Enzymes and binding proteins affecting retinoic acid concentrations

miR-1 as a Key Epigenetic Regulator in Early Differentiation of Cardiac Sinoatrial Region

A large diversity of epigenetic factors, such as microRNAs and histones modifications, are known to be capable of regulating gene expression without altering DNA sequence itself. In particular, miR-1 is considered the first essential microRNA in cardiac development. In this study, miR-1 potential role in early cardiac chamber differentiation was analyzed through specific signaling pathways. For this, we performed in chick embryos functional experiments by means of miR-1 microinjections into the posterior cardiac precursors-of both primitive endocardial tubes-committed to sinoatrial region fates. Subsequently, embryos were subjected to whole mount in situ hybridization, immunohistochemistry and RT-qPCR analysis. As a relevant novelty, our results revealed that miR-1 increased Amhc1, Tbx5 and Gata4, while this microRNA diminished Mef2c and Cripto expressions during early differentiation of the cardiac sinoatrial region. Furthermore, we observed in this developmental context that miR-1 upregulated CrabpII and Rarß and downregulated CrabpI, which are three crucial factors in the retinoic acid signaling pathway. Interestingly, we also noticed that miR-1 directly interacted with Hdac4 and Calm1/Calmodulin, as well as with Erk2/Mapk1, which are three key factors actively involved in Mef2c regulation. Our study shows, for the first time, a key role of miR-1 as an epigenetic regulator in the early differentiation of the cardiac sinoatrial region through orchestrating opposite actions between retinoic acid and Mef2c, fundamental to properly assign cardiac cells to their respective heart chambers. A better understanding of those molecular mechanisms modulated by miR-1 will definitely help in fields applied to therapy and cardiac regeneration and repair.

Cancer Stem Cell Markers-Clinical Relevance and Prognostic Value in High-Grade Serous Ovarian Cancer (HGSOC) Based on The Cancer Genome Atlas Analysis

Cancer stem cells (CSCs) may contribute to an increased risk of recurrence in ovarian cancer (OC). Further research is needed to identify associations between CSC markers and OC patients' clinical outcomes with greater certainty. If they prove to be correct, in the future, the CSC markers can be used to help predict survival and indicate new therapeutic targets. This study aimed to determine the CSC markers at mRNA and protein levels and their association with clinical presentation, outcome, and risk of recurrence in HGSOC (High-Grade Serous Ovarian Cancer). TCGA (The Cancer Genome Atlas) database with 558 ovarian cancer tumor samples was used for the evaluation of 13 CSC markers (ALDH1A1, CD44, EPCAM, KIT, LGR5, NES, NOTCH3, POU5F1, PROM1, PTTG1, ROR1, SOX9, and THY1). Data on mRNA and protein levels assessed by microarray and mass spectrometry were retrieved from TCGA. Models to predict chemotherapy response and survival were built using multiple variables, including epidemiological data, expression levels, and machine learning methodology. ALDH1A1 and LGR5 mRNA expressions indicated a higher platinum sensitivity (p = 3.50 × 10-3; p = 0.01, respectively). POU5F1 mRNA expression marked platinum-resistant tumors (p = 9.43 × 10-3). CD44 and EPCAM mRNA expression correlated with longer overall survival (OS) (p = 0.043; p = 0.039, respectively). THY1 mRNA and protein levels were associated with worse OS (p = 0.019; p = 0.015, respectively). Disease-free survival (DFS) was positively affected by EPCAM (p = 0.004), LGR5 (p = 0.018), and CD44 (p = 0.012). In the multivariate model based on CSC marker expression, the high-risk group had 9.1 months longer median overall survival than the low-risk group (p < 0.001). ALDH1A1, CD44, EPCAM, LGR5, POU5F1, and THY1 levels in OC may be used as prognostic factors for the primary outcome and help predict the treatment response.

Targeting the retinoic acid signaling pathway as a modern precision therapy against cancers

Retinoic acid (RA) is a vital metabolite derived from vitamin A. RA plays a prominent role during development, which helps in embryological advancement and cellular differentiation. Mechanistically, RA binds to its definite nuclear receptors including the retinoic acid receptor and retinoid X receptor, thus triggering gene transcription and further consequences in gene regulation. This functional heterodimer activation later results in gene activation/inactivation. Several reports have been published related to the detailed embryonic and developmental role of retinoic acids and as an anti-cancer drug for specific cancers, including acute promyelocytic leukemia, breast cancer, and prostate cancer. Nonetheless, the other side of all-trans retinoic acid (ATRA) has not been explored widely yet. In this review, we focused on the role of the RA pathway and its downstream gene activation in relation to cancer progression. Furthermore, we explored the ways of targeting the retinoic acid pathway by focusing on the dual role of aldehyde dehydrogenase (ALDH) family enzymes. Combination strategies by combining RA targets with ALDH-specific targets make the tumor cells sensitive to the treatment and improve the progression-free survival of the patients. In addition to the genomic effects of ATRA, we also highlighted the role of ATRA in non-canonical mechanisms as an immune checkpoint inhibitor, thus targeting the immune oncological perspective of cancer treatments in the current era. The role of ATRA in activating independent mechanisms is also explained in this review. This review also highlights the current clinical trials of ATRA in combination with other chemotherapeutic drugs and explains the future directional insights related to ATRA usage.

A Pan-ALDH1A Inhibitor Induces Necroptosis in Ovarian Cancer Stem-like Cells

Ovarian cancer is typified by the development of chemotherapy resistance. Chemotherapy resistance is associated with high aldehyde dehydrogenase (ALDH) enzymatic activity, increased cancer "stemness," and expression of the stem cell marker CD133. As such, ALDH activity has been proposed as a therapeutic target. Although it remains controversial which of the 19 ALDH family members drive chemotherapy resistance, ALDH1A family members have been primarily linked with chemotherapy resistant and stemness. We identified two ALDH1A family selective inhibitors (ALDH1Ai). ALDH1Ai preferentially kills CD133+ ovarian cancer stem-like cells (CSCs). ALDH1Ai induce necroptotic CSC death, mediated, in part, by the induction of mitochondrial uncoupling proteins and reduction in oxidative phosphorylation. ALDH1Ai is highly synergistic with chemotherapy, reducing tumor initiation capacity and increasing tumor eradication in vivo. These studies link ALDH1A with necroptosis and confirm the family as a critical therapeutic target to overcome chemotherapy resistance and improve patient outcomes.

The success and the challenge of all-trans retinoic acid in the treatment of cancer

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, plays important roles in cell proliferation, cell differentiation, apoptosis, and embryonic development. The effects of ATRA are mediated by nuclear retinoid receptors as well as non-genomic signal pathway, such as MAPK and PKA. The great success of differentiation therapy with ATRA in acute promyelocytic leukemia (APL) not only improved the prognosis of APL but also spurred the studies of ATRA in the treatment of other tumors. Since the genetic and physiopathological simplicity of APL is not common in human malignancies, the combination of ATRA with other agents (chemotherapy, epigenetic modifiers, and arsenic trioxide, etc) had been extensively investigated in a variety of tumors. In this review, we will discuss in details about ATRA and its role in cancer treatment.

