Evaluation of the influence of growth medium composition on cell elasticity

Novel insights from 3D models: the pivotal role of physical symmetry in epithelial organization

3D tissue culture models are utilized to study breast cancer and other pathologies because they better capture the complexity of in vivo tissue architecture compared to 2D models. However, to mimic the in vivo environment, the mechanics and geometry of the ECM must also be considered. Here, we studied the mechanical environment created in two 3D models, the overlay protocol (OP) and embedded protocol (EP). Mammary epithelial acini features were compared using OP or EP under conditions known to alter acinus organization, i.e. collagen crosslinking and/or ErbB2 receptor activation. Finite element analysis and active microrheology demonstrated that OP creates a physically asymmetric environment with non-uniform mechanical stresses in radial and circumferential directions. Further contrasting with EP, acini in OP displayed cooperation between ErbB2 signalling and matrix crosslinking. These differences in acini phenotype observed between OP and EP highlight the functional impact of physical symmetry in 3D tissue culture models.

Effects of temperature and cellular interactions on the mechanics and morphology of human cancer cells investigated by atomic force microscopy

Cell mechanics plays an important role in cellular physiological activities. Recent studies have shown that cellular mechanical properties are novel biomarkers for indicating the cell states. In this article, temperature-controllable atomic force microscopy (AFM) was applied to quantitatively investigate the effects of temperature and cellular interactions on the mechanics and morphology of human cancer cells. First, AFM indenting experiments were performed on six types of human cells to investigate the changes of cellular Young's modulus at different temperatures and the results showed that the mechanical responses to the changes of temperature were variable for different types of cancer cells. Second, AFM imaging experiments were performed to observe the morphological changes in living cells at different temperatures and the results showed the significant changes of cell morphology caused by the alterations of temperature. Finally, by co-culturing human cancer cells with human immune cells, the mechanical and morphological changes in cancer cells were investigated. The results showed that the co-culture of cancer cells and immune cells could cause the distinct mechanical changes in cancer cells, but no significant morphological differences were observed. The experimental results improved our understanding of the effects of temperature and cellular interactions on the mechanics and morphology of cancer cells.

Characterization of Dynamic Behaviour of MCF7 and MCF10A Cells in Ultrasonic Field Using Modal and Harmonic Analyses

Treatment options specifically targeting tumour cells are urgently needed in order to reduce the side effects accompanied by chemo- or radiotherapy. Differences in subcellular structure between tumour and normal cells determine their specific elasticity. These structural differences can be utilised by low-frequency ultrasound in order to specifically induce cytotoxicity of tumour cells. For further evaluation, we combined in silico FEM (finite element method) analyses and in vitro assays to bolster the significance of low-frequency ultrasound for tumour treatment. FEM simulations were able to calculate the first resonance frequency of MCF7 breast tumour cells at 21 kHz in contrast to 34 kHz for the MCF10A normal breast cells, which was due to the higher elasticity and larger size of MCF7 cells. For experimental validation of the in silico-determined resonance frequencies, equipment for ultrasonic irradiation with distinct frequencies was constructed. Differences for both cell lines in their response to low-frequent ultrasonic treatment were corroborated in 2D and in 3D cell culture assays. Treatment with ~ 24.5 kHz induced the death of MCF7 cells and MDA-MB-231 metastases cells possessing a similar elasticity; frequencies of > 29 kHz resulted in cytotoxicity of MCF10A. Fractionated treatments by ultrasonic irradiation of suspension myeloid HL60 cells resulted in a significant decrease of viable cells, mostly significant after threefold irradiation in intervals of 3 h. Most importantly in regard to a clinical application, combined ultrasonic treatment and chemotherapy with paclitaxel showed a significantly increased killing of MCF7 cells compared to both monotherapies. In summary, we were able to determine for the first time for different tumour cell lines a specific frequency of low-intensity ultrasound for induction of cell ablation. The cytotoxic effect of ultrasonic irradiation could be increased by either fractionated treatment or in combination with chemotherapy. Thus, our results will open new perspectives in tumour treatment.

