Decreased Expression of Transporters Reduces Folate Uptake across Renal Absorptive Surfaces in Experimental Alcoholism

Folate pathways mediating the effects of ethanol in tumorigenesis

Folate and alcohol are dietary factors affecting the risk of cancer development in humans. The interaction between folate status and alcohol consumption in carcinogenesis involves multiple mechanisms. Alcoholism is typically associated with folate deficiency due to reduced dietary folate intake. Heavy alcohol consumption also decreases folate absorption, enhances urinary folate excretion and inhibits enzymes pivotal for one-carbon metabolism. While folate metabolism is involved in several key biochemical pathways, aberrant DNA methylation, due to the deficiency of methyl donors, is considered as a common downstream target of the folate-mediated effects of ethanol. The negative effects of low intakes of nutrients that provide dietary methyl groups, with high intakes of alcohol are additive in general. For example, low methionine, low-folate diets coupled with alcohol consumption could increase the risk for colorectal cancer in men. To counteract the negative effects of alcohol consumption, increased intake of nutrients, such as folate, providing dietary methyl groups is generally recommended. Here mechanisms involving dietary folate and folate metabolism in cancer disease, as well as links between these mechanisms and alcohol effects, are discussed. These mechanisms include direct effects on folate pathways and indirect mediation by oxidative stress, hypoxia, and microRNAs.

FASD: folic acid and formic acid — an unholy alliance in the alcohol abusing mother

Alcohol consumption during pregnancy remains a significant cause of preventable birth defects and developmental disabilities; however, the mechanism of toxicity remains unclear. Methanol is present as a congener in many alcoholic beverages and is formed endogenously. Because ethanol is preferentially metabolized over methanol, it has been found in the sera and cerebro-spinal fluid of alcoholics. Toxicity resulting from methanol has been attributed to formic acid. Formic acid is present in significantly higher quantities in the biofluids of alcoholics. These higher levels can be cytotoxic and cause neuronal cell death. However, the adverse effects can be mitigated by adequate levels of hepatic folic acid, because formic acid elimination depends on folic acid. During pregnancy, folate concentrations are at least 2-fold higher in cord blood then in maternal blood, owing to increased folate requirements. The reverse has been demonstrated in pregnancies with alcohol abuse, suggesting downregulation of folate transporters and low fetal folate levels. Moreover, formic acid can cross the placenta and its adverse effects can be mitigated by folic acid. Thus, the combination of low fetal folate levels and presence of formic acid form a potent cytotoxic combination that may play a significant role in the etiology of fetal alcohol spectrum disorder.

Detrimental effects of alcohol exposure around conception: putative mechanisms

In western countries, alcohol consumption is widespread in women of reproductive age, and in binge quantities. These countries also continue to have high incidences of unplanned pregnancies, with women often reported to cease drinking after discovering their pregnancy. This suggests the early embryo may be highly exposed to the detrimental effects of alcohol during the periconception period. The periconception and pre-implantation windows, which include maturation of the oocyte, fertilisation, and morphogenesis of the pre-implantation embryo, are particularly sensitive times of development. Within the oviduct and uterus, the embryo is exposed to a unique nutritional environment to facilitate its development and establish de-novo expression of the genome through epigenetic reprogramming. Alcohol has wide-ranging effects on cellular stress, as well as hormonal, and nutrient signalling pathways, which may affect the development and metabolism of the early embryo. In this review, we summarise the adverse developmental outcomes of early exposure to alcohol (prior to implantation in animal models) and discuss the potential mechanisms for these outcomes that may occur within the protected oviductal and uterine environment. One interesting candidate is reduced retinoic acid synthesis, as it is implicated in the control of epigenetic reprogramming and cell lineage commitment, processes that have adverse consequences for the formation of the placenta, and subsequently, fetal programming.

