Nfkb1 Plays a Tumor-Suppressing Role in BCR-ABL-Induced Leukemias

Abstract 1666

NF-κB activation has been linked to the promotion of an assortment of malignancies. While in vitro studies have supported a tumor-promoting role for NF-κB in leukemias, in vivo evidence has not been collected. NF-κB has also been proposed as a therapeutic target for Philadelphia chromosome-positive leukemias including chronic myeloid leukemia (CML) and B-cell acute lymphoblastic leukemia (B-ALL). In this study, we investigate the role the Nfkb1 gene plays in CML and B-ALL using mouse models for these leukemias induced by the BCR-ABL oncogene. Contrary to conventional thinking, we find a tumor-suppressing role for Nfkb1 in BCR-ABL leukemias. To induced CML, we transduced bone marrow cells from 5-FU-primed wild type or Nfkb1−/− mice with BCR-ABL-GFP retrovirus, followed by transplantation of the transduced cells into lethally irradiated recipient mice. We show that recipients of BCR-ABL-transduced Nfkb1−/− donor cells developed CML more rapidly than the wild type recipients, correlating to elevated percentages (82.3% vs 38.5%) and total numbers of Gr-1+GFP+ cells in peripheral blood of the mice. We next tested whether the loss of Nfkb1 also accelerated B-ALL development under the B-ALL-inducing conditions, and show that recipients of BCR-ABL-transduced Nfkb1−/− donor cells succumbed to B-ALL more quickly than the wild type recipients. These results suggest that Nfkb1 does not stimulate leukemia growth, and instead, it plays a tumor-suppressing role in CML and B-ALL. Because these findings are opposite from what people believe in the field, we confirmed our findings by conducting a rescue experiment. The Nfkb1 gene encodes the p105 protein; roughly half of p105 product is selectively proteolyzed to yield the p50 transcription factor. In this rescue experiment, we transduced Nfkb1−/− donor cells with BCR-ABL alone or BCR-ABL and p50 of Nfkb1 to induce leukemia, and we show that acceleration of leukemia development caused by the Nfkb1 loss is partially reversed by p50. To study the underlying mechanism, we focused on CARD11, because it plays a critical role in mediating NF-κB signaling in lymphocytes. We find that CARD11 expression is induced in myeloid and increased in lymphoid cell lines expressing BCR-ABL. Furthermore, we induced B-ALL using Card1−/− mice. We find that similar to the loss of Nfkb1, the loss of Card11 expression also accelerates the development of B-ALL. To increase human relevance of our findings, we overexpressed p50 in human K562 cells by retroviral transduction with p50-IRES-GFP or empty vector, and find that FACS-sorted p50/GFP K562 cells grow significantly slower than empty vector-transduced cells. These findings support an idea that Nfkb1 signaling suppresses BCR-ABL-induce leukemia at least partially through a CARD11-dependent mechanism. In summary, our results demonstrate that Nfkb1 plays a tumor-suppressing role in BCR-ABL-induced leukemias, arguing that we should be careful when targeting of Nfkb1 in leukemia treatment.