Physiological insights into all-trans-retinoic acid biosynthesis

Retinoids in Cutaneous Squamous Cell Carcinoma

Animal studies as early as the 1920s suggested that vitamin A deficiency leads to squamous cell metaplasia in numerous epithelial tissues including the skin. However, humans usually die from vitamin A deficiency before cancers have time to develop. A recent long-term cohort study found that high dietary vitamin A reduced the risk of cutaneous squamous cell carcinoma (cSCC). cSCC is a form of nonmelanoma skin cancer that primarily occurs from excess exposure to ultraviolet light B (UVB). These cancers are expensive to treat and can lead to metastasis and death. Oral synthetic retinoids prevent the reoccurrence of cSCC, but side effects limit their use in chemoprevention. Several proteins involved in vitamin A metabolism and signaling are altered in cSCC, which may lead to retinoid resistance. The expression of vitamin A metabolism proteins may also have prognostic value. This article reviews what is known about natural and synthetic retinoids and their metabolism in cSCC.

Antiepileptogenic Effect of Retinoic Acid

Retinoic acid, a metabolite of vitamin A, acts through either genomic or nongenomic actions. The genomic action of retinoids exerts effects on gene transcription through interaction with retinoid receptors such as retinoic acid receptors (RARα, β, and γ) and retinoid X receptors (RXRα, β, and γ) that are primarily concentrated in the amygdala, pre-frontal cortex, and hippocampal areas in the brain. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks. Previous experimental studies have reported that retinoic acid exerts an antiepileptogenic effect through diverse mechanisms, including the modulation of gap junctions, neurotransmitters, long-term potentiation, calcium channels and some genes. To our knowledge, there are no previous or current clinical trials evaluating the use of retinoic acid for seizure control.

Vitamin A and Its Multi-Effects on Pancreas: Recent Advances and Prospects

Vitamin A (VA), which is stored in several forms in most tissues, is required to maintain metabolite homeostasis and other processes, including the visual cycle, energy balance, epithelial cell integrity, and infection resistance. In recent years, VA molecules, also known as retinoids, have been extensively explored and used in the treatment of skin disorders and immune-related tumors. To date, several observational and interventional studies have explored the relationship between VA status and the pathogenesis of diabetes. In particular, VA micronutrients have been shown to regulate pancreatic development, β-cell function, pancreatic innate immune responses, and pancreatic stellate cells phenotypes through multiple mechanisms. However, there are still many problems to be proven or resolved. In this review, we summarize and discuss recent and available evidence on VA biological metabolism in the pancreas. Analysis of the effects of VA on metabolism in the pancreas will contribute to our understanding of the supportive physiological roles of VA in pancreas protection.

Aldehyde Oxidase Contributes to All-Trans-Retinoic Acid Biosynthesis in Human Liver

All-trans-retinoic acid (atRA) is a critical endogenous signaling molecule. atRA is predominantly synthesized from retinaldehyde by aldehyde dehydrogenase 1A1 (ALDH1A1), but aldehyde oxidase (AOX) may also contribute to atRA biosynthesis. The goal of this study was to test the hypothesis that AOX contributes significantly to atRA formation in human liver. Human recombinant AOX formed atRA from retinaldehyde (K_m ∼1.5 ± 0.4 µM; k_cat ∼3.6 ± 2.0 minute^-1). In human liver S9 fractions (HLS9), atRA formation was observed in the absence of NAD⁺, suggesting AOX contribution to atRA formation. In the presence of NAD⁺, Eadie-Hofstee plots of atRA formation in HLS9 indicated that two enzymes contributed to atRA formation. The two enzymes were identified as AOX and ALDH1A1 based on inhibition of atRA formation by AOX inhibitor hydralazine (20%-50% inhibition) and ALDH1A1 inhibitor WIN18,446 (50%-80%inhibition). The expression of AOX in HLS9 was 9.4-24 pmol mg^-1 S9 protein, whereas ALDH1A1 expression was 156-285 pmol mg^-1 S9 protein measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) quantification of signature peptides. The formation velocity of atRA in the presence of NAD⁺ correlated significantly with the expression of ALDH1A1 and AOX protein. Taken together, the data show that both AOX and ALDH1A1 contribute to atRA biosynthesis in the human liver, with ALDH1A1 being the high-affinity, low-capacity enzyme and AOX being the low-affinity, high-capacity enzyme. The results suggest that in the case of ALDH1A dysfunction or excess vitamin A, AOX may play an important role in regulating hepatic vitamin A homeostasis and that inhibition of AOX may alter atRA biosynthesis and signaling. SIGNIFICANCE STATEMENT: This study provides direct evidence to show that human AOX converts retinaldehyde to atRA and contributes to hepatic atRA biosynthesis. The finding that AOX may be responsible for 20%-50% of overall hepatic atRA formation suggests that alterations in AOX activity via drug-drug interactions, genetic polymorphisms, or disease states may impact hepatic atRA concentrations and signaling and alter vitamin A homeostasis.

Retinoic acid synthesis by a population of choroidal stromal cells

Choroidal all- trans -retinoic acid (atRA) may play a key role in the control of postnatal eye growth in a variety of vertebrates through modulation of scleral extracellular matrix synthesis and may therefore play a crucial role in the development of myopia. In the chick eye, choroidal atRA synthesis is exclusively regulated by its synthesizing enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In chicks and humans, RALDH2 has been detected in a population of hitherto uncharacterized choroidal cells.Therefore, the aim of this study was to identify the RALDH2+ cell type(s) in the choroid and determine how these cells modulate atRA concentrations during periods of visually guided eye growth. Chicks wore translucent goggles on one eye for 10 days and choroids were analyzed for RALDH activity and RALDH2 protein expression at days 0, 1, 4, 7, 15 following removal of the goggle ("recovery"); choroids from contralateral eyes served as controls. The presence of RALDH2+ cells was assessed in chick choroid wholemounts using multiphoton microscopy. RALDH2 protein expression was measured by western blot and RALDH2 activity was assessed via HPLC quantification of atRA. Cell proliferation was assessed by BrdU-labelling in combination with RALDH2-immunohistochemistry. For characterization of RALDH2+ cells, immunohistochemistry for various tissue specific markers was applied in chicken (Ia antigen, CD5, Col1-propeptide, desmin, IgY, L-Cam, Cadherin1, MHC-II; Tcr-γδ, vimentin) and human donor tissue (α-smooth-muscle-actin, CD's 31/34/68/146, desmin, IBA1, LYVE-1, PGP9.5, vimentin) followed by confocal microscopy. In the chick and human choroid, RALDH2+ cells with variable morphology were present in the stroma and adjacent to choroidal blood vessels. In chick wholemounts, RALDH2+ cells were concentrated toward the choriocapillaris, and their number increased nearly linearly between 1 and 7 days of recovery and plateaued between 7 and 15 days compared to corresponding controls. A significant increase in choroidal RALDH2 protein concentration and atRA synthetic activity was observed by four days of recovery (↑107% and ↑120%) by western blot and HPLC, respectively. A 3-fold increase in RALDH2+/BrDU+ cells was observed following 4 days of recovery compared to controls (12.43 ± 0.73% of all RALDH2+ cells in recovering eyes as compared with 4.46 ± 0.63% in control eyes, p < 0.001). In chick choroids, the vast majority of RALDH2+ cells co-expressed Col1-propetide, but did not co-label with any other antibodies tested. In human choroid, some, but not all RALDH2+ cells colocalized with vimentin, but were negative for all other antibodies tested. RALDH2+ cells represent a novel cell type in the chick and human choroid. Our findings that some human RALDH2+ cells were positive for vimentin and all chick RALDH2+ cells were positive for Col1, suggest that RALDH2+ cells most closely resemble perivascular and stromal fibroblasts. The increased number of RALDH2+/BRDU+ cells following 4 days of recovery suggests that choroidal atRA concentrations are partially controlled by proliferation of RALDH2+ cells. The identification of this choroidal cell type will provide a broader understanding of the cellular events responsible for the regulation of postnatal ocular growth, and may provide new avenues for specifically targeted strategies for the treatment of myopia.

