Hydrogel Models with Stiffness Gradients for Interrogating Pancreatic Cancer Cell Fate

Combined effects of matrix stiffness and obesity-associated signaling directs progressive phenotype in PANC-1 pancreatic cancer cells in vitro

Obesity is a leading risk factor of pancreatic ductal adenocarcinoma (PDAC) that contributes to poor disease prognosis and outcomes. Retrospective studies have identified this link, but interactions surrounding obesity and PDAC are still unclear. Research has shifted to contributions of fibrosis (desmoplasia) on malignancy, which involves increased deposition of collagens and other extracellular matrix (ECM) molecules and increased ECM crosslinking, all of which contribute to increased tissue stiffening. However, fibrotic stiffening is underrepresented as a model feature in current PDAC models. Fibrosis is shared between PDAC and obesity, and can be leveraged for in vitro model design, as current animal obesity models of PDAC are limited in their ability to isolate individual components of fibrosis to study cell behavior. In the current study, methacrylated type I collagen (PhotoCol®) was photo-crosslinked to pathological stiffness levels to recapitulate fibrotic ECM stiffening. PANC-1 cells were encapsulated within PhotoCol®, and the tumor-tissue constructs were prepared to represent normal (healthy) (~600 Pa) and pathological (~2000 Pa) tissues. Separately, human mesenchymal stem cells were differentiated into adipocytes representing lean (2D differentiation) and obese fat tissue (3D collagen matrix differentiation), and conditioned media was applied to PANC-1 tumor-tissue constructs. Conditioned media from obese adipocytes showed increased vimentin expression, a hallmark of invasiveness and progression, that was not seen after exposure to media from lean adipocytes or control media. Characterization of the obese adipocyte secretome suggested that some PANC-1 differences may arise from increased interleukin-8 and -10 compared to lean adipocytes. Additionally, high matrix stiffness associated induced an amoeboid morphology in PANC-1 cells that was not present at low stiffness. Amoeboid morphology is an accessory to epithelial-to-mesenchymal transition and is used to navigate complex ECM environments. This plasticity has greater implications for treatment efficacy of metastatic cancers. Overall, this work 1) highlights the importance of investigating PDAC-obesity interactions to study the effects on disease progression and persistence, 2) establishes PhotoCol® as a matrix material that can be leveraged to study amoeboid morphology and invasion in PDAC, and 3) emphasizes the importance of integrating both biophysical and biochemical interactions associated within both pathologies for in vitro PDAC models.

Dynamic Magneto-Softening of 3D Hydrogel Reverses Malignant Transformation of Cancer Cells and Enhances Drug Efficacy

High extracellular matrix stiffness is a prominent feature of malignant tumors associated with poor clinical prognosis. To elucidate mechanistic connections between increased matrix stiffness and tumor progression, a variety of hydrogel scaffolds with dynamic changes in stiffness have been developed. These approaches, however, are not biocompatible at high temperature, strong irradiation, and acidic/basic pH, often lack reversibility (can only stiffen and not soften), and do not allow study on the same cell population longitudinally. In this work, we develop a dynamic 3D magnetic hydrogel whose matrix stiffness can be wirelessly and reversibly stiffened and softened multiple times with different rates of change using an external magnet. With this platform, we found that matrix stiffness increased tumor malignancy including denser cell organization, epithelial-to-mesenchymal transition and hypoxia. More interestingly, these malignant transformations could be halted or reversed with matrix softening (i.e., mechanical rescue), to potentiate drug efficacy attributing to reduced solid stress from matrix and downregulation of cell mechano-transductors including YAP1. We propose that our platform can be used to deepen understanding of the impact of matrix softening on cancer biology, an important but rarely studied phenomenon.

Cell-Biomaterial Interactions

In animals, the extracellular matrix (ECM) forms a three-dimensional network occupying the intercellular spaces (interstitial matrix) or serving as physical and biochemical support for cells and tissues (basement membrane) [...].

A Step-by-Step Methodological Guide for Developing Zonal Multicellular Scaffold-Based Pancreatic Cancer Models

The tumor microenvironment (TME), a complex heterogeneous mixture of various cellular, physical, and biochemical components and signals, is a major player in the process of tumor growth and its response to therapeutic methods. In vitro 2D monocellular cancer models are unable to mimic the complex in vivo characteristics of cancer TME involving cellular heterogeneity, presence of extracellular matrix (ECM) proteins, as well as spatial orientation and organization of different cell types forming the TME. In vivo animal-based studies have ethical concerns, are expensive and time-consuming, and involve models of non-human species. In vitro 3D models are capable of tiding over several issues associated with both 2D in vitro and in vivo animal models. We have recently developed a novel zonal multicellular 3D in vitro model for pancreatic cancer involving cancer cells, endothelial cells, and pancreatic stellate cells. Our model (i) can provide long-term culture (up to 4 weeks), (ii) can control the ECM biochemical configuration in a cell specific manner, (iii) shows large amounts of collagen secretion by the stellate cells mimicking desmoplasia, and (iv) expresses cell-specific markers throughout the whole culture period. This chapter describes the experimental methodology to form our hybrid multicellular 3D model for pancreatic ductal adenocarcinoma, including the immunofluorescence staining on the cell culture.

