Knockdown of p53 by gene-specific siRNA inhibits Nutlin-3–induced apoptosis. (A) ARPE-19 cells were transfected with control or p53-specific siRNA. Cell lysates were analyzed by Western blot for the levels of p53, p73, p21Cip1, Bcl-2, and Bcl-xL using specific antibodies. (B) Cells transfected with control or p53 siRNA were treated with 20 μM CPT for 3 hours or with 60 μM Nutlin-3 for 2 hours. Cell extracts were analyzed to determine the levels of PUMA, Siva-1, active caspase-3, and actin by Western blot analysis.