Fas Ligand Elicits a Caspase-Independent Proinflammatory Response in Human Keratinocytes: Implications for Dermatitis

When cell death goes wrong: inflammatory outcomes of failed apoptosis and mitotic cell death

Apoptosis is a regulated cellular pathway that ensures that a cell dies in a structured fashion to prevent negative consequences for the tissue or the organism. Dysfunctional apoptosis is a hallmark of numerous pathologies, and treatments for various diseases are successful based on the induction of apoptosis. Under homeostatic conditions, apoptosis is a non-inflammatory event, as the activation of caspases ensures that inflammatory pathways are disabled. However, there is an increasing understanding that under specific conditions, such as caspase inhibition, apoptosis and the apoptotic machinery can be re-wired into a process which is inflammatory. In this review we discuss how the death receptor and mitochondrial pathways of apoptosis can activate inflammation. Furthermore, we will highlight how cell death due to mitotic stress might be a special case when it comes to cell death and the induction of inflammation.

Adipose-derived stem cells decolonize skin Staphylococcus aureus by enhancing phagocytic activity of peripheral blood mononuclear cells in the atopic rats

Staphylococcus aureus (S. aureus) is known to induce apoptosis of host immune cells and impair phagocytic clearance, thereby being pivotal in the pathogenesis of atopic dermatitis (AD). Adipose-derived stem cells (ASCs) exert therapeutic effects against inflammatory and immune diseases. In the present study, we investigated whether systemic administration of ASCs restores the phagocytic activity of peripheral blood mononuclear cells (PBMCs) and decolonizes cutaneous S. aureus under AD conditions. AD was induced by injecting capsaicin into neonatal rat pups. ASCs were extracted from the subcutaneous adipose tissues of naïve rats and administered to AD rats once a week for a month. Systemic administration of ASCs ameliorated AD-like symptoms, such as dermatitis scores, serum IgE, IFN-γ⁺/IL-4⁺ cell ratio, and skin colonization by S. aureus in AD rats. Increased FasL mRNA and annexin V⁺/7-AAD⁺ cells in the PBMCs obtained from AD rats were drastically reversed when co-cultured with ASCs. In contrast, both PBMCs and CD163⁺ cells bearing fluorescent zymosan particles significantly increased in AD rats treated with ASCs. Additionally, the administration of ASCs led to an increase in the mRNA levels of antimicrobial peptides, such as cathelicidin and β-defensin, in the skin of AD rats. Our results demonstrate that systemic administration of ASCs led to decolonization of S. aureus by attenuating apoptosis of immune cells in addition to restoring phagocytic activity. This contributes to the improvement of skin conditions in AD rats. Therefore, administration of ASCs may be helpful in the treatment of patients with intractable AD.

Cytokines and apoptosis in atopic dermatitis

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease. AD affects 10-20% of children worldwide and persists into adulthood in a minority of cases, affecting approximately 2-3% of the adult population, with an increased prevalence over the past decades in developed countries. Atopy is a genetic tendency to overproduce IgE class antibodies in response to common antigens found in the environment. Concurrence of different atopy such as allergic rhinitis or asthma in children with AD is estimated at 80%. AD is characterized by a vicious cycle of an allergic immune response. The emerging picture of the AD is a complex disorder with barrier dysfunction, immunological, genetic and environmental factors all playing key roles. Patients with severe or persistent disease and their families experience significant impairment in their quality of life, and in addition, AD places a heavy economic burden on society as a whole. Pathogenesis, the role of the epidermal barrier, mechanisms of cells apoptosis, the role of T cells and cytokines in AD are discussed in this article.

Increased MxA protein expression and dendritic cells in spongiotic dermatitis differentiates dermatomyositis from eczema in a single-center case-control study

BACKGROUND

Dermatomyositis (DM) is conventionally characterized by interface dermatitis (ID) on skin histopathology. A subset of DM patients has skin biopsies showing spongiotic dermatitis (SD), a histopathology more commonly seen in eczema. In this study, we aimed to (a) identify the percentage of clinically diagnosed DM patients with SD skin biopsies, (b) identify cytokine and cell markers that can help determine if a SD skin biopsy is consistent with DM.

METHODS

In this case-control study, biopsy specimens from ten DM patients with SD (DM-SD) were compared to specimens from ten healthy controls, ten patients with eczema, and 12 patients with DM with ID (DM-ID). Specimens were stained by immunohistochemistry for MxA, IFN-β, CD11c, and BDCA2. One-way ANOVA with Bonferroni's multiple comparison test was used to compare protein expression between groups.

RESULTS

Eleven of 164 (6.7%) patients with a clinical diagnosis of DM at our tertiary care center were identified as having SD. MxA, IFN-β, CD11c, and BDCA2 protein expression was significantly higher in DM-SD compared to eczema and healthy controls. Expressions of MxA, IFN-β, and BDCA2 were not significantly different between DM-SD and DM-ID.

CONCLUSION

Increased MxA, IFN-β, CD11c, and BDCA2 protein expression may aid in distinguishing between DM-SD and eczema and warrants further investigation.

