Peroxisome proliferator activated receptor alpha/gamma dual agonist tesaglitazar attenuates diabetic nephropathy in db/db mice

Lipid Toxicity in the Cardiovascular-Kidney-Metabolic Syndrome (CKMS)

Recent studies of Cardiovascular-Kidney-Metabolic Syndrome (CKMS) indicate that elevated concentrations of derivatives of phospholipids (ceramide, sphingosine), oxidized LDL, and lipoproteins (a, b) are toxic to kidney and heart function. Energy production for renal proximal tubule resorption of critical fuels and electrolytes is required for homeostasis. Cardiac energy for ventricular contraction/relaxation is preferentially supplied by long chain fatty acids. Metabolism of long chain fatty acids is accomplished within the cardiomyocyte cytoplasm and mitochondria by means of the glycolytic, tricarboxylic acid, and electron transport cycles. Toxic lipids and excessive lipid concentrations may inhibit cardiac function. Cardiac contraction requires calcium movement from the sarcoplasmic reticulum from a high to a low concentration at relatively low energy cost. Cardiac relaxation involves calcium return to the sarcoplasmic reticulum from a lower to a higher concentration and requires more energy consumption. Diastolic cardiac dysfunction occurs when cardiomyocyte energy conversion is inadequate. Diastolic dysfunction from diminished ATP availability occurs in the presence of inadequate blood pressure, glycemia, or lipid control and may lead to heart failure. Similar disruption of renal proximal tubular resorption of fuels/electrolytes has been found to be associated with phospholipid (sphingolipid) accumulation. Elevated concentrations of tissue oxidized low-density lipoprotein cholesterols are associated with loss of filtration efficiency at the level of the renal glomerular podocyte. Macroscopically excessive deposits of epicardial and intra-nephric adipose are associated with vascular pathology, fibrosis, and inhibition of essential functions in both heart and kidney. Chronic triglyceride accumulation is associated with fibrosis of the liver, cardiac and renal structures. Successful liver, kidney, or cardiac allograft of these vital organs does not eliminate the risk of lipid toxicity. Lipid lowering therapy may assist in protecting vital organ function before and after allograft transplantation.

AdipoRon ameliorates the progression of heart failure with preserved ejection fraction via mitigating lipid accumulation and fibrosis

INTRODUCTION

Obesity and imbalance in lipid homeostasis contribute greatly to heart failure with preserved ejection fraction (HFpEF), the dominant form of heart failure. Few effective therapies exist to control metabolic alterations and lipid homeostasis.

OBJECTIVES

We aimed to investigate the cardioprotective roles of AdipoRon, the adiponectin receptor agonist, in regulating lipid accumulation in the two-hit HFpEF model.

METHODS

HFpEF mouse model was induced using 60 % high-fat diet plus L-NAME drinking water. Then, AdipoRon (50 mg/kg) or vehicle were administered by gavage to the two-hit HFpEF mouse model once daily for 4 weeks. Cardiac function was evaluated using echocardiography, and Postmortem analysis included RNA-sequencing, untargeted metabolomics, transmission electron microscopy and molecular biology methods.

RESULTS

Our study presents the pioneering evidence that AdipoR was downregulated and impaired fatty acid oxidation in the myocardia of HFpEF mice, which was associated with lipid metabolism as indicated by untargeted metabolomics. AdipoRon, orally active synthetic adiponectin receptor agonist, could upregulate AdipoR1/2 (independently of adiponectin) and reduce lipid droplet accumulation, and alleviate fibrosis to restore HFpEF phenotypes. Finally, AdipoRon primarily exerted its effects through restoring the balance of myocardial fatty acid intake, transport, and oxidation via the downstream AMPKα or PPARα signaling pathways. The protective effects of AdipoRon in HFpEF mice were reversed by compound C and GW6471, inhibitors of AMPKα and PPARα, respectively.

CONCLUSIONS

AdipoRon ameliorated the HFpEF phenotype by promoting myocardial fatty acid oxidation, decreasing fatty acid transport, and inhibiting fibrosis via the upregulation of AdipoR and the activation of AdipoR1/AMPKα and AdipoR2/PPARα-related downstream pathways. These findings underscore the therapeutic potential of AdipoRon in HFpEF. Importantly, all these parameters get restored in the context of continued mechanical and metabolic stressors associated with HFpEF.

Alternatively Spliced Landscape of PPARγ mRNA in Podocytes Is Distinct from Adipose Tissue

Podocytes are highly differentiated epithelial cells, and their structural and functional integrity is compromised in a majority of glomerular and renal diseases, leading to proteinuria, chronic kidney disease, and kidney failure. Traditional agonists (e.g., pioglitazone) and selective modulators (e.g., GQ-16) of peroxisome-proliferator-activated-receptor-γ (PPARγ) reduce proteinuria in animal models of glomerular disease and protect podocytes from injury via PPARγ activation. This indicates a pivotal role for PPARγ in maintaining glomerular function through preservation of podocytes distinct from its well-understood role in driving insulin sensitivity and adipogenesis. While its transcriptional role in activating adipokines and adipogenic genes is well-established in adipose tissue, liver and muscle, understanding of podocyte PPARγ signaling remains limited. We performed a comprehensive analysis of PPARγ mRNA variants due to alternative splicing, in human podocytes and compared with adipose tissue. We found that podocytes express the ubiquitous PPARγ Var 1 (encoding γ1) and not Var2 (encoding γ2), which is mostly restricted to adipose tissue and liver. Additionally, we detected expression at very low level of Var4, and barely detectable levels of other variants, Var3, Var11, VartORF4 and Var9, in podocytes. Furthermore, a distinct podocyte vs. adipocyte PPAR-promoter-response-element containing gene expression, enrichment and pathway signature was observed, suggesting differential regulation by podocyte specific PPARγ1 variant, distinct from the adipocyte-specific γ2 variant. In summary, podocytes and glomeruli express several PPARγ variants, including Var1 (γ1) and excluding adipocyte-specific Var2 (γ2), which may have implications in podocyte specific signaling and pathophysiology. This suggests that that new selective PPARγ modulators can be potentially developed that will be able to distinguish between the two forms, γ1 and γ2, thus forming a basis of novel targeted therapeutic avenues.

Protocol for a preclinical systematic review and meta-analysis of pharmacological targeting of peroxisome proliferator-activated receptors in experimental renal injury

INTRODUCTION

Impaired lipid metabolism in the renal tubule plays a prominent role in the progression of renal fibrosis following acute kidney injury (AKI) and in chronic kidney disease (CKD). Peroxisome proliferator-activated receptors (PPARs) are promising druggable targets to mitigate renal fibrosis by redirecting metabolism, including restoration of fatty acid oxidation (FAO) capacity. We aim to synthesise evidence from preclinical studies of pharmacological PPAR targeting in experimental renal injury, and inform the design of future studies evaluating PPAR-mediated restoration of FAO in AKI and CKD.

METHODS AND ANALYSIS

Studies reporting on the impact of pharmacological PPAR modulation in animal models of renal injury will be collected from MEDLINE (Ovid), Embase and Web of Science databases. Predefined eligibility criteria will exclude studies testing medications which are not specific ligands of one or more PPARs and studies involving multimodal pharmacological treatment. The Systematic Review Centre for Laboratory Animal Experimentation risk of bias tool and Collaborative Approach to Meta-Analysis and Review of Animal Experimental Studies checklist will be used to assess quality of the included studies. Data extraction will be followed by a narrative synthesis of the data and meta-analysis where feasible. Analysis will be performed separately for AKI, CKD and renal transplant models. Subgroup analyses will be performed based on study design characteristics, PPAR isotype(s) targeted, and classes of PPAR-targeting medications used. Risk of publication bias will be assessed using funnel plotting, Egger's regression and trim-and-fill analysis.

ETHICS AND DISSEMINATION

Ethical approval is not required. Findings will be published in a peer-reviewed journal and presented at scientific meetings.

PROSPERO REGISTRATION NUMBER

CRD42021265550.

PEGylated AdipoRon derivatives improve glucose and lipid metabolism under insulinopenic and high-fat diet conditions

The pleiotropic actions of adiponectin in improving cell survival and metabolism have motivated the development of small-molecule therapeutic agents for treating diabetes and lipotoxicity. AdipoRon is a synthetic agonist of the adiponectin receptors, yet is limited by its poor solubility and bioavailability. In this work, we expand on the protective effects of AdipoRon in pancreatic β-cells and examine how structural modifications could affect the activity, pharmacokinetics, and bioavailability of this small molecule. We describe a series of AdipoRon analogs containing amphiphilic ethylene glycol (PEG) chains. Among these, AdipoRonPEG5 induced pleiotropic effects in mice under insulinopenic and high-fat diet (HFD) conditions. While both AdipoRon and AdipoRonPEG5 substantially attenuate palmitate-induced lipotoxicity in INS-1 cells, only AdipoRonPEG5 treatment is accompanied by a significant reduction in cytotoxic ceramides. In vivo, AdipoRonPEG5 can substantially reduce pancreatic, hepatic, and serum ceramide species, with a concomitant increase in the corresponding sphingoid bases and improves insulin sensitivity of mice under HFD feeding conditions. Furthermore, hyperglycemia in streptozotocin (STZ)-induced insulinopenic adiponectin-null mice is also attenuated upon AdipoRonPEG5 treatment. Our results suggest that AdipoRonPEG5 is more effective in reducing ceramides and dihydroceramides in the liver of HFD-fed mice than AdipoRon, consistent with its potent activity in activating ceramidase in vitro in INS-1 cells. Additionally, these results indicate that the beneficial effects of AdipoRonPEG5 can be partially attributed to improved pharmacokinetics as compared with AdipoRon, thus suggesting that further derivatization may improve affinity and tissue-specific targeting.

New insights into renal lipid dysmetabolism in diabetic kidney disease

Lipid dysmetabolism is one of the main features of diabetes mellitus and manifests by dyslipidemia as well as the ectopic accumulation of lipids in various tissues and organs, including the kidney. Research suggests that impaired cholesterol metabolism, increased lipid uptake or synthesis, increased fatty acid oxidation, lipid droplet accumulation and an imbalance in biologically active sphingolipids (such as ceramide, ceramide-1-phosphate and sphingosine-1-phosphate) contribute to the development of diabetic kidney disease (DKD). Currently, the literature suggests that both quality and quantity of lipids are associated with DKD and contribute to increased reactive oxygen species production, oxidative stress, inflammation, or cell death. Therefore, control of renal lipid dysmetabolism is a very important therapeutic goal, which needs to be archived. This article will review some of the recent advances leading to a better understanding of the mechanisms of dyslipidemia and the role of particular lipids and sphingolipids in DKD.

Danthron ameliorates obesity and MAFLD through activating the interplay between PPARα/RXRα heterodimer and adiponectin receptor 2

Obesity and associated metabolic associated fatty liver diseases (MAFLD) are strongly associated with dysfunction of glucose and lipid metabolism. AMPKα and PPARα are key regulators in the lipid and glucose homeostasis, indicating that novel agents to activate them are promising therapeutic approaches for metabolic syndrome. Noticeably, as a natural anthraquinone derivative extracted from rhubarb, danthron can activate AMPKα in vitro. However, the protective effect of danthron on obesity and associated MAFLD in vivo, as well as the underlying mechanism remains unknown. In this study, obesity and associated MAFLD was induced in C57BL/6J mice by high fat diet (HFD), which were subjected to evaluations on the parameters of systematic metabolism. Simultaneously, the molecular mechanism of danthron on lipid metabolism was investigated in 3T3-L1-derived adipocytes and HepG2 cells in vitro. In vivo, danthron significantly attenuated the obesity and MAFLD by enhancing hepatic fatty acid oxidation, decreasing lipid synthesis, and promoting mitochondrial homeostasis. Mechanistically, danthron significantly promoted combination of RXRα and PPARα, enhanced the binding of RXRα/PPARα heterodimer to the promoter of adiponectin receptor 2 (AdipoR2), by which activating the AMPKα and PPARα pathway. Moreover, PPARα and AdipoR2 can interplay in a loop style. Collectively, this study demonstrates that danthron can substantially ameliorate obesity and associated hepatic steatosis via AdipoR2-mediated dual PPARα/AMPKα activation, which suggests that danthron might be a novel therapeutic approach for inhibition of obesity and hepatic steatosis.

Nuclear receptors in podocyte biology and glomerular disease

Nuclear receptors have a broad spectrum of biological functions in normal physiology and in the pathology of various diseases, including glomerular disease. The primary therapies for many glomerular diseases are glucocorticoids, which exert their immunosuppressive and direct podocyte protective effects via the glucocorticoid receptor (GR). As glucocorticoids are associated with important adverse effects and a substantial proportion of patients show resistance to these therapies, the beneficial effects of selective GR modulators are now being explored. Peroxisome proliferator-activated receptor-γ (PPARγ) agonism using thiazolidinediones has potent podocyte cytoprotective and nephroprotective effects. Repurposing of thiazolidinediones or identification of novel PPARγ modulators are potential strategies to treat non-diabetic glomerular disease. Retinoic acid receptor-α is the key mediator of the renal protective effects of retinoic acid, and repair of the endogenous retinoic acid pathway offers another potential therapeutic strategy for glomerular disease. Vitamin D receptor, oestrogen receptor and mineralocorticoid receptor modulators regulate podocyte injury in experimental models. Further studies are needed to better understand the mechanisms of these nuclear receptors, evaluate their synergistic pathways and identify their novel modulators. Here, we focus on the role of nuclear receptors in podocyte biology and non-diabetic glomerular disease.

Fenofibrate decreased microalbuminuria in the type 2 diabetes patients with hypertriglyceridemia

Background

This study was to research the efficacy of fenofibrate in the treatment of microalbuminuria in the patients with type 2 diabetes mellitus (T2DM) and hypertriglyceridemia.

Methods

Type 2 diabetic patients (56) with microalbuminuria and hypertriglyceridemia aged 30 to 75 were randomly divided into the fenofibrate treatment group(n = 28) and the control group (n = 28) for 180 days. Urinary microalbumin /creatinine ratio (UACR) and other metabolic parameters were compared at baseline, during treatment and after treatment.

Results

After 180 days, the reduction of level of fasting blood glucose (FBG) and glycosylated hemoglobin (HbA1c) between two groups showed no difference. In the treatment group, uric acid (UA) (296.42 ± 56.41 vs 372.46 ± 72.78), triglyceride (TG) [1.51(1.17, 2.06) vs 3.04(2.21, 3.29)], and UACR [36.45 (15.78,102.41) vs 129.00 (53.00, 226.25)] were significantly decreased compared with the baseline. The high-density lipoprotein cholesterol (HDL-C) levels were significantly increased (1.22 ± 0.26 vs 1.09 ± 0.24) compared with the baseline. The decrease in UACR [− 44.05(− 179.47, − 12.16) vs − 8.15(− 59.69, 41.94)]in treatment group was significantly higher compared with the control group. The decrease in UACR was positively associated with the decreases in TG (r = 0.447, P = 0.042) and UA (r = 0.478, P = 0.024) after fenofibrate treatment.

Conclusion

In the patients with hypertriglyceridemia and type 2 diabetes mellitus, fenofibrate can improve microalbuminuria and do not increase the deterioration of glomerular filtration rate.

Trial registration

ClinicalTrials.gov identifier: NCT02314533, 2014.12.9

An In Silico Approach Towards Investigation of Possible Effects of Essential Oils Constituents on Receptors Involved in Cardiovascular Diseases (CVD) and Associated Risk Factors (Diabetes Mellitus and Hyperlipidemia)

AIM

Aromatherapy products, hydrosol beverages and distillates containing essential oils are widely used for cardiovascular conditions. Investigation of the possible activity of their major constituents with the cardiovascular-related receptors may lead to developing new therapeutics. It also may prevent unwanted side effects and drug-herb interactions.

MATERIALS AND METHODS

A list of 243 volatile molecules (mainly monoterpene and sesquiterpene) was prepared from a literature survey in Scopus and PubMed (2000-2019) on hydrosols and essential oils which are used for Cardiovascular Diseases (CVD) and its risk factors (diabetes mellitus and hyperlipidemia). The PDB files of the receptors (229 native PDB files) included alpha-glucosidase, angiotensin- converting enzymes, beta-2 adrenergic receptor, glucocorticoid, HMG-CoA reductase, insulin, mineralocorticoid, potassium channel receptors and peroxisome proliferator-activated receptoralpha, were downloaded from Protein Data Bank. An in silico study using AutoDock 4.2 and Vina in parallel mode was performed to investigate possible interaction of the molecules with the receptors. Drug likeliness of the most active molecules was investigated using DruLiTo software.

RESULTS

Spathulenol, bisabolol oxide A, bisabolone oxide, bergapten, bergamotene, dill apiole, pcymene, methyl jasmonate, pinocarveol, intermedeol, α-muurolol, S-camphor, ficusin, selinen-4-ol, iso-dihydrocarveol acetate, 3-thujanone, linanool oxide and cadinol isomers made a better interaction with some of the named receptors. All of the named molecules had an acceptable dug likeliness except for α-bergamotene. In addition, all of the named molecules had the ability to pass the bloodbrain barrier and it is possible to produce unwanted side effects.

CONCLUSION

Some ingredients of essential oils might be active on cardiovascular-related receptors.

White sweet potato ameliorates hyperglycemia and regenerates pancreatic islets in diabetic mice

Background

White sweet potato (WSP) has many potential beneficial effects on metabolic control and on diabetes-related insulin resistance. The antihyperglycemic effects of Tainung No. 10 (TNG10), a variety of WSP in Taiwan, warrant investigation.

Objective

To investigate the antidiabetic activity of WSP (Ipomoea batatas L. TNG10) and the mechanisms for interventions using whole leaves or tubers of WSP in diabetic mice.

Design

Mice were co-administered with streptozotocin and nicotinamide to induce diabetes and then treated with an experimental diet including either 10% WSP tuber (10%-T) and 30% WSP tuber (30%-T) or 0.5% WSP leaf (0.5%-L) and 5% WSP leaf (5%-L). After 8 weeks’ treatment, their plasma glycemic parameters, lipid profiles, and inflammatory marker were analyzed. Their pancreases were removed for histopathologic image analysis; proteins were also extracted from their muscles for phosphoinositide 3-kinase pathway analysis.

Results

The 30%-T or 5%-L mice had lower plasma glucose, insulin, glucose area under the curve (AUC), homeostatic model assessment of insulin resistance (HOMA-IR), alanine transaminase, triglyceride, and tumor necrosis factor alpha levels. In all diabetic mice, their Langerhans’s area was reduced by 60%; however, after 30% WSP-T or 5% WSP-L diets, the mice demonstrated significant restoration of the Langerhans’s areas (approximately 30%). Only in 5%-L mice, slightly increased expression of insulin-signaling pathway-related proteins, phosphorylated insulin receptor and protein kinase B and membrane glucose transporter 4 was noted.

Conclusions

WSP has antihyperglycemic effects by inducing pancreatic islet regeneration and insulin resistance amelioration. Therefore, WSP has potential applications in dietary diabetes management.

Adiponectin in renal fibrosis

Renal fibrosis is an inevitable consequence of parenchymal scarring and is the common final pathway that mediates almost all progressive renal diseases. Adiponectin, a hormone produced by adipose tissue, possesses potent anti-insulin, anti-inflammatory, and anti-fibrotic properties. Reportedly, adiponectin serves as an important messenger that facilitates complex interactions between adipose tissue and other metabolically related organs. In recent years, a growing body of evidence supports adiponectin involvement in renal fibrosis. These studies provide a deeper understanding of the molecular mechanism of action of adiponectin in renal fibrosis and also offer a potential preventive and therapeutic target for renal fibrosis. In this review, the physiological role of adiponectin is briefly introduced, and then the mechanism of adiponectin-mediated renal fibrosis and the related signaling pathways are described. Finally, we summarize the findings regarding the clinical value of adiponectin in renal fibrotic diseases and prospected its application potential.

In vivo liposomal delivery of PPARα/γ dual agonist tesaglitazar in a model of obesity enriches macrophage targeting and limits liver and kidney drug effects

Macrophages are important regulators of obesity-associated inflammation and PPARα and -γ agonism in macrophages has anti-inflammatory effects. In this study, we tested the efficacy with which liposomal delivery could target the PPARα/γ dual agonist tesaglitazar to macrophages while reducing drug action in common sites of drug toxicity: the liver and kidney, and whether tesaglitazar had anti-inflammatory effects in an in vivo model of obesity-associated dysmetabolism. Methods: Male leptin-deficient (ob/ob) mice were administered tesaglitazar or vehicle for one week in a standard oral formulation or encapsulated in liposomes. Following the end of treatment, circulating metabolic parameters were measured and pro-inflammatory adipose tissue macrophage populations were quantified by flow cytometry. Cellular uptake of liposomes in tissues was assessed using immunofluorescence and a broad panel of cell subset markers by flow cytometry. Finally, PPARα/γ gene target expression levels in the liver, kidney, and sorted macrophages were quantified to determine levels of drug targeting to and drug action in these tissues and cells. Results: Administration of a standard oral formulation of tesaglitazar effectively treated symptoms of obesity-associated dysmetabolism and reduced the number of pro-inflammatory adipose tissue macrophages. Macrophages are the major cell type that took up liposomes with many other immune and stromal cell types taking up liposomes to a lesser extent. Liposome delivery of tesaglitazar did not have effects on inflammatory macrophages nor did it improve metabolic parameters to the extent of a standard oral formulation. Liposomal delivery did, however, attenuate effects on liver weight and liver and kidney expression of PPARα and -γ gene targets compared to oral delivery. Conclusions: These findings reveal for the first time that tesaglitazar has anti-inflammatory effects on adipose tissue macrophage populations in vivo. These data also suggest that while nanoparticle delivery reduced off-target effects, yet the lack of tesaglitazar actions in non-targeted cells such (as hepatocytes and adipocytes) and the uptake of drug-loaded liposomes in many other cell types, albeit to a lesser extent, may have impacted overall therapeutic efficacy. This fulsome analysis of cellular uptake of tesaglitazar-loaded liposomes provides important lessons for future studies of liposome drug delivery.

NPY1R-targeted peptide-mediated delivery of a dual PPARα/γ agonist to adipocytes enhances adipogenesis and prevents diabetes progression

Objective

PPARα/γ dual agonists have been in clinical development for the treatment of metabolic diseases including type 2 diabetes and dyslipidemia. However, severe adverse side effects led to complications in clinical trials. As most of the beneficial effects rely on the compound activity in adipocytes, the selective targeting of this cell type is a cutting-edge strategy to develop safe anti-diabetic drugs. The goal of this study was to strengthen the adipocyte-specific uptake of the PPARα/γ agonist tesaglitazar via NPY₁R-mediated internalization.

Methods

NPY₁R-preferring peptide tesaglitazar-[F⁷, P³⁴]-NPY (tesa-NPY) was synthesized by a combination of automated SPPS and manual couplings. Following molecular and functional analyses for proof of concept, cell culture experiments were conducted to monitor the effects on adipogenesis. Mice treated with peptide drug conjugates or vehicle either by gavage or intraperitoneal injection were characterized phenotypically and metabolically. Histological analysis and transcriptional profiling of the adipose tissue were performed.

Results

In vitro studies revealed that the tesaglitazar-[F⁷, P³⁴]-NPY conjugate selectively activates PPARγ in NPY₁R-expressing cells and enhances adipocyte differentiation and adiponectin expression in adipocyte precursor cells. In vivo studies using db/db mice demonstrated that the anti-diabetic activity of the peptide conjugate is as efficient as that of systemically administered tesaglitazar. Additionally, tesa-NPY induces adipocyte differentiation in vivo.

Conclusions

The use of the tesaglitazar-[F7, P34]-NPY conjugate is a promising strategy to apply the beneficial PPARα/γ effects in adipocytes while potentially omitting adverse effects in other tissues.

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Tesaglitazar-NPY targets adipocytes via NPY₁R receptor-mediated internalization.

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Peptide-drug conjugate is specifically delivered to NPY₁R-expressing cells.

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Release of tesaglitazar in adipocytes activates PPARγ.

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Drug delivery enhances adipocyte differentiation and adiponectin expression.

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Peptide conjugate exhibits antidiabetic activity in vivo.

Local miscommunications between glomerular cells as potential therapeutic targets for crescentic glomerulonephritides

Necrotizing and crescentic rapidly progressive glomerulonephritis or crescentic glomerulonephritis is one of the severest forms of acquired glomerular diseases with significant mortality. Risk of end-stage renal failure at 5 years is near 30%, with a number of patients developing chronic kidney disease. Currently, autoimmune crescentic glomerulonephritides are treated with broad-spectrum immunosuppression inducing remission of the injury in the majority of patients. However, treatment is associated with significant side effects and by the time remission is achieved the majority of patients have developed renal tissue damage and significant impairment of their kidney function with a steep slope of deterioration within the first weeks following initiation of immunosuppression. It is therefore important to develop complementary strategies that would be immediately active on the common process of destructive epithelial processes. We have worked to identify the major cellular pathways contributing to glomerular destruction in this context by a systematic comparison of patient tissues and experimental models. Our studies demonstrate the pivotal role of local intra- and intercellular communications in orchestrating the global glomerular tolerance to a severe rapidly progressive glomerulonephritis model with excellent anatomoclinical correlative expressions in kidney biopsies of individuals diagnosed with crescentic glomerulonephritis, irrespectively of the causal immune disorder. We hope that such approaches deciphering mechanisms of cellular adaptation that underlie kidney damage control in response to vasculitides, integrating both stress and damage responses, will delineate novel complementary therapies.

PPARs and Metabolic Disorders Associated with Challenged Adipose Tissue Plasticity

Peroxisome proliferator-activated receptors (PPARs) are members of a family of nuclear hormone receptors that exert their transcriptional control on genes harboring PPAR-responsive regulatory elements (PPRE) in partnership with retinoid X receptors (RXR). The activation of PPARs coordinated by specific coactivators/repressors regulate networks of genes controlling diverse homeostatic processes involving inflammation, adipogenesis, lipid metabolism, glucose homeostasis, and insulin resistance. Defects in PPARs have been linked to lipodystrophy, obesity, and insulin resistance as a result of the impairment of adipose tissue expandability and functionality. PPARs can act as lipid sensors, and when optimally activated, can rewire many of the metabolic pathways typically disrupted in obesity leading to an improvement of metabolic homeostasis. PPARs also contribute to the homeostasis of adipose tissue under challenging physiological circumstances, such as pregnancy and aging. Given their potential pathogenic role and their therapeutic potential, the benefits of PPARs activation should not only be considered relevant in the context of energy balance-associated pathologies and insulin resistance but also as potential relevant targets in the context of diabetic pregnancy and changes in body composition and metabolic stress associated with aging. Here, we review the rationale for the optimization of PPAR activation under these conditions.

