Nuclear receptors for retinoic acid and thyroid hormone regulate transcription of keratin genes

A short review on the role of thyroxine in fast wound healing and tissue regeneration

Wound healing is a multiplex interaction process that involves extracellular matrix, blood vessels, proteases, cytokines, and chemokine. So far, a number of studies have been performed to understand the basis of the wound-healing process and multiple wound-healing products have been designed. However, significant morbidity and mortality incidents still occurred due to poor wound healing. Thus, there is a dire need to understand the effects of topical applications of various therapeutic options that lead to fast wound healing. Thyroxine is one great panacea for wound healing that has been vigorously mooted throughout the years but a conclusive result regarding its effectiveness is still not achieved. This review is intended to find a rational basis for its positive role in wound healing. To accomplish the objective, this review highlights the different aspects of thyroxine's role in wound healing like keratin synthesis, skin thickening, and pro-angiogenesis, the basis of controversy on its wound healing ability and its potential to be used as a wound healing agent. This study will be helpful for researchers and surgeons to assess the importance of thyroxine as a candidate to comprehensively research to develop a potent, effective, and affordable wound healing drug.

Common tumor-suppressive signaling of thyroid hormone receptor beta in breast and thyroid cancer cells

The thyroid hormone receptor beta (TRβ) is a tumor suppressor in multiple types of solid tumors, most prominently in breast and thyroid cancer. An increased understanding of the molecular mechanisms by which TRβ abrogates tumorigenesis will aid in understanding the core tumor-suppressive functions of TRβ. Here, we restored TRβ expression in the MDA-MB-468 basal-like breast cancer cell line and perform RNA-sequencing to determine the TRβ-mediated changes in gene expression and associated signaling pathways. The TRβ expressing MDA-MB-468 cells exhibit a more epithelial character as determined by principle component analysis-based iterative PAM50 subtyping score and through reduced expression of mesenchymal cytokeratins. The epithelial to mesenchymal transition pathway is also significantly reduced. The MDA-MB-468 data set was further compared with RNA sequencing results from TRβ expressing thyroid cancer cell line SW1736 to determine which genes are TRβ correspondingly regulated across both cell types. Several pathways including lipid metabolism and chromatin remodeling processes were observed to be altered in the shared gene set. These data provide novel insights into the molecular mechanisms by which TRβ suppresses breast tumorigenesis.

Expression of the carcinoma-associated keratin K6 and the role of AP-1 proto-oncoproteins

The normal pattern of keratin expression in epidermis is altered in carcinomas as well as in nonmalignant diseases such as psoriasis and wound healing. Under these circumstances, the transcription of differentiation-specific keratins K1 and K10 is suppressed, whereas the activation- and hyperproliferation-associated keratins K6 and K16 are induced. Very little is known regarding transcriptional regulators involved in this switch. To investigate the nuclear factors that participate in regulation of expression of the K6 gene, we have characterized the binding sites for nuclear proteins on the promoter DNA of the K6 gene by gel retardation assays and site-specific deletion mutagenesis. We found four nuclear protein binding sites in the K6 gene promoter. Two are near the TATA box, but their ability to bind HeLa or keratinocyte nuclear extracts is independent of the TATA box-binding protein complex. The third binding site is a large palindrome. The sequences of these three sites do not correspond to any described target sequences for characterized transcriptional factors. The fourth is an AP-1 site, the target sequence for the proto-oncoproteins fos and jun. All four sites are independent of the previously characterized epidermal growth factor-responsive element, EGF-RE. These findings suggest that there may be two parallel pathways of induction of K6 transcription. One proceeds through the EGF-RE, which may be involved in nonmalignant hyperproliferation processes; the other, through the AP-1 site and the fos-jun proto-oncoproteins, may be related to induction in malignant processes.

On the role of AP2 in epithelial-specific gene expression

Transcription factor AP2 plays an important role in transcription of keratin genes, and it has been suggested that AP2 confers epithelial specificity. Promoters of keratin genes contain AP2 sites, usually within tight clusters of binding sites for other nuclear transcription factors. The role of AP2 was examined by in vitro gel shift analysis, AP2 binding site mutagenesis, and stable and transient transfection experiments. Nonepithelial cells, such as GM10 fibroblasts and melanocytes, neither express keratin nor become phenotypically epithelial when transfected with an AP2-expressing vector. However, in 3T3 and HeLa cells, co-transfection of an AP2-expressing vector increases the level of transcription from keratin gene promoters. This increase requires an intact AP2 binding site. Thus, the role of AP2 in keratin gene expression is quantitative rather than qualitative. AP2 interacts with other transcription factors and may convey extracellular regulatory signals to the transcription complex in the promoters of keratin genes.

Retinoic acid signaling regulates Krt5 and Krt14 independently of stem cell markers in submandibular salivary gland epithelium

BACKGROUND

Retinoic acid (RA), the active metabolite of vitamin A, has been demonstrated to be important for growth and branching morphogenesis of mammalian embryonic salivary gland epithelium. However, it is not known whether RA functions directly within epithelial cells or in associated tissues that influence morphogenesis of salivary epithelium. Moreover, downstream targets of RA regulation have not been identified.

RESULTS

Here, we show that canonical RA signaling occurs in multiple tissues of embryonic mouse salivary glands, including epithelium, associated parasympathetic ganglion neurons, and nonneuronal mesenchyme. By culturing epithelium explants in isolation from other tissues, we demonstrate that RA influences epithelium morphogenesis by direct action in that tissue. Moreover, we demonstrate that inhibition of RA signaling represses cell proliferation and expression of FGF10 signaling targets, and upregulates expression of basal epithelial keratins Krt5 and Krt14. Importantly, we show that the stem cell gene Kit is regulated inversely from Krt5/Krt14 by RA signaling.

CONCLUSIONS

RA regulates Krt5 and Krt14 expression independently of stem cell character in developing salivary epithelium. RA, or chemical inhibitors of RA signaling, could potentially be used for modulating growth and differentiation of epithelial stem cells for the purpose of re-populating damaged glands or generating bioengineered organs. Developmental Dynamics 246:135-147, 2017. © 2016 Wiley Periodicals, Inc.

The Retinol Circulating Complex Releases Hormonal Ligands During Acute Stress Disorders

Intensive care workers actively participate in very hot debates aiming at defining the true metabolic, hormonal and nutritional requirements of critically ill patients, the contributory roles played by thyroid and retinoid ligands being largely underestimated. The present article makes up for redressing the balance on behalf of these last hormonal compounds. The retinol circulating complex is transported in the bloodstream in the form of a trimolecular edifice made up of transthyretin (TTR), retinol-binding protein (RBP) and its retinol ligand. TTR reflects the size of the lean body mass (LBM) and is one of the 3 carrier-proteins of thyroid hormones whereas RBP is the sole conveyor of retinol in human plasma. In acute inflammatory disorders, both TTR and RBP analytes experience abrupt cytokine-induced suppressed hepatic synthesis whose amplitude is dependent on the duration and severity of the inflammatory burden. The steep drop in TTR and RBP plasma values releases thyroxine and retinol ligands in their physiologically active forms, creating free pools estimated to be 10-20 times larger than those described in healthy subjects. The peak endocrine influence is reached on day 4 and the freed ligands undergo instant cellular overconsumption and urinary leakage of unmetabolized fractions. As a result of these transient hyperthyroid and hyperretinoid states, helpful stimulatory and/or inhibitory processes are set in motion, operating as second frontlines fine-tuning the impulses primarily initiated by cytokines. The data explain why preexisting protein malnutrition, as assessed by subnormal LBM and TTR values, impairs the development of appropriate recovery processes in critically ill patients. These findings have survival implications, emphasizing the need for more adapted therapeutic strategies in intensive care units.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.

