Disruption of transforming growth factor beta signaling and profibrotic responses in normal skin fibroblasts by peroxisome proliferator-activated receptor gamma

Fibrosis in systemic sclerosis: emerging concepts and implications for targeted therapy

Systemic sclerosis (SSc) is a complex and incompletely understood disease associated with fibrosis in multiple organs. Recent findings identify transforming growth factor-ß (TGF-ß), Wnt ligands, toll-like receptor-mediated signaling, hypoxia, type I interferon, type 2 immune responses and mechanical stress as extracellular cues that modulate fibroblast function and differentiation, and as potential targets for therapy. Moreover, fibrillin-1 has a major role in storing and regulating the bioavailability of TGF-ß and other cytokines, and fibrillin-1 mutations are implicated in a congenital form of scleroderma called stiff skin syndrome. Fibrosis is due not only to the activation of tissue-resident fibroblasts and their transdifferentiation into myofibroblasts, but also the differentiation of bone marrow-derived fibrocytes, and transition of endothelial and epithelial cells, pericytes and adipocytes into activated mesenchymal cells. These responses are modulated by signaling mediators and microRNAs that amplify or inhibit TGF-ß and Wnt signaling. Gain-of-function and loss-of-function abnormalities of these mediators may account for the characteristic activated phenotype of SSc fibroblasts. The nuclear orphan receptor PPAR-γ plays a particularly important role in limiting the duration and intensity of fibroblast activation and differentiation, and impaired PPAR-γ expression or function in SSc may underlie the uncontrolled progression of fibrosis. Identifying the perturbations in signaling pathways, mediators and differentiation programs that are responsible for SSc tissue damage allows their selective targeting. This in turn opens the door for therapies utilizing novel compounds, or drug repurposing by innovative uses of already-approved drugs. In view of the heterogeneous clinical presentation and unpredictable course of SSc, as well as its complex pathogenesis, only robust clinical trials incorporating the judicious application of biomarkers will be able to clarify the clinical utility of these innovative approaches.

Leptin promotes fibroproliferative acute respiratory distress syndrome by inhibiting peroxisome proliferator-activated receptor-γ

RATIONALE

Diabetic patients have a lower incidence of acute respiratory distress syndrome (ARDS), and those who develop ARDS are less likely to die. The mechanisms that underlie this protection are unknown.

OBJECTIVES

To determine whether leptin resistance, a feature of diabetes, prevents fibroproliferation after lung injury.

METHODS

We examined lung injury and fibroproliferation after the intratracheal instillation of bleomycin in wild-type and leptin-resistant (db/db) diabetic mice. We examined the effect of leptin on transforming growth factor (TGF)-β(1)-mediated transcription in primary normal human lung fibroblasts. Bronchoalveolar lavage fluid (BAL) samples from patients with ARDS and ventilated control subjects were obtained for measurement of leptin and active TGF-β(1) levels.

MEASUREMENTS AND MAIN RESULTS

Diabetic mice (db/db) were resistant to lung fibrosis. The db/db mice had higher levels of peroxisome proliferator-activated receptor-γ (PPARγ), an inhibitor of the transcriptional response to TGF-β(1), a cytokine critical in the pathogenesis of fibroproliferative ARDS. In normal human lung fibroblasts, leptin augmented the transcription of profibrotic genes in response to TGF-β(1) through a mechanism that required PPARγ. In patients with ARDS, BAL leptin levels were elevated and correlated with TGF-β(1) levels. Overall, there was no significant relationship between BAL leptin levels and clinical outcomes; however, in nonobese patients, higher BAL leptin levels were associated with fewer intensive care unit- and ventilator-free days and higher mortality.

CONCLUSIONS

Leptin signaling is required for bleomycin-induced lung fibrosis. Leptin augments TGF-β(1) signaling in lung fibroblasts by inhibiting PPARγ. These findings provide a mechanism for the observed protection against ARDS observed in diabetic patients.

PPARγ downregulation by TGFß in fibroblast and impaired expression and function in systemic sclerosis: a novel mechanism for progressive fibrogenesis

The nuclear orphan receptor peroxisome proliferator-activated receptor-gamma (PPAR-γ) is expressed in multiple cell types in addition to adipocytes. Upon its activation by natural ligands such as fatty acids and eicosanoids, or by synthetic agonists such as rosiglitazone, PPAR-γ regulates adipogenesis, glucose uptake and inflammatory responses. Recent studies establish a novel role for PPAR-γ signaling as an endogenous mechanism for regulating transforming growth factor-ß (TGF-ß)-dependent fibrogenesis. Here, we sought to characterize PPAR-γ function in the prototypic fibrosing disorder systemic sclerosis (SSc), and delineate the factors governing PPAR-γ expression. We report that PPAR-γ levels were markedly diminished in skin and lung biopsies from patients with SSc, and in fibroblasts explanted from the lesional skin. In normal fibroblasts, treatment with TGF-ß resulted in a time- and dose-dependent down-regulation of PPAR-γ expression. Inhibition occurred at the transcriptional level and was mediated via canonical Smad signal transduction. Genome-wide expression profiling of SSc skin biopsies revealed a marked attenuation of PPAR-γ levels and transcriptional activity in a subset of patients with diffuse cutaneous SSc, which was correlated with the presence of a "TGF-ß responsive gene signature" in these biopsies. Together, these results demonstrate that the expression and function of PPAR-γ are impaired in SSc, and reveal the existence of a reciprocal inhibitory cross-talk between TGF-ß activation and PPAR-γ signaling in the context of fibrogenesis. In light of the potent anti-fibrotic effects attributed to PPAR-γ, these observations lead us to propose that excessive TGF-ß activity in SSc accounts for impaired PPAR-γ function, which in turn contributes to unchecked fibroblast activation and progressive fibrosis.

PPAR modulation of kinase-linked receptor signaling in physiology and disease

Kinase-linked receptors and nuclear receptors connect external cues to gene transcription. Among nuclear receptors, peroxisome proliferator-activated receptors (PPARs) are of special interest in relation to widespread human diseases. Mapping out connections between PPARs and kinase-linked receptor signaling is central to better understand physiological and pathophysiological processes and to better define therapeutic strategies. This is the aim of the present review.

Activation of PPARs α, β/δ, and γ Impairs TGF-β1-Induced Collagens' Production and Modulates the TIMP-1/MMPs Balance in Three-Dimensional Cultured Chondrocytes

Background and Purpose. We investigated the potency of Peroxisome Proliferators-Activated Receptors (PPARs) α, β/δ, and γ agonists to modulate Transforming Growth Factor-β1 (TGF-β1-) induced collagen production or changes in Tissue Inhibitor of Matrix Metalloproteinase- (TIMP-) 1/Matrix Metalloproteinase (MMP) balance in rat chondrocytes embedded in alginate beads. Experimental Approach. Collagen production was evaluated by quantitative Sirius red staining, while TIMP-1 protein levels and global MMP (-1, -2, -3, -7, and -9) or specific MMP-13 activities were measured by ELISA and fluorigenic assays in culture media, respectively. Levels of mRNA for type II collagen, TIMP-1, and MMP-3 & 13 were quantified by real-time PCR. Key Results. TGF-β1 increased collagen deposition and type II collagen mRNA levels, while inducing TIMP-1 mRNA and protein expression. In contrast, it decreased global MMP or specific MMP-13 activities, while decreasing MMP-3 or MMP-13 mRNA levels. PPAR agonists reduced most of the effects of TGF-β1 on changes in collagen metabolism and TIMP-1/MMP balance in rat in a PPAR-dependent manner, excepted for Wy14643 on MMP activities. Conclusions and Implications. PPAR agonists reduce TGF-β1-modulated ECM turnover and inhibit chondrocyte activities crucial for collagen biosynthesis, and display a different inhibitory profile depending on selectivity for PPAR isotypes.

