The components of VARA, a nutrient-metabolite combination of vitamin A and retinoic acid, act efficiently together and separately to increase retinyl esters in the lungs of neonatal rats

Genetic dissection in mice reveals a dynamic crosstalk between the delivery pathways of vitamin A

Vitamin A is distributed within the body to support chromophore synthesis in the eyes and retinoid signaling in most other tissues. Two pathways exist for the delivery of vitamin A: the extrinsic pathway transports dietary vitamin A in lipoproteins from intestinal enterocytes to tissues, while the intrinsic pathway distributes vitamin A from hepatic stores bound to serum retinol-binding protein. Previously, the transcription factor ISX and the retinol binding protein receptor STRA6 were identified as gatekeepers of these pathways; however, it is not clear how mutations in the corresponding genes affect retinoid homeostasis. Here, we used a genetic dissection approach in mice to examine the contributions of these proteins in select tissues. We observed that ISX-deficiency increased utilization of both preformed and pro-vitamin A. We found that increased storage of retinoids in peripheral tissues of ISX-deficient mice was dependent on STRA6 and induced by retinoid signaling. In addition, double mutant mice exhibited a partial rescue of the Stra6 mutant ocular phenotype. This rescue came at the expense of a massive accumulation of vitamin A in other tissues, demonstrating that vitamin A is randomly distributed when present in excessive amounts. Remarkably, pro-vitamin A supplementation of mutant mice induced the expression of the retinol-binding protein receptor 2 in the liver and was accompanied by increased hepatic retinyl ester stores. Taken together, these findings indicate dynamic crosstalk between the delivery pathways for this essential nutrient and suggest that hepatic reuptake of vitamin A takes place when excessive amounts circulate in the blood.

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.

Priming with Retinoic Acid, an Active Metabolite of Vitamin A, Increases Vitamin A Uptake in the Small Intestine of Neonatal Rats

Given that combined vitamin A (VA) and retinoic acid (RA) supplementation stimulated the intestinal uptake of plasma retinyl esters in neonatal rats, we administrated an RA dose as a pretreatment before VA supplementation to investigate the distinct effect of RA on intestinal VA kinetics. On postnatal days (P) 2 and 3, half of the pups received an oral dose of RA (RA group), while the remaining received canola oil as the control (CN). On P4, after receiving an oral dose of ³H-labeled VA, pups were euthanized at selected times (n = 4–6/treatment/time) and intestine was collected. In both CN and RA groups, intestinal VA mass increased dramatically after VA supplementation; however, RA-pretreated pups had relatively higher VA levels from 10 h and accumulated 30% more VA over the 30-h study. Labeled VA rapidly peaked in the intestine of CN pups and then declined from 13 h, while a continuous increase was observed in the RA group, with a second peak at 10 h and nearly twice the accumulation of ³H-labeled VA compared to CN. Our findings indicate that RA pretreatment may stimulate the influx of supplemental VA into the intestine, and the increased VA accumulation suggests a potential VA storage capacity in neonatal intestine.

Metabolism of Neonatal Vitamin A Supplementation: A Systematic Review

A systematic review was conducted to summarize the absorption, transport, storage, and metabolism of oral neonatal vitamin A supplementation (NVAS). This review focused specifically on the neonatal period (first 28 d of life for humans) to inform guidance by WHO on recommendations related to NVAS. A systematic search of international and regional databases was conducted. Inclusion criteria were human or animal studies that gave oral vitamin A as a single or limited number of doses to apparently healthy neonates. Studies evaluating fortification or food-based approaches, dosing with retinoic acid, or studies of neonatal models of disease were excluded. The search retrieved 8847 unique records. After screening by title and abstract, 88 were screened using the full text, and 35 records met inclusion criteria: 13 human and 22 animal studies. Studies indicate that high-dose NVAS is absorbed well by neonates, typically mirroring fat absorption. Doses were primarily stored in the liver and transiently increased in the lung, kidney, spleen, adrenal glands, brain, skin, and adipose tissue, generally with a dose-response. Serum retinol and retinyl esters also transiently increased following NVAS. Although minimal acute adverse effects are noted, there is a lack of data supporting NVAS for improving organ maturation or sustained delivery to target organs. Research gaps include the physiological effects of the short-term increase of vitamin A concentrations in extrahepatic tissues, or whether there are unknown adverse effects over time.

