COVID-19 and drugs: pathophysiology and therapeutic approaches

This review provides an update on the different therapeutic approaches that have been used to treat SARS-CoV-2 infection, as well as, the resulting paradoxical inflammation disorders.

Phosphorylation of Merlin by Aurora A kinase appears necessary for mitotic progression

Although Merlin's function as a tumor suppressor and regulator of mitogenic signaling networks such as the Ras/rac, Akt, and Hippo pathways is well-documented, in mammals as well as in insects, its role during cell cycle progression remains unclear. In this study, using a combination of approaches, including FACS analysis, time-lapse imaging, immunofluorescence microscopy, and co-immunoprecipitation, we show that Ser-518 of Merlin is a substrate of the Aurora protein kinase A during mitosis and that its phosphorylation facilitates the phosphorylation of a newly discovered site, Thr-581. We found that the expression in HeLa cells of a Merlin variant that is phosphorylation-defective on both sites leads to a defect in centrosomes and mitotic spindles positioning during metaphase and delays the transition from metaphase to anaphase. We also show that the dual mitotic phosphorylation not only reduces Merlin binding to microtubules but also timely modulates ezrin interaction with the cytoskeleton. Finally, we identify several point mutants of Merlin associated with neurofibromatosis type 2 that display an aberrant phosphorylation profile along with defective α-tubulin-binding properties. Altogether, our findings of an Aurora A-mediated interaction of Merlin with α-tubulin and ezrin suggest a potential role for Merlin in cell cycle progression.

[Cell complexity should be placed at the heart of cancer research]

Genetic and most likely epigenetic alterations occurring during tumor progression and metastatic process lead to a broad deregulation of major cellular functions. However, the molecular mechanisms involved are still poorly understood. To understand them, the cell, the basic unit of life, remains more than ever the essential level to integrate the functional impact of genetics and epigenetics processes in the light of the global economy of the normal and cancerous cell, and of its interactions with its microenvironment.

AMOTL1 Promotes Breast Cancer Progression and Is Antagonized by Merlin

The Hippo signaling network is a key regulator of cell fate. In the recent years, it was shown that its implication in cancer goes well beyond the sole role of YAP transcriptional activity and its regulation by the canonical MST/LATS kinase cascade. Here we show that the motin family member AMOTL1 is an important effector of Hippo signaling in breast cancer. AMOTL1 connects Hippo signaling to tumor cell aggressiveness. We show that both canonical and noncanonical Hippo signaling modulates AMOTL1 levels. The tumor suppressor Merlin triggers AMOTL1 proteasomal degradation mediated by the NEDD family of ubiquitin ligases through direct interaction. In parallel, YAP stimulates AMOTL1 expression. The loss of Merlin expression and the induction of Yap activity that are frequently observed in breast cancers thus result in elevated AMOTL1 levels. AMOTL1 expression is sufficient to trigger tumor cell migration and stimulates proliferation by activating c-Src. In a large cohort of human breast tumors, we show that AMOTL1 protein levels are upregulated during cancer progression and that, importantly, the expression of AMOTL1 in lymph node metastasis appears predictive of the risk of relapse. Hence we uncover an important mechanism by which Hippo signaling promotes breast cancer progression by modulating the expression of AMOTL1.

Excellent translational research in oncology: A journey towards novel and more effective anti-cancer therapies

Comprehensive Cancer Centres (CCCs) serve as critical drivers for improving cancer survival. In Europe, we have developed an Excellence Designation System (EDS) consisting of criteria to assess "excellence" of CCCs in translational research (bench to bedside and back), with the expectation that many European CCCs will aspire to this status.

Concomitant Notch activation and p53 deletion trigger epithelial-to-mesenchymal transition and metastasis in mouse gut

Epithelial-to-mesenchymal transition-like (EMT-like) is a critical process allowing initiation of metastases during tumour progression. Here, to investigate its role in intestinal cancer, we combine computational network-based and experimental approaches to create a mouse model with high metastatic potential. Construction and analysis of this network map depicting molecular mechanisms of EMT regulation based on the literature suggests that Notch activation and p53 deletion have a synergistic effect in activating EMT-like processes. To confirm this prediction, we generate transgenic mice by conditionally activating the Notch1 receptor and deleting p53 in the digestive epithelium (NICD/p53(-/-)). These mice develop metastatic tumours with high penetrance. Using GFP lineage tracing, we identify single malignant cells with mesenchymal features in primary and metastatic tumours in vivo. The development of such a model that recapitulates the cellular features observed in invasive human colorectal tumours is appealing for innovative drug discovery.

Conditional expression of fascin increases tumor progression in a mouse model of intestinal cancer

While absent from normal epithelia, an actin bundling protein, fascin, becomes expressed in invasive carcinoma of different origins. It is highly enriched at the tumors' invasive front suggesting that it could play a role in cancer invasion. Multiple studies have shown that fascin, through its role in formation of cellular protrusions such as filopodia and invadopodia, enhances cancer cell migration and invasion in vitro. However, the role of fascin in vivo remains unknown. We have generated a compound transgenic mouse model that allows expression of fascin in the intestinal epithelium in the Apc-mutated background. Conditional expression of fascin led to decrease in mice survival and increase in tumor burden compared to control animals. Induction of fascin expression in adult tumor-bearing animals accelerated tumor progression and led to formation of invasive adenocarcinoma. Altogether, our study shows that fascin can promote tumor progression in vivo, but also unravels an unexpected role of fascin in tumor initiation.

