A PET-Surrogate Signature for the Interrogation of the Metabolic Status of Breast Cancers

Metabolic alterations in cancers can be exploited for diagnostic, prognostic, and therapeutic purposes. This is exemplified by 18F-fluorodeoxyglucose (FDG)-positron emission tomography (FDG-PET), an imaging tool that relies on enhanced glucose uptake by tumors for diagnosis and staging. By performing transcriptomic analysis of breast cancer (BC) samples from patients stratified by FDG-PET, a 54-gene signature (PETsign) is identified that recapitulates FDG uptake. PETsign is independently prognostic of clinical outcome in luminal BCs, the most common and heterogeneous BC molecular subtype, which requires improved stratification criteria to guide therapeutic decision-making. The prognostic power of PETsign is stable across independent BC cohorts and disease stages including the earliest BC stage, arguing that PETsign is an ab initio metabolic signature. Transcriptomic and metabolomic analysis of BC cells reveals that PETsign predicts enhanced glycolytic dependence and reduced reliance on fatty acid oxidation. Moreover, coamplification of PETsign genes occurs frequently in BC arguing for their causal role in pathogenesis. CXCL8 and EGFR signaling pathways feature strongly in PETsign, and their activation in BC cells causes a shift toward a glycolytic phenotype. Thus, PETsign serves as a molecular surrogate for FDG-PET that could inform clinical management strategies for BC patients.

Breast cancers as ecosystems: a metabolic perspective

Breast cancer (BC) is the most frequently diagnosed cancer and one of the major causes of cancer death. Despite enormous progress in its management, both from the therapeutic and early diagnosis viewpoints, still around 700,000 patients succumb to the disease each year, worldwide. Late recurrency is the major problem in BC, with many patients developing distant metastases several years after the successful eradication of the primary tumor. This is linked to the phenomenon of metastatic dormancy, a still mysterious trait of the natural history of BC, and of several other types of cancer, by which metastatic cells remain dormant for long periods of time before becoming reactivated to initiate the clinical metastatic disease. In recent years, it has become clear that cancers are best understood if studied as ecosystems in which the impact of non-cancer-cell-autonomous events-dependent on complex interaction between the cancer and its environment, both local and systemic-plays a paramount role, probably as significant as the cell-autonomous alterations occurring in the cancer cell. In adopting this perspective, a metabolic vision of the cancer ecosystem is bound to improve our understanding of the natural history of cancer, across space and time. In BC, many metabolic pathways are coopted into the cancer ecosystem, to serve the anabolic and energy demands of the cancer. Their study is shedding new light on the most critical aspect of BC management, of metastatic dissemination, and that of the related phenomenon of dormancy and fostering the application of the knowledge to the development of metabolic therapies.

Endocytosis in the context-dependent regulation of individual and collective cell properties

Endocytosis allows cells to transport particles and molecules across the plasma membrane. In addition, it is involved in the termination of signalling through receptor downmodulation and degradation. This traditional outlook has been substantially modified in recent years by discoveries that endocytosis and subsequent trafficking routes have a profound impact on the positive regulation and propagation of signals, being key for the spatiotemporal regulation of signal transmission in cells. Accordingly, endocytosis and membrane trafficking regulate virtually every aspect of cell physiology and are frequently subverted in pathological conditions. Two key aspects of endocytic control over signalling are coming into focus: context-dependency and long-range effects. First, endocytic-regulated outputs are not stereotyped but heavily dependent on the cell-specific regulation of endocytic networks. Second, endocytic regulation has an impact not only on individual cells but also on the behaviour of cellular collectives. Herein, we will discuss recent advancements in these areas, highlighting how endocytic trafficking impacts complex cell properties, including cell polarity and collective cell migration, and the relevance of these mechanisms to disease, in particular cancer.

AMBRA1 regulates cyclin D to guard S-phase entry and genomic integrity

Mammalian development, adult tissue homeostasis and the avoidance of severe diseases including cancer require a properly orchestrated cell cycle, as well as error-free genome maintenance. The key cell-fate decision to replicate the genome is controlled by two major signalling pathways that act in parallel-the MYC pathway and the cyclin D-cyclin-dependent kinase (CDK)-retinoblastoma protein (RB) pathway^1,2. Both MYC and the cyclin D-CDK-RB axis are commonly deregulated in cancer, and this is associated with increased genomic instability. The autophagic tumour-suppressor protein AMBRA1 has been linked to the control of cell proliferation, but the underlying molecular mechanisms remain poorly understood. Here we show that AMBRA1 is an upstream master regulator of the transition from G1 to S phase and thereby prevents replication stress. Using a combination of cell and molecular approaches and in vivo models, we reveal that AMBRA1 regulates the abundance of D-type cyclins by mediating their degradation. Furthermore, by controlling the transition from G1 to S phase, AMBRA1 helps to maintain genomic integrity during DNA replication, which counteracts developmental abnormalities and tumour growth. Finally, we identify the CHK1 kinase as a potential therapeutic target in AMBRA1-deficient tumours. These results advance our understanding of the control of replication-phase entry and genomic integrity, and identify the AMBRA1-cyclin D pathway as a crucial cell-cycle-regulatory mechanism that is deeply interconnected with genomic stability in embryonic development and tumorigenesis.

