RING finger mutations that abolish c-Cbl-directed polyubiquitination and downregulation of the EGF receptor are insufficient for cell transformation

CBL mutations in chronic myelomonocytic leukemia often occur in the RING domain with multiple subclones per patient: Implications for targeting

Chronic myelomonocytic leukemia (CMML) is a rare blood cancer of older adults (3 in every 1,000,000 persons) characterized by poor survival and lacking effective mutation-specific therapy. Mutations in the ubiquitin ligase Cbl occur frequently in CMML and share biological and molecular features with a clonal disease occurring in children, juvenile myelomonocytic leukemia (JMML). Here we analyzed the clinical presentations, molecular features and immunophenotype of CMML patients with CBL mutations enrolled in a prospective Phase II clinical trial stratified according to molecular markers. Clinically, CBL mutations were associated with increased bone marrow blasts at diagnosis, leukocytosis and splenomegaly, similar to patients harboring NRAS or KRAS mutations. Interestingly, 64% of patients presented with more than one CBL variant implying a complex subclonal architecture, often with co-occurrence of TET2 mutations. We found CBL mutations in CMML frequently clustered in the RING domain in contrast to JMML, where mutations frequently involve the linker helix region (P<0.0001). According to our comparative alignment of available X-ray structures, mutations in the linker helix region such as Y371E give rise to conformational differences that could be exploited by targeted therapy approaches. Furthermore, we noted an increased percentage of CMML CD34+ stem and progenitor cells expressing CD116 and CD131 in all CBL mutant cases and increased CD116 receptor density compared to healthy controls, similar to CMML overall. In summary, our data demonstrate that CBL mutations are associated with distinct molecular and clinical features in CMML and are potentially targetable with CD116-directed immunotherapy.

Immunological and hematological findings as major features in a patient with a new germline pathogenic CBL variant

Casitas B-lineage lymphoma (CBL) encodes an adaptor protein with E3-ligase activity negatively controlling intracellular signaling downstream of receptor tyrosine kinases. Somatic CBL mutations play a driver role in a variety of cancers, particularly myeloid malignancies, whereas germline defects in the same gene underlie a RASopathy having clinical overlap with Noonan syndrome (NS) and predisposing to juvenile myelomonocytic leukemia and vasculitis. Other features of the disorder include cardiac defects, postnatal growth delay, cryptorchidism, facial dysmorphisms, and predisposition to develop autoimmune disorders. Here we report a novel CBL variant (c.1202G>T; p.Cys401Phe) occurring de novo in a subject with café-au-lait macules, feeding difficulties, mild dysmorphic features, psychomotor delay, autism spectrum disorder, thrombocytopenia, hepatosplenomegaly, and recurrent hypertransaminasemia. The identified variant affects an evolutionarily conserved residue located in the RING finger domain, a known mutational hot spot of both germline and somatic mutations. Functional studies documented enhanced EGF-induced ERK phosphorylation in transiently transfected COS1 cells. The present findings further support the association of pathogenic CBL variants with immunological and hematological manifestations in the context of a presentation with only minor findings reminiscent of NS or a clinically related RASopathy.

Deciphering the Selectivity of CBL-B Inhibitors Using All-Atom Molecular Dynamics and Machine Learning

We employ a combination of accelerated molecular dynamics and machine learning to unravel how the dynamic characteristics of CBL-B and C-CBL confer their binding affinity and selectivity for ligands from subtle structural disparities within their binding pockets and dissociation pathways. Our predictive model of dissociation rate constants (k off) demonstrates a moderate correlation between predicted k off and experimental IC50 values, which is consistent with experimental k off and τ-random accelerated molecular dynamics (τRAMD) results. By employing a linear regression of dissociation trajectories, we identified key amino acids in binding pockets and along the dissociation paths responsible for activity and selectivity. These amino acids are statistically significant in achieving activity and selectivity and contribute to the primary structural discrepancies between CBL-B and C-CBL. Moreover, the binding free energies calculated from molecular mechanics with generalized Born and surface area solvation (MM/GBSA) highlight the ΔG difference between CBL-B and C-CBL. The k off prediction, together with the key amino acids, provides important guides for designing drugs with high selectivity.

An Independent Assessment of a Commercial Clinical Interpretation Software Indicates That Software Can Mitigate Variation in Human Assessment

Comprehensive next-generation sequencing (NGS) panels for cancer diagnostics create a bottleneck for interpretation. QIAGEN Clinical Insights Interpret One (QCI) is a clinical decision support software that supports molecular pathologists in the classification of oncology-related variants. This study compares variant assessments by QCI to assessments utilizing current laboratory methods. Eight laboratories were recruited by the external quality assessment organization GenQA. The laboratories submitted VCFs from sequencing studies performed on both hematological disorders and solid tumors for analysis by QCI and an independent laboratory. Results were compared and conflicts were resolved using a panel of experts. In total, 14/149 variants (9%) reported as Tier 1 or Tier 2 by either QCI or the submitting laboratory were found to be discordant after expert panel review. In contrast, 41/149 variants (28%) reflected discrepancy among human reviewers. The expert panel was unable to reach resolution on eight variants. QCI demonstrates high concordance in the classification of actionable mutations with independent laboratory methods and expert assessment. The rate of disagreement among laboratories and the expert panel was greater than the disagreement between QCI and expert assessment. Disagreement among experts highlights the subjectivity of classifying variants. The study demonstrates that QCI interpretation supports streamlining and standardization of NGS variant interpretation.

Molecular Pathogenesis of BCR-ABL-Negative Atypical Chronic Myeloid Leukemia

Atypical chronic myeloid leukemia is a rare disease whose pathogenesis has long been debated. It currently belongs to the group of myelodysplastic/myeloproliferative disorders. In this review, an overview on the current knowledge about diagnosis, prognosis, and genetics is presented, with a major focus on the recent molecular findings. We describe here the molecular pathogenesis of the disease, focusing on the mechanisms of action of the main mutations as well as on gene expression profiling. We also present the treatment options focusing on emerging targeted therapies.

Somatic Variation as an Incidental Finding in the Pediatric Next Generation Sequencing Era

The methodologic approach used in next-generation sequencing (NGS) affords a high depth of coverage in genomic analysis. Inherent in the nature of genomic testing, there exists potential for identifying genomic findings that are incidental or secondary to the indication for clinical testing, with the frequency dependent on the breadth of analysis and the tissue sample under study. The interpretation and management of clinically meaningful incidental genomic findings is a pressing issue particularly in the pediatric population. Our study describes a 16-mo-old male who presented with profound global delays, brain abnormality, progressive microcephaly, and growth deficiency, as well as metopic craniosynostosis. Clinical exome sequencing (ES) trio analysis revealed the presence of two variants in the proband. The first was a de novo variant in the PPP2R1A gene (c.773G > A, p.Arg258His), which is associated with autosomal dominant (AD) intellectual disability, accounting for the proband's clinical phenotype. The second was a recurrent hotspot variant in the CBL gene (c.1111T > C, p.Tyr371His), which was present at a variant allele fraction of 11%, consistent with somatic variation in the peripheral blood sample. Germline pathogenic variants in CBL are associated with AD Noonan syndrome–like disorder with or without juvenile myelomonocytic leukemia. Molecular analyses using a different tissue source, buccal epithelial cells, suggest that the CBL alteration may represent a clonal population of cells restricted to leukocytes. This report highlights the laboratory methodologic and interpretative processes and clinical considerations in the setting of acquired variation detected during clinical ES in a pediatric patient.

