Porcine reproductive and respiratory syndrome virus increases SOCS3 production via activation of p38/AP-1 signaling pathway to promote viral replication

Highlights

• PRRSV infection induces SOCS3 upregulation.

• AP-1 signaling pathways are indispensable for PRRSV-induced SOCS3 expression.

• SOCS3 facilitates PRRSV replication.

Abstract

SOCS3 belongs to the suppressor of cytokine signaling (SOCS) family, which function as negative factors in host immune responses. Prior studies have noted the importance of SOCS family proteins in immunosuppression induced by some viruses. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important swine-borne viruses and has threatened the global swine industry with huge economic losses since it was first described in the 1980s. PRRSV is the etiological agent of PRRS, which causes reproductive failure and respiratory disorders. PRRSV causes immunosuppression thus establishing persistent infection. In this study, it was observed that SOCS3 was upregulated in PRRSV-infected primary porcine alveolar macrophages (PAMs) and Marc-145 cells with dose-dependent effects, which depends on virus replication. Deletion of AP-1 binding motif located in SOCS3 promoter inhibited promoter activities, which indicates that AP-1 is essential for PRRSV-induced SOCS3. This result was confirmed by experiments using AP-1 inhibitor, whose pretreatment suppressed SOCS3 mRNA and protein expression. Further research showed that p38 was crucial for PRRSV-induced SOCS3 production. Importantly, SOCS3 enhanced PRRSV replication during infection. Taken together, this study indicates that PRRSV infection induced SOCS3 expression through p38/AP-1 signaling pathway. These results revealed the molecular basis of SOCS3 upregulation and would advance further understanding of the strategy for viral immune evasion.

Introduction

Porcine reproductive and respiratory syndrome virus (RRRSV) is one of the most wide-spread swine viruses that lead to enormous economic losses in swine production worldwide (Albina, 1997; Holtkamp et al., 2013). PRRSV is an enveloped, non-segmented, single-stranded, positive-sense RNA virus that belongs to the genus Porartevirus, the family Arteriviridae, and the order Nidovirales, and is divided into PRRSV-1 and PRRSV-2 (Adams et al., 2017). The PRRSV genome is approximately 15.4 kb in length and contains at least 11 open reading frames (ORFs) which constitutes about 75 % of the genome in the 5′ -region and encodes at least 16 non-structural proteins (nsps) (Fang et al., 2012; Dokland, 2010; Fang and Snijder, 2010). The successful control of PRRSV infection is dependent on the immune response that is orchestrated by many cytokines and chemokines involved in the activation and recruitment of cells to the site of infection (Wang et al., 2016a; Shabir et al., 2018; Senthilkumar et al., 2019). However, PRRSV infection may impair immune response by modulating some negative regulators of immunity to counteract cellular antiviral response to establish a persistent infection (Wongyanin et al., 2012; Chen et al., 2017).

A family of regulatory proteins called suppressors of cytokine signaling (SOCS) could mediate immunosuppression through the inhibition of type Ⅰ IFN and proinflammatory cytokine signaling (Elliott and Johnston, 2004; Yoshimura et al., 2007). SOCS proteins, induced by cytokines or other stimuli, act as negative feedback regulators and maintain the cell in a homeostatic state (Yoshimura et al., 2018; Linossi et al., 2018; Chikuma et al., 2017). The SOCS family consists of eight proteins, SOCS1−7 and CIS. They share a common domain architecture, with a variable N-terminal region, a central SRC homology 2 (SH2) domain, an N-terminal extended SH2 subdomain (ESS), and a conserved C-terminal SOCS box domain (Elliott and Johnston, 2004; Yoshimura et al., 2007). The SH2 domain is responsible for determining the targeted substrates through binding with phosphorylated tyrosine residues, while an N-terminal ESS enhances the interaction with a substrate (Yoshimura et al., 2007). The SOCS box recruits Elongin B and C, Cullin-5, RING-box-2 (Rbx2), and E2 ubiquitin transferase to form an E3 ligase complex that mediates proteasome-mediated degradation of targeted proteins (Piessevaux et al., 2008). In addition, SOCS1 and SOCS3 could utilize a unique kinase inhibitory region (KIR) as pseudosubstrate to JAKs to inhibit kinase activity (Yasukawa et al., 1999). SOCS3 is relatively conserved among the common species. The porcine SOCS3 protein exhibited an amino acid sequence identity of 97.3 and 96.9 % comparing with that of human and monkey, respectively (Babon et al., 2006). These observations suggest that SOCS3 functions are conserved in porcine and humans as well as in other species. Recently, accumulating data supported a central role of SOCS3 proteins in viral replication (Collins et al., 2014; Koeberlein et al., 2010; Okumura et al., 2015; Pauli et al., 2008). However, little is known about the effects of SOCS3 expression after PRRSV infection and the underlying mechanisms.

In this study, an investigation was carried out to determine whether PRRSV infection could induce SOCS3 production. The results obtained show that PRRSV induced SOCS3 expression by p38/AP-1 signaling pathway, which in turn promoted virus replication.