Viruses and apoptosis
Lawrence S Young, Christopher W Dawson and Aristides G Eliopoulos
CRC Institute for Cancer Studies, University of Birmingham Medical School, Birmingham, UK
Virus infection and replication are often associated with apoptosis and this effect is
likely to be responsible for much of the pathology associated with infectious
disease. Many viruses encode proteins which can inhibit apoptosis thereby either
prolonging the survival of infected cells such that the production of progeny virus is
maximised or facilitating the establishment of virus persistence.These viral proteins
target the cellular pathways responsible for regulating apoptosis and have been
instrumental in furthering our understanding of the apoptotic process. Many of the
viruses associated with oncogenic transformation have adopted strategies for
blocking apoptosis highlighting the centrality of this effect in carcinogenesis.
Understanding the mechanisms by which viruses regulate apoptosis may lead to the
development of novel therapies for both infectious disease and cancer.
Correspondence fo.
Prof Lawrence S Young,
CRC Institute for
Cancer Studies,
University of Birmingham
Medical School,
Birmingham B75 2TJ, UK
Apoptosis or programmed cell death is an actively controlled process of
cell suicide characterised by distinctive morphological and biochemical
changes1. Regulation of apoptosis is essential for normal embryonic
development and for homeostasis in adult tissues. This altruistic process
is also important in eliminating cells whose survival might otherwise
prove harmful to the organism as a whole, thereby providing a defense
against viral infection and the development of cancer. Whilst virusinfected cells can often be recognised and destroyed by apoptotic
processes initiated by either virus-specific cytotoxic T lymphocytes or
certain cytokines, many viruses directly induce apoptosis during
infection. Indeed, it is this virally-induced apoptosis that is considered
to be at least partially responsible for the various pathologies associated
with virus infection. Many viruses have evolved mechanisms to block the
premature apoptosis of infected cells facilitating either the establishment
and maintenance of persistent infection or prolonging the survival of
lytically-infected cells such that the production of progeny virus is
maximised (Table 1). Indeed, these viral anti-apoptotic strategies can
also contribute to the pathogenesis of virus infection and, in extreme
situations, promote the oncogenic capacity of certain viruses. Recent
studies of the various mechanisms used by viruses to suppress apoptosis
have shed light on the fundamental biochemical pathways responsible
for regulating programmed cell death.
Bntah Medical Bulletin 1997;53 (No 3) 509-521
©Th« Brituh Council 1997
Apoptoiis
Table!
Viral inhibitors of apoptosis
Virus
Gens product
Function
Adenovirus
E1B19K
Bcl-2 homologue
1,67,13,14,17
EBV
BHRF1
Bd-2 homologue
22,24-27
References
ASFV
LMW5-HL
Bd-2 homologue
47
HHV-8
f
Bd-2 homologue
48
Adenovirus
E1B55K
InoctTvation of p53
5,7,28
SV40
Large T antigen
Inochvation of p53
28
HPV
E6
InactivaKon of p53
28
CowpoX VMVt
crmA
ICE inhibitor
31
Boculo virus
p35
ICE inhibitor
32,33
Boculo virus
lap
ICE inhibitor
32,34-36
Interacts with TRAFs
EBV
LMP1
Induces Bcl-2 in B calls
22,37-40
Into rocts with TRAFs
The role of Bcl-2 and its viral homologues during virus
infection
Many viruses induce apoptosis upon infection of certain cell types but
the precise mechanisms involved remain obscure. Bcl-2, the prototype
inhibitor of apoptosis, can block the cell death induced by Sindbis virus
thereby converting an acute lytic infection into a persistent infection2.
The apoptosis and concomitant cytopathic effects induced by acute
influenza infection are also suppressed in the presence of Bcl-23. HIV
infection of Epstein-Barr virus (EBV)-transformed lymphoblastoid cell
lines induces apoptosis apparently as a consequence of the downregulation in Bcl-2 expression4. Thus, the modulation of endogenous
Bcl-2 expression may influence the outcome of virus infection highlighting the Bcl-2 pathway as a potentially important target for viruses.
It is, therefore, not surprising that certain DNA viruses have evolved
specific virus genes with homology to Bcl-2 which are important in
controlling the host cell response to virus infection.
The adenovirus El A and E1B19K proteins
Whilst adenovirus infection results in relatively benign infections in man,
it is the ability of adenoviruses to transform primary cells in vitro that
has provided a paradigm for understanding the cellular pathways
involved in the deregulated cell growth associated with oncogenesis.
