PPAR-γ ligands repress TGFβ-induced myofibroblast differentiation by targeting the PI3K/Akt pathway: implications for therapy of fibrosis

Dissecting fibrosis: therapeutic insights from the small-molecule toolbox

Fibrosis, which leads to progressive loss of tissue function and eventual organ failure, has been estimated to contribute to ~45% of deaths in the developed world, and so new therapeutics to modulate fibrosis are urgently needed. Major advances in our understanding of the mechanisms underlying pathological fibrosis are supporting the search for such therapeutics, and the recent approval of two anti-fibrotic drugs for idiopathic pulmonary fibrosis has demonstrated the tractability of this area for drug discovery. This Review examines the pharmacology and structural information for small molecules being evaluated for lung, liver, kidney and skin fibrosis. In particular, we discuss the insights gained from the use of these pharmacological tools, and how these entities can inform, and probe, emerging insights into disease mechanisms, including the potential for future drug combinations.

Normal Human Lung Epithelial Cells Inhibit Transforming Growth Factor-β Induced Myofibroblast Differentiation via Prostaglandin E2

Introduction

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive disease with very few effective treatments. The key effector cells in fibrosis are believed to be fibroblasts, which differentiate to a contractile myofibroblast phenotype with enhanced capacity to proliferate and produce extracellular matrix. The role of the lung epithelium in fibrosis is unclear. While there is evidence that the epithelium is disrupted in IPF, it is not known whether this is a cause or a result of the fibroblast pathology. We hypothesized that healthy epithelial cells are required to maintain normal lung homeostasis and can inhibit the activation and differentiation of lung fibroblasts to the myofibroblast phenotype. To investigate this hypothesis, we employed a novel co-culture model with primary human lung epithelial cells and fibroblasts to investigate whether epithelial cells inhibit myofibroblast differentiation.

Measurements and Main Results

In the presence of transforming growth factor (TGF)-β, fibroblasts co-cultured with epithelial cells expressed significantly less α-smooth muscle actin and collagen and showed marked reduction in cell migration, collagen gel contraction, and cell proliferation compared to fibroblasts grown without epithelial cells. Epithelial cells from non-matching tissue origins were capable of inhibiting TGF-β induced myofibroblast differentiation in lung, keloid and Graves’ orbital fibroblasts. TGF-β promoted production of prostaglandin (PG) E₂ in lung epithelial cells, and a PGE₂ neutralizing antibody blocked the protective effect of epithelial cell co-culture.

Conclusions

We provide the first direct experimental evidence that lung epithelial cells inhibit TGF-β induced myofibroblast differentiation and pro-fibrotic phenotypes in fibroblasts. This effect is not restricted by tissue origin, and is mediated, at least in part, by PGE₂. Our data support the hypothesis that the epithelium plays a crucial role in maintaining lung homeostasis, and that damaged and/ or dysfunctional epithelium contributes to the development of fibrosis.

Immunogenetics of systemic sclerosis: Defining heritability, functional variants and shared-autoimmunity pathways

Systemic sclerosis (SSc) is a clinically heterogeneous connective tissue disorder of complex etiology. The development of large-scale genetic studies, such as genome-wide association studies (GWASs) or the Immunochip platform, has achieved remarkable progress in the knowledge of the genetic background of SSc. Herein, we provide an updated picture SSc genetic factors, offering an insight into their role in pathogenic mechanisms that characterize the disease. We review the most recent findings in the HLA region and the well-established non-HLA loci. Up to 18 non-HLA risk factors fulfilled the selected criteria and they were classified according to their role in the innate or adaptive immune response, in apoptosis, autophagy or fibrosis. Additionally, SSc heritability has remained as a controversial question since twin studies provided low SSc heritability estimates. However, we have recalculated the lower bond of narrow sense SSc heritability using GWAS data. Remarkably, our results suggest a greater influence of genetics on SSc than previously reported. Furthermore, we also offer a functional classification of SSc-associated SNPs and their proxies, based on annotated data, to provide clues for the identification of causal variants in these loci. Finally, we explore the genetic overlap between SSc and other autoimmune diseases (ADs). The vast majority of SSc risk loci are shared with at least one additional AD, being the overlap between SSc and systemic lupus erythematous the largest. Nevertheless, we found that an important portion of SSc risk factors are also common to rheumatoid arthritis or primary biliary cirrhosis. Considering all these evidences, we are confident that future research will be successful in understanding the relevant altered pathways in SSc and in identifying new biomarkers and therapeutic targets for the disease.

Chronic hyperglycemia induces trans-differentiation of human pancreatic stellate cells and enhances the malignant molecular communication with human pancreatic cancer cells

Background

Diabetes mellitus is linked to pancreatic cancer. We hypothesized a role for pancreatic stellate cells (PSC) in the hyperglycemia induced deterioration of pancreatic cancer and therefore studied two human cell lines (RLT-PSC, T3M4) in hyperglycemic environment.

Methodology/Principal Findings

The effect of chronic hyperglycemia (CHG) on PSCs was studied using mRNA expression array with real-time PCR validation and bioinformatic pathway analysis, and confirmatory protein studies. The stress fiber formation (IC: αSMA) indicated that PSCs tend to transdifferentiate to a myofibroblast-like state after exposure to CHG. The phosphorylation of p38 and ERK1/2 was increased with a consecutive upregulation of CDC25, SP1, cFOS and p21, and with downregulation of PPARγ after PSCs were exposed to chronic hyperglycemia. CXCL12 levels increased significantly in PSC supernatant after CHG exposure independently from TGF-β1 treatment (3.09-fold with a 2.73-fold without TGF-β1, p<0.05). The upregualtion of the SP1 transcription factor in PSCs after CHG exposure may be implicated in the increased CXCL12 and IGFBP2 production. In cancer cells, hyperglycemia induced an increased expression of CXCR4, a CXCL12 receptor that was also induced by PSC’s conditioned medium. The receptor-ligand interaction increased the phosphorylation of ERK1/2 and p38 resulting in activation of MAP kinase pathway, one of the most powerful stimuli for cell proliferation. Certainly, conditioned medium of PSC increased pancreatic cancer cell proliferation and this effect could be partially inhibited by a CXCR4 inhibitor. As the PSC conditioned medium (normal glucose concentration) increased the ERK1/2 and p38 phosphorylation, we concluded that PSCs produce other factor(s) that influence(s) pancreatic cancer behaviour.

Conclusions

Hyperglycemia induces increased CXCL12 production by the PSCs, and its receptor, CXCR4 on cancer cells. The ligand-receptor interaction activates MAP kinase signaling that causes increased cancer cell proliferation and migration.

Activation of peroxisome proliferator-activated receptor γ ameliorates monocrotaline-induced pulmonary arterial hypertension in rats

Activation of peroxisome proliferator-activated receptor γ (PPARγ) suppresses the proliferation of pulmonary artery smooth muscle cells (PASMCs) and vascular remodeling in rats and humans, and therefore improves the development of pulmonary arterial hypertension (PAH). However, molecular mechanisms underlying these effects have not been completely understood. In the present study, the effects of PPARγ activation in monocrotaline (MCT)-induced pulmonary artery remodeling in rats were investigated. Eighteen Sprague-Dawley (SD) rats were randomly assigned into three groups (n=6): Control (Con), PAH and PAH treated with rosiglitazone (MCT + Rosi). The right ventricular systolic pressure (RVSP), the ratio of the right to left ventricle plus septum weight [RV/(LV + S)], the percentage of medial wall thickness (%MT) and wall area (%WA) were used to evaluate the development of PAH. Tissue morphology was measured using hematoxylin and eosin staining. The protein levels of the phosphatase and tensin homologue deleted on chromosome ten (PTEN), Akt (ser473) phosphorylation (p-Akt) and total Akt in intrapulmonary arteries were determined by western blot analysis. MCT treatment significantly increased the RVSP, which was reduced by rosiglitazone treatment. The ratio of RV/(LV + S), %MT and %WA induced by MCT were similarly inhibited, which was associated with the increase of PTEN expression and the inhibition of Akt phosphorylation levels by rosiglitazone. In conclusion, activation of PPARγ ameliorates the proliferation of PASMCs and vascular remodeling by regulating the PTEN/PI3K/Akt pathway, suggesting that the activation of PPARγ has potential benefits for PAH.

