Vitamin B6 modulates expression of albumin gene by inactivating tissue-specific DNA-binding protein in rat liver

Potential Role of Amino Acid/Protein Nutrition and Exercise in Serum Albumin Redox State

Albumin is the major protein in the serum of mammals. It is synthesized exclusively in the liver, before being secreted into the circulation. Similar to skeletal muscle protein, albumin synthesis is stimulated by dietary amino acids and proteins as well as exercise. Albumin has three isoforms based on the redox states of the free cysteine residue at position 34. The redox state of serum albumin has long been extensively investigated in terms of oxidative stress-related chronic diseases, with the redox state of serum albumin having been regarded as a marker of systemic oxidative stress. However, according to recent animal studies, the redox state of serum albumin is modulated by albumin turnover and may also reflect amino acid/protein nutritional status. Furthermore, as the redox state of serum albumin is modulated by exercise training, measuring the pre- and post-exercise redox states of serum albumin in athletes may be useful in assessing amino acid/protein nutritional status and exercise-induced oxidative stress, which are closely associated with skeletal muscle adaptive responses. This article extensively reviews serum albumin and the redox state of albumin in the context of amino acid/protein nutritional status and exercise training.

Growth suppression and cell death by pyridoxal is dependent on p53 in the human breast cancer cell line MCF-7

Vitamin B6 compound, pyridoxine (PN), has shown antitumor action. Our previous experiments showed that PN induces expression of insulin-like growth factor binding protein-3 to arrest proliferation and induce cell death. This induction is inhibited by the p53-specific inhibitor pifithrin-α. Here, we report that another B6 compound, pyridoxal (PL), strongly inhibited MCF-7 cell growth compared to PN. PL induced the G0/G1 arrest and the accumulation of subG1 population. Although p53 mRNA was not changed by PL, 0.5 mM PL increased the protein level in MCF-7 cells. The cell growth suppression by 0.5 mM PL did not occur when p53 expression was knocked down using siRNA. Together, these data suggest that PL accumulate p53 and PL-induced cell growth suppression is dependent on p53 in MCF-7 breast cancer cells.

Vitamin B6Suppresses Growth of the Feline Mammary Tumor Cell Line FRM

Growth of FRM cells was inhibited by the addition of pyridoxine in a dose-dependent manner. Use of 5 mM pyridoxine caused an almost complete arrest of cell growth. Pyridoxal was as effective as pyridoxine, but pyridoxamine showed weak inhibitory action. Electron-microscopic examination of control cells revealed large nuclei and cellular membranes with villi, but, in pyridoxine-treated cells, condensed or degraded nuclei were observed. Many vacuoles and cholesterol crystals were widely distributed inside the cellular membrane of pyridoxine-treated cells. One of the vacuoles was identified as a lipid droplet. The DNA ladder was observed in the pyridoxine-treated cells. It is suggested that pyridoxine treatment of FRM cells causes cytolysis of cells by apoptosis.

Modulation of gene expression by vitamin B6

The physiologically active form of vitamin B6, pyridoxal 5'-phosphate (PLP), is known to function as a cofactor in many enzymic reactions in amino acid metabolism. Recent studies have shown that, apart from its role as a coenzyme, PLP acts as a modulator of steroid hormone receptor-mediated gene expression. Specifically, elevation of intracellular PLP leads to a decreased transcriptional response to glucocorticoid hormones, progesterone, androgens, and oestrogens. For example, the induction of cytosolic aspartate aminotransferase (cAspAT) in rat liver by hydrocortisone is suppressed by the administration of pyridoxine. The suppression of the cAspAT induction by pyridoxine is caused by a decrease in the expression of the cAspAT gene, which is brought about by inactivation of the binding activity of the glucocorticoid receptor to the glucocorticoid-responsive element in the regulatory region of the cAspAT gene. Vitamin B6 has recently been found to modulate gene expression not only for steroid hormone-responsive or PLP-dependent enzymes but also for steroid- and PLP-unrelated proteins such as serum albumin. Albumin gene expression was found to be modulated by vitamin B6 through a novel mechanism that involves inactivation of tissue-specific transcription factors, such as HNF-1 or C/EBP, by direct interaction with PLP in a similar manner to glucocorticoid receptor. Enhancement of albumin gene expression in the liver by an increased supply of amino acids can be explained by elevated binding of HNF-1 and C/EBP to their DNA-binding sites which, in turn, is caused by a decrease in the intracellular level of PLP by the increased amino acid supply. These findings that vitamin B6 acts as a physiological modulator of gene expression add a new dimension to the hitherto recognized function of vitamin B6 as a cofactor of enzyme action.