Changes to histone modifications following prenatal alcohol exposure: An emerging picture

Epigenetic mechanisms are important for facilitating gene-environment interactions in many disease etiologies, including Fetal Alcohol Spectrum Disorders (FASD). Extensive research into the role of DNA methylation and miRNAs in animal models has illuminated the complex role of these mechanisms in FASD. In contrast, histone modifications have not been as well researched, due in part to being less stable than DNA methylation and less well-characterized in disease. It is now apparent that even changes in transient marks can have profound effects if they alter developmental trajectories. In addition, many histone methylations are now known to be relatively stable and can propagate themselves. As technologies and knowledge have advanced, a small group has investigated the role of histone modifications in FASD. Here, we synthesize the data on the effects of prenatal alcohol exposure (PAE) on histone modifications. Several key points are evident. AS with most alcohol-induced outcomes, timing and dosage differences yield variable effects. Nevertheless, these studies consistently find enrichment of H3K9ac, H3K27me2,3, and H3K9me2, and increased expression of histone acetyltransferases and methyltransferases. The consistency of these alterations may implicate them as key mechanisms underlying FASD. Histone modification changes do not often correlate with gene expression changes, though some important examples exist. Encouragingly, attempts to reproduce specific histone modification changes are very often successful. We comment on possible directions for future studies, focusing on further exploration of current trends, expansion of time-point and dosage regimes, and evaluation of biomarker potential.

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.

Mechanistic insights of intestinal absorption and renal conservation of folate in chronic alcoholism

Folate mediated one-carbon metabolism is of fundamental importance for various cellular processes, including DNA synthesis and methylation of biological molecules. Due to the exogenous requirement of folate in mammals, there exists a well developed epithelial folate transport system for regulation of normal folate homeostasis. The intestinal and renal folate uptake is tightly and diversely regulated and disturbances in folate homeostasis like in alcoholism have pathological consequences. The study was sought to delineate the regulatory mechanism of folate uptake in intestine and reabsorption in renal tubular cells that could evaluate insights of malabsorption during alcoholism. The folate transporters PCFT and RFC were found to be associated with lipid rafts of membrane surfaces in intestine and kidney. Importantly, the observed lower intestinal and renal folate uptake was associated with decreased levels of folate transporter viz. PCFT and RFC in lipid rafts of intestinal and renal membrane surfaces. The decreased association of folate transporters in lipid rafts was associated with decreased protein and mRNA levels. In addition, immunohistochemical studies showed that alcoholic conditions deranged that localization of PCFT and RFC. These findings could explain the possible mechanistic insights that may result in folate malabsorption during alcoholism.

Adaptive transport of folic acid across renal epithelia in folate-deficient rats

Folate (vitamin B(9)) is an essential vitamin for a wide spectrum of biochemical reactions; however, unlike bacteria and plants, mammals are devoid of folate biosynthesis and thus must obtain this cofactor from exogenous sources. The activities of folate transporters on the kidneys play an important role in conserving folate excretion and reabsorption across the apical membrane of the renal proximal tubules. The different transport system activities may become identifiable in response to external stimuli, such as folate availability and exposure to chemotherapeutic agents. We have explored the effect of folate deficiency on the activity and expression of folate transporters in rat kidneys. Wistar rats were fed a folate-containing diet (2 mg folic acid kg(-1) diet) or a folic acid-free diet over a 3-month period, and mechanisms of folate transport were studied in renal brush border membrane vesicles and basolateral membrane vesicles. The renal folate uptake process is saturable and pH dependent, and it involves the folate receptor and reduced folate carrier (RFC) systems and possibly the proton coupled folate transporter (PCFT) system. We found that folate deficiency increased the renal brush border membrane and basolateral folate uptake by increasing the number of transporter molecules. The observed up-regulation of mRNA expression was also associated with a significant increase in RFC and PCFT expression at the protein 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.

