Three-dimensional matrix stiffness and adhesive ligands affect cancer cell response to toxins

Recent Developments in Glioblastoma-On-A-Chip for Advanced Drug Screening Applications

Glioblastoma (GBM) is an aggressive form of cancer, comprising ≈80% of malignant brain tumors. However, there are no effective treatments for GBM due to its heterogeneity and the presence of the blood-brain barrier (BBB), which restricts the delivery of therapeutics to the brain. Despite in vitro models contributing to the understanding of GBM, conventional 2D models oversimplify the complex tumor microenvironment. Organ-on-a-chip (OoC) models have emerged as promising platforms that recapitulate human tissue physiology, enabling disease modeling, drug screening, and personalized medicine. There is a sudden increase in GBM-on-a-chip models that can significantly advance the knowledge of GBM etiology and revolutionize drug development by reducing animal testing and enhancing translation to the clinic. In this review, an overview of GBM-on-a-chip models and their applications is reported for drug screening and discussed current challenges and potential future directions for GBM-on-a-chip models.

Tunable Viscoelasticity of Alginate Hydrogels via Serial Autoclaving

Alginate hydrogels are widely used as biomaterials for cell culture and tissue engineering due to their biocompatibility and tunable mechanical properties. Reducing alginate molecular weight is an effective strategy for modulating hydrogel viscoelasticity and stress relaxation behavior, which can significantly impact cell spreading and fate. However, current methods like gamma irradiation to produce low molecular weight alginates suffer from high cost and limited accessibility. Here, a facile and cost-effective approach to reduce alginate molecular weight in a highly controlled manner using serial autoclaving is presented. Increasing the number of autoclave cycles results in proportional reductions in intrinsic viscosity, hydrodynamic radius, and molecular weight of the polymer while maintaining its chemical composition. Hydrogels fabricated from mixtures of the autoclaved alginates exhibit tunable mechanical properties, with inclusion of lower molecular weight alginate leading to softer gels with faster stress relaxation behaviors. The method is demonstrated by establishing how viscoelastic relaxation affects the spreading of encapsulated fibroblasts and glioblastoma cells. Results establish repetitive autoclaving as an easily accessible technique to generate alginates with a range of molecular weights and to control the viscoelastic properties of alginate hydrogels, and demonstrate utility across applications in mechanobiology, tissue engineering, and regenerative medicine.

Effects of TET2-mediated methylation reconstruction on A2058 melanoma cell sensitivity to matrix stiffness in a 3D culture system

Matrix stiffness is a crucial factor in the tumor microenvironment, impacting tumor progression and development. TET2 is vital for epigenetic regulation in melanoma and is significantly reduced in advanced melanomas compared with nevi and thin melanomas. However, it is unclear how TET2 mediates the effect of matrix stiffness on melanoma cells. This study utilized A2058 cell lines and prepared different stiffness collagen hydrogels to evaluate TET2 overexpression (TET2OE) and mutant (TET2M) melanoma cells' activity, proliferation, and invasion. A2058 melanoma cells' viability and invasion decreased with increased matrix stiffness, with TET2OE cells experiencing a more significant impact than TET2M cells. Methylation analysis revealed that TET2 determines gene methylation levels, influencing cell-ECM interactions. Transcriptome analysis confirmed that TET2 promotes matrix stiffness's effect on melanoma cell fate. This research provides promising directions and opportunities for melanoma treatment.

Varying the RGD concentration on a hyaluronic acid hydrogel influences dormancy versus proliferation in brain metastatic breast cancer cells

A majority of breast cancer deaths occur due to metastasis of cancer cells to distant organs. In particular, brain metastasis is very aggressive with an extremely low survival rate. Breast cancer cells that metastasize to the brain can enter a state of dormancy, which allows them to evade death. The brain microenvironment provides biophysical, biochemical, and cellular cues, and plays an important role in determining the fate of dormant cancer cells. However, how these cues influence dormancy remains poorly understood. Herein, we employed hyaluronic acid (HA) hydrogels with a stiffness of ~0.4 kPa as an in vitro biomimetic platform to investigate the impact of biochemical cues, specifically alterations in RGD concentration, on dormancy versus proliferation in MDA-MB-231Br brain metastatic breast cancer cells. We applied varying concentrations of RGD peptide (0, 1, 2, or 4 mg/mL) to HA hydrogel surfaces and confirmed varying degrees of surface functionalization using a fluorescently labeled RGD peptide. Post functionalization, ~10,000 MDA-MB-231Br cells were seeded on top of the hydrogels and cultured for 5 days. We found that an increase in RGD concentration led to changes in cell morphology, with cells transitioning from a rounded to spindle-like morphology as well as an increase in cell spreading area. Also, an increase in RGD concentration resulted in an increase in cell proliferation. Cellular dormancy was assessed using the ratio of phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK) to phosphorylated p38 (p-p38) positivity, which was significantly lower in hydrogels without RGD and in hydrogels with lowest RGD concentration compared to hydrogels functionalized with higher RGD concentration. We also demonstrated that the HA hydrogel-induced cellular dormancy was reversible. Finally, we demonstrated the involvement of β1 integrin in mediating cell phenotype in our hydrogel platform. Overall, our results provide insight into the role of biochemical cues in regulating dormancy versus proliferation in brain metastatic breast cancer cells.

