Zinc-dependent dimers observed in crystals of human endostatin

Structural Insights into Endostatin-Heparan Sulfate Interactions Using Modeling Approaches

Glycosaminoglycans (GAGs) play a key role in a variety of biological processes in the extracellular matrix (ECM) via interactions with their protein targets. Due to their high flexibility, periodicity and electrostatics-driven interactions, GAG-containing complexes are very challenging to characterize both experimentally and in silico. In this study, we, for the first time, systematically analyzed the interactions of endostatin, a proteolytic fragment of collagen XVIII known to be anti-angiogenic and anti-tumoral, with heparin (HP) and representative heparan sulfate (HS) oligosaccharides of various lengths, sequences and sulfation patterns. We first used conventional molecular docking and a docking approach based on a repulsive scaling-replica exchange molecular dynamics technique, as well as unbiased molecular dynamic simulations, to obtain dynamically stable GAG binding poses. Then, the corresponding free energies of binding were calculated and the amino acid residues that contribute the most to GAG binding were identified. We also investigated the potential influence of Zn2+ on endostatin-HP complexes using computational approaches. These data provide new atomistic details of the molecular mechanism of HP's binding to endostatin, which will contribute to a better understanding of its interplay with proteoglycans at the cell surface and in the extracellular matrix.

Biological aspects in controlling angiogenesis: current progress

All living beings continue their life by receiving energy and by excreting waste products. In animals, the arteries are the pathways of these transfers to the cells. Angiogenesis, the formation of the arteries by the development of pre-existed parental blood vessels, is a phenomenon that occurs naturally during puberty due to certain physiological processes such as menstruation, wound healing, or the adaptation of athletes' bodies during exercise. Nonetheless, the same life-giving process also occurs frequently in some patients and, conversely, occurs slowly in some physiological problems, such as cancer and diabetes, so inhibiting angiogenesis has been considered to be one of the important strategies to fight these diseases. Accordingly, in tissue engineering and regenerative medicine, the highly controlled process of angiogenesis is very important in tissue repairing. Excessive angiogenesis can promote tumor progression and lack of enough angiogensis can hinder tissue repair. Thereby, both excessive and deficient angiogenesis can be problematic, this review article introduces and describes the types of factors involved in controlling angiogenesis. Considering all of the existing strategies, we will try to lay out the latest knowledge that deals with stimulating/inhibiting the angiogenesis. At the end of the article, owing to the early-reviewed mechanical aspects that overshadow angiogenesis, the strategies of angiogenesis in tissue engineering will be discussed.

Deciphering the focal role of endostatin in Alzheimer's disease

Alzheimer's disease (AD) is a paramount chronic neurodegenerative condition that has been affecting elderly people since the 1900s. It causes memory loss, disorientation, and poor mental function. AD is considered to be one of the most serious problems that dementia sufferers face. Despite extensive investigation, the pathological origin of Alzheimer's disease remains a mystery. The amyloid cascade theory and the vascular hypothesis, which stresses the buildup of Aβ plaques, have dominated research into dementia and aging throughout history. However, research into this task failed to yield the long-awaited therapeutic miracle lead for Alzheimer's disease. Perhaps a hypothetical fragility in the context of Alzheimer's disease was regarded as a state distinct from aging in general, as suggested by the angiogenesis hypothesis, which suggests that old age is one state associated with upregulation of angiogenic growth factors, resulting in decreased microcirculation throughout the body. There has also been evidence that by controlling or inhibiting the components involved in the sequence of events that cause angiogenesis, there is a visible progression in AD patients. In Alzheimer's disease, one such antiangiogenic drug is being used.

Effect of Disulfide Bond Incorporation on the Structure and Activity of Endostatin Peptide

The structure and function of a 27-a.a. fragment of the N-terminal sequence of human endostatin (ES-Zn) were compared to those of the mutant peptide (ES-SSZn) obtained by adding Cys-Pro-Ala to the endostatin N-terminus and substituting Asn16 for Cys ensuring formation of a disulfide bond. Structural comparison of ES-Zn and ES-SSZn by far-UV circular dichroism (CD), intrinsic fluorescence, and molecular dynamics simulation methods revealed significant structural perturbations in ES-SSZn, such as elimination of the β-sheet conformer, modification of the N-terminal loop structure, and reorganization of dynamic properties of the entire peptide backbone. ES-SSZn was approximately 2 and 3 times less efficient than ES-Zn and the full-length human endostatin, respectively, in the induction of caspase-3-dependent apoptosis in human umbilical vein endothelial cells (HUVECs) in vitro (p < 0.05). In contrast, treatment of metastatic 4T1 breast tumors in mice with ES-Zn and ES-SSZn (5 mg/kg body weight daily) for 14 days resulted in similar regression of tumor size, comparable downregulation of angiogenesis (CD31 and CD34) and cell proliferation (Ki67), and therefore, the same extent of apoptosis induction (TUNEL, p53, and Bcl-2) for both peptides (as compared to the untreated controls). Western blot analysis of HUVEC and 4T1 tumor lysates revealed the same levels of suppression of key signaling mediators Akt and ERK1/2 by ES-Zn and ES-SSZn. Contrary to the earlier studies, our results showed that the function of the 1-27 endostatin fragment is independent of its overall structure. Stabilization of the N-terminal loop structure by the disulfide bond incorporation causes relief from structural deviations.

