Suppressor of cytokine signaling 6 (SOCS6) negatively regulates Flt3 signal transduction through direct binding to phosphorylated tyrosines 591 and 919 of Flt3

BRCC36 associates with FLT3-ITD to regulate its protein stability and intracellular signaling in acute myeloid leukemia

Fms-like tyrosine kinase-3 (FLT3) is a commonly mutated gene in acute myeloid leukemia (AML). The two most common mutations are the internal-tandem duplication domain (ITD) mutation and the tyrosine kinase domain (TKD) mutation. FLT3-ITD and FLT3-TKD exhibit distinct protein stability, cellular localization, and intracellular signaling. To understand the underlying mechanisms, we performed proximity labeling with TurboID to identify proteins that regulate FLT3-ITD or -TKD differently. We found that BRCA1/BRCA2-containing complex subunit 36 (BRCC36), a specific K63-linked polyubiquitin deubiquitinase, was exclusively associated with ITD, not the wild type of FLT3 and TKD. Knockdown of BRCC36 resulted in decreased signal transducers and activators of transcription 5 phosphorylation and cell proliferation in ITD cells. Consistently, treatment with thiolutin, an inhibitor of BRCC36, specifically suppressed cell proliferation and induced cell apoptosis in ITD cells. Thiolutin efficiently affected leukemia cell lines expressing FLT3-ITD cell viability and exhibited mutual synergies with quizartinib, a standard clinical medicine for AML. Furthermore, mutation of the lysine at 609 of ITD led to significant suppression of K63 polyubiquitination and decreased its stability, suggesting that K609 is a critical site for K63 ubiquitination specifically recognized by BRCC36. These data indicate that BRCC36 is a specific regulator for FLT3-ITD, which may shed light on developing a novel therapeutic approach for AML.

TET2-mediated mRNA demethylation regulates leukemia stem cell homing and self-renewal

TET2 is recurrently mutated in acute myeloid leukemia (AML) and its deficiency promotes leukemogenesis (driven by aggressive oncogenic mutations) and enhances leukemia stem cell (LSC) self-renewal. However, the underlying cellular/molecular mechanisms have yet to be fully understood. Here, we show that Tet2 deficiency significantly facilitates leukemogenesis in various AML models (mediated by aggressive or less aggressive mutations) through promoting homing of LSCs into bone marrow (BM) niche to increase their self-renewal/proliferation. TET2 deficiency in AML blast cells increases expression of Tetraspanin 13 (TSPAN13) and thereby activates the CXCR4/CXCL12 signaling, leading to increased homing/migration of LSCs into BM niche. Mechanistically, TET2 deficiency results in the accumulation of methyl-5-cytosine (m5C) modification in TSPAN13 mRNA; YBX1 specifically recognizes the m5C modification and increases the stability and expression of TSPAN13 transcripts. Collectively, our studies reveal the functional importance of TET2 in leukemogenesis, leukemic blast cell migration/homing, and LSC self-renewal as an mRNA m5C demethylase.

Role of SOCS and VHL Proteins in Neuronal Differentiation and Development

The basic helix-loop-helix factors play a central role in neuronal differentiation and nervous system development, which involve the Notch and signal transducer and activator of transcription (STAT)/small mother against decapentaplegic signaling pathways. Neural stem cells differentiate into three nervous system lineages, and the suppressor of cytokine signaling (SOCS) and von Hippel-Lindau (VHL) proteins are involved in this neuronal differentiation. The SOCS and VHL proteins both contain homologous structures comprising the BC-box motif. SOCSs recruit Elongin C, Elongin B, Cullin5(Cul5), and Rbx2, whereas VHL recruits Elongin C, Elongin B, Cul2, and Rbx1. SOCSs form SBC-Cul5/E3 complexes, and VHL forms a VBC-Cul2/E3 complex. These complexes degrade the target protein and suppress its downstream transduction pathway by acting as E3 ligases via the ubiquitin-proteasome system. The Janus kinase (JAK) is the main target protein of the E3 ligase SBC-Cul5, whereas hypoxia-inducible factor is the primary target protein of the E3 ligase VBC-Cul2; nonetheless, VBC-Cul2 also targets the JAK. SOCSs not only act on the ubiquitin-proteasome system but also act directly on JAKs to suppress the Janus kinase-signal transduction and activator of transcription (JAK-STAT) pathway. Both SOCS and VHL are expressed in the nervous system, predominantly in brain neurons in the embryonic stage. Both SOCS and VHL induce neuronal differentiation. SOCS is involved in differentiation into neurons, whereas VHL is involved in differentiation into neurons and oligodendrocytes; both proteins promote neurite outgrowth. It has also been suggested that the inactivation of these proteins may lead to the development of nervous system malignancies and that these proteins may function as tumor suppressors. The mechanism of action of SOCS and VHL involved in neuronal differentiation and nervous system development is thought to be mediated through the inhibition of downstream signaling pathways, JAK-STAT, and hypoxia-inducible factor-vascular endothelial growth factor pathways. In addition, because SOCS and VHL promote nerve regeneration, they are expected to be applied in neuronal regenerative medicine for traumatic brain injury and stroke.

A Receptor Tyrosine Kinase Inhibitor Sensitivity Prediction Model Identifies AXL Dependency in Leukemia

Despite incredible progress in cancer treatment, therapy resistance remains the leading limiting factor for long-term survival. During drug treatment, several genes are transcriptionally upregulated to mediate drug tolerance. Using highly variable genes and pharmacogenomic data for acute myeloid leukemia (AML), we developed a drug sensitivity prediction model for the receptor tyrosine kinase inhibitor sorafenib and achieved more than 80% prediction accuracy. Furthermore, by using Shapley additive explanations for determining leading features, we identified AXL as an important feature for drug resistance. Drug-resistant patient samples displayed enrichment of protein kinase C (PKC) signaling, which was also identified in sorafenib-treated FLT3-ITD-dependent AML cell lines by a peptide-based kinase profiling assay. Finally, we show that pharmacological inhibition of tyrosine kinase activity enhances AXL expression, phosphorylation of the PKC-substrate cyclic AMP response element binding (CREB) protein, and displays synergy with AXL and PKC inhibitors. Collectively, our data suggest an involvement of AXL in tyrosine kinase inhibitor resistance and link PKC activation as a possible signaling mediator.

Regulation of mi-RNAs Target Cancer Genes Between Exercise and Non-exercise in Rat Rheumatoid Arthritis Induction: Pilot Study

Introduction:

Rheumatoid arthritis is associated with various cancers. Many studies have investigated physical exercise interventions as health improvements to ameliorate the risk of cancer during rheumatoid arthritis diagnosis. Recently, microRNAs were used as biomarkers for health assessment and cancer prediction in rheumatoid arthritis patients.

Methods:

The effects of exercise interventions on serum microRNAs were investigated in pristane-induced arthritis (PIA) rat models. Twelve Sprague-Dawley male rats were divided into 4 groups including non-exercise without PIA (N-EX), non-exercise with PIA (N-EX + PIA), exercise without PIA (EX) and exercise with PIA (EX + PIA). Blood samples were collected at the end of the study period to analyze miRNA biomarkers and target cancer gene predictions.

