Retinoic acid in alveolar development, maintenance and regeneration

Effect of vitamin A on adult lung function: a triangulation of evidence approach

BACKGROUND

Vitamin A, an essential micronutrient obtained through the diet, plays a crucial role in lung development and contributes to lung regeneration. We aimed to investigate its effect on adult lung function using triangulation of evidence from both observational and genetic data.

METHODS

Using data on 150 000 individuals from UK Biobank and correcting for measurement error (generalised structural equation modelling), we first investigated the association of dietary vitamin A intake (total vitamin A, carotene and retinol) with lung function (forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1)/FVC)). We then assessed the causality of these associations using Mendelian randomisation (MR), and we investigated the effects on adult lung function of 39 genes related to vitamin A, and their interaction with vitamin A intake.

FINDINGS

Our observational analysis suggests a positive association between carotene intake and FVC only (13.3 mL per 100 µg/day; p=2.9×10-9), with stronger associations in smokers, but no association of retinol intake with FVC or FEV1/FVC. The MR similarly shows a beneficial effect of serum beta-carotene on FVC only, with no effect of serum retinol on FVC nor FEV1/FVC. Nine of the vitamin A-related genes were associated with adult lung function, six of which have not been previously identified in genome-wide studies and three (NCOA2, RDH10, RXRB) in any type of genetic study of lung function. Five genes showed possible gene-vitamin A intake interactions.

INTERPRETATION

Our triangulation study provides convincing evidence for a causal effect of vitamin A, carotene in particular, on adult lung function, suggesting a beneficial effect of a carotene-rich diet on adult lung health.

Palovarotene (Sohonos), a synthetic retinoid for reducing new heterotopic ossification in fibrodysplasia ossificans progressiva: history, present, and future

Retinoids are metabolic derivatives of vitamin A and play crucial roles in the regulation of various tissues and organs during prenatal and postnatal development. Active retinoids, like all-trans-retinoic acid, are synthesized in the cytoplasm and subsequently interact with nuclear retinoic acid receptors (RARα, RARβ, and RARγ) to enhance transcription of specific genes. In the absence of retinoids, RARs can still bind to response elements of target genes but repress their transcription. Chondrogenic cell differentiation and cartilage maturation in the growth plate require the absence of retinoid signaling and transcriptional repression by unliganded RARs. This led to the hypothesis that synthetic retinoid agonists may be pharmacological agents to inhibit those cellular processes and counter the excessive formation of cartilage and bone in conditions like heterotopic ossification (HO). HO can be instigated by diverse culprits including trauma, invasive surgeries, inflammatory disorders, or genetic conditions. One such genetic disease is fibrodysplasia ossificans progressiva (FOP), a rare disorder driven by activating mutations in the ACVR1 gene. Patients with FOP have severe and progressive HO formation in soft tissues, leading to extensive permanent loss of mobility and increased mortality. Synthetic retinoid agonists selective for RARα or RARγ showed efficacy against injury-induced and genetic HO in mouse models. The RARγ agonists showed the highest effectiveness, with palovarotene being selected for clinical trials in patients with FOP. Post hoc analyses of phase II and phase III clinical trials showed that palovarotene has significant disease-modifying effects for FOP, but with significant risks such as premature growth plate closure in some younger subjects. This review provides an overview of retinoid and RAR roles in skeletal development and discusses the identification of palovarotene as a potential FOP therapy, the clinical data supporting its regulatory approval in some countries, and the potential applications of this drug for other relevant disorders besides FOP.

Vitamin A Status Modulates Epithelial Mesenchymal Transition in the Lung: The Role of Furin

Vitamin A deficiency (VAD) induced TGF-β hyperactivation and reduced expression of cell adhesion proteins in the lung, suggesting that the disruption of retinoic acid (RA) signaling leads to epithelial-mesenchymal transition (EMT). To elucidate the role of lung vitamin A status in EMT, several EMT markers and the expression of the proprotein convertase furin, which activates TGF-β, were analyzed in two experimental models. Our in vivo model included control rats, VAD rats, and both control rats and VAD rats, treated with RA. For the in vitro studies, human bronchoalveolar epithelial cells treated with RA were used. Our data show that EMT and furin are induced in VAD rats. Furthermore, furin expression continues to increase much more markedly after treatment of VAD rats with RA. In control rats and cell lines, an acute RA treatment induced a significant increase in furin expression, concomitant with changes in EMT markers. A ChIP assay demonstrated that RA directly regulates furin transcription. These results emphasize the importance of maintaining vitamin A levels within the physiological range since both levels below and above this range can cause adverse effects that, paradoxically, could be similar. The role of furin in EMT is discussed.

Intracellular Drug Delivery Process of Am80-Encapsulated Lipid Nanoparticles Aiming for Alveolar Regeneration

Chronic obstructive pulmonary disease (COPD) results in obstructive ventilatory impairment caused by emphysema, and current treatment is limited to symptomatic therapy or lung transplantation. Therefore, the development of new treatments to repair alveolar destruction is especially urgent. Our previous study revealed that 1.0 mg/kg of synthetic retinoid Am80 had a repair effect on collapsed alveoli in a mouse model of elastase-induced emphysema. From these results, however, the clinical dose calculated in accordance with FDA guidance is estimated to be 5.0 mg/60 kg, and it is desirable to further reduce the dose to allow the formulation of a powder inhaler for clinical application. To efficiently deliver Am80 to the retinoic acid receptor in the cell nucleus, which is the site of action, we focused on SS-cleavable proton-activated lipid-like material O-Phentyl-P4C2COATSOME^®SS-OP, hereinafter referred to as "SS-OP"). In this study, we investigated the cellular uptake and intracellular drug delivery process of Am80-encapsulated SS-OP nanoparticles to elucidate the mechanism of Am80 by nanoparticulation. Am80-encapsulated SS-OP nanoparticles were taken up into the cells via ApoE, and then Am80 was efficiently delivered into the nucleus via RARα. These results indicated the usefulness of SS-OP nanoparticles as drug delivery system carriers of Am80 for COPD treatment.

Am80-Encapsulated Lipid Nanoparticles, Developed with the Aim of Achieving Alveolar Regeneration, Have an Improvement Effect on Pulmonary Emphysema

Chronic obstructive pulmonary disease (COPD) is characterized by chronic bronchitis and emphysema, and current drug treatments target its symptoms. Thus, the development of a therapeutic drug to repair alveolar destruction is urgently needed. Our previous research revealed that the synthetic retinoic acid Am80 (1.0 mg/kg) showed a repairing effect on collapsed alveoli in a mouse model of elastase-induced emphysema. However, a further reduction in the dose is desirable to facilitate the development of a powder inhalation formulation for clinical application. We, therefore, focused on SS-OP to deliver Am80 efficiently. As a result, 0.01 mg/kg of Am80-encapsulated SS-OP nanoparticles repaired collapsed alveoli and improved the respiratory function in the mouse model of elastase induced emphysema. The results suggested that, with the use of SS-OP, the Am80 dose could be reduced. This could contribute to the development of a powder inhalation system as a curative medicine for COPD.