Effect of cell adhesiveness of Cell Dome shell on enclosed HeLa cells

The Cell Dome is a dome-shaped structure (diameter: 1 mm, height: 270 μm) with cells enclosed within a cavity, covered by a hemispherical hydrogel shell, and immobilized on a glass plate. Given that the cells within Cell Dome are in contact with the inner walls of the hydrogel shell, the properties of the shell are anticipated to influence cell behavior. To date, the impact of the hydrogel shell properties on the enclosed cells has not been investigated. In this study, we explored the effects of the cell adhesiveness of hydrogel shell on the behavior of enclosed cancer cells. Hydrogel shells with varying degrees of cell adhesiveness were fabricated using aqueous solutions containing either an alginate derivative with phenolic hydroxyl moieties exclusively or a mixture of alginate and gelatin derivatives with phenolic hydroxyl moieties. Hydrogel formation was mediated by horseradish peroxidase. We used the HeLa human cervical cancer cell line, which expresses fucci2, a cell cycle marker, to observe cell behavior. Cells cultured in hydrogel shells with cell adhesiveness proliferated along the inner wall of the hydrogel shell. Conversely, cells in hydrogel shells without cell adhesiveness grew uniformly at the bottom of the cavities. Furthermore, cells in non-adhesive hydrogel shells had a higher percentage of cells in the G1/G0 phase compared to those in adhesive shells and exhibited increased resistance to mitomycin hydrochloride when the cavities became filled with cells. These results highlight the need to consider the cell adhesiveness of the hydrogel shell when selecting materials for constructing Cell Dome.

Mechanobiology of cancer cell responsiveness to chemotherapy and immunotherapy: Mechanistic insights and biomaterial platforms

Mechanical forces are central to how cancer treatments such as chemotherapeutics and immunotherapies interact with cells and tissues. At the simplest level, electrostatic forces underlie the binding events that are critical to therapeutic function. However, a growing body of literature points to mechanical factors that also affect whether a drug or an immune cell can reach a target, and to interactions between a cell and its environment affecting therapeutic efficacy. These factors affect cell processes ranging from cytoskeletal and extracellular matrix remodeling to transduction of signals by the nucleus to metastasis of cells. This review presents and critiques the state of the art of our understanding of how mechanobiology impacts drug and immunotherapy resistance and responsiveness, and of the in vitro systems that have been of value in the discovery of these effects.

Sandwich Culture Platforms to Investigate the Roles of Stiffness Gradients and Cell–Matrix Adhesions in Cancer Cell Migration

Given the key role of cell migration in cancer metastasis, there is a critical need for in vitro models that better capture the complexities of in vivo cancer cell microenvironments. Using both two-dimensional (2D) and three-dimensional (3D) culture models, recent research has demonstrated the role of both matrix and ligand densities in cell migration. Here, we leveraged our previously developed 2.5D sandwich culture platform to foster a greater understanding of the adhesion-dependent migration of glioblastoma cells with a stiffness gradient. Using this model, we demonstrated the differential role of stiffness gradients in migration in the presence and absence of adhesion moieties. Furthermore, we observed a positive correlation between the density of cell adhesion moieties and migration, and a diminished role of stiffness gradients at higher densities of adhesion moieties. These results, i.e., the reduced impact of stiffness gradients on adhesion-dependent migration relative to adhesion-independent migration, were confirmed using inhibitors of both mechanotransduction and cell adhesion. Taken together, our work demonstrates the utility of sandwich culture platforms that present stiffness gradients to study both adhesion-dependent and -independent cell migration and to help expand the existing portfolio of in vitro models of cancer metastasis.

Pushing the Natural Frontier: Progress on the Integration of Biomaterial Cues toward Combinatorial Biofabrication and Tissue Engineering

The engineering of fully functional, biological-like tissues requires biomaterials to direct cellular events to a near-native, 3D niche extent. Natural biomaterials are generally seen as a safe option for cell support, but their biocompatibility and biodegradability can be just as limited as their bioactive/biomimetic performance. Furthermore, integrating different biomaterial cues and their final impact on cellular behavior is a complex equation where the outcome might be very different from the sum of individual parts. This review critically analyses recent progress on biomaterial-induced cellular responses, from simple adhesion to more complex stem cell differentiation, looking at the ever-growing possibilities of natural materials modification. Starting with a discussion on native material formulation and the inclusion of cell-instructive cues, the roles of shape and mechanical stimuli, the susceptibility to cellular remodeling, and the often-overlooked impact of cellular density and cell-cell interactions within constructs, are delved into. Along the way, synergistic and antagonistic combinations reported in vitro and in vivo are singled out, identifying needs and current lessons on the development of natural biomaterial libraries to solve the cell-material puzzle efficiently. This review brings together knowledge from different fields envisioning next-generation, combinatorial biomaterial development toward complex tissue engineering.

