Expression of retinoid-X receptors (-alpha,-beta,-gamma) and retinoic acid receptors (-alpha,-beta,-gamma) in normal human skin: an immunohistological evaluation

Cosmetic retinoid use in photoaged skin: A review of the compounds, their use and mechanisms of action

The inevitable attrition of skin due to ultraviolet radiation, termed photoaging, can be partially restored by treatment with retinoid compounds. Photoaged skin in lightly pigmented individuals, clinically presents with the appearance of wrinkles, increased laxity, and hyper- and hypopigmentation. Underlying these visible signs of ageing are histological features such as epidermal thinning, dermal-epidermal junction flattening, solar elastosis and loss of the dermal fibrillin microfibrillar network, fibrillar collagen and glycosaminoglycans. Retinoid compounds are comprised of three main generations with the first generation (all-trans retinoic acid, retinol, retinaldehyde and retinyl esters) primarily used for the clinical and cosmetic treatment of photoaging, with varying degrees of efficacy, tolerance and stability. All-trans retinoic acid is considered the 'gold standard' for skin rejuvenation; however, it is a prescription-only product largely confined to clinical use. Therefore, retinoid derivatives are readily incorporated into cosmeceutical formulations. The literature reported in this review suggests that retinol, retinyl esters and retinaldehyde that are used in many cosmeceutical products, are efficacious, safe and well-tolerated. Once in the skin, retinoids utilize a complex signalling pathway that promotes remodelling of photoaged epidermis and dermis and leads to the improvement of the cutaneous signs of photoaging.

Skin as an endocrine organ: A narrative review

Skin being the largest organ of the body, is equipped with numerous functional properties. Over the past few years, intricate research into the biology of skin has led to a gamut of discoveries. Skin is now regarded as one of the most vital endocrine organs. The skin contains equivalents of the hypothalamo-pituitary-adrenal axis, hypothalamo-pituitary-thyroid axis and the appendages produce multiple hormones such as Vitamin D, sex steroids, retinoids and opioids. In this article, we will explore the role of skin as a target and source of some of the hormones of the human body, and briefly touch on the clinical applications.

Current Strategies for the Gene Therapy of Autosomal Recessive Congenital Ichthyosis and Other Types of Inherited Ichthyosis

Mutations in genes such as transglutaminase-1 (TGM1), which are responsible for the formation and normal functioning of a lipid barrier, lead to the development of autosomal recessive congenital ichthyosis (ARCI). ARCIs are characterized by varying degrees of hyperkeratosis and the presence of scales on the body surface since birth. The quality of life of patients is often significantly affected, and in order to alleviate the manifestations of the disease, symptomatic therapy with moisturizers, keratolytics, retinoids and other cosmetic substances is often used to improve the condition of the patients' skin. Graft transplantation is commonly used to correct defects of the eye. However, these approaches offer symptomatic treatment that does not restore the lost protein function or provide a long-term skin barrier. Gene and cell therapies are evolving as promising therapy for ARCIs that can correct the functional activity of altered proteins. However, these approaches are still at an early stage of development. This review discusses current studies of gene and cell therapy approaches for various types of ichthyosis and their further prospects for patient treatment.

Topical Vitamin D Receptor Antagonist/Partial-Agonist Treatment Induces Epidermal Hyperproliferation via RARγ Signaling Pathways

Vitamin D and A derivatives are well-known endogenous substances responsible for skin homeostasis. In this study we topically treated shaved mouse skin with a vitamin D agonist (MC903) or vitamin D antagonist/partial agonist (ZK159222) and compared the changes with acetone (control treatment) treatment for 14 days. Topical treatment with ZK159222 resulted in increased expression of genes involved in retinoic acid synthesis, increased retinoic acid concentrations and increased expression of retinoid target genes. Clustering the altered genes revealed that heparin-binding epidermal growth factor-like growth factor, the main driver of epidermal hyperproliferation, was increased via RARγ-mediated pathways, while other clusters of genes were mainly decreased which were comparable to the changes seen upon activation of the RARα-mediated pathways. In summary, we conclude that epidermal hyperproliferation of mouse skin in response to a topically administered vitamin D receptor antagonist/partial agonist (ZK159222) is induced via increased retinoic acid synthesis, retinoic acid levels and increased RARγ-mediated pathways.

