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Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses to the pork industry worldwide each year. Our previous research demonstrated that heme oxygenase-1 (HO-1) can suppress PRRSV replication via an unknown molecular mechanism. In this study, inhibition of PRRSV replication was demonstrated to be mediated by carbon monoxide (CO), a downstream metabolite of HO-1. Using several approaches, we demonstrate that CO significantly inhibited PRRSV replication in both a PRRSV permissive cell line, MARC-145, and the predominant cell type targeted during in vivo PRRSV infection, porcine alveolar macrophages (PAMs). Our results showed that CO inhibited intercellular spread of PRRSV; however, it did not affect PRRSV entry into host cells. Furthermore, CO was found to suppress PRRSV replication via the activation of the cyclic GMP/protein kinase G (cGMP/PKG) signaling pathway. CO significantly inhibits PRRSV-induced NF-B activation, a required step for PRRSV replication. Moreover, CO significantly reduced PRRSV-induced proinflammatory cytokine mRNA levels. In conclusion, the present study demonstrates that CO exerts its anti-PRRSV effect by activating the cellular cGMP/PKG signaling pathway and by negatively regulating cellular NF-B signaling. These findings not only provide new insights into the molecular mechanism of HO-1 inhibition of PRRSV replication but also suggest potential new control measures for future PRRSV outbreaks.IMPORTANCE PRRSV causes great economic losses each year to the swine industry worldwide. Carbon monoxide (CO), a metabolite of HO-1, has been shown to have antimicrobial and antiviral activities in infected cells. Our previous research demonstrated that HO-1 can suppress PRRSV replication. Here we show that endogenous CO produced through HO-1 catalysis mediates the antiviral effect of HO-1. CO inhibits PRRSV replication by activating the cellular cGMP/PKG signaling pathway and by negatively regulating cellular NF-B signaling. These findings not only provide new insights into the molecular mechanism of HO-1 inhibition of PRRSV replication but also suggest potential new control measures for future PRRSV outbreaks.

P orcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important viruses affecting the swine industry worldwide, resulting in significant economic losses each year (1-3). PRRSV is a small, enveloped, linear, single positive-stranded RNA virus and a member of the order Nidovirales, family Arteriviridae (4). Current vaccination strategies cannot effectively control PRRSV infection because of the high antigenic heterogeneity (5, 6), the replication in and destruction of lung alveolar macrophages (7-9), and the observed antibody-dependent enhancement of PRRSV (10, 11). Therefore, it is imperative to study PRRSV pathogenesis mechanisms so that more effective control measures can be developed.Heme oxygenase-1 (HO-1) is the rate-limiting enzyme of heme degradation, and it functions to catabolize free heme into biliverdin, carbon monoxide, and iron. HO-1 and its end products have antioxidant, anti-inflammatory, and antiviral properties, and it is known to be a pivotal cytoprotective enzyme (12). Upregulation of HO-1 expression suppresses replication of a number of viruses, including hepatitis C virus (HCV), HIV-1, hepatitis B virus (HBV), and influenza virus (13-17). Our previous work showed that PRRSV significantly downregulates HO-1

Pseudorabies virus (PRV) infection brings about great economic losses to the swine industry worldwide, as there are currently no effective therapeutic agents or vaccines against this disease, and mutations in endemic wild virulent PRV strains result in immune failure of traditional vaccines. Heme oxygenase-1 (HO-1) catalyzes the conversion of heme into biliverdin (BV), iron and carbon monoxide (CO), all of which have been demonstrated to protect cells from various stressors. However, the role of HO-1 in PRV replication remains unknown. Thus, the present study aimed to investigate the effect of HO-1 on PRV replication and determine its underlying molecular mechanisms. The results demonstrated that induction of HO-1 via cobalt-protoporphyrin (CoPP) markedly suppressed PRV replication, while HO-1 specific small interfering RNA or inhibitor zinc-protoporphyrin partially reversed the inhibitory effect of CoPP on PRV replication. Furthermore, overexpression of HO-1 notably inhibited PRV replication, while knockdown of endogenous HO-1 expression promoted PRV replication. Mechanism analyses indicated that the HO-1 downstream metabolites, CO and BV/BR partially mediated the virus suppressive effect of HO-1. Taken together, the results of the present study suggest that HO-1 may be developed as a novel endogenous antiviral factor against PRV, and the HO-1/BV/CO system may constitute a unique antiviral protection network during PRV infection and interaction with host cells.

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Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important viral pathogens in the swine industry. Current antiviral strategies do not effectively prevent and control PRRSV. Recent reports show that microRNAs (miRNAs) play vital roles in viral infections by post transcriptionally regulating the expression of viral or host genes. Our previous research showed that non-muscle myosin heavy chain 9 (MYH9) is an essential factor for PRRSV infection. Using bioinformatic prediction and experimental verification, we demonstrate that MYH9 expression is regulated by the miRNA let-7f-5p, which binds to the MYH9 mRNA 3′UTR and may play an important role during PRRSV infection. To understand how let-7f-5p regulates PRRSV infection, we analyzed the expression pattern of both let-7f-5p and MYH9 in porcine alveolar macrophages (PAMs) after infection with either highly pathogenic PRRSV (HP-PRRSV) or classical type PRRSV (N-PRRSV) using a deep sequencing approach with quantitative real-time PCR validation. Our results showed that both HP-PRRSV and N-PRRSV infection reduced let-7f-5p expression while also inducing MYH9 expression. Furthermore, let-7f-5p significantly inhibited PRRSV replication through suppression of MYH9 expression. These findings not only provide new insights into the pathogenesis of PRRSV, but also suggest potential new antiviral strategies against PRRSV infection.

BackgroundPorcine reproductive and respiratory syndrome virus (PRRSV) could lead to pandemic diseases and huge financial losses to the swine industry worldwide. Curcumin, a natural compound, has been reported to serve as an entry inhibitor of hepatitis C virus, chikungunya virus and vesicular stomatitis virus. In this study, we investigated the potential effect of curcumin on early stages of PRRSV infection.ResultsCurcumin inhibited infection of Marc-145 cells and porcine alveolar macrophages (PAMs) by four different genotype 2 PRRSV strains, but had no effect on the levels of major PRRSV receptor proteins on Marc-145 cells and PAMs or on PRRSV binding to Marc-145 cells. However, curcumin did block two steps of the PRRSV infection process: virus internalization and virus-mediated cell fusion.ConclusionsOur results suggested that an inhibition of genotype 2 PRRSV infection by curcumin is virus strain-independent, and mainly inhibited by virus internalization and cell fusion mediated by virus. Collectively, these results demonstrate that curcumin holds promise as a new anti-PRRSV drug.