Endostatin: A Promising Drug for Antiangiogenic Therapy

Angiogenesis, the formation of new blood vessels from existing capillaries, is critical for tumors to grow beyond a few in size. Tumor cells produce one or more angiogenic factors including fibroblast growth factor and vascular endothelial growth factor. Surprisingly, antiangiogenic factors or angiogenesis inhibitors have been isolated from tumors. Some angiogenesis inhibitors, such as angiostatin, are associated with tumors while others, such as platelet-factor 4 and interferon-alpha are not. Endostatin, a C-terminal product of collagen XVIII, is a specific inhibitor of endothelial cell proliferation, migration and angiogenesis. The mechanism by which endostatin inhibits endothelial cell proliferation and migration is unknown. Endostatin was originally expressed in a prokaryotic system and, late, in a yeast system, thanks to which it is possible to obtain a sufficient quantity of the protein in a soluble and refolded form to be used in preclincial and clinical trials.

Review of Three New Agents that Target Angiogenesis, Matrix Metalloproteinases, and Cyclin-Dependent Kinases

Background

Many potential new antineoplastic agents are currently in various stages of clinical development. Three areas of drug development include antiangiogenic compounds, agents that inhibit matrix metalloproteinases, and agents that modulate cyclin-dependent kinases.

Methods

The authors reviewed the available data for endostatin, COL-3, and flavopiridol, each of which is being developed with one of the above-mentioned proposed mechanisms of action. These agents are among the first drugs to reach clinical testing that is focusing on these novel targets.

Results

Endostatin has finished preclinical testing and the first human trials are about to be initiated. COL-3 is in phase I testing in several locations. Phase I studies for flavopiridol have been completed and several phase II studies are underway. It is unknown at this point if any of these agents will provide clinical benefit to patients at doses that do not cause unacceptable toxicity.

Conclusions

These agents are currently at various stages of clinical testing. Albeit promising as potential modulators in molecular and biochemical pathways, continued research is needed into the toxicities and clinical usefulness of these agents.

Optimization of high-concentration endostatin formulation: Harmonization of excipients' contributions on colloidal and conformational stabilities

Recently, increasing research efforts have been devoted into developing high-concentration protein drugs for subcutaneous injection, especially for those with short half-lives and high-dose requirement. Proteins at high concentrations normally present increased colloidal and structural instability, such as aggregation, fibrillation and gelation, which significantly challenges the high-concentration formulation development of protein drugs. Here we used endostatin, a 20kD recombinant protein, as a model drug for high-concentration formulation optimization. The colloidal and conformational stability of endostatin at high concentration of 30mg/mL were investigated in formulations containing various excipients, including saccharides (mannitol, sorbitol and sucrose), salts (ArgHCl and NaCl), and surfactants (tween 20 and 80). Protein fibrillation was characterized and semi-quantified by optical polarized light microscopy and transmission electron microscopy, and the amount of fiber formation at elevated temperature of 40°C was determined. The soluble protein aggregates were characterized by dynamic and static light scattering before and after dilution. The conformational stability were characterized by polyacrylamide gel electrophoresis, fluorescence, circular dichroism, and differential scanning calorimetry. We observed that the soluble aggregation, fibrillation and gelation, induced by conformational and colloidal instabilities of the protein solution, could be substantially optimized by using suitable stabilizers such as combinations of saccharides and surfactants; while formation of gel and soluble aggregates at high protein concentration (e.g., 30mg/mL) and elevated temperature (40°C) could be prevented by avoiding the usage of salts. It's worth emphasizing that some stabilizers, such as salts and surfactants, could show opposite contributions in conformational and colloidal stabilities of endostatin. Therefore, cautions are needed when one attempts to correlate the colloidal stability of high-concentration proteins with their conformational stability, and the colloidal and conformational protein stabilities must be harmonized by a balanced selection of various types of excipients.

The Challenges of Recombinant Endostatin in Clinical Application: Focus on the Different Expression Systems and Molecular Bioengineering

Angiogenesis plays an essential role in rapid growing and metastasis of the tumors. Inhibition of angiogenesis is a putative strategy for cancer therapy. Endostatin (Es) is an attractive anti-angiogenesis protein with some clinical application challenges including; short half-life, instability in serum and requirement to high dosage. Therefore, production of recombinant endostatin (rEs) is necessary in large scale. The production of rEs is difficult because of its structural properties and is high-cost. Therefore, this review focused on the different expression systems that involved in rEs production including; mammalian, baculovirus, yeast, and Escherichia coli (E. coli) expression systems. The evaluating of the results of different expression systems declared that none of the mentioned systems can be considered to be generally superior to the other. Meanwhile with considering the advantages and disadvantage of E. coli expression system compared with other systems beside the molecular properties of Es, E. coli expression system can be a preferred expression system for expressing of the Es in large scale. Also, the molecular bioengineering and sustained release formulations that lead to improving of its stability and bioactivity will be discussed. Point mutation (P125A) of Es, addition of RGD moiety or an additional zinc biding site to N-terminal of Es , fusing of Es to anti-HER2 IgG or heavy-chain of IgG, and finally loading of the endostar by PLGA and PEG- PLGA nanoparticles and gold nano-shell particles are the effective bioengineering methods to overcome to clinical changes of endostatin.

