Solubilization and purification of the retinol-binding protein receptor from human placental membranes

Structure of the STRA6 receptor for retinol uptake

Vitamin A homeostasis is critical to normal cellular function. Retinol-binding protein (RBP) is the sole specific carrier in the bloodstream for hydrophobic retinol, the main form in which vitamin A is transported. The integral membrane receptor STRA6 mediates cellular uptake of vitamin A by recognizing RBP-retinol to trigger release and internalization of retinol. We present the structure of zebrafish STRA6 determined to 3.9-angstrom resolution by single-particle cryo-electron microscopy. STRA6 has one intramembrane and nine transmembrane helices in an intricate dimeric assembly. Unexpectedly, calmodulin is bound tightly to STRA6 in a noncanonical arrangement. Residues involved with RBP binding map to an archlike structure that covers a deep lipophilic cleft. This cleft is open to the membrane, suggesting a possible mode for internalization of retinol through direct diffusion into the lipid bilayer.

Retinoid content, visual responses, and ocular morphology are compromised in the retinas of mice lacking the retinol-binding protein receptor, STRA6

PURPOSE

We report generation of a mouse model in which the STRA6 gene has been disrupted functionally to facilitate the study of visual responses, changes in ocular morphology, and retinoid processing under STRA6 protein deficiency.

METHODS

A null mouse line, stra6 -/-, was generated. Western Blot and immunocytochemistry were used to determine expression of STRA6 protein. Visual responses and morphological studies were performed on 6-week, 5-month and 10-month-old mice. The retinoid content of eye tissues was evaluated in dark-adapted mice by high performance liquid chromatography.

RESULTS

STRA6 protein was not detectable in stra6 -/- null mice, which had a consistent reduction, but not total ablation of their visual responses. The mice also showed significant depletion of their retinoid content in retinal pigment epithelium (RPE) and neurosensory retina, including a 95% reduction in retinyl esters. At the morphological level, a reduction in thickness of the neurosensory retina due to shortening of the rod outer and inner segments was observed when compared to control litter mates with a commensurate reduction in rod a- and b-wave amplitudes. In addition, there was a reduction in cone photoreceptor cell number and cone b-wave amplitude. A typical hallmark in stra6 -/- null eyes was the presence of a persistent primary hypertrophic vitreous, an optically dense vascularized structure located in the vitreous humor between the posterior surface of the lens and neurosensory retina.

CONCLUSIONS

Our studies of stra6 -/- null mice established the importance of the STRA6 protein for the uptake, intracellular transport, and processing of retinol by the RPE. In its absence, rod photoreceptor outer and inner segment length was reduced, and cone cell numbers were reduced, as were scotopic and photopic responses. STRA6 also was required for dissolution of the primary vitreous. However, it was clear from these studies that STRA6 is not the only pathway for retinol uptake by the RPE.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

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.

Techniques to study specific cell-surface receptor-mediated cellular vitamin A uptake

STRA6 is a multitransmembrane domain protein that was recently identified as the cell-surface receptor for plasma retinol-binding protein (RBP), the vitamin A carrier protein in the blood. STRA6 binds to RBP with high affinity and mediates cellular uptake of vitamin A from RBP. It is not homologous to any known receptors, transporters, and channels, and it represents a new class of membrane transport protein. Consistent with the diverse physiological functions of vitamin A, STRA6 is widely expressed in diverse adult organs and throughout embryonic development. Mutations in human STRA6 that abolish its vitamin A uptake activity cause severe pathological phenotypes in many human organs including the eye, brain, lung, and heart. This chapter describes functional assays for STRA6 in live cells and on cellular membranes. These assays can be employed to study the mechanism of this new membrane transport mechanism and its roles in the physiology and pathology of many organs.

