Studies on the Interaction Between Prealbumin, Retinol-binding Protein, and Vitamin A

The potential role of upregulated PARP-1/RIPK1 expressions in amikacin-induced oxidative damage and nephrotoxicity in Wistar rats

This study aimed to investigate the gene expression levels associated with nephrotoxic action of amikacin, as well as the post-treatment effect of diuretics on its nephrotoxic effects. Sixty male rats were divided equally into six groups, including the control group receiving saline intra-peritoneally (ip), and the five treated groups including therapeutic and double therapeutic dose groups, injected ip (15 and 30 mg/kg b.wt./day) respectively for seven days, and another two rat groups treated as therapeutic and double therapeutic dose groups then administered the diuretic orally for seven days and the last group received amikacin ip at a rate of 15 mg/kg/day for seven days, then given free access to water without diuretics for another seven days and was kept as a self-recovery group. Amikacin caused kidney injury, which was exacerbated by the double therapeutic dose, as evidenced by abnormal serum renal injury biomarkers, elevated renal MDA levels, inhibition of renal catalase and SOD enzyme activities, with renal degenerative and necrotic changes. Moreover, comet assays also revealed renal DNA damage. Interestingly, amikacin administration markedly elevated expression levels of the PARP-1, RIP1, TNF-α, IL-1β, and iNOS genes as compared to the control group. However, compared to the self-recovery group, post-amikacin diuretic treatment modulates amikacin-induced altered findings and alleviates amikacin nephrotoxic effects more efficiently. Our findings suggested the potential role of PARP-1 and RIPK1 expressions that influence the expression of proinflammatory cytokines such as IL-1β and TNF-α by exaggerating oxidative stress which may contribute to the pathogenesis of amikacin-induced nephrotoxicity.

Revisiting PINI Scoring in Light of Recent Biological Advances

The prognostic inflammatory and nutritional index (PINI) is a simple scoring formula allowing the follow-up of dietary protein restriction and infectious complications affecting critically ill patients hospitalized in medical and surgical wards. The World Health organization (WHO) has recently recommended using the binary CRP (C-reactive protein) and AGP (α1-acid glycoprotein) numerators of the PINI formula in underprivileged inhabitants of developing countries to evaluate their (sub)clinical infectious states making their chronic malnutrition worse. These studies, mainly located in Africa and Asia, demonstrate that children and women enduring the combined effects of infectious burden and (micro)nutrient deprivation (principally retinol and iron) usually manifest persistent refractoriness and slackened recovery throughout dietary rehabilitation. The additive measurement of ALB (albumin) and TTR (transthyretin) composing the denominator of the PINI formula is shown to be helpful in grading the downsizing of lean body mass (LBM), a cornerstone of bodybuilding. The confrontation of these four objective parameters thus allows the quantification of the respective importance of nutritional and inflammatory components of any disease process, taking into account that TTR is the sole plasma protein remaining highly correlated to the fluctuations of LBM. The below review highlights the prevailing roles played by protein nutritional states in the release of plasma retinol to target tissues and to the restoration of iron-deficient anemias.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

Proteomic changes of aqueous humor in proliferative diabetic retinopathy patients treated with different intravitreal anti-VEGF agents

Anti-VEGF-based treatment have been regularly used in recent years in proliferative diabetic retinopathy (PDR) patients. However, some of these patients fail to respond effectively to anti-VEGF. Given that VEGF is not the sole factor influencing PDR pathogenesis and that different anti-VEGF pharmaceuticals are likely to differentially impact these underlying pathophysiological processes, we performed a prospective analysis of the protein profiles of the aqueous humor (AH) in PDR patients before and after treatment with three intravitreal anti-VEGF drugs (ranibizumab, aflibercept, and conbercept) to assess and compare the short-term impacts of these agents. Liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomic methods were used to evaluate the AH protein profiles of PDR patients using paired pre- and 7 days post-anti-VEGF treatment samples (ranibizumab [IVR]: n = 10; conbercept [IVC]: n = 10; aflibercept [IVA]: n = 5). Gene ontology (GO) annotation, KEGG pathway analyses, and protein-protein interaction (PPI) networks were then used to explore the functional relevance of proteins that were differentially expressed between groups. Here, a total of 874 proteins from 25 patients (50 AH samples) were identified in the three patient groups. Different and common clusters of regulated proteins for each group were identified. We identified RARRES1, ALDH3A1, and RBP4 as being specifically regulated following treatment with all three tested anti-VEGF agents. We further found that VEGFR1, VEGFR2, APOM, hornerin, and HSP90B1 were differentially expressed in different anti-VEGF agent groups. In summary, we discovered that ALDH3A1 was a previously unreported protein that was related to angiogenesis and was differentially expressed in the three anti-VEGF treatment groups, suggesting that it may be a new target for PDR therapy. The described proteomic changes in the AH of PDR patients treated with different anti-VEGF agents provide novel targets which may explain the heterogeneity of anti-VEGF treatment responses in these patients, providing a robust foundation for future studies of PDR pathogenesis.

