Thyroid hormone dependent pituitary tumor cell growth in serum-free chemically defined culture. A new regulatory role for apotransferrin

Pathophysiological aspects of transferrin-A potential nano-based drug delivery signaling molecule in therapeutic target for varied diseases

Transferrin (Tf), widely known for its role as an iron-binding protein, exemplifies multitasking in biological processes. The role of Tf in iron metabolism involves both the uptake of iron from Tf by various cells, as well as the endocytosis mediated by the complex of Tf and the transferrin receptor (TfR). The direct conjugation of the therapeutic compound and immunotoxin studies using Tf peptide or anti-Tf receptor antibodies as targeting moieties aims to prolong drug circulation time and augment efficient cellular drug uptake, diminish systemic toxicity, traverse the blood-brain barrier, restrict systemic exposure, overcome multidrug resistance, and enhance therapeutic efficacy with disease specificity. This review primarily discusses the various biological actions of Tf, as well as the development of Tf-targeted nano-based drug delivery systems. The goal is to establish the use of Tf as a disease-targeting component, accentuating the potential therapeutic applications of this protein.

In vitro and in silico approach to study the hormonal activities of the alternative plasticizer tri-(2-ethylhexyl) trimellitate TEHTM and its metabolites

Tri-(2-ethylhexyl) trimellitate (TEHTM) is a plasticizer for polyvinyl chloride (PVC) material used in medical devices. It is an alternative to di-(2-ethylhexyl) phthalate (DEHP), a well-known reprotoxic and endocrine disruptor. As plasticizers are known to easily migrate when in contact with fatty biological fluids, patient exposure to TEHTM is highly probable. However, there is currently no data on the potential endocrine-disrupting effects of its human metabolites. To evaluate the effects of TEHTM metabolites on endocrine activity, they were first synthesized and their effects on estrogen, androgen and thyroid receptors, as well as steroid synthesis, were investigated by combining in vitro and in silico approaches. Among the primary metabolites, only 4-MEHTM (4-mono-(2-ethylhexyl) trimellitate) showed agonist activities on ERs and TRs, while three diesters were TR antagonists at non-cytotoxic concentrations. These results were completed by docking experiments which specified the ER and TR isoforms involved. A mixture of 2/1-MEHTM significantly increased the estradiol level and reduced the testosterone level in H295R cell culture supernatants. The oxidized secondary metabolites of TEHTM had no effect on ER, AR, TR receptors or on steroid hormone synthesis. Among the fourteen metabolites, these data showed that two of them (4-MEHTM and 2/1-MEHTM) induced effect on hormonal activities in vitro. However, by comparing the concentrations of the primary metabolites found in human urine with the active concentrations determined in bioassays, it can be suggested that the metabolites will not be active with regard to estrogen, androgen, thyroid receptors and steroidogenesis-mediated effects.

Comparative Effects of Di-(2-ethylhexyl)phthalate and Di-(2-ethylhexyl)terephthalate Metabolites on Thyroid Receptors: In Vitro and In Silico Studies

Plasticizers added to polyvinylchloride (PVC) used in medical devices can be released into patients' biological fluids. Di-(2-ethylhexyl)phthalate (DEHP), a well-known reprotoxic and endocrine disruptor, must be replaced by alternative compounds. Di-(2-ethylhexyl) terephthalate (DEHT) is an interesting candidate due to its lower migration from PVC and its lack of reprotoxicity. However, there is still a lack of data to support the safety of its human metabolites with regard to their hormonal properties in the thyroid system. The effects of DEHT metabolites on thyroid/hormone receptors (TRs) were compared in vitro and in silico to those of DEHP. The oxidized metabolites of DEHT had no effect on T3 receptors whereas 5-hydroxy-mono-(ethylhexyl)phthalate (5-OH-MEHP) appeared to be primarily an agonist for TRs above 0.2 µg/mL with a synergistic effect on T3. Monoesters (MEHP and mono-(2-ethylhexyl)terephthalate, MEHT) were also active on T3 receptors. In vitro, MEHP was a partial agonist between 10 and 20 µg/mL. MEHT was an antagonist at non-cytotoxic concentrations (2-5 µg/mL) in a concentration-dependent manner. The results obtained with docking were consistent with those of the T-screen and provide additional information on the preferential affinity of monoesters and 5-OH-MEHP for TRs. This study highlights a lack of interactions between oxidized metabolites and TRs, confirming the interest of DEHT.