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

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

Carotenoid metabolism at the intestinal barrier

Carotenoids exert a rich variety of physiological functions in mammals and are beneficial for human health. These lipids are acquired from the diet and metabolized to apocarotenoids, including retinoids (vitamin A and its metabolites). The small intestine is a major site for their absorption and bioconversion. From here, carotenoids and their metabolites are distributed within the body in triacylglycerol-rich lipoproteins to support retinoid signaling in peripheral tissues and photoreceptor function in the eyes. In recent years, much progress has been made in identifying carotenoid metabolizing enzymes, transporters, and binding proteins. A diet-responsive regulatory network controls the activity of these components and adapts carotenoid absorption and bioconversion to the bodily requirements of these lipids. Genetic variability in the genes encoding these components alters carotenoid homeostasis and is associated with pathologies. We here summarize the advanced state of knowledge about intestinal carotenoid metabolism and its impact on carotenoid and retinoid homeostasis of other organ systems, including the eyes, liver, and immune system. The implication of the findings for science-based intake recommendations for these essential dietary lipids is discussed. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Proteomic Evaluation of the Natural History of the Acute Radiation Syndrome of the Gastrointestinal Tract in a Non-human Primate Model of Partial-body Irradiation with Minimal Bone Marrow Sparing Includes Dysregulation of the Retinoid Pathway

Exposure to ionizing radiation results in injuries of the hematopoietic, gastrointestinal, and respiratory systems, which are the leading causes responsible for morbidity and mortality. Gastrointestinal injury occurs as an acute radiation syndrome. To help inform on the natural history of the radiation-induced injury of the partial body irradiation model, we quantitatively profiled the proteome of jejunum from non-human primates following 12 Gy partial body irradiation with 2.5% bone marrow sparing over a time period of 3 wk. Jejunum was analyzed by liquid chromatography-tandem mass spectrometry, and pathway and gene ontology analysis were performed. A total of 3,245 unique proteins were quantified out of more than 3,700 proteins identified in this study. Also a total of 289 proteins of the quantified proteins showed significant and consistent responses across at least three time points post-irradiation, of which 263 proteins showed strong upregulations while 26 proteins showed downregulations. Bioinformatic analysis suggests significant pathway and upstream regulator perturbations post-high dose irradiation and shed light on underlying mechanisms of radiation damage. Canonical pathways altered by radiation included GP6 signaling pathway, acute phase response signaling, LXR/RXR activation, and intrinsic prothrombin activation pathway. Additionally, we observed dysregulation of proteins of the retinoid pathway and retinoic acid, an active metabolite of vitamin A, as quantified by liquid chromatography-tandem mass spectrometry. Correlation of changes in protein abundance with a well-characterized histological endpoint, corrected crypt number, was used to evaluate biomarker potential. These data further define the natural history of the gastrointestinal acute radiation syndrome in a non-human primate model of partial body irradiation with minimal bone marrow sparing.

Knockout of Cyp26a1 and Cyp26b1 during postnatal life causes reduced lifespan, dermatitis, splenomegaly, and systemic inflammation in mice

All-trans-retinoic acid (atRA), the active metabolite of vitamin A, is an essential signaling molecule in all chordates. Global knockouts of the atRA clearing enzymes Cyp26a1 or Cyp26b1 are embryonic lethal. In adult rodents, inhibition of Cyp26a1 and Cyp26b1 increases atRA concentrations and signaling. However, postnatal knockout of Cyp26a1 does not cause a severe phenotype. We hypothesized that Cyp26b1 is the main atRA clearing Cyp in postnatal mammals. This hypothesis was tested by generating tamoxifen-inducible knockout mouse models of Cyp26b1 alone or with Cyp26a1. Both mouse models showed dermatitis, blepharitis, and splenomegaly. Histology showed infiltration of inflammatory cells including neutrophils and T lymphocytes into the skin and hyperkeratosis/hyperplasia of the nonglandular stomach. The mice lacking both Cyp26a1 and Cyp26b1 also had a reduced lifespan, failed to gain weight, and showed fat atrophy. There were significant changes in vitamin A homeostasis. Postnatal knockout of Cyp26b1 resulted in increased atRA concentrations in the skin while the postnatal knockout of both Cyp26a1 and Cyp26b1 resulted in increased atRA concentrations in the liver, serum, skin, spleen, and intestines. This study demonstrates the paramount role of Cyp26b1 in regulating retinoid homeostasis in postnatal life.

Collecting duct cells show differential retinoic acid responses to acute versus chronic kidney injury stimuli

Retinoic acid (RA) activates RA receptors (RAR), resulting in RA response element (RARE)-dependent gene expression in renal collecting duct (CD). Emerging evidence supports a protective role for this activity in acute kidney injury (AKI) and chronic kidney disease (CKD). Herein, we examined this activity in RARE-LacZ transgenic mice and by RARE-Luciferase reporter assays in CD cells, and investigated how this activity responds to neurotransmitters and mediators of kidney injury. In RARE-LacZ mice, Adriamycin-induced heavy albuminuria was associated with reduced RA/RAR activity in CD cells. In cultured CD cells, RA/RAR activity was repressed by acetylcholine, albumin, aldosterone, angiotensin II, high glucose, cisplatin and lipopolysaccharide, but was induced by aristolochic acid I, calcitonin gene-related peptide, endothelin-1, gentamicin, norepinephrine and vasopressin. Compared with age-matched normal human CD cells, CD-derived renal cystic epithelial cells from patients with autosomal recessive polycystic kidney disease (ARPKD) had significantly lower RA/RAR activity. Synthetic RAR agonist RA-568 was more potent than RA in rescuing RA/RAR activity repressed by albumin, high glucose, angiotensin II, aldosterone, cisplatin and lipopolysaccharide. Hence, RA/RAR  in CD cells is a convergence point of regulation by neurotransmitters and mediators of kidney injury, and may be a novel therapeutic target.

Prognostic Impact of DHRS9 Overexpression in Pancreatic Cancer

Purpose

Pancreatic cancer (PC) has poor prognosis despite systemic treatment. Dehydrogenase/reductase member 9 (DHRS9) has been reported to be involved in many events of tumorigenesis, but its prognostic impact in PC remains undetermined. Thus, this study aimed to explore the association between DHRS9 expression and the prognosis of PC and investigate the possible mechanism by which DHRS9 is involved in PC progression.

Patients and Methods

The study used data: from Gene Expression Omnibus (GEO) and our institution to compare the DHRS9 expression between PC and adjacent normal tissues; from The Cancer Genome Atlas (TCGA) and our institution to assess the clinicopathological characteristics and prognosis of PC patients in high and low DHRS9 expression groups; and from TCGA to predict the potential mechanism of DHRS9 in PC. Western blot assay was used to identify DHRS9 expression in specimens collected from five patients who underwent surgery in our institute. Furthermore, immunohistochemistry (IHC) was then used to identify DHRS9 expression in the specimens of 109 patients who underwent surgery at our institute. Kaplan–Meier and Cox regression analyses were used to assess the prognostic significance of DHRS9 expression among PC patients.

Results

All the IHC, Western blot, and GEO datasets indicated that compared to normal tissues, DHRS9 was significantly overexpressed in PC tissues. IHC results demonstrated that the strong intensity of DHRS9 expression was significantly correlated with vascular infiltration (P < 0.05). Further, high DHRS9 expression was identified as a prognostic risk factor for overall survival. Functional analysis of DHRS9 co-expressed genes indicated that DHRS9 was involved in mitogen-activated protein kinases/extracellular signal-regulated kinase (MAPK/ERK) signaling pathway.

Conclusion

DHRS9 is upregulated in PC tissue, and high DHRS9 expression is correlated with poor prognosis in PC. DHRS9 may affect the oncological process of PC through MAPK/ERK pathway.