The effects of alcohol on the metabolism and toxicology of anti-psoriasis drugs

INTRODUCTION

Alcohol has long been suspected to be a triggering and precipitating factor of psoriasis. Alcohol misuse is common in patients with moderate-to-severe psoriasis and appears to impair treatment outcome.

AREAS COVERED

In this article, the authors review the available data regarding the metabolic and toxicological interactions between anti-psoriasis systemic drugs and ethanol and/or alcoholic beverages. Special attention is given to the influence of alcohol consumption on the hepatotoxic risk of some anti-psoriasis drugs. The article was prepared using a MEDLINE literature search.

EXPERT OPINION

The available knowledge highlights the existence of a few significant pharmacological interactions, such as the reduced exposure to cyclosporine by red wine, the possible increase of cyclosporine levels following a heavy acute alcohol intake, and, especially, the conversion of acitretin to etretinate, in the presence of ethanol, with important implications in females of child-bearing potential. There are limited data on the contributing role of alcohol in the hepatotoxicity induced by some anti-psoriasis drugs and the existing information on this topic is still controversial. However, further investigation is needed to assess the relevance of interactions between alcohol consumption and drug therapy for psoriasis, under both pharmacological and toxicological perspectives. Long-term prospective studies on large cohorts of patients are warranted to disclose the actual significance of such potential interactions in clinical practice.

Folate malabsorption and its influence on DNA methylation during cancer development

The folate transport across the epithelial of the intestine, colon, kidney, and liver is essential for folate homeostasis. The relative localization of transporters in membranes is an important determinant for the vectorial flow of substrates across the epithelia. Folate deficiency is a highly prevalent vitamin deficiency in the world, and alcohol ingestion has been the major contributor. It can develop because of folate malabsorption in tissues, increased renal excretion dietary inadequacy, and altered hepatobiliary metabolism. Additionally, folate-mediated one-carbon metabolism is important for various cellular processes, including DNA synthesis and methylation. In this regard, the contribution of alcohol-associated and dietary folate deficiency to methylation patterns is under intense investigation, especially in cancer. The epigenetic events have increasing relevance in the development of strategies for early diagnosis, prevention, and treatment of cancer.

Effect of chronic alcohol exposure on folate uptake by liver mitochondria

Mammalian cells obtain folate, a water-soluble vitamin, from their surroundings via transport across cell membrane. Intracellular folate is compartmentalized between the cytoplasm and the mitochondria. Transport of folate from the cytoplasm into the mitochondria is via a specific carrier-mediated process involving the mitochondrial folate transporter (MFT). Chronic alcohol use negatively impacts folate homeostasis, but its effect on mitochondrial folate uptake is not clear. We addressed this issue using mitochondrial preparations isolated from the liver of rats chronically fed an alcohol liquid diet and from human liver HepG2 cells chronically exposed to alcohol. The results showed that chronic alcohol feeding of rats leads to a significant inhibition in mitochondrial carrier-mediated folate uptake. This inhibition was associated with a significant reduction in the level of expression of the MFT protein, mRNA, and heterogenous nuclear RNA (hnRNA). Similarly, chronic alcohol exposure (96 h) of HepG2 cells led to significant inhibition in mitochondrial carrier-mediated folate uptake, which was associated with a marked reduction in the level of expression of the human MFT (hMFT). To determine whether the latter effect is, in part, being exerted at the transcriptional level, we cloned the 5'-regulatory region of the human SLC25A32 gene (which encodes the hMFT) and showed that chronic alcohol exposure of HepG2 cells leads to a significant inhibition in its promoter activity. These studies show for the first time that chronic alcohol feeding/exposure leads to a significant inhibition in mitochondrial carrier-mediated folate uptake and that the inhibition is, in part, being exerted at the level of transcription of the SLC25A32 gene.