The nutritional biochemistry of wool and hair follicles

The rôle of various classes of nutrients (energy substrates, vitamins, minerals, amino acids) in the production of wool and hair from follicles, is considered for a variety of animal species. The wool and hair follicle have evolved a number of interesting features of carbohydrate metabolism including glutaminolysis, aerobic glycolysis, significant activity of the pentose phosphate pathway, and storage and mobilisation of glycogen. Presumably the necessity to continue to produce fibre despite fluctuations in the supply of oxygen and nutrients has resulted in some of these unique features, while others reflect the high level of DNA and protein synthesis occurring in the follicle. While it is considered that energy does not normally limit fibre growth, the relative contributions of aerobic and anerobic metabolism will greatly influence the amount of ATP available for follicle activity, such that energy availability may at times alter fibre growth. Alopecia and deficient fibre growth are consistent outcomes of deficiencies of biotin, riboflavin, pyridoxine, folate and pantothenic acid, but the precise rôles of these vitamins in follicle function await elucidation. Folate, in particular appears to play an important rôle in wool production, presumably reflecting its involvement in methionine metabolism. Cholecalciferol (vitamin D) significantly alters fibre growth in cultured follicles; vitamin D receptors are located in the outer root sheath, bulb, and dermal papilla of the follicle; and alopecia occurs in humans with defects in the vitamin D receptor. Retinol (vitamin A), too, appears to influence follicle function by altering keratinocyte proliferation and differentiation, with direct effects on the expression of keratin genes. The receptors for the retinoids are present in the keratogenous zone, the outer root sheath, the bulb, and the sebaceous glands. Vitamin A may also act indirectly on follicle function by influencing the activity of the insulin-like and epidermal growth factors and by altering vitamin D activity. At present there is little evidence implicating alpha-tocopherol (vitamin E) or phytylmenaquinone (vitamin K) in follicular events. Of the minerals, only copper and zinc have been shown to have direct effects on follicle function, independent of effects on food intake. Copper has direct effects on the activity of an unidentified enzyme on oxidation of thiol groups to form disulphide linkages. Wool produced by copper-deficient sheep lacks crimp, is weak and lustrous. Copper is also necessary for the activity of tyrosinase and the tyrosinase-related proteins involved in melanin synthesis. Zinc, like copper, is required for the normal keratinization of fibres but again, the precise rôle has yet to be elucidated. While the importance of amino acid supply for wool growth has long been established, there are still some unaswered questions such as; what are the effects of amino acids on fibre growth in animals other than sheep; what are the characteristics of the amino acid transport genes and proteins operating in the wool and hair follicle; and what are the specific rôles for amino acids in follicle function.

The effect of two endogenous retinoids on the mRNA expression profile in human primary keratinocytes, focusing on genes causing autosomal recessive congenital ichthyosis

Retinoids (natural forms and synthetic derivatives of vitamin A) are used as therapeutic agents for numerous skin diseases such as keratinization disorders (e.g. ichthyoses) and psoriasis. Two endogenous ligands for retinoic acid receptors exist, retinoic acid (atRA) and 3,4-didehydroretinoic acid (ddRA). In primary human epidermal keratinocytes many transcriptional targets for atRA are known, whereas the targets for ddRA are unknown. In an attempt to determine the targets, we compared the effect of atRA and ddRA on transcriptional profiles in undifferentiated and differentiating human primary keratinocytes. First, as expected, many genes were induced or suppressed in response to keratinocyte differentiation. Furthermore, the two retinoids affected substantially more genes in differentiated keratinocytes (>350) than in proliferating keratinocytes (≈20). In differentiating keratinocytes markers of cornification were suppressed suggesting a de-differentiating effect by the two retinoids. When comparing the expression profile of atRA to that of ddRA, no differently regulated genes were found. The array analysis also found that a minor number of miRNAs and a large number of non-coding transcripts were changed during differentiation and in response to the two retinoids. Furthermore, the expression of all, except one, genes known to cause autosomal recessive congenital ichthyosis (ARCI) were found to be induced by differentiation. These results comprehensively document that atRA and ddRA exert similar transcriptional changes in keratinocytes and also add new insights into the molecular mechanism influenced by retinoids in the epidermis. Furthermore, it suggests which ARCI patients could benefit from therapy with retinoids.