Transformation of cells by adenoviruses type 2, 5 and 12 is dependent on
the early region 1 portion of the viral genome which contains the
transcription units for the E1A and E1B proteins5. The E1A protein
510
Bnhih Medico/ Bulletin 1997,53 (No. 3)
Viruses and apoptosis
activates resting cells into the cell cycle stimulating host cell DNA
synthesis via diverse interactions with numerous cellular proteins
including the retinoblastoma gene product pRb5. This creates a
proliferative cellular environment in which viral DNA replication can
occur. However, it was noted several years ago that infection with
adenoviras mutants lacking the 19K protein encoded by the ElB region
resulted in the degradation of both viral and host cell DNA leading to
premature cell death5. This effect has more recently been characterised
as ElA-induced apoptosis occurring in the absence of a functional ElB
19K protein6. In common with other apoptosis-inducing factors, ElA
expression results m the accumulation of p53 and this response mediates
the ability of ElA to induce apoptosis7'8. Thus, the outcome of ElA
expression is dependent on the p53 status of cells and consequently
mutant p53 blocks the induction of apoptosis by ElA 7 and ElA is able
to transform p53-deficient mouse embryo fibroblasts9. Furthermore,
baby rat kidney (BRK) cells transformed by ElA and a temperaturesensitive p53 proliferate at the restrictive temperature when p53 is
predominantly in the mutant conformation but undergo apoptosis at the
permissive temperature when p53 is predominantly wild type7. Thus,
apoptosis may result from conflicting cell growth signals with ElA
stimulating both cell proliferation and elevated p53 which inhibits cell
cycle progression. This contention is supported by data demonstrating
that the regions of ElA that function to induce cell proliferation
cosegregate with those that are responsible for apoptosis and apoptosis
is only induced by ElA when cell proliferation is inhibited10. These
effects of ElA resemble those of the c-myc protein which can induce
apoptosis under conditions where proliferation is blocked11. More recent
work suggests a possible role for the transcription factor NF-KB in ElAinduced apoptosis12.
ElA-induced p53-mediated apoptosis is suppressed by products of the
adenovirus ElB transcription unit. Two major protein products are
encoded by this region, the ElB 19K or the ElB 55K proteins, and both
of these proteins are able to block ElA-induced apoptosis and cooperate
with ElA for complete growth transformation6-7. The ElB 55K protein
blocks the function of p53 s whilst the precise mechanism by which ElB
19K protein inhibits p53-mediated apoptosis remains unknown. However, the ElB 19K protein shares significant sequence and functional
homology with Bcl-2 suggesting that these two proteins operate to
inhibit apoptosis by similar mechanisms6'13-14. In this regard, Bcl-2 can
substitute for ElB 19K protein during adenovirus infection of human
cells14 and in the cooperation with ElA to transform rodent cells6. Both
ElB 19K and Bcl-2 proteins block p53-dependent apoptosis and can
overcome the transcriptional repression activity of p53 J . Both proteins
inhibit cell death induced by different apoptotic stimuli, including the
Bnfis/iA<Wico/Bu//.tinl997;53(No 3)
511
Apoptosis
cytotoxic drug cisplatin, tumour necrosis factor a (TNFa) and the Fas
antigen although the ElB 19K protein appears to be more effective1.
Recent data show that ElB 19K can antagonise the stimulatory effect of
E1A on NF-KB and similar effects on the activity of this transcription
factor have been observed with Bcl-215-16. In addition to the functional
similarities between ElB 19K and Bcl-2, both these proteins are
membrane-anchored, contain three short regions with sequence homology and interact with a common set of cellular proteins (Nipl,2,3)13. A
comparison of the amino acid sequence of Bcl-2 and ElB 19K revealed
homology over a region of 19K known to be important for structure and
function of the protein with particularly striking homology within the
Bcl-2 homology region 1 (BH1), a functionally important domain
required for heterodimerisation of Bcl-2 with another member of the
Bcl-2 family, Bax1 (Fig. 1). NH1 and NH2 have been identified as other
regions of homology between ElB 19K and Bcl-2 and domain swapping
studies between these proteins have emphasised the significance of the
NH1 domain as a common functional domain in survival promoting
members of the Bcl-2 family17. Two cell death promoting members of
the Bcl-2 family, Bik and Bak, have been identified by their interaction
with ElB 19K and share another common domain designated BH3
which is also present in Bax18-19. Recent work demonstrates that the BH3
region of Bax mediates the interaction of Bax with ElB 19K and that this
heterodimerisation inhibits p53-mediated apoptosis20. In this scenario,
the stabilisation of p53 in response to E1A expression or other stimuli
results in the transcriptional activation of genes required for both growth
arrest (p21/Waf-l/Cip-l) and death (Bax) with ElB 19K or Bcl-2 acting
downstream of p53-mediated transactivation to overcome apoptosis
induced by p53. The identification of specific domains such as BH3
which mediate the interaction of survival promoting Bcl-2 family
members with Bax, Bik and Bak may be useful targets for the
development of agents that can abrogate this interaction and thereby
promote the apoptosis of virus-infected cells and of cancer cells.
