Herpesviruses: hijacking the Ras signaling pathway.

Abstract

Cancer is the final result of the accumulation of several genetic alterations occurring in a cell. Several herpesviruses and especially gamma-herpesviruses have played an important role in Cancer Biology, contributing significantly to our comprehension of cell signaling and growth control pathways which lead to malignancy. Unlike other infectious agents, herpesviruses persist in the host by establishing a latent infection, so that they can reactivate periodically. Interestingly, some herpesviruses are able to either deliver or induce the expression of cellular oncogenes. Such alterations can result in the derailment of the normal cell cycle and ultimately shift the balance between continuous proliferation and programmed cell death. Herpesvirus infection employs key molecules of cellular signaling cascades mostly to enhance viral replication. However, most of these molecules are also involved in essential cellular functions, such as proliferation, cellular differentiation and migration, as well as in DNA repair mechanisms. Ras proteins are key molecules that regulate a wide range of cellular functions, including differentiation, proliferation and cell survival. A broad field of medical research is currently focused on elucidating the role of ras oncogenes in human tumor initiation as well as tumor progression and metastasis. Upon activation, Ras proteins employ several downstream effector molecules such as phosphatidylinositol 3-kinase (PI3-K) and Raf and Ral guanine nucleotide-dissociation stimulators (RALGDS) to regulate a cascade of events ranging from cell proliferation and survival to apoptosis and cellular death. In this review, we give an overview of the impact that herpesvirus infection has on the host-cell Ras signaling pathway, providing an outline of their interactions with the key cascade molecules with which they associate. Several of these interactions of viral proteins with member of the Ras signaling pathway may be crucial in determining herpesviruses' oncogenic potential or their oncomodulatory behavior. The questions that emerge concern the potential role of these molecules as therapeutic targets both for viral infections and cancer. Understanding the means by which viruses may cause oncogenesis would therefore provide a deeper knowledge of the overall oncogenic process.

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Review
Herpesviruses: Hijacking the Ras signaling pathway
Harilaos Filippakis, Demetrios A. Spandidos, George Sourvinos⁎
Department of Clinical Virology, Faculty of Medicine, University of Crete, Heraklion 71003, Crete, Greece
a b s t r a c ta r t i c l ei n f o
Article history:
Received 20 November 2009
Received in revised form 24 February 2010
Accepted 10 March 2010
Available online 18 March 2010
Keywords:
Herpesvirus
Ras
HSV-1
HSV-2
VZV
EBV
HCMV
HHV6
HHV7
HHV8
Cancer
Cancer is the final result of the accumulation of several genetic alterations occurring in a cell. Several
herpesviruses and especially gamma-herpesviruses have played an important role in Cancer Biology,
contributing significantly to our comprehension of cell signaling and growth control pathways which lead to
malignancy. Unlike other infectious agents, herpesviruses persist in the host by establishing a latent infection,
so that they can reactivate periodically. Interestingly, some herpesviruses are able to either deliver or induce
the expression of cellular oncogenes. Such alterations can result in the derailment of the normal cell cycle and
ultimately shift the balance between continuous proliferation and programmed cell death. Herpesvirus
infection employs key molecules of cellular signaling cascades mostly to enhance viral replication. However,
most of these molecules are also involved in essential cellular functions, such as proliferation, cellular
differentiation and migration, as well as in DNA repair mechanisms. Ras proteins are key molecules that
regulate a wide range of cellular functions, including differentiation, proliferation and cell survival. A broad
field of medical research is currently focused on elucidating the role of ras oncogenes in human tumor
initiation as well as tumor progression and metastasis. Upon activation, Ras proteins employ several
downstream effector molecules such as phosphatidylinositol 3-kinase (PI3-K) and Raf and Ral guanine
nucleotide-dissociation stimulators (RALGDS) to regulate a cascade of events ranging from cell proliferation
and survival to apoptosis and cellular death. In this review, wegive anoverview of the impact thatherpesvirus
infection has on the host-cell Ras signaling pathway, providing an outline of their interactions with the key
cascade molecules with which they associate. Several of these interactions of viral proteins with member of
the Ras signaling pathway may be crucial in determining herpesviruses' oncogenic potential or their
