Quantitative Measurements of Intercellular Adhesion Strengths between Cancer Cells with Different Malignancies Using Atomic Force Microscopy

Micro tensile tester measurement of biomechanical properties and adhesion force of microtubule-polymerization-inhibited cancer cells

Both mechanical and adhesion properties of cancer cells are complex and reciprocally related to migration, invasion, and metastasis with large cell deformation. Therefore, we evaluated these properties for human cervical cancer cells (HeLa) simultaneously using our previously developed micro tensile tester system. For efficient evaluation, we developed image analysis software to modify the system. The software can analyze the tensile force in real time. The modified system can evaluate the tensile stiffness of cells to which a large deformation is applied, also evaluate the adhesion strength of cancer cells that adhered to a culture substrate and were cultured for several days with their adhesion maturation. We used the modified system to simultaneously evaluate the stiffness of the cancer cells to which a large deformation was applied and their adhesion strength. The obtained results revealed that the middle phase of tensile stiffness and adhesion force of the microtubule-depolymerized group treated with colchicine (an anti-cancer drug) (stiffness, 13.4 ± 7.5 nN/%; adhesion force, 460.6 ± 258.2 nN) were over two times larger than those of the control group (stiffness, 5.0 ± 3.5 nN/%; adhesion force, 168.2 ± 98.0 nN). Additionally, the same trend was confirmed with the detailed evaluation of cell surface stiffness using an atomic force microscope. Confocal fluorescence microscope observations showed that the stress fibers (SFs) of colchicine-treated cells were aligned in the same direction, and focal adhesions (FAs) of the cells developed around both ends of the SFs and aligned parallel to the developed direction of the SFs. There was a possibility that the microtubule depolymerization by the colchicine treatment induced the development of SFs and FAs and subsequently caused an increment of cell stiffness and adhesion force. From the above results, we concluded the modified system would be applicable to cancer detection and anti-cancer drug efficacy tests.

Multicellular dynamics on structured surfaces: Stress concentration is a key to controlling complex microtissue morphology on engineered scaffolds

Tissue engineers have utilised a variety of three-dimensional (3D) scaffolds for controlling multicellular dynamics and the resulting tissue microstructures. In particular, cutting-edge microfabrication technologies, such as 3D bioprinting, provide increasingly complex structures. However, unpredictable microtissue detachment from scaffolds, which ruins desired tissue structures, is becoming an evident problem. To overcome this issue, we elucidated the mechanism underlying collective cellular detachment by combining a new computational simulation method with quantitative tissue-culture experiments. We first quantified the stochastic processes of cellular detachment shown by vascular smooth muscle cells on model curved scaffolds and found that microtissue morphologies vary drastically depending on cell contractility, substrate curvature, and cell-substrate adhesion strength. To explore this mechanism, we developed a new particle-based model that explicitly describes stochastic processes of multicellular dynamics, such as adhesion, rupture, and large deformation of microtissues on structured surfaces. Computational simulations using the developed model successfully reproduced characteristic detachment processes observed in experiments. Crucially, simulations revealed that cellular contractility-induced stress is locally concentrated at the cell-substrate interface, subsequently inducing a catastrophic process of collective cellular detachment, which can be suppressed by modulating cell contractility, substrate curvature, and cell-substrate adhesion. These results show that the developed computational method is useful for predicting engineered tissue dynamics as a platform for prediction-guided scaffold design. STATEMENT OF SIGNIFICANCE: Microfabrication technologies aiming to control multicellular dynamics by engineering 3D scaffolds are attracting increasing attention for modelling in cell biology and regenerative medicine. However, obtaining microtissues with the desired 3D structures is made considerably more difficult by microtissue detachments from scaffolds. This study reveals a key mechanism behind this detachment by developing a novel computational method for simulating multicellular dynamics on designed scaffolds. This method enabled us to predict microtissue dynamics on structured surfaces, based on cell mechanics, substrate geometry, and cell-substrate interaction. This study provides a platform for the physics-based design of micro-engineered scaffolds and thus contributes to prediction-guided biomaterials design in the future.

