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).

Single-stranded DNA-binding protein enhances the stability of CTG triplet repeats in Escherichia coli

The stability of CTG triplet repeats was analyzed in Escherichia coli to identify processes responsible for their genetic instability. Using a biochemical assay for stability, we show that the absence of single-stranded-DNA-binding protein leads to an increase in the frequency of large deletions within the triplet repeats.

TTA.TAA triplet repeats in plasmids form a non-H bonded structure

CTG.CAG, CGG.CCG, and AAG.CTT triplet repeats proximal to or in disease genes expand by a non-Mendelian genetic process to cause several human hereditary syndromes. As part of our physical, biological, and genetic studies on the 10 possible triplet repeats, we discovered that the TTA.TAA repeat, isolated from the upstream region of the variant surface glycoprotein gene of Trypanosoma brucei, shows a propensity to adopt a non-H bonded structure under appropriate conditions. The other nine triplet repeat sequences do not exhibit this property. (TTA.TAA)n, where n = 90, 60, 30, and 18, cloned into pUC19 was studied by chemical and enzymatic probes as well as two-dimensional gel electrophoretic analyses under a variety of conditions. The helix opening was observed for all four inserts in supercoiled plasmids as a function of temperature, pH, metal ions, and buffer conditions using OsO4, diethyl pyrocarbonate, and chloroacetaldehyde probes. This unusual property of the TTA.TAA repeat suggests that it plays a different role from the other nine triplet repeats in gene expression.

Cloning, characterization, and properties of seven triplet repeat DNA sequences

Several neuromuscular and neurodegenerative diseases are caused by genetically unstable triplet repeat sequences (CTG.CAG, CGG.CCG, or AAG.CTT) in or near the responsible genes. We implemented novel cloning strategies with chemically synthesized oligonucleotides to clone seven of the triplet repeat sequences (GTA.TAC, GAT.ATC, GTT.AAC, CAC.GTG, AGG.CCT, TCG.CGA, and AAG.CTT), and the adjoining paper (Ohshima, K., Kang, S., Larson, J. E., and Wells, R. D.(1996) J. Biol. Chem. 271, 16784-16791) describes studies on TTA.TAA. This approach in conjunction with in vivo expansion studies in Escherichia coli enabled the preparation of at least 81 plasmids containing the repeat sequences with lengths of approximately 16 up to 158 triplets in both orientations with varying extents of polymorphisms. The inserts were characterized by DNA sequencing as well as DNA polymerase pausings, two-dimensional agarose gel electrophoresis, and chemical probe analyses to evaluate the capacity to adopt negative supercoil induced non-B DNA conformations. AAG.CTT and AGG.CCT form intramolecular triplexes, and the other five repeat sequences do not form any previously characterized non-B structures. However, long tracts of TCG.CGA showed strong inhibition of DNA synthesis at specific loci in the repeats as seen in the cases of CTG.CAG and CGG.CCG (Kang, S., Ohshima, K., Shimizu, M., Amirhaeri, S., and Wells, R. D.(1995) J. Biol. Chem. 270, 27014-27021). This work along with other studies (Wells, R. D.(1996) J. Biol. Chem. 271, 2875-2878) on CTG.CAG, CGG.CCG, and TTA.TAA makes available long inserts of all 10 triplet repeat sequences for a variety of physical, molecular biological, genetic, and medical investigations. A model to explain the reduction in mRNA abundance in Friedreich's ataxia based on intermolecular triplex formation is proposed.

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.

CTG triplet repeats from the myotonic dystrophy gene are expanded in Escherichia coli distal to the replication origin as a single large event

The expansion and contraction of CTG and CGG trinucleotide repeat sequences have been associated with several heritable genetic diseases. We developed a system for investigating the expansion of triplet repeat sequences in Escherichia coli in order to elucidate molecular mechanisms. Analysis of expanded regions using the interrupting CTA triplet sequence as a location marker within the CTG tract revealed that the expansion of large CTG repeats is one event rather than an accumulation of multiple small expansions and that the expansions occur more frequently in the region distal from the replication origin. Also, we showed that a loss of interruptions increases the expansion frequency. Thus, the instability of large triplet repeats in hereditary diseases occurs by a mechanism different from the instability in microsatellite sequences caused by defects in mismatch repair systems for certain sporadic cancers and hereditary non-polyposis colorectal cancers.

