Differentiation and cell death: lessons from the immune system

DMSO Modifies Structural and Functional Properties of RPMI-8402 Cells by Promoting Programmed Cell Death

Apoptosis in lymphoid cells can be induced in different ways depending on cell type and acquired signal. Biochemical modifications occur at an early phase of cell death while at late times the typical morphological features of apoptosis can be visualized. The aim of this study is to verify by multiparametric analyses the plasma membrane fluidity, the intracellular Ca2+ concentration and the nitric oxide synthase (NOS) activity during cell death progression induced by DMSO treatment. The RPMI-8402 human pre-T lymphoblastoid cell line was induced to cell death by DMSO. Analyses rescued at early times of treatment prove a substantial modification of plasma membrane fluidity associated with an increase of intracellular Ca2+. Moreover, these modifications are associated with an up regulation of NOS activity. Our results are consistent with the hypothesis that programmed cell death can be induced by up regulation of the intracellular Ca2+ associated with an increase of cell membrane fluidity. The apoptotic mechanisms seem to involve not only membrane damage and increased intracellular calcium levels but also production of nitric oxide.

Odontoblast-targeted Bcl-2 overexpression impairs dentin formation

Apoptosis has been described extensively in tooth development, which is under tight control of multiple apoptosis regulators, including anti-apoptotic protein Bcl-2. However, it is totally unclear how Bcl-2 is related to odontogenesis, especially dentinogenesis. Using a transgenic mouse Col2.3Bcl-2 in which human Bcl-2 was overexpressed in odontoblasts, the effect of Bcl-2 on dentinogenesis was investigated. Overexpression of Bcl-2 was detected by immunohistochemistry and Western blot. Odontoblast apoptosis was evaluated by TUNEL and Western blot detection of cleaved caspase-3. Odontoblast differentiation was assessed by real-time PCR detection of dentin matrix expression. Dentin mineralization was evaluated by micro-CT in vivo, and alizarin red S staining and calcium content analysis in vitro. Bcl-2 was found to be overexpressed in odontoblasts and prevent their apoptosis. Odontoblast differentiation and mineralization was inhibited by Bcl-2, as evidenced by lower expressions of DMP-1, OC, and DSPP, and decreased odontoblast mineralization in vitro, as well as decreased dentin thickness and mineral density in vivo when compared to the wild-type animals. Inhibition of odontoblast differentiation by Bcl-2 occurs, at least partially, via a suppression of MEK-ERK1/2 signaling pathway. In conclusion, Bcl-2 overexpression prevents odontoblast apoptosis and impairs dentin formation, partially via an inhibition of odontoblast differentiation. This study revealed some novel functions of Bcl-2 in dentinogenesis in addition to its anti-apoptotic effect, which shed some light on the genetic complexity of tooth development.

Bone-targeted overexpression of Bcl-2 increases osteoblast adhesion and differentiation and inhibits mineralization in vitro

Apoptosis is a process important for the development and homeostasis of self-renewing tissues, including bone. However, little is known about the function of Bcl-2, a key player of apoptosis, in the regulation of osteoblast activity. Ex vivo cultures of osteoblasts from Col2.3Bcl-2 mice, in which human Bcl-2 was targeted to bone by the 2.3 kb fragment of the type I collagen promoter, were used to study the effect of Bcl-2 in osteoblasts. During 35 days of culture, hBcl-2 expression increased without any effect on endogenous mouse Bcl-2 and Bax expression. Adhesion of transgenic (TG) osteoblasts was twofold more than that of wild-type (WT) cells, with significantly higher expression of integrins alpha(1), alpha(2), and alpha(5) but similar levels of alpha(v) and beta(1) relative to WT cells. Proliferation of osteoblasts was not affected. Overexpression of hBcl-2 promoted the differentiation of osteoblasts, as shown by increased message levels of alkaline phosphatase, type I collagen, bone sialoprotein, and osteocalcin in the TG compared to WT cells throughout the culture period. The two transcription factors essential for osteoblast differentiation, core binding factor alpha 1 (Cbfa-1) and osterix, had significantly higher expression in TG than WT cells during the early culture period. ss-Catenin, a central player in the canonical Wnt pathway, also had higher expression in TG than WT cultures. Mineralization was significantly decreased in TG cultures, with less osteoblast apoptosis, compared to WT. Thus, Bcl-2 seems to have multiple roles in modulating osteoblast activities.

Essential role of PI3Kdelta and PI3Kgamma in thymocyte survival

Class 1 phosphoinositide 3-kinases (PI3Ks), consisting of PI3Kalpha, beta, gamma, and delta, are a family of intracellular signaling molecules that play important roles in cell-mediated immune responses. In thymocytes, however, their role is less clear, although PI3Kgamma is postulated to partially contribute to pre-TCR-dependent differentiation. We now report that PI3Kdelta, in conjunction with PI3Kgamma, is required for thymocyte survival and ultimately for T-cell production. Surprisingly, genetic deletion of the p110delta and p110gamma catalytic subunits resulted in a dramatic reduction in thymus size, cellularity, and lack of corticomedullary differentiation. Total thymocyte counts in these animals were 27-fold lower than in wild-type (WT) controls because of a diminished number of CD4+ CD8+ double-positive (DP) cells and were associated with T-cell depletion in blood and in secondary lymphoid organs. Moreover, this alteration in the DP population was intrinsic to thymocytes, because the reconstitution of p110gammadelta-/- animals with WT fetal liver cells restored the proportions of all thymocyte populations to those in WT controls. The observed defects were related to massive apoptosis in the DP population; TCRB expression, pre-TCR selection, and generation of DP cells appeared relatively unperturbed. Thus, class 1 PI3Ks work in concert to protect developing thymocytes from apoptosis.

Inverse relationship between the ratio of ICAM-1 expressing lymphocytes and serum TGF-β1 concentrations in acute rheumatic fever

Autoimmunity in acute rheumatic fever (ARF) is triggered by group-A beta hemolytic streptococci (GAS). Although most of the recent work has focused on the major impact of lymphocytes, the exact immunopathogenesis is still unresolved. Regulation of self-tolerance in response to GAS has been investigated in various animal experiments. This study aimed to associate the ratio of lymphocytes bearing adhesion/costimulatory molecules, Bcl-2/CD95 and serum TGF-beta1 concentrations with clinical stages of ARF. Thirty-five patients were assigned according to the clinical stages. Bcl-2 expression on CD19+ and CD3+ lymphocytes was similar within patient groups and controls. CD62p expression was higher in patients with carditis. The ratio of ICAM-1 bearing lymphocytes was significantly different between patient groups and controls and was increased through acute to remission stages longitudinally. In contrast, a gradual and significant decrease in TGF-beta1 concentrations was observed longitudinally from acute to chronic stages. A positive correlation has been documented between ICAM-1+ lymphocyte ratios and Fas+ cytotoxic T cell ratios supported by a prominent increase in CD95+ T cells. These data draw our attention to the role of ICAM-1, Fas and TGF-beta1 in ARF pathogenesis through the perspective of self-tolerance in a clinical setting.

