The conserved N-terminal BH4 domain of Bcl-2 homologues is essential for inhibition of apoptosis and interaction with CED-4

The central effectors of cell death in the immune system

The immune system relies on cell death to maintain lymphoid homeostasis and avoid disease. Recent evidence has indicated that the caspase family of cysteine proteases is a central effector in apoptotic cell death and is absolutely responsible for many of the morphological features of apoptosis. Cell death, however, can occur through caspase-independent and caspase-dependent pathways. In the case of cells that are irreversibly neglected or damaged, death occurs even in the absence of caspase activity. In contrast, healthy cells require caspase activation to undergo cell death induced by surface receptors. This review summarizes the current understanding of these two pathways of cell death in the immune system.

p75 neurotrophin receptor-mediated neuronal death is promoted by Bcl-2 and prevented by Bcl-xL

The p75 neurotrophin receptor (p75NTR) has been shown to mediate neuronal death through an unknown pathway. We microinjected p75NTR expression plasmids into sensory neurons in the presence of growth factors and assessed the effect of the expressed proteins on cell survival. We show that, unlike other members of the TNFR family, p75NTR signals death through a unique caspase-dependent death pathway that does not involve the "death domain" and is differentially regulated by Bcl-2 family members: the anti-apoptotic molecule Bcl-2 both promoted, and was required for, p75NTR killing, whereas killing was inhibited by its homologue Bcl-xL. These results demonstrate that Bcl-2, through distinct molecular mechanisms, either promotes or inhibits neuronal death depending on the nature of the death stimulus.

The potential for monocyte-mediated immunotherapy during infection and malignancy. Part I: apoptosis induction and cytotoxic mechanisms

The mononuclear phagocyte system consists of peripheral blood monocytes and tissue macrophages that collectively play a major role in host immunity. Far from existing solely as phagocytic scavengers of cell debris and foreign matter, monocytes are highly active and responsive to inflammatory and immunological signals that activate their microbicidal and tumoricidal functions. Cytokines that are secreted as an integral component of the innate immune response such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and -IFN may directly activate the functions of the monocyte system. A key mediator of the effector functions of monocytes is tumour necrosis factor (TNF) which transduces its signals upon binding to specific transmembrane receptors. TNF is highly cytotoxic to micro-organisms and susceptible malignant cells and in most cases delivers its cytotoxic signal to tumour cells by highly regulated mechanisms of programmed cell death or apoptosis. We believe that the numerous functions of the monocyte system may be harnessed for therapeutic gain both in the context of microbiological infection and malignant disease. In this review, the mechanisms by which secreted and monocyte cell-membrane-associated TNF induce apoptosis will be discussed. In addition, the cell-associated and secretory immunological mechanisms employed by monocytes in host defence will be discussed in the context of the their ability to combat infection and neoplasia.

Bak BH3 peptides antagonize Bcl-xL function and induce apoptosis through cytochrome c-independent activation of caspases

The Bcl-2 homology 3 (BH3) domain is crucial for the death-inducing and dimerization properties of pro-apoptotic members of the Bcl-2 protein family, including Bak, Bax, and Bad. Here we report that synthetic peptides corresponding to the BH3 domain of Bak bind to Bcl-xL, antagonize its anti-apoptotic function, and rapidly induce apoptosis when delivered into intact cells via fusion to the Antennapedia homeoprotein internalization domain. Treatment of HeLa cells with the Antennapedia-BH3 fusion peptide resulted in peptide internalization and induction of apoptosis within 2-3 h, as indicated by caspase activation and subsequent poly(ADP-ribose) polymerase cleavage, as well as morphological characteristics of apoptosis. A point mutation within the BH3 peptide that blocks its ability to bind to Bcl-xL abolished its apoptotic activity, suggesting that interaction of the BH3 peptide with Bcl-2-related death suppressors, such as Bcl-xL, may be critical for its activity in cells. While overexpression of Bcl-xL can block BH3-induced apoptosis, treatment with BH3 peptides resensitized Bcl-xL-expressing cells to Fas-mediated apoptosis. BH3-induced apoptosis was blocked by caspase inhibitors, demonstrating a dependence on caspase activation, but was not accompanied by a dramatic early loss of mitochondrial membrane potential or detectable translocation of cytochrome c from mitochondria to cytosol. These findings demonstrate that the BH3 domain itself is capable of inducing apoptosis in whole cells, possibly by antagonizing the function of Bcl-2-related death suppressors.

