RGNNV-induced cell cycle arrest at G1/S phase enhanced viral replication via p53-dependent pathway in GS cells

Interference of pseudorabies virus infection on functions of porcine granulosa cells via apoptosis modulated by MAPK signaling pathways

BACKGROUND

Pseudorabies virus (PRV) is one of the major viral pathogens leading to reproductive disorders in swine. However, little is known about the effects of PRV infection on porcine reproductive system. Ovarian granulosa cells are somatic cells surrounding oocytes in ovary and required for folliculogenesis. The present study aimed to investigate the interference of PRV on functions of porcine ovarian granulosa cells in vitro.

METHODS

Primary granulosa cells were isolated from porcine ovaries. To investigate the PRV infectivity, transmission electron microscopy (TEM) was used to check the presence of viral particles, and the expression of viral gE gene was detected by quantitative real-time PCR (qPCR) in PRV-inoculated cells. After PRV infection, cell viability was detected by MTS assay, Ki67 for proliferative status was determined by immunofluorescence assay (IFA), cell cycle and apoptosis were detected by flow cytometry, and progesterone (P4) and estradiol (E2) were determined by radioimmunoassay. The checkpoint genes of cell cycle and apoptosis-related proteins were studied by qPCR and western blotting.

RESULTS

Virus particles were observed in the nucleus and cytoplasm of PRV-infected granulosa cells by TEM imaging, and the expression of viral gE gene increased in a time-dependent manner post infection. PRV infection inhibited cell viability and blocked cell cycle at S phase in porcine granulosa cells, accompanied by decreases in expression of Ki67 protein and checkpoint genes related to S phase. Radioimmunoassay revealed decreased levels in P4 and E2, and the expressions of key steroidogenic enzymes were also down-regulated post PRV-infection. In addition, PRV induced apoptosis with an increase in Bax expression and activation of caspase 9, and the phosphorylation of JNK, ERK and p38 MAPKs were significantly up-regulated in porcine ovarian granulosa cells post PRV infection.

CONCLUSIONS

The data indicate that PRV causes infection on porcine ovarian granulosa cells and interferes the cell functions through apoptosis, and the MAPK signaling pathway is involved in the viral pathogenesis.

Red-grouper nervous necrosis virus B1 protein inhibits fish IFN response by targeting Ser5-phosphorylated RNA polymerase II to promote viral replication

Nervous necrosis virus (NNV) could infect more than 200 fish species worldwide, with almost 100% mortality in affected larvae and juvenile fish. Among different genotypes of NNV, the red-grouper nervous necrosis virus (RGNNV) genotype is the most widely reported with the highest number of susceptible species. Interferon (IFN) is a crucial antiviral cytokine and RGNNV needs to develop some efficient strategies to resist host IFN-stimulated antiviral immune. Although considerable researches on RGNNV, whether RGNNV B1 protein participates in regulating the host's IFN response remains unknown. Here, we reported that B1 protein acted as a transcript inhibition factor to suppress fish IFN production. We firstly found that ectopic expression of B1 protein significantly decreased IFN and IFN-stimulated genes (ISGs) mRNA levels and IFNφ1 promoter activity induced by polyinosinic:polycytidylic acid [poly (I:C)]. Further studies showed that B1 protein inhibited the IFNφ1 promoter activity stimulated by the key RIG-I-like receptors (RLRs) factors, including MDA5, MAVS, TBK1, IRF3, and IRF7 and decreased their protein levels. Moreover, B1 protein significantly inhibited the activity of constitutively active cytomegalovirus (CMV) promoter, which suggested that B1 protein was a transcription inhibitor. Western blot indicated that B1 protein decreased the Ser5 phosphorylation of RNA polymerase II (RNAP II) C-terminal domain (CTD). Together, our data demonstrated that RGNNV B1 protein was a host transcript antagonist, which intervened RNAP II Ser5-phosphorylation, inhibiting host IFN response and facilitating RGNNV replication.

