Purification and characterization of human Syk produced using a baculovirus expression system

Expression of Syk and MAP4 proteins in ovarian cancer

Purpose

We have previously reported on the prognostic importance of the calpain family of proteins in ovarian cancer, especially calpain-2. Spleen tyrosine kinase (Syk) phosphorylates a variety of cytoskeletal proteins with studies suggesting potential interactions between Syk and conventional calpains. Microtubule-associated protein 4 (MAP4) has been reported to be regulated by Syk.

Methods

The current study assessed Syk and MAP4 protein expression, by immunohistochemistry on a tissue microarray comprised of cores from primary ovarian carcinomas (n = 575), to evaluate associations with patient clinical outcomes and other clinicopathological factors and sought to determine whether there were any correlations between the expression of Syk, MAP4 and the calpain system.

Results

MAP4 expression was significantly associated with ovarian cancer histological subtype (P < 0.001), stage (P = 0.001), grade (P < 0.001) and residual tumour (P = 0.005). Despite this finding, we found no significant association existing between MAP4 expression and overall survival. Syk expression was also found significantly associated with histological subtype (P < 0.001). Syk seems to play a contradictory role with respect to tumour progression: low cytoplasmic Syk expression was significantly associated with low stage (P = 0.013), and low nuclear Syk expression with chemo-resistance in patients treated with taxane-containing therapy (P = 0.006). Interestingly, despite the lack of association in the whole cohort, high nuclear Syk expression was significantly associated with better overall survival in certain subgroups (P = 0.001).

Conclusions

The current study indicates a lack of correlation between calpain-2 expression and Syk and MAP4. Syk, MAP4 and calpain-1 appeared to significantly correlate with each other in the whole cohort, with calpain-1 being more highly associated with MAP4 and Syk in mucinous carcinomas. Overall, the current results suggest that Syk, MAP4, and calpain-1 expression are correlated with each other and these proteins may be involved in early stages of tumour spread.

Electronic supplementary material

The online version of this article (10.1007/s00432-019-02856-9) contains supplementary material, which is available to authorized users.

Selective inhibition of spleen tyrosine kinase (SYK) with a novel orally bioavailable small molecule inhibitor, RO9021, impinges on various innate and adaptive immune responses: implications for SYK inhibitors in autoimmune disease therapy

Introduction

Spleen tyrosine kinase (SYK) is a key integrator of intracellular signals triggered by activated immunoreceptors, including Bcell receptors (BCR) and Fc receptors, which are important for the development and function of lymphoid cells. Given the clinical efficacy of Bcell depletion in the treatment of rheumatoid arthritis and multiple sclerosis, pharmacological modulation of Bcells using orally active small molecules that selectively target SYK presents an attractive alternative therapeutic strategy.

Methods

A SYK inhibitor was developed and assayed in various in vitro systems and in the mouse model of collagen-induced arthritis (mCIA).

Results

A novel ATP-competitive inhibitor of SYK, 6-[(1R,2S)-2-Amino-cyclohexylamino]-4-(5,6-dimethyl-pyridin-2-ylamino)-pyridazine-3-carboxylic acid amide, designated RO9021, with an adequate kinase selectivity profile and oral bioavailability, was developed. In addition to suppression of BCR signaling in human peripheral blood mononuclear cells (PBMC) and whole blood, FcγR signaling in human monocytes, and FcϵR signaling in human mast cells, RO9021 blocked osteoclastogenesis from mouse bone marrow macrophages in vitro. Interestingly, Toll-like Receptor (TLR) 9 signaling in human Bcells was inhibited by RO9021, resulting in decreased levels of plasmablasts, immunoglobulin (Ig) M and IgG upon B-cell differentiation. RO9021 also potently inhibited type I interferon production by human plasmacytoid dendritic cells (pDC) upon TLR9 activation. This effect is specific to TLR9 as RO9021 did not inhibit TLR4- or JAK-STAT-mediated signaling. Finally, oral administration of RO9021 inhibited arthritis progression in the mCIA model, with observable pharmacokinetics (PK)-pharmacodynamic (PD) correlation.

