FTI-induced apoptosis in CML cells does not involve FasR/FasL signaling. (A) FTIs did not modify FasR and FasL expression in CML cells. A representative flow cytometric analysis of FasR and FasL expression, quantified as mean fluorescence intensity, in LAMA cells without and with in vitro exposure to SCH (1 μM) is shown. (B) FasR and FasL expression on CD34⁺ CML cells did not correlate with susceptibility to FTI-mediated inhibition of CML cell viability. Cumulative mean ± SEM CD34⁺ FasR+ and C34+ FasL+: 37% ± 1% and 29.9% ± 6% in FTI-sensitive CML versus 40% ± 3% and 36.6% ± 13% in FTI-resistant CML; P = .7 and P = .6, respectively. Horizontal bars represent mean values; vertical bars, SEM. (C) FTIs did not induce activation of caspase-1 and caspase-8. CML cells were cultured for 48 hours in medium alone containing the same concentration of DMSO used to dissolve SCH (untreated) or treated with a representative FTI (SCH [1 μM]). Caspase-1 and caspase-8 activities were measured by flow cytometry as described in “Materials and methods.” At least 1 × 10⁴ cells were examined in each assay. (D) Inhibition of FasR triggering and caspase-1 and -8 did not revert FTI-mediated inhibition of CML cell growth. CML cells were grown for 48 hours with FTIs (IC50) and with FTIs + Fas-receptor–triggering inhibitor Fas:Fc, caspase-8 inhibitor IETD-FMK, or caspase-1 inhibitor ZYVAD-FMK (all used at 50 μM). CML cell viability was measured by the colorimetric MTT assay. Bars represent cumulative mean ± SEM of cell viability of 9 experiments performed with any FTI: 41.6% ± 3% versus 43.3% ± 3% without and with Fas:Fc, respectively (P = .6); 48.1% ± 1% versus 50.3% ± 3% without and with IETD-FMK, respectively (P = .3); 62.3% ± 3% versus 61.6% ± 3% without and with ZYVAD-FMK, respectively (P = .7).