Retinoic acid promotes tissue vitamin A status and modulates adipose tissue metabolism of neonatal rats exposed to maternal high-fat diet-induced obesity

Maternal obesity may compromise the micronutrient status of the offspring. Vitamin A (VA) is an essential micronutrient during neonatal development. Its active metabolite, retinoic acid (RA), is a key regulator of VA homeostasis, which also regulates adipose tissue (AT) development in obese adults. However, its role on VA status and AT metabolism in neonates was unknown and it was determined in the present study. Pregnant Sprague-Dawley rats were randomised to a normal fat diet (NFD) or a high fat diet (HFD). From postnatal day 5 (P5) to P20, half of the HFD pups received oral RA every 3 d (HFDRA group). NFD pups and the remaining HFD pups (HFD group) received placebo. Six hours after dosing on P8, P14 and P20, n 4 pups per group were euthanised for different measures. It was found that total retinol concentration in neonatal liver and lung was significantly lower in the HFD group than the NFD group, while the concentrations were significantly increased in the HFDRA group. The HFD group exhibited significantly higher body weight (BW) gain, AT mass, serum leptin and adiponectin, and gene expression of these adipokines in white adipose tissue compared with the NFD group; these measures were significantly reduced in the HFDRA group. BAT UCP2 and UCP3 gene expression were significantly higher in pups receiving RA. In conclusion, repeated RA treatment during the suckling period improved the tissue VA status of neonates exposed to maternal obesity. RA also exerted a regulatory effect on neonatal obesity development by reducing BW gain and adiposity and modulating AT metabolism.

All-trans retinoic acid increases the pathogenicity of the H9N2 influenza virus in mice

BACKGROUND

The H9N2 virus can infect not only birds but also humans. The pathogenicity of H9N2 virus infection is determined by an excessive immune response in the lung. All-trans retinoic acid (ATRA), the active metabolite of vitamin A, plays an important regulatory role and has been widely used in the clinical practice. This study was aimed to investigate whether ATRA could regulate the immune response to H9N2 virus infection in the lungs of mice, thereby reducing the pathogenicity of the H9N2 virus in mice.

METHODS

Mice were infected intranasally with H9N2 virus, and injected intraperitoneally with 0.2 mL of ATRA at low (1 mg/kg), medium (5 or 10 mg/kg), or high therapeutic dose (20 mg/kg), and toxic dose (40, 60, or 80 mg/kg), once per day for 10 days. Clinical signs, survival rates, and lung gross pathology were compared between the ATRA-treated H9N2-infected group, the ATRA group, and the H9N2-infected group, to investigate the effect of different doses of ATRA on the pathogenicity of H9N2 virus. Additionally, the viral load and cytokine concentration of lungs were measured at 3, 5, 7, and 9 days after infection, to investigate the potential mechanism of ATRA in affecting the pathogenicity of the H9N2 virus. Expression levels of cellular retinoic acid-binding protein 1 (CRABP1), cellular retinoic acid-binding protein 2 (CRABP2), and Retinoic acid-inducible gene-I (RIG-I) were detected using Western blotting.

RESULTS

The ATRA-treated H9N2-infected mice showed more severe clinical signs compared with the H9N2-infected group. The medium and high therapeutic doses of ATRA reduced the survival rates, aggravated lung tissue damage, decreased the expression of interferon beta (IFN-β), and increased the concentrations of interleukin-1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and C-C motif chemokine ligand 2 (CCL2) in the lungs of the H9N2-infected mice. At the same time, the expression patterns of CRABP1, CRABP2, and RIG-I were changed in mice infected by H9N2 and treated with different concentrations of ATRA.

CONCLUSIONS

Our findings suggest that the therapeutic dose of ATRA can increase the pathogenicity of the H9N2 virus. Therefore, the consequences of those infected by influenza virus would be more severe after ATRA treatment.

Serum vitamin A levels as a novel predictor for respiratory distress syndrome in neonates: is it beneficial?

BACKGROUND

Neonatal respiratory distress syndrome (RDS) is a very important dangerous disease in neonates and is accompanied by neonatal complications and death.

OBJECTIVE

Early determination of neonatal serum vitamin A levels as a possible novel predictor for neonatal RDS.

PATIENTS AND METHODS

A case-control study, with the identification number TCTR20210206001 was done on 100 neonates who were admitted to the neonatal intensive care unit (NICU) of Tanta University Hospital (TUH) suffering from RDS (patient group) and 100 healthy neonates (control group). Estimation of serum vitamin A (retinol) was occurred for all neonates of this study within the first four hours after birth before any nutrition or therapies.

RESULTS

Showed no significant difference between serums retinol (RT) levels in grade 1&2 RDS neonates (patient group) and the serum RT levels in the control group while there was significant difference between serums retinol levels in grade 3&4 RDS neonates (patient group) and the serum retinol levels in the control group where the P-value was 0.006, 0.002 respectively.

CONCLUSION

Serum vitamin A levels in neonates were decreased in RDS neonates and could be used as a predictor for the development and severity of RDS.

RECOMMENDATION

Routine estimation of serum vitamin A levels in neonates as a novel predictor for neonatal RDS.

Mechanistic aspects of carotenoid health benefits - where are we now?

Dietary intake and tissue levels of carotenoids have been associated with a reduced risk of several chronic diseases, including cardiovascular diseases, type 2 diabetes, obesity, brain-related diseases and some types of cancer. However, intervention trials with isolated carotenoid supplements have mostly failed to confirm the postulated health benefits. It has thereby been speculated that dosing, matrix and synergistic effects, as well as underlying health and the individual nutritional status plus genetic background do play a role. It appears that our knowledge on carotenoid-mediated health benefits may still be incomplete, as the underlying mechanisms of action are poorly understood in relation to human relevance. Antioxidant mechanisms - direct or via transcription factors such as NRF2 and NF-κB - and activation of nuclear hormone receptor pathways such as of RAR, RXR or also PPARs, via carotenoid metabolites, are the basic principles which we try to connect with carotenoid-transmitted health benefits as exemplified with described common diseases including obesity/diabetes and cancer. Depending on the targeted diseases, single or multiple mechanisms of actions may play a role. In this review and position paper, we try to highlight our present knowledge on carotenoid metabolism and mechanisms translatable into health benefits related to several chronic diseases.

Exploring the role of microRNAs in axolotl regeneration

The axolotl, Ambystoma mexicanum, is used extensively for research in developmental biology, particularly for its ability to regenerate and restore lost organs, including in the nervous system, to full functionality. Regeneration in mammals typically depends on the healing process and scar formation with limited replacement of lost tissue. Other organisms, such as spiny mice (Acomys cahirinus), salamanders, and zebrafish, are able to regenerate some damaged body components. Blastema is a tissue that is formed after tissue injury in such organisms and is composed of progenitor cells or dedifferentiated cells that differentiate into various cell types during regeneration. Thus, identifying the molecules responsible for initiation of blastema formation is an important aspect for understanding regeneration. Introns, a major source of noncoding RNAs (ncRNAs), have characteristic sizes in the axolotl, particularly in genes associated with development. These ncRNAs, particularly microRNAs (miRNAs), exhibit dynamic regulation during regeneration. These miRNAs play an essential role in timing and control of gene expression to order and organize processes necessary for blastema creation. Master keys or molecules that underlie the remarkable regenerative abilities of the axolotl remain to be fully explored and exploited. Further and ongoing research on regeneration promises new knowledge that may allow improved repair and renewal of human tissues.

