PDGF and FGF induce focal adhesion kinase (FAK) phosphorylation at Ser-910: Dissociation from Tyr-397 phosphorylation and requirement for ERK activation

Fibronectin fragments generated by pancreatic trypsin act as endogenous inhibitors of pancreatic tumor growth

BACKGROUND

The pancreatic microenvironment has a defensive role against cancer but it can acquire tumor-promoting properties triggered by multiple mechanisms including alterations in the equilibrium between proteases and their inhibitors. The identification of proteolytic events, targets and pathways would set the basis for the design of new therapeutic approaches.

METHODS AND RESULTS

Here we demonstrate that spheroids isolated from human and murine healthy pancreas and co-transplanted orthotopically with pancreatic ductal adenocarcinoma (PDAC) in mouse pancreas inhibited tumor growth. The effect was mediated by trypsin-generated fibronectin (FN) fragments released by pancreatic spheroids. Tumor inhibition was observed also in a model of acute pancreatitis associated with trypsin activation. Mass spectrometry proteomic analysis of fragments and mAb against different FN epitopes identified the FN type III domain as responsible for the activity. By inhibiting integrin α5β1, FAK and FGFR1 signaling, the fragments induced tumor cell detachment and reduced cell proliferation. Consistent with the mutual relationship between the two pathways, FGF2 restored both FGFR1 and FAK signaling and promoted PDAC cell adhesion and proliferation. FAK and FGFR inhibitors additively inhibited PDAC growth in vitro and in orthotopic in vivo models.

CONCLUSIONS

This study identifies a novel role for pancreatic trypsin and fibronectin cleavage as a mechanism of protection against cancer by the pancreatic microenvironment. The finding of a FAK-FGFR cross-talk in PDAC support the combination of FAK and FGFR inhibitors for PDAC treatment to emulate the protective effect of the normal pancreas against cancer.

The enigmatic roles of epithelial gasdermin B: Recent discoveries and controversies

Gasdermin B (GSDMB) belongs to a family of structurally related proteins [(i.e., gasdermins (GSDMs)]. It distinguishes itself from other members by the lack of autoinhibition but clear bioactivity of its full-length form, its preference to bind to phosphatidylinositol phosphates and sulfatides, and the ability to promote both lytic and nonlytic cellular functions. It is the only gasdermin that lacks a mouse ortholog, making in vivo mechanistic studies challenging to perform. GSDMB is abundantly expressed in epithelial cells lining organs that directly interface with the external environment, such as the gastrointestinal tract, with emerging evidence supporting its role in enteric infections, inflammatory bowel disease (IBD), and colorectal cancer. This review discusses the unique features of GSDMB among other gasdermin family members and controversies surrounding GSDMB-dependent mammalian inflammatory cell death (i.e., pyroptosis), including recent discoveries revealing both lytic and nonlytic functions of epithelial-derived GSDMB, particularly during gut health and disease.

Perlecan: Roles in osteoarthritis and potential treating target

Osteoarthritis (OA) is the most common joint disease, affecting hundreds of millions of people globally, which leads to a high cost of treatment and further medical care and an apparent decrease in patient prognosis. The recent view of OA pathogenesis is that increased vascularity, bone remodeling, and disordered turnover are influenced by multivariate risk factors, such as age, obesity, and overloading. The view also reveals the gap between the development of these processes and early stage risk factors. This review presents the latest research on OA-related signaling pathways and analyzes the potential roles of perlecan, a typical component of the well-known protective structure against osteoarthritic pericellular matrix (PCM). Based on the experimental results observed in end-stage OA models, we summarized and analyzed the role of perlecan in the development of OA. In normal cartilage, it plays a protective role by maintaining the integrin of PCM and sequesters growth factors. Second, perlecan in cartilage is required to not only activate vascular epithelium growth factor receptor (VEGFR) signaling of endothelial cells for vascular invasion and catabolic autophagy, but also for different signaling pathways for the catabolic and anabolic actions of chondrocytes. Finally, perlecan may participate in pain sensitization pathways.

Regulation of FGF-2, FGF-18 and Transcription Factor Activity by Perlecan in the Maturational Development of Transitional Rudiment and Growth Plate Cartilages and in the Maintenance of Permanent Cartilage Homeostasis

The aim of this study was to highlight the roles of perlecan in the regulation of the development of the rudiment developmental cartilages and growth plate cartilages, and also to show how perlecan maintains permanent articular cartilage homeostasis. Cartilage rudiments are transient developmental templates containing chondroprogenitor cells that undergo proliferation, matrix deposition, and hypertrophic differentiation. Growth plate cartilage also undergoes similar changes leading to endochondral bone formation, whereas permanent cartilage is maintained as an articular structure and does not undergo maturational changes. Pericellular and extracellular perlecan-HS chains interact with growth factors, morphogens, structural matrix glycoproteins, proteases, and inhibitors to promote matrix stabilization and cellular proliferation, ECM remodelling, and tissue expansion. Perlecan has mechanotransductive roles in cartilage that modulate chondrocyte responses in weight-bearing environments. Nuclear perlecan may modulate chromatin structure and transcription factor access to DNA and gene regulation. Snail-1, a mesenchymal marker and transcription factor, signals through FGFR-3 to promote chondrogenesis and maintain Acan and type II collagen levels in articular cartilage, but prevents further tissue expansion. Pre-hypertrophic growth plate chondrocytes also express high Snail-1 levels, leading to cessation of Acan and CoI2A1 synthesis and appearance of type X collagen. Perlecan differentially regulates FGF-2 and FGF-18 to maintain articular cartilage homeostasis, rudiment and growth plate cartilage growth, and maturational changes including mineralization, contributing to skeletal growth.

GSDMB is increased in IBD and regulates epithelial restitution/repair independent of pyroptosis

Gasdermins are a family of structurally related proteins originally described for their role in pyroptosis. Gasdermin B (GSDMB) is currently the least studied, and while its association with genetic susceptibility to chronic mucosal inflammatory disorders is well established, little is known about its functional relevance during active disease states. Herein, we report increased GSDMB in inflammatory bowel disease, with single-cell analysis identifying epithelial specificity to inflamed colonocytes/crypt top colonocytes. Surprisingly, mechanistic experiments and transcriptome profiling reveal lack of inherent GSDMB-dependent pyroptosis in activated epithelial cells and organoids but instead point to increased proliferation and migration during in vitro wound closure, which arrests in GSDMB-deficient cells that display hyper-adhesiveness and enhanced formation of vinculin-based focal adhesions dependent on PDGF-A-mediated FAK phosphorylation. Importantly, carriage of disease-associated GSDMB SNPs confers functional defects, disrupting epithelial restitution/repair, which, altogether, establishes GSDMB as a critical factor for restoration of epithelial barrier function and the resolution of inflammation.

Metformin inhibition of colorectal cancer cell migration is associated with rebuilt adherens junctions and FAK downregulation

E-cadherin, a central component of the adherens junction (AJ), is a single-pass transmembrane protein that mediates cell-cell adhesion. The loss of E-cadherin surface expression, and therefore cell-cell adhesion, leads to increased cell migration and invasion. Treatment of colorectal cancer (CRC)-derived cells (SW-480 and HT-29) with 2.0 mM metformin promoted a redistribution of cytosolic E-cadherin to de novo formed puncta along the length of the contacting membranes of these cells. Metformin also promoted translocation from the cytosol to the plasma membrane of p120-catenin, another core component of the AJs. Furthermore, E-cadherin and p120-catenin colocalized with β-catenin at cell-cell contacts. Western blot analysis of lysates of CRC-derived cells revealed a substantial metformin-induced increase in the level of p120-catenin as well as E-cadherin phosphorylation on Ser838/840 , a modification associated with β-catenin/E-cadherin interaction. These modifications in E-cadherin, p120-catenin and β-catenin localization suggest that metformin induces rebuilding of AJs in CRC-derived cells. Those modifications were accompanied by the inhibition of focal adhesion kinase (FAK), as revealed by a significant decrease in the phosphorylation of FAK at Tyr397 and paxillin at Tyr118 . These changes were associated with a reduction in the numbers, but an increase in the size, of focal adhesions and by the inhibition of cell migration. Overall, these observations indicate that metformin targets multiple pathways associated with CRC development and progression.

