Retinoic acid receptors regulate expression of retinoic acid 4-hydroxylase that specifically inactivates all-trans retinoic acid in human keratinocyte HaCaT cells

Preclinical assessment of dual CYP26[A1/B1] inhibitor, DX308, as an improved treatment for keratinization disorders

Background

Retinoid‐based therapies are commonly used in the treatment of disorders of keratinization and other skin disorders but can result in non‐specific effects and adverse reactions. Use of retinoic acid metabolism blocking agents (RAMBAs) such as DX308 may address these shortcomings.

Objectives

Characterize the therapeutic potential of recently discovered, CYP26‐selective RAMBA, DX308.

Materials and Methods

Preliminary in vitro assessment of potential off‐target activity, metabolic and toxicologic profiling. Studies to assess safety and efficacy of topical treatment in correcting abnormal skin morphology in rhino mice. Extensive gene expression profiling by RNA sequencing and qPCR in 3D epidermis grown with keratinocytes (KCs) from keratinization disorders and healthy controls, to investigate modulation of retinoid biopathways.

Results

In vitro, DX308 does not interact with off‐target nuclear receptors or CYP450s, is not genotoxic, and is stable in skin, despite vigorous hepatic metabolism. In vivo, topical DX308 induces comedolysis and epidermal thickening without apparent adverse effects. Gene expression profiling shows potent modulation of retinoid‐responsive genes by DX308 in both healthy and keratinization disorder KCs. Pathway analysis suggests DX308 may inhibit inflammatory and immune responses in KCs.

Conclusions

These preliminary studies suggest that DX308 is an efficacious topical therapeutic with a favourable metabolic and safety profiles. DX308 may present an improved therapeutic alternative for the treatment of keratinization disorders and other retinoid‐responsive skin ailments.

Characterization of CYP26B1-Selective Inhibitor, DX314, as a Potential Therapeutic for Keratinization Disorders

Inhibition of CYP450-mediated retinoic acid (RA) metabolism by RA metabolism blocking agents increases endogenous retinoids and is an alternative to retinoid therapy. Currently available RA metabolism blocking agents (i.e., liarozole and talarozole) tend to have fewer adverse effects than traditional retinoids but lack target specificity. Substrate-based inhibitor DX314 has enhanced selectivity for RA-metabolizing enzyme CYP26B1 and may offer an improved treatment option for keratinization disorders such as congenital ichthyosis and Darier disease. In this study, we used RT-qPCR, RNA sequencing, pathway, upstream regulator, and histological analyses to demonstrate that DX314 can potentiate the effects of all-trans-RA in healthy and diseased reconstructed human epidermis. We unexpectedly discovered that DX314, but not all-trans-RA or previous RA metabolism blocking agents, appears to protect epidermal barrier integrity. In addition, DX314-induced keratinization and epidermal proliferation effects are observed in a rhino mice model. Altogether, the results indicate that DX314 inhibits all-trans-RA metabolism with minimal off-target activity and shows therapeutic similarity to topical retinoids in vitro and in vivo. Findings of a barrier-protecting effect require further mechanistic study but may lead to a unique strategy in barrier-reinforcing therapies. DX314 is a promising candidate compound for further study and development in the context of keratinization disorders.

Fetal Alcohol Spectrum Disorder: Embryogenesis Under Reduced Retinoic Acid Signaling Conditions

Fetal Alcohol Spectrum Disorder (FASD) is a complex set of developmental malformations, neurobehavioral anomalies and mental disabilities induced by exposing human embryos to alcohol during fetal development. Several experimental models and a series of developmental and biochemical approaches have established a strong link between FASD and reduced retinoic acid (RA) signaling. RA signaling is involved in the regulation of numerous developmental decisions from patterning of the anterior-posterior axis, starting at gastrulation, to the differentiation of specific cell types within developing organs, to adult tissue homeostasis. Being such an important regulatory signal during embryonic development, mutations or environmental perturbations that affect the level, timing or location of the RA signal can induce multiple and severe developmental malformations. The evidence connecting human syndromes to reduced RA signaling is presented here and the resulting phenotypes are compared to FASD. Available data suggest that competition between ethanol clearance and RA biosynthesis is a major etiological component in FASD.

Transcriptional Profiles in Liver from Mice Treated with Hepatotumorigenic and Nonhepatotumorigenic Triazole Conazole Fungicides: Propiconazole, Triadimefon, and Myclobutanil

Conazoles are environmental and pharmaceutical fungicides. The present study relates the toxicological effects of conazoles to alterations of gene and pathway transcription and identifies potential modes of tumorigenic action. In a companion study employing conventional toxicological bioassays ( Allen et al., 2006 ), male CD-1 mice were fed triadimefon, propiconazole, or myclobutanil in a continuous oral-dose regimen for 4, 30, or 90 days. These conazoles were found to induce hepatomegaly, to induce high levels of hepatic pentoxyresorufin-O-dealkylase activity, to increase hepatic cell proliferation, to decrease serum cholesterol, and to increase serum triglycerides. Differentially expressed genes and pathways were identified using Affymetrix GeneChips. Gene-pathway associations were obtained from the Kyoto Encyclopedia of Genes and Genomes, Biocarta, and MetaCore compendia. The pathway profiles of each conazole were different at each time point. In general, the number of altered metabolism, signaling, and growth pathways increased with time and dose and were greatest with propiconazole. All conazoles had effects on nuclear receptors as evidenced by increased expression and enzymatic activities of a series of related cytochrome P450s (CYP). A subset of altered genes and pathways distinguished the three conazoles from each other. Triadimefon and propiconazole both altered apoptosis, cell cycle, adherens junction, calcium signaling, and EGFR signaling pathways. Triadimefon produced greater changes in cholesterol biosynthesis and retinoic acid metabolism genes and in selected signaling pathways. Propiconazole had greater effects on genes responding to oxidative stress and on the IGF/P13K/AKt/PTEN/mTor and Wnt-β-catenin pathways. In conclusion, while triadimefon, propiconazole, and myclobutanil had similar effects in mouse liver on hepatomegaly, histology, CYP activities, cell proliferation, and serum cholesterol, genomic analyses revealed major differences in their gene expression profiles.