Aberrant retinoic acid production in the decidua: Implications for pre-eclampsia

Fine-tuning of the endometrium during the evanescent 'window of implantation' relies upon an array of diverse and redundant signaling molecules, particularly the ovarian steroids E2 and P4, but also growth factors, eicosanoids, and vitamins including the vitamin A compounds (retinoids). Pregnancy complications such as preeclampsia (PE) can result from aberrations in the production or function of these molecules that arise during this critical period of decidual development. Such aberrations may be reflected by incomplete decidualization, reduced spiral artery modification, and/or loss of immune tolerance to the developing fetus. Our understanding of the role of the active retinoid metabolite all-trans retinoic acid (RA) in maintaining immune balance in certain tissues, along with data describing its role in decidualization, present a compelling argument that aberrant RA signaling in the decidua can play a significant role in the etiology of PE. Recent findings that decidualization and expression of the anti-angiogenic gene product, 'soluble fms-like tyrosine kinase-1' (sFLT1) are negatively correlated and that sFLT1 expression is directly inhibited by RA, provide additional evidence of the critical role of this retinoid in regulating early vascular development in the decidua. This review provides insight into the production and function of RA in the decidua and how modifications in its metabolism and signaling might lead to certain pregnancy disorders such as PE.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Identifying vitamin A signaling by visualizing gene and protein activity, and by quantification of vitamin A metabolites

Vitamin A (retinol) is an essential nutrient for embryonic development and adult homeostasis. Signaling by vitamin A is carried out by its active metabolite, retinoic acid (RA), following a two-step conversion. RA is a small, lipophilic molecule that can diffuse from its site of synthesis to neighboring RA-responsive cells where it binds retinoic acid receptors within RA response elements of target genes. It is critical that both vitamin A and RA are maintained within a tight physiological range to protect against developmental disorders and disease. Therefore, a series of compensatory mechanisms exist to ensure appropriate levels of each. This strict regulation is provided by a number synthesizing and metabolizing enzymes that facilitate the precise spatiotemporal control of vitamin A metabolism, and RA synthesis and signaling. In this chapter we describe protocols that (1) biochemically isolate and quantify vitamin A and its metabolites and (2) visualize the spatiotemporal activity of genes and proteins involved in the signaling pathway.

Vitamin A and its natural derivatives

Vitamin A and derivatives, the natural retinoids, underpin signaling pathways of cellular differentiation, and are key chromophores in vision. These functions depend on transfer across membranes, and carrier proteins to shuttle retinoids to specific cell compartments. Natural retinoids, ultimately derived from plant carotenoids by metabolism to all-trans retinol, are lipophilic and consist of a cyclohexenyl (β-ionone) moiety linked to a polyene chain. This structure constrains the orientation of retinoids within lipid membranes. Cis-trans isomerization at double bonds of the polyene chain and s-cis/s-trans rotational isomerization at single bonds define the functional dichotomy of retinoids (signaling/vision) and specificities of interactions with specific carrier proteins and receptors. Metabolism of all-trans retinol to 11-cis retinal, transfer to photoreceptors, and removal and recycling of all-trans retinal generated by photoreceptor irradiation, is the key process underlying vision. All-trans retinol transferred into cells is metabolized to all-trans retinoic acid and shuttled to the cell nucleus to regulate gene expression controlling organ, tissue and cell differentiation, and cellular homeostasis. Research methods need to address the potential of photoisomerization in vitro to confound research results, and data should be interpreted in the context of membrane-association properties of retinoids and physiological concentrations in vivo. Despite a century of research, there are many fundamental questions of retinoid cellular biochemistry and molecular biology still to be answered. Computational modeling techniques will have an important role for understanding the nuances of vitamin A signaling and function.

Inhibition of GSK3 Represses the Expression of Retinoic Acid Synthetic Enzyme ALDH1A2 via Wnt/β-Catenin Signaling in WiT49 Cells

Organogenesis, including renal development, requires an appropriate retinoic acid concentration, which is established by differential expression of aldehyde dehydrogenase 1 family member A2 (ALDH1A2) and cytochrome P450 family 26 subfamily A/B/C member 1 (CYP26A1/B1/C1). In the fetal kidney, ALDH1A2 expresses in the developing stroma and renal vesicle and its derivatives but does not present in the ureteric bud. It remains unclear what may contribute to this expression pattern. Here we show that the glycogen synthase kinase 3 alpha/beta (GSK3A/B) inhibitor CHIR99021 significantly represses ALDH1A2 expression in WiT49, which is a Wilms’ tumor cell line that exhibits “triphasic” differential potential and is used as a fetal kidney cell model. CHIR99021 fails to suppress ALDH1A2 as β-catenin is inhibited, suggesting that the downregulation of ALDH1A2 by CHIR99021 is through Wnt/β-catenin signaling. Ectopic expression of mouse Wnt1, Wnt3a, Wnt4, and Wnt9b represses ALDH1A2 expression in WiT49 cells. Using immunohistochemistry, we show an inverse correlation of Aldh1a2 expression with β-catenin in rat E18.5 kidney. ChIP demonstrated that β-catenin is recruited to the ALDH1A2 promoter, the conserved intron1G, and another site within intron 1 of ALDH1A2. Using a luciferase assay, we further show that the ALDH1A2 promoter and the intron1G element are involved in the repression of ALDH1A2 expression by CHIR99021. Our work demonstrates that ALDH1A2 expression can be directly repressed by the Wnt/β-catenin signaling in fetal kidney cells, suggesting that Wnt/β-catenin may play a role in maintaining the expression pattern of ALDH1A2 in the fetal kidney, thus controlling the availability and localization of retinoic acid and regulating aspects of kidney development.

Post-natal all-trans-retinoic acid biosynthesis

Generation of the autacoid all-trans-retinoic acid (ATRA) from retinol (vitamin A) relies on a complex metabolon that includes retinol binding-proteins and enzymes from the short-chain dehydrogenase/reductase and aldehyde dehydrogenase gene families. Serum retinol binding-protein delivers all-trans-retinol (vitamin A) from blood to cells through two membrane receptors, Stra6 and Rbpr2. Stra6 and Rbpr2 convey retinol to cellular retinol binding-protein type 1 (Crbp1). Holo-Crbp1 delivers retinol to lecithin: retinol acyl transferase (Lrat) for esterification and storage. Lrat channels retinol directly into its active site from holo-Crbp1 by protein-protein interaction. The ratio apo-Crbp1/holo-Crbp1 directs flux of retinol into and out of retinyl esters, through regulating esterification vs ester hydrolysis. Multiple retinol dehydrogenases (Rdh1, Rdh10, Dhrs9, Rdhe2, Rdhe2s) channel retinol from holo-Crbp1 to generate retinal for ATRA biosynthesis. β-Carotene oxidase type 1 generates retinal from carotenoids, delivered by the scavenger receptor-B1. Retinal reductases (Dhrs3, Dhrs4, Rdh11) reduce retinal into retinol, thereby restraining ATRA biosynthesis. Retinal dehydrogenases (Raldh1, 2, 3) dehydrogenate retinal irreversibly into ATRA. ATRA regulates its own concentrations by inducing Lrat and ATRA degradative enzymes. ATRA exhibits hormesis. Its effects relate to its concentration as an inverted J-shaped curve, transitioning from beneficial in the "goldilocks" zone to toxicity, as concentrations increase. Hormesis has distorted understanding physiological effects of ATRA post-nataly using chow-diet fed, ATRA-dosed animal models. Cancer, immune deficiency and metabolic abnormalities result from mutations and/or insufficiency in Crbp1 and retinoid metabolizing enzymes.

Cyp1b1 directs Srebp-mediated cholesterol and retinoid synthesis in perinatal liver; Association with retinoic acid activity during fetal development

Background

Cytochrome P450 1b1 (Cyp1b1) deletion and dietary retinol deficiency during pregnancy (GVAD) affect perinatal liver functions regulated by Srebp. Cyp1b1 is not expressed in perinatal liver but appears in the E9.5 embryo, close to sites of retinoic acid (RA) signaling.

Hypothesis

Parallel effects of Cyp1b1 and retinol on postnatal Srebp derive from effects in the developing liver or systemic signaling.

Approach

Cluster postnatal increases in hepatic genes in relation to effects of GVAD or Cyp1b1 deletion. Sort expression changes in relation to genes regulated by Srebp1 and Srebp2.Test these treatments on embryos at E9.5, examining changes at the site of liver initiation. Use in situ hybridization to resolve effects on mRNA distributions of Aldh1a2 and Cyp26a1 (RA homeostasis); Hoxb1 and Pax6 (RA targets). Assess mice lacking Lrat and Rbp4 (DKO mice) that severely limits retinol supply to embryos.

Results

At birth, GVAD and Cyp1b1 deletion stimulate gene markers of hepatic stellate cell (HSC) activation but also suppress Hamp. These treatments then selectively prevent the postnatal onset of genes that synthesize cholesterol (Hmgcr, Sqle) and fatty acids (Fasn, Scd1), but also direct cholesterol transport (Ldlr, Pcsk9, Stard4) and retinoid synthesis (Aldh1a1, Rdh11). Extensive support by Cyp1b1 is implicated, but with distinct GVAD interventions for Srebp1 and Srebp2. At E9.5, Cyp1b1 is expressed in the septum transversum mesenchyme (STM) with β-carotene oxygenase (Bco1) that generates retinaldehyde. STM provides progenitors for the HSC and supports liver expansion. GVAD and Cyp1b1-/- do not affect RA-dependent Hoxb1 and Pax6. In DKO embryos, RA-dependent Cyp26a1 is lost but Hoxb1 is sustained with Cyp1b1 at multiple sites.

