Expression of CRABP-I and -II in human epidermal cells. Alteration of relative protein amounts is linked to the state of differentiation

Topical apigenin improves epidermal permeability barrier homoeostasis in normal murine skin by divergent mechanisms

The beneficial effects of certain herbal medicines on cutaneous function have been appreciated for centuries. Among these agents, chrysanthemum extract, apigenin, has been used for skin care, particularly in China, for millennia. However, the underlying mechanisms by which apigenin benefits the skin are not known. In this study, we first determined whether topical apigenin positively influences permeability barrier homoeostasis, and then the basis thereof. Hairless mice were treated topically with either 0.1% apigenin or vehicle alone twice daily for 9 days. At the end of the treatments, permeability barrier function was assessed with either an electrolytic water analyzer or a Tewameter. Our results show that topical apigenin significantly enhanced permeability barrier homoeostasis after tape stripping, although basal permeability barrier function remained unchanged. Improved barrier function correlated with enhanced filaggrin expression and lamellar body production, which was paralleled by elevated mRNA levels for the epidermal ABCA12. The mRNA levels for key lipid synthetic enzymes also were upregulated by apigenin. Finally, both cathelicidin-related peptide and mouse beta-defensin 3 immunostaining were increased by apigenin. We conclude that topical apigenin improves epidermal permeability barrier function by stimulating epidermal differentiation, lipid synthesis and secretion, as well as cutaneous antimicrobial peptide production. Apigenin could be useful for the prevention and treatment of skin disorders characterized by permeability barrier dysfunction, associated with reduced filaggrin levels and impaired antimicrobial defenses, such as atopic dermatitis.

The transcriptional effects of the vesicants lewisite and sulfur mustard on human epidermal keratinocytes

Sulfur mustard (SM) injury is a complex process that begins with extensive alkylation of critical cellular components and culminates in cell death and skin vesication. The mechanism of toxicity is not well understood since SM broadly alkylates cellular nucleophiles. However, two events appear integral-the formation of DNA cross-links and the release of proteases into the extracellular matrix. To identify genes directly involved in vesication, the transcriptional profile of SM was compared to the vesicant lewisite (L). Similarly, to identify genes directly involved in DNA damage, the transcriptional profile of SM was compared to the genotoxic agent cisplatin (c-Pt). Microarrays containing 7,075 sequence-verified human cDNAs were screened with mRNA from human epidermal keratinocytes treated with 200 mu M agent for 2 h. A large number of differentially expressed genes were identified, with many similarities observed between agents. Many genes not previously associated with SM and L injury were also identified, including a large percentage of unknown function. A comparison of the differential expression profiles revealed that L had the broadest and most robustly altered expression. Apoptotic transcripts were clearly evident in L but not in SM, suggesting a late stage in L injury.

Increased retinoic acid levels through ablation of Cyp26b1 determine the processes of embryonic skin barrier formation and peridermal development

The process by which the periderm transitions to stratified epidermis with the establishment of the skin barrier is unknown. Understanding the cellular and molecular processes involved is crucial for the treatment of human pathologies, where abnormal skin development and barrier dysfunction are associated with hypothermia and perinatal dehydration. For the first time, we demonstrate that retinoic acid (RA) levels are important for periderm desquamation, embryonic skin differentiation and barrier formation. Although excess exogenous RA has been known to have teratogenic effects, little is known about the consequences of elevated endogenous retinoids in skin during embryogenesis. Absence of cytochrome P450, family 26, subfamily b, polypeptide 1 (Cyp26b1), a retinoic-acid-degrading enzyme, results in aberrant epidermal differentiation and filaggrin expression, defective cornified envelopes and skin barrier formation, in conjunction with peridermal retention. We show that these alterations are RA dependent because administration of exogenous RA in vivo and to organotypic skin cultures phenocopy Cyp26b1(-/-) skin abnormalities. Furthermore, utilizing the Flaky tail (Ft/Ft) mice, a mouse model for human ichthyosis, characterized by mutations in the filaggrin gene, we establish that proper differentiation and barrier formation is a prerequisite for periderm sloughing. These results are important in understanding pathologies associated with abnormal embryonic skin development and barrier dysfunction.

Retinoids suppress cysteine-rich protein 61 (CCN1), a negative regulator of collagen homeostasis, in skin equivalent cultures and aged human skin in vivo

Alterations in connective tissue collagen are prominent features of both chronologically aged and photoaged (ageing because of sun exposure) human skin. These age-related abnormalities are mediated in part by cysteine-rich protein 61 (CCN1). CCN1 is elevated in the dermis of both chronologically aged and photoaged human skin in vivo and promotes aberrant collagen homeostasis by down-regulating type I collagen, the major structural protein in skin, and promoting collagen degradation. Vitamin A and its metabolites have been shown to improve chronologically aged and photoaged skin by promoting deposition of new collagen and preventing its degradation. Here, we investigated regulation of CCN1 expression by retinoids in skin equivalent cultures and chronologically aged and photoaged human skin in vivo. In skin equivalent cultures, all-trans retinoic acid (RA), the major bioactive form of vitamin A in skin, significantly increased type I procollagen and reduced collagenase (matrix metalloproteinases-1, MMP-1). Addition of recombinant human CCN1 to skin equivalent cultures significantly reduced type I procollagen and increased MMP-1. Importantly, RA significantly reduced CCN1 expression in skin equivalent cultures. Topical treatment with retinol (vitamin A, 0.4%) for 7days significantly reduced CCN1 mRNA and protein expression in both chronologically aged (80+years) and photoaged human skin in vivo, compared to vehicle-treated skin. These data indicate that the mechanism by which retinoids improve aged skin, through increased collagen production, involves down-regulation of CCN1.

