Changes in levels of cellular retinol- and retinoic-acid-binding proteins of liver and lung during perinatal development of rat

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.

Retinoic Acid: A Key Regulator of Lung Development

Retinoic acid (RA) is a key molecular player in embryogenesis and adult tissue homeostasis. In embryo development, RA plays a crucial role in the formation of different organ systems, namely, the respiratory system. During lung development, there is a spatiotemporal regulation of RA levels that assures the formation of a fully functional organ. RA signaling influences lung specification, branching morphogenesis, and alveolarization by regulating the expression of particular target genes. Moreover, cooperation with other developmental pathways is essential to shape lung organogenesis. This review focuses on the events regulated by retinoic acid during lung developmental phases and pulmonary vascular development; also, it aims to provide a snapshot of RA interplay with other well-known regulators of lung development.

Using Experimental Models to Identify Pathogenic Pathways and Putative Disease Management Targets in Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is a common and serious complication of preterm birth. Limited pharmacological and other medical interventions are currently available for the management of severely affected, very preterm infants. BPD can be modelled in preclinical studies using experimental animals, and experimental animal models have been extremely valuable in the development of hallmark clinical management strategies for BPD, including pulmonary surfactant replacement and single-course antenatal corticosteroids. A gradual move away from large animal models of BPD in favor of term-born rodents has facilitated the identification of a multitude of new mechanisms of normal and stunted lung development, but this has also potentially limited the utility of experimental animal models for the identification of pathogenic pathways and putative disease management targets in BPD. Indeed, more recent pharmacological interventions for the management of BPD that have been validated in randomized controlled trials have relied very little on preclinical data generated in experimental animal models. While rodent-based models of BPD have tremendous advantages in terms of the availability of genetic tools, they also have considerable drawbacks, including limited utility for studying breathing mechanics, gas exchange, and pulmonary hemodynamics; and they have a less relevant clinical context where lung prematurity and a background of infection are now rarely present in the pathophysiology under study. There is a pressing need to refine existing models to better recapitulate pathological processes at play in affected infants, in order to better evaluate new candidate pharmacological and other interventions for the management of BPD.

Modulation of retinoid signaling: therapeutic opportunities in organ fibrosis and repair

The vitamin A metabolite, retinoic acid, is an important signaling molecule during embryonic development serving critical roles in morphogenesis, organ patterning and skeletal and neural development. Retinoic acid is also important in postnatal life in the maintenance of tissue homeostasis, while retinoid-based therapies have long been used in the treatment of a variety of cancers and skin disorders. As the number of people living with chronic disorders continues to increase, there is great interest in extending the use of retinoid therapies in promoting the maintenance and repair of adult tissues. However, there are still many conflicting results as we struggle to understand the role of retinoic acid in the multitude of processes that contribute to tissue injury and repair. This review will assess our current knowledge of the role retinoic acid signaling in the development of fibroblasts, and their transformation to myofibroblasts, and of the potential use of retinoid therapies in the treatment of organ fibrosis.

A retinoid X receptor partial agonist attenuates pulmonary emphysema and airway inflammation

Background

Retinoid X receptors (RXRs) are members of the nuclear receptor (NR) superfamily that mediate signalling by 9-cis retinoic acid, a vitamin A derivative. RXRs play key roles not only as homodimers but also as heterodimeric partners, e.g., for retinoic acid receptors, vitamin D receptors, and peroxisome proliferator-activated receptors. The NR family may also play important roles in the development of emphysema. However, the role of RXRs in the pathogenesis of emphysema is not well defined.

Methods

We developed a novel RXR partial agonist (NEt-4IB) and investigated its effect and mechanism compared to a full agonist (bexarotene) in a murine model of emphysema. For emphysema induction, BALB/c mice received intraperitoneal cigarette smoke extract (CSE) or intratracheal porcine pancreas elastase (PPE). Treatment with RXR agonists was initiated before or after emphysema induction.

Results

Treatment with NEt-4IB significantly suppressed the increase in static lung compliance and emphysematous changes in CSE-induced emphysema and PPE-induced established and progressive emphysema. NEt-4IB significantly suppressed PPE-induced neutrophilic airway inflammation and the levels of keratinocyte chemoattractant (KC), C-X-C motif ligand5 (CXCL5), interferon (IFN)-γ and IL-17. NEt-4IB also improved the matrix metalloproteinase-9 (MMP-9)/tissue inhibitor of metalloproteinase-1 (TIMP-1) imbalance and the reduced anti-oxidant activity in bronchoalveolar lavage (BAL) fluid. NEt-4IB suppressed PPE-induced vascular endothelial growth factor (VEGF) expression in the airway. Treatment with NEt-4IB and bexarotene significantly suppressed the increase in static lung compliance and emphysematous changes. However, adverse effects of RXR agonists, including hypertriglyceridemia and hepatomegaly, were observed in bexarotene-treated mice but not in NEt-4IB-treated mice.

Conclusion

These data suggest that RXRs play crucial roles in emphysema and airway inflammation, and novel partial RXR agonists could be potential therapeutic strategies for the treatment of PPE- and CSE-induced emphysema.

Electronic supplementary material

The online version of this article (10.1186/s12931-018-0963-0) contains supplementary material, which is available to authorized users.

Integrated analysis of methylome, transcriptome and miRNAome of three pig breeds

AIM

Integrated analysis of methylome and transcriptome may help understand the molecular basis of the different breeds with different traits of commercial interest.

MATERIALS & METHODS

We obtained the first genome-wide methylome with single-base resolution, miRNAome and transcriptome from three swine breeds.

RESULTS

We displayed the landscape of the three omics in the whole-genome level. Integrated outcomes of methylome with genetic selection, miRNAome and transcriptome are also provided. Finally, we identified 11 candidate differentially methylated genes associated with phenotype variance in pigs.

CONCLUSION

DNA methylation not only suppresses transcriptome but also miRNAome. The different -omics data have complicated interaction in directly or indirectly and exhibited close relations with the distinct phenotypic traits of growth, disease resistance and energy metabolism.

