Published online before print June 3, 2004, doi: 10.1161/01.RES.0000134630.68877.2F
© 2004 American Heart Association, Inc.
Molecular Medicine |
Origins of Serum Semicarbazide-Sensitive Amine Oxidase
From the MediCity Research Laboratory (C.M.S., G.G.Y., R.K., S.J.), University of Turku and National Public Health Institute, Turku, Finland; the Department of Oncology (P.B.), Helsinki University Central Hospital, Helsinki, Finland; and the Molecular/Cancer Biology Laboratory (K.A.), University of Helsinki, Helsinki, Finland.
Correspondence to Craig M. Stolen, PhD, MediCity Research Laboratory, University of Turku, Tykistökatu 6A, FIN-20520, Turku, Finland. E-mail craig.stolen{at}utu.fi
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Abstract |
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Semicarbazide-sensitive amine oxidases (SSAO) are enzymes that are capable of deaminating primary amines to produce aldehyde, ammonia, and hydrogen peroxide. This activity has been associated with vascular adhesion protein-1 (VAP-1) and is found in the serum, endothelium, adipose, and smooth muscle of mammals. Circulating SSAO activity is increased in congestive heart failure, diabetes, and inflammatory liver diseases. To investigate the origin of circulating SSAO activity, two transgenic mouse models were created with full-length human VAP-1 (hVAP-1) expressed on either endothelial (mTIEhVAP-1) or adipose tissues (aP2hVAP-1), with tie-1 and adipocyte P2 promoters, respectively. Under normal conditions a circulating form of hVAP-1 was found at high levels in the serum of mice with endothelium-specific expression and at low levels in the serum of mice with adipose specific expression. The level of circulating hVAP-1 in the transgenic mice varied with gender, transgene zygosity, diabetes, and fasting. Serum SSAO activity was absent from VAP-1 knockout mice and endothelial cell–specific expression of human VAP-1 restored SSAO activity to the serum of VAP-1 knockout mice. Together, these experiments show that in the mouse VAP-1 is the only source of serum SSAO, that under physiological conditions vascular endothelial cells can be a major source of circulating VAP-1 protein and SSAO, and that serum VAP-1 can originate from both endothelial cells and adipocytes during experimental diabetes. An increased endothelial cell capacity for lymphocyte binding and altered expression of redox-sensitive proteins was also associated with the mTIEhVAP-1 transgene.
Key Words: diabetes • hydrogen peroxide • transgenic mice • vascular disease • vascular endothelium
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Introduction |
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Vascular adhesion protein-1 (VAP-1) was first identified as a novel adhesion molecule that is critically involved in the process of leukocyte trafficking to sites of inflammation in man.1–3 Subsequent cloning and sequencing of the cDNAs encoding murine and human VAP-1 revealed the marked homology of VAP-1 with enzymes called semicarbazide-sensitive amine oxidases (SSAO).4 In fact, VAP-1 has dual functions: besides its adhesive properties, it also possesses SSAO enzyme activity.5 SSAOs belong to the widely distributed copper-containing monoamine oxidase class (EC 1.4.3.6) and catalyze the oxidative deamination of primary amines to produce aldehydes, ammonium, and hydrogen peroxide (RCH2NH2+O2+H2O
RCHO+NH3+H2O2).6 The VAP-1 molecule is expressed on the surface of endothelial cells and adipocytes, where its enzyme activity can directly regulate lymphocyte rolling7 and stimulate glucose transport,8 respectively. The products of the SSAO reaction may also have other biological consequences. For example, the oxidative deamination of endogenous methylamine results in the formation of formaldehyde, which is an extremely reactive chemical that is capable of causing protein crosslinking9 and increasing advanced glycation end product formation.10,11 In addition, hydrogen peroxide is a major reactive oxygen species that can cause cell damage and function as a signaling molecule.12–14
In addition to the membrane-associated VAP-1, a catalytically active form of this protein circulates in the blood stream as a soluble molecule.15 A number of clinical reports have suggested a role for the circulating SSAO activity in specific pathological conditions, including diabetes mellitus,16–19 congestive heart failure,20,21 and inflammatory liver diseases.22 The cellular source of this circulating enzyme is unknown. Whether it is derived from shedding of the transmembrane spanning VAP-1 protein or is a product of a different gene is also unknown.
