Enhanced Replication of Mouse Adenovirus Type 1 following Virus-Induced Degradation of Protein Kinase R (PKR)

Abstract

Protein kinase R (PKR) plays a major role in activating host immunity during infection by sensing double-stranded RNA (dsRNA) produced by viruses. Once activated by dsRNA, PKR phosphorylates the translation factor eukaryotic initiation factor 2α (eIF2α), halting cellular translation. Many viruses have methods of inhibiting PKR activation or its downstream effects, circumventing protein synthesis shutdown. These include sequestering dsRNA or producing proteins that bind to and inhibit PKR activation. Here we describe our finding that in multiple cell types, PKR was depleted during mouse adenovirus type 1 (MAV-1) infection. MAV-1 did not appear to be targeting PKR at the transcriptional or translational level, because total PKR mRNA levels and levels of PKR mRNA bound to polysomes were unchanged or increased during MAV-1 infection. However, inhibiting the proteasome reduced the PKR depletion seen in MAV-1-infected cells, whereas inhibiting the lysosome had no effect. This suggests that proteasomal degradation alone is responsible for PKR degradation during MAV-1 infection. Time course experiments indicated that the degradation occurs early after infection. Infecting cells with UV-inactivated virus prevented PKR degradation, whereas inhibiting viral DNA replication did not. Together, these results suggest that an early viral gene is responsible. Degradation of PKR is a rare mechanism to oppose PKR activity, and it has been described in only six RNA viruses. To our knowledge, this is the first example of a DNA virus counteracting PKR by degrading it.IMPORTANCEThe first line of defense in cells during viral infection is the innate immune system, which is activated by different viral products. PKR is a part of this innate immune system and is induced by interferon and activated by dsRNA produced by DNA and RNA viruses. PKR is such an important part of the antiviral response that many viral families have gene products to counteract its activation or the resulting effects of its activity. Although a few RNA viruses degrade PKR, this method of counteracting PKR has not been reported for any DNA viruses. MAV-1 does not encode virus-associated RNAs, a human adenoviral defense against PKR activation. Instead, MAV-1 degrades PKR, and it is the first DNA virus reported to do so. The innate immune evasion by PKR degradation is a previously unidentified way for a DNA virus to circumvent the host antiviral response.

Quantitative Proteomics Analysis of Lytic KSHV Infection in Human Endothelial Cells Reveals Targets of Viral Immune Modulation

“…The importance of the antiviral function of PKR is emphasized by the finding that many RNA and DNA viruses, including multiple Herpesviridae family members, counteract PKR-mediated shutoff of protein synthesis. Depletion of PKR (e.g., through proteasome degradation) helps prevent host protein synthesis shutoff, as shown for several viruses, e.g., mouse adenovirus (Goodman et al, 2019), Toscana virus (Kalveram and Ikegami, 2013), and Rift Fever Valley virus (Mudhasani et al, 2016). aand b-herpesviruses counteract PKR by (1) preventing its activation by direct binding of viral proteins (Budt et al, 2009;Hakki et al, 2006;Valchanova et al, 2006;Ziehr et al, 2016), (2) degrading or shielding dsRNA (Poppers et al, 2000;Sciortino et al, 2013), and (3) interfering with phosphorylation of eIF2a (Li et al, 2011).…”

Inhibition of PKR by Viruses

“…However, their PKR antagonist activity may also depend on their influence on subcellular RNA trafficking (Spurgeon and Ornelles, 2009). Proteasome-dependent degradation is, however, more likely as this mechanism was recently documented in the case of the mouse adenovirus type 1 (Goodman et al, 2019).…”

Cellular Stress: Modulator of Regulated Cell Death

“…Viral proteins, for instance, influenza NS1 [ 143 ] and vaccinia virus E3L [ 129 ] protein, can directly bind with PKR to prevent PKR-mediated eIF2α phosphorylation. In addition, viral proteins can degrade PKR [ 123 , 124 , 125 ] by mechanisms that are incompletely understood for efficient viral replication. During rift valley fever virus [ 123 , 124 ] and mouse adenovirus type 1 infection [ 125 ], PKR is degraded in the proteasome.…”

“…In addition, viral proteins can degrade PKR [ 123 , 124 , 125 ] by mechanisms that are incompletely understood for efficient viral replication. During rift valley fever virus [ 123 , 124 ] and mouse adenovirus type 1 infection [ 125 ], PKR is degraded in the proteasome. Overall, viruses evolved mechanisms to inhibit PKR activation avoiding SG formation for efficient virus replication.…”