Effect of osteoblast-targeted expression of bcl-2 in bone: differential response in male and female mice

Transgenic mice (Col2.3Bcl-2) with osteoblast-targeted human Bcl-2 expression were established. Phenotypically, these mice were smaller than their wildtype littermates and showed differential effects of the transgene on bone parameters and osteoblast activity dependent on sex. The net effect was an abrogation of sex differences normally observed in wildtype mice and an inhibition of bone loss with age. Ex vivo osteoblast cultures showed that the transgene had no effect on osteoblast proliferation, but decreased bone formation. Estrogen was shown to stimulate endogenous Bcl-2 message levels. These studies suggest a link between Bcl-2 and sex regulation of bone development and age-related bone loss.

INTRODUCTION

Whereas Bcl-2 has been shown to be an important regulator of apoptosis in development, differentiation, and disease, its role in bone homeostasis and development is not well understood. We have previously showed that the induction of glucocorticoid-induced apoptosis occurred through a dose-dependent decrease in Bcl-2. Estrogen prevented glucocorticoid-induced osteoblast apoptosis in vivo and in vitro by preventing the decrease in Bcl-2 in osteoblasts. Therefore, Bcl-2 may be an important regulator of bone growth through mechanisms that control osteoblast longevity and function.

MATERIALS AND METHODS

Col2.3Bcl-2 mice were developed carrying a 2.3-kb region of the type I collagen promoter driving 1.8 kb of human Bcl-2 (hBcl-2). Tissue specific expression of hBcl-2 in immunoassays validated the transgenic animal model. Histomorphometry and DXA were performed. Proliferation, mineralization, and glucocorticoid-induced apoptosis were examined in ex vivo cultures of osteoblasts. The effect of estrogen on mouse Bcl-2 in ex vivo osteoblast cultures was assayed by RT-PCR and Q-PCR.

RESULTS AND CONCLUSIONS

Two Col2.3Bcl-2 (tg/+) founder lines were established and appeared normal except that they were smaller than their nontransgenic wildtype (+/+) littermates at 1, 2, and 6 months of age, with the greatest differences at 2 months. Immunohistochemistry showed hBcl-2 in osteoblasts at the growth plate and cortical surfaces. Nontransgenic littermates were negative. Western blots revealed hBcl-2 only in type I collagen-expressing tissues. Histomorphometry of 2-month-old mice showed a significant decrease in tg/+ calvaria width with no significant differences in femoral trabecular area or cortical width compared with +/+. However, tg/+ males had significantly more trabecular bone than tg/+ females. Female +/+ mice showed increased bone turnover with elevated osteoblast and osteoclast parameters compared with +/+ males. Col2.3Bcl-2 mice did not show such significant differences between sexes. Male tg/+ mice had a 76.5 +/- 1.5% increase in ObS/BS with no significant differences in bone formation rate (BFR) or mineral apposition rate (MAR) compared with male +/+ mice. Transgenic females had a significant 48.4 +/- 0.1% and 20.1 +/- 5.8% decrease in BFR and MAR, respectively, compared with +/+ females. Osteoclast and osteocyte parameters were unchanged. By 6 months, femurs from female and male +/+ mice had lost a significant amount of their percent of trabecular bone compared with 2-month-old mice. There was little to no change in femoral bone in the tg/+ mice with age. Ex vivo cultures of osteoblasts from +/+ and Col2.3Bcl-2 mice showed a decrease in mineralization, no effect on proliferation, and an inhibition of glucocorticoid-induced apoptosis in Col2.3Bcl-2 cultures. Estrogen was shown to increase mouse Bcl-2 transcript levels in osteoblast cultures of wildtype mice, supporting a role for Bcl-2 in the sex-related differences in bone phenotype regulated by estrogen. Therefore, Bcl-2 differentially affected bone phenotype in male and female transgenic mice, altered bone cell activity associated with sex-related differences, and decreased bone formation, suggesting that apoptosis is necessary for mineralization. In addition, Bcl-2 targeted to mature osteoblasts seemed to delay bone development, producing a smaller transgenic mouse compared with wildtype littermates. These studies suggest that expression of Bcl-2 in osteoblasts is important in regulating bone mass in development and in the normal aging process of bone.

The c-myc gene regulates the polyamine pathway in DMSO-induced apoptosis

It is accepted that apoptosis is a gene-controlled process of cellular self-destruction. It occurs during physiological regulation and in pathological situations in the life of a cell. In the immune system, several different intracellular and extracellular factors have been associated with the induction of apoptosis, and the final responses depend on the cell system and the acquired signals. In lymphoid cells, dexamethasone-induced apoptosis is associated with c-myc downregulation in cells that remain in G0-G1 until the point of death. Ornithine decarboxylase (ODC), a key enzyme involved in polyamine biosynthesis, is regulated by c-myc, which is a transcriptional activator implicated not only in the control of cell proliferation and differentiation but also in programmed cell death. As dimethylsulphoxide (DMSO) induces apoptosis in the RPMI-8402 human pre-T cell line, the present study analysed the involvement of the c-myc proto-oncogene and polyamine pathway as mediators of apoptosis. Cell growth, programmed cell death, c-myc expression, ODC activity and intracellular polyamine content were detected after DMSO and difluoromethylornithine (DFMO) treatment. DMSO-treated cells exhibit a decrease in ODC activity and polyamine levels associated with cell growth arrest and programmed cell death induction. The expression of c-myc proto-oncogene, as its mRNA or protein, is specifically down-regulated. DFMO, a well defined polyamine biosynthesis inhibitor, completely blocks ODC activity, resulting in growth inhibition but not apoptosis. Moreover, in these samples no evidence of changes of c-myc expression were found. The results obtained suggest that, in RPMI-8402 cells, DMSO provokes a c-myc-dependent decrease of ODC activity followed by a depletion of intracellular polyamine levels, associated with programmed cell death and cell growth arrest.

Apoptosis in the immune system: 1. Fas-induced apoptosis in monocytes-derived human dendritic cells

Dendritic cells (DC) are cells of the hematopoietic system specialized in capturing antigens and initiating T cell-mediated immune responses. We show here that human DC generated from adherent peripheral blood mononuclear cells (PBMC) after in vitro stimulation with granulocyte macrophage colony stimulating factor (GM-CSF) and interleukin-4 (IL-4) express Fas antigen (APO-1, CD95) and can undergo apoptosis upon triggering of Fas by monoclonal antibodies. Immature monocytes-derived dendritic cells (MDDC) upregulate CD86 and HLA-DR expression and develop dendrites and veiled processes. Flow cytometry analysis revealed CD95 expression in approx. 40% of these MDDC and incubation with anti-CD95 mAb (0.5 microg/ml) induced apoptosis when compared to untreated controls. The extent of apoptosis induced by the agonist anti-Fas antibody strongly related to the percentage of cells expressing CD 95. Upon tumor necrosis factor alpha (TNF-alpha) additional stimulation, MDDC assumed a characteristic mature dendritic cells morphology showing prolonged veils, CD83 expression, and high levels of HLA-DR. These cells have downregulated their Fas receptors (to approx. 20%) and undergo apoptosis to a lesser extent when treated with anti-CD 95, as demonstrated by the hardly noticeable effect of this antibody on the viability of cultured cells as compared to controls. Thus, upon TNF-alpha induced maturation, MDDC became resistant to Fas-induced apoptosis. The apoptotic episodes surrounding the earlier stage of DC differentiation appeared to be mediated by Fas. In contrast, a Fas independent pathway is probably responsible for the apoptotic events associated with terminally differentiated DC.