RNA-seq reveals downregulated osteochondral genes potentially related to tibia bacterial chondronecrosis with osteomyelitis in broilers

Background

Bacterial chondronecrosis with osteomyelitis (BCO) develops in the growth plate (GP) of the proximal femur and tibia and is initiated by damage to the less mineralized chondrocytes followed by colonization of opportunistic bacteria. This condition affects approximately 1% of all birds housed, being considered one of the major causes of lameness in fast growing broilers. Although several studies have been previously performed aiming to understand its pathogenesis, the molecular mechanisms involved with BCO remains to be elucidated. Therefore, this study aimed to generate a profile of global differential gene expression involved with BCO in the tibia of commercial broilers, through RNA sequencing analysis to identity genes and molecular pathways involved with BCO in chickens.

Results

Our data showed 192 differentially expressed (DE) genes: 63 upregulated and 129 downregulated in the GP of the tibia proximal epiphysis of BCO-affected broilers. Using all DE genes, six Biological Processes (BP) were associated with bone development (connective tissue development, cartilage development, skeletal system development, organ morphogenesis, system development and skeletal system morphogenesis). The analyses of the upregulated genes did not indicate any significant BP (FDR < 0.05). However, with the downregulated genes, the same BP were identified when using all DE genes in the analysis, with a total of 26 coding genes explaining BCO in the tibia: ACAN, ALDH1A2, CDH7, CHAD, CHADL, COL11A1, COMP, CSGALNACT1, CYR61, FRZB, GAL3ST1, HAPLN1, IHH, KIF26B, LECT1, LPPR1, PDE6B, RBP4A, SERINC5, SFRP1, SOX8, SOX9, TENM2, THBS1, UCHL1 and WFIKKN2. In addition, seven transcription factors were also associated to BCO: NFATC2, MAFB, HIF1A-ARNT, EWSR1-FLI1, NFIC, TCF3 and NF-KAPPAB.

Conclusions

Our data show that osteochondral downregulated genes are potential molecular causes of BCO in broilers, and the bacterial process seems to be, in fact, a secondary condition. Sixteen genes responsible for bone and cartilage formation were downregulated in BCO-affected broilers being strong candidate genes to trigger this disorder.

Proteomics and Metabolomics for AKI Diagnosis

Acute kidney injury (AKI) is a severe and frequent condition in hospitalized patients. Currently, no efficient therapy of AKI is available. Therefore, efforts focus on early prevention and potentially early initiation of renal replacement therapy to improve the outcome in AKI. The detection of AKI in hospitalized patients implies the need for early, accurate, robust, and easily accessible biomarkers of AKI evolution and outcome prediction because only a narrow window exists to implement the earlier-described measures. Even more challenging is the multifactorial origin of AKI and the fact that the changes of molecular expression induced by AKI are difficult to distinguish from those of the diseases associated or causing AKI as shock or sepsis. During the past decade, a considerable number of protein biomarkers for AKI have been described and we expect from recent advances in the field of omics technologies that this number will increase further in the future and be extended to other sorts of biomolecules, such as RNAs, lipids, and metabolites. However, most of these biomarkers are poorly defined by their AKI-associated molecular context. In this review, we describe the state-of-the-art tissue and biofluid proteomic and metabolomic technologies and new bioinformatics approaches for proteomic and metabolomic pathway and molecular interaction analysis. In the second part of the review, we focus on AKI-associated proteomic and metabolomic biomarkers and briefly outline their pathophysiological context in AKI.

The interrelationship between bile acid and vitamin A homeostasis

Vitamin A is a fat-soluble vitamin important for vision, reproduction, embryonic development, cell differentiation, epithelial barrier function and adequate immune responses. Efficient absorption of dietary vitamin A depends on the fat-solubilizing properties of bile acids. Bile acids are synthesized in the liver and maintained in an enterohepatic circulation. The liver is also the main storage site for vitamin A in the mammalian body, where an intimate collaboration between hepatocytes and hepatic stellate cells leads to the accumulation of retinyl esters in large cytoplasmic lipid droplet hepatic stellate cells. Chronic liver diseases are often characterized by disturbed bile acid and vitamin A homeostasis, where bile production is impaired and hepatic stellate cells lose their vitamin A in a transdifferentiation process to myofibroblasts, cells that produce excessive extracellular matrix proteins leading to fibrosis. Chronic liver diseases thus may lead to vitamin A deficiency. Recent data reveal an intricate crosstalk between vitamin A metabolites and bile acids, in part via the Retinoic Acid Receptor (RAR), Retinoid X Receptor (RXR) and the Farnesoid X Receptor (FXR), in maintaining vitamin A and bile acid homeostasis. Here, we provide an overview of the various levels of "communication" between vitamin A metabolites and bile acids and its relevance for the treatment of chronic liver diseases.