Tissue regeneration as next-generation therapy for COPD--potential applications

COPD is a major cause of chronic morbidity and mortality worldwide, and there is a need to develop more effective therapeutic strategies to replace specialized treatment such as lung transplantation. Recent studies suggest that recognition of apoptotic lung epithelial or endothelial cells may result in growth factors to stimulate cell replacement, and defects in these processes may contribute to the pathogenesis of COPD. Furthermore, recent animal and human studies have revealed that tissue-specific stem cells and bone marrow-derived cells contribute to lung tissue regeneration and protection, and thus administration of exogenous stem/progenitor cells or humoral factors responsible for activation of endogenous stem/progenitor cells may be a potent next-generation therapy for COPD.

Feasibility of retinoids for the treatment of emphysema study

BACKGROUND

Retinoids promote alveolar septation in the developing lung and stimulate alveolar repair in some animal models of emphysema.

METHODS

One hundred forty-eight subjects with moderate-to-severe COPD and a primary component of emphysema, defined by diffusing capacity of the lung for carbon monoxide (Dlco) [37.1 +/- 12.0% of predicted] and CT density mask (38.5 +/- 12.8% of voxels <- 910 Hounsfield units) [mean +/- SD] were enrolled into a randomized, double-blind, feasibility study at five university hospitals. Participants received all-trans retinoic acid (ATRA) at either a low dose (LD) [1 mg/kg/d] or high dose (HD) [2 mg/kg/d], 13-cis retinoic acid (13-cRA) [1 mg/kg/d], or placebo for 6 months followed by a 3-month crossover period.

RESULTS

No treatment was associated with an overall improvement in pulmonary function, CT density mask score, or health-related quality of life (QOL) at the end of 6 months. However, time-dependent changes in Dlco (initial decrease with delayed recovery) and St. George Respiratory Questionnaire (delayed improvement) were observed in the HD-ATRA cohort and correlated with plasma drug levels. In addition, 5 of 25 participants in the HD-ATRA group had delayed improvements in their CT scores that also related to ATRA levels. Retinoid-related side effects were common but generally mild.

CONCLUSIONS

No definitive clinical benefits related to the administration of retinoids were observed in this feasibility study. However, time- and dose-dependent changes in Dlco, CT density mask score, and health-related QOL were observed in subjects treated with ATRA, suggesting the possibility of exposure-related biological activity that warrants further investigation.

Vertebrate and invertebrate carotenoid-binding proteins

In invertebrates and vertebrates, carotenoids are ubiquitous colorants, antioxidants, and provitamin A compounds that must be absorbed from dietary sources and transported to target tissues where they are taken up and stabilized to perform their physiological functions. These processes occur in a specific and regulated manner mediated by high-affinity carotenoid-binding proteins. In this mini-review, we examine the published literature on carotenoid-binding proteins in vertebrate and invertebrate systems, and we report our initial purification and characterization of a novel lutein-binding protein isolated from liver of Japanese quail (Coturnix japonica).

Retinoic acid in alveolar development, maintenance and regeneration

Recent data suggest that exogenous retinoic acid (RA), the biologically active derivative of vitamin A, can induce alveolar regeneration in a rat model of experimental emphysema. Here, we describe a mouse model of disrupted alveolar development using dexamethasone administered postnatally. We show that the effects of dexamethasone are concentration dependent, dose dependent, long lasting and result in a severe loss of alveolar surface area. When RA is administered to these animals as adults, lung architecture and the surface area per unit of body weight are completely restored to normal. This remarkable effect may be because RA is required during normal alveolar development and administering RA re-awakens gene cascades used during development. We provide evidence that RA is required during alveologenesis in the mouse by showing that the levels of the retinoid binding proteins, the RA receptors and two RA synthesizing enzymes peak postnatally. Furthermore, an inhibitor of RA synthesis, disulphiram, disrupts alveologenesis. We also show that RA is required throughout life for the maintenance of lung alveoli because when rats are deprived of dietary retinol they lose alveoli and show the features of emphysema. Alveolar regeneration with RA may therefore be an important novel therapeutic approach to the treatment of respiratory diseases characterized by a reduced gas-exchanging surface area such as bronchopulmonary dysplasia and emphysema for which there are currently no treatments.

Retinoic acid fails to reverse emphysema in adult mouse models

BACKGROUND

Previous work has shown that all-trans-retinoic acid reverses elastase induced emphysema in rats. Since there is currently no effective treatment for pulmonary emphysema, the effect of retinoic acid should be further investigated in other adult species. A study was undertaken using two murine models of emphysema to evaluate the effect of retinoic acid.

METHODS

The models used were an elastase induced emphysema model for acute alveolar destruction and a tumour necrosis factor (TNF)-alpha transgenic mouse which exhibits chronic air space enlargement, loss of elastic recoil, increased lung volume, and pulmonary hypertension comparable to human pulmonary emphysema. All-trans-retinoic acid (2 mg/kg) was injected for 12 successive days after the establishment of emphysema. The effects of treatment were evaluated using physiological and morphometric analyses.

RESULTS

In contrast to the rat, administration of all-trans-retinoic acid in these murine models did not improve the emphysema. Moreover, worsening of emphysema was observed in TNF-alpha transgenic mice treated with all-trans-retinoic acid. The level of keratinocyte chemoattractant (KC), a CXC chemokine, in bronchoalveolar lavage fluid was increased in TNF-alpha transgenic mice following retinoic acid treatment. These data raise the possibility that retinoic acid causes deterioration of emphysema by promoting inflammation in this model.

CONCLUSIONS

In these models, retinoic acid did not show positive effects on emphysema. The effect of retinoic acid in the treatment of pulmonary emphysema remains controversial, and further studies are required to determine its physiological effects under a variety of experimental conditions.

All-trans retinoic acid modulates the balance of matrix metalloproteinase-9 and tissue inhibitor of metalloproteinase-1 in patients with emphysema

STUDY OBJECTIVE

The balance between proteases and antiproteases plays an essential role in the pathogenesis of emphysema. This study was designed to evaluate the impact of all-trans retinoic acid (ATRA) on the balance of matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1) in patients with emphysema.

DESIGN AND SETTING

As part of a clinical study, ATRA was administered to 20 patients with emphysema for 12 weeks and evaluated for its effects on plasma levels of MMP-9 and TIMP-1. Plasma MMP-9 levels were also measured in a separate cohort of patients with emphysema and matched control subjects to evaluate the relationship of circulating enzyme levels to lung disease. To further investigate the effects of ATRA on protease activity within the lung microenvironment, alveolar macrophages (AM) recovered from the lungs of active smokers with COPD were cultured with ATRA in vitro.

