ÖSSZEFOGLALÓ KÖZLEMÉNY
Molecular and cellular basis of fibrosis: recent insights to the pathomechanism of scleroderma from animal models and fibroblast studies
(A fibrosis molekuláris és celluláris mechanizmusa: újabb eredmények a scleroderma patogenezisének kutatásában)
Gabriella Lakos, Shinsuke Takagawa, John Varga
 
 
 
 


GABRIELLA LAKOS, MD (levelező szerző/correspondent); SHINSUKE TAKAGAWA, MD; JOHN VARGA, MD; University of Illinois at Chicago College of Medicine, Section of Rheumatology (M/C733), Room 1158 Molecular Biology Research Building, 900 S. Ashland Avenue, Chicago, IL 60607. Phone: (1) 312-413-9310, fax: (1) 312-413-9271, e-mail: lakosg@hotmail.com

Magyar Immunol/Hun Immunol 2002;1 (4): 4-14.

Érkezett: 2002. november 10.
Elfogadva: 2002. november 15.



ÖSSZEFOGLALÁS

A scleroderma krónikus autoimmun megbetegedés, amelynek legfőbb jellegzetessége a fibrosis. A fibrosis lényege a kötőszöveti rostok, elsősorban az I. típusú kollagén fokozott szintézise és lerakódása az érintett szervekben, elsősorban a bőrben, a tüdőben, a szívben, a vesében és a gastrointestinalis traktusban, ami ezen szervek progresszív elégtelenségéhez vezet. A fibrosis patogenezise ma még nem tisztázott, és kellően hatékony terápia sem áll rendelkezésre. Az utóbbi időben azonban számos, a fibrosis kialakulásában szerepet játszó tényezőt sikerült azonosítani. Közülük a legfontosabb szerepe valószínűleg a transzformáló növekedési faktor-b-nak (TGF-b, transforming growth factor b) van. A legújabb kutatások a fibrosis kialakulásában szerepet játszó citokinek és növekedési faktorok azonosítására, kölcsönhatására és jelátviteli útvonalaira koncentrálnak, ezáltal keresve potenciális célpontokat a specifikus antifibrotikus terápia számára. Ez az összefoglaló az ezen a területen született legújabb eredményeket foglalja össze, elsősorban állatmodellek és fibroblastokon végzett kísérletek alapján.

scleroderma, fibrosis, TGF-béta



 
 

Figure 1. Scleroderma. 

a: Dermal fibrosis in scleroderma (hematoxylin and eosin staining, 100×). The dermis is acellular, and shows a paucity of adnexal structures. Note the enormously thickened dermis, tightly packed and homogenized collagen bundles.  

b: Perivascular mononuclear cell infiltrate in early scleroderma (hematoxylin and eosin staining, 400×)

 

Scleroderma (systemic sclerosis, SSc) is a chronic, progressive connective tissue disorder of  unknown etiology, and without effective treatment (1). SSc is a uniquely is a complex disease, featuring inflammation and fibrosis, vascular injury, and immunologic abnormalities. Affected individuals possess circulating serum autoantibodies against certain nuclear proteins. There are two main form of the disease, according to the severity and extent of skin fibrosis, involvement of internal organs, and autoantibody pattern: diffuse, and limited form of systemic sclerosis. Fibrosis is a hallmark of the disease: it is characterized by uncontrolled, excessive synthesis and deposition of extracellular matrix (ECM) components, mainly type I collagen (Figure 1. a). It affects the skin and multiple organs, such as lungs, kidneys, gastrointestinal tract and heart. Accumulation of ECM molecules results in progressive replacement of normal tissue architecture by acellular connective tissue, and consequent organ dysfunction. Vascular injury sometimes precedes the onset of the disease even by years (Raynaud's phenomenon) (2). The mechanism and pathogenesis of this complex disease remains incompletely understood, however, it is clear, that there is an imbalance between the production and degradation of ECM. Inflammation is an early event (Figure 1. b), followed by increased collagen synthesis by fibroblasts. The central role of transforming growth factor-beta (TGF-beta) in the pathogenesis of SSc was first proposed in 1990 (3). Signaling by TGF-beta elicits potent profibrotic responses in tissue fibroblast. There is growing knowledge on identifying the cytokine and growth factor mediators of fibrosis, and in delineating the cellular and molecular signaling pathways that are activated by these mediators. This review focuses on the new information obtained by in vitro and in vivo studies, providing insight in the pathogenesis of fibrosis in scleroderma.
 