Mechanical Properties of the Extracellular Environment of Human Brain Cells Drive the Effectiveness of Drugs in Fighting Central Nervous System Cancers

The evaluation of nanomechanical properties of tissues in health and disease is of increasing interest to scientists. It has been confirmed that these properties, determined in part by the composition of the extracellular matrix, significantly affect tissue physiology and the biological behavior of cells, mainly in terms of their adhesion, mobility, or ability to mutate. Importantly, pathophysiological changes that determine disease development within the tissue usually result in significant changes in tissue mechanics that might potentially affect the drug efficacy, which is important from the perspective of development of new therapeutics, since most of the currently used in vitro experimental models for drug testing do not account for these properties. Here, we provide a summary of the current understanding of how the mechanical properties of brain tissue change in pathological conditions, and how the activity of the therapeutic agents is linked to this mechanical state.

Glioblastoma spheroid growth and chemotherapeutic responses in single and dual-stiffness hydrogels

Glioblastoma (GBM) is the deadliest brain tumor for which there is no cure. Bioengineered GBM models, such as hydrogel-encapsulated spheroids, that capture both cell-cell and cell-matrix interactions could facilitate testing of much needed therapies. Elucidation of specific microenvironment properties on spheroid responsiveness to therapeutics would enhance the usefulness of GBM models as predictive drug screening platforms. Here, GBM spheroids consisting of U87 or patient-derived GBM cells were encapsulated in soft (∼1 kPa), stiff (∼7 kPa), and dual-stiffness polyethylene glycol-based hydrogels, with GBM spheroids seeded at the stiffness interface. Spheroids were cultured for 7 days and examined for viability, size, invasion, laminin expression, hypoxia, proliferation, and response to the chemotherapeutic temozolomide (TMZ). We noted excellent cell viability in all hydrogels, and higher infiltration in soft compared to stiff hydrogels for U87 spheroids. In dual gels spheroids mostly infiltrated away from the stiffness interface with minimal crossing over it and some individual cell migration along the interface. U87 spheroids were equally responsive to TMZ in the soft and stiff hydrogels, but cell viability in the spheroid periphery was higher than the core for stiff hydrogels whereas the opposite was true for soft hydrogels. HIF1A expression was higher in the core of spheroids in the stiff hydrogels, while there was no difference in cell proliferation between spheroids in the stiff vs soft hydrogels. Patient-derived GBM spheroids did not show stiffness-dependent drug responses. U87 cells showed similar laminin expression in soft and stiff hydrogels with higher expression in the spheroid periphery compared to the core. Our results indicate that microenvironment stiffness needs to be considered in bioengineered GBM models including those designed for use in drug screening applications. STATEMENT OF SIGNIFICANCE: Recent work on tumor models engineered for use in drug screening has highlighted the potential of hydrogel-encapsulated spheroids as a simple, yet effective platform that show drug responses similar to native tumors. It has also been shown that substrate stiffness, in vivo and in vitro, affects cancer cell responses to drugs. This is particularly important for glioblastoma (GBM), the deadliest brain cancer, as GBM cells invade by following the stiffer brain structures such as white matter tracks and the perivascular niche. Invading cells have also been associated with higher resistance to chemotherapy. Here we developed GBM spheroid models using soft, stiff and dual-stiffness hydrogels to explore the connection between substrate stiffness, spheroid invasion and drug responsiveness in a controlled environment.

Materials-driven approaches to understand extrinsic drug resistance in cancer

Metastatic cancer has a poor prognosis, because it is broadly disseminated and associated with both intrinsic and acquired drug resistance. Critical unmet needs in effectively killing drug resistant cancer cells include overcoming the drug desensitization characteristics of some metastatic cancers/lesions, and tailoring therapeutic regimens to both the tumor microenvironment and the genetic profiles of the resident cancer cells. Bioengineers and materials scientists are developing technologies to determine how metastatic sites exclude therapies, and how extracellular factors (including cells, proteins, metabolites, extracellular matrix, and abiotic factors) at metastatic sites significantly affect drug pharmacodynamics. Two looming challenges are determining which feature, or combination of features, from the tumor microenvironment drive drug resistance, and what the relative impact is of extracellular signals vs. intrinsic cell genetics in determining drug response. Sophisticated systems biology tools that can de-convolve a crowded network of signals and responses, as well as controllable microenvironments capable of providing discrete and tunable extracellular cues can help us begin to interrogate the high dimensional interactions governing drug resistance in patients.