Effects of ethanol consumption on the B-group vitamin contents of liver, blood and urine in rats

Several studies have shown that blood vitamin levels are lower in alcoholic patients than in control subjects. Acute ethanol exposure enhances the release of vitamins from liver cells in vitro. The aim of the present study is to confirm the effects of ethanol consumption on vitamin contents in vivo. We compared the contents of B-group vitamins in the liver, blood and urine between ethanol-fed and control rats fed a diet containing a sufficient- and low-vitamin mixture. The experimental rats were fed a 15 % ethanol solution freely for 28 d, and then 24 h urine samples were collected, after which the animals were killed. The B-group vitamin contents in the liver, blood and urine were measured. No differences in liver, blood and urine contents were observed between the control and ethanol-fed rats fed a diet containing a sufficient-vitamin mixture. On the contrary, in rats fed a diet containing a low-vitamin mixture, consumption of ethanol caused a decrease in the contents of vitamins B1, B2 and pantothenic acid in the liver; however, the contents of the other vitamins did not decrease. In the blood, the contents of vitamins B1, B2, B6 and pantothenic acid were lower in the ethanol-fed rats than in the controls. Urinary excretion of the B-group vitamins, except for niacin, was lower in the ethanol-fed rats. These results show that ethanol consumption affects the absorption, distribution and excretion of each of the vitamins in rats fed a diet containing a low-vitamin mixture.

Alcohol-associated folate disturbances result in altered methylation of folate-regulating genes

Folate plays a critical role in maintaining normal metabolic, energy, differentiation and growth status of all mammalian cells. The steady-state accumulation of folate seems to depend on the activity of two enzymes: folylpolyglutamate synthetase (FPGS), which adds glutamate residues, and gamma-glutamyl hydrolase (GGH), which removes them, enabling it to be transported across the biological membranes. Overexpression of GGH and downregulation of FPGS would be expected to decrease intracellular folate in its polyglutamylated form, thereby increasing efflux of folate and its related molecules, which might lead to resistance to drugs or folate deficiency. The study was sought to delineate the activity of GGH and expression FPGS in tissues involved in folate homeostasis during alcoholism and the epigenetic regulation of these enzymes and transporters regulating intracellular folate levels. We determined the activity of GGH and expression of FPGS in tissues after 3 months of ethanol feeding to rats at 1 g/kg body weight/day. The results showed that there was not any significant change in the activity of folate hydrolyzing enzyme GGH in ethanol-fed rats while there was significant down regulation in the expression of FPGS. Ethanol feeding decreased the total as well as polyglutamated folate levels. There was tissue-specific hyper/hypo methylation of folate transporter genes viz. PCFT and RFC by chronic ethanol feeding. Moreover, hypermethylation of FPGS gene was observed in intestine and kidney without any change in methylation levels of GGH in the ethanol-fed rats. In conclusion, the initial deconjugation of polyglutamylated folate by GGH was not impaired in ethanol-fed rats while the conversion of monoglutamylated folate to polyglutamylated form might be impaired. There was tissue-specific altered methylation of folate transporter genes by chronic ethanol feeding.

Biochemical and molecular mechanisms of folate transport in rat pancreas; interference with ethanol ingestion

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 mechanistic insights of reduced folate absorption in pancreas during chronic alcoholism. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20% solution) orally for 3 months and the mechanisms of alcohol associated reduced folate uptake was studied in pancreas. The folate transport system in the pancreatic plasma membrane (PPM) was found to be acidic pH dependent one. The transporters proton coupled folate transporter (PCFT) and reduced folate carrier (RFC) are involved in folate uptake across PPM. The folate transporters were found to be associated with lipid raft microdomain of the PPM. Ethanol ingestion decreased the folate transport by reducing the levels of folate transporter molecules in lipid rafts at the PPM. The decreased transport efficiency of the PPM was reflected as reduced folate levels in pancreas. The chronic ethanol ingestion led to decreased pancreatic folate uptake. The decreased levels of PCFT and RFC expression in rat PPM were due to decreased association of these proteins with lipid rafts (LR) at the PPM.

