The role of the enterohepatic cycle in folate supply to tumour in rats

Gene specific epigenetic regulation of hepatic folate transport system is responsible for perturbed cellular folate status during aging and exogenous modulation

SCOPE

The present study was designed to identify the molecular mechanism of folate modulation and aging on aberrant liver folate transporter system.

METHODS AND RESULTS

An in vivo rat model was used, in which weanling, young and adult rats were given folate deficient diet for 3 and 5 months and after 3 months of folate deficiency, one group received physiological folate repletion (2 mg/kg diet) and another group received over supplemented folate diet (8 mg/kg diet) for another 2 months. In adult group, 3 and 5 months of folate deficiency decreased serum and tissue folate levels with decreased uptake of folate, further associated with decreased expression levels of reduced folate carrier (RFC) and increased expression levels of folate exporter (ABCG2) at both mRNA and protein levels, which in turn regulated by promoter hypermethylation of RFC and promoter hypomethylation of ABCG2 gene.

CONCLUSION

Promoter hypermethylation of RFC and promoter hypomethylation of ABCG2 may be attributed to the down regulation of RFC and up regulation of ABCG2 at mRNA and protein levels in conditions of 3 and 5 months of folate deficiency in the adult group.

Regulation at multiple levels control the expression of folate transporters in liver cells in conditions of ethanol exposure and folate deficiency

Complex regulatory mechanisms control the expression of folate transporters within cells. Liver is the primary reserve of the folate stores within the body. As excessive alcohol consumption or inefficient dietary folate intake are known to create folate deficiency, so therefore the current study was designed to explore various regulatory mechanisms controlling the expression of folate transport in liver cells in conditions of ethanol exposure and folate deficiency. In order to see whether the effects mediated by the treatments are reversible or not, ethanol removal, and folate repletion was done after ethanol exposure and folate deficiency treatment respectively. Folate deficiency resulted an increase, whereas ethanol treatment decreased the folic acid uptake within the cells. The alterations in folic acid uptake were in agreement with the observed changes in the expression of folate transporters. Ethanol exposure resulted an increase in promoter methylation of reduced folate carrier; however, folate deficiency had no effect. The effects produced by ethanol exposure and folate deficiency were found to be reversible in nature as depicted in case of ethanol removal and folate repletion group. Rate of synthesis of folate transporters was found to be increased whereas half lives of mRNA of folate transporters was found to be decreased on folate deficiency treatment and reverse was the case on ethanol treatment. Overall, alteration in the expression of folate transporters under ethanol exposure and folate deficient conditions can be attributed to those regulatory mechanisms which work at the mRNA level.

Decreased activity of folate transporters in lipid rafts resulted in reduced hepatic folate uptake in chronic alcoholism in rats

Folic acid is an essential nutrient that is required for one-carbon biosynthetic processes and for methylation of biomolecules. Deficiency of this micronutrient leads to disturbances in normal physiology of cell. Chronic alcoholism is well known to be associated with folate deficiency, which is due in part to folate malabsorption. The present study deals with the regulatory mechanisms of folate uptake in liver during chronic alcoholism. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20 % solution) orally for 3 months, and the molecular mechanisms of folate uptake were studied in liver. The characterization of the folate transport system in liver basolateral membrane (BLM) suggested it to be a carrier mediated and acidic pH dependent, with the major involvement of proton coupled folate transporter and folate binding protein in the uptake. The folate transporters were found to be associated with lipid raft microdomain of liver BLM. Moreover, ethanol ingestion decreased the folate transport by altering the Vmax of folate transport process and downregulated the expression of folate transporters in lipid rafts. The decreased transporter levels were associated with reduced protein and mRNA levels of these transporters in liver. The deranged folate uptake together with reduced folate transporter levels in lipid rafts resulted in reduced folate levels in liver and thereby to its reduced levels in serum of ethanol-fed rats. The chronic ethanol ingestion led to decreased folate uptake in liver, which was associated with the decreased number of transporter molecules in the lipid rafts that can be ascribed to the reduced synthesis of these transporters.

