human normal and pathological tissues. Primary cultures of human microvascular endothelial cells (HMVECs) likewise express CXCR3, although this expression is limited to the S/G2-M phase of their cell cycle. Both IP-10 and Mig, as well as the IFN-gamma-inducible T-cell alpha chemoattractant (I-TAC), which all share high-affinity binding for CXCR3, block HMVEC proliferation in vitro, an effect that can be inhibited by an anti-CXCR3 antibody. These data provide definitive evidence of CXCR3 expression by HMVEC and open new avenues for therapeutic interventions in all conditions in which an angiostatic effect may be beneficial.

Chemokine receptors and their role in inflammation and infectious diseases

Chemokines are small peptides that are potent activators and chemoattractants for leukocyte subpopulations and some nonhemopoietic cells. Their actions are mediated by a family of 7-transmembrane G-protein–coupled receptors, the size of which has grown considerably in recent years and now includes 18 members. Chemokine receptor expression on different cell types and their binding and response to specific chemokines are highly variable. Significant advances have been made in understanding the regulation of chemokine receptor expression and the intracellular signaling mechanisms used in bringing about cell activation. Chemokine receptors have also recently been implicated in several disease states including allergy, psoriasis, atherosclerosis, and malaria. However, most fascinating has been the observation that some of these receptors are used by human immunodeficiency virus type 1 in gaining entry into permissive cells. This review will discuss structural and functional aspects of chemokine receptor biology and will consider the roles these receptors play in inflammation and in infectious diseases.

Chemokine receptors and their role in inflammation and infectious diseases

Chemokines are small peptides that are potent activators and chemoattractants for leukocyte subpopulations and some nonhemopoietic cells. Their actions are mediated by a family of 7-transmembrane G-protein–coupled receptors, the size of which has grown considerably in recent years and now includes 18 members. Chemokine receptor expression on different cell types and their binding and response to specific chemokines are highly variable. Significant advances have been made in understanding the regulation of chemokine receptor expression and the intracellular signaling mechanisms used in bringing about cell activation. Chemokine receptors have also recently been implicated in several disease states including allergy, psoriasis, atherosclerosis, and malaria. However, most fascinating has been the observation that some of these receptors are used by human immunodeficiency virus type 1 in gaining entry into permissive cells. This review will discuss structural and functional aspects of chemokine receptor biology and will consider the roles these receptors play in inflammation and in infectious diseases.

Chemokine receptors and their role in vascular biology

Chemokines play an important role in the process of leukocyte recruitment and activation at sites of inflammation. Until recently, the actions of chemokines and the expression of their receptors have only been described on different leukocyte populations. However, increasing evidence has suggested that non-haematopoietic cell types are capable of binding and responding to a number of chemokines. The functional expression of certain chemokine receptors has recently been described on vascular endothelial and smooth muscle cells. These findings provide new insight into the activities of chemokines and indicate that these molecules have a more widespread cellular target than first envisaged. Studies carried out to date indicate that chemokines and their respective receptors play an important role in the regulation of angiogenesis and angiostasis. They may also be involved in developmental and pathological processes such as organ vascularization, embryogenesis and arteriosclerosis.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

Inhibition of In Vitro Angiogenesis by Platelet Factor-4–Derived Peptides and Mechanism of Action

In this study, we examined in detail the interaction of platelet factor-4 (PF-4) with fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF) and the effect of PF-4–derived synthetic peptides. We show that a peptide between amino acids 47 and 70 that contains the heparin-binding lysine-rich site inhibits FGF-2 or VEGF function. This is based on the following observations: PF-4 peptide 47-70 inhibited FGF-2 or VEGF binding to endothelial cells; it inhibited FGF-2 or VEGF binding to FGFRs or VEGFRs in heparan sulfate–deficient CHO cells transfected with FGFR1 (CHOFGFR1) or VEGFR2 (CHOmVEGFR2) cDNA; it blocked proliferation or tube formation in three-dimensional angiogenesis assays; and, finally, it competed with the direct association of 125I-PF-4 with FGF-2 or VEGF, respectively, and inhibited heparin-induced FGF-2 dimerization. A shorter C-terminal peptide (peptide 58-70), which still contained the heparin-binding lysin-rich site, had no effect. Peptide 17-58, which is located in the central part of the molecule, although it does not inhibit FGF-2 or VEGF binding or biologic activity in endothelial cells, inhibited heparin-dependent binding of125I-FGF-2 or 125I-VEGF to CHOmFGFR1 or CHOmVEGFR2 cells, respectively. Shorter peptides (peptides 34-58 and 47-58) did not show any of these effects.

