Regulation of keratin gene expression: the role of the nuclear receptors for retinoic acid, thyroid hormone, and vitamin D3

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.

Nuclear Receptors, Ligands and the Mammalian B Cell

Questions concerning the influences of nuclear receptors and their ligands on mammalian B cells are vast in number. Here, we briefly review the effects of nuclear receptor ligands, including estrogen and vitamins, on immunoglobulin production and protection from infectious diseases. We describe nuclear receptor interactions with the B cell genome and the potential mechanisms of gene regulation. Attention to the nuclear receptor/ligand regulation of B cell function may help optimize B cell responses, improve pathogen clearance, and prevent damaging responses toward inert- and self-antigens.

Serum lipids, retinoic acid and phenol red differentially regulate expression of keratins K1, K10 and K2 in cultured keratinocytes

Abnormal keratinocyte differentiation is fundamental to pathologies such as skin cancer and mucosal inflammatory diseases. The ability to grow keratinocytes in vitro allows the study of differentiation however any translational value is limited if keratinocytes get altered by the culture method. Although serum lipids (SLPs) and phenol red (PR) are ubiquitous components of culture media their effect on differentiation is largely unknown. We show for the first time that PR and SLP themselves suppress expression of differentiation-specific keratins K1, K10 and K2 in normal human epidermal keratinocytes (NHEK) and two important cell lines, HaCaT and N/TERT-1. Removal of SLP increased expression of K1, K10 and K2 in 2D and 3D cultures, which was further enhanced in the absence of PR. The effect was reversed for K1 and K10 by adding all-trans retinoic acid (ATRA) but increased for K2 in the absence of PR. Furthermore, retinoid regulation of differentiation-specific keratins involves post-transcriptional mechanisms as we show KRT2 mRNA is stabilised whilst KRT1 and KRT10 mRNAs are destabilised in the presence of ATRA. Taken together, our results indicate that the presence of PR and SLP in cell culture media may significantly impact in vitro studies of keratinocyte differentiation.

Enhanced CD103 Expression and Reduced Frequencies of Virus-Specific CD8+ T Cells Among Airway Lymphocytes After Influenza Vaccination of Mice Deficient in Vitamins A + D

Previous research has evaluated antibody responses toward an influenza virus vaccine in the context of deficiencies for vitamins A and D (VAD+VDD). Results showed that antibodies and antibody-forming cells in the respiratory tract were reduced in VAD+VDD mice. However, effectors were recovered when oral supplements of vitamins A + D were delivered at the time of vaccination. Here we address the question of how vaccine-induced CD8+ T cell responses are affected by deficiencies for vitamins A + D. VAD+VDD and control mice were vaccinated with an intranasal, cold-adapted influenza virus A/Puerto Rico/8/34 vaccine, with or without oral supplements of vitamins A + D. Results showed that the percentages of vaccine-induced CD8+ T cell and total CD4+ T cell responses were low among lymphocytes in the airways of VAD+VDD animals compared to controls. The CD103 membrane marker, a protein that binds e-cadherin (expressed on respiratory tract epithelial cells), was unusually high on virus-specific T cells in VAD+VDD mice compared to controls. Interestingly, when T cells specific for the PA224-233/Db epitope were compared with T cells specific for the NP366-374/Db epitope, the former population was more strongly positive for CD103. Preliminary experiments revealed normal or above-normal percentages for vaccine-induced T cells in airways when VAD+VDD animals were supplemented with vitamins A + D at the time of vaccination and on days 3 and 7 after vaccination. Our results suggest that close attention should be paid to levels of vitamins A and D among vaccine recipients in the clinical arena, as low vitamin levels may render individuals poorly responsive to vaccines.

Management of Plantar KeratodermasLessons from Pachyonychia Congenita

Plantar keratodermas can arise due to a variety of genetically inherited mutations. The need to distinguish between different plantar keratoderma disorders is becoming increasingly apparent because there is evidence that they do not respond identically to treatment. Diagnosis can be aided by observation of other clinical manifestations, such as palmar keratoderma, more widespread hyperkeratosis of the epidermis, hair and nail dystrophies, or erythroderma. However, there are frequent cases of plantar keratoderma that occur in isolation. This review focuses on the rare autosomal dominant keratin disorder pachyonychia congenita, which presents with particularly painful plantar keratoderma for which there is no specific treatment. Typically, patients regularly trim/pare/file/grind their calluses and file/grind/clip their nails. Topical agents, including keratolytics (eg, salicylic acid, urea) and moisturizers, can provide limited benefit by softening the skin. For some patients, retinoids help to thin calluses but may lead to increased pain. This finding has stimulated a drive for alternative treatment options, from gene therapy to alternative nongenetic methods that focus on novel findings regarding the pathogenesis of pachyonychia congenita and the function of the underlying genes.

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.

