Viral manipulation of cell polarity signalling

Cell polarity refers to the asymmetric distribution of biomacromolecules that enable the correct orientation of a cell in a particular direction. It is thus an essential component for appropriate tissue development and function. Viral infections can lead to dysregulation of polarity. This is associated with a poor prognosis due to viral interference with core cell polarity regulatory scaffolding proteins that often feature PDZ (PSD-95, DLG, and ZO-1) domains including Scrib, Dlg, Pals1, PatJ, Par3 and Par6. PDZ domains are also promiscuous, binding to several different partners through their C-terminal region which contain PDZ-binding motifs (PBM). Numerous viruses encode viral effector proteins that target cell polarity regulators for their benefit and include papillomaviruses, flaviviruses and coronaviruses. A better understanding of the mechanisms of action utilised by viral effector proteins to subvert host cell polarity sigalling will provide avenues for future therapeutic intervention, while at the same time enhance our understanding of cell polarity regulation and its role tissue homeostasis.

Matriptase-dependent epidermal pre-neoplasm in zebrafish embryos caused by a combination of hypotonic stress and epithelial polarity defects

Aberrantly up-regulated activity of the type II transmembrane protease Matriptase-1 has been associated with the development and progression of a range of epithelial-derived carcinomas, and a variety of signaling pathways can mediate Matriptase-dependent tumorigenic events. During mammalian carcinogenesis, gain of Matriptase activity often results from imbalanced ratios between Matriptase and its cognate transmembrane inhibitor Hai1. Similarly, in zebrafish, unrestrained Matriptase activity due to loss of hai1a results in epidermal pre-neoplasms already during embryogenesis. Here, based on our former findings of a similar tumor-suppressive role for the Na+/K+-pump beta subunit ATP1b1a, we identify epithelial polarity defects and systemic hypotonic stress as another mode of aberrant Matriptase activation in the embryonic zebrafish epidermis in vivo. In this case, however, a different oncogenic pathway is activated which contains PI3K, AKT and NFkB, rather than EGFR and PLD (as in hai1a mutants). Strikingly, epidermal pre-neoplasm is only induced when epithelial polarity defects in keratinocytes (leading to disturbed Matriptase subcellular localization) occur in combination with systemic hypotonic stress (leading to increased proteolytic activity of Matriptase). A similar combinatorial effect of hypotonicity and loss of epithelial polarity was also obtained for the activity levels of Matriptase-1 in human MCF-10A epithelial breast cells. Together, this is in line with the multi-factor concept of carcinogenesis, with the notion that such factors can even branch off from one and the same initiator (here ATP1a1b) and can converge again at the level of one and the same mediator (here Matriptase). In sum, our data point to tonicity and epithelial cell polarity as evolutionarily conserved regulators of Matriptase activity that upon de-regulation can constitute an alternative mode of Matriptase-dependent carcinogenesis in vivo.

Membrane recruitment of the polarity protein Scribble by the cell adhesion receptor TMIGD1

Scribble (Scrib) is a multidomain polarity protein and member of the leucine-rich repeat and PDZ domain (LAP) protein family. A loss of Scrib expression is associated with disturbed apical-basal polarity and tumor formation. The tumor-suppressive activity of Scrib correlates with its membrane localization. Despite the identification of numerous Scrib-interacting proteins, the mechanisms regulating its membrane recruitment are not fully understood. Here, we identify the cell adhesion receptor TMIGD1 as a membrane anchor of Scrib. TMIGD1 directly interacts with Scrib through a PDZ domain-mediated interaction and recruits Scrib to the lateral membrane domain in epithelial cells. We characterize the association of TMIGD1 with each Scrib PDZ domain and describe the crystal structure of the TMIGD1 C-terminal peptide complexed with PDZ domain 1 of Scrib. Our findings describe a mechanism of Scrib membrane localization and contribute to the understanding of the tumor-suppressive activity of Scrib.

Scribble scrambles parathyroid hormone receptor interactions to regulate phosphate and vitamin D homeostasis

G protein-coupled receptors, including PTHR, are pivotal for controlling metabolic processes ranging from serum phosphate and vitamin D levels to glucose uptake, and cytoplasmic interactors may modulate their signaling, trafficking, and function. We now show that direct interaction with Scribble, a cell polarity-regulating adaptor protein, modulates PTHR activity. Scribble is a crucial regulator for establishing and developing tissue architecture, and its dysregulation is involved in various disease conditions, including tumor expansion and viral infections. Scribble co-localizes with PTHR at basal and lateral surfaces in polarized cells. Using X-ray crystallography, we show that colocalization is mediated by engaging a short sequence motif at the PTHR C-terminus using Scribble PDZ1 and PDZ3 domain, with binding affinities of 31.7 and 13.4 μM, respectively. Since PTHR controls metabolic functions by actions on renal proximal tubules, we engineered mice to selectively knockout Scribble in proximal tubules. The loss of Scribble impacted serum phosphate and vitamin D levels and caused significant plasma phosphate elevation and increased aggregate vitamin D3 levels, whereas blood glucose levels remained unchanged. Collectively these results identify Scribble as a vital regulator of PTHR-mediated signaling and function. Our findings reveal an unexpected link between renal metabolism and cell polarity signaling.

