Human skin levels of retinoic acid and cytochrome P-450-derived 4-hydroxyretinoic acid after topical application of retinoic acid in vivo compared to concentrations required to stimulate retinoic acid receptor-mediated transcription in vitro

Trifarotene: A Current Review and Perspectives in Dermatology

Retinoids have numerous applications in inflammatory, dyskeratotic, and oncohematology diseases. Retinoids have now reached the fourth generation, progressively reducing toxicity whilst increasing their efficacy. Trifarotene is a new fourth-generation retinoid with a selective action on RAR-γ. In this review, we reported the trials—both concluded and in progress—including the use of trifarotene in dermatological diseases. Studies were identified by searching electronic databases (MEDLINE, EMBASE, PubMed, Cochrane, Trials.gov) from 2012 to today and reference lists of respective articles. Only articles published in English language were included. Randomized trials evaluating trifarotene tolerability, safety, and efficacy in congenital ichthyosis and acne have demonstrated great results and mild side effects, leading to the approval by the FDA of trifarotene for the treatment of lamellar ichthyosis in 2014, and of acne vulgaris in October 2019. No high-quality randomized clinical trials have evaluated the treatment of primary cutaneous lymphomas with trifarotene. Finally, we are hypothesizing future perspectives in the treatment of non-melanoma skin cancers, fungal infections, photoaging, and hand-foot skin reactions with trifarotene.

Oily Skin: A review of Treatment Options

One of the most common dermatologic concerns is oily skin, and the demand for effective treatment options is ever apparent. This review article addresses numerous topical treatment options such as retinoids, olumacostat glasaretil, and various cosmeceutical agents. several systemic and procedural techniques that incorporate isotretinoin, spironolactone, oral contraceptives, botulinum toxin, photodynamic therapy, and lasers are reviewed as well. Each treatment option is analyzed in terms of the proposed mechanism of action, efficacy reported in the literature, and potential adverse effects.

Tretinoin peel: a critical view

The tretinoin peel, also known as retinoic acid peel, is a superficial peeling often performed in dermatological clinics in Brazil. The first study on this was published in 2001, by Cuce et al., as a treatment option for melasma. Since then, other studies have reported its applicability with reasonable methodology, although without a consistent scientific background and consensus. Topical tretinoin is used for the treatment of various dermatoses such as acne, melasma, scars, skin aging and non-melanoma skin cancer. The identification of retinoids cellular receptors was reported in 1987, but a direct cause-effect relation has not been established. This article reviews studies evaluating the use of topical tretinoin as agent for superficial chemical peel. Most of them have shown benefits in the treatment of melasma and skin aging. A better quality methodology in the study design, considering indication and intervention is indispensable regarding concentration, vehicle and treatment regimen (interval and number of applications). Additionally, more controlled and randomized studies comparing the treatment with tretinoin cream versus its use as a peeling agent, mainly for melasma and photoaging, are necessary.

Noninvasive Facial Rejuvenation. Part 1: Patient-Directed

A proper knowledge of noninvasive facial rejuvenation is integral to the practice of a cosmetic surgeon. Noninvasive facial rejuvenation can be divided into patient- versus physician-directed modalities. Patient-directed facial rejuvenation combines the use of facial products such as sunscreen, moisturizers, retinoids, α-hydroxy acids, and various antioxidants to both maintain youthful skin and rejuvenate damaged skin. Physicians may recommend and often prescribe certain products, but the patients are in control of this type of facial rejuvenation. On the other hand, physician-directed facial rejuvenation entails modalities that require direct physician involvement, such as neuromodulators, filler injections, laser resurfacing, microdermabrasion, and chemical peels. With the successful integration of each of these modalities, a complete facial regimen can be established and patient satisfaction can be maximized. This article is the first in a three-part series describing noninvasive facial rejuvenation. The authors focus on patient-directed facial rejuvenation. It is important, however, to emphasize that even in a patient-directed modality, a physician's involvement through education and guidance is integral to its success.

Topical retinoids in skin ageing: a focused update with reference to sun-induced epidermal vitamin A deficiency

Vitamin A is an important constituent of the epidermis, where it plays a crucial role in epidermal turnover. A deficiency of epidermal vitamin A may be the consequence of nutritional vitamin A deficiency, exposure to sunlight or any UV source, oxidative stress or chronological ageing. As a consequence, any treatment aiming at increasing epidermal vitamin A would exert a protective effect against these deleterious conditions. Retinoids may counteract some deleterious actions of UV radiation by physical and biological mechanisms. Topical natural retinoic acid precursors such as retinaldehyde or retinol are less irritant than acidic retinoids and may prevent epidermal vitamin A deficiency due to nutritional deficiency, exposure to sunlight or any condition leading to free radical production. Retinoids may be combined with other compounds with complementary actions against ageing, nutritional deficiency and cancer, such as antioxidants, to potentiate their beneficial effects in the skin.

Biphasic pro-melanogenic and pro-apoptotic effects of all-trans-retinoic acid (ATRA) on human melanocytes: time-course study

BACKGROUND

The effects of retinoids on melanogenesis and their mechanism as depigmenting agents in topical therapy have not been fully elucidated. Conflicting data about their impact on melanogenic pathways have been reported.

OBJECTIVE

To investigate the effects of all-trans-retinoic acid (ATRA) on normal human melanocytes from Caucasian subjects.

METHODS

We assessed ATRA's cytotoxicity by measuring viability with a cell proliferation assay, and apoptotic effects using Annexin V and γ-H2AX markers. ATRA's melanogenic activity was investigated based on spectrophotometric measurement of melanin content and tyrosinase enzymatic activity. Tyrosinase expression was assessed by Western blotting. We tested the antioxidant activity of superoxide dismutase (SOD) and catalase (CAT) in melanocytes using a spectrophotometric assay.

