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Sección de Nutrición, Bromatología y Dietética, Facultad de Ciencias Experimentales y de la Salud, Universidad San Pablo CEU, Madrid, Spain
2To whom correspondence should be addressed. E-mail: nubeda{at}ceu.es.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • valproate • methionine adenosyltransferase • glutathione • vitamin B-6 • rats.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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,2
) show two types of hepatotoxic effects, i.e., one that is dose related and reversible, and another that is dose independent and irreversible. The first effect is characterized by an elevation in liver enzyme activities, mainly transaminases, and no clinical symptoms. The second effect is much more serious and can lead to hyperammonemia, hypoglycemia, coma and death.
During pregnancy, VPA therapy results in a high incidence of spina bifida aperta, 5- to 20-fold higher than in an untreated population (3
–5
). In animal models, VPA administration has been frequently used as a common strategy to induce neural tube defects (NTD) (6
–8
). VPA reduces blood folate levels and causes folate deficiency in both humans (9
) and laboratory animals (10
,11
).
The mechanisms underlying the toxic effects of VPA are unclear and have been investigated in different studies. Hepatotoxicity has been suggested to be a consequence of carnitine deficiency [reviewed in (12
)] and also to be an oxidative effect. VPA treatment has been associated with a deficiency of selenium, which is required for glutathione peroxidase activity (13
). Furthermore, depletion of glutathione has been observed in animals treated with VPA (14
,15
).
To explain VPA teratogenesis, an interaction with folic acid metabolism has been suggested (11
,16
). In nonepileptic humans, folate supplementation prevented 70% of NTD cases (17
,18
). However, when the supplementation was assayed in VPA-induced NTD in animal models, results were unsatisfactory (19
–21
), suggesting that folate might not be the only metabolite participating in VPA teratogenesis.
Folate serves as a methyl donor to homocysteine, which is converted to methionine, in a reaction catalyzed by the vitamin B-12–dependent methionine synthase (MS). This is one of the main reactions in the methionine cycle, a metabolic route that leads to the synthesis of S-adenosylmethionine (AdoMet) through the enzyme methionine adenosyltransferase (MAT). AdoMet is the main methyl group donor in many transmethylation reactions (e.g., DNA-, proteins, lipids), yielding S-adenosylhomocysteine (AdoHcy). AdoHcy is hydrolyzed to homocysteine, which can then be remethylated to methionine [via MS or betaine-homocysteine methyltransferase (BHMT), in hepatic tissue], thus completing the cycle, or be degraded via the transsulfuration pathway in which vitamin B-6 participates and leads to glutathione synthesis (22
) (Fig. 1
).
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,24
), but this hypothesis has not been sufficiently studied after exposure to VPA. In mice, Wegner and Nau (16
) demonstrated that VPA administration during organogenesis interfered with folate metabolism and decreased the levels of formylated folates. This might explain some aspects of VPA teratogenesis. More recently, Hishida and Nau (25
) reported that a reduction in methionine levels, accompanied by inhibition of homocysteine remethylation, increases in aminothiols and decreases in glutathione, was associated with the induction of VPA-induced NTD in mice. In pregnant rats, we showed that VPA affects methionine synthesis by altering methionine synthase activity, an effect that impairs methionine availability and disrupts the methylation cycle, thereby inducing maternal and fetal DNA hypomethylation (26
). All of the studies showed a relationship between VPA administration and an alteration in the methionine cycle that could explain VPA-induced teratogenesis. In the present study, we decided to examine this relationship more fully in an animal model using a single dose of VPA. A detailed time course of biochemical changes after acute exposure to the drug could be very valuable not only to define the mechanism of VPA-induced teratogenesis, but also to further understand VPA-induced hepatotoxicity.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Rats were anesthetized with CO2 and killed by decapitation at different time points according to the experimental groups (1, 3, 6, 9 and 24 h after injection). Whole blood was collected from all rats and the plasma and serum were separately obtained by centrifugation (1,100 x g, 4°C, for 15 min) and kept at -20°C until analyzed. Livers were promptly removed, frozen in liquid nitrogen and stored at -80°C for further analyses.
All animal experiments were undertaken according to the guidelines from the European Community Council (27
).
Amino acids and derivatives.
