A human membrane-associated folate binding protein is anchored by a glycosyl-phosphatidylinositol tail

Recent Advances in Folates and Autoantibodies against Folate Receptors in Early Pregnancy and Miscarriage

Though firstly identified in cerebral folate deficiency, autoantibodies against folate receptors (FRAbs) have been implicated in pregnancy complications such as miscarriage; however, the underlying mechanism needs to be further elaborated. FRAbs can be produced via sensitization mediated by folate-binding protein as well as gene mutation, aberrant modulation, or degradation of folate receptors (FRs). FRAbs may interfere with folate internalization and metabolism through blocking or binding with FRs. Interestingly, different types of FRs are expressed on trophoblast cells, decidual epithelium or stroma, and macrophages at the maternal-fetal interface, implying FRAbs may be involved in the critical events necessary for a successful pregnancy. Thus, we propose that FRAbs may disturb pregnancy establishment and maintenance by modulating trophoblastic biofunctions, placental development, decidualization, and decidua homeostasis as well as the functions of FOLR2+ macrophages. In light of these findings, FRAbs may be a critical factor in pathological pregnancy, and deserve careful consideration in therapies involving folic acid supplementation for pregnancy complications.

Next-generation antibody-drug conjugates for breast cancer: Moving beyond HER2 and TROP2

Antibody-drug conjugates (ADCs) have reshaped the treatment of several malignancies, including breast cancer. Two ADCs are currently approved for the treatment of each breast cancer subtype, including the HER2 targeted ADCs trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd), and the TROP2-targeted ADC sacituzumab govitecan. Each of the ADC components (antibody, linker, and payload) plays a key role in determining the efficacy and toxicity profile of an individual ADC, and their modification can lead to major changes in the clinical profile of these agents. Leveraging the knowledge from three decades of development in the field, several novel ADCs are currently being investigated. Some approaches include targeting different antigens beyond the established HER2/TROP2, or evaluating innovative constructs, such as bispecific ADCs, ADCs with dual payload, immune-modulating ADCs, radionuclide drug conjugates, and masked ADCs, among others. In this review article we discuss the evolving landscape of novel ADCs, highlighting opportunities and challenges emerging in the field.

Protein-Labeling Fluorescent Probe for Folate Receptor α

GPI-anchored folate receptor α (FRα) is an attractive anticancer drug target and diagnosis marker in fundamental biology and medical research due to its significant expression on many cancer cells. Currently, analyses of FRα expression levels are usually achieved using immunological methods. Due to the continual FRα synthesis and degradation, immunological methods are not suitable for studying real-time dynamic activities of FRα in living cells. In this paper, we introduce a rapid and specific FRα protein-labeling fluorescent probe, FR1, to facilitate the study of the dynamics of expression and degradation processes of endogenous FRα in living cells. With this labeling probe, insights on FRα protein lifetime and shedding from the cell surface can be obtained using fluorescence live-cell imaging and electrophoresis techniques. We revealed that FRα undergoes soluble domain release and endocytosis degradation simultaneously. Imaging results showed that most of the membrane FRα are transported to the lysosomes after 2 h of incubation. Furthermore, we also showed that the secretion of a FRα soluble domain into the environment is most likely accomplished by phospholipase. We believe that this protein-labeling approach can be an important tool for analyzing various dynamic processes involving FRα.

Mirvetuximab Soravtansine in Platinum-Resistant Ovarian Cancer

Ovarian cancer is the second leading cause of death from gynecologic malignancies worldwide. Management of platinum-resistant disease is challenging and clinical outcomes with standard chemotherapy are poor. Over the past decades, significant efforts have been made to understand drug resistance and develop strategies to overcome treatment failure. Antibody drug conjugates (ADCs) are a rapidly growing class of oncologic therapeutics, which combine the ability to target tumor-specific antigens with the cytotoxic effects of chemotherapy. Mirvetuximab soravtansine is an ADC comprising an IgG1 monoclonal antibody against the folate receptor alpha (FRα) conjugated to the cytotoxic maytansinoid effector molecule DM4 that has shown promising clinical activity in patients with FR-α-positive ovarian cancer. This review summarizes current evidence of mirvetuximab soravtansine in platinum-resistant ovarian cancer, focusing on clinical activity, toxicity, and future directions.

Folate receptor alpha in ovarian cancer tissue and patient serum is associated with disease burden and treatment outcomes

BACKGROUND

Survival rates for ovarian cancer remain poor, and monitoring and prediction of therapeutic response may benefit from additional markers. Ovarian cancers frequently overexpress Folate Receptor alpha (FRα) and the soluble receptor (sFRα) is measurable in blood. Here we investigated sFRα as a potential biomarker.

METHODS

We evaluated sFRα longitudinally, before and during neo-adjuvant, adjuvant and palliative therapies, and tumour FRα expression status by immunohistrochemistry. The impact of free FRα on the efficacy of anti-FRα treatments was evaluated by an antibody-dependent cellular cytotoxicity assay.

RESULTS

Membrane and/or cytoplasmic FRα staining were observed in 52.7% tumours from 316 ovarian cancer patients with diverse histotypes. Circulating sFRα levels were significantly higher in patients, compared to healthy volunteers, specifically in patients sampled prior to neoadjuvant and palliative treatments. sFRα was associated with FRα cell membrane expression in the tumour. sFRα levels decreased alongside concurrent tumour burden in patients receiving standard therapies. High concentrations of sFRα partly reduced anti-FRα antibody tumour cell killing, an effect overcome by increased antibody doses.

CONCLUSIONS

sFRα may present a non-invasive marker for tumour FRα expression, with the potential for monitoring patient response to treatment. Larger, prospective studies should evaluate FRα for assessing disease burden and response to systemic treatments.

