A dual effect of porcine reproductive and respiratory syndrome virus replication on the phosphatidylinositol-3-kinase-dependent Akt pathway

Antiviral mechanism of Fuzhengjiedu San against porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a viral infectious disease that can cause infection in pigs of different ages. The condition known as porcine reproductive and respiratory syndrome poses a serious risk to the world's pig business and results in significant financial losses. Fuzhengjiedu San (FZJDS) is a traditional Chinese medicine compound, the main components include:Radix Isatidis, Radix Astragali and Herba Epimedii. It has been widely used in clinical and experimental studies, showing a wide range of biological activity. However, it is not clear whether FZJDS has anti-PRRSV activity. We observed that FZJDS had significant antiviral activity in Marc-145 cells. And FZJDS could inhibit viral infection in the stages of viral internalization and replication. Furthermore, FZJDS can inhibit PRRSV replication by inhibiting the p53 signaling pathway to affect autophagy, and FZJDS can also inhibit PRRSV replication by inhibiting the PI3K/Akt pathway.We showed in this work that FZJDS inhibits PRRSV replication in vitro and offers a novel therapeutic approach for PRRSV infection.

Fangchinoline Inhibits African Swine Fever Virus Replication by Suppressing the AKT/mTOR/NF-κB Signaling Pathway in Porcine Alveolar Macrophages

African swine fever (ASF), caused by the African swine fever virus (ASFV), is one of the most important infectious diseases that cause high morbidity and mortality in pigs and substantial economic losses to the pork industry of affected countries due to the lack of effective vaccines. The need to develop alternative robust antiviral countermeasures, especially anti-ASFV agents, is of the utmost urgency. This study shows that fangchinoline (FAN), a bisbenzylisoquinoline alkaloid found in the roots of Stephania tetrandra of the family Menispermaceae, significantly inhibits ASFV replication in porcine alveolar macrophages (PAMs) at micromolar concentrations (IC50 = 1.66 µM). Mechanistically, the infection of ASFV triggers the AKT/mTOR/NF-κB signaling pathway. FAN significantly inhibits ASFV-induced activation of such pathways, thereby suppressing viral replication. Such a mechanism was confirmed using an AKT inhibitor MK2206 as it inhibited AKT phosphorylation and ASFV replication in PAMs. Altogether, the results suggest that the AKT/mTOR pathway could potentially serve as a treatment strategy for combating ASFV infection and that FAN could potentially emerge as an effective novel antiviral agent against ASFV infections and deserves further in vivo antiviral evaluations.

Research Progress of Porcine Reproductive and Respiratory Syndrome Virus NSP2 Protein

Porcine reproductive and respiratory syndrome virus (PRRSV) is globally prevalent and seriously harms the economic efficiency of pig farming. Because of its immunosuppression and high incidence of mutant recombination, PRRSV poses a great challenge for disease prevention and control. Nonstructural protein 2 (NSP2) is the most variable functional protein in the PRRSV genome and can generate NSP2N and NSP2TF variants due to programmed ribosomal frameshifts. These variants are broad and complex in function and play key roles in numerous aspects of viral protein maturation, viral particle assembly, regulation of immunity, autophagy, apoptosis, cell cycle and cell morphology. In this paper, we review the structural composition, programmed ribosomal frameshift and biological properties of NSP2 to facilitate basic research on PRRSV and to provide theoretical support for disease prevention and control and therapeutic drug development.

