Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Dual clathrin and integrin signaling systems regulate growth factor receptor activation

The crosstalk between growth factor and adhesion receptors is key for cell growth and migration. In pathological settings, these receptors are drivers of cancer. Yet, how growth and adhesion signals are spatially organized and integrated is poorly understood. Here we use quantitative fluorescence and electron microscopy to reveal a mechanism where flat clathrin lattices partition and activate growth factor signals via a coordinated response that involves crosstalk between epidermal growth factor receptor (EGFR) and the adhesion receptor β5-integrin. We show that ligand-activated EGFR, Grb2, Src, and β5-integrin are captured by clathrin coated-structures at the plasma membrane. Clathrin structures dramatically grow in response to EGF into large flat plaques and provide a signaling platform that link EGFR and β5-integrin through Src-mediated phosphorylation. Disrupting this EGFR/Src/β5-integrin axis prevents both clathrin plaque growth and dampens receptor signaling. Our study reveals a reciprocal regulation between clathrin lattices and two different receptor systems to coordinate and enhance signaling. These findings have broad implications for the regulation of growth factor signaling, adhesion, and endocytosis.

Genetic deletion of endothelial microRNA-15a/16-1 promotes cerebral angiogenesis and neurological recovery in ischemic stroke through Src signaling pathway

Cerebral angiogenesis is tightly controlled by specific microRNAs (miRs), including the miR-15a/16-1 cluster. Recently, we reported that endothelium-specific conditional knockout of the miR-15a/16-1 cluster (EC-miR-15a/16-1 cKO) promotes post-stroke angiogenesis and improves long-term neurological recovery by increasing protein levels of VEGFA, FGF2, and their respective receptors VEGFR2 and FGFR1. Herein, we further investigated the underlying signaling mechanism of these pro-angiogenic factors after ischemic stroke using a selective Src family inhibitor AZD0530. EC-miR-15a/16-1 cKO and age- and sex-matched wild-type littermate (WT) mice were subjected to 1 h middle cerebral artery occlusion (MCAO) and 28d reperfusion. AZD0530 was administered daily by oral gavage to both genotypes of mice 3-21d after MCAO. Compared to WT, AZD0530 administration exacerbated spatial cognitive impairments and brain atrophy in EC-miR-15a/16-1 cKO mice following MCAO. AZD0530 also attenuated long-term recovery of blood flow and inhibited the formation of new microvessels, including functional vessels with blood circulation, in the penumbra of stroked cKO mice. Moreover, AZD0530 blocked the Src signaling pathway by downregulating phospho-Src and its downstream mediators (p-Stat3, p-Akt, p-FAK, p-p44/42 MAPK, p-p38 MAPK) in post-ischemic brains. Collectively, our data demonstrated that endothelium-targeted deletion of the miR-15a/16-1 cluster promotes post-stroke angiogenesis and improves long-term neurological recovery via activating Src signaling pathway.

Biological Responses of Stem Cells to Photobiomodulation Therapy

BACKGROUND

Stem cells have attracted the researchers interest, due to their applications in regenerative medicine. Their self-renewal capacity for multipotent differentiation, and immunomodulatory properties make them unique to significantly contribute to tissue repair and regeneration applications. Recently, stem cells have shown increased proliferation when irradiated with low-level laser therapy or Photobiomodulation Therapy (PBMT), which induces the activation of intracellular and extracellular chromophores and the initiation of cellular signaling. The purpose of this study was to evaluate this phenomenon in the literature.

METHODS

The literature investigated the articles written in English in four electronic databases of PubMed, Scopus, Google Scholar and Cochrane up to April 2019. Stem cell was searched by combining the search keyword of "low-level laser therapy" OR "low power laser therapy" OR "low-intensity laser therapy" OR "photobiomodulation therapy" OR "photo biostimulation therapy" OR "LED". In total, 46 articles were eligible for evaluation.

RESULTS

Studies demonstrated that red to near-infrared light is absorbed by the mitochondrial respiratory chain. Mitochondria are significant sources of reactive oxygen species (ROS). Mitochondria play an important role in metabolism, energy generation, and are also involved in mediating the effects induced by PBMT. PBMT may result in the increased production of (ROS), nitric oxide (NO), adenosine triphosphate (ATP), and cyclic adenosine monophosphate (cAMP). These changes, in turn, initiate cell proliferation and induce the signal cascade effect.

CONCLUSION

The findings of this review suggest that PBMT-based regenerative medicine could be a useful tool for future advances in tissue engineering and cell therapy.

Squalene synthase promotes the invasion of lung cancer cells via the osteopontin/ERK pathway

Cholesterol is the major component of lipid rafts. Squalene synthase (SQS) is a cholesterol biosynthase that functions in cholesterol biosynthesis, modulates the formation of lipids rafts and promotes lung cancer metastasis. In this study, we investigated the lipid raft-associated pathway of SQS in lung cancer. Gene expression microarray data revealed the upregulation of secreted phosphoprotein 1 (SPP1; also known as osteopontin, OPN) in CL1-0/SQS-overexpressing cells. Knockdown of OPN in SQS-overexpressing cells inhibits their migration and invasion, whereas an OPN treatment rescues the migration and invasion of SQS knockdown cells. High OPN expression is associated with lymph node status, advanced stage and poor prognosis in patients with lung cancer. Moreover, patients with high SQS expression and high OPN expression show poor survival compared with patients with low SQS expression and low OPN expression. SQS induces the phosphorylation of Src and ERK1/2 via OPN, resulting in increased expression of MMP1 and subsequent metastasis of lung cancer cells. Based on our findings, SQS expression increases the expression of OPN and phosphorylation of Src through cholesterol synthesis to modulate the formation of lipid rafts. SQS may represent a therapeutic strategy for lung cancer.

PKA and Ube3a regulate SK2 channel trafficking to promote synaptic plasticity in hippocampus: Implications for Angelman Syndrome

The ubiquitin ligase, Ube3a, plays important roles in brain development and functions, since its deficiency results in Angelman Syndrome (AS) while its over-expression increases the risk for autism. We previously showed that the lack of Ube3a-mediated ubiquitination of the Ca²⁺-activated small conductance potassium channel, SK2, contributes to impairment of synaptic plasticity and learning in AS mice. Synaptic SK2 levels are also regulated by protein kinase A (PKA), which phosphorylates SK2 in its C-terminal domain, facilitating its endocytosis. Here, we report that PKA activation restores theta burst stimulation (TBS)-induced long-term potentiation (LTP) in hippocampal slices from AS mice by enhancing SK2 internalization. While TBS-induced SK2 endocytosis is facilitated by PKA activation, SK2 recycling to synaptic membranes after TBS is inhibited by Ube3a. Molecular and cellular studies confirmed that phosphorylation of SK2 in the C-terminal domain increases its ubiquitination and endocytosis. Finally, PKA activation increases SK2 phosphorylation and ubiquitination in Ube3a-overexpressing mice. Our results indicate that, although both Ube3a-mediated ubiquitination and PKA-induced phosphorylation reduce synaptic SK2 levels, phosphorylation is mainly involved in TBS-induced endocytosis, while ubiquitination predominantly inhibits SK2 recycling. Understanding the complex interactions between PKA and Ube3a in the regulation of SK2 synaptic levels might provide new platforms for developing treatments for AS and various forms of autism.

HB-EGF Improves the Hair Regenerative Potential of Adipose-Derived Stem Cells via ROS Generation and Hck Phosphorylation

Although adipose-derived stem cells (ASCs) have hair regenerative potential, their hair inductive capabilities are limited. The mitogenic and hair inductive effects of heparin binding-epidermal growth factor-like growth factor (HB-EGF) on ASCs were investigated in this study and the underlying mechanism of stimulation was examined. Cell growth, migration, and self-renewal assays, as well as quantitative polymerase chain reactions and immunostaining, were carried out. Telogen-to-anagen transition and organ culture using vibrissa follicles were also conducted. HB-EGF significantly increased ASC motility, including cell proliferation, migration, and self-renewal activity. The preconditioning of ASCs with HB-EGF induced telogen-to-anagen transition more rapidly in vivo, and injected PKH26-ASCs survived for longer periods of time. Conditioned medium obtained from HB-EGF-treated ASCs promoted hair growth in vivo, upregulating growth factors. In particular, thrombopoietin (THPO) also induced hair growth in vivo, stimulating dermal papilla cells (DPCs). Reactive oxygen species (ROS) appeared to play a key role in ASC stimulation as the inhibition of ROS generation and NOX4 knockout attenuated ASC stimulation and THPO upregulation by HB-EGF. In addition, the Hck phosphorylation pathway mediated the stimulation of ASCs by HB-EGF. In summary, HB-EGF increased the motility and paracrine effects of ASCs releasing THPO growth factor and THPO promoted hair growth-stimulating DPCs. ROS generation and Hck phosphorylation are key factors in HB-EGF-induced ASC stimulation. Therefore, combination therapy involving HB-EGF and ASCs may provide a novel solution for hair-loss treatment.

