The Bmx tyrosine kinase is activated by IL-3 and G-CSF in a PI-3K dependent manner

BMX deletion mitigates neuroinflammation induced by retinal ischemia/reperfusion through modulation of the AKT/ERK/STAT3 signaling cascade

Aims

Retinal ischemia/reperfusion (I/R) injury is implicated in the etiology of various ocular disorders. Prior research has demonstrated that bone marrow tyrosine kinase on chromosome X (BMX) contributes to the advancement of ischemic disease and inflammatory reactions. Consequently, the current investigation aims to evaluate BMX's impact on retinal I/R injury and clarify its implied mechanism of action.

Main methods

This study utilized male and female systemic BMX knockout (BMX-/-) mice to conduct experiments. The utilization of Western blot assay and immunofluorescence labeling techniques was employed to investigate variations in the expression of protein and tissue localization. Histomorphological changes were observed through H&E staining and SD-OCT examination. Visual function changes were assessed through electrophysiological experiments. Furthermore, apoptosis in the retina was identified using the TUNEL assay, as well as the ELISA technique, which has been utilized to determine the inflammatory factors level.

Key findings

Our investigation results revealed that the knockdown of BMX did not yield a significant effect on mouse retina. In mice, BMX knockdown mitigated the negative impact of I/R injury on retinal tissue structure and visual function. BMX knockdown effectively reduced apoptosis, suppressed inflammatory responses, and decreased inflammatory factors subsequent to I/R injury. The outcomes of the current investigation revealed that BMX knockdown partially protected the retina through downregulating phosphorylation of AKT/ERK/STAT3 pathway.

Significance

Our investigation showed that BMX-/- reduces AKT, ERK, and STAT3 phosphorylation, reducing apoptosis and inflammation. Thus, this strategy protected the retina from structural and functional damage after I/R injury.

Bruton tyrosine kinase inhibitors for multiple sclerosis

Current therapies for multiple sclerosis (MS) reduce both relapses and relapse-associated worsening of disability, which is assumed to be mainly associated with transient infiltration of peripheral immune cells into the central nervous system (CNS). However, approved therapies are less effective at slowing disability accumulation in patients with MS, in part owing to their lack of relevant effects on CNS-compartmentalized inflammation, which has been proposed to drive disability. Bruton tyrosine kinase (BTK) is an intracellular signalling molecule involved in the regulation of maturation, survival, migration and activation of B cells and microglia. As CNS-compartmentalized B cells and microglia are considered central to the immunopathogenesis of progressive MS, treatment with CNS-penetrant BTK inhibitors might curtail disease progression by targeting immune cells on both sides of the blood-brain barrier. Five BTK inhibitors that differ in selectivity, strength of inhibition, binding mechanisms and ability to modulate immune cells within the CNS are currently under investigation in clinical trials as a treatment for MS. This Review describes the role of BTK in various immune cells implicated in MS, provides an overview of preclinical data on BTK inhibitors and discusses the (largely preliminary) data from clinical trials.

Indigenous Nigeria medicinal herbal remedies: A potential source for therapeutic against rheumatoid arthritis

Rheumatoid arthritis (RA) is a debilitating disease associated with locomotion impairment, and conventional therapeutic drugs are not optimal for managing RA. There is an avalanche of medications used for the management of RA. Still, studies have shown that they are associated with severe side effects, including hepatotoxicity, retinopathy, and cardiotoxicity disorders of the central nervous system (CNS), skin, blood, and infections. Complementary and alternative medicine (CAM) is currently gaining attention as a novel panacea for managing debilitating diseases, such as RA. Nigerian folk herbal remedies are replete with a plethora of curative medicine, albeit unvalidated scientifically but with seemingly miraculous provenance. Studies of the identification of bioactive compounds present in these botanicals using advanced spectral analytical techniques have enhanced our understanding of the role of Nigerian herbal remedies in the treatment and management of RA. Interestingly, experimental studies abound that the bioactive compounds present in the extracts of plant botanicals protected animals from the development of RA in different experimental models and reduced the toxicity associated with conventional therapeutics. Validated mechanisms of RA amelioration in human and animal models include suppression of the expression of NF-κB, IL-1β, TNF-α, IL-6, IL-8, IL-17, IL-23, chemokines, TGF-β, RANKL, RANK, iNOS, arginase, COX-2, VEGFA, VEGFR, NFATC1, and TRAP in the synoviocytes. Decreased ROS, NO, MDA, carbonyl groups, and PGE₂ in the synovial fluid increased the expression of PPARα/γ; antioxidant and anti-inflammatory molecules also improve RA etiology. In this mini-review, we discuss the global burden of RA, the novel role of plant-based botanicals as potential therapeutics against signaling pathways in RA. Also addressed is the possible repurposing/reprofiling of plant botanicals to increase their therapeutic index among RA patients that patronize traditional healers in Nigeria with a global projection.

