Effect of D-glucose on nitric oxide release from glomerular endothelial cells

Logic-Based Modeling of Inflammatory Macrophage Crosstalk with Glomerular Endothelial Cells in Diabetic Kidney Disease

Diabetic kidney disease is a complication in 1 out of 3 patients with diabetes. Aberrant glucose metabolism in diabetes leads to an immune response causing inflammation and to structural and functional damage in the glomerular cells of the kidney. Complex cellular signaling lies at the core of metabolic and functional derangement. Unfortunately, the mechanism underlying the role of inflammation in glomerular endothelial cell dysfunction during diabetic kidney disease is not fully understood. Computational models in systems biology allow the integration of experimental evidence and cellular signaling networks to understand mechanisms involved in disease progression. We built a logic-based ordinary differential equations model to study macrophage-dependent inflammation in glomerular endothelial cells during diabetic kidney disease progression. We studied the crosstalk between macrophages and glomerular endothelial cells in the kidney using a protein signaling network stimulated with glucose and lipopolysaccharide. The network and model were built using the open-source software package Netflux. This modeling approach overcomes the complexity of studying network models and the need for extensive mechanistic details. The model simulations were fitted and validated against available biochemical data from in vitro experiments. The model identified mechanisms responsible for dysregulated signaling in macrophages and glomerular endothelial cells during diabetic kidney disease. In addition, we investigated the influence of signaling interactions and species that on glomerular endothelial cell morphology through selective knockdown and downregulation. We found that partial knockdown of VEGF receptor 1, PLC-γ, adherens junction proteins, and calcium partially recovered the endothelial cell fenestration size. Our model findings contribute to understanding signaling and molecular perturbations that affect the glomerular endothelial cells in the early stage of diabetic kidney disease.

High Glucose Dialysis Solutions Increase Synthesis of Vascular Endothelial Growth Factors by Peritoneal Vascular Endothelial Cells

♦ Objective

Increased peritoneal vasculature has been reported in long-term peritoneal dialysis (PD), and vascular endothelial growth factors (VEGFs) have been found in dialysate. High concentrations of glucose or lactate, glucose degradation products (GDPs), and low pH of dialysis solutions are all possible factors in increased peritoneal VEGF synthesis. In this study, we investigated the effects of high glucose dialysis solutions on VEGF synthesis by peritoneal vascular endothelial cells (PVECs).

♦ Methods

The PVECs were isolated from rat omentum and were incubated for 4 hours in three different culture media [M199 media (control), conventional dialysis solutions containing 4.25% glucose diluted with an equal volume of M199 media (HGD), and M199 media containing 118 mmol/L mannitol as an osmolar control (mannitol)]. Levels of VEGF protein in the culture supernatant were measured by ELISA, and mRNA expression was determined by Northern blot analysis. Data are presented as percent of control.

♦ Results

After incubation for 4 hours, the number of cells did not differ between the 3 groups. Levels of VEGF in culture supernatant were significantly higher in the HGD group (124% ± 19%, p = 0.006) as compared with the control and mannitol (85% ± 10%) groups. The mRNA expression of VEGF appeared to be higher in the HGD group (128% ± 49%) than in the control and mannitol (94% ± 18%) groups.

♦ Conclusion

High glucose dialysis solutions increased VEGF synthesis by PVECs. The relationship between VEGF synthesis by PVECs and neovascularization of the peritoneum observed in long-term peritoneal dialysis patients has to be studied further.

Poor glycemic control is related to increased nitric oxide activity within the renal circulation of patients with type 2 diabetes

OBJECTIVE

Experimental studies have shown that glucose releases endothelial nitric oxide (NO) and that NO contributes to renal hyperperfusion in models of diabetes. To examine whether this translates into the human condition, we studied the relationship between glycemic control and renal NO activity in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 113 patients with type 2 diabetes and a wide range of HbA1c concentrations were included. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were determined by constant infusion input clearance. Functional NO activity in the renal circulation was determined as change of RPF to infusion of the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA) (4.25 mg/kg). As additional markers, we measured urinary excretion of NO (UNOx) and L-arginine-to-asymmetrical dimethylarginine (ADMA) ratio in plasma.

RESULTS

Subjects within the highest tertile of HbA1c concentration had increased RPF (low, medium, and high tertiles 576 ± 17 vs. 585 ± 22 vs. 627 ± 33 mL/min/m(2), P = 0.05 by one-way ANOVA), while GFR was similar across tertiles. The response of RPF to NOS blockade was augmented in subjects with higher HbA1c levels (-55 ± 7 vs. -64 ± 8 vs. -86 ± 8 mL/min, P = 0.04 by one-way ANOVA). Further, L-arginine-to-ADMA ratio and UNOx were increased in subjects with higher HbA1c levels.

CONCLUSIONS

In line with experimental evidence, we could demonstrate in humans that poor glycemic control is related to higher NO activity and hyperperfusion of the kidney. The renal NO system may thus be a novel therapeutic target for improving renal hemodynamics in patients with diabetes.