The important role of epidermal triacylglycerol metabolism for maintenance of the skin permeability barrier function

Survival in a terrestrial, dry environment necessitates a permeability barrier for regulated permeation of water and electrolytes in the cornified layer of the skin (the stratum corneum) to minimize desiccation of the body. This barrier is formed during cornification and involves a cross-linking of corneocyte proteins as well as an extensive remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by various hydrolytic enzymes generates ceramides, cholesterol, and non-esterified fatty acids for the extracellular lipid lamellae in the stratum corneum. However, the important role of epidermal triacylglycerol (TAG) metabolism during formation of a functional permeability barrier in the skin was only recently discovered. Humans with mutations in the ABHD5/CGI-58 (α/β hydrolase domain containing protein 5, also known as comparative gene identification-58, CGI-58) gene suffer from a defect in TAG catabolism that causes neutral lipid storage disease with ichthyosis. In addition, mice with deficiencies in genes involved in TAG catabolism (Abhd5/Cgi-58 knock-out mice) or TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2, Dgat2 knock-out mice) also develop severe skin permeability barrier dysfunctions and die soon after birth due to increased dehydration. As a result of these defects in epidermal TAG metabolism, humans and mice lack ω-(O)-acylceramides, which leads to malformation of the cornified lipid envelope of the skin. In healthy skin, this epidermal structure provides an interface for the linkage of lamellar membranes with corneocyte proteins to maintain permeability barrier homeostasis. This review focuses on recent advances in the understanding of biochemical mechanisms involved in epidermal neutral lipid metabolism and the generation of a functional skin permeability barrier. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Endogenous retinoids in the hair follicle and sebaceous gland

Vitamin A and its derivatives (retinoids) are critically important in the development and maintenance of multiple epithelial tissues, including skin, hair, and sebaceous glands, as shown by the detrimental effects of either vitamin A deficiency or toxicity. Thus, precise levels of retinoic acid (RA, active metabolite) are needed. These precise levels of RA are achieved by regulating several steps in the conversion of dietary vitamin A (retinol) to RA and RA catabolism. This review discusses the localization of RA synthesis to specific sites within the hair follicle and sebaceous gland, including their stem cells, during both homeostasis and disease states. It also discusses what is known about the specific roles of RA within the hair follicle and sebaceous gland. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

An update on the role of the sebaceous gland in the pathogenesis of acne

The pathogenesis of acne, a disease of the pilosebaceous follicle and one of the most common chronic skin disorders, is attributed to multiple factors such as increased sebum production, alteration of the quality of sebum lipids, inflammatory processes, dysregulation of the hormone microenvironment, interaction with neuropeptides, follicular hyperkeratinisation and the proliferation of Propionibacterium acnes within the follicle. In particular, the sebaceous gland plays an exquisite role in the initiation of the disease as it possesses all the enzyme machinery for the production of hormones and cytokines. In addition, in response to the altered tissue environment in the pilosebaceous follicle as well as in answer to emotional fret, stress response system mechanisms with induction of central and local expression of neuropeptides, are also initiated. This review summarises the latest advances in understanding the role of sebaceous gland cells in the pathomechanism of acne.

Sebaceous gland receptors

Receptors are proteins, embedded in a cell or cytoplasmic membrane, to which a mobile signaling molecule may attach. Receptor ligands may be peptides (such as neurotransmitters), hormones, pharmaceutical drugs and/or a toxins, whereas "binding" ordinarily initiates a cellular response. Human sebocytes are biologically and metabolically very active cells and consequently express numerous receptors. Three of four groups of peptide/neurotransmitter receptors, the so-called serpentine receptor group are present (corticotropin-releasing hormone receptors 1 and 2, melanocortin-1 and 5 receptors, mu-opiate receptors, VPAC receptors, cannabinoid receptors 1 and 2, vascular endothelial growth factor receptor and histamine 1 receptor). The single-transmembrane domain receptors are represented by the insulin-like growth factor-I receptor and the third group, which does not possess intrinsic tyrosine kinase activity, by the growth factor receptor. Nuclear receptors expressed in sebocytes are grouped into two major subtypes. From the steroid receptor family, the androgen receptor and the progesterone receptor are expressed. The thyroid receptor family includes the estrogen receptors (alpha and beta isotypes), the retinoic acid receptors (isotypes alpha and gamma) and retinoid X receptors (isotypes alpha, beta, gamma), the vitamin D receptor, the peroxisome proliferator-activated receptors (isotypes alpha, delta and gamma) and the liver X receptors (alpha and beta isotypes). The vanilloid receptor belongs to the transient ion channels and is expressed in differentiating human sebocytes. Further sebocyte receptors, which may influence their function are fibroblast growth factor receptor 2, epidermal growth factor receptor, c-MET, CD14, Toll-like receptor 2, Toll-like receptor 4 and Toll-like receptor 6. Receptor-ligand interactions control sebocyte proliferation, differentiation and lipid synthesis. However, not every ligand that binds to a sebocyte receptor also activates it, such ligands are receptor antagonists and inverse agonists.

