Up to now this protein has received only little attention in liver (patho-)physiology and most of its functional roles are deduced from non-hepatic tissue. However, as anticipated from the reaction of CTGF in non-liver fibrotic tissues, it is not surprising that this protein is increasingly produced during human and experimental liver fibrogenesis (93). The transcript levels assessed by Northern blotting, reverse transcriptase-polymerase chain reaction, RNase protection assay and in situ hybridization as well as the tissue protein concentration of CTGF are significantly upregulated in human liver fibrosis/cirrhosis of various aetiologies (101–108). However, cellular distribution and transcripts, respectively, of CTGF in liver fibrosis are still a matter of debate and possibly dependent on the aetiology and time course of the disease. Whereas in situ hybridization in progressive fibrosis owing to biliary atresia reveals increased CTGF mRNA in HSC and PC (105), no appreciable staining was found in PC, HSC and sinusoidal endothelial cells in human liver with idiopathic portal hypertension (104). The most intense staining was noticed in proliferating bile ducts (104). Similarly, in experimental biliary fibrosis, proliferating bile duct epithelial cells were identified as the predominant source of CTGF mRNA (108) but CTGF transcripts were also noticed in desmin-positive HSC cells. Another report emphasizes strong CTGF mRNA expression in fibroblasts/MFBs of fibrous septa, in HSC, endothelial cells and ductular epithelial cells (102). HSC and fibroblasts were suggested in this report as the main source of CTGF, a conclusion that is supported by findings of a combined experimental and human in situ hybridization study (103). Surprisingly, the role of PC as a cell type contributing to CTGF expression in diseased liver remains controversial. Conflicting data were reported showing either no mRNA signal (106), no appreciable transcript staining (104) or marked expression (105). Other in situ hybridization studies did not mention PC at all (102, 103, 108). However, a recent immunocytochemical analysis of CTGF in CCl4-induced fibrotic rat liver clearly demonstrates patch-like positive immunostaining of CTGF in PC, which coincides with that of cytokeratin 18 but not with that of desmin (109) (Fig. 4). The competence of PC for the synthesis of CTGF was recently shown by detailed cell culture studies, which clearly demonstrate CTGF expression in parenchymal liver cells, which is sensitively upregulated by exogenous TGF-β (109, 110) but also occurs spontaneously under TGF-β-free culture conditions owing to intracellular activation of latent TGF-β (111) (Fig. 4). Thus, PC are now recognized as a quantitatively important source of CTGF, that responds to TGF-β. Cultured human and rat HSC are an established cell type for CTGF production (101, 109, 112–115). In contrast to PC, which express only the full-length CTGF, cultured HSC were shown to produce the C-terminal fragment (109) additionally, which increases during culture time (Fig. 5). It is suspected that the proteolytic fragmentation occurs intracellularly but details require further analysis (109). The expression level can be upregulated by TGF-β (60, 109, 115), endothelin-1 (109), glucocorticoids (112), VEGF, acetaldehyde, lipid peroxidation products (hydroxynonenal, malondialdehyde) and PDGF-BB (114) (Fig. 6). Additionally, under diseased liver hypoxic conditions, oxidative stress during inflammation, shear stress and cell stretch during scarring might contribute to CTGF upregulation in PC and HSC. Reduction of CTGF expression in HSC is accomplished by PPAR-γ ligands (60) and the potent antioxidant curcumin through increasing the cellular content of reduced glutathione (116). The HCV-core protein was recently reported to be pathophysiological potentially important stimulator of CTGF expression in HSC (117). Co-culture of HSC and a stable transfected HepG2-HCV core cell line significantly increased not only CTGF but also TGF-β1, suggesting a prominent mechanism in HCV-related fibrogenesis (117). In addition to CTGF, HSC were shown to express NOV (CCN3) increasingly during activation of HSC in culture (118). Taken together, in addition to mesenchymal liver cells [HSC, (myo-)fibroblasts, endothelial cells], epithelial cell types (PC, bile duct) are newly recognized sources of CTGF in diseased liver. This is potentially an important finding with respect to the ability of epithelial cells to change the phenotype during EMT to (myo-)fibroblasts. The expression of CTGF might serve as an important sensitizer, which promotes the TGF-β-driven phenotypic switch (see above). Whether CTGF also exerts mitogenic activity not only for HSC (76) but also for PC and bile duct epithelial cells remains an interesting aspect of future studies (119).
Figure 6. Schematic presentation of liver cell types that produce CTGF, secrete it into the sinusoidal blood stream and remove it from the circulation via LRP-1. The most important exogenous stimulators (+) and inhibitors (−) are summarized. Several effector molecules are depicted from non-hepatic tissue; their relevance for the liver is not yet established. CTGF follows paracrine, autocrine and potentially endocrine routes. Possible excretion of CTGF into the bile is indicated. PC, hepatocytes; SEC, sinusoidal endothelial cells; HSC, hepatic stellate cells; BEC, biliary epithelial cells; LPOx, lipid oxidation products; LRP, low-density lipoprotein receptor-related protein; ROS, reactive oxygen species; PG, prostaglandin; CTGF, connective tissue growth factor.
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Up to now, no data are available on SNPs and promoter polymorphisms of CTGF in fibrosing liver diseases. Such an analysis, however, would be valuable, because the information might have prognostic value for the assessment of the progression rate of fibrosis as done previously for TGF-β1 (120–122). In addition to CTGF production, the liver is likely to play a major role as a clearance organ for circulating CTGF. LRP, the unique multifunctional endocytosis receptor involved in uptake of CTGF (see above), is abundantly expressed by PC (69) (Fig. 6). HSC in addition express LRP and bind full-length CTGF and the C-terminal fragment (115). The isolated module IV is not bound to LRP. Cell surface proteoheparan sulphate of PC and HSC, respectively, are binding sites for CTGF without endocytotic activity (Fig. 2). As mentioned above binding of CTGF to LRP (6) interferes with Wnt/β-catenin signalling (70), which might have profound effects on the cross-talk of intracellular signalling pathways. Recently, anti-adipogenic Wnt signalling during HSC-activation was shown and, hence, the therapeutic potential of Wnt antagonism for liver fibrosis was suggested (123). Following reported data, suppression of Wnt signalling by CTGF (70) would inhibit HSC-activation.