Fascin plays a role in stress fiber organization and focal adhesion disassembly

Migrating cells nucleate focal adhesions (FAs) at the cell front and disassemble them at the rear to allow cell translocation. FAs are made of a multiprotein complex, the adhesome, which connects integrins to stress fibers made of mixed-polarity actin filaments [1-5]. Myosin II-driven contraction of stress fibers generates tensile forces that promote adhesion growth [6-9]. However, tension must be tightly controlled, because if released, FAs disassemble [3, 10-12]. Conversely, excess tension can cause abrupt cell detachment resulting in the loss of a major part of the adhesion [9, 12]. Thus, both adhesion growth and disassembly depend on tensile forces generated by stress fiber contraction, but how this contractility is regulated remains unclear. Here, we show that the actin-bundling protein fascin crosslinks the actin filaments into parallel bundles at the stress fibers' termini. Fascin prevents myosin II entry at this region and inhibits its activity in vitro. In fascin-depleted cells, polymerization of actin filaments at the stress fiber termini is slower, the actin cytoskeleton is reorganized into thicker stress fibers with a higher number of myosin II molecules, FAs are larger and less dynamic, and consequently, traction forces that cells exert on their substrate are larger. We also show that fascin dissociation from stress fibers is required to allow their severing by cofilin, leading to efficient disassembly of FAs.

Proteomic screening identifies a YAP-driven signaling network linked to tumor cell proliferation in human schwannomas

BACKGROUND

Inactivation of the NF2 gene predisposes to neurofibromatosis type II and the development of schwannomas. In vitro studies have shown that loss of NF2 leads to the induction of mitogenic signaling mediated by receptor tyrosine kinases (RTKs), MAP kinase, AKT, or Hippo pathways. The goal of our study was to evaluate the expression and activity of these signaling pathways in human schwannomas in order to identify new potential therapeutic targets.

METHODS

Large sets of human schwannomas, totaling 68 tumors, were analyzed using complementary proteomic approaches. RTK arrays identified the most frequently activated RTKs. The correlation between the expression and activity of signaling pathways and proliferation of tumor cells using Ki67 marker was investigated by reverse-phase protein array (RRPA). Finally, immunohistochemistry was used to evaluate the expression pattern of signaling effectors in the tumors.

RESULTS

We showed that Her2, Her3, PDGFRß, Axl, and Tie2 are frequently activated in the tumors. Furthermore, RRPA demonstrated that Ki67 levels are linked to YAP, p-Her3, and PDGFRß expression levels. In addition, Her2, Her3, and PDGFRß are transcriptional targets of Yes-associated protein (YAP) in schwannoma cells in culture. Finally, we observed that the expression of these signaling effectors is very variable between tumors.

CONCLUSIONS

Tumor cell proliferation in human schwannomas is linked to a signaling network controlled by the Hippo effector YAP. Her2, Her3, PDGFRß, Axl, and Tie2, as well as YAP, represent potentially valuable therapeutic targets. However, the variability of their expression between tumors may result in strong differences in the response to targeted therapy.

Myosin VI regulates actin dynamics and melanosome biogenesis

Myosin VI has been implicated in various steps of organelle dynamics. However, the molecular mechanism by which this myosin contributes to membrane traffic is poorly understood. Here, we report that myosin VI is associated with a lysosome-related organelle, the melanosome. Using an actin-based motility assay and video microscopy, we observed that myosin VI does not contribute to melanosome movements. Myosin VI expression regulates instead the organization of actin networks in the cytoplasm. Using a cell-free assay, we showed that myosin VI recruited actin at the surface of isolated melanosomes. Myosin VI is involved in the endocytic-recycling pathway, and this pathway contributes to the transport of a melanogenic enzyme to maturing melanosomes. We showed that depletion of myosin VI accumulated a melanogenic enzyme in enlarged melanosomes and increased their melanin content. We confirmed the requirement of myosin VI to regulate melanosome biogenesis by analysing the morphology of melanosomes in choroid cells from of the Snell's waltzer mice that do not express myosin VI. Together, our results provide new evidence that myosin VI regulates the organization of actin dynamics at the surface of a specialized organelle and unravel a novel function of this myosin in regulating the biogenesis of this organelle.

Clathrin is required for Scar/Wave-mediated lamellipodium formation

The Scar/Wave complex (SWC) generates lamellipodia through Arp2/3-dependent polymerisation of branched actin networks. In order to identify new SWC regulators, we conducted a screen in Drosophila cells combining proteomics with functional genomics. This screen identified Clathrin heavy chain (CHC) as a protein that binds to the SWC and whose depletion affects lamellipodium formation. This role of CHC in lamellipodium formation can be uncoupled from its role in membrane trafficking by several experimental approaches. Furthermore, CHC is detected in lamellipodia in the absence of the adaptor and accessory proteins of endocytosis. We found that CHC overexpression decreased membrane recruitment of the SWC, resulting in reduced velocity of protrusions and reduced cell migration. By contrast, when CHC was targeted to the membrane by fusion to a myristoylation sequence, we observed an increase in membrane recruitment of the SWC, protrusion velocity and cell migration. Together these data suggest that, in addition to its classical role in membrane trafficking, CHC brings the SWC to the plasma membrane, thereby controlling lamellipodium formation.

Ezrin regulates microvillus morphogenesis by promoting distinct activities of Eps8 proteins

The membrane cytoskeleton linker ezrin differentially regulates the activity of Eps8 and Eps8L1a in microvillar actin-F assembly. Eps8L1a displays F-actin capping activity, therefore controlling microvillus length, whereas, as previously shown, Eps8 displays bundling activity.

