Sensitization of Leukemic Cells for Glucocorticoids by Inhibition of NFκB Activity.

Based on their immunosuppressive and cytotoxic effects, glucocorticoids (GCs) find widespread use as immune-modulatory agents and in (childhood) leukemia, lymphomas and myelomas. Unfortunately, resistance to GCs is a major adverse prognostic factor and occurs in about 20% in newly diagnosed childhood acute lymphoblastic leukemia (ALL) and in more than 50% in relapsed ALL, while acute myeloid leukemia (AML) is largely unresponsive to GCs. In addition, adult AML is more often GC resistant compared to childhood AML. Recently, we observed (van der Heijden et al, Ann Rheum Dis 63: 131-137,2004) that prolonged exposure of human CEM-C7 T cell leukemia cells to the anti-rheumatic drug sulfasalazine (SSZ) sensitized these primary sensitive cells even further for GCs. SSZ is an inhibitor of activation of NFκB that is often therapeutically used in combination with methotrexate and prednisone. Following these observations, two human myeloid leukemia cell lines (THP-1 and U937) with inherent resistance to GCs (IC50 for prednisolone > 250 μM, IC50 dexamethasone > 25 μM) were exposed to SSZ to establish whether this would provoke GC sensitization. Indeed, prolonged SSZ exposure sensitized both myeloid leukemia cells for prednisolone (IC50 < 1 μM) and dexamethasone (IC50 < 0.05 μM). In order to elicit the mechanistic basis for the GC sensitizing effect of SSZ expression levels of Glucocorticoid Receptor α (GRα), NFκB p65 and Inhibitor of NFκB (IκBα) were analyzed. In contrast to GC-sensitive CEM-C7 cells, parental THP-1 and U937 cells had no detectable and constitutive expression of GRα and NFκB p65 protein, even though mRNA levels for both parameters were readily detectable. In SSZ-exposed THP-1 and U937 cells protein expression of NFκB p65 was markedly increased although it does not represent the active form of NFκB since protein expression of IκB increased concomitantly. Consistent with the sensitization for GCs in SSZ-exposed THP-1 and U937 cells was a marked increase in GRα protein expression, suggesting that GRα is post-transcriptionally prevented from degradation. In line with this concept is that co-incubation with a 26S proteasome inhibitor Bortezomib further enhanced (5-fold) GC sensitivity in SSZ-exposed THP-1 and U937 cells. Collectively, this study reveals that SSZ-exposure impairs activation of NFκB and facilitates stabilization of GRα protein, conditions which confer GC-sensitization in leukemic cells with primary resistance to GCs. In conclusion, modulation of GC resistance in acute leukemias by NFκB inhibitors is a novel, clinically attractive approach that warrants further preclinical and subsequent clinical investigations.