- Email: [email protected]
Targeting of transcriptional cofactors by the HPV E6 protein: another tale of David and Goliath
COMMENT Vi e w p o i n t s
Targeting of transcriptional cofactors by the HPV E6 protein: another tale of David and Goliath Mark J. O’Connor
C
BP [cAMP-response-elementbinding protein (CREB)binding protein] and the closely related protein p300 are large transcriptional regulators comprising ~2400 amino acids that are thought to play a fundamental role in a variety of signalmodulated cellular events1. Evidence from studies of CBP/p300associated disease and knockout mice suggests that the cellular levels of these proteins are rate limiting. This idea is supported by the fact that different signal transduction pathways with a mutual dependency for CBP and p300 antagonize one another except when intracellular levels of these proteins are artificially raised. CBP and p300 might, therefore, act as integrators of different signalling pathways because their sequestration by particular transcription factors will select which set of genes are expressed2,3. Interestingly, although CBP and p300 are very large proteins, the majority of transcription factors that bind to these proteins do so at ‘hot spots’, such as the C/H1, KIX and C/H3 regions (Fig. 1). In the case of the C/H3 region, a single, 19-amino-acid sequence termed the transcriptional adapter motif (TRAM) has been identified as the target of many cellular and viral transcription factors, which increases the potential for competitive binding4. CBP and p300 regulate a number of genes involved in both cell-cycle regulation and differentiation, and there is growing evidence that these proteins have a role as tumour suppressors5, not least because they act as cofactors of p53-dependent transcription6,7.
The DNA tumour viruses, including simian virus 40 (SV40), adenoviruses (Ads) and human papillomaviruses (HPVs), subvert cellular pathways to induce entry into the S phase of the cell cycle, thus providing an optimal environment for viral replication. One common target for these viruses is p53 (Refs 6,8). It has been shown that both the Ad E1A protein and the SV40 T antigen (TAg) repress p53-dependent transcription by targeting the cofactors CBP and p300 (Refs 6,9). In the case of E1A, repression of p53 activity involves competitive binding to the CBP or p300 TRAM (Ref. 4). The SV40 TAg also binds to the C/H3 domain, but until recently there was no indication that HPV proteins were targeting CBP or p300. This has now changed with the recent publication of two papers showing that the HPV E6 oncoprotein interacts with, and regulates the function of, CBP and p300 (Refs 10,11). HPV E6 is a relatively small protein (comprising ~150 amino acids) but remarkably, given its size, it has the capacity to bind a plethora of cellular regulators (reviewed in Ref. 12). One of the main targets of E6 is p53. In the retinal photoreceptor cells of transgenic mice in which pRB has been deregulated, by for example the expression of the HPV-16 E7 protein, p53-dependent apoptosis is observed. Those cells expressing M.J. O’Connor is in KuDOS Pharmaceuticals Ltd, 327 Cambridge Science Park, Milton Road, Cambridge, UK CB4 4WG. tel: 144 1223 719719, fax: 144 1223 719720, e-mail: [email protected]
0966-842X/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. TRENDS
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HPV-16 E7 but not p53 do not undergo apoptosis and instead could go on to form retinoblastomas13. Identical results have been observed in cells in which pRB is deregulated and HPV-16 E7 and HPV-16 E6 are coexpressed (Ref. 14). These results suggest a requirement for HPV E6-dependent abolition of p53 activity if the virus is to avoid inducing host cell death after pRB function has been abrogated. Until recently, the downregulation of p53 transcriptional activity by E6 has been attributed to the induced degradation of p53 via the ubiquitination pathway15, a process that involves the formation of a complex with a cellular ubiquitin ligase, E6-AP (Ref. 16). In the first paper to describe an E6–CBP/p300 interaction, Zimmermann et al.10 provided evidence that E6-mediated downregulation of p53 transcriptional activity is dependent upon E6 targeting of CBP or p300, and not E6-AP-promoted p53 degradation10. These