Inhibition of Human Umbilical Vein Endothelial Cell Proliferation by the CXC Chemokine, Platelet Factor 4 (PF4), Is Associated With Impaired Downregulation of p21Cip1/WAF1

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 on epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cell cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E–cyclin-dependent kinase 2 (cdk2) activity with resulting attenuation of retinoblastoma protein phosphorylation. PF4-dependent downregulation of cyclin E-cdk2 activity was associated with increased binding of the cyclin-dependent kinase inhibitor, p21Cip1/WAF1, to the cyclin E-cdk2 complex. Analysis of total cellular p21Cip1/WAF1 showed that in the presence of PF4, p21Cip1/WAF1 levels were sustained at time points when p21Cip1/WAF1 was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21Cip1/WAF1 and provide the first evidence for interference with cell cycle mechanisms by a chemokine.

Inhibition of Human Umbilical Vein Endothelial Cell Proliferation by the CXC Chemokine, Platelet Factor 4 (PF4), Is Associated With Impaired Downregulation of p21Cip1/WAF1

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 on epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cell cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E–cyclin-dependent kinase 2 (cdk2) activity with resulting attenuation of retinoblastoma protein phosphorylation. PF4-dependent downregulation of cyclin E-cdk2 activity was associated with increased binding of the cyclin-dependent kinase inhibitor, p21Cip1/WAF1, to the cyclin E-cdk2 complex. Analysis of total cellular p21Cip1/WAF1 showed that in the presence of PF4, p21Cip1/WAF1 levels were sustained at time points when p21Cip1/WAF1 was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21Cip1/WAF1 and provide the first evidence for interference with cell cycle mechanisms by a chemokine.

Angiogenesis modulates the tumour immune response

Outgrowth of solid tumors and metastases is dependent on the process of angiogenesis. Tumors escape from the formation of an effective infiltrate by downregulation of endothelial adhesion molecules. This downregulation of adhesion receptors is governed by the exposure to angiogenic factors. In recent years proof for this has been provided by demonstrating that freshly isolated tumor endothelial cells exhibit a decreased expression of ICAM-1 and -2 as compared to endothelial cells in normal tissue. In addition, adhesion molecules are downregulated on normal tissue endothelial cells when cultured with angiogenesis stimulators such as basic fibroblast growth factor and vascular endothelial cell growth factor, while under these conditions endothelial cells become less responsive to cytokines such as tumor necrosis factor-alpha with respect to the upregulation of endothelial adhesion molecules. Very recently it has been demonstrated that this harmful endothelial cell anergy can be counteracted by inhibitors of angiogenesis.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

Endothelial cells differentially express functional CXC-chemokine receptor-4 (CXCR-4/fusin) under the control of autocrine activity and exogenous cytokines

Analysis of endothelial cell (EC) chemokine receptor expression by RT-PCR revealed that EC essentially do not express CC-chemokine receptors whereas they express all known CXC-chemokine receptors. Endotheliotropic functions of ligands for CXCR-1, CXCR-2, and CXCR-3 have previously been described. We have consequently performed a detailed analysis of endothelial CXCR-4 expression. CXCR-4 is constitutively expressed by quiescent, resting EC. Cytokine stimulation revealed that bFGF upregulates endothelial CXCR-4 expression, whereas TNF alpha downregulates endothelial CXCR-4 expression. Expression of CXCR-4 mRNA as well as protein is also upregulated in autocrine activated, migrating bovine aortic endothelial cells (BAEC). Furthermore, migrating BAEC preferentially present CXCR-4 on the cell surface as evidenced by cytochemistry and FACS analysis. Lastly, the monospecific CXCR-4 ligand SDF-1 was found to act as a potent inducer of EC chemotaxis. In summary, the data indicate that the CXCR-4/SDF-1 receptor ligand interaction may be an important regulator of activated endothelial cell functions as they occur during vascular remodeling and angiogenesis.