Subcellular localization and distribution of the reduced folate carrier in normal rat tissues

The reduced folate carrier (Rfc1; Slc19a1) mediated transport of reduced folates and antifolate drugs such as methotrexate (MTX) play an essential role in physiological folate homeostasis and MTX cancer chemotherapy. As no systematic reports are as yet available correlating Rfc1 gene expression and protein levels in all tissues crucial for folate and antifolate uptake, storage or elimination, we investigated gene and protein expression of rat Rfc1 (rRfc1) in selected tissues. This included the generation of a specific anti-rRfc1 antibody. Rabbits were immunised with isolated rRfc1 peptides producing specific anti-rRfc1 antiserum targeted to the intracellular C-terminus of the carrier. Using RT-PCR analysis, high rRfc1 transcript levels were detected in colon, kidney, brain, thymus, and spleen. Moderate rRfc1 gene expression was observed in small intestine, liver, bone marrow, lung, and testes whereas transcript levels were negligible in heart, skeletal muscle or leukocytes. Immunohistochemical analyses revealed strong carrier expression in the apical membrane of tunica mucosa epithelial cells of small intestine and colon, in the brush-border membrane of choroid plexus epithelial cells or in endothelial cells of small vessels in brain and heart. Additionally, high rRfc1 protein levels were localized in the basolateral membrane of renal tubular epithelial cells, in the plasma membrane of periportal hepatocytes, and sertoli cells of the testes. Taken together, our results demonstrated that rRfc1 is expressed almost ubiquitously but to very different levels. The predominant tissue distribution supports the essential role of Rfc1 in physiological folate homeostasis. Moreover, our results may contribute to understand antifolate pharmacokinetics and selected organ toxicity associated with MTX chemotherapy.

Reduced levels of folate transporters (PCFT and RFC) in membrane lipid rafts result in colonic folate malabsorption in chronic alcoholism

We studied the effect of chronic ethanol ingestion on folate transport across the colonic apical membranes (CAM) in rats. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20%) solution orally for 3 months and folate transport was studied in the isolated colon apical membrane vesicles. The folate transport was found to be carrier mediated, saturable, with pH optima at 5.0. Chronic ethanol ingestion reduced the folate transport across the CAM by decreasing the affinity of transporters (high Km) for the substrate and by decreasing the number of transporter molecules (low Vmax) on the colon luminal surface. The decreased transport activity at the CAM was associated with down-regulation of the proton-coupled folate transporter (PCFT) and the reduced folate carrier (RFC) which resulted in decreased PCFT and RFC protein levels in the colon of rats fed alcohol chronically. Moreover, the PCFT and the RFC were found to be distributed in detergent insoluble fraction of the CAM in rats. Floatation experiments on Optiprep density gradients demonstrated the association of the PCFT and the RFC protein with lipid rafts (LR). Chronic alcoholism decreased the PCFT and the RFC protein levels in the CAM LR in accordance with the decreased synthesis. Hence, we propose that downregulation in the expression of the PCFT and the RFC in colon results in reduced levels of these transporters in colon apical membrane LR as a mechanism of folate malabsorption during chronic alcoholism.

Regulatory mechanisms of intestinal folate uptake in a rat model of folate oversupplementation

