POINT OF VIEW

IL-6 and extracellular matrix remodeling

J. A. Solís Herruzo, P. de la Torre, T. Díaz Sanjuán, I. García Ruiz and T. Muñoz Yagüe

Department of Medicine, Gastroenterology. Research Center. Hospital Universitario 12 de Octubre. Madrid, Spain

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Solís Herruzo JA, de la Torre P, Díaz Sanjuán T, García Ruiz I, Muñoz Yagüe T. IL-6 and extracellular matrix remodeling. Rev Esp Enferm Dig 2005; 97: 575-595.

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Acknowledgment: This study was supported in part by a Grant for Medical Research from the "Fundación Mutua Madrileña" (Spain, 2004).

Recibido: 04-01-05.
Aceptado: 08-01-05.

Correspondencia: José A. Solís Herruzo. Servicio de Patología Digestiva. Hospital 12 de Octubre. Avda. de Córdoba, s/n. 28041 Madrid. Fax: 91 3908280.
e-mail: jsolis.hdoc@salud.madrid.org

 

ABBREVIATIONS

AP1: activator protein 1; CAT: Chloramphenicol acetyl transferase; ECM: Extracellular matrix; HSCs: Hepatic stellate cells; Jak: Janus tyrosine kinases; JNK: c-Jun N-terminal kinase; MMPs: Matrix metalloproteinases; PP2A: Protein phosphatase 2A; TIMPs: Tissue inhibitor MMPs; IL-6: Interleukin-6.

INTRODUCTION

Hepatic fibrosis is a characteristic feature of chronic liver diseases. It is the result from the excessive deposition of extracellular matrix (ECM) proteins into the liver in response to a number of liver injuries, including chronic ethanol ingestion, viral hepatitis and iron deposition. Hepatic fibrosis has important physiopathologic consequences on the liver function and structure. First, it may contribute to increase intrahepatic vascular resistances to the portal flow and to originate portal hypertension. Activated hepatic stellate cells may contribute to this effect, since these cells exhibit contractile capacity and can modulate blood flow at the sinusoidal level (1). Second, perisinusoidal fibrosis represents a barrier to the exchange of nutrients and metabolites between sinusoidal blood and hepatocytes. Furthermore, deposition of ECM in the space of Disse induces the loss of fenestrae of the sinusoidal endothelial cells (2). Third, ECM is a physiological reservoir for cytokines and growth factors in the close cell surrounding. The binding of these factors to ECM component modulates their biological activity and protects them from proteolysis (3).

EMC of the liver is composed of three distinct types of proteins including collagen, noncollagenous glycoproteins, and proteoglycans (Fig. 1). Collagen type I, III and V are heterotrimer molecules that form a triple helical structure with a tendency to form supramolecular aggregates (4,5). Type V, a pericellular collagen in the normal liver, is also present in increased amount in septa and portal tracts. Collagen IV is the major collagen of basement membrane. It is an unusual collagen type in that molecules cross-link at their terminal domains to form a network. This sheet of collagen is physically intermeshed or directly bound to other components of the basement membrane like laminins, nidogen, or perlecan.

In the hepatic fibrosis and cirrhosis, all types of collagens are increased. When hepatic fibrosis is mild, collagen type I and III are equally increased. However, when hepatic fibrosis is severe, the predominant collagen in the connective tissue bands is type I, and the ratio of collagen I to collagen III increased to 4:1 (5). In these conditions, collagen IV is deposited mainly in the space of Disse, transforming sinusoids into capillaries. Although collagen I becomes the predominant ECM constituent in fibrosis and cirrhosis, many other proteins also are part of the fibrous complex. Thus, glycoproteins (fibronectin, laminins, tenascin, undulin, thrombospondin, SPARC) and proteoaminoglycans (perlecan, biglycan, decorin, aggrecan) also are increased in scars and septa.

Although it is likely that each cell population in the liver plays a role in the production of ECM components, hepatic stellate cells (HSCs) appear to be the major fibrogenic cell type (6). HSCs are resident nonparenchymal cells located in the subendothelial space between hepatocytes and sinusoidal endothelial cells. In normal liver, HSCs are relatively quiescent with respect to ECM synthesis (7,8). However, during liver injury, they undergo a complex process that transforms quiescent HSC to one activated cell that is proliferative, fibrogenic, and contractile (9,10). HSC activation is associated with changes in the metabolism (increased DNA synthesis, cellular proliferation), the pattern of gene expression [collagen I, III, IV, laminin, fibronectin, MMPs (Matrix Metalloproteinases), TIMPs (Tissue Inhibitor of MMPs)], and the capacity to synthesize cytokines, growth factors and their receptors.

