Carnosine metabolism in diabetes is altered by reactive metabolites

Serum carnosinase 1, an early indicator for incident microalbuminuria in type 1 diabetes

Aims

Carnosinase (CN1) polymorphisms have been linked to diabetic kidney disease (DKD), as CN1 degrades dipeptides which scavenge oxidative metabolites and prevent the formation of advanced glycation end-products. In this work, we studied the association between serum CN1, the systemic redox status and long-term renal outcome in type 1 diabetes.

Methods

Serum CN1 was measured in a prospective type 1 diabetes cohort (n = 218) with a 16-year follow-up. A total of 218 patients treated at the Diabetes Outpatient Clinic of the Weezenlanden Hospital (nowadays Isala Hospital, Zwolle, The Netherlands) were included in this analysis. We assessed whether serum CN1 was associated with renal function and development of DKD as well as other diabetic complications.

Results

At baseline, age, systemic redox status and N-terminal pro brain-natriuretic peptide (NT-proBNP) were associated with serum CN1 concentration (p < 0.05). During follow-up, CN1 concentration in the middle tertile was associated with less incident microalbuminuria (odds ratio = 0.194, 95% C.I.: 0.049-0.772, p = 0.02) after adjustment for age, systemic redox status, NT-proBNP and sex.

Discussion

Serum CN1 could predict incident microalbuminuria and may be used as a novel parameter to identify patients at risk for DKD.

State of the Art in the Development of Human Serum Carnosinase Inhibitors

Human serum carnosinase is an enzyme that operates the preferential hydrolysis of dipeptides with a C-terminus histidine. Only higher primates excrete such an enzyme in serum and cerebrospinal fluid. In humans, the serum hydrolytic rate has high interindividual variability owing to gene polymorphism, although age, gender, diet, and also diseases and surgical interventions can modify serum activity. Human genetic diseases with altered carnosinase activity have been identified and associated with neurological disorders and age-related cognitive decline. On the contrary, low peripheral carnosinase activity has been associated with kidney protection, especially in diabetic nephropathy. Therefore, serum carnosinase is a druggable target for the development of selective inhibitors. However, only one molecule (i.e., carnostatine) has been discovered with the purpose of developing serum carnosinase inhibitors. Bestatin is the only inhibitor reported other than carnostatine, although its activity is not selective towards serum carnosinase. Herein, we present a review of the most critical findings on human serum carnosinase, including enzyme expression, localization and substrate selectivity, along with factors affecting the hydrolytic activity, its implication in human diseases and the properties of known inhibitors of the enzyme.

Anserine and Carnosine Induce HSP70-Dependent H2S Formation in Endothelial Cells and Murine Kidney

Anserine and carnosine have nephroprotective actions; hydrogen sulfide (H₂S) protects from ischemic tissue damage, and the underlying mechanisms are debated. In view of their common interaction with HSP70, we studied possible interactions of both dipeptides with H₂S. H₂S formation was measured in human proximal tubular epithelial cells (HK-2); three endothelial cell lines (HUVEC, HUAEC, MCEC); and in renal murine tissue of wild-type (WT), carnosinase-1 knockout (Cndp1-KO) and Hsp70-KO mice. Diabetes was induced by streptozocin. Incubation with carnosine increased H₂S synthesis capacity in tubular cells, as well as with anserine in all three endothelial cell lines. H₂S dose-dependently reduced anserine/carnosine degradation rate by serum and recombinant carnosinase-1 (CN1). Endothelial Hsp70-KO reduced H₂S formation and abolished the stimulation by anserine and could be restored by Hsp70 transfection. In female Hsp70-KO mice, kidney H₂S formation was halved. In Cndp1-KO mice, kidney anserine concentrations were several-fold and sex-specifically increased. Kidney H₂S formation capacity was increased 2-3-fold in female mice and correlated with anserine and carnosine concentrations. In diabetic Cndp1-KO mice, renal anserine and carnosine concentrations as well as H₂S formation capacity were markedly reduced compared to non-diabetic Cndp1-KO littermates. Anserine and carnosine induce H₂S formation in a cell-type and Hsp70-specific manner within a positive feedback loop with CN1.