RESULTS

Of the concentrations tested in this 72h time-course study, the 1.0μM ATRA had a well-defined two-stage pro-melanogenic and pro-apoptotic effect on melanocytes. In the first 6h, treated cells showed significant increase (p≤0.01) of melanin content, tyrosinase, SOD, and CAT activities compared to the controls. While overall tyrosinase expression was not affected by ATRA, all other tested parameters decreased progressively beyond the short-term point of 6h. ATRA treatment of over 6h induced melanocyte apoptosis, as shown by the time-dependent decrease in cell viability, coupled with significant increase in Annexin V positive cells and nuclear accumulation of γ-H2AX foci.

CONCLUSION

The results obtained using this testing platform show a biphasic ATRA action: immediate pro-melanogenic effect and longer-term exposure pro-apoptotic activity. These data qualify ATRA as a potent tool to better understand the mechanisms that regulate the pigmentary system.

Polymeric micelles of PEG-PE as carriers of all-trans retinoic acid for stability improvement

The topical application of all-trans retinoic acid (ATRA) is an effective treatment for several skin disorders, including photo-aging. Unfortunately, ATRA is susceptible to light, heat, and oxidizing agents. Thus, this study aimed to investigate the ability of polymeric micelles prepared from polyethylene glycol conjugated phosphatidylethanolamine (PEG-PE) to stabilize ATRA under various storage conditions. ATRA entrapped in polymeric micelles with various PEG and PE structures was prepared. The critical micelle concentrations were 97-243 μM, depending on the structures of the PEG and PE molecules. All of the micelles had particle diameters of 6-20 nm and neutral charges. The highest entrapment efficiency (82.7%) of the tested micelles was exhibited by ATRA in PEG with a molecular weight of 750 Da conjugated to dipalmitoyl phosphatidylethanolamine (PEG(750)-DPPE) micelles. The PEG(750)-DPPE micelle could significantly retard ATRA oxidation compared to ATRA in 75% methanol/HBS solution. Up to 87% of ATRA remained in the PEG(750)-DPPE micelle solution after storage in ambient air for 28 days. This result suggests that PEG(750)-DPPE micelle can improve ATRA stability. Therefore, ATRA in PEG(750)-DPPE micelle is an interesting alternative structure for the development of cosmeceutical formulations.

Study on cytochrome p-450 dependent retinoic Acid metabolism and its inhibitors as potential agents for cancer therapy

The relative lack of clinical success with conventional anticancer agents may be due in part to the traditional concept of cancer being a biological state rather than a dynamic process. Redefining cancer as a dynamic disease commencing with carcinogenesis introduces the possibility of chemoprevention. Retinoids offer the promise of a therapeutic option based on differentiation of premalignant as well as malignant cells. Research to date has concentrated on the use of exogenous retinoids in cancer. Although this research continues with new retinoid derivatives, an alternative approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450- mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs which increase the level of endogenous retinoic acid (RA) within the tumor cells by blocking their metabolism. This approach presents several theoretic advantages.

In the present study a wide range of established P-450 inhibitors has been screened to examine their inhibitory activity on all-trans-Retinoic acid (ATRA) metabolism. Forty-one known P450 inhibitors were tested for their inhibitory activity against RA metabolism. Most of them are nitrogen-containing compounds. The results showed that among these compounds only six compounds (N-benzyl-2-phenylethanamine, itraconazole, chlorpromazine, 5-chloro-1,3-benzoxazol-2-amine, proadifen and furazolidone) showed inhibition of RA metabolism which was > 50%. Ketoconazole and liarozole were also screened as standard potent inhibitors in the same system and gave 87.5% and 89% inhibition, respectively. The results indicate that mostly azoles with substituents in positions other than the 1-position on the ring are very weak inhibitors of RA metabolism. The most effective inhibitors (ketoconazole, itraconazole, bifonazole and clotrimazole) are 1-substituted and possess relatively large aromatic groups in the molecule. 1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution in the other position of the imidazole decreases the binding affinity.

Metabolic activation of heterocyclic amines and expression of CYP1A1 in the tongue

Xenobiotic metabolism in oral tissues, especially in the tongue, has never been reported. In the present study, the metabolic activation/detoxification ability of promutagens in the tongue and the expression levels of related enzymes were investigated. Quantitative PCR analysis of rat tongue demonstrated constitutive messenger RNA (mRNA) expression of numerous drug-metabolizing enzymes. In particular, we detected mRNA, protein expression, and enzymatic activity of cytochrome P450 (CYP)1A1 in the tongue tissue. Metabolic activation of promutagens in the tongue was estimated using benzo[a]pyrene or heterocyclic amines (HCAs), found in cooked meat and tobacco products. Metabolic activation levels of HCAs in the tongue were comparable to those in the liver. In contrast, the expression levels of glutathione-S-transferase (GST) and uridine diphosphate-glucuronosyltransferase (UGT) in the tongue were considerably lower compared with those in the liver, and as a result, the mutagenic activity in the tongue was not decreased by GST- or UGT-dependent conjugation. Treatment of rats with sudan III, a typical inducer of CYP1A1, resulted in markedly increased CYP1A1 mRNA, protein expressions, and CYP1A-dependent enzymatic and mutagenic activities. In addition, CYP1A1 mRNA expression in carcinoma cells (SAS) was induced by sudan III exposure. In conclusion, mutagenic activation of xenobiotics and an increased risk of cancer in the tongue were observed in this study. Furthermore, ingestion of drug-metabolizing enzyme inducers has the potential to increase the metabolic activation in the tongue tissue and increase the risk of biomolecular attack by promutagens.

