The promoter of the HaCaT keratinocyte differentiation-related gene keratin 4 contains a functional AP-2 binding site

The nuclear transcription factor AP-2 appears to be a key regulator mediating programmed gene expression during embryonic morphogenesis and adult cell differentiation. AP-2 has also been considered to be involved in epidermal gene regulation, but its precise role is not yet defined. The level of AP-2 transcripts increases during culturing of HaCaT keratinocytes preceding the expression of the differentiation-related gene keratin 4 (K4). The current study was aimed at investigating whether AP-2 transactivates K4 transcription. We cloned and sequenced the promoter region of K4 and found, in addition to canonical sequences, an AP-2 consensus site in the vicinity of the transcriptional start. In order to provide functional evidence for a regulation of K4 transcription by AP-2, we cloned various parts, which did or did not contain the AP-2 site of the K4 upstream sequence, into Cat reporter plasmids. These constructs were ballistically transfected into differentiating HaCaT keratinocytes. The determination of the resulting Cat activity revealed that the AP-2 site in the vicinity of the transcriptional start was functional for K4 transcription. Thus, the role of AP-2 in the process of keratinocyte differentiation appears to be considerable. In addition, further regulatory elements were found to be necessary for full transcription of K4.

Upregulation of CD40 and CD40 ligand expression in IgE-associated cutaneous diseases

In order to better understand the immunological processes connected with IgE-associated cutaneous disease, we have examined the expression of CD40 and its ligand CD40L, required for the induction of IgE synthesis in B-cells, as well as of IgE and its receptors in various dermatoses (atopic dermatitis (AD), scabies, chronic recurrent urticaria) versus normal skin, and in one dermopathic lymph node versus normal lymphatic tissue by immunohistochemistry. Compared to normal skin, cells expressing IgE, Fc epsilon RI, Fc epsilon RII, CD40, CD40L and L26 were increased in the dermis, partly also in the epidermis, from patients with AD and scabies, but not in chronic urticaria. CD40 and CD40L were detected on numerous cells in lymphatic tissue from both normal donors and a patient with AD, whereas large numbers of IgE- and Fc epsilon RI-positive cells were only found in the dermopathic lymph node from the AD patient, in contrast to very few in normal lymphatic tissue. These results with selectively increased IgE/Fc epsilon RI and associated CD40/CD40L expression in the skin of AD and scabies suggest that cutaneous tissue, in addition to dermopathic lymphatic tissue, might contribute to IgE synthesis.

The mast cell products histamine and serotonin stimulate and TNF-alpha inhibits the proliferation of murine epidermal keratinocytes in situ

To better characterize the proposed growth modulatory functions of mast cells and their products in the regulation of epidermal homeostasis, we have assessed keratinocyte proliferation by incorporation of [3H]thymidine in murine skin organ cultures after incubation with different concentrations of histamine, serotonin or TNF-alpha. Both histamine and serotonin significantly stimulate the proliferation of keratinocytes as compared to controls (P < 0.001), reaching maxima within a range from micro- to nanomolar concentrations. Even at subnanomolar concentration, TNF-alpha significantly inhibits the incorporation of labelled thymidine by epidermal keratinocytes in murine skin organ culture (P < 0.001). These observations support the concept of mast cell products as modulators of keratinocyte proliferation, and suggest a role for mast cells in the regulation of epidermal tissue turnover under physiological conditions.

Induction of MHC class II antigen expression on human HMC-1 mast cells

Mast cells are not only the primary effector cells of immediate type immune reactions, but they have recently also been considered to contribute to the induction of an immune response. Data on the ability of the cells to express major histocompatibility complex (MHC) antigens and the mechanisms involved are however controversial or unclear. We have therefore studied the expression of MHCI and the induction of MHCII molecules on leukemic HMC-1 mast cells by immunohistochemistry. Cells were incubated for up to 72 h in the presence of IFN gamma, TNF alpha and IL-4, and immunohistochemical staining was done with monoclonal antibodies with specificity for HLA class I heavy chain, HLA-DQ (Tü22), HLA-DR (Tü36) and HLA-DQ, -R and -P (Tü35). All unstimulated mast cells expressed MHC class I, but almost no class II antigens. Incubation with IFN gamma caused a rapid, dose-dependent induction of MHC class II molecules, with Tü35 staining maximally one third of the cells within 24 h at the highest dose tested (100 IU/ml), with decline on extended culture. TNF alpha (2 ng/ml) was less effective but caused more persistent induction with time. IL-4 (200 ng/ml) had hardly any effects at all. Staining with Tü22 and Tü36 was always lower than with Tü35, and additive or even synergistic results were obtained when cells were stimulated with a combination of IFN gamma and TNF alpha. These findings support the concept that mast cells can facultatively participate in immune recognition processes depending on the type of pathological conditions in their microenvironment which allow expression of MHC class II molecules.