data come from the use of a mutant HPV-16 E6 protein that no longer binds E6-AP or promotes p53 degradation in vitro or in vivo. However, this mutant does retain the ability to bind CBP and p300 and can downregulate p53 transcriptional activity as well as the wild-type protein is able to. Additionally, there is a correlation between those HPV-16 E6 domains that can bind CBP or p300 and the repression of p53dependent transcription10. Interestingly, the E6 proteins of HPV-16 and other ‘high-risk’ viruses associated with cervical cancer, but not E6 proteins of ‘low-risk’ HPVs associated with benign lesions, are found to interact with the same CBP/p300 sequence PII: S0966-842X(99)01672-8 FEBRUARY 2000
COMMENT
SV40 TAg Ad E1A HPV E6
Ad E1A HPV E6 1
461 662
Ad E1A HPV E6
1621 1877
CBP
2441
HAT Bromo
C/H2
C/H3 TRAM
C/H1 KIX 1
340 413
1763 1811
p300
2414
HAT RAR ER p53 GR NF-κB TR STAT2 HIF-1
SRC1 p53 CDK2 MyoD TIF2 FOS JUNB pp90RSK TFIIB p53 P/CAF YY1 E2F JUN trends in Microbiology
JUN CREB TAN Myb SAP1A ELK
Fig. 1. A schematic diagram of the transcriptional adapters CBP and p300 and their interacting proteins. CBP and p300 share extensive homology, especially in the domains depicted in colour. The C/H1, KIX and C/H3 domains represent ‘hot spots’ for the interaction of transcriptional regulators, as do the amino and carboxy termini. Also indicated are the bromo domain and the HAT domain, which is responsible for acetyl-transferase activity. The binding sites of the DNA tumour virus proteins Ad E1A, SV40 TAg and HPV E6 are indicated above the schematics and the binding sites for cellular transcription factors are indicated below the schematics. Abbreviations: Ad, adenovirus; CBP, cAMP-response-element-binding protein (CREB)-binding protein; CDK2, cyclin-dependent kinase 2; ER, estrogen receptor; GR, glucocorticoid receptor; HIF- 1, hypoxia-inducible factor 1; HPV, human papilloma virus; NF-kB, nuclear factor kB; pp90RSK, 90-kDa ribosomal S6 kinase; P/CAF, p300/CBP-associated factor; RAR, retinoic acid receptor; SRC1, steroid receptor coactivator 1; STAT2, signal transducers and activators of transcription 2; SV40, simian virus 40; TAN, translocation-associated Notch homologue; TFIIB, transcription factor IIB; TIF2, transcription intermediary factor 2; TR, thyroid hormone receptor; TRAM, transcriptional adapter motif; YY1, ying yang 1.
as that targeted by E1A, namely the C/H3 TRAM. Like E1A, these E6 proteins can inhibit the interaction of p53 with the TRAM and are able to repress p53 activity. Thus, the E6 proteins of high-risk HPVs target p53 in two ways. The first, which could be a more immediate response, is the abrogation of p53 transcriptional activity by binding to CBP or p300. The second is the removal of p53 via E6-APdependent degradation. Together, these complementary functions of E6 could facilitate the effective elimination of cellular p53 activity. In the second paper describing the interaction of E6 with CBP/p300, Patel et al.11 have confirmed the interaction of high-risk E6 proteins with the C/H3 domain. Moreover, they extended these observations
TRENDS
IN
to include interactions with the C/H1 domain (observed for both high-risk and, to a lesser extent, low-risk HPVs) as well as with carboxy-terminal sequences (observed only for high-risk HPV E6 proteins). In addition to the repression of p53 transcriptional activity, Patel et al. also demonstrated the repression of nuclear factor (NF)kB promoter elements by HPV-16 E6. The authors suggest that this could play a role in the downregulation of local immune responses and facilitate persistent infection. However, the E6 protein from the low-risk virus HPV-6 does not repress NF-kB-dependent transcription11. So what is the significance of the finding that HPV E6 proteins target CBP and p300? Firstly, both
