Nucleocapsid protein of SARS-CoV activates the expression of cyclooxygenase-2 by binding directly to regulatory elements for nuclear factor-kappa B and CCAAT/enhancer binding protein

Modulation of signaling pathways by RNA virus capsid proteins

Capsid proteins are structural components of virus particles. They are nucleic acid-binding proteins whose main recognized function is to package viral genomes into protective structures called nucleocapsids. Research over the last 10 years indicates that in addition to their role as genome guardians, viral capsid proteins modulate host cell signaling networks. Disruption or alteration of intracellular signaling pathways by viral capsids may benefit replication of the virus by affecting innate immunity and in some cases, may underlie disease progression. In this review, we describe how the capsid proteins from medically relevant RNA viruses interact with host cell signaling pathways.

Cell type-specific cleavage of nucleocapsid protein by effector caspases during SARS coronavirus infection

The epidemic outbreak of severe acute respiratory syndrome (SARS) in 2003 was caused by a novel coronavirus (CoV), designated SARS-CoV. The RNA genome of SARS-CoV is complexed by the nucleocapsid protein (N) to form a helical nucleocapsid. Besides this primary function, N seems to be involved in apoptotic scenarios. We show that upon infection of Vero E6 cells with SARS-CoV, which elicits a pronounced cytopathic effect and a high viral titer, N is cleaved by caspases. In contrast, in SARS-CoV-infected Caco-2 cells, which show a moderate cytopathic effect and a low viral titer, this processing of N was not observed. To further verify these observations, we transiently expressed N in different cell lines. Caco-2 and N2a cells served as models for persistent SARS-CoV infection, whereas Vero E6 and A549 cells did as prototype cell lines lytically infected by SARS-CoV. The experiments revealed that N induces the intrinsic apoptotic pathway, resulting in processing of N at residues 400 and 403 by caspase-6 and/or caspase-3. Of note, caspase activation is highly cell type specific in SARS-CoV-infected as well as transiently transfected cells. In Caco-2 and N2a cells, almost no N-processing was detectable. In Vero E6 and A549 cells, a high proportion of N was cleaved by caspases. Moreover, we examined the subcellular localization of SARS-CoV N in these cell lines. In transfected Vero E6 and A549 cells, SARS-CoV N was localized both in the cytoplasm and nucleus, whereas in Caco-2 and N2a cells, nearly no nuclear localization was observed. In addition, our studies indicate that the nuclear localization of N is essential for its caspase-6-mediated cleavage. These data suggest a correlation among the replication cycle of SARS-CoV, subcellular localization of N, induction of apoptosis, and the subsequent activation of caspases leading to cleavage of N.

Severe acute respiratory syndrome-associated coronavirus nucleocapsid protein interacts with Smad3 and modulates transforming growth factor-beta signaling

Severe acute respiratory syndrome (SARS) is an acute infectious disease with significant mortality. A typical clinical feature associated with SARS is pulmonary fibrosis and the associated lung failure. However, the underlying mechanism remains elusive. In this study, we demonstrate that SARS-associated coronavirus (SARS-CoV) nucleocapsid (N) protein potentiates transforming growth factor-β (TGF-β)-induced expression of plasminogen activator inhibitor-1 but attenuates Smad3/Smad4-mediated apoptosis of human peripheral lung epithelial HPL1 cells. The promoting effect of N protein on the transcriptional responses of TGF-β is Smad3-specific. N protein associates with Smad3 and promotes Smad3-p300 complex formation while it interferes with the complex formation between Smad3 and Smad4. These findings provide evidence of a novel mechanism whereby N protein modulates TGF-β signaling to block apoptosis of SARS-CoV-infected host cells and meanwhile promote tissue fibrosis. Our results reveal a novel mode of Smad3 action in a Smad4-independent manner and may lead to successful strategies for SARS treatment by targeting the TGF-β signaling molecules.

