Metabolism to a response pathway selective retinoid ligand during axial pattern formation

Abstract

We report identification of 9-cis-4-oxo-retinoic acid (9-cis-4-oxo-RA) as an in vivo retinoid metabolite in Xenopus embryos. 9-Cis-4-oxo-RA bound receptors (RARs) ␣, ␤, and ␥ as well as retinoid X receptors (RXRs) ␣, ␤, and ␥ in vitro. However, this retinoid displayed differential RXR activation depending on the response pathway used. Although it failed to activate RXRs in RXR homodimers, it activated RXRs and RARs synergistically in RAR-RXR heterodimers. 9-Cis-4-oxo-RA thus acted as a dimer-specific agonist. Considering that RAR-RXR heterodimers are major functional units involved in transducing retinoid signals during embryogenesis and that 9-cis-4-oxo-RA displayed high potency for modulating axial pattern formation in Xenopus, metabolism to 9-cis-4-oxo-RA may provide a mechanism to target retinoid action to this and other RAR-RXR heterodimer-mediated processes.Much evidence implicates retinoids (vitamin A and metabolites) in regulating embryonic pattern formation as well as growth, differentiation, reproduction, metabolism, and homeostasis (1-4). Many important effects of retinoids are mediated by two families of nuclear receptors: retinoic acid (RA) receptors (RARs) and retinoid X receptor (RXRs), each consisting of three types (␣, ␤, and ␥), there being several isoforms of each type (5-7). RARs and RXRs are retinoid ligand-dependent transcription factors, which can act via specific DNA response elements consisting of direct repeats of the hexameric motif RG(G͞ T)TCA. Two main retinoid response pathways are known. RARs and RXRs heterodimerize and then activate transcription via RA response elements consisting of direct repeats (DRs) spaced by 2 (DR-2) or 5 (DR-5) base pairs. Both the RAR and RXR partners of the heterodimer can be ligand activated in vivo, resulting in synergistic activation (8-13). RXRs also homodimerize, and this homodimerization contrasts with RAR-RXR heterodimerization in being ligand stimulated (14-17). RXR homodimers activate transcription from retinoid X response elements (RXREs) consisting of DRs spaced by 1 (DR-1) base pair. In addition to these two main pathways, retinoid receptors interact with other signaling pathways, either via heterodimerization between RXR and other nuclear hormone receptors or via crosstalk with AP-1.Paralleling the multiplicity of retinoid receptors, several natural retinoids are known to act as retinoid receptor ligands (4, 18-23). These fall into two groups. Ligands that activate RARs only include all-trans-RA, all-trans-3,4-didehydroretinoic acid (ddRA), all-trans-4-oxo-retinoic acid (4-oxo-RA), all-trans-4-oxoretinal, and all-trans-4-oxo-retinol. These ligands activate only RAR-RXR heterodimers and act via the RAR part of the heterodimer. The second group, including 9-cis-RA and 9-cis-3,4-didehydroretinoic acid, activates both RARs and RXRs. 9-Cis-RA has been shown to activate RAR-RXR heterodimers more efficiently than natural RAR ligands, suggesting that both partners of the heterodimer are activated by this ligand (8, 12). 9-Cis-RA also activat...

Metabolism and regulation of gene expression by 4‐oxoretinol versus all‐trans retinoic acid in normal human mammary epithelial cells

“…4E). Although 4-oxo-RA can activate the RARs (Gaemers et al, 1996; Idres et al, 2001; Idres et al, 2002; Pijnappel et al, 1998; Pijnappel et al, 1993; Sani et al, 1996; Sonneveld et al, 1999), our data show that most of the 4-oxo-RA produced from 4-oxo-ROL is not present within HMECs. Instead, the majority of 4-oxo-RA is in the cell culture medium (Fig.…”

Active repression of RAR signaling is required for head formation

“…In contrast, CYP26 converts RA to 4-OH RA and is thought to mediate the breakdown and clearance of retinoic acid (White et al 1996;Maden 1999). However, it is interesting to note that 4-OH and 4-oxo RAs are as active as RA in embryos (Pijnappel et al 1993(Pijnappel et al , 1998, whereas 4-OH and 4-oxo retinol and retinaldehyde are more biologically active than retinol or retinaldehyde (Achkar et al 1996;Blumberg et al 1996). RALDH-2 is expressed predominantly in the posterior mesoderm with a sharp anterior border in the gastrula at what will become the level of the first somite (Chen et al 2001).…”

“…A variety of endogenous bioactive retinoids exist in embryos (Durston et al 1989;Pijnappel et al 1993Pijnappel et al , 1998Blumberg et al 1996) although their spatial distributions are not well characterized. Two critical enzymes regulate the availability of RA and consequently its role in A-P patterning.…”

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

“…CYP2J4 also produces RA [212]. Novel 4-oxo acid and aldehyde derivatives are also powerful and abundant RAR ligands [213,214], and they may be produced by CYP1A2 [106].…”