The mechanisms that regulate actin filament polymerization resulting in the morphogenesis of the brush border microvilli in epithelial cells remain unknown. Eps8, the prototype of a family of proteins capable of capping and bundling actin filaments, has been shown to bundle the microvillar actin filaments. We report that Eps8L1a, a member of the Eps8 family and a novel ezrin-interacting partner, controls microvillus length through its capping activity. Depletion of Eps8L1a leads to the formation of long microvilli, whereas its overexpression has the opposite effect. We demonstrate that ezrin differentially modulates the actin-capping and -bundling activities of Eps8 and Eps8L1a during microvillus assembly. Coexpression of ezrin with Eps8 promotes the formation of membrane ruffles and tufts of microvilli, whereas expression of ezrin and Eps8L1a induces the clustering of actin-containing structures at the cell surface. These distinct morphological changes are neither observed when a mutant of ezrin defective in its binding to Eps8/Eps8L1a is coexpressed with Eps8 or Eps8L1a nor observed when ezrin is expressed with mutants of Eps8 or Eps8L1a defective in the actin-bundling or -capping activities, respectively. Our data show a synergistic effect of ezrin and Eps8 proteins in the assembly and organization of actin microvillar filaments.

A new role for the architecture of microvillar actin bundles in apical retention of membrane proteins

The bundled architecture of actin filaments is not needed for intestinal microvillar morphogenesis, as shown in knockout mice devoid of microvillar actin-bundling proteins. This architecture is essential for the apical anchorage of digestive proteins, probably via the recruitment of key players in apical retention, such as myosin-1a, and, as a result, for intestinal physiology.

Actin-bundling proteins are identified as key players in the morphogenesis of thin membrane protrusions. Until now, functional redundancy among the actin-bundling proteins villin, espin, and plastin-1 has prevented definitive conclusions regarding their role in intestinal microvilli. We report that triple knockout mice lacking these microvillar actin-bundling proteins suffer from growth delay but surprisingly still develop microvilli. However, the microvillar actin filaments are sparse and lack the characteristic organization of bundles. This correlates with a highly inefficient apical retention of enzymes and transporters that accumulate in subapical endocytic compartments. Myosin-1a, a motor involved in the anchorage of membrane proteins in microvilli, is also mislocalized. These findings illustrate, in vivo, a precise role for local actin filament architecture in the stabilization of apical cargoes into microvilli. Hence, the function of actin-bundling proteins is not to enable microvillar protrusion, as has been assumed, but to confer the appropriate actin organization for the apical retention of proteins essential for normal intestinal physiology.

Notch signaling in intestinal homeostasis across species: the cases of Drosophila, Zebrafish and the mouse

Notch signaling has been recently shown to have a fundamental role in stem cell maintenance and control of proper homeostasis in the intestine of different species. Here, we briefly review the current literature on Notch signals in the intestine of Drosophila, Zebrafish and the mouse, and try to highlight conserved and divergent Notch functions across species. Notch signals show a remarkably conserved role in skewing cell fate choices in intestinal lineages throughout evolution. Genetic analysis demonstrates that loss of Notch signaling invariably leads to increased numbers of secretory cells and loss of enterocytes, while gain of Notch function will completely block secretory cell differentiation. Finally, we discuss the potential contribution of Notch signaling to the initiation of colorectal cancer by controlling the maintenance of the undifferentiated state of intestinal neoplastic cells and speculate on the therapeutic consequences of affecting cancer stem cells.

The ERM proteins interact with the HOPS complex to regulate the maturation of endosomes

In the degradative pathway, the progression of cargos through endosomal compartments involves a series of fusion and maturation events. The HOPS (homotypic fusion and protein sorting) complex is part of the machinery that promotes the progression from early to late endosomes and lysosomes by regulating the exchange of small GTPases. We report that an interaction between subunits of the HOPS complex and the ERM (ezrin, radixin, moesin) proteins is required for the delivery of EGF receptor (EGFR) to lysosomes. Inhibiting either ERM proteins or the HOPS complex leads to the accumulation of the EGFR into early endosomes, delaying its degradation. This impairment in EGFR trafficking observed in cells depleted of ERM proteins is due to a delay in the recruitment of Rab7 on endosomes. As a consequence, the maturation of endosomes is perturbed as reflected by an accumulation of hybrid compartments positive for both early and late endosomal markers. Thus, ERM proteins represent novel regulators of the HOPS complex in the early to late endosomal maturation.

Actin, microtubules, and vimentin intermediate filaments cooperate for elongation of invadopodia

Microtubules and intermediate filaments cooperate with actin and other components of filopodia during invadopodia maturation.

Invasive cancer cells are believed to breach the basement membrane (BM) using specialized protrusions called invadopodia. We found that the crossing of a native BM is a three-stage process: invadopodia indeed form and perforate the BM, elongate into mature invadopodia, and then guide the cell toward the stromal compartment. We studied the remodeling of cytoskeleton networks during invadopodia formation and elongation using ultrastructural analysis, spatial distribution of molecular markers, and RNA interference silencing of protein expression. We show that formation of invadopodia requires only the actin cytoskeleton and filopodia- and lamellipodia-associated proteins. In contrast, elongation of invadopodia is mostly dependent on filopodial actin machinery. Moreover, intact microtubules and vimentin intermediate filament networks are required for further growth. We propose that invadopodia form by assembly of dendritic/diagonal and bundled actin networks and then mature by elongation of actin bundles, followed by the entry of microtubules and vimentin filaments. These findings provide a link between the epithelial to mesenchymal transition and BM transmigration.

