A secreted tumor-suppressor, mac25, with activin-binding activity

Background: mac25 is a follistatin (FS)-like protein that has a growth-supp ressing effect on a p53-deficient osteosarcoma cell line (Saos-2). The prot ein exhibits a strong homology to FS, an activin-binding protein, and part of its sequence includes the consensus sequence of the member of the Kazal serine protease inhibitor family. Materials and Methods: Localization of mac25 protein was analyzed using mac 25 protein fused with green fluorescent protein (GFP). Recombinant mac25 pr otein was expressed in E. coli and purified. The recombinant mac25 protein was added in culture medium for analysis of growth suppression and cell cyc le analysis. Binding of mac25 protein to activin A was studied by immunopre cipitation and Western blots analysis. Results: mac25 protein was localized in the cytoplasm and secreted into cul ture medium. Addition of recombinant mac25 protein (10(-7) M) into the cult ure medium induced significant suppression of the growth of human cervical carcinoma cells (HeLa) and murine embryonic carcinoma cells (P19), as well as osteosarcoma cells (Saos-2). mac25 protein was coimmunoprecipitated with activin A, a result that suggests that mac25 may be a secreted tumor-suppr essor that binds activin A. Conclusion: mac25 exhibits homology to insulinlike growth factor-binding pr oteins (IGF-BPs) and to fibroblast growth factor receptor. The multifunctio nal nature of mac25 protein may be important for growth-suppression and/or cellular senescence.