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.

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.

Retinoic acid signaling in vascular development

Formation of the vasculature is an essential developmental process, delivering oxygen and nutrients to support cellular processes needed for tissue growth and maturation. Retinoic acid (RA) and its downstream signaling pathway is vital for normal pre- and post-natal development, playing key roles in the specification and formation of many organs and tissues. Here, we review the role of RA in blood and lymph vascular development, beginning with embryonic yolk sac vasculogenesis and remodeling and discussing RA's organ-specific roles in angiogenesis and vessel maturation. In particular, we highlight the multi-faceted role of RA signaling in CNS vascular development and acquisition of blood-brain barrier properties.

Vitamin A Deficiency and the Lung

Vitamin A (all-trans-retinol) is a fat-soluble micronutrient which together with its natural derivatives and synthetic analogues constitutes the group of retinoids. They are involved in a wide range of physiological processes such as embryonic development, vision, immunity and cellular differentiation and proliferation. Retinoic acid (RA) is the main active form of vitamin A and multiple genes respond to RA signalling through transcriptional and non-transcriptional mechanisms. Vitamin A deficiency (VAD) is a remarkable public health problem. An adequate vitamin A intake is required in early lung development, alveolar formation, tissue maintenance and regeneration. In fact, chronic VAD has been associated with histopathological changes in the pulmonary epithelial lining that disrupt the normal lung physiology predisposing to severe tissue dysfunction and respiratory diseases. In addition, there are important alterations of the structure and composition of extracellular matrix with thickening of the alveolar basement membrane and ectopic deposition of collagen I. In this review, we show our recent findings on the modification of cell-junction proteins in VAD lungs, summarize up-to-date information related to the effects of chronic VAD in the impairment of lung physiology and pulmonary disease which represent a major global health problem and provide an overview of possible pathways involved.

Vitamin A Supplementation by Endotracheal Application of a Nano-encapsulated Preparation Is Feasible in Ventilated Preterm Lambs

BACKGROUND

Vitamin A (VA) is crucial for lung growth and development. In premature infants, inadequate VA levels are associated with an increased risk of bronchopulmonary dysplasia (BPD). Intramuscular VA supplementation has been shown to decrease the incidence of BPD, but is not widely used in the clinical setting due to concerns about feasibility and pain. We studied VA kinetics, distribution, and the induction of early genetic expression of retinoid homeostatic genes in the lung after endotracheal and intravenous application in a preterm lamb model.

METHODS

Lambs were delivered prematurely after 85% of gestation, intubated, and ventilated for 3 hours. The animals were randomized to receive no VA ("control"), a bolus of VA intravenously ("i.v."), or VA endotracheally directly after administration of surfactant ("e.t.").

RESULTS

Animals treated with VA endotracheally directly after administration of surfactant showed significant increases of VA in serum and lung compared to controls. Animals treated with a bolus of VA intravenously showed significant increases of VA in serum, lung, and liver; however, peak serum concentrations and mRNA levels of homeostatic genes raised concerns about toxicity in this group.

CONCLUSIONS

Endotracheal VA supplementation in preterm lambs is feasible and might offer advantages in comparison to i.v. Further studies are warranted to explore biological effects in the context of BPD.

Lung development, regeneration and plasticity: From disease physiopathology to drug design using induced pluripotent stem cells

Lungs have a complex structure composed of different cell types that form approximately 17 million airway branches of gas-delivering bronchioles connected to 500 million gas-exchanging alveoli. Airways and alveoli are lined by epithelial cells that display a low rate of turnover at steady-state, but can regenerate the epithelium in response to injuries. Here, we review the key points of lung development, homeostasis and epithelial cell plasticity in response to injury and disease, because this knowledge is required to develop new lung disease treatments. Of note, canonical signaling pathways that are essential for proper lung development during embryogenesis are also involved in the pathophysiology of most chronic airway diseases. Moreover, the perfect control of these interconnected pathways is needed for the successful differentiation of induced pluripotent stem cells (iPSC) into lung cells. Indeed, differentiation of iPSC into airway epithelium and alveoli is based on the use of biomimetics of normal embryonic and fetal lung development. In vitro iPSC-based models of lung diseases can help us to better understand the impaired lung repair capacity and to identify new therapeutic targets and new approaches, such as lung cell therapy.

Inhibition of pulmonary β-carotene 15, 15'-oxygenase expression by glucocorticoid involves PPARα

β-carotene 15,15'-oxygenase (BCO1) catalyzes the first step in the conversion of dietary provitamin A carotenoids to vitamin A. This enzyme is expressed in a variety of developing and adult tissues, suggesting that its activity may regulate local retinoid synthesis. Vitamin A and related compounds (retinoids) are critical regulators of lung epithelial development, integrity, and injury repair. A balance between the actions of retinoids and glucocorticoids (GCs) promotes normal lung development and, in particular, alveolarization. Alterations in this balance, including vitamin A deficiency and GC excess, contribute to the development of chronic lung disorders. Consequently, we investigated if GCs counteract retinoid effects in alveolar epithelial cells by mechanisms involving BCO1-dependent local vitamin A metabolism. We demonstrate that BCO1 is expressed in human fetal lung tissue and human alveolar epithelial-like A549 cells. Our results indicate A549 cells metabolize β-carotene to retinal and retinoic acid (RA). GCs exposure using dexamethasone (DEX) decreases BCO1 mRNA and protein levels in A549 cells and reduces BCO1 promoter activity via inhibiting peroxisome proliferator-activated receptor γ (PPARγ) DNA binding. DEX also induces expression of PPARα, which in turn most likely causes a decrease in PPARγ/RXRα heterodimer binding to the bco1 gene promoter and consequent inhibition of bco1 gene expression. PPARα knockdown with siRNA abolishes DEX-induced suppression of BCO1 expression, confirming the requirement for PPARα in this DEX-mediated BCO1 mechanism. Taken together, these findings provide the first evidence that GCs regulate vitamin A (retinoid) signaling via inhibition of bco1 gene expression in a PPARα-dependent manner. These results explicate novel aspects of local GC:retinoid interactions that may contribute to alveolar tissue remodeling in chronic lung diseases that affect children and, possibly, adults.

Retinoic acid in developmental toxicology: Teratogen, morphogen and biomarker

This review explores the usefulness retinoic acid (RA) related physiological factors as possible biomarkers of embryotoxicity. RA is involved in the morphogenesis of the early embryo as well as in the development and maturation of a wide variety of organ anlagen. The region-specific homeostasis of RA in the embryo is in many ways the driving force determining developmental cell proliferation versus differentiation. As a consequence, RA concentrations are carefully controlled in time and space in the developing embryo. RA deficiency and overdosing both result in characteristic patterns of malformations that may involve many different organ systems. The central role of RA in embryo development provides us with a set of sensitive biomarkers that may be employed in developmental toxicity testing. This includes the synthesizing and metabolizing enzymes of RA, but also a myriad of related morphogenetic factors and their genes, of which the expression may be affected by changes in RA balance. Several examples of embryotoxicants interfering with the homeostasis of RA and related parameters have been described. A preliminary adverse outcome pathway framework for RA mediated malformations has been published. Expansion of this framework and its application in developmental toxicity testing may allow the detection of a large variety of embryotoxicants with diverse modes of action. RA homeostasis therefore provides a promising set of molecular tools that may be employed in the advancement of mode of action driven animal-free developmental toxicity testing.

