Xestospongin C, a novel blocker of IP3 receptor, attenuates the increase in cytosolic calcium level and degranulation that is induced by antigen in RBL-2H3 mast cells

Abstract

1. We evaluated the role of the cross-linking of FcεRI-mediated inositol 1,4,5-triphosphate (IP3) in the increase in cytosolic Ca2+ level ([Ca2+]i) using xestospongin C, a selective membrane permeable blocker of IP3 receptor, in RBL-2H3 mast cells. 2. In the cells sensitized with anti-dinitrophenol (DNP) IgE, DNP-human serum albumin (DNP-HSA) and thapsigargin induced degranulation of β-hexosaminidase and a sustained increase in [Ca2+]i. Xestospongin C (3-10 μM) inhibited both of these changes that were induced by DNP-HSA without changing those induced by thapsigargin. 3. In the absence of external Ca2+, DNP-HSA induced a transient increase in [Ca2+]i. Xestospongin C (3-10 μM) inhibited this increase in [Ca2+]i. 4. In the cells permeabilized with β-escin, the application of IP3 decreased Ca2+ in the endoplasmic reticulum (ER) as evaluated by mag-fura-2. Xestospongin C (3-10 μM) inhibited the effect of IP3. 5. After the depletion of Ca2+ stores due to stimulation with DNP-HSA or thapsigargin, the addition of Ca2+ induced capacitative calcium entry (CCE). Xestospongin C (3-10 μM) inhibited the DNP-HSA-induced CCE, whereas it did not affect the thapsigargin-induced CCE. 6. These results suggest that FcεRI-mediated generation of IP3 contributes to Ca2+ release not only in the initial phase but also in the sustained phase of the increase in [Ca2+]i, resulting in prolonged Ca2+ depletion in the ER. The ER Ca2+ depletion may subsequently activate CCE to achieve a continuous [Ca2+]i increase, which is necessary for degranulation in the RBL-2H3 mast cells. Xestospongin C may inhibit Ca2+ release and consequently may attenuate degranulation.