The Adiponectin Receptor Agonist AdipoRon Ameliorates Diabetic Nephropathy in a Model of Type 2 Diabetes

Adiponectin exerts renoprotective effects against diabetic nephropathy (DN) by activating the AMP-activated protein kinase (AMPK)/peroxisome proliferative-activated receptor-α (PPARα) pathway through adiponectin receptors (AdipoRs). AdipoRon is an orally active synthetic adiponectin receptor agonist. We investigated the expression of AdipoRs and the associated intracellular pathways in 27 patients with type 2 diabetes and examined the effects of AdipoRon on DN development in male C57BLKS/J db/db mice, glomerular endothelial cells (GECs), and podocytes. The extent of glomerulosclerosis and tubulointerstitial fibrosis correlated with renal function deterioration in human kidneys. Expression of AdipoR1, AdipoR2, and Ca²⁺/calmodulin-dependent protein kinase kinase-β (CaMKKβ) and numbers of phosphorylated liver kinase B1 (LKB1)- and AMPK-positive cells significantly decreased in the glomeruli of early stage human DN. AdipoRon treatment restored diabetes-induced renal alterations in db/db mice. AdipoRon exerted renoprotective effects by directly activating intrarenal AdipoR1 and AdipoR2, which increased CaMKKβ, phosphorylated Ser⁴³¹LKB1, phosphorylated Thr¹⁷²AMPK, and PPARα expression independently of the systemic effects of adiponectin. AdipoRon-induced improvement in diabetes-induced oxidative stress and inhibition of apoptosis in the kidneys ameliorated relevant intracellular pathways associated with lipid accumulation and endothelial dysfunction. In high-glucose-treated human GECs and murine podocytes, AdipoRon increased intracellular Ca²⁺ levels that activated a CaMKKβ/phosphorylated Ser⁴³¹LKB1/phosphorylated Thr¹⁷²AMPK/PPARα pathway and downstream signaling, thus decreasing high-glucose-induced oxidative stress and apoptosis and improving endothelial dysfunction. AdipoRon further produced cardioprotective effects through the same pathway demonstrated in the kidney. Our results show that AdipoRon ameliorates GEC and podocyte injury by activating the intracellular Ca²⁺/LKB1-AMPK/PPARα pathway, suggesting its efficacy for treating type 2 diabetes-associated DN.

Induction of cytochrome P450 4A14 contributes to angiotensin II-induced renal fibrosis in mice

Angiotensin II (AngII) plays an important role in the pathogenesis of hypertension and associated renal injuries. To elucidate the molecular mechanism by which AngII induces renal damage, we found that AngII infusion significantly induced CYP4A14 expression in renal proximal tubule cells (RPTCs) with marked increases in blood pressure and proteinuria. Renal production of the major CYP4A metabolite, 20-HETE, was also significantly increased in the AngII-treated mice. Compared to wild-type (WT) mice, CYP4A14 knockout (CYP4A14^-/-) mice exhibited significantly lower levels of blood pressure, renal 20-HETE production, proteinuria and renal fibrosis following AngII infusion. Furthermore, AngII-induced renal expression of profibrotic genes and proinflammatory genes was significantly attenuated in CYP4A14^-/- mice. In vitro studies using cultured RPTCs demonstrated that AngII significantly induced CYP4A14 expression and 20-HETE production via the MAPK signaling pathway. AngII treatment increased TGF-β and collagen expression, which was attenuated by the CYP4A inhibitor, TS-011. Moreover, 20-HETE treatment potently induced CYP4A14 expression and TGF-β and collagen levels. Collectively, these findings suggest that attenuated renal fibrosis in AngII-treated CYP4A14^-/- mice may result from both reduced systemic blood pressure and renal 20-HETE production. Therefore, CYP4A14 may represent a useful target for the treatment of AngII-associated renal damage.

The effects of dual PPARα/γ agonism compared with ACE inhibition in the BTBRob/ob mouse model of diabetes and diabetic nephropathy

The leptin‐deficient BTBRob/ob mouse develops progressive albuminuria and morphological lesions similar to human diabetic nephropathy (DN), although whether glomerular hyperfiltration, a recognized feature of early DN that may contribute to renal injury, also occurs in this model is not known. Leptin replacement has been shown to reverse the signs of renal injury in this model, but in contrast, the expected renoprotection by angiotensin‐converting enzyme (ACE) inhibition in BTBRob/ob mice seems to be limited. Therefore, to investigate the potential renal benefits of improved metabolic control in this model, we studied the effect of treatment with the dual peroxisome proliferator‐activated receptor (PPAR) α/γ agonist AZD6610 and compared it with the ACE inhibitor enalapril. AZD6610 lowered plasma glucose and triglyceride concentrations and increased liver size, but had no significant effect in reducing albuminuria, whereas enalapril did have an effect. Nephrin and WT1 mRNA expression decreased in the kidneys of BTBRob/ob mice, consistent with podocyte injury and loss, but was unaffected by either drug treatment: at the protein level, both nephrin and WT1‐positive cells per glomerulus were decreased. Mesangial matrix expansion was reduced in AZD6610‐treated mice. GFR, measured by creatinine clearance, was increased in BTBRob/ob mice, but unaffected by either treatment. Unexpectedly, enalapril‐treated mice showed intrarenal arteriolar vascular remodeling with concentric thickening of vessel walls. In summary, we found that the BTBRob/ob mouse model shows some similarities to the early changes seen in human DN, but that ACE inhibition or PPARα/γ agonism afforded limited or no kidney protection.

High glucose instigates tubulointerstitial injury by stimulating hetero-dimerization of adiponectin and angiotensin II receptors

Abnormal expression and dysfunction of adiponectin and the cognate receptors are involved in diabetes and diabetic kidney disease (DKD), whereas angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEIs) alleviate diabetic albuminuria and prevent development of DKD through upregulation of adiponectin expression. Here we report that high glucose stimulates expression of angiotensin II (AngII) receptors (AT1 and AT2) in renal proximal tubular epithelial cells (NRK-52E). These receptors underwent hetero-dimerization with adiponectin receptor AdipoR1 and AdipoR2, respectively. High glucose inhibited the dimerization between AT1 and AT2. Interestingly, these hetero-dimers instigated tubulointerstitial injury by inhibiting the cytoprotective action of the adiponectin receptors. These modes of receptor-receptor hetero-dimerization may contribute to high glucose-induced renal tubulointerstitial injury and could be potential therapeutic targets.

Adiponectin and Its Receptors in Diabetic Kidney Disease: Molecular Mechanisms and Clinical Potential

Diabetic kidney disease (DKD) is a major complication for diabetic patients. Adiponectin is an insulin sensitizer and anti-inflammatory adipokine and is mainly secreted by adipocytes. Two types of adiponectin receptors, AdipoR1 and AdipoR2, have been identified. In both type 1 and type 2 diabetes (T2D) patients with DKD, elevated adiponectin serum levels have been observed, and adiponectin serum level is a prognostic factor of end-stage renal disease. Renal insufficiency and tubular injury possibly play a contributory role in increases in serum and urinary adiponectin levels in diabetic nephropathy by either increasing biodegradation or elimination of adiponectin in the kidneys, or enhancing production of adiponectin in adipose tissue. Increases in adiponectin levels resulted in amelioration of albuminuria, glomerular hypertrophy, and reduction of inflammatory response in kidney tissue. The renoprotection of adiponectin is associated with improvement of the endothelial dysfunction, reduction of oxidative stress, and upregulation of endothelial nitric oxide synthase expression through activation of adenosine 5'-monophosphate-activated protein kinase by AdipoR1 and activation of peroxisome proliferator-activated receptor (PPAR)-α signaling pathway by AdipoR2. Several single nucleotide polymorphisms in the AdipoQ gene, including the promoter, are associated with increased risk of the development of T2D and DKD. Renin-angiotensin-aldosterone system blockers, adiponectin receptor agonists, and PPAR agonists (e.g., tesaglitazar, thiazolidinediones, fenofibrate), which increase plasma adiponectin levels and adiponectin receptors expression, may be potential therapeutic drugs for the treatment of DKD.

Novel Anti-fibrotic Therapies

Fibrosis is a major player in cardiovascular disease, both as a contributor to the development of disease, as well as a post-injury response that drives progression. Despite the identification of many mechanisms responsible for cardiovascular fibrosis, to date no treatments have emerged that have effectively reduced the excess deposition of extracellular matrix associated with fibrotic conditions. Novel treatments have recently been identified that hold promise as potential therapeutic agents for cardiovascular diseases associated with fibrosis, as well as other fibrotic conditions. The purpose of this review is to provide an overview of emerging antifibrotic agents that have shown encouraging results in preclinical or early clinical studies, but have not yet been approved for use in human disease. One of these agents is bone morphogenetic protein-7 (BMP7), which has beneficial effects in multiple models of fibrotic disease. Another approach discussed involves altering the levels of micro-RNA (miR) species, including miR-29 and miR-101, which regulate the expression of fibrosis-related gene targets. Further, the antifibrotic potential of agonists of the peroxisome proliferator-activated receptors will be discussed. Finally, evidence will be reviewed in support of the polypeptide hormone relaxin. Relaxin is long known for its extracellular remodeling properties in pregnancy, and is rapidly emerging as an effective antifibrotic agent in a number of organ systems. Moreover, relaxin has potent vascular and renal effects that make it a particularly attractive approach for the treatment of cardiovascular diseases. In each case, the mechanism of action and the applicability to various fibrotic diseases will be discussed.