In this study, we used transgenic technology to specifically express the full-length human VAP-1 protein either on the surface of endothelial cells or on adipocytes. Using these mice together with VAP-1 knockout mice, we show that VAP-1 is the only source of circulating SSAO in the mouse, that circulating VAP-1/SSAO can be derived from the human VAP-1 gene and that, under physiological conditions, the endothelium can be major source of circulating, catalytically active VAP-1/SSAO, whereas in times of biological stress, such as diabetes, both adipocytes and endothelial cells can release serum hVAP-1. We also show VAP-1 expression increases the capacity of endothelial cells to bind lymphocytes and that its over-expression promotes the expression of redox sensitive proteins.
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Materials and Methods |
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Human VAP-1 ELISA
Transgenic human VAP-1 levels were analyzed using an ELISA exactly as described.15 Briefly, the serum and tissue samples were diluted in blocking solution and tested in triplicate. The VAP-1 content was quantified by absorbing anti–VAP-1 mAb, TK8–18, onto the bottom of 96-well microplates, incubating with the diluted samples, and detecting the bound VAP-1 with another anti–VAP-1 mAb, biotinylated TK8–14, followed by peroxidase-conjugated streptavidin and BM Chemiluminescence ELISA Reagent (Boehringer-Mannheim). The VAP-1 concentration was determined for each sample by subtracting the mean value of luminescence (Tecan Ultra Luminometer) obtained with an irrelevant detecting mAb, biotinylated Hermes-3, from the VAP-1–specific intensity followed by conversion to ng/mL using linear regression and known standards included on each plate.
Measurement of SSAO Activity
Total amine oxidase activity, derived from both endogenous and transgenic sources, was assayed either radiochemically using [7-14C]benzylamine hydrochloride (specific activity 57 mCi/mmol, Amersham) as a substrate22 or by fluorometric determination of the reaction product H2O2 using Amplex Red reagent.7
Induction of Diabetes
Streptozotocin (STZ) was dissolved in 0.05 mol/L sodium citrate (pH 4.5) and immediately injected intraperitoneally. The glucose levels of arteriovenous blood collected from tail vessels was measured at regular intervals using a MediSense Precision Xtra Plus sensor (Abbot Oy). An optimal streptozotocin "high dose" of 240 mg/kg body weight was selected during a pilot study for its capacity to consistently induce stable hyperglycemia in this strain of mice (FVB/NTac; Taconic M&B, Denmark). All experiments were conducted in accordance with the University of Turku and Finnish National guidelines for animal care and use. Mice that failed to become hyperglycemic (<13 mmol/mL blood) after a single high dose of STZ were given a second STZ injection at an equal or lower concentration (240 or 120 mg/kg). Littermate controls, were injected with vehicle only. In the "multiple low dose" STZ experiment, mice were injected daily for 5 days during the first week with STZ (60 mg/kg body weight) or vehicle only and then similarly for 5 consecutive days during week 7.
Stamper-Woodruff Adhesion Assay
Nonstatic ex vivo lymphocyte binding assays were performed as described earlier.23 For a brief description, see the expanded Materials and Methods section, found in the online data supplement available at http://circres.ahajournals.org.
Two-Dimensional PAGE and Peptide Sequencing
Isoelectric focusing and mass-spectrometry were performed at the University of Turku Center for Biotechnology. A brief description of the protocol can be found with the expanded Materials and Methods section.
Additional details of the materials and methods used can be found in the online data supplement.
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Results |
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Generation of Transgenic Mice That Express Human VAP-1 in Specific Tissues
To determine the source and regulation of serum VAP-1 (sVAP), transgenic mice were created with either a mouse tie-1 promoter or a murine aP2 enhancer/promoter driving the expression of full-length human VAP-1 (online Figure 1 in the online data supplement), including the transmembrane domain, specifically to endothelial and adipose cells respectively. Transgenic founder mice were identified by PCR and confirmed by Southern blot analysis using transgene-specific primers and probes. Two independent lines of mice with each transgene were selected for further analyses (Table).