Thyroid hormone action on skin

PURPOSE OF REVIEW

To review the current understanding regarding thyroid hormone action on skin. To provide a historical context for the recent findings.

RECENT FINDINGS

Although direct thyroid hormone actions have been demonstrated on multiple aspects of cutaneous biology, rigorous study remains scant. Still, there is a slowly evolving literature supporting the concept that thyroid hormone can directly stimulate epidermis, dermis, and hair. That action may be accessed to treat cutaneous disease.

SUMMARY

Here, we review the literature regarding thyroid hormone action on skin along with skin manifestations of thyroid disease. We provide context for more recent findings of direct thyroid hormone stimulation of cutaneous cell proliferation in vitro and in vivo which may portend the use of thyroid hormone to treat cutaneous pathologies.

Triiodothyronine represses MUC5AC expression by antagonizing Sp1 binding to its promoter in human bronchial epithelial HBE16 cells

Mucus hypersecretion is a distinguished feature of chronic inflammatory airway diseases. Interestingly, in this condition thyroid function is impaired with decreased level of triiodothyronine (T3), indicating potential link between low level of T3 and mucus hypersecretion. But the underlying mechanisms are poorly understood. In this study we aimed to elucidate the effect of T3 on MUC5AC secretion in human bronchial epithelial HBE16 cells and further investigate how T3 regulates MUC5AC gene expression at transcriptional level. By RT-PCR and ELISA we showed that T3 inhibited MUC5AC mRNA expression and protein secretion in HBE16 cells. Furthermore, luciferase assay and site-directed mutagenesis analysis demonstrated that T3 repressed MUC5AC expression at transcriptional level and the mechanism might partly lie in the specific inhibition of Sp1 binding to the promoter. Our results suggest that decreased T3 level leads to the release of repression of MUC5AC expression and thus contributes to mucus hypersecretion.

The thyroid hormone receptors modulate the skin response to retinoids

Background

Retinoids play an important role in skin homeostasis and when administered topically cause skin hyperplasia, abnormal epidermal differentiation and inflammation. Thyroidal status in humans also influences skin morphology and function and we have recently shown that the thyroid hormone receptors (TRs) are required for a normal proliferative response to 12-O-tetradecanolyphorbol-13-acetate (TPA) in mice.

Methodology/Principal findings

We have compared the epidermal response of mice lacking the thyroid hormone receptor binding isoforms TRα1 and TRβ to retinoids and TPA. Reduced hyperplasia and a decreased number of proliferating cells in the basal layer in response to 9-cis-RA and TPA were found in the epidermis of TR-deficient mice. Nuclear levels of proteins important for cell proliferation were altered, and expression of keratins 5 and 6 was also reduced, concomitantly with the decreased number of epidermal cell layers. In control mice the retinoid (but not TPA) induced parakeratosis and diminished expression of keratin 10 and loricrin, markers of early and terminal epidermal differentiation, respectively. This reduction was more accentuated in the TR deficient animals, whereas they did not present parakeratosis. Therefore, TRs modulate both the proliferative response to retinoids and their inhibitory effects on skin differentiation. Reduced proliferation, which was reversed upon thyroxine treatment, was also found in hypothyroid mice, demonstrating that thyroid hormone binding to TRs is required for the normal response to retinoids. In addition, the mRNA levels of the pro-inflammatory cytokines TNFα and IL-6 and the chemotactic proteins S1008A and S1008B were significantly elevated in the skin of TR knock-out mice after TPA or 9-cis-RA treatment and immune cell infiltration was also enhanced.

Conclusions/significance

Since retinoids are commonly used for the treatment of skin disorders, these results demonstrating that TRs regulate skin proliferation, differentiation and inflammation in response to these compounds could have not only physiological but also therapeutic implications.

Thyroid hormone action on skin

The skin characteristics associated with thyroid hormone are classic. The name "myxedema" refers to the associated skin condition caused by increased glycosaminoglycan deposition in the skin. Generalized myxedema is still the classic cutaneous sign of hypothyroidism. It is caused by deposition of dermal acid mucopolysaccharides, notably hyaluronic acid. Despite its appearance, the skin does not pit with pressure.

Plasma cortisol and thyroid hormone concentrations in pre-weaning Australian fur seal pups

The hormonal factors that influence development from birth to weaning in otariid seals is still largely unknown. In the present study, a suite of thyroid hormones and cortisol were measured in Australian fur seal pups in order to determine baseline concentrations as well as to describe their endocrinology over this critical developmental period. A cross-section of newborn pups from a breeding colony located on Kanowna Island, Australia were sampled at six different times over the course of the 10 month lactation period. Sample times were designed to correspond to periods of heightened physiological change during pre-weaning development: post-natal, pre-molt, the initiation of molt, mid-molt, period of peak milk intake and weaning. Results indicate that the greatest hormonal changes were associated with the post-natal stage and molt, with molt showing the greatest changes, as has been reported for several species of pinnipeds. Two forms of thyroid hormones analyzed (Total T(4), and Free T(3)), increased with the initiation of the molt, and Free T(3) exhibited a second increase that was associated with the period of peak milk intake. The T(3):T(4) ratio was significantly lower during the initiation of molt than either pre- or mid-molt. The study was able to describe physiological change during the first year of life in Australian fur seals as well as document basal concentrations of thyroid hormones and cortisol in pups of this species.

Nexus between epidermolysis bullosa and transcriptional regulation by thyroid hormone in epidermal keratinocytes

Abstract Thyroid hormone, T3, through the interaction of its receptor with the recognition sequences in the DNA, regulates gene expression. This regulation includes the promoter activity of keratin genes. The receptor shares coregulators with other members of the nuclear receptor family, including RXR. Intending to define the transcriptional effects of thyroid hormones in keratinocytes, we used Affymetrix microarrays to comprehensively compare the genes expressed in T3-treated and untreated human epidermal keratinocytes. The transcriptomes were compared at 1, 4, 24, 48, and 72 hours. Surprisingly, T3 induced only 9 and suppressed 28 genes, much fewer than expected. Significantly, genes associated with epidermolysis bullosa, a set of inherited blistering skin diseases, were found statistically highly overrepresented among the suppressed genes. These genes include Integrin beta4, Plectin, Collagen XVII, MMP1, MMP3, and MMP14. The data imply that in keratinocytes T3 could suppresses the remodeling by, attachment to, and production of extracellular matrix. The results suggest that topical treatment with T3 may be effective for alleviation of symptoms in patients with epidermolysis bullosa.

