Increased primary amine oxidase expression and activity in white adipose tissue of obese and diabetic db-/- mice

Monoamine oxidase mediated oxidative stress: a potential molecular and biochemical crux in the pathogenesis of obesity

Obesity has become a global health concern with an increasing prevalence as years pass by but the researchers have not come to a consensus on the exact pathophysiological mechanism underlying this disease. In the past three decades, Monoamine Oxidases (MAO), has come into limelight for a possible involvement in orchestrating the genesis of obesity but the exact mechanism is not well elucidated. MAO is essentially an enzyme involved in the catabolism of neurotransmitters and other biogenic amines to form a corresponding aldehyde, hydrogen peroxide (H2O2) and ammonia. This review aims to highlight the repercussions of MAO's catabolic activity on the redox balance, carbohydrate metabolism and lipid metabolism of adipocytes which ultimately leads to obesity. The H2O2 produced by these enzymes seems to be the culprit causing oxidative stress in pre-adipocytes and goes on to mimic insulin's activity independent of its presence via the Protein Kinase B Pathway facilitating glucose influx. The H2O2 activates Sterol regulatory-element binding protein-1c and peroxisome proliferator activated receptor gamma crucial for encoding enzymes like fatty acid synthase, acetyl CoA carboxylase 1, Adenosine triphosphate-citrate lyase, phosphoenol pyruvate carboxykinase etc., which helps promoting lipogenesis at the same time inhibits lipolysis. More reactive oxygen species production occurs via NADPH Oxidases enzymes and is also able activate Nuclear Factor kappa B leading to inflammation in the adipocyte microenvironment. This chronic inflammation is the seed for insulin resistance.

Extracellular Vesicles as Carriers of Adipokines and Their Role in Obesity

Extracellular vesicles (EVs) have lately arisen as new metabolic players in energy homeostasis participating in intercellular communication at the local and distant levels. These nanosized lipid bilayer spheres, carrying bioactive molecular cargo, have somehow changed the paradigm of biomedical research not only as a non-classic cell secretion mechanism, but as a rich source of biomarkers and as useful drug-delivery vehicles. Although the research about the role of EVs on metabolism and its deregulation on obesity and associated pathologies lagged slightly behind other diseases, the knowledge about their function under normal and pathological homeostasis is rapidly increasing. In this review, we are focusing on the current research regarding adipose tissue shed extracellular vesicles including their characterization, size profile, and molecular cargo content comprising miRNAs and membrane and intra-vesicular proteins. Finally, we will focus on the functional aspects attributed to vesicles secreted not only by adipocytes, but also by other cells comprising adipose tissue, describing the evidence to date on the deleterious effects of extracellular vesicles released by obese adipose tissue both locally and at the distant level by interacting with other peripheral organs and even at the central level.

Brown Adipose Tissue Sheds Extracellular Vesicles That Carry Potential Biomarkers of Metabolic and Thermogenesis Activity Which Are Affected by High Fat Diet Intervention

Brown adipose tissue (BAT) is a key target for the development of new therapies against obesity due to its role in promoting energy expenditure; BAT secretory capacity is emerging as an important contributor to systemic effects, in which BAT extracellular vesicles (EVs) (i.e., batosomes) might be protagonists. EVs have emerged as a relevant cellular communication system and carriers of disease biomarkers. Therefore, characterization of the protein cargo of batosomes might reveal their potential as biomarkers of the metabolic activity of BAT. In this study, we are the first to isolate batosomes from lean and obese Sprague–Dawley rats, and to establish reference proteome maps. An LC-SWATH/MS analysis was also performed for comparisons with EVs secreted by white adipose tissue (subcutaneous and visceral WAT), and it showed that 60% of proteins were exclusive to BAT EVs. Precisely, batosomes of lean animals contain proteins associated with mitochondria, lipid metabolism, the electron transport chain, and the beta-oxidation pathway, and their protein cargo profile is dramatically affected by high fat diet (HFD) intervention. Thus, in obesity, batosomes are enriched with proteins involved in signal transduction, cell communication, the immune response, inflammation, thermogenesis, and potential obesity biomarkers including UCP1, Glut1, MIF, and ceruloplasmin. In conclusion, the protein cargo of BAT EVs is affected by the metabolic status and contains potential biomarkers of thermogenesis activity.