Abstract 5113: Filopodial actin machinery cooperates with microtubules and vimentin intermediate filaments in the elongation of invadopodia

Invasive cancer cells breach the basement membrane (BM) using specialized protrusions, called invadopodia. Here, we found that the crossing of a native BM is a three-stage process: invadopodia form and perforate the BM; elongate into mature invadopodia; and then guide the cell towards the stromal compartment. We have studied the remodeling of cytoskeleton networks during invadopodia formation and elongation using ultrastructural analysis, spatial distribution of molecular markers and RNAi silencing of protein expression. We show that formation of invadopodia requires only the actin cytoskeleton, both filopodia- and lamellipodia-associated proteins. On contrary, using a novel in vitro assay, we demonstrate that elongation of invadopodia is mostly dependent on filopodial actin machinery, microtubules and vimentin intermediate filaments. These findings indicate that invadopodia form by assembly of dendritic/diagonal and bundled actin networks and then maturate by elongation of actin bundles together with the infiltration of microtubules and vimentin intermediate filaments.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5113.

Plastin 1 binds to keratin and is required for terminal web assembly in the intestinal epithelium

Plastin 1 (I-plastin, fimbrin) along with villin and espin is a prominent actin-bundling protein of the intestinal brush border microvilli. We demonstrate here that plastin 1 accumulates in the terminal web and interacts with keratin 19, possibly contributing to anchoring the rootlets to the keratin network. This prompted us to investigate the importance of plastin 1 in brush border assembly. Although in vivo neither villin nor espin is required for brush border structure, plastin 1-deficient mice have conspicuous ultrastructural alterations: microvilli are shorter and constricted at their base, and, strikingly, their core actin bundles lack true rootlets. The composition of the microvilli themselves is apparently normal, whereas that of the terminal web is profoundly altered. Although the plastin 1 knockout mice do not show any overt gross phenotype and present a normal intestinal microanatomy, the alterations result in increased fragility of the epithelium. This is seen as an increased sensitivity of the brush border to biochemical manipulations, decreased transepithelial resistance, and increased sensitivity to dextran sodium sulfate-induced colitis. Plastin 1 thus emerges as an important regulator of brush border morphology and stability through a novel role in the organization of the terminal web, possibly by connecting actin filaments to the underlying intermediate filament network.

Epithelial morphogenesis and intestinal cancer: new insights in signaling mechanisms

In this review, the major signal transduction pathways that have been shown to play an important role in intestinal homeostasis are highlighted. Each of them, the Wnt, Notch, Hedgehog, and Bone Morphogenetic Protein, as well as growth-factor regulated Receptor Tyrosine Kinases are depicted with a special emphasis through their involvement in stem cell maintenance and their role in intestinal tumorigenesis. Finally, we discuss recent data on the final steps of tumor progression, notably the formation of distant metastases. This multistep process is highly complex and still far from being understood while being of major importance for the survival of patients with digestive cancer.

Different microtubule motors move early and late endocytic compartments

Important progress has been made during the past decade in the identification of molecular motors required in the distribution of early and late endosomes and the proper trafficking along the endocytic pathway. There is little direct evidence, however, that these motors drive movement of the endosomes. To evaluate the contributions of kinesin-1, dynein and kinesin-2 to the movement of early and late endosomes along microtubules, we made use of a cytosol-free motility assay using magnetically isolated early and late endosomes as well as biochemical analyses and live-cell imaging. By making use of specific antibodies, we confirmed that kinesin-1 and dynein move early endosomes and we found that kinesin-2 moves both early and late endosomes in the cell-free assay. Unexpectedly, dynein did not move late endosomes in the cell-free assay. We provide evidence from disruption of dynein function and latrunculin A treatment, suggesting that dynein regulates late endosome movement indirectly, possibly through a mechanism involving the actin cytoskeleton. These data provide new insights into the complex regulation of endosomes' motility and suggest that dynein is not the major motor required to move late endosomes toward the minus end of microtubules.

Interaction of ezrin with the novel guanine nucleotide exchange factor PLEKHG6 promotes RhoG-dependent apical cytoskeleton rearrangements in epithelial cells

The mechanisms underlying functional interactions between ERM (ezrin, radixin, moesin) proteins and Rho GTPases are not well understood. Here we characterized the interaction between ezrin and a novel Rho guanine nucleotide exchange factor, PLEKHG6. We show that ezrin recruits PLEKHG6 to the apical pole of epithelial cells where PLEKHG6 induces the formation of microvilli and membrane ruffles. These morphological changes are inhibited by dominant negative forms of RhoG. Indeed, we found that PLEKHG6 activates RhoG and to a much lesser extent Rac1. In addition we show that ezrin forms a complex with PLEKHG6 and RhoG. Furthermore, we detected a ternary complex between ezrin, PLEKHG6, and the RhoG effector ELMO. We demonstrate that PLEKHG6 and ezrin are both required in macropinocytosis. After down-regulation of either PLEKHG6 or ezrin expression, we observed an inhibition of dextran uptake in EGF-stimulated A431 cells. Altogether, our data indicate that ezrin allows the local activation of RhoG at the apical pole of epithelial cells by recruiting upstream and downstream regulators of RhoG and that both PLEKHG6 and ezrin are required for efficient macropinocytosis.

