PKR (Protein Kinase-R) is a 68-kDa serine-threonine kinase that appears to play a primary role in mediating the antiviral activities of infected cells. PKR mediates apoptosis induced by many different stimuli, such as LPS (Lipopolysaccharides), TNF-Alpha (Tumour Necrosis Factor-Alpha), viral infection, or serum starvation. Viral infection leads to the increased expression and secretion of the cytokine Ifn-Gamma (Interferon- Gamma) from host cells. Ifn-Gamma induces the dsRNA (Double-Stranded RNA)-dependent PKR. dsRNA, produced during viral replication, is an active component of a viral infection that stimulates antiviral responses in infected cells (Ref.1). The PKR protein is enzymatically inactive unless it is activated by binding to dsRNA. PKR activates as a kinase upon binding to dsRNA. Two DRBD (dsRNA Binding Domains) termed DRBDI and DRBDII, located in the NH2-terminal half of PKR, are necessary for its activation. The DRBD is a 70 amino acid motif that is conserved in a large family of dsRNA binding proteins and plays a key role in PKR function and regulation. Although their exact role in PKR activation is still not clear, the dsRNA binding domains might interact with dsRNA in a cooperative manner and contribute to optimal activation of PKR by facilitating PKR autophosphorylation and homodimerization. Binding to dsRNA induces PKR dimerization, autophosphorylation and activation. Once activated, PKR inhibits protein translation by phosphorylating the Alpha-subunit of the eIF2 (Eukaryotic Initiation Factor-2). Phosphorylation of eIF2-Alpha inhibits translation by sequestration of the guanine nucleotide exchange factor eIF2-Beta (Ref.2). In addition, PKR has been found to participate in transcriptional regulation of the E-Selectin and immunoglobulin Kappa light chain genes. Besides, it also controls the activation of several transcription factors such as NF-KappaB (Nuclear Factor-KappaB), p53, or STATs (Signal Transducer and Activator of Transcription) (Ref.3).
Bacterial products or cellular stress can also activate PKR by an endogenous gene product called PACT (Protein Activator of Interferon-Induced Protein Kinase). The binding of PACT to PKR promotes conformational changes that allow PKR to activate the downstream signaling pathways leading to the activation of NF-KappaB (Nuclear Factor-KappaB). Rel/NF-KappaB transcription factors are a family of structurally related eukaryotic transcription factors that are involved in the control of a large number of normal cellular and organismal processes, such as immune and inflammatory responses, developmental processes, cellular growth, and apoptosis (Ref.4). In mammals, these proteins include p50 (NF-KappaB1), p52 (NF-KappaB2), p65/RelA, RelB and c-Rel. These proteins share homology within a 300-amino-acid Rel homology domain, which mediates homo- and heterodimerization, DNA binding activity, and nuclear localization. A large number of stimuli including proinflammatory cytokines, antigen stimulation of T and B-Cells L, bacterial LPS, UV irradiation, ionizing radiation, viral infection, phorbol esters, reactive oxygen intermediates and bacterial or viral products can activate NF-KappaB and its target genes. NF-KappaB is found in the cytoplasm sequestered in a complex with I-KappaB (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells). Phosphorylation of I-KappaB triggers Ubiquitin-dependent degradation, and subsequent release of NF-KappaB. NF-KappaB is then free to translocate to the nucleus to activate gene transcription (Ref.5).
PKR has been shown to activate NF-KappaB, either by directly phosphorylating I-KappaB, or indirectly by activating IKK (I-KappaB kinase). PKR physically interacts with IKK. IKK is a large multicomponent enzyme complex consisting minimally of three components: two closely related kinase subunits, IKK-Alpha and IKK-Beta, which have identical structural domains and interact as a heterodimer; and a third regulatory subunit termed IKK-Gamma (or NEMO). In the case of proinflammatory stimuli, including those that signal through PKR, activation of the IKK complex is dependent on IKK-Beta phosphorylation. PKR appears to associate with IKK through its catalytic domain, the dsRNA signal appears to be mediated through IKK-Beta. Clearly, PKR is required for the efficient activation of NF-KappaB through the activation of IKK
and degradation of I-KappaB. By phosphorylating I-KappaB, PKR functions within dsRNA- and IFN-signaling pathways to induce NF-KappaB-dependent transcription. NF-KappaB complexes induced by PKR are composed primarily of p50-p65 heterodimers and also of c-Rel-p50 heterodimers. The level of PKR activity in a cell is an important parameter contributing to cell fate decisions such as growth, differentiation and apoptosis. PKR has emerged as an important signaling mediator in a wide variety of cellular responses to extracellular stimuli. This kinase is required for the maintenance and propagation of signals necessary to sustain balanced host responses in proinflammatory and apoptotic situations (Ref.6).