Topical retinoids in the treatment of photoaging

A large number of different substances comprise the family of retinoids, which are traditionally described as vitamin A derivatives. By exerting their action through nuclear and cytoplasmic receptors they may improve photoaging. Tretinoin is the best studied retinoid in the treatment of photoaging. Others such as isotretinoin, retinaldehyde, and tazarotene, although less well studied, have given promising results.

Proposed mechanisms of action for retinoid derivatives in the treatment of skin aging

Skin aging (intrinsic aging) and photoaging (extrinsic aging) involve a similar process that leads to the typical creased appearance of the skin, with the progressive loss of its physical and biologic properties. Photoaging is a premature skin aging caused by long-term exposure to the ultraviolet B radiations of the sun, and is more frequently associated to skin cancer than intrinsic aging. Retinoids are natural and synthetic vitamin A derivatives. They are lipophilic molecules and penetrate the epidermis easily. Their biologically active forms can modulate gene expression by binding to nuclear receptors and then to specific DNA sequences. Because of their ability to modulate genes involved in cellular differentiation and proliferation, they appear as good candidates to treat and prevent photoaging. Hyaluronate and collagen, two major constituents of the dermis, are progressively decreased and altered during aging. Various retinoids were shown to increase their synthesis and concentration in the skin and reduce their rate of degradation. Furthermore, retinoids share a common chemical structure containing several conjugated double bonds that enable them to trap free radicals and absorb UV radiations from the sun, thereby protecting cellular targets such as DNA, lipid membranes, or proteins by preventing direct photochemical damage or UV-induced oxidative stress. Therefore, retinoids may be beneficial in treating skin aging and photoaging because of their biologic, chemical, and physical properties, which act at several levels.

The Göttingen minipig for assessment of retinoid efficacy in the skin: comparison of results from topically treated animals with results from organ-cultured skin

Göttingen minipigs were treated topically for 6 d with a novel retinoid (MDI 301) at concentrations ranging from 0.3% to 30% in cream vehicle. Treatment of the minipigs did not adversely affect their health (hematological and necropsy parameters) or produce changes in the skin suggestive of retinoid-induced skin irritation. After killing the animals, skin samples from each treatment site were excised and maintained in organ culture for 6 d. In addition, untreated skin was also maintained in organ culture and treated with MDI 301 (0.1-5 microg/ml). After 3 d, the culture supernatants were collected and analyzed for levels of collagen type I and for matrix metalloproteinases (MMPs). Both skin samples treated in vivo and skin samples exposed to MDI 301 in culture demonstrated increased collagen production. Only slight changes in levels of MMP-2 (gelatinase A) or MMP-9 (gelatinase B) were seen. After 6 d, the organ-cultured skin was fixed in formalin and prepared for histology. The organ-cultured skin was compared to skin that was fixed at killing after in vivo treatment. Epidermal hyperplasia was quantified at various MDI 301 concentrations. In vivo and in vitro treatments showed similar results-although the thickness was not substantially changed on average, there were focal areas of hyperplasia at higher retinoid concentrations. Taken together, these data suggest that MDI 301 enhances collagen production in minipig skin, without irritation. Furthermore, these studies suggest that minipig skin exposed to the retinoid in organ culture is equally predictive as topically treated skin. The in vitro organ culture approach may provide a cost-effective alternative model to that of the intact animal for skin retinoid testing.

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.

Topical treatment with CYP26 inhibitor talarozole (R115866) dose dependently alters the expression of retinoid-regulated genes in normal human epidermis

BACKGROUND

An alternative approach to retinoid therapy is to inhibit the cytochrome P450 (CYP)-mediated catabolism of endogenous all-trans retinoic acid in the skin by applying retinoic acid metabolism blocking agents such as talarozole (R115866).

OBJECTIVES

To study the effects of topical talarozole on retinoid biomarkers in normal skin in a randomized phase I trial.

METHODS

Gels containing talarozole (0.35% or 0.07%) and vehicle were applied once daily for 9 days on either buttock of 16 healthy volunteers. Epidermal shave biopsies (for mRNA analysis) and punch biopsies (for histology and immunofluorescence analysis) were collected from the treatment areas. Genes encoding the following were studied by quantitative real-time polymerase chain reaction: cellular retinoic acid binding protein 2 (CRABP2), cytokeratins (KRT2 and KRT4), CYP26A1, CYP26B1, CYP26C1 and CYP2S1, two enzymes in the retinol metabolism (retinal dehydrogenase-2 and retinol acyltransferase) and two proinflammatory cytokines [interleukin (IL)-1alpha and tumour necrosis factor-alpha].

RESULTS

Talarozole treatment increased the mRNA expression of CRABP2, KRT4, CYP26A1 and CYP26B1 dose dependently, and decreased the expression of KRT2 and IL-1alpha compared with vehicle-treated skin. No mRNA change in retinol-metabolizing enzymes was obtained. There was no induction of epidermal thickness or overt skin inflammation in talarozole-treated skin. Immunofluorescence analysis confirmed an upregulation of KRT4 protein, but no upregulation of CYP26A1 and CYP26B1 expression was detected.

CONCLUSIONS

Talarozole influences the biomarker pattern consistently with increased retinoic acid stimulation. The low irritancy of talarozole at the two examined dosages is a possible advantage over topical retinoids.