Inhibitory effects of voluntary wheel exercise on apoptosis in splenic lymphocyte subsets of C57BL/6 mice

Two-month-old mice were placed in cages with (Ex) or without exercise running wheels with free access to the wheel 24 h/day for 10 mo. An equal amount of food for both groups was provided daily. Ex mice ran an average of 33.67 km/wk initially, and exercise decreased gradually with age. Ex mice had gained an average of 43.5% less body weight at the end of the experiment. Although serum lipid peroxides were not altered by exercise, superoxide dismutase and glutathione peroxidase activities in serum were significantly increased. Flow cytometric analysis of spleen cells revealed an increased percentage of CD8+ T cells and a decreased percentage of CD19+ B cells in Ex mice (P < 0.05). Exercise decreased apoptosis in total splenocytes and CD4+ cells incubated with medium alone or with H(2)O(2), dexamethasone, tumor necrosis factor-alpha (TNF-alpha), or anti-CD3 monoclonal antibody (P < 0.05) and CD8+ cells with medium alone or with TNF-alpha (P < 0.05). Even though exercise did not alter the intracellular cytokines (TNF-alpha and interleukin-2) or Fas ligand, it did significantly lower interferon-gamma in CD4+ and CD8+ cells (P < 0.05). In summary, voluntary wheel exercise appears to decrease H(2)O(2)-induced apoptosis in immune cells as well as decrease interferon-gamma production.

Cytotoxic activity of Justicia spicigera is inhibited by bcl-2 proto-oncogene and induces apoptosis in a cell cycle dependent fashion

Identification of organic compounds from plants is of clinical significance because of the effect that they might have in patients with haematopoietic disorders. We studied the effect of the plant extract Justicia spicigera (Acanthaceae) in different haematopoietic cells: human leukaemic cell lines, umbilical cord blood cells, and mouse bone marrow cells. By examining colony formation and performing the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay it was shown that the plant extract of Justicia spicigera contains cytotoxic factors for leukaemic cells and has no proliferative activity on normal haematopoietic progenitor cells. Our results show that this plant extract induces apoptosis in the human leukaemia cell line TF-1, but not in the bcl-2 transfectant cell line TB-1. Similar results were obtained using a haemopoietic cell line 32D and 32DBcl2. The cultures of umbilical cord blood cells and mouse bone marrow that contain granulocyte-macrophage colony-stimulating factor (GM-CSF) do not proliferate or become terminally differentiated in the presence of the infusion of Justicia spicigera. GM-CSF that acts by abrogating programmed cell death is not sufficient to inhibit the apoptotic stimulus in TF-1 and 32D cells. Moreover mouse fibroblasts (3T3) and two cervical carcinoma cell lines CALO and INBL, undergo apoptosis in the presence of different concentrations of an infusion from the plant. Our data show that there is a strong correlation between the cytotoxic effect and cell proliferation. Together, these results indicate that the plant infusion of Justicia spicigera does not contain any haematopoietic activity, induces apoptosis inhibited by bcl-2 and is linked to cell proliferation.

Steel factor sustains SCL expression and the survival of purified CD34+ bone marrow cells in the absence of detectable cell differentiation

CD34+ cells express the basic helix-loop-helix transcription factor SCL, which is essential for blood cell formation in vivo. In addition, their survival is critically dependent on hemopoietic growth factors. We therefore compared the effects of Steel factor (SF) and GM-CSF on the survival, proliferation, and differentiation of primary human CD34+ cells, as well as the role of SCL during these processes. GM-CSF suppresses apoptosis in CD34+ cells, which proliferate and differentiate into mature granulocytic and monocytic cells (CD34-CD13+) and loose SCL expression. In contrast, SF suppresses apoptosis without a significant increase in cell numbers, and the cells remain CD34+ and SCL+ with a blast-like morphology. Examination of apoptosis by the terminal deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling (TUNEL) reaction and of the cell cycle status indicated that SF is both a survival factor and a mitogenic factor for CD34+ cells. There was, however, constant cell death in a fraction of the population, which could be rescued by GM-CSF. Co-addition of SF and GM-CSF prevents the downregulation of SCL observed in the presence of GM-CSF by itself, allows for prolonged survival and expansion of CD34+ cells in culture, inhibits monocytic differentiation and impairs granulocytic differentiation. Finally, exposure to an antisense SCL but not a control oligonucleotide decreases SCL protein levels and prevents the suppression of apoptosis by SF without affecting GM-CSF-dependent cell survival. These observations suggest that the hemopoietic transcription factor SCL regulates the survival of CD34+ cells in response to SF.

Regulation of Differentiation, Proliferation and Drug-Induced Apoptosis in HT58 Lymphoma Cells

Recently, it has been suggested, that differentiated cells are more resistant to the apoptotic effect of DNA damaging agents possibly due to the decreased activity of "damage detecting/apoptosis triggering" mechanism. Previously, we have shown, that PMA pretreatment reduced etoposide-(ETO) but enhanced staurosporine- (STA) -induced apoptosis in HT58 cells. Data presented here show that the HT58 human, "mature" B-lymphoma cells exposed to PMA secrete more IgM into the supernatant indicating commitment of cells to perform differentiated function. The sensitivity of HT58 cells to ETO- or STA-induced apoptosis is influenced diversely with PMA pre- or posttreatment. Interestingly, the DNA damage (gamma radiation, bleomycin, ETO) or okadaic acic (30 nM) reduced the [PMA+STA] induced apoptosis.

Derangement of apoptosis-related lymphocyte homeostasis in systemic sclerosis

OBJECTIVES

Both increased and decreased apoptosis may be involved in generating autoimmunity. This study addressed the question of whether apoptosis and apoptosis-regulating proteins are altered in systemic sclerosis (SSc). Patients and methods. Peripheral lymphocytes of 39 SSc patients and 47 healthy control persons were studied for apoptosis, Bcl-2 and Bax levels, expression of Fas (CD95) and activation markers (CD25, HLA-DR) as determined by fluorocytometry. Serum Fas and Fas ligand were measured by ELISA.

RESULTS

SSc lymphocytes (mainly CD4(+)) expressed increased amounts of Bcl-2, while Bax was not elevated. Apoptosis rates of SSc lymphocytes were increased in unsupplemented medium, but returned to normal in the presence of autologous plasma. SSc patients had increased percentages of activated and CD95(+) lymphocytes and elevated soluble Fas and soluble FasL levels in serum. Activating anti-CD95 antibodies further increased the apoptosis rate.

CONCLUSIONS

Increased in vitro apoptosis, elevated lymphocytic Bcl-2 content and the increased number of Fas-positive T cells are not specific for peripheral blood from SSc patients, but indicate deregulation of lymphocyte homeostasis in this disease.

