Regulation of the Resident Chromosomal Copy of c- myc by c-Myb Is Involved in Myeloid Leukemogenesis

Abstract

ABSTRACT c- myb is a frequent target of retroviral insertional mutagenesis in murine leukemia virus-induced myeloid leukemia. Induction of the leukemogenic phenotype is generally associated with inappropriate expression of this transcriptional regulator. Despite intensive investigations, the target genes of c- myb that are specifically involved in development of these myeloid lineage neoplasms are still unknown. In vitro assays have indicated that c- myc may be a target gene of c-Myb; however, regulation of the resident chromosomal gene has not yet been demonstrated. To address this question further, we analyzed the expression of c- myc in a myeloblastic cell line, M1, expressing a conditionally active c-Myb–estrogen receptor fusion protein (MybER). Activation of MybER both prevented the growth arrest induced by interleukin-6 (IL-6) and rapidly restored c- myc expression in nearly terminal differentiated cells that had been exposed to IL-6 for 3 days. Restoration occurred in the presence of a protein synthesis inhibitor but not after a transcriptional block, indicating that c- myc is a direct, transcriptionally regulated target of c-Myb. c- myc is a major target that transduces Myb's proliferative signal, as shown by the ability of a c-Myc–estrogen receptor fusion protein alone to also reverse growth arrest in this system. To investigate the possibility that this regulatory connection contributes to Myb's oncogenicity, we expressed a dominant negative Myb in the myeloid leukemic cell line RI-4-11. In this cell line, c- myb is activated by insertional mutagenesis and cannot be effectively down regulated by cytokine. Myb's ability to regulate c- myc 's expression was also demonstrated in these cells, showing a mechanism through which the proto-oncogene c- myb can exert its oncogenic potential in myeloid lineage hematopoietic cells.