Retinoic acid isomers applied to human skin in vivo each induce a 4-hydroxylase that inactivates only trans retinoic acid

9-cis-13,14-Dihydroretinoic Acid Is an Endogenous Retinoid Acting as RXR Ligand in Mice

The retinoid X receptors (RXRs) are ligand-activated transcription factors which heterodimerize with a number of nuclear hormone receptors, thereby controlling a variety of (patho)-physiological processes. Although synthetic RXR ligands are developed for the treatment of various diseases, endogenous ligand(s) for these receptors have not been conclusively identified. We show here that mice lacking cellular retinol binding protein (Rbp1-/-) display memory deficits reflecting compromised RXR signaling. Using HPLC-MS and chemical synthesis we identified in Rbp1-/- mice reduced levels of 9-cis-13,14-dihydroretinoic acid (9CDHRA), which acts as an RXR ligand since it binds and transactivates RXR in various assays. 9CDHRA rescues the Rbp1-/- phenotype similarly to a synthetic RXR ligand and displays similar transcriptional activity in cultured human dendritic cells. High endogenous levels of 9CDHRA in mice indicate physiological relevance of these data and that 9CDHRA acts as an endogenous RXR ligand.

Daily nutrition, in addition to being a source of energy, contains micronutrients, a class of nutrients including vitamins which are essential for life and which act by orchestrating a vast number of developmental and physiological processes. During metabolism, micronutrients are frequently transformed into their bioactive forms. Nuclear hormone receptors are a family of proteins functioning as ligand-regulated transcription factors which can sense such bioactive molecules and translate those signals into transcriptional, adaptive responses. Retinoid X receptors occupy a central place in this signaling as they directly interact, and thereby control, activities of several nuclear hormone receptors. We report here the identification of a novel bioactive form of vitamin A, which is the first endogenous form of this vitamin capable to bind and activate retinoid X receptors. Accordingly, we show that this single molecule displays biological activity similar to synthetic agonists of retinoid X receptors and coordinates transcriptional activities of several nuclear receptor signaling pathways. Those findings may have immediate biomedical implications, as retinoid X receptors are implicated in the control of a number of physiological functions and their pathology.

Mutual exclusivity analysis of genetic and epigenetic drivers in melanoma identifies a link between p14 ARF and RARβ signaling

Melanoma genomes contain thousands of alterations including: mutations, copy number alterations, structural aberrations, and methylation changes. The bulk of this variation is stochastic and functionally neutral, with only a small minority representing "drivers" that contribute to the genesis and maintenance of tumors. Drivers are often directly or inversely correlated across tumors, reflecting the molecular and regulatory signaling pathways in which they operate. Here, a profile of genetic and epigenetic drivers in 110 human melanoma cell lines was generated and searched for non-random distribution patterns. Statistically significant mutual exclusivity was revealed among components of each of the p16(INK4A)-CDK4-RB, RAS-RAF-MEK-ERK and PI3K-AKT signaling pathways. In addition, an inverse correlation was observed between promoter hypermethylation of retinoic acid receptor β (RARB) and CDKN2A alterations affecting p14(ARF) (P < 0.0001), suggesting a functional link between RARβ signaling and the melanoma-suppressive activities of p14(ARF). Mechanistically, all-trans retinoic acid (ATRA) treatment increased the expression of p14(ARF) in primary human melanocytes and the steady-state levels of p14(ARF) in these cells were shown to be regulated via RARβ. Furthermore, the ability of ATRA to induce senescence is reduced in p14(ARF)-depleted melanocytes, and we provide proof-of-concept that ATRA can induce irreversible growth arrest in melanoma cells with an intact RARβ-p14(ARF) signaling axis, independent of p16(INK4A) and p53 status.

IMPLICATIONS

These data highlight the power of mutual exclusivity analysis of cancer drivers to unravel molecular pathways and establish a previously unrecognized cross-talk between RARβ and p14(ARF) with potential implications for melanoma treatment.

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.

Oral R115866 in the treatment of moderate to severe plaque-type psoriasis

BACKGROUND

R115866 (Rambazole) is a new generation all-trans retinoic acid metabolism blocking agent, highly specific against the retinoic acid 4-hydroxylase. The drug alleviates hyperproliferation and normalizes differentiation of the epidermis in animal models of psoriasis.

OBJECTIVE

To explore the efficacy, safety and tolerability of systemic R115866 in patients with moderate to severe plaque-type psoriasis.

PATIENTS AND METHODS

In this open label, single-arm trial, patients were treated with R115866, 1 mg/day for 8 weeks, followed by a 2-week treatment-free follow-up period. Patients were monitored for efficacy and safety.

RESULTS

Nineteen patients (intent-to-treat population) were treated and 14 completed the entire study. Two patients discontinued due to lack of efficacy and three due to adverse events. At the end of the treatment, 26% of the patients showed at least 50% reduction in Psoriasis Area Severity Index (PASI) compared to baseline. Further improvement was observed at the end of the 2-week follow-up period where 47% of the patients showed a 50% or greater reduction in PASI. Kinetic data showed no evidence of accumulation of either R115866 or retinoic acid in plasma. The most common adverse events were pruritus, xerosis, cheilitis and an increase in blood triglycerides. The majority of adverse events were mild to moderate. No deaths or serious adverse events were reported.

CONCLUSION

Eight-week daily treatment with 1 mg R115866 resulted in a significant reduction in PASI from baseline to end of therapy. Additional improvement was seen after the 2-week follow-up period. The drug was well tolerated. R115866 merits further evaluation to optimize its clinical efficacy and safety profile in moderate to severe plaque-type psoriasis.

13-cis retinoic acid and isomerisation in paediatric oncology--is changing shape the key to success?