Expression, localisation and functional activation of NFAT-2 in normal human skin, psoriasis, and cultured keratocytes

Ciclosporin A (CsA) is widely utilized for the treatment of inflammatory skin diseases such as psoriasis. The therapeutic effects of CsA are thought to be mediated via its immunosuppressive action on infiltrating lymphocytes in skin lesions. CsA and tacrolimus block T cell activation by inhibiting the phosphatase calcineurin and preventing translocation from the cytoplasm to the nucleus of the transcription factor Nuclear Factor of Activated T cells (NFAT). As calcineurin and NFAT 1 have been shown to be functionally active in cultured human keratocytes, expression of other NFAT family members such as NFAT-2 and possible functional activation was investigated in human keratocytes. RT-PCR and Western Analysis were used to investigate the presence of NFAT-2 mRNA and protein in human keratocytes. Tissue culture of human keratocytes and immunostaining of cells on coverslips and confocal microscopy were used to assess the degree of nuclear localisation of NFAT-2 in cultured cells. Keratome biopsies were taken from patients with psoriasis (lesional and non-lesional skin) and normal skin and immunohistochemistry was used to assess the NFAT-2 localisation in these biopsies using a well characterized anti-NFAT-2 antibody. The NFAT-2 mRNA and protein expression was demonstrated using RT-PCR and Western blotting. Moreover, the expression of NFAT-2 in normal skin, non-lesional and lesional psoriasis showed a striking basal staining suggesting a role for NFAT-2 in keratocytes proliferation. A range of cell types in the skin express NFAT-2. The expression of NFAT-2 in human keratocytes and response to different agonists provides perhaps a unique opportunity to examine the regulation, subcellular localization and kinetics of translocation of different NFATs in primary cultured human cells. In these experiments the author assessed the expression, localization of NFAT-2 in cultured human keratocytes and measured the degree of nuclear localisaion of NFAT-2 using immunofluorescence and confocal microscopy and whether CsA and tacrolimus inhibit NFAT-2 nuclear translocation. As with NFAT 1, differentiation-promoting agents that increase intracellular calcium concentration induced nuclear translocation of NFAT-2 in cultured keratocytes but with different kinetics. These data provide the first evidence of that NFAT-2 is expressed in normal skin, psoriasis and that NFAT-2 functionally active in human keratocytes and that nuclear translocation of NFAT-2 in human skin cells has different kinetics than NFAT 1 suggesting that NFAT-2 may play an important role in regulation of keratocytes proliferation and differentiation at a different stage. Inhibition of this pathway in human epidermal keratocytes many account, in part for the therapeutic effects of CsA and tacrolimus in skin disorders such as psoriasis. Thus, supporting our previous work data that calcineurin/NFAT is functionally active not only in T cells, but in skin cells.

Topical thyroid hormone accelerates wound healing in mice

Although the physiologic role of thyroid hormone in skin is not well understood, mounting evidence suggests that T3 plays an important role in epidermal proliferation. The goal of this project was to evaluate whether the topical application of supraphysiologic doses of T3 could accelerate wound healing. We evaluated mice treated with topical T3 vs. the same mice receiving vehicle alone (Novasome A). Ten-millimeter diameter (79 mm2) dorsal skin wounds were established in all animals, and wounds were remeasured 4 d after injury. All animals were evaluated twice: once with the T3 treatment and once with the vehicle alone. Daily topical application of 150 ng T3 resulted in 58% greater wound closure relative to wounds on the same animals receiving vehicle alone (P < 0.001). Furthermore, we determined that wound healing-associated keratin 6 protein expression in hair follicle keratinocytes increased in a dose-dependent manner in vivo during topical T3 treatment. The data support our previous hypothesis that T3 is necessary for optimal wound healing. Now, we further suggest that topical thyroid hormone may be an inexpensive agent to hasten healing of certain wounds.

The Book of Opposites: The Role of the Nuclear Receptor Co-regulators in the Suppression of Epidermal Genes by Retinoic Acid and Thyroid Hormone Receptors

Transcriptional regulation by nuclear receptors occurs through complex interactions that involve DNA response elements, co-activators/co-repressors, and histone modifying enzymes. Very little is known about how molecular interplay of these components may determine tissue specificity of hormone action. We have shown previously that retinoic acid (RA) and thyroid hormone (T3) repress transcription of a specific group of epidermal keratin genes through a novel mechanism that utilizes receptors homodimers. In this paper, we have analyzed the epidermal specificity of RA/T3 action by testing the role of co-repressors and co-activators in regulation of epidermal genes. Using transient co-transfections, northern blots, antisense oligonucleotides, and a histone deacetylase (HDAC) inhibitor, trichostatin A, we found that in the context of specific keratin RE (KRE), co-activators and histone acetylase become co-repressors of the RA/T3 receptors in the presence of their respective ligands. Conversely, co-repressors and HDAC become co-activators of unliganded T3Ralpha. The receptor-co-activator interaction is intact and occurs through the NR-box. Therefore, the role of co-activator is to associate with liganded receptors whereas the KRE-receptor interaction determines specific transcriptional signal, in this case repression. This novel molecular mechanism of transcriptional repression conveys how RA and T3 target specific groups of epidermal genes, thus exerting intrinsic tissue specificity.

From an Enhanceosome to a Repressosome: Molecular Antagonism between Glucocorticoids and EGF Leads to Inhibition of Wound Healing

Wound healing in its complexity depends on the concerted activity of many signaling pathways. Here, we analyzed how the simultaneous presence of glucocorticoids (GC), retinoic acid (RA) and epidermal growth factor (EGF) affect wound healing at the molecular, cellular and tissue levels. We found that GC inhibit wound healing by inhibiting keratinocyte migration, whereas RA does not. Furthermore, GC block EGF-mediated migration, whereas RA does not. On the molecular level, these compounds target expression of one of the earliest markers of wound healing, cytoskeletal components, keratins K6 and K16. Both GC and RA repress their transcription, whereas EGF induces it. Interestingly, the GC inhibition is mediated by a repressosome complex consisting of four monomers of the GC receptor, beta-catenin and coactivator-associated-arginine-methyltransferase-1. GC are dominant, EGF cannot rescue GC-mediated inhibition. Pre-treatment of keratinocytes with GC shifts the balance towards the repressosome, allowing for dominant inhibition of K6 even in the presence of EGF or c-fos/c-jun. Although RA receptor gamma and glucocorticoid receptor bind to the same response element repressing transcription of keratins K6/K16, RA receptor interacts with the components of the EGF-enhanceosome (co-activators: glucocorticoid-receptor-interactive protein-1(GRIP-1)/steroid-receptors coactivator-1 (SRC-1)) without breaking it. Consequently, RA has a co-dominant effect with EGF: when present simultaneously, their effects balance each other. When keratinocytes are pre-treated with mitogen-activated protein kinase (MAPK) inhibitor, thus blocking EGF, the balance is shifted towards the RA repression. Similar to clinical findings, pre-treatment of keratinocytes with RA blocks GC-mediated inhibition. In summary, our results identify complex molecular mechanisms through which RA alleviates GC-mediated inhibition of wound healing.

