- Email: [email protected]
Mcl-1
The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Molecules in focus
Mcl-1 Jorg Michels, Peter W. M. Johnson, Graham Packham∗ Cancer Research UK Oncology Unit, The Somers Cancer Research Building (MP824), University of Southampton School of Medicine, Southampton General Hospital, Southampton SO16 6YD, UK Received 1 March 2004; received in revised form 2 April 2004; accepted 2 April 2004
Abstract Mcl-1 is a Bcl-2 family protein which can act as an apical molecule in apoptosis control, promoting cell survival by interfering at an early stage in a cascade of events leading to release of cytochrome c from mitochondria. Mcl-1 has a short half life and is a highly regulated protein, induced by a wide range of survival signals and also rapidly down regulated during apoptosis. Mcl-1 can also readily be cleaved by caspases during apoptosis to produce a cell death promoting molecule. The multiple levels of control of Mcl-1 expression suggest that Mcl-1 plays a critical role in controlling life and death decisions in response to rapidly changing environmental cues and Mcl-1 is required for embryonic development and the function of the immune system. Expression of Mcl-1 may be useful in informing decision making in the treatment of various cancers, and countering Mcl-1 function may be an attractive therapeutic strategy in malignancy, inflammatory conditions and infectious disease where Mcl-1 may play a major role in suppressing apoptosis. © 2004 Elsevier Ltd. All rights reserved. Keywords: Apoptosis; Bcl-2; Regulation; Caspase; Structure
1. Introduction Apoptosis is an evolutionary conserved cell death pathway essential for tissue homeostasis, development and removal of damaged cells. Deregulation of apoptosis contributes to human disease, including malignancies and neurodegenerative disorders (Thompson, 1995). The Bcl-2 protein family comprises key regulators of cell survival which can suppress (e.g., Bcl-2, Bcl-XL ) or promote (e.g., Bad, Bax) apoptosis (Gross et al., 1999). A wide range of apoptotic stimuli trigger the release of apoptogenic factors, such as cytochrome c, from the mitochondria, resulting in the activation
of caspases. Cleavage of specific protein substrates by caspases leads to the morphological and biochemical feature of apoptosis and Bcl-2 related proteins appear to modulate apoptosis by regulating cytochrome c release. Mcl-1 (myeloid cell leukaemia-1) is an anti-apoptotic member of the Bcl-2 family protein, originally identified in 1993 in differentiating myeloid cells (Kozopas et al., 1993). Since then Mcl-1 has been shown to be expressed in multiple cell lineages and has emerged as a key member of this family of apoptosis control molecules.
2. Structure ∗ Corresponding author. Tel.: +44-23-8079-6184; fax: +44-23-8079-5152. E-mail address: [email protected] (G. Packham).
The human Mcl-1 gene is located on chromosome 1q21 (see Fig. 1 for molecular organisation). The
1357-2725/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2004.04.007
268
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Fig. 1. Molecular organisation of Mcl-1 The human Mcl-1 gene (top) is located on chromosome 1q21 and comprises three exons all of which contribute protein-coding information (highlighted). There are at least two polyadenylation sites for Mcl-1 mRNAs; the 3 site is shown. The nucleotide co-ordinates of the exon boundaries and 3 polyadenylation site (from GenBank accession AF198614) are given relative to transcriptional start site at position 1 (Bingle et al., 2000). Alternate splicing gives rise to distinct Mcl-1 mRNAs (middle) either containing or lacking exon 2 and encoding the Mcl-1L and Mcl-1S/TM isoforms, respectively (Bae et al., 2000; Bingle et al., 2000). The structure and size (in amino-acid residues) of the Mcl-1L and S/TM isoforms are shown (bottom). The PEST, BH (Bcl-2 homology) and TM (transmembrane) domains are indicated, along with the caspase cleavage sites at Asp127 and Asp157 (Michels et al., in press). These residues are also present in Mcl-1S/TM but it is not known whether this isoform is similarly cleaved by caspases. The exon 1/exon 3 splicing giving rise to Mcl-1S/TM causes exon 3 sequences to be translated in a different reading frame and do not encode a TM domain, so sequences at the C-terminus of this isoform are unique to this isoform (shown as TM). Finally, the Mcl-1 cleavage products generated by caspases mediated cleavage within the PEST domain at Asp127 are shown.