Ultrasmall Folate Receptor Alpha Targeted Enzymatically Cleavable Silica Nanoparticle Drug Conjugates Augment Penetration and Therapeutic Efficacy in Models of Cancer

To address the key challenges in the development of next-generation drug delivery systems (DDS) with desired physicochemical properties to overcome limitations regarding safety, in vivo efficacy, and solid tumor penetration, an ultrasmall folate receptor alpha (FRα) targeted silica nanoparticle (C'Dot) drug conjugate (CDC; or folic acid CDC) was developed. A broad array of methods was employed to screen a panel of CDCs and identify a lead folic acid CDC for clinical development. These included comparing the performance against antibody-drug conjugates (ADCs) in three-dimensional tumor spheroid penetration ability, assessing in vitro/ex vivo cytotoxic efficacy, as well as in vivo therapeutic outcome in multiple cell-line-derived and patient-derived xenograft models. An ultrasmall folic acid CDC, EC112002, was identified as the lead candidate out of >500 folic acid CDC formulations evaluated. Systematic studies demonstrated that the lead formulation, EC112002, exhibited highly specific FRα targeting, multivalent binding properties that would mediate the ability to outcompete endogenous folate in vivo, enzymatic responsive payload cleavage, stability in human plasma, rapid in vivo clearance, and minimal normal organ retention organ distribution in non-tumor-bearing mice. When compared with an anti-FRα-DM4 ADC, EC112002 demonstrated deeper penetration into 3D cell-line-derived tumor spheroids and superior specific cytotoxicity in a panel of 3D patient-derived tumor spheroids, as well as enhanced efficacy in cell-line-derived and patient-derived in vivo tumor xenograft models expressing a range of low to high levels of FRα. With the growing interest in developing clinically translatable, safe, and efficacious DDSs, EC112002 has the potential to address some of the critical limitations of the current systemic drug delivery for cancer management.

High expression of folate receptor alpha is associated with poor prognosis in patients with cervical cancer

OBJECTIVE

Folate receptor α (FRα) is a membrane protein expressed in various solid tumors but has limited expression in normal cells. Therefore, FRα is an attractive target for cancer treatment. This study aimed to investigate the relationship between FRα expression and the clinicopathological characteristics and survivals of cervical cancer.

METHODS

This retrospective study included patients with cervical cancer who underwent primary surgery between 2000 and 2020 at our institution. Immunohistochemical staining of FRα was performed using an anti-folate-binding protein/FBP antibody. FRα-positive staining was defined as ≥5% of tumor staining and FRα-high as ≥50% tumor staining with ≥2+ intensity. The association between FRα expression and survival was assessed using multivariate Cox regression analysis, adjusting for established prognostic factors.

RESULTS

Overall, 123 patients were identified, and 140 tumor samples, including 17 paired primary and metastatic samples, were evaluated. As histological types, 67 patients had squamous cell carcinoma (SCC), and 56 patients had non-SCC. All primary tumors were FRα-positive. High FRα expression was observed in 25% of the cases and differed according to histology (SCC vs. non-SCC, 14.9% vs. 37.5%, p=0.004). FRα expression was significantly higher in metastatic tumors than in primary (170 [IQR, 140-205] vs. 125 [IQR, 110-150], p=0.0006). High FRα expression was significantly associated with worse overall survival (hazard ratio, 6.73; 95% confidence interval, 2.21-20.53; p=0.001).

CONCLUSION

In cervical cancer, FRα expression was elevated in metastatic tumors and high expression was associated with a worse prognosis. Our study supports the development of FRα-targeted therapy for advanced cervical cancer.

CD9 and folate receptor overexpression are not sufficient for VSV-G-independent lentiviral transduction

Extracellular vesicles have become a research focus for their potential as therapeutic vehicles that carry cargo substances. Extracellular vesicles may origin from the endosomal compartment and share several characteristics with the envelope of lentiviruses. A previous study reported that constitutive expression of the tetraspanin CD9, an extracellular vesicle marker, not only increases vesicle secretion from cells, but has also a positive effect on lentiviral transduction efficiency. Moreover, it was shown that expression of CD9 on the viral envelope in absence of viral glycoproteins was sufficient for the transduction of mammalian cells. In this study, we investigate the effect of CD9 and folate receptor alpha, a GPI-anchored protein, on biosynthesis and transduction efficiency of vesicles carrying lentiviral vectors. We demonstrate that neither CD9 nor FRα nor the combination of both were able to mediate a significant transduction of therapeutic vesicles carrying lentiviral RNA. Further studies are required to identify endogenous mammalian proteins that can be used for pseudotyping of viral envelopes to improve viral targeting without inducing immune responses.

High-grade extracellular vesicles preparation by combined size-exclusion and affinity chromatography

Extracellular vesicles (EVs) have recently gained growing interest for their diagnostic and therapeutic potential. Despite this, few protocols have been reported for the isolation of EVs with preserved biological function. Most EV purification methods include a precipitation step that results in aggregation of vesicles and most available techniques do not efficiently separate the various types of EVs such as exosomes and ectosomes, which are involved in distinct biological processes. For this reason, we developed a new two-step fast performance liquid chromatography (FPLC) protocol for purification of large numbers of EVs. The method comprises size exclusion chromatography followed by immobilized metal affinity chromatography, which is enabled by expression of poly-histidine tagged folate receptor α in the parental cells. Characterisation and comparison of the EVs obtained by this method to EVs purified by differential centrifugation, currently the most common method to isolate EVs, demonstrated higher purity and more selective enrichment of exosomes in EV preparations using our FPLC method, as assessed by comparison of marker proteins and density distribution. Our studies reveal new possibilities for the isolation of defined subpopulations of EVs with preserved biological function that can easily be upscaled for production of larger amounts of EVs.

Antibody-drug conjugates in solid tumors: a look into novel targets

Antibody-drug conjugates (ADCs) are a relatively new class of anticancer agents designed to merge the selectivity of monoclonal antibodies with cell killing properties of chemotherapy. They are commonly described as the "Trojan Horses" of therapeutic armamentarium, because of their capability of directly conveying cytotoxic drug (payloads) into the tumor space, thus transforming chemotherapy into a targeted agent. Three novel ADCs have been recently approved, i.e., trastuzumab deruxtecan, sacituzumab govitecan and enfortumab vedotin, respectively, targeting HER2, Trop2 and Nectin4. Thanks to progressive advances in engineering technologies these drugs rely on, the spectrum of diseases sensitive to these drugs as well as their indications are in continuous expansion. Several novel ADCs are under evaluation, exploring new potential targets along with innovative payloads. This review aims at providing a summary of the technology behind these compounds and at presenting the latest ADCs approved in solid tumors, as well as at describing novel targets for ADCs under investigation and new strategies to optimize their efficacy in solid tumors.

A bird's eye view of the advanced approaches and strategies for overshadowing triple negative breast cancer

Triple negative breast cancer (TNBC) is one of the most aggressive form of breast cancer. It is characterized by the absence of estrogen, progesterone and human epidermal growth factor receptors. The main issue with TNBC is that it exhibits poor prognosis, high risk of relapse, short progression-free survival and low overall survival in patients. This is because the conventional therapy used for managing TNBC has issues pertaining to poor bioavailability, lower cellular uptake, increased off-target effects and development of resistance. To overcome such pitfalls, several other approaches are explored. In this context, the present manuscript showcases three of the most widely used approaches which are (i) nanotechnology-based approach; (ii) gene therapy approach and (iii) Phytochemical-based approach. The ultimate focus is to present and explain the insightful reports based on these approaches. Further, the review also expounds on the identified molecular targets and novel targeting ligands which are explored for managing TNBC effectively. Thus, in a nutshell, the review tries to highlight these existing treatment approaches which might inspire for future development of novel therapies with a potential of overshadowing TNBC.

