Molekulare Mechanismen der Glukokortikoid-abhängigen Hemmung des IgE-Rezeptor-Signalweges in Mastzellen

Abstract

Glucocorticoids are well known for their potent anti-inflammatory and anti-allergic properties. In mast cells, glucocorticoids inhibit the release of inflammatory mediators and the expression of pro-inflammatory genes. These processes are triggered by antigenic crosslinking of the high affinity receptor for IgE FceRI and are associated with an activation of the MAP kinases ERK1/2, JNK and p38 MAPK. FceRI surface expression and antigen-induced phosphorylation of MAP kinases are inhibited by glucocorticoids. In the work presented here, the mechanisms of glucocorticoid-dependent down-regulation of IgE receptor surface expression and negative regulation of p38 MAP kinase and ERK1/2 activity in bone marrow derived mast cells (BMMC) were analysed. Down-regulation of FCERI surface expression is a relatively late effect of glucocorticoids which occurred after 16 hours treatment of BMMC with dexamethasone. Dexamethasone did not affect the mRNA levels of the FceRI subunits FceRIα, β and y, but decreased the β subunit at the protein level. Using different inhibitors it was shown, that reduction of FceRI(3 requires the glucocorticoid receptor, de novo protein synthesis and the activity of the proteasome pathway. BMMC, treated for 2-24 hours with dexamethasone, showed an enhanced expression of the MAP kinase phosphatases MKP-1 and MKP-2 and the protein tyrosine phosphatase PEP. By analyzing BMMC derived from PEP and MKP-1 knockout (KO) mice it was shown that the repression of phosphorylation of ERK1/2 by dexamethasone is independent of PEP and MKP-1. Moreover PEP KO BMMC treated with glucocorticoid also showed a decreased phosphorylation of p38 MAPK compared to the control, whereas in MKP-1 deficient BMMC dexamethasone failed to repress the activity of this kinase. This MKP-1-mediated inhibition of p38 MAPK phosphorylation was only observed four and eight hours after glucocorticoid treatment. MKP-1 KO BMMC cultured for a longer time in the presence of dexamethsone showed a decrease in p38 MAPK phosphorylation similar to the wild-type cells, indicating that the contribution of MKP-1 to the mediation of glucocorticoid action is only restricted to a relatively short time after glucocorticoid treatment. A reduction of MKP-2 expression in wild-type BMMC by MKP-2 siRNA did not abolish the repression of p38 MAPK and ERK1/2 phosphorylation by dexamethasone. Similarly the glucocorticoid-mediated down-regulation of expression of the pro-inflammatory genes TNF-a, IL-6, MCP-1 and MMP13 was not altered in MKP-1 KO BMMC and BMMC transfected with MKP-2 siRNA. MKP-1 knockout mice were however highly susceptible to systemic anaphylaxis. Nevertheless dexamethasone was able to suppress the IgE/antigen-induced anaphylactic reaction. Taken together, the findings of these studies demonstrate that inhibition of IgE receptor-dependent p38 MAPK-phosphorylation by dexamethasone is mediated by induction of MKP-1. However MKP-1 does not suffice for the mediation of the negative regulatory actions of glucocorticoids in BMMC.