Selective autophagy maintains centrosome integrity and accurate mitosis by turnover of centriolar satellites

The centrosome is the master orchestrator of mitotic spindle formation and chromosome segregation in animal cells. Centrosome abnormalities are frequently observed in cancer, but little is known of their origin and about pathways affecting centrosome homeostasis. Here we show that autophagy preserves centrosome organization and stability through selective turnover of centriolar satellite components, a process we termed doryphagy. Autophagy targets the satellite organizer PCM1 by interacting with GABARAPs via a C-terminal LIR motif. Accordingly, autophagy deficiency results in accumulation of large abnormal centriolar satellites and a resultant dysregulation of centrosome composition. These alterations have critical impact on centrosome stability and lead to mitotic centrosome fragmentation and unbalanced chromosome segregation. Our findings identify doryphagy as an important centrosome-regulating pathway and bring mechanistic insights to the link between autophagy dysfunction and chromosomal instability. In addition, we highlight the vital role of centriolar satellites in maintaining centrosome integrity.

KRAS-Driven Metabolic Rewiring Reveals Novel Actionable Targets in Cancer

Tumors driven by mutant KRAS are among the most aggressive and refractory to treatment. Unfortunately, despite the efforts, targeting alterations of this GTPase, either directly or by acting on the downstream signaling cascades, has been, so far, largely unsuccessful. However, recently, novel therapeutic opportunities are emerging based on the effect that this oncogenic lesion exerts in rewiring the cancer cell metabolism. Cancer cells that become dependent on KRAS-driven metabolic adaptations are sensitive to the inhibition of these metabolic routes, revealing novel therapeutic windows of intervention. In general, mutant KRAS fosters tumor growth by shifting cancer cell metabolism toward anabolic pathways. Depending on the tumor, KRAS-driven metabolic rewiring occurs by up-regulating rate-limiting enzymes involved in amino acid, fatty acid, or nucleotide biosynthesis, and by stimulating scavenging pathways such as macropinocytosis and autophagy, which, in turn, provide building blocks to the anabolic routes, also maintaining the energy levels and the cell redox potential (1). This review will discuss the most recent findings on mutant KRAS metabolic reliance in tumor models of pancreatic and non-small-cell lung cancer, also highlighting the role that these metabolic adaptations play in resistance to target therapy. The effects of constitutive KRAS activation in glycolysis elevation, amino acids metabolism reprogramming, fatty acid turnover, and nucleotide biosynthesis will be discussed also in the context of different genetic landscapes.

Abstract 261: L1-MET transcription silencing modulates MET and EGFR gene and their protein expression and induces apoptosis and cell-death in different types of cancer cells

The activation of the LINE-1 sequence located within the second intron of MET leads to the onset of L1-MET transcript. We recently characterized the full L1-MET structure in breast cancer and showed that high levels of this transcript recognize a subset of more aggressive breast carcinomas, mainly of triple negative phenotype. However, at present, the relationship between L1-MET and MET is still poorly understood. In order to elucidate this function, we silenced L1-MET transcription using cells expressing different levels of L1-MET/MET, including lung cancer (A549 and EBC1), gastric cancer (GTL16), and breast cancer (MDA-MB231), that were transiently transfected with Gapmers-LNA (Exiqon), specifically targeting L1-MET sequence. Cell viability and apoptosis were evaluated after 24 h by cell count, Cell Titer Glow (Promega) and propidium iodide/annexin based-cytofluorimeter assays. RNA was purified from sample and control cells to assess the L1-METsilencing by qRT-PCR and to evaluate the gene-expression of a subset of cancer-related genes using Nanostring Technology, whereas western blot analyses were carried out to measure the protein expressions. A significant decrease of cell viability was detected in A549, EBC1 and GTL16, but not in MDA-MB231 cells, characterized by the lowest level of L1-MET overall. In parallel, the highly expressing L1-MET cells showed an increased rate of early and late apoptosis together with a strong reduction of MET gene and its protein expression. On the contrary, in MDA-MB231 cells L1-MET silencing induced only a slight MET gene and protein impairment. Overall, L1-MET knock-down caused a decrease expression of a conserved gene cluster, including a marked reduction of EGFR protein expression. Moreover, L1-MET silenced cells showed lower MET and EGFR phosphorylation, with a downstream silencing effect on pERK and pAKT. Results of cell treatment with the inhibitors of lysosome and proteasome activity bafilomycin and MG-132 ruled out the interaction of L1-MET silencing with protein degradation pathway. This is the first study investigating the function of the L1-MET transcript in cancer models. Our results show that although L1-MET is unable to encode for a protein, its silencing exerts a strong phenotypic effect on different tumor cell types, suggesting potential regulations at the transcriptional level.

Note: This abstract was not presented at the meeting.