oncomodulatory behavior. The questions that emerge concern the potential role of these molecules as
therapeutic targets both for viral infections and cancer. Understanding the means by which viruses may cause
oncogenesis would therefore provide a deeper knowledge of the overall oncogenic process.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Ras signaling pathway
Ras proteins are membrane-bound molecules that regulate a wide
range of cellular functions, including differentiation, proliferation and
cell survival [1]. A broad field of medical research is currently focused
on elucidating the role of ras oncogenes in human tumor initiation as
well as tumor progression and metastasis [2]. More specifically, Ras
proteins are found to be mutated in many tumors such as melanoma,
ovarian and lung carcinoma [3,4]. Genetic alterations of ras genes
usually shift the balance between cell proliferation and cell death,
towards continuing proliferation and differentiation. The role of ras
genes during normal cellular function is dictated by the post-
translational modification to which the proteins are subjected, which
is mainly farnesylation. This process determines the final location of
the proteins in the cell, primarily the plasma membrane, as well as
their functionality. The enzyme farnesyl transferase links a farnesyl
group (15-carbon isoprenoid) covalently to a cysteine residue located
in the carboxy-terminal CAAX motif of Ras, allowing Ras to be
Biochimica et Biophysica Acta 1803 (2010) 777–785
Abbreviations: BL, Burkitt's lymphoma; COX-2, cytochrome c oxidase subunit II; E,
Early; EBNA, Epstein–Barr nuclear antigen; EBV, Epstein–Barr virus; EGF, Epidermal
Growth Factor; EGFR, Epidermal Growth Factor Receptor; Elk, Ets-related transcription
factor; ERK, extracellular regulated kinase; GDP, guanosine diphosphate; GTP,
guanosine triphosphate; HCMV, Human Cytomegalovirus; HHV, Human Herpes
Virus; HL, Hodgkin's lymphoma; HSV, Herpes Simplex Virus; IE, Immediate Early; IL,
interleukin; JNK, c-Jun amino-terminal kinases; KSHV, Kaposi Sarcoma-associated
Herpes Virus; L, Late; LAT, latency-associated transcript; LNA, latent nuclear antigen 1;
MAPK, Mitogen-Activated Protein Kinases; MAPKK, MAPK kinase; MAPKKK, MAPK
kinase kinase; MCD, multicentric variant of Castleman Disease; NF-κB, Nuclear Factor
kappa-light-chain-enhancer of activated B cells; NGF, Neuronal Growth Factor; NPC,
Nasopharyngeal Carcinoma; ORF, Open Reading Frame; PEL, Primary Effusion
Lymphoma; PI3-K, phosphatidylinositol 3-kinase; PKR, RNA-activated protein kinase;
RA, retinoic acid; RALGDS, Ral guanine nucleotide-dissociation stimulators; RTA, viral
replication and transcriptional activation protein; TNF-α, tumor necrosis factor; TPA,
12-O-tetradecanoyl-phorbol-13-acetate; VZV, Varicella Zoster Virus
⁎ Corresponding author. Tel.: +30 2810 394 835.
E-mail address: sourvino@med.uoc.gr (G. Sourvinos).
0167-4889/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.bbamcr.2010.03.007
Contents lists available at ScienceDirect
Biochimica et Biophysica Acta
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anchored to the plasma membrane. It is known that the mis-positioned
Ras proteins are non-functional, probably due to the inability of these
proteins to employ their target enzymes and initiate signal transduc-
tion. Binding of ligands to tyrosine/kinase transmembrane receptors,
autophosphorylates their tyrosine residues in the cytosol. The tyrosine
residues serve as intracellular docking sites specific for several adaptor
molecules such as members of the SOS family. Ras proteins remain
inactive in the cell when they are bound to guanosine diphosphate
(GDP), but become activated when bound to guanosine triphosphate
(GTP) [5,6]. Ras is activated when ligand-bound receptors nucleate a
complex including adapter molecules [e.g., Src homology and collagen
(Shc; protein), Gab2, and growth factor receptor binding protein 2
(Grb2)], the phosphatase SHP-2, and guanine nucleotide exchange
factors (e.g., SOS). Guanine nucleotide exchange factors bind to Ras
and catalyze guanine nucleotide dissociation, which results in
increased Ras-GTP levels. Ras activation is terminated by hydrolysis
of GTP to GDP. This reaction is greatly accelerated by the GTPase-
activating proteins p120GAP and neurofibromin.
ActivationofRasisresponsibleforthesequentialphosphorylationof
downstream molecules which amplify and transduce signals from the
cellsurfacetothenucleus.Morespecifically,Rasproteinscanemployup
to 20 downstream effector molecules such as phosphatidylinositol 3-
kinase (PI3-K) and Raf and Ral guanine nucleotide-dissociation
stimulators (RALGDS) to regulate a cascade of events ranging from
cellproliferationandsurvivaltoapoptosisandcellulardeath[7–12].The
activated Raf kinase, in particular, activates the MAPK cascade (Fig. 1).
