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conducted in PBMCs from acute myeloid leukemia and
healthy patients have shown epigenetic variability of
nc886 (128), suggesting that the expression levels of this
regulator will be variable, and may be decisive in diseases
where PKR activity is deregulated.
3.4. PKR regulation by compartmental localisation and
post-translational modifications: identification of a novel
PKR modification by Sumoylation
Although PKR has been identified in nuclear and
cytoplasmic fractions, most activities attributed to PKR
occur in the cytoplasm; hence, the role of nuclear PKR
remains unclear. However, recent studies have attributed
clinical and pathological significance to nuclear PKR,
mainly in neurodegeneratives diseases (21, 129).
Moreover, we and others have shown that PKR can be
sequestered in the nucleus by nucleophosmin (1, 2). In
acute leukemia, it has been found that PKR exists in
diverse molecular weight forms in the nucleus, suggesting
that this variance in protein weight is the result of post-
translational modifications (129). In fact, PKR can be
regulated by different post-translational modifications,
including phosphorylation, ubiquitination, and ISGylation.
Recent observations resulting from the cooperation
between our group and that of Dr Rivas reveal a post-
translational modification of PKR by SUMOylation, with a
direct role in PKR activation and control of virus infection
(130). These results indicate that PKR is modified by both
SUMO1 and SUMO2, in vitro and in vivo. We identified
lysine residues Lys-60, Lys-150, and Lys-440 as
SUMOylation sites in PKR. Moreover, these results show
that SUMO is required for efficient PKR-dsRNA binding,
PKR dimerisation, and eIF2a phosphorylation. SUMO
potentiated the inhibition of protein synthesis induced by
PKR in response to dsRNA, whereas a PKR SUMOylation
mutant was impaired in its ability to inhibit protein
synthesis and showed reduced capability to control
vesicular stomatitis virus replication and to induce
apoptosis in response to vesicular stomatitis virus infection and complex regulatory relationship between both
(130). Hence, the analysis of the post-translational
regulation and compartmentalisation of PKR offer
interesting possibilities to control their role in various
diseases.
4. PKR INVOLVEMENT IN SEVERAL DISEASES:
ITS POTENTIAL AS A BIOMARKER AND
THERAPEUTIC TARGET
The study of the mechanism of action and regulation of
PKR has opened the door to a new understanding of the
implications and therapeutic possibilities of this protein in response to DNA damage stress. In addition, PKR could
several diseases as we have recently published in FASEB
Journal in the review titled “The impact of PKR
activation: from neurodegeneration to cancer” under my
leadership as corresponding and last author (131).
Although further analysis are required involving patient
samples, PKR translational research, carried out only in
recent years, shows great potential for this kinase as a
biomarker and therapeutic target.
My incorporation at the University Hospital
154
María Ángel García Chaves
Complex of Granada as a Researcher of the National
Health System (Miguel Servet Program) was in 2009, with
the aim of transferring our knowledge of PKR and
Interferon to certain diseases, conducting studies with
patients in order to define their clinical utility. Today, I
lead a line of research based on the study of Interferon and
PKR in several diseases at the Institute of Biomedical
Research of Granada (ibs.GRANADA)/University
Hospital Complex of Granada, in the group directed by Dr
Marchal, collaborating also in the discovery of novel
antitumour drugs.
4.1. The role of PKR in Cancer
In addition to its well-established role in the interferon
response, PKR is involved in many cellular pathways
exerting various functions on cell growth and
tumourigenesis (1, 2, 131). However, PKR's exact role in
cancer biology remains controversial. Initially, PKR was
thought to be a tumour suppressor. The first evidence that
PKR controls cell growth, and consequently may function
as an inhibitor of cell proliferation, was obtained after
overexpression of PKR in mammalian, insect, and yeast
cells, where PKR was observed to suppress cell growth
(132, 133). Conversely, the expression of several PKR
dominant-negative mutants leads to malignant
transformation of NIH 3T3 cells, and is able to cause
tumourigenesis in nude mice (134, 135). The apoptotic
role of PKR was also in agreement with the notion that this
protein could be a tumour suppressor. PKR is able to
activate the intrinsic and extrinsic apoptotic routes in
cancer cells in response to several stimuli, including
antitumour drugs (22, 76). Moreover, PKR has been
suggested as an essential part of the antitumour activity of
tumour suppressors, such as p53 and PTEN (62, 63, 136).
The link between PKR and p53 has been described
fundamentally in cancer cells where there is a bidirectional
proteins. PKR interacts directly with the C-terminal part of
p53 and phosphorylates p53 at the Ser392 residue (1, 2,
62). In addition, PKR is able to promote the proteasomal
degradation of p53 in association with GSK-3ßand mouse
double minute 2 homologue (Mdm2), independently of
translational control (131). 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 (1, 2). In fact, it has been
suggested that PKR is a p53 target gene that plays an
important role in the tumour-suppressor function of p53 in
be transcriptionally regulated by p53 activity in response
to some genotoxic stresses (63). On the other hand, we
have evaluated the antitumour activity of overexpressed
PKR using VV recombinant (Figure 1) with interesting
oncolytic results in mice xenotransplanted with prostate
cancer cells, publishing these data as first author in 2010 in
Anales de la Real Academia Nacional de Farmacia (137).
PKR overexppresion was able to reduce tumour volume
with the advantage of minimising the specific antibodies
induced by the animal against the viral vector in
@Real Academia Nacional de Farmacia. Spain