GENERATION OF CYTOKINES AND ARACHIDONIC ACID IN MAST CELLS

Heart, Lung, And Blood Institute

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Abstract

We have previously shown that release of free arachidonic acid in antigen-stimulated RBL-2H3 cells is dependent on phosphorylation of cytosolic phospholipase A2 (PLA2) by the MAP kinase, ERK2. A p38 MAP kinase-dependent induction of cyclooxygenase-2 (COX-2) then promotes synthesis of prostaglandins from arachidonic acid (see last years report). We now find that constitutive expression of PLA2 and other enzymes involved in eicosanoid metabolism such as 5-lipoxygenase is also dependent on p38 MAP kinase even though these enzymes are not induced by antigen. Furthermore, expression of these enzymes as well as the induction of COX-2 can be blocked by inhibitors of p38 MAP kinase even when p38 MAP kinase activity is at basal or minimally elevated levels to suggest that regulation of the eicosanoid-related enzymes occurs at low p38 MAP kinase activity. This kinase, like other MAP kinases (i.e., ERK and JNK), exhibits a rapid burst in activity followed by sustained but minimally elevated level of activity following antigen stimulation. Physiologic concentrations of glucocorticoids suppress activation of all three MAP kinases, induction of COX-2, and release of eicosanoids consistent with the idea that different MAP kinases regulate distinct steps in the eicosanoid cascade. Current and previous studies now indicate that dexamethasone, and presumably other glucocorticoids, act via the glucocorticoid receptor to cause disassociation of heat shock protein 70 and 90 from molecular complexes of the initiating kinases (i.e., Raf-1, MEKK-1, TAK)in the MAP kinase pathways. The kinases, bereft of heat shock proteins, fail to activate and translocate to the cell membrane. These actions of dexamethasone are accompanied by extensive dephosphorylation of the heat shock proteins although the mechanism, for example altered expression of a kinase or phosphatase, is as yet undetermined. These findings indicate a novel mechanism of action of glucocorticoids that could contribute to their antiinflammatory/immunosuppressive activities. Complementary studies have revealed that the array of signaling and functional responses in antigen-stimulated human mast cells (see ZO1 HL00993-14)are similar to those in RBL-2H3 cells although there are some distinctions. Signals induced by growth factors such as kit-ligand, IL-3, and IL-6 are clearly discernable in the human mast cells in contrast to RBL-2H3 cells and other tumor mast-cell lines whose growth-factor receptors are constitutively active. For this reason, we have investigated whether there are critical differences in signaling responses to antigen and growth factors. All stimulants cause rapid activation of MAP and phosphatidylinositol kinases and phosphorylation of downstream substrates in the human mast cells. These signals, however, are sustained only when cells are stimulated by antigen in the presence of growth factors. Also, the growth factors, individually or in combination, induce relatively small increases in cytosolic calcium whereas antigen causes robust and sustained elevations of cytosolic calcium even in the absence of growth factors. Another, possibly critical, difference is that induction and phosphorylation of the MAP kinase substrates, c-Jun and AFT-2, are minimally stimulated by growth factors when compared to antigen. The responses to antigen, however, are markedly potentiated by the presence of growth factors. The human mast cell may thus provide a useful model to elucidate linkages between signaling events and functional read-outs.