Generation Of Cytokines And Arachidonic Acid In Mast Cel

Heart, Lung, And Blood Institute

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Abstract

We have previously shown that secretion of granules is primarily regulated by protein kinase (PK) C and a calcium signal (see accompanying report) whereas the generation of inflammatory lipids and cytokines in antigen-stimulated RBL-2H3 cells is primarily dependent on MAP kinases.With respect to the inflammatory lipids, present and past work indicate that p38 MAP kinase regulates expression of phospholipase (PL)A2 (the enzyme responsible for production of arachidonic acid) in addition to other enzymes that catalyze formation of eicosanoids from arachidonic acid such as cyclooxygenase-2 (COX-2) and 5-lipoxygenase. The phosphorylation and activation of PLA2 is dependent on ERK. With respect to the cytokines, our recent work with human mast cells that were derived from peripheral blood pluripotent CD34+ cells suggest a role for PKC in addition to ERK. Inhibitors of both kinases block production of THFalpha and IL-6 and comparison of responses to stem cell factor (a mast cell growth factor) and antigen show that antigen uniquely stimulates PKC as well as the PKC-dependent induction/phosphorylation of AP1 components and phosphorylation of ATF-2 and the IkapaB/NFkappaB system. In addition, antigen unlike stem cell factor stimulates production of an array of inflammatory cytokines. Nevertheless, stem cell factor activates other signalling pathways common to antigen stimulation such as PLC/calcium mobilization, phosphatidylinositolphosphate 3-kinase, all MAP kinases, and enhances responses when added in combination with antigen. We have also investigated the actions of low-dose glucocorticoids on the signaling pathways and functional responses in antigen stimulated cells. As reported in previous reports, physiologic concentrations of glucocorticoids suppress activation of all MAP kinases and, as a consequence, induction of COX-2, release of eicosanoids, and the generation of inflammatory cytokines. The MAP kinase pathways are inhibited at the level of the MAP kinase, kinase, kinase and, at least in the case of Raf1 and MEKK1, pretreatment with nanomolar concentrations of dexamethasone results in disassembly of the kinase complex with heat shock proteins and failure to translocate to the plasma membrane on stimulation. At the moment the mechanisms are uncertain because other critical systems are disrupted as well by dexamethasone. These include reduced expression of the linker protein, Lat, impaired phosphorylation/activation of the regulatory Csk homolog, Chk, and suppressed activation of phosphtidylinositol 3-kinase. In addition expression of certain protein tyrosine phosphatases are significantlt increased in dexamethasone-treated cells. Whether the primary action of dexamethasone is to reduce expression of critical anchoring proteins thereby preventing proper assembly of signaling complexes or to enhance tyrosine phosphatase expression/activity thereby dampening tyrosine kinase pathways is under investigation. Regardless of this uncertainty, our findings reveal novel actions of glucocorticoids on biochemical pathways in addition to the known direct actions of glucocorticoids on transcriptional regulation.