Improvement of endothelial nitric oxide synthase activity retards the progression of diabetic nephropathy in db/db mice

Crosstalk among podocytes, glomerular endothelial cells and mesangial cells in diabetic kidney disease: an updated review

Diabetic kidney disease (DKD) is a long-term and serious complication of diabetes that affects millions of people worldwide. It is characterized by proteinuria, glomerular damage, and renal fibrosis, leading to end-stage renal disease, and the pathogenesis is complex and involves multiple cellular and molecular mechanisms. Among three kinds of intraglomerular cells including podocytes, glomerular endothelial cells (GECs) and mesangial cells (MCs), the alterations in one cell type can produce changes in the others. The cell-to-cell crosstalk plays a crucial role in maintaining the glomerular filtration barrier (GFB) and homeostasis. In this review, we summarized the recent advances in understanding the pathological changes and interactions of these three types of cells in DKD and then focused on the signaling pathways and factors that mediate the crosstalk, such as angiopoietins, vascular endothelial growth factors, transforming growth factor-β, Krüppel-like factors, retinoic acid receptor response protein 1 and exosomes, etc. Furthermore, we also simply introduce the application of the latest technologies in studying cell interactions within glomerular cells and new promising mediators for cell crosstalk in DKD. In conclusion, this review provides a comprehensive and updated overview of the glomerular crosstalk in DKD and highlights its importance for the development of novel intervention approaches.

Optimizing diabetic kidney disease animal models: Insights from a meta-analytic approach

Diabetic kidney disease (DKD) is a prevalent complication of diabetes, often leading to end-stage renal disease. Animal models have been widely used to study the pathogenesis of DKD and evaluate potential therapies. However, current animal models often fail to fully capture the pathological characteristics of renal injury observed in clinical patients with DKD. Additionally, modeling DKD is often a time-consuming, costly, and labor-intensive process. The current review aims to summarize modeling strategies in the establishment of DKD animal models by utilizing meta-analysis related methods and to aid in the optimization of these models for future research. A total of 1215 articles were retrieved with the keywords of "diabetic kidney disease" and "animal experiment" in the past 10 years. Following screening, 84 articles were selected for inclusion in the meta-analysis. Review manager 5.4.1 was employed to analyze the changes in blood glucose, glycosylated hemoglobin, total cholesterol, triglyceride, serum creatinine, blood urea nitrogen, and urinary albumin excretion rate in each model. Renal lesions shown in different models that were not suitable to be included in the meta-analysis were also extensively discussed. The above analysis suggested that combining various stimuli or introducing additional renal injuries to current models would be a promising avenue to overcome existing challenges and limitations. In conclusion, our review article provides an in-depth analysis of the limitations in current DKD animal models and proposes strategies for improving the accuracy and reliability of these models that will inspire future research efforts in the DKD research field.

Renal intrinsic cells remodeling in diabetic kidney disease and the regulatory effects of SGLT2 Inhibitors

Diabetic kidney disease (DKD) is a prevalent complication of diabetes and a major secondary factor leading to end-stage renal disease. The kidney, a vital organ, is composed of a heterogeneous group of intrinsic cells, including glomerular endothelial cells, podocytes, mesangial cells, tubular epithelial cells, and interstitial fibroblasts. In the context of DKD, hyperglycemia elicits direct or indirect injury to these intrinsic cells, leading to their structural and functional changes, such as cell proliferation, apoptosis, and transdifferentiation. The dynamic remodeling of intrinsic cells represents an adaptive response to stimulus during the pathogenesis of diabetic kidney disease. However, the persistent stimulus may trigger an irreversible remodeling, leading to fibrosis and functional deterioration of the kidney. Sodium-glucose cotransporter 2 (SGLT2) inhibitors, a new class of hypoglycemic drugs, exhibit efficacy in reducing blood glucose levels by curtailing renal tubular glucose reabsorption. Furthermore, SGLT2 inhibitors have been shown to modulate intrinsic cell remodeling in the kidney, ameliorate kidney structure and function, and decelerate DKD progression. This review will elaborate on the intrinsic cell remodeling in DKD and the underlying mechanism of SGLT2 inhibitors in modulating it from the perspective of the renal intrinsic cell, providing insights into the pathogenesis of DKD and the renal protective action of SGLT2 inhibitors.

Tetrahydrobiopterin (BH4) Supplementation Prevents the Cardiorenal Effects of Diabetes in Mice by Reducing Oxidative Stress, Inflammation and Fibrosis

Background: The effects of diabetes on the cardiovascular system as well as in the kidney are profound, which include hypertrophy and fibrosis. Diabetes also induces oxidative stress, at least in part due to the uncoupling of nitric oxide synthase (NOS); this is a shift in NO production toward superoxide production due to reduced levels of the NOS cofactor tetrahydrobiopterin (BH4). With this in mind, we tested the hypothesis that BH4 supplementation may prevent the development of diabetic cardiomyopathy and nephropathy. Methods: Diabetes was induced in Balb/c mice with streptozotocin. Then, diabetic mice were divided into two groups: one group provided with BH4 (sapropterin) in drinking water (daily doses of 15 mg/kg/day, during eight weeks) and the other that received only water. A third group of normoglycemic mice that received only water were used as the control. Results: Cardiac levels of BH4 were increased in mice treated with BH4 (p = 0.0019). Diabetes induced cardiac hypertrophy, which was prevented in the group that received BH4 (p < 0.05). In addition, hypertrophy was evaluated as cardiomyocyte cross-sectional area. This was reduced in diabetic mice that received BH4 (p = 0.0012). Diabetes induced cardiac interstitial fibrosis that was reduced in mice that received BH4 treatment (p < 0.05). We also evaluated in the kidney the impact of BH4 treatment on glomerular morphology. Diabetes induced glomerular hypertrophy compared with normoglycemic mice and was prevented by BH4 treatment. In addition, diabetic mice presented glomerular fibrosis, which was prevented in mice that received BH4. Conclusions: These results suggest that chronic treatment with BH4 in mice ameliorates the cardiorenal effects of diabetes,, probably by restoring the nitroso−redox balance. This offers a possible new alternative to explore a BH4-based treatment for the organ damage caused by diabetes.

Restoration of Autophagic Flux Improves Endothelial Function in Diabetes Through Lowering Mitochondrial ROS-Mediated eNOS Monomerization

Endothelial nitric oxide synthase (eNOS) monomerization and uncoupling play crucial roles in mediating vascular dysfunction in diabetes, although the underlying mechanisms are still incompletely understood. Increasing evidence indicates that autophagic dysregulation is involved in the pathogenesis of diabetic endothelial dysfunction; however, whether autophagy regulates eNOS activity through controlling eNOS monomerization or dimerization remains elusive. In this study, autophagic flux was impaired in the endothelium of diabetic db/db mice and in human endothelial cells exposed to advanced glycation end products or oxidized low-density lipoprotein. Inhibition of autophagic flux by chloroquine or bafilomycin A1 were sufficient to induce eNOS monomerization and lower nitric oxide bioavailability by increasing mitochondrial reactive oxygen species (mtROS). Restoration of autophagic flux by overexpressing transcription factor EB (TFEB), a master regulator of autophagy and lysosomal biogenesis, decreased endothelial cell oxidative stress, increased eNOS dimerization, and improved endothelium-dependent relaxations (EDRs) in db/db mouse aortas. Inhibition of mammalian target of rapamycin kinase (mTOR) increased TFEB nuclear localization, reduced mtROS accumulation, facilitated eNOS dimerization, and enhanced EDR in db/db mice. Moreover, calorie restriction also increased TFEB expression, improved autophagic flux, and restored EDR in the aortas of db/db mice. Taken together, the findings of this study reveal that mtROS-induced eNOS monomerization is closely associated with the impaired TFEB-autophagic flux axis leading to endothelial dysfunction in diabetic mice.

Newly established LC-MS/MS method for measurement of plasma BH4 as a predictive biomarker for kidney injury in diabetes

OBJECTIVE

The clinical research on BH4 is limited because of the difficulties on its measurement. In this study, we used our own established LC-MS/MS method to examine the plasma BH4 levels in diabetes to determine whether it could be used as a biomarker for the prediction of kidney injury in those patients.

METHODS

Hospitalized diabetes patients in Renmin Hospital of Wuhan University from Jan to Aug 2021 were recruited. To assess the association between plasma BH4 with ACR or eGFR in diabetes, a total of 142 patients with type 2 diabetes (T2DM) were enrolled. They were divided into three groups by albuminuria levels: normoalbuminuria (n = 68), microalbuminuria (n = 48), and macroalbuminuria (n = 26) according to ACR; or into two groups by eGFR: eGFR≥90 or eGFR<90 ml/min for correlation and logistic regression analysis. Plasma BH4 level was measured by LC-MS/MS along with other biochemical indices.

