Differential expression of Bcl-2 homologs in human CD34(+) hematopoietic progenitor cells induced to differentiate into erythroid or granulocytic cells

The Bcl-2 family of proteins has been shown to play a central role in the r egulation of apoptosis. We have examined the expression of several Bcl-2 ho mologs upon stimulation of CD34(+) human hematopoietic progenitor cells. CD 34+ cells were induced to differentiate into predominantly erythroid cells in the presence of erythropoietin (Epo) and stem cell factor (SCF), while t he addition of G-CSF and SCF led to differentiation predominantly into gran ulocytic cells, as demonstrated by immunophenotyping and morphological exam ination of cultured cells. In Epo- and SCF-stimulated cells, we found a mar ked increase in the level of Bcl-x(L) protein expression and downregulation of Bar expression, apparent from day 4 and more pronounced on days 8 and 2 1, In contrast, Bcl-xL protein expression was downregulated in G-CSF- and S CF-stimulated cells compared with cells cultured in medium alone, whereas t here nas no sign of change in the level of Bax. Mcl-1 expression showed a b iphasic expression pattern in both early erythropoiesis and early granulopo iesis, but with an inverse regulation. Thus, Mcl-1 levels initially decreas ed in granulocytic progenitor cells and increased in erythroid progenitor c ells, Finally, Bcl-2 expression was significantly downregulated in both Epo and SCF and G-CSF- and SCP-stimulated cells. The role of the distinct upregulation of Bcl-x(L) in early erythroid differ entiation was further examined by use of specific ribozymes against Bcl-x(L ). Addition of Bcl-x(L) ribozymes promoted a clear increase in cell death o f Epo- and SCF-stimulated cells, while erythroid differentiation was not af fected. In conclusion, we found a distinct regulation of several Bcl-2 fami ly members in CD34(+) cells dependent on the cytokine stimulation given, Th e use of Bcl-x(L)-specific ribozymes suggested that Bcl-x(L) is important f or survival but not for differentiation of erythroid progenitor cells.