ATM kinase activity down-regulates FLIP protein levels and sensitizes Hodgkin lymphoma cell lines to Fas-induced apoptosis. (A) L428 HL cells were transiently transfected with the indicated constructs along with GFP. Twenty-four hours after transfection, GFP-positive cells were isolated by FACS sorting, and incubated for an additional 12 hours. For immunoblotting, 80 to 100 μg protein extract were separated by SDS-PAGE and transferred on nitrocellulose. The proteins of interest and their phosphorylation were revealed by immunoblotting with specific antibodies. (B) L428 HL cells were transiently transfected with the indicated constructs along with GFP. Twenty-four hours after transfection, cells were stimulated to undergo apoptosis with 250 ng/mL anti-Fas mAb. Apoptosis was determined by the analysis of annexin V binding 24 hours after anti-Fas treatment, upon FACS sorting of GFP-positive cells.