FLT3 LIGAND STIMULATES PROLIFERATION AND INHIBITS APOPTOSIS OF ACUTE MYELOID-LEUKEMIA CELLS - REGULATION OF BCL-2 AND BAX

Flt3/flk-2 ligand (flt3-L) is a potent costimulator of normal bone mar row (BM) myeloid progenitors. Flt3-L is produced by BM stromal cells a nd its receptor is expressed in the majority of acute myeloid leukemia (AML) cases. Therefore, flt3-L may play a role in the paracrine and/o r autocrine loops sustaining leukemic cell growth. We evaluated the ef fects of recombinant human flt3-L on proliferation, apoptosis, and Bcl -2 and Bax expression in primary AML cells and compared them with thos e of stem cell factor (SCF). Mononuclear BM cells from patients with n ewly diagnosed AML were cultured in serum-free conditions with flt3-L, SCF, granulocyte colony-stimulating factor (G-CSF) and granulocyte ma crophage-colony-stimulating factor (GM-CSF) alone and in combination. In 9 of 10 samples, flt3-L significantly increased [H-3]thymidine upta ke (geometric mean stimulation index, 7.5; range, 2.4 to 41.5). Flt3-L also increased the number of AML blast colonies by 126% (range, 61% t o 181%). In these 9 samples, flt3-L significantly enhanced the prolife rative response triggered by G-CSF or GM-CSF. Flt3-L prevented apoptos is in AML blasts. It reduced the number of apoptotic cells by 36% +/- 3.9% compared with control cultures. Combining flt3-L with G-CSF or GM -CSF doubled the antiapoptotic effect. Cellular Bcl-2 and Bax levels w ere determined separately for apoptotic and nonapoptotic cells by flow cytometry. Cells undergoing spontaneous apoptosis had low Bcl-2 and h igh Bax levels, whereas nonapoptotic cells had high Bcl-2 and low Bax levels. Flt3-L alone or in combination with G-CSF or GM-CSF did not up regulate Bcl-2. However, Bax expression decreased in viable cells in t he presence of these cytokines and the lowest level was achieved when a combination of flt3 and GM-CSF was used. Proliferative and viability effects of flt3-L were similar to those of SCF. Our results demonstra te that flt3-L acts as a stimulatory factor for primary AML cells. The antiapoptotic effects of flt3-L or its combinations with G-CSF or GM- CSF correlate with their ability to prevent upregulation of Bax. (C) 1 996 by The American Society of Hematology.