Mechanisms associated with thymocyte apoptosis induced by simian immunodeficiency virus

Despite considerable research, the mechanisms by which HIV disrupts thymic function remain controversial. We have described the phenotypic changes that occur in the thymus of SIV-infected macaques during acute SIV infection. In this study, we analyzed the effects of SIV infection on apoptotic pathways in thymic tissue from newborn macaques infected with SIV. Thymocyte apoptosis was accompanied by a modest increase in surface Fas expression, a profound decrease in the frequency of bcl-2-positive cells, as well as the amount of bcl-2 per cell. With control of viral replication, levels of bcl-2 and Fas returned to baseline together with a return to basal levels of apoptosis. In the thymus, SIV infection resulted in depletion of CD4+CD8+ thymocytes, an increase in apoptosis of thymocytes, and a down-regulation of MHC class I molecules. These changes peaked 14-21 days after infection at or just after peak viremia. This data further suggests disruption of the antiapoptotic pathway regulated by bcl-2 plays a critical role in SIV-induced apoptosis of thymocytes.

Expression of activated CDC42 induces T cell apoptosis in thymus and peripheral lymph organs via different pathways

CDC42, a Ras-related small GTP binding protein, is involved in diverse cellular functions in lymphocytes. We generated transgenic mice expressing constitutively active murine CDC42 (Q61L) under the control of the human CD2 promoter. Transgenic mice showed smaller thymi with a dramatic reduction of CD4+CD8+, CD4+ and CD8+ thymocytes and with increase of CD4-CD8- thymocytes at CD25-CD44+ and CD25+ stage. A high percentage of the transgenic thymocytes were apoptotic, explaining the reduction of cellularity and size of the thymus. Mature T cells (TCR alphabeta+) in peripheral lymph organs, spleen and lymph node, were also dramatically reduced, and exhibited massive apoptosis. Expression of Fas and Fas ligand on both thymocytes and peripheral T cells was upregulated in transgenic mice, but the increased apoptosis in the thymus was independent of Fas (CD95), whereas peripheral spleen and lymph node T cell apoptosis was Fas dependent. Thus, activated CDC42 triggers distinct apoptotic pathways in thymocytes and peripheral T cells.

Induction of apoptosis and apoptotic mediators in Balb/C splenic lymphocytes by dietary n-3 and n-6 fatty acids

The present study was designed to investigate the effect of dietary n-6 and n-3 polyunsaturated fatty acids (PUFA) on anti-CD3 and anti-Fas antibody-induced apoptosis and its mediators in mouse spleen cells. Nutritionally adequate semipurified diets containing either 5% w/w corn oil (n-6 PUFA) or fish oil (n-3 PUFA) were fed to weanling female Balb/C mice, and 24 wk later mice were sacrificed. In n-3 PUFA-fed mice, serum and splenocyte lipid peroxides were increased by 20 and 28.3% respectively, compared to n-6 PUFA-fed mice. Further, serum vitamin E levels were decreased by 50% in the n-3 PUFA-fed group, whereas higher anti-Fas- and anti-CD3-induced apoptosis (65 and 66%) and necrosis (17 and 25%), compared to the n-6 PUFA-fed group, were found when measured with Annexin V and propidium iodide staining, respectively. In addition, decreased Bcl-2 and increased Fas-ligand (Fas-L) also were observed in the n-3 PUFA-fed group compared to the n-6 PUFA-fed group. No difference in the ratio of splenocyte subsets nor their Fas expression was observed between the n-3 PUFA-fed and n-6 PUFA-fed groups, whereas decreased proliferation of splenocytes was found in n-3 PUFA-fed mice compared to n-6 PUFA-fed mice. In conclusion, our results indicate that dietary n-3 PUFA induces higher apoptosis by increasing the generation of lipid peroxides and elevating Fas-L expression along with decreasing Bcl-2 expression. A reduced proliferative response of immune cells also was observed in n-3 PUFA-fed mice.

Distinct glucocorticoid receptor transcriptional regulatory surfaces mediate the cytotoxic and cytostatic effects of glucocorticoids

Glucocorticoids act through the glucocorticoid receptor (GR), which can function as a transcriptional activator or repressor, to elicit cytostatic and cytotoxic effects in a variety of cells. The molecular mechanisms regulating these events and the target genes affected by the activated receptor remain largely undefined. Using cultured human osteosarcoma cells as a model for the GR antiproliferative effect, we demonstrate that in U20S cells, GR activation leads to irreversible growth inhibition, apoptosis, and repression of Bcl2. This cytotoxic effect is mediated by GR's transcriptional repression function, since transactivation-deficient mutants and ligands still bring about apoptosis and Bcl2 down-regulation. In contrast, the antiproliferative effect of GR in SAOS2 cells is reversible, does not result in apoptosis or repression of Bcl2, and is a function of the receptor's ability to stimulate transcription. Thus, the cytotoxic versus cytostatic outcome of glucocorticoid treatment is cell context dependent. Interestingly, the cytostatic effect of glucocorticoids in SAOS2 cells involves multiple GR activation surfaces. GR mutants and ligands that disrupt individual transcriptional activation functions (activation function 1 [AF-1] and AF-2) or receptor dimerization fail to fully inhibit cellular proliferation and, remarkably, discriminate between the targets of GR's cytostatic action, the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). Induction of p21(Cip1) is agonist dependent and requires AF-2 but not AF-1 or GR dimerization. In contrast, induction of p27(Kip1) is agonist independent, does not require AF-2 or AF-1, but depends on GR dimerization. Our findings indicate that multiple GR transcriptional regulatory mechanisms that employ distinct receptor surfaces are used to evoke either the cytostatic or cytotoxic response to glucocorticoids.

Bcl-2-Induced Changes in E2F Regulatory Complexes Reveal the Potential for Integrated Cell Cycle and Cell Death Functions

Proliferation and cell death are tightly linked fates during cell and tissue differentiation. In the past few years, it has been shown that Bcl-2 exhibits a potent cell cycle inhibitory effect, in addition to its better known role in the antagonism of cell death. In the present study, we show that the cell cycle effects of Bcl-2 apparently occur at the level of E2F control of gene transcription. Under conditions of normal cell growth, or under conditions that lead to cell death in the absence of Bcl-2, bcl-2 expression results in a reduction of free (active) E2F isoforms and in an increase in the formation of higher-order (inactive) complexes. Bcl-2-induced changes in E2F complex formation are paralleled by an apparent increase in pRb regulatory activity, by the up-regulation of p130 protein expression, and by the formation of E2F/p130 complexes at the expense of those consisting of E2F/p107. Cells lacking bcl-2 expression respond to growth factor withdrawal in the opposite manner, by the liberation of E2F from inactivating complexes and by continued cell cycle leading to cell death. These analyses reveal a mechanism for cell cycle regulation by Bcl-2 that occurs at the level of E2F transcriptional activity. Further, since specific E2F activities are clearly linked to the induction of cell death, these findings may help to consolidate the cell survival and cell cycle effects of Bcl-2 through a common transcriptional mechanism.