Bioscaffold Stiffness Mediates Aerosolized Nanoparticle Uptake in Lung Epithelial Cells

In this study, highly porous, ultrasoft polymeric mats mimicking human tissues were formed from novel polyurethane soft dendritic colloids (PU SDCs). PU SDCs have a unique fibrillar morphology controlled by antisolvent precipitation. When filtered from suspension, PU SDCs form mechanically robust nonwoven mats. The stiffness of the SDC mats can be tuned for physiological relevance. The unique physiochemical characteristics of the PU SDC particles dictate the mechanical properties resulting in tunable elastic moduli ranging from 200 to 800 kPa. The human lung A549 cells cultured on both stiff and soft PU SDC membranes were found to be viable, capable of supporting the air-liquid interface (ALI) cell culture, and maintained barrier integrity. Furthermore, A549 cellular viability and uptake efficiency of aerosolized tannic acid-coated gold nanoparticles (Ta-Au) was found to depend on elastic modulus and culture conditions. Ta-Au nanoparticle uptake was twofold and fourfold greater on soft PU SDCs, when cultured at submerged and ALI conditions, respectively. The significant increase in endocytosed Ta-Au resulted in a 20% decrease in viability, and a 4-fold increase in IL-8 cytokine secretion when cultured on soft PU SDCs at ALI. Common tissue culture materials exhibit super-physiological elastic moduli, a factor found to be critical in analyzing nanomaterial cellular interactions and biological responses.

Personalized models of heterogeneous 3D epithelial tumor microenvironments: Ovarian cancer as a model

Intractable human diseases such as cancers, are context dependent, unique to both the individual patient and to the specific tumor microenvironment. However, conventional cancer treatments are often nonspecific, targeting global similarities rather than unique drivers. This limits treatment efficacy across heterogeneous patient populations and even at different tumor locations within the same patient. Ultimately, this poor efficacy can lead to adverse clinical outcomes and the development of treatment-resistant relapse. To prevent this and improve outcomes, it is necessary to be selective when choosing a patient's optimal adjuvant treatment. In this review, we posit the use of personalized, tumor-specific models (TSM) as tools to achieve this remarkable feat. First, using ovarian cancer as a model disease, we outline the heterogeneity and complexity of both the cellular and extracellular components in the tumor microenvironment. Then we examine the advantages and disadvantages of contemporary cancer models and the rationale for personalized TSM. We discuss how to generate precision 3D models through careful and detailed analysis of patient biopsies. Finally, we provide clinically relevant applications of these versatile personalized cancer models to highlight their potential impact. These models are ideal for a myriad of fundamental cancer biology and translational studies. Importantly, these approaches can be extended to other carcinomas, facilitating the discovery of new therapeutics that more effectively target the unique aspects of each individual patient's TME. STATEMENT OF SIGNIFICANCE: In this article, we have presented the case for the application of biomaterials in developing personalized models of complex diseases such as cancers. TSM could bring about breakthroughs in the promise of precision medicine. The critical components of the diverse tumor microenvironments, that lead to treatment failures, include cellular- and extracellular matrix- heterogeneity, and biophysical signals to the cells. Therefore, we have described these dynamic components of the tumor microenvironments, and have highlighted how contemporary biomaterials can be utilized to create personalized in vitro models of cancers. We have also described the application of the TSM to predict the dynamic patterns of disease progression, and predict effective therapies that can produce durable responses, limit relapses, and treat any minimal residual disease.

Self-Assembling Polypeptide Hydrogels as a Platform to Recapitulate the Tumor Microenvironment

Simple Summary

The tumor microenvironment is characterized by increased tissue stiffness, low (acidic) pH, and elevated temperature, all of which contribute to the development of cancer. Improving our in vitro models of cancer, therefore, requires the development of cell culture platforms that can mimic these microenvironmental properties. Here, we study a new biomaterial composed of short amino acid chains that self-assemble into a fibrous hydrogel network. This material enables simultaneous and independent tuning of substrate rigidity, extracellular pH, and temperature, allowing us to mimic both healthy tissues and the tumor microenvironment. We used this platform to study the effect of these conditions on pancreatic cancer cells and found that high substrate rigidity and low pH promote proliferation and survival of cancer cells and activate important signaling pathways associated with cancer progression.

Abstract

The tumor microenvironment plays a critical role in modulating cancer cell migration, metabolism, and malignancy, thus, highlighting the need to develop in vitro culture systems that can recapitulate its abnormal properties. While a variety of stiffness-tunable biomaterials, reviewed here, have been developed to mimic the rigidity of the tumor extracellular matrix, culture systems that can recapitulate the broader extracellular context of the tumor microenvironment (including pH and temperature) remain comparably unexplored, partially due to the difficulty in independently tuning these parameters. Here, we investigate a self-assembled polypeptide network hydrogel as a cell culture platform and demonstrate that the culture parameters, including the substrate stiffness, extracellular pH and temperature, can be independently controlled. We then use this biomaterial as a cell culture substrate to assess the effect of stiffness, pH and temperature on Suit2 cells, a pancreatic cancer cell line, and demonstrate that these microenvironmental factors can regulate two critical transcription factors in cancer: yes-associated protein 1 (YAP) and hypoxia inducible factor (HIF-1A).