RESULTS

Plasma BH4 concentrations were decreased as ACR progressed. BH4 (r = -0.55, P < 0.001) and 2h C-Peptide (CP-2h) (r = -0.248, P = 0.003) levels were negatively correlated with ACR. Moreover, multivariable logistic regression analysis showed BH4 concentrations (B = -0.468, P < 0.001) and CP-2h (B = -0.257, P = 0.028) were independently associated with ACR progression. ROC curve showed that BH4 level has a predictive value on ACR (95%CI 0.686-0.841, sensitivity 69.1%, specificity 73%). Moreover, in diabetes patients with eGFR≥90 ml/min, plasma BH4 level (P = 0.008) is higher than those in diabetes with eGFR<90 ml/min and BH4 was remained independently associated with eGFR after multivariable logistic regression analysis (B = -0.193, P = 0.048).

CONCLUSION

Our established LC-MS/MS method could be used on human plasma BH4 measurements and our data suggested that BH4 level can be used as a biomarker for kidney injury in diabetes indicated by its association with ACR progression and early renal function decline.

Role of Endothelial Glucocorticoid Receptor in the Pathogenesis of Kidney Diseases

Glucocorticoids, as multifunctional hormones, are widely used in the treatment of various diseases including nephrological disorders. They are known to affect immunological cells, effectively treating many autoimmune and inflammatory processes. Furthermore, there is a growing body of evidence demonstrating the potent role of glucocorticoids in non-immune cells such as podocytes. Moreover, novel data show additional pathways and processes affected by glucocorticoids, such as the Wnt pathway or autophagy. The endothelium is currently considered as a key organ in the regulation of numerous kidney functions such as glomerular filtration, vascular tone and the regulation of inflammation and coagulation. In this review, we analyse the literature concerning the effects of endothelial glucocorticoid receptor signalling on kidney function in health and disease, with special focus on hypertension, diabetic kidney disease, glomerulopathies and chronic kidney disease. Recent studies demonstrate the potential role of endothelial GR in the prevention of fibrosis of kidney tissue and cell metabolism through Wnt pathways, which could have a protective effect against disease progression. Another important aspect covered in this review is blood pressure regulation though GR and eNOS. We also briefly cover potential therapies that might affect the endothelial glucocorticoid receptor and its possible clinical implications, with special interest in selective or local GR stimulation and potential mitigation of GC treatment side effects.

Endothelial TRPV4-eNOS coupling as a vital therapy target for treatment of hypertension

BACKGROUND AND PURPOSE

Reduced nitric oxide (NO) level and activity are signs of endothelial dysfunction, which is important in mediating blood pressure up-regulation. Previously, we demonstrated that transient receptor potential channel V4 (TRPV4) could form functional complex with other proteins to mediate vasodilation in the Endothelial cells (ECs). But how TRPV4 interacts with the NO pathway in larger arteries requires further exploration.

EXPERIMENTAL APPROACH

We used single-cell RNA-sequencing to find the CD106⁺ TRPV4^high NOS3^high ECs. The TRPV4-eNOS interaction was verified by co-immunoprecipitation and Immunofluorescence resonance energy transfer (FRET), and their binding site was found by site-directed mutagenesis. Endothelium-specific TRPV4 knockout (TRPV4_EC ^-/- ) mice were used to study the effect of the TRPV4-eNOS interaction on blood pressure. A small molecule, JNc-463 was designed through molecular docking technology.

KEY RESULTS

We uncovered CD106⁺ TRPV4^high NOS3^high ECs in the mouse aorta, which they could regulate vasodilation via a TRPV4-eNOS interaction, and they were essential to regulate blood pressure. The TRPV4-eNOS interaction markedly decreased during the process of hypertension. We further attempted to identify the molecules re-join the TRPV4-eNOS interaction and develop a small-molecule drug, JNc-463, which could increase the TRPV4-eNOS interaction to enhance vasodilation, and exert antihypertensive effects in mice.

CONCLUSION AND IMPLICATIONS

This is the first study integrating single-cell RNA-Seq, single-cell functional study and drug screening in aorta. We identified a subpopulation of CD106⁺ TRPV4^high NOS3^high ECs, in which an impaired TRPV4-eNOS interaction was important in the progress of hypertension and we designed a small molecule, JNc-463 to improve the impaired TRPV4-eNOS interaction in hypertension.

Coagulation, Protease-Activated Receptors, and Diabetic Kidney Disease: Lessons from eNOS-Deficient Mice

Endothelial nitric oxide synthase (eNOS) dysfunction is known to exacerbate the progression and prognosis of diabetic kidney disease (DKD). One of the mechanisms through which this is achieved is that low eNOS levels are associated with hypercoagulability, which promotes kidney injury. In the extrinsic coagulation cascade, the tissue factor (factor III) and downstream coagulation factors, such as active factor X (FXa), exacerbate inflammation through activation of the protease-activated receptors (PARs). Recently, it has been shown that the lack of or reduced eNOS expression in diabetic mice, as a model of advanced DKD, increases renal tissue factor levels and PAR1 and 2 expression in their kidneys. Furthermore, pharmaceutical inhibition or genetic deletion of coagulation factors or PARs ameliorated inflammation in DKD in mice lacking eNOS. In this review, we summarize the relationship between eNOS, coagulation, and PARs and propose a novel therapeutic option for the management of patients with DKD.

The vascular endothelial growth factor trap aflibercept induces vascular dysfunction and hypertension via attenuation of eNOS/NO signaling in mice

Aflibercept, as a soluble decoy vascular endothelial growth factor receptor, Which has been used as a first-line monotherapy for cancers. Aflibercept often causes cardiovascular toxicities including hypertension, but the mechanisms underlying aflibercept-induced hypertension remain unknown. In this study we investigated the effect of short-term and long-term administration of aflibercept on blood pressure (BP), vascular function, NO bioavailability, oxidative stress and endothelin 1 (ET-1) in mice and cultured endothelial cells. We showed that injection of a single-dose of aflibercept (18.2, 36.4 mg/kg, iv) rapidly and dose-dependently elevated BP in mice. Aflibercept treatment markedly impaired endothelial-dependent relaxation (EDR) and resulted in NADPH oxidases 1 (NOX1)- and NADPH oxidases 4 (NOX4)-mediated generation of ROS, decreased the activation of protein kinase B (Akt) and endothelial nitric oxide synthase (eNOS) concurrently with a reduction in nitric oxide (NO) production and elevation of ET-1 levels in mouse aortas; these effects were greatly attenuated by supplementation of L-arginine (L-arg, 0.5 or 1.0 g/kg, bid, ig) before aflibercept injection. Similar results were observed in L-arg-pretreated cultured endothelial cells, showing markedly decreased ROS accumulation and AKT/eNOS/NO signaling impairment induced by aflibercept. In order to assess the effects of long-term aflibercept on hypertension and to evaluate the beneficial effects of L-arg supplementation, we administered these two drugs to WT mice for up to 14 days (at an interval of two days). Long-term administration of aflibercept resulted in a sustained increase in BP and a severely impaired EDR, which are associated with NOX1/NOX4-mediated production of ROS, increase in ET-1, inhibition of AKT/eNOS/NO signaling and a decreased expression of cationic amino acid transporter (CAT-1). The effects caused by long-term administration were greatly attenuated by L-arg supplementation in a dose-dependent manner. We conclude that aflibercept leads to vascular dysfunction and hypertension by inhibiting CAT-1/AKT/eNOS/NO signaling, increasing ET-1, and activating NOX1/NOX4-mediated oxidative stress, which can be suppressed by supplementation of L-arg. Therefore, L-arg could be a potential therapeutic agent for aflibercept-induced hypertension.

Diabetic kidney disease in type 2 diabetes: a review of pathogenic mechanisms, patient-related factors and therapeutic options

The global prevalence of diabetic kidney disease is rapidly accelerating due to an increasing number of people living with type 2 diabetes. It has become a significant global problem, increasing human and financial pressures on already overburdened healthcare systems. Interest in diabetic kidney disease has increased over the last decade and progress has been made in determining the pathogenic mechanisms and patient-related factors involved in the development and pathogenesis of this disease. A greater understanding of these factors will catalyse the development of novel treatments and influence current practice. This review summarises the latest evidence for the factors involved in the development and progression of diabetic kidney disease, which will inform better management strategies targeting such factors to improve therapeutic outcomes in patients living with diabetes.

Glomerular Endothelial Cells as Instigators of Glomerular Sclerotic Diseases

Glomerular endothelial cell (GEnC) dysfunction is important in the pathogenesis of glomerular sclerotic diseases, including Focal Segmental Glomerulosclerosis (FSGS) and overt diabetic nephropathy (DN). GEnCs form the first cellular barrier in direct contact with cells and factors circulating in the blood. Disturbances in these circulating factors can induce GEnC dysfunction. GEnC dysfunction occurs in early stages of FSGS and DN, and is characterized by a compromised endothelial glycocalyx, an inflammatory phenotype, mitochondrial damage and oxidative stress, aberrant cell signaling, and endothelial-to-mesenchymal transition (EndMT). GEnCs are in an interdependent relationship with podocytes and mesangial cells, which involves bidirectional cross-talk via intercellular signaling. Given that GEnC behavior directly influences podocyte function, it is conceivable that GEnC dysfunction may culminate in podocyte damage, proteinuria, subsequent mesangial activation, and ultimately glomerulosclerosis. Indeed, GEnC dysfunction is sufficient to cause podocyte injury, proteinuria and activation of mesangial cells. Aberrant gene expression patterns largely contribute to GEnC dysfunction and epigenetic changes seem to be involved in causing aberrant transcription. This review summarizes literature that uncovers the importance of cross-talk between GEnCs and podocytes, and GEnCs and mesangial cells in the context of the development of FSGS and DN, and the potential use of GEnCs as efficacious cellular target to pharmacologically halt development and progression of DN and FSGS.