Keratinocyte detachment-differentiation connection revisited, or anoikis-pityriasi nexus redux

Epidermis, a continuously self-renewing and differentiating organ, produces a protective stratum corneum that shields us from external chemical, physical and microbial threats. Epidermal differentiation is a multi-step process regulated by influences, some unknown, others insufficiently explored. Detachment of keratinocytes from the basement membrane is one such pro-differentiation stimulus. Here, we define the transcriptional changes during differentiation, especially those caused by detachment from the substratum. Using comprehensive transcriptional profiling, we revisited the effects of detachment as a differentiation signal to keratinocytes. We identified the genes regulated by detachment, the corresponding ontological categories and, using metaanalysis, compared the genes and categories to those regulated by other pro-differentiating stimuli. We identified 762 genes overexpressed in suspended keratinocyte, including known and novel differentiation markers, and 1427 in attached cells, including basal layer markers. Detachment induced epidermis development, cornification and desmosomal genes, but also innate immunity, proliferation inhibitors, transcription regulators and MAPKs; conversely the attached cells overexpressed cell cycle, anchoring, motility, splicing and mitochondrial genes, and both positive and negative regulators of apoptosis. Metaanalysis identified which detachment-regulated categories overlap with those induced by suprabasal location in vivo, by reaching confluency in vitro, and by inhibition of JUN kinases. Attached and in vivo basal cells shared overexpression of mitochondrial components. Interestingly, melanosome trafficking components were also overexpressed in the attached and in vivo basal keratinocytes. These results suggest that specific pro-differentiation signals induce specific features of the keratinization process, which are in vivo orchestrated into harmonious epidermal homeostasis.

Cutaneous retinoic acid levels determine hair follicle development and downgrowth

Retinoic acid (RA) is essential during embryogenesis and for tissue homeostasis, whereas excess RA is well known as a teratogen. In humans, excess RA is associated with hair loss. In the present study, we demonstrate that specific levels of RA, regulated by Cyp26b1, one of the RA-degrading enzymes, are required for hair follicle (hf) morphogenesis. Mice with embryonic ablation of Cyp26b1 (Cyp26b1(-/-)) have excessive endogenous RA, resulting in arrest of hf growth at the hair germ stage. The altered hf development is rescued by grafting the mutant skin on immunodeficient mice. Our results show that normalization of RA levels is associated with reinitiation of hf development. Conditional deficiency of Cyp26b1 in the dermis (En1Cre;Cyp26b1f/-) results in decreased hair follicle density and specific effect on hair type, indicating that RA levels also influence regulators of hair bending. Our results support the model of RA-dependent dermal signals regulating hf downgrowth and bending. To elucidate target gene pathways of RA, we performed microarray and RNA-Seq profiling of genes differentially expressed in Cyp26b1(-/-) skin and En1Cre;Cyp26b1f/- tissues. We show specific effects on the Wnt-catenin pathway and on members of the Runx, Fox, and Sox transcription factor families, indicating that RA modulates pathways and factors implicated in hf downgrowth and bending. Our results establish that proper RA distribution is essential for morphogenesis, development, and differentiation of hfs.

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.

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.

Activation of LXRα induces lipogenesis in HaCaT cells

The oxysterol nuclear receptors, LXRα (liver X receptor α; NR1H3) and LXRβ (NR1H2), coordinately regulate the expression of genes involved in lipid metabolism, anti-inflammation, and cholesterol transport. Previous studies have demonstrated that ligands of LXRα are important in the maintenance of the normal epidermal barrier function and keratinocyte differentiation. In this study, we examined whether LXRα and its ligands regulate lipid synthesis in HaCaT cells, a spontaneously transformed human keratinocyte cell line. When HaCaT cells were treated with the LXRα ligand TO901317, lipid droplets accumulated in the majority of cells, which were stained by Oil Red O. A luciferase reporter construct containing the LXR response element was activated about fourfold in HaCaT cells by TO901317 treatment, suggesting that LXR has a role in lipid synthesis in these cells. The expression of LXRα target genes, such as those encoding sterol regulatory binding protein and fatty acid synthase, were induced time dependently by TO901317, as measured by RT-PCR and western blotting. The expression of PPAR-α, -β, and -γ which regulate lipid metabolism, was also increased by TO901317 treatment. In contrast, TO901317 reduced the lipopolysaccharide-induced expression of cyclooxygenase 2 and inducible nitric oxide synthase in HaCaT cells. These results indicate that LXRα activation leads to lipogenesis in keratinocytes, which may enhance the epidermal barrier function of the skin.

