Abstract A15: Megakaryocyte differentiation is regulated by p21-activated kinase 2

Abstract

Abstract Megakaryocytes (MKs) mature and differentiate into multi-nucleated cells in the bone marrow, and migrate from the osteoblastic niche to the vascular niche by means of an SDF-1a gradient, culminating in the release of platelets into circulation. Failure to do so results in myeloproliferative disorders, which can manifest with extremely reduced platelet counts. Regulation of this process is controlled by MAPK signaling, cytoskeletal and microtubule-associated proteins and GTPases. p21-activated kinase 2 (Pak2) regulates the cytoskeleton through the MAPK and RhoA signaling pathways, both required for MK differentiation and cytokinesis. In our study, we demonstrate that Pak2 is essential for proper MK differentiation and polyploidization, in vitro and in vivo. Pak2 deletion, upon cre induction specifically in hematopoietic tissues (Mx1-cre), reduces peripheral blood platelet counts (thrombocytopenia) by 50%. Bone marrow histology demonstrates hyperplastic MKs coinciding with reduced platelet counts. Analysis of bone marrow by flow cytometry demonstrates altered MK differentiation, observed by elevated MK stem cell precursors in the bone marrow (CD150+CD41+). Increased stem cell potential of MKs deficient for Pak2 results in increased colony forming units (CFU-Mks). Additionally, Pak2-null MKs have increased levels of polyploidization. Signaling to MLC2 and GEF-H1 are altered in Pak2-deficient MKs, both previously shown to regulate MK differentiation and polyploidization, respectively. In summary, Pak2 is a negative regulator of MK stem cell differentiation as well as polyploidization. Since Pak2 negatively regulates polyploidization, this leads to therapeutic uses for Pak family inhibitors in the treatment for acute megakaryoblastic leukemia, a fatal disorder which presents with an accumulation of immature MKs, due to a failure to differentiate into mature MKs. Citation Format: Rachelle E. Kosoff, Jonathan Chernoff. Megakaryocyte differentiation is regulated by p21-activated kinase 2. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr A15. doi: 10.1158/1557-3125.RASONC14-A15