E3 ligase-inactivation rewires CBL interactome to elicit oncogenesis by hijacking RTK-CBL-CIN85 axis

Casitas B-lineage lymphoma (CBL) is a ubiquitin ligase (E3) that becomes activated upon Tyr371-phosphorylation and targets receptor protein tyrosine kinases for ubiquitin-mediated degradation. Deregulation of CBL and its E3 activity is observed in myeloproliferative neoplasms and other cancers, including breast, colon, and prostate cancer. Here, we explore the oncogenic mechanism of E3-inactive CBL mutants identified in myeloproliferative neoplasms. We show that these mutants bind strongly to CIN85 under normal growth conditions and alter the CBL interactome. Lack of E3 activity deregulates CIN85 endosomal trafficking, leading to an altered transcriptome that amplifies signaling events to promote oncogenesis. Disruption of CBL mutant interactions with EGFR or CIN85 reduces oncogenic transformation. Given the importance of the CBL-CIN85 interaction in breast cancers, we examined the expression levels of CIN85, CBL, and the status of Tyr371-phosphorylated CBL (pCBL) in human breast cancer tissue microarrays. Interestingly, pCBL shows an inverse correlation with both CIN85 and CBL, suggesting that high expression of inactivated CBL could coordinate with CIN85 for breast cancer progression. Inhibition of the CBL-CIN85 interaction with a proline-rich peptide of CBL that binds CIN85 reduced the proliferation of MDA-MB-231 cells. Together, these results provide a rationale for exploring the potential of targeting the EGFR-CBL-CIN85 axis in CBL-inactivated mutant cancers.

C-Cbl regulates c-MPL receptor trafficking and its internalization

Thrombocyte formation from megakaryocyte and their progenitor cells is tightly regulated by thrombopoietin (TPO) and its receptor c‐MPL, thereby maintaining physiological functionality and numbers of circulating platelets. In patients, dysfunction of this regulation could cause thrombocytopenia or myeloproliferative syndromes. Since regulation of this pathway is still not completely understood, we investigated the role of the ubiquitin ligase c‐Cbl which was previously shown to negatively regulated c‐MPL signalling. We developed a new conditional mouse model using c‐Cbl^fl/flPf4^Cre mice and demonstrated that platelet‐specific knockout of c‐Cbl led to severe microthrombocytosis and impaired uptake of TPO and c‐MPL receptor internalization. Furthermore, we characterized a constitutive STAT5 activation c‐Cbl KO platelets. This study identified c‐Cbl as a potential player in causing megakaryocytic and thrombocytic disorders.

c-Src and EGFR Inhibition in Molecular Cancer Therapy: What Else Can We Improve?

The proto-oncogene c-Src is a non-receptor tyrosine kinase playing a key role in many cellular pathways, including cell survival, migration and proliferation. c-Src de-regulation has been observed in several cancer types, making it an appealing target for drug discovery efforts. Recent evidence emphasizes its crucial role not only in promoting oncogenic traits, but also in the acquisition and maintenance of cancer resistance to various chemotherapeutic or molecular target drugs. c-Src modulates epidermal growth factor receptor (EGFR) activation and amplifies its downstream oncogenic signals. In this review, we report several studies supporting c-Src kinase role in the intricate mechanisms of resistance to EGFR tyrosine kinase inhibitors (TKIs). We further highlighted pre- and clinical progresses of combined treatment strategies made in recent years. Several pre-clinical data have encouraged the use of c-Src inhibitors in combination with EGFR inhibitors. However, clinical trials provided controversial outcomes in some cancer types. Despite c-Src inhibitors showed good tolerability in cancer patients, no incontrovertible and consistent clinical responses were recorded, supporting the idea that a better selection of patients is needed to improve clinical outcome. Currently, the identification of biological markers predictive of therapy response and the accurate molecular screening of cancer patients aimed to gain most clinical benefits become decisive and mandatory.

After 95 years, it's time to eRASe JMML

Juvenile myelomonocytic leukaemia (JMML) is a rare clonal disorder of early childhood. Constitutive activation of the RAS pathway is the initial event in JMML. Around 90% of patients diagnosed with JMML carry a mutation in the PTPN11, NRAS, KRAS, NF1 or CBL genes. It has been demonstrated that after this first genetic event, an additional somatic mutation or epigenetic modification is involved in disease progression. The available genetic and clinical data have enabled researchers to establish relationships between JMML and several clinical conditions, including Noonan syndrome, Ras-associated lymphoproliferative disease, and Moyamoya disease. Despite scientific progress and the development of more effective treatments, JMML is still a deadly disease: the 5-year survival rate is ~50%. Here, we report on recent research having led to a better understanding of the genetic and molecular mechanisms involved in JMML.

Identification and characterization of novel double zinc fingers encoded by putative proteins in genome of white spot syndrome virus

White spot syndrome virus (WSSV), is a major viral pathogen affecting the shrimp culture industry worldwide. Studies in understanding the mechanisms of WSSV pathogenicity has led to the identification of The Really Interesting New Gene (RING) finger domains in WSSV encoded proteins that have been shown to function as E3 ligase modulating the host-ubiquitin pathway. In this study, we report two proteins encoded by the WSSV genome to harbor a double zinc finger domain, one each in its N- and C-terminal region. Sequence and structural analysis of the two domains showed the N- and C-terminal domains to be similar to known RING1 and RING2 domains of eukaryotic RBR (RING-between-RING) ligases respectively. This is the first report wherein genes within WSSV are shown to encode for double RING domains, which could pave way in understanding further, the function of these proteins and their role in the pathogenic mechanisms of the virus.

Triad1 regulates the expression and distribution of EHD1 contributing to the neurite outgrowth of neurons after spinal cord injury

Traumatic spinal cord injury is a common and severe complication after an accident. As we all know that neurite outgrowth of neurons is difficult after a spinal cord injury. Endosome system is associated with cargoes transportation and contributes in promoting the neuronal capability for neurite outgrowth. EH domain-containing protein 1 (EHD1) transports proteins through the endosome system, especially in the recycling endosomes and regulating the neurite outgrowth. In mammalian cells, the involvement of the ubiquitin-proteasome system in endosomal sorting has been well established. Two RING fingers and a DRIL (double RING finger-linked) 1 (Triad1) plays an important role in membrane trafficking and its mutant results in the wrong accumulation of receptors in endosomes and plasma membrane. In this current study, we reasonably integrated the results of the above research and investigated the regulating function of Triad1 to EHD1 following the spinal cord injury. We characterized the upregulated expression and distribution of Triad1 and EHD1 in the neurons after SCI and declared the interaction between Triad1 with EHD1 both in vitro and in vivo. Triad1 regulated the interaction between itself and the full-length or EH domain of EHD1, which influenced the neurite outgrowth of PC12 cells. Our data delineate a novel interaction between Triad1 and EHD1 that may contribute to the regulation of neurite outgrowth for neurons after the spinal cord injury.

