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

The Logistical Backbone of Photoreceptor Cell Function: Complementary Mechanisms of Dietary Vitamin A Receptors and Rhodopsin Transporters

In this review, we outline our current understanding of the mechanisms involved in the absorption, storage, and transport of dietary vitamin A to the eye, and the trafficking of rhodopsin protein to the photoreceptor outer segments, which encompasses the logistical backbone required for photoreceptor cell function. Two key mechanisms of this process are emphasized in this manuscript: ocular and systemic vitamin A membrane transporters, and rhodopsin transporters. Understanding the complementary mechanisms responsible for the generation and proper transport of the retinylidene protein to the photoreceptor outer segment will eventually shed light on the importance of genes encoded by these proteins, and their relationship on normal visual function and in the pathophysiology of retinal degenerative diseases.

The molecular aspects of absorption and metabolism of carotenoids and retinoids in vertebrates

Vitamin A is an essential nutrient necessary for numerous basic physiological functions, including reproduction and development, immune cell differentiation and communication, as well as the perception of light. To evade the dire consequences of vitamin A deficiency, vertebrates have evolved specialized metabolic pathways that enable the absorption, transport, and storage of vitamin A acquired from dietary sources as preformed retinoids or provitamin A carotenoids. This evolutionary advantage requires a complex interplay between numerous specialized retinoid-transport proteins, receptors, and enzymes. Recent advances in molecular and structural biology resulted in a rapid expansion of our understanding of these processes at the molecular level. This progress opened new avenues for the therapeutic manipulation of retinoid homeostasis. In this review, we summarize current research related to the biochemistry of carotenoid and retinoid-processing proteins with special emphasis on the structural aspects of their physiological actions. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Comparative pharmacokinetics and pharmacodynamics of the advanced Retinol-Binding Protein 4 antagonist in dog and cynomolgus monkey

Accumulation of lipofuscin bisretinoids in the retina contributes to pathogenesis of macular degeneration. Retinol-Binding Protein 4 (RBP4) antagonists reduce serum retinol concentrations thus partially reducing retinol delivery to the retina which decreases bisretinoid synthesis. BPN-14136 is a novel RBP4 antagonist with good in vitro potency and selectivity and optimal rodent pharmacokinetic (PK) and pharmacodynamic (PD) characteristics. To select a non-rodent species for regulatory toxicology studies, we conducted PK and PD evaluation of BPN-14136 in dogs and non-human primates (NHP). PK properties were determined following oral and intravenous administration of BPN-14136 in beagle dogs and cynomolgus monkeys. Dynamics of plasma RBP4 reduction in response to compound administration was used as a PD marker. BPN-14136 exhibited favorable PK profile in both species. Dose-normalized exposure was significantly higher in NHP than in dog. Baseline concentrations of RBP4 were considerably lower in dog than in NHP, reflecting the atypical reliance of canids on non-RBP4 mechanisms of retinoid trafficking. Oral administration of BPN-14136 to NHP induced a strong 99% serum RBP4 reduction. Dynamics of RBP4 lowering in both species correlated with compound exposure. Despite adequate PK and PD characteristics of BPN-14136 in dog, reliance of canids on non-RBP4 mechanisms of retinoid trafficking precludes evaluation of on-target toxicities for RBP4 antagonists in this species. Strong RBP4 lowering combined with good PK attributes and high BPN-14136 exposure achieved in NHP, along with the biology of retinoid trafficking that is similar to that of humans, support the choice of NHP as a non-rodent safety species.

Serum proteome mapping of EGF transgenic mice reveal mechanistic biomarkers of lung cancer precursor lesions with clinical significance for human adenocarcinomas

