Fenofibrate attenuates tubulointerstitial fibrosis and inflammation through suppression of nuclear factor-κB and transforming growth factor-β1/Smad3 in diabetic nephropathy

Inflammation and the pathogenesis of diabetic nephropathy

The most problematic issue in clinical nephrology is the relentless and progressive increase in patients with ESRD (end-stage renal disease) worldwide. The impact of diabetic nephropathy on the increasing population with CKD (chronic kidney disease) and ESRD is enormous. Three major pathways showing abnormality of intracellular metabolism have been identified in the development of diabetic nephropathy: (i) the activation of polyol and PKC (protein kinase C) pathways; (ii) the formation of advanced glycation end-products; and (iii) intraglomerular hypertension induced by glomerular hyperfiltration. Upstream of these three major pathways, hyperglycaemia is the major driving force of the progression to ESRD from diabetic nephropathy. Downstream of the three pathways, microinflammation and subsequent extracellular matrix expansion are common pathways for the progression of diabetic nephropathy. In recent years, many researchers have been convinced that the inflammation pathways play central roles in the progression of diabetic nephropathy, and the identification of new inflammatory molecules may link to the development of new therapeutic strategies. Various molecules related to the inflammation pathways in diabetic nephropathy include transcription factors, pro-inflammatory cytokines, chemokines, adhesion molecules, Toll-like receptors, adipokines and nuclear receptors, which are candidates for the new molecular targets for the treatment of diabetic nephropathy. Understanding of these molecular pathways of inflammation would translate into the development of anti-inflammation therapeutic strategies.

Differential effects of low-dose fenofibrate treatment in diabetic rats with early onset nephropathy and established nephropathy

We have previously shown that low-dose fenofibrate treatment has an ability to prevent diabetes-induced nephropathy in rats. We investigated here the comparative pre- and post-treatment effects of low-dose fenofibrate (30 mg/kg/day p.o.) in diabetes-induced onset of nephropathy. Rats were made diabetics by single administration of streptozotocin (STZ, 55 mg/kg i.p.). The development of diabetic nephropathy was assessed biochemically and histologically. Moreover, lipid profile and renal oxidative stress were assessed. Diabetic rats after 8 weeks of STZ-administration developed apparent nephropathy by elevating serum creatinine, blood urea nitrogen and microproteinuria, and inducing glomerular-capsular wall distortion, mesangial expansion and tubular damage and renal oxidative stress. Fenofibrate (30 mg/kg/day p.o., 4 weeks) pretreatment (4 weeks after STZ-administration) markedly prevented diabetes-induced onset of diabetic nephropathy, while the fenofibrate (30 mg/kg/day p.o., 4 weeks) post-treatment (8 weeks after STZ-administration) was less-effective. However, both pre-and post fenofibrate treatments were effective in preventing diabetes-induced renal oxidative stress and lipid alteration in diabetic rats though the pretreatment was slightly more effective. Conversely, both pre-and post fenofibrate treatments did not alter elevated glucose levels in diabetic rats. It may be concluded that diabetes-induced oxidative stress and lipid alteration, in addition to a marked glucose elevation, play a detrimental role in the onset of nephropathy in diabetic rats. The pretreatment with low-dose fenofibrate might be a potential therapeutic approach in preventing the onset of nephropathy in diabetic subjects under the risk of renal disease induction. However, low-dose fenofibrate treatment might not be effective in treating the established nephropathy in diabetic subjects.

Fibrates: therapeutic potential for diabetic nephropathy?

Despite intensive glucose-lowering treatment and advanced therapies for cardiovascular risk factors, such as hypertension and dyslipidaemia, diabetes mellitus with its macro- and microvascular complications remains a major health problem. Especially diabetic nephropathy is a leading cause of morbidity and mortality, and its prevalence is increasing. Peroxisome proliferator-activated receptor-α (PPAR-α), a member of a large nuclear receptor superfamily, is expressed in several tissues including the kidney. Recently, experimental data have suggested that PPAR-α activation plays a pivotal role in the regulation of fatty acid oxidation, lipid metabolism, inflammatory and vascular responses, and might regulate various metabolic and intracellular signalling pathways that lead to diabetic microvascular complications. This review examines the role of PPAR-α activation in diabetic nephropathy and summarises data from experimental and clinical studies on the emerging therapeutic potential of fibrates in diabetic nephropathy.