Retinoid-responsive transcriptional changes in epidermal keratinocytes

Retinoids (RA) have been used as therapeutic agents for numerous skin diseases, from psoriasis to acne and wrinkles. While RA is known to inhibit keratinocyte differentiation, the molecular effects of RA in epidermis have not been comprehensively defined. To identify the transcriptional targets of RA in primary human epidermal keratinocytes, we compared the transcriptional profiles of cells grown in the presence or absence of all-trans retinoic acid for 1, 4, 24, 48, and 72 h, using large DNA microarrays. As expected, RA suppresses the protein markers of cornification; however the genes responsible for biosynthesis of epidermal lipids, long-chain fatty acids, cholesterol, and sphingolipids, are also suppressed. Importantly, the pathways of RA synthesis, esterification and metabolism are activated by RA; therefore, RA regulates its own bioavailability. Unexpectedly, RA regulates many genes associated with the cell cycle and programmed cell death. This led us to reveal novel effects of RA on keratinocyte proliferation and apoptosis. The response to RA is very fast: 315 genes were regulated already after 1 h. More than one-third of RA-regulated genes function in signal transduction and regulation of transcription. Using in silico analysis, we identified a set of over-represented transcription factor binding sites in the RA-regulated genes. Many psoriasis-related genes are regulated by RA, some induced, others suppressed. These results comprehensively document the transcriptional changes caused by RA in keratinocytes, add new insights into the molecular mechanism influenced by RA in the epidermis and demonstrate the hypothesis-generating power of DNA microarray analysis.

Thyroid hormones directly alter human hair follicle functions: anagen prolongation and stimulation of both hair matrix keratinocyte proliferation and hair pigmentation

CONTEXT

Both insufficient and excess levels of thyroid hormones (T3 and T4) can result in altered hair/skin structure and function (e.g. effluvium). However, it is still unclear whether T3 and T4 exert any direct effects on human hair follicles (HFs), and if so, how exactly human HFs respond to T3/T4 stimulation.

OBJECTIVE

Our objective was to asses the impact of T3/T4 on human HF in vitro.

METHODS

Human anagen HFs were isolated from skin obtained from females undergoing facelift surgery. HFs from euthyroid females between 40 and 69 yr (average, 56 yr) were cultured and treated with T3/T4.

RESULTS

Studying microdissected, organ-cultured normal human scalp HFs, we show here that T4 up-regulates the proliferation of hair matrix keratinocytes, whereas their apoptosis is down-regulated by T3 and T4. T4 also prolongs the duration of the hair growth phase (anagen) in vitro, possibly due to the down-regulation of TGF-beta2, the key anagen-inhibitory growth factor. Because we show here that human HFs transcribe deiodinase genes (D2 and D3), they may be capable of converting T4 to T3. Intrafollicular immunoreactivity for the recognized thyroid hormone-responsive keratins cytokeratin (CK) 6 and CK14 is significantly modulated by T3 and T4 (CK6 is enhanced, CK14 down-regulated). Both T3 and T4 also significantly stimulate intrafollicular melanin synthesis.

CONCLUSIONS

Thus, we present the first evidence that human HFs are direct targets of thyroid hormones and demonstrate that T3 and/or T4 modulate multiple hair biology parameters, ranging from HF cycling to pigmentation.

Inefficacy of topical thyroid hormone analogue TriAc in plaque psoriasis: results of a double-blind placebo-controlled trial

BACKGROUND

Thyroid hormone receptors are expressed in human skin and are believed to be involved in the regulation of epidermal proliferation and differentiation, i.e. processes which are disturbed in psoriatic skin lesions. Ligands of the thyroid hormone receptors have so far not been tested as antipsoriatic agents. TriAc (3,3',5-triiodo-thyroacetic acid) is a well-known thyroid hormone analogue with much reduced cardiac thyrotoxic activity compared with the classical thyroid hormones.

OBJECTIVES

To determine the effectiveness and side-effects of topical TriAc in patients with chronic plaque psoriasis.

METHODS

Twelve patients with mild to moderate psoriasis were treated with TriAc (0.1% in hydrophilic ointment) and placebo applied twice daily to either of two (or several) bilaterally symmetrical plaques for 8 weeks. The patients and investigator were blinded as to the content of the tubes. Every 2 weeks the treated plaques were evaluated by the patient (using a balanced visual analogue scale for a right-left comparison) and by the investigator (using a psoriasis severity index and a global assessment of each plaque).

RESULTS

After 8 weeks of treatment, more than 33% improvement of the psoriasis index occurred in 10 of 12 TriAc-treated and nine of 12 placebo-treated plaques. There were no statistically significant differences between the treatments in terms of reduction of the scores for erythema, scaling, induration or pruritus during the study. Half of the patients considered TriAc superior to placebo, whereas three of 12 were of the opposite opinion (P > 0.05). The global assessment showed marked improvement or remission in six TriAc-treated and five placebo-treated cases (P > 0.05 for difference). No adverse effects were noted.

CONCLUSIONS

TriAc in the dosage and formulation studied was safe but no more effective than placebo in treating plaque psoriasis. However, newer thyroid hormone analogues (agonists or antagonists) might be more active and should be further explored in this context.

