Plasma amine oxidase: a postulated cardiovascular risk factor in nondiabetic obese patients

Vascular adhesion protein-1 (VAP-1) in vascular inflammatory diseases

Summary: Vascular adhesion protein-1 (VAP-1) also known as amino oxidase copper containing 3 (AOC3) is a pro-inflammatory and versatile molecule with adhesive and enzymatic properties. VAP-1 is a primary amine oxidase belonging to the semicarbazide-sensitive amine oxidase (SSAO) family, which catalyzes the oxidation of primary amines leading to the production of ammonium, formaldehyde, methylglyoxal, and hydrogen peroxide. VAP-1 is mainly expressed by endothelial cells, smooth muscle cells, adipocytes and pericytes. It is involved in a repertoire of biological functions, e.g., immune cell extravasation, angiogenesis, and vascularization. Research into VAP-1 has intensified within the last decade on its role as a novel clinical biomarker and as a potential therapeutic target of vascular inflammatory disorders such as atherosclerosis, stroke, diabetes, neurovascular disorders (e.g., Alzheimer’s Disease), hepatic disease (e.g., non-alcoholic steatohepatitis), and skin conditions (e.g., psoriasis). This is the most up-to-date and comprehensive review on VAP-1 focusing on the translational aspects of VAP-1. Compared to recent reviews, our review provides novel insights on VAP-1 and heart failure, stroke and frailty, diabetes, endometriosis, osteoarthritis, COVID-19, conjunctivitis associated systemic lupus erythematosus, hematopoietic stem cells, gliomas, treatment of colorectal cancer with a novel VAP-1 inhibitor (U-V269), promoting recovery of motor functions and habit learning with a novel VAP-1 inhibitor (PXS-4681A), and 68Ga-DOTA-Siglec-9, a labelled peptide of Siglec-9 (a VAP-1 ligand), which appears to be a safe PET tracer for inflammation in rheumatoid arthritis. Finally, we present the emerging role of VAP-1 in pregnancy as a gatekeeper of immune cells, which are critical for spiral arterial remodeling, the deficiency of which could lead to vascular disorders of pregnancy such as preeclampsia. Future research should prioritize clinical trials on VAP-1 small-molecule inhibitors and monoclonal antibodies, thus, maximizing the potential of VAP-1 targeted therapy as well as research into sVAP-1 as a clinical biomarker of diseases and its prognosis.

Role of oxidative stress in calcific aortic valve disease and its therapeutic implications

Calcific aortic valve disease (CAVD) is the end result of active cellular processes that lead to the progressive fibrosis and calcification of aortic valve leaflets. In western populations, CAVD is a significant cause of cardiovascular morbidity and mortality, and in the absence of effective drugs, it will likely represent an increasing disease burden as populations age. As there are currently no pharmacological therapies available for preventing, treating, or slowing the development of CAVD, understanding the mechanisms underlying the initiation and progression of the disease is important for identifying novel therapeutic targets. Recent evidence has emerged of an important causative role for reactive oxygen species (ROS)-mediated oxidative stress in the pathophysiology of CAVD, inducing the differentiation of valve interstitial cells into myofibroblasts and then osteoblasts. In this review, we focus on the roles and sources of ROS driving CAVD and consider their potential as novel therapeutic targets for this debilitating condition.

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.

SSAO/VAP-1 in Cerebrovascular Disorders: A Potential Therapeutic Target for Stroke and Alzheimer's Disease

The semicarbazide-sensitive amine oxidase (SSAO), also known as vascular adhesion protein-1 (VAP-1) or primary amine oxidase (PrAO), is a deaminating enzyme highly expressed in vessels that generates harmful products as a result of its enzymatic activity. As a multifunctional enzyme, it is also involved in inflammation through its ability to bind and promote the transmigration of circulating leukocytes into inflamed tissues. Inflammation is present in different systemic and cerebral diseases, including stroke and Alzheimer’s disease (AD). These pathologies show important affectations on cerebral vessels, together with increased SSAO levels. This review summarizes the main roles of SSAO/VAP-1 in human physiology and pathophysiology and discusses the mechanisms by which it can affect the onset and progression of both stroke and AD. As there is an evident interrelationship between stroke and AD, basically through the vascular system dysfunction, the possibility that SSAO/VAP-1 could be involved in the transition between these two pathologies is suggested. Hence, its inhibition is proposed to be an interesting therapeutical approach to the brain damage induced in these both cerebral pathologies.

Semicarbazide-Sensitive Amine Oxidase Increases in Calcific Aortic Valve Stenosis and Contributes to Valvular Interstitial Cell Calcification

Introduction

Calcific aortic valve stenosis (CAVS) is a common disease associated with aging. Oxidative stress participates in the valve calcification process in CAVS. Semicarbazide-sensitive amine oxidase (SSAO), also referred to as vascular adhesion protein 1 (VAP-1), transforms primary amines into aldehydes, generating hydrogen peroxide and ammonia. SSAO is expressed in calcified aortic valves, but its role in valve calcification has remained largely unexplored. The aims of this study were to characterize the expression and the activity of SSAO during aortic valve calcification and to establish the effects of SSAO inhibition on human valvular interstitial cell (VIC) calcification.

Methods

Human aortic valves from n = 80 patients were used for mRNA extraction and expression analysis, Western blot, SSAO activity determination, immunohistochemistry, and the isolation of primary VIC cultures.

Results

SSAO mRNA, protein, and activity were increased with increasing calcification within human aortic valves and localized in the vicinity of the calcified zones. The valvular SSAO upregulation was consistent after stratification of the subjects according to cardiovascular and CAVS risk factors associated with increased oxidative stress: body mass index, diabetes, and smoking. SSAO mRNA levels were significantly associated with poly(ADP-ribose) polymerase 1 (PARP1) in calcified tissue. Calcification of VIC was inhibited in the presence of the specific SSAO inhibitor LJP1586.

