Contribution of Src-FAK signaling to the induction of connective tissue growth factor in renal fibroblasts

Expression of connective tissue growth factor (CTGF) is sensitive to reorganization of the actin cytoskeleton, but also to alterations in cell morphology due to extracellular forces, for example, cyclic stretching or mechanical loading. Dynamic alterations of focal adhesion proteins were thus proposed to modulate CTGF induction. Immortalized human renal fibroblasts were cultured in or on top of preformed collagen-1 gels. Proteins were detected by immunofluorescence and quantified by Western blotting. Fibroblasts cultured in/on collagen gels resembled cells in vivo by their spindle-like morphology, absence of actin stress fibers, small punctiform focal contacts, and low levels of CTGF expression. Disassembly of microtubules by short-term treatment with colchicine induced cell rounding, cortical recruitment of patchy F-actin, reorganization of focal contacts into strong clusters, and upregulation of CTGF, all of which were dependent on RhoA-Rho-kinase signaling. Clustering of focal adhesion sites activated Src-family kinases and focal adhesion kinase (FAK). Interference with Src activity by PP2 had no effect on the morphological alterations but decreased tyrosine phosphorylation of focal adhesion proteins and almost completely prevented upregulation of CTGF. Furthermore, inhibition of phosphatidylinositol 3-kinase reduced CTGF expression. On the other hand, when the fibroblasts were cultured on a rigid matrix, that is collagen-coated plates, strong focal complexes prevented the dynamic alterations, and RhoA-mediated upregulation of CTGF expression was independent of Src-FAK signaling. Assembly of focal adhesion proteins regulates CTGF expression, providing a link between actin network, adhesion receptors, and CTGF-mediated functions such as synthesis of extracellular matrix proteins.