Platelet Factor 4 Modulates Fibroblast Growth Factor 2 (FGF-2) Activity and Inhibits FGF-2 Dimerization

Platelet factor 4 (PF-4) inhibits angiogenesis in vitro and in vivo. The mechanism of inhibition is poorly understood. We have investigated the mechanism of inhibition by examining the interaction of PF-4 and the fibroblast growth factor-2 (FGF-2)/fibroblast growth factor receptor (FGFR) system. PF-4 inhibited the binding of FGF-2 to high-affinity and low-affinity binding sites in murine microvascular endothelial cells (LEII cells) and proliferation. Maximum inhibition of binding to endothelial FGF receptors was observed at PF-4 concentrations between 5 and 10 μg/mL (half maximum inhibition at 0.6 μg/mL), and proliferation was completely inhibited at 2 μg/mL. At this concentration, PF-4 reduced internalization of125I–FGF-2 by threefold and delayed degradation. To gain insight into the mechanism of inhibition, we have analyzed the interaction of PF-4 with FGF-2/FGFR by using mutant heparan sulfate–deficient Chinese hamster ovary (CHO) cells transfected with the FGFR-1 cDNA (CHOm–FGFR-1) and by examining the direct interaction with FGF-2. In the absence of heparin, PF-4 inhibited binding of 125I–FGF-2 to CHOm–FGFR-1 cells in a concentration-dependent manner, although not completely. In the presence of heparin, PF-4 abolished totally the stimulatory effect of heparin. Furthermore, PF-4 complexed to FGF-2 and inhibited endogenous or heparin-induced FGF-2 dimerization. These results indicate that PF-4 interacts with FGF-2 by complex formation, inhibiting FGF-2 dimerization, binding to FGF receptors, and internalization. This mechanism most likely contributes to the antiangiogenic properties of PF-4.

Platelet Factor 4 Modulates Fibroblast Growth Factor 2 (FGF-2) Activity and Inhibits FGF-2 Dimerization

Platelet factor 4 (PF-4) inhibits angiogenesis in vitro and in vivo. The mechanism of inhibition is poorly understood. We have investigated the mechanism of inhibition by examining the interaction of PF-4 and the fibroblast growth factor-2 (FGF-2)/fibroblast growth factor receptor (FGFR) system. PF-4 inhibited the binding of FGF-2 to high-affinity and low-affinity binding sites in murine microvascular endothelial cells (LEII cells) and proliferation. Maximum inhibition of binding to endothelial FGF receptors was observed at PF-4 concentrations between 5 and 10 μg/mL (half maximum inhibition at 0.6 μg/mL), and proliferation was completely inhibited at 2 μg/mL. At this concentration, PF-4 reduced internalization of125I–FGF-2 by threefold and delayed degradation. To gain insight into the mechanism of inhibition, we have analyzed the interaction of PF-4 with FGF-2/FGFR by using mutant heparan sulfate–deficient Chinese hamster ovary (CHO) cells transfected with the FGFR-1 cDNA (CHOm–FGFR-1) and by examining the direct interaction with FGF-2. In the absence of heparin, PF-4 inhibited binding of 125I–FGF-2 to CHOm–FGFR-1 cells in a concentration-dependent manner, although not completely. In the presence of heparin, PF-4 abolished totally the stimulatory effect of heparin. Furthermore, PF-4 complexed to FGF-2 and inhibited endogenous or heparin-induced FGF-2 dimerization. These results indicate that PF-4 interacts with FGF-2 by complex formation, inhibiting FGF-2 dimerization, binding to FGF receptors, and internalization. This mechanism most likely contributes to the antiangiogenic properties of PF-4.

Chemokine receptors in human endothelial cells. Functional expression of CXCR4 and its transcriptional regulation by inflammatory cytokines

Chemokines play an important role in the regulation of endothelial cell (EC) function, including proliferation, migration and differentiation during angiogenesis, and re-endothelialization after injury. In this study, reverse transcriptase-polymerase chain reaction was used to reveal expression of various CXC and CC chemokine receptors in human umbilical vein EC. Northern analysis showed that CXCR4 was selectively expressed in vascular EC, but not in smooth muscle cells. Compared with other chemokines, stromal cell-derived factor-1alpha (SDF-1alpha), the known CXCR4 ligand, was an efficacious chemoattractant for EC, causing the migration of approximately 40% input cells with an EC50 of 10-20 nM. Of the chemokines tested, only SDF-1alpha induced a rapid, though variable mobilization of intracellular Ca2+ in EC. Experiments with actinomycin D demonstrated that CXCR4 transcripts were short-lived, indicating a rapid mRNA turnover. Interferon-gamma (IFN-gamma) caused a pronounced down-regulation of CXCR4 mRNA in a concentration- and time-dependent manner. In a striking functional correlation, IFN-gamma treatment also attenuated the chemotactic response of EC to SDF-1alpha. IL-1beta, tumor necrosis factor-alpha, and lipopolysaccharide produced a time course-dependent biphasic effect on CXCR4 transcription. Expression of CXCR4 in EC is significant, more so as it and several CC chemokine receptors have been shown to serve as fusion co-receptors along with CD4 during human immunodeficiency virus infection. Taken together, these findings provide evidence of chemokine receptor expression in EC and offer an explanation for the action of chemokines like SDF-1alpha on the vascular endothelium.