prototypical Mcl-1 protein (sometimes referred to as Mcl-1L) comprises 350 amino-acid residues and contains regions of similarity to other Bcl-2 family proteins, termed BH (Bcl-2 homology) domains (Kozopas et al., 1993). BH domains are short motifs which mediate protein:protein interactions between family proteins and are important for apoptosis regulation (Gross et al., 1999). Mcl-1 contains BH domains 1–3, but appears to lack the N-terminal BH4 domain present in Bcl-2 and Bcl-XL . Like many other Bcl-2 family proteins, Mcl-1 also contains a C-terminal transmembrane (TM) domain that serves to localise Mcl-1 to various intracellular membranes, most notably the outer mitochondrial membrane (Yang et al., 1995). This localisation is consistent with a role for Mcl-1 in controlling key mitochondrial events during
apoptosis, although localisation of Mcl-1 to other intracellular membranes has also been observed. The N-terminal parts of Mcl-1 contain two PEST domains, rich in proline, glutamic acid, serine and threonine amino-acid residues. PEST domains are often found in rapidly turned over proteins and Mcl-1 has a short half-life in cells (typically in the range of one to a few hours) (Craig, 2002; Cuconati et al., 2003; Nijhawan et al., 2003). Degradation via the proteasome appears to be the major route responsible for the rapid turnover of Mcl-1. Alternate splicing via skipping of the second Mcl-1 exon gives rise to a second protein isoform, Mcl-1S/TM (Fig. 1). The N-terminal parts of this 271 amino-acid residue protein (including the PEST and BH3 domains) are identical to Mcl-1L, but
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Mcl-1S/TM lacks the BH1, 2 and transmembrane domains (Bae et al., 2000; Bingle et al., 2000). Although the significance of this isoform remains to be determined, the structure of Mcl-1S/TM resembles certain pro-apoptotic “BH3 only” proteins (Gross et al., 1999) and, in marked contrast to Mcl-1L, overexpression of Mcl-1S/TM promotes cell death. Other isoforms of Mcl-1 can be detected by immunoblotting and these may arise from phosphorylation, alternate translation initiation and/or caspase cleavage. However, with the exception of caspase cleavage (see below), these are not characterised in detail.
3. Synthesis and degradation Mcl-1 is expressed in a wide variety of cell types, in the adult and during embryonic development with tissue- and differentiation-specific variations in expression levels. High levels of Mcl-1 are detected in the more differentiated apical layers of epithelia (e.g., in prostate, breast, colon and lung epithelia) while Bcl-2 expression tends to be higher in the basal cell layer (Krajewski et al., 1995). In the lymphoid system, Mcl-1 is abundantly expressed in the germinal centre B-cell compartment in contrast to Bcl-2 which is found in clonally selected B-cells of the mantle zone (Krajewski et al., 1995). These differences suggest unique roles for Mcl-1 and Bcl-2 in apoptosis control. Mcl-1 expression is highly induced by survival and differentiation signals such as cytokines and growth factors (Craig, 2002). Mitogen-activated protein kinase (MAPK), phosphatidylinositol-3 (PI3K) and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) dependant pathways have all been implicated in stimulation of Mcl-1 transcription, acting via specific transcription factor response elements in the Mcl-1 promoter (Craig, 2002; Townsend et al., 1999). However, direct phosphorylation of Mcl-1 may also play a role in controlling Mcl-1 expression/function (Craig, 2002; Inoshita et al., 2002). Mcl-1 expression is also downregulated during apoptosis in many cell systems, often in contrast to the anti-apoptotic Bcl-2 and Bcl-XL proteins (Craig, 2002; Michels et al., in press). Caspase mediated cleavage might contribute to this downregulation of Mcl-1 expression since caspases (in particular, effec-
269
tor caspases, such as caspase 3) can cleave Mcl-1 following two aspartic acid residues (Asp127 and Asp157 in human Mcl-1) (Fig. 1) (Michels et al., in press; Snowden et al., 2003). These residues lie within the PEST domains and are conserved in Mcl-1 proteins in mammals and zebrafish. In cells, Asp127 may be the preferred site of cleavage. The C-terminal fragment that results from cleavage at Asp127 is a potent cell death promoting protein, so caspase cleavage of Mcl-1 simultaneously deprives cells of a survival molecule and generates an effective cell killer (Michels et al., in press). Decreased promoter activity and/or translation might also contribute to downregulation of Mcl-1 during apoptosis in some settings, e.g., following irradiation or in virally infected cells (Cuconati et al., 2003; Gojo et al., 2002; Iglesias-Serret et al., 2003; Nijhawan et al., 2003).
