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

CYP2C7 expression in rat liver and hepatocytes: regulation by retinoids

Rats deficient in vitamin A express low levels of P4502C7 mRNA in the liver. Administration of all-trans retinoic acid (at-RA) or growth hormone (GH) to deficient animals only partially restored the expression whereas the combined treatment returned the P4502C7 mRNA levels to that observed in normal rats. That a retinoid is the predominant inducer of P4502C7 at the cellular level is evident from studies performed with primary hepatocytes, but it became clear that GH is a prerequisite for the vitamin A effect in vivo. The at-RA induction of P4502C7 mRNA in primary rat hepatocytes was inhibited by ketoconazole, an inhibitor of P450 activity, and by cycloheximide, blocking ongoing protein synthesis. In contrast, the at-RA induction of RAR-beta2 mRNA was not affected by any of these compounds. This could indicate previously not recognized mechanisms of at-RA action. Interestingly, at-4-oxo-RA, an at-RA metabolite formed by a P450 catalyzed reaction, also induced P4502C7 mRNA. Induction of P4502C7 mRNA by the retinoic acid receptor (RAR) selective agonist TTNPB indicated that this pathway is preferred over the retinoid X receptor (RXR) pathway. In addition, analysis of RA metabolites in liver cell extracts revealed the formation of several as yet unidentified metabolites. The formation of some of these metabolites was inhibited by ketoconazole and they could therefore constitute potential inducers of CYP2C7. We suggest that metabolism of at-RA, possibly by a P450 enzyme, is an important step in the at-RA induction of P4502C7.

Retinoic acid stimulates essential fatty acid-supplemented human keratinocytes in culture

The effect of all-trans retinoic acid on the proliferation of essential fatty acid (EFA)-deficient and of EFA-supplemented adult human keratinocytes was investigated. EFA-deficient cell strains were supplied with one of four different fatty acid-supplemented media at the P0 to P1 passage. All-trans retinoic acid at 0.5 or 1.0 microM was added to the cultures at the P1 to P2 passage. At passage P3, and 3 and 7 d thereafter, the cell growth rate was determined. The fatty acid content of cultures grown in each medium was measured using gas chromatography. All the EFA media "normalized" the cellular fatty acid composition and drastically decreased the cell number and total DNA and protein of the cultures. All-trans retinoic acid at 1 microM prevented the loss of cell viability and growth usually associated with EFA supplementation but did not affect the control (EFA deficient) or 18:1 fatty acid-supplemented cultures. All-trans retinoic acid at 1 microM altered the fatty acid content of the EFA-supplemented cultures. A statistically significant increase in 14:0, 14:1, 16:1, 18:1, and 20:4 fatty acids occurred, whereas the amounts of 18:0 and 18:2 fatty acids decreased. The largest changes were in 16:1 fatty acid (8-14%) and 18:2 fatty acid (12-5%). All-trans retinoic acid at 0.5 microM also affected both cell growth and fatty acid composition without induction of the CRABP II message. These studies demonstrate that all-trans retinoic acid stimulates the growth of EFA-supplemented keratinocyte cultures while also altering the fatty acid composition of the cells.

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.

Current use and future potential role of retinoids in dermatology

Since their introduction 15 years ago, retinoids have been increasingly used for topical and systemic treatment of psoriasis and other hyperkeratotic and parakeratotic skin disorders, keratotic genodermatoses, severe acne and acne-related dermatoses, and also for therapy and/or chemoprevention of skin cancer and other neoplasia. Oxidative metabolites of vitamin A (retinol) are natural retinoids present at low levels in the peripheral blood. Synthetic retinoids are classified into 3 generations including nonaromatic, monoaromatic and polyaromatic compounds. They are detectable in plasma 30-60 minutes after systemic administration, and reach maximum concentrations 2 to 4 hours later. Elimination half-life is 10 to 20 hours for isotretinoin, 80 to 175 days for etretinate and 2 to 4 days for, trans-acitretin; the latter, however, partially converts into etretinate. Retinoid concentrations in skin are rather low in contrast to subcutaneous fat tissue. Intracellularly, retinoids interact with cytosolic proteins and specific nuclear receptors. Two classes of nuclear receptors have been suggested to mediate retinoid activity at the molecular level, RARs and RXRs. The expression of retinoid receptors is tissue specific; skin mainly espresses RAR gamma and RXR alpha. Retinoids affect epidermal cell growth and differentiation as well as sebaceous gland activity and exhibit immunomodulatory and anti-inflammatory properties. Current retinoid research targets the development of receptor-selective retinoids for tailoring and/or improving their therapeutic profile. Currently, tretinoin is used systemically for acute promyelocytic leukaemia, etretinate and acitretin for psoriasis and related disorders, as well as other disorders of keratinisation and isotretinoin for seborrhoea, severe acne, rosacea and acneiform dermatoses. Systemic retinoids are also applied for chemoprevention of epithelial skin cancer and cutaneous T cell lymphoma. The major adverse effect of retinoids is teratogenicity; all other adverse effects are dose-dependent and controllable. Contraception is, therefore, essential during retinoid treatment in women of child-bearing age. Clinical monitoring requires physical examination for adverse effects every 3 to 4 weeks and proper laboratory investigations, also including analysis of retinoid bioavailability in selected cases. Topical retinoids are rapidly developing at present and seem promising for the future; their clinical application includes acne, aging, photodamage, precanceroses, skin cancer and disorders of skin pigmentation. The development of receptor-specific retinoids for topical treatment of psoriasis and/or acne may lead to interesting new compounds based on our current concepts of retinoid function.