Investigation on protein dimerization and evaluation of medicine effects by single molecule force spectroscopy

Monitoring the dimerization state of the mesenchymal-epithelial transition factor (Met) was essential for in-depth understanding of the tumor signal transduction network. At present, the dimerization activation pathway of Met protein was mainly studied at the macro level, while the research at the single molecule level was far from comprehensive. Herein, the dimerization activation of Met protein's extracellular domain induced by ligand hepatocyte growth factor (HGF) was dynamically studied by single-molecule force spectroscopy. Met protein was immobilized on a biomimetic lipid membrane for ensuring its physiological environment, and then the Met dimers were recognized by bivalent probe which was formed by two Met-binding aptamers. Then the dimeric state of Met protein could be distinguished from monomeric state of Met protein through some parameters, (such as unimodal ratio, bimodal ratio and separation work). The unimodal indicates the occurrence of single molecule binding event, and the bimodal represents the occurrence of double binding event (also represents the presence of Met dimer). Before HGF treatment, most of the Met protein on the lipid membrane was still in the form of monomer, so the unimodal ratio in the force curve was larger (78.8 ± 5.2%), and the bimodal ratio was smaller (17.0 ± 4.1%). After HGF treatment, the unimodal ratio decreased to 54.0 ± 7.4%, and the bimodal ratio increased to 43.2 ± 7.3%. It was due to the formation of dimers after the binding of Met protein on the fluidity lipid membrane with HGF. In addition, the average separation work increased to about 2 times after HGF treatment. Given that studies of Met protein dimerization inhibitors have contributed to the development of more potent and safe inhibitors to significantly inhibit tumor metastasis, the effects of different medicines (including anticoagulant medicines, different antibiotics and anti-cancer medicines) on the dimerization activation of Met protein were then explored by the platform described above. The results showed that anticoagulant medicines heparin and its analogs can significantly inhibit HGF-mediated Met protein activation, while different antibiotics and anticancer medicines had no significant effect on the dimerization of Met protein. This work provided a platform for studying protein dimerization as well as for screening Met protein dimerization inhibitors at the single-molecule level.

Contacts with Macrophages Promote an Aggressive Nanomechanical Phenotype of Circulating Tumor Cells in Prostate Cancer

Aggressive tumors of epithelial origin shed cells that intravasate and become circulating tumor cells (CTC). The CTCs that are able to survive the stresses encountered in the bloodstream can then seed metastases. We demonstrated previously that CTCs isolated from the blood of prostate cancer patients display specific nanomechanical phenotypes characteristic of cell endurance and invasiveness and patient sensitivity to androgen deprivation therapy. Here we report that patient-isolated CTCs are nanomechanically distinct from cells randomly shed from the tumor, with high adhesion as the most distinguishing biophysical marker. CTCs uniquely coisolated with macrophage-like cells bearing the markers of tumor-associated macrophages (TAM). The presence of these immune cells was indicative of a survival-promoting phenotype of "mechanical fitness" in CTCs based on high softness and high adhesion as determined by atomic force microscopy. Correlations between enumeration of macrophages and mechanical fitness of CTCs were strong in patients before the start of hormonal therapy. Single-cell proteomic analysis and nanomechanical phenotyping of tumor cell-macrophage cocultures revealed that macrophages promoted epithelial-mesenchymal plasticity in prostate cancer cells, manifesting in their mechanical fitness. The resulting softness and adhesiveness of the mechanically fit CTCs confer resistance to shear stress and enable protective cell clustering. These findings suggest that selected tumor cells are coached by TAMs and accompanied by them to acquire intermediate epithelial/mesenchymal status, thereby facilitating survival during the critical early stage leading to metastasis. SIGNIFICANCE: The interaction between macrophages and circulating tumor cells increases the capacity of tumor cells to initiate metastasis and may constitute a new set of blood-based targets for pharmacologic intervention.