Uteroplacental Insufficiency Alters the Retinoid Pathway and Lung Development in Newborn Rats

BACKGROUND

Intrauterine growth retardation (IUGR) is associated with reduced lung function during infancy and perhaps throughout adulthood. The retinoic acid (RA) signaling pathway modulates pre- and postnatal lung development. This study was conducted to test our hypothesis that uteroplacental insufficiency alters the elements of the retinoid pathway in developing lungs.

METHODS

On Gestation Day 18, either uteroplacental insufficiency was induced through bilateral uterine vessel ligation (IUGR group) or sham surgery (control group) was performed. Lung tissues from the offspring were examined through Western blotting, immunohistochemistry, and morphometry on Postnatal Day 3 and Postnatal Day 7.

RESULTS

Compared with control rats, the IUGR rats exhibited significantly lower body weights on Postnatal Day 3 and Postnatal Day 7 and significantly lower lung weights on Postnatal Day 3. Uteroplacental insufficiency significantly increased RA receptor (RAR)-β protein expression on Postnatal Day 3. The expression of RAR-α, RAR-γ, cellular RA-binding protein-1, and cellular RA-binding protein-2 between the control and IUGR rats was comparable on Postnatal Day 3 and Postnatal Day 7. Compared with the control rats, the IUGR rats exhibited a significantly higher volume fraction of alveolar airspace on Postnatal Day 3 and Postnatal Day 7 and a significantly lower volume fraction of alveolar walls on Postnatal Day 3.

CONCLUSION

Uteroplacental insufficiency causes defective alveolarization and transient increases in RAR-β expression in the lungs of newborn rats. The retinoid pathway may be one of the probable pathways mediating lung abnormalities caused by uteroplacental insufficiency.

Ultraperformance convergence chromatography-high resolution tandem mass spectrometry for lipid biomarker profiling and identification

Lipids represent biologically ubiquitous and highly dynamic molecules in terms of abundance and structural diversity. Whereas the potential for lipids to inform on disease/injury is promising, their unique characteristics make detection and identification of lipids from biological samples analytically demanding. We report the use of ultraperformance convergence chromatography (UPC² ), a variant of supercritical fluid chromatography, coupled to high-resolution, data-independent tandem mass spectrometry for characterization of total lipid extracts from mouse lung tissue. The UPC² platform resulted in lipid class separation and when combined with orthogonal column chemistries yielded chromatographic separation of intra-class species based on acyl chain hydrophobicity. Moreover, the combined approach of using UPC² with orthogonal column chemistries, accurate mass measurements, time-aligned low- and high-collision energy total ion chromatograms, and positive and negative ion mode product ion spectra correlation allowed for confident lipid identification. Of great interest was the identification of differentially expressed ceramides that were elevated 24 h post whole thorax lung irradiation. The identification of lipids that were elevated 24 h post-irradiation signifies a unique opportunity to investigate early mechanisms of action prior to the onset of clinical symptoms in the whole thorax lung irradiation mouse model.

Should we still use vitamin A to prevent bronchopulmonary dysplasia?

Bronchopulmonary dysplasia (BPD) is associated with significant short- and long-term morbidity in preterm infants, and it can be prevented in some infants with vitamin A prophylaxis. Vitamin A, once widely used in neonatal intensive care, was scarce for the last few years, but has become available again at a much higher price, leading to dilemmas about its routine use. In this review we discuss experimental, clinical and socioeconomic evidence related to BPD, and provide a framework for clinicians and policy-makers to evaluate the value of vitamin A treatment and make decisions about its use for prevention of BPD.

Bridging Lung Development with Chronic Obstructive Pulmonary Disease. Relevance of Developmental Pathways in Chronic Obstructive Pulmonary Disease Pathogenesis

Chronic obstructive pulmonary disease (COPD) is characterized by chronic airflow limitation. This generic term encompasses emphysema and chronic bronchitis, two common conditions, each having distinct but also overlapping features. Recent epidemiological and experimental studies have challenged the traditional view that COPD is exclusively an adult disease occurring after years of inhalational insults to the lungs, pinpointing abnormalities or disruption of the pathways that control lung development as an important susceptibility factor for adult COPD. In addition, there is growing evidence that emphysema is not solely a destructive process because it is also characterized by a failure in cell and molecular maintenance programs necessary for proper lung development. This leads to the concept that tissue regeneration required stimulation of signaling pathways that normally operate during development. We undertook a review of the literature to outline the contribution of developmental insults and genes in the occurrence and pathogenesis of COPD, respectively.

Regulation of Retinoic Acid Receptor Beta by Interleukin-15 in the Lung during Cigarette Smoking and Influenza Virus Infection

Virus-induced exacerbations often lead to further impairment of lung function in chronic obstructive pulmonary disease. IL-15 is critical in antiviral immune responses. Retinoic acid (RA) signaling plays an important role in tissue maintenance and repair, particularly in the lung. We studied RA signaling and its relation to IL-15 in the lung during cigarette smoke (CS) exposure and influenza virus infection. In vivo studies show that RA signaling is diminished by long-term CS exposure or influenza virus infection alone, which is further attenuated during infection after CS exposure. RA receptor β (RARβ) is specifically decreased in the lung of IL-15 transgenic (overexpression; IL-15Tg) mice, and a greater reduction in RARβ is found in these mice compared with wild-type (WT) mice after infection. RARβ is increased in IL-15 knockout (IL-15KO) mice compared with WT mice after infection, and the additive effect of CS and virus on RARβ down-regulation is diminished in IL-15KO mice. IL-15 receptor α (IL-15Rα) is increased and RARβ is significantly decreased in lung interstitial macrophages from IL-15Tg mice compared with WT mice. In vitro studies show that IL-15 down-regulates RARβ in macrophages via IL-15Rα signaling during influenza virus infection. These studies suggest that RA signaling is significantly diminished in the lung by CS exposure and influenza virus infection. IL-15 specifically down-regulates RARβ expression, and RARβ may play a protective role in lung injury caused by CS exposure and viral infections.

Retinoid Homeostatic Gene Expression in Liver, Lung and Kidney: Ontogeny and Response to Vitamin A-Retinoic Acid (VARA) Supplementation from Birth to Adult Age

Vitamin A (VA, retinol) metabolism is homeostatically controlled, but little is known of its regulation in the postnatal period. Here, we determined the postnatal trajectory of VA storage and metabolism in major compartments of VA metabolism-plasma, liver, lung, and kidney from postnatal (P) day 1 to adulthood. We also investigated the response to supplementation with VARA, a combination of VA and 10% all-trans-retinoic acid that previously was shown to synergistically increase retinol uptake and storage in lung. Nursling pups of dams fed a VA-marginal diet received an oral dose of oil (placebo) or VARA on each of four neonatal days: P1, P4, P7, and P10; and again as adults. Tissues were collected 6 h after the final dosing on P1, P4, P10, and at adult age. Gene transcripts for Lrat and Rbp4 in liver and Raldh-1 and Raldh-3 in lung, did not differ in the neonatal period but were higher, P<0.05, in adults, while Cyp26B1, Stra6, megalin, and Raldh-2 in lung did not differ from perinatal to adult ages. VARA supplementation increased total retinol in plasma, liver and lung, with a dose-by-dose accumulation in neonatal liver and lung, while transcripts for Lrat in liver, megalin in kidney, Cyp26A1/B1 in liver and lung, respectively, and Stra6 in lung, were all increased, suggesting pathways of VA uptake, storage and RA oxidation were each augmented after VARA. VARA decreased hepatic expression of Rbp4, responsible for VA trafficking from liver to plasma, and, in lung, of Raldh-1 and Raldh-2, which function in RA production. Our results define retinoid homeostatic gene expression from neonatal and adult age and show that while supplementation with VARA acutely alters retinol content and retinoid homeostatic gene expression in neonatal and adult lung, liver and kidney, VARA supplementation of neonates increased adult-age VA content only in the liver.

