Overexpression of SLAP2 inhibits triple-negative breast cancer progression by promoting macrophage M1-type polarization

Breast cancer is the most common malignant tumor in women, and triple-negative breast cancer (TNBC) is a specific subtype of breast cancer characterized by high invasiveness, high metastatic potential, ease of recurrence, and poor prognosis. Src-like adaptor protein 2 (SLAP2), which can be involved in the regulation of multiple signaling pathways, may be a key target for TNBC. The aim of this study was to investigate the effect of overexpression of SLAP2 on TNBC and to explore the underlying mechanisms. First, we constructed and transfected SLAP2 overexpressing lentivirus based on MDA-MB-231 human TNBC cell line, screened for differential downstream target genes in combination with mRNA high-throughput sequencing (RNA-Seq), and predicted their functions and enriched pathways in conjunction with bioinformatics analysis. The effects of SLAP2 overexpression on macrophage polarization, as well as on tumor proliferation and apoptosis, were assessed by tail vein injection of a stable transfection line of 4T1 cells transfected with SLAP2 overexpressing lentivirus. The effect of SLAP2 on macrophage polarization was assessed by inducing M1/M2 polarization and transfecting SLAP2 overexpressing lentivirus. Meanwhile, a transwell co-culture system was constructed between differently treated macrophages and 4T1 cells to assess the effect of SLAP2 overexpression on the malignant behavior of the cells via macrophage polarization. Overexpression of SLAP2 revealed 179 genes up-regulated and 74 genes down-regulated by mRNA high-throughput sequencing, and the enriched functions and pathways of differential genes were mainly related to immunity response. In vivo experiments revealed that overexpression of SLAP2 inhibited the growth of tumor in nude mice, decreased the expression of ki67 in tumor tissues, and increased the rate of apoptosis in tumor tissues. Meanwhile, we found that overexpression of SLAP2 promoted macrophage polarization toward M1 type and inhibited M2 type polarization in tumors. In vitro experiments further verified its effect on M1/M2 polarization by transfecting SLAP2 overexpressing lentivirus. By transwell co-culture system, we further demonstrated that overexpression of SLAP2 inhibits cell proliferation and invasion, promotes apoptosis, up-regulates the expression of Bax in cells, and down-regulates the expression of Bcl-2 in cells by promoting macrophage M1-type polarization. Overexpression of SLAP2 inhibits TNBC progression by promoting macrophage M1-type polarization.

Protein tyrosine phosphatase receptor type E (PTPRE) regulates the activation of wild-type KIT and KIT mutants differently

Activation of receptor tyrosine kinases needs tight control by tyrosine phosphatases to keep their normal function. In this study, we investigated the regulation of activation of the type III receptor tyrosine kinase KIT by protein tyrosine phosphatase receptor type E (PTPRE). We found that PTPRE can associate with wild-type KIT and inhibit KIT activation in a dose-dependent manner, although the activation of wild-type KIT is dramatically inhibited even when PTPRE is expressed at low level. The D816V mutation of KIT is the most frequently found oncogenic mutation in mastocytosis, and we found that PTPRE can associate and inhibit the activation of KIT/D816V in a dose dependent manner, but the inhibition is much weaker compared with wild-type KIT. Similar to mastocytosis, KIT mutations are the main oncogenic mutations in gastrointestinal stromal tumors (GISTs) although GISTs carry different types of KIT mutations. We further studied the regulation of the activation of GISTs-type KIT mutants and other mastocytosis-type KIT mutants by PTPRE. Indeed, PTPRE can almost block the activation of GISTs-type KIT mutants, while the activation of mastocytosis-type KIT mutants is more resistant to the inhibition of PTPRE. Taken together, our results suggest that PTPRE can associate with KIT, and inhibit the activation of both wild-type KIT and GISTs-type KIT mutants, while the activation of mastocytosis-type KIT mutants is more resistant to PTPRE.

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PTPRE associates with wild-type KIT and KIT mutants.