Net flux of folates and vitamin B12 through the gastrointestinal tract and the liver of lactating dairy cows

In study 1, four cows had a ruminal canula, a catheter in the right ruminal vein and an ultrasonic flow probe around the right ruminal artery; a catheter was placed in the auricular artery on experimental days. Blood samples were taken every 10 min from -20 to 60 min after ruminal infusion of 5.79 mmol pteroylmonoglutamic acid and cyanocobalamin. There was a net release of these vitamins across the rumen wall following the infusion (P=0.06). In studies 2 and 3, four cows had catheters in the portal, one hepatic and two mesenteric veins and one mesenteric artery. Plasma flow was determined using p-aminohippurate. In study 2, blood samples were taken before and every 30 min for 6 h after feeding 0 or 4 mg of pteroylmonoglutamic acid. Flow of folates through the portal-drained viscera (PDV) and the total splanchnic tissues (TSP) tended to increase with the ingestion of pteroylmonoglutamic acid (P=0.19). In study 3, blood samples were collected every 30 min for the first 3 h to calculate plasma flow and basal net fluxes of folates and vitamin B12. The cows were fed 2.6 g pteroylmonoglutamic acid and 500 mg cyanocobalamin; blood samples were taken every 2 h for 24 h. Vitamin supplements increased the net release of folates and vitamin B12 from PDV (P=0.04) and TSP (P=0.13). The present results demonstrate that, in dairy cows, at doses reported to improve animal performance, passage of pteroylmonoglutamic acid to the portal blood appears during the 6 h following its ingestion, whereas for cyanocobalamin, it is a slow process, not yet completed 24 h after its ingestion.

Folic acid

Folic acid is an essential nutrient from the B complex group of vitamins. Folate, as a cofactor, is involved in numerous intracellular reactions, and this is reflected in the various derivatives that have been isolated from biological sources. Folic acid is involved in single carbon transfer reactions and serves as a source of single carbon units in different oxidative states. The processes involved in the absorption, transport, and intracellular metabolism of this cofactor are complex. Much of folate is bound tightly to enzymes, indicating that there is not excess of this cofactor and that its cellular availability is protected as well as being strictly regulated. In animals, the liver controls the supply of folate through first pass metabolism, biliary secretion, enterohepatic recirculation, as well as through senescent erythrocyte recycling. Deficiencies of folate can occur for many reasons, including reduced intake, increased metabolism, and/or increased requirements as well as through genetic defects. The effects of folate deficiency include hyperhomocysteinemia, megaloblastic anemia, and mood disorders. Folate deficiency has also been implicated in disorders associated with neural tube defects. Supplementation of grain products such as cereals has been undertaken in several countries as a cost-effective means of reducing the prevelance of neural tube defects. Recently, common polymorphisms have been discovered in several genes associated with folate pathways that may play a role in diseases associated with folate deficiency, particularly mild folate deficiency.

Kinetic profile of overall elimination of 5-methyltetrahydropteroylglutamate in rats

The in vivo biliary and urinary excretion kinetics of 5-methyltetrahydropteroylglutamate (5-CH3-H4PteGlu) were studied in rats. During infusion at various rates (48-965 nmol. h-1. kg-1), the total body clearance (CLtotal) of 5-CH3-H4PteGlu could be attributed almost entirely to the sum of the biliary and urinary (CLurine,p) excretion clearances. After a 4-h infusion at the highest rate, the 5-CH3-H4PteGlu in the liver was 10 times higher than the endogenous level, whereas its polyglutamate form did not increase, suggesting that most of the infused 5-CH3-H4PteGlu is not incorporated in the polyglutamate pool but is eliminated by excretion. The parallel increase in CLtotal and CLurine,p with the increase in infusion rate might result from saturation of reabsorption at the renal proximal tubules, since the urinary excretion clearance, defined with respect to the kidney concentration, also increased while the biliary excretion clearance, defined with respect to the liver concentration, remained almost constant. We conclude that the hepatobiliary excretion is a relatively low-affinity process with a constant clearance, whereas the renal tubular reabsorption is saturated at higher plasma 5-CH3-H4PteGlu concentration ( approximately 0.5 microM). Urinary excretion becomes the predominant elimination route for any excess 5-CH3-H4PteGlu in the body.

