Plasma Arginine and Citrulline are Elevated in Diabetic Retinopathy.

PURPOSE: To determine if plasma levels of six arginine-related and citrulline-related metabolites (arginine, citrulline, asymmetric dimethylarginine [ADMA], ornithine, proline, and argininosuccinate) differ between patients with type 2 diabetes and diabetic retinopathy (DR) and type 2 diabetic controls or between patients with proliferative DR (PDR) and non-proliferative DR (NPDR). DESIGN: Cross-sectional study. METHODS: Adults with type 2 diabetes were recruited from the Vanderbilt Eye Institute. Exclusion criteria included non-diabetic retinal disease. Plasma metabolite levels were quantified in 159 diabetic controls and 156 DR patients (92 NPDR, 64 PDR) using isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS). Metabolite levels were compared using Wilcoxon Rank Sum test and logistic regressions adjusting for age, sex, hemoglobin A1c, diabetes duration, statin use, and angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker use. A secondary analysis that included creatinine in the regression model was performed for the subset of patients with available creatinine values (135 diabetic controls, 100 DR patients [58 NPDR, 42 PDR]). RESULTS: Multivariable logistic regression analyses determined that arginine (OR = 1.20, [1.06-1.38], P = .0067) and citrulline (OR = 1.53, [1.20-1.98], P = .0025) were significantly elevated in DR patients compared to diabetic controls. While ADMA differed between NPDR and PDR patients in the primary analysis (OR = 1.56, [1.15-2.16], P = .0051), it was not significantly different when adjusting for creatinine (OR = 1.30, [0.90-1.91], P = .15). CONCLUSIONS: Plasma arginine and citrulline were significantly elevated in type 2 diabetic patients with DR compared to diabetic controls. None of the tested metabolites significantly differed between NPDR and PDR patients in the adjusted analysis.

Direct Detection of Isolevuglandins in Tissues using a D11 scFv-Alkaline Phosphatase Fusion Protein and Immunofluorescence.

Isolevuglandins (IsoLGs) are highly reactive gamma ketoaldehydes formed from H2-isoprostanes through lipid peroxidation and crosslink proteins leading to inflammation and various diseases including hypertension. Detection of IsoLG accumulation in tissues is crucial in shedding light on their involvement in the disease processes. However, measurement of IsoLGs in tissues is extremely difficult, and currently available tools, including mass spectrometry analysis, are laborious and extremely expensive. Here we describe a novel method for in situ detection of IsoLGs in tissues using alkaline phosphatase-conjugated D11 ScFv and a recombinant phage-display antibody produced in E. coli by immunofluorescent microscopy. Four controls were used for validating the staining: (1) staining with and without D11, (2) staining with bacterial periplasmic extract with the alkaline phosphatase linker, (3) irrelevant scFV antibody staining, and (4) competitive control with IsoLG prior to the staining. We demonstrate the effectiveness of the alkaline phosphatase-conjugated D11 in both human and mouse tissues with or without hypertension. This method will likely serve as an important tool to study the role of IsoLGs in a wide variety of disease processes.

Glycine-Conjugated Bile Acids Protect RPE Tight Junctions against Oxidative Stress and Inhibit Choroidal Endothelial Cell Angiogenesis In Vitro.

We previously demonstrated that the bile acid taurocholic acid (TCA) inhibits features of age-related macular degeneration (AMD) in vitro. The purpose of this study was to determine if the glycine-conjugated bile acids glycocholic acid (GCA), glycodeoxycholic acid (GDCA), and glycoursodeoxycholic acid (GUDCA) can protect retinal pigment epithelial (RPE) cells against oxidative damage and inhibit vascular endothelial growth factor (VEGF)-induced angiogenesis in choroidal endothelial cells (CECs). Paraquat was used to induce oxidative stress and disrupt tight junctions in HRPEpiC primary human RPE cells. Tight junctions were assessed via transepithelial electrical resistance and ZO-1 immunofluorescence. GCA and GUDCA protected RPE tight junctions against oxidative damage at concentrations of 100-500 µM, and GDCA protected tight junctions at 10-500 µM. Angiogenesis was induced with VEGF in RF/6A macaque CECs and evaluated with cell proliferation, cell migration, and tube formation assays. GCA inhibited VEGF-induced CEC migration at 50-500 µM and tube formation at 10-500 µM. GUDCA inhibited VEGF-induced CEC migration at 100-500 µM and tube formation at 50-500 µM. GDCA had no effect on VEGF-induced angiogenesis. None of the three bile acids significantly inhibited VEGF-induced CEC proliferation. These results suggest glycine-conjugated bile acids may be protective against both atrophic and neovascular AMD.