Extensive fibrosis in experimental liver injury may resolve upon withdrawal of the injurious agent. Thus, fibrosis or cirrhosis induced by biliary obstruction or carbon tetrachloride intoxication in rat is reversible over a period of one to three months (11). Likewise, reversible fibrosis has been shown in mouse and rabbit models of hepatosplenic schistosomiasis (12). Current evidence indicates that regression of liver fibrosis and advanced micronodular cirrhosis is a real phenomenon if the original cause of the liver injury is effectively removed (13-17). However, this process may be incomplete over a prolonged period and results in the development of maconodular cirrhosis. Remodeling of this residual lesion is limited by matrix cross-linking (18). Transglutaminase-mediated cross-linking of collagen provides matrix proteins with resistance to MMP-mediated degradadation (19). Thus, these linking may limit matrix degradation even in the presence of active MMPs.

Degradation of the ECM in the hepatic fibrosis is a complex process involving MMPs, specific TIMPs, and enzymes that activate latent MMPs (20). In an experimental model of liver fibrosis, transient MMPs overexpression in the liver effectively attenuates established fibrosis (21).

MMPs constitute a large family of structurally related zinc-dependent endopeptidases capable of degrading a wide variety of ECM components (22-24), and, therefore, they may play a key role in the resolution of liver fibrosis upon withdrawal of the injurious agent. MMP-1 and MMP-13 cleave interstitial collagens (types I, II, III and X). MMP-2 (gelatinase A) and MMP-9 (gelatinase B) degrade denatured interstitial collagen (gelatins), type V and IV collagens, and noncollagen proteins (fibronectin, laminin, elastin). MMP-3 (stromelysin-1), MMP-10 (stromelysin-2) and MMP-7 (matrilysin) degrade a broad range of substratres, including proteoglycans, and collagen and noncollagen proteins. Membrane-type MMPs (MMP-14 to MMP-25) are anchored to the surface of cells through their carboxyl-terminal transmembrane domain. These membrane-type MMPs degrade collagen and noncolagenous proteins, proteoglycans and play an important role in the activation of MMP-2 and MMP-13. In rats and mice, there is only one interstitial MMP, the MMP-13, which shares 86% homology with human MMP-13. MMP-13 is capable of degrading type I, II, III, IV, X, and XIV collagens, tenascin, fibronectina and the aggrecan protein (25).

MMPs are secreted as inactive proenzymes; therefore they need to be subsequently activated by several mechanisms including enzymatic cleavage (serine protease plasmin, stromelysin-1, membrane-type MMPs) (20,26). Activated MMPs produce a limited number of cleavages within the triple helix of the three a-chain of interstitial collagen molecules, e.g. in type I collagen, cleavage occurs between Gly⁷⁷⁵ and Ile⁷⁷⁶ in the a₁ chain and the corresponding Gly/Leu in the a₂ chain, to yield typical three-fourths length amino-terminal and one-fourth length carboxyl-terminal fragments. This initial single site cleavage of interstitial collagens results in their partial unfolding, which then renders them susceptible to degradation to small fragments by gelatinases. The activity of MMPs is regulated by a complex mechanism at the level of gene expression, proenzyme activation, or binding of proenzyme or active enzyme to TIMPs.

TIMPs are low molecular weight glycoproteins capable of irreversibly binding to proenzyme or active form of MMPs with a 1:1 stoichiometry and inactivating a wide variety of MMPs degrading enzymes. Because of the one-to-one interactions, small changes in levels of TIMPs can lead to significant changes in the biological activity of MMPs. Excess production of TIMPs relative to MMPs could contribute to the progression of liver fibrosis, while the enhancement of matrix breakdown may be achieved by either reducing the synthesis of TIMPs or increasing the expression of MMPs. All members of this family are able to inhibit all of the MMPs. However, some TIMPs have specific relationship with individual MMPs (27,28). TIMPs are multifunctional molecules that not only inhibit MMPs but also stimulate proliferation of various cell types and can protect cells from apoptosis (29,30).