Evaluation of intercellular cross-linking abilities correlated with cytotoxicities of bispecific antibodies with domain rearrangements using AFM force-sensing

Bispecific antibodies (bsAbs) are a promising engineered antibody format; thus, technologies for the fabrication and evaluation of functional bsAbs are attracting increasing attention. Here, based on atomic force microscopy (AFM) force-sensing integrated with a metal cup-attached AFM chip (cup-chip) to ensure efficient capture of a target cell on a cantilever, we established a novel method for measuring cross-linking ability that is correlated with the cytotoxicities of bsAbs targeting two cells. We previously reported that domain rearrangements of bsAbs affected their cytotoxicities; however, no differences in cross-linking ability for soluble antigens were observed by surface plasmon resonance. We predicted that there would be differences in molecular configurations to avoid steric hindrance in the cross-linking of the two whole target cells. A picked-up T cell lymphoma cell on the cantilever using a cup-chip was moved to approach a cancer cell adhered to a dish, and force-curve measurements were performed. The resulting forces mediated by the cross-linking of bsAbs with different domain orders were well-correlated with their cytotoxicities. The AFM force-sensing method established herein may reflect steric hindrance of intercellular cross-linking, and thus has the potential to evaluate the net function of bsAbs and contribute to the generation of functional bsAbs.

Organotypic platform for studying cancer cell metastasis

Metastasis is the leading cause of mortality in cancer patients. To migrate to distant sites, cancer cells would need to adapt their behaviour in response to different tissue environments. Thus, it is essential to study this process in models that can closely replicate the tumour microenvironment. Here, we evaluate the use of organotypic liver and brain slices to study cancer metastasis. Morphological and viability parameters of the slices were monitored daily over 3 days in culture to assess their stability as a realistic 3D tissue platform for in vitro metastatic assays. Using these slices, we evaluated the invasion of MDA-MB-231 breast cancer cells and of a subpopulation that was selected for increased motility. We show that the more aggressive invasion of the selected cells likely resulted not only from their lower stiffness, but also from their lower adhesion to the surrounding tissue. Different invasion patterns in the brain and liver slices were observed for both subpopulations. Cells migrated faster in the brain slices (with an amoeboid-like mode) compared to in the liver slices (where they migrated with mesenchymal or collective migration-like modes). Inhibition of the Ras/MAPK/ERK pathway increased cell stiffness and adhesion forces, which resulted in reduced invasiveness. These results illustrate the potential for organotypic tissue slices to more closely mimic in vivo conditions during cancer cell metastasis than most in vitro models.

Influence of Nivolumab for Intercellular Adhesion Force between a T Cell and a Cancer Cell Evaluated by AFM Force Spectroscopy

The influence of nivolumab on intercellular adhesion forces between T cells and cancer cells was evaluated quantitatively using atomic force microscopy (AFM). Two model T cells, one expressing high levels of programmed cell death protein 1 (PD-1) (PD-1^high Jurkat) and the other with low PD-1 expression levels (PD-1^low Jurkat), were analyzed. In addition, two model cancer cells, one expressing programmed death-ligand 1 (PD-L1) on the cell surface (PC-9, PD-L1⁺) and the other without PD-L1 (MCF-7, PD-L1⁻), were also used. A T cell was attached to the apex of the AFM cantilever using a cup-attached AFM chip, and the intercellular adhesion forces were measured. Although PD-1^high T cells adhered strongly to PD-L1⁺ cancer cells, the adhesion force was smaller than that with PD-L1⁻ cancer cells. After the treatment of PD-1^high T cells with nivolumab, the adhesion force with PD-L1⁺ cancer cells increased to a similar level as with PD-L1⁻ cancer cells. These results can be explained by nivolumab influencing the upregulation of the adhesion ability of PD-1^high T cells with PD-L1⁺ cancer cells. These results were obtained by measuring intercellular adhesion forces quantitatively, indicating the usefulness of single-cell AFM analysis.