Cbl Proteins Research Articles

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 125I-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.

Cbl regulates the activity of SIRT2

SIRT2 is a member of the sirtuin family of NAD+-dependent protein deacetylases. It is involved in metabolic homeostasis and has been linked to the progression of age-related diseases. Casitas B-lineage lymphoma (Cbl) proteins regulate signal transduction through many pathways and, consequently, regulate cell function and development. Cbl proteins are ubiquitin ligases that ubiquitinate and target many signaling molecules for degradation. The function of SIRT2 is modulated by post-translational modifications. However, the precise molecular signaling mechanism of SIRT2 through interactions with Cbl proteins has not yet been established. In this study, we investigated the potential regulation of SIRT2 function by the Cbl mammalian family members Cbl-b and c-Cbl. We found that Cbl-b and c-Cbl increased the protein level and stability of SIRT2 and that Cbl-b and c-Cbl interact with SIRT2. They were also found to regulate the deacetylase activity of SIRT2. Further investigation revealed that Cbl-mediated SIRT2 regulation occurred via ubiquitination of SIRT2.

Abstract 4441: Ube2e inhibits the ubiquitination and degradation of EGFR mediated by Cbl and Ube2d

Abstract Receptor tyrosine kinases (RTKs) have a demonstrated role as drivers of malignant transformation. Many RTKs (e.g., EGFR, MET, FLT3) are negatively regulated by ubiquitination and degradation mediated by Cbl proteins a family of RING finger (RF) ubiquitin ligases (E3s). There are three mammalian family members Cbl, Cbl-b and Cbl-c. All Cbl proteins have a highly conserved N-terminal tyrosine kinase binding domain and a C3HC4 (RF) domain. Loss of Cbl protein function has be associated with malignant transformation drive by increased RTK activity. E3s such as the Cbl proteins confer specificity to the ubiquitination process and direct the conjugation of ubiquitin to one or more lysines on the specific target proteins in collaboration with ubiquitin-conjugating enzymes (E2s). The E2s bind to the E3 and together they mediate ubiquitination of specific substrates. There are approximately 30 human E2s. All of the E2s have a highly conserved catalytic domain (UBC domain). In this study, we characterized the interaction of Cbl with E2s. Using an in vitro E3 assay, we found that only the Ube2d family (Ube2d1, Ube2d2, and Ube2d3) of E2s mediates autoubiquitination of the three Cbl proteins. Subsequently, using the yeast two-hybrid system, we found that Ube2e1, Ube2e2, Ube2e3, Ube2L3, Ube2u, Ube2w and Ube2z can interact with Cbl even though they do not support autoubiquitination of Cbl in vitro E3 assay. Among these E2s, three Ube2e family members and Ube2w bind to the RF domain of Cbl as demonstrated by the loss of interaction when the RF domain is disrupted. Further, we demonstrated that Ube2e family members are involved in regulation of EGFR activity mediated by Cbl in HeLa cells. Knockdown of Ube2e increases down-regulation and ubiquitination of EGFR in HeLa cells. We further showed that three Ube2e inhibit autoubiquitination of Cbl mediated by Ube2d2 in vitro. Together these data suggest that RTK degradation can be modulated by competition between E2s for the Cbl E3s. Citation Format: Ke Ma, Stanley Lipkowitz, Rachel Klevit, Mariya Sergeyevna Liyasova. Ube2e inhibits the ubiquitination and degradation of EGFR mediated by Cbl and Ube2d. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4441. doi:10.1158/1538-7445.AM2014-4441

