Retinoid binding to retinol-binding protein and the interference with the interaction with transthyretin

Tetrameric Transthyretin as a Protective Factor Against Alzheimer's Disease

Transthyretin (TTR) is a tetrameric protein traditionally recognized for its role in transporting thyroxine and retinol. Recent research has highlighted the potential neuroprotective functions of TTR in the setting of Alzheimer's disease (AD), which is the most common form of dementia and is caused by the deposition of amyloid beta (Aβ) and the resulting cytotoxic effects. This paper explores the mechanisms of TTR protective action, including its interaction with Aβ to prevent fibril formation and promote Aβ clearance from the brain. It also synthesizes experimental evidence suggesting that enhanced TTR stability may mitigate neurodegeneration and cognitive decline in AD. Potential therapeutic strategies such as small molecule stabilizers of TTR are discussed, highlighting their role in enhancing TTR binding to Aβ and facilitating its clearance. By consolidating current knowledge and proposing directions for future research, this review aims to underscore the significance of TTR as a neuroprotective factor in AD and the potential implications for future research.

Fenretinide in Young Women at Genetic or Familial Risk of Breast Cancer: A Placebo-Controlled Biomarker Trial

Fenretinide, a retinoid with a low-toxicity profile that accumulates in the breast, has been shown to prevent second breast cancer in young women. Fenretinide exhibits apoptotic and antiinvasive properties and it improves insulin sensitivity in overweight premenopausal women with insulin resistance. This study aimed to further characterize its role in cancer prevention by measuring circulating biomarkers related to insulin sensitivity and breast cancer risk.Sixty-two women, ages 20 to 46 years, healthy or who had already undergone breast cancer surgery, with a known BRCA1/2 mutation or a likelihood of mutation ≥20% according to the BRCAPRO model, were randomly assigned to receive fenretinide (200 mg/day) or placebo for 5 years (trial registration: EudraCT No. 2009-010260-41). Fasting blood samples were drawn at baseline, 12 and 36 months, and the following biomarkers were analyzed: retinol, leptin, adiponectin, retinol-binding protein 4 (RBP-4), total cholesterol, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, triglycerides, glucose, insulin, insulin-like growth factor (IGF-1), IGF-binding protein 3, sex hormone binding globulin (SHBG), testosterone, and vascular endothelial growth factor (VEGF).After 12 months of treatment, we observed a favorable effect of fenretinide on glucose (decrease; P = 0.005), insulin (decrease; P = 0.03), homeostatic model assessment index (decrease; P = 0.004), HDL cholesterol (increase; P = 0.002), even though these effects were less prominent after 36 months. Retinol and retinol-binding protein 4 markedly decreased (P < 0.0001) throughout the study. None of the other measured biomarkers changed.

PREVENTION RELEVANCE

Fenretinide exhibits beneficial effects on the metabolic profile, supporting its clinical use in breast cancer prevention especially in premenopausal women with a positive family history and pathogenic variants in BRCA1/2 genes. This finding requires further investigations in larger trials to confirm its role in breast cancer prevention.

Recent Advances in Age-Related Macular Degeneration Therapies

Age-related macular degeneration (AMD) was described for the first time in the 1840s and is currently the leading cause of blindness for patients over 65 years in Western Countries. This disease impacts the eye’s posterior segment and damages the macula, a retina section with high levels of photoreceptor cells and responsible for the central vision. Advanced AMD stages are divided into the atrophic (dry) form and the exudative (wet) form. Atrophic AMD consists in the progressive atrophy of the retinal pigment epithelium (RPE) and the outer retinal layers, while the exudative form results in the anarchic invasion by choroidal neo-vessels of RPE and the retina. This invasion is responsible for fluid accumulation in the intra/sub-retinal spaces and for a progressive dysfunction of the photoreceptor cells. To date, the few existing anti-AMD therapies may only delay or suspend its progression, without providing cure to patients. However, in the last decade, an outstanding number of research programs targeting its different aspects have been initiated by academics and industrials. This review aims to bring together the most recent advances and insights into the mechanisms underlying AMD pathogenicity and disease evolution, and to highlight the current hypotheses towards the development of new treatments, i.e., symptomatic vs. curative. The therapeutic options and drugs proposed to tackle these mechanisms are analyzed and critically compared. A particular emphasis has been given to the therapeutic agents currently tested in clinical trials, whose results have been carefully collected and discussed whenever possible.

The Journey of Human Transthyretin: Synthesis, Structure Stability, and Catabolism

Transthyretin (TTR) is a homotetrameric protein mainly synthesised by the liver and the choroid plexus whose function is to carry the thyroid hormone thyroxine and the retinol-binding protein bound to retinol in plasma and cerebrospinal fluid. When the stability of the tetrameric structure is lost, it breaks down, paving the way for the aggregation of TTR monomers into insoluble fibrils leading to transthyretin (ATTR) amyloidosis, a progressive disorder mainly affecting the heart and nervous system. Several TTR gene mutations have been characterised as destabilisers of TTR structure and are associated with hereditary forms of ATTR amyloidosis. The reason why also the wild-type TTR is intrinsically amyloidogenic in some subjects is largely unknown. The aim of the review is to give an overview of the TTR biological life cycle which is largely unknown. For this purpose, the current knowledge on TTR physiological metabolism, from its synthesis to its catabolism, is described. Furthermore, a large section of the review is dedicated to examining in depth the role of mutations and physiological ligands on the stability of TTR tetramers.

Retinoid Homeostasis and Beyond: How Retinol Binding Protein 4 Contributes to Health and Disease

Retinol binding protein 4 (RBP4) is the specific transport protein of the lipophilic vitamin A, retinol, in blood. Circulating RBP4 originates from the liver. It is secreted by hepatocytes after it has been loaded with retinol and binding to transthyretin (TTR). TTR association prevents renal filtration due to the formation of a higher molecular weight complex. In the circulation, RBP4 binds to specific membrane receptors, thereby delivering retinol to target cells, rendering liver-secreted RBP4 the major mechanism to distribute hepatic vitamin A stores to extrahepatic tissues. In particular, binding of RBP4 to 'stimulated by retinoic acid 6' (STRA6) is required to balance tissue retinoid responses in a highly homeostatic manner. Consequently, defects/mutations in RBP4 can cause a variety of conditions and diseases due to dysregulated retinoid homeostasis and cover embryonic development, vision, metabolism, and cardiovascular diseases. Aside from the effects related to retinol transport, non-canonical functions of RBP4 have also been reported. In this review, we summarize the current knowledge on the regulation and function of RBP4 in health and disease derived from murine models and human mutations.

