Mast cells play a pivotal role in the pathogenesis of asthma and other allergic diseases. These reactions are generally initiated by antigen-dependent aggregation of the high affinity IgE receptor (Fc-epsilon-RI) expressed on the cell surface and subsequent release of pro-inflammatory mediators (e.g. histamine, prostanoids, proteases and cytokines). However, ligands for other receptors such as KIT and various GPCRs may serve to prime mast cells for, or act as co-activators of, antigen-mediated mast cell activation. The signaling pathways linking Fc-epsilon-RI aggregation to human mast cell activation and function and how other receptors modify these Fc-mediated signaling events not been fully elucidated. Thus the primary focus of the research is the elucidation of signaling mechanisms associated with the activation of mast cells via the Fc-epsilon-RI and especially how the signaling pathways initiated by other receptors may integrate with those initiated by the Fc-epsilon-RI for synergistic mast cell activation and/or inhibition. The ability of mast cells to impact disease states in vivo also depends on their growth and differentiation from their progenitor cells, migration of the mast cells to their resident tissues, and their survival at these sites. Therefore, the integrated receptor-mediated signaling events regulating these processes are also being examined. The following observations were made since the last report: i. Mammalian Target of Rapamycin Complexes 1 and 2 (mTORC1 and mTORC2) differentially regulate homeostasis of neoplastic and non-neoplastic human mast cells Our studies revealed that mTORC1 and mTORC2 play differential roles in the expansion and homeostasis of neoplastic and non-neoplastic mast cells. Whereas mTORC1 may contribute to mast cell survival, mTORC2 is critical for regulating mast cell division by controlling progression through the cell cycle. Due to the terminally differentiated, non-dividing nature of mature mast cells, the function of mTORC2 in the homeostasis of these cells thus becomes redundant. Therefore, selectively targeting mTORC2 activity may allow the selective reduction of mast cell numbers associated with mast cell clonal disorders. In addition, as we have also demonstrated that mTORC1 and mTORC2 contributes to the regulation mast cell chemotaixs, targeting these complexes may help reduce the influx of mast cells to sites of allergic inflammation. ii. Glycogen synthase kinase-3-beta (GSK3-beta) activation is a pre-requisite signal for cytokine production and chemotaxis in human mast cells This study provided evidence that GSK3-beta is a key regulator of mast cell homeostasis in both neoplastic and primary cultured human mast cells through prevention of apoptosis. The data also indicated that the myeloproliferative capacity of the neoplastic HMC1.2 cells requires, at least in part, GSK3-beta activity and that a small molecule inhibitor (CHIR 99021) of GSK3-beta activity effectively reduces HMC1.2 cell survival. Thus, targeting GSK3-beta may provide a mechanism for modulating mast cell survival and apoptotic pathways in myeloproliferative disorders as well as in the allergic inflammatory response. iii. Targeting the KIT Activating Switch Control Pocket: A Novel Mechanism to Inhibit Neoplastic Mast Cell Proliferation and Mast Cell Activation As a collaborative effort with Dr. Todd Wilson (MCBS/LAD) and Daniel Flynn and Scott Wise of Deciphera Pharmaceuticals, the ability of two KIT """"""""switch pocket"""""""" inhibitors, DP-2976 and DP-4851, to potently and selectively inhibit both wild type (WT) and KIT D816V activation and consequently proliferation of both neoplastic and non-neoplastic mast cell populations was demonstrated. These inhibitors were also shown to effectively block the ability of SCF to enhance antigen-mediated mast cell activation. These studies demonstrated that, by targeting the switch pocket, this class of KIT inhibitors possesses a novel mechanism of inhibition whose dual suppression of KIT D816V neoplastic proliferation and SCF enhanced mast cell activation advocate future clinical development.
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