Fas/FasL pathway and cytokines in keratinocytes in atopic dermatitis - Manipulation by the electromagnetic field

BACKGROUND

Atopic dermatitis (AD) is one of the most frequent skin diseases. Changes of the keratinocytes functionality play a major role in the development of AD. For example, activation of the Fas (CD95)/FasL (CD178) pathway in AD does not lead to extensive apoptosis in skin. Binding of the Fas receptor to its protein ligand-FasL, which are present on the (AD)-modified keratinocytes, should result in the sequential induction of cell death, but there is no evidence of extensive apoptosis of these cells. This suggests that non-apoptotic mechanism of Fas/FasL pathway is commonly encountered, although not examined in the case of AD, phenomenon. An electromagnetic field, which was used to influence cultured cells in this study, can modulate proliferation, apoptosis, differentiation, and metabolism in various cells.

OBJECTIVE

Here, we evaluate the possibility to manipulate the immune activation of AD keratinocytes and their response to the electromagnetic field, which was not tested before.

METHODS

Keratinocytes isolated from the skin of healthy subjects (n = 20) and patients with atopic dermatitis (n = 20) as well as HaCaT and PCS-200-010 cell were exposed to the 900 MHz electromagnetic field for 60 minutes. Cytometric analysis of viability, Fas/FasL, p-ERK, p-p38 and p-JNK expression and Luminex analysis of cytokine concentration were performed in two-time points: 4 and 24 hours after the exposition.

RESULTS

This research has shown upregulated Fas, FasL, p-ERK, p-p38, and p-JNK expression along with increased cytokine secretion (IL-1β, IL-4, IL-8, IL-10, IL-12p70, IL-13, IL-17A, IL-31 and TNFα) by keratinocytes derived from the skin of patients with the AD when compared with healthy control. Exposure of keratinocyte cultures obtained from AD patients to EMF resulted in a decrease of 1β, IL-4, IL-10, IL-12, I L-13, IL-17, IL-31 and TNFα levels. Keratinocytes derived from the skin of AD patients are characterized by elevated Fas and FasL expression when compared to healthy control.

CONCLUSION

Apoptotic and nonapoptotic activation of the Fas/FasL-dependent signaling pathway may play a significant role in the pathogenesis of AD, by adjusting the local cytokine and chemokine environment at the site of inflammation. Moreover, the electromagnetic field exhibits strong immunomodulatory effects on AD-modified keratinocytes.

Involvement of Fas/FasL pathway in the murine model of atopic dermatitis

Objective and design

The aim of this study was to elucidate the role of apoptosis mediated through Fas/FasL pathway using the mouse model of atopic dermatitis (AD).

Materials and treatment

AD was induced by epicutaneous application of ovalbumin (OVA) in wild-type C57BL/6, B6. MRL-Faslpr/J (Fas−) and B6Smn.C3-Faslgld/J (FasL−) mouse strains.

Methods

Skin samples were subjected to staining for Fas/FasL expression, M30 epitope and assessment of inflammatory response via immunohistochemical staining. Cytokine and chemokine production was assessed by real-time PCR.

Results

In comparison to wild-type mice, OVA sensitization of Fas- and FasL-deficient mice led to increased epidermal and dermal thickness, collagen deposition and local inflammation consisting of macrophages, neutrophils and CD4+ T cells. Fas- and FasL-deficient mice showed increased total counts of regulatory T cells (Tregs) and IgE levels in blood as well as increased expression of IL-1β, IL-4, IL-5, IL-13 and TGF-1β mRNA in comparison to wild-type mice. On the other hand, expression of CXCL9 and CXCL10, IL-17 mRNAs in the skin samples in Fas- and FasL-deficient mice was decreased.

Conclusions

Our results show that lack of the Fas-induced apoptosis leads to exacerbation of AD characteristics such as Th2 inflammation and dermal thickening. Therefore, Fas receptor can play an important role in AD pathogenesis by controlling development of the local inflammation.

Fas and TRAIL 'death receptors' as initiators of inflammation: Implications for cancer

Fas (CD95/APO-1) and TRAIL (CD253, TNFSF10, APO2) are members of a subset of the TNF receptor superfamily known as 'death receptors'. To date, the overwhelming majority of studies on Fas and TRAIL (TNF-related apoptosis-inducing ligand) have explored the role of these receptors as initiators of apoptosis. However, sporadic reports also suggest that engagement of the Fas and TRAIL receptors can lead to other outcomes such as cytokine and chemokine production, cell proliferation, cell migration and differentiation. Indeed, although transformed cells frequently express Fas and TRAIL, most do not undergo apoptosis upon engagement of these receptors and significant effort has been devoted toward exploring how to sensitize such cells to the pro-apoptotic effects of 'death receptor' stimulation. Moreover, the expression of Fas and TRAIL receptors is greatly elevated in many cancer types such as hepatocellular carcinoma, renal carcinoma and ovarian cancer, suggesting that such tumors benefit from the expression of these receptors. Furthermore, several studies have shown that tumor proliferation, progression and invasion can be impaired through blocking or downregulation of Fas expression, but the mechanistic basis for these effects is largely unknown. Thus, the characterization of Fas and TRAIL as 'death receptors' is a gross oversimplification, especially in the context of cancer. It is becoming increasingly clear that 'death receptor' engagement can lead to outcomes, other than apoptosis, that become subverted by certain tumors to their benefit. Here we will discuss death-independent outcomes of Fas and TRAIL signaling and their implications for cancer.