The apoptosome: heart and soul of the cell death machine

Apoptosis is a fundamental biologic process by which metazoan cells orchestrate their own self-demise. Genetic analyses of the nematode C elegans identified three core components of the suicide apparatus which include CED-3, CED-4, and CED-9. An analogous set of core constituents exists in mammalian cells and includes caspase-9, Apaf-1, and bcl-2/xL, respectively. CED-3 and CED-4, along with their mammalian counterparts, function to kill cells, whereas CED-9 and its mammalian equivalents protect cells from death. These central components biochemically intermingle in a ternary complex recently dubbed the "apoptosome." The C elegans protein EGL-1 and its mammalian counterparts, pro-apoptotic members of the bcl-2 family, induce cell death by disrupting apoptosome interactions. Thus, EGL-1 may represent a primordial signal integrator for the apoptosome. Various biochemical processes including oligomerization, adenosine triphosphate ATP/dATP binding, and cytochrome c interaction play a role in regulating the ternary death complex. Recent studies suggest that cell death receptors, such as CD95, may amplify their suicide signal by activating the apoptosome. These mutual associations by core components of the suicide apparatus provide a molecular framework in which diverse death signals likely interface. Understanding the apoptosome and its cellular connections will facilitate the design of novel therapeutic strategies for cancer and other disease states in which apoptosis plays a pivotal role.

Expression and purification of full-length human Bax alpha

Bax is a proapoptotic ion channel forming protein of the Bcl-2 family. In cells the protein is found in the cytosol and in the mitochondria membrane where it presumably is involved during apoptosis in disruption of the mitochondrial membrane potential and release of cytochrome c. The protein has a hydrophobic domain at the C-terminus, which renders it a limited solubility. Thus, all studies on recombinant Bax has so far been performed on C-terminal truncated protein. We have expressed and purified the full-length human Bax alpha. The protein was expressed with a His tag at the N-terminus and purified by affinity chromatography on Ni-NTA-agarose followed by ion-exchange chromatography on Q-Sepharose. The protein was more than 98% pure on SDS-PAGE and in the presence of 1% (w/v) octyl glucoside it could be concentrated up to 0.5 mg/ml. Full-length Bax was 25-fold more efficient, compared to C-terminal truncated Bax, in forming ion channels and trigger carboxyfluorescein release from liposomes.

Boo, a novel negative regulator of cell death, interacts with Apaf-1

In this report, we describe the cloning and characterization of Boo, a novel anti-apoptotic member of the Bcl-2 family. The expression of Boo was highly restricted to the ovary and epididymis implicating it in the control of ovarian atresia and sperm maturation. Boo contains the conserved BH1 and BH2 domains, but lacks the BH3 motif. Like Bcl-2, Boo possesses a hydrophobic C-terminus and localizes to intracellular membranes. Boo also has an N-terminal region with strong homology to the BH4 domain found to be important for the function of some anti-apoptotic Bcl-2 homologues. Chromosomal localization analysis assigned Boo to murine chromosome 9 at band d9. Boo inhibits apoptosis, homodimerizes or heterodimerizes with some death-promoting and -suppressing Bcl-2 family members. More importantly, Boo interacts with Apaf-1 and forms a multimeric protein complex with Apaf-1 and caspase-9. Bak and Bik, two pro-apoptotic homologues disrupt the association of Boo and Apaf-1. Furthermore, Boo binds to three distinct regions of Apaf-1. These results demonstrate the evolutionarily conserved nature of the mechanisms of apoptosis. Like Ced-9, the mammalian homologues Boo and Bcl-xL interact with the human counterpart of Ced-4, Apaf-1, and thereby regulate apoptosis.

