Integrins can collaborate with growth factors for phosphorylation of receptor tyrosine kinases and MAP kinase activation: roles of integrin aggregation and occupancy of receptors

Akt1 promotes focal adhesion disassembly and cell motility through phosphorylation of FAK in growth factor-stimulated cells

The crosstalk between spatial adhesion signals and temporal soluble signals is key in regulating cellular responses such as cell migration. Here we show that soluble growth factors enhance integrin signaling through Akt phosphorylation of FAK at Ser695 and Thr700. PDGF treatment or overexpression of active Akt1 in fibroblasts increased autophosphorylation of FAK at Tyr397, an essential event for integrin turnover and cell migration. Phosphorylation-defective mutants of FAK (S695A and T700A) underwent autophosphorylation at Tyr397 and promoted cell migration in response to the integrin ligand fibronectin, but importantly, not in response to PDGF. This study has unveiled a novel function of Akt as an 'ignition kinase' of FAK in growth factor signaling and may shed light on the mechanism by which growth factors regulate integrin signaling.

Trask loss enhances tumorigenic growth by liberating integrin signaling and growth factor receptor cross-talk in unanchored cells

The cell surface glycoprotein Trask/CDCP1 is phosphorylated during anchorage loss in epithelial cells in which it inhibits integrin clustering, outside-in signaling, and cell adhesion. Its role in cancer has been difficult to understand, because of the lack of a discernible pattern in its various alterations in cancer cells. To address this issue, we generated mice lacking Trask function. Mammary tumors driven by the PyMT oncogene and skin tumors driven by the SmoM2 oncogene arose with accelerated kinetics in Trask-deficient mice, establishing a tumor suppressing function for this gene. Mechanistic investigations in mammary tumor cell lines derived from wild-type or Trask-deficient mice revealed a derepression of integrin signaling and an enhancement of integrin-growth factor receptor cross-talk, specifically in unanchored cell states. A similar restrictive link between anchorage and growth in untransformed epithelial cells was observed and disrupted by elimination of Trask. Together our results establish a tumor-suppressing function in Trask that restricts epithelial cell growth to the anchored state.

Site-specific S-nitrosylation of integrin α6 increases the extent of prostate cancer cell migration by enhancing integrin β1 association and weakening adherence to laminin-1

The increased mortality in prostate cancer is usually the result of metastatic progression of the disease from the organ-confined location. Among the major events in this progression cascade are enhanced cell migration and loss of adhesion. Moreover, elevated levels of nitric oxide (NO) and inducible nitric oxide synthase (iNOS) found within the tumor microenvironment are hallmarks of progression of this cancer. To understand the role of nitrosative stress in prostate cancer progression, we investigated the effects of NO and iNOS on prostate cancer cell migration and adhesion. Our results indicate that ectopic expression of iNOS in prostate cancer cells increased the extent of cell migration, which could be blocked by selective ITGα6 blocking antibody or iNOS inhibitors. Furthermore, iNOS was found to cause S-nitrosylation of ITGα6 at Cys86 in prostate cancer cells. By comparing the activities of wild-type ITGα6 and a Cys86 mutant, we showed that treatment of prostate cancer cells with NO increased the level of ITGα6 heterodimerization with ITGβ1 but not with ITGβ4. Finally, S-nitrosylation of ITGα6 weakened its binding to laminin-β1 and weakened the adhesion of prostate cancer cells to laminin-1. In conclusion, S-nitrosylation of ITGα6 increased the extent of prostate cancer cell migration, which could be a potential mechanism of NO- and iNOS-induced enhancement of prostate cancer metastasis.

The extracellular matrix and mitogenic growth factors control G1 phase cyclins and cyclin-dependent kinase inhibitors

Most cell types require both mitogenic growth factors and cell adhesion to the extracellular matrix (ECM) for proliferation. Over the past few years, these growth requirements have received renewed attention and can now be explained by studies showing that signals provided by growth factors and the ECM are jointly required to stimulate the cyclin-dependent kinases (CDKs) that mediate cell-cycle progression through G1 phase. This article summarizes our current understanding of the control of G1 cyclins and CDK inhibitors by growth factors and the ECM. In addition, we have highlighted one or two signal-transduction pathways that presently seem closely linked to regulation of the G1 phase cyclin-CDK system.

Epidermal growth factor (EGF) regulates α5β1 integrin activation state in human cancer cell lines through the p90RSK-dependent phosphorylation of filamin A

BACKGROUND

Regulation of integrin activation has important implications for tumor cell invasion and metastasis.

RESULTS

EGF activates ERK/p90RSK and Rho/Rho kinase signaling in A431 and DiFi colon cancer cells, leading to phosphorylation of filamin A (FLNa) and inactivation of the α5β1 integrin receptor.

CONCLUSION

EGF promotes α5β1 inactivation through the p90RSK-dependent phosphorylation of FLNa.

SIGNIFICANCE

We have identified a novel EGF-dependent mechanism controlling the α5β1 integrin activation state. Cell adhesion, motility, and invasion are regulated by the ligand-binding activity of integrin receptors, transmembrane proteins that bind to the extracellular matrix. Integrins whose conformation allows for ligand binding and appropriate functional activity are said to be in an active state. Integrin activation and subsequent ligand binding are dynamically regulated by the association of cytoplasmic proteins with integrin intracellular domains. In this study, we evaluated the role of EGF in the regulation of the activation state of the α5β1 integrin receptor for fibronectin. The addition of EGF to either A431 squamous carcinoma cells or DiFi colon cancer cells resulted in loss of α5β1-dependent adhesion to fibronectin but no loss of integrin from the cell surface. EGF activated the EGF receptor/ERK/p90RSK and Rho/Rho kinase signaling pathways. Blocking either pathway inhibited EGF-mediated loss of adhesion, suggesting that they work in parallel to regulate integrin function. EGF treatment also resulted in phosphorylation of filamin A (FLNa), which binds and inactivates β1 integrins. EGF-mediated FLNa phosphorylation was completely blocked by an inhibitor of p90RSK and partially attenuated by an inhibitor of Rho kinase, suggesting that both pathways converge on FLNa to regulate integrin function. A431 clonal cell lines expressing non-phosphorylated dominant-negative FLNa were resistant to the inhibitory effects of EGF on integrin function, whereas clonal cell lines overexpressing wild-type FLNa were more sensitive to the inhibitory effect of EGF. These data suggest that EGF-dependent inactivation of α5β1 integrin is regulated through FLNa phosphorylation and cellular contractility.