Blockade of Fas signaling in breast cancer cells suppresses tumor growth and metastasis via disruption of Fas signaling-initiated cancer-related inflammation

Mechanisms for cancer-related inflammation remain to be fully elucidated. Non-apoptotic functions of Fas signaling have been proposed to play an important role in promoting tumor progression. It has yet to be determined if targeting Fas signaling can control tumor progression through suppression of cancer-related inflammation. In the current study we found that breast cancer cells with constitutive Fas expression were resistant to apoptosis induction by agonistic anti-Fas antibody (Jo2) ligation or Fas ligand cross-linking. Higher expression of Fas in human breast cancer tissue has been significantly correlated with poorer prognosis in breast cancer patients. To determine whether blockade of Fas signaling in breast cancer could suppress tumor progression, we prepared an orthotopic xenograft mouse model with mammary cancer cells 4T1 and found that blockade of Fas signaling in 4T1 cancer cells markedly reduced tumor growth, inhibited tumor metastasis in vivo, and prolonged survival of tumor-bearing mice. Mechanistically, blockade of Fas signaling in cancer cells significantly decreased systemic or local recruitment of myeloid derived suppressor cells (MDSCs) in vivo. Furthermore, blockade of Fas signaling markedly reduced IL-6, prostaglandin E2 production from breast cancer cells by impairing p-p38, and activity of the NFκB pathway. In addition, administration of a COX-2 inhibitor and anti-IL-6 antibody significantly reduced MDSC accumulation in vivo. Therefore, blockade of Fas signaling can suppress breast cancer progression by inhibiting proinflammatory cytokine production and MDSC accumulation, indicating that Fas signaling-initiated cancer-related inflammation in breast cancer cells may be a potential target for treatment of breast cancer.

Fas/CD95-induced chemokines can serve as "find-me" signals for apoptotic cells

Apoptosis is commonly thought to represent an immunologically silent or even anti-inflammatory mode of cell death, resulting in cell clearance in the absence of explicit activation of the immune system. However, here we show that Fas/CD95-induced apoptosis is associated with the production of an array of cytokines and chemokines, including IL-6, IL-8, CXCL1, MCP-1, and GMCSF. Fas-induced production of MCP-1 and IL-8 promoted chemotaxis of phagocytes toward apoptotic cells, suggesting that these factors serve as "find-me" signals in this context. We also show that RIPK1 and IAPs are required for optimal production of cytokines and chemokines in response to Fas receptor stimulation. Consequently, a synthetic IAP antagonist potently suppressed Fas-dependent expression of multiple proinflammatory mediators and inhibited Fas-induced chemotaxis. Thus, in addition to provoking apoptosis, Fas receptor stimulation can trigger the secretion of chemotactic factors and other immunologically active proteins that can influence immune responsiveness toward dying cells.

Activation of the aryl hydrocarbon receptor sensitises human keratinocytes for CD95L- and TRAIL-induced apoptosis

In this study, we have analysed the apoptotic effects of the ubiquitous environmental toxin benzo[a]pyrene (BP) in HaCaT cells and human keratinocytes. Although prolonged exposure to BP was not cytotoxic on its own, a strong enhancement of CD95 (Fas)-mediated apoptosis was observed with BP at concentrations activating the aryl hydrocarbon receptor (AhR). Importantly, the ultimately mutagenic BP-metabolite, that is, (+)-anti-BP-7,8-diol-9,10-epoxide (BPDE), failed to enhance CD95-mediated cell death, suggesting that the observed pro-apoptotic effect of BP is neither associated with DNA adducts nor DNA-damage related signalling. CD95-induced apoptosis was also enhanced by β-naphtoflavone, a well-known agonist of the AhR that does not induce DNA damage, thus suggesting a crucial role for AhR activation. Consistently, BP failed to sensitise for CD95L-induced apoptosis in AhR knockdown HaCaT cells. Furthermore, inhibition of CYP1A1 and/or 1B1 expression did not affect the pro-apoptotic crosstalk. Exposure to BP did not increase expression of CD95, but led to augmented activation of caspase-8. Enhancement of apoptosis was also observed with the TRAIL death receptors that activate caspase-8 and apoptosis by similar mechanisms as CD95. Together, these observations indicate an interference of AhR signalling with the activity of receptor-associated signalling intermediates that are shared by CD95 and TRAIL receptors. Our data thus suggest that AhR agonists can enhance cytokine-mediated adversity upon dermal exposure.

A perspective on mammalian caspases as positive and negative regulators of inflammation

Members of the caspase family of cysteine proteases coordinate the morphological and biochemical events that typify apoptosis. However, neutralization of caspase activity in mammals fails to block death in response to most proapoptotic stimuli. This is because many cell death triggers provoke mitochondrial dysfunction upstream of caspase activation as a consequence of BAX/BAK channel opening. Although genetic or pharmacological inactivation of caspases fails to block cell death in most instances, it does convert the phenotype from apoptosis to necrosis. This has important implications for how the immune system responds to such cells, as necrotic cells provoke inflammation whereas apoptotic cells typically do not. Here, we propose an alternative perspective on apoptosis-associated caspase function by suggesting that these proteases are activated, not to kill, but to extinguish the proinflammatory properties of dying cells. This perspective unifies the mammalian caspase family as either positive or negative regulators of inflammation.

