Regulation of Integrin β 1 recycling to lipid rafts by Rab1a to promote cell migration

Lovastatin enhances adenovirus-mediated TRAIL induced apoptosis by depleting cholesterol of lipid rafts and affecting CAR and death receptor expression of prostate cancer cells

Oncolytic adenovirus and apoptosis inducer TRAIL are promising cancer therapies. Their antitumor efficacy, when used as single agents, is limited. Oncolytic adenoviruses have low infection activity, and cancer cells develop resistance to TRAIL-induced apoptosis. Here, we explored combining prostate-restricted replication competent adenovirus-mediated TRAIL (PRRA-TRAIL) with lovastatin, a commonly used cholesterol-lowering drug, as a potential therapy for advanced prostate cancer (PCa). Lovastatin significantly enhanced the efficacy of PRRA-TRAIL by promoting the in vivo tumor suppression, and the in vitro cell killing and apoptosis induction, via integration of multiple molecular mechanisms. Lovastatin enhanced PRRA replication and virus-delivered transgene expression by increasing the expression levels of CAR and integrins, which are critical for adenovirus 5 binding and internalization. Lovastatin enhanced TRAIL-induced apoptosis by increasing death receptor DR4 expression. These multiple effects of lovastatin on CAR, integrins and DR4 expression were closely associated with cholesterol-depletion in lipid rafts. These studies, for the first time, show correlations between cholesterol/lipid rafts, oncolytic adenovirus infection efficiency and the antitumor efficacy of TRAIL at the cellular level. This work enhances our understanding of the molecular mechanisms that support use of lovastatin, in combination with PRRA-TRAIL, as a candidate strategy to treat human refractory prostate cancer in the future.

[Advances in the relationship between tumor cell metabolism and tumor metastasis]

Intracellular nutrients and the rate of energy flowing in tumor cells are often higher than that in normal cells due to the prolonged stress of tumor-specific microenvironment. In this context, the metabolism of tumor cells provides the fuel of bio-synthesis and energy required for tumor metastasis. Consistent with this, the abnormal metabolism such as extremely active glucose metabolism and excessive accumulating of fatty acid is also discovered in metastatic tumors. Previous Studies have confirmed that the regulation of tumor metabolism can affect the tumor metastasis, and some of these have been successfully applied in clinical effective, positive way. Thus, targeting metabolism of tumor cells might be an effectively positive way to prevent the metastasis of tumor. So, our review is focused on the research development of the relationship between tumor metabolism and metastasis as well as the underlying mechanism.

Rab1A regulates anterograde melanosome transport by recruiting kinesin-1 to melanosomes through interaction with SKIP

Melanosomes are lysosome-related organelles in melanocytes that are transported from the perinucleus to the cell periphery by coordination between bidirectional (anterograde and retrograde) microtubule-dependent transport and unidirectional actin-dependent transport. Although the molecular machineries that mediate retrograde transport and actin-dependent transport have already been identified, little is known about the anterograde transport complex on microtubules in mammalian cells. Here we discovered that small GTPase Rab1A on melanosomes recruits SKIP/PLEKHM2 as a Rab1A-specific effector and that Rab1A, SKIP, and a kinesin-1/(Kif5b+KLC2) motor form a transport complex that mediates anterograde melanosome transport in melanocytes. Interestingly, Arl8, Arf-like small GTPase that also interacts with SKIP, is specifically localized at lysosomes and regulates their anterograde transport in melanocytes. Our findings suggest that the anterograde microtubule-dependent transport of melanosomes and lysosomes are differently regulated by independent cargo receptors, i.e., Rab1A and Arl8, respectively, but that a SKIP–kinesin-1 mechanism is responsible for the transport of both.