Functionalized Fluorescent Silica Nanoparticles for Bioimaging of Cancer Cells

Functionalized fluorescent silica nanoparticles were designed and synthesized to selectively target cancer cells for bioimaging analysis. The synthesis method and characterization of functionalized fluorescent silica nanoparticles (50-60 nm), as well as internalization and subcellular localization in HeLa cells is reported here. The dye, rhodamine 101 (R101) was physically embedded during the sol-gel synthesis. The dye loading was optimized by varying the synthesis conditions (temperature and dye concentration added to the gel) and by the use of different organotriethoxysilanes as a second silica precursor. Additionally, R101, was also covalently bound to the functionalized external surface of the silica nanoparticles. The quantum yields of the dye-doped silica nanoparticles range from 0.25 to 0.50 and demonstrated an enhanced brightness of 230-260 fold respect to the free dye in solution. The shell of the nanoparticles was further decorated with PEG of 2000 Da and folic acid (FA) to ensure good stability in water and to enhance selectivity to cancer cells, respectively. In vitro assays with HeLa cells showed that fluorescent nanoparticles were internalized by cells accumulating exclusively into lysosomes. Quantitative analysis showed a significantly higher accumulation of FA functionalized fluorescent silica nanoparticles compared to nanoparticles without FA, proving that the former may represent good candidates for targeting cancer cells.

Exploiting the folate receptor α in oncology

Folate receptor α (FRα) came into focus as an anticancer target many decades after the successful development of drugs targeting intracellular folate metabolism, such as methotrexate and pemetrexed. Binding to FRα is one of several methods by which folate is taken up by cells; however, this receptor is an attractive anticancer drug target owing to the overexpression of FRα in a range of solid tumours, including ovarian, lung and breast cancers. Furthermore, using FRα to better localize effective anticancer therapies to their target tumours using platforms such as antibody-drug conjugates, small-molecule drug conjugates, radioimmunoconjugates and, more recently, chimeric antigen receptor T cells could further improve the outcomes of patients with FRα-overexpressing cancers. FRα can also be harnessed for predictive biomarker research. Moreover, imaging FRα radiologically or in real time during surgery can lead to improved functional imaging and surgical outcomes, respectively. In this Review, we describe the current status of research into FRα in cancer, including data from several late-phase clinical trials involving FRα-targeted therapies, and the use of new technologies to develop FRα-targeted agents with improved therapeutic indices.

Characteristics of a Folate Receptor-α Anchored into a Multilipid Bilayer Obtained from Atomistic Molecular Dynamics Simulations

Thorough computational description of the properties of membrane-anchored protein receptors, which are important for example in the context of active targeting drug delivery, may be achieved by models representing as close as possible the immediate environment of these macromolecules. An all-atom bilayer, including 35 different lipid types asymmetrically distributed among the two monolayers, is suggested as a model neoplastic cell membrane. One molecule of folate receptor-α (FRα) is anchored into its outer leaflet, and the behavior of the system is explored by atomistic molecular dynamics simulations. The total number of atoms in the model is ∼185 000. Three 1-μs-long simulations are carried out, where physiological conditions (310 K and 1 bar) are maintained with three different pressure scaling schemes. To evaluate the structure and the phase state of the membrane, the density profiles of the system, the average area per lipid, and the deuterium order parameter of the lipid tails are calculated. The bilayer is in liquid ordered state, and the specific arrangement varies between the three trajectories. The changes in the structure of FRα are investigated and are found time- and ensemble-dependent. The volume of the ligand binding pocket fluctuates with time, but this variation remains independent of the more global structural alterations. The latter are mostly "waving" motions of the protein, which periodically approaches and retreats from the membrane. The semi-isotropic pressure scaling perturbs the receptor most significantly, while the isotropic algorithm induces rather slow changes. Maintaining constant nonzero surface tension leads to behavior closest to the experimentally observed one.

Protozoan Parasites Glycosylphosphatidylinositol Anchors: Structures, Functions and Trends for Drug Discovery

BACKGROUND

Glycosylphosphatidylinositol (GPI) anchors are molecules located on cell membranes of all eukaryotic organisms. Proteins, enzymes, and other macromolecules which are anchored by GPIs are essential elements for interaction between cells, and are widely used by protozoan parasites when compared to higher eukaryotes.

METHODS

More than one hundred references were collected to obtain broad information about mammalian and protozoan parasites' GPI structures, biosynthetic pathways, functions and attempts to use these molecules as drug targets against parasitic diseases. Differences between GPI among species were compared and highlighted. Strategies for drug discovery and development against protozoan GPI anchors were discussed based on what has been reported on literature.

RESULTS

There are many evidences that GPI anchors are crucial for parasite's survival and interaction with hosts' cells. Despite all GPI anchors contain a conserved glycan core, they present variations regarding structural features and biosynthetic pathways between organisms, which could offer adequate selectivity to validate GPI anchors as drug targets. Discussion was developed with focus on the following parasites: Trypanosoma brucei, Trypanosoma cruzi, Leishmania, Plasmodium falciparum and Toxoplasma gondii, causative agents of tropical neglected diseases.

CONCLUSION

This review debates the main variances between parasitic and mammalian GPI anchor biosynthesis and structures, as well as clues for strategic development for new anti-parasitic therapies based on GPI anchors.

Identification of a target protein of Hydra actinoporin-like toxin-1 (HALT-1) using GST affinity purification and SILAC-based quantitative proteomics

Hydra actinoporin-like toxin-1 (HALT-1) is a 20.8 kDa pore-forming toxin isolated from Hydra magnipapillata. HALT-1 shares structural similarity with actinoporins, a family that is well known for its haemolytic and cytolytic activity. However, the precise pore-forming mechanism of HALT-1 remains an open question since little is known about the specific target binding for HALT-1. For this reason, a comprehensive proteomic analysis was performed using affinity purification and SILAC-based mass spectrometry to identify potential protein-protein interactions between mammalian HeLa cell surface proteins and HALT-1. A total of 4 mammalian proteins was identified, of which only folate receptor alpha was further verified by ELISA. Our preliminary results highlight an alternative-binding mode of HALT-1 to the human plasma membrane. This is the first evidence showing that HALT-1, an actinoporin-like protein, binds to a membrane protein, the folate receptor alpha. This study would advance our understanding of the molecular basis of toxicity of pore-forming toxins and provide new insights in the production of more potent inhibitors for the toxin-membrane receptor interactions.

