Relationships between EGFR signaling-competent and endocytosis-competent membrane microdomains

Acute ligand-independent Src activation mimics low EGF-induced EGFR surface signalling and redistribution into recycling endosomes

Src, a non-receptor tyrosine kinase, is a key signal transduction partner of epidermal growth factor (EGF) receptor (EGFR). In human breast cancer, EGFR and Src are frequently over-expressed and/or over-activated. Although reciprocal activation is documented, mechanisms underlying Src:EGFR interactions are incompletely understood. We here exploited ts/v-Src thermo-activation in MDCK monolayers to test whether acute Src activation impacts on signalling and trafficking of non-liganded EGFR. We found that thermo-activation caused rapid Src recruitment to the plasma membrane, concomitant association with EGFR, and its phosphorylation at Y845 and Y1173 predominantly at the cell surface. Like low EGF concentrations, activated Src (i) decreased EGF surface binding without affecting the total EGFR pool; (ii) triggered EGFR endocytosis via clathrin-coated vesicles; (iii) and led to its sequestration in perinuclear/recycling endosomes with avoidance of multivesicular bodies and lysosomal degradation. Combined Src activation and EGF were synergistic for EGFR-Y845 and -Y1173 phosphorylation at some endosomes. We conclude that acute effects of Src in MDCK cells may mimic those of low EGF on EGFR activation and redistribution. Src:EGFR interactions may be sufficient to trigger EGFR activation and might contribute to its local signalling, without requiring either soluble extracellular signal or receptor over-expression.

The epidermal growth factor receptor (EGFR) promotes uptake of influenza A viruses (IAV) into host cells

Influenza A viruses (IAV) bind to sialic-acids at cellular surfaces and enter cells by using endocytotic routes. There is evidence that this process does not occur constitutively but requires induction of specific cellular signals, including activation of PI3K that promotes virus internalization. This implies engagement of cellular signaling receptors during viral entry. Here, we present first indications for an interplay of IAV with receptor tyrosine kinases (RTKs). As representative RTK family-members the epidermal growth factor receptor (EGFR) and the c-Met receptor were studied. Modulation of expression or activity of both RTKs resulted in altered uptake of IAV, showing that these receptors transmit entry relevant signals upon virus binding. More detailed studies on EGFR function revealed that virus binding lead to clustering of lipid-rafts, suggesting that multivalent binding of IAV to cells induces a signaling platform leading to activation of EGFR and other RTKs that in turn facilitates IAV uptake.

The antimicrobial peptide cathelicidin enhances activation of lung epithelial cells by LPS

Epithelial cells (ECs) are usually hyporesponsive to various microbial products. Detection of lipopolysaccharide (LPS), the major component of gram-negative bacteria, is impeded, at least in part, by intracellular sequestration of its receptor, Toll-like receptor-4 (TLR4). In this study, using human bronchial ECs (hBECs) as a model of mucosal epithelium, we tested the hypothesis that the human LPS-binding, membrane-active cationic host defense peptide cathelicidin LL-37 augments epithelial response to LPS by facilitating its delivery to TLR4-containing intracellular compartments. We found that LL-37 significantly increases uptake of LPS by ECs with subsequent targeting to cholera toxin subunit B-labeled structures and lysosomes. This uptake is peptide specific, dose and time dependent, and involves the endocytotic machinery, functional lipid rafts, and epidermal growth factor receptor signaling. Cathelicidin-dependent LPS internalization resulted in significant increased release of the inflammatory cytokines IL-6 and IL-8. This indicates that, in ECs, this peptide may replace LPS-binding protein functions. In polarized ECs, the effect of LL-37 was restricted to the basolateral compartment of the epithelial membrane, suggesting that LL-37-mediated activation of ECs by LPS may be relevant to disease conditions associated with damage to the epithelial barrier. In summary, our study identified a novel role of LL-37 in host-microbe interactions as a host factor that licenses mucosal ECs to respond to LPS.

Tea catechins as inhibitors of receptor tyrosine kinases: mechanistic insights and human relevance

Receptor tyrosine kinases (RTKs) play important roles in the control of fundamental cellular processes, influencing the balance between cell proliferation and death. RTKs have emerged as molecular targets for the treatment of various cancers. Green tea and its polyphenolic compounds, the catechins, exhibit chemopreventive and chemotherapeutic properties in many human cancer cell types, as well as in various carcinogenicity models in vivo. Epidemiological studies are somewhat less convincing, but some positive correlations have been observed. The tea catechins, including (-)-epigallocatechin-3-gallate (EGCG), have pleiotropic effects on cellular proteins and signaling pathways. This review focuses on the ability of the tea constituents to suppress RTK signaling, and summarizes the mechanisms by which EGCG and other catechins might exert their protective effects towards dysregulated RTKs in cancer cells. The findings are discussed in the context of ongoing clinical trials with RTK inhibitors, and the possibility for drug/nutrient interactions enhancing therapeutic efficacy.

Plasma membrane contributes to the formation of pre-autophagosomal structures

Autophagy is a catabolic process in which lysosomes degrade intracytoplasmic contents transported in double-membraned autophagosomes. Autophagosomes are formed by the elongation and fusion of phagophores, which derive from pre-autophagosomal structures. The membrane origins of autophagosomes are unclear and may involve multiple sources, including the endoplasmic reticulum and mitochondria. Here we show in mammalian cells that the heavy chain of clathrin interacts with Atg16L1 and is involved in the formation of Atg16L1-positive early autophagosome precursors. Atg16L1 associated with clathrin-coated structures, and inhibition of clathrin-mediated internalization decreased the formation of both Atg16L1-positive precursors and mature autophagosomes. We tested and demonstrated that the plasma membrane contributes directly to the formation of early Atg16L1-positive autophagosome precursors. This may be particularly important during periods of increased autophagosome formation, because the plasma membrane may serve as a large membrane reservoir that allows cells periods of autophagosome synthesis at levels many-fold higher than under basal conditions, without compromising other processes.

Compartmentalization of EGFR in cellular membranes: role of membrane rafts

There is now abundant evidence that the intracellular concentration of the EGFR and many other receptors for peptide hormones and growth factors is important for the temporal and spatial regulation of cell signaling. Spatial control is achieved by the selective compartmentalization of signaling components into endosomes. However further control may be effected by sequestration into sub-domains within a given organelle such as membrane rafts which are dynamic, nano scale structures rich in cholesterol and sphingolipids. Current data suggest the presence of EGFRs in non-caveolae membrane rafts. High doses of EGF seem to promote the sorting of EGFR to late endosomes through a raft/cholesterol dependant mechanism, implicating them in EGFR degradation. However our work and that of others has led us to propose a model in which membrane rafts in late endosomes sequester highly active EGFR leading to the recruitment and activation of MAPK in this compartment.

