1,25-dihydroxyvitamin D3, transforming growth factor beta1, calcium, and ultraviolet B radiation induce apoptosis in cultured human keratinocytes

Wnt/CTNNB1 Signal Transduction Pathway Inhibits the Expression of ZFP36 in Squamous Cell Carcinoma, by Inducing Transcriptional Repressors SNAI1, SLUG and TWIST

The Wnt/CTNNB1 pathway is often deregulated in epithelial tumors. The ZFP36 gene, encoding the mRNA binding protein Tristetraprolin (TTP), is downregulated in several cancers, where it has been described to behave as a tumor suppressor. By this report, we show that Wnt/CTNNB1 pathway is constitutively activated, and ZFP36 expression is downregulated in Squamous Cell Carcinoma (SCC) cell lines compared to normal keratinocytes. Moreover, we suggest that the decrease of ZFP36 expression might depend on the activity of transcriptional repressors SNAI1, SLUG and TWIST, whose expression is induced by Wnt/CTNNB1, highlighting a potential regulatory mechanism underlying ZFP36 downregulation in epithelial cancers.

1,25(OH)2D3 regulates the proangiogenic activity of pericyte through VDR-mediated modulation of VEGF production and signaling of VEGF and PDGF receptors

We have previously demonstrated that the active form of vitamin D (calcitriol; 1,25(OH)₂D₃) is a potent inhibitor of retinal neovascularization. However, the underlying molecular and cellular mechanisms involved remained poorly understood. Perivascular supporting cells including pericytes (PC) play important roles during angiogenesis, vascular maturation, and stabilization of blood vessels. How 1,25(OH)₂D₃ affects retinal PC proliferation and migration, and whether these effects are mediated through vitamin D receptor (VDR), are unknown. Here, we determined the impact of 1,25(OH)₂D₃ on retinal PC prepared from wild‐type (Vdr+/+) and VDR‐deficient (Vdr−/−) mice. Retinal PC expressed significantly higher VDR levels compared to retinal endothelial cells (EC). Unlike retinal EC, 1,25(OH)₂D₃ significantly decreased PC proliferation and migration and resulted in a G₀/G₁ cell cycle arrest. Although 1,25(OH)₂D₃ did not inhibit the proliferation of Vdr−/− PC, it did inhibit their migration. PC adhesion to various extracellular matrix (ECM) proteins and ECM production were also affected by incubation of PC with 1,25(OH)₂D₃. Vdr−/− PC were more adherent compared with Vdr+/+ cells. Mechanistically, incubation of Vdr+/+ PC with 1,25(OH)₂D₃ resulted in an increased expression of vascular endothelial growth factor (VEGF) and attenuation of signaling through VEGF‐R2 and platelet‐derived growth factor receptor‐beta. Incubation with soluble VEGF‐R1 (sFlt‐1) partially reversed the effect of VEGF on Vdr+/+ PC. In addition, incubation of Vdr+/+ PC with VEGF or inhibition of VEGF‐R2 increased VDR expression. Together, these results suggest an important role for retinal PC as a target for vitamin D and VDR action for attenuation of angiogenesis.

Topical Vitamin D3Is Effective in Treating Senile Warts Possibly by Inducing Apoptosis

Vitamin D3 ointments containing active forms of vitamin D3 are widely used to treat inflammatory keratotic dermatoses such as psoriasis. Senile wart or seborrheic keratosis is a benign tumor which occurs mainly in the elderly. It has traditionally been treated with surgical procedures, freezing with liquid nitrogen, or laser therapy. We treated senile warts with topical vitamin D3 ointments (tacalcitol, calcipotriol or maxacalcitol). Out of 116 cases treated for 3 to 12 months, 35 (30.2%) showed complete disappearance or more than an 80% decrease in the volume of the tumor, 54 cases (46.6%) showed a decrease in the volume between 40 and 80%, and no remarkable changes or decreases of less than 40% were seen in 27 cases (23.3%). The tumors faded without any inflammatory changes such as erythema or swelling. An organ culture experiment using senile wart as a material with several concentrations of tacalcitol revealed that tacalcitol induced apoptosis in the tissue. On the other hand, only sporadic apoptotic cells were seen in the controls (p<0.001). Vitamin D3 may affect senile warts by inducing apoptosis. Clearance of senile warts, especially on exposed areas without pain, may improve the quality of life (QOL) of the elderly.

Mechanisms of Mycotoxin-induced Dermal Toxicity and Tumorigenesis Through Oxidative Stress-related Pathways

Among the many mycotoxins, T-2 toxin, citrinin (CTN), patulin (PAT), aflatoxin B1 (AFB1) and ochratoxin A (OTA) are known to have the potential to induce dermal toxicity and/or tumorigenesis in rodent models. T-2 toxin, CTN, PAT and OTA induce apoptosis in mouse or rat skin. PAT, AFB1 and OTA have tumor initiating properties, and OTA is also a tumor promoter in mouse skin. This paper reviews the molecular mechanisms of dermal toxicity and tumorigenesis induced in rodent models by these mycotoxins especially from the viewpoint of oxidative stress-mediated pathways.

Vitamin D and death by sunshine

Exposure to sunlight is the major cause of skin cancer. Ultraviolet radiation (UV) from the sun causes damage to DNA by direct absorption and can cause skin cell death. UV also causes production of reactive oxygen species that may interact with DNA to indirectly cause oxidative DNA damage. UV increases accumulation of p53 in skin cells, which upregulates repair genes but promotes death of irreparably damaged cells. A benefit of sunlight is vitamin D, which is formed following exposure of 7-dehydrocholesterol in skin cells to UV. The relatively inert vitamin D is metabolized to various biologically active compounds, including 1,25-dihydroxyvitamin D3. Therapeutic use of vitamin D compounds has proven beneficial in several cancer types, but more recently these compounds have been shown to prevent UV-induced cell death and DNA damage in human skin cells. Here, we discuss the effects of vitamin D compounds in skin cells that have been exposed to UV. Specifically, we examine the various signaling pathways involved in the vitamin D-induced protection of skin cells from UV.

An investigation of the effects of dithranol-induced apoptosis in a human keratinocyte cell line

Objectives

Dithranol, one of the most successful topical agents for the treatment of psoriasis, has been shown to exert its therapeutic effect by inducing keratinocyte apoptosis. To gain further insights into dithranol-induced apoptotic events in vitro, a detailed investigation of its time- and dose-dependent effects has been performed through the evaluation of selected apoptotic markers, using a human keratinocyte cell line (HaCaT) as a model.

Methods

The time- and dose-dependent effects of dithranol on a human keratinocyte cell line (HaCaT) were investigated through the evaluation of a series of apoptotic markers; morphological changes (electron microscopy), phosphatidylserine externalisation (flow cytometry), and caspase-3/7 activation.

Key findings

The dithranol-induced apoptotic cascade was found to follow a well-defined dose and time-course, with the concentration and the period of exposure to the drug acting as the two major factors influencing the events and nature of cell death. The earliest apoptotic event detected was caspase activation (after 6 h), followed by the occurrence of phosphatidylserine externalisation (after 9 h) and subsequently the morphological characteristics associated with early and late stage apoptosis/necrosis (after 12 h).

Conclusions

This study has elucidated the dose- and time-response effects of dithranol-induced apoptosis in human keratinocytes in vitro.