Ultrasonic spectrometry of aqueous solutions of alkyl maltosides: kinetics of micelle formation and head-group isomerization

At frequencies between 100 kHz and 400 MHz, ultrasonic attenuation spectra are measured at 25 degrees C for aqueous solutions of hexyl-, heptyl-, octyl-, nonyl-, and decyl-beta-D-maltopyranoside as well as of decyl-alpha-D-maltopyranoside. The spectra with surfactant concentration c above the relevant critical micelle concentration (cmc) display three relaxation terms with discrete relaxation times. That with a relaxation time between 0.1 and 1.2 micros is due to exchange of monomers between micelles and the suspending phase. It is discussed in the light of the Teubner-Kahlweit-Aniansson-Wall model of the formation/decay kinetics of systems with Gaussian size distribution of micelles. The relaxation parameters are compared to those for solutions of other non-ionic surfactants, such as alkyl monoglycosides and poly(ethylene glycol) monoalkyl ethers. At c < cmc this low-frequency relaxation term is missing and at c approximately = cmc it is broadened, as is characteristic of solutions of oligomeric molecular structures rather than proper micelles. The relaxation terms with relaxation times between 6 and 15 ns and 0.7 and 2.3 ns reveal head-group rotations around glycosidic angles and isomerization of the exocyclic hydroxymethyl group, respectively. These unimolecular reactions are also examined with a view to solutions of alkyl monoglycosides as well as of glucose and maltose.

An essential ligand-binding domain in the membrane receptor for retinol-binding protein revealed by large-scale mutagenesis and a human polymorphism

Plasma retinol-binding protein (RBP), the principal carrier of vitamin A in the blood, delivers vitamin A from liver, the site of storage, to distant organs that need vitamin A, such as the eye, brain, placenta, and testis. STRA6 is a high-affinity membrane receptor for RBP and mediates vitamin A uptake in these target organs. STRA6 is a 74-kDa multi-transmembrane domain protein that represents a new class of membrane transport protein. In this study, we used an unbiased strategy by analyzing >900 random mutants of STRA6 to study its structure and function, and we identified an essential RBP-binding domain in STRA6. Mutations in any of the three essential residues in this domain can almost completely abolish binding of STRA6 to RBP and its vitamin A uptake activity from holo-RBP without affecting its cell surface expression. We have also functionally characterized the mutations in human STRA6 that cause severe birth defects as well as several human polymorphisms. All STRA6 mutants associated with severe birth defects have largely abolished vitamin A uptake activity, consistent with the severe clinical phenotypes. In addition, we have identified a human polymorphism that significantly reduces the vitamin A uptake activity of STRA6. Interestingly, the residue affected by this polymorphism is located in the RBP-binding domain we identified, and the polymorphism causes decreased vitamin A uptake by reducing RBP binding. This study identifies an essential functional domain in STRA6 and a human polymorphism in this domain that leads to reduced vitamin A uptake activity.

RPE65 of Retinal Pigment Epithelium, A Putative Receptor Molecule for Plasma Retinol-Binding Protein, is Expressed in Human Keratinocytes

Retinoids are important modulators for cell growth and differentiation of normal skin. In plasma, retinol is transported coupled to plasma retinol-binding protein. In this study, we investigated gene and protein expression of RPE65, a putative receptor for plasma retinol-binding protein in human epidermal keratinocytes. We performed real-time PCR analysis to evaluate expression of RPE65 mRNA in proliferating and differentiating keratinocytes. Immunoblotting with anti-RPE65 antibody shows distinct reactivity to a 61-kDa protein. Indirect immunofluorescence on normal human epidermis reveals cell surface labeling of keratinocytes. Laser scan microscopy exhibits colocalization of plasma retinol-binding protein and RPE65 on cultured keratinocytes. Internalization experiments with [3H]retinoic acid-retinol-binding protein complex in the presence and absence of excess of retinol-binding protein indicates receptor-dependent uptake of retinoids. We further show isolation of RPE65 protein by affinity chromatography from lysates of keratinocytes using a retinol-binding protein-matrix gel column. In summary, we demonstrate mRNA and protein expression of RPE65 in epidermal keratinocytes. Colocalization of plasma retinol-binding protein with RPE65 and affinity binding suggest a direct interaction of RPE65 with plasma retinol-binding protein in cultured human keratinocytes that might be involved in retinoid uptake of keratinocytes.