Overexpression of myosin VI in prostate cancer cells enhances PSA and VEGF secretion, but has no effect on endocytosis

Tissue expression microarrays, employed to determine the players and mechanisms leading to prostate cancer development, have consistently shown that myosin VI, a unique actin-based motor, is upregulated in medium-grade human prostate cancers. Thus, to understand the role of myosin VI in prostate cancer development, we have characterized its intracellular localization and function in the prostate cancer cell line LNCaP. Using light and electron microscopy, we identified myosin VI on Rab5-positive early endosomes, as well as on recycling endosomes and the trans-Golgi network. Intracellular targeting seems to involve two myosin VI-interacting proteins, GIPC and LMTK2, both of which can be co-immunoprecipitated with myosin VI from LNCaP cells. The absence of Disabled-2 (Dab2), a tumour suppressor and myosin VI-binding partner, inhibits recruitment of myosin VI to endocytic structures at the plasma membrane in LNCaP cells, but interestingly has no effect on endocytosis. Small interfering RNA-mediated downregulation of myosin VI expression results in a significant reduction in prostate-specific antigen (PSA) and vascular endothelial growth factor (VEGF) secretion in LNCaP cells. Our results suggest that in prostate cancer cells, myosin VI regulates protein secretion, but the overexpression of myosin VI has no major impact on clathrin-mediated endocytosis.

Immunotherapy of cancer

Major advances have been made in the field of immunology in the past two decades. A better understanding of the molecular and cellular mechanisms controlling the immune system has opened the door to many innovative and promising new cancer therapies that manipulate the immune response. For instance, toll-like receptor agonists have been shown to boost immune responses toward tumors. Also, a wide array of cell-based immunotherapies utilizing T cells, NK cells, and dendritic cells have been established. Furthermore, a rapidly expanding repertoire of monoclonal antibodies is being developed to treat tumors, and many of the available antibodies have demonstrated impressive clinical responses. Here, we examine some of these immunotherapeutic approaches currently in use or testing to treat cancer, and we examine available evidence with regards to mechanism and efficacy of these treatments.

Loss of myosin VI no insert isoform (NoI) induces a defect in clathrin-mediated endocytosis and leads to caveolar endocytosis of transferrin receptor

Myosin VI is a motor protein that moves toward the minus end of actin filaments. It is involved in clathrin-mediated endocytosis and associates with clathrin-coated pits/vesicles at the plasma membrane. In this article the effect of the loss of myosin VI no insert isoform (NoI) on endocytosis in nonpolarized cells was examined. The absence of myosin VI in fibroblasts derived from the Snell's waltzer mouse (myosin VI knock-out) gives rise to defective clathrin-mediated endocytosis with shallow clathrin-coated pits and a strong reduction in the internalization of clathrin-coated vesicles. To compensate for this defect in clathrin-mediated endocytosis, plasma membrane receptors such as the transferrin receptor (TfR) are internalized by a caveola-dependent pathway. Moreover the clathrin adaptor protein, AP-2, necessary for TfR internalization, follows the receptor and relocalizes in caveolae in Snell's waltzer fibroblasts.

Lipid rafts and caveolin-1 coordinate interleukin-1beta (IL-1beta)-dependent activation of NFkappaB by controlling endocytosis of Nox2 and IL-1beta receptor 1 from the plasma membrane

Recent evidence suggests that signaling by the proinflammatory cytokine interleukin-1beta (IL-1beta) is dependent on reactive oxygen species derived from NADPH oxidase. Redox signaling in response to IL-1beta is known to require endocytosis of its cognate receptor (IL-1R1) following ligand binding and the formation of redox-active signaling endosomes that contain Nox2 (also called redoxosomes). The consequent generation of reactive oxygen species by redoxosomes is responsible for the downstream recruitment of IL-1R1 effectors (IRAK, TRAF6, and IkappaB kinase kinases) and ultimately for activation of the transcription factor NFkappaB. Despite this knowledge of the signaling events that occur downstream of redoxosome formation, an understanding of the mechanisms that coordinate the genesis of redoxosomes following IL-1beta stimulation has been lacking. Here, we demonstrate that lipid rafts play an important role in this process. We show that Nox2 and IL-1R1 localize to plasma membrane lipid rafts in the unstimulated state and that IL-1beta signals caveolin-1-dependent endocytosis of both proteins into the redoxosome. We also show that inhibiting lipid raft-mediated endocytosis prevents NFkappaB activation. Finally, we demonstrate that Vav1, a Rac1 guanine exchange factor and activator of Nox2, is recruited to lipid rafts following IL-1beta stimulation and that it is required for NFkappaB activation. Our results fill in an important mechanistic gap in the understanding of early IL-1R1 and Nox2 signaling events that control NFkappaB activation, a redox-dependent process important in inflammation.

Direct antigen presentation and gap junction mediated cross-presentation during apoptosis

MHC class I molecules present peptides from endogenous proteins. Ags can also be presented when derived from extracellular sources in the form of apoptotic bodies. Cross-presentation of such Ags by dendritic cells is required for proper CTL responses. The fate of Ags in cells initiated for apoptosis is unclear as is the mechanism of apoptosis-derived Ag transfer into dendritic cells. Here we show that novel Ags can be generated by caspases and be presented by MHC class I molecules of apoptotic cells. Since gap junctions function until apoptotic cells remodel to form apoptotic bodies, transfer and cross-presentation of apoptotic peptides by neighboring and dendritic cells occurs. We thus define a novel phase in classical Ag presentation and cross-presentation by MHC class I molecules: presentation of Ags created by caspase activities in cells in apoptosis.

The extracellular domain of the TGFβ type II receptor regulates membrane raft partitioning

Cell-surface TGFβ (transforming growth factor β) receptors partition into membrane rafts and the caveolin-positive endocytic compartment by an unknown mechanism. In the present study, we investigated the determinant in the TGFβ type II receptor (TβRII) that is necessary for membrane raft/caveolar targeting. Using subcellular fractionation and immunofluorescence microscopy techniques, we demonstrated that the extracellular domain of TβRII mediates receptor partitioning into raft and caveolin-positive membrane domains. Pharmacological perturbation of glycosylation using tunicamycin or the mutation of Mgat5 [mannosyl(α-1,6)-glycoprotein β-1,6-N-acetylglucosaminyltransferase V] activity interfered with the raft partitioning of TβRII. However, this was not due to the glycosylation state of TβRII, as a non-glycosylated TβRII mutant remained enriched in membrane rafts. This suggested that other cell-surface glycoproteins associate with the extracellular domain of TβRII and direct their partitioning in membrane raft domains. To test this we analysed a GMCSF (granulocyte/macrophage colony-stimulating factor)–TβRII chimaeric receptor, which contains a glycosylated GMCSF extracellular domain fused to the transmembrane and intracellular domains of TβRII. This chimaeric receptor was found to be largely excluded from membrane rafts and caveolin-positive structures. Our results indicate that the extracellular domain of TβRII mediates receptor partitioning into membrane rafts and efficient entrance into caveolin-positive endosomes.