EARLY VITAMIN A SUPPLEMENTATION IMPROVES THE OUTCOME OF RETINOPATHY OF PREMATURITY IN EXTREMELY PRETERM INFANTS

PURPOSE

This study assessed the efficacy and safety of early vitamin A (VA) supplementation to improve outcomes of retinopathy of prematurity in extremely preterm infants.

METHODS

A total of 262 eligible extremely preterm infants underwent randomization; of these, 132 were assigned to the VA group and 130 to the control group. The infants were administered a solution of VA (1,500 IU/day), added to their enteral feeds as soon as minimal feeding was introduced and continued for 28 days or until discharge.

RESULTS

With no adverse effects occurring, serum VA of the VA-supplemented infants on Days 14, 28, and postmenstrual 36 weeks was higher than that of the placebo group (P < 0.001). No signs of VA toxicity or increased intracranial pressure were reported. The VA group had lower unadjusted rates of Type 1 retinopathy of prematurity (1.6 vs. 6.9%, P = 0.030) and bronchopulmonary dysplasia (18.9 vs. 33.8%, P = 0.008) than the control group. Regression analysis revealed an association between serum VA levels and risk of Type 1 retinopathy of prematurity (beta = -2.37).

CONCLUSION

Vitamin A supplementation reduced VA deficiency in extremely preterm infants; it was associated with a decreased incidence of Type 1 retinopathy of prematurity and may also have a positive impact on reducing bronchopulmonary dysplasia.

Vitamin A supplement after neonatal Streptococcus pneumoniae pneumonia inhibits the progression of experimental asthma by altering CD4+T cell subsets

Studies demonstrated that pneumonia can decrease vitamin A productions and vitamin A reduction/deficiency may promote asthma development. Our previous study showed that neonatal Streptococcus pneumoniae (S. pneumoniae) infection promoted asthma development. Whether neonatal S. pneumoniae pneumonia induced asthma was associated with vitamin A levels remains unclear. The aim of this study was to investigate the effects of neonatal S. pneumoniae pneumonia on vitamin A expressions, to explore the effects of vitamin A supplement after neonatal S. pneumoniae pneumonia on adulthood asthma development. Non-lethal S. pneumoniae pneumonia was established by intranasal inoculation of neonatal (1-week-old) female BALB/c mice with D39. S. pneumoniae pneumonia mice were supplemented with or without all-trans retinoic acid 24 hours after infection. Vitamin A concentrations in lung, serum and liver were measured post pneumonia until early adulthood. Four weeks after pneumonia, mice were sensitized and challenged with OVA to induce allergic airway disease (AAD). Twenty-four hours after the final challenge, the lungs and bronchoalveolar lavage fluid (BALF) were collected to assess AAD. We stated that serum vitamin A levels in neonatal S. pneumoniae pneumonia mice were lower than 0.7µmol/L from day 2-7 post infection, while pulmonary vitamin A productions were significantly lower than those in the control mice from day 7-28 post infection. Vitamin A supplement after neonatal S. pneumoniae pneumonia significantly promoted Foxp3⁺Treg and Th1 productions, decreased Th2 and Th17 cells expressions, alleviated airway hyperresponsiveness (AHR) and inflammatory cells infiltration during AAD. Our data suggest that neonatal S. pneumoniae pneumonia induce serum vitamin A deficiency and long-time lung vitamin A reduction, vitamin A supplement after neonatal S. pneumoniae pneumonia inhibit the progression of asthma by altering CD4⁺T cell subsets.

