Protective effect of nicotinamide on high glucose/palmitate-induced glucolipotoxicity to INS-1 beta cells is attributed to its inhibitory activity to sirtuins

SIRT3: A potential therapeutic target for liver fibrosis

Sirtuin3 (SIRT3) is a nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase located in the mitochondria, which mainly regulates the acetylation of mitochondrial proteins. In addition, SIRT3 is involved in critical biological processes, including oxidative stress, inflammation, DNA damage, and apoptosis, all of which are closely related to the progression of liver disease. Liver fibrosis characterized by the deposition of extracellular matrix is a result of long termed or repeated liver damage, frequently accompanied by damaged hepatocytes, the recruitment of inflammatory cells, and the activation of hepatic stellate cells. Based on the functions and pharmacology of SIRT3, we will review its roles in liver fibrosis from three aspects: First, the main functions and pharmacological effects of SIRT3 were investigated based on its structure. Second, the roles of SIRT3 in major cells in the liver were summarized to reveal its mechanism in developing liver fibrosis. Last, drugs that regulate SIRT3 to prevent and treat liver fibrosis were discussed. In conclusion, exploring the pharmacological effects of SIRT3, especially in the liver, may be a potential strategy for treating liver fibrosis.

Silibinin alleviates ferroptosis of rat islet β cell INS-1 induced by the treatment with palmitic acid and high glucose through enhancing PINK1/parkin-mediated mitophagy

Type 2 diabetes (T2DM) is induced by the abundance of glucose and lipids, which causes glucolipotoxicity to the pancreatic β-cells. Silibinin is a natural flavonoid possessing the regulatory activity on insulin production and therapeutic activity in diabetic mice; however, its effect on glucolipotoxicity is not fully explained. This in vitro study investigates the effects of silibinin on palmitic acid (PA) and high glucose (HG)-induced cell loss and ferroptosis of rat insulinoma INS-1 cells. In the cells treated with PA and HG, expressions of glucose transporter 4 (Glut4) and carnitine acyltransferase I (CPT1) for β-oxidation of fatty acids are reduced. Mitochondria are the metabolic organelles for glucose and fatty acids. The mitochondrial membrane potential (MMP) and ATP production were decreased, while the ROS level was elevated in the cells treated with PA and HG, indicating an induction of mitochondrial disorder. Cell loss was partially rescued by ferroptosis inhibition, suggesting an involvement of ferroptosis in the cells treated with PA and HG. More importantly, the increases in total iron, lipid ROS, MDA and COX-2, and the decrease in ferroptosis inhibitory molecules GSH, GPX4 and FSP1 appeared in the cells treated with PA and HG, confirming the occurrence of ferroptosis. Moreover, PINK1/parkin-mediated mitophagy, a vital process for selective elimination of damaged mitochondria, was blocked. Interestingly, silibinin rescued the mitochondria, restricted the ferroptosis and restored the mitophagy. By using the pharmacological stimulator and inhibitor of mitophagy, and si-RNA transfection to silence PINK1 expression, silibinin's protective effect against ferroptosis caused by PA and HG treatment was found to depend on mitophagy. Collectively, our current study reveals the new mechanisms for the protection of silibinin against the injury of INS-1 cells treated with PA and HG, elucidates the participation of ferroptosis in glucolipotoxicity, highlighting the involvement of mitophagy in defense against ferroptotic cell death.

Role of SIRT5 in cancer. Friend or Foe?

Cancer is one of the main diseases currently afflicting mankind, being difficult to treat and generating thousands of deaths per year. As a result, researchers around the world are constantly searching for new therapeutic strategies to increase the survival rate of patients. In this regard, SIRT5 may be a promising therapeutic target due to its involvement in many metabolic pathways. Notably, SIRT5 has a dual role in the context of cancer, being able to act as a tumor suppressor in some types of cancer and behaving as an oncogene in others. Interestingly, the performance of SIRT5 is not specific and is highly dependent on the cellular context. As a tumor suppressor, SIRT5 prevents the Warburg effect, increases protection against ROS and reduces cell proliferation and metastasis, while as an oncogene it has the opposite effects as well as increasing resistance to chemotherapeutics and/or radiation. In this way, the aim of this work was to identify in which cancers SIRT5 has beneficial effects and in which deleterious ones based on their molecular characteristics. Furthermore, it was analyzed whether it is feasible to use this protein as a therapeutic target, either enhancing its activity or inhibiting it as appropriate.