Rational design and synthesis of a novel anti-leukemic agent targeting Bruton's tyrosine kinase (BTK), LFM-A13 [alpha-cyano-beta-hydroxy-beta-methyl-N-(2, 5-dibromophenyl)propenamide]

In a systematic effort to design potent inhibitors of the anti-apoptotic tyrosine kinase BTK (Bruton's tyrosine kinase) as anti-leukemic agents with apoptosis-promoting and chemosensitizing properties, we have constructed a three-dimensional homology model of the BTK kinase domain. Our modeling studies revealed a distinct rectangular binding pocket near the hinge region of the BTK kinase domain with Leu460, Tyr476, Arg525, and Asp539 residues occupying the corners of the rectangle. The dimensions of this rectangle are approximately 18 x 8 x 9 x 17 A, and the thickness of the pocket is approximately 7 A. Advanced docking procedures were employed for the rational design of leflunomide metabolite (LFM) analogs with a high likelihood to bind favorably to the catalytic site within the kinase domain of BTK. The lead compound LFM-A13, for which we calculated a Ki value of 1.4 microM, inhibited human BTK in vitro with an IC50 value of 17.2 +/- 0.8 microM. Similarly, LFM-A13 inhibited recombinant BTK expressed in a baculovirus expression vector system with an IC50 value of 2.5 microM. The energetically favorable position of LFM-A13 in the binding pocket is such that its aromatic ring is close to Tyr476, and its substituent group is sandwiched between residues Arg525 and Asp539. In addition, LFM-A13 is capable of favorable hydrogen bonding interactions with BTK via Asp539 and Arg525 residues. Besides its remarkable potency in BTK kinase assays, LFM-A13 was also discovered to be a highly specific inhibitor of BTK. Even at concentrations as high as 100 micrograms/ml (approximately 278 microM), this novel inhibitor did not affect the enzymatic activity of other protein tyrosine kinases, including JAK1, JAK3, HCK, epidermal growth factor receptor kinase, and insulin receptor kinase. In accordance with the anti-apoptotic function of BTK, treatment of BTK+ B-lineage leukemic cells with LFM-A13 enhanced their sensitivity to ceramide- or vincristine-induced apoptosis. To our knowledge, LFM-A13 is the first BTK-specific tyrosine kinase inhibitor and the first anti-leukemic agent targeting BTK.

Increased susceptibility of thymocytes to apoptosis in mice lacking AIM, a novel murine macrophage-derived soluble factor belonging to the scavenger receptor cysteine-rich domain superfamily

Apoptosis of cells must be regulated both positively and negatively in response to a variety of stimuli in the body. Various environmental stresses are known to initiate apoptosis via differential signal transduction cascades. However, induction of signals that may inhibit apoptosis is poorly understood, although a number of intracellular molecules that mediate inhibition of apoptosis have been identified. Here we present a novel murine macrophage-specific 54-kD secreted protein which inhibits apoptosis (termed AIM, for apoptosis inhibitor expressed by macrophages). AIM belongs to the macrophage scavenger receptor cysteine-rich domain superfamily (SRCR-SF), members of which share a highly homologous conserved cysteine-rich domain. In AIM-deficient mice, the thymocyte numbers were diminished to half those in wild-type mice, and CD4/CD8 double-positive (DP) thymocytes were strikingly more susceptible to apoptosis induced by both dexamethasone and irradiation in vivo. Recombinant AIM protein significantly inhibited cell death of DP thymocytes in response to a variety of stimuli in vitro. These results indicate that in the thymus, AIM functions in trans to induce resistance to apoptosis within DP cells, and thus supports the viability of DP thymocytes before thymic selection.

Bruton's tyrosine kinase as an inhibitor of the Fas/CD95 death-inducing signaling complex

Bruton's tyrosine kinase (BTK) is a member of the Src-related Tec family of protein tyrosine kinases. Mutations in the btk gene have been linked to severe developmental blocks in human B-cell ontogeny leading to X-linked agammaglobulinemia. Here, we provide unique biochemical and genetic evidence that BTK is an inhibitor of the Fas/APO-1 death-inducing signaling complex in B-lineage lymphoid cells. The Src homology 2, pleckstrin homology (PH), and kinase domains of BTK are all individually important and apparently indispensable, but not sufficient, for its function as a negative regulator of Fas-mediated apoptosis. BTK associates with Fas via its kinase and PH domains and prevents the FAS-FADD interaction, which is essential for the recruitment and activation of FLICE by Fas during the apoptotic signal. Fas-resistant DT-40 lymphoma B-cells rendered BTK-deficient through targeted disruption of the btk gene by homologous recombination knockout underwent apoptosis after Fas ligation, but wild-type DT-40 cells or BTK-deficient DT-40 cells reconstituted with wild-type human btk gene did not. Introduction of an Src homology 2 domain, a PH domain, or a kinase domain mutant human btk gene into BTK-deficient cells did not restore the resistance to Fas-mediated apoptosis. Introduction of wild-type BTK protein by electroporation rendered BTK-deficient DT-40 cells resistant to the apoptotic effects of Fas ligation. BTK-deficient RAMOS-1 human Burkitt's leukemia cells underwent apoptosis after Fas ligation, whereas BTK-positive NALM-6-UM1 human B-cell precursor leukemia cells expressing similar levels of Fas did not. Treatment of the anti-Fas-resistant NALM-6-UM1 cells with the leflunomide metabolite analog alpha-cyano-beta-methyl-beta-hydroxy-N-(2, 5-dibromophenyl)propenamide, a potent inhibitor of BTK, abrogated the BTK-Fas association without affecting the expression levels of BTK or Fas and rendered them sensitive to Fas-mediated apoptosis. The ability of BTK to inhibit the pro-apoptotic effects of Fas ligation prompts the hypothesis that apoptosis of developing B-cell precursors during normal B-cell ontogeny may be reciprocally regulated by Fas and BTK.

Modulation of antioxidant enzymes and apoptosis in mice by dietary lipids and treadmill exercise

The current experiments were designed to study the effect of dietary n-6 and n-3 polyunsaturated fatty acids on antioxidant enzyme activity and dexamethasone (DEX)-induced apoptosis in spleen cells of sedentary (Sed) and treadmill-exercised (Ex) ICR male mice. Two-month-old mice maintained on AIN 76 formula diet, supplemented with either 5% corn oil (CO) or 5% fish oil (FO) diets, were trained on a treadmill to run from 45 to 50 min 1 km/day, 6 days a week for 12 weeks. After 12 weeks of exercise, both Sed and Ex groups were sacrificed. Blood and various tissues, including spleen, were collected asceptically. Increased serum and spleen homogenate peroxide [malondialdehyde (MDA)] levels were observed in mice fed FO (n-3 PUFA) diets, compared to mice fed CO (n-6 PUFA). However, exercise did not alter MDA levels in either CO- or FO-fed mice. Feeding n-3 PUFA significantly increased superoxide dismutase (SOD), catalase, and glutathione peroxidase activity of spleen homogenates. Exercise also significantly increased SOD and peroxidase in CO-fed animals, whereas catalase, glutathione peroxidase, and glutathione transferase were higher in FO-fed mice, compared to the Sed group. Apoptosis and necrosis were quantitated in splenocytes incubated with or without 1 microM Dex in RPMI medium for 8 and 24 hr. Cells were stained with Annexin V and propidium iodide (PI) for apoptotic and necrotic cells. FO-fed mice showed higher apoptosis (64 vs 50%) and necrosis (40 vs 22%) in spleen cells than CO-fed mice. Cells from FO-fed mice, incubated in medium alone, showed increased apoptosis (112%) 24 hr after incubation, and necrosis (37 and 70%) at 8 and 24 hr of incubation, compared to CO-fed mice. In Ex group, apoptosis was increased in both CO- and FO-fed mice only at 24 hr after incubation. In summary, these results indicate that FO (n-3 PUFA-enriched) diets increase apoptosis and antioxidant enzyme activity in spleen cells, probably due to elevated lipid peroxides.