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Topical stabilized retinol treatment induces the expression of HAS genes and HA production in human skin in vitro and in vivo

Skin Aging manifests primarily with wrinkles, dyspigmentations, texture changes, and loss of elasticity. During the skin aging process, there is a loss of moisture and elasticity in skin resulting in loss of firmness finally leading to skin sagging. The key molecule involved in skin moisture is hyaluronic acid (HA), which has a significant water-binding capacity. HA levels in skin decline with age resulting in decrease in skin moisture, which may contribute to loss of firmness. Clinical trials have shown that topically applied ROL effectively reduces wrinkles and helps retain youthful appearance. In the current study, ROL was shown to induce HA production and stimulates the gene expression of all three forms of hyaluronic acid synthases (HAS) in normal human epidermal keratinocytes monolayer cultures. Moreover, in human skin equivalent tissues and in human skin explants, topical treatment of tissues with a stabilized-ROL formulation significantly induced the gene expression of HAS mRNA concomitant with an increased HA production. Finally, in a vehicle-controlled human clinical study, histochemical analysis confirmed increased HA accumulation in the epidermis in ROL-treated human skin as compared to vehicle. These results show that ROL increases skin expression of HA, a significant contributing factor responsible for wrinkle formation and skin moisture, which decrease during aging. Taken together with the activity to increase collagen, elastin, and cell proliferation, these studies establish that retinol provides multi-functional activity for photodamaged skin.

Effect of alcohol dehydrogenase 1C (ADH1C) genotype on vitamin A restriction and marbling in Korean native steers

Objective

This work was to find the correlation of alcohol dehydrogenase 1C (ADH1C) genotype with vitamin A reduction and carcass traits during the vitamin A restriction period.

Methods

In study 1, 60 Korean native steers were fed a diet (890 IU/kg) with 8,000 IU and 0 IU of supplemental premix vitamin A/kg of dry matter (DM) for control and treatment group, respectively. The levels of serum vitamin A were analyzed through high preparative performance liquid chromatography, and the ADH1C genotype was analyzed based on polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP; 78.1% TT type, 21.9% TC type); however, CC type was not found. Then, the interaction between ADH1C and carcass traits on the vitamin A restriction was investigated in study 2. A total of 136 Korean native steers were fed a diet that included 930 IU/kg vitamin A of DM.

Results

Serum vitamin A in treatment was reduced to 112.4 IU/dL in steers with TT type of ADH1C, while for steers with TC type the concentration of serum vitamin A was dropped to 79.5 IU/dL (p<0.1) in study 1. This showed that TC type had the potential to lower serum vitamin A concentration during vitamin A restriction compared to TT type. In study 2 we found that eye muscle area, marbling and carcass weight in Korean native steers with TC type were higher than in steers with TT type (p<0.05).

Conclusion

The interaction between vitamin A restriction and TC type of ADH1C gene could have the potential of increasing the marbling in Korean native steers. These results indicated that steers with TC type of the ADH1C gene were more sensitive to the change of serum vitamin A than TT types. Furthermore, this finding has the potential to enable a higher marbling score under the condition of vitamin A restriction in Korean native steers.

Postnatal Chick Choroids Exhibit Increased Retinaldehyde Dehydrogenase Activity During Recovery From Form Deprivation Induced Myopia

Purpose

Increases in retinaldehyde dehydrogenase 2 (RALDH2) transcript in the chick choroid suggest that RALDH2 may be responsible for increases observed in all-trans-retinoic acid (atRA) synthesis during recovery from myopic defocus. The purpose of the present study was to examine RALDH2 protein expression, RALDH activity, and distribution of RALDH2 cells in control and recovering chick ocular tissues.

Methods

Myopia was induced in White Leghorn chicks for 10 days, followed by up to 15 days of unrestricted vision (recovery). Expression of RALDH isoforms in chick ocular tissues was evaluated by Western blot. Catalytic activity of RALDH was measured in choroidal cytosol fractions using an in vitro atRA synthesis assay together with HPLC quantification of synthesized atRA. Distribution of RALDH2 cells throughout the choroid was evaluated by immunohistochemistry.

Results

RALDH2 was expressed predominately in the chick choroid (P < 0.001) and increased after 24 hours and 4 days of recovery (76%, 74%, and 165%, respectively; P < 0.05). Activity of RALDH was detected solely in the choroid and was elevated at 3 and 7 days of recovery compared to controls (70% and 48%, respectively; P < 0.05). The number of RALDH2 immunopositive cells in recovering choroids was increased at 24 hours and 4 to 15 days of recovery (P < 0.05) and were concentrated toward the RPE side compared to controls.

Conclusions

The results of this study suggest that RALDH2 is the major RALDH isoform in the chick choroid and is responsible for the increased RALDH activity seen during recovery.

Use of fast HPLC multiple reaction monitoring cubed for endogenous retinoic acid quantification in complex matrices

Retinoic acid (RA), an essential active metabolite of vitamin A, controls numerous physiological processes. In addition to the analytical challenges owing to its geometric isomers, low endogenous abundance, and often localized occurrence, nonspecific interferences observed during liquid chromatography (LC) multiple reaction monitoring (MRM) quantification methods have necessitated lengthy chromatography to obtain accurate quantification free of interferences. We report the development and validation of a fast high performance liquid chromatography (HPLC) multiplexing multiple reaction monitoring cubed (MRM(3)) assay for selective and sensitive quantification of endogenous RA from complex matrices. The fast HPLC separation was achieved using an embedded amide C18 column packed with 2.7 μm fused-core particles which provided baseline resolution of endogenous RA isomers (all-trans-RA, 9-cis-RA, 13-cis-RA, and 9,13-di-cis-RA) and demonstrated significant improvements in chromatographic efficiency compared to porous particle stationary phases. Multiplexing technology further enhanced sample throughput by a factor of 2 by synchronizing parallel HPLC systems to a single mass spectrometer. The fast HPLC multiplexing MRM(3) assay demonstrated enhanced selectivity for endogenous RA quantification in complex matrices and had comparable analytical performance to robust, validated LC-MRM methodology for RA quantification. The quantification of endogenous RA using the described assay was validated on a number of mouse tissues, nonhuman primate tissues, and human plasma samples. The combined integration of fast HPLC, MRM(3), and multiplexing yields an analysis workflow for essential low-abundance endogenous metabolites that has enhanced selectivity in complex matrices and increased throughput that will be useful in efficiently interrogating the biological role of RA in larger study populations.

An alternative retinoic acid-responsive Stra6 promoter regulated in response to retinol deficiency

Cellular uptake of vitamin A (retinol) is essential for many biological functions. The Stra6 protein binds the serum retinol-binding protein, RBP4, and acts in conjunction with the enzyme lecithin:retinol acyltransferase to facilitate retinol uptake in some cell types. We show that in embryonic stem (ES) cells and in some tissues, the Stra6 gene encodes two distinct mRNAs transcribed from two different promoters. Whereas both are all-trans-retinoic acid (RA)-responsive in ES cells, the downstream promoter contains a half-site RA response element (RARE) and drives an ∼ 13-fold, RA-associated increase in luciferase reporter activity. We employed CRISPR-Cas9 genome editing to show that the endogenous RARE is required for RA-induced transcription of both Stra6 isoforms. We further demonstrate that in ES cells, 1) both RARγ and RXRα are present at the Stra6 RARE; 2) RA increases co-activator p300 (KAT3B) binding and histone H3 Lys-27 acetylation at both promoters; 3) RA decreases Suz12 levels and histone H3 Lys-27 trimethylation epigenetic marks at both promoters; and 4) these epigenetic changes are diminished in the absence of RARγ. In the brains of WT mice, both the longer and the shorter Stra6 transcript (Stra6L and Stra6S, respectively) are highly expressed, whereas these transcripts are found only at low levels in RARγ(-/-) mice. In the brains of vitamin A-deficient mice, both Stra6L and Stra6S levels are decreased. In contrast, in the vitamin A-deficient kidneys, the Stra6L levels are greatly increased, whereas Stra6S levels are decreased. Our data show that kidneys respond to retinol deficiency by differential Stra6 promoter usage, which may play a role in the retention of retinol when vitamin A is low.