The extracellular matrix

Organization of the extracellular matrix

The extracellular matrix (ECM) is a highly organized network of macromolecules that provide tissues with both structural support and information. It is composed of proteoglycans like decorin and fibromodulin; fibrous proteins like collagen, elastin and fibrillin; adhesion molecules like fibronectin and laminin; and different types of matrix metalloproteinases. The major component of ECM in skin is type I collagen that is composed of two glycine- and proline-rich alpha1 and one alpha2 chain (COL1A1 and COL1A2). Procollagens are secreted by the fibroblasts through the Golgi apparatus in the extracellular space, where the N-terminal and C-terminal propeptides are cleaved by specific proteases. It is apparent that a careful equilibrium bet-ween synthesis and degradation of the ECM must be continuously maintained.
 

Regulation of extracellular matrix accumulation

The cells residing in the matrix, particularly fibroblasts and related mesenchyme-derived cells, are key players in maintaining the balance between matrix accumulation and degradation. These cells have the capacity to generate the molecular components of the ECM, as well as the enzymes for its degradation (matrix metalloproteinases), along with other molecules that regulate the activity of these matrix-degrading enzymes. Several ligands that can stimulate production of matrix components by fibroblasts have been described (Table 1). Among them, TGF-beta1 and connective tissue growth factor (CTGF) are considered to be the most important. TGF-beta is a potent chemoattractant for fibroblasts, and a strong inducer of collagen synthesis (4). Moreover, it can stimulate the production of CTGF and fibroblast growth factor (FGF), as well as its own production (autoinduction). FGF promotes fibroblast proliferation (5), while CTGF further increases collagen production.
 

Table 1. Cytokines, growth factors and other signals implicated in the development of fibrosis. 
 

Adhesive receptors (integrins) on fibroblasts also play an important role in ECM turnover (6). As well as mediating cell attachment, these heteromeric transmembrane receptors convey signals from the ECM to the cell, modifying cell differentiation, proliferation, survival and ECM production and degradation (7, 8). Thus, signals from the ECM induce cellular responses that are required for tissue remodeling during physiological processes such as development, growth and wound repair.

Given the high degree of complexity in the bidirectional communication network between cells and the matrix, even modest and transient perturbation at any level may results in the development of pathological tissue fibrosis. Increased productions of ECM molecules, or decreased ECM turnover are regarded as the cardinal pathways for the development of fibrosis. Both of these processes are implicated in SSc, and fibroblasts are the key effector cells. While unifying concept to explain the pathogenesis of fibrosis has not yet emerged, multiple alterations result in the development of pathological tissue fibrosis have been identified.
 

Transforming growth factor-beta

Sources, synthesis, activities

The ability of transforming growth factor-beta (TGF-beta) to stimulate collagen production in fibroblasts and its association with multiple diseases characterized by tissue fibrosis led to the proposal that this cytokine mediates the fibrosis in SSc (3). TGF-beta1 is a prototypic member of a larger TGF-beta superfamily of proteins that include several bone morphogenetic proteins (BMP), activin, and the recently identified lefty family. Over 40 TGF-beta superfamily members have been identified, which affect growth and differentiation in many cell types (9, 10). TGF-beta1-3 are the three mammalian TGF-beta isoforms. Numerous studies have shown TGF-beta1 to be a potent modulator of immune responses (11), and mice lacking TGF-beta1 develop an early onset multifocal inflammation resulting in death around three weeks of age (12, 13). TGF-beta is produced by monocytes/macrophages, platelets, endothelial, epithelial cells, and fibroblasts. Besides of stimulating collagen synthesis, TGF-betas have many profibrotic activities, summarized in Table 2. Although the three TGF-beta isoforms utilize the same cellular receptors and have similar effect in vitro, they are differentially expressed during embryogenesis, and are not functionally redundant (14).
 

Table 2. Profibrotic activities of TGF-beta1. 
 

