Stimulation of in vivo angiogenesis using dual growth factor-loaded crosslinked glycosaminoglycan hydrogels

Strategies to Promote Vascularization in 3D Printed Tissue Scaffolds: Trends and Challenges

Three-dimensional (3D) printing techniques for scaffold fabrication have shown promising advancements in recent years owing to the ability of the latest high-performance printers to mimic the native tissue down to submicron scales. Nevertheless, host integration and performance of scaffolds in vivo have been severely limited owing to the lack of robust strategies to promote vascularization in 3D printed scaffolds. As a result, researchers over the past decade have been exploring strategies that can promote vascularization in 3D printed scaffolds toward enhancing scaffold functionality and ensuring host integration. Various emerging strategies to enhance vascularization in 3D printed scaffolds are discussed. These approaches include simple strategies such as the enhancement of vascular in-growth from the host upon implantation by scaffold modifications to complex approaches wherein scaffolds are fabricated with their own vasculature that can be directly anastomosed or microsurgically connected to the host vasculature, thereby ensuring optimal integration. The key differences among the techniques, their pros and cons, and the future opportunities for utilizing each technique are highlighted here. The Review concludes with the current limitations and future directions that can help 3D printing emerge as an effective biofabrication technique to realize tissues with physiologically relevant vasculatures to ultimately accelerate clinical translation.

Therapeutic angiogenesis in coronary artery disease: a review of mechanisms and current approaches

INTRODUCTION

Despite tremendous advances, the shortcomings of current therapies for coronary disease are evidenced by the fact that it remains the leading cause of death in many parts of the world. There is hence a drive to develop novel therapies to tackle this disease. Therapeutic approaches to coronary angiogenesis have long been an area of interest in lieu of its incredible, albeit unrealized potential.

AREAS COVERED

This paper offers an overview of mechanisms of native angiogenesis and a description of angiogenic growth factors. It progresses to outline the advances in gene and stem cell therapy and provides a brief description of other investigational approaches to promote angiogenesis. Finally, the hurdles and limitations unique to this particular area of study are discussed.

EXPERT OPINION

An effective, sustained, and safe therapeutic option for angiogenesis truly could be the paradigm shift for cardiovascular medicine. Unfortunately, clinically meaningful therapeutic options remain elusive because promising animal studies have not been replicated in human trials. The sheer complexity of this process means that numerous major hurdles remain before therapeutic angiogenesis truly makes its way from the bench to the bedside.

A systematic review on the effect of inorganic surface coatings in large animal models and meta-analysis on tricalcium phosphate and hydroxyapatite on periimplant bone formation

The aim of the present systematic review was to analyse studies using inorganic implant coatings and, in a meta‐analysis, the effect of specifically tricalcium phosphate (TCP) and hydroxyapatite (HA) implant surface coatings on bone formation according to the PRISMA criteria. Inclusion criteria were the comparison to rough surfaced titanium implants in large animal studies at different time points of healing. Forty studies met the inclusion criteria for the systematic review. Fifteen of these analyzed the bone‐to‐implant contact (BIC) around the most investigated inorganic titanium implant coatings, namely TCP and HA, and were included in the meta‐analysis. The results of the TCP group show after 14 days a BIC being 3.48% points lower compared with the reference surface. This difference in BIC decreases to 0.85% points after 21–28 days. After 42–84 days, the difference in BIC of 13.79% points is in favor of the TCP‐coatings. However, the results are not statistically significant, in part due to the fact that the variability between the studies increased over time. The results of the HA group show a significant difference in mean BIC of 6.94% points after 14 days in favor of the reference surface. After 21–28 days and 42–84 days the difference in BIC is slightly in favor of the test group with 1.53% points and 1.57% points, respectively, lacking significance. In large animals, there does not seem to be much effect of TCP‐coated or HA‐coated implants over uncoated rough titanium implants in the short term.

Neovascularization of engineered tissues for clinical translation: Where we are, where we should be?

One of the key challenges in engineering three-dimensional tissue constructs is the development of a mature microvascular network capable of supplying sufficient oxygen and nutrients to the tissue. Recent angiogenic therapeutic strategies have focused on vascularization of the constructed tissue, and its integration in vitro; these strategies typically combine regenerative cells, growth factors (GFs) with custom-designed biomaterials. However, the field needs to progress in the clinical translation of tissue engineering strategies. The article first presents a detailed description of the steps in neovascularization and the roles of extracellular matrix elements such as GFs in angiogenesis. It then delves into decellularization, cell, and GF-based strategies employed thus far for therapeutic angiogenesis, with a particularly detailed examination of different methods by which GFs are delivered in biomaterial scaffolds. Finally, interdisciplinary approaches involving advancement in biomaterials science and current state of technological development in fabrication techniques are critically evaluated, and a list of remaining challenges is presented that need to be solved for successful translation to the clinics.

Histologic evaluation of therapeutic responses in ischemic myocardium elicited by dual growth factor delivery from composite glycosaminoglycan hydrogels

In this project, the ability of dual growth factor-preloaded, silk-reinforced, composite hyaluronic acid-based hydrogels to elicit advantageous histologic responses when secured to ischemic myocardium was evaluated in vivo. Reinforced hydrogels containing both Vascular Endothelial Growth Factor (VEGF) and Platelet-derived Growth Factor (PDGF) were prepared by crosslinking chemically modified hyaluronic acid and heparin with poly(ethylene glycol)-diacrylate around a reinforcing silk mesh. Composite patches were sutured to the ventricular surface of ischemic myocardium in Sprague-Dawley rats, and the resulting angiogenic response was followed for 28 days. The gross appearance of treated hearts showed significantly reduced ischemic area and fibrous deposition compared to untreated control hearts. Histologic evaluation showed growth factor delivery to restore myofiber orientation to pre-surgical levels and to significantly increase elicited microvessel density and maturity by day 28 in infarcted myocardial tissue (p < 0.05). In addition, growth factor delivery reduced cell apoptosis and decreased the density of elicited mast cells and both CD68+ and anti-inflammatory CD163+ macrophages. These findings suggest that HA-based, dual growth factor-loaded hydrogels can successfully induce a series of beneficial responses in ischemic myocardium, and offer the potential for therapeutic improvement of ischemic myocardial remodeling.