Syntheses, antiproliferative activity and theoretical characterization of acitretin-type retinoids with changes in the lipophilic part

Acitretin analogs, incorporating changes in the lipophilic part, were efficiently synthesized from commercially available aromatic aldehydes or methyl ketones using the Wittig or Horner-Wadsworth-Emmons reaction. Their antiproliferative activity was evaluated against human breast MCF-7 epithelial cells. Analogs 3, 4, 8 and 11 exhibited strong, dose-dependent, antiproliferative activity on the tested cell line. Analog 3, incorporating three methoxy groups in the aromatic ring, exhibited the strongest inhibitory effect at 10 μM. High-level all electron conventional ab initio and density functional theory quantum chemical calculations were performed to obtain the molecular structure, electron charge distribution and polarization properties of all compounds of interest in this work. The most active analogs were planar and were characterized by larger dipole moments than the other synthesized molecules. Another factor of importance to the analysis of the activity of these molecules is the dipole polarizability.

The consequences of scoring docked ligand conformations using free energy correlations

Ligands from a set of 19 protein-ligand complexes were re-docked with AutoDock, GOLD and FlexX using the scoring algorithms native to these programs supplemented by analysis using the HINT free energy force field. A HINT scoring function was calibrated for this data set using a simple linear regression of total HINT score for crystal-structure complexes vs. measured free energy of binding. This function had an r(2) of 0.84 and a standard error of +/-0.42 kcal mol(-1). The free energies of binding were calculated for the best poses using the AutoDock, GOLD and FlexX scoring functions. The AutoDock and GoldScore algorithms estimated more than half of the binding free energies within the reported calibration standard errors for these functions, while that of FlexX did not. In contrast, the calibrated HINT scoring function identified optimized poses with standard errors near +/-0.5 kcal mol(-1). When the metric of success is minimum RMSD (vs. crystallographic coordinates) the three docking programs were more successful, with mean RMSDs for the top-ranking poses in the 19 complexes of 3.38, 2.52 and 2.62 A for AutoDock, GOLD and FlexX, respectively. Two key observations in this study have general relevance for computational medicinal chemistry: first, while optimizing RMSD with docking score functions is clearly of value, these functions may be less well optimized for free energy of binding, which has broader applicability in virtual screening and drug discovery than RMSD; second, scoring functions uniquely calibrated for the data set or sets under study should nearly always be preferable to universal scoring functions. Due to these advantages, the poses selected by the HINT score also required less post-docking structure optimization to produce usable molecular models. Most of these features may be achievable with other scoring functions.

Homozygous deletion of the CRABPI gene in AB1 embryonic stem cells results in increased CRABPII gene expression and decreased intracellular retinoic acid concentration

The cellular retinoic acid (RA) binding proteins I and II (CRABPI and CRABPII), intracellular proteins which bind retinoic acid with high affinity, are involved in the actions of RA, though their exact roles are not fully understood. We have generated several genetically engineered AB1 cell lines in which both alleles of the CRABPI gene have been deleted by homologous recombination. We have used these CRABPI knockout cell lines to examine the consequences of functional loss of CRABPI on RA-induced gene expression and RA metabolism in the murine embryonic stem cell line, AB1, which undergoes differentiation in response to RA. Complete lack of CRABPI results in decreased intracellular [3H]RA concentrations under conditions in which external concentrations of [3H]RA are low (1-10nM) and in an altered distribution of [3H] polar metabolites of [3H]RA in the cell and in the medium. Fewer [3H] polar metabolites are retained within the CRABPI(-/-) cells compared to the wild-type cells. These data suggest that CRABPI functions to regulate the intracellular concentrations of retinoic acid and to maintain high levels of oxidized retinoic acid metabolites such as 4-oxoretinoic acid within cells.

Topical retinoic acid alters the expression of cellular retinoic acid-binding protein-I and cellular retinoic acid-binding protein-II in non-lesional but not lesional psoriatic skin

Therapeutic retinoids have profound effects on psoriatic skin pathology but their interactions with various retinoid-binding proteins in lesional vs non-lesional skin have not been investigated. Using quantitative real-time PCR the mRNA expression of cellular retinol-binding protein I (CRBPI) and retinoic acid-binding protein I/II (CRABPI/CRABPII) was studied in psoriatic and healthy control (=normal) skin after 4 days of occlusive RA/vehicle treatment (n=6). Untreated psoriatic lesions showed a markedly elevated CRABPII/CRABPI ratio, while the CRBPI level was reduced in lesional and non-lesional skin as compared to normal skin. In RA-treated normal and non-lesional skin, the mRNA expression of CRBPI was unaltered while that of CRABPI and CRABPII was reduced by approximately 80% and increased approximately 5-fold, respectively, as compared to vehicle-treated skin. In contrast, lesional skin exposed to RA showed an almost 90% increase in CRBPI transcripts but unaltered expression of CRABPI and CRABPII, yet, the mRNA expression of several inflammatory mediators, e.g. inducible nitric oxide synthase, interferon-gamma and interleukin-1beta, was clearly reduced. Immunohistochemistry localized CRABPII to suprabasal keratinocytes in normal skin and revealed markedly elevated levels in lesional skin. RA treatment induced CRABPII protein expression in normal and non-lesional skin, to similar levels as in untreated lesions. The results indicate that the effects of RA differ in normal/non-lesional psoriatic skin and lesional skin. Whether the high expression of CRABPII in psoriatic skin lesions is due to increased amounts of endogenous retinoids in lesional skin or reflects an abnormal regulation of the CRABPII gene in psoriasis remains to be studied.

