An interleukin-1 beta-converting enzyme-like protease is a common mediator of apoptosis in thymocytes

Sperm freezing damage: the role of regulated cell death

Substantial progress in research on sperm cryopreservation has occurred since the twentieth century, especially focusing on improving sperm freezing procedures and optimizing semen extenders. However, the cellular biological mechanisms of sperm freezing damage are still unclear, which greatly restricts the promotion and development of sperm cryopreservation. An essential component of sperm freezing damage is the occurrence of cell death. Considering the existence of multiple types of cell death pathways, this review discusses connections between characteristics of regulated cell death (e.g., apoptosis and ferroptosis), and accidental cell death (e.g., intracellular ice crystals) with sperm freezing damage and explores possible future research directions in this field.

Exploring the Potent Anticancer Activity of Essential Oils and Their Bioactive Compounds: Mechanisms and Prospects for Future Cancer Therapy

Cancer is one of the leading causes of death worldwide, affecting millions of people each year. Fortunately, the last decades have been marked by considerable advances in the field of cancer therapy. Researchers have discovered many natural substances, some of which are isolated from plants that have promising anti-tumor activity. Among these, essential oils (EOs) and their constituents have been widely studied and shown potent anticancer activities, both in vitro and in vivo. However, despite the promising results, the precise mechanisms of action of EOs and their bioactive compounds are still poorly understood. Further research is needed to better understand these mechanisms, as well as their effectiveness and safety in use. Furthermore, the use of EOs as anticancer drugs is complex, as it requires absolute pharmacodynamic specificity and selectivity, as well as an appropriate formulation for effective administration. In this study, we present a synthesis of recent work on the mechanisms of anticancer action of EOs and their bioactive compounds, examining the results of various in vitro and in vivo studies. We also review future research prospects in this exciting field, as well as potential implications for the development of new cancer drugs.

Small RNA-sequencing reveals the involvement of microRNA-132 in benzo[a]pyrene-induced toxicity in primary human blood cells

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed environmental contaminants, triggering deleterious effects such as carcinogenicity and immunosuppression, and peripheral blood mononuclear cells (PBMCs) are among the main cell types targeted by these pollutants. In the present study, we sought to identify the expression profiles and function of miRNAs, gene regulators involved in major cellular processes recently linked to environmental pollutants, in PBMC-exposed to the prototypical PAH, benzo [a]pyrene (B [a]P). Using small RNA deep sequencing, we identified several B [a]P-responsive miRNAs. Bioinformatics analyses showed that their predicted targets could modulate biological processes relevant to cell death and survival. Further studies of the most highly induced miRNA, miR-132, showed that its up-regulation by B [a]P was time- and dose-dependent and required aryl hydrocarbon receptor (AhR) activation. By evaluating the role of miR-132 in B [a]P-induced cell death, we propose a mechanism linking B [a]P-induced miR-132 expression and cytochromes P-450 (CYPs) 1A1 and 1B1 mRNA levels, which could contribute to the apoptotic response of PBMCs. Altogether, this study increases our understanding of the roles of miRNAs induced by B [a]P and provides the basis for further investigations into the mechanisms of gene expression regulation by PAHs.

Stearate-derived very long-chain fatty acids are indispensable to tumor growth

Reprogramming of lipid metabolism is emerging as a hallmark of cancer, yet involvement of specific fatty acids (FA) species and related enzymes in tumorigenesis remains unclear. While previous studies have focused on involvement of long-chain fatty acids (LCFAs) including palmitate in cancer, little attention has been paid to the role of very long-chain fatty acids (VLCFAs). Here, we show that depletion of acetyl-CoA carboxylase (ACC1), a critical enzyme involved in the biosynthesis of fatty acids, inhibits both de novo synthesis and elongation of VLCFAs in human cancer cells. ACC1 depletion markedly reduces cellular VLCFA but only marginally influences LCFA levels, including palmitate that can be nutritionally available. Therefore, tumor growth is specifically susceptible to regulation of VLCFAs. We further demonstrate that VLCFA deficiency results in a significant decrease in ceramides as well as downstream glucosylceramides and sphingomyelins, which impairs mitochondrial morphology and renders cancer cells sensitive to oxidative stress and cell death. Taken together, our study highlights that VLCFAs are selectively required for cancer cell survival and reveals a potential strategy to suppress tumor growth.

Glucose-6-phosphate dehydrogenase exerts antistress effects independently of its enzymatic activity

G6PD (glucose-6-phosphate dehydrogenase) is the rate-limiting enzyme in the oxidative pentose phosphate pathway that can generate cytosolic NADPH for biosynthesis and oxidative defense. Since cytosolic NADPH can be compensatively produced by other sources, the enzymatic activity deficiency alleles of G6PD are well tolerated in somatic cells but the effect of null mutations is unclear. Herein, we show that G6PD KO sensitizes cells to the stresses induced by hydrogen peroxide, superoxide, hypoxia, and the inhibition of the electron transport chain. This effect can be completely reversed by the expressions of natural mutants associated with G6PD deficiency, even without dehydrogenase activity, exactly like the WT G6PD. Furthermore, we demonstrate that G6PD can physically interact with AMPK (AMPK-activated protein kinase) to facilitate its activity and directly bind to NAMPT (nicotinamide phosphoribosyltransferase) to promote its activity and maintain the NAD(P)H/NAD(P)⁺ homeostasis. These functions are necessary to the antistress ability of cells but independent of the dehydrogenase activity of G6PD. In addition, the WT G6PD and naturally inactive mutant also can similarly regulate the metabolism of glucose, glutamine, fatty acid synthesis, and GSH and interact with the involved enzymes. Therefore, our findings reveal the previously unidentified functions of G6PD that can act as the important physiological neutralizer of stresses independently of its enzymatic activity.

SETDB1-Mediated Cell Fate Transition between 2C-Like and Pluripotent States

Known as a histone H3K9 methyltransferase, SETDB1 is essential for embryonic development and pluripotent inner cell mass (ICM) establishment. However, its function in pluripotency regulation remains elusive. In this study, we find that under the "ground state" of pluripotency with two inhibitors (2i) of the MEK and GSK3 pathways, Setdb1-knockout fails to induce trophectoderm (TE) differentiation as in serum/LIF (SL), indicating that TE fate restriction is not the direct target of SETDB1. In both conditions, Setdb1-knockout activates a group of genes targeted by SETDB1-mediated H3K9 methylation, including Dux. Notably, Dux is indispensable for the reactivation of 2C-like state genes upon Setdb1 deficiency, delineating the mechanistic role of SETDB1 in totipotency restriction. Furthermore, Setdb1-null ESCs maintain pluripotent marker (e.g., Nanog) expression in the 2i condition. This "ground state" Setdb1-null population undergoes rapid cell death by activating Ripk3 and, subsequently, RIPK1/RIPK3-dependent necroptosis. These results reveal the essential role of Setdb1 between totipotency and pluripotency transition.