Immediate effects of retinoic acid on gene expression in primary murine osteoblasts

Consistent with clinical observations demonstrating that hypervitaminosis A is associated with increased skeletal fracture risk, we have previously found that dietary retinol deprivation partially corrects the bone mineralization defects in a mouse model of X-linked hypophosphatemic rickets. That retinol-dependent signaling pathways impact the skeleton is further supported by various findings demonstrating a negative influence of retinoic acid (RA) on bone-forming osteoblasts. We hypothesized that RA would directly regulate the expression of specific target genes in osteoblasts, and we aimed to identify these by genome-wide expression analyses. Here we show that high dietary retinol intake in mice causes low bone mass associated with increased osteoclastogenesis and decreased osteoblastogenesis, but intact bone matrix mineralization. We additionally found that short-term treatment of primary osteoblasts with RA causes a rapid induction of specific genes involved in either retinol-dependent signaling (i.e. Rara, Crabp2) or skeletal remodeling (i.e. Twist2, Tnfsf11). In contrast, neither expression of established osteoblast differentiation markers nor the proliferation rate was immediately affected by RA administration. Collectively, our data suggest that the negative effects of vitamin A on skeletal integrity are explainable by an immediate influence of RA signaling on specific genes in osteoblasts that in turn influence bone remodeling.

Stevens-Johnson syndrome and toxic epidermal necrolysis (SJS/TEN): could retinoids play a causative role?

Stevens-Johnson syndrome and toxic epidermal necrolysis (SJS/TEN) are overlapping manifestations on a spectrum of acute drug-induced conditions associated with severe blistering, skin peeling, and multi-organ damage. TEN is an eruption resembling severe scalding, with ≥30% skin detachment. SJS is a mild form of TEN, characterized histologically by epidermal keratinocyte apoptosis with dermo-epidermal separation and extensive small blisters with <10% body surface skin detachment. The syndrome can be induced by numerous medications and typically occurs 1–4 weeks after the initiation of therapy. Granulysin is found in the lesions of patients with SJS/TEN and plays a significant pathogenic role in the condition, but the overall mechanisms linking medications, granulysin, and disease manifestations remain obscure. This paper reviews evidence suggesting that the different medications implicated in SJS/TEN have the common property of interacting and synergizing with endogenous retinoids (vitamin A and its congeners), in many instances causing the latter to accumulate in and damage the liver, the main storage organ for vitamin A. It is hypothesized that liver damage leads to the spillage of toxic retinoid compounds into the circulation, resulting in an endogenous form of hypervitaminosis A and cytotoxicity with widespread apoptosis, mediated by granulysin and recognized as SJS/TEN. Subject to testing, the model suggests that symptom worsening could be arrested at onset by lowering the concentration of circulating retinoids and/or granulysin via phlebotomy or plasmapheresis or by pharmacological measures to limit their expression.

Cyp26b1 regulates retinoic acid-dependent signals in T cells and its expression is inhibited by transforming growth factor-β

Background

The vitamin A metabolite, retinoic acid (RA), plays important roles in the regulation of lymphocyte properties. Dendritic cells in gut-related lymphoid organs can produce RA, thereby imprinting gut-homing specificity on T cells and enhancing transforming growth factor (TGF)-β-dependent induction of Foxp3⁺ regulatory T cells upon antigen presentation. In general, RA concentrations in cells and tissues are regulated by its degradation as well. However, it remained unclear if T cells could actively catabolize RA.

Methodology/Principal Findings

We assessed the expression of known RA-catabolizing enzymes in T cells from mouse lymphoid tissues. Antigen-experienced CD44⁺ T cells in gut-related lymphoid organs selectively expressed Cyp26b1, a member of the cytochrome P450 family 26. However, T cells in the spleen or skin-draining lymph nodes did not significantly express Cyp26b1. Accordingly, physiological levels of RA (1–10 nM) could induce Cyp26b1 expression in naïve T cells upon activation in vitro, but could not do so in the presence of TGF-β. Overexpression of Cyp26b1 significantly suppressed the RA effect to induce expression of the gut-homing receptor CCR9 on T cells. On the other hand, knocking down Cyp26b1 gene expression with small interfering RNA or inhibiting CYP26 enzymatic activity led to enhancement of the RA-induced CCR9 expression.

Conclusions/Significance

Our data demonstrate a role for CYP26B1 in regulating RA-dependent signals in activated T cells but not during TGF-β-dependent differentiation to Foxp3⁺ regulatory T cells. Aberrant expression of CYP26B1 may disturb T cell trafficking and differentiation in the gut and its related lymphoid organs.

Gene regulation of CYP4F11 in human keratinocyte HaCaT cells

Mechanisms regulating CYP4F genes remain under investigation, although characterization of CYP4F regulatory modalities would facilitate the discovery of new drug targets. This present study shows that all-trans- and 9-cis-retinoic acids can inhibit CYP4F11 expression in human keratinocyte-derived HaCaT cells. Transrepression of many genes by retinoic acids is mediated by interactions between retinoid receptors and the activator protein 1 (AP-1) complex. Proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin 1beta, which can activate the AP-1 complex, induce CYP4F11 transcription in HaCaT cells. The c-Jun N-terminal kinase (JNK)-specific inhibitor 1,9-pyrazoloanthrone (SP600125) blocked the induction of CYP4F11 by both cytokines, indicating involvement of the JNK pathway. Furthermore, TNF-alpha failed to induce CYP4F11 transcription when HaCaT cells were preincubated with retinoic acids. Retinoic acids are ligands for the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). The RXR agonist 6-(1(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)cyclopropyl) nicotinic acid (LG268) greatly induced CYP4F11 transcription, whereas the RAR agonist 4-(2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl)benzoic acid (TTNPB) markedly inhibited CYP4F11 transcription, indicating that down-regulation of CYP4F11 transcription by retinoic acid is mediated by RARs and may also be related to ligand competition for RXRs. Thus, the CYP4F11 gene is positively regulated by multiple signaling pathways in HaCaT keratinocytes, including RXR and JNK signaling pathways.

Retinoic acid 4-hydroxylase inducibility and clinical response to isotretinoin in patients with acne

BACKGROUND

The cytochrome P450 (CYP) enzyme CYP26 (retinoic acid [RA] 4-hydroxylase) initiates the catabolism of all-trans RA (tRA) and limits the effects of tRA. The CYP26 enzyme acts specifically on tRA, but not 13-cis RA (isotretinoin), a retinoid used to treat severe acne. However, 13-cis RA can isomerize to tRA, which can then be metabolized by CYP26.

OBJECTIVE

In healthy individuals, we assessed the variability of CYP26 enzymatic activity. We then investigated whether response to oral 13-cis RA among patients with acne correlates with variability in CYP26 expression.

METHODS

In healthy individuals, we isolated microsomal fractions from the epidermis of keratome biopsy specimens and measured CYP26 enzymatic activity in untreated skin and skin treated with tRA. Enzymatic activity was determined based on rate of formation of 4-hydroxy RA (pg/min/mg microsomal protein). Using real-time polymerase chain reaction we quantified CYP26 messenger RNA induction after tRA application in patients with acne who responded or did not respond to one course of 13-cis RA.