Spatial recruitment and activation of the Fes kinase by ezrin promotes HGF-induced cell scattering

The remodeling of epithelial monolayers induced by hepatocyte growth factor (HGF) results in the reorganization of actin cytoskeleton and cellular junctions. We previously showed that the membrane-cytoskeleton linker ezrin plays a major role in HGF-induced morphogenic effects. Here we identified a novel partner of phosphorylated ezrin, the Fes kinase, that acts downstream of ezrin in HGF-mediated cell scattering. We found that Fes interacts directly, through its SH2 domain, with ezrin phosphorylated at tyrosine 477. We show that in epithelial cells, activated Fes localizes either to focal adhesions or cell-cell contacts depending on cell confluency. The recruitment and the activation of Fes to the cell-cell contacts in confluent cells depend on its interaction with ezrin. When this interaction is impaired, Fes remains in focal adhesions and as a consequence the cells show defective spreading and scattering in response to HGF stimulation. Altogether, these results provide a novel mechanism whereby ezrin/Fes interaction at cell-cell contacts plays an essential role in HGF-induced cell scattering and implicates Fes in the cross-talk between cell-cell and cell-matrix adhesion.

Wnt/beta-catenin is essential for intestinal homeostasis and maintenance of intestinal stem cells

The Wnt signaling pathway is deregulated in over 90% of human colorectal cancers. beta-Catenin, the central signal transducer of the Wnt pathway, can directly modulate gene expression by interacting with transcription factors of the TCF/LEF family. In the present study we investigate the role of Wnt signaling in the homeostasis of intestinal epithelium by using tissue-specific, inducible beta-catenin gene ablation in adult mice. Block of Wnt/beta-catenin signaling resulted in rapid loss of transient-amplifying cells and crypt structures. Importantly, intestinal stem cells were induced to terminally differentiate upon deletion of beta-catenin, resulting in a complete block of intestinal homeostasis and fatal loss of intestinal function. Transcriptional profiling of mutant crypt mRNA isolated by laser capture microdissection confirmed those observations and allowed us to identify genes potentially responsible for the functional preservation of intestinal stem cells. Our data demonstrate an essential requirement of Wnt/beta-catenin signaling for the maintenance of the intestinal epithelium in the adult organism. This challenges attempts to target aberrant Wnt signaling as a new therapeutic strategy to treat colorectal cancer.

Fascin, a novel target of beta-catenin-TCF signaling, is expressed at the invasive front of human colon cancer

Cancer cells become metastatic by acquiring a motile and invasive phenotype. This step requires remodeling of the actin cytoskeleton and the expression of exploratory, sensory organelles known as filopodia. Aberrant beta-catenin-TCF target gene activation plays a major role in colorectal cancer development. We identified fascin1, a key component of filopodia, as a target of beta-catenin-TCF signaling in colorectal cancer cells. Fascin1 mRNA and protein expression were increased in primary cancers in a stage-dependent manner. Fascin1 was exclusively localized at the invasive front of tumors also displaying nuclear beta-catenin. Forced expression of fascin1 in colorectal cancer cells increased their migration and invasion in cell cultures and caused cell dissemination and metastasis in vivo, whereas suppression of fascin1 expression by small interfering RNA reduces cell invasion. Although expression of fascin1 in primary tumors correlated with the presence of metastases, fascin1 was not expressed in metastases. Our studies show that fascin1 expression is tightly regulated during development of colon cancer metastases and is a novel target of beta-catenin-TCF signaling. We propose that transient up-regulation of fascin1 in colorectal cancer promotes the acquisition of migratory and invasive phenotypes that lead to metastasis. Moreover, the expression of fascin1 is down-regulated when tumor cells reach their metastatic destination where migration ceases and proliferation is enhanced. Although metastasis to vital organs is often the cause of mortality, only limited success has been attained in developing effective therapeutics against metastatic disease. We propose that genes involved in cell migration and invasion, such as fascin1, could serve as novel targets for metastasis prevention.

APC inactivation associates with abnormal mitosis completion and concomitant BUB1B/MAD2L1 up-regulation

BACKGROUND & AIMS

Chromosomal instability, a hallmark of most colorectal cancers, has been related to altered chromosome segregation and the consequent deficit in genetic integrity. A role for the tumor suppressor gene APC has been proposed in colorectal cancer that leads to compromised chromosome segregation even though the molecular mechanism is not yet understood. Here, we tackled the genetic basis for the contribution of APC to chromosomal instability in familial adenomatous polyposis and sporadic colorectal cancer.

METHODS

We have used video-microscopy of primary cultures and molecular genetic methods to address these issues in human samples and in genetically defined mouse models that either recapitulate the familial adenomatous polyposis syndrome (Apc(1638N)), or develop tumors in the absence of APC mutations (pvillin-KRASV12G).

RESULTS

Mutations in APC were associated with an increased incidence in cell cycle defects during the completion of cytokinesis. Transcriptome analysis performed on mouse models indicated a significant up-regulation of genes that regulate accurate mitosis. Notably, we identified up-regulated expression of BUB1B and MAD2L1, 2 genes that are involved in the mitotic checkpoint, but have so far not been implicated in chromosomal instability induced by APC loss of function. In vitro modulation of APC expression suggested a causal association for this upregulation, which was consistently found in sporadic and familial adenomatous polyposis lesions, as an early event in colorectal tumorigenesis.

CONCLUSIONS

In addition to the known function of APC during correct spindle assembly and positioning, we propose a concomitant involvement of APC in the surveillance mechanism of accurate mitosis.