Results:

Four significant Rattus norvegicus (rno-microRNAs) may purpose as tumor suppressors were identified as potential target cancer gene candidate expressions within the 4 comparative interventional exercise groups. One rno-microRNA and target cancer gene candidate was up-regulated and 3 rno-microRNAs and their target cancer genes were down-regulated.

Conclusions:

Exercise interventions affected rno-miRNAs regulated target cancer gene candidates ITPR3, SOCS6, ITGA6, and NKX2-1 as biomarkers for cancer prognosis in rheumatoid arthritis diagnosis.

Down-regulation of SOCS6: an unfavorable prognostic factor for gastrointestinal stromal tumor proven by survival analysis

Background

Many studies reporting that down-regulation of SOCS6 plays vital roles in promoting progression of malignant tumors have been published. The present study was performed to evaluate whether SOCS6 was significantly associated with prognosis of GIST patients.

Methods

Immunohistochemical staining was accomplished to evaluate the expression levels of SOCS6 among GIST patients. The impacts of SOCS6 expression on overall survival (OS) and recurrence-free survival (RFS) of GIST patients were assessed by Cox proportional hazard regression model analysis and Kaplan-Meier curve analysis.

Results

It was demonstrated that the expression level of SOCS6 was significantly associated with tumor size (P=0.001). Then according to Kaplan-Meier curve analysis, low expression of SOCS6 was significantly correlated with worse OS and RFS of GIST patients. Ultimately, it was revealed by Cox proportional regression model analysis that low expression of SOCS6 was an independent predictive factor for OS and RFS.

Conclusions

Low expression of SOCS6 was an independent prognostic factor for GIST, suggesting its potential as a novel biomarker predicting survival of GIST patients.

The Emerging Role of Suppressors of Cytokine Signaling (SOCS) in the Development and Progression of Leukemia

Cytokines are pleiotropic signaling molecules that execute an essential role in cell-to-cell communication through binding to cell surface receptors. Receptor binding activates intracellular signaling cascades in the target cell that bring about a wide range of cellular responses, including induction of cell proliferation, migration, differentiation, and apoptosis. The Janus kinase and transducers and activators of transcription (JAK/STAT) signaling pathways are activated upon cytokines and growth factors binding with their corresponding receptors. The SOCS family of proteins has emerged as a key regulator of cytokine signaling, and SOCS insufficiency leads to constitutive activation of JAK/STAT signaling and oncogenic transformation. Dysregulation of SOCS expression is linked to various solid tumors with invasive properties. However, the roles of SOCS in hematological malignancies, such as leukemia, are less clear. In this review, we discuss the recent advances pertaining to SOCS dysregulation in leukemia development and progression. We also highlight the roles of specific SOCS in immune cells within the tumor microenvironment and their possible involvement in anti-tumor immunity. Finally, we discuss the epigenetic, genetic, and post-transcriptional modifications of SOCS genes during tumorigenesis, with an emphasis on leukemia.

A Cas-BCAR3 co-regulatory circuit controls lamellipodia dynamics

Integrin adhesion complexes regulate cytoskeletal dynamics during cell migration. Adhesion activates phosphorylation of integrin-associated signaling proteins, including Cas (p130Cas, BCAR1), by Src-family kinases. Cas regulates leading-edge protrusion and migration in cooperation with its binding partner, BCAR3. However, it has been unclear how Cas and BCAR3 cooperate. Here, using normal epithelial cells, we find that BCAR3 localization to integrin adhesions requires Cas. In return, Cas phosphorylation, as well as lamellipodia dynamics and cell migration, requires BCAR3. These functions require the BCAR3 SH2 domain and a specific phosphorylation site, Tyr 117, that is also required for BCAR3 downregulation by the ubiquitin-proteasome system. These findings place BCAR3 in a co-regulatory positive-feedback circuit with Cas, with BCAR3 requiring Cas for localization and Cas requiring BCAR3 for activation and downstream signaling. The use of a single phosphorylation site in BCAR3 for activation and degradation ensures reliable negative feedback by the ubiquitin-proteasome system.

RNA Sequencing Reveals Distinct Immune Responses in the Chorioamniotic Membranes of Women with Preterm Labor and Microbial or Sterile Intra-amniotic Inflammation

Preterm labor precedes premature birth, the leading cause of neonatal morbidity and mortality worldwide. Preterm labor can occur in the context of either microbe-associated intra-amniotic inflammation (i.e., intra-amniotic infection) or intra-amniotic inflammation in the absence of detectable microorganisms (i.e., sterile intra-amniotic inflammation). Both intra-amniotic infection and sterile intra-amniotic inflammation trigger local immune responses that have deleterious effects on fetal life. Yet, the extent of such immune responses in the fetal tissues surrounding the amniotic cavity (i.e., the chorioamniotic membranes) is poorly understood. By using RNA sequencing (RNA seq) as a discovery approach, we found that there were significant transcriptomic differences involving host response to pathogens in the chorioamniotic membranes of women with intra-amniotic infection compared to those from women without inflammation. In addition, the sterile or microbial nature of intra-amniotic inflammation was associated with distinct transcriptomic profiles in the chorioamniotic membranes. Moreover, the immune response in the chorioamniotic membranes of women with sterile intra-amniotic inflammation was milder in nature than that induced by microbes and involved the upregulation of alarmins and inflammasome-related molecules. Lastly, the presence of maternal and fetal inflammatory responses in the placenta was associated with the upregulation of immune processes in the chorioamniotic membranes. Collectively, these findings provide insight into the immune responses against microbes or alarmins that take place in the fetal tissues surrounding the amniotic cavity, shedding light on the immunobiology of preterm labor and birth.

SOCS6 promotes radiosensitivity and decreases cancer cell stemness in esophageal squamous cell carcinoma by regulating c-Kit ubiquitylation

Background

Radiotherapy is a major treatment for esophageal squamous cell carcinoma (ESCC). However, HPV infection related radioresistance caused poor prognosis of ESCC. The function of SOCS6, which has been shown to be a tumor suppressor in several cancers, has not been fully investigated up till now. In this manuscript, we aim to further investigate the role of SOCS6 in regulating ESCC radioresistance.

Methods

Fifty-seven ESCC patients were enrolled for survival analysis. SOCS6 was stably overexpressed in HPV⁺ ESCC and ESCC cells, and cells were treated with radiation and then subjected to colony formation assays. Expression of DNA damage repair regulating proteins were examined by Western blotting. Cell growth, cell migration and cisplatin sensitivity were then analyzed. Sphere formation assays and flow cytometry were used to investigate changes in cancer stem cell (CSC) properties. Immunofluorescent staining and confocal microscopy were used to locate SOCS6 and c-Kit. Ubiquitylation level of c-Kit were analyzed after immunoprecipitation. Then, coimmunoprecipitation (CoIP) of SOCS6 and c-Kit were performed. In vivo, xenograft animal models were treated with radiation to examine the radiosensitivity.

