The microenvironmental stromal cells abrogate NF-κB inhibitor induced apoptosis in chronic lymphocytic leukemia

NF-κB is known to play an important role in the pathogenesis of chronic lymphocytic leukemia. Several NF-κB inhibitors have been shown to successfully induce apoptosis of chronic lymphocytic leukemia cells in vitro. Since the microenvironment is known to be crucial for the survival of chronic lymphocytic leukemia cells, we tested here whether NF-κB inhibition may still induce apoptosis in these leukemic cells in the presence of protective stromal interaction. We used the specific NF-κB inhibitor Dehydroxymethylepoxyquinomicin. Microenvironmental support was mimicked by co-culturing chronic lymphocytic leukemia cells with bone marrow-derived stroma cell lines (HS-5 and M2-10B4). NF-κB inhibition by Dehydroxymethylepoxyquinomicin in chronic lymphocytic leukemia cells could be confirmed in both, the mono- and co-culture setting. In line with previous reports, NF-κB inhibition induced apoptosis in the monoculture setting by activating the intrinsic apoptotic pathway resulting in PARP-cleavage. However, it was unable to induce apoptosis in leukemic cells co-cultured with stroma cells. Similarly, siRNA mediated RELA downregulation induced apoptosis of chronic lymphocytic leukemia cells cultured alone but not in the presence of supportive stroma cells. BAFF could be identified as potential signal of the microenvironment protecting the leukemic cells from NF-κB inhibition-induced apoptosis. Finally, we show improved sensitivity of stroma-supported chronic lymphocytic leukemia cells to NF-κB-inhibition when combining the NF-κB inhibitor with the SYK inhibitor R406 or the BTK inhibitor Ibrutinib, agents known to inhibit the stroma-leukemia crosstalk. We conclude that NF-κB inhibitors are not promising as monotherapies in chronic lymphocytic leukemia, but may represent attractive therapeutic partners for Ibrutinib and R406.