Roles of retinoic acid signaling in normal and abnormal development of the palate and tongue

Palatogenesis involves various developmental events such as growth, elevation, elongation and fusion of opposing palatal shelves. Extrinsic factors such as mouth opening and subsequent tongue withdrawal are also needed for the horizontal elevation of palate shelves. Failure of any of these steps can lead to cleft palate, one of the most common birth defects in humans. It has been shown that retinoic acid (RA) plays important roles during palate development, but excess RA causes cleft palate in fetuses of both rodents and humans. Thus, the coordinated regulation of retinoid metabolism is essential for normal palatogenesis. The endogenous RA level is determined by the balance of RA-synthesizing (retinaldehyde dehydrogenases: RALDHs) and RA-degrading enzymes (CYP26s). Cyp26b1 is a key player in normal palatogenesis. In this review, we discuss recent progress in the study of the pathogenesis of RA-induced cleft palate, with special reference to the regulation of endogenous RA levels by RA-degrading enzymes.

The retinol esterifying enzyme LRAT supports cell signaling by retinol-binding protein and its receptor STRA6

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP). At some tissues, holo-RBP is recognized by a plasma membrane receptor termed STRA6, which serves a dual role: it mediates transport of retinol from RBP into cells, and it functions as a cytokine receptor that, on binding holo-RBP, activates JAK2/STAT5 signaling. As STAT target genes include SOCS3, an inhibitor of insulin receptor, holo-RBP suppresses insulin responses in STRA6-expressing cells. We have shown previously that the two functions of STRA6 are interdependent. These observations suggest factors that regulate STRA6-mediated retinol transport may also control STRA6-mediated cell signaling. One such factor is retinol metabolism, which enables cellular uptake of retinol by maintaining an inward-directed concentration gradient. We show here that lecithin:retinol acyl transferase (LRAT), which catalyzes esterification of retinol to its storage species retinyl esters, is necessary for activation of the STRA6/JAK2/STAT5 cascade by holo-RBP. In accordance, LRAT-null mice are protected from holo-RBP-induced suppression of insulin responses. Hence, STRA6 signaling, which requires STRA6-mediated retinol transport, is supported by LRAT-catalyzed retinol metabolism. The observations demonstrate that STRA6 regulates key cellular processes by coupling circulating holo-RBP levels and intracellular retinol metabolism to cell signaling.

Reversal by RARα agonist Am580 of c-Myc-induced imbalance in RARα/RARγ expression during MMTV-Myc tumorigenesis

INTRODUCTION

Retinoic acid signaling plays key roles in embryonic development and in maintaining the differentiated status of adult tissues. Recently, the nuclear retinoic acid receptor (RAR) isotypes α, β and γ were found to play specific functions in the expansion and differentiation of the stem compartments of various tissues. For instance, RARγ appears to be involved in stem cell compartment expansion, while RARα and RARβ are implicated in the subsequent cell differentiation. We found that over-expressing c-Myc in normal mouse mammary epithelium and in a c-Myc-driven transgenic model of mammary cancer, disrupts the balance between RARγ and RARα/β in favor of RARγ.

METHODS

The effects of c-Myc on RAR isotype expression were evaluated in normal mouse mammary epithelium, mammary tumor cells obtained from the MMTV-Myc transgenic mouse model as well as human normal immortalized breast epithelial and breast cancer cell lines. The in vivo effect of the RARα-selective agonist 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carboxamido]benzoic acid (Am580) was examined in the MMTV-Myc mouse model of mammary tumorigenesis.

RESULTS

Modulation of the RARα/β to RARγ expression in mammary glands of normal mice, oncomice, and human mammary cell lines through the alteration of RAR-target gene expression affected cell proliferation, survival and tumor growth. Treatment of MMTV-Myc mice with the RARα-selective agonist Am580 led to significant inhibition of mammary tumor growth (~90%, P<0.001), lung metastasis (P<0.01) and extended tumor latency in 63% of mice. Immunocytochemical analysis showed that in these mice, RARα responsive genes such as Cyp26A1, E-cadherin, cellular retinol-binding protein 1 (CRBP1) and p27, were up-regulated. In contrast, the mammary gland tumors of mice that responded poorly to Am580 treatment (37%) expressed significantly higher levels of RARγ. In vitro experiments indicated that the rise in RARγ was functionally linked to promotion of tumor growth and inhibition of differentiation. Thus, activation of the RARα pathway is linked to tumor growth inhibition, differentiation and cell death.

CONCLUSIONS

The functional consequence of the interplay between c-Myc oncogene expression and the RARγ to RARα/β balance suggests that prevalence of RARγ over-RARα/β expression levels in breast cancer accompanied by c-Myc amplification or over-expression in breast cancer should be predictive of response to treatment with RARα-isotype-specific agonists and warrant monitoring during clinical trials.

Physiological and genomic consequences of adrenergic deficiency during embryonic/fetal development in mice: impact on retinoic acid metabolism

Adrenergic hormones are essential for early heart development. To gain insight into understanding how these hormones influence heart development, we evaluated genomic expression changes in embryonic hearts from adrenergic-deficient and wild-type control mice. To perform this study, we used a mouse model with targeted disruption of the Dopamine β-hydroxylase (Dbh) gene, whose product is responsible for enzymatic conversion of dopamine into norepinephrine. Embryos homozygous for the null allele (Dbh(-/-)) die from heart failure beginning as early as embryonic day 10.5 (E10.5). To assess underlying causes of heart failure, we isolated hearts from Dbh(-/-) and Dbh(+/+) embryos prior to manifestation of the phenotype and examined gene expression changes using genomic Affymetrix 430A 2.0 arrays, which enabled simultaneous evaluation of >22,000 genes. We found that only 22 expressed genes showed a significant twofold or greater change, representing ~0.1% of the total genes analyzed. More than half of these genes are associated with either metabolism (31%) or signal transduction (22%). Remarkably, several of the altered genes encode for proteins that are directly involved in retinoic acid (RA) biosynthesis and transport. Subsequent evaluation showed that RA concentrations were significantly elevated by an average of ~3-fold in adrenergic-deficient (Dbh(-/-)) embryos compared with controls, thereby suggesting that RA may be an important downstream mediator of adrenergic action during embryonic heart development.

Cross talk between signaling and vitamin A transport by the retinol-binding protein receptor STRA6

The plasma membrane protein STRA6 transports vitamin A from its blood carrier retinol binding protein (RBP) into cells, and it also functions as a cytokine receptor which activates JAK/STAT signaling. We show here that, unlike other cytokine receptors, phosphorylation of STRA6 is not simply induced upon binding of its extracellular ligand. Instead, activation of the receptor is triggered by STRA6-mediated translocation of retinol from serum RBP to an intracellular acceptor, the retinol-binding protein CRBP-I. The observations also demonstrate that the movement of retinol from RBP to CRBP-I, and thus activation of STRA6, is critically linked to the intracellular metabolism of the vitamin. Furthermore, the data show that STRA6 phosphorylation is required for retinol uptake to proceed. Hence, the observations demonstrate that STRA6 orchestrates a multicomponent "machinery" that couples vitamin A homeostasis and metabolism to activation of a signaling cascade and that, in turn, STRA6 signaling regulates the cellular uptake of the vitamin. STRA6 appears to be a founding member of a new class of proteins that may be termed "cytokine signaling transporters."

