Syk expression and novel function in a wide variety of tissues

Identification of natural product-based effective inhibitors of spleen tyrosine kinase (SYK) through virtual screening and molecular dynamics simulation approaches

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase that plays an essential role in signal transduction across different cell types. In the context of allergy and autoimmune disorders, it is a crucial regulator of immune receptor signaling in inflammatory cells such as B cells, mast cells, macrophages, and neutrophils. Developing SYK kinase inhibitors has gained significant interest for potential therapeutic applications in neurological and cancer-related conditions. The clinical use of the most advanced SYK inhibitor, Fostamatinib, has been limited due to its unwanted side effects. Thus, a more targeted approach to SYK inhibition would provide a more comprehensive treatment window. In this study, we used a virtual screening approach to identify potential SYK inhibitors from natural compounds from the IMPPAT database. We identified two compounds, Isolysergic acid and Michelanugine, which showed strong affinity and specificity for the SYK binding pocket. All-atom molecular dynamics (MD) simulations were also performed to explore the stability, conformational changes, and interaction mechanism of SYK in complexes with the identified compounds. The identified compounds might have the potential to be developed into promising SYK inhibitors for the treatment of various diseases, including autoimmune disorders, cancer, and inflammatory diseases. This work aims to identify potential phytochemicals to develop a new protein kinase inhibitor for treating advanced malignancies by providing an updated understanding of the role of SYK.Communicated by Ramaswamy H. Sarma.

Biochemical characterization of spleen tyrosine kinase (SYK) isoforms in platelets

Alternate splicing is among the regulatory mechanisms imparting functional diversity in proteins. Studying protein isoforms generated through alternative splicing is therefore critical for understanding protein functions in many biological systems. Spleen tyrosine kinase (Syk) plays an essential role in ITAM/hemITAM signaling in many cell types, including platelets. However, the spectrum of Syk isoforms expressed in platelets has not been characterized. Syk has been shown to have a full-length long isoform SykL and a shorter SykS lacking 23 amino acid residues within its interdomain B. Furthermore, putative isoforms lacking another 23 amino acid-long sequence or a combination of the two deletions have been postulated to exist. In this report, we demonstrate that mouse platelets express full-length SykL and the previously described shorter isoform SykS, but lack other shorter isoforms, whereas human platelets express predominantly SykL. These results both indicate a possible role of alternative Syk splicing in the regulation of receptor signaling in mouse platelets and a difference between signaling regulation in mouse and human platelets.

Cathepsin K inhibition induces Raptor destabilization and mitochondrial dysfunction via Syk/SHP2/Src/OTUB1 axis-mediated signaling

The Raptor signaling pathway is a critical point of intervention in the invasion and progression of cancer. The non-receptor tyrosine kinase Src-mediated phosphorylation of OTUB1-Y26 plays a critical role in Raptor stabilization, whereas cathepsin K inhibitor (odanacatib; ODN) and knockdown (siRNA) induce Raptor destabilization. However, the mechanisms involved in cathepsin K inhibition-induced OTUB1-Y26 phosphorylation in Raptor stabilization have not been yet elucidated. This study showed that cathepsin K inhibition activates SHP2, a tyrosine phosphatase, that dephosphorylates OTUB1 and destabilizes Raptor, whereas SHP2 deletion and pharmacological inhibition increase OTUB1-Y26 phosphorylation and Raptor expression. SHP2 deletion also led to the inhibition of ODN-induced mitochondrial ROS, fusion, and dysfunction. Furthermore, cathepsin K inhibition phosphorylated spleen tyrosine kinase (Syk) at Y525 and Y526, resulting in the SHP2-mediated dephosphorylation of OTUB1-Y26. Collectively, our findings identified Syk not only as an upstream tyrosine kinase required for SHP2 activation but also showed a critical mechanism that regulates ODN-induced Raptor downregulation and mitochondrial dysfunction. In conclusion, Syk/SHP2/Src/OTUB1 axis-mediated signaling can act as a therapeutic target in cancer management.

Spleen tyrosine kinase (SYK) signals are implicated in cardio-cerebrovascular diseases

Post-translational modifications regulate numerous biochemical reactions and functions through covalent attachment to proteins. Phosphorylation, acetylation and ubiquitination account for over 90% of all reported post-translational modifications. As one of the tyrosine protein kinases, spleen tyrosine kinase (SYK) plays crucial roles in many pathophysiological processes and affects the pathogenesis and progression of various diseases. SYK is expressed in tissues outside the hematopoietic system, especially the heart, and is involved in the progression of various cardio-cerebrovascular diseases, such as atherosclerosis, heart failure, diabetic cardiomyopathy, stroke and others. Knowledge on the role of SYK in the progress of cardio-cerebrovascular diseases is accumulating, and many related mechanisms have been discovered and validated. This review summarizes the role of SYK in the progression of various cardio-cerebrovascular diseases, and aims to provide a theoretical basis for future experimental and clinical research targeting SYK as a therapeutic option for these diseases.

Immunoregulatory mechanism of acute kidney injury in sepsis: A Narrative Review

Sepsis acute kidney injury (SAKI) is a common complication of sepsis, accounting for 26-50 % of all acute kidney injury (AKI). AKI is an independent risk factor for increased mortality risk in patients with sepsis. The excessive inflammatory cascade reaction in SAKI is one of the main causes of kidney damage. Both the innate immune system and the adaptive immune system are involved in the inflammation process of SAKI. Under the action of endotoxin, neutrophils, monocytes, macrophages, T cells and other complex immune network reactions occur, and a large number of endogenous inflammatory mediators are released, resulting in the amplification and loss of control of the inflammatory response. The study of immune cells in SAKI will help improve the understanding of the immune mechanisms of SAKI, and will lay a foundation for the development of new diagnostic and therapeutic targets. This article reviews the role of known immune mechanisms in the occurrence and development of SAKI, with a view to finding new targets for SAKI treatment.

A novel dual epigenetic approach targeting BET proteins and HDACs in Group 3 (MYC-driven) Medulloblastoma

Background

Medulloblastoma (MB) patients with MYC oncogene amplification or overexpression exhibit extremely poor clinical outcomes and respond poorly to current therapies. Epigenetic deregulation is very common in MYC-driven MB. The bromodomain extra-terminal (BET) proteins and histone deacetylases (HDACs) are epigenetic regulators of MYC transcription and its associated tumorigenic programs. This study aimed to investigate the therapeutic potential of inhibiting the BET proteins and HDACs together in MB.

Methods

Using clinically relevant BET inhibitors (JQ1 or OTX015) and a pan-HDAC inhibitor (panobinostat), we evaluated the effects of combined inhibition on cell growth/survival in MYC-amplified MB cell lines and xenografts and examined underlying molecular mechanism(s).

Results

Co-treatment of JQ1 or OTX015 with panobinostat synergistically suppressed growth/survival of MYC-amplified MB cells by inducing G2 cell cycle arrest and apoptosis. Mechanistic investigation using RNA-seq revealed that co-treatment of JQ1 with panobinostat synergistically modulated global gene expression including MYC/HDAC targets. SYK and MSI1 oncogenes were among the top 50 genes synergistically downregulated by JQ1 and panobinostat. RT-PCR and western blot analyses confirmed that JQ1 and panobinostat synergistically inhibited the mRNA and protein expression of MSI1/SYK along with MYC expression. Reduced SYK/MSI expression after BET (specifically, BRD4) gene-knockdown further confirmed the epigenetic regulation of SYK and MSI1 genes. In addition, the combination of OTX015 and panobinostat significantly inhibited tumor growth in MYC-amplified MB xenografted mice by downregulating expression of MYC, compared to single-agent therapy.