Effects of TNF-α and IL-1β on the Activation of Genes Related to Inflammatory, Immune Responses and Cell Death in Immortalized Human HaCat Keratinocytes

The present experiments were designed to characterize by microarray analysis the transcriptional responses of human keratinocytes (HaCat) to TNF-α and IL-1β, given alone or in combination, in order to better understand the mechanisms underlying inflammatory, immune responses and cell death in which both cytokines play a pathophysiological role. Significant differences in the percentage and quality of genes dysregulated by TNF-α and IL-1β were shown. Both cytokines activated a series of genes involved in inflammatory, immune response as well as in cell death. In our experimental conditions, TNF-α, in contrast to IL-1β, did not induce a significant level of apoptosis in keratinocytes. However, given together both cytokines produced a significant decrease in apoptotic cells and synergistic transcriptional response which was due to the activation of several specific genes occurring after application of each cytokine. TNF-α and IL-1β evoked apoptotic effect and transcriptional responses were linked to the stimulation of their specific receptors since a pre-treatment with monoclonal antibodies vs TNF-α and/or IL-1β receptors was able to significantly reduce them.

Matrix metalloproteinase-21 expression is associated with keratinocyte differentiation and upregulated by retinoic acid in HaCaT cells

In the skin, expression of several matrix metalloproteinases (MMPs) occurs in response to tissue injury, tumorigenesis, angiogenesis, apoptosis, and inflammation. The recently cloned MMP-21 has been implicated in skin development and various epithelial cancers. In this study, we found that it is also expressed by differentiated keratinocytes (KCs) in various benign skin disorders, in which it was not associated with KC apoptosis or proliferation, and in organotypic cultures. Furthermore, MMP-21 was induced in keratinocytes in association with increased calcium and presence of the differentiation marker filaggrin. In stably transfected A431 and HEK293 cell lines, MMP-21 increased invasion of cells but did not associate with increased apoptosis, proliferation, or epithelial-to-mesenchymal transition. Of various agents tested in HaCaT cell cultures, only retinoic acid (10(-6) M) and staurosporine (2.5 x 10(-8) M) upregulated MMP-21 mRNA and protein expression, whereas tumor promoters, hormones, or dexamethasone were without effect. Our results suggest that MMP-21 may be an important protease in the terminal differentiation of keratinocytes.

Targets of vitamin D receptor signaling in the mammary gland

Since the discovery of the vitamin D receptor (VDR) in mammary cells, the role of the vitamin D signaling pathway in normal glandular function and in breast cancer has been extensively explored. In vitro studies have shown that the VDR ligand, 1,25-dihydroxyvitamin D (1,25D), modulates key proteins involved in signaling proliferation, differentiation, and survival of normal mammary epithelial cells. Similar anti-proliferative and pro-differentiating effects of 1,25D have been observed in VDR-positive breast cancer cells, indicating that transformation per se does not abolish vitamin D signaling. However, many transformed breast cancer lose sensitivity to 1,25D secondary to alterations in vitamin D metabolizing enzymes or downregulation of VDR function. Over the years, our laboratory has focused on three general areas: (1) defining mechanisms of vitamin D-mediated apoptosis in breast cancer cells, (2) examining changes in the vitamin D signaling pathway during transformation, including the development of vitamin D resistance, and (3) using mouse models to study the impact of the VDR on growth regulatory pathways in the context of development and tumorigenesis in vivo. Recent developments include detection of megalin-mediated uptake of vitamin D-binding protein (DBP) and identification of CYP27B1 and CYP24 metabolizing enzymes in mammary cells, demonstration of precocious mammary gland development in VDR-null mice, and identification of novel pathways triggered by 1,25D during apoptosis. Our preclinical studies have been complemented by emerging data from other groups suggesting that human breast cancer may be influenced by VDR genotype and vitamin D status. Collectively, these studies have reinforced the need to further define the regulation and function of the vitamin D pathway in cells in relation to prevention and treatment of breast cancer.

Radiation-induced alterations in cytokine production by skin cells

Ionizing radiation exposure of skin results in a cutaneous radiation reaction comprising all pathophysiological reactions and clinical symptoms in irradiated skin. Biological responses of skin occur in a characteristic temporal pattern and mainly depend on radiation quality, dose rate, total dose, and cellular conditions. Immediately after irradiation, production of cytokines by skin cells is initiated and continues as a cascade during all stages of the cutaneous radiation syndrome leading to progressive late symptoms, the predominant of which is fibrosis. Cytokines are important signaling molecules mediating communicative interactions both locally between different cell types within dermal tissues and distantly between organs. Although during recent years much progress has been made in dissecting the complex cytokine network, the role of cytokines in the pathophysiology of the cutaneous radiation reaction is only beginning to be elucidated. Previous studies indicate that the major cytokines in the response of skin cells to ionizing radiation include IL (interleukin)-1, IL-6, tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, and the chemokines IL-8 and eotaxin. In this paper, existing data on the radiation-induced modulation of cytokine expression by skin cells are reviewed.

Procyanidin Oligomers Counteract TGF-β1- and TGF-β2-Induced Apoptosis in Hair Epithelial Cells: An Insight into Their Mechanisms

Procyanidin oligomers are polyphenol compounds we have identified in apples and barley which have hair growth stimulant effects, and which are able to promote hair epithelial cell growth and induce anagen induction of the hair cycle in the in vivo murine model. For the purpose of examining the hair-growing mechanisms of procyanidin oligomers, we examined their relationship to the TGF-beta signal pathway, known to be a regulator of catagen induction, and the mitogen-activated protein kinase cascade linked to cell proliferation. Addition of TGF-beta(1) or TGF-beta(2) to hair epithelial cell cultures dose-dependently decreased cell growth and induced apoptosis; however, addition of procyanidin B-2 to the culture neutralized the growth-inhibiting effects of both TGF-beta(1) and TGF-beta(2) and protected the cells from apoptosis. The same effects were observed with procyanidin B-3. We confirmed that procyanidin B-2 upregulates the expression of MEK-1/2 in cultured murine hair epithelial cells. We speculate that the hair-growing activity of procyanidin oligomers is at least linked to their growth-promoting effects on hair epithelial cells that follow MEK activation and their protective action on TGF-beta(1)- or TGF-beta(2)-induced apoptosis that is assumed to trigger catagen induction in the hair cycle.

Keratinocyte apoptosis in epidermal development and disease

Keratinocyte (KC) apoptosis plays a critical role in regulating epidermal development and restraining carcinogenesis. Apoptosis balances proliferation to maintain epidermal thickness, contributes to stratum corneum formation and may eliminate pre-malignant cells. Apart from the normal developmental program, KC apoptosis can be triggered by UV light and other stimuli. Dysfunctional apoptosis occurs in some skin diseases, such as psoriasis and skin cancer. Here we review the current state of knowledge of KC apoptosis, with particular focus on apoptotic signaling pathways and molecular mechanisms of apoptosis control, and discuss new insights into the complex role of apoptosis in skin carcinogenesis that are emerging from mouse models.

Repair of the three main types of bipyrimidine DNA photoproducts in human keratinocytes exposed to UVB and UVA radiations

Induction of DNA damage by solar UV radiation is a key event in the development of skin cancers. Bipyrimidine photoproducts, including cyclobutane pyrimidine dimers (CPDs), (6-4) photoproducts (64 PPs) and their Dewar valence isomers, have been identified as major UV-induced DNA lesions. In order to identify the predominant and most persistent lesions, we studied the repair of the three types of photolesions in primary cultures of human keratinocytes. Specific and quantitative data were obtained using HPLC associated with tandem mass spectrometry. As shown in other cell types, 64 PPs are removed from UVB-irradiated keratinocytes much more efficiently than CPDs. In contrast, CPDs are still present in high amounts when cells recover their proliferation capacities after cell cycle arrest and elimination of a part of the population by apoptosis. The predominance of CPDs is still maintained when keratinocytes are exposed to a combination of UVB and UVA. Under these conditions, 64 PPs are converted into their Dewar valence isomers that are as efficiently repaired as their (6-4) precursors. Exposure of cells to pure UVA radiation generates thymine cyclobutane dimers that are slightly less efficiently repaired than CPDs produced upon UVB irradiation. Altogether, our results show that CPDs are the most frequent and the less efficiently repaired bipyrimidine photoproducts irrespectively of the applied UV treatment.

