Retinol-binding protein and transthyretin expressed in HeLa cells form a complex in the endoplasmic reticulum in both the absence and the presence of retinol

Adipocyte HSL is required for maintaining circulating vitamin A and RBP4 levels during fasting

Vitamin A (retinol) is distributed via the blood bound to its specific carrier protein, retinol-binding protein 4 (RBP4). Retinol-loaded RBP4 is secreted into the circulation exclusively from hepatocytes, thereby mobilizing hepatic retinoid stores that represent the major vitamin A reserves in the body. The relevance of extrahepatic retinoid stores for circulating retinol and RBP4 levels that are usually kept within narrow physiological limits is unknown. Here, we show that fasting affects retinoid mobilization in a tissue-specific manner, and that hormone-sensitive lipase (HSL) in adipose tissue is required to maintain serum concentrations of retinol and RBP4 during fasting in mice. We found that extracellular retinol-free apo-RBP4 induces retinol release by adipocytes in an HSL-dependent manner. Consistently, global or adipocyte-specific HSL deficiency leads to an accumulation of retinoids in adipose tissue and a drop of serum retinol and RBP4 during fasting, which affects retinoid-responsive gene expression in eye and kidney and lowers renal retinoid content. These findings establish a novel crosstalk between liver and adipose tissue retinoid stores for the maintenance of systemic vitamin A homeostasis during fasting.

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.

Vitamin A signaling and homeostasis in obesity, diabetes, and metabolic disorders

Much evidence has accumulated in the literature over the last fifteen years that indicates vitamin A has a role in metabolic disease prevention and causation. This literature proposes that vitamin A can affect obesity development and the development of obesity-related diseases including insulin resistance, type 2 diabetes, hepatic steatosis and steatohepatitis, and cardiovascular disease. Retinoic acid, the transcriptionally active form of vitamin A, accounts for many of the reported associations. However, a number of proteins involved in vitamin A metabolism, including retinol-binding protein 4 (RBP4) and aldehyde dehydrogenase 1A1 (ALDH1A1, alternatively known as retinaldehyde dehydrogenase 1 or RALDH1), have also been identified as being associated with metabolic disease. Some of the reported effects of these vitamin A-related proteins are proposed to be independent of their roles in assuring normal retinoic acid homeostasis. This review will consider both human observational data as well as published data from molecular studies undertaken in rodent models and in cells in culture. The primary focus of the review will be on the effects that vitamin A per se and proteins involved in vitamin A metabolism have on adipocytes, adipose tissue biology, and adipose-related disease, as well as on early stage liver disease, including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).

Rescue of retinal morphology and function in a humanized mouse at the mouse retinol-binding protein locus

Retinol-binding protein RBP4 is the specific carrier for retinol in the blood. We previously produced a Rbp4-deficient (Rbp4^-/-) mouse that showed electroretinogram (ERG) abnormalities, accompanied by histological and electron-microscopic changes such as fewer synapses in the inner plexiform layer in the central retina. To address whether human RBP4 gene expression can rescue the phenotypes observed in Rbp4^-/- mice, we produced a humanized (Rbp4^{hRBP4orf/ hRBP4orf}) mouse with a human RBP4 open reading frame in the mouse Rbp4 locus using a Cre-mutant lox recombination system. In Rbp4^{hRBP4orf/hRBP4orf} mice, the tissue-specific expression pattern of hRBP4orf was roughly the same as that of mouse Rbp4. ERG and morphological abnormalities observed in Rbp4^-/- mice were rescued in Rbp4^{hRBP4orf/hRBP4orf} mice as early as 7 weeks of age. The temporal expression pattern of hRBP4orf in the liver of Rbp4^{hRBP4orf/hRBP4orf} mice was similar to that of mouse Rbp4 in Rbp4^+/+mice. In contrast, hRBP4orf expression levels in eyes were significantly lower at 6 and 12 weeks of age compared with mouse Rbp4 but were restored to the control levels at 24 weeks. The serum hRBP4 levels in Rbp4^{hRBP4orf/hRBP4orf} mice were approximately 30% of those in Rbp4^+/+ at all ages examined. In accordance with this finding, the plasma retinol levels remained low in Rbp4^{hRBP4orf/hRBP4orf} mice. Retinol accumulation in the liver occurred in control and Rbp4^{hRBP4orf/hRBP4orf} mice but was higher in Rbp4^{hRBP4orf/hRBP4orf} mice at 30 weeks of age. Mouse transthyretin expression was not altered in Rbp4^-/- or Rbp4^{hRBP4orf/hRBP4orf} mice. Taken together, 30% of the serum RBP4 level was sufficient to correct the abnormal phenotypes observed in Rbp4^-/- mice.

