Upregulation of type I collagen by TGF-beta in mesangial cells is blocked by PPARgamma activation

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.

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.

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.

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.

Homocysteine and Hypertension in Diabetes: Does PPARgamma Have a Regulatory Role?

Dysfunction of macro- and microvessels is a major cause of morbidity and mortality in patients with cardio-renovascular diseases such as atherosclerosis, hypertension, and diabetes. Renal failure and impairment of renal function due to vasoconstriction of the glomerular arteriole in diabetic nephropathy leads to renal volume retention and increase in plasma homocysteine level. Homocysteine, which is a nonprotein amino acid, at elevated levels is an independent cardio-renovascular risk factor. Homocysteine induces oxidative injury of vascular endothelial cells, involved in matrix remodeling through modulation of the matrix metalloproteinase (MMP)/tissue inhibitor of metalloproteinase (TIMP) axis, and increased formation and accumulation of extracellular matrix protein, such as collagen. In heart this leads to increased endothelial-myocyte uncoupling resulting in diastolic dysfunction and hypertension. In the kidney, increased matrix accumulation in the glomerulus causes glomerulosclerosis resulting in hypofiltration, increased renal volume retention, and hypertension. PPARγ agonist reduces tissue homocysteine levels and is reported to ameliorate homocysteine-induced deleterious vascular effects in diabetes. This review, in light of current information, focuses on the beneficial effects of PPARγ agonist in homocysteine-associated hypertension and vascular remodeling in diabetes.

Identifying advanced glycation end products as a major source of oxidants in aging: implications for the management and/or prevention of reduced renal function in elderly persons

Aging is characterized by increasing inflammation and oxidant stress (OS). Reduced renal function was present in more than 20% of normal-aged individuals sampled in the National Health and Nutrition Examination Survey (NHANES) cross-sectional study of the US population. Longitudinal studies in the United States and Italy showed that renal function does not decline in some individuals, suggesting that a search for causes of the loss of renal function in some persons might be indicated and interventions to reduce this outcome should be sought. Because advanced glycation end products (AGEs) induce both inflammation and OS, accumulate with age, and primarily are excreted by the kidney, one outcome of reduced renal function in aging could be decreased AGE disposal. The build-up of AGEs with reduced renal function could contribute to inflammation, increased oxidant stress, and accumulation of AGEs in aging. In fact, results from a longitudinal study of normal aging adults in Italy showed that the most significant correlation with mortality was the level of renal function. A clear link between inflammation, OS, AGEs, and chronic disease was shown in studies of mice that showed that reduction of AGE levels by drugs or decreased intake of AGEs reduces chronic kidney disease (CKD) and cardiovascular disease of aging. The data support a role for AGEs in the development of renal lesions in aging mice and reveal that AGEs in the diet are very important contributors to renal and cardiovascular lesions. AGEs signal through two receptors, one of which is anti-inflammatory (AGER1) and the other is proinflammatory (RAGE). Overexpression of AGER1 protects against OS and acute vascular injury. The reduction of AGEs in the diet is as efficient in preventing aging-related cardiovascular and renal lesions in mice as that seen with calorie restriction. Studies in normal adults of all ages and those with CKD suggest that the findings in mice may be directly applicable to both aging and CKD. Namely, the dietary content of AGEs determines the serum levels of AGEs and inflammatory mediators and urine AGE levels in both normal subjects and CKD patients. Importantly, reduction of AGEs controls these changes in both normal subjects and CKD patients, and the phenotypic changes in AGER1 are reduced in CKD patients by decreasing the amount of AGEs consumed with the diet. These data suggest that the changes in renal function in normal aging may be subject to control and this subject deserves renewed attention.

PPARgamma agonists inhibit TGF-beta-PKA signaling in glomerulosclerosis

AIM

To study the probable mechanisms of the anti-glomerulosclerosis effects induced by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists in rat intraglomerular mesangial cells (MCs).

METHODS

Cells were transfected with the pTAL-PPRE-tk-Luc(+) plasmid and then treated with different concentrations of PPARgamma agonist, either troglitazone or telmisartan, for the indicated times. Promega luciferase assays were subsequently used for the detection of PPARgamma activation. Protein expression levels were assessed by Western blot, and PepTag assays were used for the non-radioactive detection of protein kinase A (PKA) activity. The deposition of alpha-smooth muscle actin (alpha-SMA) and p-cyclic AMP responsive element binding protein (pCREB) were analyzed by confocal laser scanning.

RESULTS

Both troglitazone and telmisartan remarkably inhibit the PKA activation and pCREB expression that is stimulated by TGF-beta. The PPARgamma agonists also inhibited alpha-SMA and collagen IV protein expression by blocking PKA activation.

CONCLUSION

PPARgamma ligands effectively suppress the activation of MCs and the accumulation of collagen IV stimulated by TGF-beta in vitro. The renal protection provided by PPARgamma agonists is partly mediated via their blockade of TGF-beta/PKA signaling.

Omacor, n-3 polyunsaturated fatty acid, attenuated albuminuria and renal dysfunction with decrease of SREBP-1 expression and triglyceride amount in the kidney of type II diabetic animals

BACKGROUND

We assumed that n-3 polyunsaturated fatty acid (n-3 PUFA) would attenuate the tissue dyslipidemic condition through suppression of sterol regulatory element-binding protein (SREBP-1) in the kidney and would prevent renal progression in diabetic animals.

METHODS

We gavaged Omacor, composed of docosahexaenoic acid and eicosapentaenoic acid, to db/db mice for 2 weeks (0.2 g/100 g/day). We measured the markers of renal function, triglyceride amount and expressions of SREBP-1, liver X-activated receptor alpha (LXRalpha), collagen IV and TGFbeta-1 in kidney lysate, and performed immunohistochemical staining for SREBP-1, desmin and WT-1 in the renal sections. We measured collagen IV in primary mesangial cells cultured with high glucose media (25 mM), both with and without a transient transfection of small interfering RNA (siRNA) SREBP-1.

RESULTS

Omacor decreased the concentration of serum free fatty acid, and the amount of renal triglyceride, which was associated with decreased expression of SREBP-1 in the kidney, albuminuria and renal dysfunction in db/db mice. Omacor attenuated the expansion of mesangial matrix and the expression of desmin, preserved the WT-1 positive cells, and inhibited the phosphorylation of nuclear factor kappaB in renal tissue. In mesangial cells cultured in high glucose media, the suppression of SREBP-1 expression decreased the collagen IV in the cells.

