Does B cell receptor signaling in chronic lymphocytic leukaemia cells differ from that in other B cell types?

Abstract

Chronic lymphocytic leukaemia (CLL) is an incurable malignancy of mature B cells. CLL is important clinically in Western countries because of its commonality and because of the significant morbidity and mortality associated with the progressive form of this incurable disease. The B cell receptor (BCR) expressed on the malignant cells in CLL contributes to disease pathogenesis by providing signals for survival and proliferation, and the signal transduction pathway initiated by engagement of this receptor is now the target of several therapeutic strategies. The purpose of this review is to outline current understanding of the BCR signal cascade in normal B cells and then question whether this understanding applies to CLL cells. In particular, this review studies the phenomenon of anergy in CLL cells, and whether certain adaptations allow the cells to overcome anergy and allow full BCR signaling to take place. Finally, this review analyzes how BCR signals can be therapeutically targeted for the treatment of CLL.

Figure 1

Induction of proximal BCR signaling. Illustration of the most proximal signals initiated during BCR engagement. (a) Antigen engagement of the BCR induces Lyn-mediated phosphorylation of CD79 on tyrosine residues within and outside the ITAM motif. This attracts adaptor molecules such as LAB, Nck, BCAP, and BLNK, as well as the tyrosine kinase Syk. (b) Syk binding to tyrosine phosphorylated CD79 induces its activation, and it phosphorylates BLNK, BCAP, and LAB. Phospho-BCAP attracts and activates PI3Kδ and converts PIP₂ to PIP₃. The presence of PIP₃ on the plasma membrane attracts PDK1 and Akt; PDK1 phosphorylates and activates Akt, and this is followed by a second phosphorylation event by mTorc2. PIP₃ on the plasma membrane also attracts Btk, which then binds to phospho-BLNK and exposes a phosphorylation site for Syk leading to autophosphorylation and full activation of Btk [17]. Phospho-BLNK also acts as a scaffold for PLCγ2. (c) Active Btk phosphorylates and activates PLCγ2 and catalyzes the conversion of PIP₂ to DAG and IP₃ (which acts on ER to release intracellular Ca²⁺). DAG and Ca²⁺ then act to activate PKCβ. (d) Phospho-BLNK and phospho-LAB serve as scaffolds for Vav, which is also attracted to the plasma membrane by the presence of PIP₃. Syk is then able to phosphorylate and activate Vav, which then acts as a guanine exchange factor to convert GDP-Rac1/2 to GTP-Rac1/2. This results in cytoskeletal changes and induction of BCR internalization.

Figure 2

Signal integration following BCR engagement. Schematic of distal signaling events following BCR engagement. (a) Phospho-BLNK serves as a scaffold for Grb2, and this acts to prime activation of Sos. Active PKCβ phosphorylates RasGRP3 to facilitate its activation by DAG. The guanine exchange factor function of RasGRP3 converts GDP-Ras to GTP-Ras, which then acts to augment the guanine exchange factor function of Sos. Together RasGRP3 and Sos produce sufficient GTP-Ras to power activation of the MAPK cascade illustrated here by Raf, MEK, and ERK. (b) Active PKCβ acts to phosphorylate CARMA1 to induce assembly of the CARMA1-Bcl10-MALT1 complex. This allows activation of TAK1 which facilitates activation of the NFκB pathway by phosphorylating IKK and of the JNK pathway by phosphorylating MKK4 and MKK7. Active PKCβ also acts in a feedback inhibition loop by phosphorylating Btk on serine 180. This catalyzes the removal of Btk from the plasma membrane away from its substrate, PLCγ2.

Figure 3

Possible roles of ZAP70, RhoH, and HS1 in BCR signaling in CLL cells. Schematic showing the potential role of ZAP70 as a scaffolding protein for Lck, RhoH, talin, and HS1 at the immunological synapse formed when the BCR is cross-linked on CLL cells. HS1 becomes hyperphosphorylated by Lyn, Syk, and Lck and plays a role with RhoH in regulating cytoskeletal remodeling.

Figure 4

Points of inhibition for ibrutinib, idelalisib, fostamatinib, and Lck-i in the BCR signaling pathway. Illustration of proximal signals initiated during BCR engagement and points where well described inhibitors act. (a) Lck-i acts to inhibit the most proximal signaling event of SFK-mediated phosphorylation of tyrosine residues within CD79. Ibrutinib acts to inhibit the ability of Btk to phosphorylate and activate PLCγ2. (b) Fostamatinib acts to inhibit Syk kinase activity as well as that of Lyn. Idelalisib works to inhibit PI3Kδ to limit the formation of PIP₃ and has the effect of blocking membrane interaction of signaling proteins containing PH domains such as PDK1, Akt, and Btk.