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Ad E1A and SV40 TAg depend upon their interaction with CBP or p300 for their cellular transformation properties. It would seem likely that the same is true for E6 proteins of HPVs associated with malignant lesions. The identification of CBP/p300-bindingdeficient E6 mutants should provide the wherewithal to test the importance of this interaction. At this stage, it is still not clear which, if any, of the E6–CBP/p300 domain interactions might be involved in the transformation process. It will therefore be necessary to identify E6 mutants that are capable of selectively inhibiting binding to one or more of the different regions of CBP/p300 (Ref. 11). Given that the C/H3 TRAM is the target of Ad E1A, SV40 TAg and the E6 proteins of HPVs associated with cancer, this interaction might play a pivotal role in the transformation process. Perhaps the binding to other regions of CBP or p300 by the E6 proteins of high- and low-risk HPVs might have other consequences for the regulation of host cell fate. One advantage of targeting important transcriptional co-activators such as CBP and p300 is that it could result in the control of multiple signalling pathways. Such a strategy appears to have been adopted by Ad E1A and could also apply to HPV E6 proteins. Although only small in stature, the E6 proteins’ targeting of a recognized transcriptional ‘giant’ might represent an important victory in the attempt of HPV to control host cell functions. References 1 Janknecht, R. and Hunter, T. (1996) Transcription: a growing coactivator network. Nature 383, 22–23 2 Chakravarti, D. et al. (1996) Role of CBP/p300 in nuclear receptor signalling. Nature 383, 99–103 3 Kamei, Y. et al. (1996) A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors. Cell 85, 403–414 4 O’Connor, M.J. et al. (1999) Characterization of an E1A–CBP interaction defines a novel transcriptional adapter motif (TRAM) in CBP/p300. J. Virol. 73, 3574–3581 5 Giles, R.H. et al. (1998) Conjunction dysfunction: CBP/p300 in human disease. Trends Genet. 14, 178–183
FEBRUARY 2000
COMMENT
6 Lill, N.L. et al. (1997) Binding and modulation of p53 by p300/CBP coactivators. Nature 387, 823–827 7 Gu, W. et al. (1997) Synergistic activation of transcription by CBP and p53. Nature 387, 819–823 8 Mietz, J.A. et al. (1992) The transcriptional transactivation function of wild-type p53 is inhibited by SV40 large T-antigen and by HPV-16 E6 oncoprotein. EMBO J. 11, 5013–5020 9 Eckner, R. et al. (1996) Association of p300 and CBP with simian virus 40 large T antigen. Mol. Cell. Biol. 16, 3454–3464 10 Zimmermann, H. et al. (1999) The human papillomavirus type 16 E6
oncoprotein can downregulate p53 activity by targeting the transcriptional coactivator CBP/p300. J. Virol. 73, 6209–6219 11 Patel, D. et al. (1999) The E6 protein of human papillomavirus type 16 binds to and inhibits coactivation by CBP and p300. EMBO J. 18, 5061–5072 12 Myers, G. and Androphy, E.J. (1995) The E6 protein. In Human Papillomaviruses (Myers, G. et al., eds), pp. 47–57, Los Alamos National Laboratory, Los Alamos, NM, USA 13 Howes, K.A. et al. (1994) Apoptosis or retinoblastoma: alternative fates of photoreceptors expressing the HPV-16
E7 gene in the presence or absence of p53. Genes Dev. 8, 1300–1310 14 Pan, H. and Griep, A.E. (1994) Altered cell-cycle regulation in the lens of HPV16 E6 or E7 transgenic mice: implications for tumor suppressor gene function in development. Genes Dev. 8, 1285–1299 15 Scheffner, M. et al. (1990) The E6 oncoprotein encoded by human papillomavirus types 16 and 18 promotes the degradation of p53. Cell 63, 1129–1136 16 Huibregtse, J.M. et al. (1991) A cellular protein mediates association of p53 with the E6 oncoprotein of human papillomavirus types 16 or 18. EMBO J. 10, 4129–4135
Exploitation of macrophages as a replication niche by Legionella pneumophila Michele S. Swanson and Sheila Sturgill-Koszycki
N
ormally a parasite of freshwater amoebae, Legionella pneumophila can wreak havoc in humans when inhaled into the lung. Outbreaks of Legionnaires’ disease are the conspicuous examples of what is more typically a sporadic, communityacquired or nosocomial pneumonia. L. pneumophila owes its virulence to its remarkable capacity to divert membrane traffic within professional phagocytes1,2. Indeed, a paradoxical consequence of natural selection is that macrophages, normally potent mediators of innate resistance to infections, provide a safe haven for L. pneumophila and other pathogens, including mycobacteria, Leishmania spp., Toxoplasma spp. and Chlamydia spp. Consequently, immunologists, microbiologists and cell biologists are keen to understand the biochemical mechanisms governing the fate of microorganisms once they have been ingested by macrophages.