Results

SOCS6 is correlated with better prognosis in ESCC patients. Radioresistance is impaired by SOCS6 upregulation, which inhibited cell growth, migration and increased sensitivity to cisplatin. SOCS6 significantly decreased the population of CSCs expressing the surface biomarker CD271 or CD24^low/CD44^high and their ability of sphere formation. SOCS6 and c-Kit were collocated in the cytoplasm. Blotting of ubiquitin and CoIP experiments indicated that the mechanism was related to ubiquitylation and degradation of the receptor c-Kit. Xenograft tumor mouse model showed that SOCS6 inhibited tumor growth and promoted radiosensitivity in vivo.

Conclusions

Our findings suggest that SOCS6 can promote the radiosensitivity of HPV⁺ ESCC and ESCC cells and reduce their stemness via ubiquitylation and degradation of c-Kit. Thus, SOCS6 is a potential target for overcoming radioresistance of ESCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-01859-2.

Phosphorylation regulates cullin-based ubiquitination in tumorigenesis

Cullin-RING ligases (CRLs) recognize and interact with substrates for ubiquitination and degradation, and can be targeted for disease treatment when the abnormal expression of substrates involves pathologic processes. Phosphorylation, either of substrates or receptors of CRLs, can alter their interaction. Phosphorylation-dependent ubiquitination and proteasome degradation influence various cellular processes and can contribute to the occurrence of various diseases, most often tumorigenesis. These processes have the potential to be used for tumor intervention through the regulation of the activities of related kinases, along with the regulation of the stability of specific oncoproteins and tumor suppressors. This review describes the mechanisms and biological functions of crosstalk between phosphorylation and ubiquitination, and most importantly its influence on tumorigenesis, to provide new directions and strategies for tumor therapy.

Megakaryocytic Expansion in Gilteritinib-Treated Acute Myeloid Leukemia Patients Is Associated With AXL Inhibition

Numerous recurrent genetic mutations are known to occur in acute myeloid leukemia (AML). Among these common mutations, Fms-like tyrosine kinase 3 remains as one of the most frequently mutated genes in AML. We observed apparent marrow expansion of megakaryocytes in three out of six patients with Flt3-mutated AML following treatment with a recently FDA-approved Flt3 inhibitor, gilteritinib which possesses activity against internal tandem duplication and tyrosine kinase domain Flt3 mutations and also inhibits tyrosine kinase AXL. To assess whether biopsy findings can be attributed to promotion of megakaryocytic (Mk) differentiation with gilteritinib, we devised a cellular assay by overexpressing double mutated Flt3-ITD^Y591F/Y919F in chronic myeloid leukemia cell line K562 to study Mk differentiation in the presence of Flt3 and AXL inhibitors with non-mutually exclusive mechanisms. These experiments demonstrated the lack of direct effect Flt3 inhibitors gilteritinib and quizartinib on megakaryocytic differentiation at either transcriptional or phenotypic levels, and highlighted antileukemic effects of AXL receptor tyrosine kinase inhibitor and its potential role in megakaryocytic development.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Deletion of SOCS2 Reduces Post-Colitis Fibrosis via Alteration of the TGFβ Pathway

Inflammatory bowel disease (IBD) is an immunologically mediated chronic intestinal disorder. Growth hormone (GH) administration enhances mucosal repair and decreases intestinal fibrosis in patients with IBD. In the present study, we investigated the effect of cellular sensitivity to GH via suppressor of cytokine signaling 2 (SOCS2) deletion on colitis and recovery. To induce colitis, wild type and SOCS2 knockout (SOCS2−/−) mice were treated with 3% dextran sodium sulphate (DSS), followed by a recovery period. SOCS2−/− mice showed higher disease activity during colitis with increased mRNA expression of the pro-inflammatory cytokines nitric oxide synthase 2 (NOS2) and interleukin 1 β (IL1-β). At recovery time point, SOCS2−/− showed better recovery with less fibrosis measured by levels of α-SMA and collagen deposition. Protein and mRNA expressions of transforming growth factor beta β1 (TGF-β1) receptors were significantly lower in SOCS2−/− mice compared to wild-type littermates. Using an in vivo bromodeoxyuridine (BrdU) proliferation assay, SOCS2−/− mice showed higher intestinal epithelial proliferation compared to wild-type mice. Our results demonstrated that deletion of the SOCS2 protein results in higher growth hormone sensitivity associated with higher pro-inflammatory signaling; however, it resulted in less tissue damage with less fibrotic lesions and higher epithelial proliferation, which are markers of GH-protective effects in IBD. This suggests a pleiotropic effect of SOCS2 and multiple cellular targets. Further study is required to study role of SOCS2 in regulation of TGFβ-mothers against the decapentaplegic homolog (Smad) pathway.

Targeting PRMT1-mediated FLT3 methylation disrupts maintenance of MLL-rearranged acute lymphoblastic leukemia

Relapse remains the main cause of MLL-rearranged (MLL-r) acute lymphoblastic leukemia (ALL) treatment failure resulting from persistence of drug-resistant clones after conventional chemotherapy treatment or targeted therapy. Thus, defining mechanisms underlying MLL-r ALL maintenance is critical for developing effective therapy. PRMT1, which deposits an asymmetric dimethylarginine mark on histone/non-histone proteins, is reportedly overexpressed in various cancers. Here, we demonstrate elevated PRMT1 levels in MLL-r ALL cells and show that inhibition of PRMT1 significantly suppresses leukemic cell growth and survival. Mechanistically, we reveal that PRMT1 methylates Fms-like receptor tyrosine kinase 3 (FLT3) at arginine (R) residues 972 and 973 (R972/973), and its oncogenic function in MLL-r ALL cells is FLT3 methylation dependent. Both biochemistry and computational analysis demonstrate that R972/973 methylation could facilitate recruitment of adaptor proteins to FLT3 in a phospho-tyrosine (Y) residue 969 (Y969) dependent or independent manner. Cells expressing R972/973 methylation-deficient FLT3 exhibited more robust apoptosis and growth inhibition than did Y969 phosphorylation-deficient FLT3-transduced cells. We also show that the capacity of the type I PRMT inhibitor MS023 to inhibit leukemia cell viability parallels baseline FLT3 R972/973 methylation levels. Finally, combining FLT3 tyrosine kinase inhibitor PKC412 with MS023 treatment enhanced elimination of MLL-r ALL cells relative to PKC412 treatment alone in patient-derived mouse xenografts. These results indicate that abolishing FLT3 arginine methylation through PRMT1 inhibition represents a promising strategy to target MLL-r ALL cells.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

miR-1260b, mediated by YY1, activates KIT signaling by targeting SOCS6 to regulate cell proliferation and apoptosis in NSCLC

Non-small cell lung cancer (NSCLC) is one of the most common aggressive malignancies. miRNAs have been identified as important biomarkers and regulators of NSCLC. However, the functional contributions of miR-1260b to NSCLC cell proliferation and apoptosis have not been studied. In this study, miR-1260b was upregulated in NSCLC plasma, tissues, and cell lines, and its high expression was correlated with tumor size and progression. Functionally, miR-1260b overexpression promoted cell proliferation and cell cycle, conversely inhibited cell apoptosis and senescence. Mechanically, miR-1260b negatively regulated SOCS6 by directly binding to its 3′-UTR. Furthermore, miR-1260b-mediated suppression of SOCS6 activated KIT signaling. Moreover, YY1 was an upstream regulator of miR-1260b. This study is the first to illustrate that miR-1260b, mediated by YY1, activates KIT signaling by targeting SOCS6 to regulate NSCLC cell proliferation and apoptosis, and is a potential biomarker and therapeutic target for NSCLC. In sum, our work provides new insights into the molecular mechanisms of NSCLC involved in cell proliferation and apoptosis.

