Perivascular signals alter global gene expression profile of glioblastoma and response to temozolomide in a gelatin hydrogel

Glioblastoma (GBM) is the most common primary malignant brain tumor, with patients exhibiting poor survival (median survival time: 15 months). Difficulties in treating GBM include not only the inability to resect the diffusively-invading tumor cells, but also therapeutic resistance. The perivascular niche (PVN) within the GBM tumor microenvironment contributes significantly to tumor cell invasion, cancer stem cell maintenance, and has been shown to protect tumor cells from radiation and chemotherapy. In this study, we examine how the inclusion of non-tumor cells in culture with tumor cells within a hydrogel impacts the overall gene expression profile of an in vitro artificial perivascular niche (PVN) comprised of endothelial and stromal cells directly cultured with GBM tumor cells within a methacrylamide-functionalized gelatin hydrogel. Using RNA-seq, we demonstrate that genes related to angiogenesis and extracellular matrix remodeling are upregulated in the PVN model compared to hydrogels containing only tumor or perivascular niche cells, while downregulated genes are related to cell cycle and DNA damage repair. Signaling pathways and genes commonly implicated in GBM malignancy, such as MGMT, EGFR, PI3K-Akt signaling, and Ras/MAPK signaling are also upregulated in the PVN model. We describe the kinetics of gene expression within the PVN hydrogels over a course of 14 days, observing the patterns associated with tumor cell-mediated endothelial network co-option and regression. We finally examine the effect of temozolomide, a frontline chemotherapy used clinically against GBM, on the PVN culture. Notably, the PVN model is less responsive to TMZ compared to hydrogels containing only tumor cells. Overall, these results demonstrate that inclusion of cellular and matrix-associated elements of the PVN within an in vitro model of GBM allows for the development of gene expression patterns and therapeutic response relevant to GBM.

Biomechanical forces in tissue engineered tumor models

Solid tumors are complex three-dimensional (3D) networks of cancer and stromal cells within a dynamic extracellular matrix. Monolayer cultures fail to recapitulate the native microenvironment and therefore are poor candidates for pre-clinical drug studies and studying pathways in cancer. The tissue engineering toolkit allows us to make models that better recapitulate the 3D architecture present in tumors. Moreover, the role of the mechanical microenvironment, including matrix stiffness and shear stress from fluid flow, is known to contribute to cancer progression and drug resistance. We review recent developments in tissue engineered tumor models with a focus on the role of the biomechanical forces and propose future considerations to implement to improve physiological relevance of such models.

The Feasibility of Encapsulated Embryonic Medullary Reticular Cells to Grow and Differentiate Into Neurons in Functionalized Gelatin-Based Hydrogels

The study of the behavior of embryonic neurons in controlled in vitro conditions require methodologies that take advantage of advanced tissue engineering approaches to replicate elements of the developing brain extracellular matrix. We report here a series of experiments that explore the potential of photo-polymerized gelatin hydrogels to culture primary embryonic neurons. We employed large medullary reticular neurons whose activity is essential for brain arousal as well as a library of gelatin hydrogels that span a range of mechanical properties, inclusion of brain-mimetic hyaluronic acid, and adhesion peptides. These hydrogel platforms showed inherent capabilities to sustain neuronal viability and were permissive for neuronal differentiation, resulting in the development of neurite outgrowth under specific conditions. The maturation of embryonic medullary reticular cells took place in the absence of growth factors or other exogenous bioactive molecules. Immunocytochemistry labeling of neuron-specific tubulin confirmed the initiation of neural differentiation. Thus, this methodology provides an important validation for future studies of nerve cell growth and maintenance.

Hypoxia activates enhanced invasive potential and endogenous hyaluronic acid production by glioblastoma cells