The role of nuclear receptors in the kidney in obesity and metabolic syndrome

Nuclear receptors are ligand-activated transcriptional regulators of several key aspects of renal physiology and pathophysiology. As such, nuclear receptors control a large variety of metabolic processes, including kidney lipid metabolism, drug clearance, inflammation, fibrosis, cell differentiation, and oxidative stress. Derangement of nuclear receptor regulation, that is, mainly due to obesity may induce metabolic syndrome, may contribute to the pathogenesis and progression of chronic renal disease and may result in end-stage renal disease. This places nuclear receptors at the forefront of novel therapeutic approaches for a broad range of kidney disorders and diseases, including glomerulosclerosis, tubulointerstitial disease, renal lipotoxicity, kidney fibrosis, and hypertension. This review focuses on the importance of the transcription factors peroxisome proliferator-activated receptor alpha, peroxisome proliferator-activated receptor beta, peroxisome proliferator-activated receptor gamma, liver X receptors, farnesoid X receptor, and the pregnane X receptor/steroid and xenobiotic receptor (PXR) on the physiology and pathophysiology of renal diseases associated with obesity and metabolic syndrome.

Myocardial infarction impairs renal function, induces renal interstitial fibrosis, and increases renal KIM-1 expression: implications for cardiorenal syndrome

Progressive decline in renal function coexists with myocardial infarction (MI); however, little is known about its pathophysiology. This study aimed to systematically identify post-MI renal changes (functional, histological, and molecular) over time in a rat MI model and examine potential mechanisms that may underlie these changes. Rats were randomized into three groups: nonoperated, sham, and MI. Cardiac and renal function was assessed before death at 1, 4, 8, 12, and 16 wk with tissues collected for histological, protein, and gene studies. Tail-cuff blood pressure was lower in MI than sham and nonoperated animals only at 1 wk (P < 0.05). Systolic function was reduced (P < 0.0001) while heart/body weight and left ventricle/body weight were significantly greater in MI animals at all time points. Glomerular filtration rate decreased following MI at 1 and 4 wk (P < 0.05) but not at 8 and 12 wk and then deteriorated further at 16 wk (P = 0.052). Increased IL-6 gene and transforming growth factor (TGF)-β protein expression as well as macrophage infiltration in kidney cortex was detected at 1 wk (P < 0.05). Renal cortical interstitial fibrosis was significantly greater in MI animals from 4 wk, while TGF-β bioactivity (phospho-Smad2) was upregulated at all time points. The degree of fibrosis increased and was maximal at 16 wk. In addition, kidney injury molecule-1-positive staining in the tubules was more prominent in MI animals, maximal at 1 wk. In conclusion, renal impairment occurs early post-MI and is associated with hemodynamic and structural changes in the kidney possibly via activation of the Smad2 signaling pathway.

The renoprotective actions of peroxisome proliferator-activated receptors agonists in diabetes

Pharmaceutical agonists of peroxisome proliferator-activated receptors (PPARs) are widely used in the management of type 2 diabetes, chiefly as lipid-lowering agents and oral hypoglycaemic agents. Although most of the focus has been placed on their cardiovascular effects, both positive and negative, these agents also have significant renoprotective actions in the diabetic kidney. Over and above action on metabolic control and effects on blood pressure, PPAR agonists also appear to have independent effects on a number of critical pathways that are implicated in the development and progression of diabetic kidney disease, including oxidative stress, inflammation, hypertrophy, and podocyte function. This review will examine these direct and indirect actions of PPAR agonists in the diabetic kidney and explore recent findings of clinical trials of PPAR agonists in patients with diabetes.

Targeting of a platinum-bound sunitinib analog to renal proximal tubular cells

Background

Activated proximal tubular cells play an important role in renal fibrosis. We investigated whether sunitinib and a kidney-targeted conjugate of sunitinib were capable of attenuating fibrogenic events in tubulointerstitial fibrosis.

Methods

A kidney-targeted conjugate was prepared by linkage of a sunitinib analog (named 17864) via a platinum-based linker to the kidney-specific carrier lysozyme. Pharmacological activity of 17864-lysozyme was evaluated in human kidney proximal tubular cells (HK-2); the capability of the kidney-directed conjugate to accumulate in the kidneys was studied in mice. Potential antifibrotic effects of a single-dose treatment were evaluated in the unilateral ureteral obstruction (UUO) model in mice.