Characterization of Nervous Necrosis Virus (NNV) Nonstructural Protein B2 and Its Enhancement on Virus Proliferation

The nerve necrosis virus (NNV), a pathogen of viral nervous necrosis disease in several important mariculture economic fish species, causes economic loss. Its nonstructural protein B2 encoded by the sub-genomic RNA3 affects the amplification of the virus. In this study, the B2 protein was recombinantly expressed, the polyclonal antibodies were produced and the dynamics of the B2 protein and genomes were measured in vivo and in vitro after NNV infection. Then, the effects of the overexpressed B2 protein on virus proliferation were investigated. The results showed that the polyclonal antibodies can recognize the B2 protein in both SSN-1 cells and the brain/eye of the grouper. The RNA3 expression significantly increased at 12 h and kept rising till the end of the experiment; it was 10^6.9 copies/μL at 120 h. The B2 protein could be first detected at 3 h post-infection, which was earlier than the capsid protein was first detected (12 h post-infection). The B2 protein can be detected in the brain, eye and heart on day 3 and the copy number of genomes reached a maximum at 6 d post-infection. There was a low expression of NNV genomes in the liver, spleen and kidney, and no virus was detected in the gill, stomach and intestine. In the meantime, the B2 protein was successfully expressed in GF-1 cells and significantly enhanced virus proliferation, which produced an earlier cytopathic effect and higher cell death rates after 3 d post-infection than the control. In conclusion, the B2 protein acts as an early expressed protein during virus replication and proliferation and is involved in the early infection of NNV. The results may provide insight into the early stage of virus infection and prevention of the disease.

Modulatory effects of curcumin on Singapore grouper iridovirus infection-associated apoptosis and autophagy in vitro

Singapore grouper iridovirus (SGIV) with high pathogenicity can cause great economic losses to aquaculture industry. Thus, it is of urgency to find effective antiviral strategies to combat SGIV. Curcumin has been demonstrated effective antiviral activity on SGIV infection. However, the molecular mechanism behind this action needs to be further explanations. In view of the fact that apoptosis (type I programmed cell death) and autophagy (type II programmed cell death) were key regulators during SGIV infection, we aimed to investigate the relevance between antiviral activity of curcumin and SGIV-associated programmed and clarify the role of potential signaling pathways. Our results showed that curcumin suppressed SGIV-induced apoptosis. At the same time, the activities of caspase-3/8/9 and activating protein-1 (AP-1), P53, nuclear factor-κB (NF-ΚB) promoters were inhibited. Besides, the activation of extracellular regulated protein kinases (ERK), c-Jun N-terminal kinase (JNK) and p38 mitogen activate protein kinase (p38 MAPK) signal pathways were suppressed in curcumin-treated cells. On the other hand, curcumin down-regulated protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway to promote autophagy representing by increased LC3 II and Beclin1 expression. Curcumin also hindered the transition of cells from G1 to S phase, as well as down-regulating the expression of CyclinD1. Our findings revealed the resistance curcumin induced to the effects of DNA virus on cell apoptosis and autophagy and the insights gained from this study may be of assistance to understand the molecular mechanism of curcumin against DNA virus infection.

Establishment and characterization of a liver cell line, ALL, derived from yellowfin sea bream, Acanthopagrus latus, and its application to fish virology

Yellowfin sea bream (Acanthopagrus latus) is an important economic fish, which is seriously threatened by various fish viruses. In this study, a cell line designated as ALL derived from the liver of yellowfin sea bream was developed and characterized. The cell line grew well in Dulbecco's modified Eagle's medium containing 10%-20% foetal bovine serum at 28°C. Amplification of the cytochrome B gene indicated that ALL cells originated from yellowfin sea bream. The modal chromosome number of ALL cells was 48. ALL cells were efficiently transfected with pEGFP-N3 plasmids, indicating the potential application of ALL cells in exogenous gene manipulation studies. ALL cells were susceptive to three main fish viruses, including viral haemorrhagic septicaemia virus (VHSV), red-spotted grouper nervous necrosis virus (RGNNV) and largemouth bass virus (LMBV). The replication of VHSV, RGNNV and LMBV in ALL cells was confirmed by quantitative real-time polymerase chain reaction, virus titre and transmission electron microscopy assays. Moreover, ALL cells could respond to VHSV, RGNNV and LMBV infections, as indicated by the differential expression of antiviral genes involving in the innate immune response. In conclusion, the newly established ALL cell line will be an excellent in vitro platform for the study of the virus-yellowfin sea bream interaction.