Glioblastoma (GBM) is the most common, aggressive, and deadly form of adult brain cancer, and is associated with a short survival rate (median 12-15 months, 5+ year less than 5%). The complex tumor microenvironment includes matrix transitions at the tumor margin, such as gradations in hyaluronic acid (HA). In addition, metabolic stress induced by decreased oxygen content across the tumor may contribute to tumor progression. However, cross-talk between matrix composition and metabolic stress remains unclear. In this study, we fabricated an in vitro brain memetic HA-decorated gelatin hydrogel platform incorporating variable oxygen concentrations to mimic intra-tumoral hypoxia. We observed that EGFR status (wildtype vs. a constitutively active EGFRvIII mutant) of U87 GBM cells affected proliferation and metabolic activity in response to hypoxia and matrix-bound HA. The use of an invasion assay revealed that invasion was significantly enhanced in both cell types under hypoxia. Moreover, we observed compensatory secretion of soluble HA in cases of enhanced GBM cell invasion, consistent with our previous findings using other GBM cell lines. Interestingly, U87 GBM cells adapted to hypoxia by shifting toward a more anaerobic metabolic state, a mechanism that may contribute to GBM cell invasion. Collectively, these data demonstrate that the use of a three-dimensional hydrogel provides a robust method to study the impact of matrix composition and metabolic challenges on GBM cell invasion, a key factor contributing to the most common, aggressive, and deadly form of adult brain cancer.

Photo-immobilized EGF chemical gradients differentially impact breast cancer cell invasion and drug response in defined 3D hydrogels

Breast cancer cell invasion is influenced by growth factor concentration gradients in the tumor microenvironment. However, studying the influence of growth factor gradients on breast cancer cell invasion is challenging due to both the complexities of in vivo models and the difficulties in recapitulating the tumor microenvironment with defined gradients using in vitro models. A defined hyaluronic acid (HA)-based hydrogel crosslinked with matrix metalloproteinase (MMP) cleavable peptides and modified with multiphoton labile nitrodibenzofuran (NDBF) was synthesized to photochemically immobilize epidermal growth factor (EGF) gradients. We demonstrate that EGF gradients can differentially influence breast cancer cell invasion and drug response in cell lines with different EGF receptor (EGFR) expression levels. Photopatterned EGF gradients increase the invasion of moderate EGFR expressing MDA-MB-231 cells, reduce invasion of high EGFR expressing MDA-MB-468 cells, and have no effect on invasion of low EGFR-expressing MCF-7 cells. We evaluate MDA-MB-231 and MDA-MB-468 cell response to the clinically tested EGFR inhibitor, cetuximab. Interestingly, the cellular response to cetuximab is completely different on the EGF gradient hydrogels: cetuximab decreases MDA-MB-231 cell invasion but increases MDA-MB-468 cell invasion and cell number, thus demonstrating the importance of including cell-microenvironment interactions when evaluating drug targets.

Focal adhesion signaling affects regeneration by human nucleus pulposus cells in collagen- but not carbohydrate-based hydrogels

Hydrogel-based 3D cell cultures are an emerging strategy for the regeneration of cartilage. In an attempt to regenerate dysfunctional intervertebral discs, nucleus pulposus (NP) cells can be cultured in hydrogels of various kinds and physical properties. Stiffness sensing through focal adhesions is believed to direct chondrogenesis, but the mechanisms by which this works are largely unknown. In this study we compared focal adhesion formation and glycosaminoglycan (GAG) deposition by NP cells in a range of hydrogels. Using a focal adhesion kinase (FAK) inhibitor, we demonstrated that focal adhesion signaling is involved in the response of NP cells in hydrogels that contain integrin binding sites (i.e. methacrylated gelatin (gelMA) and type II collagen), but not in hydrogels deplete from integrin binding sites such as alginate and agarose, or CD44-binding hydrogels based on hyaluronic acid. As a result of FAK inhibition we observedenhanced proteoglycan production in gelMA, but decreased production in type II collagen hydrogels, which could be explained by alteration in cell fate as supported by the increase in the adipogenic marker peroxisome proliferator-activated receptor gamma (PPARy). Furthermore, GAG deposition was inversely proportional to polymer concentration in integrin-binding gelMA, while no direct relationship was found for the non-integrin binding gels alginate and agarose. This corroborates our finding that focal adhesion formation plays an important role in NP cell response to its surrounding matrix.

STATEMENT OF SIGNIFICANCE

Biomaterials are increasingly being investigated for regenerative medicine applications, including regeneration of the nucleus pulposus. Cells interact with their environment and are influenced by extracellular matrix or polymer properties. Insight in these interactions can improve regeneration and helps to understand degeneration processes. The role of focal adhesion formation in the regenerative response of nucleus pulposus cells is largely unknown. Therefore, the relation between materials, stiffness and focal adhesion formation is studied here.

