Cell death by apoptosis and its protective role against disease

Cadmium induces apoptosis in a human T cell line

Cadmium, a potent toxic metal, poses a serious environmental threat but the mechanisms of its toxicity remain unclear. In the present study, we investigated the nature of cadmium-induced cell death in the human T cell line CEM-C12. Cadmium was time- and dose-dependently toxic for CEM-C12 cells, cell death being preceded by chromatin condensation and DNA fragmentation. Quantification of the latter indicated an increase above 4 microM cadmium, with maximal fragmentation at 8 to 10 microM. By contrast, when CEM-C12 cells were exposed to higher cadmium concentrations (50 microM), cell death increased without concomitant chromatin condensation or DNA fragmentation. Thus, cadmium at low and high concentration kills CEM-C12 cells by apoptosis and necrosis, respectively. Addition of cycloheximide reduced the apoptotic effect of cadmium, suggesting that cadmium-induced apoptosis is an process depending on protein synthesis. Verapamil, a calcium/potassium channel blocker, markedly increased the viability of CEM-C12 cells treated by low cadmium concentrations and prevented DNA fragmentation. The apoptotic effect of cadmium suggests a possible mechanism for lymphocyte damage occurring after in vivo exposure to cadmium.

Apoptotic cell death analyzed at the molecular level by two-dimensional gel electrophoresis

The pattern of protein expression and phosphorylation after an apoptotic stimulus has been studied in two systems. Bovine aortic endothelial cells were induced to undergo apoptotic cell death by a combination of a cytokine (tumor necrosis factor, TNF) and inhibitors of protein synthesis, like cycloheximide. Two-dimensional (2-DE) electrophoresis of proteins from such cells revealed specific proteolysis of distinct proteins, some at an early stage of apoptosis and some at a later stage. These proteins may have antiapoptotic properties. In rat IPC-81 promyelocytic leukemia cells, cAMP induced apoptosis. 2-DE of such cells pulse-labeled with [35S]methionine revealed two "novel" protein spots (of 30 kDa and 46 kDa, respectively), induced very rapidly by a posttranscriptional mechanism. It is proposed that "dysphosphorylation" may accompany apoptosis in general, since both endothelial cells treated with TNF/cycloheximide and IPC-81 cells treated with cAMP analog or the apoptosis-inducing phosphatase inhibitors okadaic acid or calyculin A all showed altered protein phosphorylation patterns, as revealed by 2-DE electrophoresis of proteins from cells prelabeled with 32Pi.

The survival of developing neurons: a review of afferent control

Developmental cell death is a major event of neurogenesis, and emphasis has systematically been placed on the roles of either the peripheral targets or central postsynaptic neurons in the control of neuronal survival. In this article, the main types of experimental design used to test the control of neuronal death by the afferent supply are compared with analogous data indicating neurotrophic support by the targets. It is argued that targets and afferents may have equivalent roles and interact in the control of neuron numbers during development of the vertebrate nervous system. Possible mechanisms of anterograde trophic control include contact-mediated cell interactions, activity-dependent processes mediated by neurotransmitters or neuromodulators, modulation of the levels of cytoplasmic free calcium and the involvement of neurotrophic factors.

Detection of apoptosis-associated DNA strand breaks in fine-needle aspiration biopsies by in situ end labeling of fragmented DNA

The predominant mode of either spontaneous or drug-induced death of cells in tumors is apoptosis. A flow cytometric method was developed in our laboratory to identify apoptotic cells, based on labeling DNA strand breaks, which appear as a result of extensive DNA cleavage by the apoptosis-associated endonuclease, with biotinylated dUTP in the reaction catalyzed by exogenous terminal deoxynucleotidyl transferase. The aim of this study was to reveal whether this methodology can be applied to human solid tumors sampled by fine-needle biopsy. Twenty-two tumors, consisting of 11 breast carcinomas; three metastatic anaplastic carcinomas; three adenocarcinomas of colon, endometrium, and lung; two metastatic lymph node squamous cell carcinomas of the larynx; and three malignant lymphomas were examined. It was possible to identify cells with DNA strand breaks in all these tumors. Extremely high variability in the proportion of cells with DNA strand breaks was observed between the individual tumors. In diploid tumors (n = 12) the percentage of cells with DNA strand breaks varied from 1% to 43%, and the mean value was 19%. In aneuploid tumors this percentage varied from 15% to 51% and the mean value was 37%. In the latter tumors the presence of cells with DNA strand breaks was limited to the DNA aneuploid cell population; very few diploid, presumably tumor infiltrating or stromal cells, showed the presence of DNA strand breaks. No correlation was observed between the percent of cells in S phase and those with DNA strand breaks. The data indicate that apoptosis is more frequent in populations of tumor cells than among normal cells of the same organs.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanistic considerations in chemopreventive drug development