Conclusion

Cyp1b1-/- suppresses genes supported by Srebp. GVAD effects distinguish Srebp1 and Srebp2 mediation. Srebp regulation overlaps appreciably in cholesterol and retinoid homeostasis. Bco1/Cyp1b1 partnership in the STM may contribute to this later liver regulation.

Retinoic Acid Is Required for Oligodendrocyte Precursor Cell Production and Differentiation in the Postnatal Mouse Corpus Callosum

Myelination of the CNS relies on the production and differentiation of oligodendrocyte (OL) precursor cells (OPCs) into mature OLs. During the first month of postnatal life, OPCs that populate the corpus callosum (CC) arise from neural stem cells (NSCs) in the subcallosal subventricular zone (SVZ), and then differentiate to generate myelinating OLs. However, the signals that regulate these processes are not fully understood.

Myelination of the CNS relies on the production and differentiation of oligodendrocyte (OL) precursor cells (OPCs) into mature OLs. During the first month of postnatal life, OPCs that populate the corpus callosum (CC) arise from neural stem cells (NSCs) in the subcallosal subventricular zone (SVZ), and then differentiate to generate myelinating OLs. However, the signals that regulate these processes are not fully understood. In this study, we show that endogenous expression of the retinoic acid (RA)-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2) is required for OPC generation and differentiation in the postnatal subcortical white matter. In male and female pups, conditional deletion of Raldh2 reduced OPC numbers and differentiation. Moreover, decreased OPC numbers coincided with reductions in NSC survival and expression of the sonic hedgehog (SHH) signaling effector protein Gli1 in the SVZ. Additionally, GFAP expression in the CC was decreased, and cortical neuron numbers were altered. Our work suggests a role for endogenous RALDH2-dependent RA synthesis in OPC production and differentiation in the CC, as well as in the development of other cell types derived from NSCs in the embryonic ventricular zone (VZ) and SVZ, as well as the postnatal subcallosal SVZ.

The Antagonist of Retinoic Acid Receptor α, ER-50891 Antagonizes the Inhibitive Effect of All-Trans Retinoic Acid and Rescues Bone Morphogenetic Protein 2-Induced Osteoblastogenic Differentiation

Background

Hypervitaminosis A, alcoholism or medical treatment for acute promyelocytic leukaemia may cause unphysiologically high accumulation of all-trans retinoic acid (ATRA), which could inhibit osteoblastogenesis, thereby triggering osteoporosis. We have shown that bone morphogenetic protein-2 (BMP-2) can only partially antagonize the inhibitive effects of ATRA. In this study, we hypothesized that antagonists of retinoic acid receptors (RARs) could further antagonize the inhibitive effect of ATRA and rescue BMP2-induced osteoblastogenesis.

Materials and Methods

We first screened the dose-dependent effects of the specific antagonists of RAR α, β and γ and transforming growth factor-beta receptor (ER-50891, LE-135, MM11253, and SB-43142, respectively) on ATRA-induced inhibition of the total cell metabolic activity and proliferation of preosteoblasts. We selected ER-50891 and tested its effects on osteoblastogenesis with the presence or absence of 1 μM ATRA and/or 200 ng/mL BMP-2. We measured the following parameters: Alkaline phosphatase activity (ALP), osteocalcin (OCN) expression and extracellular matrix mineralization as well as the level of phosphorylated Smad1/5.

Results

ER-50891 but not LE-135, MM11253, or SB-431542 significantly antagonized the inhibition of ATRA and enhanced the total cell metabolic activity and proliferation of preosteoblasts. Dose-dependent assays show ER-50891 could also rescue ATRA inhibited OCN expression and mineralization with or without the induction of BMP. ER-50891 also suppressed the ALP activity that was synergistically enhanced by BMP and ATRA. Neither ATRA, nor ER-50891 or their combination significantly affected the level of BMP-induced phosphorylated Smad1/5.

Conclusion

The antagonist of RARα, ER-50891 could significantly attenuate ATRA’s inhibitive effects on BMP 2-induced osteoblastogenesis.

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.

New Insights into the Control of Cell Fate Choices and Differentiation by Retinoic Acid in Cranial, Axial and Caudal Structures

Retinoic acid (RA) signaling is an important regulator of chordate development. RA binds to nuclear RA receptors that control the transcriptional activity of target genes. Controlled local degradation of RA by enzymes of the Cyp26a gene family contributes to the establishment of transient RA signaling gradients that control patterning, cell fate decisions and differentiation. Several steps in the lineage leading to the induction and differentiation of neuromesodermal progenitors and bone-producing osteogenic cells are controlled by RA. Changes to RA signaling activity have effects on the formation of the bones of the skull, the vertebrae and the development of teeth and regeneration of fin rays in fish. This review focuses on recent advances in these areas, with predominant emphasis on zebrafish, and highlights previously unknown roles for RA signaling in developmental processes.

Biochemical and physiological importance of the CYP26 retinoic acid hydroxylases

The Cytochrome P450 (CYP) family 26 enzymes contribute to retinoic acid (RA) metabolism and homeostasis in humans, mammals and other chordates. The three CYP26 family enzymes, CYP26A1, CYP26B1 and CYP26C1 have all been shown to metabolize all-trans-retinoic acid (atRA) it's 9-cisRA and 13-cisRA isomers and primary metabolites 4-OH-RA and 4-oxo-RA with high efficiency. While no crystal structures of CYP26 enzymes are available, the binding of various ligands has been extensively explored via homology modeling. All three CYP26 enzymes are inducible by treatment with atRA in various prenatal and postnatal tissues and cell types. However, current literature shows that in addition to regulation by atRA, CYP26 enzyme expression is also regulated by other endogenous processes and inflammatory cytokines. In humans and in animal models the expression patterns of CYP26 enzymes have been shown to be tissue and cell type specific, and the expression of the CYP26 enzymes is believed to regulate the formation of critical atRA concentration gradients in various tissue types. Yet, very little data exists on direct disease associations of altered CYP26 expression or activity. Nevertheless, data is emerging describing a variety of human genetic variations in the CYP26 enzymes that are associated with different pathologies. Interestingly, some of these genetic variants result in increased activity of the CYP26 enzymes potentially leading to complex gene-environment interactions due to variability in dietary intake of retinoids. This review highlights the current knowledge of structure-function of CYP26 enzymes and focuses on their role in human retinoid metabolism in different tissues.

Cycles, waves, and pulses: Retinoic acid and the organization of spermatogenesis

Background

Spermatogenesis in mammals is organized in a manner that maximizes sperm production. The central aspect of this organization is the cycle of the seminiferous epithelium that is characterized by an asynchronous repeating series of germ cell associations. These cell associations are the result of a fixed point of entry into the cycle at regular short time intervals and the longer time required for cells to fully differentiate and exit the cycle.

Objective

This review will examine the current information on the action and metabolism of retinoic acid in the testis, the interaction of retinoic acid (RA) with the cycle and the spermatogenic wave, and the mechanisms that can lead to synchronous spermatogenesis. Finally, the unique applications of synchronous spermatogenesis to the study of the cycle and the mass isolation of specific germ cell populations are described.

Materials and methods

Retinoic acid metabolism and spermatogonial differentiation have been examined by gene deletions, immunocytochemistry, chemical inhibitors, and mass spectrometry.

Results, discussion, and conclusion

Both the Sertoli cells and the germ cells have the capacity to synthesize retinoic acid from retinol and in the mouse the entry into the cycle of the seminiferous epithelium, and the subsequent conversion of undifferentiated spermatogonia into differentiating spermatogonia is governed by a peak of RA synthesis occurring at stages VIII‐IX of the cycle. Normal asynchronous spermatogenesis can be modified by altering RA levels, and as a result the entire testis will consist of a few closely related stages of the cycle.