Absence of endogenous carnosine synthesis does not increase protein carbonylation and advanced lipoxidation end products in brain, kidney or muscle

Carnosine and other histidine-containing dipeptides are expected to be important anti-oxidants in vertebrates based on various in vitro and in vivo studies with exogenously administered carnosine or its precursor β-alanine. To examine a possible anti-oxidant role of endogenous carnosine, mice lacking carnosine synthase (Carns1^−/−) had been generated and were examined further in the present study. Protein carbonylation increased significantly between old (18 months) and aged (24 months) mice in brain and kidney but this was independent of the Carns1 genotype. Lipoxidation end products were not increased in 18-month-old Carns1^−/− mice compared to controls. We also found no evidence for compensatory increase of anti-oxidant enzymes in Carns1^−/− mice. To explore the effect of carnosine deficiency in a mouse model known to suffer from increased oxidative stress, Carns1 also was deleted in the type II diabetes model Lepr^db/db mouse. In line with previous studies, malondialdehyde adducts were elevated in Lepr^db/db mouse kidney, but there was no further increase by additional deficiency in Carns1. Furthermore, Lepr^db/db mice lacking Carns1 were indistinguishable from conventional Lepr^db/db mice with respect to fasting blood glucose and insulin levels. Taken together, Carns1 deficiency appears not to reinforce oxidative stress in old mice and there was no evidence for a compensatory upregulation of anti-oxidant enzymes. We conclude that the significance of the anti-oxidant activity of endogenously synthesized HCDs is limited in mice, suggesting that other functions of HCDs play a more important role.

Metabolomic and pharmacologic analyses of brain substances associated with sleep pressure in mice

Sleep pressure, the driving force of the homeostatic sleep regulation, is accumulated during wakefulness and dissipated during sleep. Sleep deprivation (SD) has been used as a method to acutely increase animal's sleep pressure for investigating the molecular changes under high sleep pressure. However, SD induces changes not only reflecting increased sleep pressure but also inevitable stresses and prolonged wake state itself. The Sik3^Sleepy mutant mice (Sleepy) exhibit constitutively high sleep pressure despite sleeping longer, and have been useful as a model of increased sleep pressure. Here we conducted a cross-comparison of brain metabolomic profiles between SD versus ad lib slept mice, as well as Sleepy mutant versus littermate wild-type mice. Targeted metabolome analyses of whole brains quantified 203 metabolites in total, of which 43 metabolites showed significant changes in SD, whereas three did in Sleepy mutant mice. The large difference in the number of differential metabolites highlighted limitations of SD as methodology. The cross-comparison revealed that a decrease in betaine and an increase in imidazole dipeptides are associated with high sleep pressure in both models. These metabolites may be novel markers of sleep pressure at the whole-brain level. Furthermore, we found that intracerebroventricular injection of imidazole dipeptides increased subsequent NREM sleep time, suggesting the possibility that imidazole dipeptides may participate in the regulation of sleep in mice.

Carnosine alleviates podocyte injury in diabetic nephropathy by targeting caspase-1-mediated pyroptosis

Diabetic nephropathy (DN) is a main complication of diabetes and often develops into end-stage nephropathy. Histologically, DN progresses as the gradual loss of podocytes with the loss of glomerular podocytes being the earliest sign of DN. Pyroptosis is a new type of programmed cell death and has been mechanistically correlated with podocyte injury in DN. The current study aimed to evaluate the protective effects of carnosine on glomerular podocytes in DN, both in vivo and in vitro. Using high glucose-treated cultured MPC5 cells and a streptozotocin (STZ)-induced diabetic mouse model, we evaluated the effects of carnosine on alleviating podocyte injury in DN. We found that carnosine significantly reversed albuminuria and histopathological lesions and alleviated renal inflammatory and pyroptosis responses in STZ-induced diabetic mice for 12 weeks. The results also showed that carnosine strongly inhibited podocyte inflammation and podocyte pyroptosis in vitro. Cellular Thermal Shift Assay (CETSA) and molecular docking results revealed that mechnaistically caspase-1 was the target of carnosine. We then found that silencing caspase-1 eliminated the protective effect of carnosine. Interestingly, we also found that caspase-1 and gasdermin D expression were increased in renal biopsy tissue of patients with DN. Our study is the first to demonstrate the novel role of carnosine in alleviating podocyte injury by inhibiting pyroptosis via the targeting of caspase-1. Carnosine may have potential as a therapeutic agent in treating DN by targeting caspase-1.

Increased Hypothalamic Anti-Inflammatory Mediators in Non-Diabetic Insulin Receptor Substrate 2-Deficient Mice