Retinoic acid isomers have been used with some success as chemotherapeutic agents, most recently with 13-cis retinoic acid showing impressive clinical efficacy in the paediatric malignancy neuroblastoma. The aim of this commentary is to review the evidence that 13-cis retinoic acid is a pro-drug, and consider the implications of retinoid metabolism and isomerisation for the further development of retinoic acid for cancer therapy. The low binding affinity of 13-cis retinoic acid for retinoic acid receptors, low activity in gene expression assays and the accumulation of the all-trans isomer in cells treated with 13-cis retinoic acid, coupled with the more-favourable pharmacokinetic profile of 13-cis retinoic acid compared to other isomers, suggest that intracellular isomerisation to all-trans retinoic acid is the key process underlying the biological activity of 13-cis retinoic acid. Intracellular metabolism of all-trans retinoic acid by a positive auto-regulatory loop may result in clinical resistance to retinoic acid. Agents that block or reduce the metabolism of all-trans retinoic acid are therefore attractive targets for drug development. Devising strategies to deliver 13-cis retinoic acid to tumour cells and facilitate the intracellular isomerisation of 13-cis retinoic acid, while limiting metabolism of all-trans retinoic acid, may have a major impact on the efficacy of 13-cis retinoic acid in paediatric oncology.

Regulation of hepatic retinol metabolism: perspectives from studies on vitamin A status

Liver vitamin A (retinol) is obtained from several sources and is subject to multiple fates. Lecithin:retinol acyltransferase (LRAT), a microsomal enzyme present in liver and several other retinol-metabolizing tissues, esterifies retinol that is associated with a cellular retinol-binding protein, CRBP or CRBP-II. Recent research has shown that LRAT mRNA expression and enzyme activity are regulated in a tissue-specific manner. In vitamin A-deficient liver, both LRAT mRNA and activity are significantly down-regulated as well as rapidly induced after the administration of vitamin A or its principal hormonal metabolite, retinoic acid (RA). In long-term feeding studies and the metabolic steady state, liver LRAT is expressed dose-dependently across a wide range of dietary vitamin A. Additionally, an RA-inducible cytochrome P450, P450RAI or CYP26, is down-regulated in liver during vitamin A deficiency and up-regulated dose-dependently by dietary vitamin A and exogenous RA. Based on these results, we propose that LRAT and CYP26 serve as two molecular mechanisms, coordinately regulated by all-trans-RA, to control the availability of retinol and RA, respectively. The LRAT reaction, besides providing a readily retrievable storage form of vitamin A, may regulate the availability of retinol to other pathways, while the CYP26 reaction may serve to prevent a detrimental "overshoot" of RA concentration. Moreover, retinoid metabolism in the liver is likely to be closely integrated with that in peripheral tissues through the rapid interorgan transfer and recycling of retinoids, affecting the whole-body economy of vitamin A.

Assessment of dermatopharmacokinetic approach in the bioequivalence determination of topical tretinoin gel products

BACKGROUND

A new dermatopharmacokinetic (DPK) approach has been proposed for bioequivalence determination of topical drug products by comparing the drug content kinetics in stratum corneum.

OBJECTIVE

We sought to establish any correlation between clinical safety/efficacy and DPK approach in bioequivalence determination of tretinoin gel 0.025%.

METHODS

Tretinoin and isotretinoin were quantified in human volar forearm stratum corneum as a function of time with 3 tretinoin gel 0.025% products in 49 patients. Stratum corneum layers were harvested using multiple adhesive disks, which were subsequently extracted and quantified for both isomers by high-performance liquid chromatography.

RESULTS

Products with similar composition and therapeutic equivalence were found bioequivalent, and products with different composition and clinical profiles were found bioinequivalent by DPK methodology.

CONCLUSIONS

There is a direct correlation between DPK parameters in healthy patients and clinical safety/efficacy of tretinoin gel products in patients with acne. Data support the use of DPK parameters and methodology in the bioequivalence assessment of topical tretinoin gel products.

Expression pattern and biochemical characteristics of a major epidermal retinol dehydrogenase

The biological functions of vitamin A in the epidermis are mediated by all-trans retinoic acid, which is biosynthesized from retinol in two oxidative reactions. The first step involves enzymatic conversion of retinol to retinaldehyde. The physiological significance and relative contributions of the various retinol dehydrogenases to the oxidation of retinol in epidermal cells remain unclear. We report the characterization of a retinol dehydrogenase/reductase of the SDR superfamily, hRoDH-E2, which is abundantly expressed in the epidermis, epidermal appendages and in cultured epidermal keratinocytes. Both in live keratinocytes and in isolated keratinocyte microsomes, where the enzyme normally localizes, hRoDH-E2 functions as a bona fide retinol dehydrogenase. In the prevailing oxidative reaction it recognizes both free- and CRBP-bound retinol, and shows preference toward NADP as a co-substrate. In comparison, hRoDH-E2 retinol dehydrogenase activity in the simple epithelial HEK 293 cells is much lower and in CHO cells is non-existent. hRoDH-E2 transcripts are distributed throughout the epidermal layers but are more abundant in the basal cells. In contrast, the protein is detected predominantly in the basal and the most differentiated living layers. Its synthesis is negatively regulated by retinoic acid. The biochemical properties and the differential expression of hRoDH-E2 in the strata where retinoic acid signaling is critical for epidermal homeostasis support a conclusion that hRoDH-E2 bears the characteristics of the major microsomal retinol dehydrogenase activity in the epidermal keratinocytes in physiological circumstances.

Retinoic acid decreases the viability of mouse blastocysts in vitro

BACKGROUND

This study was designed to examine the cytotoxic effect of retinoic acid on the blastocyst stage of mouse embryos and on subsequent early postimplantation embryo development in vitro.