Vitamin A Protects the Preterm Lamb Diaphragm Against Adverse Effects of Mechanical Ventilation

Background: Preterm infants are deficient in vitamin A, which is essential for growth and development of the diaphragm. Preterm infants often require mechanical ventilation (MV) for respiratory distress. In adults, MV is associated with the development of ventilation-induced diaphragm dysfunction and difficulty weaning from the ventilator. We assessed the impact of MV on the preterm diaphragm and the protective effect of vitamin A during MV.

Methods: Preterm lambs delivered operatively at ∼131 days gestation (full gestation: 150 days) received respiratory support by synchronized intermittent mandatory ventilation for 3 days. Lambs in the treated group received daily (24 h) enteral doses of 2500 IU/kg/day vitamin A combined with 250 IU/kg/day retinoic acid (VARA) during MV, while MV control lambs received saline. Unventilated fetal reference lambs were euthanized at birth, without being allowed to breathe. The fetal diaphragm was collected to quantify mRNA levels of myosin heavy chain (MHC) isoforms, atrophy genes, antioxidant genes, and pro-inflammatory genes; to determine ubiquitin proteasome pathway activity; to measure the abundance of protein carbonyl, and to investigate metabolic signaling.

Results: Postnatal MV significantly decreased expression level of the neonatal MHC gene but increased expression level of MHC IIx mRNA level (p < 0.05). Proteasome activity increased after 3 days MV, accompanied by increased MuRF1 mRNA level and accumulated protein carbonyl abundance. VARA supplementation decreased proteasome activity and FOXO1 signaling, down-regulated MuRF1 expression, and reduced reactive oxidant production.

Conclusion: These findings suggest that 3 days of MV results in abnormal myofibrillar composition, activation of the proteolytic pathway, and oxidative injury of diaphragms in mechanically ventilated preterm lambs. Daily enteral VARA protects the preterm diaphragm from these adverse effects.

Direct and indirect vitamin A supplementation strategies result in different plasma and tissue retinol kinetics in neonatal rats

Many questions remain regarding vitamin A (VA) supplementation of infants. Herein we compared direct oral VA supplementation of the neonate and indirect treatment through maternal dietary VA (M-VA) treatment on VA status and kinetics in neonatal rats. Treatments included direct VA combined with retinoic acid (RA) [D-VARA; VA (6 mg/kg) + 10% RA, given orally to neonates on postnatal day (P)2 and P3] and indirect VA supplementation through increased M-VA, compared with each other and oil-treated neonates. [(3)H]retinol was administered orally to all neonates on P4. Plasma and tissue [(3)H]retinol kinetics were determined from 1 h to 14 days post-dosing. D-VARA versus placebo dramatically increased liver and lung retinol, but only in the first 8-10 days. In M-VA neonates, liver and lung VA increased progressively throughout the study. Compartmental modeling of plasma [(3)H]retinol showed that both D-VARA and indirect M-VA reduced retinol recycling between plasma and tissues. Compartmental models of individual tissues predicted that D-VARA stimulated the uptake of VA in chylomicrons to extrahepatic tissues, especially intestine, while the uptake was not observed in M-VA neonates. In conclusion, indirect maternal supplementation had a greater sustained effect than D-VARA on neonatal VA status, while also differentially affecting plasma and tissue retinol kinetics.

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.

Dynamics of gut microbiota in autoimmune lupus

Gut microbiota has been recognized as an important environmental factor in health, as well as in metabolic and immunological diseases, in which perturbation of the host gut microbiota is often observed in the diseased state. However, little is known on the role of gut microbiota in systemic lupus erythematosus. We investigated the effects of host genetics, sex, age, and dietary intervention on the gut microbiome in a murine lupus model. In young, female lupus-prone mice resembling women at childbearing age, a population with the highest risk for lupus, we found marked depletion of lactobacilli, and increases in Lachnospiraceae and overall diversity compared to age-matched healthy controls. The predicted metagenomic profile in lupus-prone mice showed a significant enrichment of bacterial motility- and sporulation-related pathways. Retinoic acid as a dietary intervention restored lactobacilli that were downregulated in lupus-prone mice, and this correlated with improved symptoms. The predicted metagenomes also showed that retinoic acid reversed many lupus-associated changes in microbial functions that deviated from the control. In addition, gut microbiota of lupus-prone mice were different between sexes, and an overrepresentation of Lachnospiraceae in females was associated with an earlier onset of and/or more severe lupus symptoms. Clostridiaceae and Lachnospiraceae, both harboring butyrate-producing genera, were more abundant in the gut of lupus-prone mice at specific time points during lupus progression. Together, our results demonstrate the dynamics of gut microbiota in murine lupus and provide evidence to suggest the use of probiotic lactobacilli and retinoic acid as dietary supplements to relieve inflammatory flares in lupus patients.