CONCLUSIONS

Our study results demonstrated that n-3 PUFA prevented renal progression with attenuation of SREBP-1 and reduction of triglyceride in the diabetic kidney. This suggests that the regulation of dyslipidemic signals in the kidney could be a possible mechanism by which PUFA preserves renal function in the diabetic condition.

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.

Role of PPARgamma in renoprotection in Type 2 diabetes: molecular mechanisms and therapeutic potential

DN (diabetic nephropathy) is a chronic disease characterized by proteinuria, glomerular hypertrophy, decreased glomerular filtration and renal fibrosis with loss of renal function. DN is the leading cause of ESRD (end-stage renal disease), accounting for millions of deaths worldwide. TZDs (thiazolidinediones) are synthetic ligands of PPARgamma (peroxisome-proliferator-activated receptor gamma), which is involved in many important physiological processes, including adipose differentiation, lipid and glucose metabolism, energy homoeostasis, cell proliferation, inflammation, reproduction and renoprotection. A large body of research over the past decade has revealed that, in addition to their insulin-sensitizing effects, TZDs play an important role in delaying and preventing the progression of chronic kidney disease in Type 2 diabetes. Although PPARgamma activation by TZDs is in general considered beneficial for the amelioration of diabetic renal complications in Type 2 diabetes, the underlying mechanism(s) remains only partially characterized. In this review, we summarize and discuss recent findings regarding the renoprotective effects of PPARgamma in Type 2 diabetes and the potential underlying mechanisms.

[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.

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists attenuate the profibrotic response induced by TGF-beta1 in renal interstitial fibroblasts

BACKGROUND

Studies have shown that peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists could ameliorate renal fibrotic lesions in both diabetic nephropathy and nondiabetic chronic kidney diseases. In order to elucidate the antifibrotic mechanism of PPAR-gamma agonists, we investigated the effects of PPAR-gamma activation on TGF-beta1-induced renal interstitial fibroblasts.

METHODS

In rat renal interstitial fibroblasts (NRK/49F), the mRNA expression of TGF-beta1-induced alpha-smooth muscle actin (alpha-SMA), connective tissue growth factor (CTGF), fibronectin (FN) and collagen type III (Col III) were observed by reverse transcriptase-polymerase chain reaction (RT-PCR). The protein expressions of FN and Smads were observed by Western blot.

RESULTS

In NRK/49F, TGF-beta1 enhanced CTGF, FN and Col III mRNA expression in a dose- and time-dependent manner. alpha-SMA, CTGF, FN and Col III mRNA and FN protein expression in 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2)-troglitazone- and ciglitazone-pretreated groups, respectively, were significantly decreased compared with the TGF-beta1-stimulated group. TGF-beta1 (5 ng/mL) enhanced p-Smad2/3 protein expression in a time-dependent manner. Compared with the TGF-beta1-stimulated group, p-Smad2/3 protein induced by TGF-beta1 in PPAR-gamma agonists-pretreated groups significantly decreased with no statistical difference amongst the three pretreated groups.

CONCLUSION

PPAR-gamma agonists could inhibit TGF-beta1-induced renal fibroblast activation, CTGF expression and ECM synthesis through abrogating the TGF-beta1/Smads signaling pathway.

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.

Rosiglitazone increases matrix production and quenches inflammation: studies in human cells

BACKGROUND

Type 2 diabetes (T2D) is characterized by an accelerated atherogenesis, a process to which both proliferative and inflammatory responses contribute. Peroxisome proliferator-activated receptors-gamma (PPARgamma) agonists have both anti-proliferative and anti-inflammatory properties. We tested the effect of therapeutic doses of rosiglitazone on proliferative and inflammatory pathways in fibroblasts (HF) from five controls (C) and five T2D patients, and in aortic smooth muscle cells (hSMC).

METHODS

Transforming growth factor-beta (TGFbeta) and interleukin-6 (IL-6) expression, and IL-6, laminin and fibronectin release were measured. To identify the involved intracellular signalling, extracellular signal-regulated kinases (ERK)1/2 phosphorylation and p38 activation were evaluated.

RESULTS

Both phorbol 12-myristate 13-acetate (PMA) [a protein kinase C (PKC) activator] and rosiglitazone increased TGFbeta expression and fibronectin and laminin release in C and T2D patients. Rosiglitazone effect was reversed by its specific inhibitor Sr202. The combination PMA + rosiglitazone was additive in C, but not in T2D patients. IL-6 production was stimulated by PMA in both C and T2D patients; this effect was prevented by rosiglitazone in a Sr202-inhibitable manner. Experiments performed in hSMC yielded the same results. Rosiglitazone increased p38 activation more in C than in T2D patients; PMA-induced phosphorylation of ERK1/2 was similarly reduced in both cells.

CONCLUSIONS

In HF and hSMC, rosiglitazone stimulates the synthesis of matrix components via enhanced TGFbeta expression; when combined with PMA, the resulting PKC activation is mediated by enhanced p38 phosphorylation. On the other hand, rosiglitazone quenches inflammation in both cell types, by counteracting PMA-induced phosphorylation of ERK1/2.

Rhein reverses the diabetic phenotype of mesangial cells over-expressing the glucose transporter (GLUT1) by inhibiting the hexosamine pathway

BACKGROUND AND PURPOSE

Rhein, an anthraquinone compound isolated from rhubarb, has been proved effective in treatment of experimental diabetic nephropathy (DN). To explore the mechanism of its therapeutic effect on DN, rhein was tested for its effect on the hexosamine pathway.

EXPERIMENTAL APPROACH

The influence of rhein on cellular hypertrophy, fibronectin synthesis, glucose uptake, glutamine: fructose 6-phosphate aminotransferase (GFAT) activity, UDP-N-acetylglucosamine (UDP-GlcNAc) level and TGF-beta1 and p21 expression was evaluated in MCGT1 cells, a GLUT1 transgenic rat mesangial cell line. GFAT activity in normal rat mesangial cells in high glucose concentrations and in vitro was also measured.

KEY RESULTS

Significantly increased fibronectin synthesis, cellular hypertrophy, much higher GFAT activity and UDP-GlcNAc level and increased TGF-beta1 and p21 expression were found in MCGT1 cells cultured in normal glucose concentration. Rhein treatment decreased all these features of MCGT1 cells but did not exert a direct effect on GFAT enzymatic activity.

