Directional Migration of MDA-MB-231 Cells Under Oxygen Concentration Gradients

Biofabrication approaches and regulatory framework of metastatic tumor-on-a-chip models for precision oncology

The complexity of the tumor microenvironment (TME) together with the development of the metastatic process are the main reasons for the failure of conventional anticancer treatment. In recent years, there is an increasing need to advance toward advanced in vitro models of cancer mimicking TME and simulating metastasis to understand the associated mechanisms that are still unknown, and to be able to develop personalized therapy. In this review, the commonly used alternatives and latest advances in biofabrication of tumor‐on‐chips, which allow the generation of the most sophisticated and optimized models for recapitulating the tumor process, are presented. In addition, the advances that have allowed these new models in the area of metastasis, cancer stem cells, and angiogenesis are summarized, as well as the recent integration of multiorgan‐on‐a‐chip systems to recapitulate natural metastasis and pharmacological screening against it. We also analyze, for the first time in the literature, the normative and regulatory framework in which these models could potentially be found, as well as the requirements and processes that must be fulfilled to be commercially implemented as in vitro study model. Moreover, we are focused on the possible regulatory pathways for their clinical application in precision medicine and decision making through the generation of personalized models with patient samples. In conclusion, this review highlights the synergistic combination of three‐dimensional bioprinting systems with the novel tumor/metastasis/multiorgan‐on‐a‐chip systems to generate models for both basic research and clinical applications to have devices useful for personalized oncology.

Oxygen control: the often overlooked but essential piece to create better in vitro systems

Variations in oxygen levels play key roles in numerous physiological and pathological processes, but are often not properly controlled in in vitro models, introducing a significant bias in experimental outcomes. Recent developments in microfluidic technology have introduced a paradigm shift by providing new opportunities to better mimic physiological and pathological conditions, which is achieved by both regulating and monitoring oxygen levels at the micrometre scale in miniaturized devices. In this review, we first introduce the nature and relevance of oxygen-dependent pathways in both physiological and pathological contexts. Subsequently, we discuss strategies to control oxygen in microfluidic devices, distinguishing between engineering approaches that operate at the device level during its fabrication and chemical approaches that involve the active perfusion of fluids oxygenated at a precise level or supplemented with oxygen-producing or oxygen-scavenging materials. In addition, we discuss readout approaches for monitoring oxygen levels at the cellular and tissue levels, focusing on electrochemical and optical detection schemes for high-resolution measurements directly on-chip. An overview of different applications in which microfluidic devices have been utilized to answer biological research questions is then provided. In the final section, we provide our vision for further technological refinements of oxygen-controlling devices and discuss how these devices can be employed to generate new fundamental insights regarding key scientific problems that call for emulating oxygen levels as encountered in vivo. We conclude by making the case that ultimately emulating physiological or pathological oxygen levels should become a standard feature in all in vitro cell, tissue, and organ models.

Further Evidence that Gradients of Extracellular pH Direct Migration of MDA-MB-231 Cells In Vitro

We hypothesised that concentration gradients of O2/H+ within tissue guide migration of primary cancer cells toward intra-tumour microvessels, thus promoting intravasation and eventual haematogenous metastasis of cancer cells. Previously, we demonstrated in vitro that MDA-MB-231 cells under pH and O2 gradients (0.2-0.3 units/mm and ~ 6%/mm, respectively) migrate toward higher pH/O2 regions. The present study was designed to address questions yet unanswered in the previous one, i.e., (1) whether extracellular O2 gradients could be a cue for directional cell migration in physiologically relevant O2 environments, and (2) whether average pH level in the bulk extracellular medium affects directional cell migration. In the absence of pH gradients, directional cell migration was not demonstrated at a physiological O2 level (<5%). We demonstrated that both the migration velocity and directionality are significantly affected by the average extracellular pH level. This result is consistent with our model for directional cell migration that does not necessitate sensing of pH gradient at a single cell level. Thus, in this study, we demonstrated further evidence that gradients of extracellular pH direct migration of MDA-MB-231 cells in vitro.

Hypoxia directed migration of human naïve monocytes is associated with an attenuation of cytokine release: indications for a key role of CCL26

Background

Numerous tissue-derived factors have been postulated to be involved in tissue migration of circulating monocytes. The aim of this study was to evaluate whether a defined hypoxic gradient can induce directed migration of naïve human monocytes and to identify responsible autocrine/paracrine factors.