This overview of the potential mechanisms of chemopreventive activity will provide the conceptual groundwork for chemopreventive drug discovery, leading to structure-activity and mechanistic studies that identify and evaluate new agents. Possible mechanisms of chemopreventive activity with examples of promising agents include carcinogen blocking activities such as inhibition of carcinogen uptake (calcium), inhibition of formation or activation of carcinogen (arylalkyl isothiocyanates, DHEA, NSAIDs, polyphenols), deactivation or detoxification of carcinogen (oltipraz, other GSH-enhancing agents), preventing carcinogen binding to DNA (oltipraz, polyphenols), and enhancing the level or fidelity of DNA repair (NAC, protease inhibitors). Chemopreventive antioxidant activities include scavenging reactive electrophiles (GSH-enhancing agents), scavenging oxygen radicals (polyphenols, vitamin E), and inhibiting arachidonic acid metabolism (glycyrrhetinic acid, NAC, NSAIDs, polyphenols, tamoxifen). Antiproliferation/antiprogression activities include modulation of signal transduction (glycyrrhetinic acid, NSAIDs, polyphenols, retinoids, tamoxifen), modulation of hormonal and growth factor activity (NSAIDs, retinoids, tamoxifen), inhibition of aberrant oncogene activity (genistein, NSAIDs, monoterpenes), inhibition of polyamine metabolism (DFMO, retinoids, tamoxifen), induction of terminal differentiation (calcium, retinoids, vitamin D3), restoration of immune response (NSAIDs, selenium, vitamin E), enhancing intercellular communication (carotenoids, retinoids), restoration of tumor suppressor function, induction of programmed cell death (apoptosis) (butyric acid, genistein, retinoids, tamoxifen), correction of DNA methylation imbalances (folic acid), inhibition of angiogenesis (genistein, retinoids, tamoxifen), inhibition of basement membrane degradation (protease inhibitors), and activation of antimetastasis genes. A systematic drug development program for chemopreventive agents is only possible with continuing research into mechanisms of action and thoughtful application of the mechanisms to new drug design and discovery. One approach is to construct pharmacological activity profiles for promising agents. These profiles are compared among the promising agents and with untested compounds to identify similarities. Classical structure-activity studies are used to find optimal agents (high efficacy with low toxicity) based on good lead agents. Studies evaluating tissue-specific and pharmacokinetic parameters are very important. A final approach is design of mechanism-based assays and identification of mechanism-based intermediate biomarkers for evaluation of chemopreventive efficacy.

Apoptotic cell death during treatment of leukemias

The apoptosis-associated DNA strand breaks were detected in situ, in individual leukemic cells in peripheral blood and bone marrow of over 110 patients with different types of leukemia (ALL, AML, CML in blastic crisis, APL), prior to and during routine chemotherapy. The DNA strand breaks were labeled with digoxigenin- or biotin-conjugated dUTP in the reaction catalyzed by exogenous terminal deoxynucleotidyl transferase, and the cells, counterstained for DNA, were analyzed by bivariate flow cytometry. The proportion of cells with DNA strand breaks prior to therapy, most likely reflecting spontaneous apoptosis, varied from 0.1 to 16%, but in the large majority of cases was below 3%. Administration of drugs of different classes, which included DNA topoisomerase I (Topotecan) and II (mitoxantrone, VP-16) inhibitors, antimetabolite (ara-C) or microtubule poison (Taxol), all triggered the appearance of cells with extensive DNA breakage, typical of apoptosis, to up to 80%. The peak of the response, measured as maximal percent of cells with DNA strand breaks, which varied between individual patients by as much as factor 10, was generally seen between 8 to 24 h after the initial administration of DNA topoisomerase inhibitors, and somewhat later (48-72 h) during the response to Taxol or ara-C. Thus, the data show that the response to treatment with a variety of drugs, in terms of induction of apoptosis, can be conveniently measured by the present method. The prognostic value of the apoptotic index, before, as well as during treatment, is being estimated for each type of leukemia, in the ongoing prospective studies.

Down-modulation of CD4 antigen during programmed cell death in U937 cells

It has been hypothesized that programmed cell death (PCD), an active cell suicide process occurring in place of necrosis, can be associated with the pathogenesis of acquired immunodeficiency syndrome (AIDS). The entry of human immunodeficiency virus (HIV) into competent cells is mediated by the CD4 molecule present on the surface of certain lymphocyte subpopulations as well as on some cultured cell lines, e.g. U937 myelomonocytic cells. The present paper focuses on some specific aspects of PCD induced by the cytokine tumor necrosis factor (TNF). The results obtained indicate that the exposure of U937 cells to cycloheximide facilitates TNF-mediated PCD via a short term cell death program and modifies the expression of CD4 surface molecules. This change in surface antigen expression, manifested by internalization of the CD4 molecule, occurs in cells in which apoptosis has been triggered, but not in cells undergoing necrosis. These results indicate that the progression of cell death could be associated with specific alterations of certain surface molecules and could have a role in the entry of HIV into cells.