Sub-cellular force microscopy in single normal and cancer cells

This work investigates the biomechanical properties of sub-cellular structures of breast cells using atomic force microscopy (AFM). The cells are modeled as a triple-layered structure where the Generalized Maxwell model is applied to experimental data from AFM stress-relaxation tests to extract the elastic modulus, the apparent viscosity, and the relaxation time of sub-cellular structures. The triple-layered modeling results allow for determination and comparison of the biomechanical properties of the three major sub-cellular structures between normal and cancerous cells: the up plasma membrane/actin cortex, the mid cytoplasm/nucleus, and the low nuclear/integrin sub-domains. The results reveal that the sub-domains become stiffer and significantly more viscous with depth, regardless of cell type. In addition, there is a decreasing trend in the average elastic modulus and apparent viscosity of the all corresponding sub-cellular structures from normal to cancerous cells, which becomes most remarkable in the deeper sub-domain. The presented modeling in this work constitutes a unique AFM-based experimental framework to study the biomechanics of sub-cellular structures.

Three-dimensional cage-like microscaffolds for cell invasion studies

Cancer cell motility is one of the major events involved in metastatic process. Tumor cells that disseminate from a primary tumor can migrate into the vascular system and, being carried by the bloodstream, transmigrate across the endothelium, giving rise to a new tumor site. However, during the invasive process, tumor cells must pass through the extracellular matrix, whose structural and mechanical properties define the parameters of the migration process. Here, we propose 3D-complex cage-like microstructures, realized by two-photon (TP) direct laser writing (DLW), to analyze cell migration through pores significantly smaller than the cell nucleus. We found that the ability to traverse differently sized pores depends on the metastatic potential and on the invasiveness of the cell lines, allowing to establish a pore-area threshold value able to discriminate between non-tumorigenic and tumorigenic human breast cells.

The fractional viscoelastic response of human breast tissue cells

The mechanical response of a living cell is notoriously complicated. The complex, heterogeneous characteristics of cellular structure introduce difficulties that simple linear models of viscoelasticity cannot overcome, particularly at deep indentation depths. Herein, a nano-scale stress-relaxation analysis performed with an atomic force microscope reveals that isolated human breast cells do not exhibit simple exponential relaxation capable of being modeled by the standard linear solid (SLS) model. Therefore, this work proposes the application of the fractional Zener (FZ) model of viscoelasticity to extract mechanical parameters from the entire relaxation response, improving upon existing physical techniques to probe isolated cells. The FZ model introduces a new parameter that describes the fractional time-derivative dependence of the response. The results show an exceptional increase in conformance to the experimental data compared to that predicted by the SLS model, and the order of the fractional derivative (α) is remarkably homogeneous across the populations, with a median value of 0.48 ± 0.06 for the malignant population and 0.51 ± 0.07 for the benign. The cells' responses exhibit power-law behavior and complexity not associated with simple relaxation (SLS, α = 1) that supports the application of a fractional model. The distributions of some of the FZ parameters also preserve the distinction between the malignant and benign sample populations seen from the linear model and previous results while including the contribution of fast-relaxation behavior. The resulting viscosity, measured by a composite relaxation time, exhibits considerably less dispersion due to residual error than the distribution generated by the linear model and therefore serves as a more powerful marker for cell differentiation.

Stiffness of cancer cells measured with an AFM indentation method

The stiffness of cancer cells and its changes during metastasis are very important for understanding the pathophysiology of cancer cells and the mechanisms of metastasis of cancer. As the first step of the studies on the mechanics of cancer cells during metastasis, we determined the elasticity and stiffness of cancer cells with an indentation method using an atomic force microscope (AFM), and compared with those of normal cells. In most of the past AFM studies, Young׳s elastic moduli of cells have been calculated from force-indentation data using Hertzian model. As this model is based on several important assumptions including infinitesimal strain and Hooke׳s linear stress-strain law, in the exact sense it cannot be applied to cells that deform very largely and nonlinearly. To overcome this problem, we previously proposed an equation F=a[exp(bδ)-1] to describe relations between force (F) and indentation (δ), where a and b are parameters relating with cellular stiffness. In the present study, we applied this method to cancer cells instead of Young׳s elastic modulus. The conclusions obtained are: 1) AFM indentation test data of cancer cells can be very well described by the above equation, 2) cancer cells are softer than normal cells, and 3) there are no significant locational differences in the stiffness of cancer cells between the central and the peripheral regions. These methods and results are useful for studying the mechanics of cancer cells and the mechanisms of metastasis.