Topical tretinoin (retinoic acid) improves early stretch marks

BACKGROUND AND DESIGN

Stretch marks are disfiguring lesions usually caused by excessive stretching of skin. We investigated the response of early, clinically active stretch marks to topical 0.1% tretinoin (retinoic acid) cream. In a double-blind, randomized, vehicle-controlled study, 22 patients applied either 0.1% tretinoin (n = 10) or vehicle (n = 12) daily for 6 months to the affected areas. Patients were evaluated by physical examination monthly and by analysis of biopsy specimens of stretch marks obtained before and at the end of therapy in comparison with untreated normal skin.

RESULTS

After 2 months, patients treated with tretinoin had significant improvements in severity scores of stretch marks compared with patients who received vehicle (P < .05). After 6 months, eight (80%) of the 10 tretinoin-treated patients had definite or marked improvement compared with one (8%) of the 12 vehicle-treated patients (P = .002). Targeted stretch marks in patients treated with tretinoin had a decrease in mean length and width of 14% and 8%, respectively, compared with an increase of 10% (P < .001) and 24% (P = .008), respectively, in patients who received vehicle. There were no significant differences in various measures of quality and quantity of dermal collagen and elastic fibers in stretch marks when tretinoin and vehicle treatments were compared.

CONCLUSIONS

Topical application of tretinoin significantly improves the clinical appearance of early, active stretch marks. The processes that are responsible for the clinical improvement remain unknown.

Adenomyosis: specificity of 5 mm as the maximum normal uterine junctional zone thickness in MR images

OBJECTIVE

Our objective was to investigate the specificity of the criterion stating that a diagnosis of adenomyosis can be made confidently from MR images of the uterus when the junctional zone is thicker than 5 mm.

SUBJECTS AND METHODS

Twenty clinically normal women volunteers (chosen by strict criteria) underwent MR imaging of the uterus. Images were independently evaluated by two experienced observers who measured the greatest thickness of the junctional zone in the anterior, posterior, right and left walls of the uterine corpus and at the top of the fundus.

RESULTS

The mean value of the greatest junctional zone thickness (averaged for both observers) was 4.4 +/- 2.1 mm (mean +/- SD, range, 1-9 mm) for the anterior and posterior walls of the corpus, 4.3 +/- 2.1 mm (range, 1-10 mm) for the right and left walls, and 2.3 +/- 1.1 mm (range, 1-5 mm) for the fundus. At least one region of the junctional zone was found to be thicker than 5 mm by at least one observer in 13 (65%) of the 20 subjects; at least one region was found to be thicker than 5 mm by both observers in eight (40%) subjects. At least two regions were measured as thicker than 5 mm by both observers in six (30%) subjects. Serial studies of three volunteers showed transient irregular focal or diffuse thickening (up to 12 mm) of the junctional zone and focal myometrial bulging, probably due to myometrial contractions.

CONCLUSION

Our results suggest that if a diagnosis of adenomyosis is based solely on junctional zone thickness in MR images, 5 mm should not be assumed as the upper limit of normal, because this assumption may result in a high false-positive rate.

Pharmacology and molecular action of retinoids and vitamin D in skin

All-trans retinoic acid and 1,25-dihydroxyvitamin D3 are biologically active hormones derived from metabolism of vitamins A and D, respectively. Their cellular effects in physiology and pharmacology are mediated mainly through their nuclear receptors, retinoic acid receptors (RARs), and vitamin D receptor (VDR), which are members of the steroid/thyroid hormone receptor superfamily. The nuclear receptor superfamily is a group of hormone (ligand)-dependent transcription factors that bind to specific DNA recognition sequences located in the promotor region of target genes and modulate the gene transcription. Both RARs and VDR bind to DNA in the form of heterodimers with retinoid X receptor (RXR) as the key partner. In human skin epidermis, endogenous RAR and VDR bind to their respective enhancer elements as heterodimers (RAR-RXR, VDR-RXR) and not as homodimers. 9-cis retinoic acid is the natural ligand for RXRs. Because RXR heterodimerizes with either RARs or VDR, it functions as a key protein in the overall retinoid or vitamin D response of a given biological system. The contribution of RXRs in coregulating these signaling pathways is likely to depend on the abundance of the RXRs within target cells, the relative amounts of other receptors with which they can dimerize, and the availability of their ligands (hormones). Understanding each of these components in human skin in vivo will lead to practical applications in dermatologic therapeutics.

