Respiratory Syncytial Virus Limits α Subunit of Eukaryotic Translation Initiation Factor 2 (eIF2α) Phosphorylation to Maintain Translation and Viral Replication

Abstract

The impact of respiratory syncytial virus (RSV) on morbidity and mortality is significant in that it causes bronchiolitis in infants, exacerbations in patients with obstructive lung disease, and pneumonia in immunocompromised hosts. RSV activates protein kinase R (PKR) Respiratory syncytial virus (RSV) 2 is a ubiquitous pathogen that causes upper respiratory infections in healthy adults, bronchiolitis and pneumonia in young children, exacerbations in patients with obstructive lung disease, and life-threatening pneumonia in immunosuppressed patients. RSV infection early in life has been associated with the subsequent development of asthma (2-9). RSV is a member of the Paramyxoviridae family and consists of a negative strand RNA genome in a nucleocapsid surrounded by an envelope (10). Production of dsRNA is part of the replicative cycle for RNA viruses like RSV, and RNA serves as a template for both transcription and replication (11). Entry into the host respiratory epithelium is by cell surface fusion, and infection leads to viral replication and subsequent host inflammatory responses (12-17).,We previously showed that RSV increases the amount of protein kinase R (PKR) in airway epithelial cells (1). When activated by phosphorylation, PKR inhibits cellular translation through its ability to phosphorylate eIF2␣ on the Ser-51 regulatory site (18). eIF2␣ is a GTP-binding protein that delivers the initiator methionyl-tRNA to the small ribosomal subunit in translation initiation. Phosphorylation of eIF2␣ converts eIF2 from a substrate to an inhibitor of its GDP-GTP exchange factor eIF2B, blocking protein synthesis (19). If the phosphorylation of PKR during RSV infection triggers activation of eIF2␣, then cellular and viral protein translation should markedly decrease. However, viral protein translation occurs in a robust fashion during RSV infection.Viruses have different strategies to maintain viral protein translation, and PKR plays a limited role with some viruses and cell types. Subgenomic hepatitis C virus RNA replicates more efficiently in PKR knock-out mouse embryonic fibroblasts than in wild type mouse embryonic fibroblasts. The suppression of PKR activity by small interfering RNA enhances the level of hepatitis C virus RNA replication, suggesting PKR controls hepatitis C virus replication, likely via eIF2␣ phosphorylation (20). These findings were confirmed in PKR knockdown cells (21). However, inhibition of PKR by antisense peptide-conjugated phosphorodiamidate morpholino oligomers has no effect on severe acute respiratory syndrome virus titers nor does it affect the severe acute respiratory syndrome-induced phosphorylation of eIF2␣ (22). These observations suggest that another eIF2␣ kinase may regulate its activation or the severe acute respiratory syndrome virus has direct inhibitory effects on eIF2␣. Additionally, cells void of PKR protein by RNA interference do not alter the growth of adenovirus, reovirus, or measles virus (22,23). Studies using alphavirus demonstrate a decrease in viral titers in PKR Ϫ/...

Species Specificity of Protein Kinase R Antagonism by Cytomegalovirus TRS1 Genes

“…The VV K3L gene encodes a PKR pseudosubstrate that mimics eIF2␣ and binds preferentially to activated PKR, preventing phosphorylation of eIF2␣ (12). Respiratory syncytial virus infection appears to allow PKR activation but blunts its eIF2␣ kinase activity, possibly as a result of the viral N protein binding to PKR and shifting its association from eIF2␣ to protein phosphatase 2A (18). Although the effects of blocking PKR's eIF2␣ kinase activity before or after autophosphorylation are similar with respect to translational initiation, it is possible that there are different consequences of the two mechanisms.…”

Respiratory Syncytial Virus: Infection, Detection, and New Options for Prevention and Treatment

“…Sequestration of O-linked N-acetylglucosamine transferase into viral IBs may be another means by which RSV prevents the formation of SGs in infected cells (128). A third mechanism employed by RSV to prevent SG assembly is inhibition of eIF2␣ phosphorylation by RSV-N binding to PKR (133). There is further evidence that SGs may be coopted by RSV replication machinery, but this is still under debate.…”

“…When a cell is infected with a virus, PKR is a critical mediator of the innate antiviral response that acts to shut down host and, therefore, virus protein synthesis. However, the RSV-N protein is capable of binding to PKR and preventing it from phosphorylating eIF2␣ (133). This allows protein synthesis to be maintained during RSV replication.…”

Human Metapneumovirus Impairs Apoptosis of Nasal Epithelial Cells in Asthma via HSP70

“…RSV can suppress apoptosis by the function of the 2 non-structural proteins NS1 and NS2 [29] , and is known to induce the peroxidation of lipids in AECs through oxidative stress [30] , which was recently associated with non-apoptotic cell death [31] . RSV was also a poor inducer of ER stress compared to hMPV, potentially due to the ability of RSV to prevent phosphorylation of eIF2α, and therefore the UPR, in order to ensure continuous global protein translation [32] . RSV is less prevalent in the exacerbation of adult asthma than of child asthma; this corresponds with our findings, in that RSV infection was not elevated in NECs from asthmatic subjects.…”