Liver-specific increase of UTP and UDP-sugar concentrations in rats induced by dietary vitamin B6-deficiency and its relation to complex N-glycan structures of liver membrane-proteins

This is the first known report on the influence of vitamin B6-deficiency on the concentration of UDP-sugars and other uracil nucleotides in rats. Animals aged 3 weeks or 2 months were fed a vitamin B6-free diet for periods varying from 3 days to 7 weeks. Nucleotides were quantified by enzymatic-photometry and by SAX-high precision liquid chromatography. In 3 week-old rats, vitamin B6-deficiency resulted in an up to 6.3-fold increase in the concentrations of UTP, UDP, UMP and UDP-sugars and less of CTP in rat liver, while no changes were observed in older rats. In young rats, the concentration of uracil nucleotides started to increase after 1 week diet, with a maximum after 2 weeks. After 5 weeks, the concentrations returned to normal values. In heart, lungs, kidney and brain, concentrations were measured after 2 weeks diet in young rats. In contrast to liver, the heart muscle uracil nucleotide concentrations were decreased by 40%. In kidney, the sum of UTP, UDP and UMP showed a decrease of 40%, whereas UDP-sugars were increased 1.4-fold. In the lungs, nucleotide concentrations were mostly unaffected by vitamin B6-deficiency, despite a 70% increase of UDP-GA. In brain, UDP-Glc, UDP-Gal and the sum of CTP and CDP showed an increase of 30-50%. We became surprised that the increased UDP-sugar concentrations did not influence the structure of liver plasma membrane-N-glycans. Despite the 4 to 6-fold increase of UTP and UDP-sugars, no changes in the complexity or sialylation of these N-glycans could be detected. This study demonstrates that, especially in liver, pyridoxal phosphate is closely involved in the control of uracil nucleotides during a defined period of development. In contrast to in vitro experiments, in vivo N-glycan biosynthesis in liver is regulated by a more complex or higher mechanism than substrate concentrations.

Effect of High Dose of Pyridoxine on Mammary Tumorigenesis

The effect of high-dose pyridoxine (PN) on mammary tumorigenesis was examined in female Sprague-Dawley rats. The first mammary tumors appeared between 84 and 90 days after 7,12-dimethylbenzanthracene treatment. There was no effect of PN level on tumor incidence at 90 days but at 98, 104, and 111 days. Tumor incidence was lower in the high-dose group (35 mg PN/kg diet) compared with the controls (7 mg PN/kg diet). All tumors were identified as adenocarcinoma and most as papillary type. The number of microcarcinomas in mammary glands of the 35-mg PN group tended to be reduce than that of the 7-mg group. The number of proliferating Ki67-positive cells was significantly reduced by supplementation with PN.

Activities of hepatic cytosolic and mitochondrial forms of serine hydroxymethyltransferase and hepatic glycine concentration are affected by vitamin B-6 intake in rats

Serine hydroxymethyltransferase (SHMT) is a pyridoxal phosphate (PLP)-dependent enzyme that exists as cytosolic and mitochondrial isozymes that catalyze the reversible interconversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methyleneTHF. SHMT is a major source of one-carbon units for cellular metabolism, but its sensitivity to various degrees of altered vitamin B-6 nutritional status has not been determined. In this study, cytosolic and mitochondrial SHMT activities were measured in liver from rats fed dietary pyridoxine (PN) ranging from adequate to deficient levels (2, 1, 0.5, 0.1, and 0 mg PN/kg diet; n = 10 per group). Both mitochondrial and cytosolic SHMT activities increased (P < 0.001) with increasing dietary PN over this range, and activities were a linear function of liver PLP concentration. Mitochondrial SHMT comprised approximately 70% of total activity. Assays conducted with and without in vitro addition of PLP indicated that total SHMT (apo- and holoenzyme forms) varied with dietary PN for each isoform, but that the proportion of each present as the apoenzyme was not affected by PN intake. This aspect of SHMT nutritional regulation differs from that of many other PLP-dependent enzymes. Hepatic glycine concentration was inversely related to vitamin B-6 intake (P < 0.05), which suggests a functional effect of altered SHMT activity. Overall these results demonstrate the potential for disruption of SHMT-mediated one-carbon metabolism by inadequate vitamin B-6 intake.