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

The impact of vitamin A restriction and ADH1C genotype on marbling in feedlot steers

A novel SNP was discovered within the promoter region of alcohol dehydrogenase 1C (ADH1C c.-64T>C), the C allele eliminating a potential binding site for the transcription factor C/EPBα. The purpose of this study was to examine if an interaction between this SNP and vitamin A restriction had an effect on carcass characteristics in beef cattle. Following backgrounding on a β-carotene-deficient diet, 130 steers (50 TT, 50 CT, and 30 CC) were finished for 5 mo and received either no supplemental vitamin A (unsupplemented) or 750,000 IU/mo (supplemented). A subgroup of 5 steers • genotype(-1) • treatment(-1) was randomly selected for pre- and postfinishing liver biopsies to assess vitamin A status and measure gene expression. Unsupplemented steers (Bos taurus) had significantly greater (P < 0.05) marbling scores than supplemented steers. There was a significant interaction between genotype and vitamin A supplementation on ether-extractable intramuscular fat (IMF). Within the unsupplemented treatment, TT steers had nearly 23% greater IMF than CC steers. Additionally, unsupplemented TT steers had over 24% greater IMF than supplemented TT steers. Expression of ADH1C in the liver was additive with each additional T allele, potentially due to the elimination of a possible binding site for C/EBPα. It is plausible that CC cattle have reduced ability to metabolize retinol to retinaldehyde (and subsequently retinoic acid) and that a phenotypic effect is only observed when vitamin A is limiting. Therefore, ADH1C c.-64T>C genotype, in combination with reduced vitamin A supplementation, could potentially be implemented in marker-assisted management to maximize marbling in finishing cattle.

Gene expression signatures differentiate ovarian/peritoneal serous carcinoma from breast carcinoma in effusions

Ovarian/primary peritoneal carcinoma and breast carcinoma are the gynaecological cancers that most frequently involve the serosal cavities.With the objective of improving on the limited diagnostic panel currently available for the differential diagnosis of these two malignancies,as well as to define tumour-specific biological targets, we compared their global gene expression patterns. Gene expression profiles of 10 serous ovarian/peritoneal and eight ductal breast carcinoma effusions were analysed using the HumanRef-8 BeadChip from Illumina.Differentially expressed candidate genes were validated using quantitative real-time PCR and immunohistochemistry. Unsupervised hierarchical clustering using all 54,675 genes in the array separated ovarian from breast carcinoma samples. We identified 288 unique probes that were significantly differentially expressed in the two cancers by greater than 3.5-fold, of which 81 and 207 were overexpressed in breast and ovarian/peritoneal carcinoma, respectively. SAM analysis identified 1078 differentially expressed probes with false discovery rate less than 0.05. Genes overexpressed in breast carcinoma included TFF1, TFF3, FOXA1, CA12, GATA3, SDC1, PITX1, TH, EHFD1, EFEMP1, TOB1 and KLF2. Genes overexpressed in ovarian/peritoneal carcinoma included SPON1, RBP1, MFGE8, TM4SF12, MMP7, KLK5/6/7, FOLR1/3,PAX8, APOL2 and NRCAM. The differential expression of 14 genes was validated by quantitative real-time PCR, and differences in 5 gene products were confirmed by immunohistochemistry. Expression profiling distinguishes ovarian/peritoneal carcinoma from breast carcinoma and identifies genes that are differentially expressed in these two tumour types. The molecular signatures unique to these cancers may facilitate their differential diagnosis and may provide a molecular basis for therapeutic target discovery.

Gene expression signatures differentiate ovarian/peritoneal serous carcinoma from breast carcinoma in effusions

Ovarian/primary peritoneal carcinoma and breast carcinoma are the gynaecological cancers that most frequently involve the serosal cavities. With the objective of improving on the limited diagnostic panel currently available for the differential diagnosis of these two malignancies, as well as to define tumour-specific biological targets, we compared their global gene expression patterns. Gene expression profiles of 10 serous ovarian/peritoneal and eight ductal breast carcinoma effusions were analysed using the HumanRef-8 BeadChip from Illumina. Differentially expressed candidate genes were validated using quantitative real-time PCR and immunohistochemistry. Unsupervised hierarchical clustering using all 54,675 genes in the array separated ovarian from breast carcinoma samples. We identified 288 unique probes that were significantly differentially expressed in the two cancers by greater than 3.5-fold, of which 81 and 207 were overexpressed in breast and ovarian/peritoneal carcinoma, respectively. SAM analysis identified 1078 differentially expressed probes with false discovery rate less than 0.05. Genes overexpressed in breast carcinoma included TFF1, TFF3, FOXA1, CA12, GATA3, SDC1, PITX1, TH, EHFD1, EFEMP1, TOB1 and KLF2. Genes overexpressed in ovarian/peritoneal carcinoma included SPON1, RBP1, MFGE8, TM4SF12, MMP7, KLK5/6/7, FOLR1/3, PAX8, APOL2 and NRCAM. The differential expression of 14 genes was validated by quantitative real-time PCR, and differences in 5 gene products were confirmed by immunohistochemistry. Expression profiling distinguishes ovarian/peritoneal carcinoma from breast carcinoma and identifies genes that are differentially expressed in these two tumour types. The molecular signatures unique to these cancers may facilitate their differential diagnosis and may provide a molecular basis for therapeutic target discovery.

Expressions of cellular retinoic acid binding proteins I and retinoic acid receptor-β in the guinea pig eyes with experimental myopia

AIM

All-trans retinoic acid (RA) is the only extrinsic biochemical candidate known to date that could act as a growth controller, the aim of this study was to investigate the expression cellular retinoic acid binding proteins I (CRABP-I) and retinoic acid receptor-β (RAR-β) in retina of the guinea pig eyes with experimental myopia.

METHODS

Ninety guinea pigs aged 14 days were equally and randomly divided into three groups: form deprivation (FD), -5D lens, and control. The diffusers for FD were white translucent hemispheres, and -5D lenses were used to introduce hyperopic defocus. Refraction was measured with streak retinoscopy after cycloplegia, and axial length was calculated with Cinescan A/B ultrasonography. Retina harvested at different time points were used to measure RA level with HPLC and expressions of cellular retinoic acid binding proteins I (CRABP-I) and RA receptor-β (RAR-β) were assayed with Western blot and Real-time PCR. SPSS13.0 software was used for statistical analysis.

RESULTS

Up-regulations of CRABP-I and RAR-β in ocular tissues correlated with changes in the refractive status and growth rate of the guinea pig eye (P<0.05). 14 days of monocular form-deprivation led to -5.14D myopia and a 0.281mm axial elongation; 14 days of monocular defocus produced -3.64D myopia and a 0.163 mm axial elongation. The level of retinal RA started to elevate in 7 days (P<0.05) after visual manipulation in both FD and -5D lens groups and became more prominent by 14 days (P<0.01) . The expressions of CRABP-I and RAR-β increased by 14 days after visual manipulation (P<0.05), the mRNA level of RAR-β, however, increased by 7 days after visual manipulation (P<0.05), which suggested that changes of expressions of CRABP-I and RAR-β might lag behind the change of RA.

CONCLUSION

The levels of CRABP-I and RAR-β were elevated in retina of the guinea pig eye with experimental myopia. During the progression of experimental myopia, the retinal RA level increased rapidly, and there might be a positive feedback between the increase of RA and up-regulation of RAR-β.

All trans retinoic acid and cancer

All-trans retinoic acid (ATRA) is an active metabolite of vitamin A under the family retinoid. Retinoids, through their cognate nuclear receptors, exert potent effects on cell growth, differentiation and apoptosis, and have significant promise for cancer therapy and chemoprevention. Differentiation therapy with ATRA has marked a major advance and become the first choice drug in the treatment of acute promyelocytic leukemia (APL). Conversions of 13-cis-retinoic acid and 9-cis-retinoic acid to all-trans-retinoic acid is very rapid. Currently, two distinct families of retinoid responsive nuclear receptors have been identified and characterized: retinoic acid receptors (RARs) and retinoid receptors (RXRs), each of which include three isoforms, α,β,and γ. ATRA is being increasingly included in anti-tumour therapeutical schemes for the treatment of various tumoral diseases such as Kaposi's sarcoma, head and neck squamous cell carcinoma, ovarian carcinoma, bladder cancer, neuroblastoma and has shown antiangiogenic effects in several systems, inhibiting proliferation in vascular smooth muscle cells (VSMCs) and anti-inflammatory in rheumatoid arthritis. This review helps to understand in details about the ATRA and its role on cancer and it is predicted that modulating the activity of ATRA will soon provide novel prevention and treatment approaches for the cancer patients.