Results

The 17864-lysozyme conjugate and its metabolites strongly inhibited tyrosine kinase activity. Upon intravenous injection, 17864-lysozyme rapidly accumulated in the kidneys and provided sustained renal drug levels for up to 3 days after a single dose. Renal drug level area under the curve was increased 28-fold versus an equimolar dose of sunitinib malate. Daily treatment of UUO mice with a high dose of sunitinib malate (50 mg/kg) resulted in antifibrotic responses, but also induced drug-related toxicity. A single dose of 17864-lysozyme (equivalent to 1.8 mg/kg sunitinib) was safe but showed no antifibrotic effects.

Conclusion

Multikinase inhibitors like sunitinib can be of benefit in the treatment of fibrotic diseases, provided that their safety can be improved by strategies as presented in this paper, and sustained renal levels can be achieved.

Are PPAR alpha agonists a rational therapeutic strategy for preventing abnormalities of the diabetic kidney?

The uncontrolled diabetes mellitus may result in the induction of diabetic nephropathy, one of the detrimental microvascular complications of diabetes mellitus. Diabetic nephropathy is associated with glomerular hypertrophy, glomerulosclerosis, tubulointerstitial fibrosis, mesangial cell expansion, followed by albuminuria and reduction in glomerular filtration rate. Indeed, no promising therapeutic options are available in the present clinical scenario to manage efficiently the diabetic nephropathy. Nevertheless, angiotensin converting enzyme inhibitors and angiotensin-II-AT(1) receptor blockers are currently employed to improve structural and functional status of the diabetic kidney. These interventions, however, are not optimal in improving overall outcomes of diabetic nephropathy. Hence, there is a continuing need of developing promising therapeutic interventions to manage this insidious condition adequately. Recent bench and clinical studies strongly suggest the potentials of peroxisome proliferator-activated receptor alpha (PPARα) agonists in the management of diabetic nephropathy by keeping the view that renal lipid accumulation-induced lipotoxicity is one of risk factors for nephropathy during chronic diabetes mellitus. As inflammation, oxidative stress and dyslipidemia are common consequences of renal dysfunction, PPARα agonists could serve as promising therapeutic agents for controlling the progression of diabetic nephropathy. In fact, fenofibrate, a hypolipidemic agent acts as a PPARα agonist, reduced renal lipotoxicity, inflammation, fibrosis and oxidative stress, and subsequently prevented the symptoms of diabetic nephropathy. However, fenofibrate has been shown to cause renal dysfunction in established renal disorders. The present review addressed the rationale of employing PPARα agonists in the management of diabetic nephropathy.

Fluvastatin inhibits angiotensin II-induced nuclear factor kappa B activation in renal tubular epithelial cells through the p38 MAPK pathway

3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors has been shown to reduce the progression of renal disease independent of cholesterol-lowering effect, but the mechanism of potential protective effect remains unclear. Here, we investigate the effect of fluvastatin on activation of nuclear factor-κB (NF-κB) induced by angiotensin II (AngII) in rat kidney tubule epithelial cells (NRK-52E). Electrophoretic mobility shift assays (EMSA) was used to detect NF-κB activation. Phosphorylation of cellular p38 mitogen-activated protein kinase (p38MAPK) was determined by western blot analysis. AngII stimulated the DNA-binding activity of NF-κB and phosphorylation of p38MAPK in cultured NRK-52E cells in a dose-dependent (10(-9)-10(-6) mol/l) manner (P < 0.01). AngII (10(-6) mol/l) induced a rapid (5 min) increase of the p38MAPK phosphorylation. NF-κB DNA-binding activity was increased at as early as 30 min, peaked at 2 h after AngII treatment. This stimulatory effect of AngII on NF-κB was blocked by SB203580 (a specific inhibitor of p38MAPK). Incubation of cells with fluvastatin significantly inhibited the AngII-induced NF-κB activation in a dose-dependent (10(-7)-10(-5) mol/l) manner (P < 0.05). Exogenous mevalonate (10(-4)mol/l) prevented the effect of fluvastatin on NF-κB activation. These results suggest the fluvastatin reduced AngII-induced NF-κB activation via the p38MAPK pathway in NRK-52E cells. The effect is at least partly due to blocking the biosynthesis of mevalonate.