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.

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.

Production of functional human vitamin A transporter/RBP receptor (STRA6) for structure determination

STRA6 is a plasma membrane protein that mediates the transport of vitamin A, or retinol, from plasma retinol binding protein (RBP) into the cell. Mutations in human STRA6 are associated with Matthew-Wood syndrome, which is characterized by severe developmental defects. Despite the obvious importance of this protein to human health, little is known about its structure and mechanism of action. To overcome the difficulties frequently encountered with the production of membrane proteins for structural determination, STRA6 has been expressed in Pichia pastoris as a fusion to green fluorescent protein (GFP), a strategy which has been a critical first step in solving the crystal structures of several membrane proteins. STRA6-GFP was correctly targeted to the cell surface where it bound RBP. Here we report the large-scale expression, purification and characterisation of STRA6-GFP. One litre of culture, corresponding to 175 g cells, yielded about 1.5 mg of pure protein. The interaction between purified STRA6 and its ligand RBP was studied by surface plasmon resonance-based binding analysis. The interaction between STRA6 and RBP was not retinol-dependent and the binding data were consistent with a transient interaction of 1 mole RBP/mole STRA6.

Vitamin A: a multifunctional tool for development

Extensive research carried out over the last 100 years has established that the fat-soluble organic compound vitamin A plays crucial roles in early development, organogenesis, cell proliferation, differentiation and apoptosis as well as in tissue homeostasis. Given its importance during development, the delivery of vitamin A to the embryo is very tightly regulated with perturbations leading to severe malformations. This review discusses the roles of vitamin A during human development and the molecular mechanisms controlling its biological effects, hence bridging the gap between human development and molecular genetic work carried out in animal models. Vitamin A delivery during pregnancy and its developmental teratology in humans are thus discussed alongside work on model organisms, such as chicken or mice, revealing the molecular layout and functions of vitamin A metabolism and signaling. We conclude that, during development, vitamin A-derived signals are very tightly controlled in time and space and that this complex regulation is achieved by elaborate autoregulatory loops and by sophisticated interactions with other signaling cascades.

Rapid T cell-based identification of human tumor tissue antigens by automated two-dimensional protein fractionation

Identifying the antigens that have the potential to trigger endogenous antitumor responses in an individual cancer patient is likely to enhance the efficacy of cancer immunotherapy, but current methodologies do not efficiently identify such antigens. This study describes what we believe to be a new method of comprehensively identifying candidate tissue antigens that spontaneously cause T cell responses in disease situations. We used the newly developed automated, two-dimensional chromatography system PF2D to fractionate the proteome of human tumor tissues and tested protein fractions for recognition by preexisting tumor-specific CD4+ Th cells and CTLs. Applying this method using mice transgenic for a TCR that recognizes an OVA peptide presented by MHC class I, we demonstrated efficient separation, processing, and cross-presentation to CD8+ T cells by DCs of OVA expressed by the OVA-transfected mouse lymphoma RMA-OVA. Applying this method to human tumor tissues, we identified MUC1 and EGFR as tumor-associated antigens selectively recognized by T cells in patients with head and neck cancer. Finally, in an exemplary patient with a malignant brain tumor, we detected CD4+ and CD8+ T cell responses against two novel antigens, transthyretin and calgranulin B/S100A9, which were expressed in tumor and endothelial cells. The immunogenicity of these antigens was confirmed in 4 of 10 other brain tumor patients. This fast and inexpensive method therefore appears suitable for identifying candidate T cell antigens in various disease situations, such as autoimmune and malignant diseases, without being restricted to expression by a certain cell type or HLA allele.

Factors that influence retinol-binding protein 4-transthyretin interaction are not altered in overweight subjects and overweight subjects with type 2 diabetes mellitus

Retinol-binding protein 4 (RBP4) is an adipokine bound in plasma to transthyretin (TTR), which prevents its glomerular filtration and subsequent catabolism in the kidney. Alterations of this interaction have been suggested to be implicated in the elevation of RBP4 that are thought to contribute to the development of insulin resistance associated with obesity and type 2 diabetes mellitus (T2DM). However, the factors linking RBP4 to TTR in humans are not clear. Therefore, this study evaluated parameters influencing the RBP4-TTR interaction and their relation to obesity and T2DM. The RBP4 and TTR levels were quantified in plasma of 16 lean controls, 28 overweight controls, and 14 overweight T2DM patients by enzyme-linked immunosorbent assay. Transthyretin isoforms involved in RBP4 binding were determined by linear matrix-assisted laser desorption/ionization-time of flight-mass spectrometry after RBP4 coimmunoprecipitation. Holo-RBP4 (retinol-bound) and apo-RBP4 (retinol-free) were assessed by immunoblotting using nondenaturating polyacrylamide gel electrophoresis. Plasma levels of both RBP4 and TTR did not differ among the groups of lean controls, overweight controls, and overweight T2DM subjects. Using RBP4 immunoprecipitation, 4 mass signals were observed for TTR representing native, S-cysteinylated, S-cysteinglycinylated, and S-glutathionylated TTR. No differences in peak intensity of TTR isoforms were observed among the groups. Moreover, no differences in the ratio of holo- and apo-RBP4 were evident. The results suggest that circulating RBP4 and TTR were not affected by human obesity or T2DM, which might be attributed to the absence of alterations of TTR isoforms and the ratio of holo- and apo-RBP4 that might modify the TTR-RBP4 interaction.