EGCG Restricts PRRSV Proliferation by Disturbing Lipid Metabolism

Porcine reproductive and respiratory syndrome virus (PRRSV) infection leads to late-term reproductive failure and respiratory illness that affect the global swine industry. Epigallocatechin gallate (EGCG) is a polyphenolic compound from green tea that exerts antiviral activity against diverse viruses. This study aimed to report an uncharacterized mechanism of how EGCG restricted PRRSV proliferation. EGCG showed no significant effects on cell viability, cell cycle progression, and apoptosis in porcine alveolar macrophages and MARC-145 cells. The treatment of cells with EGCG attenuated the replication of both highly pathogenic and less pathogenic PRRSV in vitro. The viral life cycle analysis demonstrated that EGCG affected PRRSV replication and assembly, but not viral attachment, entry, or release. Interestingly, EGCG treatment abrogated the increased lipid droplets formation and lipid content induced by PRRSV infection. We further demonstrated that EGCG blocked PRRSV-stimulated expression of the key enzymes in lipid synthesis. In addition, EGCG attenuated PRRSV-induced autophagy that is critical for PRRSV proliferation. The supplementation of oleic acid restored PRRSV replication and assembly under EGCG treatment. Together, our results support that EGCG inhibits PRRSV proliferation through disturbing lipid metabolism. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is an enveloped single-positive-stranded RNA virus that causes acute respiratory distress in piglets and reproductive failure in sows, resulting in huge economic losses to the global swine industry. Several lines of evidence have suggested the crucial roles of lipids in PRRSV proliferation. Our previous report demonstrated that PRRSV activated lipophagy to facilitate viral replication through downregulating the expression of N-Myc downstream-regulated gene 1. The manipulation of lipid metabolism may be a new perspective to prevent PRRSV spread. In the present study, we reported that epigallocatechin-3-gallate (EGCG), the major component of green tea catechins, significantly attenuated PRRSV infection through inhibiting lipid synthesis and autophagy. Given that natural products derived from plants have helped in the prevention and treatment of various infectious diseases, EGCG has a great potential to serve as a safe and environmentally friendly natural compound to treat PRRSV infection.

Porcine Reproductive and Respiratory Syndrome Virus Interferes with Swine Influenza A Virus Infection of Epithelial Cells

Respiratory infections are still a major concern in pigs. Amongst the involved viruses, the porcine reproductive and respiratory syndrome virus (PRRSV) and the swine influenza type A virus (swIAV) have a major impact. These viruses frequently encounter and dual infections are reported. We analyzed here the molecular interactions between viruses and porcine tracheal epithelial cells as well as lung tissue. PRRSV-1 species do not infect porcine respiratory epithelial cells. However, PRRSV-1, when inoculated simultaneously or shortly before swIAV, was able to inhibit swIAV H1N2 infection, modulate the interferon response and alter signaling protein phosphorylations (ERK, AKT, AMPK, and JAK2), in our conditions. SwIAV inhibition was also observed, although at a lower level, by inactivated PRRSV-1, whereas acid wash treatment inactivating non-penetrated viruses suppressed the interference effect. PRRSV-1 and swIAV may interact at several stages, before their attachment to the cells, when they attach to their receptors, and later on. In conclusion, we showed for the first time that PRRSV can alter the relation between swIAV and its main target cells, opening the doors to further studies on the interplay between viruses. Consequences of these peculiar interactions on viral infections and vaccinations using modified live vaccines require further investigations.

Molecular and Cellular Mechanisms for PRRSV Pathogenesis and Host Response to Infection

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PRRSV has evolved to arm with various strategies to modify host antiviral response.

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Viral modulation of homeostatic cellular processes provides favorable conditions for PRRSV survival during infection.

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PRRSV modulation of cellular processes includes pathways for interferons, apoptosis, microRNAs, cytokines, autophagy, and viral genome recombination.

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous amounts of economic losses to the swine industry for more than three decades, but its control is still unsatisfactory. A significant amount of information is available for host cell-virus interactions during infection, and it is evident that PRRSV has evolved to equip various strategies to disrupt the host antiviral system and provide favorable conditions for survival. The current study reviews viral strategies for modulations of cellular processes including innate immunity, apoptosis, microRNAs, inflammatory cytokines, and other cellular pathways.

Signaling pathways involved in regulating apoptosis induction in host cells upon PRRSV infection

Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of porcine reproductive and respiratory syndrome (PRRS), a devastating disease of swine that poses a serious threat to the swine industry worldwide. The induction of apoptosis in host cells is suggested to be the key cellular mechanism that contributes to the pathogenesis of PRRS. Various signaling pathways have been identified to be involved in regulating PRRSV-induced apoptosis. In this review, we summarize the potential signaling pathways that contribute to PRRSV-induced apoptosis, and propose the issues that need to be addressed in future studies for a better understanding of the molecular basis underlying the pathogenesis of PRRS.