RESULTS

In normal-appearing skin (N = 118), CYP26 enzymatic activity was widely variable (1-180 pg/min/mg microsomal fraction; mean 42.7 +/- 3.5). Furthermore, CYP26 enzymatic activity was inducible in a dose-dependent manner in normal-appearing skin after tRA application, but not correlated with age or sex (N = 29). In patients with acne, CYP26 messenger RNA induction after 0.1% tRA application did not differ (P > .05) between patients who responded (N = 8, 587 +/- 325-fold) or did not respond (N = 8, 657 +/- 227-fold) to one course of 13-cis RA.

LIMITATIONS

The small number of patients with acne treated with 13-cis RA was a major limitation.

CONCLUSION

Factors other than CYP26 activity may determine response to isotretinoin in acne.

Effect of 2-(4-aminophenylmethyl)-6-hydroxy-3,4-dihydronaphthalen-1(2H)-one on all-transand 13-cis-retinoic acid levels in plasma quantified by high perfomance liquid chromatography coupled to tandem mass spectrometry

The effect of the titled tetralone as a retinoic acid metabolism blocking agent (RAMBA) in vivo in comparison with ketoconazole, a well known cytochrome P450 inhibitor, was studied. Development of a HPLC/MS/MS method for the quantification of retinoic acid levels extracted from rat plasma was used to demonstrate that ketoconazole and the tetralone (100 mg/kg) enhanced the endogenous plasma concentration of retinoic acid. Levels of retinoid were raised from a control value of 0.11 to 0.15 and 0.17 ng/mL after treatment with tetralone and ketoconazole respectively showing that the tetralone and ketoconazole lead to comparable effects, indicating an inhibitory activity of the tetralone on retinoic acid metabolism.

Some 1,2-Diphenylethane Derivatives as Inhibitors of Retinoic Acid—Metabolising Enzymes

In a search for novel inhibitors of RA-metabolising enzyme inhibitors as potential anti-cancer agents some 1,2-ethandiones, 2-hydroxyethanones and 1-ethylenedioxyethanones based on aryl-substituted 1,2-diphenylethane have been examined. Several of the compounds were weak inhibitors of the non-specific rat liver microsomal P450 enzymes and moderate inhibitors of the RA-induced enzymes in cultured human genital fibroblasts, where the RA-specific enzyme CYP26 is probably expressed. The 2-hydroxyethanone (13) with a 1-(4-dimethylaminophenyl) substituent was overall the most potent compound for rat liver microsomal enzyme (IC50 = 52.1 microM; ketoconazole, 2.8 microM) and the RA-induced enzyme (100 microM, 65.9% inhibition; ketoconazole, 20 microM, 75.0%). Modification of the dimethylamino group in (13) with more hydrophobic dialkylamino functions or separate modification of the 2-(2,4-dichlorophenyl) function did not improve potency.

Some Aryl Substituted 2-(4-Nitrophenyl)-4-oxo-4-phenylbutanoates and 3-(4-Nitrophenyl)-1-phenyl-1,4-butanediols and Related Compounds as Inhibitors of Rat Liver Microsomal Retinoic Acid Metabolising Enzymes

Some aryl substituted methyl 2-(4-nitrophenyl)-4-oxo-4-phenylbutanoates generally had poor to moderate inhibitory potency (4-73%) towards rat liver microsomal retinoic acid metabolising enzymes compared with ketoconazole (80%). Conversion to the corresponding 3-(4-nitrophenyl)-1-aryl-1,4-butanediols considerably increased potency (29-78%). The 4-iodophenyl analogue, (30) and the 4-iodo- (45) and 4-methoxyphenyl (46) analogues, were the most potent in both series respectively. The corresponding 5-membered lactones, in the three instances examined, were also potent (52%, 67%, 69%) as were the cis- and trans-isomers of the 5-membered tetrahydrofuran (77%, 65% respectively). Beckmann rearrangement of the oxime methyl 4-(2,4-dichlorophenyl)-4-hydroxyimino-2-(4-nitrophenyl)butanoate (54) gave the expected products (55) and (56), which were potent inhibitors (75%, 74% respectively) of the enzyme whereas the oxime was an activator.

Overexpression of CRABPI in suprabasal keratinocytes enhances the proliferation of epidermal basal keratinocytes in mouse skin topically treated with all-trans retinoic acid

We investigated whether ectopic expression of CRABPI, a cellular retinoic acid binding protein, influenced the actions of all-trans retinoic acid (ATRA) in transgenic (TG) mice. We targeted CRABPI to the basal vs. suprabasal layers of mouse epidermis by using the keratin 14 (K14) and keratin 10 (K10) promoters, respectively. Greater CRABPI protein levels were detected in the epidermis of adult transgenic(+) mice than in transgenic(-) mice for both transgenes. In adult mouse skin CRABPI overexpression in the basal or suprabasal keratinocytes did not cause morphological abnormalities, but did result in decreased CRABPII mRNA levels. Ectopically overexpressed CRABPI in suprabasal keratinocytes, but not in basal keratinocytes, enhanced the thickening of the epidermis induced by topical ATRA treatments (10 microM, 400 microl for 4 days) by 1.59+/-0.2-fold (p<0.05). ATRA treatment (10 microM) resulted in a 59.9+/-9.8% increase (p<0.05) in the BrdU labeling index in K10/FLAG-CRABPI TG(+) mice vs. TG(-) mice. Retinoid topical treatments reduced p27 and CYP26A1 mRNA levels in TG(+) and TG(-) mouse skin in K14 and K10/FLAG-CRABPI transgenic mice. As epidermal basal keratinocyte proliferation is stimulated by paracrine growth factors secreted by ATRA activated suprabasal keratinocytes, our results indicate that CRABPI overexpression in suprabasal keratinocytes enhances the physiological functions of ATRA.

[Retinoid metabolism and cancer]

Retinoids play important roles in cell differentiation and apoptosis, notably in epithelial tissues. Their utility in cancer therapy has been demonstrated in specific cancer types. Use of retinoic acid (RA) in the treatment of acute promyelocytic leukemia was the first successful example of retinoid-based differentiation therapy. RA has since been evaluated for treatment of other cancers, revealing variable effectiveness. The observation that expression of enzymes involved in RA biosynthesis is suppressed during tumorigenesis suggests that intra-tumor depletion in RA levels may contribute to tumor development and argues for the use of retinoids in cancer treatment. However, the induction of RA-inactivating enzymes is one of the mechanisms that may limit the efficacy of retinoid therapy and contribute to acquired resistance to RA treatment, suggesting that retinoic acid metabolism blocking agents may be effective agents in differentiation therapy.