MEASUREMENTS AND RESULTS

Administration of ATRA to patients with emphysema produced a 45 +/- 14% reduction (mean +/- SEM) in plasma MMP-9 by enzyme-linked immunosorbent assay and a similar reduction in MMP-9 enzyme activity, while having little effect on TIMP-1 levels. Baseline MMP-9 levels were higher in patients with emphysema compared to nonsmoking control subjects, suggesting a relationship between plasma levels and the presence of lung disease. In vitro, concentrations of ATRA similar to those achieved in the plasma of study subjects significantly reduced both the production and enzyme activity of MMP-9 by AM. In the same experiments, TIMP-1 levels increased significantly, resulting in a marked reduction in the MMP-9/TIMP-1 molar ratio.

CONCLUSION

We conclude that ATRA can modulate protease/antiprotease balance in a manner that may impact on disease pathogenesis.

Rough and rocky road to the retinoid revolution

Some of us who were born in the middle of Europe between World Wars I and II had to face quite a few unusual challenges that we all met in different ways. I was born and raised in Prague, Czechoslovakia, a country at the time of my birth that was governed by a Western style of democracy, which was later destroyed by the occupation by Nazi Germany and subsequently by the takeover by the equally cruel Communists. Life required special means of adaptation to the changing living conditions and a great deal of luck to survive. After graduating from the School of Technology, I started working in the Department of Medicine at Charles University in Prague as a clinical chemist in endocrinology. This work was followed with training in basic biochemistry and the study of metabolic changes in stress. This rather diversified research, due to my changing of workplaces, led to the findings that diet can change enzymatic activity of liver tryptophan oxygenase. For a short time I worked on the metabolism of cyclic AMP in Escherichia coli, and at the age of 41, I made a risky move and succeeded in escaping with my family from the "paradise of communism." The reasons for this decision will become clear. After settling in the United States, I worked on the mechanism of activation of liver tryptophan oxygenase by cyclic AMP and eventually moved to the Vanderbilt University School of Medicine. There I initially worked on the mechanism of action of steroid hormones and finally on the molecular mechanism of action of retinoids, retinol, and retinoic acid. Also in cooperation with neonatologists, I initiated studies on prematurely born human neonates which led to successful supplementation of these patients with vitamin A. The work from my laboratory and my coworkers eventually became recognized.

Retinoic acid inhibits elastase-induced injury in human lung epithelial cell lines

The protective effects of retinoic acid on elastase-induced lung epithelial cell injury were studied using elastase extracted from purulent human sputum, the BEAS-2B human bronchial epithelial cell line, A549 human type II lung cell line, and primary cultures of human tracheal epithelial cells. Elastase decreased viability of BEAS-2B cells, A549 cells, and human tracheal epithelial cells in concentration- and time-dependent fashions. Elastase also induced apoptosis of BEAS-2B cells, A549 cells, and the tracheal epithelial cells detected with cell death detection enzyme-linked immunosorbent assay and terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) methods. Retinoic acid alone did not affect the viability of BEAS-2B cells, A549 cells, or the tracheal epithelial cells, and did not induce apoptosis of the cells. However, retinoic acid prevented the decreases in the viability and reduced apoptosis of BEAS-2B cells, A549 cells, and the tracheal epithelial cells induced by elastase. Likewise, retinoic acid inhibited caspase 3 activity in BEAS-2B cells and A549 cells induced by elastase, as well as proteolytic activity of elastase. Furthermore, caspase 3 inhibitor inhibited the elastase-induced apoptosis of the cells. These findings suggest that retinoic acid may inhibit elastase-induced lung epithelial cell injury partly through the inhibition of proteolytic activity of elastase and through the inhibition of caspase 3 activity by elastase. Retinoic acid may, therefore, have protective effects against the elastase-induced lung injury and subsequent development of pulmonary emphysema.

Retinoic acid, a regeneration-inducing molecule

Retinoic acid (RA) is the biologically active metabolite of vitamin A. It is a low molecular weight, lipophilic molecule that acts on the nucleus to induce gene transcription. In amphibians and mammals, it induces the regeneration of several tissues and organs and these examples are reviewed here. RA induces the "super-regeneration" of organs that can already regenerate such as the urodele amphibian limb by respecifying positional information in the limb. In organs that cannot normally regenerate such as the adult mammalian lung, RA induces the complete regeneration of alveoli that have been destroyed by various noxious treatments. In the mammalian central nervous system (CNS), which is another tissue that cannot regenerate, RA does not induce neurite outgrowth as it does in the embryonic CNS, because one of the retinoic acid receptors, RAR beta 2, is not up-regulated. When RAR beta 2 is transfected into the adult spinal cord in vitro, then neurite outgrowth is stimulated. In all these cases, RA is required for the development of the organ, in the first place suggesting that the same gene pathways are likely to be used for both development and regeneration. This suggestion, therefore, might serve as a strategy for identifying potential tissue or organ targets that have the capacity to be stimulated to regenerate.

Invited Review: pulmonary alveoli: formation, the "call for oxygen," and other regulators

The lung's only known essential function is to provide sufficient alveolar surface to meet the organism's need for oxygen and elimination of CO(2). The importance of the magnitude of alveolar surface area (Sa) to O(2) uptake (VO(2)) is supported by the presence among mammals of a direct linear relationship between Sa and VO(2). This match has been achieved, despite the higher body mass-specific VO(2) of small organisms compared with large, by a greater subdivision of alveolar surface, not by a larger relative lung volume in small organisms. This highly conserved relationship between alveolar architecture and VO(2) suggests the presence of similarly conserved mechanisms that control the onset, rate, and cessation of alveolus formation and alveolar size, which are also influenced by retinoids and thyroid and corticosteroid hormones. Furthermore, the "call for oxygen" is met at a breathing rate and tidal volume at which the work of breathing is lowest. Thus there is a complex, fascinating, but poorly understood, signaling relationship among VO(2), the neural regulation of breathing, and lung architecture, composition, and mechanics.