Interstitial lung disease in connective tissue diseases: evolving concepts of pathogenesis and management

Interstitial lung disease (ILD) is a challenging clinical entity associated with multiple connective tissue diseases, and is a significant cause of morbidity and mortality. Effective therapies for connective tissue disease-associated interstitial lung disease (CTD-ILD) are still lacking. Multidisciplinary clinics dedicated to the early diagnosis and improved management of patients with CTD-ILD are now being established. There is rapid progress in understanding and identifying the effector cells, the proinflammatory and profibrotic mediators, and the pathways involved in the pathogenesis of CTD-ILD. Serum biomarkers may provide new insights as risk factors for pulmonary fibrosis and as measures of disease progression. Despite these recent advances, the management of patients with CTD-ILD remains suboptimal. Further studies are therefore urgently needed to better understand these conditions, and to develop effective therapeutic interventions.

Molecular pathogenesis of skin fibrosis: insight from animal models

Skin fibrosis occurs in a variety of human diseases, most notably systemic sclerosis (SSc). The end stage of scleroderma in human skin consists of excess collagen deposition in the dermis with loss of adnexal structures and associated adipose tissue. The initiating factors for this process and the early stages are believed to occur through vascular injury and immune dysfunction with a dysregulated inflammatory response. However, because of the insidious onset of the disease, this stage is rarely observed in humans and remains poorly understood. Animal models have provided a means to examine these early stages and to isolate and understand the effect of perturbations in signaling pathways, chemokines, and cytokines. This article summarizes recent progress in the understanding of the molecular pathogenesis of skin fibrosis in SSc from different animal models, both its initiation and its maintenance phases.

Proteasomal regulation of pulmonary fibrosis

It is estimated that, combined, 400,000 people are diagnosed with idiopathic pulmonary fibrosis (IPF) or acute lung injury/acute respiratory distress syndrome annually in the United States, and both diseases are associated with an unacceptably high mortality rate. Although these disorders are distinct clinical entities, they share pathogenic mechanisms that may provide overlapping therapeutic targets. One example is fibroblast activation, which occurs concomitant with acute lung injury as well as in the progressive fibrosis of IPF. Both clinical entities are characterized by elevations of the profibrotic cytokine, transforming growth factor (TGF)-beta1. Protein degradation by the ubiquitin-proteasomal system modulates TGF-beta1 expression and signaling. In this review, we highlight the effects of proteasomal inhibition in various animal models of tissue fibrosis and mechanisms by which it may regulate TGF-beta1 expression and signaling. At present, there are no effective therapies for fibroproliferative acute respiratory distress syndrome or IPF, and proteasomal inhibition may provide a novel, attractive target in these devastating diseases.

Peroxisome proliferator-activated receptor (PPAR)gamma can inhibit chronic renal allograft damage

Chronic inflammation and fibrosis are the leading causes of chronic allograft failure. The nuclear receptor peroxisome proliferator-activated receptor (PPAR)gamma is a transcription factor known to have antidiabetogenic and immune effects, and PPARgamma forms obligate heterodimers with the retinoid X receptor (RXR). We have reported that a retinoic acid (RAR)/RXR-agonist can potently influence the course of renal chronic allograft dysfunction. In this study, in a Fischer to Lewis rat renal transplantation model, administration of the PPARgamma-agonist, rosiglitazone, independent of dose (3 or 30 mg/kgBW/day), lowered serum creatinine, albuminuria, and chronic allograft damage with a chronic vascular damage score as follows: 35.0 +/- 5.8 (controls) vs. 8.1 +/- 2.4 (low dose-Rosi; P < 0.05); chronic tubulointerstitial damage score: 13.6 +/- 1.8 (controls) vs. 2.6 +/- 0.4 (low dose-Rosi; P < 0.01). The deposition of extracellular matrix proteins (collagen, fibronectin, decorin) was strikingly lower. The expression of transforming growth factor-beta1 was inhibited, whereas that of bone morphogenic protein-7 (BMP-7) was increased. Intragraft mononuclear cells and activated fibroblast numbers were reduced by 50%. In addition, the migratory and proliferative activity of these cells was significantly inhibited in vitro. PPARgamma activation diminished the number of cells expressing the proinflammatory and fibrogenic proteoglycan biglycan. In macrophages its secretion was blocked by rosiglitazone in a predominantly PPARgamma-dependent manner. The combination of PPARgamma- and RAR/RXR-agonists resulted in additive effects in the inhibition of fibrosis. In summary, PPARgamma activation was potently immunosuppressive and antifibrotic in kidney allografts, and these effects were enhanced by a RAR/RXR-agonist.

Effects of PPAR gamma ligands on TGF-beta1-induced epithelial-mesenchymal transition in alveolar epithelial cells

Background

Transforming growth factor β1 (TGF-β1)-mediated epithelial mesenchymal transition (EMT) of alveolar epithelial cells (AEC) may contribute to lung fibrosis. Since PPARγ ligands have been shown to inhibit fibroblast activation by TGF-β1, we assessed the ability of the thiazolidinediones rosiglitazone (RGZ) and ciglitazone (CGZ) to regulate TGF-β1-mediated EMT of A549 cells, assessing changes in cell morphology, and expression of cell adhesion molecules E-cadherin (epithelial cell marker) and N-cadherin (mesenchymal cell marker), and collagen 1α1 (COL1A1), CTGF and MMP-2 mRNA.

Methods

Serum-deprived A549 cells (human AEC cell line) were pre-incubated with RGZ and CGZ (1 - 30 μM) in the absence or presence of the PPARγ antagonist GW9662 (10 μM) before TGFβ-1 (0.075-7.5 ng/ml) treatment for up to 72 hrs. Changes in E-cadherin, N-cadherin and phosphorylated Smad2 and Smad3 levels were analysed by Western blot, and changes in mRNA levels including COL1A1 assessed by RT-PCR.

Results

TGFβ-1 (2.5 ng/ml)-induced reductions in E-cadherin expression were associated with a loss of epithelial morphology and cell-cell contact. Concomitant increases in N-cadherin, MMP-2, CTGF and COL1A1 were evident in predominantly elongated fibroblast-like cells. Neither RGZ nor CGZ prevented TGFβ1-induced changes in cell morphology, and PPARγ-dependent inhibitory effects of both ligands on changes in E-cadherin were only evident at submaximal TGF-β1 (0.25 ng/ml). However, both RGZ and CGZ inhibited the marked elevation of N-cadherin and COL1A1 induced by TGF-β1 (2.5 ng/ml), with effects on COL1A1 prevented by GW9662. Phosphorylation of Smad2 and Smad3 by TGF-β1 was not inhibited by RGZ or CGZ.

Conclusions

RGZ and CGZ inhibited profibrotic changes in TGF-β1-stimulated A549 cells independently of inhibition of Smad phosphorylation. Their inhibitory effects on changes in collagen I and E-cadherin, but not N-cadherin or CTGF, appeared to be PPARγ-dependent. Further studies are required to unravel additional mechanisms of inhibition of TGF-β1 signalling by thiazolidinediones and their implications for the contribution of EMT to lung fibrosis.