Protective Role of Tangshen Formula on the Progression of Renal Damage in db/db Mice by TRPC6/Talin1 Pathway in Podocytes

Tangshen Formula (TSF) is a Chinese Medicine formula that has been reported to alleviate proteinuria and protect renal function in humans and animals with diabetic kidney disease (DKD). However, little is known about its mechanism in improving proteinuria. The dysregulation of podocyte cell-matrix adhesion has been demonstrated to play an important role in the pathogenesis and progression of proteinuric kidney diseases including DKD. In the present study, the underlying protective mechanism of TSF on podocytes was investigated using the murine model of type 2 DKD db/db mice in vivo and advanced glycation end products (AGEs)-stimulated primary mice podocytes in vitro. Results revealed that TSF treatment could significantly mitigate reduction of podocyte numbers and foot process effacement, reduce proteinuria, and protect renal function in db/db mice. There was a significant increase in expression of transient receptor potential canonical channel 6 (TRPC6) and a decrease in expression of talin1 in podocytes of db/db mice. The results of AGEs-stimulated primary mice podocytes showed increased cell migration and actin-cytoskeleton rearrangement. Moreover, primary mice podocytes stimulated by AGEs displayed an increase in TRPC6-dependent Ca2+ influx, a loss of talin1, and translocation of nuclear factor of activated T cell (NFATC) 2. These dysregulations in mice primary podocytes stimulated by AGEs could be significantly attenuated after TSF treatment. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), a TRPC6 agonist, blocked the protective role of TSF on podocyte cell-matrix adherence. In conclusion, TSF could protect podocytes from injury and reduce proteinuria in DKD, which may be mediated by the regulation of the TRPC6/Talin1 pathway in podocytes.

Astragaloside IV protects against diabetic nephropathy via activating eNOS in streptozotocin diabetes-induced rats

BACKGROUND

Astragaloside IV (AS-IV) was reported to play a role in improving diabetic nephropathy (DN), however, the underlying mechanisms still remain unclear. The aim of the present study is to investigate whether AS-IV ameliorates DN via the regulation of endothelial nitric oxide synthase (eNOS).

METHODS

DN model was induced in Sprague-Dawley (SD) male rats by intraperitoneal injection of 65 mg/kg streptozotocin (STZ). Rats in the AS-IV treatment group were orally gavaged with 5 mg/kg/day or 10 mg/kg/day AS-IV for eight consecutive weeks. Body weight, blood glucose, blood urea nitrogen (BUN), Serum creatinine (Scr), proteinuria and Glycosylated hemoglobin (HbA1c) levels were measured. Hematoxylin-Eosin (HE) and Periodic Acid-Schiff (PAS) staining were used to detect the renal pathology. The apoptosis status of glomerular cells was measured by TUNEL assay. The phosphorylation and acetylation of eNOS were detected by western blot. The effects of AS-IV on high-glucose (HG)-induced apoptosis and eNOS activity were also investigated in human renal glomerular endothelial cells (HRGECs) in vitro.

RESULTS

Treatment with AS-IV apparently reduced DN symptoms in diabetic rats, as evidenced by reduced BUN, Scr, proteinuria, HbA1c levels and expanding mesangial matrix. AS-IV treatment also promoted the synthesis of nitric oxide (NO) in serum and renal tissues and ameliorated the phosphorylation of eNOS at Ser 1177 with decreased eNOS acetylation. Moreover, HG-induced dysfunction of HRGECs including increased cell permeability and apoptosis, impaired eNOS phosphorylation at Ser 1177, and decreased NO production, were all reversed by AS-IV treatment.

CONCLUSIONS

These novel findings suggest that AS-IV ameliorates functional abnormalities of DN through inhibiting acetylation of eNOS and activating its phosphorylation at Ser 1177. AS-IV could be served as a potential therapeutic drug for DN.

Endothelium structure and function in kidney health and disease

The kidney harbours different types of endothelia, each with specific structural and functional characteristics. The glomerular endothelium, which is highly fenestrated and covered by a rich glycocalyx, participates in the sieving properties of the glomerular filtration barrier and in the maintenance of podocyte structure. The microvascular endothelium in peritubular capillaries, which is also fenestrated, transports reabsorbed components and participates in epithelial cell function. The endothelium of large and small vessels supports the renal vasculature. These renal endothelia are protected by regulators of thrombosis, inflammation and complement, but endothelial injury (for example, induced by toxins, antibodies, immune cells or inflammatory cytokines) or defects in factors that provide endothelial protection (for example, regulators of complement or angiogenesis) can lead to acute or chronic renal injury. Moreover, renal endothelial cells can transition towards a mesenchymal phenotype, favouring renal fibrosis and the development of chronic kidney disease. Thus, the renal endothelium is both a target and a driver of kidney and systemic cardiovascular complications. Emerging therapeutic strategies that target the renal endothelium may lead to improved outcomes for both rare and common renal diseases.

miR-372-3p is a potential diagnostic factor for diabetic nephropathy and modulates high glucose-induced glomerular endothelial cell dysfunction via targeting fibroblast growth factor-16

INTRODUCTION

Previous studies have reported that microRNAs are implicated in the pathogenesis of diabetic nephropathy (DN). In this study, the underlying molecular mechanisms and diagnostic significance of miR-372-3p were investigated in the process of DN.

MATERIAL AND METHODS

Cell proliferation and apoptosis were measured using MTT and Annexin V-FITC double staining, respectively. RT-qPCR and western blotting were used to measure the expression levels of mRNA and protein. The diagnostic power of miR-372-3p in plasma for DN was evaluated using the receiver operating characteristics (ROC) curves and the area under the ROC curves (AUC).

RESULTS

miR microarray analysis revealed that 126 miRs were significantly differentially expressed in response to high glucose stimulation. Among these miRs, high glucose stimulated miR-372-3p expression at the highest level. In vitro experimental measurements showed that knockdown of miR-372-3p showed the ability to reverse high glucose-induced glomerular endothelial cell apoptosis and impairment of eNOS/NO bioactivity. Mechanistic analysis revealed that fibroblast growth factor-16 (FGF-16) as a direct of miR-372-3p protected against high glucose-induced glomerular endothelial cell dysfunction. ROC analysis revealed that the diagnostic value of miR-372-3p, miR-15a or miR-372-3p combined with miR-15a in type 2 diabetes mellitus patients (AUC = 0.841, p < 0.001; AUC = 0.822, p < 0.001 or AUC = 0.922, p < 0.001) with DN was better than in type 1 diabetes mellitus patients (AUC = 0.805, p < 0.001; AUC = 0.722, p < 0.001 or AUC = 0.865, p < 0.001) with DN.

CONCLUSIONS

miR-372-3p might be a valuable therapeutic target and diagnostic marker for patients with DN.

Endothelial Toxicity of High Glucose and its by-Products in Diabetic Kidney Disease

Alterations of renal endothelial cells play a crucial role in the initiation and progression of diabetic kidney disease. High glucose per se, as well as glucose by-products, induce endothelial dysfunction in both large vessels and the microvasculature. Toxic glucose by-products include advanced glycation end products (AGEs), a group of modified proteins and/or lipids that become glycated after exposure to sugars, and glucose metabolites produced via the polyol pathway. These glucose-related endothelio-toxins notably induce an alteration of the glomerular filtration barrier by increasing the permeability of glomerular endothelial cells, altering endothelial glycocalyx, and finally, inducing endothelial cell apoptosis. The glomerular endothelial dysfunction results in albuminuria. In addition, high glucose and by-products impair the endothelial repair capacities by reducing the number and function of endothelial progenitor cells. In this review, we summarize the mechanisms of renal endothelial toxicity of high glucose/glucose by-products, which encompass changes in synthesis of growth factors like TGF-β and VEGF, induction of oxidative stress and inflammation, and reduction of NO bioavailability. We finally present potential therapies to reduce endothelial dysfunction in diabetic kidney disease.

