DHEA-pretreatment attenuates oxidative stress in kidney-cortex and liver of diabetic rabbits and delays development of the disease

In view of reported discrepancies concerning antioxidant activity of dehydroepiandrosterone (DHEA), a widely used dietary supplement, the current investigation was undertaken to evaluate the antioxidant properties of DHEA in both kidney-cortex and liver of alloxan (ALX)-induced diabetic rabbits, as this diabetogenic compound exhibits the ROS-dependent action. ALX was injected to animals following 7 days of DHEA administration. Four groups of rabbits were used in the experiments: control, DHEA-treated control, diabetic and DHEA-treated diabetic. Our results show for the first time, that in kidney-cortex DHEA resulted in normalization of hydroxyl free radicals (HFR) levels and restoration of catalase (CAT) and glutathione peroxidase (GPx) activities to near the control values, while in liver DHEA prevented the malondialdehyde (MDA) accumulation and normalized glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G6PDH) activities. Moreover, in both kidney-cortex and liver DHEA supplementation prevented GSSG elevation accompanied by a decrease in GSH/GSSG ratio. Although DHEA attenuated oxidative stress in both kidney-cortex and liver of ALX-induced diabetic rabbits and significantly delayed the onset of diabetes in time, it did not protect against the final development of diabetes. In conclusion, the current investigation underscores the complexity of the antioxidant action of DHEA. The data are of clinical interest since DHEA supplementation could prevent the deleterious effects of ROS and delay, or even prevent the onset of many diseases. However, in view of the reported pro-oxidant effects of high DHEA doses, the potential use of this agent as a supplement needs a careful evaluation.

The role of gut microbiota in the pathogenesis of neuropsychiatric and neurodegenerative diseases

According to current knowledge, the number of microorganisms living in our body slightly exceeds the number of our own cells, and most of them occupy the large intestine. New methods for analyzing microorganisms residing in our intestine (intestinal microbiota) enable a better understanding of their metabolic, protective and structural functions as well as complex interactions with the host. The development of microbiota is dynamic, and its composition may change during our lifetime. Many factors can affect the composition of microbiota, such as diet, stress, age, genetic factors and antibiotic therapy. Microbiota-gut-brain communication is bi-directional and is mediated via neuronal, immunological and humoral pathways. This article focuses on gut-brain axis elements, such as the vagus nerve, hypothalamic-pituitary-adrenal axis (HPA), cytokines, neurotransmitters, hormones and intestinal peptides, allowing microbiota to contact with the central nervous system. Moreover, this article shows the mechanisms by which microbiota affects the brain functions related to our behavior, mood and cognitive processes. In addition, the role of microbiota composition disorders in the pathogenesis of central nervous system diseases (such as depression, autism spectrum disorder, schizophrenia, multiple sclerosis, Parkinson’s disease and Alzheimer’s disease) is discussed. This article also focuses on the results from studies in which probiotics have been used as potential therapeutic agents in the treatment of gastrointestinal disorders and also alleviating the symptoms of the central nervous system diseases.

Biosynthesis of Carnosine and Related Dipeptides in Vertebrates

Carnosine (β-alanyl-L-histidine) and its methylated derivatives: anserine (β-alanyl-Nπ- methyl-L-histidine) and balenine (β-alanyl-Nτ-methyl-L-histidine) are abundant constituents of excitable tissues of vertebrates. While carnosine and anserine are present at high concentrations and in variable proportions in skeletal muscle and brain of most vertebrates, balenine appears to be rather more abundant in marine mammals and certain reptilian species. Since the discovery of these compounds at the beginning of 20th century, numerous studies have been devoted to identification of the biochemical and physiological properties of carnosine and related dipeptides. These led to the discovery of the pHbuffering, metal-chelation and antioxidant, capabilities of carnosine and anserine, although no definitive ideas concerning their physiological role has yet been formulated. Only recently the molecular identities of the enzymes catalyzing synthesis of carnosine (carnosine synthase, EC 6.3.2.11) and anserine (carnosine N-methyltransferase, EC 2.1.1.22) have been elucidated, which has given a new insight into their metabolism in vertebrates. These findings have opened new research areas and provide authentic opportunities for understanding the biological function of these "enigmatic" dipeptides. This review aims to summarize recent advances in our knowledge concerning enzymes responsible for the biosynthesis of carnosine and related dipeptides and to evaluate their importance in vertebrate physiology.