An Unusual Member of the Papain Superfamily: Mapping the Catalytic Cleft of the Marasmius oreades agglutinin (MOA) with a Caspase Inhibitor

Papain-like cysteine proteases (PLCPs) constitute the largest group of thiol-based protein degrading enzymes and are characterized by a highly conserved fold. They are found in bacteria, viruses, plants and animals and involved in a number of physiological and pathological processes, parasitic infections and host defense, making them interesting targets for drug design. The Marasmius oreades agglutinin (MOA) is a blood group B-specific fungal chimerolectin with calcium-dependent proteolytic activity. The proteolytic domain of MOA presents a unique structural arrangement, yet mimicking the main structural elements in known PLCPs. Here we present the X-ray crystal structure of MOA in complex with Z-VAD-fmk, an irreversible caspase inhibitor known to cross-react with PLCPs. The structural data allow modeling of the substrate binding geometry and mapping of the fundamental enzyme-substrate interactions. The new information consolidates MOA as a new, yet strongly atypical member of the papain superfamily. The reported complex is the first published structure of a PLCP in complex with the well characterized caspase inhibitor Z-VAD-fmk.

Crude extract of Ceriporia lacerata has a protective effect on dexamethasone-induced cytotoxicity in INS-1 cells via the modulation of PI3K/PKB activity

Excessive and/or long-term glucocorticoid therapy reduces β-cell mass and induces hyperglycemia, which contribute to the development of steroid‑induced diabetes. Ceriporia (C.) lacerata is one of the white‑rot fungi and has been used in bioremediations, such as lignocellulose degradation, in nature. The pharmacologic effect of C. lacerata on steroid-induced β-cell toxicity is not known. In this study, we evaluated the effect of a crude extract from a submerged cultivation of C. lacerata on the survival and apoptosis of INS-1 rat insulin-secreting cells exposed to dexamethasone (Dex), a synthetic diabetogenic glucocorticoid. Treatment with the C. lacerata crude extract (CLCE) largely blocked the Dex-induced reduction in survival and apoptosis of INS-1 cells. Moreover, CLCE treatment inhibited Dex-induced protein kinase B (PKB) dephosphorylation without affecting Dex-induced extracellular signal-regulated protein kinase-1/2 dephosphorylation and MKP-1 upregulation. Importantly, the protective effect of CLCE on Dex-induced cytotoxicity in INS-1 cells was attenuated by LY294002, an inhibitor of PI3K/PKB. CLCE treatment, however, did not protect the INS-1 cells from the cytotoxic effects triggered by other insults, such as interleukin-1β (an inflammatory cytokine), streptozotocin (a diabetogenic drug), thapsigargin (a calcium mobilizing agent), and tunicamycin (an ER stress inducer). Collectively, these findings demonstrate for the first time the ability of CLCE to specifically protect INS-1 cells from Dex-induced cytotoxicity through the modulation of the PI3K/PKB pathway. It is suggested that CLCE may be applied for the prevention and/or treatment of steroid diabetes in which reduction of β-cell survival and induction of β-cell apoptosis play pathogenic roles.

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.

Characterization of enantiomeric bile acid-induced apoptosis in colon cancer cell lines

Bile acids are steroid detergents that are toxic to mammalian cells at high concentrations; increased exposure to these steroids is pertinent in the pathogenesis of cholestatic disease and colon cancer. Understanding the mechanisms of bile acid toxicity and apoptosis, which could include nonspecific detergent effects and/or specific receptor activation, has potential therapeutic significance. In this report we investigate the ability of synthetic enantiomers of lithocholic acid (ent-LCA), chenodeoxycholic acid (ent-CDCA), and deoxycholic acid (ent-DCA) to induce toxicity and apoptosis in HT-29 and HCT-116 cells. Natural bile acids were found to induce more apoptotic nuclear morphology, cause increased cellular detachment, and lead to greater capase-3 and -9 cleavage compared with enantiomeric bile acids in both cell lines. In contrast, natural and enantiomeric bile acids showed similar effects on cellular proliferation. These data show that bile acid-induced apoptosis in HT-29 and HCT-116 cells is enantiospecific, hence correlated with the absolute configuration of the bile steroid rather than its detergent properties. The mechanism of LCA- and ent-LCA-induced apoptosis was also investigated in HT-29 and HCT-116 cells. These bile acids differentially activate initiator caspases-2 and -8 and induce cleavage of full-length Bid. LCA and ent-LCA mediated apoptosis was inhibited by both pan-caspase and selective caspase-8 inhibitors, whereas a selective caspase-2 inhibitor provided no protection. LCA also induced increased CD95 localization to the plasma membrane and generated increased reactive oxygen species compared with ent-LCA. This suggests that LCA/ent-LCA induce apoptosis enantioselectively through CD95 activation, likely because of increased reactive oxygen species generation, with resulting procaspase-8 cleavage.

Mitochondria, endoplasmic reticulum, and alternative pathways of cell death in critical illness

Dying cells are distinguished by their biochemical and morphologic traits and categorized into three subtypes: apoptosis, oncosis (necrosis), and cell death with autophagy. Each of these types of cell death plays critical roles in tissue morphogenesis during normal development and in the pathogenesis of human diseases. Given that tissue homeostasis is controlled by the intricate balance between degeneration and regeneration, it is essential to understand the mechanisms of different forms of cell death to establish and improve therapeutic interventions for prevention and rescue of these cell death-related disorders. Critical illness, including sepsis, trauma, and burn injury, is often complicated by multiple organ dysfunction syndrome and is accompanied by increased cell death in parenchymal and nonparenchymal tissues. Accumulating evidence suggests that augmented cell death plays an important role in the organ failure in critical illness. We discuss possible therapeutic approaches for prevention of cell death, particularly apoptotic cell death.

Inactivation of Cdk1/Cyclin B in metaphase-arrested mouse FT210 cells induces exit from mitosis without chromosome segregation or cytokinesis and allows passage through another cell cycle

It is well known that inactivation of Cdk1/Cyclin B is required for cells to exit mitosis. The work reported here tests the hypothesis that Cdk1/Cyclin B inactivation is not only necessary but also sufficient to induce mitotic exit and reestablishment of the interphase state. This hypothesis predicts that inactivation of Cdk1 in metaphase-arrested cells will induce the M to G1-phase transition. It is shown that when mouse FT210 cells (in which Cdk1 is temperature-sensitive) are arrested in metaphase and then shifted to their non-permissive temperature, they rapidly exit mitosis as evidenced by reassembly of interphase nuclei, decondensation of chromosomes, and dephosphorylation of histones H1 and H3. The resulting interphase cells are functionally normal as judged by their ability to progress through another cell cycle. However, they have double the normal number of chromosomes because they previously bypassed anaphase, chromosome segregation, and cytokinesis. These results, taken together with other observations in the literature, strongly suggest that in mammalian cells, inactivation of Cdk1/cyclin B is the trigger for mitotic exit and reestablishment of the interphase state.