RAB1A promotes Vaccinia virus replication by facilitating the production of intracellular enveloped virions

Vaccinia virus (VACV) is a large double-stranded DNA virus with a complex cytoplasmic replication cycle that exploits numerous cellular proteins. This work characterises the role of a proviral cellular protein, the small GTPase RAB1A, in VACV replication. Using siRNA, we identified RAB1A as required for the production of extracellular enveloped virions (EEVs), but not intracellular mature virions (IMVs). Immunofluorescence and electron microscopy further refined the role of RAB1A as facilitating the wrapping of IMVs to become intracellular enveloped virions (IEVs). This is consistent with the known function of RAB1A in maintenance of ER to Golgi transport. VACV can therefore be added to the growing list of viruses which require RAB1A for optimal replication, highlighting this protein as a broadly proviral host factor.

Plasma membrane mechanisms in a preclinical rat model of chronic pain

We have recently shown that the prolongation of prostaglandin E2 hyperalgesia in a preclinical model of chronic pain-hyperalgesic priming-is mediated by release of cyclic adenosine monophosphate from isolectin B4-positive nociceptors and its metabolism by ectonucleotidases to produce adenosine. The adenosine, in turn, acts in an autocrine mechanism at an A1 adenosine receptor whose downstream signaling mechanisms in the nociceptor are altered to produce nociceptor sensitization. We previously showed that antisense against an extracellular matrix molecule, versican, which defines the population of nociceptors involved in hyperalgesic priming, eliminated the prolongation of prostaglandin E2 hyperalgesia. To further evaluate the mechanisms at the interface between the extracellular matrix and the nociceptor's plasma membrane involved in hyperalgesia prolongation, we interrupted a plasma membrane molecule involved in versican signaling, integrin β1, with an antisense oligodeoxynucleotide. Integrin β1 antisense eliminated mechanical hyperalgesia induced by an adenosine A1 receptor agonist, cyclopentyladenosine, in the primed rat. We also disrupted a molecular complex of signaling molecules that contains integrin β1, lipid rafts, with methyl-β-cyclodextrin, which attenuated the prolongation without affecting the acute phase of prostaglandin E2 hyperalgesia, while having no effect on cyclopentyladenosine hyperalgesia. Our findings help to define the plasma membrane mechanisms involved in a preclinical model of chronic pain.

PERSPECTIVE

The present study contributes to a further understanding of mechanisms involved in the organization of messengers at the plasma membrane that participate in the transition from acute to chronic pain.

Lipid rafts couple class A scavenger receptors to phospholipase A2 activation during macrophage adhesion

SR-A mediated macrophage adhesion to modified ECM proteins in a process that involves physical attachment of SR-A to modified ECM and activation of Lyn-PI3K and PLA2-12/15-lipoxygenase signaling pathways. Structurally, SR-A-mediated cell adhesion requires a 6-aa membrane-proximal cytoplasmic motif. However, the mechanism that couples SR-A-mediated adhesion to activation of these distinct signaling pathways is not known. For other adhesion receptors, including integrins, localization in cholesterol-rich LRs is an important mechanism for coupling the receptor with the activation of specific signaling pathways. We hypothesized that SR-A-mediated macrophage adhesion might also involve LRs. Our results demonstrate that SR-A is enriched in LRs in HEK cells that heterologously express SR-A and in macrophages that endogenously expressed the receptor. We further show that a truncated SR-A construct (SR-A(Δ1-49)), which mediates cell adhesion but not ligand internalization, is also enriched in LRs, suggesting an association between LRs and SR-A-dependent cell adhesion. To examine this association more directly, we used the cholesterol chelator MβCD to deplete cholesterol and disrupt LR function. We found that cholesterol depletion significantly decreased SR-A-mediated macrophage adhesion. We further show that decreased SR-A-dependent macrophage adhesion following cholesterol depletion results from the inhibition of PLA2 but not PI3K activation. Overall, our results demonstrate an important role for LRs in selectively coupling SR-A with PLA2 activation during macrophage adhesion.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