METHODS AND RESULTS

Mouse blastocysts were exposed for 24 h to doses of 0, 0.1 micromol/l and 10 micromol/l all-trans retinoic acid and observed for their capacity to implant and develop during the early postimplantation period in vitro. When retinoic acid-pretreated blastocysts were allowed to implant in vitro, significantly fewer embryos were able to reach a later stage of embryo development. Compared with the findings for the control blastocysts, exposure to retinoic acid resulted in a significant reduction in the average number of total cells in blastocysts and the trophectoderm/inner cell mass lineage. The effect was associated with a significant increase in the frequency of cells identified as being engaged in apoptosis by means of the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling and Annexin V techniques.

CONCLUSIONS

This is the first evidence that retinoic acid induces cell death (apoptosis) and inhibits cell proliferation in mouse blastocysts. This results in the retardation of early postimplantation blastocyst development and subsequent blastocyst death.

Human cytochrome P450s involved in the metabolism of 9-cis- and 13-cis-retinoic acids

The purpose of this work was to identify the principal human cytochrome P450s (CYPs) involved in the metabolism of the retinoic acid (RA) isomers, 9-cis- and 13-cis-RA, by using a combination of techniques including human liver microsomes (correlation of activity and inhibition), and lymphoblast microsomes expressing a single CYP. Concerning the 9-cis-RA, 4-OH- and 4-oxo-9-cis-RA were formed with human liver microsomes, and their formation correlated with activities linked to CYPs 3A4/5, 2B6, 2C8, 2A6, and 2C9. The use of lymphoblast microsomes expressing a single human CYP identified CYPs 2C9>2C8>3A7 as the most active in the formation of 4-OH-9-cis-RA. With regard to 13-cis-RA, specific P450 activities linked to CYPs 2B6, 2C8, 3A4/5, and 2A6 were correlated with the formation of 4-OH- and 4-oxo-13-cis-RA. Microsomes expressing a single CYP identified CYPs 3A7, 2C8, 4A11, 1B1, 2B6, 2C9, 2C19, 3A4 (decreasing activity) in the formation of 4-OH-13-cis-RA. The use of CYP-specific inhibitors in human liver microsomes disclosed that the formation of the 4-OH-9-cis-RA was best inhibited by sulfaphenazole (72%) and quercetin (66%), whereas ketoconazole and troleandomycin inhibited its formation by 55 and 38%, respectively; the formation of 4-OH-13-cis-RA was best inhibited by troleandomycin (54%) and ketoconazole (46%), whereas quercetin was a weak inhibitor (14%). In conclusion, adult human CYPs 2C9, 2C8, 3A4 have been identified as active in the 9-cis-RA metabolism, whereas CYPs 3A4 and 2C8 were active in 13-cis-RA metabolism. The fetal form CYP3A7 was also identified as very active in either 9-cis- or 13-cis-RA metabolism. The role of these human CYPs in the biological response or resistance to RA isomers remains to be determined.

Effect of retinoic acid on implantation and post-implantation development of mouse embryos in vitro

BACKGROUND

This study was designed to examine the embryotoxic potential of retinoic acid (RA) at the blastocyst stage and during early post-implantation development of mouse embryos in vitro.

METHODS AND RESULTS

All-trans retinoic acid (t-RA) was administered to ICR mice embryos at a dose level of 0, 0.001 micromol/l, 0.1 micromol/l and 10 micromol/l throughout in-vitro culture. A total of 404 embryos was randomly assigned to all different dose groups. The percentage of embryos in later stages of development changed depending upon the dose of RA used. Exposure to 10 micromol/l of t-RA at the blastocyst stage, implanted blastocyst stage or early oocyte stage was also found to cause different degrees of retardation of embryo development and embryo death. These findings demonstrate, for the first time, that RA exerts an adverse effect on embryo growth during the early post-implantation stages of development, in comparison with day 3 to day 8 of gestation in vivo.

CONCLUSIONS

Although isotretinoin (13-cis-retinoic acid) is effective for the therapy of cystic acne and other dermatological disorders, retinoid treatment should be avoided at the early post-implantation stage of gestation.

13-cis retinoic acid exerts its specific activity on human sebocytes through selective intracellular isomerization to all-trans retinoic acid and binding to retinoid acid receptors

Despite its potent biologic effect on human sebocytes, 13-cis retinoic acid exhibits low binding affinity for cellular retinoic acid binding proteins and nuclear retinoid receptors. Hence, 13-cis retinoic acid may represent a pro-drug possibly acting through all-trans isomerization. In this study, marked isomerization of 13-cis retinoic acid has been confirmed in cultured SZ95 sebocytes showing 2- to 15-fold higher levels of all-trans retinoic acid at 12-72 h, as measured by high performance liquid chromatography. In contrast, only low amounts of all-trans retinoic acid were converted intracellularly to its 13-cis isoform. 9-cis retinoic acid was not detected after either 13-cis retinoic acid or all-trans retinoic acid treatment. The rapid isomerization of 13-cis retinoic acid to high levels of all-trans retinoic acid was a sebocyte-specific event, as no significant isomerization of 13-cis retinoic acid to all-trans retinoic acid occurred in HaCaT keratinocytes. De novo mRNA expression of cytochrome P450 1A1, a major xenobiotic metabolizing enzyme, in SZ95 sebocytes was induced by all-trans retinoic acid, but not by 13-cis retinoic acid. In addition, mRNA levels of cellular retinoic acid-binding protein II, which is supposed to regulate the concentration of intracellular all-trans retinoic acid, rapidly increased under all-trans retinoic acid treatment (30 min-6 h), whereas the 13-cis retinoic acid effect was markedly weaker and delayed. Both 13-cis retinoic acid and all-trans retinoic acid suppressed mRNA expression of cytochrome P450 1A2. In parallel experiments, 13-cis retinoic acid significantly reduced SZ95 sebocyte proliferation at 10-7 M, show- ing 30-40% inhibition after 9 d. All-trans retinoic acid and 9-cis retinoic acid exhibited similar anti-proliferative effects. AGN 193109, a pan-antagonist of the retinoic acid receptors, antagonized the anti-proliferative activity of all retinoic acid isomers tested in a concentration-dependent manner with complete abolishment at ratios of 1:10 13-cis retinoic acid and 1:1 all-trans retinoic acid. Coincubation of SZ95 sebocytes with 13-cis retinoic acid and AGN 193109 did not alter the intracellular concentration of 13-cis retinoic acid and its isomerization profile. In contrast, the retinoid X receptor antagonist CD 3507 did not affect the inhibition of SZ95 sebocyte proliferation induced by retinoic acids. Our findings indicate: (i) a selective 13-cis retinoic acid isomerization to all-trans retinoic acid in the intracellular compartment of SZ95 sebocytes; (ii) a reduced all-trans retinoic acid inactivation process after 13-cis retinoic acid treatment as compared with treatment with all-trans retinoic acid; and (iii) a retinoic acid receptor-mediated inhibition of SZ95 sebocyte proliferation. These data explain the sebocyte-specific activity of 13-cis retinoic acid and support a pro-drug/drug relation between 13-cis retinoic acid and all-trans retinoic acid.

