Combined Targeting of JAK2 and Bcl-2/Bcl-xL to Cure Mutant JAK2-Driven Malignancies and Overcome Acquired Resistance to JAK2 Inhibitors

Abstract

SummaryTo design rational therapies for JAK2-driven hematological malignancies, we functionally dissected the key survival pathways downstream of hyperactive JAK2. In tumors driven by mutant JAK2, Stat1, Stat3, Stat5, and the Pi3k and Mek/Erk pathways were constitutively active, and gene expression profiling of TEL-JAK2 T-ALL cells revealed the upregulation of prosurvival Bcl-2 family genes. Combining the Bcl-2/Bcl-xL inhibitor ABT-737 with JAK2 inhibitors mediated prolonged disease regressions and cures in mice bearing primary human and mouse JAK2 mutant tumors. Moreover, combined targeting of JAK2 and Bcl-2/Bcl-xL was able to circumvent and overcome acquired resistance to single-agent JAK2 inhibitor treatment. Thus, inhibiting the oncogenic JAK2 signaling network at two nodal points, at the initiating stage (JAK2) and the effector stage (Bcl-2/Bcl-xL), is highly effective and provides a clearly superior therapeutic benefit than targeting just one node. Therefore, we have defined a potentially curative treatment for hematological malignancies expressing constitutively active JAK2.

Selective targeting of JAK/STAT signaling is potentiated by Bcl-xL blockade in IL-2–dependent adult T-cell leukemia

“…Navitoclax, a Bcl-2/Bcl-xL inhibitor, was identified as the most promising candidate for multicomponent therapy. It was recently reported that apoptosis induced by JAK2 inhibition was enhanced by the Bcl-2/Bcl-xL inhibitor ABT-737 in mutant JAK2-driven malignancies (20,21). We demonstrated that the combination of ruxolitinib and navitoclax showed additive/synergistic efficacy inhibiting proliferation of IL-2-dependent ATL cell lines and synergy prolonging survival in a murine model of human ATL.…”

“…Consequently, caspase 3/7 activation and an associated increase in PARP and Mcl-1 cleavage occurred upon cotreatment with ruxolitinib and navatoclax. Aspects of these results have been noted in other disorders (13,20,21,36). Gozgit et al reported that inhibition of JAK2 blocked production of the Pim1/2 kinases and resulted in a corresponding decrease in BAD phosphorylation (13), and Will et al demonstrated that apoptosis induced by JAK2 inhibition is mediated by Bim and enhanced by the BH3 mimetic ABT-737 in JAK2 mutant human erythroid cells (20).…”

“…Chen et al demonstrated that triptolide inhibited JAK2 transcription and induced apoptosis in human myeloid proliferative disorder cells bearing JAK2 V617F through caspase-mediated cleavage of Mcl-1 (36). Recently, it was reported that combined targeting of JAK2 and Bcl-2/Bcl-xL mediated prolonged disease regressions and cures in mice bearing JAK2 mutant tumors (21). Our studies and these reports suggest that disruption of JAK signaling in several hematological malignancies leads to a generalized increase in proapoptotic signatures that can enhance actions of antiapoptotic member inhibitors like navatoclax.…”

“…Several lines of evidence indicate that activation of the JAK/ STAT pathway up-regulates the expression of antiapoptotic Bcl-2 family proteins and that inhibition of this pathway induces expression of proapoptotic BH3-only proteins, thereby inducing apoptosis (13,20,21). Treatment with ruxolitinib or IL-2 withdrawal reduced expression of p-STAT5 in all IL-2-dependent ATL cell lines (SI Appendix, Fig.…”

“…S3A). The Bcl-2-specific inhibitor (ABT-199) (22) showed less activity and the Bcl-xL-specific inhibitor (WEHI 539) (21,23) showed less or equal activity in inhibition of the proliferation of ATL cell lines compared with navitoclax (SI Appendix, Fig. S3B).…”