RNAi depleted Drosophila cell extracts to dissect signaling pathways leading to actin polymerization

Dissection of signal transduction pathways leading to actin polymerization has been performed in cytosolic extracts. In such assays, the implication of an effector molecule is demonstrated by the loss of actin polymerization upon its depletion and the restoration of actin polymerization upon its add-back. Two major limitations in the wide use of this approach have been the availability of immunodepleting antibodies and the functional redundancy for many classes of effector molecules encoded by vertebrate genomes. To circumvent these limitations, we developed extracts derived from S2 Drosophila cells, which are competent for actin polymerization. In this system, depleted extracts are simply obtained from cells cultured with long double stranded RNAs in the medium. We validated the method by showing that beads coated with the C-terminal domain of Wave2 were no longer able to trigger actin polymerization in an extract depleted of the Arp2/3 complex. We also examined the complete set of Drosophila small GTPases of the Rho family for their ability to polymerize actin in such extracts, and found that only dCdc42 was able to induce actin polymerization. Using RNAi depleted extract, we confirmed that dCdc42 triggers actin polymerization in a Wasp dependent manner.

Conditional mouse models of cancer

The development of inducible and conditional technologies allowed us to generate transgenic mouse models that faithfully recapitulate human tumorigenesis. It is possible to control, in time and space, the development of tumors in almost every mouse tissue. The result is that now we have available mouse models for all major human cancers. Novel noninvasive approaches to tumor imaging will enable us to follow tumor development and metastasis in vivo, as well as the effects of candidate therapeutic drugs. Such new generation tumor models, which accurately emulate the disease state in situ, should provide a useful platform with which to experimentally test drugs targeted to specific gene products, or combinations of genes that control rate-limiting steps of tumor development. In this review, we focus on the different mouse models for colon cancer.

Heterogeneous Metastasis Efficiency of Isogenic Orthotopic Colon Cancer Xenografts Reveals Distinctive Gene Expression Profiles

Hepatic and lung metastases are the leading causes of mortality and major indicators of aggressiveness in colorectal cancer. The underlying molecular mechanisms contributing to the development of metastasis are still unclear. Here, we designed a novel approach to explore gene expression profiles associated with metastasis in human colorectal cancer (hCRC). A series of ten isogenic tumors from three different hCRC models were orthotopically implanted into nude mice. In these series, we analyzed the contribution of dynamic heterogeneity, independently of any intrinsic gene expression program predictive of metastasis. When screened for the presence of disseminated tumor cells in the lung and liver, as the most common host tissues for hCRC metastases, both high- and low-metastatic efficient tumors were found among these isogenic orthotopic series. The metastasis-specific cDNA macroarray analysis of 96 genes, in both tumor populations for each of the three hCRC models, characterized a common differential gene expression within a small group of genes. Our results suggest that, independently of a gene expression profile predictive of metastasis, the progressive acquisition of additional alterations occurs during hCRC tumorigenesis. This dynamic process might determine tumor progression, namely the metastasis dissemination.

Villin severing activity enhances actin-based motility in vivo

Villin, an actin-binding protein associated with the actin bundles that support microvilli, bundles, caps, nucleates, and severs actin in a calcium-dependant manner in vitro. We hypothesized that the severing activity of villin is responsible for its reported role in enhancing cell plasticity and motility. To test this hypothesis, we chose a loss of function strategy and introduced mutations in villin based on sequence comparison with CapG. By pyrene-actin assays, we demonstrate that this mutant has a strongly reduced severing activity, whereas nucleation and capping remain unaffected. The bundling activity and the morphogenic effects of villin in cells are also preserved in this mutant. We thus succeeded in dissociating the severing from the three other activities of villin. The contribution of villin severing to actin dynamics is analyzed in vivo through the actin-based movement of the intracellular bacteria Shigella flexneri in cells expressing villin and its severing variant. The severing mutations abolish the gain of velocity induced by villin. To further analyze this effect, we reconstituted an in vitro actin-based bead movement in which the usual capping protein is replaced by either the wild type or the severing mutant of villin. Confirming the in vivo results, villin-severing activity enhances the velocity of beads by more than two-fold and reduces the density of actin in the comets. We propose a model in which, by severing actin filaments and capping their barbed ends, villin increases the concentration of actin monomers available for polymerization, a mechanism that might be paralleled in vivo when an enterocyte undergoes an epithelio-mesenchymal transition.

APC and oncogenic KRAS are synergistic in enhancing Wnt signaling in intestinal tumor formation and progression

BACKGROUND & AIMS

Synchronous activation of the Wnt signaling pathway, mostly because of loss of function of the APC tumor suppressor, and of the oncogenic KRAS-signaling pathway is very frequent in colorectal cancer and is associated with poor prognosis.

METHODS

We have generated a compound transgenic mouse model, KRAS(V12G)/Apc(+/1638N), to recapitulate the human disease and compared it with single transgenic littermates.

RESULTS

Compound mutant mice are characterized by a 10-fold increase in tumor multiplicity and by accelerated tumor progression, resulting in strongly enhanced morbidity and mortality. Tumors from compound mutant mice proliferate faster and show decreased levels of apoptosis. Several lines of evidence indicate that the observed increase in tumor multiplicity and malignant transformation is caused by the synergistic activation of Wnt signaling in cells with oncogenic KRAS and loss-of-function Apc mutations. Activated KRAS is known to induce tyrosine phosphorylation of beta-catenin, leading to its release from E-cadherin at the adherens junction. This results in an increased beta-catenin pool in the cytoplasma, its subsequent translocation to the nucleus, and the transcriptional activation of Wnt downstream target genes. Accordingly, intestinal tumors from KRAS(V12G)/Apc(+/1638N) mice show a significant increase in cells with nuclear accumulation of beta-catenin when compared with Apc(+/1638N) animals. Moreover, Apc/KRAS-mutant embryonic stem cells show a significantly enhanced beta-catenin/T-cell factor-mediated transcriptional activation, accompanied by increased beta-catenin nuclear localization.