Copper and lipid metabolism: A reciprocal relationship

BACKGROUND

Copper and lipid metabolism are intimately linked, sharing a complex, inverse relationship in the periphery (outside of the central nervous system), which remains to be fully elucidated.

SCOPE

Copper and lipids have independently been implicated in the pathogenesis of diseases involving dyslipidaemia, including obesity, cardiovascular disease and non-alcoholic fatty liver disease and also in Wilson disease, an inherited disorder of copper overload. Here we review the relationship between copper and lipid regulatory pathways, which are potential druggable targets for therapeutic intervention.

MAJOR CONCLUSIONS

While the inverse relationship between copper and lipids is apparent, tissue-specific roles for the copper regulatory protein, ATP7B provide further insight into the association between copper and lipid metabolism.

GENERAL SIGNIFICANCE

Understanding the relationship between copper and lipid metabolism is important for identifying druggable targets for diseases with disrupted copper and/or lipid metabolism; and may reveal similar connections within the brain and in neurological diseases with impaired copper and lipid transport.

Oral Supplementation with Benzylamine Delays the Onset of Diabetes in Obese and Diabetic db-/- Mice

Substrates of semicarbazide-sensitive amine oxidase (SSAO) exert insulin-like actions in adipocytes. One of them, benzylamine (Bza) exhibits antihyperglycemic properties in several rodent models of diabetes. To further study the antidiabetic potential of this naturally occurring amine, a model of severe type 2 diabetes, the obese db-/- mouse, was subjected to oral Bza administration. To this end, db-/- mice and their lean littermates were treated at 4 weeks of age by adding 0.5% Bza in drinking water for seven weeks. Body mass, fat content, blood glucose and urinary glucose output were followed while adipocyte insulin responsiveness and gene expression were checked at the end of supplementation, together with aorta nitrites. Bza supplementation delayed the appearance of hyperglycemia, abolished polydypsia and glycosuria in obese/diabetic mice without any detectable effect in lean control, except for a reduction in food intake observed in both genotypes. The improvement of glucose homeostasis was observed in db-/- mice at the expense of increased fat deposition, especially in the subcutaneous white adipose tissue (SCWAT), without sign of worsened inflammation or insulin responsiveness and with lowered circulating triglycerides and uric acid, while NO bioavailability was increased in aorta. The higher capacity of SSAO in oxidizing Bza in SCWAT, found in the obese mice, was unaltered by Bza supplementation and likely involved in the activation of glucose utilization by adipocytes. We propose that Bza oxidation in tissues, which produces hydrogen peroxide mainly in SCWAT, facilitates insulin-independent glucose utilization. Bza could be considered as a potential agent for dietary supplementation aiming at preventing diabetic complications.

Obesity of mice lacking VAP-1/SSAO by Aoc3 gene deletion is reproduced in mice expressing a mutated vascular adhesion protein-1 (VAP-1) devoid of amine oxidase activity

