Taurine increases glucose sensitivity of UCP2-overexpressing beta-cells by ameliorating mitochondrial metabolism

Taurine promotes insulin synthesis by enhancing Isl-1 expression through miR-7a/RAF1/ERK1/2 pathway

It has been well established that the circulating taurine affects the insulin synthesis in pancreatic islet β-cells, whereas miR-7a and LIM-homeodomain transcription factor Isl-1 are important intracellular factors regulating insulin transcription and synthesis. However, it still remains unknown whether taurine regulates insulin synthesis by affecting miR-7a and/or Isl-1 expressions in mouse pancreatic islet β-cells. The present study was thus proposed to identify the effects of taurine on the expressions of miR-7a and/or Isl-1 and their relations to insulin synthesis in mouse pancreatic islet β-cells by using miR-7a2 knockout (KO) and taurine transporter (TauT) KO mouse models and the related in vitro experiments. The results demonstrated that taurine supplement significantly decreased the pancreas miR-7a expression, but sharply upregulated the pancreas Isl-1 and insulin expressions, and serum insulin levels. However, the enhanced effects of taurine on Isl-1 expression and insulin synthesis were mitigated in the TauT KO and miR-7a2 KO mice. In addition, our results confirmed that taurine markedly increased pancreas RAF1 and ERK1/2 expressions. Collectively, the present study firstly demonstrates that taurine regulates insulin synthesis through TauT/miR-7a/RAF1/ERK1/2/Isl-1 signaling pathway, which are crucial for our understanding the mechanisms of taurine affecting insulin synthesis, and also potential for establishing the therapeutic strategies for diabetes and the diseases related to metabolism.

Taurine rescues pancreatic β-cell stress by stimulating α-cell transdifferentiation

The semi-essential ubiquitous amino acid taurine has been shown to alleviate obesity and hyperglycemia in humans; however, the pathways underlying the antidiabetic actions have not been characterized. We explored the effect of chronic taurine exposure on cell biology of pancreatic islets, in degenerative type 1-like diabetes. The latter was modeled by small dose of streptozotocin (STZ) injection for 5 days in mice, followed by a 10-day administration of taurine (2% w/v, orally) in the drinking water. Taurine treatment opposed the detrimental changes in islet morphology and β-/α-cell ratio, induced by STZ diabetes, coincidentally with a significant 3.9 ± 0.7-fold enhancement of proliferation and 40 ± 5% reduction of apoptosis in β-cells. In line with these findings, the treatment counteracted an upregulation of antioxidant (Sod1, Sod2, Cat, Gpx1) and downregulation of islet expansion (Ngn3, Itgb1) genes induced by STZ, in a pancreatic β-cell line. At the same time, taurine enhanced the transdifferentiation of α-cells into β-cells by 2.3 ± 0.8-fold, echoed in strong non-metabolic elevation of cytosolic Ca²⁺ levels in pancreatic α-cells. Our data suggest a bimodal effect of dietary taurine on islet β-cell biology, which combines the augmentation of α-/β-cell transdifferentiation with downregulation of apoptosis. The dualism of action, stemming presumably from the intra- and extracellular modality of the signal, is likely to explain the antidiabetic potential of taurine supplementation.

Beneficial Effects of Taurine on Metabolic Parameters in Animals and Humans

Taurine (2-aminoethanesulfonic acid) is a non-essential amino acid mainly obtained through diet in humans. Despite the lack of research on the health effects of taurine in animals and humans, it is widely used as a dietary supplement. Evidence from human and animal studies indicates that taurine is involved in conjugation of bile acids and regulation of blood pressure and has anti-oxidative, anti-inflammatory, and anti-obesogenic properties. Taurine can benefit both human and non-human animal health in multiple ways. However, few interventional and epidemiological studies regarding the beneficial impacts of taurine in humans and other animals have been conducted. Here, we review the evidence from animal and human studies showing that taurine protects against dyslipidemia, obesity, hypertension, and diabetes mellitus.

The Role of Taurine in Mitochondria Health: More Than Just an Antioxidant

Taurine is a naturally occurring sulfur-containing amino acid that is found abundantly in excitatory tissues, such as the heart, brain, retina and skeletal muscles. Taurine was first isolated in the 1800s, but not much was known about this molecule until the 1990s. In 1985, taurine was first approved as the treatment among heart failure patients in Japan. Accumulating studies have shown that taurine supplementation also protects against pathologies associated with mitochondrial defects, such as aging, mitochondrial diseases, metabolic syndrome, cancer, cardiovascular diseases and neurological disorders. In this review, we will provide a general overview on the mitochondria biology and the consequence of mitochondrial defects in pathologies. Then, we will discuss the antioxidant action of taurine, particularly in relation to the maintenance of mitochondria function. We will also describe several reported studies on the current use of taurine supplementation in several mitochondria-associated pathologies in humans.

