Cooperation between integrins and growth factor receptors in signaling and endocytosis.

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Click Biotinylation of PLGA Template for Biotin Receptor Oriented Delivery of Doxorubicin Hydrochloride in 4T1 Cell-Induced Breast Cancer.

PLGA was functionalized with PEG and biotin using click chemistry to generate a biotin receptor targeted copolymer (biotinylated-PEG-PLGA) which in turn was used to fabricate ultrafine nanoparticles (BPNP) of doxorubicin hydrochloride (DOX) for effective delivery in 4T1 cell induced breast cancer. However, adequate entrapment of a hydrophilic bioactive like DOX in a hydrophobic polymer system made of PLGA is not usually possible. We therefore modified a conventional W/O/W emulsion method by utilizing NH4Cl in the external phase to constrain DOX in dissolved polymer phase by suppressing DOX's inherent aqueous solubility as per common ion effect. This resulted in over 8-fold enhancement in entrapment efficiency of DOX inside BPNP, which otherwise is highly susceptible to leakage due to its relatively high aqueous solubility. TEM and DLS established BPNP to be sized below 100 nm, storage stability studies showed that BPNP were stable for one month at 4 °C, and in vitro release suggested significant control in drug release. Extensive in vitro and in vivo studies were conducted to propound anticancer and antiproliferative activity of BPNP. Plasma and tissue distribution study supplemented by pertinent in vivo fluorescence imaging mapped the exact fate of DOX contained inside BPNP once it was administered intravenously. A comparative safety profile via acute toxicity studies in mice was also generated to out rightly establish usefulness of BPNP. Results suggest that BPNP substantially enhance anticancer activity of DOX while simultaneously mitigating its toxic potential due to altered spatial and temporal presentation of drug and consequently deserve further allometric iteration.

Regulation of hemidesmosome disassembly by growth factor receptors.

Hemidesmosomes (HDs) promote the stable adhesion of basal epithelial cells to the underlying basement membrane (BM). Critical for the mechanical stability of the HD is the interaction between integrin alpha6beta4 and plectin, which is destabilized when HD disassembly is required, for instance, to allow keratinocyte migration during wound healing. Growth factors such as epidermal growth factor (EGF) can trigger HD disassembly and induce phosphorylation of the beta4 intracellular domain. Whereas tyrosine phosphorylation appears to mediate cooperation with growth factor signaling pathways and invasion in carcinoma cells, serine phosphorylation seems the predominant mechanism for regulating HD destabilization. Here, we discuss recent advances that shed light on the residues involved, the identity of the kinases that phosphorylate them, and the interactions that become disrupted by these phosphorylations.

Understanding cytokine and growth factor receptor activation mechanisms.

Our understanding of the detailed mechanism of action of cytokine and growth factor receptors - and particularly our quantitative understanding of the link between structure, mechanism and function - lags significantly behind our knowledge of comparable functional protein classes such as enzymes, G protein-coupled receptors, and ion channels. In particular, it remains controversial whether such receptors are activated by a mechanism of ligand-induced oligomerization, versus a mechanism in which the ligand binds to a pre-associated receptor dimer or oligomer that becomes activated through subsequent conformational rearrangement. A major limitation to progress has been the relative paucity of methods for performing quantitative mechanistic experiments on unmodified receptors expressed at endogenous levels on live cells. In this article, we review the current state of knowledge on the activation mechanisms of cytokine and growth factor receptors, critically evaluate the evidence for and against the different proposed mechanisms, and highlight other key questions that remain unanswered. New approaches and techniques have led to rapid recent progress in this area, and the field is poised for major advances in the coming years which promise to revolutionize our understanding of this large and biologically and medically important class of receptors.

[(Cp-R)M(CO)3] (M = Re or 99mTc) conjugates for theranostic receptor targeting.

