In vivo binding of ARTS-1 to TNFR1 in human epithelial and endothelial cells. (a) ARTS-1 binds to TNFR1 in vivo. Coimmunoprecipitation experiments were performed on membrane proteins from NCI-H292 cells (left) and HUVECs (right). As shown in the top panels, immunoprecipitations were performed with either an anti-TNFR1 monoclonal antibody (+) or a murine IgG₁ isotype control (IgG₁) and immunoblotted with either anti–ARTS-1 preimmune (PI) or immune (+) serum. As shown in the bottom panels, reciprocal coimmunoprecipitations were performed with either anti-ARTS-1 preimmune (PI) or immune (+) serum and immunoblotted with either an anti-TNFR1 monoclonal antibody (+) or a murine IgG₁ isotype control (IgG₁). IP indicates the antibody used for immunoprecipitation and IB indicates the antibody used for immunoblotting. (b) Effect of ARTS-1 protein expression on in vivo binding of ARTS-1 to TNFR1 in NCI-H292 cell lines. Membrane proteins of wild-type NCI-H292 cells, mock-transfected cells, and ARTS-1 cell lines were immunoprecipitated with an anti-TNFR1 monoclonal antibody and immunoblotted with anti–ARTS-1 serum.