Pyridoxal 5'-phosphate inhibits DNA binding of HNF1

An efficient Escherichia coli expression system was constructed for the production of a variant form of HNF1 protein having the additional five amino acid residues (Asp-Arg-Trp-Gly-Ser) at the NH(2)-terminal. The cDNA encoding HNF1 was ligated to 6 x His tag and inserted into an inducible bacterial expression vector pRSET A. After expression in E. coli, the recombinant product was purified by Ni-NTA affinity column chromatography. The purified product showed expected NH(2)-terminal sequence and specific binding to the HNF1 site. The effect of pyridoxal 5'-phosphate and its analogues on the binding activity of recombinant HNF1 was examined and found that only pyridoxal 5'-phosphate effectively inhibited the DNA binding. The concentration of pyridoxal 5'-phosphate that inhibited 50% of DNA binding was around 100 microM. Furthermore, we identified Lys197 of HNF1 molecule as the essential residue of DNA binding. These observations suggest that pyridoxal 5'-phosphate directly interacts with tissue-specific transcription factor HNF1 and modulates the binding to DNA.

Thiamine deficiency decreases steady-state transketolase and pyruvate dehydrogenase but not alpha-ketoglutarate dehydrogenase mRNA levels in three human cell types

Reductions in the levels and activities of enzymes that utilize thiamine diphosphate (ThDP) as a cofactor are thought to be responsible for the tissue damage suffered during thiamine deficiency. Although loss of cofactor can account in part for loss of enzyme activity, thiamine and its phosphorylated derivatives may also regulate the expression of the genes encoding these proteins. To examine this possibility, steady-state mRNA levels for three ThDP-dependent enzymes were measured in human fibroblasts, lymphoblasts and neuroblastoma cells cultured under conditions of thiamine sufficiency and deficiency. In all three cell types, the mRNA levels of transketolase and the E1beta subunit of pyruvate dehydrogenase complex were lower in thiamine-deficient cultures. In contrast, mRNA levels for a ThDP-binding subunit of alpha-ketoglutarate dehydrogenase, the E1 subunit did not differ. These results indicate that thiamine or a thiamine metabolite regulates the expression in humans of some, but not all, genes encoding ThDP-utilizing enzymes.

Vitamin B6 suppresses growth and expression of albumin gene in a human hepatoma cell line HepG2

The effect of vitamin B6 on the growth of a human hepatoma cell line HepG2 in culture was studied. The growth of HepG2 cells and protein synthesis were almost completely inhibited in medium supplemented with 5 mM pyridoxine. Pyridoxal was as effective as pyridoxine, but pyridoxamine showed no inhibitory action. The growth inhibition of HepG2 cells by pyridoxine was accompanied by a marked inhibition of secretion of plasma proteins, particularly albumin. Northern blot analysis of albumin mRNA showed that pyridoxine caused a rapid decrease in the expression of albumin gene. The electron-microscopic examination of pyridoxine-treated HepG2 cells revealed a smoothing of nuclear membrane, a decrease in the number of nucleoli, and an appearance of aggregated heterochromatin structures. These morphological features are compatible with the depressed transcriptional activity in the pyridoxine-treated cells. The mechanism by which vitamin B6 exerts its inhibitory effect was discussed in terms of our recent finding that vitamin B6 modulates expression of albumin gene by inactivating tissue-specific DNA-binding proteins. Binding of pyridoxal phosphate with tissue-specific transcription factors may reduce the capacity of these factors to interact with the regulatory region of albumin gene, resulting in the inhibition of the gene expression.