Compartmental modeling of whole-body vitamin A kinetics in unsupplemented and vitamin A-retinoic acid-supplemented neonatal rats

Little is known about the contribution of different tissues to whole-body vitamin A (VA) kinetics in neonates. Here, we have used model-based compartmental analysis of tissue tracer kinetic data from unsupplemented (control) and VA-retinoic acid (VARA)-supplemented neonatal rats to determine VA kinetics in specific tissues under control and supplemented conditions. First, compartmental models for retinol kinetics were developed for individual tissues, and then an integrated compartmental model incorporating all tissues was developed for both groups. The models predicted that 52% of chylomicron (CM) retinyl ester was cleared by liver in control pups versus 22% in VARA-treated pups, whereas about 51% of VA was predicted to be extrahepatic in 4- to 6-day-old unsupplemented neonatal rats. VARA increased CM retinyl ester uptake by lung, carcass, and intestine; decreased the release into plasma of retinol that had been cleared by liver and lung as CM retinyl esters; stimulated the uptake of retinol from plasma holo-retinol binding protein into carcass; and decreased the retinol turnover out of the liver. Overall, neonatal VA trafficking differed from that previously described for adult animals, with a larger contribution of extrahepatic tissues to CM clearance, especially after VA supplementation, and a significant amount of VA distributed in extrahepatic tissues.

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.

Acidic retinoids in small amounts promote retinyl ester formation in neonatal lung, with transient increases in retinoid homeostatic gene expression

BACKGROUND

Mixing a small proportion, 10%, of retinoic acid (RA) into an oral dose of vitamin A (VA) has been shown to markedly increase retinol uptake and retinyl ester (RE) formation in the neonatal lung, as compared to VA given alone. Concomitantly, several retinoid homeostatic genes, lecithin:retinol acyltransferase (LRAT), RA-4-hydroxylase (CYP26B1), and stimulated by retinoic acid gene-6 (STRA6) were upregulated. However, whether multiple doses may act accumulatively and whether less than 10% RA can be used has not been determined.

METHODS

Neonatal rats were treated once on postnatal day (PD) 4 or PD14 with VA alone or VA combined with 10% RA (VARA10%) or a stable analog, Am580 (VAAm10%), or they were treated with multiple doses on PD4, 7, 11, and 14.

RESULTS

RE increased cumulatively with multiple dosing. However, LRAT, CYP26B1 and STRA6 mRNA levels were similar for single and multiple treatments, indicating a transient noncumulative impact on gene expression. Lung RE was elevated with as little as 0.5% RA (P < 0.05) in a single dosing study. Whereas all concentrations of VARA elevated lung RE in single dosing studies, only 10% RA increased lung RE after multiple dosing, suggesting an attenuation of RA action with repeated dosing. In contrast, VAAm10%, 2%, and 1% all significantly increased lung RE after multiple doses (P < 0.05), while also increasing the expression of LRAT and CYP26B1.

CONCLUSIONS

These results indicate that the neonatal lung is very sensitive to acidic retinoid exposure and suggest that a VA combined with a very small fraction of acidic retinoid could be effective in increasing the lung's storage pool of VA.