Frontiers in sebaceous gland biology and pathology

The development of experimental models for the in vitro study of human sebaceous gland turned down the theory of a phylogenetic relict and led to the identification of several, unknown or disregarded functions of this organ. Such functions are the production of foetal vernix caseosa, the influence of three-dimensional organization of the skin surface lipids and the integrity of skin barrier and the influence on follicular differentiation. In addition, the sebaceous gland contributes to the transport of fat-soluble antioxidants from and to the skin surface, the natural photoprotection, the pro- and antiinflammatory skin properties and to the innate antimicrobial activity of the skin. It is mainly responsible for skin's independent endocrine function, the hormonally induced skin ageing process, the steroidogenic function of the skin as well as its thermoregulatory and repelling properties and for selective control of the hormonal and xenobiotical actions of the skin. Interestingly, sebocytes, at least in vitro, preserve characteristics of stem-like cells despite their programming for terminal differentiation. This review reports on various sebaceous gland functions, which are currently under investigation, including its role on the hypothalamus-pituitary-adrenal-like axis of the skin, the impact of acetylcholine on sebocyte biology, the activity of ectopeptidases as new targets to regulate sebocyte function, the effects of vitamin D on human sebocytes, the expression of retinoid metabolizing cytochrome P450 enzymes and the possible role of sebum as vehicle of fragrances. These multiple homeostatic functions award the sebaceous gland the role 'brain of the skin' and the most important cutaneous endocrine gland.

Retinoic acid metabolism blocking agents (RAMBAs): a new paradigm in the treatment of hyperkeratotic disorders

Synthetic vitamin A derivatives, retinoids,have long been the mainstay of treatment for several disorders of keratinization, notably the ichthyoses and severe acne. Some forms of psoriasis also respond well. Their considerable power comes at a price. They have dose-limiting side effects and can be highly teratogenic, limiting their use in women of childbearing age.Thus, retinoids are used less often than their potential would warrant. However, the recent development of compounds that block the catabolism of endogenous vitamin A, called Retinioic Acid Metabolism Blocking Agents or RAMBAs, offers new possibilities. With these drugs, retinoid effects with less side effects and a reduction of the post-treatment teratogenicity period due to their favourable pharmacokinetic profile might be expected. In this review, we discuss how retinoids work, how they are metabolized and how RAMBAs influence this process. We also review the presently available data from clinical trials with RAMBAs.

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.

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.

Thiazolidinediones: potential as therapeutics for psoriasis and perhaps other hyperproliferative skin disease

The thiazolidinediones constitute a family of synthetic compounds that act as high-affinity ligands for persoxisome proliferator-activated receptor-gamma (PPAR-gamma), a member of the nuclear hormone receptor family. Although originally developed to facilitate glucose control in patients with Type 2 diabetes, a number of studies showed that these agents effectively inhibited epithelial cell proliferation and tissue inflammation. Many of the initial cell growth inhibition studies were conducted with malignant epithelial cells from various sites; however, in addition to malignant epithelial cells, other studies showed that rapidly proliferating epidermal keratinoctyes in culture were also sensitive to the growth-inhibiting action of these moieties. Additional studies subsequently demonstrated that some patients with plaque psoriasis responded to treatment with one or another member of the thiazolidinedione family. Due to the potential therapeutic benefit of these compounds in diseases such as psoriasis, studies have been conducted to elucidate mechanisms by which growth inhibition is achieved. Interference with a number of growth-influencing signalling pathways has been demonstrated. Of interest, some of the growth-inhibiting effects are seen under conditions in which PPAR-gamma activation may not be responsible for the activity. Based on therapeutic potential, additional ongoing studies are aimed at developing novel thiazolidinediones that may have better efficacy than the currently available agents. Other studies are aimed at identifying optimal ways to use these agents in the treatment of hyperplastic skin diseases such as psoriasis.

The role of topical retinoids in the treatment of photoaging

Aging of the skin is a complex biological process which is influenced by the interaction of several intrinsic and extrinsic factors. Intrinsic or chronological aging is an inevitable, genetically programmed process, of unclear underlying mechanism, for which no prevention or effective treatment is currently available. Photoaging refers to the gross and microscopic cutaneous changes that are induced by cumulative exposure to UV radiation and are superimposed on the background of chronological aging. Although primarily an aesthetic problem with significant psychological effects, photoaging constitutes the background for the development of precancerous and cancerous skin lesions.Overwhelming clinical and histological evidence indicate that certain structural changes induced by excessive sun exposure can be reversed, to some extent, by the use of topical retinoids. A number of retinoid compounds, for example tretinoin, isotretinoin, retinaldehyde and tazarotene, have been employed for the treatment of photoaged skin, and demonstrate beneficial clinical and histological effects. Adverse effects have been limited to an irritant reaction of variable intensity presenting with dryness, scaling and erythema. Ongoing research will enhance our understanding of the molecular mechanisms that determine the effects of retinoids on photodamaged skin and contribute to the employment of new, more effective and less irritating retinoid compounds.