Abstract 4429: Loss of function Cbl-c mutations in solid tumors

Abstract Receptor Tyrosine Kinase (RTK) signaling is essential for normal biological processes and disruption of this regulation can lead to tumor initiation and progression. Cbl proteins are a family of RING finger ubiquitin ligases that negatively regulate a variety of RTKs, including EGFR, MET, RET and components of the TCR pathway. Recent studies have identified a growing number of Cbl mutations associated with human myeloid neoplasias and epithelial tumors. Cbl-c is the most recently identified human Cbl protein and is expressed exclusively in epithelial cells. Recently, a novel cDNA has been isolated from the ductal carcinoma of a mouse mammary cancer model. This mutant has a deletion in the catalytic domain of Cbl-c, thereby resembling known Cbl family mutations associated with myeoloid neoplasias. Genomic analysis of both parental and transgenic lines shows no evidence of germline mutation indicating that this mutation is likely a somatic mutation. In overexpression studies, this mutant cDNA fails to mediate ubiquitination and degradation of activated EGFR and, acting as a dominant negative, enhances cell transformation. Furthermore, data mining reveals several Cbl family mutations associated with solid tumors in humans. Subsequent cell-based analysis demonstrates a similar loss of E3 function in some of these human mutations. Through subsequent analysis of transforming capacity and signal pathway regulation, we are assessing interacting partners of Cbl-c and the potential role of these mutations in tumorigenesis. Citation Format: Stephen C. Kales, Philip E. Ryan, Marion M. Nau, Jeffrey E. Green, Stanley Lipkowitz. Loss of function Cbl-c mutations in solid tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4429. doi:10.1158/1538-7445.AM2014-4429

CBL restricts myeloid proliferation of human AML1-ETO preleukemia cells

CBL is a multifunctional protein engaged in the regulation of intracellular signaling pathways. Recent studies have found CBL mutations and deletions in a variety of myeloid diseases. In acute myeloid leukemia (AML), CBL mutations are associated with core binding factor (CBF) leukemia, a subset of AML characterized by the presence of AML1-ETO or CBFB-MYH11 fusion genes. However, the potential cooperation between mutant CBL and CBF-fusion proteins has not been demonstrated experimentally.

Regulation of cancer stem cells by RING finger ubiquitin ligases.

Like normal stem cells, cancer stem cells (CSCs) are capable of self-renewal, either by symmetric or asymmetric cell division. They have the exclusive ability to reproduce malignant tumors indefinitely, and to confer resistance in response to radiation or chemotherapy. The ubiquitin modification system plays various roles in physiology and pathology. The key component for the specificity of this system is ubiquitin ligases (E3s). Of these E3s, the majority are RING finger proteins. Many RING finger E3s, such as the Cullin1-Skp1-F-box protein (SCF) E3s, CBL, BRCA1, MDM2 and von Hippel-Lindau tumour suppressor (VHL), are crucial in the regulation of cell-cycle progression and cell differentiation. As a result, many RING finger E3s are implicated in the positive and negative regulation of CSC maintenance. This review summarizes current knowledge in this research field.

Enigma Prevents Cbl-c-Mediated Ubiquitination and Degradation of RETMEN2A

The Cbl proteins (Cbl, Cbl-b, and Cbl-c) are a highly conserved family of RING finger ubiquitin ligases (E3s) that function as negative regulators of tyrosine kinases in a wide variety of signal transduction pathways. In this study, we identify a new Cbl-c interacting protein, Enigma (PDLIM7). This interaction is specific to Cbl-c as Enigma fails to bind either of its closely related homologues, Cbl and Cbl-b. The binding between Enigma and Cbl-c is mediated through the LIM domains of Enigma as removal of all three LIM domains abrogates this interaction, while only LIM1 is sufficient for binding. Here we show that Cbl-c binds wild-type and MEN2A isoforms of the receptor tyrosine kinase, RET, and that Cbl-c enhances ubiquitination and degradation of activated RET. Enigma blocks Cbl-c-mediated RETMEN2A ubiquitination and degradation. Cbl-c decreased downstream ERK activation by RETMEN2A and co-expression of Enigma blocked the Cbl-c-mediated decrease in ERK activation. Enigma showed no detectable effect on Cbl-c-mediated ubiquitination of activated EGFR suggesting that this effect is specific to RET. Through mapping studies, we show that Cbl-c and Enigma bind RETMEN2A at different residues. However, binding of Enigma to RETMENA prevents Cbl-c recruitment to RETMEN2A. Consistent with these biochemical data, exploratory analyses of breast cancer patients with high expression of RET suggest that high expression of Cbl-c correlates with a good outcome, and high expression of Enigma correlates with a poor outcome. Together, these data demonstrate that Cbl-c can ubiquitinate and downregulate RETMEN2A and implicate Enigma as a positive regulator of RETMEN2A through blocking of Cbl-mediated ubiquitination and degradation.