Enhanced P-selectin expression in chronic and dermographic urticaria

In both chronic and dermographic urticaria, superficial perivascular leukocytic infiltrations are seen histologically in lesional skin. We have therefore investigated the role of endothelial adhesion molecule expression in these diseases. E-selectin, P-selectin, ICAM-1 and VCAM-1 expression was examined immunohistologically, and the serum levels of the soluble forms of these adhesion molecules were determined by EIA in patients with chronic urticaria, dermographic urticaria as well as in healthy controls (n = 8 in each group) and subjects with symptomatic allergic rhinitis (n = 7) for comparison. A significant increase (p<0.01) was observed for soluble P-selectin in dermographic urticaria (mean 487+/-44 ng/ml) and chronic urticaria (mean 464+/-74 ng/ml) compared to healthy controls (mean 149+/-15 ng/ml) and rhinitis subjects (mean 177+/-30 ng/ml). In contrast, the other adhesion molecules were not significantly elevated in both urticaria groups. Immunohistologically, a strong expression of P-selectin was found in superficial vessels of lesional and nonlesional skin in dermographic urticaria with only a mild increase of the other adhesion molecules studied, supporting the findings observed with the soluble forms in the patients' sera. Since an alteration of soluble P-selectin was not seen in symptomatic allergic rhinitis, an unspecific effect due to inflammation appears to be unlikely. These results therefore point to a potentially relevant role of the endothelial P-selectin expression in the evolution of urticarial whealing.

Retinoic acid attenuates phospholipase C-mediated signaling in HaCaT keratinocytes

Release of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) generated by phospholipase C (PLC) upon receptor stimulation plays an important role in the regulation of cell growth and differentiation. A second, completely different, signal transduction system involves retinoic acid (RA) and related derivatives. Binding to intracellular receptor sites can modulate keratinocyte growth and inhibits differentiation. The present study was aimed at characterizing possible interactions between the two signalling pathways in HaCaT keratinocytes. As determined by anion exchange chromatography and HPLC analysis, HaCaT keratinocytes treated with 1 microM RA for up to 72 h showed a marked decrease in Ins(1,4,5)P3 release upon stimulation with 10 microM bradykinin or 10 microM ionomycin. Thin-layer chromatography of phosphatidylinositol phosphates, the substrates of PLC, revealed no differences between RA-treated and untreated cells. Western blot analysis of the PLC isozymes present in HaCaT cells, PLC beta 3 and PLC gamma 1, showed no alterations in the expression of these proteins in RA-treated cells as compared to vehicle-treated controls. In addition, expression of the PLC-activating G protein G alpha q was not affected by RA treatment. Our results show that RA downregulates the PLC-mediated signaling system. The point of interference of this signal transduction crosstalk has yet to be elucidated. Our results suggest, furthermore, that RA-induced attenuation of keratinocyte differentiation might be mediated at least in part by the downregulation of Ins(1,4,5)P3 release.