Deregulation of tumor suppressive ASXL1-PTEN/AKT axis in myeloid malignancies

Mutations of epigenetic regulators are pervasive in human tumors. ASXL1 is frequently mutated in myeloid malignancies. We previously found that ASXL1 forms together with BAP1 a complex that can deubiquitinylate mono-ubiquitinylated lysine 119 on histone H2A (H2AK119ub1), a Polycomb repressive mark. However, a complete mechanistic understanding of ASXL1 in transcriptional regulation and tumor suppression remains to be defined. Here, we find that depletion of Asxl1 confers murine 32D cells to IL3-independent growth at least partly due to sustained activation of PI3K/AKT signaling. Consistently, Asxl1 is critical for the transcriptional activation of Pten, a key negative regulator of AKT activity. Then we confirm that Asxl1 is specifically enriched and required for H2AK119 deubiquitylation at the Pten promoter. Interestingly, ASXL1 and PTEN expression levels are positively correlated in human blood cells and ASXL1 mutations are associated with lower expression levels of PTEN in human myeloid malignancies. Furthermore, malignant cells with ASXL1 downregulation or mutations exhibit higher sensitivity to the AKT inhibitor MK2206. Collectively, this study has linked the PTEN/AKT signaling axis to deregulated epigenetic changes in myeloid malignancies. It also provides a rationale for mechanism-based therapy for patients with ASXL1 mutations.

Signaling Pathways Regulated by UBR Box-Containing E3 Ligases

UBR box E3 ligases, also called N-recognins, are integral components of the N-degron pathway. Representative N-recognins include UBR1, UBR2, UBR4, and UBR5, and they bind destabilizing N-terminal residues, termed N-degrons. Understanding the molecular bases of their substrate recognition and the biological impact of the clearance of their substrates on cellular signaling pathways can provide valuable insights into the regulation of these pathways. This review provides an overview of the current knowledge of the binding mechanism of UBR box N-recognin/N-degron interactions and their roles in signaling pathways linked to G-protein-coupled receptors, apoptosis, mitochondrial quality control, inflammation, and DNA damage. The targeting of these UBR box N-recognins can provide potential therapies to treat diseases such as cancer and neurodegenerative diseases.

Ibrutinib inactivates BMX-STAT3 in glioma stem cells to impair malignant growth and radioresistance

Glioblastoma (GBM) is the most lethal primary brain tumor and is highly resistant to current treatments. GBM harbors glioma stem cells (GSCs) that not only initiate and maintain malignant growth but also promote therapeutic resistance including radioresistance. Thus, targeting GSCs is critical for overcoming the resistance to improve GBM treatment. Because the bone marrow and X-linked (BMX) nonreceptor tyrosine kinase is preferentially up-regulated in GSCs relative to nonstem tumor cells and the BMX-mediated activation of the signal transducer and activator of transcription 3 (STAT3) is required for maintaining GSC self-renewal and tumorigenic potential, pharmacological inhibition of BMX may suppress GBM growth and reduce therapeutic resistance. We demonstrate that BMX inhibition by ibrutinib potently disrupts GSCs, suppresses GBM malignant growth, and effectively combines with radiotherapy. Ibrutinib markedly disrupts the BMX-mediated STAT3 activation in GSCs but shows minimal effect on neural progenitor cells (NPCs) lacking BMX expression. Mechanistically, BMX bypasses the suppressor of cytokine signaling 3 (SOCS3)-mediated inhibition of Janus kinase 2 (JAK2), whereas NPCs dampen the JAK2-mediated STAT3 activation via the negative regulation by SOCS3, providing a molecular basis for targeting BMX by ibrutinib to specifically eliminate GSCs while preserving NPCs. Our preclinical data suggest that repurposing ibrutinib for targeting GSCs could effectively control GBM tumor growth both as monotherapy and as adjuvant with conventional therapies.

A novel BMX variant promotes tumor cell growth and migration in lung adenocarcinoma

The non-receptor tyrosine kinase BMX has been reported in several solid tumors. However, the alternative splicing of BMX and its clinical relevance in lung cancer remain to be elucidated. Exon1.0 array was used to identify a novel alternative splicing of BMX, BMXΔN, which was confirmed by rapid amplification of cDNA ends and reverse transcription-polymerase chain reaction. BMXΔN, resulting from exon skipping with excluding exon 1 to exon 8 of BMX gene, was found in 12% human lung adenocarcinoma specimens. BMXΔN is not found in paired pathologically normal lungs and positively correlated with EGFR mutation in lung adenocarcinomas. Moreover, BMXΔN increases cell proliferation, neoplastic transformation, and migratory property of human non-small cell lung cancer cells. The function of BMXΔN in lung cancer might be presumably due to enhanced ERK signaling.

Phosphorylation Impacts N-end Rule Degradation of the Proteolytically Activated Form of BMX Kinase

Cellular signaling leading to the initiation of apoptosis typically results in the activation of caspases, which in turn leads to the proteolytic generation of protein fragments with new or altered cellular functions. Increasing numbers of reports are demonstrating that the activity of many of these proteolytically activated protein fragments can be attenuated by their selective degradation by the N-end rule pathway. Here we report the first evidence that selective degradation of a caspase product by the N-end rule pathway can be modulated by phosphorylation. We demonstrate that the pro-apoptotic fragment of the bone marrow kinase on chromosome X (BMX) generated by caspase cleavage in the prostate cancer-derived PC3 cell line is metabolically unstable in cells because its N-terminal tryptophan targets it for proteasomal degradation via the N-end rule pathway. In addition, we have demonstrated that phosphorylation of tyrosine 566 relatively inhibits degradation of the C-terminal BMX catalytic fragment, and this phosphorylation is crucial for its pro-apoptotic function. Overall, our results demonstrate that cleaved BMX is a novel N-end rule substrate, and its degradation exhibits a novel interplay between substrate phosphorylation and N-end rule degradation, revealing an increasing complex regulatory network of apoptotic proteolytic signaling cascades.