Bcl-2 sensitive mitochondrial potassium accumulation and swelling in apoptosis

During etoposide-induced apoptosis in HL-60 cells, cytochrome c release was associated with mitochondrial swelling caused by increased mitochondrial potassium uptake. The mitochondrial permeability transition was also observed; however, it was not the primary cause of mitochondrial swelling. Potassium uptake and swelling of mitochondria were blocked by bcl-2 overexpression. As a result, cytochrome c release was reduced, and apoptosis delayed. Residual cytochrome c release in the absence of swelling in bcl-2 expressing cells could be due to observed Bax translocation into mitochondria. This study suggests several novel aspects of apoptotic signaling: (1) potassium related swelling of mitochondria; (2) inhibition of mitochondrial potassium uptake by bcl-2; (3) co-existence within one system of multiple mechanisms of cytochrome c release: mitochondrial swelling and swelling-independent permeabilization of the outer mitochondrial membrane.

Vulnerability of central neurons to secondary insults after in vitro mechanical stretch

Mild traumatic brain injuries are of major public health significance. Neurons in such injuries often survive the primary mechanical deformation only to succumb to subsequent insults. To study mechanisms of vulnerability of injured neurons to secondary insults, we used an in vitro model of sublethal mechanical stretch. Stretch enhanced the vulnerability of the neurons to excitotoxic insults, causing nuclear irregularities, DNA fragmentation, and death suggestive of apoptosis. However, the DNA degradation was not attributable to classical (caspase mediated) or caspase-independent apoptosis. Rather, it was associated with profound stretch-induced mitochondrial dysfunction and the overproduction of reactive oxygen species (ROS). Sublethally stretched neurons produced surprisingly high levels of ROS, but these in isolation were insufficient to kill the cells. To be lethal, the ROS also needed to combine with nitric oxide (NO) to form the highly reactive species peroxynitrite. Peroxynitrite was not produced after stretch alone and arose only after combining stretch with an insult capable of stimulating NO production, such as NMDA or an NO donor. This explained the exquisite sensitivity of sublethally stretched neurons to a secondary NMDA insult. ROS scavengers and NO synthase (NOS) inhibitors prevented cell death and DNA degradation. Moreover, inhibiting neuronal NOS activation by NMDA using peptides that perturb NMDA receptor-postsynaptic density-95 interactions also reduced protein nitration and cell death, indicating that the reactive nitrogen species produced were neuronal in origin. Our data explain the mechanism of enhanced vulnerability of sublethally injured neurons to secondary excitotoxic insults and highlight the importance of secondary mechanisms to the ultimate outcome of neurons in mild neurotrauma.

Physiological and pathological caspase cleavage of the neuronal RasGEF GRASP-1 as detected using a cleavage site-specific antibody

Caspases are proteases involved in various physiological and pathological processes in the nervous system, including development and pathogenesis. GRASP-1 is a recently identified neuronal substrate of caspase-3-subfamily caspases. It is a Ras-guanine nucleotide exchange factor (RasGEF) that interacts with the glutamate receptor interacting protein (GRIP). This alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor/GRIP protein complex has been proposed to be involved in AMPA receptor synaptic targeting. The caspase-3 cleavage of GRASP-1 separates the N-terminal RasGEF catalytic domain from the C-terminal GRIP-interacting region, potentially disrupting regulation of the RasGEF activity by GRIP. To examine the regulation and regional distribution of the caspase-3 cleavage of GRASP-1 in vivo, we generated a cleavage site-specific antibody, termed CGP, against the cleaved N-terminal fragment of GRASP-1. Using this antibody, we have examined the caspase cleavage of GRASP-1 during postnatal development and following ischemia in mice. We found that caspase cleavage of GRASP-1 occurs in specific brain regions in a time-dependent manner during development and ischemia. This data provides an important account of the brain areas that might require caspase-3 activity in postnatal development and ischemic damage, which has not been documented. It also demonstrates that the CGP antibody is a powerful tool for studying neuronal activity of the caspase-3-subfamily caspases in vivo.

Proliferation and apoptosis in the developing human neocortex

The cell kinetics of the developing central nervous system (CNS) is determined by both proliferation and apoptosis. In the human neocortex at week 6 of gestation, proliferation is confined to the ventricular zone, where mitotic figures and nuclear immunoreactivity for proliferating cell nuclear antigen (PCNA) are detectable. Cell division is symmetric, with both daughter cells reentering mitosis. At week 7, the subventricular zone, a secondary proliferative zone, appears. It mainly gives rise to local circuit neurons and glial cells. Around week 12, the ventricular and subventricular zones are thickest, and the nuclear PCNA label is strongest, indicating that proliferation peaks at this stage. Thereafter, asymmetric division becomes the predominant mode of proliferation, with one daughter cell reentering mitosis and the other one migrating out. Towards late gestation, the ventricular and subventricular zones almost completely disappear and proliferation shifts towards the intermediate and subplate zones, where mainly glial cells are generated. A remnant of the subventricular zone with proliferative activity persists into adulthood. In general, proliferation follows a latero-medial gradient in the neocortex lasting longer in its lateral parts. Apoptotic nuclei have been detected around week 5, occurring in low numbers in the ventricular zone at this stage. Apoptotic cell death increases around midgestation and then spreads throughout all cortical layers, with most dying cells located in the ventricular and subventricular zones. This spatial distribution of apoptosis extends into late gestation. During the early postnatal period, most apoptotic cells are still located in the subcortical layers. During early embryonic development, proliferation and apoptosis are closely related, and are probably regulated by common regulators. In the late fetal and early postnatal periods, when proliferation has considerably declined in all cortical layers, apoptosis may occur in neurons whose sprouting axons do not find their targets.

Evidence for involvement of calpain in c-Myc proteolysis in vivo

Precise control of the level of c-Myc protein is important to normal cellular homeostasis, and this is accomplished in part by degradation through the ubiquitin-proteasome pathway. The calpains are a family of calcium-dependent proteases that play important roles in proteolysis of some proteins, and their possible participation in degradation of intracellular c-Myc was therefore investigated. Activation of calpain with the cell-permeable calcium ionophore A23187 in Rat1a-myc or ts85 cells in culture induced rapid cleavage of c-Myc. This degradation was both calpain- and calcium-dependent since it was inhibited by preincubation with either the calpain-inhibitory peptide calpeptin or the calcium-chelating agent EGTA. A23187-induced c-Myc cleavage occurred in a time-dependent manner comparable to that of FAK, a known calpain substrate, and while calpeptin was able to significantly protect c-Myc from degradation, inhibitors of the proteasome or caspase proteases could not. Exposure of Rat1a-myc or ts85 cells in culture to calpeptin, or to the thiol-protease inhibitor E64d, resulted in the accumulation of c-Myc protein without an impact on ubiquitin-protein conjugates. Using an in vitro assay, calpain-mediated degradation occurred rapidly with wild-type c-Myc as the substrate, but was significantly prolonged in some c-Myc mutants with increased transforming activity derived from lymphoma patients. Those mutants with a prolonged half-life in vitro were also more resistant to A23187-induced cleavage in intact cells. These studies support a role for calpain in the control of c-Myc levels in vivo, and suggest that mutations impacting on sensitivity to calpain may contribute to c-Myc-mediated tumorigenesis.