Induction of the oxidative catabolism of retinoid acid in MCF-7 cells

Cytochrome P450-dependent oxidation is a pathway for all-trans-retinoic acid (all-trans-RA) catabolism. Induction of this catabolic pathway was studied in MCF-7 breast cancer cells. MCF-7 cells showed low constitutive all-trans-RA catabolism. Concentration-dependent induction was obtained by preincubation of the cells with all-trans-RA (10(-9) to 10(-6) M). Onset of induction was fast, being detectable within 60 min, with maximal induction (45-fold) obtained after 16 h. Enzymatic characterization of induced all-trans-RA catabolism showed an estimated Km value (Michaelis-Menten constant) of 0.33 microM and a Vmax value (maximal velocity of an enzyme-catalysed reaction) of 54.5 fmol polar all-trans-RA metabolites 10(6) cells(-1) h(-1). These kinetic parameters represent the overall formation of polar metabolites from all-trans-RA. Induction of all-trans-RA catabolism was also obtained with other retinoids, CH55 >> 13-cis-RA = all-trans-RA > 9-cis-RA > 4-keto-all-trans-RA > 4-keto-13-cis-RA > retinol. The potency of the retinoids to induce all-trans-RA catabolism was correlated to their retinoic acid receptor affinity (Crettaz et al, 1990; Repa et al, 1990; Sani et al, 1990). Induction of all-trans-RA catabolism was inhibited by actinomycin D. Furthermore, all-trans-RA did not increase cytosolic retinoic acid-binding protein (CRABP) mRNA levels. These data suggest that induction of all-trans-RA catabolism in MCF-7 cells is a retinoic acid receptor-mediated gene transcriptional event. Induced all-trans-RA catabolism was inhibited by various retinoids with decreasing potency in the order: all-trans-RA > 4-keto-all-trans-RA > 13-cis-RA > 9-cis-RA > 4-keto-13-cis-RA > retinol > CH55. The antitumoral compound liarozole-fumarate inhibited all-trans-RA catabolism with a potency similar to that of all-trans-RA.

Identification of the retinoic acid-inducible all-trans-retinoic acid 4-hydroxylase

Retinoic acid (RA) metabolites of vitamin A are key regulators of gene expression involved in embryonic development and maintenance of epithelial tissues. The cellular effects of RA are dependent upon the complement of nuclear receptors expressed (RARs and RXRs), which transduce retinoid signals into transcriptional regulation, the presence of cellular retinoid-binding proteins (CRABP and CRBP), which may be involved in RA metabolism, and the activity of RA metabolizing enzymes. We have been using the zebrafish as a model to study these processes. To identify genes regulated by RA during exogenous RA exposure, we utilized mRNA differential display. We describe the isolation and characterization of a cDNA, P450RAI, encoding a novel member of the cytochrome P450 family. mRNA transcripts for P450RAI are expressed normally during gastrulation, and in a defined pattern in epithelial cells of the regenerating caudal fin in response to exogenous RA. In COS-1 cells transfected with the P450RAI cDNA, all-trans-RA is rapidly metabolized to more polar metabolites. We have identified 4-oxo-RA and 4-OH-RA as major metabolic products of this enzyme. P450RAI represents the first enzymatic component of RA metabolism to be isolated and characterized at the molecular level and provides key insight into regulation of retinoid homeostasis.

Topical retinaldehyde increases skin content of retinoic acid and exerts biologic activity in mouse skin

Retinaldehyde, a natural metabolite of beta-carotene and retinol, has been proposed recently for topical use in humans. Because retinaldehyde does not bind to retinoid nuclear receptors, its biologic activity should result from enzymatic transformation by epidermal keratinocytes into ligands for these receptors, such as all-trans retinoic acid and 9-cis-retinoic acid. In this study, we analyzed by high performance liquid chromatography the type and amounts of tissue retinoids as well as several biologic activities resulting from topical application of either retinaldehyde or all-trans retinoic acid on mouse tail skin. Biologic activities of all-trans retinoic acid and retinaldehyde were qualitatively identical in metaplastic parameters (induction of orthokeratosis, reduction of keratin 65-kDa mRNA, increase in filaggrin and loricrin mRNAs) and hyperplastic parameters (increase in epidermal thickness, increase in bromodeoxyuridine (BrdU)-positive cells, increase in keratin 50-kDa mRNA, and reduction in keratin 70-kDa mRNA). Some quantitative differences, not all in favor of all-trans retinoic acid, were found in several indices. Cellular retinoic acid-binding protein II and cellular retinol-binding protein I mRNAs were increased by both topical retinaldehyde and all-trans retinoic acid. Whereas all-trans retinoic acid, 9-cis-retinoic acid, and 13-cis-retinoic acid were not detectable (limit 5 ng/g) in vehicle-treated skin, 0.05% retinaldehyde-treated skin contained 13 +/- 6.9 ng/g wet tissue of all-trans retinoic acid (mean +/- SD), 12.6 +/- 5.9 ng/g 13-cis-retinoic acid, and no 9-cis-retinoic acid. In contrast, 9-cis-retinoic acid was detectable in 0.05% of all-trans retinoic acid-treated skin, which also contained 25-fold more all-trans retinoic acid and 5-fold more 13-cis-retinoic acid than retinaldehyde-treated skin. Our results show that topical retinaldehyde is transformed in vivo into all-trans retinoic acid by mouse epidermis. The small amounts of ligand for retinoic acid nuclear receptors thus produced are sufficient to induce biologic effects similar to those resulting from the topical application of the ligand itself in much higher concentration.