Conclusions

Together, our findings demonstrated that dual-inhibition of BET and HDAC proteins of the epigenetic pathway can be a novel therapeutic approach against MYC-driven MB.

The role of Syk phosphorylation in Fc receptor mediated mIgM+ B lymphocyte phagocytosis in flounder (Paralichthys olivaceus)

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase, and it mediates downstream signaling of FcR-mediated immune responses. Our previous work revealed that the expression of Syk was significantly up-regulated in flounder mIgM⁺ B lymphocytes after phagocytosis of antiserum-opsonized Edwardsiella tarda, which suggested Syk might be involved in Ig-opsonized phagocytosis. In this paper, phospho-Syk (pSyk) inhibitor was used to investigate the potential role of phosphorylated Syk in FcR-mediated phagocytosis of IgM⁺ B cells. Indirect immunofluorescence assay (IFA) and Western blotting showed that the level of phosphorylated Syk in the mIgM⁺ B lymphocytes treated with pSyk inhibitor was significantly lower compared to the control group after stimulation with flounder antiserum. Flow cytometry analysis showed that after 3 h incubation with antiserum-opsonized E. tarda, the phagocytosis rates of mIgM⁺ B lymphocytes from peripheral blood, spleen and head kidney pre-treated with pSyk inhibitor were 48.1%, 40.1% and 43.6% respectively, which were significantly lower than that of the control groups with 58.7%, 53.2% and 57.4%, respectively. And likewise, after pSyk inhibitor treatment, the proportions of mIgM⁺ B lymphocytes with higher intracellular reactive oxygen species (ROS) levels in peripheral blood, spleen and head kidney decreased to 15.2%, 12.0% and 12.1% from the control level of 26.5%, 25.9% and 26.3%, respectively. Moreover, the expression of three genes affected by pSyk, including phospholipase Cγ1 (PLCγ1), phospholipase Cγ2 (PLCγ2) and phosphatidylinositol 3 kinase (PI3K) were found to be significantly down-regulated in pSyk inhibitor-treated mIgM⁺ B lymphocytes post phagocytosis. These results suggest that pSyk plays a key role in FcR-mediated phagocytosis and bactericidal activity of mIgM⁺ B lymphocytes, which promotes further understanding of the regulatory role of pSyk in teleost B cells phagocytosis.

Small-molecule screening identifies Syk kinase inhibition and rutaecarpine as modulators of macrophage training and SARS-CoV-2 infection

Biologically active small molecules can impart modulatory effects, in some cases providing extended long-term memory. In a screen of biologically active small molecules for regulators of tumor necrosis factor (TNF) induction, we identify several compounds with the ability to induce training effects on human macrophages. Rutaecarpine shows acute and long-term modulation, enhancing lipopolysaccharide (LPS)-induced pro-inflammatory cytokine secretion and relieving LPS tolerance in human macrophages. Rutaecarpine inhibits β-glucan-induced H3K4Me3 marks at the promoters of several pro-inflammatory cytokines, highlighting the potential of this molecule to modulate chromosomal topology. Syk kinase inhibitor (SYKi IV), another screen hit, promotes an enhanced response to LPS similar to that previously reported for β-glucan-induced training. Macrophages trained with SYKi IV show a high degree of resistance to influenza A, multiple variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and OC43 coronavirus infection, highlighting a potential application of this molecule and other SYKis as prophylactic treatments for viral susceptibility.

Tumor cell SYK expression modulates the tumor immune microenvironment composition in human cancer via TNF-α dependent signaling

BACKGROUND

The expression of SYK in cancer cells has been associated with both tumor promoting and tumor suppressive effects. Despite being proposed as anticancer therapeutic target, the possible role of SYK in modulating local adaptive antitumor immune responses remains uncertain. Using detailed analysis of primary human tumors and in vitro models, we reveal the immunomodulatory effect of SYK protein in human solid cancer.

METHODS

We spatially mapped SYK kinase in tumor cells, stromal cells and tumor-infiltrating leukocytes (TILs) in 808 primary non-small cell lung carcinomas (NSCLCs) from two cohorts and in 374 breast carcinomas (BCs) from two independent cohorts. We established the associations of localized SYK with clinicopathologic variables and outcomes. The immunomodulatory role of SYK on tumor cells was assessed using in vitro cytokine stimulation, transcriptomic analysis and selective SYK blockade using a small molecule inhibitor. Functional responses were assessed using cocultures of tumor cells with peripheral blood lymphocytes. T cell responses in baseline and post-treatment biopsies from patients with BC treated with a SYK inhibitor in a phase I clinical trial were also studied.

RESULTS

Elevated tumor cell or leukocyte SYK expression was associated with high CD4⁺ and CD8⁺ TILs and better outcome in both NSCLC and BC. Tumor cell SYK was associated with oncogenic driver mutations in EGFR or KRAS in lung adenocarcinomas and with triple negative phenotype in BC. In cultured tumor cells, SYK was upregulated by TNFα and required for the TNFα-induced proinflammatory responses and T cell activation. SYK blockade after nivolumab in a phase I clinical trial including three patients with advanced triple negative BC reduced TILs and T cell proliferation. Our work establishes the proinflammatory function of tumor cell SYK in lung and breast cancer. SYK signaling in cultured tumor cells is required for T cell activation and SYK blockade limits adaptive antitumor immune responses and tumor rejection in patients with cancer.

CONCLUSIONS

Together, our results establish the immunomodulatory role of SYK expression in human solid tumors. This information could be used to develop novel biomarkers and/or therapeutic strategies.

Molecular and Immunomodulatory Actions of New Antiasthmatic Agents: Exploring the Diversity of Biologics in Th2 Endotype Asthma

Asthma is a major respiratory disorder characterised by chronic inflammation and airway remodelling. It affects about 1-8% of the global population and is responsible for over 461,000 deaths annually. Until recently, the pharmacotherapy of severe asthma involved high doses of inhaled corticosteroids in combination with β-agonist for prolonged action, including theophylline, leukotriene antagonist or anticholinergic yielding limited benefit. Although the use of newer agents to target Th2 asthma endotypes has improved therapeutic outcomes in severe asthmatic conditions, there seems to be a paucity of understanding the diverse mechanisms through which these classes of drugs act. This article delineates the molecular and immunomodulatory mechanisms of action of new antiasthmatic agents currently being trialled in preclinical and clinical studies to remit asthmatic conditions. The ultimate goal in developing antiasthmatic agents is based on two types of approaches: either anti-inflammatory or bronchodilators. Biologic and most small molecules have been shown to modulate specific asthma endotypes, targeting thymic stromal lymphopoietin, tryptase, spleen tyrosine kinase (Syk), Janus kinase, PD-L1/PD-L2, GATA-3, and CD38 for the treatment and management of Th2 endotype asthma.

A concise review on tyrosine kinase targeted cancer therapy

Abstract:

The tyrosine kinase (TK) family is considered one of the important family members of the kinase family due to its important role in various cellular processes like cell growth, cell differentiation, apoptosis, etc. Mutation, overexpression, and dysfunction of tyrosine kinase receptors lead to the development of malignancy; thus, they are considered as one of the important targets for the development of anti-cancer molecules. The tyrosine kinase family is majorly divided into two classes; receptor and non-receptor tyrosine kinase. Both of the classes have an important role in the development of tumour cells. Currently, there are more than 40 FDA-approved tyrosine kinase inhibitors, which are used in the treatment of various types of cancers. Tyrosine kinase inhibitors mainly block the phosphorylation of tyrosine residue of the corresponding kinase substrate and so activation of downstream signalling pathways can be inhibited. The promising results of tyrosine kinase inhibitors in solid tumours provide a revolution in oncology research. In this article, we had summarized the role of some important members of the tyrosine kinase family in the development and progression of tumour cells and the significance of tyrosine kinase inhibitors in the treatment of various types of cancer.

