Abstract
Most thrombopoietin receptor (MPL) mutations result in abnormal megakaryocyte expansion in the spleen or bone marrow (BM), leading to progressive fibrosis. It has been reported that p21 (Rac Family Small GTPase 1 [RAC1])-activated kinase 1 (PAK1) participates in the proliferation and differentiation of megakaryoblasts. PAK1 phosphorylation increased in patients with myeloproliferative neoplasms (MPNs) and murine MPN cells with the Mplw515l mutant gene in this study; however, the function of overactivated PAK1 in MPN cells remains unclear. We found that inhibition of PAK1 caused significant changes in the biological behaviors of MPLW515L mutant cells in vitro, including arrested growth or reduced clonality and increased polyploid DNA and cell apoptosis due to upregulated cleaved caspase 3. In vivo, PAK1 inhibitor treatment caused a slow elevation of leukocytosis and hematocrit (HCT) and a reduction in hepatosplenomegaly in 6133/MPLW515L-transplanted mice, along with reduced tumor cell infiltration and prolonged survival. Further, deletion of PAK1 sustained a relatively normal HCT and platelet count at the beginning of the disease but did not completely alleviate the splenomegaly of MPLW515L mutant mice. Notably, PAK1 knockout attenuated the destruction of splenic structure, and reduced the megakaryocyte burden within the BM. These results suggest that inhibition of PAK1 may be a useful method for treating MPLW515L mutant MPN by intervening megakaryocytes.