The expression of selected proapoptotic molecules in dermatitis herpetiformis

The role of the process of apoptosis is investigated in the pathogenesis of many autoimmune diseases; however at present, there is not much information about its role in dermatitis herpetiformis. Skin biopsies were taken from 18 DH patients and from 10 healthy subjects. The localization and expression of Bax, Fas, FasL, TRAIL, TRAIL-R in skin lesions, and perilesional skin were studied by immunohistochemistry. Expression of Bax, Fas, and Fas ligand was detected in the keratinocytes in skin biopsies from DH patients. Expression of TRAIL and TRAIL receptor was confirmed in epidermis, infiltration cells, and some fibroblasts. The expression of examined molecules in biopsies from healthy people was observed only in single cells. There were statistically significant differences between lesional, perilesional, and healthy skin of control group in Bax expression analysis and between lesional skin and control group in Fas, FasL, and TRAIL expression. There were statistically significant differences between control group and perilesional skin in Bax and FasL expression. Our results show that selected proapoptotic molecules may take part in pathogenesis of dermatitis herpetiformis, but the role of apoptosis in this process is not clear.

FasL and TRAIL signaling in the skin during cutaneous leishmaniasis - implications for tissue immunopathology and infectious control

Cutaneous leishmaniasis (CL) is associated with chronic inflammation and ulceration of the skin. Tissue macrophages serve as host cells and immune activation is necessary for parasite clearance. The balance between immune-mediated tissue destruction and successful clearance of infection is delicate and ulceration has been proposed to be a result of infiltration of activated immune cells into the skin. FasL and TRAIL play a dual role in skin homeostasis through induction of apoptosis as well as proinflammatory signaling. During leishmaniasis, dysregulation of both FasL and TRAIL has been described by us and others but the resulting pathogenic effects in the skin during human leishmaniasis are not fully elucidated. Targeting disease specific immune deviations has proven to be a promising new approach for the therapy of autoimmune diseases. Potentially, targeting FasL or TRAIL in combination with microcidals could offer a future treatment strategy to reduce the disfiguring immunopathology associated with CL. In this mini review we will discuss how FasL and TRAIL-induced signaling may influence on the extent of tissue inflammation and the efficacy of parasite clearance in leishmaniasis.

Mice with defective Fas ligand are protected from crescentic glomerulonephritis

Fas ligand is a well-known inducer of apoptosis in cells expressing its receptor Fas; it also prevents autoimmunity by inducing apoptosis of activated T cells. However, Fas ligand also mediates non-apoptotic functions involving inflammatory cell migration and cytokine responses. We sought here to study the role of Fas ligand in nephrotoxic nephritis, a model of crescentic glomerulonephritis, using generalized lymphoproliferative disorder (GLD) mice on a C57BL/6 background, which have defective Fas ligand and display only mild autoimmunity. These mice were significantly protected from glomerular crescent formation, glomerular thrombosis, renal impairment, and albuminuria 15 days after the induction of glomerulonephritis in comparison with wild-type mice. There were a reduced number of apoptotic cells in the glomeruli of nephritic GLD mice but no defect in their antibody responses or splenocyte proliferation at 15 days following the induction of glomerulonephritis. Bone marrow transplantation from wild-type mice restored disease susceptibility to GLD mice; however, wild-type mice were not protected when transplanted with bone marrow from GLD mice. Mesangial cells express Fas ligand in vitro, and these cells isolated from GLD mice produced lower amounts of monocyte chemoattractive protein-1 following interleukin-1 stimulation compared with cells from wild-type mice. Thus, Fas ligand-defective mice are protected from nephrotoxic nephritis, a disease in which both circulating and intrinsic renal cells appear to have a role.

An epidermotypic model of interface dermatitis reveals individual functions of fas ligand and gamma interferon in hypergranulosis, cytoid body formation, and gene expression

Analysis of complex cutaneous reactions using animal models allows for the identification of essential or modulatory participants, that is, cyto- and chemokines or adhesion molecules. However, complex whole animal modeling is bound to obscure some specific contributions of individual players. Mouse models suggest that expression of Fas ligand (FasL) by donor T cells is essential for the cutaneous acute graft-versus-host reaction (aGvHR), a major complication after allogeneic hematopoietic stem cell transplantation. The role of FasL/Fas in human cutaneous GvHR is not known. To understand the mechanisms of cytotoxicity and inflammation in human cutaneous GvHR, we developed an organotypic model using reconstructed human epidermis (RHE) that was exposed to FasL, gamma-interferon (IFNγ), or both. The model recapitulated key histological hallmarks of cutaneous aGvHR, including interface dermatitis, appearance of cytoid bodies, hypergranulosis, and expression of ICAM-1. Cytoid body formation and expression of ICAM-1 were attributable entirely to IFNγ, whereas hypergranulosis was triggered by FasL. Both FasL and IFNγ triggered vacuolar degeneration of keratinocytes. The validity of the RHE model of GvHR was demonstrated by histological correlation with biopsied skin from patients with acute graft-versus-host disease. FasL and IFNγ each elicited potent and specific proinflammatory genomic responses in RHE. Inhibition of caspase activity dramatically augmented the FasL-induced proinflammatory responses, suggesting an "apoptosis-versus-inflammation" antagonism in cutaneous aGvHR and other lichenoid dermatoses.

Expression of TWEAK in normal human skin, dermatitis and epidermal neoplasms: association with proliferation and differentiation of keratinocytes

BACKGROUND

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) has been implicated in the pathogenesis of various inflammatory pathologies and cancer. We aimed to investigate its expression in normal human skin, inflammatory skin diseases and epidermal neoplasms.