Between death and survival: retinoic acid in regulation of apoptosis

The vitamin A metabolite all-trans-retinoic acid (RA) regulates multiple biological processes by virtue of its ability to regulate gene expression. It thus plays critical roles in embryonic development and is involved in regulating growth, remodeling, and metabolic responses in adult tissues. RA can also suppress carcinoma cell growth and is currently used in treatment of some cancers. Growth inhibition by RA may be exerted by induction of differentiation, cell cycle arrest, or apoptosis, or by a combination of these activities. Paradoxically, in the context of some cells, RA not only fails to inhibit growth but, instead, enhances proliferation and survival. This review focuses on the involvement of RA in regulating apoptotic responses. It includes brief overviews of transcriptional signaling by RA and of apoptotic pathways, and then addresses available information on the mechanisms by which RA induces apoptosis or, conversely, inhibits cell death and enhances survival.

Genome-wide analysis of binding sites and direct target genes of the orphan nuclear receptor NR2F1/COUP-TFI

Background

Identification of bona fide direct nuclear receptor gene targets has been challenging but essential for understanding regulation of organismal physiological processes.

Results

We describe a methodology to identify transcription factor binding sites and target genes in vivo by intersecting microarray data, computational binding site queries, and evolutionary conservation. We provide detailed experimental validation of each step and, as a proof of principle, utilize the methodology to identify novel direct targets of the orphan nuclear receptor NR2F1 (COUP-TFI). The first step involved validation of microarray gene expression profiles obtained from wild-type and COUP-TFI^−/− inner ear tissues. Secondly, we developed a bioinformatic tool to search for COUP-TFI DNA binding sites in genomes, using a classification-type Hidden Markov Model trained with 49 published COUP-TF response elements. We next obtained a ranked list of candidate in vivo direct COUP-TFI targets by integrating the microarray and bioinformatics analyses according to the degree of binding site evolutionary conservation and microarray statistical significance. Lastly, as proof-of-concept, 5 specific genes were validated for direct regulation. For example, the fatty acid binding protein 7 (Fabp7) gene is a direct COUP-TFI target in vivo because: i) we identified 2 conserved COUP-TFI binding sites in the Fabp7 promoter; ii) Fapb7 transcript and protein levels are significantly reduced in COUP-TFI^−/− tissues and in MEFs; iii) chromatin immunoprecipitation demonstrates that COUP-TFI is recruited to the Fabp7 promoter in vitro and in vivo and iv) it is associated with active chromatin having increased H3K9 acetylation and enrichment for CBP and SRC-1 binding in the newborn brain.

Conclusion

We have developed and validated a methodology to identify in vivo direct nuclear receptor target genes. This bioinformatics tool can be modified to scan for response elements of transcription factors, cis-regulatory modules, or any flexible DNA pattern.

P19 neuronal differentiation and retinoic acid metabolism as criteria to investigate atrazine, nitrite, and nitrate developmental toxicity

Atrazine and nitrogenous fertilizers are agrochemical contaminants frequently detected in water systems in North America. Several studies reported their ability to affect amphibian and mammalian development. Retinoids, supplied in the diet or synthesized by cells, are essential to embryogenesis. Disturbance of their homeostasis may lead to teratogenic effects. Retinoic acid (RA) is a major retinoid regulator of cell proliferation and differentiation. Previous studies reported alterations of retinoid stores in bullfrogs of Yamaska River subwatersheds (Québec, Canada), a region of intensive agricultural activities associated with atrazine, nitrate, and nitrite contaminants. These contaminants could affect RA metabolism and RA-mediated processes. Mouse P19 embryonic stem cells, which can differentiate to neurons in response to RA, were used to test this hypothesis. Cells were cultured in the absence or presence of contaminants during neuroinduction with RA and assayed by flow cytometry for expression of stage-specific embryonic antigen-1 (SSEA1) (embryonic marker) and betaIII-tubulin (neuronal marker). Cell cultures were also analyzed for RA metabolism by high performance liquid chromotagraphy (HPLC). Downregulation of SSEA1 paralleled betaIII-tubulin upregulation in an RA concentration-dependent manner. Atrazine, nitrate, and nitrite did not affect differentiation at environmentally encountered micromolar concentrations. However, low molar nitrite prevented RA-induced SSEA1 downregulation and decreased betaIII-tubulin appearance. Decreased cell viability/proliferation accompanied these differentiation effects. P19 cells metabolized RA to polar retinoids. RA metabolism was not affected at any concentration of atrazine, nitrate, or nitrite. Environmentally relevant levels of these contaminants, thus, had no gross effect on neurodifferentiation and RA catabolism of embryonic stem cells. P19 cell-based bioassays may provide valuable tools in monitoring developmental toxicity.

GS2 as a retinol transacylase and as a catalytic dyad independent regulator of retinylester accretion

GS2 (PNPLA4; iPLAeta) is the smallest member of the patatin-like family of phospholipases (PNPLA). It was initially identified by its ability to hydrolyze retinylesters (RE) in cell homogenates, and was later found to esterify retinol using a variety of acyl donors. In the present study we set out to determine its cellular function and examined its impact on RE status in 293T cells transfected with GS2, GS2-M1 (a non-translatable mutant of GS2) and empty vector, in fibroblasts isolated from normal and GS2-null donors and in SCC12b and in a somatic cell knock-out of GS2 (SCC12b-GS2(neo/-)), that we generated by homologous recombination. At 50nM medium retinol, GS2 had no significant impact on RE accumulation. However, at 2muM retinol, GS2 promoted a 1.6- to 5-fold increase in RE accumulation. To verify role of GS2 as a catalyst, RE levels were measured in 293T transfected wild type GS2, catalytic dyad mutants devoid of enzymatic activity, or alanine substitution mutants spanning the entire GS2 sequence. Surprisingly, every GS2 mutant promoted RE accumulation. This activity was also observed in the GS2 paralogues and rat orthologue. The data demonstrate that within the context of the cell GS2 promotes RE accumulation and may do so either as a catalyst or as a regulatory protein that enhances RE formation catalyzed by other acyl transferases.

Distribution of the cellular retinoic acid binding protein CRABP-I in the developing chick optic tectum

Vitamin A is a major morphogen for the visual system. Most of its effects are mediated by retinoic acid (RA), whose developmental functions include pattern formation, neuronal differentiation and possibly axonal guidance. Although RA has been suggested to regulate development of the retina and its central projection, little is known about the distribution of retinoid receptors and binding proteins in the optic tectum, which in birds is the direct target of most retinofugal axons. We investigated the spatial and temporal distribution of the cellular retinoic acid binding protein-I (CRABP-I) in the chick midbrain. While the precise role of CRABP-I is still unknown, this is an intracellular transport protein for RA, which tends to be expressed in cells that are responsive to retinoic acid. Our data show immunoreactivity of CRABP-I in the tectal anlage at E2.5 and during the entire period of embryonic development. It was found in differentiating neurons of the generative zone, in migrating cells of the prospective stratum griseum et fibrosum superficiale and in mature neurons in this layer. In addition, we detected retinoid receptors RARalpha, RARbeta, RXRalpha, RXRbeta and RXRgamma in the developing tectum. Cell culture experiments demonstrate CRABP-I expression in a subpopulation of tectal neurons as they differentiate in vitro. These results are consistent with a regulatory role of RA in tectal neurogenesis and physiology.