mTORC1 Negatively Regulates the Replication of Classical Swine Fever Virus Through Autophagy and IRES-Dependent Translation

Classical swine fever virus (CSFV) can utilize diverse host signaling pathways for its replication; however, the cross talk between mammalian target of rapamycin (mTOR) and CSFV remains unknown. Here, we describe the potential role of mTOR complex 1 (mTORC1) in promoting CSFV replication via virus-induced hypophosphorylation of the Akt/mTORC1/S6 pathway, especially at an early stage of viral infection. Conversely, activation of mTORC1 inhibited the replication of CSFV. Furthermore, we revealed the underlying mechanisms of mTORC1 pathway in mediating CSFV replication; in addition, our data also showed that CSFV-induced transient inhibition of mTORC1 elicited a negative feedback activation of PI3K/Akt/mTORC1pathway, likely contributing to maintain the dynamic balance between viral replication and host cell survival. This study has provided strong evidence showing how CSFV utilizes mTORC1 pathway for viral replication at an early stage in the viral replicative cycle and how the mTORC1 rescues itself by eliciting a feedback loop to limit viral replication and maintain cell survival.

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Akt/mTORC1 pathway negatively regulates the replication of CSFV

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CSFV induces autophagy for viral replication in an mTORC1/ULK1-dependent manner

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CSFV enhances the translation of viral proteins in an mTORC1/S6K1/eIF3-dependent manner

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Feedback activation of Akt/mTORC1 equilibrates viral replication and cell survival

Isobavachalcone inhibits post-entry stages of the porcine reproductive and respiratory syndrome virus life cycle

Porcine reproductive and respiratory syndrome virus (PRRSV) is a pathogen of great economic significance that impacts the swine industry globally. Since the first report of a porcine reproductive and respiratory syndrome (PRRS) outbreak, tremendous efforts to control this disease, including various national policies and plans incorporating the use of multiple modified live-virus vaccines, have been made. However, PRRSV is still a significant threat to the swine industry, and new variants continually emerge as a result of PRRSV evolution. Several studies have shown that pandemic PRRSV strains have enormous genetic diversity and that commercial vaccines can only provide partial protection against these strains. Therefore, effective anti-PRRSV drugs may be more suitable and reliable for PRRSV control. In this study, we observed that isobavachalcone (IBC), which was first isolated from Psoralea corylifolia, had potent anti-PRRSV activity in vitro. Although many biological activities of IBC have been reported, this is the first report describing the antiviral activity of IBC. Furthermore, after a systematic investigation, we demonstrated that IBC inhibits PRRSV replication at the post-entry stage of PRRSV infection. Thus, IBC may be a candidate for further evaluation as a therapeutic agent against PRRSV infection of swine in vivo.

Induction of Apoptosis by the Nonstructural Protein 4 and 10 of Porcine Reproductive and Respiratory Syndrome Virus

Infection by most viruses triggers apoptosis in host cells, and viruses manipulate this cell response to promote viral replication, virus spread, and cell killing. Porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to induce apoptosis both in vitro and in vivo, while the regulatory roles of PRRSV-encoded products in apoptosis are not fully understood. In the present study, we first showed a biphasic apoptosis regulation by a highly pathogenic PRRSV strain JXwn06. It was indicated that PRRSV infection delays apoptosis at early infection but activates apoptosis at late infection in MARC-145 cells. In PRRSV-infected MARC-145 cells, procaspase-8, -9 and -12 were activated at late infection, demonstrating the involvements of death receptor pathway, mitochondrial pathway and endoplasmic reticulum (ER) stress pathway in inducing apoptosis. PRRSV was also shown to induce a similar apoptosis process in pulmonary alveolar macrophages (PAMs) with an early initiation. Next, the PRRSV-encoded apoptosis inducers were screened, indicating that the nonstructural protein (Nsp) 4 and Nsp10 of PRRSV are pro-apoptotic. In the presence of Nsp4, it was confirmed that procaspase-8, -9 and -12 were cleaved, and Nsp4 facilitates the cleavage of procaspase-9 by activating B-cell lymphoma 2 interacting mediator of cell death (Bim), a pro-apoptotic protein. In addition, Nsp4 was shown to induce the degradation of an anti-apoptotic protein, B-cell lymphoma-extra large (Bcl-xL). Nsp10 was shown to activate procaspase-8 and -9 but procaspase-12 and to upregulate the expression of BH3-only pro-apoptotic protein BH3 interacting-domain death agonist (Bid) and its active form, truncated Bid (tBid). Clearly, the participation of both activated caspase-8 and Bid is required for Nsp10-induced apoptosis, indicating a crosstalk between extrinsic- and mitochondria-dependent pathways. Together, our findings suggest that PRRSV infection regulates apoptosis in a two-phase manner and activates all three apoptotic pathways; the Nsp4 and Nsp10 of PRRSV function as apoptosis inducers with different molecular basis.

