Quiescent CD34⁺ early erythroid progenitors are resistant to several erythropoietic ‘inhibitory’ cytokines; role of FLIP

Summary. In this study, quiescent bone marrow-derived CD34⁺ erythroid burst-forming units (BFU-E) were found to be resistant to the inhibitory effects of tumour necrosis factor (TNF)-α and -β as well as interferon (IFN)-α, -β and -γ, in contrast to those stimulated by a combination of erytrhropoietin (Epo) plus kit ligand (KL). Unexpectedly, we found that TNF-α also inhibited the apoptosis of quiescent normal human CD34⁺ BFU-E cells. Accordingly, TNF-α added to CD34⁺ cells cultured for 2 d in serum-free medium protected clonogeneic BFU-E from undergoing serum deprivation-mediated apoptosis. Furthermore, the prosurvival effect of TNF-α in quiescent CD34⁺ cells was consistent with its ability to induce phosphorylation of mitogen-activated protein kinase (MAPK) p42/44. However, when added to CD34⁺ cells that were stimulated by Epo + KL, TNF-α induced apoptosis and inhibited proliferation of BFU-E. To explain this intriguing differential sensitivity between unstimulated CD34⁺ cells versus those stimulated by Epo + KL, we examined the expression of apoptosis-regulating genes (FLIP, BCL-2, BCL-XL, BAD and BAX) in these cells. Of all the genes tested, FLIP became rapidly downregulated in CD34⁺ cells 24 h after stimulation with Epo + KL, suggesting that it may protect quiescent CD34⁺ BFU-E progenitors residing in the bone marrow from the inhibitory effects of inflammatory cytokines. Thus, we hypothesize that cycling cells may become more sensitive to proapoptotic stimuli (e.g. chemotherapy, inhibitory cytokines) than quiescent ones because of the downregulation of protective FLIP.