AlloFinder: a strategy for allosteric modulator discovery and allosterome analyses

Allostery tweaks innumerable biological processes and plays a fundamental role in human disease and drug discovery. Exploration of allostery has thus been regarded as a crucial requirement for research on biological mechanisms and the development of novel therapeutics. Here, based on our previously developed allosteric data and methods, we present an interactive platform called AlloFinder that identifies potential endogenous or exogenous allosteric modulators and their involvement in human allosterome. AlloFinder automatically amalgamates allosteric site identification, allosteric screening and allosteric scoring evaluation of modulator-protein complexes to identify allosteric modulators, followed by allosterome mapping analyses of predicted allosteric sites and modulators in human proteome. This web server exhibits prominent performance in the reemergence of allosteric metabolites and exogenous allosteric modulators in known allosteric proteins. Specifically, AlloFinder enables identification of allosteric metabolites for metabolic enzymes and screening of potential allosteric compounds for disease-related targets. Significantly, the feasibility of AlloFinder to discover allosteric modulators was tested in a real case of signal transduction and activation of transcription 3 (STAT3) and validated by mutagenesis and functional experiments. Collectively, AlloFinder is expected to contribute to exploration of the mechanisms of allosteric regulation between metabolites and metabolic enzymes, and to accelerate allosteric drug discovery. The AlloFinder web server is freely available to all users at http://mdl.shsmu.edu.cn/ALF/.

All-trans retinoic acid and COX-2 cross-talk to regulate BMP9-induced osteogenic differentiation via Wnt/β-catenin in mesenchymal stem cells

COX-2 specific inhibitor, which has been widely used, can delay bone fracture healing and reduce osteogenic potential of bone marrow stromal cells. However, it remains unknown how to prevent these side-effects of COX-2 inhibitor. In this study, we introduced BMP9-induced osteogenic differentiation as model to evaluate whether all-trans retinoic acid (ATRA) could ameliorate these adverse effects of COX-2 specific inhibitor on bone metabolism with in vitro and in vivo experiments, and uncover the possible mechanism underlying this process. Results showed that ATRA enhanced the potential of BMP9 to induce the osteogenic markers, such as alkaline phosphates (ALP) and mineralization; but retinoic acid receptor a (RARa) inhibitor showed the reversal effects. COX-2 specific inhibitor (NS398) reduced the osteogenic markers induced by BMP9, and ATRA almost eliminated the inhibitory effect of NS398. BMP9 up-regulated the protein level of β-catenin and promoted it translocate to nucleus, and both were reduced by NS398. On the contrary, ATRA notablely attenuated the inhibitory effect of NS398 on BMP9-increased β-catenin. Exogenous RXRa obviously ameliorated the inhibitory effect of silencing COX-2 on ectopic bone formation induced by BMP9. NS398 reduced the level of phosphorylated CREB, which was almost reversed by ATRA. Besides, RXRa interacted with phosphorylated CREB directly and both were recruited at β-catenin promoter region. Thus, we demonstrated that ATRA may reverse the side-effects of COX-2 inhibitor on bone metabolism through increasing the activation of Wnt/β-catenin pathway partly.

All-trans Retinoic Acid as a Versatile Cytosolic Signal Modulator Mediated by CRABP1

All-trans retinoic acid (AtRA), an active metabolite of vitamin A, is recognized for its classical action as an endocrine hormone that triggers genomic effects mediated through nuclear receptors RA receptors (RARs). New evidence shows that atRA-mediated cellular responses are biphasic with rapid and delayed responses. Most of these rapid atRA responses are the outcome of its binding to cellular retinoic acid binding protein 1 (CRABP1) that is predominantly localized in cytoplasm and binds to atRA with a high affinity. This review summarizes the most recent studies of such non-genomic outcomes of atRA and the role of CRABP1 in mediating such rapid effects in different cell types. In embryonic stem cells (ESCs), atRA-CRABP1 dampens growth factor sensitivity and stemness. In a hippocampal neural stem cell (NSC) population, atRA-CRABP1 negatively modulates NSC proliferation and affects learning and memory. In cardiomyocytes, atRA-CRABP1 prevents over-activation of calcium-calmodulin-dependent protein kinase II (CaMKII), protecting heart function. These are supported by the fact that CRABP1 gene knockout (KO) mice exhibit multiple phenotypes including hippocampal NSC expansion and spontaneous cardiac hypertrophy. This indicates that more potential processes/signaling pathways involving atRA-CRABP1 may exist, which remain to be identified.

CYP26A1 gene promoter is a useful tool for reporting RAR-mediated retinoid activity

Of numerous genes regulated by retinoic acid (RA), CYP26A1 is the most inducible gene by RA. In this study, we have used a shortened construct form, E4, of the CYP26A1 gene promoter, in a promoter-less vector with either luciferase or red fluorescent protein (RFP) as the reporter gene and have tested its responses to retinoids in transfected HepG2 and HEK293T cells. The promoter responded linearly to a wide concentration range of RA in cells cotransfected with retinoic acid receptors. It also responded quantitatively to retinol and other retinoids. An isolated clonal line of HEK293T cells permanently transfected with the promoter driving the expression of RFP responded to both RA and retinol, and the responses could be measured by fluorescence microscopy and flow cytometry. The promoter was used to assess the retinoid activity of 3 novel synthetic retinoid analogues, as well as of the intact serum samples of rats. Among the synthetic retinoid analogues tested, EC23 is more potent than RA at lower concentrations and was more stable than RA. The retinoid activities could be measured in control rat serum samples and were increased in the serum of RA-treated rats. This system offers a biologically-based alternative to mass-based retinoid analysis.

Characterization of Vitamin A Metabolome in Human Livers With and Without Nonalcoholic Fatty Liver Disease

Retinoids are essential endogenous compounds involved in regulation of critical biologic processes, including maintenance of metabolic homeostasis in the liver. Much of the knowledge of altered retinoid homeostasis in human disease states is derived from changes in indirect markers such as mRNA expression of retinoid-related genes and circulating concentrations of retinol or its binding protein RBP4. We hypothesized that in the human liver, concentrations of the active retinoid all-trans-retinoic acid (atRA) correlate with the concentrations of retinyl palmitate (RP), the storage form of atRA, retinol, the inactive vitamin A, and the mRNA expression of retinoid-related genes. On the basis of existing knowledge of altered vitamin A homeostasis in metabolic syndrome, we also predicted that in human livers with nonalcoholic fatty liver disease (NAFLD) retinoid concentrations would be decreased. Using novel liquid chromatography-tandem mass spectrometry methods, the hepatic vitamin A metabolome was quantified in normal human livers (n = 50) and 22 livers from donors with NAFLD. The hepatic concentrations of RP, atRA, 13-cisRA, and 4-oxo-atRA were significantly decreased in NAFLD samples in comparison with normal liver samples, whereas retinol levels remained unchanged. The concentrations of atRA were positively correlated with RP and 13-cisRA but not with retinol or the relative mRNA expression of LRAT, ALDH1A1, CYP26A1, RARα, and RARβ An active metabolite of atRA, 4-oxo-atRA was, for the first time, detected in human tissues at comparable concentration with RA isomers, suggesting this retinoid may contribute to retinoid signaling in humans. SIGNIFICANCE STATEMENT: This study shows that in NAFLD liver vitamin A homeostasis is disrupted potentially contributing to disease progression. The results show that interpretation of retinoid homeostasis on the basis of indirect markers such as retinol concentrations or mRNA data is probably misleading when evaluating human disease processes, and analysis of the broader retinoid metabolome is needed to characterize disease effects on retinoid signaling.

MAP7 and MUCL1 Are Biomarkers of Vitamin D3-Induced Tolerogenic Dendritic Cells in Multiple Sclerosis Patients

The administration of autologous tolerogenic dendritic cells (tolDC) has become a promising alternative for the treatment of autoimmune diseases, such as multiple sclerosis (MS). Specifically, the use of vitamin D3 for the generation of tolDC (vitD3-tolDC) constitutes one of the most widely studied approaches, as it has evidenced significant immune regulatory properties, both in vitro and in vivo. In this article, we generated human vitD3-tolDC from monocytes from healthy donors and MS patients, characterized in both cases by a semi-mature phenotype, secretion of IL-10 and inhibition of allogeneic lymphocyte proliferation. Additionally, we studied their transcriptomic profile and selected a number of differentially expressed genes compared to control mature and immature dendritic cells for their analysis. Among them, qPCR results validated CYP24A1, MAP7 and MUCL1 genes as biomarkers of vitD3-tolDC in both healthy donors and MS patients. Furthermore, we constructed a network of protein interactions based on the literature, which manifested that MAP7 and MUCL1 genes are both closely connected between them and involved in immune-related functions. In conclusion, this study evidences that MAP7 and MUCL1 constitute robust and potentially functional biomarkers of the generation of vitD3-tolDC, opening the window for their use as quality controls in clinical trials for MS.