Atypical adenomatous hyperplasia (AAH) of the lung is a pre-invasive lesion (PL) with high risk of progression to lung cancer (LC). However, the pathways involved are uncertain. We searched for novel mechanistic biomarkers of AAH in an EGF transgenic disease model of lung cancer. Disease regulated proteins were validated by Western immunoblotting and immunohistochemistry (IHC) of control and morphologically altered respiratory epithelium. Translational work involved clinical resection material. Collectively, 68 unique serum proteins were identified by 2DE-MALDI-TOF mass spectrometry and 13 reached statistical significance (p < 0.05). EGF, amphiregulin and the EGFR endosomal sorting protein VPS28 were induced up to 5-fold while IHC confirmed strong induction of these proteins. Furthermore, ApoA1, α-2-macroglobulin, and vitamin-D binding protein were nearly 6- and 2-fold upregulated in AAH; however, ApoA1 was oppositely regulated in LC to evidence disease stage dependent regulation of this tumour suppressor. Conversely, plasminogen and transthyretin were highly significantly repressed by 3- and 20-fold. IHC confirmed induced ApoA1, Fetuin-B and transthyretin expression to influence calcification, inflammation and tumour-infiltrating macrophages. Moreover, serum ApoA4, ApoH and ApoM were 2-, 2- and 6-fold repressed; however tissue ApoM and sphingosine-1-phosphate receptor expression was markedly induced to suggest a critical role of sphingosine-1-phosphate signalling in PL and malignant transformation. Finally, a comparison of three different LC models revealed common and unique serum biomarkers mechanistically linked to EGFR, cMyc and cRaf signalling. Their validation by IHC on clinical resection material established relevance for distinct human lung pathologies. In conclusion, we identified mechanistic biomarker candidates recommended for in-depth clinical evaluation.

Genome-wide transcriptome analysis of hypothalamus in rats with inherited stress-induced arterial hypertension

Background

The hypothalamus has an important role in the onset and maintenance of hypertension and stress responses. Rats with inherited stress-induced arterial hypertension (ISIAH), reproducing the human stress-sensitive hypertensive state with predominant involvement of the neuroendocrine hypothalamic-pituitary-adrenal and sympathoadrenal axes, were used for analysis of the hypothalamus transcriptome.

Results

RNA-seq analysis revealed 139 genes differentially expressed in the hypothalami of hypertensive ISIAH and normotensive Wistar Albino Glaxo (WAG) rats. According to the annotation in databases, 18 of the differentially expressed genes (DEGs) were associated with arterial hypertension. The Gene Ontology (GO) functional annotation showed that these genes were related to different biological processes that may contribute to the hypertension development in the ISIAH rats. The most significantly affected processes were the following: regulation of hormone levels, immune system process, regulation of response to stimulus, blood circulation, response to stress, response to hormone stimulus, transport, metabolic processes, and endocrine system development. The most significantly affected metabolic pathways were those associated with the function of the immune system and cell adhesion molecules and the metabolism of retinol and arachidonic acid. Of the top 40 DEGs making the greatest contribution to the interstrain differences, there were 3 genes (Ephx2, Cst3 and Ltbp2) associated with hypertension that were considered to be suitable for further studies as potential targets for the stress-sensitive hypertension therapy. Seven DEGs were found to be common between hypothalamic transcriptomes of ISIAH rats and Schlager mice with established neurogenic hypertension.

Conclusions

The results of this study revealed multiple DEGs and possible mechanisms specifying the hypothalamic function in the hypertensive ISIAH rats. These results provide a basis for further investigation of the signalling mechanisms that affect hypothalamic output related to stress-sensitive hypertension development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0307-8) contains supplementary material, which is available to authorized users.

Pharmacological inhibition of lipofuscin accumulation in the retina as a therapeutic strategy for dry AMD treatment

Age-related macular degeneration (AMD) is the leading cause of blindness in the western world. There is no FDA-approved treatment for the most prevalent dry (atrophic) form of AMD. Photoreceptor degeneration in dry AMD is triggered by abnormalities in the retinal pigment epithelium (RPE). It has been suggested that excessive accumulation of fluorescent lipofuscin pigment in the RPE represents an important pathogenic factor in etiology and progression of dry AMD. Cytotoxic lipofuscin bisretinoids, such as A2E, are formed in the retina in a non-enzymatic way from visual cycle retinoids. Inhibition of toxic bisretinoid production in the retina seems to be a sound treatment strategy for dry AMD. In this review we discuss the following classes of pharmacological treatments inhibiting lipofuscin bisretinoid formation in the retina: direct inhibitors of key visual cycle enzymes, RBP4 antagonists, primary amine-containing aldehyde traps, and deuterated analogs of vitamin A.

Probiotics promote rapid-turnover protein production by restoring gut flora in patients with alcoholic liver cirrhosis

BACKGROUND AND AIMS

Accumulating evidence suggests that deterioration of the gut flora contributes to the pathogenesis of alcoholic liver cirrhosis (ALC). However, the ALC flora profile and its response to probiotic treatment have not been fully examined. This double-blind placebo-controlled study aimed to evaluate whether the probiotic beverage Yakult 400 (Y400), which contains Lactobacillus casei strain Shirota, improves liver function in ALC patients, and to analyze the precise gut flora profile by real-time quantitative PCR (qPCR).