Overexpression Of Leukemia Associated CblY371H Mutation In Transgenic Mice Causes Dosage Dependent Embryonic Lethality

Juvenile Myelomonocytic Leukemia (JMML) is a devastating childhood cancer which is rapidly fatal with infiltration of myeloid cells into multiple organs (Hess, Zutter, Castleberry, & Emanuel, 1996). Based on the observation that uniparental disomy was found in the chromosomal region 11q in JMML patient samples, about 15% of patient samples were found to contain a mutation in c-Cbl (Loh et al., 2009). Moreover, mutant Cbl was also found to be a tumor suppressor gene where a germline mutation results in the predisposition for developing JMML (Niemeyer et al., 2010). The c-Cbl gene encodes a multifunctional adaptor protein which contains an N-terminal tyrosine-kinase binding (TKB) domain, a RING finger motif which contain E3 ligase activity, and a C-terminal ubiquitin-associated domain. Previous mutations in myeloid malignancies have been described where mutations occur in the RING finger domain or the linker domain (Caligiuri et al., 2007; Dunbar et al., 2008). Interestingly, a hotspot for mutations at residue 371 exists in JMML patients where 1/3 of the detected mutations are a tyrosine to histidine substitution, Y371H (Loh et al., 2009). This residue belongs in the linker region of the CBL protein, and it was previously observed that Y371 mediates the binding of c-Cbl to the p85 subunit of PI3 kinase (Blaydes et al., 2001). In vitro, CblY371H mutation does indeed destroy its ligase function resulting in prolonged signaling through the Ras pathway only when the endogenous c-Cbl gene was silenced (Niemeyer et al., 2010). However, how mutant Cbl gives rise to JMML and how it acts in concert with other genes in the pathogenesis of JMML requires further study.To address these questions, we tested the oncogenicity of the CblY371H mutation in transgenic mice. Of the 6 founder transgenic mice, we chose L5 line for further analysis because it had the highest level of expression. As expected, overexpression of CblY371H by itself in wild type mice had no phenotype since inactivation of wild type Cbl allele is seen in patients. We, then, generated transgenic mice which were heterozygous for both the BAC transgene and the cbl null mutation (CblY371H; Cbl+/-) and then bred them to Cbl heterozygous knockout mice (Cbl+/-). Because homozygous knockout mice have reduced fertility, we preferentially utilized heterozygous mice for the breeding (El Chami et al., 2005). Interestingly, out of the first 118 progeny genotyped where 1/8 of the pups is expected to be the desired genotype (CblY371H;Cbl-/-), no pups were of the desired genotype suggesting embryonic lethality. Because mice which lack both c-cbl and cbl-b are embryonic lethal, we reasoned that high expression that is achieved from the L5 line was causing a dominant negative affect on cbl-b function resulting in an embryonic lethal phenotype. We then tested if a lower level of the transgene expression would result in viable pups. We used the K5 line which was the second highest expressing transgenic line. We were now able to generate the desired genotype (CblY371H; Cbl-/-) although at less than expected numbers. Thus far, we have generated 4 mice which have the correct genotype which are being monitored for disease. We also monitored the heterozygous mice that carried the transgene (CblY371H; Cbl+/-) which were also asymptomatic without any obvious phenotype. Further studies are being done to characterize the hematopoietic phenotype in these mice by isolating fetal liver cells followed by transplantation into lethally irradiated mice and will be discussed. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Development of a Fluorescence Polarization Based High-Throughput Assay to Identify Casitas B-Lineage Lymphoma RING Domain Regulators

The E3 ubiquitin protein ligase Casitas B-lineage Lymphoma (Cbl) proteins and their binding partners play an important role in regulating signal transduction pathways. It is important to utilize regulators to study the protein-protein interactions (PPIs) between these proteins. However, finding specific small-molecule regulators of PPIs remains a significant challenge due to the fact that the interfaces involved in PPIs are not well suited for effective small molecule binding. We report the development of a competitive, homogeneous, high-throughput fluorescence polarization (FP) assay to identify small molecule regulators of Cbl (RING) domain. The FP assay was used to measure binding affinities and inhibition constants of UbCH7 peptides and small molecule regulators of Cbl (RING) domains, respectively. In order to rule out promiscuous, aggregation-based inhibition, two assay conditions were developed and compared side by side. Under optimized conditions, we screened a 10,000 natural compound library in detergent-free and detergent-present (0.01% Triton X-100) systems. The results indicate that the detergent-present system is more suitable for high-throughput screens. Three potential compounds, methylprotodioscin, leonuride and catalpol, have been identified that bind to Cbl (RING) domain and interfere with the Cbl (RING)-UbCH7 protein-protein interaction.