Two rules of phagosomal life The molecular mechanisms vacuolar pathogens use to evade degradation in macrophage lysosomes are not yet fully understood, but cell biological studies have revealed two common rules. Firstly, to interfere with phagolysosome development, it is presumed that microbial virulence factors interact directly with the phagosomal membrane. For example, maturation of phagosomes containing Leishmania donovani promastigotes is retarded by the polymer lipophosphoglycan, of which there are several million copies on the parasite surface3. Secondly, to influence its fate, a pathogen must M.S. Swanson* and S. Sturgill-Koszycki are in the Dept of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109-0629, USA. *tel: 11 734 647 7295, fax: 11 734 764 3562, e-mail: [email protected]
0966-842X/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. TRENDS
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act as the phagosome is formed. For example, when Toxoplasma gondii actively invades macrophages, it assembles a parasitophorous vacuole, a replication niche which does not become acidic and is not accessible to the endocytic pathway. However, when pathogen invasion is overridden by Fc-mediated phagocytosis, macrophages deliver the pathogen to acidic lysosomes4–6. The Dot–Icm export system Recent work by Coers and colleagues7 has indicated that L. pneumophila could also adhere to these rules: a Dot–Icm transport complex appears to act during phagosome biogenesis to prevent fusion with degradative lysosomes. The dot and icm loci were originally identified by the Isberg and Shuman laboratories, respectively, in genetic screens for mutants defective for growth within macrophages (reviewed in Ref. 8). PII: S0966-842X(99)01674-1 FEBRUARY 2000
Targeting of transcriptional cofactors by the HPV E6 protein: another tale of David and Goliath Mark J. O’Connor
C
BP [cAMP-response-elementbinding protein (CREB)binding protein] and the closely related protein p300 are large transcriptional regulators comprising ~2400 amino acids that are thought to play a fundamental role in a variety of signalmodulated cellular events1. Evidence from studies of CBP/p300associated disease and knockout mice suggests that the cellular levels of these proteins are rate limiting. This idea is supported by the fact that different signal transduction pathways with a mutual dependency for CBP and p300 antagonize one another except when intracellular levels of these proteins are artificially raised. CBP and p300 might, therefore, act as integrators of different signalling pathways because their sequestration by particular transcription factors will select which set of genes are expressed2,3. Interestingly, although CBP and p300 are very large proteins, the majority of transcription factors that bind to these proteins do so at ‘hot spots’, such as the C/H1, KIX and C/H3 regions (Fig. 1). In the case of the C/H3 region, a single, 19-amino-acid sequence termed the transcriptional adapter motif (TRAM) has been identified as the target of many cellular and viral transcription factors, which increases the potential for competitive binding4. CBP and p300 regulate a number of genes involved in both cell-cycle regulation and differentiation, and there is growing evidence that these proteins have a role as tumour suppressors5, not least because they act as cofactors of p53-dependent transcription6,7.