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PTPRE inhibits the activation of both wild-type KIT and GISTs-type KIT mutants dramatically.

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The activation of mastocytosis-type KIT mutants are more resistant to the inhibition of PTPRE.

SLAP2 Adaptor Binding Disrupts c-CBL Autoinhibition to Activate Ubiquitin Ligase Function

CBL is a RING type E3 ubiquitin ligase that functions as a negative regulator of tyrosine kinase signaling and loss of CBL E3 function is implicated in several forms of leukemia. The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL and are required for CBL-dependent downregulation of antigen receptor, cytokine receptor, and receptor tyrosine kinase signaling. Despite the established role of SLAP/SLAP2 in regulating CBL activity, the nature of the interaction and the mechanisms involved are not known. To understand the molecular basis of the interaction between SLAP/SLAP2 and CBL, we solved the crystal structure of CBL tyrosine kinase binding domain (TKBD) in complex with SLAP2. The carboxy-terminal region of SLAP2 adopts an α-helical structure which binds in a cleft between the 4H, EF-hand, and SH2 domains of the TKBD. This SLAP2 binding site is remote from the canonical TKBD phospho-tyrosine peptide binding site but overlaps with a region important for stabilizing CBL in its autoinhibited conformation. In addition, binding of SLAP2 to CBL in vitro activates the ubiquitin ligase function of autoinhibited CBL. Disruption of the CBL/SLAP2 interface through mutagenesis demonstrated a role for this protein-protein interaction in regulation of CBL E3 ligase activity in cells. Our results reveal that SLAP2 binding to a regulatory cleft of the TKBD provides an alternative mechanism for activation of CBL ubiquitin ligase function.

Loss of PI3 kinase association improves the sensitivity of secondary mutation of KIT to Imatinib

Background

KIT mutations are the predominant driver mutations in gastrointestinal stromal tumors (GISTs), and targeted therapy against KIT has improved treatment outcome dramatically. However, gaining secondary mutation of KIT confers drug resistance of GISTs leading to treatment failure.

Results

In this study, we found that secondary mutation of KIT dramatically increases the ligand-independent activation of the receptor and their resistance to the often used KIT inhibitor Imatinib in the treatment of GISTs. PI3 kinase plays essential roles in the cell transformation mediated by the primary mutation of KIT. We found that loss of PI3 kinase association, but not the inhibition of the lipid kinase activity of PI3 kinase, inhibits the ligand-independent activation of secondary mutations of KIT, and increases their sensitivity to Imatinib, and loss of PI3 kinase association inhibits secondary mutations of KIT mediated cell survival and proliferation in vitro. The in vivo assay further showed that the growth of tumors carrying secondary mutations of KIT is more sensitive to Imatinib when PI3 kinase association is blocked while inhibition of the lipid kinase activity of PI3 kinase cannot inhibit tumor growth, indicating that PI3 kinase is important for the drug resistance of secondary mutation of KIT independent of the lipid kinase activity of PI3 kinase.

Conclusions

Our results suggested that PI3 kinase is necessary for the ligand-independent activation of secondary mutations of KIT, and loss of PI3 kinase association improves the sensitivity of secondary mutations to the targeted therapy independent of the lipid kinase activity of PI3 kinase.

SLAP Is a Negative Regulator of FcεRI Receptor-Mediated Signaling and Allergic Response

