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

From immunofluorescence microscopy, the localization of c-myc tagged 3CLpro 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 3CLpro-transfected cells by both RT-PCR and ELISA, but without changes in other cytokines, i.e., IL-1β, IL-6, IL-8, IL12p40, TNF-α, and TGF-β. Furthermore, the protein levels of NF-kB decreased in 3CLpro-transfected A549 cells when compared to EGFP transfected cells.Our results suggest that the 3CLpro may suppress expression of GM-CSF in transfected A549 cells through down-regulation of NF-kB production.Severe acute respiratory syndrome (SARS) emerged as a communicable human disease in November 2002 and rapidly spread throughout the world [1]. A coronavirus, called SARS-associated coronavirus (SARS-CoV), was identified as the causative agent [2,3]. SARS-CoV is a plus-strand RNA virus featuring a large single-stranded RNA genome of approximately 29.7 kb [4] that contains 16 non-structural proteins (nsps) with multiple enzymatic functions. These are known or are predicted to include types of enzymes that are common components of the replication machinery of plus-strand RNA viruses [5,6]. In these nsps, the Nsp5 [main protease (Mpro) or 3C-like protease (3CLpro)] is responsible for the proteolytic processing of viral polypeptides into functional proteins and 3CLpro mutant, C145A, blocks the maturation process [7]. This viral protease has been the target for design of the inhibitors [7-11]. Recently, SARS-3CLpro was found to induce cell apotosis and interact with cellular vacuolar-H+ ATPase [12,13]. Several human proteins might possess a similar structure of SARS-3CLpro cleavage sites using computational methods [14]. Theses studies implied the SARS-3CLpro could involve in virus-induced host pathology.SARS-CoV causes a severe lung infection, and the epithelial cells of the upper respiratory tract are the primary targets [