Understanding angiogenesis and the role of angiogenic growth factors in the vascularisation of engineered tissues

Tissue engineering is a rapidly developing field with many potential clinical applications in tissue and organ regeneration. The development of a mature and stable vasculature within these engineered tissues (ET) remains a significant obstacle. Currently, several growth factors (GFs) have been identified to play key roles within in vivo angiogenesis, including vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), FGF and angiopoietins. In this article we attempt to build on in vivo principles to review the single, dual and multiple GF release systems and their effects on promoting angiogenesis. We conclude that multiple GF release systems offer superior results compared to single and dual systems with more stable, mature and larger vessels produced. However, with more complex release systems this raises other problems such as increased cost and significant GF-GF interactions. Upstream regulators and pericyte-coated scaffolds could provide viable alternative to circumnavigate these issues.

Development of an in situ injectable hydrogel containing hyaluronic acid for neural regeneration

In this work, a novel enzymatically crosslinked injectable hydrogel comprising hyaluronic acid (HyA), dopamine (DA), and 3-(4-hydroxyphenyl) propionic acid (HPA) conjugates was successfully developed. To the best of our knowledge, it is the first time that HPA is conjugated to a HyA-based backbone. In situ hydrogelation of HyA-DA-HPA occurred in the presence of hydrogen peroxide (H₂O₂) as an oxidant and horseradish peroxidase (HRP) as a catalyst. Proton nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to characterize the chemical reactions between HyA, DA, and HPA. Gel formation completed between 3 s to 5 min depending on the concentrations of polymer, HRP, and H₂O₂. Crosslinked HyA-DA-HPA gels acquired storage moduli ranging from ∼100 Pa to ∼20 000 Pa (at f = 2000 rad s^-1). Biocompatibility of the hydrogels was examined with human mesenchymal stem cells (hMSCs) and human induced pluripotent stem cell-derived neural stem cells. The hydrogels made of 2.0 w/v% HyA-DA-HPA hydrogels, 0.24 U ml^-1 HRP and ≤ 0.5 µmol ml^-1 H₂O₂ were found biocompatible with hMSCs cultured on and encapsulated within the hydrogels. Since HyA serves as a backbone of the extracellular matrix in the central nervous system (CNS) and DA acquires the ability to restore dopaminergic neurons, use of this injectable HyA-DA-HPA hydrogel for stem cell transplantation is a potential treatment strategy for CNS repair and regeneration.

Hydrogel vehicles for sequential delivery of protein drugs to promote vascular regeneration

In recent years, as the mechanisms of vasculogenesis and angiogenesis have been uncovered, the functions of various pro-angiogenic growth factors (GFs) and cytokines have been identified. Therefore, therapeutic angiogenesis, by delivery of GFs, has been sought as a treatment for many vascular diseases. However, direct injection of these protein drugs has proven to have limited clinical success due to their short half-lives and systemic off-target effects. To overcome this, hydrogel carriers have been developed to conjugate single or multiple GFs with controllable, sustained, and localized delivery. However, these attempts have failed to account for the temporal complexity of natural angiogenic pathways, resulting in limited therapeutic effects. Recently, the emerging ideas of optimal sequential delivery of multiple GFs have been suggested to better mimic the biological processes and to enhance therapeutic angiogenesis. Incorporating sequential release into drug delivery platforms will likely promote the formation of neovasculature and generate vast therapeutic potential.

Glomerular Function and Structural Integrity Depend on Hyaluronan Synthesis by Glomerular Endothelium

BACKGROUND

A glycocalyx envelope consisting of proteoglycans and adhering proteins covers endothelial cells, both the luminal and abluminal surface. We previously demonstrated that short-term loss of integrity of the luminal glycocalyx layer resulted in perturbed glomerular filtration barrier function.

METHODS

To explore the role of the glycocalyx layer of the endothelial extracellular matrix in renal function, we generated mice with an endothelium-specific and inducible deletion of hyaluronan synthase 2 (Has2), the enzyme that produces hyaluronan, the main structural component of the endothelial glycocalyx layer. We also investigated the presence of endothelial hyaluronan in human kidney tissue from patients with varying degrees of diabetic nephropathy.

RESULTS

Endothelial deletion of Has2 in adult mice led to substantial loss of the glycocalyx structure, and analysis of their kidneys and kidney function showed vascular destabilization, characterized by mesangiolysis, capillary ballooning, and albuminuria. This process develops over time into glomerular capillary rarefaction and glomerulosclerosis, recapitulating the phenotype of progressive human diabetic nephropathy. Using a hyaluronan-specific probe, we found loss of glomerular endothelial hyaluronan in association with lesion formation in tissue from patients with diabetic nephropathy. We also demonstrated that loss of hyaluronan, which harbors a specific binding site for angiopoietin and a key regulator of endothelial quiescence and maintenance of EC barrier function results in disturbed angiopoietin 1 Tie2.

CONCLUSIONS

Endothelial loss of hyaluronan results in disturbed glomerular endothelial stabilization. Glomerular endothelial hyaluronan is a previously unrecognized key component of the extracelluar matrix that is required for glomerular structure and function and lost in diabetic nephropathy.

Bioprinting of Vascularized Tissue Scaffolds: Influence of Biopolymer, Cells, Growth Factors, and Gene Delivery

Over the past decades, tissue regeneration with scaffolds has achieved significant progress that would eventually be able to solve the worldwide crisis of tissue and organ regeneration. While the recent advancement in additive manufacturing technique has facilitated the biofabrication of scaffolds mimicking the host tissue, thick tissue regeneration remains challenging to date due to the growing complexity of interconnected, stable, and functional vascular network within the scaffold. Since the biological performance of scaffolds affects the blood vessel regeneration process, perfect selection and manipulation of biological factors (i.e., biopolymers, cells, growth factors, and gene delivery) are required to grow capillary and macro blood vessels. Therefore, in this study, a brief review has been presented regarding the recent progress in vasculature formation using single, dual, or multiple biological factors. Besides, a number of ways have been presented to incorporate these factors into scaffolds. The merits and shortcomings associated with the application of each factor have been highlighted, and future research direction has been suggested.

Hyaluronan chemistries for three-dimensional matrix applications

Hyaluronan is a ubiquitous constituent of mammalian extracellular matrices and, because of its excellent intrinsic biocompatibility and chemical modification versatility, has been widely employed in a multitude of biomedical applications. In this article, we will survey the approaches used to tailor hyaluronan to specific needs of tissue engineering, regenerative and reconstructive medicine and overall biomedical research. We will also describe recent examples of applications in these broader areas, such as 3D cell culture, bioprinting, organoid biofabrication, and precision medicine that are facilitated by the use of hyaluronan as a biomaterial.

