Defective protein phosphorylation and Ca2+ mobilization in a low secreting variant of the rat basophilic leukemia...

High and low secreting variants of the rat basophilic leukemia cell line represent powerful tools to study the molecular basis of stimulus/secretion coupling via the high affinity receptor (Fc epsilon R1) complex for immunoglobulin E since an identification of the differences between these subclones may produce important information concerning the signaling pathways involved. A comparison between a variant supporting high mediator secretion (> 50%) and one with a 10-fold reduced response to antigen shows that the latter is associated with a defect in threonine and tyrosine phosphorylation of the subunits of the Fc epsilon R1 complex. The delayed onset and reduced mediator release in the low secretor facilitated a slow motion study of the early events following receptor activation. It showed that tyrosine phosphorylation of a 72-kDa protein is an early event preceding threonine and subsequent tyrosine phosphorylation of the gamma-chain. This points to the activation of both protein-tyrosine kinases and protein kinase(s) C as early events in signal transduction. The retarded onset and low intensity of phosphorylation in the low secreting variant is associated with reduced levels of inositol phosphate production, and this and the lack of the Ca2+ mobilization from intracellular stores indicate a defect upstream of teh activation of phospholipase C.