Extracellular matrix and matrix receptors in blood-brain barrier formation and stroke

The blood-brain barrier (BBB) is formed primarily to protect the brain microenvironment from the influx of plasma components, which may disturb neuronal functions. The BBB is a functional unit that consists mainly of specialized endothelial cells (ECs) lining the cerebral blood vessels, astrocytes, and pericytes. The BBB is a dynamic structure that is altered in neurologic diseases, such as stroke. ECs and astrocytes secrete extracellular matrix (ECM) proteins to generate and maintain the basement membranes (BMs). ECM receptors, such as integrins and dystroglycan, are also expressed at the brain microvasculature and mediate the connections between cellular and matrix components in physiology and disease. ECM proteins and receptors elicit diverse molecular signals that allow cell adaptation to environmental changes and regulate growth and cell motility. The composition of the ECM is altered upon BBB disruption and directly affects the progression of neurologic disease. The purpose of this review is to discuss the dynamic changes of ECM composition and integrin receptor expression that control BBB functions in physiology and pathology.

Nanostructures of designed geometry and functionality enable regulation of cellular signaling processes

Extracellular matrices (ECM) triggered cellular signaling processes often begin with the clustering of the cellular receptors such as integrin and FcεRI. The sizes of these initial protein complexes or clusters are tens to 100 nm in dimension; therefore, engineered nanostructures could provide effective mimics of ECM for investigation and control of the initial and downstream specific signaling processes. This current topic discusses recent advances in nanotechnology in the context of design and production of matching chemical functionality and geometry for control of specific cellular signaling processes. Two investigations are reported to demonstrate this concept: (a) how the presentation of antigen at the nanometer scale would influence the aggregation of FcεRI, which would impact the formation of activation complexes, leading to the rearrangement of actin in cytoskeleton and degranulation or activation of mast cells; (b) how the engineered nanostructure could guide the initial integrin clustering, which would impact the formation of focal adhesion and downstream cell signaling cascades, leading to polarization, migration, and morphological changes. Complementary to engineered ECMs using synthetic ligands or peptides, or topographic control at the micrometer scale, nanostructures of designed geometry and chemical functionality provide new and effective biochemical cues for regulation of cellular signaling processes and downstream behaviors.

Expression of Toll-like receptor 4 and beta 1 integrin in breast cancer

Toll-like receptor (TLR) 4 signaling pathway has been shown to support tumor cell growth in vitro and in vivo. Its stimulation on breast cancer cell lines induces β1 integrin and promotes tumor invasiveness. However, its role in predicting clinical behavior of tumor is not yet clarified. Therefore, we investigated TLR4 and β1 integrin expression on 133 primary breast cancer samples by immunohistochemistry and correlated it with overall survival and disease-free survival of patients as well as with clinicopathological characteristics of the tumor. We found higher β1 integrin expression in invasive lobular cancer in comparison with other tumor types. No significant association of TLR4 and β1 integrin expression with overall survival or disease-free survival was seen. Therefore, we conclude that expression of these markers is of biological interest but appears to be of little additional use as predictive clinical marker.

Development of transgenic cloned pig models of skin inflammation by DNA transposon-directed ectopic expression of human β1 and α2 integrin

Integrins constitute a superfamily of transmembrane signaling receptors that play pivotal roles in cutaneous homeostasis by modulating cell growth and differentiation as well as inflammatory responses in the skin. Subrabasal expression of integrins α2 and/or β1 entails hyperproliferation and aberrant differentiation of keratinocytes and leads to dermal and epidermal influx of activated T-cells. The anatomical and physiological similarities between porcine and human skin make the pig a suitable model for human skin diseases. In efforts to generate a porcine model of cutaneous inflammation, we employed the Sleeping Beauty DNA transposon system for production of transgenic cloned Göttingen minipigs expressing human β1 or α2 integrin under the control of a promoter specific for subrabasal keratinocytes. Using pools of transgenic donor fibroblasts, cloning by somatic cell nuclear transfer was utilized to produce reconstructed embryos that were subsequently transferred to surrogate sows. The resulting pigs were all transgenic and harbored from one to six transgene integrants. Molecular analyses on skin biopsies and cultured keratinocytes showed ectopic expression of the human integrins and localization within the keratinocyte plasma membrane. Markers of perturbed skin homeostasis, including activation of the MAPK pathway, increased expression of the pro-inflammatory cytokine IL-1α, and enhanced expression of the transcription factor c-Fos, were identified in keratinocytes from β1 and α2 integrin-transgenic minipigs, suggesting the induction of a chronic inflammatory phenotype in the skin. Notably, cellular dysregulation obtained by overexpression of either β1 or α2 integrin occurred through different cellular signaling pathways. Our findings mark the creation of the first cloned pig models with molecular markers of skin inflammation. Despite the absence of an overt psoriatic phenotype, these animals may possess increased susceptibility to severe skin damage-induced inflammation and should be of great potential in studies aiming at the development and refinement of topical therapies for cutaneous inflammation including psoriasis.

Regulation of integrin adhesions by varying the density of substrate-bound epidermal growth factor

Substrates coated with specific bioactive ligands are important for tissue engineering, enabling the local presentation of extracellular stimulants at controlled positions and densities. In this study, we examined the cross-talk between integrin and epidermal growth factor (EGF) receptors following their interaction with surface-immobilized Arg-Gly-Asp (RGD) and EGF ligands, respectively. Surfaces of glass coverslips, modified with biotinylated silane-polyethylene glycol, were functionalized by either biotinylated RGD or EGF (or both) via the biotin-NeutrAvidin interaction. Fluorescent labeling of the adhering A431 epidermoid carcinoma cells for zyxin or actin indicated that EGF had a dual effect on focal adhesions (FA) and stress fibers: at low concentrations (0.1; 1 ng/ml), it stimulated their growth; whereas at higher concentrations, on surfaces with low to intermediate RGD densities, it induced their disassembly, leading to cell detachment. The EGF-dependent dissociation of FAs was, however, attenuated on higher RGD density surfaces. Simultaneous stimulation by both immobilized RGD and EGF suggest a strong synergy between integrin and EGFR signaling, in FA induction and cell spreading. A critical threshold level of EGF was required to induce significant variation in cell adhesion; beyond this critical density, the immobilized molecule had a considerably stronger effect on cell adhesion than did soluble EGF. The mechanisms underlying this synergy between the adhesion ligand and EGF are discussed.

