Transcriptional regulation of the human reduced folate carrier A1/A2 promoter: Identification of critical roles for the USF and GATA families of transcription factors

Clinical Implications of Folate Transport in the Central Nervous System

Folates are essential for key biosynthetic processes in mammalian cells and play a crucial role in the maintenance of central nervous system homeostasis. Mammals lack the metabolic capacity for folate biosynthesis; hence, folate requirements are largely met through dietary sources. To date, three major folate transport pathways have been characterized: the folate receptors (FRs), reduced folate carrier (RFC), and proton-coupled folate transporter (PCFT). This article reviews current knowledge on the role of folate transport systems in mediating folate delivery to vital tissues, particularly the brain, and how these pathways are modulated by various regulatory mechanisms. We will also briefly highlight the clinical significance of cerebral folate transport in relation to neurodevelopmental disorders associated with folate deficiency.

Reduced SP1-mediated transcriptional activation decreases expression of intestinal folate transporters in response to ethanol exposure

SCOPE

The study was designed to identify the regulatory mechanisms underlying the effects of ethanol exposure on intestinal folate transport and to investigate the reversibility of such effects.

METHODS AND RESULTS

Caco-2 cells were grown in control and ethanol containing medium for 96 h. Thereafter, one subgroup of cells was shifted on ethanol free medium and grown for next 72 h. For in vivo studies, rats were given 1g ethanol/kg body weight/day either for 3 or 5 months and after 3 months of ethanol treatment, one group of rats received no ethanol for 2 months. A significant decrease in folic acid transport as well as expression of folate transporters was observed on ethanol treatment and the effects were reversible upon removal of ethanol. Ethanol exposure had no impact on CpG island methylation of the folate transporters however, an increase in their mRNA half-life was observed that seems to be a homeostatic mechanism. Chromatin immunoprecipitation assay revealed a decrease in binding of SP1 transcription factor to the promoter regions of folate transporters.

CONCLUSION

Reduced binding of SP1 to the promoter region of folate transporters may be a part of the regulatory mechanism resulting in decreased expression of folate transporters on ethanol exposure.

Epigenetic alterations in folate transport genes in placental tissue from fetuses with neural tube defects and in leukocytes from subjects with hyperhomocysteinemia

The objectives of this study were to identify tissue-specific differentially methylated regions (T-DMR’s) in the folate transport genes in placental tissue compared with leukocytes, and from placental tissues obtained from normal infants or with neural tube defects (NTDs). Using pyrosequencing, we developed methylation assays for the CpG islands (CGIs) and the CGI shore regions of the folate receptor α (FOLR1), proton-coupled folate transporter (PCFT) and reduced folate carrier 1 (RFC1) genes. The T-DMRs differed in location for each gene and the difference in methylation ranged between 2 and 54%. A higher T-DMR methylated fraction was associated with a lower mRNA level of the FOLR1 and RFC1 genes. Methylation fractions differed according to RFC1 80G > A genotype in the NTD cases and in leukocytes from subjects with high total plasma homocysteine (tHcy). There were no differences in methylated fraction of folate transporter genes between NTD cases and controls. We suggest that T-DMRs participate in the regulation of expression of the FOLR1 and RFC1 genes, that the RFC1 80G > A polymorphism exerts a gene-nutrition interaction on DNA methylation in the RFC1 gene, and that this interaction appears to be most prominent in NTD-affected births and in subjects with high tHcy concentrations.

HINT1 inhibits beta-catenin/TCF4, USF2 and NFkappaB activity in human hepatoma cells

In this study we explored the relevance of Hint, a novel tumor suppressor gene, to human hepatoma. The human hepatoma cell lines Hep3B and HepG2 express very low levels of the HINT1 protein but the Huh7 cells express a relatively high level. In Hep3B and HepG2 cells, but not in Huh7 cells, the promoter region of Hint1 is partially methylated and treatment with 5-azadcdeoxycytidine increased expression of the HINT1 protein and Hint1 mRNA in Hep3B and HepG2 cells. Increased expression of HINT1 in HepG2 cells markedly inhibited their growth. It also inhibited the transcriptional activities of beta-catenin/TCF4, and USF2, and inhibited the expression of endogenous cyclin D1 and TGFbeta2. Furthermore, HINT1 co-immunoprecipitated with USF2 in extracts of Hep2 cells. HINT1 also inhibited NFkappaB transcription factor reporter activity and inhibited translocation of the endogenous p65 protein to the nucleus of HepG2 cells. Therefore, decreased expression of the Hint1 gene through epigenetic silencing may play a role in enhancing the growth of a subset of human hepatoma by increasing the expression of genes controlled by the transcription factors beta-catenin, USF2, and NFkappaB.