TGF-betas are synthesized as 390-412 amino acid precursors. These precursors are proteolytically processed, and the mature C-terminal fragment non-covalently binds to its N-terminal pro-domain, called latency-associated peptide (LAP). In this small latent complex TGF-beta is not able to interact with its receptors (15). LAP covalently binds to a matrix protein, termed latent TGF-beta-binding protein (LTBP), forming a large latent complex. Latent TGF-beta-binding protein is important in attachment to the ECM (16). Activation of TGF-beta requires its release from the large molecular LTBP complex, a process mediated in vivo by thrombospondin-1, plasmin, and probably other extracellular enzymes (17). Recently, it has been shown that TGF-beta LAP is a ligand for integrin alphav-beta6, and alphav-beta6 expressing cells induce activation of TGF-beta (18). The mature TGF-beta molecule is a disulfide-bonded homodimer. The release of TGF-beta from its latent storage complex in the ECM represents a critical step in regulating TGF-beta bioactivity.

TGF-beta signaling

Activated TGF-beta elicits cellular responses by binding to high affinity serine/threonine kinase transmembrane receptors. Three major types of TGF-beta receptors have been identified (19). Among them, the type I and type II TGF-beta receptors (TbetaRI and TbetaRII) are the signaling receptors, expressed on the surface of almost all cell types. The type III receptors include endoglin, which is a non-signaling receptor, and its role has not been identified. Of note, it has been recently demonstrated that endoglin is mutated in patients with hereditary hemorrhagic telangiectasia 1, a disease characterized by widespread telangiectasia (20). Interestingly, the cutaneous lesions in this inherited disease are indistinguishable from those found on patients with limited cutaneous SSc/CREST. All type I and type II TGF-beta receptors exist as homodimers in the absence of ligand. TGF-beta binds to TbRII homodimer, which then recruits TbetaRI to form a heterotetrameric complex, resulting in the phosphorylation of serine and threonine residues of TbetaRI by TbetaRII (21). The phosphorylated TbetaRI then becomes activated and can phosphorylate downstream targets, the recently discovered Smads.
 

The role of Smads
 

Table 3. The vertebrate Smad family. 
 

Thus far, nine mammalian Smads heve been identified (22). Smads can be divided into three structurally and functionally distinct groups (Table 3). The group of receptor-activated Smads (R-Smads) includes Smads1, 5 and 8, which can be phosphorylated by BMP, and Smad2 and 3 for TGF-beta1 and activin. The second group contains Smad4, the common partner Smad (co-Smad), while the last group is referred as inhibitory Smads, including Smad6 and Smad7 (23, 24). Ligand-induced phosphorylation of Smad3 allows it to dissociate from the activated receptor, and increases its affinity for Smad4; interaction with Smad4 is essential for assembly of the transcriptionally-competent Smad complex in the cytoplasm. TGF-beta induces rapid Smad3 and 4 translocation from the cytoplasm into the nucleus, where it stimulates transcription (25, 26) (Figure 2).
 

 
  

Figure 2. Schematic illustration of TGF-beta/Smad signaling. Upon binding of TGF-beta, TRbetaII recruits and phosphorylates TRbetaI. Activated TRbetaI then phosphorylates receptor-activated Smads (Smad2 or Smad3). Phosphorylated Smad2 or Smad3 then associates with Smad4, and the heteromeric Smad complex translocates from the cytoplasm to the nucleus, where it binds to and regulates the transcription of TGF-beta target genes. Smad7 is induced by TGF-beta through Smad3. It interferes with Smad2 and Smad3 phosphorylation upon binding to the activated TRbeta complex, and thus blocks TGF-beta responses.

 

Smads have conserved globular mad homology domains 1 and 2 (MH1 and MH2), and a more divergent linker region connecting them (in Smad6 and 7 only the MH2 domain is conserved). The MH1 domain is the DNA-binding domain, while the MH2 domain of Smad3 serves as transcriptional activator. Another important function of the MH2 domain is to enable Smad3 to interact with transcriptional co-activators such as p300/CBP (27). Smad3 directly recognizes the CAGAC sequence found in the promoters of the human type I collagen genes (28, 29). Besides collagen I, the expression of numerous genes potentially involved in the pathogenesis of fibrosis is regulated by Smads, including COL7A1 (30), plasminogen activator inhibitor (PAI-1) (31), tissue inhibitor of metalloproteinase (TIMP) (32), platelet-derived growth factor-beta chain (PDGF-b) (33), and connective tissue growth factor (CTGF)34. TGF-beta1 also can induce the expression of Smad7 (26, 35), which binds to TbetaRI and inhibits the activation of R-Smads by this receptor, establishing a negative feedback loop. Another important inhibitor of the TGF-beta/Smad signaling pathway is interferon-gamma (IFN-gamma), a potent antifibrotic cytokine (36).
 

