Proteinase 2A pro Is Essential for Enterovirus Replication in Type I Interferon-Treated Cells

134

130

The recent outbreak of enterovirus 71 (EV71) infected millions of children and caused over 1,000 deaths. To date, neither an effective vaccine nor antiviral treatment is available for EV71 infection. Interferons (IFNs) have been successfully applied to treat patients with hepatitis B and C viral infections for decades but have failed to treat EV71 infections. Here, we provide the evidence that EV71 antagonizes type I IFN signaling by reducing the level of interferon receptor 1 (IFNAR1). We show that the host cells could sense EV71 infection and stimulate IFN-␤ production. However, the induction of downstream IFN-stimulated genes is inhibited by EV71. Also, only a slight interferon response and antiviral effects could be detected in cells treated with recombinant type I IFNs after EV71 infection. Further studies reveal that EV71 blocks the IFN-mediated phosphorylation of STAT1, STAT2, Jak1, and Tyk2 by reducing IFNAR1. Finally, we identified the 2A protease encoded by EV71 as an antagonist of IFNs and show that the protease activity is required for reducing IFNAR1 levels. Taken together, our study for the first time uncovers a mechanism used by EV71 to antagonize type I IFN signaling and provides new targets for future antiviral strategies. E nterovirus 71 (EV71) is a typical positive-strand RNA virus which belongs to the Picornaviridae family (44). The genome of EV71 is approximately 7.5 kb in length and contains a single open reading frame encoding a polyprotein precursor. This polyprotein is cleaved by two viral proteases, 2Apro and 3C pro , to form four structural proteins (VP1, VP2, VP3, and VP4) and seven nonstructural proteins (2A, 2B, 2C, 3A, 3B, 3C, and 3D) (39). EV71 infection was a major cause of outbreaks of hand, foot, and mouth disease (HFMD) in infants and young children (19,22). As a typical neurotropic virus, EV71 has a propensity to cause neurological disease during acute infection and may lead to permanent paralysis and even death (1,42,45). The outbreaks and severity of EV71 infection have been frequently reported worldwide (60), and recently EV71 has become a major threat to public health in China (56). The Chinese government reported that there were ϳ1,770,000 cases of HFMD and herpangina with over 900 deaths in 2010 (http://www.moh.gov.cn/publicfiles/business/htmlfiles/zwgkzt /pyq/list.htm). However, to date, no effective vaccine or therapy can be applied to prevent or treat EV71 infection.Type I interferon (IFN), as the first line of host immune response, is critical in mediating antiviral defense. The host recognizes viral invasion and activates the type I IFN response through the recognition of pathogen-associated molecular patterns (PAMPs) by patternrecognition receptors (PRRs) (61). Binding with the corresponding receptors, IFN receptor 1 (IFNAR1) and IFNAR2 (11, 50), the secreted IFNs activate the Janus kinases Jak1 and Tyk2 and then phosphorylate signal transducers and activators of transcription STAT1 and STAT2 (23). The phosphorylated STAT1 and STAT2 form heterodimers and bind with...

Section: Discussioncontrasting

confidence: 52%

Enterovirus 71 Disrupts Interferon Signaling by Reducing the Level of Interferon Receptor 1

Liu

Zhao

et al. 2012

134

130

“…Overall, our data suggest that type I IFN can induce a PKR-independent pathway to block VACV protein translation and that this pathway can be inhibited by K1L and C7L. Interestingly, some recent studies with norovirus and picornavirus also suggest that a PKR-independent pathway is induced by type I IFN to inhibit protein translation in cells infected by these RNA viruses (8,20). Identification of the target for K1L and C7L may lead to the identification of a novel antiviral host factor/pathway that has broad antiviral effects.…”

Section: C7lmentioning

confidence: 76%

Vaccinia Virus K1L and C7L Inhibit Antiviral Activities Induced by Type I Interferons