Unfolding and aggregation of transthyretin by the truncation of 50 N-terminal amino acids

Senile systemic amyloidosis (SSA) is caused by amyloid deposits of wild-type transthyretin in various organs. Amyloid deposits from SSA contain large amounts of the C-terminal fragments starting near amino acid residue 50 as well as full-length transthyretin. Although a number of previous studies suggest the importance of the C-terminal fragments in the pathogenesis of SSA, little is known about the structure and aggregation properties of the C-terminal fragments of transthyretin. To understand the role of C-terminal fragments in SSA, we examined the effects of the truncation of the N-terminal portions on the structure and aggregation properties of wild-type transthyretin. The deletion mutant lacking 50 N-terminal residues was largely unfolded in terms of secondary and tertiary structure, leading to self-assembly into spherical aggregations under nearly physiological conditions. By contrast, the deletion mutant lacking 37 N-terminal residues did not have a strong tendency to aggregate, although it also adopted a largely unfolded conformation. These results suggest that global unfolding of transthyretin by proteolysis near amino acid residue 50 is an important step of self-assembly into aggregations in SSA.

Regulation of transthyretin by thyroid hormones in fish

Transthyretin (TTR) is a thyroid hormone-binding protein (THBP) which in its tetrameric form transports thyroid hormones (THs), thyroxine (T(4)) and triiodothyronine (T(3)) in the blood of vertebrates. The principal site of production of TTR is the liver but in the sea bream TTR mRNA is also present in the heart, intestine and brain. The regulation of TTR is unstudied in fish and the normal circulating level of this THBP is unknown. The aim of the present study was to establish factors which regulate TTR production in fish. As a first step a number of tools were generated; sea bream recombinant TTR (sbrTTR) and specific sbrTTR antisera which were used to establish an ELISA (enzyme-linked immunosorbent assay) for measuring TTR plasma levels. Subsequently, an experiment was conducted to determine the influence of THs on TTR production. Circulating physiological levels of TTR in sea bream determined by ELISA are approximately 3.8microgml(-1). Administration of T(3) and T(4) to sea bream significantly increased (p<0.001 and p<0.005, respectively) the concentration of circulating TTR ( approximately or = 11.5microgml(-1)) in relation to control fish, but did not change gene transcription in the liver. Methimazol (MMI) an antithyroid agent, failed to significantly reduce circulating THs below control levels but significantly increased (p<0.005) plasma TTR levels (approximately or = 10.8microgml(-1)) and decreased (p<0.05) transcription in the liver. Future studies will aim to elucidate in more detail these regulatory pathways.

Transthyretin and familial amyloidotic polyneuropathy

Familial amyloidotic polyneuropathy (FAP) is an inherited autosomal dominant disease that is commonly caused by accumulation of deposits of transthyretin (TTR) amyloid around peripheral nerves. The only effective treatment for FAP is liver transplantation. However, recent studies on TTR aggregation provide clues to the mechanism of the molecular pathogenesis of FAP and suggest new avenues for therapeutic intervention. It is increasingly recognized that there are common features of a number of protein-misfolding diseases that can lead to neurodegeneration. As for other amyloidogenic proteins, the most toxic forms of aggregated TTR are likely to be the low-molecular-mass diffusible species, and there is increasing evidence that this toxicity is mediated by disturbances in calcium homeostasis. This article reviews what is already known about the mechanism of TTR aggregation in FAP and describes how recent discoveries in other areas of amyloid research, particularly Alzheimer's disease, provide clues to the molecular pathogenesis of FAP.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

The concentration of free holo-retinol binding protein is higher in vitamin A-sufficient than in deficient Nepalese women in late pregnancy