Glomerular expression of kidney injury molecule-1 and podocytopenia in diabetic glomerulopathy

BACKGROUND/AIMS

Studies have shown that kidney injury molecule-1 (KIM-1) is upregulated in damaged renal proximal tubules. In this study, we examined KIM-1 expression in glomerular epithelial cells in diabetic glomerulopathy.

METHODS

Renal histology, immunostaining and Western blot for protein level, and real-time PCR for mRNA expression of KIM-1 and podocyte markers were evaluated in untreated or losartan-treated Zucker lean (Fa/+) and Zucker diabetic fatty (Fa/Fa) rats.

RESULTS

The diabetic rats showed an increased glomerular expression of KIM-1. KIM-1 staining was localized primarily in the hyperplastic parietal epithelium of Bowman's capsule in the early stages of diabetes with subsequent increase in KIM-1-positive cells in the glomerular tuft in the more advanced stages. The increase in glomerular KIM-1 was associated with a decrease in podocytes in Fa/Fa rats. Antiproteinuric treatment with losartan attenuated podocytopenia and decreased renal expression of KIM-1 in treated diabetic rats. In an in vitro study, albumin overload increased KIM-1 protein in the primary cultures of rat glomerular epithelial cells.

CONCLUSION

These results show that glomerular KIM-1 expression was increased, in proportion to the extent of proteinuria and podocytopenia in the diabetic animals, supporting that KIM-1 could be used as a potential biomarker for glomerular injury in proteinuric kidney disease.

Aldosterone-induced kidney injury is mediated by NFκB activation

BACKGROUND

Aldosterone induces inflammation and fibrosis in the kidney, while nuclear factor κB (NFκB) plays key roles in inflammation mediated by various cytokines. Here, we determined the roles of NFκB activation in aldosterone-induced kidney injury.

METHODS

We used unilaterally nephrectomized rats with or without continuous aldosterone infusion and 0.9% saline as drinking water for 3 weeks. IMD-1041, an IKKβ inhibitor, and spironolactone were orally administered to inhibit NFκB and mineralocorticoid receptor, respectively.

RESULTS

The aldosterone-infused rats exhibited severe kidney injury, hypertension, and increased expression of pro-inflammatory and fibrotic proteins, osteopontin, fibrinogen, collagen type I, and PAI-1. Western blotting confirmed NFκB activation by aldosterone by the increased amount of p65 in the nuclear fraction of the kidney, and oral IMD-1041 prevented the kidney injury and lessened the increase in pro-inflammatory and fibrotic proteins without significant changes in blood pressures. In addition, changes in angiotensin-converting enzyme 2 (ACE2), which has been found to act as a protective factor in various kidney injury models, were examined. Immunofluorescence studies revealed the presence of ACE2 in the brush-border membrane of the proximal convoluted tubules and markedly blunted ACE2 staining in aldosterone-infused rats. The decrease in amount of ACE2 protein was confirmed by Western blotting, and IMD-1041 also prevented the decrease in ACE2. The administration of spironolactone also abolished the effects of aldosterone.

CONCLUSION

Our results suggest that aldosterone induces kidney injury via activation of NFκB and mineralocorticoid receptor, and that decreased ACE2 expression may play an important role in aldosterone-induced kidney injury.

Role of PPARα and Its Agonist in Renal Diseases

Peroxisome proliferator-activated receptor (PPAR)-α, a member of a large nuclear receptor superfamily, plays a major role in the regulation of lipid metabolism. Recently, PPARα activation has been shown to confer additional benefits on endothelial function, kidney function, and anti-inflammation, suggesting that PPARα agonists may be good candidates for treating acute renal failure. In clinical application, PPAR-α activators, such as hypolipidemic drugs in fibric acid class, were proven to have therapeutic effects on metabolic syndrome and cardiovascular disease. This paper focuses on signaling pathways, ligand selectivity, and physio-pathological roles of PPARα in kidney diseases and the therapeutic utility of PPARα modulators in the treatment of diabetes and inflammation-induced nephropathy. Implication of new and more potent PPAR-α activators could provide important insights into the overall benefits of activating PPAR-α clinically for the treatment of dyslipidemia and the prevention of diabetic or inflammation-induced nephropathy in the future.