ABR, a novel inducer of transcription factor C/EBPα, is necessary for myeloid differentiation and a favorable prognostic factor in acute myeloid leukemia

Abstract

Introduction: Active BCR related (ABR) protein is closely homologous to BCR, which acts as a tumor suppressor in leukemia. Repression of ABR expression has been reported in t(8;21) acute myeloid leukemia (AML). However, the specific function of ABR in myeloid differentiation or AML has not been studied yet. Methods: We utilized acute myeloid leukemia U937 cell line, mouse bone marrow cells and human bone marrow cells from patients with acute myeloid leukemia. Cell differentiation was assessed by flow cytometry. Cell-cycle analysis was performed with propidium iodide and analyzed by flow cytometry. RNA detection was performed by quantitative real-time PCR. Protein detection was performed by immunoblot analyses. Trans¬fections of constructs pEYFP-ABR, pcDNA3.1-E2F1, ABR siRNA or CE¬BPA siRNA in U937 cells were performed via electroporation. Results: We observed highly reduced ABR mRNA in AML patients from different karyotypes. Moreover, AML patients with high ABR expression survive significantly longer after hematopoietic stem cell transplantation. Here we found for the first time that older patients with AML that have high ABR expression benefited from azacytidine treatment, presenting an early response. Furthermore, treatment of leukemic cells with azacytidine induces ABR expression. ABR expression is increased while M-CSF and G-CSF stimulated myeloid differentiation of mouse bone marrow cells. In contrast to this, siRNA mediated block of ABR prevents myeloid differen¬tiation. We identified ABR as a novel player in myelopoiesis via increasing the expression of the myeloid transcription factor C/EBPα, a major regu¬lator of myeloid differentiation and functionally impaired in leukemia. Fi¬nally, ABR blocks cell-cycle progression and downregulates the cell-cycle activator E2F1, indicating the functional role of ABR as tumor suppressor in leukemic cells. Conclusions: Taken together, our data demonstrate that ABR plays a crit¬ical role in myelopoiesis via inducing C/EBPα and indicate the strong tu¬mor suppressor potential of ABR expression in AML. Targeted treatments that increase endogenous levels of ABR might represent novel therapeutic strategies