A role for thyroid hormone in wound healing through keratin gene expression

The importance of thyroid hormone (TH) in wound healing is not well understood. To gain insight, we evaluated the impact of TH deficiency on wound-healing genes in cultured keratinocytes. By RT-PCR, keratin 6a (K6a) and 16 (K16) gene expression in TH replete cells was 3.8- (P < 0.005) and 1.9-fold (P < 0.05) greater, respectively, than expression in TH-deficient cells. By real-time PCR, TH replete cell expression of K6a, K16, and K17 was greater than in deficient cells: 18- (P < 0.001), 10- (P < 0.001), and 4-fold (P < 0.005), respectively. To examine TH requirement for optimal wound healing, we contrasted TH-deficient vs. ip T(3)-treated mice. Four days after wounding, ip T(3)-treated mice had twice the degree of wound closure as hypothyroid mice (P < 0.001). By RT-PCR, K6a and K17 gene expression from control mouse skin was greater than from hypothyroid mouse skin: 5- (P < 0.001) and 1.7-fold (P < 0.05), respectively. T(3) is necessary for the keratinocyte proliferation required for optimal wound healing. T(3) exerts influence by stimulating expression of the wound-healing keratin genes. Thus, for hypothyroid patients undergoing surgery that cannot be delayed until euthyroidism is achieved, our data support T(3) treatment for the perioperative period.

Thyroid hormones and gamma interferon specifically increase K15 keratin gene transcription

Basal layers of stratified epithelia express keratins K5, K14, and K15, which assemble into intermediate filament networks. Mutations in K5 or K14 genes cause epidermolysis bullosa simplex (EBS), a disorder with blistering in the basal layer due to cell fragility. Nonkeratinizing stratified epithelia, e.g., in the esophagus, produce more keratin K15 than epidermis, which alleviates the esophageal symptoms in patients with K14 mutations. Hypothesizing that increasing the cellular content of K15 could compensate for the mutant K14 and thus ease skin blistering in K14 EBS patients, we cloned the promoter of the K15 gene and examined its transcriptional regulation. Using cotransfection, gel mobility shifts, and DNase I footprinting, we have identified the regulators of K15 promoter activity and their binding sites. We focused on those that can be manipulated with extracellular agents, transcription factors C/EBP, AP-1, and NF-kappaB, nuclear receptors for thyroid hormone, retinoic acid, and glucocorticoids, and the cytokine gamma interferon (IFN-gamma). We found that C/EBP-beta and AP-1 induced, while retinoic acid, glucocorticoid receptors, and NF-kappaB suppressed, the K15 promoter, along with other keratin gene promoters. However, the thyroid hormone and IFN-gamma uniquely and potently activated the K15 promoter. Using these agents, we could boost the amounts of K15 in human epidermis. Our findings suggest that treatments based on thyroid hormone and IFN-gamma could become effective agents in therapy for patients with EBS.

Changing paradigms in dermatology: nuclear hormone receptors

Nuclear hormone receptors (NHRs) are a family of proteins that function similarly as nuclear transcription factors. The NHR family includes glucocorticoid receptors, retinoic acid and retinoid receptors, vitamin D receptors, thyroxin receptors, and peroxisome proliferator activated receptors. These proteins are targets for some of the most commonly prescribed medications in dermatology, including corticosteroids, retinoids, and vitamin D analogues, all of which have limiting side effects. Advances in this field have led to better understanding of the mechanisms of NHR therapeutic and toxic effects, receptor subtypes, tissue distribution, and interaction with other molecules. New generations of more specific NHR ligands designed to increase therapeutic efficacy and limit adverse effects have dramatically expanded the clinical application of NHR-targeting drugs. The current understanding of NHRs and future directions for NHR ligands in dermatology are discussed.

Negative response elements in keratin genes mediate transcriptional repression and the cross-talk among nuclear receptors

Very little is known about the mechanisms responsible for the findings that binding of nuclear receptors (NR) to some promoter elements leads to transcriptional activation, whereas binding to others leads to repression. Case in point is the group of epidermal keratin genes and their DNA sequences responsible for repression by NR. Keratin response elements (KREs) interact with receptors for retinoic acid, thyroid hormone, and glucocorticoids. KREs, by their structure and sequence, direct the binding of retinoic acid and thyroid hormone as homodimers and glucocorticoids as monomers. Such specific DNA-receptor interactions are crucial for the repression signal of transcription. In this paper we have analyzed the interactions between the KREs and NR that lead to such repression. We have found that KREs are promoter-independent. They not only provide a docking platform for the receptors, but also play a key role in directing the receptors to bind into particular configurations and coordinating the interactions among different receptors. Both an intact KRE and an intact receptor DNA-binding domain are necessary for the regulation to occur, which emphasizes the importance of interaction between the DNA and NR for proper signaling. Furthermore, KREs allow simultaneous binding of multiple receptors, thus providing fine-tuning of transcriptional regulation. The DNA/DNA-binding domain interactions in keratin promoters exemplify tissue and gene specificity of hormone action.

Regulation of human profilaggrin promoter activity in cultured epithelial cells by retinoic acid and glucocorticoids

Vitamin A and other retinoids profoundly inhibit both morphological and biochemical aspects of epidermal differentiation in vitro. Profilaggrin, like most other markers of keratinocyte differentiation, is negatively regulated by retinoic acid in vitro, both at the level of mRNA synthesis and by inhibiting the activity of endoproteases that convert profilaggrin to filaggrin. Profilaggrin is an abundant component of keratohyalin granules and forms the precursor of filaggrin, the keratin associated protein of the stratum corneum. In this report, we identify a region of the human profilaggrin promoter that is involved in the transcriptional regulation of expression by retinoic acid (RA). A series of promoter deletions linked to the chloramphenicol acetyl transferase (CAT) reporter gene were prepared and analyzed by transfection into Hela cells and keratinocytes. We also cotransfected vectors expressing retinoic acid receptor and cultured the transfected cells in the presence and absence of ligand. The region responsive to retinoic acid was localized to a 53 bp sequence between -1109 and -1056 (relative to the mRNA start site at +1) that contains a cluster of five retinoic acid response elements with variable spacing and orientation. In vitro gel shift analysis demonstrated that nuclear retinoid receptors do not bind directly to the identified sequence, suggesting that the mode of regulation by RA may be indirect or that binding requires another cofactor in addition to retinoid receptors. Whereas in keratin genes retinoic acid and glucocorticoid responsive sequences frequently coincide, the glucocorticoid response element in the profilaggrin promoter was located downstream of the RARE cluster between -965 and -951. These studies demonstrate that RA and glucocorticoids regulate profilaggrin expression at least in part by transcriptional mechanisms, via a region of the promoter that contains both retinoid and glucocorticoid responsive elements.

Regulation of mucin gene expression in human tracheobronchial epithelial cells by thyroid hormone

We reported previously that the expression of the gene encoding MUC5AC mucin in human airway epithelial cells is controlled by retinoic acid via the retinoic acid receptor (RAR)-alpha and that 3,3',5-tri-iodothyronine (T(3)) inhibits the expression of MUC5AC. The purpose of the present study was to identify mechanisms mediating the effect of T(3). T(3) has been shown to inhibit gene expression via several mechanisms, either by enhancing or repressing the transcription of target genes or by the regulation of post-transcriptional events. Results showed that T(3) strongly inhibited MUC5AC-driven luciferase activity in normal human tracheobronchial epithelial cells that had been transiently transfected with a MUC5AC-luciferase reporter construct; however, it did not affect MUC5AC mRNA stability. These results indicate that T(3) suppresses MUC5AC expression at the transcriptional level. An analysis of deletion constructs showed that deletion of the region downstream of 3 kb resulted in markedly decreased levels of MUC5AC transcription in the absence of T(3) (i.e. under control conditions) as well as a loss of responsiveness to the inhibitory effects of T(3). This suggests that this region might contain elements important for the activation as well as the repression of MUC5AC transcription. To determine whether T(3) modulates retinoic-acid-dependent MUC5AC transcription via an alteration in the abundance of retinoid receptor proteins, we examined the type and abundance of these receptors in nuclear extracts of airway epithelial cells grown in the presence or absence of T(3). Western blots showed that T(3) markedly decreased several types of retinoid receptor while not affecting T(3) receptor proteins. Consistent with this finding were gel-shift assays revealing a decrease in RAR-retinoic acid response element complexes obtained from T(3)-treated cells. We propose that T(3) might inhibit retinoid-dependent MUC5AC expression by decreasing retinoid receptor levels and thereby decreasing the transcriptional activation of this gene for mucins.

Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differentiation, and homeostasis, are used extensively in the treatment of skin diseases. Using keratin gene expression as a paradigm of epidermal physiology and pathology, we have developed a model system to study the molecular mechanism of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an example of customizing a device for transcriptional regulation to target a specific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific keratins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-specific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed footprinting revealed that the function of nGREs is to instruct the GR to bind as four monomers. Furthermore, using cotransfection and antisense technology we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex that suppresses keratin genes, histone acetyltransferase and CBP are. In addition, we have found that GR, independently from GREs, blocks the induction of keratin gene expression by AP1. We conclude that GR suppresses keratin gene expression through two independent mechanisms: directly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.

Keratin 4 upregulation by retinoic acid in vivo: a sensitive marker for retinoid bioactivity in human epidermis

Retinoids affect keratinocyte differentiation and modulate the expression of many epidermal proteins, among them cellular retinoic acid-binding protein II and the family of cytokeratins. The upregulation of the former protein is a well-known phenomenon, whereas the retinoid-induced regulation of epidermal keratin expression is more complex and only partially understood. We studied the effect of topical retinoids on the expression in healthy skin of cellular retinoic acid-binding protein II, tazarotene-induced genes 1 and 2, several epidermal keratins (K1, K2e, and K10), and two mucous keratins (K4 and K13) known to appear in epidermis under certain abnormal conditions. Reverse transcription-polymerase chain reaction experiments showed that the K4 expression was the one most overtly induced by 2 wk of open treatment with 0.05% of retinoic acid and tazarotene. Using real-time quantitative polymerase chain reaction (TaqMan) and normalization of the mRNA values to beta-actin, the increase in K4 was found to be 100-1000-fold. In comparison, the expression of K13 and cellular retinoic acid-binding protein II was increased 10-50-fold, the K1 and K10 mRNA levels remained unchanged, and the K2e level decreased by a factor of 100-1000. In parallel biopsies, immunohistochemistry showed no change in K1, K2e, or K10 staining, but a strong de novo appearance of K4 in the granular layer after retinoid treatment. In a separate study, occlusive application of 0.025% retinoic acid in four healthy subjects produced a maximal K4 mRNA signal after 48 h and strong K4 staining after 80 h. Finally, a dose-response study showed that the de novo appearance of K4 can be utilized as a sensitive test for retinoid bioactivity in epidermis in vivo.

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

Stimulation of the myelin basic protein gene expression by 9-cis-retinoic acid and thyroid hormone: activation in the context of its native promoter

Thyroid hormone plays an important role in brain development, in part by regulating myelination. Previous studies have shown that the myelin basic protein (MBP) promoter is activated by thyroid hormone (T3) via a T3-response element (T3RE) at position -186. Surprisingly, although MBP levels are initially decreased in hypothyroid neonates, they approach later control levels, in most brain regions, despite persistent hypothyroidism. We have studied the T3-independent transcriptional regulation of this gene, using transient transfection assays. We found that, in the absence of T3, the RXR ligand, 9-cis-retinoic acid (9cRA) was able to stimulate transcription of the MBP promoter in a dose-dependent manner. This activation was unaffected by the mutation or deletion of the T3RE and required DNA sequences located between positions -162/+60. Accordingly, this MBP promoter fragment bound RXR in vitro. The 9cRA-dependent activation of the MBP promoter required the presence of both, the DNA binding and the ligand-dependent transactivation domain (AF-2) in RXR. Furthermore, as T3, 9cRA was able to stimulate MBP expression in the CG-4 cell line after differentiation to oligodendrocytes and increased the number of cells expressing the MBP protein in primary rat optic nerve glial cell cultures.

Restoration of the mucous phenotype by retinoic acid in retinoid-deficient human bronchial cell cultures: changes in mucin gene expression

Retinoid-deficient cultures of airway epithelial cells undergo squamous differentiation. Treatment of such cultures with retinoic acid (RA) leads to restoration of the mucous phenotype. The purpose of our study was to characterize the cellular and molecular changes following RA treatment of retinoid-deficient human tracheobronchial epithelial cell cultures. Of particular interest was to determine when during the conversion of the squamous to the mucous phenotype the mucin genes MUC2, MUC5AC, and MUC5B were expressed. We used cornifin alpha and secreted mucin as markers to monitor the squamous and mucous phenotypes, respectively. Our studies showed that the RA responsiveness of the cultures progressively decreased with protracted retinoid deficiency, requiring higher RA concentrations to restore the mucous phenotype. Within 12 h after the start of RA treatment, cornifin alpha expression decreased, signaling the beginning of a change in cellular phenotype. At 24 h after addition of RA to the cultures, a significant number of mucous cells appeared, and at 72 h mucin was secreted in measurable amounts. Induction of mucin gene expression occurred sequentially: MUC2, MUC5AC, and MUC5B mRNAs were upregulated at 24, 48, and 72 h, respectively. When cultures maintained in 10(-8) M RA were treated with 10(-6) M RA, MUC2 but not MUC5AC and MUC5B mRNA levels were upregulated within 6 h. Our study indicates that MUC2 mRNA is an early marker of mucous differentiation, whereas MUC5AC and MUC5B mRNAs are expressed during more advanced stages of mucous differentiation. Our studies further suggest that each of the mucin genes is regulated by distinct mechanisms.

Constitutive activation of gene expression by thyroid hormone receptor results from reversal of p53-mediated repression

Thyroid hormone receptor (T3R) is a member of the steroid hormone receptor gene family of nuclear hormone receptors. In most cells T3R activates gene expression only in the presence of its ligand, L-triiodothyronine (T3). However, in certain cell types (e.g., GH4C1 cells) expression of T3R leads to hormone-independent constitutive activation. This activation by unliganded T3R occurs with a variety of gene promoters and appears to be independent of the binding of T3R to specific thyroid hormone response elements (TREs). Previous studies indicate that this constitutive activation results from the titration of an inhibitor of transcription. Since the tumor suppresser p53 is capable of repressing a wide variety of gene promoters, we considered the possibility that the inhibitor is p53. Evidence to support this comes from studies indicating that expression of p53 blocks T3R-mediated constitutive activation in GH4C1 cells. In contrast with hormone-independent activation by T3R, p53 had little or no effect on T3-dependent stimulation which requires TREs. In addition, p53 mutants which oligomerize with wild-type p53 and interfere with its function also increase promoter activity. This enhancement is of similar magnitude to but is not additive with the stimulation mediated by unliganded T3R, suggesting that they target the same factor. Since p53 mutants are known to target wild-type p53 in the cell, this suggests that T3R also interacts with p53 in vivo and that endogenous levels of p53 act to suppress promoter activity. Evidence supporting both functional and physical interactions of T3R and p53 in the cell is presented. The DNA binding domain (DBD) of T3R is important in mediating constitutive activation, and the receptor DBD appears to functionally interact with the N terminus of p53 in the cell. In vitro binding studies indicate that the T3R DBD is important for interaction of T3R with p53 and that this interaction is reduced by T3. These findings are consistent with the in vivo studies indicating that p53 blocks constitutive activation but not ligand-dependent stimulation. These studies provide insight into mechanisms by which unliganded nuclear hormone receptors can modulate gene expression and may provide an explanation for the mechanism of action of the v-erbA oncoprotein, a retroviral homolog of chicken T3R alpha.