Spleen tyrosine kinase regulates keratinocyte inflammasome activation and skin inflammation induced by UVB irradiation

UVB can induce inflammatory responses contributing to diverse skin damage. UVB-triggered inflammasome activation of human keratinocytes underlies UVB-induced skin sunburn reaction. Pleiotropic functions of spleen tyrosine kinase (Syk) have rendered it as a potential therapeutic target. In immunocytes, Syk modulates immunoreceptor signaling and NLRP3 inflammasome activation. In skin, Syk mediates EGFR signaling, regulates keratinocyte differentiation and is involved in inflammatory disorders. However, roles of Syk in UVB-induced inflammasome activation in keratinocytes remain elusive. We investigated roles of keratinocyte Syk in UVB-triggered photo-responses. Primary normal human epidermal keratinocytes (NHEKs) isolated from skin were used. Syk knockdown or Syk inhibitor R406 was applied to investigate functions of keratinocyte Syk in UVB photobiology. The possible in vivo role of Syk was evaluated by checking UVB-induced skin damage in R406-treated mice. UVB was able to induce Syk phosphorylation in NHEKs that could be regulated by reactive oxygen species (ROS) generation and EGFR. Syk knockdown or Syk inhibitor (R406) treatment reduced UVB-triggered apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC) crosslinking, procaspase-1 cleavage, active IL-1β formation, and gasdermin D activation, indicating roles of Syk in UVB-triggered inflammasome activation in keratinocytes. UVB-induced production of IL-8, TNF-α, ROS, and phosphorylation of JNK and p38 were attenuated after Syk knockdown or inhibition. R406 ameliorated UVB-induced mouse skin damage, including erythema and transepidermal water loss (TEWL). Thus, Syk participated in UVB-induced inflammasome activation and inflammatory response in vitro and in vivo, suggesting potential photo-protective effects of Syk inhibition in UVB-induced skin inflammation.

Single-cell analysis of skin immune cells reveals an Angptl4-ifi20b axis that regulates monocyte differentiation during wound healing

The persistent inflammatory response at the wound site is a cardinal feature of nonhealing wounds. Prolonged neutrophil presence in the wound site due to failed clearance by reduced monocyte-derived macrophages delays the transition from the inflammatory to the proliferative phase of wound healing. Angiopoietin-like 4 protein (Angptl4) is a matricellular protein that has been implicated in many inflammatory diseases. However, its precise role in the immune cell response during wound healing remains unclear. Therefore, we performed flow cytometry and single-cell RNA sequencing to examine the immune cell landscape of excisional wounds from Angptl4+/+ and Angptl4−/− mice. Chemotactic immune cell recruitment and infiltration were not compromised due to Angptl4 deficiency. However, as wound healing progresses, Angptl4−/− wounds have a prolonged neutrophil presence and fewer monocyte-derived macrophages than Angptl4+/+ and Angptl4LysM−/− wounds. The underlying mechanism involves a novel Angptl4-interferon activated gene 202B (ifi202b) axis that regulates monocyte differentiation to macrophages, coordinating neutrophil removal and inflammation resolution. An unbiased kinase inhibitor screen revealed an Angptl4-mediated kinome signaling network involving S6K, JAK, and CDK, among others, that modulates the expression of ifi202b. Silencing ifi202b in Angptl4−/− monocytes, whose endogenous expression was elevated, rescued the impaired monocyte-to-macrophage transition in the in vitro reconstituted wound microenvironment using wound exudate. GSEA and IPA functional analyses revealed that ifi202b-associated canonical pathways and functions involved in the inflammatory response and monocyte cell fate were enriched. Together, we identified ifi202b as a key gatekeeper of monocyte differentiation. By modulating ifi202b expression, Angptl4 orchestrates the inflammatory state, innate immune landscape, and wound healing process.

Targeting SYK of monocyte-derived macrophages regulates liver fibrosis via crosstalking with Erk/Hif1α and remodeling liver inflammatory environment

Liver fibrosis is a danger signal indicating a huge risk of liver cancer occurrence, but there is still no effective clinical means to regulate the progress of liver fibrosis. Although a variety of drugs targeting SYK have been developed for tumors and autoimmune diseases, the mechanism and specific efficacy of SYK's role in liver fibrosis are not yet clear. Our studies based on chronic CCL4, bile duct ligation, and subacute TAA mouse models show that SYK in monocyte-derived macrophages (MoMFs) is fully dependent on phosphorylation of Erk to up-regulate the expression of Hif1α, thereby forming the crosstalk with SYK to drive liver fibrosis progress. We have evaluated the ability of the small molecule SYK inhibitor GS9973 in a variety of models. Contrary to previous impressions, high-frequency administration of GS9973 will aggravate CCL4-induced liver fibrosis, which is especially unsuitable for patients with cholestasis whose clinical features are bile duct obstruction. In addition, we found that inhibition of MoMFs SYK impairs the expression of CXCL1, on one hand, it reduces the recruitment of CD11bhiLy6Chi inflammatory cells, and on the other hand, it promotes the phenotype cross-dress process of pro-resolution MoMFs, thereby remodeling the chronic inflammatory environment of the fibrotic liver. Our further findings indicate that on the basis of the administration of CCR2/CCR5 dual inhibitor Cenicriviroc, further inhibiting MoMFs SYK may give patients with fibrosis additional benefits.

miR-96-5p is the tumor suppressor in osteosarcoma via targeting SYK

BACKGROUND

Osteosarcoma is a multiple malignant tumor in adolescents. MicroRNAs (MiRNAs) have been found to express abnormally in OS tissues and are considered as potential targets for OS prognosis and treatment.

METHODS

MiR-96-5p and SYK expression in clinical samples, osteoblast and OS cell lines were detected. The changes of cell proliferation, apoptosis, adhesion and metastasis of OS cells were detected by CCK-8, BrdU, caspase-3 activity and transwell assay. Dual luciferase report analysis and RNA pull-down were used to confirm binding relation of miR-96-5p and SYK.

RESULTS

MiR-96-5p was increased in OS tissue and cells. Moreover, miR-96-5p inhibits proliferation, adhesion and migration of HOS and Saos-2 cells, and promotes cell apoptosis. SYK has been identified to be targeted by miR-96-5p. Overexpressed SYK inhibits the suppressive impact of miR-96-5 on OS cells.

CONCLUSION

MiR-96-5p may function as an effective target in OS treatment.