Irofulven induces apoptosis in breast cancer cells regardless of caspase-3 status

Caspase-3 deficiency can limit the efficiency of pro-apoptotic anticancer treatments. Irofulven (hydroxymethylacyl-fulvene, HMAF. MGI 114, NSC 683863) is an antitumor drug, currently in a Phase III and multiple Phase II trials, which can differentiate between tumor and normal cells in apoptosis induction. This study investigated whether apoptosis induced by irofulven requires caspase-3. Irofulven action was compared in breast cancer cells differing in caspase-3 status: deficient MCF-7 cells and proficient MDA-MB-231 cells and in normal human mammary epithelial cells, HMEC. Irofulven induces significant, concentration and time-dependent apoptotic DNA fragmentation in breast cancer cell lines, regardless of caspase-3 status. After 12, 24 and 48 h incubation at 1 microM irofulven (approximately 3 x GI50), fragmented DNA comprised 3.7, 14.1 and 34.6% and 8.4, 12.6 and 20.3% of total DNA in MCF-7 and MDA-MB-231 cells, respectively. Cell viability (trypan blue exclusion) remained largely unaffected during the first 24 h but decreased markedly after 48 h, indicating secondary necrosis. Net losses in cell numbers were apparent at 48 h. Normal HMEC cells were refractory to 1 microM drug with only approximately 3-9% fragmented DNA after 12-48 h, although apoptosis was observed at drug levels >3 microM. The broad-spectrum caspase inhibitor Z-VAD-fmk inhibited irofulven-induced apoptosis of all cell lines at 20 microM with nearly complete abrogation of apoptosis at 100 microM. Irofulven treatment resulted in marginal caspase-3 processing in MDA-MB-231 and HMEC cells. These results indicate that whereas the caspase cascade mediates irofulven- induced apoptosis, caspase-3 is dispensable (supported by NIH CA70091 and CA78706).

Caspase inhibition supports proper gene expression in ex vivo mouse limb cultures

We standardized conditions for ex vivo mouse limb culture to study cartilage maturation and joint formation. We compared 12.5 d.p.c. mouse forelimbs that were cultured either mounted or freely rotating for up to 72 h. Limb outgrowth progressed ex vivo at a variable rate as compared to its development in vivo, spanning approximately 48 h. Although cartilage maturation and joint formation developed grossly normal, aberrant expression of skeletal marker genes was seen. Interestingly, no regression of the interdigital webs took place in mounted cultures, in contrast to limited webbing under freely rotating conditions. Caspase inhibition, by addition of zVAD-fmk to the culture medium of freely rotating limbs, supported proper gene expression associated with skeletal development, and prevented interdigital web regression. Taken together, a freely rotating ex vivo culture for mouse limb outgrowth that is combined with caspase inhibition provides a good model to study cartilage maturation and joint formation.

Role of PI 3-kinase, Akt and Bcl-2-related proteins in sustaining the survival of neurotrophic factor-independent adult sympathetic neurons

By adulthood, sympathetic neurons have lost dependence on NGF and NT-3 and are able to survive in culture without added neurotrophic factors. To understand the molecular mechanisms that sustain adult neurons, we established low density, glial cell-free cultures of 12-wk rat superior cervical ganglion neurons and manipulated the function and/or expression of key proteins implicated in regulating cell survival. Pharmacological inhibition of PI 3-kinase with LY294002 or Wortmannin killed these neurons, as did dominant-negative Class IA PI 3-kinase, overexpression of Rukl (a natural inhibitor of Class IA PI 3-kinase), and dominant-negative Akt/PKB (a downstream effector of PI 3-kinase). Phospho-Akt was detectable in adult sympathetic neurons grown without neurotrophic factors and this was lost upon PI 3-kinase inhibition. The neurons died by a caspase-dependent mechanism after inhibition of PI 3-kinase, and were also killed by antisense Bcl-xL and antisense Bcl-2 or by overexpression of Bcl-xS, Bad, and Bax. These results demonstrate that PI 3-kinase/Akt signaling and the expression of antiapoptotic members of the Bcl-2 family are required to sustain the survival of adult sympathetic neurons.

Oncogenic Ras blocks anoikis by activation of a novel effector pathway independent of phosphatidylinositol 3-kinase

Activated Ras, but not Raf, causes transformation of RIE-1 rat intestinal epithelial cells, demonstrating the importance of Raf-independent effector signaling in mediating Ras transformation. To further assess the contribution of Raf-dependent and Raf-independent function in oncogenic Ras transformation, we evaluated the mechanism by which oncogenic Ras blocks suspension-induced apoptosis, or anoikis, of RIE-1 cells. We determined that oncogenic versions of H-, K-, and N-Ras, as well as the Ras-related proteins TC21 and R-Ras, protected RIE-1 cells from anoikis. Surprisingly, our analyses of Ras effector domain mutants or constitutively activated effectors indicated that activation of Raf-1, phosphatidylinositol 3-kinase (PI3K), or RalGDS alone is not sufficient to promote Ras inhibition of anoikis. Treatment of Ras-transformed cells with the U0126 MEK inhibitor caused partial reversion to an anoikis-sensitive state, indicating that extracellular signal-regulated kinase activation contributes to inhibition of anoikis. Unexpectedly, oncogenic Ras failed to activate Akt, and treatment of Ras-transformed RIE-1 cells with the LY294002 PI3K inhibitor did not affect anoikis resistance or growth in soft agar. Thus, while important for Ras transformation of fibroblasts, PI3K may not be involved in Ras transformation of RIE-1 cells. Finally, inhibition of epidermal growth factor receptor kinase activity did not overcome Ras inhibition of anoikis, indicating that this autocrine loop essential for transformation is not involved in anoikis protection. We conclude that a PI3K- and RalGEF-independent Ras effector(s) likely cooperates with Raf to confer anoikis resistance upon RIE-1 cells, thus underscoring the complex nature by which Ras transforms cells.

Elevated extracellular [K+] inhibits death-receptor- and chemical-mediated apoptosis prior to caspase activation and cytochrome c release

Efflux of intracellular K(+) and cell shrinkage are features of apoptosis in many experimental systems, and a regulatory role has been proposed for cytoplasmic [K(+)] in initiating apoptosis. We have investigated this in both death-receptor-mediated and chemical-induced apoptosis. Using Jurkat T cells pre-loaded with the K(+) ion surrogate (86)Rb(+), we have demonstrated an efflux of intracellular K(+) during apoptosis that was concomitant with, but did not precede, other apoptotic changes, including phosphatidylserine externalization, mitochondrial depolarization and cell shrinkage. To further clarify the role of K(+) ions in apoptosis, cytoprotection by elevated extracellular [K(+)] was studied. Induction of apoptosis by diverse death-receptor and chemical stimuli in two cell lines was inhibited prior to phosphatidylserine externalization, mitochondrial depolarization, cytochrome c release and caspase activation. Using a cell-free system, we have demonstrated a novel mechanism by which increasing [K(+)] inhibited caspase activation. In control dATP-activated lysates, Apaf-1 oligomerized to a biologically active caspase processing approximately 700 kDa complex and an inactive approximately 1.4 MDa complex. Increasing [K(+)] inhibited caspase activation by preventing formation of the approximately 700 kDa complex, but not of the inactive complex. Thus intracellular and extracellular [K(+)] markedly affect caspase activation and the initiation of apoptosis induced by both death-receptor ligation and chemical stress.

