Intracellular lipid binding proteins and nuclear receptors involved in branched-chain fatty acid signaling

Branched-chain fatty acids are potent regulators of gene expression. Among them are the vitamin A-derived retinoic acids, which are involved in cell growth and differentiation, and the chlorophyll-derived phytol metabolites such as phytanic and pristanic acids, which affect catabolic lipid metabolism. Gene expression regulated by these signaling molecules is mediated by two protein families. These are, on the one hand, the intracellular lipid binding proteins, i.e. cellular retinoic acid binding protein and liver-type fatty acid binding protein, which are responsible for ligand-transport to the nucleus. On the other hand are the ligand-activated nuclear receptors, i.e. the retinoic acid receptors for retinoic acids and the peroxisome proliferator-activated receptors for the phytol metabolites. In this review, we discuss the cross-talk between the two protein families and how this cross-talk contributes to targeted signaling with branched-chain fatty acids.     

Expression of cellular retinoic acid-binding protein I and II (CRABP I and II) in embryonic mouse hearts treated with retinoic acid

Cellular retinoic acid binding proteins are considered to be involved in retinoic acid (RA) signaling pathways. Our aim was to compare the expression and localization of cellular retinoic acid binding proteins I and II (CRABP I and II) in embryonic mouse hearts during normal development and after a single teratogenic dose of RA. Techniques such as real-time PCR, RT-PCR, Western blots and immunostaining were employed to examine hearts from embryos at 9-17 dpc. RA treatment at 8.5dpc affects production of CRABP I and II in the heart in the 48-h period. Changes in expression of mRNA for retinaldehyde dehydrogenase II (Raldh2), Crabp1 and Crabp2 genes also occur within the same time window (i.e. 10-11dpc) after RA treatment. In the embryonic control heart these proteins are localized in groups of cells within the outflow tract (OT), and the atrioventricular endocardial cushions. A gradient of labeling is observed with CRABP II but not for CRABP I along the myocardium of the looped heart at 11 dpc; this gradient is abolished in hearts treated with RA, whereas an increase of RALDH2 staining has been observed at 10 dpc in RA-treated hearts. Some populations of endocardial endothelial cells were intensively stained with anti-CRABP II whereas CRABP I was negative in these structures. These results suggest that CRABP I and II are independently regulated during heart development, playing different roles in RA signaling, essential for early remodeling of the heart tube and alignment of the great arteries to their respective ventricles.     

Ultraviolet irradiation of human skin causes functional vitamin A deficiency, preventable by all-trans retinoic acid pre-treatment

We report here that ultraviolet irradiation substantially reduced the mRNA and protein of the two major nuclear retinoid receptors, RAR-gamma and RXR-alpha, in human skin in vivo. Pre-treatment with retinoic acid mitigated this loss of nuclear retinoid receptors. Ultraviolet irradiation caused a near-total loss of retinoic acid induction of two RAR/RXR target genes, cellular retinoic acid binding protein-II and RA 4-hydroxylase, but did not affect 1,25-dihydroxyvitamin D3 induction of the vitamin D receptor/RXR-regulated gene vitamin D 24-hydroxylase. In effect, ultraviolet irradiation causes a functional vitamin A deficiency that may have deleterious effects on skin function, contributing to skin photo-aging and carcinogenesis.     

Regionalized metabolic activity establishes boundaries of retinoic acid signalling.

The competence of a cell to respond to the signalling molecule retinoic acid (RA) is thought to depend largely on its repertoire of cognate zinc finger nuclear receptors. XCYP26 is an RA hydroxylase that is expressed differentially during early Xenopus development. In Xenopus embryos, XCYP26 can rescue developmental defects induced by application of exogenous RA, suggesting that the enzymatic modifications introduced inhibit RA signalling activities in vivo. Alterations in the expression pattern of a number of different molecular markers for neural development induced upon ectopic expression of XCYP26 reflect a primary function of RA signalling in hindbrain development. Progressive inactivation of RA signalling results in a stepwise anteriorization of the molecular identity of individual rhombomeres. The expression pattern of XCYP26 during gastrulation appears to define areas within the prospective neural plate that develop in response to different concentrations of RA. Taken together, these observations appear to reflect an important regulatory function of XCYP26 for RA signalling; XCYP26-mediated modification of RA modulates its signalling activity and helps to establish boundaries of differentially responsive and non-responsive territories.     