Epstein -Barr
virus and the BHRFI protein
Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus which is
predominantly found as an asymptomatic persistent infection. However,
under certain circumstances, EBV can contribute to the development of
tumours of B cell origin (Burkitt's lymphoma, immunoblastic lymphoma) or of epithelial cell origin (nasopharyngeal carcinoma, gastric
adenocarcinomas)21. A unique characteristic of EBV is its ability to
transform resting B lymphocytes in vitro into permanently growing
512
Bnh%h Medical Bullmtm 1997,53 (No 3)
Viruses and apoptosis
BH3
BCL2
APQ AAAOF.
BHRFl
ElB19k
HEANECLEDF
SAVRNLLEQS
HH1
SNSTi
BH1
BCL2
Fig. 1
Amino acid
BHRFl
sequsncs homology
ElB19k
between Bd-2, BHRFl
and adenovirui5 E1B19K
proteins. Sequences were
BH2
retrieved From the Gen
Bank a n d EMBL d a t a
BCL2
OVM.
.v|s VHREMSPL..
banks a n d alignments
BHRFl
_STLCC
w e r e performed using
ElB19k
SFIKDKWsfs
NQSTPYYWD
THLSO
WRHKH
Pileup software. G a p s
m a d e in individual
sequences to optimize
alignment are indicated
by dots Identical residues
or conservative changes
BCL2
,YO.
.D.
BHRFl
ElB19k
LALVOAC
ILTLSLL
RLLLLSSVRP AX
are shown on a grey
IQQQ
IPRE
background.The Bcl-2
homology domains BH1,
BH2 a n d BH3 and the E1B
19K homologous region 1
(NH1) a r e also
shown1-13-17-"-23
BCL2
BHRFl
ESR ORH
ElB19k
lymphoblastoid cell lines in which every cell expresses a limited set of socalled virus latent proteins comprising a group of six nuclear antigens
(EBNAs) and two membrane proteins (LMP1 and LMP2)22. Apart from
their coordinate role in the immortalisation of B cells, these latent
proteins have also been implicated in the protection of B cells from
apoptosis and thus may contribute to the persistence of EBV infection23.
The role of LMP1 in the regulation of apoptosis will be considered later.
When EBV-infected cells enter the virus productive cycle, a cascade of
additional EBV genes are expressed culminating in the lysis of the
infected cell and release of progeny virus. One of the EBV proteins
expressed during virus replication is BHRFl, an immediate early EBV
antigen with sequence homology to Bcl-222 (Fig. 1). Although
dispensable for both virus-induced cell growth transformation in vitro
and virus replication22, BHRFl has a proven ability to act as a cell
survival gene. EBV-negative Burkitt lymphoma cells expressing BHRFl
are rendered more resistant to apoptosis under experimental conditions
of growth factor withdrawal and rodent hamster fibroblasts expressing
BHRFl display increased resistance to the apoptotic inducing effects of
British Mtdical Bulletin 199753 (No 3)
513
Apoptosis
DNA-damaging drugs and of a mutant adenovirus defective in the ElB
19K protein 24 ' 25 . These studies suggest that whilst BHRFl is not
consistently expressed in EBV-associated tumours, it is possible that
expression of this protein at an early stage in the oncogemc process may
influence the development of these malignancies. To date, the only in
vivo lesion where BHRFl is abundantly expressed is oral 'hairy'
leukoplakia (HL), a benign lesion of oral tongue mucosa which
represents a focus of chronic EBV replication with absence of detectable
latent gene expression 21 . Our recent data demonstrating that BHRFl can
delay the terminal differentiation of epithelial cells through the
prevention of apoptosis suggests that this protein may be responsible
for HL pathology but may normally function to delay cell death during
EBV replication, so that full virus maturation can occur26.