Participation of CYP2C8 in retinoic acid 4-hydroxylation in human hepatic microsomes

Cytochromes P450 (CYPs) catalyze the 4-hydroxylation of all-trans-retinoic acid (ATRA), an agent used in the treatment of certain malignancies. Literature studies have implicated several CYPs in this reaction, but the relative importance of individual CYPs is unclear. Human microsomal CYPs that contribute to the activity were evaluated by correlation with activities of hepatic drug-metabolizing CYPs, the capacity of cDNA-derived CYPs to catalyze the reaction, and inhibition of the microsomal activity by chemicals. 4-HydroxyATRA formation in microsomes varied 7-fold (8.7 to 61 pmol/mg protein/min) and correlated partially with activities mediated by CYPs 3A, 2C, and 1A (p = 0.53 to 0.66). cDNA-derived CYPs 2C8, 2C9, and 3A4, but not 1A1 or 1A2, catalyzed ATRA 4-hydroxylation (2.53, 4.68, and 1.29 pmol/pmol CYP/hr). The Km for the reaction was 9 +/- 3 microM in hepatic microsomes (N = 3) and 6 microM in microsomes containing cDNA-derived CYP2C8; by comparison, Km values for the activity mediated by CYPs 2C9 and 3A4 were 100 and 74 microM, respectively. Inhibition of microsomal ATRA 4-hydroxylation was elicited by chemicals that interact with CYP2C8 (paclitaxel and diclofenac), but not those that interact with CYP2C9 (sulfaphenazole, tolbutamide, and torasemide). The CYP3A inhibitor troleandomycin and an anti-CYP3A IgG inhibited the activity slightly. Greater inhibition was produced by the less selective CYP3A inhibitors parathion, quinidine, and ketoconazole; CYP1A inhibitors were ineffective. These findings suggest that CYP2C8 is a major contributor to ATRA 4-hydroxylation in human liver and that 3A subfamily CYPs may be minor participants. Individual variation in CYP2C8 and 3A4 expression may influence ATRA pharmacokinetics and drug interactions during therapy.

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.

Cloning and characterization of retinol dehydrogenase transcripts expressed in human epidermal keratinocytes

The normal growth and differentiation of the epidermis require an adequate supply of vitamin A. The active form of vitamin A for normal epidermal homeostasis is retinoic acid (RA). Retinoic acid controls the expression of retinoid-responsive genes via interactions of the retinoic acid/nuclear receptor complexes at specific DNA sequences in their control regions. The message conveyed by RA is likely modulated by the concentration of the ligand available for binding to the receptors. Following the uptake of plasma retinol, epidermal keratinocytes synthesize retinoic acid via two sequential reactions with retinaldehyde as an intermediate. Several retinol dehydrogenase (RDH) enzymes, members of the short-chain dehydrogenase/reductase (SDR) gene superfamily, catalyze the first and rate-limiting step that generates retinaldehyde from retinol bound to cellular retinol-binding protein (holo-CRBP). However, little is known about these enzymes and their genes in the epidermal cells. Our work describes the first member of the RDH family found in epidermis. We show that this gene is expressed predominantly in the differentiating spinous layers and that it is under positive, feed-forward regulation by retinoic acid. It encodes a protein that, using NAD+ as a preferred cofactor, utilizes free and CRBP-bound all-trans-retinol and steroids as substrates.

Ultraviolet irradiation of human skin causes functional vitamin A deficiency, preventable by all-trans retinoic acid pre-treatment

We report here that ultraviolet irradiation substantially reduced the mRNA and protein of the two major nuclear retinoid receptors, RAR-gamma and RXR-alpha, in human skin in vivo. Pre-treatment with retinoic acid mitigated this loss of nuclear retinoid receptors. Ultraviolet irradiation caused a near-total loss of retinoic acid induction of two RAR/RXR target genes, cellular retinoic acid binding protein-II and RA 4-hydroxylase, but did not affect 1,25-dihydroxyvitamin D3 induction of the vitamin D receptor/RXR-regulated gene vitamin D 24-hydroxylase. In effect, ultraviolet irradiation causes a functional vitamin A deficiency that may have deleterious effects on skin function, contributing to skin photo-aging and carcinogenesis.

