Enzymes and binding proteins affecting retinoic acid concentrations

Crystal structure of apo-cellular retinoic acid-binding protein type II (R111M) suggests a mechanism of ligand entry

The crystal structure of unliganded mutant R111M of human cellular retinoic acid-binding protein type II (apo-CRABPII (R111M)) has been determined at 2.3 A and refined to a crystallographic R-factor of 0. 18. Although the mutant protein has lower affinity for all-trans-retinoic acid (RA) than the wild-type, it is properly folded, and its conformation is very similar to the wild-type. apo-CRABPII (R111M) crystallizes in space group P1 with two molecules in the unit cell. The two molecules have high structural similarity except that their alpha2 helices differ strikingly. Analyses of the molecular conformation and crystal packing environment suggest that one of the two molecules assumes a conformation compatible with RA entry. Three structural elements encompassing the opening of the binding pocket exhibit large conformational changes, when compared with holo-CRABPII, which include the alpha2 helix and the betaC-betaD and betaE-betaF hairpin loops. The alpha2 helix is unwound at its N terminus, which appears to be essential for the opening of the RA-binding pocket. Three arginine side-chains (29, 59, and 132) are found with their guanidino groups pointing into the RA-binding pocket. A three-step mechanism of RA entry has been proposed, addressing the opening of the RA entrance, the electrostatic potential that directs entry of RA into the binding pocket, and the intramolecular interactions that stabilize the RA.CRABPII complex via locking the three flexible structural elements when RA is bound.

Regulation of the CRABP-I gene during mouse embryogenesis

The cellular retinoic acid binding protein type I (CRABP-I) shows a highly specific expression pattern during mouse embryonic development. The tissues that express CRABP-I, i.e. the central nervous system (CNS), neural crest, branchial arches, limb bud and frontonasal mass, coincide with those that are most sensitive to unphysiological retinoic acid (RA) concentrations. We have investigated the transcriptional elements that are responsible for the spatiotemporal regulation of CRABP-I expression in the mouse embryo. We show here that a 16 kb fragment harbours all the elements needed for the correct spatiotemporal expression pattern. Upon further dissection of this fragment we have found that expression in the CNS is driven by elements in the upstream region of the gene, while expression in mesenchymal and neural crest tissue is regulated via element(s) located downstream of exon II of the gene. Two distinct fragments in the upstream region are required for expression in the CNS, as neither of these fragments alone is able to drive correct expression of a reporter gene in transgenic mice. DNAseI footprinting analysis of the two upstream fragments revealed the presence of a number of protected elements. One of these regulatory elements has the hallmarks of an RA response element, suggesting that CRABP-I expression in neural tissue can be directly modulated by RA via the RARs/RXRs.

Unoccluded retinol penetrates human skin in vivo more effectively than unoccluded retinyl palmitate or retinoic acid

The formation of all-trans retinoic acid is an oxidative process whereby retinol is converted to retinaldehyde and then to retinoic acid. Because retinol causes qualitative molecular changes similar to those produced by retinoic acid, we compared potency of retinol, retinaldehyde, and retinyl palmitate to retinoic acid and assessed the effects of occlusion. Retinoids were prepared in an experimental vehicle of 95% ethanol:propylene glycol (7:3) with anti-oxidant. Induction of retinoic acid 4-hydroxylase activity was the end point for comparison. Retinoic acid concentrations from 0.001% to 0.05% under occlusion produced a linear dose-response induction of 4-hydroxylase activity. The concentrations of the other retinoids under occlusion required to achieve significant induction of enzyme activity were 0.6% retinyl palmitate, 0.025% retinol, and 0.01% retinaldehyde. The linear dose-response was lost with retinoid concentrations in excess of 0.25% retinol or 0.5% retinaldehyde. Statistical analyses showed no difference in 4-hydroxylase activity between unoccluded and occluded retinol treated sites. By contrast, however, unoccluded sites treated with retinoic acid or retinyl palmitate had less induction of 4-hydroxylase activity than occluded sites. Retinol, retinaldehyde, and retinyl palmitate did not produce erythema but did increase epidermal thickness. Although retinol is a weaker retinoid than retinoic acid, the increased penetration of unoccluded retinol in comparison to unoccluded retinoic acid with this prototypic vehicle confers on retinol a more effective delivery of a retinoidal effect than unoccluded retinoic acid. Retinol at 0.25% may be a useful retinoid for application without occlusion because it does not irritate but does induce cellular and molecular changes similar to those observed with application of 0.025% retinoic acid.

