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

Abstract

We have cloned a mouse cDNA homolog of P450RAI, a cytochrome P450 belonging to a new family (CYP26), which has previously been isolated from zebrafish and human cDNAs and found to encode a retinoic acid-inducible retinoic acid hydroxylase activity. The crossspecies conservation of the amino acid sequence is high, particularly between the mouse and the human enzymes, in which it is over 90%. Like its human and zibrafish counterparts, the mouse P450RAI cDNA catalyzes metabolism of retinoic acid into 4-OH-retinoic acid, 4-oxo-retinoic acid, 18-OH-retinoic acid, and unidentified water-soluble metabolites when transfected into COS-1 cells. Retinoic acid-inducible retinoic acid metabolism has previously been observed in F9 murine embryonal carcinoma cells and some derivatives lacking retinoid receptors. We were interested in determining whether P450RAI could be responsible for retinoic acid metabolism in F9 cells and in studying the effect of retinoid receptor ablation on P450RAI expression. In wild-type F9 cells and derivatives lacking RAR␥, RAR␣, and/or RXR␣, we observed a direct relationship between the level of retinoic acid metabolic activity and retinoic acid-induced P450RAI mRNA. These experiments, as well as others using synthetic receptor subtype-specific retinoids, suggest that the RAR␥ and RXR␣ receptors mediate the effects of retinoic acid on the expression of the P450RAI gene.

The role of retinoic acid in embryonic and post-embryonic development

“…The first type of enzyme, the retinol dehydrogenases (ROLDH), convert retinol to retinaldehyde which is used in the visual cycle, and the second type of enzyme, the retinal dehydrogenases (RALDH), convert retinaldehyde to RA (Duester, 1996;Napoli, 1996). In addition, there is a cytochrome P450 enzyme known as CYP26 which is thought to break down all-trans-RA to 4-oxo-RA, 4-hydroxyRA and 18-hydroxyRA (Abu-Abed et al 1998;White et al 1996White et al , 1997 or 5,8-epoxy-RA (Fujii et al 1997), and these breakdown products were thought to be inactive metabolites on their way to being excreted. However, 4-oxo-RA is a potent bioactive retinoid which respecifies the headto-tail axis of the Xenopus embryo (Pijnappel et al 1993), and the overexpression of CYP26 in embryonal carcinoma cells induces neuronal differentiation .…”

Identification of the human cytochrome P450, P450RAI-2, which is predominantly expressed in the adult cerebellum and is responsible for all-trans-retinoic acid metabolism

“…tory feedback-loop plays an important role in balancing alltrans-RA levels in certain developing tissues (6,(10)(11)(12)15).…”

The effectiveness of retinoic acid treatment in bladder cancer: Impact on recurrence, survival and TGFα and VEGF as end-point biomarkers.

“…Cyp450 is the major retinoid resistance-developing and degradation pathway, known to be induced by atRA to its maximum within a few hours. [32][33] Rationale of the present study was based on the mutual relationship of the bladder and retinoids as evident from: (1) The expression of not only the complete retinoid signal transduction cascade, synthesizing and catabolising enzymes but also retinoid storing ability. [34][35][36][37][38] (2) Abnormalities at the genetic and epigenetic levels involving these components correlated unfavorable bladder cancer prognosis and early disease.…”