Extraction of human epidermis treated with retinol yields retro-retinoids in addition to free retinol and retinyl esters

Vitamin A, all-trans-retinol, is metabolized to retinoic acid in vivo by a tightly controlled two-step conversion. Retinoic acid then binds to nuclear receptors and modulates cellular proliferation and differentiation. Because only a small fraction of retinol applied topically can be metabolized to retinoic acid, alternative pathways of retinol metabolism in skin were investigated. Retinol (0.4%) was applied to adult human skin under occlusion for 6 h to 4 d. The conversion of retinol into various metabolites such as 14-hydroxy-4,14-retro-retinol, anhydroretinol, 4-oxo-retinol, retinyl esters, and retinyl glucuronides was investigated. The level of 14-hydroxy-retro-retinol was increased from undetectable at time 0 to 326 ng/g wet weight of tissue at 6 h (6% of the retinol level) and maintained approximately the same concentration at 24 h to 409 ng/g wet weight (1.9% of the retinol level); it decreased to 48 ng/g wet weight of tissue (12% of its maximum level) by 4 d. Anhydroretinol was undetectable at time 0, increased only slightly at 6 h, and remained at the same level. We did not detect 4-oxo-retinol. Because 14-hydroxy-retro-retinol was found in the retinol-treated areas, its effects on epidermis were compared with those of retinol. Topical application of trans-retinol (0.3%) significantly increased both epidermal thickness and cellular retinoic acid binding protein II mRNA, whereas 14-hydroxy-4,14-retro-retinol (0.3%) did not increase either of these well-characterized cutaneous retinoid responses. Retinol, when applied topically in pharmacologic doses to human epidermis, remained as free retinol, was metabolized primarily to retinol ester, and was metabolized to a lesser extent to retro-retinoids and didehydroretinol.

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).

Auto-regulation of retinoic acid biosynthesis through regulation of retinol esterification in human keratinocytes

In this report, we describe an auto-regulatory loop in human keratinocytes, whereby all-trans retinoic acid (retinoic acid) regulates its own biosynthesis from all-trans retinol (retinol) through regulation of retinol esterification. Retinol esterification activity was low in normal proliferating human keratinocytes, cultured in retinoid-free media. Treatment of keratinocytes with retinoic acid induced retinol esterifying activity (8-fold). Induction of retinol esterifying activity was blocked by either actinomycin D or cycloheximide. Based on substrate specificity and inhibitor sensitivity, lecithin:retinol acyltransferase (LRAT) was identified as the retinoic acid-inducible retinol esterifying enzyme. Induction of LRAT by retinoic acid reduced conversion of retinol to retinoic acid by 50%. This reduction in retinoic acid synthesis resulted from sequestration of retinol as retinyl esters, since inhibition of LRAT restored retinoic acid synthesis to control levels. In normal human skin, undifferentiated keratinocytes, in the lowest cell layer, esterified retinol 4 times greater, than differentiating keratinocytes, in upper cell layers, reflecting an induced state, under conditions of retinol sufficiency. Regulation of LRAT activity by retinoic acid provides a novel mechanism through which retinoic acid can regulate its own level by controlling availability of retinol for conversion to retinoic acid. In human skin in vivo, retinyl esters synthesized in basal keratinocytes could undergo hydrolysis during differentiation and thus serve as a source of retinol for keratinocytes in upper layers of skin.

All-trans retinoic acid reduces membrane fluidity of human dermal fibroblasts. Assessment by fluorescence redistribution after photobleaching

All-trans retinoic acid (RA) preserves human dermal fibroblast viability and stimulates proliferation in vitro. These effects are mediated, at least in part, by reducing the extracellular Ca2+ requirement. The same concentrations of RA that reduce the extracellular Ca2+ requirement also interrupt movement of Ca 2+ across the fibroblast plasma membrane. Based on these observations, we have examined the effects of RA on membrane properties that could influence Ca2+ movement. Fibroblasts were labeled with 1-acyl-2-(N-4- nitrobenzo-2-oxa-1,3 diazole)-amino-caproyl phosphatidyl-choline (a fluorescent phospholipid analogue) and examined for fluorescence redistribution after photobleaching (FRAP) with a pulse of intense light as a measure of membrane fluidity. Using this approach, we observed that membrane fluidity was higher when the cells were incubated in medium containing a low (sub-optimal) level of extracellular Ca2+ (0.15 mmol/L) than in a medium containing an optimal concentration (1.4 mmol/L). Treatment of the cells with 3 micromol/L RA reduced membrane fluidity of the cells under both high- and low-Ca2+ conditions. These findings demonstrate that RA has a direct effect on the plasma membrane of human dermal fibroblasts. This provides a possible mechanism for the previously identified inhibition of Ca2+ movement across the membrane of the same cells and for the previously identified protective effects against lysis under low-Ca2+ conditions.