Overexpression of TRIM44 is related to invasive potential and malignant outcomes in esophageal squamous cell carcinoma

Recent studies have shown that some members of the tripartite motif-containing protein family function as important regulators for carcinogenesis. In this study, we investigated whether tripartite motif-containing protein 44 acts as a cancer-promoting gene through its overexpression in esophageal squamous cell carcinoma. We analyzed esophageal squamous cell carcinoma cell lines to evaluate malignant potential and also analyzed 68 primary tumors to evaluate clinical relevance of tripartite motif-containing protein 44 protein in esophageal squamous cell carcinoma patients. Expression of the tripartite motif-containing protein 44 protein was detected in esophageal squamous cell carcinoma cell lines (8/14 cell lines; 57%) and primary tumor samples of esophageal squamous cell carcinoma (39/68 cases; 57%). Knockdown of tripartite motif-containing protein 44 expression in esophageal squamous cell carcinoma cells using several specific small interfering RNAs inhibited cell migration and invasion, but not cell proliferation. Immunohistochemical analysis demonstrated that the overexpression of the tripartite motif-containing protein 44 protein in the tumor infiltrated region was associated with the status of lymph node metastasis ( p = 0.049), and the overall survival rates were significantly worse among patients with tripartite motif-containing protein 44-overexpressing tumors than those with non-expressing tumors ( p = 0.029). Moreover, multivariate Cox regression model identified that overexpression of the tripartite motif-containing protein 44 protein was an independent worse prognostic factor (hazard ratio = 2.815; p = 0.041), as well as lymphatic invasion (hazard ratio = 2.735; p = 0.037). These results suggest that tripartite motif-containing protein 44 protein could play a crucial role in tumor invasion through its overexpression and highlight its usefulness as a predictor and potential therapeutic target in esophageal squamous cell carcinoma.

Cbl-mediated K63-linked ubiquitination of JAK2 enhances JAK2 phosphorylation and signal transduction

JAK2 activation is crucial for cytokine receptor signal transduction and leukemogenesis. However, the underlying processes that lead to full activation of JAK2 are unclear. Here, we report a positive role for ubiquitination of JAK2 during GM-CSF-induced activation. Upon GM-CSF stimulation, JAK2 ubiquitination is significantly enhanced through K63-linked poly-ubiquitination. Studies employing both knockout and overexpression of Cbl, an E3 ubiquitin ligase, led to the conclusion that Cbl specifically promotes JAK2 ubiquitination, and this was further confirmed in vitro using a Cbl ubiquitination assay. Moreover, following GM-CSF stimulation, the levels of phospho-JAK2 and -STAT5 and a STAT5 luciferase reporter assay were all reduced in Cbl knockout cells and this effect could be rescued by Cbl expression. Mechanistically, Cbl can interact with, and ubiquitinate JAK2 FERM and kinase domains via the Cbl TKB domain. Using lysine-to-arginine site-directed mutagenesis, K970 in the kinase domain of JAK2 was identified as the ubiquitination site important for promoting full JAK2 activation by Cbl via K63-conjugated poly-ubiquitination. Our study suggests that GM-CSF-induced JAK2 activation is enhanced by Cbl-mediated ubiquitination of JAK2. Targeting ubiquitination of JAK2 might offer a novel therapeutic strategy against JAK2-mediated disorders.

Harnessing LRIG1-mediated inhibition of receptor tyrosine kinases for cancer therapy

Leucine-rich repeats and immunoglobulin-like domains containing protein 1 (LRIG1) is an endogenous feedback regulator of receptor tyrosine kinases (RTKs) and was recently shown to inhibit growth of different types of malignancies. Additionally, this multifaceted RTK inhibitor was reported to be a tumor suppressor, a stem cell regulator, and a modulator of different cellular phenotypes. This mini-review provides a concise and up-to-date summary about the known functions of LRIG1 and its related family members, with a special emphasis on underlying molecular mechanisms and the opportunities for harnessing its therapeutic potential against cancer.

Structural Determinants of the Gain-of-Function Phenotype of Human Leukemia-associated Mutant CBL Oncogene

Mutations of the tyrosine kinase-directed ubiquitin ligase CBL cause myeloid leukemias, but the molecular determinants of the dominant leukemogenic activity of mutant CBL oncogenes are unclear. Here, we first define a gain-of-function attribute of the most common leukemia-associated CBL mutant, Y371H, by demonstrating its ability to increase proliferation of hematopoietic stem/progenitor cells (HSPCs) derived from CBL-null and CBL/CBL-B-null mice. Next, we express second-site point/deletion mutants of CBL-Y371H in CBL/CBL-B-null HSPCs or the cytokine-dependent human leukemic cell line TF-1 to show that individual or combined Tyr → Phe mutations of established phosphotyrosine residues (Tyr-700, Tyr-731, and Tyr-774) had little impact on the activity of the CBL-Y371H mutant in HSPCs, and the triple Tyr → Phe mutant was only modestly impaired in TF-1 cells. In contrast, intact tyrosine kinase-binding (TKB) domain and proline-rich region (PRR) were critical in both cell models. PRR deletion reduced the stem cell factor (SCF)-induced hyper-phosphorylation of the CBL-Y371H mutant and the c-KIT receptor and eliminated the sustained p-ERK1/2 and p-AKT induction by SCF. GST fusion protein pulldowns followed by phospho-specific antibody array analysis identified distinct CBL TKB domains or PRR-binding proteins that are phosphorylated in CBL-Y371H-expressing TF-1 cells. Our results support a model of mutant CBL gain-of-function in which mutant CBL proteins effectively compete with the remaining wild type CBL-B and juxtapose TKB domain-associated PTKs with PRR-associated signaling proteins to hyper-activate signaling downstream of hematopoietic growth factor receptors. Elucidation of mutant CBL domains required for leukemogenesis should facilitate targeted therapy approaches for patients with mutant CBL-driven leukemias.

Cardiac ubiquitin ligases: Their role in cardiac metabolism, autophagy, cardioprotection and therapeutic potential

Both the ubiquitin-proteasome system (UPS) and the lysosomal autophagy system have emerged as complementary key players responsible for the turnover of cellular proteins. The regulation of protein turnover is critical to cardiomyocytes as post-mitotic cells with very limited regenerative capacity. In this focused review, we describe the emerging interface between the UPS and autophagy, with E3's regulating autophagy at two critical points through multiple mechanisms. Moreover, we discuss recent insights in how both the UPS and autophagy can alter metabolism at various levels, to present new ways to think about therapeutically regulating autophagy in a focused manner to optimize disease-specific cardioprotection, without harming the overall homeostasis of protein quality control. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan F.C. Glatz.

Enzymatic Activity of the Scaffold Protein Rapsyn for Synapse Formation

Neurotransmission is ensured by a high concentration of neurotransmitter receptors at the postsynaptic membrane. This is mediated by scaffold proteins that bridge the receptors with cytoskeleton. One such protein is rapsyn (receptor-associated protein at synapse), which is essential for acetylcholine receptor (AChR) clustering and NMJ (neuromuscular junction) formation. We show that the RING domain of rapsyn contains E3 ligase activity. Mutation of the RING domain that abolishes the enzyme activity inhibits rapsyn- as well as agrin-induced AChR clustering in heterologous and muscle cells. Further biological and genetic studies support a working model where rapsyn, a classic scaffold protein, serves as an E3 ligase to induce AChR clustering and NMJ formation, possibly by regulation of AChR neddylation. This study identifies a previously unappreciated enzymatic function of rapsyn and a role of neddylation in synapse formation, and reveals a potential target of therapeutic intervention for relevant neurological disorders.

Casitas B-lineage lymphoma linker helix mutations found in myeloproliferative neoplasms affect conformation

BACKGROUND

Casitas B-lineage lymphoma (Cbl or c-Cbl) is a RING ubiquitin ligase that negatively regulates protein tyrosine kinase (PTK) signalling. Phosphorylation of a conserved residue (Tyr371) on the linker helix region (LHR) between the substrate-binding and RING domains is required to ubiquitinate PTKs, thereby flagging them for degradation. This conserved Tyr is a mutational hotspot in myeloproliferative neoplasms. Previous studies have revealed that select point mutations in Tyr371 can potentiate transformation in cells and mice but not all possible mutations do so. To trigger oncogenic potential, Cbl Tyr371 mutants must perturb the LHR-substrate-binding domain interaction and eliminate PTK ubiquitination. Although structures of native and pTyr371-Cbl are available, they do not reveal how Tyr371 mutations affect Cbl's conformation. Here, we investigate how Tyr371 mutations affect Cbl's conformation in solution and how this relates to Cbl's ability to potentiate transformation in cells.