Vitamin A Absorption, Storage and Mobilization

It is well established that chylomicron remnant (dietary) vitamin A is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. Since retinol-binding protein 4 (RBP4), a protein that is required for mobilizing stored vitamin A, is synthesized solely by hepatocytes and not HSCs, it similarly is not known how vitamin A is transferred from HSCs to hepatocytes. Although it has long been thought that RBP4 is absolutely essential for delivering vitamin A to tissues, recent research has proven that this notion is incorrect since total RBP4-deficiency is not lethal. In addition to RBP4, vitamin A is also found in the circulation bound to lipoproteins and as retinoic acid bound to albumin. It is not known how these different circulating pools of vitamin A contribute to the vitamin A needs of different tissues. In our view, better insight into these three issues is required to better understand vitamin A absorption, storage and mobilization. Here, we provide an up to date synthesis of current knowledge regarding the intestinal uptake of dietary vitamin A, the storage of vitamin A within the liver, and the mobilization of hepatic vitamin A stores, and summarize areas where our understanding of these processes is incomplete.

The diversity of mechanisms influenced by transthyretin in neurobiology: development, disease and endocrine disruption

Transthyretin (TTR) is a protein that binds and distributes thyroid hormones (THs). TTR synthesised in the liver is secreted into the bloodstream and distributes THs around the body, whereas TTR synthesised in the choroid plexus is involved in movement of thyroxine from the blood into the cerebrospinal fluid and the distribution of THs in the brain. This is important because an adequate amount of TH is required for normal development of the brain. Nevertheless, there has been heated debate on the role of TTR synthesised by the choroid plexus during the past 20 years. We present both sides of the debate and how they can be reconciled by the discovery of TH transporters. New roles for TTR have been suggested, including the promotion of neuroregeneration, protection against neurodegeneration, and involvement in schizophrenia, behaviour, memory and learning. Recently, TTR synthesis was revealed in neurones and peripheral Schwann cells. Thus, the synthesis of TTR in the central nervous system (CNS) is more extensive than previously considered and bolsters the hypothesis that TTR may play wide roles in neurobiological function. Given the high conservation of TTR structure, function and tissue specificity and timing of gene expression, this implies that TTR has a fundamental role, during development and in the adult, across vertebrates. An alarming number of 'unnatural' chemicals can bind to TTR, thus potentially interfering with its functions in the brain. One role of TTR is delivery of THs throughout the CNS. Reduced TH availability during brain development results in a reduced IQ. The combination of the newly discovered sites of TTR synthesis in the CNS, the increasing number of neurological diseases being associated with TTR, the newly discovered functions of TTR and the awareness of the chemicals that can interfere with TTR biology render this a timely review on TTR in neurobiology.

Carrier-mediated thyroid hormone transport into placenta by placental transthyretin

CONTEXT

The serum protein transthyretin (TTR) plays an important role in the transport of thyroid hormone and retinol, which are critical for normal development of the human fetus. TTR is not only synthesized and secreted into the circulation by the liver and other tissues but is also synthesized by placental trophoblasts, which separate the maternal and fetal circulations. Whether it is secreted or taken up by these cells and whether it carries thyroid hormone is unknown.

OBJECTIVE AND METHODS

Our objective was to study placental handling of TTR and determine whether TTR participates in placental thyroid hormone transport. We investigated the capacity of human placenta and choriocarcinoma cell lines to secrete and internalize TTR and its ligands by Western blotting, immunofluorescence, and uptake of radiolabeled TTR.

RESULTS

Human placental explants and TTR expressing JEG-3 cells secrete TTR. JEG-3 cells grown in bicameral chambers secrete TTR, predominantly from the apical surface. Human placental explants and JEG-3 cells internalize Alexa Fluor488-labeled TTR and (125)I-TTR. Furthermore, binding to thyroid hormones (T(4), T(3)) increases (125)I-TTR uptake by enhancing tetramer formation. Cross-linking experiments confirm internalization of the TTR-(125)I-T(4) complex.