CONCLUSIONS AND IMPLICATIONS

There was over-activity of the hexosamine pathway in MCGT1 cells, which may explain the higher expression of TGF-beta1 and p21, the cellular hypertrophy and the increased expression of extracellular matrix (ECM) components in the cells. By inhibiting the increased activity the hexosamine pathway, rhein decreased TGF-beta1 and p21 expression and thus contributed to the decreased cellular hypertrophy and ECM synthesis. Inhibition of the hexosamine pathway may be one of the mechanism through which rhein exerts its therapeutic role in diabetic nephropathy.

PPARs and the kidney in metabolic syndrome

The metabolic syndrome (MetS) is defined by a set of metabolic risk factors, including insulin resistance, central obesity, dyslipidemia, hyperglycemia, and hypertension for type 2 diabetes and cardiovascular disease. Although both retrospective and prospective clinical studies have revealed that MetS is associated with chronic renal disease, even with a nondiabetic cause, the cellular and molecular mechanisms in this association remain largely uncharacterized. Recently, increasing evidence suggests that peroxisome proliferator-activated receptors (PPARs), a subgroup of the nuclear hormone receptor superfamily of ligand-activated transcription factors, may play an important role in the pathogenesis of MetS. All three members of the PPAR nuclear receptor subfamily, PPARalpha, -beta/delta, and -gamma, are critical in regulating insulin sensitivity, adipogenesis, lipid metabolism, inflammation, and blood pressure. PPARs have also been implicated in many renal pathophysiological conditions, including diabetic nephropathy and glomerulosclerosis. Ligands for PPARs such as hypolipidemic PPARalpha activators, and antidiabetic thiazolidinedione PPARgamma agonists affect not only diverse aspects of MetS but also renal disease progression. Emerging data suggest that PPARs may be potential therapeutic targets for MetS and its related renal complications. This review focuses on current knowledge of the role of PPARs in MetS and discusses the potential therapeutic utility of PPAR modulators in the treatment of kidney diseases associated with MetS.

Knocking out peroxisome proliferator-activated receptor (PPAR) alpha inhibits radiation-induced apoptosis in the mouse kidney through activation of NF-kappaB and increased expression of IAPs

Peroxisome proliferator-activated receptor (PPAR) alpha, a member of the ligand-activated nuclear receptor superfamily, plays an important role in lipid metabolism and glucose homeostasis and is highly expressed in the kidney. The present studies were aimed at testing the hypothesis that PPARalpha knockout mice would exhibit decreased radiation-induced apoptosis due to exacerbated activation of NF-kappaB (NFKB) and expression of pro-survival factors. Thirty wild-type mice (29S1/SvImJ) and 30 PPARalpha knockout mice were irradiated with a single total-body dose 10 Gy of (137)Cs gamma rays; controls were sham-irradiated. Tissue samples were collected at 3, 6, 12, 24 and 48 h postirradiation. Apoptosis was quantified using immunohistochemical staining for apoptotic bodies and cleaved caspase 3. Radiation-induced apoptosis was observed in both mouse strains in a time-dependent manner. However, the level of apoptosis was significantly suppressed in PPARalpha knockout mice compared with wild-type mice at 6 h postirradiation (P < 0.05). This inhibition of radiation-induced apoptosis was associated with time-dependent increases in NF-kappaB DNA-binding activity, IkappaBalpha phosphorylation, and expression of other antiapoptosis factors in the PPARalpha knockout mouse kidneys but not in wild-type animals. These data support the hypothesis that the loss of PPARalpha expression leads to the suppression of radiation-induced apoptosis in the mouse kidney, mediated through activation of NF-kappaB and up-regulation of anti-apoptosis factors.

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.

PPAR-gamma agonist protects podocytes from injury

Podocyte injury and loss contribute to progressive glomerulosclerosis. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a nuclear hormone receptor, which we have found to be increased in podocytes in a variety of kidney diseases. It is not known if PPAR-gamma contributes to renal injury or if it serves as a countermeasure to limit renal injury during disease progression. We tested these possibilities utilizing the puromycin aminonucleoside (PAN) model of renal injury in immortalized mouse podocytes. The cultured podocytes expressed PPAR-gamma mRNA at baseline but this was decreased by PAN. Pioglitazone, a pharmacologic agonist of PPAR-gamma, increased both PPAR-gamma mRNA and activity in injured podocytes, as assessed by a reporter plasmid assay. Further, pioglitazone significantly decreased PAN-induced podocyte apoptosis and necrosis while restoring podocyte differentiation. The PPAR-gamma agonist significantly restored expression of the cyclin-dependent kinase inhibitor p27 and the antiapoptotic molecule Bcl-xL while significantly decreasing proapoptotic caspase-3 activity. Pioglitazone tended to decrease PAN-induced transforming growth factor-beta (TGF-beta) mRNA expression. Our study shows that PPAR-gamma is normally expressed by podocytes and its activation is protective against PAN-induced apoptosis and necrosis. We postulate that this protective effect may be mediated in part by effects on p27 and TGF-beta expression.

Thiazolidinedione ameliorates renal injury in experimental diabetic rats through anti-inflammatory effects mediated by inhibition of NF-kappaB activation

Thiazolidinedione (TZD), a ligand for peroxisome proliferator-activated receptor-gamma (PPAR-gamma), exerts anti-inflammatory effects independently of the insulin-sensitizing effect. In the present study, we tested the hypothesis that TZD prevents the progression of diabetic nephropathy by modulating the inflammatory process. Five-week-old Sprague-Dawley rats were divided into three groups: 1) nondiabetic control rats (non-DM), 2) diabetic rats (DM), and 3) diabetic rats treated with pioglitazone (DM+pio). Diabetes was induced by injection with streptozotocin (STZ). The DM+pio group received 0.0002% pioglitazone mixed in chow for 8 wk after induction of diabetes. Blood glucose and HbA1c were elevated in diabetic rats but did not change by treatment with pioglitazone. Pioglitazone reduced urinary albumin excretion and glomerular hypertrophy, suppressed the expression of transforming growth factor (TGF)-beta, type IV collagen, and ICAM-1, and infiltration of macrophages in the kidneys of diabetic rats. Furthermore, renal NF-kappaB activity was increased in diabetic rats and reduced by pioglitazone. PPAR-gamma was expressed in glomerular endothelial cells in the diabetic kidney and in cultured glomerular endothelial cells. High-glucose conditions increased the expression of ICAM-1 and the activation of NF-kappaB in cultured glomerular endothelial cells. These changes were reduced by pioglitazone, ciglitazone, and pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB. However, pioglitazone did not show the changes in the presence of PPAR-gamma antagonist GW9662. Our results suggest that the preventive effects of pioglitazone may be mediated by its anti-inflammatory actions, including inhibition of NF-kappaB activation, ICAM-1 expression, and macrophage infiltration in the diabetic kidney.

