Identification of the Retinoic Acid-inducible All-trans-retinoic Acid 4-Hydroxylase

Abstract

Retinoic acid (RA) metabolites of vitamin A are key regulators of gene expression involved in embryonic development and maintenance of epithelial tissues. The cellular effects of RA are dependent upon the complement of nuclear receptors expressed (RARs and RXRs), which transduce retinoid signals into transcriptional regulation, the presence of cellular retinoid-binding proteins (CRABP and CRBP), which may be involved in RA metabolism, and the activity of RA metabolizing enzymes. We have been using the zebrafish as a model to study these processes. To identify genes regulated by RA during exogenous RA exposure, we utilized mRNA differential display. We describe the isolation and characterization of a cDNA, P450RAI, encoding a novel member of the cytochrome P450 family. mRNA transcripts for P450RAI are expressed normally during gastrulation, and in a defined pattern in epithelial cells of the regenerating caudal fin in response to exogenous RA. In COS-1 cells transfected with the P450RAI cDNA, alltrans-RA is rapidly metabolized to more polar metabolites. We have identified 4-oxo-RA and 4-OH-RA as major metabolic products of this enzyme. P450RAI represents the first enzymatic component of RA metabolism to be isolated and characterized at the molecular level and provides key insight into regulation of retinoid homeostasis.

Retinoid metabolism: a balancing act

“…A cytochrome P450 enzyme, Cyp26, was recently identified and shown to be essential for the generation of 4-hydoxyretinoic acid and 4-oxo-retinoic acid, as well as other hydroxylated retinoid derivatives 7,8 . Studies of Cyp26-deficient mice have largely supported the conclusion that 4-hydroxy-retinoic acid and 4-oxoretinoic acid are inactive degradation products of the all-trans isomer.…”

Cyp26 enzymes generate the retinoic acid response pattern necessary for hindbrain development

“…Cloning of the zebrafish cyp26a1 ortholog has been described (White et al, 1996). We cloned zebrafish homologs of cyp26b1 and cyp26c1, and examined their expression during development.…”

“…Here, we propose such a model based on the hindbrain patterning defects caused by the prevention of RA metabolism by the cytochrome P450 enzymes of the Cyp26 class. The Cyp26 enzymes (Cyp26a1, Cyp26b1 and Cyp26c1) have been proposed to function in the regulation of RA-dependent gene expression through their ability to metabolize RA into hydroxylated polar derivatives (Fujii et al, 1997;White et al, 1996). In the mouse tailbud and limbs, loss Cyp26 enzymes generate the retinoic acid response pattern necessary for hindbrain development of Cyp26 function leads to increased RA-dependent gene expression, spina bifida and caudal agenesis similar to the teratogenic effects of high concentrations of exogenous RA (AbuAbed et al, 2001;Sakai et al, 2001;Yashiro et al, 2004).…”

Effects of acute exposure to polybrominated diphenyl ethers on retinoid signaling in zebrafish larvae

“…In target cells, after being transported by cellular retinoic acid binding protein (CRABP) into the nucleus, RA binds to RA receptors (RARs) and retinoid X receptors (RXRs), which initiates the transcription of specific genes that contribute to developmental patterning of the hindbrain, spinal cord, heart, and eye (Novák et al, 2008). However, RA in non-target cells is catalyzed to oxidized derivatives by an RA-inducible gene, cyp26a, in order to regulate RA activity and avoid RA-induced teratogenesis (Dobbs-McAuliffe et al, 2004;White et al, 1996). Meanwhile, in the eyes, the chromophore retinal binds covalently to photoreceptor opsins to form visual pigments during photoexcitation to initiates the visual transduction cascade.…”