Interleukin-6-induced inhibition of multiple myeloma cell apoptosis: support for the hypothesis that protection is mediated via inhibition of the JNK/SAPK pathway.

The mechanism by which interleukin-6 (IL-6) protects multiple myeloma (MM) plasma cells from apoptosis induced by anti-fas antibodies and dexamethasone was studied. Anti-apoptotic concentrations of IL-6 had no effect on cell-cycle distribution or activation of RAF-1 or ERK in dexamethasone- or anti-fas-treated 8226 and UCLA #1 MM cell lines. However, IL-6-dependent protection of viability correlated with an inhibition of dexamethasone- and anti-fas-induced activation of jun kinase (JNK) and AP-1 transactivation. To test the hypothesis that cytokine-induced protection was mediated through inhibition of JNK/c-jun, we also inhibited c-jun function in 8226 cells via introduction of a mutant dominant negative c-jun construct. Mutant c-jun-containing MM cells were also resistant to anti-fas-induced apoptosis but were significantly more sensitive to dexamethasone-induced apoptosis. These results support the notion that IL-6 protects MM cells against anti-fas through its inhibitory effects on JNK/c-jun but indicate protection against dexamethasone occurs through separate, yet unknown pathways.