Multidisciplinary Perspectives for Alzheimer's and Parkinson's Diseases: Hydrogels for Protein Delivery and Cell-Based Drug Delivery as Therapeutic Strategies

This review presents two intriguing multidisciplinary strategies that might make the difference in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The first proposed strategy is based on the controlled delivery of recombinant proteins known to play a key role in these neurodegenerative disorders that are released in situ by optimized polymer-based systems. The second strategy is the use of engineered cells, encapsulated and delivered in situ by suitable polymer-based systems, that act as drug reservoirs and allow the delivery of selected molecules to be used in the treatment of Alzheimer's and Parkinson's diseases. In both these scenarios, the design and development of optimized polymer-based drug delivery and cell housing systems for central nervous system applications represent a key requirement. Materials science provides suitable hydrogel-based tools to be optimized together with suitably designed recombinant proteins or drug delivering-cells that, once in situ, can provide an effective treatment for these neurodegenerative disorders. In this scenario, only interdisciplinary research that fully integrates biology, biochemistry, medicine and materials science can provide a springboard for the development of suitable therapeutic tools, not only for the treatment of Alzheimer's and Parkinson's diseases but also, prospectively, for a wide range of severe neurodegenerative disorders.

Sequential delivery of VEGF, FGF-2 and PDGF from the polymeric system enhance HUVECs angiogenesis in vitro and CAM angiogenesis

Angiogenesis is an organized series of events, beginning with vessel destabilization, followed by endothelial cell re-organization, and ending with vessel maturation. The formation of a mature vascular network requires precise spatial and temporal regulation of a large number of angiogenic factors, including vascular endothelial growth factor (VEGF), basic fibroblast growth factor-2 (FGF-2) and platelet-derived growth factor (PDGF). VEGF aids in vascular permeability and endothelial cell recruitment, FGF-2 activates endothelial cell proliferation and migration while PDGF stimulates vascular stability. Accordingly, VEGF may inhibit vessel stabilization while PDGF may inhibit endothelial cell recruitment. Therefore, a new polymeric system was prepared by the supercritical carbon dioxide foaming technology, which realized sequential delivery of two or more growth factors with the controlled dose and rate. Increased release of VEGF (71.10%) and FGF-2 (69.76%) compared to PDGF (43.17%) was observed for the first 7 days. Thereafter, up till 21 days, an increased rate of release of BMP-2 compared to VEGF 165 was observed. The effects of PDGF-PLAms/VEGF-FGF-2-PLGA scaffolds on angiogenesis were investigated by human umbilical vein endothelial cells (HUVECs) angiogenic differentiation in vitro and chorioallantoic membrane (CAM) angiogenesis in vivo. Sequential delivery of VEGF, FGF-2 and PDGF from structural polymer scaffolds with distinct kinetics resulted in significant angiogenic differentiation of HUVECs and rapid formation of mature vascular networks in chorioallantoic membrane. This study reported a composite scaffold with distinct release kinetics, and these results clearly indicated the importance of sequential delivery of multiple growth factors in tissue regeneration and engineering.

Sphingosine 1-phosphate (S1P) signalling: Role in bone biology and potential therapeutic target for bone repair

The lipid mediator sphingosine 1-phosphate (S1P) affects cellular functions in most systems. Interest in its therapeutic potential has increased following the discovery of its G protein-coupled receptors and the recent availability of agents that can be safely administered in humans. Although the role of S1P in bone biology has been the focus of much less research than its role in the nervous, cardiovascular and immune systems, it is becoming clear that this lipid influences many of the functions, pathways and cell types that play a key role in bone maintenance and repair. Indeed, S1P is implicated in many osteogenesis-related processes including stem cell recruitment and subsequent differentiation, differentiation and survival of osteoblasts, and coupling of the latter cell type with osteoclasts. In addition, S1P's role in promoting angiogenesis is well-established. The pleiotropic effects of S1P on bone and blood vessels have significant potential therapeutic implications, as current therapeutic approaches for critical bone defects show significant limitations. Because of the complex effects of S1P on bone, the pharmacology of S1P-like agents and their physico-chemical properties, it is likely that therapeutic delivery of S1P agents will offer significant advantages compared to larger molecular weight factors. Hence, it is important to explore novel methods of utilizing S1P agents therapeutically, and improve our understanding of how S1P and its receptors modulate bone physiology and repair.

The Pharmaceutical Device Prisma® Skin Promotes in Vitro Angiogenesis through Endothelial to Mesenchymal Transition during Skin Wound Healing

Glycosaminoglycans are polysaccharides of the extracellular matrix supporting skin wound closure. Mesoglycan is a mixture of glycosaminoglycans such as chondroitin-, dermatan-, heparan-sulfate and heparin and is the main component of Prisma^® Skin, a pharmaceutical device developed by Mediolanum Farmaceutici S.p.a. Here, we show the in vitro effects of this device in the new vessels formation by endothelial cells, since angiogenesis represents a key moment in wound healing. We found a strong increase of migration and invasion rates of these cells treated with mesoglycan and Prisma^® Skin which mediate the activation of the pathway triggered by CD44 receptor. Furthermore, endothelial cells form longer capillary-like structures with a great number of branches, in the presence of the same treatments. Thus, the device, thanks to the mesoglycan, leads the cells to the Endothelial-to-Mesenchymal Transition, suggesting the switch to a fibroblast-like phenotype, as shown by immunofluorescence assays. Finally, we found that mesoglycan and Prisma^® Skin inhibit inflammatory reactions such as nitric oxide secretion and NF-κB nuclear translocation in endothelial cells and Tumor Necrosis Factor-α production by macrophages. In conclusion, based on our data, we suggest that Prisma^® Skin may be able to accelerate angiogenesis in skin wound healing, and regulate inflammation avoiding chronic, thus pathological, responses.

Biomaterial-driven in situ cardiovascular tissue engineering-a multi-disciplinary perspective

There is a persistent and growing clinical need for readily-available substitutes for heart valves and small-diameter blood vessels. In situ tissue engineering is emerging as a disruptive new technology, providing ready-to-use biodegradable, cell-free constructs which are designed to induce regeneration upon implantation, directly in the functional site. The induced regenerative process hinges around the host response to the implanted biomaterial and the interplay between immune cells, stem/progenitor cell and tissue cells in the microenvironment provided by the scaffold in the hemodynamic environment. Recapitulating the complex tissue microstructure and function of cardiovascular tissues is a highly challenging target. Therein the scaffold plays an instructive role, providing the microenvironment that attracts and harbors host cells, modulating the inflammatory response, and acting as a temporal roadmap for new tissue to be formed. Moreover, the biomechanical loads imposed by the hemodynamic environment play a pivotal role. Here, we provide a multidisciplinary view on in situ cardiovascular tissue engineering using synthetic scaffolds; starting from the state-of-the art, the principles of the biomaterial-driven host response and wound healing and the cellular players involved, toward the impact of the biomechanical, physical, and biochemical microenvironmental cues that are given by the scaffold design. To conclude, we pinpoint and further address the main current challenges for in situ cardiovascular regeneration, namely the achievement of tissue homeostasis, the development of predictive models for long-term performances of the implanted grafts, and the necessity for stratification for successful clinical translation.

