Mutations in the Pro-apoptotic Effector Bcl2 Family Proteins
Immunology. 2005 Apr; 114(iv): 441–449.
Bodyguards and assassins: Bcl-2 family proteins and apoptosis control in chronic lymphocytic leukaemia
Graham Packham
1Cancer Research United kingdom of great britain and northern ireland Oncology Unit, University of Southampton School of Medicine, Southampton General Hospital, Southampton, UK
Freda Thousand Stevenson
iiMolecular Immunology Group, Cancer Sciences Division, University of Southampton School of Medicine, Southampton Full general Hospital, Southampton, U.k.
Received 2004 Dec six; Revised 2004 Dec 14; Accepted 2004 December 14.
Abstract
Chronic lymphocytic leukaemia (CLL) is the about common B-jail cell malignancy in the Western world and exists every bit subtypes with very different clinical courses. CLL is more often than not described as a disease of failed apoptosis. Apoptosis resistance may stem from a combination of microenvironmental survival signals as well as from intrinsic alterations in the apoptotic machinery within the CLL cell. The molecular mechanism involved in controlling apoptosis in CLL is circuitous and is influenced by many factors, including Bcl-2 family proteins, which are critical regulators of prison cell death. Here nosotros review the significance of apoptosis dysregulation in CLL, focusing on the role of Bcl-two and related Bcl-2 family proteins, such as Bax and Mcl-1. The differential properties of the newly described subsets of CLL are besides highlighted.
Keywords: apoptosis, Bax, Bcl-ii, chronic lymphocytic leukaemia, Mcl-1
Introduction
Chronic lymphocytic leukaemia (CLL) is the most mutual B-cell malignancy in the Western world and is characterized by the accumulation of CD5-positive monoclonal B cells in the claret, bone marrow and peripheral lymphoid organs.one Although long considered as a unmarried disease, the clinical course of CLL is heterogeneous. Some patients have ambitious disease with relatively rapid increases in the number of cancerous cells in the blood, and crave treatment, most commonly with cytotoxic therapy. Relapses are common and CLL remains incurable. Past dissimilarity, other patients accept stable, non-progressive affliction that often requires no treatment other than careful monitoring.
Studies of the B-jail cell receptor (BCR) take provided important clues to the mechanisms that underlie the various clinical behaviour of CLL.2 , iii Most notably, the presence of somatic mutations in the variable region of heavy (VH) and lite (VL) immunoglobulin chains is a powerful prognostic marking for CLL.4 , 5 Patients with mutated immunoglobulin genes (M-CLL) have a greatly improved prognosis compared to patients with unmutated germline sequences (U-CLL).
The accumulation of variable-region somatic mutations is a hallmark of antigen-driven affinity maturation within germinal centres, suggesting that these clinically important subsets of disease differ with respect to the stage of maturation reached by the B cell of origin. The usage of selected VH genes in CLL provides evidence for antigenic pressure during the development of both U-CLL and M-CLL.4 , 6 Still, the nature and outcome of this antigenic stimulation appears to vary between the subsets. Although gene expression profiling has provided evidence to suggest that all CLLs may arise from (or near closely resemble) memory B cells,7 the relatively small-scale differences in the factor-expression profiles of naïve and memory B cells, and evidence for continued activation, limits the conclusions that can be drawn from these analyses.3 An alternative proposal is that U-CLL could arise from B cells undergoing a T-contained stimulation by low-ardor (super)antigen, outside germinal centres.2 , 3 By dissimilarity, high-avidity antigen is more than likely to contribute to the development of Chiliad-CLL via normal T-cell-dependent events within germinal centres (Fig. ane). The expression of activation markers in both subsets of CLL suggests that environmental antigen continues to play a role in the maintenance of the CLL clones.ii Expression of the cell-surface activation mark CD38 has also been linked to disease heterogeneity in CLL.five , 8 , ix Relatively loftier levels of CD38 are associated with poor prognosis and, although controversial, CD38 expression appears to exist largely independent of VH mutational status.
A model for the development of chronic lymphocytic leukaemia (CLL) subtypes. CLL with unmutated, germline immunoglobulin sequences (U-CLL) may develop in response to low-avidity antigens (potentially superantigens) via T-prison cell-independent events outside the germinal centre. CLL with mutated, germline immunoglobulin sequences (M-CLL) may develop in response to high-avidity antigens that drive T-cell-dependent germinal centre formation and are therefore associated with the accumulation of immunoglobulin mutations. In that location is evidence for continued antigen interaction in both subsets of CLL. Whereas M-CLL cells appear to be relatively anergic, U-CLL cells are more often than not more responsive to B-jail cell receptor signalling. This may be a result of the nature of the antigen (i.eastward. low avidity) and differences between B cells of differing maturation in the propensity to become anergic. Resistance to apoptosis will probably contribute to both types of CLL. Interpatient heterogeneity in the susceptibility to apoptosis/expression of apoptosis regulators may impact nigh significantly on the response to therapy, rather than disease progression. Adjusted from Stevenson and Caligaris-Cappio.3
The power of CLL cells from unlike patients to transmit signals via the BCR varies between the subsets.10 – 12 Whereas U-CLL cells are generally able to transmit signals following cross-linking of the BCR with antibodies to immunoglobulin M (anti-IgM), M-CLL cells tend to be unresponsive. This might relate to the presence of zeta-associated protein 70 (ZAP-seventy), a critical T-cell receptor signalling molecule that has been shown to be overexpressed in U-CLL/bespeak-responsive CLL.12 – fourteen Nosotros have hypothesized that the differential response to BCR ligation reflects variable levels of anergy in CLL, with the extent of anergy existence influenced by the stage of B-cell maturation and possibly past the nature of the antigen (Fig. 1).3 Anergy may exist associated with improved clinical outcome because anergic cells are less responsive to proliferative/survival signals driven by antigen stimulation of the BCR in specific microenvironments.