Measurement of apoptosis

The cell dying by apoptosis undergoes a sequence of morphological, biochemical, and molecular changes which are characteristic, and often unique, to this mode of cell death. Specific features of apoptotic cells resulting from these changes, which serve as markers used to reveal the apoptotic mode of cell death and to quantify the extent of apoptosis in cultures or in tissue, are reviewed. Analysis of these features by flow or image cytometry is the most commonly used approach to detect, quantify, and study various aspects of apoptosis. Flow or laser scanning cytometry also offer all the advantages of rapid, accurate and multiparametric measurements to investigate the biological processes associated with cell death. Numerous methods have been developed to identify apoptotic and necrotic cells, which are widely used in various disciplines, particularly in oncology and immunology. The methods based on changes in cell morphology, plasma membrane molecular structure and transport function, function of cell organelles, DNA stability to denaturation and endonucleolytic DNA degradation are reviewed and their applicability in the research laboratory and in the clinical setting is discussed. The most common pitfalls and improper use of the methodology in analysis of cell death and in data interpretation are also discussed.

Bruton's tyrosine kinase (BTK) as a dual-function regulator of apoptosis

Multiple counterregulatory mechanisms have been identified in B-cell precursors that operate to regulate cell survival and growth, thereby ensuring the orderly development and differentiation of B-cells. Inappropriate apoptosis may underlie the pathogenesis of immunodeficiencies, as well as pathogenesis and drug/radiation resistance of human leukemias and lymphomas, which makes control of apoptosis an important potential target for therapeutic interventions. Therefore, identification of the molecular regulators of apoptosis is an area of intense investigation. Bruton's tyrosine kinase (BTK) is the first tyrosine kinase to be identified as a dual-function regulator of apoptosis, which promotes radiation-induced apoptosis but inhibits Fas-activated apoptosis in B-cells. BTK functions in a pro-apoptotic manner when B-cells are exposed to reactive oxygen intermediates, at least in part, by down-regulating the anti-apoptotic activity of STAT-3 transcription factor. In contrast, BTK associates with the death receptor Fas and impairs its interaction with Fas-associated protein with death domain (FADD), which is essential for the recruitment and activation of FLICE by Fas during the apoptotic signal, thereby preventing the assembly of a pro-apoptotic death inducing signaling complex (DISC) after Fas-ligation. The identification of BTK as a dual-function regulator of apoptosis will significantly increase our understanding of both the biological processes involved in programmed cell death and the diseases associated with dysregulation of apoptosis. New agents with BTK-modulatory activity may have clinical potential in the treatment of B-cell malignancies (in particular acute lymphoblastic leukemia, the most common form of childhood cancer), as well as B-cell immunodeficiencies.

Transcription factor SCL is required for c-kit expression and c-Kit function in hemopoietic cells

In normal hemopoietic cells that are dependent on specific growth factors for cell survival, the expression of the basic helix-loop-helix transcription factor SCL/Tal1 correlates with that of c-Kit, the receptor for Steel factor (SF) or stem cell factor. To address the possibility that SCL may function upstream of c-kit, we sought to modulate endogenous SCL function in the CD34(+) hemopoietic cell line TF-1, which requires SF, granulocyte/macrophage colony-stimulating factor, or interleukin 3 for survival. Ectopic expression of an antisense SCL cDNA (as-SCL) or a dominant negative SCL (dn-SCL) in these cells impaired SCL DNA binding activity, and prevented the suppression of apoptosis by SF only, indicating that SCL is required for c-Kit-dependent cell survival. Consistent with the lack of response to SF, the level of c-kit mRNA and c-Kit protein was significantly and specifically reduced in as-SCL- or dn-SCL- expressing cells. c-kit mRNA, c-kit promoter activity, and the response to SF were rescued by SCL overexpression in the antisense or dn-SCL transfectants. Furthermore, ectopic c-kit expression in as-SCL transfectants is sufficient to restore cell survival in response to SF. Finally, enforced SCL in the pro-B cell line Ba/F3, which is both SCL and c-kit negative is sufficient to induce c-Kit and SF responsiveness. Together, these results indicate that c-kit, a gene that is essential for the survival of primitive hemopoietic cells, is a downstream target of the transcription factor SCL.

Apoptosis and the cell cycle

Apoptosis is a genetically controlled response by which eukaryotic cells undergo programmed cell death. This phenomenon plays a major role in developmental pathways (1), provides a homeostatic balance of cell populations, and is deregulated in many diseases including cancer. Control of cell number is determined by an intricate balance of cell death and cell proliferation. Accumulation of cells through suppression of death can contribute to cancer and to persistent viral infections, while excessive death can result in impaired development and in degenerative diseases. Identification of genes that control cell death, and understanding of the impact of apoptosis in both development and disease has advanced our knowledge of apoptosis in the past few years. There appears to be a linkage between apoptosis and cell cycle control mechanisms. Elucidating the mechanisms that link cell cycle control with apoptosis will be of key importance in understanding tumour progression and designing new models of effective tumour therapy.

Cutting Edge: p21-Activated Kinase (PAK) Is Required for Fas-Induced JNK Activation in Jurkat Cells

The process of apoptosis is a critical component of normal immune system development and homeostasis, and in many cells this involves signaling through the c-Jun amino terminal kinase (JNK) pathway. In Jurkat T cells, Fas-induced JNK activity is dependent upon activation of the caspase cascades known to be central components of the apoptotic program. We show in Jurkat cell lines expressing a dominant negative PAK construct that PAK signaling is necessary for JNK activation in response to Fas receptor cross-linking. Inhibition of JNK activation induced by Fas does not impair cell death as assessed by DNA fragmentation. However, expression of the catalytically active C terminus of PAK2, which is generated through caspase action during Fas-mediated apoptosis, induces Jurkat cell apoptosis. We conclude that PAK activity resulting from caspase-mediated cleavage is a necessary component of JNK activation induced by Fas receptor signaling and that PAK2 can contribute to the induction of cell death.

Detection of apoptosis in peripheral blood cells of 31 subjects affected by Down syndrome before and after zinc therapy

Thirty-one patients affected by Down syndrome (DS) were investigated to study the presence of apoptosis in peripheral blood cells in relation to the plasma levels of zinc. Twelve patients had undergone therapy with ZnSO4, while the remaining 19 were untreated. The presence of programmed cell death was evaluated by means of electron microscopy, in situ nick translation (NT), and agarose gel electrophoresis of DNA. These approaches evidenced the presence of apoptosis in peripheral blood cells of patients before therapy with ZnSO4, while after zinc supplementation there was a reduction in the number of apoptotic cells. These results suggest that the process of programmed cell death in peripheral blood cells of patients with Down syndrome is related to the plasma levels of zinc ion.