Computational Model-Based Analysis of Strategies to Enhance Scaffold Vascularization

Stable and extensive blood vessel networks are required for cell function and survival in engineered tissues. A number of different strategies are currently being investigated to enhance biomaterial vascularization with screening primarily through extensive in vitro and in vivo experiments. In this article, we describe an agent-based model (ABM) developed to evaluate various strategies in silico, including design of optimal biomaterial structure, delivery of angiogenic factors, and application of prevascularized biomaterials. The model predictions are evaluated using experimental data. The ABM developed provides insight into different strategies currently applied for scaffold vascularization and will enable researchers to rapidly screen new hypotheses and explore alternative strategies for enhancing vascularization.

Regulation of sequential release of growth factors using bilayer polymeric nanoparticles for cardiac tissue engineering

Aim: Cardiac tissue engineering aims to develop engineered constructs for myocardial infarction repair, where a challenge is the control of growth factor (GF) sequential release. Herein, bilayer polymeric nanoparticles composed of a GF-encapsulating core surrounded by rate-regulating shell were developed for sequential GF release. Materials & methods: Single and bilayer polymeric nanoparticles were fabricated, characterized and biologically assessed. A novel ‘Geno-Neural model’ was developed and validated for rate-programming of the nanoparticles. Results: The bilayer nanoparticles featured low burst effect and time-delayed release, and allowed for sequential release of PDGF following co-release of VEGFand bFGF, which promoted angiogenesis. Conclusion: The nanoparticulate delivery system, along with the Geno-Neural model, offers great potential for spatiotemporal control of GF release for cardiovascular regenerative medicine.

Hyaluronic acid-based hydrogel scaffold without angiogenic growth factors enhances ovarian tissue function after autotransplantation in rats

One of the problems encountered during ovarian transplantation is that the number of primordial follicles in the grafts is considerably reduced 2 d after transplantation due to post-transplantation ischemia. This study investigates if the use of hyaluronic acid-based hydrogel (HABH) with and without vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) could prevent or minimize ischemia-induced follicle loss during ovarian autotransplantation and thereby restore ovarian tissue function in the rat model. In this study, twenty four female rats were subjected to bilateral ovariectomy and were randomly divided into 3 groups for ovarian tissue autotransplantation. Group A included rats with ovarian tissue without HABH, VEGF and bFGF, group B comprised rats with ovarian tissue encapsulated with HABH and group C had rats with ovarian tissue encapsulated with HABH containing VEGF and bFGF. Three days after transplantation, the grafts were assessed through histological and hormonal analyses. Apoptotic, angiogenic and maturation genes expressions were also analyzed. The mean number of follicles in all developmental stages increased in group B (P  <  0.05). The level of FSH decreased in group B (P  <  0.05) whereas, the expression level of VEGF gene increased in group B (P  <  0.05). No significant changes were observed in the expression levels of maturation and apoptotic genes in all groups. In conclusion, ovarian encapsulation with HABH alone can prevent or minimize ischemia-induced follicle loss, preserve the follicular pool, promote follicular survival, facilitate angiogenesis, and restore hormone levels. However, its efficiency in a clinical setting and in comparison with other hydrogels needs further investigation.

Restoration of ovarian tissue function and estrous cycle in rat after autotransplantation using hyaluronic acid hydrogel scaffold containing VEGF and bFGF

This study investigates the effect of hyaluronic acid (HA) containing VEGF and bFGF on restoration of ovarian function after ovarian autotransplantation. Twenty-four rats were randomly divided into three groups for ovarian autotransplantation: group A (ovaries without HA, VEGF and bFGF), group B (ovaries encapsulated with HA) and group C (ovaries encapsulated with HA containing VEGF and bFGF). The grafts were assessed using vaginal smears, histological, hormonal, and the genes expression analysis. The duration of first estrous cycle was shorter in group C than in group A (p < 0.01). The mean number of primordial follicles was protected in group C. The level of estradiol was higher in group A than in group C (p < 0.01). The expression level of Cellular-Myelocytomatosis (C-Myc) in group C was lower than in group B (p < 0.05). HA containing VEGF and bFGF can ensure follicular survival, decrease apoptosis and recover ovarian function after auto-transplantation.

A tunable hydrogel system for long-term release of cell-secreted cytokines and bioprinted in situ wound cell delivery

For many cellular therapies being evaluated in preclinical and clinical trials, the mechanisms behind their therapeutic effects appear to be the secretion of growth factors and cytokines, also known as paracrine activity. Often, delivered cells are transient, and half-lives of the growth factors that they secrete are short, limiting their long-term effectiveness. The goal of this study was to optimize a hydrogel system capable of in situ cell delivery that could sequester and release growth factors secreted from those cells after the cells were no longer present. Here, we demonstrate the use of a fast photocross-linkable heparin-conjugated hyaluronic acid (HA-HP) hydrogel as a cell delivery vehicle for sustained growth factor release, which extends paracrine activity. The hydrogel could be modulated through cross-linking geometries and heparinization to support sustained release proteins and heparin-binding growth factors. To test the hydrogel in vivo, we used it to deliver amniotic fluid-derived stem (AFS) cells, which are known to secrete cytokines and growth factors, in full thickness skin wounds in a nu/nu murine model. Despite transience of the AFS cells in vivo, the HA-HP hydrogel with AFS cells improved wound closure and reepithelialization and increased vascularization and production of extracellular matrix in vivo. These results suggest that HA-HP hydrogel has the potential to prolong the paracrine activity of cells, thereby increasing their therapeutic effectiveness in wound healing. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1986-2000, 2017.

Importance of dual delivery systems for bone tissue engineering

Bone formation is a complex process that requires concerted function of multiple growth factors. For this, it is essential to design a delivery system with the ability to load multiple growth factors in order to mimic the natural microenvironment for bone tissue formation. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and high toxicity suggest that conventional routes of administration are unlikely to be effective. Therefore, it seems that using multiple bioactive factors in different delivery systems can develop new strategies for improving bone tissue regeneration. Combination of these factors along with biomaterials that permit tunable release profiles would help to achieve truly spatiotemporal regulation during delivery. This review summarizes the various dual-control release systems that are used for bone tissue engineering.

