Spatial organization of EGF receptor transmodulation by PDGF

The therapeutic effect of methotrexate-conjugated Pluronic-based polymeric micelles on the folate receptor-rich tumors treatment

The therapeutic effect of methotrexate (MTX)-conjugated Pluronic-based polymeric mixed micelles (F127/P105-MTX) on the folate receptor-overexpressing tumors treatment was investigated in this study. Due to its high structural similarity to folic acid and the high expression of folate receptor in most solid tumors, MTX serves as not only a cytotoxic agent but also a homing ligand. Cellular uptake and the endocytic mechanism studies of MTX-conjugated mixed micelles were performed in folate receptor-rich KBv and folate receptor-deficient A-549 cancer cells. Additionally, the efficacy and safety studies of F127/P105-MTX in KBv tumor-bearing mice were evaluated. Results indicate that F127/P105-MTX significantly enhanced the cellular uptake in KBv cells as compared to that of conventional non-MTX-conjugated mixed micelles. Moreover, the results showed that F127/P105-MTX can be internalized by both caveolae- and clathrin-mediated endocytosis in energy-dependent and folate receptor-dependent manners. The in vitro and in vivo antitumor efficacies of F127/P105-MTX were significantly enhanced in comparison with MTX-entrapped mixed micelles. Furthermore, no acute toxicities to hematological system and major organs have been observed after intravenous administration during the regimen. Therefore, our results suggest that F127/P105-MTX could be an effective and safe nano-drug delivery system for cancer therapy, especially for the folate receptor-rich cancer treatment.

Modulation of pyridinium cationic lipid-DNA complex properties by pyridinium gemini surfactants and its impact on lipoplex transfection properties

The study presents the effects of blending a cationic gemini surfactant into cationic lipid bilayers and its impact on the plasmid DNA compaction and delivery process. Using nanoDSC, dynamic light scattering, zeta potential, and electrophoretic mobility measurements, together with transfection (2D- and 3D-) and viability assays, we identified the main physicochemical parameters of the lipid bilayers, liposomes, and lipoplexes that are affected by the gemini surfactant addition. We also correlated the cationic bilayer composition with the dynamics of the DNA compaction process and with transfection efficiency, cytotoxicity, and the internalization mechanism of the resultant nucleic acid complexes. We found that the blending of gemini surfactant into the cationic bilayers fluidized the supramolecular assemblies, reduced the amount of positive charge required to fully compact the plasmid DNA and, in certain cases, changed the internalization mechanism of the lipoplexes. The transfection efficiency of select ternary lipoplexes derived from cationic gemini surfactants and lipids was several times superior to the transfection efficiency of corresponding binary lipoplexes, also surpassing standard transfection systems. The overall impact of gemini surfactants into the formation and dynamic of cationic bilayers was found to depend heavily on the presence of colipids, their nature, and amount present in lipoplexes. The study confirmed the possibility of combining the specific properties of pyridinium gemini surfactants and cationic lipids synergistically to obtain efficient synthetic transfection systems with negligible cytotoxicity useful for therapeutic gene delivery.

Platelet-derived growth factor receptor alpha (PDGFRα) targeting and relevant biomarkers in ovarian carcinoma

OBJECTIVE

Platelet-derived growth factor receptor alpha (PDGFRα) is believed to be associated with cell survival. We examined (i) whether PDGFRα blockade enhances the antitumor activity of taxanes in ovarian carcinoma and (ii) potential biomarkers of response to anti-PDGFRα therapy.

METHODS

PDGFRα expression in 176 ovarian carcinomas was evaluated with tissue microarray and correlated to survival outcome. Human-specific monoclonal antibody to PDGFRα (IMC-3G3) was used for in vitro and in vivo experiments with or without docetaxel. Gene microarrays and reverse-phase protein arrays with pathway analyses were performed to identify potential predictive biomarkers.

RESULTS

When compared to low or no PDGFRα expression, increased PDGFRα expression was associated with significantly poorer overall survival of patients with ovarian cancer (P=0.014). Although treatment with IMC-3G3 alone did not affect cell viability or increase apoptosis, concurrent use of IMC-3G3 with docetaxel significantly enhanced sensitization to docetaxel and apoptosis. In an orthotopic mouse model, IMC-3G3 monotherapy had no significant antitumor effects in SKOV3-ip1 (low PDGFRα expression), but showed significant antitumor effects in HeyA8-MDR (high PDGFRα expression). Concurrent use of IMC-3G3 with docetaxel, compared with use of docetaxel alone, significantly reduced tumor weight in all tested cell lines. In protein ontology, the EGFR and AKT pathways were downregulated by IMC-3G3 therapy. MAPK and CCNB1 were downregulated only in the HeyA8-MDR model.

CONCLUSION

These data identify IMC-3G3 as an attractive therapeutic strategy and identify potential predictive markers for further development.