Taurine upregulates insulin signaling and mitochondrial metabolism in vitro but not in adipocytes of obese women

OBJECTIVES

Based on taurine's beneficial roles in metabolic diseases in rodents and obese individuals, we investigated the effects of taurine supplementation on adipose tissue using transcriptome analysis, 3T3-L1 adipocytes, and subcutaneous white adipose tissue (scWAT) of obese women.

METHODS

First, we applied bioinformatics analysis to evaluate the effect of the taurine synthesis pathway on the adipose tissue of several BXD mice strains. After that, using 3T3-L1 adipocytes, we investigated the effects of different taurine doses in proteins related to insulin signaling, lipid oxidation, and mitochondrial function. Finally, we evaluated the effects of taurine supplementation (3 grams, 8 wk) on the same proteins in the scWAT of obese women.

RESULTS

The transcriptome analysis showed that the taurine biosynthesis pathway was positively associated with insulin signaling and mitochondrial metabolism in the scWAT of BXD mice. The experiments using 3T3-L1 cells highlighted that the taurine dosage has an essential function in taurine synthesis, insulin, and mitochondrial markers. In contrast, the 8-wk taurine administration did not change the basal insulin, proteins of the taurine synthesis or insulin pathways, lipid oxidation, or mitochondrial metabolism in the scWAT of obese women.

CONCLUSIONS

For the first time, to our knowledge, we showed that supplementation with 3 g of taurine for 8 wk promoted no effect in the insulin signaling pathway in the scWAT of obese women. These findings bring new perspectives to investigate different taurine doses and the intervention period for human studies owing to the potential antiobesity activity of taurine.

Regulation of glucose and lipid metabolism by the pancreatic and extra-pancreatic actions of taurine

The beneficial actions of L-taurine (Tau) against glucose intolerance, obesity, type 2 diabetes (T2D), and non-alcoholic fat liver disease (NAFLD) have been linked to its antioxidant and anti-inflammatory effects, which ameliorate tissue insulin sensitivity. Importantly, there are several lines of evidence that indicate a direct action of Tau on the endocrine pancreas to regulate the secretion and paracrine actions of insulin, glucagon, and somatostatin. Furthermore, Tau can also ameliorate glucose metabolism through the enhancement of insulin signaling. However, some of the benefits of Tau upon intermediary metabolism may manifest via considerable antagonism of the action of insulin. Therefore, this review discusses the mechanisms of action by which Tau may regulate endocrine pancreatic morphofunction, and glucose and lipid homeostasis.

Mitochondria, the NLRP3 Inflammasome, and Sirtuins in Type 2 Diabetes: New Therapeutic Targets

SIGNIFICANCE

Type 2 diabetes mellitus and hyperglycemia can lead to the development of comorbidities such as atherosclerosis and microvascular/macrovascular complications. Both type 2 diabetes and its complications are related to mitochondrial dysfunction and oxidative stress. Type 2 diabetes is also a chronic inflammatory condition that leads to inflammasome activation and the release of proinflammatory mediators, including interleukins (ILs) IL-1β and IL-18. Moreover, sirtuins are energetic sensors that respond to metabolic load, which highlights their relevance in metabolic diseases, such as type 2 diabetes. Recent Advances: Over the past decade, great progress has been made in clarifying the signaling events regulated by mitochondria, inflammasomes, and sirtuins. Nod-like receptor family pyrin domain containing 3 (NLRP3) is the best characterized inflammasome, and the generation of oxidant species seems to be critical for its activation. NLRP3 inflammasome activation and altered sirtuin levels have been observed in type 2 diabetes. Critical Issue: Despite increasing evidence of the relationship between the NLRP3 inflammasome, mitochondrial dysfunction, and oxidative stress and of their participation in type 2 diabetes physiopathology, therapeutic strategies to combat type 2 diabetes that target NLRP3 inflammasome and sirtuins are yet to be consolidated.

FUTURE DIRECTIONS

In this review article, we attempt to provide an overview of the existing literature concerning the crosstalk between mitochondrial impairment and the inflammasome, with particular attention to cellular and mitochondrial redox metabolism and the potential role of the NLRP3 inflammasome and sirtuins in the pathogenesis of type 2 diabetes. In addition, we discuss potential targets for therapeutic intervention based on these molecular interactions. Antioxid. Redox Signal. 29, 749-791.