Role of Microenvironment in Glioma Invasion: What We Learned from In Vitro Models

The invasion properties of glioblastoma hamper a radical surgery and are responsible for its recurrence. Understanding the invasion mechanisms is thus critical to devise new therapeutic strategies. Therefore, the creation of in vitro models that enable these mechanisms to be studied represents a crucial step. Since in vitro models represent an over-simplification of the in vivo system, in these years it has been attempted to increase the level of complexity of in vitro assays to create models that could better mimic the behaviour of the cells in vivo. These levels of complexity involved: 1. The dimension of the system, moving from two-dimensional to three-dimensional models; 2. The use of microfluidic systems; 3. The use of mixed cultures of tumour cells and cells of the tumour micro-environment in order to mimic the complex cross-talk between tumour cells and their micro-environment; 4. And the source of cells used in an attempt to move from commercial lines to patient-based models. In this review, we will summarize the evidence obtained exploring these different levels of complexity and highlighting advantages and limitations of each system used.

Brain-Mimetic 3D Culture Platforms Allow Investigation of Cooperative Effects of Extracellular Matrix Features on Therapeutic Resistance in Glioblastoma

Glioblastoma (GBM) tumors exhibit potentially actionable genetic alterations against which targeted therapies have been effective in treatment of other cancers. However, these therapies have largely failed in GBM patients. A notable example is kinase inhibitors of EGFR, which display poor clinical efficacy despite overexpression and/or mutation of EGFR in >50% of GBM. In addressing this issue, preclinical models may be limited by the inability to accurately replicate pathophysiologic interactions of GBM cells with unique aspects of the brain extracellular matrix (ECM), which is relatively enriched in hyaluronic acid (HA) and flexible. In this study, we present a brain-mimetic biomaterial ECM platform for 3D culturing of patient-derived GBM cells, with improved pathophysiologic properties as an experimental model. Compared with orthotopic xenograft assays, the novel biomaterial cultures we developed better preserved the physiology and kinetics of acquired resistance to the EGFR inhibition than gliomasphere cultures. Orthogonal modulation of both HA content and mechanical properties of biomaterial scaffolds was required to achieve this result. Overall, our findings show how specific interactions between GBM cell receptors and scaffold components contribute significantly to resistance to the cytotoxic effects of EGFR inhibition.Significance: Three-dimensional culture scaffolds of glioblastoma provide a better physiological representation over current methods of patient-derived cell culture and xenograft models. Cancer Res; 78(5); 1358-70. ©2017 AACR.

Extracellular Hyaluronic Acid Influences the Efficacy of EGFR Tyrosine Kinase Inhibitors in a Biomaterial Model of Glioblastoma

3D biomaterial models have potential to explore the influence of the tumor microenvironment on aberrant signaling pathways and compensatory signals using patient-derived cells. Glioblastoma (GBM) tumors are highly heterogeneous, with both cell composition and extracellular matrix biophysical factors seen as key regulators of malignant phenotype and treatment outcomes. Amplification, overexpression, and mutation of the epidermal growth factor receptor (EGFR) tyrosine kinase have been identified in 50% of GBM patients. Here, hyaluronic acid (HA) decorated methacrylamide-functionalized gelatin (GelMA) hydrogels are used to examine the synergies between microenvironmental factors and a model EGFR tyrosine kinase inhibitor (TKI) using patient-derived xenograft cells. The in vitro behavior of 3 patient-derived xenografts that reflect a clinically relevant range of EGFR variants is characterized: GBM10 (EGFR, wild type), GBM12 (EGFR+), and GBM6 (EGFRvIII). GelMA hydrogels support xenograft culture; cells remain viable, active, respond to matrix-immobilized HA, and upregulate genes associated with matrix remodeling and tumor growth. Interestingly, matrix-immobilized HA alters the response of GBM cells to a model tyrosine kinase inhibitor, erlotinib. While constitutively activated EGFRvIII cells are sensitive to TKI in gelatin hydrogels, hyaluronic acid mediated adhesive signaling interacts with EGFRvIII signaling to increase cell metabolic activity, increase soluble hyaluronic acid synthesis, and modify response to erlotinib exposure.