Retinoid signaling by all-trans retinoic acid and all-trans retinoyl-beta-D-glucuronide is attenuated by simultaneous exposure of human keratinocytes to retinol

Retinol and retinyl esters are converted with time to slowly increasing amounts of all-trans retinoic acid (RA) in cultured human keratinocytes. Exogenous RA has been shown to limit retinol oxidation and to increase retinol esterification. Because significant amounts of retinol are present in biologic systems, we examined whether RA and all-trans-retinoyl-beta-D-glucuronide (RAG) interact with retinol in exhibiting their activities on HaCaT keratinocytes maintained in a retinoid-free culture system. RA was more potent than RAG and retinol in inducing ultrastructural changes attributed to retinoids, inhibiting cell proliferation as well as enhancing keratin 19 expression. In addition, retinoids were able to induce cellular retinoic acid-binding protein II mRNA levels in the cultures, whereas early RA and late RAG activity was detected. The described biologic effects of RA and RAG were diminished by simultaneous cell exposure to retinol. HaCaT cells quickly metabolized retinol to retinyl esters and consequently to low amounts of RA. RA treatment led to an early high peak of cellular RA followed by reduction to trace amounts. Treatment with RAG resulted in constantly high cellular RAG and low RA levels. Under the combined RA and retinol treatment retinyl esters were increased and RA was reduced in HaCaT cells, whereas extracellular RA levels were similar to those obtained by RA alone. On the other hand, the combination of RAG and retinol resulted in higher extracellular RAG, similar cellular RAG, and lower cellular RA levels than those obtained by RAG alone without any change in retinyl esters. This study demonstrates that retinoid signaling by RA and RAG is attenuated by simultaneous exposure of HaCaT keratinocytes in vitro to retinol. The presence of retinol in the medium alters the rate of RA or RAG metabolism and thus cellular RA concentrations. The intensity of retinoid signal is probably dependent on cellular RA levels. The resulting "antagonism" among retinoids is consistent with the presence of an auto-regulatory mechanism in human keratinocytes offering protection against excessive accumulation of cellular RA.

Reduced lecithin:retinol acyl transferase activity in cultured squamous cell carcinoma lines results in increased substrate-driven retinoic acid synthesis

The uptake and metabolism of retinol was compared in squamous cell carcinoma lines, SCC12b and SCC13, and in normal human keratinocytes (NHK). Long chain fatty acid esters of retinol and 3,4-didehydroretinol were the predominant metabolites formed in both cell types. Lesser amounts of unesterified retinol, 3,4-didehydroretinol, and their respective active acid forms were also observed. Despite a qualitatively similar retinoid composition, there were significant quantitative differences between cell types. Most notable was that SCC formed only about one-fourth the retinoid ester as did normal cells. In parallel with this, unesterified retinol and retinoic acid concentrations in SCC were significantly elevated over those in normal cells. This altered pattern of retinoid metabolites in SCC was found to be due to very low lecithin:retinol acyltransferase (LRAT) activity. SCC exhibited less than one-tenth the LRAT activity of normal cells. Acyl-coenzyme A:retinol acyltransferase (ARAT) and retinyl ester hydrolase activities were not different between cell types. Challenging cells with increasing medium retinol concentrations resulted in dose-dependent increases in retinol and retinoic acid within SCC. In contrast, retinol and retinoic acid concentrations in similarly challenged normal cells remained relatively low across a wide retinol concentration range. This was accomplished by the storage of retinol, via LRAT activity, as retinyl ester. Consistent with increased substrate-driven retinoic acid synthesis in SCC, the expression of transglutaminase 1 was suppressed to a greater extent in the SCCs than in NHK, when cells were exposed to equivalent medium concentrations of retinol. The data demonstrate a central role of LRAT in regulating retinoic acid synthesis via its capacity to modulate cellular levels of substrate retinol.

Dermal fibroblasts actively metabolize retinoic acid but not retinol

The accessibility of plasma retinol and retinoic acid to the epidermis may be influenced by the number and metabolic capacity of fibroblasts in papillary dermis. The metabolism of retinol-binding protein-bound all-trans-retinol, and albumin-bound all-trans-retinoic acid, by fibroblasts cultured on plastic dishes or in type I collagen gels, was examined. There were no significant differences in the metabolism of either retinoid by fibroblasts as a function of culture condition. There were large differences between retinoids, however. Retinoic acid was rapidly taken up and metabolized to unidentified polar metabolites that were released to the medium. Metabolic capacity was not saturated up to a medium retinoic acid concentration of 1 microM, and was induced further by prior exposure to retinoic acid. In contrast, retinol, although readily taken up, was not metabolized, i.e., neither retinoic acid nor retinyl ester was formed. By immunohistochemistry, the average number of fibroblasts in a 100 microm thickness of papillary dermis was estimated to be 1 x 10(6) cells per cm2. Utilizing this value, the capacity of dermal fibroblasts to metabolize retinoic acid based on fibroblast abundance in the dermis was calculated. The results suggest that fibroblasts could limit delivery of plasma retinoic acid but not retinol to the epidermis on the basis of their metabolic capacity and abundance in the dermis.