The MAPK cascade in the mammalian cell comprises three well-
characterized protein kinases that are activated by protein phosphor-
ylation:aMAPK kinasekinase(MAPKKK),a MAPKkinase(MAPKK) and
finally a MAPK. The final protein kinases (MAPKs) are the extracellular
regulated kinase (ERK1/2), p38 kinases, ERK5 and the c-Jun amino-
terminal kinases (JNK12/3). The Ras/Raf/MEK/ERK pathway is gener-
ally induced by cell surface receptors such as the Epidermal Growth
Factor Receptor (EGFR), whereas the p38 and JNK kinases respond to
stress signals as well as growth factor expression [13].
1.2. Herpesviruses and the Ras pathway
The Ras pathway molecules respond to various extracellular stimuli
which eventually determine the fate of the cell. Environmental,
chemical and infectious agents have the ability to affect the signaling
process in the cell, altering its physiological function. Human herpes-
viruses are of particular interest, since they are able to either induce
tumor initiation (oncogenic viruses), or regulate tumor behavior
(oncomodulatory viruses). Interestingly, 17.8% of worldwide cancer
cases are attributed to infectious agents, while 12.1% of the total cancer
cases are caused by viral infections [14]. Essentially, the above 12.1%
represents almost 70% of the total (17.8%) caused by infectious agents.
Bearing these epidemiological data in mind, as well as the
causative role of herpesviruses in several human diseases, involving
key signaling molecules of the host, we review this family of viruses
focusing on the Ras signaling pathway. Eight human herpesviruses
have been extensively studied and several of them have been
associated with the oncogenic Ras signaling pathway. In more detail,
Fig. 1. Host cell Ras signaling pathway. The key mediators of the pathway are represented. Interactions between herpesviruses and host cell proteins are illustrated, indicating
whether the virus up-, or down-regulates a specific cellular molecule.
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the viruses are: Herpes Simplex Virus-1 (HSV-1 or HHV-1), Herpes
Simplex Virus-2 (HSV-2 or HHV-2), Human Herpes Virus Type 3
(HHV-3) or Varicella Zoster Virus (VZV), Epstein–Barr virus (EBV or
HHV-4), Human Cytomegalovirus (HCMV or HHV-5), Human Herpes
Viruses -6 and -7 (HHV-6 and HHV-7) and finally the Kaposi Sarcoma-
associated Herpes Virus (KSHV or HHV-8).
Upon infection of a host cell, a number of cellular defense
mechanisms are employed to eliminate the threat, followed by acute
inflammation. Although inflammation is generally considered to be
beneficial for the cell, the environment of the inflammation site is a
very potent place for transformations to occur. During inflammation a
wide array of DNA-binding proteins, cytokines and chemokines are
recruited to combat the inflammatorysignal. TNF-α, NF-κB, IL-1, COX-
2 and the pro-inflammatory chemokine families CXC and CC usually
have positive effects on cell proliferation. However, their ability to
cause DNA damage in the inflammation site has been shown to have a
negative effect on normal cell proliferation and angiogenesis [15–18].
Herpesviruses possess the ability to evade the host immune response
during initial exposure, and remain in a latent state until they
reactivate to carry out lytic infection [19,20]. Additionally, some
herpesviruses can promote oncogenesis either by delivering onco-
genes in the host cell or by activating cellular oncogenes. The
activation of oncogenes in the host cell leads to the expression of
transforming growth factors that deregulate normal cellular functions
and result in malignant cell transformation [21,22].
We provide an overview on the effect of herpesviral infection to
the host-cell Ras signaling pathway, focusing on gamma-herpes-
viruses and their association with the key cascade molecules with
which they interact.
2. Alpha-herpesviruses
Alpha-herpesviruses are the only subfamily of the herpesviridae
family, known as neurotropic pathogens. Thus, HSV and VZV are able
to infect and remain latent inside nerve cells and sensory ganglia.
Upon reactivation, the two viruses trigger a painful localized vesicular
rash. For HSV-1, it is usually lesions that form around the mouth area
ofthe infectedindividual, whereasfor HSV-2theoutbreakis knownas
genital herpes. VZV, on the other hand, is the causative agent of two
distinct clinical diseases: primary infection that leads to Varicella
(chickenpox) and reactivation from a latent state in sensory ganglia
that gives rise to zoster (shingles).