The severity of hepatic fibrosis is the result of the degree of collagen synthesis and the fate of the collagen deposited into the ECM of the liver, which, in turn, depend on the balance between the production and the activation of MMPs and the levels of TIMPs. In chronic liver disease in human and in animal models of liver fibrosis, expression of MMP-1/MMP-13 does not change significantly, as hepatic fibrosis develops and progresses, but there is a striking increase in expression of TIMP-1 and TIMP-2 (31). In homogenates prepared from fibrotic livers, the level of TIMP-1 protein increases approximately five-fold in comparison with normal liver. Regression of hepatic fibrosis is characterized by degradation of ECM and restoration of normal liver histology. Following cessation of injury, there is a five-fold increase in collagenase activity in the liver, which is associated with a progressive decrease in the level of expression of TIMP-1 and TIMP-2, but no change in the level of expression of MMP-13. Studies in cultured HSCs have explored the molecular basis of the up-regulation of TIMP-1 during hepatic fibrogenesis. These studies have identified a key region in the TIMP-1 promoter that is critical to gene expression (32). However, up to now, it has not been identified the proteins interacting with this region. Many cell types, particularly HSCs, Kupffer cells and inflammatory cells, produce several MMPs in culture (20,26).

Very few studies have examined the effect of cytokines on ECM degradation in HSCs. However, it is well known that MMP production can be up-regulated by several hormones (PTH, cortisol), growth factors (platelet derived growth factor, basic fibroblast growth factor), cytokines (TNFa, IL-1a, IL-6, IL-10, IFNg, oncostatin M, macrophage migration inhibitory factor) and tumor promoters (phorbol esters). On the other hand, MMP-13 is down-regulated by transforming growth factor (TGFb), insulin-like growth factor (ILGF) (20,26,33-40). Likewise, several cytokines (TNFa, IL1b, IL-6, oncostatin M, IFNa) and growth factors (TGFb) up-regulate (41-43) or inhibit (44-47) TIMPs expression by cultured HSCs.

Interleukin-6 (IL-6) is a multifunctional glycoprotein produced by activated monocytes, macrophages, endothelial cells, and hepatic stellate cells that mediates a wide variety of functions in different cells, including fibroblasts, hepatocytes, and hepatic stellate cells (48). IL-6 promotes cell proliferation and differentiation and regulates specific gene expression (49), particularly the expression of the acute phase proteins in liver cells (48,50). Acute and chronic liver diseases, particularly alcoholic liver diseases, are similar to the acute phase response in some respects. Thus, patients with alcoholic hepatitis show fever, muscle wasting, neutrophilia, and increased production of C-reactive protein, a1-antitrypsin and amyloid A (50,51). High levels of IL-6 have been detected in the sera of patients with alcoholic liver cirrhosis (52-54) and some authors have shown a correlation between circulating concentrations of IL-6 and serum concentrations of C-reactive protein (55,56), an IL-6-induced protein. Thus, IL-6 seems to be one of the most important factors regulating inflammatory response in the liver.

We have shown that IL-6 increases MMP-13 protein levels and the steady-state levels of MMP-13 mRNA in a dose- and time-dependent fashion (Fig. 2) and that this effect is produced acting within the gene promoter (57). This effect is evident after 6 hours of treatment but it is particularly significant after 24 hours. This prolonged incubation time required by IL-6 to activate MMP-13 gene expression suggests that it may be required the synthesis of a new protein. The IL-6-induced increase in MMP-13 mRNA levels can be blocked by inhibiting the protein synthesis with cycloheximide (Fig. 2C), which supports this requirement.

This enhanced MMP-13 gene expression is associated with a marked increase in the immunoreactive MMP-13 protein (Fig. 2D). These results agree with those reported by Franchimont et al. (58) and Kusano et al. (59), who also demonstrated that IL-6, in the presence of its soluble receptor, increased MMP-13 mRNA levels, immunoreactive MMP-13 and its biological activity. It is likely that IL-6 has an effect on the extracellular metabolism of MMP-13, since the effect of this cytokine on MMP-13 secreted into the culture medium is much higher than that induced on the steady-state level of MMP-13 mRNA. Although some authors have shown that IL-6 significantly enhanced TIMP-1 production and TIMP-1 mRNA expression in human fibroblasts and other cell lines (60-64), in our experience, IL-6 also increases immunoreactive TIMP-1, but to a lesser extent (Fig. 2D). In fact, TIMP-1 and MMP-13 promoters contain common regulatory elements, i.e. an AP1 binding site (TRE) and PEA-3 (65). Therefore, factors that up-regulate MMP-13 gene expression may also stimulate TIMP-1 expression.