Photoaging and phototoxicity from long-term voriconazole treatment in a 15-year-old girl

Voriconazole is a second-generation triazole that was approved by the Food and Drug Administration in May 2002 for treatment of severe fungal infections. In clinical trials it demonstrated superior efficacy in addition to a survival benefit when compared with the then current treatment standard, amphotericin B, for primary treatment of invasive aspergillosis. Voriconazole is a highly selective inhibitor of fungal cytochrome P450 enzymes. Adverse cutaneous reactions have been reported, namely cheilitis, erythema, discoid lupus erythematosus, Stevens-Johnson syndrome, toxic epidermal necrolysis, erythema multiforme, and photosensitivity reactions. We report a case of photoaging caused by voriconazole therapy. A 15-year-old patient developed cheilitis and erythema over the sun-exposed areas of her body 5 weeks after beginning voriconazole for a severe fungal infection. The lesions showed a mild transient improvement before subsequent photodamage occurred to the back of her forearms, back of her hands, and face. Voriconazole was discontinued once the fungal infection had completely resolved. The patient's blisters, erythema, and cheilitis resolved after discontinuation of voriconazole. However, she was left with solar elastotic changes, multiple lentigines, and ephelides of sun-exposed areas. These cutaneous manifestations may represent a unique adverse event caused by a new second-generation triazole.

Pharmacology of RALGA, a Mixture of Retinaldehyde and Glycolic Acid

BACKGROUND

Retinoids and alpha-hydroxy acids (AHAs) are major compounds in topical therapy. They exert distinct but potentially complementary activities. However, their association is limited by their respective irritating potential. Recently, the first association between a retinoid and an AHA has been achieved; this formulation (RALGA) associates retinaldehyde (RAL)--a precursor of retinoic acid (RA)--and glycolic acid (GA)--an AHA.

OBJECTIVE

To study the pharmacological properties of RALGA.

METHODS

The bioavailability of RAL into the skin after topical RALGA was studied by HPLC, and its bioconversion to RA was analysed by measuring the enzyme activity of retinaldehyde dehydrogenase and the RA content in the epidermis and dermis. The retinoid activity of RALGA was studied on the modulation of Hhb4 keratin mRNA on the tail of C57BL/6 mice, and its comedolytic properties on the size and density of dermal cysts and the morphology of sebaceous glands in hairless mice.

RESULTS

Epidermal and dermal concentrations of RAL and RA were higher after RALGA treatment, as compared to both RAL 0.1% alone and RA 0.05% alone; this indicates that the presence of GA favours the bioavailability and biotransformation of RAL into RA. The retinoid activity of RALGA (suppression of Hhb4 mRNA keratin) was similar to that of RAL alone, indicating that the presence of GA does not interfere with specific retinoid activity; GA alone had no effect in this test, which confirms the specificity of Hhb4 mRNA keratin modulation for retinoid activity. The diameter and the density of dermal cysts as well as the size of sebaceous glands were significantly decreased by RALGA.

CONCLUSION

These observations indicate that the addition of an AHA such as GA to a retinoid such as RAL results in a better bioavailability of the retinoid, thus a higher delivery of RA, which potentiates the biological activities of the retinoid. This combination allows a delivery of high amounts of RA in the skin while preventing the side-effects usually observed with high concentrations of topical RA.

Retinoid-induced epidermal hyperplasia in human skin organ culture: inhibition with soy extract and soy isoflavones

Organ cultures of human skin were incubated for 8 days with 1 microg/ml 14-all trans retinoic acid (14-all trans RA) and concomitantly treated with varying concentrations of soy extract. The epidermis of organ cultures treated with 14-all trans RA alone underwent a hyperplastic response. In cultures treated with a combination of 14-all trans RA and soy extract (4-40 microg/ml), hyperplasia was reduced by 16-41%. The same concentrations of soy extract that reduced epidermal hyperplasia in organ culture also suppressed proliferation of rapidly growing keratinocytes in monolayer culture (approximately 25% reduction at 20 and 40 microg/ml). On the other hand, soy extract did not further inhibit proliferation of quiescent keratinocytes; rather, it stimulated growth (50-52% increase relative to control). When dermal fibroblasts were examined for a response to soy extract (i.e., proliferation and synthesis of type I procollagen), both responses were stimulated (proliferation: 75% increase and collagen production 114% increase relative to control). Genistein, the major isoflavone in extracts of soy also inhibited epidermal hyperplasia in organ culture (34-40% reduction relative to control). The same concentrations that reduced epidermal hyperplasia (0.5-1.0 microg/ml) also inhibited keratinocyte proliferation in monolayer culture but had little effect on fibroblast growth. Two other isoflavones (daidzein and glycetein) were also inhibitory, but were less effective than genistein. Taken together, these data suggest that use of soy extract or its constituent isoflavones in conjunction with 14-all trans RA may provide a way to mitigate unwanted epidermal effects of topical retinoid therapy without compromising beneficial retinoid effects in the dermis.

Retinoic acid increases the expression of p53 and proapoptotic caspases and sensitizes keratinocytes to apoptosis: a possible explanation for tumor preventive action of retinoids

Retinoids influence growth and differentiation of keratinocytes (KCs) and are widely used for the management of skin diseases and for prevention of nonmelanoma skin cancer (NMSC) in predisposed patients. Here we investigated the effect of all-trans-retinoic acid (ATRA) on KC apoptosis. When KCs were cultured in confluent monolayers for several days, they acquired resistance against UVB-induced apoptosis. In contrast, when the cells were treated with 1 micromol/L ATRA for 6 days and subsequently irradiated with different doses of UVB, they underwent massive apoptosis as assessed by morphology, expression of activated caspase-3, and DNA fragmentation. The same effect was observed when doxorubicin was used instead of UVB. Analysis by real-time PCR and Western blot revealed that ATRA treatment strongly increased the mRNA and protein expression of p53 and caspase-3, -6, -7, and -9, which are key regulators of apoptosis. UVB irradiation of ATRA-treated cells but not of control cells led to the accumulation of p53 protein and of its target gene Noxa. Inhibition of p53 and caspases with alpha-pifithrin and z-Val-Ala-Asp-fluoromethyl ketone, respectively, blocked UVB- and doxorubicin-induced apoptosis in ATRA-treated KCs. Analogous to the observed ATRA effects in monolayer cultures, in vitro-generated organotypic skin cultures reacted with up-regulation of p53 and proapoptotic caspases and displayed increased sensitivity to UVB-induced apoptosis. The ability of retinoic acid to regulate the expression of proapoptotic genes and to sensitize KCs to apoptosis may play a role in their prevention of NMSC in transplant patients and patients with DNA-repair deficiencies.