Intratumoral generation of photothermal gold nanoparticles through a vectorized biomineralization of ionic gold

Various cancer cells have been demonstrated to have the capacity to form plasmonic gold nanoparticles when chloroauric acid is introduced to their cellular microenvironment. But their biomedical applications are limited, particularly considering the millimolar concentrations and longer incubation period of ionic gold. Here, we describe a simplistic method of intracellular biomineralization to produce plasmonic gold nanoparticles at micromolar concentrations within 30 min of application utilizing polyethylene glycol as delivery vector for ionic gold. We have characterized this process for intracellular gold nanoparticle formation, which progressively accumulates proteins as the ionic gold clusters migrate to the nucleus. This nano-vectorized application of ionic gold emphasizes its potential biomedical opportunities while reducing the quantity of ionic gold and required incubation time. To demonstrate its biomedical potential, we further induce in-situ biosynthesis of gold nanoparticles within MCF7 tumor mouse xenografts which is followed by its photothermal remediation.

Extracellular signal regulated kinase 5 promotes cell migration, invasion and lung metastasis in a FAK-dependent manner

This study was designed to evaluate ERK5 expression in lung cancer and malignant melanoma progression and to ascertain the involvement of ERK5 signaling in lung cancer and melanoma. We show that ERK5 expression is abundant in human lung cancer samples, and elevated ERK5 expression in lung cancer was linked to the acquisition of increased metastatic and invasive potential. Importantly, we observed a significant correlation between ERK5 activity and FAK expression and its phosphorylation at the Ser⁹¹⁰ site. Mechanistically, ERK5 increased the expression of the transcription factor USF1, which could transcriptionally upregulate FAK expression, resulting in FAK signaling activation to promote cell migration. We also provided evidence that the phosphorylation of FAK at Ser⁹¹⁰ was due to ERK5 but not ERK1/2, and we then suggested a role for Ser⁹¹⁰ in the control of cell motility. In addition, ERK5 had targets in addition to FAK that regulate epithelial-to-mesenchymal transition and cell motility in cancer cells. Taken together, our findings uncover a cancer metastasis-promoting role for ERK5 and provide the rationale for targeting ERK5 as a potential therapeutic approach.

FGF Trapping Inhibits Multiple Myeloma Growth through c-Myc Degradation-Induced Mitochondrial Oxidative Stress

Multiple myeloma, the second most common hematologic malignancy, frequently relapses because of chemotherapeutic resistance. Fibroblast growth factors (FGF) act as proangiogenic and mitogenic cytokines in multiple myeloma. Here, we demonstrate that the autocrine FGF/FGFR axis is essential for multiple myeloma cell survival and progression by protecting multiple myeloma cells from oxidative stress-induced apoptosis. In keeping with the hypothesis that the intracellular redox status can be a target for cancer therapy, FGF/FGFR blockade by FGF trapping or tyrosine kinase inhibitor impaired the growth and dissemination of multiple myeloma cells by inducing mitochondrial oxidative stress, DNA damage, and apoptotic cell death that were prevented by the antioxidant vitamin E or mitochondrial catalase overexpression. In addition, mitochondrial oxidative stress occurred as a consequence of proteasomal degradation of the c-Myc oncoprotein that led to glutathione depletion. Accordingly, expression of a proteasome-nondegradable c-Myc protein mutant was sufficient to avoid glutathione depletion and rescue the proapoptotic effects due to FGF blockade. These findings were confirmed on bortezomib-resistant multiple myeloma cells as well as on bone marrow-derived primary multiple myeloma cells from newly diagnosed and relapsed/refractory patients, including plasma cells bearing the t(4;14) translocation obtained from patients with high-risk multiple myeloma. Altogether, these findings dissect the mechanism by which the FGF/FGFR system plays a nonredundant role in multiple myeloma cell survival and disease progression, and indicate that FGF targeting may represent a therapeutic approach for patients with multiple myeloma with poor prognosis and advanced disease stage. SIGNIFICANCE: This study provides new insights into the mechanisms by which FGF antagonists promote multiple myeloma cell death. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/11/2340/F1.large.jpg.

MMP Inhibition Preserves Integrin Ligation and FAK Activation to Induce Survival and Regeneration in RGCs Following Optic Nerve Damage

Purpose

Integrin adherence to the extracellular matrix (ECM) is essential for retinal ganglion cell (RGC) survival: damage causes production and release of ECM degrading matrix metalloproteinases (MMPs) that disrupt integrin ligation, leading to RGC death. The interplay of MMPs, integrins, and focal adhesion kinase (FAK) was studied in RGCs after optic nerve injury.

Methods

Optic nerve transection and optic nerve crush were used to study RGC survival and regeneration, respectively. Treatments were administered intravitreally or into the cut end of the optic nerve. RGC survival was assessed by fluorescence or confocal microscopy; cell counting, peptide levels, and localization were assessed by Western blot and immunohistochemistry.

Results

MMP-9 was most strongly increased and localized to RGCs after injury. Pan-MMP, MMP-2/-9, and MMP-3 inhibition all significantly enhanced RGC survival and increased RGC axon regeneration. FAK activation was decreased at 4 days postaxotomy, when apoptosis begins. FAK inhibition reduced RGC survival and abrogated the neuroprotective effects of MMP inhibition, whereas FAK activation increased RGC survival despite MMP activation. Integrin ligation with CD29 antibody or glycine-arginine-glycine-aspatate-serine (GRGDS) peptide increased RGC survival after axotomy.

Conclusions

ECM-integrin ligation promotes RGC survival and axon regeneration via FAK activation.

Extracellular-Regulated Kinases: Signaling From Ras to ERK Substrates to Control Biological Outcomes

The extracellular-regulated kinases ERK1 and ERK2 are evolutionarily conserved, ubiquitous serine-threonine kinases that are involved in regulating cellular signaling in both normal and pathological conditions. Their expression is critical for development and their hyperactivation is a major factor in cancer development and progression. Since their discovery as one of the major signaling mediators activated by mitogens and Ras mutation, we have learned much about their regulation, including their activation, binding partners and substrates. In this review I will discuss some of what has been discovered about the members of the Ras to ERK pathway, including regulation of their activation by growth factors and cell adhesion pathways. Looking downstream of ERK activation I will also highlight some of the many ERK substrates that have been discovered, including those involved in feedback regulation, cell migration and cell cycle progression through the control of transcription, pre-mRNA splicing and protein synthesis.

MeHg affects the activation of FAK, Src, Rac1 and Cdc42, critical proteins for cell movement in PDGF-stimulated SH-SY5Y neuroblastoma cells

Methylmercury (MeHg) is an environmental neurotoxicant that inhibits neuronal migration. This process requires several cyclic steps involving the formation of membrane protrusions (lamellipodia and filopodia) and focal adhesion turnover. FAK and Src are critical proteins that regulate both processes. The FAK-Src complex promotes the activation of Rac1 and Cdc42, two GTPases involved in the remodeling of the actin cytoskeletal network. Here, we studied the effect of MeHg (1, 10, 100, 500 and 1000nM) on cell migration, the formation of cell protrusions, focal adhesion location and the activation of FAK, Src, Rac1 and Cdc42 using the SH-SY5Y neuroblastoma cell line stimulated with PDGF-BB (PDGF). The data show that MeHg (1-500nM) inhibited PDGF-stimulated cell migration. In PDGF-stimulated cells, MeHg (100-1000nM) decreased protrusions and increased the size of the p-FAK^Y397 clusters. MeHg also inhibited PDGF-induced FAK and Src activation and, at 100nM, MeHg inhibited the activation of Rac1 and Cdc42. Altogether, the findings show that low concentrations of MeHg inhibit SH-SY5Y cell migration by disrupting the activation and disassembly of FAK. This negatively affects the activation of Src, Rac1 and Cdc42, all of which are critical proteins for the regulation of cell movement. These effects could be related to the MeHg-mediated inhibition of PDGF-induced formation of lamellipodia and filopodia, focal adhesion disassembly and PDGF-induced movement.