The Spleen Tyrosine Kinase Inhibitor, Entospletinib (GS-9973) Restores Chemosensitivity in Lung Cancer Cells by Modulating ABCG2-mediated Multidrug Resistance

Tyrosine kinase inhibitors (TKIs) are important in managing lymphoid malignancies by targeting B-cell receptor signaling pathways. Entospletinib (GS-9973) is an oral, selective inhibitor of spleen tyrosine kinase (Syk), currently in the phase II clinical trials for the treatment of chronic lymphocytic leukemia. Syk is abundantly present in the cells of hematopoietic lineage that mediates cell proliferation, differentiation, and adhesion. In this current study, we evaluated the efficacy of GS-9973 to overcome multidrug resistance (MDR) due to the overexpression of the ABCG2 transporter in the non-small cell lung cancer (NSCLC) cell line, NCI-H460/MX20. In vitro, 3 μM of GS-9973 reversed the drug resistance of NCI-H460/MX20 cell line to mitoxantrone or doxorubicin. GS-9973, at 3 μM reverses ABCG2-mediated MDR by blocking ABCG2 efflux activity and downregulating ABCG2 expression at the protein level but did not alter the ABCG2 mRNA expression and subcellular localization of the ABCG2 protein compared to drug-resistant cells incubated with the vehicle. GS-9973 produced a moderate concentration-dependent increase in the ATPase activity of ABCG2 (EC50 = 0.42 µM) and molecular docking data indicated that GS-9973 had a high affinity (-10.226 kcal/mol) for the substrate-binding site of ABCG2. Finally, HPLC analysis proved that the intracellular concentration of GS-9973 is not significantly different in both parental and resistant cell lines. In conclusion, our study suggests that in vitro, GS-9973 in combination with certain anticancer drugs, represent a strategy to overcome ABCG2-mediated MDR cancers.

Spleen Tyrosine Kinase Inhibition Ameliorates Tubular Inflammation in IgA Nephropathy

Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase involved in signal transduction in a variety of immune responses. It has been demonstrated that Syk plays a pathogenic role in orchestrating inflammatory responses and cell proliferation in human mesangial cells (HMC) in IgA nephropathy (IgAN). However, whether Syk is involved in tubular damage in IgAN remains unknown. Using human kidney biopsy specimens, we found that Syk was activated in renal tubules of biopsy-proven IgAN patients with an increase in total and phosphorylated levels compared to that from healthy control subjects. In vitro, cultured proximal tubular epithelial cells (PTECs) were stimulated with conditioned medium prepared from human mesangial cells incubated with polymeric IgA (IgA-HMC) from patients with IgAN or healthy control. Induction of IL-6, IL-8, and ICAM-1 synthesis from cultured PTECs incubated with IgA-HMC conditioned medium was significantly suppressed by treatment with the Syk inhibitor R406 compared to that from healthy control. Furthermore, R406 downregulated expression of phosphorylated p65 NF-κB and p-42/p-44 MAPK, and attenuated TNF-α-induced cytokine production in PTECs. Taken together, our findings suggest that Syk mediates IgA-HMC conditioned medium-induced inflammation in tubular cells via activation of NF-κB and p-42/p-44 MAPK signaling. Inhibition of Syk may be a potential therapeutic approach for tubulointerstitial injury in IgAN.

Comparison of SYK Signaling Networks Reveals the Potential Molecular Determinants of Its Tumor-Promoting and Suppressing Functions

Spleen tyrosine kinase (SYK) can behave as an oncogene or a tumor suppressor, depending on the cell and tissue type. As pharmacological SYK inhibitors are currently evaluated in clinical trials, it is important to gain more information on the molecular mechanisms underpinning these opposite roles. To this aim, we reconstructed and compared its signaling networks using phosphoproteomic data from breast cancer and Burkitt lymphoma cell lines where SYK behaves as a tumor suppressor and promoter. Bioinformatic analyses allowed for unveiling the main differences in signaling pathways, network topology and signal propagation from SYK to its potential effectors. In breast cancer cells, the SYK target-enriched signaling pathways included intercellular adhesion and Hippo signaling components that are often linked to tumor suppression. In Burkitt lymphoma cells, the SYK target-enriched signaling pathways included molecules that could play a role in SYK pro-oncogenic function in B-cell lymphomas. Several protein interactions were profoundly rewired in the breast cancer network compared with the Burkitt lymphoma network. These data demonstrate that proteomic profiling combined with mathematical network modeling allows untangling complex pathway interplays and revealing difficult to discern interactions among the SYK pathways that positively and negatively affect tumor formation and progression.

Inhibition of SYK kinase does not confer a pro-proliferative or pro-invasive phenotype in breast epithelium or breast cancer cells

SYK has been reported to possess both tumour promotor and repressor activities and deletion has been linked to a pro-proliferative / pro-invasive phenotype in breast tumours. It is unclear whether this is a consequence of protein deletion or loss of kinase activity. The SYK inhibitor, BI 1002494, caused no increase in proliferation in breast cancer cells or primary mammary epithelial cells in 2D or 3D cultures, nor changes in proliferation (CD1/2, CDK4, PCNA, Ki67) or invadopodia markers (MMP14, PARP, phospho-vimentin Ser56). BI 1002494 did not alter SYK protein expression. There was no change in phenotype observed in 3D cultures after addition of BI 1002494. Thirteen weeks of treatment with BI 1002494 resulted in no ductal branching or cellular proliferation in the mammary glands of mice. An in silico genetic analysis in breast tumour samples revealed no evidence that SYK has a typical tumour suppressor gene profile such as focal deletion, inactivating mutations or lower expression levels. Furthermore, SYK mutations were not associated with reduction in survival and disease-free period in breast cancer patients. In conclusion, small molecule inhibition of the kinase function of SYK does not contribute to a typical tumour suppressor profile.

Role of spleen tyrosine kinase in liver diseases

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase expressed in most hematopoietic cells and non-hematopoietic cells and play a crucial role in both immune and non-immune biological responses. SYK mediate diverse cellular responses via an immune-receptor tyrosine-based activation motifs (ITAMs)-dependent signalling pathways, ITAMs-independent and ITAMs-semi-dependent signalling pathways. In liver, SYK expression has been observed in parenchymal (hepatocytes) and non-parenchymal cells (hepatic stellate cells and Kupffer cells), and found to be positively correlated with the disease severity. The implication of SYK pathway has been reported in different liver diseases including liver fibrosis, viral hepatitis, alcoholic liver disease, non-alcoholic steatohepatitis and hepatocellular carcinoma. Antagonism of SYK pathway using kinase inhibitors have shown to attenuate the progression of liver diseases thereby suggesting SYK as a highly promising therapeutic target. This review summarizes the current understanding of SYK and its therapeutic implication in liver diseases.

The Syk Kinase Promotes Mammary Epithelial Integrity and Inhibits Breast Cancer Invasion by Stabilizing the E-Cadherin/Catenin Complex

While first discovered in immunoreceptor signaling, the Syk protein kinase behaves as a tumor and metastasis suppressor in epithelial cells. Its reduced expression in breast and other carcinomas is correlated with decreased survival and increased metastasis risk, but its action mechanism remains largely unknown. Using phosphoproteomics we found that Syk phosphorylated E-cadherin and α-, β-, and p120-catenins on multiple tyrosine residues that concentrate at intercellular junctions. Increased Syk expression and activation enhanced E-cadherin/catenin phosphorylation, promoting their association and complex stability. In human breast cancer cells, Syk stimulated intercellular aggregation, E-cadherin recruitment and retention at adherens junctions, and promoted epithelial integrity, whereas it inhibited cell migration and invasion. Opposite effects were obtained with Syk knockdown or non-phosphorylatable mutant E-cadherin expression. Mechanistically, Syk stimulated the interaction of the E-cadherin/catenin complex with zonula occludens proteins and the actin cytoskeleton. Conditional Syk knockout in the lactating mouse mammary gland perturbed alveologenesis and disrupted E-cadherin localization at adherens junctions, corroborating the observations in cells. Hence, Syk is involved in the maintenance of the epithelial integrity of the mammary gland via the phosphorylation and stabilization of the E-cadherin/catenin adherens junction complex, thereby inhibiting cell migration and malignant tumor invasion.

