I. Anti-μ antibody stimulates the phosphatidylinositol cycle and immunoglobulin secretion in a human lymphoblastoid B-cell line, LA350

Abstract

Within 5 min of the binding of anti-μ antibody (anti-μ) to surface IgM on LA350, a human lymphoblastoid B-cell line, there was a significantly enhanced incorporation of 32P into the phosphatidic acid (PA) and phosphatidylinositol (PI) fractions of cellular phospholipids and the magnitude of the early increase in PA was twice as great as that in PI. This anti-μ-enhanced incorporation of 32P into PA and PI required the binding of a divalent form of antibody (IgG or F(ab′) 2), was blocked by coincubation with micromolar concentrations of soluble IgM, was decreased by incubation of cells at temperatures lower than 37 °C, and was inhibited by coincubation with millimolar concentrations of dibutyryl cyclic AMP and theophylline. Similar incorporation studies with [ 3H]inositol demonstrated a selective and significant increase in labeling of PI. In LA350 labeled with [ 3H]inositol for 30 hr (equilibrium) and acutely stimulated by anti-μ, specific hydrolysis of phosphorylated PI (PI 4,5-bisphosphate) was measured by the significantly increased release at 15 min of radioactive inositol 1,4,5-trisphosphate, inositol 1,4 bisphosphate, and inositol 1-phosphate. The release of these inositol phosphates was significantly augmented by coincubation with 0.01 M LiCl which prevented their simultaneous enzymatic degradation. All of these findings are consistent with an activation of a linked series of metabolic events known as the PI cycle. In similar cell cultures anti-μ significantly stimulated the secretion of IgM by LA350 as measured at 48 hr in a reverse hemolytic plaque assay. Two other IgM-bearing human lymphoblastoid B-cell lines which gave no evidence of turnover of 32P in PA and PI in response to binding by anti-μ likewise failed to enhance their secretion of IgM. We conclude that the binding of surface IgM on LA350 by anti-μ results in the generation of a transmembrane signal which causes a rapid activation of the PI cycle which itself may play a role in the subsequent increase in IgM secretion.