Disulfide and amide-bridged cyclic peptide analogues of the VEGF₈₁₋₉₁ fragment: synthesis, conformational analysis and biological evaluation

The design, synthesis, conformational studies and binding affinity for VEGFR-1 receptors of a collection of linear and cyclic peptide analogues of the β-hairpin fragment VEGF(81-91) are described. Cyclic 11-mer peptide derivatives were prepared from linear precursors with conveniently located Cys, Asp or Dap residues, by the formation of disulfide and amide bridges, using solid-phase synthesis. Molecular modelling studies indicated a tendency to be structured around the central β-turn of the VEGF(81-91) β-hairpin in most synthesized cyclic compounds. This structural behavior was confirmed by NMR conformational analysis. The NHCO cyclic derivative 7 showed significant affinity for VEGFR-1, slightly higher than the native linear fragment, thus supporting the design of mimics of this fragment as a valid approach to disrupt the VEGF/VEGFR-1 interaction.     

Engineered conformation-dependent VEGF peptide mimics are effective in inhibiting VEGF signaling pathways

Angiogenesis, or formation of new blood vessels, is crucial to cancer tumor growth. Tumor growth, progression, and metastasis are critically influenced by the production of the pro-angiogenic vascular endothelial growth factor (VEGF). Promising anti-angiogenic drugs are currently available; however, their susceptibilities to drug resistance and long term toxicity are serious impediments to their use, thus requiring the development of new therapeutic approaches for safe and effective angiogenic inhibitors. In this work, peptides were designed to mimic the VEGF-binding site to its receptor VEGFR-2. The VEGF conformational peptide mimic, VEGF-P3(CYC), included two artificial cysteine residues, which upon cyclization constrained the peptide in a loop native-like conformation to better mimic the anti-parallel structure of VEGF. The engineered cyclic VEGF mimic peptide demonstrated the highest affinity to VEGFR-2 by surface plasmon resonance assay. The VEGF peptide mimics were evaluated as inhibitors in several in vitro assays in which VEGF-dependent signaling pathways were observed. All VEGF mimics inhibited VEGFR-2 phosphorylation with VEGF-P3(CYC) showing the highest inhibitory effects when compared with unstructured peptides. Additionally, we show in several angiogenic in vitro assays that all the VEGF mimics inhibited endothelial cell proliferation, migration, and network formation with the conformational VEGF-P3 (CYC) being the best. The VEGF-P3(CYC) also caused a significant delay in tumor development in a transgenic model of VEGF(+/-)Neu2-5(+/-). These results indicate that the structure-based design is important for the development of this peptidomimetic and for its anti-angiogenic effects.     

Identification of a peptide blocking vascular endothelial growth factor (VEGF)-mediated angiogenesis

Vascular endothelial growth factor (VEGF) binding to the kinase domain receptor (KDR/FLK1 or VEGFR-2) mediates vascularization and tumor-induced angiogenesis. Since there is evidence that KDR plays an important role in tumor angiogenesis, we sought to identify peptides able to block the VEGF-KDR interaction. A phage epitope library was screened by affinity for membrane-expressed KDR or for an anti-VEGF neutralizing monoclonal antibody. Both strategies led to the isolation of peptides binding KDR specifically, but those isolated by KDR binding tended to display lower reactivities. Of the synthetic peptides corresponding to selected clones tested to determine their inhibitory activity, ATWLPPR completely abolished VEGF binding to cell-displayed KDR. In vitro, this effect led to the inhibition of the VEGF-mediated proliferation of human vascular endothelial cells, in a dose-dependent and endothelial cell type-specific manner. Moreover, in vivo, ATWLPPR totally abolished VEGF-induced angiogenesis in a rabbit corneal model. Taken together, these data demonstrate that ATWLPPR is an effective antagonist of VEGF binding, and suggest that this peptide may be a potent inhibitor of tumor angiogenesis and metastasis.     

Design of the Y1-receptor-selective cyclic peptide based on the C-terminal sequence of neuropeptide Y.

