Functional Gfi-1 Is Important for BCR/ABL-Mediated Proliferation and Survival of Myeloid Cells.

Chronic myelogenous leukemia (CML) is a clonal hematopoietic disorder caused by the BCR/ABL fusion oncogene. CML typically evolves in three distinct clinical stages: chronic and accelerated stages and blast crisis. The progression of CML from chronic phase (CP) to blast crisis (BC) is characterized by the increasing failure of myeloid precursors to differentiate into mature granulocytes and aggressive proliferation of immature myeloid cells. It has been shown that CML progression from CP to BC is associated with downregulation of C/EBPs including C/EBPa and C/EBPe, critical regulators of myeloid development. Forced expression of C/EBPs suppresses transformation and restores granulocytic differentiation of BCR/ABL-expressing myeloid cells, suggesting that downregulation of C/EBPs may be involved in CML transition from CP to BC. The proto-oncogene Gfi-1 encodes a nuclear zinc-finger transcriptional repressor that is required for the survival and proliferation of myeloid cells. We recently showed that expression of a dominant negative Gfi-1 mutant, N382S, led to markedly increased expression of C/EBPe in myeloid cells, suggesting that Cebpe encoding C/EBPe is a Gfi-1 target. We investigated the possibility that inhibition of Gfi-1 function in BCR/ABL-expressing cells may increase C/EBPe expression, thereby exerting a negative effect on BCR/ABL-mediated transformation. Myeloid 32D cells transfected with BCR/ABL (32D/BCR/ABL) are independent of IL-3 for proliferation and survival. Expression of the N382S mutant in 32D/BCR/ABL cells (32D/N382S) had a significant inhibitory effect on proliferation and survival in the absence of IL-3, but only a weak effect in the presence of IL-3, suggesting that the N382S mutant specifically inhibited BCR/ABL-mediated proliferation and survival. The level of C/EBPe protein was markedly augmented in 32D/N382S cells. Interestingly, when cultured in medium containing no growth factors, 32D/N382S cells exhibited certain features of macrophage differentiation including increased cell size, adherence to the surface of culture flasks, spreading and increased surface expression of macrophage differentiation marker F4/80. 32D/N382S cells also became more sensitive to c-ABL kinase inhibitor imatinib. These data indicate that Gfi-1 may play an important role in BCR/ABL-mediated transformation and thus represent a potential therapeutic target in the treatment of CML.