Isolation of Circulating Tumor Cells from Multiple Epithelial Cancers with ApoStream(®) for Detecting (or Monitoring) the Expression of Folate Receptor Alpha

This study describes our efforts to further the field of noninvasive diagnostics, specifically in the area of liquid biopsies in oncology. We employed laser scanning cytometry using highly selective antibodies to interrogate circulating tumor cells (CTCs) that were isolated using ApoStream^® technology to identify folate receptor alpha (FRα)-positive cells. We demonstrate that FRα⁺ CTCs can be isolated from patients with metastatic cancers, including NSCLC adenocarcinoma, breast cancer, and ovarian cancer, whereas squamous cell lung cancer and normal healthy controls were devoid of FRα⁺ CTCs. We believe that the developed methodology will have applications in both the diagnosis and the monitoring of FRα-expressing cancers. Folate receptor alpha (FRα) expression may have utility as a potential diagnostic and therapeutic target in solid tumors. As tissue samples are not always available for patient screening, this study evaluated a noninvasive assay in CTCs from blood samples to detect FRα expression. The presence of FRα⁺ CTCs enriched using ApoStream^® and detected using laser capture cytometry was evaluated in blood samples from cancer patients [NSCLC adenocarcinoma (n = 14), breast cancer (n = 20), ovarian cancer (n = 6), and squamous lung cancer patients (n = 6)] and healthy subjects (n = 20). The data demonstrated that FRα⁺ CTCs were detected in blood from NSCLC adenocarcinoma, breast, and ovarian cancer patients, whereas squamous cell lung cancer patients and normal healthy controls lacked FRα⁺ CTCs as previously known. We demonstrate that CTCs captured using ApoStream^® can be used to detect FRα⁺ CTCs and may have clinical utility as a real-time liquid biopsy for assessing FRα levels in cancer patients.

Upconversion nanoparticles as versatile light nanotransducers for photoactivation applications

Upconversion nanoparticles enable use of near infrared light for spatially and temporally controlled activation of therapeutic compounds in deeper tissues.

Folate-equipped nanolipoplexes mediated efficient gene transfer into human epithelial cells

Since recombinant viral vectors have been associated with serious side effects, such as immunogenicity and oncogenicity, synthetic delivery systems represent a realistic alternative for achieving efficacy in gene therapy. A major challenge for non-viral nanocarriers is the optimization of transgene expression in the targeted cells. This goal can be achieved by fine-tuning the chemical carriers and the adding specific motifs to promote cellular penetration. Our study focuses on the development of novel folate-based complexes that contain varying quantities of folate motifs. After controlling for their physical properties, neutral folate-modified lipid formulations were compared in vitro to lipoplexes leading to comparable expression levels. In addition, no cytotoxicity was detected, unlike what was observed in the cationic controls. Mechanistically, the delivery of the transgene appeared to be, in part, due to endocytosis mediated by folate receptor targeting. This mechanism was further validated by the observation that adding free folate into the medium decreased luciferase expression by 50%. In vivo transfection with the folate-modified MM18 lipid, containing the highest amount of FA-PEG(570)-diether co-lipid (w:w; 90:10), at a neutral charge ratio, gave luciferase transgene expression. These studies indicate that modification of lipids with folate residues could enhance non-toxic, cell-specific gene delivery.

Folate receptor-mediated targeted polymeric gadolinium complexes for magnetic resonance imaging in pulmonary tumor xenografts

Targeted delivery is a highly desirable strategy for diagnostic imaging due to enhanced efficacy and reduced dosage/toxicity. The need to develop target-specific magnetic resonance imaging (MRI) contrast agents to aid in disease characterization is highly essential. In this study, a specific contrast agent, Gd-DTPA-poly-L-lysine (PL-Gd-DTPA)-folate, was synthesized and evaluated for its efficacy as a targeted agent for the imaging of tumors that overexpress the folate receptor. Folic acid was conjugated to PL-Gd-DTPA via the ε-amino groups. The receptor binding properties of folate-PL-Gd-DTPA were studied in cultured tumor cells that overexpressed the folate receptor. The tumor-selecting properties of folate-PL-Gd-DTPA were then evaluated in BALB/c mice bearing subcutaneously implanted folate receptor-positive tumors. Tissue MR signal intensities were measured at six different time-points. In the in vitro study, the folate-PL-Gd-DTPA was able to bind to these cells, which overexpressed the folate receptor, as with free folic acid. Excellent tumor selectivity was also shown in the animal model; after the success of injection of folate-PL-Gd-DTPA, a maximum intensity increase of 125.4% was observed from pre-injection compared to post-injection images of the tumor at the 48 h time-point. The liver enhancement was non-specific and the muscle signal intensity at any time-point after injection showed no statistical difference with that observed before injection. Folate-PL-Gd-DTPA is a promising, novel receptor-specific MRI contrast agent with potential applications in the imaging of human folate receptor-positive tumors.

Enzymatic and non-enzymatic antioxidative effects of folic acid and its reduced derivates

A great part of the population appears to have insufficient folate intake, especially subgroups with higher demand, as determined through more sensitive methods and parameters currently in use. As the role of folate deficiency in congenital defects, e.g. in cardiovascular and neurodegenerative diseases, and in carcinogenesis has become better understood, folate has been recognized as having great potential to prevent these many disorders through folate supplementation or fortification for the general population. Folates are essential cofactors in the transfer and utilization of one-carbon groups in the process of DNA-biosynthesis with implications for genomic repair and stability. Folate acts indirectly to lower homocysteine levels and insures optimal functioning of the methylation cycle. Homocysteine was shown to be an independent risk factor for neurodegenerative and cardiovascular disease, which includes peripheral vascular disease, coronary artery disease, cerebrovascular disease and venous thrombosis. In fact, it was long believed that the beneficial effects of folate on vascular function and disease are related directly to the mechanism of homocysteine-diminution. Recent investigations have, however, demonstrated beneficial effects of folates unrelated to homocysteine-diminution, suggesting independent properties. One such mechanism could be free radical scavenging and antioxidant activity, as it is now recognized that free radicals play an important role in the oxidative stress leading to many diseases. It was found that folic acid and, in particular, its reduced derivates act both directly and indirectly to produce antioxidant effects. Folates interact with the endothelial enzyme NO synthase (eNOS) and, exert effects on the cofactor bioavailability of NO and thus, on peroxynitrite formation. Folate metabolism provides an interesting example of gene-environmental interaction.

