Decreased cell stiffness facilitates cell detachment and cell migration from breast cancer spheroids in 3D collagen matrices of different rigidity

Purpose: Tumour-cell detachment is a critical early event in the metastatic cascade. Although several mechanisms have been reported, the role of cell mechanical properties in facilitating cell detachment and migration is not well understood. We, therefore, investigated how cells alter intracellular stiffness during these processes. Methods: MDA-MB-231 cells were embedded as 10,000-cell spheroids in 2 and 4 mg/ml collagen matrices. Using mitochondrial-based particle tracking microrheology (PTM), the intracellular stiffness of cells that have migrated different distances from the spheroid were assessed. Here, 0dC, 4dC and 6dC represented no, medium and high migration, respectively. Results: For 2 and 4 mg/ml collagen matrices, the MSD and cell stiffness of 0dC cells were larger than for migrated 4dC and 6dC cells. The MSD of 4dC and 6dC cells were similar; however, the cell stiffness of 4dC cells was smaller than that of 6dC cells. The stiffness of 0dC cells was lower for higher matrix concentration and rigidity compared to lower matrix rigidity, whereas matrix rigidity did not affect the stiffness of 4dC and 6dC cells. Conclusions: PTM was capable of quantifying intracellular mechanics during tumour detachment and migration in 3D environments. Based on our findings, it is proposed that decreased cell stiffness drives cellular detachment and migration. Increased matrix rigidity physically hinders migration and cells need to either soften or remodel the environment to migrate. The finding that matrix rigidity did not affect the stiffness of migrated cells suggests that cells facilitate migration by remodelling their environment through cleavage of matrix proteins.