The Src-like adaptor protein regulates PDGF-induced actin dorsal ruffles in a c-Cbl-dependent manner

A Systematic Compilation of Human SH3 Domains: A Versatile Superfamily in Cellular Signaling

SRC homology 3 (SH3) domains are fundamental modules that enable the assembly of protein complexes through physical interactions with a pool of proline-rich/noncanonical motifs from partner proteins. They are widely studied modular building blocks across all five kingdoms of life and viruses, mediating various biological processes. The SH3 domains are also implicated in the development of human diseases, such as cancer, leukemia, osteoporosis, Alzheimer's disease, and various infections. A database search of the human proteome reveals the existence of 298 SH3 domains in 221 SH3 domain-containing proteins (SH3DCPs), ranging from 13 to 720 kilodaltons. A phylogenetic analysis of human SH3DCPs based on their multi-domain architecture seems to be the most practical way to classify them functionally, with regard to various physiological pathways. This review further summarizes the achievements made in the classification of SH3 domain functions, their binding specificity, and their significance for various diseases when exploiting SH3 protein modular interactions as drug targets.

Characterization of unique functionalities in c-Src domains required for osteoclast podosome belt formation

Deletion of c-Src, a ubiquitously expressed tyrosine kinase, results in osteoclast dysfunction and osteopetrosis, in which bones harden into "stone." In contrast, deletion of the genes encoding other members of the Src family kinase (SFK) fails to produce an osteopetrotic phenotype. This suggests that c-Src performs a unique function in the osteoclast that cannot be compensated for by other SFKs. We aimed to identify the molecular basis of this unique role in osteoclasts and bone resorption. We found that c-Src, Lyn, and Fyn were the most highly expressed SFKs in WT osteoclasts, whereas Hck, Lck, Blk, and Fgr displayed low levels of expression. Formation of the podosome belt, clusters of unique actin assemblies, was disrupted in src^-/- osteoclasts; introduction of constitutively activated SFKs revealed that only c-Src and Fyn could restore this process. To identify the key structural domains responsible, we constructed chimeric Src-Hck and Src-Lyn constructs in which the unique, SH3, SH2, or catalytic domains had been swapped. We found that the Src unique, SH3, and kinase domains were each crucial to establish Src functionality. The SH2 domain could however be substituted with Lyn or Hck SH2 domains. Furthermore, we demonstrate that c-Src's functionality is, in part, derived from an SH3-proximal proline-rich domain interaction with c-Cbl, leading to phosphorylation of c-Cbl Tyr700. These data help clarify Src's unique functionality in the organization of the cytoskeleton in osteoclasts, required for efficient bone resorption and explain why c-Src cannot be replaced, in osteoclasts, by other SFKs.

Genetic analyses in mouse fibroblast and melanoma cells demonstrate novel roles for PDGF-AB ligand and PDGF receptor alpha

Platelet Derived Growth Factor Receptor (PDGFR) signaling is a central mitogenic pathway in development, as well as tissue repair and homeostasis. The rules governing the binding of PDGF ligand to the receptor to produce activation and downstream signaling have been well defined over the last several decades. In cultured cells after a period of serum deprivation, treatment with PDGF leads to the rapid formation of dramatic, actin-rich Circular Dorsal Ruffles (CDRs). Using CDRs as a robust visual readout of early PDGFR signaling, we have identified several contradictory elements in the widely accepted model of PDGF activity. Employing CRISPR/Cas9 gene editing to disrupt the Pdgfra gene in two different murine cell lines, we show that in addition to the widely accepted function for PDGFR-beta in CDR formation, PDGFR-alpha is also clearly capable of eliciting CDRs. Moreover, we demonstrate activity for heterodimeric PDGF-AB ligand in the vigorous activation of PDGFR-beta homodimers to produce CDRs. These findings are key to a more complete understanding of PDGF ligand-receptor interactions and their downstream signaling consequences. This knowledge will allow for more rigorous experimental design in future studies of PDGFR signaling and its contributions to development and disease.

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.

E3 Ubiquitin Ligase c-cbl Inhibits Microglia Activation After Chronic Constriction Injury

E3 ubiquitin ligase c-Caritas B cell lymphoma (c-cbl) is associated with negative regulation of receptor tyrosine kinases, signal transduction of antigens and cytokine receptors, and immune response. However, the expression and function of c-cbl in the regulation of neuropathic pain after chronic constriction injury (CCI) are unknown. In rat CCI model, c-cbl inhibited the activation of spinal cord microglia and the release of pro-inflammatory factors including tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β) and interleukin 6 (IL-6), which alleviated mechanical and heat pain through down-regulating extracellular signal-regulated kinase (ERK) pathway. Additionally, exogenous TNF-α inhibited c-cbl protein level vice versa. In the primary microglia transfected with c-cbl siRNA, when treated with TNF-α or TNF-α inhibitor, the corresponding secretion of IL-1β and IL-6 did not change. In summary, CCI down-regulated c-cbl expression and induced the activation of microglia, then activated microglia released inflammatory factors via ERK signaling to cause pain. Our data might supply a novel molecular target for the therapy of CCI-induced neuropathic pain.