Nuclear patterns of apoptotic and developing neurons of superior cervical ganglion of newborn rat

Superior cervical ganglion (SCG) neurons of female rats aged 3, 5 and 7 days revealed conspicuous nuclear changes in neurons undergoing postnatal cell death. Several qualitative and quantitative data such as nuclear size and shape, the presence of atypical chromocenters and chromatin textural features discriminated well neurons candidate to degeneration and those advancing in the direction of adult maturation. At least on morphological grounds, postnatal death of SCG neurons appears to be of apoptotic type. The sequence of nuclear events observed enables the recognition of the early stages of involution which prelude neuron degeneration.

Neuronal birth and death

Neurogenesis and cell death occur predominantly during the postnatal period in the dentate gyrus of the rat. Recent studies have shown that mitosis and apoptosis in this system are regulated by adrenal steroids, possibly through excitatory amino acids. Studies performed in other systems have identified genes that mediate cell birth and death, which may also participate in the development and maintenance of the dentate gyrus.

Apoptosis is induced by beta-amyloid in cultured central nervous system neurons

The molecular mechanism responsible for the neurodegeneration in Alzheimer disease is not known; however, accumulating evidence suggests that beta-amyloid peptide (A beta P) contributes to this degeneration. We now report that synthetic A beta Ps trigger the degeneration of cultured neurons through activation of an apoptotic pathway. Neurons treated with A beta Ps exhibit morphological and biochemical characteristics of apoptosis, including membrane blebbing, compaction of nuclear chromatin, and internucleosomal DNA fragmentation. Aurintricarboxylic acid, an inhibitor of nucleases, prevents DNA fragmentation and delays cell death. Our in vitro results suggest that apoptosis may play a role in the neuronal loss associated with Alzheimer disease.

Biochemical events in naturally occurring forms of cell death

Several molecular elements of programmed cell death and apoptosis have recently been revealed. The function of gene products which deliver the lethal 'hit' is still not known. Well-characterized and newly discovered cell surface structures (e.g. antigen receptors, FAS/APO-1), as well as transcriptional factors (steroid receptor, c-myc, P53, retinoblastoma protein and others), have been implicated in the initiation of the death pathway. Negative regulators of the process (ced-9 gene product in programmed death of cells in Caenorhabditis elegans and bcl-2 protein in apoptosis) have been described. Biochemical mechanisms responsible for the silent nature of natural deaths of cells include their rapid engulfment (mainly through integrin receptors), transglutaminase-catalyzed cross-linking of cellular proteins, and fragmentation of DNA. Several lines of evidence suggest that distinct molecular mechanisms may operate in various forms of natural cell death.

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.

N-acetylcysteine inhibits apoptosis and decreases viral particles in HIV-chronically infected U937 cells

Apoptosis or programmed cell death (PCD) is a type of death occurring in various physiological processes. Several data suggest that: (1) apoptosis may play a critical role in AIDS pathogenesis; (2) an increase of endocellular free radical levels can be associated with activation of previously latent HIV virus. Tumor necrosis factor (TNF), a cytokine capable of inducing oxygen free radicals and apoptosis, appears also to be involved in HIV activation. The present findings, which elucidate a relationship between the percentage of apoptotic cells, reduced glutathione (GSH) depletion and an increase of p24 antigenemia, suggest that pretreatment with N-acetylcysteine (NAC) is capable of decreasing the above-mentioned phenomena in HIV-infected U937 cells.

Non-genotoxic hepatocarcinogenesis in vitro: the FaO hepatoma line responds to peroxisome proliferators and retains the ability to undergo apoptosis

A range of hepatoma cell lines (RH1, HTC, FaO, 7800C1 and MH1C1), has been studied with the aim of establishing an in vitro model to investigate the molecular mechanisms of hepatocarcinogenicity induced by the peroxisome proliferator class of non-genotoxic carcinogens. In view of speculation that peroxisome proliferators suppress hepatocyte apoptosis in vivo, we have placed particular emphasis on evaluating whether hepatoma cell lines retain the ability to undergo apoptotic cell death. Expression of the liver-specific differentiation marker albumin and the peroxisome proliferator-activated receptor (PPAR) was highest in the Reuber hepatoma cell line, FaO. This cell line also demonstrated the most marked response to the peroxisome proliferator nafenopin with a 2.2-fold induction of the microsomal enzyme cytochrome p450IVA1. This response was found to display intercellular heterogeneity by immunocytochemistry. Thus, the FaO cell line maintained characteristics of hepatocytes, both in vivo and in vitro, in terms of expression of constitutive and inducible markers. However, none of the cell lines tested mirrored the hyperplastic response of hepatocytes to nafenopin, since no increase in cell growth kinetics was observed on addition of nafenopin to the growth medium. The mode of cell death in confluent FaO cultures was characterised as apoptosis, by fluorescence microscopy and agarose gel electrophoresis of extracted DNA. Cells detaching from confluent FaO cultures exhibited chromatin condensation and DNA fragmentation patterns characteristic of cels undergoing apoptotic death.Interestingly, no apoptosis was seen in monolayer cells, suggesting that apoptosis in vitro is associated with cell shrinkage and detachment similar to that documented for the liver in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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.