Evaluation of the role of inositol trisphosphate in IgE-dependent exocytosis.
A close correlation exists between inhibition by 12-O-tetradecanoylphorbol 13-acetate (TPA) of inositol trisphosphate (InsP3) formation and the rise in internal Ca2+ concentrations in IgE-stimulated rat basophilic leukemia (RBL-2H3) cells. Inhibition of both processes is dose-dependent, with half-maximal and maximal inhibition occurring at 1.5 and 10 ng of TPA/ml respectively. At a similar range of concentrations TPA does not inhibit, but rather enhances, IgE-dependent secretion. When added to antigen-activated cells. EGTA immediately abrogates secretion and stimulates InsP3 production. In contrast, EGTA has only a small inhibitory effect on IgE-induced secretion from TPA-activated cells. In antigen-activated cells, EGTA partially inhibits InsP1 formation, suggesting that, unlike InsP3, InsP1 may in part be formed directly from phosphatidylinositol in a Ca2(-)-dependent manner. Together, these findings suggest that under physiological conditions the stimulated formation of InsP3 is insufficient for triggering secretion, and that Ca2+ influx is essential. Moreover, InsP3 formation is not obligatory for IgE-mediated exocytosis, provided that the cells are activated by TPA. Secretion from TPA-activated cells, which is independent of InsP3 formation and the rise in internal Ca2+, does not require the presence of external Ca2+, implying that the presence of external Ca2+ during IgE-induced secretion is required for producing the Ca2+ signal and not for exocytosis per se.
Gat-Yablonski
G
G
Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
Sagi-Eisenberg
R
R
D000818:Animals; D000941:Antigens; D002118:Calcium / Q000502:physiology; D004533:Egtazic Acid / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects; D006636:Histamine Release; D007073:Immunoglobulin E / Q000502:physiology*; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D008094:Lithium / Q000494:pharmacology; D008407:Mast Cells / Q000502:physiology*; D051381:Rats; D011971:Receptors, Immunologic / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D000941:Antigens; D011971:Receptors, Immunologic; D012701:Serotonin; D007073:Immunoglobulin E; D004533:Egtazic Acid; D015544:Inositol 1,4,5-Trisphosphate; D008094:Lithium; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium