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A historical overview of protein kinase PKR… initiated from different entry points, as for example at the
plasma membrane upon ligation of death receptor
in mediating PKR-induced apoptosis (1, 2, 26). Our (receptor pathway/extrinsic pathway) or at the
research has provided interesting results involving all these mitochondria (mitochondrial pathway/intrinsic pathway)
factors. Initial evidence that PKR phosphorylation of eIF- level. Stimulation of death receptors of the tumour
2a is involved in apoptosis induction came from studies necrosis factor (TNF) receptor superfamily, such as CD95
showing that PKR-mediated apoptosis can be inhibited by (APO-1/Fas) or TNF-related apoptosis-inducing ligand
expressing an eIF-2a dominant-negative mutant (73). (TRAIL) receptors, results in activation of the initiator
Using the VV expression system (Figure 1), the group of caspase-8, which can propagate the apoptosis signal by
Dr Esteban showed that apoptosis induced by PKR direct cleavage of downstream effector caspases such as
expression was prevented by coexpression of an eIF-2a caspase-3. The mitochondrial pathway is initiated by the
51A mutant (73). Moreover, they described the release of apoptogenic factors such as cytochrome c,
involvement of NF-?B activation in the apoptosis induced among others, from the mitochondrial intermembrane
by PKR using proteasome inhibitors that block I?Ba space. The release of cytochrome c into the cytosol
degradation, or by coexpressing dominant negative forms triggers caspase-3 activation through formation of the
of I?Ba. These observations may appear paradoxical, as cytochrome c/Apaf-1/caspase-9-containing apoptosome
NF-?B is often classified as a prosurvival factor that complex. Links between the receptor and the
prevents apoptosis. However, there is considerable mitochondrial pathway exist at different levels. Upon
evidence for the context of NF-?B as a pro- or death receptor triggering, activation of caspase-8 may
antiapoptotic factor, depending on the stimulus that result in cleavage of Bid and Bax, Bcl-2 family proteins
triggers the apoptosis (1, 2, 41, 42). Subsequently, we which in turn translocate to mitochondria to release
contributed to the discovery of a new transcription factor cytochrome c, thereby initiating a mitochondrial
involved in the apoptosis induced by PKR (66). The ATF- amplification loop. Finally, activation of caspase-3 triggers
3 transcription factor, involved in stress-induced ß-cell the induction of cell death by apoptosis (1, 74), (Figure 3).
apoptosis, was upregulated after PKR over-expression.
Activation of endogenous PKR with a VV mutant lacking Our research on the mechanism of action involved in
the viral protein E3L, a PKR inhibitor, triggered an apoptosis induced by PKR has been very active. The group
increase in ATF-3 expression that was not observed in led by Dr Esteban showed that PKR-induced apoptosis
PKR-knockout cells. Using null cells for ATF-3 and for involves mainly the FADD/caspase 8 pathway. Expression
the p65 subunit of NF-?B, we showed that induction of of a FADD dominant-negative mutant or MC159L (from
apoptosis by PKR at late stages of infection was dependent molluscum contagiosum virus, MCV) by using VV
on ATF-3 expression and regulated by NF-?B activation recombinants blocked PKR-induced apoptosis and
(66). Other potential mediators of PKR-induced apoptosis decreased caspase 8 activity, showing that PKR triggers
are the components of the Arf/p53 pathway. The p53 apoptosis through FADDmediated activation of caspase 8
pathway is a critical regulator of apoptosis (1, 2), and the (75). In 2002, we found that PKR expression by VV
link between PKR and this pathway has been established recombinants also induces caspase 9 activation, which
at several levels. Our recently published work in the field correlates with Bax protein translocation to the
is discussed below. IRF-1 is another PKR target involved mitochondria and cytochrome c release to the cytoplasm,
in apoptosis induction (44). resulting in mitochondrion depolarisation (76). The PKR-
induced caspase 9 biochemical process occurred
To understand how the activation of PKR effectors downstream of caspase 8 activation, as treating cells with
regulates apoptosis induction, it is necessary to understand an inhibitor of caspase 9 results in partial prevention of
how the apoptotic machinery integrates these signals. PKR-induced apoptosis (76). Also a part of my doctoral
Proteolytic enzymes such as caspases are important thesis, this work showed both the extrinsic and intrinsic
effector molecules in apoptosis. Caspases are synthesised pathways that PKR induces during apoptosis (Figure 3).
as inactive proforms and, upon activation, they cleave next
to aspartate residues. Activation of caspases can be
@Real Academia Nacional de Farmacia. Spain 149
