Increased expression and genomic organization of a folate-binding protein homologous to the human placental isoform in L1210 murine leukemia cell lines with a defective reduced folate carrier

A novel splice variant of FR4 predominantly expressed in CD4+CD25+ regulatory T cells

Folate receptor 4 (FR4) is recently found as a lymphoid tissue specific protein. In this study, we have identified an alternative splicing variant of the FR4 gene from murine splenocytes, termed FR4v, which is almost identical to FR4 cDNA sequence except with the retained 108 bp intron 3 between exon 3 and 4 of FR4 gene. FR4v mRNA encodes a larger protein than FR4 and is constitutively expressed on CD4(+)CD25(+) regulatory T cell (Treg) membrane via a GPI anchor mechanism. Whether FR4v plays a redundant or unique functional role in Tregs should be investigated further in the future.

The role of folate receptor alpha in cancer development, progression and treatment: cause, consequence or innocent bystander?

Folate receptor alpha (FRalpha) is a membrane-bound protein with high affinity for binding and transporting physiologic levels of folate into cells. Folate is a basic component of cell metabolism and DNA synthesis and repair, and rapidly dividing cancer cells have an increased requirement for folate to maintain DNA synthesis, an observation supported by the widespread use of antifolates in cancer chemotherapy. FRalpha levels are high in specific malignant tumors of epithelial origin compared to normal cells, and are positively associated with tumor stage and grade, raising questions of its role in tumor etiology and progression. It has been suggested that FRalpha might confer a growth advantage to the tumor by modulating folate uptake from serum or by generating regulatory signals. Indeed, cell culture studies show that expression of the FRalpha gene, FOLR1, is regulated by extracellular folate depletion, increased homocysteine accumulation, steroid hormone concentrations, interaction with specific transcription factors and cytosolic proteins, and possibly genetic mutations. Whether FRalpha in tumors decreases in vivo among individuals who are folate sufficient, or whether the tumor's machinery sustains FRalpha levels to meet the increased folate demands of the tumor, has not been studied. Consequently, the significance of carrying a FRalpha-positive tumor in the era of folic acid fortification and widespread vitamin supplement use in countries such as Canada and the United States is unknown. Epidemiologic and clinical studies using human tumor specimens are lacking and increasingly needed to understand the role of environmental and genetic influences on FOLR1 expression in tumor etiology and progression. This review summarizes the literature on the complex nature of FOLR1 gene regulation and expression, and suggests future research directions.

Structure of the genes for porcine endometrial secreted and membrane folate binding proteins

The endometrium of the pig produces two types of folate binding proteins (FBP) which, based on their sequences, are likely to be membrane (m) and secreted (s) forms. A clone containing both a gene coding for the sFBP cDNA and a gene coding for the mFBP was isolated from a yeast artificial chromosome (YAC) library. Each gene was subcloned and sequenced. The gene for sFBP spanned 4.4 kbp and included 5 exons. The mFBP gene spanned 7.0 kbp and also contained 5 exons. Structures of the genes were very similar for the last three exons, and this similarity was shared with other known FBP/folate receptor (FR) gene sequences. Unexpectedly, portions of introns 3 and 4 of both genes were highly homologous, suggesting the possibility that sequences within these introns served some as yet unknown function. In contrast, the structures of the 5' exons differed between the two genes and other known FBP/FR genes. Comparison of putative promoter regions for the two genes with promoter regions for human FBP/FR genes revealed significant sequence homology between sFBP and human gammaFBP and between mFBP and human alphaFR. These regions of homology may play a role in control of transcription of each gene.