Mapping the CXCR4 receptor on breast cancer cells

The CXCR4 receptor triggers cell migration and, in breast cancer, promotes metastasis. To date, the dynamic assembly of CXCR4 on the cell surface as a mediator of receptor binding is not well characterized. The objective of this work is to quantify the density, spatial organization, and magnitude of binding of the CXCR4 receptor on live metastatic breast cancer (MBC) cells. We measured the Young's modulus, the CXCR4 surface density, and CXCR4 unbinding force on MBC cells by atomic force microscopy. We conclude that the CXCR4 density, spatial organization, and matrix stiffness are paramount to achieve strong binding.

A microfluidic pipette array for mechanophenotyping of cancer cells and mechanical gating of mechanosensitive channels

Micropipette aspiration measures the mechanical properties of single cells. A traditional micropipette aspiration system requires a bulky infrastructure and has a low throughput and limited potential for automation. We have developed a simple microfluidic device which is able to trap and apply pressure to single cells in designated aspiration arrays. By changing the volume flow rate using a syringe pump, we can accurately exert a pressure difference across the trapped cells for pipette aspiration. By examining cell deformation and protrusion length into the pipette under an optical microscope, several important cell mechanical properties, such as the cortical tension and the Young's modulus, can be measured quantitatively using automated image analysis. Using the microfluidic pipette array, the stiffness of breast cancer cells and healthy breast epithelial cells was measured and compared. Finally, we applied our device to examine the gating threshold of the mechanosensitive channel MscL expressed in mammalian cells. Together, the development of a microfluidic pipette array could enable rapid mechanophenotyping of individual cells and for mechanotransduction studies.

Towards Elucidating the Effects of Purified MWCNTs on Human Lung Epithelial cells

Toxicity of engineered nanomaterials is associated with their inherent properties, both physical and chemical. Recent studies have shown that exposure to multi-walled carbon nanotubes (MWCNTs) promotes tumors and tumor-associated pathologies and lead to carcinogenesis in model in vivo systems. Here in we examined the potential of purified MWCNTs used at occupationally relevant exposure doses for particles not otherwise regulated to affect human lung epithelial cells. The uptake of the purified MWCNTs was evaluated using fluorescence activated cell sorting (FACS), while the effects on cell fate were assessed using 2- (4-iodophenyl) - 3- (4-nitrophenyl) - 5-(2, 4-disulfophenyl) -2H-tetrazolium salt colorimetric assay, cell cycle and nanoindentation. Our results showed that exposure to MWCNTs reduced cell metabolic activity and induced cell cycle arrest. Our analysis further emphasized that MWCNTs-induced cellular fate results from multiple types of interactions that could be analyzed by means of intracellular biomechanical changes and are pivotal in understanding the underlying MWCNTs-induced cell transformation.

Biomechanical profile of cancer stem-like/tumor-initiating cells derived from a progressive ovarian cancer model

We herein report, for the first time, the mechanical properties of ovarian cancer stem-like/tumor-initiating cells (CSC/TICs). The represented model is a spontaneously transformed murine ovarian surface epithelial (MOSE) cell line that mimics the progression of ovarian cancer from early/non-tumorigenic to late/highly aggressive cancer stages. Elastic modulus measurements via atomic force microscopy (AFM) illustrate that the enriched CSC/TICs population (0.32±0.12kPa) are 46%, 61%, and 72% softer (P<0.0001) than their aggressive late-stage, intermediate, and non-malignant early-stage cancer cells, respectively. Exposure to sphingosine, an anti-cancer agent, induced an increase in the elastic moduli of CSC/TICs by more than 46% (0.47±0.14kPa, P<0.0001). Altogether, our data demonstrate that the elastic modulus profile of CSC/TICs is unique and responsive to anti-cancer treatment strategies that impact the cytoskeleton architecture of cells. These findings increase the chance for obtaining distinctive cell biomechanical profiles with the intent of providing a means for effective cancer detection and treatment control.

FROM THE CLINICAL EDITOR

This novel study utilized atomic force microscopy to demonstrate that the elastic modulus profile of cancer stem cell-like tumor initiating cells is unique and responsive to anti-cancer treatment strategies that impact the cytoskeleton of these cells. These findings pave the way to the development of unique means for effective cancer detection and treatment control.