Conclusions

Inhibition of SYK kinase activity impinges on various innate and adaptive immune responses. RO9021 could serve as a starting point for the development of selective SYK inhibitors for the treatment of inflammation-related and autoimmune-related disorders.

Initiation of translation at an upstream non-AUG codon accounting for N-terminally extended minor forms of recombinant proteins expressed in insect cells

When the 34 kDa kinase domain of human spleen tyrosine kinase (Syk-KD) was expressed as a C-terminally His-tagged protein in baculovirus-infected Sf-21 insect cells, the purified protein included two forms that migrated slightly differently in SDS-polyacrylamide gel electrophoresis. Intact mass analysis and LC-MS/MS peptide mapping showed that the major and faster-migrating product had the intended amino-acid sequence and 0-6 phosphorylations. This material accounted for about 95% of the purified protein. The minor product was Syk-KD with a 26 amino-acid N-terminal extension. The result suggested the existence of an upstream alternative site for the initiation of translation, and this proved to be an ACG codon derived from the pBacPAK9 vector used to express Syk-KD. The ACG codon was preceded and followed by Kozak-type sequence elements (a purine in the -3 position and a G in the +4 position) that would have enhanced the viability of initiation at ACG. The initiating amino-acid residue was Met for both minor and major products, and both forms of the protein were α-N-acetylated. For the minor product, protein intact mass analysis and peptide mapping both gave results in agreement with the sequence predicted from the DNA. A similar result with the same underlying cause was obtained with insect cell expression of full-length Syk. It appears that similar results are possible whenever this vector is used.

The Src, Syk, and Tec family kinases: distinct types of molecular switches

The Src, Syk, and Tec family kinases are three of the most well characterized tyrosine kinase families found in the human genome. Members of these kinase families function downstream of antigen and F(c) receptors in hematopoietic cells and transduce signals leading to calcium mobilization, altered gene expression, cytokine production, and cell proliferation. Over the last several years, structural and biochemical studies have begun to uncover the molecular mechanisms regulating activation of these kinases. It appears that each kinase family functions as a distinct type of molecular switch. This review discusses the activation of the Src, Syk, and Tec kinases from the perspective of structure, phosphorylation, allosteric regulation, and kinetics. The multiple factors that regulate the Src, Syk, and Tec families illustrate the important role played by each of these kinases in immune cell signaling.

Steady state kinetics of spleen tyrosine kinase investigated by a real time fluorescence assay

Spleen tyrosine kinase (Syk) is a cytoplasmic tyrosine kinase that plays an important signaling role in several types of immune cells. To improve our understanding of the enzymology and activation mechanism of Syk, we characterized the steady state kinetics of Syk substrate phosphorylation. A new real time fluorescence kinase assay was employed that utilizes a nonnatural amino acid in the peptide substrate which undergoes an enhancement in fluorescence following phosphorylation. Characterizing the steady state kinetics using a Syk kinase domain construct [Syk(360-635)] revealed that Syk employs a ternary complex kinetic mechanism involving little cooperativity between substrate binding sites and a Km(ATP) of 36 +/- 5 microM and a Km(peptide substrate) of 4.4 +/- 0.9 microM. The order of substrate binding was determined to be either random or ordered with ATP binding first, as determined in substrate analogue inhibitor studies. Utilizing the real time capabilities of the fluorescence assay, we established that Syk demonstrates no lag phase in product formation. Furthermore, a Syk mutant lacking tyrosine in the activation loop (Syk Y525F,Y526F) exhibited activity identical to that of wild-type Syk. These two findings indicate that autophosphorylation of the activation loop of Syk does not regulate Syk(360-635) activity. We also compared the activity of Syk(360-635) to that of full-length Syk and revealed that Syk(360-635) is 10-fold more active, suggesting that residues outside the catalytic domain of Syk suppress kinase activity. The findings presented here provide the first kinetic description of the Syk enzyme mechanism.