A pilot study of all-trans-retinoic acid for the treatment of human emphysema

Emphysema results from progressive destruction of alveolar septae and was considered irreversible until all-trans-retinoic acid (ATRA) was shown to reverse anatomic and physiologic signs of emphysema in a rat model. To evaluate the feasibility of ATRA as a clinical therapy, 20 patients with severe emphysema were enrolled into a randomized, double-blind, placebo-controlled pilot study. Participants included 16 male and 4 female former smokers, two with alpha(1)-antitrypsin deficiency. Patients were treated with either 3 mo of ATRA (50 mg/m(2)/d) or 3 mo of placebo, followed by a 3-mo crossover phase. Plasma drug levels were followed and outcome measures included serial pulmonary function tests, blood gases, lung compliance, computed tomography (CT) imaging, and quality of life questionnaires. In general, treatment was well tolerated and associated with only mild side effects including skin changes, transient headache, hyperlipidemia, transaminites, and musculoskeletal pains. Plasma drug levels varied considerably between subjects and decreased significantly over time in 35% of the participants. Physiologic and CT measurements did not change appreciably in response to therapy. We conclude that ATRA is well tolerated in patients with emphysema, and trials evaluating higher doses, longer treatment, or different dosing schedules are feasible.

Temporal/spatial expression of retinoid binding proteins and RAR isoforms in the postnatal lung

Endogenous retinoids have been implicated in alveologenesis in both the rat and the mouse, and exogenous retinoic acid (RA) can reverse or partially reverse experimental emphysema in adult rat and mouse models by an unknown mechanism. In this study, we examine the cellular and molecular biology of retinoid signaling during alveologenesis in the mouse. We describe the temporal and spatial expression of the retinoid binding proteins CRBP-I, CRBP-II, and CRABP-I using RT-PCR and immunohistochemistry. We identify the retinoic acid receptor isoforms RAR-alpha 1, RAR-beta 2, RAR-beta 4, and RAR-gamma 2 and describe their temporal and spatial expression using RT-PCR and in situ hybridization. We demonstrate that both retinoid binding proteins and RAR isoforms are temporally regulated and found within the alveolar septal regions during alveologenesis. These data support a role of dynamic endogenous RA signaling during alveolar formation.

Alveolar proliferation, retinoid synthesizing enzymes, and endogenous retinoids in the postnatal mouse lung. Different roles for Aldh-1 and Raldh-2

Alveoli are formed postnatally in the rat, mouse, and human. The molecular signals controlling the patterning of this developmental process are not well understood. Here we describe immunohistochemical studies that label proliferating alveolar wall cells which suggest two distinct patterns of alveologenesis: (1) a low grade, peripheral subpleural parenchymal process which occurs from P1 through to P15; and (2) a dramatic increase in central cell proliferation from P4 which is complete by P15, corresponding to the well described period of alveolar septation. We describe the temporal and spatial expression of the retinoid-synthesizing enzymes Aldh-1 and Raldh-2 in the postnatal mouse lung. Both enzymes are upregulated during the period of maximal alveolar wall cell proliferation. Aldh-1 is located in the bronchial epithelium and alveolar parenchyma, and Raldh-2 is restricted to the bronchial epithelium and pleural mesothelial cells. High-pressure liquid chromatography (HPLC) reveals that rapidly septating lungs have relatively simple chromatographic profiles; in contrast, the adult lungs have a complex profile that includes many novel retinoids. These data suggest two patterns of alveolar proliferation with temporal and spatial association of the enzymes Aldh-1 and Raldh-2 and a dynamic role for different retinoids in both the septating and adult mouse lung.

Retinoic acid attenuates cytokine-driven fibroblast degradation of extracellular matrix in three-dimensional culture

Proteolytic degradation of extracellular matrix is thought to play an important role both in emphysema and in tissue development and repair. Retinoic acid has been suggested to modify tissue injury, and in an animal model of emphysema may induce alveolar repair. Since cytokines can induce matrix metalloproteinase (MMP) production in fibroblasts and neutrophil elastase (NE) can activate MMPs, we hypothesized that retinoic acid could attenuate collagen degradation by modifying MMP production and activation. To evaluate this, human lung fibroblasts were cast into native type I collagen gels and floated in medium containing cytomix (TNF-alpha, IL-1beta, and IFN-gamma) alone or in combination with NE in the presence and absence of retinoic acid (1 microM). After 5 d, cytomix with elastase induced significant degradation of the collagen gels assessed by quantifying total hydroxyproline (41.6 +/- 1.6 microg versus 3.3 +/- 1.5 microg, P < 0.01). Retinoic acid significantly inhibited this degradation (23.3 +/- 1.5 microg versus 3.3 +/- 1.5 microg, P < 0.01). Gelatin zymography and Western blot revealed that MMP-1, MMP-3, and MMP-9 were induced by cytomix and that co-exposure to NE resulted in increased production of activated forms of these enzymes. Retinoic acid attenuated the induction and activation of MMP-1 and MMP-3. The current study, therefore, suggests that in addition to stimulating anabolic effects, retinoic acid may modulate proteolytic processes thought to contribute to tissue destruction in emphysema.

Retinoic acid treatment partially rescues failed septation in rats and in mice

Pulmonary alveoli are formed in part by subdivision (septation) of the gas-exchange saccules of the immature lung. Septation results in smaller, more numerous structures (alveoli) and is developmentally regulated in mammals including humans, rats, and mice; if it fails to occur at the appropriate time, there is no spontaneous post hoc septation nor has there been a means of inducing septation after it has failed to occur. We measured lung volume, the volume of individual alveoli, and alveolar surface area and calculated alveolar number in neonatal rats in which septation had been blocked by treatment with a glucocorticosteroid hormone and in adult tight-skin mice that have a genetic failure of septation. We tested the hypothesis that treatment with all-trans retinoic acid induces post hoc septation. In both models of failed septation, hence in two species, and in immature and adult animals, treatment with all-trans retinoic acid induced post hoc septation, offering the possibility of a similar effect in premature infants.