Bioactive sphingolipid metabolites modulate ovarian cancer cell structural mechanics

Cancer progression is associated with an increased deformability of cancer cells and reduced resistance to mechanical forces, enabling motility and invasion. This is important for metastases survival and outgrowth and as such could be a target for chemopreventive strategies. In this study, we determined the differential effects of exogenous sphingolipid metabolites on the elastic modulus of mouse ovarian surface epithelial cells as they transition to cancer. Treatment with ceramide or sphingosine-1-phosphate in non-toxic concentrations decreased the average elastic modulus by 21% (p≤ 0.001) in transitional and 15% (p≤ 0.02) in aggressive stages while exerting no appreciable effect on non-malignant cells. In contrast, sphingosine treatment on average increased the elastic modulus by 33% (p≤ 0.0002) in aggressive cells while not affecting precursor cells. These results indicate that tumor-supporting sphingolipid metabolites act by making cells softer, while the anti-cancer metabolite sphingosine partially reverses the decreased elasticity associated with cancer progression. Thus, sphingosine may be a valid alternative to conventional chemotherapeutics in ovarian cancer prevention or treatment.

Roles of bioactive sphingolipid metabolites in ovarian cancer cell biomechanics

Bioactive sphingolipid metabolites have emerged as important lipid second messengers in the regulation of cell growth, death, motility and many other events. These processes are important in cancer development and progression; thus, sphingolipid metabolites have been implicated in both cancer development and cancer prevention. Despite recent considerable progress in understanding the multi-faceted functions of these bioactive metabolites, little is known about their influence on the biomechanical property of cells. The biomechanical properties of cancer cells change during progression with aggressive and invasive cells being softer compared to their benign counterparts. In this paper, we investigated the effects of exogenous sphingolipid metabolites on the Young's modulus and cytoskeletal organization of cells representing aggressive ovarian cancer. Our findings demonstrate that the elasticity of aggressive ovarian cancer cells decreased ∼15% after treatment with ceramide and sphingosine-1-phosphate. In contrast, sphingosine treatment caused a ∼30% increase in the average elasticity which was associated with a more defined actin cytoskeleton organization. This indicates that sphingolipid metabolites differentially modulate the biomechanic properties of cancer cells which may have a critical impact on cancer cell survival and progression, and the use of sphingolipid metabolites as chemopreventive or chemo-therapeutic agents.

Mechanical properties of human amniotic fluid stem cells using nanoindentation

The aim of this study was to obtain nanomechanical properties of living cells focusing on human amniotic fluid stem (hAFS) cell using nanoindentation techniques. We modified the conventional method of atomic force microscopy (AFM) in aqueous environment for cell imaging and indentation to avoid inherent difficulties. Moreover, we determined the elastic modulus of murine osteoblast (OB6) cells and hAFS cells at the nucleus and cytoskeleton using force-displacement curves and Hertz theory. Since OB6 cell line has been widely used, it was selected to validate and compare the obtained results with the previous research studies. As a result, we were able to capture high resolution images through utilization of the tapping mode without adding protein or using fixation methods. The maximum depth of indentation was kept below 15% of the cell thickness to minimize the effect of substrate hardness. Nanostructural details on the surface of cells were visualized by AFM and fluorescence microscopy. The cytoskeletal fibers presented remarkable increase in elastic modulus as compared with the nucleus. Furthermore, our results showed that the elastic modulus of hAFS cell edge (31.6 kPa) was lower than that of OB6 cell edge (42.2 kPa). In addition, the elastic modulus of nucleus was 13.9 kPa for hAFS cell and 26.9 kPa for OB6 cells. Differences in cell elastic modulus possibly resulted from the type and number of actin cytoskeleton organization in these two cell types.

Method for quantitative measurements of the elastic modulus of biological cells in AFM indentation experiments

Here we overview and further develop a quantitative method to measure mechanics of biological cells in indentation experiments, which is based on the use of atomic force microscopy (AFM). We demonstrate how the elastic modulus of the cell body should be measured when the cellular brush is taken into account. The brush is an essential inelastic part of the cell, which surrounds all eukaryotic (the brush is mostly microvilli and glycocalyx) and gram-negative prokaryotic cells (the brush is polysaccharides). The other main feature of the described method is the use of a relatively dull AFM probe to stay in the linear stress-strain regime. In particular, we show that the elastic modulus (aka the Young's modulus) of cells is independent of the indentation depth up to 10-20% deformation for the eukaryotic cells studied here. Besides the elastic modulus, the method presented allows obtaining the parameters of cellular brush, such as the effective length and grafting density of the brush. Although the method is demonstrated on eukaryotic cells, it is directly applicable for all types of cells, and even non-biological soft materials surrounded by either a brush or any field of long-range forces.