A Skin-Like Cytochrome P450 Cocktail Activates Prohaptens to Contact Allergenic Metabolites

Allergic contact dermatitis is a complex syndrome representing immunological responses to cutaneous exposure to protein-reactive chemicals. Although many contact sensitizers directly can elicit this disorder, others (prohaptens) require activation. Knowledge regarding the activating mechanisms remains limited, but one possibility is metabolic activation by cytochrome P450 (CYP) enzymes in the skin. We have, after quantitative reverse transcriptase-PCR studies of the CYP content in 18 human skin samples, developed an enriched skin-like recombinant human (rh) CYP cocktail using CYP1A1, 1B1, 2B6, 2E1, and 3A5. To validate the rhCYP cocktail, a prohaptenic conjugated diene ((5R)-5-isopropenyl-2-methyl-1-methylene-2-cyclohexene) was investigated using: the skin-like rhCYP cocktail, a liver-like rhCYP cocktail, single rhCYP enzymes, liver microsomes, keratinocytes, and a dendritic cell (DC) assay. The diene was activated to sensitizing epoxides in all non-cell-based incubations including the skin-like rhCYP cocktail. An exocyclic epoxide metabolite ((7R)-7-isopropenyl-4-methyl-1-oxaspiro[2.5]oct-4-ene) was found to be mainly responsible for the allergenic activity of the diene. This epoxide also induced pronounced DC activation indicated by upregulation of IL-8. The skin-like rhCYP cocktail provides a simplified alternative to using skin tissue preparations in mechanistic studies of CYP-mediated skin metabolism of prohaptens and offers the future possibility of designing in vitro predictive assays for assessment of allergenic activity of prohaptens.

Vitamins A and E: metabolism, roles and transfer to offspring

Vitamins A and E are essential, naturally occurring, fat-soluble nutrients that are involved in several important biological processes such as immunity, protection against tissue damage, reproduction, growth and development. They are extremely important during the early stages of life and must be transferred adequately to the young during gestation and lactation. The present article presents an overview of their biological functions, metabolism and dynamics of transfer to offspring in mammals. Among other topics, the review focuses on the biochemical aspects of their intestinal absorption, blood transport, tissue uptake, storage and catabolism. It also describes their different roles as well as their use as preventive and therapeutic agents. Finally, the mechanisms involved in their transfer during gestation and lactation are discussed.

Skin Retinoid Concentrations Are Modulated by CYP26AI Expression Restricted to Basal Keratinocytes in Normal Human Skin and Differentiated 3D Skin Models

Cellular levels of all-trans retinoic acid (RA) are meticulously regulated utilizing an array of systems to balance uptake, biosynthesis, catabolism, and efflux transport. Metabolic transformation of all-trans RA to 4-hydroxylated RA appears to be primarily catalyzed by the cytochrome P450 (CYP) 26AI. Analysis of monolayer cultures of normal human epidermal keratinocytes (NHEKs) and dermal fibroblasts by quantitative real-time PCR and reverse transcription-PCR revealed no basal levels of CYP26AI mRNA expression, whereas specific transcripts were detectable following addition of 10(-6) M all-trans RA. Immunofluorescence and Western blot analysis showed a weak expression of CYP26AI in NHEK, which was increased by stimulation with all-trans RA. Using a newly developed peptide antibody, we further examined the localization of CYP26AI expression in normal skin and three-dimensional (3D) skin models. In contrast to cell culture monolayers where CYP26AI was only weakly detectable, strong constitutive expression of CYP26AI in vivo and in organotypic culture was found to be restricted to basal epidermal keratinocytes, as well as eccrine sweat glands and sebaceous glands. These studies verify the capacity of human skin to metabolize RA, although substantial differences exist in CYP expression between normal skin and 3D skin models compared to monolayer cultures. Complex metabolic processes that maintain retinoid homeostasis may therefore be better studied in model systems more closely resembling in vivo skin. In light of our prior studies documenting the functional activity of RA metabolites, expression of CYP26 in the sebaceous gland epithelium supports the suggestion that altered RA metabolism may be involved in the pathogenesis of acne.

Retinoic acid metabolism blocking agents (RAMBAs) for treatment of cancer and dermatological diseases

The naturally occurring retinoids and their synthetic analogs play a key role in differentiation, proliferation, and apoptosis, and their use/potential in oncology, dermatology and a variety of diseases are well documented. This review focuses on the role of all-trans-retinoic acid (ATRA), the principal endogenous metabolite of vitamin A (retinol) and its metabolism in oncology and dermatology. ATRA has been used successfully in differentiated therapy of acute promyelocytic leukemia, skin cancer, Kaposi's sarcoma, and cutaneous T-cell lymphoma, and also in the treatment of acne and psoriasis. However, its usefulness is limited by the rapid emergence of acquired ATRA resistance involving multifactoral mechanisms. A key mechanism of resistance involves ATRA-induced catabolism of ATRA. Thus, a novel strategy to overcome the limitation associated with exogenous ATRA therapy has been to modulate and/or increase the levels of endogenous ATRA by inhibiting the cytochrome P450-dependent ATRA-4-hydroxylase enzymes (particularly CYP26s) responsible for ATRA metabolism. These inhibitors are also referred to as retinoic acid metabolism blocking agents (RAMBAs). This review highlights development in the design, synthesis, and evaluation of RAMBAs. Major emphasis is given to liarozole, the most studied and only RAMBA in clinical use and also the new RAMBAs in development and with clinical potential.

Transcriptional co-operativity between distant retinoic acid response elements in regulation of Cyp26A1 inducibility

Cyp26A1 encodes an RA (retinoic acid)-catabolizing CYP (cytochrome P450) protein that plays a critical role in regulating RA distribution in vivo. Cyp26A1 expression is inducible by RA, and the locus has previously been shown to contain a RARE (RA response element), R1, within the minimal promoter [Loudig, Babichuk, White, Abu-Abed, Mueller and Petkovich (2000) Mol. Endocrinol. 14, 1483-1497]. In the present study, we report the identification of a second functional RARE (R2) located 2.0 kb upstream of the Cyp26A1 transcriptional start site. Constructs containing murine sequences encompassing both R1 and R2 showed that these elements work together to generate higher transcriptional activity upon treatment with RA than those containing R1 alone. Inclusion of R2 also dramatically enhanced the sensitivity of reporter constructs to RA, as even treatment with 10(-8) M RA resulted in a 5-fold induction of reporter activity. Mutational analysis identified R2 as the functional element responsible for the increased RA inducibility of promoter constructs. The element was shown to bind RARgamma (RA receptor gamma)/RXRalpha (retinoid X receptor alpha) heterodimers in vitro, and inclusion of nuclear receptors in transfections boosted the transcriptional response. A construct containing both R1 and R2 was used to generate a stable luciferase reporter cell line that can be used as a tool to identify factors regulating Cyp26A1 expression. The analysis of R1 and R2 has led to the proposal that the two elements work synergistically to provide a maximal response to RA and that R2 is an upstream enhancer.