Prolactin-induced PAK1 tyrosyl phosphorylation promotes FAK dephosphorylation, breast cancer cell motility, invasion and metastasis

BACKGROUND

The serine/threonine kinase PAK1 is an important regulator of cell motility. Both PAK1 and the hormone/cytokine prolactin (PRL) have been implicated in breast cancer cell motility, however, the exact mechanisms guiding PRL/PAK1 signaling in breast cancer cells have not been fully elucidated. Our lab has previously demonstrated that PRL-activated tyrosine kinase JAK2 phosphorylates PAK1 on tyrosines 153, 201, and 285, and that tyrosyl phosphorylated PAK1 (pTyr-PAK1) augments migration and invasion of breast cancer cells.

RESULTS

Here we further investigate the mechanisms by which pTyr-PAK1 enhances breast cancer cell motility in response to PRL. We demonstrate a distinct reduction in PRL-induced FAK auto-phosphorylation in T47D and TMX2-28 breast cancer cells overexpressing wild-type PAK1 (PAK1 WT) when compared to cells overexpressing either GFP or phospho-tyrosine-deficient mutant PAK1 (PAK1 Y3F). Furthermore, pTyr-PAK1 phosphorylates MEK1 on Ser298 resulting in subsequent ERK1/2 activation. PRL-induced FAK auto-phosphorylation is rescued in PAK1 WT cells by inhibiting tyrosine phosphatases and tyrosine phosphatase inhibition abrogates cell motility and invasion in response to PRL. siRNA-mediated knockdown of the tyrosine phosphatase PTP-PEST rescues FAK auto-phosphorylation in PAK1 WT cells and reduces both cell motility and invasion. Finally, we provide evidence that PRL-induced pTyr-PAK1 stimulates tumor cell metastasis in vivo.

CONCLUSION

These data provide insight into the mechanisms guiding PRL-mediated breast cancer cell motility and invasion and highlight a significant role for pTyr-PAK1 in breast cancer metastasis.

Conformational dynamics of the focal adhesion targeting domain control specific functions of focal adhesion kinase in cells

Focal adhesion (FA) kinase (FAK) regulates cell survival and motility by transducing signals from membrane receptors. The C-terminal FA targeting (FAT) domain of FAK fulfils multiple functions, including recruitment to FAs through paxillin binding. Phosphorylation of FAT on Tyr(925) facilitates FA disassembly and connects to the MAPK pathway through Grb2 association, but requires dissociation of the first helix (H1) of the four-helix bundle of FAT. We investigated the importance of H1 opening in cells by comparing the properties of FAK molecules containing wild-type or mutated FAT with impaired or facilitated H1 openings. These mutations did not alter the activation of FAK, but selectively affected its cellular functions, including self-association, Tyr(925) phosphorylation, paxillin binding, and FA targeting and turnover. Phosphorylation of Tyr(861), located between the kinase and FAT domains, was also enhanced by the mutation that opened the FAT bundle. Similarly phosphorylation of Ser(910) by ERK in response to bombesin was increased by FAT opening. Although FAK molecules with the mutation favoring FAT opening were poorly recruited at FAs, they efficiently restored FA turnover and cell shape in FAK-deficient cells. In contrast, the mutation preventing H1 opening markedly impaired FAK function. Our data support the biological importance of conformational dynamics of the FAT domain and its functional interactions with other parts of the molecule.

Heparan sulfate proteoglycans and heparin regulate melanoma cell functions

BACKGROUND

The solid melanoma tumor consists of transformed melanoma cells, and the associated stromal cells including fibroblasts, endothelial cells, immune cells, as well as, soluble macro- and micro-molecules of the extracellular matrix (ECM) forming the complex network of the tumor microenvironment. Heparan sulfate proteoglycans (HSPGs) are an important component of the melanoma tumor ECM. Importantly, there appears to be both a quantitative and a qualitative shift in the content of HSPGs, in parallel to the nevi-radial growth phase-vertical growth phase melanoma progression. Moreover, these changes in HSPG expression are correlated to modulations of key melanoma cell functions.

SCOPE OF REVIEW

This review will critically discuss the roles of HSPGs/heparin in melanoma development and progression.

MAJOR CONCLUSIONS

We have correlated HSPGs' expression and distribution with melanoma cell signaling and functions as well as angiogenesis.

GENERAL SIGNIFICANCE

The current knowledge of HSPGs/heparin biology in melanoma provides a foundation we can utilize in the ongoing search for new approaches in designing anti-tumor therapy. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Cellular kinetics of perivascular MSC precursors

Mesenchymal stem/stromal cells (MSCs) and MSC-like multipotent stem/progenitor cells have been widely investigated for regenerative medicine and deemed promising in clinical applications. In order to further improve MSC-based stem cell therapeutics, it is important to understand the cellular kinetics and functional roles of MSCs in the dynamic regenerative processes. However, due to the heterogeneous nature of typical MSC cultures, their native identity and anatomical localization in the body have remained unclear, making it difficult to decipher the existence of distinct cell subsets within the MSC entity. Recent studies have shown that several blood-vessel-derived precursor cell populations, purified by flow cytometry from multiple human organs, give rise to bona fide MSCs, suggesting that the vasculature serves as a systemic reservoir of MSC-like stem/progenitor cells. Using individually purified MSC-like precursor cell subsets, we and other researchers have been able to investigate the differential phenotypes and regenerative capacities of these contributing cellular constituents in the MSC pool. In this review, we will discuss the identification and characterization of perivascular MSC precursors, including pericytes and adventitial cells, and focus on their cellular kinetics: cell adhesion, migration, engraftment, homing, and intercellular cross-talk during tissue repair and regeneration.

Signal transduction in cerebral arteries after subarachnoid hemorrhage-a phosphoproteomic approach

After subarachnoid hemorrhage (SAH), pathologic changes in cerebral arteries contribute to delayed cerebral ischemia and poor outcome. We hypothesize such changes are triggered by early intracellular signals, targeting of which may prevent SAH-induced vasculopathy. We performed an unbiased quantitative analysis of early SAH-induced phosphorylations in cerebral arteries and evaluated identified signaling components as targets for prevention of delayed vasculopathy and ischemia. Labeled phosphopeptides from rat cerebral arteries were quantified by high-resolution tandem mass spectrometry. Selected SAH-induced phosphorylations were validated by immunoblotting and monitored over a 24-hour time course post SAH. Moreover, inhibition of key phosphoproteins was performed. Major SAH-induced phosphorylations were observed on focal adhesion complexes, extracellular regulated kinase 1/2 (ERK1/2), calcium calmodulin-dependent kinase II, signal transducer and activator of transcription (STAT3) and c-Jun, the latter two downstream of ERK1/2. Inhibition of ERK1/2 6-hour post SAH prevented increases in cerebrovascular constrictor receptors, matrix metalloprotease-9, wall thickness, and improved neurologic outcome. STAT3 inhibition partially mimicked these effects. The study shows that quantitative mass spectrometry is a strong approach to study in vivo vascular signaling. Moreover, it shows that targeting of ERK1/2 prevents delayed pathologic changes in cerebral arteries and improves outcome, and identifies SAH-induced signaling components downstream and upstream of ERK1/2.