Biological Functions of RBP4 and Its Relevance for Human Diseases

Retinol binding protein 4 (RBP4) is a member of the lipocalin family and the major transport protein of the hydrophobic molecule retinol, also known as vitamin A, in the circulation. Expression of RBP4 is highest in the liver, where most of the body’s vitamin A reserves are stored as retinyl esters. For the mobilization of vitamin A from the liver, retinyl esters are hydrolyzed to retinol, which then binds to RBP4 in the hepatocyte. After associating with transthyretin (TTR), the retinol/RBP4/TTR complex is released into the bloodstream and delivers retinol to tissues via binding to specific membrane receptors. So far, two distinct RBP4 receptors have been identified that mediate the uptake of retinol across the cell membrane and, under specific conditions, bi-directional retinol transport. Although most of RBP4’s actions depend on its role in retinoid homeostasis, functions independent of retinol transport have been described. In this review, we summarize and discuss the recent findings on the structure, regulation, and functions of RBP4 and lay out the biological relevance of this lipocalin for human diseases.

The interaction of zinc with the multi-functional plasma thyroid hormone distributor protein, transthyretin: evolutionary and cross-species comparative aspects

A considerable body of evidence has been accumulated showing the interrelationship between zinc and the plasma thyroid hormone (TH) distributor protein, transthyretin (TTR). TTR is a multi-functional protein, which emerged from 5-hydroxyisourate hydrolase (HIUHase) by neo-functionalization after gene duplication during early chordate evolution. HIUHase is also a zinc-binding protein. Most biochemical and molecular biological findings have been obtained from mammalian studies. However, in the past two decades, it has become clear that fish TTR displays zinc-dependent TH binding. After a brief introduction on plasma zinc, THs and their binding proteins, this review will focus on the role of zinc in TTR functions of various vertebrates. In particular primitive fish TTR has an extremely high zinc content, with an increased number of histidine residues which are involved in TH binding. However, zinc-dependent TH binding may have been gradually lost from TTRs during higher vertebrate evolution. Although human TTR has a low zinc content, zinc plays an essential role in TTR functions other than TH binding: the stability of TTR-holo retinol binding protein 4 (holoRBP4) complex, TTR amyloidogenesis, the sequestration of amyloid β (Aβ) fibrils and cryptic proteolytic activity. The interaction of TTR with metallothioneins may be a critical step in the exertion of some of these functions. Evolutionary and physiological insights on zinc-dependent functions of TTRs are also discussed.

Retinol binding protein IV purified from Escherichia coli using intein-mediated cleavage as a suitable replacement for serum sources

Retinol binding protein IV (RBP) functions as the principal carrier of retinol (Vitamin A) in the blood, where RBP circulates bound to another serum protein, transthyretin. Isolation of pure RBP from the transthyretin complex in human serum can be difficult, but expression of RBP in recombinant systems can circumvent these purification issues. Human recombinant RBP has previously been successfully expressed and purified from E. coli, but recovery of active protein typically requires extensive processing steps, such as denaturing and refolding, and complex purification steps, such as multi-modal chromatography. Furthermore, these methods produce recombinant proteins, often tagged, that display different functional and structural characteristics across systems. In this work, we optimized downstream processing by use of an intein-based expression system in E. coli to produce tag-free, human recombinant RBP (rRBP) with intact native amino termini at yields of up to ~15 mg/L off column. The novel method requires solubilization of inclusion bodies and subsequent oxidative refolding in the presence of retinol, but importantly allows for one-step chromatographic purification that yields high purity rRBP with no N-terminal Met or other tag. Previously reported purification methods typically require two or more chromatographic separation steps to recover tag-free rRBP. Given the interest in mechanistic understanding of RBP transport of retinol in health and disease, we characterized our purified product extensively to confirm rRBP is both structurally and functionally a suitable replacement for serum-derived RBP.

Retinol-Binding Protein Interferes with Transthyretin-Mediated β-Amyloid Aggregation Inhibition

β-Amyloid (Aβ) aggregation is causally linked to Alzheimer's disease. On the basis of in vitro and transgenic animal studies, transthyretin (TTR) is hypothesized to provide neuroprotection against Aβ toxicity by binding to Aβ and inhibiting its aggregation. TTR is a homotetrameric protein that circulates in blood and cerebrospinal fluid; its normal physiological role is as a carrier for thyroxine and retinol-binding protein (RBP). RBP forms a complex with retinol, and the holoprotein (hRBP) binds with high affinity to TTR. In this study, the role of TTR ligands in TTR-mediated inhibition of Aβ aggregation was investigated. hRBP strongly reduced the ability of TTR to inhibit Aβ aggregation. The effect was not due to competition between Aβ and hRBP for binding to TTR, as Aβ bound equally well to TTR-hRBP complexes and TTR. hRBP is known to stabilize the TTR tetrameric structure. We show that Aβ partially destabilizes TTR and that hRBP counteracts this destabilization. Taken together, our results support a mechanism wherein TTR-mediated inhibition of Aβ aggregation requires not only TTR-Aβ binding but also destabilization of TTR quaternary structure.