Ameliorative effects of taurine against diabetes: a review

Diets in rats and humans have shown promising results. Taurine improved glucagon activity, promoted glycemic stability, modified glucose levels, successfully addressed hyperglycemia via advanced glycation end-product control, improved insulin secretion and had a beneficial effect on insulin resistance. Taurine treatment performed well against oxidative stress in brain, increased the secretion of required hormones and protected against neuropathy, retinopathy and nephropathy in diabetes compared with the control. Taurine has been observed to be effective in treatments against diabetic hepatotoxicity, vascular problems and heart injury in diabetes. Taurine was shown to be effective against oxidative stress. The mechanism of action of taurine cannot be explained by one pathway, as it has many effects. Several of the pathways are the advanced glycation end-product pathway, PI3-kinase/AKT pathway and mitochondrial apoptosis pathway. The worldwide threat of diabetes underscores the urgent need for novel therapeutic measures against this disorder. Taurine (2-aminoethane sulfonic acid) is a natural compound that has been studied in diabetes and diabetes-induced complications.

Compatible osmolytes modulate mitochondrial function in a marine osmoconformer Crassostrea gigas (Thunberg, 1793)

Salinity is an important environmental factor affecting physiology of marine organisms. Osmoconformers such as marine mollusks maintain metabolic function despite changes of the osmolarity and composition of the cytosol during salinity shifts. Currently, metabolic responses to the salinity-induced changes of the intracellular milieu are not well understood. We studied the effects of osmolarity (450 vs. 900 mOsm) and compatible osmolytes (70-590 mM of taurine or betaine) on isolated gill mitochondria of a marine osmoconformer, the Pacific oyster Crassostrea gigas. Physiological concentrations of taurine enhanced mitochondrial ATP synthesis and electron transport system (ETS) capacity, increased mitochondrial coupling and stimulated the forward flux through the Complex I. Notably, the stimulatory effects of taurine were more pronounced at 900 mOsm compared to 450 mOsm. In contrast, betaine proportionally increased the rates of the mitochondrial proton leak, oxidative phosphorylation and ETS flux (with no net effect on the mitochondrial coupling) and suppressed the activity of cytochrome c oxidase in oyster mitochondria. However, the effective concentration of betaine (590 mM) was higher than typically found in bivalves, and thus betaine is not likely to affect oyster mitochondria under the physiological conditions in vivo. Our findings indicate that taurine may support the mitochondrial bioenergetics during hyperosmotic stress in oysters. Compatibility of taurine with the metabolic functions and its beneficial effects on mitochondria may have contributed to its broad distribution as an osmolyte in marine osmoconformers and might explain the earlier reports of the positive effects of taurine supplementation on energy metabolism of other organisms, including mammals.

Role of ROS and RNS Sources in Physiological and Pathological Conditions

There is significant evidence that, in living systems, free radicals and other reactive oxygen and nitrogen species play a double role, because they can cause oxidative damage and tissue dysfunction and serve as molecular signals activating stress responses that are beneficial to the organism. Mitochondria have been thought to both play a major role in tissue oxidative damage and dysfunction and provide protection against excessive tissue dysfunction through several mechanisms, including stimulation of opening of permeability transition pores. Until recently, the functional significance of ROS sources different from mitochondria has received lesser attention. However, the most recent data, besides confirming the mitochondrial role in tissue oxidative stress and protection, show interplay between mitochondria and other ROS cellular sources, so that activation of one can lead to activation of other sources. Thus, it is currently accepted that in various conditions all cellular sources of ROS provide significant contribution to processes that oxidatively damage tissues and assure their survival, through mechanisms such as autophagy and apoptosis.

Effect of ruthenium red, a ryanodine receptor antagonist in experimental diabetes induced vascular endothelial dysfunction and associated dementia in rats

Diabetes mellitus is considered as a main risk factor for vascular dementia. In the past, we have reported the induction of vascular dementia by experimental diabetes. This study investigates the efficacy of a ruthenium red, a ryanodine receptor antagonist and pioglitazone in the pharmacological interdiction of pancreatectomy diabetes (PaD) induced vascular endothelial dysfunction and subsequent vascular dementia in rats. Attentional set shifting and Morris water-maze test were used for assessment of learning and memory. Vascular endothelial function, blood brain barrier permeability, serum glucose, serum nitrite/nitrate, oxidative stress (viz. aortic superoxide anion, brain thiobarbituric acid reactive species and brain glutathione), brain calcium and inflammation (myeloperoxidase) were also estimated. PaD rats have shown impairment of endothelial function, blood brain barrier permeability, learning and memory along with an increase in brain inflammation, oxidative stress and calcium. Administration of ruthenium red and pioglitazone has significantly attenuated PaD induced impairment of learning, memory, blood brain barrier permeability, endothelial function and biochemical parameters. It may be concluded that ruthenium red, a ryanodine receptor antagonist and pioglitazone, a PPAR-γ agonist may be considered as potent pharmacological agent for the management of PaD induced endothelial dysfunction and subsequent vascular dementia. Ryanodine receptor may be explored further for their possible benefits in vascular dementia.