The product of Aoc3 gene is known as vascular adhesion protein-1 (VAP-1), a glycoprotein contributing to leukocyte extravasation and exhibiting semicarbazide-sensitive amine oxidase activity (SSAO). Regarding the immune functions of VAP-1/SSAO, it is known that mice bearing Aoc3 gene knock-out (AOC3KO) exhibit defects in leukocyte migration similar to those of mice expressing a mutated VAP-1 lacking functional SSAO activity (knock-in, AOC3KI). However, it has not been reported whether these models differ regarding other disturbances. Thus, we further compared endocrine-metabolic phenotypes of AOC3KO and AOC3KI mice to their respective control. Special attention was paid on adiposity, glucose and lipid handling, since VAP-1/SSAO is highly expressed in adipose tissue (AT). In both mouse lines, no tissue SSAO activity was found, while Aoc3 mRNA was absent in AOC3KO only. Although food consumption was unchanged, both AOC3KO and AOC3KI mice were heavier and fatter than their respective controls. Other alterations commonly found in adipocytes from both lines were loss of benzylamine insulin-like action with unchanged insulin lipogenic responsiveness and adiponectin expression. A similar downregulation of inflammatory markers (CD45, IL6) was found in AT. Glucose handling and liver mass remained unchanged, while circulating lipid profile was distinctly altered, with increased cholesterol in AOC3KO only. These results suggest that the lack of oxidase activity found in AOC3KI is sufficient to reproduce the metabolic disturbances observed in AOC3KO mice, save those related with cholesterol transport. Modulation of SSAO activity therefore constitutes a potential target for the treatment of cardiometabolic diseases, especially obesity when complicated by low-grade inflammation.

Attenuation of Weight Gain and Prevention of Associated Pathologies by Inhibiting SSAO

Obesity is a worldwide prevalent metabolic disorder that is associated with diabetes, among many other diseases. Bearing this in mind, prevention and treatment ways need to be improved. Notably, activity of the enzyme semicarbazide-sensitive amine oxidase (SSAO) is found to be elevated in overweight subjects. Moreover, SSAO inhibition has resulted in an increase of histamine activity in adipose tissue and the limitation of body fat. The current review aims to overview the risks of obesity, rationalize the molecular ways of SSAO activity, and outline the strategies of inhibiting upregulated enzyme levels. It describes the differences between SSAO inhibitors and advances the prospective agents. Based on evidence, caffeine is proposed as an effective, safe, and reliable choice to inhibit SSAO activity. Furthermore, the histamine in adipocytes has been associated with SSAO activity. Therefore, it is suggested as one of the key compounds to be studied for obesity management. To conclude, inhibiting SSAO may attenuate weight gain and prevent related diseases.

Past, Present and Future Anti-Obesity Effects of Flavin-Containing and/or Copper-Containing Amine Oxidase Inhibitors

Background: Two classes of amine oxidases are found in mammals: those with a flavin adenine dinucleotide as a cofactor, such as monoamine oxidases (MAO) and lysine-specific demethylases (LSD), and those with copper as a cofactor, including copper-containing amine oxidases (AOC) and lysyl oxidases (LOX). All are expressed in adipose tissue, including a semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1) strongly present on the adipocyte surface. Methods: Previously, irreversible MAO inhibitors have been reported to limit food intake and/or fat extension in rodents; however, their use for the treatment of depressed patients has not revealed a clear anti-obesity action. Semicarbazide and other molecules inhibiting SSAO/VAP-1 also reduce adiposity in obese rodents. Results: Recently, a LOX inhibitor and a subtype-selective MAO inhibitor have been shown to limit fattening in high-fat diet-fed rats. Phenelzine, which inhibits MAO and AOC, limits adipogenesis in cultured preadipocytes and impairs lipogenesis in mature adipocytes. When tested in rats or mice, phenelzine reduces food intake and/or fat accumulation without cardiac adverse effects. Novel amine oxidase inhibitors have been recently characterized in a quest for promising anti-inflammatory or anti-cancer approaches; however, their capacity to mitigate obesity has not been studied so far. Conclusions: The present review of the diverse effects of amine oxidase inhibitors impairing adipocyte differentiation or limiting excessive fat accumulation indicates that further studies are needed to reveal their potential anti-obesity properties.

