Characterization of the receptors for IgE on membranes isolated from rats basophilic leukemia cells

Regulation of phospholipase D and secretion in mast cells by protein kinase A and other protein kinases

Functions attributed to phospholipase (PL) D include the regulation of intracellular trafficking of Golgi-derived vesicles and secretion of granules from mast cells. We have reported that activation of PLD and secretion in a rat mast cell (RBL-2H3) line is substantially enhanced by cholera toxin, a known activator of protein kinase (PK) A. Here we review the evidence that (1) the synergistic interactions of cholera toxin and other pharmacological agents on mast cell secretion are attributable to the synergistic activation of PLD via PKA, CaM kinase II, and PKC and (2) both PLD1 and PLD2 participate in this process. For example, treatment with cholera toxin, thapsigargin, and phorbol 12-myristate 13-acetate (which activate PKA, CaM kinase II, and PKC, respectively) exhibit synergy in the stimulation of both PLD and secretion. These kinases and PLD are likely confined to membrane components, as similar synergistic interactions could be demonstrated in permeabilized cells. The regulation of PLD and secretion by these kinases is also apparent from studies of inhibitors of PKA and other kinases. Also, by overexpression of either PLD1 or PLD2 it is apparent that both isoforms respond to the same stimuli as endogenous PLD, although PLD1 is largely associated with secretory granules and PLD2 with plasma membrane. The studies reveal interesting differences in the regulation of the translocation of granules (regulated by PKA) and the fusion of these granules with the plasma membrane (regulated by Ca(2+) and PKC). The pathological/physiological implications of the regulation of PLD by PKA require further evaluation in other cell systems.

Studies with a monoclonal antibody to the beta subunit of the receptor with high affinity for immunoglobulin E

The receptor with high affinity for IgE consists of a tetrameric complex of polypeptides, one of which (alpha), contains the binding site for IgE. The function of the other chains--a single beta and two disulfide-linked gamma chains--is unknown. We report the cloning of a murine hybridoma that secretes an IgG1 antibody which specifically reacts with the beta subunit. Studies with this monoclonal antibody show that the subunit stoichiometry of the receptor is unaffected by the presence or absence of bound IgE. We also found that under certain conditions where the alpha beta gamma 2 complex dissociates, beta remains attached to the dimer of gamma chains, indicating that these chains contact each other in the native receptor. In rat basophilic leukemia cells--a neoplastic line of mucosal-type mast cells--all of the beta subunits expressed by the cells appeared to be associated with the high affinity receptor. However, in at least one cell line which has no high affinity receptors--a putative rat lymphoma line--beta or beta-like polypeptides were also expressed.

Molecular crowding on the cell surface

Strong steric interactions among proteins on crowded living cell surfaces were revealed by measurements of the equilibrium spatial distributions of proteins in applied potential gradients. The fraction of accessible surface occupied by mobile surface proteins can be accurately represented by including steric exclusion in the statistical thermodynamic analysis of the data. The analyses revealed enhanced, concentration-dependent activity coefficients, implying unanticipated thermodynamic activity even at typical cell surface receptor concentrations.

T cell derived IgE binding factors inhibit IgE-specific rosettes on basophilic leukaemic cells and lymphocytes

T cell derived IgE-binding factors inhibited the formation of IgE-specific rosettes on both rat basophilic leukaemic cells and lymphocytes. This was demonstrated by IgE-binding factors released from either IgE-pulsed T cells or adherent cells as well as from IgE-binding factor producing hybridomas. The different binding factors inhibited the IgE-rosettes to a different degree when tested on basophils and lymphocytes, most likely due to the higher affinity for IgE of the former cell type. The results indicate that rat basophils and mouse T cells may recognize common epitopes on the Fc portion of IgE.

Homology of the rat basophilic leukemia cell and the rat mucosal mast cell

Secretory granules of the rat basophilic leukemia (RBL-1) cell, a chemically generated tumor cell line maintained in tissue culture, were shown to stain with alcian blue but not with safranin counterstain and to have sparse, small, electron-dense granules. A Mr 25,000 protein was the major [3H]diisopropyl fluorophosphate-binding protein in extracts of RBL-1 cells. Double-immunodiffusion analysis of extracts revealed immunoreactivity for rat mast cell protease (RMCP)-II, a Mr 25,000 neutral protease present in the secretory granules of rat mucosal mast cells and cultured rat bone marrow-derived mast cells, but no immunoreactivity for RMCP-I, the predominant neutral protease of rat connective tissue mast cells. By radial immunodiffusion, there was 66.8 ng of RMCP-II per 10(6) cells. Whereas rat connective tissue mast cells stain with alcian blue and safranin and contain heparin proteoglycan, rat mucosal and rat bone marrow-derived mast cells stain with alcian blue only and contain a non-heparin proteoglycan and lesser amounts of histamine. Proliferation of rat mucosal mast cells in vivo and rat bone marrow-derived mast cells in vitro requires T-cell factors, whereas no comparable requirement has been observed for connective tissue mast cells. The transformed RBL-1 tumor cells, whose growth is independent of factors other than those present in standard tissue culture medium, has previously been shown to contain predominantly chondroitin sulfate di-B proteoglycans and low amounts of histamine. The similar histology and secretory granule biochemistry of the rat mucosal mast cells, rat culture-derived mast cell, and RBL-1 cell suggest that they comprise a single mast cell subclass distinct from the rat connective tissue mast cell.

Ca2+-dependent and phorbol ester activating phosphorylation of a 36K-dalton protein of rat basophilic leukemia cell membranes and immunoprecipitation of the phosphorylated protein with IgE-anti IgE system

Endogeneous phosphorylation of rat basophilic leukemia cell membranes was investigated. EGTA specifically inhibited the phosphorylation of a protein having an approximate molecular weight of 36,000 dalton (36K-Da protein). Phosphorylation of this protein was enhanced by phorbol-12-myristate-13-acetate in the presence of phosphatidylserine. The phosphorylated 36K-Da protein was specifically immunoprecipitated with IgE and anti IgE antibody. These results suggest that the phosphorylated 36K-Da protein is the beta-chain of the receptor for IgE and that protein kinase C is involved in the phosphorylation mechanism.