Superoxidation of retinoic acid

Atmospheric pressure chemical ionization mass spectroscopy (APCI-MS) was used to examine the light-induced oxidation products of retinoic acid under conditions that favor and preclude its aggregation. We observed that in conditions that favor aggregation, i.e. in aqueous solutions, retinoic acid undergoes superoxidation to yield highly oxidized species. Oxidation is limited, however, in the absence of such communication, i.e. when the polyene is fully solvated. From a comparison of the measured MS with that obtained from chemical oxidation of retinoic acid under conditions that promote radical oxidation and singlet oxygen-mediated oxidation, we conclude that superoxidation is mediated by reactive oxygen species other than singlet oxygen.

A Repressive Epigenetic Domino Effect Confers Susceptibility to Breast Epithelial Cell Transformation: Implications for Predicting Breast Cancer Risk

Retinoic acid (RA) is a master epigenetic regulator that plays a pivotal role in both breast morphogenesis and development. Here, we show for the first time that RA, via the RA receptor alpha (RARalpha), epigenetically regulates in a concerted fashion the transcription of two RA-responsive genes, the RA receptor beta2 (RARbeta2) and the cellular retinol-binding protein 1 (CRBP1). Specifically, an impaired RA signal through RARalpha in human breast epithelial cells triggers a repressive epigenetic domino effect, involving first RARbeta2 and second CRBP1. The phenotype acquired by breast epithelial cells clearly implies that the resistance to RA-mediated growth inhibition precedes the acquisition of morphological epithelial transformation, thus supporting the occurrence of sequential transcriptional silencing of first RARbeta2 and second CRBP1. The identification of this epigenetic network mechanistically linking RARbeta2 and CRBP1 transcription provides the basis for devising more accurate epigenetic tests for the prediction of breast cancer risk.

Molecular Screening for GS2 Lipase Regulators: Inhibition of Keratinocyte Retinylester Hydrolysis by TIP47

Retinoic acid at nanomolar concentrations modulates epidermal functions by serving as a transcription factor ligand. Under conditions of retinol sufficiency, it is imperative to limit retinoic acid biosynthesis from serum-derived retinol. In the epidermis, this is accomplished by esterifying retinol with long-chain fatty acids. Retinylester (RE) pools serve as a source of retinol for retinoic acid production under retinol deficiency and when required for proper differentiation. We have recently reported that GS2 lipase is expressed in keratinocytes and has the enzymatic properties of keratinocyte RE hydrolase. As GS2 lipase has a robust activity that can affect the intracellular retinol levels, we postulated that its activity must be regulated. Therefore, we screened keratinocyte cDNA expression libraries for the putative inhibitor. Herein, we report the identity of an inhibitor, TIP47, which prevents RE hydrolysis catalyzed by GS2 lipase and hormone-sensitive lipase. This protein was known to transport mannose-6-phosphate receptors from endosome to trans-Golgi and to be distributed between the cytoplasm and lipid droplets. Using a series of deletion mutants, we found two regions involved in the inhibitory activity. Residues within the carboxyl alpha3-alpha4 helices are essential in the context of the full-length protein. Residues within the amino-terminal also contribute depending on the context.

Development of a versatile reporter assay for studies of retinol uptake and metabolism in vivo

The two isomers of retinoic acid (RA), all-trans RA and 9-cis RA, are produced in several tissues in order to allow specific control of target gene transcription. Given the high potency of these receptor ligands, it seems likely that the cellular uptake and metabolic activation of the precursor, retinol (vitamin A), should be a highly regulated process. Several retinol dehydrogenases and components involved in the downstream events have been identified and partially characterized. However, less is known about the cellular uptake of retinol, and the isomerase activity giving rise to the 9-cis and 11-cis branches of the pathway. In this work, we show that the 9-cis RA biosynthesis pathway can be fully reconstituted in cultured HEK293A cells expressing a reporter system, including an endogenous isomerase activity converting all-trans retinol into 9-cis retinol. This assay allows for functional studies of known components, as well as screening for yet unidentified genes involved in the pathway. In addition to free all-trans retinol, we find that these cells can take up retinol from plasma retinol binding protein (RBP) by a mechanism that can be efficiently inhibited by blocking antibodies, suggesting that the uptake may involve a cellular receptor. We also demonstrate that overexpression of CRBPI can drive the accumulation of intracellular retinol from unbound retinol added to the medium. Thus, this versatile cellular assay can be used to study several aspects of retinol uptake and metabolism in vivo.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

Identification of a Novel Keratinocyte Retinyl Ester Hydrolase as a Transacylase and Lipase

Retinoic acid influences epidermal morphology and function through its ability to control transcription. Because the circulation presents the epidermis with micromolar amounts of retinol that can be converted to retinoic acid, regulating retinol access is imperative. In keratinocytes the majority of retinol is sequestered as long chain fatty acid esters. Although much has been learned about the major esterifying enzyme, little is known about the hydrolase that accesses retinol from its storage depot. Murine carboxylesterases and hormone sensitive lipase have been shown to have this activity. We found that their in vitro sensitivity to bis-p-nitrophenyl phosphate (BNPP), however, was not shared by the epidermal hydrolase activity. We therefore produced and screened two keratinocyte cDNA expression libraries and identified a previously sequenced gene (GS2) as a keratinocyte retinyl ester (RE) hydrolase insensitive to BNPP. The enzyme also catalyzes fattyacyl CoA-dependent and -independent retinol esterification. The hydrolysis reaction is greater at neutral pH, whereas the esterification reaction is greater at acidic pH. These activities are consistent with the increased RE content that accompanies epidermal maturation. In addition, this enzyme utilizes triolein as substrate and generates diacylglyceride and free fatty acid.

Ethanol increases retinoic acid production in cerebellar astrocytes and in cerebellum

Several characteristics of fetal alcohol syndrome (FAS) are similar to the teratogenic effects of retinoic acid (RA) exposure. It has been suggested that FAS may result from ethanol-induced alteration in endogenous RA synthesis, leading to abnormal embryonic concentrations of this morphogen. We examined whether ethanol may interfere with RA synthesis in the postnatal cerebellum, as a region of the developing CNS particularly vulnerable to both ethanol and RA teratogenesis. It was found that astrocytes are the predominant source of postnatal RA synthesis in the cerebellum. They express both retinaldehyde dehydrogenase 1 and 2. In vitro cytosolic preparations of astrocytes, as well as live cell preparations, have an increased capacity to synthesize RA in the presence of ethanol. A mechanism by which ethanol could stimulate RA synthesis is via the ethanol-activated short-chain retinol dehydrogenases, which we show to be present in the postnatal cerebellum. To determine whether ethanol stimulated RA synthesis in vivo, a sensitive and highly specific HPLC/MSn technique was used to measure cerebellar RA after administration of ethanol to postnatal day 4 rat pups. Cerebellar RA levels climbed significantly after such treatment. These results suggest that the cerebellar pathology exerted by ethanol may occur, at least in part, through increased production of RA.