Stem cell factor and NSC87877 synergism enhances c-Kit mediated proliferation of human erythroid cells

The biological mechanisms underlying the effects of stem cell factor (SCF) and an inhibitor, NSC87877 (N) of the c-Kit negative regulator (SHP-1 and SHP-2) on cell proliferation are different. Therefore, we compared the cell's response to these two either alone or in combination in K562 cells. Binding of SCF (S) to c-Kit induces dimerization that activates its kinase activity. The activated c-Kit undergoes autophosphorylation at tyrosine residues that serve as a docking site for signal transduction molecules containing SH2 domains. Predominantly, the phosphotyrosine 568 (pY568) in Juxtamembrane (JM) region of c-Kit interacts with adaptor protein APS, Src family kinase, and SHP-2, while phosphotyrosine 570 (pY570) interacts with the SHP-1 and the adaptor protein Shc. The dephosphorylation of phosphotyrosine residues by SHP-1/SHP-2 leads to inhibition of c-Kit proliferative signaling. A chemical molecule, N is reported to inhibit the enzymatic activity of SHP-1/SHP-2, but its effect on c-Kit-mediated proliferation has not been studied yet. Thus, this work aims at examining the effect of the combination of S and N on cells growth as compared to individual treatment. The present study is performed with erythroleukemic K562 cells, chosen for its mRNA expression concerning the c-Kit, and SHP-1/SHP-2. Interestingly, proliferation assay showed that combination significantly increased proliferation when G₁ sorted K562 cells were used. These changes were significantly higher when K562 cells were initially treated with N followed by S treatment. Collectively, these results give mechanistic insight into the proliferation enhancement of bone marrow transplantation through the synergistic effect of S and N by inhibiting SHP-1/SHP-2. The study gives solid evidence that S and N combination can be used to enhance cell proliferation/growth.

XK-related protein 5 (XKR5) is a novel negative regulator of KIT/D816V-mediated transformation

In order to investigate the molecular mechanisms by which the oncogenic mutant KIT/D816V causes transformation of cells, we investigated proteins that selectively bind KIT/D816V, but not wild-type KIT, as potential mediators of transformation. By mass spectrometry several proteins were identified, among them a previously uncharacterized protein denoted XKR5 (XK-related protein 5), which is related to the X Kell blood group proteins. We could demonstrate that interaction between XKR5 and KIT/D816V leads to phosphorylation of XKR5 at Tyr 369, Tyr487, and Tyr 543. Tyrosine phosphorylated XKR5 acts as a negative regulator of KIT signaling, which leads to downregulation of phosphorylation of ERK, AKT, and p38. This led to reduced proliferation and colony forming capacity in semi-solid medium. Taken together, our data demonstrate that XKR5 is a novel type of negative regulator of KIT-mediated transformation.

Src-like adaptor protein 2 (SLAP2) binds to and inhibits FLT3 signaling

Fms-like tyrosine kinase (FLT3) is a frequently mutated oncogene in acute myeloid leukemia (AML). FLT3 inhibitors display promising results in a clinical setting, but patients relapse after short-term treatment due to the development of resistant disease. Therefore, a better understanding of FLT3 downstream signal transduction pathways will help to identify an alternative target for the treatment of AML patients carrying oncogenic FLT3. Activation of FLT3 results in phosphorylation of FLT3 on several tyrosine residues that recruit SH2 domain-containing signaling proteins. We screened a panel of SH2 domain-containing proteins and identified SLAP2 as a potent interacting partner of FLT3. We demonstrated that interaction occurs when FLT3 is activated, and also, an intact SH2 domain of SLAP2 is required for binding. SLAP2 binding sites in FLT3 mainly overlap with those of SRC. SLAP2 over expression in murine proB cells or myeloid cells inhibited oncogenic FLT3-ITD-mediated cell proliferation and colony formation in vitro, and tumor formation in vivo. Microarray analysis suggests that higher SLAP2 expression correlates with a gene signature similar to that of loss of oncogene function. Furthermore, FLT3-ITD positive AML patients with higher SLAP2 expression displayed better prognosis compared to those with lower expression of SLAP2. Expression of SLAP2 blocked FLT3 downstream signaling cascades including AKT, ERK, p38 and STAT5. Finally, SLAP2 accelerated FLT3 degradation through enhanced ubiquitination. Collectively, our data suggest that SLAP2 acts as a negative regulator of FLT3 signaling and therefore, modulation of SLAP2 expression levels may provide an alternative therapeutic approach for FLT3-ITD positive AML.

Internal tandem duplication mutations in the tyrosine kinase domain of FLT3 display a higher oncogenic potential than the activation loop D835Y mutation

Acute myeloid leukemia (AML) remains the most common form of acute leukemia among adults and accounts for a large number of leukemia-related deaths. Mutations in FMS-like tyrosine kinase 3 (FLT3) is one of the most prevalent findings in this heterogeneous disease. The major types of mutations in FLT3 can be categorized as internal tandem duplications (ITD) and point mutations. Recent studies suggest that ITDs not only occur in the juxtamembrane region as originally described, but also in the kinase domain. Although the juxtamembrane ITDs have been well characterized, the tyrosine kinase domain ITDs have not yet been thoroughly studied due to their recent discovery. For this reason, we compared ITD mutations in the juxtamembrane domain with those in the tyrosine kinase domain, as well as with the most common activating point mutation in the tyrosine kinase domain, D835Y. The purpose of this study was to understand whether it is the nature of the mutation or the location of the mutation that plays the main role in leukemogenesis. The various FLT3 mutants were expressed in the murine pro-B cell line Ba/F3 and examined for their capacity to form colonies in semisolid medium. The size and number of colonies formed by Ba/F3 cells expressing either the internal tandem duplication within juxtamembrane domain of the receptor (JMD-ITD) or the tyrosine kinase domain (TKD)-ITD were indistinguishable, while Ba/F3 cells expressing D835Y/FLT3 failed to form colonies. Cell proliferation and cell survival was also significantly higher in TKD-ITD expressing cells, compared to cells expressing D835Y/FLT3. Furthermore, TKD-ITD is capable of inducing phosphorylation of STAT5, while D835Y/FLT3 fails to induce tyrosine phosphorylation of STAT5. Other signal transduction pathways such as the RAS/ERK and the PI3K/AKT pathways were activated to the same level in TKD-ITD cells as compared to D835Y/FLT3 expressing cells. Taken together, our data suggest that TKD-ITD displays similar oncogenic potential to the JMD-ITD but a higher oncogenic potential than the D835Y point mutation.