Regulation of keratin expression by retinoids

Vitamin A and its natural and synthetic metabolites (retinoids) affect growth and differentiation of human skin and among the genes affected by retinoids in epidermis are keratin genes. Keratins are intermediate filament proteins that have essential functions in maintaining the structural integrity of epidermis and its appendages. Their expressions are under strict control to produce keratins that are optimally adapted to their environment. In this article, retinoid regulation of keratin expression in cultured human epidermal keratinocytes and in human skin in vivo will be reviewed. The direct and indirect mechanisms involved will be discussed and novel therapeutic strategies will be proposed for utilizing retinoids in skin disorders due to keratin mutations (e.g., epidermolysis bullosa simplex and epidermolytic ichthyosis).

Fat in the skin: Triacylglycerol metabolism in keratinocytes and its role in the development of neutral lipid storage disease

Keratinocyte differentiation is essential for skin development and the formation of the skin permeability barrier. This process involves an orchestrated remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by β-glucocerebrosidase, sphingomyelinase, phospholipases and sterol sulfatase generates ceramides, non-esterified fatty acids and cholesterol for the lipid-containing extracellular matrix, the lamellar membranes in the stratum corneum. The importance of triacylglycerol (TAG) hydrolysis for the formation of a functional permeability barrier was only recently appreciated. Mice with defects in TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2-knock-out) or TAG catabolism (comparative gene identification-58, -CGI-58-knock-out) develop severe permeability barrier defects and die soon after birth because of desiccation. In humans, mutations in the CGI-58 gene also cause (non-lethal) neutral lipid storage disease with ichthyosis. As a result of defective TAG synthesis or catabolism, humans and mice lack ω-(O)-acylceramides, which are essential lipid precursors for the formation of the corneocyte lipid envelope. This structure plays an important role in linking the lipid-enriched lamellar membranes to highly cross-linked corneocyte proteins. This review focuses on the current knowledge of biochemical mechanisms that are essential for epidermal neutral lipid metabolism and the formation of a functional skin permeability barrier.

[The sebaceous gland]

The development and function of the sebaceous gland in the fetal and neonatal periods appear to be regulated by maternal androgens and by endogenous steroid synthesis, as well as by other morphogens. The most apparent function of the glands is to excrete sebum. A strong increase in sebum excretion occurs a few hours after birth; this peaks during the first week and slowly subsides thereafter. A new rise takes place at about age 9 years with adrenarche and continues up to age 17 years, when the adult level is reached. The sebaceous gland is a target organ but also an important formation site of hormones, and especially of active androgens. Hormonal activity is based on an hormone (ligand)-receptor interaction, whereas sebocytes express a wide spectrum of hormone receptors. Androgens are well known for their effects on sebum excretion, whereas terminal sebocyte differentiation is assisted by peroxisome proliferator-activated receptor ligands. Estrogens, glucocorticoids, and prolactin also influence sebaceous gland function. In addition, stress-sensing cutaneous signals lead to the production and release of corticotrophin-releasing hormone from dermal nerves and sebocytes with subsequent dose-dependent regulation of sebaceous nonpolar lipids. Among other lipid fractions, sebaceous glands have been shown to synthesize considerable amounts of free fatty acids without exogenous influence. Atopic dermatitis, seborrheic dermatitis, psoriasis and acne vulgaris are some of the disease on which pathogenesis and severity sebaceous lipids may or are surely involved.

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.

Retinoid receptor alpha and Beta expression in serous ovarian tumors

The expression of retinoid acid receptors alpha (RARalpha) and beta (RARbeta) and estrogen receptor alpha (ERalpha) was assessed by immunohistochemistry and Western blotting in normal ovaries, serous cystadenoma (n = 20), serous borderline (n = 14), and serous ovarian cancer (n = 47) and was correlated in cancer cases with stage, grade, progress-free survival (PFS), and survival. RARalpha was increasingly expressed in benign cystadenomas, borderline, and low-stage and advanced-stage neoplasms (p < 0.001). In stage III, G3 serous carcinoma, increased RARalpha expression was an independent prognostic factor associated with lower chemoresponse to first-line chemotherapy (taxol and carboplatin) and shorter PFS (p < 0.002).RARbeta and ERalpha expression did not correlate with RARalpha tumor characteristics or PFS and survival.