Endothelium structure and function in kidney health and disease

The kidney harbours different types of endothelia, each with specific structural and functional characteristics. The glomerular endothelium, which is highly fenestrated and covered by a rich glycocalyx, participates in the sieving properties of the glomerular filtration barrier and in the maintenance of podocyte structure. The microvascular endothelium in peritubular capillaries, which is also fenestrated, transports reabsorbed components and participates in epithelial cell function. The endothelium of large and small vessels supports the renal vasculature. These renal endothelia are protected by regulators of thrombosis, inflammation and complement, but endothelial injury (for example, induced by toxins, antibodies, immune cells or inflammatory cytokines) or defects in factors that provide endothelial protection (for example, regulators of complement or angiogenesis) can lead to acute or chronic renal injury. Moreover, renal endothelial cells can transition towards a mesenchymal phenotype, favouring renal fibrosis and the development of chronic kidney disease. Thus, the renal endothelium is both a target and a driver of kidney and systemic cardiovascular complications. Emerging therapeutic strategies that target the renal endothelium may lead to improved outcomes for both rare and common renal diseases.

Piperazine ferulate ameliorates the development of diabetic nephropathy by regulating endothelial nitric oxide synthase

Diabetic nephropathy (DN) is among the most common complications of diabetes mellitus. The disorder is associated with a decrease in the activity of the nitric oxide synthase/nitric oxide system. Piperazine ferulate (PF) is widely used for the treatment of kidney disease in China. The aim of the present study was to examine the effects of PF on streptozotocin (STZ)‑induced DN and the underlying mechanism of this process. STZ‑induced diabetic mice were intragastrically administered PF (100, 200 and 400 mg/kg/body weight/day) for 12 weeks. At the end of the treatment period, the parameters of 24‑h albuminuria and blood urea nitrogen, creatinine and oxidative stress levels were measured. Hematoxylin and eosin staining, periodic acid‑Schiff staining and electron microscopy were used to evaluate the histopathological alterations. mRNA and protein expression of endothelial nitric oxide synthase (eNOS) were measured by quantitative polymerase chain reaction and western blotting, respectively. PF significantly decreased blood urea nitrogen and creatinine levels and 24‑h albuminuria, and it alleviated oxidative stress, improved glomerular basement membrane thickness and caused an upregulation in eNOS expression and activity levels in diabetic mice. In addition, high glucose decreased eNOS expression levels, whereas PF caused a reversal in the nitric oxide (NO) levels of glomerular endothelial cells. The present results suggested that PF exhibited renoprotective effects on DN. The mechanism of its action was associated with the regulation of eNOS expression and activity.

Endothelial cell-specific overexpression of endothelial nitric oxide synthase in Ins2Akita mice exacerbates diabetic nephropathy

Previous studies demonstrated that global deficiency of eNOS in diabetic mice exacerbated renal lesions and that overexpression of eNOS may protect against tissue injury. Our study revealed for the first time overexpression of eNOS leads to disease progression rather than protection. Transgenic mice selectively expressing eNOS in endothelial cells (eNOSTg) were cross bred with Ins2Akita type-1 (AK) diabetic mice to generate eNOS overexpressing eNOSTg/AK mice. Wild type, eNOSTg, AK and eNOSTg/AK mice were assessed for kidney function and blood glucose levels. Remarkably, overexpressing eNOSTg mice showed evidence of unpredicted glomerular injury with segmental mesangiolysis and occasional microaneurysms. Notably, in eNOSTg/AK mice overexpression of eNOS led to increased glomerular/endothelial injury that was associated with increased superoxide levels and renal dysfunction. Results indicate for the first time that overexpressing eNOS in endothelial cells cannot ameliorate diabetic lesions, but paradoxically leads to progression of nephropathy likely due to eNOS uncoupling and superoxide upsurge. This novel finding has a significant impact on current therapeutic strategies to improve endothelial function and prevent progression of diabetic renal disease. Further, the eNOSTg/AK model developed in this study has significant translational potentials for elucidating the underlying mechanism implicated in the deflected function of eNOS in diabetic nephropathy.

Sepiapterin Improves Vascular Reactivity and Insulin-Stimulated Glucose in Wistar Rats

In the vasculature, sedentary behavior leads to endothelial abnormalities, resulting in elevated cardiovascular disease risk. Endothelial nitric oxide synthase (eNOS) aberrations characterize endothelial dysfunction; eNOS also regulates mitochondrial function. We hypothesized that sepiapterin (a precursor to eNOS cofactor tetrahydrobiopterin (BH₄)) supplementation would improve endothelium-dependent vascular relaxation in sedentary animals via modulation of NOS function and mitochondrial activity. Sedentary male Wistar rats were fed ad libitum for a total of 10 weeks. Sepiapterin was administered in diet during the final 5 weeks. Intraperitoneal insulin and glucose tolerance tests (IP-ITT/IP-GTT) were conducted at baseline and endpoint. Aorta was assessed for vasoreactivity and mitochondrial respiration. Insulin tolerance, determined by IP-ITT, significantly improved in rats treated with sepiapterin (p < 0.05, interaction of time and treatment). Acetylcholine- (ACh-) driven vasodilation was significantly greater in aorta from sepiapterin-treated rats as compared with control (76.4% versus 54.9% of phenylephrine contraction at 20 μM ACh, p < 0.05). Sepiapterin treatment resulted in significantly elevated state 3 (9.00 oxygen pmol/sec^∗mg versus 8.17 oxygen pmol/sec^∗mg, p < 0.05) and 4 (7.28 oxygen pmol/sec^∗mg versus 5.86 oxygen pmol/sec^∗mg, p < 0.05) aortic mitochondrial respiration with significantly lower respiratory control ratio (p < 0.05) during octanoylcarnitine-driven respiration. Vasodilation and insulin sensitivity were improved through targeting NOS via sepiapterin supplementation.

Estradiol attenuates ischemia reperfusion-induced acute kidney injury through PPAR-γ stimulated eNOS activation in rats

We investigated the involvement of peroxisome proliferator activated receptor-γ (PPAR-γ)/endothelial nitric oxide synthase (eNOS) pathway in estradiol mediated protection against ischemia reperfusion (I/R)-induced acute kidney injury (AKI) in rats. To induce AKI, rats underwent 40 min of bilateral renal ischemia followed by 24 h of reperfusion. I/R-induced kidney damage was quantified by measuring serum creatinine, creatinine clearance, urea nitrogen, uric acid, potassium, fractional excretion of sodium, microproteinuria, and renal oxidative stress (thiobarbituric acid reactive substances, superoxide anion generation, and reduced glutathione). Hematoxylin eosin stain demonstrated renal histology, while renal expression of apoptotic markers (Bcl-2, Bax), PPAR-γ and eNOS were quantified by immunohistochemistry. Estradiol (1 mg/kg, i.p.) was administered 30 min before I/R in rats. In separate groups, PPAR-γ antagonist, BADGE (30 mg/kg, i.p.), and NOS inhibitor, L-NAME (20 mg/kg, i.p.) were administered prior to estradiol treatment, which was followed by I/R in rats. I/R caused significant renal damage as demonstrated by biochemical (serum/urine), renal oxidative stress and histological changes alongwith increased expression of Bax and decreased levels of Bcl-2, PPAR-γ and eNOS, which were prevented by estradiol. Pre-treatment with BADGE and L-NAME abolished estradiol mediated renoprotection. Notably, I/R + estradiol + BADGE group revealed decreased expression of PPAR-γ and eNOS in renal tissues. In I/R + estradiol + L-NAME group, eNOS expression was reduced while PPAR-γ levels remained unchanged. These results suggest that estradiol modulates PPAR-γ which consequently regulates eNOS expression in rat kidneys. We conclude that estradiol protects against I/R-induced AKI through PPAR-γ stimulated eNOS activation in rats.

Glomerular Endothelial Cell Stress and Cross-Talk With Podocytes in Early [corrected] Diabetic Kidney Disease

Diabetic kidney disease (DKD) is one of the major causes of morbidity and mortality in diabetic patients and also the leading single cause of end-stage renal disease in the United States. A large proportion of diabetic patients develop DKD and others don't, even with comparable blood glucose levels, indicating a significant genetic component of disease susceptibility. The glomerulus is the primary site of diabetic injury in the kidney, glomerular hypertrophy and podocyte depletion are glomerular hallmarks of progressive DKD, and the degree of podocyte loss correlates with severity of the disease. We know that chronic hyperglycemia contributes to both microvascular and macrovascular complications, as well as podocyte injury. We are beginning to understand the role of glomerular endothelial injury, as well as the involvement of reactive oxygen species and mitochondrial stress, which play a direct role in DKD and in other diabetic complications. There is, however, a gap in our knowledge that links genetic susceptibility to early molecular mechanisms and proteinuria in DKD. Emerging research that explores glomerular cell's specific responses to diabetes and cell cross-talk will provide mechanistic clues that underlie DKD and provide novel avenues for therapeutic intervention.

