Suppressor of cytokine signaling 2 (SOCS2) associates with FLT3 and negatively regulates downstream signaling.

The suppressor of cytokine signaling 2 (SOCS2) is a member of the SOCS family of E3 ubiquitin ligases. SOCS2 is known to regulate signal transduction by cytokine receptors and receptor tyrosine kinases. The receptor tyrosine kinase FLT3 is of importance for proliferation, survival and differentiation of hematopoietic cells and is frequently mutated in acute myeloid leukemia. We observed that SOCS2 associates with activated FLT3 through phosphotyrosine residues 589 and 919, and co-localizes with FLT3 in the cell membrane. SOCS2 increases FLT3 ubiquitination and accelerates receptor degradation in proteasomes. SOCS2 negatively regulates FLT3 signaling by blocking activation of Erk 1/2 and STAT5. Furthermore, SOCS2 expression leads to a decrease in FLT3-ITD-mediated cell proliferation and colony formation. Thus, we suggest that SOCS2 associates with activated FLT3 and negatively regulates the FLT3 signaling pathways

Phosphorylation of the activation loop tyrosine 823 in c-Kit is crucial for cell survival and proliferation.

The receptor tyrosine kinase c-Kit, also known as the stem cell factor receptor, plays a key role in several developmental processes. Activating mutations in c-Kit lead to alteration of these cellular processes and have been implicated in many human cancers such as gastrointestinal stromal tumors (GISTs), acute myeloid leukemia (AML), testicular seminomas and mastocytosis. Regulation of the catalytic activity of several kinases is known to be governed by phosphorylation of tyrosine residues in the activation loop of the kinase domain. However, in the case of c-Kit phosphorylation of Y823 has been demonstrated to be a late event that is not required for kinase activation. However, since phosphorylation of Y823 is a ligand-activated event, we sought to investigate the functional consequences of Y823 phosphorylation. By using a tyrosine to phenylalanine mutant of tyrosine 823 we investigated the impact of Y823 on c-Kit signaling. We here demonstrate that Y823 is crucial for cell survival and proliferation and mutation of Y823 to phenylalanine leads to decreased sustained phosphorylation and ubiquitination of c-Kit as compared to the wild-type receptor. Furthermore, the mutated receptor was upon ligand-stimulation quickly internalized and degraded. Phosphorylation of the E3 ubiquitin ligase, Cbl was transient followed by a substantial reduction in phosphorylation of downstream signaling molecules such as Akt, Erk, Shc and Gab2. Thus, we propose that activation loop tyrosine 823 is crucial for activation of both the MAPK and PI3K pathways and that its disruption leads to a destabilization of the c-Kit receptor and decreased survival of cells

Deregulation of protein phosphatase expression in acute myeloid leukemia

Acute myeloid leukemia (AML) is a highly malignant disease of myeloid cell line. AML is the most frequent adult leukemia with inadequate treatment possibility. The protein phosphatases are critical regulators of cell signaling, and deregulation of protein phosphatases always contribute to cell transformation. Although many studies established a relationship between protein phosphatases and leukemia, little is known about the role of this group of proteins in AML. To address this issue, we initially identified the complete catalog of human protein phosphatase genes and used this catalog to study deregulation of protein phosphatases in AML. Using mRNA expression data of AML patients, we show that 11 protein phosphatases are deregulated in AML within 174 protein phosphatases. The GO enrichment study suggests that these genes are involved in multiple biological processes other than protein de-phosphorylation. Expression of DUSP10, PTPRC, and PTPRE was significantly higher than average expression in AML, and a linear combination of DUSP10, MTMR11, PTPN4, and PTPRE expressions provides important information about disease subtypes. Our results provide an overview of protein phosphatase deregulation in AML

Keratin 19 expression correlates with poor prognosis in breast cancer

Breast cancer expression profiling has been used for determining biomarkers. Using gene expression profiles of 2,400 patients we identified keratin 19 (KRT19) as a highly deregulated gene in breast cancer. KRT19 expression is independent of patient race but correlates with disease grade, and ER, PR or HER2 expression. Expression of TPD52, GATA3 and KRT18 was increased in KRT19 expressing patients. Furthermore, KRT19 expression was associated with ER up-regulation and Luminal B gene signatures, as well as a constitutive RAF1 signaling pathway. Finally, KRT19 expression correlated with poor overall survival. Taken together, our results suggest that KRT19 expression can be used as a prognostic marker

Protein kinase C (PKC) as a drug target in chronic lymphocytic leukemia.