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Transgene Expression Is Targeted to Specific Cell Populations
In human tissue, VAP-1 exists as a 170- to 180-kDa homodimeric sialoglycoprotein24 and in the transgenic tissues a VAP-1 protein with the appropriate sialic acid decorations and the expected molecular mass was detected (online Figure 2). To characterize the expression pattern of human VAP-1 in the mTIEhVAP-1 and aP2hVAP-1 transgenic lines, immunohistochemistry and ELISA were performed using antibodies specific for human VAP-1. Detectable human VAP-1 protein expression was found in 6 of 18 mTIEhVAP-1 lines. In lines E25 and E35, strong protein expression was restricted to the endothelial cells of all analyzed organs (liver, heart, lung, spleen, kidney, small intestine, mucosal lymph node, peripheral lymph node, eye, skin, muscle, and adipose; data not shown) and was not found in other cell types. Conversely, strong expression was found only in the adipocytes of white (Figure 1A) and brown (Figure 1C) adipose tissue from aP2hVAP-1 mice (lines A31 and A33). No VAP-1 protein expression was found in tissues other than adipose (liver, heart, lung, pancreas, spleen, kidney, small intestine, thymus, testis, muscle, and brain; data not shown). No staining of endogenous mouse VAP-1 was seen in any tissue, including smooth muscle, thus confirming the specificity of the antibodies for human VAP-1 (Figure 1B; data not shown).
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To determine if the human VAP-1 protein was on the endothelial cell surface, frozen tissue sections were prepared after the intravenous injection of a FITC-conjugated anti–human VAP-1 antibody. Human VAP-1–specific staining was found on the endothelial cell surface of vessels in the mTIEhVAP-1 liver (Figure 1 D), kidney, pancreas, and lymph node, but not in nontransgenic tissues (Figure 1 E). Liver sections were also double stained with an anti-human VAP-1 antibody and a marker for sinusoidal endothelium, anti-CD36 (Figure 1G).25 The anti–human VAP-1 antibodies stained the terminal hepatic venules and sinusoidal endothelium of mTIEhVAP-1 sections (Figure 1H) but not in the nontransgenic liver (not shown). Clear double staining of the sinusoidal endothelium was also evident in the mTIEhVAP-1 sections (Figure 1I). Thus, as predicted, the transgene expression patterns in the mTIEhVAP-1 and aP2hVAP-1 transgenic animals are different in that expression driven by the tie-1 promoter is directed to endothelial cells, whereas protein expression driven by the aP2 promoter is only directed to adipocytes.
Human VAP-1 Overexpression Increases Total SSAO Activity
We next tested whether overexpression of hVAP-1 leads to increased SSAO activity in the transgenic tissues. First, adipose tissue lysates were incubated with saturating concentrations of either benzylamine or methylamine, as appropriate enzyme substrates, and clorgyline, as an inhibitor of intracellular MAO. The generation of the SSAO reaction product H2O2 was determined by using a sensitive fluorometric assay. With this technique, increased SSAO activity was found in the adipose tissue lysates of the aP2hVAP-1 mice compared with the nontransgenic mice (Figure 2A). Second, an independent, radiochemical assay was used to measure the rate of [14C]benzylamine deamination in liver lysates. Because the application of the fluorometric assay to liver lysates is complicated by very low SSAO activity and high background from the massive release of endogenous H2O2, during the solubilization of hepatocytes, it was more suitable to use the radiochemical assay with subsaturating concentrations of [14C]benzylamine. With this assay, a significant increase in SSAO activity was also found in liver lysates from mTIEhVAP-1 mice (Figure 2B).
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Soluble Human VAP-1 Is Found in the Serum of mTIEhVAP-1 Transgenic Mice
Although SSAO activity has been found in the serum of human patients and in experimental animals, the molecular and cellular source of this activity has not been defined. To determine if full-length VAP-1 expressed on endothelial cells or adipocytes could act as a source of this activity, the serum of transgenic mice was examined. By using a quantitative sandwich ELISA, with anti-VAP-1 antibodies that are specific for the human VAP-1 protein, a soluble form of VAP-1 was found at high levels in the serum of mTIEhVAP-1 transgenic mice and at very low levels in the serum of aP2hVAP-1 mice but not in nontransgenic controls (Table). Whereas the average concentration of soluble human VAP-1 in the mTIEhVAP-1 mice (10 to 48 ng/mL; Table) was lower than that found in human AB serum samples (92.6±13.4 ng/mL, n=61), it was of a similar magnitude, suggesting normal regulation of its release. Increased SSAO activity, as determined by the rate of [14C]benzylamine deamination, was also found in the serum of the mTIEhVAP-1 mice compared with nontransgenic controls (Figure 2B) and could be diminished by immunodepletion with the human VAP-1–specific antibody, JG2.10 (data not shown). Furthermore SSAO activity is restored to the serum of VAP-1 knockout mice by the mTIEhVAP-1 transgene (Figure 3). Thus the transmembrane form of human VAP-1 expressed on endothelial cells can act as a source of sVAP-1 and SSAO activity.
