PML up-regulation correlates with TRAIL induction in MM lines. (A) TRAIL is induced only in IFN-responsive cell lines H929 and 8226. Western blot analysis of TRAIL protein following treatment of indicated lines with or without IFN-α for 48 hours (2000 U/mL). GAPDH expression is included as protein loading control. (B) TRAIL inhibits proliferation of both IFN-responsive and nonresponsive cell lines. MM cell lines were treated with the indicated amount of TRAIL and proliferation measured by MTS assay. (C) Biologically active TRAIL is secreted from IFN-α–responsive cell lines. Supernatant from IFN-α–treated and untreated H929 cells was added to IFN-α–nonresponsive Delta47 cells with or without anti-TRAIL antibodies (10 mg/mL) and proliferation measured by MTS assay. Data are presented as percent proliferation of cells treated with supernatant from untreated H929 cells as described in “Materials and methods.” Data are average of 3 independent experiments done in triplicate (± SD). (D) Dominant negative TRAIL receptor blocks IFN-α growth inhibition. The 8226 cells transfected with either vector control (8226/ctr) or DR5-DN construct (8226/DR5-DN) were stimulated with IFN-α 500 U/mL, and proliferation was measured by MTS assay. Proliferation was calculated as described in “Materials and methods.” A representative experiment (1 of 3 independent experiments) performed in triplicate (± SD) is shown.