Increased retinoid signaling in vascular smooth muscle cells by proinflammatory cytokines

Retinoids have been shown to modulate inflammation and the immune response in many cell types including macrophages, endothelial cells, and vascular smooth muscle cells. However, present knowledge of whether inflammatory mediators modulate vitamin A status in these cells is limited. To identify the role of inflammation on retinoid metabolism in vascular smooth muscle cells, the cells were exposed to a combination of proinflammatory cytokines: interleukin-1beta, interferon-gamma, and lipopolysaccharides. Without stimulation with proinflammatory cytokines, vascular smooth muscle cells expressed retinol dehydrogenases-2 and 5 mRNA detected by RT-PCR. Stimulation with the combination of cytokines induced a substantial increase of retinol dehydrogenase-5 mRNA. This was associated with increased production of ligands for retinoic acid receptors, when assayed in a retinoic acid receptor-dependent luciferase reporter system. Our results demonstrate that inflammatory mediators activate the retinoid metabolic pathway in vascular smooth muscle cells, which potentially may modulate the inflammatory response in the vascular wall.

Differential expression of chicken CYP26 in anterior versus posterior limb bud in response to retinoic acid

Multiple studies indicate that quantitative control of the levels of all-trans-retinoic acid (RA) in the vertebrate embryo is necessary for correct development. The function of RA in cells is regulated by a number of coordinated mechanisms. One of those mechanisms involves controls on the rate of RA catabolism. Recently, enzymes capable of catabolizing RA were found to constitute a new family, called CYP26, within the cytochrome P450 superfamily. CYP26 homologues have been isolated from human, mouse, zebra fish, and recently from the chick. In this study, we examined the regulation of chicken CYP26 (cCYP26) expression by RA during the early phase of chick limb outgrowth. In the anterior limb mesenchyme and apical ectodermal ridge (AER), cCYP26 expression was induced in a concentration dependent manner by implanting beads soaked in 0.1, 1, and 5 mg/ml RA. The RA-induced expression of cCYP26 in anterior limb mesenchyme and the AER was detected as early as 1 hr after treatment and was not affected by the presence of cycloheximide. In contrast to the anterior limb, the induction of cCYP26 was dramatically reduced (or absent) when RA beads were implanted in the posterior limb mesenchyme. Furthermore, induction of cCYP26 expression in the anterior mesenchyme was inhibited by transplantations of the zone of polarizing activity (ZPA) and by Shh-soaked beads. Our data suggest that different mechanisms regulate retinoid homeostasis in the AER and mesenchyme during limb bud outgrowth. J. Exp. Zool. 290:136-147, 2001.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Application of photoaffinity labeling with [(3)H] all trans- and 9-cis-retinoic acids for characterization of cellular retinoic acid--binding proteins I and II

Cellular retinoic acid-binding proteins (CRABPs) are carrier proteins thought to play a crucial role in the transport and metabolism of all-trans-retinoic acid (atRA) and its derivatives within the cell. This report describes a novel photoaffinity-based binding assay involving competition between potential ligands of CRABP and [(3)H]atRA or [(3)H]-9-cis-RA for binding to the atRA-binding sites of CRABP I and II. Photoaffinity labeling of purified CRABPs with [(3)H]atRA was light- and concentration-dependent, saturable, and protected by several retinoids in a concentration-dependent manner, indicating that binding occurred in the CRABP atRA-binding site. Structure-function relationship studies demonstrated that oxidative changes to the atRA beta-ionone ring did not affect ligand potency. However, derivatives lacking a terminal carboxyl group and some cis isomers did not bind to CRABPs. These studies also identified two novel ligands for CRABPs: 5,6-epoxy-RA and retinoyl-beta-D-glucuronide (RAG). The labeling of both CRABPs with 9-cis-RA occurred with much lower affinity. Experimental evidence excluded nonspecific binding of RAG to CRABPs and UDP-glucuronosyltransferases, the enzymes responsible for RAG synthesis. These results established that RAG is an effective ligand of CRABPs. Therefore, photoaffinity labeling with [(3)H]atRA can be used to identify new ligands for CRABP and retinoid nuclear receptors and also provide information concerning the identity of amino acid(s) localized in the atRA-binding site of these proteins.

Mouse retinol binding protein gene: cloning, expression and regulation by retinoic acid

A full-length cDNA clone encoding the retinol binding protein (RBP) was isolated from a mouse liver cDNA library by hybridization screening. The nucleotide sequence of murine RBP is 85 and 95% homologous to that of human and rat RBP, respectively, with a deduced amino acid sequence > or = 83% homologous to both species. Analysis of the tissue expression pattern of RBP mRNA in the female mouse indicated relatively abundant expression in the liver, with lesser amounts in extrahepatic tissues including adipose, kidney, spleen and uterus, suggesting that these tissues may have a significant role in retinol homeostasis. Mouse liver cell RBP regulation by retinoids was also investigated. Both all-trans retinoic acid (AT-RA) and 9-cis retinoic acid (9c-RA) induced RBP mRNA expression in a dose- and time-dependent manner. Maximal levels (up to 4-fold above controls) were observed at > or = 48 h following treatment of both mouse hepatoma cells in vitro and in vivo in mice receiving a single, oral dose of either retinoid. Interestingly, 9c-RA was more potent at RBP induction in both in vivo and in vitro systems. Given the extent and temporal pattern of RBP induction, we suggest that the RA-mediated increase in liver RBP is part of a cellular protection mechanism. Increased levels of RBP would facilitate sequestration and possibly cellular export of RA in cells receiving prolonged exposure to high levels of RA, thus minimizing toxicity.

9-cis-retinoids: biosynthesis of 9-cis-retinoic acid

Retinoids function through conformational alterations of ligand-dependent nuclear transcription factors, the retinoic acid receptors, and retinoid X receptors. 9-cis-Retinoic acid is a known biological ligand for retinoid X receptors, but its synthesis pathway in vivo is largely unknown. Recently, we identified a cis-retinol dehydrogenase (cRDH) that oxidizes 9-cis-retinol to 9-cis-retinal. Since both the expression of cRDH mRNA and its substrate are found in liver, we studied 9-cis-retinol metabolism and 9-cis-retinoic acid biosynthesis in two hepatic-derived cell types, Hep G2 hepatoma cells and HSC-T6 stellate cells. Both cell lines accumulate similar amounts of 9-cis-retinol provided in the medium. However, Hep G2 cells preferentially incorporate all-trans-retinol when equimolar concentrations of all-trans- and 9-cis-retinol were provided. In contrast, HSC-T6 cells did not exhibit a preference between all-trans- and 9-cis-retinol under the same conditions. Esterification of 9-cis-retinol occurred in both cell types, likely by acyl-CoA:retinol acyltransferase and lecithin:retinol acyltransferase. In vitro enzyme assays demonstrated that both cell types can hydrolyze 9-cis-retinyl esters via retinyl ester hydrolase(s). In Hep G2 cells, 9-cis-retinoic acid synthesis was strongly inhibited by high concentrations of 9-cis-retinol, which may explain the low levels of 9-cis-retinol in liver of mice. Cell homogenates of Hep G2 can convert all-trans-retinol to 9-cis-retinal, suggesting that the free form of all-trans-retinol may be used as a source for 9-cis-retinol and, thus, 9-cis-retinoic acid synthesis. Our studies provide the basis for identification of additional pathways for the generation of 9-cis-retinoic acid in specialized tissues.

