Studies on 'orotic acid fatty liver'

Effects of Fatty Liver Induced by Excess Orotic Acid on B-Group Vitamin Concentrations of Liver, Blood, and Urine in Rats

Fatty liver is caused when rats are given orotic acid of the pyrimidine base in large quantities. The lack of B-group vitamins suppresses the biosynthesis of fatty acids. We investigated how orotic acid-induced fatty liver affects the concentrations of liver, blood, and urine B-group vitamins in rats. The vitamin B6 and B12 concentrations of liver, blood, and urine were not affected by orotic acid-induced fatty liver. Vitamin B2 was measured only in the urine, but was unchanged. The liver, blood, and urine concentrations of niacin and its metabolites fell dramatically. Niacin and its metabolites in the liver, blood, and urine were affected as expected. Although the concentrations of vitamin B1, pantothenic acid, folate, and biotin in liver and blood were decreased by orotic acid-induced fatty liver, these urinary excretion amounts showed a specific pattern toward increase. Generally, as for the typical urinary excretion of B-group vitamins, these are excreted when the body is saturated. However, the ability to sustain vitamin B1, pantothenic acid, folate, and biotin decreased in fatty liver, which is hypothesized as a specific phenomenon. This metabolic response might occur to prevent an abnormally increased biosynthesis of fatty acids by orotic acid.

Effects of fatty liver induced by niacin-free diet with orotic acid on the metabolism of tryptophan to niacin in rats

The effects of dietary orotic acid on the metabolism of tryptophan to niacin in weaning rats was investigated. The rats were fed with a niacin-free, 20% casein diet containing 0% (control diet) or 1% orotic acid diet (test diet) for 29 d. Retardation of growth, development of fatty liver, and enlargement of liver were observed in the test group in comparison with the control group. The concentrations of NAD and NADP in liver significantly decreased, while these in blood did not decrease compared to the control group. The formation of the upper metabolites of tryptophan to niacin such as anthranilic acid, kynurenic acid, and 3-hydroxyanthranilic acid were not affected, but the quinolinic acid and beyond, such as nicotinamide, N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide, were significantly reduced by the administration of orotic acid. Therefore, the conversion ratio of tryptophan to niacin significantly decreased in the test group in comparison with the control group.

Orotic acid, a new promoter for experimental liver carcinogenesis

Male Fischer 344 rats initiated with 1,2-dimethylhydrazine 2HCl (100 mg/kg) given 18 hr after partial hepatectomy and exposed to a diet containing 1% orotic acid for 13 months developed a 100% incidence of hepatocellular carcinoma. The creation of nucleotide pool imbalances by dietary orotic acid, for e.g., an increase in uridine nucleotides and a decrease in adenine nucleotides, was considered as a possible mechanism for the promotional effect of orotic acid on liver carcinogenesis. The significance of this hypothesis is that altered nucleotide pools affect both genomic as well as membrane organization. Consistent with this hypothesis is our finding that feeding rats with a diet containing 1% orotic acid for 10 weeks resulted in a liver DNA damage as monitored by its slower sedimentation in alkaline sucrose gradients compared to the corresponding controls. To assess the general applicability of this hypothesis, nucleotide pool imbalances were created by using methods other than feeding orotic acid and their effect on the incidence of gamma-glutamyltransferase positive foci in carcinogen initiated rats was determined. The results obtained indicated that rats initiated with 1,2-dimethylhydrazine.2HCl (100 mg/kg) given 18 hr after partial hepatectomy and exposed to diet deficient in arginine, a regimen that causes an increased synthesis and excretion of orotic acid, or were fed diets containing 1% thymidine or 1% thymine developed greater number of gamma-glutamyltransferase positive foci compared to the corresponding controls fed the basal diets. These results were interpreted to indicate that orotic acid exerts its promotional effect probably by creating an imbalance in nucleotide pools. One of the mechanisms by which an imbalance of nucleotide pools influences the pathogenesis of the carcinogenic process may be by inducing perturbations in the DNA.

The regulation of ribonucleoside diphosphate reductase by the tumor promoter orotic acid in normal rat liver in vivo

Our earlier studies have shown that in normal hepatocytes, orotic acid (OA) inhibits DNA synthesis induced by several growth factors in vitro and after two-thirds partial hepatectomy (PH) in vivo. As in the normal liver OA induces an imbalance in nucleotide pools (specifically, an increase in uridine nucleotides, including deoxyuridine nucleotides, and a decrease in adenosine nucleotides, including ATP) and creation of this imbalance is crucial for the mitoinhibitory effects of OA, we hypothesized that ribonucleoside diphosphate reductase (RNR), a key enzyme in DNA synthesis that is regulated by nucleotide/deoxynucleotide levels, might be one of the targets for the inhibition of DNA synthesis by OA. To test this hypothesis, we subjected male Fischer 344 rats (130-150 g) to two-thirds PH in the absence or in the presence of OA (a 300-mg tablet of OA methyl ester implanted intraperitoneally at the time of two-thirds PH). The rats were killed at different times later, and their livers were processed for analysis of levels of RNR enzyme activity, protein, and mRNA transcripts. The results obtained indicated that treatment with OA resulted in a near-100% inhibition of RNR induced by two-thirds PH in rat liver, as monitored by enzyme activity and protein level. Furthermore, this inhibition was paralleled by a decrease in the mRNA transcripts for both the M1 and M2 subunits of RNR. Nuclear run-off assays indicated that this decrease in the levels of mRNA transcripts could not be attributed to an effect on transcription. However, administration of OA 20 h after two-thirds PH, when RNR mRNA transcripts were maximally induced, resulted in increased degradation of the RNR M1 and M2 subunits. Taken together, these results indicate that OA treatment decreases RNR levels induced by two-thirds PH, at the levels of enzyme activity, protein, and mRNA transcripts, and the decreased levels of mRNA transcripts appeared to be due to increased degradation of the transcripts.