Endothelial Bmx tyrosine kinase activity is essential for myocardial hypertrophy and remodeling

Cardiac hypertrophy accompanies many forms of heart disease, including ischemic disease, hypertension, heart failure, and valvular disease, and it is a strong predictor of increased cardiovascular morbidity and mortality. Deletion of bone marrow kinase in chromosome X (Bmx), an arterial nonreceptor tyrosine kinase, has been shown to inhibit cardiac hypertrophy in mice. This finding raised the possibility of therapeutic use of Bmx tyrosine kinase inhibitors, which we have addressed here by analyzing cardiac hypertrophy in gene-targeted mice deficient in Bmx tyrosine kinase activity. We found that angiotensin II (Ang II)-induced cardiac hypertrophy is significantly reduced in mice deficient in Bmx and in mice with inactivated Bmx tyrosine kinase compared with WT mice. Genome-wide transcriptomic profiling showed that Bmx inactivation suppresses myocardial expression of genes related to Ang II-induced inflammatory and extracellular matrix responses whereas expression of RNAs encoding mitochondrial proteins after Ang II administration was maintained in Bmx-inactivated hearts. Very little or no Bmx mRNA was expressed in human cardiomyocytes whereas human cardiac endothelial cells expressed abundant amounts. Ang II stimulation of endothelial cells increased Bmx phosphorylation, and Bmx gene silencing inhibited downstream STAT3 signaling, which has been implicated in cardiac hypertrophy. Furthermore, activation of the mechanistic target of rapamycin complex 1 pathway by Ang II treatment was decreased in the Bmx-deficient hearts. Our results demonstrate that inhibition of the cross-talk between endothelial cells and cardiomyocytes by Bmx inactivation suppresses Ang II-induced signals for cardiac hypertrophy. These results suggest that the endothelial Bmx tyrosine kinase could provide a target to attenuate the development of cardiac hypertrophy.

Deletion of the endothelial Bmx tyrosine kinase decreases tumor angiogenesis and growth

Bmx, [corrected] also known as Etk, is a member of the Tec family of nonreceptor tyrosine kinases. Bmx is expressed mainly in arterial endothelia and in myeloid hematopoietic cells. Bmx regulates ischemia-mediated arteriogenesis and lymphangiogenesis, but its role in tumor angiogenesis is not known. In this study, we characterized the function of Bmx in tumor growth using both Bmx knockout and transgenic mice. Isogenic colon, lung, and melanoma tumor xenotransplants showed reductions in growth and tumor angiogenesis in Bmx gene-deleted ((-/-)) mice, whereas developmental angiogenesis was not affected. In addition, growth of transgenic pancreatic islet carcinomas and intestinal adenomas was also slower in Bmx(-/-) mice. Knockout mice showed high levels of Bmx expression in endothelial cells of tumor-associated and peritumoral arteries. Moreover, endothelial cells lacking Bmx showed impaired phosphorylation of extracellular signal-regulated kinase (Erk) upon VEGF stimulation, indicating that Bmx contributes to the transduction of vascular endothelial growth factor signals. In transgenic mice overexpressing Bmx in epidermal keratinocytes, tumors induced by a two-stage chemical skin carcinogenesis treatment showed increased growth and angiogenesis. Our findings therefore indicate that Bmx activity contributes to tumor angiogenesis and growth.

Epigenetic regulation of CpG promoter methylation in invasive prostate cancer cells

BACKGROUND

Recently, much attention has been focused on gaining a better understanding of the different populations of cells within a tumor and their contribution to cancer progression. One of the most commonly used methods to isolate a more aggressive sub-population of cells utilizes cell sorting based on expression of certain cell adhesion molecules. A recently established method we developed is to isolate these more aggressive cells based on their properties of increased invasive ability. These more invasive cells have been previously characterized as tumor initiating cells (TICs) that have a stem-like genomic signature and express a number of stem cell genes including Oct3/4 and Nanog and are more tumorigenic compared to their 'non-invasive' counterpart. They also have a profile reminiscent of cells undergoing a classic pattern of epithelial to mesenchymal transition or EMT. Using this model of invasion, we sought to investigate which genes are under epigenetic control in this rare population of cells. Epigenetic modifications, specifically DNA methylation, are key events regulating the process of normal human development. To determine the specific methylation pattern in these invasive prostate cells, and if any developmental genes were being differentially regulated, we analyzed differences in global CpG promoter methylation.

RESULTS

Differentially methylated genes were determined and select genes were chosen for additional analyses. The non-receptor tyrosine kinase BMX and transcription factor SOX1 were found to play a significant role in invasion. Ingenuity pathway analysis revealed the methylated gene list frequently displayed genes from the IL-6/STAT3 pathway. Cells which have decreased levels of the targets BMX and SOX1 also display loss of STAT3 activity. Finally, using Oncomine, it was determined that more aggressive metastatic prostate cancers in humans also have higher levels of both Stat3 and Sox1.

CONCLUSIONS

Using this method we can begin to understand which genes are epigenetically regulated in the invasive population compared to the bulk tumor cells. These aggressive sub-populations of cells may be linked to the cancer stem cell hypothesis, making their patterns of epigenetic regulation very attractive for biomarker analysis.