Comparative Analyses of the 12 Most Abundant PCB Congeners Detected in Human Maternal Serum for Activity at the Thyroid Hormone Receptor and Ryanodine Receptor

Polychlorinated biphenyls (PCBs) pose significant risk to the developing human brain; however, mechanisms of PCB developmental neurotoxicity (DNT) remain controversial. Two widely posited mechanisms are tested here using PCBs identified in pregnant women in the MARBLES cohort who are at increased risk for having a child with a neurodevelopmental disorder (NDD). As determined by gas chromatography-triple quadruple mass spectrometry, the mean PCB level in maternal serum was 2.22 ng/mL. The 12 most abundant PCBs were tested singly and as a mixture mimicking the congener profile in maternal serum for activity at the thyroid hormone receptor (THR) and ryanodine receptor (RyR). Neither the mixture nor the individual congeners (2 fM to 2 μM) exhibited agonistic or antagonistic activity in a THR reporter cell line. However, as determined by equilibrium binding of [3H]ryanodine to RyR1-enriched microsomes, the mixture and the individual congeners (50 nM to 50 μM) increased RyR activity by 2.4-19.2-fold. 4-Hydroxy (OH) and 4-sulfate metabolites of PCBs 11 and 52 had no TH activity; but 4-OH PCB 52 had higher potency than the parent congener toward RyR. These data support evidence implicating RyRs as targets in environmentally triggered NDDs and suggest that PCB effects on the THR are not a predominant mechanism driving PCB DNT. These findings provide scientific rationale regarding a point of departure for quantitative risk assessment of PCB DNT, and identify in vitro assays for screening other environmental pollutants for DNT potential.

3,3'-Dichlorobiphenyl (PCB 11) promotes dendritic arborization in primary rat cortical neurons via a CREB-dependent mechanism

PCB 11 (3,3'-dichlorobiphenyl), a contemporary congener produced as a byproduct of current pigment production processes, has recently emerged as a prevalent worldwide pollutant. We recently demonstrated that exposure to PCB 11 increases dendritic arborization in vitro, but the mechanism(s) mediating this effect remain unknown. To address this data gap, primary cortical neuron-glia co-cultures derived from neonatal Sprague-Dawley rats were exposed for 48 h to either vehicle (0.1% DMSO) or PCB 11 at concentrations ranging from 1 fM to 1 nM in the absence or presence of pharmacologic antagonists of established molecular targets of higher chlorinated PCBs. Reporter cell lines were used to test activity of PCB 11 at the aryl hydrocarbon receptor (AhR) and thyroid hormone receptor (THR). PCB 11 lacked activity at the AhR and THR, and antagonism of these receptors had no effect on the dendrite-promoting activity of PCB 11. Pharmacologic antagonism of various calcium channels or treatment with antioxidants also did not alter PCB 11-induced dendritic arborization. In contrast, pharmacologic blockade or shRNA knockdown of cAMP response element-binding protein (CREB) significantly decreased dendritic growth in PCB 11-exposed cultures, suggesting PCB 11 promotes dendritic growth via CREB-mediated mechanisms. Since CREB signaling is crucial for normal neurodevelopment, and perturbations of CREB signaling have been associated with neurodevelopmental disorders, our findings suggest that this contemporary pollutant poses a threat to the developing brain, particularly in individuals with heritable mutations that promote CREB signaling.