Fetal serum folate concentrations and placental folate transport in obese women

OBJECTIVE

We hypothesized that fetal folate serum concentrations are lower and placental folate transport is impaired in pregnancies of obese women.

STUDY DESIGN

Umbilical vein serum and placental tissue were collected from normal weight and obese pregnant women at term. Cellular localization (immunohistochemistry) of folate receptor-α (FR-α), proton coupled folate transporter (PCFT), and reduced folate carrier (RFC) was established. Protein expression (Western blot) and transporter activity (isotope labeled methyltetrahydrofolate) were determined in syncytiotrophoblast microvillus membranes (MVM).

RESULTS

Fetal folate concentrations were similar in obese women as compared with normal weight women. Protein expression of FR-α in microvillus membranes was increased (+173%), in RFC was decreased (-41%), and in PCFT was unchanged. However, activity of FR-α, PCFT, and RFC was unaltered in obesity.

CONCLUSION

Fetal serum folate concentrations and placental folate transport activity are not altered in obesity at term, which suggests that limited availability of folate does not contribute to abnormal gene methylation and developmental programming.

Effect of paternal folate deficiency on placental folate content and folate receptor α expression in rats

We investigated the effect of paternal folate status on folate content and expression of the folate transporter folate receptor α (FRα) in rat placental tissues. Rats were mated after males were fed a diet containing 0 mg of folic acid/kg of diet (paternal folate-deficient, PD) or 8 mg folic acid/kg of diet (paternal folate-supplemented, PS) for 4 weeks. At 20 days of gestation, the litter size, placental weight, and fetal weight were measured, and placental folate content (n = 8/group) and expression of FRα (n = 10/group) were analyzed by microbiological assay and Western blot analysis, respectively. Although there was no difference observed in litter size or fetal weight, but significant reduction (10%) in the weight of the placenta was observed in the PD group compared to that in the PS group. In the PD group, placental folate content was significantly lower (by 35%), whereas FRα expression was higher (by 130%) compared to the PS group. Our results suggest that paternal folate status plays a critical role in regulating placental folate metabolism and transport.

Exploitation of the folate receptor in the management of cancer and inflammatory disease

Over the last 25+ years, the folate receptor (FR) has emerged as an attractive tumor biomarker with the potential to be exploited for therapeutic purposes. Increasing evidence suggests that this endocytosing protein can functionally mediate the cellular uptake and retention of natural folates, certain antifolates, and folate-drug conjugates; the consequences of the latter two events could result in biological modulation, including (but not limited to) tumor-targeted cytotoxicity. Because its tissue expression profile appears to be somewhat limited to either tissues responsible for whole body retention of folates (e.g., kidney and placenta), or certain pathologic tissues (e.g., tumors or activated macrophages), the FR is believed to be a useful biological target for disease management. Indeed, recent years have been peppered with reports of novel FR-targeted therapies, and many have demonstrated impressive in vivo potency, particularly against tumor xenografts, without the undesirable toxicity that often accompanies nontargeted drug regimens. This chapter will provide essential details on the properties of the FR, including where it is expressed and how it has been successfully manipulated for therapeutic benefit.

The folate receptor: what does it promise in tissue-targeted therapeutics?

For over a decade the folate receptor has been intensively investigated as a means for tumor-specific delivery of a broad range of experimental therapies including several conceptually new treatments. Despite a few set backs in clinical trials, the literature is replete with encouraging in vitro and pre-clinical studies of gynecological and other tumors and more therapeutic approaches are ready for clinical testing. Recent studies have added myelogenous leukemias to the list of candidate cancers for FR-targeted therapies. Each approach faces unique challenges in translation that could be addressed through a mechanistic understanding of the function and expression of the receptor in the appropriate experimental systems and by improvements in the technology. This review discusses FR in the context of positive recent developments in broad areas of FR-targeted therapy and attempts to highlight its potential and the anticipated challenges.

In utero physiology: role of folic acid in nutrient delivery and fetal development

Despite the isolation of placental folate receptors 25 y ago and progress in defining the mechanism of folate delivery, considerable gaps remain in the literature for each level of the maternal-placental-fetal unit. Although a critical role of placental folate receptors in maternal-to-fetal folate transport was shown by use of the isolated perfused-placental cotyledon model a decade ago, in vivo confirmation is still needed. Knockout of folate receptors in mice, and knock-down of folate receptors by delivery of antisense oligonucleotides at gestation day 8 or antibodies to folate receptor, results in profound developmental abnormalities in the fetus, ranging from neural tube defects to neurocristopathies such as cleft-lip and cleft-palate, cardiac septal defects, and eye defects. These abnormalities can be prevented by ensuring the entry of folate into cells via alternative pathways. Controlled dietary folate restriction studies also identified adverse effects on reproductive performance, implantation, and fetal growth and other subtler (microscopic) defects in murine fetal development. Longitudinal follow-up showed that gestational folate deficiency results in behavioral changes-an anxiety phenotype-during adulthood in these mice, which supports the Barker hypothesis. The extent to which these findings are relevant to humans is unclear, however. Nevertheless, the high incidence of neural tube defects among North Indian women, who chronically subsist on one-third to one-half of the optimum folate needed to prevent birth defects, underscores the magnitude of the public health problem and emphasizes the urgent challenge to define the most efficient way to ensure adequate dietary folate for hundreds of millions of such women at risk in developing countries.

The kidney in vitamin B12and folate homeostasis: characterization of receptors for tubular uptake of vitamins and carrier proteins

Over the past 10 years, animal studies have uncovered the molecular mechanisms for the renal tubular recovery of filtered vitamin and vitamin carrier proteins. Relatively few endocytic receptors are responsible for the proximal tubule uptake of a number of different vitamins, preventing urinary losses. In addition to vitamin conservation, tubular uptake by endocytosis is important to vitamin metabolism and homeostasis. The present review focuses on the receptors involved in renal tubular recovery of folate, vitamin B12, and their carrier proteins. The multiligand receptor megalin is important for the uptake and tubular accumulation of vitamin B12. During vitamin load, the kidney accumulates large amounts of free vitamin B12, suggesting a possible storage function. In addition, vitamin B12is metabolized in the kidney, suggesting a role in vitamin homeostasis. The folate receptor is important for the conservation of folate, mediating endocytosis of the vitamin. Interaction between the structurally closely related, soluble folate-binding protein and megalin suggests that megalin plays an additional role in the uptake of folate bound to filtered folate-binding protein. A third endocytic receptor, the intrinsic factor-B12receptor cubilin-amnionless complex, is essential to the renal tubular uptake of albumin, a carrier of folate. In conclusion, uptake is mediated by interaction with specific endocytic receptors also involved in the renal uptake of other vitamins and vitamin carriers. Little is known about the mechanisms regulating intracellular transport and release of vitamins, and whereas tubular uptake is a constitutive process, this may be regulated, e.g., by vitamin status.