Lipid rafts and clathrin cooperate in the internalization of PrP in epithelial FRT cells

BACKGROUND

The cellular prion protein (PrP(C)) plays a key role in the pathogenesis of Transmissible Spongiform Encephalopathies in which the protein undergoes post-translational conversion to the infectious form (PrP(Sc)). Although endocytosis appears to be required for this conversion, the mechanism of PrP(C) internalization is still debated, as caveolae/raft- and clathrin-dependent processes have all been reported to be involved.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the mechanism of PrP(C) endocytosis in Fischer Rat Thyroid (FRT) cells, which lack caveolin-1 (cav-1) and caveolae, and in FRT/cav-1 cells which form functional caveolae. We show that PrP(C) internalization requires activated Cdc-42 and is sensitive to cholesterol depletion but not to cav-1 expression suggesting a role for rafts but not for caveolae in PrP(C) endocytosis. PrP(C) internalization is also affected by knock down of clathrin and by the expression of dominant negative Eps15 and Dynamin 2 mutants, indicating the involvement of a clathrin-dependent pathway. Notably, PrP(C) co-immunoprecipitates with clathrin and remains associated with detergent-insoluble microdomains during internalization thus indicating that PrP(C) can enter the cell via multiple pathways and that rafts and clathrin cooperate in its internalization.

CONCLUSIONS/SIGNIFICANCE

These findings are of particular interest if we consider that the internalization route/s undertaken by PrP(C) can be crucial for the ability of different prion strains to infect and to replicate in different cell lines.

Signaling components of redox active endosomes: the redoxosomes

Subcellular compartmentalization of reactive oxygen species (ROS) plays a critical role in transmitting cell signals in response to environmental stimuli. In this regard, signals at the plasma membrane have been shown to trigger NADPH oxidase-dependent ROS production within the endosomal compartment and this step can be required for redox-dependent signal transduction. Unique features of redox-active signaling endosomes can include NADPH oxidase complex components (Nox1, Noxo1, Noxa1, Nox2, p47phox, p67phox, and/or Rac1), ROS processing enzymes (SOD1 and/or peroxiredoxins), chloride channels capable of mediating superoxide transport and/or membrane gradients required for Nox activity, and novel redox-dependent sensors that control Nox activity. This review will discuss the cytokine and growth factor receptors that likely mediate signaling through redox-active endosomes, and the common mechanisms whereby they act. Additionally, the review will cover ligand-independent environmental injuries, such as hypoxia/reoxygenation injury, that also appear to facilitate cell signaling through NADPH oxidase at the level of the endosome. We suggest that redox-active endosomes encompass a subset of signaling endosomes that we have termed redoxosomes. Redoxosomes are uniquely equipped with redox-processing proteins capable of transmitting ROS signals from the endosome interior to redox-sensitive effectors on the endosomal surface. In this manner, redoxosomes can control redox-dependent effector functions through the spatial and temporal regulation of ROS as second messengers.

Lipid raft-dependent FcepsilonRI ubiquitination regulates receptor endocytosis through the action of ubiquitin binding adaptors

The best characterized role for ubiquitination of membrane receptors is to negatively regulate signaling by targeting receptors for lysosomal degradation. The high affinity receptor for IgE (FcεRI) expressed on mast cells and basophils is rapidly ubiquitinated upon antigen stimulation. However, the nature and the role of this covalent modification are still largelly unknown. Here, we show that FcεRI subunits are preferentially ubiquitinated at multiple sites upon stimulation, and provide evidence for a role of ubiquitin as an internalization signal: under conditions of impaired receptor ubiquitination a decrease of receptor entry is observed by FACS analysis and fluorescence microscopy. We also used biochemical approaches combined with fluorescence microscopy, to demonstrate that receptor endocytosis requires the integrity of specific membrane domains, namely lipid rafts. Additionally, by RNA interference we demonstrate the involvement of ubiquitin-binding endocytic adaptors in FcεRI internalization and sorting. Notably, the triple depletion of Eps15, Eps15R and Epsin1 negatively affects the early steps of Ag-induced receptor endocytosis, whereas Hrs depletion retains ubiquitinated receptors into early endosomes and partially prevents their sorting into lysosomes for degradation. Our results are compatible with a scenario in which the accumulation of engaged receptor subunits into lipid rafts is required for receptor ubiquitination, a prerequisite for efficient receptor internalization, sorting and delivery to a lysosomal compartment.

Estradiol attenuates EGF-induced rapid uPAR mobilization and cell migration via the G-protein-coupled receptor 30 in ovarian cancer cells

Epidermal growth factor (EGF) stimulates proliferation and migration in ovarian cancer cells, and high tumor expression of the EGF system correlates with poor prognosis. Epidermal growth factor upregulates urokinase plasminogen activator receptor (uPAR) on the cell surface via 3 distinct mechanisms: rapid mobilization of uPAR from detergent-resistant domains, increased mRNA, and decreased degradation. G-protein-coupled receptor 30 (GPR30) is a newly identified membrane estrogen receptor (ER).The objective of this study was to explore the effects of 17beta-estradiol (E(2)) on uPAR expression and cell migration in ovarian cancer cells and further to identify the ER involved.We used 7 ovarian cancer cell lines, cell migration assay, cellular binding of (125)I-uPA, cellular degradation of (125)I-uPA/PAI-1 complex, enzyme-linked immunosorbent assay for uPAR, solid-phase enzyme immunoassay for ERalpha, and quantitative polymerase chain reaction. Estradiol attenuates the stimulatory effect of EGF on cell migration and uPAR expression. Specifically, E(2) reduces the very rapid increase of detergent extractable uPAR, which occurs within minutes of EGF stimulation and probably represents mobilization of uPAR from detergent-resistant domains such as lipid rafts. Estradiol influenced neither the amount of uPAR mRNA nor the rate of uPAR degradation or solubilization. The nuclear ER antagonists ICI 182780 and tamoxifen, which are GPR30 agonists, as well as the specifically constructed GPR30 agonist G1, mimicked the effect of E(2) on uPAR expression and cell migration. OVCAR-3 cells express mRNA for GPR30.Estradiol attenuates EGF-induced mobilization of ligated uPAR from detergent-resistant domains and subsequent migration in ovarian cancer cells. The response to various ER ligands indicates that this effect is mediated via the membrane ER GPR30.

