IDENTIFICATION OF THE RETINOIC ACID-INDUCIBLE ALL-TRANS-RETINOIC ACID4-HYDROXYLASE

Retinoic acid (RA) metabolites of vitamin A are key regulators of gene expression involved in embryonic development and maintenance of epith elial tissues, The cellular effects of RA are dependent upon the compl ement of nuclear receptors expressed (RARs and RXRs), which transduce retinoid signals into transcriptional regulation, the presence of cell ular retinoid-binding proteins (CRABP and CRBP), which may be involved in RA metabolism, and the activity of RA metabolizing enzymes, We hav e been using the zebrafish as a model to study these processes, To ide ntify genes regulated by RA during exogenous RA exposure, we utilized mRNA differential display, We describe the isolation and characterizat ion of a cDNA, P450RAI, encoding a novel member of the cytochrome P450 family, mRNA transcripts for P450RAI are expressed normally during ga strulation, and in a defined pattern in epithelial cells of the regene rating caudal fin in response to exogenous RA. In COS-1 cells transfec ted with the P450RAI cDNA, all-trans-RA is rapidly metabolized to more polar metabolites. We have identified 4-oxo-RA and 4-OH-RA as major m etabolic products of this enzyme. P450RAI represents the first enzymat ic component of RA metabolism to be isolated and characterized at the molecular level and provides key insight into regulation of retinoid h omeostasis.