Conclusion

The association of SSAO expression and activity with valvular calcification and oxidative stress as well as the decreased VIC calcification by SSAO inhibition points to SSAO as a possible marker and therapeutic target to be further explored in CAVS.

Obesity is associated with copper elevation in serum and tissues

Copper misbalance has been linked to fat accumulation in animals and experimental systems; however, information about copper homeostasis in human obesity is limited. In this study, the copper status of obese individuals was evaluated by measuring their levels of copper and cuproproteins in serum, adipose and hepatic tissues. The analysis of serum trace elements showed significant positive and element-specific correlation between copper and BMI after controlling for gender, age, and ethnicity. Serum copper also positively correlated with leptin, insulin, and the leptin/BMI ratio. When compared to lean controls, obese patients had elevated circulating cuproproteins, such as semucarbazide-sensitive amine oxidase (SSAO) and ceruloplasmin, and higher SSAO activity and copper levels in visceral fat. Although hepatic steatosis reduces copper levels in the liver, obese patients with no or mild steatosis have higher copper content in the liver compared to lean controls. In conclusion, obese patients evaluated in this study had altered copper status. Strong positive correlations of copper levels with BMI and leptin suggest that copper and/or cuproproteins may be functionally linked to fat accumulation.

Vascular Adhesion Protein-1: A Cell Surface Amine Oxidase in Translation

Significance: Vascular adhesion protein-1 (VAP-1) is an ectoenzyme that oxidates primary amines in a reaction producing also hydrogen peroxide. VAP-1 on the blood vessel endothelium regulates leukocyte extravasation from the blood into tissues under physiological and pathological conditions. Recent Advances: Inhibition of VAP-1 by neutralizing antibodies and by several novel small-molecule enzyme inhibitors interferes with leukocyte trafficking and alleviates inflammation in many experimental models. Targeting of VAP-1 also shows beneficial effects in several other diseases, such as ischemia/reperfusion, fibrosis, and cancer. Moreover, soluble VAP-1 levels may serve as a new prognostic biomarker in selected diseases. Critical Issues: Understanding the contribution of the enzyme activity-independent and enzyme activity-dependent functions, which often appear to be mediated by the hydrogen peroxide production, in the VAP-1 biology will be crucial. Similarly, there is a pressing need to understand which of the VAP-1 functions are regulated through the modulation of leukocyte trafficking, and what is the role of VAP-1 synthesized in adipose and smooth muscle cells. Future Directions: The specificity and selectivity of new VAP-1 inhibitors, and their value in animal models under therapeutic settings need to be addressed. Results from several programs studying the therapeutic potential of VAP-1 inhibition, which now are in clinical trials, will reveal the relevance of this amine oxidase in humans.

Serum vascular adhesion protein-1 is up-regulated in hyperglycemia and is associated with incident diabetes negatively

BACKGROUND/OBJECTIVES

Vascular adhesion protein-1 (VAP-1) can enhance tissue glucose uptake in cell studies and normalize hyperglycemia in animal studies. However, serum VAP-1 concentration (sVAP-1) is higher in subjects with diabetes in cross-sectional studies. In this cohort study, we test our hypothesis that sVAP-1 is increased in prediabetes to counteract hyperglycemia and is associated with incident diabetes negatively.

SUBJECTS/METHODS

From 2006 to 2012, 600 subjects without diabetes from Taiwan Lifestyle Study were included and followed regularly. Diabetes was diagnosed if FPG ≥ 126 mg/dL (7 mmol/L), 2-h plasma glucose (2hPG) during an oral glucose tolerance test (OGTT) ≥ 200 mg/dL (11.1 mmol/L), or hemoglobin A1c (HbA1c) ≥ 6.5%, or if the subject received anti-diabetic medications. Abdominal fat areas were measured by abdominal computed tomography and sVAP-1 was analyzed by ELISA.

RESULTS

sVAP-1 was higher in subjects with prediabetes (p < 0.05) and increased during an OGTT (p < 0.001). Fasting sVAP-1 was associated with the response of sVAP-1 during an OGTT (p < 0.001). Besides, sVAP-1 was associated negatively with body mass index (BMI, r = -0.1449, p = 0.003), waist circumference (r = -0.1425, p = 0.004), abdominal visceral (r = -0.1457, p = 0.003), and subcutaneous (r = -0.1025, p = 0.035) fat areas, and serum high-sensitivity C-reactive protein (hsCRP) concentration (r = -0.2035, p < 0.0001), and positively with plasma adiponectin concentration (r = 0.2086, p < 0.0001), adjusted for age and gender. After 4.7 ± 2.6 years, 73 subjects (12.2%) developed incident diabetes. High sVAP-1 predicted a lower incidence of diabetes, adjusted for age, gender, BMI, family history of diabetes, HbA1c, HOMA2-%B and HOMA2-IR (HR = 0.66, 95% CI = 0.50-0.88, p < 0.01).

CONCLUSIONS

sVAP-1 is increased in response to hyperglycemia. It is associated with obesity and serum hsCRP concentration negatively, and plasma adiponectin concentration positively. Besides, a high sVAP-1 is associated with a lower incidence of diabetes in human.