METHODS

Immunohistochemistry for TWEAK was performed in samples of healthy skin, plaque psoriasis, lichen planus, prurigo nodularis, discoid lupus erythematosus, lichen sclerosus, seborrheic keratosis, common warts, actinic keratosis, Bowen's disease, keratoacanthoma and basal and squamous cell carcinoma. Double immunofluorescence was used to investigate co-localization of TWEAK with cytokeratin-10 and proliferating cell nuclear antigen (PCNA).

RESULTS

TWEAK was robustly expressed in the epidermis of healthy skin and decreased in inflammatory conditions, both in the context of epidermal hyperplasia and atrophy. Decreased TWEAK immunoreactivity was regularly observed in common warts, actinic keratosis and Bowen's disease, particularly in areas of marked proliferation as evidenced by PCNA-positive nuclei. In squamous cell carcinoma, expression of TWEAK ranged from strong to completely absent, and it mostly corresponded with the expression of cytokeratin-10. TWEAK was absent in keratoacanthoma and basal cell carcinoma.

CONCLUSIONS

TWEAK is a constitutively expressed epidermal protein whose downregulation might be an early indicator of disturbed differentiation or pathologic proliferation of keratinocytes that accompany inflammatory and neoplastic skin diseases.

Cellular FLICE-inhibitory protein (cFLIP) isoforms block CD95- and TRAIL death receptor-induced gene induction irrespective of processing of caspase-8 or cFLIP in the death-inducing signaling complex

Death receptors (DRs) induce apoptosis but also stimulate proinflammatory "non-apoptotic" signaling (e.g. NF-κB and mitogen-activated protein kinase (MAPK) activation) and inhibit distinct steps of DR-activated maturation of procaspase-8. To examine whether isoforms of cellular FLIP (cFLIP) or its cleavage products differentially regulate DR signaling, we established HaCaT cells expressing cFLIP(S), cFLIP(L), or mutants of cFLIP(L) (cFLIP(D376N) and cFLIP(p43)). cFLIP variants blocked TRAIL- and CD95L-induced apoptosis, but the cleavage pattern of caspase-8 in the death inducing signaling complex was different: cFLIP(L) induced processing of caspase-8 to the p43/41 fragments irrespective of cFLIP cleavage. cFLIP(S) or cFLIP(p43) blocked procaspase-8 cleavage. Analyzing non-apoptotic signaling pathways, we found that TRAIL and CD95L activate JNK and p38 within 15 min. cFLIP variants and different caspase inhibitors blocked late death ligand-induced JNK or p38 MAPK activation suggesting that these responses are secondary to cell death. cFLIP isoforms/mutants also blocked death ligand-mediated gene induction of CXCL-8 (IL-8). Knockdown of caspase-8 fully suppressed apoptotic and non-apoptotic signaling. Knockdown of cFLIP isoforms in primary human keratinocytes enhanced CD95L- and TRAIL-induced NF-κB activation, and JNK and p38 activation, underscoring the regulatory role of cFLIP for these DR-mediated signals. Whereas the presence of caspase-8 is critical for apoptotic and non-apoptotic signaling, cFLIP isoforms are potent inhibitors of TRAIL- and CD95L-induced apoptosis, NF-κB activation, and the late JNK and p38 MAPK activation. cFLIP-mediated inhibition of CD95 and TRAIL DR could be of crucial importance during keratinocyte skin carcinogenesis and for the activation of innate and/or adaptive immune responses triggered by DR activation in the skin.

Role of Fas/FasL in regulation of inflammation in vaginal tissue during HSV-2 infection

To assess the role of Fas in lesion development during genital HSV-2 infection, we used a well-established HSV-2 murine model applied to MRL-Fas(lpr)/J (Fas-/-) and C3-Fasl(gld)/J (FasL-/-) C57BL6 mice. In vitro infection of murine keratinocytes and epithelial cells was used to clarify molecular details of HSV-2 infection. Despite upregulation of Fas and FasL, HSV-2-infected keratinocytes and epithelial cells showed a moderate level of apoptosis due to upregulated expression of the anti-apoptotic factors Bcl-2, Akt kinase and NF-κB. Inflammatory lesions within the HSV-2-infected epithelium of C57BL6 mice consisted of infected cells upregulating Fas, FasL and Bcl-2, uninfected cells upregulating Fas and neutrophils expressing both Fas and FasL. Apoptosis was detected in HSV-2-infected cells and to even higher extent in non-infected cells surrounding HSV-2 infection sites. HSV-2 infection of Fas- and FasL-deficient mice led to increased apoptosis and stronger recruitment of neutrophils within the infection sites. We conclude that the Fas pathway participates in regulation of inflammatory response in the vaginal epithelium at the initial stage of HSV-2 infection.

Enhanced contact allergen- and UVB-induced keratinocyte apoptosis in the absence of CD95/Fas/Apo-1

FAS/CD95/Apo-1 is a ubiquitously expressed cell-surface receptor involved in the initiation of programmed cell death. Its function in epidermal keratinocytes has been incompletely defined. Available evidence from in vitro studies points to important roles of Fas in the pathogenesis of contact dermatitis and in keratinocyte apoptosis induced by ultraviolet light. To define functions of Fas in the epidermis in vivo, we have generated mice with epidermis-specific deletion of the fas gene and tested its requirement for 2,4-dinitrofluorobenzene-induced contact dermatitis and for ultraviolet light B (UVB)-induced keratinocyte apoptosis. We report here our unexpected finding that keratinocyte apoptosis induced by both a contact allergen and UVB irradiation was significantly enhanced in Fas-negative epidermis. Expression of Fas by epidermal keratinocytes was neither necessary for the normal development of contact hypersensitivity of the skin, nor required for keratinocyte apoptosis following UVB irradiation. Our study results thus show that in the epidermis in vivo Fas exerts antiapoptotic effects that outweigh its proapoptotic role in contact hypersensitivity responses of the skin and in the tissue response of the epidermis to UVB irradiation.