Retinoid induction of CRABP II mRNA in human dermal fibroblasts: use as a retinoid bioassay

Cellular retinoic acid binding protein II (CRABP II) mRNA is selectively induced by all-trans retinoic acid in human skin and dermal fibroblasts. In order to determine whether this response can be used as a reliable measure of retinoid potency and activity, we treated human skin fibroblasts for 24 h with increasing concentrations of several natural and synthetic retinoids. CRABP II mRNA levels were measured by quantitative Northern blotting and compared, when possible, with those obtained after topical application of the same retinoids to human skin. All eight active retinoids tested induced a concentration-dependent CRABP II mRNA response in the fibroblast assay. In contrast, one known inactive retinoid (meta-carboxy TTNPB), differing from the active form only in the position of the carboxyl substituent, failed to evoke a response. The fibroblast and human skin bioassays agreed with respect to relative potency and response amplitude for three of the three retinoids tested. Retinoic acid was approximately 10-fold more potent than retinal in both assays, suggesting that oxidation to retinoic acid underlies the activity of retinol in fibroblasts as well as in intact skin. In support of this hypothesis, treatment with liarozole, an inhibitor of P450-mediated retinoic acid oxidative catabolism, significantly increased fibroblast CRABP II mRNA levels and potentiated the effects of retinol by 1.5-fold at concentrations at which it had no effect on its own. Taken together, these results identify the fibroblast CRABP II response as a reproducible measure of retinoid bioactivity with promise as a predictor of human skin responses and further suggest that metabolism is an important determinant of retinoid bioactivity in vivo.

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.

CTG triplet repeats from human hereditary diseases are dominant genetic expansion products in Escherichia coli

The relative ability of the 10 triplet repeat sequences to be expanded in Escherichia coli was determined. Surprisingly, CTG tracts are expanded at least 8 times more frequently than any of the other nine triplets. Low levels of expansion were found also for CGG, GTG, and GTC. Thus, the structure of the CTG repeats and/or their utilization by the DNA synthetic systems in vivo must be quite different from the other triplets. These data further validate this genetically defined system for elucidating molecular mechanisms of expansion and may explain why most triplet repeat hereditary neuromuscular and neurodegenerative disease genes contain CTG repeats.

Molecular basis of sun-induced premature skin ageing and retinoid antagonism

Damage to skin collagen and elastin (extracellular matrix) is the hallmark of long-term exposure to solar ultraviolet irradiation, and is believed to be responsible for the wrinkled appearance of sun-exposed skin. We report here that matrix-degrading metalloproteinase messenger RNAs, proteins and activities are induced in human skin in vivo within hours of exposure to ultraviolet-B irradiation (UVB). Induction of metalloproteinase proteins and activities occurred at UVB doses well below those that cause skin reddening. Within minutes, low-dose UVB upregulated the transcription factors AP-1 and NF-kappa B, which are known to be stimulators of metalloproteinase genes. All-trans retinoic acid, which transrepresses AP-1 (ref. 8), applied before irradiation with UVB, substantially reduced AP-1 and metalloproteinase induction. We propose that elevated metalloproteinases, resulting from activation of AP-1 and NF-kappa B by low-dose solar irradiation, degrade collagen and elastin in skin. Such damage, if imperfectly repaired, would result in solar scars, which through accumulation from a lifetime of repeated low-dose sunlight exposure could cause premature skin ageing (photoageing).

Anomalous rapid electrophoretic mobility of DNA containing triplet repeats associated with human disease genes

Eight human genetic diseases have been associated with the expansion of CTG or CGG triplet repeats. The molecular etiology behind expansion is unknown but may involve participation of an unusual DNA structure in replication, repair, or recombination. We show that DNA fragments containing CTG triplet repeats derived from the human myotonic dystrophy gene migrate up to 20% faster than expected in nondenaturing polyacrylamide gels, suggesting the presence of an unusual DNA helix structure within the CTG triplet repeats. The anomalous migration is dependent upon the number of triplet repeats, the length of the flanking DNA, and the percentage and temperature of the polyacrylamide. The effect could be reduced by the addition of actinomycin D. Applying a reptation model for electrophoresis, the results are consistent with a 20% increase in persistence length of the DNA. PCR products containing CTG or CGG repeats from the spinocerebellar ataxia type I gene (SCA1) or the fragile X FMR1 gene, respectively, also showed higher electrophoretic mobility. These are the first sequences of defined length for which a dramatic increase in mobility can be attributed to sequence-dependent structural elements in DNA.