Relaxin signaling activates peroxisome proliferator-activated receptor gamma

Relaxin is a polypeptide hormone that triggers multiple signaling pathways through its receptor RXFP1 (relaxin family peptide receptor 1). Many of relaxin's functions, including vascular and antifibrotic effects, are similar to those induced by activation of PPARgamma. In this study, we tested the hypothesis that relaxin signaling through RXFP1 would activate PPARgamma activity. In cells overexpressing RXFP1 (HEK-RXFP1), relaxin increased transcriptional activity through a PPAR response element (PPRE) in a concentration-dependent manner. In cells lacking RXFP1, relaxin had no effect. Relaxin increased both the baseline activity and the response to the PPARgamma agonists rosiglitazone and 15d-PGJ(2), but not to agonists of PPARalpha or PPARdelta. In HEK-RXFP1 cells infected with adenovirus expressing PPARgamma, relaxin increased transcriptional activity through PPRE, and this effect was blocked with an adenovirus expressing a dominant-negative PPARgamma. Knockdown of PPARgamma using siRNA resulted in a decrease in the response to both relaxin and rosiglitazone. Both relaxin and rosiglitazone increased expression of the PPARgamma target genes CD36 and LXRalpha in HEK-RXFP1 and in THP-1 cells naturally expressing RXFP1. Relaxin did not increase PPARgamma mRNA or protein levels. Treatment of cells with GW9662, an inhibitor of PPARgamma ligand binding, effectively blocked rosiglitazone-induced PPARgamma activation, but had no effect on relaxin activation of PPARgamma. These results suggest that relaxin activates PPARgamma activity, and increases the overall response in the presence of PPARgamma agonists. This activation is dependent on the presence of RXFP1. Furthermore, relaxin activates PPARgamma via a ligand-independent mechanism. These studies represent the first report that relaxin can activate the transcriptional activity of PPARgamma.

Peroxisome proliferator-activated receptor-g agonist treatment increases septation and angiogenesis and decreases airway hyperresponsiveness in a model of experimental neonatal chronic lung disease

Chronic lung disease (CLD) affects premature newborns requiring supplemental oxygen and results in impaired lung development and subsequent airway hyperreactivity. We hypothesized that the maintenance of peroxisome proliferator-activated receptor gamma (PPARgamma) signaling is important for normal lung morphogenesis and treatment with PPARgamma agonists could protect against CLD and airway hyperreactivity (AHR) following chronic hyperoxic exposure. This was tested in an established hyperoxic murine model of experimental CLD. Newborn mice and mothers were exposed to room air (RA) or moderate hyperoxia (70% oxygen) for 10 days and fed a standard diet or chow impregnated with the PPARgamma agonist rosiglitazone (ROSI) for the duration of study. Following hyperoxic exposure (HE) animals were returned to RA until postnatal day (P) 13 or P41. The accumulation of ROSI in neonatal and adult tissue was confirmed by mass spectrometry. Analyses of body weight and lung histology were performed on P13 and P41 to localize and quantitate PPARgamma expression, determine alveolar and microvessel density, proliferation and alpha-smooth muscle actin (alpha-SMA) levels as a measure of myofibroblast differentiation. Microarray analyses were conducted on P13 to examine transcriptional changes in whole lung. Pulmonary function and airway responsiveness were analyzed at P55. ROSI treatment during HE preserved septation and vascular density. Key array results revealed ontogeny groups differentially affected by hyperoxia including cell cycle, angiogenesis, matrix, and muscle differentiation/contraction. These results were further confirmed by histological evaluation of myofibroblast and collagen accumulation. Late AHR to methacholine was present in mice following HE and attenuated with ROSI treatment. These findings suggest that rosiglitazone maintains downstream PPARgamma effects and may be beneficial in the prevention of severe CLD with AHR.

A role for TGFbeta signaling in the pathogenesis of psoriasis

Deregulation of transforming growth factor-beta (TGFbeta) signaling has been reported in human psoriasis. Our recent study using a keratin 5 promoter (K5.TGFbeta1(wt)) showed that transgenic mice expressing wild-type TGFbeta1 in the epidermis developed severe skin inflammation. Additional experimental data further support a direct role for TGFbeta1 overexpression in skin inflammation. First, we temporally induced TGFbeta1 expression in keratinocytes in our gene-switch TGFbeta1(wt) transgenic mice and found inflammation severity correlated with TGFbeta1(wt) transgene expression. Second, deletion of T cells in K5.TGFbeta1(wt) mice significantly delayed skin inflammation and associated epidermal hyperplasia/hyperkeratosis. Third, therapeutic approaches effective for human psoriasis, that is, Etanercept and Rosiglitazone, are effective in alleviating the symptoms observed in K5.TGFbeta1(wt) mice. Future studies will analyze specific mechanisms and identify key factors in TGFbeta1-induced skin inflammation. Our mouse models will provide a useful tool for understanding the molecular mechanisms of inflammatory skin disorders in which TGFbeta1 is overexpressed.

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonist inhibits transforming growth factor-beta1 and matrix production in human dermal fibroblasts

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists are increasingly used in patients with diabetes, and some studies have suggested a beneficial effect on organ fibrosis, but their effects on dermal cell growth and extracellular matrix (ECM) turnover are unknown. To investigate the effect of the PPAR-gamma agonist troglitazone on cell growth and matrix production in human dermal fibroblasts (HDF), HDF were cultured and grown in a different concentration of troglitazone. PPAR-gamma expression and matrix production were measured in HDF in the presence of troglitazone. The mRNA expressions of TGF-beta1, collagen I (Col I) and fibronectin (FN) were determined by quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). The protein of transforming growth factor-beta1 (TGF-beta1) was determined by enzyme-linked immunosorbent assay (ELISA) and proteins of Col I and FN were determined by Western blotting. The mRNA expression of TGF-beta1, Col I and FN were significantly decreased in HDF in 15-30 micromol l(-1) troglitazone compared to the control group with Dulbecco's modified Eagle's medium (P<0.01). An obvious decrease of TGF-beta1 protein was found in troglitazone-treated groups as compared to the control group (P<0.05). Exposure of HDF to troglitazone reduced col I secretion (P<0.05), and fibronectin secretion (P<0.05). This study suggests that PPAR-gamma agonist will provide a novel approach with therapeutic potential in dermal fibrosis, such as hypertrophic scar, keloid and so on.

Transforming growth factor beta as a therapeutic target in systemic sclerosis

Transforming growth factor beta (TGF-beta) is a pleiotropic cytokine with vital homeostatic functions. Aberrant TGF-beta expression is implicated in the pathogenesis of fibrosis in systemic sclerosis (SSc); thus, TGF-beta represents a molecular therapeutic target in this disease. Anti-TGF-beta monoclonal antibody has been evaluated in a small trial of early SSc, with disappointing results. Antibodies against the alphavbeta6 integrin that prevent latent TGF-beta activation, however, have shown promise in preclinical studies. Small-molecule inhibitors of TGF-beta-receptor activity are effective in animal models of fibrosis. Imatinib mesylate and related tyrosine kinase inhibitors also block TGF-beta pathways and abrogate fibrotic responses. The blocking of TGF-beta activity might lead to spontaneous immune activation, epithelial hyperplasia and impaired wound healing. Loss of immune tolerance is a potential concern in an autoimmune disease such as SSc. Novel insights from microarray-based gene expression analyses and studies of genetic polymorphisms in TGF-beta signaling could aid in identifying patients who are most likely to respond to anti-TGF-beta treatment. This intervention promises to have a major impact on the treatment of SSc. Concerns regarding efficacy and safety and whether biomarkers can indicate these features, questions regarding appropriate dosing and timing of therapy, and identification of potential responders are critical challenges ahead.