Peroxisome proliferator-activated receptor (PPAR)-beta/delta stimulates differentiation and lipid accumulation in keratinocytes

Peroxisome proliferator-activated receptor (PPAR) are nuclear hormone receptors that are activated by endogenous lipid metabolites. Previous studies have demonstrated that PPAR-alpha activation stimulates keratinocyte differentiation in vitro and in vivo, is anti-inflammatory, and improves barrier homeostasis. Recent studies have shown that PPAR-beta/delta activation induces keratinocyte differentiation in vitro. This study demonstrated that topical treatment of mice with a selective PPAR-beta/delta agonist (GW1514) in vivo had pro-differentiating effects, was anti-inflammatory, improved barrier homeostasis, and stimulated differentiation in a disease model of epidermal hyperproliferation [corrected]. In contrast to PPAR-alpha activation, PPAR-beta/deltain vivo did not display anti-proliferative or pro-apoptotic effects. The pro-differentiating effects persisted in mice lacking PPAR-alpha, but were decreased in mice deficient in retinoid X receptor-alpha, the major heterodimerization partner of PPAR. Furthermore, in vitro PPAR-beta/delta activation, aside from stimulating differentiation-related genes, additionally induced adipose differentiation-related protein (ADRP) and fasting induced adipose factor (FIAF) mRNA in cultures keratinocytes, which was paralleled by increased oil red O staining indicative of lipid accumulation, the bulk of which were triglycerides (TG). Comparison of differentially expressed genes between PPAR-beta/delta and PPAR-alpha activation revealed distinct profiles. Together, these studies indicate that PPAR-beta/delta activation stimulates keratinocyte differentiation, is anti-inflammatory, improves barrier homeostasis, and stimulates TG accumulation in keratinocytes.

Nukleäre Hormonrezeptoren: Interventionsmöglichkeiten jenseits von Glukokortikoiden, Retinoiden und Vitamin D

Nuclear hormone receptors are ligand-dependent transcription factors. The class of nuclear hormone receptors that form heterodimers with retinoid X receptor includes members that are well established targets of current dermatological therapeutics such as the retinoic acid receptor (RAR) and the vitamin D receptor (VDR) as well as more recently discovered receptors including the peroxisome proliferator-activated receptors (PPAR) and the liver X receptor (LXR). After ligand activation (often lipid metabolites), these intracellular receptors exert their functions by binding to specific response elements in regulatory sequences of target genes, preferentially those involved in differentiation, energy expenditure and lipid metabolism. A number of selective activators has been developed by combinational chemistry, initially for their anti-diabetic and lipid lowering properties as well as their ability to regulate bile acid and drug metabolism. However, these activators also have marked effects on cutaneous homeostasis. Therefore, these compounds have important implications for dermatological therapy. In this review, the clinical implications of the more recently discovered members of the nuclear hormone receptor family are discussed.

Similarly potent action of 1,25-dihydroxyvitamin D3 and its analogues, tacalcitol, calcipotriol, and maxacalcitol on normal human keratinocyte proliferation and differentiation

BACKGROUND

The active vitamin D3 regulates proliferation and differentiation of epidermal keratinocytes. Recently topical vitamin D3, tacalcitol, calcipotriol, and maxacalcitol are widely used for psoriasis.

OBJECTIVE

To examine the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on cultured normal keratinocytes (NHK) and compared its effect with those of various vitamin D3 analogues.

METHODS

Cell proliferation of NHK cells was analyzed by MTS, BrdU and 3H-thymidine incorporation. The expression of involucrin, transglutaminase 1, keratin 5 and keratin 1 was investigated by western blot and PCR amplification and quantitative assay. Furthermore, we performed cornified cell envelope (CE) formation assay.

RESULTS

1,25(OH)2D3, tacalcitol, calcipotriol, and maxacalcitol decreased NHK cell proliferation in a concentration-dependent manner and the maximal effect was observed at 10(-7) M. There was no significant difference in the anti-proliferative effect among the active vitamin D3 analogues. The expression of involucrin and transglutaminase 1 were induced by 1,25(OH)2D3 and its analogues in mRNA and protein levels. CE formation was also induced by 1,25(OH)2D3 and its analogues. There was no significant difference in the potency among these chemicals. Keratin 5 and 1 expression was not altered by these active vitamin D3 analogues.

CONCLUSIONS

The present study demonstrated that active vitamin D3 analogues, tacalcitol, calcipotriol, and maxacalcitol, suppress keratinocyte proliferation and induce differentiation with similar potency.

Loricrin expression in cultured human keratinocytes is controlled by a complex interplay between transcription factors of the Sp1, CREB, AP1, and AP2 families