The Role of PPAR Gamma in Systemic Sclerosis

Fibrosis is recognized as an important feature of many chronic diseases, such as systemic sclerosis (SSc), an autoimmune disease of unknown etiology, characterized by immune dysregulation and vascular injury, followed by progressive fibrosis affecting the skin and multiple internal organs. SSc has a poor prognosis because no therapy has been shown to reverse or arrest the progression of fibrosis, representing a major unmet medical need. Recently, antifibrotic effects of PPARγ ligands have been studied in vitro and in vivo and some theories have emerged leading to new insights. Aberrant PPARγ function seems to be implicated in pathological fibrosis in the skin and lungs. This antifibrotic effect is mainly related to the inhibition of TGF-β/Smad signal transduction but other pathways can be involved. This review focused on recent studies that identified PPARγ as an important novel pathway with critical roles in regulating connective tissue homeostasis, with emphasis on skin and lung fibrosis and its role on systemic sclerosis.

Potent natural products and herbal medicines for treating liver fibrosis

Liver fibrosis is a wound-healing response to chronic liver injury characterized by progressive inflammation and deposition of extracellular matrix components. The pathological condition of liver fibrosis involves secretion of extracellular matrix proteins and formation of scar tissue. The major regulators involved in hepatic fibrogenesis are the transforming growth factor (TGF)-β1/SMAD and toll-like receptor 4 (TLR4)-initiated myeloid differentiation primary response 88 gene (MyD88)/NF-ĸB cell signaling pathways. This article reviews natural products and herbal medicines that have demonstrated activity against liver fibrosis through different mechanisms of action, including anti-hepatitis B and C virus activity, anti-inflammation, inhibition of cytokine production and nuclear receptor activation, and free radical scavenging.

IL-13 induces YY1 through the AKT pathway in lung fibroblasts

A key feature of lung fibrosis is the accumulation of myofibroblasts. Interleukin 13 (IL-13) is a pro-fibrotic mediator that directly and indirectly influences the activation of myofibroblasts. Transforming growth factor beta (TGF-β) promotes the differentiation of fibroblasts into myofibroblasts, and can be regulated by IL-13. However, IL-13’s downstream signaling pathways are not completely understood. We previously reported that the transcription factor Yin Yang 1 (YY1) is upregulated in fibroblasts treated with TGF-β and in the lungs of mice and patients with pulmonary fibrosis. Moreover, YY1 directly regulates collagen and alpha smooth muscle actin (α-SMA) expression in fibroblasts. However, it is not known if IL-13 regulates fibroblast activation through YY1 expression. We hypothesize that IL-13 up-regulates YY1 expression through regulation of AKT activation, leading to fibroblast activation. In this study we found that YY1 was upregulated by IL-13 in lung fibroblasts in a dose- and time-dependent manner, resulting in increased α-SMA. Conversely, knockdown of YY1 blocked IL-13-induced α-SMA expression in fibroblasts. Furthermore, AKT phosphorylation was increased in fibroblasts treated with IL-13, and AKT overexpression upregulated YY1, whereas blockade of AKT phosphorylation suppressed the induction of YY1 by IL-13 in vitro. In vivo YY1 was upregulated in fibrotic lungs from CC10-IL-13 transgenic mice compared to that from wild-type littermates, which was associated with increased AKT phosphorylation. Taken together, these findings demonstrate that IL-13 is a potent stimulator and activator of fibroblasts, at least in part, through AKT-mediated YY1 activation.

Intestinal myofibroblast TRPC6 channel may contribute to stenotic fibrosis in Crohn's disease

BACKGROUND

Intestinal fibrosis is a frequent complication of Crohn's disease (CD) and often leads to detrimental stricture formation. Myofibroblasts play active roles in mediating fibrotic changes in various tissues. We investigated whether transient receptor potential channels in intestinal myofibroblasts are involved in CD-associated intestinal fibrosis.

METHODS

An intestinal myofibroblast cell line (InMyoFibs) was stimulated with transforming growth factor-β1 (TGF-β1) to model excessive fibrosis. Biopsy samples from nonstenotic or stenotic intestinal regions from patients with CD were used for quantitative comparisons of transient receptor potential channel and fibrosis-associated factor expression levels.

RESULTS

TGF-β1 treatment transformed spindle-shaped InMyoFibs into filament-shaped cells with enhanced α-actin stress fiber formation, transient receptor potential canonical (TRPC) 4 and TRPC6 messenger RNA and protein expression, and basal- and agonist-induced Ca influxes. TGF-β1 also enhanced the formation of TRPC6/smooth muscle α-actin, TRPC6/N-cadherin, and TRPC4/N-cadherin coimmunoprecipitates. Inhibition of TRPC6 in InMyoFibs by RNA interference or dominant-negative mutations suppressed TGF-β1-induced Ca influxes, stress fiber formation, and smooth muscle α-actin expression, but increased COL1A1, interleukin (IL)-10, and IL-11 expression, as well as Smad-2, ERK, and p38-MAPK phosphorylation. Similar increases in phosphorylation levels were observed with TRPC and calcineurin inhibitors. In stenotic areas in patients with CD, TRPC6, ACTA2 (smooth muscle α-actin), CDH2 (N-cadherin), COL1A1, IL-10, and IL-11 were significantly increased.

CONCLUSIONS

These results suggest that augmented Ca influxes due to TRPC6 upregulation facilitate stress fiber formation and strengthen cell-cell interactions by negatively regulating the synthesis of antifibrotic factors in TGF-β1-treated myofibroblasts. Similar changes observed in stenotic areas of patients with CD suggest the therapeutic significance of targeting TRPC6.

Hallmarks of the ageing lung

Ageing is the main risk factor for major non-communicable chronic lung diseases, including chronic obstructive pulmonary disease, most forms of lung cancer and idiopathic pulmonary fibrosis. While the prevalence of these diseases continually increases with age, their respective incidence peaks at different times during the lifespan, suggesting specific effects of ageing on the onset and/or pathogenesis of chronic obstructive pulmonary disease, lung cancer and idiopathic pulmonary fibrosis. Recently, the nine hallmarks of ageing have been defined as cell-autonomous and non-autonomous pathways involved in ageing. Here, we review the available evidence for the involvement of each of these hallmarks in the pathogenesis of chronic obstructive pulmonary disease, lung cancer, or idiopathic pulmonary fibrosis. Importantly, we propose an additional hallmark, “dysregulation of the extracellular matrix”, which we argue acts as a crucial modifier of cell-autonomous changes and functions, and as a key feature of the above-mentioned lung diseases.