Collagen XVIII in tissue homeostasis and dysregulation - Lessons learned from model organisms and human patients

Collagen XVIII is a ubiquitous basement membrane (BM) proteoglycan produced in three tissue-specific isoforms that differ in their N-terminal non-collagenous sequences, but share collagenous and C-terminal non-collagenous domains. The collagenous domain provides flexibility to the large collagen XVIII molecules on account of multiple interruptions in collagenous sequences. Each isoform has a complex multi-domain structure that endows it with an ability to perform various biological functions. The long isoform contains a frizzled-like (Fz) domain with Wnt-inhibiting activity and a unique domain of unknown function (DUF959), which is also present in the medium isoform. All three isoforms share an N-terminal laminin-G-like/thrombospondin-1 sequence whose specific functions still remain unconfirmed. The proteoglycan nature of the isoforms further increases the functional diversity of collagen XVIII. An anti-angiogenic domain termed endostatin resides in the C-terminus of collagen XVIII and is proteolytically cleaved from the parental molecule during the BM breakdown for example in the process of tumour progression. Recombinant endostatin can efficiently reduce tumour angiogenesis and growth in experimental models by inhibiting endothelial cell migration and proliferation or by inducing their death, but its efficacy against human cancers is still a subject of debate. Mutations in the COL18A1 gene result in Knobloch syndrome, a genetic disorder characterised mainly by severe eye defects and encephalocele and, occasionally, other symptoms. Studies with gene-modified mice have elucidated some aspects of this rare disease, highlighting in particular the importance of collagen XVIII in the development of the eye. Research with model organisms have also helped in determining other structural and biological functions of collagen XVIII, such as its requirement in the maintenance of BM integrity and its emerging roles in regulating cell survival, stem or progenitor cell maintenance and differentiation and inflammation. In this review, we summarise current knowledge on the properties and endogenous functions of collagen XVIII in normal situations and tissue dysregulation. When data is available, we discuss the functions of the distinct isoforms and their specific domains.

Endostatin and endorepellin: A common route of action for similar angiostatic cancer avengers

Traditional cancer therapy typically targets the tumor proper. However, newly-formed vasculature exerts a major role in cancer development and progression. Autophagy, as a biological mechanism for clearing damaged proteins and oxidative stress products released in the tumor milieu, could help in tumor resolution by rescuing cells undergoing modifications or inducing autophagic-cell death of tumor blood vessels. Cleaved fragments of extracellular matrix proteoglycans are emerging as key players in the modulation of angiogenesis and endothelial cell autophagy. An essential characteristic of cancer progression is the remodeling of the basement membrane and the release of processed forms of its constituents. Endostatin, generated from collagen XVIII, and endorepellin, the C-terminal segment of the large proteoglycan perlecan, possess a dual activity as modifiers of both angiogenesis and endothelial cell autophagy. Manipulation of these endogenously-processed forms, located in the basement membrane within tumors, could represent new therapeutic approaches for cancer eradication.

Engineering of a disulfide loop instead of a Zn binding loop restores the anti-proliferative, anti-angiogenic and anti-tumor activities of the N-terminal fragment of endostatin: Mechanistic and therapeutic insights

Although considerable effort has been devoted to understanding the molecular mechanism of endostatin's anti-cancer activity, the role of its Zn bound N-terminal loop has not been completely clarified. To investigate whether Zn binding or the N-terminal loop is involved in the anti-cancer properties of endostatin, we compared the structure and biological activity of a native Zn binding endostatin peptide (ES-Zn) with three variants: a Zn free variant (ES), a variant containing both a Zn binding site and a disulfide bond (ES-SSZn), and a variant including a disulfide loop but incapable of Zn binding (ES-SS). Spectroscopic studies indicated that ES-Zn and ES-SS consist of random coil and β structures, whereas ES-SSZn and ES fold into random coils. Theoretical analysis proposed that ES-Zn and ES-SS have a similar binding site to αVβ3 integrin. The anti-proliferative activity of endostatin was retained by all peptides except ES, and the in vitro anti-angiogenic property was preserved in ES-Zn and ES-SS. Remarkably, breast tumor growth and CD31 activity were inhibited more effectively by ES-SS than by ES-Zn. Therefore, a correlation exists between the N-terminal loop and anti-cancer properties of endostatin fragment and a disulfide loop may be more promising than a Zn binding loop for inhibiting tumor growth.

Endostatin has ATPase activity, which mediates its antiangiogenic and antitumor activities

Endostatin is an endogenous angiogenesis inhibitor with broad-spectrum antitumor activities. Although the molecular mechanisms of endostatin have been extensively explored, the intrinsic biochemical characteristics of endostatin are not completely understood. Here, we revealed for the first time that endostatin embedded novel ATPase activity. Moreover, mutagenesis study showed that the ATPase activity of endostatin mutants positively correlated with effects on endothelial cell activities and tumor growth. E-M, an endostatin mutant with higher ATPase activity than that of wild-type (WT) endostatin, significantly increased endostatin-mediated inhibitory effects on endothelial cell proliferation, migration, tube formation, and adhesion. In vivo study showed that E-M displayed enhanced antitumor effects compared with WT. On the other hand, K96A, K96R, and E176A, endostatin mutants with lower ATPase activities than that of WT, showed reduced or comparable effects on targeting both in vitro endothelial cell activities and in vivo tumor angiogenesis and tumor growth. Furthermore, endostatin and its mutants exhibited distinct abilities in regulations of gene expression (Id1, Id3), cell signaling (Erk, p38, and Src phosphorylation), and intracellular ATP levels. Collectively, our study demonstrates that endostatin has novel ATPase activity, which mediates its antiangiogenic and antitumor activities, suggesting that construction of endostatin analogues with high ATPase activity may provide a new direction for the development of more potent antiangiogenic drugs.

Endostatin: A novel inhibitor of androgen receptor function in prostate cancer

Acquired resistance to androgen receptor (AR)-targeted therapies compels the development of novel treatment strategies for castration-resistant prostate cancer (CRPC). Here, we report a profound effect of endostatin on prostate cancer cells by efficient intracellular trafficking, direct interaction with AR, reduction of nuclear AR level, and down-regulation of AR-target gene transcription. Structural modeling followed by functional analyses further revealed that phenylalanine-rich α1-helix in endostatin-which shares structural similarity with noncanonical nuclear receptor box in AR-antagonizes AR transcriptional activity by occupying the activation function (AF)-2 binding interface for coactivators and N-terminal AR AF-1. Together, our data suggest that endostatin can be recognized as an endogenous AR inhibitor that impairs receptor function through protein-protein interaction. These findings provide new insights into endostatin whose antitumor effect is not limited to inhibiting angiogenesis, but can be translated to suppressing AR-mediated disease progression in CRPC.