Like Bcl-2 and ElB 19K, the BHRFl protein predominantly localises to
mitochondrial membranes, interacts with a common set of cellular proteins
(Nip 1,2,3), and via binding to Bik can suppress the death promoting
activity of this Bcl-2 family member13'18. Recent work indicates that
BHRFl does not heterodimerise with Bax but can interact with p23 R-Ras,
a member of the ras superfamily that has previously been shown to interact
with Bcl-227. The R-Ras binding region of BHRFl and the other domains
of limited homology with Bcl-2 (i.e. BH1, BH2, N H 1 , etc.) are conserved
amongst a range of different EBV isolates as is the anti-apoptotic function
of this viral protein (Khanim and Young, submitted). BHRFl mutants
which are unable to bind R-Ras retain the ability to suppress p53dependent apoptosis but also display a gain of function phenotype which
results in cell proliferation and more efficient co-operation with El A in
BRK transformation assays27. The proliferation-restraining activity of
BHRFl could be inaaivated by spontaneous mutation or by expression of
BHRFl in cells which are R-Ras negative and this may lead to the
proliferation of cells otherwise destined for apoptosis, thereby contributing
to EBV-induced oncogenesis. The ability of BHRFl to block apoptosis in
both lymphoid and epithelial cells may be important in promoting the
survival of EBV-infected cells that enter into the virus productive cycle, so
that full virus replication and maturation can occur. Thus, BHRFl
expression in Bcl-2-negative suprabasal epithelial cells or in Bcl-2-negative
B lymphocytes may be an important requirement for the efficient
production and release of mature EBV virions.
Other virus-encoded regulators of apoptosis
Apart from the viral homologues of Bcl-2, a number of viruses have
adopted different strategies to inhibit apoptosis. Thus, viral proteins
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Bnhsh Medical Bulletin 1997J3 (No 3)
Viruses and apoptosis
have been identified which impinge on specific biochemical pathways
involved in apoptosis and these have proved extremely useful in
dissecting the apoptotic machinery. The targets for these viral proteins
can be divided into three main groups which will be briefly considered.
Modifiers of the p53 pathway
Given the central role of p53 in growth arrest and apoptosis induced by
many different stimuli, it is not surprising that viral proteins have
evolved which block the function of p53. The adenovirus E1B 55K
protein and the large T antigen of SV40 virus directly interact with p53
leading to the stabilisation of the protein with consequent inhibition of
its transcriptional activity28. The E6 proteins of the oncogemc human
papillomaviruses (HPV) also bind to p53 but, rather than stabilising the
protein, this interaction results in the rapid degradation of p53 through
the ubiquitin-directed pathway28. Thus, the small DNA tumour viruses
(HPV, SV40, adenovirus) have all developed strategies to overcome the
growth restraining and apoptosis-inducing properties of p53 and this
effect contributes to the ability of these viruses to transform cells. The
hepatitis B virus oncoprotein HBx, the IE84 protein from human
cytomegalovirus and the EBNA5 protein from EBV have all been
described as able to interact with p53 and inhibit its normal function22*29.
Recent work using either the SV40 large T antigen or HPV16 E6 to
disable p53 function have highlighted the existence of p53-independent
apoptotic pathways responsible for mediating cell death in response to
genotoxic agents30.
Virus-encoded ICE inhibitors
Another mechanism used by viruses to block apoptosis is the inhibition
of the interleukin-l(3 converting enzyme (ICE) family of cysteine
proteases which have been shown to play a central role in apoptosis.
The crmA gene of cowpox virus is a potent and specific inhibitor of ICElike proteases and can protect cells against apoptosis induced by growth
factor withdrawal, Fas antigen engagement or cytotoxic T lymphocytes
(CTL)31. The insect baculoviruses contain a p35 gene which also inhibits
ICE family proteases and blocks apoptosis in insect, nematode and
mammalian cells32-33. Recombinant baculoviruses lacking the p35 gene
induce accelerated cell death leading to severely impaired virus
production32. Unlike crmA, the p35 inhibitor has a broader specificity
for the ICE-like cysteine proteases but has no effect on granzyme B, a
Bnhih Mtdxat Buffrtm 1997^3 (No 3)
515
Apoptosis
serine protease responsible for CTL-induced apoptosis33. The presence
of an ICE inhibitor-encoding gene in the genome of an insect virus
emphasises the significance of the ICE pathway as an evolutionary
conserved regulator of apoptosis.
Viral proteins and the TNF receptor pathway
Another gene encoded by different members of the baculovirus family is
IAP (inhibitor of apoptosis) which can protect against apoptosis in both
insect and mammalian cells32-34. Mammalian homologues of IAP have
recently been identified and, like the baculovirus IAPs, contain an Nterminal baculovirus IAP repeat (BIR) motif and a C-terminal zinc
binding RING finger domain34-35. The IAPs can interact with a family of
TNF receptor associated factors (TRAFs) which mediate signal
transduction through the cytoplasmic domain of certain members of
the TNF receptor superfamily35-36. The TRAFs also carry RING finger
domains and their regulation of TNF receptor signalling may be
modified by interaction with IAPs. Baculovirus-encoded IAP and certain
mammalian IAPs appear to protect against ICE-induced apoptosis35.