Virtual Screening in the Identification of Sirtuins’ Activity Modulators

Sirtuins are NAD⁺-dependent deac(et)ylases with different subcellular localization. The sirtuins’ family is composed of seven members, named SIRT-1 to SIRT-7. Their substrates include histones and also an increasing number of different proteins. Sirtuins regulate a wide range of different processes, ranging from transcription to metabolism to genome stability. Thus, their dysregulation has been related to the pathogenesis of different diseases. In this review, we discussed the pharmacological approaches based on sirtuins’ modulators (both inhibitors and activators) that have been attempted in in vitro and/or in in vivo experimental settings, to highlight the therapeutic potential of targeting one/more specific sirtuin isoform(s) in cancer, neurodegenerative disorders and type 2 diabetes. Extensive research has already been performed to identify SIRT-1 and -2 modulators, while compounds targeting the other sirtuins have been less studied so far. Beside sections dedicated to each sirtuin, in the present review we also included sections dedicated to pan-sirtuins’ and to parasitic sirtuins’ modulators. A special focus is dedicated to the sirtuins’ modulators identified by the use of virtual screening.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

5-Bromoprotocatechualdehyde Combats against Palmitate Toxicity by Inhibiting Parkin Degradation and Reducing ROS-Induced Mitochondrial Damage in Pancreatic β-Cells

Pancreatic β-cell loss is critical in diabetes pathogenesis. Up to now, no effective treatment has become available for β-cell loss. A polyphenol recently isolated from Polysiphonia japonica, 5-Bromoprotocatechualdehyde (BPCA), is considered as a potential compound for the protection of β-cells. In this study, we examined palmitate (PA)-induced lipotoxicity in Ins-1 cells to test the protective effects of BPCA on insulin-secreting β-cells. Our results demonstrated that BPCA can protect β-cells from PA-induced lipotoxicity by reducing cellular damage, preventing reactive oxygen species (ROS) overproduction, and enhancing glucose-stimulated insulin secretion (GSIS). BPCA also improved mitochondrial morphology by preserving parkin protein expression. Moreover, BPCA exhibited a protective effect against PA-induced β-cell dysfunction in vivo in a zebrafish model. Our results provide strong evidence that BPCA could be a potential therapeutic agent for the management of diabetes.

Tectorigenin enhances PDX1 expression and protects pancreatic β-cells by activating ERK and reducing ER stress

Pancreas/duodenum homeobox protein 1 (PDX1) is an important transcription factor that regulates islet β-cell proliferation, differentiation, and function. Reduced expression of PDX1 is thought to contribute to β-cell loss and dysfunction in diabetes. Thus, promoting PDX1 expression can be an effective strategy to preserve β-cell mass and function. Previously, we established a PDX1 promoter-dependent luciferase system to screen agents that can promote PDX1 expression. Natural compound tectorigenin (TG) was identified as a promising candidate that could enhance the activity of the promoter for the PDX1 gene. In this study, we first demonstrated that TG could promote the expression of PDX1 in β-cells via activating extracellular signal-related kinase (ERK), as indicated by increased phosphorylation of ERK; this effect was observed under either normal or glucotoxic/lipotoxic conditions. We then found that TG could suppress induced apoptosis and improved the viability of β-cells under glucotoxicity and lipotoxicity by activation of ERK and reduction of reactive oxygen species and endoplasmic reticulum (ER) stress. These effects held true in vivo as well: prophylactic or therapeutic use of TG could obviously inhibit ER stress and decrease islet β-cell apoptosis in the pancreas of mice given a high-fat/high-sucrose diet (HFHSD), thus dramatically maintaining or restoring β-cell mass and islet size, respectively. Accordingly, both prophylactic and therapeutic use of TG improved HFHSD-impaired glucose metabolism in mice, as evidenced by ameliorating hyperglycemia and glucose intolerance. Taken together, TG, as an agent promoting PDX1 expression exhibits strong protective effects on islet β-cells both in vitro and in vivo.

Nicotinamide reduces inflammation and oxidative stress via the cholinergic system in fructose-induced metabolic syndrome in rats

AIMS

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) have been associated with risk factors for metabolic syndrome (MetS). Our objective was to evaluate the effect of nicotinamide (NAM) on the activities, expression and protein content of cholinesterases in a MetS model.