Methods

Monocytes were isolated from peripheral blood mononuclear cells, transferred into chemotaxis chambers and subjected to a defined oxygen gradient with or without the addition of CCL26. Cell migration was recorded and secretome analyses were performed.

Results

Cell migration recordings revealed directed migration of monocytes towards the source of hypoxia. Analysis of the monocyte secretome demonstrated a reduced secretion of 70% (19/27) of the analyzed cytokines under hypoxic conditions. The most down-regulated factors were CCL26 (− 99%), CCL1 (− 95%), CX3CL1 (− 95%), CCL17 (− 85%) and XCL1 (− 83%). Administration of recombinant CCL26 abolished the hypoxia-induced directed migration of human monocytes, while the addition of CCL26 under normoxic conditions resulted in a repulsion of monocytes from the source of CCL26.

Conclusions

Hypoxia induces directed migration of human monocytes in-vitro. Autocrine/paracrine released CCL26 is involved in the hypoxia-mediated monocyte migration and may represent a target molecule for the modulation of monocyte migration in-vivo.

A Relatively Small Gradient of Extracellular pH Directs Migration of MDA-MB-231 Cells In Vitro

Hematogenous tumor metastasis begins with the invasion and spread of primary tumor cells in the local tissue leading to intravasation. We hypothesized that tumor cells might actively migrate toward intratumor vessels with the extracellular metabolic gradient acting as a guiding cue. Here, we determined in vitro whether the extracellular gradient of pH can act as a cue for directional migration in MDA-MB-231 cells. Cell migration was determined by the wound-healing assay under gradients of extracellular pH (~0.2 units/mm) and oxygen concentration (~6% O₂/mm) that were produced by a microfluidic device, gap cover glass (GCG). Without GCG, the migration of cells was spatially homogeneous; the same number of cells migrated to the rectangular wound space from the left and right boundaries. In contrast, when GCG generated pH/O₂ gradients across the wound space, the number of cells migrating to the wound space from the boundary with higher pH/O₂ values was considerably decreased, indicating a preferential movement of cells toward the region of higher pH/O₂ in the gradient. The addition of hepes in the extracellular medium abolished both the extracellular pH gradient and the directional cell migration under GCG. We conclude that relatively small gradients of pH in the extracellular medium compared to those found in Na⁺/H⁺ exchanger-driven cell migration were sufficient to guide MDA-MB-231 cells. The directional cell migration as guided by the metabolic gradient could effectively elevate the probability of intravasation and, ultimately, hematogenous metastasis.

Microfluidic platform for three-dimensional cell culture under spatiotemporal heterogeneity of oxygen tension

Cells in a tumor microenvironment are exposed to spatial and temporal variations in oxygen tension due to hyperproliferation and immature vascularization. Such spatiotemporal oxygen heterogeneity affects the behavior of cancer cells, leading to cancer growth and metastasis, and thus, it is essential to clarify the cellular responses of cancer cells to oxygen tension. Herein, we describe a new double-layer microfluidic device allowing the control of oxygen tension and the behavior of cancer cells under spatiotemporal oxygen heterogeneity. Two parallel gas channels were located above the media and gel channels to enhance gas exchange, and a gas-impermeable polycarbonate film was embedded in the device to prevent the diffusion of atmospheric oxygen. Variations in oxygen tension in the device with the experimental parameters and design variables were investigated computationally and validated by using oxygen-sensitive nanoparticles. The present device can generate a uniform hypoxic condition at oxygen levels down to 0.3% O2, as well as a linear oxygen gradient from 3% O2 to 17% O2 across the gel channel within 15 min. Moreover, human breast cancer cells suspended in type I collagen gel were introduced in the gel channel to observe their response under controlled oxygen tension. Hypoxic exposure activated the proliferation and motility of the cells, which showed a local maximum increase at 5% O2. Under the oxygen gradient condition, the increase in the cell number was relatively high in the central mild hypoxia region. These findings demonstrate the utility of the present device to study cellular responses in an oxygen-controlled microenvironment.