Binding of antigen to IgE-high affinity FcεRI complexes on mast cells and basophils results in the release of preformed mediators such as histamine and de novo synthesis of cytokines causing allergic reactions. Src-like adapter protein (SLAP) functions co-operatively with c-Cbl to negatively regulate signaling downstream of the T cell receptor, B cell receptor, and receptor tyrosine kinases (RTK). Here, we investigated the role of SLAP in FcεRI-mediated mast cell signaling, using bone marrow derived mast cells (BMMCs) from SLAP knock out (SLAP KO) mice. Mature SLAP-KO BMMCs displayed significantly enhanced antigen induced degranulation and synthesis of IL-6, TNFα, and MCP-1 compared to wild type (WT) BMMCs. In addition, SLAP KO mice displayed an enhanced passive cutaneous anaphylaxis response. In agreement with a negative regulatory role, SLAP KO BMMCs showed enhanced FcεRI-mediated signaling to downstream effector kinases, Syk, Erk, and Akt. Recombinant GST-SLAP protein binds to the FcεRIβ chain and to the Cbl-b in mast cell lysates, suggesting a role in FcεRI down regulation. In addition, the ubiquitination of FcεRIγ chain and antigen mediated down regulation of FcεRI is impaired in SLAP KO BMMCs compared to the wild type. In line with these findings, stimulation of peripheral blood human basophils with FcεRIα antibody, or a clinically relevant allergen, resulted in increased SLAP expression. Together, these results indicate that SLAP is a dynamic regulator of IgE-FcεRI signaling, limiting allergic responses.

Control of Tyrosine Kinase Signalling by Small Adaptors in Colorectal Cancer

Tyrosine kinases (TKs) phosphorylate proteins on tyrosine residues as an intracellular signalling mechanism to coordinate intestinal epithelial cell communication and fate decision. Deregulation of their activity is ultimately connected with carcinogenesis. In colorectal cancer (CRC), it is still unclear how aberrant TK activities contribute to tumour formation because TK-encoding genes are not frequently mutated in this cancer. In vertebrates, several TKs are under the control of small adaptor proteins with potential important physiopathological roles. For instance, they can exert tumour suppressor functions in human cancer by targeting several components of the oncogenic TK signalling cascades. Here, we review how the Src-like adaptor protein (SLAP) and the suppressor of cytokine signalling (SOCS) adaptor proteins regulate the SRC and the Janus kinase (JAK) oncogenic pathways, respectively, and how their loss of function in the intestinal epithelium may influence tumour formation. We also discuss the potential therapeutic value of these adaptors in CRC.

SRC-like adaptor protein negatively regulates Wnt signaling in intrahepatic cholangiocarcinoma

Currently, the molecular mechanisms underlying intrahepatic cholangiocarcinoma (IHCC) are poorly understood. In the present study, the focus was primarily on SRC-like adaptor protein (SLAP), an adaptor protein, which is aberrantly expressed in various cancer types. To the best of our knowledge, the present study was the first to demonstrate that SLAP was decreased in IHCC tissues and cells, compared with controls. Further study indicated that SLAP overexpression suppressed IHCC cell proliferation and induced cell cycle arrest, indicating the tumor suppressor role of SLAP in IHCC progression. To demonstrate the effects of SLAP on Wnt signaling, the β-catenin/T cell factor transcription reporter assay was conducted. Compared with the negative adenovirus vector control (Ad-NC), overexpression of SLAP reduced TOPflash activity, and no changes in FOPflash activity were identified. Furthermore, the expression levels of Wnt target genes, including β-catenin, c-Myc, cluster of differentiation 44, Slug, Vimentin and matrix metallopeptidase-9, were reduced in RBE and Huh28 cells overexpressing SLAP. Additionally, the effects of SLAP on IHCC cell invasion and migration were determined. Compared with the Ad-NC control, the migration and invasion capacity was reduced following overexpression of SLAP in RBE and Huh28 cells. In summary, reduced SLAP expression may enhance IHCC malignant progression by activating Wnt signaling.

XK-related protein 5 (XKR5) is a novel negative regulator of KIT/D816V-mediated transformation

In order to investigate the molecular mechanisms by which the oncogenic mutant KIT/D816V causes transformation of cells, we investigated proteins that selectively bind KIT/D816V, but not wild-type KIT, as potential mediators of transformation. By mass spectrometry several proteins were identified, among them a previously uncharacterized protein denoted XKR5 (XK-related protein 5), which is related to the X Kell blood group proteins. We could demonstrate that interaction between XKR5 and KIT/D816V leads to phosphorylation of XKR5 at Tyr 369, Tyr487, and Tyr 543. Tyrosine phosphorylated XKR5 acts as a negative regulator of KIT signaling, which leads to downregulation of phosphorylation of ERK, AKT, and p38. This led to reduced proliferation and colony forming capacity in semi-solid medium. Taken together, our data demonstrate that XKR5 is a novel type of negative regulator of KIT-mediated transformation.