Comparison of gene expression profiles in human keratinocyte mono-layer cultures, reconstituted epidermis and normal human skin; transcriptional effects of retinoid treatments in reconstituted human epidermis

In order to validate a model for predictive screening of dermatological drugs, we used a customized cDNA macro-array system containing 475 skin-related genes to analyze the gene expression patterns in human keratinocytes from different origins: (1) normal human epidermal keratinocyte mono-layer cultures, (2) the commercially available SkinEthic reconstituted human epidermis model, and (3) biopsies of normal human epidermis. Few markers of those that were detected significantly in keratinocyte mono-layers or in reconstituted epidermis were undetected or detected at very low level in the normal epidermis biopsies. A comparative expression of more than 100 markers could be evidenced in both normal epidermis and reconstituted epidermis samples; however, only 90% of these were detected in keratinocyte mono-layers: expression of several terminal differentiation markers, such as filaggrin, loricrin, and corneodesmosin were strongly detected in normal epidermis and reconstituted epidermis, but were not significantly expressed in keratinocyte mono-layers. Under the experimental conditions described herein, the reconstituted human epidermis model was found to significantly reproduce the gene expression profile of normal human epidermis. Using the same methodology, we then investigated the effects of all-trans retinoic acid, 9-cis retinoic acid, all-trans retinol and a commercialized tretinoin-containing cream (Retacnyl) on the gene expression profiles of reconstituted human epidermis. According to the nature and the length of the treatments, more than 40 genes were found significantly modified. Among the genes whose expression was decreased, we found cytokeratins 1, 10, 2E, and 6B, several cornified envelope precursors, integrins alpha 3, alpha 6, beta 1, beta 4, some components of desmosomes, of hemi-desmosomes and of the epidermal basement membrane. Transcriptional upregulation was observed for keratins 18 and 19, autocrine and paracrine growth factors such as HB-EGF, IGF 1, PDGF-A, calgranulins A and B, interleukin-1 alpha and the other IL-1-related markers, type II IL-1 receptor and type I IL-1-receptor antagonist. Our results confirm most of the known effects of retinoids on human epidermis, but also give new insights into their complex pharmacological activity on skin. The reconstituted human epidermis used proves to be a highly predictive model for efficacy evaluation of skin-targeted compounds, such as retinoids.

Induction of differentiation and apoptosis in human promyelocytic leukemia HL60 cell line by a new type of steroids

Mer-NF8054X is a new type of steroid whose structure has been established as 11-oxo-18, 22-cycloergosta-6, 8(14)-diene-3beta, 5beta, 9beta, 23S-tetraol (an 18, 22-cycloergostane), which has been reported to have antifungal activity against Aspergillus fumigatus. However, other biological activities are unknown. Herein, we reported that Mer-NF8054X inhibited cell growth of HL60 human leukemia cells, when used either singly or in combination with retinoic acid (RA). In addition, Mer-NF8054X alone induced differentiation and apoptosis of HL60 cells. The induction of differentiation of HL60 cells by Mer-NF8054X was synergistic in combination with RA. On the other hand, Emesterone A, an analogue of Mer-NF8054X which is missing a hydroxy residue from the third position, showed much lower activity than Mer-NF8054X on the inhibition of cell growth and the induction of cell differentiation and apoptosis. However, Emesterone B, an analogue of Emesterone A which is missing a hydroxy residue from the fifth position, showed higher activity than Emesterone A but lower activity than Mer-NF8054X when examined for the inhibition of cell growth and the induction of cell differentiation and apoptosis. These results suggested that Mer-NF8054X and its analogs may be a new type of differentiation inducing agent. The hydroxy residue at the third position or fifth position in Mer-NF8054X may be necessary, but not essential, for inhibition of growth and induction of both differentiation and apoptosis of HL60 cells. In addition, Mer-NF8054X enhanced the differentiation of HL60 cells induced by RA. Based on these results, Mer-NF8054X may have utility in the clinic in combination with RA for leukemia patients.

Keratinocytes and dermal factors activate CRABP-I in melanocytes

Recognition that cellular retinoic acid binding protein (CRABP)-I and CRABP-II are found in different cell types has provided additional support for the presumably divergent roles of these two proteins in mediating retinoic acid (RA) effects in human skin. CRABP-II is expressed in fibroblasts and keratinocytes, and CRABP-I in as yet unidentified cells, possibly epidermal melanocytes. Recently, we demonstrated that each of these RA-binding proteins in human skin possesses two classes of binding sites, possibly related to the state of phosphorylation of the proteins. We now characterize the cutaneous origin of CRABP-I further using an anion-exchange HPLC assay that allows effective separation of the two proteins in human skin, and a fluorescent in situ hybridization technique. We report that CRABP-I is expressed in isolated melanocytes at the mRNA level, although under these circumstances the protein has minimal RA-binding activity, and that keratinocytic and dermal influences are required for CRABP-I activity in melanocytes. This melanocyte origin for CRABP-I and the improvement by RA of the irregular hyperpigmentation associated with photoaging led us to examine the effects of RA using various cellular associations, from conventional pure cultures of melanocytes grown on plastic dishes to a pigmented skin equivalent consisting of melanocytes and keratinocytes grown on a dermal equivalent. We established that the inhibitory effects of RA on melanogenesis do not result from a direct effect on melanocytes alone but also involve keratinocytes and dermal influence. These data expand our understanding of cell-to-cell signaling in cutaneous pigmentation, and strongly suggest a role for CRABP-I in mediating RA effects on melanogenesis.