Vitamin A transport: in vitro models for the study of RBP secretion

Retinol-binding protein (RBP) is the specific plasma carrier of retinol, encharged of the vitamin transport from the liver to target cells. Ligand binding influences the RBP affinity for transthyretin (TTR), a homotetrameric protein involved in the RBP/TTR circulating complex, and the secretion rate of RBP. In fact, in vitamin A deficiency, the RBP release from the hepatocytes dramatically decreases and the protein accumulates in the cells, until retinol is available again. The mechanism is still not clear and new cellular models are needed to understand in detail how the soluble RBP can be retained inside the cell. In fish, a vitamin A transport system similar to that of higher vertebrates is emerging, although with significant differences.

Hepatic stellate cells uptake of retinol associated with retinol-binding protein or with bovine serum albumin

Retinol is stored in liver, and the dynamic balance between its accumulation and mobilization is regulated by hepatic stellate cells (HSC). Representing less than 1% total liver protein, HSC can reach a very high intracellular retinoid (vitamin-A and its metabolites) concentration, which elicits their conversion from the myofibroblast to the fat-storing lipocyte phenotype. Circulating retinol is associated with plasma retinol-binding protein (RBP) or bovine serum albumin (BSA). Here we have used the in vitro model of GRX cells to compare incorporation and metabolism of BSA versus RBP associated [(3)H]retinol in HSC. We have found that lipocytes, but not myofibroblasts, expressed a high-affinity membrane receptor for RBP-retinol complex (KD = 4.93 nM), and both cell types expressed a low-affinity one (KD = 234 nM). The RBP-retinol complex, but not the BSA-delivered retinol, could be dislodged from membranes by treatments that specifically disturb protein-protein interactions (high RBP concentrations). Under both conditions, treatments that disturb the membrane lipid layer (detergent, cyclodextrin) released the membrane-bound retinol. RBP-delivered retinol was found in cytosol, microsomal fraction and, as retinyl esters, in lipid droplets, while albumin-delivered retinol was mainly associated with membranes. Disturbing the clathrin-mediated endocytosis did not interfere with retinol uptake. Retinol derived from the holo-RBP complex was differentially incorporated in lipocytes and preferentially reached esterification sites close to lipid droplets through a specific intracellular traffic route. This direct influx pathway facilitates the retinol uptake into HSC against the concentration gradients, and possibly protects cell membranes from undesirable and potentially noxious high retinol concentrations.

Antisense down-regulation of lipocalin-interacting membrane receptor expression inhibits cellular internalization of lipocalin-1 in human NT2 cells

There is increasing experimental evidence demonstrating that many lipocalins bind to specific cell surface receptors. However, whereas the binding of lipocalins to their lipophilic ligands has now been characterized in much detail, there is a lack of knowledge about the nature of lipocalin receptors, the physiological role of receptor binding, and the molecular mechanism of ligand delivery. We previously identified a novel human membrane protein (lipocalin-1-interacting membrane receptor (LIMR)), which interacts with lipocalin-1 (Wojnar, P., Lechner, M., Merschak, P., and Redl, B. (2001) J. Biol. Chem. 276, 20206-20212). In the present study, we investigated the physiological role of LIMR and found this protein to be essential for mediating internalization of lipocalin-1 (Lcn-1) in NT2 cells, leading to its degradation. Whereas control NT2 cells rapidly internalized (125)I-Lcn-1 or fluorescein isothiocyanate-labeled Lcn-1, NT2 cells that were made LIMR deficient by cDNA antisense expression greatly accumulated Lcn-1 in the culture medium but did not internalize it. Because sequence and structure analysis indicated that proteins similar to LIMR are present in several organisms and at least two closely related orthologues are found in human and mouse, we suggest LIMR to be the prototype of a new family of endocytic receptors, which are topographically characterized by nine putative transmembrane domains and a characteristic large central cytoplasmic loop.