Hydrogels for therapeutic cardiovascular angiogenesis

Acute myocardial infarction (MI) caused by ischemia is the most common cause of cardiac dysfunction. While growth factor or cell therapy is promising, the retention of bioactive agents in the highly vascularized myocardium is limited and prevents sustained activation needed for adequate cellular responses. Various types of biomaterials with different physical and chemical properties have been developed to improve the localized delivery of growth factor and/or cells for therapeutic angiogenesis in ischemic tissues. Hydrogels are particularly advantageous as carrier systems because they are structurally similar to the tissue extracellular matrix (ECM), they can be processed under relatively mild conditions and can be delivered in a minimally invasive manner. Moreover, hydrogels can be designed to degrade in a timely fashion that coincides with the angiogenic process. For these reasons, hydrogels have shown great potential as pro-angiogenic matrices. This paper reviews a few of the hydrogel systems currently being applied together with growth factor delivery and/or cell therapy to promote therapeutic angiogenesis in ischemic tissues, with emphasis on myocardial applications.

Depot-Based Delivery Systems for Pro-Angiogenic Peptides: A Review

Insufficient vascularization currently limits the size and complexity for all tissue engineering approaches. Additionally, increasing or re-initiating blood flow is the first step toward restoration of ischemic tissue homeostasis. However, no FDA-approved pro-angiogenic treatments exist, despite the many pre-clinical approaches that have been developed. The relatively small size of peptides gives advantages over protein-based treatments, specifically with respect to synthesis and stability. While many pro-angiogenic peptides have been identified and shown promising results in vitro and in vivo, the majority of biomaterials developed for pro-angiogenic drug delivery focus on protein delivery. This narrow focus limits pro-angiogenic therapeutics as peptides, similar to proteins, suffer from poor pharmacokinetics in vivo, necessitating the development of controlled release systems. This review discusses pro-angiogenic peptides and the biomaterials delivery systems that have been developed, or that could easily be adapted for peptide delivery, with a particular focus on depot-based delivery systems.

Mimicking biological phenomena in hydrogel-based biomaterials to promote dynamic cellular responses

Novel methods to endow cell-responsiveness into hydrogels are explored and successful work is summarized.

A biomimetic approach to active self-microencapsulation of proteins in PLGA

A biomimetic approach to organic solvent-free microencapsulation of proteins based on the self-healing capacity of poly (DL)-lactic-co-glycolic acid (PLGA) microspheres containing glycosaminoglycan-like biopolymers (BPs), was examined. To screen BPs, aqueous solutions of BP [high molecular weight dextran sulfate (HDS), low molecular weight dextran sulfate (LDS), chondroitin sulfate (CS), heparin (HP), hyaluronic acid (HA), chitosan (CH)] and model protein lysozyme (LYZ) were combined in different molar and mass ratios, at 37 °C and pH7. The BP-PLGA microspheres (20-63 μm) were prepared by a double water-oil-water emulsion method with a range of BP content, and trehalose and MgCO3 to control microclimate pH and to create percolating pores for protein. Biomimetic active self-encapsulation (ASE) of proteins [LYZ, vascular endothelial growth factor165 (VEGF) and fibroblast growth factor (FgF-20)] was accomplished by incubating blank BP-PLGA microspheres in low concentration protein solutions at ~24 °C, for 48 h. Pore closure was induced at 42.5 °C under mild agitation for 42h. Formulation parameters of BP-PLGA microspheres and loading conditions were studied to optimize protein loading and subsequent release. LDS and HP were found to bind >95% LYZ at BP:LYZ>0.125 w/w, whereas HDS and CS bound >80% LYZ at BP:LYZ of 0.25-1 and <0.33, respectively. HA-PLGA microspheres were found to be not ideal for obtaining high protein loading (>2% w/w of LYZ). Sulfated BP-PLGA microspheres were capable of loading LYZ (~2-7% w/w), VEGF (~4% w/w), and FgF-20 (~2% w/w) with high efficiency. Protein loading was found to be dependent on the loading solution concentration, with higher protein loading obtained at higher loading solution concentration within the range investigated. Loading also increased with content of sulfated BP in microspheres. Release kinetics of proteins was evaluated in-vitro with complete release media replacement. Rate and extent of release were found to depend upon volume of release (with non-sink conditions observed <5 ml release volume for ~18 mg loaded BP-PLGA microspheres), ionic strength of release media and loading solution concentration. HDS-PLGA formulations were identified as having ideal loading and release characteristics. These optimal microspheres released ~73-80% of the encapsulated LYZ over 60 days, with >90% of protein being enzymatically active. Nearly 72% of immunoreactive VEGF was similarly released over 42 days, without significant losses in heparin binding affinity in the release medium.

Bioerodible system for sequential release of multiple drugs

Because many complex physiological processes are controlled by multiple biomolecules, comprehensive treatment of certain disease conditions may be more effectively achieved by administration of more than one type of drug. Thus, the objective of the present research was to develop a multilayered, polymer-based system for sequential delivery of multiple drugs. The polymers used were cellulose acetate phthalate (CAP) complexed with Pluronic F-127 (P). After evaluating morphology of the resulting CAPP system, in vitro release of small molecule drugs and a model protein was studied from both single and multilayered devices. Drug release from single-layered CAPP films followed zero-order kinetics related to surface erosion of the association polymer. Release studies from multilayered CAPP devices showed the possibility of achieving intermittent release of one type of drug as well as sequential release of more than one type of drug. Mathematical modeling accurately predicted the release profiles for both single layer and multilayered devices. The present CAPP association polymer-based multilayer devices can be used for localized, sequential delivery of multiple drugs for the possible treatment of complex disease conditions, and perhaps for tissue engineering applications, that require delivery of more than one type of biomolecule.

Controlled release strategies for bone, cartilage, and osteochondral engineering--Part II: challenges on the evolution from single to multiple bioactive factor delivery

The development of controlled release systems for the regeneration of bone, cartilage, and osteochondral interface is one of the hot topics in the field of tissue engineering and regenerative medicine. However, the majority of the developed systems consider only the release of a single growth factor, which is a limiting step for the success of the therapy. More recent studies have been focused on the design and tailoring of appropriate combinations of bioactive factors to match the desired goals regarding tissue regeneration. In fact, considering the complexity of extracellular matrix and the diversity of growth factors and cytokines involved in each biological response, it is expected that an appropriate combination of bioactive factors could lead to more successful outcomes in tissue regeneration. In this review, the evolution on the development of dual and multiple bioactive factor release systems for bone, cartilage, and osteochondral interface is overviewed, specifically the relevance of parameters such as dosage and spatiotemporal distribution of bioactive factors. A comprehensive collection of studies focused on the delivery of bioactive factors is also presented while highlighting the increasing impact of platelet-rich plasma as an autologous source of multiple growth factors.