Detection of intracellular interleukin-8 in human mast cells: flow cytometry as a guide for immunoelectron microscopy

The chemokine interleukin-8 (IL-8) mediates infiltration and adhesion of neutrophils during inflammatory processes. We have previously shown that this cytokine can be produced and released by normal and leukemic human mast cells (HMC-1 cells). To assess whether and to what extent this cytokine is stored intracellularly, we investigated production and localization of IL-8 at the single-cell level by combined use of flow cytometry (FACS) and immunoelectron microscopy. Conditions necessary for optimal fixation and permeabilization of HMC-1 cells were determined by measuring changes in cell-specific light scatter parameters and by estimating cellular uptake of propidiumiodide (PI). In this way, we were able to detect IL-8 with a monoclonal antibody in stimulated cells that were microwave-fixed with a combination of paraformaldehyde (4%) and glutaraldehyde (0.1%), followed by permeabilization with saponin (0.025%). FACS analysis revealed time-dependent synthesis of IL-8 with at most 50% positively stained cells at 8-12 hr after stimulation. For pre-embedding immunogold electron microscopy, cells were treated according to the protocol established by flow cytometry. IL-8 was found to be located in specific cytoplasmic, electron-dense granules of stimulated HMC-1 cells. These results confirm and extend our previous findings by demonstrating IL-8 expression in HMC-1 cells at the single-cell level. In addition, we propose that quantitative FACS can be reliably used in a timesaving manner to establish appropriate conditions for pre-embedding immunoelectron microscopy of intracellular antigens.

Leukosialin (CD43) is proteolytically cleaved from stimulated HMC-1 cells

Leukosialin (CD43), the major sialoprotein on circulating leukocytes, has been previously described to be down-regulated on neutrophils following activation with phorbol myristate acetate (PMA). The other single cells previously examined, blood lymphocytes, do not down-regulate CD43 when stimulated by PMA. Recently, we have characterized leukosialin on the human mast cell line HMC-1 and observed that leukosialin is down-regulated after stimulation with PMA. In the present study, we have investigated the mechanism of PMA-mediated down-regulation of CD43 on HMC-1 cells (subclone 5C6). PMA caused the release of soluble leukosialin (123 kD) during HMC-1 cell activation. The molecular weight of soluble leukosialin was nearly identical to that of the cell-membrane bound molecule, suggesting a cleavage proximal from the cell membrane. Inhibitors of serine proteases, like phenylmethylsulphonyl fluoride (PMSF), benzamidine and 3, 4-dichloroisocoumarin, blocked the PMA-mediated cleavage of CD43. In all experiments, the inhibition of CD43-down-regulation was dependent on the concentration of protease inhibitors. Treatment of HMC-1 cells with various proteases (trypsin, alpha-chymotrypsin, elastase, papain, nagarse) substantially decreased anti-CD43 binding capacity and caused the release of soluble leukosialin (116 kD) or its fragments into the supernatant. Pretreatment of HMC-1 cells with neuraminidases from Vibrio cholerae or Arthrobacter ureafaciens resulted in an increased sensitivity of CD43 against proteases, whereas the effects of PMA were not influenced. In conclusion, proteolytic cleavage of CD43 is described for the first time in a cell other than neutrophils, namely HMC-1 cells. Our results suggest that serine proteases are involved in the PMA-mediated down-regulation of leukosialin on HMC-1 cells.

Molecular regulation of human IgE synthesis

Understanding the induction and regulation of IgE synthesis in human B cells is crucial to elucidate the molecular pathogenesis of IgE-dependent diseases. Experimental data, in part supported by clinical observations, suggests that IgE regulation is a complex process involving several cellular and molecular interactions. A two-signal model is accepted for the induction of IgE synthesis in human B cells. The first signal is provided by the cytokines interleukin 4 or 13, which are secreted by T cells, mast cells, and basophils. The second signal for the induction of IgE synthesis requires cell contact between T and B cells. Engagement of the B cell antigen CD40 by the CD40 ligand (CD40L) expressed on T cells leads to subsequent isotype switching during immunoglobulin synthesis in B cells. The CD40-CD40L interaction is well established as a key signal for the induction of isotype switching while the elucidation of the role of other cell-cell interactions, for example, through adhesion molecules, needs further study. An important counteracting cytokine for IgE synthesis is interferon (IFN) gamma which is produced mainly by T lymphocytes. Several cell-contact molecules, cytokines, and various hormones have been shown to modulate IgE synthesis in vitro, suggesting a complex network of molecular events to be involved in the production of IgE. However, the relevance of these factors for IgE production in vivo requires further elucidation. Here we describe the molecular mechanisms known to be involved in the induction and regulation of human IgE synthesis and discuss the role of various molecules during this process. Furthermore, evidence is presented that the understanding of IgE synthesis provides a potential key for new therapeutic strategies in patients with IgE mediated diseases including atopic dermatitis.

