Aldo–keto reductases in retinoid metabolism: Search for substrate specificity and inhibitor selectivity Sergio Porté a , F. Xavier Ruiz a,b , Joan Giménez a , Iago Molist a , Susana Alvarez c , Marta Domínguez c , Rosana Alvarez c , Angel R. de Lera c , Xavier Parés a , Jaume Farrés a,⇑ a Department of Biochemistry and Molecular Biology, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain b Institut de Génétique et de Biologie Moléculaire et Cellulaire, 1 rue Laurent Fries, 67404 Illkirch Cedex, France c Departamento de Química Orgánica, Universidade de Vigo, E-36310 Vigo, Spain article info Article history: Available online xxxx Keywords: Aldo–keto reductase Aldose reductase Retinoid inhibitors Retinoic acid abstract Biological activity of natural retinoids requires the oxidation of retinol to retinoic acid (RA) and its bind- ing to specific nuclear receptors in target tissues. The first step of this pathway, the reversible oxidore- duction of retinol to retinaldehyde, is essential to control RA levels. The enzymes of retinol oxidation are NAD-dependent dehydrogenases of the cytosolic medium-chain (MDR) and the membrane-bound short-chain (SDR) dehydrogenases/reductases. Retinaldehyde reduction can be performed by SDR and aldo–keto reductases (AKR), while its oxidation to RA is carried out by aldehyde dehydrogenases (ALDH). In contrast to SDR, AKR and ALDH are cytosolic. A common property of these enzymes is that they only use free retinoid, but not retinoid bound to cellular retinol binding protein (CRBP). The relative contribu- tion of each enzyme type in retinoid metabolism is discussed in terms of the different subcellular local- ization, topology of membrane-bound enzymes, kinetic constants, binding affinity of CRBP for retinol and retinaldehyde, and partition of retinoid pools between membranes and cytoplasm. The development of selective inhibitors for AKR enzymes 1B1 and 1B10, of clinical relevance in diabetes and cancer, granted the investigation of some structure–activity relationships. Kinetics with the 4-methyl derivatives of ret- inaldehyde isomers was performed to identify structural features for substrate specificity. Hydrophilic derivatives were better substrates than the more hydrophobic compounds. We also explored the inhib- itory properties of some synthetic retinoids, known for binding to retinoic acid receptors (RAR) and ret- inoid X receptors (RXR). Consistent with its substrate specificity towards retinaldehyde, AKR1B10 was more effectively inhibited by synthetic retinoids than AKR1B1. A RARb/c agonist (UVI2008) inhibited AKR1B10 with the highest potency and selectivity, and docking simulations predicted that its carboxyl group binds to the anion-binding pocket. Ó 2012 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Retinoic acid (RA) is one of the most potent small biological sig- naling molecules. When binding to specific nuclear receptors, it regulates the expression of more than 500 genes. It is essential throughout embryonic development, and it is involved in many processes in the adult life [1,2]. Alteration of its normal levels has been related to many pathological conditions, including cancer. A precise RA concentration has to be present in specific cells at the exact moment for its correct hormonal action. Therefore, its amount must be strictly spatially and temporally controlled, and evidence indicates that this is mostly performed at pre-receptor le- vel, along its metabolic synthesis and degradation pathways [3]. Retinol (vitamin A) is the precursor molecule of RA, and the ret- inoid form that is transported in blood to the target tissues. In the cell, retinol has several fates (Fig. 1): (1) Retinol can be stored as retinyl ester of long chain fatty acids through the action of leci- thin–retinol acyl transferase (LRAT). (2) Most non-esterified retinol is probably bound to cellular retinol binding protein I (CRBPI), since its concentration is high in many cells and it has a very low dissociation constant for retinol (K d  0.1 nM). It is believed that CRBPI protects retinol from its oxidative metabolism since most of the enzymes are inactive towards the CRBPI–retinol complex [4–7]. (3) Retinol can enter the oxidative pathway for the synthesis of RA. First retinol is oxidized to retinaldehyde, through the action of alcohol dehydrogenases (ADH), members of the medium-chain dehydrogenase/reductase (MDR) superfamily, or retinol dehydro- genases (RDH), members of the short-chain dehydrogenase/reduc- tase (SDR) superfamily. This reaction is reversible and the rate-limiting step of the pathway [8]. NAD + -dependent ADH and 0009-2797/$ - see front matter Ó 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cbi.2012.11.014 ⇑ Corresponding author. Tel.: +34 93 581 25 57; fax: +34 93 581 12 64. E-mail address: jaume.farres@uab.cat (J. Farrés). Chemico-Biological Interactions xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Chemico-Biological Interactions journal homepage: www.elsevier.com/locate/chembioint Please cite this article in press as: S. Porté et al., Aldo–keto reductases in retinoid metabolism: Search for substrate specificity and inhibitor selectivity, Chemico-Biological Interactions (2012), http://dx.doi.org/10.1016/j.cbi.2012.11.014