CONCLUSIONS

Our results suggest that human placenta and choriocarcinoma cells secrete transthyretin, which binds extracellular T(4), and that T(4) binding results in increased internalization of TTR-T(4) complex. TTR production by trophoblasts may represent a mechanism to allow transfer of maternal thyroid hormone to the fetal circulation that could have important implications for fetal development.

Oxidative folding and assembly with transthyretin are sequential events in the biogenesis of retinol binding protein in the endoplasmic reticulum

Retinol-binding protein (RBP) is secreted out of the cell in its ligand-bound holo-form. The apo-form of RBP is selectively retained within the endoplasmic reticulum (ER) by a mechanism that remains unknown. Using isolated microsomal system, we have recapitulated the biogenesis of RBP involving its oxidative folding and assembly with transthyretin in the ER. In addition to dissecting its pathway of disulfide oxidation, we have analyzed association of its early folding intermediates with ER-chaperones. Our results show that of the three intramolecular disulfides present in RBP (4-160, 70-174, and 120-129) the smallest loop (120-129) was most critical for RBP to fold. Its absence caused RBP to aggregate into an intermolecular disulfide-linked structure. After acquisition of the small loop, formation of one of the two big disulfides (4-160 or 70-174) was sufficient for RBP to acquire a folded state. Using cross-linking in intact microsomes and sedimentation on sucrose gradients, we show that newly synthesized RBP is associated with a complex of chaperones consisting of Grp94, BiP, PDI, and calnexin. The complex was constitutively present in the ER, independent of the presence of folding substrates. RBP dissociated from this complex coincident with the formation of one of the two big disulfide loops, whereas RBP mutant lacking both the large disulfides showed persistent association. While highlighting the matrix-like characteristics of ER in isolated microsomal system our results provide insight into RBP folding and assembly mechanisms that will aid our understanding of its complex secretion properties.

Delivery of retinoid-based therapies to target tissues

Through its various metabolites, vitamin A controls essential physiological functions. Both naturally occurring metabolites and novel retinoid analogues have shown effectiveness in many clinical settings that include skin diseases and cancer, and in animal models of human conditions affecting vision. In this review, we analyze several potential retinoid-based therapies from the point of view of drug metabolism and transport to target tissues. We focus on the endogenous factors that affect the absorption, transport, and metabolism of retinoids by taking into account data obtained from the analysis of animal models that lack the enzymes or proteins involved in the storage and absorption of retinoids. We also discuss findings of toxicity associated with retinoids in an effort to improve the outcome of retinoid-based therapies. In this context, we review evidence that esterification of retinol and retinol-based drugs within target tissues provides one of the most efficient means to improve the absorption and to reduce the toxicity associated with pharmacological doses of retinoids. Future retinoid-based therapeutic strategies could involve targeted delivery mechanisms leading to lower toxicity and improved effectiveness of retinoids.

Glycosylation is essential for translocation of carp retinol-binding protein across the endoplasmic reticulum membrane

Retinoid transport is well characterized in many vertebrates, while it is still largely unexplored in fish. To study the transport and utilization of vitamin A in these organisms, we have isolated from a carp liver cDNA library retinol-binding protein, its plasma carrier. The primary structure of carp retinol-binding protein is very conserved, but presents unique features compared to those of the correspondent proteins isolated and characterized so far in other species: it has an uncleavable signal peptide and two N-glycosylation sites in the NH(2)-terminal region of the protein that are glycosylated in vivo. In this paper, we have investigated the function of the carbohydrate chains, by constructing three mutants deprived of the first, the second or both carbohydrates. The results of transient transfection of wild type and mutant retinol-binding protein in Cos cells followed by Western blotting and immunofluorescence analysis have shown that the absence of both carbohydrate moieties blocks secretion, while the presence of one carbohydrate group leads to an inefficient secretion. Experiments of carp RBP mRNA in vitro translation in a reticulocyte cell-free system in the presence of microsomes have demonstrated that N-glycosylation is necessary for efficient translocation across the endoplasmic reticulum membranes. Moreover, when Cos cells were transiently transfected with wild type and mutant retinol-binding protein (aa 1-67)-green fluorescent protein fusion constructs and semi-permeabilized with streptolysin O, immunofluorescence analysis with anti-green fluorescent protein antibody revealed that the double mutant is exposed to the cytosol, thus confirming the importance of glycan moieties in the translocation process.