Voriconazole-induced retinoid-like photosensitivity in children

Voriconazole is a new triazole antifungal agent with activity against a wide range of systemic fungal pathogens, including Aspergillus spp. Photosensitivity is a rarely reported side effect of voriconazole, hypothesized to be a result of retinoid-like effects. We report two children with chronic granulomatous disease to whom voriconazole was administered for chronic invasive aspergillosis. Severe photosensitivity occurred in both patients, one of whom had striking photodamage at the 5-month follow-up.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Vitamin D3 induces caspase-14 expression in psoriatic lesions and enhances caspase-14 processing in organotypic skin cultures

Caspase-14 is a nonapoptotic caspase family member whose expression in the epidermis is confined to the suprabasal layers, which consist of differentiating keratinocytes. Proteolytic activation of this caspase is observed in the later stages of epidermal differentiation. In psoriatic skin, a dramatic decrease in caspase-14 expression in the parakeratotic plugs was observed. Topical treatment of psoriatic lesions with a vitamin D3 analogue resulted in a decrease of the psoriatic phenotype and an increase in caspase-14 expression in the parakeratotic plugs. To investigate whether vitamin D3 directly affects caspase-14 expression levels, we used keratinocyte cell cultures. 1alpha,25-Dihydroxycholecalciferol, the biologically active form of vitamin D3, increased caspase-14 expression, whereas retinoic acid inhibited it. Moreover, retinoic acid repressed the vitamin D3-induced caspase-14 expression level. In addition, the use of organotypic skin cultures demonstrated that 1alpha,25-dihydroxycholecalciferol enhanced epidermal differentiation and caspase-14 activation, whereas retinoic acid completely blocked caspase-14 processing. Our data indicate that caspase-14 plays an important role in terminal epidermal differentiation, and its absence may contribute to the psoriatic phenotype.

Cytochrome P450 expression and activities in rat, rabbit and bovine tongue

Xenobiotic metabolism in the tongue has received little attention in the literature. In the present study, we report a comparative analysis of constitutive cytochrome P450 (CYP) expression and activities in the tongue. First we compared catalytic activities of rabbit, rat and bovine tongue samples using the probe substrates 4-nitrophenol, 1-phenylethanol, caffeine and 7-ethoxycoumarin. Rabbit tongue samples showed the highest activities for all substrates. We then compared the activities in rat and rabbit tongue with those in the rabbit liver, along with the effects of P450 inhibitors on specific activities. Combined, the activity studies indicate that CYP1A1 is active in rabbit tongue cells, but CYP1A2, CYP3A6 and CYP2E1 are below limits of detection. RT-PCR was also used to compare mRNA levels of 11 different rabbit and six different rat P450 isoforms in the tongue to those in the liver of these two species. Only CYP2E1, CYP1A1 and CYP4A4 were detected at significant levels in the rabbit tongue. None of the six rat isoforms probed were observed in the tongue. Although 4-nitrophenol activity was observed in the rabbit tongue samples, the kinetic parameter K(m) was inconsistent with the involvement of CYP2E1. We suggest that although CYP2E1 is expressed in the tongue, it is rapidly degraded in this organ, and the nitrophenol hydroxylation and caffeine hydroxylation we observe is the result of activity of CYP1A1.

Separation of retinoid-induced epidermal and dermal thickening from skin irritation

The ability of the synthetic retinoid MDI-301, in which the carboxylic acid of 9- cis-retinoic acid (9-cis-RA) is replaced with an ester linkage, to induce epidermal and dermal thickening and skin irritation (erythema and flaking) in hairless (rhino) mice following its topical application was investigated in comparison with that of 14-all- trans-retinoic acid (14-all-trans-RA) and 9-cis-RA. MDI-301 induced epidermal proliferation leading to a thickened epidermis. Treated animals also demonstrated a prominent band of organized connective tissue immediately below the epidermis. In its ability to induce epidermal thickening, MDI-301 was quantitatively similar to 14-all-trans-RA and 9-cis-RA. However, unlike 14-all-trans-RA and 9-cis-RA, which produced skin irritation associated with a perivascular influx of mononuclear leukocytes into the dermis, there was no evidence of irritation with MDI-301 and little leukocyte infiltration. Intraperitoneal injection of either 14-all-trans-RA or MDI-301 also resulted in epidermal and dermal thickening. Irritation of skin was not observed in these animals but splenomegaly was prominent in animals treated with either agent.