Cathepsin inhibition induces apoptotic death in human leukemia and lymphoma cells

We examined the effects of cathepsin inhibitor 1 (CATI-1), a selective inhibitor of cysteine cathepsins, on human leukemia and lymphoma cells. CATI-1 induced apoptosis in all 12 cell lines tested. Apoptosis of CATI-1-treated leukemia/lymphoma cells was caspase-independent, p53-independent, BAX-independent as well as MAP kinase-independent. Our findings provide unprecedented experimental evidence that cathepsins play a pivotal role for the survival of human leukemia/lymphoma cells. Therefore, cathepsin inhibitors may provide the basis for new treatment programs against leukemia and lymphoma.

Establishment of a high-throughput screening system for caspase-3 inhibitors

In most tissues, apoptosis plays a pivotal role in normal development and for regulating cell number, thus inappropriate apoptosis underlies a variety of diseases. Caspase-3 is one of a family of caspases that are mainly involved in the apoptotic signal transduction pathway, where caspase-3 acts as an effect molecule to proteolytically cleave intracellular substrates that are necessary for maintaining cell survival. Recent evidences show that apoptotic cell death can be blocked by inhibiting caspase-3, suggesting its inhibitors have potential to be therapeutic drugs for the diseases related with inappropriate apoptosis. We have established a screening system to search caspase-3 inhibitors from chemical libraries stocked in our institute. The enzyme assay is configured entirely in 96-well format, which is easily adapted for high throughput screening. Before performing mass screening, 80 in-house compounds were screened as a preliminary experiment, and we found that morin hydrate inhibited caspase-3 by 66.4% at the final concentration of 20 microM.

Caspase-induced proteolysis of the cyclin-dependent kinase inhibitor p27Kip1 mediates its anti-apoptotic activity

The caspase-mediated cleavage of a limited number of cellular proteins is a common feature of apoptotic cell death. This cleavage usually inhibits the function of the target protein or generates peptides that actively contribute to the death process. In the present study, we demonstrate that the cyclin-dependent kinase inhibitor p27Kip1 is cleaved by caspases in human leukemic cells exposed to apoptotic stimuli. We have shown recently that p27Kip1 overexpression delayed leukemic cell death in response to cytotoxic drugs. In transient transfection experiments, the p23 and the p15 N-terminal peptides generated by p27Kip1 proteolysis demonstrate an anti-apoptotic effect similar to that induced by the wild-type protein, whereas cleavage-resistant mutants have lost their protective effect. Moreover, stable transfection of a cleavage-resistant mutant of p27Kip1 sensitizes leukemic cells to drug-induced cell death. Altogether, these results indicate that proteolysis of p27Kip1 triggered by caspases mediates the anti-apoptotic activity of the protein.

Caspases-3 and -7 are activated in goniothalamin-induced apoptosis in human Jurkat T-cells

Goniothalamin, a plant styrylpyrone derivative isolated from Goniothalamus andersonii, induced apoptosis in Jurkat T-cells as assessed by the externalisation of phosphatidylserine. Immunoblotting showed processing of caspases-3 and -7 with the appearance of their catalytically active large subunits of 17 and 19 kDa, respectively. Activation of these caspases was further evidenced by detection of poly(ADP-ribose) polymerase cleavage (PARP). Pre-treatment with the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone (Z-VAD.FMK) blocked apoptosis and the resultant cleavage of these caspases and PARP. Our results demonstrate that activation of at least two effector caspases is a key feature of goniothalamin-induced apoptosis in Jurkat T-cells.

Sequential activation of caspase-1 and caspase-3-like proteases during apoptosis in myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are a group of hematopoietic disorders characterized by the concomitant presence of peripheral cytopenias and normocellular to hypercellular BM. This paradox has been proposed to be due to the presence of excessive proliferation matched by excessive intramedullary apoptosis of hematopoietic cells. When cultured in vitro MDS BM mononuclear cells (BMMC) undergo apoptosis within 4 h. We measured caspase-1-like and caspase-3-like activity in 22 MDS and 4 normal BM immediately following cell separation or after 4 h culture. When cultured in vitro, MDS BMMC demonstrated an increased apoptotic index within 4 h as measured by in situ end-labeling of fragmented DNA that was matched by a concurrent increase in caspase-3-like specific activity, and the two were significantly correlated. During the 4 h culture, a sequential activation of caspase-1-like and caspase-3-like activities was detected. Caspase-1-like specific activity was detected early and transiently at approximately 15 min, followed by a gradual increase in caspase-3-like-specific activity peaking at 2 h. When the broad-spectrum caspase inhibitor, Z-VAD.FMK, was included in the MDS BM aspirate 4 h culture, apoptosis was attenuated. We conclude that sequential activation of caspase-1-like and caspase-3-like activities may form the central biochemical pathway of apoptosis in BMMC from some MDS patients, and prevention of this process by caspase inhibitors may be of significant therapeutic value for these patients, in whom supportive care continues to be the mainstay of therapy.

The zinc finger protein A20 interacts with a novel anti-apoptotic protein which is cleaved by specific caspases

A20 is a Cys2/Cys2 zinc finger protein which is induced by a variety of inflammatory stimuli and which has been characterized as an inhibitor of cell death by a yet unknown mechanism. In order to clarify its molecular mechanism of action, we used the yeast two-hybrid system to screen for proteins that interact with A20. A cDNA fragment was isolated which encoded a portion of a novel protein (TXBP151), which was recently found to be a human T-cell leukemia virus type-I (HTLV-I) Tax-binding protein. The full-length 2386 bp TXBP151 mRNA encodes a protein of 86 kDa. Like A20, overexpression of TXBP151 could inhibit apoptosis induced by tumour necrosis factor (TNF) in NIH3T3 cells. Moreover, transfection of antisense TXBP151 partially abolished the anti-apoptotic effect of A20. Furthermore, apoptosis induced by TNF or CD95 (Fas/APO-1) was associated with proteolysis of TXBP151. This degradation could be inhibited by the broad-spectrum caspase inhibitor zVAD-fmk or by expression of the cowpox virus-derived inhibitor CrmA, suggesting that TXBP151 is a novel substrate for caspase family members. TXBP151 was indeed found to be specifically cleaved in vitro by members of the caspase-3-like subfamily, viz. caspase-3, caspase-6 and caspase-7. Thus TXBP151 appears to be a novel A20-binding protein which might mediate the anti-apoptotic activity of A20, and which can be processed by specific caspases.

Changes in nuclear morphology during apoptosis correlate with vimentin cleavage by different caspases located either upstream or downstream of Bcl-2 action

BACKGROUND

Upon Fas stimulation, procaspase-8 is recruited to the death-inducing signalling complex where autoactivation of caspase-8 occurs. Active caspase-8 can directly activate downstream caspases (e.g. caspase-3, 6, and 7) for the execution of apoptosis (mitochondria-independent pathway), while caspase-8 can also lead to executioner caspase activation through mitochondrial damage (mitochondria-dependent pathway). Caspase activation results in the dismantling of intracellular structure through specific proteolysis.