Impact of taurine depletion on glucose control and insulin secretion in mice

Taurine, an endogenous sulfur-containing amino acid, is found in millimolar concentrations in mammalian tissue, and its tissue content is altered by diet, disease and aging. The effectiveness of taurine administration against obesity and its related diseases, including type 2 diabetes, has been well documented. However, the impact of taurine depletion on glucose metabolism and fat deposition has not been elucidated. In this study, we investigated the effect of taurine depletion (in the taurine transporter (TauT) knockout mouse model) on blood glucose control and high fat diet-induced obesity. TauT-knockout (TauTKO) mice exhibited lower body weight and abdominal fat mass when maintained on normal chow than wild-type (WT) mice. Blood glucose disposal after an intraperitoneal glucose injection was faster in TauTKO mice than in WT mice despite lower serum insulin levels. Islet beta-cells (insulin positive area) were also decreased in TauTKO mice compared to WT mice. Meanwhile, overnutrition by high fat (60% fat)-diet could lead to obesity in TauTKO mice despite lower body weight under normal chow diet condition, indicating nutrition in normal diet is not enough for TauTKO mice to maintain body weight comparable to WT mice. In conclusion, taurine depletion causes enhanced glucose disposal despite lowering insulin levels and lower body weight, implying deterioration in tissue energy metabolism.

Effect of fishmeal replacement by soy protein concentrate with taurine supplementation on growth performance, hematological and biochemical status, and liver histology of totoaba juveniles (Totoaba macdonaldi)

The effect of dietary inclusion of soy protein concentrate (SPC) and simultaneous supplementation with taurine on the growth, hematology, blood biochemistry, and liver histology of totoaba (Totoaba macdonaldi) juveniles was assessed. Four isoproteic and isolipidic diets were formulated containing either 30 or 60% of SPC (diets S30 and S60), supplemented or not with 1% of taurine (diets S30T and S60T). A fishmeal-based diet formulated for totoaba nutritional requirements, without SPC and taurine supplementation, was used as a reference diet. Triplicate groups of 32 totoaba juveniles (average body weight 7.5 ± 0.6 g) were fed these diets for 45 days. Results showed that growth performance in fish fed S30, S30T, and S60T was similar to fish fed the reference diet. Red blood cells and hematocrit in fish fed with supplemented taurine in both levels of SPC (S30T and S60T) were similar to the fish fed the RD; the addition of taurine improved the state of hydration of totoaba. Plasmatic hemoglobin in fish fed the lower SPC level was similar to fish fed the RD. The mean corpuscular hemoglobin concentration in fish fed S30T was similar to fish fed the RD, taurine supplementation prevented the development of hypochromic anemia in this group of fish. Plasmatic albumin in fish fed S30 was similar to fish fed the RD. Plasmatic total protein and globulin concentration increased and AL:GLB (albumin:globulin ratio) decreased in fish fed the SPC-based diets despite taurine supplementation. The protein profile showed that taurine supplementation did not prevent a possible inflammatory process (increased globulins, decreased AL:GLB) in juvenile totoaba fed both levels of SPC. Glucose concentration was similar in fish fed S30, S30T, and S60T. The histological hepatic index was highest in fish fed S60. These results suggest that with an appropriate nutritional level, taurine may play an important modulatory role in the hematology and blood biochemistry status in totoaba fed SPC-based diets, contributing to the enhancement of an overall healthy growth performance.

Protective and therapeutic effectiveness of taurine in diabetes mellitus: a rationale for antioxidant supplementation

Taurine, 2-amino ethanesulfonic acid, is a conditionally essential β amino acid which is not utilized in protein synthesis. Taurine is one of the most abundant free amino acids in mammals tissues and is one of the three well-known sulfur-containing amino acids; the others are methionine and cysteine which are considered as the precursors for taurine synthesis. Different scientific studies emphasize on the cytoprotective properties of taurine which included antioxidation, antiapoptosis, membrane stabilization, osmoregulation, and neurotransmission. Protective and therapeutic ameliorations of oxidative stress-induced pathologies were also attributed to taurine both in experimental and human models. Data demonstrating the beneficial effectiveness of taurine against type 1 and type 2 diabetes mellitus and their complications are growing and providing a better understanding of the underlying molecular mechanisms. Although the clinical studies are limited compared to the experimental ones, the present updated systematic review of the literature is set up to provide experimental and clinical evidences regarding the effectiveness of taurine in the context of diabetes mellitus and its complications. Gathering these scientific effects of taurine on diabetes mellitus could provide the physicians and specially the endocrinologists with a comprehensive overview on possible trends in the prevention and management of the disease and its complications through antioxidant supplementation.