Cbl‐family proteins as regulators of cytoskeleton‐dependent phenomena

Although the role of Cbl-family proteins as key cellular regulators has been established, phenomena regulated in a Cbl-dependent fashion are multiple and the mechanisms mediating the effects of Cbl proteins are diverse. This finding makes it important to consider different phenomena affected by functions of Cbl proteins individually. Among effects of Cbl on various biological functions there are many cases of regulation of cellular phenomena related to cytoskeletal rearrangements, such as cell adhesion, motility and invasion. Some of these regulatory functions are mediated by adaptor-type interactions of Cbl, especially by Cbl-dependent modulation of phosphatidyl-inositol-3' kinase (PI3K), while others are caused by Cbl-dependent ubiquitylation of various cytoskeletal and regulatory proteins, identified and unidentified. The role of Cbl in regulation of cytoskeleton-dependent cellular functions is discussed in this review.

The Role of the E3 Ligase Cbl-B in Murine Dendritic Cells

Dendritic cells (DCs) are potent antigen-presenting cells with a promising potential in cancer immunotherapy. Cbl proteins are E3 ubiquitin ligases and have been implicated in regulating the functional activity of various immune cells. As an example, c-Cbl negatively affects DC activation. We here describe that another member of the Cbl-protein family (i.e. Cbl-b) is highly expressed in murine bone-marrow-derived DCs (BMDCs). Differentiation of cblb−/− bone marrow mononuclear cells into classical BMDCs is unaltered, except enhanced induction of DEC-205 (CD205) expression. When tested in mixed-lymphocyte reaction (MLR), cblb−/− BMDCs exhibit increased allo-stimulatory capacity in vitro. BMDCs were next in vitro stimulated by various toll like receptor (TLR)-agonists (LPS, Poly(I:C), CpG) and exposed to FITC-labeled dextran. Upon TLR-stimulation, cblb−/− BMDCs produce higher levels of proinflammatory cytokines (IL-1α, IL-6 and TNF-α) and exhibit a slightly higher level of FITC-dextran uptake. To further characterize the functional significance of cblb−/− BMDCs we tested them in antigen-specific T cell responses against ovalbumin (OVA) protein and peptides, activating either CD8+ OT-I or CD4+ OT-II transgenic T cells. However, cblb−/− BMDCs are equally effective in inducing antigen-specific T cell responses when compared to wildtype BMDCs both in vitro and in vivo. The migratory capacity into lymph nodes during inflammation was similarly not affected by the absence of Cbl-b. In line with these observations, cblb−/− peptide-pulsed BMDCs are equally effective vaccines against OVA-expressing B16 tumors in vivo when compared to wildtype BMDCs. We conclude that in contrast to c-Cbl, Cbl-b plays only a limited role in the induction of Ag-specific T cell responses by murine BMDCs in vitro and in vivo.

Abstract 543: A loss of function Cbl-c mutation in a murine breast cancer model.

Abstract Receptor Tyrosine Kinase (RTK) signaling is essential for normal biological processes and inappropriate RTK activity can result in malignant transformation. Cbl proteins are a family of RING finger ubiquitin ligases that negatively regulate a variety of RTKs, including EGFR, MET, RET and components of the TCR pathway and disruption of this negative regulation can lead to tumor initiation and progression. Recent studies have identified a growing number of Cbl mutations associated with human myeloid neoplasias and epithelial tumors. Cbl-c is the most recently identified human Cbl protein and is expressed exclusively in epithelial cells. Recently, a novel Cbl-c cDNA has been isolated from a ductal carcinoma which developed in a C3(1)SV40Tag mouse, a murine model of breast cancer driven by Large T antigen. This mutant cDNA encodes a Cbl-c protein which lacks the catalytic domain of Cbl-c, thereby resembling known Cbl family mutations associated with myeoloid neoplasias. Genomic analysis of both parental and transgenic lines shows no evidence of germline mutation indicating that this mutation is likely somatic and may contribute to malignant transformation in this animal model. In overexpression studies, this mutant Cbl-c protein fails to mediate ubiquitination and degradation of activated EGFR and acts as a dominant negative protein, preventing ubiquitination of the EGFR by wild type Cbl proteins. The mutant Cbl-c protein also enhances transformation of NIH 3T3 cells by SV40 Large T antigen and through subsequent analysis of transforming capacity and signal pathway regulation, we are assessing interacting partners of Cbl-c and the potential role of this mutation in tumorigenesis. This mutant Cbl-c protein is the first functional evidence for the involvement of Cbl-c in transformation. Citation Format: Stephen C. Kales, Philip E. Ryan, Marion Nau, Jeffrey E. Green, Stanley Lipkowitz. A loss of function Cbl-c mutation in a murine breast cancer model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 543. doi:10.1158/1538-7445.AM2013-543