Modifying effect of intravenous laser therapy on the protein expression of arginase and epidermal growth factor receptor in type 2 diabetic patients

The epidermal growth factor receptor (EGFR) signaling pathway may be involved in cell activation and may influence the neuronal microenvironment, microglia activation, and production of proinflammatory cytokines. Arginase and nitric oxide synthase (NOS) both use L-arginine as a common substrate. Decreasing the arginase expression may increase L-arginine consumption by NOS and increase nitric oxide (NO) synthesis. Intravenous laser blood irradiation (ILBI) is an effective systemic treatment for different pathologies including diabetes mellitus. Previous studies have shown that low-level laser therapy can have an effect on the release of certain cytokines and growth factors. The aim of this study was to evaluate the effects of ILBI on the expression of arginase and epidermal growth factor receptor in type 2 diabetic patients. We used 630 nm red laser light, 1.5 mW, continuous mode, intravenously for 30 min in 13 type 2 diabetic patients and compared their blood samples using the flow cytometry technique, before and after ILBI. The difference between the percentage of cells before and after therapy was analyzed using repeated-measures ANOVA, and the relationship between EGFR and arginase expression in blood and tissue was evaluated by calculating the Pearson correlation coefficient. We found a significant decrease in the expression of both arginase- and EGFR-positive cells after laser therapy (P < 0.01). In conclusion, laser therapy may have a beneficial effect for diabetic patients via decreasing arginase expression and activation of the NOS/NO pathway which increases NO production and vasodilation, and decreasing EGFR expression which may reduce neuroinflammation and its secondary damages.

Synaptopodin Is a Coincidence Detector of Tyrosine versus Serine/Threonine Phosphorylation for the Modulation of Rho Protein Crosstalk in Podocytes

Tyrosine and serine/threonine signal-transduction pathways influence many aspects of cell behavior, including the spatial and temporal regulation of the actin cytoskeleton. However, little is known about how input from diverse tyrosine and serine/threonine kinases is integrated to control Rho protein crosstalk and actin remodeling, which are critically important in podocyte health and disease. Here we unveil the proteolytically-regulated, actin organizing protein synaptopodin as a coincidence detector of tyrosine versus serine/threonine phosphorylation. We show that serine/threonine and tyrosine kinases duel for synaptopodin stability versus degradation. EGFR/Src-mediated tyrosine phosphorylation of synaptopodin in podocytes promotes binding to the serine/threonine phosphatase calcineurin. This leads to the loss of 14-3-3 binding, resulting in synaptopodin degradation, Vav2 activation, enhanced Rac1 signaling, and ultimate loss of stress fibers. Our studies reveal how synaptopodin, a single proteolytically-controlled protein, integrates antagonistic tyrosine versus serine/threonine phosphorylation events for the dynamic control of the actin cytoskeleton in podocytes.

Role of Redox Status in Development of Glioblastoma

Glioblastoma multiforme (GBM) is a highly aggressive neoplasia, prognosis remains dismal, and current therapy is mostly palliative. There are no known risk factors associated with gliomagenesis; however, it is well established that chronic inflammation in brain tissue induces oxidative stress in astrocytes and microglia. High quantities of reactive species of oxygen into the cells can react with several macromolecules, including chromosomal and mitochondrial DNA, leading to damage and malfunction of DNA repair enzymes. These changes bring genetic instability and abnormal metabolic processes, favoring oxidative environment and increase rate of cell proliferation. In GBM, a high metabolic rate and increased basal levels of reactive oxygen species play an important role as chemical mediators in the regulation of signal transduction, protecting malignant cells from apoptosis, thus creating an immunosuppressive environment. New redox therapeutics could reduce oxidative stress preventing cellular damage and high mutation rate accompanied by chromosomal instability, reducing the immunosuppressive environment. In addition, therapies directed to modulate redox rate reduce resistance and moderate the high rate of cell proliferation, favoring apoptosis of tumoral cells. This review describes the redox status in GBM, and how this imbalance could promote gliomagenesis through genomic and mitochondrial DNA damage, inducing the pro-oxidant and proinflammatory environment involved in tumor cell proliferation, resistance, and immune escape. In addition, some therapeutic agents that modulate redox status and might be advantageous in therapy against GBM are described.

DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly

Wnt ligands regulate heart morphogenesis but the underlying mechanisms remain unclear. Two Formin-related proteins, DAAM1 and 2, were previously found to bind the Wnt effector Disheveled. Here, since DAAM1 and 2 nucleate actin and mediate Wnt-induced cytoskeletal changes, a floxed-allele of Daam1 was used to disrupt its function specifically in the myocardium and investigate Wnt-associated pathways. Homozygous Daam1 conditional knockout (CKO) mice were viable but had misshapen hearts and poor cardiac function. The defects in Daam1 CKO mice were observed by mid-gestation and were associated with a loss of protrusions from cardiomyocytes invading the outflow tract. Further, these mice exhibited noncompaction cardiomyopathy (NCM) and deranged cardiomyocyte polarity. Interestingly, Daam1 CKO mice that were also homozygous for an insertion disrupting Daam2 (DKO) had stronger NCM, severely reduced cardiac function, disrupted sarcomere structure, and increased myocardial proliferation, suggesting that DAAM1 and DAAM2 have redundant functions. While RhoA was unaffected in the hearts of Daam1/2 DKO mice, AKT activity was lower than in controls, raising the issue of whether DAAM1/2 are only mediating Wnt signaling. Daam1-floxed mice were thus bred to Wnt5a null mice to identify genetic interactions. The hearts of Daam1 CKO mice that were also heterozygous for the null allele of Wnt5a had stronger NCM and more severe loss of cardiac function than Daam1 CKO mice, consistent with DAAM1 and Wnt5a acting in a common pathway. However, deleting Daam1 further disrupted Wnt5a homozygous-null hearts, suggesting that DAAM1 also has Wnt5a-independent roles in cardiac development.

Epidermal growth factor-stimulated Akt phosphorylation requires clathrin or ErbB2 but not receptor endocytosis

Upon ligand binding, the epidermal growth factor receptor (EGFR) activates signaling and undergoes endocytosis. EGFR signaling leading to Akt activation is impaired by perturbation of clathrin but not by inhibition of internalization through perturbation of dynamin. Clathrin may thus directly regulate receptor signaling at the cell surface.

Epidermal growth factor (EGF) binding to its receptor (EGFR) activates several signaling intermediates, including Akt, leading to control of cell survival and metabolism. Concomitantly, ligand-bound EGFR is incorporated into clathrin-coated pits—membrane structures containing clathrin and other proteins—eventually leading to receptor internalization. Whether clathrin might regulate EGFR signaling at the plasma membrane before vesicle scission is poorly understood. We compared the effect of clathrin perturbation (preventing formation of, or receptor recruitment to, clathrin structures) to that of dynamin2 (allowing formation of clathrin structures but preventing EGFR internalization) under conditions in which EGFR endocytosis is clathrin dependent. Clathrin perturbation by siRNA gene silencing, with the clathrin inhibitor pitstop2, or knocksideways silencing inhibited EGF-simulated Gab1 and Akt phosphorylation in ARPE-19 cells. In contrast, perturbation of dynamin2 with inhibitors or by siRNA gene silencing did not affect EGF-stimulated Gab1 or Akt phosphorylation. EGF stimulation enriched Gab1 and phospho-Gab1 within clathrin structures. ARPE-19 cells have low ErbB2 expression, and overexpression and knockdown experiments revealed that robust ErbB2 expression bypassed the requirement for clathrin for EGF-stimulated Akt phosphorylation. Thus clathrin scaffolds may represent unique plasma membrane signaling microdomains required for signaling by certain receptors, a function that can be separated from vesicle formation.