Alteration in susceptibility to apoptosis is an important feature of many human cancers. Decreased apoptosis contributes non only to neoplasm evolution, but also to resistance to conventional anti-cancer therapies, such every bit radiations and cytotoxic agents. CLL is frequently considered to be a disease of failed apoptosis.i , 15 , xvi Genetic alterations and changes in the expression of many apoptosis regulators take been described, at to the lowest degree in a subset of patients. For case, the loss of p53 and ATM are relatively common genetic alterations and contribute to a reduced response to DNA-damaging agents.17 The nuclear factor-kappa B (NF-κB) transcription gene and phosphatidylinositol-three kinase signalling pathways are activated in CLL and are important for jail cell survival.18 – 21 Many cytokines, including B-cell activation factor (BAFF), a proliferation inducing ligand (April), CD40 ligand and interleukin-four (IL-four) promote the survival of CLL cells,22 – 24 and CLL cells appear to be resistant to the effects of signalling via jail cell-surface death receptors.25 Antigen stimulation via the BCR is also probable to influence apoptotic pathways, although the upshot of signalling via surface IgM and immunoglobulin D (IgD) remains controversial.10 , 26 Thus, the molecular control of apoptosis in CLL is complex and, attributable to the influence of stromal cells on survival of neoplasm cells,27 , 28 is difficult to assess in vitro.
Bcl-2 family proteins are critical regulators of apoptosis and have also been implicated in CLL. This review will discuss the significance of dysregulation of apoptosis in CLL, with a particular emphasis on disease heterogeneity and the part of Bcl-2 family proteins.
Apoptosis pathways and Bcl-ii family unit proteins
The key players in the execution of the apoptotic cascade are caspases (cysteine proteases with aspartate specificity), which are activated past cleavage during apoptosis. Two major pathways of apoptosis converge on caspases (Fig. 2).29 – 31 The 'intrinsic' cell death pathway is activated by a very broad range of signals, including radiation, cytotoxic drugs, cellular stress and growth gene withdrawal, and involves the release of proteins, including cytochrome c, from the mitochondrial intermembrane space. Cytoplasmic cytochrome c combines with an adaptor molecule, Apaf-ane, and an inactive 'initiator' caspase, procaspase nine, within a multiprotein circuitous chosen the apoptosome. This leads to the activation of caspase 9 which and then triggers a pour of caspase activation, resulting in the morphological and biochemical changes associated with apoptosis (e.g. mediated by the 'effector' caspase, caspase 3). There is a 2nd cell-decease pathway called the 'extrinsic' cell-expiry pathway, which can function independently of mitochondria. This is activated by cell-surface expiry receptors, such as Fas, and straight activates the caspase cascade via the 'initiator' caspase, caspase 8, within a death-inducing signalling circuitous (DISC) (Fig. ii).
Pathways of apoptosis. The 'intrinsic' and 'extrinsic' apoptosis pathways converge on the effector caspases, such as caspase iii, which promote the biochemical and morphological characteristics of apoptosis. The 'extrinsic' pathway is activated by cell-surface death receptors, such every bit Fas, and is mediated by activation of the initiator caspase, caspase 8, inside a decease-inducing signalling complex (DISC). The 'intrinsic' pathway is activated past a wide range of stimuli which trigger the release of cytochrome c from mitochondria, leading to the activation of a distinct initiator caspase, caspase 9, within the apoptosome. In some situations, the activation of caspase eight can lead to the cleavage of Bid, which is besides able to promote cytochrome c release. Bcl-2 family proteins play a primal role in decision-making mitochondrial function associated with the 'intrinsic' cell-death pathway, either preventing or promoting the release of cytochrome c.
Bcl-2 is the prototypical member of a family of structurally related apoptosis command molecules, discovered by virtue of its activation by the t(14:18) translocation in the majority of follicular B-cell lymphomas (FL).30 , 31 The translocation fuses the Bcl-2 open up reading frame with immunoglobulin regulatory sequences, causing overexpression of the intact Bcl-2 protein. Many prison cell-based studies accept demonstrated that overexpression of Bcl-2 enhances resistance to apoptosis, and the oncogenic potential of Bcl-ii has been confirmed in mouse models. Transgenic mice overexpressing Bcl-2 in the B-cell compartment develop B-lymphoid tumours.32
Bcl-ii-related proteins can deed as cellular bodyguards or assassins to positively or negatively command apoptosis.29 – 31 Bcl-two family unit proteins are characterized by the presence of upward to four relatively brusk sequence motifs (less than 20 amino acid residues in length) termed Bcl-2 homology (BH) domains. More than 20 family members have been identified and these tin be divided into three subfamilies, based on structural and functional features (Fig. 3). The anti-apoptotic subfamily contains the Bcl-2 and Bcl-XL proteins, which suppress apoptosis and contain all four BH domains, designated BH1–iv. Mcl-1 is some other Bcl-2-related survival poly peptide, but is somewhat structurally distinct and probably lacks a 'classical' BH4 domain. Pro-apoptotic proteins, such as Bax, Bak and Bok, contain BH1–3 domains and are termed 'multidomains', whereas other pro-apoptotic proteins, such as Bim, Bad and Bid, comprise only the BH3 domain and are termed 'BH3-only'. Some Bcl-2 family proteins also comprise a carboxy-terminal transmembrane domain. The targeting of Bcl-2 family proteins to mitochondrial membranes is thought to play an important role in controlling the function of mitochondria, cardinal participants in apoptotic cell death. Bcl-2 proteins as well target other organelles: this is probably important for apoptosis control just is less well understood.
Schematic diagram of representative Bcl-2 family proteins. Examples of anti-apoptotic (Bcl-2, Bcl-XL and Mcl-1), pro-apoptotic multidomain (Bax, Bak and Bok) and pro-apoptotic BH3-only (Bid, Bim, Bad) proteins are shown. Bcl-2 homology (BH) and transmembrane (TM) domains are indicated. Note that Mcl-1 may not contain a classical BH4 domain.