Peroxisome proliferator-activated receptor α ligands and modulators from dietary compounds: Types, screening methods and functions

Peroxisome proliferator-activated receptor α (PPARα) plays a key role in lipid metabolism and glucose homeostasis and a crucial role in the prevention and treatment of metabolic diseases. Natural dietary compounds, including nutrients and phytochemicals, are PPARα ligands or modulators. High-throughput screening assays have been developed to screen for PPARα ligands and modulators in our diet. In the present review, we discuss recent advances in our knowledge of PPARα, including its structure, function, and ligand and modulator screening assays, and summarize the different types of dietary PPARα ligands and modulators.

Sildenafil Prevents Podocyte Injury via PPAR-γ-Mediated TRPC6 Inhibition

Transient receptor potential channel C6 (TRPC6) gain-of-function mutations and increased TRPC6 expression in podocytes induce glomerular injury and proteinuria. Sildenafil reduces TRPC6 expression and activity in nonrenal cell types, although the mechanism is unknown. Peroxisome proliferator-activated receptor γ (PPAR-γ) is a downstream target of sildenafil in the cyclic guanosine monophosphate (cGMP)-activated protein kinase G (PKG) axis. PPAR-γ agonists, like pioglitazone, appear antiproteinuric. We hypothesized that sildenafil inhibits TRPC6 expression in podocytes through PPAR-γ-dependent mechanisms, thereby counteracting podocyte injury and proteinuria. Treatment with sildenafil, the cGMP derivative 8-bromoguanosine 3',5'-cyclic monophosphate sodium salt (8-Br-cGMP), or pioglitazone dose-dependently downregulated podocyte injury-induced TRPC6 expression in vitro Knockdown or application of antagonists of PKG or PPAR-γ enhanced TRPC6 expression in podocytes and counteracted effects of sildenafil and 8-Br-cGMP. We observed similar effects on TRPC6 promoter activity and TRPC6-dependent calcium influx. Chromatin immunoprecipitation showed PPAR-γ binding to the TRPC6 promoter. Sildenafil or pioglitazone treatment prevented proteinuria and the increased TRPC6 expression in rats with adriamycin-induced nephropathy and mice with hyperglycemia-induced renal injury. Rats receiving PPAR-γ antagonists displayed proteinuria and increased podocyte TRPC6 expression, as did podocyte-specific PPAR-γ knockout mice, which were more sensitive to adriamycin and not protected by sildenafil. Thus, sildenafil ameliorates podocyte injury and prevents proteinuria through cGMP- and PKG-dependent binding of PPAR-γ to the TRPC6 promoter, which inhibits TRPC6 promoter activity, expression, and activity. Because sildenafil is approved for clinical use, our results suggest that additional clinical study of its antiproteinuric effect in glomerular disease is warranted.

Nuclear Factor Erythroid 2-Related Factor 2 Drives Podocyte-Specific Expression of Peroxisome Proliferator-Activated Receptor γ Essential for Resistance to Crescentic GN

Necrotizing and crescentic rapidly progressive GN (RPGN) is a life-threatening syndrome characterized by a rapid loss of renal function. Evidence suggests that podocyte expression of the transcription factor peroxisome proliferator-activated receptor γ (PPARγ) may prevent podocyte injury, but the function of glomerular PPARγ in acute, severe inflammatory GN is unknown. Here, we observed marked loss of PPARγ abundance and transcriptional activity in glomerular podocytes in experimental RPGN. Blunted expression of PPARγ in podocyte nuclei was also found in kidneys from patients diagnosed with crescentic GN. Podocyte-specific Pparγ gene targeting accentuated glomerular damage, with increased urinary loss of albumin and severe kidney failure. Furthermore, a PPARγ gain-of-function approach achieved by systemic administration of thiazolidinedione (TZD) failed to prevent severe RPGN in mice with podocyte-specific Pparγ gene deficiency. In nuclear factor erythroid 2-related factor 2 (NRF2)-deficient mice, loss of podocyte PPARγ was observed at baseline. NRF2 deficiency markedly aggravated the course of RPGN, an effect that was partially prevented by TZD administration. Furthermore, delayed administration of TZD, initiated after the onset of RPGN, still alleviated the severity of experimental RPGN. These findings establish a requirement for the NRF2-PPARγ cascade in podocytes, and we suggest that these transcription factors have a role in augmenting the tolerance of glomeruli to severe immune-complex mediated injury. The NRF2-PPARγ pathway may be a therapeutic target for RPGN.

Diabetic nephropathy research in China: Data analysis and review from the National Natural Science Foundation of China

As the largest funding agency of natural science of China, the National Natural Science Foundation of China (NSFC) has made great efforts in promoting the development of diabetic nephropathy (DN) research in recent years. The aim of the current study is to summarize the diabetic nephropathy research in China by analyzing NSFC-funded projects. Data on all projects in the DN field funded by NSFC from 1986 to 2013 were collected. The funding tendency, funding areas, and hotspots in the DN field, and major research institutions, were analyzed. As one output of this support, outstanding research groups in China, and their representative studies, are also highlighted. From 1986 to 2013, the NSFC has funded a total of 248 projects in the DN field, with a total funding amount of 91.5 million RMB (US$14.9 million). A rapid increase could be seen in the past 5 years, with an average annual 30% increase in projects numbers and a 52% increase in funding amount. All fields in DN research have been covered by the NSFC, including etiology, pathophysiology, diagnostics, and therapeutics. Along with increased funding of the DN research, there has been a growth in the papers published in Science Citation Index journals by Chinese scholars. In the past decade, the funding scale and funding budget have increased dramatically. Benefiting from this, DN research in China has also made considerable progression.

Inhibitory effects of peroxisome proliferator-activated receptor γ agonists on collagen IV production in podocytes

Peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists have beneficial effects on the kidney diseases through preventing microalbuminuria and glomerulosclerosis. However, the mechanisms underlying these effects remain to be fully understood. In this study, we investigate the effects of PPAR-γ agonist, rosiglitazone (Rosi) and pioglitazone (Pio), on collagen IV production in mouse podocytes. The endogenous expression of PPAR-γ was found in the primary podocytes and can be upregulated by Rosi and Pio, respectively, detected by RT-PCR and Western blot. PPAR-γ agonist markedly blunted the increasing of collagen IV expression and extraction in podocytes induced by TGF-β. In contrast, adding PPAR-γ antagonist, GW9662, to podocytes largely prevented the inhibition of collagen IV expression from Pio treatment. Our data also showed that phosphorylation of Smad2/3 enhanced by TGF-β in a time-dependent manner was significantly attenuated by adding Pio. The promoter region of collagen IV gene contains one putative consensus sequence of Smad-binding element (SBE) by promoter analysis, Rosi and Pio significantly ameliorated TGF-β-induced SBE4-luciferase activity. In conclusion, PPAR-γ activation by its agonist, Rosi or Pio, in vitro directly inhibits collagen IV expression and synthesis in primary mouse podocytes. The suppression of collagen IV production was related to the inhibition of TGF-β-driven phosphorylation of Smad2/3 and decreased response activity of SBEs of collagen IV in PPAR-γ agonist-treated mouse podocytes. This represents a novel mechanistic support regarding PPAR-γ agonists as podocyte protective agents.

Myocardin-Related Transcription Factor A Epigenetically Regulates Renal Fibrosis in Diabetic Nephropathy

Diabetic nephropathy (DN) is one of the most common complications associated with diabetes and characterized by renal microvascular injury along with accelerated synthesis of extracellular matrix proteins causing tubulointerstitial fibrosis. Production of type I collagen, the major component of extracellular matrix, is augmented during renal fibrosis after chronic exposure to hyperglycemia. However, the transcriptional modulator responsible for the epigenetic manipulation leading to induction of type I collagen genes is not clearly defined. We show here that tubulointerstitial fibrosis as a result of DN was diminished in myocardin-related transcription factor A (MRTF-A) -deficient mice. In cultured renal tubular epithelial cells and the kidneys of mice with DN, MRTF-A was induced by glucose and synergized with glucose to activate collagen transcription. Notably, MRTF-A silencing led to the disappearance of prominent histone modifications indicative of transcriptional activation, including acetylated histone H3K18/K27 and trimethylated histone H3K4. Detailed analysis revealed that MRTF-A recruited p300, a histone acetyltransferase, and WD repeat-containing protein 5 (WDR5), a key component of the histone H3K4 methyltransferase complex, to the collagen promoters and engaged these proteins in transcriptional activation. Estradiol suppressed collagen production by dampening the expression and binding activity of MRTF-A and interfering with the interaction between p300 and WDR5 in renal epithelial cells. Therefore, targeting the MRTF-A-associated epigenetic machinery might yield interventional strategies against DN-associated renal fibrosis.