The small GTPase Cdc42 initiates an apoptotic signaling pathway in Jurkat T lymphocytes

Apoptosis plays an important role in regulating development and homeostasis of the immune system, yet the elements of the signaling pathways that control cell death have not been well defined. When expressed in Jurkat T cells, an activated form of the small GTPase Cdc42 induces cell death exhibiting the characteristics of apoptosis. The death response induced by Cdc42 is mediated by activation of a protein kinase cascade leading to stimulation of c-Jun amino terminal kinase (JNK). Apoptosis initiated by Cdc42 is inhibited by dominant negative components of the JNK cascade and by reagents that block activity of the ICE protease (caspase) family, suggesting that stimulation of the JNK kinase cascade can lead to caspase activation. The sequence of morphological events observed typically in apoptotic cells is modified in the presence of activated Cdc42, suggesting that this GTPase may account for some aspects of cytoskeletal regulation during the apoptotic program. These data suggest a means through which the biochemical and morphological events occurring during apoptosis may be coordinately regulated.

Apoptosis of terminally differentiated chondrocytes in culture

During the process of endochondral ossification chondrocytes progress through stages of terminal differentiation culminating in apoptotic death. We have developed a serum-free suspension culture that allows terminal differentiation and facilitates the investigation of factors affecting chondrocyte apoptosis. We have found that chondrocytes not committed to terminal differentiation, i.e., those from the caudal region of chick embryo sterna, a region that remains cartilaginous for some months after the chick hatches, maintained high viability in serum-free suspension culture. A strong dependence of viability on culture density and sensitivity to induction of apoptosis with the protein kinase inhibitor, staurosporine, was consistent with the proposal that these chondrocytes, like nearly all cells, require intercellular communication for survival. Chondrocytes that were committed to terminal differentiation, i.e., those from the cephalic region of chick embryo sterna, a region that is replaced by bone before the chick hatches, expressed the hypertrophic phenotype but maintained their viability in culture for only approximately 6 days. Subsequent cell death was very consistent between cultures and shown to occur by an apoptotic process by analysis of DNA fragmentation and cell morphology. Short-term viability of hypertrophic chondrocytes was independent of culture density and relatively resistant to treatment with staurosporine. Induction of the hypertrophic phenotype in immature chondrocytes committed them to cell death and prevention of expression of the hypertrophic phenotype prevented cell death. We conclude that commitment of chondrocytes to terminal differentiation is associated with a commitment to apoptosis and apoptosis of hypertrophic chondrocytes in growth cartilage does not require initiation by external signals.

Role of Bag-1 in the survival and proliferation of the cytokine-dependent lymphocyte lines, Ba/F3 and Nb2

The expression and function of the newly identified Bcl-2- and Raf-1- binding protein, Bag-1, during the cytokine-regulated growth of B and T cell lines was examined. Immunoblot analysis of lysates from the interleukin-3 (IL-3)-dependent B cell line Ba/F3, and the PRL-dependent T cell line Nb2, revealed that variations in Bag-1 levels paralleled alterations in cellular proliferation, viability, and apoptosis induced by the presence or absence of growth factor. To test whether up-regulation of Bag-1 levels altered cellular survival and proliferation, Ba/F3 cells were transfected with a Bag-1 expression construct. The overexpression of Bag-1 in transfected Ba/F3 cells induced an IL-3-independent state. Such transfectants demonstrated sustained viability and proliferation, with minimal apoptosis, in the complete absence of exogenous IL-3. Bag-1 expression was also compared in glucocorticoid-sensitive Nb2 cells and a PRL-independent, glucocorticoid-resistant subline, SFJCD1, during culture of these lines in dexamethasone (Dex). Bag-1 levels were profoundly decreased by the addition of Dex to Nb2 cells, precedent to the onset of apoptotic cell death. In contrast, Dex treatment or PRL withdrawal had no effect on levels of Bag-1 within the SFJCD1 line. These findings establish that the overexpression of Bag-1 in the appropriate cellular context promotes cellular survival and growth, events that may result from the juxtaposition of this protein with mitogenic and antiapoptotic signaling pathways.

The common cytokine receptor gamma chain plays an essential role in regulating lymphoid homeostasis

In the immune system, there is a careful regulation not only of lymphoid development and proliferation, but also of the fate of activated and proliferating cells. Although the manner in which these diverse events are coordinated is incompletely understood, cytokines are known to play major roles. Whereas IL-7 is essential for lymphoid development, IL-2 and IL-4 are vital for lymphocyte proliferation. The receptors for each of these cytokines contain the common cytokine receptor gamma chain (gammac), and it was previously shown that gammac-deficient mice exhibit severely compromised development and responsiveness to IL-2, IL-4, and IL-7. Nevertheless, these mice exhibit an age-dependent accumulation of splenic CD4+ T cells, the majority of which have a phenotype typical of memory/activated cells. When gammac-deficient mice were mated to DO11.10 T cell receptor (TCR) transgenic mice, only the T cells bearing endogenous TCRs had this phenotype, suggesting that its acquisition was TCR dependent. Not only do the CD4+ T cells from gammac-deficient mice exhibit an activated phenotype and greatly enhanced incorporation of bromodeoxyuridine but, consistent with the lack of gammac-dependent survival signals, they also exhibit an augmented rate of apoptosis. However, because the CD4+ T cells accumulate, it is clear that the rate of proliferation exceeds the rate of cell death. Thus, surprisingly, although gammac-independent signals are sufficient to mediate expansion of CD4+ T cells in these mice, gammac-dependent signals are required to regulate the fate of activated CD4+ T cells, underscoring the importance of gammac-dependent signals in controlling lymphoid homeo-stasis.

Final checkpoint in the drug-promoted and poliovirus-promoted apoptosis is under post-translational control by growth factors

The treatment of HeLa subline (HeLa-B) cells with cycloheximide or Actinomycin D resulted in a rapid (approximately 1.5 h and approximately 2.5 h, respectively) development of morphological and biochemical signs of apoptosis. The addition of fetal bovine serum to the cycloheximide-treated or Actinomycin D-treated cells suppressed the apoptotic reaction, as evidenced by the postponement of the DNA fragmentation for at least 9 and 5 h, respectively. A similar suppressive effect was observed upon the serum addition to cells undergoing abortive infection with poliovirus, which died of apoptosis in the absence of the serum. The serum appeared to exert its anti-apoptotic effect without any appreciable lag and even immediately blocked further progress of ongoing DNA fragmentation. The epidermal growth factor also suppressed, although less efficiently and more transiently, the apoptotic reaction promoted by the metabolic inhibitors. It is concluded that growth factors may affect, without modulating either transcription or translation, the balance of pro-apoptotic and anti-apoptotic activities at a final checkpoint, just preceding the irreversible effector step of apoptosis.

