title: Regulation of the resident chromosomal copy of c-myc by c-Myb is involved in myeloid leukemogenesis.
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.
authors.affiliation: Laboratory of Cellular Oncology, National Cancer Institute, Bethesda, Maryland, USA.
authors.forename: M
authors.initials: M
authors.lastname: Schmidt
authors.forename: V
authors.initials: V
authors.lastname: Nazarov
authors.forename: L
authors.initials: L
authors.lastname: Stevens
authors.forename: R
authors.initials: R
authors.lastname: Watson
authors.forename: L
authors.initials: L
authors.lastname: Wolff
mesh_terms: D000818:Animals; D019070:Cell Lineage / Q000235:genetics; D002471:Cell Transformation, Neoplastic / Q000235:genetics*; D015972:Gene Expression Regulation, Neoplastic; D020570:Genes, myb; D016259:Genes, myc; D007951:Leukemia, Myeloid / Q000235:genetics* / Q000473:pathology; D051379:Mice; D014407:Tumor Cells, Cultured