Mechanotransduction of strain regulates an invasive phenotype in newly transformed epithelial cells

Sophie Chagnon-Lessard, Hubert Jean-Ruel, Michel Godin, Andrew E. Pelling

Posted on: 17 October 2019

Preprint posted on 15 September 2019

Article now published in Frontiers in Physics at http://dx.doi.org/10.3389/fphy.2021.654613

How the everyday movement of tissues affects cell-cell interactions and the initiation of cancer

Selected by William Hill

Categories: biophysics, cancer biology, cell biology

Background

Tumours are the result of outgrowths of single progenitor cells. By several rounds of mutation and selection, aberrant cells overcome the constraints imposed by tissue homeostasis. Heterotypic cell-cell interactions arise between normal and mutant cells at the earliest stages of carcinogenesis and can have profound impact on both cell populations. For example, these normal-mutant cell-cell interactions drive extrusion of Ras^V12 expressing mutant cells, which has been modelled extensively using Madeline-Darby Canine Kidney (MDCK) cell lines. But this simple 2-dimensional model does not fully recapitulate the in vivo scenario.

Cells in vivo are constantly sensing and responding to mechanical forces and the changing physical properties of the extracellular matrix (ECM). A number of normal functions – breathing, digestion, muscle contraction, heart pumping and fluid pressure – all result in cells being constantly stretched and compressed. The authors set out to model this mechanically dynamic environment and investigate what effect it had on extrusion of Ras^V12 by normal neighbours.

Highlights:

The day-to-day forces that are applied to epithelial tissues may alter homeostatic mechanisms such as extrusion of mutant cells.
These dynamic mechanical strains affect mutant and normal cells differently and may actually promote an invasive Ras^V12 phenotype.
Pharmacological treatments can have opposing effect on Ras^V12 cells depending on whether the system is under strain or not.

Key Findings

Generating and characterising the system

The authors have previously characterised a pneumatic-based microfluidic stretcher. In this system, cells are plated on a thin membrane in a micro – chamber which undergoes cyclical deformation from adjacent vacuum chambers. Normal and inducible Ras^V12 cells were plated at 1:75 ratios and allowed to form a confluent monolayer before cyclical stretching and induction of Ras^V12.

Normally, the arising cell-cell interactions drive the vast majority of Ras^V12 cells to increase in cell height, undergo cytoskeletal changes and apical extrusion, with a small number forming basal protrusions. However, when the monolayer was put under cyclical stretch, Ras^V12 cells were found to be shorter than surrounding cells and basal protrusions were significantly bigger.

To investigate the mechanism of how strain alters cell behaviour, a range of pharmacological inhibitors were added to the system. The increased invasiveness induced by strain was found to involve myosin II activity and actomyosin contractility. Interestingly, inhibiting focal adhesion kinase (FAK) only reduced Ras^V12 invasiveness during dynamic stretching. The authors propose that strain promotes increased protrusion size and adhesion with the substrate, making it more difficult to force a cell out apically.

Orientation of protrusions

The cyclical stretching was also found to have an effect on the surrounding normal cells. Basal actin stress fibres were found to be promoted in normal cells but not Ras^V12 cells following strain. This could alter normal-normal cell-cell interactions and facilitate Ras^V12cell invasion.

Protrusion growth is known to facilitate and guide invasion and so the authors also looked at the orientation of protrusions. They found protrusion direction was not related to strain direction, but appeared to grow parallel to the strain-induced basal stress fibre network in normal cells and preferentially along normal cell interfaces. The authors propose that cyclical strain modulates monolayers of normal cells to promote basal protrusions.

Why this paper is cool

I think it’s super interesting to think about the day-to-day forces that are applied to the tissues in our body and how they may alter homeostatic mechanisms such as the elimination of less fit cells.

In the preprint the authors also find that pharmacological inhibitors can have different effects depending on whether cells are under cyclical strain or not. An interesting consideration for moving from in vitro to in vivo systems.