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.

SLAP displays tumour suppressor functions in colorectal cancer via destabilization of the SRC substrate EPHA2

The adaptor SLAP is a negative regulator of receptor signalling in immune cells but its role in human cancer is ill defined. Here we report that SLAP is abundantly expressed in healthy epithelial intestine but strongly downregulated in 50% of colorectal cancer. SLAP overexpression suppresses cell tumorigenicity and invasiveness while SLAP silencing enhances these transforming properties. Mechanistically, SLAP controls SRC/EPHA2/AKT signalling via destabilization of the SRC substrate and receptor tyrosine kinase EPHA2. This activity is independent from CBL but requires SLAP SH3 interaction with the ubiquitination factor UBE4A and SLAP SH2 interaction with pTyr594-EPHA2. SRC phosphorylates EPHA2 on Tyr594, thus creating a feedback loop that promotes EPHA2 destruction and thereby self-regulates its transforming potential. SLAP silencing enhances SRC oncogenicity and sensitizes colorectal tumour cells to SRC inhibitors. Collectively, these data establish a tumour-suppressive role for SLAP in colorectal cancer and a mechanism of SRC oncogenic induction through stabilization of its cognate substrates.

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.

CBL controls a tyrosine kinase network involving AXL, SYK and LYN in nilotinib-resistant chronic myeloid leukaemia

A tyrosine kinase network composed of the TAM receptor AXL and the cytoplasmic kinases LYN and SYK is involved in nilotinib-resistance of chronic myeloid leukaemia (CML) cells. Here, we show that the E3-ubiquitin ligase CBL down-regulation occurring during prolonged drug treatment plays a critical role in this process. Depletion of CBL in K562 cells increases AXL and LYN protein levels, promoting cell resistance to nilotinib. Conversely, forced expression of CBL in nilotinib-resistant K562 cells (K562-rn) dramatically reduces AXL and LYN expression and resensitizes K562-rn cells to nilotinib. A similar mechanism was found to operate in primary CML CD34(+) cells. Mechanistically, the E3-ligase CBL counteracts AXL/SYK signalling, promoting LYN transcription by controlling AXL protein stability. Surprisingly, the role of AXL in resistance was independent of its ligand GAS6 binding and its TK activity, in accordance with a scaffold activity for this receptor being involved in this cellular process. Collectively, our results demonstrate a pivotal role for CBL in the control of a tyrosine kinase network mediating resistance to nilotinib treatment in CML cells.

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.

Induction of membrane circular dorsal ruffles requires co-signalling of integrin-ILK-complex and EGF receptor

Integrin and receptor tyrosine kinase signalling networks cooperate to regulate various biological functions. The molecular details underlying the integration of both signalling networks remain largely uncharacterized. Here we identify a signalling module composed of a fibronectin-α5β1-integrin-integrin-linked-kinase (ILK) complex that, in concert with epidermal growth factor (EGF) cues, cooperatively controls the formation of transient actin-based circular dorsal ruffles (DRs) in fibroblasts. DR formation depends on the precise spatial activation of Src at focal adhesions by integrin and EGF receptor signals, in an ILK-dependent manner. In a SILAC-based phosphoproteomics screen we identified the tumour-suppressor Cyld as being required for DR formation induced by α5β1 integrin and EGF receptor co-signalling. Furthermore, EGF-induced Cyld tyrosine phosphorylation is controlled by integrin-ILK and Src as a prerequisite for DR formation. This study provides evidence for a novel function of integrin-ILK and EGF signalling crosstalk in mediating Cyld tyrosine phosphorylation and fast actin-based cytoskeletal rearrangements.

Src-Like adaptor protein (SLAP) binds to the receptor tyrosine kinase Flt3 and modulates receptor stability and downstream signaling

Fms-like tyrosine kinase 3 (Flt3) is an important growth factor receptor in hematopoiesis. Gain-of-function mutations of the receptor contribute to the transformation of acute myeloid leukemia (AML). Src-like adaptor protein (SLAP) is an interaction partner of the E3 ubiquitin ligase Cbl that can regulate receptor tyrosine kinases-mediated signal transduction. In this study, we analyzed the role of SLAP in signal transduction downstream of the type III receptor tyrosine kinase Flt3. The results show that upon ligand stimulation SLAP stably associates with Flt3 through multiple phosphotyrosine residues in Flt3. SLAP constitutively interacts with oncogenic Flt3-ITD and co-localizes with Flt3 near the cell membrane. This association initiates Cbl-dependent receptor ubiquitination and degradation. Depletion of SLAP expression by shRNA in Flt3-transfected Ba/F3 cells resulted in a weaker activation of FL-induced PI3K-Akt and MAPK signaling. Meta-analysis of microarray data from patient samples suggests that SLAP mRNA is differentially expressed in different cancers and its expression was significantly increased in patients carrying the Flt3-ITD mutation. Thus, our data suggest a novel role of SLAP in different cancers and in modulation of receptor tyrosine kinase signaling apart from its conventional role in regulation of receptor stability.

