Introduction
In the complex landscape of cancer biology, it is important to understand crosstalk intricacies between tumour cells and other non-malignant cells within their microenvironment in order to better understand disease progression and treatment options. Tumours exist in a dynamic ecosystem comprising of a heterogenous milieu of various cell types, extracellular matrix (ECM) components and signalling molecules that all play roles in oncogenic development, tumour progression and response to environmental changes [1]. One example are cancer-associated fibroblasts, transformed stromal cells that act to provide structural support to the tumour via remodelling of the ECM and contribute to oncogenesis via secretion of growth factors and immune modulators [2]. Recent advancements in biomaterial science have provided innovative tools that researchers can utilise to study the tumour microenvironment (TME) in greater detail to better understand the networks of cellular interactions that exist within the tumour ecosystem [3]. One particular biomaterial that has attracted attention in recent years for the potential applications in cancer biology are hydrogels, which will be discussed in more detail in this article.
Engineering the tumour microenvironment in vitro
In order to elucidate the complex dynamics of the TME in vivo, it is imperative that in vitro models must recapitulate key elements and have defined aspects comparable to cancer biology within the body. Whilst conventional 2D models of humans are the most commonly used in vitro method for studying cancer biology, they suffer from limitations of oversimplifying tumour characteristics and being unpredictable in their anticipation of drug responses in humans [4]. In contrast, 3D tumour models have been shown to possess more relevant cell behaviours, morphology and gene expression that better resemble tumours compared to 2D monolayer culture [5]. For example, a pivotal study over 30 years ago first demonstrated breast epithelial cells expressed tumour-like traits when cultured in 2D, but when cultured in 3D basement membranes they reverted back to normal growth behaviours [6]. Various bioengineering methodologies have been developed to contribute to creating more faithful 3D models of the in vivo interplay between cancer and other TME cells, including stromal, immune and endothelial cells, using naturally occurring or synthetically produced materials. Naturally occurring ECM proteins are currently the most commonly used biomaterials for 3D culture of cancer cells, containing a mixture of proteins such as collagen, laminin and fibrin [7]. These materials benefit from their biocompatibility and ability to be remodelled and adhered to by cells, but also possess disadvantages that could potentially hinder their use for in vitro 3D cancer research. ECM derived from animals suffer from batch-to-batch variability in mechanical and biochemical properties due to different compositional configurations of ECM proteins and other contaminants from each individual animal, resulting in a lack of reproducibility in data that requires unaltered variables [8]. This has led to increasing interest in recent years to alternatives using synthetically-derived 3D scaffolds that also contain biocompatible and biomimetic characteristics, such as self-assembling peptide hydrogels.
Modelling the tumour microenvironment using synthetic hydrogels
Hydrogels are generally defined as a polymeric material composed of 3D crosslinked hydrophilic networks that have the ability to retain water within its structure due to functional groups attached to the polymer, but does not dissolve due to the polymer chain crosslinking [9]. Hydrogels present many benefits within in vitro research that aims to closely mimic in vivo conditions, such as possessing similar stiffnesses to those found in human tissue, and the permissive ability to transport oxygen, nutrients, other soluble factors and waste amongst cells and their surroundings [3]. Synthetic hydrogels are chemically defined and can therefore be altered via the chemical composition and molecular weight of the polymer, as well as the crosslinking density and process of polymerisation to finely tune the mechanical and physical properties of the 3D scaffold to mimic the exact structure of TMEs that can influence cellular behaviour [10]. In addition, these synthetic hydrogels can be chemically modified with motifs, such as amino acid sequence RGD that is responsible for cellular adhesion to the ECM via integrin binding, or modified with entrapment of ECM proteins and molecules that can be compositionally altered to match in vivo expressions of each type of TME [11]. Furthermore, this functionalisation with biochemical cues can be constrained to mimic the spatial and temporal heterogeneity found within the tumour ecosystem, for example creating a gradient or localised concentration of a particular signalling molecule [3]. These reproducible and defined modifications open opportunities for controlled, measurable changes in ECM construction and degradation within these TME models that could prove beneficial for various applications in cancer biology, such as development of new therapeutic applications.
Hydrogels and therapeutic opportunities
Polymer chemistry advances in recent years have driven the evolution of hydrogels being used as an increasingly popular platform for modelling the tumour ecosystem and therefore show real promise in its application in cancer biology. It’s ability to mimic many in vivo TME characteristics including its structure and dynamic cellular feedback allows it to be a sophisticated tool to measure processes such as cancer migration through the ECM, invasion through the basement membrane, bi-directional crosstalk between non-malignant cells and changes to the tumour’s 3D morphology [12]. This can be utilised to not only better understand the complex nature of this heterogeneous disease, but also help with potential approaches for therapeutic interventions. In addition, 3D cell culture systems such as hydrogels have the ability to be used for high-throughput assays, enabling rapid evaluation of large cohorts of drug candidates or genetic modifications on their effects on the TME in parallel screenings of multiple experiment conditions [13]. The use of more relevant disease models that can be investigated in an exceedingly efficient manner presents the opportunity to create highly specialised patient-derived organoid models that recapitulate the complexity of each individual’s disease in terms of their genomic, transcriptomic and phenotypic characteristics, allowing more tailored treatment strategies in the future [3].
Conclusion
In conclusion, in order to better elucidate the complex mechanisms behind the tumour ecosystem to advance our understanding of cancer biology, new research advances are required to study the TME. Hydrogels within the field of biomaterials offer unique advantages by providing a biomimetic environment with tuneable physical properties and high-throughput screening capabilities. This empowers researchers to develop innovative therapeutics quicker that can be more personalised to individuals, potentially influencing patient prognosis and mitigating side effects. As the field of cancer research continues to evolve, hydrogel-based approaches could offer great promise in the near future for study of other cells of the TME such as stromal and immune populations, targeting non-malignant cells that are less prone to drug resistance due to their genomic stability [14].
About the Author
Mikayla is a cancer research scientist who recently completed her PhD at Leeds Beckett University. Her PhD project focused on the bidirectional crosstalk between melanoma and cells of the tumour microenvironment via secretion of extracellular vesicles. This project involved study of a wide variety of cellular phenotypic and expression changes, as well as determination of cargo within the vesicles. She also has a background as a bioassay scientist in industry in a multitude of client projects.
References
[1] The evolving tumor microenvironment: From cancer initiation to metastatic outgrowth - ScienceDirect
[2] A framework for advancing our understanding of cancer-associated fibroblasts | Nature Reviews Cancer
[3] Biomaterials and Emerging Anticancer Therapeutics: Engineering the Microenvironment - PMC (nih.gov)
[4] 2D and 3D cell cultures – a comparison of different types of cancer cell cultures - PMC (nih.gov)
[6] Interaction with basement membrane serves to rapidly distinguish growth and differentiation pattern of normal and malignant human breast epithelial cells. - PMC (nih.gov)
[8] IJMS | Free Full-Text | The ECM: To Scaffold, or Not to Scaffold, That Is the Question (mdpi.com)
[11] The Synthesis of RGD-functionalized Hydrogels as a Tool for Therapeutic Applications - PMC (nih.gov)