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

Application of all-trans retinoic acid to human skin for 4 d under occlusion produces a marked increase in retinoic acid 4-hydroxylase activity. In this study, the possible induction of other hydroxylase in response to 9-cis and 13-cis retinoic acid application to adult human skin in vivo was determined. Application of 0.1% all-trans, 0.1% 9-cis, and 0.1% 13-cis retinoic acid to human skin for 2 d resulted in induction of only all-trans retinoic acid 4-hydroxylase activity. The 4-hydroxylase activity in microsomes from the treated tissue ranged from 838 +/- 46 to 531 +/- 59 pg of 4- hydroxy all-trans retinoic acid formed/min/mg protein (n=6). These same preparations were unable to use 9-cis or 13-cis retinoic acid as substrate for the hydroxylation reaction. Extraction of the retinoic acid isomers from epidermis 48 h after application of 0.1% solution of each isomer yielded significant amounts of all-trans retinoic acid (36-72%) regardless of the isomer applied. The all-trans isomer produced by isomerization of both 9-cis and 13-cis retinoic acids is the likely inducer of the 4-hydroxylase. All-trans retinol and all-trans retinal were unable to compete with all-trans retinoic acid as substrate for 4-hydroxylase enzyme. The 4-hydroxylase induced in response to pharmacological doses of retinoic acids is specific for the all-trans isomer. The inability of 9-cis or 13-cis retinoic acid to induce their own hydroxylation and inactivation or act as substrate for the 4-hydroxylase in skin may have considerable implications in light of the clinical use of retinoids in the treatment of various diseases including cancers.

Application of retinol to human skin in vivo induces epidermal hyperplasia and cellular retinoid binding proteins characteristic of retinoic acid but without measurable retinoic acid levels or irritation

We investigated the clinical, histologic, and molecular responses of normal human skin to all-trans-retinol (ROL) application, compared to those induced by topical all-trans-retinoic acid (RA), and measured ROL-derived metabolites. Up to 1.6% ROL, 0.025% RA in vehicle (70% ethanol/30% propylene glycol), or vehicle alone were applied in a double-blind fashion to normal buttock skin and occluded for 4 d. ROL produced from none to only trace erythema, which was clinically and statistically insignificant, whereas RA induced a significant 3.7-fold increase in erythema score compared to vehicle (n = 10, p < 0.01). However, ROL induced significant epidermal thickening (1.5-fold at 1.6% ROL, p < 0.01), similar to RA (1.6-fold at 0.025% RA, p < 0.01), relative to the vehicle. ROL, compared with vehicle, also increased mRNA levels of cellular retinoic acid binding protein (CRABP-II) and cellular retinol binding protein (CRBP) genes as determined by Northern analysis (5-6-fold and 6-7-fold, respectively) and riboprobe in situ hybridization. CRABP-II and CRBP protein levels were also higher following ROL than vehicle treatment, as measured by ligand binding (3.2-fold, p < 0.001; n = 7) and Western analysis (3.6-fold, p < 0.003; n = 6), respectively. Epidermal retinyl ester (RE) content, measured after removal of stratum corneum, rose 240-fold (p < 0.005, n = 5) by 24 h of ROL occlusion. RA content, however, was undetectable or detectable only at trace amounts in all samples obtained at 0, 6, 24, and 96 h after ROL occlusion. Detectability of RA was not correlated with ROL treatment (compared to untreated normal skin, p = 0.86) or baseline skin ROL levels (average r = -0.1, p > 0.3). These data demonstrate that ROL application 1) produces trace erythema not significantly different from vehicle, whereas RA causes erythema; 2) induces epidermal thickening and enhances expression of CRABP-II and CRBP mRNAs and proteins as does RA; 3) causes marked accumulation of retinyl ester; and 4) does not significantly increase RA levels. Taken together, the data are compatible with the idea that ROL may be a prohormone of RA, because it produces changes in skin similar to those produced by RA but without measurable RA or irritation.

Chromatographic and electrophoretic analysis of biomedically important retinoids

The determination of retinol (vitamin A) and its metabolites, as well as synthetic retinoids, in biological samples is a challenging task due to the sensitivity of these compounds to light, heat and oxygen, high protein binding, separation of geometric isomers and determination of low endogenous levels. Numerous procedures for sample preparation have been published for biological fluids and tissues, consisting of solvent extraction, solid-phase extraction (off-line) and HPLC with column switching (on-line solid-phase extraction). The last-mentioned technique has several advantages, including a high degree of automation, no evaporation of extraction solvents, protection from light and higher sensitivity. Due to the favourable UV characteristics of most retinoids, HPLC with UV detection is most often employed, and photodiode array detection is becoming more and more popular. Fluorescence and electrochemical detection have found only a limited field of application, but the use of LC-MS resulted in a few highly sensitive methods. Reconsideration of GC through the use of better deactivated columns and cold on-column injection and evaluation of new promising separation methods, such as supercritical fluid chromatography and capillary electrophoresis, have shown preliminary encouraging results, but appear to reach the required sensitivity only by coupling to MS. Therefore, HPLC with UV detection is still the method of choice for highly sensitive and selective retinoid determination, as well as for high sample throughput and robustness.