Plasma homocysteine levels were determined by HPLC, using a Chromsystems Reagent Kit for HPLC analysis of homocysteine in plasma (Chromsystems, Munich, Germany), which uses a simple isocratic HPLC system with an attached fluorescence detector (
Ex 385 nm; Em 515 nm). During sample preparation, homocysteine is released from protein by a reduction step; after precipitation, it is subjected to a precolumn derivatization. Methionine was quantified using serum samples deproteinized by ultrafiltration and analyzed using a Beckman System 6300 High performance Amino Acid Analyzer (Beckman Instruments, Palo Alto, CA) following the modifications described by Andersson et al. (28
). Hepatic AdoMet and AdoHcy levels were determined by HPLC according to the method described by Fell et al. (29
), with some modifications (30
). Aliquots of frozen liver (
100 mg) were homogenized in 4 volumes of 0.4 mol/L perchloric acid, and then centrifuged at 10,000 x g, 4°C, for 10 min. The clear supernatants were removed, filtered and appropriate aliquots were analyzed for AdoMet and AdoHcy.
Vitamins.
Serum folate levels were measured by chemiluminescence, using an Automated Chemiluminescence System (Ciba-Corning ACS, Madrid, Spain). Vitamin B-6 is the generic term for three natural pyridine derivatives and their 5-phosphate esters. Pyridoxal (PL)-5-phosphate is the active coenzyme form and is the main component of several variants in plasma. Plasma PL-5-phosphate concentration (now referred to as vitamin B-6) was determined by HPLC, using a Chromsystems Reagent Kit for HPLC analysis of vitamin B-6 (Chromsystems), which uses a simple isocratic HPLC system with an attached fluorescence detector ( Ex 300 nm; Em 400 nm). With this method, the derivatization necessary for the detection of vitamin B-6 takes place automatically when the sample is applied to the HPLC column because the derivatization reagent is present in the mobile phase.
Glutathione.
Hepatic reduced (GSH) and oxidized (GSSG) glutathione were measured by HPLC according to the method described by Reed et al. (31
). Livers were homogenized in 4 volumes of 50 mmol/L Tris buffer (pH 8) containing 0.3 mol/L sucrose and 0.1 mmol/L EGTA. The homogenate was mixed with 0.6 mol/L trichloroacetic acid and 100 mmol/L iodoacetic acid and incubated in the dark for 30 min. Dinitrofluorobenzene (1.5%) was added and the mixture was incubated overnight at 4°C. The homogenate was then centrifuged at 4000 x g for 10 min and the supernatant was again centrifuged at 13,000 x g for 100 min to obtain the supernatant for GSH and GSSG assays by HPLC.
Enzymes.
MAT, BHMT and MS activities were measured in liver extracts using radioenzymatic assays as described by Martín-Duce et al. (32
), Finkelstein and Mudd (33
) and Keating et al. (34
), respectively. Total protein content in liver extracts was measured by the method of Bradford (35
).
Valproate.
Serum valproate was determined by fluorescence polarization immunoassay using a TDx System and an Abbot Reagent kit (South Pasadena, CA).
Statistics.
Results were expressed as means ± SEM, n = 6. Data were subjected to one-way ANOVA and when the F-test was significant (P < 0.05), multiple comparisons between means were done using Student’s t test with the Bonferroni adjustment for inequality and the Student-Newman-Keuls test. Significant differences were further analyzed by the Kruskal-Wallis nonparametric test (SYSTAT Version 5.0, Systat, Chicago, IL and Excel for Windows, Microsoft Office Version 7.0).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). One hour after VPA administration (group t1), there was a significant increase in hepatic AdoHcy concentration (Table 1
). This was still evident 3 h after treatment (group t3), but AdoHcy levels were not different from controls in the t6, t9 and t24 groups. AdoMet concentration tended to be lower than in controls in the t1 group (P = 0.11). The slight reduction in AdoMet and the significant increase in AdoHcy were reflected in a significant reduction in the AdoMet/AdoHcy concentration ratio 1 h after VPA administration.
|
|
). GSH concentration did not differ from the control in any of the VPA-treated groups but the GSH concentration was lower in groups t3 and t9 than in group t1 (Table 2
). The GSSG concentration was greater in group t1 than in all other groups.
|
|
). Plasma homocysteine concentrations were significantly reduced 3 and 24 h after VPA injection compared with group t1 but none of the VPA-treated groups differed. Serum folate concentrations and hepatic activities of MS and BHMT were not affected by VPA treatment (Table 3)
.