Retinoic acid signaling cascade in differentiating murine epidermal keratinocytes: alterations in papilloma- and carcinoma-derived cell lines

The retinoic acid (RA) signaling pathway was investigated by transient transfection of a chloramphenicol acetyltransferase (CAT) reporter gene construct containing the RA response element (RARE) of the murine (m) RARbeta2 gene into murine primary epidermal keratinocytes (PEK), papilloma-derived SP1 cells, and carcinoma-derived 3P2 cells. Murine PEK transfected in a low-Ca2+ medium (0.05 mM Ca2+) exhibited a strong transactivation of the CATgene after exposure of the cells to 0.1 microM RA. Transactivation of the CATgene could, however, also be achieved by shifting RAREbeta2-transfected low-Ca2+ PEK to high-Ca2+ conditions (0.15-1.2 mM Ca2+). Concomitantly, the Ca2+ raise also led to the induction of both cellular retinol (ROL)-binding protein I (CRBPI) and cellular RA-binding protein II (CRABPII), whereas expression of cellular RA-binding protein I (CRABPI) was not observed. Moreover, induction of in vitro differentiation also activated the ROL-->RA converting enzyme system in PEK. These findings suggest the following sequence of events involved in the high Ca2+-mediated activation of RAREbeta2. First, high Ca2+ induces the synthesis of mCRBPI, which binds ROL released from retinyl ester stores and makes it accessible to the ROL-RA converting enzyme system. Enzymatically generated RA is taken over by mCRABPII and transported to the nucleus, where it acts as ligand for nuclear receptors, which complex with RAREbeta2 to activate the reporter gene. This hypothetical cascade of RA signaling was supported by our findings that inhibition of the ROL-->RA converting enzyme system by citral abolished the Ca2+-mediated transactivation of the CAT gene in a nontoxic manner. Studies in transformed murine cell lines revealed that Ca2+-induced activation of RAREbeta2 was essentially maintained in papilloma-derived SP1 cells, although all parameters of the Ca2+-dependent RAREbeta2 activation cascade were induced to a much lower extent. In contrast, strong RAREbeta2 activity was already observed in low-Ca2+ carcinoma-derived 3P2 cells. Low-Ca2+ 3P2 cells also expressed high levels of both mCRBPI and mCRABPII and possessed a highly active ROL-->RA converting enzyme system. Again, inhibition of the enzyme by citral abolished RAREbeta2 activity in low-Ca2+ 3P2 cells. Our data show that Ca2+-induced differentiation in cultured murine PEK entails a series of events that ultimately lead to the activation of RARE-containing genes. These properties are maintained in transformed epidermal keratinocytes. However, with increasing malignant potential of the cells, the respective signaling pathway becomes independent from a differentiation stimulus and leads to constitutive activation of RARE-controlled genes.

The isolation and characterization of purified heterocomplexes of recombinant retinoic acid receptor and retinoid X receptor ligand binding domains

Retinoic acid exerts many of its biological effects by interaction with heterocomplexes of nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). To further examine this interaction, a glutathione S-transferase (GST) fusion protein containing the ligand binding domain of human RXR alpha has been used to copurify the ligand binding domain of human RAR gamma by affinity chromatography over glutathione-agarose. Complexes of recombinant RAR-RXR ligand binding domains retaining full ligand binding capacity were purified, and their interactions with various retinoids were characterized by fluorometric titration and photoaffinity labeling. Analyses of the distribution of limiting amounts of [3H]-all-trans-retinoic acid between cytoplasmic retinoic acid binding proteins, CRABP-I and CRABP-II, and the purified heterocomplexes indicate that all-trans-retinoic acid binds with comparable affinity to CRABP-I and the heterocomplexes, but with approximately 10-fold less affinity to CRABP-II. The aromatic retinoid acitretin, which is used in the treatment of psoriasis, binds relatively poorly to the purified heterocomplexes, although it binds with high affinity to the CRABPs. Acitretin displaces [3H]-all-trans-retinoic acid from the CRABPs and increases retinoic acid occupancy of the heterocomplexes. These results suggest that certain retinoids could potentially perturb the distribution of endogenous retinoic acid between the CRABPs and the nuclear receptors and thus affect retinoid signaling. The purified recombinant complexes should provide a useful model system for further structural analysis of the dimerization interface between the RAR and RXR ligand binding domains.

An analysis of retinoic acid-induced gene expression and metabolism in AB1 embryonic stem cells

Murine embryonic stem cells such as the AB1 cell line undergo differentiation in the presence of retinoic acid (RA) into an extraembryonic epithelial cell type. This results in the activation of genes such as Hoxa-1, Hoxb-1, laminin, collagen IV(alpha1), tissue plasminogen activator, RARbeta, and CRABPII. The CRABPI gene is regulated in an unusual fashion; CRABPI message and protein levels are induced at low concentrations of RA, but induction is diminished at higher concentrations. AB1 cells take up RA rapidly from the medium, and the addition of low, exogenous concentrations of RA to the culture medium results in very high intracellular RA concentrations. For example, AB1 stem cells cultured in 5 nM [3H]RA have an internal [3H]RA concentration of 1-2 microM within the first hour. AB1 cells also metabolize [3H]RA to more polar RA derivatives. The half-life of RA in AB1 cells not previously exposed to RA is about 2-2.5 h versus 40-45 min in cells cultured for 2-3 days in 1 microM exogenous RA. Thus, the enzyme(s) which metabolize RA are induced or activated by RA. Furthermore, the local concentration of RA required to elicit some biological responses may be higher than previously thought.