RESULTS

To explore how Tyr371 mutations affect Cbl's properties, we used surface plasmon resonance to measure Cbl mutant binding affinities for E2 conjugated with ubiquitin (E2-Ub), small angle X-ray scattering studies to investigate Cbl mutant conformation in solution and focus formation assays to assay Cbl mutant transformation potential in cells. Cbl Tyr371 mutants enhance E2-Ub binding and cause Cbl to adopt extended conformations in solution. LHR flexibility, RING domain accessibility and transformation potential are associated with the extent of LHR-substrate-binding domain perturbation affected by the chemical nature of the mutation. More disruptive mutants like Cbl Y371D or Y371S are more extended and the RING domain is more accessible, whereas Cbl Y371F mimics native Cbl in solution. Correspondingly, the only Tyr371 mutants that potentiate transformation in cells are those that perturb the LHR-substrate-binding domain interaction.

CONCLUSIONS

c-Cbl's LHR mutations are only oncogenic when they disrupt the native state and fail to ubiquitinate PTKs. These findings provide new insights into how LHR mutations deregulate c-Cbl.

The Ubiquitin Ligases c-Cbl and Cbl-b Negatively Regulate Platelet-derived Growth Factor (PDGF) BB-induced Chemotaxis by Affecting PDGF Receptor β (PDGFRβ) Internalization and Signaling

Protein ubiquitination controls protein stability and subcellular localization of tyrosine kinase receptors, hence affecting signaling both quantitatively and qualitatively. In this report, we demonstrate that, after ligand stimulation, the PDGF β receptor (PDGFRβ) becomes ubiquitinated in a manner requiring both the c-Cbl and Cbl-b ubiquitin ligases. Simultaneous depletion of c-Cbl and Cbl-b resulted in reduced ligand-induced PDGFRβ clearance from the cell surface because of reduced endocytosis of the receptor. Cbl-b formed a complex with c-Cbl, as well as with the PDGFRβ, in response to PDGF-BB stimulation. We were unable to find a direct interaction between the receptor and c-Cbl, raising the possibility that Cbl-b is necessary for c-Cbl to interact with PDGFRβ. Phosphorylated Tyr-1021 in PDGFRβ was the primary interaction site for Cbl-b, with some contribution from Tyr-1009. Depletion of c-Cbl and Cbl-b led to an increased ligand-induced tyrosine phosphorylation of the receptor. Several tyrosine residues with elevated phosphorylation (i.e. Tyr-579, Tyr-581, Tyr-1009, and Tyr-1021) have previously been shown to interact with Src kinases and PLCγ. Indeed, in cells depleted of c-Cbl and Cbl-b, both Src and PLCγ phosphorylation were enhanced, whereas activation of other pathways, such as Erk1/2 MAP kinase and Akt, were not affected. In addition, Stat3 phosphorylation, which has been connected to Src activity, was also elevated in cells lacking c-Cbl and Cbl-b. Functionally, we found that cells depleted of c-Cbl and Cbl-b were more prone to migrate toward PDGF-BB, whereas no reproducible effect on cell proliferation could be observed. In conclusion, internalization as well as signaling via PDGFRβ are controlled by ubiquitination.

Juvenile myelomonocytic leukemia due to a germline CBL Y371C mutation: 35-year follow-up of a large family

Juvenile myelomonocytic leukemia (JMML) is a pediatric myeloproliferative neoplasm that arises from malignant transformation of the stem cell compartment and results in increased production of myeloid cells. Somatic and germline variants in CBL (Casitas B-lineage lymphoma proto-oncogene) have been associated with JMML. We report an incompletely penetrant CBL Y371C mutation discovered by whole-exome sequencing in three individuals with JMML in a large pedigree with 35 years of follow-up. The Y371 residue is highly evolutionarily conserved among CBL orthologs and paralogs. In silico bioinformatics prediction programs suggested that the Y371C mutation is highly deleterious. Protein structural modeling revealed that the Y371C mutation abrogated the ability of the CBL protein to adopt a conformation that is required for ubiquitination. Clinically, the three mutation-positive JMML individuals exhibited variable clinical courses; in two out of three, primary hematologic abnormalities persisted into adulthood with minimal clinical symptoms. The penetrance of the CBL Y371C mutation was 30% for JMML and 40% for all leukemia. Of the 8 mutation carriers in the family with available photographs, only one had significant dysmorphic features; we found no evidence of a clinical phenotype consistent with a "CBL syndrome". Although CBL Y371C has been previously reported in familial JMML, we are the first group to follow a complete pedigree harboring this mutation for an extended period, revealing additional information about this variant's penetrance, function and natural history.

Molecular Diversity and Associated Phenotypic Spectrum of Germline CBL Mutations

Noonan syndrome (NS) is a relatively common developmental disorder with a pleomorphic phenotype. Mutations causing NS alter genes encoding proteins involved in the RAS-MAPK pathway. We and others identified Casitas B-lineage lymphoma proto-oncogene (CBL), which encodes an E3-ubiquitin ligase acting as a tumor suppressor in myeloid malignancies, as a disease gene underlying a condition clinically related to NS. Here, we further explored the spectrum of germline CBL mutations and their associated phenotype. CBL mutation scanning performed on 349 affected subjects with features overlapping NS and no mutation in NS genes allowed the identification of five different variants with pathological significance. Among them, two splice-site changes, one in-frame deletion, and one missense mutation affected the RING domain and/or the adjacent linker region, overlapping cancer-associated defects. A novel nonsense mutation generating a v-Cbl-like protein able to enhance signal flow through RAS was also identified. Genotype-phenotype correlation analysis performed on available records indicated that germline CBL mutations cause a variable phenotype characterized by a relatively high frequency of neurological features, predisposition to juvenile myelomonocytic leukemia, and low prevalence of cardiac defects, reduced growth, and cryptorchidism. Finally, we excluded a major contribution of two additional members of the CBL family, CBLB and CBLC, to NS and related disorders.

Molecular pathways: cbl proteins in tumorigenesis and antitumor immunity-opportunities for cancer treatment

The Cbl proteins are a family of ubiquitin ligases (E3s) that regulate signaling through many tyrosine kinase-dependent pathways. A predominant function is to negatively regulate receptor tyrosine kinase (RTK) signaling by ubiquitination of active RTKs, targeting them for trafficking to the lysosome for degradation. Also, Cbl-mediated ubiquitination can regulate signaling protein function by altered cellular localization of proteins without degradation. In addition to their role as E3s, Cbl proteins play a positive role in signaling by acting as adaptor proteins that can recruit signaling molecules to the active RTKs. Cbl-b, a second family member, negatively regulates the costimulatory pathway of CD8 T cells and also negatively regulates natural killer cell function. The different functions of Cbl proteins and their roles both in the development of cancer and the regulation of immune responses provide multiple therapeutic opportunities. Mutations in Cbl that inactivate the negative E3 function while maintaining the positive adaptor function have been described in approximately 5% of myeloid neoplasms. An improved understanding of how the signaling pathways [e.g., Fms-like tyrosine kinase 3 (Flt3), PI3K, and signal transducer and activator of transcription (Stat)] are dysregulated by these mutations in Cbl has helped to identify potential targets for therapy of myeloid neoplasms. Conversely, the loss of Cbl-b leads to increased adaptive and innate antitumor immunity, suggesting that inhibiting Cbl-b may be a means to increase antitumor immunity across a wide variety of tumors. Thus, targeting the pathways regulated by Cbl proteins may provide attractive opportunities for treating cancer.