Suppression of miRNA-708 by polycomb group promotes metastases by calcium-induced cell migration

The progression of cancer to metastatic disease is a major cause of death. We identified miR-708 being transcriptionally repressed by polycomb repressor complex 2-induced H3K27 trimethylation in metastatic breast cancer. miR-708 targets the endoplasmic reticulum protein neuronatin to decrease intracellular calcium level, resulting in reduction of activation of ERK and FAK, decreased cell migration, and impaired metastases. Ectopic expression of neuronatin refractory to suppression by miR-708 rescued cell migration and metastasis defects. In patients with breast cancer, miR-708 expression was decreased in lymph node and distal metastases, suggesting a metastasis-suppressive role. Our findings uncover a mechanistic role for miR-708 in metastasis and provide a rationale for developing miR-708 as a therapeutic agent against metastatic breast cancer.

Tools for protein posttranslational modifications analysis: FAK, a case study

Recent advances in mass spectrometry have resulted in an exponential increase in annotation of posttranslational modifications (PTMs). Just in the Swiss-Prot Knowledgebase, there are 89,931 of a total of 27 characterized PTM types reported experimentally. A single protein can be dynamically modified during its lifetime for regulation of its function. Considering a PTM can occur at different levels and the number of different PTMs described, the number of possibilities for a single protein is unthinkable. Narrowing the study to a single PTM can be rather unmerited considering that most proteins are heavily modified. Currently crosstalk between PTMs is plentifully reported in the literature. The example of amino acids serine and threonine on one hand and lysine on the other hand, as targets of different modifications, demand a more global analysis approach of a protein. Besides the direct competition for the same amino acid, a PTM can directly or indirectly influence other PTMs in the same protein molecule by for example steric hindrance due to close proximity between the modifications or creation of a binding site such as an SH2 binding domain for protein recruitment and further modifications. Given the complexity of PTMs a number of tools have been developed to archive, analyze, and visualize modifications. VISUALPROT is presented here to demonstrate the usefulness of visualizing all annotated protein features such as amino acid content, domains, amino acid modification sites and single amino acid polymorphisms in a single image. VISUALPROT application is demonstrated for the protein focal adhesion kinase (FAK) as an example. FAK is a highly phosphorylated cytoplasmatic tyrosine kinase comprising different domains and regions. FAK is crucial for integrating signals from integrins and receptor tyrosine kinases in processes such as cell survival, proliferation, and motility.

SHOC2 and CRAF mediate ERK1/2 reactivation in mutant NRAS-mediated resistance to RAF inhibitor

ERK1/2 signaling is frequently dysregulated in tumors through BRAF mutation. Targeting mutant BRAF with vemurafenib frequently elicits therapeutic responses; however, durable effects are often limited by ERK1/2 pathway reactivation via poorly defined mechanisms. We generated mutant BRAF(V600E) melanoma cells that exhibit resistance to PLX4720, the tool compound for vemurafenib, that co-expressed mutant (Q61K) NRAS. In these BRAF(V600E)/NRAS(Q61K) co-expressing cells, re-activation of the ERK1/2 pathway during PLX4720 treatment was dependent on NRAS. Expression of mutant NRAS in parental BRAF(V600) cells was sufficient to by-pass PLX4720 effects on ERK1/2 signaling, entry into S phase and susceptibility to apoptosis in a manner dependent on the RAF binding site in NRAS. ERK1/2 activation in BRAF(V600E)/NRAS(Q61K) cells required CRAF only in the presence of PLX4720, indicating a switch in RAF isoform requirement. Both ERK1/2 activation and resistance to apoptosis of BRAF(V600E)/NRAS(Q61K) cells in the presence of PLX4720 was modulated by SHOC-2/Sur-8 expression, a RAS-RAF scaffold protein. These data show that NRAS mutations confer resistance to RAF inhibitors in mutant BRAF cells and alter RAF isoform and scaffold molecule requirements to re-activate the ERK1/2 pathway.

Combination of integrin-binding peptide and growth factor promotes cell adhesion on electron-beam-fabricated patterns

Understanding and controlling cell adhesion on engineered scaffolds is important in biomaterials and tissue engineering. In this report we used an electron-beam (e-beam) lithography technique to fabricate patterns of a cell adhesive integrin ligand combined with a growth factor. Specifically, micron-sized poly(ethylene glycol) (PEG) hydrogels with aminooxy- and styrene sulfonate-functional groups were fabricated. Cell adhesion moieties were introduced using a ketone-functionalized arginine-glycine-aspartic acid (RGD) peptide to modify the O-hydroxylamines by oxime bond formation. Basic fibroblast growth factor (bFGF) was immobilized by electrostatic interaction with the sulfonate groups. Human umbilical vein endothelial cells (HUVECs) formed focal adhesion complexes on RGD- and RGD and bFGF-immobilized patterns as shown by immunostaining of vinculin and actin. In the presence of both bFGF and RGD, cell areas were larger. The data demonstrate confinement of cellular focal adhesions to chemically and physically well-controlled microenvironments created by a combination of e-beam lithography and "click" chemistry techniques. The results also suggest positive implications for addition of growth factors into adhesive patterns for cell-material interactions.

A novel function of p38-regulated/activated kinase in endothelial cell migration and tumor angiogenesis

The p38 mitogen-activated protein kinase (MAPK) pathway has been implicated in both suppression and promotion of tumorigenesis. It remains unclear how these 2 opposite functions of p38 operate in vivo to impact cancer development. We previously reported that a p38 downstream kinase, p38-regulated/activated kinase (PRAK), suppresses tumor initiation and promotion by mediating oncogene-induced senescence in a murine skin carcinogenesis model. Here, using the same model, we show that once the tumors are formed, PRAK promotes the growth and progression of skin tumors. Further studies identify PRAK as a novel host factor essential for tumor angiogenesis. In response to tumor-secreted proangiogenic factors, PRAK is activated by p38 via a vascular endothelial growth factor receptor 2 (VEGFR2)-dependent mechanism in host endothelial cells, where it mediates cell migration toward tumors and incorporation of these cells into tumor vasculature, at least partly by regulating the phosphorylation and activation of focal adhesion kinase (FAK) and cytoskeletal reorganization. These findings have uncovered a novel signaling circuit essential for endothelial cell motility and tumor angiogenesis. Moreover, we demonstrate that the tumor-suppressing and tumor-promoting functions of the p38-PRAK pathway are temporally and spatially separated during cancer development in vivo, relying on the stimulus, and the tissue type and the stage of cancer development in which it is activated.

How focal adhesion kinase achieves regulation by linking ligand binding, localization and action

Focal adhesion kinase (FAK) has an astonishing number of ligands and functions, which enable it to contribute to embryonic development and human health. FAK can promote different effects in similar cellular environments or similar effects in different cellular environments. Recent advances in structural and cellular analysis of FAK are starting to reveal the interrelationships between the conformations, localizations, interactions, and functions of FAK. This review focuses on our emerging understanding of how the structural framework of FAK mechanistically allows it to integrate manifold stimuli into environment-specific functions.

Serine-910 phosphorylation of focal adhesion kinase is critical for sarcomere reorganization in cardiomyocyte hypertrophy

AIMS

Tyrosine-phosphorylated focal adhesion kinase (FAK) is required for the hypertrophic response of cardiomyocytes to growth factors and mechanical load, but the role of FAK serine phosphorylation in this process is unknown. The aims of the present study were to characterize FAK serine phosphorylation in cultured neonatal rat ventricular myocytes (NRVM), analyse its functional significance during hypertrophic signalling, and examine its potential role in the pathogenesis of human dilated cardiomyopathy (DCM).

METHODS AND RESULTS

Endothelin-1 (ET-1) and other hypertrophic factors induced a time- and dose-dependent increase in FAK-S910 phosphorylation. ET-1-induced FAK-S910 phosphorylation required ET(A)R-dependent activation of PKCδ and Src via parallel Raf-1 → MEK1/2 → ERK1/2 and MEK5 → ERK5 signalling pathways. Replication-deficient adenoviruses expressing wild-type (WT) FAK and a non-phosphorylatable, S910A-FAK mutant were then used to examine the functional significance of FAK-S910 phosphorylation. Unlike WT-FAK, S910A-FAK increased the half-life of GFP-tagged paxillin within costameres (as determined by total internal reflection fluorescence microscopy and fluorescence recovery after photobleaching) and increased the steady-state FAK-paxillin interaction (as determined by co-immunoprecipitation and western blotting). These alterations resulted in reduced NRVM sarcomere reorganization and cell spreading. Finally, we found that FAK was serine-phosphorylated at multiple sites in non-failing, human left ventricular tissue. FAK-S910 phosphorylation and ERK5 expression were dramatically reduced in patients undergoing heart transplantation for end-stage DCM.

