P-21 Activated Kinase 1 (Pak1) Is Critical for Normal Mast Cell Degranulation and Cytokine Production in Response to Antigen Stimulation of the High Affinity IgE Receptor.

Mast cells participate in normal and pathogenic inflammatory processes, including innate host defense, allergy, and asthma. Stimulation of the mast cell high-affinity IgE receptor (FcεR) by receptor cross-linking activates multiple downstream signaling pathways resulting in degranulation and de novo synthesis of multiple cytokines. However, the molecular mechanisms underlying these processes are incompletely defined. It is known that Rac2 deficient murine bone marrow derived mast cells (BMMCs) have impaired degranulation, but the downstream effectors that modulate this function are unknown. We hypothesized that p-21 activated kinase 1 (Pak1), a downstream effector of Rac proteins, is important in degranulation and de novo cytokine synthesis. Mature BMMCs from wild-type (WT) and Pak1 KO mice were sensitized with anti-DNP IgE then stimulated with DNP-HSA to stimulate FcεR. Interestingly, Pak1 KO BMMCs showed significant impairment in degranulation, as demonstrated by a 3-fold reduction in the percent of B-hexosaminidase released upon IgE stimulation. IgE stimulation of mast cell results not only in degranulation, but also in the production of TNFα, which is critical in the early recruitment of neutrophils to sites of acute inflammation. TNFαsynthesis is influenced by a number of transcription factors, many which are regulated by Erk. Since Pak1 has been shown, in overexpression systems, to phosphorylate Raf and Mek to activate Erk, we examined Erk activation in WT and Pak1 KO BMMCs in response to IgE stimulation. Pak1 KO BMMCs have a 50% reduction in phospho-Erk as compared to controls. We then tested another MAPK member important in mast cell cytokine synthesis, p38, and found phospho-p38 to be decreased in Pak1 KO BMMCs as well. Further, IgE-stimulated Pak1 KO BMMCs produce only 20–30% as much TNFαas controls. To define the role of Pak1 in cytokine production as specific for TNFαversus a more global defect, we also studied IL-6 synthesis and are able to report a 50% reduction in IL-6 production by Pak1 KO BMMCs. Our results indicate that Pak1 is important in BMMC degranulation, cytokine production, and MAPK activation in response to FcεR stimulation. These studies identify Pak1 as a potential therapeutic target in pathologic inflammation. Mechanisms by which Pak1 may be influencing mast cell degranulation as well as further study of transcription factors important in mast cell cytokine production are under current investigation.