Expression of GADS enhances FLT3-induced mitogenic signaling

GADS is a member of a family of SH2 and SH3 domain-containing adaptors that functions in tyrosine kinase-mediated signaling cascades. Its expression is largely restricted to hematopoietic tissues and cell lines. Therefore, GADS is mainly involved in leukocyte-specific protein tyrosine kinase signaling. GADS is known to interact with tyrosine-phosphorylated SHC, BCR-ABL and KIT. The SH2 domain of GADS has a similar binding specificity to that of GRB2 but its SH3 domain displays a different binding specificity, and thus it is involved in other downstream signaling pathways than GRB2. In the present study, we examined the role of GADS in FLT3 signaling. FLT3 is a type III receptor tyrosine kinase, which is mutated in more than 30% of acute myeloid leukemia (AML) and the most common mutations is the internal tandem duplication (ITD) mutations. We observed that expression of GADS enhanced oncogenic FLT3-ITD-induced cell proliferation and colony formation in vitro. In a mouse xenograft model, GADS accelerated FLT3-ITD-dependent tumor formation. Furthermore, expression of GADS induced a transcriptional program leading to upregulation of MYC and mTORC1 target genes. GADS localizes to the cell membrane and strongly binds to ligand-stimulated wild-type FLT3 or is constitutively associated with the oncogenic mutant FLT3-ITD. We mapped the binding sites in FLT3 to pY955 and pY969 which overlaps with the GRB2 binding sites. Expression of GADS enhanced FLT3-mediated phosphorylation of AKT, ERK1/2, p38 and STAT5. Taken together, our data suggests that GADS is an important downstream component of FLT3 signaling and expression of GADS potentiates FLT3-mediated mitogenic signaling.

The Src family kinase LCK cooperates with oncogenic FLT3/ITD in cellular transformation

The non-receptor tyrosine kinase LCK belongs to the SRC family of kinases. SRC family kinases are proto-oncogenes that have long been known to play key roles in cell proliferation, motility, morphology and survival. Here we show that LCK regulates the function of the type III receptor tyrosine kinase FLT3 in murine pro-B cells. We observed that expression of LCK significantly enhances the colony forming capacity of the constitutively active FLT3 mutant FLT3-ITD (internal tandem duplication). Furthermore, cells expressing LCK developed tumor earlier compared to cells transfected with empty control vector. Staining of the tissues from mouse xenografts showed higher Ki67 staining in cells expressing LCK suggesting that expression of LCK enhances the FLT3-ITD-mediated proliferative capacity. LCK expression did not affect either FLT3-WT or FLT3-ITD -induced AKT, ERK1/2 or p38 phosphorylation. However, LCK expression significantly enhanced FLT3-ITD-mediated STAT5 phosphorylation. Taken together, our data suggest that LCK cooperates with oncogenic FLT3-ITD in cellular transformation.

miR-494-3p overexpression promotes megakaryocytopoiesis in primary myelofibrosis hematopoietic stem/progenitor cells by targeting SOCS6

Primary myelofibrosis (PMF) is a chronic Philadelphia-negative myeloproliferative neoplasm characterized by hematopoietic stem cell-derived clonal myeloproliferation, involving especially the megakaryocyte lineage. To better characterize how the altered expression of microRNAs might contribute to PMF pathogenesis, we have previously performed the integrative analysis of gene and microRNA expression profiles of PMF hematopoietic stem/progenitor cells (HSPCs), which allowed us to identify miR-494-3p as the upregulated microRNA predicted to target the highest number of downregulated mRNAs.To elucidate the role of miR-494-3p in hematopoietic differentiation, in the present study we demonstrated that miR-494-3p enforced expression in normal HSPCs promotes megakaryocytopoiesis. Gene expression profiling upon miR-494-3p overexpression allowed the identification of genes commonly downregulated both after microRNA overexpression and in PMF CD34+ cells. Among them, suppressor of cytokine signaling 6 (SOCS6) was confirmed to be a miR-494-3p target by luciferase assay. Western blot analysis showed reduced level of SOCS6 protein as well as STAT3 activation in miR-494-3p overexpressing cells. Furthermore, transient inhibition of SOCS6 expression in HSPCs demonstrated that SOCS6 silencing stimulates megakaryocytopoiesis, mimicking the phenotypic effects observed upon miR-494-3p overexpression. Finally, to disclose the contribution of miR-494-3p upregulation to PMF pathogenesis, we performed inhibition experiments in PMF HSPCs, which showed that miR-494-3p silencing led to SOCS6 upregulation and impaired megakaryocyte differentiation.Taken together, our results describe for the first time the role of miR-494-3p during normal HSPC differentiation and suggest that its increased expression, and the subsequent downregulation of its target SOCS6, might contribute to the megakaryocyte hyperplasia commonly observed in PMF patients.

SOCS2 Binds to and Regulates EphA2 through Multiple Mechanisms

Suppressors of cytokine signaling (SOCS) proteins inhibit signaling by serving as substrate receptors for the Cullin5-RING E3 ubiquitin ligase (CRL5) and through a variety of CRL5-independent mechanisms. CRL5, SOCS2 and SOCS6 are implicated in suppressing transformation of epithelial cells. We identified cell proteins that interact with SOCS2 and SOCS6 using two parallel proteomics techniques: BioID and Flag affinity purification mass spectrometry. The receptor tyrosine kinase ephrin type-A receptor 2 (EphA2) was identified as a SOCS2-interacting protein. SOCS2-EphA2 binding requires the SOCS2 SH2 domain and EphA2 activation loop autophosphorylation, which is stimulated by Ephrin A1 (EfnA1) or by phosphotyrosine phosphatase inhibition. Surprisingly, EfnA1-stimulated EphA2-SOCS2 binding is delayed until EphA2 has been internalized into endosomes. This suggests that SOCS2 binds to EphA2 in the context of endosomal membranes. We also found that SOCS2 overexpression decreases steady state levels of EphA2, consistent with increased EphA2 degradation. This effect is indirect: SOCS2 induces EfnA1 expression, and EfnA1 induces EphA2 down-regulation. Other RTKs have been reported to bind, and be regulated by, over-expressed SOCS proteins. Our data suggest that SOCS protein over-expression may regulate receptor tyrosine kinases through indirect and direct mechanisms.