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Human VAP-1 Is Released From Adipocytes and Endothelial Cells During Biological Stress
Having determined that the transmembrane form of VAP-1 can act as a source of circulating SSAO, we next investigated whether the levels present in mouse serum could vary. A higher level of human sVAP-1 was found in mTIEhVAP-1 homozygous versus heterozygous mice, indicating that this level could be influenced by gene dose (Table). The age of the mTIEhVAP-1 mice did not correlate with sVAP-1 levels (heterozygous males r=–0.09, P=0.6; females r=–0.20, P=0.4); however, higher human sVAP-1 levels were found in the serum of male versus female mice (Table). The zygosity and gender-specific differences were found in two independent lines of mTIEhVAP-1 mice (E25 and E35) and in one line of aP2hVAP-1 mice (A33), indicating that the differences are not due to the site of transgene integration, but are more likely due to differential promoter activity or protein shedding. No gender-specific difference in serum SSAO activity was found in nontransgenic samples (male, 22.7±5.6 pmol/hr per mL, n=11 versus female, 26.7±2.2 pmol/hr per mL, n=11).
Previous studies have found increased levels of sVAP-1 and SSAO in the plasma of patients with diabetes and in animal models of diabetes. Thus, we decided to test whether STZ treatment would lead to increased SSAO activity and sVAP-1 levels in the mTIEhVAP-1 and aP2hVAP-1 transgenic mice. We used two different STZ regimens to induce experimental diabetes.
First, high-dose STZ exposure was used to induce ß-cell death and acute hyperglycemia. After 2 to 5 days of hyperglycemia, serum and tissue samples were collected. Using a radiochemical assay, increased SSAO activity was found in the serum of the STZ injected nontransgenic, mTIEhVAP-1, and aP2hVAP-1 mice compared with buffer injected controls (Figure 4A). A marked increase in hVAP-1 was also found in the serum of both mTIEhVAP-1 and in aP2hVAP-1 mice (Figure 4B), thus indicating that sVAP-1 can be released from both endothelial cells and adipocytes in diabetes. No change was found in the total hVAP-1 content of heart, liver, kidney, and pancreatic lysates of the same mTIEhVAP-1 mice (data not shown).
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Second, a multiple low-dose regimen of STZ was used to induce pancreatic insulitis and thus avoid any immediate effects from STZ itself.26,27 In this model, the onset of hyperglycemia was delayed but then maintained for several weeks before sample collection at 13 weeks. Using this model, dramatically elevated sVAP-1 levels were again found in the serum of the hyperglycemic mTIEhVAP-1 mice several weeks after exposure to STZ (Figure 4B).
Finally, we postulated that ketone bodies might regulate sVAP-1 release. To elevate the blood level of ketones, we fasted the mice for 44 to 48 hours and then compared their sVAP-1 levels to littermate controls that had been similarly handled and fed ad libitum. Although there was an increase in sVAP-1 levels in the fasted mTIEhVAP-1 mice compared with their fed transgenic littermates (paired t test of mean litter values, P=0.03; Figure 5) and compared with all fed female mTIEhVAP-1 mice (unpaired t test P=0.001; Figure 5), there was no correlation with ketone body levels (r=0.20, P=0.5). To determine if fasting could also trigger sVAP-1 release from adipose cells, we measured the sVAP-1 level in plasma taken from aP2hVAP-1 mice before and after a 40-hour fasting period. There was no detectable increase in sVAP-1 found (data not shown).