Taurine supplementation increases K(ATP) channel protein content, improving Ca2+ handling and insulin secretion in islets from malnourished mice fed on a high-fat diet

Pancreatic β-cells are highly sensitive to suboptimal or excess nutrients, as occurs in protein-malnutrition and obesity. Taurine (Tau) improves insulin secretion in response to nutrients and depolarizing agents. Here, we assessed the expression and function of Cav and KATP channels in islets from malnourished mice fed on a high-fat diet (HFD) and supplemented with Tau. Weaned mice received a normal (C) or a low-protein diet (R) for 6 weeks. Half of each group were fed a HFD for 8 weeks without (CH, RH) or with 5% Tau since weaning (CHT, RHT). Isolated islets from R mice showed lower insulin release with glucose and depolarizing stimuli. In CH islets, insulin secretion was increased and this was associated with enhanced KATP inhibition and Cav activity. RH islets secreted less insulin at high K(+) concentration and showed enhanced KATP activity. Tau supplementation normalized K(+)-induced secretion and enhanced glucose-induced Ca(2+) influx in RHT islets. R islets presented lower Ca(2+) influx in response to tolbutamide, and higher protein content and activity of the Kir6.2 subunit of the KATP. Tau increased the protein content of the α1.2 subunit of the Cav channels and the SNARE proteins SNAP-25 and Synt-1 in CHT islets, whereas in RHT, Kir6.2 and Synt-1 proteins were increased. In conclusion, impaired islet function in R islets is related to higher content and activity of the KATP channels. Tau treatment enhanced RHT islet secretory capacity by improving the protein expression and inhibition of the KATP channels and enhancing Synt-1 islet content.

Maternal taurine supplementation in rats partially prevents the adverse effects of early-life protein deprivation on β-cell function and insulin sensitivity

Dietary protein restriction during pregnancy and lactation in rats impairs β-cell function and mass in neonates and leads to glucose intolerance in adult offspring. Maternal taurine (Tau) supplementation during pregnancy in rats restores β-cell function and mass in neonates, but its long-term effects are unclear. The prevention of postnatal catch-up growth has been suggested to improve glucose tolerance in adult offspring of low-protein (LP)-fed mothers. The objective of this study was to examine the relative contribution of β-cell dysfunction and insulin resistance to impaired glucose tolerance in 130-day-old rat offspring of LP-fed mothers and the effects of maternal Tau supplementation on β-cell function and insulin resistance in these offspring. Pregnant rats were fed i) control, ii) LP, and iii) LP+Tau diets during gestation and lactation. Offspring were given a control diet following weaning. A fourth group consisting of offspring of LP-fed mothers, maintained on a LP diet following weaning, was also studied (LP-all life). Insulin sensitivity in the offspring of LP-fed mothers was reduced in females but not in males. In both genders, LP exposure decreased β-cell function. Tau supplementation improved insulin sensitivity in females and β-cell function in males. The LP-all life diet improved β-cell function in males. We conclude that i) maternal Tau supplementation has persistent effects on improving glucose metabolism (β-cell function and insulin sensitivity) in adult rat offspring of LP-fed mothers and ii) increasing the amount of protein in the diet of offspring adapted to a LP diet after weaning may impair glucose metabolism (β-cell function) in a gender-specific manner.

Taurine ameliorates hyperglycemia and dyslipidemia by reducing insulin resistance and leptin level in Otsuka Long-Evans Tokushima fatty (OLETF) rats with long-term diabetes

This study aimed to determine whether taurine supplementation improves metabolic disturbances and diabetic complications in an animal model for type 2 diabetes. We investigated whether taurine has therapeutic effects on glucose metabolism, lipid metabolism, and diabetic complications in Otsuka Long-Evans Tokushima fatty (OLETF) rats with long-term duration of diabetes. Fourteen 50-week-old OLETF rats with chronic diabetes were fed a diet supplemented with taurine (2%) or a non-supplemented control diet for 12 weeks. Taurine reduced blood glucose levels over 12 weeks, and improved OGTT outcomes at 6 weeks after taurine supplementation, in OLETF rats. Taurine significantly reduced insulin resistance but did not improve β-cell function or islet mass. After 12 weeks, taurine significantly decreased serum levels of lipids such as triglyceride, cholesterol, high density lipoprotein cholesterol, and low density lipoprotein cholesterol. Taurine significantly reduced serum leptin, but not adiponectin levels. However, taurine had no therapeutic effect on damaged tissues. Taurine ameliorated hyperglycemia and dyslipidemia, at least in part, by improving insulin sensitivity and leptin modulation in OLETF rats with long-term diabetes. Additional study is needed to investigate whether taurine has the same beneficial effects in human diabetic patients.