A proteolytic pathway that controls glucose uptake in fat and muscle

Insulin regulates glucose uptake by controlling the subcellular location of GLUT4 glucose transporters. GLUT4 is sequestered within fat and muscle cells during low-insulin states, and is translocated to the cell surface upon insulin stimulation. The TUG protein is a functional tether that sequesters GLUT4 at the Golgi matrix. To stimulate glucose uptake, insulin triggers TUG endoproteolytic cleavage. Cleavage accounts for a large proportion of the acute effect of insulin to mobilize GLUT4 to the cell surface. During ongoing insulin exposure, endocytosed GLUT4 recycles to the plasma membrane directly from endosomes, and bypasses a TUG-regulated trafficking step. Insulin acts through the TC10α GTPase and its effector protein, PIST, to stimulate TUG cleavage. This action is coordinated with insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases, and with other signals to direct overall GLUT4 targeting. Data support the idea that the N-terminal TUG cleavage product, TUGUL, functions as a novel ubiquitin-like protein modifier to facilitate GLUT4 movement to the cell surface. The C-terminal TUG cleavage product is extracted from the Golgi matrix, which vacates an "anchoring" site to permit subsequent cycles of GLUT4 retention and release. Together, GLUT4 vesicle translocation and TUG cleavage may coordinate glucose uptake with physiologic effects of other proteins present in the GLUT4-containing vesicles, and with potential additional effects of the TUG C-terminal product. Understanding this TUG pathway for GLUT4 retention and release will shed light on the regulation of glucose uptake and the pathogenesis of type 2 diabetes.

Prognostic value of a nine-gene signature in glioma patients based on mRNA expression profiling

INTRODUCTION

Gliomas are the most common primary brain tumors in adults and a significant cause of cancer-related mortality. A 9-gene signature was identified as a novel prognostic model reflecting survival situation obviously in gliomas.

AIMS

To identify an mRNA expression signature to improve outcome prediction for patients with different glioma grades.

RESULTS

We used whole-genome mRNA expression microarray data of 220 glioma samples of all grades from the Chinese Glioma Genome Atlas (CGGA) database (http://www.cgga.org.cn) as a discovery set and data from Rembrandt and GSE16011 for validation sets. Data from every single grade were analyzed by the Kaplan-Meier method with a two-sided log-rank test. Univariate Cox regression and linear risk score formula were applied to derive a gene signature with better prognostic performance. We found that patients who had high risk score according to the signature had poor overall survival compared with patients who had low risk score. Highly expressed genes in the high-risk group were analyzed by gene ontology (GO) and gene set variation analysis (GSVA). As a result, the reason for the divisibility of gliomas was likely due to cell life processes and adhesion.

CONCLUSION

This 9-gene-signature prediction model provided a more accurate predictor of prognosis that denoted patients with high risk score have poor outcome. Moreover, these risk models based on defined molecular profiles showed the considerable prospect in personalized cancer management.

Lipid rafts control human melanoma cell migration by regulating focal adhesion disassembly

Tumor cell migration is a crucial step in the metastatic cascade, and interruption of this step is considered to be logically effective in preventing tumor metastasis. Lipid rafts, distinct liquid ordered plasma membrane microdomains, have been shown to influence cancer cell migration, but the underlying mechanisms are still not well understood. Here, we report that lipid rafts regulate the dynamics of actin cytoskeleton and focal adhesion in human melanoma cell migration. Disrupting the integrity of lipid rafts with methyl-β cyclodextrin enhances actin stress fiber formation and inhibits focal adhesion disassembly, accompanied with alterations in cell morphology. Furthermore, actin cytoskeleton, rather than microtubules, mediates the lipid raft-dependent focal adhesion disassembly by regulating the dephosphorylation of focal adhesion proteins and the internalization of β3 integrin. We also show that Src-RhoA-Rho kinase signaling pathway is responsible for lipid raft disruption-induced stress fiber formation. Taken together, these observations provide a new mechanism to further explain how lipid rafts regulate the migration of melanoma cell and suggest that lipid rafts may be novel and attractive targets for cancer therapy.