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

Tissue levels of all-trans retinoic acid (RA) are maintained through coordinated regulation of biosynthesis and breakdown. The major pathway for all-trans RA inactivation is initiated by 4-hydroxylation. A novel cytochrome P-450 (CYP26) that catalyzes 4-hydroxylation of all-trans RA has recently been cloned. We have investigated regulation and properties of RA 4-hydroxylase in immortalized human keratinocyte HaCaT cells. In the absence of added retinoid, RA 4-hydroxylase (CYP26) mRNA and protein were minimally detected. Addition of all-trans RA rapidly induced RA 4-hydroxylase mRNA (within 2 h) and activity (within 6 h). Induction of both mRNA and activity was transient, returning to baseline within 48 h, and completely dependent on mRNA synthesis (i.e., blocked by actinomycin D). The synthetic retinoid CD367, which specifically activates nuclear RA receptors, also rapidly induced RA 4-hydroxylase activity. This induction, however, unlike that of all-trans RA, was long-lived (>48 h). This difference was attributable to lack of metabolic inactivation of CD367 in HaCaT cells. CD2665, which inhibits RA receptor-dependent gene transcription, blocked retinoid induction of RA 4-hydroxylase, indicating that it is mediated by RA receptors. Addition of excess unlabeled substrates specific for 10 distinct mammalian P-450 subfamilies did not compete with all-trans RA for RA 4-hydroxylase activity. RA 4-hydroxylase did not hydroxylate 9-cis RA or 13-cis RA. Inhibition of RA 4-hydroxylase activity by ketoconazole potentiated activation of RA receptors by all-trans RA. In summary, RA 4-hydroxylase is a unique, highly specific cytochrome P-450 isoenzyme, whose expression is regulated by its natural substrate, all-trans RA, through activation of RA receptors. RA 4-hydroxylase functions to limit the levels, and thereby the biologic activity of all-trans RA in HaCaT cells.

The antiproliferative activity of all-trans-retinoic acid catabolites and isomers is differentially modulated by liarozole-fumarate in MCF-7 human breast cancer cells

The clinical use of all-trans-retinoic acid (ATRA) in the treatment of cancer is significantly hampered by the prompt emergence of resistance, believed to be caused by increased ATRA catabolism. Inhibitors of ATRA catabolism may therefore prove valuable for cancer therapy. Liarozole-fumarate is an anti-tumour drug that inhibits the cytochrome P450-dependent catabolism of ATRA. ATRA, but also its naturally occurring catabolites, 4-oxo-ATRA and 5,6-epoxy-ATRA, as well as its stereoisomers, 9-cis-RA and 13-cis-RA, show significant antiproliferative activity in MCF-7 human breast cancer cells. To further elucidate its mechanism of action, we investigated whether liarozole-fumarate was able to enhance the antiproliferative activity of ATRA catabolites and isomers. Liarozole-fumarate alone up to a concentration of 10(-6) M had no effect on MCF-7 cell proliferation. However, in combination with ATRA or the ATRA catabolites, liarozole-fumarate (10(-6) M) significantly enhanced their antiproliferative activity. On the contrary, liarozole-fumarate (10(-6) M) was not able to potentiate the antiproliferative activity of the ATRA stereoisomers, most probably because of the absence of cytochrome P450-dependent catabolism. Together, these findings show that liarozole-fumarate acts as a versatile inhibitor of retinoid catabolism in that it not only blocks the breakdown of ATRA, but also inhibits the catabolic pathway of 4-oxo-ATRA and 5,6-epoxy-ATRA, thereby enhancing their antiproliferative activity.

Retinoic acid receptor-gamma in human epidermis preferentially traps all-trans retinoic acid as its ligand rather than 9-cis retinoic acid

The biologic activity of retinoids is mediated through nuclear retinoic acid receptors (RAR), which are ligand-activated transcription factors. RAR directly bind and are activated by two naturally occurring isomers of retinoic acid (RA), all-trans retinoic acid (t-RA) and 9-cis retinoic acid (9c-RA). Human skin predominantly expresses RAR-gamma (approximately 87%) and RAR-alpha makes up the remainder. Recombinant RAR-gamma preferentially binds t-RA over 9c-RA in cell-free assays containing mixtures of the two retinoic acid isomers. We have investigated the ligand-binding properties of RAR in human epidermis. [3H]All-trans retinol (t-ROL) added to suspensions of intact epidermal cells was metabolically converted to [3H]t-RA, which bound to RAR. No binding of [3H]9c-RA to RAR was detected. Binding of [3H]t-RA, formed from [3H]t-ROL, was abolished by adding unlabeled t-RA, but was unaffected by adding unlabeled 9c-RA. Intact epidermal cells were incubated with mixtures of [3H]9c-RA and [3H]t-RA in varying ratios, and the amount of each labeled retinoid bound to RAR was measured. At ratios of 9c-RA to t-RA of 3:1 or lower, only [3H]t-RA was bound by RAR. Incubation of cells with [3H]9c-RA alone resulted in substantial (38%) binding of [3H]t-RA to RAR, in addition to binding of [3H]9c-RA, due to isomerization of [3H]9c-RA to [3H]t-RA. RAR in nuclear extracts from epidermal cells also displayed strong preferential binding of t-RA over 9c-RA. Competition studies revealed that 9c-RA was 6-fold less effective than t-RA at displacing [3H]t-RA bound to RAR in nuclear extracts. At ratios of 9c-RA to t-RA of 4:1 or lower, RAR in nuclear extracts bound t-RA exclusively. At higher ratios, [3H]9c-RA binding increased steeply. RAR-alpha in nuclear extracts bound both 9c-RA and t-RA without preference, whereas RAR-gamma displayed strong preferential binding of t-RA over 9c-RA. The level of endogenous t-RA exceeds that of 9c-RA in human skin in vivo, and significant isomerization of topically applied 9c-RA and 13c-RA to t-RA occurs. The relative abundance of t-RA in human skin, and preferential binding of t-RA by RAR-gamma, indicate that t-RA is the primary ligand mediating RAR-dependent responses in human skin under physiologic conditions, and under pharmacologic conditions when t-RA, 9c-RA, or 13c-RA are applied to skin.

Mouse P450RAI (CYP26) expression and retinoic acid-inducible retinoic acid metabolism in F9 cells are regulated by retinoic acid receptor gamma and retinoid X receptor alpha

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 cross-species 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 gamma, RAR alpha, and/or RXR alpha, 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 gamma and RXR alpha receptors mediate the effects of retinoic acid on the expression of the P450RAI gene.