Increasing the length and hydrophobicity of the C-terminal sequence of transthyretin strengthens its binding affinity to retinol binding protein

Transthyretin (TTR) is a transporter for thyroid hormone (TH) and retinol, the latter via binding with retinol binding protein (RBP). Both the N‐terminal and C‐terminal regions of the TTR subunit are located in close proximity to the central binding channel for ligands. During the evolution of vertebrates, these regions changed in length and hydropathy. The changes in the N‐terminal sequence were demonstrated to affect the binding affinities for THs and RBP. Here, the effects of changes in the C‐terminal sequence were determined. Three chimeric TTRs, namely pigC/huTTR (human TTR with the C‐terminal sequence changed to that of Sus scrofa TTR), xenoN/pigC/huTTR (human TTR with the N‐terminal and C‐terminal sequences changed to those of Xenopus laevis and S. scrofa, respectively), and pigC/crocTTR (Crocodylus porosus TTR with the C‐terminal sequence changed to that of S. scrofa TTR), were constructed and their binding affinities for human RBP were determined at low TTR/RBP molar ratio using chemiluminescence immunoblotting. The binding dissociation constant (K_d) values of pigC/huTTR, xenoN/pigC/huTTR and pigC/crocTTR were 3.20 ± 0.35, 1.53 ± 0.38 and 0.31 ± 0.04 μm, respectively, and the K_d values of human and C. porosus TTR were 4.92 ± 0.68 and 1.42 ± 0.45 μm, respectively. These results demonstrate chimeric TTRs bound RBP with a higher strength than wild‐type TTRs, and the changes in the C‐terminal sequence of TTR had a positive effect on its binding affinity for RBP. In addition, changes to the N‐terminal and C‐terminal sequences showed comparable effects on the binding affinity.

Novel interactions of domain III from the envelope glycoprotein of dengue 2 virus with human plasma proteins

Blood cells and plasma are important media for the four serotypes of dengue virus (DENV1-4) spreading into an infected person. Thus, interactions with human plasma proteins are expected to be decisive in the course of the viral infection. Affinity purification followed by MS analysis (AP/MS) was used to isolate and identify plasma-derived proteins capable to interact with a recombinant protein comprising the domain III of the envelope protein of DENV2 (DIIIE2). The elution of the AP potently inhibits DENV2 infection. Twenty-nine proteins were identified using a label-free approach as specifically captured by DIIIE2. Of these, a direct interaction with C reactive protein, thrombin and Inter-alpha-inhibitor complexes was confirmed by ELISA. Results provide further evidence of a significant representation of proteins from complement and coagulation cascades on DENV2 interactome in human plasma and stand out the domain III of the viral envelope protein as participant on these interactions. A functional clustering analysis highlights the presence of three structural motifs among putative DIIIE2-binding proteins: hydroxylation and EGF-like calcium-binding- and Gla domains.

BIOLOGICAL SIGNIFICANCE

Early cycles of dengue virus replication take place in human blood cells. Thus, the characterization of the interactome of dengue virus proteins in human plasma can lead to the identification of pivotal interactions for the infection that can eventually constitute the target for the development of methods to control dengue virus-caused disease. In this work we identified 29 proteins from human plasma that potentially interact with the envelope protein of dengue 2 virus either directly or through co-complex formation. C reactive protein, thrombin and Inter-alpha-inhibitor complexes were validated as interactors of the domain III of the envelope protein of dengue 2. Results highlight the presence of three structural motifs among putative DIIIE2-binding proteins: hydroxylation and EGF-like calcium-binding- and Gla domains. This finding together with the participation of domain III of the envelope protein on the interactions with human plasma proteins should contribute to a better understanding of dengue virus interactome in human plasma. Such knowledge can contribute to the development of more effective treatments to infected persons.

Biochemical Basis for Dominant Inheritance, Variable Penetrance, and Maternal Effects in RBP4 Congenital Eye Disease

Gestational vitamin A (retinol) deficiency poses a risk for ocular birth defects and blindness. We identified missense mutations in RBP4, encoding serum retinol binding protein, in three families with eye malformations of differing severity, including bilateral anophthalmia. The mutant phenotypes exhibit dominant inheritance, but incomplete penetrance. Maternal transmission significantly increases the probability of phenotypic expression. RBP normally delivers retinol from hepatic stores to peripheral tissues, including the placenta and fetal eye. The disease mutations greatly reduce retinol binding to RBP, yet paradoxically increase the affinity of RBP for its cell surface receptor, STRA6. By occupying STRA6 nonproductively, the dominant-negative proteins disrupt vitamin A delivery from wild-type proteins within the fetus, but also, in the case of maternal transmission, at the placenta. These findings establish a previously uncharacterized mode of maternal inheritance, distinct from imprinting and oocyte-derived mRNA, and define a group of hereditary disorders plausibly modulated by dietary vitamin A.

Investigation of oral fenretinide for treatment of geographic atrophy in age-related macular degeneration

BACKGROUND

Excessive accumulation of retinol-based toxins has been implicated in the pathogenesis of geographic atrophy (GA). Fenretinide, an orally available drug that reduces retinol delivery to the eye through antagonism of serum retinol-binding protein (RBP), was used in a 2-year trial to determine whether retinol reduction would be effective in the management of geographic atrophy.

METHODS

The efficacy of fenretinide (100 and 300 mg daily, orally) to slow lesion growth in geographic atrophy patients was examined in a 2-year, placebo-controlled double-masked trial that enrolled 246 patients at 30 clinical sites in the United States.

RESULTS

Fenretinide treatment produced dose-dependent reversible reductions in serum RBP-retinol that were associated with trends in reduced lesion growth rates. Patients in the 300 mg group who achieved serum retinol levels of ≤ 1 μM (≤ 2 mg/dL RBP) showed a mean reduction of 0.33 mm in the yearly lesion growth rate compared with subjects in the placebo group (1.70 mm/year vs. 2.03 mm/year, respectively, P = 0.1848). Retinol-binding protein reductions <2 mg/dL correlated with further reductions in lesion growth rates (r = 0.478). Fenretinide treatment also reduced the incidence of choroidal neovascularization (approximately 45% reduction in incidence rate in the combined fenretinide groups vs. placebo, P = 0.0606). This therapeutic effect was not dose dependent and is consistent with anti-angiogenic properties of fenretinide, which have been observed in other disease states.

CONCLUSION

The findings of this study and the established safety profile of fenretinide in chronic dosing regimens warrant further study of fenretinide in the treatment of geographic atrophy.