Adaptive regulation of intestinal folate uptake: effect of dietary folate deficiency

Folate is an essential micronutrient that, in mammals, must be obtained from exogenous sources via intestinal absorption. Previous studies have characterized different aspects of the mechanism of the intestinal folate uptake process. Much less, however, is known about regulation of this process. In this study, we examined the effect of dietary folate deficiency on intestinal folate uptake using the rat as an animal model. The results showed that dietary folate deficiency leads to a significant (P < 0.01) and specific upregulation in the transepithelial transport of folic acid. The upregulation in transepithelial folate transport 1) was found to be due to an induction in carrier-mediated folate uptake across the brush-border membrane (BBM) and was mediated via a significant (P < 0.01) increase in the maximal velocity but not the apparent Michaelis constant of the uptake process, 2) was associated with a marked increase in the steady-state mRNA level of reduced folate carrier-1 and in the level of the expressed protein at the intestinal BBM, and 3) was associated with a marked (>10-fold) increase in the activity of the intestinal BBM form of folate hydrolase. Results of this study demonstrate, for the first time, that dietary folate deficiency leads to a marked upregulation in intestinal folate uptake and in the activity of folate hydrolase. Furthermore, the upregulation in folate uptake is associated with an increase in mRNA and protein levels of folate carrier, suggesting possible involvement of a transcriptional regulatory mechanism(s) in the upregulation.

Tissue folate binding protein levels in transgenic mice with tumors and in non-transgenic controls

Localized folate deficiency may be a risk factor for cancer. Since, folate binding proteins (FBP) and reduced folate carrier proteins (RFC) mediate cellular transport of folate, we compared FBP concentrations in several organs from tumor-bearing transgenic (TBT) mice and tumor-free non-transgenic controls (NTC) of the same strain, age, and fed identical diets. Liver, spleen, brain, small intestine and kidney were individually homogenized in phosphate-buffered saline (PBS) and separated into membrane, cytoplasmic, mitochondrial/lysomal and nuclear fractions (confirmed with marker enzymes). Homogenates and fractions was analyzed for total protein, and FBP. We used rabbit anti-bovine milk antibody and ELISA to measure FBP. FBP concentrations in kidney, small intestine, and spleen of TBT mice were higher than those of NTC mice; the opposite was true in liver and lung. FBP seemed to be upregulated in kidneys (all fractions), small intestine (all fractions), and spleen (cytoplasmic and nuclear fractions only) of TBT mice compared to NTC mice; the opposite appeared true in liver (all fractions) and lung (all fractions). FBP concentrations in brain, heart, and muscle of TBT mice were not different from those in brain, heart and muscle of NTC mice. A longitudinal study will determine if these changes in FBP concentrations precede tumor onset.

Retinoic acid-dependent upregulation of mouse folate receptor-alpha expression in embryonic stem cells, and conservation of alternative splicing patterns

The action of retinoic acid (RA) in normal development and differentiation is mediated by changes in the expression of RA-responsive target genes. We used differential display reverse transcriptase polymerase chain reaction (RT-PCR) to identify RA-responsive genes expressed in embryonic stem (ES) cells and found that murine folate receptor-alpha (FR-alpha) expression is rapidly induced by RA treatment. The observed increase in FR-alpha expression occurs within 3h, is independent of protein synthesis and does not occur when ES cells are differentiated by removal of leukaemia inhibitory factor (LIF), evidence that the response to RA is both direct and specific. Two messenger RNA (mRNA) isoforms of FR-alpha featuring novel sequence in the 5' untranslated region (UTR) were cloned, and both were found to be upregulated by RA. Other splice variants in both the 5' UTR and 3' UTR of FR-alpha mRNA were also identified. There is a striking similarity between these splicing patterns and those reported for human FR-alpha, which also generates multiple isoforms by alternative splicing in the 5' and 3' UTR. The conservation of these splicing patterns in the non-coding regions of the FR-alpha gene between mouse and human suggests that these regions, and in particular the 5' UTR, play an important role in regulating expression of this gene.

Concentrating capacity of the human reduced folate carrier (hRFC1) in human ZR-75 breast cancer cell lines

