c-Cbl regulates migration of v-Abl-transformed NIH 3T3 fibroblasts via Rac1

MicroRNA-155 regulates the proliferation, cell cycle, apoptosis and migration of colon cancer cells and targets CBL

MicroRNA-155 (miR-155) is a well-studied miR and acts as an oncomiR in numerous cancer types. However, the biological functions of miR-155 in colon cancer as well as its target genes have remained to be fully elucidated. In order to investigate the biological functions of miR-155, MTT, colony formation and wound healing assays, cell cycle analysis and detection of apoptosis were performed. The results demonstrated that miR-155 promoted the proliferation of colon cancer cells and enhanced their colony formation capacity, promoted their cell cycle progression and inhibited apoptosis. miR-155 also promoted the migration of colon cancer cells. In the present study, casitas B-lineage lymphoma was identified as a novel target of miR-155. The present study suggested that miR-155 functions as an oncomiR in colon cancer cells and may become a promising therapeutic target for colon cancer therapy.

SLI-1 Cbl inhibits the engulfment of apoptotic cells in C. elegans through a ligase-independent function

The engulfment of apoptotic cells is required for normal metazoan development and tissue remodeling. In Caenorhabditis elegans, two parallel and partially redundant conserved pathways act in cell-corpse engulfment. One pathway, which includes the small GTPase CED-10 Rac and the cytoskeletal regulator ABI-1, acts to rearrange the cytoskeleton of the engulfing cell. The CED-10 Rac pathway is also required for proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. The second pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Cbl, the mammalian homolog of the C. elegans E3 ubiquitin ligase and adaptor protein SLI-1, interacts with Rac and Abi2 and modulates the actin cytoskeleton, suggesting it might act in engulfment. Our genetic studies indicate that SLI-1 inhibits apoptotic cell engulfment and DTC migration independently of the CED-10 Rac and CED-1 pathways. We found that the RING finger domain of SLI-1 is not essential to rescue the effects of SLI-1 deletion on cell migration, suggesting that its role in this process is ubiquitin ligase-independent. We propose that SLI-1 opposes the engulfment of apoptotic cells via a previously unidentified pathway.

c-Cbl regulates glioma invasion through matrix metalloproteinase 2

c-Cbl, a multifunctional adaptor and an E3 ubiquitin ligase, plays a role in such cytoskeleton-mediated events as cell adhesion and migration. Invasiveness of human glioma is dependent on cell adhesion, migration, and degradation of extracellular matrix (ECM). However, the function of c-Cbl in glioma invasion has never been investigated. We report here, for the first time, that c-Cbl plays a positive role in the invasion of ECM by SNB19 glioma cells. RNAi-mediated depletion of c-Cbl decreases SNB19 cell invasion and expression of matrix metalloproteinase 2 (MMP2). Consistent with these findings, SNB19 cells expressing wild-type, but not mutant c-Cbl show increased invasion and MMP2 expression. We demonstrate that the observed role of c-Cbl in invasion of SNB19 cells is not mediated by the previously shown effects of c-Cbl on cell adhesion and migration or on EGFR signaling. Together, our results suggest that c-Cbl promotes glioma invasion through up-regulation of MMP2.

The role of ubiquitylation and degradation in RhoGTPase signalling

Rho-like guanosine triphosphatases (RhoGTPases) control many aspects of cellular physiology through their effects on the actin cytoskeleton and on gene transcription. Signalling by RhoGTPases is tightly coordinated and requires a series of regulatory proteins, including guanine-nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and guanine-nucleotide dissociation inhibitors (GDIs). GEFs and GAPs regulate GTPase cycling between the active (GTP-bound) and inactive (GDP-bound) states, whereas GDI is a cytosolic chaperone that binds inactive RhoGTPases. Like many other proteins, RhoGTPases are subject to degradation following the covalent conjugation of ubiquitin. There have been increasing indications that ubiquitylation of small GTPases occurs in a regulated fashion, primarily upon activation, and is an important means to control signalling output. Recent work has identified cellular proteins that control RasGTPase and RhoGTPase ubiquitylation and degradation, allowing us to amend the canonical model for GTPase (in)activation. Moreover, accumulating evidence for indirect regulation of GTPase function through the ubiquitylation of GTPase regulators makes this post-translational modification a key feature of GTPase-dependent signalling pathways. Here, we will discuss these recent insights into the regulation of RhoGTPase ubiquitylation and their relevance for cell signalling.