All-trans retinoic acid inhibits fluctuations in intracellular Ca2+ resulting from changes in extracellular Ca2+

Previous studies have shown that all-trans retinoic acid (RA) preserves fibroblast viability and stimulates their proliferation, in part, by reducing the extracellular Ca2+ requirement (Am J Pathol 1990, 130:1275). Based on this observation, we have in the present study examined the effects of RA on Ca2+ mobilization in human dermal fibroblasts. For these studies we used the Ca(2+)-binding dyes, Fluo-3 and Indo-1. Using fluorescence of Fluo-3-loaded cells or Indo-1-loaded cells as indicators of intracellular free Ca2+, we observed that treatment of the cells with RA did no, by itself, alter the concentration of intracellular Ca2+. Nor did it interfere with the rapid, transient rise in intracellular Ca2+ induced by treatment with ionomycin. However, treatment of the cells with RA prevented re-equilibration of intracellular Ca2+ when the cells were initially equilibrated in low Ca2+ (0.15 mmol/L) culture medium and then switched to high Ca2+ (1.4 mmol/L) medium or when cells were first equilibrated in high Ca2+ medium and then switched to low Ca2+ medium. This effect of RA could be seen within seconds after treatment and the effect was observed 1 day after treatment (longest time point examined). The effect was concentration dependent and concentrations of RA that modulated Ca2+ re-equilibration (0.3 to 3.0 mumol/L) were the same as those that have previously been shown to promote fibroblast survival and growth. A biologically inactive retinoid did not have this effect. Specificity of the response was suggested by the finding that concentrations of RA that modulated Ca2+ movement had no effect on Ba2+ transport. These data suggest that RA prevents re-equilibration of intracellular Ca2+ in human dermal fibroblasts by interfering with Ca2+ movement across the plasma membrane.

Evaluation of topical retinoids for cutaneous pharmacological activity in Yucatan microswine

The pharmacological effects of retinoids on skin have been studied primarily in test systems using small animals, such as mice and rabbits. Because of potentially significant differences in skin permeation and metabolism between small animals and humans, we have used Yucatan microswine as an alternative model for testing topical retinoids. Microswine skin resembles human skin, functionally and anatomically, more closely than most other species. In these studies, microswine skin was treated topically with retinoids for 5 consecutive days per week for 5 weeks. We found microswine epidermis to be functionally responsive to retinoids in that it undergoes hyperplasia and shows an increase in transepidermal water loss (TEWL). All-trans-retinoic acid, and its analogs, 13-cis-retinoic acid, 4-hydroxy-retinoic acid and 4-oxo-retinoic acid all caused epidermal thickening and increased TEWL. The three analogs were less potent than all-trans-retinoic acid. A synthetic retinoid, TTNPB, potently induced epidermal hyperplasia and increased TEWL, but a close structural analog, m-carboxy-TTNPB, which is also inactive on nuclear retinoic acid receptors, was without effects on microswine epidermis. These findings show that microswine are useful for evaluating the cutaneous effects of topical retinoids. This model could be of value in identifying retinoids with potential clinical activity.

Induction of retinoid resistance by all-trans retinoic acid in acute promyelocytic leukemia after remission

Acute promyelocytic leukemia (APL) results from a malignant process that leads to the accumulation in the blood and the bone marrow of myeloid precursor cells characterized by an abnormal behavior and a differentiation arrest. It aroused considerable interest well beyond the hematologic field during the last five years since APL has two unique features i) the remission of the disease obtained with all-trans retinoic acid (ATRA) treatment ii) the presence in APL blasts of an abnormal protein, the promyelocytic myeloid leukemia/retinoic acid receptor (PML/RAR alpha) protein. APL is characterized cytogenetically by a t(15;17) translocation which involves both the PML gene on chromosome 15 and the RAR alpha gene on chromosome 17 and gives rise to the PML/RAR alpha fusion protein.

Tretinoin. A review of its pharmacological properties and clinical efficacy in the topical treatment of photodamaged skin

Tretinoin (all-trans-retinoic acid) is a retinol (vitamin A) derivative which has been evaluated as a topical treatment for the symptoms of photodamaged skin. In several well-controlled clinical trials, the proportion of patients showing improvement was significantly higher with 0.01 or 0.05% tretinoin cream than with placebo for criteria such as global assessment, fine and coarse wrinkling, pigmentation and roughness. Improvements in the overall severity of photodamage were also significantly greater with tretinoin than with placebo. The extent of clinical improvement with tretinoin has generally been moderate, but cytological and histological studies have shown that extensive changes in the epidermis and dermis occur during treatment. However, the permanency and clinical significance of these changes has yet to be fully evaluated. Topical tretinoin has also demonstrated potential for the treatment and eradication of premalignant skin growths such as actinic keratoses, and may be useful as combination therapy with fluorouracil in this indication. Dermatitis (the retinoid skin reaction) is the most common adverse event experienced by patients receiving topical tretinoin; this condition may persist for up to 3 months, but is usually mild or moderate in nature. Thus, topical tretinoin has been shown to be an effective form of treatment for the characteristic signs of photodamaged skin. Its ability to produce significant, albeit moderate, clinical improvements in symptoms such as fine wrinkling, roughness and pigmentation, together with its relatively mild or moderate adverse event profile, suggests that it is likely to be of considerable value in this indication. The treatment and eradication of potentially malignant growths such as actinic keratoses may also prove to be an important application for topical tretinoin.

