BBA - Molecular Cell Research (v.1644, #2-3)

Role of Bcl-2 family members in invertebrates by Tatsushi Igaki; Masayuki Miura (73-81).
Proteins belonging to the Bcl-2 family function as regulators of ‘life-or-death’ decisions in response to various intrinsic and extrinsic stimuli. In mammals, cell death is controlled by pro- and anti-apoptotic members of the Bcl-2 family, which function upstream of the caspase cascade. Structural and functional homologues of the Bcl-2 family proteins also exist in lower eukaryotes, such as nematodes and flies. In nematodes, an anti-apoptotic Bcl-2 family protein, CED-9, functions as a potent cell death inhibitor, and a BH3-only protein, EGL-1, acts as an inhibitor of CED-9 to facilitate the spatio-temporal regulation of programmed cell death. On the other hand, the Drosophila genome encodes two Bcl-2 family proteins, Drob-1/Debcl/dBorg-1/dBok and Buffy/dBorg-2, both of which structurally belong to the pro-apoptotic group, despite abundant similarities in the cell death mechanisms between flies and vertebrates. Drob-1 acts as a pro-apoptotic factor in vitro and in vivo, and Buffy/dBorg-2 exhibits a weak anti-apoptotic function. The ancestral role of the Bcl-2 family protein may be pro-apoptotic, and the evolution of the functions of this family of proteins may be closely linked with the contribution of mitochondria to the cell death pathway.
Keywords: Bcl-2 family; Caenorhabditis elegans; Drosophila; Mitochondrion; Cell death; Apoptosis;

Structural biology of the Bcl-2 family of proteins by Andrew M. Petros; Edward T. Olejniczak; Stephen W. Fesik (83-94).
The proteins of the Bcl-2 family are important regulators of programmed cell death. Structural studies of Bcl-2 family members have provided many important insights into their molecular mechanism of action and how members of this family interact with one another. To date, structural studies have been performed on six Bcl-2 family members encompassing both anti- (Bcl-xL, Bcl-2, KSHV-Bcl-2, Bcl-w) and pro-apoptotic (Bax, Bid) members. They all show a remarkably similar fold despite an overall divergence in amino acid sequence and function (pro-apoptotic versus anti-apoptotic). The three-dimensional structures of Bcl-2 family members consist of two central, predominantly hydrophobic α-helices surrounded by six or seven amphipathic α-helices of varying lengths. A long, unstructured loop is present between the first two α-helices. The structures of the Bcl-2 proteins show a striking similarity to the overall fold of the pore-forming domains of bacterial toxins. This finding led to experiments which demonstrated that Bcl-xL, Bcl-2, and Bax all form pores in artificial membranes. A prominent hydrophobic groove is present on the surface of the anti-apoptotic proteins. This groove is the binding site for peptides that mimic the BH3 region of various pro-apoptotic proteins such as Bak and Bad. Structures of Bcl-xL in complex with these BH3 peptides showed that they bind as an amphipathic α-helix and make extensive hydrophobic contacts with the protein. These data have not only helped to elucidate the interactions important for hetero-dimerization of Bcl-2 family members but have also been used to guide the discovery of small molecules that block Bcl-xL and Bcl-2 function. In the recently determined structure of the anti-apoptotic Bcl-w protein, the protein was also found to have a hydrophobic groove on its surface capable of binding BH3-containing proteins and peptides. However, in the native protein an additional carboxy-terminal α-helix interacts with the hydrophobic groove. This is reminiscent of how the carboxy-terminal α-helix of the pro-apoptotic protein Bax binds into its hydrophobic groove. This interaction may play a regulatory role and for Bax may explain why it is found predominately in the cytoplasm prior to activation. The hydrophobic groove of the pro-apoptotic protein, Bid protein, is neither as long nor as deep as that found in Bcl-xL, Bcl-2, or Bax. In addition, Bid contains an extra α-helix, which is located between α1 and α2 with respect to Bcl-xL, Bcl-2, and Bax. Although there are still many unanswered questions regarding the exact mechanism by which the Bcl-2 family of proteins modulates apoptosis, structural studies of these proteins have deepened our understanding of apoptosis on the molecular level.
Keywords: Apoptosis; Bcl-xL; Bcl-2; Bcl-w; Bax; Bak; Bid;