The relative expression (or activity) of various anti-apoptotic and pro-apoptotic Bcl-ii family proteins is a critical determinant of apoptosis sensitivity. Precisely how the functions of the dissever subfamilies and individual Bcl-2 proteins are coordinated to control apoptosis is far from clear, but one primal target appears to exist the release of cytochrome c from mitochondria and, thereby, downstream caspase activation. Bcl-2 prevents mitochondrial cytochrome c release, whereas the improver of Bax or BH3 peptides to isolated mitochondria is sufficient to promote cytochrome c release. Bcl-ii family proteins act in a coordinated way to command apoptosis, and diverse homotypic and heterotypic interactions, mediated via the BH domains of these proteins, have been described. This is considered key for their biological role, every bit mutation of BH domains prevents dimerization and inactivates Bcl-ii family proteins. The basic mode of interaction of Bcl-2 family unit proteins is the insertion of an blastoff-helical BH3 domain into a shallow binding groove formed past the BH1, 2 and 3 domains of a second Bcl-two family poly peptide (Fig. 4).33 Through the control of cytochrome release, Bcl-2 proteins modulate the 'intrinsic' cell-death pathway, but straight regulation of caspases, for example, by physical sequestration, may likewise play a role.34 Under some conditions, death receptors which tin activate the 'extrinsic' pathway can also cross-talk to the 'intrinsic' pathway, by caspase-mediated cleavage of a BH3-only Bcl-ii family molecule, Bid, which targets the mitochondria (Fig. ii). Therefore, in some jail cell settings, Bcl-ii proteins can besides command susceptibility to death receptor-induced apoptosis.
Three-dimensional construction of Bcl-2 family proteins. Bcl-XL structure showing insertion of the Bak BH3 alpha-helix (shown in black) into a binding groove formed by domains BH1–iii of Bcl-Ten50.33 Due north, amino-terminus; C, carboxy-terminus.
Apoptosis resistance in CLL: bad influences or the devil within?
CLL is commonly considered as a epitome for a malignancy of failed apoptosis, equally CLL cells circulating in the blood are largely non-proliferating and arrested in the G0/G1 phase of the cell bicycle. Prison cell division must occur, presumably in 'proliferation centres' in tissue microenvironments, accounting for the inexorable ascension in white claret jail cell counts in some patients and evidenced by the shortening of telomeres.35 Still, lack of apoptosis is considered a major component of the dysregulation of normal B-cell homeostasis in all subsets of this malignancy.
A critical question is to what extent the apoptotic resistance machinery of CLL cells are autonomous, for instance, owing to genetic or epigenetic alterations in the expression of apoptosis regulators inside the CLL cell, or are driven by environmental signals that are received past the CLL in vivo. The office of the microenvironment is clear from the well-recognized propensity of CLL cells to undergo apoptosis when placed in ex vivo culture,36 but intrinsic cellular alterations could likewise contribute.
CLL cells generally undergo rapid 'spontaneous' apoptosis post-obit ex vivo culture.36 , 37 Clearly the CLL cell does non have autonomous survival capability and must depend, to some extent, on environmental signals. However, does this preclude a role for autonomous genetic/epigenetic alterations that confer further apoptosis resistance? Put some other style, without these changes perchance a CLL cell could accept died even more than rapidly post-obit removal from the body. Follicular lymphoma cells likewise undergo apoptosis when placed in culture,38 even though they carry the t(14:18) translocation, the authentication genetic amending conferring cellular resistance to apoptosis. Thus, even cells which have genetically deranged apoptosis control mechanisms can readily undergo cell death ex vivo. There is besides considerable patient-to-patient heterogeneity in the rate of CLL cell apoptosis ex vivo, suggesting that intrinsic susceptibility to apoptosis may differ. It will be difficult to establish the relative contribution of cell autonomous and surroundings-derived survival signals for apoptosis dysregulation in the CLL cell, without identifying the elusive normal B-cell counterpart. Fifty-fifty then, comparisons would exist difficult, as the CLL cell may be nudged towards apoptosis by oncogene activation/genomic harm. Although surround-derived signals are important, information technology may exist a combination of jail cell autonomous and exogenous survival signals that underlie defective apoptosis in CLL.
Bcl-2 and Bax in CLL
Essentially all CLL cells express loftier levels of Bcl-2. Expression generally appears to exist equivalent to or to exceed that in normal peripheral blood lymphocytes, or even that in cells containing the t(xiv:18) translocation.39 The mechanisms that mediate Bcl-2 expression in CLL remain unclear. Information technology has been suggested that a small proportion of patients incorporate the t(fourteen:eighteen) translocation that is commonly found in FL, although information technology remains controversial as to whether these cases are truly CLL.40 In the majority of individuals, the promoter region for Bcl-2 is hypomethylated, which may contribute to increased transcription and Bcl-ii protein expression in CLL.39 In FL, the t(14:xviii) translocation provides strong show that activation of Bcl-2 is a tumour-associated consequence. However, the absence of an analogous genetic alteration in the majority of CLL means that information technology is unclear whether the abundant expression of Bcl-2 in CLL is caused during leukaemogenesis or reflects the origin of CLL from B cells that ordinarily limited Bcl-ii.
Despite this unanswered question, the expression of Bcl-2 and Bax and, peradventure more than significantly, the relative expression of these functional antagonists, is an of import variable in CLL. CLL cells tin can have increased Bcl-2/Bax ratios (favouring cell survival) compared to normal controls in at to the lowest degree some individuals.41 , 42 Individual variation in the expression of Bcl-2/Bax correlates with apoptosis and clinical outcome. For example, decreased Bcl-2/Bax ratios are associated with increased sensitivity to cytotoxic drugs in vitro and improved responses to chemotherapy in patients.41 – 43 The relevance of these differences for apoptosis command is demonstrated by experiments using antisense oligodeoxynucleotides (Every bit-ODN). Those aimed specifically to interfere with the expression of Bcl-2, enhanced spontaneous apoptosis and sensitivity to chlorambucil.44 , 45 By dissimilarity, Bax Every bit-ODN delayed spontaneous apoptosis.46 Even so, other proteins are also likely to contribute to apoptosis control in CLL. For case, the expression of other Bcl-2 family unit proteins, such as Mcl-1 (see below) and unrelated survival proteins (Bag-1), may besides correlate with response to cytotoxic therapy in patients.42 , 47 Notably, variation in susceptibility to apoptosis and in the expression of the regulatory proteins, Bcl-2, Bax and Mcl-1, exercise not announced to correlate consistently with stage.42 , 47 Correlations with disease subtypes identified past the presence of VH mutations have non been reported.