Detection of Folate Binding Protein with Enhanced Sensitivity Using a Functionalized Quartz Crystal Microbalance Sensor

In this report, we describe the development of a quartz crystal microbalance biosensor for detection of folate binding protein (FBP). Using a simple folate-BSA conjugate adsorbed onto a Au-coated quartz sensor, a detection limit of 30 nM was achieved. Binding of FBP to the sensor surface could be blocked at concentrations as high as 1 microM with a 100-fold excess of folic acid, indicating the specificity of the folate-FBP interaction and the absence of nonspecific binding to the functionalized surface. Moreover, capture could be achieved in the presence of blood serum, making the assay amenable to the analysis of bodily fluids. Further signal enhancement based on an anti-FBP antibody and protein-A-coated gold nanosphere sandwich assay extended the detection limit to 50 pM (approximately 3 orders-of-magnitude improvement). Given the overexpression of FBP in certain malignancies and inflammatory disorders, we expect the methodology described here to be useful to detect FBP as a possible biomarker for disease diagnosis.

Detection and quantitation of serum mesothelin, a tumor marker for patients with mesothelioma and ovarian cancer

PURPOSE

To determine whether mesothelin, a cell surface protein highly expressed in mesothelioma and ovarian cancer, is shed into serum and if so to accurately measure it.

EXPERIMENTAL DESIGN

We developed a sandwich ELISA using antibodies reacting with two different epitopes on human mesothelin. To quantitate serum mesothelin levels, a standard curve was generated using a mesothelin-Fc fusion protein. Sera from 24 healthy volunteers, 95 random hospital patients, 56 patients with mesothelioma, and 21 patients with ovarian cancer were analyzed. Serum mesothelin levels were also measured before and after surgical cytoreduction in six patients with peritoneal mesothelioma.

RESULTS

Elevated serum mesothelin levels were noted in 40 of 56 (71%) patients with mesothelioma and in 14 of 21 (67%) patients with ovarian cancer. Serum mesothelin levels were increased in 80% and 75% of the cases of mesothelioma and ovarian cancer, respectively, in which the tumors expressed mesothelin by immunohistochemistry. Out of the six patients with peritoneal mesothelioma who underwent surgery, four had elevated serum mesothelin levels before surgery. Out of these four patients, three had cytoreductive surgery and the serum mesothelin level decreased by 71% on postoperative day 1 and was undetectable by postoperative day 7.

CONCLUSIONS

We developed a serum mesothelin assay that shows that mesothelin is elevated in patients with mesothelioma and ovarian cancer. The rapid decrease in mesothelin levels after surgery in patients with peritoneal mesothelioma suggests that serum mesothelin may be a useful test to monitor treatment response in mesothelin-expressing cancers.

Enhancement of folate receptor alpha expression in tumor cells through the glucocorticoid receptor: a promising means to improved tumor detection and targeting

The utility of the folate receptor (FR) type alpha, in a broad range of targeted therapies and as a diagnostic serum marker in cancer, is confounded by its variable tumor expression levels. FR-alpha, its mRNA and its promoter activity were coordinately up-regulated by the glucocorticoid receptor (GR) agonist, dexamethasone. Optimal promoter activation which occurred at <50 nmol/L dexamethasone was inhibited by the GR antagonist, RU486, and was enhanced by coactivators, supporting GR mediation of the dexamethasone effect. The dexamethasone response of the FR-alpha promoter progressed even after dexamethasone was withdrawn, but this delayed effect required prior de novo protein synthesis indicating an indirect regulation. The dexamethasone effect was mediated by the G/C-rich (Sp1 binding) element in the core P4 promoter and was optimal in the proper initiator context without associated changes in the complement of major Sp family proteins. Histone deacetylase (HDAC) inhibitors potentiated dexamethasone induction of FR-alpha independent of changes in GR levels. Dexamethasone/HDAC inhibitor treatment did not cause de novo FR-alpha expression in a variety of receptor-negative cells. In a murine HeLa cell tumor xenograft model, dexamethasone treatment increased both tumor-associated and serum FR-alpha. The results support the concept of increasing FR-alpha expression selectively in the receptor-positive tumors by brief treatment with a nontoxic dose of a GR agonist, alone or in combination with a well-tolerated HDAC inhibitor, to increase the efficacy of various FR-alpha-dependent therapeutic and diagnostic applications. They also offer a new paradigm for cancer diagnosis and combination therapy that includes altering a marker or a target protein expression using general transcription modulators.

Fumonisins disrupt sphingolipid metabolism, folate transport, and neural tube development in embryo culture and in vivo: a potential risk factor for human neural tube defects among populations consuming fumonisin-contaminated maize

Fumonisins are a family of toxic and carcinogenic mycotoxins produced by Fusarium verticillioides (formerly Fusarium moniliforme), a common fungal contaminant of maize. Fumonisins inhibit ceramide synthase, causing accumulation of bioactive intermediates of sphingolipid metabolism (sphinganine and other sphingoid bases and derivatives) as well as depletion of complex sphingolipids, which interferes with the function of some membrane proteins, including the folate-binding protein (human folate receptor alpha). Fumonisin causes neural tube and craniofacial defects in mouse embryos in culture. Many of these effects are prevented by supplemental folic acid. Recent studies in LMBc mice found that fumonisin exposure in utero increases the frequency of developmental defects and administration of folate or a complex sphingolipid is preventive. High incidences of neural tube defects (NTD) occur in some regions of the world where substantial consumption of fumonisins has been documented or plausibly suggested (Guatemala, South Africa, and China); furthermore, a recent study of NTD in border counties of Texas found a significant association between NTD and consumption of tortillas during the first trimester. Hence, we propose that fumonisins are potential risk factors for NTD, craniofacial anomalies, and other birth defects arising from neural crest cells because of their apparent interference with folate utilization.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Antibodies to folate receptors impair embryogenesis and fetal development in the rat

BACKGROUND

Folic acid (FA) supplementation reduces neural tube defects (NTDs) by 70%. However, the cause of most NTDs cannot be attributed to folate deficiency, to mutations of genes that encode folate pathway enzymes, and folate receptors (FRs) that mediate cellular folate uptake. Mouse embryos nullizygous for the ortholog of the FRalpha gene have lethal congenital abnormalities that are preventable by administration of folinic acid to the dams. To determine whether antibodies to FRs are similarly teratogenic, we studied a rat model.