Folic acid is essential for numerous biological functions, ranging from nucleotide biosynthesis to the remethylation of homocysteine. Folic acid is unable to cross the biological membranes by simple diffusion, so there exists a well-developed epithelial folate transport system for the regulation of normal folate homeostasis in the intestine. Any perturbances in the folate uptake system might lead to a state of folate deficiency, which in turn is strongly associated with the risk of various cancers, birth defects and CVD. Countries with obligatory folate fortification of food (USA and Canada) have documented a significant decrease in neural tube defects in newborns. However, the effect of folate oversupplementation on the intestinal absorption of folic acid has not been studied. We studied the process of folate transport and the expression of folate transporters in the rat intestine after folate oversupplementation. Rats were oversupplemented with tenfold the normal requirement of folic acid for periods of 10 and 60 d. Folate uptake in intestinal brush-border membrane vesicles followed saturable kinetics with pH optimum at 5·5. Acute, but not chronic, folate oversupplementation led to a significant down-regulation in intestinal folate uptake at acidic pH optima and was associated with a decrease in Vmax without any significant change in the Km of the folate uptake process. The decrease in folate uptake was also associated with the down-regulation in the protein levels of major folate transporters, proton-coupled folate transporter (PCFT) and reduced folate carrier (RFC), without altering their mRNA levels. Hence, it was concluded that acute folate oversupplementation results in a significant decrease in intestinal folate uptake by down-regulating the expressions of RFC and PCFT, via some post-transcriptional or translational mechanisms.

Effect of chronic alcohol feeding on physiological and molecular parameters of renal thiamin transport

The renal thiamin reabsorption process plays an important role in regulating thiamin body homeostasis and involves both thiamin transporters-1 and -2 (THTR1 and THTR2). Chronic alcohol use is associated with thiamin deficiency. Although a variety of factors contribute to the development of this deficiency, effects of chronic alcohol use on renal thiamin transport have not been thoroughly examined. We addressed this issue by examining the effect of chronic alcohol feeding of rats with liquid diet on physiological and molecular parameters of renal thiamin transport. Chronic alcohol feeding caused a significant inhibition in carrier-mediated thiamin transport across the renal brush-border membrane and was evident as early as 2 wk after initiation of alcohol feeding. Similarly, thiamin transport across the renal basolateral membrane was significantly inhibited by chronic alcohol feeding. The inhibition in renal thiamin transport was associated with a marked decrease in the level of expression of THTR1 and -2 proteins, mRNAs, and heterogeneous nuclear RNAs. Chronic alcohol feeding also caused a significant reduction in the level of expression of thiamin pyrophosphokinase but not that of the mitochondrial thiamin pyrophosphate transporter. These studies show that chronic alcohol feeding inhibits the entry and exit of thiamin in the polarized renal epithelial cells and that the effect is, at least in part, mediated at the transcriptional level. These findings also suggest that chronic alcohol feeding interferes with the normal homeostasis of thiamin in renal epithelial cells.

Genome-wide association study of alcohol dependence

CONTEXT

Alcohol dependence is a serious and common public health problem. It is well established that genetic factors play a major role in the development of this disorder. Identification of genes that contribute to alcohol dependence will improve our understanding of the mechanisms that underlie this disorder.

OBJECTIVE

To identify susceptibility genes for alcohol dependence through a genome-wide association study (GWAS) and a follow-up study in a population of German male inpatients with an early age at onset.

DESIGN

The GWAS tested 524,396 single-nucleotide polymorphisms (SNPs). All SNPs with P < 10(-4) were subjected to the follow-up study. In addition, nominally significant SNPs from genes that had also shown expression changes in rat brains after long-term alcohol consumption were selected for the follow-up step.

SETTING

Five university hospitals in southern and central Germany.

PARTICIPANTS

The GWAS included 487 male inpatients with alcohol dependence as defined by the DSM-IV and an age at onset younger than 28 years and 1358 population-based control individuals. The follow-up study included 1024 male inpatients and 996 age-matched male controls. All the participants were of German descent.

MAIN OUTCOME MEASURES

Significant association findings in the GWAS and follow-up study with the same alleles.

RESULTS

The GWAS produced 121 SNPs with nominal P < 10(-4). These, together with 19 additional SNPs from homologues of rat genes showing differential expression, were genotyped in the follow-up sample. Fifteen SNPs showed significant association with the same allele as in the GWAS. In the combined analysis, 2 closely linked intergenic SNPs met genome-wide significance (rs7590720, P = 9.72 x 10(-9); rs1344694, P = 1.69 x 10(-8)). They are located on chromosome region 2q35, which has been implicated in linkage studies for alcohol phenotypes. Nine SNPs were located in genes, including the CDH13 and ADH1C genes, that have been reported to be associated with alcohol dependence.

