Vitamin A Homeostasis and Activation into atRA

Intestinal absorptive cells absorb dietary carotenoids and retinol during the bile-acid-mediated process of lipid absorption. Within the enterocyte, central cleavage by a soluble 63-kDa carotene 15,15'-monooxy-genase catalyzes the principal route of carotenoid metabolism (Figure 4). Carotene 15,15'-monooxy-genase belongs to the same gene family as RPE65 (the mouse proteins have only 37% amino acid identity, however), suggesting a family of proteins/ enzymes dedicated to transport/metabolism of highly hydrophobic substances. Intestine expresses the carotene 15,15'-monooxygenase mRNA, but kidney and liver show much more intense expression, and the testis curiously shows most intense expression, consistent with the ability of tissues other than the intestine to cleave carotenoids. Carotene 15,15'-monooxygenase also metabolizes car-otenoids without provitamin A activity, such as lycopene, although with lower efficiency.

CRBP(II) sequesters atRCHO generated from car-otenoids and allows its reduction into atROH, catalyzed by an ER retinal reductase (uncharacterized). In contrast to CRBP(I), CRBP(II) does not allow oxidation/dehydrogenation of its ligands. LRAT accesses the CRBP(II)-atROH complex and produces atRE for incorporation into chylomicrons. During conversion into remnants by lipoprotein lipase in adipose, chylomicrons retain most of their RE, as they do cholesterol esters.

Figure 4 Model of atRA biogeneration in mammals. REH, retinyl ester hydrolase (e.g., ES4 and ES10); TTR, transthyretin; RAR-RXR, the heterodimer of retinoic acid receptors with retinoid X receptors; atRCHO, all-trans-retinal; atROH, all-trans-retinol; CRBP(I), cellular retinol binding protein, type I; LRAT, lecithin:retinol acyltransferase; SRBP, serum retinol binding protein. CRBP(I), CRABP(I), and CRABP(II) have been placed in the same cell for simplicity. This does not necessarily occur in vivo.

Figure 4 Model of atRA biogeneration in mammals. REH, retinyl ester hydrolase (e.g., ES4 and ES10); TTR, transthyretin; RAR-RXR, the heterodimer of retinoic acid receptors with retinoid X receptors; atRCHO, all-trans-retinal; atROH, all-trans-retinol; CRBP(I), cellular retinol binding protein, type I; LRAT, lecithin:retinol acyltransferase; SRBP, serum retinol binding protein. CRBP(I), CRABP(I), and CRABP(II) have been placed in the same cell for simplicity. This does not necessarily occur in vivo.

Hepatocytes sequester RE and cholesteryl esters by receptor-mediated endocytosis of chylomicron remnants. Substantial RE hydrolysis apparently occurs before engulfing of the remnants by lysosomes. CRBP(I) sequesters the atROH released and allows esterification by LRAT but protects from esterifica-tion via other acyltransferases, just like CRBP(II) functions in the intestine. Ultimately, liver stellate cells accumulate most of the RE. CRBP(I) seems necessary for retinoid transfer from hepatocytes to stellate cells because the CRBP(I) null mouse does not accumulate RE in stellate cells. The mechanism of transfer, however, has not been established.

Liver senses local and extrahepatic need for atRA biosynthesis by an unknown signal (possibly atRA) and need for atROH in the visual cycle, and it responds by mobilizing RE to maintain serum retinol levels. ES-10 and ES-4, two neutral ER-localized, bile salt-independent carboxyesterases, provide at least 94% of the RE hydrolysis activity in liver. Kidney also expresses ES-4, and kidney, testis, lung, and skin express ES-10. Either SRBP or CRBP(I) sequesters the atROH released. SRBP delivers atROH to serum, whereas CRBP(I), unlike CRBP(II), allows dehydrogenation into atRCHO to support atRA biosynthesis. (Figure 4 shows atRA biogeneration only in target cells, but it also occurs in liver; likewise, RE storage also occurs in target cells in animals exposed to higher dietary atROH, suggesting that the liver does not initially sequester all dietary retinoids.) What keeps the CRBP(I)-bound atROH from undergoing futile cycling back to atRE? Apo-CRBP(I) stimulates endogenous microsomal RE hydrolysis and inhibits LRAT (Figure 5). Note that apo-CRBP(I) exerts potent effects at concentrations of ^2.5 mM—well within the range of the CRBP(I) expressed in liver. Thus, the ratio apo-CRBP/holo-CRBP signals atROH status and directs atROH flux into or out of atRE.