New Insights into the PPAR γ Agonists for the Treatment of Diabetic Nephropathy

Diabetic nephropathy (DN) is a severe complication of diabetes and serves as the leading cause of chronic renal failure. In the past decades, angiotensin-converting enzyme inhibitors (ACEIs)/angiotensin II receptor blockers (ARBs) based first-line therapy can slow but cannot stop the progression of DN, which urgently requests the innovation of therapeutic strategies. Thiazolidinediones (TZDs), the synthetic exogenous ligands of nuclear receptor peroxisome proliferator-activated receptor- γ (PPAR γ ), had been thought to be a promising candidate for strengthening the therapy of DN. However, the severe adverse effects including fluid retention, cardiovascular complications, and bone loss greatly limited their use in clinic. Recently, numerous novel PPAR γ agonists involving the endogenous PPAR γ ligands and selective PPAR γ modulators (SPPARMs) are emerging as the promising candidates of the next generation of antidiabetic drugs instead of TZDs. Due to the higher selectivity of these novel PPAR γ agonists on the regulation of the antidiabetes-associated genes than that of the side effect-associated genes, they present fewer adverse effects than TZDs. The present review was undertaken to address the advancements and the therapeutic potential of these newly developed PPAR γ agonists in dealing with diabetic kidney disease. At the same time, the new insights into the therapeutic strategies of DN based on the PPAR γ agonists were fully addressed.

The protective effect of telmisartan in Type 2 diabetes rat kidneys is related to the downregulation of thioredoxin-interacting protein

BACKGROUND

Thioredoxin-interacting protein (Txnip), an inhibitor of thioredoxin (Trx), increases in diabetic nephropathy and promotes oxidative stress. The angiotensin II (Ang II) receptor blocker telmisartan may protect renal function in diabetic models and patients via multiple effects including antioxidation. However, its mechanism has not been fully elucidated, and its relationship to Txnip remains unclear.

AIM

This study aimed to investigate whether telmisartan ameliorates oxidative stress by regulating Txnip and Trx expression in Type 2 diabetic rat kidneys and explore the possible relationship between renoprotection by telmisartan and Txnip.

METHODS

Twenty-one rats were equally divided into control (C), streptozotocin-induced diabetic (D), and telmisartan- treated diabetic (T) groups. Txnip and Trx expression in rat kidneys was analyzed by immunohistochemistry, RTPCR, and western blot. Peroxisome proliferator-activated receptor- γ (PPARγ), NADPH oxidase activity, and parameters of renal function and oxidative stress were also measured.

RESULTS

Trx and PPARγ were significantly decreased, and Txnip expression and NADPH oxidase activity markedly increased, in the D and T groups compared to the C group. After telmisartan treatment, Trx and PPARγ were upregulated, while Txnip expression and NADPH oxidase activity were downregulated. Parameters of renal function and oxidative stress were improved by telmisartan.

CONCLUSION

Telmisartan ameliorates oxidative stress and protects renal function in Type 2 diabetic rat kidneys. The downregulation of Txnip by telmisartan may be associated with PPARγ activation.

Pancreatic angiotensin-converting enzyme 2 improves glycemia in angiotensin II-infused mice

An overactive renin-angiotensin system (RAS) is known to contribute to type 2 diabetes mellitus (T2DM). Although ACE2 overexpression has been shown to be protective against the overactive RAS, a role for pancreatic ACE2, particularly in the islets of Langerhans, in regulating glycemia in response to elevated angiotensin II (Ang II) levels remains to be elucidated. This study examined the role of endogenous pancreatic ACE2 and the impact of elevated Ang II levels on the enzyme's ability to alleviate hyperglycemia in an Ang II infusion mouse model. Male C57bl/6J mice were infused with Ang II or saline for a period of 14 days. On the 7th day of infusion, either an adenovirus encoding human ACE2 (Ad-hACE2) or a control adenovirus (Ad-eGFP) was injected into the mouse pancreas. After an additional 7-8 days, glycemia and plasma insulin levels as well as RAS components expression and oxidative stress were assessed. Ang II-infused mice exhibited hyperglycemia, hyperinsulinemia, and impaired glucose-stimulated insulin secretion from pancreatic islets compared with control mice. This phenotype was associated with decreased ACE2 expression and activity, increased Ang II type 1 receptor (AT1R) expression, and increased oxidative stress in the mouse pancreas. Ad-hACE2 treatment restored pancreatic ACE2 expression and compensatory activity against Ang II-mediated impaired glycemia, thus improving β-cell function. Our data suggest that decreased pancreatic ACE2 is a link between overactive RAS and impaired glycemia in T2DM. Moreover, maintenance of a normal endogenous ACE2 compensatory activity in the pancreas appears critical to avoid β-cell dysfunction, supporting a therapeutic potential for ACE2 in controlling diabetes resulting from an overactive RAS.

Dual therapy of rosiglitazone/pioglitazone with glimepiride on diabetic nephropathy in experimentally induced type 2 diabetes rats

Diabetic nephropathy is a major cause of end-stage renal disease (ESRD) in the general population. It is estimated that diabetic nephropathy will eventually develop in about 40% of all patients with diabetes; therefore, prevention is critical for delaying the development and progression of diabetic kidney disease. Despite extensive efforts, medical advances are still not successful enough to prevent the progression of the disease. In the present study, we focused on the comparison of combination therapies and whether they offered additional renoprotection. Type 2 diabetes mellitus was induced by intraperitoneally administering streptozotocin (90 mg/kg) in neonatal rats and then these rats were treated with rosiglitazone (1.0 mg/kg) in combination with glimepiride (0.5 mg/kg) or with pioglitazone (2.5 mg/kg) in combination with glimepiride (0.5 mg/kg). Diabetic nephropathy markers were evaluated by biochemical and ELISA kits and renal structural changes were examined by light microscopy and transmission electron microscopy. Results show that the combination of pioglitazone with glimepiride is more effective in amelioration of diabetic nephropathy than rosiglitazone with glimepiride drug therapy due to glycemic control, suppressing albumin excretion rate, total protein excretion rate and augmented TNF-a signaling during the development of streptozotocin induced type 2 diabetic nephropathy.

Role of lipotoxicity in endothelial dysfunction

Vascular endothelial dysfunction is determined by both genetic and environmental factors that cause decreased bioavailability of the vasodilator nitric oxide. This is a hallmark of atherosclerosis, hypertension, and coronary heart disease, which are major complications of metabolic disorders, including diabetes and obesity. Several therapeutic interventions, including changes in lifestyle as well as pharmacologic treatments, are useful for improving endothelial dysfunction in the face of lipotoxicity. This review discusses the current understanding of molecular and physiologic mechanisms underlying lipotoxicity-mediated endothelial dysfunction as well as relevant therapeutic approaches to ameliorate dyslipidemia and consequent endothelial dysfunction that have the potential to improve cardiovascular and metabolic outcomes.

PPARγ as a therapeutic target in diabetic nephropathy and other renal diseases

PURPOSE OF REVIEW

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear transcription factor that regulates many important physiological processes including glucose and lipid metabolism, energy homeostasis, cell proliferation, inflammation, immunity and reproduction. The current review aims to summarize and discuss recent findings evaluating the protective effects of PPARγ against kidney diseases with a focus on diabetic nephropathy. We will also delineate the potential underlying mechanisms.

RECENT FINDINGS

PPARγ plays important roles in renal physiology and pathophysiology. Agonists of PPARγ exert protective effects against various kidney diseases including diabetic nephropathy, ischemic renal injury, IgA nephropathy, chemotherapy-associated kidney damage, polycystic kidney diseases and age-related kidney diseases via both systemic and renal actions.

SUMMARY

PPARγ agonists are effective in delaying and even preventing the progression of many renal diseases, especially diabetic nephropathy. PPARγ may represent a promising target for the treatment of renal diseases.

Pharmacokinetic-pharmacodynamic assessment of the interrelationships between tesaglitazar exposure and renal function in patients with type 2 diabetes mellitus

The effects of tesaglitazar on renal function (assessed as urinary clearance of 125I-sodium iothalamate or estimated by the modification of diet in renal disease formula) were studied in a 24-week open-label trial in type 2 diabetes mellitus patients randomized to daily doses of either tesaglitazar 2 mg or pioglitazone 45 mg. The aim of the analysis was to develop a population pharmacokinetic-pharmacodynamic model that could simultaneously describe the interrelationship between tesaglitazar exposure and reduction in renal function over time in patients with type 2 diabetes mellitus. The pharmacokinetic-pharmacodynamic model could adequately describe the interplay between tesaglitazar and glomerular filtration rate. A one-compartment model in which the apparent clearance was influenced by glomerular filtration rate characterized the pharmacokinetics of tesaglitazar. An indirect-response model was used for the slow time course of change in glomerular filtration rate, which decreased from 100 to 78 mL/min/1.73m(2) after 12 weeks of treatment. All tesaglitazar-treated patients had a reduction in glomerular filtration rate, and available demographic variables could not explain differences in response. Patients treated with an angiotensin converting enzyme inhibitor were more sensitive to tesaglitazar and had larger glomerular filtration rate decrease compared to nontreated patients. Approximately 8 weeks after discontinuing treatment, mean glomerular filtration rate had returned towards baseline. The model and data give valuable insights into the dynamic changes in glomerular filtration rate over time.