Evaluation of neonicotinoid insecticides for oestrogenic, thyroidogenic and adipogenic activity reveals imidacloprid causes lipid accumulation

Few studies have investigated non-target effects of neonicotinoid insecticides on mammalian physiology. This is largely due to the widespread perception that their weak affinity for nicotinic acetylcholine receptor subtypes in vertebrates makes mammalian exposures unlikely to pose health risks. To the best of our knowledge, we describe the first investigation evaluating the interaction of seven principal neonicotinoid insecticides (thiamethoxam, imidacloprid, clothianidin, flupyradifurone, dinotefuran, nitenpyram, thiacloprid) with oestrogen and thyroid hormone receptors, as well as their adipogenic effects, in mammalian cell culture assay systems. An E-Screen with MCF-7 and T-Screen with GH3 cells respectively showed a lack of oestrogen and thyroid hormone receptor agonist effects for any of the neonicotinoids tested. Adipogenicity was assessed by the ability to stimulate lipid accumulation in adipocyte differentiated 3T3-L1 cells, with only imidacloprid scoring positive in this assay causing triglyceride accumulation from a concentration of 50 mg l^-1 . Data mining of ToxCast high-throughput screening assays revealed that this adipogenic effect of imidacloprid is probably mediated via the pregnane X receptor.

A microscale protocol for the isolation of transferrin directly from serum

A microscale procedure for the isolation of transferrin directly from human serum (hTf) is described in this study. The protocol is based on three precipitation steps without application of chromatography. It lasts 90min with the initial sample volume of 250μL. The yield of the isolated hTf is 58%, which is considerable in biochemical terms. The purity of the isolated hTf is 97%, as assessed by three methods: electrophoresis followed by protein staining, immunoblotting and HPLC. Immunoblotting with antibodies against other major serum proteins indicated that isolated hTf does not contain albumin, immunoglobulin G or alpha-2-macroglobulin. Lectin dot-blot demonstrated that isolated hTf preserved its glycan moieties. Fluorescent emission spectroscopy of the isolated hTf has shown no changes in tertiary structure. Isolated hTf was approximately 26% saturated with iron ion, which is comparable to physiological value (although a degree of saturation decreases to some extent during isolation procedure). Finally, co-immunoprecipitation experiment confirmed that isolated hTf retained its ligand characteristics crucial for the ligand-receptor type of interaction with the hTf receptor. To conclude, the procedure described in this work, is time and cost-effective, allows multiple sample handling and provides high-purity hTf isolate with preserved structural and functional properties.

Comparison of in vitro hormone activities of novel flame retardants TBB, TBPH and their metabolites TBBA and TBMEPH using reporter gene assays

The anti-androgenic and anti-thyroid hormonal activities of the two novel brominated flame retardants, TBB and TBPH and of their metabolites TBBA and TBMEPH have been compared using the luciferase reporter gene assays. Only the parent compounds TBB and TBPH exhibited anti-glucocorticoid activity with IC50 values of 1.9 μM and 0.3 μM. Furthermore, mode of action for these two compounds is by direct competing to the glucocorticoid receptor (GR) with IC50 values of 0.03 μM and 0.002 μM. All four tested compounds possess anti-androgenic and anti-thyroid hormonal activities, without agonist activities on the respective receptors. Anti-androgenic activities with IC50 values of 43.5 μM, 0.1 μM, 47.5 μM and 1.3 μM were found for TBB, TBPH, TBBA and TBMEPH. The anti-thyroid hormonal IC50 values of 37.5 μM, 0.1 μM, 22.8 μM and 32.3 μM for TBB, TBPH, TBBA and TBMEPH, together with the above quoted results, indicate that metabolism can modify anti-androgenic, anti-glucocorticoid and anti-thyroid hormonal effects of these novel brominated flame retardants. Furthermore, the parent flame retardants are shown to be able to disrupt the function of the GR as antagonists by direct competition to the receptor.