SRC-like adaptor protein 2 (SLAP2) is a negative regulator of KIT-D816V-mediated oncogenic transformation

KIT is a receptor tyrosine kinase (RTK) involved in several cellular processes such as regulation of proliferation, survival and differentiation of early hematopoietic cells, germ cells and melanocytes. Activation of KIT results in phosphorylation of tyrosine residues in the receptor, and recruitment of proteins that mediate downstream signaling and also modulate receptor signaling. Here we show that the SRC-like adaptor protein 2 (SLAP2) binds to wild-type KIT in a ligand-dependent manner and is furthermore found constitutively associated with the oncogenic mutant KIT-D816V. Peptide fishing analysis mapped pY568 and pY570 as potential SLAP2 association sites in KIT, which overlaps with the SRC binding sites in KIT. Expression of SLAP2 in cells expressing the transforming mutant KIT-D816V led to reduced cell viability and reduced colony formation. SLAP2 also partially blocked phosphorylation of several signal transduction molecules downstream of KIT such as AKT, ERK, p38 and STAT3. Finally, SLAP2 expression enhanced ubiquitination of KIT and its subsequent degradation. Taken together, our data demonstrate that SLAP2 negatively modulates KIT-D816V-mediated transformation by enhancing degradation of the receptor.

Src-like adaptor protein 2 (SLAP2) binds to and inhibits FLT3 signaling

Fms-like tyrosine kinase (FLT3) is a frequently mutated oncogene in acute myeloid leukemia (AML). FLT3 inhibitors display promising results in a clinical setting, but patients relapse after short-term treatment due to the development of resistant disease. Therefore, a better understanding of FLT3 downstream signal transduction pathways will help to identify an alternative target for the treatment of AML patients carrying oncogenic FLT3. Activation of FLT3 results in phosphorylation of FLT3 on several tyrosine residues that recruit SH2 domain-containing signaling proteins. We screened a panel of SH2 domain-containing proteins and identified SLAP2 as a potent interacting partner of FLT3. We demonstrated that interaction occurs when FLT3 is activated, and also, an intact SH2 domain of SLAP2 is required for binding. SLAP2 binding sites in FLT3 mainly overlap with those of SRC. SLAP2 over expression in murine proB cells or myeloid cells inhibited oncogenic FLT3-ITD-mediated cell proliferation and colony formation in vitro, and tumor formation in vivo. Microarray analysis suggests that higher SLAP2 expression correlates with a gene signature similar to that of loss of oncogene function. Furthermore, FLT3-ITD positive AML patients with higher SLAP2 expression displayed better prognosis compared to those with lower expression of SLAP2. Expression of SLAP2 blocked FLT3 downstream signaling cascades including AKT, ERK, p38 and STAT5. Finally, SLAP2 accelerated FLT3 degradation through enhanced ubiquitination. Collectively, our data suggest that SLAP2 acts as a negative regulator of FLT3 signaling and therefore, modulation of SLAP2 expression levels may provide an alternative therapeutic approach for FLT3-ITD positive AML.

The role of small adaptor proteins in the control of oncogenic signalingr driven by tyrosine kinases in human cancer

Protein phosphorylation on tyrosine (Tyr) residues has evolved as an important mechanism to coordinate cell communication in multicellular organisms. The importance of this process has been revealed by the discovery of the prominent oncogenic properties of tyrosine kinases (TK) upon deregulation of their physiological activities, often due to protein overexpression and/or somatic mutation. Recent reports suggest that TK oncogenic signaling is also under the control of small adaptor proteins. These cytosolic proteins lack intrinsic catalytic activity and signal by linking two functional members of a catalytic pathway. While most adaptors display positive regulatory functions, a small group of this family exerts negative regulatory functions by targeting several components of the TK signaling cascade. Here, we review how these less studied adaptor proteins negatively control TK activities and how their loss of function induces abnormal TK signaling, promoting tumor formation. We also discuss the therapeutic consequences of this novel regulatory mechanism in human oncology.