The Influence of Hyaluronic Acid and Glioblastoma Cell Coculture on the Formation of Endothelial Cell Networks in Gelatin Hydrogels

Glioblastoma (GBM) is the most common and deadly form of brain cancer. Interactions between GBM cells and vasculature in vivo contribute to poor clinical outcomes, with GBM-induced vessel co-option, regression, and subsequent angiogenesis strongly influencing GBM invasion. Here, elements of the GBM perivascular niche are incorporated into a methacrylamide-functionalized gelatin hydrogel as a means to examine GBM-vessel interactions. The complexity of 3D endothelial cell networks formed from human umbilical vein endothelial cells and normal human lung fibroblasts as a function of hydrogel properties and vascular endothelial growth factor (VEGF) presentation is presented. While overall length and branching of the endothelial cell networks decrease with increasing hydrogel stiffness and incorporation of brain-mimetic hyaluronic acid, it can be separately altered by changing the vascular cell seeding density. It is shown that covalent incorporation of VEGF supports network formation as robustly as continuously available soluble VEGF. The impact of U87-MG GBM cells on the endothelial cell networks is subsequently investigated. GBM cells localize in proximity to the endothelial cell networks and hasten network regression in vitro. Together, this in vitro platform recapitulates the close association between GBM cells and vessel structures as well as elements of vessel co-option and regression preceding angiogenesis in vivo.

Proangiogenic Activity of Endometrial Epithelial and Stromal Cells in Response to Estradiol in Gelatin Hydrogels

Biomaterial vascularization remains a major focus in the field of tissue engineering. Biomaterial culture of endometrial cells is described as a platform to inform the design of proangiogenic biomaterials. The endometrium undergoes rapid growth and shedding of dense vascular networks during each menstrual cycle mediated via estradiol and progesterone in vivo. Cocultures of endometrial epithelial and stromal cells encapsulated within a methacrylamide-functionalized gelatin hydrogel are employed. It is reported that proangiogenic gene expression profiles and vascular endothelial growth factor production are hormone dependent in endometrial epithelial cells, but that hormone signals have no effect on human telomerase reverse transcriptase (hTERT)-immortalized endometrial stromal cells. This study subsequently examines whether the magnitude of epithelial cell response is sufficient to induce changes in human umbilical vein endothelial cell network formation. Incorporation of endometrial stromal cells improves vessel formation, but co-culture with endometrial epithelial cells leads to a decrease in vascular formation, suggesting the need for stratified cocultures of endometrial epithelial and stromal cells with endothelial cells. Given the transience of hormonal signals within 3D biomaterials, the inclusion of sex hormone binding globulin (SHBG) to alter the bioavailability of estradiol within the hydrogel is reported, demonstrating a strategy to reduce diffusive losses via SHBG-mediated estradiol sequestration.

The Combined Influence of Hydrogel Stiffness and Matrix-Bound Hyaluronic Acid Content on Glioblastoma Invasion

Glioblastoma (GBM) is the most common and lethal form of brain cancer. Its high mortality is associated with its aggressive invasion throughout the brain. The heterogeneity of stiffness and hyaluronic acid (HA) content within the brain makes it difficult to study invasion in vivo. A dextran-bead assay is employed to quantify GBM invasion within HA-functionalized gelatin hydrogels. Using a library of stiffness-matched hydrogels with variable levels of matrix-bound HA, it is reported that U251 GBM invasion is enhanced in softer hydrogels but reduced in the presence of matrix-bound HA. Inhibiting HA-CD44 interactions reduces invasion, even in hydrogels lacking matrix-bound HA. Analysis of HA biosynthesis suggests that GBM cells compensate for a lack of matrix-bound HA by producing soluble HA to stimulate invasion. Together, a robust method is showed to quantify GBM invasion over long culture times to reveal the coordinated effect of matrix stiffness, immobilized HA, and compensatory HA production on GBM invasion.

Patterning Three-Dimensional Hydrogel Microenvironments Using Hyperbranched Polyglycerols for Independent Control of Mesh Size and Stiffness

The extracellular matrix is an environment rich with structural, mechanical, and molecular signals that can impact cell biology. Traditional approaches in hydrogel biomaterial design often rely on modifying the concentration of cross-linking groups to adjust mechanical properties. However, this strategy provides limited capacity to control additional important parameters in 3D cell culture such as microstructure and molecular diffusivity. Here we describe the use of multifunctional hyperbranched polyglycerols (HPGs) to manipulate the mechanical properties of polyethylene glycol (PEG) hydrogels while not altering biomolecule diffusion. This strategy also provides the ability to separately regulate spatial and temporal distribution of biomolecules tethered within the hydrogel. The functionalized HPGs used here can also react through a copper-free click chemistry, allowing for the encapsulation of cells and covalently tethered biomolecules within the hydrogel. Because of the hyperbranched architecture and unique properties of HPGs, their addition into PEG hydrogels affords opportunities to locally alter hydrogel cross-linking density with minimal effects on global network architecture. Additionally, photocoupling chemistry allows micropatterning of bioactive cues within the three-dimensional gel structure. This approach therefore enables us to tailor mechanical and diffusive properties independently while further allowing for local modulation of biomolecular cues to create increasingly complex cell culture microenvironments.