Retinoic acid signaling cascade in differentiating murine epidermal keratinocytes: alterations in papilloma- and carcinoma-derived cell lines

The retinoic acid (RA) signaling pathway was investigated by transient transfection of a chloramphenicol acetyltransferase (CAT) reporter gene construct containing the RA response element (RARE) of the murine (m) RARbeta2 gene into murine primary epidermal keratinocytes (PEK), papilloma-derived SP1 cells, and carcinoma-derived 3P2 cells. Murine PEK transfected in a low-Ca2+ medium (0.05 mM Ca2+) exhibited a strong transactivation of the CATgene after exposure of the cells to 0.1 microM RA. Transactivation of the CATgene could, however, also be achieved by shifting RAREbeta2-transfected low-Ca2+ PEK to high-Ca2+ conditions (0.15-1.2 mM Ca2+). Concomitantly, the Ca2+ raise also led to the induction of both cellular retinol (ROL)-binding protein I (CRBPI) and cellular RA-binding protein II (CRABPII), whereas expression of cellular RA-binding protein I (CRABPI) was not observed. Moreover, induction of in vitro differentiation also activated the ROL-->RA converting enzyme system in PEK. These findings suggest the following sequence of events involved in the high Ca2+-mediated activation of RAREbeta2. First, high Ca2+ induces the synthesis of mCRBPI, which binds ROL released from retinyl ester stores and makes it accessible to the ROL-RA converting enzyme system. Enzymatically generated RA is taken over by mCRABPII and transported to the nucleus, where it acts as ligand for nuclear receptors, which complex with RAREbeta2 to activate the reporter gene. This hypothetical cascade of RA signaling was supported by our findings that inhibition of the ROL-->RA converting enzyme system by citral abolished the Ca2+-mediated transactivation of the CAT gene in a nontoxic manner. Studies in transformed murine cell lines revealed that Ca2+-induced activation of RAREbeta2 was essentially maintained in papilloma-derived SP1 cells, although all parameters of the Ca2+-dependent RAREbeta2 activation cascade were induced to a much lower extent. In contrast, strong RAREbeta2 activity was already observed in low-Ca2+ carcinoma-derived 3P2 cells. Low-Ca2+ 3P2 cells also expressed high levels of both mCRBPI and mCRABPII and possessed a highly active ROL-->RA converting enzyme system. Again, inhibition of the enzyme by citral abolished RAREbeta2 activity in low-Ca2+ 3P2 cells. Our data show that Ca2+-induced differentiation in cultured murine PEK entails a series of events that ultimately lead to the activation of RARE-containing genes. These properties are maintained in transformed epidermal keratinocytes. However, with increasing malignant potential of the cells, the respective signaling pathway becomes independent from a differentiation stimulus and leads to constitutive activation of RARE-controlled genes.

Differences in metabolism and isomerization of all-trans-retinoic acid and 9-cis-retinoic acid between human endothelial cells and hepatocytes

Retinoic acid stimulates the expression of tissue-type plasminogen activator (t-PA) in vascular endothelial cells in vitro and enhances t-PA levels in plasma and tissues in vivo. Compared with the in vivo situation, high retinoic acid concentrations are required to induce optimally t-PA expression in vitro. These findings led us to study retinoic acid metabolism in cultured human endothelial cells. For comparison, these studies were also performed in the human hepatoma cell line, HepG2, and key experiments were repeated with human primary hepatocytes. Both hepatocyte cultures gave very similar results. Human endothelial cells were shown to possess an active retinoic acid metabolizing capacity, which is quantitatively comparable to that of hepatocytes, but different from that of hepatocytes in several qualitative aspects. Our results demonstrate that all-trans-retinoic acid is quickly metabolized by both endothelial cells and hepatocytes. All-trans-retinoic acid induces its own metabolism in endothelial cells but not in hepatocytes. 9-cis-Retinoic acid is degraded slowly by endothelial cells, whereas hepatocytes metabolize 9-cis-retinoic acid very quickly. Furthermore, our data show that hepatocytes, but not endothelial cells, detectably isomerise all-trans-retinoic acid to 9-cis-retinoic acid and vice versa. In both endothelial cells and hepatocytes all-trans-retinoic acid metabolism was inhibitable by the cytochrome P-450 inhibitors liarozole (10 microM) and ketoconazole (10 microM), albeit to different extents and with different specificities. In the presence of the most potent retinoic acid metabolism inhibitor in endothelial cells, liarozole, at least 10-fold lower all-trans-retinoic acid concentrations were required than in the absence of the inhibitor to obtain the same induction of t-PA. In conclusion, our results clearly demonstrate that all-trans-retinoic acid and 9-cis retinoic acid are actively but differently metabolized and isomerised by human endothelial cells and hepatocytes. The rapid metabolism of retinoic acid explains the relatively high concentrations of retinoic acid required to induce t-PA in cultured endothelial cells.

Head and Neck Chemoprevention: Recent Advances

BACKGROUND: Head and neck cancers are important to human life and health in both developed and underdeveloped countries. Management of established cancers is difficult, and there is great interest in evaluating methods to prevent these tumors from developing. METHODS: The biology of carcinogenesis, including field carcinogenesis, is reviewed, together with the biology and pharmacology of the retinoids. Intervention studies of premalignant lesions have led to prospective clinical trials of the capability of various retinoids to reduce the incidence of new second cancers. RESULTS: High-dose 13-cis-retinoic acid (13cRA) has significant activity in reversing oral leukoplakia but at a cost of substantial toxicity, and relapses occur early. An ongoing intergroup trial is underway to evaluate the capacity of low-dose (30 mg/d) 13cRA given for three years to reduce the incidence of second primary tumors in patients with "cured" squamous head and neck cancers. CONCLUSIONS: Molecular studies of loss of heterozygosity and p53 gene mutations are advancing our understanding of field carcinogenesis and the biology, pharmacology, and effects of the retinoids used in cancer prevention. Translation of early clinical trials into large-scale intervention trials to prevent second cancers are now underway.

