Calreticulin is an interleukin‐3‐sensitive calcium‐binding protein in human basophil leukocytes

Abstract

IL-3 enhances basophil histamine release upon stimulation with any known secretagogue. The molecular mechanism behind this regulation is not known, although some observations suggest that IL-3 modulates the calcium part of the signal transduction mechanism. The inhibitory action of glucocorticoids on basophils can be reversed by stimulation with IL-3. Calcium-binding proteins in the basophil cell line KU812 were identified by two-dimensional gel electrophoresis, Calcium-overlay assay, N-terminal sequence analysis, and mass spectometry. The presence of the same proteins in purified human basophil leukocytes was established by comigration of KU812 and human basophil proteins on the two-dimensional gels. The expression of the calcium-binding proteins in the absence and presence of IL-3 and/or anti-IgE was determined by densitometric measurement of the spots on the two-dimensional gels. Calreticulin was identified on the two-dimensional gel of KU812 proteins. A protein with exactly the same migration pattern was found on the gels of proteins from purified human basophils. Immunoblotting with a specific antihuman calreticulin antibody confirmed that this protein was calreticulin. Subsequent analysis showed that the expression of calreticulin in the basophils is upregulated twofold upon stimulation with rhIL-3, even in doses below those needed for enhancement of histamine release. The expression of calreticulin in human basophil leukocytes is regulated by IL-3. Calreticulin is known to modulate IP3-dependent Ca2+ influx in different cell systems, and calreticulin overexpression inhibits steroid-induced transcriptional activation. Therefore, modulation of calreticulin expression may be one mechanism by which IL-3 exerts its effects on human basophils.