From LipidomicsWiki
Vitamin A and its derivatives are essential for vision and via regulation of the expression of target genes, they influence proliferation and differentiation of many cells types, support maintenance of epithelial surfaces, immune competence, reproduction and embryonic growth and development. Vitamin A insufficiency is characterized clinically by several ocular features and impaired resistance to infection. Hypervitaminosis caused by excessive intake of vitamin A however leads to serious impairments in skin, nervous system, musculo-skeletal system and organ function. The term vitamin A is reserved for all compounds that possess the biological activity of retinol. The term retinoids refers to all compounds that consist of four isoprenoid units joined in a head-tail manner and that are derived from a monocyclic parent compound containing five carbon-carbon double bonds and a functional terminal group at the terminus of the acyclic portion [International Union of Pure and Applied Chemistry - International Union of Biochemistry (IUPAC-IUB), 1982]. The parent retinoid compound all-trans retinol is the alcohol form of vitamin A of molecular weight of 286. In animal tissues the predominant retinoid is retinyl palmitate but also retinyl oleate and stearate have been found. Within cells retinol is either esterified with long chain fatty acids or with the ethanolamine groups of phosphatidylethanolamine for storage or converted to active metabolites, including retinal, retinoic acid and hydroxyretroretinols.
Conversion of retinol to retinal is reversible and conversion of retinal to retinoic acid is irreversible. Retinoic acid is responsible for many but not all effects of vitamin A and is inactivated by further oxidation to form polar metabolites (Li & Tso, 2003). All-trans retinol is highly unstable in the presence of antioxidants and light, what leads to its oxidative degradation.
In the retina, retinal pigment cells take up all-trans retinol from the blood. Afterwards all-trans retinol is subsequently esterified by LRAT, converted to 11-cis retinol and either stored as 11-cis retinyl ester or or oxidized by 11-cis retinol dehydrogenase to 11-cis retinal. 11-cis retinal is transferred to photoreceptor cells, where rhodopsin is manufactured from opsin and 11-cis retinal in the Golgi of the rod inner segment and transported to rod outer segment discs. Upon light absorption the 11-cis form of retinal is converted to an all-trans form, which reacts with phosphatidylethanolamine (PE) to form the Schiff-base product N-retinylidene-PE (N-RPE).
ABCA4 is thought to flip N-RPE to the outer leaflet of the disc membrane. There, all-trans retinal is generated by hydrolysis of N-RPE and subsequently reduced to all-trans retinol by retinol-dehydrogenase prior to its delivery to the retinal pigment epithelial cells and re-esterification. Under the effect of short-wave light or in ABCA4 deficiency, all-trans retinal accumulates, causing photooxidative damage and generation of toxic A2E (N-retinyl-N-retinylidene ethanolamine) (Schmitz & Langmann, 2003).