METHODS

Immunohistochemistry with an antiserum to rat FRs was used to identify the receptors on reproductive tissues and embryos. Gestation day (GD) 8 rats received intraperitoneal injections of antiserum to the FRs, and their embryos were examined 2-9 days later. Some rats received pharmacologic doses of folinic acid or dexamethasone before the antiserum was administered.

RESULTS

The FRs are present on oocytes, the oviduct, and uterine epithelial cells, and in the embryo at all stages examined between GD4 and GD15. The antiserum has a dose-related effect on embryo viability and organogenesis. Folinic acid prevented teratogenicity resulting from smaller doses of antiserum, but not that caused by larger doses. Resorption of embryos with the larger doses of the antiserum was prevented by dexamethasone.

CONCLUSIONS

FRs are expressed on oocytes, epithelial cells of reproductive organs, and embryonic and extraembryonic tissues. Antiserum to FRs administered to pregnant rats causes embryonic damage. Embryo lethality with smaller doses of antiserum is preventable by administration of folinic acid, while larger doses cause embryo damage by immune-mediated cell lysis, which can be prevented by dexamethasone.

Reversion of transformed phenotype in ovarian cancer cells by intracellular expression of anti folate receptor antibodies

The alpha-folate receptor (FR) is selectively overexpressed in 90% of nonmucinous ovarian carcinomas, whereas no expression is detectable in normal ovarian surface epithelium (OSE). Indirect evidence suggests that FR expression is associated with tumor progression and affects cell proliferation. To evaluate better the role of FR, we developed an approach based on intracellular expression of single-chain (sc) antibodies (intrabody) to downmodulate membrane expression of FR in ovary cancer cells. IGROV-1 and SKOV3 ovarian carcinoma cell lines were transfected with an anti-FR intrabody. Transfectants and parental cells were tested for FR, integrins and anti-FR intrabody expression by fluorescence-activated cell sorting (FACS), reverse transcription and polymerase chain reaction (RT-PCR) and/or immunoblotting. Cell growth characteristics and adhesion properties were evaluated in liquid, semisolid and organotypic cultures. The anti-FR scFv inhibited FR expression from 60 to 99%. At physiological concentrations of folate, proliferation varied directly as a function of FR expression. FR downmodulation was accompanied by reduced colony-forming ability in soft agar, morphological change of the cells, significant enhanced adhesion to laminin or Matrigel, a two- to three-fold increase in alpha6beta4 integrin expression, and a marked reduction in laminin production. In three-dimensional organotypic cultures, anti-FR intrabody-transfected IGROV1 cells grew as a single-ordered layer, reminiscent of normal OSE growth in vivo. In conclusion, the anti-FR intrabody reverses the transformed phenotype in ovary cancer cells and may provide an efficient means to inhibit selectively the growth of these cells.

Modulation of the folate receptor type beta gene by coordinate actions of retinoic acid receptors at activator Sp1/ets and repressor AP-1 sites

Folate receptor (FR) type beta is a promising target for therapeutic intervention in acute myelogenous leukemia (AML) owing particularly to its specific up-regulation in AML cells by all-trans retinoic acid (ATRA). Here we identify functional elements in the FR-beta gene and examine the molecular mechanism of transcriptional induction of FR-beta by ATRA. The basal promoter activity of FR-beta resulted from synergistic interaction between Sp1 and ets binding sites (EBSs) and repression by upstream AP-1-like elements, whose action required EBSs. A minimal promoter containing the Sp1 and ets elements was ATRA-responsive. The repressor elements bound Fos family proteins; association of the proteins with the repressor elements correlated negatively with FR-beta expression in peripheral blood neutrophils and monocytes and also in KG-1 (AML) cells grown in the absence or in the presence of ATRA. Furthermore, down-regulation of FR-beta in KG-1 cells treated with O-tetradecanoylphorbol 13-acetate (TPA) was accompanied by increased AP-1 binding to the repressor elements. From chromatin immunoprecipitation (ChIP) assays, the nuclear retinoic acid receptor alpha (RARalpha) associated with the Sp1 region, and RARs beta and gamma associated with the AP-1 and Sp1 regions; treatment of KG-1 cells with ATRA did not alter Sp1 binding but increased the association of RARalpha and decreased the association of RARs beta and gamma. ATRA also decreased RAR expression levels. The results suggest that the FR-beta gene is a target for multiple coordinate actions of nuclear receptors for ATRA directly and indirectly acting on a transcriptional complex containing activating Sp1/ets and inhibitory AP-1 proteins. The multiple mechanisms favor the prediction that ATRA will induce FR-beta expression in a broad spectrum of AML cells. Further, optimal FR-beta induction may be expected when all 3 RAR subtypes bind agonist.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

The half-life of the transcript encoding the folate receptor alpha in KB cells is reduced by cytosolic proteins expressed in folate-replete and not in folate-depleted cells

The KB cell, a transformed human cell line, constitutively expresses a high level of the glycosylphosphatidylinositol (GPI) anchored folate receptor protein alpha (FR alpha) and thereby can grow in medium containing <1 nM folate. When transferred from a folate-replete (FR) medium to one folate-deficient (FD), intracellular folate diminishes about 50-fold and expression of the FR alpha increases 6-fold. This up-regulation is mediated by a 4.5-fold increase in the initial transcription rate and a 2.4-fold prolongation of the mRNA half-life that together provide a higher level of the steady-state mRNA abundance. An RNA gel -shift assay of a 5' region of the mRNA that includes all of the non-coding and 24 nt of coding sequence, and a 3' region comprised only of coding sequence, identified unique complexes with cytosolic proteins from the FR-KB cells that were not observed with the cytosol from FD-KB cells. It appears, therefore, that expression of these folate-dependent cytosolic trans-active proteins function to maintain a shorter half-life of the mRNA in the FR-KB cells by binding to 5' and 3' cis elements, reducing the stability of this transcript.

