REGULATION OF PROLIFERATION AND CYTOKINE EXPRESSION OF BONE-MARROW FIBROBLASTS - ROLE OF C-MYB

The c-myb protooncogene plays a major role in regulating the process o f in vitro and in vivo hematopoiesis via its activity as transcription al regulator in hematopoietic progenitor cells. Since the bone marrow microenvironment appears to regulate in vivo hematopoiesis by maintain ing the growth of multipotent progenitors via secretion of specific cy tokines, we asked whether c-myb is also required for the proliferation of and/or cytokine production by stromal cells that generate fibrobla st-like colonies (fibroblast colony-forming units [CFU-F]). Using the reverse transcriptase polymerase chain reaction technique, we detected low levels of c-myb mRNA transcripts in human normal bone marrow fibr oblasts. Treatment of these cells with c-myb antisense oligodeoxynucle otides caused downregulation of c-myb expression, decrease in the numb er of marrow CFU-F colonies (approximately 54% inhibition) and in the cell number within residual colonies (approximately 80%), and downregu lation of granulocyte/macrophage colony-stimulating factor (GM-CSF) an d stem cell factor (SCF) mRNA expression. Transfection of T98G gliobla stoma cells, in which expression of c-myb, GM-CSF, and SCF mRNAs is un detectable or barely detectable, with a plasmid containing a full-leng th c-myb cDNA under the control of the SV40 promoter induced the expre ssion of biologically active SCF and GM-CSF in these cells. Regulation of GM-CSF expression by c-myb was due in part to transactivation of t he GM-CSF promoter. These results indicate that, in addition to regula ting hematopoietic cell proliferation, c-myb is also required for prol iferation of and cytokines synthesis by bone marrow fibroblasts.