Integrin signaling in mammary epithelial cells and breast cancer

Cells sense and respond to the extracellular matrix (ECM) by way of integrin receptors, which facilitate cell adhesion and intracellular signaling. Advances in understanding the mammary epithelial cell hierarchy are converging with new developments that reveal how integrins regulate the normal mammary gland. But in breast cancer, integrin signaling contributes to the development and progression of tumors. This paper highlights recent studies which examine the role of integrin signaling in mammary epithelial cells and their malignant counterparts.

New insights into cartilage repair - the role of migratory progenitor cells in osteoarthritis

Osteoarthritis is one of the most common musculo-skeletal diseases with a complex patholoy and a strong impact on cell biology, differentiation and migration behavior of mesenchymal stem cell-derived progenitor cells. In this review, we elucidate the influence of the pathologically altered extracellular matrix on progenitor cell behavior. Moreover, we discuss the modulation of progenitor cells especially of previously characterized chondrogenic progenitor cells (Koelling et al., 2009) in situ to enhance their regeneration potential. These options comprise the application of growth factors like fibroblast growth factor-2, a Runx-2 knock down and a contemporary anti-inflammatory therapy. This supports endogenous regeneration on behalf of the diseased osteoarthritic cartilage, which otherwise results mainly in an insufficient fibro-cartilaginous repair tissue. Furthermore, new results indicate a role of pericytes in osteoarthritis for these repair attempts. We discuss the biological mechanisms potentially leading to new therapeutic options in osteoarthritis to enhance regeneration in situ.

Synergistic control of mesenchymal stem cell differentiation by nanoscale surface geometry and immobilized growth factors on TiO2 nanotubes

The aim of this study is to elucidate whether combined environmental signals provided by nanoscale topography and by growth factors control cell behavior of mesenchymal stem cells (MSCs) in a synergistic or simply additive manner. Chondrogenic and osteogenic differentiation of MSCs is studied on vertically aligned TiO(2) nanotubes of size 15 and 100 nm with and without immobilized bone morphogenetic protein-2 (BMP-2). Although BMP-2 coating stimulates both chondrogenic and osteogenic differentiation of MSCs, the response strongly depends on the surface nanoscale geometry of the BMP-2-coated nanotubes. Chondrogenic differentiation is strongly supported on 100 nm BMP-2-coated nanotubes, but not on 15 nm nanotubes, which induce spreading and de-differentiation of chondrocytes. A similar response is observed with primary chondrocytes, which maintain their chondrogenic phenotype on BMP-2-coated 100 nm nanotubes, but de-differentiate on 15 nm nanotubes. In contrast, osteogenic differentiation is greatly enhanced on 15 nm but not on 100 nm BMP-2-coated nanotubes as shown previously. Furthermore, covalent immobilization of BMP-2 rescues MSCs from apoptosis occurring on uncoated 100 nm TiO(2) nanotube surfaces. Thus, combined signals provided by BMP-2 immobilized to a defined lateral nanoscale spacing geometry seem to contain environmental cues that are able to modulate a lineage-specific decision of MSC differentiation and cell survival in a synergistic manner.

Cilengitide: a prototypic integrin inhibitor for the treatment of glioblastoma and other malignancies

Integrins are critical intermediaries in a wide spectrum of cancer cell activities and thus represent a highly attractive target in oncology therapy. Nonetheless, successful exploitation of anti-integrin therapeutics has proven challenging to date for cancer patients. In this review, we will focus on cilengitide, an RGD pentapeptide inhibitor of α V integrins. Although several integrin inhibitors are under clinical evaluation, cilengitide is the most clinically advanced and is emerging as a prototype for this class of anticancer therapy. A foundation of encouraging preclinical studies led to a well-designed clinical development plan that culminated in a pivotal phase III study of cilengitide in combination with radiation therapy and temozolomide chemotherapy for newly diagnosed glioblastoma patients. Accrual to this study recently completed, while phase II studies of cilengitide are ongoing for head and neck cancer as well as lung cancer. Important future considerations for cilengitide and other integrin-targeting agents will likely include the identification of optimal combinatorial regimens and the delineation of biomarkers associated with efficacy.

Cooperative effects in differentiation and proliferation between PDGF-BB and matrix derived synthetic peptides in human osteoblasts

Background

Enhancing osteogenic capabilities of bone matrix for the treatment of fractures and segmental defects using growth factors is an active area of research. Recently, synthetic peptides like AC- 100, TP508 or p-15 corresponding to biologically active sequences of matrix proteins have been proven to stimulate bone formation. The platelet-derived growth factor (PDGF) BB has been identified as an important paracrine factor in early bone healing. We hypothesized that the combined use of PDGF-BB with synthetic peptides could result in an increase in proliferation and calcification of osteoblast-like cells.

Methods

Osteoblast-like cell cultures were treated with PDGF and synthetic peptides, singly and as combinations, and compared to non-treated control cell cultures. The cultures were evaluated at days 2, 5, and 10 in terms of cell proliferation, calcification and gene expression of alkaline phosphate, collagen I and osteocalcin.

Results

Experimental findings revealed that the addition of PDGF, p-15 and TP508 and combinations of PDGF/AC-100, PDGF/p-15 and PDGF/TP508 resulted in an increase in proliferating osteoblasts, especially in the first 5 days of cultivation. Proliferation did not significantly differ between single factors and factor combinations (p > 0.05). The onset of calcification in osteoblasts occurred earlier and was more distinct compared to the corresponding control or PDGF stimulation alone. Significant difference was found for the combined use of PDGF/p-15 and PDGF/AC-100 (p < 0.05).

Conclusions

Our findings indicate that PDGF exhibits cooperative effects with synthetic peptides in differentiation and proliferation. These cooperative effects cause a significant early calcification of osteoblast-like cells (p < 0.05). We suggest the combination of synthetic peptides and PDGF as a potential clinical approach for accelerating bone healing or coating osteosynthesis materials.