Do tumour-associated events as well underlie variation in the expression of Bax in CLL? Bax mutations accept been described in cancerous B cells, leading to a lack of expression or functional inactivation of the protein.48 – l In some cases, these are clearly associated with microsatellite instability [where unmarried base insertions or deletions in a K(8) tract within the Bax open-reading frame outcome in a frameshift mutation and premature termination of translation], merely the absenteeism of pregnant microsatellite instability in CLL and direct sequence analysis does non support the thought that mutations of the coding sequence alter the function/expression of Bax in CLL.51
Sequence alterations in the Bax promoter may contribute to decreased expression in some cases of CLL. In a study of 34 patients, Saxena et al. described a G-to-A polymorphism in the Bax 5′ untranslated region, at position −248 relative to the translation starting time site and 146 nucleotides downstream from the TATA box.51 A heterozygous polymorphism was present in 69% of stage I–Four CLL, but in just half-dozen% of stage 0 CLL and 4% of controls. The polymorphism was homozygous in RL lymphoma cells, which did not limited Bax protein. In CLL, the presence of the polymorphism was associated with decreased Bax expression (P= 0·05), progression beyond stage 0 (P = 0·0002) and failure to achieve consummate remission (P = 0·038).51 Although of significant interest, further work is required to determine the frequency of the polymorphism in larger cohorts of CLL patients and normal individuals and to determine directly whether the G-to-A change directly modulates Bax expression, due east.chiliad. via modulating RNA stability or promoter activeness. Given the familial component to CLL it would be interesting to determine the frequency of the polymorphism in CLL twins and families.
Mcl-i: new kid on the block
More recent analyses accept revealed an important function for some other Bcl-2 family protein, Mcl-1, in CLL. Like Bcl-2, Mcl-1 is a survival protein, but one with singled-out features. It was first discovered in differentiating myeloid cells where it is thought to play a transient role in promoting prison cell survival (reviewed in refs 52 , 53), simply has since been shown to be expressed in various tissues and malignant cells, including CLL, where its expression is significantly associated with a failure to accomplish consummate remission following cytotoxic therapy.42 , 47 Overexpression of Mcl-1 in transgenic mice predisposes to lymphomagenesis, and antisense ablation experiments have demonstrated that Mcl-one is required for the survival of multiple myeloma and lymphoma cells.52 – 54 Interestingly, the incidence of tumours is higher in mice expressing an Mcl-1 transgene than in those expressing Bcl-2, and the tumours that develop more than closely resemble lymphomas. Mcl-1 is an essential gene in the mouse, and deletion results in very early preimplantation lethality.55 Experiments with chimeric mice demonstrate that Mcl-1 is as well essential for B- and T-cell development.56
One of the remarkable features of Mcl-one is its rapid regulation, and Mcl-one is thought to play a critical role in regulating apoptosis in response to rapidly irresolute environmental cues.52 , 53 For example, Mcl-one expression is rapidly induced via control of transcription and protein stability by various growth factors and survival signals. Compared to other Bcl-2 family survival proteins, Mcl-i is structurally distinct, containing PEST sequences in its amino-terminal regions. Mcl-one poly peptide has a rapid turnover, and is targeted for degradation past the proteasome through ubiquitination. Mcl-1 typically has a half-life of a few hours, a common feature of highly modulated proteins. Although PEST sequences are often important determinants of degradation, the office of the Mcl-1 PEST sequences in determining rapid poly peptide turnover remains unclear.57
Mcl-one tin can also be negatively controlled, down-regulated through control of transcription, translation and stability in response to a wide range of pro-apoptotic signals. Early down-regulation of Mcl-1 may be essential for initiation of the apoptotic cascade in response to DNA harm, in at to the lowest degree some systems.58 , 59
Although the rapid turnover of Mcl-1 in healthy cells is largely caused past proteasome-mediated deposition, other proteolytic fates look Mcl-1. During apoptosis, Mcl-one is a very efficient substrate for caspases.53 , 60 – 62 Caspase cleavage of Mcl-1 simultaneously inactivates the survival function of this protein and converts Mcl-one into a cell death-promoting molecule, activating a positive feedback loop that results in increased caspase activation.53 Therefore, Mcl-ane acts as a molecular switch during apoptosis, converted from a molecular bodyguard to assassin, by proteolytic cleavage. This mechanism has also been described for Bcl-2 and Bcl-XFifty, but it appears that Mcl-1 is a particularly efficient substrate for caspases. In CLL cells undergoing apoptosis, both caspase cleavage53 , 60 and down-regulation of expression, via mechanisms obviously contained of caspase cleavage,63 , 64 accept been described, possibly influenced past the nature of the apoptotic betoken.
As discussed above, microenvironment signals are idea to play a critical role in CLL. How are microenvironment-derived survival signals communicated to the apoptosis control machinery? A peachy many signals tin can promote the survival of CLL cells in civilization, including cytokines and 'nurse' and other stroma-similar cells.27 , 65 Although it is hard to know which of these signals might play a predominant function in promoting CLL survival in specific microenvironments inside the body, the up-regulation of Mcl-one appears to play a key role in at least ane of these systems. The HK jail cell line resembles follicular dendritic cells and is able to promote the survival of CLL cells for extended periods of time.28 , 66 HK-promoted survival is mediated, in function, past CD44, and is associated with an increased expression of Mcl-i. Antisense ablation of Mcl-1 reverses the survival mediated past HK cells, confirming the importance of Mcl-one in this organization. However, in other systems, alternate apoptosis control molecules may play a predominant role. CLL-associated microenvironments are rich in T cells, and T-jail cell-derived survival signals mediated by engagement of the CD40 receptor on CLL cells could provide another relevant pathway of CLL survival in vivo. In contrast to HK cells, CD40 up-regulates the expression of Bcl-XFifty and as well of survivin, an inhibitor of caspases.28 , 67 In complex tissues, multiple microenvironment-derived signals may act on the CLL cells to co-ordinately modulate cell survival.