Free holo-retinol binding protein (RBP) [i.e., unbound to transthyretin (TTR)] plays a role in transporting vitamin A across the placenta during pregnancy. In a cross-sectional study of clinically healthy urban women, we assessed the association among clinical and biochemical factors on estimated concentrations of free holo-RBP during the last trimester of pregnancy. Serum samples obtained from a subsample of women (n = 259), who had participated in the Night Vision Threshold Test study in Nepal, were analyzed for determinations of retinol by HPLC, and RBP, TTR, and alpha-1 acid glycoprotein by radial immunodiffusion. Free holo-RBP concentrations were calculated using dissociation constants for free holo- and apo-RBP. Among these women, 30% were vitamin A deficient based on either the RBP:TTR index < or = 0.36 or serum retinol < 1.05 micromol/L. Using stepwise regression analyses, the RBP:TTR index explained 75% of the variance in free holo-RBP concentrations, whereas retinol explained only 14%. Women were classified as vitamin A sufficient (n = 185) or deficient (n = 74) using the RBP:TTR index and were stratified into 3 gestational groups (I: 24-28 wk, II: 29-33 wk, III: >33 wk). Concentrations of free holo-RBP were higher in vitamin A-sufficient women than in vitamin A-deficient women (mean +/- SEM, 48.1 +/- 1.2 vs. 27.6 +/- 0.8 nmol/L; P < 0.001), and in a 3 x 2 factorial analysis, the interaction between gestational group and vitamin A status was significant. These results demonstrate that the RBP:TTR index is a useful proxy for free holo-RBP concentration and that vitamin A status affects its distribution.

Interactions amongst plasma retinol-binding protein, transthyretin and their ligands: implications in vitamin A homeostasis and transthyretin amyloidosis

Retinol transport complex consisting of retinol-binding protein (RBP) and transthyretin (TTR) is involved in the transport of retinol (vitamin A) and thyroxine (T(4)) in the human plasma. RBP is a 21-kDa single polypeptide chain protein, synthesized in the liver, which binds and transports retinol to the target organs. The circulating RBP binds to another protein called TTR, a 55-kDa homotetrameric T(4) transport protein. Such protein-protein complex formation is thought to prevent glomerular filtration of low molecular mass RBP. Misfolding and aggregation of TTR is implicated in amyloid disorders such as familial amyloid polyneuropathy (FAP) and senile systemic amyloidosis (SSA). Recent observations suggest that both RBP and T(4), the physiological ligands of TTR, prevent its misfolding and amyloid fibril formation, suggesting yet another structure-function relationship to this protein-protein complex. TTR2, a poorly characterized protein, was also found bound to RBP in human and pig plasma but its significance remains to be understood. Furthermore, knockout models of both RBP and TTR unequivocally demonstrated the importance of this protein-protein complex in retinoid transport. Thus, interactions amongst multiple components of retinol transport play critical roles in vitamin A homeostasis and TTR amyloidosis.

Endocrine disrupting chemicals: interference of thyroid hormone binding to transthyretins and to thyroid hormone receptors

We examined the effects of industrial, medical and agricultural chemicals on 3,5,3'-L-[125I]triiodothyronine ([125I]T(3)) binding to transthyretins (TTRs) and thyroid hormone receptors (TRs). Among the chemicals investigated diethylstilbestrol (DES) was the most powerful inhibitor of [125I]T(3) binding to chicken and bullfrog TTR (cTTR and bTTR). Inhibition of [125I]T(3) binding was more apparent in cTTR than bTTR. Scatchard analysis revealed DES, pentachlorophenol and ioxynil as competitive inhibitors of [125I]T(3) binding to cTTR and bTTR. However, cTTR's affinity for the three chemicals was higher than its affinity for T(3). A miticide dicofol (10(-10)-10(-7) M) activated [125I]T(3) binding to bTTR up to 170%. However, at 4x10(-5) M it inhibited [125I]T(3) binding by 83%. Dicofol's biphasic effect upon [125I]T(3) binding was not detected in TTRs from other species. DES and pentachlorophenol, in the presence of plasma, increased cellular uptake of [125I]T(3) in vitro, by displacing [125I]T(3) from its plasma binding sites. These chemicals did not, however, affect the association of cTTR with chicken retinol-binding protein. All chemicals investigated had little or no influence on [125I]T(3) binding to chicken TR (cTR) and bullfrog TR (bTR). Several endocrine disrupting chemicals that were tested interfered with T(3) binding to TTR rather than to TR. Binding of the endocrine disrupting chemicals to TTR may weaken their intrinsic effects on target cells by depressing their free concentrations in plasma. However, this may affect TH homeostasis in vivo by altering the free concentrations of plasma THs.

Transthyretin in fish: state of the art

Relatively little is known about thyroid hormone-binding proteins in fish and, until recently, the thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), had only been found in fish plasma bound to albumin and lipoproteins. Recently, transthyretin (TTR) was cloned in a teleost fish, the sea bream (sb); it is composed of 130 amino acids and shares 47-54% sequence similarity with other vertebrate TTR and binds preferentially T3. Homology modelling of sbTTR based upon the crystallographic structure of TTR in human, rat and chicken reveals similar monomer-monomer and dimer-dimer interfaces and a conserved tetrameric structure. In sbTTR, a single amino acid substitution in the thyroid hormone binding site (Ser 117 in human by Thr in sea bream) may explain the higher affinity of this tetramer for T3 rather than T4. The principal site of production of TTR in the sea bream is the liver but transcripts are also present in the intestine, brain, skin, heart, skeletal muscle, kidney, testis, gills and pituitary (in descending order of abundance). The function of TTR in fish remains to be studied but we have recently carried out studies which suggest it may be involved in TH balance during food shortage.