c-Cbl inhibits angiogenesis and tumor growth by suppressing activation of PLCγ1

Angiogenesis is regulated by highly coordinated function of various proteins with pro- and anti-angiogenic functions. Among the many cytoplasmic signaling proteins that are activated by VEGFR-2, activation of PLCγ1 is considered to have a pivotal role in angiogenic signaling. In previous study we have identified c-Cbl as a negative regulator of PLCγ1 in endothelial cells, the biochemical and biological significance of c-Cbl, however, in angiogenesis in vivo and molecular mechanisms involved were remained elusive. In this study, we report that genetic inactivation of c-Cbl in mice results in enhanced tumor angiogenesis and retinal neovascularization. Endothelial cells derived from c-Cbl null mice displayed elevated cell proliferation and tube formation in response to VEGF stimulation. Loss of c-Cbl also resulted in robust activation of PLCγ1 and increased intracellular calcium release. c-Cbl-dependent ubiquitination selectively inhibited tyrosine phosphorylation of PLCγ1 and mostly refrained from ubiquitin-mediated degradation. Hence, we propose c-Cbl as an angiogenic suppressor protein where upon activation it uniquely modulates PLCγ1 activation by ubiquitination and subsequently inhibits VEGF-driven angiogenesis.

c-Cbl facilitates cytoskeletal effects in v-Abl transformed fibroblast through Rac1- and Rap1-mediated signaling

c-Cbl functions as a multifunctional adaptor and an E3 ubiquitin ligase. Several studies have shown that c-Cbl is involved in cytoskeleton-mediated events, but the molecular mechanisms linking c-Cbl to cytoskeletal rearrangements remain to be elucidated. Our previous results indicated that c-Cbl facilitates spreading and migration of v-Abl-transformed NIH 3T3 fibroblasts and suggested that small GTPases play important roles in the cytoskeletal effects of c-Cbl in this system. To elucidate the individual contributions of small GTPases to these effects, we assessed the roles of endogenous Rac1, RhoA and Rap1 in the c-Cbl-dependent spreading and migration of v-Abl-transformed fibroblasts overexpressing c-Cbl, using RNAi. Furthermore, since it has been shown that Rap1 can act as an upstream regulator of Rac1 in inducing cell spreading, we analyzed the interplay between Rap1 and Rac1 in the signaling pathways connecting c-Cbl to the cytoskeletal events. Our results indicate that Rac1 is essential for cell migration and spreading, whereas activation of RhoA exerts a negative effect. We have also shown that Rap1 is essential for cell spreading, although not for migration in our experimental system. Furthermore, we provide evidence that Rap1 is located upstream of Rac1 in one of the signaling pathways that regulate c-Cbl-facilitated cell spreading. Overall, our findings are consistent with the model describing the connection of c-Cbl to the cytoskeletal rearrangements via two pathways, one of which is mediated by PI3K and Rac1, and the other, by CrkL/C3G, Rap1 and Rac1.

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

The Src-like adaptor protein (SLAP) belongs to the subfamily of adapter proteins that negatively regulate cellular signalling initiated by tyrosine kinases. SLAP has a unique, myristylated N-terminus, followed by SH3 and SH2 domains with high homology to Src family tyrosine kinases (SFK) and a unique C-terminal tail, which is important for c-Cbl binding. We have previously shown that SLAP negatively regulates platelet-derived growth factor (PDGF)-induced mitogenesis in fibroblasts and we now report that it regulates F-actin assembly for dorsal ruffles formation. c-Cbl mediated SLAP inhibition towards actin remodelling. Moreover, SLAP enhanced PDGF-induced c-Cbl phosphorylation by SFK. In contrast, SLAP mitogenic inhibition was not mediated by c-Cbl, but it rather involved a competitive mechanism with SFK for PDGF-receptor (PDGFR) association and mitogenic signalling. Accordingly, phosphorylation of the Src mitogenic substrates Stat3 and Shc were reduced by SLAP. Thus, we concluded that SLAP regulates PDGFR signalling by two independent mechanisms: a competitive mechanism for PDGF-induced Src mitogenic signalling and a non-competitive mechanism for dorsal ruffles formation mediated by c-Cbl.