CONCLUSIONS

This is the first GWAS and follow-up study to identify a genome-wide significant association in alcohol dependence. Further independent studies are required to confirm these findings.

New perspectives on folate transport in relation to alcoholism-induced folate malabsorption--association with epigenome stability and cancer development

Folates are members of the B-class of vitamins, which are required for the synthesis of purines and pyrimidines, and for the methylation of essential biological substances, including phospholipids, DNA, and neurotransmitters. Folates cannot be synthesized de novo by mammals; hence, an efficient intestinal absorption process is required. Intestinal folate transport is carrier-mediated, pH-dependent and electroneutral, with similar affinity for oxidized and reduced folic acid derivatives. The various transporters, i.e. reduced folate carrier, proton-coupled folate transporter, folate-binding protein, and organic anion transporters, are involved in the folate transport process in various tissues. Any impairment in uptake of folate can lead to a state of folate deficiency, the most prevalent vitamin deficiency in world, affecting 10% of the population in the USA. Such impairments in folate transport occur in a variety of conditions, including chronic use of ethanol, some inborn hereditary disorders, and certain diseases. Among these, ethanol ingestion has been the major contributor to folate deficiency. Ethanol-associated folate deficiency can develop because of dietary inadequacy, intestinal malabsorption, altered hepatobiliary metabolism, enhanced colonic metabolism, and increased renal excretion. Ethanol reduces the intestinal and renal uptake of folate by altering the binding and transport kinetics of folate transport systems. Also, ethanol reduces the expression of folate transporters in both intestine and kidney, and this might be a contributing factor for folate malabsorption, leading to folate deficiency. The maintenance of intracellular folate homeostasis is essential for the one-carbon transfer reactions necessary for DNA synthesis and biological methylation reactions. DNA methylation is an important epigenetic determinant in gene expression, in the maintenance of DNA integrity and stability, in chromosomal modifications, and in the development of mutations. Ethanol, a toxin that is consumed regularly, has been found to affect the methylation of DNA. In addition to its effect on DNA methylation due to folate deficiency, ethanol could directly exert its effect through its interaction with one-carbon metabolism, impairment of methyl group synthesis, and affecting the enzymes regulating the synthesis of S-adenosylmethionine, the primary methyl group donor for most biological methylation reactions. Thus, ethanol plays an important role in the pathogenesis of several diseases through its potential ability to modulate the methylation of biological molecules. This review discusses the underlying mechanism of folate malabsorption in alcoholism, the mechanism of methylation-associated silencing of genes, and how the interaction between ethanol and folate deficiency affects the methylation of genes, thereby modulating epigenome stability and the risk of cancer.

Folate status in various pathophysiological conditions

Folate is the generic term for compounds that have vitamin activity similar to that of pteroylglutamic acid. Folate acts as a coenzyme in several single carbon transfers involved in biosynthesis of purine nucleotides and deoxythymidylic acid essential for DNA and RNA synthesis. In addition, folate provides one-carbon unit for methylation of a wide variety of biological substances including DNA, proteins, phospholipids, and neurotransmitters, thereby regulating their function. Recent epidemiological-clinical and experimental studies suggest the association of folate deficiency with the risk of various cancers, birth defects, and cardiovascular diseases. Thus, it is important to consider the conditions that are associated with altered folate status and their consequences. The impairment in folate status has been found in number of pathophysiological conditions like inflammatory bowel disease, cancer, alcoholism, pregnancy, neonatal growth, and during administration of some drugs. The recent advances dealing with mechanistic aspects of impaired folate status in these conditions have been discussed in this review.