Non-receptor tyrosine kinase Etk is involved in the apoptosis of small cell lung cancer cells

Epithelial and endothelial tyrosine kinase (Etk), also known as Bmx (bone marrow X kinase) plays an important role in apoptosis of epithelial cells. The goal of this study was to investigate whether Etk is involved in apoptosis of small cell lung cancer (SCLC) cells and correlated with the expression levels of apoptosis-associated proteins such as Bcl-2, Bcl-X(L) and p53. One hundred and seventy-one cases of lung cancer specimens including seventy-one SCLCs and one hundred NSCLCs were immunostained for Etk, Bcl-2, Bcl-X(L) and p53. Parental SCLC H446 cell line, and its subline (H446-Etk) that overexpresses Etk, were used to study the role of Etk in apoptosis induced by doxorubicin. It was found that high expression of Etk occurs in 74.6% of SCLC cases, but only in 40% of NSCLC cases, and there is marked difference in the expression levels of Bcl-2, Bcl-X(L) and p53 between Etk-positive and Etk-negative SCLC cases. Furthermore, the levels of Bcl-2 and Bcl-X(L) significantly increased in H446-Etk cells than that in H446 cells after doxorubicin treatment, and were positively associated with Etk expression. However, p53 did not correspond with Etk expression although its expression decreased greatly with apoptosis both in H446-Etk and H446 cells. After doxorubicin treatment, the cell viability was significantly higher in H446-Etk cells than in parental H446 cells. Downregulation of Etk by Etk siRNA sensitized H446 cells to doxorubicin. Our results indicate that upregulation of tyrosine kinase Etk may be a new mechanism involved in protection of SCLC cells from apoptosis. Bcl-2 and Bcl-X(L) but not p53 may contribute to doxorubicin-induced apoptosis through Etk pathway.

Bruton's tyrosine kinase (Btk): function, regulation, and transformation with special emphasis on the PH domain

Bruton's agammaglobulinemia tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important in B-lymphocyte development, differentiation, and signaling. Btk is a member of the Tec family of kinases. Mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Activation of Btk triggers a cascade of signaling events that culminates in the generation of calcium mobilization and fluxes, cytoskeletal rearrangements, and transcriptional regulation involving nuclear factor-kappaB (NF-kappaB) and nuclear factor of activated T cells (NFAT). In B cells, NF-kappaB was shown to bind to the Btk promoter and induce transcription, whereas the B-cell receptor-dependent NF-kappaB signaling pathway requires functional Btk. Moreover, Btk activation is tightly regulated by a plethora of other signaling proteins including protein kinase C (PKC), Sab/SH3BP5, and caveolin-1. For example, the prolyl isomerase Pin1 negatively regulates Btk by decreasing tyrosine phosphorylation and steady state levels of Btk. It is intriguing that PKC and Pin1, both of which are negative regulators, bind to the pleckstrin homology domain of Btk. To this end, we describe here novel mutations in the pleckstrin homology domain investigated for their transforming capacity. In particular, we show that the mutant D43R behaves similar to E41K, already known to possess such activity.

G-CSF stimulates Jak2-dependent Gab2 phosphorylation leading to Erk1/2 activation and cell proliferation

Granulocyte colony-stimulating factor (G-CSF), the major cytokine regulator of neutrophilic granulopoiesis, stimulates both the proliferation and differentiation of myeloid precursors. A variety of signaling proteins have been identified as mediators of G-CSF signaling, but understanding of their specific interactions and organization into signaling pathways for particular cellular effects is incomplete. The present study examined the role of the scaffolding protein Grb2-associated binding protein-2 (Gab2) in G-CSF signaling. We found that a chemical inhibitor of Janus kinases inhibited G-CSF-stimulated Gab2 phosphorylation. Transfection with Jak2 antisense and dominant negative constructs also inhibited Gab2 phosphorylation in response to G-CSF. In addition, G-CSF enhanced the association of Jak2 with Gab2. In vitro, activated Jak2 directly phosphorylated specific Gab2 tyrosine residues. Mutagenesis studies revealed that Gab2 tyrosine 643 (Y643) was a major target of Jak2 in vitro, and a key residue for Jak2-dependent phosphorylation in intact cells. Mutation of Gab2 Y643 inhibited G-CSF-stimulated Erk1/2 activation and Shp2 binding to Gab2. Loss of Y643 also inhibited Gab2-mediated G-CSF-stimulated cell proliferation. Together, these results identify a novel signaling pathway involving Jak2-dependent Gab2 phosphorylation leading to Erk1/2 activation and cell proliferation in response to G-CSF.

Bone marrow X kinase-mediated signal transduction in irradiated vascular endothelium

Radiation-induced activation of the phosphatidyl inositol-3 kinase/Akt signal transduction pathway requires Akt binding to phosphatidyl-inositol phosphates (PIP) on the cell membrane. The tyrosine kinase bone marrow X kinase (Bmx) binds to membrane-associated PIPs in a manner similar to Akt. Because Bmx is involved in cell growth and survival pathways, it could contribute to the radiation response within the vascular endothelium. We therefore studied Bmx signaling within the vascular endothelium. Bmx was activated rapidly in response to clinically relevant doses of ionizing radiation. Bmx inhibition enhanced the efficacy of radiotherapy in endothelial cells as well as tumor vascular endothelium in lung cancer tumors in mice. Retroviral shRNA knockdown of Bmx protein enhanced human umbilical vascular endothelial cell (HUVEC) radiosensitization. Furthermore, pretreatment of HUVEC with a pharmacologic inhibitor of Bmx, LFM-A13, produced significant radiosensitization of endothelial cells as measured by clonogenic survival analysis and apoptosis as well as functional assays including cell migration and tubule formation. In vivo, LFM-A13, when combined with radiation, resulted in significant tumor microvascular destruction as well as enhanced tumor growth delay. Bmx therefore represents a molecular target for the development of novel radiosensitizing agents.