Annexin A2 regulates β1 integrin internalization and intestinal epithelial cell migration

The gastrointestinal epithelium functions as an important barrier that separates luminal contents from the underlying tissue compartment and is vital in maintaining mucosal homeostasis. Mucosal wounds in inflammatory disorders compromise the critical epithelial barrier. In response to injury, intestinal epithelial cells (IECs) rapidly migrate to reseal wounds. We have previously observed that a membrane-associated, actin binding protein, annexin A2 (AnxA2), is up-regulated in migrating IECs and plays an important role in promoting wound closure. To identify the mechanisms by which AnxA2 promotes IEC movement and wound closure, we used a loss of function approach. AnxA2-specific shRNA was utilized to generate IECs with stable down-regulation of AnxA2. Loss of AnxA2 inhibited IEC migration while promoting enhanced cell-matrix adhesion. These functional effects were associated with increased levels of β1 integrin protein, which is reported to play an important role in mediating the cell-matrix adhesive properties of epithelial cells. Because cell migration requires dynamic turnover of integrin-based adhesions, we tested whether AnxA2 modulates internalization of cell surface β1 integrin required for forward cell movement. Indeed, pulse-chase biotinylation experiments in IECs lacking AnxA2 demonstrated a significant increase in cell surface β1 integrin that was accompanied by decreased β1 integrin internalization and degradation. These findings support an important role of AnxA2 in controlling dynamics of β1 integrin at the cell surface that in turn is required for the active turnover of cell-matrix associations, cell migration, and wound closure.

Expression profiles of subtracted mRNAs during cellular senescence in human mesenchymal stem cells derived from bone marrow

Cellular senescence is an irreversible cell cycle arrest that limits the replicative lifespan of cells. Senescence suppresses development of tumors by regulating aging factors, such as cyclin dependent kinase inhibitor (CKI) and telomerase. Suppression subtractive hybridization (SSH) was used to identify genes that were differentially expressed between young human mesenchymal stem cells (Y-hMSCs) and senescent human mesenchymal stem cells (S-hMSCs). We selected positive clones that were functionally characterized by referring to public databases using NCBI BLAST tool. This search revealed that 19 genes were downregulated, and 43 genes were upregulated in S-hMSCs relative to Y-hMSCs. Among subtracted clones in Y-hMSCs, most of genes markedly were related to metabolic functions. These genes, PDIA3, WDR1, FSTL1, COPG1, LMAN1, and PDIA6, significantly downregulated. Conversely, genes for subtracted clones in S-hMSCs were mostly associated with cell adhesion. In particular, the expression levels of 9 genes, HSP90B1, EID1, ATP2B4, DDAH1, PRNP, RAB1A, PGS5, TM4SF1 and SSR3, gradually increased during senescence. These genes have not previously been identified as being related to cellular senescence, but they seemed to be potentially affected during cellular senescence.

Expression of Tax-interacting protein 1 (TIP-1) facilitates angiogenesis and tumor formation of human glioblastoma cells in nude mice

Glioblastoma is the most common and fatal type of primary brain tumors featured with hyperplastic blood vessels. Here, we performed meta-analyses of published data and established a correlation between high TIP-1 expression levels and the poor prognosis of glioblastoma patients. Next, we explored the biological relevance of TIP-1 expression in the pathogenesis of glioblastoma. By using orthotopic and heterotopic mouse models of human glioblastomas, this study has characterized TIP-1 as one contributing factor to the tumor-driven angiogenesis. In vitro and in vivo functional assays, along with biochemical analyses with microarrays and antibody arrays, have demonstrated that TIP-1 utilizes multiple pathways including modulating fibronectin gene expression and uPA protein secretion, to establish or maintain a pro-angiogenic microenvironment within human glioblastoma. In conclusion, this work supports one hypothesis that TIP-1 represents a novel prognostic biomarker and a therapeutic target of human glioblastoma.

Antibiotics increase gut metabolism and antioxidant proteins and decrease acute phase response and necrotizing enterocolitis in preterm neonates

Background

The appropriate use of antibiotics for preterm infants, which are highly susceptible to develop necrotizing enterocolitis (NEC), is not clear. While antibiotic therapy is commonly used in neonates with NEC symptoms and sepsis, it remains unknown how antibiotics may affect the intestine and NEC sensitivity. We hypothesized that broad-spectrum antibiotics, given immediately after preterm birth, would reduce NEC sensitivity and support intestinal protective mechanisms.