MAIN METHODS

MetS was induced in male rats administrating 40% fructose to the drinking water for 16 weeks. Additionally, from 5th week onward, the carbohydrate solution was replaced by NAM, at several concentrations for 5 h each morning for the next 12 weeks. In the 15th week, the glucose tolerance test was conducted, and blood pressure was measured. After the treatment period had concluded, the biochemical profile; oxidant stress; proinflammatory markers; and the activity, quantity and expression of cholinesterases were evaluated, and molecular docking analysis was performed.

KEY FINDINGS

The MetS group showed anthropometric, hemodynamic and biochemical alterations and increased cholinesterase activity, inflammation and stress markers. In the liver, cholinesterase activity and mRNA, free fatty acid, tumor necrosis factor-alpha (TNF-α), and thiobarbituric acid-reactive substance (TBARS) levels were increased, while reduced glutathione (GSH) levels were decreased. NAM partially or totally decreased risk factors for MetS, markers of stress and inflammation, and the activity (serum and liver) and expression (liver) of cholinesterases. Molecular docking analysis showed that NAM has a greater affinity for cholinesterases than acetylcholine (ACh), suggesting NAM as an inhibitor of cholinesterases.

SIGNIFICANCE

Supplementation with 40% fructose induced MetS, which increased the activity and expression of cholinesterases, oxidative stress and the inflammation. NAM attenuated these MetS-induced alterations and changes in cholinesterases.

Involvement of Estrogen Receptor-α in the Activation of Nrf2-Antioxidative Signaling Pathways by Silibinin in Pancreatic β-Cells

Silibinin exhibits antidiabetic potential by preserving the mass and function of pancreatic β-cells through up-regulation of estrogen receptor-α (ERα) expression. However, the underlying protective mechanism of silibinin in pancreatic β-cells is still unclear. In the current study, we sought to determine whether ERα acts as the target of silibinin for the modulation of antioxidative response in pancreatic β-cells under high glucose and high fat conditions. Our in vivo study revealed that a 4-week oral administration of silibinin (100 mg/kg/day) decreased fasting blood glucose with a concurrent increase in levels of serum insulin in high-fat diet/streptozotocin-induced type 2 diabetic rats. Moreover, expression of ERα, NF-E2-related factor 2 (Nrf2), and heme oxygenase-1 (HO-1) in pancreatic β-cells in pancreatic islets was increased by silibinin treatment. Accordingly, silibinin (10 μM) elevated viability, insulin biosynthesis, and insulin secretion of high glucose/palmitate-treated INS-1 cells accompanied by increased expression of ERα, Nrf2, and HO-1 as well as decreased reactive oxygen species production in vitro. Treatment using an ERα antagonist (MPP) in INS-1 cells or silencing ERα expression in INS-1 and NIT-1 cells with siRNA abolished the protective effects of silibinin. Our study suggests that silibinin activates the Nrf2-antioxidative pathways in pancreatic β-cells through regulation of ERα expression.

The role of different SIRT1-mediated signaling pathways in toxic injury

Common environmental pollutants and drugs encountered in everyday life can cause toxic damage to the body through oxidative stress, inflammatory stimulation, induction of apoptosis, and inhibition of energy metabolism. Silent information regulator 1 (SIRT1), a nicotinamide adenine dinucleotide-dependent deacetylase, is a member of the evolutionarily highly conserved Sir2 (silent information regulator 2) superprotein family, which is located in the nucleus and cytoplasm. It can deacetylate protein substrates in various signal transduction pathways to regulate gene expression, cell apoptosis and senescence, participate in the process of neuroprotection, energy metabolism, inflammation and the oxidative stress response in living organisms, and plays an important role in toxic damage caused by toxicants and in the process of SIRT1 activator/inhibitor antagonized toxic damage. This review summarizes the role that SIRT1 plays in toxic damage caused by toxicants via its interactions with protein substrates in certain signaling pathways.