Advances in 3D peptide hydrogel models in cancer research

In vitro cell culture models on monolayer surfaces (2D) have been widely adapted for identification of chemopreventive food compounds and food safety evaluation. However, the low correlation between 2D models and in vivo animal models has always been a concern; this gap is mainly caused by the lack of a three-dimensional (3D) extracellular microenvironment. In 2D models, cell behaviors and functionalities are altered, resulting in varied responses to external conditions (i.e., antioxidants) and hence leading to low predictability. Peptide hydrogel 3D scaffolding technologies, such as PGmatrix for cell culture, have been recently reported to grow organoid-like spheroids physiologically mimicking the 3D microenvironment that can be used as an in vitro 3D model for investigating cell activities, which is anticipated to improve the prediction rate. Thus, this review focuses on advances in 3D peptide hydrogels aiming to introduce 3D cell culture tools as in vitro 3D models for cancer-related research regarding food safety and nutraceuticals.

In vitro biomimetic models for glioblastoma-a promising tool for drug response studies

The poor response of glioblastoma to current treatment protocols is a consequence of its intrinsic drug resistance. Resistance to chemotherapy is primarily associated with considerable cellular heterogeneity, and plasticity of glioblastoma cells, alterations in gene expression, presence of specific tumor microenvironment conditions and blood-brain barrier. In an attempt to successfully overcome chemoresistance and better understand the biological behavior of glioblastoma, numerous tri-dimensional (3D) biomimetic models were developed in the past decade. These novel advanced models are able to better recapitulate the spatial organization of glioblastoma in a real time, therefore providing more realistic and reliable evidence to the response of glioblastoma to therapy. Moreover, these models enable the fine-tuning of different tumor microenvironment conditions and facilitate studies on the effects of the tumor microenvironment on glioblastoma chemoresistance. This review outlines current knowledge on the essence of glioblastoma chemoresistance and describes the progress achieved by 3D biomimetic models. Moreover, comprehensive literature assessment regarding the influence of 3D culturing and microenvironment mimicking on glioblastoma gene expression and biological behavior is also provided. The contribution of the blood-brain barrier as well as the blood-tumor barrier to glioblastoma chemoresistance is also reviewed from the perspective of 3D biomimetic models. Finally, the role of mathematical models in predicting 3D glioblastoma behavior and drug response is elaborated. In the future, technological innovations along with mathematical simulations should create reliable 3D biomimetic systems for glioblastoma research that should facilitate the identification and possibly application in preclinical drug testing and precision medicine.

Cancer Stem Cell Microenvironment Models with Biomaterial Scaffolds In Vitro

Defined by its potential for self-renewal, differentiation and tumorigenicity, cancer stem cells (CSCs) are considered responsible for drug resistance and relapse. To understand the behavior of CSC, the effects of the microenvironment in each tissue are a matter of great concerns for scientists in cancer biology. However, there are many complicated obstacles in the mimicking the microenvironment of CSCs even with current advanced technology. In this context, novel biomaterials have widely been assessed as in vitro platforms for their ability to mimic cancer microenvironment. These efforts should be successful to identify and characterize various CSCs specific in each type of cancer. Therefore, extracellular matrix scaffolds made of biomaterial will modulate the interactions and facilitate the investigation of CSC associated with biological phenomena simplifying the complexity of the microenvironment. In this review, we summarize latest advances in biomaterial scaffolds, which are exploited to mimic CSC microenvironment, and their chemical and biological requirements with discussion. The discussion includes the possible effects on both cells in tumors and microenvironment to propose what the critical factors are in controlling the CSC microenvironment focusing the future investigation. Our insights on their availability in drug screening will also follow the discussion.

Matrix Stiffness-Regulated Growth of Breast Tumor Spheroids and Their Response to Chemotherapy

Interactions between tumor cells and the extracellular matrix (ECM) are an important factor contributing to therapy failure in cancer patients. Current in vitro breast cancer spheroid models examining the role of mechanical properties on spheroid response to chemotherapy are limited by the use of two-dimensional cell culture, as well as simultaneous variation in hydrogel matrix stiffness and other properties, e.g., hydrogel composition, pore size, and cell adhesion ligand density. In addition, currently used hydrogel matrices do not replicate the filamentous ECM architecture in a breast tumor microenvironment. Here, we report a collagen-alginate hydrogel with a filamentous architecture and a 20-fold variation in stiffness, achieved independently of other properties, used for the evaluation of estrogen receptor-positive breast cancer spheroid response to doxorubicin. The variation in hydrogel mechanical properties was achieved by altering the degree of cross-linking of alginate molecules. We show that soft hydrogels promote the growth of larger MCF-7 tumor spheroids with a lower fraction of proliferating cells and enhance spheroid resistance to doxorubicin. Notably, the stiffness-dependent chemotherapeutic response of the spheroids was temporally mediated: it became apparent at sufficiently long cell culture times, when the matrix stiffness has influenced the spheroid growth. These findings highlight the significance of decoupling matrix stiffness from other characteristics in studies of chemotherapeutic resistance of tumor spheroids and in development of drug screening platforms.