MDA-MB-231 Breast Cancer Cells and Their CSC Population Migrate Towards Low Oxygen in a Microfluidic Gradient Device

Most cancer deaths are caused by secondary tumors formed through metastasis, yet due to our limited understanding of this process, prevention remains a major challenge. Recently, cancer stem cells (CSCs) have been proposed as the source of metastases, but only little is known about their migratory behavior. Oxygen gradients in the tumor have been linked to directional migration of breast cancer cells. Here, we present a method to study the effect of oxygen gradients on the migratory behavior of breast CSCs using a microfluidic device. Our chip contains a chamber in which an oxygen gradient can be generated between hypoxic (<1%) and ambient (21%) conditions. We tracked the migration of CSCs obtained from MDA-MB-231 breast cancer cells, and found that their migration patterns do not differ from the average MDA-MB-231 population. Surprisingly, we found that the cells migrate towards low oxygen levels, in contrast with an earlier study. We hypothesize that in our device, migration is exclusively due to the pure oxygen gradient, whereas the effects of oxygen in earlier work were obscured by additional cues from the tumor microenvironment (e.g., nutrients and metabolites). These results open new research directions into the role of oxygen in directing cancer and CSC migration.

Metastasis in context: modeling the tumor microenvironment with cancer-on-a-chip approaches

Most cancer deaths are not caused by the primary tumor, but by secondary tumors formed through metastasis, a complex and poorly understood process. Cues from the tumor microenvironment, such as the biochemical composition, cellular population, extracellular matrix, and tissue (fluid) mechanics, have been indicated to play a pivotal role in the onset of metastasis. Dissecting the role of these cues from the tumor microenvironment in a controlled manner is challenging, but essential to understanding metastasis. Recently, cancer-on-a-chip models have emerged as a tool to study the tumor microenvironment and its role in metastasis. These models are based on microfluidic chips and contain small chambers for cell culture, enabling control over local gradients, fluid flow, tissue mechanics, and composition of the local environment. Here, we review the recent contributions of cancer-on-a-chip models to our understanding of the role of the tumor microenvironment in the onset of metastasis, and provide an outlook for future applications of this emerging technology.

Summary: This Review evaluates the recent contributions of cancer-on-a-chip models to our understanding of the tumor microenvironment and its role in the onset of metastasis. The authors also provide an outlook for future applications of this emerging technology.

Exposure to 60% oxygen promotes migration and upregulates angiogenesis factor secretion in breast cancer cells

Peri-operative factors, including anaesthetic drugs and techniques, may affect cancer cell biology and clinical recurrence. In breast cancer cells, we demonstrated that sevoflurane promotes migration and angiogenesis in high fractional oxygen but not in air. Follow-up analysis of the peri-operative oxygen fraction trial found an association between high inspired oxygen during cancer surgery and reduced tumor-free survival. Here we evaluated effects of acute, high oxygen exposure on breast cancer cell viability, migration and secretion of angiogenesis factors in vitro. MDA-MB-231 and MCF-7 breast cancer cells were exposed to 21%, 30%, 60%, or 80% v/v O₂ for 3 hours. Cell viability at 24 hours was determined by MTT and migration at 24 hours with the Oris™ Cell Migration Assay. Secretion of angiogenesis factors at 24 hours was measured via membrane-based immunoarray. Exposure to 30%, 60% or 80% oxygen did not affect cell viability. Migration of MDA-MB-231 and MCF-7 cells was increased by 60% oxygen (P = 0.012 and P = 0.007, respectively) while 30% oxygen increased migration in MCF-7 cells (P = 0.011). These effects were reversed by dimethyloxaloylglycine. In MDA-MB-231 cells high fractional oxygen increased secretion of angiogenesis factors monocyte chemotactic protein 1, regulated on activation normal T-cell expressed and vascular endothelial growth factor. In MCF-7 cells, interleukin-8, angiogenin and vascular endothelial growth factor secretion was significantly increased by high fractional oxygen. High oxygen exposure stimulates migration and secretion of angiogenesis factors in breast cancer cells in vitro.

Simple and inexpensive technique for measuring oxygen consumption rate in adherent cultured cells

Measurement of cellular oxygen consumption rate (OCR) is essential in assessing roles of mitochondria in physiology and pathophysiology. Classical techniques, in which polarographic oxygen electrode measures the extracellular oxygen concentration in a closed measuring vessel, require isolation and suspension of the cell. Because cell functions depend on the extracellular milieu including the extracellular matrix, isolation of cultured cells prior to the measurement may significantly affect the OCR. More recent techniques utilize optical methods in which oxygen-dependent quenching of fluorophores determines oxygen concentration in the medium at a few microns above the surface of the cultured cells. These techniques allow the OCR measurement in cultured cells adhered to the culture dish. However, this technique requires special equipment such as a fluorescence lifetime microplate reader or specialized integrated system, which are usually quite expensive. Here, we introduce a simple and inexpensive technique for measuring OCR in adherent cultured cells that utilizes conventional fluorescence microscopy and a glassware called a gap cover glass.