CYP26 Inhibitor R115866 Increases Retinoid Signaling in Intimal Smooth Muscle Cells

Abstract

Objective-Intimal smooth muscle cells (SMCs) are dedifferentiated SMCs that have a powerful ability to proliferate and migrate. This cell-type is responsible for the development of intimal hyperplasia after vascular angioplasty. Retinoids, especially all-trans retinoid acid, are known to regulate many processes activated at sites of vascular injury, including modulation of SMC phenotype and inhibition of SMC proliferation. Intracellular levels of active retinoids are under firm control. A key enzyme is the all-trans retinoic acid-degrading enzyme cytochrome p450 isoform 26 (CYP26 10 Retinoids inhibit intimal hyperplasia after vascular injury 6,9 by reducing SMC proliferation. 11 They are known to influence differentiation of various cell types, and previous studies indicate that retinoids can alter the phenotype of vascular SMCs. 9,12 To date, a number of genes have been identified as markers of intimal SMCs including tropomyosin 4, 13 cytokeratins, 14 and cellular retinol binding protein-1 (CRBP-1). 15 Interestingly, CRBP-1 is involved in retinoid metabolism, 16 suggesting an altered retinoid metabolism in the intima. Indeed, we previously demonstrated that intimal SMCs have higher capacity to metabolize retinol into the active metabolite, all-trans retinoic acid (atRA), than medial SMCs. 17 The inhibition of DNA synthesis in intimal SMCs, in contrast to increased DNA synthesis in medial SMCs on retinol stimulation, 17 further supports the concept of differential retinoid metabolism in these subsets of cells.The metabolism of retinoids is complex and involves multiple binding proteins and metabolizing enzymes. The synthesis of retinoic acid (RA) is a 2-step reaction involving substrate-specific enzymes and cellular retinol binding proteins. The first step is the reversible oxidation of retinol (ROH) into retinal by retinol dehydrogenases (RDH). 16,18 The second step, the oxidation of retinal into the biologically active RA, is carried out by retinal dehydrogenases (RalDH). RA activates 2 families of nuclear receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are ligand-inducible transcription factors. Retinoids exert their effects on gene transcription mainly through 2 active stereoisomers, atRA and 9-cis retinoic acid (9-cisRA). 19

CYP26B1 Plays a Major Role in the Regulation of All-<i>trans</i>-Retinoic Acid Metabolism and Signaling in Human Aortic Smooth Muscle Cells

“…Therefore, our results showing that inhibition of CYP26 and more specifically, CYP26B1, increases retinoid signaling in human vascular cells adds new insight into the role of CYP26 in vascular cells and suggests a potential novel way to circumvent the development of retinoid resistance. The effects of R115866 on retinoid catabolism in AOSMCs demonstrated in this study are in agreement with our previous study in rat-derived intimal SMCs [27]. Interestingly, whereas retinoid levels in rat intimal SMCs were declining 24 h after R115866 stimuli [27], this decrease in cellular retinoic acid levels was not observed in AOSMCs.…”

“…The effects of R115866 on retinoid catabolism in AOSMCs demonstrated in this study are in agreement with our previous study in rat-derived intimal SMCs [27]. Interestingly, whereas retinoid levels in rat intimal SMCs were declining 24 h after R115866 stimuli [27], this decrease in cellular retinoic acid levels was not observed in AOSMCs. The increased retinoic acid degradation in rat intimal SMCs may reflect a stronger and faster CYP26 induction in these cells compared to human AOSMCs.…”

“…We previously demonstrated that CYP26 is expressed by rat intimal SMCs and that inhibition of CYP26 results in increased retinoid availability and increased retinoid signaling [27]. This finding suggests CYP26 inhibition as a new potential therapeutic approach for regulating endogenous retinoid levels in human vascular pathology.…”

A CYP26B1 Polymorphism Enhances Retinoic Acid Catabolism and May Aggravate Atherosclerosis

“…We and others previously showed that pharmacological inhibition of CYP26, or RNA interference (RNAi)-mediated CYP26 silencing, increases levels of retinoic acid and retinoid signaling (27,28,42). Hence, because our data indicate that the CYP26B1 enzyme was present and upregulated in atherosclerosis, regulation of CYP26B1 activity may influence atherosclerosis development by altering the local availability of retinoid ligands.…”

“…CYP26B1 has 41% amino acid identity with CYP26A1 but similar functional activity (24,25). In subsets of vascular and immune cells, interference with CYP26 has profound effects on atRA levels (26)(27)(28), and increased levels of endogenous atRA result in induction of a number of retinoid-responsive genes in vascular cells (27). To date, little is known about the significance of genetic polymorphisms that occur in the CYP26 enzymes.…”