Activation of JAZ by Small Molecules in Human Leukemia Cells.

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 protein. JAZ belongs to a new class of evolutionarily conserved C₂H₂-type ZFPs that feature an unusually long linker sequence and preferentially bind to double-stranded RNA. JAZ localizes in the nucleus and its human gene is localized to the chromosome 5q35-qter, a specific chromosomal region at which deletions and translocations occur in leukemia and lymphoma. We have recently discovered JAZ as a novel direct, positive regulator of p53 transcriptional activity. The mechanism involves direct binding to p53’s C-terminal (negative) regulatory domain to activate “latent” p53 in response to non-genotoxic stress signals (such as interleukin-3 growth factor withdrawal). Thus, we have explored JAZ as a potentially novel molecular target in human leukemia by identifying small molecules to activate JAZ-mediated leukemic cell killing. A structure-based drug design approach was employed to screen small molecules that potentially bind and activate JAZ. Cytotoxicity assays were then carried out with a set of candidate compounds in a dose- and time-dependent manner in various human leukemia cell lines including REH, HL-60, U937, HEL and K562 cells. Several JAZ “activating” compounds were selected, which display differential abilities to induce leukemic cell killing (IC₅₀ = < 1 to 100 μM). Interestingly, biochemical analysis shows that the J-compound(s) can mediate cell death in p53 expressing leukemia cells in association with upregulation of expression of JAZ, p53 and the proapoptotic p53 target gene BAX, indicating activation of p53. Furthermore, the J-compound(s) synergizes induction of leukemic cell death when combined with other known p53 activating agents including cisplatin and nutlin-3, a recently developed small molecule MDM2 antagonist that disrupts the p53-MDM2 interaction. Therefore, the J-compound(s) may mediate leukemic cell death in a mechanism involving activation of “latent”, wild-type p53 by targeting JAZ. However, the J-compound-mediated cell death was also observed in some p53-deficient leukemia cell lines. This suggests that depending on the cell type the J-compounds may also act by a p53-independent mechanism. Since JAZ was recently reported to be a cargo protein for exportin-5, the nuclear export receptor for pre-microRNAs, this dsRNA-binding ZFP may also have an unknown p53-independent function via which the J-compound(s) may possibly act. While these possible mechanisms remain to be further investigated, the J-compound(s) points the way to develop a potentially novel therapeutic strategy targeting JAZ to treat human leukemia.