Integrating the glioblastoma microenvironment into engineered experimental models

Glioblastoma (GBM) is the most lethal cancer originating in the brain. Its high mortality rate has been attributed to therapeutic resistance and rapid, diffuse invasion - both of which are strongly influenced by the unique microenvironment. Thus, there is a need to develop new models that mimic individual microenvironmental features and are able to provide clinically relevant data. Current understanding of the effects of the microenvironment on GBM progression, established experimental models of GBM and recent developments using bioengineered microenvironments as ex vivo experimental platforms that mimic the biochemical and physical properties of GBM tumors are discussed.

Regulating dynamic signaling between hematopoietic stem cells and niche cells via a hydrogel matrix

Hematopoietic stem cells (HSC) reside in unique bone marrow niches and are influenced by signals from surrounding cells, the extracellular matrix (ECM), ECM-bound or diffusible biomolecules. Here we describe the use of a three-dimensional hydrogel to alter the balance of HSC-generated autocrine feedback and paracrine signals generated by co-cultured niche-associated cells. We report shifts in HSC proliferation rate and fate specification in the presence of lineage positive (Lin⁺) niche cells. Hydrogels promoting autocrine feedback enhanced expansion of early hematopoietic progenitors while paracrine signals from Lin⁺ cells increased myeloid differentiation. We report thresholds where autocrine vs. paracrine cues alter HSC fate transitions, and were able to selectively abrogate the effects of matrix diffusivity and niche cell co-culture via the use of inhibitory cocktails of autocrine or paracrine signals. Together, these results suggest diffusive biotransport in three-dimensional biomaterials are a critical design element for the development of a synthetic stem cell niche.

Impact of matrix stiffness on fibroblast function

Chronic non-healing wounds, caused by impaired production of growth factors and reduced vascularization, represent a significant burden to patients, health care professionals, and health care system. While several wound dressing biomaterials have been developed, the impact of the mechanical properties of the dressings on the residing cells and consequently on the healing of the wounds is largely overlooked. The primary focus of this study is to explore whether manipulation of the substrate mechanics can regulate the function of fibroblasts, particularly in the context of their angiogenic activity. A photocrosslinkable hydrogel platform with orthogonal control over gel modulus and cell adhesive sites was developed to explore the quantitative relationship between ECM compliance and fibroblast function. Increase in matrix stiffness resulted in enhanced fibroblast proliferation and stress fiber formation. However, the angiogenic activity of fibroblasts was found to be optimum when the cells were seeded on compliant matrices. Thus, the observations suggest that the stiffness of the wound dressing material may play an important role in the progression of wound healing.

Effect of matrix metalloproteinase-mediated matrix degradation on glioblastoma cell behavior in 3D PEG-based hydrogels

Glioblastoma (GBM) is the most common and aggressive form of primary brain tumor with median survival of 12 months. To improve clinical outcomes, it is critical to develop in vitro models that support GBM proliferation and invasion for deciphering tumor progression and screening drug candidates. A key hallmark of GBM cells is their extreme invasiveness, a process mediated by matrix metalloproteinase (MMP)-mediated degradation of the extracellular matrix. We recently reported the development of a MMP-degradable, poly(ethylene-glycol)-based hydrogel platform for culturing GBM cells. In the present study, we modulated the percentage of MMP-degradable crosslinks in 3D hydrogels to analyze the effects of MMP-degradability on GBM fates. Using an immortalized GBM cell line (U87) as a model cell type, our results showed that MMP-degradability was not required for supporting GBM proliferation. All hydrogel formulations supported robust GBM proliferation, up to 10 fold after 14 days. However, MMP-degradability was essential for facilitating tumor spreading, and 50% MMP-degradable hydrogels were sufficient to enable both robust tumor cell proliferation and spreading in 3D. The findings of this study highlight the importance of modulating MMP-degradability in engineering 3D in vitro brain cancer models and may be applied for engineering in vitro models for other cancer types. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 770-778, 2017.