Metabolic Effects of Oral Phenelzine Treatment on High-Sucrose-Drinking Mice

Phenelzine has been suggested to have an antiobesity effect by inhibiting de novo lipogenesis, which led us to investigate the metabolic effects of oral chronic phenelzine treatment in high-sucrose-drinking mice. Sucrose-drinking mice presented higher body weight gain and adiposity versus controls. Phenelzine addition did not decrease such parameters, even though fat pad lipid content and weights were not different from controls. In visceral adipocytes, phenelzine did not impair insulin-stimulated de novo lipogenesis and had no effect on lipolysis. However, phenelzine reduced the mRNA levels of glucose transporters 1 and 4 and phosphoenolpyruvate carboxykinase in inguinal white adipose tissue (iWAT), and altered circulating levels of free fatty acids (FFA) and glycerol. Interestingly, glycemia was restored in phenelzine-treated mice, which also had higher insulinaemia. Phenelzine-treated mice presented higher rectal temperature, which was associated to reduced mRNA levels of uncoupling protein 1 in brown adipose tissue. Furthermore, unlike sucrose-drinking mice, hepatic malondialdehyde levels were not altered. In conclusion, although de novo lipogenesis was not inhibited by phenelzine, the data suggest that the ability to re-esterify FFA is impaired in iWAT. Moreover, the effects on glucose homeostasis and oxidative stress suggest that phenelzine could alleviate obesity-related alterations and deserves further investigation in obesity models.

Identification and validation of reference genes for quantitative real-time PCR studies in long yellow daylily, Hemerocallis citrina Borani

Gene expression analysis using reverse transcription quantitative real-time PCR (RT-qPCR) requires the use of reference gene(s) in the target species. The long yellow daylily, Hemerocallis citrina Baroni. is rich in beneficial secondary metabolites and is considered as a functional vegetable. It is widely cultivated and consumed in East Asian countries. However, reference genes for use in RT-qPCR in H. citrina are not available. In the present study, six potential reference genes, actin (ACT), AP-4 complex subunit (AP4), tubulin (TUB), ubiquitin (UBQ), 18S and 60S ribosomal RNA, were selected and their expression stability in different developmental stages, organs and accessions was evaluated using four statistical software packages (geNorm, NormFinder, BestKeeper, and RefFinder). For commercial flower buds of different landraces, the combination of 60S, TUB, and AP4 was appropriate whereas ACT and 60S was suitable for normalization of different organs. In addition, AP4 exhibited the most stable expression in flower buds among different developmental stages. UBQ was less stable than the other reference genes under the experimental conditions except under different organs was 18S. The relative expression levels of two genes, primary-amine oxidase (HcAOC3) and tyrosine aminotransferase (HcTAT) which play important roles in alkaloid biosynthesis were also examined in different organs of the 'Datong' landrace, which further confirmed the results of selected reference genes. This is the first report to evaluate the stability of reference genes in the long yellow daylily that can serve as a foundation for RT-qPCR analysis of gene expression in this species.

Identification and validation of reference genes for quantitative real-time PCR studies in long yellow daylily, Hemerocallis citrina Borani

Gene expression analysis using reverse transcription quantitative real-time PCR (RT-qPCR) requires the use of reference gene(s) in the target species. The long yellow daylily, Hemerocallis citrina Baroni. is rich in beneficial secondary metabolites and is considered as a functional vegetable. It is widely cultivated and consumed in East Asian countries. However, reference genes for use in RT-qPCR in H. citrina are not available. In the present study, six potential reference genes, actin (ACT), AP-4 complex subunit (AP4), tubulin (TUB), ubiquitin (UBQ), 18S and 60S ribosomal RNA, were selected and their expression stability in different developmental stages, organs and accessions was evaluated using four statistical software packages (geNorm, NormFinder, BestKeeper, and RefFinder). For commercial flower buds of different landraces, the combination of 60S, TUB, and AP4 was appropriate whereas ACT and 60S was suitable for normalization of different organs. In addition, AP4 exhibited the most stable expression in flower buds among different developmental stages. UBQ was less stable than the other reference genes under the experimental conditions except under different organs was 18S. The relative expression levels of two genes, primary-amine oxidase (HcAOC3) and tyrosine aminotransferase (HcTAT) which play important roles in alkaloid biosynthesis were also examined in different organs of the 'Datong' landrace, which further confirmed the results of selected reference genes. This is the first report to evaluate the stability of reference genes in the long yellow daylily that can serve as a foundation for RT-qPCR analysis of gene expression in this species.