Gene expression of cellular retinoid-binding proteins: modulation by retinoic acid and dexamethasone in postnatal rat lung

In rats, septation of gas-exchange saccules occurs during the first 2 postnatal weeks; dexamethasone (DEX) treatment irreversibly impairs septation, and treatment with all-trans retinoic acid (RA) prevents the DEX-induced inhibition of septation. Cellular retinoic acid-binding protein I (CRABP I) and cellular retinol-binding protein I (CRBP I) are important modulators of the cellular metabolism of retinoids. In the present study, therefore, we measured the mRNA concentration of CRABP I and CRBP I in lungs of neonatal rats. In untreated rats, CRABP I and CRBP I mRNA peaked at postnatal d 8, indicating that CRABP I and CRBP I are developmentally regulated at least in part at a pretranslational level during lung septation. Daily treatment of 3- to 8-d-old rats with RA (500 microg/kg/d) had no effect on the level of CRABP I mRNA; treatment with DEX (0.25 microg/d) from d 4 to 8 caused a decrease in CRABP I mRNA that was not prevented by concomitant treatment with RA. These findings suggest that a decrease in CRABP I expression may be important in the DEX-induced block of septation but not in the prevention by RA of DEX-induced inhibition of septation. RA treatment caused an increase of CRBP I mRNA; conversely, treatment with DEX caused a decrease in CRBP I mRNA that was prevented by concomitant treatment with RA. These data suggest CRBP I may play a role in RA-induced septation, in the inhibition of septation caused by DEX, and in the ability of RA to prevent DEX-blocked septation.

Retinoic acid treatment abrogates elastase-induced pulmonary emphysema in rats

Pulmonary emphysema is a common disease in which destruction of the lung's gas-exchange structures (alveoli) leads to inadequate oxygenation, disability and frequently death; lung transplantation provides its only remediation. Because treatment of normal rats with all-trans-retinoic acid increases the number of alveoli, we tested whether a similar effect would occur in rats with emphysema. Elastase was instilled into rat lungs, producing changes characteristic of human and experimental emphysema: increased lung volume reflecting a loss of lung elastic recoil, larger but fewer alveoli and diminished volume-corrected alveolar surface area due to destruction of alveolar walls. Treatment with all-trans-retinoic acid reversed these changes providing nonsurgical remediation of emphysema and suggesting the possibility of a similar effect in humans.

The pulmonary lipofibroblast (lipid interstitial cell) and its contributions to alveolar development

The pulmonary lipofibroblast is located in the alveolar interstitium and is recognizable by its characteristic lipid droplets. During alveolar development it participates in the synthesis of extracellular matrix structural proteins, such as collagen and elastin, and as an accessory cell to the type II pneumocyte, in the synthesis of surfactant. The lipofibroblast contains cortical contractile filaments and is thereby related to the contractile interstitial cells that are normally found at the alveolar septal tips and after lung injury. The morphologic, immunologic, and biochemical characteristics of the lipofibroblast and its probable physiologic functions are reviewed. The retinoid and lipid metabolism of the lipofibroblast is compared with that of the hepatic lipocyte and the adipocyte. Although the functions of the lipofibroblast remain incompletely characterized, this cell type is emerging as an important contributor to pulmonary alveolar septal development.

Postnatal rat lung retinoic acid receptor (RAR) mRNA expression and effects of dexamethasone on RAR beta mRNA

Retinoids exert multiple effects upon lung differentiation and growth. Although the mechanisms involved are presently poorly understood, increasing evidence points to a central role of nuclear retinoic acid receptors (RAR). The purpose of this study was to determine RAR mRNA expression profile during postnatal alveolarization, compared with the expression in prenatal and adult rat lung, and to describe the effects of dexamethasone (DEX) and oxygen on postnatal lung RAR gene expression. Total RNA was isolated from lungs of Sprague-Dawley rats on prenatal day 19, on postnatal days 1, 3, 7, 10, and 14 of life, and from adults. One subgroup of littermate pups was treated with DEX daily for 3 or 7 days. In a second experiment, rats were exposed to room air or to 95% oxygen for 72 hours, and received either DEX or saline. Northern hybridization showed that the levels of all RAR subtypes in fetal lung were 45% or less of levels at postnatal day 1. The 3.7 kb RAR alpha transcript levels were lower than day 1 on days 10 and 14 (relative to day 1, day 10 = 0.54 +/- 0.05; day 14 = 0.54 +/- 0.08), but there was no change in a 2.7 kb RAR alpha transcript over this time period. By contrast, RAR beta mRNA levels were significantly higher at days 3, 10, and 14 compared with day 1 (day 3 = 1.79 +/- 0.19; day 10 = 1.41 +/- 0.14; day 14 = 1.53 +/- 0.05). Similarly, RAR gamma mRNA expression levels were higher on day 10 (1.45 +/- 0.09), but by day 14 there was no difference from day 1. Adult lung 3.7 kb RAR alpha, 2.7 kb RAR alpha, and RAR gamma were lower than day 1, but RAR beta was significantly greater (3.7 alpha = 0.52 +/- 0.05; 2.7 alpha = 0.49 +/- 0.26; gamma = 0.74 +/- 0.06; beta = 1.63 +/- 0.22). Treatment with DEX prevented the rise in RAR beta mRNA occurring on day 3 and significantly lowered (0.65 +/- 0.06) the amount of RAR beta mRNA in day 7 lung. Exposure of rat pups to oxygen caused an increase in RAR beta mRNA (1.21 +/- 0.03). DEX treatment again decreased RAR beta mRNA in both control (0.55 +/- 0.06) and oxygen-exposed pups (0.67 +/- 0.12). In addition, 2.7 kb RAR alpha mRNA was decreased with the combination of DEX and oxygen exposure (0.63 +/- 0.06). The differential gene expression profiles and the response to DEX and oxygen of the various members of the RAR family suggest that each subtype may have a specific role during the period of alveolarization in rat lung.