RESULTS

We have found that an intermediate filament protein, vimentin, is cleaved at multiple sites by caspases at an early stage of apoptosis in Jurkat cells. The sequences of the two major cleavage sites in vimentin (IDVD/V and DSVD/F) suggested that these sites are cleaved by caspase-8 and caspase-3, respectively, or by close homologues of these proteases. The IDVD/V site can be cleaved by caspase-8 in vitro, and its cleavage is less sensitive to DEVD-CHO and Bcl-2 over-expression than that of the DSVD/F site in Jurkat cells. Over-expression of a mutant vimentin which was insensitive to caspase cleavage at these sites delayed the appearance of apoptotic nuclei in Jurkat cells.

CONCLUSION

The specific cleavage of vimentin can be used as an apoptotic marker of both apical- and mitochondria-dependent caspase activation. Apoptotic cleavage of vimentin most likely results in disruption of its filamentous structure, which may facilitate nuclear condensation and subsequent fragmentation through disruption of the cytoskeletal network.

Caspase-3 is necessary and sufficient for cleavage of protein synthesis eukaryotic initiation factor 4G during apoptosis

Induction of apoptosis BJAB cells is accompanied by the rapid cleavage of protein synthesis eukaryotic initiation factor 4G and the appearance of a fragment of approximately 76 kDa. Inhibition of apoptotic proteases (caspases) has previously been shown to prevent the cleavage of eukaryotic initiation factor 4G. In MCF-7 breast carcinoma cells, which are deficient in caspase-3, eukaryotic initiation factor 4G is not cleaved but in vivo expression of caspase-3 restores eukaryotic initiation factor 4G cleavage following induction of apoptosis. Recombinant caspase-3 can also cleave eukaryotic initiation factor 4G to yield the 76 kDa fragment both in cell extracts and when the eukaryotic initiation factor 4G is presented in a purified eukaryotic initiation factor 4F complex. These results indicate that caspase-3 activity is necessary and sufficient for eukaryotic initiation factor 4G degradation.

Benzodiazepine receptor agonists modulate thymocyte apoptosis through reduction of the mitochondrial transmembrane potential

Peripheral-type benzodiazepines have been shown to exert immunological effects. In this study, we examined the effects of the peripheral-type benzodiazepines on murine thymocytes. Murine thymocytes that were incubated with the peripheral-type benzodiazepines underwent apoptosis associated with the collapse of mitochondrial transmembrane potential (delta psi(m)). The drugs stimulated dexamethasone- and etoposide-induced apoptosis with the enhanced collapse of delta psi(m). The central-type benzodiazepines had no effect on either the delta psi(m) or apoptosis. The reduction of delta psi(m) depended on protein synthesis and protein phosphorylation. These results suggest that the immunomodulating effect of benzodiazepines is in part due to the modulation of thymocyte apoptosis associated with the collapse of delta psi(m).

Two types of death of poliovirus-infected cells: caspase involvement in the apoptosis but not cytopathic effect

The death of poliovirus-infected cells may occur in two forms: canonical cytopathic effect (CPE) (on productive infections) or apoptosis (when the viral reproduction is hindered by certain drugs or some other restrictive conditions). Morphological manifestations of the CPE and apoptosis, being distinct, share some traits (e.g., chromatin condensation and nuclear deformation). It was shown here that a permeable caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone (zVAD.fmk), prevented the development of the poliovirus-induced apoptosis on abortive infection. The apoptotic pathway could be dissected by an inhibitor of chymotrypsin-like serine proteases, N-tosyl-l-phenylalanine chloromethyl ketone (TPCK), which prevented the cleavage of DNA to oligonucleosome-sized pieces and nuclear fragmentation but did not suppress cellular shrinkage, cytoplasmic blebbing, and partial chromatin condensation. These results demonstrate that caspase activation is involved in the execution phase of the viral apoptosis and suggest that a nuclear subset of the apoptotic program is under a separate control, involving a TPCK-sensitive event. Neither zVAD.fmk nor TPCK, at the concentrations affecting the apoptotic response, exerted appreciable influence on the virus growth or cellular pathological changes on productive infection, indicating that the pathways leading to the poliovirus-evoked CPE and apoptosis are different.

Role of superoxide in apoptosis induced by growth factor withdrawal

We have examined the role of reactive oxygen species (ROS) in apoptosis induced by growth factor deprivation in primary cultures of mouse proximal tubular (MPT) cells. When confluent monolayers of MPT cells are deprived of all growth factors, the cells die by apoptosis over a 10- and 14-day period. Both epidermal growth factor (EGF) and high-dose insulin directly inhibit apoptosis of MPT cells deprived of growth factors. Growth factor deprivation results in an increase in the cellular levels of superoxide anion while apoptosis of MPT cells induced by growth factor withdrawal is inhibited by a number of antioxidants and scavengers of ROS. Growth factor deprivation also results in activation of caspase activity, which is inhibited by EGF and high-dose insulin as well as by the ROS scavengers and antioxidants that inhibit apoptosis. The cell-permeant caspase inhibitor, z-Val-Ala-Asp-CH2F (zVAD-fmk), prevents the increase in caspase activity and markedly inhibits apoptosis induced by growth factor deprivation. However, zVAD-fmk had no effect on the increased levels of superoxide associated with growth factor deprivation. Thus we provide novel evidence that ROS play an important role in mediating apoptosis associated with growth factor deprivation. ROS appear to act upstream of caspases in the apoptotic pathway. We hypothesize that oxidant stress, induced by growth factor withdrawal, represents a signaling mechanism for the default pathway of apoptosis.

The role of caspases in apoptosis

The process of apoptosis, or programmed cell death, is fundamental during normal development and homeostasis and aberrant apoptosis has been implicated in a number of human diseases. The cellular machinery involved in the execution of apoptosis includes a family of cysteine proteases termed caspases. Caspases exhibit the rare substrate preference of cleavage C-terminal to aspartate residues, a property shared only by the cytotoxic lymphocyte serine protease, granzyme B. Experimental evidence demonstrates a vital role for caspase activation in the apoptotic pathway, and, as such, caspases are a target for the development of agents that can modulate their activity. This article reviews the members of the caspase family and the role that each contributes to the execution of cell death induced by apoptotic stimuli.

Caspase-Independent Cell Death Induced by Anti-CD2 or Staurosporine in Activated Human Peripheral T Lymphocytes

We examined the effects of the cell-permeable, broad spectrum peptide caspase inhibitors, benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD.fmk), and BOC-Asp(OMe)-fluoromethyl ketone (BOC-D.fmk), on apoptosis induced by anti-CD2, anti-Fas, and the protein kinase inhibitor staurosporine in activated human peripheral T lymphocytes. We monitored ultrastructural, flow cytometric, and biochemical apoptotic changes, including externalization of phosphatidylserine, cleavage of poly(ADP-ribose) polymerase (PARP) and lamins, activation of caspase-3 and caspase-7, decrease in mitochondrial membrane potential, and DNA fragmentation. Z-VAD.fmk and BOC-D.fmk completely inhibited all the biochemical and ultrastructural changes of apoptosis in anti-Fas-treated cells. In marked contrast, neither Z-VAD.fmk nor BOC-D.fmk inhibited CD2- or staurosporine-mediated cell shrinkage, dilatation of the endoplasmic reticulum (seen in anti-CD2-treated cells), externalization of phosphatidylserine, and loss of mitochondrial membrane potential that accompanied cell death. However, these inhibitors did inhibit the cleavage of PARP and lamins and the formation of hypodiploid cells, and partially inhibited chromatin condensation. These results demonstrate that in activated T cells, anti-CD2 and staurosporine induce a caspase-independent cell death pathway that exhibits prominent cytoplasmic features of apoptosis. However, caspase activation is required for the proteolytic degradation of nuclear substrates such as PARP and lamins together with the DNA fragmentation and extreme chromatin condensation that occur in apoptotic cells.