Finally, the contempo identification of modest sequence insertions in the Mcl-1 promoter in a subset of CLL has provided further evidence that Mcl-ane tin can play an important role in CLL. The study of Saxena et al. demonstrated the presence of half dozen- or xviii-base pair (bp) insertions in the Mcl-one promoter in 17/58 (29%) patients with CLL, simply in none of the controls.68 The insertion site and the sequences of the half dozen- or xviii-bp inserted regions were the aforementioned in all affected individuals. Although the consequence of the insertions on promoter activeness was not tested, the presence of insertions was correlated with elevated Mcl-1 expression at both RNA and poly peptide levels. The presence of insertions was correlated with rapid disease progression (P = 0·012) and, more strongly, with a poor response to chemotherapy (P = 0·001) and shorter disease-specific survival (P < 0·001). The insertions were more than commonly found in CD38-negative patients, suggesting that they may nowadays a poor prognostic mark in what is otherwise a relatively favourable group.
Conclusions
Although widely considered every bit a paradigm for a malignancy of failed apoptosis, the mechanisms and significance of resistance to apoptosis in CLL remain poorly defined. In this review, nosotros have focused on the function of Bcl-2 family proteins in apoptosis command in CLL. The presence of loftier levels of Bcl-2 in the majority of patients, and the genetic alterations of Mcl-1 in a subset, contribute to an emerging moving picture that these disquisitional control molecules are dysregulated in CLL. However, it is important to consider that other apoptosis regulators, such as p53 and ATM, are besides altered in CLL cells.
The first central question is, 'To what extent does apoptosis resistance in CLL stem from cell autonomous or exogenous survival signals?'. The well-described propensity of CLL cells to undergo rapid apoptosis when removed from the torso clearly shows that survival is non 'hard wired' in CLL cells and that these cells require specific signals in the torso for survival. The touch on of a multitude of potential microenvironment-derived signals on CLL cells has been described in detail, but the precise nature of the signals relevant for CLL survival in the body remain unclear. It is likely that multiple environmental factors and target genes contribute to apoptosis command in CLL cells.
In contrast to FL, where the recurrent t(fourteen:xviii) translocation provides directly evidence for tumour-associated dysregulation of apoptosis, it is unclear whether the high-level expression of Bcl-2 in CLL is acquired during tumorigenesis or whether information technology reflects abundant Bcl-2 expression in the cell(s) of origin of CLL. However, as at that place is now evidence for genetic alteration of apoptosis command molecules in at least some CLL (p53, ATM, Bax and Mcl-1 promoter alterations), it seems probable that the intrinsic apoptosis sensitivity is altered in at to the lowest degree some CLL, and this may contribute to patient-to-patient heterogeneity.
Resistance to apoptosis can contribute to the development of cancer and decide the response to therapy. A second key question is therefore, 'To what extent does variation in apoptosis sensitivity bear on on the clinical behaviour of CLL?'. Although CLL exists as at least ii distinct subsets of disease (Fig. 1), in that location is no compelling show that consequent differences in apoptosis regulation underlie the variable clinical course associated with these subtypes. By contrast, differences in BCR signalling capacity do announced to correlate. We favour a model where an underlying resistance to apoptosis, potentially driven by both environmental and/or jail cell autonomous events, contributes to all CLL, providing a foundation on which different (super)antigen-driven events (such as anergy) make a significant contribution to variation in the clinical course. Notwithstanding, interpatient variability in the expression of Bcl-2 family unit proteins has been demonstrated in CLL. Although identified in some studies, these variables are not consistently associated with disease stage, suggesting that they are non important determinants of clinical progression, and direct correlations with VH status have not been reported. By dissimilarity, these changes are relatively strongly correlated with differences in response to cytotoxic therapy. For example, differences in expression of the Bcl-2/Bax ratio, Mcl-1 expression and the presence of potentially activating insertions in the Mcl-one promoter, all correlate with response to therapy.41 , 42 , 47 , 68 Therefore, we advise that a degree of apoptosis resistance provides a stage on which antigen/BCR-mediated events modulate the clinical course of the disease. Even so, once the illness progresses and therapy is initiated, individual differences in apoptosis susceptibility, mediated in role by Bcl-2 family unit proteins but also influenced by deficits in p53 and ATM, may play a significant role in determining response to therapy.
The expression and functional part of Bcl-2 and related survival proteins in CLL cells suggests that targeting these molecules has therapeutic potential. Indeed, results of antisense ablation of Bcl-2 in preclinical experiments accept provided some encouragement and trials are ongoing. However, commitment and specificity remain problematic. A new generation of small-molecule inhibitors of Bcl-2-related proteins, the so-called BH3-mimetics, which can neutralize survival proteins by mimicking the binding of proapoptotic BH3-only proteins, may offer a new avenue.69 Fine tuning of the structures of these molecules may permit the inhibition of specific Bcl-ii proteins. Their potential activity in CLL must be weighed against the risk of toxicity; for example, deletion of Mcl-1 is lethal in mice. Regardless, they volition be exciting agents to appraise in clinical trials and valuable chemical tools for using to further investigate the function of Bcl-two family unit proteins in CLL.
Acknowledgments
Nosotros wish to thank Kathy Potter, Ian Mockridge, Myra Armstrong, Louise Neville, Christian Ottensmeier, Andrew Duncombe, Debbie Richardson and Becky Pickering for invaluable contributions. Piece of work on CLL in the Stevenson and Packham laboratories is funded by Cancer Research Uk, Tenovus and the Leukaemia Research Fund. We apologise for not being able to cite all main literature; this is because of space constraints.