Insulin receptor substrate (IRS) 2 is a key mediator of insulin signaling and IRS-2 knockout (IRS2^−/−) mice are a preclinical model to study the development of diabetes, as they develop peripheral insulin resistance and beta-cell failure. The differential inflammatory profile and insulin signaling in the hypothalamus of non-diabetic (ND) and diabetic (D) IRS2^−/− mice might be implicated in the onset of diabetes. Because the lipid profile is related to changes in inflammation and insulin sensitivity, we analyzed whether ND IRS2^−/− mice presented a different hypothalamic fatty acid metabolism and lipid pattern than D IRS2^−/− mice and the relationship with inflammation and markers of insulin sensitivity. ND IRS2^−/− mice showed elevated hypothalamic anti-inflammatory cytokines, while D IRS2^−/− mice displayed a proinflammatory profile. The increased activity of enzymes related to the pentose-phosphate route and lipid anabolism and elevated polyunsaturated fatty acid levels were found in the hypothalamus of ND IRS2^−/− mice. Conversely, D IRS2^−/− mice have no changes in fatty acid composition, but hypothalamic energy balance and markers related to anti-inflammatory and insulin-sensitizing properties were reduced. The data suggest that the concurrence of an anti-inflammatory profile, increased insulin sensitivity and polyunsaturated fatty acids content in the hypothalamus may slow down or delay the onset of diabetes.

Urinary Carnosinase-1 Excretion is Associated with Urinary Carnosine Depletion and Risk of Graft Failure in Kidney Transplant Recipients: Results of the TransplantLines Cohort Study

Carnosine affords protection against oxidative and carbonyl stress, yet high concentrations of the carnosinase-1 enzyme may limit this. We recently reported that high urinary carnosinase-1 is associated with kidney function decline and albuminuria in patients with chronic kidney disease. We prospectively investigated whether urinary carnosinase-1 is associated with a high risk for development of late graft failure in kidney transplant recipients (KTRs). Carnosine and carnosinase-1 were measured in 24 h urine in a longitudinal cohort of 703 stable KTRs and 257 healthy controls. Cox regression was used to analyze the prospective data. Urinary carnosine excretions were significantly decreased in KTRs (26.5 [IQR 21.4–33.3] µmol/24 h versus 34.8 [IQR 25.6–46.8] µmol/24 h; p < 0.001). In KTRs, high urinary carnosinase-1 concentrations were associated with increased risk of undetectable urinary carnosine (OR 1.24, 95%CI [1.06–1.45]; p = 0.007). During median follow-up for 5.3 [4.5–6.0] years, 84 (12%) KTRs developed graft failure. In Cox regression analyses, high urinary carnosinase-1 excretions were associated with increased risk of graft failure (HR 1.73, 95%CI [1.44–2.08]; p < 0.001) independent of potential confounders. Since urinary carnosine is depleted and urinary carnosinase-1 imparts a higher risk for graft failure in KTRs, future studies determining the potential of carnosine supplementation in these patients are warranted.

Human carnosinase 1 overexpression aggravates diabetes and renal impairment in BTBROb/Ob mice

Objective

To assess the influence of serum carnosinase (CN1) on the course of diabetic kidney disease (DKD).

Methods

hCN1 transgenic (TG) mice were generated in a BTBR^Ob/Ob genetic background to allow the spontaneous development of DKD in the presence of serum carnosinase. The influence of serum CN1 expression on obesity, hyperglycemia, and renal impairment was assessed. We also studied if aggravation of renal impairment in hCN1 TG BTBR^Ob/Ob mice leads to changes in the renal transcriptome as compared with wild-type BTBR^Ob/Ob mice.

Results

hCN1 was detected in the serum and urine of mice from two different hCN1 TG lines. The transgene was expressed in the liver but not in the kidney. High CN1 expression was associated with low plasma and renal carnosine concentrations, even after oral carnosine supplementation. Obese hCN1 transgenic BTBR^Ob/Ob mice displayed significantly higher levels of glycated hemoglobin, glycosuria, proteinuria, and increased albumin-creatinine ratios (1104 ± 696 vs 492.1 ± 282.2 μg/mg) accompanied by an increased glomerular tuft area and renal corpuscle size. Gene-expression profiling of renal tissue disclosed hierarchical clustering between BTBR^Ob/Wt, BTBR^Ob/Ob, and hCN1 BTBR^Ob/Ob mice. Along with aggravation of the DKD phenotype, 26 altered genes have been found in obese hCN1 transgenic mice; among them claudin-1, thrombospondin-1, nephronectin, and peroxisome proliferator–activated receptor-alpha have been reported to play essential roles in DKD.

Conclusions

Our data support a role for serum carnosinase 1 in the progression of DKD. Whether this is mainly attributed to the changes in renal carnosine concentrations warrants further studies.

Key messages

Increased carnosinase 1 (CN1) is associated with diabetic kidney disease (DKD).

BTBR^Ob/Ob mice with human CN1 develop a more aggravated DKD phenotype.

Microarray revealed alterations by CN1 which are not altered by hyperglycemia.

These genes have been described to play essential roles in DKD.

Inhibiting CN1 could be beneficial in DKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01957-0) contains supplementary material, which is available to authorized users.

Carnosine and Kidney Diseases: What We Currently Know?