Nuclear-translocated endostatin downregulates hypoxia inducible factor-1α activation through interfering with Zn(II) homeostasis

Hypoxia‑inducible factor‑1α (HIF‑1α) is key in tumor progression and aggressiveness as it regulates a series of genes involved in angiogenesis and anaerobic metabolism. Previous studies have shown that the transcriptional levels of HIF‑1α may be downregulated by endostatin. However, the molecular mechanism by which endostatin represses HIF‑1α expression remains unknown. The current study investigated the mechanism by which nuclear‑translocated endostatin suppresses HIF‑1α activation by disrupting Zn(II) homeostasis. Endostatin was observed to downregulate HIF‑1α expression at mRNA and protein levels. Blockage of endostatin nuclear translocation by RNA interference of importin α1/β1 or ectopic expression of NLS‑deficient mutant nucleolin in human umbilical vein endothelial cells co‑transfected with small interfering (si)‑nucleolin siRNA compromises endostatin‑reduced HIF‑1α expression. Nuclear‑translocated apo‑endostatin, but not holo‑endostatin, significantly disrupts the interaction between CBP/p300 and HIF‑1α by disturbing Zn(II) homeostasis, which leads to the transcriptional inactivation of HIF‑1α. The results reveal mechanistic insights into the method by which nuclear‑translocated endostatin downregulates HIF‑1α activation and provides a novel way to investigate the function of endostatin in endothelial cells.

The high-molecular-weight kininogen domain 5 is an intrinsically unstructured protein and its interaction with ferritin is metal mediated

High-molecular-weight kininogen domain 5 (HK5) is an angiogenic modulator that is capable of inhibiting endothelial cell proliferation, migration, adhesion, and tube formation. Ferritin can bind to a histidine-glycine-lysine-rich region within HK5 and block its antiangiogenic effects. However, the molecular intricacies of this interaction are not well understood. Analysis of the structure of HK5 using circular dichroism and nuclear magnetic resonance [(1) H, (15) N]-heteronuclear single quantum coherence determined that HK5 is an intrinsically unstructured protein, consistent with secondary structure predictions. Equilibrium binding studies using fluorescence anisotropy were used to study the interaction between ferritin and HK5. The interaction between the two proteins is mediated by metal ions such as Co(2+) , Cd(2+) , and Fe(2+) . This metal-mediated interaction works independently of the loaded ferrihydrite core of ferritin and is demonstrated to be a surface interaction. Ferritin H and L bind to HK5 with similar affinity in the presence of metals. The ferritin interaction with HK5 is the first biological function shown to occur on the surface of ferritin using its surface-bound metals.

Extended interaction network of procollagen C-proteinase enhancer-1 in the extracellular matrix

PCPE-1 (procollagen C-proteinase enhancer-1) is an extracellular matrix glycoprotein that can stimulate procollagen processing by procollagen C-proteinases such as BMP-1 (bone morphogenetic protein 1). PCPE-1 interacts with several proteins in addition to procollagens and BMP-1, suggesting that it could be involved in biological processes other than collagen maturation. We thus searched for additional partners of PCPE-1 in the extracellular matrix, which could provide new insights into its biological roles. We identified 17 new partners of PCPE-1 by SPR (surface plasmon resonance) imaging. PCPE-1 forms a transient complex with the β-amyloid peptide, whereas it forms high or very high affinity complexes with laminin-111 (KD=58.8 pM), collagen VI (KD=9.5 nM), TSP-1 (thrombospondin-1) (KD1=19.9 pM, KD2=14.5 nM), collagen IV (KD=49.4 nM) and endostatin, a fragment of collagen XVIII (KD1=0.30 nM, KD2=1.1 nM). Endostatin binds to the NTR (netrin-like) domain of PCPE-1 and decreases the degree of superstimulation of PCPE-1 enhancing activity by heparin. The analysis of the PCPE-1 interaction network based on Gene Ontology terms suggests that, besides its role in collagen deposition, PCPE-1 might be involved in tumour growth, neurodegenerative diseases and angiogenesis. In vitro assays have indeed shown that the CUB1CUB2 (where CUB is complement protein subcomponents C1r/C1s, urchin embryonic growth factor and BMP-1) fragment of PCPE-1 inhibits angiogenesis.

Genetically engineered endostatin-lidamycin fusion proteins effectively inhibit tumor growth and metastasis

Background

Endostatin (ES) inhibits endothelial cell proliferation, migration, invasion, and tube formation. It also shows antiangiogenesis and antitumor activities in several animal models. Endostatin specifically targets tumor vasculature to block tumor growth. Lidamycin (LDM), which consists of an active enediyne chromophore (AE) and a non-covalently bound apo-protein (LDP), is a member of chromoprotein family of antitumor antibiotics with extremely potent cytotoxicity to cancer cells. Therefore, we reasoned that endostatin-lidamycin (ES-LDM) fusion proteins upon energizing with enediyne chromophore may obtain the combined capability targeting tumor vasculature and tumor cell by respective ES and LDM moiety.

Methods

In this study, we designed and obtained two new endostatin-based fusion proteins, endostatin-LDP (ES-LDP) and LDP-endostatin (LDP-ES). In vitro, the antiangiogenic effect of fusion proteins was determined by the wound healing assay and tube formation assay and the cytotoxicity of their enediyne-energized analogs was evaluated by CCK-8 assay. Tissue microarray was used to analyze the binding affinity of LDP, ES or ES-LDP with specimens of human lung tissue and lung tumor. The in vivo efficacy of the fusion proteins was evaluated with human lung carcinoma PG-BE1 xenograft and the experimental metastasis model of 4T1-luc breast cancer.