Citation Format: Enrico Berrino, Umberto Miglio, Valentina Miano, Letizia Lanzetti, Silvia Benvenuti, Carla Debernardi, MIchele De Bortoli, Caterina Marchio, Tiziana Venesio, Anna Sapino. L1-MET transcription silencing modulates MET and EGFR gene and their protein expression and induces apoptosis and cell-death in different types of cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 261.

High USP6NL Levels in Breast Cancer Sustain Chronic AKT Phosphorylation and GLUT1 Stability Fueling Aerobic Glycolysis

USP6NL, also named RN-tre, is a GTPase-activating protein involved in control of endocytosis and signal transduction. Here we report that USP6NL is overexpressed in breast cancer, mainly of the basal-like/integrative cluster 10 subtype. Increased USP6NL levels were accompanied by gene amplification and were associated with worse prognosis in the METABRIC dataset, retaining prognostic value in multivariable analysis. High levels of USP6NL in breast cancer cells delayed endocytosis and degradation of the EGFR, causing chronic AKT (protein kinase B) activation. In turn, AKT stabilized the glucose transporter GLUT1 at the plasma membrane, increasing aerobic glycolysis. In agreement, elevated USP6NL sensitized breast cancer cells to glucose deprivation, indicating that their glycolytic capacity relies on this protein. Depletion of USP6NL accelerated EGFR/AKT downregulation and GLUT1 degradation, impairing cell proliferation exclusively in breast cancer cells that harbored increased levels of USP6NL. Overall, these findings argue that USP6NL overexpression generates a metabolic rewiring that is essential to foster the glycolytic demand of breast cancer cells and promote their proliferation.Significance: USP6NL overexpression leads to glycolysis addiction of breast cancer cells and presents a point of metabolic vulnerability for therapeutic targeting in a subset of aggressive basal-like breast tumors.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/13/3432/F1.large.jpg Cancer Res; 78(13); 3432-44. ©2018 AACR.

Emerging functions of the EGFR in cancer

The physiological function of the epidermal growth factor receptor (EGFR) is to regulate epithelial tissue development and homeostasis. In pathological settings, mostly in lung and breast cancer and in glioblastoma, the EGFR is a driver of tumorigenesis. Inappropriate activation of the EGFR in cancer mainly results from amplification and point mutations at the genomic locus, but transcriptional upregulation or ligand overproduction due to autocrine/paracrine mechanisms has also been described. Moreover, the EGFR is increasingly recognized as a biomarker of resistance in tumors, as its amplification or secondary mutations have been found to arise under drug pressure. This evidence, in addition to the prominent function that this receptor plays in normal epithelia, has prompted intense investigations into the role of the EGFR both at physiological and at pathological level. Despite the large body of knowledge obtained over the last two decades, previously unrecognized (herein defined as 'noncanonical') functions of the EGFR are currently emerging. Here, we will initially review the canonical ligand-induced EGFR signaling pathway, with particular emphasis to its regulation by endocytosis and subversion in human tumors. We will then focus on the most recent advances in uncovering noncanonical EGFR functions in stress-induced trafficking, autophagy, and energy metabolism, with a perspective on future therapeutic applications.

Behind the Scenes: Endo/Exocytosis in the Acquisition of Metastatic Traits

Alterations of endo/exocytic proteins have long been associated with malignant transformation, and genes encoding membrane trafficking proteins have been identified as bona fide drivers of tumorigenesis. Focusing on the mechanisms underlying the impact of endo/exocytic proteins in cancer, a scenario emerges in which altered trafficking routes/networks appear to be preferentially involved in the acquisition of prometastatic traits. This involvement in metastasis frequently occurs through the integration of programs leading to migratory/invasive phenotypes, survival and resistance to environmental stresses, epithelial-to-mesenchymal transition, and the emergence of cancer stem cells. These findings might have important implications in the clinical setting for the development of metastasis-specific drugs and for patient stratification to optimize the use of available therapies. Cancer Res; 77(8); 1813-7. ©2017 AACR.

Deep Sequencing Reveals a Novel miR-22 Regulatory Network with Therapeutic Potential in Rhabdomyosarcoma

Current therapeutic options for the pediatric cancer rhabdomyosarcoma have not improved significantly, especially for metastatic rhabdomyosarcoma. In the current work, we performed a deep miRNA profiling of the three major human rhabdomyosarcoma subtypes, along with cell lines and normal muscle, to identify novel molecular circuits with therapeutic potential. The signature we determined could discriminate rhabdomyosarcoma from muscle, revealing a subset of muscle-enriched miRNA (myomiR), including miR-22, which was strongly underexpressed in tumors. miR-22 was physiologically induced during normal myogenic differentiation and was transcriptionally regulated by MyoD, confirming its identity as a myomiR. Once introduced into rhabdomyosarcoma cells, miR-22 decreased cell proliferation, anchorage-independent growth, invasiveness, and promoted apoptosis. Moreover, restoring miR-22 expression blocked tumor growth and prevented tumor dissemination in vivo Gene expression profiling analysis of miR-22-expressing cells suggested TACC1 and RAB5B as possible direct miR-22 targets. Accordingly, loss- and gain-of-function experiments defined the biological relevance of these genes in rhabdomyosarcoma pathogenesis. Finally, we demonstrated the ability of miR-22 to intercept and overcome the intrinsic resistance to MEK inhibition based on ERBB3 upregulation. Overall, our results identified a novel miR-22 regulatory network with critical therapeutic implications in rhabdomyosarcoma. Cancer Res; 76(20); 6095-106. ©2016 AACR.