Mismatch repair in Escherichia coli enhances instability of (CTG)n triplet repeats from human hereditary diseases

Long CTG triplet repeats which are associated with several human hereditary neuromuscular disease genes are stabilized in ColE1-derived plasmids in Escherichia coli containing mutations in the methyl-directed mismatch repair genes (mutS, mutL, or mutH). When plasmids containing (CTG)180 were grown for about 100 generations in mutS, mutL, or mutH strains, 60-85% of the plasmids contained a full-length repeat, whereas in the parent strain only about 20% of the plasmids contained the full-length repeat. The deletions occur only in the (CTG)180 insert, not in DNA flanking the repeat. While many products of the deletions are heterogeneous in length, preferential deletion products of about 140, 100, 60, and 20 repeats were observed. We propose that the E. coli mismatch repair proteins recognize three-base loops formed during replication and then generate long single-stranded gaps where stable hairpin structures may form which can be bypassed by DNA polymerase during the resynthesis of duplex DNA. Similar studies were conducted with plasmids containing CGG repeats; no stabilization of these triplets was found in the mismatch repair mutants. Since prokaryotic and human mismatch repair proteins are similar, and since several carcinoma cell lines which are defective in mismatch repair show instability of simple DNA microsatellites, these mechanistic investigations in a bacterial cell may provide insights into the molecular basis for some human genetic diseases.

Pausing of DNA synthesis in vitro at specific loci in CTG and CGG triplet repeats from human hereditary disease genes

Several human hereditary neuromuscular disease genes are associated with the expansion of CTG or CGG triplet repeats. The DNA syntheses of CTG triplets ranging from 17 to 180 and CGG repeats from 9 to 160 repeats in length were studied in vitro. Primer extensions using the Klenow fragment of DNA polymerase I, the modified T7 DNA polymerase (Sequenase), or the human DNA polymerase beta paused strongly at specific loci in the CTG repeats. The pausings were abolished by heating at 70 degrees C. As the length of the triplet repeats in duplex DNA, but not in single-stranded DNA, was increased, the magnitude of pausing increased. The location of the pause sites was determined by the distance between the site of primer hybridization and the beginning of the triplet repeats. CGG triplet repeats also showed similar, but not identical, patterns of pausings. These results indicate that appropriate lengths of the triplets adopt a non-B conformation(s) that blocks DNA polymerase progression; the resultant idling polymerase may catalyze slippages to give expanded sequences and hence provide the molecular basis for this non-Mendelian genetic process. These mechanisms, if present in human cells, may be related to the etiology of certain neuromuscular diseases such as myotonic dystrophy and Fragile X syndrome.

The PWL host specificity gene family in the blast fungus Magnaporthe grisea

The PWL2 gene, isolated from a Magnaporthe grisea rice pathogen, prevents this fungus from infecting a second host grass, weeping lovegrass. We have investigated the distribution of sequences homologous to PWL2 in M. grisea strains isolated from diverse grass species. Multiple PWL2 homologs with varying degrees of sequence homology were identified. The presence of PWL2 homologs does not correlate with an avirulent phenotype on weeping lovegrass in many cases: some strains were fully pathogenic on weeping lovegrass although they carry multiple PWL2 homologs. Three weakly hybridizing PWL2 homologs were cloned and characterized. One of these, the PWL1 gene previously identified by genetic analysis, functioned to prevent infection of weeping lovegrass. Cloned PWL3 and PWL4 genes were nonfunctional, although PWL4 became functional if its expression was driven by either the PWL1 or the PWL2 promoter. The PWL1, PWL2, and PWL3/PWL4 genes map to different genomic locations. The amino acid sequences of the predicted PWL1, PWL3, and PWL4 proteins have 75, 51, and 57% identity, respectively, to the PWL2 protein. Our studies indicate that PWL genes are members of a dynamic, rapidly evolving gene family.

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.