The pharmacology of apoptosis

Apoptosis is an area of intense scientific interest, which encompasses the study of and triggers mechanisms involved in mediating the cell biology of programmed cell death. A number of low molecular weight compounds have been used to inhibit or enhance this fundamental cellular process and so apoptosis has now become amenable to pharmacological manipulation. In this review Ross Kinloch, Mark Treherne, Mike Furness and Iradj Hajimohamadreza will focus on the current literature describing the pharmacology of apoptosis, with particular reference to the therapeutic potential that could arise from the development of pro- and anti-apoptotic drugs. The pivotal role of apoptosis in such diverse pathological processes as tumour growth, the immune response and neurodegeneration suggests that an understanding of how apoptosis can be regulated by drugs will become increasingly important to the pharmaceutical industry.

Neuroprotection with Bcl-2(20-34) peptide against trauma

We tested the neuroprotective potential of the Bcl-2(20-34) peptide sequence in hippocampal slices. Treatment with Bcl-2 after fluid percussion trauma significantly improved recovery of CA1 antidromic PS to a mean of 92%+/-1 of initial amplitude, compared with only 16%+/-2 in unmedicated slices. The EC50 for trauma protection was 84 microM Bcl-2(20-34). Protection with Bcl-2(20-34) also extended to long-term potentiation. No protection was seen with the reverse sequence of Bcl-2(20-34). Treatment with Bcl-2(20-34) also protected against hypoxic damage, with treated slices recovering to 98%+/-2, while unmedicated slices recovered to 14%+/-2. Similar protection was seen against AMPA, NMDA and nitric oxide. These findings indicate that Bcl-2(20-34) provides specific neuroprotection against acute CA1 neuronal injury.

Role of Bcl-2 family proteins in apoptosis: apoptosomes or mitochondria?

Apoptosis is an essential physiological process for the selective elimination of cells, which is involved in a variety of biological events. The Bcl-2 family is the best characterized protein family involved in the regulation of apoptotic cell death, consisting of anti-apoptotic and pro-apoptotic members. The anti-apoptotic members of this family, such as Bcl-2 and Bcl-XL, prevent apoptosis either by sequestering proforms of death-driving cysteine proteases called caspases (a complex called the apoptosome) or by preventing the release of mitochondrial apoptogenic factors such as cytochrome c and AIF (apoptosis-inducing factor) into the cytoplasm. After entering the cytoplasm, cytochrome c and AIF directly activate caspases that cleave a set of cellular proteins to cause apoptotic changes. In contrast, pro-apoptotic members of this family, such as Bax and Bak, trigger the release of caspases from death antagonists via heterodimerization and also by inducing the release of mitochondrial apoptogenic factors into the cytoplasm via acting on mitochondrial permeability transition pore, thereby leading to caspase activation. Thus, the Bcl-2 family of proteins acts as a critical life-death decision point within the common pathway of apoptosis.

Blocking cytochrome c activity within intact neurons inhibits apoptosis

Cytochrome c has been shown to play a role in cell-free models of apoptosis. During NGF withdrawal-induced apoptosis of intact rat superior cervical ganglion (SCG) neurons, we observe the redistribution of cytochrome c from the mitochondria to the cytoplasm. This redistribution is not inhibited by the caspase inhibitor Z-Val-Ala-Asp-fluoromethylketone (ZVADfmk) but is blocked by either of the neuronal survival agents 8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate (CPT-cAMP) or cycloheximide. Moreover, microinjection of SCG neurons with antibody to cytochrome c blocks NGF withdrawal-induced apoptosis. However, microinjection of SCG neurons with cytochrome c does not alter the rate of apoptosis in either the presence or absence of NGF. These data suggest that cytochrome c is an intrinsic but not limiting component of the neuronal apoptotic pathway.