Calpains: a physiological regulator of the endothelial barrier?

The intracellular protease calpain, abundant in endothelial cells (EC), is assumed to be inactive under physiological conditions but may account for Ca2+ -linked pathophysiological events. However, nonstimulated EC contained autolyzed, activated calpain. Adding 12-48 microM calpain inhibitor I (CI) or 0.5-1 microM of the novel, membrane-permeable conjugate of calpastatin peptide-penetratin (CPP) caused rapid rounding and retraction of cultured EC (phase contrast, capacitance) and translocation of Syk, Rac, and Rho to the membrane, signifying activation upon inhibition of calpain. Isolated hearts (guinea pig) perfused with 12 microM CI or 0.5 muM CPP developed coronary leak. We conclude that calpain is constitutively active in EC and regulates vascular permeability by governing cell-cell attachment.

Targeting Syk as a treatment for allergic and autoimmune disorders

Recent advances in our understanding of allergic and autoimmune disorders have begun to translate into novel, effective and safe medicines for these common maladies. Examples include an anti-IgE monoclonal antibody recently approved for severe asthmatics and the TNF-alpha antagonists that have demonstrated their ability to suppress rheumatoid arthritis, Crohn's disease and other chronic inflammatory processes. However, protein therapies are difficult and expensive to develop, manufacture and administer. Clearly, there is also a need for small-molecule inhibitors of novel targets that have safe and effective characteristics. Syk is an intracellular protein tyrosine kinase that was discovered 15 years ago as a key mediator of immunoreceptor signalling in a host of inflammatory cells including B cells, mast cells, macrophages and neutrophils. These immunoreceptors, including Fc receptors and the B-cell receptor, are important for both allergic diseases and antibody-mediated autoimmune diseases and thus pharmacologically interfering with Syk could conceivably treat these disorders. In addition, as Syk is positioned upstream in the cell signalling pathway, therapies targeting Syk may be more advantageous relative to drugs that inhibit a single downstream event. Syk inhibition during an allergic or asthmatic response will block three mast cell functions: the release of preformed mediators such as histamine, the production of lipid mediators such as leukotrienes and prostaglandins and the secretion of cytokines. In contrast, commonly used antihistamines or leukotriene receptor antagonists target only a single mediator of this complex cascade. Despite its expression in platelets and other non-haematopoietic cells, the role of Syk in regulating vascular homeostasis and other housekeeping functions is minimal or masked by redundant Syk-independent pathways. This suggests that targeting Syk would be an optimal approach to effectively treat a multitude of chronic inflammatory diseases without undue toxicity.

WIP regulates signaling via the high affinity receptor for immunoglobulin E in mast cells

Wiskott-Aldrich syndrome protein-interacting protein (WIP) stabilizes actin filaments and is important for immunoreceptor-mediated signal transduction leading to actin cytoskeleton rearrangement in T and B cells. Here we report a role for WIP in signaling pathways downstream of the high affinity receptor for immunoglobulin (Ig)E (FcepsilonRI) in mast cells. WIP-deficient bone marrow-derived mast cells (BMMCs) were impaired in their capacity to degranulate and secrete interleukin 6 after FcepsilonRI ligation. Calcium mobilization, phosphorylation of Syk, phospholipase C-g2, and c-Jun NH2-terminal kinase were markedly decreased in WIP-deficient BMMCs. WIP was found to associate with Syk after FcepsilonRI ligation and to inhibit Syk degradation as evidenced by markedly diminished Syk levels in WIP-deficient BMMCs. WIP-deficient BMMCs exhibited no apparent defect in their subcortical actin network and were normal in their ability to form protrusions when exposed to an IgE-coated surface. However, the kinetics of actin changes and the cell shape changes that follow FcepsilonRI signaling were altered in WIP-deficient BMMCs. These results suggest that WIP regulates FcepsilonRI-mediated mast cell activation by regulating Syk levels and actin cytoskeleton rearrangement.