TGF-beta-regulated collagen type I accumulation: role of Src-based signals

Transforming growth factor-beta (TGF-beta) stimulates myofibroblast transdifferentiation, leading to type I collagen accumulation and fibrosis. We investigated the function of Src in TGF-beta-induced collagen I accumulation. In human mesangial cells, PTyr416 Src (activated Src) was 3.3-fold higher in TGF-beta-treated cells than in controls. Src activation by TGF-beta was blocked by rottlerin and by a dominant negative mutant of protein kinase Cdelta (PKCdelta), showing that TGF-beta activates Src by a PKCdelta-based mechanism. Pharmacological inhibitors and a dominant negative Src mutant prevented the increase in collagen type I secretion in cells exposed to TGF-beta. Similarly, on-target Src small interference RNA (siRNA) prevented type I collagen secretion in response to TGF-beta, but off-target siRNA complexes had no effect. It is well established in mesangial cells that upregulation of type I collagen by TGF-beta requires extracellular signal-regulated kinase 1/2 (ERK1/2), and we found that activation of ERK1/2 by TGF-beta requires Src. In conclusion, these results suggest that stimulation of collagen type I secretion by TGF-beta requires a PKCdelta-Src-ERK1/2 signaling motif.

Protection of the kidney by thiazolidinediones: An assessment from bench to bedside

The global epidemic of diabetes mellitus has led to a continuous increase in the prevalence of diabetic nephropathy over the past years. Thus, diabetic nephropathy is currently the number one cause of end-stage renal disease in the Western world. It represents a major public health problem for which more effective prevention and treatment strategies are needed. Thiazolidinediones (TZDs) are a class of agents that lower blood glucose through reduction of insulin resistance in patients with type 2 diabetes. Growing evidence support the concept that TZDs have several beneficial effects on the cardiovascular system beyond their effects on glycemic control. These benefits include: blood pressure lowering, triglyceride reduction, high-density lipoprotein-cholesterol elevation, and reduction in subclinical vascular inflammation. Moreover, data from several animal and human studies support the notion that TZDs reduce urine albumin excretion and may prevent development of renal injury. The relative lack of evidence, however, demonstrating the effects of TZDs on hard renal outcomes mandates the need for well-designed trials with this particular objective. This paper summarizes all the data from clinical and experimental studies relevant to a possible renoprotective effect of TZDs and discusses actions of these compounds that may contribute toward this effect.

Salutary effect of pigment epithelium-derived factor in diabetic nephropathy: evidence for antifibrogenic activities

Diabetic nephropathy is a major complication of diabetes and a leading cause of end-stage renal diseases in the U.S. Pigment epithelium-derived factor (PEDF) is a potent angiogenic inhibitor that has been extensively studied in diabetic retinopathy. Recently, we reported that PEDF is expressed at high levels in normal kidneys and that PEDF levels are decreased in kidneys of streptozotocin (STZ)-induced diabetic rats. In the present study, we injected STZ-diabetic rats with an adenovirus expressing PEDF (Ad-PEDF) to evaluate its effects in diabetes. The results showed that increased expression of PEDF in the kidney in response to Ad-PEDF delivery significantly alleviated microalbuminuria in early stages of diabetes. Administration of Ad-PEDF was found to prevent the overexpression of two major fibrogenic factors, transforming growth factor-beta (TGF-beta)1 and connective tissue growth factor (CTGF), and to significantly reduce the production of an extracellular matrix (ECM) protein in the diabetic kidney. Moreover, PEDF upregulated metalloproteinase-2 expression in diabetic kidney, which is responsible for ECM degradation. In cultured human mesangial cells, PEDF significantly inhibited the overexpression of TGF-beta1 and fibronectin induced by angiotensin II. PEDF also blocked the fibronectin production induced by TGF-beta1 through inhibition of Smad3 activation. These findings suggest that PEDF functions as an endogenous anti-TGF-beta and antifibrogenic factor in the kidney. A therapeutic potential of PEDF in diabetic nephropathy is supported by its downregulation in diabetes; its prevention of the overexpression of TGF-beta, CTGF, and ECM proteins in diabetic kidney; and its amelioration of proteinuria in diabetic rats following Ad-PEDF injection.

The complex role of PPARgamma in renal dysfunction in obesity: managing a Janus-faced receptor

Obesity is frequently accompanied by insulin resistance, type II diabetes, hypertension and atherosclerosis, a cluster of pathologies that are the major components of the metabolic syndrome. Obesity is a known cause for renal dysfunction that leads to two major renal pathologies: hypertension and glomerular and tubulointerstitial injury. Peroxizome proliferator activated receptors (PPARs) are transcription factors belonging to the nuclear hormone receptor superfamily with important functions in the regulation of metabolism. The role of PPARgamma isoforms in adipogenesis and vascular inflammation associated to obesity has been vastly studied and is well recognized, albeit not completely mechanistically understood. Also, the effect of various PPARgamma agonists on blood pressure reduction in different forms of hypertension, including obesity related hypertension has been reported, but the mechanisms involved are only beginning to be studied. Even less clear is the concurrent beneficial effect of PPARgamma agonists thiazolinendiones (TZD) on blood pressure reduction in different forms of hypertension and, at the same time, in some cases, the significant water retention leading to edema and heart failure. The occurrence of both these apparently opposite effects on the renal water and sodium handling suggests a complex role of PPARgamma in the kidney that is likely related to the metabolic state. Also, PPARgamma activation leads to a reduction in mesangial cell proliferation while stimulating apoptosis. TZD treatment reduces albuminuria in obese and diabetic humans and rodent models suggesting protective effects against renal tubuloglomerular injury. The focus of this review is to present and critically discuss the recent findings on the roles of PPARgamma in the kidney in direct relation to renal function and renal injury in obesity and obesity-initiated diabetes.