Endocytic trafficking of activated EGFR is AP-2 dependent and occurs through preformed clathrin spots

The removal of the epidermal growth factor receptor (EGFR) from the cell surface by endocytosis is triggered by receptor activation, but many facets of EGFR trafficking remain unresolved. We employed total internal fluorescence microscopy to elucidate the dynamics of activated EGFR at the cell surface through live-cell imaging. The results of these studies demonstrate that: (1) EGFR does not localize to caveolae in live cells either before or after activation; (2) EGFR does localize to clathrin-coated pits, but only after activation; (3) activation does not result in the formation of new clathrin-coated pits; (4) activated EGFR clusters at sites of preformed clathrin lattices; (5) The AP-2 complex is involved in the internalization of activated EGFR. Using imaging techniques to show the endocytic sorting of activated EGFR for the first time in live cells, these studies suggest a refinement of the model for EGFR entry.

Systems biology of growth factor-induced receptor endocytosis

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

Sorting of EGF and transferrin at the plasma membrane and by cargo-specific signaling to EEA1-enriched endosomes

The biological function of receptors is determined by their appropriate trafficking through the endosomal pathway. Following internalization, the transferrin (Tf) receptor quantitatively recycles to the plasma membrane, whereas the epidermal growth factor (EGF) receptor undergoes degradation. To determine how Tf and EGF engage these two different pathways we imaged their binding and early endocytic pathway in live cells using total internal reflection fluorescence microscopy (TIRF-M). We find that EGF and Tf bind to distinct plasma membrane regions and are incorporated into different endocytic vesicles. After internalization, both EGF-enriched and Tf-enriched vesicles interact with endosomes containing early endosome antigen 1 (EEA1). EGF is incorporated and retained in these endosomes, while Tf-containing vesicles rapidly dissociate and move to a juxtanuclear compartment. Endocytic vesicles carrying EGF recruit more Rab5 GTPase than those carrying Tf, which, by strengthening their association with EEA1-enriched endosomes, may provide a mechanism for the observed cargo-specific sorting. These results reveal pre-endocytic sorting of Tf and EGF, a specialized role for EEA1-enriched endosomes in EGF trafficking, and a potential mechanism for cargo-specified sorting of endocytic vesicles by these endosomes.

EGF induces coalescence of different lipid rafts

The suggestion that microdomains may function as signaling platforms arose from the presence of growth factor receptors, such as the EGFR, in biochemically isolated lipid raft fractions. To investigate the role of EGFR activation in the organization of lipid rafts we have performed FLIM analyses using putative lipid raft markers such as ganglioside GM1 and glycosylphosphatidylinositol (GPI)-anchored GFP (GPI-GFP). The EGFR was labeled using single domain antibodies from Llama glama that specifically bind the EGFR without stimulating its kinase activity. Our FLIM analyses demonstrate a cholesterol-independent colocalization of GM1 with EGFR, which was not observed for the transferrin receptor. By contrast, a cholesterol-dependent colocalization was observed for GM1 with GPI-GFP. In the resting state no colocalization was observed between EGFR and GPI-GFP, but stimulation of the cell with EGF resulted in the colocalization at the nanoscale level of EGFR and GPI-GFP. Moreover, EGF induced the enrichment of GPI-GFP in a detergent-free lipid raft fraction. Our results suggest that EGF induces the coalescence of the two types of GM1-containing microdomains that might lead to the formation of signaling platforms.

(-)-Epigallocatechin gallate causes internalization of the epidermal growth factor receptor in human colon cancer cells

We recently found that the inhibitory effect of (-)-epigallocatechin gallate (EGCG) on epidermal growth factor (EGF) binding to the epidermal growth factor receptor (EGFR) is associated with alterations in lipid organization in the plasma membrane of colon cancer cells. Since changes in lipid organizations are thought to play a role in the trafficking of several membrane proteins, in this study we examined the effects of EGCG on cellular localization of the EGFR in SW480 cells. Treatment of the cells for 30 min with as little as 1 microg/ml of EGCG caused a decrease in cell surface-associated EGFRs and this was associated with internalization of EGFRs into endosomal vesicles. Similar effects were seen with a green fluorescent protein (GFP)-EGFR fusion protein. As expected, the EGFR protein was phosphorylated at tyrosine residues, ubiquitinated and partially degraded when the cells were treated with EGF, but treatment with EGCG caused none of these effects. The loss of EGFRs from the cell surface induced by treating the cells with EGF for 30 min persisted for at least 2 h. However, the loss of EGFRs from the cell surface induced by temporary exposure to EGCG was partially restored within 1-2 h. These studies provide the first evidence that EGCG can induce internalization of EGFRs into endosomes, which can recycle back to the cell surface. This sequestrating of inactivated EGFRs into endosomes may explain, at least in part, the ability of EGCG to inhibit activation of the EGFR and thereby exert anticancer effects.

ATP depletion inhibits the endocytosis of ClC-2

The chloride channel, ClC-2 is expressed ubiquitously and participates in multiple physiological processes. In particular, ClC-2 has been implicated in the regulation of neuronal chloride ion homeostasis and mutations in ClC-2 are associated with idiopathic generalized epilepsy. Despite the physiological and pathophysiological significance of this channel, its regulation remains incompletely understood. The functional expression of ClC-2 at the cell surface has been shown to be enhanced by depletion of cellular ATP, implicating its possible role in cellular energy sensing. In the present study, biochemical assays of cell surface expression suggest that this gain of function reflects, in part, an increase in channel number due to the reduction in ClC-2 internalization by endocytosis. Cell surface expression of the disease-causing mutant: G715E, thought to lack wild-type nucleotide binding affinity, is similarly affected, suggesting that ATP-depletion modifies the function of proteins in the endocytic pathway rather than ClC-2 directly. Using a combination of immunofluorescence and biochemical studies, we confirmed that ClC-2 is internalized via dynamin-dependent endocytosis and that the change in surface expression evoked by ATP depletion is partially mimicked by inhibition of dynamin function using a dynamin dominant-negative mutant (DynK44A). Furthermore, trafficking via the early endosomal compartment occurs in part through rab5-associated vesicles and recycling of ClC-2 to the cell surface occurs through a rab11 dependent pathway. In summary, we have determined that the internalization of ClC-2 by endocytosis is inhibited by metabolic stress, highlighting the importance for understanding the molecular mechanisms mediating the endosomal trafficking of this channel.

