B16 Melanoma Cancer Cells with Higher Metastatic Potential are More Deformable at a Whole-Cell Level

Abstract

Introduction

Metastasis is a process in which cancer cells spread from the primary focus site to various other organ sites. Many studies have suggested that reduced stiffness would facilitate passing through extracellular matrix when cancer cells instigate a metastatic process. Here we investigated the compressive properties of melanoma cancer cells with different metastatic potentials at the whole-cell level. Differences in their compressive properties were analyzed by examining actin filament structure and actin-related gene expression.

Methods

Compressive tests were carried out for two metastatic B16 melanoma variants (B16-F1 and B16-F10) to characterize global compressive properties of cancer cells. RNA-seq analysis and fluorescence microscopic imaging were performed to clarify contribution of actin filaments to the global compressive properties.

Results

RNA-seq analysis and fluorescence microscopic imaging revealed the undeveloped structure of actin filaments in B16-F10 cells. The Young’s modulus of B16-F10 cells was significantly lower than that of B16-F1 cells. Disruption of the actin filaments in B16-F1 cells reduced the Young’s modulus to the same level as that of B16-F10 cells, while the Young’s modulus in B16-F10 cells remained the same regardless of the disruption.

Conclusions

In B16 melanoma cancer cell lines, cells with higher metastatic potential were more deformable at the whole-cell level with undeveloped actin filament structure, even when highly deformed. These results imply that invasive cancer cells may gain the ability to inhibit actin filament development.

Appendix

The hyperelastic Tatara model was adopted to describe the mechanical behavior of cells. Here we used the model originally developed by Tatara36 and modified by Hu et al.10 to represent cellular elastic behaviors during the compression test. The hyperelastic Tatara model describes the relationship between the applied force F and compressive strain ε as

$$ F = Eg\left( \varepsilon \right) $$

(A1)

and $g\left( \varepsilon \right)$ is a function of strain ε given by

$$ g\left( \varepsilon \right) = \left\{ {\frac{{3 \left( {1 - \nu^{2} } \right)A}}{{2 \left( {L_{0} - L_{\text{c}} } \right)}}\left( {1 + \frac{{2Ba^{2} }}{{D_{\text{deform}}^{2} }}} \right)\frac{1}{a} - \frac{2A}{{\pi \left( {L_{0} - L_{\text{c}} } \right)}}\left( {1 + \frac{{4Ba^{2} }}{{5 D_{\text{deform}}^{2} }}} \right)\frac{1}{f\left( a \right)}} \right\}^{ - 1} $$

(A2)

where A and B are the function of strain ε as described below. In Eq. (A2), L₀ and L_c are the lengths of the cell before and after compression, respectively. D_deform is cell deformed diameter (Fig. A1), and ν is a Poisson’s ratio, a is the contact radius, f(a) is the characteristic length of non-spherical geometry after compression, A and B (related to hyperelastic correction) are

Figure A1

Geometrical parameters for fitting the hyperelastic Tatara model. L₀ and L_c are the lengths of the cell before and after compression, respectively. D_deform is the cell deformed diameter, and a is the contact radius.

$$ a = \frac{1}{2}\left( {\sqrt {L_{0}^{2} - L_{c}^{2} } + D_{\text{deform}} - L_{0} } \right) $$

(A3)

$$ f\left( a \right) = \left\{ {\frac{{\left( {1 + \nu } \right)L_{0}^{2} }}{{2\left( {a^{2} + L_{0}^{2} } \right)^{3/2} }} + \frac{{1 - \nu^{2} }}{{\left( {a^{2} + L_{0}^{2} } \right)^{1/2} }}} \right\}^{ - 1} $$

(A4)

$$ A = \frac{{\left( {1 - \varepsilon } \right)^{2} }}{{1 - \varepsilon + \frac{{\varepsilon^{2} }}{3}}} $$

(A5)

$$ B = \frac{{1 - \frac{\varepsilon }{3}}}{{1 - \varepsilon + \frac{{\varepsilon^{2} }}{3}}} $$

(A6)

The hyperelastic Tatara model was fitted to the F − ε data by the least squared method to obtain Young’s modulus, E, under the assumption of material incompressibility (ν = 0.5).