The anabolic benefits of venous blood flow restriction training may be induced by muscle cell swelling

Venous blood flow restriction (VBFR) combined with low intensity resistance exercise (20-30% concentric 1-RM) has been observed to result in skeletal muscle hypertrophy, increased strength, and increased endurance. Knowledge of the mechanisms behind the benefits seen with VBFR is incomplete, but the benefits have traditionally been thought to occur from the decreased oxygen and accumulation of metabolites. Although many of the proposed mechanisms appear valid and are likely true with VBFR combined with resistance exercise, there are certain situations in which benefits are observed without a large accumulation of metabolites and/or large increases in fast twitch fiber type recruitment. Cell swelling appears to be a likely mechanism that appears to be present throughout all studies. VBFR may be able to induce cell swelling through a combination of blood pooling, accumulation of metabolites, and reactive hyperemia following the removal of VBFR which may contribute to skeletal muscle adaptations that occur with VBFR. We hypothesize that cell swelling is important for muscle growth and strength adaptation but when coupled with higher metabolic accumulation, this adaptation is even greater.

Contra-regulation of calcium- and integrin-binding protein 1-induced cell migration on fibronectin by PAK1 and MAP kinase signaling

Calcium- and integrin-binding protein 1 (CIB1) has been shown to be involved in cell spreading and migration. The signaling events regulated by CIB1 during cell migration are poorly understood. Here we found that accumulation of CIB1 at the tip of the filopodia requires an intact cytoskeleton. Depletion of CIB1 using shRNA affects formation of FAK- and phosphotyrosine-rich focal adhesions without affecting stress fiber formation. Overexpression of CIB1 results in cell migration on fibronectin and Erk1/2 MAP kinase activation. CIB1-induced cell migration is dependent upon Erk1/2 activation, since it is inhibited by the MEK-specific inhibitor PD98059. Furthermore, CIB1-induced cell migration, as well as Erk1/2 activation, is dependent on PKC, Src family kinases as well as PI-3 kinase as it is inhibited by bisindolylmaleimide 1, PP2, and wortmannin, respectively, in a dose-dependent manner. Co-expression of dominant-negative Cdc42 completely abolished CIB1-induced cell migration. Additionally, co-expression of constitutively active, but not dominant negative PAK1, a CIB1 binding protein, inhibited CIB1-induced cell migration. These results suggest that CIB1 positively regulates cell migration and is necessary for the recruitment of FAK to the focal adhesions. Furthermore, CIB1-induced cell migration is dependent on MAP kinase signaling and its function is attenuated by PAK1.

Involvement of lipid rafts in adhesion-induced activation of Met and EGFR

Background

Cell adhesion has been shown to induce activation of certain growth factor receptors in a ligand-independent manner. However, the mechanism for such activation remains obscure.

Methods

Human epidermal carcinoma A431 cells were used as a model to examine the mechanism for adhesion-induced activation of hepatocyte growth factor receptor Met and epidermal growth factor receptor (EGFR). The cells were suspended and replated on culture dishes under various conditions. The phosphorylation of Met at Y1234/1235 and EGFR at Y1173 were used as indicators for their activation. The distribution of the receptors and lipid rafts on the plasma membrane were visualized by confocal fluorescent microscopy and total internal reflection microscopy.

Results

We demonstrate that Met and EGFR are constitutively activated in A431 cells, which confers proliferative and invasive potentials to the cells. The ligand-independent activation of Met and EGFR in A431 cells relies on cell adhesion to a substratum, but is independent of cell spreading, extracellular matrix proteins, and substratum stiffness. This adhesion-induced activation of Met and EGFR cannot be attributed to Src activation, production of reactive oxygen species, and the integrity of the cytoskeleton. In addition, we demonstrate that Met and EGFR are independently activated upon cell adhesion. However, partial depletion of Met and EGFR prevents their activation upon cell adhesion, suggesting that overexpression of the receptors is a prerequisite for their self-activation upon cell adhesion. Although Met and EGFR are largely distributed in 0.04% Triton-insoluble fractions (i.e. raft fraction), their activated forms are detected mainly in 0.04% Triton-soluble fractions (i.e. non-raft fraction). Upon cell adhesion, lipid rafts are accumulated at the cell surface close to the cell-substratum interface, while Met and EGFR are mostly excluded from the membrane enriched by lipid rafts.

Conclusions

Our results suggest for the first time that cell adhesion to a substratum may induce a polarized distribution of lipid rafts to the cell-substratum interface, which may allow Met and EGFR to be released from lipid rafts, thus leading to their activation in a ligand-independent manner.

Diabetic lower extremity wounds: the rationale for growth factors-based infiltration treatment

Repair machinery and local infection control failure contribute to wound chronicity and lower extremity amputation in diabetic patients. In these wounds, inflammation is a proximal condition which disrupts wound matrix turnover and the local redox balance. Contemporary therapeutic interventions are relatively broad including drugs, devices and surgical procedures. However, clinical efficacy remains modest and recurrences are frequent. Recombinant growth factors advent was followed by their premature and empiric introduction in the clinical practice. Its topical administration is still challenged by local kinetic and pharmacodynamic limitations related to the hostile microenvironment of chronic wounds. The rationale of infiltrating epidermal growth factor (EGF) down inside complex diabetic wounds as an alternative treatment modality is described here. The concept emerged from two experimental evidences: (a) locally infiltrated EGF prevented trophic ulcers and limb necrosis upon denervation, (b) acute, controlled experimental wounds' exudate exhibited proteolytic activity. Depositing EGF in deep cells' responsive strata allows for two main pharmacological actions indispensable for chronic wounds healing: cyto-protection and proliferation of fibroblasts and endothelial cells, thus inducing progressive granulation. Ten years of clinical experience have validated laboratory and theoretical concepts, while most importantly have improved quality-of-life to thousands of diabetic patients.