Identification of chondromodulin I as a novel endothelial cell growth inhibitor. Purification and its localization in the avascular zone of epiphyseal cartilage

Cartilage is unique among tissues of mesenchymal origin in that it is resistant to vascular invasion due to an intrinsic angiogenic inhibitor. During endochondral bone formation, however, calcified cartilage formed in the center of the cartilaginous bone rudiment allows vascular invasion, which initiates the replacement of cartilage by bone. The transition of cartilage from the angioresistant to the angiogenic status thus plays a key role in bone formation. However, the molecular basis of this phenotypic transition of cartilage has been obscure. We report here purification of an endothelial cell growth inhibitor from a guanidine extract of bovine epiphyseal cartilage. The N-terminal amino acid sequence indicated that the inhibitor was identical to chondromodulin I (ChM-I), a cartilage-specific growth-modulating factor. Purified ChM-I inhibited DNA synthesis and proliferation of vascular endothelial cells as well as tube morphogenesis in vitro. Expression of ChM-I cDNA in COS7 cells indicated that mature ChM-I molecules were secreted from the cells after post-translational modifications and cleavage from the transmembrane precursor at the predicted processing signal. Recombinant ChM-I stimulated DNA synthesis and proteoglycan synthesis of cultured growth plate chondrocytes, but inhibited tube morphogenesis of endothelial cells. In situ hybridization and immunohistochemical studies indicated that ChM-I is specifically expressed in the avascular zone of cartilage in developing bone, but not present in calcifying cartilage. These results suggest a regulatory role of ChM-I in vascular invasion during endochondral bone formation.

Pieces of eight: bioactive fragments of extracellular proteins as regulators of angiogenesis

Specific stages of angiogenesis are regulated by extracellular proteins. This review discusses the endogenous proteolysis of eight of these proteins and the release of polypeptide fragments that have biological activities different from those of the native, parent protein. The generation of natural cleavage products could provide a precise mechanism for the regulation of angiogenesis. Although further experimental confirmation of this mechanism is needed, this leitmotif offers an attractive explanation, in part, for the complex role of proteolysis in vascular morphogenesis.

Platelet Factor 4 and Other CXC Chemokines Support the Survival of Normal Hematopoietic Cells and Reduce the Chemosensitivity of Cells to Cytotoxic Agents

The effects of platelet factor 4 (PF4) on the viability and chemosensitivity of normal hematopoietic cells and cancer cell lines were studied to determine the mechanisms whereby PF4 functions as either an inhibitor or a protector and to evaluate its clinical significance. Two other chemokines, interleukin-8 (IL-8) and neutrophil-activating peptide-2 (NAP-2), were also studied in comparison to PF4. Using a tetrazolium salt assay for cell viability, we observed that PF4 at 1 to 50 μg/mL supported the viability of normal human bone marrow cells. Approximately 45% of cells cultured for 48 hours survived, whereas 80% or more survived in the presence of PF4 5 μg/mL. PF4 also supported the viability of CD34+ cord blood (CB) cells and protected them from apoptosis induced by transforming growth factor β1 (TGFβ1) and cytotoxic drugs. Pretreatment of CD34+ cells by PF4, but not by TGFβ1, caused an increase in the number of megakaryocyte colonies after these cells were replated in secondary cultures. Flow cytometry analysis showed that when CD34+ cells were preincubated with PF4 or TGFβ1 for 12 days in hematopoietic growth factor–rich medium, an increased number of remaining CD34+ cells was observed only for PF4-treated cells. Furthermore, PF4 significantly reduced the chemosensitivity of bone marrow cells, as shown by its ability to increase the 50% inhibition concentration (IC50) of several cytotoxic agents. Like PF4, IL-8 and NAP-2 at 0.1, 0.6, and 1 μg/mL supported the survival of myeloid progenitors, including colony-forming units granulocyte, erythroblast, monocyte, megakaryocyte (CFU-GEMM), CFU-megakaryocyte (CFU-MK), CFU–granulocyte/macrophage (CFU-GM), and burst-forming units–erythroblast (BFU-E), and reduced their sensitivity to the toxicity of etoposide (ETP). Protamine sulfate at 1 to 100 μg/mL showed no such activity of PF4. Interestingly, the three chemokines failed to affect significantly the viability and chemosensitivity of three leukemic and two other tumor cell lines. Based on these results, we conclude for the first time that PF4 and IL-8 and NAP-2 support the survival of normal hematopoietic precursors and protect them from the toxicity of chemotherapeutic agents. Because such activities are unique to normal hematopoietic cells but not to the cancer cell lines evaluated, a potential clinical application of these molecules in the treatment of cancer is suggested.