Bmx tyrosine kinase regulates TLR4-induced IL-6 production in human macrophages independently of p38 MAPK and NFkapp}B activity

Chronic inflammation, as seen in conditions such as rheumatoid arthritis and Crohn disease, is in part driven by discordant production of inflammatory cytokines, such as tumor necrosis factor-alpha and interleukin-6 (IL-6). Tyrosine kinase activity is essential to lipopolysaccharide-induced cytokine production in monocytes, and previous studies by us and others have implicated a role for the Tec kinase Bruton's tyrosine kinase (Btk) in inflammatory cytokine production. Here we show that knockdown of Btk using RNA interference results in decreased tumor necrosis factor-alpha, but not IL-6 production. Further investigations into the signaling mechanisms regulating IL-6 production led to the discovery that the Tec kinase bone marrow tyrosine kinase gene in chromosome X (Bmx) regulates Toll-like receptor-induced IL-6 production. Our data further showed that Bmx-dependent super-induction of IL-6 does not involve nuclear factor-kappaB activity. More detailed investigations of pathways downstream of Bmx signaling revealed that Bmx targets the IL-6 3' untranslated region to increase mRNA stabilization via a novel, thus far undefined, p38 mitogen activated protein kinase-independent pathway. These data have important implications for the design of therapeutics targeted against specific cytokines and their regulators in inflammatory disease.

Activation of nonreceptor tyrosine kinase Bmx/Etk mediated by phosphoinositide 3-kinase, epidermal growth factor receptor, and ErbB3 in prostate cancer cells

Pathways activated downstream of constitutively active phosphatidylinositol (PI) 3-kinase in PTEN-deficient prostate cancer (PCa) cells are possible therapeutic targets. We found that the nonreceptor Tec family tyrosine kinase Bmx/Etk was activated by tyrosine phosphorylation downstream of Src and PI 3-kinase in PTEN-deficient LNCaP and PC3 PCa cells and that Bmx down-regulation by short interfering RNA markedly inhibited LNCaP cell growth. Bmx also associated with ErbB3 in LNCaP cells, and heregulin-beta1 enhanced this interaction and further stimulated Bmx activity. Epidermal growth factor (EGF) similarly stimulated an interaction between Bmx and EGF receptor and rapidly increased Bmx kinase activity. Bmx stimulation in response to heregulin-beta1 and EGF was Src-dependent, and heregulin-beta1 stimulation of Bmx was also PI 3-kinase-dependent. In contrast, the rapid tyrosine phosphorylation and activation of Bmx in response to EGF was PI 3-kinase-independent. Taken together, these results demonstrate that Bmx is a critical downstream target of the constitutively active PI 3-kinase in PTEN-deficient PCa cells and further show that Bmx is recruited by the EGF receptor and ErbB3 and activated in response to their respective ligands. Therefore, Bmx may be a valuable therapeutic target in PCa and other epithelial malignancies in which PI 3-kinase or EGF receptor family pathways are activated.

Bmx is a downstream Rap1 effector in VEGF-induced endothelial cell activation

We had previously shown that Rap1 mediates certain of the signaling pathways involved in VEGF-induced endothelial cell migration, although the downstream Rap1 effectors are not known. Towards the goal of identifying those effectors, we utilized a commercially available antibody array filter to identify proteins that either directly interact with Rap1 or interact indirectly through a multi-protein complex. The protocol identified 10 possible Rap1-interacting proteins, including the Bmx non-receptor tyrosine kinase. The conclusion that VEGF treatment leads to a Rap1/Bmx complex was confirmed by an experiment in which cell lysates from VEGF and control cells were immunoprecipitated with Bmx antibodies and Western blotting was done using anti-Rap1 antibodies. VEGF treatment led to the recruitment of Bmx to the CAS scaffolding protein, and inhibition of the Bmx kinase blocked VEGF-induced cell migration. Formation of a Rap1/Bmx complex was not observed in cells transfected with an expression vector for a dominant-negative Rap1, indicating that Bmx is a downstream Rap1 effector in VEGF-induced endothelial cell activation.

Bmx tyrosine kinase transgene induces skin hyperplasia, inflammatory angiogenesis, and accelerated wound healing

The Bmx gene, a member of the Tec family of nonreceptor protein tyrosine kinases, is expressed in arterial endothelium and in certain hematopoietic and epithelial cells. Previous in vitro studies have implicated Bmx signaling in cell migration and survival and suggested that it contributes to the progression of prostate carcinomas. However, the function of Bmx in normal tissues in vivo is unknown. We show here that Bmx expression is induced in skin keratinocytes during wound healing. To analyze the role of Bmx in epidermal keratinocytes in vivo, we generated transgenic mice overexpressing Bmx in the skin. We show that Bmx overexpression accelerates keratinocyte proliferation and wound reepithelialization. Bmx expression also induces chronic inflammation and angiogenesis in the skin, and gene expression profiling suggests that this occurs via cytokine-mediated recruitment of inflammatory cells. Our studies provide the first data on Bmx function in vivo and form the basis of evaluation of its role in epithelial neoplasia.