Connective tissue growth factor

Connective tissue growth factor (CTGF) was first described as an endothelial cell product, but was shown to be synthesized by other cells, including fibroblasts. CTGF plays an important role in embryonic development (37). In adults it is intimately involved in tissue repair and wound healing processes (38). CTGF is a member of the CCN family of immediate-early genes, characterized by the presence of highly conserved cysteine-rich motifs (39). CTGF have two main functional activities: it stimulates mesenchymal cells to proliferate and produce connective tissue components (40), moreover, acting on endothelial cells it promotes angiogenesis (41). CTGF is constitutively expressed in endothelial cells, but induced only by treatment with TGF-beta in normal fibroblast (42).
 

Hypoxia

Hypoxia is a physiological feature of the early phases of wound healing, and has been shown to stimulate angiogenesis through upregulating of several growth factors (43, 44). Hypoxia also upregulates the synthesis of TGF-beta1 by human dermal fibroblasts (45). Moreover, low oxygen tension leads to a dose dependent increase in both COL1A1 mRNA and protein synthesis (46, 47). Upregulation of COL1A1 mRNA levels in hypoxia can be blocked by TGF-beta1 anti-sense oligonucleotide, and fails to occur in fibroblasts from TGF-beta1-konckout mice (47). These results suggest that hypoxia is a potent physiologic stimulus for collagen synthesis. Microvascular injury is an early feature in scleroderma. The consequent tissue hypoxia may play a role in the initiating of uncontrolled collagen production and fibrosis.
 

Animal models of scleroderma

The pathogenesis of SSc is not completely understood, and there is no effective treatment of the disease. There are several different experimental animal models for SSC that show some of its features, however, they can't produce all of the characteristics of the disease. In addition these models provide insight in the pathogenesis of SSc, they also help test potential new interventions in human scleroderma.
 

The TSK mice

Several models of SSc result from genetic alterations, reflecting the importance of genetic background of scleroderma. Tight-skin1 (TSK1) mouse is genetically originated by a dominant mutation (tandem duplication) within the fibrillin-1 gene (48, 49). The TSK1 mutation is lethal in homozygous form, while heterozygous animals display cutaneous and certain visceral abnormalities resembling human scleroderma. The most characteristic alteration is the strong subcutaneous fibrosis. This histological evidence has been confirmed by biochemical studies, showing increased collagen biosynthesis of cultured dermal fibroblasts (50). The most prominent visceral changes occur in the lungs and heart. The lung abnormalities are characterized by distended lungs, resembling human emphysema, but with little fibrosis (51). The lack of pulmonary fibrosis in the Tsk mice indicates a striking difference of this model comparing to human scleroderma. Myocardial hypertrophy is due to markedly increased type I collagen synthesis in the myocardium (52). Immunologic abnormalities are also present: antinuclear antibodies have been detected in about 50% of the mice, and autoantibodies to fibrillin-1 and RNA polymerase I have been shown to produce by the affected animals (53, 54). However, unlikely to human scleroderma, mononuclear inflammatory cell infiltration of affected organs is not seen in the TSK1/+ mouse. The role of the immune system in developing the characteristic TSK/+ phenotype is controversial. Although it was shown that neutralizing antibodies to IL-4 or a null mutation in this gene could prevent the development of dermal fibrosis in mice (55), others demonstrated that the TSK phenotype is not dependent on the presence of mature T and B lymphocytes (56), moreover, it can be developed even in scid mice (57). TSK1/+ mice produce both normal and mutant fibrillin-1 molecules. Mutant fibrillin-1 molecules are incorporated in the ECM, and it was hypothesized that the abnormal protein would change the homeostasis of the ECM (58).