Mechanism of p27 upregulation induced by downregulation of cathepsin B and uPAR in glioma

Cathepsin B and urokinase plasminogen activator receptor (uPAR) are overexpressed in gliomas. Deregulation of the G1 phase cell cycle machinery is a common feature of cancers. p27(Kip1) (p27) is one of the major cyclin-CDK regulators in the G1 phase. uPAR and cathepsin B downregulation was recently shown to induce p27 expression through PI3K/Akt/FOXO3a signaling. Since uPAR and cathepsin B knockdown also decreased phosphorylation of ERK, we hypothesized that ERK also has a role to play in p27 induction. As induction of p27 is due to an increase in gene transcription, we investigated the roles of c-Myc and E2F1 transcription factors which have been shown to potently affect p27 promoter activity. In the present study, shRNA against cathepsin B and uPAR as well as specific inhibitors, Wortmannin (10 μM) and U0126 (10 μM), were used to determine the roles of AKT and ERK signaling on p27 expression. Immunoblot analysis demonstrated that downregulation of both p-ERK and p-AKT downstream of EGFR and β1 integrin are involved in the p27 upregulation. Cathepsin B and uPAR downregulation induced E2F1 and decreased phosphorylaion of pocket proteins and c-Myc expression. CHIP analysis and luciferase expression studies confirmed the functional association of transcription factor E2F1 to the p27 promoter. Further, c-Myc-Max interaction inhibitor studies showed an inverse pattern of c-Myc and p27 expression. Also, cathepsin B and uPAR downregulation reduced tumor growth and increased p27 nuclear expression in vivo. In summary, cathepsin B and uPAR downregulation reduced p-ERK levels and c-Myc expression, increased expression of E2F1 and FOXO3a, decreased phosphorylation of pocket proteins and thus upregulated p27 expression in glioma cells.

Cilengitide: an RGD pentapeptide ανβ3 and ανβ5 integrin inhibitor in development for glioblastoma and other malignancies

Cilengitide, a cyclicized arginine-glycine-aspartic acid-containing pentapeptide, potently blocks ανβ3 and ανβ5 integrin activation. Integrins are upregulated in many malignancies and mediate a wide variety of tumor-stroma interactions. Cilengitide and other integrin-targeting therapeutics have preclinical activity against many cancer subtypes including glioblastoma (GBM), the most common and deadliest CNS tumor. Cilengitide is active against orthotopic GBM xenografts and can augment radiotherapy and chemotherapy in these models. In Phase I and II GBM trials, cilengitide and the combination of cilengitide with standard temozolomide and radiation demonstrate consistent antitumor activity and a favorable safety profile. Cilengitide is currently under evaluation in a pivotal, randomized Phase III study (Cilengitide in Combination With Temozolomide and Radiotherapy in Newly Diagnosed Glioblastoma Phase III Randomized Clinical Trial [CENTRIC]) for newly diagnosed GBM. In addition, randomized controlled Phase II studies with cilengitide are ongoing for non-small-cell lung cancer and squamous cell carcinoma of the head and neck. Cilengitide is the first integrin inhibitor in clinical Phase III development for oncology.

The effect of cell-ECM adhesion on signalling via the ErbB family of growth factor receptors

Integrins and growth factor receptors of the ErbB family are involved in the regulation of cellular interactions with the extracellular microenvironment. Cross-talk between these two groups of transmembrane receptors is essential for cellular responses and can be regulated through the formation of multimolecular complexes. Tetraspanins as facilitators and building blocks of specialized microdomains may be involved in this process. In the present study, we demonstrated that, in contrast with previous reports, integrin-mediated adhesion did not stimulate ligand-independent activation of ErbB receptors in epithelial cells. However, integrin-dependent adhesion potentiated ligand-induced activation of EGFR (epidermal growth factor receptor) and ErbB2 and facilitated receptor homo- and hetero-dimerization. The actin cytoskeleton appeared to play a critical role in this phenomenon.

Essential roles of fibronectin in the development of the left-right embryonic body plan

Studies in Xenopus laevis suggested that cell-extracellular matrix (ECM) interactions regulate the development of the left-right axis of asymmetry; however, the identities of ECM components and their receptors important for this process have remained unknown. We discovered that FN is required for the establishment of the asymmetric gene expression pattern in early mouse embryos by regulating morphogenesis of the node, while cellular fates of the nodal cells, canonical Wnt and Shh signaling within the node were not perturbed by the absence of FN. FN is also required for the expression of Lefty 1/2 and activation of SMADs 2 and 3 at the floor plate, while cell fate specification of the notochord and the floor plate, as well as signaling within and between these two embryonic organizing centers remained intact in FN-null mutants. Furthermore, our experiments indicate that a major cell surface receptor for FN, integrin α5β1, is also required for the development of the left-right asymmetry, and that this requirement is evolutionarily conserved in fish and mice. Taken together, our studies demonstrate the requisite role for a structural ECM protein and its integrin receptor in the development of the left-right axis of asymmetry in vertebrates.

Regenerative potentials of platelet-rich plasma enhanced by collagen in retrieving pro-inflammatory cytokine-inhibited chondrogenesis

This study was undertaken to evaluate the role of collagen matrix to enhance platelet-rich plasma (PRP) effects on pro-inflammatory cytokine-induced arthritic model. We have previously demonstrated the highly regenerative roles of PRP to restore disc degeneration and osteoporosis. In this study, PRP modulated by collagen matrix was used as a regenerative and anti-inflammatory mediator to rescue the chondrocyte degeneration induced by pro-inflammatory cytokines IL-1β (10 ng/ml)+TNF-α (20 ng/ml). First, the MTT result indicated that 1 ng/ml TGF-β1 in PRP showed an optimal dosage for chondrocytes proliferation. The chondrogenic-specific gene expressions were rescued by PRP from the inhibition of IL-1β+TNF-α, especially under the modulation of collagen matrix. The inflammatory molecules activated by IL-1β+TNF-α were also significantly diminished by PRP with collagen matrix. The membrane receptors integrin α1β1 and CD44 were strongly inhibited by IL-1β+TNF-α, while this inhibition was then recovered by PRP in collagen coating condition. In a 3D model encapsulated with collagen, PRP-induced chondrogenesis were highly enhanced, such as strong restoration of type II collagen and proteoglycan from the inhibition of IL-1β+TNF-α. The result indicated that collagen matrix enhances the effect of PRP on chondrogenesis in response to pro-inflammatory cytokines. The combination of PRP and collagen matrix might facilitate a physiological microenvironment beneficial for maintaining chondrocyte homeostasis and represents an advanced osteoarthritis therapy for clinical applications.

