CELLULAR AND MOLECULAR ASPECTS OF HUMAN CD34+
CD38-/low PRECURSORS: ANALYSIS OF A PRIMITIVE HEMATOPOIETIC
POPULATION
Xiao M, Dooley D.
American Red
Cross,
Stem Cell
Laboratory, Northwest Regional Blood Services,
Portland, OR. USA
Objective:
Human
hematopoietic CD34+CD38- cells are highly enriched
for primitive precursors. In the present study, we analyzed the regulation
of apoptosis (programmed cell death) in CD34+CD38-/low cells. Apoptosis is modulated in large
measure by the Bcl-2 family of cytoplasmic proteins. Some proteins (Bcl-2 and Bcl-x) in
this family block apoptosis while others (Bax and Bad) drive it. Expression
of these proteins is regulated by positive and negative factors such as transforming
growth factor-beta 1 (TGF-b1) and flt3
ligand (FL). In this way,
apoptotic pathways appear to control the production of hematopoietic cells.
Methods: Apoptosis and expression of Bcl-2,
Bcl-x, Bax and Bad by CD34+CD38+ cells and CD34+CD38-/low
cells treated with or without TGF-b1 and FL were
analyzed.
Results: Freshly isolated CD34+CD38+
and CD34+CD38-/low cells expressed Bcl-2 at similar
low levels. However, after 3 days, Bcl-2 expression was markedly higher in
cultured CD34+CD38+ cells. TGF-b1
significantly increased Bax expression in both fractions (p = 0.0034).
Thus, addition of TGFb-1 further
reduced the already low Bcl-2: Bax ratio
in CD34+CD38-/low cells and promoted apoptosis in
these cells after 3 days cultivation.
Conclusion: Compared to CD34+CD38+ cells, CD34+CD38-/low
cells were slow to upregulate expression of Bcl-2 during ex vivo
culture. TGF-b1 upregulated Bax
expression by both CD34+CD38+ and CD34+CD38-/low
cells and promoted apoptosis in the latter fraction. This suggests that the preferential
induction of apoptosis in primitive cells by TGF-b1 may be due to its
further reduction of the Bcl-2: Bax ratio.