Human RFC1 (hRFC1) transfected in transport-deficient methotrexate MTXR(R)ZR-75-1 human breast carcinoma cells (MTX(R)ZR-75/RFC) were used to investigate the impact of hRFC1 overexpression on influx and concentrative transport of methotrexate (MTX). Eight-fold overexpression of hRFC1, as determined by northern analysis, resulted in a 4-fold increase in MTX influx accompanied by a 2.4-fold increase in the steady-state level of free drug as compared with wild-type ZR-75-1 cells when the extracellular MTX level was 0.5 microM. When extracellular MTX was increased to 10 microM, the increase in influx equaled the increase in the transmembrane chemical gradient for MTX in the transfectant relative to wild-type cells. By 50 min, approximately 16-20 and 25% of the intracellular 3H represented MTX polyglutamates by HPLC analysis at [MTX]e = 0.5 and 10 microM in wild-type and transfected cells, respectively. Overexpression of hRFC1 enhanced sensitivity to MTX in MTX(R)ZR-75-1 cells by more than 250-fold. The data indicate that overexpression of hRFC1 in human cells results in comparable increases in influx and transmembrane gradients. This is different from what was reported when mouse RFC1 was transfected into murine leukemia cells, resulting in large, more symmetrical increases in the MTX bidirectional transport kinetics with a much smaller change in steady-state levels. The changes in the human cells transfected with hRFC1 however, were similar to what has been observed by other investigators when RFC1 expression is increased by low folate selective pressure.

Characterization of folate transport mediated by a low pH route in mouse L1210 leukemia cells with defective reduced folate carrier function

Folate influx at low pH was characterized in MTXrA cells, an L1210 mouse leukemia cell line with a functional defect in the reduced folate carrier. Folic acid influx in MTXrA cells was negligible at pH 7.5, increased 13-fold as the pH was decreased to 6.0, and was indistinguishable from that in L1210 cells. In contrast, while methotrexate (MTX) influx in MTXrA cells at pH 6.0 was 15-fold higher than at pH 7.5, in L1210 cells it was decreased by half. Influx of MTX, folic acid, 5-methyltetrahydrofolate and 5-formyltetrahydrofolate in MTXrA cells was increased at pH < 7.0, but their pH optima and profile differed substantially. Influx of MTX and 5-methyltetrahydrofolate at pH 6.0 showed saturability, with a Kt of 2.65 and 0.56 microM, and a Vmax of 0.45 and 0.083 nmol/g dry wt/min, respectively. MTX influx mediated by the low pH transporter was insensitive to the anionic composition of the transport buffer and affected minimally (approximately 20%) by Na+ substitution. The anion transport inhibitors sulfobromophthalein, diisothiocyanatostilbene disulfonic acid, and acetamidoisothiocyanatostilbene disulfonic acid were not effective inhibitors of the low pH route. MTX transport at low pH did not increase in MTXrA-R16 cells, an MTXrA derivative with 10-fold overexpression of the reduced folate carrier (RFC) due to transfection with RFC1 cDNA. Inhibition of reduced folate carrier activity with acetamidoisothiocyanatostilbene disulfonic acid resulted in identical MTX influx in L1210, MTXrA, and MTXrA-R16 cells at pH 5.5. Finally, low pH-mediated MTX influx was reduced by energy inhibitors and partially inhibited by ionophores (nigericin > monensin >> valinomycin). The data indicate that L1210 and MTXrA cells express similar activities of a low pH folate transporter that has properties distinct from, and independent of, the reduced folate carrier.

Neural tube and craniofacial defects with special emphasis on folate pathway genes

Neural tube and orofacial defects are common congenital malformations in humans. While etiologically heterogeneous, they are for the most part multifactorial in their pathogenesis, having both genetic and environmental components in their development. In recent years, there has been a great deal of epidemiologic evidence demonstrating that women who received multivitamins containing folic acid periconceptionally had significantly reduced occurrence and recurrence risks for producing infants with such malformations. This risk reduction is not observed in all populations, further suggestive of a genetic regulation of this phenomenon. Unfortunately, the mechanisms underlying the beneficial effects of folic acid are not well-understood. In this article, we review the relevant epidemiologic data on both neural tube defects and orofacial malformations, the fundamental embryological processes involved in closing the neural tube, and the development of the craniofacies, and propose a working hypothesis for susceptibility to these malformations. This hypothesis is based on the interworkings of cellular folate transport, focusing on the key elements involved in potocytosis. We propose that infants with mutations in the folate receptor alpha gene might be at increased risk for congenital anomalies due to a reduced binding affinity for 5-methyltetrahydrofolate, the physiologic form of folic acid. Various experimental approaches to test the working hypothesis are considered.