It has been hypothesized that retinoid deregulation or deficiency may play a role in the etiology of neurodegenerative and mood disorders. This observation is based on the fact that genes coding for retinoic acid nuclear receptors RAR, RXR, retinoid transport proteins and retinoid metabolizers are located in proximity of genes linked to schizophrenia (Goodman, 2006). Human and animals are not able to synthesize retinoids de novo and they have to obtain vitamin A from the diet. There are two dietary sources of vitamin A in humans: as preformed vitamin A from animal sources and as provitamin A carotenoids from plants, which are converted within the body to retinal and retinol. ß-carotene is one of the most abundant carotenoid found in human diet and the most potent vitamin A precursor (During & Harrison, 2004). Two pathways have been described for the cleavage of ß-carotene to retinoids: central and eccentric. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, beta-carotene 15,15'-monooxygenase 1 (BCMO1), which cleaves ß-carotene at its central double bond to yield retinal, a direct precursor of retinol (vitamin A), and retinoic acid (an active form of vitamin A). Two mechanisms for the enzymatic central cleavage of ß-carotene have been proposed. The first is a dioxygenase reaction that requires molecular oxygen and yields an unstable dioxetane intermediate that is rapidly converted into retinal. The second pathway of ß-carotene metabolism is the eccentric cleavage, which occurs at double bonds other than the central 15,15´-double bond of the polyene chain of ß-carotene to produce ß-apo-carotenals with different chain lengths. Based on in vitro observations, it was suggested that eccentric cleavage could occur preferentially under oxidative conditions (when antioxidants are insufficient) such as smoking and diseases involving an oxidative stress and/or in the presence of high ß-carotene levels (Yeum et al., 2000). In contrast, under normal physiological conditions (when antioxidants are present), central cleavage would be the predominant pathway. The two major sites of ß-carotene conversion in humans are the intestine and liver. The in vivo intestinal absorption of carotenoids involves several crucial steps: (1) release of carotenoids from the food matrix, (2) solubilization of carotenoids into mixed lipid micelles in the lumen, (3) cellular uptake of carotenoids by intestinal mucosal cells, (4) incorporation of carotenoids into chylomicrons (CM), and (5) secretion of carotenoids and their metabolites associated with CM into the lymph (Yeum & Russell, 2002). Mechanisms involved in the digestion and absorption of dietary retinyl esters require the participation of several proteins. Dietary retinyl esters are hydrolysed in the intestine by the pancreatic enzyme, pancreatic triglyceride lipase, and intestinal brush border enzyme, phospholipase B. Unesterified retinol taken up by the enterocyte is complexed with cellular retinol-binding protein type 2 (CRBP-II) and the complex serves as a substrate for reesterification of retinol with fatty acids (mainly palmitate) by the enzyme lecithin: retinol acyltransferase (LRAT) in the enterocytes after absorption. In intestinal mucosa cells, retinyl esters are incorporated into chylomicrons, together with other dietary lipids, such as triglycerides, phospholipids, free and esterified cholesterol, and apolipoprotein B. The Chylomicrons containing the newly absorbed retinyl esters are then secreted into the lymph. Following secretion into the lymph, chylomicrons are transported into the circulation, where triacylglycerol hydrolysis and apolipoprotein exchange result in the formation of chylomicron remnants and nearly all retinyl esters remain with chylomicrons. The liver takes up most of the postprandial chylomicron-associated retinyl esters. Within hepatocytes, retinyl esters are hydrolyzed and retinol is either transferred to hepatic stellate cells (HSC) for storage, or bound to retinol binding protein (RBP) and secreted into circulation, or metabolized by oxidative enzymes. Binding of retinol to RBP initiates translocation of retinol-RBP complexes from the endoplasmatic reticulum to the Golgi complex, and secretion into the plasma. RBP is a physiological carrier of vitamin A from the blood to the organs and is responsible for evolutionary important adaptation of vertebrates to fluctuation of vitamin A.
HSC exist in the space between liver parenchymal cells and sinusoidal endothelial cells of the hepatic lobule, and quiescent HSC store 50-80% of the body’s total retinol as retinyl-palmitate in lipid droplets in the cytoplasm (Senoo, 2004; Kawaguchi et al., 2007). The transcription factors PPARγ, C/EBP, SREBP-1c and LXR retain the quiescent state of HSC and their expression is lost along with depletion of lipids in activated HSC (Tsukamoto, 2005).
In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homeostasis; they express specific receptors for RBP on their cell surface, and take up the complex of retinol and RBP by receptor-mediated endocytosis. (Senoo et al., 2007). Cellular binding protein (CRBP-I) and lecithin: retinol acyltransferase (LRAT) are highly expressed in HSC and play a key role in the storage of retinyl esters (Batten et al., 2004). Although under normal dietary conditions much of the dietary vitamin A is absorbed via the chylomicron/lymphatic route, it is also clear that under some circumstances there is substantial absorption of unesterified retinol via the portal vein route. Evidence supports the idea that lipid transporters mediate cellular uptake and efflux of unesterified retinol by enterocytes (Harrison, 2005). Only recently, it has been reported that STRA6, a widely expressed multitransmembrane domain protein, fulfils the criteria of a RBP receptor that mediates cellular uptake of vitamin A (Kawaguchi et al., 2007). The high expression of STRA6 in various tissues is consistent with the role of RBP to deliver vitamin A to these tissues.