Uncoupling of stem cell inhibition from monocyte chemoattraction in MIP-1alpha by mutagenesis of the proteoglycan binding site

We have studied the role of proteoglycans in the function of Macrophage Inflammatory Protein-1 alpha (MIP-1alpha), a member of the proteoglycan binding chemokine family. Sequence and peptide analysis has identified a basic region within MIP-1alpha which appears to be the major determinant of proteoglycan binding and we have now produced a mutant of MIP-1alpha lacking the basic charges on two of the amino acids within this proteoglycan binding site. This mutant (Hep Mut) appears to have lost the ability to bind to proteoglycans. Bioassay of Hep Mut indicates that it has retained stem cell inhibitory properties but has a compromised activity as a monocyte chemoattractant, thus suggesting uncoupling of these two properties of MIP-1alpha. Receptor studies have indicated that the inactivity of Hep Mut on human monocytes correlates with its inability to bind to CCR1, a cloned human MIP-1alpha receptor. In addition, studies using proteoglycan deficient cells transfected with CCR1 have indicated that the proteoglycan binding site in MIP-1alpha is a site that is also involved in the docking of MIP-1alpha to the monocyte receptor. The site for interaction with the stem cell receptor must therefore be distinct, suggesting that MIP-1alpha utilizes different receptors for these two different biological processes.

Inhibition of endothelial cell growth by macrophage-like U-937 cell-derived oncostatin M, leukemia inhibitory factor, and transforming growth factor beta1

Conditioned media were collected from phorbol ester-treated human macrophage-like U-937 cells and analyzed for the presence of inhibitors of endothelial cell (EC) proliferation. By a combination of ion exchange and reverse-phase liquid chromatography, three inhibitors were purified to homogeneity as ascertained by microsequencing of 14-17 N-terminal amino acids. These inhibitors were identified as oncostatin M (OSM), leukemia inhibitory factor (LIF), and transforming growth factor beta1 (TGF-beta1). The identities of the three EC growth inhibitors were confirmed by demonstrating that recombinant human OSM, LIF, and TGF-beta1 were inhibitory in the same concentration range. Inhibition of EC proliferation by OSM was a newly described property of this cytokine. OSM was the most potent inhibitor with a half-maximal inhibition by recombinant material of 0.15-.2 ng/ml compared with 0.6-0.9 and 0. 9-1.0 ng/ml for LIF and TGF-beta1, respectively. The three factors inhibited basal, vascular endothelial cell growth factor-stimulated, and fibroblast growth factor 2-stimulated EC proliferation. Interleukin-6 and ciliary neurotrophic factor, two cytokines related structurally to OSM and LIF, were not active as EC growth inhibitors. It was concluded that macrophage-like cells secrete a variety of potent EC growth inhibitors and that one of these, OSM, is among the most potent EC growth inhibitors yet reported.

A potent inhibitor of endothelial cell proliferation is generated by proteolytic cleavage of the chemokine platelet factor 4

Platelet factor 4 (PF-4) is an archetype of the "chemokine" family of low molecular weight proteins that play an important role in injury responses and inflammation. From activated human leukocyte culture supernatants, we have isolated a form of PF-4 that acts as a potent inhibitor of endothelial cell proliferation. The PF-4 derivative is generated by peptide bond cleavage between Thr-16 and Ser-17, a site located downstream from the highly conserved and structurally important CXC motif. The unique cleavage leads to a loss of one of the structurally important large loops in the PF-4 molecule and generation of an N terminus with basic residues that have the potential to interact with the acidic extracellular domain of the G-protein-coupled chemokine receptor. The N-terminal processed PF-4 exhibited a 30- to 50-fold greater growth inhibitory activity on endothelial cells than PF-4. Since endothelial cell growth inhibition is the only known cellular activity of the cleaved PF-4, we have designated this chemokine endothelial cell growth inhibitor. The N-terminal processing of PF-4 may represent an important mechanism for modulating PF-4 activity on endothelial cells during tissue injury, inflammation, and neoplasia.

