Dexamethasone-induced apoptotic mechanisms in myeloma cells investigated by analysis of mutant glucocorticoid receptors

Abstract

The mechanism by which the glucocorticoid (GC) dexamethasone induces apo-ptosis in multiple myeloma (MM) cells is unknown, although previous work suggests that either transactivation through the glucocorticoid response element (GRE), transrepression of NF-κB, phosphorylation of RAFTK (Pyk2), or induction of Bim is important. We studied this question by ectopically expressing mutant glucocorticoid receptors (GRs) in the dexamethasone-resistant MM1R cell line, which has lost its GR. Lentiviral-mediated reexpression of wild-type GR restored GRE transactivation, NF-κB transrepression, RAFTK phosphorylation, Bim induction, and dexamethasone-induced apopto-sis. We then reexpressed 4 GR mutants, each possessing various molecular effects, into MM1R cells. A perfect correlation was present between induction of GRE transactivation and induction of apo-ptosis. In contrast, NF-κB transrepres-sion and RAFTK phosphorylation were not required for apoptosis. Although not required for dexamethasone-mediated apoptosis, NF-κB inhibition achieved by gene transfer suggested that NF-κB transrepression could contribute to apoptosis in dexamethasone-treated cells. Dexa-methasone treatment of MM1R cells expressing a mutant incapable of inducing apoptosis successfully resulted in RAFTK (Pyk2) phosphorylation and Bim induction indicating the latter GR-mediated events were not sufficient to induce apo-ptosis. MM1R cells expressing mutant GRs will be helpful in defining the molecular mechanisms of dexamethasone-induced apoptosis of myeloma cells. © 2008 by The American Society of Hematology.