Porcine circovirus type 2 capsid protein induces unfolded protein response with subsequent activation of apoptosis

Abstract

Porcine circovirus type 2 (PCV2) has recently been reported to elicit the unfolded protein response (UPR) via activation of the PERK/eIF2α (RNA-activated protein kinase-like endoplasmic reticulum (ER) kinase/eukaryotic initiation factor 2α) pathway. This study attempted to examine which viral protein might be involved in inducing UPR and whether this cellular event would lead to apoptosis of the cells expressing the viral protein. By transient expression, we found that both replicase (Rep) and capsid (Cap) proteins of PCV2 could induce ER stress as shown by increased phosphorylation of PERK with subsequent activation of the eIF2α-ATF4 (activating transcription factor 4)-CHOP (CCAAT/enhancer-binding protein homologous protein) axis. Cap expression, but not Rep, significantly reduced antiapoptotic B-cell lymphoma-2 (Bcl-2) and increased caspase-3 cleavage, possibly due to increased expression of CHOP. Since knockdown of PERK by RNA interference clearly reduced Cap-induced CHOP expression, caspase-3 cleavage, and apoptotic cell death possibly by partially rescuing Bcl-2 expression, we propose that there is connection between Cap-induced UPR and apoptosis via the PERK/eIF2α/ATF4/CHOP/Bcl-2 pathway. This study, together with our earlier studies, provides insight into the mechanisms underlying PCV2 pathogenesis.

Porcine Circovirus Type 2 ORF5 Protein Induces Autophagy to Promote Viral Replication via the PERK-eIF2α-ATF4 and mTOR-ERK1/2-AMPK Signaling Pathways in PK-15 Cells

“…PCV2 infection induces ERS and activates the PERK/eIF2α pathway in UPR, and ERS may play a role in PCV2 infection as part of autophagy and apoptosis , as well as the PERK pathway also regulates autophagosome formation . The Rep and Cap proteins of PCV2 induced UPR by the phosphorylation of PERK with subsequent activation of the eIF2α-ATF4 pathway (Zhou et al, 2017). In the present study, we showed that ORF5 protein activates eIF2α-ATF4 expression by phosphorylating PERK which is consistent with the above studies.…”

Porcine Circovirus Type 2 Induces ORF3-Independent Mitochondrial Apoptosis via PERK Activation and Elevation of Cytosolic Calcium

“…These findings, together with earlier observations of immune stimulation of viral replication (3), suggest that stress factors that perturb cellular responses are most likely in favor of PCV2 replication. Previous work in our laboratory showed that PCV2 infection initiated autophagy via activation of CaMKK␤ by increased cytosolic Ca 2ϩ levels (27) and that PCV2 utilizes its Cap protein to induce ER stress by PERK activation, which eventually leads to an apoptotic response (28). Because the ER is the major reservoir of Ca 2ϩ , we postulate that PCV2-induced ER stress leads to a cellular Ca 2ϩ imbalance that affects important cellular responses, including apoptosis.…”

“…The cell viability of PK-15 and PAM cells treated with the chemicals was assayed with cell counting kit 8 (Dojindo), following the manufacturer's instruction. Previous studies showed that treatment with these chemicals did not have significant effects on the viability of PK-15 and PAM cells (15,27,28).…”

Caspase-Dependent Apoptosis Induction via Viral Protein ORF4 of Porcine Circovirus 2 Binding to Mitochondrial Adenine Nucleotide Translocase 3

“…Likewise, PCV2 ORF4 protein was also reported to be associated with apoptosis by inducing the activities of casp-3 and casp-8 (19,45). In addition, the PCV2 ORF3 protein was observed to activate casp-9 and casp-8 for induction of apoptosis (44), and PCV2 Cap was considered to promote apoptosis via decreasing antiapoptotic B-cell lymphoma-2, involved in regulation of mitochondrial apoptosis (65,66). Lymphopenia has been reported as a main histopathological characteristic in the lymph nodes of PCV2-infected organisms (36,(67)(68)(69).…”