Epidermal Growth Factor Receptor (EGFR) Crosstalks in Liver Cancer

Hepatocarcinogenesis is a complex multistep process in which many different molecular pathways have been implicated. Hepatocellular carcinoma (HCC) is refractory to conventional chemotherapeutic agents, and the new targeted therapies are meeting with limited success. Interreceptor crosstalk and the positive feedback between different signaling systems are emerging as mechanisms of targeted therapy resistance. The identification of such interactions is therefore of particular relevance to improve therapeutic efficacy. Among the different signaling pathways activated in hepatocarcinogenesis the epidermal growth factor receptor (EGFR) system plays a prominent role, being recognized as a “signaling hub” where different extracellular growth and survival signals converge. EGFR can be transactivated in response to multiple heterologous ligands through the physical interaction with multiple receptors, the activity of intracellular kinases or the shedding of EGFR-ligands. In this article we review the crosstalk between the EGFR and other signaling pathways that could be relevant to liver cancer development and treatment.

Molecular architecture and function of matrix adhesions

Cell adhesions mediate important bidirectional interactions between cells and the extracellular matrix. They provide an interactive interface between the extracellular chemical and physical environment and the cellular scaffolding and signaling machinery. This dynamic, reciprocal regulation of intracellular processes and the matrix is mediated by membrane receptors such as the integrins, as well as many other components that comprise the adhesome. Adhesome constituents assemble themselves into different types of cell adhesion structures that vary in molecular complexity and change over time. These cell adhesions play crucial roles in cell migration, proliferation, and determination of cell fate.

Thrombospondin-1 opens the paracellular pathway in pulmonary microvascular endothelia through EGFR/ErbB2 activation

Thrombospondin-1 (TSP1) is a multidomain protein that contains epidermal growth factor (EGF)-like repeats that indirectly activate the EGF receptor (EGFR) and selected downstream signaling pathways. In these studies, we show that TSP1 opens the paracellular pathway in human lung microvascular endothelial cells (HMVEC-Ls) in a dose-, time-, and protein tyrosine kinase (PTK)-dependent manner. TSP1 increased tyrosine phosphorylation of proteins enriched to intercellular boundaries including the zonula adherens (ZA) proteins, vascular endothelial-cadherin, γ-catenin, and p120 catenin. In HMVEC-Ls, EGFR and ErbB2 are expressed at low levels, and both heterodimerize and tyrosine autophosphorylate in response to TSP1. Prior EGFR-selective PTK inhibition with AG1478 or ErbB2-selective PTK inhibition with AG825 protected against TSP1-induced tyrosine phosphorylation of ZA proteins and barrier disruption. Preincubation of HMVEC-Ls with an EGFR ectodomain-blocking antibody also prevented TSP1-induced opening of the paracellular pathway. Therefore, in HMVEC-Ls, TSP1 increases tyrosine phosphorylation of ZA proteins and opens the paracellular pathway, in part, through EGFR/ErbB2 activation. Surprisingly, recombinant TSP1 EGF-like repeats 1-3 and the high-affinity EGFR ligands, EGF, TGF-α, and amphiregulin, each failed to increase paracellular permeability. However, HMVEC-Ls in which EGFR was overexpressed became responsive to the EGF-like repeats of TSP1 as well as to EGF. These studies indicate that TSP1 disrupts the endothelial barrier through EGFR/ErbB2 activation although additional signals are necessary in cells with low receptor expression.

Mesenchymal stem cell migration is regulated by fibronectin through α5β1-integrin-mediated activation of PDGFR-β and potentiation of growth factor signals

Cell migration during vascular remodelling is regulated by crosstalk between growth factor receptors and integrin receptors, which together coordinate cytoskeletal and motogenic changes. Here, we report extracellular matrix (ECM)-directed crosstalk between platelet-derived growth factor receptor (PDGFR)-β and α5β1-integrin, which controls the migration of mesenchymal stem (stromal) cells (MSCs). Cell adhesion to fibronectin induced α5β1-integrin-dependent phosphorylation of PDGFR-β in the absence of growth factor stimulation. Phosphorylated PDGFR-β co-immunoprecipitated with α5-integrin and colocalised with α5β1-integrin in the transient tidemarks of focal adhesions. Adhesion to fibronectin also strongly potentiated PDGF-BB-induced PDGFR-β phosphorylation and focal adhesion kinase (FAK) activity, in an α5β1-integrin-dependent manner. PDGFR-β-induced phosphoinositide 3-kinase (PI3K) and Akt activity, actin reorganisation and cell migration were all regulated by fibronectin and α5β1-integrin. This synergistic relationship between α5β1-integrin and PDGFR-β is a fundamental determinant of cell migration. Thus, fibronectin-rich matrices can prime PDGFR-β to recruit mesenchymal cells at sites of vascular remodelling.

Peptide inhibition of human cytomegalovirus infection

BACKGROUND

Human cytomegalovirus (HCMV) is the most prevalent congenital viral infection in the United States and Europe causing significant morbidity and mortality to both mother and child. HCMV is also an opportunistic pathogen in immunocompromised individuals, including human immunodeficiency virus (HIV)- infected patients with AIDS, and solid organ and allogeneic stem cell transplantation recipients. Current treatments for HCMV-associated diseases are insufficient due to the emergence of drug-induced resistance and cytotoxicity, necessitating novel approaches to limit HCMV infection. The aim of this study was to develop therapeutic peptides targeting glycoprotein B (gB), a major glycoprotein of HCMV that is highly conserved across the Herpesviridae family, that specifically inhibit fusion of the viral envelope with the host cell membrane preventing HCMV entry and infection.

RESULTS

Using the Wimley-White Interfacial Hydrophobicity Scale (WWIHS), several regions within gB were identified that display a high potential to interact with lipid bilayers of cell membranes and hydrophobic surfaces within proteins. The ability of synthetic peptides analogous to WWIHS-positive sequences of HCMV gB to inhibit viral infectivity was evaluated. Human foreskin fibroblasts (HFF) were infected with the Towne-GFP strain of HCMV (0.5 MOI), preincubated with peptides at a range of concentrations (78 nm to 100 μM), and GFP-positive cells were visualized 48 hours post-infection by fluorescence microscopy and analyzed quantitatively by flow cytometry. Peptides that inhibited HCMV infection demonstrated different inhibitory concentration curves indicating that each peptide possesses distinct biophysical properties. Peptide 174-200 showed 80% inhibition of viral infection at a concentration of 100 μM, and 51% and 62% inhibition at concentrations of 5 μM and 2.5 μM, respectively. Peptide 233-263 inhibited infection by 97% and 92% at concentrations of 100 μM and 50 μM, respectively, and 60% at a concentration of 2.5 μM. While peptides 264-291 and 297-315, individually failed to inhibit viral infection, when combined, they showed 67% inhibition of HCMV infection at a concentration of 0.125 μM each.

