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Abstract
Differentiation in the hematopoietic system involves, among other changes, altered expression of antigens, including the CD34 and CD38 surface antigens. In normal hematopoiesis, the most immature stem cells have the CD34+CD34 m phenotype. In acute myeloid leukemia (AML), although blasts from most patients are CD38+, some are CD38 m. AML blasts are blocked at early stages of differentiation; in some leukemic cells this block can be overcome by a variety of agents, including retinoids, that induce maturation into macrophages and granulocytes both in vitro and in vivo. Retinoids can also induce CD38 expression. In the present study, we investigated the relationship between induction of CD38 expression and induction of myeloid differentiation by retinoic acid (RA) in normal and leukemic human hematopoietic cells. In the promyelocytic (PML) CD34 m cell lines, HL60 and CB-1, as well as in normal CD34+CD34 m hematopietic progenitor cells RA induced both CD38 expression as well as morphological and functional myeloid differentiation that resulted in loss of self-renewal. In contrast, in the myeloblastic CD34+ leukemic cell lines, ML-1 and KG-1a, as well as in primary cultures of cells derived from CD34+-AML (M 0 and M 1 ) patients, RA caused an increase in CD38+ that was not associated with significant differentiation. Yet, long exposure of ML-1, but not KG-1, cells to RA resulted in loss of self-renewal. The results suggest that while in normal hematopoietic cells and in PML CD34 m cells induction of CD38 antigen expression by RA results in terminal differentiation along the myeloid lineage, in early myeloblastic leukemic CD34+ cells, induction of CD38 and differentiation are not functionally related. Since, several lines of evidence suggest that the CD38 m cells are the targets of leukemic transformation, transition of these cells into CD38+ phenotype by RA or other drugs may have therapeutic effect, either alone or in conjunction with cytotoxic drugs, regardless the ability of the cells to undergo differentiation.