Catalytic domains of tyrosine kinases determine the phosphorylation sites within c-Cbl

Src kinase and Syk activation initiate PI3K signaling by a chimeric latent membrane protein 1 in Epstein-Barr virus (EBV)+ B cell lymphomas

The B lymphotrophic γ-herpesvirus EBV is associated with a variety of lymphoid- and epithelial-derived malignancies, including B cell lymphomas in immunocompromised and immunosuppressed individuals. The primary oncogene of EBV, latent membrane protein 1 (LMP1), activates the PI3K/Akt pathway to induce the autocrine growth factor, IL-10, in EBV-infected B cells, but the mechanisms underlying PI3K activation remain incompletely understood. Using small molecule inhibition and siRNA strategies in human B cell lines expressing a chimeric, signaling-inducible LMP1 protein, nerve growth factor receptor (NGFR)-LMP1, we show that NGFR-LMP1 utilizes Syk to activate PI3K/Akt signaling and induce IL-10 production. NGFR-LMP1 signaling induces phosphorylation of BLNK, a marker of Syk activation. Whereas Src kinases are often required for Syk activation, we show here that PI3K/Akt activation and autocrine IL-10 production by NGFR-LMP1 involves the Src family kinase Fyn. Finally, we demonstrate that NGFR-LMP1 induces phosphorylation of c-Cbl in a Syk- and Fyn-dependent fashion. Our results indicate that the EBV protein LMP1, which lacks the canonical ITAM required for Syk activation, can nevertheless activate Syk, and the Src kinase Fyn, resulting in downstream c-Cbl and PI3K/Akt activation. Fyn, Syk, and PI3K/Akt antagonists thus may present potential new therapeutic strategies that target the oncogene LMP1 for treatment of EBV+ B cell lymphomas.

ABL tyrosine kinases: evolution of function, regulation, and specificity

ABL-family proteins comprise one of the best conserved branches of the tyrosine kinases. Each ABL protein contains an SH3-SH2-TK (Src homology 3-Src homology 2-tyrosine kinase) domain cassette, which confers autoregulated kinase activity and is common among nonreceptor tyrosine kinases. This cassette is coupled to an actin-binding and -bundling domain, which makes ABL proteins capable of connecting phosphoregulation with actin-filament reorganization. Two vertebrate paralogs, ABL1 and ABL2, have evolved to perform specialized functions. ABL1 includes nuclear localization signals and a DNA binding domain through which it mediates DNA damage-repair functions, whereas ABL2 has additional binding capacity for actin and for microtubules to enhance its cytoskeletal remodeling functions. Several types of posttranslational modifications control ABL catalytic activity, subcellular localization, and stability, with consequences for both cytoplasmic and nuclear ABL functions. Binding partners provide additional regulation of ABL catalytic activity, substrate specificity, and downstream signaling. Information on ABL regulatory mechanisms is being mined to provide new therapeutic strategies against hematopoietic malignancies caused by BCR-ABL1 and related leukemogenic proteins.

Cbl-b is a novel physiologic regulator of glycoprotein VI-dependent platelet activation

Cbl-b, a member of the Cbl family of E3 ubiquitin ligases, plays an important role in the activation of lymphocytes. However, its function in platelets remains unknown. We show that Cbl-b is expressed in human platelets along with c-Cbl, but in contrast to c-Cbl, it is not tyrosine-phosphorylated upon glycoprotein VI (GPVI) stimulation. Cbl-b, unlike c-Cbl, is not required for Syk ubiquitylation downstream of GPVI activation. Phospholipase Cgamma2 (PLCgamma2) and Bruton's tyrosine kinase (BTK) are constituently associated with Cbl-b. Cbl-b-deficient (Cbl-b(-/-)) platelets display an inhibition in the concentration-response curve for GPVI-specific agonist-induced aggregation, secretion, and Ca(2+) mobilization. A parallel inhibition is found for activation of PLCgamma2 and BTK. However, Syk activation is not affected by the absence of Cbl-b, indicating that Cbl-b acts downstream of Syk but upstream of BTK and PLCgamma2. When Cbl-b(-/-) mice were tested in the ferric chloride thrombosis model, occlusion time was increased and clot stability was reduced compared with wild type controls. These data indicate that Cbl-b plays a positive modulatory role in GPVI-dependent platelet signaling, which translates to an important regulatory role in hemostasis and thrombosis in vivo.

c-Cbl promotes T cell receptor-induced thymocyte apoptosis by activating the phosphatidylinositol 3-kinase/Akt pathway