Involvement of unfolded protein response, p53 and Akt in modulation of porcine reproductive and respiratory syndrome virus-mediated JNK activation

Our previous study has shown that activation of JNK plays a critical role in Porcine reproductive and respiratory syndrome virus (PRRSV)-mediated apoptosis. In this follow-up study, we further investigated the mechanisms involved in modulation of PRRSV-mediated JNK activation and apoptosis. We found that unfolded protein response (UPR) was induced in response to PRRSV infection which in turn triggered JNK activation and apoptosis. We also found that p53 and Akt were activated at the early stage of infection and functioned as negative regulator of JNK activation to counteract the PRRSV-mediated apoptosis. Furthermore, induction of UPR, p53 and Akt was not only involved in modulation of PRRSV-mediated apoptosis, but also contributed to the virus replication. Our findings indicated that multiple signaling pathways were involved in modulation of PRRSV-mediated apoptosis of the host cells via regulating JNK signaling pathway and provided novel insights into understanding the mechanisms of pathogenesis of PRRSV infection.

Control of the PI3K/Akt pathway by porcine reproductive and respiratory syndrome virus

Phosphatidylinositol-3-kinase (PI3K)/Akt is an important cellular pathway that has been shown to participate in various replication steps of multiple viruses. In the present study, we compared the phosphorylation status of Akt during infection of MARC-145 cells and porcine alveolar macrophages (PAMs) with highly pathogenic PRRSV (HP-PRRSV) strain HuN4. We observed that biphasic activation of Akt was induced in at both the early stage (5, 15 and 30 min postinfection) and the late stage (12 and 24 h postinfection) of HP-PRRSV infection of MARC-145 cells, while an early-phase activation of Akt was found exclusively in virus-infected PAMs in vitro. Analysis with the PI3K-specific inhibitor LY294002 confirmed that PI3K acted as the upstream activator for the virus-induced activation of Akt. UV-irradiation-inactivated virus still induced the early event in PAMs but not in MARC-145 cells, suggesting that different mechanisms are employed for the early-stage induction of phosphorylated Akt within different cell cultures. We further demonstrated that FoxO1 and Bad, which serve as downstream targets of Akt, were phosphorylated in virus-infected MARC-145 cells. Moreover, the suppression of phosphorylated Akt with LY294002 significantly inhibited the virus-induced cytopathic effect (CPE) on MARC-145 cells, but it had a negligible effect on virus propagation. Collectively, our data provide new evidence of a novel role for the PI3K/Akt pathway in PRRSV infection of MARC-145 cells.

Induction of interleukin-10 is dependent on p38 mitogen-activated protein kinase pathway in macrophages infected with porcine reproductive and respiratory syndrome virus

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) causes reproductive failure and respiratory illness in pigs and usually establishes a persistent infection. Previous studies suggested that interleukin-10 (IL-10) could play a critical role in PRRSV-induced immunosuppression. However, the ability of PRRSV to induce IL-10 in infected cells is controversial. In this study, we further investigated this issue using PRRSV strain CH-1a, which is the first North American genotype strain isolated in China.

