Differentiation of human epidermal keratinocytes is accompanied by increased expression of CRABP-II and increased cellular concentration of retinoic acids: retention of newly synthesized retinoic acids by CRABP-II

Correlation of Orexin-A and brain-derived neurotrophic factor levels in metabolic syndrome and cognitive impairment in schizophrenia treated with clozapine

Orexin-A and brain-derived neurotrophic factor (BDNF) are implicated in regulating metabolic syndrome (MetS) and cognitive impairment of schizophrenia. However, the associations among them remains unclear. Here, we aimed to investigate the relationship between Orexin-A levels, BDNF, MetS, clinical symptom profile, and cognitive function in schizophrenia patients following long-term clozapine treatment. We measured Orexin-A and BDNF levels in 140 schizophrenia patients with and without MetS. We assessed clinical symptoms on the Positive and Negative Syndrome Scale and cognitive function by the assessment of Neuropsychological Status (RBANS), and examined their associations with Orexin-A. Patients with MetS had significantly lower Orexin-A levels and higher coding test, attention span and delayed retention in RBANS (P < 0.05). Correlation analysis showed that Orexin-A was associated with BDNF, TG, HDLC, PANSS active social avoidance and emotional withdrawal significantly. Besides, Orexin-A significantly interacted with BDNF for metabolic and cognitive profiles including waist circumference, delayed retention and list recognition. Logistic regression analysis showed that Orexin-A level (odds ratio [OR]= 0.380, 95% confidence interval [CI]: 0.151-0.952, P = 0.039) and total illness duration (OR = 0.932, 95% CI: 0.875-0.991, P = 0.025) were predictive variables of MetS. However, there was no significant relationship between Orexin-A and cognitive function after adjustment for age, sex and educational levels. Totally, a lower plasma Orexin-A level seems to be related to metabolic parameters more than cognitive profiles. The interaction of Orexin-A with BDNF may be partly responsible for worse MetS and better cognition of elderly schizophrenia, but the causal relationship needs further clarification.

Retinol dehydrogenase 10 but not retinol/sterol dehydrogenase(s) regulates the expression of retinoic acid-responsive genes in human transgenic skin raft culture

Retinoic acid is essential for skin growth and differentiation, and its concentration in skin is controlled tightly. In humans, four different members of the short-chain dehydrogenase/reductase (SDR) superfamily of proteins were proposed to catalyze the rate-limiting step in the biosynthesis of retinoic acid (the oxidation of retinol to retinaldehyde). Epidermis contains at least three of these enzymes, but their relative importance for retinoic acid biosynthesis and regulation of gene expression during growth and differentiation of epidermis is not known. Here, we investigated the effect of the four human SDRs on retinoic acid biosynthesis, and their impact on growth and differentiation of keratinocytes using organotypic skin raft culture model of human epidermis. The results of this study demonstrate that ectopic expression of retinol dehydrogenase 10 (RDH10, SDR16C4) in skin rafts dramatically increases proliferation and inhibits differentiation of keratinocytes, consistent with the increased steady-state levels of retinoic acid and activation of retinoic acid-inducible genes in RDH10 rafts. In contrast, SDRs with dual retinol/sterol substrate specificity, namely retinol dehydrogenase 4 (RoDH4, SDR9C8), RoDH-like 3α-hydroxysteroid dehydrogenase (RL-HSD, SDR9C6), and RDH-like SDR (RDHL, SDR9C4) do not affect the expression of retinoic acid-inducible genes but alter the expression levels of several components of extracellular matrix. These results reveal essential differences in the metabolic contribution of RDH10 versus retinol/sterol dehydrogenases to retinoic acid biosynthesis and provide the first evidence that non-retinoid metabolic products of retinol/sterol dehydrogenases affect gene expression in human epidermis.

Keratinocyte differentiation induced by calcium, phorbol ester or interferon-gamma elicits distinct changes in the retinoid signalling pathways

BACKGROUND

Retinoids influence keratinocyte proliferation and differentiation via binding to nuclear retinoic acid receptors (RARalpha, -gamma) and retinoid X receptor alpha (RXRalpha). The effect of keratinocyte differentiation on expression of nuclear retinoid receptors and on the conversion of retinol into retinoic acid has not been examined earlier in depth.

OBJECTIVES

Our aim was to examine the expression of retinoid receptors and a retinoid-regulated gene CRABPII, as well as the metabolism of exogenous [(3)H]retinol in cultured human keratinocytes induced to differentiate by exposure to either calcium, phorbol 12-myristate 13-acetate (PMA), or interferon-gamma (IFNgamma).

METHODS

Normal human keratinocytes were cultured and exposed to differentiation-inducing agents. The mRNA and protein expression of retinoid receptors were examined using real-time PCR and Western blot. [(3)H]Retinol uptake and metabolism was monitored by HPLC with on-line radioactivity detection.