Peroxisome proliferator-activated receptor-gamma abrogates Smad-dependent collagen stimulation by targeting the p300 transcriptional coactivator

Ligands of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) abrogate the stimulation of collagen gene transcription induced by transforming growth factor-beta (TGF-beta). Here, we delineate the mechanisms underlying this important novel physiological function for PPAR-gamma in connective tissue homeostasis. First, we demonstrated that antagonistic regulation of TGF-beta activity by PPAR-gamma ligands involves cellular PPAR-gamma, since 15-deoxy-Delta12,14-prostaglandin J(2) (15d-PGJ(2)) failed to block TGF-beta-induced responses in either primary cultures of PPAR-gamma-null murine embryonic fibroblasts, or in normal human skin fibroblasts with RNAi-mediated knockdown of PPAR-gamma. Next, we examined the molecular basis underlying the abrogation of TGF-beta signaling by PPAR-gamma in normal human fibroblasts in culture. The results demonstrated that Smad-dependent transcriptional responses were blocked by PPAR-gamma without preventing Smad2/3 activation. In contrast, the interaction between activated Smad2/3 and the transcriptional coactivator and histone acetyltransferase p300 induced by TGF-beta, and the accumulation of p300 on consensus Smad-binding DNA sequences and histone H4 hyperacetylation at the COL1A2 locus, were all prevented by PPAR-gamma. Wild-type p300, but not a mutant form of p300 lacking functional histone acetyltransferase, was able to restore TGF-beta-induced stimulation of COL1A2 in the presence of PPAR-gamma ligands. Collectively, these results indicate that PPAR-gamma blocked Smad-mediated transcriptional responses by preventing p300 recruitment and histone H4 hyperacetylation, resulting in the inhibition of TGF-beta-induced collagen gene expression. Pharmacological activation of PPAR-gamma thus may represent a novel therapeutic approach to target p300-dependent TGF-beta profibrotic responses such as stimulation of collagen gene expression.

Strategies for blocking the fibrogenic actions of connective tissue growth factor (CCN2): From pharmacological inhibition in vitro to targeted siRNA therapy in vivo

Connective tissue growth factor (CCN2) is a major pro-fibrotic factor that frequently acts downstream of transforming growth factor beta (TGF-beta)-mediated fibrogenic pathways. Much of our knowledge of CCN2 in fibrosis has come from studies in which its production or activity have been experimentally attenuated. These studies, performed both in vitro and in animal models, have demonstrated the utility of pharmacological inhibitors (e.g. tumor necrosis factor alpha (TNF-alpha), prostaglandins, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists, statins, kinase inhibitors), neutralizing antibodies, antisense oligonucleotides, or small interfering RNA (siRNA) to probe the role of CCN2 in fibrogenic pathways. These investigations have allowed the mechanisms regulating CCN2 production to be more clearly defined, have shown that CCN2 is a rational anti-fibrotic target, and have established a framework for developing effective modalities of therapeutic intervention in vivo.

Rosiglitazone abrogates bleomycin-induced scleroderma and blocks profibrotic responses through peroxisome proliferator-activated receptor-gamma

The nuclear hormone receptor, peroxisome proliferator-activated receptor (PPAR)-gamma, originally identified as a key mediator of adipogenesis, is expressed widely and implicated in diverse biological responses. Both natural and synthetic agonists of PPAR-gamma abrogated the stimulation of collagen synthesis and myofibroblast differentiation induced by transforming growth factor (TGF)-beta in vitro. To characterize the role of PPAR-gamma in the fibrotic process in vivo, the synthetic agonist rosiglitazone was used in a mouse model of scleroderma. Rosiglitazone attenuated bleomycin-induced skin inflammation and dermal fibrosis as well as subcutaneous lipoatrophy and counteracted the up-regulation of collagen gene expression and myofibroblast accumulation in the lesioned skin. Rosiglitazone treatment reduced the induction of the early-immediate transcription factor Egr-1 in situ without also blocking the activation of Smad2/3. In both explanted fibroblasts and skin organ cultures, rosiglitazone prevented the stimulation of collagen gene transcription and cell migration elicited by TGF-beta. Rosiglitazone-driven adipogenic differentiation of both fibroblasts and preadipocytes was abrogated in the presence of TGF-beta; this effect was accompanied by the concomitant down-regulation of cellular PPAR-gamma mRNA expression. Collectively, these results indicate that rosiglitazone treatment attenuates inflammation, dermal fibrosis, and subcutaneous lipoatrophy via PPAR-gamma in a mouse model of scleroderma and suggest that pharmacological PPAR-gamma ligands, widely used as insulin sensitizers in the treatment of type-2 diabetes mellitus, may be potential therapies for scleroderma.

[Pathophysiological relevance of peroxisome proliferators activated receptors (PPAR) to joint diseases - the pro and con of agonists]

Peroxisome proliferators activated receptors (PPAR) are ligand-inducible nuclear transacting factors comprising three subtypes, PPARalpha, PPARbeta/delta and PPARgamma, which play a key role in lipids and glucose homeostasis. All PPAR subtypes have been identified in joint or inflammatory cells and their activation resulted in a transcriptional repression of pro-inflammatory cytokines (IL-1, TNFalpha), early inflammatory genes (NOS(2), COX-2, mPGES-1) or matrix metalloproteases (MMP-1, MMP-13), at least for the gamma subtype. PPAR full agonists were also shown to stimulate IL-1 receptor antagonist (IL-1Ra) production by cytokine-stimulated articular cells in a subtype-dependent manner. These anti-inflammatory and anti-catabolic properties were confirmed in animal models of joint diseases where PPAR agonists reduced synovial inflammation while preventing cartilage destruction or inflammatory bone loss, although many effects required much higher doses than needed to restore insulin sensitivity or to lower circulating lipid levels. However, these promising effects of PPAR full agonists were hampered by their ability to reduce the growth factor-dependent synthesis of extracellular matrix components or to induce chondrocyte apoptosis, by the possible contribution of immunosuppressive properties to their anti-arthritic effects, by the increased adipocyte differentiation secondary to prolonged stimulation of PPARgamma, and by a variable contribution of PPAR subtypes depending on the system. Clinical data are scarce in rheumatoid arthritis (RA) patients whereas thousands of patients worldwilde, treated with PPAR agonists for type 2 diabetes or dyslipidemia, are paradoxically prone to suffer from osteoarthritis (OA). Whereas high dosage of full agonists may expose RA patients to cardiovascular adverse effects, the proof of concept that PPAR agonists have therapeutical relevance to OA may benefit from an epidemiological follow-up of joint lesions in diabetic or hyperlipidemic patients treated for long periods of time with glitazones or fibrates. Additionally, cellular and animal studies are required to assess whether partial agonists of PPAR (SPPARMs) may preserve therapeutical properties with potentially less safety concern.

Troglitazone suppresses transforming growth factor-beta1-induced collagen type I expression in keloid fibroblasts

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR)-gamma agonists are increasingly used in patients with diabetes and some studies have suggested a beneficial effect on organ fibrosis. However their effects on dermal fibrosis in keloids are unknown.

OBJECTIVE

To investigate the effect of the PPAR-gamma agonist troglitazone on transforming growth factor (TGF)-beta1-induced collagen type I expression in keloid fibroblasts.

METHODS

Keloid fibroblasts were cultured and exposed to different concentrations of troglitazone in the presence of TGF-beta1. The mRNA expression of PPAR-gamma was determined by semiquantitative reverse transcriptase-polymerase chain reaction. The protein of PPAR-gamma, Smad2, Smad3, phoshpo-Smad2/3 and collagen type I was determined by Western blotting and collagen synthesis was evaluated by measuring (3)H-proline incorporation. The effect of troglitazone on cell viability was evaluated by the colorimetric conversion of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide.