HPLC-UV assays for evaluation of inhibitors of mono and diamine oxidases using novel phenyltetrazolylalkanamine substrates

Recently, we have described an HPLC-UV assay for the evaluation of inhibitors of plasma amine oxidase (PAO) using 6-(5-phenyl-2H-tetrazol-2-yl)hexan-1-amine (4) as a new type of substrate. Now we studied, whether this compound or homologues of it can also function as substrate for related amine oxidases, namely diamine oxidase (DAO), monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B). Among these substances, 4 was converted by DAO with the highest rate. The best substrate for MAO A and B was 4-(5-phenyl-2H-tetrazol-2-yl)butan-1-amine (2). To validate the new assays, the inhibition values of known enzyme inhibitors were determined and the data were compared with those obtained with the substrate benzylamine, which is often used in amine oxidase assays. For the DAO inhibitor 2-(4-phenylphenyl)acetohydrazide an about 10fold lower IC₅₀-value against DAO was obtained when benzylamine was applied instead of 4, indicating that 4 binds to the enzyme with higher affinity than benzylamine. The IC₅₀-values of clorgiline and selegiline against MAO A and B, respectively, also decreased (two- and 30fold) replacing 2 by benzylamine. The discrepancies largely disappeared, when the enzymes were pre-incubated with the inhibitors for 15 min. This can be explained with the covalent inhibition mechanism of the inhibitors.

Inactivation of bovine plasma amine oxidase by 1,1,1-trihalo-3-aminopropanes

In this paper, we report the inactivation of copper containing bovine plasma amine oxidase (BPAO) by a series of saturated alkylamines containing halogen atoms at γ-position, which are 1,1,1-trihalo-3-aminopropane, 1,1,1-trifluoro-2-hydroxy-3-aminopropane, 1,1,1-trichloro-2-hydroxy-3-aminopropane, and 1,1,1-trichloro-2-(2-phenethyloxy)-3-aminopropane. The trihalo-2-hydroxypropylamine analogs exhibited a time-dependent inactivation behavior of BPAO, with 1,1,1-trifluoro-2-hydroxy-3-aminopropane as the most efficient inactivator. The incorporation of a OH group at β-position increased inactivation efficiency by 10-fold within the trifluoro analogs, and the incorporation of a phenethyloxy group at β-position exhibited a higher efficiency by 3-fold within the trichloro analogs based on I₇₅ values. All four compounds were found to be irreversible inactivators for BPAO.

Association between metabolically healthy central obesity in women and levels of soluble receptor for advanced glycation end products, soluble vascular adhesion protein-1, and the activity of semicarbazide-sensitive amine oxidase

AIM

To determine the levels of circulating soluble receptor for advanced glycation end products (sRAGE), as a biomarker of risk of metabolic syndrome and cardiovascular disease development in centrally obese (CO) women considered metabolically healthy (COH) in comparison with those metabolically unhealthy (COU).

METHODS

47 lean healthy, 17 COH (presenting waist-to-height ratio ≥0.5 but not elevated blood pressure, atherogenic lipid profile, and insulin resistance), and 50 COU (CO presenting ≥2 risk factors) women aged 40-45 years were included. Anthropometric characteristics, blood chemistry and hematology data, adipokines, markers of inflammation, sRAGE, soluble vascular adhesion protein-1 (sVAP-1), and the activity of semicarbazide sensitive amine oxidase (SSAO) were determined.

RESULTS

Central obesity associated with low sRAGE levels (lean healthy: 1503±633 pg/mL; COH: 1103±339 pg/mL, P<0.05; COU: 1106±367 ng/mL, P<0.0.1), hyperleptinemia, and elevated markers of inflammation irrespective of the presence or absence of cardiometabolic risk factors. COU women presented high adiponectin levels. SVAP-1 concentrations and the activity of SSAO were similar in all 3 groups.

CONCLUSION

COH women present abnormalities in non-standard markers of cardiometabolic risk (sRAGE, leptin, high sensitive C-reactive protein), supporting the view that there is no healthy pattern of obesity. The clinical impact of our findings for future prognosis of metabolically healthy obese subjects remains to be elucidated in longitudinal studies.

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.

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.

Semicarbazide-sensitive amine oxidase/vascular adhesion protein-1: a patent survey

INTRODUCTION

Vascular adhesion protein-1 (VAP-1)/semicarbazide-sensitive amine oxidase (SSAO) is an adhesion protein involved in leukocyte trafficking and inflammatory processes, with a special amine oxidase activity. Inhibitors have been mainly developed for treating chronic inflammatory disorders. The utility of inhibitors as antiangiogenic agents in ophthalmological and oncological diseases is currently under evaluation. SSAO substrates may mimic several insulin effects, although their utility for the treatment of diabetes is still far from being fully understood.

AREAS COVERED

This paper reviews the patent literature of SSAO/VAP-1 inhibitors and substrates, for the period of 1990 - 2010. The current stage of SSAO/VAP-1-interacting agents published in patents is described, along with their chemical structures and pharmacological uses.

EXPERT OPINION

SSAO/VAP-1 is a promising anti-inflammatory target. Another important field for therapeutic application of these inhibitors may be ophthalmology, due to their antiangiogenic effects. SSAO substrates might also be of therapeutic value in the treatment of diabetes; however, more extensive research has to be undertaken to validate this approach.