Inhibition of transglutaminase 2, a novel target for pulmonary fibrosis, by two small electrophilic molecules

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive fibrotic destruction of normal lung architecture. Due to a lack of effective treatment options, new treatment approaches are needed. We previously identified transglutaminase (TG)2, a multifunctional protein expressed by human lung fibroblasts (HLFs), as a positive driver of fibrosis. TG2 catalyzes crosslinking of extracellular matrix proteins, enhances cell binding to fibronectin and integrin, and promotes fibronectin expression. We investigated whether the small electrophilic molecules 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) and 15-deoxy-delta-12,14-prostaglandin J2 (15d-PGJ2) inhibit the expression and profibrotic functions of TG2. CDDO and 15d-PGJ2 reduced expression of TG2 mRNA and protein in primary HLFs from control donors and donors with IPF. CDDO and 15d-PGJ2 also decreased the in vitro profibrotic effector functions of HLFs including collagen gel contraction and cell migration. The decrease in TG2 expression did not occur through activation of the peroxisome proliferator activated receptor γ or generation of reactive oxidative species. CDDO and 15d-PGJ2 inhibited the extracellular signal-regulated kinase pathway, resulting in the suppression of TG2 expression. This is the first study to show that small electrophilic compounds inhibit the expression and profibrotic effector functions of TG2, a key promoter of fibrosis. These studies identify new and important antifibrotic activities of these two small molecules, which could lead to new treatments for fibrotic lung disease.

Prostacyclin analogue beraprost inhibits cardiac fibroblast proliferation depending on prostacyclin receptor activation through a TGF β-Smad signal pathway

Previous studies showed that prostacyclin inhibited fibrosis. However, both receptors of prostacyclin, prostacyclin receptor (IP) and peroxisome proliferator-activated receptor (PPAR), are abundant in cardiac fibroblasts. Here we investigated which receptor was vital in the anti-fibrosis effect of prostacyclin. In addition, the possible mechanism involved in protective effects of prostacyclin against cardiac fibrosis was also studied. We found that beraprost, a prostacyclin analogue, inhibited angiotensin II (Ang II)-induced neonatal rat cardiac fibroblast proliferation in a concentration-dependent and time-dependent manner. Beraprost also suppressed Ang II-induced collagen I mRNA expression and protein synthesis in cardiac fibroblasts. After IP expression was knocked down by siRNA, Ang II-induced proliferation and collagen I synthesis could no longer be rescued by beraprost. However, treating cells with different specific inhibitors of PPAR subtypes prior to beraprost and Ang II stimulation, all of the above attenuating effects of beraprost were still available. Moreover, beraprost significantly blocked transforming growth factor β (TGF β) expression as well as Smad2 phosphorylation and reduced Smad-DNA binding activity. Beraprost also increased phosphorylation of cAMP response element binding protein (CREB) at Ser133 in the nucleus. Co-immunoprecipitation analysis revealed that beraprost increased CREB but decreased Smad2 binding to CREB-binding protein (CBP) in nucleus. In conclusion, beraprost inhibits cardiac fibroblast proliferation by activating IP and suppressing TGF β-Smad signal pathway.

Effects of vitamin E on peroxisome proliferator-activated receptor γ and nuclear factor-erythroid 2-related factor 2 in hypercholesterolemia-induced atherosclerosis

Atherosclerosis and associated cardiovascular complications such as stroke and myocardial infarction are major causes of morbidity and mortality. We have previously reported a significant increase in mRNA levels of the scavenger receptor CD36 in aortae of cholesterol-fed rabbits and shown that vitamin E treatment attenuated increased CD36 mRNA expression. In the present study, we further investigated the redox signaling pathways associated with protection against atherogenesis induced by high dietary cholesterol and correlated these with CD36 expression and the effects of vitamin E supplementation in a rabbit model. Male albino rabbits were assigned to either a control group fed with a low vitamin E diet alone or a test group fed with a low vitamin E diet containing 2% cholesterol in the absence or presence of daily intramuscular injections of vitamin E (50mg/kg). To elucidate the mechanisms by which vitamin E supplementation alters the effects of hypercholesterolemia in rabbit aortae, we measured peroxisome proliferator-activated receptor γ (PPARγ), ATP-binding cassette transporter A1 (ABCA1), and matrix metalloproteinase-1 (MMP-1) mRNA levels by quantitative RT-PCR and the expression of MMP-1, nuclear factor-erythroid 2-related factor 2 (Nrf2), and glutathione S-transferase α (GSTα) protein by immunoblotting. The increased MMP-1 and decreased GSTα expression observed suggests that a cholesterol-rich diet contributes to the development of atherosclerosis, whereas vitamin E supplementation affords protection by decreasing MMP-1 and increasing PPARγ, GSTα, and ABCA1 levels in aortae of rabbits fed a cholesterol-rich diet. Notably, protein expression of Nrf2, the antioxidant transcription factor, was increased in both the cholesterol-fed and the vitamin E-supplemented groups. Although Nrf2 activation can promote CD36-mediated cholesterol uptake by macrophages, the increased induction of Nrf2-mediated antioxidant genes is likely to contribute to decreased lesion progression. Thus, our study demonstrates that Nrf2 can mediate both pro- and antiatherosclerotic effects.

Calponin expression in renal tubulointerstitial fibrosis induced in rats by Cisplatin

Renal tubulointerstitial fibrosis is the common feature of chronic renal failure, regardless of its etiology. Myofibroblasts play important roles in progression of the fibrosis and are characterized by expressions of various cytoskeletons such as vimentin, desmin and α-smooth muscle actin (α-SMA). To pursue the characteristics of the cells, we immunohistochemically investigated the relationship between calponin (a marker of terminal smooth muscles) expression and myofibroblasts in cisplatin-induced rat renal tubulointerstitial fibrosis. Calponin-expressing interstitial cells increased with fibrosis and reacted simultaneously to vimentin or α-SMA (a marker of well-differentiated myofibroblasts) but not desmin or Thy-1 (a marker of myofibroblasts at the early stage). The present study shows that calponin may be expressed transiently in relatively well-developed myofibroblasts in rat renal fibrosis. Calponin could become a marker for myofibroblast development in chronic renal toxicity in rats.

Reversal of myofibroblast differentiation: a review

It has long been considered that fibrosis and fibroblast-to-myofibroblast differentiation are irreversible processes. However, recent data obtained indicates that tissue fibrosis and fibroblast-to-myofibroblast differentiation can indeed be reversed, which offers the possibility of a new therapeutic approach for fibrotic disorders. Here, we discuss the origin of the myofibroblasts and different aspects of their differentiation, especially the key mediators and TGFβ-induced signaling pathways. We also report here a few factors involved in myofiroblast dedifferentiation and several compounds which can reverse the established dedifferentiated myofibroblast, as examples that provide the reader a glimpse of the current trends of approach for discovering useful anti-fibrotic drugs.

The wound healing, chronic fibrosis, and cancer progression triad

For decades tumors have been recognized as "wounds that do not heal." Besides the commonalities that tumors and wounded tissues share, the process of wound healing also portrays similar characteristics with chronic fibrosis. In this review, we suggest a tight interrelationship, which is governed as a concurrence of cellular and microenvironmental reactivity among wound healing, chronic fibrosis, and cancer development/progression (i.e., the WHFC triad). It is clear that the same cell types, as well as soluble and matrix elements that drive wound healing (including regeneration) via distinct signaling pathways, also fuel chronic fibrosis and tumor progression. Hence, here we review the relationship between fibrosis and cancer through the lens of wound healing.

Inhibitory effects of PPARγ ligands on TGF-β1-induced corneal myofibroblast transformation

Corneal scarring, whether caused by trauma, laser refractive surgery, or infection, remains a significant problem for humans. Certain ligands for peroxisome proliferator-activated receptor gamma (PPARγ) have shown promise as antiscarring agents in a variety of body tissues. In the cornea, their relative effectiveness and mechanisms of action are still poorly understood. Here, we contrasted the antifibrotic effects of three different PPARγ ligands (15-deoxy-Δ12,14-prostaglandin J2, troglitazone, and rosiglitazone) in cat corneal fibroblasts. Western blot analyses revealed that all three compounds reduced transforming growth factor (TGF)-β1-driven myofibroblast differentiation and up-regulation of α-smooth muscle actin, type I collagen, and fibronectin expression. Because these effects were independent of PPARγ, we ascertained whether they occurred by altering phosphorylation of Smads 2/3, p38 mitogen-activated protein kinase, stress-activated protein kinase, protein kinase B, extracellular signal-regulated kinase, and/or myosin light chain 2. Only p38 mitogen-activated protein kinase phosphorylation was significantly inhibited by all three PPARγ ligands. Finally, we tested the antifibrotic potential of troglitazone in a cat model of photorefractive keratectomy-induced corneal injury. Topical application of troglitazone significantly reduced α-smooth muscle actin expression and haze in the stromal ablation zone. Thus, the PPARγ ligands tested here showed great promise as antifibrotics, both in vitro and in vivo. Our results also provided new evidence for the signaling pathways that may underlie these antifibrotic actions in corneal fibroblasts.