2.1. Herpes Simplex Virus type-1 and type-2 (HSV-1 and HSV-2)
Productive infection by HSV-1 involves the timely expression of
approximately 80 viral genes. These genes are expressed in three
sequential phases: Immediate Early (IE), Early (E) and Late (L). Many
products of these viral genes have been shown to interact with host
encoded proteins during infection in order to ablate the host-cell
defenses and finally replicate [23–27]. In addition, HSV-1 has the
ability to overcome cellular defense mechanisms in tumor as well as
in normal cells [28–32]. More specifically, experiments carried out on
H-rastransformedcell lineshave shownthatHSV-1replicatesin these
cells by inhibiting the virus-induced activation of the double-stranded
RNA-activated protein kinase (PKR) [33]. Phosphorylation of PKR
results in the phosphorylation of the translation initiation factor eIF-
2α, resulting in inhibition of the viral transcripts in the host cell. PKR
activation is essentially the major defense of the host cell against viral
infections.
PKR is also impaired by the expression of the HSV-1 neuroviru-
lence protein ICP34.5. ICP34.5 ablates host-cell PKR function by
inducing the dephosphorylation of eIF2α [34,35]. Inhibition of PKR
activity allows for the favorable expression of viral proteins, resulting
in impaired host-cell defense. Furthermore, HSV-1 establishes a latent
infection in sensory neurons in a Ras-dependent fashion [36].
Infection in these cells by HSV-1 is characterized by the over-
expression of a series of transcripts, termed latency-associated
transcripts (LATs). Both Ras and Raf molecules are activated in
response to Neuronal Growth Factor (NGF) stimulation, which leads
to the activation of LATs and the establishment of latent infection [37].
In lytically infected cells, both HSV-1 and HSV-2 possess an anti-
apoptotic activity, as was previously described [38–40]. The two
viruses inhibit apoptosis in a cell type-specific manner. This inhibition
is attributed to the US3 gene, as well as to the US5 and ICP27 genes of
HSV-1 [41,42]. More specifically, expression of the HSV-1 ICP27
protein results in the enhanced activation of the p38 and JNK cellular
pathways [42]. Activation of the p38 pathway by ICP27 results in the
induction of apoptosis, whereas activation of the JNK pathway by
ICP27 requires the co-expression of one or more early or late viral
proteins. ICP27 appears to play a role as a pro-apoptotic factor for the
host cell since ICP27 expression alone cannot inhibit the apoptotic
signaling that takes place after p38 activation [43]. Induction of
apoptosis in cells infected by ICP27 mutant viruses, probably involves
the destabilization of Bcl-2, which is mediated by p38/MAPK pathway
activation. Bcl-2 loss of function is induced by p38 expression and
results in sensitization and induction of apoptotic cell death processes
[44]. Bcl-2 function in HSV-1 infected cells is crucial for the fate of the
host cell, and may prove to be a key mediator molecule for HSV-1
infection.
Interestingly, the HSV-2 tegument protein ICP10 PK [45–47],
appears to mediate activation of the host-cell Ras/Raf/MEK/ERK
pathway. Indeed, the levels of activated MAPK1/2 significantly
increased in hippocampal cultures infected by HSV-2 when compared
to mock-infected cultures [48]. The inhibition of MAPKs by the
chemical inhibitor UO126 caused a dose-dependent increase in
apoptotic cells, suggesting that HSV-2 infection is actually mediated
through the cellular MAPK pathway, as shown in Fig. 1 [49]. Hence,
the infection of HSV-2 is mediated and enhanced by the host-cell ras
pathway since the viral ICP10 PK activates Ras, MEK and MAPK while
protecting the cell from apoptotic cell death [50,51].
Several studies have used mutant forms of HSV-1 and HSV-2 to
target tumor cells [52,53]. The mutant HSV forms are known as
oncolytic viruses since they are genetically modified to express their
proteins and replicate in tumor but not in normal cells [53,54]. These
conditionally replicating viruses infect and eventually kill tumor cells
either by inducing apoptosis, or by destroying these cells with the
cytolytic activity they possess [55,56]. For instance, the oncolytic
G207 HSV-1 strain is able to achieve enhanced permissiveness to
malignant peripheral nerve sheath tumors (MPNSTs). It is believed
that over-activation of Ras in MPNSTs leads to increased viral
replication in these cells and not in the normal ras expressing cells
[57].