We designed four cyclic peptides which are mimics of the C-terminal region of human neuropeptide Y (NPY) on the basis of the structural model of NPY. One of these cyclic peptides, c[D-Cys29-L-Cys34]NPY Ac-29-36 (YM-42454), exhibited significantly higher affinity for the Y1-receptor than the corresponding C-terminal linear fragment, NPY Ac-28-36. Interestingly, YM-42454 showed binding affinity for the Y1-receptor in spite of the lack of the N-terminal sequence of NPY, whereas it did not show any binding affinity for the Y2-receptor. This conformationally restricted Y1-selective peptide would provide some insights into the bioactive conformation of the C-terminal region of NPY.     

Vascular endothelial growth factor receptor-2 and neuropilin-1 form a receptor complex that is responsible for the differential signaling potency of VEGF(165) and VEGF(121)

The two most abundant secreted isoforms of vascular endothelial growth factor A (VEGF(165) and VEGF(121)) are formed as a result of differential splicing of the VEGF-A gene. VEGF(165) and VEGF(121) share similar affinities at the isolated VEGF receptor (VEGFR)-2 but have been previously demonstrated to have differential ability to activate VEGFR-2-mediated effects on endothelial cells. Herein we investigate whether the recently described VEGF(165) isoform-specific receptor neuropilin-1 (Npn-1) is responsible for the difference in potency observed for these ligands. We demonstrate that although VEGFR-2 and Npn-1 form a complex, this complex does not result in an increase in VEGF(165) binding affinity. Therefore, the differential activity of VEGF(165) and VEGF(121) cannot be explained by a differential binding affinity for the complex. Using an antagonist that competes for VEGF(165) binding at the VEGFR-2.Npn-1 complex, we observe specific antagonism of VEGF(165)-meditated phosphorylation of VEGFR-2 without affecting the VEGF(121) response. These data indicate that the formation of the complex is responsible for the increased potency of VEGF(165) versus VEGF(121). Taken together, these data suggest a receptor-clustering role for Npn-1, as opposed to Npn-1 behaving as an affinity-converting subunit.     

Low molecular weight fucoidan increases VEGF165-induced endothelial cell migration by enhancing VEGF165 binding to VEGFR-2 and NRP1

Therapeutic induction of angiogenesis is a potential treatment for chronic ischemia. Heparan sulfate proteoglycans are known to play an important role by their interactions with proangiogenic growth factors such as vascular endothelial growth factor (VEGF). Low molecular weight fucoidan (LMWF), a sulfated polysaccharide from brown seaweeds that mimic some biological activities of heparin, has been shown recently to promote revascularization in rat critical hindlimb ischemia. In this report, we first used cultured human endothelial cells (ECs) to investigate the possible ability of LMWF to enhance the actions of VEGF(165). Data showed that LMWF greatly enhances EC tube formation in growth factor reduced matrigel. LMWF is a strong enhancer of VEGF(165)-induced EC chemotaxis, but not proliferation. In addition, LMWF has no effect on VEGF(121)-induced EC migration, a VEGF isoform that does not bind to heparan sulfate proteoglycans. Then, with binding studies using (125)I-VEGF(165), we observed that LMWF enhances the binding of VEGF(165) to recombinant VEGFR-2 and Neuropilin-1 (NRP1), but not to VEGFR-1. Surface plasmon resonance analysis showed that LMWF binds with high affinity to VEGF(165) (1.2 nm) and its receptors (5-20 nm), but not to VEGF(121). Pre-injection of LMWF on immobilized receptors shows that VEGF(165) has the highest affinity for VEGFR-2 and NRP1, as compared with VEGFR-1. Overall, the effects of LMWF were much more pronounced than those of LMW heparin. These findings suggested an efficient mechanism of action of LMWF by promoting VEGF(165) binding to VEGFR-2 and NRP1 on ECs that could help in stimulating therapeutic revascularization.     