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.     

维甲酸致小鼠胚胎畸形发生过程中维甲酸受体α/β及β-catenin和caspase-3基因的表达变化

流行病学研究显示,在胚胎发育过程中摄入过多维甲酸可致各种发育缺陷,其中神经管畸形最为常见. 因此有必要探明维甲酸致各种发育缺陷的发生机制,以便为各种生长缺陷的预防和治疗提供实验依据. 用 RT-PCR 及蛋白质印迹技术,探测了过量维甲酸对昆明小鼠胚胎神经管中维甲酸受体α/β及β-catenin 和 caspase-3 基因表达的调整. 结果显示,在神经管闭合期过量维甲酸显著降低了维甲酸受体α/β及β-catenin 和 caspase-3 的基因表达,神经管闭合后,维甲酸受体β、β-catenin 及 caspase-3 的基因表达又出现了一个明显的回升过程. 提示,过量维甲酸改变了昆明小鼠胚胎神经管中维甲酸受体α/β及β-catenin 和 caspase-3 基因的正常时间表达模式,这种异常的基因表达模式可能参与了维甲酸致昆明小鼠胚胎畸形的发生机制.     

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

The vitamin A metabolite, retinoic acid, carries out essential and conserved roles in vertebrate heart development. Retinoic acid signals via retinoic acid receptors (RAR)/retinoid X receptors (RXRs) heterodimers to induce the expression of genes that control cell fate specification, proliferation, and differentiation. Alterations in retinoic acid levels are often associated with congenital heart defects. Therefore, embryonic levels of retinoic acid need to be carefully regulated through the activity of enzymes, binding proteins and transporters involved in vitamin A metabolism. Here, we review evidence of the complex mechanisms that control the fetal uptake and synthesis of retinoic acid from vitamin A precursors. Next, we highlight recent evidence of the role of retinoic acid in orchestrating myocardial compact zone growth and coronary vascular development.     

Colloquium 14: New Functions for Retinoic Acid in the Developing CNS. The role of retinoic acid in the patterning of the developing nervous system

Retinoic acid (RA) is metabolised from its precursor, retinol (vitamin A). In mammalian embryos, retinol is provided by the mother via the placenta and in birds retinol comes from the yolk. We have studied the role of RA in CNS development in quail embryos by depriving adult quails of retinol in the diet which results in them laying eggs which have no retinol stores. The resulting embryos are therefore retinol and RA deficient. The CNS of these embryos is abnormal in three regards; patterning, neural crest production and neurite outgrowth. With regard to patterning, at an early stage of development prior to somitogenesis, hindbrain patterning genes are not induced which leads to the respecification of the posterior hindbrain territory. This region is not lost from the embryo but instead becomes transformed into an enlarged anterior hindbrain. Another aspect of patterning that is abnormal in these RA deficient embryos is the dorsoventral gene expression domains in the anterior spinal cord. These domains are required for the proper specification of motor neurons, sensory neurons and various classes of interneurons. Consequently these neuronal classes are mis‐specified in the RA deficient embryos. With regard to the neural crest, these cells often fail to migrate correctly and then die in the absence of RA. With regard to neurite outgrowth, very little outgrowth seems to take place in these deficient embryos which suggests that RA is involved in neurite outgrowth. Taking these experiments into the adult to examine the role of RA in neurite regeneration, we have had success in inducing neurite outgrowth in vitro from adult mouse spinal cord by manipulating the retinoic acid receptors which transduce the RA signal at the level of the nucleus.     

Retinoids and their targets in vertebrate development

The elaboration of the effect of retinoic acid on limb morphogenesis has prompted renewed investigation into the teratology of retinoic acid treatment, with the hope that such analysis might give insight into mechanisms of vertebrate patterning. Retinoids, their nuclear receptors and their cytoplasmic binding proteins are now known to be deployed throughout development, but the extent to which they are natural agents of morphogenesis remains obscure. The study of retinoic acid receptors may offer molecular insight into gene regulation underlying vertebrate pattern formation.