Interaction of ubiquitin ligase CBL with LMP2A protein of Epstein-Barr virus occurs via PTB domain of CBL and does not depend on adaptor ITSN1

Previously Latent membrane protein 2A (LMP2A) of Epstein-Barr virus was found to be ubiquitylated by CBL ubiquitin ligase but no direct interaction of LMP2A with CBL was reported. We aimed to explore this interaction and study a possibility of adaptor protein involvement. Taking into consideration that both LMP2A and CBL were shown to interact with endocytic adaptor protein intersectin 1 (ITSN1), we assumed that the latter could serve as a scaffold for LMP2A/CBL complex. Methods. We used an immunofluorescence and coimmunoprecipitation approaches to test a mutual complex formation of ITSN1, CBL and LMP2A proteins. Results. LMP2A coimmunoprecipitated with CBL while LMP2A did not interact with CBL G306E mutant harboring inactive phosphotyrosine-binding domain. We observed a triple colocalization of ITSN1, CBL and LMP2A signals in MCF-7 cells as well as coprecipitation of all mentioned proteins. Overexpression of ITSN1 did not affect the efficiency of complex formation of LMP2A with CBL. Moreover, LMP2A mutant unable to interact with ITSN1 was readily precipitated with CBL. Conclusions. LMP2A can be engaged in the complex together with endocytic adaptor ITSN1 and ubiquitin ligase CBL. We show that PTB domain of CBL is responsible for interaction with LMP2A. ITSN1 is not required for LMP2A recruiting to CBL.

Oncogenic Signaling by Leukemia-Associated Mutant Cbl Proteins

Members of the Cbl protein family (Cbl, Cbl-b, and Cbl-c) are E3 ubiquitin ligases that have emerged as critical negative regulators of protein tyrosine kinase (PTK) signaling. This function reflects their ability to directly interact with activated PTKs and to target them as well as their associated signaling components for ubiquitination. Given the critical roles of PTK signaling in driving oncogenesis, recent studies in animal models and genetic analyses in human cancer have firmly established that Cbl proteins function as tumor suppressors. Missense mutations or small in-frame deletions within the regions of Cbl protein that are essential for its E3 activity have been identified in nearly 5% of leukemia patients with myelodysplastic/myeloproliferative disorders. Based on evidence from cell culture studies, in vivo models and clinical data, we discuss the potential signaling mechanisms of mutant Cbl-driven oncogenesis. Mechanistic insights into oncogenic Cbl mutants and associated animal models are likely to enhance our understanding of normal hematopoietic stem cell homeostasis and provide avenues for targeted therapy of mutant Cbl-driven cancers.

The Role of E3 Ubiquitin Ligase Cbl Proteins in Interleukin-2-Induced Jurkat T-Cell Activation