Retinol binding protein 4 affects the adipogenesis of porcine preadipocytes through insulin signaling pathways

Retinol binding protein 4 (RBP4), a novel cytokine, is mainly secreted by hepatocytes and adipocytes. RBP4 reportedly induces insulin resistance and RBP4 secretion is increased in the adipocytes of animals or humans with type 2 diabetes, obesity, and metabolic syndrome, but its role in preadipocyte differentiation remains unclear. In this study, we investigated the effect of RBP4 on the differentiation of porcine preadipocytes into adipocytes. The results suggest that RBP4 significantly suppresses the differentiation of porcine preadipocytes into adipocytes, including those treated with the hormone cocktail methylisobutylxanthine-dexamethasone-insulin. RBP4 also weakened the activity of normal threonine 308, the phosphorylation of serine/threonine kinase AKT, and downstream insulin signaling, including the mammalian target of rapamycin (mTOR) and β-catenin. Moreover, the activation of insulin signaling mediated by knockdown RBP4 in porcine preadipocytes was recovered in the suppression of LY294002. RBP4 also had a suppressive effect on the differentiation of porcine preadipocytes by decreasing the activation of insulin signaling pathways.

Exome analysis identified a novel mutation in the RBP4 gene in a consanguineous pedigree with retinal dystrophy and developmental abnormalities

Retinitis Pigmentosa (RP) is a common form of retinal degeneration characterized by photoreceptor degeneration and retinal pigment epithelium (RPE) atrophy causing loss of visual field and acuities. Exome sequencing identified a novel homozygous splice site variant (c.111+1G>A) in the gene encoding retinol binding protein 4 (RBP4). This change segregated with early onset, progressive, and severe autosomal recessive retinitis pigmentosa (arRP) in an eight member consanguineous pedigree of European ancestry. Additionally, one patient exhibited developmental abnormalities including patent ductus arteriosus and chorioretinal and iris colobomas. The second patient developed acne from young age and extending into the 5^th decade. Both patients had undetectable levels of RBP4 in the serum suggesting that this mutation led to either mRNA or protein instability resulting in a null phenotype. In addition, the patients exhibited severe vitamin A deficiency, and diminished serum retinol levels. Circulating transthyretin levels were normal. This study identifies the RBP4 splice site change as the cause of RP in this pedigree. The presence of developmental abnormalities and severe acne in patients with retinal degeneration may indicate the involvement of genes that regulate vitamin A absorption, transport and metabolism.

Fenretinide derivatives act as disrupters of interactions of serum retinol binding protein (sRBP) with transthyretin and the sRBP receptor

Serum retinol binding protein (sRBP) is released from the liver as a complex with transthyretin (TTR), a process under the control of dietary retinol. Elevated levels of sRBP may be involved in inhibiting cellular responses to insulin and in generating first insulin resistance and then type 2 diabetes, offering a new target for therapeutic attack for these conditions. A series of retinoid analogues were synthesized and examined for their binding to sRBP and their ability to disrupt the sRBP-TTR and sRBP-sRBP receptor interactions. A number inhibit the sRBP-TTR and sRBP-sRBP receptor interactions as well as or better than Fenretinide (FEN), presenting a potential novel dual mechanism of action and perhaps offering a new therapeutic intervention against type 2 diabetes and its development. Shortening the chain length of the FEN derivative substantially abolished binding to sRBP, indicating that the strength of the interaction lies in the polyene chain region. Differences in potency against the sRBP-TTR and sRBP-sRBP receptor interactions suggest variant effects of the compounds on the two loops of sRBP guarding the entrance of the binding pocket that are responsible for these two protein-protein interactions.

The membrane receptor for plasma retinol-binding protein, a new type of cell-surface receptor

Vitamin A is essential for diverse aspects of life ranging from embryogenesis to the proper functioning of most adult organs. Its derivatives (retinoids) have potent biological activities such as regulating cell growth and differentiation. Plasma retinol-binding protein (RBP) is the specific vitamin A carrier protein in the blood that binds to vitamin A with high affinity and delivers it to target organs. A large amount of evidence has accumulated over the past decades supporting the existence of a cell-surface receptor for RBP that mediates cellular vitamin A uptake. Using an unbiased strategy, this specific cell-surface RBP receptor has been identified as STRA6, a multitransmembrane domain protein with previously unknown function. STRA6 is not homologous to any protein of known function and represents a new type of cell-surface receptor. Consistent with the diverse functions of vitamin A, STRA6 is widely expressed in embryonic development and in adult organ systems. Mutations in human STRA6 are associated with severe pathological phenotypes in many organs such as the eye, brain, heart, and lung. STRA6 binds to RBP with high affinity and mediates vitamin A uptake into cells. This review summarizes the history of the RBP receptor research, its expression in the context of known functions of vitamin A in distinct human organs, structure/function analysis of this new type of membrane receptor, pertinent questions regarding its very existence, and its potential implication in treating human diseases.

Trapping of palindromic ligands within native transthyretin prevents amyloid formation

Transthyretin (TTR) amyloidosis is a fatal disease for which new therapeutic approaches are urgently needed. We have designed two palindromic ligands, 2,2'-(4,4'-(heptane-1,7-diylbis(oxy))bis(3,5-dichloro-4,1-phenylene)) bis(azanediyl)dibenzoic acid (mds84) and 2,2'-(4,4'-(undecane-1,11-diylbis(oxy))bis(3,5-dichloro-4,1-phenylene)) bis(azanediyl)dibenzoic acid (4ajm15), that are rapidly bound by native wild-type TTR in whole serum and even more avidly by amyloidogenic TTR variants. One to one stoichiometry, demonstrable in solution and by MS, was confirmed by X-ray crystallographic analysis showing simultaneous occupation of both T4 binding sites in each tetrameric TTR molecule by the pair of ligand head groups. Ligand binding by native TTR was irreversible under physiological conditions, and it stabilized the tetrameric assembly and inhibited amyloidogenic aggregation more potently than other known ligands. These superstabilizers are orally bioavailable and exhibit low inhibitory activity against cyclooxygenase (COX). They offer a promising platform for development of drugs to treat and prevent TTR amyloidosis.