A humanized mouse model for the reduced folate carrier

The ubiquitously expressed reduced folate carrier (RFC) or SLC19A1 is recognized to be an essential transport system for folates in mammalian cells and tissues. In addition to its generalized role as a folate transporter, RFC provides specialized tissue functions including absorption across intestinal/colonic epithelia, transport across the basolateral membrane of renal proximal tubules, transplacental transport of folates, and folate transport across the blood-brain barrier. The human RFC (hRFC) gene is regulated by five major upstream non-coding regions (designated A1/A2, A, B, C, and D), each transcribed from a unique promoter. Altogether, at least 14 distinct hRFC transcripts can be envisaged in which different 5' untranslated regions (UTRs) are fused to a common splice acceptor region (positions -1 to -49) within the first coding exon with a common 1776bp coding sequence. The 5' non-coding regions are characterized by alternate transcription start sites, multiple splice forms, and selective tissue distributions. Alternate 5' UTRs impact mRNA stabilities and translation efficiencies, and result in synthesis of modified hRFC proteins translated from upstream AUGs. In this report, we describe production and characterization of transgenic mice (TghRFC1) containing a functional hRFC gene and of humanized mice in which the mRFC gene is inactivated and an active hRFC gene has been introduced. The mice appear to be healthy and to breed well. Analysis of tissue specificity of expression in both the TghRFC1 and humanized hRFC mice by real-time RT-PCR demonstrates that the hRFC gene is expressed with a specificity closely resembling that seen in human tissues. For the humanized hRFC mice, levels of B and A1/A2 5' UTRs predominated in all mice/tissues, thus resembling results in normal human tissues. Lower levels of A and C 5' UTRs were also detected. The availability of humanized mouse models for hRFC will permit investigators to address critical unanswered questions pertinent to human health and disease. These include the ability to analyze the hRFC gene in vivo, to control dietary and other environmental conditions that may impact levels of gene expression, and to control the genetics of the mice in order to assess the effects of hRFC gene alterations on tissue folate uptake and distribution, none of which can be easily achieved in human populations.

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

Cloning and functional characterization of a folate transporter from the nematode Caenorhabditis elegans

Two putative orthologs to the human reduced folate carrier (hRFC), folt-1 and folt-2, which share a 40 and 31% identity, respectively, with the hRFC sequence, have been identified in the Caenorhabditis elegans genome. Functional characterization of the open reading frame of the putative folt-1 and folt-2 showed folt-1 to be a specific folate transporter. Transport of folate by folt-1 expressed in a heterologous expression system showed an acidic pH dependence, saturability (apparent K(m) of 1.23 +/- 0.18 microM), a similar degree of inhibition by reduced and substituted folate derivatives, sensitivity to the anti-inflammatory drug sulfasalazine (apparent K(i) of 0.13 mM), and inhibition by anion transport inhibitors, e.g., DIDS. Knocking down (silencing) or knocking out the folt-1 gene led to a significant inhibition of folate uptake by intact living C. elegans. We also cloned the 5'-regulatory region of the folt-1 gene and confirmed promoter activity of the construct in vivo in living C. elegans. With the use of the transcriptional fusion construct (i.e., folt-1::GFP), the expression pattern of folt-1 in different tissues of living animal was found to be highest in the pharynx and intestine. Furthermore, folt-1::GFP expression was developmentally and adaptively regulated in vivo. These studies demonstrate for the first time the existence of a specialized folate uptake system in C. elegans that has similar characteristics to the folate uptake process of the human intestine. Thus C. elegans provides a genetically tractable model that can be used to study integrative aspects of the folate uptake process in the context of the whole animal level.