Bombyx mori Nucleopolyhedrovirus (BmNPV) Induces G2/M Arrest to Promote Viral Multiplication by Depleting BmCDK1

Simple Summary

Baculoviruses arrest the cell cycle in the S or G2/M phase in insect cells, but the exact mechanism of this process still remains obscure. Bombyx mori nucleopolyhedrovirus (BmNPV), one of the best characterized baculoviruses, is an important pathogen in silkworms. In the present study, we determined that downregulation of BmCDK1 and BmCyclin B expression was required for BmNPV-mediated G2/M phase arrest, which plays an essential role in facilitating BmNPV replication. Further investigations showed that BmNPV IAP1 interacted with BmCDK1. The overexpression of the BmNPV iap1 gene led to the accumulation of cells in the G2/M phase, and BmNPV iap1 gene knockdown attenuated the effect of BmNPV-mediated G2/M phase arrest. These findings enhance the understanding of BmNPV pathogenesis, and indicate a novel mechanism through which baculoviruses impact the cell cycle progression.

Abstract

Understanding virus–host interaction is very important for delineating the mechanism involved in viral replication and host resistance. Baculovirus, an insect virus, can cause S or G2/M phase arrest in insect cells. However, the roles and mechanism of Baculovirus-mediated S or G2/M phase arrest are not fully understood. Our results, obtained using flow cytometry (FCM), tubulin-labeling, BrdU-labeling, and CellTiter 96^® AQueous One Solution Cell Proliferation Assay (MTS), showed that Bombyx mori nucleopolyhedrovirus (BmNPV) induced G2/M phase arrest and inhibited cellular DNA replication as well as cell proliferation in BmN-SWU1 cells. We found that BmNPV induced G2/M arrest to support its replication and proliferation by reducing the expression of BmCDK1 and BmCyclin B. Co-immunoprecipitation assays confirmed that BmNPV IAP1 interacted with BmCDK1. BmNPV iap1 was involved in the process of BmNPV-induced G2/M arrest by reducing the content of BmCDK1. Taken together, our results improve the understanding of the virus–host interaction network, and provide a potential target gene that connects apoptosis and the cell cycle.

Effects of rrm1 on NNV Resistance Revealed by RNA-seq and Gene Editing

Viral nervous necrosis (VNN) disease caused by the nervous necrosis virus (NNV) is a major disease, leading to a huge economic loss in aquaculture. Previous GWAS and QTL mapping have identified a major QTL for NNV resistance in linkage group 20 in Asian seabass. However, no causative gene for NNV resistance has been identified. In this study, RNA-seq from brains of Asian seabass fingerlings challenged with NNV at four time points (5, 10, 15 and 20 days post-challenge) identified 1228, 245, 189 and 134 DEGs, respectively. Eight DEGs, including rrm1, were located in the major QTL for NNV resistance. An association study in 445 survived and 608 dead fingerlings after NNV challenge revealed that the SNP in rrm1 were significantly associated with NNV resistance. Therefore, rrm1 was selected for functional analysis, as a candidate gene for NNV resistance. The expression of rrm1 was significantly increased in the gill, liver, spleen and muscle, and was suppressed in the brain, gut and skin after NNV challenge. The rrm1 protein was localized in the nuclear membrane. Over-expression of rrm1 significantly decreased viral RNA and titer in NNV-infected Asian seabass cells, whereas knock-down of rrm1 significantly increased viral RNA and titer in NNV-infected Asian seabass cells. The rrm1 knockout heterozygous zebrafish was more susceptible to NNV infection. Our study suggests that rrm1 is one of the causative genes for NNV resistance and the SNP in the gene may be applied for accelerating genetic improvement for NNV resistance.

Coronavirus Porcine Epidemic Diarrhea Virus Nucleocapsid Protein Interacts with p53 To Induce Cell Cycle Arrest in S-Phase and Promotes Viral Replication