The DNA tumour viruses, including simian virus 40 (SV40), adenoviruses (Ads) and human papillomaviruses (HPVs), subvert cellular pathways to induce entry into the S phase of the cell cycle, thus providing an optimal environment for viral replication. One common target for these viruses is p53 (Refs 6,8). It has been shown that both the Ad E1A protein and the SV40 T antigen (TAg) repress p53-dependent transcription by targeting the cofactors CBP and p300 (Refs 6,9). In the case of E1A, repression of p53 activity involves competitive binding to the CBP or p300 TRAM (Ref. 4). The SV40 TAg also binds to the C/H3 domain, but until recently there was no indication that HPV proteins were targeting CBP or p300. This has now changed with the recent publication of two papers showing that the HPV E6 oncoprotein interacts with, and regulates the function of, CBP and p300 (Refs 10,11). HPV E6 is a relatively small protein (comprising ~150 amino acids) but remarkably, given its size, it has the capacity to bind a plethora of cellular regulators (reviewed in Ref. 12). One of the main targets of E6 is p53. In the retinal photoreceptor cells of transgenic mice in which pRB has been deregulated, by for example the expression of the HPV-16 E7 protein, p53-dependent apoptosis is observed. Those cells expressing M.J. O’Connor is in KuDOS Pharmaceuticals Ltd, 327 Cambridge Science Park, Milton Road, Cambridge, UK CB4 4WG. tel: 144 1223 719719, fax: 144 1223 719720, e-mail: [email protected]
0966-842X/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. TRENDS
IN
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45
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NO. 2
HPV-16 E7 but not p53 do not undergo apoptosis and instead could go on to form retinoblastomas13. Identical results have been observed in cells in which pRB is deregulated and HPV-16 E7 and HPV-16 E6 are coexpressed (Ref. 14). These results suggest a requirement for HPV E6-dependent abolition of p53 activity if the virus is to avoid inducing host cell death after pRB function has been abrogated. Until recently, the downregulation of p53 transcriptional activity by E6 has been attributed to the induced degradation of p53 via the ubiquitination pathway15, a process that involves the formation of a complex with a cellular ubiquitin ligase, E6-AP (Ref. 16). In the first paper to describe an E6–CBP/p300 interaction, Zimmermann et al.10 provided evidence that E6-mediated downregulation of p53 transcriptional activity is dependent upon E6 targeting of CBP or p300, and not E6-AP-promoted p53 degradation10. These data come from the use of a mutant HPV-16 E6 protein that no longer binds E6-AP or promotes p53 degradation in vitro or in vivo. However, this mutant does retain the ability to bind CBP and p300 and can downregulate p53 transcriptional activity as well as the wild-type protein is able to. Additionally, there is a correlation between those HPV-16 E6 domains that can bind CBP or p300 and the repression of p53dependent transcription10. Interestingly, the E6 proteins of HPV-16 and other ‘high-risk’ viruses associated with cervical cancer, but not E6 proteins of ‘low-risk’ HPVs associated with benign lesions, are found to interact with the same CBP/p300 sequence PII: S0966-842X(99)01672-8 FEBRUARY 2000
COMMENT
SV40 TAg Ad E1A HPV E6
Ad E1A HPV E6 1
461 662
Ad E1A HPV E6
1621 1877
CBP
2441
HAT Bromo
C/H2
C/H3 TRAM
C/H1 KIX 1
340 413
1763 1811
p300
2414
HAT RAR ER p53 GR NF-κB TR STAT2 HIF-1
SRC1 p53 CDK2 MyoD TIF2 FOS JUNB pp90RSK TFIIB p53 P/CAF YY1 E2F JUN trends in Microbiology
JUN CREB TAN Myb SAP1A ELK
Fig. 1. A schematic diagram of the transcriptional adapters CBP and p300 and their interacting proteins. CBP and p300 share extensive homology, especially in the domains depicted in colour. The C/H1, KIX and C/H3 domains represent ‘hot spots’ for the interaction of transcriptional regulators, as do the amino and carboxy termini. Also indicated are the bromo domain and the HAT domain, which is responsible for acetyl-transferase activity. The binding sites of the DNA tumour virus proteins Ad E1A, SV40 TAg and HPV E6 are indicated above the schematics and the binding sites for cellular transcription factors are indicated below the schematics. Abbreviations: Ad, adenovirus; CBP, cAMP-response-element-binding protein (CREB)-binding protein; CDK2, cyclin-dependent kinase 2; ER, estrogen receptor; GR, glucocorticoid receptor; HIF- 1, hypoxia-inducible factor 1; HPV, human papilloma virus; NF-kB, nuclear factor kB; pp90RSK, 90-kDa ribosomal S6 kinase; P/CAF, p300/CBP-associated factor; RAR, retinoic acid receptor; SRC1, steroid receptor coactivator 1; STAT2, signal transducers and activators of transcription 2; SV40, simian virus 40; TAN, translocation-associated Notch homologue; TFIIB, transcription factor IIB; TIF2, transcription intermediary factor 2; TR, thyroid hormone receptor; TRAM, transcriptional adapter motif; YY1, ying yang 1.
