PS-341 sensitizes MM cells to DNA-damaging chemotherapy.
(A) MM.1S cells were pretreated with doxorubicin (Doxo; 40 nM), melphalan (Mel; 1 μM), or dexamethasone (Dex; 0.5 μM) for 24 hours, and then PS-341 (2 nM) was added for additional 24 hours (black bars, without PS-341; white bars, with PS-341). PS-341 sensitizes MM.1S cells to DNA-damaging chemotherapy. (B) Dose-response analysis for the effect of doxorubicin on MM.1S cells in the presence (▪) or absence (♦) of PS-341 (2 nM) reveals that PS-341 decreases the LD50 of doxorubicin from 150 to 26 nM. (C) MM.1S cells were pretreated with doxorubicin (50 ng/mL) for 24 hours, and then PS-341 (2 nM) was added for an additional 24 hours (i), or pretreated with PS-341 for 24 hours and then doxorubicin for an additional 24 hours (ii), or treated with PS-341 and doxorubicin together for 24 hours (iii). In all cases a synergistic effect is found, but the strongest synergy is observed when the cells are pretreated with doxorubicin followed by PS-341 treatment (black bars, control; white bars, doxorubicin alone; grid bars, PS-341; gray bars, doxorubicin plus PS-341). (D) RPMI-8226/S, ARP-1, S6B45, NCH-H929, and INA6 cells were pretreated with doxorubicin (50 ng/mL) for 24 hours and then with PS-341 (2 nM) for an additional 24 hours (black bars, control; white bars, doxorubicin alone; grid bars, PS-341; gray bars, doxorubicin plus PS-341). (E) Primary MM cells from 4 PS-341–naive patients were pretreated with doxorubicin (50 ng/mL) for 24 hours and then with PS-341 (2 nM) for an additional 24 hours. PS-341 sensitizes all MM cells to doxorubicin (black bars, control; white bars, doxorubicin alone; grid bars, PS-341; gray bars, doxorubicin plus PS-341). (F) Doxorubicin-resistant RPMI-Dox40 cells were pretreated with (white bars) or without (black bars) doxorubicin (800 ng/mL) for 24 hours, and then PS-341 (2-10 nM) was added for an additional 24 hours (black bars, without doxorubicin; white bars, with doxorubicin). PS-341 sensitizes RPMI-Dox40 cells to doxorubicin. (G) Melphalan-resistant LR5 cells were pretreated with or without melphalan (5 μM) for 24 hours, and then PS-341 (2 nM) was added for an additional 24 hours. PS-341 sensitizes LR5 cells to melphalan. (H) MM cells isolated from a patient who had relapsed following treatment with PS-341 were pretreated with (white bars) or without (black bars) doxorubicin (100 ng/mL) for 24 hours, and then PS-341 (5-20 nM) was added for an additional 24 hours. Pretreatment with doxorubicin overcomes resistance to PS-341. (I) MM.1S cells were treated for 24 hours with doxorubicin (100-200 ng/mL) in wells coated with (white bars) or without (black bars) fibronectin (FN). PS-341 (10 nM) was added for additional 24 hours. In all cases, percentage of cell survival (mean ± SD) is quantified by MTT. All experiments were repeated at least 3 times, and each experimental condition was repeated at least in quadruplicate wells in each experiment. Results from representative experiments are shown.