The epidermis: genes on - genes off

The epidermal keratinocyte stem cell is distinguished by a relatively undifferentiated phenotype and an ability to proliferate. As part of a carefully orchestrated process, the offspring of these stem cells lose the ability to proliferate and begin a process of morphologic and biochemical transformation that results in their conversion into corneocytes. This process requires the coordinated expression of a host of cellular genes. The mechanisms responsible for regulation of these genes is an area of intense interest. In keratinocytes, as in other cell types, the expression of most genes is regulated at the transcriptional level by a class of proteins called transcription factors. Transcription factors are nuclear proteins that regulate transcription by mediating the final steps in the relay of information from the cell surface to the nucleus and the gene. These factors bind to specific DNA sequence elements located within the target gene. In this brief review we summarize evidence implicating activator protein 1 (AP1), AP2, Sp1, POU domain, CCAAT enhancer binding protein, and several other transcription factors as regulators of expression of keratinocyte genes.

Specific organization of the negative response elements for retinoic acid and thyroid hormone receptors in keratin gene family

Retinoic acid and thyroid hormone are important regulators of epidermal growth, differentiation, and homeostasis. Retinoic acid is extensively used in the treatment of many epidermal disorders ranging from wrinkles to skin cancers. Retinoic acid and thyroid hormone directly control the transcription of differentiation-specific genes including keratins. Their effect is mediated through nuclear receptors RAR and T3R. We have previously identified the response element in the K14 gene, K14RARE/TRE, to which these receptors bind, and found that it consists of a cluster of five half-sites with variable spacing and orientation. To determine whether this specific structure is found in other keratin genes, we have mapped and analyzed the RARE/TRE elements in three additional epidermal keratin genes: K5, K6, and K17. We used three different approaches to identify these elements: co-transfection of promoter deletion constructs, gel-shift assays, and site-specific mutagenesis. We localized the RARE/TRE elements relatively close to the TATA box in all three promoters. All three RARE/TRE elements have a similar structural organization: they consist of clusters of 3-6 half-sites with variable spacing and orientation. This means that the clustered structure of the RARE/TREs is a common characteristic for keratin genes. RARE and TRE in the K5 promoter are adjacent to each other whereas in the K17 promoter they overlap. All three keratin REs bind specifically both RAR and T3R in gel-shift assays. Interestingly, addition of ligand to the receptor changes the binding pattern ofthe T3R from homodimer to monomer, reflecting the change in regulation from induction to inhibition.

Cytokeratin and proliferative cell nuclear antigen expression in superior limbic keratoconjunctivitis

PURPOSE

Superior limbic keratoconjunctivitis (SLK) is a disease of unknown etiology which showed various degrees of keratinization. The cytokeratins (CKs) are known to be the hallmark of epithelial differentiations and each CK has a characteristic distribution dependent on the type and the differentiation status of epithelium. The authors have studied the expression of cytokeratins (CKs) as well as proliferating cell nuclear antigen (PCNA) in the conjunctiva of SLK.

METHODS

Three superior limbic conjunctivae of patients with SLK and two normal conjunctivae were examined immunohistochemically using monoclonal antibodies against CKs and PCNA.

RESULTS

In SLK conjunctivae, increased expression of a basal cell marker (CK14) was observed throughout the epithelium. Sporadic positive staining with CK10, which is a specific marker for keratinization, was correlated to the severity of the disease. The disappearance of CK13 staining, the marker for nonkeratinized stratified epithelia, at the basal cell layer of SLK was noted. Moreover, increased expression of PCNA in SLK was observed.

CONCLUSIONS

The altered expression of CKs in SLK suggests an abnormality of differentiation in the conjunctival epithelium of the disease. Upregulated proliferation of conjunctival epithelial cells may be another feature of the disease.

Regulation of gene expression of a binding protein for fibroblast growth factors by retinoic acid

Retinoids are potent regulators of growth and differentiation and have shown promise as chemotherapeutic agents against selected cancers in particular squamous cell carcinoma (SCC). Earlier studies from our laboratory showed that a secreted binding protein for fibroblast growth factors (BP) is expressed at high levels in SCC cell lines and tissue samples. Here we investigate whether retinoids affect BP gene expression in SCC. In six different human SCC cell lines, we found that all-trans-retinoic acid (tRA) down-regulated BP mRNA by 39-89% within 24 h. From this group of cell lines, we selected the ME-180 cell line for more detailed studies of the mechanisms of this regulation. tRA down-regulated BP mRNA in a time- and dose-dependent manner. The effect of tRA was reversible, and BP mRNA returned to control levels within 24 h after removal of tRA. We also measured BP mRNA half-life and performed nuclear run-on experiments to study if tRA down-regulates BP by destabilizing the mRNA and/or by decreasing the rate of transcription. BP mRNA in ME-180 cells is very stable with a half-life of >16 h, and tRA decreased BP mRNA with a half-time of 5 h. Actinomycin D and cycloheximide blocked the tRA effect, suggesting that transcriptional regulation as well as de novo protein synthesis contribute to this post-transcriptional regulation of BP mRNA levels. In addition, tRA decreased the rate of BP gene transcription by 2- to 3-fold within 1 h. We conclude that retinoids down-regulate BP gene expression by post-transcriptional as well as by transcriptional mechanisms.