Up-regulation of the Cbl family of ubiquitin ligases is involved in ATRA and bufalin-induced cell adhesion but not cell differentiation

The Casitas B-lineage Lymphoma (Cbl) family of ubiquitin ligases is multifunctional proteins that play important roles in different cell signaling pathways. It has been reported that c-Cbl and Cbl-b mRNAs are up-regulated during TPA-induced U937 and HL-60 cell differentiation. But the mechanism of the up-regulation and the roles of the Cbl family of ubiquitin ligases still remain unclear. In the present study, we demonstrated that bufalin enhanced all-trans retinoic acid (ATRA) induced differentiation of HL-60 cells, accompanied by up-regulation of the Cbl family of ubiquitin ligases. CsA, an inhibitor of calcium mobilization, reversed this up-regulation. Pretreatment with CsA and PS-341 did not affect the expression of CD11b, but suppressed the percentage of adherent cells. Lipid raft localization of Cbl-b enhanced cell adhesion, while C-terminal deletion partially suppressed the effect. Moreover, the expression of the adhesion-related kinases Pyk2 and Paxillin was up-regulated in parallel with the increase of Cbl proteins. These results suggested that up-regulation of c-Cbl and Cbl-b was involved in the regulation of ATRA and bufalin-induced HL-60 cell adhesion rather than cell differentiation, which might be mediated by lipid raft localization, ubiquitin ligase activity and C-terminal structure of Cbl proteins. Meanwhile, up-regulation of proline-rich tyrosine kinase (Pyk2) and Paxillin might also be implicated in this regulation.

Quantitative time-resolved phosphoproteomic analysis of mast cell signaling

Mast cells play a central role in type I hypersensitivity reactions and allergic disorders such as anaphylaxis and asthma. Activation of mast cells, through a cascade of phosphorylation events, leads to the release of mediators of the early phase allergic response. Understanding the molecular architecture underlying mast cell signaling may provide possibilities for therapeutic intervention in asthma and other allergic diseases. Although many details of mast cell signaling have been described previously, a systematic, quantitative analysis of the global tyrosine phosphorylation events that are triggered by activation of the mast cell receptor is lacking. In many cases, the involvement of particular proteins in mast cell signaling has been established generally, but the precise molecular mechanism of the interaction between known signaling proteins often mediated through phosphorylation is still obscure. Using recently advanced methodologies in mass spectrometry, including automation of phosphopeptide enrichments and detection, we have now substantially characterized, with temporal resolution as short as 10 s, the sites and levels of tyrosine phosphorylation across 10 min of FcepsilonRI-induced mast cell activation. These results reveal a far more extensive array of tyrosine phosphorylation events than previously known, including novel phosphorylation sites on canonical mast cell signaling molecules, as well as unexpected pathway components downstream of FcepsilonRI activation. Furthermore, our results, for the first time in mast cells, reveal the sequence of phosphorylation events for 171 modification sites across 121 proteins in the MCP5 mouse mast cell line and 179 modification sites on 117 proteins in mouse bone marrow-derived mast cells.

c-Cbl-facilitated cytoskeletal effects in v-Abl-transformed fibroblasts are regulated by membrane association of c-Cbl

The multi-functional protein c-Cbl is an important modulator of actin cytoskeletal dynamics in diverse biological systems. We had previously reported that c-Cbl facilitates cell spreading and adhesion and suppresses anchorage-independent growth of v-Abl-transformed fibroblasts. To assess the importance of membrane localization of c-Cbl for the observed effects of c-Cbl in v-Abl-3T3 cells, we first mapped the membrane interactive domain(s) of c-Cbl. Our studies indicate that localization of c-Cbl to the membrane is likely to be mediated by the tyrosine kinase binding (TKB) domain and the proline-rich region of c-Cbl, whereas C-terminal tyrosine phosphorylation does not play a role. The association of v-Cbl, which encompasses the TKB domain, with the membrane was unusual as it was not entirely dependent on SH2-phosphotyrosine interactions. Our studies further demonstrate that Src-like adaptor protein (SLAP), which binds to v-Cbl in a tyrosine phosphorylation-independent manner, facilitates membrane association of Cbl. The interaction between c-Cbl and SLAP in v-Abl-3T3 cells positively influenced c-Cbl-mediated spreading and adhesion of these cells. SLAP appears to exert its effects not simply by increasing the amount of c-Cbl in the membrane but by facilitating binding of p85-phosphatidylinositol-3-kinase (PI3K) with membrane-associated c-Cbl.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.