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.

Analysis of ligand-dependent recruitment of coactivator peptides to RXRbeta in a time-resolved fluorescence resonance energy transfer assay

Because RXR plays a significant role in nuclear receptor signaling as a common heterodimeric partner for TR, PPAR, RAR, VDR, LXR and others, the ability of RXRbeta ligand binding domain (LBD) to interact with coregulator peptides bearing LXXLL or other interaction motifs was investigated using time-resolved fluorescence resonance energy transfer (TR-FRET). The random phage display peptide D22 and peptides derived from PGC1alpha, SRC1-4, SRC2-3, PRIP/RAP250 and RIP140 yielded the highest TR-FRET signal with RXRbeta LBD in the presence of saturating 9-cis retinoic acid (9-cisRA). Several peptides including D22, PGC1alpha, SRC3-2, PRIP/RAP250 and SRC1-4 also formed a complex with RXRbeta LBD in the presence of all-trans retinoic acid (at-RA) and the fatty acids, phytanic acid (PA) and docosahexaenoic acid (DHA). Determination of the dose dependency (EC50) of these compounds to recruit D22 to RXRbeta LBD indicated that the rank order potency was 9-cisRA>PA>at-RA>DHA. The ligands 9-cisRA and at-RA yielded an overall higher fold-change in D22 recruitment to RXRbeta LBD suggesting that more RXRbeta LBD-D22 complex was formed in the presence of these ligands under the assay conditions tested. The statistical parameter Z' factor for 9-cisRA-induced recruitment of D22 to RXRbeta LBD was 0.6 after 2h incubation, indicating a robust methodology that could be applied to high throughput screening. These results demonstrate that RXRbeta occupied with the fatty acid ligands, DHA and PA, can recruit coactivator peptides in a ligand-dependent manner.

Physiological Role of Retinyl Palmitate in the Skin

The skin is similar to other organs in how it absorbs, stores, and metabolizes vitamin A. However, because of the anatomical location of skin and the specialized physiological roles it plays, there are ways in which the skin is rather unique. The stratified structure of the epidermis results from the orchestration of retinoid-influenced cellular division and differentiation. Similarly, many of the physiological responses of the skin, such as dermal aging, immune defense, and wound healing, are significantly affected by retinoids. While much is known about the molecular events through which retinoids affect the skin's responses, more remains to be learned. Interest in the effects of retinol, retinyl palmitate, and other retinoids on the skin, fueled in part by the promise of improved dermatologic and cosmetic products, will undoubtedly make the effects of retinoids on skin a subject for continued intense investigation.

Towards dissecting the pathogenesis of retinoid-induced hair loss: all-trans retinoic acid induces premature hair follicle regression (catagen) by upregulation of transforming growth factor-beta2 in the dermal papilla

Diffuse hair loss ranks among the most frequent and psychologically most distressing adverse effects of systemic therapy with retinoids, which severely limits their therapeutic use even where clinically desired. Since the underlying mechanisms of retinoid-induced effluvium are as yet unknown, we have investigated the influence of the prototypic retinoid all-trans retinoic acid (ATRA, tretinoin) on the growth of human scalp hair follicles (HF) in culture. HF in the anagen VI stage of the hair cycle were cultured in the presence of 10(-8) or 10(-10) M ATRA. Compared with controls, hair shaft elongation declined significantly already after 2 d in the ATRA-treated group, and approximately 80% of the ATRA-treated HF had prematurely entered catagen-like stage at day 6, compared with 30% in the control group. This corresponded to an upregulation of apoptotic and a downregulation of Ki67-positive cells in ATRA-treated HF. Since transforming growth factor (TGF)-beta has been implicated as a key inducer of catagen, we next studied whether ATRA treatment had any effect on follicular expression. TGF-beta2 immunoreactivity was detected in the outer root sheath of anagen VI scalp HF. In catagen follicles, TGF-beta2 was also expressed in the regressing epithelial strand. After 4 d of ATRA treatment, TGF-beta2 was significantly upregulated in anagen HF in the dermal papilla (DP) and the dermal sheath, 7, and TGF-beta neutralizing antibody partially abrogated at RA induced hair growth inhibition. Real-time PCR confirmed a significant upregulation of TGF-beta2 transcripts in ATRA-treated hair bulbs. This study is the first to provide direct evidence that ATRA can indeed induce a catagen-like stage in human HF and suggests that this occurs, at least in part, via upregulation of TGF-beta2 in the DP. Therefore, topical TGF-beta2/TGF-beta receptor II antagonists deserve to be explored for the prevention and management of retinoid-induced hair loss.