The PPARα ligand fenofibrate: meeting multiple targets in diabetic nephropathy

Fibrate peroxisome proliferator-activated receptor (PPAR)-alpha ligands are mainly used as hypolipidemic drugs. But this commentary highlights their potential in treating insulin resistance, dyslipidemia, and hypertension and in preventing diabetic nephropathy, inflammation, and cardiovascular disease. Because diabetes is a major contributor to chronic kidney disease and cardiovascular disease, PPAR-alpha agonists may provide greater opportunities for hitting multiple targets in this complex metabolic disease.

PPARα agonist fenofibrate improves diabetic nephropathy in db/db mice

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the ligand-activated nuclear receptor superfamily, and plays an important role in lipid metabolism and glucose homeostasis. The purpose of this study is to determine whether the activation of PPARalpha by fenofbrate would improve diabetes and its renal complications in type II diabetes mellitus. Male C57 BLKS db/db mice and db/m controls at 8 weeks of age were divided to receive either a regular diet chow (db/db, n=8; db/m, n=6) or a diet containing fenofibrate (db/db, n=8; db/m, n=7). Mice were followed for 8 weeks. Fenofibrate treatment dramatically reduced fasting blood glucose (P<0.001) and HbA1c levels (P<0.001), and was associated with decreased food intake (P<0.01) and slightly reduced body weight. Fenofibrate also ameliorated insulin resistance (P<0.001) and reduced plasma insulin levels (P<0.05) in db/db mice. Hypertrophy of pancreatic islets was decreased and insulin content markedly increased (P<0.05) in fenofibrate-treated diabetic animals. In addition, fenofibrate treatment significantly reduced urinary albumin excretion (P<0.001). This was accompanied by dramatically reduced glomerular hypertrophy and mesangial matrix expansion. Furthermore, the addition of fenofibrate to cultured mesangial cells, which possess functional active PPARalpha, decreased type I collagen production. Taken together, the PPARalpha agonist fenofibrate dramatically improves hyperglycemia, insulin resistance, albuminuria, and glomerular lesions in db/db mice. The activation of PPARalpha by fenofibrate in mesangial cells may partially contribute to its renal protection. Thus, fenofibrate may serve as a therapeutic agent for type II diabetes and diabetic nephropathy.

Involvement of PPAR nuclear receptors in tissue injury and wound repair

Tissue damage resulting from chemical, mechanical, and biological injury, or from interrupted blood flow and reperfusion, is often life threatening. The subsequent tissue response involves an intricate series of events including inflammation, oxidative stress, immune cell recruitment, and cell survival, proliferation, migration, and differentiation. In addition, fibrotic repair characterized by myofibroblast transdifferentiation and the deposition of ECM proteins is activated. Failure to initiate, maintain, or stop this repair program has dramatic consequences, such as cell death and associated tissue necrosis or carcinogenesis. In this sense, inflammation and oxidative stress, which are beneficial defense processes, can become harmful if they do not resolve in time. This repair program is largely based on rapid and specific changes in gene expression controlled by transcription factors that sense injury. PPARs are such factors and are activated by lipid mediators produced after wounding. Here we highlight advances in our understanding of PPAR action during tissue repair and discuss the potential for these nuclear receptors as therapeutic targets for tissue injury.

Protective effect of peroxisome proliferator activated receptor gamma agonists on diabetic and non-diabetic renal diseases

Peroxisome proliferator activated receptor gamma (PPARgamma) agonist has not only antidiabetic effect but also a protective effect against various types of injury of the kidney. The protective effects of PPARgamma agonists are observed in diabetic nephropathy and non-diabetic renal diseases such as 5/6 ablation model of renal failure, experimental glomerulonephritis, ischemia-reperfusion injury, hypertensive nephropathy and cyclosporin-induced renal injury. The mechanism of renoprotection by PPARgamma agonist is multifactorial. Anti-fibrotic and anti-inflammatory effects, suppression of renin-angiotensin system, vascular protective effect and antiapoptotic effect were proposed.

hepatocyte growth factor is a downstream effector that mediates the antifibrotic action of peroxisome proliferator-activated receptor-gamma agonists

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-dependent transcription factor that plays an important role in the regulation of insulin sensitivity and lipid metabolism. Evidence shows that PPAR-gamma agonists also ameliorate renal fibrotic lesions in both diabetic nephropathy and nondiabetic chronic kidney disease. However, little is known about the mechanism underlying their antifibrotic action. This study demonstrated that PPAR-gamma agonists could exert their actions by inducing antifibrotic hepatocyte growth factor (HGF) expression. Incubation of mesangial cells with natural or synthetic PPAR-gamma agonists 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) or troglitazone and ciglitazone suppressed TGF-beta1-mediated alpha-smooth muscle actin, fibronectin, and plasminogen activator inhibitor-1 expression. PPAR-gamma agonists also induced HGF mRNA expression and protein secretion. Transfection studies revealed that 15d-PGJ2 stimulated HGF gene promoter activity, which was dependent on the presence of a novel peroxisome proliferator response element. Treatment of mesangial cells with 15d-PGJ2 induced the binding of PPAR-gamma to the peroxisome proliferator response element in the HGF promoter region. PPAR-gamma agonists also activated c-met receptor tyrosine phosphorylation, induced Smad transcriptional co-repressor TG-interacting factor expression, and blocked TGF-beta/Smad-mediated gene transcription in mesangial cells. Furthermore, ablation of c-met receptor through the LoxP-Cre system in mesangial cells abolished the antifibrotic effect of 15d-PGJ2. PPAR-gamma activation also induced HGF expression in renal interstitial fibroblasts and repressed TGF-beta1-mediated myofibroblast activation. Both HGF and 15d-PGJ2 attenuated Smad nuclear translocation in response to TGF-beta1 stimulation in renal fibroblasts. Together, these findings suggest that HGF may act as a downstream effector that mediates the antifibrotic action of PPAR-gamma agonists.

Nuclear receptors and their coregulators in kidney

Nuclear receptors are transcription factors that are essential in embryonic development, maintenance of differentiated cellular phenotypes, metabolism, and apoptosis. Dysfunction of nuclear receptor signaling leads to a wide spectra of proliferative, reproductive, and metabolic diseases, including cancers, infertility, obesity, and diabetes. In addition, many proteins have been identified as coregulators which can be recruited by DNA-binding nuclear receptors to affect transcriptional regulation. The cellular level of coregulators is crucial for nuclear receptor-mediated transcription and many coregulators have been shown to be targets for diverse intracellular signaling pathways and posttranslational modifications. This review provides a general overview of the roles and mechanism of action of nuclear receptors and their coregulators. Since progression of renal diseases is almost always associated with inflammatory processes and/or involve metabolic disorders of lipid and glucose, cell proliferation, hypertrophy, apoptosis, and hypertension, the importance of nuclear receptors and their coregulators in these contexts will be addressed.