Phospholipase C-mediated signaling is altered during HaCaT cell proliferation and differentiation

To elucidate the signaling mechanisms associated with keratinocyte differentiation, we studied in vitro phospholipase C-mediated signal transduction, which results in the generation of inositol phosphates, comparing proliferating versus differentiated HaCaT cells, a human keratinocyte line. Bradykinin- or A23187-induced formation of inositol 1,4,5-trisphosphate, inositol 1,4-bisphosphate, and inositol monophosphates, as determined by anion exchange high performance liquid chromatography, were found to be highest in the early logarithmic growth phase of the cells. In more highly differentiated HaCaT cells, which expressed maximal amounts of the differentiation marker involucrin, inositol phosphate formation was reduced to about one third of that in proliferating cells. Thin layer chromatography of membrane phosphatidylinositol phosphates revealed that this reduction was associated with a steady decrease in phospholipase C substrates. Immunoblot analysis of phospholipase C isozymes, however, and of expression of Gq alpha, the G protein subunit that activates phospholipase C beta, revealed no decrease during the differentiation phase. The results suggest that the inositol-phospholipid signal transduction pathway is involved in keratinocyte proliferation and in the induction of differentiation, with attenuated signal transduction activity via phospholipase C-coupled receptors in more differentiated keratinocytes.

GM-CSF downregulates expression of tryptase, Fc epsilon RI and histamine in HMC-1 mast cells

Fibroblast-derived growth factors like SCF are able to upregulate the expression of mast cell characteristics in human multilineage hematopoietic progenitor cells. Other factors, like GM-CSF, have been reported to inhibit this process, probably by the competitive recruitment of cells not belonging to the mast cell lineage. In this study, we investigated the influence of GM-CSF on immature mast cells of the HMC-1 cell line which already show low-level expression of mast cell tryptase, histamine and Fc epsilonRI alpha. Culture of HMC-1 cells with mast-cell-conditioning medium, containing fibroblast supernatants, upregulated tryptase activity, histamine contents and expression of Fc epsilonRI alpha. However, addition of GM-CSF (10 ng/ml) markedly downregulated these mast cell markers, without affecting proliferation and viability of cells. Thus, GM-CSF may provide an inhibitory signal during mast cell differentiation and probably even downregulates mast cell characteristics in more differentiated cells.

Human leukaemic (HMC-1) and normal skin mast cells express beta 2-integrins: characterization of beta 2-integrins and ICAM-1 on HMC-1 cells

Mast cells are bone marrow-derived, ubiquitous connective tissue resident cells. However, their mechanisms of migration, the distribution of immature and mature cells and their interaction with other inflammatory cells are largely unclarified. Possibly, beta 2-integrins play an important role in these processes. In the present investigation, the authors studied the expression and regulation of the beta 2-integrins LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), p150,95 (CD11c/CD18) and of the LFA-1/Mac-1 counter-receptor intercellular adhesion molecule-1 (ICAM-1; CD54) on leukaemic (HMC-1 cell subclone 5C6) and on normal mature human skin mast cells. The HMC-1 cells clearly expressed CD11a, CD18 and CD54, while expression of CD11b and CD11c was low. The apparent molecular weights were 180 kDa (CD11a), 95 kDa (CD18) and 90 kDa (CD54) as determined by Western blot analysis. Phorbol myristate acetate (PMA) induced a time- and dose-dependent up-regulation of CD11a, CD11b, CD11c, CD18 and CD54 that was inhibited by cycloheximide, suggesting a dependence on de novo protein synthesis. Enhanced expression of CD11a, CD11b, CD11c and CD18 could also be confirmed at the gene level as demonstrated by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Increased expression of LFA-1/ICAM-1 in response to PMA was accompanied by strong enhancement of homotypic cell aggregation, suggesting that newly synthesized LFA-1/ICAM-1 is functionally active. In order to determine a physiologically relevant way of mast cell beta 2-integrin modulation, several cytokines and chemotactic mediators (interleukin-4, IL-4; nerve growth factor beta, NGF beta; C5a; and leukotriene B4, LTB4) were tested for their influence on adhesion molecule cell surface density. Only LTB4 was shown specifically to up-regulate CD11a and CD18, but not CD11b or CD11c. The presence of CD11a, CD11c and CD18 could be confirmed on a low percentage of normal skin mast cells by immunofluorescence, using a double staining technique. In comparison to normal skin, a significantly higher percentage of CD18+ mast cells was found in inflammatory dermatoses such as psoriasis vulgaris, atopic dermatitis and lichen planus. Therefore, mast cell beta 2-integrins possibly play an important role during homing of immature mast cells as well as during the interaction of activated mast cells with other inflammatory cells.

