Protein phosphatase inhibitors induce the release of serotonin from rat basophilic leukemia cells (RBL-2H3)

Preliminary characterization of the role of protein serine/threonine phosphatases in the regulation of human lung mast cell function

1. Okadaic acid, a cell permeant inhibitor of protein serine/threonine phosphatases (PPs), attenuated the IgE-dependent release of mediators from human lung mast cells (HLMC). The concentration of okadaic acid required to inhibit by 50% (IC50) the IgE-dependent release of histamine was 0.2 microM. Okadaic acid also inhibited the IgE-mediated generation of prostaglandin D2 (PGD2) and sulphopeptidoleukotrienes (sLT) with IC50 values of 0.2 microM and 0.6 microM respectively. 2. The IgE-mediated generation of histamine, PGD2 and sLT was inhibited by okadaic acid and two analogues of okadaic acid, okadaol and okadaone, with the following rank order of activity; okadaic acid > okadaol > okadaone. This order of activity for the inhibition of mediator release parallels the activity of these compounds as inhibitors of isolated PPs. 3. Extracts of HLMC liberated 32P from radiolabelled glycogen phosphorylase and this PP activity was inhibited by the PP inhibitors (all at 3 microM), okadaic acid (73 +/- 4% inhibition, P < 0.0005), okadaol (26 +/- 7% inhibition, P < 0.05) and okadaone (8 +/- 7% inhibition, P = 0.52). The rank order of activity of okadaic acid > okadaol > okadaone parallels the activity of these compounds as inhibitors of isolated PPs. 4. Dephosphorylation of radiolabelled glycogen phosphorylase by extracts of HLMC was inhibited by 15 +/- 3% (P < 0.001) by a low (2 nM) concentration of okadaic acid and by 88 +/- 4% (P < 0.0005) by a higher (5 microM) concentration of okadaic acid. Because 2 nM okadaic acid may act selectively to inhibit PP2A whereas 5 microM okadaic acid inhibits both PP1 and PP2A, these data suggest that both PP1 and PP2A are present in HLMC. 5. Inhibitor 2, a PP1-selective inhibitor, attenuated (71 +/- 3% inhibition, P < 0.05) PP activity in extracts of HLMC suggesting that HLMC contain PP1 and that it may constitute 71% of the phosphorylase PP activity in extracts of HLMC. 6. Radiolabelled casein, a PP2A-restricted substrate, was dephosphorylated by extracts of purified HLMC and this activity was inhibited (81 +/- 8% inhibition, P < 0.005) by 2 nM okadaic acid suggesting that PP2A is resident in HLMC. 7. Collectively, these data suggest that both PP1 and PP2A are resident in HLMC. However, although the data suggest that okadaic acid regulates responses in HLMC by interacting with PPs, it has not been possible to determine whether either PP1 or PP2A or both PPs are involved in the okadaic acid-induced inhibition of mediator release from HLMC.

Regulation of human basophil function by phosphatase inhibitors

1. Okadaic acid, a cell permeant inhibitor of protein serine/threonine phosphatases (PPs), attenuated the IgE-mediated release of the pre-formed mediator, histamine from human basophils in a time- and dose-dependent manner. Optimal inhibition (77 +/- 4%, P < 0.0001) of histamine release was observed following a 2 h incubation with 1 microM okadaic acid. 2. Okadaic acid and two analogues of okadaic acid were also studied and were found to inhibit the IgE-dependent release of histamine. Concentrations required to inhibit release by 50% (IC50) were 0.6 microM for okadaic acid and 7.5 microM for okadaol, whereas okadaone was inactive. 3. The structurally-unrelated PP inhibitor, calyculin A, also inhibited IgE-dependent histamine release from basophils dose-dependently and was approximately six fold more potent than okadaic acid. 4. The IgE-mediated generation of sulphopeptidoleukotrienes (sLT) from basophils was inhibited by okadaic acid and related analogues with the following rank order of potency; okadaic acid (approx. IC50 0.3 microM) > okadaol (3 microM) > okadaone (inactive). 5. Okadaic acid, okadaol and okadaone (all at 3 microM) inhibited the IgE-mediated generation of the cytokine interleukin 4 (IL4) from human basophils by 67 +/- 9% (P < 0.002), 48 +/- 14% (P < 0.05) and 8 +/- 7% (P = 0.31), respectively. 6. Extracts of purified human basophils liberated 32P from radiolabelled glycogen phosphorylase and this PP activity was inhibited by 17 +/- 3% (P < 0.0005) by a low (2 nM) concentration of okadaic acid and was inhibited by 96 +/- 1% (P < 0.0001) by a higher (5 microM) concentration of okadaic acid. Because a low (2 nM) concentration of okadaic acid inhibits PP2A selectively whereas a higher (5 microM) concentration inhibits both PP1 and PP2A, these findings suggest that both PP1 and PP2A are present in basophils. 7. In total these data suggest that PPs are resident in human basophils and that PPs may be important in the regulation of basophil function.

Distinct functions of the Fc epsilon R1 gamma and beta subunits in the control of Fc epsilon R1-mediated tyrosine kinase activation and signaling responses in RBL-2H3 mast cells

In RBL-2H3 rat tumor mast cells, cross-linking the high affinity IgE receptor, Fc epsilon R1, activates the protein-tyrosine kinases Lyn and Syk and initiates a series of responses including protein-tyrosine phosphorylation, inositol 1,4,5-trisphosphate synthesis, Ca2+ mobilization, secretion, membrane ruffling, and actin plaque assembly. The development of chimeric receptors containing cytoplasmic domains of individual subunits of the heterotrimeric (alpha beta gamma 2) Fc epsilon R1 has simplified analyses of early signaling events in RBL-2H3 cells. Here, RBL-2H3 cells were transfected with cDNAs encoding the extracellular and transmembrane domains of the interleukin-2 receptor alpha subunit (the Tac antigen) joined to the C-terminal cytoplasmic domains of the Fc epsilon R1 gamma and beta subunits (TT gamma and TT beta). Both sequences contain tyrosine activation motifs implicated in antigen receptor signal transduction. TT gamma and TT beta are expressed independently of the native Fc epsilon R1, as demonstrated by the ability of Tac cross-linking agents to trigger the clustering and internalization through coated pits of both chimeric receptors without co-clustering the Fc epsilon R1. A full range of signaling activities is induced by TT gamma cross-linking; the TT gamma-induced responses are slower and, except for Lyn activation, smaller than the Fc epsilon R1-induced responses. In striking contrast, TT beta cross-linking elicits no tyrosine phosphorylation or signaling responses, it impairs basal activities measured in secretion and anti-PY (anti-phosphotyrosine antibody) immune complex kinase assays, and it antagonizes Fc epsilon R1-induced Lyn and Syk activation, protein-tyrosine phosphorylation, and signaling responses. We hypothesize that the isolated beta subunit binds a specific kinase or coupling protein(s) required for signaling activity, sequestering it from the signal-transducing gamma subunit. Binding the same kinase or coupling protein to the beta subunit of the intact Fc epsilon R1 may serve instead to present it to the adjacent gamma subunit, resulting in enhanced kinase activation and signaling responses.