Inhibition of AKT Signaling Potently Inhibits the Growth of JAK and MPL-Mutant Cells in the Myeloproliferative Neoplasms In Vitro and In Vivo,

Abstract 3862

Somatic mutations in JAK2 and MPL are associated with the BCR-ABL negative myeloproliferative neoplasms (MPNs) including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). While oral JAK2 inhibitors improve peripheral blood counts and splenomegaly, these drugs show varying effects on JAK2 mutant allele burden and do not eliminate the malignant clone in humans or in animal models of MPN. Although much of the research to date has focused on JAK/STAT signaling, other pathways downstream of the class I cytokine receptors, including PI3K/AKT and ERK are also activated in MPNs. Our hypothesis is that persistent activation of these signaling pathways contributes to the progression of myeloproliferative neoplasms (MPNs).

Multiple studies have shown that overexpression of activated JAK2 and MPL mutants (e.g. JAK2 V617F or MPL W515L) in primary murine bone marrow cells causes robust AKT, STAT3, and STAT5, JNK, ERK phosphorylation. To determine the extent to which these signaling pathways are involved in the disease, we cultured JAK2 V617F and MPL W515L expressing clones of the G1ME and 6133 megakaryocytic cell lines with a panel of small molecule kinase inhibitors. We discovered that inhibition of the PI3K/AKT and MAPK/JNK signaling pathways with triciribine and SP600125 respectively, potently suppressed growth of these cells by inducing G1 arrest and apoptosis. In contrast, inhibitors of Ras, ERK, Src family tyrosine kinase, and PKC failed to significantly inhibit proliferation of the JAK2 or MPL mutant expressing cells. Murine bone marrow cells transduced with MPL W515L show a dramatic expansion of megakaryocytes (CFU-MKs) and this expansion was abrogated in the presence of PI3K/AKT inhibitors suggesting a requirement for this pathway in aberrant megakaryocyte expansion.

Since AKT inhibition showed the strongest effect, we assayed the activity of MK-2206, a potent and selective allosteric AKT inhibitor, on multiple models of MPNs. We discovered that MK-2206 induced proliferative arrest and apoptosis accompanied by suppression of PI3K/AKT signaling in G1ME and 6133 cells expressing MPLW515L. MK-2206 also potently inhibited liquid culture growth and colony formation of PMF patient CD34+ cells in vitro. Finally, in preliminary murine transplants experiments with MPLW515L expressing bone marrow progenitors, treatment with MK-2206 led to significant reductions in peripheral blood leukocytosis and extramedullary hematopoiesis. Together, these findings demonstrate that the PI3K/AKT axis represents a rational target for therapy in human myeloproliferative neoplasms.