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Abstract

Unjamming transitions from a solid-like to a fuid-like state are a gateway to breast epithelial cancer invasion. However, the mechanical interplay between phase transitions and dimension transitions, in particular wetting, remains elusive, despite being critical for understanding the onset of metastatic dissemination. In addition, invading cancer cells undergo continuous confinement in the tumor microenvironment due to cellular overcrowding and/or a dense extracellular matrix (ECM). Accumulating evidence suggests that the resulting biomechanical stresses can mediate cancer progression.

This study shows that unjamming, mediated by the RAB5A GTPase, alters carcinoma spheroid fluidity and rigidity, and rewires adhesion mechanics to drive supracellular active wetting as a new mode of tumor expansion. Spheroid fluiuidification enhances the selective expression of integrin subunits and increases focal adhesion dynamics, inducing a fluid-like spreading behavior on specific matrix ligands. Notably, nanoscale regulation of integrin clustering can select for distinct phase transitions at the collective scale upon wetting. In this framework, fluidized spheroids polarize into cohesive "supra-cells", and maintain a stiff peripheral actin bundle as measured by nanomechanical mapping. Furthermore, a combination of Brillouin microscopy and 2.5D traction force analysis reveals a mechanical switch within the spheroid core, characterized by significant cell softening and a reduction in compressive forces exerted on the substrate, thereby mimicking the wetting of a liquid droplet.

Furthermore, we employ polydimethylsiloxane (PDMS)-based microchannels to confine DCIS cancer spheroids, and examine the mechanical signatures of confined spheroid migration and dissemination. We demonstrate that high level of RAB5A expression enhances confined spheroid dissemination in adhesive microchannels, while depending on the channel size. Notably, in confined anti-adhesive environment, RAB5A expression enhances a collective amoeboid mode of spheroid locomotion. In absence of cell-substrate interactions, we show that spheroid actin cortex remodeling is precluded, thereby abrogating dissemination. Next, we observe that RAB5A-mediated spheroid invasion under confinement is accompanied by tissue-level softening — albeit to a lesser extent than in non-confined settings — which extent depends on cell-matrix adhesion strength, whereas single-cell nuclei exhibit more resistance to deformation.

These findings establish unjamming-driven active wetting as a key mechanism to comprehend the molecular and biophysical underpinnings of solid tumor invasion, and its interplay with confinement.