Transthyretin as a thyroid hormone carrier: function revisited

Thyroid hormones are essential for normal mammalian development and for normal metabolism. Thyroxine (T4) is the principal product synthesized by the thyroid follicles, and triiodothyronine (T3), the biologically active hormone, derives mainly from tissue T4 deiodination. More than 99% of the circulating hormone is bound to plasma proteins, mainly to thyroxine-binding globulin, transthyretin and albumin in man, and to transthyretin and albumin in rodents. The role of plasma proteins in the transport of hormones to target tissues has, for a long time, been controversial. The liver and the choroid plexus are the major sites of transthyretin synthesis, tissues from which transthyretin is secreted into the blood and the cerebrospinal fluid, respectively. Transthyretin has been proposed to mediate thyroid hormone transfer into the tissues, particularly into the brain across the choroid-plexus-cerebrospinal fluid barrier. Studies in a transthyretin-null mice strain have shown conclusively that transthyretin is not indespensable for thyroid hormones' entry into the brain and other tissues, nor for the maintenance of an euthyroid status. An euthyroid status is also observed in man totally deprived of thyroxine-binding globulin and in rats without albumin. Taken together, these results exclude dependence of thyroid hormone homeostasis on any major plasma carrier per se. This evidence agrees with the free hormone hypothesis which states that the biologically significant fraction, that is taken up by the tissues, is the free circulating hormone.

Support for the multigenic hypothesis of amyloidosis: the binding stoichiometry of retinol-binding protein, vitamin A, and thyroid hormone influences transthyretin amyloidogenicity in vitro

The amyloidoses are a large group of protein misfolding diseases. Genetic and biochemical evidence support the hypothesis that amyloid formation from wild-type or 1 of 80 sequence variants of transthyretin causes the human amyloid diseases senile systemic amyloidosis or familial amyloid polyneuropathy, respectively. The late onset and variable penetrance of these diseases has led to their designation as multigenic--implying that the expression levels and alleles of multiple gene products influence the course of pathology. Here we show that the binding stoichiometry of three interacting molecules, retinol-binding protein, vitamin A, and L-thyroxine, notably influenced transthyretin amyloidogenicity in vitro. At least 70 genes control retinol-binding protein, vitamin A, and L-thyroxine levels in plasma and have the potential to modulate the course of senile systemic amyloidosis or familial amyloid polyneuropathy.

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.

Evolution of the thyroid hormone-binding protein, transthyretin

Transthyretin (TTR) belongs to a group of proteins, which includes thyroxine-binding globulin and albumin, that bind to and transport thyroid hormones in the blood. TTR is also indirectly implicated in the carriage of vitamin A through the mediation of retinol-binding protein (RBP). It was first identified in 1942 in human serum and cerebrospinal fluid and was formerly called prealbumin for its ability to migrate faster than serum albumin on electrophoresis of whole plasma. It is a single polypeptide chain of 127 amino acids (14,000 Da) and is present in the plasma as a tetramer of noncovalently bound monomers. The major sites of synthesis of TTR in eutherian mammals, marsupials, and birds are the liver and choroid plexus but in reptiles it is synthesised only in the choroid plexus. The observation that TTR is strongly expressed in the choroid plexus but not in the liver of the stumpy-tailed lizard and the strong conservation of expression in the choroid plexus from reptiles to mammals have been taken as evidence to suggest that extrahepatic synthesis of TTR evolved first. The identification and cloning of TTR from the liver of an amphibian, Rana catesbeiana, and a teleost fish, Sparus aurata, and its absence from the choroid plexus of both species suggest an alternative model for its evolution. Protein modelling studies are presented that demonstrate differences in the electrostatic characteristics of the molecule in human, rat, chicken, and fish, which may explain why, in contrast to TTR from human and rat, TTR from fish and birds preferentially binds triiodo-l-thyronine.

Assessment of protein energy malnutrition in older persons, Part II: Laboratory evaluation

A large proportion of chronic diseases affecting older persons can be either prevented or significantly improved by improving nutrition. This places an increased burden on health care professionals caring for older persons. Screening for malnutrition at an early stage allows the intervention to be most successful. History, physical examination, and anthropometric measurements are essential parts of any nutritional evaluation. However, these tools can be highly subjective and rely heavily on the knowledge and experience of the evaluator. Incorporating biochemical measurements in the routine nutritional assessment provides an often-needed objective dimension. Interpreting these measurements must take into consideration the normal biological changes seen with aging. In this article, we review many of the biochemical parameters used in nutritional assessment and their relation to morbidity and mortality, with a special focus on normal changes seen with aging.