Activation of retinoic acid receptor-dependent transcription by all-trans-retinoic acid metabolites and isomers

We have shown that four metabolites of all-trans-retinoic acid (ATRA) (4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA) can induce maturation of NB4 promyelocytic leukemia cells (Idres, N., Benoit, G., Flexor, M. A., Lanotte, M., and Chabot, G. G. (2001) Cancer Res. 61, 700-705). To better understand the mechanism of action of ATRA metabolites and isomers, we assessed their binding to retinoic acid receptors (RARs) and activation of RAR-mediated transcription via a retinoic acid response element (RARE). Competition binding experiments with tritiated ATRA showed that all metabolites could bind to RARs with variable affinity. For transactivation studies, COS-7 cells were cotransfected with RAR alpha, beta, or gamma expression vectors and the reporter plasmid RARE-tk-Luc, and the retinoid concentrations for half-maximal luciferase activity (EC(50)) were determined. All retinoids tested could activate the three RAR isotypes. The lowest EC(50) value for RAR alpha was with 9-cis-RA (13 nM), followed by 4-oxo-RA (33 nM), 5,6-epoxy-RA (77 nM), 13-cis-RA (124 nM), 18-OH-RA (162 nM), ATRA (169 nM), and 4-OH-RA (791 nM). For RAR beta, the EC(50) values increased as follows: 4-oxo-RA (8 nM), ATRA (9 nM), 18-OH-RA (14 nM), 5,6-epoxy-RA (35 nM), 13-cis-RA (47 nM), 4-OH-RA (64 nM), and 9-cis-RA (173 nM). For RAR gamma the EC(50) values were: ATRA (2 nM), 5,6-epoxy-RA (4 nM), 18-OH-RA (14 nM), 13-cis-RA (36 nM), 9-cis-RA (58 nM), 4-oxo-RA (89 nM), and 4-OH-RA (94 nM). By comparing the -fold induction of luciferase activity, all retinoids tested were equipotent at transactivating RARE-tk-Luc whatever the RAR considered. However, the best induction of the transcription was obtained for RAR alpha, which was 5-fold higher than for RAR beta and 10-fold higher than for RAR gamma. In conclusion, these data show that ATRA metabolites can bind to and activate the three RARs with variable relative affinity but with similar efficacy. These results suggest that ATRA metabolites may activate several signaling pathways and probably play an important role in cellular physiology and cancer therapy.

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.

Heparin-binding epidermal-growth-factor-like growth factor activation of keratinocyte ErbB receptors Mediates epidermal hyperplasia, a prominent side-effect of retinoid therapy

Sun-protected human skin was maintained in organ culture and treated with all-trans retinoic acid in the presence or absence of reversible or irreversible pharmacologic antagonists of c-erbB receptor tyrosine kinase activity. In the absence of these inhibitors, all-trans retinoic acid induced epidermal hyperplasia comparable to that induced in intact skin by all-trans retinol or all-trans retinoic acid itself. There was a strong correlation between inhibition of epidermal hyperplasia in organ culture and inhibition of epidermal-growth-factor-dependent keratinocyte growth in monolayer culture. In additional studies it was shown that all-trans retinoic acid could overcome the known inhibitory effects of calcium on expression of HB-EGF-like growth factor mRNA in organ-cultured skin. Further, it was shown that an antibody to HB-EGF-like growth factor inhibited retinoid-stimulated epidermal hyperplasia in organ culture and reduced proliferation in cultured keratinocytes. In contrast, the c-erbB receptor tyrosine kinase antagonists and the neutralizing HB-EGF-like growth factor antibody were ineffective in inhibiting all-trans-retinoic-acid-dependent survival and proliferation of human dermal fibroblasts. Taken together, these data indicate (i) that retinoid-induced epidermal hyperplasia in human skin proceeds through c-erbB, and (ii) that HB-EGF-like growth factor is one of the c-erbB ligands mediating this effect.

Muco-cutaneous retinoid-effects and facial erythema related to the novel triazole antifungal agent voriconazole

Voriconazole is a new azole antifungal drug with activity against a wide range of systemic fungal pathogens, including Aspergillus spp. Five patients with chronic invasive aspergillosis were treated for 12-58 weeks with voriconazole, 200 mg twice daily and developed facial erythema and cheilitis. One who received 58 weeks of therapy also developed discoid lupus erythematosus-like lesions on both sides of her neck. Both erythema and cheilitis resolved after discontinuation of voriconazole. Serum retinoids were elevated in the three patients in whom they were measured. Voriconazole has the potential for retinoid-like side-effects and facial erythema.

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.

Drug treatment of photoaged skin

Although the prevention of skin aging is a holy grail of the cosmetic and pharmaceutical industries, this venture may be misplaced. The predominant clinical and biochemical features of aged skin are mostly attributable to photoaging rather than chronology. For instance chronic sun exposure is the major determinant of age spots (actinic lentigines) and wrinkles. Surgical approaches to the treatment of photoaging include face-lift, dermabrasion, chemical peeling, collagen and botulinum toxin injections, and laser re-surfacing. These approaches all have benefit and improve the clinical features of facial photoaging. Drug or pharmaceutical prevention and treatment of photoaged skin is still in its infancy. The main pharmaceutical approach to prevention of photoaging lies in the assiduous use of sunscreens. Recent evidence points to the importance of ultraviolet A (UVA) radiation as well as ultraviolet B (UVB) radiation in the aetiology of photoaging and thus the need for sunscreens that block both UVB and UVA. Drug treatment of photoaged skin can be categorised as antioxidants, alpha-hydroxy acids and topical retinoids. Of these 3 approaches only topical retinoids, particularly tretinoin (all-trans retinoic acid), have a well documented ability to repair photoaged skin at the clinical, histological and molecular level. Furthermore, the use of topical retinoids may actually prevent photoaging. The current interest in pharmaceutical modulation of the photoaging process has attracted considerable research into the mechanisms of photoaging and cutaneous aging. It is likely that treatment for, or prevention of, the chronological aging process may result from such research.

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.

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.