ERV3 and related sequences in humans: structure and RNA expression

The ERV3 locus at chromosome 7q11 is a much studied human endogenous retroviral (HERV) sequence, owing to an env open reading frame (ORF) and placental RNA and protein expression. An analysis of the human genome demonstrated that ERV3 is one of a group of 41 highly related elements (ERV3-like HERVs) which use proline, isoleucine, or arginine tRNA in their primer binding sites. In addition to elements closely related to ERV3, the group included the previously known retinoic acid-inducible element, RRHERVI, also referred to as HERV15, but was separate from the related HERV-E elements. The ERV3-like elements are defective. The only element with an ORF among gag, pro, pol, and env genes was the env ORF of the original ERV3 locus. A search in dbEST revealed ERV3 RNA expression in placenta, skin, carcinoid tumor, and adrenal glands. Expression was also studied with newly developed real-time quantitative PCRs (QPCR) of ERV3 and HERV-E(4-1) env sequences. Results from a novel histone 3.3 RNA QPCR result served as the expression control. QPCR results for ERV3 were compatible with previously published results, with a stronger expression in adrenal gland and placenta than in 15 other human tissues. The expression of the envelope (env) of ERV3 at chromosome 7q11 was also studied by using stringent in situ hybridization. Expression was found in corpus luteum, testis, adrenal gland, Hassal's bodies in thymus, brown fat, pituitary gland, and epithelium of the lung. We conclude that ERV3 env is most strongly expressed in adrenal and sebaceous glands as well as in placenta.

Retinoid Receptor Subtypes in Sebaceous Cell Carcinoma of the Eyelid

PURPOSE

To study retinoid receptor expression in sebaceous cell carcinoma of the eyelid.

METHODS

Expression of retinoid receptors (RAR alpha, beta, and gamma and RXR alpha, beta, and gamma) in tumor specimens from 10 patients with sebaceous cell carcinoma of the eyelid and in 3 normal incidental tarsus specimens from healthy adults without cancer was studied immunohistochemically by using antiretinoid receptor antibodies.

RESULTS

In all 3 specimens of normal tarsus, all 6 retinoid receptors were expressed. RARalpha expression was absent in 3 tumors and was decreased in 3 tumors compared with expression in the control tissues. RARbeta expression in carcinomas was primarily perinuclear; 6 tumors showed increased RARbeta expression compared with controls. RARgamma expression was absent in 4 tumors and was decreased in 2 tumors compared with controls. RXRalpha nuclear expression was decreased compared with controls in 5 tumors. RXRbeta expression was low in 7 tumors. RXRgamma expression was absent in 6 tumors.

CONCLUSIONS

Aberrant expression of retinoid receptors in sebaceous cell carcinoma of the eyelid might play a role in the pathogenesis and progression of this carcinoma.

Evidence of increased apoptosis and reduced proliferation in basal cell carcinomas treated with tazarotene

A preliminary clinical experience suggested tazarotene, a new acetylenic retinoid, as an effective alternative topical treatment of basal cell carcinomas (BCC). The mechanisms of action of this synthetic retinoid, however, have not been yet clarified. In this work we assessed the in vivo effects of daily application of tazarotene for 24 wk, on 30 small superficial and nodular BCC, and the in vitro effects of tazarotene on immortalized basal and squamous tumor epidermal cells. Cellular proliferation, apoptosis and changes in expression of retinol and retinoic acid receptors (RAR), p53, bcl-2, and bax were studied by immunohistochemistry, western blotting and PCR. Overall, 76.7% of treated tumors showed >50% regression. Complete healing was observed in 46.7% of all treated BCC, without recurrences at 2-y observation. Regression was associated with reduced proliferation and increased apoptosis, demonstrated by Ki-67- and TdT-mediated dUTP-biotin nick-end labelling-positive nuclear staining, and with enhanced RAR-beta and bax expression, with RAR-alpha and -gamma expression unchanged. In vitro, tazarotene induced a concentration-dependent increase of RAR-beta and bax associated with a greater rate of apoptosis and growth inhibition in basaloid tumor cells compared with squamous tumor cells. Our studies provide convincing evidence that tazarotene induces BCC regression possibly by synergistic RAR-beta-dependent anti-proliferative and pro-apoptotic pathways.