The ability of thymocytes to assess T cell receptor (TCR) signaling strength and initiate the appropriate downstream response is crucial for determining their fate. We have previously shown that a c-Cbl RING finger mutant knock-in mouse, in which the E3 ubiquitin ligase activity of c-Cbl is inactivated, is highly sensitive to TCR-induced death signals that cause thymic deletion. This high intensity signal involves the enhanced tyrosine phosphorylation of the mutant c-Cbl protein promoting a marked increase in the activation of Akt. Here we show that this high intensity signal in c-Cbl RING finger mutant thymocytes also promotes the enhanced induction of two mediators of TCR-directed thymocyte apoptosis, Nur77 and the pro-apoptotic Bcl-2 family member, Bim. In contrast, a knock-in mouse harboring a mutation at Tyr-737, the site in c-Cbl that activates phosphatidylinositol 3-kinase, shows reduced TCR-mediated responses including suppression of Akt activation, a reduced induction of Nur77 and Bim, and greater resistance to thymocyte death. These findings identify tyrosine-phosphorylated c-Cbl as a critical sensor of TCR signal strength that regulates the engagement of death-promoting signals.

Roles of E3 ubiquitin ligases in cell adhesion and migration

Recent studies have demonstrated that a number of E3 ubiquitin ligases, including Cbl, Smurf1, Smurf2, HDM2, BCA2, SCF(beta-TRCP) and XRNF185, play important roles in cell adhesion and migration. Cbl negatively regulates cell adhesion via alpha integrin and Rap1 and inhibits actin polymerization by ubiquitinating mDab1 and WAVE2. Smurf1 regulates cell migration through ubiquitination of RhoA, talin head domain and hPEM2, while Smurf2 ubiquitinates Smurf1, TGFbeta type I receptor and RaplB to modulate cell migration and adhesion. HDM2 negatively regulates cell migration by targeting NFAT (a transcription factor) for ubiquitination and degradation, while SCF(beta-TRCP) ubiquitinates Snail (a transcriptional repressor of E-cadherin) to inhibit cell migration. TRIM32 promotes cell migration through ubiquitination of Abl interactor 2 (Abi2), a tumor suppressor. RNF5 and XRNF185 modulate cell migration by ubiquitinating paxillin. Thus, these E3 ubiquitin ligases regulate cell adhesion and (or) migration through ubiquitination of their specific substrates.

Organization of F-actin via concerted regulation of Kette by PTP61F and dAbl

We identify Kette, a key regulator of actin polymerization, as a substrate for Drosophila protein tyrosine phosphatase PTP61F, as well as for dAbl tyrosine kinase. We further show that dAbl is a direct substrate for PTP61F. Therefore, Kette phosphotyrosine levels are regulated both directly and indirectly by PTP61F. Kette and PTP61F genetically interact in the regulation of F-actin organization in pupal eye discs, suggesting that tyrosine phosphorylation is essential for the proper regulation of Kette-mediated actin dynamics. This hypothesis was confirmed by demonstrating the loss of Kette-mediated F-actin organization and lamella formation in S2 cells in a Kette Y482F mutant in which the dAbl phosphorylation site was eliminated. Our results establish for the first time that PTP61F and dAbl ensure proper actin organization through the coordinated and reversible tyrosine phosphorylation of Kette.

Intrinsic differences between the catalytic properties of the oncogenic NUP214-ABL1 and BCR-ABL1 fusion protein kinases

The NUP214-ABL1 fusion kinase has recently been identified in 6% of patients with T-cell acute lymphoblastic leukemia. In contrast to the more common oncogenic ABL1 fusion BCR-ABL1, NUP214-ABL1 localizes to the nuclear pore complexes and has attenuated transforming properties in hematopoietic cells and in mouse bone marrow transplant models. We have performed a thorough biochemical comparative analysis of NUP214-ABL1 and BCR-ABL1 and show that, despite their common tyrosine kinase domain, the two fusion proteins differ in many critical catalytic properties. NUP214-ABL1 has lower in vitro tyrosine kinase activity, which is in agreement with the absence of phosphorylation on its activation loop. NUP214-ABL1 was more sensitive to imatinib (Glivec) than BCR-ABL1 in vitro and in cells, indicating a different activation state and conformation of the two ABL1 fusion kinases. Using a peptide array, we identified differences in the spectrum and efficiency of substrate peptide phosphorylation and a differential involvement of Src kinases in downstream signaling. These results clearly indicate that different fusion partners of the same kinase can determine not only localization, but also critical functional properties of the enzyme such as inhibitor sensitivity and substrate preference, with subsequent differences in downstream signaling effectors and likely consequences in disease pathogenesis.