Cyclopentadienyl complexes of 99mTc became accessible via a retro Diels-Alder synthetic approach of dimerized cyclopentadiene derivatives. So far, this approach was limited to derivatives comprising a carboxylic acid group, directly conjugated to the Cp-ring, leading to complexes [(C5H5COOH)99mTc(CO)3] and [(C5H5CONH-R)99mTc(CO)3], respectively. The introduction of an -NCO group via Curtius rearrangement and subsequent in situ reactions with alcohols or amines gave [(C5H5NHCO-OR)2] and [(C5H5NHCO-NHR)2]. To increase the spacer lengths between the Cp-ring and the functional groups, methylene and ethylene spacers were introduced to yield C5H5-CH2COOH and C5H5-C2H4COOH respectively. The latter Cp-derivatives reacted with [99mTcO4)]- and in the presence of CO releasing/reducing agents to the corresponding [(C5H5-spacer-COOH)99mTc(CO)3] complexes. The carboxylato groups can be derivatized with targeting functions, leading to structurally altered receptor binding complexes, with 99mTc for imaging and with rhenium for therapy. The nature of the 99mTc complexes was assessed by HPLC comparison with the corresponding rhenium compounds.

One step synthesis of gold-loaded radial mesoporous silica nanospheres and supported lipid bilayer functionalization: towards bio-multifunctional sensors.

A simple synthetic route is developed to achieve gold functionalized radial mesoporous silica nanoparticles (Au-MsNP) synthesized by a one step procedure fully compatible with basic conditions required for the preparation of monodispersed nanospheres. In a second step, Au-MsNP particles have been coated with phospholipid bilayers in order to design an advanced biofunctional platform with the gold metallic nanoparticles previously grown into the pore channels and responsible for a plasmonic activity relevant for biosensing. The size of Au-MsNP is checked by dynamic light scattering while zeta potential measurements reflect their surface charge. The particle morphology is characterized by transmission and scanning electron microscopy and the Si/Au ratios are obtained from energy dispersive X-ray analysis. The textural properties of Au-MsNP, specific surface area and pore size, are determined from N(2) adsorption. The supported bilayers are achieved from vesicles of different phospholipids incubated with Au-MsNP particles. The coating efficiency is investigated by zeta potential and cryo- transmission electron microscopy. The plasmonic activities of bare Au-MsNP particles and coated lipid bilayer Au-MsNP platform are evidenced for two model systems: direct adsorption of bovine serum albumin and molecular recognition events between avidin molecules and biotin receptors integrated in the supported lipid bilayer.

Molecular Docking of Phytochemicals Targeting GFRs as Therapeutic Sites for Cancer: an In Silico Study.

Drug delivery in a safe manner is a major challenge in the drug development process. Growth factor receptors (GFRs) are known to have profound roles in the growth and progression of cancerous cells making these receptors a therapeutic target in the effective treatment of cancer. This work focused on exploring bioactive compounds that can target GFRs using in silico method. In this study, 50 bioactive compounds from different plant sources were screened as anticancer agent against GFRs using drug likeness parameters of Lipinski's rule of five. The molecular docking was performed between phytochemicals and GFRs. Ligands with acceptable drug likeness and binding energy comparable to the standard drugs were further screened to determine their pharmacokinetic activities. This study showed phytochemicals with the binding energy comparable with the standard drugs (Dovitinib and Gefitinib), while ADME, bioactivity score, and bioavailability radar analysis gave further insight on these compounds as potent anticancer agents.

The HGF/MET pathway as target for the treatment of multiple myeloma and B-cell lymphomas.