ABL2 suppresses FLT3-ITD-induced cell proliferation through negative regulation of AKT signaling

The type III receptor tyrosine kinase FLT3 is one of the most commonly mutated oncogenes in acute myeloid leukemia (AML). Inhibition of mutated FLT3 in combination with chemotherapy has displayed promising results in clinical trials. However, one of the major obstacles in targeting FLT3 is the development of resistant disease due to secondary mutations in FLT3 that lead to relapse. FLT3 and its oncogenic mutants signal through associating proteins that activate downstream signaling. Thus, targeting proteins that interact with FLT3 and their downstream signaling cascades can be an alternative approach to treat FLT3-dependent AML. We used an SH2 domain array screen to identify novel FLT3 interacting proteins and identified ABL2 as a potent interacting partner of FLT3. To understand the role of ABL2 in FLT3-mediated biological and cellular events, we used the murine pro-B cell line Ba/F3 as a model system. Overexpression of ABL2 in Ba/F3 cells expressing an oncogenic mutant of FLT3 (FLT3-ITD) resulted in partial inhibition of FLT3-ITD-dependent cell proliferation and colony formation. ABL2 expression did not alter the kinase activity of FLT3, its ubiquitination or its stability. However, it partially blocked FLT3-induced AKT phosphorylation without affecting ERK1/2 and p38 activation. Taken together our data suggest that ABL2 acts as negative regulator of signaling downstream of FLT3.

Tyrosine 842 in the activation loop is required for full transformation by the oncogenic mutant FLT3-ITD

The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation.

FYN expression potentiates FLT3-ITD induced STAT5 signaling in acute myeloid leukemia

FYN is a non-receptor tyrosine kinase belonging to the SRC family of kinases, which are frequently over-expressed in human cancers, and play key roles in cancer biology. SRC has long been recognized as an important oncogene, but little attention has been given to its other family members. In this report, we have studied the role of FYN in FLT3 signaling in respect to acute myeloid leukemia (AML). We observed that FYN displays a strong association with wild-type FLT3 as well as oncogenic FLT3-ITD and is dependent on the kinase activity of FLT3 and the SH2 domain of FYN. We identified multiple FYN binding sites in FLT3, which partially overlapped with SRC binding sites. To understand the role of FYN in FLT3 signaling, we generated FYN overexpressing cells. We observed that expression of FYN resulted in slightly enhanced phosphorylation of AKT, ERK1/2 and p38 in response to ligand stimulation. Furthermore, FYN expression led to a slight increase in FLT3-ITD-dependent cell proliferation, but potent enhancement of STAT5 phosphorylation as well as colony formation. We also observed that FYN expression is deregulated in AML patient samples and that higher expression of FYN, in combination with FLT3-ITD mutation, resulted in enrichment of the STAT5 signaling pathway and correlated with poor prognosis in AML. Taken together our data suggest that FYN cooperates with oncogenic FLT3-ITD in cellular transformation by selective activation of the STAT5 pathway. Therefore, inhibition of FYN, in combination with FLT3 inhibition, will most likely be beneficial for this group of AML patients.

Targeting FLT3-ITD signaling mediates ceramide-dependent mitophagy and attenuates drug resistance in AML

Signaling pathways regulated by mutant Fms-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD), which mediate resistance to acute myeloid leukemia (AML) cell death, are poorly understood. Here, we reveal that pro-cell death lipid ceramide generation is suppressed by FLT3-ITD signaling. Molecular or pharmacologic inhibition of FLT3-ITD reactivated ceramide synthesis, selectively inducing mitophagy and AML cell death. Mechanistically, FLT3-ITD targeting induced ceramide accumulation on the outer mitochondrial membrane, which then directly bound autophagy-inducing light chain 3 (LC3), involving its I35 and F52 residues, to recruit autophagosomes for execution of lethal mitophagy. Short hairpin RNA (shRNA)-mediated knockdown of LC3 prevented AML cell death in response to FLT3-ITD inhibition by crenolanib, which was restored by wild-type (WT)-LC3, but not mutants of LC3 with altered ceramide binding (I35A-LC3 or F52A-LC3). Mitochondrial ceramide accumulation and lethal mitophagy induction in response to FLT3-ITD targeting was mediated by dynamin-related protein 1 (Drp1) activation via inhibition of protein kinase A-regulated S637 phosphorylation, resulting in mitochondrial fission. Inhibition of Drp1 prevented ceramide-dependent lethal mitophagy, and reconstitution of WT-Drp1 or phospho-null S637A-Drp1 but not its inactive phospho-mimic mutant (S637D-Drp1), restored mitochondrial fission and mitophagy in response to crenolanib in FLT3-ITD⁺ AML cells expressing stable shRNA against endogenous Drp1. Moreover, activating FLT3-ITD signaling in crenolanib-resistant AML cells suppressed ceramide-dependent mitophagy and prevented cell death. FLT3-ITD⁺ AML drug resistance is attenuated by LCL-461, a mitochondria-targeted ceramide analog drug, in vivo, which also induced lethal mitophagy in human AML blasts with clinically relevant FLT3 mutations. Thus, these data reveal a novel mechanism which regulates AML cell death by ceramide-dependent mitophagy in response to FLT3-ITD targeting.

Kinase inhibition, competitive binding and proteasomal degradation: resolving the molecular function of the suppressor of cytokine signaling (SOCS) proteins

The suppressor of cytokine signaling (SOCS) family of proteins are key negative regulators of cytokine and growth factor signaling. They act at the receptor complex to modulate the intracellular signaling cascade, preventing excessive signaling and restoring homeostasis. This regulation is critical to the normal cessation of signaling, highlighted by the complex inflammatory phenotypes exhibited by mice deficient in SOCS1 or SOCS3. These two SOCS proteins remain the best characterized of the eight family members (CIS, SOCS1-7), and in particular, we now possess a sound understanding of the mechanism of action for SOCS3. Here, we review the mechanistic role of the SOCS proteins and identify examples where clear, definitive data have been generated and discuss areas where the information is less clear. From this functional viewpoint, we discuss how the SOCS proteins achieve exquisite and specific regulation of cytokine signaling and highlight outstanding questions regarding the function of the less well-studied SOCS family members.

BEX1 acts as a tumor suppressor in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease of the myeloid lineage. About 35% of AML patients carry an oncogenic FLT3 mutant making FLT3 an attractive target for treatment of AML. Major problems in the development of FLT3 inhibitors include lack of specificity, poor response and development of a resistant phenotype upon treatment. Further understanding of FLT3 signaling and discovery of novel regulators will therefore help to determine additional pharmacological targets in FLT3-driven AML. In this report, we identified BEX1 as a novel regulator of oncogenic FLT3-ITD-driven AML. We showed that BEX1 expression was down-regulated in a group of AML patients carrying FLT3-ITD. Loss of BEX1 expression resulted in poor overall survival (hazard ratio, HR = 2.242, p = 0.0011). Overexpression of BEX1 in mouse pro-B and myeloid cells resulted in decreased FLT3-ITD-dependent cell proliferation, colony and tumor formation, and in increased apoptosis in vitro and in vivo. BEX1 localized to the cytosolic compartment of cells and significantly decreased FLT3-ITD-induced AKT phosphorylation without affecting ERK1/2 or STAT5 phosphorylation. Our data suggest that the loss of BEX1 expression in FLT3-ITD driven AML potentiates oncogenic signaling and leads to decreased overall survival of the patients.