Nanoparticle uptake and gene transfer efficiency for MSCs on chitosan and chitosan-hyaluronan substrates

Nanoparticles (NPs) are usually surface modified to increase endocytosis for applications in cellular imaging and gene delivery. The influence of cell culture substrates on endocytosis remains relatively unexplored. This study investigated the substrate-mediated effects on the uptake of NPs by mesenchymal stem cells (MSCs). Two types of NPs were employed, negatively charged paramagnetic iron oxide (Fe(3)O(4)) NPs (~5 nm) and bare plasmid DNA pTRE-Tight-DsRED2 (3.3 kb, ~5 nm), each of which were poorly endocytosed by the adipose-derived MSCs grown on tissue culture polystyrene (TCPS). When cells were cultured on chitosan or hyaluronan-modified chitosan (chitosan-HA) membranes, significant increases (>5-fold) in the intracellular uptake of Fe(3)O(4) NPs as well as transfectability of plasmid DNA were demonstrated. The enhancement in transgene expression was more pronounced than that using the transfection agent. The beneficial effects were not caused by elevated proliferation or a change in the differentiation state of interacting MSCs. On chitosan and chitosan-HA, cells moved fast and formed spheroids. The cytoskeletal arrangement associated with the up-regulated RhoA activity during spheroid formation may have accounted for the increased endocytosis. Using different inhibitors, the endocytosis pathways were further clarified. Both Fe(3)O(4) NPs and plasmid DNA were taken up primarily by clathrin-mediated endocytosis on chitosan (~50%). The caveolae-mediated endocytosis on chitosan-HA was more evident (~30-40%) than that on chitosan (<25%). For plasmid DNA but not Fe(3)O(4) NPs, macropinocytosis also occurred on both substrates. Chitosan and chitosan-HA as cell culture substrates may activate different endocytic pathways of MSCs to increase NP internalization or plasmid transfection. The substrate-mediated endocytosis described here may represent a new and potentially attractive approach to facilitate stem cell labeling or to improve gene delivery efficiency without altering cell viability and differentiation.

Human SCARB2-mediated entry and endocytosis of EV71

Enterovirus (EV) 71 infection is known to cause hand-foot-and-mouth disease (HFMD) and in severe cases, induces neurological disorders culminating in fatality. An outbreak of EV71 in South East Asia in 1997 affected over 120,000 people and caused neurological disorders in a few individuals. The control of EV71 infection through public health interventions remains minimal and treatments are only symptomatic. Recently, human scavenger receptor class B, member 2 (SCARB2) has been reported to be a cellular receptor of EV71. We expressed human SCARB2 gene in NIH3T3 cells (3T3-SCARB2) to study the mechanisms of EV71 entry and infection. We demonstrated that human SCARB2 serves as a cellular receptor for EV71 entry. Disruption of expression of SCARB2 using siRNAs can interfere EV71 infection and subsequent inhibit the expression of viral capsid proteins in RD and 3T3-SCARB2 but not Vero cells. SiRNAs specific to clathrin or dynamin or chemical inhibitor of clathrin-mediated endocytosis were all capable of interfering with the entry of EV71 into 3T3-SCARB2 cells. On the other hand, caveolin specific siRNA or inhibitors of caveolae-mediated endocytosis had no effect, confirming that only clathrin-mediated pathway was involved in EV71 infection. Endocytosis of EV71 was also found to be pH-dependent requiring endosomal acidification and also required intact membrane cholesterol. In summary, the mechanism of EV71 entry through SCARB2 as the receptor for attachment, and its cellular entry is through a clathrin-mediated and pH-dependent endocytic pathway. This study on the receptor and endocytic mechanisms of EV71 infection is useful for the development of effective medications and prophylactic treatment against the enterovirus.

Sunitinib malate and figitumumab in solitary fibrous tumor: patterns and molecular bases of tumor response

Antiangiogenic treatment activity has been reported in solitary fibrous tumor (SFT), a rare and little chemosensitive sarcoma. We explored the activity of sunitinib malate (SM) in SFT and studied receptor tyrosine kinase (RTK) activation profile. Eleven patients with progressive metastatic SFT resistant to chemotherapy were treated with continuous-dosing 37.5 mg/d SM on a named-use basis. One of them also received the insulin-like growth factor I receptor (IGFIR) inhibitor figitumumab after developing secondary resistance to SM. Besides, biochemical, molecular, and fluorescence in situ hybridization analyses were done in eight naïve SFTs whose cryopreserved material was available to clarify RTK upstream and downstream signaling. In two cases treated with SM and belonging to the naïve series, both pretreatment and posttreatment samples were available. Ten patients were evaluable for response to SM. The best response according to the Choi criteria was six partial response (all with Response Evaluation Criteria in Solid Tumors stable disease), one stable disease, and three progressive disease. Responses lasted >6 months in five patients. The eight naïve samples showed high expression/phosphorylation of PDGFRB, epidermal growth factor receptor, and IGFIR/IR, in the presence of their cognate ligands. Downstream pathways revealed expression/activation of Akt, extracellular signal-regulated kinase 1-2 and, closely related to SFT subtypes, of S6 and 4E-BP1. In two patients, whose pretreatment and posttreatment clinical and molecular status were available, biochemical data confirmed the activity of SM, although they also suggested a possible time-dependent shift of dominant RTK from PDGFRB to IGFIR/insulin receptor. A Response Evaluation Criteria in Solid Tumors partial response to figitumumab corroborated these findings. SM has antitumor activity in SFT, possibly through a PDGFRB-mediated mechanism, but treatments with IGFIR/insulin receptor and possibly epidermal growth factor receptor inhibitors are worth testing.