Precise Tuning of Facile One-Pot Gelatin Methacryloyl (GelMA) Synthesis

Gelatin-methacryloyl (GelMA) is one of the most commonly used photopolymerizable biomaterials in bio-applications. However, GelMA synthesis remains suboptimal, as its reaction parameters have not been fully investigated. The goal of this study is to establish an optimal route for effective and controllable GelMA synthesis by systematically examining reaction parameters including carbonate-bicarbonate (CB) buffer molarity, initial pH adjustment, MAA concentration, gelatin concentration, reaction temperature, and reaction time. We employed several analytical techniques in order to determine the degree of substitution (DS) and conducted detailed structural analysis of the synthesized polymer. The results enabled us to optimize GelMA synthesis, showing the optimal conditions to balance the deprotonation of amino groups with minimizing MAA hydrolysis, which led to nearly complete substitution. The optimized conditions (low feed ratio of MAA to gelatin (0.1 mL/g), 0.25 M CB buffer at pH 9, and a gelatin concentration of 10-20%) enable a simplified reaction scheme that produces GelMA with high substitution with just one-step addition of MAA in one pot. Looking forward, these optimal conditions not only enable facile one-pot GelMA synthesis but can also guide researchers to explore the efficient, high methacrylation of other biomacromolecules.

Strategies of Mesenchymal Invasion of Patient-derived Brain Tumors: Microenvironmental Adaptation

The high mortality in glioblastoma multiforme (GBM) patients is primarily caused by extensive infiltration into adjacent tissue and subsequent rapid recurrence. There are no clear therapeutic strategies that target the infiltrative subpopulation of GBM mass. Using mesenchymal mode of invasion, the GBM is known to widely infiltrate by interacting with various unique components within brain microenvironment such as hyaluronic acid (HA)-rich matrix and white matter tracts. However, it is unclear how these GBM microenvironments influence the strategies of mesenchymal invasion. We hypothesize that GBM has different strategies to facilitate such invasion through adaptation to their local microenvironment. Using our in vitro biomimetic microenvironment platform for three-dimensional GBM tumorspheres (TSs), we found that the strategies of GBM invasion were predominantly regulated by the HA-rich ECM microenvironment, showing marked phenotypic changes in the presence of HA, which were mainly mediated by HA synthase (HAS). Interestingly, after inhibition of the HAS gene, GBM switched their invasion strategies to a focal adhesion (FA)-mediated invasion. These results demonstrate that the microenvironmental adaptation allowed a flexible invasion strategy for GBM. Using our model, we suggest a new inhibitory pathway for targeting infiltrative GBM and propose an importance of multi-target therapy for GBM, which underwent microenvironmental adaptation.

A double-network poly(Nɛ-acryloyl L-lysine)/hyaluronic acid hydrogel as a mimic of the breast tumor microenvironment

To mimic the structure of breast tumor microenvironment, novel double-network poly(Nɛ-acryloyl L-lysine)/hyaluronic acid (pLysAAm/HA) hydrogels were fabricated by a two-step photo-polymerization process for in vitro three-dimensional (3D) cell culture. The morphology, mechanical properties, swelling and degradation behaviors of pLysAAm/HA hydrogels were investigated. The growth behavior and function of MCF-7 cells cultured on the hydrogels and standard 2D culture plates were compared. The results showed that pLysAAm/HA hydrogels had a highly porous microstructure with a double network and that their mechanical properties, swelling ratio and degradation rate depended on the degree of methacrylation of HA. The results of in vitro studies revealed that the pLysAAm/HA hydrogels could support MCF-7 cell adhesion, promote cell proliferation, and induce the diversification of cell morphologies and overexpression of VEGF, IL-8 and bFGF. The MCF-7 cells cultured on 3D hydrogels showed significantly increased migration and invasion abilities as compared to 2D-cultured cells. Preliminary in vivo results confirmed that the 3D culture of MCF-7 cells resulted in greater tumorigenesis than their 2D culture. These results indicate that the pLysAAm/HA hydrogels can provide a 3D microenvironment for MCF-7 cells that is more representative of the in vivo breast cancer.

STATEMENT OF SIGNIFICANCE

Traditional 2D cell cultures cannot ideally represent their in vivo physiological conditions. In this work, we reported a method for preparing double-network poly(Nɛ-acryloyl L-lysine)/hyaluronic acid hydrogel, and demonstrated its suitability for use in mimicing breast tumor microenvironment. Results showed the prepared hydrogels had controllable mechanical properties, swelling ratio and degradation rate. The MCF-7 cells cultured in hydrogels expressed much higher levels of pro-angiogenic growth factors and displayed significantly enhanced migration and invasion abilities. The tumorigenic capability of MCF-7 cells pre-cultured in 3D hydrogels was enhanced significantly. Therefore, the novel hydrogel may provide a more physiologically relevant 3D in vitro model for breast cancer research. To our knowledge, this is the first report assessing a HA-based double-network hydrogel used as a tumor model.