Constitutive and inducible expression of drug metabolizing enzymes in cultured human keratinocytes

Drug metabolizing enzymes, particularly those involved in the metabolism of carcinogenic chemicals, were characterized in cultured human keratinocytes. Using immunoblotting experiments, we analysed the expression of phase I enzymes, cytochrome P4501A1 (CYP1A1) and NADPH reductase, and phase II enzymes, phenol UDP-glucuronosyltransferase (UGT) and glutathione S-transferase (GST) isoform pi, in the presence of either classical inducers (i.e. 3-methylcholanthrene, dimethylbenz[a]anthracene, phenobarbital, and clofibrate) or all-trans retinoic acid (RA). This study has shown that the expression of CYP1A1 and UGT is concomitantly induced by 3-methylcholanthrene, dimethylbenz[a]anthracene, and RA, and that of NADPH reductase is only enhanced by phenobarbital and RA. In contrast, the expression of GST pi was not affected by the inducers. Using the reverse transcriptase-polymerase chain reaction, we have demonstrated that the effects of 3-methylcholanthrene, dimethylbenz[a]anthracene and RA on CYP1A1 expression correlate with an increase of CYP1A1 mRNA level. Our results indicate that, with the exception of clofibrate, xenobiotics and RA differentially modulate the expression of drug metabolizing enzymes.

Human in vivo pharmacology of topical retinoids

All-trans retinoic acid is used topically for treating a variety of dermatologic conditions ranging from acne to photoaged skin. Although the clinical effects of retinoic acid treatment are often considerable, relatively little is known about the basic mechanisms underlying such effects. With the development of an in vivo human assay we have investigated the pleiotypic effects of topical retinoids from the histologic to the molecular. Histologically, retinoic acid induces epidermal proliferation and differentiation coupled with dermal fibroblast production of collagen. Immunologic effects include stimulation of the antigen-presenting capacity of Langerhans cells and induction of keratinocyte ICAM-1 expression. At the biochemical level, retinoic acid regulates transglutaminase and tyrosinase activities and activates protein kinase C. Both polar metabolites and stereoisomers of all-trans retinoic acid are also biologically active. Molecular biologic techniques have revealed that elevation of mRNA for cellular retinoic acid binding protein II is a retinoid-related event and that nuclear receptors such as retinoic acid receptors and retinoid X-receptors may transduce the retinoid response.

Topical retinaldehyde on human skin: biologic effects and tolerance

The present study was designed to explore if *etinaldehyde, a natural metabolite of vitamin A, has any biologic activity and is tolerated by human skin. Biologic activity was shown by the induction of cellular retinoic acid-binding protein type 2 (CRABP 2) mRNA and protein; the rank order for CRABP-2 increase was retinoic acid > retinaldehyde > 9 cis retinoic acid > retinol > beta carotene. In volunteers treated 1-3 months with 0.5, 0.1, and 0.05% retinaldehyde, there was a dose-dependent and significant increase in epidermal thickness, keratin 14 immunoreactivity, and Ki67-positive cells. The area of distribution of involucrin, transglutaminase, and filaggrin immunoreactivity was also increased in a dose-dependent manner, and keratin 4 immunoreactivity was induced in the upper epidermis. In pilot clinical tolerance studies, 229 patients received topical retinaldehyde at different concentrations; the 1% preparation was tolerated by up to 70% of the treated subjects; tolerance of the 0.5% preparation was slightly better, whereas both 0.1 and 0.05% preparations applied on facial skin were well tolerated and allowed prolonged use (up to 3 years) in patients with inflammatory dermatoses. These findings indicate that topical retinaldehyde has biologic activity and is well tolerated on human skin.

All-trans retinoic acid (RA) stimulates events in organ-cultured human skin that underlie repair. Adult skin from sun-protected and sun-exposed sites responds in an identical manner to RA while neonatal foreskin responds differently

Adult human skin from a sun-protected site (hip) and from a sun-exposed site (forearm) was maintained in organ culture for 12 d in the presence of a serum-free, growth factor-free basal medium. Cultures were incubated under conditions optimized for keratinocyte growth (i.e., in 0.15 mM extracellular Ca2+) or for fibroblast growth (i.e., in 1.4 mM extracellular Ca2+). Treatment with all-trans retinoic acid (RA) induced histological changes in the organ-cultured skin under both conditions which were similar to the changes seen in intact skin after topical application. These included expansion of the viable portion of the epidermis and activation of cells in the dermis. In sun-damaged skin samples, which were characterized by destruction of normal connective tissue elements and presence of thick, dark-staining elastotic fibers, a zone of healthy connective tissue could be seen immediately below the dermo-epidermal junction. This zone was more prominent in RA-treated organ cultures than in matched controls. Associated with these histological changes was an increase in overall protein and extracellular matrix synthesis. In concomitant studies, it was found that RA treatment enhanced survival and proliferation of adult keratinocytes and adult dermal fibroblasts under both low- and high-Ca2+ conditions. In all of these assays, responses of sun-protected and sun-exposed skin were identical. In contrast, responses of neonatal foreskin to RA were similar to those of adult skin in the presence of low-Ca2+ culture medium, but under conditions of high extracellular Ca2+ RA provided little or no additional stimulus. Together these studies suggest that the ability of RA to enhance repair of sun-damaged skin (documented in previous studies) may reflect its ability to influence the behavior of skin in a manner that is age dependent but independent of sun-exposure status.