Cbl-b and c-Cbl negatively regulate osteoblast differentiation by enhancing ubiquitination and degradation of Osterix

E3 ubiquitin ligase Cbl-b and c-Cbl play important roles in bone formation and maintenance. Cbl-b and c-Cbl regulate the activity of various receptor tyrosine kinases and intracellular protein tyrosine kinases mainly by regulating the degradation of target proteins. However, the precise mechanisms of how Cbl-b and c-Cbl regulate osteoblast differentiation are not well known. In this study, we investigated potential targets of Cbl-b and c-Cbl. We found that Cbl-b and c-Cbl inhibit BMP2-induced osteoblast differentiation in mesenchymal cells. Among various osteogenic transcription factors, we identified that Cbl-b and c-Cbl suppress the protein stability and transcriptional activity of Osterix. Our results suggest that Cbl-b and c-Cbl inhibit the function of Osterix by enhancing the ubiquitin-proteasome-mediated degradation of Osterix. Taken together, we propose novel regulatory roles of Cbl-b and c-Cbl during osteoblast differentiation in which Cbl-b and c-Cbl regulate the degradation of Osterix through the ubiquitin-proteasome pathway.

Src enhances osteogenic differentiation through phosphorylation of Osterix

Osterix, a zinc-finger transcription factor, is required for osteoblast differentiation and new bone formation during embryonic development. The c-Src of tyrosine kinase is involved in a variety of cellular signaling pathways, leading to the induction of DNA synthesis, cell proliferation, and cytoskeletal reorganization. Src activity is tightly regulated and its dysregulation leads to constitutive activation and cellular transformation. The function of Osterix can be also modulated by post-translational modification. But the precise molecular signaling mechanisms between Osterix and c-Src are not known. In this study we investigated the potential regulation of Osterix function by c-Src in osteoblast differentiation. We found that c-Src activation increases protein stability, osteogenic activity and transcriptional activity of Osterix. The siRNA-mediated knockdown of c-Src decreased the protein levels and transcriptional activity of Osterix. Conversely, Src specific inhibitor, SU6656, decreased the protein levels and transcriptional activity of Osterix. The c-Src interacts with and phosphorylates Osterix. These results suggest that c-Src signaling modulates osteoblast differentiation at least in part through Osterix.

Neuroblastoma tyrosine kinase signaling networks involve FYN and LYN in endosomes and lipid rafts

Protein phosphorylation plays a central role in creating a highly dynamic network of interacting proteins that reads and responds to signals from growth factors in the cellular microenvironment. Cells of the neural crest employ multiple signaling mechanisms to control migration and differentiation during development. It is known that defects in these mechanisms cause neuroblastoma, but how multiple signaling pathways interact to govern cell behavior is unknown. In a phosphoproteomic study of neuroblastoma cell lines and cell fractions, including endosomes and detergent-resistant membranes, 1622 phosphorylated proteins were detected, including more than half of the receptor tyrosine kinases in the human genome. Data were analyzed using a combination of graph theory and pattern recognition techniques that resolve data structure into networks that incorporate statistical relationships and protein-protein interaction data. Clusters of proteins in these networks are indicative of functional signaling pathways. The analysis indicates that receptor tyrosine kinases are functionally compartmentalized into distinct collaborative groups distinguished by activation and intracellular localization of SRC-family kinases, especially FYN and LYN. Changes in intracellular localization of activated FYN and LYN were observed in response to stimulation of the receptor tyrosine kinases, ALK and KIT. The results suggest a mechanism to distinguish signaling responses to activation of different receptors, or combinations of receptors, that govern the behavior of the neural crest, which gives rise to neuroblastoma.

Neuroblastoma is a childhood cancer for which therapeutic progress has been slow. We analyzed a large number phosphorylated proteins in neuroblastoma cells to discern patterns that indicate functional signal transduction pathways. To analyze the data, we developed novel techniques that resolve data structure and visualize that structure as networks that represent both protein interactions and statistical relationships. We also fractionated neuroblastoma cells to examine the location of signaling proteins in different membrane fractions and organelles. The analysis revealed that signaling pathways are functionally and physically compartmentalized into distinct collaborative groups distinguished by phosphorylation patterns and intracellular localization. We found that two related proteins (FYN and LYN) act like central hubs in the tyrosine kinase signaling network that change intracellular localization and activity in response to activation of different receptors.

Redox biology in normal cells and cancer: restoring function of the redox/Fyn/c-Cbl pathway in cancer cells offers new approaches to cancer treatment

This review discusses a unique discovery path starting with novel findings on redox regulation of precursor cell and signaling pathway function and identification of a new mechanism by which relatively small changes in redox status can control entire signaling networks that regulate self-renewal, differentiation, and survival. The pathway central to this work, the redox/Fyn/c-Cbl (RFC) pathway, converts small increases in oxidative status to pan-activation of the c-Cbl ubiquitin ligase, which controls multiple receptors and other proteins of central importance in precursor cell and cancer cell function. Integration of work on the RFC pathway with attempts to understand how treatment with systemic chemotherapy causes neurological problems led to the discovery that glioblastomas (GBMs) and basal-like breast cancers (BLBCs) inhibit c-Cbl function through altered utilization of the cytoskeletal regulators Cool-1/βpix and Cdc42, respectively. Inhibition of these proteins to restore normal c-Cbl function suppresses cancer cell division, increases sensitivity to chemotherapy, disrupts tumor-initiating cell (TIC) activity in GBMs and BLBCs, controls multiple critical TIC regulators, and also allows targeting of non-TICs. Moreover, these manipulations do not increase chemosensitivity or suppress division of nontransformed cells. Restoration of normal c-Cbl function also allows more effective harnessing of estrogen receptor-α (ERα)-independent activities of tamoxifen to activate the RFC pathway and target ERα-negative cancer cells. Our work thus provides a discovery strategy that reveals mechanisms and therapeutic targets that cannot be deduced by standard genetics analyses, which fail to reveal the metabolic information, isoform shifts, protein activation, protein complexes, and protein degradation critical to our discoveries.

Nexus of signaling and endocytosis in oncogenesis driven by non-small cell lung cancer-associated epidermal growth factor receptor mutants

Epidermal growth factor receptor (EGFR) controls a wide range of cellular processes, and aberrant EGFR signaling as a result of receptor overexpression and/or mutation occurs in many types of cancer. Tumor cells in non-small cell lung cancer (NSCLC) patients that harbor EGFR kinase domain mutations exhibit oncogene addiction to mutant EGFR, which confers high sensitivity to tyrosine kinase inhibitors (TKIs). As patients invariably develop resistance to TKIs, it is important to delineate the cell biological basis of mutant EGFR-induced cellular transformation since components of these pathways can serve as alternate therapeutic targets to preempt or overcome resistance. NSCLC-associated EGFR mutants are constitutively-active and induce ligand-independent transformation in nonmalignant cell lines. Emerging data suggest that a number of factors are critical for the mutant EGFR-dependent tumorigenicity, and bypassing the effects of TKIs on these pathways promotes drug resistance. For example, activation of downstream pathways such as Akt, Erk, STAT3 and Src is critical for mutant EGFR-mediated biological processes. It is now well-established that the potency and spatiotemporal features of cellular signaling by receptor tyrosine kinases such as EGFR, as well as the specific pathways activated, is determined by the nature of endocytic traffic pathways through which the active receptors traverse. Recent evidence indicates that NSCLC-associated mutant EGFRs exhibit altered endocytic trafficking and they exhibit reduced Cbl ubiquitin ligase-mediated lysosomal downregulation. More recent work has shown that mutant EGFRs undergo ligand-independent traffic into the endocytic recycling compartment, a behavior that plays a key role in Src pathway activation and oncogenesis. These studies are beginning to delineate the close nexus between signaling and endocytic traffic of EGFR mutants as a key driver of oncogenic processes. Therefore, in this review, we will discuss the links between mutant EGFR signaling and endocytic properties, and introduce potential mechanisms by which altered endocytic properties of mutant EGFRs may alter signaling and vice versa as well as their implications for NSCLC therapy.