A genome-wide association study follow-up suggests a possible role for PPARG in systemic sclerosis susceptibility

Introduction

A recent genome-wide association study (GWAS) comprising a French cohort of systemic sclerosis (SSc) reported several non-HLA single-nucleotide polymorphisms (SNPs) showing a nominal association in the discovery phase. We aimed to identify previously overlooked susceptibility variants by using a follow-up strategy.

Methods

Sixty-six non-HLA SNPs showing a P value <10^-4 in the discovery phase of the French SSc GWAS were analyzed in the first step of this study, performing a meta-analysis that combined data from the two published SSc GWASs. A total of 2,921 SSc patients and 6,963 healthy controls were included in this first phase. Two SNPs, PPARG rs310746 and CHRNA9 rs6832151, were selected for genotyping in the replication cohort (1,068 SSc patients and 6,762 healthy controls) based on the results of the first step. Genotyping was performed by using TaqMan SNP genotyping assays.

Results

We observed nominal associations for both PPARG rs310746 (P_MH = 1.90 × 10^-6, OR, 1.28) and CHRNA9 rs6832151 (P_MH = 4.30 × 10^-6, OR, 1.17) genetic variants with SSc in the first step of our study. In the replication phase, we observed a trend of association for PPARG rs310746 (P value = 0.066; OR, 1.17). The combined overall Mantel-Haenszel meta-analysis of all the cohorts included in the present study revealed that PPARG rs310746 remained associated with SSc with a nominal non-genome-wide significant P value (P_MH = 5.00 × 10^-7; OR, 1.25). No evidence of association was observed for CHRNA9 rs6832151 either in the replication phase or in the overall pooled analysis.

Conclusion

Our results suggest a role of PPARG gene in the development of SSc.

Localization of ανβ6 integrin-TGF-β1/Smad3, mTOR and PPARγ in experimental colorectal fibrosis

A simultaneous action of several pro-fibrotic mediators appears relevant in the development of fibrosis. There are evidences that transforming growth factor-β (TGF-β)/Smad3 pathway forms with αvβ6 integrin, mammalian target of Rapamycin (mTOR) and peroxisome proliferator-activated receptor-γ (PPARγ) a complex signalling network with extensive crosstalk and strong effects on fibrosis development. The present study evaluated the expression of TGFβ, Smad3, αvβ6 integrin, mTOR and PPARγ in 2, 4, 6-trinitrobenzenesulphonic acid (TNBS)-induced colorectal fibrosis in Smad3 wild-type (WT) and null mice. Smad3 WT mice treated with TNBS developed a marked colorectal fibrosis and showed a concomitant up-regulation of TGFβ, Smad3, αvβ6 and mTOR and a reduction of PPARγ expression. On the other hand, Smad3 Null mice similarly treated with TNBS did not develop fibrosis and showed a very low or even absent expression of TGFβ, Smad3, αvβ6 and mTOR and a marked over-expression of PPARγ. At the same time the expression of α-smooth muscle actin (a marker of activated myofibroblasts), collagen I-III and connective tissue growth factor (a downstream effector of TGFβ/Smad3-induced extracellular matrix proteins) were up-regulated in Smad3 WT mice treated with TNBS compared to Null TNBS-treated mice. These preliminary results suggest a possible interaction between these pro-fibrotic molecules in the development of intestinal fibrosis.

Novel investigational agents for the treatment of scleroderma

INTRODUCTION

The purpose of this article is to highlight novel therapies that are being used in scleroderma (SSc). Therapeutic interventions in SSc generally target at least one of three ongoing biological processes characteristic of the disease: vasculopathy, autoimmunity and tissue fibrosis. Treatment decisions in SSc are determined by the level of disease activity and the degree of specific organ involvement. Traditional therapy has primarily focused on organ-specific management without clear evidence of overall disease modification.

AREAS COVERED

The authors provide a review of a variety of agents, which are already used for other autoimmune diseases, that are now being used to treat active SSc skin or lung disease, including rituximab, tocilizumab and IVIG. Several agents studied in vitro and in animal models of fibrosis have shown promise, including bortezomib, LPA-1 antagonists, anti-CCN2 therapy, anti-IL-13 and thrombin antagonists. The authors also provide details on targeting intracellular molecular pathways and matricellular proteins, which is another novel area of investigation.

EXPERT OPINION

Combination therapy may be necessary to control the complex biological network active in SSc. Most of the current evidence that suggest benefit of these agents is based on small population studies. Ultimately well-designed clinical trials are required to define the role of these agents in treating SSc.

Nrf2 acts cell-autonomously in endothelium to regulate tip cell formation and vascular branching

Angiogenesis, in which new blood vessels form via endothelial cell (EC) sprouting from existing vessels, is a critical event in embryonic development and multiple disease processes. Many insights have been made into key EC receptors and ligands/growth factors that govern sprouting angiogenesis, but intracellular molecular mechanisms of this process are not well understood. NF-E2-related factor 2 (Nrf2) is a transcription factor well-known for regulating the stress response in multiple pathologic settings, but its role in development is less appreciated. Here, we show that Nrf2 is increased and activated during vascular development. Global deletion of Nrf2 resulted in reduction of vascular density as well as EC sprouting. This was also observed with specific deletion of Nrf2 in ECs, but not with deletion of Nrf2 in the surrounding nonvascular tissue. Nrf2 deletion resulted in increased delta-like ligand 4 (Dll4) expression and Notch activity in ECs. Blockade of Dll4 or Notch signaling restored the vascular phenotype in Nrf2 KOs. Constitutive activation of endothelial Nrf2 enhanced EC sprouting and vascularization by suppression of Dll4/Notch signaling in vivo and in vitro. Nrf2 activation in ECs suppressed Dll4 expression under normoxia and hypoxia and inhibited Dll4-induced Notch signaling. Activation of Nrf2 blocked VEGF induction of VEGFR2-PI3K/Akt and downregulated HIF-2α in ECs, which may serve as important mechanisms for Nrf2 inhibition of Dll4 and Notch signaling. Our data reveal a function for Nrf2 in promoting the angiogenic sprouting phenotype in vascular ECs.

Regulation of myofibroblast differentiation by poly(ADP-ribose) polymerase 1

Poly(ADP-ribosyl)ation (PARylation) is a post-translational protein modification effected by enzymes belonging to the poly(ADP-ribose) polymerase (PARP) superfamily, mainly by PARP-1. The key acceptors of poly(ADP-ribose) include PARP-1 itself, histones, DNA repair proteins, and transcription factors. Because many of these factors are involved in the regulation of myofibroblast differentiation, we examined the role of PARylation on myofibroblast differentiation. Overexpression of PARP-1 with an expression plasmid activated expression of the α-SMA gene (Acta2), a marker of myofibroblast differentiation in lung fibroblasts. Suppression of PARP-1 activity or gene expression with PARP-1 inhibitors or siRNA, respectively, had the opposite effect on these cells. PARP-1-deficient cells also had reduced α-SMA gene expression. DNA pyrosequencing identified hypermethylated regions of the α-SMA gene in PARP-1-deficient cells, relative to wild-type cells. Interestingly, and of potential relevance to human idiopathic pulmonary fibrosis, PARP activity in lung fibroblasts isolated from idiopathic pulmonary fibrosis patients was significantly higher than that in cells isolated from control subjects. Furthermore, PARP-1-deficient mice exhibited reduced pulmonary fibrosis in response to bleomycin-induced lung injury, relative to wild-type controls. These results suggest that PARylation is important for myofibroblast differentiation and the pathogenesis of pulmonary fibrosis.