Increased concentrations of 3,4-didehydroretinol and retinoic acid-binding protein (CRABPII) in human squamous cell carcinoma and keratoacanthoma but not in basal cell carcinoma of the skin

Retinoids are biologic response modifiers that are present in normal skin and may possibly be perturbed in carcinogenesis. To examine this possibility in human skin, we analyzed vitamin A and cytosolic retinoid binding proteins (cellular retinol binding protein and cellular retinoic acid binding protein [CRABP]) in a total of 38 non-melanoma skin tumors and 25 healthy skin samples using high performance liquid chromatography, radioligand electrophoresis, and reverse transcriptase-polymerase chain reaction. The mean +/- SEM retinol concentration was normal in basal cell carcinoma (0.60 +/- 0.10 microM) and seborrheic keratosis (0.47 +/- 0.07 microM), but increased in keratoacanthoma (1.60 +/- 0.41 microM) and squamous cell carcinoma (1.17 +/- 0.28 microM) (p < 0.05 for both). Also, the concentrations of 3,4-didehydroretinol, a major vitamin A metabolite produced in human skin, were markedly elevated (6-7 times normal) in keratoacanthoma and squamous cell cancer. All types of tumors showed moderately increased levels of cellular retinol binding protein. In addition, keratoacanthoma and squamous cell cancer showed markedly increased levels (6-7 times normal) of CRABPII protein. Transcriptional activity of the CRABPII gene was demonstrated in both normal and neoplastic epidermis, but clear CRABPI mRNA expression was found only in basal cell carcinoma. The data indicate that characteristic perturbations of the vitamin A and retinoid binding protein levels occur in squamous cell-derived skin tumors, but whether these reflect intrinsic errors in retinoid metabolism or are secondary to abnormal cellular differentiation is unknown.

Ichthyosiform dermatosis with superficial blister formation and peeling: evidence for a desmosomal anomaly and altered epidermal vitamin A metabolism

We describe a man with generalized congenital ichthyosiform dermatosis, severe cheilitis, and palmar and plantar hyperkeratosis with superficial blistering. Low-dose acitretin therapy induced areas of peeling skin, similar to that seen in the peeling skin syndrome. Histologically, the skin was moderately hyperkeratotic and the palmar blisters were subcorneal. Electron microscopy revealed that the splitting occurred within the desmosomal plaque. Ultrastructural and biochemical investigations indicated epidermal hypervitaminosis A, probably related to alteration of epidermal retinoic acid metabolism. This disease is proposed as a hitherto unreported variant of the peeling skin syndrome.

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.

A role for interleukin-1 in epidermal differentiation: regulation by expression of functional versus decoy receptors

Although human epidermis contains levels of interleukin-1 (IL-1) up to 100 times higher than other tissues, the role of this cytokine in epidermal biology is unknown. Here, we show that interleukin-1 regulates the expression of mRNAs for two proteins associated with the differentiated phenotype of human keratinocytes, cellular retinoic acid-binding protein type II (CRABP II) and small, proline rich protein 1 (SPRR1). The ability of IL-1 to induce these transcripts correlates directly with keratinocyte expression of the IL-1 receptor type I (IL-1 RI) during differentiation and inversely with the expression of the type II IL-1 receptor (IL-1 RII), shown in other cell types to be a nonfunctional, decoy receptor. Furthermore, addition to keratinocyte cultures of an IL-1 RI-blocking, but not an IL-1 RII-blocking, antibody reduces the levels of CRABP II and SPRR1 mRNAs in these cells. These data suggest that epidermal IL-1 functions to promote keratinocyte differentiation and that a change in the IL-1 receptor profile of these cells initiates this IL-1 response through a relative enhanced expression of functional IL-1 receptors.

Gene expression of retinoic acid receptors and cellular retinoic acid-binding proteins in rhino and hairless mouse skin

The rhino mouse comedolytic model and the hairless mouse photoaging model are established animal models for screening the in vivo activity of retinoids. However, the expression of the retinoic acid receptors (RARs) and cellular retinoic acid-binding proteins (CRABPs), known to regulate retinoid activity, is not completely understood in these mouse mutants. For this purpose, mRNA was isolated from rhino and hairless mouse skin and the gene expression of the RARs and CRABPs was measured by Northern blot hybridization. Results showed that RAR gamma was the predominantly expressed RAR in both mouse strains. Two isoforms of RAR gamma, RAR gamma 1 and RAR gamma 2, were detected with RAR gamma 1 being the more strongly expressed. RAR alpha was also detected, but to a lesser degree than RAR gamma. RAR beta expression was not detectable by our methodology. Additionally, topical treatment of these mice with 0.1% all-trans-retinoic acid (tRA) cream resulted in no significant alteration in the expression of the RAR genes. By contrast, CRABP-II was induced 2-4 fold by topical tRA treatment. CRABP-I, expressed to a lesser degree than CRABP-II, was not inducible. The relative expression of the RARs, CRABPs, and inducibility of CRABP-II by tRA in both rhino and hairless mouse skin paralleled that reported for human and mouse skin. These observations suggest that the altered phenotype observed in the rhino mouse most likely does not result from an altered expression level of these genes. The results also support these two animals as models for evaluating the therapeutic potential of retinoids.