Subversion of the host cell cycle to facilitate viral replication is a common feature of coronavirus infections. Coronavirus nucleocapsid (N) protein can modulate the host cell cycle, but the mechanistic details remain largely unknown. Here, we investigated the effects of manipulation of porcine epidemic diarrhea virus (PEDV) N protein on the cell cycle and the influence on viral replication. Results indicated that PEDV N induced Vero E6 cell cycle arrest at S-phase, which promoted viral replication (P < 0.05). S-phase arrest was dependent on the N protein nuclear localization signal S⁷¹NWHFYYLGTGPHADLRYRT⁹⁰ and the interaction between N protein and p53. In the nucleus, the binding of N protein to p53 maintained consistently high-level expression of p53, which activated the p53-DREAM pathway. The key domain of the N protein interacting with p53 was revealed to be S¹⁷¹RGNSQNRGNNQGRGASQNRGGNN¹⁹⁴ (N^S171-N194), in which G¹⁸³RG¹⁸⁵ are core residues. N^S171-N194 and G¹⁸³RG¹⁸⁵ were essential for N-induced S-phase arrest. Moreover, small molecular drugs targeting the N^S171-N194 domain of the PEDV N protein were screened through molecular docking. Hyperoside could antagonize N protein-induced S-phase arrest by interfering with interaction between N protein and p53 and inhibit viral replication (P < 0.05). The above-described experiments were also validated in porcine intestinal cells, and data were in line with results in Vero E6 cells. Therefore, these results reveal the PEDV N protein interacts with p53 to activate the p53-DREAM pathway, and subsequently induces S-phase arrest to create a favorable environment for virus replication. These findings provide new insight into the PEDV-host interaction and the design of novel antiviral strategies against PEDV. IMPORTANCE Many viruses subvert the host cell cycle to create a cellular environment that promotes viral growth. PEDV, an emerging and reemerging coronavirus, has led to substantial economic loss in the global swine industry. Our study is the first to demonstrate that PEDV N-induced cell cycle arrest during the S-phase promotes viral replication. We identified a novel mechanism of PEDV N-induced S-phase arrest, where the binding of PEDV N protein to p53 maintains consistently high levels of p53 expression in the nucleus to mediate S-phase arrest by activating the p53-DREAM pathway. Furthermore, a small molecular compound, hyperoside, targeted the PEDV N protein, interfering with the interaction between the N protein and p53 and, importantly, inhibited PEDV replication by antagonizing cell cycle arrest. This study reveals a new mechanism of PEDV-host interaction and also provides a novel antiviral strategy for PEDV. These data provide a foundation for further research into coronavirus-host interactions.

Sulforaphane induces S-phase arrest and apoptosis via p53-dependent manner in gastric cancer cells

Sulforaphane (SFN) extracted from broccoli sprout has previously been investigated for its potential properties in cancers, however, the underlying mechanisms of the anticancer activity of SFN remain not fully understood. In the present study, we investigate the effects of SFN on cell proliferation, cell cycle, cell apoptosis, and also the expression of several cell cycle and apoptosis-related genes by MTT assay, flow cytometry and western blot analysis in gastric cancer (GC) cells. The results showed that SFN could impair the colony-forming ability in BGC-823 and MGC-803 cell lines compared with the control. In addition, SFN significantly suppressed cell proliferation by arresting the cell cycle at the S phase and enhancing cell apoptosis in GC cells in a dose-dependent manner. Western blot results showed that SFN treatment significantly increased the expression levels of p53, p21 and decreased CDK2 expression, which directly regulated the S phase transition. The Bax and cleaved-caspase-3 genes involved in apoptosis executive functions were significantly increased in a dose-dependent manner in BGC-823 and MGC-803 cells. These results suggested that SFN-induced S phase cell cycle arrest and apoptosis through p53-dependent manner in GC cells, which suggested that SFN has a potential therapeutic application in the treatment and prevention of GC.

Lappaconitine sulfate induces apoptosis and G0/G1 phase cell cycle arrest by PI3K/AKT signaling pathway in human non-small cell lung cancer A549 cells

Lappaconitine sulfate (LS) has good solubility and bioavailability. We have previously studied the anti-proliferative activity of LS on colon cancer HT-29 cell, but its anti-proliferative activity and molecular mechanism on human non-small cell lung cancer A549 cells are still unclear. This study was to investigate the effects of LS on proliferation, cell cycle and apoptosis in human non-small cell lung cancer A549 cells, and its possible molecular mechanisms. Cell proliferation activity was measured by Cell Counting Kit-8 (CCK-8) and 5-Ethynyl-2'- deoxyuridine (EdU) cell proliferation kit. Cell cycle was detected by propidium iodide (PI) flow cytometry. Apoptosis was detected by Annexin-V-FITC/PI method. Western blot was used to detect cycle and apoptosis-related proteins expression. These results showed that the proliferation activity of LS was significantly decreased in A549 cells, showing a dose- and time-dependent manner (p < 0.05). LS could increase the proportion of G0/G1 phase cells and decrease the proportion of cells in S phase, showing obvious G0/G1 phase arrest. LS significantly inhibited the expression of p-PI3K/PI3K, p-AKT/AKT, Cyclin D1 and Bcl-2 proteins (p < 0.05), and increased the expression of p53, p21, Bax, caspase 3 and caspase 9 (p < 0.05). Moreover, PI3K inhibitor (LY294002) significantly decreased A549 cell viability rate induced by LS, abrogated the activation of p-PI3K/PI3K and p-AKT/AKT in the presence of LS. These results indicated that LS could block A549 cells in the G0/G1 phase, induce apoptosis, and inhibit cell proliferation through the PI3K/AKT signaling pathway.

