Loss of Pak1 Corrects Multiple Gain of Function Phenotypes in Nf1+/− Mast Cells.

Neurofibromatosis type 1, caused by mutations in the tumor suppressor gene NF1, is the most common genetic disorder in man and is characterized by the development of tumors called neurofibromas. Studies in genetically engineered Nf1 knockout mice have demonstrated that loss of the Nf1 tumor suppressor in Schwann cells, the known tumorigenic cell in neurofibromas, is necessary but not sufficient for tumor formation. By using adoptive transfer, we have now established that haploinsufficiency of Nf1 in the hematopoietic microenvironment (specifically mast cells) is necessary and sufficient for tumor progression in a Nf1 conditional knockout model. It is known that mast cells promote tumor progression via the secretion of growth factors and proteinases. Thus molecules that target mast cell function may be useful therapeutically. We have previously demonstrated that haploinsufficiency of Nf1 results in hyperproliferation and hyperactivation of the mitogen-activated protein kinase (MAPK) pathway in bone marrow-derived mast cells in response to kit-ligand (kit-L). Cross-cascade signaling between phosphoinositide 3-kinase activated Rho-GTPases and the classical MAPK pathway has been shown to be responsible for these abnormal phenotypes. p21-activated kinases (Paks) are downstream mediators of Rho-GTPase proteins that have been implicated in immortalized cells as positive regulators of MAPK pathway members and as modulators of cell growth and cytoskeletal dynamics. Utilizing an intercross of Pak 1−/− mice with Nf1+/− mice, we studied how Pak1 in regulates cellular functions associated with haploinsufficiency at Nf1 in primary mast cells. Loss of Pak1 corrects the gains of function in proliferation and migration found in Nf1+/− mast cells back to wild type levels via differential regulation of MAPKs. Pak1 regulates proliferation via a Pak/Erk pathway, while a Pak1/p38 pathway is critical for migration. The increased release of preformed inflammatory mediators in Nf1 haploinsufficient mast cells was also abrogated by genetic disruption of Pak1. Pak1 was found to contribute to MAPK signaling by modulating both ERK and p38 activation. Loss of Pak1 reduces the hyperactivation of ERK and p38 found in Nf1+/− mast cells to that of wild type controls. Further, using a model that stimulates local proliferation of mast cells in vivo, we confirm that loss of Pak1 corrects the accumulation and degranulation of Nf1+/− mast cells in vivo to wild type levels. Thus, Pak1 is a novel mediator of Ras-dependent mast cell functions, and treatments that target Pak1 may be useful as a targeted therapy for treatment of neurofibromas.