ATM kinase activity down-regulates FLIP protein stability. (A) RT-PCR analysis of FLIP-L and FLIP-S RNA expression levels was performed in the indicated cell lines. Amplified actin was used as an internal control. (B) C3ABR-FLAG-FLIP-L stably transfected cells were incubated with NCS for different times to trigger ATM kinase activation. Protein extract (80-100 μg) was separated by SDS-PAGE and transferred on nitrocellulose, and endogenous (*) and transfected FLIP (**) expression was revealed using specific anti-FLIP antibodies. (C) Cells were pretreated with KU-55933 O/N, washed, and then incubated with CHX for the indicated times.