Methodology/Principal Findings

Preterm pigs were treated with antibiotics for 5 d (oral and systemic doses of gentamycin, ampicillin and metrodinazole; AB group) and compared with untreated pigs. Only the untreated pigs showed evidence of NEC lesions and reduced digestive function, as indicated by lowered villus height and activity of brush border enzymes. In addition, 53 intestinal and 22 plasma proteins differed in expression between AB and untreated pigs. AB treatment increased the abundance of intestinal proteins related to carbohydrate and protein metabolism, actin filaments, iron homeostasis and antioxidants. Further, heat shock proteins and the complement system were affected suggesting that all these proteins were involved in the colonization-dependent early onset of NEC. In plasma, acute phase proteins (haptoglobin, complement proteins) decreased, while albumin, cleaved C3, ficolin and transferrin increased.

Conclusions/Significance

Depressed bacterial colonization following AB treatment increases mucosal integrity and reduces bacteria-associated inflammatory responses in preterm neonates. The plasma proteins C3, ficolin, and transferrin are potential biomarkers of the colonization-dependent NEC progression in preterm neonates.

Functional involvement of Rab1A in microtubule-dependent anterograde melanosome transport in melanocytes

Melanosomes are transported to the cell periphery of melanocytes by coordination between bidirectional microtubule-dependent movements and unidirectional actin-dependent movement. Although both the mechanism of the actin-dependent melanosome transport and the mechanism of the microtubule-dependent retrograde melanosome transport in mammalian skin melanocytes have already been determined, almost nothing is known about the mechanism of the microtubule-dependent anterograde melanosome transport. Small GTPase Rab proteins are common regulators of membrane traffic in all eukaryotes, and in this study we performed genome-wide screening for Rab proteins that are involved in anterograde melanosome transport by expressing 60 different constitutive active (and negative) mutants, and succeeded in identifying Rab1A, originally described as a Golgi-resident Rab, as a prime candidate. Endogenous Rab1A protein was found to be localized to mature melanosomes in melanocytes, and its functional ablation either by siRNA-mediated knockdown or by overexpression of a cytosolic form of Rab1A-GTPase-activating protein/TBC1D20 induced perinuclear melanosome aggregation. The results of time-lapse imaging further revealed that long-range anterograde melanosome movements were specifically suppressed in Rab1A-deficient melanocytes, whereas retrograde melanosome transport occurred normally. Taken together, these findings indicate that Rab1A is the first crucial component of the anterograde melanosome transport machinery to be identified in mammalian skin melanocytes.

CD24 regulates cell proliferation and transforming growth factor β-induced epithelial to mesenchymal transition through modulation of integrin β1 stability

To determine the role of CD24 in breast cancer cells, we knocked down CD24 in MCF-7 human breast cancer cells by retroviral delivery of shRNA. MCF-7 cells with knocked down CD24 (MCF-7 hCD24 shRNA) exhibited decreased cell proliferation and cell adhesion as compared to control MCF-7 mCD24 shRNA cells. Decreased proliferation of MCF-7 hCD24 shRNA cells resulted from the inhibition of cell cycle progression from G1 to S phase. The specific inhibition of MEK/ERK signaling by CD24 ablation might be responsible for the inhibition of cell proliferation. Phosphorylation of Src/FAK and TGF-β1-mediated epithelial to mesenchymal transition was also down-regulated in MCF-7 hCD24 shRNA cells. Reduced Src/FAK activity was caused by a decrease in integrin β1 bound with CD24 and subsequent destabilization of integrin β1. Our results suggest that down-regulation of Raf/MEK/ERK signaling via Src/FAK may be dependent on integrin β1 function and that this mechanism is largely responsible for the CD24 ablation-induced decreases in cell proliferation and epithelial to mesenchymal transition.