A mutant form of vascular endothelial growth factor (VEGF) that lacks VEGF receptor-2 activation retains the ability to induce vascular permeability

Vascular endothelial growth factor (VEGF) is a major mediator of vasculogenesis and angiogenesis both during development and in pathological conditions. VEGF has a variety of effects on vascular endothelium, including the ability to stimulate endothelial cell mitogenesis, and the potent induction of vascular permeability. These activities are at least in part mediated by binding to two high affinity receptors, VEGFR-1 and VEGFR-2. In this study we have made mutations of mouse VEGF in order to define the regions that are required for VEGFR-2-mediated functions. Development of a bioassay, which responds only to signals generated by cross-linking of VEGFR-2, has allowed evaluation of these mutants for their ability to activate VEGFR-2. One mutant (VEGF0), which had amino acids 83-89 of VEGF substituted with the analogous region of the related placenta growth factor, demonstrated significantly reduced VEGFR-2 binding compared with wild type VEGF, indicating that this region was required for VEGF-VEGFR-2 interaction. Intriguingly, when this mutant was evaluated in a Miles assay for its ability to induce vascular permeability, no difference was found when compared with wild type VEGF. In addition we have shown that the VEGF homology domain of the structurally related growth factor VEGF-D is capable of binding to and activating VEGFR-2 but has no vascular permeability activity, indicating that VEGFR-2 binding does not correlate with permeability activity for all VEGF family members. These data suggest different mechanisms for VEGF-mediated mitogenesis and vascular permeability and raise the possibility of an alternative receptor mediating vascular permeability.     

The effect of cyclization of magainin 2 and melittin analogues on structure, function, and model membrane interactions: implication to their mode of action

The amphipathic alpha-helical structure is a common motif found in membrane binding polypeptides including cell lytic peptides, antimicrobial peptides, hormones, and signal sequences. Numerous studies have been undertaken to understand the driving forces for partitioning of amphipathic alpha-helical peptides into membranes, many of them based on the antimicrobial peptide magainin 2 and the non-cell-selective cytolytic peptide melittin, as paradigms. These studies emphasized the role of linearity in their mode of action. Here we synthesized and compared the structure, biological function, and interaction with model membranes of linear and cyclic analogues of these peptides. Cyclization altered the binding of melittin and magainin analogues to phospholipid membranes. However, at similar bound peptide:lipid molar ratios, both linear and cyclic analogues preserved their high potency to permeate membranes. Furthermore, the cyclic analogues preserved approximately 75% of the helical structure of the linear peptides when bound to membranes. Biological activity studies revealed that the cyclic melittin analogue had increased antibacterial activity but decreased hemolytic activity, whereas the cyclic magainin 2 analogue had a marked decrease in both antibacterial and hemolytic activities. The results indicate that the linearity of the peptides is not essential for the disruption of the target phospholipid membrane, but rather provides the means to reach it. In addition, interfering with the coil-helix transition by cyclization, while maintaining the same sequence of hydrophobic and positively charged amino acids, allows a separated evaluation of the hydrophobic and electrostatic contributions to binding of peptides to membranes.     

Synthesis and biological evaluation of cyclic and branched peptide analogues as ligands for cholecystokinin type 1 receptor

A library of cyclic CCK8 analogues, containing unnatural amino acids in the peptide sequence, is prepared using solid-phase synthesis. The structure of these cyclic peptides is based on a previously synthesised compound, cyclo-CCK8, selective for CCK(1) receptor. Structure-activity investigations are performed by evaluating the binding properties of the new analogues. In particular, the binding ability of the cyclic CCK8 analogues is tested by nuclear medicine studies on cell line transfected with CCK(1) receptor. Compounds named cyclo-A4-cyclo-A7 show binding constant in the range 6.0-8.0 microM, with an improved affinity over the previous described cyclo-CCK8, but almost comparable IC(50) values among new analogues towards CCK(1) were obtained.     

Effects of CD4 synthetic peptides on HIV type I envelope glycoprotein function.