Influence of metabolism on endocrine activities of bisphenol S

Bisphenol S (BPS; bis[4-hydroxyphenyl]sulfone) is commonly used as a replacement for bisphenol A in numerous consumer products. The main goal of this study was to examine the influence of different metabolic reactions that BPS undergoes on the endocrine activity. We demonstrate that hydroxylation of the aromatic ring of BPS, catalyzed mainly by the cytochrome P450 enzymes CYP3A4 and CYP2C9, is its major in-vitro phase I biotransformation. Nevertheless, coupled oxidative-conjugative reactions analyses revealed that glucuronidation and formation of BPS glucuronide is the predominant BPS metabolic pathway. BPS reactive metabolites that can be tracked as glutathione conjugates were not detected in the present study. Two in-vitro systems were used to evaluate the endocrine activity of BPS and its two main metabolites, BPS glucuronide and hydroxylated BPS 4-(4-hydroxy-benzenesulfonyl)-benzene-1,2-diol (BPSM1). In addition, we have tested two structural analogs of BPS, bis[4-(2-hydroxyetoxy)phenyl]sulfone (BHEPS) and 4,4-sulfonylbis(2-methylphenol) (dBPS). The test systems were yeast cells, for evaluating estrogenic and androgenic activities, and the GH3.TRE-Luc reporter cell line for measuring thyroid hormone activity. BPS and BPSM1 were weak agonists of the estrogen receptor, EC50 values of 8.4 × 10(-5) M and 6.7 × 10(-4) M, respectively. Additionally, BPSM1 exhibited weak antagonistic activity toward the thyroid hormone receptor, with an IC50 of 4.3 × 10(-5) M. In contrast to BPSM1, BPS glucuronide was inactive in these assays, inhibiting neither the estrogen nor the thyroid hormone receptors. Hence, glucuronidation appears to be the most important pathway for both BPS metabolism and detoxification.

T-screen and yeast assay for the detection of the thyroid-disrupting activities of cadmium, mercury, and zinc

In the present study, a two-hybrid yeast bioassay and a T-screen were used to screen for the thyroid receptor (TR)-disrupting activity of select metallic compounds (CdCl2, ZnCl2, HgCl2, CuSO4, MnSO4, and MgSO4). The results reveal that none of the tested metallic compounds showed TR-agonistic activity, whereas ZnCl2, HgCl2, and CdCl2 demonstrated TR antagonism. For the yeast assay, the dose-response relationship of these metallic compounds was established, and the concentrations producing 20 % of the maximum effect of ZnCl2, HgCl2, and CdCl2 were 9.1 × 10(-5), 3.2 × 10(-6), and 1.2 × 10(-6) mol/L, respectively. The T-screen also supported the finding that ZnCl2, HgCl2, and CdCl2 decreased the cell proliferation at concentrations ranging from 10(-6) to 10(-4) mol/L. Furthermore, the thyroid-disrupting activity of metallic compounds in environmental water samples collected from the Guanting Reservoir, Beijing, China was evaluated. Solid-phase extraction was used to separate the organic extracts, and a modified two-hybrid yeast bioassay revealed that the metallic compounds in the water samples could affect thyroid hormone-induced signaling by decreasing the binding of the thyroid hormone. The addition of ethylenediaminetetraacetic acid (30 mg/L) could eliminate the effects. Thus, the cause(s) of the thyroid toxicity in the water samples appeared to be partly related to the metallic compounds.

Pituitary specific retinoid-X receptor ligand interactions with thyroid hormone receptor signaling revealed by high throughput reporter and endogenous gene responses

Disruption of thyroid hormone (TH) signaling can compromise vital processes both during development and in the adult. We previously reported on high-throughput screening experiments for man-made TH disruptors using a stably integrated line of rat pituitary cells, GH3.TRE-Luc, in which a thyroid hormone receptor (TR) response element drives luciferase (Luc) expression. In these experiments, several retinoid/rexinoid compounds activated the reporter. Here we show that all-trans and 13-cis retinoic acid appear to function through the heterodimer partners of TRs, retinoid-X receptors (RXRs), as RXR antagonists abrogated retinoid-induced activation. The retinoids also induced known endogenous TR target genes, showing good correlation with Luc activity. Synthetic RXR-specific agonists significantly activated all tested TR target genes, but interestingly, retinoid/rexinoid activation was more consistent between genes than the extent of T3-induced activation. In contrast, the retinoids neither activated the Luc reporter construct in transient transfection assays in the human hepatocarcinoma cell line HuH7, nor two of the same T3-induced genes examined in pituitary cells. These data demonstrate the suitability and sensitivity of GH3.TRE-Luc cells for screening chemical compound libraries for TH disruption and suggest that the extent of disruption can vary on a cell type and gene-specific bases, including an underappreciated contribution by RXRs.