References
1. Caligaris-Cappio F, Hamblin TJ. B-cell chronic lymphocytic leukemia: a bird of a different plume. J Clin Oncol. 1999;17:399–408. 2. [PubMed] [Google Scholar]
2. Chiorazzi Due north, Ferrarini M. B cell chronic lymphocytic leukemia: lessons learned from studies of the B cell antigen receptor. Annu Rev Immunol. 2003;21:841–94. [PubMed] [Google Scholar]
3. Stevenson FK, Caligaris-Cappio F. Chronic lymphocytic leukemia: revelations from the B-prison cell receptor. Blood. 2004;103:4389–95. [PubMed] [Google Scholar]
four. Hamblin TJ, Davis Z, Gardiner A, Oscier DG, Stevenson FK. Unmutated Ig V (H) genes are associated with a more aggressive form of chronic lymphocytic leukemia. Claret. 1999;94:1848–54. [PubMed] [Google Scholar]
v. Damle RN, Wasil T, Fais F, et al. Ig V factor mutation status and CD38 expression as novel prognostic indicators in chronic lymphocytic leukemia. Claret. 1999;94:1840–vii. [PubMed] [Google Scholar]
6. Fais F, Ghiotto F, Hashimoto S, et al. Chronic lymphocytic leukemia B cells express restricted sets of mutated and unmutated antigen receptors. J Clin Invest. 1998;102:1515–25. [PMC free article] [PubMed] [Google Scholar]
vii. Klein U, Tu Y, Stolovitzky GA, et al. Factor expression profiling of B cell chronic lymphocytic leukemia reveals a homogeneous phenotype related to memory B cells. J Exp Med. 2001;194:1625–38. [PMC free article] [PubMed] [Google Scholar]
8. Morabito F, Mangiola M, Oliva B, et al. Peripheral blood CD38 expression predicts survival in B-cell chronic lymphocytic leukemia. Leuk Res. 2001;25:927–32. [PubMed] [Google Scholar]
9. Hamblin TJ, Orchard JA, Ibbotson RE, Davis Z, Thomas PW, Stevenson FK, Oscier DG. CD38 expression and immunoglobulin variable region mutations are independent prognostic variables in chronic lymphocytic leukemia, but CD38 expression may vary during the class of the disease. Blood. 1999;94:1840–7. [PubMed] [Google Scholar]
ten. Zupo S, Isnardi L, Megna Thou, Massara R, Malavasi F, Dono K, Cosulich Eastward, Ferrarini M. CD38 expression distinguishes two groups of B-jail cell chronic lymphocytic leukemias with different responses to anti-IgM antibodies and propensity to apoptosis. Blood. 1996;88:1365–74. [PubMed] [Google Scholar]
11. Lanham S, Hamblin T, Oscier D, Ibbotson R, Stevenson F, Packham G. Differential signaling via surface IgM is associated with VH gene mutational status and CD38 expression in chronic lymphocytic leukemia. Blood. 2003;101:1087–93. [PubMed] [Google Scholar]
12. Chen L, Widhopf G, Huynh L, Rassenti L, Rai KR, Weiss A, Kipps TJ. Expression of ZAP-seventy is associated with increased B-cell receptor signaling in chronic lymphocytic leukemia. Claret. 2002;100:4609–14. [PubMed] [Google Scholar]
13. Rosenwald A, Alizadeh AA, Widhopf One thousand, et al. Relation of gene expression phenotype to immunoglobulin mutation genotype in B jail cell chronic lymphocytic leukemia. J Exp Med. 2001;194:1639–47. [PMC complimentary article] [PubMed] [Google Scholar]
14. Chen 50, Apgar J, Huynh L, Dicker F, Giago-McGahan T, Rassenti L, Weiss A, Kipps TJ. ZAP-70 directly enhances IgM signaling in chronic lymphocytic leukemia. Blood. 2004 Epub. [PubMed] [Google Scholar]
15. Schimmer AD, Munk-Pedersen I, Minden MD, Reed JC. Bcl-2 and apoptosis in chronic lymphocytic leukemia. Curr Treat Options Oncol. 2003;4:211–eight. [PubMed] [Google Scholar]
16. Jewell AP. Function of apoptosis in the pathogenesis of B-jail cell chronic lymphocytic leukaemia. Br J Biomed Sci. 2002;59:235–viii. [PubMed] [Google Scholar]
17. Pettitt AR, Sherrington PD, Stewart 1000, Cawley JC, Taylor AM, Stankovic T. Dysfunction in B-cell chronic lymphocytic leukemia: inactivation of ATM equally an culling to TP53 mutation. Blood. 2001;98:814–22. [PubMed] [Google Scholar]
eighteen. Cuni Due south, Perez-Aciego P, Perez-Chacon G, et al. A sustained activation of the PI3K/NF-kappaB pathway is critical for the survival of chronic lymphocytic leukemia B cells. Leukemia. 2004;18:1391–400. [PubMed] [Google Scholar]
19. Ringshausen I, Schneller F, Bogner C, Hipp Due south, Duyster J, Peschel C, Decker T. Constitutively activated phosphatidylinositol-3 kinase (PI-3K) is involved in the defect of apoptosis in B-CLL: association with protein kinase Cdelta. Blood. 2002;100:3741–8. [PubMed] [Google Scholar]
20. Barragan M, Bellosillo B, Campas C, Colomer D, Pons G, Gil J. Involvement of poly peptide kinase C and phosphatidylinositol 3-kinase pathways in the survival of B-cell chronic lymphocytic leukemia cells. Blood. 2002;99:2969–76. [PubMed] [Google Scholar]
21. Romano MF, Lamberti A, Tassone P, et al. Triggering of CD40 antigen inhibits fludarabine-induced apoptosis in B chronic lymphocytic leukemia cells. Claret. 1998;92:990–5. [PubMed] [Google Scholar]
22. Kern C, Cornuel JF, Billard C, et al. Interest of BAFF and APRIL in the resistance to apoptosis of B-CLL through an autocrine pathway. Blood. 2004;103:679–88. [PubMed] [Google Scholar]
23. Schattner EJ. CD40 ligand in CLL pathogenesis and therapy. Leuk Lymphoma. 2000;37:461–72. [PubMed] [Google Scholar]