Benzylated derivatives of a peptide (CD4(81-92)) representing the CDR3-like region of CD4 were previously found to inhibit gp120 binding, HIV-1 infectivity, and syncytium formation. These results have been interpreted to indicate a role for the corresponding CD4 region in these processes. The peptide (TbYICbEbVEDQKAcEE) is the prototype of a series of similar CD4(81-92) derivatives. We report that this peptide noncompetitively inhibits binding to CD4 of both gp120 and a mAb (MAX.16H5), both of which recognize the CDR2-like region of CD4. The binding of an antibody (Leu 3a) that is directed against a different area of the D1 domain of CD4 was also inhibited. The peptide derivative inhibited both HIV-1- and HTLV-1-mediated syncytium formation in the same concentration range. Nonbenzylated cyclic and linear peptides representing the CDR3-like region of CD4 (CD4(84-101)) had only minor effects on gp120 binding which were not sequence specific. The results of this study suggest that the effects of benzylated CD4(81-92) derivatives on HIV-1 binding or fusion should not be used to reach conclusions about the function of the corresponding CD4 region.     

A novel snake venom vascular endothelial growth factor (VEGF) predominantly induces vascular permeability through preferential signaling via VEGF receptor-1

Vascular endothelial growth factor (VEGF)/vascular permeability factor induces both angiogenesis and vascular permeability mainly through VEGF receptor (VEGFR)-2 activation. VEGF binds VEGFR-1 as well, but the importance of VEGFR-1 signaling in vascular permeability has been largely neglected. Here, we report the purification and characterization of a novel VEGF-like protein from Trimeresurus flavoviridis Habu snake venom. The Habu snake has a venom-specific VEGF-like molecule, T. flavoviridis snake venom VEGF (TfsvVEGF), in addition to VEGF-A. TfsvVEGF has almost 10-fold less mitotic activity than VEGF(165), a predominant isoform of human VEGF-A, but a similar effect on vascular permeability. TfsvVEGF bound VEGFR-1 and induced its autophosphorylation to almost the same extent as VEGF(165), but bound VEGFR-2 weakly and induced its autophosphorylation almost 10-fold less effectively than VEGF(165). This unique binding affinity for VEGFR-1 and VEGFR-2 leads to the vascular permeability-dominant activity of TfsvVEGF. These results suggest that Habu snakes have acquired a highly purposive molecule for a toxin, which enhances the toxicity in envenomation without inducing effective angiogenesis and the following regeneration of damaged tissues, taking advantage of the difference in signaling properties involving VEGFR-1 and VEGFR-2 between vascular permeability and angiogenesis. TfsvVEGF is thus a potent inducing factor selective for vascular permeability through preferential signaling via VEGFR-1. These data strongly indicate the importance of VEGFR-1 signaling in vascular permeability.     

Monoclonal antibodies to vascular endothelial growth factor-D block its interactions with both VEGF receptor-2 and VEGF receptor-3.

Vascular endothelial growth factor-D (VEGF-D), the most recently discovered mammalian member of the VEGF family, is an angiogenic protein that activates VEGF receptor-2 (VEGFR-2/Flk1/KDR) and VEGFR-3 (Flt4). These receptor tyrosine kinases, localized on vascular and lymphatic endothelial cells, signal for angiogenesis and lymphangiogenesis. VEGF-D consists of a central receptor-binding VEGF homology domain (VHD) and N-terminal and C-terminal propeptides that are cleaved from the VHD to generate a mature, bioactive form consisting of dimers of the VHD. Here we report characterization of mAbs raised to the VHD of human VEGF-D in order to generate VEGF-D antagonists. The mAbs bind the fully processed VHD with high affinity and also bind unprocessed VEGF-D. We demonstrate, using bioassays for the binding and cross-linking of VEGFR-2 and VEGFR-3 and biosensor analysis with immobilized receptors, that one of the mAbs, designated VD1, is able to compete potently with mature VEGF-D for binding to both VEGFR-2 and VEGFR-3 for binding to mature VEGF-D. This indicates that the binding epitopes on VEGF-D for these two receptors may be in close proximity. Furthermore, VD1 blocks the mitogenic response of human microvascular endothelial cells to VEGF-D. The anti-(VEGF-D) mAbs raised to the bioactive region of this growth factor will be powerful tools for analysis of the biological functions of VEGF-D.     