RESULTS

In calcium-exposed cells, increased expression of RARgamma and RXRalpha, enhanced metabolism of [(3)H]retinol to 3,4-didehydro-RA (ddRA), and an induction of CRABPII mRNA and protein was noted. In contrast, treatment with PMA and IFNgamma reduced the RARgamma and RXRalpha protein expression (preventable by the proteasome inhibitor MG132), increased the accumulation of [(3)H]RA and/or [(3)H]ddRA in the cells, and changed the CRABPII transcription.

CONCLUSIONS

Retinoid signalling is profoundly altered upon differentiation of keratinocytes and the effects depend on how cellular differentiation is initiated.

Immunolocalization of enzymes, binding proteins, and receptors sufficient for retinoic acid synthesis and signaling during the hair cycle

Retinoic acid (RA) is essential for maintenance of most epithelial tissues. One RA biosynthesis pathway consists of cellular retinol-binding protein (Crbp), retinol dehydrogenase (Dhrs9/eRoldh), retinal dehydrogenase 1-3 (Aldh1a1-3), and cellular RA-binding protein 2 (Crabp2). Previously, we localized Aldh1a2 and Aldh1a3 to both epithelial and mesenchymal cells within the hair follicle throughout the hair cycle. This study expands that observation by examining the complete pathway of RA biosynthesis and signaling via RA receptors alpha, beta, and gamma by immunohistochemistry in C57BL/6J mice wax-stripped to initiate and synchronize the cycle. This pathway of RA biosynthesis and signaling localized to the majority of layers of the hair follicle, sebaceous gland, and interfollicular epidermis in a hair cycle-dependent manner, suggesting that RA biosynthesis within the hair follicle is regulated in both a spatial and temporal manner. This localization pattern also revealed insights into epithelial-mesenchymal interactions and differentiation state differences within the RA biosynthesis and signaling pathway, as well as novel observations on nuclear versus cytoplasmic localization of Crabp2 and RA receptors. This complex pattern of RA biosynthesis and signaling identified by immunolocalization suggests that endogenous RA regulates specific aspects of hair follicle growth, differentiation, and cycling.

Dynamic changes in nicotinamide pyridine dinucleotide content in normal human epidermal keratinocytes and their effect on retinoic acid biosynthesis

The function of many enzymes that regulate metabolism and transcription depends critically on the nicotinamide pyridine dinucleotides. To understand the role of NAD(P)(H) in physiology and pathophysiology, it is imperative to estimate both their amount and ratios in a given cell type. In human epidermis and in cultured epidermal keratinocytes, we found that the total dinucleotide content is in the low millimolar range. The dinucleotide pattern changes during proliferation and maturation of keratinocytes in culture. Differences in the concentrations of NAD(P)(H) of 1.5- to 12-fold were observed. This resulted in alteration of the NAD(P)H/NAD(P) ratio, which could impact the differential regulation of both transcriptional and metabolic processes. In support of this notion, we provide evidence that the two-step oxidation of retinol to retinoic acid, a nuclear hormone critical for epidermal homeostasis, can be regulated by the relative physiological amounts of the pyridine dinucleotides.

Unique domain functions of p63 isotypes that differentially regulate distinct aspects of epidermal homeostasis

p63 is critical for squamous development and exists as multiple isotypes of two subclasses, TA and DeltaN. DeltaNp63 isotypes can antagonize transcription by TAp63 and p53, and are highly expressed in squamous cell cancers. Using mouse keratinocytes as a biological model of squamous epithelium, we show that multiple p63 isotypes, DeltaN- and TA-containing, are expressed and differentially modulated during in vitro murine keratinocyte differentiation. DeltaNp63alpha declines with Ca2+-induced differentiation, while a smaller DeltaN-form, DeltaNp63s, persists, suggesting unique functions of the two DeltaN-forms. To investigate the impact of dysregulated p63 expression that is observed in cancers and to define the biological contribution of the different domains of the p63 isotypes, DeltaNp63alpha, DeltaNp63p40, TAp63alpha, TAp63gamma or beta-galactosidase were overexpressed in primary murine keratinocytes. Microarray, RT-PCR and western blot analyses revealed that overexpression of DeltaNp63p40, which lacks the entire alpha-tail present in DeltaNp63alpha, permits expression of a full panel of differentiation markers. This is in contrast to overexpression of the full-length DeltaNp63alpha, which blocks induction of keratin 10, loricrin and filaggrin. These findings support a role for the alpha-tail of DeltaNp63alpha in blocking differentiation-specific gene expression. Overexpression of either TAp63 isotype permits keratin 10 and loricrin expression, thus the alpha-terminus requires the cooperation of the DeltaN domain in blocking early differentiation. However, both TA isotypes block filaggrin induction. The DeltaN-terminus is sufficient to maintain keratinocytes in a proliferative state, as both DeltaN forms block Ca2+-mediated p21WAF1 induction and S-phase arrest, while sustaining elevated PCNA levels. No alteration in cell cycle regulation was observed in keratinocytes overexpressing TAp63alpha or TAp63gamma. Clarifying the functional distinctions between p63 isotypes and domains will help to elucidate how their dysregulation impacts tumor biology and may suggest novel therapeutic strategies for modulating behavior of tumor cells with altered expression of p53 family members.