The Src family kinase LCK cooperates with oncogenic FLT3/ITD in cellular transformation

The non-receptor tyrosine kinase LCK belongs to the SRC family of kinases. SRC family kinases are proto-oncogenes that have long been known to play key roles in cell proliferation, motility, morphology and survival. Here we show that LCK regulates the function of the type III receptor tyrosine kinase FLT3 in murine pro-B cells. We observed that expression of LCK significantly enhances the colony forming capacity of the constitutively active FLT3 mutant FLT3-ITD (internal tandem duplication). Furthermore, cells expressing LCK developed tumor earlier compared to cells transfected with empty control vector. Staining of the tissues from mouse xenografts showed higher Ki67 staining in cells expressing LCK suggesting that expression of LCK enhances the FLT3-ITD-mediated proliferative capacity. LCK expression did not affect either FLT3-WT or FLT3-ITD -induced AKT, ERK1/2 or p38 phosphorylation. However, LCK expression significantly enhanced FLT3-ITD-mediated STAT5 phosphorylation. Taken together, our data suggest that LCK cooperates with oncogenic FLT3-ITD in cellular transformation.

ABL2 suppresses FLT3-ITD-induced cell proliferation through negative regulation of AKT signaling

The type III receptor tyrosine kinase FLT3 is one of the most commonly mutated oncogenes in acute myeloid leukemia (AML). Inhibition of mutated FLT3 in combination with chemotherapy has displayed promising results in clinical trials. However, one of the major obstacles in targeting FLT3 is the development of resistant disease due to secondary mutations in FLT3 that lead to relapse. FLT3 and its oncogenic mutants signal through associating proteins that activate downstream signaling. Thus, targeting proteins that interact with FLT3 and their downstream signaling cascades can be an alternative approach to treat FLT3-dependent AML. We used an SH2 domain array screen to identify novel FLT3 interacting proteins and identified ABL2 as a potent interacting partner of FLT3. To understand the role of ABL2 in FLT3-mediated biological and cellular events, we used the murine pro-B cell line Ba/F3 as a model system. Overexpression of ABL2 in Ba/F3 cells expressing an oncogenic mutant of FLT3 (FLT3-ITD) resulted in partial inhibition of FLT3-ITD-dependent cell proliferation and colony formation. ABL2 expression did not alter the kinase activity of FLT3, its ubiquitination or its stability. However, it partially blocked FLT3-induced AKT phosphorylation without affecting ERK1/2 and p38 activation. Taken together our data suggest that ABL2 acts as negative regulator of signaling downstream of FLT3.

Tyrosine 842 in the activation loop is required for full transformation by the oncogenic mutant FLT3-ITD

The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation.

FYN expression potentiates FLT3-ITD induced STAT5 signaling in acute myeloid leukemia

FYN is a non-receptor tyrosine kinase belonging to the SRC family of kinases, which are frequently over-expressed in human cancers, and play key roles in cancer biology. SRC has long been recognized as an important oncogene, but little attention has been given to its other family members. In this report, we have studied the role of FYN in FLT3 signaling in respect to acute myeloid leukemia (AML). We observed that FYN displays a strong association with wild-type FLT3 as well as oncogenic FLT3-ITD and is dependent on the kinase activity of FLT3 and the SH2 domain of FYN. We identified multiple FYN binding sites in FLT3, which partially overlapped with SRC binding sites. To understand the role of FYN in FLT3 signaling, we generated FYN overexpressing cells. We observed that expression of FYN resulted in slightly enhanced phosphorylation of AKT, ERK1/2 and p38 in response to ligand stimulation. Furthermore, FYN expression led to a slight increase in FLT3-ITD-dependent cell proliferation, but potent enhancement of STAT5 phosphorylation as well as colony formation. We also observed that FYN expression is deregulated in AML patient samples and that higher expression of FYN, in combination with FLT3-ITD mutation, resulted in enrichment of the STAT5 signaling pathway and correlated with poor prognosis in AML. Taken together our data suggest that FYN cooperates with oncogenic FLT3-ITD in cellular transformation by selective activation of the STAT5 pathway. Therefore, inhibition of FYN, in combination with FLT3 inhibition, will most likely be beneficial for this group of AML patients.