CONCLUSIONS

This KRAS-induced increase in Wnt/beta-catenin signaling may enhance the plasticity and self-renewal capacity of the tumor, thus resulting in the drastically augmented tumor multiplicity and malignant behavior in compound mutant animals.

In vivo tumor targeting using a novel intestinal pathogen-based delivery approach

Efficient methods for tumor targeting are eagerly awaited and must satisfy several challenges: molecular specificity, transport through physiologic barriers, and capacity to withstand extracellular or intracellular degradation and inactivation by the immune system. Through interaction with its hosts, the intestinal pathogen-produced Shiga toxin has evolved molecular properties that are of interest in this context. Its nontoxic B-subunit binds to the cellular toxin receptor, glycosphingolipid Gb3, which is highly expressed on human cancers and has recently been reported to be involved in the formation of metastasis in colorectal cancers. Its function as a target for cancer therapy has already been addressed in xenograft experiments. We here show that after oral or i.v. injections in mice, the B-subunit targets spontaneous digestive Gb3-expressing adenocarcinomas. The nontumoral mucosa is devoid of labeling, with the exception of rare enteroendocrine and CD11b-positive cells. As opposed to other delivery tools that are often degraded or recycled on cancer cells, the B-subunit stably associates with these cells due to its trafficking via the retrograde transport route. This can be exploited for the in vivo delivery of contrast agents to tumors, as exemplified using fibered confocal fluorescence endoscopy and positron emission tomography (PET) imaging. In conclusion, the data presented in this manuscript lay the groundwork for a novel delivery technology that, in addition to its use for molecular imaging applications such as noninvasive PET, could also be exploited for targeted tumor therapies.

ERM proteins in epithelial cell organization and functions

ERM (Ezrin, Radixin, Moesin) proteins are membrane-cytoskeleton linkers that regulate the structure and the function of specific domains of the plasma membrane. ERM proteins are expressed in all metazoan analyzed so far. Genetic analysis of ERM protein functions has recently been performed simultaneously in three different organisms, mouse, Drosophila melanogaster and C. elegans. These studies have revealed a remarkable conservation of the protein functions through evolution. Moreover they have shed light on the crucial role these proteins play in various physiological processes that occur in epithelial cells.

Myosin Ib modulates the morphology and the protein transport within multi-vesicular sorting endosomes

Members of at least four classes of myosin (I, II, V and VI) have been implicated in the dynamics of a large variety of organelles. Despite their common motor domain structure, some of these myosins, however, are non processive and cannot move organelles along the actin tracks. Here, we demonstrate in the human pigmented MNT-1 cell line that, (1) the overexpression of one of these myosins, myosin 1b, or the addition of cytochalasin D affects the morphology of the sorting multivesicular endosomes; (2) the overexpression of myosin 1b delays the processing of Pmel17 (the product of murine silver locus also named GP100), which occurs in these multivesicular endosomes; (3) myosin 1b associated with endosomes coimmunoprecipitates with Pmel17. All together, these observations suggest that myosin 1b controls the traffic of protein cargo in multivesicular endosomes most probably through its ability to modulate with actin the morphology of these sorting endosomes.

Shigella flexneri infection is dependent on villin in the mouse intestine and in primary cultures of intestinal epithelial cells

Villin is an actin-binding protein present in intestinal and kidney brush borders. Villin has been shown to present in vitro Ca(2+)-dependent bundling and severing F-actin properties. The study of villin knock-out mice allowed us to show that while bundling of F-actin microfilaments is unaffected, this protein is important for the reorganization of the actin cytoskeleton elicited by various signals during both physiological and pathological conditions. Here, we studied the role of villin during infection by Shigella flexneri, the causative agent of bacillary dysentery. This bacterium induces the reorganization of the host actin cytoskeleton to penetrate into epithelial cells and spread from cell to cell. In vivo, we show that unlike newborn vil+/+ mice, which are sensitive to Shigella invasion, resulting in a destructive inflammatory response of the intestinal mucosa following intragastric inoculation, newborn vil-/- mice appear fully resistant to infection. Using primary cultures of intestinal epithelial cells derived from vil+/+ or vil -/- mice, we demonstrate that villin plays an essential role in S. flexneri entry and cell-to-cell dissemination. Villin expression is thus critical for Shigella infection through its ability to remodel the actin cytoskeleton.

Mouse models of K-ras-initiated carcinogenesis

Activating mutations of the oncogene K-ras are found in one third of all human cancers. Much of our knowledge on K-ras signal transduction and its influence on tumor initiation and progression comes from in vitro studies with cell lines. However, mouse models of human cancer allow a much more faithful recapitulation of the human disease, and the in vivo perspective is crucial for our understanding of neoplasia. In recent years, several new murine models for K-ras-induced tumorigenesis have been described. They allow new insights into the specific role that oncogenic K-ras proteins play in different solid tumors, and they permit the molecular dissection of the pathways that are initiated by somatic mutations in subsets of cells. Key advances have been made by the use of tissue-specific and inducible control of expression, which is achieved by the Cre/LoxP technology or the tetracycline system. from these sophisticated models, a common picture emerges: The effects of K-ras on tumor initiation depend strongly on the cellular context, and different tissues vary in their susceptibility to K-ras transformation.