Muscle expression of human retinol-binding protein (RBP). Suppression of the visual defect of RBP knockout mice

Mice lacking retinol-binding protein (RBP) have low circulating retinol levels. They have severe visual defects due to a low content of retinol or retinyl esters in the eye. A transgenic mouse strain that expresses human RBP under the control of the muscle creatine kinase promoter in the null background was generated. The exogenous protein bound retinol and transthyretin in the circulation and effectively delivered retinol to the eye. Thus, RBP expressed from an ectopic source suppresses the visual phenotype, and retinoids accumulate in the eye. No human RBP was found in the retinal pigment epithelium of the transgenic mice, indicating that retinol uptake by the eye does not entail endocytosis of the carrier RBP.

The transfer of transthyretin and receptor-binding properties from the plasma retinol-binding protein to the epididymal retinoic acid-binding protein

Members of the lipocalin superfamily share a common structural fold, but differ from each other with respect to the molecules with which they interact. They all contain eight beta-strands (A-H) that fold to form a well-defined beta-barrel, which harbours a binding pocket for hydrophobic ligands. These strands are connected by loops that vary in size and structure and make up the closed and open ends of the pocket. In addition to binding ligands, some members of the family interact with other macromolecules, the specificity of which is thought to be associated with the variable loop regions. Here, we have investigated whether the macromolecular-recognition properties can be transferred from one member of the family to another. For this, we chose the prototypical lipocalin, the plasma retinol-binding protein (RBP) and its close structural homologue the epididymal retinoic acid-binding protein (ERABP). RBP exhibits three molecular-recognition properties: it binds to retinol, to transthyretin (TTR) and to a cell-surface receptor. ERABP binds retinoic acid, but whether it interacts with other macromolecules is not known. Here, we show that ERABP does not bind to TTR and the RBP receptor, but when the loops of RBP near the open end of the pocket (L-1, L-2 and L-3, connecting beta-strands A-B, C-D and E-F, respectively) were substituted into the corresponding regions of ERABP, the resulting chimaera acquired the ability to bind TTR and the receptor. L-2 and L-3 were found to be the major determinants of the receptor- and TTR-binding specificities respectively. Thus we demonstrate that lipocalins serve as excellent scaffolds for engineering novel biological functions.

Molecular cloning of a novel lipocalin-1 interacting human cell membrane receptor using phage display

Human lipocalin-1 (Lcn-1, also called tear lipocalin), a member of the lipocalin structural superfamily, is produced by a number of glands and tissues and is known to bind an unusually large array of hydrophobic ligands. Apart from its specific function in stabilizing the lipid film of human tear fluid, it is suggested to act as a physiological scavenger of potentially harmful lipophilic compounds, in general. To characterize proteins involved in the reception, detoxification, or degradation of these ligands, a cDNA phage-display library from human pituitary gland was constructed and screened for proteins interacting with Lcn-1. Using this method an Lcn-1 interacting phage was isolated that expressed a novel human protein. Molecular cloning and analysis of the entire cDNA indicated that it encodes a 55-kDa protein, lipocalin-1 interacting membrane receptor (LIMR), with nine putative transmembrane domains. The cell membrane location of this protein was confirmed by immunocytochemistry and Western blot analysis of membrane fractions of human NT2 cells. Independent biochemical investigations using a recombinant N-terminal fragment of LIMR also demonstrated a specific interaction with Lcn-1 in vitro. Based on these data, we suggest LIMR to be a receptor of Lcn-1 ligands. These findings constitute the first report of cloning of a lipocalin interacting, plasma membrane-located receptor, in general. In addition, a sequence comparison supports the biological relevance of this novel membrane protein, because genes with significant nucleotide sequence similarity are present in Takifugu rubripes, Drosophila melanogaster, Caenorhabditis elegans, Mus musculus, Bos taurus, and Sus scrofa. According to data derived from the human genome sequencing project, the LIMR-encoding gene has to be mapped on human chromosome 12, and its intron/exon organization could be established. The entire LIMR-encoding gene consists of about 13.7 kilobases in length and contains 16 introns with a length between 91 and 3438 base pairs.

