The aim of our research is to elucidate the roles of actin cytoskeletal remodeling and mechanotransduction in ordering of epithelial cell population during epithelial tubule formation. We identified Farp1 as a Rho-GEF that is involved in the matrix stiffness-induced transformation of mammary epithelial MCF10A cells. We found that Farp1 binds to integrin via the N-terminal FERM domain, facilitates cell adhesion to the extracellular matrix, and induces the adhesion-dependent cell extension and proliferation. We also identified 11 Rho-GEFs that are involved in cyclic stretch-induced reorientation of vascular endothelial cells. We focused on Solo, one of these Rho-GEFs, and found that Solo is involved in cadherin-dependent, cell-adhesion mediated mechanical signal transduction during cyclic stretch-induced endothelial cell reorientation and that Solo is required for tensile force-induced RhoA activation in MDCK epithelial cells (J. Cell Sci., 2015). We searched for the binding proteins of Solo in epithelial cells and found that Solo binds to the keratin-8/18 filament, one of simple epithelial cell-specific intermediate filaments and that Solo is required for the proper organization of the keratin-8/18 network in MDCK cells. It is known that tensional force application reinforces and generates stress fibers in a direction parallel to the force application in cells. Knockdown of Solo or keratin-18 caused the suppression of tensional force-induced stress fiber formation, and the binding of Solo to keratin-8/18 filaments is required for force-induced RhoA activation and stress fiber formation (Mol. Biol. Cell, 2016). We further studied the roles of Solo in the ordering of epithelial cell population in collective migration and HGF-induced tubule formation of MDCK cells. We found that Solo is involved in the regulation of the velocity of collective migration and the lumen size of the tubules of MDCK cells (unpublished data).