METHODS

A total of 37 hospitalized ALC patients were randomly allocated to Y400 (n = 18) and placebo (n = 19) groups. Y400 or placebo was served twice a day during the first half of the four-week study. Serum concentrations of rapid-turnover proteins (i.e., transthyretin and transferrin), hypersensitive C-reactive protein, and interleukin-6 were measured weekly. qPCR analysis of fecal bacteria was performed biweekly; stocked fecal samples from 46 healthy subjects were used as references.

RESULTS

Serum transthyretin levels significantly increased in the Y400 group in week 3; similar-although statistically insignificant-increases were seen for transferrin and albumin. Levels of hypersensitive C-reactive protein, but not interleukin-6, significantly decreased in week 4. Before treatment, populations of obligate anerobic bacteria were significantly smaller and those of Enterobacteriaceae were larger in patients than in healthy subjects examined in a previous study. Y400 corrected this imbalance.

CONCLUSIONS

This is the first report describing the unique gut flora of ALC patients. Y400 treatment normalized the gut flora and improved liver function. These promising findings warrant further investigation in larger, multicenter studies.

Liver retinol transporter and receptor for serum retinol-binding protein (RBP4)

Vitamin A (retinol) is absorbed in the small intestine, stored in liver, and secreted into circulation bound to serum retinol-binding protein (RBP4). Circulating retinol may be taken up by extrahepatic tissues or recycled back to liver multiple times before it is finally metabolized or degraded. Liver exhibits high affinity binding sites for RBP4, but specific receptors have not been identified. The only known high affinity receptor for RBP4, Stra6, is not expressed in the liver. Here we report discovery of RBP4 receptor-2 (RBPR2), a novel retinol transporter expressed primarily in liver and intestine and induced in adipose tissue of obese mice. RBPR2 is structurally related to Stra6 and highly conserved in vertebrates, including humans. Expression of RBPR2 in cultured cells confers high affinity RBP4 binding and retinol transport, and RBPR2 knockdown reduces RBP4 binding/retinol transport. RBPR2 expression is suppressed by retinol and retinoic acid and correlates inversely with liver retinol stores in vivo. We conclude that RBPR2 is a novel retinol transporter that potentially regulates retinol homeostasis in liver and other tissues. In addition, expression of RBPR2 in liver and fat suggests a possible role in mediating established metabolic actions of RBP4 in those tissues.

Evolutionary changes to transthyretin: structure-function relationships

Transthyretin is one of the three major thyroid hormone-binding proteins in plasma and/or cerebrospinal fluid of vertebrates. It transports retinol via binding to retinol-binding protein, and exists mainly as a homotetrameric protein of approximately 55 kDa in plasma. The first 3D structure of transthyretin was an X-ray crystal structure from human transthyretin. Elucidation of the structure-function relationship of transthyretin has been of significant interest since its highly conserved structure was shown to be associated with several aspects of metabolism and with human diseases such as amyloidosis. Transthyretin null mice do not have an overt phenotype, probably because transthyretin is part of a network with other thyroid hormone distributor proteins. Systematic study of the evolutionary changes of transthyretin structure is an effective way to elucidate its function. This review summarizes current knowledge about the evolution of structural and functional characteristics of vertebrate transthyretins. The molecular mechanism of evolutionary change and the resultant effects on the function of transthyretin are discussed.

A 28-day oral dose toxicity study enhanced to detect endocrine effects of a purified technical pentabromodiphenyl ether (pentaBDE) mixture in Wistar rats

A 28-day subacute oral toxicity study was performed in Wistar rats with a purified preparation of the commercial pentabromodiphenyl ether (pentaBDE), DE-71. The applied OECD407 protocol was enhanced for endocrine and immune parameters, and to enable benchmark dose analysis. A vehicle control group and 7 dose groups were included, which received 0.27, 0.82, 2.47, 7.4, 22.2, 66.7 or 200 mg pentaBDE/kg bw/d (mkd). The liver appeared to be a key target organ, showing a marked increase of weight and centrilobular hepatocellular hypertrophy, probably due to the observed induction of P450 enzymes, notably CYP1A and CYP2B. A marked decrease of circulating total thyroxine (TT4) and an increase of plasma cholesterol were probably secondary to the liver effects. Furthermore, dose-dependently decreased weight of epididymis, seminal vesicles, and prostate, as well as sperm head deformities in males, and induction of CYP17 activity in adrenals in females were observed, all possibly related to anti-androgenic activity. Finally, we observed a substantial increase of large unstained cells in the blood and a decrease of apolar retinoids in the liver. All these effects had benchmark doses at the lower confidence bound (BMDL) in the low- or mid-dose range, but particular sensitive, potentially adverse effects were TT4 decrease (BMDLs 1.1 in males and 1.8 mkd in females), and decrease of hepatic apolar retinoids (BMDLs 0.5 mkd in males and 2.3 mkd in females). These results contribute to refinement of the hazard identification of pentaBDE and improved risk assessment of human exposure to this industrial chemical and environmental pollutant.