CD43 (leukosialin, sialophorin) expression is differentially regulated by retinoic acids

CD43 (leukosialin, sialophorin), a cell-surface associated mucin that is constitutively expressed at high levels on most leukocytes, is thought to be involved in cell activation and adhesion. We here provide evidence that the vitamin A metabolites all-trans and 13-cis retinoic acid up-regulate CD43 on human leukemic (HMC-1) mast cells, as determined by flow cytometry, Western blot analysis, and by semiquantitative reverse transcriptase-polymerase chain reaction. Enhanced CD43 expression was accompanied by a strong increase in anti-CD43-mediated, LFA-1-dependent homotypic aggregation of HMC-1 cells, demonstrating that newly synthesized CD43 is functionally active in transmitting signals across the plasma membrane which result in enhanced cellular adhesion. CD43 expression was also enhanced in response to retinoic acids on isolated human skin mast cells and human monocytes, but not on cells of the basophilic cell line KU-812 and promyelocytic HL-60 cells, indicating that these agents might act in a cell-type specific manner. These combined result-point to a novel aspect in the regulation of CD43. Possibly, vitamin A metabolites act directly on the CD43 gene, since putative retinoic acid response elements have been detected within its regulatory regions.

C3a and C5a stimulate chemotaxis of human mast cells

The factors that control migration of mast cells to sites of inflammation and tissue repair remain largely undefined. Whereas several recent studies have described chemotactic factors that induce migration of murine mast cells, only stem cell factor (SCF) is known to induce migration of human mast cells. We report here that the anaphylatoxins C3a and C5a are chemotactic factors for the human mast cell line HMC-1, human cord blood-derived mast cells (CBMC) and cutaneous mast cells in vitro. The presence of an extracellular matrix protein, laminin, was required for chemotaxis in response to complement peptides. Migration of mast cells towards C3a and C5a was dose-dependent, peaking at 1 microg/mL (100 nmol/L), and was inhibited by specific antibodies. Pretreatment with pertussis toxin inhibited the anaphylatoxin-mediated migration of HMC-1 cells, indicating that Gi proteins are involved in complement-activated signal transduction pathways in human mast cells. Both C3a and C5a also induced a rapid and transient mobilization of intracellular free calcium ([Ca2+]i) in HMC-1 cells. Besides SCF, other chemotactic factors tested, such as interleukin-3, nerve growth factor, transforming growth factor beta, RANTES (regulated upon activation, normal Tcell expressed and secreted), monocyte chemotactic protein-1 (MCP-1), MCP-2, MCP-3, macrophage inflammatory protein-1alpha (MIP-1alpha), and MIP-1beta, failed to stimulate migration of human mast cells. In summary, these findings indicate that C3a and C5a serve as chemotaxins for human mast cells. Anaphylatoxin-mediated recruitment of mast cells might play an important role in hypersensitivity and inflammatory processes.