Single-chain Fv/folate conjugates mediate efficient lysis of folate-receptor-positive tumor cells

Bispecific antibodies that bind to a tumor antigen and the T cell receptor (TCR) redirect cytotoxic T lymphocytes (CTL) to lyse tumor cells which have escaped normal immune recognition mechanisms. One well-characterized tumor antigen, the folate receptor (FR), is expressed on most ovarian carcinomas and some types of brain cancer. Recently, it was shown that conjugates of folate and anti-TCR antibodies are extremely potent bispecific agents that target tumor cells expressing the high-affinity folate receptor, but not normal cells expressing only the reduced folate carrier protein. In this paper, it is shown that the size of these conjugates can be reduced to the smallest bispecific agent yet described (30 kDa) by attaching folate to a single-chain antibody, scFv, of the anti-TCR antibody KJ16. The scFv/folate conjugates are as effective as IgG/folate conjugates in mediating lysis of FR4 tumor cells by CTL. The optimal folate density was in the range of 5-15 folate molecules per scFv or IgG molecule, which yielded half-maximal lysis values (EC50) of approximately 40 pM (1.2 ng/mL for scFv). Finally, the scFv/folate conjugates could efficiently target tumor cells even in the presence of free folic acid at concentrations that are normally found in serum. Compared to conventional bispecific antibodies, the small size of scFv/folate conjugates may prove advantageous in the ability to penetrate tumors and in reduced immunogenicity.

The divergent 5' termini of the alpha human folate receptor (hFR) mRNAs originate from two tissue-specific promoters and alternative splicing: characterization of the alpha hFR gene structure

The human KB cell or alpha folate receptor (alpha hFR) is a membrane glycoprotein of 42 kDa that participates in the internalization of folates and antifolates. Seven independent alpha hFR cDNA isoforms have been reported that contain unique 5' termini but share a common open reading frame (ORF). To investigate the molecular basis of these heterogeneous 5' sequences, we determined the sequence of the alpha hFR gene from two clones isolated from a human lymphocyte lambda DASH genomic library. The gene is composed of seven exons that span 6.8 kb. The ORF is encoded by exons 4 through 7 while the reported 5' termini of the cDNA isoforms (including two novel cDNAs designated KB2 and KB4) are encoded by exons 1 through 4. Using RNase protection assays, we demonstrate that transcripts corresponding to the KB1 and KB4 cDNAs originate from promoters upstream from exon 1 and exon 4, designated P1 and P4, respectively, and that these mRNA isoforms are the most abundant transcripts expressed in KB cells and selected normal tissues (including kidney, lung, and cerebellum). We observed a heterogeneous start site within exon 1 from the P1 promoter while transcripts from the P4 promoter originate from a single site. In addition, we detected tissue specificity for the P1 and P4 promoter utilization. Transcripts originating from the P1 promoter are the most abundant transcripts expressed by human cerebellum and kidney. In contrast, transcripts from the P4 promoter are the most abundant transcripts expressed by human KB cells and lung. Total RNA from KB cells also protects a 66 bp fragment of an exon 3 riboprobe that is consistent with an alternatively spliced transcript. To examine the functional activity of the predicted P1 and P4 promoters, alpha hFR promoter-CAT chimeric plasmids were constructed using sequences flanking exon 1 and exon 4. We observed a 7.5- and 10-fold increase in CAT activity in HeLa cells transiently transfected with the P1 and P4 promoter constructs, respectively. These data demonstrate that a single gene encodes the divergent 5' termini of the alpha hFR cDNAs and that the alpha hFR transcripts are transcribed from two promoters that are activated in a tissue-specific manner.

pH dependence of methotrexate transport by the reduced folate carrier and the folate receptor in L1210 leukemia cells. Further evidence for a third route mediated at low pH