Results

ES-LDP and LDP-ES disrupted the formation of endothelial tube structures and inhibited endothelial cell migration. Evidently, ES-LDP accumulated in the tumor and suppressed tumor growth and metastasis. ES-LDP and ES show higher binding capability than LDP to lung carcinoma; in addition, ES-LDP and ES share similar binding capability. Furthermore, the enediyne-energized fusion protein ES-LDP-AE demonstrated significant efficacy against lung carcinoma xenograft in athymic mice.

Conclusions

The ES-based fusion protein therapy provides some fundamental information for further drug development. Targeting both tumor vasculature and tumor cells by endostatin-based fusion proteins and their enediyne-energized analogs probably provides a promising modality in cancer therapy.

The nuclear translocation of endostatin is mediated by its receptor nucleolin in endothelial cells

Endostatin, the C-terminal fragment of collagen XVIII, is a potent anti-angiogenic factor that significantly modulates the gene expression pattern in endothelial cells. Upon cell surface binding, endostatin can not only function extracellularly, but also translocate to the nucleus within minutes. However, the mechanism by which this occurs is partially understood. Here we systematically investigated the nuclear translocation mechanism of endostatin. By chemical inhibition and RNA interference, we firstly observed that clathrin-mediated endocytosis, but not caveolae-dependent endocytosis or macropinocytosis, is essential for the nuclear translocation of endostatin. We then indentified that nucleolin and integrin α5β1, two widely accepted endostatin receptors, mediate this clathrin-dependent uptake process, which also involves urokinase plasminogen activator receptor (uPAR). Either mutagenesis study, fluorescence resonance energy transfer assay, or fluorescence cell imaging demonstrates that nucleolin and integrin α5β1 interact with uPAR simultaneously upon endostatin stimulation. Blockade of uPAR decreases not only the interaction between nucleolin and integrin α5β1, but also the uptake process, suggesting that the nucleolin/uPAR/integrin α5β1 complex facilitates the internalization of endostatin. After endocytosis, nucleolin further regulates the nuclear transport of endostatin. RNA interference and mutational analysis revealed that the nuclear translocation of endostatin involves the association of nucleolin with importin α1β1 via the nuclear localization sequence. Taken together, this study reveals the pathway by which endostatin translocates to the nucleus and the importance of nucleolin in this process, providing a new perspective for the functional investigation of the nuclear-translocated endostatin in endothelial cells.

Antiangiogenic and anticancer molecules in cartilage

Cartilage is one of the very few naturally occurring avascular tissues where lack of angiogenesis is the guiding principle for its structure and function. This has attracted investigators who have sought to understand the biochemical basis for its avascular nature, hypothesising that it could be used in designing therapies for treating cancer and related malignancies in humans through antiangiogenic applications. Cartilage encompasses primarily a specialised extracellular matrix synthesised by chondrocytes that is both complex and unique as a result of the myriad molecules of which it is composed. Of these components, a few such as thrombospondin-1, chondromodulin-1, the type XVIII-derived endostatin, SPARC (secreted protein acidic and rich in cysteine) and the type II collagen-derived N-terminal propeptide (PIIBNP) have demonstrated antiangiogenic or antitumour properties in vitro and in vivo preclinical trials that involve several complicated mechanisms that are not completely understood. Thrombospondin-1, endostatin and the shark-cartilage-derived Neovastat preparation have also been investigated in human clinical trials to treat several different kinds of cancers, where, despite the tremendous success seen in preclinical trials, these molecules are yet to show success as anticancer agents. This review summarises the current state-of-the-art antiangiogenic characterisation of these molecules, highlights their most promising aspects and evaluates the future of these molecules in antiangiogenic applications.

Matrikines and the lungs

The extracellular matrix is a complex network of fibrous and nonfibrous molecules that not only provide structure to the lung but also interact with and regulate the behaviour of the cells which it surrounds. Recently it has been recognised that components of the extracellular matrix proteins are released, often through the action of endogenous proteases, and these fragments are termed matrikines. Matrikines have biological activities, independent of their role within the extracellular matrix structure, which may play important roles in the lung in health and disease pathology. Integrins are the primary cell surface receptors, characterised to date, which are used by the matrikines to exert their effects on cells. However, evidence is emerging for the need for co-factors and other receptors for the matrikines to exert their effects on cells. The potential for matrikines, and peptides derived from these extracellular matrix protein fragments, as therapeutic agents has recently been recognised. The natural role of these matrikines (including inhibitors of angiogenesis and possibly inflammation) make them ideal targets to mimic as therapies. A number of these peptides have been taken forward into clinical trials. The focus of this review will be to summarise our current understanding of the role, and potential for highly relevant actions, of matrikines in lung health and disease.

Tunicate cytostatic factor TC14-3 induces a polycomb group gene and histone modification through Ca(2+) binding and protein dimerization

Background

As many invertebrate species have multipotent cells that undergo cell growth and differentiation during regeneration and budding, many unique and interesting homeostatic factors are expected to exist in those animals. However, our understanding of such factors and global mechanisms remains very poor. Single zooids of the tunicate, Polyandrocarpa misakiensis, can give off as many as 40 buds during the life span. Bud development proceeds by means of transdifferentiation of very limited number of cells and tissues. TC14-3 is one of several different but closely related polypeptides isolated from P. misakiensis. It acts as a cytostatic factor that regulates proliferation, adhesion, and differentiation of multipotent cells, although the molecular mechanism remains uncertain. The Polycomb group (PcG) genes are involved in epigenetic control of genomic activity in mammals. In invertebrates except Drosophila, PcG and histone methylation have not been studied so extensively, and genome-wide gene regulation is poorly understood.