Nicotinamide prevents sweet beverage-induced hepatic steatosis in rats by regulating the G6PD, NADPH/NADP+ and GSH/GSSG ratios and reducing oxidative and inflammatory stress

The disruption of redox state homeostasis, the overexpression of lipogenic transcription factors and enzymes, and the increase in lipogenic precursors induced by sweetened beverages are determinants of the development of nonalcoholic fatty liver disease. This study evaluated the action of nicotinamide (NAM) on the expression of glucose-6-phosphate dehydrogenase (G6PD) and redox, oxidative, and inflammatory states in a model of nonalcoholic hepatic steatosis induced by high and chronic consumption of carbohydrates. Male rats were provided drinking water with 30% glucose or fructose ad libitum for 12 weeks. Additionally, 30 days after the beginning of carbohydrate administration, some rats were simultaneously provided water with 0.06% or 0.12% NAM for 5h daily over the next 8 weeks. Biochemical profiles and expression levels of G6PD, tumor necrosis factor α (TNFα), and NADPH oxidase 4 (NOX4) were evaluated together with glutathione/glutathione disulfide (GSH/GSSG) and reduced nicotinamide adenine dinucleotide (phosphate)/nicotinamide adenine dinucleotide (phosphate) [NAD(P)H/NAD(P)] ratios and thiobarbituric acid reactive substances (TBARS). The results showed that hepatic steatosis induced by the chronic consumption of glucose or fructose was associated with body weight gain and increased levels of serum glucose, insulin, triacylglycerols, free fatty acids, transaminases, and TBARS. In the liver, the expression and activity of G6PD increased along with the GSSG, TBARS, and TG concentrations. These alterations were reduced by NAM treatment through the attenuation of increases in G6PD expression and activity and in the NADPH/NADP⁺ ratio, thereby slowing liver steatosis. NAM prevents redox, oxidative, and inflammatory alterations induced by high carbohydrate consumption.

Nicotinamide is an inhibitor of SIRT1 in vitro, but can be a stimulator in cells

Nicotinamide (NAM), a form of vitamin B3, plays essential roles in cell physiology through facilitating NAD+ redox homeostasis and providing NAD+ as a substrate to a class of enzymes that catalyze non-redox reactions. These non-redox enzymes include the sirtuin family proteins which deacetylate target proteins while cleaving NAD+ to yield NAM. Since the finding that NAM exerts feedback inhibition to the sirtuin reactions, NAM has been widely used as an inhibitor in the studies where SIRT1, a key member of sirtuins, may have a role in certain cell physiology. However, once administered to cells, NAM is rapidly converted to NAD+ and, therefore, the cellular concentration of NAM decreases rapidly while that of NAD+ increases. The result would be an inhibition of SIRT1 for a limited duration, followed by an increase in the activity. This possibility raises a concern on the validity of the interpretation of the results in the studies that use NAM as a SIRT1 inhibitor. To understand better the effects of cellular administration of NAM, we reviewed published literature in which treatment with NAM was used to inhibit SIRT1 and found that the expected inhibitory effect of NAM was either unreliable or muted in many cases. In addition, studies demonstrated NAM administration stimulates SIRT1 activity and improves the functions of cells and organs. To determine if NAM administration can generate conditions in cells and tissues that are stimulatory to SIRT1, the changes in the cellular levels of NAM and NAD+ reported in the literature were examined and the factors that are involved in the availability of NAD+ to SIRT1 were evaluated. We conclude that NAM treatment can hypothetically be stimulatory to SIRT1.

Exogenous nicotinamide supplementation and moderate physical exercise can attenuate the aging process in skeletal muscle of rats

Nicotinamide (NAM) could enhance the availability of NAD+ and be beneficial to cell function. However, NAM can inhibit the activities of SIRT1 and PARP. The effect of NAM supplementation on the aging process is not well known. In the present study exogenous NAM (1-0.5% in drinking water) was supplemented for 5 weeks and in the last 4 weeks moderate treadmill running was given to 5 mo and 28 mo old rats. The content of SIRT1 was not effected by NAM treatment alone. However, the activity of SIRT1, judged from the acetylated p53/p53 ratio, increased in both NAM treated age groups, suggesting beneficial effects of exogenous NAM. This was confirmed by the finding of increased PGC-1α and pCREB/CREB ratio in the gastrocnemius muscle of old but not young NAM treated animals. Our data suggest NAM administration can attenuate the aging process in skeletal muscle of rats, but NAM administration together with exercise training might be too great challenge to cope with in the old animals, since it leads to decreased levels of SIRT1.