Regulation of CRABP II mRNA expression in human keratinocytes

Cultured human neonatal keratinocytes were used to study the mechanisms and factors involved in the regulation of CRABP II gene expression. Post-confluent, relatively differentiated keratinocyte cultures had higher levels of CRABP II mRNA, but nuclear run-on experiments detected no sustained increase in CRABP II gene transcription rate between pre-confluent and post-confluent cells. Also, our studies could detect no change in the long half-life (> 32 hours) of this message in pre- and post-confluent cultures. Hydrocortisone was found to reduce the confluency-related increase in CRABP II mRNA in keratinocyte cultures. Because corticosteroids are known to reduce the effect of various cytokines, a series of epidermal cytokines were examined for a modulating effect on CRABP II mRNA content in cultured keratinocytes. IL1 alpha produced the greatest increase and IL6 the strongest reduction in the level of this message in cells grown in serum-free, defined medium. These data support a role for CRABP II in the proliferation and differentiation of human keratinocytes and suggest that epidermal cytokines may at least in part regulate the expression of the CRABP II gene at the mRNA level.

Mice deficient in cellular retinoic acid binding protein II (CRABPII) or in both CRABPI and CRABPII are essentially normal

We have disrupted the CRABPII gene using homologous recombination in embryonic stem cells, and shown that this disruption results in a null mutation. CRABPII null mutant mice are essentially indistinguishable from wild-type mice as judged by their normal development, fertility, life span and general behaviour, with the exception of a minor limb malformation. Moreover, CRABPI−/−/CRABPII−/− double mutant mice also appear to be essentially normal, and both CRABPII−/− single mutant and CRABPI−/−/CRABPII−/− double mutant embryos are not more sensitive than wild-type embryos to retinoic acid excess treatment in utero. Thus, CRABPI and CRABPII are dispensable both during mouse development and adult life. Our present results demonstrate that CRABPs are not critically involved in the retinoic acid signaling pathway, and that none of the functions previously proposed for CRABPs are important enough to account for their evolutionary conservation.

The distribution of cellular retinoic acid-binding protein I (CRABPI) and cellular retinol-binding protein I (CRBPI) during molar tooth development and eruption in the rat

The distribution of cellular retinoic acid-binding protein (CRABPI) and cellular retinol binding protein (CRBPI) was studied in a series of prenatal and early postnatal rats, covering the main stages of development and eruption of the molar teeth. CRABPI positive cells were found in the mesenchymal cells of the dental follicle from the cap stage and in the dental papilla from the early bell stage. In the dental papilla, CRABPI positive cells were situated adjacent to the enamel organ in the cervical loop region and in the subodontoblastic region. Newly formed odontoblasts were CRABPI positive for a short period of time. The enamel organ was CRBPI and CRABPI negative, except for the presence of CRABPI positive cells in the internal enamel epithelium over the tip of cusps and in parts of the stratum intermedium. During root formation, CRABPI positive cells were found in the developing periodontal ligament, in the dental papilla adjacent to the epithelial root sheath and in the subodontoblastic zone. During crown formation, CRBPI positive cells were mainly localized to the mesenchymal cells of the dental papilla during the cap stage of crown development. The periosteum of the developing mandible contained CRABPI positive cells while some osteoclasts appeared to show a weak but positive reaction to CRBPI. The findings were considered in terms of the possible significance of retinoid-binding proteins during tooth and bone development.

Crystal structures of cellular retinoic acid binding proteins I and II in complex with all-trans-retinoic acid and a synthetic retinoid

BACKGROUND

Retinoic acid (RA) plays a fundamental role in diverse cellular activities. Cellular RA binding proteins (CRABPs) are thought to act by modulating the amount of RA available to nuclear RA receptors. CRABPs and cellular retinol-binding proteins (CRBPs) share a unique fold of two orthogonal beta-sheets that encapsulate their ligands. It has been suggested that a trio of residues are the prime determinants defining the high specificity of CRBPs and CRABPs for their physiological ligands.

RESULTS

Bovine/murine CRABP I and human CRABP II have been crystallized in complex with their natural ligand, all-trans-RA. Human CRABP II has also been crystallized in complex with a synthetic retinoid, 'compound 19'. Their structures have been determined and refined at resolutions of 2.9 A, 1.8 A and 2.2 A, respectively.

CONCLUSIONS

The retinoid-binding site in CRABPs differs significantly from that observed in CRBP. Structural changes in three juxtaposed areas of the protein create a new, displaced binding site for RA. The carboxylate of the ligand interacts with the expected trio of residues (Arg132, Tyr134 and Arg111; CRABP II numbering). The RA ligand is almost flat with the beta-ionone ring showing a significant deviation (-33 degrees) from a cis conformation relative to the isoprene tail. The edge atoms of the beta-ionone ring are accessible to solvent in a suitable orientation for presentation to metabolizing enzymes. The bulkier synthetic retinoid causes small conformational changes in the protein structure.

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.