Retinoic acid imprints gut-homing specificity on T cells

For a preferential homing of T cells to the gut, expression of the integrin alpha4beta7 and the chemokine receptor CCR9 is essential and is induced by antigenic stimulation with dendritic cells from the gut-associated lymphoid organs. Here, we show that the vitamin A (retinol) metabolite, retinoic acid, enhances the expression of alpha4beta7 and CCR9 on T cells upon activation and imprints them with the gut tropism. Dendritic cells from the gut-associated lymphoid organs produced retinoic acid from retinol. The enhanced alpha4beta7 expression on T cells by antigenic stimulation with these dendritic cells was suppressed by the retinal dehydrogenase inhibitor citral and the retinoic acid receptor antagonist LE135. Accordingly, vitamin A deficiency caused a reduction in alpha4beta7(+) memory/activated T cells in lymphoid organs and a depletion of T cells from the intestinal lamina propria. These findings revealed a novel role for retinoic acid in the imprinting of gut-homing specificity on T cells.

Transient activation of FOXN1 in keratinocytes induces a transcriptional programme that promotes terminal differentiation: contrasting roles of FOXN1 and Akt

The forkhead transcription factor FOXN1 is required for normal cutaneous and thymic epithelial development. Mutations in FOXN1 give rise to the nude phenotype in mice, rats and man. However, the genes that are regulated by FOXN1 are unknown. To investigate FOXN1 function we expressed an inducible form of the protein, FOXN1ER, that is activated by 4-hydroxytamoxifen in primary human epidermal keratinocytes. Transient activation of FOXN1 decreased the proportion of keratinocytes that formed actively growing clones attributable to stem cell founders and increased the number of abortive clones, without inducing apoptosis. Within 24 hours the majority of cells had initiated terminal differentiation, as assessed by involucrin expression. We performed a cDNA microarray experiment to analyse changes in the transcription of approximately 6000 genes. Following FOXN1 activation we detected increases of two fold or greater in the RNA levels of over 30 genes. Genes promoting growth arrest, survival and differentiation featured prominently and markers of early events in keratinocyte differentiation were also detected. Since one of the induced genes was Akt we investigated whether Akt played a role in terminal differentiation. Activation of PI 3-kinase but not Akt was necessary for FOXN1-induced differentiation. In reconstituted epidermis FOXN1 promoted early stages of terminal differentiation whereas Akt activation was sufficient to induce late stages, including formation of the cornified layers. These results establish a role for FOXN1 in initiation of terminal differentiation and implicate Akt in subsequent events.

The action of a dietary retinoid on gene expression and cancer induction in electron-irradiated rat skin

Current models of radiation carcinogenesis generally assume that the DNA is damaged in a variety of ways by the radiation and that subsequent cell divisions contribute to the conversion of the damage to heritable mutations. Cancer may seem complex and intractable, but its complexity provides multiple opportunities for preventive interventions. Mitotic inhibitors are among the strongest cancer preventive agents, not only slowing the growth rate of preneoplasias but also increasing the fidelity of DNA repair processes. Ionizing radiation, including electrons, is a strong inducer of cancer in rat skin, and dietary retinoids have shown potent cancer preventive activity in the same system. A non-toxic dietary dose of retinyl acetate altered gene expression levels 24 hours after electron irradiation of rat skin. Of the 8740 genes on an Affymetrix rat expression array, the radiation significantly (5 fold or higher) altered 188, while the retinoid altered 231, including 16 radiation-altered genes that were reversely altered. While radiation strongly affected the expression of stress response, immune/inflammation and nucleic acid metabolism genes, the retinoid most strongly affected proliferation-related genes, including some significant reversals, such as, keratin 14, retinol binding protein, and calcium binding proteins. These results point to reversal of proliferation-relevant genes as a likely basis for the anti-radiogenic effects of dietary retinyl acetate.

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.

Epidermal growth factor signaling pathway influences retinoid metabolism by reduction of retinyl ester hydrolase activities in normal and malignant keratinocytes

The effects of EGFR signaling on retinol metabolism were evaluated in the squamous cell carcinoma cell lines defective in LRAT. In a 24-h incubation, the presence of EGF resulted in a 20-25% increase in retinyl ester accumulation. Assessment of retinol esterification and retinyl ester utilization (hydrolysis), in cell cultures and in cell homogenates, revealed that the increase in retinyl ester mass was the result of a reduction in retinyl ester hydrolysis. When grown in the absence of EGF, the cultures used about 40% of their retinyl esters, compared to about 21% in cultures grown with EGF. This effect of EGF was blocked by an EGF receptor-neutralizing antibody, an EGF receptor tyrosine-kinase inhibitor (PD153035), and a specific inhibitor of MEK kinase influencing the mitogen-activated protein kinase (MAPK) cascade (PD98059). Both transcription and translation were required, suggesting that signaling from the EGF receptor through the MAPK cascade controls the expression of modulators or inhibitors of the retinyl ester hydrolase(s). Thus EGFR signaling can alter the intracellular concentration of retinol by suppressing the access to the retinyl ester pool. Similar EGF effects were seen in cultures of normal keratinocytes.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

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

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

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

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