The triterpenoid CDDO-Me inhibits bleomycin-induced lung inflammation and fibrosis

Pulmonary Fibrosis (PF) is a devastating progressive disease in which normal lung structure and function is compromised by scarring. Lung fibrosis can be caused by thoracic radiation, injury from chemotherapy and systemic diseases such as rheumatoid arthritis that involve inflammatory responses. CDDO-Me (Methyl 2-cyano-3,12-dioxooleana-1,9(11)dien-28-oate, Bardoxolone methyl) is a novel triterpenoid with anti-fibrotic and anti-inflammatory properties as shown by our in vitro studies. Based on this evidence, we hypothesized that CDDO-Me would reduce lung inflammation, fibrosis and lung function impairment in a bleomycin model of lung injury and fibrosis. To test this hypothesis, mice received bleomycin via oropharyngeal aspiration (OA) on day zero and CDDO-Me during the inflammatory phase from days -1 to 9 every other day. Bronchoalveolar lavage fluid (BALF) and lung tissue were harvested on day 7 to evaluate inflammation, while fibrosis and lung function were evaluated on day 21. On day 7, CDDO-Me reduced total BALF protein by 50%, alveolar macrophage infiltration by 40%, neutrophil infiltration by 90% (p≤0.01), inhibited production of the inflammatory cytokines KC and IL-6 by over 90% (p≤0.001), and excess production of the pro-fibrotic cytokine TGFβ by 50%. CDDO-Me also inhibited α-smooth muscle actin and fibronectin mRNA by 50% (p≤0.05). On day 21, CDDO-Me treatment reduced histological fibrosis, collagen deposition and αSMA production. Lung function was significantly improved at day 21 by treatment with CDDO-Me, as demonstrated by respiratory rate and dynamic compliance. These new findings reveal that CDDO-Me exhibits potent anti-fibrotic and anti-inflammatory properties in vivo. CDDO-Me is a potential new class of drugs to arrest inflammation and ameliorate fibrosis in patients who are predisposed to lung injury and fibrosis incited by cancer treatments (e.g. chemotherapy and radiation) and by systemic autoimmune diseases.

A synthetic PPAR-γ agonist triterpenoid ameliorates experimental fibrosis: PPAR-γ-independent suppression of fibrotic responses

BACKGROUND

Persistent fibroblast activation initiated by transforming growth factor β (TGF-β) is a fundamental event in the pathogenesis of systemic sclerosis, and its pharmacological inhibition represents a potential therapeutic strategy. The nuclear receptor, peroxisome proliferator-activated receptor γ (PPAR-γ), exerts potent fibrotic activity. The synthetic oleanane triterpenoid, 2-cyano-3,12-dioxo-olean-1,9-dien-28-oic acid (CDDO), is a PPAR-γ agonist with potential effects on TGF-β signalling and dermal fibrosis.

OBJECTIVE

To examine the modulation of fibrogenesis by CDDO in explanted fibroblasts, skin organ cultures and murine models of scleroderma.

MATERIAL AND METHODS

The effects of CDDO on experimental fibrosis induced by bleomycin injection or by overexpression of constitutively active type I TGF-β receptor (TgfbR1ca) were evaluated. Modulation of fibrotic gene expression was examined in human skin organ cultures. To delineate the mechanisms underlying the antifibrotic effects of CDDO, explanted skin fibroblasts cultured in two-dimensional monolayers or in three-dimensional full-thickness human skin equivalents were studied.

RESULTS

CDDO significantly ameliorated dermal fibrosis in two complementary mouse models of scleroderma, as well as in human skin organ cultures and in three-dimensional human skin equivalents. In two-dimensional monolayer cultures of explanted normal fibroblasts, CDDO abrogated fibrogenic responses induced by TGF-β. These CDDO effects occurred via disruption of Smad-dependent transcription and were associated with inhibition of Akt activation. In scleroderma fibroblasts, CDDO attenuated the elevated synthesis of collagen. Remarkably, the in vitro antifibrotic effects of CDDO were independent of PPAR-γ.

CONCLUSIONS

The PPAR-γ agonist triterpenoid CDDO attenuates fibrogenesis by antagonistically targeting canonical TGF-β/Smad and Akt signalling in a PPAR-γ-independent manner. These findings identify this synthetic triterpenoid as a potential new therapy for the control of fibrosis.

Papillary fibroblasts differentiate into reticular fibroblasts after prolonged in vitro culture

The dermis can be divided into two morphologically different layers: the papillary and reticular dermis. Fibroblasts isolated from these layers behave differently when cultured in vitro. During skin ageing, the papillary dermis decreases in volume. Based on the functional differences in vitro, it is hypothesized that the loss of papillary fibroblasts contributes to skin ageing. In this study, we aimed to mimic certain aspects of skin ageing by using high-passage cultures of reticular and papillary fibroblasts and investigated the effect of these cells on skin morphogenesis in reconstructed human skin equivalents. Skin equivalents generated with reticular fibroblasts showed a reduced terminal differentiation and fewer proliferating basal keratinocytes. Aged in vitro papillary fibroblasts had increased expression of biomarkers specific to reticular fibroblasts. The phenotype and morphology of skin equivalents generated with high-passage papillary fibroblasts resembled that of reticular fibroblasts. This demonstrates that papillary fibroblasts can differentiate into reticular fibroblasts in vitro. Therefore, we hypothesize that papillary fibroblasts represent an undifferentiated phenotype, while reticular fibroblasts represent a more differentiated population. The differentiation process could be a new target for anti-skin-ageing strategies.

Peroxisome proliferator-activated receptor γ B cell-specific-deficient mice have an impaired antibody response

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. PPARγ, a ligand-activated transcription factor, has important anti-inflammatory and antiproliferative functions, and it has been associated with diseases including diabetes, scarring, and atherosclerosis, among others. PPARγ is expressed in most bone marrow-derived cells and influences their function. PPARγ ligands can stimulate human B cell differentiation and promote Ab production. A knowledge gap is that the role of PPARγ in B cells under physiological conditions is not known. We developed a new B cell-specific PPARγ (B-PPARγ) knockout mouse and explored the role of PPARγ during both the primary and secondary immune response. In this article, we show that PPARγ deficiency in B cells decreases germinal center B cells and plasma cell development, as well as the levels of circulating Ag-specific Abs during a primary challenge. Inability to generate germinal center B cells and plasma cells is correlated to decreased MHC class II expression and decreased Bcl-6 and Blimp-1 levels. Furthermore, B-PPARγ-deficient mice have an impaired memory response, characterized by low titers of Ag-specific Abs and low numbers of Ag-experienced, Ab-secreting cells. However, B-PPARγ-deficient mice have no differences in B cell population distribution within primary or secondary lymphoid organs during development. This is the first report, to our knowledge, to show that, under physiological conditions, PPARγ expression in B cells is required for an efficient B cell-mediated immune response as it regulates B cell differentiation and Ab production.