Engineering the regenerative microenvironment with biomaterials

Modern synthetic biomaterials are being designed to integrate bioactive ligands within hydrogel scaffolds for cells to respond and assimilate within the matrix. These advanced biomaterials are only beginning to be used to simulate the complex spatio-temporal control of the natural healing microenvironment. With increasing understanding of the role of growth factors and cytokines and their interactions with components of the extracellular matrix, novel biomaterials are being developed that more closely mimic the natural healing environments of tissues, resulting in increased efficacy in applications of tissue repair and regeneration. Herein, the important aspects of the healing microenvironment, and how these features can be incorporated within innovative hydrogel scaffolds, are presented.

Role of hyaluronan in angiogenesis and its utility to angiogenic tissue engineering

Angiogenesis represents the outgrowth of new blood vessels from existing ones, a physiologic process that is vital to supply nourishment to newly forming tissues during development and tissue remodeling and repair (wound healing). Regulation of angiogenesis in the healthy body occurs through a fine balance of angiogenesis-stimulating factors and angiogenesis inhibitors. When this balance is disturbed, excessive or deficient angiogenesis can result and contribute to development of a wide variety of pathological conditions. The therapeutic stimulation or suppression of angiogenesis could be the key to abrogating these diseases. In recent years, tissue engineering has emerged as a promising technology for regenerating tissues or organs that are diseased beyond repair. Among the critical challenges that deter the practical realization of the vision of regenerating functional tissues for clinical implantation, is how tissues of finite size can be regenerated and maintained viable in the long-term. Since the diffusion of nutrients and essential gases to cells, and removal of metabolic wastes is typically limited to a depth of 150-250 microm from a capillary (3-10 cells thick), tissue constructs must mandatorily permit in-growth of a blood capillary network to nourish and sustain the viability of cells within. The purpose of this article is to provide an overview of the role and significance of hyaluronan (HA), a glycosaminoglycan (GAG) component of connective tissues, in physiologic and pathological angiogenesis, its applicability as a therapeutic to stimulate or suppress angiogenesis in situ within necrotic tissues in vivo, and the factors determining its potential utility as a pro-angiogenic stimulus that will enable tissue engineering of neo-vascularized and functional tissue constructs for clinical use.

Biomimetic hydrogels for controlled biomolecule delivery to augment bone regeneration

The regeneration of large bone defects caused by trauma or disease remains a significant clinical problem. Although osteoinductive growth factors such as bone morphogenetic proteins have entered clinics, transplantation of autologous bone remains the gold standard to treat bone defects. The effective treatment of bone defects by protein therapeutics in humans requires quantities that exceed the physiological doses by several orders of magnitude. This not only results in very high treatment costs but also bears considerable risks for adverse side effects. These issues have motivated the development of biomaterials technologies allowing to better control biomolecule delivery from the solid phase. Here we review recent approaches to immobilize biomolecules by affinity binding or by covalent grafting to biomaterial matrices. We focus on biomaterials concepts that are inspired by extracellular matrix (ECM) biology and in particular the dynamic interaction of growth factors with the ECM. We highlight the value of synthetic, ECM-mimicking matrices for future technologies to study bone biology and develop the next generation of 'smart' implants.

Hydrogels for protein delivery in tissue engineering

Tissue defects caused by diseases or trauma present enormous challenges in regenerative medicine. Recently, a better understanding of the biological processes underlying tissue repair led to the establishment of new approaches in tissue engineering which comprise the combination of biodegradable scaffolds and appropriate cells together with specific environmental cues, such as growth or adhesive factors. These factors (in fact proteins) have to be loaded and sustainably released from the scaffolds in time. This review provides an overview of the various hydrogel technologies that have been proposed to control the release of bioactive molecules of interest for tissue engineering applications. In particular, after a brief introduction on bioactive protein drugs that have remarkable relevance for tissue engineering, this review will discuss their release mechanisms from hydrogels, their encapsulation and immobilization methods and will overview the main classes of hydrogel forming biomaterials used in vitro and in vivo to release them. Finally, an outlook on future directions and a glimpse into the current clinical developments are provided.

The use of calcium phosphate-based biomaterials in implant dentistry

Since calcium phosphates (CaPs) were first proposed, a wide variety of formulations have been developed and continuously optimized, some of which (e.g. calcium phosphate cements, CPCs) have been successfully commercialized for clinical applications. These CaP-based biomaterials have been shown to be very attractive bone substitutes and efficient drug delivery vehicles across diverse biomedical applications. In this article, CaP biomaterials, principally CPCs, are addressed as alternatives/complements to autogenous bone for grafting in implant dentistry and as coating materials for enhancing the osteoinductivity of titanium implants, highlighting their performance benefits simultaneously as carriers for growth factors and as scaffolds for cell proliferation, differentiation and penetration. Different strategies for employing CaP biomaterials in dental implantology aim to ultimately reach the same goal, namely to enhance the osseointegration process for dental implants in the context of immediate loading and to augment the formation of surrounding bone to guarantee long-term success.

Covalently grafted VEGF(165) in hydrogel models upregulates the cellular pathways associated with angiogenesis

Angiogenesis is an important biological response known to be involved in many physiological and pathophysiological situations. Cellular responses involved in the formation of new blood vessels, such as increases in endothelial cell proliferation, cell migration, and the survival of apoptosis-inducing events, have been associated with vascular endothelial growth factor isoform 165 (VEGF(165)). Current research in the areas of bioengineering and biomedical science has focused on developing polyethylene glycol (PEG)-based systems capable of initiating and sustaining angiogenesis in vitro. However, a thorough understanding of how endothelial cells respond at the molecular level to VEGF(165) incorporated into these systems has not yet been established in the literature. The goal of the current study was to compare the upregulation of key intracellular proteins involved in angiogenesis in human umbilical vein endothelial cells (HUVEC) and human microvascular endothelial cells (HMEC) seeded on PEG hydrogels containing grafted VEGF(165) and adhesion peptides Arg-Gly-Asp-Ser (RGDS). Our data suggest that the covalent incorporation of VEGF(165) into PEG hydrogels encourages the upregulation of signaling proteins responsible for increases in endothelial cell proliferation, cell migration, and the survival after apoptosis-inducing events.