RESULTS

PPAR-gamma was expressed at a moderate level in keloid fibroblasts. Troglitazone depressed TGF-beta1-stimulated collagen type I expression and collagen synthesis in keloid fibroblasts in a concentration-dependent manner. Moreover, troglitazone inhibited expression and phosphorylation of TGF-beta1-induced Smad2/3. Cell viability was unaffected. These inhibitory effects of troglitazone were reversed by the PPAR-gamma-specific antagonist GW9662.

CONCLUSIONS

Our data suggest that PPAR-gamma is present in keloid fibroblasts and PPAR-gamma activation inhibits TGF-beta1-induced collagen type I expression at least in part by decreasing collagen synthesis. PPAR-gamma may be a promising therapeutic target for keloids.

Constitutive Smad signaling and Smad-dependent collagen gene expression in mouse embryonic fibroblasts lacking peroxisome proliferator-activated receptor-gamma

Transforming growth factor-beta (TGF-beta), a potent inducer of collagen synthesis, is implicated in pathological fibrosis. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a nuclear hormone receptor that regulates adipogenesis and numerous other biological processes. Here, we demonstrate that collagen gene expression was markedly elevated in mouse embryonic fibroblasts (MEFs) lacking PPAR-gamma compared to heterozygous control MEFs. Treatment with the PPAR-gamma ligand 15d-PGJ(2) failed to down-regulate collagen gene expression in PPAR-gamma null MEFs, whereas reconstitution of these cells with ectopic PPAR-gamma resulted in their normalization. Compared to control MEFs, PPAR-gamma null MEFs displayed elevated levels of the Type I TGF-beta receptor (TbetaRI), and secreted more TGF-beta1 into the media. Furthermore, PPAR-gamma null MEFs showed constitutive phosphorylation of cellular Smad2 and Smad3, even in the absence of exogenous TGF-beta, which was abrogated by the ALK5 inhibitor SB431542. Constitutive Smad2/3 phosphorylation in PPAR-gamma null MEFs was associated with Smad3 binding to its cognate DNA recognition sequences, and interaction with coactivator p300 previously implicated in TGF-beta responses. Taken together, these results indicate that loss of PPAR-gamma in MEFs is associated with upregulation of collagen synthesis, and activation of intracellular Smad signal transduction, due, at least in part, to autocrine TGF-beta stimulation.

In perspective: murine models of scleroderma

An understanding of the complex pathogenesis of systemic sclerosis (SSc) has been slow to emerge, due in large part to the lack of an animal model recapitulating the three cardinal attributes of SSc: autoimmunity, vasculopathy, and fibrosis. Experimental manipulations in inbred murine strains can lead to conditions that mimic SSc fibrosis. Furthermore, genetic engineering has enabled the creation of novel murine strains that spontaneously develop fibrosis or are protected from fibrosis development. Studies of these mice shed light on the cell types, cell interactions, molecules, and pathways that contribute to SSc manifestations. High-throughput discovery technologies such as DNA microarrays in animal models can identify novel genes and regulatory networks that are important for disease manifestations and that may be targets for therapy. In this brief review, we highlight recent progress in the field and attempt to place the strengths and limitations of popular SSc murine models in perspective.

The synthetic peroxisome proliferator-activated receptor-gamma agonist ciglitazone attenuates neuroinflammation and accelerates encapsulation in bacterial brain abscesses

Brain abscesses result from a pyogenic parenchymal infection commonly initiated by Gram-positive bacteria such as Staphylococcus aureus. Although the host immune response elicited following infection is essential for effective bacterial containment, this response also contributes to the significant loss of brain parenchyma by necrosis that may be reduced by modulating the inflammatory response. Ciglitazone, a PPAR-gamma agonist with anti-inflammatory properties, was evaluated for its ability to influence the course of brain abscess development when treatment was initiated 3 days following infection. Interestingly, abscess-associated bacterial burdens were significantly lower following ciglitazone administration, which could be explained, in part, by the finding that ciglitazone enhanced S. aureus phagocytosis by microglia. In addition, ciglitazone attenuated the expression of select inflammatory mediators during brain abscess development including inducible NO synthase, TNF-alpha, IL-1beta, CXCL2, and CCL3. Unexpectedly, ciglitazone also accelerated brain abscess encapsulation, which was typified by the heightened expression of fibronectin and alpha-smooth muscle actin-positive myofibroblasts. Collectively, through its ability to attenuate excessive inflammation and accelerate abscess encapsulation, ciglitazone may effectively sequester brain abscesses and limit bacterial dissemination.

PPAR-gamma agonists inhibit profibrotic phenotypes in human lung fibroblasts and bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is characterized by alterations in fibroblast phenotypes resulting in excessive extracellular matrix accumulation and anatomic remodeling. Current therapies for this condition are largely ineffective. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a member of the nuclear hormone receptor superfamily, the activation of which produces a number of biological effects, including alterations in metabolic and inflammatory responses. The role of PPAR-gamma as a potential therapeutic target for fibrotic lung diseases remains undefined. In the present study, we show expression of PPAR-gamma in fibroblasts obtained from normal human lungs and lungs of patients with idiopathic interstitial pneumonias. Treatment of lung fibroblasts and myofibroblasts with PPAR-gamma agonists results in inhibition of proliferative responses and induces cell cycle arrest. In addition, PPAR-gamma agonists, including a constitutively active PPAR-gamma construct (VP16-PPAR-gamma), inhibit the ability of transforming growth factor-beta1 to induce myofibroblast differentiation and collagen secretion. PPAR-gamma agonists also inhibit fibrosis in a murine model, even when administration is delayed until after the initial inflammation has largely resolved. These observations indicate that PPAR-gamma is an important regulator of fibroblast/myofibroblast activation and suggest a role for PPAR-gamma ligands as novel therapeutic agents for fibrotic lung diseases.

Peroxisome proliferator-activated receptors (PPARs) and the human skin: importance of PPARs in skin physiology and dermatologic diseases

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily that regulate lipid, glucose, and amino acid metabolism. More recently, PPARs and corresponding ligands have been shown in skin and other organs to regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. These new functions identify PPARs and corresponding ligands as potential targets for the treatment of various skin diseases and other disorders. It has been shown that in inflammatory skin disorders, including hyperproliferative psoriatic epidermis and the skin of patients with atopic dermatitis, the expression of both PPARalpha and PPARgamma is decreased. This observation suggests the possibility that PPARalpha and PPARgamma activators, or compounds that positively regulate PPAR gene expression, may represent novel NSAIDs for the topical or systemic treatment of common inflammatory skin diseases such as atopic dermatitis, psoriasis, and allergic contact dermatitis. Moreover, recent findings indicate that PPAR-signaling pathways may act as a promising therapeutic target for the treatment of hyperproliferative skin diseases including skin malignancies. Studies in non-diabetic patients suggest that oral thiazolidinediones, which are synthetic ligands of PPARgamma, not only exert an antidiabetic effect but also may be beneficial for moderate chronic plaque psoriasis by suppressing proliferation and inducing differentiation of keratinocytes; furthermore, they may even induce cell growth arrest, apoptosis, and terminal differentiation in various human malignant tumors. It has been reported that PPARalpha immunoreactivity is reduced in human keratinocytes of squamous cell carcinoma (SCC) and actinic keratosis (AK), while PPARdelta appears to be upregulated. Additionally, the microvessel density is significantly higher in AK and SCC that express high levels of PPARdelta. PPARdelta has been demonstrated to have an anti-apoptotic role and to maintain survival and differentiation of epithelial cells, whereas PPARalpha and PPARgamma activators induce differentiation and inhibit proliferation and regulate apoptosis. In melanoma, the growth inhibitory effect of PPARgamma activation is independent of apoptosis and seems to occur primarily through induction of cell cycle arrest in the G1 phase of the cell cycle or induction of re-differentiation. PPARalpha activation causes inhibition of migration of melanoma cells and anchorage-independent growth, whereas primary tumor growth remains unaltered. In clinical trials of gemfibrozil, a PPARalpha ligand, significantly fewer patients treated with this lipid-lowering drug were diagnosed with melanoma as compared to those in the control group. In conclusion, an increasing body of evidence indicates that PPAR signaling pathways may represent interesting therapeutic targets for a broad variety of skin disorders, including inflammatory skin diseases such as psoriasis and atopic dermatitis, and skin malignancies.