Internalization of transthyretin. Evidence of a novel yet unidentified receptor-associated protein (RAP)-sensitive receptor

Transthyretin (TTR) is a plasma carrier of thyroxine and retinol-binding protein (RBP). Though the liver is the major site of TTR degradation, its cellular uptake is poorly understood. We explored TTR uptake using hepatomas and primary hepatocytes and showed internalization by a specific receptor. RBP complexed with TTR led to a 70% decrease of TTR internalization, whereas TTR bound to thyroxine led to a 20% increase. Different TTR mutants showed differences in uptake, suggesting receptor recognition dependent on the structure of TTR. Cross-linking studies using hepatomas and (125)I-TTR revealed a approximately 90-kDa complex corresponding to (125)I-TTR bound to its receptor. Given previous evidence that a fraction of TTR is associated with high-density lipoproteins (HDL) and that in the kidney, megalin, a member of the low-density lipoprotein receptor family (LDLr) internalizes TTR, we hypothesized that TTR and lipoproteins could share related degradation pathways. Using lipid-deficient serum in uptake assays, no significant changes were observed showing that TTR uptake is not lipoprotein-dependent or due to TTR-lipoprotein complexes. However, competition studies showed that lipoproteins inhibit TTR internalization. The scavenger receptor SR-BI, a HDL receptor, and known LDLr family hepatic receptors did not mediate TTR uptake as assessed using different cellular systems. Interestingly, the receptor-associated protein (RAP), a ligand for all members of the LDLr, was able to inhibit TTR internalization. Moreover, the approximately 90-kDa TTR-receptor complex obtained by cross-linking was sensitive to the presence of RAP. To confirm that RAP sensitivity observed in hepatomas did not represent a mechanism absent in normal cells, primary hepatocytes were tested, and similar results were obtained. The RAP-sensitive TTR internalization together with displacement of TTR uptake by lipoproteins, further suggests that a common pathway might exist between TTR and lipoprotein metabolism and that an as yet unidentified RAP-sensitive receptor mediates TTR uptake.

Mammalian odorant binding proteins

Odorant binding proteins (OBPs) pertain to one of the most abundant classes of proteins found in the olfactory apparatus. OBPs are a sub-class of lipocalins, defined by their property of reversibly binding volatile chemicals, that we call 'odorants'. Numerous sequences of OBPs are now available, derived from protein sequencing from nasal mucus material, or from DNA sequences. The structural knowledge of OBPs has been improved too in recent years, with the availability of two X-ray structures. The physiological role of OBPs remains, however, essentially hypothetical, and most probably, not linked to a function of odor transport. The present knowledge on OBP biochemistry, sequence and structure will be examined here in relation to the different functional hypotheses proposed for OBPs.

Beyond the superfamily: the lipocalin receptors

Lipocalins are characterized by multiple molecular recognition properties including the ability to bind to cell surface receptors. Receptors for a number of lipocalins have been identified. These include receptors for alpha-1-microglobulin, insecticyanin, glycodelin, retinol-binding protein, alpha-1-acid glycoprotein, beta-lactoglobulin and odorant-binding protein. The properties of these receptors are summarized and discussed.

Retinoid-binding proteins: mediators of retinoid action

Active vitamin A metabolites, known as retinoids, are essential for multiple physiological processes, ranging from vision to embryonic development. These small hydrophobic compounds associate in vivo with soluble proteins that are present in a variety of cells and in particular extracellular compartments, and which bind different types of retinoids with high selectivity and affinity. Traditionally, retinoid-binding proteins were viewed as transport proteins that act by solubilizing and protecting their labile ligands in aqueous spaces. It is becoming increasingly clear, however, that, in addition to this general role, retinoid-binding proteins have diverse and specific functions in regulating the disposition, metabolism and activities of retinoids. Some retinoid-binding proteins appear to act by sequestering their ligands, thereby generating concentration gradients that allow cells to take up retinoids from extracellular pools and metabolic steps to proceed in energetically unfavourable directions. Other retinoid-binding proteins regulate the metabolic fates of their ligands by protecting them from some enzymes while allowing metabolism by others. In these cases, delivery of a bound retinoid from the binding protein to the 'correct' enzyme is likely to be mediated by direct and specific interactions between the two proteins. One retinoid-binding protein was reported to enhance the ability of its ligand to regulate gene transcription by directly delivering this retinoid to the transcription factor that is activated by it. 'Channelling' of retinoids between their corresponding binding protein and a particular protein target thus seems to be a general theme through which some retinoid-binding proteins exert their effects.