CONCLUSIONS

Peptides designed to target putative fusogenic domains of gB provide a basis for the development of novel therapeutics that prevent HCMV infection.

Autocrine-controlled formation and function of tissue-like aggregates by primary hepatocytes in micropatterned hydrogel arrays

The liver carries out a variety of essential functions regulated in part by autocrine signaling, including hepatocyte-produced growth factors and extracellular matrix (ECM). The local concentrations of autocrine factors are governed by a balance between receptor-mediated binding at the cell surface and diffusion into the local matrix and are thus expected to be influenced by the dimensionality of the cell culture environment. To investigate the role of growth factor and ECM-modulated autocrine signaling in maintaining appropriate primary hepatocyte survival, metabolic functions, and polarity, we created three-dimensional cultures of defined geometry using micropatterned semisynthetic polyethylene glycol-fibrinogen hydrogels to provide a mechanically compliant, nonadhesive material platform that could be modified by cell-secreted factors. We found that in the absence of exogenous peptide growth factors or ECM, hepatocytes retain the epidermal growth factor (EGF) receptor ligands (EGF and transforming growth factor-α) and the proto-oncogenic mesenchymal epithelial transition factor (c-MET) ligand hepatocyte growth factor (HGF), along with fibronectin. Further, hepatocytes cultured in this three-dimensional microenvironment maintained high levels of liver-specific functions over the 10-day culture period. Function-blocking inhibitors of α5β1 or EGF receptor dramatically reduced cell viability and function, suggesting that signaling by both these receptors is needed for in vitro survival and function of hepatocytes in the absence of other exogenous signals.

Basement membranes: cell scaffoldings and signaling platforms

Basement membranes are widely distributed extracellular matrices that coat the basal aspect of epithelial and endothelial cells and surround muscle, fat, and Schwann cells. These extracellular matrices, first expressed in early embryogenesis, are self-assembled on competent cell surfaces through binding interactions among laminins, type IV collagens, nidogens, and proteoglycans. They form stabilizing extensions of the plasma membrane that provide cell adhesion and that act as solid-phase agonists. Basement membranes play a role in tissue and organ morphogenesis and help maintain function in the adult. Mutations adversely affecting expression of the different structural components are associated with developmental arrest at different stages as well as postnatal diseases of muscle, nerve, brain, eye, skin, vasculature, and kidney.

Fibronectin growth factor-binding domains are required for fibroblast survival

Fibronectin (FN) is required for embryogenesis, morphogenesis, and wound repair, and its Arg-Gly-Asp-containing central cell-binding domain (CCBD) is essential for mesenchymal cell survival and growth. Here, we demonstrate that FN contains three growth factor-binding domains (FN-GFBDs) that bind platelet-derived growth factor-BB (PDGF-BB), a potent fibroblast survival and mitogenic factor. These sites bind PDGF-BB with dissociation constants of 10-100 nM. FN-null cells cultured on recombinant CCBD (FNIII(8-11)) without a FN-GFBD demonstrated minimal metabolism and underwent autophagy at 24 hours, followed by apoptosis at 72 hours, even in the presence of PDGF-BB. In contrast, FN-null cells plated on FNIII(8-11) contiguous with FN-GFBD survived without, and proliferated with, PDGF-BB. FN-null cell survival on FNIII(8-11) and noncontiguous arrays of FN-GFBDs required these domains to be adsorbed on the same surface, suggesting the existence of a mesenchymal cell-extracellular matrix synapse. Thus, fibroblast survival required GF stimulation in the presence of a FN-GFBD, as well as adhesion to FN through the CCBD. The findings that fibroblast survival is dependent on FN-GFBD underscore the critical importance of pericellular matrix for cell survival and have significant implications for cutaneous wound healing and regeneration.

Integrins: versatile receptors controlling melanocyte adhesion, migration and proliferation

From the onset of melanocyte specification from the neural crest, throughout their migration during embryogenesis and until they reside in their niche in the basal keratinocyte layer, melanocytes interact in dynamic ways with the extracellular environment of the growing embryo. To recognize and to adhere to their environment, melanocytes depend on heterodimeric cell surface receptors of the family of integrins. In addition to the control of adhesive interactions between melanocytes and the extracellular matrix scaffold secreted by fibroblasts and keratinocytes, the integrin receptors allow cells also to sense the mechanical condition of the extracellular environment, responding by intracellular signaling, triggering cell survival, proliferation or migration events. In this review, we summarize the recently emerged concepts that explain integrin-dependent adhesion and how this adhesion system interfaces with integrin-dependent signaling events. The gained information will help to understand melanocyte behavior in pathological situations such as melanoma growth and metastasis formation.

bFGF regulates PI3-kinase-Rac1-JNK pathway and promotes fibroblast migration in wound healing

Fibroblast proliferation and migration play important roles in wound healing. bFGF is known to promote both fibroblast proliferation and migration during the process of wound healing. However, the signal transduction of bFGF-induced fibroblast migration is still unclear, because bFGF can affect both proliferation and migration. Herein, we investigated the effect of bFGF on fibroblast migration regardless of its effect on fibroblast proliferation. We noticed involvement of the small GTPases of the Rho family, PI3-kinase, and JNK. bFGF activated RhoA, Rac1, PI3-kinase, and JNK in cultured fibroblasts. Inhibition of RhoA did not block bFGF-induced fibroblast migration, whereas inhibition of Rac1, PI3-kinase, or JNK blocked the fibroblast migration significantly. PI3-kinase-inhibited cells down-regulated the activities of Rac1 and JNK, and Rac1-inhibited cells down-regulated JNK activity, suggesting that PI3-kinase is upstream of Rac1 and that JNK is downstream of Rac1. Thus, we concluded that PI3-kinase, Rac1, and JNK were essential for bFGF-induced fibroblast migration, which is a novel pathway of bFGF-induced cell migration.