Characteristics of dengue virus-infected peripheral blood mononuclear cell death that correlates with the severity of illness

The pathogenic mechanism of the severe form of dengue is complicated. Recent reports indicate that apoptotic death of various tissues or organs may be associated with vascular leakage, and ultimately leads to the death of DENV-infected patients. In the present study, we provide additional evidence supporting the detrimental role of apoptosis in DENV infection. A comparison of the rate of apoptosis in PBMCs isolated from patients suffering DF, a mild form of the disease, and the rate in patients with DHF, a life-threatening disease, revealed that PBMCs from DHF patients underwent apoptosis at a significantly higher rate than those suffering from DF alone. This suggests that the severity of natural DENV infection correlates with PBMC apoptosis. In addition, this cell death was induced not only by DENV itself, but also by the apoptotic activities of pro-inflammatory cytokines, such as TNF-alpha, and IL-1beta, that were upregulated in DHF patients. The death of these mononuclear cells that function in an innate immune system may explain the higher viral load in DHF patients than in DF patients. Interestingly, a gene expression profile pattern elucidated that apoptosis occurring during natural DENV infection involved mainly the extrinsic apoptosis pathway, which is mediated via both caspase-dependent and caspase-independent mechanisms. In conclusion, our data highlight the adverse effect of apoptosis induced by DENV and by pro-inflammatory cytokines during natural DENV infection.

Th17 cytokines stimulate CCL20 expression in keratinocytes in vitro and in vivo: implications for psoriasis pathogenesis

T helper (Th) 17 cells have recently been implicated in psoriasis pathogenesis, but mechanisms of how these cells traffic into inflamed skin are unknown. By immunostaining for interleukin (IL)-17A and IL-22, we show numerous cells present in psoriasis lesions that produce these cytokines. We next found that Th17 cytokines (IL-17A, IL-22, and tumor necrosis factor (TNF)-alpha) markedly increased the expression of CC chemokine ligand (CCL) 20, a CC chemokine receptor (CCR)6 ligand, in human keratinocyte monolayer and raft cultures in a dose- and time-dependent manner. Lastly, we showed in mice that subcutaneous injection with recombinant IL-17A, IL-22, or TNF-alpha led to the upregulation of both CCL20 and CCR6 expression in skin as well as cutaneous T-cell infiltration. Taken together, these data show that Th17 cytokines stimulate CCL20 production in vitro and in vivo, and thus provide a potential explanation of how CCR6-positive Th17 cells maintain their continual presence in psoriasis through a positive chemotactic feedback loop.

The contact allergen nickel sensitizes primary human endothelial cells and keratinocytes to TRAIL-mediated apoptosis

Primary endothelial cells are fully resistant to TNF-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Here, we demonstrate that certain environmental conditions, such as exposure to the widespread allergen nickel, can dramatically increase the susceptibility of naturally resistant primary endothelial cells or keratinocytes to TRAIL-induced apoptosis. While nickel treatment increased surface expression of the apoptosis-inducing TRAIL receptors TRAIL-R1 and TRAIL-R2, it also up-regulated the apoptosis-deficient TRAIL-R4, suggesting that modulation of TRAIL receptor expression alone is unlikely to fully account for the dramatic sensitization effect of nickel. Further analysis of candidate mediators revealed that nickel strongly repressed c-FLIP at mRNA and protein levels. Accordingly, increased activation of Caspase-8 and Caspase-3 following nickel treatment was observed. Importantly, depletion of c-FLIP by RNA interference could largely recapitulate the effect of nickel and sensitize endothelial cells to TRAIL-dependent apoptosis in the absence of nickel pre-treatment. Conversely, ectopic expression of c-FLIP_L largely protected nickel-treated cells from TRAIL-mediated apoptosis. Our data demonstrate that one key mechanism of sensitization of primary human endothelial cells or keratinocytes is transcriptional down-regulation of c-FLIP. We hypothesize that environmental factors, exemplified by the contact allergen nickel, strongly modulate death ligand sensitivity of endothelial cells and keratinocytes thus influencing vascular and epidermal function and integrity under physiological and pathophysiological conditions.

Mouse macrophages primed with alendronate down-regulate monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) production in response to Toll-like receptor (TLR) 2 and TLR4 agonist via Smad3 activation

Alendronate is one of the nitrogen-containing bisphosphonates (NBPs) used as anti-bone resorptive drugs. However, NBPs have inflammatory side effects including osteomyelitis and osteonecrosis of the jaw. In the present study, we examined the effects of alendronate on chemokine production by the macrophage-like cell line, J774.1, when incubated with Pam(3)CSK(4) (a Toll-like receptor (TLR) 2 agonist) and Lipid A (a TLR4 agonist). Pretreatment of J774.1 cells with alendronate decreased the production of TLR ligand-induced monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) but did not influence nuclear factor-kappaB (NF-kappaB) activation. While this agent induced caspase-8 activation, a caspase-8 inhibitor did not affect the decrease in MCP-1 production by alendronate and TLR ligands. Thus, the alendronate-mediated decrease in chemokine production was independent of NF-kappaB and caspase-8 activation. Although transforming growth factor-beta1 (TGF-beta1) is known to inhibit chemokine production by various cell types via Smad3 activation, pretreatment with alendronate did not increase TGF-beta1 production by J774.1 cells incubated in the presence or absence of TLR ligands. However, alendronate directly activated Smad3. These results suggest that by down-regulating MCP-1 and MIP-1alpha production via Smad3, long-term use of alendronate might inhibit normal activation and migration of osteoclasts and cause osteonecrosis.