Molecular basis of antifolate resistance

Folates play a key role in one-carbon metabolism essential for the biosynthesis of purines, thymidylate and hence DNA replication. The antifolate methotrexate has been rationally-designed nearly 60 years ago to potently block the folate-dependent enzyme dihydrofolate reductase (DHFR) thereby achieving temporary remissions in childhood acute leukemia. Recently, the novel antifolates raltitrexed and pemetrexed that target thymidylate synthase (TS) and glycineamide ribonucleotide transformylase (GARTF) were introduced for the treatment of colorectal cancer and malignant pleural mesothelioma. (Anti)folates are divalent anions which predominantly use the reduced folate carrier (RFC) for their cellular uptake. (Anti)folates are retained intracellularly via polyglutamylation catalyzed by folylpoly-gamma-glutamate synthetase (FPGS). As the intracellular concentration of antifolates is critical for their pharmacologic activity, polyglutamylation is a key determinant of antifolate cytotoxicity. However, anticancer drug resistance phenomena pose major obstacles towards curative cancer chemotherapy. Pre-clinical and clinical studies have identified a plethora of mechanisms of antifolate-resistance; these are frequently associated with qualitative and/or quantitative alterations in influx and/or efflux transporters of (anti)folates as well as in folate-dependent enzymes. These include inactivating mutations and/or down-regulation of the RFC and various alterations in the target enzymes DHFR, TS and FPGS. Furthermore, it has been recently shown that members of the ATP-binding cassette (ABC) superfamily including multidrug resistance proteins (MRP/ABCC) and breast cancer resistance protein (BCRP/ABCG2) are low affinity, high capacity ATP-driven (anti)folate efflux transporters. This transport activity is in addition to their established facility to extrude multiple cytotoxic agents. Hence, by actively extruding antifolates, overexpressed MRPs and/or BCRP confer antifolate resistance. Moreover, down-regulation of MRPs and/or BCRP results in decreased folate efflux thereby leading to expansion of the intracellular folate pool and antifolate resistance. This chapter reviews and discusses the panoply of molecular modalities of antifolate-resistance in pre-clinical tumor cell systems in vitro and in vivo as well as in cancer patients. Currently emerging novel strategies for the overcoming of antifolate-resistance are presented. Finally, experimental evidence is provided that the identification and characterization of the molecular mechanisms of antifolate-resistance may prove instrumental in the future development of rationally-based novel antifolates and strategies that could conceivably overcome drug-resistance phenomena.

Effects of 5' untranslated region diversity on the posttranscriptional regulation of the human reduced folate carrier

The human RFC (hRFC) gene is regulated by five major 5' non-coding exons, characterized by alternate transcription start sites and splice forms. The result is up to 14 hRFC transcripts for which different 5' untranslated regions (UTRs) are fused to a common coding sequence. By in vitro translation assays with hRFC constructs corresponding to the major transcript forms, most of the forms were translated poorly. Upon expression of the 5'UTR-hRFC constructs in hRFC-null HeLa cells, a range of steady state hRFC proteins and transcripts were detected that reflected relative transcript stabilities and, to a lesser extent, translation efficiencies. Transcripts including 5' UTRs derived from non-coding exon A encoded a modified hRFC protein translated from an upstream initiation site. When this modified hRFC protein was expressed in hRFC-null K562 cells, there were only minor differences in surface targeting, stability, or transport function from wild type hRFC. Our results demonstrate an important role for posttranscriptional determinants of cellular hRFC levels and activity.

Transcriptional regulation of the human reduced folate carrier in childhood acute lymphoblastic leukemia cells

PURPOSE

The transcriptional regulation of the human reduced folate carrier (hRFC), involved in cellular uptake of methotrexate and reduced folates, was studied in childhood acute lymphoblastic leukemia (ALL). The hRFC gene is regulated by six noncoding exons (A1/A2 and A to E) and multiple promoters. In ALL, hRFC-A1/A2 and hRFC-B are the major transcript forms.

EXPERIMENTAL DESIGN

RNAs from 18 ALL lymphoblast specimens and 10 nonobese diabetic/severe combined immunodeficient ALL xenografts were assayed by real-time reverse transcription-PCR for hRFC-A1/A2 and hRFC-B transcripts and for transcripts encoding USF1, GATA1, Sp1, and Ikaros transcription factors. For the xenografts, gel shift and chromatin immunoprecipitation assays assessed transcription factor binding to the hRFC-A1/A2 and hRFC-B promoters. CpG methylation density within a 334-bp region, including the core hRFC-B promoter, was established by bisulfite sequencing. hRFC-A1/A2 and hRFC-B promoter polymorphisms were assayed by DNA sequencing.

RESULTS

For the 28 ALLs, hRFC-A1/A2 and hRFC-B transcripts spanned a 546-fold range. By chromatin immunoprecipitation and gel shift assays, binding was confirmed for USF1 and GATA1 for hRFC-A1/A2, and for Sp1, USF1, and Ikaros for hRFC-B. hRFC transcript levels correlated with those for GATA1 and USF1 for hRFC-A1/A2 and with Sp1 and USF1 transcripts for hRFC-B. CpG methylation in ALL did not correlate with hRFC-B transcripts. In 40 ALL and 17 non-ALL specimens, 2 cosegregating high-frequency polymorphisms (T-1309/C-1217 and C-1309/T-1217; allelic frequencies of 36% and 64%, respectively) were detected in the A1/A2 promoter; none were detected in promoter B. The hRFC-A1/A2 polymorphisms only slightly affected promoter activity.

CONCLUSIONS

Our results show a complex regulation of hRFC in ALL involving the hRFC-A1/A2 and hRFC-B promoters and noncoding exons. Although Sp1, USF1, and GATA1 levels are critical determinants of hRFC transcription in ALL, neither DNA methylation nor promoter polymorphisms contribute to differences in hRFC expression.