Central role of G protein-coupled PI3Kγ in mast cell function

Abstract

Mast cells are primary effector cells in the allergic and inflammatory cascades. Mast cell activation is achieved by antigen/IgE-mediated cross-linking of the FceR1 receptor. Mast cells also possess an array of G protein-coupled receptors (GPCRs) that relay signals from C5a and C3a of the complement system, as well as from adenosine and chemokines. Adenosine is released by various cells including mast cells, during ischemia and inflammation and in damaged tissue. It can activate mast cells, rendering them hyper-reactive to allergen/IgE complexes. However, the downstream events that mediate the exacerbation by adenosine and other GPCR ligands on mast cell hyper-reactivity remain to be elucidated. In an elegant study by Laffargue et al [1xPhosphoinositide 3-kinase-γ is an essential amplifier of mast cell function. Laffargue, M. et al. Immunity. 2002; 16: 441–451Abstract | Full Text | Full Text PDF | PubMed | Scopus (235)See all References][1], the signals from GPCRs were found to require functional phosphoinositide 3-kinase-γ (PI3Kγ).The first clue that PI3Kγ might be essential for the amplification of mast cell function came from experiments using bone-marrow derived mast cells (BMMCs) from mice that are deficient in PI3Kγ. Laffargue et al. found that mast cell degranulation by IgE/antigen stimulation was markedly abrogated in cells from PI3Kγ−/− mice compared with wild-type cells. To explore whether adenosine initiated a PI3Kγ-dependent sensitization of mast cells, both wild type and PI3Kγ−/− BMMC were challenged with IgE, antigen and adenosine in parallel. These experiments revealed that the adenosine co-stimulatory signal was fully dependent on the presence of a functional PI3Kγ. The attenuated response of BMMC correlated with a reduction in both the adenosine-stimulated synthesis of PI(3,4,5)P3 (a product of PI3K activation) and the PI(3,4,5)P3-dependent phosphorylation of protein kinase B (PKB). By contrast, both biochemical events could be robustly stimulated by adenosine in wildtype BMMCs.The use of adenosine receptor-selective agonists revealed that the adenosine-dependent hyper-reactivity of BMMCs is chiefly mediated by the A3 adenosine receptor, which uses mainly trimeric Gi proteins to activate PI3Kγ. This in turn produces PI(3,4,5)P3, which is necessary for triggering the release of histamine-containing granules. Laffargue et al. also showed that many other GPCR agonists that are released during tissue damage and anaphylactic shock can induce similar effects to adenosine. Of particular interest, was the observation that the chemokines macrophage inflammatory protein-1α (MIP-1α) and RANTES amplified allergen-dependent granule release that was sensitive to pertussis toxin, thus indicating dependence on Gi proteins. The chemokine responses also correlated with PI3Kγ-dependent activation of PKB. Importantly, not all GPCR-coupled receptors share this feature, as platelet activating factor appears to be coupled to pertussis toxin-resistant biochemical events and does not act synergistically with allergen to enhance granule release. Thus, only those GPCRs that use Gi proteins and, in turn, couple to PI3Kγ can enhance allergen-dependent granule release.The report also examined the in vivo role of PI3Kγ in a mouse model of adenosine-induced mast cell activation. These experiments revealed that mice lacking PI3Kγ did not form edema after intradermal injection of adenosine and when challenged by passive systemic anaphylaxis. The results from in vitro and in vivo experiments presented in this study point toward a central role for PI3Kγ in relaying signals from GPCR that are required to reinforce the IgE/antigen response in mast cells.