The Bcl-2 protein family: arbiters of cell survival

Bcl-2 and related cytoplasmic proteins are key regulators of apoptosis, the cell suicide program critical for development, tissue homeostasis, and protection against pathogens. Those most similar to Bcl-2 promote cell survival by inhibiting adapters needed for activation of the proteases (caspases) that dismantle the cell. More distant relatives instead promote apoptosis, apparently through mechanisms that include displacing the adapters from the pro-survival proteins. Thus, for many but not all apoptotic signals, the balance between these competing activities determines cell fate. Bcl-2 family members are essential for maintenance of major organ systems, and mutations affecting them are implicated in cancer.

Cleavage of BID by caspase 8 mediates the mitochondrial damage in the Fas pathway of apoptosis

We report here that BID, a BH3 domain-containing proapoptotic Bcl2 family member, is a specific proximal substrate of Casp8 in the Fas apoptotic signaling pathway. While full-length BID is localized in cytosol, truncated BID (tBID) translocates to mitochondria and thus transduces apoptotic signals from cytoplasmic membrane to mitochondria. tBID induces first the clustering of mitochondria around the nuclei and release of cytochrome c independent of caspase activity, and then the loss of mitochondrial membrane potential, cell shrinkage, and nuclear condensation in a caspase-dependent fashion. Coexpression of BclxL inhibits all the apoptotic changes induced by tBID. Our results indicate that BID is a mediator of mitochondrial damage induced by Casp8.

Bcl-2-family proteins: the role of the BH3 domain in apoptosis

Bcl-2-related proteins have come to occupy a prominent position in the realm of programmed cell death. Members of this fast-growing family are highly related in one or more specific regions, commonly referred to as Bcl-2 homology (BH) domains. BH domains contribute at multiple levels to the function of these proteins in cell death and survival. Particularly intriguing is the emergence of the BH3 domain as a potent 'death domain' and of a growing subclass of pro-apoptotic proteins with no similarity to Bcl-2 beyond their BH3 homology. Here, the authors classify proteins of the Bcl-2 family on the basis of function and domain organization, discuss the importance of the BH3 domain in protein-protein interactions and in cell death and provide possible explanations for the perceived redundancy in the expression of this subclass of death promoters.

Cell death: shadow baxing

Bcl-2, one of a family of key regulators of apoptosis, was the first cell-death machinery component to be identified, but how the family functions is still not clear. Mammalian Bax, a pro-apoptotic family member, can cause yeast cells to die, and two recent yeast genetic screens shed light on how Bax might function.

Antiapoptotic activity of the herpesvirus saimiri-encoded Bcl-2 homolog: stabilization of mitochondria and inhibition of caspase-3-like activity

Viruses have evolved different strategies to interfere with host cell apoptosis. Herpesvirus saimiri (HVS) and other lymphotropic herpesviruses code for proteins that are homologous to the cellular antiapoptotic Bcl-2. In this study HVS-Bcl-2 was stably expressed in the human leukemia cell line Jurkat and in the murine T-cell hybridoma DO to assess its antiapoptotic spectrum and to gain further insight into its mode of action. HVS- Bcl-2 prevented apoptosis that occurs as a result of a disturbance of intracellular homeostasis by, for example, DNA damage or menadione, which gives rise to oxygen radicals. In Jurkat cells, HVS-Bcl-2 also inhibited apoptosis mediated by the death receptor CD95. In DO cells, HVS-Bcl-2 did not interfere with CD95-mediated apoptosis but blocked dexamethasone-induced cell death. Mitochondrial damage is a central coordinating event in apoptosis induced by different stimuli. To assess the integrity of mitochondria, we used rhodamine 123, which is released upon disturbance of the mitochondrial membrane potential, and determined the release of cytochrome c into the cytosol. Both signs of mitochondrial damage were prevented by HVS-Bcl-2. This viral protein also inhibited the generation of caspase-3-like DEVDase activity and blocked the cleavage of poly(ADP-ribose) polymerase, a natural substrate of caspase-3-like proteases. In conclusion, HVS-Bcl-2 protects against a great variety of apoptotic stimuli, stabilizes mitochondria, and acts upstream of the generation of caspase-3-like activity.