Reduced folate derivatives are endogenous substrates for cMOAT in rats

We examined the role of the canalicular multispecific organic anion transporter (cMOAT) in the biliary excretion of reduced folate derivatives in vivo and in vitro using normal [Sprague-Dawley rats (SDR)] and mutant [Eisai hyperbilirubinemic rats (EHBR)] rats whose cMOAT is hereditarily deficient. In vivo, the biliary excretion of endogenous tetrahydrofolate (H4PteGlu), 5-methyltetrahydrofolate (5-CH3-H4PteGlu), and 5,10-methylenetetrahydrofolate (5, 10-CH2-H4PteGlu) in EHBR was reduced to 8.2%, 1.9%, and 5.5% of those in SDR, respectively, whereas that of 10-formyltetrahydrofolate (10-HCO-H4PteGlu) was detected only in SDR and not in EHBR. Bile drainage caused reduction of endogenous plasma folate concentrations in SDR but not in EHBR. In vitro, significant ATP-dependent uptake of 3H-labeled 5-CH3-H4PteGlu into canalicular membrane vesicles was observed only in SDR. This ATP-dependent uptake was saturable with a Michaelis constant (Km) value of 126 microM, which was comparable with its inhibitor constant (Ki) value of 121 microM for the ATP-dependent uptake of a typical cMOAT substrate, 2,4-dinitrophenyl-S-glutathione (DNP-SG). Vice versa, DNP-SG inhibited the uptake of 5-CH3-H4PteGlu with a Ki of 35 microM, which was similar to its Km value. In addition, H4PteGlu and 5, 10-CH2-H4PteGlu also inhibited the ATP-dependent uptake of DNP-SG. These results indicate that 5-CH3-H4PteGlu and other derivatives are transported via cMOAT. Therefore, reduced folate derivatives are the first endogenous substrates for cMOAT that do not contain glutathione, glucuronide, or sulfate moieties.

Biliary excretion of biotin and biotin metabolites is quantitatively minor in rats and pigs

We sought to determine whether the biliary excretion of biotin contributes substantially to the overall excretion of the vitamin in mammals, and hence, whether metabolism by gut microorganisms could account for some metabolism of biotin administered parenterally. [carbonyl-14C]Biotin was injected intravenously into six rats; bile and urine were collected for 24 h after injection. In a study of five pigs, serum and bile were analyzed for endogenous biotin and metabolites. In rat bile and urine, biotin, bisnorbiotin, biotin-d,l-sulfoxide, bisnorbiotin methyl ketone and two unidentified compounds were quantitated. In bile, these six compounds accounted for only 1.9 +/- 0.2% of the administered 14C, but in urine they accounted for 60.6 +/- 4.1%. The metabolite and time profiles in bile were also strikingly different from those in urine. Only biotin, bisnorbiotin and biotin-d,l-sulfoxide were quantitated in pig bile and serum. The concentrations of biotin, bisnorbiotin and biotin-d,l-sulfoxide in bile were 6.9-14.7 times the concentrations in serum. However, the bile to serum ratios of biotin and metabolites were >99% less than those of bilirubin, which is actively excreted. These data provide evidence that the biliary excretion of biotin and metabolites is quantitatively negligible.

Identification of 10-formyltetrahydrofolate, tetrahydrofolate and 5-methyltetrahydrofolate as major reduced folate derivatives in rat bile

Reduced folate derivatives in rat bile were examined using high-performance liquid chromatography with electrochemical detection (HPLC-ED). Three peaks of folate compounds were observed on the chromatograms. From the retention-time profiles and hydrodynamic voltammograms, and the profiles of ultraviolet (UV) absorbance spectra obtained by HPLC with photodiode array detection, these 3 peaks were identified as 10-formyltetrahydrofolate (10-HCO-H4PteGlu), tetrahydrofolate (H4PteGlu) and 5-methyltetrahydrofolate (5-CH3-H4PteGlu). The rates of bile secretion of 10-HCO-H4PteGlu, H4PteGlu and 5-CH3H4PteGlu were 314 +/- 181, 321 +/- 179 and 449 +/- 198 ng/h (mean +/- S.D.), respectively. 10-HCO-H4PteGlu and H4PteGlu together wtih 5-CH3-H4-PteGlu are found to be the major folate derivatives in rat bile. The nonmethylated folates, 10-HCO-H4PteGlu and H4PteGlu, may also play an important role in folate homeostasis.