Thus, rather than binding to and inhibiting ICE proteases, it may be that
IAPs are involved in regulating receptor-mediated signals that are
required for the processing and activation of cysteine proteases.
The LMP1 protein of EBV is oncogenic in rodent fibroblasts and
induces phenotypic changes in human B lymphocytes characteristic of
activated cells including induction of DNA synthesis and up-regulation
of various cell surface activation markers and adhesion molecules22.
LMP1 is essential for the in vitro growth transformation of B cells.
Stable LMP1 expression can enhance the survival of B cells in response
to serum withdrawal or p53-induced apoptosis through up-regulation of
Bcl-237-38. The induction of Bcl-2 expression by LMP1 is not observed in
epithelial cells where stable LMP1 expression also results in phenotypic
changes, including the inhibition of terminal differentiation22-39. The
ability of LMP1 to activate the transcription factor NF-KB is responsible
for many of its phenotypic effects including the induction of the A20 zinc
finger protein which affords protection from the cytotoxic effects of
TNF-a22. Fiigh level expression of LMP1 can induce growth arrest and
apoptosis and the regions of the protein responsible for this effect
coincide with those that are required for rodent cell transformation22-39.
Many of the effects induced by LMP1 resemble those observed in
response to stimulation of B cells or epithelial cells through the TNF
receptor or CD40, another member of the TNF receptor family. It is,
therefore, not surprising that the cytoplasmic carboxy terminus of LMP1
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British Mtdical Bull»hn 1997,53 |No 3}
Viruses and apoptosis
interacts with members of the TRAF family, thereby impinging on the
same biochemical pathways activated by TNF receptor or CD40
signalling22. Our recent work demonstrates that LMP1 expression
inhibits epithelial cell growth in a manner mimicked by CD40 ligation
and that this effect is partly mediated by TRAF340. This inhibition of
epithelial cell growth by LMP1 or through CD40 sensitises cells to
subsequent apoptosis induced by cytotoxic drugs, TNF-a, Fas or
ceramide. Thus, the effect of LMP1 on cell growth and apoptosis
depends on the level and stability of LMP1 expression and on the
cellular environment.
HIV and apoptosis
Infection by the HIV (human immunodeficiency virus) retrovirus leads to
acquired immune deficiency syndrome (AIDS). Early events following
HTV infection include functional defects of T cells, characterised by the
inability of T cells to proliferate in vitro in response to T cell receptor
(TcR) stimulation, followed by loss of CD4+ and the decline of CD8+ T
cells with the onset of ADDS. It has been proposed that the loss of CD4+
T cells in HTV-infected patients is a result of inappropriate induction of
apoptotic cell death. Indeed, there is increasing evidence which now
suggests that apoptosis induced by HTV is central to the pathogenesis of
AIDS41. Thus, freshly isolated T cells from HTV-infected but not from
healthy individuals undergo spontaneous and activation-mediated
apoptosis. This phenomenon may also occur in vivo, as high levels of
CD4+ and CD8+ T cell death have been observed in lymph nodes of HTVinfected patients. More importantly, although a significant correlation
between CD4+ T cell depletion and apoptosis has been reported in a
number of different animal models of AIDS, apoptosis is not seen in
chimpanzees, where viral replication does not result in AIDS. It,
therefore, appears that HTV infection results in CD4+ T cell apoptosis
both in vitro and in vivo and that this is a major factor in the
development of AIDS. In addition to infected T lymphocytes, HTV may
induce apoptosis in uninfected CD4+ T cells via an indirect mechanism.
This could explain the dramatic loss of CD4+ T lymphocytes in HTV
patients despite the fact that only a small percentage of peripheral blood
mononuclear cells are actively infected. In this context it is also known
that HTV can induce cell death in the absence of infection in cell types
other than T cells, including neurons and haemopoietic progenitors in
the bone marrow.
Two main mechanisms have been proposed for HFV-induced
apoptosis in uninfected T lymphocytes42. Firstly, engagement of the
Bnft.hM.cf.ca/Bulletin 1997;33 (No. 3)
517
Apoptosis
CD4 molecule on the surface of uninfected T cells by the HIV env
proteins expressed on the surface of infected macrophages and CD4+ T
cells has been proposed to result to apoptosis. Indeed, CD4 cross-linking
by anti-CD4 antibodies can induce apoptosis in uninfected CD4+ T cells
both in vitro and in vivo and anti-env antibodies have been shown to
block induction of apoptosis in response to HIV infection in vitro.
