4-Oxoretinol, a new natural ligand and transactivator of the retinoic acid receptors.

Abstract

All-trans-retinoic acid (at-RA) induces cell differentiation in a wide variety of cell types, including F9 embryonic teratocarcinoma cells, and can influence axial pattern formation during embryonic development. We now identify a novel retinoid synthetic pathway in differentiating F9 cells that results in the intracellular production of 4-oxoretinol (4-oxo-ROL) from retinol (vitamin A). Approximately 10-15% of the total retinol in the culture is metabolized to 4-hydroxyretinol and 4-oxo-ROL by the at-RA-treated, differentiating F9 cells over an 18-hr period, but no detectable metabolism of all-trans-retinol to at-RA or 9-cis-retinoic acid is observed in these cells. Remarkably, we show that 4-oxo-ROL can bind and activate transcription of the retinoic acid receptors whereas all-rans-retinol shows neither activity. Low doses of 4-oxo-ROL (e.g., 10-9 or 10-1o M) can activate the retinoic acid receptors even though, unlike at-RA, 4-oxo-ROL does not contain an acid moiety at the carbon 15 position. 4-oxo-ROL does not bind or transcriptionally activate the retinoid X receptors. Treatment of F9 cells with 4-oxo-ROL induces differentiation without conversion to the acid and 4-oxo-ROL is active in causing axial truncation when administered toXenopus embryos at the blastula stage. Thus, 4-oxo-ROL is a natural, biologically active retinoid that is present in differentiated F9 cells. Our data suggest that 4-oxo-ROL may be a novel signaling molecule and regulator of cell differentiation.Retinoids are derivatives of retinol (vitamin A) that are required for the appropriate functioning of a wide number of cell types involved in processes such as cell differentiation, axial pattern formation in embryogenesis, reproduction, and vision (for reviews, see refs. 1-3). Retinoids also exhibit cancer preventive actions and are used to treat some types of cancers (for review, see ref.2). The biological and transcriptional effects of retinoids are mediated by their interactions with the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) (for review, see ref. 4). RARs and RXRs can heterodimerize and bind response elements in a variety of target genes (for reviews, see refs. 1 and 4). The ligands for these receptors are generally thought to be acid derivatives of retinol; all-trans-retinoic acid (at-RA) has been shown to bind to the RARs with high affinity and 9-cis-retinoic acid (9-cis-RA) binds and activates the RXRs as well the RARs (for reviews, see refs. 4 and 5). The N-terminal portion of the ligand binding domain (E domain) is essential for the recognition of at-RA by various RARs (6).Much recent research in the vitamin A field has focused on at-RA and its isomers. Retinol has been reported to be metabolized to at-RA in a large number of different cultured cells and in various tissues from developing embryos (3, 5). However, the complexity of the biological actions of vitamin A suggests that retinol derivatives other than at-RA mediate some of these actions. Consistent with this idea, two nonacidic retinol d...

Genetic evidence that oxidative derivatives of retinoic acid are not involved in retinoid signaling during mouse development

“…In addition, variations in the residual levels of retinoic acid may account for the variability of the phenotypic rescue. The present data do not exclude any possible role for 4-oxo-retinol 17,18 .…”

Mouse P450RAI (CYP26) Expression and Retinoic Acid-inducible Retinoic Acid Metabolism in F9 Cells Are Regulated by Retinoic Acid Receptor γ and Retinoid X Receptor α

“…The correlation that we observed between RA metabolism and P450RAI expression does not preclude the possibility that other enzymes are involved in the observed RA metabolism. RA-inducible retinol metabolism has been reported in F9 cells (45,46), and because P450RAI does not metabolize retinol in transfected-COS cells (28), a different enzyme may be involved in retinol metabolism. The enzyme responsible for this activity has not been identified, but it may also have the ability to metabolize RA.…”

Effects of dietary retinoids and carotenoids on immune development

“…By activation of these nuclear receptors retinoic acids can influence the transcription of various retinoid-response genes (De Luca, 1991). In addition to the retinoic acids several other retinoids, such as 13,14-dihydroretinoic acid (Moise et al 2004(Moise et al , 2005, 3,4-didehydroretinoic acids (Allenby et al 1993), 4-oxo-retinol (Achkar et al 1996) and also 4-oxoretinoic acids (Baron et al 2005), have been found to be potent activators of RAR (Fig. 2).…”