Taurocholic acid inhibits features of age-related macular degeneration in vitro.

Previous metabolomics studies from our lab found altered plasma levels of bile acids in patients with age-related macular degeneration (AMD) compared to controls. In this study, we investigated the ability of the bile acid taurocholic acid (TCA) to inhibit features of AMD modeled in vitro. Paraquat was used to induce oxidative stress in HRPEpiC primary retinal pigment epithelial (RPE) cells. Cells were treated with 300 µM paraquat alone or with TCA (10, 50, 100, 200, or 500 µM). RPE tight junction integrity was assessed via ZO-1 immunofluorescence and transepithelial electrical resistance (TEER) measurements. RF/6A macaque choroidal endothelial cells were treated with 100 ng/mL vascular endothelial growth factor (VEGF) to induce angiogenesis. The effect of TCA on VEGF-induced angiogenesis was evaluated with cell proliferation, cell migration, and tube formation assays. Addition of TCA at 100 (P = 8.6 × 10-4), 200 (P = 0.0035), and 500 (P = 2.1 × 10-4) µM resulted in significant preservation of TEER in paraquat treated cells. In RF/6A cells, TCA did not significantly affect VEGF-induced cell proliferation. VEGF-induced migration of RF/6A cells was significantly inhibited at TCA concentrations of 100 (P = 0.010), 200 (P = 0.023) and 500 (P = 0.0049) µM. VEGF-induced tube formation was significantly inhibited when treated with 200 (P = 0.014) and 500 (P = 7.1 × 10-4) µM TCA. In vitro, TCA promoted RPE cell integrity and diminished VEGF-induced choroidal endothelial cell migration and tube formation. This suggests that TCA may have protective effects against both degenerative and neovascular AMD.

Arginine and Carnitine Metabolites Are Altered in Diabetic Retinopathy.

Purpose: To determine plasma metabolite and metabolic pathway differences between patients with type 2 diabetes with diabetic retinopathy (DR) and without retinopathy (diabetic controls), and between patients with proliferative DR (PDR) and nonproliferative DR (NPDR). Methods: Using high-resolution mass spectrometry with liquid chromatography, untargeted metabolomics was performed on plasma samples from 83 DR patients and 90 diabetic controls. Discriminatory metabolic features were identified through partial least squares discriminant analysis, and linear regression was used to adjust for age, sex, diabetes duration, and hemoglobin A1c. Pathway analysis was performed using Mummichog 2.0. Results: In the adjusted analysis, 126 metabolic features differed significantly between DR patients and diabetic controls. Pathway analysis revealed alterations in the metabolism of amino acids, leukotrienes, niacin, pyrimidine, and purine. Arginine, citrulline, glutamic γ-semialdehyde, and dehydroxycarnitine were key contributors to these pathway differences. A total of 151 features distinguished PDR patients from NPDR patients, and pathway analysis revealed alterations in the ß-oxidation of saturated fatty acids, fatty acid metabolism, and vitamin D3 metabolism. Carnitine was a major contributor to the pathway differences. Conclusions: This study demonstrates that arginine and citrulline-related pathways are dysregulated in DR, and fatty acid metabolism is altered in PDR patients compared with NPDR patients.

High dietary salt-induced dendritic cell activation underlies microbial dysbiosis-associated hypertension.

