Pretreatment by low-dose fibrates protects against acute free fatty acid-induced renal tubule toxicity by counteracting PPARα deterioration

WWP2 Regulates Renal Fibrosis and the Metabolic Reprogramming of Profibrotic Myofibroblasts

Key Points

WWP2 expression is elevated in the tubulointerstitium of fibrotic kidneys and contributes to CKD pathogenesis and progression. WWP2 uncouples the profibrotic activation and cell proliferation in renal myofibroblasts. WWP2 controls mitochondrial respiration in renal myofibroblasts through the metabolic regulator peroxisome proliferator-activated receptor gamma coactivator 1-alpha.

Background

Renal fibrosis is a common pathologic end point in CKD that is challenging to reverse, and myofibroblasts are responsible for the accumulation of a fibrillar collagen–rich extracellular matrix. Recent studies have unveiled myofibroblasts' diversity in proliferative and fibrotic characteristics, which are linked to different metabolic states. We previously demonstrated the regulation of extracellular matrix genes and tissue fibrosis by WWP2, a multifunctional E3 ubiquitin–protein ligase. Here, we investigate WWP2 in renal fibrosis and in the metabolic reprograming of myofibroblasts in CKD.

Methods

We used kidney samples from patients with CKD and WWP2 -null kidney disease mice models and leveraged single-cell RNA sequencing analysis to detail the cell-specific regulation of WWP2 in fibrotic kidneys. Experiments in primary cultured myofibroblasts by bulk-RNA sequencing, chromatin immunoprecipitation sequencing, metabolomics, and cellular metabolism assays were used to study the metabolic regulation of WWP2 and its downstream signaling.

Results

The tubulointerstitial expression of WWP2 was associated with fibrotic progression in patients with CKD and in murine kidney disease models. WWP2 deficiency promoted myofibroblast proliferation and halted profibrotic activation, reducing the severity of renal fibrosis in vivo . In renal myofibroblasts, WWP2 deficiency increased fatty acid oxidation and activated the pentose phosphate pathway, boosting mitochondrial respiration at the expense of glycolysis. WWP2 suppressed the transcription of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a metabolic mediator of fibrotic response, and pharmacologic inhibition of PGC-1α partially abrogated the protective effects of WWP2 deficiency on myofibroblasts.

Conclusions

WWP2 regulates the metabolic reprogramming of profibrotic myofibroblasts by a WWP2-PGC-1α axis, and WWP2 deficiency protects against renal fibrosis in CKD.

Transition of acute kidney injury to chronic kidney disease: role of metabolic reprogramming

Acute kidney injury (AKI) is a global public health concern associated with high morbidity and mortality. Although advances in medical management have improved the in-hospital mortality of severe AKI patients, the renal prognosis for AKI patients in the later period is not encouraging. Recent epidemiological investigations have indicated that AKI significantly increases the risk for the development of chronic kidney disease (CKD) and end-stage renal disease (ESRD) in the future, further contributing to the economic burden on health care systems. The transition of AKI to CKD is complex and often involves multiple mechanisms. Recent studies have suggested that renal tubular epithelial cells (TECs) are more prone to metabolic reprogramming during AKI, in which the metabolic process in the TECs shifts from fatty acid β-oxidation (FAO) to glycolysis due to hypoxia, mitochondrial dysfunction, and disordered nutrient-sensing pathways. This change is a double-edged role. On the one hand, enhanced glycolysis acts as a compensation pathway for ATP production; on the other hand, long-term shut down of FAO and enhanced glycolysis lead to inflammation, lipid accumulation, and fibrosis, contributing to the transition of AKI to CKD. This review discusses developments and therapies focused on the metabolic reprogramming of TECs during AKI, and the emerging questions in this evolving field.

When the Renal (Function) Begins to Fall: A Mini-Review of Acute Kidney Injury Related to Acute Respiratory Distress Syndrome in Critically Ill Patients

Acute kidney injury (AKI) is one of the most frequent causes of organ failure encountered in patients in the intensive care unit (ICU). Because of its predisposition to occur in the most critically ill patients, it is not surprising to observe a high frequency of AKI in patients with acute respiratory distress syndrome (ARDS). However, few studies have been carried out to assess the epidemiology of AKI in subgroups of ARDS patients using recommended KDIGO criteria. Moreover, the mechanisms involved in the physio-pathogenesis of AKI are still poorly understood, in particular the impact of mechanical ventilation on the kidneys. We carried out a review of the literature, focusing on the epidemiology and physiopathology of AKI in patients with ARDS admitted to the ICU. We addressed the importance of clinical management, focusing on mechanical ventilation for improving outcomes, on AKI. Finally, we also propose candidate treatment strategies and management perspectives. Our literature search showed that AKI is particularly common in ICU patients with ARDS. In association with the classic risk factors for AKI, such as comorbidities and iatrogeny, changes in mechanical ventilation parameters, which have been exclusively evaluated for their outcomes on respiratory function and death, must be considered carefully in terms of their impact on the short-term renal prognosis.