All-trans retinoic acid blocks the antiproliferative prodifferentiating actions of 1,25-dihydroxyvitamin D3 in normal human keratinocytes

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] and all-trans retinoic acid (RA), the active metabolites of vitamins D and A respectively, regulate the proliferation and differentiation of keratinocytes. Both the vitamin D receptor (VDR) and the retinoic acid receptor family (RAR) bind to DNA response elements as heterodimers with the retinoic X receptor (RXR), suggesting that there are pathways of action that are shared by both compounds. Therefore, we examined the interactions of 1,25(OH)2D3 and RA upon the proliferation and differentiation of normal human keratinocytes (NHK) and of a squamous cell carcinoma cell line, SCC4. Although both 1,25(OH)2D3 and RA were each able to inhibit NHK proliferation in a dose-dependent manner, when they were administered in combination, proliferation was stimulated, suggesting mutual antagonism. In contrast, SCC4 cells proved insensitive in terms of proliferation to 1,25(OH)2D3 and to all but the highest concentration (10(-6) M) of RA. 1,25(OH)2D3 exerted a biphasic effect on transglutaminase (TGase) and involucrin (INV) mRNA levels, with maximal stimulation at 10(-9) M. RA inhibited TGase and INV mRNA levels and antagonized the stimulation by 1,25(OH)2D3. A similar pattern was observed for TGase protein, but, RA, which, by itself, reduced INV, markedly enhanced the ability of 1,25(OH)2D3 to raise INV levels, possibly by inhibiting 1,25(OH)2D3-stimulated TGase activity and cross-linking of soluble INV into the insoluble cornified envelope (CE). Thus, in NHK cells, RA antagonizes the antiproliferative prodifferentiating actions of 1,25(OH)2D3, but assessment of a single marker, such as INV protein, may be misleading.

Unoccluded retinol penetrates human skin in vivo more effectively than unoccluded retinyl palmitate or retinoic acid

The formation of all-trans retinoic acid is an oxidative process whereby retinol is converted to retinaldehyde and then to retinoic acid. Because retinol causes qualitative molecular changes similar to those produced by retinoic acid, we compared potency of retinol, retinaldehyde, and retinyl palmitate to retinoic acid and assessed the effects of occlusion. Retinoids were prepared in an experimental vehicle of 95% ethanol:propylene glycol (7:3) with anti-oxidant. Induction of retinoic acid 4-hydroxylase activity was the end point for comparison. Retinoic acid concentrations from 0.001% to 0.05% under occlusion produced a linear dose-response induction of 4-hydroxylase activity. The concentrations of the other retinoids under occlusion required to achieve significant induction of enzyme activity were 0.6% retinyl palmitate, 0.025% retinol, and 0.01% retinaldehyde. The linear dose-response was lost with retinoid concentrations in excess of 0.25% retinol or 0.5% retinaldehyde. Statistical analyses showed no difference in 4-hydroxylase activity between unoccluded and occluded retinol treated sites. By contrast, however, unoccluded sites treated with retinoic acid or retinyl palmitate had less induction of 4-hydroxylase activity than occluded sites. Retinol, retinaldehyde, and retinyl palmitate did not produce erythema but did increase epidermal thickness. Although retinol is a weaker retinoid than retinoic acid, the increased penetration of unoccluded retinol in comparison to unoccluded retinoic acid with this prototypic vehicle confers on retinol a more effective delivery of a retinoidal effect than unoccluded retinoic acid. Retinol at 0.25% may be a useful retinoid for application without occlusion because it does not irritate but does induce cellular and molecular changes similar to those observed with application of 0.025% retinoic acid.

Retinoic acid modulates the anti-proliferative effect of 1,25-dihydroxyvitamin D3 in cultured human epidermal keratinocytes

Both 1,25-dihydroxyvitamin D3 (VD) and retinoids have potent effects on keratinocyte proliferation. Parallelism in their action as steroid hormones, which involves interaction of their receptors, and in their therapeutic efficacy for hyper-proliferative skin diseases provides a rationale to investigate their combined action on proliferation in pre-confluent human epidermal keratinocyte cultures. As shown by [3H]thymidine incorporation, all-trans retinoic acid (atRA) at subpharmacologic concentrations and 9-cis retinoic acid (9cRA) diminished the anti-proliferative effect of VD. Pre-incubation of the cells with the retinoids clearly enhanced this effect. Cell-cycle analysis revealed G1 arrest upon VD treatment that was attenuated by retinoic acid (RA). Moreover, Northern and Western blot analysis demonstrated that retinoic acid opposed VD-induced accumulation of transforming growth factor-beta1, p21WAF1, and p27KIP1. Finally, retinoic acid reduced VD-elicited hypophosphorylation of the retinoblastoma protein. AtRA at micromolar concentrations conversely potentiated most of the aforementioned VD-dependent actions. In addition, atRA and 9cRA (but not VD) caused a rapid, sustained reduction of RXR alpha protein. VD receptor protein was induced by VD regardless of the presence of RA. In conclusion, RA modulates VD-dependent effects at different levels of keratinocyte proliferation. This could have implications for the use of combinations of both drugs for skin diseases.