Meng

Jiang

Arsenio

et al. 2009

Cellular tropism of vaccinia virus (VACV) isVaccinia virus (VACV) is the prototypical member of the poxvirus family of large, complex, double-stranded DNA viruses (21). VACV has a very broad host range and is capable of infecting many vertebrate animal species. Its host range, however, can be significantly narrowed by deleting from its genome some of the so-called host range genes, the most important of which are E3L, K1L, and C7L (17). VACV mutants deleted of E3L (⌬E3L) or both K1L and C7L (⌬K1L⌬C7L) replicate abortively and express only a subset of viral genes in most mammalian cell lines (3, 24). These mutants are highly attenuated in animal hosts but are capable of eliciting immune responses, making them attractive vaccine vectors for infectious diseases and cancers (27,28). NYVAC, a VACV strain derived through deletion of 18 genes, including both K1L and C7L (27), has been used as the vector for an AIDS vaccine (2).The functions of E3L and the host factors that restrict the replication of the ⌬E3L mutant have been studied extensively. E3L encodes a 20-kDa and a 25-kDa protein that bind doublestranded RNA (dsRNA) and Z form DNA (6, 15). The E3 proteins antagonize the dsRNA-dependent protein kinase PKR (5), which exists as an inactive form in the cells and undergoes autophosphorylation and activation upon binding to dsRNA. The activated PKR phosphorylates the ␣ subunit of eukaryotic initiation factor 2 (eIF2␣), resulting in a block in protein translation at the initiation step. The infection of most mammalian cells by the ⌬E3L mutant leads to the activation of PKR and a block in translating viral mRNAs (16). The replication of the ⌬E3L mutant in nonpermissive HeLa cells can be rescued by silencing PKR expression (32), while its replication in permissive Huh7 (human hepatoma) cells can be blocked by upregulating PKR expression with interferon (IFN) treatment (1). In addition to affecting PKR, E3 has also been shown to inactivate IFN-stimulated gene 15 (ISG15) (14), another IFN effector that plays a role in host defense against VACV.Like the replication of the ⌬E3L mutant, the replication of the ⌬K1L⌬C7L mutant in nonpermissive HeLa cells is blocked at the translation of viral mRNA. However, the host factors that restrict the replication of the ⌬K1L⌬C7L mutant and the molecular functions of K1L or C7L remain a mystery. K1 and C7 share no amino acid sequence homology, but either K1L or C7L can complement the replication defect of the ⌬K1L⌬C7L mutant in most cell lines. The exception is rabbit RK13 cells, where K1L but not C7L can complement (24). K1L is present in only a few orthopoxviruses, while C7L or a functional homologue of C7L is present in almost all mammalian poxviruses (18). K1 comprises multiple ankyrin repeats, a protein motif that is involved in protein-ligand interaction. It was shown to prevent the degradation of IB␣ and to thus inhibit host . C7L has no homologue outside the poxvirus family, and its molecular function remains unknown. It may play a role in inhibiting cellular apoptosis in respons...

“…Remarkably, 2A protein of cardioviruses, being nonessential though important (69,90,108), also exhibits antihost functions by inhibiting cell translation (44). 2A protease proteins of enterovirus and rhinovirus (unrelated to 2A proteins of cardioviruses), though apparently essential (70), also perform a set of anti-defensive functions very similar to those exhibited by L proteins of cardioviruses and aphthoviruses, namely the inhibition of host translation (59), the disruption of controllable nucleocytoplasmic traffic (12,75), the suppression of interferon action (71), and others (56). We propose that L proteins of aphthoviruses as well as 2A proteins of cardioviruses, enteroviruses, and rhinoviruses also be included in the group of security proteins.…”

Section: Apoptosis-triggering and Apoptosis-preventing Functions Of Cmentioning

confidence: 99%

Antiapoptotic Activity of the Cardiovirus Leader Protein, a Viral “Security” Protein

Romanova

Lidsky

Kolesnikova

et al. 2009

Apoptosis is a common antiviral defensive mechanism that potentially limits viral reproduction and spread. Many viruses possess apoptosis-suppressing tools. Here, we show that the productive infection of HeLa cells with encephalomyocarditis virus (a cardiovirus) was not accompanied by full-fledged apoptosis (although the activation of caspases was detected late in infection) but rather elicited a strong antiapoptotic state, as evidenced by the resistance of infected cells to viral and nonviral apoptosis inducers. The development of the antiapoptotic state appeared to depend on a function(s) of the viral leader (L) protein, since its mutational inactivation resulted in the efflux of cytochrome c from mitochondria, the early activation of caspases, and the appearance of morphological and biochemical signs of apoptosis in a significant proportion of infected cells. Infection with both wild-type and L-deficient viruses induced the fragmentation of mitochondria, which in the former case was not accompanied with cytochrome c efflux. Although the exact nature of the antiapoptotic function(s) of cardioviruses remains obscure, our results suggested that it includes previously undescribed mechanisms operating upstream and possibly downstream of the mitochondrial level, and that L is involved in the control of these mechanisms. We propose that cardiovirus L belongs to a class of viral proteins, dubbed here security proteins, whose roles consist solely, or largely, in counteracting host antidefenses. Unrelated L proteins of other picornaviruses as well as their highly variable 2A proteins also may be security proteins. These proteins appear to be independent acquisitions in the evolution of picornaviruses, implying multiple cases of functional (though not structural) convergence.