The major protein component of the cornified cell envelope barrier structure of the epidermis is loricrin, and it is expressed late during terminal differentiation in epidermal keratinocytes. We have previously shown that an AP1 site located in the proximal promoter region (position -55) is essential for human loricrin promoter activity (Rossi, A., Jang, S-I., Ceci, R., Steinert, P. M., and Markova, N. G. (1998) J. Invest. Dermatol. 110, 34-40). In this study we show that its regulation requires complex cooperative and competitive interactions between multiple transcription factors in keratinocytes located in different compartments of the epidermis. We show that as few as 154 base pairs of 5'-upstream sequences from the cap site can direct the keratinocyte-specific expression in cultured keratinocytes. Mutation and DNA-protein analyses show that Sp1, c-Jun, an unidentified regulator, and the co-activator p300/CREB-binding protein up-regulate whereas Sp3, CREB-1/CREMalpha/ATF-1, Jun B, and an AP2-like protein (termed the keratinocyte-specific repressor-1 (KSR-1)) suppress loricrin promoter activity. We show that CREB protein can compete with c-Jun for the AP1 site and repress loricrin promoter activity. We show here that the protein kinase A pathway can activate loricrin expression by manipulation of the Sp1, Sp3, and KSR-1 levels in the nucleus. Thus, in undifferentiated cells, loricrin expression is suppressed by Jun B, Sp3, and KSR-1 proteins. But in advanced differentiated cells, levels of Sp3, KSR-1, and CREB proteins are lower; the unidentified regulator protein can bind; Sp1 and c-Jun are increased; and then p300/CBP is recruited. Together, these events allow loricrin transcription to proceed. Indeed, the synergistic effects of the Sp1, c-Jun, and p300 factors indicate that p300/CBP might act as bridge to form an active transcription complex.

Lack of the vitamin D receptor is associated with reduced epidermal differentiation and hair follicle growth

The active vitamin D metabolite, 1,25-dihydroxyvitamin D, acting through the vitamin D receptor, regulates the expression of genes in a variety of vitamin D-responsive tissues, including the epidermis. To investigate the role of the vitamin D receptor in mediating epidermal differentiation, we examined the histomorphology and expression of differentiation markers in the epidermis of vitamin D receptor knockout mice generated by gene targeting. The homozygous knockout mouse displayed a phenotype that closely resembles vitamin D-dependent rickets type II in humans, including the development of rickets and alopecia. Hair loss developed by 3 mo after birth and gradually led to nearly total hair loss by 8 mo. Histologic analysis of the skin of homozygous knockout mice revealed dilation of the hair follicles with the formation of dermal cysts starting at the age of 3 wk. These cysts increased in size and number with age. Epidermal differentiation markers, including involucrin, profilaggrin, and loricrin, detected by immunostaining and in situ hybridization, showed decreased expression levels in homozygous knockout mice from birth until 3 wk, preceding the morphologic changes observed in the hair follicles. Keratin 10 levels, however, were not reduced. At the ultrastructural level, homozygous knockout mice showed increased numbers of small dense granules in the granular layer with few or no surrounding keratin bundles and a loss of keratohyalin granules. Thus, both the interfollicular epidermis and the hair follicle appear to require the vitamin D receptor for normal differentiation. The temporal abnormalities between the two processes reflect the apparent lack of requirement for the vitamin D receptor during the anagen phase of the first (developmental) hair cycle, but with earlier effects on the terminal differentiation of the interfollicular epidermis.

Xerosis in hypothyroidism: a potential role for the use of topical thyroid hormone in euthyroid patients

Xerosis is an extraordinarily common problem in dermatology. Despite the knowledge of well recognized aggravating factors, its etiology is an enigma, and the management of the condition is often suboptimal. Dry skin may be a manifestation of hypothyroidism. The nature of this association is reviewed, culminating in the speculation that topical thyroid hormone may represent a useful modality in euthyroid patients with xerosis or other disorders of keratinization.

1,25-dihydroxyvitamin D3 induces phospholipase D-1 expression in primary mouse epidermal keratinocytes

The hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) elicits the programmed pattern of differentiation in epidermal keratinocytes. Based on data indicating a potential role of phospholipase D (PLD) in mediating keratinocyte differentiation, we investigated the effect of 1,25(OH)2D3 on PLD expression. A 24-h exposure to 1, 25(OH)2D3 stimulated PLD-1, but not PLD-2, mRNA expression. This 1, 25(OH)2D3-enhanced expression was accompanied by increased total PLD and PLD-1 activity. Time course studies indicated that 1,25(OH)2D3 induced PLD-1 expression by 8 h, with a maximal increase at 20-24 h. Exposure to 1,25(OH)2D3 inhibited proliferation over the same time period with similar kinetics. Expression of the early (spinous) differentiation marker keratin 1 decreased in response to 1, 25(OH)2D3 over 12-24 h. Treatment with 1,25(OH)2D3 enhanced the activity of transglutaminase, a late (granular) differentiation marker, by 12 h with a maximal increase after 24 h. In situ hybridization studies demonstrated that the highest levels of PLD-1 expression are in the more differentiated (spinous and granular) layers of the epidermis, with little expression in basal keratinocytes. Our results suggest a role for PLD expression/activity during keratinocyte differentiation.