Specific targeting of folate-dendrimer MRI contrast agents to the high affinity folate receptor expressed in ovarian tumor xenografts

The need to develop target-specific MRI contrast agents to aid in disease characterization remains highly essential. In this study, we present a generation four polyamidoamine (PAMAM) folate-dendrimer that specifically targets the high affinity folate receptor (hFR) overexpressed on more than 80% of ovarian tumors. In vitro, mouse erythroleukemia cells expressing the hFR bind the radiolabeled folate-dendrimer chelate resulting in over 2700% increase in binding compared with untreated cells. The binding was inhibited by free folic acid to levels observed on folate-receptor-negative cells. In vivo, ovarian tumor xenografts resulted in a 33% contrast enhancement, following the folate-dendrimer chelate administration, that was significantly different compared with results obtained with a non-specific, extracellular fluid space agent, Gd-HP-DO3A. In addition, this contrast enhancement was absent in saline-treated animals, folate-receptor-negative tumors, and was inhibited by free folic acid. Results suggest that a macromolecular, dendrimeric MRI agent with high molecular relaxivities (1646 mM(-1) s(-1)) can be used in specifically targeting the hFR on tumor cells and ovarian tumors.

The rate of folate receptor alpha (FR alpha) synthesis in folate depleted CHL cells is regulated by a translational mechanism sensitive to media folate levels, while stable overexpression of its mRNA is mediated by gene amplification and an increase in transcript half-life

DC-3F/FA3 cells (FA3) were obtained by selection of Chinese hamster lung fibroblasts for growth in folic acid free media, supplemented with 15 pM [6S]-5-formyltetrahydrofolic acid. These cells, as a result of low level gene amplification and RNA stabilization, were found to overexpress folate receptor alpha (FR alpha) mRNA by more than five hundred fold. The expression level of the receptor, a 43 kDa GPI-linked plasma membrane glycoprotein, was found to be inversely related to changes in media folate concentrations while its steady state mRNA level remained unaffected. In low folate, the rate of receptor synthesis was found to increase by more than three fold, while its half-life stabilized as compared to that observed in high folate media. Although DC-3F cells were found to contain low amounts of FR alpha mRNA, receptor expression was undetectable, and changing media folate concentrations had no effect on the expression of either. Hence, while selection for growth in low folate leads to stable overexpression of FR alpha mRNA, receptor expression is regulated at the level of protein synthesis by a mechanism sensitive to media folate levels.

In vitro demonstration of IgE antibody to folate-albumin in anaphylaxis from folic acid

BACKGROUND

Folic acid (the synthetic form of folate B vitamins in foods) is widely used in vitamin supplements. Anaphylaxis from ingestion or injection of folic acid suggests IgE antibody-mediated mechanisms, but this has not been demonstrated previously in vitro.

OBJECTIVE

This study was conducted to better define the mechanism of folic acid hypersensitivity and cross-reactivity among folic acid congeners.

METHODS

Skin testing was performed with folic acid congeners in a woman who developed anaphylaxis after ingestion of 2 different multivitamin preparations containing folic acid. In vitro immunologic serum studies were conducted using a folate-human serum albumin (HSA) conjugate prepared by a novel application of carbodiimide condensation.

RESULTS

The patient had positive immediate-type skin test reactions to folic acid and several folate analogues including leucovorin (folinic acid). Urticaria developed during graded oral test dosing with leucovorin. Using a dot immunoblot assay or an ELISA for IgE antibody to folate-HSA, results of the patient's serum testing were positive, whereas results of sera from normal control subjects were negative, the first in vitro demonstration of IgE to a folic acid-protein conjugate. By ELISA, the positive result of the patient's serum was inhibited significantly by serum coincubation with folate-HSA, but not HSA or folic acid.

CONCLUSIONS

Immediate hypersensitivity to folic acid and possibly other vitamins can be mediated by IgE antibody to conjugates formed between vitamins and self-proteins or polypeptides. Leucovorin can have clinically important immunologic cross-reactivity with folic acid. A diet rich in natural folates (pteroylpolyglutamates) appears useful as a management strategy for providing adequate nutrition to patients with folic acid hypersensitivity.

Development of a tumor-targeting MR contrast agent using the high-affinity folate receptor: work in progress

RATIONALE AND OBJECTIVES

Macromolecular contrast agents enhance tumors by means of active or passive targeting. Active targeting requires surface receptors. Many tumors of epithelial origin express the high-affinity folate receptor (hFR), including ovarian tumors. The objective of this research was to enhance tumors that express hFR using macromolecular contrast agents conjugated to folic acid.

METHODS

The authors prepared a folate-conjugated dendrimer polychelate by attaching folic acid to a fourth-generation ammonia-core polyamidoamine dendrimer. The remaining amines were reacted with 2-(4-isothiocyanatobenzyl)-6-methyldiethylenetriamine pentaacetic acid. Relaxivity measurements (r1 and r2) and MRI were conducted at 4.7 T.

RESULTS

The dendrimer r2 exceeded that of Gd-HP-DO3A by 8.2 times at 4.7 T. It increased the tumor percentage contrast enhancement, 24 hours after injection, of T2-weighted images by 33%.

CONCLUSIONS

This new agent accumulates in tumors expressing hFR. These results do not differentiate between active and passive targeting mechanisms. Receptor-negative tumors suggest a mechanism other than a nonspecific blood pool effect.

Isolation of complementary deoxyribonucleic acids encoding putative secreted and membrane-bound folate binding proteins from endometrium of swine

Two distinct forms of endometrial folate binding protein (FBP) cDNAs were isolated using reverse transcription-polymerase chain reaction and 3' and 5' rapid amplification of cDNA ends (RACE) procedures. On the basis of the absence or presence of an intact glycophosphatidylinositol linkage site in the C terminus of the predicted amino acid sequences, the two forms appear to encode secreted and membrane-bound forms of FBP. The cDNAs for the putative secreted and membrane forms encoded 252- and 249-amino acid proteins, respectively, that were 73% identical with each other and were 66-82% identical with other known FBPs. However, the nucleotide sequences within the 5' untranslated region and from codons 224 and 223 of the secreted and membrane forms, respectively, to the 3' ends of each RNA, were divergent. The divergence in the 3' ends of the two cDNAs was exploited to determine changes in concentrations of each mRNA in the endometrium during the estrous cycle and early pregnancy. Northern blots of endometrial total RNA probed with a putative secreted FBP specific probe indicated that mRNA concentrations do not change during early pregnancy. In contrast, blots probed with a putative membrane FBP specific probe indicated that mRNA concentrations increase dramatically from Day 15 to Day 24 of pregnancy. Finally, N-terminal amino acid sequencing of FBP purified from Day 15 pregnant uterine flushings matched the secreted form of FBP mRNA. These data are consistent with a role for putative secreted and membrane-bound forms of FBPs in the transport of folate to the developing swine conceptus during early pregnancy.