Myeloid leukemia cells, but not noncancerous monocytes, are resistant to TNF-induced cell death. (A) Primary monocytes, THP-1, HL60, U937, and AML 193 cells were treated with 10 μM BAY 11-7082 30 minutes before treatment with the indicated concentrations of TNF for 24 hours. Cell number was then assessed by MTS assay as described in “Proliferation/death and apoptotic assays.” (B) Primary monocytes, THP-1 cells, and HL60 cells were transfected with p65 siRNA 24 hours before stimulation with the indicated concentrations of TNF for 24 hours (inset, p65 protein knockdown 24 hours after transfection). Cell number was then assessed by MTS assay. (C) Apoptotic cells morphology detected by epifluorescence after staining with Hoechst 33342. Values indicate the mean plus or minus SEM from 3 independent experiments (* statistical significance, P < .01, between the different treatment groups).