Sensitization of interferon-gamma induced apoptosis in human osteosarcoma cells by extracellular S100A4. S100A4 is a small Ca2+-binding protein of the S100 family with metastasis-promoting properties. Recently, secreted S100A4 protein has been shown to possess a number of functions, including induction of angiogenesis, stimulation of cell motility and neurite extension.Cell cultures from two human osteosarcoma cell lines, OHS and its anti-S100A4 ribozyme transfected counterpart II-11b, was treated with IFN-gamma and recombinant S100A4 in order to study the sensitizing effects of extracellular S100A4 on IFN-gamma mediated apoptosis. Induction of apoptosis was demonstrated by DNA fragmentation, cleavage of poly (ADP-ribose) polymerase and Lamin B.In the present work, we found that the S100A4-expressing human osteosarcoma cell line OHS was more sensitive to IFN-gamma-mediated apoptosis than the II-11b cells. S100A4 protein was detected in conditioned medium from OHS cells, but not from II-11b cells, and addition of recombinant S100A4 to the cell medium sensitized II-11b cells to apoptosis induced by IFN-gamma. The S100A4/IFN-gamma-mediated induction of apoptosis was shown to be independent of caspase activation, but dependent on the formation of reactive oxygen species. Furthermore, addition of extracellular S100A4 was demonstrated to activate nuclear factor-kappa B (NF-kappa B).In conclusion, we have shown that S100A4 sensitizes osteosarcoma cells to IFN-gamma-mediated induction of apoptosis. Additionally, extracellular S100A4 activates NF-kappa B, but whether these events are causally related remains unknown.