Wild-type HSV-2 on the other hand is able to selectively replicate
and lyse tumor cells [58], such as neuroblastoma, renal, breast and
pancreatic cancers [59–61]. The abovementioned ICP10 PK protein of
HSV-2 is responsible for activating the Ras pathway leading to
deregulation of the cell cycle and uncontrolled proliferation.
Deletion of the PK subunit of the viral ICP10 protein impairs the
ability of HSV-2 to infect normal cells that have an inactive Ras
pathway [62]. Recent experimental data using recombinant Signal-
Smart (SS1) HSV-1 viruses, show that upon exposure of ELK-
overexpressing prostate cancer cells to the recombinant virus, the
cells' proliferation, invasiveness and colony formation capabilities
are significantly decreased, whereas the rate of apoptosis/necrosis
functions is increased [63]. Additionally, increased Ras signaling cells
exposed to the recombinant virus, showed a conspicuous G1 cell
cycle arrest when compared to cells infected with parental HSV-1.
Oncolytic HSV-1 viruses provide an excellent tool to study oncogenic
signals and open novel paths in using these biological agents for
detection and targeting cancer cells, with augmented specificity and
efficiency.
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2.2. Varicella Zoster Virus (VZV)
The Varicella Zoster Virus genome encodes approximately 70 gene
products, expressed in a timely fashion similar to the expression of
HSV proteins (IE, E and L). Lytic infection from the virus leads to
Varicella, a clinical manifestation also known as chickenpox that
includes symptoms such as fever and a generalized vesicular rash.
Reactivation of the virus from its latent state usually occurs during
adulthood and presents a distinct, localized vesicular rash that is
uncommon in herpesviral infections [64].
As with all herpesvirus infections, VZV-induced infections are
known to utilize host-cell signaling pathways in order to achieve
increased permissiveness [65]. Upon infection, the cell responds by
secreting, among others, interleukins (ILs). Interleukins and particu-
larly IL-8, play a crucial role in the immune response of the host cell,
sinceitactsasakeymediatorduringacuteinflammationwithavariety
of actions on several tissues [66]. Secretion of IL-8 in VZV-infected T
cells is dependent of the status of both the JNK/SAPK and p38/MAPK
pathways. Hence, an active p38/MAPK pathway is required for IL-
8secretionandasuccessfulimmuneresponseinTcells[67].Moreover,
infectionbyVZVleadstotheactivationoftheMAPKpathwaybyatwo-
fold increase of p38/MAPK phosphorylation (Fig. 1), and ultimately to
the deregulation of the normal cell cycle and proliferation processes
[68–70]. Although there is a direct association of VZV infection with
the MAPK pathway, it remains unclear whether the virus interacts
with upstream molecules such as Ras, since current studies have
merely focused on the phosphorylation levels of MAPK, as well as the
characterization of two Open Reading Frames (ORF49 and ORF61)
[71,72]. ORF61 encodes proteins with transregulatory activities such
as the activation/phosphorylation of MAPKs in the host cell, whereas
ORF49issynthesizedatlateinfectionstagessinceitispartofthevirion
component.
3. Beta-herpesviruses
The beta-herpesviruses subfamily consists of the members HCMV,
HHV-6 and HHV-7. In order to gain entry into the host cell, beta-
herpesviruses, as with all herpesviruses, use virion envelope proteins
to adhere to the cell membrane and depose the virion components
into the cytoplasm. HCMV in particular, appears to infect a wide range
of cell types, such as fibroblasts, endothelial cells, epithelial cells,
monocytes/macrophages, smooth muscle cells, neuronal cells, neu-
trophils, stromal cells and hepatocytes [73,74]. The broad cellular
tropism that HCMV demonstrates in the human body, justifies the
increased manifestation of the virus in the worldwide population. On
the other hand,HHV-6 and HHV-7 are restricted to T lymphocytes and
certain cells of the myeloid lineage [69].
3.1. Human Cytomegalovirus (HCMV)
Human Cytomegalovirus is a widespread pathogen; an estimated
50–85% of the population worldwide is currently infected by HCMV
[75]. In particular, HCMV is able to remain latent in the monocyte-
macrophage cell lineage for extended periods of time, and is
reactivated when the individual is immuno-compromised [76]. Data
suggest that gene products of Human Cytomegalovirus can modulate
tumor cell biology, promoting mutagenesis, cell cycle progression,
angiogenesis, cell invasion and immune evasion [77,78].