Hydrogels for Salivary Gland Tissue Engineering

Mimicking the complex architecture of salivary glands (SGs) outside their native niche is challenging due their multicellular and highly branched organization. However, significant progress has been made to recapitulate the gland structure and function using several in vitro and ex vivo models. Hydrogels are polymers with the potential to retain a large volume of water inside their three-dimensional structure, thus simulating extracellular matrix properties that are essential for the cell and tissue integrity. Hydrogel-based culture of SG cells has seen a tremendous success in terms of developing platforms for cell expansion, building an artificial gland, and for use in transplantation to rescue loss of SG function. Both natural and synthetic hydrogels have been used widely in SG tissue engineering applications owing to their properties that support the proliferation, reorganization, and polarization of SG epithelial cells. While recent improvements in hydrogel properties are essential to establish more sophisticated models, the emphasis should still be made towards supporting factors such as mechanotransduction and associated signaling cues. In this concise review, we discuss considerations of an ideal hydrogel-based biomaterial for SG engineering and their associated signaling pathways. We also discuss the current advances made in natural and synthetic hydrogels for SG tissue engineering applications.

Advances in bioengineering pancreatic tumor-stroma physiomimetic Biomodels

Pancreatic cancer exhibits a unique bioarchitecture and desmoplastic cancer-stoma interplay that governs disease progression, multi-resistance, and metastasis. Emulating the biological features and microenvironment heterogeneity of pancreatic cancer stroma in vitro is remarkably complex, yet highly desirable for advancing the discovery of innovative therapeutics. Diverse bioengineering approaches exploiting patient-derived organoids, cancer-on-a-chip platforms, and 3D bioprinted living constructs have been rapidly emerging in an endeavor to seamlessly recapitulate major tumor-stroma biodynamic interactions in a preclinical setting. Gathering on this, herein we showcase and discuss the most recent advances in bio-assembling pancreatic tumor-stroma models that mimic key disease hallmarks and its desmoplastic biosignature. A reverse engineering perspective of pancreatic tumor-stroma key elementary units is also provided and complemented by a detailed description of biodesign guidelines that are to be considered for improving 3D models physiomimetic features. This overview provides valuable examples and starting guidelines for researchers envisioning to engineer and characterize stroma-rich biomimetic tumor models. All in all, leveraging advanced bioengineering tools for capturing stromal heterogeneity and dynamics, opens new avenues toward generating more predictive and patient-personalized organotypic 3D in vitro platforms for screening transformative therapeutics targeting the tumor-stroma interplay.

Patterned photocrosslinking to establish stiffness anisotropies in fibrous 3D hydrogels

Cells are known to constantly interact with their local extracellular matrix (ECM) and respond to a variety of biochemical and mechanical cues received from the ECM. Nonetheless, comprehensive understanding of cell-ECM interactions has been elusive. Many studies rely on analysis of cell behavior on 2D substrates, which do not reflect a natural cell environment. Further, lack of dynamic control over local stiffness anisotropies and fiber alignment hinders progress in studies in naturally derived fibrous 3D cultures. Here, we present a cell-safe method of patterned photocrosslinking, which can aid in studying biological hypotheses related to mechanotransduction in 3D hydrogels. As previously described by our group, ruthenium-catalyzed photocrosslinking (RCP) of selected ECM regions promotes localized increase in stiffness mediated by focused blue laser light in a confocal microscope. In this study, we further demonstrate that RCP can induce localized strain stiffening and fiber alignment outside of the selected crosslinked region and induce stiffness anisotropy biased towards the direction of fiber alignment. MDA-MB-231 cells are shown to respond to RCP-induced changes in local ECM architecture and display directional bias towards the direction of fiber alignment, as compared to control cells. Further, the effect of patterned crosslinking on a stiffness landscape is measured using multi-axes optical tweezers active microrheology (AMR) with backscattered laser beam illumination. AMR validates RCP as a suitable tool for creating distinct stiffness anisotropies which promote directed migration of cells, further underscoring the usefulness of RCP in cell-ECM studies. STATEMENT OF SIGNIFICANCE: Studies on cell-ECM interactions in 3D cultures have often been hindered by the lack of available tools to dynamically alter local ECM stiffness and fiber alignment. Here, we present a non-invasive, cell-safe and easily applicable method of patterned photocrosslinking, which can aid in studying biological hypotheses in fibrous 3D hydrogels. Ruthenium-catalyzed crosslinking (RCP) of selected fibrin ECM regions promotes localized increase in stiffness and creates distinct stiffness anisotropies in the presence of the focused blue laser light. Outside of the crosslinked region, RCP causes fiber alignment and strain stiffening in the ECM, verified using multi-axes optical tweezers active microrheology (AMR). Following RCP, human breast cancer MDA-MB-231 exhibit directed cell migration, validating usefulness of this method in cell-ECM studies.