Dissecting lipid raft facilitated cell signaling pathways in cancer

Cancer is one of the most devastating disorders in our lives. Higher rate of proliferation than death of cells is one of the essential factors for development of cancer. The dynamicity of cell membrane plays some vital roles in cell survival and cell death, including protection, endocytosis, signaling, and increases in mechanical stability during cell division, as well as decrease of shear forces during separation of two cells after division, and cell separation from tissues for cancer metastasis. Within the membrane, there are specialized domains, known as lipid rafts. A raft can coordinate various signaling pathways. Recent data on the proteomics of lipid rafts/caveolae have highlighted the enigmatic role of various signaling proteins in cancer development. Analysis of these data of raft proteome from various tumors, cancer tissues, and cell lines cultured without and with therapeutic agents, as well as from model rafts revealed that there may be two subsets of raft assemblage in cell membrane. One subset of raft is enriched with cholesterol-sphingomyeline-ganglioside-cav-1/Src/EGFR (hereafter, "chol-raft") that is involved in normal cell signaling, and when dysregulated promotes cell transformation and tumor progression; another subset of raft is enriched with ceramide-sphingomyeline-ganglioside-FAS/Ezrin (hereafter, "cer-raft") that generally promotes apoptosis. In view of this, and to focus insight into the cancer cell physiology caused by the lipid rafts mediated signals and their receptors, and the downstream transmitters, either proliferative (for example, EGF and EGFR) or death-inducing (for example, FASL and FAS), and the precise roles of some therapeutic drugs and endogenous acid sphingomylenase in this scenario in in situ transformation of "chol-raft" into "cer-raft" are summarized and discussed in this contribution.

Regulation of raft-dependent endocytosis

Raft-dependent endocytosis is in large part defined as the cholesterol-sensitive, clathrin-independent internalization of ligands and receptors from the plasma membrane. It encompasses the endocytosis of caveolae, smooth plasmalemmal vesicles that form a subdomain of cholesterol and sphingolipid-rich lipid rafts and that are enriched for caveolin-1. While sharing common mechanisms, like cholesterol sensitivity, raft endocytic routes show differential regulation by various cellular components including caveolin-1, dynamin-2 and regulators of the actin cytoskeleton. Dynamin-dependent raft pathways, mediated by caveolae and morphologically equivalent non-caveolin vesicular intermediates, are referred to as caveolae/raft-dependent endocytosis. In contrast, dynamin-independent raft pathways are mediated by non-caveolar intermediates. Raft-dependent endocytosis is regulated by tyrosine kinase inhibitors and, through the regulation of the internalization of various ligands, receptors and effectors, is also a determinant of cellular signaling. In this review, we characterize and discuss the regulation of raft-dependent endocytic pathways and the role of key regulators such as caveolin-1.

Plasma membrane domain organization regulates EGFR signaling in tumor cells

Macromolecular complexes exhibit reduced diffusion in biological membranes; however, the physiological consequences of this characteristic of plasma membrane domain organization remain elusive. We report that competition between the galectin lattice and oligomerized caveolin-1 microdomains for epidermal growth factor (EGF) receptor (EGFR) recruitment regulates EGFR signaling in tumor cells. In mammary tumor cells deficient for Golgi β1,6N-acetylglucosaminyltransferase V (Mgat5), a reduction in EGFR binding to the galectin lattice allows an increased association with stable caveolin-1 cell surface microdomains that suppresses EGFR signaling. Depletion of caveolin-1 enhances EGFR diffusion, responsiveness to EGF, and relieves Mgat5 deficiency–imposed restrictions on tumor cell growth. In Mgat5^+/+ tumor cells, EGFR association with the galectin lattice reduces first-order EGFR diffusion rates and promotes receptor interaction with the actin cytoskeleton. Importantly, EGFR association with the lattice opposes sequestration by caveolin-1, overriding its negative regulation of EGFR diffusion and signaling. Therefore, caveolin-1 is a conditional tumor suppressor whose loss is advantageous when β1,6GlcNAc-branched N-glycans are below a threshold for optimal galectin lattice formation.

Lipid rafts and caveolae in signaling by growth factor receptors

Lipid rafts and caveolae are microdomains of the plasma membrane enriched in sphingolipids and cholesterol, and hence are less fluid than the remainder of the membrane. Caveolae have an invaginated structure, while lipid rafts are flat regions of the membrane. The two types of microdomains have different protein compositions (growth factor receptors and their downstream molecules) suggesting that lipid rafts and caveolae have a role in the regulation of signaling by these receptors. The purpose of this review is to discuss this model, and the implications that it might have regarding a potential role for lipid rafts and caveolae in human cancer. Particular attention will be paid to the epidermal growth factor receptor, for which the largest amount of information is available. It has been proposed that caveolins act as tumor suppressors. The role of lipid rafts is less clear, but they seem to be capable of acting as 'signaling platforms', in which signal initiation and propagation can occur efficiently.

The Tom1L1-clathrin heavy chain complex regulates membrane partitioning of the tyrosine kinase Src required for mitogenic and transforming activities

Compartmentalization of Src tyrosine kinases (SFK) plays an important role in signal transduction induced by a number of extracellular stimuli. For example, Src mitogenic signaling induced by platelet-derived growth factor (PDGF) is initiated in cholesterol-enriched microdomain caveolae. How this Src subcellular localization is regulated is largely unknown. Here we show that the Tom1L1-clathrin heavy chain (CHC) complex negatively regulates the level of SFK in caveolae needed for the induction of DNA synthesis. Tom1L1 is both an interactor and a substrate of SFK. Intriguingly, it stimulates Src activity without promoting mitogenic signaling. We found that, upon association with CHC, Tom1L1 reduced the level of SFK in caveolae, thereby preventing its association with the PDGF receptor, which is required for the induction of mitogenesis. Similarly, the Tom1L1-CHC complex reduced also the level of oncogenic Src in cholesterol-enriched microdomains, thus affecting both its capacity to induce DNA synthesis and cell transformation. Conversely, Tom1L1, when not associated with CHC, accumulated in caveolae and promoted Src-driven DNA synthesis. We concluded that the Tom1L1-CHC complex defines a novel mechanism involved in negative regulation of mitogenic and transforming signals, by modulating SFK partitioning at the plasma membrane.

SGIP1α Is an Endocytic Protein That Directly Interacts with Phospholipids and Eps15

SGIP1 has been shown to be an endophilin-interacting protein that regulates energy balance, but its function is not fully understood. Here, we identified its splicing variant of SGIP1 and named it SGIP1alpha. SGIP1alpha bound to phosphatidylserine and phosphoinositides and deformed the plasma membrane and liposomes into narrow tubules, suggesting the involvement in vesicle formation during endocytosis. SGIP1alpha furthermore bound to Eps15, an important adaptor protein of clathrin-mediated endocytic machinery. SGIP1alpha was colocalized with Eps15 and the AP-2 complex. Upon epidermal growth factor (EGF) stimulation, SGIP1alpha was colocalized with EGF at the plasma membrane, indicating the localization of SGIP1alpha at clathrin-coated pits/vesicles. SGIP1alpha overexpression reduced transferrin and EGF endocytosis. SGIP1alpha knockdown reduced transferrin endocytosis but not EGF endocytosis; this difference may be due to the presence of redundant pathways in EGF endocytosis. These results suggest that SGIP1alpha plays an essential role in clathrin-mediated endocytosis by interacting with phospholipids and Eps15.

