Getting Syk: spleen tyrosine kinase as a therapeutic target

Abstract

Spleen tyrosine kinase (Syk) is a cytoplasmic protein tyrosine kinase well known for its ability to couple immune cell receptors to intracellular signaling pathways that regulate cellular responses to extracellular antigens and antigen-immunoglobulin (Ig) complexes of particular importance to the initiation of inflammatory responses. Thus, Syk is an attractive target for therapeutic kinase inhibitors designed to ameliorate the symptoms and consequences of acute and chronic inflammation. Its more recently recognized role as a promoter of cell survival in numerous cancer cell types ranging from leukemia to retinoblastoma has attracted considerable interest as a target for a new generation of anticancer drugs. This review discusses the biological processes in which Syk participates that have made this kinase such a compelling drug target.

Keywords: allergic asthma; fostamatinib; leukemia; protein kinase inhibitor; rheumatoid arthritis; tyrosine phosphorylation.

Figure 1

Domain organization of Syk. Syk contains an N-terminal, tandem pair of SH2 domains connected by linker A and separated by a linker B region from the catalytic (kinase) domain. Aromatic residues present in linker A, linker B (tyrosines 348 and 352; blue circles), and the catalytic domain interact to form a “linker-kinase sandwich” (dotted circle) that stabilizes the inactive form of the kinase.

Figure 2

Syk couples FcεRI, the high affinity receptor for IgE, to degranulation in mast cells. Following aggregation of FcεRI by IgE-antigen complexes (not pictured), Lyn initiates the phosphorylation of ITAM tyrosines leading to the recruitment of Syk to the receptor in an interaction mediated by its tandem pair of SH2 domains. Syk becomes phosphorylated in trans by Lyn and by other Syk molecules recruited to the clustered receptor. Active Syk phosphorylates adaptor proteins LAT and then SLP-76, recruited to LAT via GADS (G), to generate binding sites for PLCγ and Btk (not pictured). The phosphorylation of PLCγ by Btk and Syk leads to its activation and the hydrolysis of phosphoinositide 4,5-bisphosphate (PIP₂) to generate the second messengers diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). The binding of IP₃ to IP₃ receptors on the ER triggers the release of calcium from intracellular stores leading to the entry of extracellular calcium to trigger the release of inflammatory mediators stored in intracellular granules.

Figure 3

Examples of Syk kinase inhibitors. R788/fostamatinib is the produrg form of R406 and is the best characterized of the Syk inhibitors in patient studies. R343 is an inhaled Syk inhibitor designed for the treatment of allergic asthma and R112 an intranasal inhibitor tested for the alleviation of seasonal allergies. P505-15 is an orally available, highly selective Syk inhibitor. R788, R406, R343 and R112 were developed by Rigel Pharmaceuticals and P505-15 by Portola Pharmaceuticals. All are ATP-binding site inhibitors. Piceatannol, a natural stilbene, is a relatively low affinity inhibitor of Syk, but one that binds competitively with the phosphoacceptor substrate.

Figure 4

Syk couples the BCR to the activation of mTOR and stabilization of the antiapoptotic protein Mcl-1. Binding of antigen to the surface IgM (sIgM) component of the BCR complex leads to the phosphorylation of ITAM tyrosines on the cytoplasmic tails of Igα (CD79a) and Igβ (CD79b) and the recruitment of Syk. Activated phosphorylates adaptor proteins such as CD19 and BCAP to create docking sites for the SH2 domain of the p85 subunit of phosphoinositide 3-kinase (PI3K), which is associated with the p110 catalytic subunit. PI3K catalyzes the formation within the plasma membrane of phosphoinositide 3,4,5-trisphosphate (PIP₃), a ligand for the plexkstrin homology (PH) domain of Akt. Activated Akt catalyzes the phosphorylation of and inhibition of the tuberous sclerosis complex, a negative regulator of mammalian target of rapamycin (mTOR). Akt also phosphorylates and inhibits glycogen synthase kinase 3 (GSK3), blocking the phosphorylation of Mcl-1 and thus enhancing its stability.