Identification of thyroid hormone receptor active compounds using a quantitative high-throughput screening platform

To adapt the use of GH3.TRE-Luc reporter gene cell line for a quantitative high-throughput screening (qHTS) platform, we miniaturized the reporter gene assay to a 1536-well plate format. 1280 chemicals from the Library of Pharmacologically Active Compounds (LOPAC) and the National Toxicology Program (NTP) 1408 compound collection were analyzed to identify potential thyroid hormone receptor (TR) agonists and antagonists. Of the 2688 compounds tested, eight scored as potential TR agonists when the positive hit cut-off was defined at ≥10% efficacy, relative to maximal triiodothyronine (T₃) induction, and with only one of those compounds reaching ≥20% efficacy. One common class of compounds positive in the agonist assays were retinoids such as all-trans retinoic acid, which are likely acting via the retinoid-X receptor, the heterodimer partner with the TR. Five potential TR antagonists were identified, including the antiallergy drug tranilast and the anxiolytic drug SB 205384 but also some cytotoxic compounds like 5-fluorouracil. None of the inactive compounds were structurally related to T₃, nor had been reported elsewhere to be thyroid hormone disruptors, so false negatives were not detected. None of the low potency (>100µM) TR agonists resembled T₃ or T₄, thus these may not bind directly in the ligand-binding pocket of the receptor. For TR agonists, in the qHTS, a hit cut-off of ≥20% efficacy at 100 µM may avoid identification of positives with low or no physiological relevance. The miniaturized GH3.TRE-Luc assay offers a promising addition to the in vitro test battery for endocrine disruption, and given the low percentage of compounds testing positive, its high-throughput nature is an important advantage for future toxicological screening.

Transferrin induces interleukin-18 expression in chronic myeloid leukemia cell line, K-562

Transferrin is an iron carrier protein involved in iron uptake and the regulation of cell growth. Although highly proliferative cells express transferrin and its receptor, little is known about the role of transferrin in the cellular response to cytokine production. The non-iron-bound form of transferrin (apo-transferrin) was administered to human chronic myeloid leukemia cell line, K-562 cells to assess whether it could induce interleukin-18 (IL-18). Apo-transferrin enhanced IL-18 mRNA and protein and, moreover, IL-18 secretion was increased by treatment with apo-transferrin. In conclusion, apo-transferrin regulates IL-18 expression and we suggest that it is involved in cytokine production.

T-screen to quantify functional potentiating, antagonistic and thyroid hormone-like activities of poly halogenated aromatic hydrocarbons (PHAHs)

The present study investigates chemical thyroid hormone disruption at the level of thyroid hormone receptor (TR) functioning. To this end the (ant)agonistic action of a series of xenobiotics was tested in the newly developed T-screen. This assay makes use of a GH3 rat pituitary cell line, that specifically proliferates when exposed to 3,3',5-triiodo-L-thyronine (T3). The growth stimulatory effect is mediated via T3-receptors. (Ant)agonistic and potentiating action of compounds was studied in absence and presence of T3 at its EC50 level (0.25 nM). The compounds tested included the specific TR-antagonist amiodarone, as well as a series of brominated diphenyl ethers (BDEs), including specifically synthesized BDEs with a structural resemblance to 3,5-diiodo-L-thyronine (T2), T3 and T4 (3,3',5,5'-tetraiodo-L-thyronine). The results obtained reveal that only BDE206 and amiodarone are specific antagonists. Interestingly some compounds which did not respond in the T-screen in absence of T3, potentiated effects when tested in combination with T3. This points at possibilities for disruption at the TR in vivo, where exposure generally occurs in presence of T3. Altogether the results of the present study show that the newly developed T-screen can be used as a valuable tool for identification and quantification of compounds active in disturbing thyroid hormone homeostasis at the level of TR-functioning.

Transferrin: structure, function and potential therapeutic actions

There are many proteins that can multi-task. Transferrin, widely known as an iron-binding protein, is one such example of a multi-tasking protein. In this review, the multiple biological actions of transferrin, including its growth and cytoprotective activities, are discussed with the view of highlighting the potential therapeutic applications of this protein.

