MG-132, a proteosome inhibitor, suppresses ATRA-induced cyclin D1 degradation and cell-cycle arrest. (A) NB4 cells were cultured in the presence of 0.5 μM ATRA for 24 hours and then treated with or without 5 μM MG-132 for 30 minutes. After being incubated in ATRA for another 24 hours, the cells were lysed and cellular cyclin D1 protein level was analyzed using Western-blot analysis as described in Figure 4B. The degradation of cyclin D1 in ATRA-treated cells is significantly suppressed by MG-132 incubation (n = 3, *P < .05). (B) Treatment with MG-132 promotes NB4 cell proliferation in the presence of ATRA. NB4 cells were cultured in liquid medium containing 0.5 μM ATRA and treated with MG-132 as described in Figure 6A. Cell proliferation was assessed as described in Figure 2B. Bars indicate mean (± SD, n = 3, *P < .05). (C) MG-132 suppresses ATRA-induced G0/G1 cell-cycle arrest. NB4 cells were treated with ATRA and MG-132 as described in panel A. Cell-cycle analysis was carried out as described in Figure 5A. The figure shows the results of a representative experiment. (D) The percentage of cells arrested in G0/G1 phase was quantified using a ModFIT software as described in Figure 5B. Data presented are the means (± SD) of 3 independent experiments. P < .01 versus MG132-untreated cells.