24. Pu QQ, Bezwoda WR. Interleukin-4 prevents spontaneous in vitro apoptosis in chronic lymphatic leukaemia just sensitizes B-CLL cells to melphalan cytotoxicity. Br J Haematol. 1997;98:413–7. [PubMed] [Google Scholar]
25. MacFarlane Yard, Harper N, Snowden RT, Dyer MJ, Barnett GA, Pringle JH, Cohen GM. Mechanisms of resistance to TRAIL-induced apoptosis in primary B cell chronic lymphocytic leukaemia. Oncogene. 2002;21:6809–eighteen. [PubMed] [Google Scholar]
26. Bernal A, Pastore RD, Asgary Z, Keller SA, Cesarman E, Liou HC, Schattner EJ. Survival of leukemic B cells promoted by engagement of the antigen receptor. Claret. 2001;98:3050–7. [PubMed] [Google Scholar]
27. Burger JA, Tsukada N, Burger M, Zvaifler NJ, Dell'Aquila M, Kipps TJ. Blood-derived nurse-similar cells protect chronic lymphocytic leukemia B cells from spontaneous apoptosis through stromal cell-derived gene-i. Claret. 2000;96:2655–63. [PubMed] [Google Scholar]
28. Pedersen IM, Kitada S, Leoni LM, et al. Protection of CLL B cells by a follicular dendritic prison cell line is dependent on induction of Mcl-1. Blood. 2002;100:1795–801. [PubMed] [Google Scholar]
29. Dark-green DR, Kroemer G. The pathophysiology of mitochondrial prison cell decease. Science. 2004;305:626–9. [PubMed] [Google Scholar]
30. Cory S, Huang DC, Adams JM. The Bcl-2 family: roles in cell survival and oncogenesis. Oncogene. 2003;22:8590–607. [PubMed] [Google Scholar]
31. Kirkin Five, Joos South, Zornig One thousand. The role of Bcl-2 family unit members in tumorigenesis. Biochim Biophys Acta. 2004;1644:229–49. [PubMed] [Google Scholar]
32. Adams JM, Harris AW, Strasser A, Ogilvy Southward, Cory South. Transgenic models of lymphoid neoplasia and evolution of a pan-hematopoietic vector. Oncogene. 1999;18:5268–77. [PubMed] [Google Scholar]
33. Sattler Chiliad, Liang H, Nettesheim D, et al. Structure of Bcl-xL-Bak peptide complex: recognition betwixt regulators of apoptosis. Science. 1997;275:983–6. [PubMed] [Google Scholar]
34. Marsden VS, O'Connor L, O'Reilly LA, et al. Apoptosis initiated by Bcl-2-regulated caspase activation independently of the cytochrome c/Apaf-ane/caspase-nine apoptosome. Nature. 2002;419:634–vii. [PubMed] [Google Scholar]
35. Damle RN, Batliwalla FM, Ghiotto F, et al. Telomere length and telomerase activity delineate distinctive replicative features of the B-CLL subgroups defined by immunoglobulin V gene mutations. Blood. 2004;103:375–82. [PubMed] [Google Scholar]
36. Ghia P, Caligaris-Cappio F. The indispensable role of the microenvironment in the natural history of low-course B-cell neoplasms. Adv Cancer Res. 2000;79:157–73. [PubMed] [Google Scholar]
37. Collins RJ, Verschuer LA, Harmon BV, Prentice RL, Pope JH, Kerr JF. Spontaneous programmed expiry (apoptosis) of B-chronic lymphocytic leukaemia cells following their culture in vitro. Br J Haematol. 1989;71:343–50. [PubMed] [Google Scholar]
38. Ghia P, Boussiotis VA, Schultze JL, Cardoso AA, Dorfman DM, Gribben JG, Freedman Equally, Nadler LM. Unbalanced expression of bcl-two family proteins in follicular lymphoma: contribution of CD40 signaling in promoting survival. Claret. 1998;91:244–51. [PubMed] [Google Scholar]
39. Hanada M, Delia D, Aiello A, Stadtmauer Due east, Reed JC. bcl-ii cistron hypomethylation and high-level expression in B-cell chronic lymphocytic leukemia. Blood. 1993;82:1820–8. [PubMed] [Google Scholar]
40. Nelson BP, Variakojis D, Peterson LC. Leukemic phase of B-cell lymphomas mimicking chronic lymphocytic leukemia and variants at presentation. Modern Pathol. 2002;15:1111–twenty. [PubMed] [Google Scholar]
41. Pepper C, Hoy T, Bentley DP. Bcl-2/Bax ratios in chronic lymphocytic leukaemia and their correlation with in vitro apoptosis and clinical resistance. Br J Cancer. 1997;76:935–viii. [PMC free article] [PubMed] [Google Scholar]
42. Saxena A, Viswanathan S, Moshynska O, Tandon P, Sankaran Thou, Sheridan DP. Mcl-1 and Bcl-2/Bax ratio are associated with treatment response simply not with Rai phase in B-cell chronic lymphocytic leukemia. Am J Hematol. 2004;75:22–33. [PubMed] [Google Scholar]
43. Thomas A, El Rouby S, Reed JC, Krajewski S, Silber R, Potmesil 1000, Newcomb EW. Drug-induced apoptosis in B-jail cell chronic lymphocytic leukemia: relationship between p53 gene mutation and bcl-2/bax proteins in drug resistance. Oncogene. 1996;12:1055–62. [PubMed] [Google Scholar]
44. Pepper C, Thomas A, Hoy T, Cotter F, Bentley P. Antisense-mediated suppression of Bcl-2 highlights its pivotal role in failed apoptosis in B-cell chronic lymphocytic leukaemia. Br J Haematol. 1999;107:611–5. [PubMed] [Google Scholar]
45. Pepper C, Hooper K, Thomas A, Hoy T, Bentley P. Bcl-two antisense oligonucleotides enhance the cytotoxicity of chlorambucil in B-cell chronic lymphocytic leukaemia cells. Leuk Lymphoma. 2001;42:491–8. [PubMed] [Google Scholar]
46. Pepper C, Thomas A, Hoy T, Bentley P. Antisense oligonucleotides complementary to Bax transcripts reduce the susceptibility of B-prison cell chronic lymphocytic leukaemia cells to apoptosis in a bcl-ii independent fashion. Leuk Lymphoma. 2002;43:2003–9. [PubMed] [Google Scholar]