Nicotinamide riboside kinases display redundancy in mediating nicotinamide mononucleotide and nicotinamide riboside metabolism in skeletal muscle cells

Objective

Augmenting nicotinamide adenine dinucleotide (NAD⁺) availability may protect skeletal muscle from age-related metabolic decline. Dietary supplementation of NAD⁺ precursors nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) appear efficacious in elevating muscle NAD⁺. Here we sought to identify the pathways skeletal muscle cells utilize to synthesize NAD⁺ from NMN and NR and provide insight into mechanisms of muscle metabolic homeostasis.

Methods

We exploited expression profiling of muscle NAD⁺ biosynthetic pathways, single and double nicotinamide riboside kinase 1/2 (NRK1/2) loss-of-function mice, and pharmacological inhibition of muscle NAD⁺ recycling to evaluate NMN and NR utilization.

Results

Skeletal muscle cells primarily rely on nicotinamide phosphoribosyltransferase (NAMPT), NRK1, and NRK2 for salvage biosynthesis of NAD⁺. NAMPT inhibition depletes muscle NAD⁺ availability and can be rescued by NR and NMN as the preferred precursors for elevating muscle cell NAD⁺ in a pathway that depends on NRK1 and NRK2. Nrk2 knockout mice develop normally and show subtle alterations to their NAD+ metabolome and expression of related genes. NRK1, NRK2, and double KO myotubes revealed redundancy in the NRK dependent metabolism of NR to NAD⁺. Significantly, these models revealed that NMN supplementation is also dependent upon NRK activity to enhance NAD⁺ availability.

Conclusions

These results identify skeletal muscle cells as requiring NAMPT to maintain NAD⁺ availability and reveal that NRK1 and 2 display overlapping function in salvage of exogenous NR and NMN to augment intracellular NAD⁺ availability.

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NRK1 and NRK2 are expressed in skeletal muscle and display redundancy in converting NR and NMN to NAD⁺.

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NRK1 and NRK2 are dispensable for maintaining basal skeletal muscle cell NAD⁺.

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Exogenous NMN salvage to NAD⁺ is NRK dependent.

Trehalose supplementation reduces hepatic endoplasmic reticulum stress and inflammatory signaling in old mice

The accumulation of damaged proteins can perturb cellular homeostasis and provoke aging and cellular damage. Quality control systems, such as the unfolded protein response (UPR), inflammatory signaling and protein degradation, mitigate the residence time of damaged proteins. In the present study, we have examined the UPR and inflammatory signaling in the liver of young (~6 months) and old (~28 months) mice (n=8/group), and the ability of trehalose, a compound linked to increased protein stability and autophagy, to counteract age-induced effects on these systems. When used, trehalose was provided for 4 weeks in the drinking water immediately prior to sacrifice (n=7/group). Livers from old mice were characterized by activation of the UPR, increased inflammatory signaling and indices of liver injury. Trehalose treatment reduced the activation of the UPR and inflammatory signaling, and reduced liver injury. Reductions in proteins involved in autophagy and proteasome activity observed in old mice were restored following trehalose treatment. The autophagy marker, LC3B-II, was increased in old mice treated with trehalose. Metabolomics analyses demonstrated that reductions in hexosamine biosynthetic pathway metabolites and nicotinamide in old mice were restored following trehalose treatment. Trehalose appears to be an effective intervention to reduce age-associated liver injury and mitigate the need for activation of quality control systems that respond to disruption of proteostasis.

Using mitochondrial sirtuins as drug targets: disease implications and available compounds

Sirtuins are an evolutionary conserved family of NAD(+)-dependent protein lysine deacylases. Mammals have seven Sirtuin isoforms, Sirt1-7. They contribute to regulation of metabolism, stress responses, and aging processes, and are considered therapeutic targets for metabolic and aging-related diseases. While initial studies were focused on Sirt1 and 2, recent progress on the mitochondrial Sirtuins Sirt3, 4, and 5 has stimulated research and drug development for these isoforms. Here we review the roles of Sirtuins in regulating mitochondrial functions, with a focus on the mitochondrially located isoforms, and on their contributions to disease pathologies. We further summarize the compounds available for modulating the activity of these Sirtuins, again with a focus on mitochondrial isoforms, and we describe recent results important for the further improvement of compounds. This overview illustrates the potential of mitochondrial Sirtuins as drug targets and summarizes the status, progress, and challenges in developing small molecule compounds modulating their activity.