Retinoic acid and the transcription factor MafB act together and differentially to regulate aggrecan and matrix metalloproteinase gene expression in neonatal chondrocytes

Vitamin A (VA) and its active form, retinoic acid (RA), are regulators of skeletal development and chondrogenesis. MafB, a transcription factor, has been identified as an important mediator in monocyte and osteoclast differentiation. However, the presence and function of MafB in chondrocytes is not clear. In this study, MafB gene expression was regulated by both the VA status of the mother (VA-marginal, adequate, and supplemented diets) and by direct oral supplementation of the neonates with VARA (VA mixed with 10% RA). Expression was highest in neonates of VA-supplemented versus VA-marginal dams (P < 0.05), and in VARA-treated versus placebo-treated neonates across all diet groups (P < 0.05). To examine cellular changes, primary chondrocytes derived from neonatal rat ribs were cultured in the presence of RA for up to 48 h. MafB mRNA exhibited a time- and dose-dependent increase in response to RA, while the induction of MafB mRNA was attenuated by BMS-493, a pan-RAR inverse agonist, implicating RAR signaling in the regulation of MafB. The genetic knockdown of MafB in chondrocytes using siRNA (MafB(SI) chondrocytes) abrogated the RA-induced increase in MafB expression. MafB(SI) chondrocytes expressed higher levels of aggrecan mRNA. Additionally, the increased matrix metalloproteinase (MMP)3 and MMP13 gene expression due to RA was attenuated in MafB(SI) chondrocytes, while total extracellular matrix staining was increased. These results support a role for MafB as a regulator of chondrocyte gene expression and matrix formation via control of aggrecan, MMP3 and MMP13 expression, and indicate an important role for RA in the regulation of MafB.

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.

Perinatal exposure to vitamin A differentially regulates chondrocyte growth and the expression of aggrecan and matrix metalloprotein genes in the femur of neonatal rats

Vitamin A (VA) and its active form, retinoic acid (RA), are regulators of skeletal development. In the present study, we investigated if maternal VA intake during pregnancy and lactation, as well as direct oral supplementation of neonates with VA + RA (VARA) in early life, alters neonatal bone formation and chondrocyte gene expression. Offspring of dams fed 3 levels of VA (marginal, adequate, and supplemented) for 10 wk were studied at birth (P0) and postnatal day 7 (P7). One-half of the newborns received an oral supplement of VARA on P1, P4, and P7. Tissues were collected on P0 and 6 h after the last dose on P7. Pup plasma and liver retinol concentrations were increased by both maternal VA intake and VARA (P < 0.01). Although maternal VA did not affect bone mineralization as assessed by von Kossa staining, newborn femur length was increased with maternal VA (P < 0.05). VARA supplementation of neonates increased the length of the hypertrophic zone only in VA-marginal pups, close to that in neonates from VA-adequate dams, suggesting VARA caused a catching up of growth that was limited by low maternal VA intake. Maternal diet did not alter type X nor type II collagen mRNA. However, VARA-treated pups from VA-supplemented dams had reduced mRNA for aggrecan, a major component of cartilage matrix, and increased mRNA for matrix metalloproteinase (MMP)13, which catalyzes the degradation of aggrecan and collagens. These results suggest that moderately high maternal VA intake combined with neonatal VARA supplementation can reduce the ratio of aggrecan:MMP, which may unfavorably alter early bone development.

Vitamin A and retinoic acid act synergistically to increase lung retinyl esters during normoxia and reduce hyperoxic lung injury in newborn mice

We have shown that vitamin A (VA) and retinoic acid (RA) synergistically increase lung retinyl ester content in neonatal rats. To confirm whether this biochemical synergism attenuates early neonatal hyperoxic lung injury in mice, we exposed newborn C57BL/6 mice to 95% O2 or air from birth to 4 d. The agent [vehicle, VA, RA, or the combination vitamin A+retinoic acid (VARA)] was given orally daily. Lung and liver retinyl ester content was measured, and lung injury and development were evaluated. We observed that lung, but not liver, retinyl ester levels were increased more by VARA than by VA or RA alone. Hyperoxic lung injury was reduced by VA and RA, and more so by VARA. VARA attenuated the hyperoxia-induced increases in macrophage inflammatory protein (MIP)-2 mRNA and protein expression, but did not alter hyperoxia-induced effects on peptide growth factors (PDGF, VEGF, and TGF-beta1). The 4-d exposure to hyperoxia or retinoids did not lead to observable differences in lung development. We conclude that the VARA combination has synergistic effects on lung retinyl ester concentrations and on the attenuation of hyperoxia-induced lung injury in newborn mice, possibly by modulation of inflammatory mediators.