Expression of serine proteinases and metalloproteinases in organ-cultured human skin. Altered levels in the presence of retinoic acid and possible relationship to retinoid-induced loss of epidermal cohesion

Neonatal human foreskin obtained at circumcision was cut into 2 x 2-mm pieces and placed in organ culture. Culture medium consisted of a serum-free, growth factor-free basal medium containing either 0.15 mmol/L Ca2+ or 1.4 mmol/L Ca2+. Some cultures were left as control, whereas others were treated with 3 mumol/L all-trans retinoic acid (RA). In the presence of RA, epidermal cohesion was disrupted and the upper layers separated from the viable epidermis beneath. This effect was observed under both low Ca2+ and high Ca2+ conditions. At 2-day intervals, culture fluids were collected and analyzed for serine and metalloproteinase activities. Serine proteinase activity was detected in the culture fluids and virtually all of the detected activity was dependent on the presence of plasminogen. Activity was elevated in the RA-treated tissues and this was due to increased amounts of both urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA). Elastase and cathepsin G were not detected in either control or RA-treated cultures. Increased plasminogen activator levels were also detected in RA-treated keratinocytes and fibroblasts in monolayer culture. Significant amounts of t-PA (though not u-PA) were found in fibroblast culture fluids, whereas both t-PA and u-PA were detected in culture fluids from keratinocytes. Metalloproteinase activity was also detected in the culture fluids of control and RA-treated tissues but in contrast to plasminogen activator, metalloproteinase activity decreased in the presence of RA. Casein and gelatin zymographic studies indicated the presence of both 92- and 72-kd gelatinases and stromelysin-1 and suggested that the decreased activity was primarily due to reduction in the 92- and 72-kd gelatinases. When serine proteinase inhibitors (aprotinin and soybean trypsin inhibitor) were included in the culture medium throughout the incubation period, epidermal discohesion was reduced. A metalloproteinase inhibitor, tissue inhibitor of metalloproteinase-2, did not have this effect. Taken together, these data show that a number of proteolytic enzymes are produced during organ culture of human skin. They suggest that these proteases may influence the structural integrity of the tissue.

Inhibition of microsomal 17 beta-hydroxysteroid oxidoreduction activities in rat liver by all-trans-, 9-cis- and 13-cis-retinoic acid

Retinoids and steroid hormones mediate their biological effects through nuclear receptors. However, retinoids may alter the intracellular availability of ligands for steroid receptor activation by modulating the activity of biotransformation enzymes. This study investigated the modulation of NAD(H)-linked steroid oxidoreductases in rat hepatic microsomes by retinoids. 13-cis-Retinoic acid inhibited testosterone 17 beta-dehydrogenation (Ki 2.4 +/- 0.5 microM; Km/Ki ratio 0.34 +/- 0.06) but androstenedione reduction was less susceptible to inhibition (Ki 27 +/- 13 microM; Km/Ki ratio 0.045 +/- 0.12). All-trans-retinoic acid was less potent than the 13-cis-isomer and 9-cis-retinoic acid was of intermediate potency. In vivo administration of all-trans-retinoic acid (60 mg/kg i.p. for 7 days) decreased hepatic microsomal oxidoreduction activity, but exposure over shorter periods and 13-cis-retinoic acid were without effect. Thus, all-trans-retinoic acid elicits direct inhibition and may also alter the normal regulation of the oxidoreductase under certain conditions. Three geometric isomers of retinal were potent inhibitors of testosterone dehydrogenation (IC50s approximately 6 microM), but were ineffective against androstenedione reduction. These findings suggest that certain anti-hormonal effects of retinoids may be attributable in part to modulation of endobiotic biotransformation prior to receptor binding and activation.

Determination of orally administered all-trans-retinoic acid in human plasma by high-performance liquid chromatography

All-trans-retinoic acid (RA) is used successfully in the treatment of acute promyelocytic leukaemia (APL), although unexplained relapses occur in many of the patients. Pharmacokinetic studies may help in understanding the mechanism of resistance to RA and a simple and rapid procedure for its determination in biological samples may be advantageous. A high-performance liquid chromatographic procedure is described, involving one-step extraction of RA from plasma, isocratic elution from a reversed-phase column (LiChrosorb RP-18, 5-microns particle size) and UV detection at 340 nm. The calibration graph is linear over a wide range and the limit of detection is approximately 10 ng/ml, using 0.5 ml of human plasma. The method is selective for RA, accurate and robust and thus suitable for the routine analysis of plasma samples from patients undergoing RA therapy. Analysis of plasma in a patient on RA therapy (45 mg/m2 per day) confirmed that during continuous treatment with RA the drug plasma concentrations are markedly lower at the time of relapse than on the first day of treatment.

Liarozole fumarate inhibits the metabolism of 4-keto-all-trans-retinoic acid

The metabolism of 4-keto-all-trans-retinoic-acid (4-keto-RA), a biologically active oxygenated metabolite of all-trans-retinoic (RA), has been examined. In vitro, incubation of [14C]4-keto-RA with hamster liver microsomes in the presence of NADPH produced two major radioactive metabolites which were more polar than the parent compound. Following isolation, appropriate derivatization and analysis by GC-MS, these compounds were tentatively identified as 2-hydroxy- and 3-hydroxy-4-ketoretinoic acid. Formation of both hydroxy-keto derivatives was suppressed by the imidazole-containing P450 inhibitor liarozole fumarate (IC50, 1.3 microM). In vitro, an i.v. injection of 4-keto-RA (20 micrograms) into rats was followed by rapid disappearance of the retinoid from plasma with a half-life of 7 min. Pretreatment with liarozole fumarate (40 mg/kg, -60 min) reduced the elimination rate of 4-keto-RA: it prolonged the plasma half-life of the retinoid to 12 min, without affecting its distribution volume. These results indicate the important role of the P450 enzyme system in the metabolism of 4-keto-RA both in vitro and in vivo. The inhibitory effect of liarozole fumarate on this metabolic process may contribute to the reported retinoid-mimetic activity of this drug.