Molecular pathways involved in the anti-apoptotic effect of 1,25-dihydroxyvitamin D3 in primary human keratinocytes

We previously reported that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] protects primary human keratinocytes against ultraviolet (UV)B-induced apoptosis. Here, we confirmed the anti-apoptotic effect of 1,25(OH)2D3 in keratinocytes, using cisplatin and doxorubicin as apoptotic triggers. We further showed that 1,25(OH)2D3 activates two survival pathways in keratinocytes: the MEK/extracellular signal regulated kinase (ERK) and the phosphatidylinositol 3-kinase (PI-3K)/Akt pathway. Activation of ERK and Akt by 1,25(OH)2D3 was transient, required a minimal dose of 10(-9) M and could be blocked by actinomycin D and cycloheximide. Moreover, inhibition of Akt or ERK activity with respectively a PI-3K inhibitor (LY294002) or MEK inhibitors (PD98059, UO126), partially or totally suppressed the anti-apoptotic capacity of 1,25(OH)2D3. Finally, 1,25(OH)2D3 changed the expression of different apoptosis regulators belonging to the Bcl-2 family. Indeed, 1,25(OH)2D3 treatment increased levels of the anti-apoptotic protein Bcl-2 and decreased levels of the pro-apoptotic proteins Bax and Bad in a time- and dose-dependent way. Induction of Bcl-2 by 1,25(OH)2D3 was further shown to be mediated by ERK and, to a lesser extent, by Akt. In conclusion, 1,25(OH)2D3 clearly protects keratinocytes against apoptosis (1) by activating the MEK/ERK and the PI-3K/Akt survival pathways and (2) by increasing the Bcl-2 to Bax and Bad ratio.

The therapeutic effects of vitamin D3 and its analogues in psoriasis

Psoriasis is a common skin disease which is characterised by the proliferation and abnormal differentiation of keratinocytes, coupled with complex immune disturbances. The beneficial effects of vitamin D derivatives in this disease are due to their capacity to inhibit proliferation, their ability to induce normal differentiation and their immunomodulatory properties. Since the systemic administration of these compounds is limited by their effect on calcium metabolism, topical preparations have become available in most countries. Topical calcipotriol and/or tacalcitol are now considered as first-line treatment for mild-to-moderate psoriasis and can be taken in combination with other systemic therapies in more severe cases of the disease. Novel orally active vitamin D analogues, with minimal calcitropic effercts, are, however, required for more effective treatment.

Leptomycin B Reduces Matrix Metalloproteinase-9 Expression and Suppresses Cutaneous Inflammation

Matrix metalloproteinase-9 (MMP-9), a type of gelatinase, plays many roles in tissue metabolism, especially in inflammation, and many regulatory elements have been reported in the promoter region of its encoding gene. Leptomycin B, which regulates the nucleo-cytoplasmic trafficking of proteins, including transcription-factor-related ones, has the potential to exert important biological effects. The addition of leptomycin B to keratinocytes in culture had no effect on matrix metalloproteinase-2 (another gelatinase) but caused the selective down-regulation of MMP-9 during the stimulation of differentiation with high Ca(2+) or transforming growth factor-beta, as well as during the stimulation of inflammation by tumor necrosis factor-alpha or interleukin-1alpha. This down-regulation depended on multiple regulatory elements in the promoter of MMP-9 including KRE-M9 (which we have recently identified), and a classical 12-o-tetradecanoyl-phorbol-13-acetate-responsive element. The topical application of leptomycin B to murine skin also effectively suppressed inflammation, including MMP-9 expression, after ultraviolet B irradiation. These results suggest that the application of leptomycin B and/or its derivatives could be useful for treating many inflammatory conditions.

Matrix Metalloproteinase 9 Expression is Coordinately Modulated by the KRE-M9 and 12-O-Tetradecanoyl-Phorbol-13-Acetate Responsive Elements

To investigate the pathophysiologic role of matrix metalloproteinase 9 (MMP-9), we analyzed the mechanism of its transcriptional regulation in keratinocytes and in HT1080 fibrosarcoma cells in culture. The KRE-M9 element, which is located between the 12-O-tetradecanoyl-phorbol-13-acetate responsive element (TRE) and the transcription initiation site in the MMP-9 promoter, is essential for MMP-9 transcription in the absence of the TRE. The KRE-M9 binding protein, however, is shown to be a repressor of transcription rather than an activator; we found several times higher transcriptional activity when the KRE-M9 element was mutated. In contrast, activator protein 1 proteins (AP-1) are shown to activate transcription of MMP-9 by binding to the TRE, which is located adjacent to the KRE-M9 element. Moreover, we found that the KRE-M9 binding protein could serve as a differentiation repressing factor 1 (DRF-1) as shown by the decrease in levels of this protein with differentiation. In addition, the TRE binding protein is able to bind to the KRE-M9 to some extent. These results indicate that the coordinated modulation of MMP-9 transcription via the TRE and the KRE-M9 elements is important in epidermal and mesenchymal tissues. Our findings could facilitate consideration of the molecular mechanism in a variety of pathophysiologic conditions with which MMP-9 is involved.

SUPPRESSIVE EFFECTS OF ASCORBATE DERIVATIVES ON ULTRAVIOLET-B–INDUCED INJURY IN HACAT HUMAN KERATINOCYTES

The aging of skin, including sunburning, is caused by ultraviolet (UV) irradiation. Here, we examined the inhibitory effect of ascorbic acid (AsA) and its derivatives AsA 2-phosphate (AA-2P) and AsA 2-glucoside (AA-2G) on UV-B- induced cytotoxicity in HaCaT keratinocytes. Results show that cell viability significantly decreased when exposed to UV-B at 0.1-0.4 J/cm2 in a dose-dependent manner. In this study, AsA could not inhibit cytotoxicity, but AA-2P and AA-2G was able to cancel the harmful effect of UV-B when treated at high levels of 0.5-5 mM. These results indicate that the masking of the C-2 OH group may be an effective modification for AsA to inhibit UV-B-induced cytotoxicity in human keratinocytes.

Modulation of X-ray-induced apoptosis in human keratinocytes (HaCaT) by 1,25-dihydroxyvitamin D3

Possible effects of 1,25-dihydoxyvitamin D3 (vitamin D) on ionizing radiation-induced cell damage have been unknown until now. The task of the present study was to analyze, in a human keratinocyte cell line (HaCaT), the effects of a preincubation with vitamin D on the X-ray-induced mRNA expression of different genes related to apoptosis (gene array). The first results show that ionizing radiation leads to a down-regulation of various apoptosis-relevant genes in HaCaT cells pretreated with vitamin D. Therefore it can be speculated that vitamin D could prove to be a promising radioprotective substance.

Induction of matrix metalloproteinase-9 secretion from human keratinocytes in culture by ultraviolet B irradiation

BACKGROUND

Matrix metalloproteinases (MMPs) are known as important enzymes involved in tissue metabolism. Among them, MMP-2 and MMP-9 are termed gelatinases, but their specific roles in vivo are still unknown, including their expression patterns following ultraviolet (UV) irradiation.