The role of tyrosine kinase Etk/Bmx in EGF-induced apoptosis of MDA-MB-468 breast cancer cells

Etk/Bmx, a member of the Tec family of tyrosine kinases, mediates various signaling pathways and confers several cellular functions. In the present study, we have explored the functional role of Etk in mediating EGF-induced apoptosis, using MDA-MB-468 cell line as a model. We first demonstrated that EGF treatment induces Etk tyrosine phosphorylation in both HeLa and MDA-MB-468 cells. Overexpression of Etk by recombinant adenovirus in MDA-MB-468 cells potentiates the extent of EGF-induced cell apoptosis. The observed Etk-enhanced MDA-MB-468 cell apoptosis is associated with the Stat1 activation, as demonstrated by electrophoresis mobility shift assays and reporter gene assays. By contrast, a kinase domain deletion mutant EtkDeltaK, functioning as a dominant-negative mutant, ameliorates EGF-induced Stat1 activation and apoptosis in MDA-MB-468 cells. To explore whether the activated Etk alone is sufficient for inducing apoptosis, a conditionally activated Etk (DeltaEtk-ER), a chimeric fusion protein of PH domain-truncated Etk and ligand-binding domain of estrogen receptor, was introduced into MDA-MB-468 cells. Upon beta-estradiol ligand activation, the DeltaEtk-ER could stimulate Stat1 activity and confer cell apoptosis independent of EGF treatment. Taken together, our findings indicate that Etk is a downstream signaling molecule of EGF receptor and suggest that Etk activation is essential for transducing the EGF-induced apoptotic signaling.

p130Cas Couples the tyrosine kinase Bmx/Etk with regulation of the actin cytoskeleton and cell migration

Bmx/Etk, a member of the Tec/Btk family of nonreceptor kinases, has recently been shown to mediate cell motility in signaling pathways that become activated upon integrin-mediated cell adhesion (Chen, R., Kim, O., Li, M., Xiong, X., Guan, J. L., Kung, H. J., Chen, H., Shimizu, Y., and Qiu, Y. (2001) Nat Cell Biol. 3, 439-444). The molecular mechanisms of Bmx-induced cell motility have so far remained unknown. Previous studies by us and others have demonstrated that a complex formation between the docking protein p130Cas (Cas) and the adapter protein Crk is instrumental in connecting several stimuli to the regulation of actin cytoskeleton and cell motility. We demonstrate here that expression of Bmx leads to an interaction between Bmx and Cas at membrane ruffles, which are sites of active actin remodeling in motile cells. Expression of Bmx also enhances tyrosine phosphorylation of Cas and Cas.Crk complex formation, and coexpression of Bmx with Cas results in an enhanced membrane ruffling and haptotactic cell migration. Importantly, a mutant form of Bmx that fails to interact with Cas also fails to induce cell migration. Furthermore, expression of a dominant-negative form of Cas that is incapable of interacting with Crk inhibits Bmx-induced membrane ruffling and cell migration. These studies suggest that Bmx-Cas interaction, phosphorylation of Cas by Bmx, and subsequent Cas.Crk complex formation functionally couple Bmx to the regulation of actin cytoskeleton and cell motility.

Gene and protein expression profiling during differentiation of neuroblastoma cells triggered by 13-cis retinoic acid

PURPOSE

The precise changes in RNA and protein expression that accompany neuroblastoma differentiation remain unknown. The authors used microarray technologies to screen molecules associated with the differentiation of neuroblastoma (NB) cells induced by 13-cis retinoic acid.

METHODS

The authors quantified the expression of 2,061 RNA transcripts related to oncogenesis and of 380 proteins expressed in SK-N-SH and CHP-134 NB cell lines in the presence or absence of 13-cis retinoic acid.

RESULTS

Hierarchical clustering captured gene expression altered during neuroblastoma differentiation induced by 13-cis retinoic acid. Several genes were further abstracted based on P values below 0.0017 or protein chips observed in both NB cell lines. The altered expressions of gene products revealed by both DNA and protein chips were in agreement. The expressions of N-myc, cyclin D3, and Wnt10B were downregulated, whereas those of retinoblastoma (RB) and related genes (p107, RB2/p130, p300/CBP, E2F-1, DP-1) as well as others were upregulated.

CONCLUSIONS

These results suggest that microarray technology can screen for genes that are important in neuroblastoma differentiation.

Endothelial cell heterogeneity

OBJECTIVE

To review recent advances in the field of endothelial cell heterogeneity, and to apply this knowledge to an understanding of site-specific vasculopathy, including acute lung injury.

DATA SOURCES AND STUDY SELECTION

Published research and review articles in the English language related to endothelial cell biology and endothelial cell heterogeneity.

DATA EXTRACTION AND SYNTHESIS

The results of published studies have been used to provide a perspective of endothelial cell phenotypes in health and disease.

CONCLUSIONS

The structure and function of endothelial cells are differentially regulated in space and time. Far from being a giant monopoly of homogeneous cells, the endothelium represents a consortium of smaller enterprises of cells located within blood vessels of different tissues. Although united in certain functions, each enterprise is uniquely adapted to meet the demands of the underlying tissue. The endothelium may also vary in its response to pathophysiologic stimuli and therefore contribute to the focal nature of vasculopathic disease states. In acute lung injury, the unique properties of the endothelium may conspire with systemic imbalances to localize pathology to the pulmonary vasculature.