Inhibition of poly(ADP-ribose) polymerase rescues human T lymphocytes from methylmercury-induced apoptosis

The objective of this investigation was to determine the role of poly(ADP-ribose) polymerase (PARP) in methylmercuric chloride (MeHgCl)-induced T-cell apoptosis. Following exposure of human T-cells to 2.5 microM MeHgCl, we observed PARP activation within 45 min. Maximal activation was observed at 90 min after MeHgCl treatment; thereafter, PARP activity declined. The loss in enzyme activity was coincidental with the cleavage of 116-kDa intact PARP protein to an 85-kDa fragment. To address the relationship between PARP activation and induction of apoptosis, we first examined the redox status of T cells treated with MeHgCl. We found that exposure of T cells to low concentrations of this toxicant resulted in decreased levels of reduced pyridine nucleotides and an increase in the relative amounts of oxidized flavoproteins. Thus, the possibility exists that activation of PARP leads to NAD+ depletion and thereby alters mitochondrial redox status. To determine if PARP activation is indeed part of the proapoptotic (destructive) response or a component of the antiapoptotic (protective) response, we employed two inhibitors: 3-aminobenzamide and nicotinamide. Pretreatment of T cells with these inhibitors protected cells from MeHgCl-induced apoptosis; this was seen as a reduction in the uptake of Hoechst 33258 and DNA fragmentation. Moreover, these inhibitors blocked MeHgCl-induced oxidative stress as evidenced by a reduction in reactive oxygen species (ROS) generation. These agents, however, failed to block MeHgCl-dependent decline in mitochondrial transmembrane potential (delta psi m). We conclude that PARP activation leads to proapoptotic events that contribute to MeHgCl-induced cell death.

TNF, apoptosis and autoimmunity: a common thread?

A subset of cytokine mediators belonging to the tumor necrosis factor (TNF) family cause apoptosis, acting through receptors and signaling pathways that have recently come to light. Further, at least one autoimmune disease results from a defined defect of apoptosis (mutations of the Fas ligand or its receptor). It is offered that many, and perhaps most autoimmune diseases may result from primary defects of apoptosis. Such defects may cause reflexive overproduction of TNF and other pro-apoptotic cytokines. The collateral damage produced by these mediators may be of pathogenetic importance in complex autoimmune disorders such as rheumatoid arthritis and Crohn disease, wherein TNF blockade is known to have ameliorative effects.

STAT1 is inactivated by a caspase

Apoptosis involves the activation of a cascade of interleukin-1beta converting enzyme-like proteases (caspases), a group of cysteine proteases related to the prototype interleukin-1beta-converting enzyme (caspase-1). These proteases cleave specific intracellular targets such as poly(ADP-ribose) polymerase, DNA-dependent protein kinase, and nuclear lamins. We show here that apoptosis can be induced by double-stranded RNA. The induction of apoptosis by double-stranded RNA and other agents leads to the cleavage by a caspase of the signal transducer and activator of transcription factor, STAT1 which is pivotal in the signal transduction pathways of the interferons and many other cytokines and growth factors. The product of this cleavage is no longer able to mediate interferon-activated signal transduction and the cleavage event may play a role in regulating the apoptosis response itself.

Inhibition of caspase activity induces a switch from apoptosis to necrosis

The role of caspases in B lymphocyte cell death was investigated by using two broad spectrum inhibitors of the caspase family, Z-Asp-cmk and Z-VAD-fmk. They totally prevented spontaneous and drug-induced apoptosis and inhibited the CPP32/caspase-3-like activity exhibited by apoptotic cells. However, the suppression of apoptosis was not associated with a long-term increase of cell survival, but conversely, with a switch from apoptotic death to the necrotic form. These results strongly suggest that apoptosis and necrosis share common initiation pathways, the final issue being determined by the presence of an active caspase.

Antigen receptor-induced B lymphocyte apoptosis mediated via a protease of the caspase family

An extensive body of data, in a variety of systems, denoted the caspase family of proteases as a key player in the execution of programmed cell death. This family consists of cysteine proteases that cleave after asparagine-containing motifs. It is well established that the caspases are essential for the apoptosis mediated by Fas (CD95) and TNF receptor p55, molecules that contain the "death domain" in the cytoplasmic tail. However, little is known about the mechanisms underlying the antigen receptor-mediated cell death in B lymphocytes, a process instrumental in negative selection of potentially autoreactive B cells. Here, we investigated the involvement of caspases in cell death triggered via the antigen receptor in B lymphocytes (BCR) by using specific inhibitors. Initially, we used a well-established cell line, CH31, which is a model of B cell tolerance, to demonstrate that these proteases indeed participate in the BCR-induced apoptotic pathway. Next, we confirmed the physiological relevance of the caspase-mediated cell death pathway in splenic B cell populations isolated ex vivo that were induced to undergo apoptosis by extensive cross-linking of their BCR. Most interestingly, our data demonstrated that caspases regulate not only the nuclear DNA fragmentation, but also the surface membrane phosphatidylserine translocation as well as the degradation of a specific nuclear substrate. Taken together, this report supports the hypothesis that regulation of the caspase family is crucial in controlling the life/death decision in B lymphocytes mediated by the antigen receptor signal transduction.

ICE-like protease (caspase) is involved in transforming growth factor beta1-mediated apoptosis in FaO rat hepatoma cell line

Transforming growth factor-beta1 (TGF-beta1) arrests growth and/or stimulates apoptosis of a variety of cells. The biochemical pathways involved in the apoptotic processes, however, remain poorly defined. TGF-beta1 induces DNA fragmentation together with morphological changes, which are characteristic of apoptosis in the FaO rat hepatoma cell line. Histones were remarkably enriched in lysates of these cells during TGF beta1-induced apoptosis. We identified U1-70 kd as a death substrate which is cleaved following TGF-beta1 treatment. The tetrapeptide caspase inhibitor carbobenzoxy-valyl-alanly-aspartyl-(beta-O-methyl)-fluoromethyl ketone (ZVAD-FMK) prevented TGF beta1-induced apoptotic DNA fragmentation and cleavage of the U1-70 kd protein, showing that caspase(s) are involved in TGF beta1-mediated apoptosis. To identify specific caspases involved in apoptosis induced by TGF-beta1 in FaO cells, proteolytic activation of several of these caspases and their substrates were studied as a function of time following TGF beta1-treatment. TGF beta1-treatment induced the progressive proteolytic processing of caspase-2 (ICH-1L/Nedd-2), whereas caspase-1 itself did not show any cleavage from the precursor. Pretreatment with ZVAD-FMK abrogated the maturation of caspase-2 and blocked the apoptotic progress. These results suggest that caspase-2, but not caspase-1, may play a crucial role in TGF beta1-induced apoptosis in these cells.