Carnosine (beta-alanyl-L-histidine) is an endogenously synthesised dipeptide which is present in different human tissues e.g. in the kidney. Carnosine is degraded by enzyme serum carnosinase, encoding by CNDP1 gene. Carnosine is engaged in different metabolic pathways in the kidney. It reduces the level of proinflammatory and profibrotic cytokines, inhibits advanced glycation end products' formation, moreover, it also decreases the mesangial cell proliferation. Carnosine may also serve as a scavenger of peroxyl and hydroxyl radicals and a natural angiotensin-converting enzyme inhibitor. This review summarizes the results of experimental and human studies concerning the role of carnosine in kidney diseases, particularly in chronic kidney disease, ischemia/reperfusion-induced acute renal failure, diabetic nephropathy and also drug-induced nephrotoxicity. The interplay between serum carnosine concentration and serum carnosinase activity and polymorphism in the CNDP1 gene is discussed. Carnosine has renoprotective properties. It has a promising potential for the treatment and prevention of different kidney diseases, particularly chronic kidney disease which is a global public health issue. Further studies of the role of carnosine in the kidney may offer innovative and effective strategies for the management of kidney diseases.

L-carnosine and its Derivatives as New Therapeutic Agents for the Prevention and Treatment of Vascular Complications of Diabetes

Vascular complications are among the most serious manifestations of diabetes. Atherosclerosis is the main cause of reduced life quality and expectancy in diabetics, whereas diabetic nephropathy and retinopathy are the most common causes of end-stage renal disease and blindness. An effective therapeutic approach to prevent vascular complications should counteract the mechanisms of injury. Among them, the toxic effects of Advanced Glycation (AGEs) and Lipoxidation (ALEs) end-products are well-recognized contributors to these sequelae. L-carnosine (β-alanyl-Lhistidine) acts as a quencher of the AGE/ALE precursors Reactive Carbonyl Species (RCS), which are highly reactive aldehydes derived from oxidative and non-oxidative modifications of sugars and lipids. Consistently, L-carnosine was found to be effective in several disease models in which glyco/lipoxidation plays a central pathogenic role. Unfortunately, in humans, L-carnosine is rapidly inactivated by serum carnosinase. Therefore, the search for carnosinase-resistant derivatives of Lcarnosine represents a suitable strategy against carbonyl stress-dependent disorders, particularly diabetic vascular complications. In this review, we present and discuss available data on the efficacy of L-carnosine and its derivatives in preventing vascular complications in rodent models of diabetes and metabolic syndrome. We also discuss genetic findings providing evidence for the involvement of the carnosinase/L-carnosine system in the risk of developing diabetic nephropathy and for preferring the use of carnosinase-resistant compounds in human disease. The availability of therapeutic strategies capable to prevent both long-term glucose toxicity, resulting from insufficient glucoselowering therapy, and lipotoxicity may help reduce the clinical and economic burden of vascular complications of diabetes and related metabolic disorders.

The Disposal of Reactive Carbonyl Species through Carnosine Conjugation: What We Know Now

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Reactive Carbonyl Species are electrophiles generated by the oxidative cleavage of lipids and sugars. Such compounds have been described as important molecules for cellular signaling, whilst their accumulation has been found to be cytotoxic as they may trigger aberrant modifications of proteins (a process often referred to as carbonylation).

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A correlation between carbonylation of proteins and human disease progression has been shown in ageing, diabetes, obesity, chronic renal failure, neurodegeneration and cardiovascular disease. However, the fate of reactive carbonyl species is still far from being understood, especially concerning the mechanisms responsible for their disposal as well as the importance of this in disease progression.

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In this context, some data have been published on phase I and phase II deactivation of reactive carbonyl species. In the case of phase II mechanisms, the route involving glutathione conjugation and subsequent disposal of the adducts has been extensively studied both in vitro and in vivo for some of the more representative compounds, e.g. 4-hydroxynonenal.

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There is also emerging evidence of an involvement of carnosine as an endogenous alternative to glutathione for phase II conjugation. However, the fate of carnosine conjugates is still poorly investigated and, unlike glutathione, there is little evidence of the formation of carnosine adducts in vivo. The acquisition of such data could be of importance for the development of new drugs, since carnosine and its derivatives have been proposed as potential therapeutic agents for the mitigation of carbonylation associated with disease progression.

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Herein, we wish to review our current knowledge of the binding of reactive carbonyl species with carnosine together with the disposal of carnosine conjugates, emphasizing those aspects still requiring investigation such as conjugation reversibility and enzyme assisted catalysis of the reactions.