Studies on the cellular uptake of retinol binding protein and retinol

The uptake and release of (125)I-RBP and of holoRBP labeled with [(3)H]retinol ((3)H-ROH) were studied in two cell lines which synthesize and secrete RBP, the HepG2 hepatocarcinoma cell line and the Caki-1 kidney adenocarcinoma cell line, and in HeLa cells that do not express the endogenous RBP gene. In all three cell lines a part of endocytosed (125)I-RBP is recycled to the extracellular medium and part is degraded. Nonspecific endocytosis of (125)I-RBP was estimated to be approximately 10% of total endocytosed (125)I-RBP. In HepG2 cells the (3)H-ROH from the [(3)H]retinol-RBP complex ((3)H-ROH-RBP) is recycled bound to RBP into serum-free chase medium. This (3)H-ROH recycling is blocked in HepG2 cells by cyclohexymide and by brefeldin A, an inhibitor of protein export from the main secretory route, and is absent in HeLa cells, which do not synthesize RBP. These data suggest that at least part of retinol taken up from exogenous holoRBP is delivered to newly synthesized RBP. (3)H-ROH recycled by HeLa cells is bound to serum albumin, as is a portion of that recycled by HepG2 cells. Transfer of (3)H-ROH from RBP to serum albumin does not occur in the absence of cells. We conclude that RBP is endocytosed through a specific pathway and that the RBP-associated retinol is transferred to newly synthesized RBP or to serum albumin.

Retinol-binding protein is produced by rabbit chondrocytes and responds to parathyroid hormone (PTH)/PTH-related peptide-cyclic adenosine monophosphate pathway

PTH and dibutyryl cAMP [(Bu)2cAMP] induced the expression of a 19-kDa protein in the conditioned media of rabbit growth plate chondrocyte cultures. The 19-kDa protein was identified as plasma retinol-binding protein (RBP) by aminoterminal sequence analysis and immunoblot analysis with an anti-RBP monoclonal antibody. Northern blot analysis showed that PTH, PTH-related peptide (PTHrP), and (Bu)2cAMP increased the RBP messenger RNA (mRNA) level in chondrocyte cultures. Further, both PTH and (Bu)2cAMP markedly induced the expression of RBP mRNA by about 10-fold at 3 h and by about 40-fold at 24 h, indicating a pretranslational regulation. The level of the mRNA expression induced by PTH, PTHrP, and (Bu)2cAMP was as high as that by retinoic acid (RA), known as a potent inducer of RBP in hepatoma cells. RBP mRNA was also detected in cartilage tissues at higher levels than in the other tissues examined except liver. Both RBP and PTH/PTHrP inhibited the dedifferentiative activity of RA on growth plate chondrocytes when added to the culture medium. These results demonstrate that chondrocytes synthesize and secrete RBP in vivo and in vitro and suggest that PTH/PTHrP modulates the effect of RA by means of RBP production in chondrocytes.

A receptor for the lipocalin placental protein 14 on human monocytes

Human placental protein 14 (PP14), a member of the lipocalin structural superfamily, is an abundant amniotic fluid glycoprotein with documented immunoinhibitory activities. While receptors have been characterized for several other lipocalins, none have been reported to date for PP14. In the present study, two-color immunofluorescence and flow cytometry was used to screen peripheral blood mononuclear cell subpopulations for their capacity to engage fluoresceinated recombinant PP14. The tagged PP14 bound strongly in a specific and saturable fashion to CD14+ (monocyte lineage) cells, but not to CD20+ (B cell lineage) or CD3+ (T cell lineage) cells. This binding was both pH- and temperature-sensitive, and was reduced by proteolytic pre-digestion of the cells with trypsin or proteinase K. Scatchard analysis demonstrated a single class of receptors on CD14+ cells, with a K(D) of approximately 1 x 10(-8) and approximately 10-35,000 receptors per cell. These findings constitute the first report of a cell surface-associated binding protein for PP14 and set the stage for exploring the molecular mechanisms of PP14-mediated signaling and immunomodulation.

