Researchers at the National University of Singapore (NUS) have used atomic force microscopy (AFM) to compare the viscoelasticity of normal and malignant human mammary epithelial cells in both adhered and suspended states. They undertook this study to better understand breast cancer cells from a cell mechanics approach, and determined that malignant cells have lower viscosity and are more deformable than normal healthy cells (Converg. Sci. Phys. Oncol. 3 034003)

Biological cells are viscoelastic, meaning that they exhibit a time-dependent response to stress and strain due to internal physical interactions of the cells' contents, such as organelles, membrane and cytoskeletal components. AFM is one of the most robust techniques for measuring mechanical properties of biological cells, with published studies using AFM showing that most types of cancer cells are generally softer than normal, healthy cells.

The Singapore team used AFM to characterize mammary epithelial cells with different metastatic potential, specifically normal cells (MCF-10A), malignant cells (MCF-7) and highly invasive malignant cells (MDA-MB-231). They measured the Young's modulus of elasticity (the ability of a material to withstand changes in length under lengthwise tension or compression, equal to the longitudinal stress divided by the strain) and the viscosity of the cells in both adhered and suspended states.

Chwee Teck Lim, NUSS Professor in the Department of Biomedical Engineering, and colleagues stated that while almost all prior AFM studies were performed only on adhered cells, studying cells in their suspended state is equally important. It provides a means to better understand how cells' mechanical properties change as they detach themselves from a primary tumour and metastasize to a distant location in a suspended state as circulating tumour cells.

With respect to elasticity, the research team determined that as the invasiveness of breast cancer cells decreases, the cell stiffness increases. Their findings suggest that as the cells' metastatic potential increases compared to normal cells, the malignant cells become more invasive and also softer. The authors justified this finding by investigating the cells' intrinsic cytoskeletal organizations as the main attributor to elasticity. While normal cells have a highly developed and polymerized meshwork of actin filaments, as the metastatic potential of a cell increases, the actin structures become less organized and more fragmented. This was observed using AFM. The team also investigated F-actin density, and showed that actin density decreased as metastatic potential increased.

Evaluating the elasticity of adhered cells showed that elasticity decreases as cells become more invasive. Examining suspended cells revealed that the cells appeared much softer in their suspended form than their adherent form, due to lack of polymerized actin filaments. The authors noted that the congenital difference in the stiffness of cells with various metastatic potentials might not be as evident in their suspended state. They also attributed changes seen in the elasticity of suspended cells to intrinsic differences in cell and nucleus size.

In both adherent and suspended cells, normal cells had the highest viscosity. The authors believe that actin distribution and higher nucleus-to-cytoplasmic ratio in cancer cells were the two main factors in determining cell viscosity. They identified a decreasing trend in viscosity as a cell became more invasive. However, there was no significant difference between the viscosity of the two malignant cell lines. They explained that the higher viscosity of normal cells could be due to the higher concentration of cystoskeletal elements compared with malignant cells. Viscosity decreased in all cell lines when going from an adhered to a suspended state.

Lim told medicalphysicsweb that the research team is now looking into performing similar tests on tumour cells obtained from cancer patients, both from tumour biopsy as well as liquid biopsy (circulating tumour cells). "This research on clinical samples will verify how the clinically obtained results differ from that of cancer cell lines, and the correlation between viscoelasticity and actual conditions of malignancy," he said.

This study expands the current cancer mechanics database for cells in their suspended state. The researchers recommend that future studies should be conducted that incorporate effects of local environments, to determine the impact that these may have on cancer cell mechanics.

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