Proteasome inhibitors target FoxM1 protein and overexpression of FoxM1 partially protects cancer cell lines from proteasome inhibitor-induced apoptosis.

<p>(A) Proteasome inhibitors induce apoptosis and inhibit FoxM1 protein expression in multiple myeloma, leukemia and osteosarcoma cell lines. U266 and RPMI8226 multiple myeloma, HL-60 leukemia and U2OS-C3 osteosarcoma cell lines were treated with proteasome inhibitors MG115, MG132 and bortezomib (BOR). Apoptosis was assessed by immunoblotting for cleaved caspase-3 and FoxM1 protein levels were detected by immunoblotting. β-actin was used as the loading control. (B) Proteasome inhibitor-induced apoptosis was further quantified by using Annexin V -PE/7AAD staining. U266 and RPMI8226 multiple myeloma, HL-60 leukemia and U2OS-C3 osteosarcoma cell lines were grown in the absence or presence of indicated concentrations of proteasome inhibitors MG115, MG132 and bortezomib (BOR) for 24 hours. Following drug treatment cells were stained with Annexin V- PE/7AAD and then analyzed by flow cytometry. Percentage of apoptotic cells is shown in brackets. Concentrations of drugs are the same as in the Fig 2A. (C) Overexpression of FoxM1 specifically protects against cell death induced by Bortezomib. Overexpression of FoxM1 protected against cell death induced by increasing amount of bortezomib (BOR), but not that of doxorubicin. Immunoblotting was carried out with antibodies specific for cleaved caspase-3 and β-actin.