CONCLUSION

FAK undergoes S910 phosphorylation via PKCδ and Src-dependent pathways that are important for cell spreading and sarcomere reorganization. Reduced FAK-S910 phosphorylation may contribute to sarcomere disorganization in DCM.

EGF-induced ERK-activation downstream of FAK requires rac1-NADPH oxidase

Reactive oxygen species (ROS) function as signaling molecules mainly by reversible oxidation of redox-sensitive target proteins. ROS can be produced in response to integrin ligation and growth factor stimulation through Rac1 and its effector protein NADPH oxidase. One of the central roles of Rac1-NADPH oxidase is actin cytoskeletal rearrangement, which is essential for cell spreading and migration. Another important regulator of cell spread is focal adhesion kinase (FAK), a coordinator of integrin and growth factor signaling. Here, we propose a novel role for NADPH oxidase as a modulator of the FAK autophosphorylation site. We found that Rac1-NADPH oxidase enhanced the phosphorylation of FAK at Y397. This site regulates FAK's ability to act as a scaffold for EGF-mediated signaling, including activation of ERK. Accordingly, we found that EGF-induced activation of FAK at Y925, the following activation of ERK, and phosphorylation of FAK at the ERK-regulated S910-site depended upon NADPH oxidase. Furthermore, the inhibition of NADPH oxidase caused excessive focal adhesions, which is in accordance with ERK and FAK being modulators of focal adhesion dissociation. Our data suggest that Rac1 through NADPH oxidase is part of the signaling pathway constituted by FAK, Rac1, and ERK that regulates focal adhesion disassembly during cell spreading.

Endothelial cell-specific molecule 2 (ECSM2) localizes to cell-cell junctions and modulates bFGF-directed cell migration via the ERK-FAK pathway

Background

Despite its first discovery by in silico cloning of novel endothelial cell-specific genes a decade ago, the biological functions of endothelial cell-specific molecule 2 (ECSM2) have only recently begun to be understood. Limited data suggest its involvement in cell migration and apoptosis. However, the underlying signaling mechanisms and novel functions of ECSM2 remain to be explored.

Methodology/Principal Findings

A rabbit anti-ECSM2 monoclonal antibody (RabMAb) was generated and used to characterize the endogenous ECSM2 protein. Immunoblotting, immunoprecipitation, deglycosylation, immunostaining and confocal microscopy validated that endogenous ECSM2 is a plasma membrane glycoprotein preferentially expressed in vascular endothelial cells (ECs). Expression patterns of heterologously expressed and endogenous ECSM2 identified that ECSM2 was particularly concentrated at cell-cell contacts. Cell aggregation and transwell assays showed that ECSM2 promoted cell-cell adhesion and attenuated basic fibroblast growth factor (bFGF)-driven EC migration. Gain or loss of function assays by overexpression or knockdown of ECSM2 in ECs demonstrated that ECSM2 modulated bFGF-directed EC motility via the FGF receptor (FGFR)-extracellular regulated kinase (ERK)-focal adhesion kinase (FAK) pathway. The counterbalance between FAK tyrosine phosphorylation (activation) and ERK-dependent serine phosphorylation of FAK was critically involved. A model of how ECSM2 signals to impact bFGF/FGFR-driven EC migration was proposed.

Conclusions/Significance

ECSM2 is likely a novel EC junctional protein. It can promote cell-cell adhesion and inhibit bFGF-mediated cell migration. Mechanistically, ECSM2 attenuates EC motility through the FGFR-ERK-FAK pathway. The findings suggest that ECSM2 could be a key player in coordinating receptor tyrosine kinase (RTK)-, integrin-, and EC junctional component-mediated signaling and may have important implications in disorders related to endothelial dysfunction and impaired EC junction signaling.

Live Cell Imaging of Src/FAK Signaling by FRET

The Src/FAK complex is involved in many signaling pathways and plays crucial roles in cell adhesion/migration. It becomes clear that the subcellular localization of Src and FAK is crucial for their activities and functions. In this article, we first overview the molecular mechanisms and functions of Src and FAK involved in cell adhesion/migration. We then introduce the development of genetically encoded biosensors based on fluorescence resonance energy transfer (FRET) to visualize the activities of Src and FAK in live cells with high spatiotemporal resolutions. Different kinds of signal peptides targeting subcellular compartments are also discussed. FRET-based biosensors fused with these targeting signals peptides are further introduced to provide an overview on how these targeting signals can facilitate the localization of biosensors to continuously monitor the local activity of Src and FAK at subcellular compartments. In summary, genetically-encoded FRET biosensors integrated with subcellular compartment-targeting signals can provide powerful tools for the visualization of subcellular Src and FAK activities in live cells and advance our in-depth understanding of Src/FAK functions at different subcellular compartments.

Akt directly regulates focal adhesion kinase through association and serine phosphorylation: implication for pressure-induced colon cancer metastasis

Although focal adhesion kinase (FAK) is typically considered upstream of Akt, extracellular pressure stimulates cancer cell adhesion via Akt-dependent FAK activation. How Akt regulates FAK is unknown. We studied Akt-FAK interaction in colon cancer cells under 15 mmHg increased extracellular pressure. Pressure enhanced Akt-FAK association, blocked by inhibiting FAK or silencing Akt1 but not Akt2, and stimulated FAK serine phosphorylation in Caco-2 and human colon cancer cells from surgical specimens Akt1-dependently. FAK includes three serine (S517/601/695) and one threonine (T600)-containing consensus sequences for Akt phosphorylation. Studying S->A nonphosphorylatable point mutants suggests that these sites coordinately upregulate FAK Y397 tyrosine phosphorylation, which conventionally initiates FAK activation, and mediate pressure-induced cancer cell adhesion. FAK(T600A) mutation did not prevent pressure-induced FAK(Y397) phosphorylation or adhesion. Akt1 appeared to directly bind FAK, and this binding did not depend on the FAK autophosphorylation site (Y397). In addition, our results demonstrated that Akt phosphorylated FAK at three novel serine phosphorylation sites, which were also not required for FAK-Akt binding. This novel interaction suggests that FAK and Akt may be dual kinase targets to prevent cancer cell adhesion and metastasis.

Hyperactivation of MEK-ERK1/2 signaling and resistance to apoptosis induced by the oncogenic B-RAF inhibitor, PLX4720, in mutant N-RAS melanoma cells

Activating mutations in B-RAF and N-RAS occur in ∼60 and ∼15% of melanomas, respectively. The most common mutation in B-RAF is V600E, which activates B-RAF and the downstream MEK-ERK1/2 pathway. Thus, B-RAF(V600E) is a viable therapeutic target. PLX4720 is a selective inhibitor of mutant B-RAF and its analog, PLX4032, is currently undergoing clinical trials in melanoma. However, the effects of PLX4720 across the genotypic spectrum in melanoma remain unclear. Here, we describe that PLX4720 treatment rapidly induces hyperactivation of the MEK-ERK1/2 pathway in mutant N-RAS melanoma cells. Furthermore, we demonstrate that C-RAF is the major RAF isoform involved in this process. Importantly, PLX4720-induced hyperactivation of the MEK-ERK1/2 pathway promotes resistance to apoptosis in both non-invasive and invasive mutant N-RAS melanoma cells but does not enhance cell cycle properties. These findings underscore the need to genotypically stratify melanoma patients before enrollment on a mutant B-RAF inhibitor trial.