The members of the Bcl-2 family of proteins are crucial regulators of apoptosis. In order to determine cell fate, these proteins must be targeted to distinct intracellular membranes, including the mitochondrial outer membrane (MOM), the membrane of the endoplasmic reticulum (ER) and its associated nuclear envelope. The targeting sequences and mechanisms that mediate the specificity of these proteins for a particular cellular membrane remain poorly defined. Several Bcl-2 family members have been reported to be tail-anchored via their predicted hydrophobic COOH-terminal transmembrane domains (TMDs). Tail-anchoring imposes a posttranslational mechanism of membrane insertion on the already folded protein, suggesting that the transient binding of cytosolic chaperone proteins to the hydrophobic TMD may be an important regulatory event in the targeting process. The TMD of certain family members is initially concealed and only becomes available for targeting and membrane insertion in response to apoptotic stimuli. These proteins either undergo a conformational change, posttranslational modification or a combination of these events enabling them to translocate to sites at which they are functional. Some Bcl-2 family members lack a TMD, but nevertheless localize to the MOM or the ER membrane during apoptosis where they execute their functions. In this review, we will focus on the intracellular targeting of Bcl-2 family members and the mechanisms by which they translocate to their sites of action. Furthermore, we will discuss the posttranslational modifications which regulate these events.
Keywords: Bcl-2 family; Subcellular localization; Tail-anchoring; Membrane insertion; Posttranslational modification; Translocation;

Control of mitochondrial permeability by Bcl-2 family members by Juanita C. Sharpe; Damien Arnoult; Richard J. Youle (107-113).
Programmed cell death (apoptosis) is regulated by the Bcl-2 family of proteins. Although it remains unclear how these family members control apoptosis, they clearly have the capacity to regulate the permeability of intracellular membranes to ions and proteins. Proapoptotic members of the Bcl-2 family, especially Bax and Bid, have been extensively analyzed for the ability to form channels in membranes and to regulate preexisting channels. Anti-apoptotic members of the family tend to have the opposing effects on membrane channel formation. The molecular mechanisms of the different models for the permeabilization of membranes by the Bcl-2 family members and the regulation of Bcl-2 family member subcellular localizations are discussed.
Keywords: Bax; Bid; Mitochondrion; Channel;

There is more to life and death than mitochondria: Bcl-2 proteins at the endoplasmic reticulum by Matthew G. Annis; Jeremy A. Yethon; Brian Leber; David W. Andrews (115-123).
Proteins of the Bcl-2 family are important regulators of cell fate. The role of these proteins in controlling mitochondrial apoptotic processes has been extensively investigated, although exact molecular mechanisms are incompletely understood. However, mounting evidence indicates that these proteins also function at the endoplasmic reticulum and other locations within the cell. Both pro- and anti-apoptotic Bcl-2 family members can regulate endoplasmic reticulum calcium, cellular pH and endoplasmic reticulum resident proteins. In this review, we discuss the activities and potential targets of Bcl-2 family members at the endoplasmic reticulum and other cellular locations.
Keywords: Bcl-2; Endoplasmic reticulum; Calcium; Bap31; Bax;

Two principal pathways for apoptosis initiation exist. One pathway, which is also termed the ‘extrinsic’ pathway, is mediated by death receptors, a subgroup of the TNF receptor superfamily. The second pathway, which is also referred to as the ‘intrinsic’ pathway is controlled by members of the Bcl-2 family. A long standing discussion revolves around the question of how these two pathways influence each other in regulating the decision about life or death of a cell. Here, we review our current knowledge about the interactions between these two pathways and discuss current models which could help to resolve previous apparently contradictory results.
Keywords: Apoptosis; Bcl-2 family; Death receptor;