Acidic retinoids synergize with vitamin A to enhance retinol uptake and STRA6, LRAT, and CYP26B1 expression in neonatal lung

Vitamin A (VA) is essential for fetal lung development and postnatal lung maturation. VA is stored mainly as retinyl esters (REs), which may be mobilized for production of retinoic acid (RA). This study was designed 1) to evaluate several acidic retinoids for their potential to increase RE in the lungs of VA-supplemented neonatal rats, and 2) to determine the expression of retinoid homeostatic genes related to retinol uptake, esterification, and catabolism as possible mechanisms. When neonatal rats were treated with VA combined with any one of several acidic retinoids (RA, 9-cis-RA, or Am580, a stable analog of RA), lung RE increased approximately 5-7 times more than after an equal amount of VA alone. Retinol uptake and esterification during the period of absorption correlated with increased expression of both STRA6 (retinol-binding protein receptor) and LRAT (retinol esterification), while a reduction in RE after 12 h in Am580-treated, VA-supplemented rats correlated with a strong and persistent increase in CYP26B1 (RA hydroxylase). We conclude that neonatal lung RE can be increased synergistically by VA combined with both natural and synthetic acidic retinoids, concomitant with induction of the dyad of STRA6 and LRAT. However, the pronounced and prolonged induction of CYP26B1 by Am580 may counteract lung RE accumulation after the absorption process is completed.

Effects of maternal retinoic acid administration in a congenital diaphragmatic hernia rabbit model

Maternal retinoid administration has beneficial effects on lung development in the nitrofen rodent toxic model of congenital diaphragmatic hernia (DH). We wanted to investigate the effects in a surgical model, where the retinoid signaling pathway is not primarily disrupted by the toxic agent. We created DH in fetal rabbits at day 23 of gestation, administrated to the does all trans-retinoic acid (ATRA) or vehicle (VHC) intramuscularly for 8 consecutive days and harvested normal and operated (DH) fetuses at 31 d (n = 7 in each group). Normal lungs exposed to ATRA had increased surfactant protein mRNA levels without change in type II pneumocyte density. There was no measurable effect on lung-to-body weight ratio and airway morphometry by ATRA. In DH lungs (DH/VHC) surfactant protein mRNA levels were increased, as well as the density of type II pneumocytes. When supplemented with ATRA (DH/ATRA) these parameters returned to normal (VHC). Cell proliferation or apoptosis were not influenced by ATRA supplementation. In conclusion, maternal ATRA supplementation does not affect gross anatomic, morphologic or proliferation indices in hypoplastic lungs related to surgically induced DH in rabbit. However, ATRA lowers surfactant protein expression and normalizes type I/II pneumocyte ratio to what is observed in normal lungs.

Retinol combined with retinoic acid increases retinol uptake and esterification in the lungs of young adult rats when delivered by the intramuscular as well as oral routes

The lungs require an adequate supply of vitamin A for normal embryonic development, postnatal maturation, and maintenance and repair during adult life. We have previously shown that a nutrient-metabolite combination of vitamin A admixed with a small proportion (10%) of retinoic acid (RA), referred to as VARA, acts synergistically to increase lung retinyl ester (RE) concentration in neonatal rats. A series of studies was designed to test whether VARA increases RE in adult lungs, and whether VARA is more effective than vitamin A when given by the i.m. route. Orally administered VARA increased RE in the lungs of vitamin A-marginal adult rats more than either vitamin A or RA alone (P < 0.05). In vitamin A-deficient young adult rats, lung RE was increased by VARA when administered by the i.m. route. When a tracer of (3)H-retinol was added to the placebo (oil), vitamin A, and VARA doses, total (3)H and (3)H-RE increased in the lungs more with VARA than vitamin A alone, for oral and i.m. dosing. Nevertheless, when VARA and vitamin A were given by the oral route, they were more effective in increasing RE in the liver. Plasma retinol was increased similarly in vitamin A-deficient rats after administration of VARA and vitamin A, by either the oral or the i.m. route. Overall, VARA can increase retinol uptake and esterification in adult lungs when delivered intramuscularly as well as orally.