Keratinocyte differentiation is stimulated by activators of the nuclear hormone receptor PPARalpha

Peroxisome proliferator activated receptors (PPAR) belong to the superfamily of nuclear hormone receptors that heterodimerize with the retinoid X receptor and regulate transcription of several genes involved in lipid metabolism and adipocyte differentiation. Because of the role of 1,25-dihydroxyvitamin D3 and retinoic acid working through similar receptors (the vitamin D receptor and retinoic acid receptor, respectively) on keratinocyte differentiation, we have examined the effects of activators of PPARalpha on keratinocyte differentiation. The rate of cornified envelope formation was increased 3-fold in keratinocytes maintained in low calcium (0.03 mM) and incubated in the presence of clofibric acid, a potent PPARalpha activator. Involucrin, a cornified envelope precursor, and the cross-linking enzyme transglutaminase, were increased at both the message level (2-7-fold) and the protein level (4-12-fold) by clofibric acid. Furthermore, physiologic doses of the fatty acids oleic acid, linoleic acid, and eicosatetraynoic acid, which are also activators of PPARalpha, also induced involucrin and transglutaminase protein and mRNA. In contrast, the PPARgammaligand prostaglandin J2 had no effect on protein or mRNA levels of involucrin or transglutaminase. Levels of involucrin and transglutaminase mRNA and protein were induced by clofibric acid in keratinocytes incubated in 1.2 mM calcium, a concentration which by itself induces keratinocyte differentiation. Finally, PPARalpha activators inhibit DNA synthesis. This study demonstrates that PPARalpha activators, including putative endogenous ligands such as fatty acids, induce differentiation and inhibit proliferation in keratinocytes, and suggests a regulatory role for the PPARalpha in epidermal homeostasis.

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.

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.

Interactive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and retinoids on proliferation and differentiation in cultured human keratinocytes: quantification of cross-linked envelope formation

Dioxins are potent inducers of chloracne in humans. This skin aberration can be interpreted as an altered differentiation pattern of acinar sebaceous base cells and a change in the rate of terminal differentiation of the keratinocytes. We measured this rate induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in primary cultures of human keratinocytes. As parameters for differentiation, we quantified the 35S-methionine incorporation into cross-linked envelopes (revealing the total CLE biomass), as well as the number of microscopically visible CLEs. It was shown that TCDD is a very potent inducer of both CLE biomass and number with a half-maximal effect concentration (EC50) of 1.4 nM. CLE biomass was maximally increased 10-fold and the number of cells in culture producing a CLE was increased from 15% in control cultures to maximally 75% of the cells in TCDD-treated cultures. Both effects were Ca(2+)-dependent and increased with elevated cell density, being optimal in post-confluent cultures. Retinoic acid dose-dependently decreased the effect of 10(-8) M TCDD, 10(-6) M having a nearly complete antagonistic action. This interaction of retinoic acid with TCDD-induced differentiation was non-competitive. Retinol was equally potent as an antagonist of the TCDD-induced elevation of CLE formation as compared with retinoic acid. Retinyl palmitate and etretinate were not very effective as TCDD antagonists. Supplementation of hydrocortisone suppressed the TCDD-induced keratinocyte differentiation. It was concluded that CLE biomass quantification provides a reliable and sensitive parameter for keratinocyte differentiation. In this in vitro system it is shown that TCDD strongly induces a switch from proliferation to terminal differentiation and that this effect can be antagonized effectively by retinoic acid and retinol.

Topical application of calcitriol alters expression of filaggrin but not keratin K1 in mouse epidermis

Calcitriol (1 alpha,25-dihydroxyvitamin D3) and its analogues are antiproliferative agents which promote epidermal differentiation in vitro, possibly reflecting their modes of action in the treatment of psoriasis. We examined the effect of calcitriol on early and late terminal differentiation in mouse epidermis in vivo using an immunofluorescence assay to detect keratin K1 and filaggrin expression. Pulse labelling with the tymidine analogue 5-bromo-2-deoxyuridine (BrdUrd) was performed by intraperitoneal injection of mice immediately or 16 h after a single topical application of 0.72 nmol calcitriol. The BrdUrd labelling index (LI) and keratin K1 or filaggrin expression of postmitotic cell cohorts were scored by paired immunofluorescence staining for up to 72 h after BrdUrd labelling. Calcitriol induced cell proliferation as shown by a 100% increase in the BrdUrd LI 17 h after application. The onset of keratin K1 expression in the postmitotic period was, however, unchanged in both series after calcitriol treatment. Filaggrin expression appeared earlier after calcitriol treatment than in control epidermis, probably reflecting altered cell kinetics with increased epidermal turnover. The results suggest that calcitriol only influences the later stages of the keratinocyte differentiation programme, possibly secondarily to its hyperproliferative effect.