F2-MTX'A is an L1210 leukemia cell line with a functional detect in the reduced folate carrier and high level expression of folate receptor beta. The pH dependence of methotrexate (MTX) influx by folate receptor beta in F2-MTX'A cells was characterized and compared with that of the reduced folate carrier in parental L1210 cells. MTX influx by folate receptor beta had a pH optimum of 6.5, whereas influx mediated by the reduced folate carrier showed a pH optimum of 7.5. Increased folate receptor beta-mediated MTX influx at pH 6.5 relative to pH 7.5 was accompanied by a 5-fold increase in binding affinity of the receptor for MTX without a change in the number of binding sites. At pH 6.2, approximately 24% of MTX influx in F2-MTX'A cells proceeded by another mechanism. This transport route became active at pH < 7.5, operated optimally at pH 6.0 to 6.5, and, unlike folate receptor beta-mediated MTX influx, was insensitive to the presence of low levels of folic acid (100 nM). MTX influx by the low pH system showed saturability, with a Ki of 5.3 microM and a Vmax of 1.53 nmol/g dry wt/min, was energy dependent, was inhibited by sulfobromophthalein with a Ki of 148 microM, and had similar relative affinities for folic acid, leucovorin, and 5 methyltetrahydrofolate. Influx of 5-methyltetrahydrofolate was also mediated by this route. The data provide further confirmatory evidence for an MTX influx route in F2-MTX'A cells, optimal at low pH and distinct from the reduced folate carrier or the folate receptor.

Comparison of transport properties of the reduced folate carrier and folate receptor in murine L1210 leukemia cells

This laboratory previously described an L1210 murine leukemia cell line with a functional defect in the reduced folate carrier and increased expression of folate receptor-beta (F2-MTXrA). This cell line was used to characterize methotrexate (MTX) influx mediated by folate receptor-beta and to compare this with influx mediated by the reduced folate carrier in L1210 parental cells. Influx of 0.2 microM MTX in F2-MTXrA cells was one-third that of L1210 cells and was abolished by very low concentrations of folic acid. Kinetic analysis revealed that MTX transport mediated by folate receptor-beta exhibited an influx kappa t one-third, and an influx Vmax one-fourth, that of the reduced folate carrier. Metabolic inhibitors markedly suppressed influx in F2-MTXrA cells but had no effect on MTX influx in L1210 cells. MTX influx in both cell lines was inhibited by the organic anions probenecid, sulfobromophthalein, and CI-920, but to a lesser extent in F2-MTXrA cells. The inhibitory effects of these anions on transport in F2-MTXrA cells could be attributed to their inhibition of MTX binding to the folate receptor. Although MTX influx in both cell lines was not sodium dependent, removal of extracellular chloride increased influx 2-fold in L1210 cells while markedly inhibiting influx in F2-MTXrA cells. Substitution of Cl- with isethionate or NO3- partially restored influx in the latter cells, whereas SO4(2-) was inhibitory. Anions enhanced MTX binding to folate receptor-beta with isethionate > SO4(2-) > Cl-. Decreasing the buffer pH to 6.2 produced a 69% reduction, and a 260% increase, in MTX influx in L1210 cells and F2-MTXrA cells, respectively. The data indicate that folate receptor-beta-mediated MTX influx has properties fundamentally different from transport mediated by the reduced folate carrier in terms of energy, ion, and pH dependence. There was no evidence indicating that these processes are functionally linked.

Characterization of a mutation in the reduced folate carrier in a transport defective L1210 murine leukemia cell line

This laboratory previously described an L1210 leukemia cell line (MTXrA) selected for resistance to methotrexate by virtue of impaired transport due to a functional defect in the translocation process. We now report on the sequence analysis of cDNAs encoding the reduced folate carrier from this line and identify a single mutation that results in the substitution of a proline for an alanine in a highly conserved transmembrane region of the protein. Transfection of the parental reduced folate carrier into MTXrA cells resulted in a cell line which exhibited a complete restoration of methotrexate uptake and an enhanced sensitivity to methotrexate. Northern analysis and specific [3H]MTX cell surface binding indicated that expression of the reduced folate carrier was elevated approximately 5-fold in the transfectant compared to parental and MTXrA cells. The MTX influx properties of the transfectant cell line were identical to those of the well characterized reduced folate carrier from parental L1210 cells in terms of: 1) patterns of sensitivity to competing folates, 2) sensitivity to the organic anion sulfobromophthalein, 3) lack of energy dependence, and 4) capacity for trans-stimulation. We also provide new data which suggests that the nucleotide sequence 5' of the predicted ATG initiation codon may encode additional protein information in the form of a leader sequence. Finally, we demonstrate that the MTXrA line has both the mutant and the parental reduced folate carrier alleles but that expression appears to be restricted to the mutant allele. Thus, the methotrexate transport phenotype and resultant drug resistance in this cell line result from genetic/regulatory events at both alleles.