Modulation of Immune Cell Functions by the E3 Ligase Cbl-b

Maintenance of immunological tolerance is a critical hallmark of the immune system. Several signaling checkpoints necessary to balance activating and inhibitory input to immune cells have been described so far, among which the E3 ligase Cbl-b appears to be a central player. Cbl-b is expressed in all leukocyte subsets and regulates several signaling pathways in T cells, NK cells, B cells, and different types of myeloid cells. In most cases, Cbl-b negatively regulates activation signals through antigen or pattern recognition receptors and co-stimulatory molecules. In line with this function, cblb-deficient immune cells display lower activation thresholds and cblb knockout mice spontaneously develop autoimmunity and are highly susceptible to experimental autoimmunity. Interestingly, genetic association studies link CBLB-polymorphisms with autoimmunity also in humans. Vice versa, the increased activation potential of cblb-deficient cells renders them more potent to fight against malignancies or infections. Accordingly, several reports have shown that cblb knockout mice reject tumors, which mainly depends on cytotoxic T and NK cells. Thus, targeting Cbl-b may be an interesting strategy to enhance anti-cancer immunity. In this review, we summarize the findings on the molecular function of Cbl-b in different cell types and illustrate the potential of Cbl-b as target for immunomodulatory therapies.

Regulation of T cell development by c-Cbl: essential role of Lck

A canonical pre-TCR/TCR signaling pathway critical for thymic T cell development involves sequential phosphorylation and signaling through Lck, Zap70, Lat and Slp76. However, we and others have previously reported that genomic deletion of c-Cbl (Cbl) partially or completely reverses the defects in thymic development in mice deficient in Zap70, Slp76, Lat or Vav1, indicating the presence of alternative pathways normally suppressed by Cbl. To further elucidate pre-TCR/TCR signaling pathways involved in thymic development, we characterized the effect of Cbl inactivation on developmental and signaling defects in mice deficient in proximal signaling molecules Lck and Zap70. Inactivation of Cbl partially reversed defective T cell development in Zap70 (-/-) mice and reversed defects in phosphorylation of Erk, Plc-γ1, Vav1 and Akt, in TCR-stimulated Cbl (-/-) Zap70 (-/-) thymocytes. Recent reports identified an essential role of Lck in associating with CD4 and CD8 coreceptors and mediating the requirement for MHC restriction in TCR recognition. Since TCR recognition has been shown to be MHC-restricted in Cbl (-/-) mice, it was of interest to determine whether the requirement for Lck remained unmodified by Cbl deletion. Indeed, in contrast to the effect of Cbl inactivation in partially or fully bypassing requirements for other TCR signaling components, inactivation of Cbl did not reverse either defective T cell development or defective phosphorylation of TCR signaling molecules in Lck (-/-) mice. Thus, Lck, which plays a unique role in enforcing MHC restriction, is essential for thymic development in presence or absence of Cbl, ensuring MHC restriction of T cells derived from either pathway.

Cbl-family ubiquitin ligases and their recruitment of CIN85 are largely dispensable for epidermal growth factor receptor endocytosis

Members of the casitas B-lineage lymphoma (Cbl) family (Cbl, Cbl-b and Cbl-c) of ubiquitin ligases serve as negative regulators of receptor tyrosine kinases (RTKs). An essential role of Cbl-family protein-dependent ubiquitination for efficient ligand-induced lysosomal targeting and degradation is now well-accepted. However, a more proximal role of Cbl and Cbl-b as adapters for CIN85-endophilin recruitment to mediate ligand-induced initial internalization of RTKs is supported by some studies but refuted by others. Overexpression and/or incomplete depletion of Cbl proteins in these studies is likely to have contributed to this dichotomy. To address the role of endogenous Cbl and Cbl-b in the internalization step of RTK endocytic traffic, we established Cbl/Cbl-b double-knockout (DKO) mouse embryonic fibroblasts (MEFs) and demonstrated that these cells lack the expression of both Cbl-family members as well as endophilin A, while they express CIN85. We show that ligand-induced ubiquitination of EGFR, as a prototype RTK, was abolished in DKO MEFs, and EGFR degradation was delayed. These traits were reversed by ectopic human Cbl expression. EGFR endocytosis, assessed using the internalization of (125)I-labeled or fluorescent EGF, or of EGFR itself, was largely retained in Cbl/Cbl-b DKO compared to wild type MEFs. EGFR internalization was also largely intact in Cbl/Cbl-b depleted MCF-10A human mammary epithelial cell line. Inducible shRNA-mediated knockdown of CIN85 in wild type or Cbl/Cbl-b DKO MEFs had no impact on EGFR internalization. Our findings, establish that, at physiological expression levels, Cbl, Cbl-b and CIN85 are largely dispensable for EGFR internalization. Our results support the model that Cbl-CIN85-endophilin complex is not required for efficient internalization of EGFR, a prototype RTK.

Novel somatic and germline mutations in intracranial germ cell tumours

Intracranial germ cell tumours (IGCTs) are a group of rare heterogeneous brain tumours that are clinically and histologically similar to the more common gonadal GCTs. IGCTs show great variation in their geographical and gender distribution, histological composition and treatment outcomes. The incidence of IGCTs is historically five- to eightfold greater in Japan and other East Asian countries than in Western countries, with peak incidence near the time of puberty. About half of the tumours are located in the pineal region. The male-to-female incidence ratio is approximately 3-4:1 overall, but is even higher for tumours located in the pineal region. Owing to the scarcity of tumour specimens available for research, little is currently known about this rare disease. Here we report the analysis of 62 cases by next-generation sequencing, single nucleotide polymorphism array and expression array. We find the KIT/RAS signalling pathway frequently mutated in more than 50% of IGCTs, including novel recurrent somatic mutations in KIT, its downstream mediators KRAS and NRAS, and its negative regulator CBL. Novel somatic alterations in the AKT/mTOR pathway included copy number gains of the AKT1 locus at 14q32.33 in 19% of patients, with corresponding upregulation of AKT1 expression. We identified loss-of-function mutations in BCORL1, a transcriptional co-repressor and tumour suppressor. We report significant enrichment of novel and rare germline variants in JMJD1C, which codes for a histone demethylase and is a coactivator of the androgen receptor, among Japanese IGCT patients. This study establishes a molecular foundation for understanding the biology of IGCTs and suggests potentially promising therapeutic strategies focusing on the inhibition of KIT/RAS activation and the AKT1/mTOR pathway.