Identification of the fenretinide metabolite 4-oxo-fenretinide present in human plasma and formed in human ovarian carcinoma cells through induction of cytochrome P450 26A1

PURPOSE

The synthetic retinoid fenretinide (4-HPR) exhibits preventive and therapeutic activity against ovarian tumors. An unidentified polar metabolite was previously found in 4-HPR-treated subjects and in A2780 human ovarian carcinoma cells continuously treated with 4-HPR (A2780/HPR). The metabolite and the enzyme involved in its formation in tumor cells are herein identified.

EXPERIMENTAL DESIGN

The metabolite was identified by mass spectrometry in A2780/HPR cell extracts and in plasma from 11 women participating in a phase III trial and treated with 200 mg/d 4-HPR for 5 years. The expression of proteins involved in retinoid metabolism and transport, cytochrome P450 26A1 (CYP26A1), cellular retinol-binding protein I (CRBP-I), and cellular retinoic acid-binding protein I and II (CRABP-I, CRABP-II) were evaluated in tumor cells by reverse transcription-PCR and Western blot analyses. Overexpression of CYP26A1 and retinoic acid receptors (RARs) in A2780 cells were obtained by cDNAs transfection.

RESULTS

The polar metabolite was 4-oxo-N-(4-hydroxyphenyl)retinamide (4-oxo-4-HPR) i.e., an oxidized form of 4-HPR with modification in position 4 of the cyclohexene ring. 4-oxo-4-HPR plasma levels were slightly lower (0.52 +/- 0.17 micromol/L) than those of the parent drug (0.84 +/- 0.53 micromol/L) and of the already identified metabolite N-(4-methoxyphenyl)retinamide (1.13 +/- 0.85 micromol/L). In A2780/HPR cells continuously treated with 4-HPR and producing 4-oxo-4-HPR, CYP26A1 and CRBP-I were markedly up-regulated compared with A2780 untreated cells. In A2780 cells, not producing 4-oxo-4-HPR, overexpression of CYP26A1 caused formation of 4-oxo-4-HPR, which was associated with no change in 4-HPR sensitivity. Moreover, the addition of 4-oxo-4-HPR to A2780 cells inhibited cell proliferation. Elevated levels of CYP26A1 protein and metabolism of 4-HPR to 4-oxo-4-HPR were found in A2780 cells transfected with RARbeta and to a lesser extent in those transfected with RARgamma.

CONCLUSIONS

A new metabolite of 4-HPR, 4-oxo-4-HPR, present in human plasma and in tumor cells, has been identified. The formation of this biologically active metabolite in tumor cells was due to CYP26A1 induction and was influenced by RAR expression. Moreover evidence was provided that 4-HPR up-modulates the expression of CRBP-I transcript, which is lost during ovarian carcinogenesis.

Transcriptional modulations by RXR agonists are only partially subordinated to PPARalpha signaling and attest additional, organ-specific, molecular cross-talks

Nuclear hormone receptors (NR) are important transcriptional regulators of numerous genes involved in diverse pathophysiological and therapeutic functions. Following ligand activation, class II NR share the ability to heterodimerize with the retinoid X receptor (RXR). It is established that RXR activators, rexinoids, transactivate several peroxisome proliferator-activated receptor alpha (PPARalpha) target genes in a PPARalpha-dependent manner. We hypothesized that, once activated, RXR might signal through quiescent NR other than PPARalpha, in an organ-specific manner. To study this putative phenomenon in vivo, we developed an array of 120 genes relevant to the class II NR field. The genes were selected using both published data and high-density screenings performed on RXR or PPARalpha agonist-treated mice. Wild-type C57BL/6J and PPARalpha-deficient mice were treated with fenofibrate (PPARalpha activator) or LGD1069 (RXR activator). Using our customized array, we studied the hepatic, cardiac, and renal expression of this panel of 120 genes and compared them in both murine genotypes. The results obtained from this study confirmed the ability of an RXR agonist to modulate PPARalpha-restricted target genes in the liver and the kidney. Furthermore, we show that various organ-specific regulations occurring in both genotypes (PPARalpha +/+ or -/-) are highly indicative of the ability of RXR to recruit other class II NR pathways. Further development of this molecular tool may lead to a better understanding of the permissiveness of class II nuclear receptor dimers in vivo.

Delivery of all trans-retinoic acid (RA) to hepatocyte cell line from RA/galactosyl α-cyclodextrin inclusion complex

All trans-retinoic acid (RA) plays a role in regulation of P450RAI gene expression. In this study, hepatocyte cell line (HepG2) was used to study an effect of RA released from RA/galactosyl alpha-cyclodextrin (GCD) inclusion complex on regulation of P450RAI gene expression. A delivery system composed of RA/GCD inclusion complex was applied because RA is poorly water soluble, and organic solvents used to dissolve it often interfere with cytotoxicity. Solubility of RA in water was increased by forming complex with GCD. Inclusion complex between GCD and RA was checked by (1)H-nuclear magnetic resonance, Fourier transformation infrared (FT-IR) spectroscopy and X-ray diffraction (XRD). The chemical shifts of the interior and exterior GCD protons in the presence of RA indicated that the RA was included within the GCD macrocycle cavity. The carbonyl band of RA and crystalline peak of RA in RA/GCD inclusion complex disappeared from FT-IR and XRD measurements, respectively, indication of inclusion complex between RA and GCD. From the observation of fluorescence micrograph of hepatocytes and flow cytometry measurement of HepG2, the internalization of fluorescein isothiocyanate-GCD by the hepatocyte occurred. Gene expression of P450RAI in HepG2 by delivery of RA from RA/GCD complex was observed.

Role of the IGF-II receptor in mediating acute, non-genomic effects of retinoids and IGF-II on keratinocyte cell death

In this study, we have examined the effects of retinoic acid (RA) on the human immortalized keratinocyte cell line (HaCaT). A significant twofold (P < 0.01) increase in apoptotic cell death compared with the control was found within 24 h of treatment with 10-5 M of RA. Apoptosis was confirmed by flow cytometry. Cycloheximide did not inhibit this acute RA-induced apoptosis. Interestingly, insulin-like growth factor-II (IGF-II, 50 ng/ml) was able to significantly (67.3%; P < 0.05) reduce RA effects, whereas IGF-I (50 ng/ml) and insulin (75 ng/ml) were without effect. Furthermore, analogues of IGF-II [leu27 IGF-II and Des(1-6) IGF-II], with altered affinities for the IGF-I receptor and IGF-binding proteins (IGFBPs), but retained affinities for the IGF-II receptor, also completely inhibited (100%; P < 0.01) RA-induced apoptosis, while an IGF-I receptor antagonist did not reduce the survival effects of IGF-II. Insulin pretreatment negates the survival effect of IGF-II. In contrast, mannose 6 phosphate (M6P) did not alter RA or IGF-II actions. These results indicate that rapid induction of cell death by RA is independent of production or secretion of new proteins. The inhibition of RA action by IGF-II was independent of its ability to signal through the IGF-I receptor or to interact with IGFBPs.

