20S proteasomal proteolytic activity. The pMTH1-U937 and asCD11b-U937 cells were treated with 5nM TPA for 4 h up to 72 h. At the time points indicated the cells were harvested and nuclear extracts were measured by a fluorometric proteolytic assay to obtain the 20 S proteasomal activity. The relative proteasomal activity of pMTH1-U937 control cells in steady-state was set to 100%. Data represent the mean ± s.d. of three independent experiments.