Vitamin D receptor and retinoid X receptor interactions in motion

Vitamin D receptor (VDR) and retinoid X receptor (RXR) are members of the nuclear receptor superfamily and they bind target DNA sequences as heterodimers to regulate transcription. This article surveys the latest findings regarding the roles of dimerizing RXR in VDR function and emphasizes potential areas for future developments. We first highlight the importance of dimerization with RXR for both the ligand-independent (hair growth) and ligand-dependent functions of VDR (calcium homeostasis, bone development and mineralization, control of cell growth and differentiation). Emerging information regarding the regulatory control of dimerization based on biochemical, structural, and genetic studies is then presented. Finally, the main focus of this article is a new dynamic perspective of dimerization functions, based on recent research with fluorescent protein chimeras in living cells by microscopy. These studies revealed that both VDR and RXR constantly shuttle between the cytoplasm and the nucleus and between subnuclear compartments, and showed the transient nature of receptor--DNA and receptor--coregulator interactions. Because RXR dimerizes with most of the nuclear receptors, regulation of receptor dynamics by RXR has a broad significance.

The levels of retinoid RARβ receptors correlate with galectin-1, -3 and -8 expression in human cholesteatomas

Cholesteatoma is a benign disease characterized by the presence of an unrestrained growth and the accumulation of keratin debris in the middle ear cavity. This often recurs, even when surgical resection is thought to be complete. In a previous study we showed that cholesteatomas with the highest apoptotic indices recurred more rapidly and also exhibited a high level of p53 immunopositive cells. In view of their relevance to the characterization of the cell differentiation status, the present study focuses on the expression of retinoid acid receptors (RARs) and galectins in human cholesteatomas. Retinoids control the differentiation processes in keratinocytes while galectins play strikingly modulatory roles at apoptosis and cell adhesion levels in a wide variety of tissue (embryonic, normal and neoplastic). To clarify the expression of these two protein families in human cholesteatomas we examined and quantified the levels of immunohistochemical expression of RARalpha, beta and gamma, and also galectin-1, -3 and -8 in a series of 70 human cholesteatomas. Our data show clearly that predominantly RARbeta and galectin-1 were expressed. The RARgamma concentration was significantly lower than that of the RARalpha; this was also observed for the galectin-8 concentration in comparison with the galectin-3 one. Furthermore, the level of RARbeta expression correlated highly (P=0.00001) with the level of galectin-8 expression, which also correlated significantly with the level of RARalpha and RARgamma expression. In addition, this parameter also correlated with the level of galectin-1 and galectin-3 expression. These data suggest that cholesteatomas may originate in an undifferentiated population of keratinocytes, and that a relation may exist between retinoid activity and galectins.

Histochemistry and cytochemistry of nuclear receptors

Receptors of steroid hormones, thyroid hormones and several kinds of vitamins have been shown to act as nuclear transcription factors and to form a nuclear receptor (NR) family. Histochemical techniques including autoradiography using radio-labeled ligands, immunohistochemistry and in situ hybridization histochemistry, have displayed that target cells of these receptors are distributed not only in the classical target organs but also widely in a variety of tissues; these techniques can demonstrate the presence of receptor proteins and mRNAs, even though they are expressed in a small cell population of tissues. On the other hand, many studies have been performed to demonstrate the interaction between NRs and nuclear and cytoplasmic proteins, and to clarify the mechanism of transcriptional regulation through NRs in artificial conditions which are created in gene transfer experiments or under cell-free conditions. Some data coincide with those obtained from histochemical techniques, however, some histochemical data do not support the results of studies in vitro. This review focuses on the following topics: histochemical methodologies to detect NRs, the distribution and function of NRs in the tissues, the intracellular and intranuclear localization of NRs, roles of gonadal steroid receptors and their ligands on developing tissues including cell communications such as mesenchymal-stromal interaction, and the interaction between other cellular components and NRs. In addition, the agreement and disagreement between the results of histochemical studies and those from the experiments in the model systems or in vitro are discussed.