Effect of dexamethasone and oxygen exposure on neonatal rat lung retinoic acid receptor proteins

Retinol deficiency in animal models results in histopathologic airway changes that appear similar to those found in human premature infants with bronchopulmonary dysplasia (BPD). Dexamethasone (DEX), a steroid now often used in the treatment of BPD, might potentially affect lung vitamin A homeostasis since it alters serum and liver retinoid stores in certain models. Our objective was to determine the effect of DEX on neonatal rat lung retinoid status and the binding of retinoic acid (RA) to cytosolic and nuclear receptor proteins. We examined this effect both in room air and when the animals breathed 95% oxygen (O2). Twenty-four 1-day-old rat pups received either 1 microgram/g DEX subcutaneously, an equal volume of normal saline (NS) subcutaneously at 0 (start experiment time), 24, and 48 hours, or no injection at all, and were sacrificed at 72 hours. Twelve rats in each treatment group were housed in room air and 12 in each group were exposed to > 95% O2 for the 3 day period. Lung and liver were analyzed for retinyl palmitate (RP). Nuclear retinoic acid receptor (RAR) and cellular retinoic acid binding protein (CRABP) were measured by specific binding assays. DEX decreased liver RP by 33-55% and rat pup lung RP by over 60%; it also decreased lung RAR binding (mean dpm/microgram protein +/- SEM) in both room air and oxygen groups: Air (11.2 +/- 1.0) vs. Air/DEX (4.6 +/- 1.3, n = 6; P < 0.01), and O2 (18.2 +/- 0.6) vs. O2/DEX (3.2 +/- 0.6, n = 6; P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoids and the skin

Retinoids are a group of naturally occurring and synthetic compounds with vitamin A-like biological activity. They play an important role in vision, reproduction, growth, and epithelial cell differentiation. Recent discoveries of specific retinoid cellular binding proteins and nuclear receptors have led to a better (although not complete) understanding of the complex mechanisms of retinoid action. Numerous clinical studies have demonstrated beneficial effects of retinoids on skin diseases such as acne, psoriasis, ichthyoses, keratodermas, skin cancers and their precursors, as well as a reversal of the effects of photoaging.

The function of vitamin A in cellular growth and differentiation, and its roles during pregnancy and lactation

Recent advances in the molecular biology of the retinoids have provided a mechanistic explanation for the observations, first made several decades ago, that vitamin A profoundly influences the differentiation of tissues throughout the body. A central concept has recently emerged, namely that retinoids seldom exist "free" in solution but, rather, are nearly always associated with specific retinoid-binding proteins. In plasma, these include RBP and the chylomicron whereas, in cells two distinct classes of retinoid-binding proteins exist: the cellular (cytoplasmic) proteins (CRBPs and CRABPs) and the nuclear receptors proteins (RARs and RXRs). Whereas the cellular retinoid-binding proteins serve as buffers and as chaperones during metabolism (Ross, 1993b), the nuclear receptors are now recognized to be the direct mediators of retinoid actions on the genome. Both the cytoplasmic and nuclear classes of retinoid-binding proteins are expressed early in development and are proposed to control the concentration of retinoic acid and the transcription of retinoid-responsive genes, respectively. Given the profound effects of retinoic deficiency or excess on the developing fetus, it is not surprising that mechanisms have evolved to control the placental transfer of vitamin A. Transfer is nearly uniform over a rather wide range of maternal dietary vitamin A intake. The importance of RBP in transporting retinol to tissues is suggested by the observations that the visceral yolk sac and the liver of the fetus transcribe and translate RBP. In comparison to pregnancy, vitamin A transport during lactation is much more responsive to variations in maternal vitamin A intake. The young of mothers with good vitamin A nutriture may thus accumulate significant retinol reserves during the suckling period. Conversely, young nursed by mothers with poor vitamin A status and low intake during lactation may fail to develop adequate stores and be vulnerable to vitamin A deficiency if the post-weaning diet is also poor in vitamin A. In populations with low vitamin A status, the lactation period provides an excellent window of opportunity for supplementing mothers and, indirectly, their offspring, with vitamin A to replenish the mother's vitamin A reserves and assure that the infant's growth and development are not limited by an inadequate quantity of this essential nutrient.

Retinoids and photodamage

Extensive well-controlled clinical studies performed over the past 5 years have demonstrated a consistent, dose-dependent, statistically significant improvement in the appearance of photodamaged skin after 3-6 months of daily treatment with topical 0.001-0.1% tretinoin cream. Clinical changes included decreases in surface roughness, irregular pigmentation, fine and coarse wrinkling, and sallowness. Actinic keratoses have also been reported to decrease in size and number. Blinded analysis of biopsies from more than 500 subjects showed that there was compaction of the stratum corneum, an increase in the number of granular layers, thickening of the epidermis and a decrease in epidermal melanin. There were no detectable histological changes in any dermal parameters. The specific cellular mechanisms by which retinoic acid (RA) exerts its beneficial effect on photodamaged skin are currently the subject of intensive investigation. It is well established that RA enters the nucleus where it binds to an RA receptor (RAR), and that the RA-RAR complex then binds to specific RA response elements in the DNA, modulating the expression of target genes. It is thus likely that RA improves at least some aspects of photoageing by modifying cellular differentiation programmes, as retinoids have been shown to do during embryogenesis, in malignantly transformed cells and in skin affected by certain dermatoses.

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Morphological differences elicited by two weak acids, retinoic and valproic, in rat embryos grown in vitro

We compared in rat whole-embryo culture the morphological changes elicited by valproic acid (VPA) with those elicited by trans-retinoic acid (RA). Rat embryos explanted on day 9.5 of gestation were treated on day 10 with RA or VPA at concentrations producing equivalent reductions in embryonic protein. The concentrations selected for morphological assessment by scanning and transmission electron microscopy, 2.3 and 800 microM, respectively, for RA and VPA, produced approximately a 50% incidence of abnormally open anterior neuropores in initial range-finding experiments in the culture system. Protein and DNA analyses were also performed on corresponding groups of embryos at three different doses. With concurrent control groups used as reference standards, the two treatment groups were compared for differences in external and internal morphology, protein and DNA contents, and growth indices. While certain variables responded similarly in the two treatment groups, e.g., the growth variables, protein and DNA contents, each drug produced selective morphological effects. Whereas treatment with RA produced underdeveloped branchial arches, symmetrically cleft cranial defects resulting in openings in rhombencephalic and prosencephalic regions, and exteriorized neural tissue in the caudal neuropore region, VPA produced irregular clefts with wavy margins along the entire length of the neural tube, and an open caudal neuropore without eversion of the neuroepithelium, while producing no detectable effect on the branchial arches. The similar effects of these two drugs on protein and DNA contents suggest comparable degrees of overall toxicity; however, the dissimilar effects on neural tube and branchial arches, coupled with the large difference in concentration of the drug required to produce the effects, add to the evidence that their mechanisms for elicitation of abnormal development are qualitatively different.