All-trans–retinol is the major source of active retinoid metabolites. The synthesis of all-trans-retinoic acid from all-trans-retinol occurs in a two-step reaction. The rate-limiting step is the oxidation of retinol to retinal catalyzed by cytosolic medium-chain alcohol dehydrogenases 1, 3 and 4 (ADH1,3,4) and members of the short-chain dehydrogenase (SDR) family: RDH1, RDH5, RDH11. The final oxidation of retinal to retinoic acid is catalyzed by retinal dehydrogenase 1 (RALDH1). The major function of retinoic acid is the activation of transcription factors to which retinoic acid binds as an activating small molecular ligand.
Catabolism of retinoids The catabolism of retinoic acid is an important mechanism for controlling the level of retinoic acid in tissues and cells. The cytochrome P450 enzymes CYP26A1, CYP26B1 and CYP26C1 were found to degrade retinoic acid (White et al., 1997; White et al., 2000; Taimi et al., 2004). Retinoic acid is metabolized to its polar metabolites including 4-hydroxy retinoic acid, 4-oxo retinoic acid, 18-hydroxy retinoic acid, 5,6-epoxy retinoic acid and 5,8-retinoic acid. Glucuronides may be also formed from retinol and retinoic acid, excreted later into bile and urine. The catabolizing enzymes show different tissue expression patterns suggesting individual roles for each of them in the catabolism of retinoic acid. Cellular retinoic acid binding protein type I (CRABP-I) seems to play a role in the regulation of retinoic acid catabolism. Overexpression of CRABP-I accelerates the degradation of retinoic acid (Boylan & Gudas, 1992).
RETINOID X RECEPTORS, THE COMMON HETERODIMER PARTNERS
The most intensively studied members of the orphan nuclear receptor subfamilies are the retinoid X receptors (RXR). The identification of the vitamin A derivative, 9-cis retinoic acid, as an endogenous ligand for the RXRs represented the discovery of the first true orphan nuclear receptor ligand and ushered in the age of orphan nuclear receptors (Mangelsdorf and Evans, 1995). Subsequent studies have shown that RXRs also can be activated by a variety of dietary lipids, including docosahexaenoic acid (DHA), a toxic plant lipid called phytanic acid, and the insecticide derivative methoprene acid (Giguere, 1999; de Urquiza et al., 2000). In terms of nuclear receptor signaling, one of the most important advances to come from the discovery of the RXRs was the finding that they function as obligate heterodimer partners for other nuclear receptors (Mangelsdorf and Evans, 1995). Thus, RXRs typically do not function alone, but rather serve as master regulators of several crucial regulatory pathways. The evolution of the heterodimeric nuclear receptors has permitted a unique, but simple, mechanism for expanding the repertoire of lipid signaling pathways. Therefore, it is not surprising that the lipid sensing receptors that have been identified thus far are all RXR heterodimers. The recognition that some RXR heterodimers are permissive for activation by RXR ligands has led to the finding that potent synthetic RXR agonists (called rexinoids) have dramatic effects on lipid homeostasis (Mukherjee et al., 1998; Repa et al., 2000).