DHEA supplementation to dexamethasone-treated rabbits alleviates oxidative stress in kidney-cortex and attenuates albuminuria

Our recent study has shown that dehydroepiandrosterone (DHEA) administered to rabbits partially ameliorated several dexamethasone (dexP) effects on hepatic and renal gluconeogenesis, insulin resistance and plasma lipid disorders. In the current investigation, we present the data on DHEA protective action against dexP-induced oxidative stress and albuminuria in rabbits. Four groups of adult male rabbits were used in the in vivo experiment: (1) control, (2) dexP-treated, (3) DHEA-treated and (4) both dexP- and DHEA-treated. Administration of dexP resulted in accelerated generation of renal hydroxyl free radicals (HFR) and malondialdehyde (MDA), accompanied by diminished superoxide dismutase (SOD) and catalase activities and a dramatic rise in urinary albumin/creatinine ratio. Treatment with DHEA markedly reduced dexP-induced oxidative stress in kidney-cortex due to a decline in NADPH oxidase activity and enhancement of catalase activity. Moreover, DHEA effectively attenuated dexP-evoked albuminuria. Surprisingly, dexP-treated rabbits exhibited elevation of GSH/GSSG ratio, accompanied by a decrease in glutathione peroxidase (GPx) and glutathione-S-transferase (GST) activities as well as an increase in glucose-6-phosphate dehydrogenase (G6PDH) activity. Treatment with DHEA resulted in a decline in GSH/GSSG ratio and glutathione reductase (GR) activity, accompanied by an elevation of GPx activity. Interestingly, rabbits treated with both dexP and DHEA remained the control values of GSH/GSSG ratio. As the co-administration of DHEA with dexP resulted in (i) reduction of oxidative stress in kidney-cortex, (ii) attenuation of albuminuria and (iii) normalization of glutathione redox state, DHEA might limit several undesirable renal side effects during chronic GC treatment of patients suffering from allergies, asthma, rheumatoid arthritis and lupus. Moreover, its supplementation might be particularly beneficial for the therapy of patients with glucocorticoid-induced diabetes.

Proinsulin C-peptide potentiates the inhibitory action of insulin on glucose synthesis in primary cultured rabbit kidney-cortex tubules: Metabolic studies

Effects of equimolar concentrations of proinsulin C-peptide and insulin on glucose synthesis were studied in primary cultures of rabbit kidney-cortex tubules grown in the presence of alanine, glycerol, and octanoate. The rhodamine-labeled C-peptide entered renal tubular cells and localized in nuclei, both in the presence and absence of insulin; preincubations with the unlabeled compound inhibited internalization. C-peptide did not affect glucose formation when added alone but potentiated the inhibitory action of insulin by about 20% due to a decrease in flux through glucose-6-phosphate isomerase (GPI) and (or) glucose-6-phosphatase (G6Pase). GPI inhibition was caused by: (i) increased intracellular contents of fructose-1,6-bisphosphate and fructose-1-phosphate, inhibitors of the enzyme and (ii) reduced level of the phosphorylated GPI, which exhibits higher enzymatic activity in the presence of casein kinase 2. A decrease in flux through G6Pase, due to diminished import of G6P by G6P-transporter from the cytoplasm into endoplasmic reticulum lumen, is also suggested. The data show for the first time that in the presence of insulin and C-peptide, both GPI and G6P-ase may act as regulatory enzymes of renal gluconeogenic pathway.