Epidemiology of the diabetic foot in Africa

Diabetes mellitus is the most common, non-communicable chronic disease globally. In African countries, peripheral neuropathy underlies most diabetic foot complications; however, peripheral vascular disease appears to be increasing, presumably a reflection of increasing urbanisation. We conducted this study to ascertain the occurrence of foot complications among diabetes patients in the African continent. Using various keyword combinations, we searched Medline (PubMed) databases and the archives of Muhimbili National Hospital, the major teaching hospital in Tanzania, for obscure diabetes literature on diabetes in Africa. We also reviewed available non-English literature and obtained relevant translations where appropriate. We found articles encompassing years 1960-2003. Foot complications such as ulceration, infection, or gangrene were generally associated with considerable long-term disability and pre-mature mortality. Rates of complications varied by country-foot ulcers: 4-19%; peripheral neuropathy: 4-84.4%; peripheral vascular disease: 2.9-78.7%; frequency of patients presenting with gangrenous foot ulcers: 0.6-69%; foot amputation rates: 0.3-45%. A study of diabetic patients in Tanzania showed mortality rates >50% among patients with severe foot ulcers, who did not undergo surgery. Other published data from Tanzania suggest that surgical intervention after the onset of gangrene may be too late to prevent death. Prevention and control programmes are needed to stem the rising occurrence of diabetic foot complications in Africa. Gangrenous diabetic feet require aggressive management and early surgical intervention. Early presentation by patients and prompt surgical intervention during less severe rather than during later stages of an ulcer may improve patients outcome and reduce mortality rates.

Notch signals control the fate of immature progenitor cells in the intestine

The Notch signalling pathway plays a crucial role in specifying cellular fates in metazoan development by regulating communication between adjacent cells. Correlative studies suggested an involvement of Notch in intestinal development. Here, by modulating Notch activity in the mouse intestine, we directly implicate Notch signals in intestinal cell lineage specification. We also show that Notch activation is capable of amplifying the intestinal progenitor pool while inhibiting cell differentiation. We conclude that Notch activity is required for the maintenance of proliferating crypt cells in the intestinal epithelium.

Rab8 regulates the actin-based movement of melanosomes

Rab GTPases have been implicated in the regulation of specific microtubule- and actin-based motor proteins. We devised an in vitro motility assay reconstituting the movement of melanosomes on actin bundles in the presence of ATP to investigate the role of Rab proteins in the actin-dependent movement of melanosomes. Using this assay, we confirmed that Rab27 is required for the actin-dependent movement of melanosomes, and we showed that a second Rab protein, Rab8, also regulates this movement. Rab8 was partially associated with mature melanosomes. Expression of Rab8Q67L perturbed the cellular distribution and increased the frequency of microtubule-independent movement of melanosomes in vivo. Furthermore, anti-Rab8 antibodies decreased the number of melanosomes moving in vitro on actin bundles, whereas melanosomes isolated from cells expressing Rab8Q67L exhibited 70% more movements than wild-type melanosomes. Together, our observations suggest that Rab8 is involved in regulating the actin-dependent movement of melanosomes.

The membrane cytoskeletal crosslinker ezrin is required for metastasis of breast carcinoma cells

Introduction

The membrane cytoskeletal crosslinker ezrin participates in several functions including cell adhesion, motility and cell survival, and there is increasing evidence that it regulates tumour progression. However, the role played by ezrin in breast cancer metastasis has not been clearly delineated.

Methods

We examined the role of ezrin in metastasis using a highly metastatic murine mammary carcinoma cell line, namely AC2M2. Stable cell clones that overexpress wild-type ezrin or a dominant-negative amino-terminal domain of ezrin were selected. They were then tested for cell motility and invasion in vitro, and metastasis in a mouse in vivo tumour transplantation model.

Results

Parental AC2M2 cells and cells overexpressing wild-type ezrin were transplanted into the mammary fat pad of syngeneic recipient mice; these animals subsequently developed lung metastases. In contrast, expression of the dominant-negative amino-terminal ezrin domain markedly inhibited lung metastasis. Consistent with this effect, we observed that the expression of amino-terminal ezrin caused strong membrane localization of cadherin, with increased cell–cell contact and a decrease in cell motility and invasion, whereas cells expressing wild-type ezrin exhibited strong cytoplasmic expression of cadherins and pseudopodia extensions. In addition, inhibitors of phosphatidylinositol 3-kinase and c-Src significantly blocked cell motility and invasion of AC2M2 cells expressing wild-type ezrin. We further found that overexpression of amino-terminal ezrin reduced levels of Akt pS473 and cytoskeletal-associated c-Src pY418 in AC2M2 cells, which contrasts with the high levels of phosphorylation of these proteins in cells expressing wild-type ezrin. Phosphorylated Erk1/2 was also reduced in amino-terminal ezrin expressing cells, although a mitogen-activated protein kinase kinase (MEK) inhibitor had no detectable effect on cell motility or invasion in this system.

Conclusion

Our findings indicate that ezrin is required for breast cancer metastasis, and that c-Src and phosphatidylinositol 3-kinase/Akt are effectors of ezrin in the cell motility and invasion stages of the metastatic process. Together, these results suggest that blocking ezrin function may represent a novel and effective strategy for preventing breast cancer metastasis.