IN BONE-MARROW-DERIVED BAF-3 CELLS, INHIBITION OF APOPTOSIS BY IL-3 IS MEDIATED BY 2 INDEPENDENT PATHWAYS

The inhibition of cell death by growth factors plays a key role in the maintenance of the haematopoietic system homeostasis. However the mec hanisms involved in this inhibition are still poorly understood. In or der to determine if inhibition of apoptosis by growth factors is depen dent only on the expression of survival genes, we have studied that pr ocess in the bone marrow derived IL-3 dependent cell line Baf-3. We sh ow that, following IL-3 starvation, mRNA and protein levels of Bcl-X b ut not Bcl-2 decrease rapidly preceeding the onset of death. The death of IL-3 starved cells is asynchronous, starting between 6 to 8 h with 50% death being reached after 10 to 12 h. At any time point, apoptosi s can be rapidly inhibited by growth factor re-addition, This has allo wed us to determine that the inhibition of apoptosis by growth factor takes place at two levels. The first one, which we have called short t erm inhibition, is independent of mRNA and protein synthesis i.e. it t akes place in the absence of survival gene neosynthesis and can be dem onstrated during the first 6 h following growth factor re-addition. Th e second one corresponds to long-term survival-more than 24 h survival -and is strongly correlated with the induction of Bcl-X but not Bcl-2 gene expression. This induction of Bcl-X by IL3 is shown to be depende nt on MAP-kinase activation.