Tankyrase inhibition impairs directional migration and invasion of lung cancer cells by affecting microtubule dynamics and polarity signals

BACKGROUND

Tankyrases are poly(adenosine diphosphate)-ribose polymerases that contribute to biological processes as diverse as modulation of Wnt signaling, telomere maintenance, vesicle trafficking, and microtubule-dependent spindle pole assembly during mitosis. At interphase, polarized reshaping of the microtubule network fosters oriented cell migration. This is attained by association of adenomatous polyposis coli with the plus end of microtubules at the cortex of cell membrane protrusions and microtubule-based centrosome reorientation towards the migrating front.

RESULTS

Here we report a new function for tankyrases, namely, regulation of directional cell locomotion. Using a panel of lung cancer cell lines as a model system, we found that abrogation of tankyrase activity by two different, structurally unrelated small-molecule inhibitors (one introduced and characterized here for the first time) or by RNA interference-based genetic silencing weakened cell migration, invasion, and directional movement induced by the motogenic cytokine hepatocyte growth factor. Mechanistically, the anti-invasive outcome of tankyrase inhibition could be ascribed to sequential deterioration of the distinct events that govern cell directional sensing. In particular, tankyrase blockade negatively impacted (1) microtubule dynamic instability; (2) adenomatous polyposis coli plasma membrane targeting; and (3) centrosome reorientation.

CONCLUSIONS

Collectively, these findings uncover an unanticipated role for tankyrases in influencing at multiple levels the interphase dynamics of the microtubule network and the subcellular distribution of related polarity signals. These results encourage the further exploration of tankyrase inhibitors as therapeutic tools to oppose dissemination and metastasis of cancer cells.

Regulation of the microtubular cytoskeleton by Polycystin-1 favors focal adhesions turnover to modulate cell adhesion and migration

Background

Polycystin-1 (PC-1) is a large plasma membrane receptor, encoded by the PKD1 gene, which is mutated in most cases of Autosomal Dominant Polycystic Kidney Disease (ADPKD). The disease is characterized by renal cysts. The precise function of PC-1 remains elusive, although several studies suggest that it can regulate the cellular shape in response to external stimuli. We and others reported that PC-1 regulates the actin cytoskeleton and cell migration.

Results

Here we show that cells over-expressing PC-1 display enhanced adhesion rates to the substrate, while cells lacking PC-1 have a reduced capability to adhere. In search for the mechanism responsible for this new property of PC-1 we found that this receptor is able to regulate the stability of the microtubules, in addition to its capability to regulate the actin cytoskeleton. The two cytoskeletal components are acting in a coordinated fashion. Notably, we uncovered that PC-1 regulation of the microtubule cytoskeleton impacts on the turnover rates of focal adhesions in migrating cells and we link all these properties to the capability of PC-1 to regulate the activation state of Focal Adhesion Kinase (FAK).

Conclusions

In this study we show several new features of the PC-1 receptor in modulating microtubules and adhesion dynamics, which are essential for its capability to regulate migration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-015-0059-3) contains supplementary material, which is available to authorized users.

Abstract A48: Tankyrase inhibitors impair directional cell migration of cancer cells by affecting microtubule dynamics and polarity signals

Tumor colonization of distant sites underlies the vast majority of cancer-related deaths worldwide, despite current therapeutic efforts. Unfortunately, the emergence of metastasis-competent clones characterizes the rapid outgrowth of highly aggressive tumors, such as lung cancer [1]. Therefore, dissecting the molecular mechanisms that drive metastatic dissemination is the obligatory groundwork to discover novel therapeutic targets.

In an endeavor to identify potential anti-invasive compounds, we performed a large-scale drug screen coupled to cell-based strategies, including wound healing and Transwell assays, in a panel of non-small cell lung cancer (NSCLC) cell lines. This approach led to the identification of two small molecules, namely XAV939 and JNJ928, both displaying remarkable inhibitory effects on cell invasiveness, in the absence of overt cytotoxicity. Both compounds are known to inhibit tankyrases 1&2, two poly(ADP)-ribosylases that are gaining interest due to their involvement in cancer-related processes, ranging from wnt signaling to telomere maintenance and microtubule-based mitotic dynamics [2]. A scrutiny of polarity signaling pathways known to be involved in cell locomotion allowed us to allocate tankyrases within an accredited route governing directional cell migration that entails cdc42-dependent phosphorylation of GSK3β, the ensuing recruitment of APC to the plasma membrane, and the establishment of microtubule-dependent pulling forces that reorient the microtubule organizing center (MTOC) towards the leading edge [3]. Notably, all these spatially orchestrated events were compromised by tankyrase blockade, likely explaining the reduced chemotactic response of XAV939- and JNJ928-treated cells. Concurrently, tankyrase inhibition seemed to impinge on microtubule dynamics by stabilizing the microtubule network. Notably, dynamic instability of microtubules is needed for their “search and capture” function to generate an asymmetric microtubule array and facilitate cell movement [4]. Conceivably, suppression of dynamic instability by tankyrase blockade might prevent microtubules from probing outward, leading to reduced receptiveness of chemotactic inputs. Efforts are currently being placed to uncover the mechanistic details of such interconnections, which seem to adhere to the established notion that tankyrases bind to and regulate the activity of several microtubule-related proteins.

