Inhibition of USP9X Downregulates JAK2-V617F and Induces Apoptosis Synergistically with BH3 Mimetics Preferentially in Ruxolitinib-Persistent JAK2-V617F-Positive Leukemic Cells.

JAK2-V617F plays a key role in the pathogenesis of myeloproliferative neoplasm. However, its inhibitor ruxolitinib has shown limited clinical efficacies because of the ruxolitinib-persistent proliferation of JAK2-V617F-positive cells. We here demonstrate that the USP9X inhibitor WP1130 or EOAI3402143 (G9) inhibited proliferation and induced apoptosis more efficiently in cells dependent on JAK2-V617F than on cytokine-activated JAK2. WP1130 preferentially downregulated activated and autophosphorylated JAK2-V617F by enhancing its K63-linked polyubiquitination and inducing its aggresomal translocation to block downstream signaling. Furthermore, JAK2-V617F associated physically with USP9X in leukemic HEL cells. Induction of apoptosis by inhibition of USP9X was mediated through the intrinsic mitochondria-mediated pathway, synergistically enhanced by BH3 mimetics, prevented by overexpression of Bcl-xL, and required oxidative stress to activate stress-related MAP kinases p38 and JNK as well as DNA damage responses in HEL cells. Although autophosphorylated JAK2-V617F was resistant to WP1130 in the ruxolitinib-persistent HEL-R cells, these cells expressed Bcl-2 and Bcl-xL at lower levels and showed an increased sensitivity to WP1130 as well as BH3 mimetics as compared with ruxolitinib-naive HEL cells. Thus, USP9X represents a promising target along with anti-apoptotic Bcl-2 family members for novel therapeutic strategies against JAK2-V617F-positive myeloproliferative neoplasms, particularly under the ruxolitinib persistence conditions.