Germline mutations of KIT in gastrointestinal stromal tumor (GIST) and mastocytosis

Somatic mutations of KIT are frequently found in mastocytosis and gastrointestinal stromal tumor (GIST), while germline mutations of KIT are rare, and only found in few cases of familial GIST and mastocytosis. Although ligand-independent activation is the common feature of KIT mutations, the phenotypes mediated by various germline KIT mutations are different. Germline KIT mutations affect different tissues such as interstitial cells of Cajal (ICC), mast cells or melanocytes, and thereby lead to GIST, mastocytosis, or abnormal pigmentation. In this review, we summarize germline KIT mutations in familial mastocytosis and GIST and discuss the possible cellular context dependent transforming activity of KIT mutations.

The Emerging and Diverse Roles of Src-Like Adaptor Proteins in Health and Disease

Although Src-like adaptor proteins (SLAP-1 and SLAP-2) were mainly studied in lymphocytes, where they act as negative regulators and provide fine control of receptor signaling, recently, several other functions of these proteins were discovered. In addition to the well-characterized immunoregulatory functions, SLAP proteins appear to have an essential role in the pathogenesis of type I hypersensitivity, osteoporosis, and numerous malignant diseases. Both adaptor proteins are expressed in a wide variety of tissues, where they have mostly inhibitory effects on multiple intracellular signaling pathways. In this review, we summarize the diverse effects of SLAP proteins.

Role of SRC-like adaptor protein (SLAP) in immune and malignant cell signaling

SRC-like adaptor protein (SLAP) is an adaptor protein structurally similar to the SRC family protein kinases. Like SRC, SLAP contains an SH3 domain followed by an SH2 domain but the kinase domain has been replaced by a unique C-terminal region. SLAP is expressed in a variety of cell types. Current studies suggest that it regulates signaling of various cell surface receptors including the B cell receptor, the T cell receptor, cytokine receptors and receptor tyrosine kinases which are important regulator of immune and cancer cell signaling. SLAP targets receptors, or its associated components, by recruiting the ubiquitin machinery and thereby destabilizing signaling. SLAP directs receptors to ubiquitination-mediated degradation and controls receptors turnover as well as signaling. Thus, SLAP appears to be an important component in regulating signal transduction required for immune and malignant cells.

Oncogenic Kit signals on endolysosomes and endoplasmic reticulum are essential for neoplastic mast cell proliferation

Kit is a receptor-type tyrosine kinase found on the plasma membrane. It can transform mast cells through activating mutations. Here, we show that a mutant Kit from neoplastic mast cells from mice, Kit(D814Y), is permanently active and allows cells to proliferate autonomously. It does so by activating two signalling pathways from different intracellular compartments. Mutant Kit from the cell surface accumulates on endolysosomes through clathrin-mediated endocytosis, which requires Kit's kinase activity. Kit(D814Y) is constitutively associated with phosphatidylinositol 3-kinase, but the complex activates Akt only on the cytoplasmic surface of endolysosomes. It resists destruction because it is under-ubiquitinated. Kit(D814Y) also appears in the endoplasmic reticulum soon after biosynthesis, and there, can activate STAT5 aberrantly. These mechanisms of oncogenic signalling are also seen in rat and human mast cell leukemia cells. Thus, oncogenic Kit signalling occurs from different intracellular compartments, and the mutation acts by altering Kit trafficking as well as activation.