The PDZ protein TIP-1 facilitates cell migration and pulmonary metastasis of human invasive breast cancer cells in athymic mice

Tax-interacting protein 1 (TIP-1, also known as Tax1bp3) inhibited proliferation of colon cancer cells through antagonizing the transcriptional activity of beta-catenin. However, in this study, elevated TIP-1 expression levels were detected in human invasive breast cancers. Studies with two human invasive breast cancer cell lines indicated that RNAi-mediated TIP-1 knockdown suppressed the cell adhesion, proliferation, migration and invasion in vitro, and inhibited tumor growth in mammary fat pads and pulmonary metastasis in athymic mice. Biochemical studies showed that TIP-1 knockdown had moderate and differential effects on the beta-catenin-regulated gene expression, but remarkably down regulated the genes for cell adhesion and motility in breast cancer cells. The decreased expression of integrins and paxillin was accompanied with reduced cell adhesion and focal adhesion formation on fibronectin-coated surface. In conclusion, this study revealed a novel oncogenic function of TIP-1 suggesting that TIP-1 holds potential as a prognostic biomarker and a therapeutic target in the treatment of human invasive breast cancers.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

The significance of differential expression of genes and proteins in human primary cells caused by microgrooved biomaterial substrata

We demonstrate that etched microgrooves, with truncated V-shape in cross-section and subsequent acid etching, on titanium substrata alter the expression of various genes and proteins in human primary cells. Etched microgrooves with 30 or 60 μm width and 10 μm depth promoted human gingival fibroblast proliferation and significantly enhanced the osteoblast differentiation of human bone marrow-derived mesenchymal stem cells and human periodontal ligament cells by inducing differential expression of various genes involved in cell adhesion, migration, proliferation, mitosis, cytoskeletal reorganization, translation initiation, vesicular trafficking, proton transportation, transforming growth factor-β signaling, mitogen-activated protein kinase signaling, simvastatin's anabolic effect on bone, inhibitory guanine nucleotide binding protein (G protein)'s action, sumoylation pathway, survival/apoptosis, mitochondrial distribution, type I collagen production, osteoblast differentiation, and bone remodeling that were verified by the differential display PCR and quantitative real-time PCR. The most influential genes on the enhancement of fibroblast proliferation or osteoblast differentiation were determined by multiple regression analysis, and the expression of relevant proteins was confirmed by western blotting and protein quantitation.

Acheron regulates vascular endothelial proliferation and angiogenesis together with Id1 during wound healing

RNA binding protein acheron has proved to be either the mediator of integrin-extracellular matrix interactions or the regulatory factor that participates in vertebrate development, cell differentiation and cell death. We report the role of acheron in vascular endothelial proliferation, angiogenesis and wound healing post-trauma. Co-immunoprecipitation showed that Acheron forms a ternary complex with β1 integrin and Id1 in human umbilical vein endothelial cells following stimulation with serious trauma serum. Acheron, vascular endothelial growth factor (VEGF), and β1 integrin mRNA expression was apparently inhibited, and capillary density and wound healing rate also were reduced in Id1-deficient mice trauma model. Acheron together with Id1 significantly induces VEGF, not CD105 level inhibition by serious trauma serum for 24 h. In conclusion, we have demonstrated that acheron may be an effective mediator of promoting endothelial proliferation, angiogenesis and wound healing probably by regulating VEGF together with Id1.

Localisation to lipid rafts correlates with increased function of the Gal/GalNAc lectin in the human protozoan parasite, Entamoeba histolytica