Anatomical distribution of primary amine oxidase activity in four adipose depots and plasma of severely obese women with or without a dysmetabolic profile

Semicarbazide-sensitive amine oxidase (SSAO), identical to primary amine oxidase or vascular adhesion protein-1, is a membrane enzyme that generates hydrogen peroxide. SSAO is highly expressed at the adipocyte surface, and its plasma levels increase with type 2 diabetes. Since visceral adipose tissue (AT) is more tightly associated with obesity complications than subcutaneous (SC) abdominal fat, we compared SSAO activity in plasma and 4 distinct AT locations in 48 severely obese women (body mass index (BMI), averaging 54 ± 11 kg/m²), with or without a dysmetabolic profile. Higher glucose and triacylglycerol levels vs lower high-density lipoprotein (HDL)-cholesterol characterized dysmetabolic women (DYS; n = 25) from non-dysmetabolic (NDYS; n = 23), age- and weight-matched subjects. SC, mesenteric (ME), omental (OM), and round ligament (RL) fat locations were collected during bariatric surgery. SSAO capacity to oxidize up to 1 mM benzylamine was determined in AT and plasma with radiometric and fluorimetric methods. Plasma SSAO was higher in the DYS group. SSAO activity was higher in fat than in plasma, when expressed as radiolabeled benzaldehyde per milligram of protein. In ATs from DYS women, protein content was 10 % higher, and basal hydrogen peroxide release lower than in NDYS subjects, except for RL location. The SSAO affinity towards benzylamine did not exhibit regional variation and was not altered by a dysmetabolic profile (K _m averaging 184 ± 7 μM; n = 183). Although radiometric and fluorimetric methods gave different estimates of oxidase activity, both indicated that AT SSAO activity did not vary according to anatomical location and/or metabolic status in severely obese women.

Piceatannol and resveratrol share inhibitory effects on hydrogen peroxide release, monoamine oxidase and lipogenic activities in adipose tissue, but differ in their antilipolytic properties

Piceatannol is a hydroxylated derivative of resveratrol. While both dietary polyphenols coexist in edible plants and fruits, and share equivalent concentrations in several wines, the influence of piceatannol on adiposity has been less studied than that of resveratrol. Though resveratrol is now recognized to limit fat deposition in various obesity models, the benefit of its dietary supplementation remains under debate regarding human obesity treatment or prevention. The research for more potent resveratrol analogs is therefore still undergoing. This prompted us to compare various effects of piceatannol and resveratrol directly on human adipose tissue (hAT). Hydrogen peroxide release was measured by Amplex Red-based fluorescence in subcutaneous hAT samples from obese patients. Interactions of stilbenes with human amine oxidases and quinone reductase were assessed by radiometric methods, computational docking and electron paramagnetic resonance. Influences on lipogenic and lipolytic activities were compared in mouse adipocytes. Resveratrol and piceatannol inhibited monoamine oxidase (MAO) with respective IC50 of 18.5 and 133.7 μM, but not semicarbazide-sensitive amine oxidase (SSAO) in hAT. For both stilbenes, the docking scores were better for MAO than for SSAO. Piceatannol and resveratrol similarly hampered hydrogen peroxide detection in assays with and without hAT, while they shared pro-oxidant activities when incubated with purified quinone reductase. They exhibited similar dose-dependent inhibition of adipocyte lipogenic activity. Only piceatannol inhibited basal and stimulated lipolysis when incubated at a dose ≥100 μM. Thus, piceatannol exerted on fat cells dose-dependent effects similar to those of resveratrol, except for a stronger antilipolytic action. In this regard, piceatannol should be useful in limiting the lipotoxicity related to obesity when ingested or administered alone - or might hamper the fat mobilization induced by resveratrol when simultaneously administered with it.