4. Biological functions The rapid induction and degradation of Mcl-1 suggests it plays an important role in apoptotic control in multiple cell types in response to rapidly changing environmental cues (Craig, 2002). Consistent with this Mcl-1 is essential for embryogenesis (Rinkenberger et al., 2000) and for development and maintenance of both B and T lymphocytes in animals (Opferman et al., 2003). Mcl-1 also plays a critical role in the survival of malignant cells since depletion of Mcl-1 via antisense oligodeoxynucleotides triggers apoptosis in cancer cells (Derenne et al., 2002; Michels et al., in press). The exact molecular mechanism by which Mcl-1 promotes cell survival is not completely understood but is thought to involve suppression of cytochrome c release from mitochondria, possibly via heterodimerisation with and neutralisation of pro-apoptotic Bcl-2 family proteins, for example, Bim or Bak (Cuconati et al., 2003; Opferman et al., 2003) (Fig. 2). Interestingly, Mcl-1 may be an apical player in apoptosis control, modulating early events in a cascade leading to cytochrome c release. Therefore, at least in some systems, rapid downregulation of Mcl-1 expression may be required for initiation of the apoptosis cascade (Nijhawan et al., 2003). In addition to its survival promoting functions, Mcl-1 may also play a positive role in apoptosis. The cell death promoting protein that re-
270
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Fig. 2. Regulation of Apoptosis by Mcl-1. (a) Induction of Mcl-1 expression by survival signals may contribute to resistance to apoptosis. (b) Rapid downregulation of Mcl-1 expression following removal of survival factors or exposure to other pro-apoptotic signals may contribute to apoptosis by promoting cytochrome c release. (c) Caspases activated during apoptosis can cleave remaining Mcl-1, generating a potent cell death promoting protein.
sults from caspase cleavage of Mcl-1 may participate in a positive feedback loop leading to further caspase activation (Michels et al., in press). It is possible that Mcl-1 has additional functions, allowing it to impinge directly on cell differentiation and cell cycle control. For example, Mcl-1 binds proliferating cell nuclear antigen (PCNA) causing cell cycle arrest while the transcription factor E2F1, a key cell cycle regulator, represses Mcl-1 expression (Fujise et al., 2000; Croxton et al., 2002).
5. Potential medical applications Given the clear association between defective apoptosis and cancer it is perhaps not surprising that much of the research on Mcl-1 and disease is in the context of malignancy. Most notably, overexpression of Mcl-1 in transgenic mice results in a high incidence of lymphoma, demonstrating that Mcl-1 can directly contribute to the development of malignancies (Zhou et al., 2001). Mcl-1 is widely expressed in human malignant cells and data suggest that Mcl-1 might be
a useful prognostic/predictive marker (Kitada et al., 1998). Since ablating Mcl-1 function is sufficient to promote apoptosis in cancer cells, Mcl-1 might be a target for new anti-cancer therapies, for example small molecules interfering with critical BH domains (Oxford et al., 2004). However, the benefits of targeting Mcl-1 will need to be carefully considered in light of results of knock-out experiments, suggesting potential toxicity in the lymphoid compartment. In addition to its role in malignancy, upregulation of Mcl-1 has been implicated in inappropriate survival of virally or bacterially infected cells and in inflammatory conditions, suggesting that interfering with Mcl-1 might be therapeutically beneficial in many other disease settings (Moulding et al., 1998; Sly et al., 2003).