The other murine tight skin mutation was a result of administration of the mutagenic agent, ethylnitrosourea. TSK2/+ mice develop a tight skin phenotype, however, in contrast to TSK1/+ mice, a mononuclear cell infiltration is present in the dermis and adipose tissue (59). The TSK2 mutation is at particular interest, because it was caused by a toxic agent. Studies to identify the mechanisms responsible for the connective tissue abnormalities in the TSK2/+ mice may therefore provide information regarding the role of environmental exposures in the pathogenesis of SSc.
 

The murine sclerodermatous graft versus host disease

Murine sclerodermatous graft versus host disease (Scl GVHD) is induced by transplantation of bone marrow and spleen cells across minor histocompatibility loci. The disease in mice described by Gulliam's lab, highly resembles the human chronic GVHD occurs in some patients after bone marrow transplantation (BMT). Transplanting B10.D2 bone marrow and spleen cells into BALB/c mice after lethal irradiation of the recipients results in a disease characterized by remarkable skin thickening and pulmonary fibrosis 21 days after BMT. Type I collagen mRNA and protein, as well as TGF-beta1 mRNA levels are upregulated. By 14 days after BMT many monocyte/macrophages and CD3+ T lymphocytes infiltrate the skin. The infiltrating immune cells are of donor origin. Increased macrophage scavenger receptor and H-2d class II molecule expression suggest the activated status of the macrophages (60). There are a lot of clinical, immunologic and histopathologic similarities between Scl GVHD and human scleroderma, however, there is no vascular injury, and autoantibodies can't be detected. Moreover, the cells initiating the fibrotic process are not of host origin. Therapeutic interventions were tested on this model. Intravenous administration of anti-TGF-bETA1 blocking antibody at days one and six after BMT successfully prevented the development of Scl GVHD. Antibodies effectively blocked the influx of immune cells, as well as the upregulation of TGF-bETA1 and collagen synthesis (61). Halofuginone also prevented skin thickening, and decreased type I collagen synthesis (62).
 

Bleomycin-induced pulmonary and skin fibrosis

Bleomycin, an antibiotic widely used in cancer treatment has a side effect of inducing pulmonary fibrosis and scleroderma-like conditions (63, 64). Bleomycin is used to induce pulmonary fibrosis in experimental animal models (65, 66). The prominent infiltration of lung parenchyma with chronic inflammatory cells suggests that the resulting fibrosis is immune-mediated. Expression of TbRI and TbRII was found to be altered, and the TGF-beta signal transduction pathway seems to be involved in the pathogenesis of lung fibrosis (67, 68). Recently, a mouse model of scleroderma was established by repeated subcutaneous bleomycin injections (69). Daily injections at a dose of >10 mikrogramm/ml for three weeks (in C3H mice) or four weeks (in BALB/c mice) induce dermal sclerosis and lung fibrosis. The skin shows early inflammatory infiltrates of mononuclear cells, increased number of myofibrocytes, and thickened homogenous dermal collagen bundles (69, 70) (Figure 3). Transient TGF-beta1 mRNA upregulation can be shown at early stages of the disease, while TGF-beta2 mRNA upregulation after four week, when the fibrosis is prominent.
 

  
  

Figure 3. Bleomycin induced scleroderma in mouse (hematoxylin and eosin staining).  
a: Mononuclear cell infiltrate in the dermis after three days in bleomycin-injected skin (100×).  
b: Thickened homogeneous collagen bundles in the dermis after three weeks in bleomycin-injected skin (400×). 

 

Our recent results on this model show that the inflammatory infiltrate mainly consists of monocytes, showing activation of TGF-beta/Smad signaling pathway. Smad3 expression localizes in the nuclei, and reactivity with anti-phospho-Smad2 and 3 antibody also indicates an activated status. The upregulation of Smad3 and the lack of upregulation of Smad7 in the lesional dermis suggest an altered balance of stimulatory and inhibitory signals during TGF-beta1-induced signal transduction in this model (Takagawa, Lakos, Varga. Submitted). Several therapeutic interventions were tested on this model. Intravenous administration of anti-TGF-beta antibody after subcutaneous bleomycin injections significantly decreased dermal sclerosis, as well as immune cell infiltration (71). Daily injection of superoxide dismutase also inhibited the bleomycin-induced skin sclerosis (72). However, intralesional injections of IFN-gamma although partially reduced dermal sclerosis, but did not decrease the cellular infiltration (73).
 