Alternatively, it has been proposed that env-mediated crosslinking of
CD4 induces expression and secretion of soluble Fas ligand (FasL) from
uninfected CD4+ T cells and macrophages, which could then promote
apoptosis in bystander lymphocytes via a Fas-FasL interaction. Fas is a
type I membrane protein belonging to the TNF receptor family43. It is
expressed in a wide range of tissues and its ligation by certain
monoclonal antibodies or FasL provides a rapid apoptotic response.
The importance of the Fas/FasL interaction in regulating normal
immune responses was revealed by the demonstration that the
lymphoproliferative diseases in Ipr/lpr and gld/gld mice can be attributed
to defects in the genes encoding Fas and FasL, respectively. Interestingly,
whilst in vivo engagement of CD4 by specific anti-CD4 antibodies causes
depletion of CD4+ T cells in normal mice, lymphocytes from Ipr/lpr mice
are not affected44, suggesting that CD4 cross-linking primes or induces
apoptosis in a Fas-dependent manner. In this context, it has been shown
that CD4 cross-linking may up-regulate Fas expression in T lymphocytes. The significance of Fas/FasL interaction in AIDS is endorsed by the
observations that T lymphocytes from HTV-infected individuals show
high levels of Fas expression and are more susceptible to Fas-mediated
apoptosis45. HTV-infected human macrophages also show elevated levels
of expression of Fas and, interestingly, FasL46. Overall, it appears that
the depletion of CD4+ T cells by HIV may involve both direct and
indirect pathways. Thus, CD4 cross-linking by the HIV env proteins may
induce apoptosis of CD4+ T cells directly or may provide a stimulatory
signal for Fas-mediated cell death.
Conclusions
Virus infection and replication commonly result in apoptosis and this
effect may be responsible for much of the pathology associated with
infectious disease. Viruses have adopted diverse strategies for inhibiting
apoptosis and, thereby, prolonging the life of the infected cell, such that
virus replication, spread and persistence is maximised. These strategies
target the cellular pathways that mediate and regulate apoptosis and
thus the viral proteins have proved useful in dissecting the biochemical
mechanisms of cell death. In this context, it is interesting that diverse
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Bnhti.M«dK:a/6u//«hn 1997^3 (No 3)
Viruses and apoptosis
cellular and viral factors which result in growth inhibition and apoptosis
(TNFa, CD40 stimulation, expression of E1A or LMP1) also activate the
NF-KB transcription factor and that these effects are antagonised by
survival promoting Bcl-2 family members such as Bcl-2, adenovirus E1B
19K and EBV BHRFl«,i5,i« (Eliopoulos and Young, unpublished
observations). Viral homologues of Bcl-2 have also been identified in
African swine fever virus and the newly described Kaposi's sarcomaassociated human herpesvirus 847>48. It is likely that other viral proteins
which can modulate host apoptotic responses will be identified and these
will continue to help in elucidating the pathways governing apoptosis.
The regulation of apoptosis by viruses has also provided insight into
oncogenic mechanisms and further understanding of these processes are
likely to provide novel targets for cancer therapy.
Acknowledgements
Work performed in the authors' laboratory is supported by the Medical
Research Council and the Cancer Research Campaign.
References
1 White E. Life, death and the pursuit of apoptosis. Genes Dev 1996; 10: 1-15
2 Levine B, Huang Q, Isaacs JT, Reed JC, Griffin DE, Hardwick JM. Conversion of lytic to
persistent alphavirus infection by the bcl-2 cellular oncogene Nature 1993; 361: 739-42
3 Hinshaw VS, Olsen CW, Dybdahl-Sissoko N, Evans D. Apoptosis: a mechanism of cell killing
by influenza A and B viruses / Virol 1994; 68 3667-73
4 de Rossi A, Ometto L, Roncella S et al. HTV-1 induced down-regulation of bcl-2 expression and
death by apoptosis of EBV-immortalised B cells a model for a persistent 'self-limiting' HTV-1
infection. Virology 1994; 198: 234-44
5 Shenk T Adenovindae: the viruses and their replication. In: Fields BN, Knipe DM, Howley
PM, eds. Fields Virology. 3rd edn. Philadelphia: Lippincott-Raven, 1996: 2111-48
6 Rao LM, Debbas M, Sabbanni P, Hockenberry D, Korsmeyer S, White E. The adenovirus E1A
proteins induce apoptosis which is inhibited by the E1B-19K and Bcl-2 proteins. Proc Natl
