Ubiquitination of Lyn-kinase in rat basophilic leukemia RBL-2H3 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.

Src family kinases: regulation of their activities, levels and identification of new pathways

While the Src family of protein tyrosine kinases (SFK), and the main ancillary molecules involved in their regulation, have been studied for many years, the details of their interplay are not fully understood and thus remain under active investigation. Additionally, new players that coordinate their regulation and direct their signalling cascades are also being uncovered, shedding new light on the complexity of these signalling networks. Through the utilization of novel interaction assays, several new interconnecting mediators that are helping to show the elegance of Src family kinase regulation have been discovered. This review outlines SFK regulation, the discovery of the Csk binding protein (Phosphoprotein Associated with Glycosphingolipid-enriched microdomains, Cbp/PAG), and its role in regulating SFK kinase activity status, as well as protein levels. Further, details of the methods used to identify this dual mode of regulation can be applied to delineate the full gamut of SH2/SH3-directed SFK pathways and, indeed, those of any tyrosine kinase. Using Lyn as a model SFK, we and others have shown that Cbp recruits negative regulators of COOH-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk) after Lyn is activated and bound to Cbp. Lyn phosphorylates Cbp on multiple tyrosine residues, including two that can bind Lyn's SH2 domain with high affinity. Lyn also phosphorylates Y314, which recruits Csk/Ctk to phosphorylate Lyn at its Y508 negative site, allowing an inactive conformation to form. However, the pY508 site has a low affinity for Lyn's SH2 domain, while the Cbp sites have high affinity. Thus, until these Cbp sites are dephosphorylated, Lyn can remain active. Intriguingly, phosphorylated Y314 also binds the suppressor of cytokine signalling 1 (SOCS1), resulting in elevated ubiquitination and degradation of Lyn. Thus, a single phosphotyrosine residue within Cbp co-ordinates a two-phase process involving distinct negative regulatory pathways that allow inactivation, followed by degradation, of SFKs.

Csk-binding protein mediates sequential enzymatic down-regulation and degradation of Lyn in erythropoietin-stimulated cells

We have shown previously that the Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors. The importance of Lyn to red cell maturation has been highlighted by Lyn-/- mice developing anemia. Here we show that Lyn interacts with C-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators C-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk). Lyn phosphorylated Cbp on several tyrosine residues, including Tyr314, which recruited Csk/Ctk to suppress Lyn kinase activity. Intriguingly, phosphorylated Tyr314 also bound suppressor of cytokine signaling 1 (SOCS1), another well characterized negative regulator of cell signaling, resulting in elevated ubiquitination, and degradation of Lyn. In Epo-responsive primary cells and cell lines, Lyn rapidly phosphorylated Cbp, suppressing Lyn kinase activity via Csk/Ctk within minutes of Epo stimulation; hours later, SOCS1 bound to Cbp and was involved in the ubiquitination and turnover of Lyn protein. Thus, a single phosphotyrosine residue on Cbp coordinates a two-phase process involving distinct negative regulatory pathways to inactivate, then degrade, Lyn.

Chronic lymphocytic leukemia B cells contain anomalous Lyn tyrosine kinase, a putative contribution to defective apoptosis

B cell chronic lymphocytic leukemia (B-CLL) is a neoplastic disorder characterized by accumulation of B lymphocytes due to uncontrolled growth and resistance to apoptosis. Analysis of B cells freshly isolated from 40 patients with chronic lymphocytic leukemia demonstrated that the Src kinase Lyn, the switch molecule that couples the B cell receptor to downstream signaling, displays anomalous properties. Lyn is remarkably overexpressed at the protein level in leukemic cells as compared with normal B lymphocytes, with a substantial aliquot of the kinase anomalously present in the cytosol. Whereas in normal B lymphocytes Lyn activation is dependent on B cell-receptor stimulation, in resting malignant cells, the constitutive activity of the kinase accounts for high basal protein tyrosine phosphorylation and low responsiveness to IgM ligation. Addition of the Lyn inhibitors PP2 and SU6656 to leukemic cell cultures restores cell apoptosis, and treatment of malignant cells with drugs that induce cell apoptosis decreases both activity and amount of the tyrosine kinase. These findings suggest a direct correlation between high basal Lyn activity and defects in the induction of apoptosis in leukemic cells. They also support a critical role for Lyn in B-CLL pathogenesis and identify this tyrosine kinase as a potential therapeutic target.

Negative regulation of Lyn protein-tyrosine kinase by c-Cbl ubiquitin-protein ligase in Fc epsilon RI-mediated mast cell activation

BACKGROUND

Recent studies have demonstrated that c-Cbl functions as a ubiquitin-protein ligase toward immune receptors and non-receptor protein-tyrosine kinase Syk by facilitating their ubiquitination and subsequent targeting to proteasomes. However, it was not clear whether Src family kinase Lyn is regulated by the Cbl family of ubiquitin-protein ligases.

RESULTS

Aggregation of the high affinity IgE receptor (Fc epsilon RI) induces the rapid ubiquitination of Lyn in rat basophilic leukaemia RBL-2H3 cells. Treatment of cells with a proteasome inhibitor enhances the ubiquitination of Lyn. Stimulation of Fc epsilon RI results in the association of Lyn with c-Cbl and Cbl-b, both of which then become tyrosine phosphorylated. Co-transfection study shows that both c-Cbl and Cbl-b could induce the ubiquitination of activated Lyn in COS cells. Furthermore, over-expression of membrane-anchored form of c-Cbl inhibits the Fc epsilon RI-mediated degranulation and cytokine gene production in RBL-2H3 cells by the down-regulation of the kinase activity of Lyn through the enhanced ubiquitination.

CONCLUSIONS

These results demonstrate that Lyn is down-regulated by c-Cbl-mediated ubiquitination and subsequent degradation in proteasome after Fc epsilon RI stimulation in mast cells. Targeting of c-Cbl in the lipid raft results in the inhibition of Fc epsilon RI-mediated mast cell activation.

On the role of proteasomes in cell biology and proteasome inhibition as a novel frontier in the development of immunosuppressants

The proteasome, a large protease complex in cells, is the major machinery for protein degradation. It was previously considered a humble garbage collector, performing housekeeping duties to remove misfolded or spent proteins. Until recently, the interests of immunologists in proteasomes were focused largely on its role in antigen processing. Its real importance in cell biology has only been revealed contemporarily due to the availability of relatively specific inhibitors. It has now become increasingly clear that many aspects of immune responses highly depend on proper proteasome activity. Recently, a proteasome inhibitor has been successfully used to prevent acute as well as ongoing heart allograft rejection in mice. Such inhibitors are also efficacious in treating several autoimmune diseases, such as arthritis, psoriasis, and probably type I diabetes, in animal models. Phase II and III clinical trials of proteasome inhibitors in treating various tumors have shown promising results, and the side-effects of these drugs are tolerable. Therefore, proteasome inhibition represents a new and promising frontier in immunosuppressant development.