Tryptophan-niacin metabolism in liver cirrhosis rat caused by carbon tetrachloride

Acute liver failure increases kynurenic acid production in rat brain via changes in tryptophan metabolism in the periphery

The tryptophan metabolite, kynurenic acid (KYNA), is a preferential antagonist of the α7 nicotinic acetylcholine receptor and N-methyl-d-aspartic acid receptor at endogenous brain concentrations. Recent studies have suggested that increased brain KYNA levels are involved in psychiatric disorders such as schizophrenia and depression. Most of the brain kynurenine (KYN), the KYNA precursor, comes from the periphery, and the liver has a central role in the peripheral tryptophan metabolism. In this study, the effect of acute liver failure (ALF) on brain KYNA production and on the peripheral tryptophan metabolism was investigated in rats. ALF was induced by administration of the hepatotoxin, thioacetamide (TAA). Brain KYNA levels were increased by TAA-induced ALF, and these increases were consistent with KYN levels in the brain, serum and liver. These results suggest that the ALF-induced increase in serum KYN contributes to the increase in brain KYNA via elevated KYN uptake within the brain. This increase in serum KYN level can be caused by the changes in tryptophan-2,3-dioxygenase activity in the liver and the immune-related activation of indoleamine-2,3-dioxygenase in extrahepatic tissues. These findings suggest that hepatic dysfunction may contribute to neurological and psychiatric diseases associated with increased KYNA levels.

Effect of dietary phytol on the expression of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase, a key enzyme of tryptophan-niacin metabolism, in rats

α-Amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) plays a key role in the regulation of NAD biosynthesis or the production of quinolinate from tryptophan (Trp). We investigated in this study the effect of phytol, a phytochemical known as a peroxisome proliferator-activated receptor α (PPARα) ligand, on NAD synthesis and ACMSD expression in rats. Male Sprague-Dawley rats were fed a diet containing 0.5%, 1%, or 2% phytol for 7 d. Phytol decreased the ACMSD activity and its mRNA expression in a dose-dependent manner in the liver. Phytol similarly and significantly suppressed ACMSD mRNA expression in primary rat hepatocytes. However, the mRNA expression of ACO (a known PPARα target gene) was higher in the low-phytol groups than in the high-phytol group in vivo and in vitro. Phytol increased the blood NAD level by suppressing ACMSD mRNA expression in the liver of the rats. It is possible that this mechanism occurred by the activation of PPARα and also of other transcriptional factors.

Down-regulation of alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase by polyunsaturated fatty acids in hepatocytes is not mediated by PPARalpha

BACKGROUND

alpha-Amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD) is a key enzyme in NAD biosynthesis from tryptophan. Dietary polyunsaturated fatty acids (PUFA) have been shown to suppress hepatic ACMSD activity and its mRNA level in rat. However the mechanism of the suppressive action has not been clarified yet. Although the phenomena that fatty acids suppress the expression of ACMSD in rat liver have been established in vivo experiment, it is still obscure whether the effect of fatty acids on the expression of the enzyme is caused by its direct or indirect action, because there have been very few investigations performed in vitro.

AIM OF THE STUDY

In this study, to examine whether down-regulation of ACMSD mRNA by PUFA involves peroxisome proliferator-activated receptor (PPAR) alpha mediated mechanism or not, we investigated the effect of PUFA on the ACMSD expression by using primary cultured rat hepatocytes.

METHODS

For this purpose we investigated the effect of PUFA (linoleic acid and eicosapentanoic acid) on the ACMSD mRNA level in primary-cultured rat hepatocytes and compared its effect with that of WY-14,643 (a PPARalpha agonist). After the incubation of hepatocytes with fatty acids, WY-14,643 and/or MK886 (a PPARalpha antagonist), mRNA levels of ACMSD and a peroxisome marker enzyme acyl-CoA oxidase (ACO) were determined by competitive reverse transcription-polymerase chain reaction (RT-PCR) method.

RESULTS

ACMSD mRNA level in primary hepatocytes were decreased by the incubation with high concentrations of linoleic acid, eicosapentaenoic acid (EPA) and WY-14,643. The appearance of ACO mRNA by WY-14,643 was remarkably increased, and those by linoleic acid and EPA were increased less than that by WY-14,643. Moreover, the suppression of ACMSD mRNA and the augmentation of ACO mRNA by WY-14,643 were inhibited by MK886, but the suppression by PUFA was not substantially affected by MK886.

CONCLUSIONS

The present study suggesting that the mechanism of decrease in ACMSD mRNA level by PUFA was different from that by WY-14,643, and that there would be any pathway other than PPARalpha mediated one for PUFA to regulate ACMSD expression.

Differential effects of dietary fatty acids on rat liver alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase activity and gene expression

Hepatic alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD; formerly termed picolinic carboxylase) [EC4.1.1.45] plays a key role in regulating NAD biosynthesis and the generation of quinolinate (quinolinic acid) from tryptophan. Quinolinate is a potent endogenous excitotoxin of neuronal cells. We previously reported that ingestion of fatty acids by rats leads to a decrease in their hepatic ACMSD activity. However, the mechanism of this phenomenon is not clarified. We previously purified ACMSD and cloned cDNA encoding rat ACMSD. Therefore, in this study, we examined the differential effect of fatty acids on ACMSD mRNA expression by Northern blot. Moreover, we measured quinolinic acid concentration in rats fed on fatty acid. When diets containing 2% level of fatty acid were given to male Sprague-Dawley rats (4 weeks old) for 8 days, long-chain saturated fatty acids and oleic acid did not affect ACMSD mRNA expression in the liver. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) strongly suppressed the liver ACMSD mRNA expression. In rats fed with high linoleic acid diet for 8 days, serum quinolinic acid was significantly increased as compared with the rats fed on a fatty acid-free diet under the condition of the approximately same calorie ingestion. These results suggest that the transcription level of ACMSD is modulated by polyunsaturated fatty acids, and suppressive potency of ACMSD mRNA is n-3 fatty acid family>linoleic acid (n-6 fatty acid)>saturated fatty acid. Moreover, this study provides the information that a high polyunsaturated fatty acid diet affects the production of quinolinic acid in serum by suppressing the ACMSD activity.

Dietary Linoleic Acid Suppresses Gene Expression of Rat Liver α-Amino-β-Carboxymuconate-ε-Semialdehyde Decarboxylase (ACMSD) and Increases Quinolinic Acid in Serum

Hepatic ACMSD [EC4.1.1.45] plays a key role in regulating NAD biosynthesis from tryptophan. We previously reported that ingestion of polyunsaturated fatty acids by rats leads to a decrease in their hepatic ACMSD activity. We purified ACMSD and cloned cDNA encoding rat ACMSD. Therefore, in this study, we examined whether dietary linoleic acid altered ACMSD gene expression and its protein level. Moreover we measured the tryptophan catabolite quinolinic acid level in rats. In the rats fed with linoleic acid, ACMSD mRNA and its protein levels in the liver were strongly suppressed and serum quinolinic acid was significantly increased as compared with the rats fed on a fat-free diet. These results suggest that the transcription level of ACMSD is modulated by linoleic acids or their metabolites and probably there is an inverse relationship between ACMSD activity and the production of quinolinic acid converted from tryptophan.