Grouper IFIT1 inhibits iridovirus and nodavirus infection by positively regulating interferon response

Interferon-induced protein with tetratricopeptide repeats 1 (IFIT1), one of the interferon stimulated genes (ISGs), is strongly induced by type I interferon (IFN), double-stranded RNAs and virus infection. To investigate the actions of fish IFIT1 in response to virus infection, we cloned an IFIT1 homolog from orange spotted grouper (EcIFIT1) and clarified its function in this study. The full-length cDNA of EcIFIT1 is 1839 bp, which is composed of 436 amino acid (aa) residues, with 77.8% and 22.8% identity to IFIT1 homolog of yellow perch (Perca flavescens) and humans (homo sapiens), respectively. Sequence alignment analysis showed that EcIFIT1 contained three tetratricopeptide repeats (TPRs). Tissue distribution analysis indicated that EcIFIT1 was abundant in intestine, spleen, liver, and heart. Moreover, EcIFIT1 was significantly up-regulated by Singapore grouper iridovirus (SGIV) or red-spotted grouper nervous necrosis virus (RGNNV) infection, and polyinosinic-polycytidylic acid (poly I:C) or lipopolysaccharide (LPS) treatment in vitro. Under fluorescence microscopy, EcIFIT1 was found to localize throughout the cytoplasm in transfected cells. EcIFIT1 overexpression significantly suppressed the replication of SGIV and RGNNV, demonstrated by decreasing the cytopathic effect (CPE) severity, viral gene transcription and the virus titers. Further studies showed that the ectopic expression of EcIFIT1 increased the transcription level of IFN related molecules, including IFN regulatory factor (IRF) 3, IRF7, IFN stimulated gene (ISG) 15 and myxovirus resistance gene (MX) I. Meanwhile, the expression levels of pro-inflammation cytokines were differently regulated by the ectopic expression of EcIFIT1. In addition, flow cytometry analysis suggested that EcIFIT1 overexpression affected cell cycle progression by mediating S/G2 transition. Taken together, our results indicated that EcIFIT1 might exert antiviral function against fish virus by up-regulating interferon response or affecting cell cycle.

Fish Autophagy Protein 5 Exerts Negative Regulation on Antiviral Immune Response Against Iridovirus and Nodavirus

Autophagy is an important biological activity that maintains homeostasis in eukaryotic cells. However, little is known about the functions of fish autophagy-related genes (Atgs). In this study, we cloned and characterized Atg5, a key gene in the autophagy gene superfamily, from orange-spotted grouper (Epinephelus coioides) (EcAtg5). EcAtg5 encoded a 275-amino acid protein that shared 94 and 81% identity to seabass (Lates calcarifer) and humans (Homo sapiens), respectively. The transcription level of EcAtg5 was significantly increased in cells infected with red-spotted grouper nervous necrosis virus (RGNNV). In cells infected with Singapore grouper iridovirus (SGIV), EcAtg5 expression declined during the early stage of infection and increased in the late stage. Fluorescence microscopy revealed that EcAtg5 mainly localized with a dot-like pattern in the cytoplasm of grouper cells. Overexpression of EcAtg5 significantly increased the replication of RGNNV and SGIV at different levels of detection, as indicated by increased severity of the cytopathic effect, transcription levels of viral genes, and levels of viral proteins. Knockdown of EcAtg5 decreased the replication of RGNNV and SGIV. Further studies showed that overexpression EcAtg5 activated autophagy, decreased expression levels of interferon related cytokines or effectors and pro-inflammatory factors, and inhibited the activation of nuclear factor κB, IFN-sensitive response element, and IFNs. In addition, ectopic expression of EcAtg5 affected cell cycle progression by hindering the G1/S transition. Taken together, our results demonstrated that fish Atg5 exerted a crucial role in virus replication by promoting autophagy, down-regulating antiviral IFN responses, and affecting the cell cycle.