Assay of retinol-binding protein-transthyretin interaction and techniques to identify competing ligands

The principles of fluorescence resonance energy transfer have been utilized to develop a high-throughput assay which detects compounds that interfere with interaction between retinol-binding protein (RBP) and transthyretin (TTR). In this assay, the intrinsic fluorescence from the RBP-retinol complex excites a probe molecule which is covalently coupled to TTR. Generation of an emission signal from the TTR probe indicates interaction between RBP-retinol and TTR. Importantly, the inclusion of retinol in the assay allows discrimination of test compounds which bind RBP versus those which bind to TTR. Thus, compounds which bind to RBP must compete with retinol in order to affect RBP-TTR interaction. This feature of the assay will be useful to identify test compounds which are more likely to have an effect in vivo.

The interaction between retinol-binding protein and transthyretin analyzed by fluorescence anisotropy

The retinol carrier retinol-binding protein (RBP) forms in blood a complex with the thyroid hormone carrier transthyretin (TTR). The interactions of retinoid-RBP complexes, as well as of unliganded RBP, with TTR can be investigated by means of fluorescence anisotropy. RBP represents the prototypic lipocalin, in the internal cavity of which the retinol molecule is accommodated. Due to the tight binding of retinol within a substantially apolar binding site, an intense fluorescence emission characterizes the RBP-bound vitamin. The addition of TTR to the retinol-RBP complex (holoRBP) causes a marked increase in the fluorescence anisotropy of the RBP-bound retinol within the system, due to the formation of the holoRBP-TTR complex, which allows the interaction between the two proteins to be monitored. The fluorescence anisotropy technique is also suitable to study the interaction of TTR with apoRBP and RBP in complex with non-fluorescent retinoids. In the latter cases, the fluorescence signal is provided by a fluorescent probe covalently linked to TTR rather than by RBP-bound retinol. We report here on the preparation of recombinant human RBP and TTR, the covalent labeling of TTR with the fluorescent dansyl probe, and fluorescence anisotropy titrations for RBP and TTR.

Structural and mutational analyses of protein-protein interactions between transthyretin and retinol-binding protein

Transthyretin is a tetrameric binding protein involved in the transport of thyroid hormones and in the cotransport of retinol by forming a complex in plasma with retinol-binding protein. In the present study, we report the crystal structure of a macromolecular complex, in which human transthyretin, human holo-retinol-binding protein and a murine anti-retinol-binding protein Fab are assembled according to a 1 : 2 : 2 stoichiometry. The main interactions, both polar and apolar, between retinol-binding protein and transthyretin involve the retinol hydroxyl group and a limited number of solvent exposed residues. The relevance of transthyretin residues in complex formation with retinol-binding protein has been examined by mutational analysis, and the structural consequences of some transthyretin point mutations affecting protein-protein recognition have been investigated. Despite a few exceptions, in general, the substitution of a hydrophilic for a hydrophobic side chain in contact regions results in a decrease or even a loss of binding affinity, thus revealing the importance of interfacial hydrophobic interactions and a high degree of complementarity between retinol-binding protein and transthyretin. The effect is particularly evident when the mutation affects an interacting residue present in two distinct subunits of transthyretin participating simultaneously in two interactions with a retinol-binding protein molecule. This is the case of the amyloidogenic I84S replacement, which abolishes the interaction with retinol-binding protein and is associated with an altered retinol-binding protein plasma transport in carriers of this mutation. Remarkably, some of the residues in mutated human transthyretin that weaken or abolish the interaction with retinol-binding protein are present in piscine transthyretin, consistent with the lack of interaction between retinol-binding protein and transthyretin in fish.

Application of an allosteric model to describe the interactions among retinol binding protein 4, transthyretin, and small molecule retinol binding protein 4 ligands

Retinol binding protein 4 (RBP4) is a serum protein that serves as the major transport protein for retinol (vitamin A). Recent reports suggest that elevated levels of RBP4 are associated with insulin resistance and that insulin sensitivity may be improved by reducing serum RBP4 levels. This can be accomplished by administration of small molecules, such as fenretinide, that compete with retinol for binding to RBP4 and disrupt the protein-protein interaction between RBP4 and transthyretin (TTR), another serum protein that protects RBP4 from renal clearance. We developed a fluorescence resonance energy transfer (FRET) assay that measures the interaction between RBP4 and TTR and can be used to determine the binding affinities of RBP4 ligands. We present an allosteric model that describes the pharmacology of interaction among RBP4, TTR, retinol, and fenretinide, and we show data that support the model. We show that retinol increases the affinity of RBP4 for TTR by a factor of 4 and determine the affinity constants of fenretinide and retinyl acetate. The assay may be useful for characterizing small molecule ligands that bind to RBP4 and disrupt its interaction with TTR. In addition, such a model could be used to describe other protein-protein interactions that are modulated by small molecules.

Retinol-binding protein-4 in experimental and clinical metabolic disease

Retinol-binding protein-4 (RBP4), a 21-kDa protein synthesized in the liver and adipose tissue, has recently been described as a murine adipokine involved in the development of insulin resistance. The expression of the gene encoding RBP4 was increased in the adipose tissue, but not in the liver, of insulin-resistant adipose GLUT4(-/-) mice and five other mouse models of obesity and insulin resistance. In addition, intraperitoneal injection or transgenic overexpression of RBP4 in mice induced insulin resistance. While experimental clinical approaches (mostly applying clamp techniques) in humans confirmed correlations of RBP4 with insulin resistance, studies in larger groups out of clinical routine failed to demonstrate a connection with alternative measures of insulin sensitivity. Yet, significant associations of RBP4 with atherogenic lipids were found and a focus of future studies should be the influence on atherosclerosis and related complications. Based on current data, the function of RBP4 as an adipokine exerting metabolic effects in glucose metabolism in humans remains uncertain and might be restricted to rodent models.

