ZAP-70 enhances B-cell–receptor signaling despite absent or inefficient tyrosine kinase activation in chronic lymphocytic leukemia and lymphoma B cells

Abstract

IntroductionB-cell chronic lymphocytic leukemia (CLL) is a malignancy of mature monoclonal CD5 ϩ B cells that typically express low levels of surface IgM. The clinical course is highly variable, characterized by progressive disease and unfavorable prognosis in approximately half the cases and a relatively indolent disease and normal life span in the remaining patients. 1 Significant associations have recently been observed between the clinical course and certain features of the B-cell receptor (BCR), indicating that antigen stimulation and signaling may play an important role in the pathogenesis of this disease. 2,3 In particular, the leukemic cells of patients with progressive disease typically display BCRs encoded by unmutated immunoglobulin variable heavy-chain genes (IgV H ) and express the protein tyrosine kinase (PTK) ZAP-70, which plays an essential role in antigen receptor signaling in T cells. In contrast, CLL B cells from most patients with stable disease express mutated IgV H genes and lack ZAP-70. [4][5][6][7][8] The 2 subsets also appear to differ in their capacity to transmit BCR-derived stimuli because IgM ligation induced stronger signaling responses in CLL B cells from patients with a poor prognosis. [9][10][11] ZAP-70 is a key signaling molecule in T cells, where it couples the antigen-activated T-cell receptor (TCR) to downstream signaling pathways. 12 It is structurally homologous to Syk, a PTK that is involved in proximal BCR signaling. In normal B cells, stimulation of the BCR by antigen leads to phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within the cytoplasmic tails of Ig␣ and Ig␤. Syk is subsequently recruited to these motifs and, following its activation by tyrosine phosphorylation, propagates the signal by activation of downstream signaling molecules, such as phosphatidylinositol 3-kinase (PI3K) and phospholipase C␥2 (PLC␥2). PI3K generates the key second messenger phosphatidylinositol-3,4,5-triphosphate, which recruits other BCR-signaling molecules to the membrane and activates the kinase Akt. Activation of PLC␥2 leads to the release of intracellular Ca 2ϩ and activation of protein kinase C (PKC). The signaling cascade proceeds with activation of mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinase (ERK), c-JUN NH 2 -terminal kinase (JNK) and p38 MAPK, and transcription factors, including nuclear factor-B (NF-B) and nuclear factor of activated T cells (NFAT). 13 The balance among the activation of these signaling molecules determines the B-cell response, which can include proliferation, survival, differentiation, or cell death.The functional significance of ZAP-70 expression in CLL B cells is still unclear. Experiments with avian lymphoma B cells have shown that ZAP-70 can restore some BCR-signaling events when Syk is not expressed, indicating a certain degree of functional homology between the 2 PTKs. 14 More recently, ZAP-70 was shown to undergo tyrosine phosphorylation and to associate with the BCR complex in antige...

LCK Is an Important Mediator of B-Cell Receptor Signaling in Chronic Lymphocytic Leukemia Cells

“…However, using a phospho-ZAP70 antibody (anti-pY 319 -ZAP70) that also cross reacts with phospho-Syk at an analogous site (pY 352 ), we found that although LCK is involved in the induction of Syk phosphorylation following BCR cross-linking of CLL cells, ZAP70 remains unphosphorylated. This result agrees with a published report showing that BCR cross-linking induces Syk but not ZAP70 phosphorylation in CLL cells (19). That LCK is directly able to phosphorylate Syk on Y 352 in BCR-stimulated CLL cells is not shown by our data.…”

“…Phosphorylation at this residue acts to enhance the catalytic function of ZAP70, as well as play a scaffolding role within ZAP70 by providing a binding site for LCK and other proteins such as PI3K, Grb2, and CrkII (54,55). The malignant cells from a subset of patients with CLL also express ZAP70 (17) where it functions to enhance BCR signaling (19,20). Thus, ZAP70 may be a target of LCK in CLL cells that express this protein.…”

“…Work investigating the role of ZAP70 in CLL cells has shown that BCR signaling is enhanced in cells expressing high levels of this kinase (19,20), partially explaining why ZAP70 expression is associated with poor prognosis. Another TCR signaling protein that may have a role in CLL cells is the SFK LCK.…”

Divergence in CD19-Mediated Signaling Unfolds Intraclonal Diversity in Chronic Lymphocytic Leukemia, Which Correlates with Disease Progression

“…These data emphasize the complexity of the crosstalk between the sIgM and CD19, in which BCR activation is capable of increasing CD19 surface expression and signaling. One of the major signaling responses triggered by the BCR is the phosphorylation of Akt (33). As shown in Fig.…”

Redundant role for Zap70 in B cell development and activation

“…Alternatively, the expression of Zap70 may be important for the pathology of U-CLL. It has been shown that expression of Zap70 in U-CLL cells enhances BCR signaling, and this may contribute to the more aggressive course of the disease [25][26][27].…”