Melatonin attenuates cisplatin-induced acute kidney injury in mice: Involvement of PPARα and fatty acid oxidation

The present study focused on the protective effects of melatonin against cisplatin-induced acute kidney injury in mice and its possible mechanism of action in relation to the major regulator of fatty acid oxidation (FAO), peroxidase proliferative receptor α (PPARα). The experiment consisted of the following four groups: vehicle control, cisplatin (15 mg/kg), cisplatin & melatonin (20 mg/kg/day), and melatonin (20 mg/kg/day). Concomitant administration of melatonin significantly ameliorated cisplatin-induced acute kidney injury in mice by decreasing serum levels of triglyceride, blood urea nitrogen and creatinine, reducing the number and size of lipid droplets in tubular epithelial cells, and decreasing the incidence of histopathological changes including tubular cell apoptosis. Moreover, melatonin administration protected kidney tissue by significantly upregulating the levels of PPARα reduced by cisplatin injection, resulting in increased FAO pathway-associated genes (PGC-1a, Acadm, Acat1, Acsm2, Acsm3, Bdh2, Echs and Pecr) as well as reducing protein levels of caspase-3, -9 and Bax. Melatonin not only partially modulated FAO via PPARα signaling, but also decreased cisplatin-induced apoptosis by inhibiting the caspase-3, -9 and Bax pathways. Our findings suggest that melatonin prevents cisplatin-induced acute kidney injury in mice, possibly by upregulating the expression of PPARα, resulting in enhanced FAO and anti-apoptotic properties.

Metabolic Reprogramming and Renal Fibrosis

There are several causes of chronic kidney disease, but all of these patients have renal fibrosis. Although many studies have examined the pathogenesis of renal fibrosis, there are still no effective treatments. A healthy and balanced metabolism is necessary for normal cell growth, proliferation, and function, but metabolic abnormalities can lead to pathological changes. Normal energy metabolism is particularly important for maintaining the structure and function of the kidneys because they consume large amounts of energy. We describe the metabolic reprogramming that occurs during renal fibrosis, which includes changes in fatty acid metabolism and glucose metabolism, and the relationship of these changes with renal fibrosis. We also describe the potential role of novel drugs that disrupt this metabolic reprogramming and the development of fibrosis, and current and future challenges in the treatment of fibrosis.

The role of metabolic reprogramming in tubular epithelial cells during the progression of acute kidney injury

Acute kidney injury (AKI) is one of the most common clinical syndromes. AKI is associated with significant morbidity and subsequent chronic kidney disease (CKD) development. Thus, it is urgent to develop a strategy to hinder AKI progression. Renal tubules are responsible for the reabsorption and secretion of various solutes and the damage to this part of the nephron is a key mediator of AKI. As we know, many common renal insults primarily target the highly metabolically active proximal tubular cells (PTCs). PTCs are the most energy-demanding cells in the kidney. The ATP that they use is mostly produced in their mitochondria by fatty acid β-oxidation (FAO). But, when PTCs face various biological stresses, FAO will shut down for a time that outlives injury. Recent studies have suggested that surviving PTCs can adapt to FAO disruption by increasing glycolysis when facing metabolic constraints, although PTCs do not perform glycolysis in a normal physiological state. Enhanced glycolysis in a short period compensates for impaired energy production and exerts partial renal-protective effects, but its long-term effect on renal function and AKI progression is not promising. Deranged FAO and enhanced glycolysis may contribute to the AKI to CKD transition through different molecular biological mechanisms. In this review, we concentrate on the recent pathological findings of AKI with regards to the metabolic reprogramming in PTCs, confirming that targeting metabolic reprogramming represents a potentially effective therapeutic strategy for the progression of AKI.