Dasatinib targets B-lineage cells but does not provide an effective therapy for myeloproliferative disease in c-Cbl RING finger mutant mice

This study aimed to determine whether the multi-kinase inhibitor dasatinib would provide an effective therapy for myeloproliferative diseases (MPDs) involving c-Cbl mutations. These mutations, which occur in the RING finger and linker domains, abolish the ability of c-Cbl to function as an E3 ubiquitin ligase and downregulate activated protein tyrosine kinases. Here we analyzed the effects of dasatinib in a c-Cbl RING finger mutant mouse that develops an MPD with a phenotype similar to the human MPDs. The mice are characterized by enhanced tyrosine kinase signaling resulting in an expansion of hematopoietic stem cells, multipotent progenitors and cells within the myeloid lineage. Since c-Cbl is a negative regulator of c-Kit and Src signaling we reasoned that dasatinib, which targets these kinases, would be an effective therapy. Furthermore, two recent studies showed dasatinib to be effective in inhibiting the in vitro growth of cells from leukemia patients with c-Cbl RING finger and linker domain mutations. Surprisingly we found that dasatinib did not provide an effective therapy for c-Cbl RING finger mutant mice since it did not suppress any of the hematopoietic lineages that promote MPD development. Thus we conclude that dasatinib may not be an appropriate therapy for leukemia patients with c-Cbl mutations. We did however find that dasatinib caused a marked reduction of pre-B cells and immature B cells which correlated with a loss of Src activity. This study is therefore the first to provide a detailed characterization of in vivo effects of dasatinib in a hematopoietic disorder that is driven by protein tyrosine kinases other than BCR-ABL.

CD2-associated protein (CD2AP) enhances casitas B lineage lymphoma-3/c (Cbl-3/c)-mediated Ret isoform-specific ubiquitination and degradation via its amino-terminal Src homology 3 domains

Ret is the receptor tyrosine kinase for the glial cell line-derived neurotrophic factor (GDNF) family of neuronal growth factors. Upon activation by GDNF, Ret is rapidly polyubiquitinated and degraded. This degradation process is isoform-selective, with the longer Ret51 isoform exhibiting different degradation kinetics than the shorter isoform, Ret9. In sympathetic neurons, Ret degradation is induced, at least in part, by a complex consisting of the adaptor protein CD2AP and the E3-ligase Cbl-3/c. Knockdown of Cbl-3/c using siRNA reduced the GDNF-induced ubiquitination and degradation of Ret51 in neurons and podocytes, suggesting that Cbl-3/c was a predominant E3 ligase for Ret. Coexpression of CD2AP with Cbl-3/c augmented the ubiquitination of Ret51 as compared with the expression of Cbl-3/c alone. Ret51 ubiquitination by the CD2AP·Cbl-3/c complex required a functional ring finger and TKB domain in Cbl-3/c. The SH3 domains of CD2AP were sufficient to drive the Cbl-3/c-dependent ubiquitination of Ret51, whereas the carboxyl-terminal coiled-coil domain of CD2AP was dispensable. Interestingly, activated Ret induced the degradation of CD2AP, but not Cbl-3/c, suggesting a potential inhibitory feedback mechanism. There were only two major ubiquitination sites in Ret51, Lys(1060) and Lys(1107), and the combined mutation of these lysines almost completely eliminated both the ubiquitination and degradation of Ret51. Ret9 was not ubiquitinated by the CD2AP·Cbl-3/c complex, suggesting that Ret9 was down-regulated by a fundamentally different mechanism. Taken together, these results suggest that only the SH3 domains of CD2AP were necessary to enhance the E3 ligase activity of Cbl-3/c toward Ret51.

Internalizing MHC class II-peptide complexes are ubiquitinated in early endosomes and targeted for lysosomal degradation

As sentinels of the immune system, dendritic cells (DCs) continuously generate and turnover antigenic peptide-MHC class II complexes (pMHC-II). pMHC-II generation is a complex process that involves many well-characterized MHC-II biosynthetic intermediates; however, the mechanisms leading to MHC-II turnover/degradation are poorly understood. We now show that pMHC-II complexes undergoing clathrin-independent endocytosis from the DC surface are efficiently ubiquitinated by the E3 ubiquitin ligase March-I in early endosomes, whereas biosynthetically immature MHC-II-Invariant chain (Ii) complexes are not. The inability of MHC-II-Ii to serve as a March-I substrate is a consequence of Ii sorting motifs that divert the MHC-II-Ii complex away from March-I(+) early endosomes. When these sorting motifs are mutated, or when clathrin-mediated endocytosis is inhibited, MHC-II-Ii complexes internalize by using a clathrin-independent endocytosis pathway and are now ubiquitinated as efficiently as pMHC-II complexes. These data show that the selective ubiquitination of internalizing surface pMHC-II in March-I(+) early endosomes promotes degradation of "old" pMHC-II and spares forms of MHC-II that have not yet loaded antigenic peptides or have not yet reached the DC surface.

Cbl ubiquitination of p85 is essential for Epo-induced EpoR endocytosis

Erythropoietin (Epo) binding to the Epo receptor (EpoR) elicits downstream signaling that is essential for red blood cell production. One important negative regulatory mechanism to terminate Epo signaling is Epo-induced EpoR endocytosis and degradation. Defects in this mechanism play a key role in the overproduction of erythrocytes in primary familial and congenital polycythemia (PFCP). Here we have identified a novel mechanism mediating Epo-dependent EpoR internalization. Epo induces Cbl-dependent ubiquitination of the p85 regulatory subunit of PI3K, which binds to phosphotyrosines on EpoR. Ubiquitination allows p85 to interact with the endocytic protein epsin-1, thereby driving EpoR endocytosis. Knockdown of Cbl, expression of its dominant negative forms, or expression of an epsin-1 mutant devoid of ubiquitin-interacting motifs all compromise Epo-induced EpoR internalization. Mutated EpoRs mimicking those from PFCP patients cannot bind p85, co-localize with epsin-1, or internalize on Epo stimulation and exhibit Epo hypersensitivity. Similarly, knockdown of Cbl also causes Epo hypersensitivity in primary erythroid progenitors. Restoring p85 binding to PFCP receptors rescues Epo-induced epsin-1 co-localization and EpoR internalization and normalizes Epo hypersensitivity. Our results uncover a novel Cbl/p85/epsin-1 pathway in EpoR endocytosis and show that defects in this pathway contribute to excessive Epo signaling and erythroid hyperproliferation in PFCP.

Emerging roles of E3 ubiquitin ligases in autophagy

Autophagy is an evolutionarily conserved intracellular catabolic process that delivers cytoplasmic components to lysosomes for degradation and recycling. Although originally considered to be a non-selective pathway, it is now recognized that autophagy is involved in selective processes, including the turnover of organelles, removal of protein aggregates, and elimination of intracellular pathogens. This specificity implies that cargo recognition and processing by the autophagy machinery are tightly regulated processes. In support of this, various forms of post-translational modification have been implicated in the regulation of autophagy, one of which is the ubiquitin-proteasome system. Here we review current understanding of the role of ubiquitylation in the control of autophagy.

Hepatitis B Virus-Encoded X Protein Downregulates EGFR Expression via Inducing MicroRNA-7 in Hepatocellular Carcinoma Cells

Hepatitis B virus (HBV) infection accounts for over a half of cases of hepatocellular carcinoma (HCC), the most frequent malignant tumor of the liver. HBV-encoded X (HBx) plays critical roles in HBV-associated hepatocarcinogenesis. However, it is unclear whether and how HBx regulates the expression of epidermal growth factor receptor (EGFR), an important gene for cell growth. Therefore, the study aimed to investigate the association between HBx and EGFR expression. In this study, we found that HBx upregulates miR-7 expression to target 3′UTR of EGFR mRNA, which in turn results in the reduction of EGFR protein expression in HCC cells. HBx-mediated EGFR suppression renders HCC cells a slow-growth behavior. Deprivation of HBx or miR-7 expression or restoration of EGFR expression can increase the growth rate of HCC cells. Our data showed the miR-7-dependent EGFR suppression by HBx, supporting an inhibitory role of HBx in the cell growth of HCC. These findings not only identify miR-7 as a novel regulatory target of HBx, but also suggest HBx-miR-7-EGFR as a critical signaling in controlling the growth rate of HCC cells.