PPAR-gamma agonists and diabetic nephropathy

Diabetic nephropathy is a clinical syndrome of albuminuria, declining glomerular filtration rate, and increased risk of cardiovascular disease. Multiple mechanisms have been implicated in its pathogenesis. Although current therapies appear to be effective, treatment of diabetic nephropathy remains suboptimal. This review summarizes the recently emerging evidence suggesting that peroxisome proliferator-activated receptor-gamma agonists may prove to be effective therapeutic agents in the treatment of diabetic renal complications.

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.

Glomerular aging in females is a multi-stage reversible process mediated by phenotypic changes in progenitors

The glomeruli of postmenopausal C57BL6 mice, and age-matched males, show progressive hypertrophy and glomerulosclerosis. We asked whether this was a multistage process, was due to alterations in glomerular progenitors, and was reversible in female mice. Using cross bone marrow transplants (BMT) between young and old females, we found that BMT delivered a phenotype that was donor age-specific. The fact that lesions in young recipients were more severe if the donors were in late rather than early menopause suggested that new progenitor phenotypes had appeared. Postmenopausal recipients of BMT from young donors had reduced glomerular hypertrophy and sclerosis, implying that the aging lesions in females were reversible and that progenitors, rather than the local environment, determined the glomerular profile. The altered phenotype included increased extracellular matrix synthesis and decreased matrix metalloproteinase-2 levels as well as cell hypertrophy. The mechanism of the cellular hypertrophy was due to uncoupling of hypertrophy from proliferation, resulting from elevated p27 levels. Thus, glomerular hypertrophy and sclerosis in aging females is a multistage process, is reversible, and may be determined by the phenotype of bone marrow-derived progenitor cells.

Pioglitazone increases renal tubular cell albumin uptake but limits proinflammatory and fibrotic responses

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, which are known to be critical factors in lipid metabolism, have also been reported to reduce proteinuria. The mechanism and its relevance to progressive nephropathy have not been determined. The aims of this study were to assess the direct effects of a PPARgamma agonist on tubular cell albumin uptake, proinflammatory and profibrotic markers of renal pathology, using an opossum kidney model of proximal tubular cells.

METHODS

Cells were exposed to pioglitazone (10 micromol/L) in the presence and absence of low-density lipoprotein (LDL) 100 microg/mL +/- exposure to albumin 1 mg/mL. Results were expressed relative to control (5 mmol/L glucose) conditions.

RESULTS

Pioglitazone caused a dose-dependent increase in tubular cell albumin uptake (P < 0.0001). Despite the increase in albumin reabsorption, no concurrent increase in inflammatory or profibrotic markers were observed. Exposure to LDL increased monocyte chemoattractant protein-1 (MCP-1) (P < 0.05) and transforming growth factor-beta1 (TGF-beta1) (P < 0.05) production, which were reversed in the presence of pioglitazone. LDL induced increases in MCP-1 and TGF-beta1 were independent of nuclear factor-kappaB (NF-kappaB) transcriptional activity. In contrast, tubular exposure to albumin increased tubular protein uptake, in parallel with an increase in MCP-1 (P= 0.05), TGF-beta1 (P < 0.02) and NF-kappaB transcriptional activity (P < 0.05), which were unaffected by concurrent exposure to pioglitazone.

CONCLUSION

These findings suggest that dyslipidemia potentiates renal pathology through mechanisms that may be modified by PPARgamma activation independent of NF-kappaB transcriptional activity. In contrast, tubular exposure to protein induces renal damage through NF-kappaB-dependent mechanisms that are unaffected by PPARgamma activation.

The glomerulosclerosis of aging in females: contribution of the proinflammatory mesangial cell phenotype to macrophage infiltration

Age-associated renal changes may be an important cause of renal failure. We recently found that aged female B6 mice developed progressive glomerular lesions. This was associated with macrophage infiltration, a frequent finding in glomerulosclerosis. We used these mice as a model for studying the mechanisms of glomerular aging. We compared the gene expression profile of intact glomeruli from late postmenopausal (28-month-old) mice to that of intact glomeruli from premenopausal (5-month-old) mice. We found that inflammation-related genes, especially those expressed by activated macrophages, were up-regulated in the glomeruli of 28-month-old mice, a result correlating with the histological observation of glomerular macrophage infiltration. The mechanism for macrophage recruitment could have been stable phenotypic changes in mesangial cells because we found that mesangial cells isolated from 28-month-old mice expressed higher levels of RANTES and VCAM-1 than cells from 5-month-old mice. The elevated serum tumor necrosis factor (TNF)-alpha levels present in aged mice may contribute to increased RANTES and VCAM-1 expression in mesangial cells. Furthermore, cells from 28-month-old mice were more sensitive to TNF-alpha-induced RANTES and VCAM-1 up-regulation. The effect of TNF-alpha on RANTES expression was mediated by TNF receptor 1. Interestingly, mesangial cells isolated from 28-month-old mice had increased nuclear factor-kappaB transcriptional activity. Inhibition of nuclear factor-kappaB activity decreased baseline as well as TNF-alpha-induced RANTES and VCAM-1 expression in mesangial cells isolated from 28-month-old mice. Thus, phenotypic changes in mesangial cells may predispose them to inflammatory stimuli, such as TNF-alpha, which would contribute to glomerular macrophage infiltration and inflammatory lesions in aging.

Pleiotropic effects of thiazolidinediones: taking a look beyond antidiabetic activity

Thiazolidinediones (TZD) [Troglitazone (TRO), Pioglitazone (PGZ), Rosiglitazone, (RGZ)] are a novel class of antidiabetic drugs for patients with Type-2 diabetes mellitus (T2DM) able to decrease blood glucose, working through a reduction of insulin resistance. The family of TZD exerts its effect specifically bound to peroxisome proliferator-activated receptor y (PPARy). This is a member of the nuclear hormone receptor superfamily of ligand-dependent transcription factors, together with PPARalpha and deltabeta. Although PPARgamma is essentially expressed in adipose tissue, it has also been found in endothelial cells, macrophages, vascular smooth muscle cells, glomerular mesangial cells, hepatic stellate cells and in several cancer cell lines. In these cells, the PPARgamma activation by TZD determines modulatory effects on growth factor release, production of cytokine, cell proliferation and migration, extracellular matrix remodeling and control on cell cycle progression and differentiation. In addition, TZD have been shown to have a potent antioxidant effect. This review, taking a quick look beyond the antidiabetic activity of PPARgamma, shows the dramatic ranging of medical implications that the use of TZD could have modulating the PPARgamma activity in several diseases with a strong social impact, such as insulin resistance syndrome, chronic inflammation, atherosclerosis and cancer.