OBJECTIVE

To elucidate the effects of UV irradiation on the skin, we analyzed the expression of MMP-2 and MMP-9 by primary human keratinocytes in culture.

METHODS

We evaluated the enzymatic functions of MMP-2 and MMP-9 by gelatin-zymography, and of MMP-9 expression by immunofluorescence, using cultured keratinocytes after UV irradiation.

RESULTS

The secretion of MMP-2 (72 kDa) remained at low levels under all conditions examined. Although MMP-9 (92 kDa) secretion was not induced by UVA, it was stimulated by UVB irradiation in a dose-dependent manner. In addition, an enzyme-linked immunosorbent assay showed the tendency to increase for the involucrin expression following UVB exposure. Cell viability was decreased by UVB irradiation in contrast to the induction of MMP-9 and involucrin.

CONCLUSION

These results suggest that the induction of MMP-9 secretion is related to the inflammation including apoptosis of keratinocytes resulting from UVB irradiation.

Phosphatidic acid has a potential to promote hair growth in vitro and in vivo, and activates mitogen-activated protein kinase/extracellular signal-regulated kinase kinase in hair epithelial cells

Phospholipids have recently been discovered to play an important role in cellular regulation. In this study, we focused on phosphatidic acid and lysophosphatidic acid, which are phospholipids known to possess growth-hormonal effects on several types of cells, and examined their growth-promoting effects on murine hair epithelial cells. We discovered that phosphatidic acid possesses intensive growth-promotional effects on hair epithelial cells and epidermal keratinocytes. In contrast, lyso-phosphatidic acid showed lower growth-promoting effects on hair epithelial cells relative to phosphatidic acid and showed minimal or no growth-promoting activity on epidermal keratinocytes. Phosphatidic acid was also shown to have hair-growing activity to induce the anagen phase of the hair cycle in the in vivo murine model. For the purpose of examining the hair-growing mechanisms of phosphatidic acid, we examined its relationship to the mitogen-activated protein kinase cascade linked to cell proliferation and the transforming growth factor beta signal pathway known to be a regulator of catagen induction. We confirmed that phosphatidic acid activates MEK-1/2 and upregulates the expression of MEK-1/2 in cultured murine hair epithelial cells. Addition of transforming growth factor beta1 to hair epithelial cell cultures concentration-dependently decreased cell growth and induced apoptosis; however, addition of phosphatidic acid to the culture neutralized the growth-inhibiting effects of transforming growth factor beta1 and protected the cells from apoptosis. We speculate that the hair-growing activity of phosphatidic acid is at least linked to its growth-promoting effects on hair epithelial cells that follow mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation and its protective action on transforming-growth-factor-beta1-induced apoptosis that is assumed to trigger catagen induction in the hair cycle.

1,25-Dihydroxyvitamin D3 inhibits ultraviolet B-induced apoptosis, Jun kinase activation, and interleukin-6 production in primary human keratinocytes

We investigated the capacity of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] to protect human keratinocytes against the hazardous effects of ultraviolet B (UVB)-irradiation, recognized as the most important etiological factor in the development of skin cancer. Cytoprotective effects of 1,25(OH)(2)D(3) on UVB-irradiated keratinocytes were seen morphologically and quantified using a colorimetric survival assay. Moreover, 1,25(OH)(2)D(3) suppressed UVB-induced apoptotic cell death. An ELISA, detecting DNA-fragmentation, demonstrated that pretreatment of keratinocytes with 1,25(OH)(2)D(3) 1 microM for 24 h reduced UVB-stimulated apoptosis by 55-70%. This suppression required pharmacological concentrations 1,25(OH)(2)D(3) and a preincubation period of several hours. In addition, 1,25(OH)(2)D(3) also inhibited mitochondrial cytochrome c release (90%), a hallmark event of UVB-induced apoptosis. Furthermore, we demonstrated that 1,25(OH)(2)D(3) reduced two important mediators of the UV-response, namely, c-Jun-NH(2)-terminal kinase (JNK) activation and interleukin-6 (IL-6) production. As shown by Western blotting, pretreatment of keratinocytes with 1,25(OH)(2)D(3) 1 microM diminished UVB-stimulated JNK activation with more than 30%. 1,25(OH)(2)D(3) treatment (1 microM) reduced UVB-induced IL-6 mRNA expression and secretion with 75-90%. Taken together, these findings suggest the existence of a photoprotective effect of active vitamin D(3) and create new perspectives for the pharmacological use of active vitamin D compounds in the prevention of UVB-induced skin damage and carcinogenesis.

The role of H2O2 as a mediator of UVB-induced apoptosis in keratinocytes

Apoptosis is an active form of cell death that is initiated by a variety of stimuli, including reactive oxygen species (ROS) and ultraviolet (UV) radiation. Previously, it has been reported that UVB-irradiation of keratinocytes leads to intracellular generation of hydrogen peroxide (H2O2) and that antioxidants can inhibit ROS-induced apoptosis. Although both UVB-irradiation and H2O2-incubation led to increased intracellular H2O2 levels, the antioxidants catalase and glutathione monoester (GME), inhibited apoptosis only when induced by H2O2, not by UVB. Furthermore, extracellular signal-regulated kinase (ERK), a prominent member of the mitogen-activated protein kinase (MAPK) family, was found to be activated by treatment with both UVB and H2O2. Inhibition of ERK phosphorylation by pre-treatment with PD98059 resulted in enhanced apoptosis after H2O2-exposure. However,no significant difference of apoptosis was observed between cells with and without inhibitor pre-treatment upon UVB-irradiation. DNA damage in the form of cyclobutane pyrimidine dimers was observed after exposure to UVB, but no photoproducts were found in H2O2-treated cells. These results suggest a ROS-independent pathway of UVB-induced apoptosis. Although UVB-irradiation causes moderate increase in H2O2, the generation of H2O2 does not contribute to the induction of apoptosis. Moreover, activation of ERK only blocks H2O2-dependent apoptosis but has no impact on UVB-induced apoptosis.

Estrogen-independent expression of neuropsin, a serine protease in the vagina of mice exposed neonatally to diethylstilbestrol

Perinatal treatment of female mice with natural or synthetic estrogens including diethylstilbestrol (DES) results in estrogen-independent persistent proliferation and cornification of the vaginal epithelium. However, the molecular mechanisms of the estrogen-independent changes have not been elucidated. To analyze the mechanism of estrogen-independent cell proliferation and cornification of the vaginal epithelium, we used differential display and determined specific genes expressed in neonatally DES-treated vagina. A candidate clone that designated DDV5 was identical to the serine protease, neuropsin that is reportedly expressed in the mouse central nervous system. We then analyzed the expression pattern of DDV5/neuropsin using Northern blot analysis. We found: (1). DDV5/neuropsin mRNA is expressed in vaginae from neonatally DES-treated ovariectomized mice but not in vaginae from ovariectomized control mice, (2). its expression is not detected in uteri from neonatally DES-treated mice, (3). DDV5/neuropsin is expressed in vaginae from normal intact mice during estrus. Furthermore, we found that DDV5/neuropsin mRNA rapidly decreased in vaginae after ovariectomy. DDV5/neuropsin was detected in vaginae from ovariectomized mice 48 h after estrogen treatment. These results suggest that DDV5/neuropsin is expressed in estrogen-stimulated mouse vagina, and its gene expression is regulated by estrogen. Neonatal DES exposure affects transcriptional control of DDV5/neuropsin in the mouse vagina, which results in persistent expression of DDV5/neuropsin even after ovariectomy, thus, DDV5/neuropsin may play a role in estrogen-independent persistent proliferation and cornification of the vaginal epithelium. Using in situ hybridization method, we found DDV5/neuropsin mRNA localized in epithelial cells but not stromal cells in vaginae. This is the first report on the gene expression of a serine-protease neuropsin in the mouse vagina, and as a marker of the estrogen-independent persistent proliferation and cornification of the vaginal epithelium.