Keratin 17 gene expression during the murine hair cycle

Keratin 17 (K17) expression is currently considered to be associated with hyperplastic or malignant growth of epithelial cells. The functions of this keratin in normal skin physiology and the regulation of its gene expression, however, are still unclear. As one possible approach to further explore K17 functions, we have studied the differential patterns of mouse K17 (MK17) transcription during the murine hair cycle by means of in situ hybridization, using a digoxigenin-labeled riboprobe. Cycling hair follicles in the skin of C57BL/6 mice were found to be the only skin structures expressing MK17 under physiologic conditions. MK17 transcripts were constantly observed throughout all hair cycle stages in the suprainfundibular outer root sheath (ORS). The MK17 expression was also evident in the isthmus part of the ORS, where it was expressed weakly and was spatially restricted during telogen, with an increase in early anagen and stable expression during mid- and late anagen, localizing to the zone of so-called trichilemmal keratinization. In addition, in early anagen, a group of epithelial cells in or next to the bulge region stained weakly for MK17. With progressing anagen development, MK17 expression in this region increased and was consistently localized to keratinocytes at the advancing front of the emerging epithelial hair bulb. In mid- and late anagen, this zone of MK17 expression spread along the proximal ORS, with a maximal level of expression in the innermost cell layer of the ORS. Overall, these findings provide data on the MK17 expression profile of normal murine skin and demonstrate hair-cycle-dependent regulation of MK17 expression.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD) affects keratin 1 and keratin 17 gene expression and differentially induces keratinization in hairless mouse skin

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes chloracne in humans by mechanisms that are as yet poorly understood. Because TCDD is known to affect keratinocyte differentiation in vitro, we have studied TCDD-dependent morphologic changes and the expression of murine keratin 1 (MK1; differentiation associated) and keratin 17 (MK17; presumably hyperproliferation associated) in HRS/J hr/hr hairless mouse skin. TCDD (0.2 microg in acetone) applied topically to the dorsal skin caused epidermal acanthosis and hyperkeratosis of the dermal cysts as well as an involution of the utricles and the sebaceous glands. By means of in situ hybridization with digoxigenin-labeled riboprobes of sections from untreated and vehicle (control)-treated skin, we localized MK1 mRNA to the epidermal spinous cell compartment. MK17 transcripts were detected only in the derivatives of the hair follicle-utricle epithelium and dermal cysts. No spatial overlap was observed between MK1 and MK17 expression. After TCDD application, MK17 was newly expressed in the upper spinous cell layers of the interfollicular epidermis, although it was suppressed in the involuting utricles. In contrast, MK1 expression in the interfollicular epidermis was not affected by TCDD. Furthermore, MK1 expression was induced in the epithelium of the utricle remnants and in some dermal cysts. These data suggest that increased keratinization of the part of the follicular epithelium corresponding to the dermal cyst epithelium of hairless mice most probably explains the pathogenesis of TCDD-induced chloracne. The results demonstrate, furthermore, that TCDD can differentially affect keratinocyte differentiation in vivo as well as in vitro.

Phacomatosis pigmentokeratotica: a patient with the rare melanocytic-epidermal twin nevus syndrome

We describe a 10-year-old girl affected with a speckled lentiginous nevus and an epidermal nevus of the organoid type on corresponding parts of the body. On histopathological examination, the lesions showed epidermal hyperpigmentation and melanocytic hyperplasia on the one hand and verrucous epidermal acanthosis with sebaceous hyperplasia on the other hand. Except for a minor deviation of the spine, the patient had no obvious extracutaneous symptoms. Happle et al. have recently interpreted the rare co-occurrence of these two types of nevi in spatial proximity as an example of twin spotting in human skin and proposed the name 'phacomatosis pigmentokeratotica'. In most cases, additional skeletal or neurological anomalies are found. These are dissimilar from the extracutaneous symptoms of the sebaceous nevus syndrome, from which phacomatosis pigmentokeratotica should be distinguished. Molecular studies are needed to prove the concept of twin spotting and to reveal a link to the extracutaneous manifestations.

Immunopathological events of adverse cutaneous reactions to coumarin and heparin

We here describe a female patient with thromboembolic disease, who exhibited allergic reactions to heparin and who developed a large necrotic area on the abdomen when coumarin treatment was instituted. On immunohistology of the necrotic lesion, tumour necrosis factor alpha was markedly expressed, with decreasing intensity towards the central necrotic part of the lesion. Furthermore, endothelial cell adhesion molecule expression was upregulated, particularly in the haemorrhagic zone at the periphery of the lesion. These findings suggest that the pathology of coumarin necrosis is mediated via tumour necrosis factor alpha-associated inflammatory events, after activation of the coagulation pathway due to an inherited or transiently induced, acquired protein C-deficiency. In view of these findings, we propose that patients be treated in the future with tumour necrosis factor alpha antagonists such as pentoxifylline.