Fas signal promotes lung cancer growth by recruiting myeloid-derived suppressor cells via cancer cell-derived PGE2

Fas/FasL system has been extensively investigated with respect to its capacity to induce cellular apoptosis. However, accumulated evidences show that Fas signaling also exhibits nonapoptotic functions, such as induction of cell proliferation and differentiation. Lung cancer is one of cancer's refractory to the immunotherapy, however, the underlying mechanisms remain to be fully understood. In this study, we show that Fas overexpression does not affect in vitro growth of 3LL cells, but promotes lung cancer growth in vivo. However, such tumor-promoting effect is not observed in FasL-deficient (gld) mice, and also not observed in the immune competent mice once inoculation with domain-negative Fas-overexpressing 3LL cells, suggesting the critical role of Fas signal in the promotion of lung cancer growth in vivo. More accumulation of myeloid-derived suppressor cells (MDSC) and Foxp3(+) regulatory T cells is found in tumors formed by inoculation with Fas-overexpressing 3LL cells, but not domain-negative Fas-overexpressing 3LL cells. Accordingly, Fas-ligated 3LL lung cancer cells can chemoattract more MDSC but not regulatory T cells in vitro. Furthermore, Fas ligation induces 3LL lung cancer cells to produce proinflammatory factor PGE(2) by activating p38 pathway, and in turn, 3LL cells-derived PGE(2) contribute to the Fas ligation-induced MDSC chemoattraction. Furthermore, in vivo administration of cyclooxygenase-2 inhibitor can significantly reduce MDSC accumulation in the Fas-overexpressing tumor. Therefore, our results demonstrate that Fas signal can promote lung cancer growth by recruiting MDSC via cancer cell-derived PGE(2), thus providing new mechanistic explanation for the role of inflammation in cancer progression and immune escape.

Cell death in the skin

The skin is the largest organ of the body and protects the organism against external physical, chemical and biological insults, such as wounding, ultraviolet radiation and micro-organisms. The epidermis is the upper part of the skin that is continuously renewed. The keratinocytes are the major cell type in the epidermis and undergo a specialized form of programmed cell death, called cornification, which is different from classical apoptosis. In keep with this view, several lines of evidence indicate that NF-kB is an important factor providing protection against keratinocyte apoptosis in homeostatic and inflammatory conditions. In contrast, the hair follicle is an epidermal appendage that shows cyclic apoptosis-driven involution, as part of the normal hair cycle. The different cell death programs need to be well orchestrated to maintain skin homeostasis. One of the major environmental insults to the skin is UVB radiation, causing the occurrence of apoptotic sunburn cells. Deregulation of cell death mechanisms in the skin can lead to diseases such as cancer, necrolysis and graft-versus-host disease. Here we review the apoptotic and the anti-apoptotic mechanisms in skin homeostasis and disease.

Reactive oxygen species are involved in FasL-induced caspase-independent cell death and inflammatory responses

Fas-mediated caspase-dependent cell apoptosis has been well investigated. However, recent studies have shown that Fas can induce nonapoptotic caspase-independent cell death (CICD) when caspase activity is inhibited. Currently, the molecular mechanism of this alternative cell death mediated by Fas remains unclear. In this study, we investigated the signaling pathway of Fas-induced CICD in mouse embryonic fibroblasts (MEFs) whose caspase function was disrupted by the pan-caspase inhibitor Z-VAD-FMK and its coupling to inflammatory responses. Our results revealed that receptor-interacting protein 1 and tumor necrosis factor receptor-associated factor 2 play important roles in FasL-induced CICD. This death is associated with intracellular reactive oxygen species (ROS) production from mitochondria, as a ROS scavenger (BHA), antioxidants (trolox, NAC), and a mitochondrial respiratory chain uncoupler (rotenone) could prevent this event. Furthermore, delayed and sustained JNK activation, mitochondrial membrane potential breakdown, and loss of intracellular GSH were observed. In addition to CICD, FasL also induces cyclooxygenase-2 and MIP-2 gene upregulation, and both responses are attributed to ROS-dependent JNK activation. Taken together, these results demonstrate alternative signaling pathways of Fas upon caspase inhibition in MEFs that are unrelated to the classical apoptotic pathway, but steer cells toward necrosis and an inflammatory response through ROS production.

Cornulin, a marker of late epidermal differentiation, is down-regulated in eczema

BACKGROUND

Eczema is a common chronic inflammatory skin disorder which shows strong genetic predisposition. To identify new potential molecular determinants of the disease pathogenesis, we performed a gene expression study in an eczema mouse model. This analysis identified a marked down regulation of the cornulin gene (CRNN), a member of the epidermal differentiation complex, in the eczema-like skin. We then investigated CRNN as an eczema candidate gene and studied its polymorphism and the expression in the skin of eczema patients.