Injectable biomaterials for treatment of glioblastoma

Despite ongoing advancements in the field of medicine, glioblastoma multiforme (GBM) is presently incurable, making this advanced brain tumor the deadliest tumor type in the central nervous system. The primary treatment strategies for GBM (i.e. surgical resection, radiation therapy, chemotherapy, and newly incorporated targeted therapies) fail to overcome the challenging characteristics of highly aggressive GBM tumors and are presently given with the goal of increasing the quality of life for patients. With the aim of creating effective treatment solutions, research has shifted toward utilizing injectable biomaterial adjuncts to minimize invasiveness of treatment, provide spatiotemporal control of therapeutic delivery, and engage with cells through material-cell interfaces. This review aims to summarize the limitations of the current standard of care for GBM, discuss how these limitations can be addressed by local employment of injectable biomaterial systems, and highlight developments in the field of biomaterials for these applications.

Integrating Biophysics in Toxicology

Integration of biophysical stimulation in test systems is established in diverse branches of biomedical sciences including toxicology. This is largely motivated by the need to create novel experimental setups capable of reproducing more closely in vivo physiological conditions. Indeed, we face the need to increase predictive power and experimental output, albeit reducing the use of animals in toxicity testing. In vivo, mechanical stimulation is essential for cellular homeostasis. In vitro, diverse strategies can be used to model this crucial component. The compliance of the extracellular matrix can be tuned by modifying the stiffness or through the deformation of substrates hosting the cells via static or dynamic strain. Moreover, cells can be cultivated under shear stress deriving from the movement of the extracellular fluids. In turn, introduction of physical cues in the cell culture environment modulates differentiation, functional properties, and metabolic competence, thus influencing cellular capability to cope with toxic insults. This review summarizes the state of the art of integration of biophysical stimuli in model systems for toxicity testing, discusses future challenges, and provides perspectives for the further advancement of in vitro cytotoxicity studies.

A Soft Mechanical Phenotype of SH-SY5Y Neuroblastoma and Primary Human Neurons Is Resilient to Oligomeric Aβ(1-42) Injury

Aggregated amyloid beta (Aβ) is widely reported to cause neuronal dystrophy and toxicity through multiple pathways: oxidative stress, disrupting calcium homeostasis, and cytoskeletal dysregulation. The neuro-cytoskeleton is a dynamic structure that reorganizes to maintain cell homeostasis in response to varying soluble and physical cues presented from the extracellular matrix (ECM). Due this relationship between cell health and the ECM, we hypothesize that amyloid toxicity may be directly influenced by physical changes to the ECM (stiffness and dimensionality) through mechanosensitive pathways, and while previous studies demonstrated that Aβ can distort focal adhesion signaling with pathological consequences, these studies do not address the physical contribution from a physiologically relevant matrix. To test our hypothesis that physical cues can adjust Aβ toxicity, SH-SY5Y human neuroblastoma and primary human cortical neurons were plated on soft and stiff, 2D polyacrylamide matrices or suspended in 3D collagen gels. Each cell culture was exposed to escalating concentrations of oligomeric or fibrillated Aβ(1-42) with MTS viability and lactate dehydrogenase toxicity assessed. Actin restructuring was further monitored in live cells by atomic force microscopy nanoindentation, and our results demonstrate that increasing either matrix stiffness or exposure to oligomeric Aβ promotes F-actin polymerization and cell stiffening, while mature Aβ fibrils yielded no apparent cell stiffening and minor toxicity. Moreover, the rounded, softer mechanical phenotype displayed by cells plated onto a compliant matrix also demonstrated a resilience to oligomeric Aβ as noted by a significant recovery of viability when compared to same-dosed cells plated on traditional tissue culture plastic. This recovery was reproduced pharmacologically through inhibiting actin polymerization with cytochalasin D prior to Aβ exposure. These studies indicate that the cell-ECM interface can modify amyloid toxicity in neurons and the matrix-mediated pathways that promote this protection may offer unique targets in amyloid pathologies like Alzheimer's disease.

Microstructural densification and alignment by aspiration-ejection influence cancer cell interactions with three-dimensional collagen networks

Extracellular matrix microstructure and mechanics are crucial to breast cancer progression and invasion into surrounding tissues. The peritumor collagen network is often dense and aligned, features which in vitro models lack. Aspiration of collagen hydrogels led to densification and alignment of microstructure surrounding embedded cancer cells. Two metastasis-derived breast cancer cell lines, MDA-MB-231 and MCF-7, were cultured in initially 4 mg/ml collagen gels for 3 days after aspiration, as well as in unaspirated control hydrogels. Videomicroscopy during aspiration, and at 0, 1, and 3 days after aspiration, epifluorescence microscopy of phalloidin-stained F-actin cytoskeleton, histological sections, and soluble metabolic byproducts from constructs were collected to characterize effects on the embedded cell morphology, the collagen network microstructure, and proliferation. Breast cancer cells remained viable after aspiration-ejection, proliferating slightly less than in unaspirated gels. Furthermore, MDA-MB-231 cells appear to partially relax the collagen network and lose alignment 3 days after aspiration. Aspiration-ejection generated aligned, compact collagen network microstructure with immediate cell co-orientation and higher cell number density apparently through purely physical means, though cell-collagen contact guidance and network remodeling influence cell organization and collagen network microstructure during subsequent culture. This study establishes a platform to determine the effects of collagen density and alignment on cancer cell behavior, with translational potential for anticancer drug screening in a biomimetic three-dimensional matrix microenvironment, or implantation in preclinical models.