Molecular pathways as novel therapeutic targets in systemic sclerosis

PURPOSE OF REVIEW

Systemic sclerosis is a complex disease characterized by immune/inflammatory, vascular and fibrotic processes. To date, no treatment has proven effective in modifying the course of the disease. Recent studies have begun to yield insights into the nature and interrelationship among these processes, and their cellular and molecular components.

RECENT FINDINGS

Novel intracellular molecular pathways have been characterized that positively or negatively regulate fibroblast responses contributing to the process of fibrosis. These include signaling mediators that specify and amplify transforming growth factor-beta responses, or inhibit collagen stimulation and block these responses in vitro and in animal models. Various gain of function or loss of function abnormalities in these mediators have been identified in systemic sclerosis, and may account for the characteristic activated phenotype of systemic sclerosis fibroblasts.

SUMMARY

The identification of novel signaling pathways and mediators that are altered in systemic sclerosis and contribute to tissue damage allows their selective targeting. This in turn opens the door for novel therapeutic strategies utilizing novel compounds, or innovative ways of using already-approved drugs. In light of the complex pathogenesis of systemic sclerosis, however, only carefully designed clinical trials with appropriate biomarkers and outcome measures will be able to clarify the clinical utility of these innovative approaches.

Fibrosis in systemic sclerosis

This article reviews current understanding of the pathophysiology of fibrosis in systemic sclerosis. It highlights recent discoveries, insights, and emerging research, and potential opportunities for the development of targeted antifibrotic therapies.

Thematic review series: skin lipids. Peroxisome proliferator-activated receptors and liver X receptors in epidermal biology

The epidermis is a very active site of lipid metabolism, and all peroxisome proliferator-activated receptor (PPAR) and liver X receptor (LXR) isoforms are expressed in the epidermis. Activation of PPARalpha, -beta/delta, or -gamma or LXRs stimulates keratinocyte differentiation. Additionally, activation of these receptors also improves permeability barrier homeostasis by a number of mechanisms, including stimulating epidermal lipid synthesis, increasing lamellar body formation and secretion, and increasing the activity of enzymes required for the extracellular processing of lipids in the stratum corneum, leading to the formation of lamellar membranes that mediate permeability barrier function. The stimulation of keratinocyte differentiation and permeability barrier formation also occurs during fetal development, resulting in accelerated epidermal development. PPAR and LXR activation regulates keratinocyte proliferation and apoptosis, and studies have shown that these receptors play a role in cutaneous carcinogenesis. Lastly, PPAR and LXR activation is anti-inflammatory, reducing inflammation in animal models of allergic and irritant contact dermatitis. Because of their broad profile of beneficial effects on skin homeostasis, PPAR and LXR have great potential to serve as drug targets for common skin diseases such as psoriasis, atopic dermatitis, and skin cancer.

The transcriptional coactivator and acetyltransferase p300 in fibroblast biology and fibrosis

The transcriptional coactivator p300 is a ubiquitous nuclear phosphoprotein and transcriptional cofactor with intrinsic acetyltransferase activity. p300 controls the expression of numerous genes in cell-type and signal-specific manner, and plays a pivotal role in cellular proliferation, apoptosis, and embryogenesis. By catalyzing acetylation of histones and transcription factors, p300 plays a significant role in epigenetic regulation. Recent evidence suggests that abnormal p300 function is associated with deregulated target gene expression, and is implicated in inflammation, cancer, cardiac hypertrophy, and genetic disorders such as the Rubinstein-Taybi syndrome. The activity of p300 is regulated at multiple levels, including developmental stage-specific expression, post-translational modifications, subcellular localization, and cell-type and gene-specific interactions with transcription factors. Although p300 has been investigated extensively in epithelial and hematopoietic cells, its role in fibroblast biology and tissue repair has received little attention to date. Recent studies implicate p300 in the regulation of collagen synthesis by transforming growth factor-beta (TGF-beta). Both the acetyltransferase activity of p300 and its inducible interaction with Smad3 are essential for mediating TGF-beta-induced stimulation of collagen synthesis. As a signal integrator whose availability for intracellular interactions with transcription factors is strictly limiting, p300 mediates the antagonistic regulation of TGF-beta-induced collagen synthesis by IFN-gamma and TNF-alpha via intracellular competition for limiting amount of p300. Significantly, p300 is itself a direct transcriptional target of TGF-beta in normal fibroblasts, and its levels are significantly elevated in fibrotic lesions as well as in experimental models of fibrosis. The emerging appreciation of the importance of p300 in extracellular matrix (ECM) remodeling and fibrosis and novel insights concerning the regulation, mechanism of action, and significance of p300 in fibroblast biology are discussed in this minireview.

Peroxisome proliferator-activated receptors and inflammation: take it to heart

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors acting as key regulators of lipid metabolism as well as modulators of inflammation. The role of PPARalpha and PPARgamma in cardiac ischaemia-reperfusion injury, infarct healing and hypertrophy is the subject of intense research. Due to the later development of PPARdelta-specific ligands, the role of this PPAR isoform in cardiac disease remains to be established. Although many studies point to salutatory effects of PPAR ligands in cardiac disease, the exact molecular mechanism is still largely unsolved. Both the metabolic (via transactivation) and the more recently discovered anti-inflammatory (via transrepression) effects of PPARs are likely to play a role. In this review the reported, and sometimes contradictory, effects of PPAR ligands on ischaemia-reperfusion, infarct healing and cardiac hypertrophy are critically evaluated. In particular the role of inflammation in these disease processes, the ability of PPARs to interfere with pro-inflammatory processes, and the mechanisms of transrepression are discussed. Currently, the significance of PPARs as therapeutic targets in cardiovascular disease is receiving widespread attention. Accordingly, detailed understanding of the mechanisms controlling the activity of these nuclear hormone receptors is essential.

Effect of overexpression of PPARgamma on the healing process of corneal alkali burn in mice

Wound healing involves both local cells and inflammatory cells. Alkali burn of ocular surface tissue is a serious clinical problem often leading to permanent visual impairment resulting from ulceration, scarring and neovascularization during healing. Behaviors of corneal cells and inflammatory cells are orchestrated by growth factor signaling networks that have not been fully uncovered. Here we showed that adenoviral gene introduction of peroxisome proliferator-activated receptor-gamma (PPARgamma) inhibits activation of ocular fibroblasts and macrophages in vitro and also induced anti-inflammatory and anti-fibrogenic responses in an alkali-burned mouse cornea. PPARgamma overexpression suppressed upregulation of inflammation/scarring-related growth factors and matrix metalloproteinases (MMPs) in macrophages. It also suppressed expression of such growth factors and collagen Ialpha2 and myofibroblast generation upon exposure to TGFbeta1. Exogenous PPARgamma did not alter phosphorylation of Smad2, but inhibited its nuclear translocation. PPARgamma overexpression enhanced proliferation of corneal epithelial cells, but not of fibroblasts in vitro. Epithelial cell expression of MMP-2/-9 and TGFbeta1 and its migration were suppressed by PPARgamma overexpression. In vivo experiments showed that PPARgamma gene introduction suppressed monocytes/macrophages invasion and suppressed the generation of myofibroblasts, as well as upregulation of cytokines/growth factors and MMPs in a healing cornea. In vivo re-epitheliazation with basement membrane reconstruction in the healing, burned, cornea was accelerated by PPARgamma-Ad expression, although PPARgamma overexpression was considered to be unfavorable for cell migration. Together, these data suggest that overexpression of PPARgamma may represent an effective new strategy for treatment of ocular surface burns.