RTK SLAP down: the emerging role of Src-like adaptor protein as a key player in receptor tyrosine kinase signaling

SLAP (Src like adaptor protein) contains adjacent Src homology 3 (SH3) and Src homology 2 (SH2) domains closely related in sequence to that of cytoplasmic Src family tyrosine kinases. Expressed most abundantly in the immune system, SLAP function has been predominantly studied in the context of lymphocyte signaling, where it functions in the Cbl dependent downregulation of antigen receptor signaling. However, accumulating evidence suggests that SLAP plays a role in the regulation of a broad range of membrane receptors including members of the receptor tyrosine kinase (RTK) family. In this review we highlight the role of SLAP in the ubiquitin dependent regulation of type III RTKs PDGFR, CSF-1R, KIT and Flt3, as well as Eph family RTKs. SLAP appears to bind activated type III and Eph RTKs via a conserved autophosphorylated juxtamembrane tyrosine motif in an SH2-dependent manner, suggesting that SLAP is important in regulating RTK signaling.

SOCS proteins in regulation of receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTKs) are a family of cell surface receptors that play critical roles in signal transduction from extracellular stimuli. Many in this family of kinases are overexpressed or mutated in human malignancies and thus became an attractive drug target for cancer treatment. The signaling mediated by RTKs must be tightly regulated by interacting proteins including protein-tyrosine phosphatases and ubiquitin ligases. The suppressors of cytokine signaling (SOCS) family proteins are well-known negative regulators of cytokine receptors signaling consisting of eight structurally similar proteins, SOCS1-7, and cytokine-inducible SH2-containing protein (CIS). A key feature of this family of proteins is the presence of an SH2 domain and a SOCS box. Recent studies suggest that SOCS proteins also play a role in RTK signaling. Activation of RTK results in transcriptional activation of SOCS-encoding genes. These proteins associate with RTKs through their SH2 domains and subsequently recruit the E3 ubiquitin machinery through the SOCS box, and thereby limit receptor stability by inducing ubiquitination. In a similar fashion, SOCS proteins negatively regulate mitogenic signaling by RTKs. It is also evident that RTKs can sometimes bypass SOCS regulation and SOCS proteins can even potentiate RTKs-mediated mitogenic signaling. Thus, apart from negative regulation of receptor signaling, SOCS proteins may also influence signaling in other ways.

SOCS6 is a selective suppressor of receptor tyrosine kinase signaling

The suppressors of cytokine signaling (SOCS) are well-known negative regulators of cytokine receptor signaling. SOCS6 is one of eight members of the SOCS family of proteins. Similar to other SOCS proteins, SOCS6 consists of an uncharacterized extended N-terminal region followed by an SH2 domain and a SOCS box. Unlike other SOCS proteins, SOCS6 is mainly involved in negative regulation of receptor tyrosine kinase signaling. SOCS6 is widely expressed in many tissues and is found to be downregulated in many cancers including colorectal cancer, gastric cancer, lung cancer, ovarian cancer, stomach cancer, thyroid cancer, hepatocellular carcinoma, and pancreatic cancer. SOCS6 is involved in negative regulation of receptor signaling by increasing degradation mediated by ubiquitination of receptors or substrate proteins and induces apoptosis by targeting mitochondrial proteins. Therefore, SOCS6 turns out as an important regulator of survival signaling and its activity is required for controlling receptor tyrosine kinase signaling.

Grb10 is a dual regulator of receptor tyrosine kinase signaling

The adaptor protein Grb10 is a close homolog of Grb7 and Grb14. These proteins are characterized by an N-terminal proline-rich region, a Ras-GTPase binding domain, a PH domain, an SH2 domain and a BPS domain in between the PH and SH2 domains. Human Grb10 gene encodes three splice variants. These variants show differences in functionality. Grb10 associates with multiple proteins including tyrosine kinases in a tyrosine phosphorylation dependent or independent manner. Association with multiple proteins allows Grb10 to regulate different signaling pathways resulting in different biological consequences.