Our results might add to the evolving landscape of tankyrases as drivers of many aspect of the transformed phenotype, holding promise for prospective – albeit still immature – employment of tankyrase inhibitors as anti-metastatic therapeutics in NSCLC.

[1] D.X. Nguyen, P.D. Bos, J. Massagué, Metastasis: from dissemination to organ-specific colonization, Nat Rev Cancer 9 (2009) 274-284.

[2] J.L. Riffell, C.J. Lord, A. Ashworth, Tankyrase-targeted therapeutics: expanding opportunities in the PARP family, Nat Rev Drug Discov 11 (2012) 923-936.

[3] S. Etienne-Manneville, Polarity proteins in migration and invasion, Oncogene 27 (2008) 6970-6980.

[4] S. Etienne-Manneville, Microtubules in cell migration, Annu Rev Cell Dev Biol 29 (2013) 471-499.

Citation Format: Barbara Lupo, Jorge Vialard, Andrea Bertotti, Letizia Lanzetti, Livio Trusolino. Tankyrase inhibitors impair directional cell migration of cancer cells by affecting microtubule dynamics and polarity signals. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr A48.

TNF-α promotes invasive growth through the MET signaling pathway

The inflammatory cytokine Tumor Necrosis Factor Alpha (TNF-α) is known to trigger invasive growth, a physiological property for tissue healing, turning into a hallmark of progression in cancer. However, the invasive response to TNF-α relies on poorly understood molecular mechanisms. We thus investigated whether it involves the MET oncogene, which regulates the invasive growth program by encoding the tyrosine kinase receptor for Hepatocyte Growth Factor (HGF). Here we show that the TNF-α pro-invasive activity requires MET function, as it is fully inhibited by MET-specific inhibitors (small-molecules, antibodies, and siRNAs). Mechanistically, we show that TNF-α induces MET transcription via NF-κB, and exploits MET to sustain MEK/ERK activation and Snail accumulation, leading to E-cadherin downregulation. We then show that TNF-α not only induces MET expression in cancer cells, but also HGF secretion by fibroblasts. Consistently, we found that, in human colorectal cancer tissues, high levels of TNF-α correlates with increased expression of both MET and HGF. These findings suggest that TNF-α fosters a HGF/MET pro-invasive paracrine loop in tumors. Targeting this ligand/receptor pair would contribute to prevent cancer progression associated with inflammation.

The GTPase-activating protein RN-tre controls focal adhesion turnover and cell migration

BACKGROUND

Integrin-mediated adhesion of cells to the extracellular matrix (ECM) relies on the dynamic formation of focal adhesions (FAs), which are biochemical and mechanosensitive platforms composed of a large variety of cytosolic and transmembrane proteins. During migration, there is a constant turnover of ECM contacts that initially form as nascent adhesions at the leading edge, mature into FAs as actomyosin tension builds up, and are then disassembled at the cell rear, thus allowing for cell detachment. Although the mechanisms of FA assembly have largely been defined, the molecular circuitry that regulates their disassembly still remains elusive.

RESULTS

Here, we show that RN-tre, a GTPase-activating protein (GAP) for Rabs including Rab5 and Rab43, is a novel regulator of FA dynamics and cell migration. RN-tre localizes to FAs and to a pool of Rab5-positive vesicles mainly associated with FAs undergoing rapid remodeling. We found that RN-tre inhibits endocytosis of β1, but not β3, integrins and delays the turnover of FAs, ultimately impairing β1-dependent, but not β3-dependent, chemotactic cell migration. All of these effects are mediated by its GAP activity and rely on Rab5.

CONCLUSIONS

Our findings identify RN-tre as the Rab5-GAP that spatiotemporally controls FA remodeling during chemotactic cell migration.

Neuropilin-1-dependent regulation of EGF-receptor signaling

Neuropilin-1 (NRP1) is a coreceptor for multiple extracellular ligands. NRP1 is widely expressed in cancer cells and in advanced human tumors; however, its functional relevance and signaling mechanisms are unclear. Here, we show that NRP1 expression controls viability and proliferation of different cancer cells, independent of its short intracellular tail. We found that the extracellular domain of NRP1 interacts with the EGF receptor (EGFR) and promotes its signaling cascade elicited upon EGF or TGF-α stimulation. Upon NRP1 silencing, the ability of ligand-bound EGFR to cluster on the cell surface, internalize, and activate the downstream AKT pathway is severely impaired. EGFR is frequently activated in human tumors due to overexpression, mutation, or sustained autocrine/paracrine stimulation. Here we show that NRP1-blocking antibodies and NRP1 silencing can counteract ligand-induced EGFR activation in cancer cells. Thus our findings unveil a novel molecular mechanism by which NRP1 can control EGFR signaling and tumor growth.

