Electron microscopic observations of dyskeratosis, apoptosis, colloid bodies and fibrillar degeneration after skin irritation with dithranol

Apoptosis: Activation and Inhibition in Health and Disease

There are many types of cell death, each involving multiple and complex molecular events. Cell death can occur accidentally when exposed to extreme physical, chemical, or mechanical conditions, or it can also be regulated, which involves a genetically coded complex machinery to carry out the process. Apoptosis is an example of the latter. Apoptotic cell death can be triggered through different intracellular signalling pathways that lead to morphological changes and eventually cell death. This is a normal and biological process carried out during maturation, remodelling, growth, and development in tissues. To maintain tissue homeostasis, regulatory, and inhibitory mechanisms must control apoptosis. Paradoxically, these same pathways are utilized during infection by distinct intracellular microorganisms to evade recognition by the immune system and therefore survive, reproduce and develop. In cancer, neoplastic cells inhibit apoptosis, thus allowing their survival and increasing their capability to invade different tissues and organs. The purpose of this work is to review the generalities of the molecular mechanisms and signalling pathways involved in apoptosis induction and inhibition. Additionally, we compile the current evidence of apoptosis modulation during cancer and Leishmania infection as a model of apoptosis regulation by an intracellular microorganism.

IL-1 Receptor Antagonist Reduced Chemical-Induced Keratinocyte Apoptosis through Antagonism to IL-1α/IL-1β

Extracellular interleukin 1 alpha (IL-1α) released from keratinocytes is one of the endpoints for in vitro assessments of skin irritancy. Although cells dying via primary skin irritation undergo apoptosis as well as necrosis, IL-1α is not released in apoptotic cells. On the other hand, active secretion has been identified in interleukin-1 receptor antagonist (IL-1ra), which was discovered to be a common, upregulated, differentially-expressed gene in a microarray analysis performed with keratinocytes treated using cytotoxic doses of chemicals. This study examined whether and how IL-1ra, particularly extracellularly released IL-1ra, was involved in chemically-induced keratinocyte cytotoxicity and skin irritation. Primary cultured normal adult skin keratinocytes were treated with cytotoxic doses of chemicals (hydroquinone, retinoic acid, sodium lauryl sulfate, or urshiol) with or without recombinant IL-1ra treatment. Mouse skin was administered irritant concentrations of hydroquinone or retinoic acid. IL-1ra (mRNA and/or intracellular/extracellularly released protein) levels increased in the chemically treated cultured keratinocytes with IL-1α and IL-1β mRNAs and in the chemically exposed epidermis of the mouse skin. Recombinant IL-1ra treatment significantly reduced the chemically-induced apoptotic death and intracellular/extracellularly released IL-1α and IL-1β in keratinocytes. Collectively, extracellular IL-1ra released from keratinocytes could be a compensatory mechanism to reduce the chemically-induced keratinocyte apoptosis by antagonism to IL-1α and IL-1β, suggesting potential applications to predict skin irritation.

S100B as a potential biomarker for the detection of cytotoxicity of melanocytes

Skin irritation is one of the most common adverse reactions in hydroquinone (HQ) and retinoic acid (RA). Although melanocytes have rarely been considered to be involved in skin irritation, RA and particularly HQ could induce melanocyte toxicity, resulting in depigmentation. We chose S100B as a candidate gene for melanocytotoxicity from a genome-wide transcriptional profiling analysis after applying irritant doses of HQ, RA and sodium lauryl sulphate (SLS) to cultures of keratinocytes and/or melanocytes. In this study, the role of S100B on melanocyte viability and cytotoxicity was examined. S100B was detected in melanocytes, but not in keratinocytes or fibroblasts. Melanocytes after treatment with increasing concentrations of HQ, RA, SLS and urushiol showed significant increases in intracellular and extracellular S100B expression with reduced viable cell number and increased release of lactate dehydrogenase. No RAGE expression and no significant function of CD166/ALCAM in melanocyte survival and cytotoxicity favoured the role of intracellular S100B in chemically irritated melanocytes. S100B knock-down increased apoptosis through inhibition of PI3K/AKT, NF-κB and ERK activation, suggesting the increased intracellular S100B expression by chemical irritation as a compensatory reaction to reduce cytotoxicity. The numerical decrease in S100B/c-kit-double-positive melanocytes was also examined in human skin epidermis irritated by HQ or RA with stronger staining intensities of S100B. Collectively, the decrease in viable cell number by reduced intracellular S100B levels in vitro and by chemical irritation in vivo suggests that S100B could be a potential biomarker for melanocytes cytotoxicity.

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.

Ultrastructural studies on dengue virus type 2 infection of cultured human monocytes

Background

Early interaction of dengue virus and monocyte/macrophages could be an important feature for virus dissemination after its initial entry via the mosquito vector. Since ultrastructural analysis of this interaction has not been reported, dengue type 2 (DEN2) virus-infected human monocyte cultures were studied at 1, 2, 4 and 6 hours after infection.