Rat MMP-13 promoter displays a general organization similar to that of other members of the MMP family (66,67), particularly to human (68) and rabbit MMP-1 genes (69). All share a common 10-exon organization (67,68,70) and contain a typical TATA box in addition to TRE and polyomavirus enhancer activator 3 (PEA-3) consensus site in their promoter region (67,71-74), suggesting a common regulatory mechanism of gene transcription. Rat MMP-13 gene also contains several consensus transcription factor recognition sequences such as C/EBP, Cbfa1, p53, AP-2, and AP1 (75) (Fig. 3). In cells transfected with plasmids whose activity is driven by portions of different length of the MMP-13 gene promoter, we found that sequences upstream of the base pair -79 are dispensable in IL-6-medited stimulation of MMP-13 gene expression. These experiments also show that integrity of the TRE site appears to be essential for the IL-6 induced response (Fig. 4). The TRE site has been implicated in the expression of many of the MMP genes (76,77), including MMP-13 (77). In cells transfected with a luciferase construct containing two copies of the TRE upstream of a minimal promoter, we were able to confirm this stimulatory effect of IL-6 on the TRE site (Fig. 5). Very little information exists about the effect of IL-6 on the activation of genes with a TRE. However, while Daffada et al. (78) found that IL-6 had no effect on the expression of pTRE-CAT in transient transfected cells, suggesting that AP1 is not induced by IL-6 treatment, Melamed et al. (79) showed that IL-6 induced the formation of a TRE-binding complex, which was abrogated by anti-Jun specific antibodies.

Upregulation of gene expression is usually induced by the binding of transcrition factors to specific cis-acting elements in gene promoters. Our study suggested that IL-6 upregulates MMP-13 gene expression via transcription factor AP1 and through the cis-acting element TRE. Thus, gel DNA retardation experiments show that IL-6 promotes a dose- and time-dependent binding of nuclear proteins to TRE site (Fig. 6). Nuclear protein-DNA complexes induced by the IL-6 treatment contain phosphorylated c-Jun as shown by the incubation of nuclear extract with a phosphorylated c-Jun-specific antibody prior to the gel retardation assay, which lead to the formation of two supershifted complexes. These results support the role played by TRE and AP1 in mediating the effect of IL-6 on rat MMP-13 gene expression.

The AP1 transcription factor is organized into Jun-Jun, Jun-Fos, or Jun-ATF dimmers, and the presence of Jun family members in these dimmers enables AP1 to bind cis-acting elements of gene promoters. These proteins form a variety of homo- and heterodimers that bind to a common DNA recognition site, the TRE (58,80,81). As figures 7 shows, culture of fibroblasts with IL-6 induces c-Jun, JunB and c-Fos mRNA (Fig. 7A), c-jun and c-fos promoters (Figs. 7B and 7C) and c-Jun and c-Fos proteins (Figs. 8A and 8B). Other authors have also shown that IL-6 stimulated junB gene expression in a variety of cells (82-86) and Cressman et al. (87) have shown that the expression of junB is markedly reduced in the liver of IL-6-deficient mice.

Transcriptional activity of AP1 depends not only on the quantity of AP1 components and their ability to bind DNA but also on the degree of phosphorylation of these proteins (80,88). Phosphorylation of c-Jun in its activation domain at serine 63 and 73 prolongs its half-life and potentiates the ability of c-Jun to activate transcription as either a homodimer or as a heterodimer with c-Fos (80). Western blot using a specific monoclonal antibody for serine 63 phosphorylated c-Jun demonstrates that IL-6 induces an increase in this form of c-Jun, which is particularly marked after 12 hours of treatment (Fig. 8C). Lütticken et al. (84) also showed that IL-6 triggers a delayed phosphorylation of STAT3 at serine residues. A variety of protein kinases, including JNK (c-Jun N-terminal kinase), pp42, pp54, pp44-, and p38-mitogen-activated protein kinases (89), p34cdc2, protein kinase C, casein kinase II, efficiently phosphorylates c-Jun (90). JNK, also known as stress-activated protein kinase (SAPK), is the most efficient member of the mitogen-activated protein kinase (MAPK) family involved in c-Jun phosphorylation on serines 63 and 73 of c-Jun and in potentiating its transactivation function (91,92). Moreover, JNK is a critical MAPK pathway for IL-1 induced collagenase gene expression in a variety of cell types (93). However, our study indicates that IL-6 does not induce the phosphorylation of c-Jun by stimulating JNK activity. As figure 9A shows incubation of cells with 10 and 20 ng/ml IL-6 for 15 minutes, but not for 5 minutes, decreased JNK activity to 84 and 68%, respectively, of that of control cells. Moreover, Western blots using specific polyclonal antibodies against JNK reveal one single band located at 46 kDa, the density of which does not change after incubation with IL-6. Likewise, this treatment do not modify phosphorylated JNK, as reveal Western blots using specific monoclonal IgG antibody raised against phosphorylated JNK (Fig. 9B). As dual phosphorylation of serine/threonine amino residues by the MAP kinase kinase (MKK4 and MKK7) is necessary for JNK activation (94), these results support the conclusion that this kinase is not involved in the IL-6-induced increase in phosphorylated c-Jun and MMP-13 gene expression. These results concurs with those reported by a number of authors showing that IL-6 has no detectable effects on JNK (95,96) or even inhibits the activation of this MAP kinase (97,98).