Functional mapping of receptor tyrosine kinases in myxoid liposarcoma

PURPOSE

The aim of this study was to analyze receptor tyrosine kinases (RTK) and their downstream signaling activation profile in myxoid liposarcomas (MLS) by investigating 14 molecularly profiled tumors: 7 naive and 7 treated with conventional chemotherapy/radiotherapy or the new drug trabectedin.

EXPERIMENTAL DESIGN

Frozen and matched formalin-fixed, paraffin-embedded material from surgical specimens were analyzed using biochemical, molecular, and molecular/cytogenetic approaches, complemented by immunohistochemistry and confocal microscopy.

RESULTS

In the absence of any RTK and downstream effector deregulation, the naive cases revealed epidermal growth factor receptor, platelet-derived growth factor receptor B, RET, and MET activation sustained by autocrine/paracrine loops, and RTK cross-talk as a result of heterodimerization. Interestingly, RET and MET activation seems to play a major role in the pathogenesis of MLS by involving different targets through different mechanisms. RET activation (which may activate MET) involves the tumoral vascular component by means of RET/MET cross-talk and VEGFA (vascular endothelial growth factor A)/GFRalpha3 (glial cell-derived neurotrophic factor family receptor alpha3)/artemin-mediated signaling as revealed by VEGF receptor 2/RET coimmunoprecipitation. MET activation involves the cellular tumor component by means of a direct ligand-dependent loop and indirect GFRalpha3 (RET coreceptor)/artemin-mediated signaling. About downstream signaling, the association of AKT activation with the round cell variant is interesting. No relevant changes in the original RTK activation profiles were observed in the posttreatment cases, a finding that is in keeping with the nontargeted treatments used.

CONCLUSIONS

These findings highlight the particular cell-specific activation profile of RET/GFRalpha3 and MET in MLS, and the close correlation between AKT activation and the round cell variant, thus opening up new therapeutic perspectives for MET/AKT inhibitors and antagonistic small molecules binding GFRalpha3.

Analysis of receptor tyrosine kinases (RTKs) and downstream pathways in chordomas

We have previously demonstrated that chordomas express activated platelet-derived growth factor receptor (PDGFRB) and that treatment with imatinib, which is capable of switching off the activation of various receptor tyrosine kinases (RTKs) including PDGFRB, benefits a number of patients. The aim of this study was to identify the possible presence of other activated RTKs and their downstream signaling effectors. Cryopreserved material from 22 naïve sporadic chordomas was investigated for the presence of activated RTKs and their cognate ligands and downstream signaling effectors by means of human phospho-RTK antibody arrays, Western blotting, and molecular analysis; immunohistochemistry and fluorescence in situ hybridization were used to analyze the corresponding formalin-fixed and paraffin-embedded samples. We detected activated PDGFRB, FLT3, and colony stimulating factor 1 receptor (CSF1R) of the PDGFR family and highly phosphorylated EGFR, HER2/neu, and (to a lesser extent) HER4 of the EGFR family. The detection of PDGFRB/PDGFB confirmed our previous data. The presence of activated EGFR was paralleled by the finding of high levels of epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha) and PDGFB co-expression and PDGFRB co-immunoprecipitation. Of the downstream effectors, the PI3K/AKT and RAS/MAPK pathways were both activated, thus leading to the phosphorylation of mammalian target of rapamycin (mTOR) and 4E-BP1 among the regulators involved in translational control. Taken together, our results (i) provide a rationale for tailored treatments targeting upstream activated receptors, including the PDGFR and EGFR families; (ii) support the idea that a combination of upstream antagonists and mTOR inhibitors enhances the control of tumor growth; and (iii) indicate that the 4E-BP1/eIF4E pathway is a major regulator of protein synthesis in chordoma.

Quantitative analysis of the responses of murine bone marrow mesenchymal stem cells to EGF, PDGF-BB and fibronectin by factorial design methodology

A 2-level full factorial design was firstly employed to explore the responses of murine bone marrow mesenchymal stem cells stimulated by various combinations of EGF, PDGF-BB, and fibronectin. EGF and PDGF-BB individually, rather than fibronectin, had significant effects on cell proliferation. The synergism between PDGF-BB and fibronectin, as well as the antagonism between EGF and PDGF-BB were also detected. Moreover, cells changed from a flattened and spread to a smaller and elongated shape with addition of factors. The factors also moved cells out of G0/G1 phase, increasing the fractions of cells in S and G2/M phases.