Entamoeba histolytica is the causative agent of dysentery and liver abscess and is prevalent in developing countries. Adhesion to the host is critical to infection and is mediated by amoebic surface receptors. One such receptor, the Gal/GalNAc lectin, binds to galactose or N-acetylgalactosamine residues on host components and consists of heavy (Hgl), light (Lgl) and intermediate (Igl) subunits. The mechanism by which the lectin assembles into a functional complex is not known. The parasite also relies on cholesterol-rich domains (lipid rafts) for adhesion. Therefore, it is conceivable that rafts regulate the assembly or function of the lectin. To test this, amoebae were loaded with cholesterol and lipid rafts were purified and characterised. Western blotting showed that cholesterol loading resulted in co-compartmentalisation of all three subunits in rafts. This co-compartmentalisation was accompanied by an increase in the ability of the amoebae to bind to host cells in a galactose-specific manner, suggesting that there is a correlation between location and function of the Gal/GalNAc lectin. Cholesterol loading did not increase the surface levels of the lectin subunits. Therefore, the cholesterol-induced increase in adhesion was not the result of externalisation of an internal pool of subunits. A mutant cell line that modestly responded to cholesterol with a slight increase in adhesion exhibited only a slight enrichment of Hgl and Lgl in rafts. This supports the connection between location and function of the Gal/GalNAc lectin. Actin can also influence the interaction of proteins with rafts. Therefore, the sub-membrane distribution of the lectin subunits was also assessed after treatment with an actin depolymerising agent, cytochalasin D. Cytochalasin D-treatment had no effect on the submembrane distribution of the subunits, suggesting that actin does not prevent the association of lectin subunits with rafts in this system. Together, these data provide insight into the molecular mechanisms regulating the location and function of this adhesin.

A novel strategy for the comprehensive analysis of the biomolecular composition of isolated plasma membranes

We manufactured a novel type of lipid-coated superparamagnetic nanoparticles that allow for a rapid isolation of plasma membranes (PMs), enabling high-resolution proteomic, glycomic and lipidomic analyses of the cell surface. We used this technology to characterize the effects of presenilin knockout on the PM composition of mouse embryonic fibroblasts. We found that many proteins are selectively downregulated at the cell surface of presenilin knockout cells concomitant with lowered surface levels of cholesterol and certain sphingomyelin species, indicating defects in specific endosomal transport routes to and/or from the cell surface. Snapshots of N-glycoproteomics and cell surface glycan profiling further underscored the power and versatility of this novel methodology. Since PM proteins provide many pathologically relevant biomarkers representing two-thirds of the currently used drug targets, this novel technology has great potential for biomedical and pharmaceutical applications.

Gambogic acid inhibits tumor cell adhesion by suppressing integrin β1 and membrane lipid rafts-associated integrin signaling pathway

Cell adhesion plays an important role in the steps of cancer metastasis. Regulation of cell-cell (intercellular) and cell-matrix adhesion is a promising strategy for cancer progression. Gambogic acid is a xanthone derived from the resin of the Chinese plant Garciania hanburyi, with potent anti-metastasis activity on highly metastatic cells. The aim of this study was to investigate the function and mechanism of gambogic acid on tumor adhesion. We found that gambogic acid strongly inhibited the adhesion of human cancer cells to fibronectin. This inhibition was associated with the deformation of focal adhesion complex, which was mediated by suppressing the expression of integrin β1 and integrin signaling pathway. In vitro, cell lipid rafts clustering was inhibited following treatment of gambogic acid, which induced the suppression of integrin β1 and focal adhesion complex proteins colocalization within rafts. Moreover, gambogic acid significantly decreased cellular cholesterol content, whereas cholesterol replenishment lessened the inhibitory effect of gambogic acid on cell adhesion. Real-time PCR analysis showed that gambogic acid reduced mRNA levels of hydroxymethylglutaryl-CoA reductase and sterol regulatory element binding protein-2, while increased acetyl-CoA acetyltransferase-1/2. Taken together, these results demonstrate that gambogic acid inhibits cell adhesion via suppressing integrin β1 abundance and cholesterol content as well as the membrane lipid raft-associated integrin function, which provide new evidence for the anti-cancer activity of gambogic acid.

Raft protein clustering alters N-Ras membrane interactions and activation pattern