A novel function of Rab5 in mitosis

Several membrane trafficking proteins have been shown to participate in spindle assembly and stability during mitosis. Despite the fact that the role of some of them has been clarified, the requirement for these molecules in mitosis is still poorly understood.

Role of Varp, a Rab21 exchange factor and TI-VAMP/VAMP7 partner, in neurite growth

The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP/VAMP7) was previously shown to mediate an exocytic pathway involved in neurite growth, but its regulation is still largely unknown. Here we show that TI-VAMP interacts with the Vps9 domain and ankyrin-repeat-containing protein (Varp), a guanine nucleotide exchange factor (GEF) of the small GTPase Rab21, through a specific domain herein called the interacting domain (ID). Varp, TI-VAMP and Rab21 co-localize in the perinuclear region of differentiating hippocampal neurons and transiently in transport vesicles in the shaft of neurites. Silencing the expression of Varp by RNA interference or expressing ID or a form of Varp deprived of its Vps9 domain impairs neurite growth. Furthermore, the mutant form of Rab21, defective in GTP hydrolysis, enhances neurite growth. We conclude that Varp is a positive regulator of neurite growth through both its GEF activity and its interaction with TI-VAMP.

Phosphoinositide 3-kinase p110beta activity: key role in metabolism and mammary gland cancer but not development

The phosphoinositide 3-kinase (PI3K) pathway crucially controls metabolism and cell growth. Although different PI3K catalytic subunits are known to play distinct roles, the specific in vivo function of p110beta (the product of the PIK3CB gene) is not clear. Here, we show that mouse mutants expressing a catalytically inactive PIK3CB(K805R) mutant survived to adulthood but showed growth retardation and developed mild insulin resistance with age. Pharmacological and genetic analyses of p110beta function revealed that p110beta catalytic activity is required for PI3K signaling downstream of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors as well as to sustain long-term insulin signaling. In addition, PIK3CB(K805R) mice were protected in a model of ERBB2-driven tumor development. These findings indicate an unexpected role for p110beta catalytic activity in diabetes and cancer, opening potential avenues for therapeutic intervention.

Endocytosis and cancer: an 'insider' network with dangerous liaisons

From the signaling point of view, endocytosis has long been regarded as a major mechanism of attenuation, through the degradation of signaling receptors and, in some cases, of their ligands. This outlook has changed, over the past decade, as it has become clear that signaling persists in the endocytic route, and that intracellular endocytic stations (the 'signaling endosomes') actually contribute to the sorting of signals in space and time. Endocytosis-mediated recycling of receptors and of signaling molecules to specific regions of the plasma membrane is also coming into focus as a major mechanism in the execution of spatially restricted functions, such as cell motility. In addition, emerging evidence connects endocytosis as a whole, or individual endocytic proteins, to complex cellular programs, such as the control of the cell cycle, mitosis, apoptosis and cell fate determination. Thus, endocytosis seems to be deeply ingrained into the cell regulation blueprint and its subversion is predicted to play an important role in human diseases: first and foremost, cancer.

The primate-specific protein TBC1D3 is required for optimal macropinocytosis in a novel ARF6-dependent pathway

The generation of novel genes and proteins throughout evolution has been proposed to occur as a result of whole genome and gene duplications, exon shuffling, and retrotransposition events. The analysis of such genes might thus shed light into the functional complexity associated with highly evolved species. One such case is represented by TBC1D3, a primate-specific gene, harboring a TBC domain. Because TBC domains encode Rab-specific GAP activities, TBC-containing proteins are predicted to play a major role in endocytosis and intracellular traffic. Here, we show that the TBC1D3 gene originated late in evolution, likely through a duplication of the RNTRE locus, and underwent gene amplification during primate speciation. Despite possessing a TBC domain, TBC1D3 is apparently devoid of Rab-GAP activity. However, TBC1D3 regulates the optimal rate of epidermal growth factor-mediated macropinocytosis by participating in a novel pathway involving ARF6 and RAB5. In addition, TBC1D3 binds and colocalize to GGA3, an ARF6-effector, in an ARF6-dependent manner, and synergize with it in promoting macropinocytosis, suggesting that the two proteins act together in this process. Accordingly, GGA3 siRNA-mediated ablation impaired TBC1D3-induced macropinocytosis. We thus uncover a novel signaling pathway that appeared after primate speciation. Within this pathway, a TBC1D3:GGA3 complex contributes to optimal propagation of signals, ultimately facilitating the macropinocytic process.

