Loss of syk kinase during IgE-mediated stimulation of human basophils

Ongoing secretion from human basophils is a balance of activation and deactivation events. Recent studies have focused on downregulatory steps that appear to modify the presence of the activated state of various signaling molecules. We now examine downregulation regulated by mechanisms related to proteasome processing. To determine the long-term effects of FcεRI aggregation on expression of syk kinase. Peripheral blood basophils were examined for changes in the expression of syk kinase after stimulation with optimal and suboptimal stimulation. Stimulation results in a 20% loss of syk in 1 hour and an 80% loss of syk in longer incubations (>18 hours). Loss of syk in this time frame can occur at levels of stimulation that do not result in observable mediator release. Loss of syk occurs after stimulation with either anti-IgE antibody or antigen. Activation is shown to result in c-Cbl phosphorylation, and its association with syk and immunoblotting reveals the appearance of a ladder of syk species with molecular weights that are consistent with ubiquitylation of syk. Stimulation in the presence of a proteasome inhibitor such as lactacystin A results in the sustained presence of very high-molecular-weight ubiquitylated species, although it does not alter the presence of the syk ladder. Although the loss of syk is probably too slow to account for downregulation of ongoing secretion of histamine or leukotriene C4 release, it may lead to longer-term alterations in basophil function that explain characteristics of clinical procedures like rapid drug desensitization.