Computational modeling reveals a vital role for proximity-driven additive and synergistic cell-cell interactions in increasing cancer invasiveness

Solid-tumor cell invasion typically occurs by collective migration of attached cell-cohorts, yet we show here that indirect cell-interactions through the substrate can also drive invasiveness. We have previously shown that well-spaced, invasive cancer cells push-into and indent gels to depths of 10 pm, while closely adjacent, non-contacting cancer cells may reach up to 18 pm, potentially relying on cell-cell interactions through the gel-substrate. To test that, we developed finite element models of indenting cells, using ex-perimental gel mechanics, cell mechanostructure, and force magnitudes. We show that under 50-350 nN of combined traction and normal forces, a stiff nucleus-region is essential in facilitating 5-10 pm single-cell indentations, while uniformly soft cells attain 1.6-fold smaller indentations. We observe that indentation depths of cells in close proximity (0.5-50 pm distance) increase relative to well-spaced cells, due to additive, continuum mechanics-driven contributions. Specifically, 2-3 cells applying 220 nN nor-mal forces gained up to 3% in depth, which interestingly increased to 7.8% when two cells, 10 pm apart, applied unequal force-magnitudes (i.e., 220 and 350 nN). Such additive, energy-free contributions can re-duce cell mechanical energy-output required for invasiveness, yet the experimentally observed 10-18 pm depths likely necessitate synergistic, mechanobiological changes, which may be mechanically triggered. We note that nucleus stiffening or cytoplasm softening by 25-50% increased indentation depths by only 1-7%, while depths increase nearly linearly with force-magnitude even to two-fold levels. Hence, cell -proximity triggered, synergistic and additive cell-interactions through the substrate can drive collective cancer-cell invasiveness, even without direct cell-cell interactions.Statement of significanceMetastatic cancer invasion typically occurs collectively in attached cell-cohorts. We have previously shown increased invasiveness in closely adjacent cancer cells that are able to push-into and indent soft -gels more deeply than single, well-spaced cells. Using finite element models, we reveal mechanisms of cell-proximity driven invasiveness, demonstrating an important role for the stiff nucleus. Cell-proximity can additively induce small increase in indentation depth via continuum mechanics contributions, espe-cially when adjacent cells apply unequal forces, and without requiring increased cell-mechanical-energy -output. Concurrently, proximity-triggered synergistic interactions that produce changes in cell mechanics or capacity for increased force-levels can facilitate deep invasive-indentations. Thus, we reveal concur-rent additive and synergistic mechanisms to drive collective cancer-cell invasiveness even without direct cell-cell interactions. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Solid-tumor cell invasion often occurs through collective migration of attached cell-cohorts. However, this study reveals that cell-interactions through the substrate can also drive invasiveness. Finite element models and experiments show that close proximity of cells induces additive contributions to indentation depth, especially when unequal forces are applied. These findings suggest the importance of cell-proximity and mechanical factors in driving collective cancer-cell invasiveness.