Role of NF-κB in the rescue of multiple myeloma cells from glucocorticoid-induced apoptosis by bcl-2

The molecular mechanisms by which multiple myeloma (MM) cells evade glucocorticoid-induced apoptosis have not been delineated. Using a human IgAκ MM cell line (ARP-1), we found that dexamethasone (Dex)-induced apoptosis is associated with decreased NF-κB DNA binding and κB-dependent transcription. Both nuclear p50:p50 and p50:p65 NF-κB complexes are detected in ARP-1 cells by supershift electrophoretic mobility shift assay (EMSA). Dex-mediated inhibition of NF-κB DNA binding precedes a notable increase in annexin V binding, thereby indicating that diminished NF-κB activity is an early event in Dex-induced apoptosis. Overexpression of bcl-2 in ARP-1 cells prevents Dex-mediated repression of NF-κB activity and apoptosis. Sustained NF-κB DNA binding is also observed in two previously characterized Dex- resistant MM cell lines (RPMI8226 and ARH-77) that express moderate levels of endogenous bcl-2 and IκBα proteins. In addition, enforced bcl-2 expression in ARP-1 cells did not prevent the augmentation of IκBα protein by Dex. We also noted a possible association between Dex-mediated downregulation of NF- κB in freshly obtained primary myeloma cells and the patients' responsiveness to glucocorticoid-based chemotherapy. Collectively, our data suggest that the protective effects of bcl-2 in MM cells act upstream in the NF-κB activation-signaling pathway and the potential use of NF-κB as a biomarker in progressive MM.