Impact of Nutrition on Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) remains a common morbidity of prematurity. Although the pathogenesis of BPD is recognized to be both multifactorial and complex, the role of nutrition in the pathophysiology of BPD is typically limited to management after a diagnosis has been made. Infants born small for gestational age and those who experience postnatal growth failure are more likely to have BPD. Therapies for lung disease, such as fluid restriction, diuretics, and corticosteroids, can negatively impact postnatal growth. Future research is needed to optimize nutritional strategies in the neonatal intensive care unit and following hospital discharge.

VEGF and endothelium-derived retinoic acid regulate lung vascular and alveolar development

Prevention or treatment of lung diseases caused by the failure to form, or destruction of, existing alveoli, as observed in infants with bronchopulmonary dysplasia and adults with emphysema, requires understanding of the molecular mechanisms of alveolar development. In addition to its critical role in gas exchange, the pulmonary circulation also contributes to alveolar morphogenesis and maintenance by the production of paracrine factors, termed "angiocrines," that impact the development of surrounding tissue. To identify lung angiocrines that contribute to alveolar formation, we disrupted pulmonary vascular development by conditional inactivation of the Vegf-A gene during alveologenesis. This resulted in decreased pulmonary capillary and alveolar development and altered lung elastin and retinoic acid (RA) expression. We determined that RA is produced by pulmonary endothelial cells and regulates pulmonary angiogenesis and elastin synthesis by induction of VEGF-A and fibroblast growth factor (FGF)-18, respectively. Inhibition of RA synthesis in newborn mice decreased FGF-18 and elastin expression and impaired alveolarization. Treatment with RA and vitamin A partially reversed the impaired vascular and alveolar development induced by VEGF inhibition. Thus we identified RA as a lung angiocrine that regulates alveolarization through autocrine regulation of endothelial development and paracrine regulation of elastin synthesis via induction of FGF-18 in mesenchymal cells.

Involvement of Different networks in mammary gland involution after the pregnancy/lactation cycle: Implications in breast cancer

Early pregnancy is associated with a reduction in a woman's lifetime risk for breast cancer. However, different studies have demonstrated an increase in breast cancer risk in the years immediately following pregnancy. Early and long-term risk is even higher if the mother age is above 35 years at the time of first parity. The proinflammatory microenvironment within the mammary gland after pregnancy renders an "ideal niche" for oncogenic events. Signaling pathways involved in programmed cell death and tissue remodeling during involution are also activated in breast cancer. Herein, the major signaling pathways involved in mammary gland involution, signal transducer and activator of transcription (STAT3), nuclear factor-kappa B (NF-κB), transforming growth factor beta (TGFβ), and retinoid acid receptors (RARs)/retinoid X receptors (RXRs), are reviewed as part of the complex network of signaling pathways that crosstalk in a contextual-dependent manner. These factors, also involved in breast cancer development, are important regulatory nodes for signaling amplification after weaning. Indeed, during involution, p65/p300 target genes such as MMP9, Capn1, and Capn2 are upregulated. Elevated expression and activities of these proteases in breast cancer have been extensively documented. The role of these proteases during mammary gland involution is further discussed. MMPs, calpains, and cathepsins exert their effect by modification of the extracellular matrix and intracellular proteins. Calpains, activated in the mammary gland during involution, cleave several proteins located in cell membrane, lysosomes, mitochondria, and nuclei favoring cell death. Besides, during this period, Capn1 is most probably involved in the modulation of preadipocyte differentiation through chromatin remodeling. Calpains can be implicated in cell anchoring loss, providing a proper microenvironment for tumor growth. A better understanding of the role of any of these proteases in tumorigenesis may yield novel therapeutic targets or prognostic markers for breast cancer.

Retinoic acid promotes primary fetal alveolar epithelial type II cell proliferation and differentiation to alveolar epithelial type I cells

Retinoic acid (RA) plays an important role in lung development and maturation. Many stimuli can induce alveolar epithelial cell damage which will result in the injury of lung parenchyma. The aim of this study was to observe the effect of RA on the proliferation and differentiation of primary fetal alveolar epithelial type II cells (fAECIIs). Primary fAECIIs were isolated from fetal rats at 19 d of gestation and purified by a differential centrifugation and adhesion method. The cells were randomly divided into control (dimethyl sulfoxide, DMSO) and RA groups. Cell proliferation, viability, apoptosis, cycle, and expression of target protein were examined at 24, 48, and 72 h. We found that the proliferation and viability of cells in the RA-exposed group significantly increased compared with the DMSO control group. The proportion (%) of cells in the G2 and S phases in the RA group was significantly higher than that in control group cells. The proportion (%) of both early apoptotic cells and late apoptotic cells decreased significantly in cells exposed to RA compared with cells exposed to DMSO. RA significantly enhanced the expression of aquaporin 5 (AQP5). The expression level of pulmonary surfactant C (SPC) was elevated after cells were exposed to RA for 24 and 72 h but was inhibited when cells were exposed to RA for 48 h. These results suggest that RA promotes fAECII proliferation by improving cell viability, promoting S phase entry and inhibiting apoptosis and RA promotes fAECIIs differentiation to alveolar epithelial type I cells (AECIs).

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

Retinoic acid induced repair in the lung of adult hyperoxic mice, reducing transforming growth factor-β1 (TGF-β1) mediated abnormal alterations

The aim of the study was to determine the effects of retinoic acid on lung alveolar repair in adult hyperoxic mice and to investigate the relationship between TGF-β1 and retinoic acid during the repair processes. Adult mice were divided into 4 groups. Two groups were given daily intraperitoneal injections of peanut oil/dimethylsulfoxide mixture and retinoic acid (50mg/kg body weight, 50 μl of volume) dissolved in peanut oil/dimethylsulfoxide mixture for 12 days with a 2-day break on days 6 and 7. Following hyperoxia (100% oxygen) for 72 h the remaining two groups were treated in the same manner as already described: peanut oil/dimethylsulfoxide mixture and retinoic acid. Lung structure was investigated by light microscopy. TGF-β1 and Smad protein expressions in the lung were assayed by biochemical methods. Hyperoxic mice exhibited damage to the alveolar walls, increased cell proliferation and induced Smad3/TGF-β1 signaling. Smad2 and phospho-Smad2 protein expressions were unchanged in all groups. Retinoic acid administration improved the degenerative alterations caused by hyperoxia and helped in alveolar repair. This positive effect of retinoic acid resulted from the inhibition of Smad3/TGF-β1 signaling via reduced Smad4 mRNA and increased Smad7 protein expression. Retinoic acid also induced alveolarization and restricted Smad3/TGF-β1 signaling by decreasing Smad4 mRNA in healthy mice. Thus, retinoic acid helped repair Smad3/TGF-β1-induced lung damage in hyperoxic mice.