Results

Typical dengue particles and fuzzy coated viral particles were 35 to 42 nm and 74 to 85 nm respectively. Viruses were engulfed by phagocytosis and macropicnocytosis leading to huge vacuoles and phagosomes inside the monocytes. Interaction of monocytes with DEN2 virus induced apoptosis, characterized by nuclear condensation and fragmentation, cellular shrinkage, blebbing and budding phenomena and phagocytosis of apoptotic cells by neighboring monocytes. This finding was confirmed by TUNEL. Ultrastructural features associated to DEN2 virus replication were not observed.

Conclusion

These data suggest that clearance of the virus by monocytes and cellular death are the main features during the initial interaction of DEN2 virus and monocytes and this could be important in the rapid elimination of the virus after infection by mosquito vector.

The antipsoriatic drug anthralin accumulates in keratinocyte mitochondria, dissipates mitochondrial membrane potential, and induces apoptosis through a pathway dependent on respiratory competent mitochondria

Anthralin is a potent topical drug, inducing clearance of psoriatic plaques. Anthralin disrupts mitochondrial function and structure, but its mechanism of action remains undefined. This study aimed to determine whether anthralin induced keratinocyte apoptosis as well as to investigate molecular mechanisms and the role of mitochondria. We studied human keratinocytes and human 143B rho(0) cells, which lack mitochondrial DNA and a functional respiratory chain. We show that anthralin disrupts mitochondrial membrane potential (DeltaPsim) and causes endogenous cytochrome c release, resulting in the activation of caspase-3 and characteristic morphological changes of apoptosis. Disruption of DeltaPsim and cytochrome c release were independent of mitochondrial permeability transition or caspase activation. Human 143B rho(0) cells were resistant to anthralin-induced cell death, disruption of DeltaPsim, and cytochrome c release compared with the isogenic 143B rho+ cell line. Using the intrinsic fluorescence of anthralin, rapid accumulation within mitochondria was observed independent of DeltaPsim. Using assays that measure individual respiratory chain complexes, we show that anthralin specifically interacts with ubiquinone pool. These data indicate that anthralin induces apoptosis through a novel mitochondrial pathway dependent on oxidative respiration and involving electron transfer with the ubiquinone pool. These studies identify keratinocyte apoptosis as a potentially important mechanism involved in the clearance of psoriasis.

Involvement of oxidative stress in apoptosis induced by a mixture of isothiazolinones in normal human keratinocytes

A 3:1 combination of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) is widely used to preserve cosmetic products. We show here that CMI/MI induced apoptosis in normal human keratinocytes (NHK) as at low concentrations (0.001-0.05% documented by subdiploid DNA content and phosphatidylserine exposure, while at the highest concentration (0.1% as supplied, 15 p.p.m.) the response was necrosis. Various molecular events accompanied the cytotoxic effects of CMI/MI. Generation of ROS and hyperpolarization of mitochondrial transmembrane potential (DeltaPsim) were early events, followed by increased Fas expression and activation of caspase-8, and then activation of caspase-3 and -9. The drop in DeltaPsim occurred only later in the cell death pathway, when NHK showed signs of apoptosis. Pretreatment of cells for 2 h with the redox-active agent N-acetyl-L-cysteine conferred complete protection against the CMI/MI-induced cytotoxic effects, DeltaPsim loss, and apoptosis. The pan-caspase inhibitor Z-Val-Ala-Asp(OMe)-CH2F blocked the CMI/MI-induced apoptosis without preventing ROS generation and the drop in DeltaPsim. These results indicate that the generation of ROS plays an important part in mediating apoptosis and necrosis associated with CMI/MI treatment. This new aspect of the in vitro toxicity of CMI/MI may provide important information about the relationship between the preservative's in vitro apoptotic activity and its in vivo toxicity.

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.

Cell proliferation activity and the expression of cell cycle regulatory proteins in oral lichen planus

BACKGROUND

In oral lichen planus (OLP), destruction of the basal cell layer, which is one of the characteristic histological features, is seen and many changes in cell proliferation, cell repair and cell death occur in the injured mucosal epithelium.

METHODS

We studied mucosal tissues from 19 patients of OLP and 10 controls, with immunohistochemistry for Ki-67, p53, cyclin dependent kinase inhibitors (CDKI) and cyclins. Mitotic count was calculated. TUNEL assay was also performed for evaluation of apoptotic cell death.

RESULTS

Mitotic count, Ki-67 and cyclin D1 labeling indices in the basal and parabasal layers of OLP mucosa were elevated in comparison with those of controls. p53, p21Cip1 and TUNEL indices of OLP mucosa were also increased.

CONCLUSIONS

These complex changes, which concomitantly occur in the injured mucosal epithelium, could contribute to the development and maintenance of characteristic mucosal epithelial architectures seen in OLP.