Hepatic synthesis, maturation and complex formation between retinol-binding protein and transthyretin

The retinol/retinol-binding protein/transthyretin complex, that carries and delivers hydrophobic retinol molecules to target cells, is assembled in the hepatocyte endoplasmic reticulum. In this paper, we review data related to events that lead to the formation of this complex, including transthyretin oligomerization and retinol-binding protein secretion. Our studies on transthyretin oligomerization have demonstrated that cleavage of signal peptide and the environment of endoplasmic reticulum influence transthyretin oligomerization. In vitro, mutated transthyretin without signal sequence fails to form dimers, while wild-type transthyretin is translocated into the microsomes where it forms dimers and small amounts of tetramers. In vivo, tetramers were detected in HepG2 cells but not in transfected Cos cells, suggesting that tissue-specific factors affect tetramer stability. In vitamin A deficiency, retinol-binding protein secretion is blocked and the protein accumulates in the endoplasmic reticulum, from where it is promptly released after retinol repletion. We use MMH cells to identify factors involved in complex formation, retention and secretion, the crucial steps to understand the molecular mechanisms underlying vitamin A homeostasis. In parallel, studies on vitamin A transport in fish are in progress; retinol-binding protein and transthyretin have already been characterized in different fish species.

Biochemical basis for depressed serum retinol levels in transthyretin-deficient mice

Transthyretin (TTR) acts physiologically in the transport of retinol in the circulation. We previously reported the generation and partial characterization of TTR-deficient (TTR(-)) mice. TTR(-) mice have very low circulating levels of retinol and its specific transport protein, retinol-binding protein (RBP). We have examined the biochemical basis for the low plasma retinol-RBP levels. Cultured primary hepatocytes isolated from wild type (WT) and TTR(-) mice accumulated RBP in their media to an identical degree, suggesting that RBP was being secreted from the hepatocytes at the same rate. In vivo experiments support this conclusion. For the first 11 h after complete nephrectomy, the levels retinol and RBP rose in the circulations of WT and TTR(-) mice at nearly identical rates. However, human retinol-RBP injected intravenously was more rapidly cleared from the circulation (t(12) = 0.5 h for TTR(-) versus t(12) >6 h for WT) and accumulated faster in the kidneys of TTR(-) compared with WT mice. The rate of infiltration of the retinol-RBP complex from the circulation to tissue interstitial fluids was identical in both strains. Taken together, these data indicate that low circulating retinol-RBP levels in TTR(-) mice arise from increased renal filtration of the retinol-RBP complex.

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.

MMH cells: An in vitro model for the study of retinol-binding protein secretion regulated by retinol

The untransformed stable cell line Met murine hepatocytes (MMH), generated from liver explants of transgenic mice expressing a constitutively active truncated form of the human hepatocyte growth factor receptor (cyto-Met), represents an innovative tool for in vitro studies of liver function. In the present report, we show that the MMH-D3 line isolated from the liver of a 3-day-old mouse is a useful model to investigate the regulation of the synthesis and secretion of retinol-binding protein (RBP) by retinol (vitamin A alcohol). Experiments with Northern blot hybridization, metabolic labeling of cellular proteins followed by immunoprecipitation, and Western blot analysis demonstrated that, similarly to the in vivo situation, in MMH-D3 cells the presence of retinol does not affect transcriptional and translational rate of the RBP gene but is essential for regulating the secretion rate of the protein. Unlike HepG2 human hepatocarcinoma cells used thus far in studies of retinoid metabolism, including the synthesis and secretion of RBP, vitamin A deficiency causes, in MMH-D3 cells, the inhibiton of RBP secretion and the protein accumulation in the cell, whereas retinol repletion promptly results in RBP secretion. This model will be very useful in future studies on vitamin A distribution in the organism.

Retinol-induced secretion of human retinol-binding protein in yeast

Retinol-binding protein (RBP) functions as a transporter for retinol (vitamin A) in plasma in higher eukaryotes. We have successfully expressed human RBP in Saccharomyces cerevisiae, and its secretion was found to be induced by retinol also in this lower eukaryote. Reduced induction of secretion by retinol in a temperature-sensitive sec18-1 mutant that is blocked in secretion at the restricted temperature suggests that as in mammalian cells, RBP can be released from the endoplasmic reticulum upon addition of retinol. Thus, the molecular mechanism involved in retinol-dependent secretion of RBP appears to be conserved in yeast, and this points to yeast as a putative model system for studying retinol-regulated secretion of RBP. RBP purified from yeast was found to be indistinguishable from RBP purified from human plasma in several functional assays.