Peroxisome proliferator-activated receptor gamma interacts with CIITA x RFX5 complex to repress type I collagen gene expression

Recent reports demonstrate that peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the nuclear receptor superfamily, acts as a repressor of type I collagen synthesis. Our data demonstrate that exogenously expressed PPARgamma down-regulates collagen expression in a dose-responsive manner in human lung fibroblast cells. Silencing PPARgamma using lentiviruses expressing short hairpin RNAs partially reverses interferon-gamma (IFN-gamma)-induced repression and activates collagen mRNA levels. Previous studies indicate that IFN-gamma represses collagen gene expression and induces major histocompatibility complex II (MHC II) expression by activating the formation of a regulatory factor for X-box 5 (RFX5) complex with class II transactivator (CIITA). This report demonstrates that PPARgamma is within the RFX5.CIITA complex as judged by co-immunoprecipitation and DNA affinity precipitation studies. Most importantly, occupancy of PPARgamma on the collagen transcription start site and MHC II promoter increases with IFN-gamma treatment. The PPARgamma agonist, troglitazone, sensitizes the cells to IFN-gamma treatment by increasing recruitment of PPARgamma to collagen gene while repressing collagen expression, and these effects are blocked by the PPARgamma antagonist T0070907. PPARgamma may mediate IFN-gamma-stimulated collagen transcription down-regulation and MHC II up-regulation by interacting with CIITA as well as regulating CIITA expression. Therefore, PPARgamma is a critical target for investigations into therapeutics of diseases involving extracellular matrix remodeling and the immune response.

LRP1 functions as an atheroprotective integrator of TGFbeta and PDFG signals in the vascular wall: implications for Marfan syndrome

Background

The multifunctional receptor LRP1 controls expression, activity and trafficking of the PDGF receptor-β in vascular smooth muscle cells (VSMC). LRP1 is also a receptor for TGFβ1 and is required for TGFβ mediated inhibition of cell proliferation.

Methods and Principal Findings

We show that loss of LRP1 in VSMC (smLRP⁻) in vivo results in a Marfan-like syndrome with nuclear accumulation of phosphorylated Smad2/3, disruption of elastic layers, tortuous aorta, and increased expression of the TGFβ target genes thrombospondin-1 (TSP1) and PDGFRβ in the vascular wall. Treatment of smLRP1⁻ animals with the PPARγ agonist rosiglitazone abolished nuclear pSmad accumulation, reversed the Marfan-like phenotype, and markedly reduced smooth muscle proliferation, fibrosis and atherosclerosis independent of plasma cholesterol levels.

Conclusions and Significance

Our findings are consistent with an activation of TGFβ signals in the LRP1-deficient vascular wall. LRP1 may function as an integrator of proliferative and anti-proliferative signals that control physiological mechanisms common to the pathogenesis of Marfan syndrome and atherosclerosis, and this is essential for maintaining vascular wall integrity.

Systemic sclerosis: a prototypic multisystem fibrotic disorder

A unique feature of systemic sclerosis (SSc) that distinguishes it from other fibrotic disorders is that autoimmunity and vasculopathy characteristically precede fibrosis. Moreover, fibrosis in SSc is not restricted to a single organ, but rather affects many organs and accounts for much of the morbidity and mortality associated with this disease. Although immunomodulatory drugs have been used extensively in the treatment of SSc, no therapy to date has been able to reverse or slow the progression of tissue fibrosis or substantially modify the natural progression of the disease. In this Review, we highlight recent studies that shed light on the cellular and molecular mechanisms underlying the fibrotic process in SSc and that identify cellular processes and intra- and extracellular proteins as potential novel targets for therapy in this prototypic multisystemic fibrotic disease.

Agonists of peroxisome proliferators-activated receptors (PPAR) alpha, beta/delta or gamma reduce transforming growth factor (TGF)-beta-induced proteoglycans' production in chondrocytes

OBJECTIVE

To investigate the potency of selective agonists of peroxisome proliferators-activated receptors' (PPAR) isotypes (alpha, beta/delta or gamma) to modulate the stimulating effect of transforming growth factor-beta1 (TGF-beta1) on proteoglycans' (PGs) synthesis in chondrocytes.

METHOD

Rat chondrocytes embedded in alginate beads and cultured under low serum conditions were exposed to TGF-beta1 (10 ng/ml), alone or in combination with the following agonists: Wy14643 for PPARalpha, GW501516 for PPARbeta/delta, rosiglitazone (ROSI) for PPARgamma, in the presence or absence of PPAR antagonists (GW6471 for PPARalpha, GW9662 for PPARgamma). PGs' synthesis was evaluated by radiolabelled sulphate incorporation and glycosaminoglycans' (GAGs) content by Alcian blue staining of beads and colorimetric 1.9 dimethyl-methylene blue assay after beads' solubilization. Phosphorylation of Extracellular Signal-related Kinase1/2 (ERK1/2), Smad2/3 and p38-MAPK was assessed by Western Blot and production of prostaglandin E2 (PGE2) by Enzyme immuno-assay (EIA). Levels of mRNA for PPAR target genes [acyl-CoA oxidase (ACO) for PPARalpha; mitochondrial carnitin palmitoyl transferase-1 (CPT-1) for PPARbeta/delta and adiponectin for PPARgamma], aggrecan, TGF-beta1 and genes controlling GAGs' side chains' synthesis were quantified by real time polymerase chain reaction and normalized over RP29 housekeeping gene.

RESULTS

ACO was selectively up-regulated by 100 microM of Wy14643, CPT-1 by 100 nM of GW501516 and adiponectin by 10 microM of ROSI without cell toxicity. TGF-beta1 increased PGs' synthesis by four-fold, GAGs' content and deposition by 3.5-fold and six-fold, respectively, while inducing aggrecan expression around 10-fold without modifying mRNA levels of GAGs' controlling enzymes. PPAR agonists inhibited the stimulating effect of TGF-beta1 by 24-44% on PGs' synthesis and over 75% on aggrecan, GAGs' content and deposition with the following rank order of potency: ROSI>GW501516> or =Wy14643. TGF-beta1-induced phosphorylation of Smad2/3 and ERK1/2 was reduced by ROSI over GW501516 but not by Wy14643 whereas stimulated PGE2 production was inhibited by Wy14643 over GW501516 but not by ROSI. The effect of PPAR agonists on PPAR target genes and TGF-beta1-induced aggrecan expression was reversed selectively by PPAR antagonists.

CONCLUSION

In chondrocytes' beads, PPAR agonists reduced the stimulating effect of TGF-beta1 on PGs by inhibiting TGF-beta1-induced aggrecan expression in an isotype-selective manner. Thus, PPAR agonists could be deleterious in situation of cartilage repair although being protective in situation of cartilage degradation.

Regulation of inflammation by PPARs: a future approach to treat lung inflammatory diseases?