Interleukin- (IL-) 2 is the major growth factor for T-cell activation and proliferation. IL-2 has multiple functions in the regulation of immunological processes. Although most studies focus on T-cell immunomodulation, T-cell activation by IL-2 is the foundation of priming the feedback loop. Here, we investigated the effect of MAPK/ERK and PI3K/Akt signaling pathways on IL-2-induced cell activation and the regulatory mechanisms of upstream ubiquitin ligase Cbl-b and c-Cbl. Morphological analysis of Jurkat T cells was performed by cytospin preparations with Wright-Giemsa stain. CD25 expression on Jurkat T cells was determined by flow cytometry. Changes in cell activation proteins such as p-ERK, ERK, p-Akt, Akt, and ubiquitin ligase Casitas B-cell Lymphoma (Cbl) proteins were analyzed by western blot. Following IL-2-induced activation of Jurkat T cells, p-ERK expression was upregulated, while there was no change in p-Akt, ERK, or Akt expression. Thus, the MAPK/ERK signaling pathway, but not PI3K/Akt, was involved in IL-2-induced T-cell activation. Either using PD98059 (a specific inhibitor for p-ERK) or depletion of ERK with small interfering RNA (siRNA) reduced the expression of CD25. This study also showed that ubiquitin ligase proteins Cbl-b and c-Cbl might be involved in IL-2-induced Jurkat T-cell activation by negatively regulating the MAPK/ERK signaling pathway.

The Cbl family of ubiquitin ligases regulates FcεRI expression and mast cell activation

Antigen interaction with specific IgE bound to the high-affinity Fc receptor for IgE, constitutively expressed on the cell-surface of mast cells, generates signals that cause a shift in the resting state equilibrium of phosphorylation and dephosphorylation events that serves to maintain homeostasis. The outcome of this activated state is the release of a wide array of preformed and newly synthesized pro-inflammatory mediators. During the past few years, the existence of a negative feedback loop initiated upon FcεRI engagement has also been envisaged. This negative signal involves the coordinated action of adaptors, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals, thus modulating mast cell functions. Relevant to this, others and we have demonstrated that Cbl family proteins control the amplitude of FcεRI-generated signals by specific ubiquitin modification of activated receptor subunits and associated protein tyrosine kinases. In this article, we review advances in our understanding of the molecular mechanisms through which Cbl proteins regulate FcεRI expression and signaling.

The PI3K Inhibitor GDC-0941 Attenuates Disease in a KrasG12D Mouse Model of CMML and JMML.

AbstractAbstract 2862▪▪This icon denotes a clinically relevant abstractChronic and juvenile myelomonocytic leukemias (CMML and JMML) are overlap myelodysplastic/myeloproliferative neoplasia (MDS/MPN) syndromes that respond poorly to conventional treatment regimens. Both diseases are characterized by aberrant N-Ras, K-Ras, Cbl, and SHP-2 proteins, which are not attractive drug target candidates. Focus has shifted to downstream effector pathways, which include Raf/MEK/ERK, phosphoinositide-3-OH kinase (PI3K)/Akt, and Ral-GDS/Ral-A cascades. However, it is unclear which pathways, if any, should be targeted. In part to address this question, we previously developed a mouse model of CMML and JMML by expressing a conditional “knock-in” KrasG12D oncogene in bone marrow. Our earlier studies showed that inhibition of MEK yields a significant reduction in disease burden in this model, including reduced leukocytosis, improved anemia and enhanced survival. Here, we interrogate the role of the PI3K/Akt pathway in KrasG12D driven MPN and further explore which specific pathways are responsible for leukemogenesis due to hyperactive Ras. We administered GDC-0941, a selective PI3K inhibitor, to Mx1-Cre, KrasG12D mutant mice Mice with well established MPN and wild-type (WT) littermates were randomly chosen to receive daily oral administration of GDC-0941 or a control vehicle. Treated mice exhibited dramatic corrections of leukocytosis and anemia as well as decrease in splenomegaly. Flow cytometry of bone marrow and peripheral populations also imply that GDC-0941 treatment corrects the aberrant proliferation, amends differentiation of bone marrow progenitors, and revives ineffective erythropoiesis found in KrasG12D mice. Treatment also resulted in markedly improved survival of KrasG12D mice; virtually all KrasG12Dmice in the treatment arm outlived their control counterparts. Our data suggest PI3K inhibition may play a role in suppressing hematologic dysfunction in JMML and CMML patients. Potential crosstalk between PI3K and MEK signaling further suggest that combinatorial activities of PI3K and MEK inhibition should be investigated. [Display omitted] Disclosures:Friedman:Genentech, Inc.: Employment. Sampath:Genentech, Inc.: Employment.