Comparative stability and clearance of [Met30]transthyretin and [Met119]transthyretin

[Met119]Transthyretin has been described as a non-amyloidogenic transthyretin variant. In Portugal, it has also been found in compound heterozygotic individual carriers of [Met30]transthyretin, the most prevalent variant associated with familial amyloidotic polyneuropathy. In these individuals, the evolution of the disease seems to be more benign than in typical [Met30]transthyretin carriers, suggesting a protective effect of [Met119]transthyretin on the pathogenic effects of [Met30]transthyretin. To study the mechanisms of this protective effect, we performed comparative in vivo clearance studies. Heterotetrameric [Met119]transthyretin showed a slower clearance, whereas homotetrameric [Met30]transthyretin presented a faster clearance. These data correlate with the relative TTR levels present in carriers of these mutations. Comparative analyses of the resistance to dissociation into monomers of serum transthyretin by 4M urea isoelectric focusing suggested a higher tetrameric stability of transthyretin in [Met119]transthyretin carriers, in contrast to a lower stability in [Met30]transthyretin carriers. The compound heterozygotes presented a pattern similar to the normal individuals. Our results suggest that the protective clinical effect of the Met119 mutation possibly involves the stabilisation of the tetrameric structure of transthyretin. Whether this behaviour correlates with the different metabolism found for the two variants is not known. The approaches reported here open some possibilities for the study and development of future therapeutic agents of familial amyloidotic polyneuropathy.

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

The retinol carrier retinol-binding protein (RBP) forms a complex with the thyroid hormone binding protein transthyretin in the plasma of a number of vertebrate species. The interactions of retinoid-RBP complexes, as well as of unliganded RBP, with transthyretin have been investigated by means of fluorescence anisotropy studies. The presence of two independent and equivalent RBP binding sites per transthyretin molecule has been established for proteins purified from species distant in evolution. Although the natural ligand retinol participates in the interaction between retinol-RBP and transthyretin, its binding to RBP is not a prerequisite for protein-protein interaction. The dissociation constants of human transthyretin binding liganded and unliganded forms of human RBP were determined to be: all-trans retinol-RBP, Kd approximately 0.2 microM; apoRBP, Kd approximately 1.2 microM; all-trans retinoic acid-RBP, Kd approximately 0.8 microM; all-trans retinyl methyl ether-RBP, Kd approximately 6 microM. The complex of RBP with the synthetic retinoid fenretinide, which bears the bulky hydroxyphenyl end group, exhibits negligible affinity for transthyretin. The replacement of RBP-bound retinol with synthetic retinoids affects RBP-transthyretin recognition to an extent that appears to be well correlated with the nature and steric hindrance of the groups substituting the retinol hydroxyl group, consistent with their location at the interface between the contact areas of RBP and transthyretin.

Studies on the metabolism of retinol and retinol-binding protein in transthyretin-deficient mice produced by homologous recombination

Tissue needs for retinoids are believed to be satisfied through the delivery in the circulation of retinol by its specific plasma transport protein, retinol-binding protein (RBP), which circulates as a 1-to-1 protein complex with transthyretin (TTR). The binding of RBP to TTR is thought to prevent filtration of retinol-RBP in the kidney and to play a role in secretion of RBP from hepatocytes. Recently a strain of mice (TTR-) that totally lacks immunoreactive TTR was produced by targeted mutagenesis. We have explored the effects of TTR deficiency on retinol and RBP metabolism in this mutant strain. In pooled plasma from the TTR- mice retinol levels averaged 6% of those of wild type animals. Similarly, plasma RBP in the TTR- mice was found to be 5% of wild type levels. Hepatic retinol and retinyl ester levels were similar for mutant and wild type mice, suggesting that the mutation affects neither the uptake nor storage of dietary retinol. Levels of retinol and retinyl esters in testis, kidney, spleen, and eye cups from TTR- mice were normal. Plasma all-trans-retinoic acid levels for the TTR- mice were 2.3-fold higher than those of wild type (425 versus 190 ng/dl). Kidney RBP levels were similar for the mutant and wild type mice and we were unable to detect intact RBP in urine from TTR- mice. Hepatic RBP levels in the TTR- mice were 60% higher than those of wild type mice (39.8 versus 25.0 micrograms of RBP/g of tissue). These data may suggest that there is a partial blockage in RBP secretion from TTR- hepatocytes that leads to lessened plasma levels of retinol-RBP.