Tyrosine phosphorylation of NEDD4 activates its ubiquitin ligase activity

Ligand binding to the receptor tyrosine kinase fibroblast growth factor (FGF) receptor 1 (FGFR1) causes dimerization and activation by transphosphorylation of tyrosine residues in the kinase domain. FGFR1 is ubiquitylated by the E3 ligase NEDD4 (also known as NEDD4-1), which promotes FGFR1 internalization and degradation. Although phosphorylation of FGFR1 is required for NEDD4-dependent endocytosis, NEDD4 directly binds to a nonphosphorylated region of FGFR1. We found that activation of FGFR1 led to activation of c-Src kinase-dependent tyrosine phosphorylation of NEDD4, enhancing the ubiquitin ligase activity of NEDD4. Using mass spectrometry, we identified several FGF-dependent phosphorylated tyrosines in NEDD4, including Tyr(43) in the C2 domain and Tyr(585) in the HECT domain. Mutating these tyrosines to phenylalanine to prevent phosphorylation inhibited FGF-dependent NEDD4 activity and FGFR1 endocytosis and enhanced cell proliferation. Mutating the tyrosines to glutamic acid to mimic phosphorylation enhanced NEDD4 activity. Moreover, the NEDD4 C2 domain bound the HECT domain, and the presence of phosphomimetic mutations inhibited this interaction, suggesting that phosphorylation of NEDD4 relieves an inhibitory intra- or intermolecular interaction. Accordingly, activation of FGFR1 was not required for activation of NEDD4 that lacked its C2 domain. Activation of c-Src by epidermal growth factor (EGF) also promoted tyrosine phosphorylation and enhanced the activity of NEDD4. Thus, we identified a feedback mechanism by which receptor tyrosine kinases promote catalytic activation of NEDD4 and that may represent a mechanism of receptor crosstalk.

Cell surface epidermal growth factor receptors increase Src and c-Cbl activity and receptor ubiquitylation

There is an established role for the endocytic pathway in regulation of epidermal growth factor receptor (EGFR) signaling to downstream effectors. However, because ligand-mediated EGFR endocytosis utilizes multiple "moving parts," dissecting the spatial versus temporal contributions has been challenging. Blocking all endocytic trafficking can have unintended effects on other receptors as well as give rise to compensatory mechanisms, both of which impact interpretation of EGFR signaling. To overcome these limitations, we used epidermal growth factor (EGF) conjugated to polystyrene beads (EGF beads). EGF beads simultaneously activate the EGFR while blocking its endocytosis and allow analysis of EGFR signaling from the plasma membrane. Human telomerase immortalized corneal epithelial (hTCEpi) cells were used to model normal epithelial cell biology. In hTCEpi cells, both cell surface and intracellular EGFRs exhibited dose-dependent increases in effector activity after 15 min of ligand stimulation, but only the serine phosphorylation of signal transducer and activator of transcription 3 (STAT3) was statistically significant when accounting for receptor phosphorylation. However, over time with physiological levels of receptor phosphorylation, cell surface receptors produced either enhanced or sustained mitogen-activated protein kinase kinase (MEK), Casitas B-lineage lymphoma (c-Cbl), and the pro-oncogene Src activity. These increases in effector communication by cell surface receptors resulted in an increase in EGFR ubiquitylation with sustained ligand incubation. Together, these data indicate that spatial regulation of EGFR signaling may be an important regulatory mechanism in receptor down-regulation.

Antagonizing c-Cbl enhances EGFR-dependent corneal epithelial homeostasis

PURPOSE

In many cell types, the E3 ubiquitin ligase, c-Cbl, induces ligand-dependent ubiquitylation of the epidermal growth factor receptor (EGFR) and targets the receptor for lysosomal degradation. The goal of this study was to determine whether c-Cbl is a negative regulator of EGFR in the corneal epithelium and if it can be inhibited to promote corneal epithelial homeostasis.

METHODS

Expression and activity of c-Cbl were blocked in immortalized human corneal epithelial cells (hTCEpi) using RNAi and pharmacological agents ([4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-d-3,4-pyrimidine] or PP1). Following c-Cbl inhibition, cells were assessed for ligand-dependent receptor ubiquitylation, receptor phosphorylation, and in vitro wound healing. Subsequent experiments used PP1 in hTCEpi cells and monitored in vivo murine corneal epithelial wound healing.

RESULTS

Knockdown and inhibition of c-Cbl decreased ligand-dependent ubiquitylation of the EGFR and prolonged receptor activity as measured by tyrosine phosphorylation. Further, these treatments also increased the extent of ligand-dependent corneal epithelial wound healing in vitro and in vivo.

CONCLUSION

Manipulating the duration of EGFR activity can enhance the rate of restoration of the corneal epithelial layer. Based on our findings, c-Cbl is a new therapeutic target to enhance EGFR-mediated corneal epithelial homeostasis that bypasses the limitations of previous approaches.

Chemokines and squamous cancer of the head and neck: targets for therapeutic intervention?

The biological properties of squamous carcinoma cells are intimately regulated by a multitude of cytokines and growth factors; the most well studied of these include epidermal growth factor receptor agonists and members of the transforming growth factor-beta family. The recent explosion of research in the field of chemokine function as a mediator of tumor progression has led to the possibility that these small, immunomodulatory proteins also play key roles in squamous carcinogenesis and may, therefore, be potential targets for novel therapeutic approaches.

EphA2 signaling following endocytosis: role of Tiam1

Eph receptors and their membrane-bound ligands, the ephrins, represent a complex subfamily of receptor tyrosine kinases (RTKs). Eph/ephrin binding can lead to various and opposite cellular behaviors such as adhesion versus repulsion, or cell migration versus cell-adhesion. Recently, Eph endocytosis has been identified as one of the critical steps responsible for such diversity. Eph receptors, as many RTKs, are rapidly endocytosed following ligand-mediated activation and traffic through endocytic compartments prior to degradation. However, it is becoming obvious that endocytosis controls signaling in many different manners. Here we showed that activated EphA2 are degraded in the lysosomes and that about 35% of internalized receptors are recycled back to the plasma membrane. Our study is also the first to demonstrate that EphA2 retains the capacity to signal in endosomes. In particular, activated EphA2 interacted with the Rho family GEF Tiam1 in endosomes. This association led to Tiam1 activation, which in turn increased Rac1 activity and facilitated Eph/ephrin endocytosis. Disrupting Tiam1 function with RNA interference impaired both ephrinA1-dependent Rac1 activation and ephrinA1-induced EphA2 endocytosis. In summary, our findings shed new light on the regulation of EphA2 endocytosis, intracellular trafficking and signal termination and establish Tiam1 as an important modulator of EphA2 signaling.

c-Cbl, a ubiquitin E3 ligase that targets active β-catenin: a novel layer of Wnt signaling regulation

Regulation of transcriptionally active nuclear β-catenin during the Wnt-on phase is crucial to ensure controlled induction of Wnt target genes. Several ubiquitin E3 ligases are known to regulate cytosolic β-catenin during the Wnt-off phase, but little is known about the fate of active nuclear β-catenin in the Wnt-on phase. We now describe ubiquitination of active β-catenin in the Wnt-on phase by a RING finger ubiquitin E3 ligase, Casitas B-lineage lymphoma (c-Cbl) in endothelial cells. c-Cbl binds preferentially to nuclearly active β-catenin in the Wnt-on phase via the armadillo repeat region. Wild-type c-Cbl suppresses and E3 ligase-deficient c-Cbl-70Z increases Wnt signaling. Wnt induces nuclear translocation of c-Cbl where it ubiquitinates nuclear β-catenin. Deletion of the c-Cbl UBA domain abrogates its dimerization, binding to β-catenin, Wnt-induced c-Cbl nuclear translocation, and ubiquitination of nuclear β-catenin. c-Cbl activity inhibits pro-angiogenic Wnt targets IL-8 and VEGF levels and angiogenesis in a β-catenin-dependent manner. This study defines for the first time c-Cbl as a ubiquitin E3 ligase that targets nuclearly active β-catenin in the Wnt-on phase and uncovers a novel layer of regulation of Wnt signaling.