Constitutive c-jun N-terminal kinase activity in acute myeloid leukemia derives from Flt3 and affects survival and proliferation

OBJECTIVE

c-jun N-terminal kinase (JNK) has been implicated in proliferation and survival downstream from the tyrosine kinase oncogene, p210 BCR-ABL, in chronic myeloid leukemia. We studied whether a similar relationship between JNK and FMS-like tyrosine kinase 3 (Flt3) describes acute myeloid leukemia (AML).

METHODS

By immunoprecipitation, Flt3 was found to be activated and identified as the potential origin of JNK activity in a heavy majority of JNK+ve AML blasts tested. Often, Flt3 activity is associated with activating mutation of the gene locus. However, statistical linkage tied JNK activity with Flt3 expression levels rather than with mutation. An adaptor network to describe the signal cascade Flt3-to-JNK was uncovered.

RESULTS

Active Flt3 was linked to p85 phosphoinositide-3 (PI-3) kinase, and p85 with cbl and CrkII/CrkL by co-immunoprecipitaton assays from lysates of model cell lines and primary AML blasts. JNK1 co-immunoprecipitated from such lysates with p85-cbl-crkII/L and bound to Crk species SH3 domain in pull-down assay. siRNA-mediated depletion of Flt3 or of cbl, the adaptor at the nexus of this signaling group, inhibited JNK activity on substrate c-jun. Within AML blast cells influenced by Flt3 signaling, selective inhibition of JNK by a small molecule inhibitor, led to proliferative inhibition, apoptosis, and sensitizing cells to the anthracycline, daunorubicin. These effects occurred upon JNK inhibition without off-target inhibition of extracellular signal-regulated kinase or AKT pathways, and p38-kinase activation, an effector in the p53/p14 arf tumor suppressor pathway, was also maintained or augmented.

CONCLUSION

JNK is a bonafide signaling pathway from Flt3 in AML whose function for proliferation and survival is required in a significant AML cohort with active Flt3 signaling, by mutation or overexpression of Flt3.

Identification and functional characterization of an Src homology domain 3 domain-binding site on Cbl

Cbl is an adaptor protein and ubiquitin ligase that binds and is phosphorylated by the nonreceptor tyrosine kinase Src. We previously showed that the primary interaction between Src and Cbl is mediated by the Src homology domain 3 (SH3) of Src binding to proline-rich sequences of Cbl. The peptide Cbl RDLPPPPPPDRP(540-551), which corresponds to residues 540-551 of Cbl, inhibited the binding of a GST-Src SH3 fusion protein to Cbl, whereas RDLAPPAPPPDR(540-551) did not, suggesting that Src binds to this site on Cbl in a class I orientation. Mutating prolines 543-548 reduced Src binding to the Cbl 479-636 fragment significantly more than mutating the prolines in the PPVPPR(494-499) motif, which was previously reported to bind Src SH3. Mutating Cbl prolines 543-548 to alanines substantially reduced Src binding to Cbl, Src-induced phosphorylation of Cbl, and the inhibition of Src kinase activity by Cbl. Expressing the mutated Cbl in osteoclasts induced a moderate reduction in bone-resorbing activity and increased amounts of Src protein. In contrast, disabling the tyrosine kinase-binding domain of full-length Cbl by mutating glycine 306 to glutamic acid, and thereby preventing the previously described binding of the tyrosine kinase-binding domain to the Src phosphotyrosine 416, had no effect on Cbl phosphorylation, the inhibition of Src activity by full-length Cbl, or bone resorption. These data indicate that the Cbl RDLPPPP(540-546) sequence is a functionally important binding site for Src.

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

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

Loss of c-Cbl RING finger function results in high-intensity TCR signaling and thymic deletion

Signaling from the T-cell receptor (TCR) in thymocytes is negatively regulated by the RING finger-type ubiquitin ligase c-Cbl. To further investigate this regulation, we generated mice with a loss-of-function mutation in the c-Cbl RING finger domain. These mice exhibit complete thymic deletion by young adulthood, which is not caused by a developmental block, lack of progenitors or peripheral T-cell activation. Rather, this phenotype correlates with greatly increased expression of the CD5 and CD69 activation markers and increased sensitivity to anti-CD3-induced cell death. Thymic loss contrasts the normal fate of the c-Cbl-/- thymus, even though thymocytes from both mutant mice show equivalent enhancement in proximal TCR signaling, Erk activation and calcium mobilization. Remarkably, only the RING finger mutant thymocytes show prominent TCR-directed activation of Akt. We show that the mutant c-Cbl protein itself is essential for activating this pathway by recruiting the p85 regulatory subunit of PI 3-kinase. This study provides a unique model for analyzing high-intensity TCR signals that cause thymocyte deletion and highlights multiple roles of c-Cbl in regulating this process.