Differential expression of transcripts encoding retinoid binding proteins and retinoic acid receptors during placentation of the mouse

We report the distribution of transcripts from genes encoding the retinol binding protein (RBP), the cellular retinol binding proteins (CRBP I, II) and retinoic acid binding proteins (CRABP I, II), the retinaldehyde dehydrogenase type 2 (RALDH-2), the retinoic acid receptors (RARs), and the retinoid X receptors (RXRs) in mouse placental tissues from 6.5 to 19.5 days postcoitum (dpc). During early placentation, RBP and RALDH-2 gene expression are restricted to the endoderm of the visceral yolk sac and the outer uterine epithelium, respectively, whereas CRBP I transcripts are detected in the visceral yolk sac and in the presumptive chorioallantoic placenta. By 15.5 dpc, CRBP I expression is down-regulated in the yolk sac where CRBP II becomes strongly expressed. Expression of CRBP II is also detected in the trophoblastic giant cells. Throughout placentation, the expression patterns of the CRABP I and II genes partly overlap in the decidual tissue and the vacuolar zones of the decidua, suggesting a role for these binding proteins in sequestering free retinoic acid from maternal blood, thus regulating its availability to the embryo. RAR alpha is ubiquitously expressed in all placental tissues, except in trophoblastic giant cells, at all stages studied. During early placentation, RAR beta and RAR gamma are co-expressed in the decidua but differentially expressed in the chorionic region (RAR beta, 10.5 to 12.5 dpc) and the presumptive labyrinth (RAR gamma, 7.5 to 12.5 dpc). During the same stages, RXR alpha is strongly expressed in the presumptive placenta. RAR gamma remains weakly expressed in the labyrinth until 15.5 dpc, whereas RXR alpha exhibits a strong expression in this zone until birth, suggesting a role for these receptors in the development and function of the definitive placenta.

Retinoid metabolism and all-trans retinoic acid-induced growth inhibition in head and neck squamous cell carcinoma cell lines

Retinoids can reverse potentially premalignant lesions and prevent second primary tumours in patients with head and neck squamous cell carcinoma (HNSCC). Furthermore, it has been reported that acquired resistance to all-trans retinoic acid (RA) in leukaemia is associated with decreased plasma peak levels, probably the result of enhanced retinoid metabolism. The aim of this study was to investigate the metabolism of retinoids and relate this to growth inhibition in HNSCC. Three HNSCC cell lines were selected on the basis of a large variation in the all-trans RA-induced growth inhibition. Cells were exposed to 9.5 nM (radioactive) for 4 and 24 h, and to 1 and 10 microM (nonradioactive) all-trans RA for 4, 24, 48 and 72 h, and medium and cells were analysed for retinoid metabolites. At all concentrations studied, the amount of growth inhibition was proportional to the extent at which all-trans-, 13- and 9-cis RA disappeared from the medium as well as from the cells. This turnover process coincided with the formation of a group of as yet unidentified polar retinoid metabolites. The level of mRNA of cellular RA-binding protein II (CRABP-II), involved in retinoid homeostasis, was inversely proportional to growth inhibition. These findings indicate that for HNSCC retinoid metabolism may be associated with growth inhibition.

Cloning of a cDNA encoding an aldehyde dehydrogenase and its expression in Escherichia coli. Recognition of retinal as substrate

The biosynthesis of the hormone retinoic acid from retinol (vitamin A) involves two sequential steps, catalyzed by retinol dehydrogenases and retinal dehydrogenases, respectively. This report describes the cloning of a cDNA encoding a heretofore unknown aldehyde dehydrogenase from a rat testis library and its expression in Escherichia coli. This enzyme has been designated retinal dehydrogenase, type II, RalDH(II). The deduced amino acid sequence of RalDH(II) had the highest identity with mammalian aldehyde dehydrogenases that feature low Km values (microM) for retinal: human ALDH1 (72.2%), rat retinal dehydrogenase, type I (71.5%), bovine retina (72.7%), and mouse AHD-2 (71.5%). RalDH(II) expressed in E. coli recognizes as substrates free retinal, with a Km of approximately 0.7 microM, and cellular retinol-binding protein-bound retinal, with a Km of approximately 0.2 microM. RalDH(II) also can utilize as substrate retinal generated in situ by microsomal retinol dehydrogenases, from the physiologically most abundant substrate: retinol bound to cellular retinol-binding protein. Rat testis expresses RalDH(II) mRNA most abundantly, followed by (relative to testis): lung (6.7%), brain (6.3%), heart (5.2%), liver (4.4%), and kidney (2.7%). RalDH(II) does not recognize citral, benzaldehyde, acetaldehyde, and propanal efficiently as substrates, but does metabolize octanal and decanal efficiently. These data support a function for RalDH(II) in the pathway of retinoic acid biogenesis.