Dissecting the molecular function of reggie/flotillin proteins

Reggie-1/flotillin-2 and reggie-2/flotillin-1 are ubiquitously expressed, well-conserved proteins that are associated with membrane microdomains known as rafts. Studies from us and others have suggested a role in various cellular processes such as insulin signaling, T cell activation, membrane trafficking, phagocytosis, and epidermal growth factor receptor signaling. Recent findings also demonstrate that reggie-1 is associated with cell motility and transformation. However, the exact function of reggie proteins remains to be clarified. In this review, we will focus on some recent findings that have shed new light on the elusive molecular function of these highly interesting proteins. We will especially discuss the emerging role of reggie proteins in membrane receptor signaling and membrane trafficking, with emphasis on the regulation of the molecular function of reggies by post-translational modifications such as phosphorylation and lipid modifications.

Mapping ErbB receptors on breast cancer cell membranes during signal transduction

Distributions of ErbB receptors on membranes of SKBR3 breast cancer cells were mapped by immunoelectron microscopy. The most abundant receptor, ErbB2, is phosphorylated, clustered and active. Kinase inhibitors ablate ErbB2 phosphorylation without dispersing clusters. Modest co-clustering of ErbB2 and EGFR, even after EGF treatment, suggests that both are predominantly involved in homointeractions. Heregulin leads to dramatic clusters of ErbB3 that contain some ErbB2 and EGFR and abundant PI 3-kinase. Other docking proteins, such as Shc and STAT5, respond differently to receptor activation. Levels of Shc at the membrane increase two- to five-fold with EGF, whereas pre-associated STAT5 becomes strongly phosphorylated. These data suggest that the distinct topography of receptors and their docking partners modulates signaling activities.

T6BP and NDP52 are myosin VI binding partners with potential roles in cytokine signalling and cell adhesion

Myosin VI has been implicated in many cellular processes including endocytosis, secretion, membrane ruffling and cell motility. We carried out a yeast two-hybrid screen and identified TRAF6-binding protein (T6BP) and nuclear dot protein 52 (NDP52) as myosin VI binding partners. Myosin VI interaction with T6BP and NDP52 was confirmed in vitro and in vivo and the binding sites on each protein were accurately mapped. Immunofluorescence and electron microscopy showed that T6BP, NDP52 and myosin VI are present at the trans side of the Golgi complex, and on vesicles in the perinuclear region. Although the SKICH domain in T6BP and NDP52 does not mediate recruitment into membrane ruffles, loss of T6BP and NDP52 in RNAi knockdown cells results in reduced membrane ruffling activity and increased stress fibre and focal adhesion formation. Furthermore, we observed in these knockdown cells an upregulation of constitutive secretion of alkaline phosphatase, implying that both proteins act as negative regulators of secretory traffic at the Golgi complex. T6BP was also found to inhibit NF-kappaB activation, implicating it in the regulation of TRAF6-mediated cytokine signalling. Thus myosin VI-T6BP interactions may link membrane trafficking pathways with cell adhesion and cytokine-dependent cell signalling.

Nerve growth factor stimulates the concentration of TrkA within lipid rafts and extracellular signal-regulated kinase activation through c-Cbl-associated protein

Nerve growth factor (NGF) acts through its receptor, TrkA, to elicit the neuronal differentiation of PC12 cells through the action of extracellular signal-regulated kinase 1 (ERK1) and ERK2. Upon NGF binding, TrkA translocates and concentrates in cholesterol-rich membrane microdomains or lipid rafts, facilitating formation of receptor-associated signaling complexes, activation of downstream signaling pathways, and internalization into endosomes. We have investigated the mechanisms responsible for the localization of TrkA within lipid rafts and its ability to activate ERK1 and ERK2. We report that NGF treatment results in the translocation of activated forms of TrkA to lipid rafts, and this localization is important for efficient activation of the ERKs. TrkA is recruited and retained within lipid rafts through its association with flotillin, an intrinsic constituent of these membrane microdomains, via the adapter protein, c-Cbl associated protein (CAP). Mutant forms of CAP that lack protein interaction domains block TrkA localization to lipid rafts and attenuate ERK activation. Importantly, suppression of endogenous CAP expression inhibited NGF-stimulated neurite outgrowth from primary dorsal root ganglion neurons. These data provide a mechanism for the lipid raft localization of TrkA and establish the importance of the CAP adaptor protein for NGF activation of the ERKs and neuronal differentiation.

Compartmentalization of signaling-competent epidermal growth factor receptors in endosomes

In this study, the preparation of detergent-resistant membranes (DRMs) and the immunoisolation of intracellular vesicles enriched in raft markers were used to investigate the effect of physiological doses of epidermal growth factor (EGF) in vivo on the compartmentalization and activation of EGF receptor (EGFR) in rat liver endosomes. Both of these techniques show that after EGF administration, a distinctive population of intracellular EGFR, which was characterized by a high level of tyrosine phosphorylation, accumulated in endosomes. EGFR recruited to early endosomes were more tyrosine phosphorylated than those from late endosomes. However, the level of tyrosine phosphorylation of EGFR in DRMs isolated from early and late endosomes was comparable, suggesting that EGFR in endosomal DRMs are more resistant to tyrosine dephosphorylation. In accordance with the higher level of Tyr phosphorylation, EGF induced an augmented recruitment of Grb2 and Shc to endosomal DRMs compared with whole endosomes. Furthermore, a proteomic analysis identified a selective increase of many alpha-subunits of heterotrimeric G proteins in endosomal DRMs in response to EGF. These observations suggest that a distinctive pool of endocytic EGFR, potentially competent for signaling, is actively trafficking through intracellular compartments with the characteristic of lipid rafts.

Clathrin-Coated Pits: Vive La Différence?

Because of the discovery of coated pits and vesicles more than 40 years ago and the identification of clathrin as a major component of the coat, it has been assumed that clathrin-coated pits (CCPs) are responsible for the uptake of most plasma membrane receptors undergoing internalization. The recent molecular characterization of clathrin-independent routes of endocytosis confirms that several alternative endocytic pathways operate at the plasma membrane of mammalian cells. This heterogeneous view of endocytosis has been expanded still further by recent studies, suggesting that different subpopulations of CCPs responsible for the internalization of specific sets of cargo may coexist. In the present review, we have discussed the experimental evidence in favor or against the existence of distinct parallel clathrin-dependent pathways at the plasma membrane.