Lung inflammatory diseases, such as acute lung injury (ALI), asthma, chronic obstructive pulmonary disease (COPD) and lung fibrosis, represent a major health problem worldwide. Although glucocorticoids are the most potent anti-inflammatory drug in asthma, they exhibit major side effects and have poor activity in lung inflammatory disorders such as ALI or COPD. Therefore, there is growing need for the development of alternative or new therapies to treat inflammation in the lung. Peroxisome proliferator-activated receptors (PPARs), including the three isotypes PPARalpha, PPARbeta (or PPARdelta) and PPARgamma, are transcription factors belonging to the nuclear hormone receptor superfamily. PPARs, and in particular PPARalpha and PPARgamma, are well known for their critical role in the regulation of energy homeostasis by controlling expression of a variety of genes involved in lipid and carbohydrate metabolism. Synthetic ligands of the two receptor isotypes, the fibrates and the thiazolidinediones, are clinically used to treat dyslipidaemia and type 2 diabetes, respectively. Recently however, PPARalpha and PPARgamma have been shown to exert a potent anti-inflammatory activity, mainly through their ability to downregulate pro-inflammatory gene expression and inflammatory cell functions. The present article reviews the current knowledge of the role of PPARalpha and PPARgamma in controlling inflammation, and presents different findings suggesting that PPARalpha and PPARgamma activators may be helpful in the treatment of lung inflammatory diseases.

PPAR-gamma agonist, ciglitazone, increases pigmentation and migration of human melanocytes

Peroxisome proliferator-activated receptors (PPARs) play an important role in cellular responses. It was reported that three subtypes of PPAR are expressed in human melanocytes. In this study, we investigated the effects of the PPAR-gamma agonist, ciglitazone, on pigmentation and migration of human melanocytes. Ciglitazone stimulated the melanin content of cells and cultured skin. This increase in pigmentation was due to the stimulation of tyrosinase activity and expression of tyrosinase and microphthalmia-associated transcription factor protein of the melanocytes. Migration was increased after ciglitazone treatment, which was observed by the Boyden chamber checkerboard analysis and a scratch motility assay. These results suggest the regulatory role of PPAR-gamma in pigmentation and migration of human melanocytes.

Thiazolidinediones: potential as therapeutics for psoriasis and perhaps other hyperproliferative skin disease

The thiazolidinediones constitute a family of synthetic compounds that act as high-affinity ligands for persoxisome proliferator-activated receptor-gamma (PPAR-gamma), a member of the nuclear hormone receptor family. Although originally developed to facilitate glucose control in patients with Type 2 diabetes, a number of studies showed that these agents effectively inhibited epithelial cell proliferation and tissue inflammation. Many of the initial cell growth inhibition studies were conducted with malignant epithelial cells from various sites; however, in addition to malignant epithelial cells, other studies showed that rapidly proliferating epidermal keratinoctyes in culture were also sensitive to the growth-inhibiting action of these moieties. Additional studies subsequently demonstrated that some patients with plaque psoriasis responded to treatment with one or another member of the thiazolidinedione family. Due to the potential therapeutic benefit of these compounds in diseases such as psoriasis, studies have been conducted to elucidate mechanisms by which growth inhibition is achieved. Interference with a number of growth-influencing signalling pathways has been demonstrated. Of interest, some of the growth-inhibiting effects are seen under conditions in which PPAR-gamma activation may not be responsible for the activity. Based on therapeutic potential, additional ongoing studies are aimed at developing novel thiazolidinediones that may have better efficacy than the currently available agents. Other studies are aimed at identifying optimal ways to use these agents in the treatment of hyperplastic skin diseases such as psoriasis.

Peroxisome proliferator-activated receptors-gamma activator, ciglitazone, inhibits human melanocyte growth through induction of apoptosis

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. All three PPAR subtypes, PPAR-alpha, PPAR-beta/delta and PPAR-gamma are expressed in human melanocytes. In this study, we investigated the effects of PPAR-gamma activator on melanocyte growth, and apoptosis. The PPAR-gamma activators ciglitazone, troglitazone, and 15-deoxy-prostaglandin J2 inhibited melanocyte growth in a dose-dependent manner. This inhibitory effect of ciglitazone seemed to occur through induction of apoptosis. Apoptosis was increased after ciglitazone treatment, which was observed by the TUNEL method and flow cytometry. We noted a decrease in extracellular signal regulated kinase protein expression under ciglitazone treatment. Western blot analysis revealed an apparent time-dependent reduction in Bcl-2 protein levels in ciglitazone-treated melanocytes. In terms of Bax expression, a difference was not found. The expression of caspase-3 proteins was increased time-dependently with ciglitazone treatment. These results indicate that melanocyte growth and apoptosis may be modulated through PPAR-gamma and that ciglitazone, a PPAR-gamma activator, inhibits growth of human melanocytes by inducing apoptosis.

Turning down the system: counter-regulatory mechanisms in bone and adaptive immunity

Major advances have been made in recent years toward the identification of transcription factors that control cell-type-specific gene expression in the skeletal and adaptive immune systems. However, the identification of factors necessary and sufficient to drive production of effector cell proteins such as matrix components and cytokines represents the first step toward understanding how cells in bone and the adaptive system achieve their highly specialized functions. Here, we provide selected examples of counter-regulatory mechanisms that serve to turn down cells involved in extracellular matrix biosynthesis and adaptive immunity at the level of the transcription factors Runx2 and nuclear factor for the activation of T cells.

Inhibition of skin sclerosis by 15deoxy delta12,14-prostaglandin J2 and retrovirally transfected prostaglandin D synthase in a mouse model of bleomycin-induced scleroderma

Hematopoietic prostaglandin D synthase (PGDS) is a key enzyme involved in production of the PGD and J series, which have various role in inflammation and immunity. We evaluated the effect of treatment with 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) or the injection of prostaglandin D(2) synthase (PGDS) cDNA expressing-retrovirally transfected fibroblasts on bleomycin (BLM)-induced scleroderma-like skin sclerosis. Daily injection of BLM (30 microg) for 4 weeks induced histological evidence of dermal sclerosis in C3H mice. We examined the effect of injection of 15d-PGJ(2) (30 ng twice a day) or PGDS expressing-retrovirally transfected fibroblast on BLM-induced dermal sclerosis. Administration of 15d-PGJ(2) (a nonenzymatic metabolite of PGD(2)) injection of PGDS cDNA-expressing fibroblasts significantly reduced dermal sclerosis, the hydroxyproline content, and dermal thickness. Moreover, 15-d PGJ2 down-regulation of the expression of transforming growth factor beta(1) and connective tissue growth factor which had been induced by BLM. Mast cells were also increased in the skin by BLM injection and there was prominent degranulation of these mast cells along with elevated plasma histamine levels. 15-d PGJ(2) and PGDS-expressing cells also suppressed degranulation of cultured mast cells and histamine release by these cells. These results show that 15-d PGJ(2) and PGDS-expressing cells can prevent experimental skin sclerosis induced by BLM and raise the possibility of therapeutic approaches targeting of PPARgamma for the skin lesion of scleroderma.

Scleroderma, fibroblasts, signaling, and excessive extracellular matrix

Excessive extracellular matrix (ECM) deposition in the skin, lung, and other organs is a hallmark of systemic sclerosis (SSc). The pathogenesis of SSc is still poorly understood, but increasing evidence suggests that various cytokines such as transforming growth factor (TGF)-beta and their signaling pathways are key mediators of tissue fibrosis as a consequence of ECM accumulation in the pathogenesis of fibrosis such as SSc. TGF-beta regulates diverse biologic activities including cell growth, cell death or apoptosis, cell differentiation, and ECM synthesis. TGF-beta is known to induce the expression of ECM proteins in mesenchymal cells, and to stimulate the production of protease inhibitors that prevent enzymatic breakdown of the ECM. This paper focuses on the possible role of ECM, various cytokines, especially TGF-beta signal transduction pathways in the pathogenesis of fibrosis in SSc.