Fig. 4.

UVB-induced p21 degradation is independent of ATR/ATM pathways. p21 protein levels were determined as in Fig. 2A. (A) Cells were pretreated with caffeine (1 or 5 mM) for 30 min and then exposed to UVB radiation (20 mJ cm⁻²). Cells were then collected at 1.5 h post-UVB. (B) Cells were transfected with siRNA targeting ATR or ATM. Non-targeting siRNA was used as a negative control (NC) for siRNA transfection. The protein levels of ATR or ATM were determined by Western blotting using specific anti-ATR and anti-ATM antibodies. (C) Cells were transfected with siRNA targeting ATR, ATM, the combination of both (ATR/ATM), or NC, and then exposed to UVB (5 and 20 mJ cm⁻²). Cells were collected at 1.5 h post-UVB.