Hepatocyte growth factor (HGF) and its receptor MET are essential during embryonic development and throughout postnatal life. However, aberrant MET activation, due to overexpression, mutations, or autocrine ligand production, contributes to the development and progression of a variety of human cancers, often being associated with poor clinical outcome and drug resistance. B cell malignancies arise from B cells that are clonally expanded at different stages of differentiation. Despite major therapeutic advances, most mature B cell malignancies remain incurable and biologically-oriented therapeutic strategies are urgently needed. This review addresses the role of the HGF/MET pathway during B cell development and discusses how its aberrant activation contributes to the development of B cell lymphoproliferative disorders, with particular emphasis on multiple myeloma and diffuse large B cell lymphoma. These insights, combined with the recent development of clinical-grade agents targeting the MET pathway, provide the rationale to envision the HGF/MET pathway as a new promising target for the treatment of B cell malignancies.

Revising the mechanism of p75NTR activation: intrinsically monomeric state of death domains invokes the "helper" hypothesis.

The neurotrophin receptor p75NTR plays crucial roles in neuron development and regulates important neuronal processes like degeneration, apoptosis and cell survival. At the same time the detailed mechanism of signal transduction is unclear. One of the main hypotheses known as the snail-tong mechanism assumes that in the inactive state, the death domains interact with each other and in response to ligand binding there is a conformational change leading to their exposure. Here, we show that neither rat nor human p75NTR death domains homodimerize in solution. Moreover, there is no interaction between the death domains in a more native context: the dimerization of transmembrane domains in liposomes and the presence of activating mutation in extracellular juxtamembrane region do not lead to intracellular domain interaction. These findings suggest that the activation mechanism of p75NTR should be revised. Thus, we propose a novel model of p75NTR functioning based on interaction with "helper" protein.

Computational modeling of structurally conserved cancer mutations in the RET and MET kinases: the impact on protein structure, dynamics, and stability.

Structural and biochemical characterization of protein kinases that confer oncogene addiction and harbor a large number of disease-associated mutations, including RET and MET kinases, have provided insights into molecular mechanisms associated with the protein kinase activation in human cancer. In this article, structural modeling, molecular dynamics, and free energy simulations of a structurally conserved mutational hotspot, shared by M918T in RET and M1250T in MET kinases, are undertaken to quantify the molecular mechanism of activation and the functional role of cancer mutations in altering protein kinase structure, dynamics, and stability. The mechanistic basis of the activating RET and MET cancer mutations may be driven by an appreciable free energy destabilization of the inactive kinase state in the mutational forms. According to our results, the locally enhanced mobility of the cancer mutants and a higher conformational entropy are counterbalanced by a larger enthalpy loss and result in the decreased thermodynamic stability. The computed protein stability differences between the wild-type and cancer kinase mutants are consistent with circular dichroism spectroscopy and differential scanning calorimetry experiments. These results support the molecular mechanism of activation, which causes a detrimental imbalance in the dynamic equilibrium shifted toward the active form of the enzyme. Furthermore, computer simulations of the inhibitor binding with the oncogenic and drug-resistant RET mutations have also provided a plausible molecular rationale for the observed differences in the inhibition profiles, which is consistent with the experimental data. Finally, structural mapping of RET and MET cancer mutations and the computed protein stability changes suggest a similar mechanism of activation, whereby the cancer mutations which display the higher oncogenic activity tend to have the greatest destabilization effect on the inactive kinase structure.

Characterization of type I receptors for transforming growth factor-beta and activin.

Transforming growth factor-beta (TGF-beta) and activin exert their effects by binding to heteromeric complexes of type I and type II receptors. The type II receptors for TGF-beta and activin are transmembrane serine-threonine kinases; a series of related receptors, denoted activin receptor-like kinase (ALK) 1 to 5, have recently been identified, and ALK-6 is described here. ALK-5 has been shown to be a functional TGF-beta type I receptor. A systematic analysis revealed that most ALKs formed heteromeric complexes with the type II receptors for TGF-beta and activin after overexpression in COS cells; however, among the six ALKs, only ALK-5 was a functional TGF-beta type I receptor for activation of plasminogen activator inhibitor-1, and only ALK-2 and ALK-4 bound activin with high affinity.

Molecular recognition of BMP-2 and BMP receptor IA.