Determination of a cut-off value for the molar ratio of retinol-binding protein to transthyretin (RBP:TTR) in Bangladeshi patients with low hepatic vitamin A stores

The purpose of this study was to determine a cut-off value of the molar ratio of retinol-binding protein to transthyretin (RBP:TTR) to indicate marginal vitamin A (VA) deficiency. Plasma RBP and TTR were measured by radial immunodiffusion in two groups of patients, i.e., surgical patients with known hepatic VA stores, and a cohort of children residing in a malaria-endemic area of Papua New Guinea who had received placebo or 210 micro mol of VA every 3 mo for 9 mo. A RBP:TTR ratio < or =0.36 selectively detected five of seven patients (71% sensitivity) with hepatic VA stores < or =69.9 nmol/g of tissue (i.e., < or =20 micro g/g), indicative of marginal VA deficiency. Using this cut-off value, 28% (n = 245) of children from Papua New Guinea had marginal VA deficiency before supplementation. After 7 mo, a low ratio persisted in 29% (n = 92) of placebo-treated children but in only 11% (n = 83) of those receiving VA supplements (chi(2), P < 0.01). At the end of the study, 13 mo after initiation or 4 mo after the last dose of VA, the percentage of children with a low ratio was still lower (chi(2), P < 0.02) in the VA group, 42.5% (n = 113) than in the placebo group, 58.6% (n = 118). These results demonstrate that a cut-off value < or =0.36 is indicative of marginal VA deficiency and can be used as an indirect method of VA assessment.

Three-dimensional structure of the transthyretin-retinol-binding protein complex

Transthyretin (TTR), formerly called prealbumin, one of the transporters of the hormone thyroxine and retinol-binding protein (RBP), the specific carrier of vitamin A, forms, under physiological conditions, a macromolecular complex that prevents glomerular filtration of the low-molecular-weight RBP in the kidneys. This paper describes briefly the three-dimensional structure of the two proteins as determined by X-ray diffraction analysis of single crystals and that of the complex that the two molecules form in plasma and discusses the non-crystallographic evidence that supports the model of the macromolecular complex.

Naso-maxillary deformity due to frontonasal expression of human transthyretin gene in transgenic mice

BACKGROUND

Retinoic acid, a metabolic product of retinol, is essential for craniofacial morphogenesis. Transthyretin (TTR) is a plasma protein delivering retinol to tissues. We produced several transgenic mouse lines using the human mutant TTR (hTTRMet30) gene to establish a mouse model of familial amyloidotic polyneuropathy. One of the lines showed an autosomal dominant inheritance of naso-maxillary deformity termed Nax.

RESULTS

The Nax malformation was characterized by a hypoplastic developmental defect of the frontonasal region. Homozygous mice with higher transgene expressions showed more severe phenotypes, but a subline, in which the copy number and expression of the transgene was reduced, showed a normal phenotype, indicating that the hTTRMet30 expression caused the malformation. Nax mice began to express the hTTRMet30 gene in the nasal placode from embryonic day 10.5 (E10.5), which was 2 days earlier than in the other transgenic lines with a normal phenotype. Excessive cell death was observed in the nasal placode of the E10.5 Nax embryos. In addition, the forced expression of hTTRMet30 in the nasal placode of transgenic mice resulted in similar phenotypes.

CONCLUSION

The expression of the hTTRMet30 gene in the nasal placode at E10.5 induced apoptotic cell death, leading to hypoplastic deformity in the frontonasal region.

