Modulation of the expression of connective tissue growth factor (CTGF) by alterations of the cytoskeleton

Abstract

Introduction In closing wounds or developing fibrotic tissue, fibroblasts are exposed to mechanical stress, which leads to alterations in cell morphology and reorganization of the cytoskeleton. The impact of morphological changes on gene expression was investigated in the present study. As a model system, we used a human renal fibroblast cell line and studied the expression of connective tissue growth factor (CTGF). CTGF is a downstream mediator of TGF‐β mediating many of the pro‐fibrotic actions of this growth factor and has also been shown to be up‐regulated by static or dynamic pressure. The molecular mechanisms regulating CTGF expression related to stress are not yet known.Materials and methods A human renal fibroblast cell line was kindly provided by Dr Müller, Göttingen, Germany. These cells express CTGF upon treatment with soluble stimuli (Heusinger‐Ribeiro et al. 2001; Eberlein et al. 2001). CTGF expression was detected by northern and Western blot analysis. The actin cytoskeleton was visualized by rhodamine phalloidin staining, and microtubules were detected by immunocytochemistry.Results Low concentrations of the microtubule disrupting agents nocodazole and colchicine strongly up‐regulated CTGF mRNA and protein expression in the human renal fibroblast cell line TK173. The up‐regulation was prevented by stabilization of the microtubules by paclitaxel (taxol). As a consequence of microtubule disruption, the small GTPase RhoA was activated and the actin stress fibers were stabilized. Both effects were related to CTGF induction: interference with RhoA signalling by simvastatin, toxin B and Y27632 prevented up‐regulation of CTGF. The important role of RhoA was supported by an increased CTGF expression upon overexpression of constitutively active RhoA. Direct disassembly of the actin cytoskeleton by latrunculin B interfered with colchicine‐mediated up‐regulation of CTGF expression. Disassembly of actin fibers by cytochalasin D unexpectedly increased CTGF expression. This indicated that the content of F‐actin per se was not the major determinant for CTGF gene expression. It has been shown, however, that cytochalasin D sequesters G‐actin, whereas latrunculin increases the level of G‐actin. Our data are thus in accordance with an inverse correlation between G‐actin levels and CTGF expression.Discussion These data link alterations in the microtubule and actin cytoskeleton to the expression of CTGF. Recently, decreased levels of G‐actin were observed in vascular smooth muscle cells in response to increased vascular pressure (Cipolla et al. 2002). Our findings thus provide a molecular basis for the observation that CTGF is up‐regulated in cells exposed to mechanical stress.