MULTIPLE MECHANISMS OF REGULATION OF THE HUMAN C-MYB GENE DURING MYELOMONOCYTIC DIFFERENTIATION

Alterations in expression of c-myb can have profound effects on the growth and differentiation of hematopoietic cells. Thus, it is important to understand the mechanisms by which c-myb is regulated during hematopoietic cell differentiation. Previous studies pertaining to the regulation of c-myb have been carried out with the avian and murine forms of the gene; the current studies were designed to determine the mechanisms of regulation of the human form of c-myb. Transcriptional analysis by nuclear run-on assays revealed that an attenuation of transcription was the means of primary regulation during retinoic acid- and vitamin D3-induced differentiation of HL-60 cells, while other mechanisms in addition to attenuation were active during dimethyl sulfoxide (DMSO)- and phorbol ester-induced differentiation. Densitometric analysis of the changes in c.myb transcription caused by phorbol ester suggested that the c-myb promoter may be down-regulated during phorbol ester-induced differentiation of HL-60. Additional studies exhibited post-transcriptional regulation by phorbol ester. DMSO was also shown to regulate c-myb at the post-transcriptional level. Interestingly, the post-transcriptional regulation of c-myb by DMSO required continuous transcription. This requirement was shared for c-myc but not ornithine decarboxylase expression. The transcriptional dependency of c-myb post-transcriptional regulation did not equate to translational dependency, thus a novel post-transcriptional regulatory mechanism may control c-myb gene expression. The multiple levels of regulation of c-myb suggest the importance of proper control for hematopoietic cell differentiation.