Downregulation of Src enhances the cytotoxic effect of temozolomide through AKT in glioma

Src is an attractive target since it is overexpressed in a number of malignancies, including glioma. However, the mechanism of Src signaling as well as its silencing effect on temozolomide in glioma is not well known. We hypothesized that downregulation of Src may enhance the cytotoxic effect of temozolomide on glioma. As expected, Src was overexpressed in glioblastoma multiforme (GBM) compared with normal brain tissues. Src silencing suppressed tumor proliferation and induced apoptosis in glioma. In addition, Src silencing combined with temozolomide treatment resulted in significant inhibition of tumor growth. These effects may be mediated by AKT which is a downstream effector of Src, since downregulation of AKT exhibited a similar effect as Src siRNA when combined with temozolomide. Finally, we demonstrated that overexpression of AKT suppressed the enhanced cytotoxic effect of temozolomide mediated by Src silencing. Thus, the present study demonstrated that Src plays a biologically significant role in tumor proliferation and apoptosis and enhances the cytotoxic effect of temozolomide through AKT supression in glioma.

The role of Src kinase in the biology and pathogenesis of Acanthamoeba castellanii

Background

Acanthamoeba species are the causative agents of fatal granulomatous encephalitis in humans. Haematogenous spread is thought to be a primary step, followed by blood–brain barrier penetration, in the transmission of Acanthmaoeba into the central nervous system, but the associated molecular mechanisms remain unclear. Here, we evaluated the role of Src, a non-receptor protein tyrosine kinase in the biology and pathogenesis of Acanthamoeba.

Methods

Amoebistatic and amoebicidal assays were performed by incubating amoeba in the presence of Src kinase-selective inhibitor, PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) and its inactive analog, PP3 (4-amino-7-phenylpyrazolo[3,4-d]pyrimidine). Using this inhibitor, the role of Src kinase in A. castellanii interactions with Escherichia coli was determined. Zymographic assays were performed to study effects of Src kinase on extracellular proteolytic activities of A. castellanii. The human brain microvascular endothelial cells were used to determine the effects of Src kinase on A. castellanii adhesion to and cytotoxicity of host cells.

Results

Inhibition of Src kinase using a specific inhibitor, PP2 (4-amino-5-(4 chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d] pyrimidine) but not its inactive analog, PP3 (4-amino-7-phenylpyrazolo[3,4-d] pyrimidine), had detrimental effects on the growth of A. castellanii (keratitis isolate, belonging to the T4 genotype). Interestingly, inhibition of Src kinase hampered the phagocytic ability of A. castellanii, as measured by the uptake of non-invasive bacteria, but, on the contrary, invasion by pathogenic bacteria was enhanced. Zymographic assays revealed that inhibition of Src kinases reduced extracellular protease activities of A. castellanii. Src kinase inhibition had no significant effect on A. castellanii binding to and cytotoxicity of primary human brain microvascular endothelial cells, which constitute the blood–brain barrier.

Conclusions

For the first time, these findings demonstrated that Src kinase is involved in A. castellanii proliferation, protease secretions and phagocytic properties. Conversely, invasion of Acanthamoeba by pathogenic bacteria was stimulated by Src kinase inhibition.

Cross-talk between mitogenic Ras/MAPK and survival PI3K/Akt pathways: a fine balance

In the present paper, we describe multiple levels of cross-talk between the PI3K (phosphoinositide 3-kinase)/Akt and Ras/MAPK (mitogen-activated protein kinase) signalling pathways. Experimental data and computer simulations demonstrate that cross-talk is context-dependent and that both pathways can activate or inhibit each other. Positive influence of the PI3K pathway on the MAPK pathway is most effective at sufficiently low doses of growth factors, whereas negative influence of the MAPK pathway on the PI3K pathway is mostly pronounced at high doses of growth factors. Pathway cross-talk endows a cell with emerging capabilities for processing and decoding signals from multiple receptors activated by different combinations of extracellular cues.

The ubiquitin/proteasome system mediates entry and endosomal trafficking of Kaposi's sarcoma-associated herpesvirus in endothelial cells

Ubiquitination, a post-translational modification, mediates diverse cellular functions including endocytic transport of molecules. Kaposi's sarcoma-associated herpesvirus (KSHV), an enveloped herpesvirus, enters endothelial cells primarily through clathrin-mediated endocytosis. Whether ubiquitination and proteasome activity regulates KSHV entry and endocytosis remains unknown. We showed that inhibition of proteasome activity reduced KSHV entry into endothelial cells and intracellular trafficking to nuclei, thus preventing KSHV infection of the cells. Three-dimensional (3-D) analyses revealed accumulation of KSHV particles in a cytoplasmic compartment identified as EEA1+ endosomal vesicles upon proteasome inhibition. KSHV particles are colocalized with ubiquitin-binding proteins epsin and eps15. Furthermore, ubiquitination mediates internalization of both KSHV and one of its receptors integrin β1. KSHV particles are colocalized with activated forms of the E3 ligase c-Cbl. Knock-down of c-Cbl or inhibition of its phosphorylation reduced viral entry and intracellular trafficking, resulting in decreased KSHV infectivity. These results demonstrate that ubiquitination mediates internalization of both KSHV and one of its cognate receptors integrin β1, and identify c-Cbl as a potential E3 ligase that facilitates this process.

Ubiquitination, a post-translational modification, mediates important cellular functions including endocytic transport of molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus linked to the development of Kaposi's sarcoma, an endothelial malignancy commonly found in AIDS patients, and several other malignancies. KSHV enters endothelial cells primarily through clathrin-mediated endocytosis. In this study, we show that the proteasome activity is required for KSHV entry into endothelial cells and intracellular trafficking to nuclei. Inhibition of proteasome activity reduced KSHV infectivity and led to the accumulation of KSHV particles in EEA1+ early endosomal vesicles. Furthermore, we show that ubiquitination mediates the internalization of both KSHV and one of its receptors integrin β1. KSHV particles are colocalized with ubiquitin-binding proteins epsin and eps15, as well as activated forms of the E3 ligase c-Cbl. Knock-down of c-Cbl or inhibition of its phosphorylation blocked KSHV entry and trafficking, thus preventing KSHV infection of endothelial cells. Together, these results illustrate the essential role of ubiquitination during the internalization of KSHV and its cognate receptor integrin β1. The identification of an E3 ligase that mediates the ubiquitination of KSHV and its cognate receptor integrin β1 leading to viral entry provide a potential therapeutic target for this oncogenic virus.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Signaling-mediated control of ubiquitin ligases in endocytosis

Ubiquitin-dependent regulation of endocytosis plays an important part in the control of signal transduction, and a critical issue in the understanding of signal transduction therefore relates to regulation of ubiquitination in the endocytic pathway. We discuss here what is known of the mechanisms by which signaling controls the activity of the ubiquitin ligases that specifically recognize the targets of ubiquitination on the endocytic pathway, and suggest alternative mechanisms that deserve experimental investigation.

PTK6 inhibits down-regulation of EGF receptor through phosphorylation of ARAP1

PTK6 (also known as Brk) is a non-receptor-tyrosine kinase containing SH3, SH2, and catalytic domains, that is expressed in more than 60% of breast carcinomas but not in normal mammary tissues. To analyze PTK6-interacting proteins, we have expressed Flag-tagged PTK6 in HEK293 cells and performed co-immunoprecipitation assays with Flag antibody-conjugated agarose. A 164-kDa protein in the precipitated fraction was identified as ARAP1 (also known as centaurin delta-2) by MALDI-TOF mass analysis. ARAP1 associated with PTK6 in an EGF/EGF receptor (EGFR)-dependent manner. In addition, the SH2 domain of PTK6, particularly the Arg(105) residue that contacts the phosphate group of the tyrosine residue, was essential for the association. Moreover, PTK6 phosphorylated residue Tyr(231) in the N-terminal domain of ARAP1. Expression of ARAP1, but not of the Y231F mutant, inhibited the down-regulation of EGFR in HEK293 cells expressing PTK6. Silencing of endogenous PTK6 expression in breast carcinoma cells decreased EGFR levels. These results demonstrate that PTK6 enhances EGFR signaling by inhibition of EGFR down-regulation through phosphorylation of ARAP1 in breast cancer cells.