Distinguishing between folate receptor-alpha-mediated transport and reduced folate carrier-mediated transport in L1210 leukemia cells

L1210 leukemia cells transport reduced folates and methotrexate via a well defined reduced folate carrier system and, in the absence of low folate selective pressure, do not express an alternate endocytotic route mediated by cell surface folate receptors. This laboratory previously described an L1210 leukemia cell line, MTXrA, with acquired resistance to methotrexate (MTX) due to the loss of mobility of the reduced folate carrier. We now report on the transfection of MTXrA with a cDNA encoding the murine homolog of the human folate receptor isoform of KB cells to produce MTXrA-TF1, which constitutively expresses high levels of FR-alpha. MTXrA-TF1 and L1210 cells were utilized to compare transport of methotrexate mediated by FR-alpha and the reduced folate carrier, respectively. Methotrexate influx in the two lines was similar when the extracellular level was 0.1 microM, but as the methotrexate concentration increased, influx via the reduced folate carrier increased in comparison to influx mediated by FR-alpha. Transport kinetics indicated both a approximately 20-fold lower influx Kb and Vmax for MTXrA-TF1 as compared to L1210 cells. The two cell lines exhibited distinct influx properties. Methotrexate influx in MTXrA-TF1 was markedly inhibited by 50 nM folic acid and metabolic poisons. In L1210 cells, 1.0 microM folic acid did not affect MTX influx, and metabolic poisons either had no effect on or increased methotrexate influx. Removal of extracellular chloride markedly inhibited transport in MTXrA-TF1 but stimulated influx in L1210 cells. When the pH was decreased to 6.2, methotrexate influx was not altered in MTXrA-TF1 but was reduced in L1210 cells. Probenecid and sulfobromophthalein inhibit methotrexate influx in both L1210 and MTXrA-TF1 cell lines; however, inhibition in MTXrA-TF1 could be accounted for on the basis of inhibition of methotrexate binding to FR-alpha. The data indicate that the reduced folate carrier and FR-alpha function independently and exhibit distinct properties. FR-alpha expressed at sufficient levels can mediate influx of MTX and folates into cells at rates comparable to the reduced folate carrier and hence has pharmacologic and physiologic importance.

Selective expression of the high-affinity isoform of the folate receptor (FR-alpha) in the human placental syncytiotrophoblast and choriocarcinoma cells

The folate receptor (FR), an essential component in the process of folate uptake in various cells, is known to exist in three isoforms, FR-alpha, FR-beta and FR-gamma, with differential tissue expression. Transfer of folate across the human placenta from mother to fetus involves participation of a folate receptor expressed in the syncytiotrophoblast, but the isoform identity of this receptor has not been established. Based on the tissue/cell type from which these isoforms have been cloned, it is currently believed that FR-alpha is the isoform expressed in adult tissues whereas FR-beta is the isoform expressed in fetal tissues including placenta. The present study, undertaken primarily to establish the isoform identity of the FR expressed in the placental syncytiotrophoblast, does not support this currently prevailing nomenclature. Reverse transcription coupled with polymerase chain reaction (RT-PCR) of total/poly(A)+ RNA from placenta, cultured trophoblast cells and JAR choriocarcinoma cells with primer pairs specific for either FR-alpha or FR-beta reveals that while both isoforms are detectable in the whole placental tissue, only FR-alpha is present in the normal trophoblast cells and in the choriocarcinoma cells. Northern analysis with probes designed to distinguish between the mRNA transcripts coding for these two isoforms corroborate the RT-PCR findings. Furthermore, the nucleotide sequences of the PCR products obtained from the trophoblast cells and JAR cells are identical to the nucleotide sequence of the FR-alpha cDNA. These studies establish that it is the FR-alpha isoform, and not the FR-beta isoform, which is selectively expressed in the placental trophoblast cells. FR-beta, which is known to be present in the placenta, most likely arises from the maternal decidua normally associated with this tissue.