Severe acute respiratory syndrome coronavirus as an agent of emerging and reemerging infection

Before the emergence of severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) in 2003, only 12 other animal or human coronaviruses were known. The discovery of this virus was soon followed by the discovery of the civet and bat SARS-CoV and the human coronaviruses NL63 and HKU1. Surveillance of coronaviruses in many animal species has increased the number on the list of coronaviruses to at least 36. The explosive nature of the first SARS epidemic, the high mortality, its transient reemergence a year later, and economic disruptions led to a rush on research of the epidemiological, clinical, pathological, immunological, virological, and other basic scientific aspects of the virus and the disease. This research resulted in over 4,000 publications, only some of the most representative works of which could be reviewed in this article. The marked increase in the understanding of the virus and the disease within such a short time has allowed the development of diagnostic tests, animal models, antivirals, vaccines, and epidemiological and infection control measures, which could prove to be useful in randomized control trials if SARS should return. The findings that horseshoe bats are the natural reservoir for SARS-CoV-like virus and that civets are the amplification host highlight the importance of wildlife and biosecurity in farms and wet markets, which can serve as the source and amplification centers for emerging infections.

The membrane protein of SARS-CoV suppresses NF-kappaB activation

Severe acute respiratory syndrome coronavirus (SARS‐CoV) infects many organs, such as lung, liver, and immune organs and causes life‐threatening atypical pneumonia, SARS causes high morbidity and mortality rates. The molecular mechanism of SARS pathogenesis remains elusive. Inflammatory stimuli can activate IκB kinase (IKK) signalsome and subsequently the nuclear factor kappa B (NF‐κB), which influences gene expression of cyclooxygenase‐2 (Cox‐2) along with other transcription factors. In this work, we found that the membrane (M) protein of SARS‐CoV physically interacted with IKKβ using a co‐immunoprecipitation assay (IPA). Expression of M suppressed tumor necrosis factor alpha (TNF‐α) induced NF‐κB activation using a luciferase reporter assay. Further investigation showed M protein suppressed Cox‐2 expression using a luciferase reporter gene assay, RT‐PCR and Western blot analysis. The carboxyl terminal of M protein was sufficient for the M protein function. Together, these results indicate that SARS‐CoV M suppresses NF‐κB activity probably through a direct interaction with IKKβ, resulting in lower Cox‐2 expression. Suppression of NF‐κB activity and Cox‐2 expression may contribute to SARS pathogenesis. J. Med. Virol. 79:1431–1439, 2007. © Wiley‐Liss, Inc.

Regulation of cell death during infection by the severe acute respiratory syndrome coronavirus and other coronaviruses

Both apoptosis and necrosis have been observed in cells infected by various coronaviruses, suggesting that the regulation of cell death is important for viral replication and/or pathogenesis. Expeditious research on the severe acute respiratory syndrome (SARS) coronavirus, one of the latest discovered coronaviruses that infect humans, has provided valuable insights into the molecular aspects of cell-death regulation during infection. Apoptosis was observed in vitro, while both apoptosis and necrosis were observed in tissues obtained from SARS patients. Viral proteins that can regulate apoptosis have been identified, and many of these also have the abilities to interfere with cellular functions. Occurrence of cell death in host cells during infection by other coronaviruses, such as the mouse hepatitis virus and transmissible porcine gastroenteritis virus, has also being extensively studied. The diverse cellular responses to infection revealed the complex manner by which coronaviruses affect cellular homeostasis and modulate cell death. As a result of the complex interplay between virus and host, infection of different cell types by the same virus does not necessarily activate the same cell-death pathway. Continuing research will lead to a better understanding of the regulation of cell death during viral infection and the identification of novel antiviral targets.

NS3 protein of hepatitis C virus regulates cyclooxygenase-2 expression through multiple signaling pathways

Hepatitis C virus (HCV) causes chronic hepatitis, which often results in the development of liver cirrhosis and hepatocellular carcinoma (HCC) worldwide. In this study, we demonstrated that the non-structural protein NS3 of HCV enhances cyclooxygenase-2 (COX-2) gene promoter activity, COX-2 mRNA expression, COX-2 protein production, and prostaglandin E2 (PGE2) release in HepG2 cells in a concentration-dependent fashion. We also showed that transcription factor NF-κB is required for the activation of COX-2 regulated by NS3. In addition, multiple signaling pathways are involved cooperatively in the expression of COX-2 activated by the viral protein in a calcium-independent manner, which requires signaling components including JNK, ERK, and PKD2. A thorough investigation of mechanism involved in the activation of COX-2 regulated by HCV would provide insights into our understanding the processes of liver inflammatory response and hepatocellular carcinoma development caused by the viral infection and also into the development of novel therapeutics against HCV infection.