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Taurine in adipocytes prevents insulin-mediated H2O2 generation and activates Pka and lipolysis

Among many actions assigned to taurine (Tau), the most abundant amino acid in numerous mammalian tissues, it prevents high-fat diet-induced obesity with increasing resting energy expenditure. To sustain this Tau action, the goal of the present study was to explore the acute effects of Tau on baseline and on adrenaline, insulin and their second messengers to modulate lipolysis in white adipose tissue (WAT) cells from rat epididymis. The Tau effects in this topic were compared with those recorded with Gly, Cys and Met. Tau on its own did not modify baseline lipolysis. Tau raised isoproterenol- and dibutyryl-cAMP (Bt2cAMP)-activated glycerol release. Gly diminished Bt2cAMP-activated glycerol release, and Cys and Met had no effect. Cyclic AMP-dependent activation of protein kinase A (PKA) in cell-free extracts decreased slightly by Gly and was unaltered by Cys, Met, and Tau. PKA catalytic activity in cell-free extracts was stimulated by Tau and unchanged by Cys, Gly and Met. Gly and Tau effects on PKA disappeared when these amino acids were withdrawn by gel filtration. Insulin-mediated NADPH-oxidase (NOX) raises H2O2 pool, which promotes PKA subunit oxidation, and precludes its cAMP activation; thus, lipolysis is diminished. Tau, but not Cys, Gly and Met, inhibited, by as much as 70%, insulin-mediated H2O2 pool increase. These data suggested that Tau raised lipolysis in adipocytes by two mechanisms: stimulating cAMP-dependent PKA catalytic activity and favoring PKA activation by cAMP as a consequence of lowering the H2O2 pool.

The potential usefulness of taurine on diabetes mellitus and its complications

Taurine (2-aminoethanesulfonic acid) is a free amino acid found ubiquitously in millimolar concentrations in all mammalian tissues. Taurine exerts a variety of biological actions, including antioxidation, modulation of ion movement, osmoregulation, modulation of neurotransmitters, and conjugation of bile acids, which may maintain physiological homeostasis. Recently, data is accumulating that show the effectiveness of taurine against diabetes mellitus, insulin resistance and its complications, including retinopathy, nephropathy, neuropathy, atherosclerosis and cardiomyopathy, independent of hypoglycemic effect in several animal models. The useful effects appear due to the multiple actions of taurine on cellular functions. This review summarizes the beneficial effects of taurine supplementation on diabetes mellitus and the molecular mechanisms underlying its effectiveness.

The intrauterine metabolic environment modulates the gene expression pattern in fetal rat islets: prevention by maternal taurine supplementation

AIMS/HYPOTHESIS

Events during fetal life may in critical time windows programme tissue development leading to organ dysfunction with potentially harmful consequences in adulthood such as diabetes. In rats, the beta cell mass of progeny from dams fed with a low-protein (LP) diet during gestation is decreased at birth and metabolic perturbation lasts through adulthood even though a normal diet is given after birth or after weaning. Maternal and fetal plasma taurine levels are suboptimal. Maternal taurine supplementation prevents these induced abnormalities. In this study, we aimed to reveal changes in gene expression in fetal islets affected by the LP diet and how taurine may prevent these changes.

METHODS

Pregnant Wistar rats were fed an LP diet (8% [wt/wt] protein) supplemented or not with taurine in the drinking water or a control diet (20% [wt/wt] protein). At 21.5 days of gestation, fetal pancreases were removed, digested and cultured for 7 days. Neoformed islets were collected and transcriptome analysis was performed.

RESULTS

Maternal LP diet significantly changed the expression of more than 10% of the genes. Tricarboxylic acid cycle and ATP production were highly targeted, but so too were cell proliferation and defence. Maternal taurine supplementation normalised the expression of all altered genes.

CONCLUSIONS/INTERPRETATION

Development of the beta cells and particularly their respiration is modulated by the intrauterine environment, which may epigenetically modify expression of the genome and programme the beta cell towards a pre-diabetic phenotype. This mis-programming by maternal LP diet was prevented by early taurine intervention.

