Page 30 - 82_02

regulator of IFN protein synthesis has been also supported
by other studies demonstrating that PKR plays a non-
redundant role in the IFN response to viral infections (54).
On the other hand, it is well-known that PKR is a
potent activator of NF-kB, thereby inducing IFN
transcription in concerted action with interferon regulatory
factors 3 and 7 (IRF-3/7). PKR has been also implicated in
the activation of interferon regulatory factor 1 (IRF 1),
whose expression is strongly upregulated upon viral
infection, and which acts mainly as a transcriptional
activator of IFN-a/ ß gene expression (5, 44, 54). PKR also
controls IFN and dsRNA signalling pathways by
modulation of STAT1 and STAT3 transcription factors.
PKR-knockout cells are defective in STAT1
phosphorylation on Ser727, resulting in a 4-fold decrease
in STAT1 dependent transactivation (58). STAT1 is also a
target for PKR-mediated activation in response to
lipopolysaccharide (LPS) in glial cells (59). PKR also
associates to STAT3, and is required for full STAT3
activation in response to platelet-derived growth factor
(PDGF). As proposed for STAT1, PKR regulates the Erk
activation ultimately involved in STAT3 phosphorylation
(60). On the other hand, since it also regulates STATs
transcription factors, PKR has recently been described to
be involved in the differentiation of chondrocytes through
the modulation of STAT1 and Sox-9 expression (61).
In addition to its well-established role in the interferon apoptotic process can lead to pathological conditions such
response, PKR is involved in many cellular pathways
exerting various functions on cell growth and
tumourigenesis (1, 2, 26). For example, the link between
PKR and p53 has been described fundamentally in cancer
cells where there is a bidirectional and complex regulatory
relationship between both proteins. PKR interacts directly
with the C-terminal part of p53 and phosphorylates p53 at
the Ser392 residue (62). In addition, PKR is able to
promote the proteasomal degradation of p53 in association
with GSK-3ß and mouse double minute 2 homologue
(Mdm2), independently of translational control (57).
Moreover, it has been demonstrated that the ability of p53
to cause cell cycle arrest and regulate transcription of
target genes was impaired in PKR-knockout cells (62). In
fact, it has been suggested that PKR is a p53 target gene
that plays an important role in the tumour-suppressing
function of p53 (63).
Moreover, PKR is an activator for signalling cascades
involving stress-activated protein kinases, and is described
to mediate Jun kinase (JNK) and mitogen-activated protein
kinase p38 (MAPK) activation in response to specific
stimuli (1, 26). For full activation in response to LPS or
cytokines such as IFN-?, interleukin (IL)-1, or tumour
necrosis factor (TNF)-a, both p38 and JNK are dependent
on PKR (64). Moreover, PKR interacts with and activates
mitogen-activated protein kinase kinase 6 (MKK6) in
response to double-stranded RNA stimulation (65).
In order to improve the state of knowledge about new
host genes affected by PKR, we used human cDNA
microarrays to identify, in infected cells, genes
differentially expressed after PKR expression, and
148
María Ángel García Chaves
analysed the requirements of catalytic activity of the
enzyme (66). To express PKR, we used vaccinia virus
recombinants producing wild type PKR and the
catalytically inactive mutant K296R (Figure 1). Most
regulated genes were classified according to biological
function, including apoptosis, stress, defence, and immune
response. A total of 111 genes were regulated specifically
by PKR catalytic activity, highlighting the upregulation of
the ATF-3 transcription factor, involved in stress-induced
ß-cell apoptosis. Using null cells for ATF-3 and for the
p65 subunit of NF-?B, we showed that induction of
apoptosis by PKR at late times of infection was dependent
on ATF-3 expression and regulated by NF-?B activation.
The host genes affected by PKR, identified using human
cDNA microarrays, together the ATF3 implication were
published by Dr Guerra and co-workers in 2005 under the
direction of Dr Esteban in the Journal of Biological
Chemistry (66).
2.5. PKR is a potent pro-apoptotic protein: caspases
activation by intrinsic and extrinsic routes
Cell death by apoptosis is a genetic program of
multicellular organisms which implements the ordered
removal of damaged or unwanted cells during
development and in adult life. Deregulation of the
as cancer, autoimmunity, and neurodegeneration (67).
Induction of apoptosis is a common response to viral
infection. Although it may represent an antiviral
mechanism that acts by rapidly eliminating infected cells
and preventing viral spread, virus-induced apoptosis can
also have important pathological implications. Moreover,
apoptosis cell death is an important event during some
chemotherapy treatments in cancer diseases.
The first evidence that PKR was involved in apoptosis
was suggested by Dr Esteban’s group in 1994 using HeLa
cells infected with a VV recombinant vector that expressed
the enzyme under inducible conditions (Figure 1), (68).
The role of PKR in apoptosis was reinforced by studies
developed by other groups with 3T3 cells expressing a
noncatalytic mutant PKR or using MEF derived from
PKR-/- mice (69, 70). Since then, it has been clearly
demonstrated that PKR mediates the apoptosis induced by
several viruses such us poxviruses, influenza, EMCV,
VSV, etc., probably through dsRNA production (1, 2).
PKR also regulates apoptosis induced in the absence of
viral infection. PKR was shown to mediate the apoptosis
observed during Alzheimer's disease (71) and induced by
oncogenes such as IRF1 or E2F-1, or triggered in response
to dsRNA, TNFa, LPS, tunicamycin, serum starvation, or
IL-3 withdrawal (1, 2). Many of the stimuli that trigger
PKR-dependent apoptosis in the absence of viral infection
rely on PACT/RAX activation. PACT/RAX mediates PKR
activation and subsequent apoptosis in response not only to
cytokines and serum withdrawal, but also to
chemotherapy, ethanol, and viral infection (23, 24, 72).
Analysis of the role of PKR effectors in mediating cell
death suggests an intricate pathway. To distinct degrees, at
least eIF-2a, NF-kB, ATF-3, and p53 have been implicated
@Real Academia Nacional de Farmacia. Spain