Hideki Nakagoshi

Inactivation of a c-Myb/estrogen receptor fusion protein in transformed primary cells leads to granulocyte/macrophage differentiation and down regulation of c- kit but not c- myc or cdc2

Primary murine fetal hemopoietic cells were transformed with a fusion protein consisting of the ligand-binding domain of the estrogen receptor and a carboxyl-terminally truncated c-Myb protein (ERMYB). The ERMYB-transformed hemopoietic cells exhibit an immature myeloid phenotype when grown in the presence of β-estradiol. Upon removal of β-estradiol, the ERMYB cells display increased adherence, decreased clonogenicity and differentiate to cells exhibiting granulocyte or macrophage morphology. The expression of the c-myc, c-kit, cdc2 and bcl-2 genes, which are putatively regulated by Myb, was investigated in ERMYB cells grown in the presence or absence of β-estradiol. Neither c-myc nor cdc2 expression was down-regulated after removal of β-estradiol demonstrating that differentiation is not a consequence of decreased transactivation of these genes by ERMYB. While bcl-2 expression was reduced by 50% in ERMYB cells grown in the absence of β-estradiol, there was no increase in DNA laddering, suggesting that Myb was not protecting ERMYB cells from apoptosis. In contrast, a substantial (200-fold) decrease in c-kit mRNA level was observed following differentiation of ERMYB cells, and c-kit mRNA could be partially re-induced by the re-addition of β-estradiol. Furthermore, a reporter construct containing the c-kit promoter was activated when cotransfected with a Myb expression vector, providing further evidence of a role for Myb in the regulation of c-kit.

Human A-myb gene encodes a transcriptional activator containing the negative regulatory domains

The myb gene family has three members, c‐myb, A‐myb, and B‐myb. A‐myb mRNA is mainly expressed in testis and peripheral blood leukocytes. A‐Myb can activate transcription from the promoter containing Myb‐binding sites in all cells examined. In addition to the two domains (a DNA‐binding domain and a transcriptional activation domain), two negative regulatory domains have been identified in A‐Myb. These results indicate that A‐Myb functions as a transcriptional activator mainly in testis and peripheral blood cells, and the regulatory mechanism of A‐Myb activity is similar to that of c‐Myb.