The intracellular vitamin A-binding proteins: an overview of their functions

The intracellular retinoid-binding proteins bind retinol, retinaldehyde, and retinoic acid for purposes of protection against decomposition, solubilize them in aqueous medium, render them nontoxic, and transport them within cells to their site of action. These binding proteins also function by presenting the retinoids to the appropriate enzymes for metabolism. The cytosolic retinol and retinoic acid-binding proteins--CRBP, CRABP, CRBP (II)--function in transport and metabolism of retinoids within parenchymal, intestinal, reproductive, and fetal cells and across blood-organ barriers. A different group of retinoid-binding proteins, more related to serum retinol-binding protein, functions in epididymis and uterus. Retinaldehyde-binding protein aids in the oxidation-reduction reaction of 11-cis-retinol-11-cis-retinaldehyde in the retina, where the interphotoreceptor retinol-binding protein transports retinol between pigment epithelium and photoreceptors. Finally, a group of retinoic acid-binding proteins termed "receptors" functions in the nucleus by attaching to promoter regions of a number of specific genes to stimulate their transcription and thus affect growth, development, and differentiation.

Detection of cellular retinoic acid-binding protein in chick embryonic tissues by monoclonal antibodies

We describe the production of monoclonal antibodies (MAbs) directed against cellular retinoic acid-binding protein (CRABP) and their application for the quantitation and localization of CRABP during the development and growth of chick embryo. Three MAbs, classified as D-10 and H-6 (IgG1-isotype), and G-4 (IgM-isotype), exhibited the highest degree of immunoreactivity for chick embryo CRABP. The antibodies showed partial reactivity to CRABP from rat testis. None of the MAbs showed cross-reactivity with cellular retinol-binding protein or fatty acid-binding protein which are structurally similar to CRABP. The antigen-specificity was confirmed by immunoblot analysis as well as by fast protein liquid chromatographic analysis. The radioimmunoassay developed for MAb (D-10) provided a detectability range of 0.5-5.0 ng of CRABP in the standard displacement curves. An abundance of CRABP was found in embryonic skin, brain, testis and eye. Several other tissues (heart, lung, liver), previously reported to have undetectable levels of CRABP, showed significant amounts of the binding protein. The levels of CRABP peaked in early (4-6-day-old) and late (11-14-day-old) stages of embryo development. Immunolocalization of CRABP in chick embryo skin demonstrates a specific intense staining for the antigen in the dense areas of mesenchyme cells (mesodermal layers); little or no staining was apparent in the differentiated cells of epidermis and peridermis as well as in the loose connective tissues. The MAbs are useful not only in the purification of CRABP as an affinity adsorbent, but also in the elucidation of the possible role of CRABP in the transfer of the ligand to its nuclear receptors and in the morphogenetic gradient formation of RA in chick embryo tissues.

Experimental craniofacial malformations induced by retinoids and resembling branchial arch syndromes

A syndrome which showed similarities to human branchial arch syndromes could be induced in Sprague-Dawley rat embryos by exposing them to retinoids prenatally. Treatment of pregnant rats with 40 mg/kg retinoic acid or 10 mg/kg etretinate on pregnancy day 8.5-9 resulted in craniofacial defects in 100% of the embryos. A scanning electron microscopic investigation of the early stages in the development of these malformations showed abnormal skull form, disorganised surface epithelium with "cell blebbing", lateral facial clefts, facial fistulas, narrowed skull-base and reduced size of the nasal and maxillary complexes. Histological examination confirmed these findings and supported the hypothesis that a main reason for this syndrome is hindrance of migration of the cranial neural crest cells to the facial processes during early craniofacial formation.

Retinoids increase perinatal spinal cord neuronal survival and astroglial differentiation

In this report we demonstrate that retinol and retinoic acid (RA) increase the survival and morphological differentiation of rat spinal cord neurons in vitro. Micromolar amounts of retinol and RA increased the number of surviving neurons by 2- to 3-fold and affected neuritic density resulting in increased secondary and tertiary processes compared to untreated sister cultures. A marked morphological differentiation of the astrocyte population in conjunction with an antiproliferative effect in the presence of retinoids were apparent. These trophic effects occurred mainly after 5 days in vitro, a time that corresponds to the time of birth in vivo. Retinoic acid exerted a direct trophic effect on spinal cord neurons in the absence of glial cells while retinol lost its effectiveness. Metabolic labeling suggested that retinol is converted to the biologically active RA within astrocytes but not in neurons. Taken together, our results have demonstrated direct trophic effects of RA on spinal cord neurons and have suggested another role for astrocytes in the maintenance of normal neural physiology by regulating RA concentrations through the oxidation of retinol.

Developmental changes in vitamin A level and lack of retinyl palmitate in chick lungs

1. Developmental changes in retinol and retinyl palmitate contents in lungs of chick embryos and posthatch chicks were investigated. 2. Remarkable changes in the lung retinol levels were found during development of chicks. Embryonic lungs 5 days prior to hatching contained the highest content of retinol. The level then declined rapidly and was lowest on 1 day before hatching. 3. Its level then rose substantially within 7 days after hatching. 4. No retinyl palmitate in chick lungs was detectable at any of the developmental stages examined, nor even in adult hen. 5. Serum retinol level changed in parallel with the lung retinol. 6. The patterns of changes in liver retinol and retinyl palmitate were remarkably different from that occurring in the lung retinol. In chick embryonic livers, the levels of them were low, followed by a rapid increase after hatching. 7. The high level and its rapid decrease of lung retinol content during development of chick embryos may be functionally connected with retinol action in embryonic lungs for cellular differentiation and maturation.