Synthesis, in vitro activity, and three-dimensional quantitative structure-activity relationship of novel hydrazine inhibitors of human vascular adhesion protein-1

Vascular adhesion protein-1 (VAP-1) belongs to the semicarbazide-sensitive amine oxidases (SSAOs) that convert amines into aldehydes. SSAOs are distinct from the mammalian monoamine oxidases (MAOs), but their substrate specificities are partly overlapping. VAP-1 has been proposed as a target for anti-inflammatory drug therapy because of its role in leukocyte adhesion to endothelium. Here, we describe the synthesis and in vitro activities of novel series of VAP-1 selective inhibitors. In addition, the molecular dynamics simulations performed for VAP-1 reveal that the movements of Met211, Ser496, and especially Leu469 can enlarge the ligand-binding pocket, allowing larger ligands than those seen in the crystal structures to bind. Combining the data from molecular dynamics simulations, docking, and in vitro measurements, the three-dimensional quantitative structure-activity relationship (3D QSAR) models for VAP-1 (q(2)(LOO): 0.636; r(2): 0.828) and MAOs (q(2)(LOO): 0.749, r(2): 0.840) were built and employed in the development of selective VAP-1 inhibitors.

The unique substrate specificity of human AOC2, a semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidases (SSAOs) catalyze oxidative deamination of primary amines, but the true physiological function of these enzymes is still poorly understood. Here, we have studied the functional and structural characteristics of a human cell-surface SSAO, AOC2, which is homologous to the better characterized family member, AOC3. The preferred in vitro substrates of AOC2 were found to be 2-phenylethylamine, tryptamine and p-tyramine instead of methylamine and benzylamine, the favored substrates of AOC3. Molecular modeling suggested structural differences between AOC2 and AOC3, which provide AOC2 with the capability to use the larger monoamines as substrates. Even though AOC2 mRNA was expressed in many tissues, the only tissues with detectable AOC2-like enzyme activity were found in the eye. Characterization of AOC2 will help in evaluating the contribution of this enzyme to the pathological processes attributed to the SSAO activity and in designing specific inhibitors for the individual members of the SSAO family.

p53 phosphorylation is involved in vascular cell death induced by the catalytic activity of membrane-bound SSAO/VAP-1

Semicarbazide sensitive amine oxidase (SSAO) is a multifunctional enzyme present mainly in adipocytes, endothelial and smooth muscle cells. It metabolizes primary aliphatic and aromatic amines generating products able to contribute to cellular oxidative stress. SSAO is expressed in a membrane-bound form and is also present as a soluble enzyme in plasma. Both isoforms are increased in several pathologies, and the catalytic products generated by the soluble enzymatic activity can induce cytotoxicity of vascular cells in culture. We have analyzed whether the transmembrane form of the enzyme is able to produce a cytotoxic effect through methylamine oxidation. Since cells in culture lose the expression of this enzyme, we used an SSAO stably transfected smooth muscle cell line. Herein we report that cell treatment with the substrate methylamine induced a dose and time dependent cytotoxic effect. The tumor suppressor protein p53 played an important role in the molecular pathway involved in this cell death. Moreover, we also observed the induction of PUMA-alpha expression with mitochondrial Bcl-2 family proteins being affected, and final effector caspases being activated.

A novel method for quantification of the folding of elastic laminae in elastic arteries

A transgenic mouse overexpressing the human form of semicarbazide-sensitive amine oxidase (SSAO) is known to have an abnormal structure of the elastic laminae and the elastic fibres in the aorta. Compared to the non-transgenic littermates, the elastic laminae are less folded. In order to quantify the undulation of this structure, an image analysis program that identified the elastic laminae was developed. The program measures the area fraction in different sectors from a plane parallel to the aorta wall. Images were taken from unstained aorta specimens where the elastic laminae were visualised with phase contrast microscopy. A contextual operation of the images produced a local orientation estimation for every linear structure. The image was then thresholded in eight sectors from 0 degrees to 180 degrees , with different orientation angles. The results show that the area fraction of the elastic laminae was significantly lower for the transgenic mouse in all sectors measured except for two. At 0-25 degrees , no difference was seen. In the sector at 160-180 degrees , parallel to the aorta wall, the area fraction of elastic laminae was instead significantly higher in the transgenic mouse. A novel method is presented, developed for detection and quantification of pathological changes in the elastic laminae in the aorta wall. The method gave reliable results and is considered to be a useful tool for morphometric studies of aorta with this kind of altered morphology concerning the elastic laminae. When compared with tangent count, the control group had a significantly larger mean curvature.

The origin of mammalian plasma amine oxidases

Mammalian blood plasma contains considerable activity of soluble copper-containing amine oxidase (AOC) referred to as plasma or serum amine oxidase (SAO). The identity and origin of SAO was investigated based on the recent characterization of four porcine AOC genes with AOC1 encoding diamine oxidase (DAO), AOC2 retina-specific amine oxidase (RAO), AOC3 vascular adhesion protein-1 (VAP-1), and AOC4 a VAP-1 homologue that is expressed mainly in the liver and has a signal peptide sequence instead of a transmembrane domain at its N-terminus. Purification and characterization of the major amine oxidase activity from porcine serum showed that it is the product of the AOC4 gene. Intriguingly, all mammals possessing a functional AOC4 gene exhibit high plasma amine oxidase activity. Humans and rodents lack a functional AOC4 gene and have comparably low plasma amine oxidase activity that is probably derived from partial proteolytic release of the membrane-associated AOC3 gene product VAP-1.

