Suboptimal Cross-linking of Antigen Receptor Induces Syk-dependent Activation of p70S6 Kinase through Protein Kinase C and Phosphoinositol 3-Kinase

Abstract

Ligation of the B cell antigen receptor (BCR) induces a cascade of signaling pathways that lead to clonal expansion, differentiation, or abortive activation-induced apoptosis of B lymphocytes. BCR-mediated cross-linking induces the rapid phosphorylation of protein tyrosine kinases. However, the pathways leading to the activation of downstream serine/threonine kinases such as mitogen-activated protein kinase, p90Rsk , and p70S6 kinase (p70 S6k ) that mediate reorganization of the actin cytoskeleton, cell cycle progression, gene transcription, and protein synthesis have not been delineated. We recently demonstrated that cross-linking of BCR leads to activation of p70 S6k in B lymphocytes. In this report, we demonstrate that multiple protein tyrosine kinase-dependent signal transduction pathways induced by BCR lead to the activation of p70 S6k . These distinct pathways exhibit different thresholds with respect to the extent of receptor cross-linking required for their activation. Activation of p70S6k by suboptimal doses of anti-Ig is Sykdependent and is mediated by protein kinase C and phosphoinositol 3-kinase. Moreover, the activation of p70S6k results in phosphorylation of S6 protein which is important for ribosomal protein synthesis and may be coupled to BCR-induced protein and DNA synthesis in primary murine B cells. Ligation of the antigen receptor (BCR)1 can lead to clonal expansion, differentiation, or abortive activation-induced apoptosis of B lymphocytes. Several early signaling events induced by ligation of BCR have been described including the rapid activation of protein tyrosine kinases, calcium mobilization, and activation of downstream serine/threonine kinases (1-8). These early events are followed by cytoskeletal reorganization, induced gene expression, and increased mRNA and protein synthesis (9). The relationship between the defined early signal transduction events, intermediate signaling events, and the fate of B cells is not clear.We have been investigating the protein tyrosine kinase-dependent regulation of BCR-mediated activation of serine/threonine kinases. We demonstrated that ligation of BCR on the avian B cell line DT40 results in activation of mitogen-activated protein kinase (MAPK) and members of two families of ribosomal S6 kinases, p90Rsk and p70S6 kinase (p70 S6k ) (10). Ribosomal S6 kinases are highly conserved proteins that are critical for translational regulation particularly of genes containing poly-pyrimidine tracts in their 5Ј-untranslated regions that encode essential components of the protein synthesis apparatus (11-15). These kinases can regulate mRNA translation through phosphorylation of ribosomal S6 protein. Although both p90Rsk and p70 S6k can phosphorylate S6 in vitro, p70S6k

Proliferation and Survival of Activated B Cells Requires Sustained Antigen Receptor Engagement and Phosphoinositide 3-Kinase Activation

“…Rapamycin is known to inhibit both basal and stimulated S6K1 phosphorylation. In agreement with published results (23), BCR engagement increased S6K1 phosphorylation at 5 min (our unpublished observations) and 1 h, and this was abrogated by pretreatment with the PI3K inhibitors LY294002 or wortmannin or by the mTOR inhibitor rapamycin (Fig. 3A).…”

Rapamycin is active against B-precursor leukemiain vitroandin vivo, an effect that is modulated by IL-7-mediated signaling

“…Rapamycin inhibits secretion of soluble CD23, an autocrine B cell growth factor (55). Crosslinking of B cell receptor leads to p70 S6 kinase activation, triggering protein synthesis by activation of ribosomal proteins (56). These data suggest that rapamycin may affect early B lineage cells, but an effect of mTOR inhibitors and IL-7-mediated signaling on early precursor B cells or on B cell progenitor malignancies has not been described.…”

The tyrosine kinases Syk and Lyn exert opposing effects on the activation of protein kinase Akt/PKB in B lymphocytes

“…However, the regulation of the downstream serine/threonine kinases in B cells that may mediate cell survival, reorganization of the actin cytoskeleton, cell cycle progression, gene transcription, and protein translation is poorly understood. Several serine/ threonine kinases are activated in response to ligation of B cell receptor including mitogen-activated protein kinases (MAPK) (6,7), two ribosomal S6 kinases, p90Rsk and p70S6k (8), and various isoforms of protein kinase C. We recently established that the signaling pathways leading to the activation of p90Rsk, p70S6k and MAPK could be distinguished based on their requirements for the activation of upstream protein tyrosine kinases (8,9).…”