Photoaging therapy with topical tretinoin: an evidence-based analysis

Topical tretinoin is established as an effective treatment for photoaging. Yet some confusion still exists about the proper way to use this medication, confusion that can misguide physicians in their clinical approaches and patients in their treatment regimens. Most of the misinformation about tretinoin has been perpetuated from the early days of the drug, when its efficacy for treating the effects of photoaging was still in dispute. Significant advances in clinical and basic research in this area have dispelled much of the confusion, clearing the way for an evidence-based medical approach to tretinoin therapy for photoaging. This review summarizes recent relevant advances in tretinoin therapy to guide physicians in treating patients with this safe and effective hormone. To date, tretinoin remains the only therapeutic agent proved to repair photodamage.

Mutations in the E-Domain of RARα Portion of the PML/RARα Chimeric Gene May Confer Clinical Resistance to All-transRetinoic Acid in Acute Promyelocytic Leukemia

The binding of all-trans retinoic acid (ATRA) to the ligand-binding region in the E-domain of retinoic acid receptor-α (RARα) modifies the transcriptional activity of RARα protein. ATRA probably induces differentiation of acute promyelocytic leukemia (APL) cells by binding to the E-domain of the RARα portion (RARα/E-domain) of PML/RARα chimeric protein. Therefore, molecular alteration in the RARα/E-domain of the chimeric gene is one mechanism by which patients with APL may acquire resistance to ATRA therapy. In this study using reverse transcription-polymerase chain reaction and single-strand conformation polymorphism, DNA segments amplified from the RARα/E-domain in fresh APL cells of 23 APL patients (8 males and 15 females from 4 to 76 years of age) were screened for mutations. Of those patients, 3 patients (1 with de novo and 2 with relapse) had clinical resistance to ATRA therapy. We found mutations in the RARα/E-domain of PML/RARα chimeric gene exclusively in the 2 patients who exhibited ATRA-resistance at relapse, whereas the mutations were not detected at their initial onset. Interestingly, these patients received a prolonged or intermittent administration of ATRA before relapse with ATRA-resistance. The mutations lead to the change of amino acid in the ligand-binding region of RARα/E-domain, Arg272Gln, or Met297Leu according to the amino acid sequence of RARα, respectively. Further study demonstrated that the in vitro ligand-dependent transcriptional activity of the mutant PML/RARα protein was significantly decreased as compared with that of wild-type PML/RARα. These findings suggest that mutations in the RARα/E-domain of the PML/RARα chimeric gene may confer clinical resistance to ATRA therapy in patients with APL.

Mutations in the E-Domain of RARα Portion of the PML/RARα Chimeric Gene May Confer Clinical Resistance to All-transRetinoic Acid in Acute Promyelocytic Leukemia

The binding of all-trans retinoic acid (ATRA) to the ligand-binding region in the E-domain of retinoic acid receptor-α (RARα) modifies the transcriptional activity of RARα protein. ATRA probably induces differentiation of acute promyelocytic leukemia (APL) cells by binding to the E-domain of the RARα portion (RARα/E-domain) of PML/RARα chimeric protein. Therefore, molecular alteration in the RARα/E-domain of the chimeric gene is one mechanism by which patients with APL may acquire resistance to ATRA therapy. In this study using reverse transcription-polymerase chain reaction and single-strand conformation polymorphism, DNA segments amplified from the RARα/E-domain in fresh APL cells of 23 APL patients (8 males and 15 females from 4 to 76 years of age) were screened for mutations. Of those patients, 3 patients (1 with de novo and 2 with relapse) had clinical resistance to ATRA therapy. We found mutations in the RARα/E-domain of PML/RARα chimeric gene exclusively in the 2 patients who exhibited ATRA-resistance at relapse, whereas the mutations were not detected at their initial onset. Interestingly, these patients received a prolonged or intermittent administration of ATRA before relapse with ATRA-resistance. The mutations lead to the change of amino acid in the ligand-binding region of RARα/E-domain, Arg272Gln, or Met297Leu according to the amino acid sequence of RARα, respectively. Further study demonstrated that the in vitro ligand-dependent transcriptional activity of the mutant PML/RARα protein was significantly decreased as compared with that of wild-type PML/RARα. These findings suggest that mutations in the RARα/E-domain of the PML/RARα chimeric gene may confer clinical resistance to ATRA therapy in patients with APL.

All-trans-retinoic acid metabolites significantly inhibit the proliferation of MCF-7 human breast cancer cells in vitro

All-trans-retinoic acid (ATRA) is well known to inhibit the proliferation of human breast cancer cells. Much less is known about the antiproliferative activity of the naturally occurring metabolites and isomers of ATRA. In the present study, we investigated the antiproliferative activity of ATRA, its physiological catabolites 4-oxo-ATRA and 5,6-epoxy-ATRA and isomers 9-cis-RA and 13-cis-RA in MCF-7 human breast cancer cells by bromodeoxyuridine incorporation. MCF-7 cells were grown in steroid- and retinoid-free medium supplemented with growth factors. Under these culture conditions, ATRA and its naturally occurring catabolites and isomers showed significant antiproliferative activity in MCF-7 cells in a concentration-dependent manner (10[-11] M to 10[-6] M). The antiproliferative activity of ATRA catabolites and isomers was equal to that of the parent compound ATRA at concentrations of 10(-8) M and 10(-7) M. Only at 10(-6) M were the catabolites and the stereoisomer 13-cis-RA less potent. The stereoisomer 9-cis-RA was as potent as ATRA at all concentrations tested (10[-11] M to 10[-6] M). In addition, we show that the catabolites and isomers were formed from ATRA to only a limited extent. Together, our findings suggest that in spite of their high antiproliferative activity the catabolites and isomers of ATRA cannot be responsible for the observed growth inhibition induced by ATRA.

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.