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Human VAP-1 Overexpression Increases Lymphocyte Adhesion
To assess the functional significance of increased VAP-1 expression, a Stamper-Woodruff assay was used to evaluate the effect of VAP-1 on lymphocyte binding to lymph node high endothelial venules. Initially, in the absence of blocking antibodies, both mouse and human lymphocytes preferentially adhered to the transgenic venules when compared with nontransgenic sections (Figure 6). By pretreating the transgenic lymph node sections with antibodies specific to human VAP-1 (TK8–14 and 1B2), the binding of both mouse and human lymphocytes was inhibited back to the nontransgenic level.
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Human VAP-1 Overexpression Increases Hepatic Expression of Redox-Sensitive Proteins
A transgene-specific increase in total liver mass was found in the mTIEhVAP-1 mice after a 15-month challenge with a high-fat diet, oral methylamine, or a control diet (ANOVA transgene effect, P=0.0005; for the study design, see Stolen et al)11 In particular, there was a 22% transgene-specific increase in liver mass when the mice were fed a high fat diet (Student t test, P=0.04). Although histological examination of liver sections taken from the mice fed a high-fat diet revealed fatty liver changes (ie, globular intracytoplasmic clear spaces and Oil red-O lipid staining), these changes were not specific to the transgenic mice and no other pathological changes were identified in any group of mice. To further investigate the molecular basis of this phenotype, hepatic proteins were separated by two-dimensional gel electrophoresis and differentially expressed proteins were identified by peptide sequencing (Figure 7). A large number of differentially expressed redox-sensitive proteins were found, including peroxiredoxin 1, glutathione S-transferase P, glyoxalase I, superoxide dismutase-Mn, and aldolase 2-B isoform, when comparing the transgenic to nontransgenic samples.
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Discussion |
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Human VAP-1 is an important cell adhesion molecule with high homology to copper-dependent SSAOs4 and abundant expression on endothelial cells, adipocytes, smooth muscle cells, and the dendritic cells of germinal centers. A form of human VAP-1 is also found circulating in the blood stream.15 Until now the exact cellular origin of this circulating molecule and the biological triggers that regulate its release have remained elusive. In the present study, we used transgenic mouse technology to specifically direct expression of human VAP-1 to either endothelial cells or adipocytes. This approach allowed us to determine the following things: (1) all detectable serum SSAO in the mouse is derived from VAP-1; (2) serum SSAO activity can be derived from expression of the full-length VAP-1 gene; (3) vascular endothelial cells are a major source of circulating VAP-1 protein and SSAO activity under normal physiological conditions; (4) under conditions of biological stress, the release of additional sVAP-1/SSAO is differentially regulated so that it can originate from both endothelial cells and adipocytes, as found in experimental diabetes, or solely from endothelial cells, as was found with fasting; (5) overexpression of human VAP-1 increases the capacity of mouse endothelial cells to bind both human and mouse lymphocytes; and (6) overexpression of human VAP-1 alters hepatic expression of redox-sensitive proteins.
In human diabetes and in animal models of diabetes, the plasma levels of SSAO are increased, and a significant positive correlation between SSAO activity and the development of late diabetic complications has been found.16,17 Because the deamination of endogenous substrates by SSAO produces substances that can directly damage endothelial cells, potentiate oxidative glycation, and increase oxidative stress,28,29 it is possible that this activity creates or exacerbates the damage leading to many of the various diabetic complications, such as retinopathy, nephropathy, neuropathy, and accelerated atherosclerosis. These toxic effects may be mitigated in part by the differential expression of redox-sensitive genes. In this study, we found that the expression levels of several redox-regulating proteins were changed in mice overexpressing VAP-1/SSAO (Figure 7). These changes may result from a direct signaling action of the VAP-1 molecule or the reaction byproducts: aldehyde, ammonia, and H2O2. Hydrogen peroxide in particular is a known regulator of gene expression.12–14 Alternatively, because these mice have an increased propensity for H2O2 production (Figure 2) and because H2O2 is a major reactive oxygen species that can be converted to hydroxyl-free radicals via the Fenton reaction the changes in protein expression may be secondary to SSAO-induced oxidative stress.