Visually induced changes in components of the retinoic acid system in fundal layers of the chick

Eye growth is visually regulated via messengers that are released from the retina. The retina involves a yet unknown algorithm to analyse the projected image so that the appropriate growth rates for the back of the eye are ensured. One biochemical candidate that could act as a growth controller, is retinoic acid (RA). Previous work (Seko, Shimokawa and Tokoro, 1996; Mertz et al., 1999) has shown that retinal and choroidal RA levels are indeed predictably changed by visual conditions that cause myopia or hyperopia, respectively. We have studied in which fundal tissues aldehyde dehydrogenase-2 (AHD2) and retinaldehyde dehydrogenase-2 (RALDH2), enzymes involved in RA synthesis, are expressed and at which levels the effects of vision on RA levels may be controlled. Using Northern blot analysis, we have found that the retinal mRNA level of the AHD2 is up-regulated after 3 days of treatment with negative lenses (negative lenses place the image behind the retina). The abundance of the retinal mRNA of a RA receptor, RAR-beta, was up-regulated already after 6 hr of treatment with positive lenses (positive lenses place the image in front of the retina). The up-regulation persisted for at least 1 week. Finally, we have studied the effects of an inhibitor of RA synthesis, disulfiram, on the visual control of eye growth. We found inhibition of myopia as induced by frosted goggles ('deprivation myopia') but no significant inhibitory effects on refractive errors induced by +7D or -7D lenses. Our results are in line with the hypothesis that RA may play a role in the visual control of eye growth. The RA system differs from a number of other candidates (dopamine, cholinergic agents, opiates) in that it distinguishes between positive and negative defocus, similar to the immediate early gene ZENK (Stell et al., 1999). The exact time kinetics of the changes have still to be worked out since it is possible that the changes in RA relate to already occurring changes in growth rather than to initial steps of the signaling cascade.

Retinoic acid hydroxylase (CYP26) is a key enzyme in neuronal differentiation of embryonal carcinoma cells

Besides nuclear retinoid receptors and cellular retinoid binding proteins also retinoic acid (RA)-synthesizing enzymes (using all-trans-retinal as substrate) and RA-catabolizing enzymes (producing hydroxylated products) may explain the specific effects of retinoids. In the past we have established an active role for 4-hydroxy-RA and 4-oxo-RA, which originally were considered to be inactive retinoids, but in fact are highly active modulators of positional specification in Xenopus development. Here we present evidence for a specific role of hydroxylated RA metabolites in the onset of neuronal differentiation. 4-Hydroxy- and 18-hydroxy-RA are products of the hydroxylation of RA by a novel cytochrome P450 (CYP)-type of enzyme, CYP26, expression of which is rapidly induced by RA. P19 embryonal carcinoma (EC) cell lines stably expressing hCYP26 undergo extensive and rapid neuronal differentiation in monolayer at already low concentrations of RA, while normally P19 cells under these conditions differentiate only in endoderm-like cells. Our results indicate that the effects on growth inhibition and RARbeta transactivation of P19 EC cells are mediated directly by RA, while the onset of neuronal differentiation and the subsequent expression of neuronal markers is mediated by hCYP26 via the conversion of RA to its hydroxylated products.

Forebrain induction, retinoic acid, and vulnerability to schizophrenia: insights from molecular and genetic analysis in developing mice

Schizophrenia is thought to be a disease of early development that ultimately affects forebrain neurons and circuits. There may be a relationship between disrupted forebrain development; malformations of the limb, face, and heart; and signaling via the steroid-like hormone retinoic acid (RA) in some schizophrenic patients. The limbs, face, heart, and forebrain all develop from sites where neural crest-derived, RA-producing mesenchyme contributes to induction and differentiation of adjacent epithelia. Induction between neural crest-derived, RA-producing mesenchyme, the anterior neural tube, and the anterior surface epithelium of the embryo guides regional differentiation and pathway formation during forebrain development. Furthermore, there are at least two mouse mutations--in the Pax-6 and Gli-3 genes--that cause peripheral malformations and specifically disrupt neural crest mediated, RA-dependent induction and differentiation in the forebrain. These observations suggest that induction might provide a common target for genes that alter morphogenesis of peripheral structures, disrupt RA-signaling, and compromise forebrain development. In the forebrain, some of these disruptions might influence the numbers or cellular properties of neurons and circuits. Such changes might be reflected in the aberrant forebrain function that characterizes schizophrenia.

Local activation and inactivation of thyroid hormones: the deiodinase family

Tissue-specific activation and inactivation of ligands of nuclear receptors which belong to the steroid retinoid-thyroid hormone superfamily of transcription factors represents an important principle of development- and tissue-specific local modulation of hormone action. Recently, several enzyme families have been identified which act as 'guardians of the gate' of ligand-activated transcription modulation. Three monodeiodinase isoenzymes which are involved in activation the 'prohormone' L-thyroxine (T4), the main secretory product of the thyroid gland, have been identified, characterized, and cloned. Both, type I and type II 5'-deiodinase generate the thyromimetically active hormone 3,3',5-triiodothyronine (T3) by reductive deiodination of the phenolic ring of T4. Inactivation of T4 and its product T3 occurs by deiodination of iodothyronines at the tyrosyl ring. This reaction is catalyzed both the type III 5-deiodinase and also by the type I enzyme, which has a broader substrate specificity. The three deiodinases appear to constitute a newly discovered family of selenocysteine-containing proteins and the presence of selenocysteine in the protein is critical for enzyme activity. Whereas the selenoenzyme characteristics of the type I and type III deiodinases are definitively established some controversy still exists for the type II 5'-deiodinase in mammals. The mRNA probably encoding the type II 5'-deiodinase subunit is markedly longer than those of the two other deiodinases and its selenocysteine-insertion element is located more than 5 kB downstream of the UGA-codon in the 3'-untranslated region. The three deiodinase isoenzymes show a distinct development- and tissue-specific pattern of expression, operate at individual optimal substrate levels, are differently regulated and modulated by hormones, cytokines, signaling pathways, natural factors, and pharmaceuticals. Whereas circulating T3 mainly originates from hepatic production via the type I 5'-deiodinase, the local cellular thyroid hormone concentration in various tissues including the central nervous system is controlled by complex para-, auto-, and intracrine interactions of all three deiodinases. Local thyroid hormone availability is further modulated by conjugation reactions of the phenolic 4'-OH-group of iodothyronines, which also inactivate the thyroid hormones.

Retinoid signaling by all-trans retinoic acid and all-trans retinoyl-beta-D-glucuronide is attenuated by simultaneous exposure of human keratinocytes to retinol

Retinol and retinyl esters are converted with time to slowly increasing amounts of all-trans retinoic acid (RA) in cultured human keratinocytes. Exogenous RA has been shown to limit retinol oxidation and to increase retinol esterification. Because significant amounts of retinol are present in biologic systems, we examined whether RA and all-trans-retinoyl-beta-D-glucuronide (RAG) interact with retinol in exhibiting their activities on HaCaT keratinocytes maintained in a retinoid-free culture system. RA was more potent than RAG and retinol in inducing ultrastructural changes attributed to retinoids, inhibiting cell proliferation as well as enhancing keratin 19 expression. In addition, retinoids were able to induce cellular retinoic acid-binding protein II mRNA levels in the cultures, whereas early RA and late RAG activity was detected. The described biologic effects of RA and RAG were diminished by simultaneous cell exposure to retinol. HaCaT cells quickly metabolized retinol to retinyl esters and consequently to low amounts of RA. RA treatment led to an early high peak of cellular RA followed by reduction to trace amounts. Treatment with RAG resulted in constantly high cellular RAG and low RA levels. Under the combined RA and retinol treatment retinyl esters were increased and RA was reduced in HaCaT cells, whereas extracellular RA levels were similar to those obtained by RA alone. On the other hand, the combination of RAG and retinol resulted in higher extracellular RAG, similar cellular RAG, and lower cellular RA levels than those obtained by RAG alone without any change in retinyl esters. This study demonstrates that retinoid signaling by RA and RAG is attenuated by simultaneous exposure of HaCaT keratinocytes in vitro to retinol. The presence of retinol in the medium alters the rate of RA or RAG metabolism and thus cellular RA concentrations. The intensity of retinoid signal is probably dependent on cellular RA levels. The resulting "antagonism" among retinoids is consistent with the presence of an auto-regulatory mechanism in human keratinocytes offering protection against excessive accumulation of cellular RA.

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

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