Upregulation of cytokeratins 8 and 18 in human breast cancer T47D cells is retinoid-specific and retinoic acid receptor-dependent

The mamary gland is chiefly composed of luminal epithelial cells expressing cytokeratins (K) 8, 18 and 19, and basal/myoepithelial cells expressing cytokeratins 5 and 14. Human breast cancer T47D cells have a luminal phenotype and are growth-inhibited by retinoids, a class of compounds known to regulate cytokeratin expression. To extend our knowledge of retinoid action in breast cancer, we have studied the retinoid regulation of cytokeratin expression in the T47D model. We found that retinoid inhibition of T47D cell growth was accompanied by increases in K8, K18 and K19 mRNA steady-state levels (Northern blot analysis). The effect on K8 was studied in greater detail. This effect was seen with as low as 1 nM all-trans retinoic acid (tRA) and was maximal (up to 7 fold over control) with 1 microM tRA (the highest dose tested). Time-course studies revealed a detectable effect at 1 h and a maximal effect at 8-24 h. Non-retinoidal growth inhibitors (tamoxifen, BrcAMP and genistein) did not modulate K8 expression, demonstrating that the effect of tRA was specific, K8 mRNA upregulation was blocked by actinomycin D and cycloheximide, suggesting, in accordance with other studies, that tRA exerted a transcriptional effect that was secondary to de novo protein synthesis. Five retinoids known to activate retinoic acid receptor (RAR) and/or retinoid X receptor (RXR) - tRA; 9-cis-retinoic acid, 9cRA; 13-cis RA, 13cRA; retinyl acetate; and N-(4-hydroxyphenyl) retinamide 4HPR - inhibited T47D cell growth and increased K8 expression, whereas an arotinoid (Ro-40-8757) that is not a RAR activator caused growth inhibition but did not upregulate K8. Activation of RAR alpha contributed to K8 upregulation, since this effect was partially blocked by the RAR alpha-selective antagonist Ro-41-5253. Analogous results were obtained throughout when blots were reprobed with K18 cDNA. Western blot and immunocytochemistry experiments demonstrated that protein levels of K8 and K18 increased by 2 days of treatment with 1 microM tRA. These results show that retinoids enhance the expression of cognate cytokeratin markers of luminal differentiation in T47D breast cancer cells.

Coordinate control of growth and cytokeratin 13 expression by retinoic acid

Retinoic acid (RA) modulates the growth and differentiation of various normal and malignant cells. These effects are most likely mediated by changes in gene expression. Genes whose expression is modulated by RA may be useful as markers of growth responsiveness to retinoids. Using differential cDNA cloning we identified 10 genes regulated by RA in the head and neck squamous cell carcinoma cell line MDA886Ln. Keratin (K) 13 gene expression was the gene expression most related to the degree of sensitivity of growth to RA, as K13 was not expressed in a series of RA-resistant cell lines. Our data suggest that low K13 expression may be mechanistically related to resistance to RA-induced growth inhibition.

Inhibition of TGF-alpha gene expression by vitamin A in airway epithelium

The autocrine/paracrine growth mechanism has been implicated in the regulation of bronchial epithelial cell proliferation. By inhibiting the expression of the transforming growth factor-alpha (TGF-alpha) gene product, vitamin A is able to suppress the proliferation of tracheobronchial epithelial cells in culture. Similar repressions in TGF-alpha mRNA levels by retinol were observed in airway explant cultures and in a cell line immortalized from normal human bronchial epithelial cells. Both the nuclear run-on transcriptional assay and the transfection study with the chimeric construct of the TGF-alpha promoter and chloramphenicol acetyltransferase reporter gene partly suggest a transcriptional downregulation mechanism of TGF-alpha gene expression by the retinol treatment; however, this inhibition at the transcriptional level cannot account for the total inhibition at the mRNA level. These results suggest that a downregulation of the expression of the TGF-alpha gene at the transcriptional and post-transcriptional levels by vitamin A may precede the essential event associated with the homeostasis of normal conducting airway epithelium.

Novel regulation of keratin gene expression by thyroid hormone and retinoid receptors

Expression of keratin proteins, markers of epidermal differentiation and pathology, is uniquely regulated by the nuclear receptors for retinoic acid (RAR) and thyroid hormone (T3R) and their ligands: it is constitutively activated by unliganded T3R, but it is suppressed by ligand-occupied T3R or RAR. This regulation was studied using gel mobility shift assays with purified receptors and transient transfection assays with vectors expressing various receptor mutants. Regulation of keratin gene expression by RAR and T3R occurs through direct binding of these receptors to receptor response elements of the keratin gene promoters. The DNA binding "C" domain of these receptors is essential for both ligand-dependent and -independent regulation. However, the NH2-terminal "A/B" domain of T3R is not required for either mode of regulation of keratin gene expression. Furthermore, v-ErbA, an oncogenic derivative of cT3R, also activates keratin gene expression. In contrast to the previously described mechanism of gene regulation by T3R, heterodimerization with the retinoid X receptor is not essential for activation of keratin gene expression by unliganded T3R. These findings indicate that the mechanism of regulation of keratin genes by RAR and T3R differs significantly from the mechanisms described for other genes modulated by these receptors.

Characterization of nuclear protein binding sites in the promoter of keratin K17 gene

Keratin K17, while not present in healthy skin, is expressed under various pathological conditions, including psoriasis and cutaneous allergic reactions. The regulatory circuits involved in transcription of the human keratin K17 gene are poorly understood. To begin an analysis of the molecular mechanisms that regulate K17 gene transcription, we have studied the interactions between the nuclear proteins and the promoter region of the human K17 gene. That promoter region comprised 450 bp upstream from the translation initiation site. For these studies, we used electrophoretic mobility-shift assays, computer analysis, site-directed mutagenesis, and DNA-mediated cell transfection. In addition to the previously characterized interferon-gamma-responsive elements, we identified eight protein binding sites in the promoter. Five of them bind the known transcription factors NF1, AP2, and Sp1 and three others bind still unidentified proteins. Using site-directed mutagenesis, we have demonstrated the importance of the protein binding sites for the promoter function involved in both constitutive and interferon-induced expression of the K17 keratin gene.

Retinoid receptors and keratinocytes

In 1987, a tremendous boost in our understanding of the action of dietary vitamin A occurred with the discovery and characterization of nuclear receptors for retinoic acid, the active form of the vitamin, in the laboratories of P. Chambon and R. Evans. They have shown that the nuclear receptors are ligand-activated transcription factors capable of specific gene regulation. Since that discovery, it has been determined that there are at least six retinoic acid receptors belonging to two families, RARs and RXRs, that they are differentially expressed in various mammalian tissues, and that they act as homo- and heterodimers interacting with other ligand-activated nuclear receptors. The domain structure of the receptors has been described, and their DNA-binding, ligand-binding, dimerization, and transcriptional activation regions characterized. Among the most important retinoid-regulated genes are the homeobox proteins, regulatory transcription factors which are responsible for body axis formation, patterning, limb formation, and other crucial processes during development. Retinoic acid and its receptors also regulate many differentiation markers which are particularly important in stratified epithelia, such as skin and oral epithelia. Our increased understanding led to improved therapy of a large number of skin disorders, ranging from acne to wrinkles and including epidermal and oral carcinomas.

Elements controlling the expression and induction of the skin hyperproliferation-associated keratin K6

The suprabasal keratin 6 (K6) is remarkable among the keratins as, in addition to being constitutively expressed in different stratified epithelia, it is induced in epidermis under hyperproliferative conditions, such as benign or malignant tumors, psoriasis, and wound healing. In addition, this keratin is also induced in skin treated with 12-O-tetradecanoylphorbol-13-acetate or retinoic acid (RA). These characteristics make the study of K6 regulatory elements an especially interesting issue, in particular because these elements could be useful in designing gene constructs for the therapy of skin diseases. We have analyzed by mobility shift and footprinting experiments the cell type-specific enhancer of the bovine K6 beta gene (Blessing, M., Jorcano, J. L., and Franke, W. W. (1989) EMBO J. 8, 117-126) and have identified an AP-2-like element, two AP-1 elements (one of them composite), and a retinoic acid-responsive element (RARE). Mutagenesis experiments and cotransfections with retinoic acid receptors show that the RARE mediates enhancer activation by RA. Chloramphenicol acetyltransferase assays show that under normal culture conditions, the AP-1 element retains most of the enhancer transcriptional activity, while the RARE and AP-2 are weakly active. However, following RA treatment, the AP-1 element is repressed and the RARE is activated, resulting in an overall stimulation of the enhancer by RA in the BMGE+H cells used in our study. These results explain in part the complex and sometimes contradictory response of keratin 6 to hyperproliferative stimuli.