Vascular adhesion protein-1: Role in human pathology and application as a biomarker

Vascular adhesion protein-1 (VAP-1) is a member of the copper-containing amine oxidase/semicarbazide-sensitive amine oxidase (AOC/SSAO) enzyme family. SSAO enzymes catalyze oxidative deamination of primary amines, which results in the production of the corresponding aldehyde, hydrogen peroxide and ammonium. VAP-1 is continuously expressed as a transmembrane glycoprotein in the vascular wall during development and facilitates the accumulation of inflammatory cells into the inflamed environment in concert with other leukocyte adhesion molecules. The soluble form of VAP-1 is released into the circulation mainly from vascular endothelial cells. Over- and under-expression of sVAP-1 result in alterations of the reported reaction product levels, which are involved in the pathogenesis of multiple human diseases. The combination of enzymatic and adhesion capacities as well as its strong association with inflammatory pathologies makes VAP-1 an interesting therapeutic target for drug discovery. In this article, we will review the general characteristics and biological functions of VAP-1, focusing on its important role as a prognostic biomarker in human pathologies. In addition, the potential therapeutic application of VAP-1 inhibitors will be discussed.

Glitazones inhibit human monoamine oxidase but their anti-inflammatory actions are not mediated by VAP-1/semicarbazide-sensitive amine oxidase inhibition

Glitazones are peroxisome proliferator-activated receptor gamma (PPARγ) agonists widely used as antidiabetic drugs also known as thiazolidinediones. Most of them exert other effects such as anti-inflammatory actions via mechanisms supposed to be independent from PPARγ activation (e.g., decreased plasma monocyte chemoattractant protein-1 (MCP-1) levels). Recently, pioglitazone has been shown to inhibit the B form of monoamine oxidase (MAO) in mouse, while rosiglitazone and troglitazone were described as non-covalent inhibitors of both human MAO A and MAO B. Since molecules interacting with MAO might also inhibit semicarbazide-sensitive amine oxidase (SSAO), known as vascular adhesion protein-1 (VAP-1), and since VAP-1/SSAO inhibitors exhibit anti-inflammatory activity, our aim was to elucidate whether VAP-1/SSAO inhibition could be a mechanism involved in the anti-inflammatory behaviour of glitazones. To this aim, MAO and SSAO activities were measured in human subcutaneous adipose tissue biopsies obtained from overweight women undergoing plastic surgery. The production of hydrogen peroxide, an end-product of amine oxidase activity, was determined in tissue homogenates using a fluorometric method. The oxidation of 1 mM tyramine was inhibited by pargyline and almost resistant to semicarbazide, therefore predominantly MAO-dependent. Rosiglitazone was more potent than pioglitazone in inhibiting tyramine oxidation. By contrast, benzylamine oxidation was only abolished by semicarbazide: hence SSAO-mediated. Pioglitazone hampered SSAO activity only when tested at 1 mM while rosiglitazone was inefficient. However, rosiglitazone exhibited anti-inflammatory activity in human adipocytes by limiting MCP-1 expression. Our observations rule out any involvement of VAP-1/SSAO inhibition and subsequent limitation of leukocyte extravasation in the anti-inflammatory action of glitazones.

Resveratrol directly affects in vitro lipolysis and glucose transport in human fat cells

Resveratrol is a naturally occurring polyphenol found in many dietary sources and red wine. Recognized as a cancer chemoprevention agent, an anti-inflammatory factor and an antioxidant molecule, resveratrol has been proposed as a potential anti-obesity compound and to be beneficial in diabetes. Most of the studies demonstrating the anti-adipogenic action of resveratrol were performed as long-term treatments on cultured preadipocytes. The aim of this study was to analyse the acute effects of resveratrol on glucose uptake and lipolysis in human mature adipocytes. Samples of subcutaneous abdominal adipose tissue were obtained from overweight humans and immediately digested by liberase. Fat cells were incubated (from 45 min to 4 h) with resveratrol 1 μM-1 mM. Then, glycerol release or hexose uptake was determined. Regarding lipolysis, the significant effects of resveratrol were found at 100 μM, consisting in a facilitation of isoprenaline stimulation and an impairment of insulin antilipolytic action. At 1 and 10 μM, resveratrol only tended to limit glucose uptake. Resveratrol 100 μM did not change basal glucose uptake but impaired its activation by insulin or by benzylamine. This inhibition was not found with other antioxidants. Such impairment of glucose uptake activation in fat cells may led to a reduced availability of glycerol phosphate and then to a decreased triacylglycerol assembly. Therefore, resveratrol increased triacylglycerol breakdown triggered by β-adrenergic activation and impaired lipogenesis. Consequently, our data indicate that resveratrol can be considered as limiting fat accumulation in human fat cells and further support its use for the mitigation of obesity.