As mentioned above, in addition to JNK, a variety of protein kinases, including ERK, p38 kinase, casein kinase II, and PKC (99) can efficiently phosphorylate c-Jun (90,100). Moreover, in some cell lines, ERK pathway seems to play a role in mediating some effects of IL-6 (98). However, blocking these kinases with specific inhibitors does not abrogate the stimulatory effect of IL-6 on c-Jun phosphorylation (Fig. 10A), AP1 binding to DNA (Fig. 10B) or MMP-13 gene expression (Fig. 10C). In a recent study (101), we blocked ERK with PD98059, p38 MAP kinase with SB203580, protein kinase C (PKC) with H7 and GF 109203X, protein kinase A with H8 and H89, calmoduline-dependent protein kinase with calmidazolium, and phosphatidylinositol-3-kinase with wortmannin. Taken together, these results indicate that none of these protein kinases is likely required for the effect of IL-6 on MMP-13 gene expression.

Phosphorylation state of c-Jun is a dynamic process controlled by both serine/threonine protein kinases and protein phosphatases 2A (PP2A) (102). Therefore, the increase in phosphorylated c-Jun may result not only by an enhanced kinase activity but also by a reduced PP2A activity. In fact, a number of studies have clearly demonstrated that inhibition of protein phosphatases 2A results in an induction of collagenase, JunB, and c-Fos mRNA and a potent activation of AP1, through serine/threonine phosphorylation (103-106). There are four classes of serine/threonine-specific protein phosphatases. These include PP1, PP2A, PP2B (calcineurin), and PP2C. PP2A and PP1 are widely distributed in the cytoplasm of mammalian cells and have been reported to be involved in signalling pathways, modifying the activity of a variety of protein kinases (107). Measurement of the PP2A activity in fibroblasts treated with increasing concentrations of IL-6 shows that this cytokine decreased this activity in a dose dependent fashion (Fig. 11A). Likewise, incubation of cells with 20 ng/ml IL-6 for 15 minutes to 24 hours resulted in a decrease in serine/threonine phosphatase activity, which was particularly marked between three and six hours. At 12 and 24 hours, this activity remained under the control activity (Fig. 11B).

The key role of PP2A in the IL-6-induced MMP-13 gene expression is also supported by experiments in which Rat-1 fibroblasts are pretreated for 20 minutes with 25 nM okadaic acid, a potent inhibitor of protein phosphatase 1 and PP2A (108). In this experimental condition, MMP-13-mRNA levels increase markedly and the addition of 20 ng/ml IL-6 for 24 hours does not modify this effect (Fig. 12). In contrast, as expected, inhibition of tyrosine phosphatases with 0.1 mM pervanadate does not change the stimulatory effects of IL-6 on MMP-13 gene expression. Furthermore, okadaic acid mimics the effects of IL-6 on MMP-13 gene expression. Many other authors have shown these effects of the okadaic acid on MMP gene expression, as well as on c-Jun phosphorylation and AP1 binding activity (105,109,110). I₂^PP2A, another potent inhibitor of PP2A, have produced similar effects (106). This decrease in PP2A activity is not due to a reduced amount of PP2A protein in fibroblasts exposed to IL-6. Thus, measurement of these levels in cell extracts from fibroblasts treated with 20 ng/ml IL-6 for three to sixteen hours demonstrates that this cytokine did not induce any change in intracellular PP2A levels.