Human IAP-like protein regulates programmed cell death downstream of Bcl-xL and cytochrome c

The gene encoding human IAP-like protein (hILP) is one of several mammalian genes with sequence homology to the baculovirus inhibitor-of-apoptosis protein (iap) genes. Here we show that hILP can block apoptosis induced by a variety of extracellular stimuli, including UV light, chemotoxic drugs, and activation of the tumor necrosis factor and Fas receptors. hILP also protected against cell death induced by members of the caspase family, cysteine proteases which are thought to be the principal effectors of apoptosis. hILP and Bcl-xL were compared for their ability to affect several steps in the apoptotic pathway. Redistribution of cytochrome c from mitochondria, an early event in apoptosis, was not blocked by overexpression of hILP but was inhibited by Bcl-xL. In contrast, hILP, but not Bcl-xL, inhibited apoptosis induced by microinjection of cytochrome c. These data suggest that while Bcl-xL may control mitochondrial integrity, hILP can function downstream of mitochondrial events to inhibit apoptosis.

Interleukin-1 beta converting enzyme-like protease involvement in Fas-induced and activation-induced peripheral blood T cell apoptosis in HIV infection. TNF-related apoptosis-inducing ligand can mediate activation-induced T cell death in HIV infection

Apoptosis of peripheral blood T cells has been suggested to play an important role in the pathogenesis of human immunodeficiency virus (HIV) infection. Spontaneous, Fas (CD95)-induced and activation-induced T cell apoptosis have all been described in peripheral blood mononuclear cell cultures of HIV-infected individuals. We have previously shown that activation-induced T cell apoptosis is Fas independent in peripheral blood T cells from HIV+ individuals. In this study, we extend and confirm these observations by using an inhibitor of interleukin-1 beta converting enzyme (ICE) homologues. We show that z-VAD-fmk, a tripeptide inhibitor of ICE homologues, can inhibit Fas-induced apoptosis of peripheral blood CD4(+) and CD8+ T cells from asymptomatic HIV+ individuals. z-VAD-fmk also inhibited activation (anti-CD3)- induced CD4+ and CD8+ T cell apoptosis (AICD) in some but not all asymptomatic HIV+ individuals. Apoptosis was measured by multiparameter flow cytometry. The z-VAD-fmk inhibitor also enhanced survival of T cells in anti-Fas or anti-CD3 antibody-treated cultures and inhibited DNA fragmentation. AICD that could be inhibited by z-VAD-fmk was Fas independent and could be inhibited with a blocking monoclonal antibody to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a recently described member of the TNF/nerve growth factor ligand family. The above findings show that Fas-induced T cell apoptosis is ICE dependent in HIV infection. AICD can be blocked by ICE inhibitors in some patients, and this AICD is mediated by TRAIL. These results show that TRAIL can be a mediator of AICD in T cells. These different mechanisms of peripheral blood T cell apoptosis may play different roles in the pathogenesis of HIV infection.

Cleavage of rabaptin-5 blocks endosome fusion during apoptosis

Cells undergoing apoptosis exhibit striking changes in membrane organization, including plasma membrane blebbing and invagination, vacuolation and fragmentation of organelles, and alterations in the surface expression of receptors. The underlying mechanisms for these changes are unknown, though alterations in vesicular fusion are likely to play a role. Using a cell-free system based on Xenopus laevis egg extracts we have found that endosome fusion is blocked during apoptosis. Inhibition of fusion is prevented by Bcl-2 or Bcl-xL, two negative regulators of apoptosis, or by specific inhibitors of members of the caspase family of apoptotic proteases. Selective cleavage of Rabaptin-5, an essential and rate-limiting component of endosome fusion, is responsible for the loss of fusion activity. Cleavage of Rabaptin-5 also occurs in cellular models for apoptosis. These results suggest that inactivation of Rabaptin-5 and inhibition of vesicle transport lead to fragmentation of endosomes and inhibition of the endocytic pathway during the execution phase of apoptosis. We propose that parallel changes to other membrane transport pathways would give rise to general membrane fragmentation in apoptotic cells. These changes are likely to play an important role in the generation of apoptotic bodies and their recognition by phagocytosing cells.

Caspases: the executioners of apoptosis

Apoptosis is a major form of cell death, characterized initially by a series of stereotypic morphological changes. In the nematode Caenorhabditis elegans, the gene ced-3 encodes a protein required for developmental cell death. Since the recognition that CED-3 has sequence identity with the mammalian cysteine protease interleukin-1 beta-converting enzyme (ICE), a family of at least 10 related cysteine proteases has been identified. These proteins are characterized by almost absolute specificity for aspartic acid in the P1 position. All the caspases (ICE-like proteases) contain a conserved QACXG (where X is R, Q or G) pentapeptide active-site motif. Capases are synthesized as inactive proenzymes comprising an N-terminal peptide (prodomain) together with one large and one small subunit. The crystal structures of both caspase-1 and caspase-3 show that the active enzyme is a heterotetramer, containing two small and two large subunits. Activation of caspases during apoptosis results in the cleavage of critical cellular substrates, including poly(ADP-ribose) polymerase and lamins, so precipitating the dramatic morphological changes of apoptosis. Apoptosis induced by CD95 (Fas/APO-1) and tumour necrosis factor activates caspase-8 (MACH/FLICE/Mch5), which contains an N-terminus with FADD (Fas-associating protein with death domain)-like death effector domains, so providing a direct link between cell death receptors and the caspases. The importance of caspase prodomains in the regulation of apoptosis is further highlighted by the recognition of adapter molecules, such as RAIDD [receptor-interacting protein (RIP)-associated ICH-1/CED-3-homologous protein with a death domain]/CRADD (caspase and RIP adapter with death domain), which binds to the prodomain of caspase-2 and recruits it to the signalling complex. Cells undergoing apoptosis following triggering of death receptors execute the death programme by activating a hierarchy of caspases, with caspase-8 and possibly caspase-10 being at or near the apex of this apoptotic cascade.

Apoptosis signaling pathway in T cells is composed of ICE/Ced-3 family proteases and MAP kinase kinase 6b

Fas/APO-1(CD95) ligation activates programmed cell death, a cellular process that plays an important role in the maturation of the host immune response. We show that activation of a specific MAP kinase kinase (MKK), MKK6b, is necessary and sufficient for Fas-induced apoptosis of Jurkat T cells. MKK6b activation occurs downstream of an interleukin-1 converting enzyme-like (ICE-like) protease(s), while execution of the apoptotic pathway by MKK6b requires both ICE- and CPP32-like proteases. Surprisingly, the p38 MAP kinase protein, a known substrate of MKK6b, does not participate in Fas/MKK6b-mediated apoptosis. These findings indicate a divergence of the MKK6b signaling pathways, one of which activates p38 and leads to regulation of gene expression, and one of which activates the ICE/Ced-3 family of proteases and leads to cell death. These studies represent a demonstration of an apoptotic pathway that is comprised of both the ICE/Ced-3 family of proteases and MAP kinase kinase 6.