Regulation of the resident chromosomal copy of c-myc by c-Myb is involved in myeloid leukemogenesis

c-myb is a frequent target of retroviral insertional mutagenesis in murine leukemia virus-induced myeloid leukemia. Induction of the leukemogenic phen otype is generally associated with inappropriate expression of this transcr iptional 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 a nalyzed the expression of c-myc in a myeloblastic cell line, M1, expressing a conditionally active c-Myb-estrogen receptor fusion protein (MybER), Act ivation of MybER both prevented the growth arrest induced by interleukin-6 (IL-6) and rapidly restored c-myc expression in nearly terminal differentia ted 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 transcriptio nal block, indicating that c-myc is a direct, transcriptionally regulated t arget 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 protei n alone to also reverse growth arrest in this system. To investigate the po ssibility 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 regulat e c-myc's expression was also demonstrated in these cells, showing a mechan ism through which the protooncogene c-myb can exert its oncogenic potential in myeloid lineage hematopoietic cells.