The retinoic acid hydroxylase Cyp26a1 has minor effects on postnatal vitamin A homeostasis, but is required for exogenous atRA clearance

The all-trans-retinoic acid (atRA) hydroxylase Cyp26a1 is essential for embryonic development and may play a key role in regulating atRA clearance also in adults. We hypothesized that loss of Cyp26a1 activity via inducible knockout in juvenile or adult mice would result in decreased atRA clearance and increased tissue atRA concentrations and atRA-related adverse effects. To test these hypotheses, Cyp26a1 was knocked out in juvenile and adult male and female Cyp26a1 floxed mice using standard Cre-Lox technology and tamoxifen injections. Biochemical and histological methods were used to study the effects of global Cyp26a1 knockout. The Cyp26a1 knockout did not result in consistent histopathological changes in any major organs. Cyp26a1 ^-/- mice gained weight normally and exhibited no adverse phenotypes for up to 1 year after loss of Cyp26a1 expression. Similarly, atRA concentrations were not increased in the liver, testes, spleen, or serum of these mice, and the Cyp26a1 knockout did not cause compensatory induction of lecithin:retinol acetyltransferase (Lrat) or retinol dehydrogenase 11 (Rdh11) mRNA or a decrease in aldehyde dehydrogenase 1a1 (Aldh1a1) mRNA in the liver compared with tamoxifen-treated controls. However, the Cyp26a1 ^-/- mice showed increased bone marrow cellularity and decreased frequency of erythroid progenitor cells in the bone marrow consistent with a retinoid-induced myeloid skewing of hematopoiesis. In addition, the Cyp26a1 knockout decreased clearance of exogenous atRA by 70% and increased atRA half-life 6-fold. These findings demonstrate that despite lacking a major impact on endogenous atRA signaling, Cyp26a1 critically contributes as a barrier for exogenous atRA exposure.

Ultrafast photoisomerisation of an isolated retinoid

The photoinduced excited state dynamics of gas-phase trans-retinoate (deprotonated trans-retinoic acid, trans-RA^-) are studied using tandem ion mobility spectrometry coupled with laser spectroscopy, and frequency-, angle- and time-resolved photoelectron imaging. Photoexcitation of the bright S₃(ππ*) ← S₀ transition leads to internal conversion to the S₁(ππ*) state on a ≈80 fs timescale followed by recovery of S₀ and concomitant isomerisation to give the 13-cis (major) and 9-cis (minor) photoisomers on a ≈180 fs timescale. The sub-200 fs stereoselective photoisomerisation parallels that for the retinal protonated Schiff base chromophore in bacteriorhodopsin. Measurements on trans-RA^- in methanol using the solution photoisomerisation action spectroscopy technique show that 13-cis-RA^- is also the principal photoisomer, although the 13-cis and 9-cis photoisomers are formed with an inverted branching ratio with photon energy in methanol when compared with the gas phase, presumably due to solvent-induced modification of potential energy surfaces and inhibition of electron detachment processes. Comparison of the gas-phase time-resolved data with transient absorption spectroscopy measurements on retinoic acid in methanol suggest that photoisomerisation is roughly six times slower in solution. This work provides clear evidence that solvation significantly affects the photoisomerisation dynamics of retinoid molecules.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

β-Carotene in the human body: metabolic bioactivation pathways - from digestion to tissue distribution and excretion

β-Carotene intake and tissue/blood concentrations have been associated with reduced incidence of several chronic diseases. Further bioactive carotenoid-metabolites can modulate the expression of specific genes mainly via the nuclear hormone receptors: retinoic acid receptor- and retinoid X receptor-mediated signalling. To better understand the metabolic conversion of β-carotene, inter-individual differences regarding β-carotene bioavailability and bioactivity are key steps that determine its further metabolism and bioactivation and mediated signalling. Major carotenoid metabolites, the retinoids, can be stored as esters or further oxidised and excreted via phase 2 metabolism pathways. In this review, we aim to highlight the major critical control points that determine the fate of β-carotene in the human body, with a special emphasis on β-carotene oxygenase 1. The hypothesis that higher dietary β-carotene intake and serum level results in higher β-carotene-mediated signalling is partly questioned. Alternative autoregulatory mechanisms in β-carotene / retinoid-mediated signalling are highlighted to better predict and optimise nutritional strategies involving β-carotene-related health beneficial mediated effects.

Proteomic Evaluation of the Acute Radiation Syndrome of the Gastrointestinal Tract in a Murine Total-body Irradiation Model

Radiation exposure to the gastrointestinal system contributes to the acute radiation syndrome in a dose- and time-dependent manner. Molecular mechanisms that lead to the gastrointestinal acute radiation syndrome remain incompletely understood. Using a murine model of total-body irradiation, C57BL/6J male mice were irradiated at 8, 10, 12, and 14 Gy and assayed at day 1, 3, and 6 after exposure and compared to nonirradiated (sham) controls. Tryptic digests of gastrointestinal tissues (upper ileum) were analyzed by liquid chromatography-tandem mass spectrometry on a Waters nanoLC coupled to a Thermo Scientific Q Exactive hybrid quadrupole-orbitrap mass spectrometer. Pathway and gene ontology analysis were performed with Qiagen Ingenuity, Panther GO, and DAVID databases. A number of trends were identified in our proteomic data including pronounced protein changes as well as protein changes that were consistently up regulated or down regulated at all time points and dose levels interrogated. Time- and dose-dependent protein changes, canonical pathways affected by irradiation, and changes in proteins that serve as upstream regulators were also identified. Additionally, proteins involved in key processes including inflammation, radiation, and retinoic acid signaling were identified. The proteomic profiling conducted here represents an untargeted systems biology approach to identify acute molecular events that will be useful for a greater understanding of animal models and may be potentially useful toward the development of medical countermeasures and/or biomarkers.

Acute Proteomic Changes in the Lung After WTLI in a Mouse Model: Identification of Potential Initiating Events for Delayed Effects of Acute Radiation Exposure

Radiation-induced lung injury is a delayed effect of acute radiation exposure resulting in pulmonary pneumonitis and fibrosis. Molecular mechanisms that lead to radiation-induced lung injury remain incompletely understood. Using a murine model of whole-thorax lung irradiation, C57BL/6J mice were irradiated at 8, 10, 12, and 14 Gy and assayed at day 1, 3, and 6 postexposure and compared to nonirradiated (sham) controls. Tryptic digests of lung tissues were analyzed by liquid chromatography-tandem mass spectrometry on a Waters nanoLC instrument coupled to a Thermo Scientific Q Exactive hybrid quadrupole-orbitrap mass spectrometer. Pathway and gene ontology analysis were performed with Qiagen Ingenuity, Panther GO, and DAVID databases. A number of trends were identified in the proteomic data, including protein changes greater than 10 fold, protein changes that were consistently up regulated or down regulated at all time points and dose levels interrogated, time and dose dependency of protein changes, canonical pathways affected by irradiation, changes in proteins that serve as upstream regulators, and proteins involved in key processes including inflammation, radiation, and retinoic acid signaling. The proteomic profiling conducted here represents an untargeted systems biology approach to identify acute molecular events that could potentially be initiating events for radiation-induced lung injury.

Involvement of the retinoic acid signaling pathway in sex differentiation and pubertal development in the European sea bass Dicentrarchus labrax

Retinoic Acid (RA) is a vitamin A derivative present in many biological processes including embryogenesis, organ development and cell differentiation. The RA signaling pathway is essential for the onset of meiosis in tetrapods, although its role in fish reproduction needs further evidence. This study reports the expression profiles of several genes involved in this pathway during sex differentiation and the first reproductive season in European sea bass (Dicentrarchus labrax) gonads. The assessed genes are representative of several steps of the pathway including retinol transport, RA synthesis, nuclear receptors, RA transport and degradation. The study includes a synteny analysis of stra8, a tetrapod meiosis gatekeeper, in several taxa. The results show that, these genes were overexpressed during early gonad development and their expression decreased during meiosis progression in males and during vitellogenesis in females. Specifically, a decrease of cyp26a1, involved in RA degradation, together with an increase of aldh1a2 and aldh1a3, in charge of RA-synthesis, might ensure the availability of high RA levels at the time of meiosis in males and females. Moreover, the absence of stra8 in the European sea bass genome, as well as the conserved genomic neighbourhood found in other taxa, suggest a stra8 independent signaling for RA during meiosis. Taken together, our results might help to better understand the role of RA signaling in teleost gonad development.