Programmed cell death (PCD) is essential for normal development and maintenance of tissue homeostasis in multicellular organisms. While it is now evident that PCD can take many different forms, apoptosis is probably the most well-defined cell death programme. The characteristic morphological and biochemical features associated with this highly regulated form of cell death have until recently been exclusively attributed to the caspase family of cysteine proteases. As a result, many investigators affiliate apoptosis with its pivotal execution system, i.e. caspase activation. However, it is becoming increasingly clear that PCD or apoptosis can also proceed in a caspase-independent manner and maintain key characteristics of apoptosis. Mitochondrial integrity is central to both caspase-dependent and-independent cell death. The release of pro-apoptotic factors from the mitochondrial intermembrane space is a key event in a cell's commitment to die and is under the tight regulation of the Bcl-2 family. However, the underlying mechanisms governing the efflux of these pro-death molecules are largely unknown. This review will focus on the regulation of mitochondrial integrity by Bcl-2 family members with particular attention to the controlled release of factors involved in caspase-independent cell death.
Keywords: Bcl-2 family; Mitochondrion; Apoptosis; Caspase-independent; AIF;

Bcl-2 family members are central to the control of cell survival. Work of the last years has established that the function of these proteins can be regulated by mitogenic signaling cascades. Within the scope of this review, we will discuss the contribution of Bcl-2-dependent signaling pathways to cell survival by Raf kinases and also address the underlying mechanisms.
Keywords: Raf kinase; Bcl-2; Apoptosis suppression;

Control of proliferation by Bcl-2 family members by Nathalie Bonnefoy-Berard; Abdel Aouacheria; Claire Verschelde; Laurence Quemeneur; Antoine Marçais; Jacqueline Marvel (159-168).
The anti-proliferative effect of Bcl-2 acts mainly at the level of the G0/G1 phase of the cell cycle. Deletions and point mutations in the bcl-2 gene show that the anti-proliferative activity of Bcl-2, can in some cases, be dissociated from its anti-apoptotic function. This indicates that the effect of Bcl-2 on cell cycle progression can be a direct effect and not only a consequence of its anti-apoptotic activity. Bcl-2 appears to mediate its anti-proliferative effect by acting on both signal transduction pathways (NFAT, ERK) and on specific cell cycle regulators (p27, p130).
Keywords: Bcl-2; Cell-cycle; p27; NFAT; p130; Apoptosis;

Bcl-2 family members and disease by Christine M. Sorenson (169-177).
Apoptosis plays an important role during development and in the maintenance of multicellular organisms. Bcl-2 family members affect cell death in either a positive or negative fashion. Although some redundancy exists between family members, expression of certain family members is important during development in an organ-specific manner. The founding family member bcl-2 tends to be highly expressed in the embryo and declines postnatally following differentiation and maturation. Altered expression of bcl-2, as well as other family members, has been observed in disease states potentially affecting treatment modalities. Here we examine the distribution and role death repressors bcl-2, bcl-xL and bcl-w as well as death effectors bax and bak play regulating apoptosis in a tissue-specific manner. Understanding the normal role of these proteins during embryogenesis and in the mature organ will give us important insight into what goes awry in various disease states.
Keywords: bcl-2; bcl-w; bcl-xL; bax; bak;

Role of Bcl-2 family members in immunity and disease by Nathalie M. Droin; Douglas R. Green (179-188).
The different members of the Bcl-2 family are essential regulators of programmed cell death. These different members share one or more Bcl-2 homology domains, required for their ability to regulate each other. In this review, we describe current knowledge of the functions of different Bcl-2 members and their potential roles in disease and immunity.
Keywords: Apoptosis; Bcl-2; BH3-only protein; Regulation; Disease; Immunity;

Bcl-2 family regulation of neuronal development and neurodegeneration by Rizwan S. Akhtar; Jayne M. Ness; Kevin A. Roth (189-203).
Neuronal cell death is a key feature of both normal nervous system development and neuropathological conditions. The Bcl-2 family, via its regulation of both caspase-dependent and caspase-independent cell death pathways, is uniquely positioned to critically control neuronal cell survival. Targeted gene disruptions of specific bcl-2 family members and the generation of transgenic mice overexpressing anti- or pro-apoptotic Bcl-2 family members have confirmed the importance of the Bcl-2 family in the nervous system. Data from studies of human brain tissue and experimental animal models of neuropathological conditions support the hypothesis that the Bcl-2 family regulates cell death in the mature nervous system and suggest that pharmacological manipulation of Bcl-2 family action could prove beneficial in the treatment of human neurological conditions such as stroke and neurodegenerative diseases.
Keywords: Apoptosis; Programmed cell death; Caspase; Bcl-X; Bax; Neuropathology;