Programmed cell death of peripheral myeloid precursor cells in Down patients: effect of zinc therapy

Hemopoietic stem cell differentiation represents the primary rule of self-renewal, proliferation, and specialization modulated by several mechanisms, including growth factors, cell interactions, and bioavailability of various ions, especially Ca2+ and Zn2+. Apoptotic death, during normal cell turnover, has been widely studied and is recognized as an important pathway for clonal deletion in the hemopoietic system. Multiparametric analyses have shown that subjects with Down syndrome show low levels of plasmic zinc associated with the presence of immature myeloid cells in the peripheral blood. This arrangement is repaired by in vivo zinc therapy. This study presents morphological and biochemical analyses to show that ZnSO4 therapy induces the disappearance of peripheral myeloid precursor cells by a programmed cell death mechanism. The programmed zinc-therapy-induced cell death presumably provides a simple way to regulate the myeloid differentiation selecting appropriate cells.

A mouse Fas-associated protein with homology to the human Mort1/FADD protein is essential for Fas-induced apoptosis

The Fas cell surface receptor belongs to the tumor necrosis factor receptor family and can initiate apoptosis in a variety of cell types. Using the Fas cytoplasmic domain as bait in a yeast two-hybrid screening, we isolated a mouse cDNA encoding a 205-amino-acid protein. Its predicted protein sequence shows 68% identity and 80% similarity with the sequence of recently described human Mort/FADD. This protein, most likely the mouse homolog of human FADD, associates with Fas in vivo only upon the induction of cell death. A fraction of this protein is highly phosphorylated at serine/threonine residues, with both phosphorylated and unphosphorylated forms being capable of binding to FAS. Stable expression of a truncated form of the Mort/FADD protein protects cells from Fas-mediated apoptosis by interfering with the wild-type protein-Fas interaction. Thus, mouse Mort/FADD is an essential downstream component that mediates Fas-induced apoptosis.

BCL2 regulates neural differentiation

A main function attributed to the BCL2 protein is its ability to confer resistance against apoptosis. In addition to the constitutively high expression of BCL2, caused by gene rearrangement in follicular lymphomas, elevated expression of the BCL2 gene has been found in differentiating hematopoietic, neural, and epithelial tissues. To address the question of whether the expression of BCL2 is a cause or consequence of cell differentiation, we used a human neural-crest-derived tumor cell line, Paju, that undergoes spontaneous neural differentiation in vitro. The Paju cell line displays moderate expression of BCL2, the level of which increases in parallel with further neural differentiation induced by treatment with phorbol 12-myristate 13-acetate. Transfection of normal human BCL2 cDNA in sense and antisense orientations had a dramatic impact on the differentiation of the Paju cells. Overexpression of BCL2 cDNA induced extensive neurite outgrowth, even in low serum concentrations, together with an increased expression of neuron-specific enolase. Paju cells expressing the anti-sense BCL2 cDNA construct, which reduced the endogenous levels of BCL2, did not undergo spontaneous neural differentiation. These cells acquired an epithelioid morphology and up-regulated the intermediate filament protein nestin, typically present in primitive neuroectodermal cells. The manipulated levels of BCL2 did not have appreciable impact on cell survival in normal culture. Our findings demonstrate that the BCL2 gene product participates in the regulation of neural differentiation.

Apoptosis and the maintenance of homoeostasis in the immune system

The regulation of cell proliferation and the selection against autoreactive cells in the lymphoid system both occur through the induction of apoptosis. Many of the signals that induce apoptosis in lymphocytes are now well defined. Interactions between Fas and its ligand have emerged as a major mechanism for the deletion of activated peripheral T cells and autoreactive B cells. Although the signal-transduction pathway leading from engagement of Fas to apoptosis is not entirely clear, significant advances have been made recently. There has also been progress in the elucidation of the mechanisms that regulate apoptosis in the immune system.

Dimethyl sulfoxide induces programmed cell death and reversible G1 arrest in the cell cycle of human lymphoid pre-T cell line

In human B- and T-differentiated lymphoid cell lines DMSO was found to arrest the proliferation at the G1 stage of the cell cycle, without any detectable differentiation and DMSO itself was found to prevent apoptosis. Programmed cell death, or apoptosis, is now thought to be an important regulatory process in normal hemopoiesis and in the lymphoid system this program is started in the immune process such as autoreactive T-cell elimination in the thymus, and antigen-driven B-cell selection in the terminal centre. For this purpose, we have analysed the effect of DMSO using undifferentiated pre-B (KM-3) and pre-T (RPMI-8402) human lymphoid cells. Results obtained by multiparametric analyses show that DMSO affect only the pre-T cell line inducing a reversible G1 arrest of the cell cycle with a significant presence of apoptotic cells and modification of terminal transferase (TdT) expression. Pre-B cell line is resistant to DMSO treatment. These data provide evidence of a new model for the study of the selective cell type depending effect of DMSO in the immune system.

Bax promotes neuronal survival and antagonises the survival effects of neurotrophic factors

Bcl-2, Bcl-x and Bax are members fo a family of cytoplasmic proteins that influence cell survival. Whereas increased expression of Bcl-2 or Bcl-x promotes cell survival following withdrawal of survival factors, increased expression of Bax is thought to suppress survival. To investigate the potential roles of these proteins in regulating the survival of developing neurons, we compared the effects of overexpressing these proteins in embryonic neurons deprived of different neurotrophic factors in vitro. Surprisingly, overexpression of Bax rescued populations of sensory neurons deprived of nerve growth factor, as did overexpression of Bcl-2 and two Bcl-x variants, Bcl-XL and Bcl-Xbeta. Bax also enhanced the survival of ciliary neurons deprived of ciliary neurotrophic factor, although this effect was short-lived. Whereas Bcl-2 overexpression did not affect the survival response of neurons to neurotrophic factors, Bax overexpression partially inhibited the action of neurotrophic factors. Co-injection of Bcl-2 and Bax expression vectors promoted the survival of neurotrophic factor-deprived neurons if either was in excess, but failed to rescue neurons if they injected at a 1:1 ratio. Our findings demonstrate that Bax can promote the survival of neurotrophic factor-deprived neurons and that its effect on survival is dominant to that of neurotrophic factors. Our results also argue that the relative amounts of Bcl-2 and Bax are critical in regulating neuronal survival.

The Bcl-2 family of proteins, and the regulation of neuronal survival

Bcl-2 is the founder member of a growing family of cytoplasmic proteins that modulate the responses of many cell types to the diverse extracellular signals that affect their survival. Although knowledge of the functions of these proteins has come largely from studying cells of the immune system, increasing evidence implicates these proteins in modulating neuronal survival. Several of these proteins are expressed in the nervous system, and experimental overexpression of Bcl-2 prevents the death of neurones deprived of particular neurotrophic factors in vitro, and rescues developing neurones that would otherwise die in vivo.

Reactive oxygen species as cellular messengers

Reactive oxygen species (ROSs) have recently been found to be important signaling molecules in several cellular responses. Individual species have characteristic reactive properties, yet are easily interconverted, making it difficult to identify the ROSs involved in each response.