CNDP1 knockout in zebrafish alters the amino acid metabolism, restrains weight gain, but does not protect from diabetic complications

The gene CNDP1 was associated with the development of diabetic nephropathy. Its enzyme carnosinase 1 (CN1) primarily hydrolyzes the histidine-containing dipeptide carnosine but other organ and metabolic functions are mainly unknown. In our study we generated CNDP1 knockout zebrafish, which showed strongly decreased CN1 activity and increased intracellular carnosine levels. Vasculature and kidneys of CNDP1-/- zebrafish were not affected, except for a transient glomerular alteration. Amino acid profiling showed a decrease of certain amino acids in CNDP1-/- zebrafish, suggesting a specific function for CN1 in the amino acid metabolisms. Indeed, we identified a CN1 activity for Ala-His and Ser-His. Under diabetic conditions increased carnosine levels in CNDP1-/- embryos could not protect from respective organ alterations. Although, weight gain through overfeeding was restrained by CNDP1 loss. Together, zebrafish exhibits CN1 functions, while CNDP1 knockout alters the amino acid metabolism, attenuates weight gain but cannot protect organs from diabetic complications.

Serum Carnosinase-1 and Albuminuria Rather than the CNDP1 Genotype Correlate with Urinary Carnosinase-1 in Diabetic and Nondiabetic Patients with Chronic Kidney Disease

BACKGROUND

Carnosinase-1 (CN-1) can be detected in 24 h urine of healthy individuals and patients with type 2 diabetes (T2DM). We aimed to assess whether urinary CN-1 is also reliably measured in spot urine and investigated its association with renal function and the albumin/creatinine ratio (ACR). We also assessed associations between the CNDP1 (CTG) _n genotype and CN-1 concentrations in serum and urine.

METHODS

Patients with T2DM (n = 85) and nondiabetic patients with chronic kidney disease (CKD) (n = 26) stratified by albuminuria (ACR ≤ 300 mg/g or ACR > 300 mg/g) recruited from the nephrology clinic and healthy subjects (n = 24) were studied.

RESULTS

Urinary CN-1 was more frequently detected and displayed higher concentrations in patients with ACR > 300 mg/g as compared to those with ACR ≤ 300 mg/g irrespective of the baseline disease (T2DM: 554 ng/ml [IQR 212-934 ng/ml] vs. 31 ng/ml [IQR 31-63 ng/ml] (p < 0.0001) and nondiabetic CKD: 197 ng/ml [IQR 112-739] vs. 31 ng/ml [IQR 31-226 ng/ml] (p = 0.015)). A positive correlation between urinary CN-1 and ACR was found (r = 0.68, p < 0.0001). Multivariate linear regression analysis revealed that ACR and serum CN-1 concentrations but not eGFR or the CNDP1 genotype are independent predictors of urinary CN-1, explaining 47% of variation of urinary CN-1 concentrations (R ² = 0.47, p < 0.0001).

CONCLUSION

These results confirm and extend previous findings on urinary CN-1 concentrations, suggesting that assessment of CN-1 in spot urine is as reliable as in 24 h urine and may indicate that urinary CN-1 in macroalbuminuric patients is primarily serum-derived and not locally produced.

LC-MS-Based Metabolomics and Lipidomics Study of High-Density-Lipoprotein-Modulated Glucose Metabolism with an apoA-I Knockout Mouse Model

Type 2 diabetes mellitus (T2DM) has become a tremendous problem in public health nowadays. High-density lipoprotein (HDL) refers to a group of heterogeneous particles that circulate in blood, and a recent research finds that HDL acts a pivotal part of glucose metabolism. To understand systemic metabolic changes correlated with HDL in glucose metabolism, we applied LC-MS-based metabolomics and lipidomics to detect metabolomic and lipidomic profiles of plasma from apoA-I knockout mice fed a high-fat diet. Multivariate analysis was applied to differentiate apoA-I knockout mice and controls, and potential biomarkers were found. Pathway analysis demonstrated that several metabolic pathways such as aminoacyl-tRNA biosynthesis, arginine and proline metabolism, and phenylalanine, tyrosine, and tryptophan biosynthesis were dysregulated in apoA-I knockout mice. This study may provide a new insight into the underlying pathogenesis in T2DM and prove that LC-MS-based metabolomics and lipidomics are powerful approaches in finding potential biomarkers and disturbed pathways.