Acknowledgements Work on Mcl-1 in the authors’ laboratory is supported by Cancer Research UK. We apologise for being unable to cite all original work due to limitations in space.
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
References Bae, J. et al., (2000). MCL-1S, a splicing variant of the antiapoptotic BCL-2 family member MCL-1, encodes a proapoptotic protein possessing only the BH3 domain. Journal of Biological Chemistry, 275, 25255–25261. Bingle, C. D. et al., (2000). Exon skipping in Mcl-1 results in a bcl-2 homology domain 3 only gene product that promotes cell death. Journal of Biological Chemistry, 275, 22136–22146. Craig, R. W. (2002). MCL-1 provides a window on the role of the BCL2 family in cell proliferation, differentiation and tunorigenesis. Leukemia, 16, 444–454. Croxton, R. et al., (2002). Direct repression of the Mcl-1 promoter by E2F1. Oncogene, 21, 1359–1369. Cuconati, A. et al., (2003). DNA damage response and MCL-1 destruction initiate apoptosis in adenovirus-infected cells. Genes and Development, 17, 2922–2932. Derenne, S. et al., (2002). Antisense strategy shows that Mcl-1 rather than Bcl-2 or Bcl-x(L) is an essential survival protein of human myeloma cells. Blood, 100, 194–199. Fujise, K. et al., (2000). Regulation of apoptosis and cell cycle progression by MCL1. Differential role of proliferating cell nuclear antigen. Journal of Biological Chemistry, 275, 39458– 39465. Gojo, I. et al., (2002). The cyclin-dependent kinase inhibitor flavopiridol induces apoptosis in multiple myeloma cells through transcriptional repression and down-regulation of Mcl-1. Clinical Cancer Research, 8, 3527–3538. Gross, A. et al., (1999). BCL-2 family members and the mitochondria in apoptosis. Genes and Development, 13, 1899– 1911. Iglesias-Serret, D. et al., (2003). Transcriptional and translational control of Mcl-1 during apoptosis. Archives of Biochemistry Biophysics, 417, 141–152. Inoshita, S. et al., (2002). . Phosphorylation and inactivation of myeloid cell leukemia 1 by JNK in response to oxidative stress. Journal of Biological Chemistry, 277, 43730–43734. Kitada, S. et al., (1998). Expression of apoptosis-regulating proteins in chronic lymphocytic leukemia: Correlations with in vitro and in vivo chemoresponses. Blood, 91, 3379–3389. Kozopas, K. M. et al., (1993). MCL1, a gene expressed in programmed myeloid cell differentiation, has sequence similarity to BCL2. Proceedings of National Academy of Sciences of United States of America, 90, 3516–3520.
271
Krajewski, S. et al., (1995). Immunohistochemical analysis of Mcl-1 protein in human tissues. Differential regulation of Mcl-1 and Bcl-2 protein production suggests a unique role for Mcl-1 in control of programmed cell death in vivo. American Journal of Pathology, 146, 1309–1319. Michels, J., et al. (in press). Mcl-1 is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule by efficient caspase-mediated cleavage. Oncogene. Moulding, D. A. et al., (1998). Mcl-1 expression in human neutrophils: Regulation by cytokines and correlation with cell survival. Blood, 92, 2495–2502. Nijhawan, D. et al., (2003). Elimination of Mcl-1 is required for the initiation of apoptosis following ultraviolet irradiation. Genes and Development, 17, 1475–1486. Opferman, J. T. et al., (2003). Development and maintenance of B and T lymphocytes requires antiapoptotic MCL-1. Nature, 426, 671–676. Oxford, S. M. E. et al., (2004). Current strategies to target the anti-apoptotic Bcl-2 protein in cancer cells. Current Medicinal Chemistry, 11, 1031–1039. Rinkenberger, J. L. et al., (2000). Mcl-1 deficiency results in peri-implantation embryonic lethality. Genes and Development, 14, 23–27. Sly, L. M. et al., (2003). Survival of Mycobacterium tuberculosis in host macrophages involves resistance to apoptosis