Retinal Axon Guidance Requires Integration of Eya and the Jak/Stat Pathway into Phosphotyrosine-Based Signaling Circuitries in Drosophila

The transcriptional coactivator and phosphatase eyes absent (Eya) is dynamically compartmentalized between the nucleus and cytoplasm. Although the nuclear transcriptional circuits within which Eya operates have been extensively characterized, understanding of its cytoplasmic functions and interactions remains limited. Our previous work showed that phosphorylation of Drosophila Eya by the Abelson tyrosine kinase can recruit Eya to the cytoplasm and that eya-abelson interactions are required for photoreceptor axons to project to correct layers in the brain. Based on these observations, we postulated that photoreceptor axon targeting might provide a suitable context for identifying the cytoplasmic signaling cascades with which Eya interacts. Using a dose-sensitive eya misexpression background, we performed an RNA interference-based genetic screen to identify suppressors. Included among the top 10 hits were nonreceptor tyrosine kinases and multiple members of the Jak/Stat signaling network (hop, Stat92E, Socs36E, and Socs44A), a pathway not previously implicated in axon targeting. Individual loss-of-function phenotypes combined with analysis of axonal projections in Stat92E null clones confirmed the importance of photoreceptor autonomous Jak/Stat signaling. Experiments in cultured cells detected cytoplasmic complexes between Eya and Hop, Socs36E and Socs44A; the latter interaction required both the Src homology 2 motif in Socs44A and tyrosine phosphorylated Eya, suggesting direct binding and validating the premise of the screen. Taken together, our data provide new insight into the cytoplasmic phosphotyrosine signaling networks that operate during photoreceptor axon guidance and suggest specific points of interaction with Eya.

The role of HOXB2 and HOXB3 in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous aggressive disease and the most common form of adult leukemia. Mutations in the type III receptor tyrosine kinase FLT3 are found in more than 30% of AML patients. Drugs against FLT3 have been developed for the treatment of AML, but they lack specificity, show poor response and lead to the development of a resistant phenotype upon treatment. Therefore, a deeper understanding of FLT3 signaling will facilitate identification of additional pharmacological targets in FLT3-driven AML. In this report, we identify HOXB2 and HOXB3 as novel regulators of oncogenic FLT3-ITD-driven AML. We show that HOXB2 and HOXB3 expression is upregulated in a group of AML patients carrying FLT3-ITD. Overexpression of HOXB2 or HOXB3 in mouse pro-B cells resulted in decreased FLT3-ITD-dependent cell proliferation as well as colony formation and increased apoptosis. Expression of HOXB2 or HOXB3 resulted in a significant decrease in FLT3-ITD-induced AKT, ERK, p38 and STAT5 phosphorylation. Our data suggest that HOXB2 and HOXB3 act as tumor suppressors in FLT3-ITD driven AML.

Brain-Expressed X-linked (BEX) proteins in human cancers

The Brain-Expressed X-linked (BEX) family proteins are comprised of five human proteins including BEX1, BEX2, BEX3, BEX4 and BEX5. BEX family proteins are expressed in a wide range of tissues and are known to play a role in neuronal development. Recent studies suggest a role of BEX family proteins in cancers. BEX1 expression is lost in a subgroup of patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). Expression of BEX1 controls cell surface receptor signaling and restores imatinib response in resistant cells. BEX2 is overexpressed in a group of breast cancer patients and also in gliomas. Increased BEX2 expression led to enhanced NF-κB signaling as well as cell proliferation. Although BEX2 acts as tumor promoter in a subset of breast cancer, BEX3 expression displayed an opposite role. Overexpression of BEX3 resulted in inhibition of tumor formation in breast cancer mouse xenograft models. The role of BEX4 and BEX5 in cancer has not yet been defined. Collectively this suggests that BEX family members have distinct roles in cancers. While BEX1 and BEX3 act as tumor suppressors, BEX2 seems to act as an oncogene.

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.

SOCS6 is a selective suppressor of receptor tyrosine kinase signaling

The suppressors of cytokine signaling (SOCS) are well-known negative regulators of cytokine receptor signaling. SOCS6 is one of eight members of the SOCS family of proteins. Similar to other SOCS proteins, SOCS6 consists of an uncharacterized extended N-terminal region followed by an SH2 domain and a SOCS box. Unlike other SOCS proteins, SOCS6 is mainly involved in negative regulation of receptor tyrosine kinase signaling. SOCS6 is widely expressed in many tissues and is found to be downregulated in many cancers including colorectal cancer, gastric cancer, lung cancer, ovarian cancer, stomach cancer, thyroid cancer, hepatocellular carcinoma, and pancreatic cancer. SOCS6 is involved in negative regulation of receptor signaling by increasing degradation mediated by ubiquitination of receptors or substrate proteins and induces apoptosis by targeting mitochondrial proteins. Therefore, SOCS6 turns out as an important regulator of survival signaling and its activity is required for controlling receptor tyrosine kinase signaling.

miR-17-5p promotes proliferation by targeting SOCS6 in gastric cancer cells

This study aimed to test the exact functions and potential mechanisms of miR-17-5p in gastric cancer. Using real-time PCR, miR-17-5p was found to be expressed more highly in gastric cancer compared with-normal tissues. Gain- and loss-of-function assays demonstrated that miR-17-5p increased the proliferation and growth of gastric cancer cells in vitro and in vivo. Through reporter gene and western blot assays, SOCS6 was shown to be a direct target of miR-17-5p, and proliferative assays confirmed that SOCS6 exerted opposing function to that of miR-17-5p in gastric cancer. In short, miR-17-5p might function as a pro-proliferative factor by repressing SOCS6 in gastric cancer.

The role of suppressors of cytokine signalling in human neoplasms

Suppressors of cytokine signalling 1-7 (SOCS1-7) and cytokine-inducible SH2-containing protein (CIS) are a group of intracellular proteins that are well known as JAK-STAT and several other signalling pathways negative feedback regulators. More recently several members have been identified as tumour suppressors and dysregulation of their biological roles in controlling cytokine and growth factor signalling may contribute to the development of many solid organ and haematological malignancies. This review explores their biological functions and their possible tumour suppressing role in human neoplasms.

Activating FLT3 mutants show distinct gain-of-function phenotypes in vitro and a characteristic signaling pathway profile associated with prognosis in acute myeloid leukemia

About 30% of patients with acute myeloid leukemia (AML) harbour mutations of the receptor tyrosine kinase FLT3, mostly internal tandem duplications (ITD) and point mutations of the second tyrosine kinase domain (TKD). It was the aim of this study to comprehensively analyze clinical and functional properties of various FLT3 mutants.