T-Screen as a tool to identify thyroid hormone receptor active compounds

The T-Screen represents an in vitro bioassay based on thyroid hormone dependent cell proliferation of a rat pituitary tumour cell line (GH3) in serum-free medium. It can be used to study interference of compounds with thyroid hormone at the cellular level, thus bridging the gap between limitations of assays using either isolated molecules (enzymes, transport proteins) or complex in vivo experiments with all the complex feedback mechanisms present. Compounds are tested both in the absence and presence of thyroid hormone (EC(50) concentration of T(3)) to test for both agonistic and antagonistic potency. Thyroid hormones (3,3'-5-triiodothyronine: T(3) and 3,3',5,5'-tetraiodothyroxine: T(4)) and compounds resembling the structure of thyroid hormones (3,3'-5-triiodothyroacetic acid: Triac; 3,3',5,5'-tetraiodothyroacetic acid: Tetrac) induced cell growth, with the rank order Triac > T(3) > Tetrac > T(4) (relative potency = 1.35 > 1 > 0.29 > 0.07), which is identical to published affinities of these compounds for nuclear thyroid hormone receptors. Exposure to 5,5'-diphenylhydantoin (DPH) in the presence of 0.25nM T(3) resulted in up to 60% decreased cell growth at 200μM DPH. No effect of DPH on basal metabolic activity of GH3 cells was observed at this concentration. Fentinchloride (IC(50) = 21nM) decreased cell growth induced by 0.25nM T(3), whereas parallel exposure to these concentrations in the absence of T(3) did not alter basal metabolic activities of GH3 cells. Apolar sediment extracts from the Dommel (34%) and Terneuzen (14%) decreased cell growth in the presence of 0.25nM T(3), whereas the extract from Hoogeveen increased cell growth (26%) and the extract from North Sea Channel had no effect. The T-Screen proved to be a fast and functional assay for assessing thyroid hormone receptor active potencies of pure chemicals or environmental mixtures.

Expression, purification, and characterization of authentic monoferric and apo-human serum transferrins

Transferrin is a bilobal protein with the ability to bind iron in two binding sites situated at the bottom of a cleft in each lobe. We have previously described the production of recombinant non-glycosylated human serum transferrins (hTF-NG), containing a factor Xa cleavage site and a hexa-His tag at the amino-terminus. Constructs in this background that contain strategic mutations to completely prevent iron binding in each lobe or in both lobes have now been produced. These monoferric hTFs will allow dissection of the contribution of each lobe to transferrin function. In addition, the construct completely lacking in the ability to bind iron in either lobe provides an opportunity to assess whether hTF has any other functions in addition to iron transport. Following insertion of the His-tagged hTF molecules into the pNUT vector, transfection into baby hamster kidney cells and selection with methotrexate, the secreted recombinant proteins were isolated from the tissue culture medium and characterized with regard to their iron binding properties. Significant improvements over our previous protocol include: (1) addition of butyric acid at a level of 1mM which leads to a substantial increase in protein production (as much as a 65% increase compared to control cells); and (2) elimination of an anion exchange column prior to isolation on a Qiagen Ni-NTA column which makes purification of the His-tagged constructs faster and therefore more efficient. These improvements should be applicable to expression of other recombinant proteins in mammalian cells.

Myelin membranes isolated from rats intracranially injected with apotransferrin are more susceptible to in vitro peroxidation

Purified myelin obtained from 17 day old rats intracranially injected with aTf at 3 days of age was submitted to in vitro peroxidation using Fe + ascorbic acid (FeA) or Cu + H2O2 (CuH), to investigate the susceptibility of this membrane to in vitro metal catalyzed peroxidation. There was an increase in thiobarbituric acid-reactive-substances (TBARS) (60%) and in protein-associated carbonyls (PAC) (20%) in the myelin from aTf injected rats in comparison to myelin from controls, indicating a higher susceptibility to peroxidation. Desferoxamine (DFX) injected simultaneously with aTf did not change the response of myelin to aTf. CNS myelin is highly vulnerable to oxidative stress, and its susceptibility to peroxidation increases in myelin isolated from aTf injected rats. This increased liability to peroxidation as well as the previously reported aTf-dependent increment in certain myelin proteins and lipids and in the expression of specific myelin mRNAS, does not appear to be due to an increased amount of iron bound to the injected aTf. The changes in composition that we have previously reported could result in an abnormal myelin, allowing the peroxidative system to act upon the membrane more easily than in normal circumstances.