dependent upon induction of antiapoptotic Bcl-2 family member Mcl-1. Journal of Immunology, 170, 430–437. Snowden, R. T. et al., (2003). Bisindolylmaleimide IX is a potent inducer of apoptosis in chronic lymphocytic leukaemic cells and activates cleavage of Mcl-1. Leukemia, 17, 1981– 1989. Thompson, C. B. (1995). Apoptosis in the pathogenesis and treatment of disease. Science, 267, 1456–1462. Townsend, K. J. et al., (1999). Regulation of MCL1 through a serum response factor/Elk-1-mediated mechanism links expression of a viability-promoting member of the BCL2 family to the induction of hematopoietic cell differentiation. Journal of Biological Chemistry, 274, 1801–1813. Yang, T. et al., (1995). The intracellular distribution and pattern of expression of Mcl-1 overlap with, but are not identical to, those of Bcl-2. Journal of Cell Biology, 128, 1173–1184. Zhou, P. et al., (2001). MCL1 transgenic mice exhibit a high incidence of B-cell lymphoma manifested as a spectrum of histologic subtypes. Blood, 97, 3902–3909.
Molecules in focus
Mcl-1 Jorg Michels, Peter W. M. Johnson, Graham Packham∗ Cancer Research UK Oncology Unit, The Somers Cancer Research Building (MP824), University of Southampton School of Medicine, Southampton General Hospital, Southampton SO16 6YD, UK Received 1 March 2004; received in revised form 2 April 2004; accepted 2 April 2004
Abstract Mcl-1 is a Bcl-2 family protein which can act as an apical molecule in apoptosis control, promoting cell survival by interfering at an early stage in a cascade of events leading to release of cytochrome c from mitochondria. Mcl-1 has a short half life and is a highly regulated protein, induced by a wide range of survival signals and also rapidly down regulated during apoptosis. Mcl-1 can also readily be cleaved by caspases during apoptosis to produce a cell death promoting molecule. The multiple levels of control of Mcl-1 expression suggest that Mcl-1 plays a critical role in controlling life and death decisions in response to rapidly changing environmental cues and Mcl-1 is required for embryonic development and the function of the immune system. Expression of Mcl-1 may be useful in informing decision making in the treatment of various cancers, and countering Mcl-1 function may be an attractive therapeutic strategy in malignancy, inflammatory conditions and infectious disease where Mcl-1 may play a major role in suppressing apoptosis. © 2004 Elsevier Ltd. All rights reserved. Keywords: Apoptosis; Bcl-2; Regulation; Caspase; Structure
1. Introduction Apoptosis is an evolutionary conserved cell death pathway essential for tissue homeostasis, development and removal of damaged cells. Deregulation of apoptosis contributes to human disease, including malignancies and neurodegenerative disorders (Thompson, 1995). The Bcl-2 protein family comprises key regulators of cell survival which can suppress (e.g., Bcl-2, Bcl-XL ) or promote (e.g., Bad, Bax) apoptosis (Gross et al., 1999). A wide range of apoptotic stimuli trigger the release of apoptogenic factors, such as cytochrome c, from the mitochondria, resulting in the activation
of caspases. Cleavage of specific protein substrates by caspases leads to the morphological and biochemical feature of apoptosis and Bcl-2 related proteins appear to modulate apoptosis by regulating cytochrome c release. Mcl-1 (myeloid cell leukaemia-1) is an anti-apoptotic member of the Bcl-2 family protein, originally identified in 1993 in differentiating myeloid cells (Kozopas et al., 1993). Since then Mcl-1 has been shown to be expressed in multiple cell lineages and has emerged as a key member of this family of apoptosis control molecules.
2. Structure ∗ Corresponding author. Tel.: +44-23-8079-6184; fax: +44-23-8079-5152. E-mail address: [email protected] (G. Packham).