Excess dietary salt contributes to inflammation and hypertension via poorly understood mechanisms. Antigen presenting cells including dendritic cells (DCs) play a key role in regulating intestinal immune homeostasis in part by surveying the gut epithelial surface for pathogens. Previously, we found that highly reactive γ-ketoaldehydes or isolevuglandins (IsoLGs) accumulate in DCs and act as neoantigens, promoting an autoimmune-like state and hypertension. We hypothesized that excess dietary salt alters the gut microbiome leading to hypertension and this is associated with increased immunogenic IsoLG-adduct formation in myeloid antigen presenting cells. To test this hypothesis, we performed fecal microbiome analysis and measured blood pressure of healthy human volunteers with salt intake above or below the American Heart Association recommendations. We also performed 16S rRNA analysis on cecal samples of mice fed normal or high salt diets. In humans and mice, high salt intake was associated with changes in the gut microbiome reflecting an increase in Firmicutes, Proteobacteria and genus Prevotella bacteria. These alterations were associated with higher blood pressure in humans and predisposed mice to vascular inflammation and hypertension in response to a sub-pressor dose of angiotensin II. Mice fed a high salt diet exhibited increased intestinal inflammation including the mesenteric arterial arcade and aorta, with a marked increase in the B7 ligand CD86 and formation of IsoLG-protein adducts in CD11c+ myeloid cells. Adoptive transfer of fecal material from conventionally housed high salt-fed mice to germ-free mice predisposed them to increased intestinal inflammation and hypertension. These findings provide novel insight into the mechanisms underlying inflammation and hypertension associated with excess dietary salt and may lead to interventions targeting the microbiome to prevent and treat this important disease.

Mitochondrial Haplogroups Modify the Effect of Diabetes Duration and HbA1c on Proliferative Diabetic Retinopathy Risk in Patients With Type 2 Diabetes.

Purpose: We previously demonstrated an association between European mitochondrial haplogroups and proliferative diabetic retinopathy (PDR). The purpose of this study was to determine how the relationship between these haplogroups and both diabetes duration and hyperglycemia, two major risk factors for diabetic retinopathy (DR), affect PDR prevalence. Methods: Our population consisted of patients with type 2 diabetes with (n = 377) and without (n = 480) DR. A Kruskal-Wallis test was used to compare diabetes duration and hemoglobin A1c (HbA1c) among mitochondrial haplogroups. Logistic regressions were performed to investigate diabetes duration and HbA1c as risk factors for PDR in the context of European mitochondrial haplogroups. Results: Neither diabetes duration nor HbA1c differed among mitochondrial haplogroups. Among DR patients from haplogroup H, longer diabetes duration and increasing HbA1c were significant risk factors for PDR (P = 0.0001 and P = 0.011, respectively). Neither diabetes duration nor HbA1c was a significant risk factor for PDR in DR patients from haplogroup UK. Conclusions: European mitochondrial haplogroups modify the effects of diabetes duration and HbA1c on PDR risk in patients with type 2 diabetes. In our patient population, longer diabetes duration and higher HbA1c increased PDR risk in patients from haplogroup H, but did not affect PDR risk in patients from haplogroup UK. This relationship has not been previously demonstrated and may explain, in part, why some patients with nonproliferative DR develop PDR and others do not, despite similar diabetes duration and glycemic control.

Mitochondrial Haplogroups Affect Severity But Not Prevalence of Diabetic Retinopathy.

Purpose: We previously reported European mitochondrial haplogroup H to be a risk factor for and haplogroup UK to be protective against proliferative diabetic retinopathy (PDR) among Caucasian patients with diabetic retinopathy (DR). The purpose of this study was to determine whether these haplogroups are also associated with the risk of having DR among Caucasian patients with diabetes. Methods: Deidentified medical records for 637 Caucasian patients with diabetes (223 with DR) were obtained from BioVU, Vanderbilt University's electronic, deidentified DNA databank. An additional 197 Caucasian patients with diabetes (98 with DR) were enrolled from the Vanderbilt Eye Institute (VEI). We tested for an association between European mitochondrial haplogroups and DR status. Results: The percentage of diabetes patients with DR did not differ across the haplogroups (P = 0.32). The percentage of patients with nonproliferative DR (NPDR; P = 0.0084) and with PDR (P = 0.027) significantly differed across the haplogroups. In logistic regressions adjusting for sex, age, diabetes type, duration of diabetes, and hemoglobin A1c, neither haplogroup H nor haplogroup UK had a significant effect on DR compared with diabetic controls. Haplogroup UK was a significant risk factor (OR = 1.72 [1.13-2.59], P = 0.010) for NPDR compared with diabetic controls in the unadjusted analysis, but not in the adjusted analysis (OR = 1.29 [0.79-2.10], P = 0.20). Conclusions: Mitochondrial haplogroups H and UK were associated with severity, but not presence, of DR. These data argue that the effect of these haplogroups is related to ischemia and neovascularization, the defining features of PDR.