Ultraviolet B radiation induces activation of neutral and acidic sphingomyelinases and ceramide generation in cultured normal human keratinocytes

The sphingomyelin pathway is an ubiquitous, evolutionary conserved signaling system which transduces an extracellular signal into the cell. During the past few years increasing evidence has shown that the sphingolipid ceramide may play a role as a second messenger in intracellular signal transduction. The ceramide generation via sphingomyelinase (SMase) is followed by three major cellular responses: cell growth arrest, induction of cell differentiation and/or induction of programmed cell death or apoptosis. The aim of this study is to investigate whether activation of SMases and generation of ceramide can be induced by UVB radiation in normal human keratinocytes. The present data show that exposure to UVB radiation results in rapid generation of ceramide. The ceramide accumulation starts 15 min after UV exposure and progressively increases up to 24 h. In vitro measurement of SMase activity following exposure to UVB evidences an activation of both neutral and acidic SMases. Moreover, UVB induces apoptosis in normal human keratinocytes as shown by TUNEL technique and FACS analysis. These data indicate that UVB induced ceramide generation and activation of both neutral and acidic SMases, suggesting that sphingolipids metabolism may be involved in the UVB signaling pathway.

Keratinocytes enriched for stem cells are protected from anoikis via an integrin signaling pathway in a Bcl-2 dependent manner

Because inhibition of integrin signaling induces apoptosis, we investigated whether keratinocytes expressing beta1 and alpha6beta4 integrins (enriched for stem cells) are protected from cell death. Keratinocytes rapidly adhering to type IV collagen expressed highest levels of beta1 and alpha6beta4 and of the anti-apoptotic stem cell marker p63. Apoptotic cells were significantly higher in slowly adhering than in rapidly adhering keratinocytes. Anti-beta1 integrin caused a significant increase in apoptotic cells, while it decreased Bcl-2 levels in stem keratinocytes. Bax and Bad proteins were higher in slowly adhering than in rapidly adhering cells. By contrast, Bcl-2, Bcl-x and Mcl-1 proteins were highest in rapidly adhering keratinocytes and nearly absent in slowly adhering cells. After addition of anti-beta1 integrin, the apoptotic rate was significantly higher in HaCaT cells not expressing Bcl-2 than in controls. These results indicate that keratinocytes enriched for stem cells are protected from apoptosis via beta1 integrin, in a Bcl-2 dependent manner.

The sunburn cell revisited: an update on mechanistic aspects

The sunburn cell (SBC), with its pyknotic nucleus and eosinophilic cytoplasm, is characteristic of mammalian epidermis after exposure to UVC and UVB radiation or UVA radiation in the presence of psoralens. SBC may be regarded as an example of apoptosis: controlled individual cell death. Since the discovery of apoptosis over thirty years ago, there has been a considerable increase in the knowledge of mechanisms involved in this process. DNA damage has been shown to be a major determinant of SBC production both in a p53-dependent and -independent manner. Extranuclear events such as activation of membrane bound death receptors also contribute to SBC formation. The development of new technologies and techniques has resulted in a better understanding of the mechanisms and machinery involved in apoptosis, triggered by various stimuli and in different cell types. Of particular importance has been the elucidation of regulatory molecules such as caspases, inhibitor of apoptosis proteins (IAP) and the role of mitochondria as key to the process of apoptosis and consequent production of SBC. This review attempts to give an update on those mechanisms involved and the occurrence and relevance of SBC in mammalian skin are discussed.

Antiproliferative action of vitamin D

During the past few years, it has become apparent that vitamin D may play an important role in malignant transformation. Epidemiological studies suggest that low vitamin D serum concentration increases especially the risk of hormone-related cancers. Experimentally, vitamin D suppresses the proliferation of normal and malignant cells and induces differentiation and apoptosis. In the present review we discuss the mechanisms whereby vitamin D regulates cell proliferation and whether it could be used in prevention and treatment of hyperproliferative disorders like cancers.

Detection of poly(ADP-ribose) by immunocytochemistry: a sensitive new method for the early identification of UVB- and H2O2-induced apoptosis in keratinocytes

Apoptosis is an active form of cell death that is initiated by a variety of stimuli, including reactive oxygen species (ROS) and ultraviolet (UV) radiation. Poly (ADP-ribose) (PAR) is formed upon activation of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), and therefore was suggested as a new marker of apoptosis. Since DNA of epidermal cells represents a well-known chromophore for UVB irradiation, and UVB is known to generate H2O2 in keratinocytes, we hypothesized that PAR is a very sensitive marker of UVB- and H2O2-induced apoptosis in keratinocytes. In order to test this hypothesis, human immortalized keratinocytes (HaCaT) were UVB-irradiated or treated with H2O2, and subsequently apoptosis was identified by comparing conventional parameters such as morphological analysis, DNA laddering, and TUNEL assay, with PAR formation. Both, UVB and H2O2 treatment induced PAR formation in HaCaT cells in a dose-dependent manner, and its formation was detected as early as 4 h after irradiation, and at lower UVB doses (10 mJ/cm2) than observed by DNA laddering and the TUNEL assay. In conclusion, the detection of PAR formation is a very sensitive and early method for the identification of apoptotic cells in UVB-induced apoptosis of human keratinocytes.

1Alpha,25-dihydroxyvitamin D3 protects human keratinocytes from apoptosis by the formation of sphingosine-1-phosphate

Owing to its ability to induce growth arrest and differentiation of keratinocytes, 1alpha,25-dihydroxyvitamin D3 and its analogs are useful for the treatment of hyperproliferative skin diseases, such as psoriasis vulgaris. It has been implicated that the 1alpha,25-dihydroxyvitamin D3-induced differentiation of keratinocytes is mediated, at least in part, by the formation of ceramides; however, ceramides have also been identified to induce apoptosis in many cells, including keratinocytes. Therefore, it was of interest to investigate the influence of 1alpha,25-dihydroxyvitamin D3 on apoptosis in keratinocytes. Most interestingly, physiological concentrations of 1alpha,25-dihydroxyvitamin D3 did not induce apoptosis in keratinocytes, despite the formation of ceramides. Moreover, 1alpha,25-dihydroxyvitamin D3 appeared cytoprotective and made keratinocytes resistant to apoptosis induced by ceramides, ultraviolet irradiation, or tumor necrosis factor-alpha. The cytoprotective effect was accompanied by the formation of the sphingolipid breakdown product sphingosine-1-phosphate, which prevented apoptosis in analogy to 1alpha,25-dihydroxyvitamin D3. The effect of 1alpha,25-dihydroxyvitamin D3 was specific as the almost inactive precursor cholecalciferol neither induced sphingosine-1-phosphate formation nor prevented cells from apoptosis. Besides this, the cytoprotective aptitude of 1alpha,25-dihydroxyvitamin D3 was completely abolished by the sphingosine kinase inhibitor N,N-dimethylsphingosine, which blocked sphingosine-1-phosphate formation. Moreover, sphingosine-1-phosphate was able to restore the cytoprotective effect of 1alpha,25-dihydroxyvitamin D3 in the presence of N,N-dimethylsphingosine. Taken together, here we report for the first time that 1alpha,25-dihydroxyvitamin D3 protects keratinocytes from apoptosis and additionally this cytoprotection is mediated via the formation of sphingosine-1-phosphate.