5-hydroxytryptamine actions in adipocytes: involvement of monoamine oxidase-dependent oxidation and subsequent PPARγ activation

Serotonin (5-HT) is a brain neurotransmitter instrumental for the antidepressant action of selective inhibitors of serotonin reuptake (SSRIs) while it also plays important roles in peripheral organs. Recently, the 5-HT oxidation products, 5-hydroxyindoleacetate and 5-methoxy-indoleacetate, have been shown to bind to peroxisome proliferator-activated receptor γ (PPARγ) and to enhance lipid accumulation in preadipocytes. Since we already reported that adipocytes exhibit elevated monoamine oxidase (MAO) and primary amine oxidase activities, we verified how adipocytes readily oxidize 5-HT, with the objective to determine whether such oxidation promotes PPARγ activation and lipid storage. To this aim, serotonin was tested on cultured 3T3 F442A preadipocytes and on human adipocytes. Results showed that 5-HT was oxidized by MAO in both models. Daily treatment of 3T3 F442A preadipocytes for 8 days with 100-500 μM 5-HT promoted triglyceride accumulation and emergence of adipogenesis markers. At 250 μM, 5-HT alone reproduced half of 50 nM insulin-induced adipogenesis, and exhibited an additive differentiating effect when combined with insulin. Moreover, the 5-HT-induced expression of PPARγ-responsive genes (PEPCK, aP2/FABP4) was blocked by GW 9662, a PPARγ-inhibitor, or by pargyline, a MAO-inhibitor. In human fat cells, 6-h exposure to 100 μM 5-HT increased PEPCK expression as did the PPARγ-agonist rosiglitazone. Since hydrogen peroxide, another amine oxidation product, did not reproduce such enhancement, we propose that serotonin can promote PPARγ activation in fat cells, via the indoleacetate produced during MAO-dependent oxidation. Such pathway could be involved in the adverse effects of several antidepressant SSRIs on body weight gain.

Benzylamine antihyperglycemic effect is abolished by AOC3 gene invalidation in mice but not rescued by semicarbazide-sensitive amine oxidase expression under the control of aP2 promoter

Semicarbazide-sensitive amine oxidase (SSAO) is a transmembrane enzyme that metabolizes primary amines from endogenous or dietary origin. SSAO is highly expressed in adipose, smooth muscle and endothelial cells. In each of these cell types, SSAO is implicated in different biological functions, such as glucose transport activation, extracellular matrix maturation and leucocyte extravasation, respectively. However, the physiological functions of SSAO and their involvement in pathogenesis remain uncompletely characterized. To better understand the role of adipose tissue SSAO, we investigated whether it was necessary and/or sufficient to produce the antihyperglycemic effect of the SSAO-substrate benzylamine, already reported in mice. Therefore, we crossed SSAO-deficient mice invalidated for AOC3 gene and transgenic mice expected to express human SSAO in an adipocyte-specific manner, under the control of aP2 promoter. The aP2-human AOC3 construct (aP2-hAOC3) was equally expressed in the adipose tissue of mice expressing or not the native murine form and almost absent in other tissues. However, the corresponding SSAO activity found in adipose tissue represented only 20 % that of control mice. As a consequence, the benzylamine antihyperglycemic effect observed during glucose tolerance test in control was abolished in AOC3-KO mice but not rescued in mice expressing aP2-hAOC3. The capacity of benzylamine or methylamine to activate glucose uptake in adipocytes exhibited parallel variations in the corresponding genotypes. Although the aP2-hAOC3 construct did not allow a total rescue of SSAO activity in adipose tissue, it could be assessed from our observations that adipocyte SSAO plays a pivotal role in the increased glucose tolerance promoted by pharmacological doses of benzylamine.