There is also upcoming evidence that retinoids, such as vitamin A and its derivatives influence triglyceride and cholesterol metabolism as well as antiatherosclerotic mechanisms by limiting oxidation of LDL particles and controlling ABC transporter expression, which prevents foam cell formation (Esterbauer et al., 1992; Witztum & Steinberg, 1991). In the plasma, vitamin A is bound to RBP and after cellular uptake; retinol is converted to all-trans retinoic acid (ATRA) and 9-cis retinoic acid (9-cis RA) by retinol dehydrogenases (Giguere, 1994). Retinoids exert their biological effects via high affinity binding to nuclear receptors (Mangelsdorf, 1994). RXR are mainly activated by 9-cis RA, whereas retinoic-acid-receptors (RAR) bind 9-cis RA and ATRA (Allenby et al., 1993). Although retinol and β-carotene are only marginally metabolized to ATRA and 9-cis RA, these retinoids display a very high biological activity. Three different isoforms of RAR and RXR, α, β, and γ have been described with one single gene for each subtype (Chambon, 1996). Upon activation, both receptors after heterodimerization are capable of binding to retinoic acid responsive elements (RARE).
The global gene expression pattern of primary human monocytes and in vitro differentiated macrophages stimulated with physiological concentrations of different retinoids identified a synchronously upregulated cluster of genes important for lipid metabolism including ABCA1, ABCG1, CYP27A1 and LXRα (Costet et al., 2003). In line with these data, other reports have shown that ABCA1 and LXRα can be induced at the mRNA and protein level by incubation of murine macrophages and THP-1 cells with ATRA (Costet et al., 2003; Wagsater et al., 2003). Although Wagsater et al. reported that the amount of ABCG1 mRNA was not changed by treatment with ATRA (Wagsater et al., 2003), the results clearly demonstrate that 9-cis RA and ATRA are potent inducers of ABCG1 transcription in primary human monocytes as well as in THP-1 cells. Interestingly, the CYP27A1 gene, which is critically important for the mitochondrial conversion of cholesterol to 27-hydroxycholesterol in macrophages, was highly induced by 9-cis RA, ATRA, and β-carotene via an RARE (Szanto et al., 2004). Since LXRα is also upregulated by retinoids this could imply that an indirect oxysterol-dependent activation of ABCA1 and ABCG1 via LXRα and 27-hydroxycholesterol as newly synthesized ligand could be elicited by retinoids. Furthermore, direct binding of RAR/RXR heterodimers to the DR4 element in the ABCA1 promoter could be a signaling pathway bypassing LXR-dependent mechanisms. A major side effect of retinoids including ATRA that is used as therapeutic agent for the treatment of several forms of cancer (Soprano et al., 2004) is the development of hypertriglyceridemia.
Yang et al. observed a common induction of fatty acid metabolism genes (fatty acid synthase, fatty acid desaturase 2, monoglyceride lipase), apolipoprotein genes (apoC-I, apoC-II, ApoC-IV, and apoE) and SREBP1c, which is in agreement with previous results, demonstrating that hepatic apoC-III transcription is mediated via the RXR-specific synthetic agonist LG1069 (Yang et al., 2001). In this line, Costet et al. have also noticed a slight increase in SREBP1c transcription in mouse macrophages stimulated with ATRA (Costet et al., 2003). Furthermore, upregulation of ACAT1 by ATRA in THP-1 cells may cause storage of cholesteryl esters in lipid droplets (Yang et al., 2001). These findings collectively have encouraged the hypothesis that retinoids and RAR/RXR-mediated gene induction may promote foam cell formation and thereby favor the development of atherosclerosis. However, the recent findings that genes important for lipid efflux such as ABCA1, ABCG1, CYP27A1, LXRα, and apoE are effectively upregulated by retinoids point out that the net effect of stimulation with these compounds may not result in an augmented uptake and storage of lipids in macrophages. Macrophages incubated in the presence of 9- cis RA and ATRA showed a strong induction of both phospholipid and cholesterol efflux to apoA-I as an acceptor counterbalancing potential esterification and storage mechanisms (Barbier et al., 2002). In conclusion, retinoids are potent inducers of genes related to human lipid metabolism and especially of genes involved in reverse cholesterol transport associated with increased macrophage lipid efflux such as ABCA1 and ABCG1.
Technology
Analysis methods
- Retinoid determination - Gunderson et al.
- Retinoid determination - Kane et al.
- Retinoid determination - Wang et al.
Chemical synthesis
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