We also show an increase in lymphocyte binding to the high endothelial venules of the transgenic lymph nodes (Figure 6). The adhesion of mouse lymphocytes to transgenic human VAP-1 illustrates the conservation and importance of the extravasation cascade during evolution. These studies when combined with other transgenic studies11,30 verify the multifunctional nature of VAP-1/SSAO and indicate that increased SSAO is not just a benign byproducts of specific disease states but that it has real physiological and pathological consequences. Although the many functions of VAP-1/SSAO appear unrelated, they are not mutually exclusive. We propose that the SSAO enzymatic activity normally acts to form transient but covalent crosslinks between endothelial cells and amines presented on the leukocyte cell surface,7 and that in times of crisis (ie, diabetes), its ability to produce H2O2 is utilized in an attempt to stimulate the insulin signaling pathway.8 Finally, when this activity is chronically elevated, in inflammation or diabetes,31 it alters redox sensitive protein expression and promotes vascular complications.11
Previous studies have suggested several possible sources of serum SSAO activity. In cattle, a gene encoding a soluble form of SSAO has been cloned (BSAO),32 and in sheep, kinetic data suggest the existence of two different soluble SSAO enzymes.33,34 On the other hand, in humans, the N-terminal sequence of isolated serum SSAO is identical to the membrane distal sequence of VAP-1,35 and when VAP-1 is depleted from the serum of patients with diabetes all detectable SSAO activity simultaneously disappears,31 suggesting that a VAP-1 cleavage product may be the sole source. In transgenic mice that have the entire human VAP-1 coding sequence expressed with a smooth muscle 
-actin promoter, an increase in serum SSAO activity was found.30 In the experiments presented in this study, we show that circulating VAP-1 and serum SSAO activity may be derived from endothelial cell expression of the full-length form of the VAP-1 gene (Figure 2). This SSAO activity is absent from the serum of VAP-1 knockout mice but is restored by the mTIEhVAP-1 transgene (Figure 3), suggesting that VAP-1 is responsible for all serum SSAO activity. In this study, we have also found that with the induction of experimental diabetes both endothelial cells and adipocytes could release substantial amounts of VAP-1 into the serum (Figure 4), suggesting the existence of supplementary release triggering mechanisms during biological stress.
By having determined the source of serum SSAO, work can now be done to inhibit or regulate its release and thus attenuate its action in inflammation and vascular pathology. The differences in circulating VAP-1 levels (Table) that were found between the male and female mice may reflect gender-specific differences in human VAP-1 protein cleavage. Because the differences were found in both mTIEhVAP-1 and aP2hVAP-1 transgenic mice, it is unlikely to be a result of gender-specific promoter activity. Although similar differences in sVAP-1 or SSAO have not been detected in nontransgenic mice or in human patients,15 VAP-1 overexpression may accentuate a real gender difference. Interestingly, increased SSAO production in males could be hypothesized to contribute to the increased rate of atherosclerosis formation found in males. Although ketone bodies were not found to directly regulate sVAP-1 levels, there does appear to be similarities between diabetes and fasting in that both lead to VAP-1 release from endothelial cells. Recent clinical studies have found that circulating human VAP-1/SSAO levels rapidly respond, in an inverse manner, to changes in plasma insulin,31 thus suggesting that insulin could be the common regulator in both diabetes and fasting.
In conclusion, the use of transgenic mouse technology for these experiments has enabled us to circumvent the lack of good antibodies for directly assessing endogenous VAP-1 in animal models. In addition, by using the human VAP-1 transgene, we were allowed to use the species specificity of our antibodies to identify the cellular source of circulating VAP-1, something that would not be possible using human VAP-1 antibodies alone. This approach has also provided physiologically relevant information that verifies the role of VAP-1 in lymphocyte binding and indicates that VAP-1/SSAO activity can regulate redox-sensitive protein expression.
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Acknowledgments |
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This work was supported by grants from NIH, the Juvenal Diabetes Foundation International, the European Union (QLG1-CT-1999-00295), the Finnish Academy, the Sigrid Juselius Foundation, and the Technology Development Center of Finland. The authors wish to acknowledge the expert technical assistance of Maritta Pohjansalo and Suvi Nevalainen. Jussi Niemelä is also thanked for sharing his tie-1 promoter sequence data.
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Footnotes |
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Original received January 14, 2004; revision received May 13, 2004; accepted May 21, 2004.
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References |
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