Lipotoxicity in the pancreatic beta cell: not just survival and function, but proliferation as well?

Free fatty acids (FFAs) exert both positive and negative effects on beta cell survival and insulin secretory function, depending on concentration, duration, and glucose abundance. Lipid signals are mediated not only through metabolic pathways, but also through cell surface and nuclear receptors. Toxicity is modulated by positive signals arising from circulating factors such as hormones, growth factors and incretins, as well as negative signals such as inflammatory mediators and cytokines. Intracellular mechanisms of lipotoxicity include metabolic interference and cellular stress responses such as oxidative stress, endoplasmic reticulum (ER) stress, and possibly autophagy. New findings strengthen an old hypothesis that lipids may also impair compensatory beta cell proliferation. Clinical observations continue to support a role for lipid biology in the risk and progression of both type 1 (T1D) and type 2 diabetes (T2D). This review summarizes recent work in this important, rapidly evolving field.

Toxicity generated through inhibition of pyruvate carboxylase and carnitine palmitoyl transferase-1 is similar to high glucose/palmitate-induced glucolipotoxicity in INS-1 beta cells

This work was initiated to determine whether toxicity generated through inhibition of mitochondrial fuel metabolism is similar to high glucose/palmitate (HG/PA)-induced glucolipotoxicity. Influx of glucose and free fatty acids into the tricarboxylic acid (TCA) cycle was inhibited by treatment with the pyruvate carboxylase (PC) inhibitor phenylacetic acid (PAA) and carnitine palmitoyl transferase-1 (CPT-1) inhibitor etomoxir (Eto), or knockdown of PC and CPT-1. Treatment of PAA/Eto or knockdown of PC/CPT-1 induced apoptotic death in INS-1 beta cells. Similar to HG/PA treatment, PAA/Eto increased endoplasmic reticulum stress responses but decreased the Akt signal. JNK inhibitor or chemical chaperone was protective against both PAA/Eto- and HG/PA-induced cell death. All attempts to reduce [Ca²⁺](i), stimulate lipid metabolism, and increase the TCA cycle intermediate pool protected PAA/Eto-induced death as well as HG/PA-induced death. These data suggest that signals induced from impaired mitochondrial fuel metabolism play a critical role in HG/PA-induced glucolipotoxicity.

Nicotinamide, a glucose-6-phosphate dehydrogenase non-competitive mixed inhibitor, modifies redox balance and lipid accumulation in 3T3-L1 cells

AIMS

Excessive energy uptake of dietary carbohydrates results in their storage as fat and requires glucose-6-phosphate dehydrogenase (G6PD)-mediated NADPH production. We sought to assess whether the nicotinamide-induced reduction of G6PD activity might modulate redox balance and lipid accumulation in 3T3-L1 cells.

MAIN METHODS

3T3-L1 preadipocytes (days 4 and 6 of differentiation) and adipocytes were cultured in the presence of 5 or 25 mM glucose. The cells cultured in 25 mM glucose were supplemented with nicotinamide (5-15 mM). Next, we evaluated the following parameters: cell viability, apoptosis, lipid accumulation, lipolysis, reducing power, reactive oxygen species (ROS), NAD(P)H and NAD(P)(+), isocitrate dehydrogenase (IDP), malic enzyme and G6PD, as well as the protein and mRNA levels of G6PD. We also analysed the kinetics of the nicotinamide-induced inhibition of G6PD.

KEY FINDINGS

G6PD mRNA levels increased at day 4 of adipocyte differentiation, whereas G6PD activity progressively increased at days 4 and 6 of differentiation and was reduced in adipocytes. Concomitantly, ROS, reducing power and lipid accumulation increased gradually as the preadipocytes matured into adipocytes. High glucose increased the activity of G6PD, which coincided with an increase in ROS, reducing power and lipid accumulation. All of these changes are prevented by nicotinamide, with the exception of lipid accumulation in adipocytes. Nicotinamide increased IDP activity without affecting NADPH levels. Lastly, nicotinamide inhibited G6PD in a non-competitive mixed way.

SIGNIFICANCE

Nicotinamide modulates G6PD via a non-competitive mixed inhibition and decreases high glucose-dependent oxidative stress and lipid accumulation. Nicotinamide maintains NADPH levels by increasing the activity of IDP.