Transferrin is an essential factor for myelination

It has been established that oligodendrocytes, the myelin forming cells, participate in iron homeostasis through the synthesis and secretion of transferrin. Here we investigated whether a correlation exists between myelination, the commonly studied function of oligodendrocytes, and that of transferrin synthesis and secretion. We used a proteolipid protein mutant, the myelin deficient rat, whose condition is characterized by severe hypomyelination. We compared the ontogenic profile for transferrin gene expression in mutants with that of unaffected rat pups through northern blot analysis and in situ hybridization. Surprisingly, transferrin synthesis was null in mutant oligodendrocytes. Next, we demonstrated that a single apo-transferrin intraparenchymal injection administered to P5 rat pups enabled mutant oligodendrocytes to synthesize myelin basic protein and to myelinate axons, indicating that transferrin effects mutant oligodendrocyte maturation regardless of its source. Thus, transferrin availability is essential for oligodendrocyte maturation and function, and oligodendrocytes are most vulnerable to transferrin deficiency during the premyelinating stage.

Cloning, characterization, and modeling of a monoclonal anti-human transferrin antibody that competes with the transferrin receptor

In this report we describe the isolation and characterization of a monoclonal antibody against human serum transferrin (Tf) and the cloning and sequencing of its cDNA. The antibody competes with the transferrin receptor (TR) for binding to human Tf and is therefore expected to bind at or very close to a region of interaction between Tf and its receptor. From the deduced amino acid sequence, we constructed a 3-dimensional model of the variable domains of the antibody based on the canonical structure model for the hypervariable loops. The proposed structure of the antibody is a first step toward a more detailed characterization of the antibody-Tf complex and possibly toward a better understanding of the Tf interaction with its receptor. The model might prove useful in guiding site-directed mutagenesis studies, simplifying the experimental elucidation of the antibody structure, and in the use of automatic procedures to dock the interacting molecules as soon as structural information about the structure of the human Tf molecule will be available.

Apotransferrin stimulation of thyroid hormone dependent rat pituitary tumor cell growth in serum-free chemically defined medium: role of FE(III) chelation

Triiodothyronine (T3) dependent growth of GH1 rat pituitary tumor cells in serum-free defined culture requires apotransferrin (apoTf) (Sirbasku et al.: Mol. Cell. Endocrinol., 77:C47-C55, 1991). Diferric transferrin (2Fe.Tf) also is necessary as an iron source (Eby et al.: Anal. Biochem., 203:317-325, 1992). Further, T3 dependence is prevented by soluble Fe(III) addition to the medium (Sato et al.: In Vitro Cell. Dev. Biol., 27A:599-602, 1991). While our data suggested that apoTf caused growth by chelation of Fe(III), direct evidence was required. We used urea polyacrylamide gel electrophoresis along with autoradiography and Western immunoblotting to measure the Fe(III) content of growing GH1 cell cultures and identify the apoTf, mono-metal transferrins and 2Fe.Tf present. We found that apoTf per se did not cause growth but instead chelated inhibitory levels of Fe(III). In fact, apoTf need not be present at all provided that Fe(III) is reduced to < or = 0.6 microM. In addition, other protein and non-protein Fe(III) chelators were shown to be as effective as apoTf. Here, we report that pituitary cells are completely inhibited by > or = 1.2 microM Fe(III), which are concentrations which might be expected in many culture media and usually are not thought to influence growth. The high sensitivity of pituitary cells to Fe(III) suggests further study to determine what cellular functions are affected and how they interfere with thyroid hormone dependence.