Serum retinol-binding protein: a link between obesity, insulin resistance, and type 2 diabetes

Insulin resistance occurs under conditions of obesity, metabolic syndrome, and type 2 diabetes. It was found to be accompanied by down-regulation of the insulin-responsive glucose transporter GLUT4. Decreased adipocyte GLUT4 caused secretion by adipocytes of the serum retinol-binding protein RBP4. Enhanced levels of serum RBP4 appeared to be the signal for the development of systemic insulin resistance both in experimental animals and in humans. In mice, increased levels of serum RBP4 led to impaired glucose uptake into skeletal muscle and increased glucose production by liver, whereas lowered serum RBP4 levels greatly enhanced insulin sensitivity. Thus, a link has been established between obesity and insulin resistance: RBP4, the vitamin A-transport protein secreted into the circulation by adipocytes.

Retinol-binding protein 4 in human obesity

Studies in mice suggest that adipocytes serve as glucose sensors and regulate systemic glucose metabolism through release of serum retinol-binding protein 4 (RBP4). This model has not been validated in humans. RBP4 was highly expressed in isolated mature human adipocytes and secreted by differentiating human adipocytes. In contrast to the animal data, RBP4 mRNA was downregulated in subcutaneous adipose tissue of obese women, and circulating RBP4 concentrations were similar in normal weight, overweight, and obese women (n = 74). RBP4 was positively correlated with GLUT4 expression in adipose tissue, independent of any obesity-associated variable. Five percent weight loss slightly decreased adipose RBP4 expression but did not influence circulating RBP4. In another set of experiments, we stratified patients (n = 14) by low or high basal fasting interstitial glucose concentrations, as determined by the microdialysis technique. Venous glucose concentrations were similar throughout oral glucose tolerance testing, and basal RBP4 expression in adipose tissue and serum RBP4 concentrations were similar in the groups with higher and lower interstitial glucose levels. Our findings point to profound differences between rodents and humans in the regulation of adipose or circulating RBP4 and challenge the notion that glucose uptake by adipocytes has a dominant role in the regulation of RBP4.

Biochemical basis for retinol deficiency induced by the I41N and G75D mutations in human plasma retinol-binding protein

Retinol-binding protein (RBP) is the retinol-specific carrier protein present in plasma, where it circulates almost entirely bound to thyroxine-binding transthyretin (TTR). Recently, depressed plasma retinol and RBP levels in carriers of the I41N and G75D RBP point mutations have been reported. We show here that although recombinant human N41 and D75 RBPs can form complexes with retinol and TTR in vitro, the retinol-mutated RBP complexes are significantly less stable than human normal holo-RBP, as revealed by the markedly facilitated retinol release by mutated holo-RBPs to phospholipid membranes, in accordance with the location of mutated residues inside the RBP retinol-binding cavity. Taken together, the data are consistent with the I41N and G75D point mutations being the cause of an altered interaction of retinol with RBP, resulting in a remarkably reduced stability of the retinol-RBP complex, which in turn can lead to the lowering of plasma retinol and RBP levels.

Serum retinol binding protein 4 contributes to insulin resistance in obesity and type 2 diabetes

In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4(-/-)) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4(-/-) mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes.

Distinctive binding and structural properties of piscine transthyretin

The thyroid hormone binding protein transthyretin (TTR) forms a macromolecular complex with the retinol-specific carrier retinol binding protein (RBP) in the blood of higher vertebrates. Piscine TTR is shown here to exhibit high binding affinity for L-thyroxine and negligible affinity for RBP. The 1.56 A resolution X-ray structure of sea bream TTR, compared with that of human TTR, reveals a high degree of conservation of the thyroid hormone binding sites. In contrast, some amino acid differences in discrete regions of sea bream TTR appear to be responsible for the lack of protein-protein recognition, providing evidence for the crucial role played by a limited number of residues in the interaction between RBP and TTR. Overall, this study makes it possible to draw conclusions on evolutionary relationships for RBPs and TTRs of phylogenetically distant vertebrates.

Plasma Retinol-Binding Protein: Structure and Interactions with Retinol, Retinoids, and Transthyretin

Retinol-binding protein (RBP) is the retinol-specific transport protein present in plasma. The available crystal structures of different forms of RBP have provided details of the interactions of this binding protein with retinol, retinoids, and transthyretin (TTR, one of the plasma carriers of thyroid hormones). The core of RBP is a beta-barrel, the cavity of which accommodates retinol, establishing with its buried portions apolar contacts. Instead, the retinol hydroxyl is near the protein surface, in the region of the entrance loops surrounding the opening of the binding cavity, and participates in polar interactions. The stability of the retinol-RBP complex appears to be further enhanced when holo-RBP is bound to TTR. Accordingly, the region of the entrance loops represents the contact area of RBP interacting with the TTR counterpart, such that the hydroxyl of the RBP-bound vitamin becomes fully buried in the holo-RBP-TTR complex. Limited protein conformational changes affecting the entrance loops, which lead to a decrease or loss of the binding affinity of RBP for TTR, have been demonstrated for apo-RBP and RBP in complex with retinoids modified in the area of the retinol hydroxyl. A relatively small number of amino acid residues of RBP, essentially confined to the region of the entrance loops, and of TTR appear to play a critical role in the formation of the RBP-TTR complex, as established by crystallographic studies, mutational analysis, and amino acid sequence analysis of phylogenetically distant RBPs and TTRs. Overall, the available evidence indicates the existence of a high degree of complementarity between RBP and TTR, the contact areas of which are highly sensitive to conformational changes and amino acid replacements.

Retinoids regulate survival and antigen presentation by immature dendritic cells

Maturation of dendritic cells (DCs) is a critical step for the induction of an immune response. We have examined the role of retinoid nuclear receptor pathways in this process. Retinoids induce DC apoptosis, in the absence of inflammatory signals, through retinoic acid receptor (RAR)alpha/retinoic X receptor (RXR) heterodimers. In contrast, via a cross talk with inflammatory cytokines, retinoids increase DNA binding activity of nuclear factor kappaB in DCs, trigger membrane major histocompatibility complex class II and costimulatory molecule expression, induce the differentiation of immature DCs into mature DCs, and enhance antigen-specific T cell response. This maturation of DCs is mediated via a RXR-dependent/RAR-independent pathway and via an RARalpha/RXR pathway distinct from the one responsible for apoptosis. Apoptosis and activation, mediated through distinct nuclear retinoid receptor pathways, can be dissociated from each other with selective synthetic retinoids. We identify a novel cellular function for retinoids and suggest that selective retinoids might be of interest for controlling antigen presentation.