|
). The concentration of the drug decreased by 51% 3 h after the injection, and continued to decrease until 24 h after treatment when it had almost disappeared from serum. Significant correlations between serum VPA levels and several variables were found: plasma vitamin B-6, r = -0.60, P < 0.001; hepatic AdoHcy, r = 0.50, P < 0.01; and the AdoMet/AdoHcy ratio, r = -0.39, P < 0.05.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In animal models, hepatotoxic agents such as carbon tetrachloride and buthionine sulfoximine have markedly reduced MAT activity and decreased hepatic glutathione and AdoMet levels (36
,37
). Recently, Lu et al. (38
) described a predisposition to liver injury and an increase in liver proliferation in a MAT knockout mouse model. MAT is the enzyme responsible for the synthesis of AdoMet using methionine and ATP as substrates (Fig. 1)
. Its importance is due to the fact that AdoMet is the main methyl donor for most of the transmethylation reactions in the cell, and therefore is involved in many critical biological functions.
The reduction of MAT activity in injured liver has not been clearly explained, but it could be related to oxidative stress. In vitro, the presence of GSSG can inhibit the enzyme activity and this effect can be modulated by GSH, although GSH alone has no effect on the enzyme. Therefore, MAT activity in vitro is regulated by the GSH/GSSG ratio (39
). In vivo, the reduction in MAT activity is concomitant with reduced cellular levels of glutathione, both in human cirrhosis and in animal models intoxicated with buthionine sulfoximine and carbon tetrachloride (36
,37
). Glutathione depletion seems to be a direct consequence of VPA treatment in rats (14
,15
), and depression of glutathione levels has been proposed recently as a possible mechanism underlying VPA teratogenesis in mice (25
). These studies, however, did not discriminate between the oxidized and reduced forms of glutathione.
It remains unclear whether the first insult is a depression of glutathione that leads to MAT inactivation or an inhibition of MAT activity that leads to glutathione depletion. In the transsulfuration pathway, homocysteine is converted to cystathionine and cysteine, which is further metabolized and excreted as inorganic sulfate, or it is used for the synthesis of glutathione (Fig. 1)
. The depletion of liver glutathione levels as a result of liver damage could lead to inactivation of MAT, but this in turn could result in a further decrease in glutathione levels and thus worsen the deficiency of MAT (37
). In our study, we found significant reductions in MAT activity 1, 3, 6 and 9 h after VPA administration. Enzyme activity had recovered by 24 h, when VPA had almost disappeared from the serum. We did not observe a depletion in total glutathione due to VPA but a significant increase in the oxidized form (GSSG) 1 h after injection. On the basis of these findings, we propose that VPA inhibits MAT in vivo. The mechanism could be related to the increase in GSSG because this oxidized form inhibits MAT in vitro (39
). Because MAT inactivation is strongly related to liver injury (see above), these metabolic conditions could both be related to the hepatotoxicity associated with VPA. Aliphatic compounds may inhibit MAT in vitro (40
) and it is possible that VPA has that effect. We used crude liver extracts for our assays and did not test for the presence of a soluble inhibitor.
The effect of VPA on plasma vitamin B-6 was also marked. There was a negative correlation between serum VPA and vitamin B-6 levels (r = -0.60, P < 0.001), with a 54% decrease in plasma B-6 concentration 1 h after VPA administration. This effect has not been reported before. We know that chronic treatment with anticonvulsant drugs in humans may induce vitamin B-6 deficiency (41
), and that vitamin B-6 deficiency impairs homocysteine/methionine metabolism (42
–44
), but these observations are due to chronic drug treatment. In our study, the effect of VPA on vitamin B-6 was acute. The explanation we propose is that the effects result from a disregulation of the methionine cycle. One of the major regulatory sites is the distribution of homocysteine between remethylation and cystathionine synthesis (22
), and an increased need for glutathione synthesis might affect this distribution (45
). In addition, the activity of cystathionine-ß-synthase is enhanced under oxidative conditions (46
). Changes in the ambient redox potential could affect the flux of homocysteine between the two competing pathways, increasing this flux two- to threefold through the transsulfuration pathway to replenish the glutathione pool, which was diminished in response to an oxidant insult (47
). In turn, homocysteine levels tended to be higher (P = 0.06) 1 h after VPA administration. These results suggest that VPA treatment and the impairment of MAT activity could be inducing increases in transsulfuration enzymes to prevent homocysteine elevation. The induction of the transsulfuration pathway could explain the reduction in vitamin B-6 levels because this vitamin acts as a cofactor for two enzymes involved in the pathway, cystathionine-ß-synthase and 
-cystathionase.