Three-dimensional biocompatible ascorbic acid-containing scaffold for bone tissue engineering

A biodegradable, biocompatible, ascorbic acid-containing three-dimensional polyurethane matrix was developed for bone tissue-engineering scaffolds. This matrix was synthesized with lysine-di-isocyanate (LDI), ascorbic acid (AA), glycerol, and polyethylene glycol (PEG). LDI-glycerol-PEG-AA prepolymer when reacted with water foamed with the liberation of CO(2) to provide a pliable, spongy urethane polymer with pore diameters of 100 to 500 microm. The LDI-glycerol-PEG-AA matrix degraded in aqueous solution and yielded lysine, glycerol, PEG, and ascorbic acid as breakdown products. The degradation products did not significantly affect the solution pH. The LDI-glycerol-PEG-AA matrix can be fabricated into diverse scaffold dimensions and the physicochemical properties of the polymer network supported in vitro cell growth. Green fluorescent protein-transgenic mouse bone marrow cells (GFP-MBMCs) attached to the polymer matrix and remained viable, and the cells became confluent cultures. Furthermore, ascorbic acid released from LDI-glycerol-PEG-AA matrix stimulated cell proliferation, type I collagen, and alkaline phosphatase synthesis in vitro. Cells grown on LDI-glycerol-PEG-AA matrix did not differ phenotypically from cells grown on tissue culture polystyrene plates as assessed by cell growth, expression of mRNA for collagen type I, and transforming growth factor beta(1). These observations suggest that AA-containing polyurethane may be useful in bone tissue-engineering applications.

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

OBJECTIVE

In fibroblasts, transforming growth factor beta (TGF beta) stimulates collagen synthesis and myofibroblast transdifferentiation through the Smad intracellular signal transduction pathway. TGF beta-mediated fibroblast activation is the hallmark of scleroderma and related fibrotic conditions, and disrupting the intracellular TGF beta/Smad signaling may provide a novel approach to controlling fibrosis. Because of its potential role in modulating inflammatory and fibrotic responses, we examined the expression of the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) in normal skin fibroblasts and its effect on TGF beta-induced cellular responses.

METHODS

The expression and activity of PPAR gamma in normal dermal fibroblasts were examined by Northern and Western blot analyses, immunocytochemistry, flow cytometry, and transient transfections with reporter constructs. The same approaches were used to evaluate the effects of PPAR gamma activation by naturally occurring and synthetic ligands on collagen synthesis and alpha-smooth muscle actin (alpha-SMA) expression. Modulation of Smad-mediated transcriptional responses was examined by transient transfection assays using wild-type and dominant-negative PPAR gamma expression constructs.

RESULTS

The PPAR gamma receptor was expressed and fully functional in quiescent normal skin fibroblasts. Whereas ligand activation of cellular PPAR gamma resulted in modest suppression of basal collagen gene expression, it abrogated TGF beta-induced stimulation in a concentration-dependent manner. This response was mimicked by overexpressing PPAR gamma in fibroblasts, and was blocked by a selective antagonist of PPAR gamma signaling or by transfection of fibroblasts with dominant-negative PPAR gamma constructs. Furthermore, PPAR gamma ligands abrogated TGF beta-induced expression of alpha-SMA, a marker of myofibroblasts. Stimulation of Smad-dependent transcriptional responses by TGF beta was suppressed by PPAR gamma despite the absence of a consensus PPAR gamma-response element in the targeted promoters. Ligand-induced activation of fibroblast PPAR gamma had no effect on protein expression of cellular Smad3 or Smad7.

CONCLUSION

By abrogating of TGF beta-induced stimulation of collagen gene expression, myofibroblast transdifferentiation, and Smad-dependent promoter activity in normal fibroblasts, PPAR gamma may play a physiologic role in the regulation of the profibrotic response. Furthermore, our results suggest that PPAR gamma activation by pharmacologic agonists may represent a novel approach to the control of fibrosis in scleroderma.

Expression of Peroxisome Proliferator-Activated Receptor Isoform Proteins in the Rat Kidney

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors mediating ligand-dependent transactivation. Among the 3 isoforms, PPAR- alpha is involved in lipid metabolism in the liver, while PPAR-gamma(-gamma1 and -gamma2) is involved in adipocyte differentiation. Recently, PPARs have been suggested to be involved in renal electrolyte metabolism as well as atherosclerosis. PPAR-alpha is known to regulate cytochrome P450 gene expression, and may possibly affect sodium retention in the kidney. Moreover, PPAR-gamma is involved in the transcription regulation of blood pressure regulatory genes, including thromboxane and angiotensin II type 1 receptors. In the kidney, although expression of PPARs has been reported, detailed immunohistochemical analyses have not been performed. We here generated isoform-specific anti-PPAR antibodies to localize their proteins in the kidney. Anti-PPAR antibodies were raised against synthetic peptides. Their isoform specificity was confirmed by immunoblot analyses, immunoprecipitations, and antibody supershift experiments by electrophoretic mobility shift assay. We therefore studied the protein expression of PPARs in the kidney of adult Sprague-Dawley rats using these antibodies. Immunoblot analyses demonstrated protein expression of PPAR-alpha and -gamma1, but not of -gamma2, in the kidney nuclear extracts. Immunohistochemical analyses demonstrated that both PPAR-alpha and -gamma1 proteins were widely expressed in the nuclei of mesangial and epithelial cells in glomeruli, proximal and distal tubules, the loop of Henle, medullary collecting ducts, and intima/media of renal vasculatures. PPAR-alpha and -gamma1 proteins are thus widely expressed along the nephron segments, and may affect gene expression at these segments. Further studies will be needed to identify additional target genes for PPARs along the nephron segments.