Synthesis and secretion of retinol-binding protein and transthyretin by cultured retinal pigment epithelium

Recent studies indicate that the retinal pigment epithelium (RPE) may serve as an extrahepatic source of retinol-binding protein (RBP) and transthyretin (TTR) for the retina by virtue of the fact that this cell layer is the exclusive retinal location for mRNA coding for these proteins [Herbert, J., et al. (1991) Invest. Ophthalmol. Vis. Sci. 32, 302-309; Cavallaro, T., et al. (1990) Invest. Ophthalmol. Vis. Sci. 31, 497-501], although the proteins themselves are present in a variety of retinal neurons. It is therefore necessary to determine whether these mRNAs are translated and whether their translated products are secreted like hepatic RBP and TTR. Metabolic labeling of cultured bovine RPE with [35S]cysteine and [35S]methionine and subsequent analysis of newly synthesized proteins in the conditioned medium by affinity chromatography, gel filtration, partial amino acid sequence analysis, and autoradiography of electrophoretograms indicate that both RBP and TTR are synthesized and secreted by the RPE. Moreover, for cells grown in chambers with permeable supports, the predominant direction for secretion was into the apical medium. The mean apical:basal ratio after 72 h of incubation was 9.2 for TTR and 4.5 for RBP. A function for these proteins in the neurosensory retina remains speculative. They could be involved in the delivery of all-trans-retinol to amacrine and Müller cells as a precursor for retinoic acid, since these cells are known to contain cellular retinoic acid binding protein [Gaur, V.P., et al. (1990) Exp. Eye Res. 50, 505-511; Milam et al. (1990) J. Comp. Neurol. 296, 123-129].(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of functional human retinol-binding protein in Escherichia coli using a secretion vector

In order to express human serum retinol-binding protein (sRBP) in Escherichia coli in a form that is structurally indistinguishable from the native protein, we placed the coding sequence of the RBP cDNA next to that of the outer membrane protein A (OmpA) signal sequence in the secretion vector, pIN-III-OmpA1. However, this construct did not generate detectable expression of RBP in E. coli. When the DNA fragment consisting of the ribosome-binding site and the OmpA-RBP fusion sequence was subcloned downstream to the T7 promoter of pKS-Bluescript, however, the resultant construct (pOmp-RBP2) gave low but detectable secretion of RBP into the periplasm. Deletion of the 3' untranslated region of the RBP cDNA (pOmp-RBP3) further improved the expression (by approx. 20-fold). After charging with retinol, the secreted RBP was purified from the periplasm on a transthyretin-affinity resin. The purified protein exhibited all the three molecular recognition properties characteristic of sRBP, i.e. it interacted with retinol, transthyretin and its cell-surface receptor. Comparison of the receptor binding properties of the recombinant RBP (rRBP) with those of the serum protein revealed that while the affinity of rRBP is similar to sRBP (50 +/- 20 nM), the Bmax of the rRBP is about 6-8-fold higher. This indicates that a major proportion of RBP, isolated from serum, is incapable of interacting with the receptor.

Disruption of the transthyretin gene results in mice with depressed levels of plasma retinol and thyroid hormone

Transthyretin (TTR) is thought to play a major role in vitamin A metabolism and thyroid hormone transport in mammals. To investigate the physiological role of the TTR protein in development of the embryo and in the adult, we used gene targeting techniques to generate a null mutation at the mouse ttr locus. The resultant mutant animals are phenotypically normal, viable, and fertile. However, levels of serum retinol, retinol-binding protein, and thyroid hormone are significantly depressed in the mutant animals. These observations demonstrate that the TTR protein maintains normal levels of these metabolites in the circulating plasma.

A genetically engineered purpurin/retinol-binding protein hybrid that binds to transthyretin

A mini-gene encoding rat retinol-binding protein (RBP) and a cDNA encoding chicken purpurin were separately transfected into HeLa cells. In contrast to RBP, expressed purpurin did not bind to transthyretin (TTR). A purpurin/RBP hybrid protein was constructed by substituting the cDNA sequence encoding the N-terminal 29 amino acids of purpurin for the corresponding part of RBP. The expressed hybrid molecule bound to the TTR-Sepharose. These results demonstrate that purpurin does not bind to TTR, that a functional purpurin/RBP hybrid can be constructed, and that the N-terminal coil of RBP is not required for TTR binding.