Direct binding of the EGF-like domain of neuregulin-1 to integrins ({alpha}v{beta}3 and {alpha}6{beta}4) is involved in neuregulin-1/ErbB signaling

Integrin-growth factor receptor cross-talk plays a role in growth factor signaling, but the specifics are unclear. In a current model, integrins and growth factor receptors independently bind to their ligands (extracellular matrix and growth factors, respectively). We discovered that neuregulin-1 (NRG1), either as an isolated EGF-like domain or as a native multi-domain form, binds to integrins αvβ3 (with a K(D) of 1.36 × 10(-7) m) and α6β4. Docking simulation predicted that three Lys residues at positions 180, 184, and 186 of the EGF-like domain are involved in integrin binding. Mutating these residues to Glu individually or in combination markedly suppressed integrin binding and ErbB3 phosphorylation. Mutating all three Lys residues to Glu (the 3KE mutation) did not affect the ability of NRG1 to bind to ErbB3 but markedly reduced the ability of NRG1 to induce ErbB3 phosphorylation and AKT and Erk1/2 activation in MCF-7 and T47D human breast cancer cells. This suggests that direct integrin binding to NRG1 is critical for NRG1/ErbB signaling. Notably, stimulation of cells with WT NRG1 induced co-precipitation of ErbB3 with α6β4 and with αvβ3 to a much lower extent. This suggests that WT NRG1 induces integrin-NRG1-ErbB3 ternary complex formation. In contrast, the 3KE mutant was much less effective in inducing ternary complex formation than WT NRG1, suggesting that this process depends on the ability of NRG1 to bind to integrins. These results suggest that direct NRG1-integrin interaction mediates integrin-ErbB cross-talk and that α6β4 plays a major role in NRG-ErbB signaling in these cancer cells.

The glycoprotein B disintegrin-like domain binds beta 1 integrin to mediate cytomegalovirus entry

Cellular integrins were identified as human cytomegalovirus (HCMV) entry receptors and signaling mediators in both fibroblasts and endothelial cells. The goal of these studies was to determine the mechanism by which HCMV binds to cellular integrins to mediate virus entry. HCMV envelope glycoprotein B (gB) has sequence similarity to the integrin-binding disintegrin-like domain found in the ADAM (a disintegrin and metalloprotease) family of proteins. To test the ability of this region to bind to cellular integrins, we generated a recombinant soluble version of the gB disintegrin-like domain (gB-DLD). The gB-DLD protein bound to human fibroblasts in a specific, dose-dependent and saturable manner that required the expression of an intact beta1 integrin ectodomain. Furthermore, a physical association between gB-DLD and beta1 integrin was demonstrated through in vitro pull-down assays. The function of this interaction was shown by the ability of cell-bound gB-DLD to efficiently block HCMV entry and the infectivity of multiple in vivo target cells. Additionally, rabbit polyclonal antibodies raised against gB-DLD neutralized HCMV infection. Mimicry of the ADAM family disintegrin-like domain by HCMV gB represents a novel mechanism for integrin engagement by a virus and reveals a unique therapeutic target for HCMV neutralization. The strong conservation of the DLD across beta- and gammaherpesviruses suggests that integrin recognition and utilization may be a more broadly conserved feature throughout the Herpesviridae.

Endothelial Cell Adhesion to the Extracellular Matrix Induces c-Src–Dependent VEGFR-3 Phosphorylation Without the Activation of the Receptor Intrinsic Kinase Activity

Rationale : Integrins cooperate with growth factor receptors to promote downstream signaling for cell proliferation and migration. However, the mechanism of receptor activation is still unknown.

Objective : To analyze the mechanism of phosphorylation of the vascular endothelial growth factor receptor (VEGFR)-3 by cell adhesion.

Methods and Results : We show that VEGFR-3 phosphorylation, induced by cell attachment to the extracellular matrix, is independent from the intrinsic kinase activity of the receptor, as evidenced from phosphorylation cell adhesion experiments with a mutant kinase dead receptor or in the presence of the specific kinase inhibitor MAZ 51. Cell adhesion experiments in the presence of the c-Src inhibitor PP2 or in fibroblast triple knockout for c-Src, Yes, and Fyn (SYF) demonstrate that VEGFR-3 phosphorylation, induced by extracellular matrix, is mediated by c-Src. Kinase assays in vitro with recombinant c-Src show that VEGFR-3 is a direct c-Src target and mass spectrometry analysis identified the sites phosphorylated by c-Src as tyrosine 830, 833, 853, 1063, 1333, and 1337, demonstrating that integrin-mediated receptor phosphorylation induces a phosphorylation pattern that is distinct from that induced by growth factors. Furthermore, pull-down assays show that integrin-mediated VEGFR-3 phosphorylation activates the recruitment to the receptor of the adaptor proteins CRKI/II and SHC inducing activation of JNK.

Conclusions : These data suggest that cell adhesion to extracellular matrix induces a downstream signaling using the tyrosine kinase receptor VEGFR-3 as scaffold.

Blood flow restriction exercise stimulates mTORC1 signaling and muscle protein synthesis in older men

The loss of skeletal muscle mass during aging, sarcopenia, increases the risk for falls and dependence. Resistance exercise (RE) is an effective rehabilitation technique that can improve muscle mass and strength; however, older individuals are resistant to the stimulation of muscle protein synthesis (MPS) with traditional high-intensity RE. Recently, a novel rehabilitation exercise method, low-intensity RE, combined with blood flow restriction (BFR), has been shown to stimulate mammalian target of rapamycin complex 1 (mTORC1) signaling and MPS in young men. We hypothesized that low-intensity RE with BFR would be able to activate mTORC1 signaling and stimulate MPS in older men. We measured MPS and mTORC1-associated signaling proteins in seven older men (age 70+/-2 yr) before and after exercise. Subjects were studied identically on two occasions: during BFR exercise [bilateral leg extension exercise at 20% of 1-repetition maximum (1-RM) with pressure cuff placed proximally on both thighs and inflated at 200 mmHg] and during exercise without the pressure cuff (Ctrl). MPS and phosphorylation of signaling proteins were determined on successive muscle biopsies by stable isotopic techniques and immunoblotting, respectively. MPS increased 56% from baseline after BFR exercise (P<0.05), while no change was observed in the Ctrl group (P>0.05). Downstream of mTORC1, ribosomal S6 kinase 1 (S6K1) phosphorylation and ribosomal protein S6 (rpS6) phosphorylation increased only in the BFR group after exercise (P<0.05). We conclude that low-intensity RE in combination with BFR enhances mTORC1 signaling and MPS in older men. BFR exercise is a novel intervention that may enhance muscle rehabilitation to counteract sarcopenia.