Sensitization of cancer stem cells in adherent culture to TRAIL. A, The TRAIL sensitive MDA-MB-231 cell line was subjected to increasing concentrations of TRAIL and in-well caspase-8 activity plotted as relative luminescence obtained by Caspase-Glo assay. Cell lines were transfected with siRNA (FLIPi and SCi) and plated in mammosphere conditions, as described in Figure 3, in the presence or absence of 1 ng/ml TRAIL. As in Figure 3, graphs represent the percentage of mammospheres (MFU) formed following seven days culture (Passage 1) and seven days after trypsinisation and re-seeding (Passage 2). B, Cell lines transfected with siRNA were treated with or without TRAIL for 18 hours while maintained in adherent monolayer culture. The surviving cell population was seeded at 20,000 cells/ml in low-serum, non-adherent culture conditions. Percentage mammosphere forming units (MFU) were calculated as previously described.