A three dimensional micropatterned tumor model for breast cancer cell migration studies

Breast cancer cell invasion is a highly orchestrated process driven by a myriad of complex microenvironmental stimuli, making it difficult to isolate and assess the effects of biochemical or biophysical cues (i.e. tumor architecture, matrix stiffness) on disease progression. In this regard, physiologically relevant tumor models are becoming instrumental to perform studies of cancer cell invasion within well-controlled conditions. Herein, we explored the use of photocrosslinkable hydrogels and a novel, two-step photolithography technique to microengineer a 3D breast tumor model. The microfabrication process enabled precise localization of cell-encapsulated circular constructs adjacent to a low stiffness matrix. To validate the model, breast cancer cell lines (MDA-MB-231, MCF7) and non-tumorigenic mammary epithelial cells (MCF10A) were embedded separately within the tumor model, all of which maintained high viability throughout the experiments. MDA-MB-231 cells exhibited extensive migratory behavior and invaded the surrounding matrix, whereas MCF7 or MCF10A cells formed clusters that stayed confined within the circular tumor regions. Additionally, real-time cell tracking indicated that the speed and persistence of MDA-MB-231 cells were substantially higher within the surrounding matrix compared to the circular constructs. Z-stack imaging of F-actin/α-tubulin cytoskeletal organization revealed unique 3D protrusions in MDA-MB-231 cells and an abundance of 3D clusters formed by MCF7 and MCF10A cells. Our results indicate that gelatin methacrylate (GelMA) hydrogel, integrated with the two-step photolithography technique, has great promise in the development of 3D tumor models with well-defined architecture and tunable stiffness.

Synthesis, properties, and biomedical applications of gelatin methacryloyl (GelMA) hydrogels

Gelatin methacryloyl (GelMA) hydrogels have been widely used for various biomedical applications due to their suitable biological properties and tunable physical characteristics. GelMA hydrogels closely resemble some essential properties of native extracellular matrix (ECM) due to the presence of cell-attaching and matrix metalloproteinase responsive peptide motifs, which allow cells to proliferate and spread in GelMA-based scaffolds. GelMA is also versatile from a processing perspective. It crosslinks when exposed to light irradiation to form hydrogels with tunable mechanical properties. It can also be microfabricated using different methodologies including micromolding, photomasking, bioprinting, self-assembly, and microfluidic techniques to generate constructs with controlled architectures. Hybrid hydrogel systems can also be formed by mixing GelMA with nanoparticles such as carbon nanotubes and graphene oxide, and other polymers to form networks with desired combined properties and characteristics for specific biological applications. Recent research has demonstrated the proficiency of GelMA-based hydrogels in a wide range of tissue engineering applications including engineering of bone, cartilage, cardiac, and vascular tissues, among others. Other applications of GelMA hydrogels, besides tissue engineering, include fundamental cell research, cell signaling, drug and gene delivery, and bio-sensing.

The use of covalently immobilized stem cell factor to selectively affect hematopoietic stem cell activity within a gelatin hydrogel

Hematopoietic stem cells (HSCs) are a rare stem cell population found primarily in the bone marrow and responsible for the production of the body's full complement of blood and immune cells. Used clinically to treat a range of hematopoietic disorders, there is a significant need to identify approaches to selectively expand their numbers ex vivo. Here we describe a methacrylamide-functionalized gelatin (GelMA) hydrogel for in vitro culture of primary murine HSCs. Stem cell factor (SCF) is a critical biomolecular component of native HSC niches in vivo and is used in large dosages in cell culture media for HSC expansion in vitro. We report a photochemistry based approach to covalently immobilize SCF within GelMA hydrogels via acrylate-functionalized polyethylene glycol (PEG) tethers. PEG-functionalized SCF retains the native bioactivity of SCF but can be stably incorporated and retained within the GelMA hydrogel over 7 days. Freshly-isolated murine HSCs cultured in GelMA hydrogels containing covalently-immobilized SCF showed reduced proliferation and improved selectivity for maintaining primitive HSCs. Comparatively, soluble SCF within the GelMA hydrogel network induced increased proliferation of differentiating hematopoietic cells. We used a microfluidic templating approach to create GelMA hydrogels containing gradients of immobilized SCF that locally direct HSC response. Together, we report a biomaterial platform to examine the effect of the local presentation of soluble vs. matrix-immobilized biomolecular signals on HSC expansion and lineage specification. This approach may be a critical component of a biomaterial-based artificial bone marrow to provide the correct sequence of niche signals to grow HSCs in the laboratory.

