Selective role of PKCβ enzymatic function in regulating cell survival mediated by B cell antigen receptor cross-linking

Abstract

Cross-linking of the B cell antigen receptor (BCR) results in the activation of several protein tyrosine kinases leading to phospholipase C-γ2-dependent phospholipid hydrolysis and Ca 2+ mobilization, followed by activation of the protein kinase C (PKC) family members. Sustained Ca 2+ release in B lymphocytes is dependent on the membrane localization and activation of the protein tyrosine kinase BTK. Ca 2+ release is a tightly regulated process involving BTK membrane localization through its phosphorylation by PKCβ. A selective role of PKCβ in B cell signaling was first revealed by the characterization of PKCβ knockout mice, which displayed decreased B cell proliferation in response to various mitogenic stimuli. However, it is not clear whether the B cell defects displayed by the PKCβ knockout mice are due a B cell developmental defect or the scaffolding function of PKCβ, resulting in a defect in the recruitment or formation of signal transducing complex molecules. Thus, in this report we investigated the effects of pharmacologic inhibition of the catalytic function of PKCβ on B cell survival and growth. Treatment of Daudi B lymphoma cell line with a selective PKCβ inhibitor, LY333531, inhibited anti-IgM-induced phosphorylation of BTK on Ser180 in a concentration-dependent manner, which was concomitant with an increase in BTK activation, and Ca 2+ mobilization. In primary splenic B cells, LY333531 inhibited BCR-induced B cell proliferation, but did not affect basal or LPS-induced proliferation. Finally, LY333531 treatment resulted in the induction of apoptosis of anti-IgM-activated B cells, which corroborated with their inability to up-regulate pro-survival factors, Bcl-X L and Bcl-2. These results support the important and selective role of the PKCβ enzymatic function in controlling Ca 2+ release during BCR signaling leading to B lymphocyte survival and growth.