Using fluorescent sphingolipid analogs to study intracellular lipid trafficking

Sphingolipids (SLs), including glycosphingolipids, are found on the plasma membrane where they play important roles in a wide variety of cell functions, including cell-cell communication, cell growth and differentiation, host-pathogen interactions, and cell-signaling events. This unit illustrates the use of fluorescent SL analogs to identify the mechanisms underlying SL endocytosis and subsequent intracellular trafficking. Techniques used to study SL domain formation at the plasma membrane, endocytic mechanisms and intracellular transport steps are highlighted. The use of biochemical treatments and dominant-negative protein expression to block specific steps in lipid trafficking are also discussed.

The reduced GM-CSF priming of ROS production in granulocytes from patients with myelodysplasia is associated with an impaired lipid raft formation

Patients with myelodysplasia (MDS) show an impaired reactive oxygen species (ROS) production in response to fMLP stimulation of GM-CSF-primed neutrophils. In this study, we investigated the involvement of lipid rafts in this process and showed that treatment of neutrophils with the lipid raft-disrupting agent methyl-beta-cyclodextrin abrogates fMLP-induced ROS production and activation of ERK1/2 and protein kinase B/Akt, two signal transduction pathways involved in ROS production in unprimed and GM-CSF-primed neutrophils. We subsequently showed that there was a decreased presence of Lyn, gp91(phox), and p22(phox) in lipid raft fractions from neutrophils of MDS. Furthermore, the plasma membrane expression of the lipid raft marker GM1, which increases upon stimulation of GM-CSF-primed cells with fMLP, was reduced significantly in MDS patients. By electron microscopy, we showed that the fMLP-induced increase in GM1 expression in GM-CSF-primed cells was a result of de novo synthesis, which was less efficient in MDS neutrophils. Taken together, these data indicate an involvement of lipid rafts in activation of signal transduction pathways leading to ROS production and show that in MDS neutrophils, an impaired lipid raft formation in GM-CSF-primed cells results in an impaired ROS production.

Different routes of bone morphogenic protein (BMP) receptor endocytosis influence BMP signaling

Endocytosis is important for a variety of functions in eukaryotic cells, including the regulation of signaling cascades via transmembrane receptors. The internalization of bone morphogenetic protein (BMP) receptor type I (BRI) and type II (BRII) and its relation to signaling were largely unexplored. Here, we demonstrate that both receptor types undergo constitutive endocytosis via clathrin-coated pits (CCPs) but that only BRII undergoes also caveola-like internalization. Using several complementary approaches, we could show that (i) BMP-2-mediated Smad1/5 phosphorylation occurs at the plasma membrane in nonraft regions, (ii) continuation of Smad signaling resulting in a transcriptional response requires endocytosis via the clathrin-mediated route, and (iii) BMP signaling leading to alkaline phosphatase induction initiates from receptors that fractionate into cholesterol-enriched, detergent-resistant membranes. Furthermore, we show that BRII interacts with Eps15R, a constitutive component of CCPs, and with caveolin-1, the marker protein of caveolae. Taken together, the localization of BMP receptors in distinct membrane domains is prerequisite to their taking different endocytosis routes with specific impacts on Smad-dependent and Smad-independent signaling cascades.

Transport across the endothelium: regulation of endothelial permeability

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways controlling endothelial barrier function have been identified. The transcellular pathway transports plasma proteins of the size of albumin or greater via the process of transcytosis in vesicle carriers originating from cell surface caveolae. Specific signalling cues are able to induce the internalisation of caveolae and their movement to the basal side of the endothelium. Caveolin-1, the primary structural protein required for the formation of caveolae, is also important in regulating vesicle trafficking through the cell by controlling the activity and localisation of signalling molecules that mediate vesicle fission, endocytosis, fusion and finally exocytosis. An important function of the transcytotic pathways is to regulate the delivery of albumin and immunoglobulins, thereby controlling tissue oncotic pressure and host-defence. The paracellular pathway induced during inflammation is formed by gaps between endothelial cells at the level of adherens and tight junctional complexes. Paracellular permeability is increased by second messenger signalling pathways involving Ca2+ influx via activation of store-operated channels, protein kinase Calpha (PKCalpha), and Rho kinase that together participate in the stimulation of myosin light chain phosphorylation, actin-myosin contraction, and disruption of the junctions. In this review of the field, we discuss the current understanding of the signalling pathways regulating paracellular and transcellular endothelial permeability.

SIGNALING NETWORKS IN LIVING CELLS

Recent advances in cell signaling research suggest that multiple sets of signal transducing molecules are preorganized and sequestered in distinct compartments within the cell. These compartments are assembled and maintained by specific cellular machinery. The molecular ecology within a compartment creates an environment that favors the efficient and accurate integration of signaling information arriving from humoral, mechanical, and nutritional sources. The functional organization of these compartments suggests they are the location of signaling networks that naturally organize into hierarchical interconnected sets of molecules through their participation in different classes of interacting units. An important goal is to determine the contribution of the compartment to the function of these networks in living cells.