Angiopoietin-2 stimulation of endothelial cells induces alphavbeta3 integrin internalization and degradation

The angiopoietins (Ang-1 and Ang-2) have been identified as agonistic and antagonistic ligands of the endothelial receptor tyrosine kinase Tie2, respectively. Both ligands have been demonstrated to induce translocation of Tie2 to cell-cell junctions. However, only Ang-1 induces Tie2-dependent Akt activation and subsequent survival signaling and endothelial quiescence. Ang-2 interferes negatively with Ang-1/Tie2 signaling, thereby antagonizing the Ang-1/Tie2 axis. Here, we show that both Ang-1 and Ang-2 recruit beta3 integrins to Tie2. This co-localization is most prominent in cell-cell junctions. However, only Ang-2 stimulation resulted in complex formation among Tie2, alphavbeta3 integrin, and focal adhesion kinase as evidenced by co-immunoprecipitation experiments. Focal adhesion kinase was phosphorylated in the FAT domain at Ser(910) upon Ang-2 stimulation and the adaptor proteins p130Cas and talin dissociated from alphavbeta3 integrin. The alphavbeta3 integrin was internalized, ubiquitinylated, and gated toward lysosomes. Taken together, the experiments define Tie2/alphavbeta3 integrin association-induced integrin internalization and degradation as mechanistic consequences of endothelial Ang-2 stimulation.

FAK phosphorylation by ERK primes ras-induced tyrosine dephosphorylation of FAK mediated by PIN1 and PTP-PEST

Activated Ras has been found in many types of cancer. However, the mechanism underlying Ras-promoted tumor metastasis remains unclear. We demonstrate here that activated Ras induces tyrosine dephosphorylation and inhibition of FAK mediated by the Ras downstream Fgd1-Cdc42-PAK1-MEK-ERK signaling cascade. ERK phosphorylates FAK S910 and recruits PIN1 and PTP-PEST, which colocalize with FAK at the lamellipodia of migrating cells. PIN1 binding and prolyl isomerization of FAK cause PTP-PEST to interact with and dephosphorylate FAK Y397. Inhibition of FAK mediated by this signal relay promotes Ras-induced cell migration, invasion, and metastasis. These findings uncover the importance of sequential modification of FAK-by serine phosphorylation, isomerization, and tyrosine dephosphorylation--in the regulation of FAK activity and, thereby, in Ras-related tumor metastasis.

Focal adhesion kinase is required for neural crest cell morphogenesis during mouse cardiovascular development

Neural crest cells (NCCs) participate in the remodeling of the cardiac outflow tract and pharyngeal arch arteries during cardiovascular development. Focal adhesion kinase (FAK) mediates signal transduction by integrin and growth factor receptors, each of which is important for normal cardiovascular development. To investigate the role of FAK in NCC morphogenesis, we deleted it in murine NCCs using Wnt1cre, yielding craniofacial and cardiovascular malformations resembling those observed in individuals with DiGeorge syndrome. In these mice, we observed normal cardiac NCC migration but reduced differentiation into smooth muscle within the aortic arch arteries and impaired cardiac outflow tract rotation, which resulted in a dextroposed aortic root. Moreover, within the conotruncal cushions, Fak-deficient NCCs formed a less organized mesenchyme, with reduced expression of perlecan and semaphorin 3C, and exhibited disorganized F-actin stress fibers within the aorticopulmonary septum. Additionally, absence of Fak resulted in reduced in vivo phosphorylation of Crkl and Erk1/2, components of a signaling pathway essential for NCC development. Consistent with this, both TGF-beta and FGF induced FAK and Crkl phosphorylation in control but not Fak-deficient NCCs in vitro. Our results indicate that FAK plays an essential role in cardiac outflow tract development by promoting the activation of molecules such as Crkl and Erk1/2.

Src family kinases are necessary for cell migration induced by extracellular HMGB1

HMGB1 is a nuclear protein that signals tissue damage, as it is released by cells dying traumatically or secreted by activated innate immunity cells. Extracellular HMGB1 elicits the migration to the site of tissue damage of several cell types, including inflammatory cells and stem cells. The identity of the signaling pathways activated by extracellular HMGB1 is not known completely: We reported previously that ERK and NF-kappaB pathways are involved, and we report here that Src is also activated. The ablation of Src or inhibition with the kinase inhibitor PP2 blocks migration toward HMGB1. Src associates to and mediates the phosphorylation of FAK and the formation of focal adhesions.

Cardiac fibroblasts require focal adhesion kinase for normal proliferation and migration

Migration and proliferation of cardiac fibroblasts (CFs) play an important role in the myocardial remodeling process. While many factors have been identified that regulate CF growth and migration, less is known about the signaling mechanisms involved in these processes. Here, we utilized Cre-LoxP technology to obtain focal adhesion kinase (FAK)-deficient adult mouse CFs and studied how FAK functioned in modulating cell adhesion, proliferation, and migration of these cells. Treatment of FAK(flox/flox) CFs with Ad/Cre virus caused over 70% reduction of FAK protein levels within a cell population. FAK-deficient CFs showed no changes in focal adhesions, cell morphology, or protein expression levels of vinculin, talin, or paxillin; proline-rich tyrosine kinase 2 (Pyk2) expression and activity were increased. Knockdown of FAK protein in CFs increased PDGF-BB-induced proliferation, while it reduced PDGF-BB-induced migration. Adhesion to fibronectin was not altered. To distinguish between the function of FAK and Pyk2, FAK function was inhibited via adenoviral-mediated overexpression of the natural FAK inhibitor FAK-related nonkinase (FRNK). Ad/FRNK had no effect on Pyk2 expression, inhibited the PDGF-BB-induced migration, but did not change the PDGF-BB-induced proliferation. FAK deficiency had only modest effects on increasing PDGF-BB activation of p38 and JNK MAPKs, with no alteration in the ERK response vs. control cells. These results demonstrate that FAK is required for the PDGF-BB-induced migratory response of adult mouse CFs and suggest that FAK could play an essential role in the wound-healing response that occurs in numerous cardiac pathologies.

Role of focal adhesion kinase Ser-732 phosphorylation in centrosome function during mitosis

Focal adhesion kinase (FAK) is the major cytoplasmic tyrosine kinase in focal adhesions and a critical mediator of integrin signaling in a variety of cells, including endothelial cells (ECs). Here we describe a new function for FAK in the regulation of centrosome functions in a Ser-732 phosphorylation-dependent manner during mitosis. Deletion of FAK in primary ECs causes increases in centrosome numbers, multipolar and disorganized spindles, and unaligned chromosomes during mitosis. Re-expression of wild-type FAK, but not S732A mutant, rescued these mitotic defects, suggesting a role for Ser-732 phosphorylation in the regulation of centrosomal functions. Consistent with this possibility, Ser-732-phosphorylated FAK was found to co-localize in centrosomes in mitotic cells. FAK also associated with cytoplasmic dynein in a Ser-732 phosphorylation-dependent manner. Further analysis in FAK-null primary ECs showed that S732A mutant could rescue EC migration but not proliferation or tubulogenesis in vitro. Last, we showed that deletion of FAK in ECs reduced tumor angiogenesis in vivo, which could be restored by re-expression of wild-type FAK but not S732A mutant. Together, these studies demonstrated a novel role for Ser-732 phosphorylation of FAK in the regulation of centrosome function during mitosis, which may contribute to EC proliferation and angiogenesis.