Treatment of normal human keratinocytes with 2,3,7,8-tetrachlorodibenzo-p-dioxin causes a reduction in cell number, but no increase in apoptosis

We have examined the effect of TCDD on the growth of normal human keratinocytes. TCDD is a ubiquitous environmental toxicant that causes a severe dermatopathology in humans, which is known as chloracne. The cell biological basis of this pathology remains unknown. We conducted growth experiments in preconfluent normal human keratinocytes with both low (0.05 mM) and standard (0.66 mM) extracellular calcium concentrations in the media. TCDD treatment reduced the number of adherent keratinocytes relative to controls in media containing 0.05 or 0.66 mM calcium. Based on these observations, we speculated that the decrease in the cell number of TCDD-treated cultures might be the result of increased apoptosis. Analysis of nucleosomal fragmentation, nuclear morphology, and caspase-3 activity in keratinocytes reveals no increase in the characteristics of apoptosis in response to TCDD treatment. We therefore conclude that TCDD impacts on keratinocyte homeostasis independent of apoptosis.

A novel mechanism of matrix metalloproteinase-9 gene expression implies a role for keratinization

To investigate the pathophysiological role of matrix metalloproteinase (MMP)-9 in the skin, we analyzed MMP-9 expression from human keratinocytes in culture. MMP-9 and the terminal differentiation marker involucrin were co-localized in the same keratinocytes with a high concentration of Ca(2+), a potent stimulator of differentiation. We identified the novel KRE-M9 element, further downstream to the previously reported TPA responsive element in the MMP-9 promoter, and both of these two elements were shown to be important for MMP-9 transcription and Ca(2+) induction. The concomitant upregulation of MMP-9 and involucrin transcripts was probably due to the very similar gene regulatory elements, KRE-M9 and KRE-4, in their respective promoters. These results indicate a novel mechanism of transcriptional regulation for MMP-9 in the process of keratinization, implying the probable association of apoptosis and differentiation of keratinocytes in epidermal skin tissue.

Low-dose ultraviolet B radiation synergizes with TNF-alpha to induce apoptosis of keratinocytes

High-dose ultraviolet B (UVB) irradiation is known to induce apoptosis of keratinocytes, but low-dose UVB dose not. In this paper we present evidence that low-dose UVB can induce TNF-alpha-dependent apoptosis of keratinocytes. In our study, 5 mJ/cm(2) doses of UVB were not sufficient by themselves to induce apoptosis of cultured human keratinocytes, but 20 mJ/cm(2) doses of UVB were. The combination of 5 mJ/cm(2) doses of UVB and exogenous TNF-alpha (15 ng/ml) induced significant apoptosis of keratinocytes, although exogenous TNF-alpha without UVB did not. This phenomenon was accompanied by enhanced clustering of tumor necrosis factor receptor 1 (TNFR1). TNF-alpha's promotion of the induction of apoptosis by low-dose UVB was seen until 30 min after irradiation but not at 1 h. We confirmed this finding using a skin organ culture system. UVB (20 mJ/cm(2)), which did not induce transformation of epidermal keratinocytes into sunburn cells, induced apoptosis when TNF-alpha was added to the culture medium. These results suggest that one of the possible mechanisms of inducing keratinocyte apoptosis by low-dose UVB and TNF-alpha is that low-dose UVB augments ligand-binding-induced TNFR1 clustering, resulting in increased apoptotic cell death.

BAG-1 p50 isoform interacts with the vitamin D receptor and its cellular overexpression inhibits the vitamin D pathway

Human BAG-1 is an anti-apoptotic protein with four protein isoforms (BAG-1 p50, p46, p33, and p29). BAG-1 p46 was originally isolated in a screen for proteins binding to the glucocorticoid receptor; it binds and modulates the action of several members of the nuclear steroid hormone receptor superfamily. The vitamin D receptor (VDR) is another member of this superfamily, and the vitamin D pathway is important for prevention and therapy of osteoporosis, renal failure, cancer, and psoriasis. Therefore, we investigated the effect of the recently isolated BAG-1 p50 on the vitamin D pathway. By use of Far Western blot analysis and glutathione S-transferase BAG-1 p50 binding assays, BAG-1 p50 was demonstrated to interact with the VDR, and the BAG-1 p50 N-terminus was required. In U87 cells that were stably transfected with BAG-1 p50, binding of the VDR to its response element in electrophoretic mobility shift assays was blocked, enhancement of transcriptional activation was inhibited, cell growth rate was enhanced, cell growth inhibition induced by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] was blocked, and 1,25(OH)2D3-mediated VDR induction was inhibited. These results suggest that BAG-1 p50 is a novel regulator of the vitamin D signaling pathway, and its overexpression may lead to cellular resistance to 1,25(OH)2D3 therapy.

Role of TGFbeta signaling in skin carcinogenesis

The TGFbeta signaling pathway is one of the most important mechanisms in the maintenance of epithelial homeostasis. Alterations leading to either the repression or enhancement of this pathway have been shown to affect cancer development. Although TGFbeta inhibits growth of normal epithelial cells, it is paradoxically overexpressed in many epithelial cancers. It has been postulated that TGFbeta acts as a tumor suppressor at the early stages of carcinogenesis, but overexpression of TGFbeta at late stages of carcinogenesis may be a critical factor for tumor invasion and metastasis. The detailed mechanisms regulating this functional switch of TGFbeta remain to be elucidated. The relevance of the TGFbeta signaling pathway to the development of primary epithelial tumors in man has been further substantiated by the discovery of mutations in TGFbeta receptors and in the downstream signaling mediators, the Smads. The epidermis is one of the major targeting tissues for TGFbeta signaling. Chemical carcinogenesis studies have revealed a paradoxical effect of TGFbeta on skin carcinogenesis: inhibition of papilloma formation but promotion of malignant conversion. In addition, deletion of the TGFbeta type II receptor accelerates skin carcinogenesis. This review focuses on our current understanding of the role of TGFbeta signaling in skin carcinogenesis.

Kinetics of apoptosis-related genes mRNA expression in the dorsal skin of hypotrichotic WBN/ILA-ht rats after topical application of T-2 toxin

The expression of apoptosis-related genes mRNAs was examined in the dorsal skin of hypotrichotic WBN/ILA-Ht rats topically applied with T-2 toxin (10 microl of 0.5 microg/microl solution). The total mRNA was obtained from skin biopsy samples from each rat at 3, 6, 12 and 24 hours after T-2 toxin treatment (HAT), and RT-PCR was carried out with pairs of oligonucleotide primers corresponding to the cDNA sequences of rat p53, bcl-2, c-ki-ras, c-fos and c-jun oncogenes. The expression of c-fos mRNA markedly increased at 3 HAT, peaked at 6 HAT, and greatly decreased at 12 HAT. However it maintained a higher level, compared with the control level, even at 24 HAT. Although not prominent, the expression of c-jun mRNA also showed significant elevation from 3 to 12 HAT. On the other hand, there were no changes in the expression of p53, bcl-2 and c-ki-ras mRNAs throughout the observation period. Judging from the present results and our previous report that epidermal cells developed apoptosis at 12 HAT (Histol Histopathol 1999; 14: 337-342), the induction of c-fos and perhaps of c-jun mRNAs may be associated with T-2 toxin-induced epidermal cell apoptosis.