The human Mcl-1 gene is located on chromosome 1q21 (see Fig. 1 for molecular organisation). The
1357-2725/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2004.04.007
268
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Fig. 1. Molecular organisation of Mcl-1 The human Mcl-1 gene (top) is located on chromosome 1q21 and comprises three exons all of which contribute protein-coding information (highlighted). There are at least two polyadenylation sites for Mcl-1 mRNAs; the 3 site is shown. The nucleotide co-ordinates of the exon boundaries and 3 polyadenylation site (from GenBank accession AF198614) are given relative to transcriptional start site at position 1 (Bingle et al., 2000). Alternate splicing gives rise to distinct Mcl-1 mRNAs (middle) either containing or lacking exon 2 and encoding the Mcl-1L and Mcl-1S/TM isoforms, respectively (Bae et al., 2000; Bingle et al., 2000). The structure and size (in amino-acid residues) of the Mcl-1L and S/TM isoforms are shown (bottom). The PEST, BH (Bcl-2 homology) and TM (transmembrane) domains are indicated, along with the caspase cleavage sites at Asp127 and Asp157 (Michels et al., in press). These residues are also present in Mcl-1S/TM but it is not known whether this isoform is similarly cleaved by caspases. The exon 1/exon 3 splicing giving rise to Mcl-1S/TM causes exon 3 sequences to be translated in a different reading frame and do not encode a TM domain, so sequences at the C-terminus of this isoform are unique to this isoform (shown as TM). Finally, the Mcl-1 cleavage products generated by caspases mediated cleavage within the PEST domain at Asp127 are shown.
prototypical Mcl-1 protein (sometimes referred to as Mcl-1L) comprises 350 amino-acid residues and contains regions of similarity to other Bcl-2 family proteins, termed BH (Bcl-2 homology) domains (Kozopas et al., 1993). BH domains are short motifs which mediate protein:protein interactions between family proteins and are important for apoptosis regulation (Gross et al., 1999). Mcl-1 contains BH domains 1–3, but appears to lack the N-terminal BH4 domain present in Bcl-2 and Bcl-XL . Like many other Bcl-2 family proteins, Mcl-1 also contains a C-terminal transmembrane (TM) domain that serves to localise Mcl-1 to various intracellular membranes, most notably the outer mitochondrial membrane (Yang et al., 1995). This localisation is consistent with a role for Mcl-1 in controlling key mitochondrial events during
apoptosis, although localisation of Mcl-1 to other intracellular membranes has also been observed. The N-terminal parts of Mcl-1 contain two PEST domains, rich in proline, glutamic acid, serine and threonine amino-acid residues. PEST domains are often found in rapidly turned over proteins and Mcl-1 has a short half-life in cells (typically in the range of one to a few hours) (Craig, 2002; Cuconati et al., 2003; Nijhawan et al., 2003). Degradation via the proteasome appears to be the major route responsible for the rapid turnover of Mcl-1. Alternate splicing via skipping of the second Mcl-1 exon gives rise to a second protein isoform, Mcl-1S/TM (Fig. 1). The N-terminal parts of this 271 amino-acid residue protein (including the PEST and BH3 domains) are identical to Mcl-1L, but
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Mcl-1S/TM lacks the BH1, 2 and transmembrane domains (Bae et al., 2000; Bingle et al., 2000). Although the significance of this isoform remains to be determined, the structure of Mcl-1S/TM resembles certain pro-apoptotic “BH3 only” proteins (Gross et al., 1999) and, in marked contrast to Mcl-1L, overexpression of Mcl-1S/TM promotes cell death. Other isoforms of Mcl-1 can be detected by immunoblotting and these may arise from phosphorylation, alternate translation initiation and/or caspase cleavage. However, with the exception of caspase cleavage (see below), these are not characterised in detail.