Targeting Tumor-Stromal Interactions in Pancreatic Cancer: Impact of Collagens and Mechanical Traits

Pancreatic ductal adenocarcinoma (PDAC) has one of the worst outcomes among cancers with a 5-years survival rate of below 10%. This is a result of late diagnosis and the lack of effective treatments. The tumor is characterized by a highly fibrotic stroma containing distinct cellular components, embedded within an extracellular matrix (ECM). This ECM-abundant tumor microenvironment (TME) in PDAC plays a pivotal role in tumor progression and resistance to treatment. Cancer-associated fibroblasts (CAFs), being a dominant cell type of the stroma, are in fact functionally heterogeneous populations of cells within the TME. Certain subtypes of CAFs are the main producer of the ECM components of the stroma, with the most abundant one being the collagen family of proteins. Collagens are large macromolecules that upon deposition into the ECM form supramolecular fibrillar structures which provide a mechanical framework to the TME. They not only bring structure to the tissue by being the main structural proteins but also contain binding domains that interact with surface receptors on the cancer cells. These interactions can induce various responses in the cancer cells and activate signaling pathways leading to epithelial-to-mesenchymal transition (EMT) and ultimately metastasis. In addition, collagens are one of the main contributors to building up mechanical forces in the tumor. These forces influence the signaling pathways that are involved in cell motility and tumor progression and affect tumor microstructure and tissue stiffness by exerting solid stress and interstitial fluid pressure on the cells. Taken together, the TME is subjected to various types of mechanical forces and interactions that affect tumor progression, metastasis, and drug response. In this review article, we aim to summarize and contextualize the recent knowledge of components of the PDAC stroma, especially the role of different collagens and mechanical traits on tumor progression. We furthermore discuss different experimental models available for studying tumor-stromal interactions and finally discuss potential therapeutic targets within the stroma.

Pancreatic Ductal Adenocarcinoma: Preclinical in vitro and ex vivo Models

Pancreatic ductal adenocarcinoma (PDAC) is one of the most overlooked cancers despite its dismal median survival time of 6 months. The biggest challenges in improving patient survival are late diagnosis due to lack of diagnostic markers, and limited treatment options due to almost complete therapy resistance. The past decades of research identified the dense stroma and the complex interplay/crosstalk between the cancer- and the different stromal cells as the main culprits for the slow progress in improving patient outcome. For better ex vivo simulation of this complex tumor microenvironment the models used in PDAC research likewise need to become more diverse. Depending on the focus of the investigation, several in vitro and in vivo models for PDAC have been established in the past years. Particularly, 3D cell culture such as spheroids and organoids have become more frequently used. This review aims to examine current PDAC in vitro models, their inherent limitations, and their successful implementations in research.

Dual Functionalization of Gelatin for Orthogonal and Dynamic Hydrogel Cross-Linking

Gelatin-based hydrogels are widely used in biomedical fields because of their abundance of bioactive motifs that support cell adhesion and matrix remodeling. Although inherently bioactive, unmodified gelatin exhibits temperature-dependent rheology and solubilizes at body temperature, making it unstable for three-dimensional (3D) cell culture. Therefore, the addition of chemically reactive motifs is required to render gelatin-based hydrogels with highly controllable cross-linking kinetics and tunable mechanical properties that are critical for 3D cell culture. This article provides a series of methods toward establishing orthogonally cross-linked gelatin-based hydrogels for dynamic 3D cell culture. In particular, we prepared dually functionalized gelatin macromers amenable for sequential, orthogonal covalent cross-linking. Central to this material platform is the synthesis of norbornene-functionalized gelatin (GelNB), which forms covalently cross-linked hydrogels via orthogonal thiol-norbornene click cross-linking. Using GelNB as the starting material, we further detail the methods for synthesizing gelatin macromers susceptible to hydroxyphenylacetic acid (HPA) dimerization (i.e., GelNB-HPA) and hydrazone bonding (i.e., GelNB-CH) for on-demand matrix stiffening. Finally, we outline the protocol for synthesizing a gelatin macromer capable of adjusting hydrogel stress relaxation via boronate ester bonding (i.e., GelNB-BA). The combination of these orthogonal chemistries affords a wide range of gelatin-based hydrogels as biomimetic matrices in tissue engineering and regenerative medicine applications.