Dimerization with retinoid X receptors promotes nuclear localization and subnuclear targeting of vitamin D receptors

The vitamin D receptor (VDR) acts as heterodimer with the retinoid X receptor alpha (RXR) to control transcriptional activity of target genes. To explore the influence of heterodimerization on the subcellular distribution of these receptors in living cells, we developed a series of fluorescent-protein chimeras. The steady-state distribution of the yellow fluorescent protein-RXR was more nuclear than the unliganded green fluorescent protein (GFP)-VDR. Coexpression of RXR-blue fluorescent protein (BFP) promoted nuclear accumulation of GFP-VDR by influencing both nuclear import and retention. Fluorescence resonance energy transfer microscopy (FRET) demonstrated that the unliganded GFP-VDR and RXR-BFP form heterodimers. The increase in nuclear heterodimer content correlated with an increase in basal transcriptional activity. FRET also revealed that calcitriol induces formation of multiple nuclear foci of heterodimers. Mutational analysis showed a correlation between hormone-dependent nuclear VDR foci formation and DNA binding. RXR-BFP also promoted hormone-dependent nuclear accumulation and intranuclear foci formation of a nuclear localization signal mutant receptor (nlsGFP-VDR) and rescued its transcriptional activity. Heterodimerization mutant RXR failed to alter GFP-VDR and nlsGFP-VDR distribution or activity. These experiments suggest that RXR has a profound effect on VDR distribution. This effect of RXR to promote nuclear accumulation and intranuclear targeting contributes to the regulation of VDR activity and probably the activity of other heterodimerization partners.

Cellular and subcellular localization of six retinoid receptors in rat testis during postnatal development: identification of potential heterodimeric receptors

Vitamin A is required in the testis for germ cell development. It acts through two families of retinoid receptors, retinoic acid receptors (RAR) and retinoid X receptors (RXR), each with three subtypes alpha, beta, and gamma. These receptors are postulated to dimerize and regulate the transcription of retinoid-responsive genes that are crucial for germ cell development. In this study, we determined the cellular and subcellular localization of six retinoid receptors in the developing rat testis to identify the specific cellular sites and times of receptor expression. Immunohistochemical results revealed the expression of RARalpha, RARbeta, RXRalpha, and RXRgamma proteins in somatic and germ cells throughout postnatal development. In contrast, the expression of RARgamma and RXRbeta did not increase until 30-35 days of age in somatic cells from the testis. Interestingly, RARalpha and RXRalpha had a similar subcellular localization pattern in Sertoli cells throughout postnatal testis development, while RARalpha and RXRgamma were both present in the nucleus of spermatocytes and elongating spermatids. These results suggest that RARalpha may potentially dimerize with RXRalpha in Sertoli cells and with RXRgamma in germ cells. In addition, we demonstrate that the only RAR in the nucleus of early meiotic germ cells is RARalpha.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Reduced expression of retinoic acid receptor beta protein (RAR beta) in human papillary thyroid carcinoma: immunohistochemical and western blot study

AIMS

To investigate the role of retinoic acid receptor beta (RAR beta) in thyroid carcinogenesis, we have investigated its expression in human thyroid samples by combined immunohistochemistry and Western blotting.

METHODS AND RESULTS

Fifty-eight paraffin-embedded thyroid samples (40 normal or benign tissues, 16 papillary and two follicular carcinomas) were analysed by immunohistochemistry using a specific monoclonal antibody. Western blotting was also carried out on 11 selected samples (seven normal or benign tissues, three papillary carcinomas and one follicular carcinoma) and two human ovarian carcinomas as controls. RAR beta immunostaining was nuclear and limited to the normal epithelial thyroid tissue. A dramatic decrease in RAR beta immunostaining was observed in all the papillary carcinomas and in one follicular carcinoma. The other follicular carcinoma exhibited strong RAR beta immunostaining. By immunoblotting, a 51 kDa signal corresponding to the RAR beta was observed in nuclear extracts from normal thyroids and for one follicular carcinoma. However, this signal was lacking in the papillary carcinomas. These results were in complete agreement with the observations obtained by immunohistochemistry on the same samples.

CONCLUSION

We present here the first demonstration of RAR beta protein in normal human thyroid follicular cells. In addition, we found that its expression is decreased in papillary thyroid carcinoma.