The retinoid X receptor agonist 9-cis-retinoic acid and the 24-hydroxylase inhibitor ketoconazole increase activity of 1,25-dihydroxyvitamin D3 in human skin in vivo

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] transactivates its target genes via the vitamin D receptor (VDR). VDR functions in physiology as a dimer complexed with retinoid X receptor (RXR), whose natural ligand is 9-cis-retinoic acid (9-c-RA). Inactivation of 1,25(OH)2D3 occurs through a cytochrome P-450 24-hydroxylase (OHase). The promoter of the human 24-OHase gene contains a 1,25(OH)2D3-responsive enhancer element (VDRE). We have used this VDRE containing gene as an endogenous reporter for vitamin D3-mediated gene activation in vivo. Normal adult human skin was keratomed after a 2-d exposure to 1,25(OH)2D3, 9-c-RA, all-trans-RA, and ketoconazole. 1,25(OH)2D3 caused a concentration-dependent increase in 24-OHase mRNA expression as determined by northern blot analysis. The activity of epidermal 24-OHase was also induced by 1,25(OH)2D3. Compared with vehicle, neither of the RA isomers nor ketoconazole alone induced 24-OHase mRNA. Addition of 9-c-RA or t-RA to 1,25(OH)2D3, however, caused a synergistic increase in 24-OHase mRNA. Similarly, 1,25(OH)2D3 plus ketoconazole increased 24-OHase mRNA synergistically. Ketoconazole inhibited ex vivo 1,25(OH)2D3-induced epidermal 24-OHase activity. Thus, 24-OHase mRNA induction is a sensitive reporter of 1,25(OH)2D3 activity in vivo; RXR bound to VDR is not a silent partner in vivo, because 9-c-RA enhances 1,25(OH)2D3-liganded RXR/VDR stimulation of the VDRE containing 24-OHase gene; ketoconazole inhibition of 24-OHase enhances 1,25(OH)2D3 activity by impeding its breakdown. Thus, the synergistic response of human skin to topical 1,25(OH)2D3 and/or 1,25(OH)2D3 analogs plus RXR retinoids and/or ketoconazole may be exploited to give a desired biologic/therapeutic response with less 1,25(OH)2D3, minimizing the potential calcemic risk from systemic absorption of 1,25(OH)2D3.

Suprabasal expression of a dominant-negative RXR alpha mutant in transgenic mouse epidermis impairs regulation of gene transcription and basal keratinocyte proliferation by RAR-selective retinoids

To determine whether 9-cis retinoic acid receptors (RXRs) regulate the biological activity of all-trans retinoic acid (tRA) and its receptors (RARs) in skin, we have targeted a dominant-negative RXR alpha (dnRXR alpha) lacking transactivation function AF-2 to differentiated suprabasal keratinocytes in the epidermis of transgenic mice. Driven by the suprabasal-specific keratin-10 gene promoter, expression of dnRXR alpha severely reduced the ability of RAR-selective ligands tRA and CD367 to induce epidermal mRNA levels of the CRABPII, CRBPI, and CRBPII genes, which contain RA-responsive elements (RAREs) DR1 and/or DR2. It also reduced gene-specific, synergistic induction of CRBPI mRNA by a combination of CD367 and RXR-selective SR11237. Like endogenous RXR alpha, dnRXR alpha in epidermal nuclear extracts from the transgenic mice competitively formed heterodimers with endogenous RAR gamma on RAREs, suggesting that dnRXR alpha impairs retinoid signaling by competing with endogenous RAR gamma-RXR alpha heterodimers. Histologically, the epidermis of dnRXR alpha mice showed no detectable developmental abnormalities. Surprisingly, in adult animals, the suprabasal expression of dnRXR alpha significantly reduced the ability of topically applied tRA to stimulate proliferation of undifferentiated keratinocytes in the basal layer of epidermis. RXR-selective ligands alone had no detectable effects on both normal and transgenic mouse epidermis. Accordingly, we suggest that in vivo: (1) in suprabasal keratinocytes, retinoids regulate gene transcription via RAR-RXR heterodimers in which RAR confers a predominant ligand response, whereas RXR AF-2 is required for liganded RAR AF-2 to efficiently trans-activate target genes, and (2) this suprabasal RXR-assisted mechanism indirectly regulates proliferation of basal keratinocytes likely via intercellular signaling.

Liarozole inhibits human epidermal retinoic acid 4-hydroxylase activity and differentially augments human skin responses to retinoic acid and retinol in vivo

Metabolic inactivation of all-trans retinoic acid to 4-hydroxy retinoic acid occurs via a cytochrome P-450 enzyme. We investigated the effects of liarozole on the retinoic acid 4-hydroxylase activity of human epidermis and its ability to modify in vivo human skin responses to retinoic acid and all-trans retinol. Retinoic acid 4-hydroxylase activity induced in vivo by 4 d treatment with retinoic acid (0.1%) was inhibited in vitro by liarozole in a concentration-dependent manner. Comparable micromolar concentrations of liarozole were extracted from stratum corneum-free epidermis treated with 3% liarozole. Retinoic acid levels in liarozole-treated skin increased to 19 +/- 5 ng/g wet wt (mean +/- SEM, p < 0.002, n = 17) at 18 h and to 6 +/- 2 ng/g wet wt (p = 0.38, n = 17) at 48 h as compared to vehicle (not detectable). At 48 h, retinoic acid 4-hydroxylase activity was induced 9-fold over vehicle (p < 0.03, n = 8). At 96 h, no significant erythema or increased epidermal thickness was found when either retinoic acid (0.001%), all-trans retinol (0.0250%), or liarozole (3%) was applied individually, but when 0.001% retinoic acid and 3% liarozole were applied together, both erythema and increased epidermal thickness occurred. In contrast, 0.025% all-trans retinol and 3% liarozole together caused increased epidermal thickness but no erythema. These data demonstrate that, at doses used here, liarozole, although an effective inhibitor of retinoic acid 4-hydroxylase, cannot function alone like a retinoid in vivo, probably because of retinoic acid 4-hydroxylase induction. In the presence of a low dose retinoic acid or all-trans retinol, however, liarozole can amplify human skin responses to each retinoid in a manner characteristic of the retinoid at a higher dose (erythema and hyperplasia with retinoic acid; no erythema but hyperplasia with all-trans retinol).