Liarozole, an antitumor drug, modulates cytokeratin expression in the Dunning AT-6sq prostatic carcinoma through in situ accumulation of all-trans-retinoic acid

Liarozole showed antitumoral activity in the Dunning AT-6sq, an androgen-independent rat prostate carcinoma. To investigate its potential mechanism of action, the effects of the drug doses (ranging from 3.75 to 80 mg/kg b.i.d.) on endogenous plasma and tissue all-trans-retinoic acid levels and on the differentiation status of the tumor cells were evaluated. To follow modulation of differentiation, cytokeratins were localized in the (un)treated tumors by immunocytochemistry and quantitatively determined by immunoblotting. Results showed that liarozole statistically significantly reduced tumor weight from 30 mg/kg upwards and induced accumulation of all-trans-retinoic acid both in plasma and tumors. In the tumors, a statistically significant accumulation was already noted from 7.5 mg liarozole/kg upwards. Concomitantly, the differentiation status shifted from a keratinizing towards a non-keratinizing squamous carcinoma, which was further confirmed by the cytokeratin profile of the carcinoma (presence of CK 8, 10, 13, 14, 18, 19). Immunoblotting revealed an overall decrease in cytokeratin content, except for CK 8. These findings suggest that the antitumoral properties of liarozole might be related to an increase in the degree of tumor differentiation through accumulation of all-trans-retinoic acid.

The A/B domain of truncated retinoic acid receptors can block differentiation and promote features of malignancy

Recently, we discovered that stable introduction of a carboxyl-terminally truncated retinoic acid receptor gamma (tRAR gamma) into an epidermal keratinocyte line blocked the ability of these cells to differentiate, as judged by their failure to express late markers of squamous differentiation. We now demonstrate a correlation between the level of residual endogenous RAR activity of tRAR gamma-expressing keratinocyte lines and degree of terminal differentiation. Mutagenesis studies localize the effects to the A/B subdomain of the truncated receptor. Despite tRAR gamma's capacity to interfere with RAR-mediated transactivation of retinoic acid response elements (RAREs) in keratinocytes, the effects of the truncated receptor are independent of its ability to bind DNA and directly interact with endogenous RARs. tRAR alpha also inhibits RARE-mediated gene expression in keratinocytes, even though its full-length counterpart enhances RARE activity in these cells. Intriguingly, both tRAR gamma and RAR gamma suppress keratin promoter activity in epidermal cells, although for tRAR gamma, the effect is mediated through the A/B domain whereas for RAR gamma, the effects require DNA binding. Taken together, these findings suggest that the truncation allows for new and aberrant interactions with transcriptional proteins/cofactors that participate in governing RARE activity. This discovery may have relevance in tumorigenesis, where genetic lesions can result in mutant RARs or in loss of receptor expression.

TGF beta promotes the basal phenotype of epidermal keratinocytes: transcriptional induction of K#5 and K#14 keratin genes

TGFbeta is an important regulator of epidermal keratinocyte function because it suppresses cell proliferation, while it induces synthesis of extracellular matrix proteins and their cells surface receptors. To examine whether TGFbeta affects synthesis of intracellular proteins as well, specifically the transcription of keratin genes, we transfected a series of DNA constructs that contain keratin gene promoters into human epidermal keratinocytes. The transfected cells were grown in the presence and absence of TGFbeta. We found that TGFbeta specifically induces transcription controlled by the promoters of K#5 and K#14 keratin genes, markers of basal cells. No other keratin gene promoters were induced. The effect of TGFbeta is concentration-dependent, can be demonstrated in HeLa cells, does not depend on keratinocyte growth conditions and can be elicited by both TGFbeta1 and TGFbeta2. We conclude that TGFbeta promotes the basal cell phenotype in stratified epithelia such as the epidermis.

Differential effects of topical retinoic acid application on keratin K1 and filaggrin expression in mouse epidermis

Retinoic acid (RA) modulates epidermal homeostasis and affects differentiation-associated proteins such as keratin K1 and filaggrin. Because results from in vitro and in vivo studies have been conflicting with respect to RA effects on keratinization, we examined the terminal differentiation of epidermal cell cohorts after RA stimulation in vivo. Pulse-labelling with 5-bromo-2-deoxyuridine (BrdU) was performed by intraperitoneal injection of mice immediately or at 16 h after a single topical application of 100 nmol RA. The cell cohort labelled at the time of RA application consisted of previously unperturbed cells exposed to RA after initiation of S-phase, whereas the cohort labelled 16 h after RA application consisted of cells stimulated into the S-phase by RA. These two cohorts of partially synchronized cells were followed for up to 72 h after BrdU labelling. Such labelling combined with keratin K1 or filaggrin expression was scored by paired immunofluorescence staining of skin sections. The onset of keratin K1 expression was unchanged in both series after RA treatment, while filaggrin appeared earlier than in controls. The differential effect of RA on the maturation markers was related to the proliferative activity, the increased cell turnover, and the shortened epidermal transit time. The onset of keratin expression appeared to be regulated before the postmitotic period, whereas filaggrin expression appeared to be regulated during the late phase of the maturation process, thus being influenced by the actual epidermal kinetics and structural alterations. These results suggested that the effect of RA on epidermal differentiation is secondary to its effect on proliferation, as determined by the altered cellular age distribution following regenerative proliferation.

Oct-6: a regulator of keratinocyte gene expression in stratified squamous epithelia

POU domain proteins have been implicated as regulators of differentiation and development, particularly in early embryogenesis and in neural morphogenesis. Given that neural and epidermal lineages originate from a common precursor (ectodermal) cell, we explored the possibility that POU proteins are involved in epidermal differentiation. Using reverse transcription-PCR and degenerate oligonucleotides, we generated several POU domain cDNAs from cultured human epidermal mRNAs. One of these encoded a sequence identical to the rodent Tst-1/SCIP/Oct-6 POU domain. Subsequently, we isolated a cDNA encoding a 45.3-kDa protein with 98% sequence identity to rat Tst-1/SCIP and 94% identity to mouse Oct-6. This protein bound specifically to the canonical octamer motif, warranting its designation as human Oct-6. By RNase protection assays, by PCR, and by immunoblot analysis, Oct-6 was expressed in cultured epidermal keratinocytes. By in situ hybridization, Oct-6 mRNA was detected not only in epidermis but also a variety of other stratified squamous epithelia and with greater signals than testis, the tissue in which this POU protein was originally discovered. Moreover, Oct-6 exerted a marked and specific negative influence on expression of the K5 and K14 genes, abundantly expressed in most dividing stratified squamous epithelial cells and downregulated as cells commit to terminally differentiate. The repressive effect was complex, but it was not observed with Oct-1, nor was it seen with a truncated Oct-6 missing the POU domain. Taken together, our studies suggest that Oct-6 may play an important role in controlling gene expression in stratified squamous epithelia, including epidermis.