MAP-kinase activity necessary for TGFbeta1-stimulated mesangial cell type I collagen expression requires adhesion-dependent phosphorylation of FAK tyrosine 397

The signals mediating transforming growth factor beta (TGFbeta)-stimulated kidney fibrogenesis are poorly understood. We previously reported TGFbeta-stimulated, Smad-mediated collagen production by human kidney mesangial cells, and that ERK MAP kinase activity optimizes collagen expression and enhances phosphorylation of the Smad3 linker region. Furthermore, we showed that disrupting cytoskeletal integrity decreases type I collagen production. Focal adhesion kinase (FAK, PTK2) activity could integrate these findings. Adhesion-dependent FAK Y397 phosphorylation was detected basally, whereas FAK Y925 phosphorylation was TGFbeta1-dependent. By immunocytochemistry, TGFbeta1 stimulated the merging of phosphorylated FAK with the ends of thickening stress fibers. Cells cultured on poly-L-lysine (pLL) to promote integrin-independent attachment spread less than those on control substrate and failed to demonstrate focal adhesion (FA) engagement with F-actin. FAK Y397 phosphorylation and ERK activity were also decreased under these conditions. In cells with decreased FAK Y397 phosphorylation from either plating on pLL or overexpressing a FAK Y397F point mutant, serine phosphorylation of the Smad linker region, but not of the C-terminus, was reduced. Y397F and Y925F FAK point mutants inhibited TGFbeta-induced Elk-Gal activity, but only the Y397F mutant inhibited TGFbeta-stimulated collagen-promoter activity. The inhibition by the Y397F mutant or by culture on pLL was prevented by co-transfection of constitutively active ERK MAP kinase kinase (MEK), suggesting that FAK Y397 phosphorylation promotes collagen expression via ERK MAP kinase activity. Finally, Y397 FAK phosphorylation, and both C-terminal and linker-region Smad3 phosphorylation were detected in murine TGFbeta-dependent kidney fibrosis. Together, these data demonstrate adhesion-dependent FAK phosphorylation promoting TGFbeta-induced responses to regulate collagen production.

Cell responses regulated by early reorganization of actin cytoskeleton

Microfilaments exist in a dynamic equilibrium between monomeric and polymerized actin and the ratio of monomers to polymeric forms is influenced by a variety of extracellular stimuli. The polymerization, depolymerization and redistribution of actin filaments are modulated by several actin-binding proteins, which are regulated by upstream signalling molecules. Actin cytoskeleton is involved in diverse cellular functions including migration, ion channels activity, secretion, apoptosis and cell survival. In this review we have outlined the role of actin dynamics in representative cell functions induced by the early response to extracellular stimuli.

The origins of human embryonic stem cells: a biological conundrum

Human inner cell mass (ICM) cells isolated from in vitro fertilized blastocysts are the progenitor cells used to establish in vitro stable human embryonic stem cells (hESCs) which are pluripotent and self-renew indefinitely. This long-term perpetuation of hESCs in the undifferentiated state is thought to be an in vitro adaptation of the ICM cells. To investigate at the molecular level how hESCs acquired their unique properties, transcriptional profiles of isolated ICM cells and undifferentiated hESCs were compared. We identified 33 genes enriched in the ICM compared to the trophectoderm and hESCs. These genes are involved in signaling cascades (SEMA7A and MAP3K10), cell proliferation (CUZD1 and MS4A7) and chromatin remodeling (H1FOO and HRMT1L4). Furthermore, primordial germ cell-specific genes (SGCA and TEX11) were detected as expressed in the ICM cells and not hESCs. We propose that the transcriptional differences observed between ICM cells and hESCs might be accounted for by adaptive reprogramming events induced by the in vitro culture conditions which are distinct from that of in vitro fertilized blastocysts. hESCs are a distinct cell type lacking in the human embryo but, nonetheless, resemble the ICM in their ability to differentiate into cells representative of the endodermal, ectodermal and mesodermal cell lineages.

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

Targeting of FAK Ser910 by ERK5 and PP1delta in non-stimulated and phorbol ester-stimulated cells

Ser910 of FAK (focal adhesion kinase) was phosphorylated in fibroblasts treated with the phorbol ester PMA and dephosphorylated by PP1d (protein phosphatase 1d), as indicated by shRNA (small-hairpin RNA) gene silencing. Ser910 of FAK was reported previously to be an ERK (extracellular-signal-regulated kinase) 1/2 target in cells treated with phorbol esters. In contrast, various approaches, including the use of the MEK (mitogen-activated protein kinase/ERK kinase) inhibitors UO126 and CI-1040 to inhibit ERK1/2 pointed to the involvement of ERK5. This hypothesis was confirmed by: (i) shRNA ERK5 gene silencing, which resulted in complete pSer910 loss in non-stimulated and PMA-stimulated cells; (ii) direct phosphorylation of recombinant FAK by ERK5; and (iii) ERK5 activation by PMA. PMA stimulation and ERK5 silencing in MDA-MB 231 and MDA-MB 361 breast cancer cells indicated Ser910 targeting by ERK5 also in these cells. Given the proximity of Ser910 to the FAT (focal adhesion targeting) regulatory domain of FAK, cell proliferation and morphology were investigated in FAK-/- cells expressing S910A mutant FAK. The cell growth rate decreased and exposure to PMA induced peculiar morphological changes in cells expressing S910A, with respect to wild-type FAK, suggesting a role for Ser910 in these processes. The present study indicates, for the first time, the phosphorylation of Ser910 of FAK by ERK5 and its dephosphorylation by PP1d, and suggested a role for Ser910 in the control of cell shape and proliferation.

FAK association with multiple signal proteins mediates pressure-induced colon cancer cell adhesion via a Src-dependent PI3K/Akt pathway

Cancer cell adhesion is traditionally viewed as random, occurring if the cell's receptors match the substrate. Cancer cells are subjected to pressure and shear during growth against a constraining stroma, surgical manipulation, and passage through the venous and lymphatic system. Cells shed into a cavity such as the abdomen postoperatively also experience increased pressure from postoperative edema. Increased extracellular pressure stimulates integrin-mediated cancer cell adhesion via FAK and Src. PI 3-kinase (PI3K) inhibitors (LY294002 or wortmannin), Akt inhibitors, or Akt1 siRNA blocked adhesion stimulated by 15 mmHg pressure in SW620 or primary human malignant colonocytes. Pressure activated PI3K, tyrosine-phosphorylated and membrane-translocated the p85 subunit, and phosphorylated Akt. PI3K inhibitor (LY294002) prevented pressure-stimulated Akt Ser473 and FAK Tyr397, but not FAK576 or Src416 phosphorylation. PP2 inhibited PI3K activity and Akt phosphorylation. FAK siRNA did not affect pressure-induced PI3K activation but blocked Akt phosphorylation. Pressure also stimulated FAK or FAKY397F mutant translocation to the membrane. Akt inhibitor IV blocked pressure-induced Akt and FAK translocation. Pressure activated Src- and PI3K-dependently induced p85 interaction with FAK, and FAK with beta1 integrin. These results delineate a novel force-activated inside-out Src/PI3K/FAK/Akt pathway by which cancer cells regulate their own adhesion. These signals may be potential targets for inhibition of metastatic adhesion.

Focal adhesion kinase and p53 signaling in cancer cells

The progression of human cancer is characterized by a process of tumor cell motility, invasion, and metastasis to distant sites, requiring the cancer cells to be able to survive the apoptotic pressures of anchorage-independent conditions. One of the critical tyrosine kinases linked to these processes of tumor invasion and survival is the focal adhesion kinase (FAK). FAK was first isolated from human tumors, and FAK mRNA was found to be upregulated in invasive and metastatic human breast and colon cancer samples. Recently, the FAK promoter was cloned, and it has been found to contain p53-binding sites. p53 inhibits FAK transcription, and recent data show direct binding of FAK and p53 proteins in vitro and in vivo. The structure of FAK and p53, proteins interacting with FAK, and the role of FAK in tumorigenesis and FAK-p53-related therapy are reviewed. This review focuses on FAK signal transduction pathways, particularly on FAK and p53 signaling, revealing a new paradigm in cell biology, linking signaling from the extracellular matrix to the nucleus.