Adipogenesis-related increase of semicarbazide-sensitive amine oxidase and monoamine oxidase in human adipocytes

A strong induction of semicarbazide-sensitive amine oxidase (SSAO) has previously been reported during murine preadipocyte lineage differentiation but it remains unknown whether this emergence also occurs during adipogenesis in man. Our aim was to compare SSAO and monoamine oxidase (MAO) expression during in vitro differentiation of human preadipocytes and in adipose and stroma-vascular fractions of human fat depots. A human preadipocyte cell strain from a patient with Simpson-Golabi-Behmel syndrome was first used to follow amine oxidase expression during in vitro differentiation. Then, human preadipocytes isolated from subcutaneous adipose tissues were cultured under conditions promoting ex vivo adipose differentiation and tested for MAO and SSAO expression. Lastly, human adipose tissue was separated into mature adipocyte and stroma-vascular fractions for analyses of MAO and SSAO at mRNA, protein and activity levels. Both SSAO and MAO were increased from undifferentiated preadipocytes to lipid-laden cells in all the models: 3T3-F442A and 3T3-L1 murine lineages, human SGBS cell strain or human preadipocytes in primary culture. In human subcutaneous adipose tissue, the adipocyte-enriched fraction exhibited seven-fold higher amine oxidase activity and contained three- to seven-fold higher levels of mRNAs encoded by MAO-A, MAO-B, AOC3 and AOC2 genes than the stroma-vascular fraction. MAO-A and AOC3 genes accounted for the majority of their respective MAO and SSAO activities in human adipose tissue. Most of the SSAO and MAO found in adipose tissue originated from mature adipocytes. Although the mechanism and role of adipogenesis-related increase in amine oxidase expression remain to be established, the resulting elevated levels of amine oxidase activities found in human adipocytes may be of potential interest for therapeutic intervention in obesity.

Plasma semicarbazide-sensitive amine oxidase is moderately decreased by pronounced exogenous hyperinsulinemia but is not associated with insulin sensitivity and body fat

OBJECTIVE

Semicarbazide-sensitive amine oxidase (SSAO) is widely expressed in adipose tissue, where it may contribute to stimulation of glucose transport via GLUT4 recruitment. We tested the relationships of soluble SSAO, as reflected by its plasma activity, with insulin sensitivity and indices of body fat, and determined whether insulin is involved in regulating plasma SSAO activity.

MATERIAL AND METHODS

In 24 non-diabetic subjects, the relationships of plasma SSAO activity with insulin sensitivity (M-value and free fatty acid (FFA) suppression during a 3-h hyperinsulinemic (8.3 microU kg-1 s-1), euglycemic clamp), body mass index (BMI 25.5 +/- 3.1 kg m-2), waist-hip ratio and fat mass were assessed. In 16 subjects, the effect of insulin infusion, administered at a rate of 8.3 microU kg-1 s-1 during 3 h, followed by 3-h insulin infusion at a high rate of 41.7 microU kg-1 s-1 on plasma SSAO activity was determined. In the other 8 subjects, the response of plasma SSAO activity to 24-h insulin infused at 8.3 microU kg-1 s-1 was assessed.

RESULTS

There were no relationships (all p > 0.10) of plasma SSAO activity (215 +/- 60 mU L-1) with the M-value or with any indices of body fat and FFA before and after insulin suppression. Plasma SSAO activity changed by -7.2 (95% CI, -14.5 to +0.2)% after 3 h (NS) and decreased by 10.1 (95% CI, 19.2 to 1)% after 6 h of insulin infusion (p < 0.05). Plasma SSAO activity did not significantly change after 8 h (change 0.4 (95% CI, -15.3 to +16.2)%, NS) and after 24 h (change -8.8 (95% CI, -27.4 to +9.7)%, NS) of insulin infusion.

CONCLUSIONS

It is unlikely that circulating SSAO is a clinically important marker of insulin sensitivity on glucose and fatty acid metabolism. The reduction in plasma SSAO activity in response to pronounced hyperinsulinemia suggests that insulin is involved in the regulation of the soluble form of this enzyme.

Increased monoamine oxidase and semicarbazide-sensitive amine oxidase activities in white adipose tissue of obese dogs fed a high-fat diet

Adipocytes express two types of amine oxidases: the cell surface semicarbazide-sensitive amine oxidase (SSAO) and the mitochondrial monoamine oxidase (MAO). In human abdominal subcutaneous adipose tissue, it has been reported that SSAO substrates stimulate glucose transport and inhibit lipolysis while MAO activity is decreased in obese patients when compared to age-matched controls. However, no information has been reported on visceral WAT. To further investigate the obesity-induced regulations of MAO and SSAO in white adipose tissue (WAT) from different anatomical locations, enzyme activities and mRNA abundance have been determined on tissue biopsies from control and high-fat fed dogs, an obesity model already described to be associated with arterial hypertension and hyperinsulinemia. MAO activity was increased in the enlarged omental WAT of diet-induced obese dogs, but not in their mesenteric WAT, another intra-abdominal fat depot. Subcutaneous WAT did not exhibit any change in MAO activity, as did the richest MAO-containing tissue: liver. Similarly, SSAO was increased in omental WAT of diet-induced obese dogs, but was not modified in other WAT and in aorta. The increase in SSAO activity observed in omental WAT likely results from an increased expression of the AOC3 gene since mRNA abundance and maximal benzylamine oxidation velocity were increased. Finally, plasma SSAO was decreased in obese dogs. Although the observed regulations differ from those found in subcutaneous WAT of obese patients, this canine model shows a tissue- and site-specific regulation of peripheral MAO and SSAO in obesity.

Soluble Semicarbazide Sensitive Amine Oxidase (SSAO) catalysis induces apoptosis in vascular smooth muscle cells

Semicarbazide sensitive amine oxidase (SSAO) metabolizes oxidative deamination of primary aromatic and aliphatic amines. It is selectively expressed in vascular cells of blood vessels, but it is also circulating in blood plasma. SSAO activity in plasma is increased in some diseases associated with vascular complications and its catalytic products may cause tissue damage. We examined the effect of the oxidation of the SSAO substrate, methylamine, on cultured smooth muscle cells. Cell incubation with methylamine plus soluble SSAO, contained in bovine serum, resulted toxic to rat aorta A7r5 and human aortic smooth muscle cells, as measured by MTT reduction. This effect was completely reverted by specific SSAO inhibitors, indicating that the toxicity was mediated by the end products generated. Moreover, SSAO-mediated deamination of methylamine induced apoptosis in A7r5 cells, detected by chromatin condensation, Caspase-3 activation, PARP cleavage and cytochrome c release to cytosol. Formaldehyde, rather than H2O2, resulted to be a strong apoptotic inducer to A7r5 cells. Taken together, the results suggest that increased plasma SSAO activity in pathological conditions, could contribute to apoptosis in smooth muscle cells, leading to vascular tissue damage.