Co-localization of the PDGF β-Receptor and Actin during PDGF Stimulation in Mouse Fibroblasts

The subcellular localization of the PDGF β-receptor was investigated in relation with PDGF-induced actin and membrane dynamics in mouse C3H10T1/2 fibroblasts. Serum-starved cells exhibit a nonhomogenous distribution of PDGF β-receptors. However, the observed pattern does not resemble the localization of PDGF-induced actin structures. Interestingly, the PDGF β-receptor showed a changed subcellular distribution in relation to the formation of PDGF-BB-induced actin structures. Upon PDGF exposure, PDGF β-receptors were found to accumulate in dorsal circular ruffles. The presence of both macropinosomes and clathrin in the induced circular ruffles suggests that the accumulation of PDGF β-receptors in circular ruffles results in the efficient internalization of PDGF β-receptors.

SLAP, a regulator of immunoreceptor ubiquitination, signaling, and trafficking

Src-like adapter proteins (SLAP and SLAP-2) constitute a family of proteins that are expressed in a variety of cell types but are studied most extensively in lymphocytes. They have been shown to associate with proximal components of the T-cell receptor (TCR) and B-cell receptor (BCR) signaling complexes. An interaction of SLAP with c-Cbl leads to the ubiquitination and degradation of phosphorylated components of the TCR- and BCR-signaling complexes. The absence of this process in immature SLAP-deficient T and B cells leads to increased immunoreceptor levels due to decreased intracellular retention and degradation. We propose a model in which SLAP-dependent regulation of immunoreceptor levels allows for finer control of immunoreceptor signaling. Thus, SLAP functions to dampen immunoreceptor signaling, thereby influencing lymphocyte development and repertoire selection.

Identification of c-Src tyrosine kinase substrates in platelet-derived growth factor receptor signaling

c-Src non-receptor tyrosine kinase is an important component of the platelet-derived growth factor (PDGF) receptor signaling pathway. c-Src has been shown to mediate the mitogenic response to PDGF in fibroblasts. However, the exact components of PDGF receptor signaling pathway mediated by c-Src remain unclear. Here, we used stable isotope labeling with amino acids in cell culture (SILAC) coupled with mass spectrometry to identify Src-family kinase substrates involved in PDGF signaling. Using SILAC, we were able to detect changes in tyrosine phosphorylation patterns of 43 potential c-Src kinase substrates in PDGF receptor signaling. This included 23 known c-Src kinase substrates, of which 16 proteins have known roles in PDGF signaling while the remaining 7 proteins have not previously been implicated in PDGF receptor signaling. Importantly, our analysis also led to identification of 20 novel Src-family kinase substrates, of which 5 proteins were previously reported as PDGF receptor signaling pathway intermediates while the remaining 15 proteins represent novel signaling intermediates in PDGF receptor signaling. In validation experiments, we demonstrated that PDGF indeed induced the phosphorylation of a subset of candidate Src-family kinase substrates - Calpain 2, Eps15 and Trim28 - in a c-Src-dependent fashion.

Src-like adaptor protein (SLAP) is upregulated in antigen-stimulated mast cells and acts as a negative regulator

Our studies in the RBL-2H3 mast cell line suggest that responses to antigen (Ag) are negatively modulated through upregulation of Src-like adaptor protein (SLAP). Ag stimulation of RBL-2H3 cells leads to increased levels of SLAP (but not SLAP2) transcripts and protein over a period of several hours. The effects of pharmacologic inhibitors indicate that the upregulation of SLAP is dependent on multiple signaling pathways. Knockdown of SLAP with anti-SLAP siRNA is associated with enhanced phosphorylation of Syk, the linker for activation of T cells (LAT), phospholipase C gamma, MAP kinases, and various transcription factors. Production of IL-3 and MCP-1, but not degranulation, is also enhanced. The upregulation of SLAP may thus serve to limit the duration of cytokine production in Ag-stimulated cells.

Mechanism of upregulation of the inhibitory regulator, src-like adaptor protein (SLAP), by glucocorticoids in mast cells

Glucocorticoids suppress mast cell activation by inhibiting signaling events as well as transcription of cytokine genes. The inhibition of signaling events has been attributed to upregulation of inhibitory regulators such as Src-like adaptor protein1 (SLAP), downstream of tyrosine kinase1 (Dok1), and dual specificity protein phospahatase1 (DUSP1). As reported here, the upregulation of SLAP and Dok1, but not DUSP1, in the RBL-2H3 mast cell line was inhibited by actinomycin D and was thus dependent on gene transcription. Examination of the gene sequences revealed a glucocorticoid response element (GRE) and a half GRE as potential regulators of the SLAP and Dok1, respectively. As indicated by luciferase reporter assays, SLAP GRE, but not the Dok1 half GRE, robustly activated gene transcription after treatment of cells with glucocorticoids. Binding of the glucocorticoid receptor to the SLAP GRE was verified by chromatin immunoprecipitation assay. These findings further support the notion that the immunosuppressive actions of glucocorticoids are exerted in part through upregulation of inhibitory regulators by various mechanisms. In the case of SLAP specifically, this requires activation of gene transcription through the interaction of the glucocorticoid receptor with GRE.