Contrary effects of sphingosine-1-phosphate on expression of α-smooth muscle actin in transforming growth factor β1-stimulated lung fibroblasts

Transforming growth factor-β1 (TGFβ1) plays a pivotal role in fibrosis in various organs including the lung. Following pulmonary injury, TGFβ1 stimulates conversion of fibroblasts to myofibroblasts that are mainly characterized by up-regulation of α-smooth muscle actin (αSMA) expression, and the resulting excess production of extracellular matrix proteins causes fibrosis with loss of alveolar function. The present study was undertaken to define the role of the sphingosine-1-phosphate (S1P) pathway in TGFβ1-induced expression of αSMA in human fetal lung fibroblasts, HFL1 cells. Analysis of mRNA revealed the existence of S1P(1), S1P(2), and S1P(3) receptor mRNAs. Treatment with TGFβ1 increased sphingosine kinase (SphK) activity and S1P(3) receptor mRNA at 24h after stimulation, and pharmacological data showed the involvement of sphingomyelinase, SphK, and S1P(3) receptor in the TGFβ1-induced up-regulation of αSMA with and without serum. Treatment with pertussis toxin and S1P(1) receptor antagonist W146 enhanced αSMA expression by TGFβ1/serum, and S1P decreased and increased αSMA levels with and without serum, respectively. TGFβ1 increased cyclooxygenase-2 expression in a manner dependent on serum and the sphingomyelinase/SphK pathway, and the response was decreased by pertussis toxin. Prostaglandin E(2), formed by TGFβ1/serum stimulation, decreased the TGFβ1-induced expression of αSMA via EP prostanoid receptor. These data suggest that S1P formed by TGFβ1 stimulation has diverse effects on the expression of αSMA, inhibition via the S1P(1) receptor-mediated and serum-dependent expression of cyclooxygenase-2 and the resulting formation of prostaglandin E(2), and stimulation via the S1P(3) receptor in a serum-independent manner.

Peroxisome proliferator-activated receptor gamma and regulations by the ubiquitin-proteasome system in pancreatic cancer

Pancreatic cancer is one of the most lethal forms of human cancer. Although progress in oncology has improved outcomes in many forms of cancer, little progress has been made in pancreatic carcinoma and the prognosis of this malignancy remains grim. Several molecular abnormalities often present in pancreatic cancer have been defined and include mutations in K-ras, p53, p16, and DPC4 genes. Nuclear receptor Peroxisome Proliferator-Activated Receptor gamma (PPARγ) has a role in many carcinomas and has been found to be overexpressed in pancreatic cancer. It plays generally a tumor suppressor role antagonizing proteins promoting carcinogenesis such as NF-κB and TGFβ. Regulation of pathways involved in pancreatic carcinogenesis is effectuated by the Ubiquitin Proteasome System (UPS). This paper will examine PPARγ in pancreatic cancer, the regulation of this nuclear receptor by the UPS, and their relationship to other pathways important in pancreatic carcinogenesis.

Synthetic oleanane triterpenoids: multifunctional drugs with a broad range of applications for prevention and treatment of chronic disease

We review the rationale for the use of synthetic oleanane triterpenoids (SOs) for prevention and treatment of disease, as well as extensive biological data on this topic resulting from both cell culture and in vivo studies. Emphasis is placed on understanding mechanisms of action. SOs are noncytotoxic drugs with an excellent safety profile. Several hundred SOs have now been synthesized and in vitro have been shown to: 1) suppress inflammation and oxidative stress and therefore be cytoprotective, especially at low nanomolar doses, 2) induce differentiation, and 3) block cell proliferation and induce apoptosis at higher micromolar doses. Animal data on the use of SOs in neurodegenerative diseases and in diseases of the eye, lung, cardiovascular system, liver, gastrointestinal tract, and kidney, as well as in cancer and in metabolic and inflammatory/autoimmune disorders, are reviewed. The importance of the cytoprotective Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1/nuclear factor (erythroid-derived 2)-like 2/antioxidant response element (Keap1/Nrf2/ARE) pathway as a mechanism of action is explained, but interactions with peroxisome proliferator-activated receptor γ (PARPγ), inhibitor of nuclear factor-κB kinase complex (IKK), janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT), human epidermal growth factor receptor 2 (HER2)/ErbB2/neu, phosphatase and tensin homolog (PTEN), the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway, mammalian target of rapamycin (mTOR), and the thiol proteome are also described. In these interactions, Michael addition of SOs to reactive cysteine residues in specific molecular targets triggers biological activity. Ultimately, SOs are multifunctional drugs that regulate the activity of entire networks. Recent progress in the earliest clinical trials with 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO) methyl ester (bardoxolone methyl) is also summarized.

TGFβ1 Controls PPARγ Expression, Transcriptional Potential, and Activity, in Part, through Smad3 Signaling in Murine Lung Fibroblasts

Transforming growth factor β1 (TGFβ1) promotes fibrosis by, among other mechanisms, activating quiescent fibroblasts into myofibroblasts and increasing the expression of extracellular matrices. Recent work suggests that peroxisome proliferator-activated receptor γ (PPARγ) is a negative regulator of TGFβ1-induced fibrotic events. We, however, hypothesized that antifibrotic pathways mediated by PPARγ are influenced by TGFβ1, causing an imbalance towards fibrogenesis. Consistent with this, primary murine primary lung fibroblasts responded to TGFβ1 with a sustained downregulation of PPARγ transcripts. This effect was dampened in lung fibroblasts deficient in Smad3, a transcription factor that mediates many of the effects of TGFβ1. Paradoxically, TGFβ1 stimulated the activation of the PPARγ gene promoter and induced the phosphorylation of PPARγ in primary lung fibroblasts. The ability of TGFβ1 to modulate the transcriptional activity of PPARγ was then tested in NIH/3T3 fibroblasts containing a PPARγ-responsive luciferase reporter. In these cells, stimulation of TGFβ1 signals with a constitutively active TGFβ1 receptor transgene blunted PPARγ-dependent reporter expression induced by troglitazone, a PPARγ activator. Overexpression of PPARγ prevented TGFβ1 repression of troglitazone-induced PPARγ-dependent gene transcription, whereas coexpression of PPARγ and Smad3 transgenes recapitulated the TGFβ1 effects. We conclude that modulation of PPARγ is controlled by TGFβ1, in part through Smad3 signals, involving regulation of PPARγ expression and transcriptional potential.

Lactic acid is elevated in idiopathic pulmonary fibrosis and induces myofibroblast differentiation via pH-dependent activation of transforming growth factor-β

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a complex disease for which the pathogenesis is poorly understood. In this study, we identified lactic acid as a metabolite that is elevated in the lung tissue of patients with IPF.

OBJECTIVES

This study examines the effect of lactic acid on myofibroblast differentiation and pulmonary fibrosis.

METHODS

We used metabolomic analysis to examine cellular metabolism in lung tissue from patients with IPF and determined the effects of lactic acid and lactate dehydrogenase-5 (LDH5) overexpression on myofibroblast differentiation and transforming growth factor (TGF)-β activation in vitro.

MEASUREMENTS AND MAIN RESULTS

Lactic acid concentrations from healthy and IPF lung tissue were determined by nuclear magnetic resonance spectroscopy; α-smooth muscle actin, calponin, and LDH5 expression were assessed by Western blot of cell culture lysates. Lactic acid and LDH5 were significantly elevated in IPF lung tissue compared with controls. Physiologic concentrations of lactic acid induced myofibroblast differentiation via activation of TGF-β. TGF-β induced expression of LDH5 via hypoxia-inducible factor 1α (HIF1α). Importantly, overexpression of both HIF1α and LDH5 in human lung fibroblasts induced myofibroblast differentiation and synergized with low-dose TGF-β to induce differentiation. Furthermore, inhibition of both HIF1α and LDH5 inhibited TGF-β-induced myofibroblast differentiation.

CONCLUSIONS

We have identified the metabolite lactic acid as an important mediator of myofibroblast differentiation via a pH-dependent activation of TGF-β. We propose that the metabolic milieu of the lung, and potentially other tissues, is an important driving force behind myofibroblast differentiation and potentially the initiation and progression of fibrotic disorders.