Affinity capillary electrophoresis using a low-concentration additive with the consideration of relative mobilities

Most affinity studies in capillary electrophoresis assume that the analyte concentration is much smaller than the additive concentration so that the migration of the analyte has no effect on the concentration of the additive in the capillary. However, in most medium- to high-affinity interactions, the additive concentration has to be kept rather low to observe the changes in analyte mobility before saturation is reached. In this paper, a mathematical model is developed to describe the migration behavior of the analyte in a system where the complex formed becomes concentrated to levels much greater than the original concentration of the additive due to the differences in the mobilities of the analyte, additive, and complex. The analyte is flurbiprofen, the additive is transthyretin, and the stoichiometry of the reaction between the two is 1:2. This study also provides a new algorithm to determine medium- to high-affinity binding interaction constants by CE.

Structure of chicken plasma retinol-binding protein

The crystal structure of the specific carrier of retinol (retinol-binding protein, RBP) purified from chicken plasma has been determined (space group P2(1)2(1)2(1), with a=46.06(5) A, b=53.56(6) A, c=73.41(8) A, and one protein molecule in the asymmetric unit). Despite being obtained from a species phylogenetically distant from mammals, chicken holoRBP has an overall structure that closely resembles the previously determined structures of mammalian holoRBPs. The lack in chicken RBP of eight carboxy-terminal amino acid residues characteristic of mammalian RBPs does not significantly affect the protein structure. A distinctive feature of the avian protein is a better definition of the loop 63-67, close to the opening of the beta-barrel cavity accommodating the retinol molecule, which is rather disordered in the structures of mammalian RBPs.

Regulation by retinoic acid of acylation-stimulating protein and complement C3 in human adipocytes

Acylation-stimulating protein (ASP), a product of complement C3, stimulates triacylglycerol synthesis in adipocytes. Previous studies have identified transthyretin, associated with chylomicrons, as a stimulator of C3 and ASP production. Since both transthyretin and chylomicrons transport retinyl ester/retinol, our goal was to investigate whether retinoic acid (RA) could be a potential hormonal mediator of the effect. Inhibitors of protein synthesis and protein secretion eliminated the stimulatory effects of chylomicrons on both C3 and ASP production in human differentiated adipocytes, suggesting that de novo protein synthesis and secretion are both required. Incubation with chylomicrons increased C3 mRNA levels (37+/-1.5%). RA alone or with chylomicrons had a stimulatory effect on C3 production (29-fold at 16.6 nM RA) and ASP production. An RA receptor antagonist blocked stimulation of C3 mRNA and C3 secretion by both RA and chylomicrons. Finally, RA and chylomicrons activated a 1.8 kb C3-promoter-luciferase construct transfected into 3T3-F442 and 3T3-L1 cells (by 41+/-0.2% and 69+/-0.3% respectively), possibly via RA receptor half-sites identified by sequence analysis. This is the first evidence documenting stimulation by RA of the C3 gene. Thus we propose RA as a novel cellular trigger in chylomicrons that subsequently results in increased ASP production by adipocytes after a meal.

The transthyretin-retinol-binding protein complex

Transthyretin (TTR, formerly called prealbumin), one of the transporters of the hormone thyroxine and the lipocalin retinol-binding protein (RBP), the specific carrier of the vitamin, are known to form, under physiological conditions, a macromolecular complex that is believed to play an important physiological role: prevention of glomerular filtration of the low molecular weight RBP in the kidneys. The physiological significance of complex formation is discussed first, followed by a brief description of the three-dimensional structure of the two participating proteins. The two X-ray models of the complex available are subsequently discussed and compared and finally the non-crystallographic evidence that supports these models is reviewed.

Amphibian choroid plexus lipocalin, Cpl1

Choroid plexus lipocalin 1 (Cpl1) has been isolated from the African clawed toad (Xenopus laevis) and the cane toad (Bufo marinus). Xcpl1 has been used as a marker for studying early neural development. Due to its retinoid binding properties and the fact that it causes dysmorphogenesis when overexpressed in the early embryo, the protein product is considered to be part of the retinoic acid signalling pathway. Later in development and during adulthood, the epithelial cell sheet of the choroid plexus which forms the blood-cerebrospinal fluid barrier expresses cpl1 as the predominant secretory protein. These data, the similarity of Cpl1 to prostaglandin D(2) synthase and its functional homology to transthyretin will be discussed.

Affinity capillary electrophoresis is a powerful tool to identify transthyretin binding drugs for potential therapeutic use in amyloidosis

In this work we used affinity capillary electrophoresis (ACE) to investigate the extent of interaction between a pool of drugs and wild-type transthyretin. After qualitative preliminary screening, attention was focused on the most promising molecules, flufenamic acid and flurbiprofen, which underwent a further stage of investigation, the determination of the binding constants, and, when possible, the assessment of the number of binding sites by ACE, frontal analysis (FA) capillary electrophoresis (CE) and parallel ultrafiltration (UF) experiments. Furthermore, our data demonstrate that FA CE is a suitable technique for identifying fibril ligands. This represents a novel CE application of pharmaceutical interest.

Structure at 1.44 A resolution of an N-terminally truncated form of the rat serum complement C3d fragment

Complement component C3 plays a key role in the complement-mediated immune defence, and occupies a central position within the complement cascade system. One of its degradation products, C3dg, was purified from rat serum and crystallised in two different crystal forms as N-terminally truncated fragment. Despite the truncation and the lack of a significant portion of the N-terminus as compared to C3d, the structure of the fragment is highly similar to that of recombinant human C3d (Nagar et al., Science 280 (1998) 1277-1281). Structural details of the reactive site have been obtained, suggesting a possible mode of thioester bond formation between Cys-1010 and Gln-1013 and thioester bond cleavage in the transacylation reaction involving His-1126. The truncation at the N-terminus of C3d leads to the exposure of a surface of the molecule that favours dimerisation, so that in both crystal forms, the fragment is present as a dimer, with monomers related by a two-fold axis.