The Importance of Tyrosine Phosphorylation Control of Cellular Signaling Pathways in Respiratory Disease: pY and pY Not

Reversible phosphorylation of proteins on tyrosine residues is an essential signaling mechanism by which diverse cellular processes are closely regulated. The tight temporal and spatial control of the tyrosine phosphorylation status of proteins by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) is critical to cellular homeostasis as well as to adaptations to the external environment. Via regulation of cellular signaling cascades involving other protein kinases and phosphatases, receptors, adaptor proteins, and transcription factors, PTKs and PTPs closely control diverse cellular processes such as proliferation, differentiation, migration, inflammation, and maintenance of cellular barrier function. Given these key regulatory roles, it is not surprising that dysfunction of PTKs and PTPs is important in the pathogenesis of human disease, including many pulmonary diseases. The roles of various PTKs and PTPs in acute lung injury and repair, pulmonary fibrosis, pulmonary vascular disease, and inflammatory airway disease are discussed in this review. It is important to note that although there is overlap among many of these proteins in various disease states, the mechanisms by which they influence the pathogenesis of these conditions differ, suggesting wide-ranging roles for these enzymes and their potential as therapeutic targets.

Role of the Spleen Tyrosine Kinase Pathway in Driving Inflammation in IgA Nephropathy

Summary: IgA nephropathy is the most common type of primary glomerulonephritis worldwide. At least 25% of patients may progress to kidney failure requiring dialysis or transplantation. Treatment of IgA nephropathy using generalized immunosuppression is controversial, with concerns regarding the balance of safety and efficacy in a nonspecific approach. This review describes the recent scientific evidence, and a current clinical trial, investigating whether spleen tyrosine kinase (SYK) may be a novel and selective therapeutic target for IgA nephropathy. SYK, a cytoplasmic tyrosine kinase, has a pivotal role as an early intermediate in intracellular signal transduction cascades for the B-cell receptor and the immunoglobulin Fc receptor, and thus is critical for B-cell proliferation, differentiation, and activation, and for mediating proinflammatory responses after Fc-receptor engagement in various cell types. In renal biopsy specimens of patients with IgA nephropathy, increased expression and phosphorylation of SYK were detected, and this correlated with the histologic features of mesangial and endocapillary proliferation. In cell culture studies, patient-derived IgA1 stimulated mesangial cell SYK activation, cell proliferation, and cytokine production, and these responses were attenuated by pharmacologic or molecular inhibition of SYK. A global, randomized, double-blind, placebo-controlled trial investigating the safety and efficacy of fostamatinib (an oral prodrug SYK inhibitor) in the treatment of patients with IgA nephropathy is ongoing, which may provide important evidence of the safety and efficacy of targeting this pathway in clinical disease.

SYK Inhibition Potentiates the Effect of Chemotherapeutic Drugs on Neuroblastoma Cells in Vitro

Neuroblastoma is a malignancy arising from the developing sympathetic nervous system and the most common and deadly cancer of infancy. New therapies are needed to improve the prognosis for high-risk patients and to reduce toxicity and late effects. Spleen tyrosine kinase (SYK) has previously been identified as a promising drug target in various inflammatory diseases and cancers but has so far not been extensively studied as a potential therapeutic target in neuroblastoma. In this study, we observed elevated SYK gene expression in neuroblastoma compared to neural crest and benign neurofibroma. While SYK protein was detected in the majority of examined neuroblastoma tissues it was less frequently observed in neuroblastoma cell lines. Depletion of SYK by siRNA and the use of small molecule SYK inhibitors significantly reduced the cell viability of neuroblastoma cell lines expressing SYK protein. Moreover, SYK inhibition decreased ERK1/2 and Akt phosphorylation. The SYK inhibitor BAY 61-3606 enhanced the effect of different chemotherapeutic drugs. Transient expression of a constitutive active SYK variant increased the viability of neuroblastoma cells independent of endogenous SYK levels. Collectively, our findings suggest that targeting SYK in combination with conventional chemotherapy should be further evaluated as a treatment option in neuroblastoma.

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.

SYK inhibition blocks proliferation and migration of glioma cells and modifies the tumor microenvironment

Background

Glioblastoma (GBM) is one of the most aggressive human brain tumors, with a median survival of 15-18 months. There is a desperate need to find novel therapeutic targets. Various receptor protein kinases have been identified as potential targets; however, response rates in clinical studies have been somewhat disappointing. Targeting the spleen tyrosine kinase (SYK), which acts downstream of a range of oncogenic receptors, may therefore show more promising results.

Methods

Kinase expression of brain tumor samples including GBM and low-grade tumors were compared with normal brain and normal human astrocytes by microarray analysis. Furthermore, SYK, LYN, SLP76, and PLCG2 protein expressions were analyzed by immunohistochemistry, western blot, and immunofluorescence of additional GBM patient samples, murine glioma samples, and cell lines. SYK was then blocked chemically and genetically in vitro and in vivo in 2 different mouse models. Multiphoton intravital imaging and multicolor flow cytometry were performed in a syngeneic immunocompetent C57BL/6J mouse GL261 glioma model to study the effect of these inhibitors on the tumor microenvironment.

Results

SYK, LYN, SLP76, and PLCG2 were found expressed in human and murine glioma samples and cell lines. SYK inhibition blocked proliferation, migration, and colony formation. Flow cytometric and multiphoton imaging imply that targeting SYK in vivo attenuated GBM tumor growth and invasiveness and reduced B and CD11b+ cell mobility and infiltration.

Conclusions

Our data suggest that gliomas express a SYK signaling network important in glioma progression, inhibition of which results in reduced invasion with slower tumor progression.

WITHDRAWN: Spleen tyrosine kinase (SYK) protects renal tubular epithelial cell against hypoxia injury in children with acute kidney injury

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Spleen tyrosine kinase from Nile tilapia (Oreochromis niloticus): Molecular characterization, expression pattern upon bacterial infection and the potential role in BCR signaling and inflammatory response

Spleen tyrosine kinase (SYK), a member of non-receptor tyrosine kinase family, plays an important role in immune responses against pathogen infection, which is capable of activating B cells signaling pathway and regulating inflammatory response. In this study, Nile tilapia (Oreochromis niloticus) ortholog (OnSYK) was identified and characterized at expression pattern against bacterial infection, function in B cells activation pathway and inflammatory response. The cDNA of OnSYK ORF contained 1851 bp of nucleotide sequence encoding polypeptides of 616 amino acids. The deduced OnSYK protein was highly homologous to other species SYK, containing two SH2 domains and a TyrKc domain. Spatial mRNA expression analysis revealed that OnSYK had wide tissue distribution and was highly expressed in the liver. After challenge of Streptococcus agalactiae (S. agalactiae) in vivo, mRNA expression of OnSYK was significantly up-regulated in the head kidney, spleen and liver. The up-regulation of OnSYK transcript was also displayed in the head kidney and spleen leukocytes stimulation with S. agalactiae and LPS in vitro, which was confirmed at protein level in the head kidney leukocytes by FACS analysis. In addition, after induction with mouse anti-OnIgM monoclonal antibody in vitro, the expressions of OnSYK and its downstream molecules (OnLYN, OnBLNK and OnAP-1) were significantly up-regulated in the head kidney leukocytes, and pharmacological inhibition of SYK activity with inhibitor (P505-15) significantly attenuated the expressions of OnLYN, OnBLNK and OnAP-1. Moreover, upon LPS challenge, the expressions of OnSYK, OnTNF-α, OnIL-6 and OnAP-1 were also up-regulated in the head kidney monocytes/macrophages. After treatment with SYK inhibitor (BAY 61-3606), the expressions of OnTNF-α, OnIL-6 and OnAP-1 were inhibited in the LPS-challenged head kidney monocytes/macrophages. Taken together, the results of this study indicated that OnSYK, playing potential roles in BCR signaling and inflammatory response, was likely to get involved in host defense against bacterial infection in Nile tilapia.