Evaluation of inducible nitric oxide synthase inhibition on kidney function and structure in high-fat diet-induced kidney disease

NEW FINDINGS

What is the central question of this study? The metabolic pathways regulating the effects of obesity on the kidney remain unknown. We sought to determine whether inducible nitric oxide synthase (iNOS) is involved in the underlying mechanisms of high-fat diet-induced kidney disease using a specific iNOS inhibitor, N6-(1-iminoethyl)-l-lysine hydrochloride (L-NIL). What is the main finding and its importance? We did not demonstrate an upregulation of iNOS renal expression after high caloric intake, suggesting that iNOS might not be a crucial player in the development of obesity-induced kidney disease. Although L-NIL treatment clearly ameliorated systemic metabolic parameters, the effect on loss of renal function, impairment of tubular integrity, oxidative stress and inflammation appeared to be more moderate. Central obesity is related to caloric excess, promoting deleterious cellular responses in targeted organs. Nitric oxide (NO) has been determined as a key player in the pathogenesis of metabolic diseases. Here, we investigated the implication of inducible NO synthase (iNOS) in the development of obesity-induced kidney disease. C57Bl/6 male mice were randomized to a low-fat diet (LFD) or a high-fat diet (HFD) and treated with N6-(1-iminoethyl)-l-lysine hydrochloride (L-NIL), a specific iNOS inhibitor, for 16 weeks. Mice fed an HFD exhibited a significant increase in body weight, fasting blood glucose and plasma concentrations of non-esterified fatty acids, triglyceride and insulin. Inhibition of iNOS prevented these changes in mice fed an HFD. Interestingly, the significant increase in albuminuria and mesangial matrix expansion were not ameliorated with L-NIL, whereas a significant decrease in proteinuria, N-acetyl-β-d-glucosaminidase excretion and renal triglyceride content were found, suggesting that iNOS inhibition is more suitable for tubular function than glomerular function. The urinary concentration of hydrogen peroxide, a stable product of reactive oxygen species production, that was found to be increased in mice fed an HFD, was significantly reduced with L-NIL. Finally, despite a moderate effect of L-NIL on inflammatory processes in the kidney, we demonstrated a positive impact of this treatment on adipocyte hypertrophy and on adipose tissue inflammation. These results suggest that inhibition of iNOS leads to a moderate beneficial effect on kidney function in mice fed an HFD. Further studies are needed for better understanding of the role of iNOS in obesity-induced kidney disease.

Modelling diabetic nephropathy in mice

Diabetic nephropathy (DN) is a leading cause of end-stage renal disease in the developed world. Accordingly, an urgent need exists for new, curative treatments as well as for biomarkers to stratify risk of DN among individuals with diabetes mellitus. A barrier to progress in these areas has been a lack of animal models that faithfully replicate the main features of human DN. Such models could be used to define the pathogenesis, identify drug targets and test new therapies. Owing to their tractability for genetic manipulation, mice are widely used to model human diseases, including DN. Questions have been raised, however, about the general utility of mouse models in human drug discovery. Standard mouse models of diabetes typically manifest only modest kidney abnormalities, whereas accelerated models, induced by superimposing genetic stressors, recapitulate key features of human DN. Incorporation of systems biology approaches and emerging data from genomics and metabolomics studies should enable further model refinement. Here, we discuss the current status of mouse models for DN, their limitations and opportunities for improvement. We emphasize that future efforts should focus on generating robust models that reproduce the major clinical and molecular phenotypes of human DN.

Direct Observation of Enhanced Nitric Oxide in a Murine Model of Diabetic Nephropathy

Uncoupling of nitric oxide synthase (NOS) secondary to redox signaling is a central mechanism in endothelial and macrophage activation. To date studies on the production of nitric oxide (NO) during the development of diabetic complications show paradoxical results. We previously showed that recoupling eNOS by increasing the eNOS cofactor tetrahydrobiopterin (BH4) could restore endothelial function and prevent kidney injury in experimental kidney transplantation. Here, we employed a diabetic mouse model to investigate the effects of diabetes on renal tissue NO bioavailability. For this, we used in vivo NO trapping, followed by electron paramagnetic resonance spectroscopy. In addition, we investigated whether coupling of NOS by supplying the cofactor BH4 could restore glomerular endothelial barrier function. Our data show that overall NO availability at the tissue level is not reduced sixteen weeks after the induction of diabetes in apoE knockout mice, despite the presence of factors that cause endothelial dysfunction, and the presence of the endogenous NOS inhibitor ADMA. Targeting uncoupled NOS with the BH4 precursor sepiapterin further increases NO availability, but did not modify renal glomerular injury. Notably, glomerular heparanase activity as a driver for loss of glomerular barrier function was not reduced, pointing towards NOS-independent mechanisms. This was confirmed by unaltered increased glomerular presence of cathepsin L, the protease that activates heparanase.

Diabetes-Induced Reactive Oxygen Species: Mechanism of Their Generation and Role in Renal Injury

Diabetes induces the onset and progression of renal injury through causing hemodynamic dysregulation along with abnormal morphological and functional nephron changes. The most important event that precedes renal injury is an increase in permeability of plasma proteins such as albumin through a damaged glomerular filtration barrier resulting in excessive urinary albumin excretion (UAE). Moreover, once enhanced UAE begins, it may advance renal injury from progression of abnormal renal hemodynamics, increased glomerular basement membrane (GBM) thickness, mesangial expansion, extracellular matrix accumulation, and glomerulosclerosis to eventual end-stage renal damage. Interestingly, all these pathological changes are predominantly driven by diabetes-induced reactive oxygen species (ROS) and abnormal downstream signaling molecules. In diabetic kidney, NADPH oxidase (enzymatic) and mitochondrial electron transport chain (nonenzymatic) are the prominent sources of ROS, which are believed to cause the onset of albuminuria followed by progression to renal damage through podocyte depletion. Chronic hyperglycemia and consequent ROS production can trigger abnormal signaling pathways involving diverse signaling mediators such as transcription factors, inflammatory cytokines, chemokines, and vasoactive substances. Persistently, increased expression and activation of these signaling molecules contribute to the irreversible functional and structural changes in the kidney resulting in critically decreased glomerular filtration rate leading to eventual renal failure.

Genetic Targeting of Arginase-II in Mouse Prevents Renal Oxidative Stress and Inflammation in Diet-Induced Obesity

Obesity is associated with development and progression of chronic kidney disease (CKD). Recent evidence demonstrates that enhanced levels of the L-arginine:ureahydrolase, including the two isoenzymes arginase-I (Arg-I) and arginase-II (Arg-II) in vascular endothelial cells promote uncoupling of endothelial nitric oxide synthase (eNOS), leading to increased superoxide radical anion and decreased NO production thereby endothelial dysfunction. Arg-II but not Arg-I is abundantly expressed in kidney and the role of Arg-II in CKD is uncertain and controversial. We aimed to investigate the role of Arg-II in renal damage associated with diet-induced obesity mouse model. Wild type (WT) C57BL/6 mice and mice deficient in Arg-II gene (Arg-II^−/−) were fed with either a normal chow (NC) or a high-fat-diet (HFD) for 14 weeks (starting at the age of 7 weeks) to induce obesity. In WT mice, HFD feeding caused frequent renal lipid accumulation, enhancement of renal reactive oxygen species (ROS) levels which could be attenuated by a NOS inhibitor, suggesting uncoupling of NOS in kidney. HFD feeding also significantly augmented renal Arg-II expression and activity. All the alterations in the kidney under HFD feeding were reduced in Arg-II^−/− mice. Moreover, mesangial expansion as analyzed by Periodic Acid Schiff (PAS) staining and renal expression of vascular adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) in HFD-fed WT mouse assessed by immunoblotting were reduced in the HFD-fed Arg-II^−/− mice, although there was no significant difference in body weight and renal weight/body weight ratio between the WT and Arg-II^−/− mice. Thus, Arg-II expression/activity is enhanced in kidney of diet-induced obesity mice. Genetic targeting of Arg-II prevents renal damage associated with obesity, suggesting an important role of Arg-II in obesity-associated renal disease development.

ROCK1 Induces Endothelial-to-Mesenchymal Transition in Glomeruli to Aggravate Albuminuria in Diabetic Nephropathy

Endothelial-to-mesenchymal transition (EndMT) can cause loss of tight junctions, which in glomeruli are associated with albuminuria. Here we evaluated the role of EndMT in the development of albuminuria in diabetic nephropathy (DN). We demonstrated that EndMT occurs in the glomerular endothelium of patients with DN, showing by a decrease in CD31 but an increase in α-SMA expression. In glomeruli of db/db mice, there was an increased ROCK1 expression in the endothelium plus a decreased CD31-positive cells. Cultured glomerular endothelial cells (GEnCs) underwent EndMT when stimulated by 30 mM glucose, and exhibited increased permeability. Meanwhile, they showed a higher ROCK1 expression and activation. Notably, inhibition of ROCK1 largely blocked EndMT and the increase in endothelial permeability under this high-glucose condition. In contrast, overexpression of ROCK1 induced these changes. Consistent alterations were observed in vivo that treating db/db mice with the ROCK1 inhibitor, fasudil, substantially suppressed the expression of α-SMA in the glomerular endothelium, and reduced albuminuria. Thus we conclude that ROCK1 is induced by high glucose and it stimulates EndMT, resulting in increased endothelial permeability. Inhibition of ROCK1 could be a therapeutic strategy for preventing glomerular endothelial dysfunction and albuminuria in developing DN.

