Role of NF-kappaB 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 IgAkappa MM cell line (ARP-1), we found that dexamethasone (Dex)-induced apoptosis is associated with decreased NF-kappaB DNA binding and kappaB-dependent transcription. Both nuclear p50:p50 and p50:p65 NF-kappaB complexes are detected in ARP-1 cells by supershift electrophoretic mobility shift assay (EMSA). Dex-mediated inhibition of NF-kappaB DNA binding precedes a notable increase in annexin V binding, thereby indicating that diminished NF-kappaB activity is an early event in Dex-induced apoptosis. Overexpression of bcl-2 in ARP-1 cells prevents Dex-mediated repression of NF-kappaB activity and apoptosis. Sustained NF-kappaB 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 IkappaBalpha proteins. In addition, enforced bcl-2 expression in ARP-1 cells did not prevent the augmentation of IkappaBalpha protein by Dex. We also noted a possible association between Dex-mediated downregulation of NF-kappaB 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-kappaB activation-signaling pathway and the potential use of NF-kappaB as a biomarker in progressive MM.