C/EBPδ expression in a BCR-ABL-positive cell line induces growth arrest and myeloid differentiation

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Abstract
CCAAT/enhancer-binding proteins (C/EBPs) are a family of highly conserved transcription factors that have important roles in normal myelopoiesis as well as associated with myeloid disorders. The chronic myelogenous leukemia (CML) cell lines, KCL22 and K562, express exceptionally low levels of endogenous C/EBPs and provide a good model to test the effects of C/EBPs on myeloid differentiation. To explore the possibility that C/EBP can promote differentiation in BCR-ABL-positive cells, we generated stable KCL22 and K562 clones that expressed an inducible C/EBP gene. C/EBP expression resulted in G0/G1 proliferative arrest and a moderate increase in apoptosis of the KCL22 and the K562 cells. Within 4 days of inducing expression of C/EBP, myeloid differentiation of the CML blast cells occurred as shown by morphologic changes and induction of secondary granule-specific genes. We also showed that during granulocytic differentiation of KCL22 cells, the C/EBP protein was detected in immunocomplexes with both Rb and E2F1. Furthermore, expression of C/EBP was associated with downregulation of c-Myc and cyclin E and upregulation of the cyclin-dependent kinase inhibitor p27Kip1 in both the KCL22 and K562 cell lines. These results show that expression of C/EBP in BCR-ABL-positive leukemic cells in blast crisis is sufficient for neutrophil differentiation and point to the therapeutic potential of ectopic induction of C/EBP in the acute phase of CML.