Protein kinase C (PKC) belongs to a family of ten serine/threonine protein kinases encoded by nine genes. This family of proteins plays critical roles in signal transduction which results in cell proliferation, survival, differentiation and apoptosis. Due to differential subcellular localization and tissue distribution, each member displays distinct signaling characteristics. In this review, we have summarized the roles of PKC family members in chronic lymphocytic leukemia (CLL). CLL is a heterogeneous hematological disorder with survival ranging from months to decades. PKC isoforms are differentially expressed in CLL and play critical roles in CLL pathogenesis. Thus, isoform-specific PKC inhibitors may be an attractive option for CLL treatment

The dual specificity PI3K/mTOR inhibitor PKI-587 displays efficacy against T-cell acute lymphoblastic leukemia (T-ALL)

Although significant improvements have been made in the treatment of acute lymphoblastic leukemia (ALL), there is a substantial subset of high-risk T-cell ALL (T-ALL) patients with relatively poor prognosis. Like in other leukemia types, alterations of the PI3K/mTOR pathway are predominant in ALL which is also responsible for treatment failure and relapse. In this study, we show that relapsed T-ALL patients display an enrichment of the PI3K/mTOR pathway. Using a panel of inhibitors targeting multiple components of the PI3K/mTOR pathway, we observed that the dual-specific PI3K/mTOR inhibitor PKI-587 was the most selective inhibitor for T-ALL cells dependent on the PI3K/mTOR pathway. Furthermore, we observed that PKI-587 blocked proliferation and colony formation of T-ALL cell lines. Additionally, PKI-587 selectively abrogated PI3K/mTOR signaling without affecting MAPK signaling both in in vitro and in vivo. Inhibition of the PI3K/mTOR pathway using PKI-587 delayed tumor progression, reduced tumor load and enhanced the survival rate in immune-deficient mouse xenograft models without inducing weight loss in the inhibitor treated mice. This preclinical study shows beneficial effects of PKI-587 on T-ALL that warrants further investigation in the clinical setting

Src-Like Adaptor Protein (SLAP) differentially regulates normal and oncogenic c-Kit signaling

The Src-Like Adaptor Protein (SLAP) is an adaptor protein sharing considerable structural homology with Src. SLAP is expressed in variety of cells regulating receptor tyrosine kinase signaling by direct association. In this report, we show that SLAP associates with both wild-type and oncogenic c-Kit (c-Kit-D816V). The association involves SLAP SH2 domain and receptor phosphotyrosine residues different from those mediating Src interaction. Association of SLAP triggers c-Kit ubiquitination which, in turn, is followed by receptor degradation. Although SLAP depletion potentiates c-Kit downstream signaling by stabilizing the receptor, it remains non-functional in c-Kit-D816V signaling. Ligand-stimulated c-Kit or c-Kit-D816V did not alter membrane localization of SLAP. Interestingly oncogenic c-Kit-D816V, but not wild-type c-Kit, phosphorylates SLAP on Y120, Y258 and Y273 residues. Physical interaction between c-Kit-D816V and SLAP is mandatory for the phosphorylation to take place. Although tyrosine phosphorylated SLAP does not affect c-Kit-D816V signaling, mutation of these tyrosine sites to phenylalanine can restore SLAP activity. Taken together the data demonstrate that SLAP negatively regulates wild-type c-Kit signaling, but not its oncogenic counterpart, indicating a possible mechanism by which the oncogenic c-Kit bypasses the normal cellular negative feedback control

SOCS proteins in regulation of receptor tyrosine kinase signaling.

Receptor tyrosine kinases (RTKs) are a family of cell surface receptors that play critical roles in signal transduction from extracellular stimuli. Many in this family of kinases are overexpressed or mutated in human malignancies and thus became an attractive drug target for cancer treatment. The signaling mediated by RTKs must be tightly regulated by interacting proteins including protein-tyrosine phosphatases and ubiquitin ligases. The suppressors of cytokine signaling (SOCS) family proteins are well-known negative regulators of cytokine receptors signaling consisting of eight structurally similar proteins, SOCS1-7, and cytokine-inducible SH2-containing protein (CIS). A key feature of this family of proteins is the presence of an SH2 domain and a SOCS box. Recent studies suggest that SOCS proteins also play a role in RTK signaling. Activation of RTK results in transcriptional activation of SOCS-encoding genes. These proteins associate with RTKs through their SH2 domains and subsequently recruit the E3 ubiquitin machinery through the SOCS box, and thereby limit receptor stability by inducing ubiquitination. In a similar fashion, SOCS proteins negatively regulate mitogenic signaling by RTKs. It is also evident that RTKs can sometimes bypass SOCS regulation and SOCS proteins can even potentiate RTKs-mediated mitogenic signaling. Thus, apart from negative regulation of receptor signaling, SOCS proteins may also influence signaling in other ways