Other animal models

alpha1beta1 and alpha2beta1 integrins contribute to the regulation of collagen synthesis in vivo by negative feedback. A knockout mouse for a1 shows upregulation of dermal collagen synthesis by 20%, and fibroblasts from a1 null mice are more sensitive to the effects of TGF-b compared with normal cells (74).

Knockout MRL/lpr mice lacking IFN-gamma receptors (MRL/lprgammaR-/-)  have many pathologic and clinical similarities to human scleroderma. Features of MRL/lprgammaR-/- mouse include mononuclear cell infiltrates in the skin, lungs, kidney, liver and heart, accompanied by abnormal accumulation of collagen (75). As ECM synthesis is driven by TGF-beta and inhibited by IFN-gamma, this mouse strain may lack an important antifibrosis signal.
 

Dysregulation of fibroblast function in scleroderma

The autocrine TGF-beta loop

While the pathogenesis of fibrosis in SSC is not completely understood, multiple alterations, which may result in the development of pathological tissue fibrosis, have been identified. Given the role of TGF-beta in ECM synthesis and fibroblast differentiation, this growth factor was proposed more, than 10 years ago to play a key role in the pathogenesis of SSc. However, although some early studies showed elevated levels of TGF-beta in fibrotic skin, others showed no such elevation (76, 77). A recent study found elevated TGF-beta mRNA levels in the leading, inflammatory edge of the lesion, but not in the fibrotic region itself (78). These findings suggest that TGF-beta may play a role in the initiation, but not the maintenance of the fibrosis. In spite of the fact that scleroderma fibroblasts secret similar amount of TGF-beta than normal fibroblasts (79), they express increased level of TbetaRI and TbetaRII mRNA and protein, which finding correlates with the elevated expression of the COL1A2 gene (79, 80). Moreover, the up-regulated collagen synthesis can be prevented by the blockade of TGF-beta signaling with anti-TGF-beta antibodies or TGF-beta1 anisense oligonucleotide (79). These results suggest that elevated production of type I collagen by scleroderma fibroblasts - which is the main feature of the so-called "scleroderma-phenotype" (80) - results from overexpression of TGF-beta receptors, and consequent activation of autocrin TGF-beta signaling. A recent study found that SSc fibroblasts showed elevated level of the non-signaling high affinity TbetaRIII endoglin (81). Transfection of endoglin in fibroblasts suppressed the TGF-beta-mediated induction of CTGF promoter activity, suggesting an endoglin-mediated possible negative feedback mechanism in an attempt to block further induction of profibrotic genes by TGF-beta in scleroderma fibroblasts (81). The involvement of additional members of the TGF-beta ligand familiy - such as activins and BMPs - in the fibrotic process has not been adequately addressed to date.
 

The role of connective tissue growth factor

High basal level of CTGF is also part of the "scleroderma-phenotype". Cultures of primary fibroblasts obtained from affected areas of skin from patients with diffuse disease exhibited elevated constitutive production of CTGF in the absence of exogenous stimuli (82). Significant correlation was found between CTGF expression using in situ hybridization and the extent of skin sclerosis in tissue biopsies from patients with diffuse SSc (83). CTGF levels are also increased in the serum of patients with SSc (84), and CTGF was found in bronchoalveolar lavage fluid from patients with scleroderma lung fibrosis and idiopathic pulmonary fibrosis (IPF) (82, 85). Although a Smad binding site was identified in the CTGF promoter, mutation of this site does not reduce the high level of CTGF promoter activity observed in dermal fibroblasts cultured from lesional areas of SSc patients, suggesting that maintenance of scleroderma phenotype is independent of Smad signaling (34). The sustained fibroblast activation supposed to be a result of autocrin/paracrin stimulation by TGF-beta/CTGF in lesional skin.
 