Uncoupling by (--)-epigallocatechin-3-gallate of ATP-sensitive potassium channels from phosphatidylinositol polyphosphates and ATP

Of green tea catechins, (--)-epigallocatechin-3-gallate (EGCG) and (--)-epicatechin-3-gallate (ECG), but not (--)-epicatechin and (--)-epigallocatechin, inhibit the activity of ATP-sensitive potassium (K(ATP)) channels at tens of micromolar concentrations, ECG being three times more effective than EGCG. Further, we found that by using cloned beta cell-type K(ATP) channels, only EGCG at 1 microM, a readily achievable plasma concentration by oral intake in humans, but not other epicatechins, significantly blocked channel reactivation after ATP wash-out, suggesting that interaction of phosphatidylinositol polyphosphates (PIP) with the channel was impaired by EGCG. In addition, a 10-fold higher concentration of EGCG reduced the channel sensitivity to ATP, but not AMP and ADP. This effect of EGCG was greater in the channel with the sulfonylurea receptor (SUR) than with the inwardly rectifying K(+) channel (Kir6.2) alone. Neomycin, a polycation, profoundly suppressed the effect of EGCG. Expectedly, glucose-stimulated cytosolic Ca(2+) elevation in rat pancreatic beta cells, and insulin secretory responses to high glucose loading in vivo were impaired by EGCG. In rabbit cardiac myocytes, dinitrophenol-induced opening of the channel was delayed by 1 microM EGCG. These results suggest that EGCG may interact with PIP-binding sites on the Kir6.2 subunit. SUR further endows EGCG with an ability to interfere with an interaction of the gamma-phosphate tail of ATP with Kir6.2. The specificity of EGCG possibly implies that 5'-OH of the B-ring on the pyrogallol moiety in the EGCG molecule may be critical for these actions of EGCG on the K(ATP) channel.

Taurine in women with a history of gestational diabetes

Taurine is the most abundant amino acid in the human body and seems to play an important role in increasing glucose-mediated insulin secretion, as well as in programming beta-cell maturation during the prenatal life in utero. To test the hypothesis that plasma taurine is related to glucose tolerance, insulin sensitivity and insulin secretion in subjects with history of beta-cell dysfunction such as women with history of gestational diabetes (GDM), we studied 72 non-diabetic women with history of GDM (n=43), impaired glucose tolerance (IGT; n=7), and normal glucose tolerance (NGT; n=22) as previously classified by a 100g-3h-OGTT performed between the 24th and the 28th gestational week. Insulin sensitivity (ISIogtt, calculated through Matsuda-DeFronzo index) and a proxy for insulin secretion (basal plasma C-peptide/fasting plasma glucose; CP/glucose) were measured during and after pregnancy. Plasma taurine was measured after a median period of 6 years (2-11 years) from index pregnancy, when glucose tolerance was retested by a 75 g-2h-OGTT. Plasma taurine was significantly lower in women who had experienced GDM and was unrelated to ISIogtt. Moreover, plasma taurine was inversely related to previous gestational area-under-curve of glucose and directly related to post-gestational CP/glucose, as well to CP/glucose measured during pregnancy (p<0.05 for both). The relative risk of altered glucose metabolism during previous pregnancies (IGT+GDM) was higher as plasma taurine decreased, even after adjusting for age, time-lag from pregnancy, body mass index and family history of diabetes (OR: 0.980; CI 95%: 0.963-0.999, p=0.003). In conclusion plasma taurine seems to be a fair marker of altered glucose metabolism during past pregnancies in women with antecedent GDM and appears to be inversely related to the previous as well as to the actual insulin secretion in these subjects.

Expression of PPARalpha modifies fatty acid effects on insulin secretion in uncoupling protein-2 knockout mice

Aims/hypothesis

In uncoupling protein-2 (UCP2) knockout (KO) mice, protection of beta cells from fatty acid exposure is dependent upon transcriptional events mediated by peroxisome proliferator-activated receptor-α (PPARα).

Methods

PPARα expression was reduced in isolated islets from UCP2KO and wild-type (WT) mice with siRNA for PPARα (siPPARα) overnight. Some islets were also cultured with oleic or palmitic acid, then glucose stimulated insulin secretion (GSIS) was measured. Expression of genes was examined by quantitative RT-PCR or immunoblotting. PPARα activation was assessed by oligonucleotide consensus sequence binding.

Results

siPPARα treatment reduced PPARα protein expression in KO and WT islets by >85%. In siPPARα-treated UCP2KO islets, PA but not OA treatment significantly decreased the insulin response to 16.5 mM glucose. In WT islets, siPPARα treatment did not modify GSIS in PA and OA exposed groups. In WT islets, PA treatment significantly increased UCP2 mRNA and protein expression. Both PA and OA treatment significantly increased PPARα expression in UCP2KO and WT islets but OA treatment augmented PPARα protein expression only in UCP2KO islets (p < 0.05). PA treatment induced carnitine palmitoyltransferase I, acyl CoA oxidase and malonyl CoA decarboxylase mRNA in UCP2KO islets.

Conclusion

These data show that the negative effect of saturated fatty acid on GSIS is mediated by PPARα/UCP2. Knockout of UCP2 protects beta-cells from PA exposure. However, in the absence of both UCP2 and PPARα even a short exposure (24 h) to PA significantly impairs GSIS.

