Keratin 8/18 Regulation of Cell Stiffness-Extracellular Matrix Interplay through Modulation of Rho-Mediated Actin Cytoskeleton Dynamics

Cell mechanical activity generated from the interplay between the extracellular matrix (ECM) and the actin cytoskeleton is essential for the regulation of cell adhesion, spreading and migration during normal and cancer development. Keratins are the intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatic epithelial cell IFs are made solely of keratins 8/18 (K8/K18), hallmarks of all simple epithelia. Notably, our recent work on these epithelial cells has revealed a key regulatory function for K8/K18 IFs in adhesion/migration, through modulation of integrin interactions with ECM, actin adaptors and signaling molecules at focal adhesions. Here, using K8-knockdown rat H4 hepatoma cells and their K8/K18-containing counterparts seeded on fibronectin-coated substrata of different rigidities, we show that the K8/K18 IF-lacking cells lose their ability to spread and exhibit an altered actin fiber organization, upon seeding on a low-rigidity substratum. We also demonstrate a concomitant reduction in local cell stiffness at focal adhesions generated by fibronectin-coated microbeads attached to the dorsal cell surface. In addition, we find that this K8/K18 IF modulation of cell stiffness and actin fiber organization occurs through RhoA-ROCK signaling. Together, the results uncover a K8/K18 IF contribution to the cell stiffness-ECM rigidity interplay through a modulation of Rho-dependent actin organization and dynamics in simple epithelial cells

Role of the actin cytoskeleton in tuning cellular responses to external mechanical stress

Mechanical forces are essential for tissue homeostasis. In adherent cells, cell-matrix adhesions connect the extracellular matrix (ECM) with the cytoskeleton and transmit forces in both directions. Integrin receptors and signaling molecules in cell-matrix adhesions transduce mechanical into chemical signals, thereby regulating many cellular processes. This review focuses on how cellular mechanotransduction is tuned by actin-generated cytoskeletal tension that balances external with internal mechanical forces. We point out that the cytoskeleton rapidly responds to external forces by RhoA-dependent actin assembly and contraction. This in turn induces remodeling of cell-matrix adhesions and changes in cell shape and orientation. As a consequence, a cell constantly modulates its response to new bouts of external mechanical stimulation. Changes in actin dynamics are monitored by MAL/MKL-1/MRTF-A, a co-activator of serum response factor. Recent evidence suggests that MAL is also involved in coupling mechanically induced changes in the actin cytoskeleton to gene expression. Compared with other, more rapid and transient signals evoked at the cell surface, this parallel mechanotransduction pathway is more sustained and provides spatial and temporal specificity to the response. We describe examples of genes that are regulated by mechanical stress in a manner depending on actin dynamics, among them the ECM protein, tenascin-C