Pemafibrate Protects against Fatty Acid-Induced Nephropathy by Maintaining Renal Fatty Acid Metabolism

As classical agonists for peroxisomal proliferator-activated receptor alpha (PPARα), fibrates activate renal fatty acid metabolism (FAM) and provide renoprotection. However, fibrate prescription is limited in patients with kidney disease, since impaired urinary excretion of the drug causes serious adverse effects. Pemafibrate (PEM), a novel selective PPARα modulator, is mainly excreted in bile, and, thus, may be safe and effective in kidney disease patients. It remains unclear, however, whether PEM actually exhibits renoprotective properties. We investigated this issue using mice with fatty acid overload nephropathy (FAON). PEM (0.5 mg/kg body weight/day) or a vehicle was administered for 20 days to 13-week-old wild-type male mice, which were simultaneously injected with free fatty acid (FFA)-binding bovine serum albumin from day 7 to day 20 to induce FAON. All mice were sacrificed on day 20 for assessment of the renoprotective effect of PEM against FAON. PEM significantly attenuated the histological findings of tubular injury caused by FAON, increased the renal expressions of mRNA and proteins related to FAM, and decreased renal FFA content and oxidative stress. Taken together, PEM exhibits renoprotective effects through the activation and maintenance of renal FAM and represents a promising drug for kidney disease.

Outcome of acute kidney injury: how to make a difference?

Background

Acute kidney injury (AKI) is one of the most frequent organ failure encountered among intensive care unit patients. In addition to the well-known immediate complications (hydroelectrolytic disorders, hypervolemia, drug overdose), the occurrence of long-term complications and/or chronic comorbidities related to AKI has long been underestimated. The aim of this manuscript is to briefly review the short- and long-term consequences of AKI and discuss strategies likely to improve outcome of AKI.

Main body

We reviewed the literature, focusing on the consequences of AKI in all its aspects and the management of AKI. We addressed the importance of clinical management for improving outcomes AKI. Finally, we have also proposed candidate future strategies and management perspectives.

Conclusion

AKI must be considered as a systemic disease. Due to its short- and long-term impact, measures to prevent AKI and limit the consequences of AKI are expected to improve global outcomes of patients suffering from critical illnesses.

The Link Between the Mitochondrial Fatty Acid Oxidation Derangement and Kidney Injury

Renal proximal tubular cells are high energy-demanding cells mainly relying on fatty acid oxidation. In stress conditions, such as transient hypoxia, fatty acid oxidation (FAO) decreases and carbohydrate catabolism fails to compensate for the energy demand. In this scenario, the surviving tubular cells exhibit the peculiar phenotype associated with fibrosis that is the histological manifestation of a process culminating in chronic and end-stage kidney disease. Genome-wide transcriptome analysis revealed that, together with inflammation, FAO is the top dysregulated pathway in kidney diseases with a decreased expression of key FAO enzymes and regulators. Another evidence that links the derangement of FAO to fibrosis is the progressive decrease of the expression of peroxisome proliferator-activated receptor α (PPARα) in aged people, that triggers the age-associated renal fibrosis. To allow FAO completion, a coordinate network of enzymes and transport proteins is required. Indeed, the mitochondrial inner membrane is impermeable to fatty acyl-CoAs and a specialized system, well known as carnitine shuttle, is needed for translocating fatty acids moieties, conjugated with carnitine, into mitochondrial matrix for the β-oxidation. The first component of this system is the carnitine palmitoyltransferase 1 (CPT1) responsible for transfer acyl moieties to carnitine. Several studies indicated that the stimulation of CPT1 activity and expression has a protective effect against renal fibrosis. Therefore, the network of enzymes and transporters linked to FAO may represent potential pharmacological targets deserving further attention in the development of new drugs to attenuate renal dysfunction.

Recent Progress on Lipid Intake and Chronic Kidney Disease

The incidence of chronic kidney disease (CKD) is associated with major abnormalities in circulating lipoproteins and renal lipid metabolism. This article elaborates on the mechanisms of CKD and lipid uptake abnormalities. The viewpoint we supported is that lipid abnormalities directly cause CKD, resulting in forming a vicious cycle. On the theoretical and experiment fronts, this inference has been verified by elaborately elucidating the role of lipid intake and accumulation as well as their influences on CKD. Taken together, these findings suggest that further understanding of lipid metabolism in CKD may lead to novel therapeutic approaches.

Defective Mitochondrial Fatty Acid Oxidation and Lipotoxicity in Kidney Diseases

The kidney is a highly metabolic organ and uses high levels of ATP to maintain electrolyte and acid-base homeostasis and reabsorb nutrients. Energy depletion is a critical factor in development and progression of various kidney diseases including acute kidney injury (AKI), chronic kidney disease (CKD), and diabetic and glomerular nephropathy. Mitochondrial fatty acid β-oxidation (FAO) serves as the preferred source of ATP in the kidney and its dysfunction results in ATP depletion and lipotoxicity to elicit tubular injury and inflammation and subsequent fibrosis progression. This review explores the current state of knowledge on the role of mitochondrial FAO dysfunction in the pathophysiology of kidney diseases including AKI and CKD and prospective views on developing therapeutic interventions based on mitochondrial energy metabolism.