Cbl-c Ubiquitin Ligase Activity Is Increased via the Interaction of Its RING Finger Domain with a LIM Domain of the Paxillin Homolog, Hic 5

Cbl proteins (Cbl, Cbl-b and Cbl-c) are ubiquitin ligases that are critical regulators of tyrosine kinase signaling. In this study we identify a new Cbl-c interacting protein, Hydrogen peroxide Induced Construct 5 (Hic-5). The two proteins interact through a novel interaction mediated by the RING finger of Cbl-c and the LIM2 domain of Hic-5. Further, this interaction is mediated and dependent on specific zinc coordinating complexes within the RING finger and LIM domain. Binding of Hic-5 to Cbl-c leads to an increase in the ubiquitin ligase activity of Cbl-c once Cbl-c has been activated by Src phosphorylation or through an activating phosphomimetic mutation. In addition, co-transfection of Hic-5 with Cbl-c leads to an increase in Cbl-c mediated ubiquitination of the EGFR. These data suggest that Hic-5 enhances Cbl-c ubiquitin ligase activity once Cbl-c has been phosphorylated and activated. Interactions between heterologous RING fingers have been shown to activate E3s. This is the first demonstration of enhancement of ubiquitin ligase activity of a RING finger ubiquitin ligase by the direct interaction of a LIM zinc coordinating domain.

Abstract 207: Casitas B-lineage lymphoma-3 is an epigenetically regulated gene whose knockdown induces mesenchymal to epithelial transition and enhances erlotinib sensitivity in lung cancer cell lines

Abstract Casitas B-lineage lymphoma (Cbl) proteins are E3 ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-3 is a distinct member of this family of 3 genes that is specifically expressed in the epithelial lining of organs, particularly the bronchial airways, whose function, especially in cancer cells, is relatively unknown. Here we report that Cbl-3 expression and promoter CpG island DNA methylation differed profoundly in epithelial compared to mesenchymal-like lung cancer cells. 5-azacytidine treatment, as well as DNA methyltransferase-1 (DNMT1) KD (but not DNMT-3a or 3b) enhanced promoter demethylation and Cbl-3 gene expression. Knock down (KD) of Cbl-3 in mesenchymal-like lung cancer cell lines strongly enhanced EGFR expression and activated downstream pathways. As a result, cells that were normally resistant to erlotinib became greatly sensitized to drug treatment by Cbl-3 KD. Additionally, Cbl-3 KD inhibited cellular migration and increased cellular adhesion to the extra cellular matrix (ECM) as well as promoted cell-to-cell interaction through enhanced E-Cadherin and FAK expression. These alterations suggested a mesenchymal to epithelial transition (MET) conferred by Cbl-3 KD. Take together, this data provided new insights into Cbl-3 gene regulation and elucidated a potential role of Cbl-3 in EGFR signaling in the context of EMT in lung cancer cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 207. doi:1538-7445.AM2012-207

Abstract 25: CBL enhances breast tumor formation by inhibiting tumor suppressive activity of TGF-beta signaling

Abstract CBL protein family functions as multifunctional adaptor proteins and E3 ubiquitin ligases that are implicated as regulators of signaling in various cell types. Recent discovery revealed mutations of proto-oncogenic CBL in the linker region and RING finger domain in human acute myeloid neoplasm and these transforming mutations induced carcinogenesis. However, the adaptor function of CBL mediated signaling pathway during tumorigenesis has not been well characterized. Here, we show that CBL highly expresses in breast cancer cells and significantly inhibits TGF-β tumor suppressive activity. Knockdown of CBL expression resulted in the increased expression of TGF-β target genes, SERPINE1 and CDK inhibitors such as p15INK4b and p21Cip1. Furthermore, we demonstrate that CBL is frequently overexpressed in human breast cancer tissues and the loss of CBL decreases a tumorigenic activity of breast cancer cells in vivo. CBL directly binds to Smad3 through its proline-rich motif, thereby preventing Smad3 from interacting with Smad4 and blocking nuclear translocation of Smad3. CBL-b, one of CBL protein family, also interacted with Smad3 and knockdown of both CBL and CBL-b further enhanced TGF-β transcriptional activity. Our findings provide evidence for a previously undescribed mechanism by which oncogenic CBL can block TGF-β tumor suppressor activity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 25. doi:1538-7445.AM2012-25