Expression of CYP4F8 (prostaglandin H 19-hydroxylase) in human epithelia and prominent induction in epidermis of psoriatic lesions

Our aim was to determine the tissue distribution of CYP4F8, which occurs in human seminal vesicles and catalyzes 19-hydroxylation of prostaglandin H(1) and H(2) in vitro (J. Bylund, M. Hidestrard, M. Ingelman-Sundberg, E.H. Oliw, J. Biol. Chem. 275 (2000) 21844-21849). Polyclonal antibodies were raised in rabbits against RVEPLG, the C-terminal end of CYP4F8, and purified by affinity chromatography. Screening of 50 human tissues for CYP4F8 immunoreactivity revealed protein expression, inter alia, in seminal vesicles, epidermis, hair follicles, sweat glands, corneal epithelium, proximal renal tubules, and epithelial linings of the gut and urinary tract. The CYP4F8 transcripts were detected by reverse transcription polymerase chain reaction and by Northern blot analysis. There was a prominent induction of CYP4F8 immunoreactivity and mRNA in psoriasis in comparison with unaffected epidermis of the same patients. The cDNA of CYP4F8 from plucked scalp hair roots was identical with the genital cDNA sequence. We conclude that CYP4F8 is present in epithelial linings and up regulated in epidermis of psoriatic lesions.

Anticancer activity and mechanism of action of retinoids in oral and pharyngeal cancer

Retinoids are the natural and synthetic derivatives of vitamin A. Epidemiological studies indicate that a low intake of vitamin A is associated with an increased risk of squamous cancer. In vitro studies on cancer cells show that exposure to retinoids results in the inhibition of growth, by blocking the cell cycle or by inducing apoptosis. With respect to the clinical efficacy of retinoids some positive effects have been observed in early stage oral and oropharyngeal cancer. Administration of retinoids has been shown to elicit responses in leukoplakia, a premalignant lesion of the oral mucosa that frequently develops into invasive cancer. Furthermore, it has been possible with a retinoid, 13-cis-retinoic acid, to delay or inhibit the development of second primary tumors in patients who have been curatively treated for a first primary tumor in the oral cavity or oropharynx. Recent trials, however, failed to show protective effects on the development of second primary tumors. Because of the short duration of the response, the intrinsic resistance to retinoids and the toxic side effects, the treatment with this class of compounds has not become a standard therapy. Recent studies have shed light on how preneoplastic and neoplastic cells defend themselves against the growth inhibiting action of retinoids. An increased retinoid breakdown and an inactivation of nuclear retinoid receptor appear to be the cause of acquired or intrinsic resistance. This knowledge can be used to develop novel tumor-selective strategies. This review gives an update on the role of retinoids in oral and oropharyngeal cancer and their precursor lesions. The focus will be on the anticancer activity, the mechanism of action and future directions.

HPLC-MS/MS analysis of the products generated from all-trans-retinoic acid using recombinant human CYP26A

Two mammalian hCYP26A expression systems have been used to analyze the metabolic products of CYP26A. Through the use of extensive HPLC, UV spectroscopy, and liquid chromatography/tandem mass spectrometry (LC-MS/MS) methodology, we have conclusively demonstrated that the complex mixture of products comprises 4-OH-all-trans-retinoic acid, 4-oxo-all-trans-retinoic acid, and 18-OH-all-trans-retinoic acid, and more polar products, partially identified as dihydroxy and mono-oxo, mono-hydroxy derivatives. These more polar products are presumed to result from multiple hydroxylations on the beta-ionone ring. The inter-relationship of initial and polar metabolites was inferred from both gene-dose and time-course experiments. Both initial and secondary metabolic steps after 4-oxo-all-trans-retinoic acid are ketoconazole-sensitive, suggesting that steps in the production of water-soluble metabolites are cytochrome P450-dependent.

all-trans-Retinoic acid-induced expression and regulation of retinoic acid 4-hydroxylase (CYP26) in human promyelocytic leukemia

all-trans-Retinoic acid (ATRA) induces complete remission in majority of patients with acute promyelocytic leukemia (APL). However, accelerated metabolism of ATRA that is induced by chronic daily administration of oral ATRA has been implicated as one of the mechanisms leading to a reduced sensitivity or resistance to ATRA therapy. We investigated the expression and regulation of CYP26, a novel p450 enzyme, which is highly specific for ATRA, in promyelocytic leukemia cells (NB4 and HL-60). We found that treatment of NB4 cells with a pharmacological concentration of ATRA (1 microM) induced rapid and dose-dependent expression of CYP26 mRNA. The CYP26 expression returned to pretreatment levels in both cells after ATRA was removed from the media. Retinoic acid receptor-alpha (RARalpha) specific antagonist (CD2503) totally abolished the ATRA-induced expression of CYP26 mRNA in HL-60 and NB4 cells. Furthermore, HL-60R, a HL-60 subclone expressing nonfunctional RAR because of a point mutation in the ligand-binding domain of RARalpha, failed to show CYP26 mRNA expression in response to ATRA. ATRA-induced expression of CYP26 was restored in HL-60R cells retrovirally transduced with RARalpha, but not in those cells transduced with the other retinoid receptors. In conclusion, ATRA induces expression of CYP26 in myeloid and promyelocytic leukemia cells and this expression is modulated by RARalpha. The induction of CYP26 expression by ATRA treatment might be related to a substrate-driven feedback mechanism to regulate intracellular concentrations of ATRA and its over expression in some clones may be partly responsible for reduced sensitivity or resistance to ATRA therapy.