Src triggers circular ruffling and macropinocytosis at the apical surface of polarized MDCK cells

We addressed the role of Src on cortical actin dynamics and polarized endocytosis in MDCK cells harboring a thermosensitive v-src mutant. Shifting monolayers established at 40 degrees C (non-permissive temperature) to 34 degrees C (permissive temperature) rapidly reactivated v-Src kinase, but tight junctions and cell polarity resisted for >6 h. At this interval, activated v-src was recruited on apical vesicles, induced cortactin-associated apical circular ruffles productive of macropinosomes, thereby accelerating apical pinocytosis by approximately fivefold. Ruffling and macropinosome formation were selectively abrogated by inhibitors of actin polymerization, phosphoinositide 3-kinase, phospholipase C, and phospholipase D, which all returned apical pinocytosis to the level observed at 40 degrees C, underscoring the distinct control of apical micropinocytosis and macropinocytosis. Src promoted microtubule-dependent fusion of macropinosomes to the apical recycling endosome (ARE), causing its strong vacuolation. However, preservation of tubulation and apical polarity indicated that its function was not affected. The ARE was labeled for v-src, Rab11, and rabankyrin-5 but not early endosome antigen 1, thus distinguishing two separate Rab5-dependent apical pathways. The mechanisms of Src-induced apical ruffling and macropinocytosis could shed light on the triggered apical enteroinvasive pathogens entry and on the apical differentiation of osteoclasts.

Actin in membrane trafficking

Actin cytoskeleton remodeling provides the forces required for a variety of cellular processes based on membrane dynamics, such as endocytosis, exocytosis, and vesicular trafficking at the Golgi. All these events are coordinated by networks of associated proteins, and some of them are functionally connected with cell migration. The site and the duration of actin polymerization, in connection with vesicle budding and fusion, are tightly controlled by both small GTPases and the large GTPase dynamin. Recent advances in the understanding of the mechanisms coupling actin dynamics with membrane trafficking at the cell surface have been brought by the combined studies of actin polymerizing factors and of the endocytic/exocytic machinery.

Regulation of the Rab5 GTPase-activating protein RN-tre by the dual specificity phosphatase Cdc14A in human cells

The Cdc14 family of dual specificity phosphatases regulates key mitotic events in the eukaryotic cell cycle. Although extensively characterized in yeast, little is known about the function of mammalian Cdc14 family members. Here we report a genetic substrate-trapping system designed to identify substrates of the human Cdc14A (hCdc14A) phosphatase. Using this approach, we identify RN-tre, a GTPase-activating protein for the Rab5 GTPase, as a novel physiological target of hCdc14A. As a Rab5 GTPase-activating protein, RN-tre has previously been implicated in control of intracellular membrane trafficking. We find that RN-tre forms a stable complex with the catalytically inactive hCdc14A C278S mutant but not with the wild type protein in human cells, indicative of a substrate/enzyme interaction. In support, we show that RN-tre is regulated by cell cycle-dependent phosphorylation peaking at mitosis, which can be antagonized by hCdc14A activity in vitro as well as in vivo. Furthermore, we show that RN-tre phosphorylation is critical for efficient hCdc14A association and that RN-tre binding can be displaced by tungstate, a competitive inhibitor that binds to the active site of hCdc14A. Consistent with the preference of hCdc14A for phosphorylations mediated by proline-directed kinases, we find that RN-tre is a direct substrate of cyclin-dependent kinase. Finally, phosphorylation of RN-tre appears to finely modulate its catalytic activity. Our findings reveal a novel connection between the cell cycle machinery and the endocytic pathway.

Rab5 is a signalling GTPase involved in actin remodelling by receptor tyrosine kinases

Rab5 is a small GTPase involved in the control of intracellular trafficking, both at the level of receptor endocytosis and endosomal dynamics. The finding that Rab5 can be activated by receptor tyrosine kinases (RTK) raised the question of whether it also participates in effector pathways emanating from these receptors. Here we show that Rab5 is indispensable for a form of RTK-induced actin remodelling, called circular ruffling. Three independent signals, originating from Rab5, phosphatidylinositol-3-OH kinase and Rac, respectively, are simultaneously required for the induction of circular ruffles. Rab5 signals to the actin cytoskeleton through RN-tre, a previously identified Rab5-specific GTPase-activating protein (GAP). Here we demonstrate that RN-tre has the dual function of Rab5-GAP and Rab5 effector. We also show that RN-tre is critical for macropinocytosis, a process previously connected to the formation of circular ruffles. Finally, RN-tre interacts with both F-actin and actinin-4, an F-actin bundling protein. We propose that RN-tre establishes a three-pronged connection with Rab5, F-actin and actinin-4. This may aid crosslinking of actin fibres into actin networks at the plasma membrane. Thus, we have shown that Rab5 is a signalling GTPase and have elucidated the major molecular elements of its downstream pathway.

