MG-132 treatment reduces caspase-3-like activity in PC-3 cells. PC-3 cells were pre-incubated with MG-132 (50 μM) for 3 hours, washed, irradiated, and incubated at 37°C. 15 minutes, 1, and 3 hours later, cells were lysed and total cellular protein was assayed for caspase-3-like activity, measuring the release of AMC from the fluorogenic caspase-3 substrate DEVD-AMC. Irradiation with 30 Gy initially decreased constitutive caspase-3 activity in PC-3 cells at 15 minutes followed by a slight increase at 1 and 3 hours after irradiation. In contrast, proteasome inhibition with MG-132 completely inhibited caspase-3 activity at 15 minutes. Activity recovered slowly reaching about 30% of baseline levels after 3 hours. This was not altered by combination of MG-132 treatment with ionizing radiation (n = 3, data expressed as mean ± standard error mean).