Growth factors and the development of diabetic nephropathy

Growth factors play an important role in the development of functional and structural changes associated with diabetic nephropathy. Although it has been known for years that these factors are important for early renal hypertrophy and subsequent development of glomerulosclerosis and tubulointerstitial fibrosis, the exact molecular mechanism of many of these factors has only recently been more elucidated. Furthermore, growth factors also link the metabolic theory of diabetic complications with renal hemodynamic changes in diabetic nephropathy because some growth factors could directly influence glomerular hemodynamics and tubular transport in diabetic nephropathy. The high glucose environment with stimulated cellular uptake of glucose and accelerated nonenzymatic reactions resulting in Amadori-modified proteins and the later-developing advanced glycation end products are the main stimulators for intrarenal induction of growth factors. Intracellular generation of reactive species is an important signal intermediate in these stimulated expressions of growth factors. Taking into consideration the pivotal role of growth factors in the development of diabetic nephropathy, a therapeutic strategy to antagonize growth factor effects appears to be straightforward. However, the pleiotropic function of many of these factors and their physiologic role in normal renal homeostasis may make this approach difficult.

A biodegradable polyurethane-ascorbic acid scaffold for bone tissue engineering

A novel, nontoxic, biodegradable, sponge-like polyurethane scaffold was synthesized from lysine-di-isocyanate (LDI) and glycerol. Ascorbic acid (AA) was copolymerized with LDI-glycerol. Our hypothesis was that the AA-containing polymer foam would enhance the biological activity of the osteoblastic precursor cell (OPCs). The LDI-glycerol-AA matrix degraded in aqueous solution to the nontoxic products of lysine, glycerol, and AA. The degradation products did not significantly affect the solution pH. The physical properties of the polymer network supported the cell growth in vitro. Mouse OPCs attached to the polymer matrix and remained viable. OPCs produced multilayered confluent cultures, a characteristic typical of bone cells. Furthermore, AA release stimulated cell proliferation, type I collagen, and alkaline phosphatase synthesis. Cells grown on the LDI-glycerol-AA matrix also showed an enhancement of mRNA expression for pro-alpha1 (I) collagen and transforming growth factor-alpha1 after 1 week. Data were tested for significance with an analysis of variance model and multiple comparison test (Fisher's Protected Least Significant Difference) at p < or = 0.05. The observations suggest that AA-containing polyurethane may be useful in bone tissue engineering applications.

c-Fos-driven transcriptional activation of transforming growth factor beta-1: inhibition of high glucose-induced promoter activity by thiazolidinediones

The peroxisome proliferator-activated receptor gamma activating compounds thiazolidinedione (TZD) have been shown to inhibit diabetes-induced glomerular transforming growth factor-beta1 (TGF-beta1) expression, thereby ameliorating diabetic nephropathy. Here we examined the hypothesis that TZDs block high glucose-induced TGF-beta1 gene activation by interaction with the activated protein kinase C-c-Fos-TGF-beta1 promoter cascade in mesangial cells. The TZD compounds troglitazone and rosiglitazone completely prevented the high glucose induction of both TGF-beta1 promoter activity and elevation in nuclear c-Fos protein levels. The scavenging properties of troglitazone were shown not to be responsible for this inhibitory action, because hydrogen peroxide-mediated stimulation of TGF-beta1 promoter activity was not blocked. TZD-treatment did not interfere with the transcriptional activity of c-Fos responsible for stimulation of the TGF-beta1 promoter. The findings suggest a molecular mechanism by which TZD-treatment reduces specifically high glucose-induced, c-Fos-mediated gene activation, since phorbol ester-induced c-Fos mRNA and protein expression and subsequent elevation of TGF-beta1 mRNA expression were not prevented by TZDs.

Diabetes and nephropathy

PURPOSE OF REVIEW

Diabetic nephropathy is the single most common disorder leading to renal failure. Its annual incidence has more than doubled in the past decade to reach 44% of all end-stage renal disease, despite recent therapeutic advances. Thus, research into diabetic nephropathy pathophysiology that could lead to new treatment approaches is urgently needed and this review aims to summarize the work performed in this area in the past year.

RECENT FINDINGS

There have been advances in the understanding of diabetic nephropathy pathology. Clearly, structural changes may be advanced before any clinical findings are apparent. Not all functional consequences of the condition are explained by current structural analyses. Genetic studies have connected the disorder risk to multiple candidate genes and a few genetic loci, but the exact genetic predisposition or protectors are not fully described. Perturbations in multiple metabolic pathways are associated with diabetic nephropathy in animals and humans, but their relative importance requires further work. Glycemia and blood pressure control are crucial for diabetic nephropathy prevention and treatment, but new modalities are needed.

SUMMARY

Recent advances in molecular biology and genetics will bring new insights to the mechanisms involved in diabetic nephropathy development. This will allow early identification of patients at risk of, or safe from, diabetic nephropathy and will hopefully lead to preventive strategies, based on the understanding of the pathophysiology of the disorder. Meanwhile, aggressive implementation of proven therapies to prevent (glycemic control) and slow (antihypertensive therapy, especially with renin-angiotensin system blockers) the progression of diabetic nephropathy are strongly recommended.

Resistance to glomerulosclerosis in B6 mice disappears after menopause

The frequency of chronic renal failure increases with age, especially in women after menopause. Glomerulosclerosis is a common cause of chronic renal failure in aging. We reported that pre-menopausal female C57BL6 (B6) mice are resistant to glomerulosclerosis, irrespective of the type of injury. However, we now show that B6 mice develop progressive glomerulosclerosis after menopause. Glomerular lesions, first recognized in 18-month-old mice, consisted of hypertrophy, vascular pole sclerosis, and mesangial cell proliferation. Diffuse but moderate mesangial sclerosis and more marked hypertrophy were present at 22 months. At 28 to 30 months the glomerulosclerosis was diffuse and increased levels of type I and type IV collagen and transforming growth factor-beta 1 mRNA were present. Urine albumin excretion was significantly increased in 30-month-old mice. Mesangial cells isolated from 28-month-old mice retained their sclerotic phenotype in vitro. Comparison of the effects of uninephrectomy (Nx) in 20-month-old and 2.5-month-old mice revealed a 1.7-fold increase in urine albumin excretion, accelerated glomerulosclerosis, and renal function insufficiency in 20-month-old Nx mice, but not in 2.5-month-old Nx mice. Glycemic levels, glucose, insulin tolerance, and blood pressure were normal at all ages. Thus, B6 mice model the increased frequency of chronic renal failure in postmenopausal women and provide a model for studying the mechanism(s) of glomerulosclerosis in aging women.