Angiotensin II stimulates proliferation of primary human keratinocytes via a non-AT1, non-AT2 angiotensin receptor

Angiotensin II is a hormone with long known cardiovascular actions. Recent studies revealed an additional role for angiotensin II in the regulation of cell proliferation. This study was performed to clarify whether skin is a target organ for these novel angiotensin actions. Radioligand binding studies identified a high-affinity angiotensin receptor on human primary keratinocytes in vitro with a Kd of 4.5 nM and a Bmax of 0.12 nM. Competition experiments with losartan and PD 123177 revealed that this receptor was not of the AT1- nor the AT2-subtype. Stimulation of human keratinocytes with angiotensin II (10(-10) to 10(-5) M) led to a dose dependent increase in 3H-thymidine incorporation, indicating that the keratinocyte receptor mediates a mitogenic effect. This effect is comparable at 10(-9) M to stimulation of keratinocytes by EGF (50ng/ml) and FGF (50ng/ml). These results demonstrate for the first time a possible physiological role for angiotensin II in human skin involving the regulation of keratinocyte proliferation.

Treatment of melanoma cells with the synthetic retinoid CD437 induces apoptosis via activation of AP-1 in vitro, and causes growth inhibition in xenografts in vivo

Human malignant melanoma is notoriously resistant to pharmacological modulation. We describe here for the first time that the synthetic retinoid CD437 has a strong dose-dependent antiproliferative effect on human melanoma cells (IC50: 5 x 10(-6) M) via the induction of programmed cell death, as judged by analysis of cell morphology, electron microscopical features, and DNA fragmentation. Programmed cell death was preceded by a strong activation of the AP-1 complex in CD437-treated cells as demonstrated by gel retardation and chloramphenicol transferase (CAT) assays. Northern blot analysis showed a time-dependent increase in the expression of c-fos and c-jun encoding components of AP-1, whereas bcl-2 and p53 mRNA levels remained constant. CD437 also exhibited a strong growth inhibitory effect on MeWo melanoma cells in a xenograft model. In tissue sections of CD437-treated MeWo tumors from these animals, apoptotic melanoma cells and c-fos overexpressing cells were colocalized by TdT-mediated deoxyuridine triphosphate-digoxigenin nick end labeling (TUNEL) staining and in situ hybridization. Taken together, this report identifies CD437 as a retinoid that activates and upregulates the transcription factor AP-1, leading eventually to programmed cell death of exposed human melanoma cells in vitro and in vivo. Further studies are needed to evaluate whether synthetic retinoids such as CD437 represent a new class of retinoids, which may open up new ways to a more effective therapy of malignant melanoma.

Retinoic acid affects the expression rate of the differentiation-related genes aryl hydrocarbon receptor, ARNT and keratin 4 in proliferative keratinocytes only

The environmental contaminant dioxin exerts most of its effects by activating the aryl hydrocarbon receptor (AhR). The AhR is considered to play not only a role in the regulation of xenobiotic metabolism, but also for development, growth, and differentiation. The transcript levels of the AhR and its associated translocator protein (ARNT) were found to increase with ongoing differentiation in the human keratinocyte cell line HaCaT. Correspondingly, in situ hybridization studies in normal human skin revealed an absence of AhR-expression in proliferating basal cells and increasing transcript levels in upper cell layers, in dependence of keratinocyte differentiation. AhR expression in differentiation-deficient hyperproliferative psoriatic skin was markedly decreased. When keratinocytes were continuously treated with 1 microM retinoic acid (RA), the upregulation of AhR- and ARNT-mRNA levels was inhibited as was keratin 4-expression, a marker of HaCaT-keratinocyte differentiation. In contrast, treatment of already differentiated cells with RA did not down-regulate these transcript levels. The mRNA levels of the prevalent retinoic acid receptors in keratinocytes, RAR gamma and RXR alpha, were not influenced by the process of differentiation or by addition of RA. Our data suggest that the regulation of AhR-, ARNT- and keratin 4-expression by RA is indirect and mediated by a yet to be identified factor.