Measurement of subnanomolar retinoic acid binding affinities for cellular retinoic acid binding proteins by fluorometric titration

Cellular retinoic acid binding protein I (CRABP-I) and cellular retinoic acid binding protein II (CRABP-II) are small, cytoplasmic proteins which bind all-trans-retinoic acid with high affinity. Both of these proteins belong to a family of intracellular proteins which bind amphiphilic lipids, including fatty acids, bile salts, and retinoids. Because CRABP-I and -II exhibit different tissue distributions and differential transcriptional regulation, they are proposed to serve different functions. The binding properties of mouse CRABP-I and -II purified from Escherichia coli were examined to further understand their role in intracellular retinoic acid processing. Fluorescence titrations were performed using nanomolar protein concentrations, near the obtained dissociation constants, and analyzed by direct mathematical fitting to raw data, in order to extend the range and accuracy of binding constant determination. The apparent dissociation constants, K'd, of mouse CRABP-I and CRABP-II binding all-trans-retinoic acid were determined to be 0.4 +/- 0.3 nM and 2 +/- 1 nM respectively, stronger binding than previously reported. The K'd of mCRABP-I and mCRABP-II complexing with acitretin, a pharmacologically active synthetic retinoid used in the treatment of psoriasis, was 3 +/- 1 nM and 15 +/- 11 nM. Both CRABPs bound 9-cis-retinoic acid with a K'd of roughly 200 nM, and neither exhibited significant binding of 13-cis-retinoic acid.

Differential expression of cyclophilin isoforms during keratinocyte differentiation

Cyclophilin A, the major intracellular binding protein for the immunosuppressive drug cyclosporin A (CsA), was studied in human keratinocytes during differentiation both in vivo and in vitro. Analysis of cyclophilin by gel-filtration radiobinding-assay with tritiated CsA showed one specific radioactive peak at 17 kDa. By this technique, the levels of cyclophilin (mean 55.23 +/- 8.43 pmol/mg protein) did not significantly differ during keratinocyte differentiation. When the protein extracts from calcium-induced differentiating keratinocytes and normal human skin were analysed by PAGE radiobinding-assay, two specific radioactive CsA-binding peaks were detected. The major peak (RF 0.13) was expressed in all samples (mean 47.32 +/- 17.53 pmol/mg protein) whereas the minor peak (RF 0.23) was dramatically decreased about 6-fold in abnormally differentiated skin (psoriasis) as well as in non-differentiated keratinocytes. At least six [3H]CsA-binding isoforms with pI values ranging from 5.58 to 7.75 were detected by isoelectrofocusing autoradio-blotting-assay in normal human skin; three of them immunoreacted with antibodies to cyclophilin. These results demonstrated the presence of several cyclophilin isoforms in human epidermal cells and an expression which correlated with the differentiation of human keratinocytes both in vivo and in vitro.

Gene cloning, overproduction and purification of a functionally active cytoplasmic fatty acid-binding protein (Sj-FABPC) from the human blood fluke Schistosoma japonicum

We report the gene cloning, molecular characterisation and purification of a 14.7-kDa functionally active recombinant (re) cytoplasmic fatty acid-binding protein (Sj-FABPC) from the Chinese strain of the human bloodfluke Schistosoma japonicum (Sj). As schistosomes are unable to synthesise long chain fatty acids and sterols de novo and must, therefore, take up these lipids from the host, Sj-FABPC is an attractive vaccine and/or drug target. Clone 39 (C39), which contains the entire Sj-FABPC gene, was isolated from a Sj lambda ZAPII cDNA expression library immunoscreened with hyperimmune rabbit serum (HRS) raised against soluble adult Sj proteins. The complete ORF (open reading frame) of Sj-FABPC encodes a protein of 132 amino acids (aa) of 14.7 kDa. The aa sequence of Sj-FABPC exhibits 91% identity to a FABP of S. mansoni (Sm14) and 45% identity to a FABP of Fasciola hepatica (Fh15), putative vaccine candidates for schistosomiasis. Sj-FABPC was subcloned into the QIAexpress vector, pQE-10, and subsequently expressed in Escherichia coli. The re-Sj-FABPC, purified under non-denaturing conditions, was recognized by sera from patients with acute and chronic schistosomiasis japonica. The purified re-Sj-FABPC was also shown to bind to palmitic acid with high affinity. The functional expression of Sj-FABPC will facilitate studies on re-Sj-FABPC to assess its potential as a drug and/or vaccine candidate.

Distribution of cellular retinoic acid-binding proteins I and II in the chick embryo and their relationship to teratogenesis

The distribution of cellular retinoic acid-binding proteins I and II (CRABP I and II) during the first 6 days of chick development has been investigated using immunoblotting. Since retinoic acid (RA) is teratogenic to some parts of the embryo, stimulatory to other parts, and has no effect on others it may be that the distribution of cytoplasmic proteins such as CRABP I and II plays some role in this differential activity. Neither protein is expressed in the day 2 embryo, but from day 3 onwards both proteins are expressed and CRABP I is in considerable excess over CRABP II. Within the day 4 embryo there is some significant variation in the distribution according to tissue type. Neural tissues, neural crest derivatives, and limb buds most strongly express CRABP I whilst other tissues contain only moderate levels, and heart and epidermis do not express CRABP I at all. CRABP II has a widespread distribution, although at a lower level than CRABP I, with the exception of somites and ectoderm which do not express it at all. In the limb buds, there is a significant variation in CRABP I levels across the anteroposterior axis which suggests that these two CRABPs may have different functions during development. The relationship of these distributions in the embryo to the role of endogenous RA and the teratogenic effects of RA is discussed.