Angiogenesis: mechanistic insights, neovascular diseases, and therapeutic prospects

This review of angiogenesis aims to describe (a) stimuli that either elicit or antagonize angiogenesis, (b) the response of the vasculature to angiogenic or anti-angiogenic stimuli, i.e., processes required for the formation of new vessels, (c) aspects of angiogenesis relating to tissue remodeling and disease, and (d) the potential of angiogenic or antiangiogenic therapeutic measures. Angiogenesis, the formation of new vessels from existing microvessels, is important in embryogenesis, wound healing, diabetic retinopathy, tumor growth, and other diseases. Hypoxia and other as yet ill-defined stimuli drive tumor, inflammatory, and connective tissue cells to generate angiogenic molecules such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), transforming growth factor-beta (TGF-beta), platelet-derived growth factor (PDGF), and others. Natural and synthetic angiogenesis inhibitors such as angiostatin and thalidomide can repress angiogenesis. Angiogenic and antiangiogenic molecules control the formation of new vessels via different mechanisms. VEGF and FGF elicit their effects mainly via direct action on relevant endothelial cells. TGF-beta and PDGF can attract inflammatory or connective tissue cells which in turn control angiogenesis. Additionally, PDGF may act differently on specific phenotypes of endothelial cells that are engaged in angiogenesis or that are of microvascular origin. Thus phenotypic traits of endothelial cells committed to angiogenesis may determine their cellular responses to given stimuli. Processes necessary for new vessel formation and regulated by angiogenic/antiangiogenic molecules include the migration and proliferation of endothelial cells from the microvasculature, the controlled expression of proteolytic enzymes, the breakdown and reassembly of extracellular matrix, and the morphogenic process of endothelial tube formation. In animal models some angiogenesis-dependent diseases can be controlled via induction or inhibition of new vessel formation. Life-threatening infantile hemangiomas are a first established indication for antiangiogenic therapy in humans. Treatment of other diseases by modulation of angiogenesis are currently tested in clinical trials. Thus the manipulation of new vessel formation in angiogenesis-dependent conditions such as wound healing, inflammatory diseases, ischemic heart and peripheral vascular disease, myocardial infarction, diabetic retinopathy, and cancer is likely to create new therapeutic options.

Platelet factor-4 inhibits the mitogenic activity of VEGF121 and VEGF165 using several concurrent mechanisms

The 121-amino acid form of vascular endothelial growth factor (VEGF121) and the 165-amino acid form (VEGF165) are mitogenic for vascular endothelial cells and induce angiogenesis in vivo. VEGF165 possesses a heparin binding ability and in the absence of heparin-like molecules does not bind efficiently to the VEGF receptors of vascular endothelial cells. The binding of 125I-VEGF165 to the VEGF receptors of endothelial cells, and the heparin-dependent binding of 125I-VEGF165 to a soluble extracellular domain of the VEGF receptor KDR/flk-1, were inhibited by the angiogenesis inhibitor platelet factor-4 (PF4). In contrast, PF4 was not able to inhibit the binding of VEGF121, a VEGF isoform which lacks a heparin binding capacity, to the VEGF receptors of the cells or to KDR/flk-1. These results indicate that PF4 may inhibit VEGF165 binding to VEGF receptors by disrupting the interaction of VEGF165 with cell surface heparan sulfates. Since PF4 mutants lacking a heparin binding ability retain their anti-angiogenic activity, alternative inhibitory mechanisms were also examined. 125I-PF4 bound with high affinity (Kd 5 x 10(-9) M) to VEGF165-coated wells. The binding of 125I-PF4 to the VEGF165-coated wells was inhibited by several types of heparin binding proteins, including unlabeled PF4 and unlabeled VEGF165. The binding was not inhibited by proteins which lack a heparin binding capacity, nor was it inhibited by VEGF121. Heparinase did not inhibit the binding of 125I-PF4 to VEGF165, indicating that heparin-like molecules are not required. These experiments suggest that PF4 can bind to heparin binding proteins such as VEGF165 leading to an inhibition of their receptor binding ability. In agreement with these results, we have observed that PF4 inhibits efficiently the VEGF165 induced proliferation of vascular endothelial cells. Unexpectedly, PF4 also inhibited efficiently the VEGF121-induced proliferation of the cells, indicating that PF4 can disrupt VEGF receptor mediated signal transduction using an unknown mechanism which does not interfere with VEGF121 binding.