DNA-dependent protein kinase catalytic subunit is cleaved during UV-induced apoptosis

Ultraviolet radiation can induce the injury of epidermal keratinocytes, resulting in sunburn cell (apoptotic cell) formation. It has been demonstrated that the protease caspase-3, a downstream molecule of the CD95 pathway, is activated in UV-exposed HaCaT cells, and that the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is cleaved by interleukin-1beta converting enzyme (ICE)-like protease during apoptosis induced by X-rays, staurosporine and etoposide. Then, we studied whether the DNA-PKcs is cleaved during UV-induced apoptosis in keratinocytes. We used the well-characterized cloned human keratinocyte cell line HaCaT, which carries p53 mutations. UVB-induced apoptotic cells were observed by TdT-mediated deoxyuridine 5'-triphosphate nick end labeling (TUNEL) assay and agarose gel electrophoresis. Western blot analysis was performed using the antibody against DNA-PKcs. The cleavage occurred during UVB-induced apoptosis in HaCaT cells. It suggests that the cleavage is associated with loss of DNA-PK activity. Thus, a functional relevance of cleavage of DNA-PKcs may be to prevent rejoining fragmented DNA during apoptosis, thereby promoting apoptotic processes. Although apoptosis was not completely blocked by the caspase-3 inhibitor, the cleavage of the DNA-PKcs was blocked. These results indicate that DNA-PKcs is cleaved by the caspase-3 for UVB-induced apoptosis in HaCaT cells.

Carboxyfullerenes protect human keratinocytes from ultraviolet-B-induced apoptosis

Carboxyfullerene, a water-soluble carboxylic acid derivative of a fullerene, which acts as a free-radical scavenger, was investigated as a protective agent against ultraviolet-light-induced damage in human keratinocytes. First, we demonstrate that carboxyfullerene is not cytotoxic for these cells. In addition, this compound significantly reduces the ultraviolet-B-induced inhibition of keratinocyte proliferation and protects keratinocytes from apoptosis caused by ultraviolet B irradiation in a time- and dose-dependent fashion. Furthermore, the percentage of cells with depolarized mitochondria is significantly lower in ultraviolet-B-irradiated keratinocytes pretreated with carboxyfullerene than in cells provided with diluent alone. Carboxyfullerene also protects human keratinocytes from apoptosis induced by exposure to deoxy-D-ribose, a sugar that causes cell death through a pathway involving oxidative stress. On the other hand, ultraviolet B downregulates bcl-2 levels in human keratinocytes, and carboxyfullerene fails to prevent this effect. These results suggest that carboxy- fullerene protects human keratinocytes from ultraviolet B damage possibly via a mechanism interfering with the generation of reactive oxygen species from depolarized mitochondria without the involvement of bcl-2.

Apoptosis and cytokine release induced by ionizing or ultraviolet B radiation in primary and immortalized human keratinocytes

We have compared the induction of apoptosis and cytokine release by UVB and gamma-radiation in primary (untransformed) and in two immortalized human epithelial/keratinocyte cell lines, HaCaT and KB (KB is now known to be a subline of the ubiquitous keratin-forming tumour cell line HeLa and we therefore designate it HeLa-KB). In both the primary and the immortalized cell lines apoptosis and release of the inflammatory cytokine interleukin-6 are induced rapidly following UVB irradiation. In contrast, only the immortalized cells undergo apoptosis and release interleukin-6 after gamma-irradiation and here the onset of apoptosis and cytokine release are delayed. The same distinction between primary and immortalized cells was observed when double-strand breaks were induced with the anticancer drug mitoxantrone, which stabilizes topoisomerase II-cleavable complexes. We suggest that immortalization may sensitize keratinocytes to the apoptogenic effect of ionizing radiation or mitoxantrone by deregulating normal cell cycle checkpoints. In both human keratinocytes and fibroblasts, cell killing, as assayed by loss of colony-forming ability, is not coupled to apoptosis. Immortalization increases resistance to gamma-radiation killing but sensitizes to apoptosis. In contrast, although immortalization also sensitizes to UVB-induced apoptosis, it does not affect UVB-induced cell killing. Apoptosis unambiguously indicates death at the single cell level but clonal cell survival integrates all the cellular and genetic processes which prevent or permit a scorable clone to develop.

Kinetics and distribution of transforming growth factor (TGF)-beta 1 mRNA in the dorsal skin of hypotrichotic WBN/ILA-Ht rats following topical application of T-2 toxin

Depression of basal cell proliferating activity and subsequent induction of basal cell apoptosis in the epidermis and infiltration of inflammatory cells including mast cells in the dermis were observed in the dorsal skin of hypotrichotic WBN/ILA-Ht rats following the topical application of T-2 toxin in our previous study (ALBARENQUE et al. 1999). In the present study, kinetics of TGF-beta 1 mRNA was investigated using the same experimental system. The level of TGF-beta 1 mRNA of the whole skin tissue measured by competitive RT-PCR method showed a slight elevation from 6 to 12 hours after treatment (HAT) and reached the significantly higher level at 24HAT compared with the control skin. The increase in signals of TGF-beta 1 mRNA detected by in situ hybridization method started at 3HAT in the epidermis and progressed thereafter both in the epidermis and in the dermis. These results suggest that the elevated level of TGF-beta 1 mRNA may have a close relation to the induction of epidermal basal cell apoptosis as well as to the intradermal infiltration of mast cells and fibroblasts following the topical application of T-2 toxin.

Inducible expression of filaggrin increases keratinocyte susceptibility to apoptotic cell death

Filaggrin is an intermediate filament associated protein that aids the packing of keratin filaments during terminal differentiation of keratinocytes. Premature aggregation of keratin filaments is prevented by filaggrin expression as the inactive precursor, profilaggrin, which is localized in keratohyalin granules in vivo. We have previously shown that filaggrin constructs, when transiently transfected into epithelial cells, lead to a collapsed keratin cytoskeletal network and dysmorphic nuclei with features of apoptosis. The apparent transfection rate is low with filaggrin constructs, supporting their disruptive role but hindering further study. To bypass this problem, we generated stable keratinocyte cell lines that express mature human filaggrin using a tetracycline-inducible promoter system. We found that cell lines expressing filaggrin, but not control cell lines, exhibited increased sensitivity to multiple apoptotic stimuli as measured by morphologic and biochemical criteria. None of the cell lines showed an increase in endogenous expression of filaggrin in response to the same stimuli. Filaggrin expression alone was insufficient to induce apoptosis in these keratinocyte cell lines. We conclude that filaggrin, due to its keratin binding ability, primes cells for apoptosis. Because filaggrin is expressed at a level of the epidermis where keratinocytes are in transition between the nucleated granular and the anucleate cornified layers, we hypothesize that filaggrin aids in the terminal differentiation process by facilitating apoptotic machinery.

TGF-beta1 and radiation fibrosis: a master switch and a specific therapeutic target?

Radiation fibrosis is a frequent sequel of therapeutic or accidental radiation overexposure in normal human tissues. One of the main fundamental problems yet unsolved in fibrotic tissues is the origin of the chronic activation of myofibroblasts within these tissues. It has been postulated that this chronic activation results from a continuous production of activating factors. In this context, fibrosis could be defined as a wound where continuous signals for tissue repair are emitted. Cytokines and growth factors probably play a central role in this process. Among them, transforming growth factor-beta1 (TGF-beta1) is considered as a master switch for the fibrotic program. This review discusses recent evidence on the critical role played by TGF-beta in the initiation, development, and persistence of radiation fibrosis. It summarizes the results concerning this factor after irradiation of various tissues and cells, with an emphasis on superficial fibrosis, including skin and subcutaneous tissues. Finally, recent data concerning the treatment of established fibrotic disorders of various etiology are presented, as well as the possible mechanisms involved in fibrosis regression, which show that the TGF-beta pathway may constitute a specific target for antifibrotic agents.