N‐terminal guanidinylation of the cyclic 1,4‐ureido‐deltorphin analogues: the synthesis,receptor binding studies,and resistance to proteolytic digestion

The synthesis of a series of N‐guanidinylated cyclic ureidopeptides, analogues of 1,4‐ureido‐deltorphin/dermorphine tetrapeptide is described. The δ‐ and μ‐opioid receptor affinity of new guanidinylated analogues and their non‐guanidinylated precursors was determined by the displacement radioligand binding experiments. Our results indicate that the guanidinylation of cyclic 1,4‐ureidodeltorphin peptide analogues does not exhibit a uniform influence on the opioid receptor binding properties, similarly as reported earlier for some linear peptides. All analogues were also tested for their in vitro resistance to proteolysis during incubation with large excess of chymotrypsin, pepsin, and papain by means of mass spectroscopy. Guanidinylated ureidopeptides 1G–4G showed mixed μ agonist/δ agonist properties and high enzymatic stability indicating their potential as therapeutic agents for treatment of pain. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Short PlGF‐derived peptides bind VEGFR‐1 and VEGFR‐2 in vitro and on the surface of endothelial cells

The placental growth factor (PlGF), a member of VEGF family, plays a crucial role in pathological angiogenesis, especially ischemia, inflammation, and cancer. This activity is mediated by its selective binding to VEGF receptor 1 (VEGFR‐1), which occurs predominantly through receptor domains 2 and 3. The PlGF β‐hairpin region spanning residues Q87 to V100 is one of the key binding elements on the protein side. We have undertaken a study on the design, preparation, and functional characterization of the peptide reproducing this region and of a set of analogues where glycine 94, occurring at the corner of the hairpin in the native protein, is replaced by charged as well as hydrophobic residues. Also, some peptides with arginine 96 replaced by other residues have been studied. We find that the parent peptide weakly binds VEGFR‐1, but replacement of G94 with residues bearing H‐bond donating residues significantly improves the affinity. Replacement of R96 instead blocks the interaction between the peptide and the domain. The strongest affinity is observed with the G94H (peptide PlGF‐2) and G94W (peptide PlGF‐10) mutants, while the peptide PlGF‐8, bearing the R96G mutation, is totally inactive. The PlGF‐1 and PlGF‐2 peptides also bind the VEGFR‐2 receptors, though with a reduced affinity, and are able to interfere with the VEGF‐induced receptor signaling on endothelial cells. The peptides also bind VEGFR‐2 on the surface of cells, while PlGF‐8 is inactive. Data suggest that these peptides have potential applications as PlGF/VEGF mimic in various experimental settings.     

Vascular endothelial growth factor (VEGF)/VEGF-C mosaic molecules reveal specificity determinants and feature novel receptor binding patterns

Vascular endothelial growth factors (VEGFs) and their receptors play key roles in angiogenesis and lymphangiogenesis. VEGF activates VEGF receptor-1 (VEGFR-1) and VEGFR-2, whereas VEGF-C activates VEGFR-2 and VEGFR-3. We have created a library of VEGF/VEGF-C mosaic molecules that contains factors with novel receptor binding profiles, notably proteins binding to all three VEGF receptors ("super-VEGFs"). The analyzed super-VEGFs show both angiogenic and lymphangiogenic effects in vivo, although weaker than the parental molecules. The composition of the VEGFR-3 binding molecules and scanning mutagenesis revealed determinants of receptor binding and specificity. VEGFR-2 and VEGFR-3 showed striking differences in their requirements for VEGF-C binding; extracellular domain 2 of VEGFR-2 was sufficient, whereas in VEGFR-3, both domains 1 and 2 were necessary.     