Influence of high-fat diet on amine oxidase activity in white adipose tissue of mice prone or resistant to diet-induced obesity

Decreased monoamine oxidase (MAO) activity has been observed in adipose tissue of obese patients. Since substrates of MAO and semicarbazide-sensitive amine oxidase (SSAO) can modify adipocyte metabolism, this work investigates whether changes in amine oxidase activity may occur during white adipose tissue (WAT) development. We evaluated MAO and SSAO activities in WAT of high-fat diet (HFD) and low-fat diet fed mice. To distinguish the effect of HFD on its own from the effect of fat mass enlargement, obesity-prone transgenic line of the FVBn strain lacking beta3-adrenergic receptors (AR) but expressing human beta3-AR and alpha2-AR (mbeta3-/-, hbeta3+/+, halpha2+/-) was compared to its obesity-resistant control (mbeta3-/-, hbeta3+/+). As already reported, the former mice became obese while the latter resisted to HFD. No significant change in SSAO or MAO activity was found in WAT of both strains after HFD when expressing oxidase activity per milligram of protein. However, when considering the overall capacity of the fat depots to oxidize tyramine or benzylamine, there was an increase in MAO and SSAO activity only in the enlarged WAT of HFD-induced obese mice. Therefore, the comparison of these models allowed to demonstrate that the higher amine oxidase capacity hold in enlarged fat stores of obese mice is more likely the consequence of increased fat cell number rather than the result of an increased expression of MAO or SSAO in the adipocyte.

Amine oxidase substrates for impaired glucose tolerance correction

Amine oxidases are widely distributed from microorganisms to vertebrates and produce hydrogen peroxide plus aldehyde when catabolizing endogenous or xenobiotic amines. Novel roles have been attributed to several members of the amine oxidase families, which cannot be anymore considered as simple amine scavengers. Semicarbazide-sensitive amine oxidase (SSAO) is abundantly expressed in mammalian endothelial, smooth muscle, and fat cells, and plays a role in lymphocyte adhesion to vascular wall, arterial fiber elastic maturation, and glucose transport, respectively. This latter role was studied in detail and the perspectives of insulin-like actions of amine oxidase substrates are discussed in the present review. Independent studies have demonstrated that SSAO substrates and monoamine oxidase substrates mimic diverse insulin effects in adipocytes: glucose transport activation, lipogenesis stimulation and lipolysis inhibition. These substrates also stimulate in vitro adipogenesis. Acute in vivo administration of amine oxidase substrates improves glucose tolerance in rats, mice and rabbits, while chronic treatments with benzylamine plus vanadate exert an antihyperglycaemic effect in diabetic rats. Dietary supplementations with methylamine, benzylamine or tyramine have been proven to influence metabolic control in rodents by increasing glucose tolerance or decreasing lipid mobilisation, without noticeable changes in the plasma markers of lipid peroxidation or protein glycation, despite adverse effects on vasculature. Thus, the ingested amines are not totally metabolized at the intestinal level and can act on adipose and vascular tissues. In regard with this influence on metabolic control, more attention must be paid to the composition or supplementation in amines in foods and nutraceutics.

The effect of endogenous formaldehyde on the rat aorta endothelial cells

Previous studies have demonstrated endogenous formaldehyde (FA) may be involved in endothelial damage, and may be a potential factor of vulnerability of atherosclerosis. However, the mechanism has not been characterized. The present studies examined DNA-protein cross-links (DPC) formation in rat aorta endothelial cells (RAECs) treated with formaldehyde, hydrogen peroxide (H2O2), or formaldehyde with equal molar concentration of H2O2, which is produced with formaldehyde in the body at the same time. Using a K+/SDS precipitation assay for DPC determination, concentration-dependent increases in DPC formation were observed 1.5 h after treatment of RAECs with 0.01-2mM FA, H2O2, or FA with equal molar concentration of H2O2. Time-dependent increases in DPC formation were also observed at 0.5-4 h time point after treatment of RAECs with 0.05 and 0.1mM FA, or 0.1mM FA with H2O2. The DPC levels reduced after treatment with FA and equal molar concentration of H2O2, compared with treatment with FA alone. FA may be less cytotoxic, as FA alone did not affect the cell viability even treating for 4h, until the treatment concentration reached 2mM. However, H2O2, and FA with H2O2 induced significant decreases of cell viability. These studies suggest that FA and H2O2 may injure endothelial cells synergistically, and low concentration of FA (0.05-0.1) may contribute to the endothelial injury in the body during aging.