Acad Sa USA 1992, 89: 7742-6
7 Debbas M, White E Wild type p53 mediates apoptosis by El A, which is inhibited by E1B.
Genes Dev 1993; 7- 546-54
8 Lowe SW, Ruley HE Stabilization of p53 tumour suppressor is induced by adenovirus E1A and
accompanies apoptosis. Genes Dev 1993; 7: 535-45
9 Lowe SW, Jacks T, Housman DE, Ruley HE. Abrogation of oncogene-associated apoptosis
allows transformation of p53-deficient cells. Proc Natl Acad Set USA 1994; 91: 2026-30
10 Mymryk JS, Shire K, Bayley ST. Induction of apoptosis by adenovirus type 5 E1A in rat cells
requires a proliferation block. Oncogene 1994; 9: 1187-93
11 Evan GI, Wyllie AH, Gilbert CS et al. Induction of apoptosis infibroblastsby c-myc protein
Cell 1992; 69: 119-28
12 Schmitz ML, Indorf A, Limbourg FP, Stadtler H, Traenckner EB, Baeuerle PA The dual effect
of adenovirus type 5 E1A 13S protein on NF-KB activation is antagonized by E1B 19K. Mol
Cell Biol 1996; 15: 4052-63
Bnh.hM.dica/BuHehn 1997^3 (No 3)
519
Apoptoiis
13 Boyd J, Malstrom S, Subramaruan T et al. Adenovirus E1B 19 kDa and bcl-2 proteins interact
with a common set of cellular proteins Cell 1994; 79 341-51
14 Chiou SK, Tseng CC, Rao L, White E. Functional complementation of the adenovirus E1B 19K
protein with Bcl-2 in the inhibition of apoptosis in infected cells. / Virol 1994; 68: 6553—66
15 Limbourg FP, Stadtler G, Chinnadurai G, Baeuerle PA, Schmitz ML A hydrophobic region of
the adenovirus E1B 19 kDa protein is necessary for the transient inhibition of NF-KB activated
by different stimuli. / Biol Chem 1996; 271: 20392-8
16 Grimm S. Bauer MKA, Baeuerle PA, Schulze-Osthoff K. Bcl-2 down-regulates the activity of
transcription factor NF-KB induced upon apoptosis. / Cell Btol 1996; 134: 13-23
17 Subramman T, Boyd JM, Chinnadurai G. Functional substitution identifies a cell survival
promoting domain common to adenovirus E1B 19 kDa and Bcl-2 proteins. Oncogene 1995; 11:
2403-9
18 Boyd JM, Gallo GJ, Elanogovan B et al Bik, a novel death-inducing protein shares a distinct
sequence motif with Bcl-2 family proteins and interacts with viral and cellular survivalpromoting proteins. Oncogene 1995; 11: 1921-8
19 Farrow SN, White JHM, Martmou I et al. Clomng of a bcl-2 homologue by interaction with
adenovirus E1B 19K. Nature 1995; 374. 731-3
20 Han J, Sabbatini P, Perez D, Rao L, Modha D, White E The E1B 19K protein blocks apoptosis
by interacting with and inhibiting the p53-inducible and death promoting Bax protein. Genes
Dev 1996, 10: 461-77
21 Rickinson AB, Kieff E. Epstein-Barr virus. In: Fields BN, Knipe DM, Howley PM, eds. Fields
Virology. 3rd edn. Philadelphia: Lippincott-Raven, 1996: 2397-446
22 Kieff E. Epstein-Barr virus and its replication. In: Fields BN, Knipe DM, Howley PM, eds.
Fields Virology. 3rd edn. Philadelphia: Lippincott-Raven, 1996: 2343-96
23 Gregory CD, Dive C, Henderson S et al. Activation of Epstein-Barr virus latent genes protects
human B cells from death by apoptosis. Nature 1991; 349: 612-4
24 Henderson S, Huen D, Rowe M, Dawson C, Johnson G, Rickinson A. Epstein-Barr virus-coded
BHRF1 protein, a viral homologue of Bcl-2, protects human B cells from programmed cell
death. Proc Natl Acad Sci USA 1993; 90 8479-83
25 Tarodi B, Subramanian T, Chinnadurai G. Epstein-Barr virus BHRF1 protein protects against
cell death induced by DNA-damaging agents and heterologous viral infection. Virology 1994;
201: 404-7
26 Dawson CW, Eliopoulos AG, Dawson J, Young LS. BHRF1, a viral homologue of the Bcl-2
oncogene, disturbs epithelial differentiation. Oncogene 1995; 9: 69-77
27 Theodorakis P, D'Sa-Eipper C, Subramanian T, Chinnadurai G. Unmasking the proliferationrestraining activity of the anu-apoptosis protein EBV BHRF1. Oncogene 1996; 12: 1707-13