3. Synthesis and degradation Mcl-1 is expressed in a wide variety of cell types, in the adult and during embryonic development with tissue- and differentiation-specific variations in expression levels. High levels of Mcl-1 are detected in the more differentiated apical layers of epithelia (e.g., in prostate, breast, colon and lung epithelia) while Bcl-2 expression tends to be higher in the basal cell layer (Krajewski et al., 1995). In the lymphoid system, Mcl-1 is abundantly expressed in the germinal centre B-cell compartment in contrast to Bcl-2 which is found in clonally selected B-cells of the mantle zone (Krajewski et al., 1995). These differences suggest unique roles for Mcl-1 and Bcl-2 in apoptosis control. Mcl-1 expression is highly induced by survival and differentiation signals such as cytokines and growth factors (Craig, 2002). Mitogen-activated protein kinase (MAPK), phosphatidylinositol-3 (PI3K) and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) dependant pathways have all been implicated in stimulation of Mcl-1 transcription, acting via specific transcription factor response elements in the Mcl-1 promoter (Craig, 2002; Townsend et al., 1999). However, direct phosphorylation of Mcl-1 may also play a role in controlling Mcl-1 expression/function (Craig, 2002; Inoshita et al., 2002). Mcl-1 expression is also downregulated during apoptosis in many cell systems, often in contrast to the anti-apoptotic Bcl-2 and Bcl-XL proteins (Craig, 2002; Michels et al., in press). Caspase mediated cleavage might contribute to this downregulation of Mcl-1 expression since caspases (in particular, effec-
269
tor caspases, such as caspase 3) can cleave Mcl-1 following two aspartic acid residues (Asp127 and Asp157 in human Mcl-1) (Fig. 1) (Michels et al., in press; Snowden et al., 2003). These residues lie within the PEST domains and are conserved in Mcl-1 proteins in mammals and zebrafish. In cells, Asp127 may be the preferred site of cleavage. The C-terminal fragment that results from cleavage at Asp127 is a potent cell death promoting protein, so caspase cleavage of Mcl-1 simultaneously deprives cells of a survival molecule and generates an effective cell killer (Michels et al., in press). Decreased promoter activity and/or translation might also contribute to downregulation of Mcl-1 during apoptosis in some settings, e.g., following irradiation or in virally infected cells (Cuconati et al., 2003; Gojo et al., 2002; Iglesias-Serret et al., 2003; Nijhawan et al., 2003).
4. Biological functions The rapid induction and degradation of Mcl-1 suggests it plays an important role in apoptotic control in multiple cell types in response to rapidly changing environmental cues (Craig, 2002). Consistent with this Mcl-1 is essential for embryogenesis (Rinkenberger et al., 2000) and for development and maintenance of both B and T lymphocytes in animals (Opferman et al., 2003). Mcl-1 also plays a critical role in the survival of malignant cells since depletion of Mcl-1 via antisense oligodeoxynucleotides triggers apoptosis in cancer cells (Derenne et al., 2002; Michels et al., in press). The exact molecular mechanism by which Mcl-1 promotes cell survival is not completely understood but is thought to involve suppression of cytochrome c release from mitochondria, possibly via heterodimerisation with and neutralisation of pro-apoptotic Bcl-2 family proteins, for example, Bim or Bak (Cuconati et al., 2003; Opferman et al., 2003) (Fig. 2). Interestingly, Mcl-1 may be an apical player in apoptosis control, modulating early events in a cascade leading to cytochrome c release. Therefore, at least in some systems, rapid downregulation of Mcl-1 expression may be required for initiation of the apoptosis cascade (Nijhawan et al., 2003). In addition to its survival promoting functions, Mcl-1 may also play a positive role in apoptosis. The cell death promoting protein that re-
270
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
Fig. 2. Regulation of Apoptosis by Mcl-1. (a) Induction of Mcl-1 expression by survival signals may contribute to resistance to apoptosis. (b) Rapid downregulation of Mcl-1 expression following removal of survival factors or exposure to other pro-apoptotic signals may contribute to apoptosis by promoting cytochrome c release. (c) Caspases activated during apoptosis can cleave remaining Mcl-1, generating a potent cell death promoting protein.
sults from caspase cleavage of Mcl-1 may participate in a positive feedback loop leading to further caspase activation (Michels et al., in press). It is possible that Mcl-1 has additional functions, allowing it to impinge directly on cell differentiation and cell cycle control. For example, Mcl-1 binds proliferating cell nuclear antigen (PCNA) causing cell cycle arrest while the transcription factor E2F1, a key cell cycle regulator, represses Mcl-1 expression (Fujise et al., 2000; Croxton et al., 2002).
