1096. FSTL_1 Is a Novel Pro_Inflammatory Protein

Abstract

Follistatin-like protein (FSTL-1) is a secreted glycoprotein with a molecular weight of 40-55 KD that contains a follistatin-like domain. FSTL-1 was first isolated from a cDNA library of mouse osteoblastic cells as a transforming growth factor-|[beta]|1-inducible gene. Its function has not yet been fully elucidated. We found increased expression of FSTL-1 mRNA by microarray analysis of joints of mice with collagen-induced arthritis (CIA). In situ hybridization and immunohistochemistry revealed increased expression along the margin of contact between the inflammatory synovium and eroding bone. We determined by immunohistochemistry that the source of FSTL-1 was fibroblast-like synoviocytes. The highest level of FSTL-1 expression was seen during the acute phase of CIA. When CIA mice received a non-replicating adenoviral vector encoding FSTL-1 (Ad-FSTL1) intravenously one week before the onset of arthritis, the mice developed earlier and more severe disease, compared to mice administered a control virus. This result suggests that over-expression of FSTL-1 exacerbates collagen-induced arthritis. To investigate the role of FSTL-1 in inflammation, normal DBA-1 mice received Ad-FSTL1 intravenously or in the footpad. Results of quantitative RT-PCR from the livers of mice treated intravenously with Ad-FSTL1 showed increased IL-1|[beta]| , IL-6, and TNF|[alpha]| expression compared to control group. Mice receiving Ad-FSTL1 in the footpad showed significant swelling and increased IL-1|[beta]| and IL-6 protein compared to the controls. In addition to in vivo experiments, we stably-transfected the human monocyte cell line, U937, with FSTL-1. Although these cells did not spontaneously produce inflammatory cytokines, addition of PMA to induce differentiation into macrophages led to secretion of IL-1|[beta]|, TNF-|[alpha]|, and IL-6. A substantial synergistic effect was observed upon addition of LPS, suggesting that FSTL-1 utilizes a distinct signaling pathway different from the LPS receptor. These results suggest that FSTL-1 is a pro- inflammatory molecule that might serve as a novel target for therapy of arthritis or other inflammatory diseases.