Tuftsin binds neuropilin-1 through a sequence similar to that encoded by exon 8 of vascular endothelial growth factor

Tuftsin, Thr-Lys-Pro-Arg (TKPR), is an immunostimulatory peptide with reported nervous system effects as well. We unexpectedly found that tuftsin and a higher affinity antagonist, TKPPR, bind selectively to neuropilin-1 and block vascular endothelial growth factor (VEGF) binding to that receptor. Dimeric and tetrameric forms of TKPPR had greatly increased affinity for neuropilin-1 based on competition binding experiments. On endothelial cells tetrameric TKPPR inhibited the VEGF(165)-induced autophosphorylation of vascular endothelial growth factor receptor-2 (VEGFR-2) even though it did not directly inhibit VEGF binding to VEGFR-2. Homology between exon 8 of VEGF and TKPPR suggests that the sequence coded for by exon 8 may stabilize VEGF binding to neuropilin-1 to facilitate signaling through VEGFR-2. Given the overlap between processes involving neuropilin-1 and tuftsin, we propose that at least some of the previously reported effects of tuftsin are mediated through neuropilin-1.     

The C-terminus of viral vascular endothelial growth factor-E partially blocks binding to VEGF receptor-1

Vascular endothelial growth factor (VEGF) family members play important roles in embryonic development and angiogenesis during wound healing and in pathological conditions such as tumor formation. Parapoxviruses express a new member of the VEGF family which is a functional mitogen that specifically activates VEGF receptor (VEGFR)-2 but not VEGFR-1. In this study, we show that deletion from the viral VEGF of a unique C-terminal region increases both VEGFR-1 binding and VEGFR-1-mediated monocyte migration. Enzymatic removal of O-linked glycosylation from the C-terminus also increased VEGFR-1 binding and migration of THP-1 monocytes indicating that both the C-terminal residues and O-linked sugars contribute to blocking viral VEGF binding to VEGFR-1. The data suggest that conservation of the C-terminal residues throughout the viral VEGF subfamily may represent a means of reducing the immunostimulatory activities associated with VEGFR-1 activation while maintaining the ability to induce angiogenesis via VEGFR-2.     

Peptide ligands can bind to distinct sites in integrin alphaIIbbeta3 and elicit different functional responses.

The spatial relationship between the binding sites for two cyclic peptides, cyclo(S,S)KYGCRGDWPC (cRGD) and cyclo(S,S)KYGCHarGDWPC (cHarGD), high affinity analogs for the RGD and HLGGAKQAGDV peptide ligands, in integrin alphaIIbbeta3 (GPIIb-IIIa) has been characterized. For this purpose, cRGD and cHarGD were labeled with fluorescein isothiocyanate and tetramethylrhodamine 5-isothiocyanate, respectively. Both cyclic peptides were potent inhibitors of fibrinogen binding to alphaIIbbeta3, particularly in the presence of Mn2+; IC50 values for cRGD and cHarGD were 1 and <0.1 nM in the presence of Mn2+. Direct binding experiments and fluorescence resonance energy transfer analysis using the purified receptor showed that both peptides interacted simultaneously with distinct sites in alphaIIbbeta3. The distance between these sites was estimated to be 6.1 +/- 0.5 nm. Although cRGD bound preferentially to one site and cHarGD to the other, the sites were not fully specific, and each cyclic peptide or its linear counterpart could displace the other to some extent. The binding affinity of the cHarGD site was dramatically affected by Mn2+. cRGD, but not cHarGD, bound to recombinant beta3-(95-373) in a cation-dependent manner, indicating that the cRGD site is located entirely within this fragment. With intact platelets, binding of c-RGD and cHarGD to alphaIIbbeta3 resulted in distinct conformational alterations in the receptor as indicated by the differential exposure of ligand-induced binding site epitopes and also induced the opposite on membrane fluidity as shown by electron paramagnetic resonance analyses using 5-doxylstearic acid as a spin probe. These data support the concept the two peptide ligands bind to distinct sites in alphaIIbbeta3 and initiate different functional consequences within the receptor itself and within platelets.     