Vitamin A and inflammatory bowel diseases: from cellular studies and animal models to human disease

Vitamin A (VA) and metabolites such as Retinoic Acid (RA) and all-trans-RA (at-RA) are crucial in the modulation of the immune system and may be determinative in the balance of the immune responses. Inflammatory bowel diseases (IBD) consist of chronic relapsing and heterogeneous disorders with not well-known etiology. Due to its role in inflammatory processes, VA may be helpful in the treatment of IBD. Area covered: As VA plays a significant role in the inflammatory processes, this review aims to show the potential role of this vitamin in IBD, searching for cellular studies, animal models, and studies with humans. Expert commentary: Many studies have described the importance of alternative therapeutic approaches for IBD. Due to its role in the immune system, VA may also exert an indispensable role in the IBD. Nevertheless, some authors have shown that these compounds could stimulate the release of pro-inflammatory cytokines. For these reasons, more studies should be performed to establish the precise mechanisms of VA and its metabolites in systemic and intestinal inflammation.

Nutrient Sensing by Hypothalamic Tanycytes

Nutritional signals have long been implicated in the control of cellular processes that take place in the hypothalamus. This includes food intake regulation and energy balance, inflammation, and most recently, neurogenesis. One of the main glial cells residing in the hypothalamus are tanycytes, radial glial-like cells, whose bodies are located in the lining of the third ventricle, with processes extending to the parenchyma and reaching neuronal nuclei. Their unique anatomical location makes them directly exposed to nutrients in the cerebrospinal fluid. Several research groups have shown that tanycytes can respond to nutritional signals by different mechanisms, such as calcium signaling, metabolic shift, and changes in proliferation/differentiation potential. Despite cumulative evidence showing tanycytes have the molecular components to participate in nutrient detection and response, there are no enough functional studies connecting tanycyte nutrient sensing with hypothalamic functions, nor that highlight the relevance of this process in physiological and pathological context. This review will summarize recent evidence that supports a nutrient sensor role for tanycytes in the hypothalamus, highlighting the need for more detailed analysis on the actual implications of tanycyte-nutrient sensing and how this process can be modulated, which might allow the discovery of new metabolic and signaling pathways as therapeutic targets, for the treatment of hypothalamic related diseases.

Elongation of Müllerian ducts and connection to urogenital sinus determine the borderline of uterine and vaginal development

In female mice, proximal, middle and caudal Müllerian ducts (MDs) differentiate into oviduct, uterus and vagina, respectively. The fates of female reproductive tract epithelia are determined by the mesenchyme. However, the mesenchymal fate determination system is still unclear. It is reported that presence or absence of retinoic acid (RA) signaling in MD mesenchyme induced uterine or vaginal mesenchyme, respectively. To analyze determination of the borderline, RA signal switching factors were found to play critical roles. Expression of a RA metabolizing enzyme, CYP26A1, was high in the epithelium of caudal MD and urogenital sinus, indicating that the enzyme causes the absence of RA signaling in the region. mRNA expression of some transcription factors regulating Aldh1a2, RA synthesis enzyme expressed in MDs, in other tissues was detected in MDs. When the transcription factor genes were overexpressed in a uterine mesenchymal cell line, C/ebpδ overexpression stimulated Aldh1a2 expression. Furthermore, C/EBPδ protein was strongly expressed in the proximal and middle regions of the MDs and bound to the Aldh1a2 promoter in vivo. Since C/ebpδ mRNA expression was maintained at the same level in proximal, middle and caudal MDs, we hypothesize that a high frequency of mitosis induces a low level protein expression in MD mesenchyme. In fact, the mitotic activity was significantly high in caudal mesenchyme, and a mathematical model showed that a gradient of protein was induced by cell proliferation. Therefore, morphogenesis of MDs controls the fate of mesenchyme via RA degradation in urogenital sinus and a gradient of proteins involved in RA synthesis.

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Degradation of RA by CYP26A1 is involved in fate determination of vaginal mesenchyme.

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C/EBPδ can directly bind to Aldh1a2 promoter and stimulates the expression.

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Müllerian ductal cell proliferation causes protein gradient involved in RA synthesis.

Early inhibition of endothelial retinoid uptake upon myocardial infarction restores cardiac function and prevents cell, tissue, and animal death

Physiologically, following myocardial infarction (MI), retinoid levels elevate locally in the infarcted area. Whereas therapeutic systemic application of retinoids was shown to reduce the progression of ventricular dilatation and the onset of heart failure, the role of acute physiologically increased retinoids in the infarction zone is unknown to date. To reveal the role of local retinoids in the MI zone is the central aim of this study. Using human cell culture and co-culture models for hypoxia as well as various assays systems, lentivirus-based transgene expression, in silico molecular docking studies, and an MI model in rats, we analysed the impact of the retinoid all-trans retinoic acid (ATRA) on cell signalling, cell viability, tissue survival, heart function, and MI-induced death in rats. Based on our results, ATRA-mediated signalling does aggravate the MI phenotype (e.g. 2.5-fold increased mortality compared to control), whereas 5'-methoxyleoligin (5ML), a new agent which interferes with ATRA-signalling rescues the ATRA-dependent phenotype. On the molecular level, ATRA signalling causes induction of TXNIP, a potent inhibitor of the physiological antioxidant thioredoxin (TRX1) and sensitizes cells to necrotic cell death upon hypoxia. 5ML-mediated prevention of ATRA effects were shown to be based on the inhibition of cellular ATRA uptake by interference with the cholesterol (and retinol) binding motif of the transmembrane protein STRA6. 5ML-mediated inhibition of ATRA uptake led to a strong reduction of ATRA-dependent gene expression, reduced ROS formation, and protection from necrotic cell death. As 5ML exerted a cardioprotective effect, also independent of its inhibition of cellular ATRA uptake, the agent likely has another cardioprotective property, which may rely on the induction of TRX1 activity. In summary, this is the first study to show i) that local retinoids in the early MI zone may worsen disease outcome, ii) that inhibition of endothelial retinoid uptake using 5ML may constitute a novel treatment strategy, and iii) that targeting endothelial and myocardial retinoid uptake (e.g. via STRA6 inhibition) may constitute a novel treatment target in acute MI.

Altered hepatic genes related to retinol metabolism and plasma retinol in patients with non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), especially non-alcoholic steatohepatitis (NASH) is a chronic liver disease commonly associated with hepatic fibrosis. NASH patients have an increased risk for hepatocellular carcinoma (HCC). An altered retinol metabolism is one of the pathways involved in the process of hepatic fibrosis, and enzymes involved in retinol metabolism have been associated with HCC. We aimed to determine the association between plasma retinol levels and hepatic expression of genes related to retinol metabolism, as well as to assess the hepatic expression of transcription factors regulated by retinoic acid in patients with NAFLD. Cross-sectional study where hepatic gene expression (Illumina microarray) and plasma retinol levels (HPLC) were measured in 17 patients with simple steatosis (SS), 15 with NASH, and 22 living liver donors (LD) as controls. Plasma retinol levels were higher in SS (1.53 ± 0.44 μmol/L) and NASH (1.51 ± 0.56 μmol/L) compared to LD (1.21 ± 0.38 μmol/L; p<0.05). AKR1B10 was highly overexpressed in NASH compared to SS (+6.2-fold) and LD (+9.9-fold; p = 4.89E-11). Retinaldehyde dehydrogenase 1 family, member A2 (ALDH1A2) and retinaldehyde dehydrogenase 1 family, member A3 (ALDH1A3), key enzymes for retinoic acid synthesis, were underexpressed in SS (-1.48 and -2.3-fold, respectively) and NASH (-1.47 and -2.6-fold, respectively) versus LD. In NASH, hepatic ALDH1A2 and ALDH1A3 were underexpressed and inversely correlated with plasma retinol levels, which may reduce retinoic acid in the liver. This, in addition to changes in expression of other genes involved in retinol metabolism, suggests a role for altered retinol homeostasis in NASH.

