Simultaneous inhibition of the constitutively activated nuclear factor κB and of the Interleukin-6 pathways is necessary and sufficient to completely overcome apoptosis resistance of human U266 myeloma cells

Abstract

Elevated Nuclear Factor κB (NFκB) levels have been reported in multiple myeloma cells derived from patients relapsing after chemotherapy. In the search of an in vitro a model with molecular features similar to relapsing lesions, we focused our attention on an IL-6 autocrine human myeloma cell line (U266), characterized by apoptosis resistance due to upregulation of two constitutive signaling pathways: NFκB and STAT-3. NFκB activity was inhibited with proteasome inhibitory agents, such as PS-341 and Withaferin A, with an IKK inhibitor (Wedelolactone) or with the adenoviral vector HD IκBαmut-IRES-EGFP encoding a mutant IκBα protein, resistant to proteasomal degradation. We observed that the NFκB intracellular dislocation at the beginning of the treatment affected therapeutic effectiveness of PS-341, Withaferin A and Wedelolactone; interestingly, the adenoviral vector was highly effective in inducing apopotosis even with NFκB being predominantly nuclear at the time of infection. We also observed that U266 treated with the Interleukin-6 antagonist Sant7 exhibited reduced STAT3 activity and preferential cytoplasmic NFκB location; moreover they became capable of undergoing apoptosis mainly from the G 1 phase. Adenoviral vector treated U266 have NFκB localized completely in the cytoplasm and also showed downregulation of nuclear phospho STAT-3. Finally, combined targeting of NFκB and STAT3 signalling pathways was the most effective treatment in inducing apoptosis. These findings suggest that combined NFκB and STAT3 targeting warrants further investigations in other apoptosis resistant MM cell lines as well as in suitable MM animal models. ©2008 Landes Bioscience.