Results

PRRSV strain CH-1a could significantly up-regulate IL-10 production both at mRNA and protein levels in porcine alveolar macrophages (PAMs), bone marrow-derived macrophages (BMDMs), and monocyte-derived macrophages (MDMs). However, up-regulation of IL-10 by PRRSV was retarded by specific inhibitors of p38 mitogen-activated protein kinase (MAPK) (SB203580) and NF-κB (BAY11-7082). Additionally, p38 MAPK and NF-κB pathways but not ERK1/2 MAPK were actually activated in PRRSV-infected BMDMs as demonstrated by western blot analysis, suggesting that p38 MAPK and NF-κB pathways are involved in the induction of IL-10 by PRRSV infection. Transfection of PAMs and PAM cell line 3D4/21 (CRL-2843) with viral structural genes showed that glycoprotein5 (GP5) could significantly up-regulate IL-10 production, which was dependent on p38 MAPK and signal transducer and activator of transcription-3 (STAT3) activation. We also demonstrated that a full-length glycoprotein was essential for GP5 to induce IL-10 production.

Conclusions

PRRSV strain CH-1a could significantly up-regulate IL-10 production through p38 MAPK activation.

Porcine reproductive and respiratory syndrome virus induces autophagy to promote virus replication

An increasing number of studies demonstrate that autophagy, an intrinsic mechanism that can degrade cytoplasmic components, is involved in the infection processes of a variety of pathogens. It can be hijacked by various viruses to facilitate their replication. In this study, we found that PRRSV infection significantly increases the number of double- or single-membrane vesicles in the cytoplasm of host cells in ultrastructural analysis. Our results showed the LC3-I was converted into LC3-II after virus infection, suggesting the autophagy machinery was activated. We further used pharmacological agents and shRNAs to confirm that autophagy promoted the replication of PRRSV in host cells. Confocal microscopy analysis showed that PRRSV inhibited the fusion between autophagosomes and lysosomes, suggesting that PRRSV induced incomplete autophagy. This suppression caused the accumulation of autophagosomes which may serve as replication site to enhance PRRSV replication. It has been shown that NSP2 and NSP3 of arterivirus are two components of virus replication complex. We also found in our studies that NSP2 colocalized with LC3 in MARC-145 cells by performing confocal microscopy analysis and continuous density gradient centrifugation. Our studies presented here indicated that autophagy was activated during PRRSV infection and enhanced PRRSV replication in host cells by preventing autophagosome and lysosome fusion.

Post-transcriptional control of type I interferon induction by porcine reproductive and respiratory syndrome virus in its natural host cells

Porcine reproductive and respiratory syndrome virus (PRRSV) is not only a poor inducer of type I interferon but also inhibits the efficient induction of type I interferon by porcine transmissible gastroenteritis virus (TGEV) and synthetic dsRNA molecules, Poly I:C. However, the mechanistic basis by which PRRSV interferes with the induction of type I interferon in its natural host cells remains less well defined. The purposes of this review are to summarize the key findings in supporting the post-transcriptional control of type I interferon in its natural host cells and to propose the possible role of translational control in the regulation of type I interferon induction by PRRSV.

Porcine reproductive and respiratory syndrome virus activates the transcription of interferon alpha/beta (IFN-α/β) in monocyte-derived dendritic cells (Mo-DC)

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to be a poor inducer of interferon alpha/beta (IFN-α/β), which may contribute to the delayed development of adaptive immunity and the resultant viral persistence. However, the exact mechanism by which PRRSV inhibits the induction of IFN-α/β during infection of its natural host cells remains less well defined. Here, we show that PRRSV efficiently activates the transcription of IFN-α/β in porcine monocyte-derived dendritic cells (Mo-DC) in a time-dependent and transient manner; and this effect is dependent on the activation of phosphatidylinositol 3-kinase (PI3K). Despite the abundant IFN-α transcripts detected in PRRSV-infected Mo-DC, little or no detectable IFN-α is found in the supernatants and cell lysates of PRRSV-infected Mo-DC, suggesting that PRRSV either blocks the translation of IFN-α or inhibits the RNA processing and transport. Furthermore, we observed that PRRSV infection significantly reduced the induction of IFN-α by Poly I:C treatment; and virus replication is essential to the effect since heat-inactivated PRRSV has no effect on IFN-α induction by Poly I:C. Overall, our data provide evidence for the possible role of PI3K in the activation of the transcription of IFN-α/β by PRRSV. We conclude that PRRSV inhibits the induction of IFN-α in Mo-DC by as yet undefined post-transcriptional mechanisms.