Figure 4. CPT dose not downregulate MKP1 by promoting its proteasome-mediated degradation. Cells were first pretreated with or without the proteasome inhibitor MG132 (10 μM) for 1 h, then co-treated with CPT for 24 h. Total cell extracts were prepared and analyzed by western blotting using anti-MKP1 (MKP1), anti-phospho-ERKs (P-ERK), anti-ERKs (ERK), and anti-PARP antibodies. Variations in protein loading among samples were controlled by monitoring actin levels.