METHODS

An eczema-like phenotype was induced in mice by allergen (Der p2) patching. Gene expression analysis was performed with the subtractive suppression hybridization method and validated by real time PCR and the transmission disequilibrium test was used to test for genetic associations in 406 multiplex eczema families.

RESULTS

Der p 2 patched mice developed a localized eczema and a Th 2 skewed systemic response. Real time PCR analysis confirmed a down regulation of CRNN mRNA in eczema-like skin in the mouse model and in human eczema. The CRNN polymorphism rs941934 was significantly associated with atopic eczema in the genetic analysis (P = 0.006), though only as part of an extended haplotype including a known associated variant (2282del4) in the filaggrin gene.

CONCLUSIONS

CRNN mRNA expression is decreased in eczematous skin. Further studies are needed to verify whether the associated cornulin polymorphism contribute to the genetic susceptibility in eczema.

FLIP ing the coin? Death receptor-mediated signals during skin tumorigenesis

Keratinocyte skin cancer is a multi-step process, during which a number of obstacles have to be overcome by the tumor cell to allow the development of a manifest tumor. Beside proliferation and immortality, apoptosis resistance is one additional and critical step during skin carcinogenesis. Over the past two decades, much has been learned about the prototypical membrane-bound inducers of apoptosis, namely the death receptors and their ligands, and the apoptosis signalling pathways activated by death receptors have been elucidated in great detail. In contrast, much less is known about the tissue-specific role of the death receptor/ligands systems during the development of skin cancer. Here, we summarize and discuss the role of this intriguing receptor family and the potential mechanistical impact of the intracellular caspase-8 inhibitor cFLIP for keratinocyte skin cancer. Given more recent data about cFLIP and its isoforms, a more complex regulatory role of cFLIP can be suspected. Indeed, cFLIP may not solely interfere with death receptor-mediated apoptosis signalling pathways, but may positively or negatively influence other, potential harmful signalling pathways such as the production of inflammatory cytokines, tumor cell migration or the activation of transcription factors such as NF-kappaB, considered crucial during skin tumorigenesis. In this respect, cFLIP may act to 'FLIP the coin' during the development of keratinocyte skin cancer.

Fas ligand-induced proinflammatory transcriptional responses in reconstructed human epidermis. Recruitment of the epidermal growth factor receptor and activation of MAP kinases

Fas ligand (FasL) exerts potent proapoptotic and proinflammatory actions on epidermal keratinocytes and has been implicated in the pathogenesis of eczema, toxic epidermal necrolysis, and drug-induced skin eruptions. We used reconstructed human epidermis to investigate the mechanisms of FasL-induced inflammatory responses and their relationships with FasL-triggered caspase activity. Caspase activity was a potent antagonist of the pro-inflammatory gene expression triggered by FasL prior to the onset of cell death. Furthermore, we found that FasL-stimulated autocrine production of epidermal growth factor receptor (EGFR) ligands, and the subsequent activation of EGFR and ERK1 and ERK2 mitogen-activated protein kinases, were obligatory extracellular steps for the FasL-induced expression of a subset of inflammatory mediators, including CXCL8/interleukin (IL)-8, ICAM-1, IL-1alpha, IL-1beta, CCL20/MIP-3alpha, and thymic stromal lymphopoietin. These results expand the known physiological role of EGFR and its ligands from promoting keratinocyte mitogenesis and survival to mediating FasL-induced epidermal inflammation.

Proinflammatory responses of human airway cells to ricin involve stress-activated protein kinases and NF-kappaB

Ricin is a potential bioweapon because of its toxicity, availability, and ease of production. When delivered to the lungs, ricin causes severe pulmonary damage with symptoms that are similar to those observed in acute lung injury and adult respiratory distress syndrome. The airway epithelium plays an important role in the pathogenesis of many lung diseases, but its role in ricin intoxication has not been elucidated. Exposure of cultured primary human airway epithelial cells to ricin resulted in the activation of SAPKs and NF-kappaB and in the increased expression of multiple proinflammatory molecules. Among the genes upregulated by ricin and identified by microarray analysis were those associated with transcription, nucleosome assembly, inflammation, and response to stress. Sequence analysis of the promoters of these genes identified NF-kappaB as one of the transcription factors whose binding sites were overrepresented. Although airway cells secrete TNF-alpha in response to ricin, blocking TNF-alpha did not prevent ricin-induced activation of NF-kappaB. Decreased levels of IkappaB-alpha in airway cells exposed to ricin suggest that translational suppression may be responsible for the activation of NF-kappaB. Inhibition of p38 MAPK by a chemical inhibitor or NF-kappaB by short interfering RNA resulted in a marked reduction in the expression of proinflammatory genes, demonstrating the importance of these two pathways in ricin intoxication. Therefore, the p38 MAPK and NF-kappaB pathways are potential therapeutic targets for reducing the inflammatory consequences of ricin poisoning.

CD95-Mediated Signals in the Skin: Going Out with an (Inflammatory) Bang?

The death ligand CD95L (Fas/Apo-1-ligand) has been viewed as a proapoptotic molecule involved in the pathogenesis of T cell-mediated skin diseases including eczema. In the presence of inhibitors of caspases, CD95L induces proinflammatory genes in keratinocytes. This more pleiotropic function of CD95L as enhancer of inflammation may be equally important to apoptosis induction of keratinocytes, at least in eczema.