Role of clathrin- and caveolae-mediated endocytosis in gene transfer mediated by lipo- and polyplexes

We investigated the effects of inhibitors of clathrin-mediated endocytosis (chlorpromazine and K(+) depletion) and of caveolae-mediated uptake (filipin and genistein) on internalization of FITC-poly-l-lysine-labeled DOTAP/DNA lipoplexes and PEI/DNA polyplexes by A549 pneumocytes and HeLa cells and on the transfection efficiencies of these complexes with the luciferase gene. Uptake of the complexes was assayed by fluorescence-activated cell sorting. Lipoplex internalization was inhibited by chlorpromazine and K(+) depletion but unaffected by filipin and genistein. In contrast, polyplex internalization was inhibited by all four inhibitors. We conclude that lipoplex uptake proceeds only by clathrin-mediated endocytosis, while polyplexes are taken up by two mechanisms, one involving caveolae and the other clathrin-coated pits. Transfection by lipoplexes was entirely abolished by blocking clathrin-mediated endocytosis, whereas inhibition of the caveolae pathway had no effect. By contrast, transfection mediated by polyplexes was completely blocked by genistein and filipin but was unaffected by inhibitors of clathrin-mediated endocytosis. Fluorescence colocalization studies with a lysosomal marker, AlexaFluor-dextran, revealed that polyplexes taken up by clathrin-mediated endocytosis are targeted to the lysosomal compartment for degradation, while the polyplexes internalized via caveolae escape this compartment, permitting efficient transfection.

The epidermal growth factor receptors and their family of ligands: their putative role in atherogenesis

The epidermal growth factor receptor is a member of type-I growth factor receptor family with tyrosine kinase activity that is activated following the binding of multiple cognate ligands. Several members of the EGF family of ligands are expressed by cells involved in atherogenesis. EGF receptor mediated processes have been well characterised within epithelial, smooth muscle and tumour cell lines in vitro, and the EGF receptor has been identified immunocytochemically on intimal smooth muscle cells within atherosclerotic plaques. There is also limited evidence for the expression of the EGF receptor family on leukocytes, although their function has yet to be clarified. In this review, we will discuss the biological functions of this receptor and its ligands and their potential to modulate the function of cells involved in the atherosclerotic process.

Gβγ Activation of Src Induces Caveolae-mediated Endocytosis in Endothelial Cells

Caveolae-mediated endocytosis in endothelial cells is stimulated by the binding of albumin to gp60, a specific albumin-binding protein localized in caveolae. The activation of gp60 induces its cell surface clustering and association with caveolin-1, the caveolar-scaffolding protein. This interaction leads to G(i)-induced Src kinase activation, which in turn signals dynamin-2-mediated fission and directed migration of caveolae-derived vesicles from apical to basal membrane. In this study, we investigated the possible role of the Gbetagamma heterodimer in signaling G(i)-induced Src activation and subsequent caveolae-mediated endocytosis. We observed using rat lung microvascular endothelial cells that expression of the C terminus of beta-adrenergic receptor kinase (ct-betaARK), an inhibitor Gbetagamma signaling, prevented gp60-dependent Src activation as well as caveolae-mediated endocytosis and transcellular transport of albumin and uptake of cholera toxin subunit B, a specific marker of caveolae internalization. Expression of ct-betaARK also prevented Src-mediated tyrosine phosphorylation of caveolin-1 and dynamin-2 and the resultant phosphorylation-dependent association of dynamin-2 and caveolin-1. Also, the direct activation of Gbetagamma using a specific cell-permeant activating peptide (myristoylated-SIRKALNILGYPDYD) simulated the effects of gp60 in inducing Src activation, caveolin-1, and dynamin-2 phosphorylation as well as caveolae-mediated endocytosis of cholera toxin subunit B. The myristoylated-SIRKALNILGYPDYD peptide-induced responses were inhibited by the expression of ct-betaARK. Taken together, our results demonstrate that Gbetagamma activation of Src signals caveolae-mediated endocytosis and transendothelial albumin transport via transcytosis.

