MODULATION OF APOPTOSIS IN HUMAN MYELOID LEUKEMIC-CELLS BY GM-CSF

Apoptosis (programmed cell death) regulates cell population size. To d etermine the mechanisms whereby hematopoietic growth factors (HGFs) mo dulate apoptosis in human myeloid leukemic cells, we evaluated the rol es of protein and mRNA synthesis for altering apoptosis in growth fact or-stimulated vs. quiescent leukemic TF1 cells. Lysates of cells from the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependen t myeloid leukemic cell line TF1 were separated into high molecular we ight (HMW) pellets of intact DNA and supernatants of fragmented low MW (LMW) DNA, and the DNA purified from these fractions was quantified. In the absence of both GM-CSF and fetal bovine serum (FBS), 70% of the DNA was fragmented after 3 days in culture, with a characteristic apo ptotic ladder-like pattern on agarose gel electrophoresis, whereas thi s proportion had initially been <5%. In contrast, less than 5% of the DNA was fragmented in cells incubated with GM-CSF plus FBS or GM-CSF a lone. Delayed addition of GM-CSF, but not FBS, permitted partial rescu e of the cells, inhibiting increasing rates of accumulation of fragmen ted DNA. When the macromolecular synthesis inhibitor cycloheximide (CH X) or actinomycin D (Act D) was present for 26 hours in the absence of GM-CSF and FBS, apoptosis was inhibited. In contrast, in the presence of GM-CSF or FBS, apoptosis was enhanced upon addition of CHX or Act D. The latter effect persisted even with the late addition of CHX. The se findings indicate that disparate mechanisms of enhancing or inhibit ing apoptosis exist in myeloid leukemic cells related to environmental conditions, including HGF-regulated cellular synthesis of distinct pr oteins and mRNA.