Retinoid signaling is attenuated by proteasome-mediated degradation of retinoid receptors in human keratinocyte HaCaT cells

The biological actions of retinoids are mediated by nuclear retinoid receptors, RAR and RXR, which are ligand-activated transcription factors. We investigated the mechanism of attenuation of retinoid receptor activity in human keratinocyte HaCaT cells. Treatment of HaCaT cells with all-trans-retinoic acid or 9-cis-retinoic acid reduced RARgamma and RXRalpha protein levels by one-half within 24 h. In contrast, retinoid treatment did not alter RARgamma or RXRalpha mRNA levels, suggesting that retinoids stimulate breakdown of their receptors. Pulse-chase studies revealed that retinoid treatment of HaCaT cells reduced RARgamma and RXRalpha half-lives by 50%, indicating that retinoids accelerate breakdown of their receptors. The proteasome inhibitor MG132 prevented retinoid-induced receptor loss. Furthermore, MG132 potentiated retinoid-induced receptor activity, as assessed by expression of the retinoid-regulated CRABP-II gene in HaCaT cells. These data demonstrate that retinoids attenuate retinoid receptor function by enhancing proteasome-mediated retinoid receptor breakdown in HaCaT cells. Proteasome-mediated degradation of RARgamma or RXRalpha in vitro was significantly reduced by the corepressor SMRT, which binds unliganded retinoid receptors. This protection from degradation was markedly diminished by ligand, which causes SMRT to dissociate from receptors. Ligand failed to relieve protection from degradation by SMRT of a mutant form of RXRalpha that binds SMRT in the presence and absence of ligand. Addition of coactivators TIF1, TIF2, and RIP140 had no effect on degradation of RARgamma or RXRalpha. In summary, ligand binding to retinoid receptors promotes proteasome-mediated receptor degradation via dissociation of SMRT. Ligand-stimulated receptor degradation results in attenuation of retinoid signaling.

Expression of retinoic acid receptor gamma correlates with retinoic acid sensitivity and metabolism in head and neck squamous cell carcinoma cell lines

Retinoids, analogues of vitamin A, can reverse premalignant lesions and prevent second primary tumors in patients with head and neck squamous cell carcinoma (HNSCC). The effects of retinoids are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which act as ligand-activated transcription factors. The regulation of cell growth, differentiation and retinoid metabolism in normal, premalignant and malignant cells by retinoids is thought to be a result of their effects on gene expression. We investigated mRNA expression of RARs (alpha, beta, and gamma) and RXR-beta by means of RNase protection and related this to retinoic acid (RA)-induced growth inhibition and RA turnover in four HNSCC cell lines (UM-SCC-14C, UM-SCC-22A, UM-SCC-35 and VU-SCC-OE). An RA-resistant subline of UM-SCC-35 was generated by exposure to increasing concentrations of RA for 8 months (designated UM-SCC-35R). RA turnover was determined on the basis of decreasing RA levels in the cells and culture medium after exposure to 1 microM RA. We found that RAR-gamma mRNA expression was strongly correlated with RA-induced growth inhibition (p = 0.016, R = 0.92) and RA turnover (p = 0.041, R = 0.86). RAR-beta transcript levels were reduced in three of five cell lines compared with normal mucosa, and these did not correlate with RA-induced growth inhibition and RA turnover. Expression of RAR-alpha and RXR-beta was not substantially altered in any of the cell lines. These findings suggest that in HNSCC cell lines RAR-gamma is the most important retinoid receptor for regulation of RA turnover rate and RA-induced growth inhibition.

Synthetic retinoid CD437 promotes rapid apoptosis in malignant human epidermal keratinocytes and G1 arrest in their normal counterparts

Four human cutaneous squamous cell carcinoma (SCC) cell lines and normal human epidermal keratinocyte (NHEK) cells from two donors were examined for sensitivity to the synthetic retinoid 6-[3-(1 -adamantyl)-4-hydroxyphenyl]-2-naph-thalene carboxylic acid (CD437) alone or in combination with other agents. CD437 promoted rapid (within 2 h) apoptosis in SCC cells and G1 arrest in NHEK cells. G1 arrest in NHEK cells was sustained for 48 h while apoptosis occurred in approximately 60% of SCC cell after 24 h. Apoptosis could not be inhibited by nuclear retinoic acid receptor antagonists or cycloheximide, indicating CD437 was functioning in a receptor-independent manner. All-trans retinoic acid not only failed to induce apoptosis in SCC cells even at 20-fold higher concentration relative to the effective concentration of CD437; it also decreased the efficacy of CD437. Because of its differential effects on normal versus malignant keratinocytes, CD437 may be useful for the prevention or treatment of cutaneous SCC.

Identification of human cytochrome P450 isoforms that contribute to all-trans-retinoic acid 4-hydroxylation

The role of specific human cytochrome P450 (CYP) isoforms in the oxidative metabolism of all-trans-retinoic acid was investigated by studies in human liver microsomes using isoform-specific chemical inhibitors and inhibitory antibodies. Studies using individual isoforms expressed in lymphoblastoid cells and correlation analysis using different microsome preparations were also performed. With expressed isoforms, evidence for a role for CYP2C8, CYP3A4, CYP2C9, and CYP1A1 in 4-hydroxylation was obtained, with the highest catalytic efficiency being observed for CYP2C8. Using inhibition studies and correlation analysis, we also concluded that CYP2C8 was the major all-trans-retinoic acid 4-hydroxylating cytochrome P450 in human liver microsomes, though CYP3A4 and, to a lesser extent CYP2C9, also made a contribution. In addition, we compared the rate of retinoic acid degredation in HepG2 cells when cultured in the absence and presence of 3-methylcholanthrene or all-trans-retinoic acid. Culture in the presence of all-trans-retinoic acid decreased the half-life twofold and resulted in an increased sensitivity of retinoic acid degredation to ketoconazole. Since no induction of either CYP1A1, CYP2C8, CYP2C9, or CYP3A4 was detected using immunoblotting and as mRNA encoding another cytochrome P450 enzyme, CYP26, has been previously demonstrated to be induced by retinoic acid treatment of HepG2 cells and to be highly sensitive to ketoconazole, this enzyme in addition to CYP2C8, CYP2C9 and CYP3A4 likely plays a role in all-trans-retinoic acid oxidation in the liver at high retinoic acid levels.

Patterns of CYP26 expression in human prenatal cephalic and hepatic tissues indicate an important role during early brain development

CYP26 (P450RAI) catalyzes catabolic retinoic acid (RA) hydroxylation and thereby appears to play a critical role in retinoid signaling pathways during development. In this study, a quantitative competitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay was developed for evaluation of CYP26 message levels in human prenatal tissues. Statistical analyses of transcription levels in 12 prenatal human brains and six prenatal human livers demonstrated good sensitivity and reproducibility. Quantitative profiles of CYP26 gene expression in early (gestational days 57-110) prenatal cephalic and hepatic tissues and comparisons with adult counterparts are reported for the first time. Prenatal cephalic tissues at days 57-67 exhibited values of 1950+/-420 (CYP26 molecules/10(6) GAPDH molecules) whereas prenatal cephalic tissues at days 105-110 exhibited values of 22300+/-4450 (CYP26 molecules/10(6) GAPDH molecules), indicating a sharp developmental increase (approximately 11-fold). Levels in human adult cephalic tissues were slightly less than the prenatal cephalic levels measured during the earliest stages of gestation and were approximately 3-fold lower than those measured in adult human hepatic tissues. Levels in human prenatal hepatic tissues at days 63-110 gestation were less than 800 (CYP26 molecules/10(6) GAPDH molecules) and did not exhibit developmental increases. Considered together, the data have strong implications for the importance of CYP26 in early development of the human brain.