Human plasma semicarbazide sensitive amine oxidase (SSAO), β-amyloid protein and aging

Semicarbazide sensitive amine oxidase (SSAO) metabolizes oxidative deamination of primary aromatic and aliphatic amines. The final products of its catalysis, ammonia, hydrogen peroxide (H2O2) and the corresponding aldehyde, may contribute to diseases involving vascular degeneration. SSAO is selectively expressed in blood vessels in the brain, but is also present in blood plasma. We have previously reported that membrane-bound SSAO is overexpressed in the cerebrovascular tissue of Alzheimer's disease (AD) patients. The aim of the present work is to study whether the circulating SSAO is also altered in this neurodegenerative disease. SSAO activity was determined in plasma of control cases (n = 23) and patients suffering sporadic Alzheimer dementia, distributed according to the Global Deterioration Scale (GDS): mild (n = 33), moderate (n = 14), moderate-severe (n = 15) and severe dementia (n = 19). Results show a clear increase of plasma SSAO activity (p < 0.001) in moderate-severe and severe AD patients, with patient age being an independent correlative factor. However, plasma SSAO activity was not altered in AD patients with mild or moderate dementia compared to controls. beta-Amyloid (Abeta) (40-42) immunoreactivity in plasma samples was also determined, and no correlation was observed between Abeta 40-42 levels and the severity of the dementia or the plasma SSAO activity. Our results suggest that an increase in circulating SSAO activity could contribute to oxidative stress and vascular damage in advanced Alzheimer's disease.

Semicarbazide-sensitive amine oxidase in transgenic mice with diabetes

Semicarbazide-sensitive amine oxidase (SSAO) activity in plasma is increased in diabetes, and in particular, in diabetic patients with vascular complications. It has been speculated that SSAO is involved in the development of such complications due to the production of cytotoxic compounds. In this work, we have induced diabetes in a previously described mouse-model, overexpressing SSAO in smooth muscle cells. SSAO activity was estimated as well as expression of the endogenous mouse gene and human transgene using real-time PCR. Diabetes induced an increase in SSAO activity in serum, kidney, and adipose tissue of transgenic animals. An inverse correlation between SSAO activity and mouse SSAO mRNA levels was observed in transgenic animals with diabetes. These results further support the suggestion of a negative feedback control of the SSAO gene expression. The increased SSAO activity in diabetes is most likely dependent on post-transcriptional modifications or activation of existing inactive enzyme molecules.

Increased methylglyoxal and advanced glycation end products in kidney from spontaneously hypertensive rats

BACKGROUND

Methylglyoxal (MG), a metabolite of glucose, causes nonenzymatic glycation of proteins to form irreversible advanced glycation end products (AGEs). The role of MG in the development of essential hypertension is unknown, although MG has been extensively studied in relation to diabetes.

METHODS

Blood pressure of spontaneously hypertensive rats (SHR) and paired Wistar Kyoto (WKY) rats was measured at 5, 8, 13, and 20 weeks of age. HPLC was used to determine the levels of plasma and kidney MG, as well as reduced or oxidized glutathione in the kidney. MG-induced AGEs, Nepsilon-carboxyethyl-lysine (CEL), and Nepsilon-carboxymethyl-lysine (CML) in the kidney were detected by immunohistochemistry. Glutathione peroxidase and reductase activities in the kidney were also determined.

RESULTS

Plasma MG levels were significantly elevated in SHR, but not in WKY rats, at 8, 13, and 20 weeks of age in parallel with blood pressure increase. Kidney MG levels in SHR were increased by 21% and 38% at 13 and 20 weeks, respectively, compared to age-matched WKY rats. There were no differences in blood pressure and MG levels in plasma and kidney between SHR and WKY rats at 5 weeks of age. Immunohistochemistry revealed more intense staining for CML and CEL in kidneys from SHR compared to WKY rats from 8 weeks onward. Most of the staining was localized to renal tubules with some staining in the glomerular vessels.

CONCLUSION

MG and AGEs formation was significantly elevated in kidney from SHR, which may cause local vascular and tubular damage, contributing to the development and complications of hypertension.

Alteration of amine oxidase activity in the adipose tissue of obese subjects

OBJECTIVE

To explore the activity of monoamine oxidases (MAOs) and semicarbazide-sensitive amine oxidases (SSAOs) in adipose tissue and blood of lean and moderately obese subjects and to study whether there is a link between these hydrogen peroxide-generating enzymes and blood markers of oxidative stress.

RESEARCH METHODS AND PROCEDURES

Nine obese male subjects (BMI 32.6 +/- 0.4 kg/m(2)) and nine controls (BMI 23.4 +/- 0.5) of 24- to 40-year-old subjects were included in the study. MAO and SSAO activities were measured on microbiopsies of abdominal subcutaneous adipose tissue by quantifying (14)C-tyramine and (14)C-benzylamine oxidation. Levels of soluble SSAO, lipid peroxidation products, and antioxidant agents were measured in plasma, whereas cytoprotective enzymes were determined in blood lysates.

RESULTS

The high MAO activity found in adipose tissue was diminished by one-half in obese subjects (maximum initial velocity of 1.2 vs. 2.3 nmol tyramine oxidized/mg protein/min). There was no change in SSAO activity, either under its adipose tissue-bound or plasma-soluble form. Plasma levels of lipid peroxidation products and antioxidant vitamins remained unmodified, as well as erythrocyte antioxidant enzymes, whereas circulating triglycerides, insulin, and leptin were increased.

DISCUSSION

Although they already exhibited several signs of endocrino-metabolic disorders, the obese men did not exhibit the increase in blood markers of oxidative stress or the decrease in antioxidant defenses reported to occur in very obese or diabetic subjects. The reduced MAO and the unchanged SSAO activities found in obesity suggest that these hydrogen peroxide-generating enzymes expressed in adipocytes are probably not involved in the onset of the oxidative stress found in severe obesity and/or in its complications.