47. Kitada South, Andersen J, Akar S, et al. Expression of apoptosis-regulating proteins in chronic lymphocytic leukemia: correlations with In vitro and In vivo chemoresponses. Blood. 1998;91:3379–89. [PubMed] [Google Scholar]
48. Meijerink JP, Mensink EJ, Wang K, Sedlak TW, Sloetjes AW, de Witte T, Waksman G, Korsmeyer SJ. Hematopoietic malignancies demonstrate loss-of-function mutations of BAX. Blood. 1998;91:2991–7. [PubMed] [Google Scholar]
49. Brimmell M, Mendiola R, Mangion J, Packham Thousand. Frameshift mutations in cell lines derived from human haemopoietic malignancies are associated with resistance to apoptosis and microsatellite instability. Oncogene. 1998;xvi:1803–12. [PubMed] [Google Scholar]
fifty. Peng H, Aiello A, Packham G, Isaacson PG, Pan L. Infrequent bax gene mutations in B-cell lymphomas. J Pathol. 1998;186:378–82. [PubMed] [Google Scholar]
51. Saxena A, Moshynska O, Sankaran K, Viswanathan S, Sheridan DP. Association of a novel single nucleotide polymorphism, K (−248) A, in the 5′-UTR of BAX gene in chronic lymphocytic leukemia with affliction progression and handling resistance. Cancer Lett. 2002;187:199–205. [PubMed] [Google Scholar]
52. Craig RW. MCL1 provides a window on the role of the BCL2 family in cell proliferation, differentiation and tumorigenesis. Leukemia. 2002;xvi:444–54. [PubMed] [Google Scholar]
53. Michels J, Johnson Pw, Packham K. Mcl-i. Int J Biochem Cell Biol. 2005;37:267–71. [PubMed] [Google Scholar]
54. Michels J, O'Neill JW, Dallman CL, et al. Mcl-i is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule past efficient caspase-mediated cleavage. Oncogene. 2004;23:4818–27. [PubMed] [Google Scholar]
55. Rinkenberger JL, Horning S, Klocke B, Roth G, Korsmeyer SJ. Mcl-1 deficiency results in peri-implantation embryonic lethality. Genes Dev. 2000;14:23–vii. [PMC complimentary commodity] [PubMed] [Google Scholar]
56. Opferman JT, Letai A, Beard C, Sorcinelli Medico, Ong CC, Korsmeyer SJ. Evolution and maintenance of B and T lymphocytes requires antiapoptotic MCL-1. Nature. 2003;426:671–half dozen. [PubMed] [Google Scholar]
57. Akgul C, Moulding DA, White MR, Edwards SW. In vivo localisation and stability of man Mcl-1 using dark-green fluorescent protein (GFP) fusion proteins. FEBS Lett. 2000;478:72–6. [PubMed] [Google Scholar]
58. Nijhawan D, Fang M, Traer East, Zhong Q, Du Gao WF, Wang X. Elimination of Mcl-1 is required for the initiation of apoptosis following ultraviolet irradiation. Genes Dev. 2003;17:1475–86. [PMC free article] [PubMed] [Google Scholar]
59. Cuconati A, Mukherjee C, Perez D, White Eastward. Deoxyribonucleic acid damage response and MCL-1 devastation initiate apoptosis in adenovirus-infected cells. Genes Dev. 2003;17:2922–32. [PMC gratuitous article] [PubMed] [Google Scholar]
60. Snowden RT, Sun XM, Dyer MJ, Cohen GM. Bisindolylmaleimide Ix is a potent inducer of apoptosis in chronic lymphocytic leukaemic cells and activates cleavage of Mcl-i. Leukemia. 2003;17:1981–nine. [PubMed] [Google Scholar]
61. Clohessy JG, Zhuang J, Brady HJ. Characterisation of Mcl-i cleavage during apoptosis of haematopoietic cells. Br J Haematol. 2004;125:655–65. [PubMed] [Google Scholar]
62. Herrant M, Luciano F, Loubat A, Auberger P. The protective effect of phorbol esters on Fas-mediated apoptosis in T cells. Transcriptional and postranscriptional regulation. Oncogene. 2002;21:4957–68. [PubMed] [Google Scholar]
63. Bellosillo B, Villamor N, Colomer D, Pons G, Montserrat Due east, Gil J. In vitro evaluation of fludarabine in combination with cyclophosphamide and/or mitoxantrone in B-jail cell chronic lymphocytic leukemia. Blood. 1999;94:2836–43. [PubMed] [Google Scholar]
64. Pepper C, Thomas A, Hoy T, Fegan C, Bentley P. Flavopiridol circumvents Bcl-2 family mediated inhibition of apoptosis and drug resistance in B-cell chronic lymphocytic leukaemia. Br J Haematol. 2001;114:70–7. [PubMed] [Google Scholar]
65. Panayiotidis P, Jones D, Ganeshaguru Thousand, Foroni L, Hoffbrand AV. Human os marrow stromal cells forestall apoptosis and back up the survival of chronic lymphocytic leukaemia cells in vitro. Br J Haematol. 1996;92:97–103. [PubMed] [Google Scholar]
66. Kim HS, Zhang X, Klyushnenkova E, Choi YS. Stimulation of germinal center B lymphocyte proliferation by an FDC-like prison cell line, HK. J Immunol. 1995;155:1101–9. [PubMed] [Google Scholar]
67. Granziero Fifty, Ghia P, Circosta P, et al. Survivin is expressed on CD40 stimulation and interfaces proliferation and apoptosis in B-prison cell chronic lymphocytic leukemia. Blood. 2001;97:2777–83. [PubMed] [Google Scholar]
68. Moshynska O, Sankaran G, Pahwa P, Saxena A. Prognostic significance of a short sequence insertion in the MCL-1 promoter in chronic lymphocytic leukemia. J Natl Cancer Inst. 2004;96:673–82. [PubMed] [Google Scholar]
69. Oxford SM, Dallman CL, Johnson Pow, Ganesan A, Packham 1000. Current strategies to target the anti-apoptotic Bcl-ii protein in cancer cells. Curr Med Chem. 2004;11:1031–9. [PubMed] [Google Scholar]
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1782118/
0 Response to "Mutations in the Pro-apoptotic Effector Bcl2 Family Proteins"
Post a Comment