Gelatin interpenetration in poly N-isopropylacrylamide network reduces the compressive modulus of the scaffold: A property employed to mimic hepatic matrix stiffness

The progression of liver disease from normal to cirrhotic state is characterized by modulation of the stiffness of the extracellular matrix (ECM). Mimicking this modulation in vitro scaffold could provide a better insight into hepatic cell behavior. In this study, interpenetrating poly(N-isopropylacrylamide-co-gelatin) cryogels were synthesized in 48 different compositions to yield scaffolds of different properties. It was observed that a high concentration of N-isopropylacrylamide (NIPAAm) leads to the formation of small pores while gelatin interpenetration on poly-NIPAAm framework renders porous structure. Swelling properties and porosity of the gels decreased with an increase in NIPAAm concentration owing to the increased compactness of the gels. Gelatin interpenetration relaxed the gels and enhanced these properties. An increase in gelatin concentration led to a reduction in compressive moduli indicating that gelatin interpenetration in the poly-NIPAAm network softens the cryogel. With the increase in NIPAAm concentration, the effect of gelatin interpenetration in reducing the compressive moduli expanded. The cytocompatibility studies indicated that the gels are cell-adherent and compatible with HepG2. Furthermore, biochemical and real-time polymerase chain reaction studies revealed that HepG2 and Huh-7 cells cultured on scaffolds mimicking the ECM stiffness of normal liver (1.5-2.5 kPa) exhibited optimum liver-specific functionalities. Increasing the stiffness to fibrotic (4-9 kPa) and cirrhotic (10-20 kPa) ECM decreases the functionality.

Emerging themes and unifying concepts underlying cell behavior regulation by the pericellular space

Cells reside in a complex three-dimensional (3D) microenvironment where physical, chemical, and architectural features of the pericellular space regulate important cellular functions like migration, differentiation, and morphogenesis. A major goal of tissue engineering is to identify which properties of the pericellular space orchestrate these emergent cell behaviors and how. In this review, we highlight recent studies at the interface of biomaterials and single cell biophysics that are lending deeper insight towards this goal. Advanced methods have enabled the decoupling of architectural and mechanical features of the microenvironment, revealing multiple mechanisms of adhesion and mechanosensing modulation by biomaterials. Such studies are revealing important roles for pericellular space degradability, hydration, and adhesion competition in cell shape, volume, and differentiation regulation. STATEMENT OF SIGNIFICANCE: Cell fate and function are closely regulated by the local extracellular microenvironment. Advanced methods at the interface of single cell biophysics and biomaterials have shed new light on regulators of cell-pericellular space interactions by decoupling more features of the complex pericellular milieu than ever before. These findings lend deeper mechanistic insight into how biomaterials can be designed to fine-tune outcomes like differentiation, migration, and collective morphogenesis.

Multi-modal characterization of polymeric gels to determine the influence of testing method on observed elastic modulus

Demand for materials that mechanically replicate native tissue has driven development and characterization of various new biomaterials. However, a consequence of materials and characterization technique diversity is a lack of consensus within the field, with no clear way to compare values measured via different modalities. This likely contributes to the difficulty in replicating findings across the research community; recent evidence suggests that different modalities do not yield the same mechanical measurements within a material, and direct comparisons cannot be made across different testing platforms. Herein, we examine whether "material properties" are characterization modality-specific by analyzing the elastic moduli determined by five typical biomaterial mechanical characterization techniques: unconfined-compression, tensiometry, rheometry, and micro-indentation at the macroscopic level, and microscopically using nanoindentation. These analyses were performed in two different polymeric gels frequently used for biological applications, polydimethylsiloxane (PDMS) and agarose. Each was fabricated to span a range of moduli, from physiologic to supraphysiologic values. All five techniques identified the same overall trend within each material group, supporting their ability to appreciate relative moduli differences. However, significant differences were found across modalities, illustrating a difference in absolute moduli values, and thereby precluding direct comparison of measurements from different characterization modalities. These observed differences may depend on material compliance, viscoelasticity, and microstructure. While determining the underlying mechanism(s) of these differences was beyond the scope of this work, these results demonstrate how each modality affects the measured moduli of the same material, and the sensitivity of each modality to changes in sample material composition.

Templated Macroporous Polyethylene Glycol Hydrogels for Spheroid and Aggregate Cell Culture

Macroporous cell-laden hydrogels have recently gained recognition for a wide range of biomedical and bioengineering applications. There are various approaches to create porosity in hydrogels, including lyophilization or foam formation. However, many do not allow a precise control over pore size or are not compatible with in situ cell encapsulation. Here, we developed novel templated macroporous hydrogels by encapsulating uniform degradable hydrogel microspheres produced via microfluidics into a hydrogel slab. The microspheres degraded completely leaving macropores behind. Microsphere degradation was dependent on the incubation medium, microsphere size, microsphere confinement in the hydrogel as well as cell encapsulation. Uniquely, the degradable microspheres were biocompatible and when laden with cells, the cells were deposited in the macropores upon microsphere degradation and formed multicellular aggregates. The hydrogel-encapsulated cell aggregates were used in a small drug screen to demonstrate the relevance of cell-matrix interactions for multicellular spheroid drug responsiveness. Hydrogel-grown spheroid cultures are increasingly important in applications such as in vitro tumor, hepatocellular, and neurosphere cultures and drug screening; hence, the templated cell aggregate-laden hydrogels described here would find utility in various applications.