PI3K/Akt-sensitive MEK-independent compensatory circuit of ERK activation in ER-positive PI3K-mutant T47D breast cancer cells

We explored the crosstalk between cell survival (phosphatidylinositol 3-kinase (PI3K)/Akt) and mitogenic (Ras/Raf/MEK/extracellular signal-regulated kinase (ERK)) signaling pathways activated by an epidermal growth factor (EGF) and analyzed their sensitivity to small molecule inhibitors in the PI3K-mutant estrogen receptor (ER)-positive MCF7 and T47D breast cancer cells. In contrast to MCF7 cells, ERK phosphorylation in T47D cells displayed resistance to MEK inhibition by several structurally different compounds, such as U0126, PD 098059 and PD 198306, MEK suppression by small interfering RNA (siRNA) and was also less sensitive to PI3K inhibition by wortmannin. Similar effect was observed in PI3K-wild type ER-positive BT-474 cells, albeit to a much lesser extent. MEK-independent ERK activation was induced only by ErbB receptor ligands and was resistant to inhibition of several kinases and phosphatases that are known to participate in the regulation of Ras/mitogen-activated protein kinase (MAPK) cascade. Although single agents against PDK1 or Akt did not affect EGF-induced ERK phosphorylation, a combination of PI3K/Akt and MEK inhibitors synergistically suppressed ERK activation and cellular growth. siRNA-mediated silencing of class I PI3K or Akt1/2 genes also significantly decreased U0126-resistant ERK phosphorylation. Our data suggest that in T47D cells ErbB family ligands induce a dynamic, PI3K/Akt-sensitive and MEK-independent compensatory ERK activation circuit that is absent in MCF7 cells. We discuss candidate proteins that can be involved in this activation circuitry and suggest that PDZ-Binding Kinase/T-LAK Cell-Originated Protein Kinase (PBK/TOPK) may play a role in mediating MEK-independent ERK activation.

c-Cbl acts as a mediator of Src-induced activation of the PI3K-Akt signal transduction pathway during TRAIL treatment

We have previously observed that TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) induces acquired TRAIL resistance by increasing Akt phosphorylation and Bcl-xL expression. In this study, we report that Src, c-Cbl, and PI3K are involved in the phosphorylation of Akt during TRAIL treatment. Data from immunoprecipitation and immunoblotting assay reveal that Src interacts with c-Cbl and PI3K. Data from immune complex kinase assay demonstrate that Src can directly phosphorylate c-Cbl and PI3K p85 subunit protein. Data from gene knockdown experiments with an RNA interference (RNAi) technique show that c-Cbl is involved in the interaction between Src and PI3K p85 during TRAIL treatment, playing an important role in TRAIL-induced Akt phosphorylation. Taken together, c-Cbl may act as a mediator to regulate the Src-PI3K-Akt signal transduction pathway during TRAIL treatment.

Involvement of Src tyrosine kinase in Escherichia coli invasion of human brain microvascular endothelial cells

Invasion of brain microvascular endothelial cells is a prerequisite for successful crossing of the blood-brain barrier by Escherichia coli (E. coli), but the underlying mechanism remains unclear. Here we showed activation of Src tyrosine kinase in E. coli K1 invasion of human brain microvascular endothelial cells (HBMEC). E. coli invasion of HBMEC and the E. coli-induced rearrangement of actin filaments were blocked by Src inhibitors. Overexpression of dominant-negative Src in HBMEC significantly attenuated E. coli invasion and the concomitant actin filaments rearrangement. Furthermore, E. coli K1-triggered phosphatidylinositol 3-kinase (PI3K) activation in HBMEC was effectively blocked by Src inhibitors and dominant-negative Src. These results demonstrated the involvement of Src and its interaction with PI3K in E. coli K1 invasion of HBMEC.

Negative feedback control of osteoclast formation through ubiquitin-mediated down-regulation of NFATc1

The regulation of NFATc1 expression is important for osteoclast differentiation and function. Herein, we demonstrate that macrophage-colony-stimulating factor induces NFATc1 degradation via Cbl proteins in a Src kinase-dependent manner. NFATc1 proteins are ubiquitinated and rapidly degraded during late stage osteoclastogenesis, and this degradation is mediated by Cbl-b and c-Cbl ubiquitin ligases in a Src-dependent manner. In addition, NFATc1 interacts endogenously with c-Src, c-Cbl, and Cbl-b in osteoclasts. Overexpression of c-Src induces down-regulation of NFATc1, and depletion of Cbl proteins blocks NFATc1 degradation during late stage osteoclastogenesis. Taken together, our data provide a negative regulatory mechanism by which macrophage-colony-stimulating factor activates Src family kinases and Cbl proteins, and subsequently, induces NFATc1 degradation during osteoclast differentiation.

Low-power laser irradiation promotes cell proliferation by activating PI3K/Akt pathway

Low-power laser irradiation (LPLI) can stimulate cell proliferation through a wide network of signals. Akt is an important protein kinase in modulating cell proliferation. In this study, using real-time single-cell analysis, we investigated the activity of Akt and its effects on cell proliferation induced by LPLI in African green monkey SV40-transformed kidney fibroblast cells (COS-7). We utilized a recombinant fluorescence resonance energy transfer (FRET) Akt probe (BKAR) to dynamically detect the activation of Akt after LPLI treatment. Our results show that LPLI induced a gradual and continuous activation of Akt. Moreover, the activation of Akt can be completely abolished by wortmannin, a specific inhibitor of PI3K, suggesting that the activation of Akt caused by LPLI is a PI3K-dependent event. Src family is involved in Akt activation as demonstrated by the part inhibition of Akt activity in samples treated with PP1 (an inhibitor of Src family). In contrast, loading Gö 6983, a PKC inhibitor, did not affect this response. Further experiments performed using GFP-Akt fluorescence imaging and Western blot analysis demonstrate that, the activation of Akt is a multi-step process in response to LPLI, involving membrane recruitment, phosphorylation, and membrane detachment. LPLI promotes cell proliferation through PI3K/Akt activation since the cell viability was significantly inhibited by PI3K inhibitor. All these studies create a concernful conclusion that PI3K/Akt signaling pathway is well involved in LPLI triggered cell proliferation that acts as a time- and dose-dependent manner. J. Cell. Physiol. 219: 553-562, 2009. (c) 2009 Wiley-Liss, Inc.

Dasatinib-induced autophagy is enhanced in combination with temozolomide in glioma

Glioblastoma is defined by its aggressive invasion, microvascular proliferation, and central necrosis. BMS-354825 (dasatinib) is an ATP-competitive small-molecule inhibitor effective in treating drug-resistant tumors with mutant BCR-ABL, KIT, and epidermal growth factor receptor by blocking tyrosine phosphorylation sites that are critical in tumorigenesis. In studying the action of dasatinib in human glioblastoma, we found that levels of phospho-SRC, AKT, and ribosomal protein S6 were decreased in cell lines treated with low nanomolar concentrations of dasatinib at baseline and following stimulation with epidermal growth factor. Furthermore, an increased sensitivity to dasatinib was noted in glioma cells with functional PTEN. Reduction of invasive potential was observed in vitro at concentrations well below the IC(50) of dasatinib, which was corroborated by immunofluorescence staining showing disruption of paxillin localization to focal adhesions and decreases in focal adhesion kinase autophosphorylation. Cell cycle analysis revealed minimal G(1) arrest but a significant increase in autophagic cell death in glioma cells treated with dasatinib as assessed by acridine orange staining and a concomitant increase in light chain 3 expression and processing. Combination treatment of glioma cells with dasatinib and temozolomide resulted in a significant increase in cell cycle disruption and autophagic cell death. Dasatinib in combination with temozolomide more effectively increased the therapeutic efficacy of temozolomide than when dasatinib was combined with carboplatin or irinotecan. These results strongly support the clinical use of dasatinib in the treatment of glioblastoma and provide a rationale for combination therapy with dasatinib and temozolomide.