An inhibitor of spleen tyrosine kinase suppresses experimental crescentic glomerulonephritis

Non-selective inhibitors of spleen tyrosine kinase (SYK) efficiently suppress disease in T cell-dependent models of crescentic glomerulonephritis. However, the therapeutic potential of selective SYK inhibitors in this disease has not been established. In addition, we lack knowledge regarding SYK expression in non-myeloid cells in glomerulonephritis. We addressed these two issues in a rat model of nephrotoxic serum nephritis (NTN) using a SYK inhibitor, GS-492429. Disease was induced in Sprague-Dawley rats (Study 1) or Wistar-Kyoto (WKY) rats (Study 2) by immunization with sheep IgG and administration of sheep anti-rat nephrotoxic serum. Animals were untreated or received GS-492429 (30 mg/kg/bid) or vehicle treatment from 2 h before nephrotoxic serum injection until being killed 3 or 24 h later (Study 1) or 14 days later (Study 2). Two-colour confocal microscopy found that SYK expression in NTN kidney was restricted to myeloid cells and platelets, with no evidence of SYK expression by T cells, mesangial cells, podocytes or tubular epithelial cells. In Study 1, GS-492429 treatment significantly reduced glomerular neutrophil and macrophage infiltration, with protection from glomerular thrombosis and proteinuria. In Study 2, GS-492429 treatment reduced glomerular crescent formation by 70% on day 14 NTN in conjunction with reduced glomerular thrombosis, glomerulosclerosis and tubular damage. This was accompanied by a marked reduction in markers of inflammation (CCL2, TNF-α, NOS2, MMP-12). Importantly, the protective effects of GS-492429 were independent of T cell infiltration and activation and independent of JAK/STAT3 signalling. In conclusion, this study demonstrates that a SYK inhibitor can suppress the development of crescentic glomerulonephritis through effects upon myeloid cells and platelets.

Genetic and Pharmacological Dissection of the Role of Spleen Tyrosine Kinase (Syk) in Intestinal Inflammation and Immune Dysfunction in Inflammatory Bowel Diseases

BACKGROUND

The DNAX adaptor protein 12 (DAP12) is a transmembrane adaptor molecule that signals through the activation of Syk (Spleen Tyrosine Kinase) in myeloid cells. The purpose of this study is to investigate the role of DAP12 and Syk pathways in inflammatory bowel diseases (IBDs).

METHODS

DAP12 deficient and DAP12 transgenic, overexpressing an increased amount of DAP12, mice and Syk deficient mice in the C57/BL6 background were used for these studies. Colitis was induced by administering mice with dextran sulfate sodium (DSS), in drinking water, or 2,4,6-trinitrobenzene sulfonic acid (TNBS), by intrarectal enema.

RESULTS

Abundant expression of DAP12 and Syk was detected in colon samples obtained from Crohn's disease patients with expression restricted to immune cells infiltrating the colonic wall. In rodents development of DSS colitis as measured by assessing severity of wasting diseases, global colitis score,and macroscopic and histology scores was robustly attenuated in DAP12-/- and Syk-/- mice. In contrast, DAP12 overexpression resulted in a striking exacerbation of colon damage caused by DSS. Induction of colon expression of proinflammatory cytokines and chemokines in response to DSS administration was attenuated in DAP12-/- and Syk-/- mice, whereas opposite results were observed in DAP12 transgenic mice. Treating wild-type mice with a DAP-12 inhibitor or a Syk inhibitor caused a robust attenuation of colitis induced by DSS and TNBS.

CONCLUSIONS

DAP12 and Syk are essential mediators in inflammation-driven immune dysfunction in murine colitides. Because DAP12 and Syk expression is upregulated in patients with active disease, present findings suggest a beneficial role for DAP12 and Syk inhibitors in IBD.

SYK kinase mediates brown fat differentiation and activation

Brown adipose tissue (BAT) metabolism influences glucose homeostasis and metabolic health in mice and humans. Sympathetic stimulation of β-adrenergic receptors in response to cold induces proliferation, differentiation, and UCP1 expression in pre-adipocytes and mature brown adipocytes. Here we show that spleen tyrosine kinase (SYK) is upregulated during brown adipocyte differentiation and activated by β-adrenergic stimulation. Deletion or inhibition of SYK, a kinase known for its essential roles in the immune system, blocks brown and white pre-adipocyte proliferation and differentiation in vitro, and results in diminished expression of Ucp1 and other genes regulating brown adipocyte function in response to β-adrenergic stimulation. Adipocyte-specific SYK deletion in mice reduces BAT mass and BAT that developed consisted of SYK-expressing brown adipocytes that had escaped homozygous Syk deletion. SYK inhibition in vivo represses β-agonist-induced thermogenesis and oxygen consumption. These results establish SYK as an essential mediator of brown fat formation and function.

Spleen protein tyrosine kinase (Syk) has so far been mainly studied in haematopoietic and immune cells. Here, the authors show that Syk also has a role in brown adipose tissue, where it regulates the formation of brown adipocytes and their thermogenic activation in response to β-adrenergic stimulation.

High mRNA expression of splice variant SYK short correlates with hepatic disease progression in chemonaive lymph node negative colon cancer patients

Objective

Overall and splice specific expression of Spleen Tyrosine Kinase (SYK) has been posed as a marker predicting both poor and favorable outcome in various epithelial malignancies. However, its role in colorectal cancer is largely unknown. The aim of this study was to explore the prognostic role of SYK in three cohorts of colon cancer patients.

Methods

Total messenger RNA (mRNA) expression of SYK, SYK(T), and mRNA expression of its two splice variants SYK short (S) and SYK long (L) were measured using quantitative reverse transcriptase (RT-qPCR) in 240 primary colon cancer patients (n = 160 patients with chemonaive lymph node negative [LNN] and n = 80 patients with adjuvant treated lymph node positive [LNP] colon cancer) and related to microsatellite instability (MSI), known colorectal cancer mutations, and disease-free (DFS), hepatic metastasis-free (HFS) and overall survival (OS). Two independent cohorts of patients with respectively 48 and 118 chemonaive LNN colon cancer were used for validation.

Results

Expression of SYK and its splice variants was significantly lower in tumors with MSI, and in KRAS wild type, BRAF mutant and PTEN mutant tumors. In a multivariate Cox regression analysis, as a continuous variable, increasing SYK(S) mRNA expression was associated with worse HFS (Hazard Ratio[HR] = 1.83; 95% Confidence Interval[CI] = 1.08–3.12; p = 0.026) in the LNN group, indicating a prognostic role for SYK(S) mRNA in patients with chemonaive LNN colon cancer. However, only a non-significant trend between SYK(S) and HFS in one of the two validation cohorts was observed (HR = 4.68; 95%CI = 0.75–29.15; p = 0.098).

Conclusion

In our cohort, we discovered SYK(S) as a significant prognostic marker for HFS for patients with untreated LNN colon cancer. This association could however not be confirmed in two independent smaller cohorts, suggesting that further extensive validation is needed to confirm the prognostic value of SYK(S) expression in chemonaive LNN colon cancer.