as that targeted by E1A, namely the C/H3 TRAM. Like E1A, these E6 proteins can inhibit the interaction of p53 with the TRAM and are able to repress p53 activity. Thus, the E6 proteins of high-risk HPVs target p53 in two ways. The first, which could be a more immediate response, is the abrogation of p53 transcriptional activity by binding to CBP or p300. The second is the removal of p53 via E6-APdependent degradation. Together, these complementary functions of E6 could facilitate the effective elimination of cellular p53 activity. In the second paper describing the interaction of E6 with CBP/p300, Patel et al.11 have confirmed the interaction of high-risk E6 proteins with the C/H3 domain. Moreover, they extended these observations
TRENDS
IN
to include interactions with the C/H1 domain (observed for both high-risk and, to a lesser extent, low-risk HPVs) as well as with carboxy-terminal sequences (observed only for high-risk HPV E6 proteins). In addition to the repression of p53 transcriptional activity, Patel et al. also demonstrated the repression of nuclear factor (NF)kB promoter elements by HPV-16 E6. The authors suggest that this could play a role in the downregulation of local immune responses and facilitate persistent infection. However, the E6 protein from the low-risk virus HPV-6 does not repress NF-kB-dependent transcription11. So what is the significance of the finding that HPV E6 proteins target CBP and p300? Firstly, both
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Ad E1A and SV40 TAg depend upon their interaction with CBP or p300 for their cellular transformation properties. It would seem likely that the same is true for E6 proteins of HPVs associated with malignant lesions. The identification of CBP/p300-bindingdeficient E6 mutants should provide the wherewithal to test the importance of this interaction. At this stage, it is still not clear which, if any, of the E6–CBP/p300 domain interactions might be involved in the transformation process. It will therefore be necessary to identify E6 mutants that are capable of selectively inhibiting binding to one or more of the different regions of CBP/p300 (Ref. 11). Given that the C/H3 TRAM is the target of Ad E1A, SV40 TAg and the E6 proteins of HPVs associated with cancer, this interaction might play a pivotal role in the transformation process. Perhaps the binding to other regions of CBP or p300 by the E6 proteins of high- and low-risk HPVs might have other consequences for the regulation of host cell fate. One advantage of targeting important transcriptional co-activators such as CBP and p300 is that it could result in the control of multiple signalling pathways. Such a strategy appears to have been adopted by Ad E1A and could also apply to HPV E6 proteins. Although only small in stature, the E6 proteins’ targeting of a recognized transcriptional ‘giant’ might represent an important victory in the attempt of HPV to control host cell functions. References 1 Janknecht, R. and Hunter, T. (1996) Transcription: a growing coactivator network. Nature 383, 22–23 2 Chakravarti, D. et al. (1996) Role of CBP/p300 in nuclear receptor signalling. Nature 383, 99–103 3 Kamei, Y. et al. (1996) A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors. Cell 85, 403–414 4 O’Connor, M.J. et al. (1999) Characterization of an E1A–CBP interaction defines a novel transcriptional adapter motif (TRAM) in CBP/p300. J. Virol. 73, 3574–3581 5 Giles, R.H. et al. (1998) Conjunction dysfunction: CBP/p300 in human disease. Trends Genet. 14, 178–183
FEBRUARY 2000
COMMENT
6 Lill, N.L. et al. (1997) Binding and modulation of p53 by p300/CBP coactivators. Nature 387, 823–827 7 Gu, W. et al. (1997) Synergistic activation of transcription by CBP and p53. Nature 387, 819–823 8 Mietz, J.A. et al. (1992) The transcriptional transactivation function of wild-type p53 is inhibited by SV40 large T-antigen and by HPV-16 E6 oncoprotein. EMBO J. 11, 5013–5020 9 Eckner, R. et al. (1996) Association of p300 and CBP with simian virus 40 large T antigen. Mol. Cell. Biol. 16, 3454–3464 10 Zimmermann, H. et al. (1999) The human papillomavirus type 16 E6