5. Potential medical applications Given the clear association between defective apoptosis and cancer it is perhaps not surprising that much of the research on Mcl-1 and disease is in the context of malignancy. Most notably, overexpression of Mcl-1 in transgenic mice results in a high incidence of lymphoma, demonstrating that Mcl-1 can directly contribute to the development of malignancies (Zhou et al., 2001). Mcl-1 is widely expressed in human malignant cells and data suggest that Mcl-1 might be
a useful prognostic/predictive marker (Kitada et al., 1998). Since ablating Mcl-1 function is sufficient to promote apoptosis in cancer cells, Mcl-1 might be a target for new anti-cancer therapies, for example small molecules interfering with critical BH domains (Oxford et al., 2004). However, the benefits of targeting Mcl-1 will need to be carefully considered in light of results of knock-out experiments, suggesting potential toxicity in the lymphoid compartment. In addition to its role in malignancy, upregulation of Mcl-1 has been implicated in inappropriate survival of virally or bacterially infected cells and in inflammatory conditions, suggesting that interfering with Mcl-1 might be therapeutically beneficial in many other disease settings (Moulding et al., 1998; Sly et al., 2003).
Acknowledgements Work on Mcl-1 in the authors’ laboratory is supported by Cancer Research UK. We apologise for being unable to cite all original work due to limitations in space.
J. Michels et al. / The International Journal of Biochemistry & Cell Biology 37 (2005) 267–271
References Bae, J. et al., (2000). MCL-1S, a splicing variant of the antiapoptotic BCL-2 family member MCL-1, encodes a proapoptotic protein possessing only the BH3 domain. Journal of Biological Chemistry, 275, 25255–25261. Bingle, C. D. et al., (2000). Exon skipping in Mcl-1 results in a bcl-2 homology domain 3 only gene product that promotes cell death. Journal of Biological Chemistry, 275, 22136–22146. Craig, R. W. (2002). MCL-1 provides a window on the role of the BCL2 family in cell proliferation, differentiation and tunorigenesis. Leukemia, 16, 444–454. Croxton, R. et al., (2002). Direct repression of the Mcl-1 promoter by E2F1. Oncogene, 21, 1359–1369. Cuconati, A. et al., (2003). DNA damage response and MCL-1 destruction initiate apoptosis in adenovirus-infected cells. Genes and Development, 17, 2922–2932. Derenne, S. et al., (2002). Antisense strategy shows that Mcl-1 rather than Bcl-2 or Bcl-x(L) is an essential survival protein of human myeloma cells. Blood, 100, 194–199. Fujise, K. et al., (2000). Regulation of apoptosis and cell cycle progression by MCL1. Differential role of proliferating cell nuclear antigen. Journal of Biological Chemistry, 275, 39458– 39465. Gojo, I. et al., (2002). The cyclin-dependent kinase inhibitor flavopiridol induces apoptosis in multiple myeloma cells through transcriptional repression and down-regulation of Mcl-1. Clinical Cancer Research, 8, 3527–3538. Gross, A. et al., (1999). BCL-2 family members and the mitochondria in apoptosis. Genes and Development, 13, 1899– 1911. Iglesias-Serret, D. et al., (2003). Transcriptional and translational control of Mcl-1 during apoptosis. Archives of Biochemistry Biophysics, 417, 141–152. Inoshita, S. et al., (2002). . Phosphorylation and inactivation of myeloid cell leukemia 1 by JNK in response to oxidative stress. Journal of Biological Chemistry, 277, 43730–43734. Kitada, S. et al., (1998). Expression of apoptosis-regulating proteins in chronic lymphocytic leukemia: Correlations with in vitro and in vivo chemoresponses. Blood, 91, 3379–3389. Kozopas, K. M. et al., (1993). MCL1, a gene expressed in programmed myeloid cell differentiation, has sequence similarity to BCL2. Proceedings of National Academy of Sciences of United States of America, 90, 3516–3520.
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