Retinol kinetics in unsupplemented and vitamin A-retinoic acid supplemented neonatal rats: a preliminary model

Vitamin A (VA) metabolism in neonates is virtually uncharacterized. Our objective was to develop a compartmental model of VA metabolism in unsupplemented and VA-supplemented neonatal rats. On postnatal day 4, pups (n = 3/time) received 11,12-[(3)H]retinol orally, in either oil (control) or VA combined with retinoic acid (VARA) [VA (∼6 mg/kg body weight) + 10% retinoic acid]. Plasma and tissues were collected at 14 time points up to 14 days after dose administration. VARA supplementation rapidly, but transiently, increased total retinol mass in plasma, liver, and lung. It decreased the peak fraction of the dose in plasma. A multi-compartmental model developed to fit plasma [(3)H]retinol data predicted more extensive recycling of retinol between plasma and tissues in neonates compared with that reported in adults (144 vs. 12-13 times). In VARA pups, the recycling number for retinol between plasma and tissues (100 times) and the time that retinol spent in plasma were both lower compared with controls; VARA also stimulated the uptake of plasma VA into extravascular tissues. A VARA perturbation model indicated that the effect of VARA in stimulating VA uptake into tissues in neonates is both dramatic and transient.

Lung regeneration and translational implications of the postpneumonectomy model

Lung regeneration research is yielding data with increasing translational value. The classical models of lung development, postnatal alveolarization, and postpneumonectomy alveolarization have contributed to a broader understanding of the cellular participants including stem-progenitor cells, cell-cell signaling pathways, and the roles of mechanical deformation and other physiologic factors that have the potential to be modulated in human and animal patients. Although recent information is available describing the lineage fate of lung fibroblasts, genetic fate mapping, and clonal studies are lacking in the study of lung regeneration and deserve further examination. In addition to increasing knowledge concerning classical alveolarization (postnatal, postpneumonectomy), there is increasing evidence for remodeling of the adult lung after partial pneumonectomy. Though limited in scope, compelling data have emerged describing restoration of lung tissue mass in the adult human and in large animal models. The basis for this long-term adaptation to pneumonectomy is poorly understood, but investigations into mechanisms of lung regeneration in older animals that have lost their capacity for rapid re-alveolarization are warranted, as there would be great translational value in modulating these mechanisms. In addition, quantitative morphometric analysis has progressed in conjunction with developments in advanced imaging, which allow for longitudinal and nonterminal evaluation of pulmonary regenerative responses in animals and humans. This review focuses on the cellular and molecular events that have been observed in animals and humans after pneumonectomy because this model is closest to classical regeneration in other mammalian systems and has revealed several new fronts of translational research that deserve consideration.

The roles of endogenous retinoid signaling in organ and appendage regeneration

The ability to regenerate injured or lost body parts has been an age-old ambition of medical science. In contrast to humans, teleost fish and urodele amphibians can regrow almost any part of the body with seeming effortlessness. Retinoic acid is a molecule that has long been associated with these impressive regenerative capacities. The discovery 30 years ago that addition of retinoic acid to regenerating amphibian limbs causes "super-regeneration" initiated investigations into the presumptive roles of retinoic acid in regeneration of appendages and other organs. However, the evidence favoring or dismissing a role for endogenous retinoids in regeneration processes remained sparse and ambiguous. Now, the availability of genetic tools to manipulate and visualize the retinoic acid signaling pathway has opened up new routes to dissect its roles in regeneration. Here, we review the current understanding on endogenous functions of retinoic acid in regeneration and discuss key questions to be addressed in future research.

Role of Fat-Soluble Vitamins A and D in the Pathogenesis of Influenza: A New Perspective

Reduced exposure to solar radiation, leading to a deficiency of vitamin D and hence impaired innate immunity, has been suggested as a trigger for influenza viral replication and as an explanation of seasonal influenza. Although this hypothesis accounts for many unexplained facts about the epidemiology of influenza, gaps remain in understanding the pathogenesis and manifestations of the disease. Several observations suggest a role for vitamin A compounds (retinoids) in the disease. This paper presents a new model of the etiopathogenesis of influenza, suggesting that host resistance and susceptibility depend importantly on the ratio of vitamin D to vitamin A activity. Retinoid concentrations within normal physiological limits appear to inhibit influenza pathogenesis whereas higher background concentrations (i.e., very low vitamin D : A ratios) increase the risk of severe complications of the disease. There is also evidence that influenza-induced or preexisting liver disease, diabetes, and obesity worsen the severity of infection, possibly via liver dysfunction and alterations in retinoid metabolism. The model could be tested by determining the presence of retinoids in the secretions of patients with influenza and by studies of retinoid profiles in patients and controls. Potential strategies for prevention and treatment are discussed.

Inflammation induced by lipopolysaccharide does not prevent the vitamin A and retinoic acid-induced increase in retinyl ester formation in neonatal rat lungs

Vitamin A (VA) plays an important role in post-natal lung development and maturation. Previously, we have reported that a supplemental dose of VA combined with 10% of all-trans-retinoic acid (VARA) synergistically increases retinol uptake and retinyl ester (RE) storage in neonatal rat lung, while up-regulating several retinoid homeostatic genes including lecithin:retinol acyltransferase (LRAT) and the retinol-binding protein receptor, stimulated by retinoic acid 6 (STRA6). However, whether inflammation has an impact on the expression of these genes and thus compromises the ability of VARA to increase lung RE content is not clear. Neonatal rats, 7- to 8-d-old, were treated with VARA either concurrently with lipopolysaccharide (LPS; Expt 1) or 12 h after LPS administration (Expt 2); in both studies, lung tissue was collected 6 h after VARA treatment, when RE formation is maximal. Inflammation was confirmed by increased IL-6 and chemokine (C–C motif) ligand 2 (CCL2) gene expression in lung at 6 h and C-reactive protein in plasma at 18 h. In both studies, LPS-induced inflammation only slightly reduced, but did not prevent the VARA-induced increase in lung RE. Quantitative RT-PCR showed that co-administration of LPS with VARA slightly attenuated the VARA-induced increase of LRAT mRNA, but not of STRA6 or cytochrome P450 26B1, the predominant RA hydroxylase in lung. By 18 h post-LPS, expression had subsided and none of these genes differed from the level in the control group. Overall, the present results suggest that retinoid homeostatic gene expression is reduced modestly, if at all, by acute LPS-induced inflammation and that VARA is still effective in increasing lung RE under conditions of moderate inflammation.