Protein Cross-Linkage Induced by Formaldehyde Derived from Semicarbazide-Sensitive Amine Oxidase-Mediated Deamination of Methylamine

Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the conversion of methylamine to formaldehyde. This enzyme is located on the surface of the cytoplasmic membrane and in the cytosol of vascular endothelial cells, smooth muscle cells, and adipocytes. Increased SSAO activity has been found in patients with diabetes mellitus, chronic heart failure, and multiple types of cerebral infarcts and is associated with obesity. Increased SSAO-mediated deamination may contribute to protein deposition, the formation of plaques, and inflammation, and thus may be involved in the pathophysiology of chronic vascular and neurological disorders, such as diabetic complications, atherosclerosis, and Alzheimer's disease. In the present study, we demonstrate the induction of cross-linkage of formaldehyde with the lysine moiety of peptides and proteins. Formaldehyde-protein adducts were reduced with sodium cyanoborohydride, hydrolyzed in hydrochloric acid, and the amino acids in the hydrolysates were derivatized with fluorenylmethyl chloroformate and then identified with high-performance liquid chromatography. We further demonstrate that incubation of methylamine in the presence of SSAO-rich tissues, e.g., human brain meninges, results in formaldehyde-protein cross-linkage of particulate bound proteins as well as of soluble proteins. This cross-linkage can be completely blocked by a selective inhibitor of SSAO. Our data support the hypothesis that the SSAO-induced production of formaldehyde may be involved in the alteration of protein structure, which may subsequently cause protein deposition associated with chronic pathological disorders.

Involvement of SSAO-mediated deamination in adipose glucose transport and weight gain in obese diabetic KKAy mice

Semicarbazide-sensitive amine oxidase (SSAO) is located on outer surfaces of adipocytes and endothelial and vascular smooth muscle cells. This enzyme catalyzes deamination of methylamine and aminoacetone, leading to production of toxic formaldehyde and methylglyoxal, respectively, as well as hydrogen peroxide and ammonium. Several lines of evidence suggest that increased SSAO activity is related to chronic inflammation and vascular disorders related to diabetic complications. We found that a highly potent and selective SSAO inhibitor, (E)-2-(4-fluorophenethyl)-3-fluoroallylamine (FPFA), was capable of reducing numbers of atherosclerotic lesions as well as weight gain in obese KKAy mice fed an atherogenic diet. SSAO inhibitors cause a moderate and long-lasting hyperglycemia. Such an increase in serum glucose is a result of reduction of glucose uptake by adipocytes. SSAO-mediated deamination of endogenous methylamine substrates induces adipocyte glucose uptake and lipogenesis. Highly selective SSAO inhibitors can effectively block induced glucose uptake. The results suggest that increased SSAO-mediated deamination may be concomitantly related to obesity and vascular disorders associated with type 2 diabetes.

Adipocytes release a soluble form of VAP-1/SSAO by a metalloprotease-dependent process and in a regulated manner

AIMS/HYPOTHESIS

Vascular adhesion protein-1 (VAP-1), which is identical to semicarbazide-sensitive amine oxidase (SSAO), is a dual-function membrane protein with adhesion properties and amine oxidase activity. A soluble form of VAP-1 is found in serum, where concentrations are enhanced in diabetes and obesity. In vitro, soluble VAP-1 enhances lymphocyte adhesion to endothelial cells, thus possibly participating in the enhanced lymphocyte adhesion capacity that is implicated in the cardiovascular complications associated with diabetes or obesity. In both, the tissue origin of the soluble VAP-1/SSAO is unknown. We examined whether adipose tissue, which has abundant expression of VAP-1/SSAO, is a source of soluble VAP-1.

METHODS

We detected VAP-1/SSAO in plasma of diabetic animals, with or without VAP-1 immunoprecipitation, and in culture medium from 3T3-L1 adipocytes and human adipose tissue explants. VAP-1 protein glycosylation was measured.

RESULTS

Diabetic and obese animals have increased plasma SSAO activity associated with VAP-1 protein. We also found that 3T3-L1 adipocytes and human adipose tissue explants release a soluble form of VAP-1/SSAO, which derives from the membrane. The release of soluble VAP-1 was enhanced by exposure of murine and human adipocytes to TNF-alpha and blocked by batimastat, a metalloprotease inhibitor. Partial ablation of adipose tissue reduced plasma SSAO activity in normal and diabetic rats.

CONCLUSIONS/INTERPRETATION

Adipose cells are a source of soluble VAP-1/SSAO released by shedding of the membrane form. The release of SSAO is regulated by TNF-alpha and insulin. By releasing VAP-1/SSAO, adipose cells could contribute to the atherogenesis and vascular dysfunction associated with diabetes and obesity.

Inhibitory activities of semicarbazide-sensitive amine oxidase and angiotensin converting enzyme of pectin hydroxamic acid

Solutions of 100 mL of 1% commercial pectin each with a different degree of esterification (DE), DE94, DE65, and DE25, were reacted with 100 mL of 2 M alkaline hydroxylamine (pH 12.0) at room temperature for 4 or 18 h. These pectin hydroxamic acids (PHAs; DE94T4, DE94T18, DE65T4, and DE25T4) were used to test the inhibitory activities against semicarbazide-sensitive amine oxidase (SSAO) and angiotensin-converting enzyme (ACE). Compared to different DE pectins (DE94, DE65, and DE25), the PHAs of DE94T4, DE94T18, DE65T4, and DE25T4 showed different inhibition activities against SSAO or ACE. Commercial pectins with different DE values showed negligible SSAO or ACE inhibitions. The order of SSAO inhibition was DE65T4 > DE94T18 approximately DE25T4 >> DE94T4. However, the order of ACE inhibition was DE94T4 > DE94T18 >> DE65T4 > DE25T4. The SSAO activity staining or ACE-hydrolyzed products on TLC chromatogram also confirmed the inhibitory activities of PHAs against SSAO or ACE.