Effects of Dietary Anaplerotic and Ketogenic Energy Sources on Renal Fatty Acid Oxidation Induced by Clofibrate in Suckling Neonatal Pigs

Maintaining an active fatty acid metabolism is important for renal growth, development, and health. We evaluated the effects of anaplerotic and ketogenic energy sources on fatty acid oxidation during stimulation with clofibrate, a pharmacologic peroxisome proliferator-activated receptor α (PPARα) agonist. Suckling newborn pigs (n = 72) were assigned into 8 dietary treatments following a 2 × 4 factorial design: ± clofibrate (0.35%) and diets containing 5% of either (1) glycerol-succinate (GlySuc), (2) tri-valerate (TriC5), (3) tri-hexanoate (TriC6), or (4) tri-2-methylpentanoate (Tri2MPA). Pigs were housed individually and fed the iso-caloric milk replacer diets for 5 d. Renal fatty acid oxidation was measured in vitro in fresh tissue homogenates using [1-14C]-labeled palmitic acid. The oxidation was 30% greater in pig received clofibrate and 25% greater (p < 0.05) in pigs fed the TriC6 diet compared to those fed diets with GlySuc, TriC5, and Tri2MPA. Addition of carnitine also stimulated the oxidation by twofold (p < 0.05). The effects of TriC6 and carnitine on palmitic acid oxidation were not altered by clofibrate stimulation. However, renal fatty acid composition was altered by clofibrate and Tri2MPA. In conclusion, modification of anaplerosis or ketogenesis via dietary substrates had no influence on in vitro renal palmitic acid oxidation induced by PPARα activation.

PPARα contributes to protection against metabolic and inflammatory derangements associated with acute kidney injury in experimental sepsis

Sepsis-associated acute kidney injury (AKI) is a significant problem in critically ill children and adults resulting in increased morbidity and mortality. Fundamental mechanisms contributing to sepsis-associated AKI are poorly understood. Previous research has demonstrated that peroxisome proliferator-activated receptor α (PPARα) expression is associated with reduced organ system failure in sepsis. Using an experimental model of polymicrobial sepsis, we demonstrate that mice deficient in PPARα have worse kidney function, which is likely related to reduced fatty acid oxidation and increased inflammation. Ultrastructural evaluation with electron microscopy reveals that the proximal convoluted tubule is specifically injured in septic PPARα deficient mice. In this experimental group, serum metabolomic analysis reveals unanticipated metabolic derangements in tryptophan-kynurenine-NAD+ and pantothenate pathways. We also show that a subgroup of children with sepsis whose genome-wide expression profiles are characterized by repression of the PPARα signaling pathway has increased incidence of severe AKI. These findings point toward interesting associations between sepsis-associated AKI and PPARα-driven fatty acid metabolism that merit further investigation.

Peroxisome proliferator-activated receptor alpha agonist suppresses neovascularization by reducing both vascular endothelial growth factor and angiopoietin-2 in corneal alkali burn

We investigated the effect of a peroxisome proliferator-activated receptor alpha (PPARα) agonist ophthalmic solution in wound healing using a rat corneal alkali burn model. After instillation of a selective agonist of PPARα, fenofibrate, onto the burned cornea, PPARα-positive cells were observed in vascular endothelial cells, and there was upregulation of mRNA of PPARα in corneal stroma. Fenofibrate suppressed expression of neutrophils and macrophages during the early phase, and development of neovascularization and myofibroblast generation during the late phase. Fenofibrate reduced not only mRNA expression of vascular endothelial growth factor-A but also angiopoietin-1 and angiopoietin-2. Furthermore, fenofibrate suppressed scar formation by reducing type III collagen expression. These data suggest that a PPARα agonist ophthalmic solution might be a new strategy for treating corneal wounds through not only anti-inflammatory effects but also by preventing neovascularization.