Coordinating Etk/Bmx activation and VEGF upregulation to promote cell survival and proliferation

Etk/Bmx, a member of the Tec family of non-receptor tyrosine kinase, is characterized by an N-terminal PH domain and has recently been shown to be involved in the regulation of various cellular processes, including proliferation, differentiation, motility and apoptosis. Since VEGF and the activation of its signaling pathway have been implicated in modulating a variety of biological responses, we characterized the role of Etk-dependent signaling pathways involved in the upregulation of VEGF expression, and explored the functional implications of this enhancement in sustaining cell proliferation and survival. Using Northern and Western analyses, transient transfections, and pharmacological agents, we demonstrate that Etk activation alone is sufficient to transcriptionally induce VEGF expression, independent of the previously identified hypoxia response element (HRE), in both Pa-4 epithelial and TR-BBB endothelial cells under normoxia. In addition, Etk utilizes both MEK/ERK and PI3-K/Pak1 signaling pathways in concert to activate VEGF transcription. Functionally, Etk activation elicits a profound stimulatory effect on TR-BBB cell proliferation and formation of capillary-like networks in Matrigel containing reduced levels of growth factors. Finally, antisense oligonucleotides against either endogenous VEGF or Etk abrogate the proliferation of Etk-activated TR-BBB cells, and exogenous VEGF treatment stimulates endogenous Etk tyrosine phosphorylation in HUVECs. Taken together, these results indicate that VEGF is both an Etk downstream target gene and an Etk upstream activator, constituting a reciprocal Etk-VEGF autoregulatory loop. These findings, to our knowledge, are the first delineation of a network of positive feedforward signaling pathways that converge on the Etk-VEGF axis, causally associating Etk-mediation of VEGF induction with enhanced cellular processes in both epithelial and endothelial cells.

Granulocyte colony-stimulating factor primes NADPH oxidase in neutrophils through translocation of cytochrome b(558) by gelatinase-granule release

Granulocyte colony-stimulating factor (GCSF) primes reduced neutrophil nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity in response to formyl peptide but does not increase oxidase activity when used alone. Both oxidase activity and degranulation require phospholipase D (PLD) activation, and exogenous C(2)-ceramide inhibits both functions through inhibition of PLD activity. We extended these observations to investigate neutrophil responses to GCSF. GCSF at a dosage of 30 to 100 ng/mL, a concentration range that primes superoxide release, stimulated a 60% to 100% increase in gelatinase release from tertiary granules but did not stimulate lactoferrin release from secondary granules. A 75% to 100% dose-dependent increase in PLD activity in GCSF-treated neutrophils was also observed. Gelatinase release and PLD activity were inhibited by 10 micromol/L C(2)-ceramide. The increase in gelatinase release in response to priming concentrations of GCSF suggests that tertiary granules contribute a component of the NADPH oxidase to the plasma membrane. Neutrophils treated with 50 ng/mL GCSF were found to contain 20% more cytochrome b(558) in the plasma membrane fraction than unstimulated cells, consistent with degranulation of only tertiary granules. Correspondingly, in the presence of 10 micromol/L C(2)-ceramide, cytochrome b(558) content in the plasma membrane did not increase after neutrophil activation. In contrast, GCSF did not lead to p47phox translocation to the plasma membrane or phosphorylation. Because phosphorylation and translocation of p47phox are required for oxidase activity, these findings account for the inability of GCSF alone to generate the respiratory burst. We conclude that translocation of cytochrome b(558) was responsible for GCSF priming of NADPH oxidase in neutrophils.

Prolactin-induced expression of cytokine-inducible SH2 signaling inhibitors in human hematopoietic progenitors

OBJECTIVE

The prolactin (PRL) receptor (PRLR) utilizes the JAK2/STAT-5 pathway and induces expression of cytokine-inducible SH2 (CIS)/JAK2 binding (JAB) signaling inhibitors. We and others recently showed that CIS-3 and JAB abolish PRLR-mediated JAK2 activation and STAT-5 activity, whereas CIS-1, CIS-2, and CIS-4 had a negligible effect. Human CD34(+) hematopoietic progenitors express PRLRs and respond to PRL in vitro by enhanced cytokine-induced colony formation. To assess the signaling mechanism(s) involved in PRL-mediated enhancement of hematopoiesis and to identify further the CIS/JAB targets for PRL-mediated cellular responses, we assayed the effect of PRL, alone or in the presence of interleukin-3 (IL-3), on activation of STAT-5 and expression of CIS/JAB RNA in human cord blood (CB) CD34(+) cells.

MATERIALS AND METHODS

Isolated CB CD34(+) cells were incubated in serum-free cultures in the absence or presence of recombinant human (rh)PRL, rhIL-3, or both. Cell lysates were subjected to Western blot analysis with anti-STAT-5 and anti-phospho-STAT-5 antibodies. Isolated RNA was subjected to semiquantitative reverse transcriptase polymerase chain reaction analysis of CIS/JAB expression.

RESULTS

STAT-5 tyrosine phosphorylation was similarly induced by PRL and IL-3, with an additive effect detected in the presence of both stimuli. Both PRL and IL-3, alone or combined, failed to induce CIS-3 or JAB RNA expression in CD34(+) cells. Interferon-gamma had no effect on CIS-3/JAB induction in these cells. However, CIS-1 was induced by PRL < IL-3 < PRL+IL-3, whereas CIS-2 expression was induced by PRL = IL-3 < PRL+IL-3.

CONCLUSIONS

Our findings show that PRL induces activation of STAT-5 and expression of similar CIS/JAB family members as IL-3 does in human CB CD34(+) cells. Because CIS-1 abolishes STAT-5 activation via the IL-3 but not the PRL receptor, the hematopoietic growth-promoting effects of PRL may involve its capacity to provide sustained STAT-5-mediated stimulatory signals to the cells.