Size-dependent internalization of particles via the pathways of clathrin- and caveolae-mediated endocytosis

Non-phagocytic eukaryotic cells can internalize particles <1 microm in size, encompassing pathogens, liposomes for drug delivery or lipoplexes applied in gene delivery. In the present study, we have investigated the effect of particle size on the pathway of entry and subsequent intracellular fate in non-phagocytic B16 cells, using a range of fluorescent latex beads of defined sizes (50-1000 nm). Our data reveal that particles as large as 500 nm were internalized by cells via an energy-dependent process. With an increase in size (50-500 nm), cholesterol depletion increased the efficiency of inhibition of uptake. The processing of the smaller particles was significantly perturbed upon microtubule disruption, while displaying a negligible effect on that of the 500 nm beads. Inhibitor and co-localization studies revealed that the mechanism by which the beads were internalized, and their subsequent intracellular routing, was strongly dependent on particle size. Internalization of microspheres with a diameter <200 nm involved clathrin-coated pits. With increasing size, a shift to a mechanism that relied on caveolae-mediated internalization became apparent, which became the predominant pathway of entry for particles of 500 nm in size. At these conditions, delivery to the lysosomes was no longer apparent. The data indicate that the size itself of (ligand-devoid) particles can determine the pathway of entry. The clathrin-mediated pathway of endocytosis shows an upper size limit for internalization of approx. 200 nm, and kinetic parameters may determine the almost exclusive internalization of such particles along this pathway rather than via caveolae.

Cross-talk between the receptor tyrosine kinases Ron and epidermal growth factor receptor

Heterogeneous receptor-receptor interactions may play a role in intracellular signaling. Accordingly, the interaction of two dissimilar tyrosine kinase receptors, Ron and epidermal growth factor receptor (EGFR) was investigated. The functional interaction of Ron and EGFR in cell scatter and oncogenic transformation was investigated in vivo. Transfection of a dominant negative form of EGFR into human embryonic kidney cells stably expressing Ron (293-Ron) dramatically reduced the scatter response induced by the Ron ligand hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL). The scatter response of the 293-Ron cells was also attenuated by treatment of the cells with the specific EGFR inhibitor AG 1478. Co-transfection of Ron and dominant-negative EGFR, or co-transfection of EGFR and a dominant-negative form of Ron reduced focus formation in NIH/3T3 cells. Western analysis of NIH/3T3 cells overexpressing murine Ron and expressing endogenous levels of EGFR was used to demonstrate that Ron and EGFR co-immunoprecipitate. Stimulation of the cells in vitro with the Ron ligand HGFL or with the EGFR ligand epidermal growth factor (EGF) appeared to induce phosphorylation of both receptors. Co-immunoprecipitation and phosphorylation of phosphatidyl inositol 3-kinase (PI3-K) was also observed. This novel finding of a functional and biochemical interaction between Ron and EGFR suggests that heterologous tyrosine kinase receptor interactions may play a role in cellular processes such as scatter and transformation.

Selective caveolin-1-dependent endocytosis of glycosphingolipids

We studied the endocytosis of fluorescent glycosphingolipid (GSL) analogs in various cell types using pathway-specific inhibitors and colocalization studies with endocytic markers and DsRed caveolin-1 (cav-1). Based on inhibitor studies, all GSLs tested were internalized predominantly (>80%) by a clathrin-independent, caveolar-related mechanism, regardless of cell type. In addition, fluorescent lactosylceramide (LacCer) colocalized with DsRed-cav-1 in vesicular structures upon endocytosis in rat fibroblasts. The internalization mechanism for GSLs was unaffected by varying the carbohydrate headgroup or sphingosine backbone chain length; however, a fluorescent phosphatidylcholine analog was not internalized via caveolae, suggesting that the GSL ceramide core may be important for caveolar uptake. Internalization of fluorescent LacCer was reduced 80-90% in cell types with low cav-1, but was dramatically stimulated by cav-1 overexpression. However, even in cells with low levels of cav-1, residual LacCer internalization was clathrin independent. In contrast, cholera toxin B subunit (CtxB), which binds endogenous GM1, was internalized via clathrin-independent endocytosis in cells with high cav-1 expression, whereas significant clathrin-dependent uptake occurred in cells with low cav-1. Fluorescent GM1, normally internalized by clathrin-independent endocytosis in HeLa cells with low cav-1, was induced to partially internalize via the clathrin pathway in the presence of CtxB. These results suggest that GSL analogs are selectively internalized via a caveolar-related mechanism in most cell types, whereas CtxB may undergo "pathway switching" when cav-1 levels are low.

Endocytic trafficking of glycosphingolipids in sphingolipid storage diseases

In this review, recent studies of membrane lipid transport in sphingolipid (SL) storage disease (SLSD) fibroblasts are summarized. Several fluorescent glycosphingolipid (GSL) analogues are internalized from the plasma membrane via caveolae and are subsequently transported to the Golgi complex of normal fibroblasts, while in 10 different SLSD cell types, these lipids accumulate in endosomes and lysosomes. Additional studies have shown that cholesterol homeostasis is perturbed in multiple SLSDs secondary to accumulation of endogenous SLs, and that mis-targeting of the GSLs is regulated by cellular cholesterol. Golgi targeting of GSLs internalized via caveolae is dependent on microtubules and phosphoinositide 3-kinase(s) and is inhibited by expression of dominant-negative rab7 and rab9 constructs. Overexpression of wild-type rab7 or rab9 (but not rab11) in Niemann-Pick C fibroblasts results in correction of lipid trafficking defects, including restoration of Golgi targeting of fluorescent lactosylceramide and endogenous GM1 ganglioside (monitored by the transport of fluorescent cholera toxin), and a dramatic reduction in accumulation of intracellular cholesterol. These results suggest an approach for restoring normal lipid trafficking in this, and perhaps other, SLSD cell types, and may provide a basis for future therapy of these diseases.