Development of a pre-vascularized 3D scaffold-hydrogel composite graft using an arterio-venous loop for tissue engineering applications

Hyaluronic acid (HA) and fibrin glue (FG) are effective hydrogels for tissue engineering applications as they support tissue in-growth, retain growth factors, and release them slowly with time. The scaffolds, in combination with a hydrogel, effectuate a successful graft. However, the survival of a graft entirely depends upon a functional vascular supply. Therefore, hydrogels must support the in-growing vasculature. To study and compare the vascular patterns, HA and FG hydrogel-containing PLDLLA-TCP-PCL scaffolds were implanted in the groin of male Lewis rats and supplied with a micro-surgically prepared arterio-venous (A-V) loop. The rats were perfused with a vascular contrast media after 4 and 8 weeks and sacrificed for further analysis. The specimens were scanned with micro-CT to find the vascular growth patterns. Corrosion casting of blood vessels followed by SEM demonstrated a high vascular density near the parent blood vessels. Histologically, HA and FG implanted animal groups showed significant angiogenetic activity, especially within the pores of the scaffold. However, formation of new blood vessels was more conspicuously observed at 4 weeks in FG than HA implants. Furthermore, by 8 weeks, the number and pattern of blood vessels were comparable between them. At this time, HA was still present indicating its slow degradation. The finding was confirmed by histomorphometric analysis. This experimental study demonstrates that HA containing composite scaffold systems permit stabile in-growth of blood vessels due to sustained degradation over 8 weeks. HA is a potential matrix for a tissue engineered composite graft.

Neural stem cell niches: roles for the hyaluronan-based extracellular matrix

Neural stem/progenitor cells capable of differentiating into the neurons and glial cells that populate the mammalian central nervous system (CNS) persist in specific neural stem cell niches that regulate stem cell proliferation, survival and differentiation. There is growing evidence that the extracellular matrix within neural stem cell niches is required for neural stem cell maintenance. Here, we review findings supporting a pivotal role for the glycosaminoglycan hyaluronan (HA) and its transmembrane receptors in neural stem/progenitor cell proliferation, differentiation and maturation. We also outline findings supporting changing roles for HA as cells become committed to distinct lineages in the brain and spinal cord.

Controllable dual-release of dexamethasone and bovine serum albumin from PLGA/β-tricalcium phosphate composite scaffolds

Localized dual-drug delivery from biodegradable scaffolds is an important strategy in tissue engineering. In this study, porous poly(L-lactide-co-glycolide) (PLGA)/β-tricalcium phosphate scaffolds containing both dexamethasone (Dex) and bovine serum albumin (BSA) were prepared by incorporating Dex-loaded and BSA-loaded microspheres into the scaffolds. PLGA microspheres containing Dex or BSA were prepared by spray-drying and double emulsion/solvent evaporation, respectively. In vitro release studies indicated that microspheres prepared from PLGA in 3:1 molar ratio of L-lactide/glycolide and 89.5 kDa relative molecular mass showed prolonged release profiles compared with those prepared from PLGA in 1:1 L-lactide/glycolide molar ratio and 30.5 kDa relative molecular mass. Additionally, introduction of poly(ethylene glycol) in the PLGA chain could improve the encapsulation efficiency and reduce the release rate. Based on the above results, controllable dual-release of Dex and BSA with relatively higher or lower release rate was achieved by incorporating Dex-loaded and BSA-loaded microspheres with different release profiles into the PLGA/β-tricalcium phosphate scaffolds.

Photocrosslinkable hyaluronan-gelatin hydrogels for two-step bioprinting

Bioprinting by the codeposition of cells and biomaterials is constrained by the availability of printable materials. Herein we describe a novel macromonomer, a new two-step photocrosslinking strategy, and the use of a simple rapid prototyping system to print a proof-of-concept tubular construct. First, we synthesized the methacrylated ethanolamide derivative of gelatin (GE-MA). Second, partial photochemical cocrosslinking of GE-MA with methacrylated hyaluronic acid (HA-MA) gave an extrudable gel-like fluid. Third, the new HA-MA:GE-MA hydrogels were biocompatible, supporting cell attachment and proliferation of HepG2 C3A, Int-407, and NIH 3T3 cells in vitro. Moreover, hydrogels injected subcutaneously in nude mice produced no inflammatory response. Fourth, using the Fab@Home printing system, we printed a tubular tissue construct. The partially crosslinked hydrogels were extruded from a syringe into a designed base layer, and irradiated again to create a firmer structure. The computer-driven protocol was iterated to complete a cellularized tubular construct with a cell-free core and a cell-free structural halo. Cells encapsulated within this printed construct were viable in culture, and gradually remodeled the synthetic extracellular matrix environment to a naturally secreted extracellular matrix. This two-step photocrosslinkable biomaterial addresses an unmet need for printable hydrogels useful in tissue engineering.

Biomimetic hydrogels with VEGF induce angiogenic processes in both hUVEC and hMEC

Angiogenesis is the process by which new blood vessels arise from the pre-existing vasculature. Human endothelial cells are known to be involved in three key cellular processes during angiogenesis: increased cell proliferation, degradation of the extracellular matrix during cell migration, and the survival of apoptosis. The above processes depend upon the presence of growth factors, such as vascular endothelial growth factor isoform 165 (VEGF(165)) that is released from the extracellular matrix as it is being degraded or secreted from activated endothelial cells. Thus, the goal of the current study is to develop a system with a backbone of polyethylene glycol (PEG) and grafted angiogenic signals to compare the initial angiogenic response of human umbilical vein endothelial cells (hUVEC) or human microvascular endothelial cells (hMEC). Adhesion ligands (PEG-RGDS) for cell attachment and PEG-modified VEGF(165) (PEG-VEGF(165)) are grafted into the hydrogels to encourage the angiogenic response. Our data suggest that our biomimetic system is equally effective in stimulating proliferation, migration, and survival of apoptosis in hMEC as compared to the response to hUVEC.

Sequential delivery of vascular endothelial growth factor and sphingosine 1-phosphate for angiogenesis

Angiogenesis is an organized series of events, beginning with vessel destabilization, followed by endothelial cell re-organization, and ending with vessel maturation. Vascular endothelial growth factor (VEGF) aids in vascular permeability and endothelial cell recruitment while sphingosine 1-phosphate (S1P) stimulates vascular stability. Accordingly, VEGF may inhibit vessel stabilization while S1P may inhibit endothelial cell recruitment. For this reason, we created a new externally-regulated delivery model that not only permits sustained release of bioactive factors, but also temporal separation of the delivery of growth factors. Using this model, sequential delivery of factors was first confirmed in vitro with associated endothelial cells responding in a dose dependent manner. Furthermore, using a modified murine Matrigel plug model, it is apparent that delivery strategies where VEGF presentation is temporally separated from S1P presentation not only led to greater recruitment of endothelial cells, but also higher maturation index of associated vessels.