The SARS-CoV nucleocapsid protein: a protein with multifarious activities

Ever since the discovery of SARS-CoV in the year 2003, numerous researchers around the world have been working relentlessly to understand the biology of this virus. As in other coronaviruses, nucleocapsid (N) is one of the most crucial structural components of the SARS-CoV. Hence major attention has been focused on characterization of this protein. Independent studies conducted by several laboratories have elucidated significant insight into the primary function of this protein, which is to encapsidate the viral genome. In addition, many reports also suggest that this protein interferes with different cellular pathways, thus implying it to be a key regulatory component of the virus too. In the first part of this review, we will discuss these different properties of the N-protein in a consolidated manner. Further, this protein has also been proposed to be an efficient diagnostic tool and a candidate vaccine against the SARS-CoV. Hence, towards the end of this article, we will discuss some recent progress regarding the possible clinically relevant use of the N-protein.

Cyclooxygenase activity is important for efficient replication of mouse hepatitis virus at an early stage of infection

Cyclooxygenases (COXs) play a significant role in many different viral infections with respect to replication and pathogenesis. Here we investigated the role of COXs in the mouse hepatitis coronavirus (MHV) infection cycle. Blocking COX activity by different inhibitors or by RNA interference affected MHV infection in different cells. The COX inhibitors reduced MHV infection at a post-binding step, but early in the replication cycle. Both viral RNA and viral protein synthesis were affected with subsequent loss of progeny virus production. Thus, COX activity appears to be required for efficient MHV replication, providing a potential target for anti-coronaviral therapy.

Nucleocapsid protein of SARS-CoV activates interleukin-6 expression through cellular transcription factor NF-kappaB

High levels of interleukin-6 (IL-6) in the acute stage associated with lung lesions were found in SARS patients. To evaluate the mechanisms behind this event, we investigated the roles of SARS-CoV proteins in the regulation of IL-6. Results showed that the viral nucleocapsid (N) protein activated IL-6 expression in a concentration-dependent manner. Promoter analyses suggested that NF-κB binding element was required for IL-6 expression regulated by N protein. Further studies demonstrated that N protein bound directly to NF-κB element on the promoter. We also showed that N protein activated IL-6 expression through NF-κB by facilitating the translocation of NF-κB from cytosol to nucleus. Mutational analyses revealed that two regions of N protein were essential for its function in the activation of IL-6. These results provided new insights into understanding the mechanism involved in the function of SARS-CoV N protein and pathogenesis of the virus.

Spike protein of SARS-CoV stimulates cyclooxygenase-2 expression via both calcium-dependent and calcium-independent protein kinase C pathways

We have previously shown that the nucleocapsid protein of SARS-associated coronavirus (SARS-CoV) activated cyclooxygenase-2 (COX-2) expression. In this study, we identified another viral protein, the spike of SARS-CoV, which played an important role in virus-stimulated COX-2 expression after screening all genes from the SARS-CoV genome. We found that an upstream calcium-dependent PKC isozyme PKC alpha that modulates the downstream ERK/NF-kappaB pathway through an influx of extracellular Ca2+ is induced by the spike protein of SARS-CoV. The ERK/NF-kappaB was identified to be involved in the activation of COX-2 promoter and production of COX-2 protein in HEK293T cells. We also demonstrated that another unusual pathway, the calcium-independent PI3K/PKC epsilon/JNK/CREB pathway, functioned in cooperation with the calcium-dependent pathway to induce COX-2 expression upon stimulation by spike protein. This pathway can be blocked by PKC epsilon-specific, small interfering RNA, PI3K/JNK kinase-specific inhibitors as well as dominant negative JNK. PKC epsilon-specific siRNA also attenuated the phosphorylation of JNK. Our results provide evidence that helps us understand the function of SRAS-CoV spike protein in SARS pathogenesis.

