Porcine circovirus type 2 (PCV2) causes several disease syndromes in grower pigs. PCV2 infection triggers endoplasmic reticulum (ER) stress, autophagy, and oxidative stress, all of which support PCV2 replication. We have recently reported that nuclear HMGB1 is an anti-PCV2 factor by binding to viral genomic DNA. However, how PCV2 manipulates host cell responses to favor its replication has not been explored. Here, we demonstrate that PCV2 infection increased expression of ERO1α, generation of reactive oxygen species (ROS), and nucleocytoplasmic migration of HMGB1 via protein kinase R-like endoplasmic reticulum kinase (PERK) activation in PK-15 cells. Inhibition of PERK or ERO1α repressed ROS production in PCV2-infected cells and increased HMGB1 retention within nuclei. These findings indicate that PCV2-induced activation of the PERK-ERO1α axis would lead to enhanced generation of ROS sufficient to decrease HMGB1 retention in the nuclei, thus derepressing viral DNA from HMGB1 sequestration. The viral Rep and Cap proteins were able to induce PERK-ERO1α-mediated ROS accumulation. Cysteine residues 107 and 305 of Rep or 108 of Cap played important roles in PCV2-induced PERK activation and distribution of HMGB1. Of the mutant viruses, only the mutant PCV2 with substitution of all three cysteine residues failed to activate PERK with reduced ROS generation and decreased nucleocytoplasmic migration of HMGB1. Collectively, this study offers novel insight into the mechanism of enhanced viral replication in which PCV2 manipulates ER to perturb its redox homeostasis via the PERK-ERO1α axis, and the ER-sourced ROS from oxidative folding is sufficient to reduce HMGB1 retention in the nuclei-hence the release of HMGB1-bound viral DNA for replication. IMPORTANCE Considering the fact that clinical porcine circovirus-associated diseases (PCVAD) mostly results from activation of latent PCV2 infection by confounding factors such as coinfection or environmental stresses, we propose that such confounding factors might impose oxidative stress to the animals, where PCV2 in infected cells might utilize the elevated reactive oxygen species (ROS) to promote HMGB1 migration out of nuclei in favor of its replication. An animal infection model with a particular stressor could be approached with or without antioxidant treatment to examine the relationship among the stressor, ROS level, HMGB1 distribution in target tissues, virus replication, and severity of PCVAD. This will help decide the use of antioxidants in the feeding regime on pig farms that suffer from PCVAD. Further investigation could examine if similar strategies are employed by DNA viruses, such as PCV3 and BFDV and if there is cross talk among endoplasmic reticulum (ER) stress, autophagy/mitophagy, and mitochondrial-sourced ROS in favor of PCV2 replication.
Keywords: endoplasmic reticulum stress; high-mobility group box 1 protein; oxidative stress; porcine circovirus type 2; viral replication.