Drug-metabolizing enzymes in ligand-modulated transcription

Genes encoding many of the so-called drug-metabolizing enzymes (DMEs) are present in both prokaryotes and eukaryotes, suggesting that these genes arose on this planet more than 3.5 billion years ago--long before animal-plant divergence (estimated to be about 1.2 billion years ago) and long before the use and commercial development of drugs. What, therefore, are the real functions of DMEs? Several years ago I proposed that DMEs are upstream in the regulatory cascade of numerous signal transduction pathways, i.e. necessary for maintaining physiologically "safe", or "acceptable", steady-state levels of all small non-protein endogenous ligands (M(r) = 250 +/- 200) in each cell. Innumerable foreign chemicals and drugs mimic these small endogenous ligands, thus binding to a particular receptor and acting either as an agonist or antagonist in activating or inhibiting genes effecting growth, differentiation, apoptosis, homeostasis and neuroendocrine functions. Discussed in this review are additional examples consistent with this theory and not described in previous reviews, including: (i) insect-plant symbiosis; (ii) "cross-talk" amongst genes in the aromatic hydrocarbon-responsive [Ah] battery; (iii) signal transduction pathways involving the arachidonic acid cascade; and (iv) the explanation in carcinogen-screening studies as to why a maximum, or half maximum, tolerated dose (MTD, MTD50) of many test compounds might cause cell division and tumorigenesis in experimental animals.

A retinoic acid-inducible skin-specific gene (RIS-1/psoriasin): molecular cloning and analysis of gene expression in human skin in vivo and cultured skin cells in vitro

A retinoic acid (RA) inducible skin-specific gene transcript (RIS-1) was isolated by differential hybridization screening of a RA-treated human skin cDNA library. The library was constructed from pooled RNA derived from normal adult human skin treated with all trans-RA for 4 h (n = 6) and 12 h (n = 6) in vivo. RIS-1 cDNA corresponded to a 0.6 kb transcript that was barely detectable in normal adult human skin but was significantly induced by 8 h in RA-treated compared to vehicle-treated skin (range 1.1-3.6 fold). Prolonged RA treatment for up to 24 h further increased relative RIS-1 mRNA levels by 1.3-5.5 fold. HPLC analysis of the RA content of 0.1% RA-treated skin in vivo revealed significant levels at 6 h (18.8-120.6 ng RA/g wet weight tissue; approximately 240 nM), immediately preceding the time point at which the increased RIS-1 mRNA level was first seen. This concentration of RA also induced the mRNA levels for cellular RA binding protein II (1.6-19 fold), a marker of RA activity in human skin. RIS-1 mRNA was detected by Northern and dot blotting only in normal skin but not in any other normal human tissues examined, indicating a tissue-specific pattern of gene expression. RIS-1 transcripts were detected at very low levels in untreated cultured human epidermal keratinocytes, while no expression was seen in dermal fibroblasts and melanocytes, the other major cell types in skin. Southern analysis of human and mouse DNA indicated the existence of evolutionarily conserved sequences for RIS-1 between these two species. The polypeptide sequence derived from the partial RIS-1 cDNA was found to be identical to the calcium binding domain found in 'psoriasin', a gene whose expression appears to be increased in the skin of psoriasis patients.

Biologic activities of retinoic acid and 3,4-didehydroretinoic acid in human keratinocytes are similar and correlate with receptor affinities and transactivation properties

The biologic activities of retinoic acid and 3,4-didehydroretinoic acid, two endogenous vitamin A derivatives in various tissues, were compared to their affinities for the nuclear retinoic acid receptors and their ability to induce transcriptional activation. Both retinoids were equipotent inducers of differentiation of F9 teratocarcinoma cells. In a morphologic assay, using reconstructed skin, retinoic acid and 3,4-didehydroretinoic acid inhibited keratinization at a concentration of 100 nM. In cultured keratinocytes, a 50% inhibition of the production of the keratinocyte transglutaminase enzyme was achieved with about 20 nM for both retinoids. The in vitro binding to the nuclear retinoic acid receptors alpha, beta, and gamma showed that retinoic acid and 3,4-didehydroretinoic acid had almost equal affinities for the receptors with Kds ranging from 3 to 47 nM. The transcriptional activation resulting from the addition of the two retinoids to cells co-transfected with alpha, beta, or gamma retinoic acid receptor expression vectors and a retinoic acid responsive element linked to the chloramphenicol acetyltransferase reporter gene was similar. Finally, it was demonstrated that retinoic acid did not metabolize to 3,4-didehydroretinoic acid, and a slow conversion of 3,4-didehydroretinoic acid into retinoic acid was not sufficient to explain the biologic effects produced by the former compound. In conclusion, the present study demonstrates that retinoic acid and 3,4-didehydroretinoic acid have the same activity in several different test systems, but their metabolism differs depending on the cell type used.