Results

When Phe⁶⁵ of TC14-3 was mutated to an acidic amino acid, the resultant mutant protein failed to dimerize. The replacement of Thr⁶⁹ with Arg⁶⁹ made dimers unstable. When Glu¹⁰⁶ was changed to Gly¹⁰⁶, the resultant mutant protein completely lost Ca²⁺ binding. All these mutant proteins lacked cytostatic activity, indicating the requirement of protein dimerization and calcium for the activity. Polyandrocarpa Eed, a component of PcG, is highly expressed during budding, like TC14-3. When wild-type and mutant TC14-3s were applied in vivo and in vitro to Polyandrocarpa cells, only wild-type TC14-3 could induce Eed without affecting histone methyltransferase gene expression. Eed-expressing cells underwent trimethylation of histone H3 lysine27. PmEed knockdown by RNA interference rescued cultured cells from the growth-inhibitory effects of TC14-3.

Conclusion

These results show that in P. misakiensis, the cytostatic activity of TC14-3 is mediated by PmEed and resultant histone modification, and that the gene expression requires both the protein dimerization and Ca²⁺-binding of TC14-3. This system consisting of a humoral factor, PcG, and histone methylation would contribute to the homeostatic regulation of cell growth and terminal differentiation of invertebrate multipotent cells.

Intrinsic disorder of the extracellular matrix

The extracellular matrix is very well organized at the supramolecular and tissue levels and little is known on the potential role of intrinsic disorder in promoting its organization. We predicted the amount of disorder and identified disordered regions in the human extracellular proteome with established computational tools. The extracellular proteome is significantly enriched in proteins comprising more than 50% of disorder compared to the complete human proteome. The enrichment is mostly due to long disordered regions containing at least 100 consecutive disordered residues. The amount of intrinsic disorder is heterogeneous in the extracellular protein families, with the most disordered being collagens and the small integrin-binding ligand N-linked glycoproteins. Although most domains found in extracellular proteins are structured, the fibronectin III domains contain a variable amount of disordered residues (up to 92%). Binding sites for heparin and integrins are found in disordered sequences of extracellular proteins. Intrinsic disorder is evenly distributed in hubs and ends in the interaction network of extracellular proteins with their extracellular partners. In contrast, extracellular hubs are significantly enriched in disorder in the network of extracellular proteins with their extracellular, membrane and intracellular partners. Disorder could thus provide the structural plasticity required for the hubs to interact with membrane and intracellular proteins. Organization and assembly of the extracellular matrix, development of mineralized tissues and cell-matrix adhesion are the biological processes overrepresented in the most disordered extracellular proteins. Extracellular disorder is associated with binding to growth factors, glycosaminoglycans and integrins at the molecular level.

Improvement in the standard treatment for experimental glioma by fusing antibody Fc domain to endostatin

OBJECT

Brain tumors pose many unique challenges to treatment. The authors hypothesized that Fc-endostatin may be beneficial. It is a newly synthesized recombinant human endostatin conjugated to the Fc domain of IgG with a long half-life (weeks) and unknown toxicity. The authors examined the efficacy of Fc-endostatin using various delivery methods.

METHODS

Efficacy was assessed using the intracranial 9L gliosarcoma rat model treated with Fc-endostatin for use in rodents (mFc-endostatin), which was administered either systemically or locally via different delivery methods. Oral temozolomide (TMZ) was administered in combination with mFc-endostatin to determine if there was a beneficial synergistic effect.

RESULTS

Intracranial delivery of mFc-endostatin via a polymer or convection-enhanced delivery 5 days after tumor implantation increased median survival, compared with the control group (p = 0.0048 and 0.003, respectively). Animals treated weekly with subcutaneous mFc-endostatin (started 5 days post-tumor implantation) also had statistically improved survival as compared with controls (p = 0.0008). However, there was no statistical difference in survival between the local and systemic delivery groups. Control animals had a median survival of 13 days. Animals treated either with subcutaneous mFc-endostatin weekly or with polymer had a median survival of 18 and 15 days, respectively, and those treated with oral TMZ for 5 days (Days 5-9) had a median survival of 21 days. Survival was further increased with a combination of oral TMZ and mFc-endostatin polymer, with a median survival of 28 days (p = 0.029, compared with TMZ alone). Subcutaneous mFc-endostatin administered every week starting 18 days before tumor implantation significantly increased median survival when compared with controls (p = 0.0007), with 12.5% of the animals ultimately becoming long-term survivors (that is, survival longer than 120 days). The addition of TMZ to either weekly or daily subcutaneous mFc-endostatin and its administration 18 days before tumor implantation significantly increased survival (p = 0.017 and 0.0001, respectively, compared with TMZ alone). Note that 12.5% of the animals treated with weekly subcutaneous mFc-endostatin and TMZ were long-term survivors.

CONCLUSIONS

Systemically or directly (local) delivered mFc-endostatin prolonged the survival of rats implanted with intracranial 9L gliosarcoma. This benefit was further enhanced when mFc-endostatin was combined with the oral chemotherapeutic agent TMZ.

