CCN2 Is Necessary for Adhesive Responses to Transforming Growth Factor-β1 in Embryonic Fibroblasts

Abstract

CCN2 is induced by transforming growth factor-␤ (TGF␤) in fibroblasts and is overexpressed in connective tissue disease. CCN2 has been proposed to be a downstream mediator of TGF␤ action in fibroblasts; however, the role of CCN2 in regulating this process unclear. By using embryonic fibroblasts isolated from ccn2؊/؊ mice, we showed that CCN2 is required for a subset of responses to TGF␤. Affymetrix genome-wide expression profiling revealed that 942 transcripts were induced by TGF␤ greater than 2-fold in ccn2؉/؉ fibroblasts, of which 345 were not induced in ccn2؊/؊ fibroblasts, including pro-adhesive and matrix remodeling genes. Whereas TGF␤ properly induced a generic Smad3-responsive promoter in ccn2؊/؊ fibroblasts, TGF␤-induced activation of focal adhesion kinase (FAK) and Akt was reduced in ccn2؊/؊ fibroblasts. Emphasizing the importance of FAK and Akt activation in CCN2-dependent transcriptional responses to TGF␤ in fibroblasts, CCN2-dependent transcripts were not induced by TGF␤ in fak؊/؊ fibroblasts and were reduced by wortmannin in wild-type fibroblasts. Akt1 overexpression in ccn2؊/؊ fibroblasts rescued the TGF␤-induced transcription of CCN2-dependent mRNA. Finally, induction of TGF␤-induced fibroblast adhesion to fibronectin and type I collagen was significantly diminished in ccn2؊/؊ fibroblasts. Thus in embryonic fibroblasts, CCN2 is a necessary cofactor required for TGF␤ to activate the adhesive FAK/Akt/phosphatidylinositol 3-kinase cascade, FAK/Akt-dependent genes, and adhesion to matrix.Growth factors intimately contribute to the normal wound healing process, regulating chemotaxis, cell proliferation, neovascularization, and extracellular matrix (ECM) 3 synthesis. CCN2 (connective tissue growth factor), a member of the CCN family of proteins, contains 38 conserved cysteine-rich residues and a heparin-binding domain and is chemotactic and mitogenic for connective tissue cells (1-4). However, the physiological role of CCN2 is largely unknown.As an initial approach to elucidate the physiological function of CCN2, mice deleted for the ccn2 gene were recently generated (5). Mice homozygous for a deletion of the ccn2 gene die soon after birth, displaying an inability of the rib cage to ossify properly (5). The phenotype of these mice is consistent with a role for CCN2 in matrix synthesis and remodeling as ccn2Ϫ/Ϫ embryos show reduction in the expression of bone-specific matrix proteins, such as aggrecan (5). We recently found that embryonic fibroblasts isolated from ccn2Ϫ/Ϫ mice showed reduced basal adhesive signaling, including a reduction of FAK and ERK phosphorylation and delays in ␣-smooth muscle actin (␣-SMA) stress fiber formation (6), suggesting that CCN2 plays a key role in mediating the formation of attachments between the cell and matrix at focal adhesions.Although CCN2 was discovered over a decade ago, the precise biological function of CCN2 has remained elusive. CCN2 is expressed in mesenchymal cells in development, is induced during wound healing (4,8), and is overexpressed in fibrosis (7-11)....

The gene expression profile induced by Wnt 3a in NIH 3T3 fibroblasts

“…Alternatively, conditioned media obtained from cell lines overexpressing Wnt 1, 4, 5 or 10 or containing empty expression vector (courtesy Cun-Yu Wang, University of Michigan) was added to DMEM/0.5% calf serum in a 1:1 ratio. RNA was then harvested using Trizol (Invitrogen) and quantified as previously described (Shi-wen et al 2004(Shi-wen et al , 2006Kennedy et al 2007). …”

Pivotal role of connective tissue growth factor in lung fibrosis: MAPK‐dependent transcriptional activation of type I collagen

“…CTGF acts through integrins and proteoglycans as an independent proadhesive molecule, and modifies adhesive responses to ECM and growth factors (9,13,14). Expression of CTGF is strongly induced by TGF␤ in fibroblasts (2,3,5) and is a mediator of the profibrotic effects of TGF␤ in this cell type (14).…”

“…CTGF acts through integrins and proteoglycans as an independent proadhesive molecule, and modifies adhesive responses to ECM and growth factors (9,13,14). Expression of CTGF is strongly induced by TGF␤ in fibroblasts (2,3,5) and is a mediator of the profibrotic effects of TGF␤ in this cell type (14). However, elevated CTGF expression in fibrotic fibroblasts cultured from the skin of scleroderma patients is independent of the autocrine actions of TGF␤ and Smad signaling, but is dependent on Sp-1 activity and the presence of endothelin 1 (15,16).…”

“…However, elevated CTGF expression in fibrotic fibroblasts cultured from the skin of scleroderma patients is independent of the autocrine actions of TGF␤ and Smad signaling, but is dependent on Sp-1 activity and the presence of endothelin 1 (15,16). Indeed, it appears that in vivo and in vitro, CTGF is a cofactor required to enhance the fibrotic responses to TGF␤ (14,17).…”

CCN2: a bona fide target for anti-fibrotic drug intervention

“…In a similar vein, compared to their wild-type counterparts, mouse embryonic fibroblasts deleted for CCN2 showed impaired adhesive signaling responses to TGFβ, suggesting that the presence of basally expressed CCN2 resulted in enhanced TGFβ signaling (Shi-wen et al 2006). These latter data are consistent with the general hypothesis that CCN family members, including CCN2, act as adhesion molecules to modify signaling responses to extracellular ligands (Chen andLau 2009, 2010).…”