Medicinal Chemistry and Actions of Dual and Pan PPAR Modulators

Peroxisome proliferator-activated receptor (PPAR) agonists are used as adjunct therapy in the treatment of diabetes mellitus. Fibrates, including fenofibrate, gemfibrozil, benzafibrate, ciprofibrate, and clofibrate act on PPAR alpha to reduce the level of hypertriglyceridemia. However, agonists (ligands) of PPAR-beta/delta receptors, such as tesaglitazar, muraglitazar, ragaglitazar, imiglitazar, aleglitazar, alter the body's energy substrate preference from glucose to lipids and hence contribute to the reduction of blood glucose level. Glitazones or thiazolidinediones on the other hand, bind to PPAR-gamma receptors located in the nuclei of cells. Activation of PPAR-gamma receptors leads to a decrease in insulin resistance and modification of adipocyte metabolism. They reduce hyperlipidaemia by increasing the level of ATP-binding cassette A1, which modifies extra-hepatic cholesterol into HDL. Dual or pan PPAR ligands stimulate two or more isoforms of PPAR and thereby reduce insulin resistance and prevent short- and long-term complications of diabetes including micro-and macroangiopathy and atherosclerosis, which are caused by deposition of cholesterol. This review examines the chemical structure, actions, side effects and future prospects of dual and pan PPAR agonists.

Fluorofenidone attenuates diabetic nephropathy and kidney fibrosis in db/db mice

BACKGROUND/AIMS

Fluorofenidone [1-(3-fluorophenyl)-5-methyl-2-(1H)-pyridone, AKF-PD], a novel pyridone agent, showed potent antifibrotic properties. The aim of the present study was to investigate the effects of AKF-PD on diabetic nephropathy and kidney fibrosis, and to obtain an insight into its mechanisms of action.

METHODS

We administered AKF-PD to diabetic db/db mice for 12 weeks. Moreover, we performed in vitro cultures using murine mesangial cells exposed to high ambient glucose concentrations.

RESULTS

AKF-PD reduced renal hypertrophy, mesangial matrix expansion and albuminuria in the db/db mice. The upregulated expression of α₁(I)- and α₁(IV)-collagen and fibronectin mRNAs, transforming growth factor-β1 (TGF-β₁), α-smooth muscle actin (α-SMA), and tissue inhibitors of metalloproteinase 1 (TIMP-1) mRNAs and proteins was inhibited by AKF-PD treatment in the renal cortex of db/db mice. The maximal effective dose of AKF-PD was about 500 mg/kg body weight. AKF-PD inhibited the upregulated expression of α₁(I)- and α₁(IV)-collagens, TGF-β₁, TIMP-1 and α-SMA induced by high glucose concentrations in cultured mesangial cells.

CONCLUSIONS

Our data indicate that AKF-PD diminishes the abnormal accumulation of mesangial matrix through the inhibition of upregulated expression of TGF-β target genes in kidneys of db/db mice, resulting in attenuation of renal fibrosis and amelioration of renal dysfunction despite persistent hyperglycemia. Therefore, AKF-PD, a potent antifibrotic agent, holds great promise in the treatment of diabetic nephropathy.

Comparison of direct action of thiazolidinediones and glucocorticoids on renal podocytes: protection from injury and molecular effects

The U.S. Food and Drug Administration-approved thiazolidinediones pioglitazone and rosiglitazone are peroxisome proliferator-activated receptor-γ (PPARγ) agonists developed to control serum glucose in patients with diabetes. They have been found to reduce proteinuria and microalbuminuria in both diabetic nephropathy and nondiabetic glomerulosclerosis. We hypothesized that the renal protective effects of thiazolidinediones result, at least in part, from their direct action on podocytes, similar to glucocorticoids. Treatment with pioglitazone, rosiglitazone, or dexamethasone significantly protected podocytes against puromycin aminonucleoside-induced injury (designed to mimic nephrotic syndrome-related injury), as determined by both cell survival and actin cytoskeletal integrity. Furthermore, we compared the ability of these drugs to modulate key signaling pathways in podocytes that may be critical to their protective effects. Rosiglitazone deactivated the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases 1/2, p38 MAPK, and stress-activated protein kinase/c-Jun NH₂-terminal kinase, whereas pioglitazone did not, and dexamethasone deactivated to some extent. Similar to dexamethasone, both thiazolidinediones increased the glucocorticoid receptor phosphorylation, and this response to rosiglitazone and possibly to pioglitazone was PPARγ-dependent. Furthermore, both drugs mimicked or enhanced the effects of dexamethasone on glucocorticoid-responsive genes in a PPARγ- and glucocorticoid receptor-dependent manner. In addition, both thiazolidinediones mimicked dexamethasone-induced effects on calcineurin activity. In summary, thiazolidinediones are able to modulate the glucocorticoid pathway and exert direct protective effects on podocytes, similar to glucocorticoids. This suggests that thiazolidinediones may have potential clinical utility as either primary or adjunctive therapy for nephrotic syndrome or other diseases treated with glucocorticoids. These findings may also lend mechanistic insight into the well established but poorly understood renal protective effects of thiazolidinediones in diabetic nephropathy.

Nuclear receptors in renal disease

Diabetes is the leading cause of end-stage renal disease in developed countries. In spite of excellent glucose and blood pressure control, including administration of angiotensin converting enzyme inhibitors and/or angiotensin II receptor blockers, diabetic nephropathy still develops and progresses. The development of additional protective therapeutic interventions is, therefore, a major priority. Nuclear hormone receptors regulate carbohydrate metabolism, lipid metabolism, the immune response, and inflammation. These receptors also modulate the development of fibrosis. As a result of their diverse biological effects, nuclear hormone receptors have become major pharmaceutical targets for the treatment of metabolic diseases. The increasing prevalence of diabetic nephropathy has led intense investigation into the role that nuclear hormone receptors may have in slowing or preventing the progression of renal disease. This role of nuclear hormone receptors would be associated with improvements in metabolism, the immune response, and inflammation. Several nuclear receptor activating ligands (agonists) have been shown to have a renal protective effect in the context of diabetic nephropathy. This review will discuss the evidence regarding the beneficial effects of the activation of several nuclear, especially the vitamin D receptor (VDR), farnesoid X receptor (FXR), and peroxisome-proliferator-associated receptors (PPARs) in preventing the progression of diabetic nephropathy and describe how the discovery and development of compounds that modulate the activity of nuclear hormone receptors may provide potential additional therapeutic approaches in the management of diabetic nephropathy. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Proteomics analysis reveals diabetic kidney as a ketogenic organ in type 2 diabetes

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease. To date, the molecular mechanisms of DN remain largely unclear. The present study aimed to identify and characterize novel proteins involved in the development of DN by a proteomic approach. Proteomic analysis revealed that 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase 2 (HMGCS2), the key enzyme in ketogenesis, was increased fourfold in the kidneys of type 2 diabetic db/db mice. Consistently, the activity of HMGCS2 in kidneys and 24-h urinary excretion of the ketone body β-hydroxybutyrate (β-HB) were significantly increased in db/db mice. Immunohistochemistry, immunofluorescence, and real-time PCR studies further demonstrated that HMGCS2 was highly expressed in renal glomeruli of db/db mice, with weak expression in the kidneys of control mice. Because filtered ketone bodies are mainly reabsorbed in the proximal tubules, we used RPTC cells, a rat proximal tubule cell line, to examine the effect of the increased level of ketone bodies. Treating cultured RPTC cells with 1 mM β-HB significantly induced transforming growth factor-β1 expression, with a marked increase in collagen I expression. β-HB treatment also resulted in a marked increase in vimentin protein expression and a significant reduction in E-cadherin protein levels, suggesting an enhanced epithelial-to-mesenchymal transition in RPTCs. Collectively, these findings demonstrate that diabetic kidneys exhibit excess ketogenic activity resulting from increased HMGCS2 expression. Enhanced ketone body production in the diabetic kidney may represent a novel mechanism involved in the pathogenesis of DN.

A susceptibility gene for kidney disease in an obese mouse model of type II diabetes maps to chromosome 8

Most mouse models of diabetes do not fully reproduce features of human diabetic nephropathy, limiting their utility in inferring mechanisms of human disease. Here we performed detailed phenotypic and genetic characterization of leptin-receptor (Lepr) deficient mice on the FVB/NJ background (FVB(db/db)), an obese model of type II diabetes, to determine their suitability to model human diabetic nephropathy. These mice have sustained hyperglycemia, significant albuminuria and characteristic diabetic renal findings including mesangial sclerosis and nodular glomerulosclerosis after 6 months of age. In contrast, equally obese, hyperglycemic Lepr/Sur1 deficient C57BL/6J (Sur1 has defective insulin secretion) mice have minimal evidence of nephropathy. A genome-wide scan in 165 Lepr deficient backcross progeny derived from FVB/NJ and C57BL/6J identified a major locus influencing nephropathy and albuminuria on chromosome 8B1-C5 (Dbnph1 locus, peak lod score 5.0). This locus was distinct from those contrasting susceptibility to beta cell hypertrophy and HIV-nephropathy between the same parental strains, indicating specificity to diabetic kidney disease. Genome-wide expression profiling showed that high and low risk Dbnph1 genotypes were associated with significant enrichment for oxidative phosphorylation and lipid clearance, respectively; molecular pathways shared with human diabetic nephropathy. Hence, we found that the FVB(db/db) mouse recapitulates many clinical, histopathological and molecular features of human diabetic nephropathy. Identifying underlying susceptibility gene(s) and downstream dysregulated pathways in these mice may provide insight into the disease pathogenesis in humans.