Release of retinol and denaturation of its plasma carrier, retinol-binding protein

BACKGROUND

Retinol is tightly packed inside the structure of its plasma carrier (retinol-binding protein, RBP). It was found that retinol release from RBP to aqueous solutions is facilitated by either very low pH or very high temperatures (i.e. by non-physiological conditions that cause protein denaturation). It was also found that alcohols induce protein conformational transitions to denatured states. On this basis, it may be suggested that retinol release in vivo is facilitated by the partial unfolding of the carrier resulting from the concerted action of the moderate local decrease of pH and the moderate local decrease of dielectric constant in proximity to the target membranes.

RESULTS

In vitro, at 37 degrees C, retinol is removed from its plasma carrier by the concerted action of the moderately low pH and the moderately low dielectric constant of solutions containing a low ionic strength buffer and methanol in variable proportions. Release of retinol is accompanied by a conformational transition of RBP from the native to the molten-globule state.

CONCLUSIONS

The physiological function of RBP-targeted delivery of retinol-is mimicked in vitro by the facilitated release of retinol (associated with a partial unfolding of the protein carrier) in solutions exhibiting pH and dielectric constant values that are within the range of values expected in the in vivo microenvironment.

The transfer of retinol from serum retinol-binding protein to cellular retinol-binding protein is mediated by a membrane receptor

The hypothesis that the cellular uptake of retinol involves the specific interaction of a plasma membrane receptor with serum retinol-binding protein (RBP) at the extracellular surface followed by ligand transfer to cytoplasmic cellular retinol-binding protein (CRBP) has been investigated. The experimental system consisted of the [3H]retinol-RBP complex, Escherichia coli-expressed recombinant apo-CRBP containing the 10 amino acid long streptavidin-binding peptide sequence at its C terminus (designated as CRBP-Strep) and permeabilized human placental membranes. [3H]Retinol transfer from RBP to CRBP-Strep was monitored by measuring the radioactivity associated with CRBP-Strep retained by an immobilized streptavidin resin. Using this assay system, we have demonstrated that optimal retinol uptake is achieved with holo-RBP, the membrane receptor and apo-CRBP. The effects are specific: other binding proteins, including beta-lactoglobulin and serum albumin, despite their ability to bind retinol, failed to substitute for either RBP or apo-CRBP. The process is facilitated by membranes containing the native receptor suggesting that this protein is an important component in the transfer mechanism. Taken together, the data suggest that the RBP receptor, through specific interactions with the binding proteins, participates (either directly or via associated proteins) in the mechanism which mediates the transfer of retinol from extracellular RBP to intracellular CRBP.

Plasma membrane receptor for beta-lactoglobulin and retinol-binding protein in murine hybridomas

The aim of the present work was to study the binding of [125I]-BLGA (beta-lactoglobulin variant A) to the plasma membrane fraction of hybrid cells. This binding increased as a function of time with on-rate and off-rate constant at 4.47 +/- 0.18 x 10(6) M-1 min-1 and 0.17 +/- 0.07 min-1, respectively (n = 3). The saturation study showed a single binding site type corresponding to a Kd at 8.26 +/- 2.98 nM and 14.02 +/- 2.61 x 10(12) sites per mg of the plasma membrane protein (n = 3). Competitive of binding BLGA was observed with BLGA, complexed with retinol and also with RBP (retinol-binding protein). Gel filtration of [125I]-BLGA incubated with Triton X-100 solubilized membrane showed the formation of a ligand-receptor complex. Cross-linking of the tracer to plasma membrane showed a complex with a M(r) at 69 kDa, suggesting a receptor M(r) of 51 kDa, as seen by autoradiography of SDS-PAGE.

