Clinical Topics & News

Signaling Pathways and Novel Inhibitors in Chronic Lymphocytic Leukemia

Fed Pract. 2014 August;31(8):18-22

Author(s):

Understanding signaling pathways in chronic lymphocytic leukemia (CLL) is critical to the development of therapeutic agents to treat this disease. Ibrutinib and idelalisib are therapeutic agents that block signaling pathways and, therefore, inhibit the growth of CLL cells.

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References

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Chronic lymphocytic leukemia (CLL) is a common hematological malignancy in the U.S. with 15,000 new patients diagnosed each year.¹ This leukemia is frequently diagnosed in veterans since it is more commonly seen in an elderly male population. The disease is characterized by a slow accumulation of mature B cells that are functionally incompetent and resist apoptosis. CLL has an indolent clinical course, but about 60% to 70% of patients require treatment. The disease also runs a variable course, and a number of genetic abnormalities and prognostic markers have been defined to subclassify CLL patients and prognosticate.^2-4 This article reviews important CLL signaling pathways and novel therapeutic agents in this leukemia.

Signaling Pathways

B-Cell Receptor Signaling

The B-cell receptor (BCR) signaling is the major signaling pathway in CLL, because it defines clinical, biologic, and prognostic characteristics of the disease.⁵ The BCR is composed of a surface transmembrane immunoglobulin that binds the antigen with CD79 alpha and beta chains. The activation of BCR results in the formation of a signaling complex or signalosome, which includes Lyn, Syk, BTK, and ZAP-70, among other components that assemble with other adaptor proteins (Figure). This assembly of proteins occurs on the cytoplasmic tails of immunoglobulin chains on regions called immunoreceptor tyrosine-based motifs (ITAMs).

With the assembly of this signaling complex, BCR stimulates a number of downstream pathways, such as phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), protein kinase C, nuclear factor-κB (NFκB), and extracellular signal-regulated kinases (ERKs) (Figure). Activation of these pathways results in cell proliferation, resistance to apoptosis, increased cell motility and migration. Recent studies have identified additional novel components of this signaling complex, including a guanine nucleotide exchange factor (GEF) RASGRF1. This GEF is activated by BCR signaling and, in turn, stimulates the ERK pathway by increasing the production of active GTP-bound Ras.⁶

The ability of BCR to activate a number of downstream signaling pathways makes it a highly relevant and investigated pathway in this leukemia. Inhibitors have been developed and/or identified against a number of signalosome components to block the BCR signaling.⁷ Syk and Lyn are Src kinases, and their phosphorylation is one of the initial events of BCR signaling. Syk is overexpressed in CLL specimens, and Syk inhibitors (R406 and P505-15, also known as PRT062607) have shown activity in CLL.^8,9 Dasatinib is a Src inhibitor that also shows activity in CLL specimens and is being studied in combination with chemotherapy drugs in refractory CLL patients.¹⁰

BTK, a component of the BCR signalosome, is required for BCR function, and loss of its function is seen in X-linked agammaglobulinemia. PCI-32765 (ibrutinib) is an oral BTK inhibitor that irreversibly inactivates this kinase and has been approved for clinical use in CLL patients.^11,12 Another signaling pathway activated by BCR is the PI3K, and a promising inhibitor (CAL-101) blocks its activity in CLL specimens.¹³ Investigative work has identified that the delta isoform of PI3K p110 is highly expressed in B cells and lymphocytes.¹⁴ This is a catalytic subunit of a class I PI3K with a role in BCR signaling. A selective inhibitor GS-1101 (CAL-101) is able to block PI3K signaling in CLL specimens and inhibits Akt phosphorylation and other downstream effectors along with induction of apoptosis.¹⁵ The clinical data with BTK and PI3K inhibitors will be discussed later in this review.

CLL and the Microenvironment

Interactions between CLL cells and the microenvironment allow CLL cells to thrive in certain niche environments.^16,17 Interaction mainly occurs via bone marrow stromal cells and nurselike cells (NLCs), which evolve from monocytes (Figure). These interactions can be divided into 2 groups. First, CLL cell growth is supported by a number of chemokine receptor-ligand interactions. CXCR4 is the receptor for CXCL12 (SDF-1) that stimulates chemotaxis and tissue homing. Another chemokine is CXCL13, which acts via its receptor CXCR5 and is involved in chemotaxis and activation of other kinases. Second, NLCs also support CLL cells by expressing TNF family members BAFF and APRIL, which interact with their receptors and activate the NFκB pathway.

Leukemic cells also express VLA-4 integrins, which further their support adhesion to the stromal cells and predict for an aggressive phenotype. Specific inhibitors that block the stimulation by chemokines and cytokines are not yet available; however, one can envision that this class of inhibitors will decrease the chemoresistance of leukemic cells and will be used in conjunction with other chemotherapy agents. Interestingly, inhibitors that block BCR-mediated signaling (BTK and PI3K inhibitors) also inhibit signaling via the microenvironment and chemokines.

Wnt-β-catenin Pathway

Wnt signaling affects developmental pathways, and its aberrant activation has major oncogenic effects as well. This pathway is activated in CLL as these leukemic cells express high levels of Wnt and frizzled along with epigenetic downregulation of Wnt pathway antagonist genes, including secreted frizzled-related protein (SFRP) family members and WIF1 (Figure).^18-20 The binding of Wnts to their cognate receptors results in inhibition of GSK3β phosphorylation and stabilization of β-catenin, which then translocates to the nucleus and interacts with lymphoid-enhancing (LEF) and T-cell transcription factors to activate transcription of Wnt-target genes. Lack of E-cadherin expression in CLL cells also results in an increase in translocation of β-catenin and upregulation of the Wnt pathway.²⁰