[The role of gut microbiota in the pathogenesis of obesity and diabetes]

The rapidly increasing number of people with obesity and type 2 diabetes is one of the most serious problems of the contemporary world. Until recently, it was thought that the main cause of this phenomenon is the change of lifestyle and dietary habits. According to recent reports, the gut microbiota may also play an important role in the "epidemic" of obesity and diabetes. Changes in its composition have been observed in people suffering from these diseases. In addition, the fact that the intestinal microbiota may affect body weight, insulin sensitivity or sugar and lipid metabolism has led to the hypothesis that these changes may contribute to the pathogenesis of obesity and diabetes. Scientists, using antibacterial drugs, pro- and prebiotics, are trying to modify the intestinal flora and thus affect its interaction with the host. The results are very promising, lead to further analysis and indicate gut microbiota as a potential therapeutic target for obesity and diabetes treatment.

[Physiology and molecular mechanism of glucocorticoid action]

Endogenous glucocorticoids (GCs) are secreted into the systemic circulation from the adrenal cortex. This release is under the control of the circadian clock and can be enhanced at any time in response to a stressor. The levels of circulating GC are regulated systemically by the hypothalamo-pituitary-adrenal axis and locally by access to target cells and pre-receptor metabolism by 11beta-hydroxysteroids dehydrogenase enzymes. GCs mediate their genomic action by binding to two different ligand-inducible transcription factors: high-affinity mineralocorticoid receptor (MR) and 10-fold lower affinity glucocorticoid receptors (GRs). Responses to GCs vary among individuals, cells, and tissues. The diversity and specificity in the steroid hormone's response in the cell is controlled at different levels, including receptor translocation, interaction with specific transcription factors and coregulators, and the regulation of receptor protein levels by microRNA. Moreover, multiple GR isoforms are generated from one single GR gene by alternative splicing and alternative translation initiation. These isoforms all have unique tissue distribution patterns and transcriptional regulatory profiles. Furthermore, each is subjected to various post-translational modifications that affect receptor function. Deciphering the molecular mechanisms of GC action is further complicated by the realization that GCs can induce rapid, non-genomic effects within the cytoplasm. A tight regulation of GC secretion and their cell-specific activity is essential for proper organism function. This is particularly seen under conditions of GC deficiency or excess, as in Addison's disease and Cushing's syndrome, respectively.

Differential effects of selenium compounds on glucose synthesis in rabbit kidney-cortex tubules and hepatocytes. In vitro and in vivo studies

Although selenium is taken with diet mainly as selenoamino acids, its hypoglycaemic action on hepatic gluconeogenesis has been studied with the use of inorganic selenium derivatives. The aim of the present investigation was to compare relative efficacies of inorganic and organic selenium compounds in reducing glucose synthesis in hepatocytes and renal tubules, significantly contributing to the glucose homeostasis. In contrast to hepatocytes, both selenite and methylselenocysteine inhibited renal gluconeogenesis by about 40-45% in control rabbits. Selenate did not affect this process, whereas selenomethionine inhibited gluconeogenesis by about 20% in both hepatocytes and renal tubules. In contrast to methylselenocysteine, selenite decreased intracellular ATP content, glutathione reduced/glutathione oxidized (GSH/GSSG) ratio and pyruvate carboxylase, PEPCK and FBPase activities, while methylselenocysteine diminished PEPCK activity due to elevation of intracellular 2-oxoglutarate and GSSG, inhibitors of this enzyme. Experiments in vivo indicate that in 3 of 9 alloxan-diabetic rabbits treated for 14 days with methylselenocysteine (0.182mg/kg body weight) blood glucose level was normalized, whereas in all diabetic rabbits plasma creatinine and urea levels decreased from 2.52+/-0.18 and 87.4+/-9.7 down to 1.63+/-0.11 and 39.0+/-2.8, respectively. In view of these data selenium supplementation might be beneficial for protection against diabetes-induced nephrotoxicity despite selenium accumulation in kidneys and liver.