Long-term activation of semicarbazide-sensitive amine oxidase lowers circulating levels of uric acid in diabetic conditions

Uric acid is involved in nitrogenous waste in animals, together with ammonia and urea. Uric acid has also antioxidant properties and is a surrogate marker of metabolic syndrome. We observed that the elevated plasma uric acid of high-fat fed mice was normalized by benzylamine treatment. Indeed, benzylamine is the reference substrate of semicarbazide-sensitive amine oxidase (SSAO), an enzyme highly expressed in fat depots and vessels, which generates ammonia when catalysing oxidative deamination. Ammonia interferes with uric acid metabolism/solubility. Our aim was therefore to investigate whether the lowering action of benzylamine on uric acid was related to an improvement of diabetic complications, or was connected with SSAO-dependent ammonia production. First, we observed that benzylamine administration lowered plasma uric acid in diabetic db/db mice while it did not modify uric acid levels in normoglycemic and lean mice. In parallel, benzylamine improved the glycemic control in diabetic but not in normoglycemic mice, while plasma urea remained unaltered. Then, uric acid plasma levels were measured in mice invalidated for AOC3 gene, encoding for SSAO. These mice were unable to oxidize benzylamine but were not diabetic and exhibited unaltered plasma uric levels. Therefore, activated or abolished ammonia production by SSAO was without influence on uric acid in the context of normoglycemia. Our observations confirm that plasma uric acid increases with diabetes and can be normalized when glucose tolerance is improved. They also show that uric acid, a multifunctional metabolite at the crossroads of nitrogen waste and of antioxidant defences, can be influenced by SSAO, in a manner apparently related to changes in glucose homeostasis.

Implication for functions of the ectopic adipocyte copper amine oxidase (AOC3) from purified enzyme and cell-based kinetic studies

AOC3 is highly expressed in adipocytes and smooth muscle cells, but its function in these cells is currently unknown. The in vivo substrate(s) of AOC3 is/are also unknown, but could provide an invaluable clue to the enzyme's function. Expression of untagged, soluble human AOC3 in insect cells provides a relatively simple means of obtaining pure enzyme. Characterization of enzyme indicates a 6% titer for the active site 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor and corrected k(cat) values as high as 7 s(-1). Substrate kinetic profiling shows that the enzyme accepts a variety of primary amines with different chemical features, including nonphysiological branched-chain and aliphatic amines, with measured k(cat)/K(m) values between 10(2) and 10(4) M(-1) s(-1). K(m)(O(2)) approximates the partial pressure of oxygen found in the interstitial space. Comparison of the properties of purified murine to human enzyme indicates k(cat)/K(m) values that are within 3 to 4-fold, with the exception of methylamine and aminoacetone that are ca. 10-fold more active with human AOC3. With drug development efforts investigating AOC3 as an anti-inflammatory target, these studies suggest that caution is called for when screening the efficacy of inhibitors designed against human enzymes in non-transgenic mouse models. Differentiated murine 3T3-L1 adipocytes show a uniform distribution of AOC3 on the cell surface and whole cell K(m) values that are reasonably close to values measured using purified enzymes. The latter studies support a relevance of the kinetic parameters measured with isolated AOC3 variants to adipocyte function. From our studies, a number of possible substrates with relatively high k(cat)/K(m) have been discovered, including dopamine and cysteamine, which may implicate a role for adipocyte AOC3 in insulin-signaling and fatty acid metabolism, respectively. Finally, the demonstrated AOC3 turnover of primary amines that are non-native to human tissue suggests possible roles for the adipocyte enzyme in subcutaneous bacterial infiltration and obesity.