Cathepsin S Cleavage of Protease-Activated Receptor-2 on Endothelial Cells Promotes Microvascular Diabetes Complications

Endothelial dysfunction is a central pathomechanism in diabetes-associated complications. We hypothesized a pathogenic role in this dysfunction of cathepsin S (Cat-S), a cysteine protease that degrades elastic fibers and activates the protease-activated receptor-2 (PAR2) on endothelial cells. We found that injection of mice with recombinant Cat-S induced albuminuria and glomerular endothelial cell injury in a PAR2-dependent manner. In vivo microscopy confirmed a role for intrinsic Cat-S/PAR2 in ischemia-induced microvascular permeability. In vitro transcriptome analysis and experiments using siRNA or specific Cat-S and PAR2 antagonists revealed that Cat-S specifically impaired the integrity and barrier function of glomerular endothelial cells selectively through PAR2. In human and mouse type 2 diabetic nephropathy, only CD68(+) intrarenal monocytes expressed Cat-S mRNA, whereas Cat-S protein was present along endothelial cells and inside proximal tubular epithelial cells also. In contrast, the cysteine protease inhibitor cystatin C was expressed only in tubules. Delayed treatment of type 2 diabetic db/db mice with Cat-S or PAR2 inhibitors attenuated albuminuria and glomerulosclerosis (indicators of diabetic nephropathy) and attenuated albumin leakage into the retina and other structural markers of diabetic retinopathy. These data identify Cat-S as a monocyte/macrophage-derived circulating PAR2 agonist and mediator of endothelial dysfunction-related microvascular diabetes complications. Thus, Cat-S or PAR2 inhibition might be a novel strategy to prevent microvascular disease in diabetes and other diseases.

Enhancing eNOS activity with simultaneous inhibition of IKKβ restores vascular function in Ins2(Akita+/-) type-1 diabetic mice

The balance of nitric oxide (NO) versus superoxide generation has a major role in the initiation and progression of endothelial dysfunction. Under conditions of high glucose, endothelial nitric oxide synthase (eNOS) functions as a chief source of superoxide rather than NO. In order to improve NO bioavailability within the vessel wall in type-1 diabetes, we investigated treatment strategies that improve eNOS phosphorylation and NO-dependent vasorelaxation. We evaluated methods to increase the eNOS activity by (1) feeding Ins2(Akita) spontaneously diabetic (type-1) mice with l-arginine in the presence of sepiapterin, a precursor of tetrahydrobiopterin; (2) preventing eNOS/NO deregulation by the inclusion of inhibitor kappa B kinase beta (IKKβ) inhibitor, salsalate, in the diet regimen in combination with l-arginine and sepiapterin; and (3) independently increasing eNOS expression to improve eNOS activity and associated NO production through generating Ins2(Akita) diabetic mice that overexpress human eNOS predominantly in vascular endothelial cells. Our results clearly demonstrated that diet supplementation with l-arginine, sepiapterin along with salsalate improved phosphorylation of eNOS and enhanced vasorelaxation of thoracic/abdominal aorta in type-1 diabetic mice. More interestingly, despite the overexpression of eNOS, the in-house generated transgenic eNOS-GFP (TgeNOS-GFP)-Ins2(Akita) cross mice showed an unanticipated effect of reduced eNOS phosphorylation and enhanced superoxide production. Our results demonstrate that enhancement of endogenous eNOS activity by nutritional modulation is more beneficial than increasing the endogenous expression of eNOS by gene therapy modalities.

Twenty years after ACEIs and ARBs: emerging treatment strategies for diabetic nephropathy

Diabetic nephropathy (DN) is a serious complication of both type 1 and type 2 diabetes mellitus. The disease is now the most common cause of end-stage kidney disease (ESKD) in developed countries, and both the incidence and prevalence of diabetes mellitus is increasing worldwide. Current treatments are directed at controlling hyperglycemia and hypertension, as well as blockade of the renin angiotensin system with angiotensin-converting enzyme inhibitors (ACEIs), and angiotensin receptor blockers. Despite these therapies, DN progresses to ESKD in many patients. As a result, much interest is focused on developing new therapies. It has been over two decades since ACEIs were shown to have beneficial effects in DN independent of their blood pressure-lowering actions. Since that time, our understanding of disease mechanisms in DN has evolved. In this review, we summarize major cell signaling pathways implicated in the pathogenesis of diabetic kidney disease, as well as emerging treatment strategies. The goal is to identify promising targets that might be translated into therapies for the treatment of patients with diabetic kidney disease.

COX-2 is involved in vascular oxidative stress and endothelial dysfunction of renal interlobar arteries from obese Zucker rats

Obesity is related to vascular dysfunction through inflammation and oxidative stress and it has been identified as a risk factor for chronic renal disease. In the present study, we assessed the specific relationships among reactive oxygen species (ROS), cyclooxygenase 2 (COX-2), and endothelial dysfunction in renal interlobar arteries from a genetic model of obesity/insulin resistance, the obese Zucker rats (OZR). Relaxations to acetylcholine (ACh) were significantly reduced in renal arteries from OZR compared to their counterpart, the lean Zucker rat (LZR), suggesting endothelial dysfunction. Blockade of COX with indomethacin and with the selective blocker of COX-2 restored the relaxations to ACh in obese rats. Selective blockade of the TXA2/PGH2 (TP) receptor enhanced ACh relaxations only in OZR, while inhibition of the prostacyclin (PGI2) receptor (IP) enhanced basal tone and inhibited ACh vasodilator responses only in LZR. Basal production of superoxide was increased in arteries of OZR and involved NADPH and xanthine oxidase activation and NOS uncoupling. Under conditions of NOS blockade, ACh induced vasoconstriction and increased ROS generation that were augmented in arteries from OZR and blunted by COX-2 inhibition and by the ROS scavenger tempol. Hydrogen peroxide (H2O2) evoked both endothelium- and vascular smooth muscle (VSM)-dependent contractions, as well as ROS generation that was reduced by COX-2 inhibition. In addition, COX-2 expression was enhanced in both VSM and endothelium of renal arteries from OZR. These results suggest that increased COX-2-dependent vasoconstriction contributes to renal endothelial dysfunction through enhanced (ROS) generation in obesity. COX-2 activity is in turn upregulated by ROS.

Arginase inhibition: a new treatment for preventing progression of established diabetic nephropathy

Our previous publication showed that inhibition of arginase prevents the development of diabetic nephropathy (DN). However, identification of targets that retard the progression of established DN-which is more clinically relevant-is lacking. Therefore, we tested the hypothesis that arginase inhibition would prevent the progression of established DN. Effects of arginase inhibition were compared with treatment with the angiotensin-converting enzyme inhibitor captopril, a current standard of care in DN. Experiments were conducted in Ins2(Akita) mice treated with the arginase inhibitor S-(2-boronoethyl)-l-cysteine (BEC) or captopril starting at 6 wk of age for 12 wk (early treatment) or starting at 12 wk of age for 6 wk (late treatment). Early and late treatment with BEC resulted in protection from DN as indicated by reduced albuminuria, histological changes, kidney macrophage infiltration, urinary thiobarbituric acid-reactive substances, and restored nephrin expression, kidney nitrate/nitrite, kidney endothelial nitric oxide synthase phosphorylation, and renal medullary blood flow compared with vehicle-treated Ins2(Akita) mice at 18 wk of age. Interestingly, early treatment with captopril reduced albuminuria, histological changes, and kidney macrophage infiltration without affecting the other parameters, but late treatment with captopril was ineffective. These findings highlight the importance of arginase inhibition as a new potential therapeutic intervention in both early and late stages of diabetic renal injury.

Renoprotective Effects of Fenofibrate via Modulation of LKB1/AMPK mRNA Expression and Endothelial Dysfunction in a Rat Model of Diabetic Nephropathy

This study was conducted to investigate whether the renoprotective effects of fenofibrate are mediated via attenuation of endothelial dysfunction and modulating the mRNA expression of adenosine monophosphate-activated protein kinase (AMPK) and its downstream kinase liver kinase B1 (LKB1) in rats with diabetic nephropathy (DN). Diabetes was induced by a single intraperitoneal injection of streptozotocin (55 mg kg(-1)). Fenofibrate (100 mg kg(-1), p.o.) was given to diabetic rats daily for 12 weeks. Treatment with fenofibrate significantly improved the renal function as revealed by the significant reductions in urinary albumin excretion and serum levels of creatinine and urea, in addition to the significant increase in creatinine clearance compared with the diabetic control group. Hyperglycemia-induced oxidative damage was ameliorated by treatment with fenofibrate as indicated by the significantly increased levels of glutathione and catalase together with the significant decrease in lipid peroxidation. Administration of fenofibrate caused significant increases in renal nitric oxide (NO) production and mRNA expression of endothelial NO synthase (eNOS), AMPK and LKB1, reflecting improvement of endothelial function. Our results give further insights into the mechanisms underlying the protective role of fenofibrate in DN via modulation of AMPK, LKB1 and eNOS mRNA expression.