The trafficking, membrane localization, and lipid raft association of Ras proteins, which are crucial oncogenic mediators, dictate their isoform-specific biological responses. Accordingly, their spatiotemporal dynamics are tightly regulated. While extensively studied for H- and K-Ras, such information on N-Ras, an etiological oncogenic factor, is limited. Here, we report a novel mechanism regulating the activation-dependent spatiotemporal organization of N-Ras, its modulation by biologically relevant stimuli, and isoform-specific effects on signaling. We combined patching/immobilization of another membrane protein with fluorescence recovery after photobleaching (patch-FRAP) and FRAP beam size analysis to investigate N-Ras membrane interactions. Clustering of raft-associated proteins, either glycosylphosphatidylinositol-anchored influenza virus hemagglutinin (HA-GPI) or fibronectin receptors, selectively enhanced the plasma membrane-cytoplasm exchange of N-Ras-GTP (preferentially associated with raft domains) in a cholesterol-dependent manner. Electron microscopy (EM) analysis showed N-Ras-GTP localization in cholesterol-sensitive clusters, from which it preferentially detached upon HA-GPI cross-linking. HA-GPI clustering enhanced the Golgi compartment (GC) accumulation and signaling of epidermal growth factor (EGF)-stimulated N-Ras-GTP. Notably, the cross-linking-mediated enhancement of N-Ras-GTP exchange and GC accumulation depended strictly on depalmitoylation. We propose that the N-Ras activation pattern (e.g., by EGF) is altered by raft protein clustering, which enhances N-Ras-GTP raft localization and depalmitoylation, entailing its exchange and GC accumulation following repalmitoylation. This mechanism demonstrates a functional signaling role for the activation-dependent differential association of Ras isoforms with raft nanodomains.

Epithelial membrane protein-2 promotes endometrial tumor formation through activation of FAK and Src

Endometrial cancer is the most common gynecologic malignancy diagnosed among women in developed countries. One recent biomarker strongly associated with disease progression and survival is epithelial membrane protein-2 (EMP2), a tetraspan protein known to associate with and modify surface expression of certain integrin isoforms. In this study, we show using a xenograft model system that EMP2 expression is necessary for efficient endometrial tumor formation, and we have started to characterize the mechanism by which EMP2 contributes to this malignant phenotype. In endometrial cancer cells, the focal adhesion kinase (FAK)/Src pathway appears to regulate migration as measured through wound healing assays. Manipulation of EMP2 levels in endometrial cancer cells regulates the phosphorylation of FAK and Src, and promotes their distribution into lipid raft domains. Notably, cells with low levels of EMP2 fail to migrate and poorly form tumors in vivo. These findings reveal the pivotal role of EMP2 in endometrial cancer carcinogenesis, and suggest that the association of elevated EMP2 levels with endometrial cancer prognosis may be causally linked to its effect on integrin-mediated signaling.

Compensatory function of Pyk2 protein in the promotion of focal adhesion kinase (FAK)-null mammary cancer stem cell tumorigenicity and metastatic activity

Mammary cancer stem cells (MaCSCs) have been identified as a rare population of cells capable of self-renewal to drive mammary tumorigenesis and metastasis. Nevertheless, relatively little is known about the intracellular signaling pathways regulating self-renewal and metastatic activities of MaCSCs in vivo. Using a recently developed breast cancer mouse model with focal adhesion kinase (FAK) deletion in mammary tumor cells (MFCKO-MT mice), here we present evidence suggesting a compensatory function of Pyk2, a FAK-related kinase, in the regulation of MaCSCs and metastasis in these mice. Increased expression of Pyk2 was found selectively in pulmonary metastatic nodules of MFCKO-MT mice, and its inhibition significantly reduced mammary tumor development and metastasis in these mice. Consistent with the idea of metastasis driven by MaCSCs, we detected selective up-regulation of Pyk2 in MaCSCs, but not bulk mammary tumor cells, of primary tumors developed in MFCKO-MT mice. We further showed that inhibition of Pyk2 in FAK-null MaCSCs significantly decreased their tumorsphere formation and migration in vitro as well as self-renewal, tumorigenicity, and metastatic activity in vivo. Last, we identified PI3K/Akt signaling as a major mediator of FAK regulation of MaCSCs as well as a target for the compensatory function of Pyk2 in FAK-null MaCSCs. Together, these results further advance our understanding of FAK and its related tyrosine kinase Pyk2 in regulation of MaCSCs in breast cancer and suggest that pharmaceutically targeting these kinases may hold promise as a novel treatment for the disease by targeting and eradicating MaCSCs.