Effect of diet on folates levels and distribution in selected tissues of the rat

A technique for the extraction, purification and concentration of folates, followed by high performance liquid chromatography assay with fluorometric detection of the three principal derivatives (tetrahydrofolic, 5-methyl-tetrahydrofolic and formyl-tetrahydrofolic acids) has been applied to the determination of tissue folates in rats. The levels found are compared to those of the microbiological assay using Lactobacillus casei. When rats were fed a diet containing 1 mg of folic acid per kg of food, levels in the intestinal mucosae, liver, whole blood and brain were 0.59, 14.87, 0.28 and 0.83 nmol/g of tissue. An exogenous supply of 200 mg of folic acid/kg of food significantly increased folate levels in all tissues studied, except for the brain: 1.51, 28.93, 0.52 and 0.99 nmol/g, respectively in the above four tissues. The separation of the various derivatives and a variable supply of folic acid have enabled the conversions of these metabolites to be studied.

Quinazoline CB 3717 and CB 3703 inhibition of folate retention and metabolism in Ehrlich ascites carcinoma cells and some organs of the host-mouse

Disturbances in folate metabolism in Ehrlich ascites carcinoma-bearing mice caused by 2 quinazoline-antifolates are described. They are manifested by diminished retention in tumour cells and the host-liver of injected radioactive folate and by a decreased extent of its conversion to coenzymatic, polyglutamate forms. Kidney retention of folate was also affected by either quinazoline, showing a severalfold decrease in mice treated with CB 3703, and a 2-fold increase in those treated with CB 3717. Kidney folylpolyglutamate content was also greatly reduced.

Cellular abnormalities of folate deficiency

To trace the development of folate-deficient abnormalities of morphology and DNA synthesis, Friend erythroleukaemia cells were grown in media containing 10(2), 10(3) and 10(4) ng of [3H]PteGlu1/ml and then transferred to folate-free media. Parameters examined were: intracellular folate levels; growth potential; morphology; dU suppression; and DNA content by flow microfluorimetry. The most sensitive indicators of folate-deficient cell growth were those related to DNA synthesis (dU and flow microfluorimetry). These became abnormal at intracellular folate levels of 0.2-0.5 ng/10(6) cells and markedly so below 0.1 ng/10(6) cells. Morphological criteria were less sensitive. Cells became megaloblastic at intracellular folate levels below 0.06 ng/10(6). The capacity of the cells to replicate in folate-free media was a function of the intracellular folate (ICF): duplications = 4.01 + ln(ICF)/0.67 (r = 0.993, P less than 0.001). These studies demonstrate that regardless of initial intracellular folate levels, cellular stigmata of folate deficiency appear when cellular folate falls below 3 X 10(5) molecules per cell (dU and flow microfluorimetry) and cells lose the capacity for further replication below 7-10 X 10(5) molecules. The intracellular folate level not only predicts early defects, but also determines the replicative capacity.

Folate utilization in Friend erythroleukemia cells

In order to study the generation, factors controlling endogenous folate pools, and their functional importance, Friend erythroleukemia cells were grown in media containing 100; 1,000; and 10,000 ng/ml of tritiated pteroylglutamic acid (3H)PteGlu1 and then studied in unlabeled media with varying amounts of PteGlu1. The intracellular folate pool was directly proportional to the PteGlu1 in which the cells were incubated. At equilibrium, greater than 95% of the labeled intracellular folate pool chromatographed as polyglutamyl folate, regardless of the exogenous folate concentration. The functional importance of the intracellular folate pool was studied by varying the endogenous pool and the exogenous (media) supply. The ability of the cells to replicate in the absence of exogenous folate was directly proportional to the intracellular polyglutamyl folate pool. The maximal rate of replication, however, required exogenous PteGlu1 in addition. The cell doubling time was the most important determinant of intracellular folate turnover; changes in the intracellular pool size and the extracellular folate concentration had no effect on the turnover time. In a rapidly proliferating tissue, the onset of functional folate deficiency will be determined by dilution of intracellular polyglutamates among progeny until a critical level is reached.