Severe acute respiratory syndrome coronavirus open reading frame (ORF) 3b, ORF 6, and nucleocapsid proteins function as interferon antagonists

The severe acute respiratory syndrome coronavirus (SARS-CoV) is highly pathogenic in humans, with a death rate near 10%. This high pathogenicity suggests that SARS-CoV has developed mechanisms to overcome the host innate immune response. It has now been determined that SARS-CoV open reading frame (ORF) 3b, ORF 6, and N proteins antagonize interferon, a key component of the innate immune response. All three proteins inhibit the expression of beta interferon (IFN-beta), and further examination revealed that these SARS-CoV proteins inhibit a key protein necessary for the expression of IFN-beta, IRF-3. N protein dramatically inhibited expression from an NF-kappaB-responsive promoter. All three proteins were able to inhibit expression from an interferon-stimulated response element (ISRE) promoter after infection with Sendai virus, while only ORF 3b and ORF 6 proteins were able to inhibit expression from the ISRE promoter after treatment with interferon. This indicates that N protein inhibits only the synthesis of interferon, while ORF 3b and ORF 6 proteins inhibit both interferon synthesis and signaling. ORF 6 protein, but not ORF 3b or N protein, inhibited nuclear translocation but not phosphorylation of STAT1. Thus, it appears that these three interferon antagonists of SARS-CoV inhibit the interferon response by different mechanisms.

Severe acute respiratory syndrome coronavirus open reading frame (ORF) 3b, ORF 6, and nucleocapsid proteins function as interferon antagonists

The severe acute respiratory syndrome coronavirus (SARS-CoV) is highly pathogenic in humans, with a death rate near 10%. This high pathogenicity suggests that SARS-CoV has developed mechanisms to overcome the host innate immune response. It has now been determined that SARS-CoV open reading frame (ORF) 3b, ORF 6, and N proteins antagonize interferon, a key component of the innate immune response. All three proteins inhibit the expression of beta interferon (IFN-beta), and further examination revealed that these SARS-CoV proteins inhibit a key protein necessary for the expression of IFN-beta, IRF-3. N protein dramatically inhibited expression from an NF-kappaB-responsive promoter. All three proteins were able to inhibit expression from an interferon-stimulated response element (ISRE) promoter after infection with Sendai virus, while only ORF 3b and ORF 6 proteins were able to inhibit expression from the ISRE promoter after treatment with interferon. This indicates that N protein inhibits only the synthesis of interferon, while ORF 3b and ORF 6 proteins inhibit both interferon synthesis and signaling. ORF 6 protein, but not ORF 3b or N protein, inhibited nuclear translocation but not phosphorylation of STAT1. Thus, it appears that these three interferon antagonists of SARS-CoV inhibit the interferon response by different mechanisms.

Preparation and characterization of a novel monoclonal antibody specific to severe acute respiratory syndrome-coronavirus nucleocapsid protein

Severe acute respiratory syndrome-coronavirus nucleocapsid (SARS-CoV N) protein has been found to be important to the processes related to viral pathogenesis, such as virus replication, interference of the cell process and modulation of host immune response; detection of the antigen has been used for the early diagnosis of infection. We have used recombinant N protein expressed in insect cells to generate 17 mAbs directed against this protein. We selected five mAbs that could be used in various diagnostic assays, and all of these mAbs recognized linear epitopes. Three IgG_2b mAbs were recognized within the N-terminus of N protein, whereas the epitope of two IgG₁ mAbs localized within the C-terminus. These mAbs were found to have significant reactivity with both non-phosphorylated and phosphorylated N proteins, which resulted in high reactivity with native N protein in virus-infected cells; however, they did not show cross-reactivity with human coronavirus. Therefore, these results suggested that these mAbs would be useful in the development of various diagnostic kits and in future studies of SARS-CoV pathology.