Ultraviolet-B exposure of human skin induces cytochromes P450 1A1 and 1B1

The cytochromes P450 belong to a multigene superfamily and are responsible for the metabolic activation of both xenobiotics and endobiotics. The expression of cytochrome P450 genes in target cells is an important determinant of human susceptibility to cancers and other chemically initiated diseases. In this study using immunohistochemistry, reverse transcription polymerase chain reaction, and western blot analysis, we investigated the cellular distribution and localization of cytochrome P450 1A1 and cytochrome P450 1B1 in human skin, and their induction by ultraviolet-B. Through the use of immunohistochemistry, cytochrome P450 1A1 was found to be primarily localized in the basal cell layer of the epidermis in non-ultraviolet-B exposed skin, whereas cytochrome P450 1B1 was localized in the epidermal cells other than the basal cell layer. Thus, localizations of cytochrome P450 1A1 and cytochrome P450 1B1 in human skin are different and may be related to keratinocyte differentiation. Ultraviolet-B exposure to solar-ultraviolet-protected skin (buttock site) resulted in an ultraviolet-B dose-dependent (0-4 minimal erythema doses) and time-dependent (0-48 h) induction of both cytochrome P450 1A1 and cytochrome P450 1B1 in the epidermis. Reverse transcription polymerase chain reaction and western blot analyses revealed that exposure of human skin to ultraviolet-B (4 minimal erythema doses) resulted in enhanced expression of mRNA and protein of both cytochrome P450 1A1 and cytochrome P450 1B1 in the epidermis. Ultraviolet-B induction of both cytochrome P450 1A1 and cytochrome P450 1B1 in human skin will probably result in enhanced bioactivation of polycyclic aromatic hydrocarbons and other environmental pollutants to which humans are exposed, which in turn could make the human skin more susceptible to ultraviolet-B-induced skin cancers or allergic and irritant contact dermatitis.

A second CYP26 P450 in humans and zebrafish: CYP26B1

A new P450 gene has been found in humans. It has 44% sequence identity to CYP26A1 from human and mouse, which places it in a new subfamily, CYP26B. There is only one human EST from a retinal library (AA012833) that matches the coding region. No homologous ESTs are found in mouse. A zebrafish EST AI721901 shows 68% identity to the human protein. This zebrafish EST is only 41% identical to the zebrafish CYP26A1 protein sequence, so it represents the homolog of the human CYP26B1 sequence. It is not known if this gene product will act on all-trans-retinoic acid like the CYP26A1 protein or if it might hydroxylate the 9-cis- or 13-cis-retinoic acid isoforms not recognized by CYP26A1. The importance of the CYP26A1 P450 in mouse and zebrafish development flags the CYP26B1 gene as a potential developmental gene.

Retinoic acid hydroxylase (CYP26) is a key enzyme in neuronal differentiation of embryonal carcinoma cells

Besides nuclear retinoid receptors and cellular retinoid binding proteins also retinoic acid (RA)-synthesizing enzymes (using all-trans-retinal as substrate) and RA-catabolizing enzymes (producing hydroxylated products) may explain the specific effects of retinoids. In the past we have established an active role for 4-hydroxy-RA and 4-oxo-RA, which originally were considered to be inactive retinoids, but in fact are highly active modulators of positional specification in Xenopus development. Here we present evidence for a specific role of hydroxylated RA metabolites in the onset of neuronal differentiation. 4-Hydroxy- and 18-hydroxy-RA are products of the hydroxylation of RA by a novel cytochrome P450 (CYP)-type of enzyme, CYP26, expression of which is rapidly induced by RA. P19 embryonal carcinoma (EC) cell lines stably expressing hCYP26 undergo extensive and rapid neuronal differentiation in monolayer at already low concentrations of RA, while normally P19 cells under these conditions differentiate only in endoderm-like cells. Our results indicate that the effects on growth inhibition and RARbeta transactivation of P19 EC cells are mediated directly by RA, while the onset of neuronal differentiation and the subsequent expression of neuronal markers is mediated by hCYP26 via the conversion of RA to its hydroxylated products.

Cytochrome P450, CYP26AI, is expressed at low levels in human epidermal keratinocytes and is not retinoic acid-inducible

Retinoids, and their synthetic analogues, are well-established regulators of the squamous differentiation programme both in vivo and in vitro. Despite this, very few studies have focused on the mechanism by which retinoid action is terminated, e.g. metabolism. Recently, a new cytochrome P450 family member (CYP26AI) was cloned. CYP26AI was reported to have substrate specificity for retinoids and to be retinoid-inducible. In this study, we have examined the expression and retinoic acid (RA) inducibility of CYP26AI in human epidermis and cultured keratinocytes. We found very low levels of CYP26AI mRNA expression in both epidermis and keratinocytes. Furthermore, we found no evidence for RA inducibility of CYP26 mRNA expression. This lack of RA inducibility was not due to inactivity of the retinoids, as we show that transglutaminase was still repressed by RA in the same cultures. Despite the low levels of CYP26AI expression in the keratinocytes, the keratinocytes were still capable of significant RA metabolism. In conclusion, our study reports, for the first time, that CYP26AI is unlikely to contribute to RA metabolism in keratinocytes. These studies also indicate that as yet unknown isoforms of cytochrome P450 may be involved in RA metabolism in keratinocytes.

Optimization of treatment conditions for studying the anticancer effects of retinoids using pancreatic adenocarcinoma as a model

Retinoids are natural differentiation-inducing compounds that are promising as anticancer agents. Cancer cell lines are valuable in the investigation of the potential of retinoids for the treatment of specific cancers. However, using different treatment conditions but the same cell lines, investigators have produced markedly contradictory results for the effectiveness of retinoids. The present study examined different factors in the treatment conditions that may have masked or interfered with the effects of retinoids and, thereby, resulted in this conflict. Our studies revealed that the effects of retinoids on cancer cell proliferation were influenced by serum, the choice of vehicle (DMSO vs ethanol) and its concentration, phenol red, the degree of cellular confluence, and the method of assessing proliferation (cell number or [3H]thymidine uptake vs the MTT assay). Optimized conditions were the use of serum-free, ethanol-free, and phenol red-free media, investigating cells in the log phase of growth, using </=0.01% DMSO as the vehicle, and monitoring proliferation by cell number or [3H]thymidine incorporation into DNA measured after TCA precipitation. Using these conditions, retinoids were found to exhibit potent antiproliferative effects in pancreatic cancer cells with a variety of degrees of differentiation, even in cell lines previously documented as being retinoid resistant. Retinoids also induced morphological changes and cellular death that may indicate terminal differentiation and apoptosis.