In 672 normal karyotype AML patients FLT3-ITD, but not FLT3-TKD mutations were associated with a worse relapse free and overall survival in multivariate analysis. In paired diagnosis-relapse samples FLT3-ITD showed higher stability (70%) compared to FLT3-TKD (30%). In vitro, FLT3-ITD induced a strong activating phenotype in Ba/F3 cells. In contrast, FLT3-TKD mutations and other point mutations – including two novel mutations – showed a weaker but clear gain-of-function phenotype with gradual increase in proliferation and protection from apoptosis. The pro-proliferative capacity of the investigated FLT3 mutants was associated with cell surface expression and tyrosine 591 phosphorylation of the FLT3 receptor. Western blot experiments revealed STAT5 activation only in FLT3-ITD positive cell lines, in contrast to FLT3-non-ITD mutants, which displayed an enhanced signal of AKT and MAPK activation. Gene expression analysis revealed distinct difference between FLT3-ITD and FLT3-TKD for STAT5 target gene expression as well as deregulation of SOCS2, ENPP2, PRUNE2 and ART3.

FLT3-ITD and FLT3 point mutations show a gain-of-function phenotype with distinct signalling properties in vitro. Although poor prognosis in AML is only associated with FLT3-ITD, all activating FLT3 mutations can contribute to leukemogenesis and are thus potential targets for therapeutic interventions.

Grb10 is a dual regulator of receptor tyrosine kinase signaling

The adaptor protein Grb10 is a close homolog of Grb7 and Grb14. These proteins are characterized by an N-terminal proline-rich region, a Ras-GTPase binding domain, a PH domain, an SH2 domain and a BPS domain in between the PH and SH2 domains. Human Grb10 gene encodes three splice variants. These variants show differences in functionality. Grb10 associates with multiple proteins including tyrosine kinases in a tyrosine phosphorylation dependent or independent manner. Association with multiple proteins allows Grb10 to regulate different signaling pathways resulting in different biological consequences.

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.

Top-down motif engineering of a cytokine receptor for directing ex vivo expansion of hematopoietic stem cells

The technique to expand hematopoietic stem cells (HSCs) ex vivo is eagerly anticipated to secure an enough amount of HSCs for clinical applications. Previously we developed a scFv-thrombopoietin receptor (c-Mpl) chimera, named S-Mpl, which can transduce a proliferation signal in HSCs in response to a cognate antigen. However, a remaining concern of the S-Mpl chimera may be the magnitude of the cellular expansion level driven by this molecule, which was significantly less than that mediated by endogenous wild-type c-Mpl. In this study, we engineered a tyrosine motif located in the intracellular domain of S-Mpl based on a top-down approach in order to change the signaling properties of the chimera. The truncated mutant (trunc.) and an amino-acid substitution mutant (Q to L) of S-Mpl were constructed to investigate the ability of these mutants to expand HSCs. The result showed that the truncated and Q to L mutants gave higher and considerably lower number of the cells than unmodified S-Mpl, respectively. The proliferation level through the truncated mutant was even higher than that of non-transduced HSCs with the stimulation of a native cytokine, thrombopoietin. Moreover, we analyzed the signaling properties of the S-Mpl mutants in detail using a pro-B cell line Ba/F3. The data indicated that the STAT3 and STAT5 activation levels through the truncated mutant increased, whereas activation of the Q to L mutant was inhibited by a negative regulator of intracellular signaling, SHP-1. This is the first demonstration that a non-natural artificial mutant of a cytokine receptor is effective for ex vivo expansion of hematopoietic cells compared with a native cytokine receptor.

The Impact of FLT3 Mutations on the Development of Acute Myeloid Leukemias

The development of the genetic studies on acute myeloid leukemias (AMLs) has led to the identification of some recurrent genetic abnormalities. Their discovery was of fundamental importance not only for a better understanding of the molecular pathogenesis of AMLs, but also for the identification of new therapeutic targets. In this context, it is essential to identify AML-associated “driver” mutations, which have a causative role in leukemogenesis. Evidences accumulated during the last years indicate that activating internal tandem duplication mutations in FLT3 (FLT3-ITD), detected in about 20% of AMLs, represents driver mutations and valid therapeutic targets in AMLs. Furthermore, the screening of FLT3-ITD mutations has also considerably helped to improve the identification of more accurate prognostic criteria and of the therapeutic selection of patients.

The tyrosine kinase CSK associates with FLT3 and c-Kit receptors and regulates downstream signaling

Type III receptor tyrosine kinases (RTKs), FLT3 and c-Kit play important roles in a variety of cellular processes. A number of SH2-domain containing proteins interact with FLT3 and c-Kit and regulate downstream signaling. The SH2-domain containing non-receptor protein tyrosine kinase CSK is mainly studied in the context of regulating Src family kinases. Here we present an additional role of this kinase in RTK signaling. We show that CSK interacts with FLT3 and c-Kit in a phosphorylation dependent manner. This interaction is facilitated through the SH2-domain of CSK. Under basal conditions CSK is mainly localized throughout the cytosolic compartment but upon ligand stimulation it is recruited to the inner side of cell membrane. CSK association did not alter receptor ubiquitination or phosphorylation but disrupted downstream signaling. Selective depletion of CSK using siRNA, or inhibition with CSK inhibitor, led to increased phosphorylation of Akt and Erk, but not p38, upon FLT3 ligand (FL) stimulation. Stem cell factor (SCF)-mediated Akt and Erk activation was also elevated by CSK inhibition. However, siRNA mediated CSK knockdown increased SCF stimulated Akt phosphorylation but decreased Erk phosphorylation. CSK depletion also significantly increased both FL- and SCF-induced SHC, Gab2 and SHP2 phosphorylation. Furthermore, CSK depletion contributed to oncogenic FLT3- and c-Kit-mediated cell proliferation, but not to cell survival. Thus, the results indicate that CSK association with type III RTKs, FLT3 and c-Kit can have differential impact on receptor downstream signaling.

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.

SOCS proteins in development and disease

Cytokine and growth factor signaling mediates essential roles in the differentiation, proliferation, survival and function of a number of cell lineages. This is achieved via specific receptors located on the surface of target cells, with ligand binding activating key intracellular signal transduction cascades to mediate the requisite cellular outcome. Effective resolution of receptor signaling is also essential, with excessive signaling having the potential for pathological consequences. The Suppressor of cytokine signaling (SOCS) family of proteins represent one important mechanism to extinguish cytokine and growth factor receptor signaling. There are 8 SOCS proteins in mammals; SOCS1-7 and the alternatively named Cytokine-inducible SH2-containing protein (CISH). SOCS1-3 and CISH are predominantly associated with the regulation of cytokine receptor signaling, while SOCS4-7 are more commonly involved in the control of Receptor tyrosine kinase (RTK) signaling. Individual SOCS proteins are typically induced by specific cytokines and growth factors, thereby generating a negative feedback loop. As a consequence of their regulatory properties, SOCS proteins have important functions in development and homeostasis, with increasing recognition of their role in disease, particularly their tumor suppressor and anti-inflammatory functions. This review provides a synthesis of our current understanding of the SOCS family, with an emphasis on their immune and hematopoietic roles.