Bmx tyrosine kinase has a redundant function downstream of angiopoietin and vascular endothelial growth factor receptors in arterial endothelium

The Bmx gene, a member of the Tec tyrosine kinase gene family, is known to be expressed in subsets of hematopoietic and endothelial cells. In this study, mice were generated in which the first coding exon of the Bmx gene was replaced with the lacZ reporter gene by a knock-in strategy. The homozygous mice lacking Bmx activity were fertile and had a normal life span without an obvious phenotype. Staining of their tissues using beta-galactosidase substrate to assess the sites of Bmx expression revealed strong signals in the endothelial cells of large arteries and in the endocardium starting between days 10.5 and 12.5 of embryogenesis and continuing in adult mice, while the venular endothelium showed a weak signal only in the superior and inferior venae cavae. Of the five known endothelial receptor tyrosine kinases tested, activated Tie-2 induced tyrosyl phosphorylation of the Bmx protein and both Tie-2 and vascular endothelial growth factor receptor 1 (VEGFR-1) stimulated Bmx tyrosine kinase activity. Thus, the Bmx tyrosine kinase has a redundant role in arterial endothelial signal transduction downstream of the Tie-2 and VEGFR-1 growth factor receptors.

Etk/Bmx activation modulates barrier function in epithelial cells

Etk/Bmx is a member of the Tec family of cytoplasmic non-receptor tyrosine kinases known to express in epithelial cells. We demonstrate herein that Etk activation in stably Etk-transfected epithelial Pa-4 cells resulted in a consistently increased transepithelial resistance (TER). After 24 h of hypoxic (1% O2) exposure, the TER and equivalent active ion transport rate ( I eq) were reduced to <5% of the normoxia control in Pa-4 cells, whereas both TER and I eqwere maintained at comparable and 60% levels, respectively, relative to their normoxic controls in cells with Etk activation. Moreover, Pa-4 cells exhibited an abundant actin stress fiber network with a diffuse distribution of β-catenin at the cell periphery. By contrast, Etk-activated cells displayed a redistribution of actin to an exclusively peripheral network, with a discrete band of β-catenin also concentrated at the cell periphery, and an altered occludin distribution profile. On the basis of these findings, we propose that Etk may be a novel regulator of epithelial junctions during physiological and pathophysiological conditions.

Proteolytic activation of ETK/Bmx tyrosine kinase by caspases

Etk/Bmx is a member of the Btk/Tec family of kinases, which are characterized by having a pleckstrin homology domain at the N terminus, in addition to the Src homology 3 (SH3), SH2, and the catalytic domains, shared with the Src family kinases. Etk, or Btk kinases in general, has been implicated in the regulation of apoptosis. To test whether Etk is the substrate for caspases during apoptosis, in vitro translated [(35)S]methionine-labeled Etk was incubated with different apoptotic extracts and recombinant caspases, respectively. Results showed that Etk was proteolyzed in all conditions tested with identical cleavage patterns. Caspase-mediated cleavage of Etk generated a C-terminal fragment, containing the complete SH2 and tyrosine kinase domains, but without intact pleckstrin homology and SH3 domains. This fragment has 4-fold higher kinase activity than that of the full-length Etk. Ectopic expression of the C-terminal fragment of Etk sensitized the PC3 prostate cancer cells to apoptosis in response to apoptosis-inducing stimuli. The finding, together with an earlier report that Etk is potentially antiapoptotic, suggests that Etk may serve as an apoptotic switch, depending on the forms of Etk existing inside the cells. To our knowledge, this is the first case where the activity of a tyrosine kinase is induced by caspase cleavage.

The Tec family of cytoplasmic tyrosine kinases: mammalian Btk, Bmx, Itk, Tec, Txk and homologs in other species

Cytoplasmic protein-tyrosine kinases (PTKs) are enzymes involved in transducing a vast number of signals in metazoans. The importance of the Tec family of kinases was immediately recognized when, in 1993, mutations in the gene encoding Bruton's tyrosine kinase (Btk) were reported to cause the human disease X-linked agammaglobulinemia (XLA). Since then, additional kinases belonging to this family have been isolated, and the availability of full genome sequences allows identification of all members in selected species enabling phylogenetic considerations. Tec kinases are endowed with Pleckstrin homology (PH) and Tec homology (TH) domains and are involved in diverse biological processes related to the control of survival and differentiation fate. Membrane translocation resulting in the activation of Tec kinases with subsequent Ca2+ release seems to be a general feature. However, nuclear translocation may also be of importance. The purpose of this essay is to characterize members of the Tec family and discuss their involvement in signaling. The three-dimensional structure, expression pattern and evolutionary aspects will also be considered.

Signaling network of the Btk family kinases

The Btk family kinases represent new members of non-receptor tyrosine kinases, which include Btk/Atk, Itk/Emt/Tsk, Bmx/Etk, and Tec. They are characterized by having four structural modules: PH (pleckstrin homology) domain, SH3 (Src homology 3) domain, SH2 (Src homology 2) domain and kinase (Src homology 1) domain. Increasing evidence suggests that, like Src-family kinases, Btk family kinases play central but diverse modulatory roles in various cellular processes. They participate in signal transduction in response to virtually all types of extracellular stimuli which are transmitted by growth factor receptors, cytokine receptors, G-protein coupled receptors, antigen-receptors and integrins. They are regulated by many non-receptor tyrosine kinases such as Src, Jak, Syk and FAK family kinases. In turn, they regulate many of major signaling pathways including those of PI3K, PLCgamma and PKC. Both genetic and biochemical approaches have been used to dissect the signaling pathways and elucidate their roles in growth, differentiation and apoptosis. An emerging new role of this family of kinases is cytoskeletal reorganization and cell motility. The physiological importance of these kinases was amply demonstrated by their link to the development of immunodeficiency diseases, due to germ-line mutations. The present article attempts to review the structure and functions of Btk family kinases by summarizing our current knowledge on the interacting partners associated with the different modules of the kinases and the diverse signaling pathways in which they are involved.