L-cysteine supplementation upregulates glutathione (GSH) and vitamin D binding protein (VDBP) in hepatocytes cultured in high glucose and in vivo in liver, and increases blood levels of GSH, VDBP, and 25-hydroxy-vitamin D in Zucker diabetic fatty rats.

SCOPE: Vitamin D binding protein (VDBP) status has an effect on and can potentially improve the status of 25(OH) vitamin D and increase the metabolic actions of 25(OH) vitamin D under physiological and pathological conditions. Diabetes is associated with lower levels of glutathione (GSH) and 25(OH) vitamin D. This study examined the hypothesis that upregulation of GSH will also upregulate blood levels of VDBP and 25(OH) vitamin D in type 2 diabetic rats. METHODS AND RESULTS: L-cysteine (LC) supplementation was used to upregulate GSH status in a FL83B hepatocyte cell culture model and in vivo using Zucker diabetic fatty (ZDF) rats. Results show that LC supplementation upregulates both protein and mRNA expression of VDBP and vitamin D receptor (VDR) and GSH status in hepatocytes exposed to high glucose, and that GSH deficiency, induced by glutamate cysteine ligase knockdown, resulted in the downregulation of GSH, VDBP, and VDR and an increase in oxidative stress levels in hepatocytes. In vivo, LC supplementation increased GSH and protein and mRNA expression of VDBP and vitamin D 25-hydroxylase (CYP2R1) in the liver, and simultaneously resulted in elevated blood levels of LC and GSH, as well as increases in VDBP and 25(OH) vitamin D levels, and decreased inflammatory biomarkers in ZDF rats compared with those in placebo-supplemented ZDF rats consuming a similar diet. CONCLUSION: LC supplementation may provide a novel approach by which to raise blood levels of VDBP and 25(OH) vitamin D in type 2 diabetes.

Can L-Cysteine and Vitamin D Rescue Vitamin D and Vitamin D Binding Protein Levels in Blood Plasma of African American Type 2 Diabetic Patients?

AIMS: Vitamin D (VD) deficiency has become a worldwide epidemic, particularly affecting African Americans (AA). VD deficiency has been implicated in the excessive rate of complications associated with diabetes in AA. Blood levels of VD binding protein (VDBP) and glutathione (GSH) are lower in AA compared with those in Caucasians. This study tested the hypothesis that lower GSH levels are linked to VDBP and VD deficiency in AA-type 2 diabetic (AA-T2D) patients. Blood was analyzed from T2D and nondiabetic subjects (N). Experiments examining GSH deficiency and l-cysteine (LC) supplementation were performed using THP-1 monocytes. RESULTS: Plasma levels of LC, GSH, VDBP, and VD were significantly lower in AA-T2D compared with age-matched AA-N or Caucasian-T2D. Lower levels of LC and GSH showed a significant positive correlation with lower VDBP and VD levels in AA-T2D. GSH deficiency investigated using an antisense approach depleted VDBP/vitamin D receptor (VDR); LC supplementation caused significant upregulation of GSH and of VDBP/VDR, while supplementation with VD+LC caused a significantly greater GSH and VDBP/VDR upregulation compared with that of VD alone in monocytes. INNOVATION AND CONCLUSION: The reported observations suggest that VD deficiency may be linked to GSH and LC status and lead to a novel hypothesis that supplementation with LC in combination with VD will be effective in increasing VD levels and reducing health disparities in AA.