Since the cloning of the bcl-2 gene in 1985, considerable progress has been made in elucidating the function of Bcl-2 and related proteins in controlling apoptosis. Although much of this work initially relied on the ectopic expression of bcl-2 gene family members in cell lines in vitro, a number of genetically manipulated mice have been generated to better understand the in vivo significance of specific family members to organ development and homeostasis. Of the many tissues that exhibit apoptosis at some point during fetal or postnatal life, the female gonads arguably possess one of the highest and most protracted incidences of apoptosis, associated with development and maturation of the germ line. Moreover, female germ cells (oocytes) are, for as-yet poorly understood reasons, extremely vulnerable to a host of pathological insults, such as anti-cancer therapies, that ultimately cause premature ovarian failure and infertility due to accelerated oocyte death. Accordingly, efforts to understand the occurrence and regulation of apoptosis in the ovary are of considerable importance from both biological and clinical perspectives. This review will highlight what is known of apoptosis in the female gonads, and the role that Bcl-2 family members play in regulating this process.
Keywords: Apoptosis; Cell death; Ovary; Germ cell; Bcl-2; Bax; Gene knockout; Fertility;

Viral Bcl-2 homologs and their role in virus replication and associated diseases by Brian M. Polster; Jonathan Pevsner; J.Marie Hardwick (211-227).
Cellular Bcl-2 family proteins regulate a critical step in the mammalian programmed cell death pathway by modulating mitochondrial permeability and function. Bcl-2 family proteins are also encoded by several large DNA viruses, including all known gamma herpesviruses, adenoviruses, and several other unrelated viruses. Viral Bcl-2 proteins can prevent cell death but often escape cellular regulatory mechanisms that govern their cellular counterparts. By evading the “altruistic” suicide of infected cells, viruses can ensure replication and propagation in the infected host, but sometimes in surprising ways. Many human cancers and other disorders are associated with viruses that encode Bcl-2 homologs. Here we consider the available mechanistic data for viral compared to cellular Bcl-2 protein function along with relevance to the virus life cycle and human disease states.
Keywords: Bcl-2; Apoptosis; Gamma herpesvirus; Adenovirus; Epstein–Barr; Kaposi's sarcoma;

The role of Bcl-2 family members in tumorigenesis by Vladimir Kirkin; Stefan Joos; Martin Zörnig (229-249).
The Bcl-2 family consists of about 20 homologues of important pro- and anti-apoptotic regulators of programmed cell death. The established mode of function of the individual members is to either preserve or disturb mitochondrial integrity, thereby inducing or preventing release of apoptogenic factors like Cytochrome c (Cyt c) from mitochondria. Recent findings also indicate further Bcl-2-controlled mitochondria-independent apoptosis pathways. Bcl-2 represents the founding member of the new and growing class of cell death inhibiting oncoproteins. In this review, we try to briefly summarize current models of Bcl-2 family function and to outline the work demonstrating the influence of deregulated Bcl-2 family member expression on tumorigenesis and cancer therapy. Since several Bcl-2 homologues, in addition to influencing apoptotic behaviour, also impinge on cell cycle progression, we discuss possible implications of this additional role for the expression of Bcl-2 family members in tumor cells.
Keywords: Bcl-2 family; Tumorigenesis; Cell cycle; Chemoresistance; Radiation; Clinical therapy;

Shooting at survivors: Bcl-2 family members as drug targets for cancer by Philippe Juin; Olivier Geneste; Eric Raimbaud; John A Hickman (251-260).
Keywords: Bcl-2; Target; Cancer;

Author Index (261-262).