Figure 5. Distinction between activation and delayed repression.

A. A validated shRNA was used to knockdown p16^INK4a in several strains of logarithmically growing HDFs and its effects on p16^INK4a and p18^INK4c were assessed by immunoblotting. B. Lifespan extension in TIG3 cells infected with a retrovirus encoding mouse Bmi1 compared to empty vector (Vec). C. The cells described in panel B were harvested at the indicated days post-infection and the levels of p16^INK4a and p18^INK4c were assessed by immunoblotting. D. Direct comparison of lysates prepared at days 50 and 64 (indicated by the arrows in panel B) from cells infected with Bmi1 virus (+) or empty vector (−).