Cellular and molecular mechanisms of intestinal fibrosis

Fibrosis is a chronic and progressive process characterized by an excessive accumulation of extracellular matrix (ECM) leading to stiffening and/or scarring of the involved tissue. Intestinal fibrosis may develop in several different enteropathies, including inflammatory bowel disease. It develops through complex cell, extracellular matrix, cytokine and growth factor interactions. Distinct cell types are involved in intestinal fibrosis, such as resident mesenchymal cells (fibroblasts, myofibroblasts and smooth muscle cells) but also ECM-producing cells derived from epithelial and endothelial cells (through a process termed epithelial- and endothelial-mesenchymal transition), stellate cells, pericytes, local or bone marrow-derived stem cells. The most important soluble factors that regulate the activation of these cells include cytokines, chemokines, growth factors, components of the renin-angiotensin system, angiogenic factors, peroxisome proliferator-activated receptors, mammalian target of rapamycin, and products of oxidative stress. It soon becomes clear that although inflammation is responsible for triggering the onset of the fibrotic process, it only plays a minor role in the progression of this condition, as fibrosis may advance in a self-perpetuating fashion. Definition of the cellular and molecular mechanisms involved in intestinal fibrosis may provide the key to developing new therapeutic approaches.

MicroRNA-21 promotes fibrosis of the kidney by silencing metabolic pathways

Scarring of the kidney is a major public health concern, directly promoting loss of kidney function. To understand the role of microRNA (miRNA) in the progression of kidney scarring in response to injury, we investigated changes in miRNA expression in two kidney fibrosis models and identified 24 commonly up-regulated miRNAs. Among them, miR-21 was highly elevated in both animal models and in human transplanted kidneys with nephropathy. Deletion of miR-21 in mice resulted in no overt abnormality. However, miR-21(-/-) mice suffered far less interstitial fibrosis in response to kidney injury, a phenotype duplicated in wild-type mice treated with anti-miR-21 oligonucleotides. Global derepression of miR-21 target mRNAs was readily detectable in miR-21(-/-) kidneys after injury. Analysis of gene expression profiles up-regulated in the absence of miR-21 identified groups of genes involved in metabolic pathways, including the lipid metabolism pathway regulated by peroxisome proliferator-activated receptor-α (Pparα), a direct miR-21 target. Overexpression of Pparα prevented ureteral obstruction-induced injury and fibrosis. Pparα deficiency abrogated the antifibrotic effect of anti-miR-21 oligonucleotides. miR-21 also regulated the redox metabolic pathway. The mitochondrial inhibitor of reactive oxygen species generation Mpv17l was repressed by miR-21, correlating closely with enhanced oxidative kidney damage. These studies demonstrate that miR-21 contributes to fibrogenesis and epithelial injury in the kidney in two mouse models and is a candidate target for antifibrotic therapies.

Mechanisms of fibrosis: therapeutic translation for fibrotic disease

Fibrosis is a pathological feature of most chronic inflammatory diseases. Fibrosis, or scarring, is defined by the accumulation of excess extracellular matrix components. If highly progressive, the fibrotic process eventually leads to organ malfunction and death. Fibrosis affects nearly every tissue in the body. Here we discuss how key components of the innate and adaptive immune response contribute to the pathogenesis of fibrosis. We also describe how cell-intrinsic changes in important structural cells can perpetuate the fibrotic response by regulating the differentiation, recruitment, proliferation and activation of extracellular matrix-producing myofibroblasts. Finally, we highlight some of the key mechanisms and pathways of fibrosis that are being targeted as potential therapies for a variety of important human diseases.

Mechanisms of fibrosis: therapeutic translation for fibrotic disease

Fibrosis is a pathological feature of most chronic inflammatory diseases. Fibrosis, or scarring, is defined by the accumulation of excess extracellular matrix components. If highly progressive, the fibrotic process eventually leads to organ malfunction and death. Fibrosis affects nearly every tissue in the body. Here we discuss how key components of the innate and adaptive immune response contribute to the pathogenesis of fibrosis. We also describe how cell-intrinsic changes in important structural cells can perpetuate the fibrotic response by regulating the differentiation, recruitment, proliferation and activation of extracellular matrix-producing myofibroblasts. Finally, we highlight some of the key mechanisms and pathways of fibrosis that are being targeted as potential therapies for a variety of important human diseases.

Troglitazone attenuates TGF-β1-induced EMT in alveolar epithelial cells via a PPARγ-independent mechanism

Peroxisome proliferator activated receptor γ (PPARγ) agonists are effective antifibrotic agents in a number of tissues. Effects of these agents on epithelial-mesenchymal transition (EMT) of primary alveolar epithelial cells (AEC) and potential mechanisms underlying effects on EMT have not been well delineated. We examined effects of troglitazone, a synthetic PPARγ agonist, on transforming growth factor (TGF)-β1-induced EMT in primary rat AEC and an alveolar epithelial type II (AT2) cell line (RLE-6TN). TGF-β1 (2.5 ng/mL) induced EMT in both cell types, as evidenced by acquisition of spindle-like morphology, increased expression of the mesenchymal marker α-smooth muscle actin (α-SMA) and downregulation of the tight junctional protein zonula occludens-1 (ZO-1). Concurrent treatment with troglitazone (or rosiglitazone), ameliorated effects of TGF-β1. Furthermore, following stimulation with TGF-β1 for 6 days, troglitazone reversed EMT-related morphological changes and restored both epithelial and mesenchymal markers to control levels. Treatment with GW9662 (an irreversible PPARγ antagonist), or overexpression of a PPARγ dominant negative construct, failed to inhibit these effects of troglitazone in AEC. Troglitazone not only attenuated TGF-β1-induced phosphorylation of Akt and glycogen synthase kinase (GSK)-3β, but also inhibited nuclear translocation of β-catenin, phosphorylation of Smad2 and Smad3 and upregulation of the EMT-associated transcription factor SNAI1. These results demonstrate inhibitory actions of troglitazone on TGF-β1-induced EMT in AEC via a PPARγ-independent mechanism likely through inhibition of β-catenin-dependent signaling downstream of TGF-β1, supporting a role for interactions between TGF-β and Wnt/β-catenin signaling pathways in EMT.

Emerging PPARγ-Independent Role of PPARγ Ligands in Lung Diseases

Peroxisome proliferator activated receptor (PPAR)-γ is a nuclear hormone receptor that is activated by multiple agonists including thiazolidinediones, prostaglandins, and synthetic oleanolic acids. Many PPARγ ligands are under investigation as potential therapies for human diseases. These ligands modulate multiple cellular pathways via both PPARγ-dependent and PPARγ-independent mechanisms. Here, we review the role of PPARγ and PPARγ ligands in lung disease, with emphasis on PPARγ-independent effects. PPARγ ligands show great promise in moderating lung inflammation, as antiproliferative agents in combination to enhance standard chemotherapy in lung cancer and as treatments for pulmonary fibrosis, a progressive fatal disease with no effective therapy. Some of these effects occur when PPARγ is pharmaceutically antagonized or genetically PPARγ and are thus independent of classical PPARγ-dependent transcriptional control. Many PPARγ ligands demonstrate direct binding to transcription factors and other proteins, altering their function and contributing to PPARγ-independent inhibition of disease phenotypes. These PPARγ-independent mechanisms are of significant interest because they suggest new therapeutic uses for currently approved drugs and because they can be used as probes to identify novel proteins and pathways involved in the pathogenesis or treatment of disease, which can then be targeted for further investigation and drug development.

