Studies On The Regulation Of TGF-β Signal Transduction By SARS-CoV Nucleocapsid Protein

Severe acute respiratory syndrome (SARS) is an acute infectious disease with significant mortality. A typical clinical feature associated with SARS is diffuse alveolar damage which may lead to pulmonary fibrosis and eventually lung failure. SARS-CoV (SARS-associated coronavirus) has been identified as the aetiological agent of the disease. Nucleocapsid (N) protein is one of the SARS-CoV structural proteins, and it has been implicated to be involved in viral replication and regulation of host cellular processes such as gene transcription, actin reorganization, cell cycle progression and apoptosis.Transforming growth factor-β(TGF-β) is a multifunctional cytokine that modulates a variety of biological processes. TGF-βinduces extracellular matrix deposition during tissue fibrosis and it also induces apoptosis of multiple types of cells including hepatocytes, lymphocytes and tumor cells. High levels of pro-inflammatory cytokines including TGF-β1 were expressed in SARS patients which is thought to be related to the acute lung injury and pathogenesis of SARS. However, the precise role of TGF-βin SARS-mediated pathogenesis remains obscure.In this study, we demonstrated that SARS-CoV N protein potentiates 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-βis Smad3-specific. N protein associates with Smad3 and interferes with the complex formation between Smad3 and Smad4. Furthermore, N protein promotes Smad3-p300 complex formation. Though there exist apoptotic cells in SARS patient lung, the viral-infected cells do not exhibit the apoptotic markers. Consistent with that, N protein rather down-regulates the expression of pro-apoptotic genes Bax and Bim and inhibits Smad3/Smad4-induced HPL1 apoptosis. These findings provide evidence for a novel mechanism whereby N protein modulates TGF-βsignaling to block apoptosis of SARS-CoV-infected host cells and meanwhile promote tissue fibrosis. Our results reveal the role of N protein in the pathogenesis of SARS and might also lead to successful strategies for targeting the TGF-βsignaling molecules in the treatment of SARS.