Anti-diabetic effects including diabetic nephropathy of anti-osteoporotic trace minerals on diabetic mice

OBJECTIVE

In our previous study to evaluate the effects of soluble silicon (Si) on bone metabolism, Si and coral sand (CS) as a natural Si-containing material suppressed peroxisome proliferator-activated receptor γ (PPARγ), which regulates both glucose and bone metabolism and increases adipogenesis at the expense of osteogenesis, leading to bone loss. In this study, we investigated the anti-diabetic effects of bone-seeking elements, Si and stable strontium (Sr), and CS as a natural material containing these elements using obese diabetic KKAy mice.

METHODS

Weanling male mice were fed diets containing 1% Ca supplemented with CaCO(3) as the control and CS, and diets supplemented with 50 ppm Si or 750 ppm Sr to control diet for 56 d. The mRNA expressions related to energy expenditure in the pancreas and kidney were quantified by real-time polymerase chain reaction.

RESULTS

At the end of feeding, plasma glucose, insulin, leptin, and adiponectin levels decreased significantly in three test groups, while pancreatic PPARγ and adiponectin mRNA expression levels increased significantly toward the normal level, improving the glucose sensitivity of β-cells and inducing a significant decrease in insulin expression. The renal PPARγ, PPARα, and adiponectin expression levels, histologic indices of diabetic glomerulopathy, and plasma indices of renal function were also improved significantly in the test groups.

CONCLUSION

Taken together, anti-osteoporotic trace minerals, Si and Sr, and CS containing them showed novel anti-diabetic effects of lowering blood glucose level, improving the tolerance to insulin, leptin, and adiponectin, and reducing the risk of glomerulopathy through modulation of related gene expression in the pancreas and kidney.

Mangiferin prevents diabetic nephropathy progression in streptozotocin-induced diabetic rats

Diabetic nephropathy is one of the most severe diabetic microangiopathies and accounting for approximately one-third of all cases of end-stage renal disease. In the present study, we investigated the effect of mangiferin, a polyphenol from Anemarrhena asphodeloides Bge. or Mangifera indica L., on diabetic nephropathy and the possible mechanisms by using a developed diabetic nephropathy rat model and cultured rat mesangial cells. Serum-advanced glycation end-products level, malonaldehyde level, sorbitol concentration of red blood cell, 24 h albuminuria excretion were significantly decreased, whereas activity of serum superoxide dismutase and glutathione peroxidase and creatinine clearance rate were increased by mangiferin. Blood glucose level remained unaffected. Mangiferin significantly inhibited glomerular extracellular matrix expansion and accumulation and transforming growth factor-beta 1 overexpression in glomeruli of diabetic nephropathy rats. Moreover, mangiferin was observed to inhibit proliferation of mesangial cells induced by high glucose and the overexpression of collagen type IV of mesangial cells induced by advanced glycation end products. In summary, mangiferin could significantly prevent progression of diabetic nephropathy and improve renal function.

Nuclear hormone receptors in diabetic nephropathy

Diabetes is the leading cause of end-stage renal disease in developed countries. In spite of glucose and blood pressure control, for example by use of angiotensin II receptor blockers, diabetic nephropathy still develops and progresses in affected patients and the development of additional protective therapeutic interventions is, therefore, required. Nuclear hormone receptors are transcription factors that regulate carbohydrate metabolism, lipid metabolism, the immune response, and inflammation. These receptors also modulate the development of fibrosis. As a result of their diverse biological effects, nuclear hormone receptors have become major pharmaceutical targets for the treatment of a host of diseases. The increasing prevalence of diabetic nephropathy has led intense investigation into the role that nuclear hormone receptors may have in slowing or preventing the progression of renal disease. This role of nuclear hormone receptors would be associated with improvements in metabolism, the immune response, and inflammation. Eight nuclear receptors have shown a renoprotective effect in the context of diabetic nephropathy. This Review discusses the evidence regarding the beneficial effects of the activation of these receptors in preventing the progression of diabetic nephropathy and describes how the discovery and development of compounds that modulate the activity of nuclear hormone receptors may provide potential additional therapeutic approaches in the management of diabetic nephropathy.

Transcription factors Krüppel-like factor 6 and peroxisome proliferator-activated receptor-{gamma} mediate high glucose-induced thioredoxin-interacting protein

We demonstrated recently that thioredoxin-interacting protein (Txnip) and the transcription factor Krüppel-like factor 6 (KLF6) were up-regulated in both in vivo and in vitro models of diabetic nephropathy, thus promoting renal injury. Conversely, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists have been shown to be renoprotective. Hence, this study was undertaken to determine whether Txnip expression is regulated by the transcription factors KLF6 and PPAR-gamma. By using siRNAs and overexpressing constructs, the role of KLF6 and PPAR-gamma in Txnip transcriptional regulation was determined in human kidney proximal tubule cells and in streptozocin-induced diabetes mellitus in Sprague-Dawley rats, in vitro and in vivo models of diabetic nephropathy, respectively. KLF6 overexpression increased Txnip expression and promoter activity, which was inhibited by concurrent exposure to PPAR-gamma agonists. In contrast, reduced expression of KLF6 by siRNA or exposure to PPAR-gamma agonists attenuated high glucose-induced Txnip expression and promoter activity. KLF6-Txnip promoter binding was decreased in KLF6-silenced cells, whereas PPAR-gamma agonists increased PPAR-gamma-Txnip promoter binding. Indeed, silencing of KLF6 increased PPAR-gamma expression, suggesting endogenous regulation of PPAR-gamma expression by KLF6. Moreover, renal KLF6 and Txnip expression increased in rats with diabetes mellitus and was inhibited by PPAR-gamma agonist treatment; however, KLF6 expression did not change in HK-2 cells exposed to PPAR-gamma agonists. Hence, Txnip expression and promoter activity are mediated via divergent effects of KLF6 and PPAR-gamma transcriptional regulation.

Prostacyclin-induced peroxisome proliferator-activated receptor-alpha translocation attenuates NF-kappaB and TNF-alpha activation after renal ischemia-reperfusion injury

Prostacyclin and peroxisome proliferator-activated receptors (PPAR) protect against ischemia-reperfusion (I/R) injury by the induction of an anti-inflammatory pathway. In this study, we examined the prostacyclin-enhanced protective effect of PPARalpha in I/R-induced kidney injury. PPAR-alpha reduced the NF-kappaB-induced overexpression of TNF-alpha and apoptosis in cultured kidney cells. In a murine model, pretreating wild-type (WT) mice with a PPAR-alpha activator, docosahexaenoic acid (DHA), significantly reduced I/R-induced renal dysfunction (lowered serum creatinine and urea nitrogen levels), apoptotic responses (decreased apoptotic cell number and caspase-3, -8 activation), and NF-kappaB activation. By comparison, I/R-induced injury was exacerbated in PPAR-alpha knockout mice. This indicated that PPAR-alpha attenuated renal I/R injury via NF-kappaB-induced TNF-alpha overexpression. Overexpression of prostacyclin using an adenovirus could also induce PPAR-alpha translocation from the cytosol into the nucleus to inhibit caspase-3 activation. This prostacyclin/PPAR-alpha pathway attenuated TNF-alpha promoter activity by binding to NF-kappaB. Using a cAMP inhibitor (CAY10441) and a prostacyclin receptor antibody, we also found that there was another prostacyclin/IP receptor/cAMP pathway that could inhibit TNF-alpha production. Taken together, our results demonstrate for the first time that prostacyclin induces the translocation of PPAR-alpha from the cytosol into the nucleus and attenuates NF-kappaB-induced TNF-alpha activation following renal I/R injury. Treatments that can augment prostacyclin, PPAR-alpha, or the associated signaling pathways may ameliorate conditions associated with renal I/R injury.

PPARgamma in Kidney Physiology and Pathophysiology

Involvement of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) in kidney physiology has been explored recently. Synthetic PPARγ ligands can ameliorate the diabetic kidney disease through different mechanisms, involving inhibition of mesangial cell growth, reduction of mesangial matrix, and cytokine production of glomerular cells as well as promoting endothelial cell survival within the kidney glomeruli. Activation of PPARγ has additional profibrotic consequences, which can contribute to wound healing in diabetic glomerulonephritis. Beside many beneficial effects, PPARγ activation, however, can lead to severe water retention, a common side effect of thiazolidinedione therapy. This unwanted effect is due to the activation of PPARγ in the mesonephric distal collecting system, where PPARγ positively regulates sodium and water resorbtion leading to the expansion of interstitial fluid volume. Recent studies indicate that PPARγ is also involved in the normal kidney development, renal lipid metabolism, and activation of the renin-angiotensin system. In this paper, we give a synopsis of the current knowledge on PPARγ functions in kidney phyisology and pathophysiology.