Regulation of Matrix Remodeling by Peroxisome Proliferator-Activated Receptor-γ: A Novel Link Between Metabolism and Fibrogenesis

The intractable process of fibrosis underlies the pathogenesis of systemic sclerosis (SSc) and other diseases, and in aggregate contributes to 45% of deaths worldwide. Because currently there is no effective anti-fibrotic therapy, a better understanding of the pathways and cellular differentiation programs underlying fibrosis are needed. Emerging evidence points to a fundamental role of the nuclear hormone receptor peroxisome proliferator activated receptor-γ (PPAR-γ) in modulating fibrogenesis. While PPAR-γ has long been known to be important in lipid metabolism and in glucose homeostasis, its role in regulating mesenchymal cell biology and its association with pathological fibrosis had not been appreciated until recently. This article highlights recent studies revealing a consistent association of fibrosis with aberrant PPAR-γ expression and activity in various forms of human fibrosis and in rodent models, and reviews studies linking genetic manipulation of the PPAR-γ pathway in rodents and fibrosis. We survey the broad range of anti-fibrotic activities associated with PPAR-γ and the underlying mechanisms. We also summarize the emerging data linking PPAR-γ dysfunction and pulmonary arterial hypertension (PAH), which together with fibrosis is responsible for the mortality in patients in SSc. Finally, we consider current and potential future strategies for targeting PPAR-γ activity or expression as a therapy for controlling fibrosis.

Different gene expression patterns in human papillary and reticular fibroblasts

The dermis contains two distinct layers: the papillary and the reticular layers. In vitro cultures of the fibroblasts from these layers show that they are different. However, no molecular markers to differentiate between the two subtypes of fibroblasts are known. We performed gene expression analysis on cultured fibroblasts isolated from the papillary and reticular dermis. In all, 116 genes were found to be expressed differentially. Of these, 13 were validated by quantitative reverse transcriptase-PCR analysis and two markers could be validated at the protein level in monolayer cultures. Three markers showed differential expression in in vivo skin sections. The identified, characteristic markers of the two fibroblast subpopulations provide useful tools to perform functional studies on reticular and papillary fibroblasts.

Evaluation of the antifibrotic effect of fenofibrate and rosiglitazone on bleomycin-induced pulmonary fibrosis in rats

Idiopathic pulmonary fibrosis is the most prevalent chronic fibrosing lung disease. Peroxisome proliferator-activated receptors-gamma agonists provide potential therapy for fibrotic diseases of the lung. Peroxisome proliferator-activated receptors-alpha agonists may be helpful in the treatment of lung inflammatory diseases, however their therapeutic potential on the "fibro-proliferative" process and extracellular matrix accumulation in idiopathic pulmonary fibrosis has been less well studied. So, the present study was conducted to evaluate the anti-fibrotic effects of fenofibrate (peroxisome proliferator-activated receptors-alpha agonist) alone and in combination with rosiglitazone (peroxisome proliferator-activated receptors-gamma agonist) on lung injury induced by bleomycin administration. Oral administration of either rosiglitazone (5 mg/kg/d) or fenofibrate (100 mg/kg/d) for 14 days, attenuated the severity of bleomycin-induced lung injury and fibrosis through decreasing lung water contents, lung fibrotic grading, lung hydroxyproline contents and lung transforming growth factor-beta1 levels; with no significant difference between them. Combined low doses of rosiglitazone (1 mg/kg/d) and fenofibrate (30 mg/kg/d) provided more benefits than full separate doses of each on the deleterious effects accompanied bleomycin administration. These findings suggested the potential use of peroxisome proliferator-activated receptors-alpha ligands as anti-fibrotic agents in lung fibrosis. Additionally, the concurrent administration of fenofibrate and rosiglitazone in low doses has synergistic effect and enhanced the beneficial effects afforded by either fenofibrate or rosiglitazone.

Levels of adiponectin, a marker for PPAR-gamma activity, correlate with skin fibrosis in systemic sclerosis: potential utility as biomarker?

INTRODUCTION

Progressive fibrosis in systemic sclerosis (SSc) is linked to aberrant transforming growth factor beta (TGF-beta) signaling. Peroxisome proliferator-activated receptor gamma (PPAR-gamma) blocks fibrogenic TGF-beta responses in vitro and in vivo. Reduced expression and function of PPAR-gamma in patients with SSc may contribute to progression of fibrosis. Here we evaluated the levels of adiponectin, a sensitive and specific index of PPAR-gamma activity, as a potential fibrogenic biomarker in SSc.

METHODS

Adiponectin levels were determined in the sera of 129 patients with SSc and 86 healthy controls, and serial determinations were performed in 27 patients. Levels of adiponectin mRNA in skin biopsies from SSc patients were assessed in an expression profiling microarray dataset. Regulation of adiponectin gene expression in explanted human subcutaneous preadipocytes and fibroblasts was examined by real-time quantitative PCR.

RESULTS

Patients with diffuse cutaneous SSc had reduced serum adiponectin levels. A significant inverse correlation between adiponectin levels and the modified Rodnan skin score was observed. In longitudinal studies changes in serum adiponectin levels were inversely correlated with changes in skin fibrosis. Skin biopsies from a subset of SSc patients showed reduced adiponectin mRNA expression which was inversely correlated with the skin score. An agonist ligand of PPAR-gamma potently induced adiponectin expression in explanted mesenchymal cells in vitro.

CONCLUSIONS

Levels of adiponectin, reflecting PPAR-gamma activity, are correlated with skin fibrosis and might have potential utility as a biomarker in SSc.

Cysteine-rich protein 1 is regulated by transforming growth factor-β1 and expressed in lung fibrosis

Transforming growth factor-β (TGF-β) is a diverse cytokine regulating growth, apoptosis, differentiation, adhesion, invasion, and extracellular matrix production. Dysregulation of TGF-β is associated with fibrotic disorders and epithelial-mesenchymal transition, and has been linked with idiopathic pulmonary fibrosis (IPF). Cysteine-rich protein 1 (CRP1) is a small LIM-domain containing protein involved in smooth muscle differentiation. Here, we show that TGF-β1 increases the expression of CRP1 protein and that CRP1 levels increase in a biphasic fashion. A rapid transient (15-45 min) increase in CRP1 is followed by a subsequent, sustained increase in CRP1 a few hours afterwards that lasts several days. We find that TGF-β1 regulates the expression of CRP1 through Smad and non-conventional p38 MAPK signaling pathways in a transcription-independent manner and that the induction occurs concomitant with an increase in myofibroblast differentiation. Using CRP1 silencing by shRNA, we identify CRP1 as a novel factor mediating cell contractility. Furthermore, we localize CRP1 to fibroblastic foci in IPF lungs and find that CRP1 is significantly more expressed in IPF as compared to control lung tissue. The results show that CRP1 is a novel TGF-β1 regulated protein that is expressed in fibrotic lesions and may be relevant in the IPF disease.

Loss of PPARγ expression by fibroblasts enhances dermal wound closure

Background

Peroxisome proliferator-activated receptor (PPAR)γ may be a key regulator of connective tissue deposition and remodeling in vivo. PPARγ expression is reduced in dermal fibroblasts isolated from fibrotic areas of scleroderma patients; PPARγ agonists suppress the persistent fibrotic phenotype of this cell type. Previously, we showed that loss of PPARγ expression in fibroblasts resulted in enhanced bleomycin-induced skin fibrosis. However, whether loss of PPARγ expression in skin fibroblasts affects cutaneous tissue repair or homeostasis is unknown.

Results

Mice deleted for PPARγ in skin fibroblasts show an enhanced rate of dermal wound closure, concomitant with elevated phosphorylation of Smad3, Akt and ERK, and increased expression of proliferating cell nuclear antigen (PCNA), collagen, α-smooth muscle actin (α-SMA) and CCN2. Conversely, dermal homeostasis was not appreciably affected by loss of PPARγ expression.

Conclusion

PPARγ expression by fibroblasts suppresses cutaneous tissue repair. In the future, direct PPARγ antagonists and agonists might be of clinical benefit in controlling chronic wounds or scarring, respectively.