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Mechanical forces are essential for connective tissue homeostasis. The extracellular matrix (ECM) plays a key role in the transmission of forces generated by the organism (e.g. muscle contraction) and externally applied (e.g. gravity). The expression of specific ECM proteins such as collagens and tenascin-C, as well as of matrix metalloproteinases, involved in their turnover, is influenced by mechanical stimuli. The precise mechanisms by which mechanical strains are translated into chemical signals and lead to differential gene expression are however not fully understood. Cell-matrix adhesion sites are good candidates for hosting a "mechanosensory switch", as they transmit forces from the ECM to the cytoskeleton and vice versa by physically linking the cytoskeleton to the ECM. Integrins, transmembrane proteins located to these adhesion sites, have been shown to trigger a set of internal signaling cascades after mechanical stimulation. We have shown that the expression level of tenascin-C directly correlates with externally applied mechanical stress, as well as with RhoA/RhoA-dependent kinase-mediated cytoskeletal tension. Presumably other genes are regulated in a similar manner. The changes in ECM composition and mechanical properties derived from mechanical stress are relevant in medical intervention after ligament and tendon injury.

Expression of the extracellular matrix (ECM) protein tenascin-C is induced in fibroblasts by growth factors as well as by tensile strain. Mechanical stress can act on gene regulation directly, or indirectly via the paracrine release of soluble factors by the stimulated cells. To distinguish between these possibilities for tenascin-C, we asked whether cyclic tensile strain and soluble factors, respectively, induced its mRNA via related or separate mechanisms. When cyclic strain was applied to chick embryo fibroblasts cultured on silicone membranes, tenascin-C mRNA and protein levels were increased twofold within 6 h compared to the resting control. Medium conditioned by strained cells did not stimulate tenascin-C mRNA in resting cells. Tenascin-C mRNA in resting cells was increased by serum; however, cyclic strain still caused an additional induction. Likewise, the effect of TGF-beta1 or PDGF-BB was additive to that of cyclic strain, whereas IL-4 or H2O2 (a reactive oxygen species, ROS) did not change tenascin-C mRNA levels. Antagonists for distinct mitogen-activated protein kinases (MAPK) inhibited tenascin-C induction by TGF-beta1 and PDGF-BB, but not by cyclic strain. Conversely, a specific inhibitor of Rho-dependent kinase strongly attenuated the response of tenascin-C mRNA to cyclic strain, but had limited effect on induction by growth factors. The data suggest that regulation of tenascin-C in fibroblasts by cyclic strain occurs independently from soluble mediators and MAPK pathways; however, it requires Rho/ROCK signaling.

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Induction of tenascin-C mRNA by cyclic strain in fibroblasts depends on RhoA and Rho dependent kinase (ROCK). Here we show that integrin-linked kinase (ILK) is required upstream of this pathway. In ILK-deficient fibroblasts, RhoA was not activated and tenascin-C mRNA remained low after cyclic strain; tenascin-C expression was unaffected by ROCK inhibition. In ILK wild-type but not ILK-/- fibroblasts, cyclic strain-induced reorganization of actin stress fibers and focal adhesions, as well as nuclear translocation of MAL, a transcriptional co-activator that links actin assembly to gene expression. These findings support a role for RhoA in ILK-mediated mechanotransduction. Rescue of ILK -/- fibroblasts by expression of wild-type ILK restored these responses to cyclic strain. Mechanosensation is not entirely abolished in ILK -/- fibroblasts, since cyclic strain activated Erk-1/2 and PKB/Akt, and induced c-fos mRNA in these cells. Conversely, lysophosphatidic acid stimulated RhoA and induced both c-fos and tenascin-C mRNA in ILK -/- cells. Thus, the signaling pathways controlling tenascin-C expression are functional in the absence of ILK, but are not triggered by cyclic strain. Our results indicate that ILK is selectively required for the induction of specific genes by mechanical stimulation via RhoA-mediated pathways.