Activation of PPARα by Oral Clofibrate Increases Renal Fatty Acid Oxidation in Developing Pigs

The objective of this study was to evaluate the effects of peroxisome proliferator-activated receptor α (PPARα) activation by clofibrate on both mitochondrial and peroxisomal fatty acid oxidation in the developing kidney. Ten newborn pigs from 5 litters were randomly assigned to two groups and fed either 5 mL of a control vehicle (2% Tween 80) or a vehicle containing clofibrate (75 mg/kg body weight, treatment). The pigs received oral gavage daily for three days. In vitro fatty acid oxidation was then measured in kidneys with and without mitochondria inhibitors (antimycin A and rotenone) using [1-¹⁴C]-labeled oleic acid (C18:1) and erucic acid (C22:1) as substrates. Clofibrate significantly stimulated C18:1 and C22:1 oxidation in mitochondria (p < 0.001) but not in peroxisomes. In addition, the oxidation rate of C18:1 was greater in mitochondria than peroxisomes, while the oxidation of C22:1 was higher in peroxisomes than mitochondria (p < 0.001). Consistent with the increase in fatty acid oxidation, the mRNA abundance and enzyme activity of carnitine palmitoyltransferase I (CPT I) in mitochondria were increased. Although mRNA of mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme A synthase (mHMGCS) was increased, the β-hydroxybutyrate concentration measured in kidneys did not increase in pigs treated with clofibrate. These findings indicate that PPARα activation stimulates renal fatty acid oxidation but not ketogenesis.

Effect of clofibrate on fatty acid metabolism in the kidney of puromycin-induced nephrotic rats

BACKGROUND

Proteinuria plays an essential role in the progression of tubulointerstitial damage, which causes end-stage renal disease. An increased load of fatty acids bound to albumin reabsorbed into proximal tubular epithelial cells (PTECs) contributes to tubulointerstitial damage. Fibrates, agonists of peroxisome proliferator-activated receptor α (PPARα), have renoprotective effects against proteinuria whereas the effects of these compounds on fatty acid metabolism in the kidney are still unknown. Therefore, the present study examined whether the renoprotective effects of clofibrate were associated with improvement of fatty acid metabolism in puromycin aminonucleoside (PAN)-induced nephrotic rats.

METHODS

Rats were allocated to the control, PAN or clofibrate-treated PAN group. Biochemical parameters, renal injury and changes in fatty acid metabolism were studied on day14.

RESULTS

PAN increased proteinuria, lipid accumulation in PTECs, excretions of N-acetyl-β-D-glucosaminidase (NAG) and 8-hydroxydeoxyguanosine (8OHdG) and the area of caspase 3-positive tubular cells. It decreased renal expressions of medium-chain acyl-CoA dehydrogenase (MCAD), cytochrome P450 (CYP)4A, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and estrogen-related receptor α (ERRα) without change of the expression of PPARα. Clofibrate reduced proteinuria, lipid accumulation, NAG excretion and the area of caspase 3-positive tubular cells. However, albumin excretion was not reduced and 8OHdG excretion was increased. Clofibrate minimized changes in MCAD, CYP4A, PGC-1α and ERRα expressions with increased PPARα, very long-chain acyl-CoA dehydrogenase (VLCAD) and long-chain acyl-CoA dehydrogenase (LCAD) expressions.

CONCLUSION

Clofibrate is protective against renal lipotoxicity in PAN nephrosis. This study indicates that clofibrate has renoprotective effects through maintaining fatty acid metabolism in the kidney of PAN-induced nephrotic rats.

Peroxisome proliferator-activated receptor α-dependent renoprotection of murine kidney by irbesartan

Activation of renal peroxisome proliferator-activated receptor α (PPARα) is renoprotective, but there is no safe PPARα activator for patients with chronic kidney disease (CKD). Studies have reported that irbesartan (Irbe), an angiotensin II receptor blocker (ARB) widely prescribed for CKD, activates hepatic PPARα. However, Irbe's renal PPARα-activating effects and the role of PPARα signalling in the renoprotective effects of Irbe are unknown. Herein, these aspects were investigated in healthy kidneys of wild-type (WT) and Ppara-null (KO) mice and in the murine protein-overload nephropathy (PON) model respectively. The results were compared with those of losartan (Los), another ARB that does not activate PPARα. PPARα and its target gene expression were significantly increased only in the kidneys of Irbe-treated WT mice and not in KO or Los-treated mice, suggesting that the renal PPARα-activating effect was Irbe-specific. Irbe-treated-PON-WT mice exhibited decreased urine protein excretion, tubular injury, oxidative stress (OS), and pro-inflammatory and apoptosis-stimulating responses, and they exhibited maintenance of fatty acid metabolism. Furthermore, the expression of PPARα and that of its target mRNAs encoding proteins involved in OS, pro-inflammatory responses, apoptosis and fatty acid metabolism was maintained upon Irbe treatment. These renoprotective effects of Irbe were reversed by the PPARα antagonist MK886 and were not detected in Irbe-treated-PON-KO mice. These results suggest that Irbe activates renal PPARα and that the resultant increased PPARα signalling mediates its renoprotective effects.