Some protein tyrosine phosphatases target in part to lipid rafts and interact with caveolin-1

A profile-based search of the SWISS-PROT database reveals that most protein tyrosine phosphatases (PTPs) contain at least one caveolin-1-binding motif. To ascertain if the presence of caveolin-binding motif(s) in PTPs corresponds to their actual localization in caveolin-1-enriched membrane fractions, we performed subcellular fractionating experiments. We found that all tested PTPs (PTP1B, PTP1C, SHPTP2, PTEN, and LAR) are actually localized in caveolin-enriched membrane fractions, despite their distribution in other subcellular sites, too. More than 1/2 of LAR and about 1/4 of SHPTP2 and PTP-1C are localized in caveolin-enriched membrane fractions whereas, in these fractions, PTP-1B and PTEN are poorly concentrated. Co-immunoprecipitation experiments with antibodies specific for each tested PTP demonstrated that all five phosphatases form molecular complexes with caveolin-1 in vivo. Collectively, our findings propose that particular PTPs could perform some of their cellular actions or are regulated by recruitment into caveolin-enriched membrane fractions.

Platelet-derived growth factor mediates tyrosine phosphorylation of the cytoplasmic domain of the low Density lipoprotein receptor-related protein in caveolae

The low density lipoprotein (LDL) receptor gene family represents a class of multifunctional, endocytic cell surface receptors. Recently, roles in cellular signaling have also emerged. For instance, the very low density lipoprotein receptor (VLDLR) and the apolipoprotein receptor-2 (apoER2) function in a developmental signaling pathway that regulates the lamination of cortical layers in the brain and involves the activation of tyrosine kinases. Furthermore, the cytoplasmic domain of the LDL receptor-related protein (LRP) was found to be a substrate for the non-receptor tyrosine kinase Src, but the physiological significance of this phosphorylation event remained unknown. Here we show that tyrosine phosphorylation of LRP occurs in caveolae and involves the platelet-derived growth factor (PDGF) receptor beta and phosphoinositide 3-kinase. Receptor-associated protein, an antagonist of ligand binding to LRP, and apoE-enriched beta-VLDL, a ligand for LRP, reduce PDGF-induced tyrosine phosphorylation of the LRP cytoplasmic domain. In the accompanying paper (Loukinova, E., Ranganathan, S., Kuznetsov, S., Gorlatova, N., Migliorini, M., Ulery, P. G., Mikhailenko, I., Lawrence, D. L., and Strickland, D. K. (2002) J. Biol. Chem. 277, 15499-15506) Loukinova et al. further demonstrate that one form of PDGF, PDGF-BB, binds specifically to LRP and that phosphorylation of LRP requires the activation of Src family kinases. Taken together, these findings provide a biochemical basis for a cellular signaling pathway that involves apoE and LRP.

Heterologous desensitization of EGF receptors and PDGF receptors by sequestration in caveolae

Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) receptors have been reported to signal via caveolin-containing membranes called caveolae. In contrast, others report that EGF and PDGF receptors are exclusively associated with caveolin-devoid membranes called rafts. Our subcellular fractionation and coimmunoprecipitation studies demonstrate that, in the absence of ligand, EGF and PDGF receptors are associated with rafts. However, in the presence of ligand, EGF and PDGF receptors transiently associate with caveolae. Surprisingly, pretreatment of cells with EGF prevents PDGF-dependent phosphorylation of PDGF receptors and extracellular signal-regulated kinase (ERK) 1/2 kinase activation. Furthermore, cells pretreated with PDGF prevent EGF-dependent phosphorylation of EGF receptors and ERK1/2 kinase activation. Radioligand binding studies demonstrate that incubation of cells with EGF or PDGF causes both EGF and PDGF receptors to be reversibly sequestered from the extracellular space. Experiments with methyl-beta-cyclodextrin, filipin, and antisense caveolin-1 demonstrate that sequestration of the receptors is dependent on cholesterol and caveolin-1. We conclude that ligand-induced stimulation of EGF or PDGF receptors can cause the heterologous desensitization of the other receptor by sequestration in cholesterol-rich, caveolin-containing membranes or caveolae.

Lipid rafts and little caves. Compartmentalized signalling in membrane microdomains

Lipid rafts are liquid-ordered membrane microdomains with a unique protein and lipid composition found on the plasma membrane of most, if not all, mammalian cells. A large number of signalling molecules are concentrated within rafts, which have been proposed to function as signalling centres capable of facilitating efficient and specific signal transduction. This review summarizes current knowledge regarding the composition, structure, and dynamic nature of lipid rafts, as well as a number of different signalling pathways that are compartmentalized within these microdomains. Potential mechanisms through which lipid rafts carry out their specialized role in signalling are discussed in light of recent experimental evidence.