Serum folate concentration did not change after VPA administration, as we showed previously after drug treatment on d 8, 9 and 10 of pregnancy (26
). Conversely, VPA has antifolate activity, in both humans (9
) and laboratory animals (10
,11
). These apparently contradictory observations suggest that VPA might indeed alter folate metabolism without reducing total folate concentration.
VPA also induced a marked and significant increase in hepatic AdoHcy concentration. The reaction catalyzed by S-adenosylhomocysteine hydrolase is reversible, thermodynamically favoring AdoHcy synthesis. However, the reaction normally functions in the opposite direction because the products, homocysteine and adenosine, are removed enzymatically (see Fig. 1
) (22
). Nevertheless, an alteration in methionine/homocysteine metabolism could lead to accumulation and conversion into AdoHcy, with concomitant elevation in AdoHcy levels. We previously showed this elevation in AdoHcy due to impaired homocysteine metabolism, in both VPA-treated rats (26
) and carbon tetrachloride–induced hepatic injury in rats (48
).
Another consequence of MAT inactivation could be a reduction in the availability of AdoMet. We did not find a significant reduction in hepatic AdoMet concentration but VPA significantly reduced the AdoMet/AdoHcy ratio, the so-called transmethylation ratio, 1 h after exposure to the drug. In the same sense, studies in humans (49
) and rats (37
) have shown that marked reductions in MAT activity do not necessarily lead to reduced AdoMet levels, possibly because the rate of AdoMet utilization is adjusted to the rate of synthesis. However, transmethylation reactions could be inhibited because of a lower availability of AdoMet and/or increased AdoHcy concentration. Moreover, a reduced AdoMet/AdoHcy ratio also inhibits transmethylation reactions (50
). In fact, we previously showed genomic DNA hypomethylation in rats exposed to VPA (26
).
The time-dependent effects of VPA on the methionine cycle deserve further comment. We observed significant correlations between serum VPA levels and the plasma vitamin B-6 decrease, the hepatic AdoHcy increase and the AdoMet/AdoHcy ratio decrease. At 24 h after VPA administration, when the drug has disappeared from serum, MAT activity and vitamin B-6 plasma levels as well as all of the other variables measured did not differ from the untreated control rats. We do not know whether the effects of VPA on MAT activity and vitamin B-6 would become irreversible due to cyclical drug effects or would even have different consequences due to the chronic administration of the drug. In a previous study, we showed that VPA administered on gestation days 8, 9 and 10 in rats also disrupted the methionine cycle, and in this case, the manifestations were decreased methionine synthase activity and methionine levels on gestation day 21 (26
).
We have reported for the first time significant reductions in hepatic MAT activity and plasma vitamin B-6 levels in rats exposed to a single dose of VPA (400 mg/kg). We propose that VPA alters methionine metabolism by inhibiting MAT activity in vivo. The mechanism could be related to the increase in GSSG because this form of glutathione inhibits MAT in vitro (39
). An alteration of methionine metabolism could be the single mechanism underlying the pathophysiology of the different potentially toxic effects of VPA, i.e., hepatotoxicity, teratogenicity and the antifolate effect, for the following reasons: 1) MAT enzyme activity is impaired in several forms of liver disease; 2) an alteration in the methionine cycle can lead to glutathione depletion, and glutathione is the major detoxificant metabolite in the cell; 3) the methionine cycle is closely related to folate metabolism; and 4) transmethylation reactions are involved in many aspects of cellular function. However, the relationship of these metabolic changes to hepatic injury and teratogenicity remains to be elucidated.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: AdoHcy, S-adenosylhomocysteine; AdoMet, S-adenosylmethionine; BHMT, betaine homocysteine methyltransferase; GSH, reduced glutathione; GSSG, oxidized glutathione; MAT, methionine adenosyltransferase; MS, methionine synthase; NTD, neural tube defects; PL, pyridoxal; VPA, valproate.
Manuscript received 8 November 2001. Initial review completed 3 January 2002. Revision accepted 28 May 2002.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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