Abstract
By screening a murine interleukin-3 (IL-3)-dependent myeloid cell cDNA library, we previously identified JAZ (Just Another Zinc Finger Protein), a novel zinc finger (ZF) protein that localizes in the nucleus and preferentially binds dsRNA rather than DNA. Forced overexpression of JAZ induces apoptosis but the mechanism was not known. JAZ is differentially expressed in CD34+ primary human and mouse bone marrow cells including mononuclear myeloid and lymphoid cells but not in multinuclear megakaryocytes, indicating that JAZ may function in hematopoietic progenitor cells. Since IL-3 withdrawal induces apoptosis in factor-dependent myeloid and lymphoid cells, we tested whether endogenous JAZ is involved. Hematopoietic cell lines including murine NFS/N1.H7, 32D myeloid and BaF3 lymphoid cells were deprived of IL-3 for 0, 0.5, 1, 2, 4, 6, 8, 12, 24 and/or 48 hr. Results reveal that JAZ expression is upregulated prior to induction of cell death. While a role for p53 in hematopoietic progenitor cell response to apoptosis-inducing stress has been postulated, the mechanism is not clear. Therefore, we assessed whether p53 expression or activation can be affected by JAZ. Results reveal that in association with JAZ upregulation, IL-3 withdrawal also induces p53 expression and importantly, up-regulates its transcriptional activity as assessed by increased BAX expression. To verify p53 dependency, p53-deficient murine M1 and human K562 leukemic cells were also tested. These p53-deficient cells are highly insensitive to serum withdrawal-induced cell death. Importantly, siRNA-mediated ‘knock-down’ of endogenous JAZ (by 70–80%) attenuates stress-induced cell death in NFS/N1.H7 but not M1 cells. These data point to a necessary role for JAZ in IL-3 or growth factor withdrawal-induced hematopoietic cell death in a p53-dependent manner. Further analysis using co-immunoprecipitation studies indicates that endogenous JAZ and p53 associate upon IL-3 withdrawal. Furthermore, using p53+/+ and p53−/− isogenic murine embryonic fibroblasts (MEFs), we clearly show that JAZ not only directly interacts with p53 but also stimulates its transcriptional activity, resulting in mediation/acceleration of stress-activated, p53-dependent apoptosis. Therefore, we propose that the nuclear factor JAZ may be a novel regulator of p53 in the hematopoietic cell response to stress leading to apoptosis. Since only about 15% of hematologic-malignancies express mutant (transcriptionally inactive) p53, it may be possible to therapeutically target wild-type p53 through a mechanism involving JAZ.
Hydrogen peroxide (H2O2) induces leukemic but not normal hematopoietic cell death in a dose-dependent manner