Parathyroid hormone-related protein (107-139) increases human osteoblastic cell survival by activation of vascular endothelial growth factor receptor-2

Parathyroid hormone-related protein (PTHrP) (107-139), in contrast to the N-terminal fragment PTHrP (1-36), has been shown to interact with the vascular endothelial growth factor (VEGF) system to modulate human osteoblast differentiation. In this study, we evaluated whether this interaction might affect human osteoblastic cell survival. Pre-incubation with PTHrP (107-139) for 1-24 h dose-dependently (0.1-100 nM) inhibited dexamethasone- or etoposide-induced cell death in human osteoblastic MG-63 cells and human osteoblast-like cells from trabecular bone. This effect, but not that elicited by PTHrP (1-36), was abolished by the VEGF receptor (VEGFR)-2 inhibitors SU5614 and SU1498 or VEGFR-2 siRNA transfection in these cells. PTHrP (107-139), but not PTHrP (1-36), at 100 nM, rapidly (within 2 min) increased VEGFR-2 tyrosine-phosphorylation in MG-63 cells; an effect unaffected by several inhibitors of metalloproteinases, neutralizing VEGF(165) or VEGFR-2 antibodies, or the VEGF binding inhibitor CBO-PP1. The latter two antagonists also failed to affect (125)I-[Tyr(116)] PTHrP (107-115) binding to these cells. Consistent with its effect on VEGFR-2 activation, PTHrP (107-139) rapidly induced extracellular signal-regulated kinase (ERK) 1/2 and Akt activaton, and both ERK and phosphatidylinsositol-3 kinase (PI3K) inhibitors abolished its pro-survival effect in human osteoblastic cells. In addition, SU5614 and the latter two types of inhibitors abrogated Runx2 activation by this peptide in MG-63 cells. Transfection with a dominant-negative Runx2 construct abolished the pro-survival effect of PTHrP (107-139), associated with a decrease in Bcl-2/Bax protein ratio. Our findings demonstrate that PTHrP (107-139) interacts with VEGFR-2 to promote human osteoblastic cell survival by a mechanism involving Runx2 activation.     

Synthesis and DNA nicking studies of a novel cyclic peptide: cyclo[Lys-Trp-Lys-Ahx-].

Two novel cyclic tetrapeptides: cyclo[Lys-Tyr-Lys-Ahx-] 7a and cyclo[Lys-Trp-Lys-Ahx-] 7b were synthesized by coupling protected amino acid in solution and the subsequent cyclization effected by the pentafluorophenyl ester method as described in previous papers. These cyclic peptides were designed and synthesized to study their interaction with DNA, based on previous reports that linear peptides Lys-Tyr-Lys and Lys-Trp-Lys could bind to various forms of DNA and cleaved supercoiled DNA at apurinic sites. Ethidium bromide displacement assay showed that the apparent DNA binding constant of linear Lys-Tyr-Lys and cyclic peptide 7a are far below 1 x 10(3) M(-1), whereas those of cyclic peptide 7b and linear Lys-Trp-Lys are 1.9 x 10(4) M(-1) and 9.5 x 10(3) M(-1), respectively. Kinetic studies using agarose gel electrophoresis showed that cyclic peptide 7b and Lys-Trp-Lys possessed DNA nicking activity on natural supercoiled phi X174 DNA with nicking rate of 50.7 and 75.6 pM min(-1) at 65 degrees C, respectively, whereas cyclic peptide 7a and linear Lys-Tyr-Lys were devoid of the corresponding activity. The DNA nicking rate increased significantly with increase in reaction temperature. At reaction temperatures lower than 65 degrees C, the DNA nicking rate of cyclic peptide 7b exceeded that of linear Lys-Trp-Lys. The addition of 1 microM ferrous ion did not give significant enhancement effect on the DNA nicking rate by the peptides. UV irradiation gave a marked rate enhancement on the DNA nicking rate of linear Lys-Trp-Lys and a moderate enhancement on the DNA nicking rate of cyclic peptide 7b.