Review: Mechanotransduction in ovarian cancer: Shearing into the unknown

Ovarian cancer remains a deadly diagnosis with an 85% recurrence rate and a 5-year survival rate of only 46%. The poor outlook of this disease has improved little over the past 50 years owing to the lack of early detection, chemoresistance and the complex tumor microenvironment. Within the peritoneal cavity, the presence of ascites stimulates ovarian tumors with shear stresses. The stiff environment found within the tumor extracellular matrix and the peritoneal membrane are also implicated in the metastatic potential and epithelial to mesenchymal transition (EMT) of ovarian cancer. Though these mechanical cues remain highly relevant to the understanding and treatment of ovarian cancers, our current knowledge of their biological processes and their clinical relevance is deeply lacking. Seminal studies on ovarian cancer mechanotransduction have demonstrated close ties between mechanotransduction and ovarian cancer chemoresistance, EMT, enhanced cancer stem cell populations, and metastasis. This review summarizes our current understanding of ovarian cancer mechanotransduction and the gaps in knowledge that exist. Future investigations on ovarian cancer mechanotransduction will greatly improve clinical outcomes via systematic studies that determine shear stress magnitude and its influence on ovarian cancer progression, metastasis, and treatment.

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.

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.

A biomaterial screening approach reveals microenvironmental mechanisms of drug resistance

Traditional drug screening methods lack features of the tumor microenvironment that contribute to resistance. Most studies examine cell response in a single biomaterial platform in depth, leaving a gap in understanding how extracellular signals such as stiffness, dimensionality, and cell-cell contacts act independently or are integrated within a cell to affect either drug sensitivity or resistance. This is critically important, as adaptive resistance is mediated, at least in part, by the extracellular matrix (ECM) of the tumor microenvironment. We developed an approach to screen drug responses in cells cultured on 2D and in 3D biomaterial environments to explore how key features of ECM mediate drug response. This approach uncovered that cells on 2D hydrogels and spheroids encapsulated in 3D hydrogels were less responsive to receptor tyrosine kinase (RTK)-targeting drugs sorafenib and lapatinib, but not cytotoxic drugs, compared to single cells in hydrogels and cells on plastic. We found that transcriptomic differences between these in vitro models and tumor xenografts did not reveal mechanisms of ECM-mediated resistance to sorafenib. However, a systems biology analysis of phospho-kinome data uncovered that variation in MEK phosphorylation was associated with RTK-targeted drug resistance. Using sorafenib as a model drug, we found that co-administration with a MEK inhibitor decreased ECM-mediated resistance in vitro and reduced in vivo tumor burden compared to sorafenib alone. In sum, we provide a novel strategy for identifying and overcoming ECM-mediated resistance mechanisms by performing drug screening, phospho-kinome analysis, and systems biology across multiple biomaterial environments.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

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.

Polymeric Biomaterials for In Vitro Cancer Tissue Engineering and Drug Testing Applications

Biomimetic polymers and materials have been widely used in tissue engineering for regeneration and replication of diverse types of both normal and diseased tissues. Cancer, being a prevalent disease throughout the world, has initiated substantial interest in the creation of tissue-engineered models for anticancer drug testing. The development of these in vitro three-dimensional (3D) culture models using novel biomaterials has facilitated the investigation of tumorigenic and associated biological phenomena with a higher degree of complexity and physiological context than that provided by established two-dimensional culture models. In this review, an overview of a wide range of natural, synthetic, and hybrid biomaterials used for 3D cancer cell culture and investigation of cancer cell behavior is presented. The role of these materials in modulating cell-matrix interactions and replicating specific tumorigenic characteristics is evaluated. In addition, recent advances in biomaterial design, synthesis, and fabrication are also assessed. Finally, the advantages of incorporating polymeric biomaterials in 3D cancer models for obtaining efficacy data in anticancer drug testing applications are highlighted.

Mechanotransduction in cancer

Tissue stiffness is tightly controlled under normal conditions, but changes with disease. In cancer, tumors often tend to be stiffer than the surrounding uninvolved tissue, yet the cells themselves soften. Within the past decade, and particularly in the last few years, there is increasing evidence that the stiffness of the extracellular matrix modulates cancer and stromal cell mechanics and function, influencing such disease hallmarks as angiogenesis, migration, and metastasis. This review briefly summarizes recent studies that investigate how cancer cells and fibrosis-relevant stromal cells respond to ECM stiffness, the possible sensing appendages and signaling mechanisms involved, and the emergence of novel substrates - including substrates with scar-like fractal heterogeneity - that mimic the in vivo mechanical environment of the cancer cell.