Effects of methylmercury and retinol palmitate co-administration in rats during pregnancy and breastfeeding: Metabolic and redox parameters in dams and their offspring

Ubiquitous low-dose methylmercury (MeHg) exposure through an increased fish consumption represents a global public health problem, especially among pregnant women. A plethora of micronutrients presented in fish affects MeHg uptake/distribution, but limited data is available. Vitamin A (VitA), another fish micronutrient is used in nutritional supplementation, especially during pregnancy. However, there is no information about the health effects arising from their combined exposure. Therefore, the present study aimed to examine the effects of both MeHg and retinyl palmitate administered on pregnant and lactating rats in metabolic and redox parameters from dams and their offspring. Thirty Wistar female rats were orally supplemented with MeHg (0,5 mg/kg/day) and retinyl palmitate (7500 µg RAE/kg/day) via gavage, either individually or in combination from the gestational day 0 to weaning. For dams (150 days old) and their offspring (31 days old), glycogen accumulation (hepatic and cardiac) and retinoid contents (plasma and liver) were analyzed. Hg deposition in liver tissue was quantified. Redox parameters (liver, kidney, and heart) were evaluated for both animals. Cytogenetic damage was analyzed with micronucleus test. Our results showed no general toxic or metabolic alterations in dams and their offspring by MeHg-VitA co-administration during pregnancy and lactation. However, increased lipoperoxidation in maternal liver and a disrupted pro-oxidant response in the heart of male pups was encountered, with apparently no particular effects in the antioxidant response in female offspring. GST activity in dam kidney was altered leading to possible redox disruption of this tissue with no alterations in offspring. Finally, the genomic damage was exacerbated in both male and female pups. In conclusion, low-dose MeHg exposure and retinyl palmitate supplementation during gestation and lactation produced a potentiated pro-oxidant effect, which was tissue-specific. Although this is a pre-clinical approach, we recommend precaution for pregnant women regarding food consumption, and we encourage more epidemiological studies to assess possible modulations effects of MeHg-VitA co-administration at safe or inadvertently used doses in humans, which may be related to specific pathologies in mothers and their children.

Searching for the Transcriptomic Signature of Immune Tolerance Induction-Biomarkers of Safety and Functionality for Tolerogenic Dendritic Cells and Regulatory Macrophages

The last years have witnessed a breakthrough in the development of cell-based tolerance-inducing cell therapies for the treatment of autoimmune diseases and solid-organ transplantation. Indeed, the use of tolerogenic dendritic cells (tolDC) and regulatory macrophages (Mreg) is currently being tested in Phase I and Phase II clinical trials worldwide, with the aim of finding an effective therapy able to abrogate the inflammatory processes causing these pathologies without compromising the protective immunity of the patients. However, there exists a wide variety of different protocols to generate human tolDC and Mreg and, consequently, the characteristics of each product are heterogeneous. For this reason, the identification of biomarkers able to define their functionality (tolerogenicity) is of great relevance, on the one hand, to guarantee the safety of tolDC and Mreg before administration and, on the other hand, to compare the results between different cell products and laboratories. In this article, we perform an exhaustive review of protocols generating human tolDC and Mreg in the literature, aiming to elucidate if there are any common transcriptomic signature or potential biomarkers of tolerogenicity among the different approaches. However, and although several effectors seem to be induced in common in some of the most reported protocols to generate both tolDC or Mreg, the transcriptomic profile of these cellular products strongly varies depending on the approach used to generate them.

The Biochemical Basis of Vitamin A Production from the Asymmetric Carotenoid β-Cryptoxanthin

Vitamin A serves essential functions in mammalian biology as a signaling molecule and chromophore. This lipid can be synthesized from more than 50 putative dietary provitamin A precursor molecules which contain at least one unsubstituted β-ionone ring. We here scrutinized the enzymatic properties and substrate specificities of the two structurally related carotenoid cleavage dioxygenases (CCDs) which catalyze this synthesis. Recombinant BCO1 split substrates across the C15,C15' double bond adjacent to a canonical β-ionone ring site to vitamin A aldehyde. Substitution of the ring with a hydroxyl group prevented this conversion. The removal of methyl groups from the polyene carbon backbone of the substrate did not impede enzyme activity. Homology modeling and site-directed mutagenesis identified amino acid residues at the entrance of the substrate tunnel, which determined BCO1's specificity for the canonical β-ionone ring site. In contrast, BCO2 split substrates across the C9,C10 double bond adjacent to assorted ionone ring sites. Kinetic analysis revealed a higher catalytic efficiency of BCO2 with substrates bearing 3-hydroxy-β-ionone rings. In the mouse intestine, the asymmetric carotenoid β-cryptoxanthin with one canonical and one 3-hydroxy-β-ionone ring site was meticulously converted to vitamin A. The tailoring of this asymmetric substrate occurred by a stepwise processing of the carotenoid substrate by both CCDs and involved a β-apo-10'-carotenal intermediate. Thus, opposite selectivity for ionone ring sites of the two mammalian CCDs complement each other in the metabolic challenge of vitamin A production from a chemically diverse set of precursor molecules.

Contribution of Hepatic Retinaldehyde Dehydrogenase Induction to Impairment of Glucose Metabolism by High-Fat-Diet Feeding in C57BL/6J Mice

Obesity and insulin resistance are associated with overexpression of retinaldehyde dehydrogenase 1 (RALDH1). We aimed to investigate the roles of hepatic RALDH1 induction in glucose metabolism impairment using mice fed with high-fat-diet (HFD). Mice were fed with HFD for 8 weeks and treated with RALDH inhibitor citral for another 4 weeks. Oral glucose tolerance test (OGTT), pyruvate tolerance test (PTT) and insulin tolerance test were performed. Expressions of phosphoenolpyruvate carboxykinase 1 (PCK1), glucokinase (GCK) and RALDH1 were measured. Therapeutic effects of citral were also documented in diabetic rats. Effects of retinaldehyde on PCK1 and GCK expressions were examined in rat primary hepatocytes and HepG2 cells. The results showed that HFD mice were characterized by hyperlipidaemia and insulin resistance, accompanied by significantly increased RALDH1 activity and expression. Citral (10 and 50 mg/kg) ameliorated HFD-induced hyperlipidaemia and insulin resistance, as demonstrated by the improved fasting glucose, insulin levels and lipid profiles. OGTT and PTT demonstrated that citral reversed HFD-induced glucose disposal impairment and glucose production enhancement. Citral also reversed the increased PCK1 expression and decreased GCK expression by HFD. Citral therapeutic effects were reconfirmed in diabetic rats. In vitro data indicated that retinaldehyde had the strongest PCK1 induction in primary hepatocytes of diabetic rats compared with HFD rats and control rats, in line with the increased RALDH1 expression. Citral reversed the retinaldehyde-induced PCK1 expression in primary rat hepatocytes and HepG2 cells. In conclusion, RALDH1 induction impaired glucose metabolism partly via modulating PCK1 and GCK expressions. Citral improved glucose metabolism through inhibiting RALDH activity.

REST, regulated by RA through miR-29a and the proteasome pathway, plays a crucial role in RPC proliferation and differentiation

One of the primary obstacles in the application of retinal progenitor cells (RPCs) to the treatment of retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), is their limited ability to proliferate and differentiate into specific retinal neurons. In this study, we revealed that repressor element-1-silencing transcription factor (REST), whose expression could be transcriptionally and post-transcriptionally mediated by retinoic acid (RA, one isomeride of a vitamin A derivative used as a differentiation-inducing agent in many disease treatments), plays a pivotal role in the regulation of proliferation and differentiation of RPCs. Our results show that direct knockdown of endogenous REST reduced RPC proliferation but accelerated RPC differentiation toward retinal neurons, which phenocopied the observed effects of RA on RPCs. Further studies disclosed that the expression level of REST could be downregulated by RA not only through upregulating microRNA (miR)-29a, which directly interacted with the 3′-untranslated region (3′-UTR) of the REST mRNA, but also through promoting REST proteasomal degradation. These results show us a novel functional protein, REST, which regulates RPC proliferation and differentiation, can be mediated by RA. Understanding the mechanisms of REST and RA in RPC fate determination enlightens a promising future for the application of REST and RA in the treatment of retinal degeneration diseases.