Core structure of PP2A consists of complexes of three subunits (102,111,112). A 36 kDa catalytic C subunit (PP2AC) is complexed with a structural A subunit of 65 kDa (PP2A-A). This dimer is associated with a third, variable regulatory B subunit, that likely influences substrate specificity or cellular localization (102). The phosphorylation on tyrosine-307 at the C-terminus of the catalytic subunit of PP2A regulates phosphatases activity (113,114). It is well known that this phosphorylation is associated with a 90% loss in activity, and thereby with a sustained effect of protein kinases (113,115), including c-Jun phosphorylation. On the contrary, dephosphorylation of PP2A_C reactivates this enzyme (102). Considering this mechanism of regulation of the PP2A activity, we could hypothesize that binding of IL-6 to specific receptors on cell surface might inactivate PP2A by increasing tyrosine phosphorylation on its catalytic subunit which, in turn, would prolong activation of AP1. Similar sequence of events has been shown following addition of insulin to skeletal muscle cells (115,116). Moreover, inhibition of PP2A with I₂^PP2A increased both the concentration and DNA binding of c-Jun and the transcriptional activity of AP1 (106). Likewise, PP2A inhibitors induce threonine phosphorylation of STAT3, other cytoplasmic transcription factor that upon activation translocate into the nucleus where they activate target genes (117,118).

Western blot analysis using specific polyclonal antityrosine 307 phosphorylated PP2A_C antibody shows a marked increase in the tyrosine phosphorylation of PP2A_C (Fig. 13A). On the other hand, while immunoprecipitation of PP2A using specific antibody against the structural subunit of PP2A demonstrates that IL-6 did not affect to the total amount of this subunit, immunodetection of tyrosine phosphorylated proteins with antiphosphotyrosine antibody indicates that treatment of cells with 20 ng/mL IL-6 for 0 to 60 minutes induces a time-dependent increase in the PP2A_C tyrosine phosphorylation (Fig. 13B). This result concurs with those reported by Choi et al. (119), who also found that IL-6 induced PP2A tyrosine phosphorylation.

IL-6 initiates its actions by binding to a specific receptor complex on the cell membrane. This complex is composed of two subunits: an 80-kDa binding protein and a 130-kDa transmembrane signal transducting component (gp130) (48). Other members of the IL-6 cytokine superfamily (IL-11, oncostatin M, leukemia inhibitory factor, ciliary neutrophic factor, cardiotropin-1, neutrophin-1/B.cell stimulating factor-3) (120-122) share the same gp130 receptor subunit. Activation of IL-6 signal transduction involved gp130-homodimerization and tyrosine phosphorylation of the cytoplasmic tail of gp130 by gp130-associated Janus tyrosine kinases (JAKs). These kinases are recruited to the plasma membrane and activated upon cell activation by cytokines and growth factors (123). All JAKs contain at least one copy of a peptide sequence, which is part of a consensus for PP2A binding (124,125). This phosphatase has been shown to form stable complexes with various protein kinases, including Jak2 (115,126,127). Thus, it is conceivable that the IL-6-induced PP2A_C phosphorylation and PP2A inactivation might be mediated by Jak2. Thus, as figure 14 shows, pre-treatment of cells for 60 minutes with 5 µM AG490, a specific inhibitor of Jak2 (128), abrogates completely the IL-6-induced increase in MMP-13 mRNA, binding of nuclear proteins to AP1 consensus sequence, c-Jun and PP2A_C phosphorylation, and PP2A activity. As far as we are aware, no further information is available on the role of Jak kinases and PP2A in mediating the effect of IL-6 on MMP-13 gene expression. However, oncostatin M, another member of IL-6 superfamily cytokine (122), also induces MMP gene expression in astrocytes, fibroblasts and osteoblasts (37,130), and this effect is associated with Jak1 and Jak2 tyrosine phosphorylation.

Thus, we can conclude that, at least "in vitro", IL-6 is an antifibrogenic cytokine that might contribute to remodelling connective tissue. IL-6 increases MMP-13 gene expression acting at transcriptional level via AP1. IL-6 increases c-jun and c-fos gene expression, enhances phosphorylated c-Jun, and promotes the binding of AP1 to the MMP-13 promoter. The increase in phosphorylated c-Jun is not due to increased protein kinase activity, including Jun N-terminal kinase, but to a decreased serine/threonine phosphatases activity. IL-6, after binding to a specific receptor on the cell membrane, activates tyrosine kinase JAK2, which phosphorylates the catalytic subunit of protein phosphatases 2A (PP2A) at the tyrosine 307 leading to inhibition of its activity and prolongs c-Jun phosphorylation (Fig. 15).

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