Bortezomib induces delayed apoptosis of bone marrow stromal cells (BMSCs). Cells were incubated in appropriate growth media, without or with bortezomib (5-500 nM), for 24-72 hr. Apoptosis was measured by flow cytometry after staining the cells for annexin V. (A) Representative flow cytometric profiles of bortezomib-treated U266 myeloma cells. (B) Representative flow cytometric profiles of bortezomib-treated MS-5 cells. (C) Apoptosis of MS-5 cells induced by bortezomib. Data are the mean±SD of 3 independent experiments. (D) Apoptosis of bortezomib-exposed BMSCs from 3 healthy individuals. Data are the mean±SD. (E) Apoptosis of bortezomib-exposed BMSCs from 5 myeloma patients. Data are the mean±SD. ^a)P<0.05, as compared with the control (no bortezomib).