Myofibroblasts

Dermal fibroblasts from patients with SSc and healthy individuals are heterogeneous for distribution of COL1A1 procollagen mRNA expression (86). However, SSc skin fibroblasts have a larger proportion of cells in the high collagen-producing mRNA subpopulation (86). This difference may be a result of either clonal selection or selective activation of fibroblasts. The clonal selection hypothesis is supported by the finding that SSc-derived fibroblasts are resistant to anti-Fas induced apoptosis compared to normal fibroblasts (87). This may lead to the propagation of certain apoptosis-resistant fibroblast subpopulations. The resistance to apoptosis was combined and correlated with the increased number of myofibroblasts among cultured SSc fibroblasts (87). Myofibroblast are characterized by the expression of a-smooth muscle actin (aSMA) (88). The conversion of fibroblasts into contractile myofibroblasts is an essential feature during the normal wound-healing process that is mediated by TGF-beta. Myofibroblasts produce elevated levels of collagen, TIMP and other ECM components in vitro. Smad3 and the transcription factors Sp1 and Sp2 are independent activators of the aSMA enhancer (89), and exogenous Smad7 attenuates the TGF-beta-induced aSMA expression (90). Increased aSMA expression has been also shown in SSc skin sections (87), and in fibroblasts derived from patients with idiopathic pulmonary hypertension (91).

Integrins on fibroblasts play an important role in ECM turnover. alpha1beta1 integrin stimulation by collagen provides negative feedback on collagen synthesis, while the major role of alpha2beta1 seem to be in positive regulation of matrix metalloprotease synthesis and in mediation of collagen gel contraction (92). Expression of both is reduced on SSc fibroblasts (93), causing the loss of normal negative feedback regulation.
 

Summary

Fibrosis of skin and internal organs is a hallmark of SSc, and tissue fibrosis contributes to the progressive failure of these organs. The pathogenesis of fibrosis remains poorly understood, and effective treatments are lacking. The recent use of DNA microarrays reveals numerous genes whose expression is altered in scleroderma fibroblasts, which may detect unique patterns of gene expression and identify novel pathways - and potential therapeutic targets - in fibrosis. The role of these newly identified molecules in fibroblast function and fibrosis needs to be established. In physiological conditions, the communication between extracellular signals, the matrix and the resident connective tissue cells allows a constant adjustment of function, resulting in maintenance of homeostasis. Wound healing is a temporary and self-limited process: once a sufficient amount of ECM accumulates, fibroblasts either revert to their questient phenotype or undergo apoptosis. In contrast, in fibrotic diseases fibroblasts activated by an unknown stimulus continue to make matrix components and profibrotic cytokines. The balance between matrix synthesis and matrix degradation is disturbed. The key to SSc will be in understanding what cellular and cytokine environments and what regulation defects lead to fibrosis, and in developing interventions to prevent or counteract fibrosis.

Supported by grants from the National Institutes of Health (AR-42309) and the Scleroderma Foundation.

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Scleroderma is a chronic, progressive connective tissue disorder featuring inflammation, fibrosis, vascular injury, and immunologic abnormalities. Fibrosis, a hallmark of the disease, is characterized by excessive synthesis and deposition of extracellular matrix components, mainly type I collagen in affected tissue. The key target organs are the skin, lungs, kidneys, gastrointestinal tract and heart. The pathogenesis of fibrosis remains poorly understood, and effective treatments are lacking. While unifying concept to explain the pathogenesis of fibrosis has not yet emerged, multiple alterations result in the development of pathological tissue fibrosis have been recently identified. Transforming growth factor-b, a potent profibrotic cytokine plays a key role in the process. There is growing knowledge on identifying the cytokine and growth factor mediators of fibrosis, characterizing their interactions, and in delineating the cellular and molecular signaling pathways that are activated by these mediators. This review summarizes recent results obtained from fibroblast studies, animal models, and gene expression experiments. A major goal of investigations into the pathomechanism of fibrosis is identifying new therapeutic targets for scleroderma.
 
Magy Immunol/Hun Immunol 2002;1(4):4-14.

Correspondence: GABRIELLA LAKOS, MD: University of Illinois at Chicago College of Medicine, Section of Rheumatology (M/C733), Room 1158 Molecular Biology Research Building, 900 S. Ashland Avenue, Chicago, IL 60607. Phone: (1) 312-413-9310, fax: (1) 312-413-9271,
e-mail: lakosg@hotmail.com 
 
scleroderma, fibrosis, TGF-beta