CBL linker region and RING finger mutations lead to enhanced granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling via elevated levels of JAK2 and LYN

Juvenile myelomonocytic leukemia (JMML) is characterized by hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF). SHP2, NF-1, KRAS, and NRAS are mutated in JMML patients, leading to aberrant regulation of RAS signaling. A subset of JMML patients harbor CBL mutations associated with 11q acquired uniparental disomy. Many of these mutations are in the linker region and the RING finger of CBL, leading to a loss of E3 ligase activity. We investigated the mechanism by which CBL-Y371H, a linker region mutant, and CBL-C384R, a RING finger mutant, lead to enhanced GM-CSF signaling. Expression of CBL mutants in the TF-1 cell line resulted in enhanced survival in the absence of GM-CSF. Cells expressing CBL mutations displayed increased phosphorylation of GM-CSF receptor βc subunit in response to stimulation, although expression of total GM-CSFR βc was lower. This suggested enhanced kinase activity downstream of GM-CSFR. JAK2 and LYN kinase expression is elevated in CBL-Y371H and CBL-C384R mutant cells, resulting in enhanced phosphorylation of CBL and S6 in response to GM-CSF stimulation. Incubation with the JAK2 inhibitor, TG101348, abolished the increased phosphorylation of GM-CSFR βc in cells expressing CBL mutants, whereas treatment with the SRC kinase inhibitor dasatinib resulted in equalization of GM-CSFR βc phosphorylation signal between wild type CBL and CBL mutant samples. Dasatinib treatment inhibited the elevated phosphorylation of CBL-Y371H and CBL-C384R mutants. Our study indicates that CBL linker and RING finger mutants lead to enhanced GM-CSF signaling due to elevated kinase expression, which can be blocked using small molecule inhibitors targeting specific downstream pathways.

Flt3 inhibitor AC220 is a potent therapy in a mouse model of myeloproliferative disease driven by enhanced wild-type Flt3 signaling

High levels of expression of wild-type Flt3 characterize many hematopoietic proliferative diseases and neoplasms, providing a potential therapeutic target. Using the c-Cbl RING finger mutant mouse as a model of a myeloproliferative disease (MPD) driven by wild-type Flt3, in the present study, we show that treatment with the Flt3 kinase inhibitor AC220 blocks MPD development by targeting Flt3+ multipotent progenitors (MPPs). We found that daily administration of AC220 caused a marked reduction in Flt3 expression, induction of quiescence, and a significant loss of MPPs within 4 days. Unexpectedly, a robust Flt3 ligand–associated proliferative recovery response soon followed, preventing further loss of MPPs. However, continued AC220 treatment limited MPP recovery and maintained reduced, steady-state levels of cycling MPPs that express low levels of Flt3. Therefore, a finely tuned balance between the opposing forces of AC220 and Flt3 ligand production was established; whereas the Flt3 ligand blunted the inhibitory effects of AC220, the disease was held in remission for as long as therapy was continued. The net effect is a potent therapy indicating that patients with c-Cbl mutations, or those with similarly enhanced Flt3 signaling, may respond well to AC220 even after the induction of high levels of Flt3 ligand.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Ubiquitination-dependent regulation of signaling receptors in cancer

Ubiquitination of signaling cell surface receptors is a key mechanism regulating the availability of these receptors to interact with extracellular ligands. Accordingly, this regulation determines the sensitivity of cells to the humoral and locally secreted regulators of cell function, proliferation, and viability. Alterations in receptor ubiquitination and degradation are often encountered in cancers. Malignant cells utilize modified ubiquitination of signaling receptors to augment or attenuate signaling pathways on the basis of whether the outcome of this signaling is conducive or not for tumor growth and survival. These mechanisms as well as their significance for the treatment of human cancers are discussed.

Structural basis for autoinhibition and phosphorylation-dependent activation of c-Cbl

Cbls are RING ubiquitin ligases that attenuate receptor tyrosine kinase (RTK) signal transduction. Cbl ubiquitination activity is stimulated by phosphorylation of a linker helix region (LHR) tyrosine residue. To elucidate the mechanism of activation, we determined the structures of human CBL, a CBL-substrate peptide complex and a phosphorylated-Tyr371-CBL-E2-substrate peptide complex, and we compared them with the known structure of a CBL-E2-substrate peptide complex. Structural and biochemical analyses show that CBL adopts an autoinhibited RING conformation, where the RING's E2-binding surface associates with CBL to reduce E2 affinity. Tyr371 phosphorylation activates CBL by inducing LHR conformational changes that eliminate autoinhibition, flip the RING domain and E2 into proximity of the substrate-binding site and transform the RING domain into an enhanced E2-binding module. This activation is required for RTK ubiquitination. Our results present a mechanism for regulation of c-Cbl's activity by autoinhibition and phosphorylation-induced activation.

Autoinhibition and phosphorylation-induced activation mechanisms of human cancer and autoimmune disease-related E3 protein Cbl-b

Cbl-b is a RING-type E3 ubiquitin ligase that functions as a negative regulator of T-cell activation and growth factor receptor and nonreceptor-type tyrosine kinase signaling. Cbl-b dysfunction is related to autoimmune diseases and cancers in humans. However, the molecular mechanism regulating its E3 activity is largely unknown. NMR and small-angle X-ray scattering analyses revealed that the unphosphorylated N-terminal region of Cbl-b forms a compact structure by an intramolecular interaction, which masks the interaction surface of the RING domain with an E2 ubiquitin-conjugating enzyme. Phosphorylation of Y363, located in the helix-linker region between the tyrosine kinase binding and the RING domains, disrupts the interdomain interaction to expose the E2 binding surface of the RING domain. Structural analysis revealed that the phosphorylated helix-RING region forms a compact structure in solution. Moreover, the phosphate group of pY363 is located in the vicinity of the interaction surface with UbcH5B to increase affinity by reducing their electrostatic repulsion. Thus, the phosphorylation of Y363 regulates the E3 activity of Cbl-b by two mechanisms: one is to remove the masking of the RING domain from the tyrosine kinase binding domain and the other is to form a surface to enhance binding affinity to E2.

ARAP1 association with CIN85 affects epidermal growth factor receptor endocytic trafficking

BACKGROUND INFORMATION

ARAP1 is an Arf (ADP-ribosylation factor)-directed GAP (GTPase-activating protein) that inhibits the trafficking of EGFR (epidermal growth factor receptor) to the early endosome. To further understand the function of ARAP1, we sought to identify proteins that interact with ARAP1.

RESULTS

Here we report that ARAP1 associates with the CIN85 (Cbl-interacting protein of 85 kDa). Arg86 and Arg90 of ARAP1 and the SH3 (Src homology 3) domains of CIN85 are necessary for the interaction. We found that a mutant of ARAP1 with reduced affinity for CIN85 does not efficiently rescue the effect of reduced ARAP1 expression on EGFR trafficking to the early endosome. Reduced expression of CIN85 has a similar effect as reduced expression of ARAP1 on traffic of the EGFR. Cbl proteins regulate the endocytic trafficking of the EGFR by mediating ubiquitination of the EGFR. Overexpression of ARAP1 reduced ubiquitination of the EGFR by Cbl and slowed Cbl-dependent degradation of the EGFR. Reduced expression of ARAP1 accelerated degradation of EGFR but did not affect the level of ubiquitination of the receptor that was detected.