Endemic goiter with iodine sufficiency: a possible role for the consumption of pearl millet in the etiology of endemic goiter

BACKGROUND

Deficiencies of iodine, iron, and vitamin A are the 3 most common micronutrient deficiencies in developing countries, although control programs, when properly implemented, can be effective.

OBJECTIVE

We investigated these deficiencies and their possible interaction in preschool children in the southern Blue Nile area of Sudan.

DESIGN

Goiter, signs of vitamin A deficiency, and biochemical markers of thyroid, vitamin A, and iron status were assessed in 984 children aged 1-6 y.

RESULTS

The goiter rate was 22. 3%. The median urinary iodine concentration was 0.79 micromol/L and 19.3% of the children had a concentration >1.57 micromol/L. Although serum thyroxine and triiodothyronine concentrations were within reference ranges, the median thyrotropin concentration was 3.78 mIU/L and 44% of the children had thyrotropin concentrations above normal. The mean urinary thiocyanate concentration was high (259 +/- 121 micromol/L). The prevalences of Bitot spots and night blindness were 2.94% and 2.64%, respectively, and 32% of the subjects had serum retinol binding protein concentrations <15 mg/L. A significant positive correlation was observed between thyrotropin and retinol binding protein. Whereas 88% of the children had hemoglobin concentrations <1.86 mmol/L, only 13.5% had serum ferritin concentrations below the cutoff of 12 microg/L and 95% had serum transferrin concentrations above the cutoff of 2.50 g/L.

CONCLUSIONS

Our results indicate that goiter is endemic in this region of Sudan despite iodine sufficiency and that both anemia and vitamin A deficiency are health problems in the area. Moreover, consumption of millet, vitamin A deficiency, and protein-energy malnutrition are possible etiologic factors in this endemic area.

Production of monoclonal antibodies and enzyme immunoassay to bovine retinol-binding protein and determination of retinol-binding protein serum levels and retinol concentrations in serum and liver in dairy cows before and after parturition

To study vitamin A transport in dairy cows and heifers around parturition, an enzyme immunoassay for bovine retinol binding protein (RBP) was developed and serum levels determined. Serum and liver concentrations of retinol were assayed by HPLC. Four weeks before expected calving the cows and heifers were divided into two groups each, and half of the animals received a protein supplementation during the dry period. The mean serum RBP concentration 4 weeks before calving was 42 mg l-1 for the cows and 44 mg l-1 for the heifers. The serum retinol concentrations were 0.53 mg l-1 for the cows and 0.42 mg l-1 for the heifers, and the liver retinol concentrations 0.30 mg l-1 and 0.13 mg g-1, respectively. In the groups without protein supplementation there was a significant decrease in serum RBP at sampling 1 week before parturition compared to initial values. The measurement of serum RBP may prove useful in assessment of amino acid availability in dairy cows.

Chylomicron-specific enhancement of acylation stimulating protein and precursor protein C3 production in differentiated human adipocytes

Acylation stimulating protein (ASP) is a potent stimulator of adipocyte triacylglycerol storage. In vivo studies have shown that ASP production by adipocytes increases locally after a fat meal. Initial in vitro studies demonstrated increased production of ASP in the presence of chylomicrons (CHYLO). The present aim was to define the CHYLO component responsible. None of the apoproteins tested (AI, AII, AIV, CI, CII, CIII, and E) were capable of stimulating C3 (the precursor protein) or ASP production. Rather, the active component is a nonlipid, loosely associated, trypsin-sensitive molecule. High pressure liquid chromatography fractionation of the CHYLO infranate proteins identified the critical protein as transthyretin (TTR), which binds retinol-binding protein and complexes thyroxine and retinol. Addition of TTR alone, with lipid emulsion, or with respun CHYLO to human differentiated adipocytes had little effect on C3 and ASP production. By contrast, when transthyretin was added to CHYLO, C3 and ASP production were substantially enhanced up to 75- and 7. 5-fold respectively, compared with the effect of native CHYLO alone. Finally, a polyclonal antibody against TTR could inhibit stimulation of C3 and ASP production by CHYLO (by 98 and 100%, respectively) and by CHYLO infranate proteins (by 99 and 94%, respectively). We hypothesize that TTR mediates the transfer of the active components from CHYLO to adipocytes, which then stimulates increased C3 and ASP production. Thus the CHYLO provides the physiologic trigger of the ASP pathway.

Secretion of N-(4-hydroxyphenyl) retinamide-retinol-binding protein from liver parenchymal cells: evidence for reduced affinity of the complex for transthyretin

The synthetic retinoid 4-HPR has been shown to markedly lower the plasma concentration of both retinol and RBP in rats and humans. We have studied the effect of 4-HPR on the secretion of retinol-RBP from liver cells in vivo and in vitro. In rats maintained with a normal diet, a vitamin A-deficient diet or a normal diet supplemented with 4-HPR, chylomicrons [3H]retinyl esters were rapidly cleared from the plasma. The secretion of chylomicron-derived [3H]retinol from tissues to the circulation, however, was different. In control rats, the lymph-derived [3H]retinol peaked after about 2 hr, whereas 4-HPR treatment effectively reduced this peak of [3H]retinol. Our results suggest that 4-HPR inhibits secretion of retinol-RBP from the liver. Therefore, we decided to study the effect of 4-HPR on the secretion of RBP using the human hepatoma cell line HepG2. Retinol and 4-HPR were found to induce the secretion of RBP. The medium from cells treated with 4-HPR was immunoprecipitated with antibodies against human RBP. HPLC analysis of the precipitated RBP revealed the presence of 4-HPR. When the medium from cells incubated with either 4-HPR or retinol was applied to a TTR affinity column, we found that RBP from cells incubated with 4-HPR had a considerably reduced affinity for TTR. We conclude that 4-HPR binds RBP and thereby induces secretion of RBP in HepG2 cells, and that the secreted 4-HPR-RBP complex has a reduced affinity for TTR. This observation may explain the 4-HPR-induced reduction of plasma retinol and RBP observed in in vivo studies.