Two Endogenous Antiangiogenic Inhibitors, Endostatin and Angiostatin, Demonstrate Biphasic Curves in their Antitumor Profiles

Angiogenesis refers to growth of blood vessels from pre-existing ones. In 1971, Folkman proposed that by choking off the blood supply to tumors, they are starved, leading to their demise. A few years ago, the monoclonal antibody Avastin became the first antiangiogenic biological approved by FDA, for treatment of cancer patients. Two other antiangiogenic endogenous protein fragments were isolated in Folkman's laboratory more than a decade ago. Here, we present a short review of data demonstrating that angiostatin and endostatin display a biphasic antitumor dose-response. This behavior is common among a large number of antiangiogenic agents and the reduced effectiveness of antiangiogenic agents at high dose rates may be due to suppression of growth of new vessels carrying the agent into the critical region around the tumor.

Studies of poly(ethylene glycol) modification of HM-3 polypeptides

An RGD modified endostatin-derived synthetic peptide, named HM-3, is a polypeptide angiogenesis inhibitor previously synthesized in our laboratory. Its robust inhibitory effects on endothelial cell migration and tumor growth have been demonstrated by in vivo and in vitro activity assays. The RGD integrin recognition sequence enables the selective binding of HM-3 and its specific targeting to tumor cells that express high levels of integrin. However, the drug has relatively short half-life in vivo, thus requiring administration twice a day to achieve its optimal in vivo antitumor efficacy. In the current study designed to prolong HM-3 half-life, we used methoxy-poly(ethylene glycol)-aldehyde (mPEG-ALD) to specifically modify its N terminus and optimized the reaction condition via monitoring the modification by reverse-phase high-performance liquid chromatograph (RP-HPLC) under varying stoichiometric ratios (n(mPEG10k-ALD):n(HM-3)), reaction times, and pH values. The maximal modification rate was achieved in a reaction when substrates mPEG10k-ALD and HM-3 were mixed at the molar ratio of 2:1 in a pH 6 phosphate buffer after 4 h incubation at room temperature. The reaction product of this optimal reaction was purified to 96% purity by RP-HPLC. Compared with HM-3, the newly modified PEG(10k)-HM-3 was shown to be more active in the inhibition of angiogenesis in the chorioallantoic membrane of chick embryos (CAM), its rate of in vitro degradation in serum was markedly reduced, and its in vivo half-life was prolonged by 5.86-fold relative to unmodified HM-3 after intravenous injection into male SD rats.

Bioinorganic aspects of angiogenesis

Angiogenesis is a physiologic process characterized by the sprouting of a new blood vessel from a pre-existing one. In mammalians the angiogenesis process is dormant, except for few physiological conditions such as wound healing and ovulation. In healthy individuals angiogenesis is finely tuned by pro- and anti-angiogenic factors. The shift from this equilibrium, under pathological conditions (pathological angiogenesis) is associated with several human diseases of high social impact. An efficient angiogenesis also requires that angiogenic factors cooperate with microenvironment derived co-factors, including metals. In this Perspective we describe the bioinorganic aspects of angiogenesis which contribute to a better understanding of the molecular mechanisms and regulation of angiogenesis. In particular, the role of metals, especially copper, metalloproteinases, and the current status on the imaging of angiogenesis targeting VEGF or VEGF receptors will be discussed.

A possible role for metallic ions in the carbohydrate cluster recognition displayed by a Lewis Y specific antibody

Background

Lewis Y (Le^y) is a blood group-related carbohydrate that is expressed at high surface densities on the majority of epithelial carcinomas and is a promising target for antibody-based immunotherapy. A humanized Le^y-specific antibody (hu3S193) has shown encouraging safety, pharmacokinetic and tumor-targeting properties in recently completed Phase I clinical trials.

Methodology/Principal Findings

We report the three-dimensional structures for both the free (unliganded) and bound (Le^y tetrasaccharide) hu3S193 Fab from the same crystal grown in the presence of divalent zinc ions. There is no evidence of significant conformational changes occurring in either the Le^y carbohydrate antigen or the hu3S193 binding site, which suggests a rigid fit binding mechanism. In the crystal, the hu3S193 Fab molecules are coordinated at their protein-protein interface by two zinc ions and in solution aggregation of Fab can be initiated by zinc, but not magnesium ions. Dynamic light scattering revealed that zinc ions could initiate a sharp transition from hu3S193 Fab monomers to large multimeric aggregates in solution.

Conclusions/Significance

Zinc ions can mediate interactions between hu3S193 Fab in crystals and in solution. Whether metallic ion mediated aggregation of antibody occurs in vivo is not known, but the present results suggest that similar clustering mechanisms could occur when hu3S193 binds to Le^y on cells, particularly given the high surface densities of antigen on the target tumor cells.

N-terminal modification increases the stability of the recombinant human endostatin in vitro

Endostar, approved for the treatment of non-small-cell lung cancer by the State Food and Drug Administration in China, is a derivative of human endostatin that is modified with an additional metal-chelating sequence (MGGSHHHHH) at the N-terminus. This modification contributes to an additional zinc-binding site in the endostatin sequence. In the present study, zinc-binding and zinc-free endostar were compared to further characterize their biochemical and structural properties. Thermally induced denaturation was determined by monitoring changes in fluorescence emission spectra. The data indicated that zinc binding significantly increased the transition temperature of endostar and contributed to a reversible change in protein conformation after recooling. Proteolysis assays demonstrated that the modified protein binding with zinc ions can stabilize the N-terminus and the C-terminus of endostar when treated with trypsin, chymotrypsin and carboxypeptidase A and B. Western-blot analyses using anti-His6 antibody confirmed that the major cleaved fragments of endostar were in the N-terminus when treated with trypsin and chymotrypsin. In the proliferation assay with human umbilical-vein endothelial cells, the zinc-binding and zinc-free endostar samples with extra zinc-binding sites displayed similar inhibiting activities.