Regulation of insulin secretion by uncoupling protein

UCPs (uncoupling proteins) can regulate cellular ATP production by uncoupling oxidative phosphorylation. UCP2 is expressed in islet beta-cells and its induction reduces glucose-stimulated insulin secretion. Under physiological conditions, superoxide, formed as a by-product of respiration, activates UCP2. This leads to reduced ATP production, which impairs closure of the ATP-dependent K+ channels to prevent insulin secretion. It is suggested that the physiological role of UCP2 is to prevent excessive superoxide generation through a feedback loop. UCP2 induction may also alter fatty acid metabolism by altering NAD/NADH or by facilitating cycling of fatty acid anions. Recently, UCP2 has been proposed to keep insulin secretion low during starvation, a function under the control of the transcription co-repressor, surtuin-1, which has been shown to bind to the UCP2 promoter. Pathological UCP2 expression or activation may suppress glucose-stimulated insulin secretion to the extent that diabetes onset is hastened. In ob/ob mice, induction of UCP2 at age 5 weeks precedes development of insulin secretion defects and hyperglycaemia. Activating protein kinase A-dependent pathways can normalize insulin secretion in UCP2-overexpressing islets. Conversely, lowering UCP2 expression may promote increased insulin secretion. UCP2 knockout mice were protected from the diabetogenic effects of a high-fat diet and their islets exhibited increased sensitivity to glucose and elevated ATP/ADP. These results support a role for UCP2 as a gene contributing to the pathogenesis of Type 2 diabetes.

Taurine (2-aminoethanesulfonic acid) deficiency creates a vicious circle promoting obesity

The relation between blood taurine (2-aminoethanesulfonic acid) concentrations and obesity was investigated. Taurine is supplied to the body by dietary ingestion as well as by de novo synthesis; it is anabolized by cysteine dioxygenase (CDO), which is abundantly expressed in liver and white adipose tissue. Overexpression of CDO in 3T3-L1 preadipocytes caused a decrease in the level of cysteine (precursor of taurine) and an increase in the level of taurine in the culture medium, suggesting that CDO is involved in biosynthesis and secretion of taurine in white adipose tissue. In high-fat diet-induced and/or genetically obese mice, a decrease in the blood taurine concentration was observed along with a decrease in CDO expression in adipose tissue but not in liver. Dietary taurine supplementation prevented high-fat diet-induced obesity with increased resting energy expenditure. Thus, taurine deficiency observed in association with obesity may create a vicious circle promoting obesity. Dietary taurine supplementation interrupts this vicious circle and may prevent obesity.

Is Taurine A Biomarker?

Taurine, a sulfur-containing amino acid present in high concentrations in mammals, plays an important role in several essential biological processes. Taurine is not incorporated into protein and is the most abundant free amino acid in the heart, retina, skeletal muscle, brain, and leukocytes. The ideal biomarker or biological measure should be reliable, reproducible, noninvasive, simple to perform, and inexpensive. Samples for biological measures should be easily obtained from physiological fluids such as blood or urine. Taurine levels in physiologic fluids have been useful for both diagnosing pathology and establishing a disease modifying therapy. In the specific case of taurine, it is important that patient information include nutritional supplementation as well as information on disease status and medications. Taurine has been measured in biological fluids due to the importance of this simple amino acid and its relative ease of determination. Taurine has been measured in animal models of disease as well as a variety of human conditions. However, it remains unclear how taurine should be used as a biomarker and in which situations this measurement would be a good prognostic or diagnostic indicator.

Taurine supplementation and diabetes mellitus

PURPOSE OF REVIEW

Taurine is a semi-essential sulphur amino acid derived from methionine and cysteine metabolism. It has been evaluated either in experimental or clinical type 1 and 2 diabetes mellitus and insulin resistance. One form of experiment has included the possibility that perinatal taurine administration could prevent diabetes mellitus and/or insulin resistance.

RECENT FINDINGS

Experimental data suggest strongly that taurine could have beneficial effects in type 1 diabetes mellitus, and could generally reduce organ lipid peroxidation and plasma lipids. Interestingly, retina, lens and nerves seem to respond better to taurine than other organs such as kidneys. It has been shown in some experimental models that in type 2 diabetes mellitus and insulin resistance there is alteration in taurine homeostasis. Taurine could prevent the onset of diabetes mellitus in NOD mice and postnatal taurine modifies the glucose-loading curves in adults. However, the clinical studies are too small and too short to have any real significance.

SUMMARY

Further experimental and clinical studies are required to evaluate taurine's possible therapeutic potential. Careful attention has to be paid in the selection of animal species, in standardization of taurine concentrations and patient selection. Moreover, care must also be given to the metabolic state, presence of complications, duration of supplementations and selection of the right end-points.