Bone morphogenetic protein-2 (BMP-2) and other members of the TGF-beta superfamily regulate the development, maintenance and regeneration of tissues and organs. Binding epitopes for these extracellular signaling proteins have been defined, but hot spots specifying binding affinity and specificity have so far not been identified. In this study, mutational and structural analyses show that epitopes of BMP-2 and the BRIA receptor form a new type of protein-protein interface. The main chain atoms of Leu 51 and Asp53 of BMP-2 represent a hot spot of binding to BRIA. The BMP-2 variant L51P was deficient in type I receptor binding only, whereas its overall structure and its binding to type II receptors and modulator proteins, such as noggin, were unchanged. Thus, the L51P substitution converts BMP-2 into a receptor-inactive inhibitor of noggin. These results are relevant for other proteins of the TGF-beta superfamily and provide useful clues for structure-based drug design.

Isoforms of the avian TrkC receptor: a novel kinase insertion dissociates transformation and process outgrowth from survival.

TrkC receptor isoforms have been identified by cDNA cloning and RT-PCR analysis of embryonic chick brain RNA. An N-terminal truncation motif is missing from the signal sequence and first cysteine cluster of the extracellular domain. Within the cytoplasmic dimain, a kinase truncation motif retains part of the kinase domain, but appeared to lack activity. Finally, a kinase insert (KI) motif introduces a 25 amino acid sequence distinct from the known mammalian inserts. KI receptors, like full-length receptors, were tyrosine phosphorylated in response to NT-3 and mediated the transformation of chick embryo fibroblasts and process outgrowth from rat PC12 cells. However, KI receptors supported little, if any, survival of serum-deprived PC12 cells. These results indicate that alternative splicing of trkC transcripts is an important mechanism for regulating cellular responses to NT-3.

Identification and purification of an agrin receptor from Torpedo postsynaptic membranes: a heteromeric complex related to the dystroglycans.

The selective concentration of neurotransmitter receptors at the postsynaptic membrane is an essential aspect of synaptic differentiation and function. Agrin is an extracellular matrix protein that is likely to direct the accumulation of acetylcholine receptors and several other postsynaptic elements at developing and regenerating neuromuscular junctions. How agrin interacts with the membrane to bring about these changes is unknown. We now report the identification and purification of a protein complex from Torpedo electric organ postsynaptic membranes that is likely to serve as an agrin receptor. The native receptor is a heteromeric complex of two membrane glycoproteins of 190 kDa and 50 kDa. The 190 kDa subunit is sufficient to bind ligand. Peptide sequence analysis revealed that the 190 kDa and 50 kDa subunits are related to the dystrophin-associated glycoproteins alpha- and beta-dystroglycan, respectively. No other candidate agrin receptors were detected. The identification of the agrin receptor opens new avenues toward a mechanistic understanding of synapse differentiation.

Quantitative super-resolution single molecule microscopy dataset of YFP-tagged growth factor receptors.

Background: Super-resolution single molecule localization microscopy (SMLM) is a method for achieving resolution beyond the classical limit in optical microscopes (approx. 200 nm laterally). Yellow fluorescent protein (YFP) has been used for super-resolution single molecule localization microscopy, but less frequently than other fluorescent probes. Working with YFP in SMLM is a challenge because a lower number of photons are emitted per molecule compared with organic dyes, which are more commonly used. Publically available experimental data can facilitate development of new data analysis algorithms. Findings: Four complete, freely available single molecule super-resolution microscopy datasets on YFP-tagged growth factor receptors expressed in a human cell line are presented, including both raw and analyzed data. We report methods for sample preparation, for data acquisition, and for data analysis, as well as examples of the acquired images. We also analyzed the SMLM datasets using a different method: super-resolution optical fluctuation imaging (SOFI). The 2 modes of analysis offer complementary information about the sample. A fifth single molecule super-resolution microscopy dataset acquired with the dye Alexa 532 is included for comparison purposes. Conclusions: This dataset has potential for extensive reuse. Complete raw data from SMLM experiments have typically not been published. The YFP data exhibit low signal-to-noise ratios, making data analysis a challenge. These datasets will be useful to investigators developing their own algorithms for SMLM, SOFI, and related methods. The data will also be useful for researchers investigating growth factor receptors such as ErbB3.

