Transforming growth factor-beta(1) induces apoptosis in CD34(+)CD38(-/low)cells that express Bcl-2 at a low level

Objective. Transforming growth factor-beta (1) (TGF-beta (1)) strongly inhi bits the proliferation and differentiation of primitive CD34(+)CD38(-) hema topoietic cells. In contrast, Flt3 ligand (FL) is a positive effector of CD 34(+)CD38(-/low) cell proliferation. Because apoptosis plays a critical rol e in hematopoietic development, TGF-beta (1) and FL were analyzed as possib le modulators of apoptosis. Specifically, this report examined expression o f apoptotic promoters Bax and Bad and apoptotic inhibitors Bcl-2 and Bcl-x (all members of the Bcl-2 protein family). Protein levels were determined i n fresh and cultured CD34(+)CD38(+) cells and CD34(+)CD38(-/low) cells with and without treatment with TGF-beta (1) and FL. Materials and Methods. Cells fractions were purified by sorting CD34(+)-enr iched mononuclear cells from mobilized peripheral blood. Expression of Bcl- 2, Bcl-x, Bax, and Bad and the extent of apoptosis were determined by now c ytometric analysis of freshly isolated cells and cells cultured with TGF-be ta (1) and FL effectors. Results. TGF-beta (1) reduced CD34(+)CD38(+) cell expansion and arrested ce ll division. Inhibition of growth was not accompanied by an increase in apo ptosis. In CD34(+)CD38(-/low) cells, serum TGF-beta (1) and added TGF-beta (1) inhibited cell growth and significantly increased apoptotic cell death. Freshly isolated CD34(+)CD38(+) and CD34(+)CD38(-/low) cells expressed Bcl -2 at similar low levels. However, after 3 days, Bcl-2 expression was marke dly higher in cultured CD34(+)CD38(+) cells. TGF-beta (1) significantly inc reased Bax expression in both fractions after 3 days cultivation (p = 0.003 4). Thus, addition of TGF beta -1 further reduced the already low Bcl-2:Bax ratio in CD34(+)CD38(-/low) cells. Conclusions. Compared to CD34(+)CD38(+) cells, CD34(+)CD38(-/low) cells wer e slow to up-regulate expression of Bcl-2 during ex vivo culture. TGF-beta (1) up-regulated Bax expression by both CD34(+)CD38(+) and CD34(+)CD38(-/lo w) cells and promoted apoptosis in the latter fraction. This suggests that the preferential induction of apoptosis in primitive cells by TGF-beta (1) may be due to its further reduction of the Bcl-2:Bax ratio. (C) 2001 Intern ational Society for Experimental Hematology. Published by Elsevier Science Inc.