Spatially gradated hydrogel platform as a 3D engineered tumor microenvironment

There is an acute need for biomaterial tools that recreate the heterogeneous brain-tumor microenvironment. A microfluidic mixing tool is reported to encapsulate glioblastoma multiforme cells within miniaturized gelatin hydrogels containing overlapping patterns of tumor-inspired matrix signals. This approach permits in situ analysis of glioma cells at the molecular and genomic level as well as the potential for clinical insight.

Bioengineered Scaffolds for 3D Analysis of Glioblastoma Proliferation and Invasion

The invasion of malignant glioblastoma (GBM) cells into healthy brain is a primary cause of tumor recurrence and associated morbidity. Here, we describe a high-throughput method for quantitative measurement of GBM proliferation and invasion in three-dimensional (3D) culture. Optically clear hydrogels composed of thiolated hyaluronic acid and gelatin were chemically crosslinked with thiol-reactive poly(ethylene glycol) polymers to form an artificial 3D tumor microenvironment. Characterization of the viscoelasticity and aqueous stability indicated the hydrogels were mechanically tunable with stiffness ranging from 18 Pa to 18.2 kPa and were resistant to hydrolysis for at least 30 days. The proliferation, dissemination and subsequent invasion of U118 and U87R GBM spheroids cultured on the hydrogels were tracked in situ with repeated fluorescence confocal microscopy. Using custom automated image processing, cells were identified and quantified through 500 µm of gel over 14 days. Proliferative and invasive behaviors were observed to be contingent on cell type, gel stiffness, and hepatocyte growth factor availability. These measurements highlight the utility of this platform for performing quantitative, fluorescence imaging analysis of the behavior of malignant cells within an artificial, 3D tumor microenvironment.

Biomimetic tissue-engineered systems for advancing cancer research: NCI Strategic Workshop report

Advanced technologies and biomaterials developed for tissue engineering and regenerative medicine present tractable biomimetic systems with potential applications for cancer research. Recently, the National Cancer Institute convened a Strategic Workshop to explore the use of tissue biomanufacturing for development of dynamic, physiologically relevant in vitro and ex vivo biomimetic systems to study cancer biology and drug efficacy. The workshop provided a forum to identify current progress, research gaps, and necessary steps to advance the field. Opportunities discussed included development of tumor biomimetic systems with an emphasis on reproducibility and validation of new biomimetic tumor models, as described in this report.

Regulation of glioma cell phenotype in 3D matrices by hyaluronic acid

Human glioblastoma multiforme (hGBM) is the most common, aggressive, and deadly form of brain cancer. A major obstacle to understanding the impact of extracellular cues on glioblastoma invasion is the absence of model matrix systems able to replicate compositional and structural elements of the glioma mass as well as the surrounding brain tissue. Contact with a primary extracellular matrix component in the brain, hyaluronan, is believed to play a pivotal role in glioma cell invasion and malignancy. In this study we report use of gelatin and poly(ethylene glycol) (PEG) based hydrogel platforms to evaluate the effect of extracellular (composition, mechanics, HA incorporation) and intracellular (epidermal growth factor receptor overexpression) factors on the malignant transformation of U87MG glioma cells. Three-dimensional culture platforms elicit significantly different responses of U87MG glioma cells versus standard 2D culture. Critically, grafting brain-mimetic hyaluronic acid (HA) into the hydrogel network was found to induce significant, dose-dependent alterations of markers of glioma malignancy versus non-grafted 3D gelatin or PEG hydrogels. Clustering of glioma cells was observed exclusively in HA containing gels and expression profiles of malignancy-associated genes were found to vary biphasically with incorporated HA content. We also found HA-induced expression of MMP-2 is blocked by +EGFR signaling, suggesting a connection between CD44 and EGFR in glioma malignancy. Together, this work describes an adaptable platform for manipulating the local extracellular microenvironment surrounding glioma cells and highlights the importance of developing such systems for investigating the etiology and early growth of glioblastoma multiforme tumors.