28 Nevins JR, Vogt PK. Cell transformation by viruses. In: Fields BN, Knipe DM, Howley PM,
eds. Fields Virology. 3rd edn. Philadelphia: Lippincott-Raven, 1996: 301^43
29 Wang XW, Gibson MK, Vermeulen W et al. Abrogation of p53-induced apoptosis by the
hepatitis B virus X gene Cancer Res 1995; 55: 6012-6
30 Wahl AF, Donaldson KL, Fairchild C et al. Loss of normal p53 function confers sensinzation to
taxol by increasing G2/M arrest and apoptosis. Nature Med 1996; 2: 72-9
31 Kumar S. ICE-hke proteases m apoptosis. Trends Biol Sci 1995; 20: 198-202
32 Clem RJ, Miller LK. Control of programmed cell death by the baculovirus genes p35 and lap.
Mol Cell Btol 1994; 14: 5212-22
33 Bump NJ, Hackett M, Hugunin M et al. Inhibition of ICE family proteases by baculovirus
annapoptoric protein p35. Science 1995; 269: 1885-8
34 Duckert CS, Nava VE, Gednch RW et al. A conserved family of cellular genes related to the
baculovirus lap gene and encoding apoptosis inhibitors. EMBO ] 1996; 15: 2685-94
35 Uren AG, Pakusch M, Hawkins CJ, Puls KL, Vaux DL. Cloning and expression of apoptosis
inhibiting protein homologs that function to inhibit apoptosis and/or bind rumor necrosis
factor receptor-associated factors. Proc Natl Acad Sa USA 1996; 93: 4974-8
36 Rothe M, Pan M-G, Henzel WJ, Ayres TM, Goeddel DV The TNFR2-TRAF signalling
complex contains two novel proteins related to baculoviral inhibition of apoptosis proteins
Cell 1995; 83. 1243-52
520
BnhjJi Mednzal Bullmtm 199733 (No 3)
Viruses and apoptosis
37 Henderson S, Rowe M, Gregory C et al. Induction of bcl-2 expression by Epstein-Barr virus
latent membrane protem 1 protects infected B cells from programmed cell death. Cell 1991; 65:
1107-15
38 Okan I, Wang Y, Chen F et al. The EBV-encoded LMP1 protein inhibits p53-tnggered
apoptosis but not growth arrest. Oncogene 1995; 11: 1027-31
39 Lu JJ-Y, Chen J-Y, Hsu T-Y, Yu WCY, Su I-J, Yang CS. Induction of apoptosis in epithelial
cells by Epstem-Barr virus latent membrane protem 1. / Gen Virol 1996; 77 1883-92
40 EJiopoulos AG, Dawson CW, Mosialos G et al. CD40-induced growth inhibition in epithelial
cells is mimicked by Epstein-Barr virus-encoded LMP1: involvement of TRAF3 as a common
mediator. Oncogene 1996; 13: 2243-54
41 Ameisen JC, Estaquier J, Idziorek T, DeBels F. The relevance of apoptosis to AIDS
pathogenesis. Trends Cell Biol 1995; 5: 27-32
42 Cohen DI, Taru Y, Tian H, Boone E, Samelson LE, Lane HC. Participation of tyrosine
phosphorylation in the cytopathic effect of human immunodeficiency virus-1 Science 1992;
256: 542-5
43 Lynch DH, Ramsdell F, Alderson MR. Fas and FasL in the homeostatic regulation of immune
responses. Immunol Today 1995; 16: 569—74
44 Wang ZQ, Dudhane A, Orlikowsky et al. CD4 engagement induces Fas antigen-dependent
apoptosis of T cells in vivo Eur J Immunol 1994; 24: 1549-52
45 Katsikis PD, Wunderhch ES, Smith CA, Herzenberg LA, Herzenberg LA. Fas antigen
stimulation induces marked apoptosis of T lymphocytes in human immunodeficiency virusinfected individuals / Exp Med 1995; 181: 2029-36
46 Badley AD, McElhinny JA, Leibson PJ, Lynch DH, Alderson MR, Paya CV. Up-regulanon of
Fas hgand expression by human immunodeficiency virus in human macrophages mediates
apoptosis of urunfected T lymphocytes. / Virol 1996; 70: 199-206
47 Afonso CL, Neilan JG, Kutish GF, Rock DL. An African swme fever virus bcl-2 homolog, 5HL, suppresses apoptotic cell death. / Virol 1996; 70: 4858-63
48 Sand R, Sato T, Bohenzky RA, Russo JJ, Moore PS, Chang Y. Kaposi's sarcoma-associated
herpesvirus (KSHV/HHV8) encodes a functional Bcl-2 homolog. Abstract 107. 21st
Herpesvirus Workshop, July 27—August 2 1996, Illinois, USA
BnhitiM«dx:o/Bu//.fln1997;53(No 3)
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