C-reactive protein as an outcome predictor for maintenance hemodialysis patients

BACKGROUND

The possible association between inflammatory processes and other outcome measures in ESRD patients led us to measure the blood C-reactive protein (CRP) concentration in a large sample of hemodialysis patients, and to evaluate its statistical relationship with other common laboratory measures and patient survival. This was performed in a prospective, observational analysis with mortality as the principal outcome measure.

METHODS

One thousand fifty-four routine blood samples, collected from as many patients during June and July 1995 (one sample per patient), were randomly selected for measurement of CRP, prealbumin, and other routine laboratory measures. Six months after the initial blood tests, patient survival was determined: Logistic regression analysis was the primary statistical tool used to evaluate laboratory associations with odds of death. Bivariate regression and correlation analyses were performed using all available data.

RESULTS

The distribution of CRP values was skewed; approximately 35% of the values exceeded the upper limit of the laboratory's reference range. Serum albumin and prealbumin concentrations both correlated with the serum creatinine concentration (r = 0.378 and r = 0.347, respectively; P's < 0.001), and were inversely associated with the CRP (r = -0.254 and r = -0.354, respectively; P's < 0.001). CRP was also inversely associated with blood hemoglobin concentrations (r = -0.235; P < 0.001). Using multiple regression analysis to further explore these relationships, the serum creatinine concentration was inversely associated with CRP (r = -0.140; P < 0.001). However, after adjustment for the linkage of the serum creatinine with the serum albumin concentration (r = -0.378; P < 0.001), no relationship with creatinine was observed. Before and after adjustment for serum albumin and prealbumin concentration, the ferritin concentration correlated directly with CRP (r = 0.148; P < 0.001). Ferritin was inversely and highly correlated with total iron binding capacity (r = -0.516; P < 0.001). Independent associations of hemoglobin with albumin (t = 7.16; P < 0.001), prealbumin (t = 2.39; P = 0.017), and CRP (t = -4.27; P < 0.001) were observed. Also, the dose of erythropoietin was directly associated with the CRP concentration, before (r = 0.081, P = 0.009) and after (t = 2.03, P = 0.042) adjustment for the serum albumin and iron concentrations. CRP correlated directly with neutrophil (r = 0.318; P < 0.001) and platelet counts (r = 0.180; P < 0.001), but was weakly and inversely correlated with the lymphocyte count (r = -0.071; P = 0.04). A logistic regression analysis performed using the laboratory variables revealed a strong, independent, and inverse relationships between the serum albumin and creatinine concentrations, total lymphocyte count, and the odds risk of death. In this model, no significant relationship was observed between the odds risk of death and CRP.

CONCLUSIONS

The data presented herein suggest that: (1) strong predictable associations exist among laboratory proxies for malnutrition, anemia, and the acute phase reaction, and (2) the pathobiology implied by these laboratory abnormalities influence patients' mortal risk primarily through depletion of vital body proteins, not inflammation.

A molecular model for the retinol binding protein-transthyretin complex

A three-dimensional model for the complex between human serum retinol binding protein and transthyretin (formerly named prealbumin) is presented. The model was obtained by interactive rigid-body computer graphics docking and the characterization of the molecular surfaces in terms of fractal dimension. Available experimental data, as well as results from molecular dynamics calculations, support the proposed model.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Crystallographic refinement of human serum retinol binding protein at 2A resolution

Human serum retinol binding protein (RBP) in complex with retinol has been crystallographically refined to an R-factor of 18.1% with 2A resolution data. The protein topology results in an anti-parallel beta-barrel that encapsulates the retinol ligand. A detailed description of the protein and the binding site is provided. Our structural work has helped to define a family of proteins, many of which are carrier proteins for smaller ligand molecules. We describe the structural basis for the conservation of sequence within the family.

Affinity chromatographic purification of serum retinol-binding protein using 4-substituted aminoretinoids

Retinol-binding protein has been purified from rabbit serum by a new affinity chromatographic phase. Human retinol-binding protein has been shown to bind with vitamin A derivatives and certain other terpenoids. Consequently, this affinity method is based upon the ability of the protein to reversibly bind to beta-ionone and employs a derivatized affinity ligand while preserving the integrity of the beta-ionone molecule via substitution of the allylic 4-position. Purification is relatively simple when compared with other known methods and the yield from serum is similar to other schemes. The protein is obtained in the apo-form and retains the ability of the native protein to bind retinol.

Purification of prealbumin from human serum

A method is presented by which prealbumin (thyroxine-binding prealbumin; tryptophan-rich prealbumin) may be purified to homogeneity from human serum. The method involves precipitation of contaminating proteins with dilute aqueous phenol, ion-exchange chromatography on DEAE-Sephacel, and gel permeation chromatography on Sephadex G-100. The yield is 25-30%, and the prealbumin is homogeneous by polyacrylamide gel electrophoresis at pH 8.9 and pH 3.6.

