IBRUTinib: BRUTe force against bortezomib-resistant myeloma cells

Abstract

Multiple Myeloma (MM) is a clonal plasma cell dyscrasia that is currently incurable.1-2 While current therapy is effective at decreasing the tumor burden, or “debulking” the tumor, virtually all patients ultimately relapse. The basis for relapse is incompletely understood, although global genomic studies suggest that MM tumors are genetically heterogenous, both between patients and within a single tumor.1 Thus, relapse could occur through selection of a clone present at diagnosis, which later expands because it can circumvent therapy. Another possibility is that a relapse clone could evolve during therapy and expand because it harbors genetic or epigenetic mechanisms to resist treatment. Alternatively, or perhaps in conjunction with clonal evolution, a clone of stem-like cells or “cancer stem cells (CSCs)” that was relatively quiescent at presentation could emerge and begin to proliferate after exposure to therapy that eradicates the more differentiated, “bulk” tumor cells. This latter mechanism, or CSC theory, posits that a rare population of less-differentiated, refractory CSCs (“tumor-initiator cells”) initiates and maintains the tumor.3 Emerging evidence suggests that master regulators, such as the high mobility group A1 (HMGA1) chromatin remodeling protein, orchestrates the assembly of Nuclear–factor kappa B (NF-κB) and transcription factor complexes to induce stem cell transcriptional networks and downstream signaling pathways that maintain CSCs.3-6 Prior studies also suggest that Nuclear–factor kappa B (NF-κB) is hyperactive in MM and leukemic stem cells (Fig. 1).1,7 Regardless of the basis for chemoresistance, successful MM treatment hinges on the discovery of molecular pathways that mediate resistance and can be targeted with therapy. Figure 1. Signaling pathways in MM. Hyperactive NF-κB induces pro-survival genes after: (1) phosphorylation of p65, (2) proteosomal degradation of IκB. In sensitive clones, bortezomib inhibits NF-κB activity by blocking proteasomal degradation ... In the XX issue of Cell Cycle, Murray et al. report a critical first step in reaching this lofty goal, not only in elucidating molecular underpinnings of resistant disease, but also in effectively targeting resistance mechanisms.2 Treatment for MM includes combination therapy with corticosteroids or cytotoxic agents along with the proteasome inhibitor, bortezomib.1-2 As noted, NF-κB signaling is up-regulated in MM, and bortezomib functions by inhibiting proteasomal degradation of the endogenous NF-κB inhibitor, IκB (Fig. 1). While single agent therapy with bortezomib results in remissions in only ~30% of patients, combination therapy has led to improved survival times and remissions in up to 60-90% of patients.1 Unfortunately, remissions are generally short-lived and most patients succumb to MM within 5-10 years of diagnosis.1-2 In order to identify relapse-specific mechanisms that could be targeted, relapse was modeled in vitro by selecting cultured MM cells resistant to bortezomib.2 Resistance in MM has been attributed to multiple factors, including increased NF-κB signaling, enhanced growth factor and/or oncogenic signaling, mutated proteasome subunits, or deregulated plasma cell maturation markers.1 In this study, proteasome activity was assessed using a functional assay and found to be increased in resistant cells. Bortezomib resulted in cytotoxicity in MM cells from naive (untreated) patients, while most resistant cells (4/6) were unaffected. Because Bruton's tyrosine kinase (BTK) activity can induce NF-κB activity, it was assessed and found to be highest in resistant cells. Following bortezomib, BTK activity decreased in sensitive cells, although there was no change in resistant cells, suggesting that BTK could be critical for bortezomib resistance. To investigate this further, BTK promoter activity, which includes 2 NF-κB binding sites, was assessed and upregulated in resistant clones. Moreover, BTK promoter was repressed by bortezomib in sensitive cell lines, but not in resistant cells. To determine whether BTK could be targeted, resistant MM cells were treated with the BTK inhibitor, ibrutinib, which resulted in cytotoxicity. When ibrutinib was administered with bortezomib, cell viability decreased further, and was dependent upon BTK. Finally, this study revealed that BTK induction was dependent upon the p65/RelA subunit of NF-κB. Together, this compelling work suggests that iBRUTinib could provide “brute force” needed to re-sensitize resistant MM clones to bortezomib and may help to eradicate relapsed disease (Fig. 1). In summary, this well-orchestrated study provides an elegant example of how elucidating molecular underpinnings of resistant disease may lead to more effective therapies. Clinical studies are needed to translate these results into effective therapy. This study also underscores the importance of p65/ReA and NF-κB signaling in relapsed MM and reveals another potential therapeutic target. Targeting chromatin remodeling proteins that recruit NF-κB to DNA could also prove to be an important adjunctive therapy in relapsed MM.3-4