Protective Actions of Anserine Under Diabetic Conditions

Background/Aims: In rodents, carnosine treatment improves diabetic nephropathy, whereas little is known about the role and function of anserine, the methylated form of carnosine. Methods: Antioxidant activity was measured by oxygen radical absorbance capacity and oxygen stress response in human renal tubular cells (HK-2) by RT-PCR and Western-Immunoblotting. In wildtype (WT) and diabetic mice (db/db), the effect of short-term anserine treatment on blood glucose, proteinuria and vascular permeability was measured. Results: Anserine has a higher antioxidant capacity compared to carnosine (p < 0.001). In tubular cells (HK-2) stressed with 25 mM glucose or 20–100 µM hydrogen peroxide, anserine but not carnosine, increased intracellular heat shock protein (Hsp70) mRNA and protein levels. In HK-2 cells stressed with glucose, co-incubation with anserine also increased hemeoxygenase (HO-1) protein and reduced total protein carbonylation, but had no effect on cellular sirtuin-1 and thioredoxin protein concentrations. Three intravenous anserine injections every 48 h in 12-week-old db/db mice, improved blood glucose by one fifth, vascular permeability by one third, and halved proteinuria (all p < 0.05). Conclusion: Anserine is a potent antioxidant and activates the intracellular Hsp70/HO-1 defense system under oxidative and glycative stress. Short-term anserine treatment in diabetic mice improves glucose homeostasis and nephropathy.

Responsiveness of Carnosine Homeostasis Genes in the Pancreas and Brain of Streptozotocin-Treated Mice Exposed to Dietary Carnosine

In excitable tissues, the endogenous dipeptide carnosine (CAR, β-Ala-l-His) sustains homeostatic responses to various challenges. By eliciting hypoglycemic effects via actions on the autonomic nervous system and protection of pancreatic beta-cells, CAR is also relevant in diabetes. We investigated the expression of genes involved in CAR biosynthesis, degradation, and membrane transport pathways, in the pancreas and brains of mice treated with streptozotocin (STZ) and then exposed to dietary CAR. We induced hyperglycemia by STZ intraperitoneal injections; then, STZ-treated mice received drinking water with or without CAR for two weeks. We report that CAR administration elicits beneficial effects on blood glucose levels and weight loss in STZ-treated mice and, remarkably, on the insulin gene products in the pancreas, preserving gene expression from STZ challenge. Also, we describe mRNA downregulation of the Slc15a2/Pept2 (dipeptide transporter) and Cndp2 (intracellular dipeptidase) genes in the pancreas of hyperglycemic mice, and dysregulation of Carns1 (CAR synthase), Pept2 and Cndp2 in brains; interestingly, dietary CAR elicits counteracting effects. These expression patterns associate with variations of CAR content in tissues of mice. Overall, our report suggests a direct role of CAR in the diabetes-affected pancreas and in the diabetes-targeted CNS, proposing (dys)regulation of CAR’s homeostasis as a marker condition.

Impact of carbonylation on glutathione peroxidase-1 activity in human hyperglycemic endothelial cells

AIMS

High levels of glucose and reactive carbonyl intermediates of its degradation pathway such as methylglyoxal (MG) may contribute to diabetic complications partly via increased generation of reactive oxygen species (ROS). This study focused on glutathione peroxidase-1 (GPx1) expression and the impact of carbonylation as an oxidative protein modification on GPx1 abundance and activity in human umbilical vein endothelial cells (HUVEC) under conditions of mild to moderate oxidative stress.

RESULTS

High extracellular glucose and MG enhanced intracellular ROS formation in HUVECs. Protein carbonylation was only transiently augmented pointing to an effective antioxidant defense in these cells. Nitric oxide synthase expression was decreased under hyperglycemic conditions but increased upon exposure to MG, whereas superoxide dismutase expression was not significantly affected. Increased glutathione peroxidase (GPx) activity seemed to compensate for a decrease in GPx1 protein due to enhanced degradation via the proteasome. Mass spectrometry analysis identified Lys-114 as a possible carbonylation target which provides a vestibule for the substrate H2O2 and thus enhances the enzymatic reaction.

INNOVATION

Oxidative protein carbonylation has so far been associated with functional inactivation of modified target proteins mainly contributing to aging and age-related diseases. Here, we demonstrate that mild oxidative stress and subsequent carbonylation seem to activate protective cellular redox signaling pathways whereas severe oxidative stress overwhelms the cellular antioxidant defense leading to cell damage.

CONCLUSIONS

This study may contribute to a better understanding of redox homeostasis and its role in the development of diabetes and related vascular complications.

Modulation of glutathione peroxidase activity by age-dependent carbonylation in glomeruli of diabetic mice

AIMS

Low levels of reactive oxygen species and resulting oxidative protein modifications may play a beneficial role in cellular function under stress conditions. Here we studied the influence of age-dependent protein carbonylation on expression and activity of the anti-oxidative selenoenzyme glutathione peroxidase (GPx) in insulin-deficient Ins2^Akita mice and type 2 diabetic obese db/db mice in context of diabetic nephropathy.

