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Regulation of retinal dehydrogenases and retinoic acid synthesis by cholesterol metabolites.

ABSTRACT: Retinoic acid (RA) constitutes the major active ingredient of vitamin A and is required for various biological processes. The tissue RA level is maintained through a cascade of metabolic reactions where retinal dehydrogenases (RALDHs) catalyze the terminal reaction of RA biosynthesis from retinal, a rate-limiting step. We showed that dietary supplement of cholesterol enhanced the expression of RALDH1 and 2 genes and the cellular RA content in vital organs such as brain, kidney, liver and heart. Consistently, the cholesterol-lowering agent (pravastatin sodium) downregulated the expression of RALDH1 and 2 genes in several organs especially the liver and in cultured liver cells. Further, cholesterol metabolites, predominantly the oxysterols, the natural ligands for liver X receptor (LXR), induced these genes via upregulation of sterol regulatory element binding protein-1c (SREBP-1c) that bound to the regulatory regions of these genes. Knockdown of LXRalpha/beta or SREBP-1c downregulated the expression of RALDH genes, which could be rescued by re-expressing SREBP-1c, suggesting SREBP-1c as a direct positive regulator for these genes. This study uncovered a novel crosstalk between cholesterol and RA biosynthesis.

SUBMITTER: Huq MD

PROVIDER: S-EPMC1500992 | biostudies-literature | 2006 Jul

REPOSITORIES: biostudies-literature

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Regulation of retinal dehydrogenases and retinoic acid synthesis by cholesterol metabolites.

Huq M D Mostaqul MD Tsai Nien-Pei NP Gupta Pawan P Wei Li-Na LN

The EMBO journal 20060608 13

Retinoic acid (RA) constitutes the major active ingredient of vitamin A and is required for various biological processes. The tissue RA level is maintained through a cascade of metabolic reactions where retinal dehydrogenases (RALDHs) catalyze the terminal reaction of RA biosynthesis from retinal, a rate-limiting step. We showed that dietary supplement of cholesterol enhanced the expression of RALDH1 and 2 genes and the cellular RA content in vital organs such as brain, kidney, liver and heart. ...[more]

PMID: 16763553

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Project description:BackgroundCytochrome 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.HypothesisParallel effects of Cyp1b1 and retinol on postnatal Srebp derive from effects in the developing liver or systemic signaling.ApproachCluster 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.ResultsAt 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.ConclusionCyp1b1-/- 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.

Project description:Xenopus laevis is among the few species that are capable of fully regenerating a lost lens de novo. This occurs upon removal of the lens, when secreted factors from the retina are permitted to reach the cornea epithelium and trigger it to form a new lens. Although many studies have investigated the retinal factors that initiate lens regeneration, relatively little is known about what factors support this process and make the cornea competent to form a lens. We presently investigate the role of Retinoic acid (RA) signaling in lens regeneration in Xenopus. RA is a highly important morphogen during vertebrate development, including the development of various eye tissues, and has been previously implicated in several regenerative processes as well. For instance, Wolffian lens regeneration in the newt requires active RA signaling. In contrast, we provide evidence here that lens regeneration in Xenopus actually depends on the attenuation of RA signaling, which is regulated by the RA-degrading enzyme CYP26. Using RT-PCR we examined the expression of RA synthesis and metabolism related genes within ocular tissues. We found expression of aldh1a1, aldh1a2, and aldh1a3, as well as cyp26a1 and cyp26b1 in both normal and regenerating corneal tissue. On the other hand, cyp26c1 does not appear to be expressed in either control or regenerating corneas, but it is expressed in the lens. Additionally in the lens, we found expression of aldh1a1 and aldh1a2, but not aldh1a3. Using an inhibitor of CYP26, and separately using exogenous retinoids, as well as RA signaling inhibitors, we demonstrate that CYP26 activity is necessary for lens regeneration to occur. We also find using phosphorylated Histone H3 labeling that CYP26 antagonism reduces cell proliferation in the cornea, and using qPCR we find that exogenous retinoids alter the expression of putative corneal stem cell markers. Furthermore, the Xenopus cornea is composed of an outer layer and inner basal epithelium, as well as a deeper fibrillar layer sparsely populated with cells. We employed antibody staining to visualize the localization of CYP26A, CYP26B, and RALDH1 within these corneal layers. Immunohistochemical staining of these enzymes revealed that all 3 proteins are expressed in both the outer and basal layers. CYP26A appears to be unique in also being present in the deeper fibrillar layer, which may contain cornea stem cells. This study reveals a clear molecular difference between newt and Xenopus lens regeneration, and it implicates CYP26 in the latter regenerative process.

Project description:BackgroundRetinoic acid (RA) is important for vertebrate eye morphogenesis and is a regulator of photoreceptor development in the retina. In the zebrafish, RA treatment of postmitotic photoreceptor precursors has been shown to promote the differentiation of rods and red-sensitive cones while inhibiting the differentiation of blue- and UV-sensitive cones. The roles played by RA and its receptors in modifying photoreceptor fate remain to be determined.ResultsTreatment of zebrafish embryos with RA, beginning at the time of retinal progenitor cell proliferation and prior to photoreceptor terminal mitosis, resulted in a significant alteration of rod and cone mosaic patterns, suggesting an increase in the production of rods at the expense of red cones. Quantitative pattern analyses documented increased density of rod photoreceptors and reduced local spacing between rod cells, suggesting rods were appearing in locations normally occupied by cone photoreceptors. Cone densities were correspondingly reduced and cone photoreceptor mosaics displayed expanded and less regular spacing. These results were consistent with replacement of approximately 25% of positions normally occupied by red-sensitive cones, with additional rods. Analysis of embryos from a RA-signaling reporter line determined that multiple retinal cell types, including mitotic cells and differentiating rods and cones, are capable of directly responding to RA. The RA receptors RXRγ and RARαb are expressed in patterns consistent with mediating the effects of RA on photoreceptors. Selective knockdown of RARαb expression resulted in a reduction in endogenous RA signaling in the retina. Knockdown of RARαb also caused a reduced production of rods that was not restored by simultaneous treatments with RA.ConclusionsThese data suggest that developing retinal cells have a dynamic sensitivity to RA during retinal neurogenesis. In zebrafish RA may influence the rod vs. cone cell fate decision. The RARαb receptor mediates the effects of endogenous, as well as exogenous RA, on rod development.

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Regulation of retinal dehydrogenases and retinoic acid synthesis by cholesterol metabolites.

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Regulation of retinal dehydrogenases and retinoic acid synthesis by cholesterol metabolites.

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