Retinal anatomy and function of the transthyretin null mouse

Vitamin A (retinol) is vital for the normal development and function of many tissues in the body including the eye. The purpose of this project was to characterize the retinal anatomy and function of the transthyretin (TTR) null mouse. Mice lacking TTR have been constructed by homologous recombination. Immunocytochemistry was performed to localize short and mid-long wavelength cone opsins as well as morphological examination of the entire retina in wild-type and TTR null mice. Visual function was assessed using the electroretinogram (ERG) and resulting waveforms were analysed in terms of receptoral and postreceptoral components. Retinal morphology of the TTR null mouse was normal. In addition, short and mid-long wavelength cone opsins were localized normally in both TTR null and wild-type retinae. Consistent with these findings, TTR null mice show no anomalies of receptoral (P3) nor post-receptoral (b-wave) ERG components compared with wild-type mice. The results suggest that although circulating plasma levels of retinol and retinol binding protein (RBP) are extremely low, this reduction has little effect on the retinal structure or function of the TTR null mouse. These data are consistent with the existence of mechanisms for the transport of retinol to the retina independent of the classical retinol-RBP-TTR 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.

Antioxidant potentials of vitamin A and carotenoids and their relevance to heart disease

Despite being one of the first vitamins to be discovered, the full range of biological activities for vitamin A remains to be defined. Structurally similar to vitamin A, carotenoids are a group of nearly 600 compounds. Only about 50 of these have provitamin A activity. Recent evidence has shown vitamin A, carotenoids and provitamin A carotenoids can be effective antioxidants for inhibiting the development of heart disease. Vitamin A must be obtained from the diet: green and yellow vegetables, dairy products, fruits and organ meats are some of the richest sources. Within the body, vitamin A can be found as retinol, retinal and retinoic acid. Because all of these forms are toxic at high concentrations, they are bound to proteins in the extracellular fluids and inside cells. Vitamin A is stored primarily as long chain fatty esters and as provitamin carotenoids in the liver, kidney and adipose tissue. The antioxidant activity of vitamin A and carotenoids is conferred by the hydrophobic chain of polyene units that can quench singlet oxygen , neutralize thiyl radicals and combine with and stabilize peroxyl radicals. In general, the longer the polyene chain, the greater the peroxyl radical stabilizing ability. Because of their structures, vitamin A and carotenoids can autoxidize when O2 tension increases, and thus are most effective antioxidants at low oxygen tensions that are typical of physiological levels found in tissues. Overall, the epidemiological evidence suggests that vitamin A and carotenoids are important dietary factors for reducing the incidence of heart disease. Although there is considerable discrepancy in the results from studies in humans regarding this relationship, carefully controlled experimental studies continue to indicate that these compounds are effective for mitigating and defending against many forms of cardiovascular disease. More work, especially concerning the relevance of how tissue concentrations, rather than plasma levels, relate to the progression of tissue damage in heart disease is required. This review assembles information regarding the basic structure and metabolism of vitamin A and carotenoids as related to their antioxidant activities. Epidemiological, intervention trials and experimental evidence about the effectiveness of vitamin A and carotenoids for reducing cardiovascular disease is also reviewed.

Mobilization of antioxidant vitamin pools and hemodynamic function after myocardial infarction

BACKGROUND

Although most previous studies have attempted to correlate plasma concentrations of vitamins with specific cardiovascular end points, metabolic considerations suggest that changes in myocardial tissue and storage organs may be better indicators of myocardial oxidative stress.

METHODS AND RESULTS

Rats fed commercial chow or a diet enriched with vitamin E for 2 weeks were subjected to either a surgical myocardial infarction (MI) or a sham procedure. Rats were hemodynamically assessed 16 weeks after surgery, and their heart, liver, kidney, and plasma were analyzed for antioxidant vitamins E (tocopherol) and A (retinol and total retinyl esters). At 16 weeks, MI rats on a control diet showed depressed peak systolic and elevated diastolic pressures in both right and left ventricles compared with their sham controls. Plasma concentrations of vitamins E and A in MI rats were not different from sham controls fed the same diet. However, concentrations of vitamin E in left ventricle and liver and of vitamin A in liver (retinol) and kidney (retinyl esters) were decreased in rats with MI compared with the sham controls. Vitamin E supplementation improved hemodynamic function in rats with MI and increased plasma, myocardial, liver, and kidney concentrations of vitamin E. The vitamin E diet also prevented the loss of total retinyl esters from the kidney but not of retinol from the liver in MI rats.

CONCLUSIONS

Dietary supplements of vitamin E can sustain better cardiac function subsequent to MI. Antioxidant vitamin levels in the myocardium or in storage organs and not in plasma may be better indicators of myocardial oxidative stress.