SRBP-atROH circulates as a complex with trans-thyretin (TTR) that protects it from degradation. The mechanism of atROH delivery from SRBP into

Figure 5 The effect of apo-CRBP(I) on the rates of retinol esterification (lecithin:retinol acyltransferase (LRAT)) and retinyl ester hydrolysis (REH).

cells has not been established. Some data suggest a specific SRBP membrane receptor, whereas other data indicate that CRBP(I) pulls atROH transfer from SRBP through the membrane. A third hypothesis is that an SRBP receptor is mainly in the eye, the quantitatively major site of atROH consumption.

Extrahepatic cell atROH supports atRA biosynthesis but also undergoes esterification if delivered in sufficient quantities. CRBP(I) allows dehydrogena-tion of atROH by an ^35-kDa ER retinol dehydrogenase (RDH) but protects atROH from dehydrogenation via other dehydrogenases. RDH (Rdh1) belongs to the SDR gene family. The SDR gene family consists of ^50 mammalian members that catalyze intermediary metabolism, and the metabolism of steroids and prostaglandins, in addition to retinoids.

The mechanism of atROH transfer from CRBP(I) or -(II) to RDH and LRAT has not been elucidated. Specific cross-linking of holo-CRBP(I) with both RDH1 and LRAT, however, indicates close proximity of CRBP(I) to the two enzymes. Notably, apo-CRBP(I) at concentrations of 1 mM prevents cytosolic dehydrogenation of atROH, even by soluble enzymes that access the CRBP(I)-atROH complex, while not impacting ER SDR until reaching much higher concentrations. This suggests the importance of membrane RDH, rather than soluble dehydrogenases, to atRA biosynthesis. Obviously, atRA biosynthesis in vivo occurs in the absence of CRBP(I), as indicated by lack of morphological pathology in the CRBP(I) null mouse. This was predicted by in vitro experiments that showed that neither RDH nor LRAT require presentation of atROH by CRBP(I). Rather, CRBP(I) operates as a molecular chaperone that restricts metabolism to enzymes that can access its atROH.

In the early 1950s, cytosolic alcohol dehydro-genases (ADHs) were suggested to metabolize atROH. This was an attempt to explain atRCHO

generation and the controversial and poorly understood putative occurrence (at the time) of atRA in tissues, when other families of dehydrogenases remained anonymous. ADHs do recognize atROH in vitro, albeit only when presented free of CRBP(I) and with comparatively low efficiencies. However, enzymes have poor substrate discrimination in vitro and do not metabolize many of the same substrates in vivo because of intracellular constraints. Mice null in both ADH class I (Adh1) and ADH class IV (Adh4) show no vitamin A-deficiency phenotype, nor do mice null in ADH class III (Adh3). Adh1 null mice show a decreased rate of metabolism of 50-100 mg/kg atROH, but this demonstrates only that extraordinary high exposure can defeat physiological controls imposed by retinoid binding proteins. Vitamin A excess has not been a problem throughout evolution (mice do not usually eat polar bear or marine fish liver): No pressure forced evolution of protective mechanisms against such exposure. Natural selection exerted the opposite pressure (i.e., evolution of retinoid binding proteins to conserve vitamin A).

Retinal dehydrogenases (RALDHs) catalyze the irreversible conversion of atRCHO into atRA and can do so in the presence of CRBP(I) using atRCHO generated in situ from CRBP(I)-atROH and RDH. These ^54-kDa soluble enzymes belong to the ADLH gene family. RALDH1 (Aldh1a1), -2 (Aldh1a2), and -3 (Aldh1a3) contribute most to atRA generation, whereas RALDH4 has much more efficient activity with 9-cis-retinal. The RALDH1 null mouse remains fertile and healthy but may have decreased ability to produce atRA in the liver. The RALDH2 null mouse dies in utero by midgestation, demonstrating its unique contribution to atRA synthesis during embryogenesis. The situation may differ in the adult. RALDH1-3 show overlapping expression patterns in the adult, with RALDH1 expressed most intensely. Interestingly, atRA regulates mRNA levels of RALDH1 differently in different tissues. For example, vitamin A sufficiency increases kidney and liver RALDH1 mRNA, whereas vitamin A insufficiency increases testis RALDH1 mRNA. This may represent a mechanism to divert atROH from liver to testis for atRA production during vitamin A scarcity.

100 Weight Loss Tips

100 Weight Loss Tips

Make a plan If you want to lose weight, you need to make a plan for it. Planning involves setting your goals both short term and long term ones. With proper planning, you would be able to have an effective guide on the steps that you want to take, towards losing pounds of weight. Aside from that, it would also keep you motivated.

Get My Free Ebook


Post a comment