Severe acute respiratory syndrome coronavirus (SARS-CoV) was identified to be the causative agent of SARS with atypical pneumonia. Angiotensin-converting enzyme 2 (ACE2) is the major receptor for SARS-CoV. It is not clear whether ACE2 conveys signals from the cell surface to the nucleus and regulates expression of cellular genes upon SARS-CoV infection. To understand the pathogenesis of SARS-CoV, human type II pneumocyte (A549) cells were incubated with the viral spike protein or with SARS-CoV virus-like particles containing the viral spike protein to examine cytokine modulation in lung cells. Results from oligonucleotide-based microarray, real-time PCR, and enzyme-linked immunosorbent assays indicated an upregulation of the fibrosisassociated chemokine (C-C motif) ligand 2 (CCL2) by the viral spike protein and the virus-like particles. The upregulation of CCL2 by SARS-CoV spike protein was mainly mediated by extracellular signal-regulated kinase 1 and 2 (ERK1/2) and AP-1 but not the IฮบBฮฑ-NF-ฮบB signaling pathway. In addition, Ras and Raf upstream of the ERK1/2 signaling pathway were involved in the upregulation of CCL2. Furthermore, ACE2 receptor was activated by casein kinase II-mediated phosphorylation in cells pretreated with the virus-like particles containing spike protein. These results indicate that SARS-CoV spike protein triggers ACE2 signaling and activates fibrosis-associated CCL2 expression through the Ras-ERK-AP-1 pathway. Copyright ยฉ 2010, American Society for Microbiology.