Selective Binders of the Tandem Src Homology 2 Domains in Syk and Zap70 Protein Kinases by DNA-Programmed Spatial Screening

Members of the Syk family of tyrosine kinases arrange Src homology 2 (SH2) domains in tandem to allow the firm binding of immunoreceptor tyrosine-based interaction motifs (ITAMs). While the advantages provided by the bivalency enhanced interactions are evident, the impact on binding specificity is less-clear. For example, the spleen tyrosine kinase (Syk) and the ζ-chain-associated protein kinase (ZAP-70) recognize the consensus sequence pYXXI/L(X)_6-8 pYXXI/L with near-identical nanomolar affinity. The nondiscriminatory recognition, on the one hand, poses a specificity challenge for the design of subtype selective protein binders and, on the other hand, raises the question as to how differential activation of Syk and ZAP-70 is ensured when both kinases are co-expressed. Herein, we identified the criteria for the design of binders that specifically address either the Syk or the Zap-70 tSH2 domain. Our approach is based on DNA-programmed spatial screening. Tyrosine-phosphorylated peptides containing the pYXXI/L motif were attached to oligonucleotides and aligned in tandem on a DNA template by means of nucleic acid hybridization. The distance between the pYXXI/L motifs and the orientation of strands were varied. The exploration exposed remarkably different recognition characteristics. While Syk tSH2 has a rather broad substrate scope, ZAP-70 tSH2 required a proximal arrangement of the phosphotyrosine ligands in defined strand orientation. The spatial screen led to the design of mutually selective, DNA-free binders, which discriminate Zap-70 and Syk tSH2 by 1 order of magnitude in affinity.

A role for spleen tyrosine kinase in renal fibrosis in the mouse obstructed kidney

AIMS

Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase involved in the signalling pathways of the B cell receptor, Fcγ-receptor and some leukocyte integrins. However, Syk can also be expressed by some non-haematopoietic cell types, although whether Syk signalling in these cells contributes to the pathogenesis of kidney disease is unknown. To address this question, we examined the function of Syk in antibody-independent renal interstitial fibrosis in the unilateral ureteric obstruction (UUO) model.

MAIN METHODS

Groups of C57BL/6J mice were treated with a selective Syk inhibitor (CC0417, 30 mg/kg/bid), vehicle, or no treatment, from the time of surgery until being killed 7 days later.

KEY FINDINGS

A substantial accumulation of interstitial Syk(+) cells was seen in the UUO kidney. Double staining identified Syk expression by infiltrating macrophages and by a subset of α-SMA(+) myofibroblasts. CC0417 treatment substantially reduced the Syk(+) cell population in conjunction with a reduction in both myofibroblast and macrophage accumulation. This was associated with a substantial reduction in collagen IV deposition and mRNA levels of pro-fibrotic (collagen I, collagen IV, fibronectin, α-SMA, TGF-β1 and PAI-1) and pro-inflammatory molecules (MCP-1, TNF-α and NOS2). CC0417 treatment reduced both PDGF-B mRNA levels and Ki67(+) proliferating interstitial cells in the UUO kidney. Furthermore, CC0417 inhibited PDGF-AB induced ERK activation and cell proliferation of cultured primary kidney fibroblasts.

SIGNIFICANCE

This study has identified a pathologic role for Syk in renal interstitial fibrosis. Syk inhibitors may have therapeutic potential in chronic fibrotic kidney disease.

Spleen Tyrosine Kinase Modulates Fibrous Airway Obliteration and Associated Lymphoid Neogenesis After Transplantation

Chronic lung allograft dysfunction, the major cause of death following lung transplantation, usually manifests as irreversible airflow obstruction associated with obliterative bronchiolitis (OB), a lesion characterized by chronic inflammation, lymphoid neogenesis, fibroproliferation and small airway obliteration. Spleen tyrosine kinase (Syk), a tyrosine kinase that regulates B cell function and innate immunity, has been implicated in the pathogenesis of chronic inflammation and tissue repair. This study evaluated the role of Syk in development of OB, using an intrapulmonary tracheal transplant model of OB with the conditional Syk-knockout Syk(flox/flox) //rosa26-CreER(T2) mice and a Syk-selective inhibitor, GSK2230413. BALB/c trachea allografts were transplanted into Syk-knockout (Syk(del/del) ) mice or wild-type C57BL/6 recipients treated with GSK2230413. At day 28, histological analysis revealed that in the Syk(del/del) and GSK2230413-treated C57BL/6 recipients, the graft lumen remained open compared with allografts transplanted into Syk-expressing (Syk(flox/flox) ) and placebo control-treated C57BL/6 recipients. Immunofluorescence showed lymphoid neogenesis with distinct B and T cell zones in control mice. In contrast, lymphoid neogenesis was absent and few B or T cells were found in Syk(del/del) and GSK2230413-treated mice. These observations suggest that inhibition of Syk may be a potential therapeutic strategy for the management of OB following lung transplantation.

Myeloid cell-mediated renal injury in rapidly progressive glomerulonephritis depends upon spleen tyrosine kinase

Antibody-dependent activation of myeloid cells within the glomerulus plays a central role in rapidly progressive forms of glomerulonephritis. The spleen tyrosine kinase (Syk) is expressed by all leukocytes, except mature T cells, and is required for signalling via the B-cell receptor, Fc receptors, and some integrins. Syk has been proposed as a therapeutic target in glomerulonephritis. However, little is known of Syk activation in human kidney disease, while studies in experimental glomerulonephritis using non-selective Syk inhibitors require validation via conditional gene deletion. The current study addressed both of these important points. Syk activation (Tyr(525/526) phosphorylation) was examined in a cohort of 96 patients with different glomerulonephritides. Syk activation was evident in infiltrating leukocytes, mainly neutrophils and macrophages, in 36/40 cases of rapidly progressive glomerulonephritis. In contrast, non-proliferative diseases showed little or no Syk activation. Glomerular and interstitial cells exhibiting Syk activation correlated with renal function and systemic inflammation. Next, we examined mice with conditional Syk gene deletion in myeloid cells (Syk(My) ) versus Syk(f/f) littermate controls in nephrotoxic serum nephritis - a model of rapidly progressive glomerulonephritis. Control Syk(f/f) mice featured a transient neutrophil influx at 3 h and severe disease on day 9 of nephrotoxic serum nephritis, with crescent formation, macrophage infiltration, inflammation, kidney fibrosis, and renal dysfunction. In contrast, Syk(My) mice had significantly reduced neutrophil and macrophage infiltration despite equivalent glomerular deposition of humoral reactants. Syk(My) mice exhibited reduced crescent formation, inflammation, and fibrosis, with improved renal function on day 9 of nephrotoxic serum nephritis. In conclusion, Syk activation is prominent in infiltrating myeloid cells in human rapidly progressive glomerulonephritis, and functional studies demonstrate that Syk deletion in myeloid cells is protective in mouse nephrotoxic serum nephritis.

Small-molecule therapeutics in rheumatoid arthritis: scientific rationale, efficacy and safety

Rheumatoid arthritis (RA) remains a formidable clinical challenge. This is despite remarkable recent advances in our understanding of pathogenesis and the introduction of a variety of novel agents, particularly biologic therapeutics that are potent inhibitors of extracellular immune pathways. Whereas the latter have brought substantial improvements in efficacy and thus outcomes, there remain significant numbers of non- or partial responders to current standard of care. The discovery of key intracellular pathways, particularly kinases that subserve the function of these pivotal cytokine and immune cell receptors implicated in RA pathogenesis, has facilitated the advent of a new phase of RA drug development. Thus, a range of kinase inhibitors has entered clinical trials and one agent has been licenced for use in some regions. Herein we summarise the chequered history of kinase inhibitor development in RA, describing successes and failures alike, and thereafter examine future trends in this exciting field.

