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

Whitaker H, Piskorz AM, Csizmadi I, Robson PJ. Conditions associated with circulating tumor-associated folate receptor 1 protein in healthy men and women

Background

Serum concentrations of the tumor-associated folate receptor 1 (FOLR1) protein may be a marker for early cancer detection, yet concentrations have also been detected in cancer-free women. We investigated the conditions associated with circulating FOLR1 protein in healthy individuals and sought to clarify the range of normal serum values.

Methods

Sera of cancer-free men and women (N = 60) enrolled in a population-based cohort study in Alberta, Canada were analyzed for FOLR1 protein using an electrochemical luminescence immunoassay. Dietary, lifestyle, medical and reproductive history information was collected by questionnaires. Differences in serum FOLR1 concentrations between groups were assessed by non-parametric tests, and predictors of serum FOLR1 concentrations were estimated using multivariable linear regression.

Results

Median serum FOLR1 concentration was higher in women (491 pg/ml, range = 327–693 pg/ml) than in men (404 pg/ml, range = 340–682 pg/ml), P = 0.001. FOLR1 concentration was also positively associated with vitamin A intake (P = 0.02), and showed positive trends with age and with oral contraceptive hormone use among women and an inverse trend with body mass index. All variables examined explained almost half of the variation in serum FOLR1 (model R² = 0.44, P = 0.04); however, the retention of gender (P = 0.003) and vitamin A intake (P = 0.03) together explained 20% (P = 0.001) of serum FOLR1 variation. No other predictor was significant at P<0.05.

Conclusions

The positive association between serum FOLR1 concentration and female gender independent of an age effect suggests caution against statements to exploit serum FOLR1 for early cancer detection without further understanding the biological underpinnings of these observations. Serum FOLR1 concentrations may be influenced by the steroid retinoic acid (vitamin A) but do not appear to be associated with folate nutritional status. These findings require confirmation in larger independent studies.

Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency

The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.

Retinoic acid treatment and cell aggregation independently regulate alternative splicing in P19 cells during neural differentiation

To induce neural differentiation of P19 cells, two different treatments, RA (retinoic acid) and cell aggregation, are required. However, there has been no report that RA treatment alone or cell aggregation alone could control alternative splicing regulation in P19 cells. Therefore, we focused on alternative splicing effects by neural induction (RA treatment and/or cell aggregation) in P19 cells. We analysed the splicing patterns of several genes, including 5-HT3R-A (5-hydroxytryptamine receptor), Actn1 (actinin alpha1), CUGBP2 (CUG-binding protein) and PTB (polypyrimidine track-binding protein), which showed different responses during the early neural induction of P19 cells. We show here that RA treatment alone changes the alternative splice mechanism of 5-HT3R-A. Cell aggregation alone controls alternative splicing regulation of Actn1. Both treatments (RA and cell aggregation) compensate and regulate the alternative splicing mechanism of CUGBP2. However, PTB is independent of RA and cell aggregation. Taken together, our results suggest that RA treatment and cell aggregation independently regulate the alternative splicing mechanism in the early stage of P19 cells during neural differentiation.

Characterization of folate uptake by choroid plexus epithelial cells in a rat primary culture model

Reduced derivatives of folic acid (folates) play a critical role in the development, function and repair of the CNS. However, the molecular systems regulating folate uptake and homeostasis in the central nervous system remain incompletely defined. Choroid plexus epithelial cells express high levels of folate receptor alpha (FRalpha) suggesting that the choroid plays an important role in CNS folate trafficking and maintenance of CSF folate levels. We have characterized 5-methyltetrahydrofolate (5-MTHF) uptake and metabolism by primary rat choroid plexus epithelial cells in vitro. Two distinct processes are apparent; one that is FRalpha dependent and one that is independent of the receptor. FRalpha binds 5-MTHF with high affinity and facilitates efficient uptake of 5-MTHF at low extracellular folate concentrations; a lower affinity FRalpha independent system accounts for increased folate uptake at higher concentrations. Cellular metabolism of 5-MTHF depends on the route of folate entry into the cell. 5-MTHF taken up via a non-FRalpha -mediated process is rapidly metabolized to folylpolyglutamates, whereas 5-MTHF that accumulates via FRalpha remains non-metabolized, supporting the hypothesis that FRalpha may be part of a pathway for transcellular movement of the vitamin. The proton-coupled folate transporter, proton-coupled folate transporter (PCFT), mRNA was also shown to be expressed in choroid plexus epithelial cells. This is consistent with the role we have proposed for proton-coupled folate transporter in FRalpha-mediated transport as the mechanism of export of folates from the endocytic compartment containing FRalpha.

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.

Maternal folate deficiency results in selective upregulation of folate receptors and heterogeneous nuclear ribonucleoprotein-E1 associated with multiple subtle aberrations in fetal tissues

BACKGROUND

Homocysteine, which increases in folate deficiency, can upregulate folate receptors (FR) at the translational level by increasing the interaction between a short cis-element in the 5'-untranslated region of FR-alpha mRNA and heterogeneous nuclear ribonucleoprotein-E1 (hnRNP-E1). Perturbation of this RNA-protein interaction on GD8.5 induces neural tube defects and neurocristopathies in mice. FR upregulation can also reduce cell proliferation independently of folate deficiency in some human cells. Accordingly, we tested the hypothesis that sustained murine maternal folate deficiency would negatively impact pregnancy outcomes, upregulate FR, and selectively reduce fetal cell proliferation.

