Down-regulation of granulocyte-macrophage colony-stimulating factor by 3C-like proteinase in transfected A549 human lung carcinoma cells

Background: Severe Acute Respiratory Syndrome (SARS) is a severe respiratory illness caused by a novel virus, the SARS coronavirus (SARS-CoV). 3C-like protease (3CL(pro)) of SARS-CoV plays a role in processing viral polypeptide precursors and is responsible of viral maturation. However, the function of 3CL(pro) in host cells remains unknown. This study investigated how the 3CL(pro) affected the secretion of cytokines in the gene-transfected cells. Results: From immunofluorescence microscopy, the localization of c-myc tagged 3CL(pro) was detected both in the cytoplasm and nucleus of transfected A549 cells. Expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) was significantly decreased in 3CL(pro)-transfected cells by both RT-PCR and ELISA, but without changes in other cytokines, i.e., IL-1 beta, IL-6, IL-8, IL12p40, TNF-alpha, and TGF-beta. Furthermore, the protein levels of NF-kB decreased in 3CLpro-transfected A549 cells when compared to EGFP transfected cells. Conclusions: Our results suggest that the 3CL(pro) may suppress expression of GM-CSF in transfected A549 cells through down-regulation of NF-kB production.