CD40 ligation alters the cell cycle of differentiating keratinocytes

CD40 is expressed in normal human keratinocytes, especially in the basal cell layer. We have recently reported that CD40 ligation strongly inhibits keratinocyte proliferation and induces their differentiation. In this study, the CD40 pathway that prevents keratinocyte growth was investigated. We first reported that interferon-gamma treatment potentiated the CD40-mediated inhibition of keratinocyte proliferation. CD40-CD40 ligand interactions, in the presence or absence of interferon-gamma, neither enhanced spontaneous keratinocyte apoptosis, nor did it enhance apoptosis induced by various agents. More importantly, we showed that CD40 signaling altered the keratinocyte cell cycle, as demonstrated by a decreasing number of cells in the G1 and S phases and an accumulation in G2/M phase of the cell cycle. Furthermore, western blot analysis of cell cycle regulatory proteins, showed a decrease in cyclin A and E expression in CD40-activated keratinocytes. Collectively, these results indicate that CD40 ligation inhibits keratinocyte renewal by a mechanism independent of cell apoptosis and that modulation of the keratinocyte cell cycle is an additional outcome of CD40 signaling.

New roles for glial cell line-derived neurotrophic factor and neurturin: involvement in hair cycle control

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN), and their receptors, GDNF family receptor alpha-1 (GFRalpha-1) and GDNF family receptor alpha-2 (GFRalpha-2), are critically important for kidney and nervous system development. However, their role in skin biology, specifically in hair growth control, is as yet unknown. We have studied expression and function of GDNF, neurturin, GFRalpha-1, and GFRalpha-2 in murine skin during the cyclic transformation of the hair follicle (HF) from its resting state (telogen) to active growth (anagen) and then through regression (catagen) back to telogen. GDNF protein and GFRalpha-1 messenger RNA are prominently expressed in telogen skin, which lacks NTN and GFRalpha-2 transcripts. Early anagen development is accompanied by a significant decline in the skin content of GDNF protein and GFRalpha-1 transcripts. During the anagen-catagen transition, GDNF, GFRalpha-1, NTN, and GFRalpha-2 transcripts reach maximal levels. Compared with wild-type controls, GFRalpha-1 (+/-) and GFRalpha-2 (-/-) knockout mice show a significantly accelerated catagen development. Furthermore, GDNF or NTN administration significantly retards HF regression in organ-cultured mouse skin. This suggests important, previously unrecognized roles for GDNF/GFRalpha-1 and NTN/GFRalpha-2 signaling in skin biology, specifically in the control of apoptosis-driven HF involution, and raises the possibility that GFRalpha-1/GFRalpha-2 agonists/antagonists might become exploitable for the treatment of hair growth disorders that are related to abnormalities in catagen development.

Calcitriol-induced apoptosis in LNCaP cells is blocked by overexpression of Bcl-2

While the role of vitamin D in bone and mineral metabolism has been investigated extensively, the role of the vitamin D receptor in other tissues is less well understood. 1,25-Dihydroxyvitamin D3 (calcitriol) can act as a differentiating agent in normal tissues and can inhibit the growth of many cancer cell lines including LNCaP prostate cancer cells. We have shown previously that calcitriol causes LNCaP cell accumulation in the G0/G1 phase of the cell cycle. In this study, we demonstrate that calcitriol also induces apoptosis of LNCaP cells. The calcitriol-induced apoptosis is accompanied by a down-regulation of Bcl-2 and Bcl-X(L) proteins, both of which protect cells from undergoing apoptosis. Other proteins important in apoptotic control, Bax, Mcl-1, and Bcl-X(S), are unaffected by calcitriol treatment. We find that overexpression of Bcl-2 blocks calcitriol-induced apoptosis and reduces, but does not eliminate, calcitriol-induced growth inhibition. We conclude that both regulation of cell cycle and the apoptotic pathway are involved in calcitriol action in prostate cancer cells.

Inhibition of cadherin function differentially affects markers of terminal differentiation in cultured human keratinocytes

Cadherin function is required for normal keratinocyte intercellular adhesion and stratification. In the present study, we have investigated whether cadherin-cadherin interactions may also modulate keratinocyte differentiation, as evidenced by alterations in the levels of several differentiation markers. Confluent keratinocyte cultures, propagated in low Ca(2+) medium in which cadherins are not active, were pre-incubated with antibodies that block the function of E-cadherin and/or P-cadherin; Ca(2+)was then elevated to 1 mM to activate the cadherins and induce differentiation. In control cultures (incubated with no antibody or with antibodies to other cell surface molecules), Ca(2+) elevation induced an increase in type 1 transglutaminase, profilaggrin, and loricrin, as measured by western blotting and in agreement with previous results. However, the concurrent addition of antibodies against both E- and P-cadherin prevented this increase in transglutaminase 1 protein. Incubation with either antibody alone had no consistent effect. Profilaggrin and loricrin, which are later markers of keratinocyte differentiation, responded differently from transglutaminase 1 to addition of antibodies. In the presence of anti-E-cadherin antibody, both loricrin and profilaggrin levels were dramatically enhanced compared to the high Ca(2+) control cells, while addition of antibody to P-cadherin slightly attenuated the Ca(2+)-induced increase. In the presence of both antibodies, loricrin and profilaggrin protein levels were intermediate between those observed in the presence of either antibody alone. The expression of involucrin, however, was unaffected by addition of antibodies. In addition, effects of the anti-cadherin antibodies were not secondary to alterations in proliferation or programmed cell death, as determined by several independent assays of these processes. Thus, the consequences of cadherin inhibition depend upon both the particular cadherin and the differentiation marker under study. Taken together, these data suggest that E-cadherin and P-cadherin contribute to the orderly progression of terminal differentiation in the epidermis in multiple ways.

Nerve growth factor protects human keratinocytes from ultraviolet-B-induced apoptosis

Ultraviolet radiation is a potent inducer of apoptosis, whereas autocrine nerve growth factor protects human keratinocytes from programmed cell death. To evaluate the role of nerve growth factor in the mechanisms of ultraviolet B-induced apoptosis, cultured human keratinocytes were ultraviolet B irradiated following pretreatment with K252, a specific inhibitor of the tyrosine kinase high-affinity nerve growth factor receptor. Here we report that the addition of K252 significantly enhanced keratinocyte apoptosis. We then transfected normal human keratinocytes with pNUT-hNGF. Nerve growth factor overexpressing keratinocytes secreted the highest amounts of nerve growth factor in culture supernatants, were more viable, and had a higher rate of proliferation than mock-transfected cells. Whereas ultraviolet B radiation downregulated nerve growth factor mRNA and protein as well as the tyrosine kinase high-affinity nerve growth factor receptor in normal keratinocytes, it failed to do so in nerve growth factor-transfected cells. Moreover, nerve growth factor overexpressing keratinocytes were partially resistant to apoptosis induced by increasing doses of ultraviolet B at 24 and 48 h. These results indicate that downregulation of nerve growth factor function plays an important part in the mechanisms of ultraviolet B-induced apoptosis in human keratinocytes. In addition, ultraviolet B caused a decrease in BCL-2 and BCL-xL expression in mock-transfected keratinocytes, but not in nerve growth factor overexpressing cells. Finally, nerve growth factor prevented the cleavage of the enzyme poly(ADP-ribose) polymerase induced in human keratinocytes by ultraviolet B. These results are consistent with a model whereby the autocrine nerve growth factor protects human keratinocytes from ultraviolet B-induced apoptosis by maintaining constant levels of BCL-2 and BCL-xL, which in turn might block caspase activation.