Calling in SYK: SYK's dual role as a tumor promoter and tumor suppressor in cancer

SYK (spleen tyrosine kinase) is well-characterized in the immune system as an essential enzyme required for signaling through multiple classes of immune recognition receptors. As a modulator of tumorigenesis, SYK has a bit of a schizophrenic reputation, acting in some cells as a tumor promoter and in others as a tumor suppressor. In many hematopoietic malignancies, SYK provides an important survival function and its inhibition or silencing frequently leads to apoptosis. In cancers of non-immune cells, SYK provides a pro-survival signal, but can also suppress tumorigenesis by restricting epithelial-mesenchymal transition, enhancing cell-cell interactions and inhibiting migration.

Functional roles of Syk in macrophage-mediated inflammatory responses

Inflammation is a series of complex biological responses to protect the host from pathogen invasion. Chronic inflammation is considered a major cause of diseases, such as various types of inflammatory/autoimmune diseases and cancers. Spleen tyrosine kinase (Syk) was initially found to be highly expressed in hematopoietic cells and has been known to play crucial roles in adaptive immune responses. However, recent studies have reported that Syk is also involved in other biological functions, especially in innate immune responses. Although Syk has been extensively studied in adaptive immune responses, numerous studies have recently presented evidence that Syk has critical functions in macrophage-mediated inflammatory responses and is closely related to innate immune response. This review describes the characteristics of Syk-mediated signaling pathways, summarizes the recent findings supporting the crucial roles of Syk in macrophage-mediated inflammatory responses and diseases, and discusses Syk-targeted drug development for the therapy of inflammatory diseases.

Blockade of Syk ameliorates the development of murine sclerodermatous chronic graft-versus-host disease

BACKGROUND

Murine sclerodermatous chronic graft-versus-host disease (Scl-cGVHD) is a model for human Scl-cGVHD and systemic sclerosis (SSc). Syk is expressed in most of hematopoietic cells, fibroblasts, and endothelial cells. Syk is a protein tyrosine kinase that has an important role in transmitting signals from a variety of cell surface receptors.

OBJECTIVE

This study aims to investigate the effect of R788 (fostamatinib sodium), an oral prodrug that is rapidly converted to a potent inhibitor of Syk, R406, on Scl-cGVHD.

METHODS

R788 was orally administered twice a day to allogeneic recipients from day 14 to day 42 after bone marrow transplantation (BMT). In vitro, proliferation of GVHD-derived CD4(+) T cells and CD11b(+) cells was analyzed by R406.

RESULTS

Allogeneic BMT increased Syk phosphorylation in T, B, and CD11b(+) cells. The administration of R788 attenuated severity and fibrosis of Scl-cGVHD. The elevated expressions of CXCR4 on T cells, B cells, and CD11b(+) cells were significantly down-regulated by R788 treatment. R788 reduced memory CD4(+) T cells (CD44(hi)CD62L(-)CD4(+)). R406 inhibited proliferation of GVHD CD4(+) T cells and CD11b(+) cells in vitro. In addition, R788 treatment, inhibited proliferation of CD11b(+) cells in Scl-cGVHD mice. R788 treatment also reduced skin mRNA expressions of MCP-1, MIP-1α, IFN-γ, IL-13, IL-17A, and TGF-β1, but not influenced RANTES, CXCL12, and TFN-α.

CONCLUSION

Blockade of Syk suppressed migration factor of immune cells and antigen-specific memory CD4(+) T cells and proliferation and activation of GVHD CD4(+) T cells and CD11b(+) cells. The current studies suggested that Syk inhibitor is a potential candidate for use in treating patients with Scl-cGVHD and SSc.

Non-receptor protein tyrosine kinases signaling pathways in normal and cancer cells

Protein tyrosine kinases (PTKs) are enzymes that transfer phosphate groups to tyrosine residues on protein substrates. Phosphorylation of proteins causes changes in their function and/or enzymatic activity resulting in specific biological responses. There are two classes of PTKs: the transmembrane receptor PTKs and the cytoplasmic non-receptor PTKs (NRTKs). NRTKs are involved in transduction of signals originating from extracellular clues, which often interact with transmembrane receptors. Thus, they are important components of signaling pathways which regulate fundamental cellular functions such as cell differentiation, apoptosis, survival, and proliferation. The activity of NRTKs is tightly regulated, and de-regulation and/or overexpression of NRTKs has been implicated in malignant transformation and carcinogenesis. Research on NRTKs has shed light on the mechanisms of a number of cellular processes including those involved in carcinogenesis. Not surprisingly, several tyrosine kinase inhibitors are in use as treatment for a number of malignancies, and more are under investigation. This review deals with the structure, function, and signaling pathways of nine main families of NRTKs in normal and cancer cells.

Discovery of a diaminoquinoxaline benzenesulfonamide antagonist of HIV-1 Nef function using a yeast-based phenotypic screen

BACKGROUND

HIV-1 Nef is a viral accessory protein critical for AIDS progression. Nef lacks intrinsic catalytic activity and binds multiple host cell signaling proteins, including Hck and other Src-family tyrosine kinases. Nef binding induces constitutive Hck activation that may contribute to HIV pathogenesis by promoting viral infectivity, replication and downregulation of cell-surface MHC-I molecules. In this study, we developed a yeast-based phenotypic screen to identify small molecules that inhibit the Nef-Hck complex.

RESULTS

Nef-Hck interaction was faithfully reconstituted in yeast cells, resulting in kinase activation and growth arrest. Yeast cells expressing the Nef-Hck complex were used to screen a library of small heterocyclic compounds for their ability to rescue growth inhibition. The screen identified a dihydrobenzo-1,4-dioxin-substituted analog of 2-quinoxalinyl-3-aminobenzene-sulfonamide (DQBS) as a potent inhibitor of Nef-dependent HIV-1 replication and MHC-I downregulation in T-cells. Docking studies predicted direct binding of DQBS to Nef which was confirmed in differential scanning fluorimetry assays with recombinant purified Nef protein. DQBS also potently inhibited the replication of HIV-1 NL4-3 chimeras expressing Nef alleles representative of all M-group HIV-1 clades.

CONCLUSIONS

Our findings demonstrate the utility of a yeast-based growth reversion assay for the identification of small molecule Nef antagonists. Inhibitors of Nef function discovered with this assay, such as DQBS, may complement the activity of current antiretroviral therapies by enabling immune recognition of HIV-infected cells through the rescue of cell surface MHC-I.

Internalization of B cell receptors in human EU12 μHC⁺ immature B cells specifically alters downstream signaling events

It has been recognized for a long time that engagement of B cell antigen receptors (BCRs) on immature B cells or mature B cells leads to completely opposite cell fate decisions. The underlying mechanism remains unclear. Here, we show that crosslinking of BCRs on human EU12 μHC⁺ immature B cells resulted in complete internalization of cell surface BCRs. After loss of cell surface BCRs, restimulation of EU12 μHC⁺ cells showed impaired Ca²⁺ flux, delayed SYK phosphorylation, and decreased CD19 and FOXO1 phosphorylation, which differ from those in mature Daudi or Ramos B cells with partial internalization of BCRs. In contrast, sustained phosphorylation and reactivation of ERK upon restimulation were observed in the EU12 μHC⁺ cells after BCR internalization. Taken together, these results show that complete internalization of cell surface BCRs in EU12 μHC⁺ cells specifically alters the downstream signaling events, which may favor receptor editing versus cell activation.