METHODS

Protein carbonylation, GPx expression and activity were examined in kidney tissue and lysates by common histological and protein biochemical methods.

RESULTS

In kidneys of Ins2^Akita mice, carbonylated proteins, GPx-1 and GPx-4 expression were mainly detected in podocytes and mesangial cells. GPx activity was increased in kidney cortex homogenates of these mice. Remarkably, young animals did not show a concomitant increase in GPx expression but enhanced GPx carbonylation. No carbonylation-dependent modification of GPx activity was detected in db/db mice. In cultured podocytes hyperglycemia induced an increase in GPx expression but had no effect on activity or carbonylation. In kidney tissue sections of type 1 or type 2 diabetes patients, GPx-1 and GPx-4 expression but not overall protein carbonylation was significantly decreased.

CONCLUSIONS

These results indicate the existence of a threshold for beneficial carbonylation-dependent redox signaling during the progression of diabetic nephropathy.

Role of protein carbonylation in diabetes

Diabetes mellitus is a metabolic disease characterized by, among others, elevated blood glucose levels. Hyperglycaemia as well as enhanced levels of glucose-derived reactive metabolites contribute to the development of diabetic complications partly via increased generation of reactive oxygen species (ROS). ROS are not only part of signaling pathways themselves but also lead to carbonylation of particular amino acid side chains by direct metal-catalyzed oxidation. In addition, carbonyl groups can be introduced into proteins indirectly by non-oxidative covalent adduction of reactive carbonyl species generated by the oxidation of lipids or carbohydrates. Both direct and indirect carbonylation mechanisms may affect protein conformation, activity, and function. Herein we introduce the different mechanisms of the carbonylation reaction, discuss degradation mechanisms, and the fate of proteins modified this way and how the overall degree of carbonylation affects protein homeostasis and function differently. The role of protein carbonylation in metabolic control systems and cell signaling are also summarized. Finally, current diagnostic and antioxidant therapeutic options in diabetes are discussed.

Carnosinase, diabetes mellitus and the potential relevance of carnosinase deficiency

Carnosinase (CN1) is a dipeptidase, encoded by the CNDP1 gene, that degrades histidine-containing dipeptides, such as carnosine, anserine and homocarnosine. Loss of CN1 function (also called carnosinase deficiency or aminoacyl-histidine dipeptidase deficiency) has been reported in a small number of patients with highly elevated blood carnosine concentrations, denoted carnosinaemia; it is unclear whether the variety of clinical symptoms in these individuals is causally related to carnosinase deficiency. Reduced CN1 function should increase serum carnosine concentrations but the genetic basis of carnosinaemia has not been formally confirmed to be due to CNDP1 mutations. A CNDP1 polymorphism associated with low CN1 activity correlates with significantly reduced risk for diabetic nephropathy, especially in women with type 2 diabetes, and may slow progression of chronic kidney disease in children with glomerulonephritis. Studies in rodents demonstrate antiproteinuric and vasculoprotective effects of carnosine, the precise molecular mechanisms, however, are still incompletely understood. Thus, carnosinemia due to CN1 deficiency may be a non-disease; in contrast, carnosine may potentially protect against long-term sequelae of reactive metabolites accumulating, e.g. in diabetes and chronic renal failure.

FL-926-16, a novel bioavailable carnosinase-resistant carnosine derivative, prevents onset and stops progression of diabetic nephropathy in db/db mice

BACKGROUND AND PURPOSE

The advanced glycation end products (AGEs) participate in the pathogenesis of diabetic nephropathy (DN) by promoting renal inflammation and injury. L-carnosine acts as a quencher of the AGE precursors reactive carbonyl species (RCS), but is rapidly inactivated by carnosinase. In this study, we evaluated the effect of FL-926-16, a carnosinase-resistant and bioavailable carnosine derivative, on the onset and progression of DN in db/db mice.

EXPERIMENTAL APPROACH

Adult male db/db mice and coeval db/m controls were left untreated or treated with FL-926-16 (30 mg·kg^-1 body weight) from weeks 6 to 20 (prevention protocol) or from weeks 20 to 34 (regression protocol).

KEY RESULTS

In the prevention protocol, FL-926-16 significantly attenuated increases in creatinine (-80%), albuminuria (-77%), proteinuria (-75%), mean glomerular area (-34%), fractional (-40%) and mean (-42%) mesangial area in db/db mice. This protective effect was associated with a reduction in glomerular matrix protein expression and cell apoptosis, circulating and tissue oxidative and carbonyl stress, and renal inflammatory markers, including the NLRP3 inflammasome. In the regression protocol, the progression of DN was completely blocked, although not reversed, by FL-926-16. In cultured mesangial cells, FL-926-16 prevented NLRP3 expression induced by RCS but not by the AGE N^ε -carboxymethyllysine.