Differential effects of vanadium, tungsten and molybdenum on inhibition of glucose formation in renal tubules and hepatocytes of control and diabetic rabbits: Beneficial action of melatonin and N-acetylcysteine

Effect of vanadyl acetylacetonate (VAc), tungstate and molybdate on gluconeogenesis has been studied in isolated hepatocytes and kidney-cortex tubules. In renal tubules of control and alloxan-diabetic animals, the rank order of the metal-compounds-induced (i) inhibition of glucose formation from alanine+glycerol+octanoate or aspartate+glycerol+octanoate, (ii) decrease in the mitochondrial membrane potential (delta psim), (iii) increase in the hydroxyl free radicals (HFR) generation and (iv) decline in glucose-6-phosphatase activity was the following: VAc > tungstate > molybdate. Moreover, in contrast to VAc, both tungstate and molybdate at 100 microM concentration did not practically decrease glucose production in hepatocytes isolated from diabetic rabbits, and significantly increased the rate of lactate formation in renal tubules. N-acetylcysteine at 2 mM concentration partially attenuated vanadium-induced alterations in glucose formation, delta psim and the cellular glutathione redox state, whereas 0.1 mM melatonin did not abolish vanadium-induced changes in gluconeogenesis despite attenuation of vanadium effects on HFR formation and delta psim decline. However, similarly to control rabbits, following 6 days of intraperitoneal administration of both VAc (1.275 mg V/kg body weight daily) and melatonin (1 mg/kg body weight daily) to alloxan-diabetic animals, vanadium-induced elevated serum creatinine and urea levels were decreased, indicating the beneficial effect of melatonin on diabetes- and vanadium-induced nephrotoxicity in rabbits. As serum glucose levels were also significantly diminished by vanadium+melatonin treatment of diabetic animals, the combination therapy of vanadium compounds and melatonin needs a careful evaluation.

AMINO-ACID-DEPENDENT, DIFFERENTIAL EFFECTS OF ETHANOL ON GLUCOSE PRODUCTION IN RABBIT KIDNEY-CORTEX TUBULES

AIMS

The effect of ethanol on glucose synthesis in kidney-cortex tubules of control and diabetic rabbits has been investigated.

METHODS

Both freshly isolated and grown in primary cultures, kidney-cortex tubules were incubated with alanine or aspartate plus lactate or glycerol plus octanoate in the absence and presence of 100 mmol/l ethanol.

RESULTS

In freshly isolated renal tubules incubated in the presence of alanine plus lactate or glycerol plus octanoate, and in tubules grown in primary culture in the medium containing alanine plus lactate plus octanoate alcohol, resulted in about 30% decrease in glucose formation. A diminished glucose production in freshly isolated tubules was accompanied by: (i) a decrease in alanine utilization, (ii) an increase in lactate or glycerol consumptions and (iii) a decline in GSH:GSSG ratio. The ethanol action was not abolished by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase (ADH). In view of ethanol-induced changes in gluconeogenic intermediates it is likely that in the presence of alanine plus glycerol plus octanoate ethanol causes a decline in flux through phosphoenolpyruvate carboxykinase, probably due to either an increase in intracellular content of 2-oxoglutarate, inhibitor of this key gluconeogenic enzyme and/or an enhanced flux through pyruvate kinase, as concluded from an increased lactate formation in the presence of glycerol in the incubation medium. In renal tubules grown in primary cultures in the presence of alanine plus lactate plus octanoate a decrease in GSH:GSSG ratio was accompanied by elevated generation of reactive oxygen species (ROS). Upon replacement of alanine by aspartate ethanol affected neither glucose production, substrate uptake, ROS accumulation nor GSH:GSSG ratio.

CONCLUSIONS

In the presence of alanine ethanol-induced decrease in glucose production and elevation of ROS might cause a limited NADPH generation resulting in a decrease in the intracellular GSH:GSSG ratio. On the contrary, aspartate might protect against ROS generation, so intensive gluconeogenesis supports NADPH generation and in consequence high values of the intracellular GSH:GSSG ratio are maintained.