Lung retinyl ester is low in young adult rats fed a vitamin A deficient diet after weaning, despite neonatal vitamin A supplementation and maintenance of normal plasma retinol

Although it is understood that plasma retinol concentration is not proportional to the concentration of vitamin A stored in liver, plasma retinol still is often used as an indicator of vitamin A status. An aim of vitamin A supplementation strategies is to maintain plasma retinol concentration in a range considered adequate, generally >1.05 micromol/L in humans, with some adjustment for age. In the present study in rats, we addressed the following question: Does lung vitamin A increase postnatally, as is observed in rats fed a vitamin A-adequate diet, if plasma retinol is maintained at approximately 1 micromol/L by supplementation at neonatal age, but the weaning diet is deficient in vitamin A? We treated rats on postnatal d 6, 7, and 8 with placebo (oil), vitamin A, retinoic acid (RA), and a nutrient-metabolite combination of vitamin A and RA, VARA, after which tissues were analyzed on d 9. Other rats treated identically as neonates were fed a vitamin A-deficient diet from 3-9 wk of age, and in parallel, another group of rats was fed a vitamin A-adequate diet. Although supplementation with vitamin A or VARA elevated liver and lung retinyl esters (RE) on d 9 (P < 0.0001), and prevented the fall in plasma retinol to <1 micromol/L by 9 wk of age, when the diet was vitamin A-deficient, lung RE fell to 28% of the concentration present in the lungs of rats fed the vitamin A-adequate diet (P < 0.0001). We infer that the lungs depend, at least in part, on the uptake of dietary vitamin A, probably from chylomicrons, to develop RE stores in the postweaning growth period.

Neonatal-age treatment with vitamin A delays postweaning vitamin A deficiency and increases the antibody response to T-cell dependent antigens in young adult rats fed a vitamin A-deficient diet

Vitamin A supplementation for infants and young children is recommended by WHO/UNICEF for countries with a high prevalence of vitamin A deficiency, and vitamin A is often administered at immunization contacts. Using a rat model, we tested whether supplementation with vitamin A or other retinoids at the time of neonatal immunization has prospective benefit in terms of preventing postweaning vitamin A deficiency and promoting antibody responses to T-cell dependent (TD) antigens administered at the neonatal stage and at the young adult stage. Rats were treated orally on postnatal d 6-8 with oil (placebo control), vitamin A, retinoic acid, or a combination of both (VARA) (n > or = 12/group), and immunized with tetanus toxoid (TT) on d 7. The primary anti-TT response was measured on d 21, after which weanling rats were fed the vitamin A-deficient diet until approximately 10 wk. At 8 wk, rats were immunized again with TT to determine the recall response, and with a novel TD antigen, keyhole limpet hemocyanin (KLH), to assess the adult primary response. None of the supplements affected the plasma titer of anti-TT immunoglobulin G (IgG) on d 21 (P = 0.25). However, neonatal-age supplementation with vitamin A or VARA at the young adult stage resulted in: >5 times higher anti-TT IgG recall response (P < 0.01); 5- and 9-times higher anti-KLH primary IgM and IgG responses, respectively (P < 0.05), and plasma retinol in the normal range (approximately 1.0 micromol/L vs. approximately 0.35 micromol/L in retinoic acid-treated and control groups, P < 0.0001). We conclude that early-life supplementation with vitamin A or VARA can prospectively benefit the primary and recall antibody responses to TD antigens administered at the young adult stage, which may involve the maintenance of normal plasma retinol levels.

Delivery of retinoid-based therapies to target tissues

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