Ligand-specific and non-specific in vivo modulation of human epidermal cellular retinoic acid binding protein (CRABP)

Retinoic acid (RA) is bound intracellularly by a specific, low molecular weight protein (CRABP), that is unrelated to its nuclear receptor and whose function and regulation are still unknown. In the present study we were able to obtain an in vivo modulation of CRABP by different stimuli in one of the major target organs of RA: the human skin. We found increased CRABP after daily application during 4 days of natural or synthetic retinoids (RA, acitretin, isotretinoin, Ro137410, retinol), that have either a high affinity to CRABP or can be transformed into RA. Only Ro150778 with no affinity and no reported transformation had no effect. No macro- or microscopical changes could be observed with any of the tested compounds. Induction of inflammatory and hyperproliferative changes in the skin by topical dithranol treatment, UVB irradiation or scotch tape stripping also induced a significant increase of CRABP 3 days after exposure. Topical diflucortolone showed not only a tendancy to decrease intrinsic CRABP levels, but significantly reduced the retinoid stimulated rise of CRABP. Thus we conclude that the increase of CRABP in a fully differentiated adult tissue seems to be a biological phenomenon following processes of inflammation and proliferation with a lag of several days, while retinoids seem to be able to induce such a rise independently of, or before, the appearance of such processes. Corticosteroids seem to be inhibitors of this reaction. We discuss the hypothesis that CRABP might function as an intracellular 'buffer' in the case of RA overload.

Stability of the glandular morphogenesis produced by retinoids in the newborn hamster cheek pouch in vitro

Retinoids can induce alterations in differentiation and morphogenesis in the hamster cheek pouch. In order to determine the stability of these changes, explants of neonatal pouch were exposed to 6 micrograms/ml of either retinyl acetate (RAc: 1.8 x 10(-5) M) or all-trans retinoic acid (RA: 2.0 x 10(-5) M) for an initial 3 of 7 days, out of a total of 21 days in organ culture. Three days of RAc or RA caused a delay in the differentiation and keratinization of the epithelium at least up to day 7 of culture. Additionally, two out of ten explants exposed to RA showed small downgrowths of epithelium into the stroma at 7 or 14 days. Seven days of exposure to either retinoid led to inhibition of epithelial keratinization, and produced a mucous metaplasia which was still seen at the end of the 21-day culture period. Periodic acid-Schiff (PAS)-positive, diastase-resistant material was present in the metaplastic epithelium, in intercellular, and in some instances, intracellular locations. An excess of either RAc or RA, for 7 days, induced persistent glandlike downgrowths of epithelium, suggesting that a stable alteration in the developmental program of the epithelium may have occurred. Many of these downgrowths possessed a lumen which was lined by cuboidal epithelium and contained PAS-positive, diastase-resistant secretory material. RA appeared more potent than RAc in inhibiting keratinization, in producing a mucous metaplasia, and in initiating glandlike downgrowths. The persistence of glandular downgrowths suggests that retinoids, either directly or indirectly, act in a manner similar to that of an embryonic inductor.

Functional studies of newly synthesized benzoic acid derivatives: identification of highly potent retinoid-like activity

Three newly synthesized benzoic acid derivatives (terephthalic acid anilides, chalcone carboxylic acid, and azobenzene carboxylic acid), with a certain structural similarity to retinoic acid, were examined for their retinoid-like bioactivity and their capacity to bind to cellular retinoid binding proteins. Two in vitro systems were used to evaluate their retinoid-like bioactivity: inhibition of adipose conversion of ST 13 murine preadipose cells and growth promotion of murine sarcoma virus (MSV)-transformed 3T3 cells in serum-free culture. All three compounds tested inhibited ST 13 adipose conversion at nanomolar concentrations in a manner similar to classical retinoids such as retinoic acid. The growth-stimulating activity of these compounds on MSV-transformed 3T3 cells was one to two orders of magnitude greater than that of retinoic acid. Simultaneous treatment with these compounds and retinoic acid produced only a barely detectable additive effect, suggesting a common mechanism of action, whereas unrelated mitogens, thrombin, and insulin worked synergistically in combination with retinoic acid. None of the compounds competed with retinol for binding to cellular retinol binding protein. However, two of the three competed with retinoic acid for binding to cellular retinoic acid binding protein. This study provides evidence that the newly synthesized compounds should be included among the retinoids and that their strong biological activity will undoubtedly contribute to the biological and medical application of retinoids.

Developmental effects of isotretinoin and 4-oxo-isotretinoin: the role of metabolism in teratogenicity

Previous observations have indicated that isotretinoin (IT), a drug in common use for therapy of cystic acne, is teratogenic in humans but possesses low embryotoxicity in pregnant mice, probably because of its shorter half-life and limited placental transfer in rodents. In human volunteers and patients, one major blood metabolite of IT is 4-oxo-isotretinoin (4-oxo-IT) which undergoes slower elimination than IT and may itself be a participant in teratogenesis. To investigate the problem of species differences displayed by IT and the role of its metabolism, embryotoxic effects of 4-oxo-IT were examined after its single or repeated intubations into pregnant ICR mice and compared with the effects of a similar regimen of IT. The two compounds were also tested for their relative ability to suppress chrondrogenesis in the in vitro cell and organ culture assays. We found that a single dose of 4-oxo-IT, 100 mg/kg, given on day 11 of gestation (plug day = day 0 of gestation) produced a moderate incidence of limb reduction defects and cleft palate (39% and 27% of surviving fetuses, respectively), while a dose of 150 mg/kg affected virtually every fetus. IT, on the other hand, produced no defects in fetuses exposed to similar dose levels. Repeated intubations with IT, however, resulted in increasing the frequencies of limb reduction defects and cleft palate to levels obtained after 4-oxo-IT administration. We found that a 3-hour interval between IT intubations was more effective in this regard than an 8-hour interval. Repeated IT intubations also uncovered sharper stage-dependency of limb and palatal defects than obtained otherwise.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver levels of vitamin A and cellular retinol-binding protein for patients with biliary atresia

We have examined whether the amount of cellular retinol-binding protein in human liver is related to the amount of vitamin A stored in the liver. Levels of vitamin A, as retinol and retinol esters, and of cellular retinol-binding protein have been determined in liver samples from 6 normal adults and 11 children with biliary atresia, with and without vitamin A treatment. The level of cellular retinol-binding protein in the liver was not related to the liver vitamin A concentration examined over a 300-fold range of vitamin A levels. Also, biliary atresia did not appear to interfere with storage of vitamin A, and the level of cellular retinol-binding protein was comparable to that observed in the liver of normal adults. The demonstration of proper vitamin A storage in treated children as well as normal levels of cellular retinol-binding protein suggest the vitamin A deficiency frequently observed in children with biliary atresia may be due primarily to faulty absorption rather than a combination of poor absorption and impaired hepatic vitamin A metabolism.