Linking cell structure to gene regulation: signaling events and expression controls on the model genes PAI-1 and CTGF.

The microtubule and microfilament cytoskeletal systems as well as cell-to-cell contacts and cell-matrix interactions are critical regulators of cell structure and function. Alterations in cell shape profoundly influence signaling events and gene expression programs that impact a spectrum of biological responses including cell growth, migration and apoptosis. These same pathways also contribute to the progression of several important pathologic conditions (e.g., arteriosclerosis, vascular fibrosis, and endothelial dysfunction). Indeed, hemodynamic forces in the vascular compartment are established modifiers of endothelial and smooth muscle cell cytoarchitecture and orchestrate complex genetic and biological responses in concert with contributions from the extracellular matrix (ECM), growth factors (e.g., EGF, and TGF-beta) and cell adhesion receptors (e.g., integrins, and cadherins). The profibrotic matricellular proteins plasminogen activator inhibitor-1 (PAI-1) and connective tissue growth factor (CTGF) are prominent members of a subset of genes the expression of which is highly responsive to cell shape-altering stimuli (i.e., disruption of the actin-based and microtubule networks, shear strain and cyclic stretch). Since both PAI-1 and CTGF are major mediators of cardiovascular fibrotic disease, understanding cell structure-linked signaling cascades provides potential avenues for focused therapy. It is increasingly evident that growth factor receptors (EGFR) are activated by changes in cytoarchitecture and that the "repressive state" of certain signaling proteins (e.g., SMAD, and Rho-GEFs) is maintained by sequestration on cell structural networks. Functional repression can be relieved by cytoskeletal perturbations (e.g., in response to treatment with network-specific drugs) resulting in activation of signaling cascades (e.g., Rho, and MAPK) with associated changes in gene reprogramming. Recent studies document a complex network of both similar and unique signaling control elements leading to the induction of PAI-1 and CTGF in response to modifications in cell shape. The purpose of this review is to highlight our current understanding of "cell deformation"-responsive signaling cascades focusing on the potential value of targeting such pathways, and their model response genes (e.g., PAI-1, and CTGF), as a therapeutic option for the treatment of fibrotic diseases.