Vascular signaling through cholesterol-rich domains: implications in hypertension

PURPOSE OF REVIEW

Lipid rafts are emerging as key players in the integration of cellular responses. Alterations in these highly regulated signaling cascades are important in structural, mechanical and functional abnormalities that underlie vascular pathological processes. The present review focuses on recent advances in signal transduction through caveolae/lipid rafts, implicated in hypertensive processes.

RECENT FINDINGS

Caveolae/lipid rafts function as sites of dynamic regulatory events in receptor-induced signal transduction. Mediators of vascular function, including G-protein coupled receptors, Src family tyrosine kinases, receptor tyrosine kinases, protein phosphatases and nitric oxide synthase, are concentrated within these microdomains. The assembly of functionally active nicotinamide adenine dinucleotide phosphate oxidase and subsequent reactive oxygen species production are also dependent on interactions within the caveolae/lipid rafts. Recent findings have also demonstrated the importance of actin-cytoskeleton and focal adhesion sites for protein interactions with caveolae/lipid raft.

SUMMARY

Many vascular signaling processes are altered in hypertension. Whether these events involve lipid rafts/caveolae remains unclear. A better understanding of how signaling molecules compartmentalize in lipid rafts/caveolae will provide further insights into molecular mechanisms underlying vascular damage in cardiovascular disease.

Myosin VI is required for sorting of AP-1B-dependent cargo to the basolateral domain in polarized MDCK cells

In polarized epithelial cells, newly synthesized membrane proteins are delivered on specific pathways to either the apical or basolateral domains, depending on the sorting motifs present in these proteins. Because myosin VI has been shown to facilitate secretory traffic in nonpolarized cells, we investigated its role in biosynthetic trafficking pathways in polarized MDCK cells. We observed that a specific splice isoform of myosin VI with no insert in the tail domain is required for the polarized transport of tyrosine motif containing basolateral membrane proteins. Sorting of other basolateral or apical cargo, however, does not involve myosin VI. Site-directed mutagenesis indicates that a functional complex consisting of myosin VI, optineurin, and probably the GTPase Rab8 plays a role in the basolateral delivery of membrane proteins, whose sorting is mediated by the clathrin adaptor protein complex (AP) AP-1B. Our results suggest that myosin VI is a crucial component in the AP-1B–dependent biosynthetic sorting pathway to the basolateral surface in polarized epithelial cells.

Interfering with interferon receptor sorting and trafficking: impact on signaling

Interferons (IFNs) and their receptors (IFN-Rs) play fundamental roles in a multitude of biological functions. Many articles and reviews emphasize that the JAK/STAT machinery is obligatory for relay of the information transmitted by IFNs after binding to their cognate receptors at the plasma membrane. In contrast, very few studies have addressed the endocytosis and the intracellular trafficking of IFN-Rs, the immediate step following IFN binding. However, recent findings have shed light on the importance of IFN-R sorting and trafficking in the control of IFN signaling. Thus, IFN-Rs can be included in the growing family of signaling receptors for which regulation of biological activity critically involves endocytosis and trafficking.

Association of herpesvirus saimiri tip with lipid raft is essential for downregulation of T-cell receptor and CD4 coreceptor

Lipid rafts are membrane microdomains that are proposed to function as platforms for both receptor signaling and trafficking. Our previous studies have demonstrated that Tip of herpesvirus saimiri (HVS), which is a T-lymphotropic tumor virus, is constitutively targeted to lipid rafts and interacts with cellular Lck tyrosine kinase and p80 WD repeat-containing endosomal protein. Through the interactions with Lck and p80, HVS Tip modulates diverse T-cell functions, which leads to the downregulation of T-cell receptor (TCR) and CD4 coreceptor surface expression, the inhibition of TCR signal transduction, and the activation of STAT3 transcription factor. In this study, we investigated the functional significance of Tip association with lipid rafts. We found that Tip expression remarkably increased lipid raft fractions in human T cells by enhancing the recruitment of lipid raft-resident proteins. Genetic analysis showed that the carboxyl-terminal transmembrane, but not p80 and Lck interaction, of Tip was required for the lipid raft localization and that lipid raft localization of Tip was necessary for the efficient downregulation of TCR and CD4 surface expression. Correlated with this, treatment with Filipin III, a lipid raft-disrupting agent, effectively reversed the downregulation of CD3 and CD4 surface expression induced by Tip. On the other hand, Tip mutants that were no longer present in lipid rafts were still capable of inhibiting TCR signaling and activating STAT3 transcription factor activity as efficiently as wild-type (wt) Tip. These results indicate that the association of Tip with lipid rafts is essential for the downregulation of TCR and CD4 surface expression but not for the inhibition of TCR signal transduction and the activation of STAT3 transcription factor. These results also suggest that the signaling and targeting activities of HVS Tip rely on functionally and genetically separable mechanisms, which may independently modulate T-cell function for viral persistence or pathogenesis.

Palmitoylation is required for efficient Fas cell death signaling

Localization of the death receptor Fas to specialized membrane microdomains is crucial to Fas-mediated cell death signaling. Here, we report that the post-translational modification of Fas by palmitoylation at the membrane proximal cysteine residue in the cytoplasmic region is the targeting signal for Fas localization to lipid rafts, as demonstrated in both cell-free and living cell systems. Palmitoylation is required for the redistribution of Fas to actin cytoskeleton-linked rafts upon Fas stimulation and for the raft-dependent, ezrin-mediated cytoskeleton association, which is necessary for the efficient Fas receptor internalization, death-inducing signaling complex assembly and subsequent caspase cascade leading to cell death.

Deoxycholic acid differentially regulates focal adhesion kinase phosphorylation: role of tyrosine phosphatase ShP2

Environmental factors, including dietary fats, are implicated in colonic carcinogenesis. Dietary fats modulate secondary bile acids including deoxycholic acid (DCA) concentrations in the colon, which are thought to contribute to the nutritional-related component of colon cancer risk. Here we demonstrate, for the first time, that DCA differentially regulated the site-specific phosphorylation of focal adhesion kinase (FAK). DCA decreased adhesion of HCA-7 cells to the substratum and induced dephosphorylation of FAK at tyrosine-576/577 (Tyr-576/577) and Tyr-925. Tyrosine phosphorylation of FAK at Tyr-397 remained unaffected by DCA stimulation. Interestingly, we found that c-Src was constitutively associated with FAK and DCA actually activated Src, despite no change in FAK-397 and an inhibition of FAK-576 phosphorylation. DCA concomitantly and significantly increased association of tyrosine phosphatase ShP2 with FAK. Incubation of immunoprecipitated FAK, in vitro, with glutathione-S-transferase-ShP2 fusion protein resulted in tyrosine dephosphorylation of FAK in a concentration-dependent manner. Antisense oligodeoxynucleotides directed against ShP2 decreased ShP2 protein levels and attenuated DCA-induced FAK dephosphorylation. Inhibition of FAK by adenoviral-mediated overexpression of FAK-related nonkinase and gene silencing of Shp2 both abolished DCA's effect on cell adhesion, thus providing a possible mechanism for inside-out signaling by DCA in colon cancer cells. Our results suggest that DCA differentially regulates focal adhesion complexes and that tyrosine phosphatase ShP2 has a role in DCA signaling.