Fenofibrate, a PPARα agonist, protect proximal tubular cells from albumin-bound fatty acids induced apoptosis via the activation of NF-kB

Albumin-bound fatty acids is the main cause of renal damage, PPARα is responsible in the metabolism of fatty acids. Previous study found that PPARα played a protective role in fatty acids overload associated tubular injury. The aim of the present study is to investigate whether fenofibrate, a PPARα ligands, could contribute to the renoprotective action in fatty acids overload proximal tubule epithelial cells. We observed in HK-2 cells that fenofibrate significantly inhibited fatty acids bound albumin (FA-BSA) induced up-regulation of MCP-1 and IL-8. Treatment with fenofibrate attenuated renal oxidative stress induced by FA-BSA as evidenced by decreased MDA level, increased SOD activity and catalase, GPx-1 expression. FA-BSA induced apoptosis of HK-2 cells were also obviously prevented by fenofibrate. Furthermore, fenofibrate significantly increased the expression of PPARα mRNA and protein in FA-BSA treated cells. Finally, the activation of NF-kB induced by FA-BSA was markedly suppressed by fenofibrate. Taken together, our study describes a renoprotective role of fenofibrate in fatty acids associated tubular toxicity, and the transcriptional activation of PPARα and suppression of NF-kB were at least partially involved.

Alteration of Fatty Acid Oxidation in Tubular Epithelial Cells: From Acute Kidney Injury to Renal Fibrogenesis

Renal proximal tubular cells are the most energy-demanding cells in the body. The ATP that they use is mostly produced in their mitochondrial and peroxisomal compartments, by the oxidation of fatty acids. When those cells are placed under a biological stress, such as a transient hypoxia, fatty acid oxidation (FAO) is shut down for a period of time that outlasts injury, and carbohydrate oxidation does not take over. Facing those metabolic constraints, surviving tubular epithelial cells exhibit a phenotypic switch that includes cytoskeletal rearrangement and production of extracellular matrix proteins, most probably contributing to acute kidney injury-induced renal fibrogenesis, thence to the development of chronic kidney disease. Here, we review experimental evidence that dysregulation of FAO profoundly affects the fate of tubular epithelial cells, by promoting epithelial-to-mesenchymal transition, inflammation, and eventually interstitial fibrosis. Restoring physiological production of energy is undoubtedly a possible therapeutic approach to unlock the mesenchymal reprograming of tubular epithelial cells in the kidney. In this respect, the benefit of the use of fibrates is uncertain, but new drugs that could specifically target this metabolic pathway, and, hopefully, attenuate renal fibrosis merit future research.

Fibrate treatment of eEOCs in murine AKI

BACKGROUND

Early endothelial outgrowth cells (eEOCs) protect mice from acute kidney injury (AKI). Peroxisome proliferator-activated receptor-alpha (PPAR-α) has been shown to mediate renoprotective effects under different experimental conditions. The aim of the study was to investigate consequences of fibrate treatment of murine eEOCs in a cell-based therapeutic approach to AKI.

METHODS

Male C57/Bl6N mice, subjected to unilateral renal ischemia (40 min) post-uninephrectomy, were systemically injected with 0.5 × 10(6) untreated or fenofibrate (FF 1, 5, 10 or 50 μm)/clofibrate (CF 1 mm) pretreated syngeneic murine eEOCs. Renal function and morphology were analyzed 48 h later. Cellular consequences of eEOC treatment with fibrates (FF 1, 5, 10, 50 μm, CF 1 mm) were evaluated using different in vitro assays (direct cell migration, apoptosis/necrosis, ELISA studies).

RESULTS

Administration of untreated eEOCs did not protect mice from AKI. Injection of eEOCs treated with CF (1 mm) or FF 50 μm did not result in any protection from ischemia-induced renal dysfunction. In vitro analysis showed reduced cellular secretion of vasoprotective vascular endothelial growth factor (VEGF), an effect that was more pronounced with CF; FF increased percentages of apoptotic/necrotic eEOCs, and both substances failed to stimulate migration of cultured cells. With lower FF concentrations (1, 5, 10 μm) cell survival was increased and 10 μm FF stimulated VEGF secretion. In vivo administration of FF-treated eEOCs (10 μm) also did not result in any renoprotective effect.