Preparation of iron-deficient tissue culture medium by deferoxamine-sepharose treatment and application to the differential actions of apotransferrin and diferric transferrin

We have shown that triiodothyronine-dependent GH1 rat pituitary cell growth in serum-free defined culture required apotransferrin (apoTf) (D. A. Sirbasku, et al., Biochemistry 30, 295-304, 7466-7477, 1991). These studies were done in "low-Fe" medium without Fe(III)/Fe(II) salts. Nonetheless, significant concentrations of iron may have been contributed by other components, making this medium unsuitable for study of the differential effects of apoTf and diferric transferrin (2Fe.Tf). Measuring residual iron in culture medium has been troublesome because the most sensitive method (i.e., atomic absorption) detected levels only in excess of 10 ng/ml and did not distinguish between the forms of iron present. To estimate the Fe(III) available to bind to apoTf, we developed a more sensitive and specific method. Urea-polyacrylamide gel electrophoresis (PAGE) separates apoTf, the two monoferric transferrins, and 2Fe.Tf. [125I]apoTf was incubated with medium, or components, and the formation of [125I]-2Fe.Tf was monitored by urea-PAGE/autoradiography. By this method, the concentration of Fe(III) in low-Fe medium was estimated at 8.4 to 20 ng/ml and the sources were identified. We next sought to remove the Fe(III). Standard chelators were ineffective or cytotoxic. In contrast, an affinity method with deferoxamine-Sepharose depleted greater than or equal to 90% of the Fe(III). In this medium, apoTf and 2Fe.Tf showed differential effects with GH1 cells and with MCF-7, MTW9/PL2, an MDCK cells. With the methods described here, the effects of apoTf and 2Fe.Tf on growth can be studied separately.

Apotransferrins from several species promote thyroid hormone-dependent rat pituitary tumor cell growth in iron-restricted serum-free defined culture

Previously, we have studied thyroid hormone-dependent growth of GH1 rat pituitary tumor cells in iron-restricted serum-free defined medium (Sirbasku, D.A., et al. (1991) Biochemistry 30, 295-304, 7466-7477). Proliferation was promoted by triiodothyronine (T3) and any of seven forms of horse serum-derived apotransferrin (apoTf). In this report, we have asked if apoTfs from other species also acted as thyromedins and if other metal ion chelators served this role. To address these issues, three thyromedins were isolated from human serum and identified as apoTf. Fe3+ depletion, and assay in low-Fe medium, gave ED50s of 1.4-1.7 nM. Fe3+ saturation abolished their activities in high-Fe medium. To ask if apoTf was the major thyromedin in human serum, hormone-depleted preparations were iron saturated and shown to no longer support T3-dependent GH1 cell growth. Next, commercially prepared human, rat, horse, dog, rabbit, guinea pig and mouse apoTfs were shown to be as active under iron-restricted conditions as those isolated from human serum. Bovine apoTf and colostrum lactoferrin were greater than 100-fold less active; human milk apo-lactoferrin and apo-ovotransferrins were inactive. Transferrins which displayed thyromedin activity blocked the binding of 125I-rat 2Fe.Tf to GH1 cell receptors while those without thyromedin activity were ineffective. Finally, the metal ion chelators EDTA, citrate and deferoxamine did not show thyromedin activity indicating that apoTfs uniquely were able to promote T3-dependent cell growth in defined culture.

Thyroid hormone and apotransferrin regulation of growth hormone secretion by GH1 rat pituitary tumor cells in iron restricted serum-free defined medium

Growth hormone (GH) production by GH1 rat pituitary tumor cells in iron restricted serum-free defined medium requires apotransferrin (apoTf) and triiodothyronine (T3). As measured by radioimmunoassay, apoTf plus T3 induced GH levels 2 to 4-fold above controls. Deletion of either apoTf or T3 arrested GH secretion. ApoTf/T3 defined medium regulated GH production as effectively as whole serum. Because glucocorticoids enhance GH secretion in serum containing cultures, the effects of dexamethasone were evaluated in apoTf/T3 defined medium. The steroid hormone showed no enhancing effects unless the cells were exposed to serum prior to incubation in apoTf/T3 defined medium. Even under these conditions, the response to dexamethasone remained T3 dependent. These observations indicate that a yet to be characterized serum factor(s), other than apoTf, regulates the response to the steroid hormone. This is the first report of thyroid hormone regulation of GH secretion by rat pituitary tumor cells under completely serum-free chemically defined conditions.