The complete amino acid sequence of human serum retinol-binding protein

The complete amino acid sequence of human serum Retinol-binding protein (RBP) including the distribution of its three disulfide bridges, has been determined. The protein consists of 182 amino acid residues, the order of which was determined following the isolation of five CNBr-fragments. Direct amino acid sequence analysis in an automatic liquid phase sequencer provided almost the entire sequences of the five CNBr-fragments. Several sets of enzymatically derived peptides of RBP were also used to elucidate the primary structure. RBP displays significant homology to bovine beta-lactoglobulin, human alpha 1-microglobulin and rat alpha 1-microglobulin. RBP contains an internal homology. Thus, residues 36 to 83 display statistically significant homology with residues 96 to 141.

Isolation and preliminary characterization of tear prealbumin from human ocular mucus

Tear prealbumin was purified from crude tear prealbumin previously isolated from the saline soluble human ocular mucus. Purification was achieved by further column chromatographies on DEAE Sephadex A-25 and Sephadex G-75. Preliminary characterization included amino acid analysis, gel electrophoresis, and isoelectric focusing. Unlike serum prealbumin, the purified tear prealbumin showed a predominance of acidic residues and a trace amount of tryptophan. It exhibited polymorphic nature, with pI values of 4.8 and 4.9. The possibility of a tear prealbumin/retinol complex was also examined. The protein was found to incorporate with 3H retinol. The 3H retinol-incorporated tear prealbumin did not exhibit the characteristic UV spectrum of retinol; however, it did display emission and excitation fluorescence spectra at high concentrations similar to serum retinol-binding protein.

Restriction of exogenous transthyretin (prealbumin) by the endothelium of the rat choriocapillaris

The endothelium of the choriocapillaris has been shown to restrict molecules with Einstein-Stokes radii greater than or equal to 3.2 nm which correspond to minimal molecular weights of approximately 64,000-68,000 daltons. The present study was undertaken to determine if the endothelium restricts exogenous transthyretin (prealbumin), a 55,000-dalton carrier of retinol-binding protein. Rats were injected with human 125I-transthyretin which was allowed to circulate for 15 and 30 min. Chromatographic analysis demonstrated that the human transthyretin did not bind to rat blood proteins. Eye tissue from injected rats was prepared for light and ultrastructural autoradiographic analysis. Autoradiographic grains were confined to areas over the lumen of the choriocapillaris with few present on Bruch's membrane, the basement membrane common to the endothelium of the choriocapillaris and the retinal pigment epithelium. These findings demonstrate that the choriocapillaris can restrict transthyretin and suggest a possible role of its endothelium in the metabolism of retinol-carrier molecules.

Purification of human plasma retinol-binding protein by hydrophobic interaction chromatography

Human plasma retinol-binding protein has been purified to homogeneity by a simple method that requires an ammonium sulfate fractionation, a hydrophobic interaction chromatography on phenyl-Sepharose, which dissociates the complex between retinol-binding protein and its carrier, transthyretin, and a gel filtration on Sephadex G-50. The yield of pure protein is comparable or higher than that obtained with the more complex procedures previously reported.

Family studies of the genetic abnormality in transthyretin (prealbumin) in Portuguese patients with familial amyloidotic polyneuropathy

Amyloid deposits in several heredofamilial forms of amyloidosis are chemically related to transthyretin (TTR, the protein usually referred to as prealbumin). A genetically abnormal TTR may be involved. Studies were conducted on TTR isolated from sera of patients with familial amyloidotic polyneuropathy (FAP), and on amyloid fibril protein (AFp) isolated from tissues of two Portuguese patients who died with FAP. AFp, purified by affinity chromatography on retinol-binding protein (RBP), resembled plasma TTR in forming a stable tetrameric structure, and in its binding affinities for both thyroxine and RBP. Purified AFp was found to comprise a TTR variant with a methionine for valine substitution at position 30. This conclusion was based upon studies that included: (i) comparative peptide mapping by reverse-phase high-performance liquid chromatography after trypsin digestion; (ii) cyanogen bromide (CNBr) cleavage studies; and (iii) amino acid microsequence analysis of selected tryptic and CNBr peptides. The variant TTR was also found to be present in serum samples from FAP patients, along with larger amounts of normal TTR. An effective, small-scale procedure was developed to determine whether or not the variant TTR was present in the plasma of an individual subject. This procedure involved isolation of TTR by affinity chromatography on RBP, followed by CNBr cleavage, and analysis for the presence of specific aberrant CNBr peptides. Studies with six kindreds, including 21 asymptomatic children of 6 patients with FAP, showed that the "abnormal" TTR can be detected and used as a preclinical marker of the disease in affected children of patients with FAP. It is likely that the variant TTR represents a point mutation within the TTR structural gene, and that the normal and mutant genes act as co-dominant alleles at a single locus in FAP. The distribution of the mutant TTR within the six families was consistent with the autosomal dominant mode of inheritance of FAP. The mutant TTR apparently selectively deposits in tissues as the amyloid characteristic of the disease.