[Implication of hyaluronic acid in normal and pathological angiogenesis. Application for cellular engineering]

Angiogenesis is a physiological process that allows the formation of new blood vessels, either from the local vascular structures, or from circulating endothelial progenitor cells, mobilized from the bone marrow, and attracted to the neovascularization site. This mechanism is controlled by pro-angiogenic molecules. It is crucial to supply oxygen and nutrients to tissues during growth, embryonic development or tissue regeneration in response to injuries. Thus, the dermis part of the skin is highly vascularized by a dense network of small and medium arteries and of capillaries and venules. In case of injury, rapid tissue repair is possible through this vascular network. However, once the vascularization is restored in tissue repair, the process of angiogenesis is negatively regulated by anti-angiogenic molecules. Controling the balance between pro-and anti-angiogenic agents is crucial and its deregulation leads to serious disease. The extracellular matrix plays an important role in controlling angiogenesis, allowing at least, the distribution of growth factors and the regulation of endothelial cell migration. Among these matrix components, hyaluronic acid plays a major role in the mechanical properties of connective tissues in ensuring their hydration. This glycosaminoglycan is a large size polymer, whose breakdown products strongly act on angiogenesis, especially in pathological situations (cancer, inflammation). Regarding its biological and mechanical properties, hyaluronic acid is used as matrix in tissue engineering, for improving the revascularization of tissues like skin.

Toward delivery of multiple growth factors in tissue engineering

Inspired by physiological events that accompany the "wound healing cascade", the concept of developing a tissue either in vitro or in vivo has led to the integration of a wide variety of growth factors (GFs) in tissue engineering strategies in an effort to mimic the natural microenvironments of tissue formation and repair. Localised delivery of exogenous GFs is believed to be therapeutically effective for replication of cellular components involved in tissue development and the healing process, thus making them important factors for tissue regeneration. However, any treatment aiming to mimic the critical aspects of the natural biological process should not be limited to the provision of a single GF, but rather should release multiple therapeutic agents at an optimised ratio, each at a physiological dose, in a specific spatiotemporal pattern. Despite several obstacles, delivery of more than one GF at rates mimicking an in vivo situation has promising potential for the clinical management of severely diseased tissues. This article summarises the concept of and early approaches toward the delivery of dual or multiple GFs, as well as current efforts to develop sophisticated delivery platforms for this ambitious purpose, with an emphasis on the application of biomaterials-based deployment technologies that allow for controlled spatial presentation and release kinetics of key biological cues. Additionally, the use of platelet-rich plasma or gene therapy is addressed as alternative, easy, cost-effective and controllable strategies for the release of high concentrations of multiple endogenous GFs, followed by an update of the current progress and future directions of research utilising release technologies in tissue engineering and regenerative medicine.

Inhibition of tumor growth and angiogenesis by a lysophosphatidic acid antagonist in an engineered three-dimensional lung cancer xenograft model

BACKGROUND

: We developed an engineered three-dimensional (3D) tumor xenograft model of nonsmall cell lung cancer (NSCLC) in nude mice, and we used this model to evaluate a dual-activity inhibitor of lysophosphatidic acid (LPA) biosynthesis and receptor activation.

METHODS

: First, BrP-LPA, a pan-antagonist for 4 LPA receptors and inhibitor of the lyosphospholipase D activity of autotaxin, was examined for inhibition of cell migration and cell invasion by human NSCLC A549 cells. Second, A549 cells were encapsulated in 3D in 3 semisynthetic extracellular matrices (ECMs) based on chemically modified glycosaminoglycans, and injected subcutaneously in nude mice. Tumor volume and vascularity were determined as a function of semisynthetic ECMs composition. Third, engineered NSCLC xenografts were formed from A549 cells in either Extracel-HP or Matrigel, and mice were treated with 4 intraperitoneal injections of 3 mg/kg of BrP-LPA.

RESULTS

: First, BrP-LPA inhibited cell migration and invasiveness of A549 cells in vitro. Second, tumor growth and microvessel formation for 3D encapsulated A549 cells in vivo in nude mice increased in the following order: buffer only < Extracel < Extracel-HP < Extracel-HP containing growth factorss plus laminin. Third, tumor volumes increased rapidly in both Matrigel and Extracel-HP encapsulated A549 cells, and tumor growth was markedly inhibited by BrP-LPA treatment. Finally, tumor vascularization was dramatically reduced in the A549 tumors treated with BrP-LPA.

CONCLUSIONS

: Engineered A549 lung tumors can be created by 3D encapsulation in an ECM substitute with user controlled composition. The engineered tumors regress and lose vascularity in response to a dual activity inhibitor of the LPA signaling pathway. Cancer 2010. (c) 2010 American Cancer Society.

Stimulation of in vivo angiogenesis by in situ crosslinked, dual growth factor-loaded, glycosaminoglycan hydrogels

As part of a study of elicited angiogenesis, hyaluronan (HA)-based hydrogels crosslinked by polyethylene glycol diacrylate (PEGDA) were loaded with combinations of the cytokine growth factors vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor (KGF) and platelet-derived growth factor (PDGF). GF release in vivo was controlled by covalent incorporation of thiol-modified heparin into thiolated HA hydrogels, which were injected into the ear pinnae of mice and allowed to crosslink in situ. GF release in vivo was controlled by covalent incorporation of thiol-modified heparin in the gels. The ears were harvested at 7 or 14 days post-implantation, and vascularization evaluated via a Neovascularization Index (NI). The study demonstrates that in situ gelling implants produced no gross inflammation, redness or swelling, and an improved tolerance compared to HA-based dry film implants. All treatments showed significantly more vascularization than either contralateral ears or ears receiving a sham surgery. The maximum response was observed after 14 days in the ears receiving 0.3% Hp, gelatin-containing gels loaded with VEGF + KGF (NI = 3.91). The study revealed injected growth factor-loaded HA-based hydrogels can successfully produce localized controllable vascularization, while minimizing tissue necrosis, polymorphonuclear leukocytes and inflammation. The ability to target and controllably release growth factors can prove a useful tool in specific diseased tissue/organ angiogenesis.