Four-helix bundle growth factors and their receptors: protein-protein interactions.

Many growth factors and cytokines promote receptor clustering on binding. At least three different protein-protein interaction sites are involved: cytokine-receptor I, cytokine-receptor II and receptor I-receptor II. Although structural data on these complexes are limited, recent structural and mutagenesis studies of the four-helix bundle class of cytokines are clarifying the nature of the complexes formed.

Cell surface receptors.

During the past year, the database of ligand-receptor complexes has essentially doubled. These new results have immeasurably extended and expanded our view of cell surface receptor structure and function. The flood of data has revealed new models for receptor cross-linking, demonstrating the potential stringency of the extracellular requirements for the initiation of intracellular signalling and highlighting unexpected interactions suggestive of higher order clustering.

Ligand-induced dimerization of growth factor receptors: variations on the theme.

Growth, differentiation, apoptosis and movement of cells are regulated, in part, by polypeptide growth factors, or cytokines, which exert their effects by binding to cell surface receptors on the target cells. Recent observations have indicated that a common mechanism of activation of several classes of such receptors is ligand-induced dimerization or oligomerization of the receptors.

A single residue of GDF-5 defines binding specificity to BMP receptor IB.

Growth and differentiation factor 5 (GDF-5), a member of the TGF-beta superfamily, is involved in many developmental processes, like chondrogenesis and joint formation. Mutations in GDF-5 lead to diseases, e.g. chondrodysplasias like Hunter-Thompson, Grebe and DuPan syndromes and brachydactyly. Similar to other TGF-beta superfamily members, GDF-5 transmits signals through binding to two different types of membrane-bound serine-/threonine-kinase receptors termed type I and type II. In contrast to the large number of ligands, only seven type I and five type II receptors have been identified to date, implicating a limited promiscuity in ligand-receptor interaction. However, in contrast to other members of the TGF-beta superfamily, GDF-5 shows a pronounced specificity in type I receptor interaction in cross-link experiments binding only to BMP receptor IB (BMPR-IB). In mice, deletion of either GDF-5 or BMPR-IB results in a similar phenotype, indicating that GDF-5 signaling is highly dependent on BMPR-IB. Here, we demonstrate by biosensor analysis that GDF-5 also binds to BMP receptor IA (BMPR-IA) but with approximately 12-fold lower affinity. Structural and mutational analyses revealed a single residue of GDF-5, Arg57 located in the pre-helix loop, being solely responsible for the high binding specificity to BMPR-IB. In contrast to wild-type GDF-5, variant GDF-5R57A interacts with BMPR-IA and BMPR-IB with a comparable high binding affinity. These results provide important insights into how receptor-binding specificity is generated at the molecular level and might be useful for the generation of receptor subtype specific activators or inhibitors.

Novel interaction partners of the TPR/MET tyrosine kinase.

A large variety of biological processes is mediated by stimulation of the receptor tyrosine kinase MET. Screening a mouse embryo cDNA library, we were able to identify several novel, putative intracellular TPR/MET-substrates: SNAPIN, DCOHM, VAV-1, Sorting nexin 2, Death associated protein kinase 3, SMC-1, Centromeric protein C, and hTID-1. Interactions as identified by yeast two-hybrid analysis were validated in vitro and in vivo by mammalian two-hybrid studies, a far-western assay and coimmunoprecipitation. Participation in apoptosis-regulating mechanisms through interaction with DAPK-3 and cell cycle control via binding to nuclear proteins such as CENPC and SMC-1 are possible new aspects of intracellular MET signaling.