METHODS

Dams were fed chow with various levels of folic acid added for eight weeks before and throughout pregnancy. Following sacrifice on GD17, dams were compared for folate and homocysteine status as well as pregnancy outcomes. Fetuses from some groups were evaluated by specific biochemical, molecular, and immunohistochemical studies for FR, hnRNP-E1, and apoptosis.

RESULTS

When compared to dams fed a folate-replete diet, those dams on a folate-depleted diet developed reduced red cell folates and hyperhomocysteinemia and an inverse dose-dependent upregulation of FR and hnRNP-E1 on GD17 without alterations in cell number in the majority of tissues. However, FR overexpression was accompanied by a significant reduction in the net number of cells in the midgut, lung, pons, tongue, and olfactory epithelium, and with premature differentiation in dorsal root ganglion cells and dysplasia of taste buds. By contrast, in the brain, spinal cord, diaphragm, and primordium of follicles of vibrissae, there was less FR expression, which accompanied a net reduction in number of cells and architectural anomalies. Subtle "immunohistochemical footprints" of apoptosis on GD17 fetuses corresponded with net cell loss in the lung and olfactory epithelium. Upregulation of FR could be explained by a homocysteine-induced RNA-protein interaction in folate-depleted fetuses that led to a proportionate increase in murine FR biosynthesis.

CONCLUSIONS

Maternal folate deficiency results in selective upregulation of FR and hnRNP-E1 associated with multiple aberrations in fetal tissues that include increased cell loss, architectural anomalies, and premature differentiation. The potential significance of these findings to explain the wide spectrum of folate-responsive birth defects in humans is discussed.

Human embryonic stem cell methyl cycle enzyme expression: modelling epigenetic programming in assisted reproduction?

To investigate a possible mechanism for inducing epigenetic defects in the preimplantation embryo, a human embryonic stem cell model was developed, and gene expression of the key methyl cycle enzymes, MAT2A, MAT2B, GNMT, SAHH, CBS, CGL, MTR, MTRR, BHMT, BHMT2, mSHMT, cSHMT and MTHFR was demonstrated, while MAT1 was barely detectable. Several potential acceptors of cycle-generated methyl groups, the DNA methyltransferases (DNMT1, DNMT3A, DNMT3B and DNMT3L), glycine methyltransferase and the polyamine biosynthetic enzymes, SAM decarboxylase and ornithine decarboxylase, were also expressed. Expression of folate receptor alpha suggests a propensity for folate metabolism. Methotrexate-induced depletion of folate resulted in elevated intracellular homocysteine concentration after 7 days in culture and a concomitant increase in cysteine and glutathione, indicating clearance of homocysteine through the transulphuration pathway. These studies indicate that altered methyl group metabolism provides a potential mechanism for inducing epigenetic changes in the preimplantation embryo.

Differentiation-independent retinoid induction of folate receptor type β, a potential tumor target in myeloid leukemia

Folate receptor (FR) type β is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-β expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-β expression in vitro in myeloid leukemia cells from patient marrow. FR-β was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D3, or transforming growth factor β. ATRA did not induce FR-β expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-β expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-β promoter–luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-β promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)α, RARβ, or RARγ, it appears that the retinoid effect on FR-β expression could be mediated by ligand binding to RARs α, β, or γ, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARα and RARγ selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-β expression. Thus, blocks in the RARα-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-β expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-β expression in myeloid leukemia cells refractory to retinoid differentiation therapy.

Differentiation-independent retinoid induction of folate receptor type β, a potential tumor target in myeloid leukemia

Folate receptor (FR) type β is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-β expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-β expression in vitro in myeloid leukemia cells from patient marrow. FR-β was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D3, or transforming growth factor β. ATRA did not induce FR-β expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-β expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-β promoter–luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-β promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)α, RARβ, or RARγ, it appears that the retinoid effect on FR-β expression could be mediated by ligand binding to RARs α, β, or γ, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARα and RARγ selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-β expression. Thus, blocks in the RARα-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-β expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-β expression in myeloid leukemia cells refractory to retinoid differentiation therapy.

Hypothesis: folate-responsive neural tube defects and neurocristopathies

BACKGROUND

What accounts for the wide spectrum of folate-responsive dysmorphogeneses? Both embryonic and fetal cells are entirely dependent on maternal folate to support their requirement for precisely timed proliferative bursts during gestation. Folate receptors (FRs) mediate transport into cells and are central to transplacental maternal-to-fetal folate transport. FRs are also critical for neural tube and neural crest development because recent murine "knock-out" and "knock-down" of FRs results in a high percentage of folate-responsive neural tube defects (NTDs) and neurocristopathies.

HYPOTHESIS

Central to our hypothesis is the fact that folate deficiency is accompanied by a reduction in the proliferative capacity of highly mitotic neural tube or neural crest cells. Therefore, depending on when in pregnancy various cohorts of highly proliferative cells are deprived of folate, and the origin of the affected cells will determine the type of developmental dysmorphogenesis. Thus, selective folate deficiency in early pregnancy of only highly proliferative neural tube or neural crest cells predisposes to NTDs or gross dysmorphogenesis, respectively. Folate deficiency that compromises placental development will predispose to small-for-date babies due to an overall nutrient deficiency, and the development of folate insufficiency later in pregnancy could predispose to more subtle midline birth defects involving atresia of neural crest cell-derived structures. Finally, a congenital folate transport defect would only be corrected by suprapharmacological doses of folate, which ensures passive diffusion.

CONCLUSION

This hypothesis can explain the results of several earlier and more recent clinical trials on folate supplementation in pregnancy, but it also raises the possibility that there may be several as yet undiscovered neurocristopathies that are folate responsive. Teratology 62:42-50, 2000. Published 2000 Wiley-Liss, Inc.