Retinoic acid regulates oral epithelial differentiation by two mechanisms

The effect of retinoic acid (RA) concentration on differentiation of oral keratinocytes and the influence of fibroblasts on RA-dependent regulation were investigated in a lifted culture system. Keratinocyte differentiation was assessed by morphology, immunohistochemistry and immunoblotting. Filaggrin/profilaggrin and keratin 1 were used as biochemical markers for cornified epithelium and keratins 13 and 19 as markers for noncornified epithelium. Cultured oral keratinocytes in RA-free conditions differentiated in a manner that closely resembled the differentiation pattern of gingival epithelia in vivo. Increasing RA concentrations altered the in vivo-like terminal differentiation of oral keratinocytes by disruption of organized stratification, inhibition of filaggrin/profilaggrin and K1 expression, and stimulation of K13 and K19 expression. Differentiation of keratinocytes from both cornified and noncornified regions of the oral cavity varied in a similar manner in response to added RA, with the exception of K19 expression. K19 was consistently expressed at higher levels in keratinocytes originating from noncornified epithelial as compared to those from cornified epithelia. The level of RA regulation was ultimately dependent on the type of fibroblasts underlying the epithelial cells. Homologous fibroblasts rendered the oral keratinocytes less sensitive to the effects of RA than skin fibroblasts. In addition, at a given RA concentration, fibroblasts from cornified oral mucosa potentiated keratinocyte expression of RA sensitive markers of keratinization as compared to the influence exerted by fibroblasts originating from noncornified oral mucosa. These results indicate that the RA regulation of oral epithelial differentiation is mediated by two separate mechanisms: a direct, RA concentration-dependent effect, and an indirect, fibroblast-mediated effect.

Retinoic acid regulates oral epithelial differentiation by two mechanisms

The effect of retinoic acid (RA) concentration on differentiation of oral keratinocytes and the influence of fibroblasts on RA-dependent regulation were investigated in a lifted culture system. Keratinocyte differentiation was assessed by morphology, immunohistochemistry and immunoblotting. Filaggrin/profilaggrin and keratin 1 were used as biochemical markers for cornified epithelium and keratins 13 and 19 as markers for noncornified epithelium. Cultured oral keratinocytes in RA-free conditions differentiated in a manner that closely resembled the differentiation pattern of gingival epithelia in vivo. Increasing RA concentrations altered the in vivo-like terminal differentiation of oral keratinocytes by disruption of organized stratification, inhibition of filaggrin/profilaggrin and K1 expression, and stimulation of K13 and K19 expression. Differentiation of keratinocytes from both cornified and noncornified regions of the oral cavity varied in a similar manner in response to added RA, with the exception of K19 expression. K19 was consistently expressed at higher levels in keratinocytes originating from noncornified epithelia as compared to those from cornified epithelia. The level of RA regulation was ultimately dependent on the type of fibroblasts underlying the epithelial cells. Homologous fibroblasts rendered the oral keratinocytes less sensitive to the effects of RA than skin fibroblasts. In addition, at a given RA concentration, fibroblasts from cornified oral mucosa potentiated keratinocyte expression of RA sensitive markers of keratinization as compared to the influence exerted by fibroblasts originating from noncornified oral mucosa. These results indicate that the RA regulation of oral epithelial differentiation is mediated by two separate mechanisms: a direct, RA concentration-dependent effect, and an indirect, fibroblast-mediated effect.

Activation of the silent human cytokeratin 17 pseudogene-promoter region by cryptic enhancer elements of the cytokeratin 17 gene

We have previously described the three loci CK-CA, CK-CB and CK-CC in the human genome that contain clustered type-I cytokeratin genes and reported the complete nucleic acid sequences of the functional cytokeratin 17 gene located in CK-CA and two closely related pseudogenes present in CK-CB and CK-CC [Troyanovsky, S.M., Leube, R.E. & Franke, W.W. (1992) Eur. J. Cell Biol. 59, 127-137]. By nucleic acid sequence analysis, we now show that extensive similarities between the functional gene and the pseudogenes exist in the 5'-upstream region. However, despite the high degree of nucleic acid identity (94%), only the 5'-upstream region of the functional gene was able to induce significant transcriptional activity in transfected cells of epithelial origin. Using chimeric upstream regions consisting of different fragments from the pseudogene and the functional gene, we made the surprising observation that cis elements in the proximal 5'-upstream region of the pseudogene promoter can cooperate with distal enhancer elements of the functional gene to induce strong chloramphenicol-O-acetyltransferase activity in transfected HeLa cells. A major site in the proximal upstream region was identified by deoxyribonuclease protection experiments to be necessary for this cooperative effect. The structure and properties of this element were further analysed by transfection of different chloramphenicol-O-acetyltransferase gene constructs, and by nucleic acid sequence comparison to corresponding regions of the related cytokeratins 14 and 16. It is concluded that the upstream regions identified in this study contribute to the strong expression of the human cytokeratin 17 gene in a coordinated fashion.

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.