Inhibition of gluconeogenesis by vanadium and metformin in kidney-cortex tubules isolated from control and diabetic rabbits

Effect of vanadyl acetylacetonate (VAc) and metformin on gluconeogenesis has been studied in isolated hepatocytes and kidney-cortex tubules of rabbit. Glucose formation from alanine+glycerol+octanoate, pyruvate or dihydroxyacetone was inhibited by 50-80% by 100 microM VAc or 500 microM metformin in renal tubules of control and alloxan-diabetic animals, while the inhibitory action of these compounds in hepatocytes was less pronounced (by about 20-30%). In contrast to VAc, metformin increased the rate of lactate formation by about 2-fold in renal tubules incubated with alanine+glycerol+octanoate. In view of VAc-induced changes in intracellular gluconeogenic intermediates and gluconeogenic enzyme activities, it is likely that this compound may decrease fluxes through pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase and glucose-6-phosphatase. In contrast to VAc, metformin-induced decrease in renal gluconeogenesis may result from a decline of cytosolic oxaloacetate level and consequently PEPCK activity. Following 6 days of VAc administration (1.275 mg Vkg(-1) body weight daily) the blood glucose level in alloxan-diabetic rabbits was normalised while blood glucose changes in control animals were not observed. On the contrary, in diabetic animals treated for 6 days with metformin (200 mg kg(-1) body weight day(-1)) a high blood glucose level was maintained. Unfortunately, VAc-treated control and diabetic rabbits exhibited elevated serum urea and creatinine levels. In VAc-treated animals vanadium was accumulated in kidney-cortex up to 7.6+/-0.6 microg Vg(-1) dry weight. In view of a potential vanadium nephrotoxicity a therapeutic application of vanadium compounds needs a critical re-evaluation.

The inhibition of gluconeogenesis by chloroquine contributes to its hypoglycaemic action

The effect of chloroquine on gluconeogenesis in isolated hepatocytes and kidney-cortex tubules of rabbit has been studied. The inhibitory action of 200 microM chloroquine was the highest in hepatocytes and renal tubules incubated with glutamine and glutamate+glycerol+octanoate, respectively, while in the presence of other substrates the drug action was less pronounced. With amino acids as substrates, the inhibition of gluconeogenesis was accompanied by a decreased glutamine production, resulting from a decline of glutamate dehydrogenase activity. A decrease in the urea production by hepatocytes incubated with chloroquine in the presence of glutamine but not NH4Cl as the source of ammonium is in agreement with this suggestion. The degree of inhibition by chloroquine of the rate of gluconeogenesis in renal tubules isolated from control rabbits was similar to that determined in diabetic animals. Chloroquine-induced changes in levels of intracellular gluconeogenic intermediates indicate a decrease in phosphoenolpyruvate carboxykinase and glucose-6-phosphatase activities probably due to increased concentration of 2-oxoglutarate, an inhibitor of these two enzymes. In view of the data, it is likely that inhibition by chloroquine of glucose formation in liver and kidney may contribute to the hypoglycaemic action of this drug. The importance of the inhibitory effect of chloroquine on glutamate dehydrogenase activity in the antihyperglycaemic action of the drug is discussed.

Inhibitory effect of vanadium compounds on glutamate dehydrogenase activity in mitochondria and hepatocytes isolated from rabbit liver

The effect of orthovanadate, vanadyl sulphate and vanadyl acetylacetonate on glutamate dehydrogenase activity was studied in liver mitochondria and isolated hepatocytes of rabbit. In permeabilized mitochondria with free access of substrates and drugs to glutamate dehydrogenase, orthovanadate and vanadyl sulphate at 200 microM concentrations decreased both glutamate synthesis and glutamate deamination by 80 and 50%, respectively, while vanadyl acetylacetonate was less potent. In view of kinetic data obtained at various ammonium concentrations, orthovanadate appeared to be a competitive inhibitor (Ki = 40 +/- 3 microM), while vanadyl sulphate was a non-competitive one (Ki = 147 +/- 10 microM). In contrast to orthovanadate, vanadyl sulphate augmented the inhibitory action of increased above 0.5 mM 2-oxoglutarate concentrations. All these effects on the enzyme activity were partially reversed in the presence of L-leucine and ADP, which are allosteric activators of glutamate dehydrogenase. Moreover, all compounds studied suppressed both glutamate formation and glutamate deamination in isolated hepatocytes incubated under various metabolic conditions, as concluded from decreased rates of glutamate and urea synthesis, respectively. In view of these observations it seems likely that vanadium-containing compounds may be potent inhibitors of glutamate metabolism in liver.