The Persistent Survival of MPN Cells to JAK2 Inhibition Is Dependent on SHP2 Activity, Which May Provide a Therapeutic Target to Enhance Current Anti-MPN Therapies

Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis, are hematopoietic stem cell disorders that affect about 300,000 people in the U.S. PV and ET can progress to myelofibrosis where patients are susceptible to bone marrow failure, increased incidence of acute leukemia, and poor survival. MPNs are driven by aberrant JAK2 activity that is a consequence of mutations in JAK2, Mpl, or CalR. The JAK1/2 inhibitor ruxolitinib, which is FDA-approved for a subset of MPN patients, is effective in reducing patient symptomology but fails to induce remission, as it has little effect on the neoplastic cell burden. However, a modest increase in survival and decrease of allele burden has been suggested following long-term ruxolitinib treatment. The quality of life benefit afforded by ruxolitinib suggests it will likely remain a mainstay of future MPN therapies. Identifying the mechanism(s) by which cells persistently survive in the presence of ruxolitinib could prove pivotal to improving JAK2 inhibitor therapies in MPN patients.

In order to identify mechanisms by which JAK2-driven MPN cells survive ruxolitinib treatment, we utilized global phospho-tyrosine proteomics to compare ruxolitinib-sensitive and persistent cells, the latter of which are drug persistent by nature due to their reacquisition of JAK2 inhibitor sensitivity upon removal of ruxolitinib. We utilized the JAK2-V617F-driven MPN model cell lines UKE1 and SET2 for this analysis. Pathway analysis from the datasets obtained indicated changes in phosphorylation of proteins involved in growth factor and cytokine signaling. Notably, we identified changes in phosphorylation of several tyrosine phosphatases, including SHP1 and SHP2. The SH2 domain-containing tyrosine phosphatase SHP2 (encoded by PTPN11) plays important roles in relaying signals from cell surface receptors, notably mediating activation of ERKs, and was the first phosphatase identified as leukemogenic. SHP2 regulates JAK2 signaling and is found mutated in ~8% of post-MPN leukemia, suggesting it may play a role in the biology of MPNs. In addition, SHP2 has been identified as a potential mediator of targeted therapy resistance in cancer. Therefore, we hypothesized that SHP2 activity may contribute to the persistent survival of MPN cells in ruxolitinib and provide a therapeutic target in MPNs. To test our hypothesis, we treated ruxolitinib-sensitive (drug naïve) and persistent cells with a specific allosteric inhibitor of SHP2, SHP099. Ruxolitinib-persistent cell growth was more sensitive to SHP2 inhibition, and SHP099 enhanced the growth inhibitory effects of ruxolitinib on ruxolitinib-sensitive cells. Treatment of UKE1 cells with a high concentration (~10X GI50) of ruxolitinib leads to the persistent survival of a small population of cells and this was prevented by SHP099. SHP2 inhibition was also deleterious to the maintenance of an existing drug persistent state in a high concentration of ruxolitinib. Interestingly, signaling pathways in cells persistently growing in ruxolitinib differentially respond to SHP2 inhibition. In both ruxolitinib-sensitive and persistent cells activation of ERK was inhibited by SHP099. However, in ruxolitinib-persistent UKE1 cells, this inhibition was more rapid, suggesting the kinetic regulation of ERK activation by SHP2 is altered in these cells. Also, SHP099 led to an increase in pSTAT3 in both ruxolitinib-sensitive and persistent cells. However, SHP099 led to an increase in pSTAT5 in ruxolitinib sensitive UKE1 cells, but not in cells persistently growing in ruxolitinib. Re-activation of signaling pathways following ruxolitinib treatment likely contributes to cell survival. We observed that ERK becomes re-activated shortly after ruxolitinib treatment, and this re-activation depends on SHP2 activity. Finally, we observed dose-dependent inhibition of the neoplastic growth of primary MPN patient cells by SHP099 and synergistic inhibition with ruxolitinib, suggesting co-targeting SHP2 may improve JAK2-targeted MPN therapies.

In conclusion, our data suggest that re-wiring of signaling in cells challenged with ruxolitinib may create greater dependence on SHP2 activity that contributes to cell survival to JAK2 inhibition. Thus, targeting SHP2 may improve the efficacy of ruxolitinib as an anti-MPN therapy.