Modulation of Rab5 and Rab7 recruitment to phagosomes by phosphatidylinositol 3-kinase

Phagosomal biogenesis is central for microbial killing and antigen presentation by leukocytes. However, the molecular mechanisms governing phagosome maturation are poorly understood. We analyzed the role and site of action of phosphatidylinositol 3-kinases (PI3K) and of Rab GTPases in maturation using both professional and engineered phagocytes. Rab5, which is recruited rapidly and transiently to the phagosome, was found to be essential for the recruitment of Rab7 and for progression to phagolysosomes. Similarly, functional PI3K is required for successful maturation. Remarkably, inhibition of PI3K did not preclude Rab5 recruitment to phagosomes but instead enhanced and prolonged it. Moreover, in the presence of PI3K inhibitors Rab5 was found to be active, as deduced from measurements of early endosome antigen 1 binding and by photobleaching recovery determinations. Though their ability to fuse with late endosomes and lysosomes was virtually eliminated by wortmannin, phagosomes nevertheless recruited a sizable amount of Rab7. Moreover, Rab7 recruited to phagosomes in the presence of PI3K antagonists retained the ability to bind its effector, Rab7-interacting lysosomal protein, suggesting that it is functionally active. These findings imply that (i) dissociation of Rab5 from phagosomes requires products of PI3K, (ii) PI3K-dependent effectors of Rab5 are not essential for the recruitment of Rab7 by phagosomes, and (iii) recruitment and activation of Rab7 are insufficient to induce fusion of phagosomes with late endosomes and lysosomes. Accordingly, transfection of constitutively active Rab7 did not bypass the block of phagolysosome formation exerted by wortmannin. We propose that Rab5 activates both PI3K-dependent and PI3K-independent effectors that act in parallel to promote phagosome maturation.

The Eps8 protein coordinates EGF receptor signalling through Rac and trafficking through Rab5

How epidermal growth factor receptor (EGFR) signalling is linked to EGFR trafficking is largely unknown. Signalling and trafficking involve small GTPases of the Rho and Rab families, respectively. But it remains unknown whether the signalling relying on these two classes of GTPases is integrated, and, if it is, what molecular machinery is involved. Here we report that the protein Eps8 connects these signalling pathways. Eps8 is a substrate of the EGFR, which is held in a complex with Sos1 by the adaptor protein E3bl (ref. 2), thereby mediating activation of Rac. Through its src homology-3 domain, Eps8 interacts with RN-tre. We show that RN-tre is a Rab5 GTPase-activating protein, whose activity is regulated by the EGFR. By entering in a complex with Eps8, RN-tre acts on Rab5 and inhibits internalization of the EGFR. Furthermore, RN-tre diverts Eps8 from its Rac-activating function, resulting in the attenuation of Rac signalling. Thus, depending on its state of association with E3b1 or RN-tre, Eps8 participates in both EGFR signalling through Rac, and trafficking through Rab5.

A novel SH3-containing human gene family preferentially expressed in the central nervous system

The Src-homology-3 domain (SH3) is an evolutionarily conserved, 50- to 60-amino-acid module carried by intracellular proteins involved in the transduction of signals for cell polarization, motility, enzymatic activation, and transcriptional regulation. The SH3 drives protein-protein interactions through binding to proline-rich ligands. This function relies on the conserved secondary structure, whereas the SH3 primary structure is highly diverse. Taking advantage of the fact that the few conserved amino acids are clustered near the N- and C-terminal ends, we designed degenerate oligonucleotides spanning these two regions and screened by PCR a variety of normal and tumor tissues for the expression of SH3-containing transcripts. Using this strategy, we have identified a novel SH3-containing human gene family of six related transcripts that map to four different chromosomes. The SH3 domain lies at the C-terminal end and shows 56-50% amino acid homology to the C-terminal SH3 of Sem-5/Drk/GRB2. The N-terminal segment of this novel SH3GL (from SH3-containing Grb2-like) gene family does not resemble any known protein. Three of these transcripts are in-frame and show a peculiar tissue distribution: SH3GL2 is preferentially expressed in the brain, SH3GL3 in brain and testis, and SH3GL1 is ubiquitous.

In the budding yeast Kluyveromyces marxianus, adenylate cyclase is regulated by Ras protein(s) in vitro

The presence of adenylate cyclase activity was first demonstrated in membrane fractions from the budding yeast Kluyveromyces marxianus. The enzyme showed a Mn(2+)- and Mg(2+)-dependent activity, with optimal pH at around 6 as observed in other yeast species. As in Saccharomyces cerevisiae, where adenylate cyclase is regulated by RAS1 and RAS2, we detected a guanyl nucleotide-dependent activity. Interestingly Y13-259 monoclonal antibody, raised against mammalian p21Ha-ras, inhibited Mg2+ plus GTP-gamma-S-dependent cAMP production, suggesting that the GTP binding proteins involved in adenylate cyclase regulation could be Ras proteins. The same antibody recognized on Western blot and immunoprecipitated a 40 kDa polypeptide from K. marxianus crude membranes. This polypeptide was not detected by an anti-RAS2 polyclonal antibody raised against S. cerevisiae RAS2 protein, suggesting that Ras proteins from the two species could be structurally different.