Molecular sequelae of proteasome inhibition in human multiple myeloma cells

TLDR

The proteasome inhibitor PS‑341 blocks IκB degradation, inhibits NF‑κB activation, and induces apoptosis in chemoresistant multiple myeloma cells, yet the detailed apoptotic cascades remain incompletely defined. This study aimed to delineate the molecular sequelae of PS‑341 treatment in MM cells and identify the pathways mediating its anticancer effects. Gene‑expression profiling was employed to characterize the transcriptional changes induced by PS‑341 in MM cells. PS‑341 down‑regulates growth/survival signaling, up‑regulates pro‑apoptotic, heat‑shock, and ubiquitin‑proteasome genes, activates mitochondrial and Fas‑mediated apoptosis, and its activity is modulated by IGF‑1, Bcl‑2, Akt, BH3 inhibitors, and Hsp90 inhibitors, supporting combination trials to improve MM outcomes.

Abstract

The proteasome inhibitor PS-341 inhibits IκB degradation, prevents NF-κB activation, and induces apoptosis in several types of cancer cells, including chemoresistant multiple myeloma (MM) cells. PS-341 has marked clinical activity even in the setting of relapsed refractory MM. However, PS-341-induced apoptotic cascade(s) are not yet fully defined. By using gene expression profiling, we characterized the molecular sequelae of PS-341 treatment in MM cells and further focused on molecular pathways responsible for the anticancer actions of this promising agent. The transcriptional profile of PS-341-treated cells involved down-regulation of growth/survival signaling pathways, and up-regulation of molecules implicated in proapoptotic cascades (which are both consistent with the proapoptotic effect of proteasome inhibition), as well as up-regulation of heat-shock proteins and ubiquitin/proteasome pathway members (which can correspond to stress responses against proteasome inhibition). Further studies on these pathways showed that PS-341 decreases the levels of several antiapoptotic proteins and triggers a dual apoptotic pathway of mitochondrial cytochrome c release and caspase-9 activation, as well as activation of Jun kinase and a Fas/caspase-8-dependent apoptotic pathway [which is inhibited by a dominant negative (decoy) Fas construct]. Stimulation with IGF-1, as well as overexpression of Bcl-2 or constitutively active Akt in MM cells also modestly attenuates PS-341-induced cell death, whereas inhibitors of the BH3 domain of Bcl-2 family members or the heat-shock protein 90 enhance tumor cell sensitivity to proteasome inhibition. These data provide both insight into the molecular mechanisms of antitumor activity of PS-341 and the rationale for future clinical trials of PS-341, in combination with conventional and novel therapies, to improve patient outcome in MM.