Differential FAK phosphorylation at Ser-910, Ser-843 and Tyr-397 induced by angiotensin II, LPA and EGF in intestinal epithelial cells

A rapid increase in the tyrosine phosphorylation of the non-receptor tyrosine kinase FAK is a prominent early event in fibroblasts stimulated by a variety of signaling molecules. However, a variety of epithelial cells, including intestinal epithelial cells, show a high basal level of tyrosine phosphorylated FAK that is only slightly further increased by addition of G protein-coupled receptor (GPCR) agonists or growth factors. In this study, we determined whether these stimuli could elicit FAK phosphorylation at serine residues, including Ser-910 and Ser-843. Our results show that multiple agonists including angiotensin II (ANGII), lysophosphatidic acid (LPA), phorbol esters and EGF induced a striking stimulation of FAK phosphorylation at Ser-910 in rat intestinal epithelial IEC-18 cells via an ERK-dependent pathway. In striking contrast, none of these stimuli promoted a significant further increase in FAK phosphorylation at Tyr-397 in these cells. These results were extended using cultures of polarized human colonic epithelial T84 cells. We found that either carbachol or EGF promoted a striking ERK-dependent phosphorylation of FAK at Ser-910, but these agonists caused only slight stimulation of FAK at Tyr-397 in T84 cells. In addition, we demonstrated that GPCR agonists also induced a dramatic increase of FAK phosphorylation at Ser-843 in either IEC-18 or T84 cells. Our results indicate that Ser-910 and Ser-843, rather than Tyr-397, are prominent sites differentially phosphorylated in response to neurotransmitters, bioactive lipids, tumor promoters and growth factors in intestinal epithelial cells.

Focal adhesion kinase protein levels in gut epithelial motility

Mucosal healing requires migration and proliferation. Most studies of focal adhesion kinase (FAK), a protein that regulates motility, proliferation, and apoptosis, have focused on rapid phosphorylation. We reported lower FAK protein levels in motile Caco-2 colon cancer cells and postulated that this reduction in FAK available for activation might impact cell migration and mucosal healing. Therefore, total and active FAK (FAK(397)) immunoreactivity was assessed at the migrating fronts of human Caco-2 and rat IEC-6 intestinal epithelial cells. Caco-2 and IEC-6 motility, quantitated as migration into linear or circular wounds, was examined following FAK protein inhibition by small interfering RNA (siRNA). FAK protein stability and mRNA expression were ascertained by cycloheximide decay, RT-PCR, and in situ hybridization in static and migrating Caco-2 cells. Cells at the migrating front of Caco-2 and IEC-6 monolayers exhibited lower immunostaining for both total and activated FAK than cells immediately behind the front. Western blot analysis also demonstrated diminished FAK protein levels in motile cells by >/=30% in both the differential density seeding and multiple scrape models. siRNA FAK protein inhibition enhanced motility in both the linear scrape (20% in Caco-2) and circular wound (16% in Caco-2 and 19% in IEC-6 cells) models. FAK protein degradation did not differ in motile and static Caco-2 cells and was unaffected by FAK(397) phosphorylation, but FAK mRNA was lower in migrating Caco-2 cells. Thus FAK protein abundance appears regulated at the mRNA level during gut epithelial cell motility and may influence epithelial cell migration coordinately with signals that modify FAK phosphorylation.

FGF2 binding, signaling, and angiogenesis are modulated by heparanase in metastatic melanoma cells

Heparanase (HPSE) and fibroblast growth factor-2 (FGF2) are critical regulators of melanoma angiogenesis and metastasis. Elevated HPSE expression contributes to melanoma progression; however, further augmentation of HPSE presence can inhibit tumorigenicity. HPSE enzymatically cleaves heparan sulfate glycosaminoglycan chains (HS) from proteoglycans. HS act as both low-affinity FGF2 receptors and coreceptors in the formation of high-affinity FGF2 receptors. We have investigated HPSE's ability to modulate FGF2 activity through HS remodeling. Extensive HPSE degradation of human metastatic melanoma cells (70W) inhibited FGF2 binding. Unexpectedly, treatment of 70W cells with low HPSE concentrations enhanced FGF2 binding. In addition, HPSE-unexposed cells did not phosphorylate extracellular signal-related kinase (ERK) or focal adhesion kinase (FAK) in response to FGF2. Conversely, in cells treated with HPSE, FGF2 stimulated ERK and FAK phosphorylation. Secondly, the presence of soluble HPSE-degraded HS enhanced FGF2 binding and ERK phosphorylation at low HS concentrations. Higher concentrations of soluble HS inhibited FGF2 binding, but FGF2 signaling through ERK remained enhanced. Soluble HS were unable to support FGF2-stimulated FAK phosphorylation irrespective of HPSE treatment. Finally, cell exposure to HPSE or to HPSE-degraded HS modulated FGF2-induced angiogenesis in melanoma. In conclusion, these effects suggest relevant mechanisms for the HPSE modulation of melanoma growth factor responsiveness and tumorigenicity.

Regulation of FAK Ser-722 phosphorylation and kinase activity by GSK3 and PP1 during cell spreading and migration

In addition to tyrosine sites, FAK (focal adhesion kinase) is phosphorylated on multiple serine residues. In the present study, the regulation of two of these sites, Ser-722 (S1) and Ser-911 (S4), was investigated. Phosphorylation of S1 (but not S4) decreased in resuspended cells, and recovered during spreading on fibronectin, indicating adhesion-dependent regulation. GSK3 (glycogen synthase kinase 3) inhibitors decreased S1 phosphorylation, and siRNA (short interfering RNA) silencing indicated further the involvement of GSK3beta. Furthermore, GSK3beta was found to become activated during cell spreading on fibronectin, and to physically associate with FAK. S1 phosphorylation was observed to decrease in wounded cell monolayers, while GSK3beta underwent inactivation and later was observed to increase to the original level within 24 h. Direct phosphorylation of S1, requiring pre-phosphorylation of Ser-726 in the +4 position, was demonstrated using purified GSK3 and a synthetic peptide containing FAK residues 714-730. An inhibitory role for S1 phosphorylation in FAK signalling was indicated by findings that both alanine substitution for S1 and dephosphorylation of S1 by PP1 (serine/threonine protein phosphatase type-1) resulted in an increase in FAK kinase activity; likewise, this role was also shown by cell treatment with the GSK3 inhibitor LiCl. The inhibitory role was confirmed by the finding that cells expressing FAK with alanine substitution for S1 displayed improved cell spreading and faster migration in wound-healing and trans-well assays. Finally, the finding that S1 phosphorylation increased in cells treated with the PP1 inhibitor okadaic acid indicated targeting of this site by PP1. These results indicate an additional mechanism for regulation of FAK activity during cell spreading and migration, involving Ser-722 phosphorylation modulated through the competing actions of GSK3beta and PP1.

FAK phosphorylation at Ser-843 inhibits Tyr-397 phosphorylation, cell spreading and migration

Multiple stimuli promote the tyrosine phosphorylation and activation of focal adhesion kinase (FAK), which ultimately facilitates migration. Little is known about the effect of adhesion-dependent signals and cytoskeleton organization on the regulation of FAK phosphorylation at serine sites, or about the role of FAK serine phosphorylation in cell migration. Here, we show that FAK phosphorylation at Ser-843 is strikingly increased when adherent cells are removed from the substratum and held in suspension or by treatment of adherent cells with cytochalasin D, conditions that disrupt the F-actin cytoskeleton and promote focal adhesion disassembly. Notably, the increase in Ser-843 phosphorylation was accompanied by a concomitant sharp decrease in Tyr-397 phosphorylation. To further examine the cause-effect relationship between these two phosphorylation sites we generated Ser-843 phosphorylation-deficient and phosphorylation-mimicking FAK mutants. We found that mutation of Ser-843 to aspartic acid (FAK[S843D]) markedly decreased FAK Tyr-397 phosphorylation in integrin-stimulated cells. While the migratory defect of FAK-deficient fibroblasts was rescued by stable re-expression of WT FAK or FAK[S843A], stable re-expression of FAK[S843D] failed to restore the ability of the cells to migrate into the denuded area of a wound. Our results indicate that increased FAK phosphorylation at Ser-843 represses FAK phosphorylation at Tyr-397, thus suggesting a mechanism of cross-talk between these phosphorylation sites that could regulate FAK-mediated cell shape and migration.