Role of nitric oxide in diabetic nephropathy

Diabetic nephropathy is the leading cause of end-stage renal disease in the Western hemisphere. Endothelial dysfunction is the central pathophysiologic denominator for all cardiovascular complications of diabetes including nephropathy. Abnormalities of nitric oxide (NO) production modulate renal structure and function in diabetes but, despite the vast literature, major gaps exist in our understanding in this field because the published studies mostly are confusing and contradictory. In this review, we attempt to review the existing literature, discuss the controversies, and reach some general conclusions as to the role of NO production in the diabetic kidney. The complex metabolic milieu in diabetes triggers several pathophysiologic mechanisms that simultaneously stimulate and suppress NO production. The net effect on renal NO production depends on the mechanisms that prevail in a given stage of the disease. Based on the current evidence, it is reasonable to conclude that early nephropathy in diabetes is associated with increased intrarenal NO production mediated primarily by constitutively released NO (endothelial nitric oxide synthase [eNOS] and neuronal nitric oxide synthase [nNOS]). The enhanced NO production may contribute to hyperfiltration and microalbuminuria that characterizes early diabetic nephropathy. On the other hand, a majority of the studies indicate that advanced nephropathy leading to severe proteinuria, declining renal function, and hypertension is associated with a state of progressive NO deficiency. Several factors including hyperglycemia, advanced glycosylation end products, increased oxidant stress, as well as activation of protein kinase C and transforming growth factor (TGF)-beta contribute to decreased NO production and/or availability. These effects are mediated through multiple mechanisms such as glucose quenching, and inhibition and/or posttranslational modification of NOS activity of both endothelial and inducible isoforms. Finally, genetic polymorphisms of the NOS enzyme also may play a role in the NO abnormalities that contribute to the development and progression of diabetic nephropathy.

High glucose augments arginase activity and nitric oxide production in the renal cortex

To clarify the interaction between arginase and nitric oxide (NO) production in the kidney with normal and high glucose levels, renal cortical slices from male Sprague-Dawley rats were incubated in Hank's solution containing various concentrations of L-norvaline (Nval; an arginase inhibitor), 500 U/mL superoxide dismutase, and either 5 mmol/L (normal) or 20 mmol/L (high) glucose (n = 5 per group). Incubation with Nval increased renal cortical NOX (nitrite + nitrate) production dose-dependently, indicating competition between arginase and NO synthase (NOS) for the substrate (L-arginine). In the basal condition without Nval, high glucose also increased NO(X) production to a rate 3 times that observed during incubation with normal glucose (P < .01). This effect of high glucose was not altered by Nval. Rather, the effects of high glucose and Nval were additive, indicating that the activity of NOS per se is enhanced by high glucose. Direct assay of arginase and NOS activities confirmed stimulation of both enzymes under the high glucose condition (P < .05, P < .01, v normal glucose, respectively). However, high glucose did not change the amount of L-arginine present in renal cortical slices. These data reveal that arginase competes with NOS for L-arginine in the renal cortex, and that high glucose increases the activity of both enzymes without affecting the amount of substrate. These results suggest that increased NOS activity, rather than altered substrate availability, may be the principal factor underlying increased NO synthesis in diabetic kidneys.

High concentration of glucose inhibits glomerular endothelial eNOS through a PKC mechanism

Kidney glomeruli are important targets of diabetic nephropathy. We hypothesized a high concentration of glucose could suppress glomerular endothelial nitric oxide synthase (eNOS) by a protein kinase C (PKC) mechanism, as has been found in other tissues. Mouse kidney slices (150-200 microm) were bathed in Hanks' solution with 100 microM L-arginine and exposed to either 5 or 20-30 mM D-glucose. Immunofluorescence identified only eNOS in normal mouse glomeruli. Measurements of glomerular NO concentration with NO-sensitive fluorescent dye (4,5-diaminofluorescein diacetate) using confocal microscopy and NO-sensitive microelectrodes verified that resting glomeruli had active production of NO that was inhibited by N(G)-nitro-L-arginine methyl ester. High-concentration (20-30 mM) D-glucose inhibited 60-70% of the NO production within 15-30 min; L-glucose at the same concentration did not have any effect. Inhibition of PKC-beta with 100 nM ruboxistaurin prevented eNOS suppression in high-glucose media. Activation of PKC with 100 nM phorbol ester also suppressed the glomerular NO concentration. We concluded that eNOS in the renal glomerular capillary endothelial cells is suppressed by activity of PKC at high-glucose concentrations comparable to those in diabetic animals and humans. The consequence is a rapid decline in the generation of NO in the glomerular endothelial cells in the presence of a high concentration of glucose.

Regulation of nitric oxide synthesis by dietary factors

Nitric oxide (NO) is synthesized from L-arginine by NO synthase (NOS). As an endothelium-derived relaxing factor, a mediator of immune responses, a neurotransmitter, a cytotoxic free radical, and a signaling molecule, NO plays crucial roles in virtually every cellular and organ function in the body. The discovery of NO synthesis has unified traditionally diverse research areas in nutrition, physiology, immunology, pathology, and neuroscience. Increasing evidence over the past decade shows that many dietary factors, including protein, amino acids, glucose, fructose, cholesterol, fatty acids, vitamins, minerals, phytoestrogens, ethanol, and polyphenols, are either beneficial to health or contribute to the pathogenesis of chronic diseases partially through modulation of NO production by inducible NOS or constitutive NOS. Although most published studies have focused on only a single nutrient and have generated new and exciting knowledge, future studies are necessary to investigate the interactions of dietary factors on NO synthesis and to define the underlying molecular mechanisms.