CONCLUSION AND IMPLICATIONS

FL-926-16 is effective at preventing the onset of DN and halting its progression in db/db mice by quenching RCS, thereby reducing the accumulation of their protein adducts and the consequent inflammatory response. In a future perspective, this novel compound may represent a promising AGE-reducing approach for DN therapy.

Carnosine scavenging of glucolipotoxic free radicals enhances insulin secretion and glucose uptake

The worldwide prevalence of diabetes has risen to 8.5% among adults, which represents a staggering rise in prevalence from 4.7% in 1980. Whilst some treatments work by increasing insulin secretion, over time their effectiveness decreases. We aim to increase insulin secretion by developing strategies that work through mechanisms independent of current treatment options. Isolated CD1 mouse islets, INS-1 pancreatic β-cells, or C2C12 mouse myotubes were incubated in standard tissue culture media, or media supplemented with 28 mM glucose, 200 μM palmitic acid, and 200 μM oleic acid as a cellular model of diabetic glucolipotoxicity. Intracellular reactive species content was assayed using 2′,7′-dichlorofluorescein diacetate dye, inducible nitric oxide synthase levels determined by Western blot, 3-nitrotyrosine and 4-hydrpxnonenal both assayed by ELISA, insulin secretion quantified using ELISA or radioimmunoassay, and glucose uptake determined through 2-deoxy glucose 6 phosphate luminescence. Our data indicate that carnosine, a histidine containing dipeptide available through the diet, is an effective scavenger of each of the aforementioned reactive species. This results in doubling of insulin secretion from isolated mouse islets or INS-1 β-cells. Crucially, carnosine also reverses glucolipotoxic inhibition of insulin secretion and enhances glucose uptake into skeletal muscle cells. Thus, carnosine, or non-hydrolysable carnosine analogs, may represent a new class of therapeutic agent to fight type 2 diabetes.

Allosteric inhibition of carnosinase (CN1) by inducing a conformational shift

In humans, low serum carnosinase (CN1) activity protects patients with type 2 diabetes from diabetic nephropathy. We now characterized the interaction of thiol-containing compounds with CN1 cysteine residue at position 102, which is important for CN1 activity. Reduced glutathione (GSH), N-acetylcysteine and cysteine (3.2 ± 0.4, 2.0 ± 0.3, 1.6 ± 0.2 µmol/mg/h/mM; p < .05) lowered dose-dependently recombinant CN1 (rCN1) efficiency (5.2 ± 0.2 µmol/mg/h/mM) and normalized increased CN1 activity renal tissue samples of diabetic mice. Inhibition was allosteric. Substitution of rCN1 cysteine residues at position 102 (Mut1^C102S) and 229 (Mut2^C229S) revealed that only cysteine-102 is influenced by cysteinylation. Molecular dynamic simulation confirmed a conformational rearrangement of negatively charged residues surrounding the zinc ions causing a partial shift of the carnosine ammonium head and resulting in a less effective pose of the substrate within the catalytic cavity and decreased activity. Cysteine-compounds influence the dynamic behaviour of CN1 and therefore present a promising option for the treatment of diabetes.

On the Relationship between Energy Metabolism, Proteostasis, Aging and Parkinson's Disease: Possible Causative Role of Methylglyoxal and Alleviative Potential of Carnosine

Recent research shows that energy metabolism can strongly influence proteostasis and thereby affect onset of aging and related disease such as Parkinson's disease (PD). Changes in glycolytic and proteolytic activities (influenced by diet and development) are suggested to synergistically create a self-reinforcing deleterious cycle via enhanced formation of triose phosphates (dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate) and their decomposition product methylglyoxal (MG). It is proposed that triose phosphates and/or MG contribute to the development of PD and its attendant pathophysiological symptoms. MG can induce many of the macromolecular modifications (e.g. protein glycation) which characterise the aged-phenotype. MG can also react with dopamine to generate a salsolinol-like product, 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinaline (ADTIQ), which accumulates in the Parkinson's disease (PD) brain and whose effects on mitochondria, analogous to MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), closely resemble changes associated with PD. MG can directly damage the intracellular proteolytic apparatus and modify proteins into non-degradable (cross-linked) forms. It is suggested that increased endogenous MG formation may result from either, or both, enhanced glycolytic activity and decreased proteolytic activity and contribute to the macromolecular changes associated with PD. Carnosine, a naturally-occurring dipeptide, may ameliorate MG-induced effects due, in part, to its carbonyl-scavenging activity. The possibility that ingestion of highly glycated proteins could also contribute to age-related brain dysfunction is briefly discussed.