Keratin: a journey of three decades

During the past three decades, major progress has been made in our understanding of the processes which lead to the formation of a keratinized epidermis in normal and pathologic situations. Stimulated by clinical studies of exfoliative dermatitis and related diseases, a series of investigations have been performed which proved initially that the pathways and controls of epidermal protein synthesis were equivalent to protein synthetic pathways in all other tissues of the body. Keratin was identified as not only an insoluble protein which makes up the vast majority of the intracellular protein in the stratum corneum, but as a member of the intermediate filament family of cytoskeletal proteins. Of all such proteins, the keratins are most complex, occurring in two families separable on the basis of size, structure and isoelectric point. The keratin intermediate filaments are heteropolymers of two paired components, one from each family. The pairs of keratins which form the intermediate filaments in basal and differentiated layers of epidermis and other epithelia have been defined and antibodies to specific keratins are now being used for diagnostic purposes. Sophisticated biochemical, physicochemical, and molecular biologic studies have led to complete definitions of almost all the epithelial keratin molecules and to cloning of their genes. These genes are currently being used in analyses of control of keratin expression and definition of the specific abnormalities associated with "keratinopathies" including epidermolysis bullosa simplex and epidermolytic hyperkeratosis.

Retinoid-dependent transcriptional suppression of cytokeratin gene expression in human epidermal squamous cell carcinoma cells

We have previously demonstrated that cytokeratin levels are coordinately regulated in normal cultured human keratinocytes. In the present study we examine the mechanism of this regulation using human squamous cell carcinoma (SCC) cells. Treatment of SCC-13 cells with 20 or 200 nM trans-retinoic acid results in nearly complete suppression of cytokeratin K5 and K6 expression. This change is accompanied by a simultaneous reduction (> 20-fold) in the level of the mRNAs encoding K5 and K6. Transcriptional analysis indicates that the transcription rate of the K5 and K6 genes drops by approximately four to fivefold in retinoid treated nuclei. Retinol (2000 nM) also promotes this change. In contrast, cytokeratin K19 does not increase in the presence of retinoic acid, thus the normal coordinate regulation of keratin gene expression by retinoids appears to be uncoupled in SCC-13 cells. However, this does not represent a general defect in positive regulation of gene expression by retinoids, since in a transient transfection assay trans-retinoic acid positively regulates a reporter plasmid containing the retinoid response element from the retinoic acid receptor-beta gene. The synthetic retinoids Ro 13-6298 (ethyl ester) and its metabolic derivative Ro 13-7410 (free acid) are both active in modulating the differentiation of normal keratinocytes. In contrast, only Ro 13-7410 is active in SCC-13 cells. As Ro 13-6298 binds poorly to the retinoic acid receptors, this suggests that SCC-13 cells, unlike normal keratinocytes, lack the ability to convert Ro 13-6298 to the active Ro 13-7410.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D3 and calcipotriol enhance the secretion of transforming growth factor-beta 1 and -beta 2 in cultured murine keratinocytes

Vitamin D3 and its analogue calcipotriol (MC 903) inhibit the proliferation of cultured keratinocytes and induce their differentiation. Since TGF beta s are very potent inhibitors of keratinocyte growth we studied the effects of vitamin D3 and calcipotriol on the secretion of TGF beta in cultured murine keratinocytes. Vitamin D3 and calcipotriol (10(-6)-10(-9) M) inhibited the DNA-synthesis of mouse keratinocytes by 50-80% in a time and dose-dependent manner as measured by [3H]-thymidine incorporation. Analysis of the conditioned medium of the keratinocytes indicated that the cells secreted into their medium activity that inhibited the growth of indicator Mv1Lu mink lung epithelial cells. Neutralizing antibodies against TGF beta 1 and TGF beta 2 decreased, and when used together, prevented the observed growth inhibition of the indicator cells. Heat treatment of the conditioned medium, which activates latent forms of TGF beta, revealed higher levels of growth inhibitory activity in the medium from vitamin D3 and calcipotriol treated than from control cultures indicating that a fraction of TGF beta was in a latent form. Active TGF beta was, however, detected considerably more in vitamin D3 and calcipotriol treated cultures than in control cultures. Immunoblotting analysis of the medium revealed enhanced secretion of TGF beta protein. These results indicate that enhanced TGF beta 1 and TGF beta 2 secretion and activity is associated with vitamin D3-mediated growth inhibition of cultured keratinocytes.

Identification of the retinoic acid and thyroid hormone receptor-responsive element in the human K14 keratin gene

The promoter of human K14 keratin gene, specific for the basal layer of stratified epithelia, is regulated by nuclear receptors for retinoic acid and thyroid hormone. However, the DNA sequences responsible for this regulation have not yet been identified. To identify the retinoic acid-responsive site, we have devised a simple site-specific mutagenesis method and introduced mutations into the K14 keratin gene promoter. These mutations identify the retinoic acid-responsive site. The site consists of a cluster of consensus palindrome half-sites in various orientations. As shown previously, retinoic acid and thyroid hormone receptors can recognize and bind common sequences in regulated genes. Here, we describe mutations that abolish regulation by both receptors. Interestingly, the hormone-dependent and -independent regulatory sites of the thyroid hormone nuclear receptor can be separated. Clusters of half-sites that share structural organization with the K14 regulatory site were found in the K5 and K10 keratin gene promoters. Similar clusters may be responsible for retinoic acid-mediated transcription regulation in epidermis.