Fas/Fas ligand interaction contributes to UV-induced apoptosis in human keratinocytes

Keratinocytes in human skin undergo apoptosis during various inflammatory processes and after ultraviolet (UV) irradiation. To determine if keratinocyte apoptosis may be mediated by the Fas/APO-1 receptor (CD95), a signal transduction pathway known to initiate programmed cell death of lymphocytes, we investigated Fas expression, modulation, and function in keratinocytes. Keratinocytes constitutively expressed the 2.5- and 1.9-kb Fas transcripts, as well as the 43-kDa Fas protein. Treatment of interferon-gamma-stimulated keratinocytes with Fas agonistic antibody significantly promoted their cell death, indicating that Fas in keratinocytes is functional. UV irradiation induced Fas mRNA expression within 16 to 24 h and Fas protein within 24 h and through 48 h after irradiation. Furthermore, keratinocytes constitutively expressed Fas ligand (FasL) mRNA and protein. UV irradiation induced FasL mRNA as early as 4 h after irradiation and elevated FasL mRNA levels were maintained for at least 24 h postirradiation. Moreover, a FasL neutralizing antibody significantly reduced UV-induced apoptosis of IFN-gamma-treated keratinocytes. Our data strongly suggest that the Fas system contributes to keratinocyte apoptosis in UV-irradiated human skin.

Induction of apoptosis in human HaCaT keratinocytes

Although cell death by apoptosis has been recognized as an important control mechanism in the maintenance of tissue homeostasis and in the elimination of cells with damaged DNA, information on the induction and characteristics of apoptosis in keratinocytes is rather scarce. Apoptotic mechanisms may play an important role in normal and disturbed homeostasis of the skin. In the present study, we therefore investigated the effects of several potential inducers of apoptosis in the human keratinocyte cell line HaCaT. Apoptosis was assessed with respect to morphological changes by light and electron microscopic examinations and to DNA integrity by a specific ELISA. UVB irradiation induced the morphology and internucleosomal DNA fragmentation characteristic of apoptosis in a dose- and time-dependent manner. Interferon-gamma caused DNA cleavage at the linker regions without producing morphological features consistent with apoptotic cell death. In contrast, treatment with dithranol and NP-40 resulted in necrotic alterations in the keratinocytes. Treatment with the calcium ionophore A23187 caused morphological changes which were similar to the characteristics of 'nonapoptotic programmed cell death'. Dexamethasone, tumor necrosis factor-alpha, transforming growth factor-beta, TPA, retinoic acid, the podophyllin derivative etoposide, the thromboxane A2 analogue U46619, cycloheximide, and the nitric oxide donors sodium nitroprusside and S-nitroso-glutathione, which are all known to induce apoptosis in other cell types, did not affect HaCaT keratinocytes. These results demonstrate that apoptosis can be induced in keratinocytes in vitro but the apoptosis differs from that in other cell types, such as haematopoietic cells, with regard to the type of inducer and/or the sensitivity of the target cells. Since keratinocytes are affected by numerous external and internal stimuli, they might posses several protective mechanisms to prevent apoptosis and to ensure the structural integrity of the outermost barrier of the body.

Cell death by apoptosis in epidermal biology

Homeostasis in continually renewing tissues is maintained by a tightly regulated balance between cell proliferation, cell differentiation, and cell death. Until recently, proliferation was thought to be the primary point of control in the regulation of normal tissue kinetic homeostasis and as such has been the major focus of both understanding the etiology of disease and developing therapeutic strategies. Now, physiologic cell death, known as apoptosis (ă-pŏp-to' sĭs, ă-po-to' sĭs [Thomas CL (ed.): Taber's Cyclopedic Medical Dictionary. F.A. Davis, Co., Philadelphia, 1989)] has gained scientific recognition as an active regulatory mechanism, complementary, but functionally opposite, to proliferation with important roles in shaping and maintaining tissue size and prevention of disease. In this review we will describe the concept of apoptosis and discuss possible molecular mechanisms of its regulation that may have implications for skin biology.

Patterns of cell death: the significance of apoptosis for dermatology

Development, function, remodelling, and senescence of multicellular organisms depend on the coordinated occurrence of physiological, actively induced cell death in two major patterns: terminal differentiation and programmed cell death (apoptosis). Apoptosis is a highly selective form of "cell suicide" with characteristic morphological and biochemical features: chromatin condensation, formation of apoptotic bodies, and DNA fragmentation by activation of endonucleases. Here, we outline the current understanding of apoptosis and its subtypes, discuss their biological functions, and delineate why apoptosis is relevant to the skin and its diseases. We distinguish apoptosis from necrosis, and discuss the regulation of apoptosis by selected genes, hormones, growth factors and cytokines. The epidermis and the regressing hair follicle offer interesting models for studying the as yet ill-understood biology of epithelial cell apoptosis. The selective manipulation of cell death programs may become part of the therapeutic arsenal of clinical dermatology.