CONCLUSION

PPAR-α activation using fibrates does not stimulate renoprotective effects of syngeneic murine eEOCs in ischemic AKI, although lower fibrate concentrations significantly activate eEOCs in vitro.

Peroxisome proliferator-activated receptor α mediates enhancement of gene expression of cerebroside sulfotransferase in several murine organs

Sulfatides, 3-O-sulfogalactosylceramides, are known to have multifunctional properties. These molecules are distributed in various tissues of mammals, where they are synthesized from galactosylceramides by sulfation at C3 of the galactosyl residue. Although this reaction is specifically catalyzed by cerebroside sulfotransferase (CST), the mechanisms underlying the transcriptional regulation of this enzyme are not understood. With respect to this issue, we previously found potential sequences of peroxisome proliferator-activated receptor (PPAR) response element on upstream regions of the mouse CST gene and presumed the possible regulation by the nuclear receptor PPARα. To confirm this hypothesis, we treated wild-type and Ppara-null mice with the specific PPARα agonist fenofibrate and examined the amounts of sulfatides and CST gene expression in various tissues. Fenofibrate treatment increased sulfatides and CST mRNA levels in the kidney, heart, liver, and small intestine in a PPARα-dependent manner. However, these effects of fenofibrate were absent in the brain or colon. Fenofibrate treatment did not affect the mRNA level of arylsulfatase A, which is the key enzyme for catalyzing desulfation of sulfatides, in any of these six tissues. Analyses of the DNA-binding activity and conventional gene expression targets of PPARα has demonstrated that fenofibrate treatment activated PPARα in the kidney, heart, liver, and small intestine but did not affect the brain or colon. These findings suggest that PPARα activation induces CST gene expression and enhances sulfatide synthesis in mice, which suggests that PPARα is a possible transcriptional regulator for the mouse CST gene.

Attenuation of kidney injuries maintains serum sulfatide levels dependent on hepatic synthetic ability: a possible involvement of oxidative stress

Serum sulfatides are the major glycosphingolipids in lipoproteins. Although serum sulfatides are mainly synthesized and secreted by the liver, they are significantly decreased when the kidneys are impaired. Our recent experimental study using a murine protein-overload nephropathy model suggested a hypothetical mechanism whereby serum sulfatides were reduced due to kidney dysfunction. This was the result of decreased hepatic expression of a sulfatide synthetic enzyme, cerebroside sulfotransferase (CST), which is associated with systemic enhancement of oxidative stress. However, there is a possibility that the experimental process, protein-overload itself, directly affected the sulfatide metabolism and oxidative stress in the liver. To determine whether kidney dysfunction actually reduces the hepatic synthesis of sulfatides via oxidative stress, we examined sulfatide levels, the hepatic content of metabolic sulfatide enzymes, and the degree of oxidative stress in protein-overload mice subjected to renoprotective therapy using clofibrate, a representative hypolipidemic medicine. Protein-overload mice exhibited marked kidney injuries, enhancement of hepatic oxidative stress, decreased levels of serum and hepatic sulfatides, and decreased expression of hepatic CST. The clofibrate treatment attenuated kidney damage and hepatic oxidative stress while maintaining serum/hepatic sulfatide levels and hepatic CST content in the mice. Because clofibrate monotherapy without protein-overload treatment only minimally affected these hepatic parameters, the hepatic synthesis of sulfatides appeared to be strongly influenced by kidney dysfunction and subsequent oxidative stress. This study suggests that the crosstalk between kidney dysfunction and hepatic sulfatide metabolism is mediated by oxidative stress. These results should help to understand the phenomenon in patients with end-stage kidney disease.

PPARα Activation Protects against Anti-Thy1 Nephritis by Suppressing Glomerular NF-κB Signaling

The vast increase of chronic kidney disease (CKD) has attracted considerable attention worldwide, and the development of a novel therapeutic option against a representative kidney disease that leads to CKD, mesangial proliferative glomerulonephritis (MsPGN) would be significant. Peroxisome proliferator-activated receptor α (PPARα), a member of the steroid/nuclear receptor superfamily, is known to perform various physiological functions. Recently, we reported that PPARα in activated mesangial cells exerted anti-inflammatory effects and that the deficiency of PPARα resulted in high susceptibility to glomerulonephritis. To investigate whether PPARα activation improves the disease activity of MsPGN, we examined the protective effects of a PPARα agonist, clofibrate, in a well-established model of human MsPGN, anti-Thy1 nephritis, for the first time. This study demonstrated that pretreatment with clofibrate (via a 0.02% or 0.1% clofibrate-containing diet) continuously activated the glomerular PPARα, which outweighed the PPARα deterioration associated with the nephritic process. The PPARα activation appeared to suppress the NF-κB signaling pathway in glomeruli by the induction of IκBα, resulting in the reduction of proteinuria and the amelioration of the active inflammatory pathologic glomerular changes. These findings suggest the antinephritic potential of PPARα-related medicines against MsPGN. PPARα-related medicines might be useful as a treatment option for CKD.