Identification of a mechanism to localize generation of retinoic acid in rat embryos

Vitamin A (retinol) is essential for normal mammalian development. However, its biological activity depends upon its conversion to retinoic acid (RA), a local mediator of cellular proliferation and differentiation. Previous studies have shown that embryonic RA is found specifically in tissues known to depend upon vitamin A for normal development and that its production follows uptake of maternal retinol. The aim of this study was to identify the mechanism for tissue-specific generation of RA in developing rat embryos. Here we show immunohistochemical localization of the retinol binding protein receptor, cellular retinol binding protein, retinol dehydrogenase and retinal dehydrogenase in rat embryos (presomitic to the 25-30 somite pair stage). These proteins are proposed to be responsible for cellular uptake of retinol, its intracellular transport and its conversion to RA. Thus, they potentially constitute the entire metabolic pathway from vitamin A to RA. All four proteins were detected specifically in tissues that are known to depend upon vitamin A for normal development including the yolk sac, heart, gut, notochord, somites, sensory placodes and the limb. Furthermore, our previous studies have demonstrated that uptake of retinol into the yolk sac depends upon a retinol binding protein receptor. Here we provide evidence that this mechanism functions also in the heart. Colocalization of cellular retinol binding protein, retinol and retinal dehydrogenase with the retinol binding protein receptor in tissues dependent upon vitamin A for normal development suggests that coordinate functioning of these proteins is responsible for cellular uptake of circulating retinol and its metabolism to RA. This is the first evidence of a tissue-specific mechanism for generation of RA from its precursor retinol in the developing embryo.

Interactions of transthyretin (TTR) and retinol-binding protein (RBP) in the uptake of retinol by primary rat hepatocytes

The mechanism by which cells take up retinol from retinol-binding protein (RBP) and the role of the RBP-transthyretin (TTR) complex remain unclear. Here we report on retinol uptake through the RBP-TTR complex by primary cultured rat hepatocytes (parenchymal cells, PC) and nonparenchymal cells (NPC) following incubation with [3H]retinol-RBP or the [3H]retinol-RBP-TTR complex under several conditions. The cellular accumulation of retinol was time and temperature dependent in both PC and NPC. Analysis by HPLC showed that the incorporated [3H]retinol in NPC was mainly converted to retinyl ester, although in PC it remained mainly as unesterified retinol. However, the amount of retinol taken up from the RBP-TTR complex was nearly twofold greater than that from RBP alone. The uptake of [3H]retinol from protein-bound retinol was inhibited by an excess of either retinol-RBP or retinol-RBP-TTR complex. Moreover, retinol uptake through the RBP-TTR complex was inhibited by an excess of free TTR. From these results we postulate that TTR may take part as a positive regulator in the delivery of RBP-bound retinol from plasma, possibly by a membrane receptor, and that retinol uptake takes place preferentially from the RBP-TTR complex into both PC and NPC. The uptake of [3H]retinol (2 microM) by PC was saturated, whereas uptake by NPC was not. These results indicate that the physiological importance of TTR in retinol delivery may be especially important to vitamin A-storing stellate (Ito) cells in the NPC fraction.

Retinol free and retinol complexed beta-lactoglobulin binding sites in bovine germ cells

A high affinity specific binding site for bovine beta-lactoglobulin (BLG) was identified in bovine germ cell plasma membrane enriched fractions. Binding was found to be reversible and pH-dependent with maximum binding occurring at pH 5. The on-rate and off-rate constants were 2.26 +/- 0.8 x 10(5) M(-1) min(-1) (n = 3) and 0.016 +/- 0.004 min(-1) (n = 3), respectively. Scatchard analysis showed a single class of binding sites, with 12.38 +/- 4.62 x 10(12) sites per mg of membrane protein (n = 3) and a dissociation constant (K(D)) estimated at 26.43 +/- 2.68 nM. There was inhibition of iodinated-BLG (variant A) (125I-BLGA) binding to germ cell plasma membrane enriched fractions in the presence of unlabelled BLG variant A, BLG variant B, retinol complexed BLGA and human retinol-binding protein. Inhibition was observed neither with BSA nor with lactoferrin. 125I-BLGA incubated with a Triton X-100 solubilized plasma membrane fraction formed a high molecular mass complex in Superose 12B gel filtration. This receptor complex disappeared in the presence of unlabelled BLGA and in the presence of 10 mM EDTA. The results suggest that germ cell plasma membrane may contain a receptor which is capable of binding either retinol free or retinol complexed BLGA.