Annexin A1 as an endogenous regulator of mast cell degranulation.
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Annexin A1 (Anx-A1) is a 37kDa protein that is secreted by some cells in response to glucocorticoids (GCs) and which mediates several of their acute anti-inflammatory effects. In addition to GCs, ‘mast cell stabilising’ cromones such as nedocromil also mobilise Anx-A1 by promoting its phosphorylation by protein kinase C (PKC) and hence its secretion, which explains their acute efficacy as anti-allergic agents. This thesis addresses a fundamental aspect of Anx-A1 in the actions of anti-allergic drugs. In this study, anti-allergic drugs such as H1 antagonists, mast cell stabilisers and ‘dual action’ drugs were first tested for their ability to enhance Anx-A1 phosphorylation in a model system using U937 cells. Biochemical and immuno-fluorescent techniques were used to study the mechanisms by which these drugs suppress mediator release from cord blood derived mast cells (CDMCs) and murine bone-marrow derived mast cells (BMDMCs) from wild type and Anx-A1 null-mice. This thesis suggest that PKC activation is crucial for Anx-A1 export in mast cells and nedocromil in the presence of dexamethasone, prolongs the duration of PKC activation and subsequently phosphorylation, externalisation and release of Anx-A1 from CDMCs. The ability of nedocromil to inhibit β-hexosaminidase, tryptase, histamine and PGD2 release are dependent on Anx-A1 in CDMCs. Interestingly, ketotifen, a ‘dual action’ drug possesses a similar pharmacological profile to nedocromil, but not promethazine, which does not act through the Anx-A1 release. Strong evidence supports the notion that the mechanisms of action of nedocromil are modulated by Anx-A1, thus the possibility that FPR2 might be involved in the acute actions of nedocromil was tested. Nedocromil inhibits the release of PGD2 through the activation of FPR2 but not the inhibition of histamine release. A possible explanation for this finding could be that Anx-A1 might be interacting with other FPR family members to exert the histamine inhibitory effects. Although only a small subset of the downstream intracellular signaling pathway of MAPK was tested, the results indicate that Anx-A1 differentially regulates the activation of p38 and JNK in CDMCs treated with nedocromil. These findings indicate a novel model system in which Anx-A1 mediates the pharmacological actions of anti-allergic drugs and thus has an important role in preventing the mast cell degranulation.
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