Retinoids and the skin

Retinoids are a group of naturally occurring and synthetic compounds with vitamin A-like biological activity. They play an important role in vision, reproduction, growth, and epithelial cell differentiation. Recent discoveries of specific retinoid cellular binding proteins and nuclear receptors have led to a better (although not complete) understanding of the complex mechanisms of retinoid action. Numerous clinical studies have demonstrated beneficial effects of retinoids on skin diseases such as acne, psoriasis, ichthyoses, keratodermas, skin cancers and their precursors, as well as a reversal of the effects of photoaging.

Purification and characterization of the human epidermal fatty acid-binding protein: localization during epidermal cell differentiation in vivo and in vitro

Epidermal fatty acid-binding protein (E-FABP) was isolated from human skin and purified to homogeneity. Its molecular mass was estimated to be 15 kDa and the pI of non-denaturing protein was 5.6. Scatchard-plot analysis revealed one class of binding site for oleic acid with a Kd of 0.46 microM. Structure-binding relation experiments revealed a high affinity of E-FABP for stearic acid which decreased on reduction of the number of carbon atoms or introduction of double bonds into the fatty acid chain. Squalene, cholesterol and retinoic acid isomers showed no affinity, suggesting that E-FABP displays high specificity for fatty acids. E-FABP is a scarce cytosolic protein (0.065% of total protein). Only trace amounts could be detected in normal human skin but up to 42.5 +/- 3.4 pmol/mg of protein was found in a non-malignant defect of keratinocyte differentiation (psoriatic lesions). E-FABP levels were low in cultured human keratinocytes grown under proliferation-stimulating conditions but increased about 2-fold on induction of differentiation by Ca2+. Immunohistochemical localization showed cytosolic staining in differentiated cells of normal and psoriatic skin, suggesting a link between E-FABP and keratinocyte differentiation. The presence of E-FABP in tissues other than skin (heart, intestine and adipose tissue) excludes its specific role in fatty acid metabolism in epithelial cells or its involvement in skin lipid-barrier function.

Expression of cytosolic retinoid-binding protein genes in human skin biopsies and cultured keratinocytes and fibroblasts

Using reverse transcription coupled to polymerase chain reaction we have studied the mRNA expression of serum retinol-binding protein and cytosolic receptors for retinol and retinoic acid in skin biopsies, and in cultured epidermal keratinocytes and dermal fibroblasts. Transcripts for cellular retinol-binding protein (CRBP) I and cellular retinoic-acid-binding protein (CRABP) I were found in normal skin, keratinocytes, and fibroblasts. CRABP II transcripts were detected in skin and keratinocytes. A decreased mRNA expression of CRABP I and an increased mRNA expression of CRABP II were found in lesional psoriatic skin compared with uninvolved skin. mRNA transcripts for serum retinol-binding protein (s-RBP) were detected in all tissues and cells. The biological importance of s-RBP expression in keratinocytes and fibroblasts is not known, but hypothetically this protein may be involved in the intracellular shuttling of retinol and retinoic acid, or in the retransportation of cellular retinoids into the extracellular space.

Topical glucocorticosteroids modulate the expression of CRABP I and II in human skin differently

Epidermal cells express two retinotic acid-binding proteins (CRABP I and II). Because CRABP II protein is strongly induced by topical retinoic acid, the respective roles of the two proteins in the pharmacological activity and toxicity of topical retinoids deserve particular attention. Since topical steroids diminish the irritation induced by retinoic acid (RA), whereas retinoic acid may counteract the atrophogenic effects of steroids, the possible interplay of both compounds in the expression of CRABP I and II appeared worth studying. We have analyzed the effects of topical application of triamcinolone acetonide (TA) on the retinoic acid-induced altered expression of CRABP I and II in normal human skin, at the protein and mRNA levels. We found that CRABP II protein and mRNA were strongly increased upon retinoic acid application: this induction was significantly inhibited by concomitant application of triamcinolone acetonide; a more potent steroid, difluocortolone valerate, was also found to diminish normal endogenous expression of CRABP II. In contrast, CRABP I protein was decreased by topical retinoic acid, and the down modulating effect of retinoic acid was counteracted by triamcinolone acetonide.

Cellular retinoic acid-binding proteins (CRABP)

Mammalian cell cytoplasm contains at least two proteins which bind retinoic acid (RA): CRABP I and CRABP II. Produced by separate genes, they differ in their affinity for RA, their transcriptional regulation by RA, their tissue distribution, and possibly their function. They intervene, probably at different stages, in the "intracine" metabolic process which controls the amount of biologically active ligand that is available for binding to the nuclear receptors of RA.

The distribution of cellular retinoic acid-binding protein I during odontogenesis in the rat incisor

Retinoids are important molecules in various aspects of embryological development. Here the distribution of cellular retinoic acid-binding protein I (CRABPI) was studied in the continuously growing incisor of adult rats using an affinity-purified rabbit polyclonal antibody. CRABPI was present throughout the presecretory and secretory ameloblast layer. The protein disappeared from that layer during its maturation phase. The adjacent dental mesenchyme of the developing pulp stained positively for CRABPI, especially in the layer immediately beneath the fully differentiated odontoblasts. Little CRABPI was present in the odontoblast layer itself. The distribution of CRABPI, both in the undifferentiated basal region of the incisor tooth and associated with the cells during hard-tissue formation, suggests a role for this molecule during differentiation and hard-tissue genesis.