Renal endothelial dysfunction in diabetic nephropathy

Endothelial dysfunction has been posited to play an important role in the pathogenesis of diabetic nephropathy (DN). Due to the heterogeneity of endothelial cells (ECs), it is difficult to generalize about endothelial responses to diabetic stimuli. At present, there are limited techniques fordirectly measuring EC function in vivo, so diagnosis of endothelial disorders still largely depends on indirect assessment of mediators arising from EC injury. In the kidney microcirculation, both afferent and efferent arteries, arterioles and glomerular endothelial cells (GEnC) have all been implicated as targets of diabetic injury. Both hyperglycemia per se, as well as the metabolic consequences of glucose dysregulation, are thought to lead to endothelial cell dysfunction. In this regard, endothelial nitric oxide synthase (eNOS) plays a central role in EC dysfunction. Impaired eNOS activity can occur at numerous levels, including enzyme uncoupling, post-translational modifications, internalization and decreased expression. Reduced nitric oxide (NO) bioavailability exacerbates oxidative stress, further promoting endothelial dysfunction and injury. The injured ECs may then function as active signal transducers of metabolic, hemodynamic and inflammatory factors that modify the function and morphology of the vessel wall and interact with adjacent cells, which may activate a cascade of inflammatory and proliferative and profibrotic responses in progressive DN. Both pharmacological approaches and potential regenerative therapies hold promise for restoration of impaired endothelial cells in diabetic nephropathy.

The Puerarin improves renal function in STZ-induced diabetic rats by attenuating eNOS expression

Diabetic nephropathy (DN) is a serious complication and it leads to kidney failure. The endothelial nitric oxide synthases (eNOS) seems to be involved in the development and progression of DN. The Puerarin is a well-known Chinese traditional formula, which is widely used in clinical practice for the treatment of kidney disease. The present study was designed to investigate the renal protective effects of Puerarin on streptozotocin (STZ)-induced diabetic rats. Thirty Sprague-Dawley (SD) male rats were divided into three groups at random. The diabetic group and the Puerarin-treated group were intraperitoneally injected with STZ 65 mg/kg and the Puerarin-treated rats were intraperitoneally injected Puerarin 100 mg/kg/day for 4 weeks. The results showed the Puerarin could improve body weight, blood sugar, BUN and SCr levels, and reduce ultrastructural changes of kidney in diabetic rats. It also attenuated eNOS expression in glomerular endothelial cells and tubular cells of diabetic rats with Puerarin treatment (p < 0.05). The Puerarin had significant renal-protective effects for the diabetic nephropathy, possibly through regulating eNOS expression, and it may be used as a potential therapeutic reagent.

A novel mouse model of advanced diabetic kidney disease

Currently available rodent models exhibit characteristics of early diabetic nephropathy (DN) such as hyperfiltration, mesangial expansion, and albuminuria yet features of late DN (hypertension, GFR decline, tubulointerstitial fibrosis) are absent or require a significant time investment for full phenotype development. Accordingly, the aim of the present study was to develop a mouse model of advanced DN with hypertension superimposed (HD mice). Mice transgenic for human renin cDNA under the control of the transthyretin promoter (TTRhRen) were employed as a model of angiotensin-dependent hypertension. Diabetes was induced in TTRhRen mice through low dose streptozotocin (HD-STZ mice) or by intercrossing with OVE26 diabetic mice (HD-OVE mice). Both HD-STZ and HD-OVE mice displayed more pronounced increases in urinary albumin levels as compared with their diabetic littermates. Additionally, HD mice displayed renal hypertrophy, advanced glomerular scarring and evidence of tubulointerstitial fibrosis. Both HD-OVE and HD-STZ mice showed evidence of GFR decline as FITC-inulin clearance was decreased compared to hyperfiltering STZ and OVE mice. Taken together our results suggest that HD mice represent a robust model of type I DN that recapitulates key features of human disease which may be significant in studying the pathogenesis of DN and in the assessment of putative therapeutics.

Endothelial nitric oxide synthase (eNOS) 4b/a polymorphism and the risk of diabetic nephropathy in type 2 diabetes mellitus: A meta-analysis

Many studies have accessed the association between eNOS-4b/a polymorphism and the risk of diabetic nephropathy (DN) among type 2 diabetic subjects. However, the results are conflicting and inconclusive. The aim of current meta-analysis was to more precisely estimate the relationship. Pubmed, Embase, the China National Knowledge Infrastructure and the Wanfang Database were searched for articles published up to May 26th, 2013 that addressed eNOS-4b/a polymorphism and the risk of DN among type 2 diabetic subjects. 18 studies were included in this meta-analysis. eNOS-4b/a polymorphisms were associated with an overall significantly increased risk of DN (allele model: OR = 1.44, 95% CI = 1.14-1.82; additive model: OR = 2.03, 95% CI = 1.14-3.62; dominant model: OR = 1.34, 95% CI = 1.07-1.68; recessive model: OR = 2.01, 95% CI = 1.12-3.61). Subgroup analysis revealed a significant association between the eNOS-4b/a polymorphism and DN in Asian population, especially in Chinese population, but not in non Asian populations. Our meta-analysis supported an association between the 4b/a polymorphism of eNOS gene and increased risk of DN in type 2 diabetes among Asians, especially in Chinese population.

Effects of folic acid on renal endothelial function in patients with diabetic nephropathy: results from a randomized trial

Endothelial dysfunction has been shown to promote podocyte injury and albuminuria in diabetes, highlighting the importance of the interaction between renal endothelial cells and podocytes. Folic acid (FA) improves nitric oxide synthase (NOS) function and reduces progression of diabetic nephropathy in animal models. We tested whether high-dose FA treatment improves renal endothelial function and albuminuria in human subjects with incipient diabetic nephropathy. Following a double-blind, randomized, cross-over design, 28 patients with Type 2 diabetes and albuminuria were allocated to 4 weeks' treatment with placebo and high-dose FA (5 mg/day). Renal nitric oxide (NO) production determined as the response of renal plasma flow (RPF) to NOS inhibition with NG-monomethyl-L-arginine (L-NMMA) (4.25 mg/kg intravenously), renal oxidant stress as response of RPF to vitamin C infusion (3 mg/kg) and albuminuria were determined after each treatment phase. Neither the reduction in RPF to L-NMMA nor the increase in RPF to vitamin C infusion differed between treatment phases (ΔRPF to

L-NMMA

-74±71 ml/min per m2 during placebo compared with -63±56 ml/min per m2 during FA, P=0.57; ΔRPF to vitamin C: +93±118 ml/min per m2 compared with +94±108 ml/min per m2; P=0.70). In line with the lack of effect on the renal endothelium, albuminuria was not affected by FA treatment (110±179 mg/day during placebo compared with 87±146 mg/day during FA; P=0.12). High-dose FA treatment does not improve renal endothelial function and fails to reduce albuminuria in human subjects with diabetic nephropathy. Novel treatment options for oxidant stress and endothelial dysfunction in patients with diabetes are urgently needed.

Nitric oxide in the normal kidney and in patients with diabetic nephropathy

Nitric oxide (NO) is a gas with biological and regulatory properties, produced from arginine by the way of nitric oxide synthases (NOS), and with a very short half-life (few seconds). A "coupled" NOS activity leads to NO generation, whereas its uncoupling produces the reactive oxygen species peroxynitrite (ONOO(-)). Uncoupling is usually due to inflammation, oxidative stress, decreased cofactor availability, or excessive NO production. Competitive inhibitors of NO production are post-translationally methylated arginine residues in proteins, which are constantly released into the circulation. NO availability is altered in many clinical conditions associated with vascular dysfunction, such as diabetes mellitus. The kidney plays an important role in body NO homeostasis. This article provides an overview of current literature, on NO production/availability, with a focus on diabetic nephropathy. In diabetes, NO availability is usually decreased (with exception of the early, hyper filtration phase of nephropathy in Type 1 diabetes), and it could constitute a factor of the generalized vasculopathy present in diabetic nephropathy. NO generation in Type 2 diabetes with nephropathy is inversely associated with the dimethyl-arginine concentrations, which are therefore important modulators of NO synthesis independently from the classic stimulatory pathways (such as the insulin effect). A disturbed NO metabolism is present in diabetes associated with nephropathy. Although modulation of NO production is not yet a common therapeutical strategy, a number of yet experimental compounds need to be tested as potential interventions to treat the vascular dysfunction and nephropathy in diabetes, as well as in other diseased states. Finally, in diabetic nephropathy NO deficiency may be associated to that of hydrogen sulfide, another interesting gaseous mediator which is increasingly investigated.