oncoprotein can downregulate p53 activity by targeting the transcriptional coactivator CBP/p300. J. Virol. 73, 6209–6219 11 Patel, D. et al. (1999) The E6 protein of human papillomavirus type 16 binds to and inhibits coactivation by CBP and p300. EMBO J. 18, 5061–5072 12 Myers, G. and Androphy, E.J. (1995) The E6 protein. In Human Papillomaviruses (Myers, G. et al., eds), pp. 47–57, Los Alamos National Laboratory, Los Alamos, NM, USA 13 Howes, K.A. et al. (1994) Apoptosis or retinoblastoma: alternative fates of photoreceptors expressing the HPV-16
E7 gene in the presence or absence of p53. Genes Dev. 8, 1300–1310 14 Pan, H. and Griep, A.E. (1994) Altered cell-cycle regulation in the lens of HPV16 E6 or E7 transgenic mice: implications for tumor suppressor gene function in development. Genes Dev. 8, 1285–1299 15 Scheffner, M. et al. (1990) The E6 oncoprotein encoded by human papillomavirus types 16 and 18 promotes the degradation of p53. Cell 63, 1129–1136 16 Huibregtse, J.M. et al. (1991) A cellular protein mediates association of p53 with the E6 oncoprotein of human papillomavirus types 16 or 18. EMBO J. 10, 4129–4135
Exploitation of macrophages as a replication niche by Legionella pneumophila Michele S. Swanson and Sheila Sturgill-Koszycki
N
ormally a parasite of freshwater amoebae, Legionella pneumophila can wreak havoc in humans when inhaled into the lung. Outbreaks of Legionnaires’ disease are the conspicuous examples of what is more typically a sporadic, communityacquired or nosocomial pneumonia. L. pneumophila owes its virulence to its remarkable capacity to divert membrane traffic within professional phagocytes1,2. Indeed, a paradoxical consequence of natural selection is that macrophages, normally potent mediators of innate resistance to infections, provide a safe haven for L. pneumophila and other pathogens, including mycobacteria, Leishmania spp., Toxoplasma spp. and Chlamydia spp. Consequently, immunologists, microbiologists and cell biologists are keen to understand the biochemical mechanisms governing the fate of microorganisms once they have been ingested by macrophages.
Two rules of phagosomal life The molecular mechanisms vacuolar pathogens use to evade degradation in macrophage lysosomes are not yet fully understood, but cell biological studies have revealed two common rules. Firstly, to interfere with phagolysosome development, it is presumed that microbial virulence factors interact directly with the phagosomal membrane. For example, maturation of phagosomes containing Leishmania donovani promastigotes is retarded by the polymer lipophosphoglycan, of which there are several million copies on the parasite surface3. Secondly, to influence its fate, a pathogen must M.S. Swanson* and S. Sturgill-Koszycki are in the Dept of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109-0629, USA. *tel: 11 734 647 7295, fax: 11 734 764 3562, e-mail: [email protected]
0966-842X/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. TRENDS
IN
MICROBIOLOGY
47
VOL. 8
NO. 2
act as the phagosome is formed. For example, when Toxoplasma gondii actively invades macrophages, it assembles a parasitophorous vacuole, a replication niche which does not become acidic and is not accessible to the endocytic pathway. However, when pathogen invasion is overridden by Fc-mediated phagocytosis, macrophages deliver the pathogen to acidic lysosomes4–6. The Dot–Icm export system Recent work by Coers and colleagues7 has indicated that L. pneumophila could also adhere to these rules: a Dot–Icm transport complex appears to act during phagosome biogenesis to prevent fusion with degradative lysosomes. The dot and icm loci were originally identified by the Isberg and Shuman laboratories, respectively, in genetic screens for mutants defective for growth within macrophages (reviewed in Ref. 8). PII: S0966-842X(99)01674-1 FEBRUARY 2000