Identification of phosphorylation sites of TOPORS and a role for serine 98 in the regulation of ubiquitin but not SUMO E3 ligase activity

TOPORS is the first example of a protein that possesses both ubiquitin and SUMO E3 ligase activity. The ubiquitination activity maps to a conserved RING domain in the N-terminal region of the protein, which is not required for sumoylation activity. Similar to other E3 ligases, it is likely that the ubiquitin and sumoylation activities of TOPORS are regulated by post-translational modifications. Therefore, we employed mass spectrometry to identify post-translational modifications of TOPORS. Several putative phosphorylated regions were identified in conserved regions of the protein. We investigated the role of phosphorylation of serine 98, which is adjacent to the RING domain, in both cells and in vitro. Mutation of serine 98 to aspartic acid resulted in an increase in the ubiquitin ligase activity of TOPORS both in cells and in vitro. In addition, this mutation increased the binding of TOPORS to the E2 enzyme UbcH5a both in vitro and in cells. Conversely, a phospho-deficient mutant (S98A) exhibited little change in ubiquitin ligase activity compared to wild-type TOPORS, both in cells and in vitro. Neither of the mutants affected the localization of TOPORS to punctate nuclear regions. In addition, neither mutant affected the SUMO ligase activity of TOPORS in cells or in vitro. Molecular modeling studies support a role for serine 98 in regulating TOPORS-E2 interactions. Our findings indicate that phosphorylation of serine 98 regulates the ubiquitin but not the SUMO ligase activity of TOPORS, consistent with a potential binary switch function for TOPORS in protein ubiquitination versus sumoylation.

Systems biology of growth factor-induced receptor endocytosis

Clathrin-mediated endocytosis sorts for degradation of more than 50 different growth factor receptors capable of relaying growth and differentiation signals by means of their cytoplasm-facing, intrinsic tyrosine kinase activity. The kinetics and alternative routings of receptor endocytosis critically regulate growth factor signaling, which underscores the importance of understanding mechanisms underlying fail-safe operation (robustness) and fidelity of the pathway. Like other robust systems, a layered hub-centric network controls receptor endocytosis. Characteristically, the modular hubs (e.g., AP2-Eps15 and Hrs) contain a membrane-anchoring lipid-binding domain, an ubiquitin-binding module, which recruits ubiquitinylated cargo, and a machinery enabling homo-assembly. Scheduled hub transitions, as well as cascades of Rab family guanosine triphosphatases and membrane bending machineries, define points of commitment to vesicle budding, thereby securing unidirectional trafficking. System's bistability permits stimulation by a growth factor, which oscillates a series of switches based on posttranslational protein modifications (i.e., phosphorylation, ubiquitinylation and neddylation), as well as transient low-affinity/high-avidity protein assemblies. Cbl family ubiquitin ligases, along with a set of phosphotyrosine-binding adaptors (e.g., Grb2), integrate receptor endocytosis into the densely wired networks of signal transduction pathways, which are involved in health and disease.

A tale of two Cbls: interplay of c-Cbl and Cbl-b in epidermal growth factor receptor downregulation

The precise role of Cbl in epidermal growth factor (EGF) receptor (EGFR) endocytosis and trafficking remains to be fully uncovered. Here, we showed that mutant EGFR1044, which was truncated after residue 1044, did not associate with c-Cbl and was not ubiquitinated initially in response to EGF but was internalized with kinetics similar to those of wild-type EGFR. This finding indicates that c-Cbl-mediated ubiquitination is not required for EGF-induced EGFR endocytosis. We also showed that the previously identified internalization-deficient mutant receptor EGFR1010LL/AA bound to c-Cbl and was fully ubiquitinated in response to EGF, which indicates that c-Cbl binding and ubiquitination are not sufficient for EGFR internalization. We next investigated EGFR trafficking following EGFR internalization. We found that c-Cbl disassociation from EGFR occurred well in advance of EGFR degradation and that this event was concurrent with the selective dephosphorylation of EGFR at Y1045. This finding suggests that once EGFR is ubiquitinated, continual Cbl association is not required for EGFR degradation. Because EGFR1044 is ubiquitinated and degraded similarly to wild-type EGFR, we examined the role of another prominent Cbl homologue, Cbl-b, and found that Cbl-b was associated with both EGFR and EGFR1044. Further study showed that Cbl-b bound to EGFR at two regions: one in the C-terminal direction from residue 1044 and one in the N-terminal direction from residue 958. Moreover, Cbl-b association with EGFR rose markedly following a decrease in c-Cbl association, corresponding to a second peak of EGFR ubiquitination occurring later in EGFR trafficking. Using RNA interference to knock down both c-Cbl and Cbl-b, we were able to abolish EGFR downregulation. This knockdown had no affect on the rate of EGF-induced EGFR internalization. We found that the two Cbls accounted for total receptor ubiquitination and that while c-Cbl and Cbl-b are each alone sufficient to effect EGFR degradation, both are involved in the physiological, EGF-mediated process of receptor downregulation. Furthermore, these data ultimately reveal a previously unacknowledged temporal interplay of two major Cbl homologues with the trafficking of EGFR.

Emergent decision-making in biological signal transduction networks

The complexity of biochemical intracellular signal transduction networks has led to speculation that the high degree of interconnectivity that exists in these networks transforms them into an information processing network. To test this hypothesis directly, a large scale model was created with the logical mechanism of each node described completely to allow simulation and dynamical analysis. Exposing the network to tens of thousands of random combinations of inputs and analyzing the combined dynamics of multiple outputs revealed a robust system capable of clustering widely varying input combinations into equivalence classes of biologically relevant cellular responses. This capability was nontrivial in that the network performed sharp, nonfuzzy classifications even in the face of added noise, a hallmark of real-world decision-making.

Ligand binding induces Cbl-dependent EphB1 receptor degradation through the lysosomal pathway

Eph receptor tyrosine kinases play a critical role in embryonic patterning and angiogenesis. In the adult, they are involved in carcinogenesis and pathological neovascularization. However, the mechanisms underlying their role in tumor formation and metastasis remain to be defined. Here, we demonstrated that stimulation of EphB1 with ephrinB1/Fc led to a marked downregulation of EphB1 protein, a process blocked by the lysosomal inhibitor bafilomycin. Following ephrinB1 stimulation, the ubiquitin ligase Cbl was recruited by EphB1 and then phosphorylated. Both Cbl phosphorylation and EphB1 ubiquitination were blocked by the Src inhibitor PP2. Overexpression of wild-type Cbl, but not of 70Z mutant lacking ligase activity, enhanced EphB1 ubiquitination and degradation. This negative regulation required the tyrosine kinase activity of EphB1 as kinase-dead EphB1-K652R was resistant to Cbl. Glutathione S-transferase binding experiments showed that Cbl bound to EphB1 through its tyrosine kinase-binding domain. In aggregate, we demonstrated that Cbl induces the ubiquitination and lysosomal degradation of activated EphB1, a process requiring EphB1 and Src kinase activity. To our knowledge, this is the first study dissecting the molecular mechanisms leading to EphB1 downregulation, thus paving the way to new means of modulating their angiogenic and tumorigenic properties.

Gab2 and Src co-operate in human mammary epithelial cells to promote growth factor independence and disruption of acinar morphogenesis

The Gab2 docking protein is a target of several oncogenic protein tyrosine kinases and potentiates activation of the Ras/extracellular signal regulated kinase and phosphatidylinositol 3-kinase (PI3-kinase) pathways. Since Gab2 is phosphorylated by c-Src, and both proteins are overexpressed in breast cancers, we have determined the biological consequences of their co-expression in the immortalized human mammary epithelial cell line MCF-10A. While overexpression of c-Src did not affect acinar morphogenesis or growth factor dependence in three-dimensional culture, c-Src co-operated with Gab2 to promote epidermal growth factor (EGF)-independent acinar growth. In contrast, expression of v-Src or the activated mutant c-SrcY527F led to a spectrum of aberrant phenotypes ranging from spheroids with incomplete luminal clearance to highly disrupted, dispersed structures. Gab2 co-expression shifted the phenotypic distribution towards the dispersed phenotype, an effect not observed with a Gab2 mutant unable to bind the p85 subunit of PI3-kinase (Gab2Deltap85). In v-Src-expressing cells, Gab2, but not Gab2Deltap85, significantly decreased E-cadherin adhesive strength without altering its surface expression. Gab2 associated with E-cadherin in the presence and absence of v-Src, indicating that the ability of Gab2 to weaken the strength of cell-cell contacts may reflect enhanced activation of PI3-kinase at adherens junctions. Gab2 also increased migration and invasion of these cells in transwell assays, but these effects were p85-independent. Overall, these findings demonstrate a novel mechanism whereby Gab2 may promote metastatic spread and indicate that Gab2 may play several roles during breast cancer progression.