Contributions of Zn(II)-binding to the structural stability of endostatin

Endostatin has a compact structure with a Zn(II)-binding site (His1, His3, His11, and Asp76) at the N-terminus. In this study, the effects of Zn(II)-binding on the folding and stability of recombinant human endostatin were studied. The results show that Zn(II)-binding largely stabilizes the structure of endostatin at physiological pH. Under some proteolytic conditions, Zn(II)-binding also contributes to the integrity of the N-terminus of endostatin, which is critical for endostatin to maintain a stable structure. Moreover, engineering an extra Zn(II)-binding peptide to the N-terminus of human endostatin makes this molecule more stable and cooperative in the presence of Zn(II).

Endostatin: Current concepts about its biological role and mechanisms of action

Endogenous inhibitors of angiogenesis are proved to be a major factor preventing the emergence of clinically manifested stages of human cancer. The protein endostatin, a 20-kD proteolytic fragment of type XVIII collagen, is one of the most active natural inhibitors of angiogenesis. Endostatin specifically inhibits the in vitro and in vivo proliferation of endothelial cells, inducing their apoptosis through inhibition of cyclin D1. On the surface of endothelial cells, endostatin binds with the integrin alpha(5)beta(1) that activates the Src-kinase pathway. The binding of endostatin with integrins also down-regulates the activity of RhoA GTPase and inhibits signaling pathways mediated by small kinases of the Ras and Raf families. All these events promote disassembly of the actin cytoskeleton, disorders in cell-matrix interactions, and decrease in endotheliocyte mobility, i.e., promote the suppression of angiogenesis. Endostatin displays a high antitumor activity in vivo: it inhibits the progression of more than 60 types of tumors. This review summarizes results of numerous studies concerning the biological activity and action mechanism of endostatin.

Endostatin therapy reveals a U-shaped curve for antitumor activity

Developing continuous systemic delivery of endostatin has been a goal of many laboratories. We have employed a method of gene therapy utilizing different viral constructs. Here, we report that a new serotype of adeno-associated viruses, which incorporates canine endostatin, provides dose-dependent transgene expression in the circulation after intramuscular injection in mice. Elevated levels of endostatin remained stable in the circulation for at least 4 months. In vitro assays determined that the protein expressed was biologically active. Antitumor activities of the above construct demonstrated a U-shape curve, where the maximum activity was observed within a certain critical concentration range. These data suggest that an optimum dose range may be required to achieve therapeutic efficacy in large animal models.

Structural basis for the functions of endogenous angiogenesis inhibitors

Tipping the angiogenic balance between pro- and antiangiogenic stimuli to favor vasculature induction and enhanced angiogenesis is a key event in the growth and progression of tumors. Recently, we demonstrated that the genetic loss of normal physiological levels of individual endogenous inhibitors of angiogenesis leads to a change in the balance between proangiogenic stimulators and their inhibitors, thus favoring enhanced angiogensis and increased tumor growth. Therefore, these endogenous angiogenesis inhibitors provide a physiological threshold against the induction of angiogenesis. The antiangiogenic activities of endostatin, tumstatin, and thrombospondin-1 are evaluated and correlated with their three-dimensional structure and active sites, deriving a structural basis for their activities. Collectively, structural analysis of all three inhibitors demonstrates that the active antiangiogenic sites on these molecules are exposed on the surface and available to bind their putative integrin receptors on proliferating endothelial cells.

Endostatin phenylalanines 31 and 34 define a receptor binding site

Endostatin has achieved much attention as a naturally occurring inhibitor of angiogenesis and tumor growth. Endostatin is derived from collagen XVIII's C-terminal domain and deleted or truncated in most patients suffering from Knobloch syndrome blindness. To evaluate the functional significance of two surface-exposed hydrophobic phenylalanines at positions 31 and 34 of endostatin and two human sequence alterations within endostatin, A48T and D104N, we applied the alkaline phosphatase fusion protein method. Replacement of F31 and F34 with alanines led to complete loss of characteristic in situ binding while heparin binding remained intact. In contrast, a non-heparin binding alkaline phosphatase-tagged human endostatin lacking R27 and R139 bound to specific tissue structures. The two Knobloch syndrome-associated endostatin sequence variants did not result in altered in situ binding to murine embryonal tissues, human endothelial cells, heparin and immobilized laminin. However, expression of the endostatin mutant A48T was significantly reduced. This observation may be explained by a lower folding efficiency due to the structural constraints of A48 residing in the hydrophobic core. Our data suggest that residues F31 and F34 form a putative receptor binding site acting independently from heparan sulfate binding and that the A48T mutation destabilizes the endostatin molecule.

A molecular dynamics study of human endostatin and its synthetic fragments with antiangiogenic properties

Human endostatin is one of the better characterized endogenous angiogenesis inhibitors, and its ability to modulate vascularization of tumours could be of great therapeutic interest. These properties are not exclusive to the full-length protein, but are shared by some of its synthetic fragments. A number of research groups have partitioned human endostatin in different peptides and have investigated their activity, in order to collect a body of experimental data which could be important in shedding new light on their structure-activity relationships. It was also reported that a small active fragment can become inactive when contained in a larger fragment, revealing an apparent discrepancy in the experimental results. Very few studies have been devoted to the computational analysis of these systems and to the rationalization of their properties using molecular modelling. Through molecular dynamics simulations of human endostatin and of four synthetic fragments, we have been able to rationalize the experimental findings. In particular, we have identified a pattern consisting of six amino acids, namely R-R(G)-A-D-R-A, which appears to be an active epitope if it is properly exposed to the solvent. Interestingly, this pattern can be already present in sequential order in the primary structure, or it can be generated by the spatial approach of two groups of residues, far apart in the primary structure, as an effect of the peptide folding. Comparing the structural features and the time evolution of all the simulated peptides we provide a coherent explanation of their activity or inactivity.