Clathrin-dependent and -independent internalization of plasma membrane sphingolipids initiates two Golgi targeting pathways

Sphingolipids (SLs) are plasma membrane constituents in eukaryotic cells which play important roles in a wide variety of cellular functions. However, little is known about the mechanisms of their internalization from the plasma membrane or subsequent intracellular targeting. We have begun to study these issues in human skin fibroblasts using fluorescent SL analogues. Using selective endocytic inhibitors and dominant negative constructs of dynamin and epidermal growth factor receptor pathway substrate clone 15, we found that analogues of lactosylceramide and globoside were internalized almost exclusively by a clathrin-independent ("caveolar-like") mechanism, whereas an analogue of sphingomyelin was taken up approximately equally by clathrin-dependent and -independent pathways. We also showed that the Golgi targeting of SL analogues internalized via the caveolar-like pathway was selectively perturbed by elevated intracellular cholesterol, demonstrating the existence of two discrete Golgi targeting pathways. Studies using SL-binding toxins internalized via clathrin-dependent or -independent mechanisms confirmed that endogenous SLs follow the same two pathways. These findings (a) provide a direct demonstration of differential SLs sorting into early endosomes in living cells, (b) provide a "vital marker" for endosomes derived from caveolar-like endocytosis, and (c) identify two independent pathways for lipid transport from the plasma membrane to the Golgi apparatus in human skin fibroblasts.

Cell biology of caveolae and caveolin

Originally described in the 1950s caveolae are morphologically identifiable as small omega-shaped plasma membrane invaginations present in most cell types. Caveolae are particularly abundant in adipocytes, fibroblasts, type 1 pneumocytes, endothelial and epithelial cells as well as in smooth and striated muscle cells. The first proposed function for caveolae was that of mediating the internalisation and transendothelial trafficking of solutes. Caveolae have been the object of intense research since the discovery of a biochemical marker protein, caveolin, in the early 1990s. Three genes encoding for caveolins have been characterised in mammals. Caveolins (18-24 kDa) are integral membrane proteins that constitute the major protein component of caveolar membrane in vivo. In addition to a structural role of caveolins in the formation of caveolae, caveolin protein interacts directly, and in a regulated manner, with a number of signalling molecules. We present here a general overview of the current knowledge on the structural role of caveolin in caveolae formation, and implication of caveolin in the control of cell signalling.

Localization of phospholipase C-gamma1 signaling in caveolae: importance in EGF-induced phosphoinositide hydrolysis but not in tyrosine phosphorylation

Upon epidermal growth factor treatment, phospholipase C-gamma1 (PLC-gamma1) translocates from cytosol to membrane where it is phosphorylated at tyrosine residues. Caveolae are small plasma membrane invaginations whose structural protein is caveolin. In this study, we show that the translocation of PLC-gamma1 and its tyrosine phosphorylation are localized in caveolae by caveolin-enriched low-density membrane (CM) preparation and immunostaining of cells. Pretreatment of cells with methyl-beta-cyclodextrin (MbetaCD), a chemical disrupting caveolae structure, inhibits the translocation of PLC-gamma1 to CM as well as phosphatidylinositol (PtdIns) turnover. However, MbetaCD shows no effect on tyrosine phosphorylation level of PLC-gamma1. Our findings suggest that, for proper signaling, PLC-gamma1 phosphorylation has to occur at PtdInsP(2)-enriched sites.

Presence of oxidized cholesterol in caveolae uncouples active platelet-derived growth factor receptors from tyrosine kinase substrates

Platelet-derived growth factor receptor beta (PDGFRbeta) in fibroblasts is concentrated in caveolae where it controls the tyrosine phosphorylation of multiple proteins. Caveolae are enriched in cholesterol and sphingolipids, but the role of these lipids in PDGFR signal transduction is unknown. We report that introduction of cholest-4-en-3-one into caveolae membranes uncouples PDGFR autophosphorylation from tyrosine phosphorylation of neighboring proteins. Cholest-4-en-3-one appears to interfere with the normal interaction between PDGFR and its partners. The results suggest that tightly packed caveolae lipids form a membrane platform that functions as a lipid scaffold for organizing the molecular interactions of multiple signaling pathways.

Tyrosine kinase signalling in breast cancer. Epidermal growth factor receptor: convergence point for signal integration and diversification

Cross-communication between different signalling systems is critical for the integration of multiple and changing environmental influences on individual cells. The epidermal growth factor receptor (EGFR) has been identified as a key element in the complex signalling network that is utilized by various classes of cell-surface receptors. This nonclassical mode of signalling system cross-talk, in distinction to receptor activation induced by cognate ligands, has been termed 'signal transactivation'. With the EGFR as the convergence point and distribution focus, this scenario may involve signals emitted by other members of the tyrosine kinase family, cytokine receptors, ion channels, G-protein-coupled receptors and integrins.