Neural cell adhesion molecule-180-mediated homophilic binding induces epidermal growth factor receptor (EGFR) down-regulation and uncouples the inhibitory function of EGFR in neurite outgrowth

The neural cell adhesion molecule (NCAM) plays important roles in neuronal development, regeneration, and synaptic plasticity. NCAM homophilic binding mediates cell adhesion and induces intracellular signals, in which the fibroblast growth factor receptor plays a prominent role. Recent studies on axon guidance in Drosophila suggest that NCAM also regulates the epidermal growth factor receptor (EGFR) (Molecular and Cellular Neuroscience, 28, 2005, 141). A possible interaction between NCAM and EGFR in mammalian cells has not been investigated. The present study demonstrates for the first time a functional interaction between NCAM and EGFR in mammalian cells and investigates the molecular mechanisms underlying this interaction. First, NCAM and EGFR are shown to play opposite roles in neurite outgrowth regulation in cerebellar granular neurons. The data presented indicate that negative regulation of EGFR is one of the mechanisms underlying the neuritogenic effect of NCAM. Second, it is demonstrated that expression of the NCAM-180 isoform induces EGFR down-regulation in transfected cells and promotes EGFR down-regulation induced by EGF stimulation. It is demonstrated that the mechanism underlying this NCAM-180-induced EGFR down-regulation involves increased EGFR ubiquitination and lysosomal EGFR degradation. Furthermore, NCAM-180-mediated EGFR down-regulation requires NCAM homophilic binding and interactions of the cytoplasmic domain of NCAM-180 with intracellular interaction partners, but does not require NCAM-mediated fibroblast growth factor receptor activation.

Mind bomb-2 is an E3 ligase that ubiquitinates the N-methyl-D-aspartate receptor NR2B subunit in a phosphorylation-dependent manner

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic plasticity. Post-translational modifications of NMDARs, such as phosphorylation, alter both the activity and trafficking properties of NMDARs. Ubiquitination is increasingly being recognized as another post-translational modification that can alter synaptic protein composition and function. We identified Mind bomb-2 as an E3 ubiquitin ligase that interacts with and ubiquitinates the NR2B subunit of the NMDAR in mammalian cells. The protein-protein interaction and the ubiquitination of the NR2B subunit were found to be enhanced in a Fyn phosphorylation-dependent manner. Immunocytochemical studies reveal that Mind bomb-2 is localized to postsynaptic sites and colocalizes with the NMDAR in apical dendrites of hippocampal neurons. Furthermore, we show that NMDAR activity is down-regulated by Mind bomb-2. These results identify a specific E3 ubiquitin ligase as a novel interactant with the NR2B subunit and suggest a possible mechanism for the regulation of NMDAR function involving both phosphorylation and ubiquitination.

EGFR and beta1 integrins utilize different signaling pathways to activate Akt

Akt, also called PKB, is a serine/threonine kinase that plays a major role in cell survival. It can be activated by several cellular receptors, including integrins and growth factor receptors, in PI3K-dependent manners. In this study, we analyzed the two current models for Akt activation upon beta1 integrin-mediated adhesion: via focal adhesion kinase and via transactivation of the EGF receptor. Distinct differences in the pathways leading to phosphorylation and activation of Akt from stimulated beta1 integrins and EGF receptor were observed, including opposing sensitivity to the tyrosine kinase inhibitors PP2 and Gefitinib. Using knockout cells and integrin mutant cells, we show that beta1 integrins can induce phosphorylation of Akt at Ser473 and Thr308 and Akt kinase activity independently of the EGF receptor activity, focal adhesion kinase, and the Src family members. In contrast to stimulation with EGF, beta1 integrin-mediated adhesion did not induce Akt tyrosine phosphorylation. Moreover, tyrosine phosphorylation of Akt was found not to be required for its catalytic activity. The results identify a previously unrecognized mechanism by which beta1 integrins activate the PI3K/Akt pathway.

Regulation of the Pro-apoptotic Scaffolding Protein POSH by Akt

POSH (Plenty of SH3 domains) binds to activated Rac and promotes apoptosis by acting as a scaffold to assemble a signal transduction pathway leading from Rac to JNK activation. Overexpression of POSH induces apoptosis in a variety of cell types, but apoptosis can be prevented by co-expressing the pro-survival protein kinase Akt. We report here that POSH is a direct substrate for phosphorylation by Akt in vivo and in vitro, and we identify a major site of Akt phosphorylation as serine 304 of POSH, which lies within the Rac-binding domain. We further show that phosphorylation of POSH results in a decreased ability to bind activated Rac, as does phosphomimetic S304D and S304E mutation of POSH. S304D mutant POSH also shows a strongly reduced ability to induce apoptosis. These findings identify a novel mechanism by which Akt promotes cell survival.

Signal transduction pathways for epidermal growth factor stimulated cyclooxygenase-2 induction in astrocytes

Cyclooxygenase-2 (COX-2) derived prostaglandins (PGs) are pathophysiological mediators in various disease states. Recently, we have demonstrated the rapid, epidermal growth factor receptor (EGFR)-dependent induction of COX-2 and PGE(2) synthesis in astrocytes following optic nerve injury and in culture. We have now investigated the signal transduction pathways activated by EGFR to accomplish the expression of COX-2 in primary optic nerve astrocytes. When astrocytes were exposed to EGF, marked, rapid gene expression of COX-2 was observed. Activation of EGFR caused an increase in the phosphorylation of extracellular signal-regulated kinase (ERK), p38 MAPK (p38) and c-Jun NH (2)-terminal kinase (JNK). Furthermore, U0126, an ERK pathway inhibitor, and SB203580, a p38 MAPK inhibitor, diminished EGF-induced COX-2 expression; whereas, a JNK inhibitor did not suppress COX-2 expression by EGF. Using inhibitors of several other signaling cascades, we found that, unlike epithelial and cancer cells, NF-kappaB, PI 3-kinase/Akt and PKC were not signaling pathways for EGFR-dependent induction of COX-2 in optic nerve astrocytes. Taken together, these data suggest that ERK and p38 are key components of the intracellular signaling switch that transduces EGFR activation into COX-2 induction and PGE(2) biosynthesis in optic nerve astrocytes.

Signal transduction pathways that contribute to myeloid differentiation

The production of mature, differentiated myeloid cells is regulated by the action of hematopoietic cytokines on progenitor cells in the bone marrow. Cytokines drive the process of myeloid differentiation by binding to specific cell-surface receptors in a stage- and lineage-specific manner. Following the binding of a cytokine to its cognate receptor, intracellular signal-transduction pathways become activated that facilitate the myeloid differentiation process. These intracellular signaling pathways may promote myelopoiesis by stimulating expansion of a progenitor pool, supporting cellular survival during the differentiation process, or by directly driving the phenotypic changes associated with differentiation. Ultimately, pathways that drive the differentiation process converge on myeloid transcription factors, including PU.1 and the C/EBP family, that are critical for differentiation to proceed. While much is known about the cytokines, cytokine receptors and transcription factors that regulate myeloid differentiation, less is known about the precise roles that specific signaling mediators play in promoting myeloid differentiation. Recently, however, the application of novel pharmacologic inhibitors, siRNA strategies, and transgenic and knockout models has begun to shed light on the involvement and function of signaling pathways in normal myeloid differentiation. This review will discuss the roles that key signaling pathways and mediators play in myeloid differentiation.