Retinoic acid rescues alveolar hypoplasia in the calorie-restricted developing rat lung

Infants born with intrauterine growth retardation (IUGR) are at increased risk of adverse pulmonary outcomes at birth, including meconium aspiration and persistent pulmonary hypertension. Preterm infants with IUGR are at especially high risk of developing bronchopulmonary dysplasia (BPD), a disease hallmarked by alveolar hypoplasia. Although vitamin A supplementation has been shown to decrease the incidence of BPD or death in preterm very low birth weight infants, its potential to reduce BPD or death in preterm infants with IUGR remains unknown. We used a well-characterized rat model of caloric restriction to mimic IUGR and determine the impact of IUGR on lung development. We hypothesized that retinoic acid treatment would preserve alveolar formation through increases in key signaling molecules of the retinoic acid signaling pathway. Our results showed that alveolar hypoplasia caused by caloric restriction can be reversed with refeeding, and that retinoic acid prevents the alveolar hypoplasia coincident with the increased expression of elastin and retinoic acid receptor-α and decreased transforming growth factor-β activity in developing rat lungs. These findings suggest that alveolar hypoplasia attributable to caloric restriction is reversible, and raises the possibility that retinoic acid therapy may prove a useful strategy to prevent adverse pulmonary sequelae such as BPD in preterm infants with IUGR.

Maternal smoking and the retinoid pathway in the developing lung

BACKGROUND

Maternal smoking is a risk factor for pediatric lung disease, including asthma. Animal models suggest that maternal smoking causes defective alveolarization in the offspring. Retinoic acid signaling modulates both lung development and postnatal immune function. Thus, abnormalities in this pathway could mediate maternal smoking effects. We tested whether maternal smoking disrupts retinoic acid pathway expression and functioning in a murine model.

METHODS

Female C57Bl/6 mice with/without mainstream cigarette smoke exposure (3 research cigarettes a day, 5 days a week) were mated to nonsmoking males. Cigarette smoke exposure continued throughout the pregnancy and after parturition. Lung tissue from the offspring was examined by mean linear intercept analysis and by quantitative PCR. Cell culture experiments using the type II cell-like cell line, A549, tested whether lipid-soluble cigarette smoke components affected binding and activation of retinoic acid response elements in vitro.

RESULTS

Compared to tobacco-naïve mice, juvenile mice with tobacco toxin exposure had significantly (P < 0.05) increased mean linear intercepts, consistent with an alveolarization defect. Tobacco toxin exposure significantly (P < 0.05) decreased mRNA and protein expression of retinoic acid signaling pathway elements, including retinoic acid receptor alpha and retinoic acid receptor beta, with the greatest number of changes observed between postnatal days 3-5. Lipid-soluble cigarette smoke components significantly (P < 0.05) decreased retinoic acid-induced binding and activation of the retinoic acid receptor response element in A549 cells.

CONCLUSIONS

A murine model of maternal cigarette smoking causes abnormal alveolarization in association with altered retinoic acid pathway element expression in the offspring. An in vitro cell culture model shows that lipid-soluble components of cigarette smoke decrease retinoic acid response element activation. It is feasible that disruption of retinoic acid signaling contributes to the pediatric lung dysfunction caused by maternal smoking.

Comparison of epithelial differentiation and immune regulatory properties of mesenchymal stromal cells derived from human lung and bone marrow

Mesenchymal stromal cells (MSCs) reside in many organs including lung, as shown by their isolation from fetal lung tissues, bronchial stromal compartment, bronchial-alveolar lavage and transplanted lung tissues. It is still controversial whether lung MSCs can undergo mesenchymal-to-epithelial-transition (MET) and possess immune regulatory properties. To this aim, we isolated, expanded and characterized MSCs from normal adult human lung (lung-hMSCs) and compared with human bone marrow-derived MSCs (BM-hMSCs). Our results show that lung-MSCs reside at the perivascular level and do not significantly differ from BM-hMSCs in terms of immunophenotype, stemness gene profile, mesodermal differentiation potential and modulation of T, B and NK cells. However, lung-hMSCs express higher basal level of the stemness-related marker nestin and show, following in vitro treatment with retinoic acid, higher epithelial cell polarization, which is anyway partial when compared to a control epithelial bronchial cell line. Although these results question the real capability of acquiring epithelial functions by MSCs and the feasibility of MSC-based therapeutic approaches to regenerate damaged lung tissues, the characterization of this lung-hMSC population may be useful to study the involvement of stromal cell compartment in lung diseases in which MET plays a role, such as in chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis.

The retinol signaling pathway in mouse pluripotent P19 cells

atRA (all-trans-retinoic acid), the active metabolite of retinol (vitamin A), is essential for embryogenesis and maintenance of cellular phenotype in adults. Chemicals that interfere with the metabolism of retinol to atRA, therefore, are a human health concern. During development of a screen for disruptors of this signaling pathway, we investigated whether the mouse pluripotent P19 cell metabolizes retinol to atRA and thus can be used in a cell-based screen for disruptors of the pathway. We found that retinol induced the identical pattern of homeobox gene expression as atRA and its precursor, retinal. Retinol was 160-fold less potent than atRA as an inducer, however. In spite of its lower potency, increased Hoxa1 gene expression was detected 30 min after retinol exposure and increased 40-fold by 2 h. Rdh10 and Aldh1a2/Raldh2, which together convert retinol to atRA in the embryo, were the predominant alcohol and aldehyde dehydrogenases expressed in P19 cells. The cell expressed high mRNA levels of retinol binding proteins, Rbp1 and Rbp4, and the 13,14-dihydroretinol saturase, Retsat. It also expressed all Rar and Rxr isotypes, Crabp1&2, the three Cyp26 genes, and both β-carotene-cleaving genes, Bcmo1 and Bco2. The basal expression levels and retinol responsiveness of 25 pathway-related genes were quantitated by RT-qPCR. A test of the Aldh1a2 inhibitor, citral, showed that the disruption of the pathway was easily detected and quantitated showing that the P19 cell provides an in vitro model system for identifying and exploring the mechanism of action of chemicals that interfere with this critical cellular pathway.

All-trans retinoic acid up-regulates Prostaglandin-E Synthase expression in human macrophages

All-trans retinoic acid (ATRA) is a potent retinoid, which has been used successfully in different clinical settings as a potential drug to treat COPD and emphysema. In the present study, we analyzed genes modulated by ATRA by performing mRNA expression array analysis on alveolar macrophages after treatment with ATRA. Here we observed a 375-fold up-regulation of Prostaglandin-E Synthase (microsomal PGES-1, NM_004878 PTGES) which mediates the conversion of prostaglandin H(2) (PGH(2)) to Prostaglandin E(2) (PGE(2)). We furthermore studied the expression of PTGES after treatment with ATRA in human monocyte-derived macrophages (MDMs) and bronchoalveolar lavage (BAL) cells. ATRA up-regulated PTGES mRNA expression in MDMs generated with M-CSF by 2500-fold whereas in M-CSF+IL-13 macrophages the up-regulation was only 20-fold. Similarly, ATRA up-regulated PTGES mRNA expression by factor 1524 in BAL cells. The up-regulation of PTGES mRNA expression by ATRA is both time and dose dependent. IL-13 suppressed the ATRA induced PTGES expression at both mRNA and protein level in MDM and BAL cells. We also observed that LPS acts synergistically with ATRA in MDMs and strongly induces PTGES expression. ATRA had little impact on cyclooxygenase-1 and -2 (COX-1 and -2) expression as compared to PTGES expression under the same experimental conditions. Furthermore, we observed an induction of PGE(2) levels by ATRA in BAL cells. These data indicate that ATRA is a potent inducer of PTGES expression in human macrophages but not in alternatively activated macrophages and suggest that the eicosanoid pathway is important for ATRA action in macrophages.