Modeling HER2 effects on cell behavior from mass spectrometry phosphotyrosine data

Cellular behavior in response to stimulatory cues is governed by information encoded within a complex intracellular signaling network. An understanding of how phenotype is determined requires the distributed characterization of signaling processes (e.g., phosphorylation states and kinase activities) in parallel with measures of resulting cell function. We previously applied quantitative mass spectrometry methods to characterize the dynamics of tyrosine phosphorylation in human mammary epithelial cells with varying human epidermal growth factor receptor 2 (HER2) expression levels after treatment with epidermal growth factor (EGF) or heregulin (HRG). We sought to identify potential mechanisms by which changes in tyrosine phosphorylation govern changes in cell migration or proliferation, two behaviors that we measured in the same cell system. Here, we describe the use of a computational linear mapping technique, partial least squares regression (PLSR), to detail and characterize signaling mechanisms responsible for HER2-mediated effects on migration and proliferation. PLSR model analysis via principal component inner products identified phosphotyrosine signals most strongly associated with control of migration and proliferation, as HER2 expression or ligand treatment were individually varied. Inspection of these signals revealed both previously identified and novel pathways that correlate with cell behavior. Furthermore, we isolated elements of the signaling network that differentially give rise to migration and proliferation. Finally, model analysis identified nine especially informative phosphorylation sites on six proteins that recapitulated the predictive capability of the full model. A model based on these nine sites and trained solely on data from a low HER2-expressing cell line a priori predicted migration and proliferation in a HER2-overexpressing cell line. We identify the nine signals as a “network gauge,” meaning that when interrogated together and integrated according to the quantitative rules of the model, these signals capture information content in the network sufficiently to predict cell migration and proliferation under diverse ligand treatments and receptor expression levels. Examination of the network gauge in the context of previous literature indicates that endocytosis and activation of phosphoinositide 3-kinase (PI3K)-mediated pathways together represent particularly strong loci for the integration of the multiple pathways mediating HER2′s control of mammary epithelial cell proliferation and migration. Thus, a PLSR modeling approach reveals critical signaling processes regulating HER2-mediated cell behavior.

Cells in the human body interpret extracellular information to “decide” on the execution of particular behaviors such as migration, proliferation, and differentiation. Many diseases, such as cancer, occur when these decision-making processes are compromised. The transfer of extracellular information to the intracellular space is often accomplished through receptor proteins whose chemical properties are altered as extracellular conditions change. These receptors transfer information in the intracellular space through the transfer of phosphate groups from one molecule to another. In particular, the transfer of phosphate groups to tyrosine sites is critical for cellular signaling. How the cell decides to execute a particular behavior on the basis of many changing phosphorylation events, however, is not understood. Here, we apply a computational approach to understand and predict how cells make the decision to migrate and proliferate as extracellular information changes. In particular, we wanted to understand the basis of decision-making processes in cells overexpressing a receptor protein called human epidermal growth factor receptor 2 (HER2). This receptor is overexpressed in ∼30% of breast cancer patients and correlates with poor prognosis. Taking advantage of a recently published dataset that quantified tyrosine phosphorylation events in HER2-overexpressing cells, we created models to understand and to predict HER2-mediated changes in migration and proliferation. The model identified small subsets of measured phosphorylation events that are predictive of changes in behavior with HER2 overexpression. Analysis of the phosphorylated subset of proteins implicated certain cellular processes as being crucial for cellular decision making, and suggested potential biomarkers and targets for therapeutic use in HER2-overexpressing cancers. Further application of our technique should aid in the understanding of cellular decision processes from large sets of cell signal and behavior data.

Functionally different pools of Shiga toxin receptor, globotriaosyl ceramide, in HeLa cells

Many studies have investigated the intracellular trafficking of Shiga toxin, but very little is known about the underlying dynamics of its cellular receptor, the glycosphingolipid globotriaosyl ceramide. In this study, we show that globotriaosyl ceramide is required not only for Shiga toxin binding to cells, but also for its intracellular trafficking. Shiga toxin induces globotriaosyl ceramide recruitment to detergent-resistant membranes, and subsequent internalization of the lipid. The globotriaosyl ceramide pool at the plasma membrane is then replenished from internal stores. Whereas endocytosis is not affected in the recovery condition, retrograde transport of Shiga toxin to the Golgi apparatus and the endoplasmic reticulum is strongly inhibited. This effect is specific, as cholera toxin trafficking on GM(1) and protein biosynthesis are not impaired. The differential behavior of both toxins is also paralleled by the selective loss of Shiga toxin association with detergent-resistant membranes in the recovery condition, and comparison of the molecular species composition of plasma membrane globotriaosyl ceramide indicates subtle changes in favor of unsaturated fatty acids. In conclusion, this study demonstrates the dynamic behavior of globotriaosyl ceramide at the plasma membrane and suggests that globotriaosyl ceramide-specific determinants, possibly its molecular species composition, are selectively required for efficient retrograde sorting on endosomes, but not for endocytosis.

Cargo Regulates Clathrin-Coated Pit Dynamics

Clathrin-coated pits (CCPs) are generally considered a uniform population of endocytic machines containing mixed constitutive and regulated membrane cargo. Contrary to this view, we show that regulated endocytosis of G protein-coupled receptors (GPCRs) occurs preferentially through a subset of CCPs. Significantly, GPCR-containing CCPs are also functionally distinct, as their surface residence time is regulated locally by GPCR cargo via PDZ-dependent linkage to the actin cytoskeleton. Such cargo-regulated CCPs show delayed recruitment of dynamin and can undergo an abortive event in which clathrin coats separate from the plasma membrane without concomitant receptor endocytosis. Segregation of cargo into CCP subsets, combined with cargo-dependent control of CCP dynamics, suggests a simple kinetic mechanism to generate functional specialization early in the endocytic pathway and reduce competition between diverse endocytic cargo.