Ribonucleotide reductase: a possible target for orotic acid induced mitoinhibition in normal hepatocytes in primary culture

The present study was designed to determine the mechanism by which orotic acid, a rat liver tumor promoter, inhibits DNA synthesis in normal hepatocytes in primary culture. Our results indicate that orotic acid inhibited the epidermal growth factor induced expression (mRNA) of both M1 and M2 subunits of ribonucleotide reductase while the expression of c-fos, c-myc, c-Ha-ras and beta-actin was not inhibited to any significant extent. These studies suggest that ribonucleotide reductase may be one target for orotic acid-induced mitoinhibition.

Effect of orotic acid on the generation of reactive oxygen and on lipid peroxidation in rat liver

The pyrimidine precursor orotic acid (OA) is a constituent of dairy products and therapeutic drugs. Several recent publications point towards a tumor promoting activity of OA in rat liver. An increased production of reactive oxygen has been discussed as a possible mechanism, leading to lipid peroxidation and DNA single strand breaks. In view of contradictory results, this postulated prooxidative action of OA was reexamined with new experimental techniques. Weanling Sprague-Dawley rats were fed 1% OA in different diets for 4-35 days. The NADPH-mediated lipid peroxidation in liver homogenate and microsomes was determined in vitro by analysis of low-level chemiluminescence (CL) and the strongly correlated formation of malondialdehyde (MDA). In no case did treatment with OA result in an increase of lipid peroxidation in vitro nor did such treatment enhance the generation of reactive oxygen as measured by lucigenin CL. In accordance, the total cytochrome P-450 content as well as the activity of individual P-450 isoenzymes were unchanged. Treatment with OA did not elevate the MDA content of fresh liver homogenate when butylated hydroxytoluene (BHT) was present in the test system. However, when the antioxidant was omitted, increased levels of thiobarbituric acid reactive material were found which correlated with the triglyceride content. This could explain some published data that have been taken as indication for a prooxidative action of OA. Evidence against an increased lipid peroxidation in vivo is given by the analysis of ethane exhalation. Furthermore, no increase in DNA single strand breaks by OA treatment could be observed by the alkaline elution technique. These results do not support the hypothesis of a prooxidative activity of OA. The observed reversible decrease of the GSH/GSSG ratio is assumed to result from the reduced size of the phosphopyridine nucleotide pool due to purine deficiency and an increased consumption of NADPH by the enhanced reductive degradation of pyrimidines.

Studies on the effect of dietary orotic acid on mouse liver carcinogenesis induced by diethylnitrosamine

The present study was designed to determine whether orotic acid, a liver tumor promoter in the rat, also promotes liver carcinogenesis in the mouse. Eight-week-old male BALB/c mice were initiated with diethylnitrosamine (90 mg/kg i.p.). One week later they were divided into 2 groups and given either a basal diet or the basal diet containing 1% orotic acid (OA). They were killed at 6 or 10 months after the administration of the carcinogen. At 6 months, no nodular lesions were seen in mice whether or not they were exposed to OA. However by 10 months 100% of mice in both groups developed hepatic nodules. OA neither shortened the latent period for the appearance of the nodular lesions not did it increase the size of the nodules. Although BALB/c mice exhibited an increase in uridine nucleotides and a decrease in adenosine nucleotides in the liver upon exposure to OA, the magnitude of the change was less compared with that seen in the rat liver. The resistance of BALB/c mouse to the tumor-promoting effects of OA may reflect in part the resistance of the mouse to OA-induced nucleotide pool imbalance.

Sex difference in the development of fatty liver by orotic acid

Effects of orotic acid on liver lipid accumulation and incorporation of methionine [methyl-14C] into liver phosphatidylcholine and protein, and into serum beta-lipoprotein were studied. Male and female rats of Wistar strain were fed a semisynthetic diet supplemented with 1 per cent orotic acid for 7 days. Feeding of orotic acid induced a marked fatty liver in female rats, but not in males. In female rats, radioactivity in liver phosphatidylcholine was significantly decreased by orotic acid, and that in liver protein was slightly decreased. In male rats, incorporation of methionine [methyl-14C] into liver phosphatidylcholine and protein was unchanged between the control and the rats fed orotic acid. Radioactivity in serum beta-lipoprotein was decreased to a greater extent in female rats than in males. These results suggest that sex difference in the development of fatty liver may be due to the difference in the effect of orotic acid on liver phosphatidylcholine biosynthesis.