Receptor-mediated cellular uptake mechanism that couples to intracellular storage

Cells are known to take up molecules through membrane transport mechanisms such as active transport, channels, and facilitated transport. We report here a new membrane transport mechanism that employs neither cellular energy like active transport nor a preexisting electrochemical gradient of the free substrate like channels or facilitated transport. Through this mechanism, cells take up vitamin A bound with high affinity to retinol binding protein (RBP) in the blood. This mechanism is mediated by the RBP receptor STRA6, which defines a new type of cell-surface receptor. STRA6 is essential for the proper functioning of multiple human organs, but the mechanisms that enable and control its cellular vitamin A uptake activity are unknown. We found that STRA6-mediated vitamin A uptake is tightly coupled to specific intracellular retinoid storage proteins, but no single intracellular protein is absolutely required for its transport activity. By developing sensitive real-time monitoring techniques, we found that STRA6 is not only a membrane receptor but also catalyzes vitamin A release from RBP. However, vitamin A released from RBP by STRA6 inhibits further vitamin A release by STRA6 unless specific intracellular retinoid storage proteins relieve this inhibition. This mechanism is responsible for its coupling to intracellular storage proteins. The coupling of uptake to storage provides high specificity in cellular uptake of vitamin A and prevents the excessive accumulation of free vitamin A. We have also identified a robust small-molecule-based technique to specifically stimulate cellular vitamin A uptake. This technique has implications in treating human diseases.

Prenatal administration of retinoic acid upregulates connective tissue growth factor in the nitrofen CDH model

PURPOSE

Recent studies have suggested that retinoids may be involved in the molecular mechanisms of pulmonary hypoplasia (PH) in congenital diaphragmatic hernia (CDH). Connective tissue growth factor (CTGF) plays a key role in foetal lung development and remodelling during later gestation. CTGF knockout mice exhibit PH with similar characteristics to the human and nitrofen-induced PH. Prenatal administration of retinoic acid (RA) has been shown to stimulate alveologenesis in nitrofen-induced PH. In vitro studies have revealed that RA can induce CTGF gene expression. We hypothesized that pulmonary gene expression of CTGF is downregulated during the later stages of lung development, and that prenatal administration of RA upregulates CTGF in the nitrofen CDH model.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 (D9) of gestation. RA was given intraperitoneally on D18, D19 and D20. Foetuses were harvested on D21 and divided into control, CDH, control + RA and CDH + RA group. Pulmonary CTGF gene and protein expression levels were determined using RT-PCR and immunohistochemistry.

RESULTS

On D21, CTGF relative mRNA expression levels were significantly downregulated in CDH group compared to controls. After RA treatment, expression levels of CTGF were significantly upregulated in CDH + RA and control + RA compared to the CDH group. Immunohistochemical studies confirmed these results.

CONCLUSION

Downregulation of pulmonary CTGF gene and protein expression during later stages of lung development may interfere with normal alveologenesis in the nitrofen CDH model. Upregulation of CTGF pulmonary gene expression after prenatal RA treatment may promote lung growth by promoting alveologenesis in the nitrofen-induced CDH model.

[A study of the metabolic pathways of vitamin A in the fetal human lung]

INTRODUCTION

Retinoic acid plays an essential role in lung development and is involved in all stages of embryogenesis and morphogenesis. We aimed to determine whether the human foetal lung is able to synthesize retinoic acid.

METHODS

ADH3, RALDH1 and CYP26B1 RNA were studied by qualitative and semi-quantitative RT-PCR in human lungs at different stages of development. In human alveolar epithelial cells (lines A549), RAR beta (induced by retinoic acid) was quantified, after treatment by retinol, by q-PCR at 24h and 48 h.

RESULTS

The expression of the RNA of ADH3, RALDH1 and CYP26B1 was detected for each of the four stages studied and in the A549 cell line. Only the level of RALDH1 RNA changed during the course of lung development. In the A549 cell line, treatment by retinol induced transcription of the RAR beta gene at 24 and 48 hours.

CONCLUSION

The presence of ADH3, RALDH1 and CYP26B1 during the four stages of normal lung development and in the A549 cell line, as well as the capacity to convert retinol to retinoic acid in these cells, indicate that foetal human lung has the ability to regulate the supply of vitamin A from the pseudoglandular stage.

Prenatal retinoic acid treatment upregulates late gestation lung protein 1 in the nitrofen-induced hypoplastic lung in late gestation

PURPOSE

Pulmonary hypoplasia (PH), the leading cause of mortality in congenital diaphragmatic hernia (CDH), is associated with arrested alveolarization. Late gestation lung protein 1 (LGL1) plays a crucial role in the regulation of alveolarization. Inhibition of LGL1 impairs alveolar maturation in fetal rat lungs. LGL1 heterozygotus knockout mice display delayed lung maturation. It is well known that prenatal administration of retinoic acid (RA) stimulates alveologenesis in nitrofen-induced PH. In vitro studies have reported that RA is a key modulator of LGL1 during alveologenesis. We hypothesized, that pulmonary gene expression of LGL1 is downregulated in the late stage of lung development, and that prenatal administration of RA upregulates pulmonary LGL1 expression in the nitrofen CDH model.

METHODS

Pregnant rats were exposed to nitrofen on day 9 (D9) of gestation. RA was given intraperitoneally on D18, D19 and D20. Fetal lungs were dissected on D21 and divided into control, control + RA, CDH and CDH + RA group. Expression levels of LGL1 were determined using RT-PCR and immunohistochemistry.

RESULTS

On D21, LGL1 relative mRNA expression levels were significantly downregulated in CDH group compared to controls. After RA treatment, gene expression levels of LGL1 were significantly upregulated in CDH + RA and control + RA compared to CDH group. Immunohistochemical studies confirmed these results.

CONCLUSION

Downregulation of pulmonary LGL1 gene expression in the late stage of lung development may interfere with normal alveologenesis. Upregulation of LGL1 pulmonary gene expression after RA treatment may promote lung growth by stimulating alveologenesis in the nitrofen CDH model.

Effect of retinoic acid on renal development in newborn mice treated with an angiogenesis inhibitor

BACKGROUND

A mouse model of impaired renal development was developed and the effect of retinoic acid (RA) was investigated in this animal model.

METHODS

An angiogenesis inhibitor (SU1498) was injected s.c. into day 3 C57BL/6 newborn mice to create a model of arrested renal development. RA (2 mg/kg) was injected i.p. for 10 days. Morphometry and immunohistochemistry were done.

RESULTS

Mice injected with SU1498 demonstrated deranged renal development in tubular structure and glomerular tuft area. Cortical thickness and area of glomerular tuft were significantly decreased after vascular endothelial growth factor (VEGF) inhibitor, and were significantly restored by RA. The length of capillary loops/glomerulus, the number of podocytes/glomerulus, and density of peritubular capillaries on CD31 immunostaining were significantly decreased by VEGF blocking and recovered by RA.

CONCLUSIONS

VEGF plays a major role in renal development, and RA reverses the inhibited development caused by an angiogenesis inhibitor.