CONCLUSION

ARAP1 interaction with CIN85 regulates endocytic trafficking of the EGFR and affects ubiquitination of EGFR. We propose a model in which the ARAP1-CIN85 complex drives exit of EGF-EGFR-Cbl complex from a pre-early endosome into a pathway distinct from the early endosome/lysosome pathway.

D-Cbl binding to Drk leads to dose-dependent down-regulation of EGFR signaling and increases receptor-ligand endocytosis

Proper control of Epidermal Growth Factor Receptor (EGFR) signaling is critical for normal development and regulated cell behaviors. Abnormal EGFR signaling is associated with tumorigenic process of various cancers. Complicated feedback networks control EGFR signaling through ligand production, and internalization-mediated destruction of ligand-receptor complexes. Previously, we found that two isoforms of D-Cbl, D-CblS and D-CblL, regulate EGFR signaling through distinct mechanisms. While D-CblL plays a crucial role in dose-dependent down-regulation of EGFR signaling, D-CblS acts in normal restriction of EGFR signaling and does not display dosage effect. Here, we determined the underlying molecular mechanism, and found that Drk facilitates the dose-dependent regulation of EGFR signaling through binding to the proline-rich motif of D-CblL, PR. Furthermore, the RING finger domain of D-CblL is essential for promoting endocytosis of the ligand-receptor complex. Interestingly, a fusion protein of the two essential domains of D-CblL, RING- PR, is sufficient to down-regulate EGFR signal in a dose-dependent manner by promoting internalization of the ligand, Gurken. Besides, RING-SH2^Drk, a fusion protein of the RING finger domain of D-Cbl and the SH2 domain of Drk, also effectively down-regulates EGFR signaling in Drosophila follicle cells, and suppresses the effects of constitutively activated EGFR. The RING-SH2^Drk suppresses EGFR signaling by promoting the endosomal trafficking of ligand-receptor complexes, suggesting that Drk plays a negative role in EGFR signaling by enhancing receptor endocytosis through cooperating with the RING domain of D-Cbl. Interfering the recruitment of signal transducer, Drk, to the receptor by the RING-SH2^Drk might further reduces EGFR signaling. The fusion proteins we developed may provide alternative strategies for therapy of cancers caused by hyper-activation of EGFR signaling.

Visualizing the role of Cbl-b in control of islet-reactive CD4 T cells and susceptibility to type 1 diabetes

The E3 ubiquitin ligase Cbl-b regulates T cell activation thresholds and has been associated with protecting against type 1 diabetes, but its in vivo role in the process of self-tolerance has not been examined at the level of potentially autoaggressive CD4(+) T cells. In this study, we visualize the consequences of Cbl-b deficiency on self-tolerance to lysozyme Ag expressed in transgenic mice under control of the insulin promoter (insHEL). By tracing the fate of pancreatic islet-reactive CD4(+) T cells in prediabetic 3A9-TCR × insHEL double-transgenic mice, we find that Cbl-b deficiency contrasts with AIRE or IL-2 deficiency, because it does not affect thymic negative selection of islet-reactive CD4(+) cells or the numbers of islet-specific CD4(+) or CD4(+)Foxp3(+) T cells in the periphery, although it decreased differentiation of inducible regulatory T cells from TGF-β-treated 3A9-TCR cells in vitro. When removed from regulatory T cells and placed in culture, Cblb-deficient islet-reactive CD4(+) cells reveal a capacity to proliferate to HEL Ag that is repressed in wild-type cells. This latent failure of T cell anergy is, nevertheless, controlled in vivo in prediabetic mice so that islet-reactive CD4(+) cells in the spleen and the pancreatic lymph node of Cblb-deficient mice show no evidence of increased activation or proliferation in situ. Cblb deficiency subsequently precipitated diabetes in most TCR:insHEL animals by 15 wk of age. These results reveal a role for peripheral T cell anergy in organ-specific self-tolerance and illuminate the interplay between Cblb-dependent anergy and other mechanisms for preventing organ-specific autoimmunity.

Myeloid leukemia development in c-Cbl RING finger mutant mice is dependent on FLT3 signaling

Although myeloid leukemias are primarily caused by leukemic stem cells, the molecular basis of their transformation remains largely unknown. Here, by analyzing mice with a mutation in the RING finger domain of c-Cbl, we show that the E3 ubiquitin ligase activity of c-Cbl is required to restrict myeloid leukemia development. These mice develop a myeloproliferative disease which progresses to leukemia and involves hematopoietic progenitors that exhibit augmented FLT3 signaling. Suppressing this signaling through matings with FLT3 ligand knockout mice prevents leukemia development. We also observe enhanced c-Kit, Akt and Erk activity, and deregulated expression of leukemia-associated transcription factors in hematopoietic progenitors. The characterization of these perturbations provides direction for therapeutics that may aid the treatment of patients with c-Cbl mutations.

Bimodal expression of Sprouty2 during the cell cycle is mediated by phase-specific Ras/MAPK and c-Cbl activities

Sprouty2 is an important inhibitor of cell proliferation and signal transduction. In this study, we found a bimodal expression of Sprouty2 protein during cell cycle progression after exit from quiescence, whereas elevated Sprouty4 expression in the G1 phase stayed high throughout the rest of the cell cycle. Induction of the mitogen-activated protein kinase via activated Ras was crucial for increased Sprouty2 expression at the G0/G1 transition. Following the first peak, accelerated proteasomal protein degradation caused a transient attenuation of Sprouty2 abundance during late G1. Since the decline in its expression was abolished by dominant negative c-Cbl and the timely restricted interaction between Sprouty2 and c-Cbl disappeared at the second peak of Sprouty2 expression, we conclude that the second phase in the cell cycle-specific expression profile of Sprouty2 is solely dependent on ubiquitination by c-Cbl. Our results suggest that Sprouty2 abundance is the result of strictly coordinated activities of Ras and c-Cbl.

The N terminus of Cbl-c regulates ubiquitin ligase activity by modulating affinity for the ubiquitin-conjugating enzyme

Cbl proteins are ubiquitin ligases (E3s) that play a significant role in regulating tyrosine kinase signaling. There are three mammalian family members: Cbl, Cbl-b, and Cbl-c. All have a highly conserved N-terminal tyrosine kinase binding domain, a catalytic RING finger domain, and a C-terminal proline-rich domain that mediates interactions with Src homology 3 (SH3) containing proteins. Although both Cbl and Cbl-b have been studied widely, little is known about Cbl-c. Published reports have demonstrated that the N terminus of Cbl and Cbl-b have an inhibitory effect on their respective E3 activity. However, the mechanism for this inhibition is still unknown. In this study we demonstrate that the N terminus of Cbl-c, like that of Cbl and Cbl-b, inhibits the E3 activity of Cbl-c. Furthermore, we map the region responsible for the inhibition to the EF-hand and SH2 domains. Phosphorylation of a critical tyrosine (Tyr-341) in the linker region of Cbl-c by Src or a phosphomimetic mutation of this tyrosine (Y341E) is sufficient to increase the E3 activity of Cbl-c. We also demonstrate for the first time that phosphorylation of Tyr-341 or the Y341E mutation leads to a decrease in affinity for the ubiquitin-conjugating enzyme (E2), UbcH5b. The decreased affinity of the Y341E mutant Cbl-c for UbcH5b results in a more rapid turnover of bound UbcH5b coincident with the increased E3 activity. These data suggest that the N terminus of Cbl-c contributes to the binding to the E2 and that phosphorylation of Tyr-341 leads to a decrease in affinity and an increase in the E3 activity of Cbl-c.