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for inhibition was then under active search in different María Ángel García Chaves
laboratories after the demonstration that a dsRNA-
dependent kinase could inhibit translation through Granada (Instituto de Investigación Biosanitaria de
phosphorylation of the alpha subunit of eukaryotic Granada, ibs.GRANADA) since 2009, in the group
initiation factor 2 (eIF2a) in rabbit reticulocyte lysates currently directed by Dr Marchal, which show the
(10). In addition to eIF2a, the kinase activity was found to importance that PKR has as a target of conventional
phosphorylate a p68 protein in human cells and a p65 chemotherapeutics and novel drugs. In addition, special
protein in murine cells (11). The generation of polyclonal consideration will be given to future studies necessary to
and monoclonal antibodies against the human p68 protein validate its use as a biomarker and a therapeutic target in
allowed determining that this was the kinase itself (12). To various pathologies.
achieve its cloning, which was performed at the Pasteur
Institute (13), this kinase first had to be purified by 2. MECHANISME OF ACTION OF PKR
immunoaffinity with specific monoclonal antibodies (14).
Then, the purified protein was injected into mice in PKR is a serine-threonine kinase composed of a kinase
presence of poly(A).poly(U) (15). Different names were domain, shared by the other eukaryotic initiation factor 2a
given to this kinase, such as p68 protein kinase, DAI (eIF2 a) kinases: general control nonrepressed 2
(Double stranded Activated Inhibitor), dsRNA-dependent serine/threonine-protein kinase (GCN2), heme-regulated
protein kinase, and P1/eIF2a kinase, until the decision was serine/threonine-protein kinase (HRI), and PKR-like
made to give PKR a consensus name: Protein Kinase endoplasmic reticulum kinase (PERK) (19). In addition,
dsRNA-dependent (16). The human PKR gene consists of PKR has two dsRNA binding domains that constitute the
17 exons (whereas the mouse gene has 16) and is encoded regulatory domain (20). Although the main direct PKR
from a single gene located on human chromosome 2p21- activator is dsRNA (produced during infection by several
22 and on mouse chromosome 17E2 (17). PKR, which is viruses and detected at low doses in mammalian cells),
expressed constitutively in mammalian cells, has an IFN- PKR is also activated by a variety of forms of cellular
stimulated response element (ISRE) in its determined stress described throughout the manuscript. PKR, in
promoter, required for transcriptional induction by type I response to specific stress signals, is activated by
IFN, but also a kinase conserved sequence (KCS) motif autophosphorylation and leads to the phosphorylation of
with an important role in basal transcription in absence of eIF-2a impairing its activity, which results in the inhibition
cytokine tratment (18). of protein synthesis and the induction of apoptosis (1, 2).
In addition to its translational regulatory function, PKR
Since these findings were made, PKR has been has a role in signal transduction and transcriptional control
extensively studied to document its relevance as a first-line through the inhibitor of ?B (IkB)/nuclear factor ?B (NF-
defence mechanism against infection and as regulator of ?B) pathways. Furthermore, PKR is involved in various
cell growth, and more recently it has also been analysed pathways that activate and engage a number of
for its role in metabolism, inflammatory processes, and transcription factors controlling the expression of multiple
age-related diseases. In fact, numerous pieces of original genes, including interferon regulatory factor 1 (IRF-1),
research and reviews regarding PKR action have been signal transducers and activators of transcription factors
published over the years. My research about the (STATs), mitogen-activated protein kinases (MAPKs), and
mechanism of action of this kinase started in close p53, among others (1, 2). All these events indicate that
collaboration with Dr Gil and co-workers in the group led PKR protein needs to be highly regulated. In fact,
by Dr Esteban at the National Centre for Biotechnology numerous studies have found PKR to be dysregulated in
(from 2000 until 2008), and has provided key insights into most cancers, neurodegenerative diseases, and other
the consequences that PKR activation has at the cellular pathologies.
level but also at the clinical and therapeutic levels. For this
reason our group was invited to review the knowledge of The main mechanisms of PKR activation by dsRNA, as
PKR in 2006-2007 (1, 2). These reviews are being well as its effects on the inhibition of protein synthesis
referenced by other studies looking into the mechanism of through the phosphorylation of eIF2a, had already been
action and the clinical implications of this kinase in several thoroughly characterised before the start of my research in
pathologies. In fact, the reviews we published under the the field. However, the mechanism by which PKR
direction of Dr Esteban, with my contribution as the first activation induces the activation of the transcription factor
author, in the journals Microbiology and Molecular NF-kB was still in the early stages of research when I
Biology Reviews and Biochimie have been cited over 600 joined Dr Esteban's group, where we made, along with Dr
times to this date (Scopus database). Gil, Dr Alcami and co-workers, some key contributions.
These researchs, along with those of other groups, will be
The present review will perform a chronological discussed in the following sections
account of our major contributions to the knowledge of the
mechanisms of action and regulation of PKR, as well as 2.1. PKR activation
the decisive contributions of several international groups.
Specific mention will be made of the studies that the Although the main direct PKR activator is dsRNA,
author leads at the University Hospital Complex of PKR is also activated by a variety of cellular stresses,
including cytokine, calcium stress, oxidative stress,
144 endoplasmic reticulum stress, lipo-stress, amyloid-ß (Aß)
peptide accumulation, polyanions such as heparin, and
several drugs among others (1, 2, 21, 22), or through the
@Real Academia Nacional de Farmacia. Spain