Fatty Acid Accumulation and Resulting PPARα Activation in Fibroblasts due to Trifunctional Protein Deficiency

To examine fatty acid accumulation and its toxic effects in cells, we analyzed skin fibroblasts from six patients with mitochondrial trifunctional protein deficiency, who had abnormalities in the second through fourth reactions in fatty acid β-oxidation system. We found free fatty acid accumulation, enhanced three acyl-CoA dehydrogenases, catalyzing the first reaction in the β-oxidation system and being assumed to have normal activities in these patients, and PPARα activation that was confirmed in the experiments using MK886, a PPARα specific antagonist and fenofibrate, a PPARα specific agonist. These novel findings suggest that the fatty acid accumulation and the resulting PPARα activation are major causes of the increase in the β-oxidation ability as probable compensation for fatty acid metabolism in the patients' fibroblasts, and that enhanced cell proliferation and increased oxidative stress due to the PPARα activation relate to the development of specific clinical features such as hypertrophic cardiomyopathy, slight hepatomegaly, and skeletal myopathy. Additionally, significant suppression of the PPARα activation by means of MK886 treatment is assumed to provide a new method of treating this deficiency.

Hepatic Cerebroside Sulfotransferase Is Induced by PPARα Activation in Mice

Sulfatides are one of the major sphingoglycolipids in mammalian serum and are synthesized and secreted mainly from the liver as a component of lipoproteins. Recent studies revealed a protective role for serum sulfatides against arteriosclerosis and hypercoagulation. Although peroxisome proliferator-activated receptor (PPAR) α has important functions in hepatic lipoprotein metabolism, its association with sulfatides has not been investigated. In this study, sulfatide levels and the expression of enzymes related to sulfatide metabolism were examined using wild-type (+/+), Ppara-heterozygous (+/−), and Ppara-null (−/−) mice given a control diet or one containing 0.1% fenofibrate, a clinically used hypolipidemic drug and PPARα activator. Fenofibrate treatment increased serum and hepatic sulfatides in Ppara (+/+) and (+/−) mice through a marked induction of hepatic cerebroside sulfotransferase (CST), a key enzyme in sulfatide synthesis, in a PPARα-dependent manner. Furthermore, increases in CST mRNA levels were correlated with mRNA elevations of several known PPARα target genes, and such changes were not observed for other sulfatide-metabolism enzymes in the liver. These results suggest that PPARα activation enhances hepatic sulfatide synthesis via CST induction and implicate CST as a novel PPARα target gene.

Geldanamycin derivative ameliorates high fat diet-induced renal failure in diabetes

Diabetic nephropathy is a serious complication of longstanding diabetes and its pathogenesis remains unclear. Oxidative stress may play a critical role in the pathogenesis and progression of diabetic nephropathy. Our previous studies have demonstrated that polyunsaturated fatty acids (PUFA) induce peroxynitrite generation in primary human kidney mesangial cells and heat shock protein 90β1 (hsp90β1) is indispensable for the PUFA action. Here we investigated the effects of high fat diet (HFD) on kidney function and structure of db/db mice, a widely used rodent model of type 2 diabetes. Our results indicated that HFD dramatically increased the 24 h-urine output and worsened albuminuria in db/db mice. Discontinuation of HFD reversed the exacerbated albuminuria but not the increased urine output. Prolonged HFD feeding resulted in early death of db/db mice, which was associated with oliguria and anuria. Treatment with the geldanamycin derivative, 17-(dimethylaminoehtylamino)-17-demethoxygeldanamycin (17-DMAG), an hsp90 inhibitor, preserved kidney function, and ameliorated glomerular and tubular damage by HFD. 17-DMAG also significantly extended survival of the animals and protected them from the high mortality associated with renal failure. The benefit effect of 17-DMAG on renal function and structure was associated with a decreased level of kidney nitrotyrosine and a diminished kidney mitochondrial Ca²⁺ efflux in HFD-fed db/db mice. These results suggest that hsp90β1 is a potential target for the treatment of nephropathy and renal failure in diabetes.