Thereis increasing evidencewhichconfirmsthepresenceofHCMV
in malignant tumors such as malignant glioblastoma [79,80], colon
cancer [81], as well as EBV-negative Hodgkin's lymphoma [82].
Studies on human colonic adenocarcinoma cells (Caco-2 cells), have
produced contradicting results. In one study the authors suggest that
Human Cytomegalovirus infection can only arise when Caco-2 cells
are in a specific state of differentiation, in which the virus does not
spread from cell to cell, and productive infection is rare, whereas the
other group supports that Caco-2 cells are infected by HCMV,
regardless of the differentiation state [83,84]. In glioblastoma, the
HCMV Immediate Early-1 (72 kDa) protein (IE1) is expressed in N90%
of the tumors analyzed. A stable IE1 expression can differentially
affect the growth of human glioblastoma cells, resulting in either
growth proliferation or cell cycle arrest [85]. Given the high HCMV
infection rate in these tumors, it can be hypothesized that a sustained
expression of critical viral genes such as IE1 may have important
biological consequences in malignant glioma cells. Moreover, these
findings provide important insights into the pathogenic mechanisms
associated with aberrant signaling pathways, transcription factors
and/or tumor suppressor functions of the host cell.
Although HCMV is not directly associated with tumor formation,
the timely expression of its proteins regulates the host-signaling
pathways so that the virus can be sustained in the cell [86]. To
accomplish this, HCMV has to bind to specific cellular receptors in
order to initiate infection. Upon infection by HCMV, the viral particles
must first enter the host cell and then travel to the nucleus in order to
initiate viral replication. Interestingly, HCMV entry into the host cell is
achieved by the synergistic action of the Epidermal Growth Factor
Receptor (EGFR) and the activation of the β3 integrin subunit. In turn,
this results in the activation of downstream molecules such as the
focal adhesion kinase, PI3K/Akt and phospholipase Cγ [87–89].
However, it is also argued that EGFR is not required for the cellular
expression of viral proteins or virus-induced signaling [90,91]. In
addition to EGFR and integrin, PDGFα and β have also been proposed
as cellular receptors for HCMV entry [92–94].
The binding of the viral particles to their respective receptors
[95,96] initiates a cascade of events that permit virus entry and
subsequent viral replication (Fig. 1). It is known that the Ras/Raf/
MEK/ERK pathway is activated when the phosphorylation of EGFR
occurs on the cell surface [97]. It is further believed that the gB
glycoprotein of HCMV shares common sequences with the Epidermal
Growth Factor (EGF), which acts as a ligand for EGFR [98,99], allowing
infection to occur.
Initial studies have shown that the steady-state levels of c-H-ras
increase in T2 cells upon RA-induced differentiation. Moreover, the
expression of exogenous oncogenic human H-ras in T2 cells allows for
HCMVinfection,aswellaschangesin cell surfaceantigensobservedin
the early stages of RA-induced differentiation [100]. Our recent
experimental data, obtained using H-ras transformed cells, further
support the enhanced permissiveness of HCMV augmenting the
progeny viral yield and the viral gene expression. Notably, HCMV
infection resulted in a further increase of the proliferation rate,
mobility and formation of cellular foci in the H-ras-activated cells
compared to mock-infected cells or the non-transformed parental cell
line (Filippakis, Spandidos, Sourvinos, unpublished data). These
observations suggest that HCMV employs the oncogenic Ras pathway
for the induction of cellular and/or viral gene expression and
enhanced viral permissiveness. The communication between viral
glycoproteins and cellular receptors is usually sufficient to induce
activation of the ras pathway. However, there are several cellular
responses that follow infection, including the phosphorylation of
signaling kinases and the subsequent activation of transcription
factors, cytokines and prostaglandins. Successful HCMV infection
requires the activation of MEK1/2 and ERK1/2, two kinases that are
crucial elements of the Ras pathway [101]. Inhibition of these cellular
proteins by the inhibitors UO126 and PD098059 blocks the productive
HCMV infection in the cell, suggesting that the two molecules are very
important for virus replication [102]. The abovementioned virus-
induced events impair the host-cell defense by interacting with
molecules of the Ras signaling cascade, thereby rendering the cell
more permissive to the virus. The relationship between HCMV and
cancer has been in the center of virology research for decades. The
frequent presence of a non oncogenic virus, such as HCMV, in
malignant tissues results in increased proliferation of HCMV-infected
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