Reduced Krüppel-like factor 2 expression may aggravate the endothelial injury of diabetic nephropathy

Kruppel-like Factor 2 (KLF2), a shear-stress inducible transcription factor, has endoprotective effects. In streptozotocin-induced diabetic rats, we found that glomerular Klf2 expression was reduced in comparison to non-diabetic rats. However, normalization of hyperglycemia by insulin treatment increased Klf2 expression to a level higher than that of non-diabetic rats. Consistent with this, we found that Klf2 expression was suppressed by high glucose but increased by insulin in cultured endothelial cells. To determine the role of KLF2 in streptozotocin-induced diabetic nephropathy, we used endothelial cell-specific Klf2 heterozygous knockout mice and found that diabetic knockout mice developed more kidney/glomerular hypertrophy and proteinuria than diabetic wide type mice. Glomerular expression of Vegfa, Flk1, and angiopoietin 2 increased but expression of Flt1, Tie2, and angiopoietin 1 decreased in diabetic knockout compared to diabetic wide type mice. Glomerular expression of ZO-1, glycocalyx, and eNOS was also decreased in diabetic knockout compared to diabetic wide type mice. These data suggest knockdown of Klf2 expression in the endothelial cells induced more endothelial cell injury. Interestingly, podocyte injury was also more prominent in diabetic knockout compared to diabetic wide type mice, indicating a crosstalk between these two cell types. Thus, KLF2 may play a role in glomerular endothelial cell injury in early diabetic nephropathy.

Vascular endothelial growth factor-receptor 1 inhibition aggravates diabetic nephropathy through eNOS signaling pathway in db/db mice

The manipulation of vascular endothelial growth factor (VEGF)-receptors (VEGFRs) in diabetic nephropathy is as controversial as issue as ever. It is known to be VEGF-A and VEGFR2 that regulate most of the cellular actions of VEGF in experimental diabetic nephropathy. On the other hand, such factors as VEGF-A, -B and placenta growth factor bind to VEGFR1 with high affinity. Such notion instigated us to investigate on whether selective VEGFR1 inhibition with GNQWFI hexamer aggravates the progression of diabetic nephropathy in db/db mice. While diabetes suppressed VEGFR1, it did increase VEGFR2 expressions in the glomerulus. Db/db mice with VEGFR1 inhibition showed more prominent features with respect to, albuminuria, mesangial matrix expansion, inflammatory cell infiltration and greater numbers of apoptotic cells in the glomerulus, and oxidative stress than that of control db/db mice. All these changes were related to the suppression of diabetes-induced increases in PI3K activity and Akt phosphorylation as well as the aggravation of endothelial dysfunction associated with the inactivation of FoxO3a and eNOS-NOx. In cultured human glomerular endothelial cells (HGECs), high-glucose media with VEGFR1 inhibition induced more apoptotic cells and oxidative stress than did high-glucose media alone, which were associated with the suppression of PI3K-Akt phosphorylation, independently of the activation of AMP-activated protein kinase, and inactivation of FoxO3a and eNOS-NOx pathway. In addition, transfection with VEGFR1 siRNA in HGECs also suppressed PI3K-Akt-eNOS signaling. In conclusion, the specific blockade of VEGFR1 with GNQWFI caused severe renal injury related to profound suppression of the PI3K-Akt, FoxO3a and eNOS-NOx pathway, giving rise to the oxidative stress-induced apoptosis of glomerular cells in type 2 diabetic nephropathy.

Diffuse glomerular nodular lesions in diabetic pigs carrying a dominant-negative mutant hepatocyte nuclear factor 1-alpha, an inheritant diabetic gene in humans

Glomerular nodular lesions, known as Kimmelstiel-Wilson nodules, are a pathological hallmark of progressive human diabetic nephropathy. We have induced severe diabetes in pigs carrying a dominant-negative mutant hepatocyte nuclear factor 1-alpha (HNF1α) P291fsinsC, a maturity-onset diabetes of the young type-3 (MODY3) gene in humans. In this model, glomerular pathology revealed that formation of diffuse glomerular nodules commenced as young as 1 month of age and increased in size and incidence until the age of 10 months, the end of the study period. Immunohistochemistry showed that the nodules consisted of various collagen types (I, III, IV, V and VI) with advanced glycation end-product (AGE) and N^ε-carboxymethyl-lysine (CML) deposition, similar to those in human diabetic nodules, except for collagen type I. Transforming growth factor-beta (TGF-β) was also expressed exclusively in the nodules. The ultrastructure of the nodules comprised predominant interstitial-type collagen deposition arising from the mesangial matrices. Curiously, these nodules were found predominantly in the deep cortex. However, diabetic pigs failed to show any of the features characteristic of human diabetic nephropathy; e.g., proteinuria, glomerular basement membrane thickening, exudative lesions, mesangiolysis, tubular atrophy, interstitial fibrosis, and vascular hyalinosis. The pigs showed only Armanni-Ebstein lesions, a characteristic tubular manifestation in human diabetes. RT-PCR analysis showed that glomeruli in wild-type pigs did not express endogenous HNF1α and HNF1β, indicating that mutant HNF1α did not directly contribute to glomerular nodular formation in diabetic pigs. In conclusion, pigs harboring the dominant-negative mutant human MODY3 gene showed reproducible and distinct glomerular nodules, possibly due to AGE- and CML-based collagen accumulation. Although the pathology differed in several respects from that of human glomerular nodular lesions, the somewhat acute and constitutive formation of nodules in this mammalian model might provide information facilitating identification of the principal mechanism underlying diabetic nodular sclerosis.

Podocyte-specific VEGF-a gain of function induces nodular glomerulosclerosis in eNOS null mice

VEGF-A and nitric oxide are essential for glomerular filtration barrier homeostasis and are dysregulated in diabetic nephropathy. Here, we examined the effect of excess podocyte VEGF-A on the renal phenotype of endothelial nitric oxide synthase (eNOS) knockout mice. Podocyte-specific VEGF(164) gain of function in eNOS(-/-) mice resulted in nodular glomerulosclerosis, mesangiolysis, microaneurysms, and arteriolar hyalinosis associated with massive proteinuria and renal failure in the absence of diabetic milieu or hypertension. In contrast, podocyte-specific VEGF(164) gain of function in wild-type mice resulted in less pronounced albuminuria and increased creatinine clearance. Transmission electron microscopy revealed glomerular basement membrane thickening and podocyte effacement in eNOS(-/-) mice with podocyte-specific VEGF(164) gain of function. Furthermore, glomerular nodules overexpressed collagen IV and laminin extensively. Biotin-switch and proximity ligation assays demonstrated that podocyte-specific VEGF(164) gain of function decreased glomerular S-nitrosylation of laminin in eNOS(-/-) mice. In addition, treatment with VEGF-A decreased S-nitrosylated laminin in cultured podocytes. Collectively, these data indicate that excess glomerular VEGF-A and eNOS deficiency is necessary and sufficient to induce Kimmelstiel-Wilson-like nodular glomerulosclerosis in mice through a process that involves deposition of laminin and collagen IV and de-nitrosylation of laminin.

Biopterin metabolism and eNOS expression during hypoxic pulmonary hypertension in mice

Tetrahydrobiopterin (BH4), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH4 pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH4) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH4 levels and eNOS expression, without modifying dihydrobiopterin (BH2, the oxidation product of BH4) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH4 availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH4/BH2 changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.

Methylglyoxal modulates endothelial nitric oxide synthase-associated functions in EA.hy926 endothelial cells

BACKGROUND

Increased levels of the sugar metabolite methylglyoxal (MG) in vivo were shown to participate in the pathophysiology of vascular complications in diabetes. Alterations of endothelial nitric oxide synthase (eNOS) activity by hypophosphorylation of the enzyme and enhanced monomerization are found in the diabetic milieu, and the regulation of this still remains undefined. Using various pharmacological approaches, we elucidate putative mechanisms by which MG modulates eNOS-associated functions of MG-stimulated superoxide O₂•⁻ production, phosphorylation status and eNOS uncoupling in EA.hy926 human endothelial cells.

METHODS

In cultured EA.hy926 endothelial cells, the effects of MG treatment, tetrahydrobiopterin (BH4; 100 μM) and sepiapterin (20 μM) supplementation, NOS inhibition by N(G)-nitro-L-arginine methyl ester (L-NAME; 50 μM), and inhibition of peroxynitrite (ONOO⁻) formation (300 μM Tempol plus 50 μM L-NAME) on eNOS dimer/monomer ratios, Ser-1177 eNOS phosphorylation and 3-nitrotyrosine (3NT) abundance were quantified using immunoblotting. O₂•⁻-dependent fluorescence was determined using a commercially available kit and tissue biopterin levels were measured by fluorometric HPLC analysis.

RESULTS

In EA.hy926 cells, MG treatment significantly enhanced O₂•⁻ generation and 3NT expression and reduced Ser-1177 eNOS phosphorylation, eNOS dimer/monomer ratio and cellular biopterin levels indicative of eNOS uncoupling. These effects were significantly mitigated by administration of BH4, sepiapterin and suppression of ONOO⁻ formation. L-NAME treatment significantly blunted eNOS-derived O₂•⁻ generation but did not modify eNOS phosphorylation or monomerization.

CONCLUSION

MG triggers eNOS uncoupling and hypophosphorylation in EA.hy926 endothelial cells associated with O₂•⁻ generation and biopterin depletion. The observed effects of the glycolysis metabolite MG presumably account, at least in part, for endothelial dysfunction in diabetes.