A Drosophila chemical screen reveals targeting MEK and DGKa mitigates Ras-driven polarity-impaired tumour growth

Elevated RAS signalling is highly prevalent in human cancer, however targeting RAS-driven cancers with RAS-pathway inhibitors often leads to undesirable side-effects and to drug resistance. Thus, identifying compounds that synergise with RAS-pathway inhibitors would enable lower doses of the RAS-pathway inhibitors to be used and also decrease the acquisition of drug resistance. Here, in a boutique chemical screen using a Drosophila model of Ras-driven cancer, we have identified compounds that reduce tumour size by synergising with subtherapeutic doses of the Ras-pathway inhibitor, Trametinib. Analysis of one of the hits, Ritanserin, and related compounds revealed that diacyl glycerol kinase alpha (DGKa) was the critical target required for synergism with Trametinib. Human epithelial cells harbouring the H-RAS oncogene and knockdown of the cell polarity gene, SCRIB, are also sensitive to treatment with Trametinib and DGKa inhibitors. Mechanistically, DGKa inhibition synergises with Trametinib, by increasing the P38 stress-response signalling pathway in H-RAS SCRIB-RNAi cells, which could lead to cell quiescence. Our results reveal that targeting RAS-driven human cancers with RAS-pathway and DGKa inhibitors should be an effective combination drug therapy.

Structural insight into the Scribble PDZ domains interaction with the oncogenic Human T-cell lymphotrophic virus-1 (HTLV-1) Tax1 PBM

Scribble (Scrib) is a highly conserved cell polarity regulator that harbours potent tumour suppressor activity and plays an important role in cell migration. Dysregulation of polarity is associated with poor prognosis during viral infections. Human T-cell lymphotrophic virus-1 (HTLV-1) encodes for the oncogenic Tax1 protein, a modulator of the transcription of viral and human proteins that can cause cell cycle dysregulation as well as a loss of genomic integrity. Previous studies established that Scribble interacts with Tax1 via its C-terminal PDZ-binding motif (PBM), leading to aggregation of polarity regulators and subsequent perturbation of host cell adhesion, proliferation, and signalling. Using isothermal titration calorimetry, we now show that all four PDZ domains of Scribble bind to Tax1 PBM. We then determined crystal structures of Scribble PDZ1, PDZ2 and PDZ3 domains bound to Tax1 PBM. Our findings establish a structural basis for Tax1-mediated subversion of Scribble-mediated cell polarity signalling and provide the platform for mechanistic studies to examine Tax1 induced mislocalization of Scribble and the associated changes in cellular architecture and subsequent tumorigenesis.

E-cadherin in developing murine T cells controls spindle alignment and progression through β-selection

A critical stage of T cell development is β-selection; at this stage, the T cell receptor β (TCRβ) chain is generated, and the developing T cell starts to acquire antigenic specificity. Progression through β-selection is assisted by low-affinity interactions between the nascent TCRβ chain and peptide presented on stromal major histocompatibility complex and cues provided by the niche. In this study, we identify a cue within the developing T cell niche that is critical for T cell development. E-cadherin mediates cell-cell interactions and influences cell fate in many developmental systems. In developing T cells, E-cadherin contributed to the formation of an immunological synapse and the alignment of the mitotic spindle with the polarity axis during division, which facilitated subsequent T cell development. Collectively, these data suggest that E-cadherin facilitates interactions with the thymic niche to coordinate the β-selection stage of T cell development.

Scribble and E-cadherin cooperate to control symmetric daughter cell positioning by multiple mechanisms

The fate of the two daughter cells is intimately connected to their positioning, which is in turn regulated by cell junction remodelling and orientation of the mitotic spindle. How multiple cues are integrated to dictate the ultimate positioning of daughters is not clear. Here, we identify novel mechanisms of regulation of daughter positioning in single MCF10A cells. The polarity protein, Scribble cooperates with E-cadherin for sequential roles in daughter positioning. First Scribble stabilises E-cadherin at the mitotic cortex as well as the retraction fibres, to mediate spindle orientation. Second, Scribble re-locates to the junction between the two daughters to allow a new E-cadherin-based-interface to form between them, influencing the width of the nascent daughter-daughter junction and subsequent cell positioning. Thus, E-cadherin and Scribble dynamically relocate to different intracellular sites during cell division to orient the mitotic spindle and control placement of the daughter cells after cell division. This article has an associated First Person interview with the first author of the paper.

Structural analysis of human papillomavirus E6 interactions with Scribble PDZ domains

The cell polarity regulator Scribble has been shown to be a critical regulator of the establishment and development of tissue architecture, and its dysregulation promotes or suppresses tumour development in a context-dependent manner. Scribble activity is subverted by numerous viruses. This includes human papillomaviruses (HPVs), who target Scribble via the E6 protein. Binding of E6 from high-risk HPV strains to Scribble via a C-terminal PDZ-binding motif leads to Scribble degradation in vivo. However, the precise molecular basis for Scribble-E6 interactions remains to be defined. We now show that Scribble PDZ1 and PDZ3 are the major interactors of HPV E6 from multiple high-risk strains, with each E6 protein displaying a unique interaction profile. We then determined crystal structures of Scribble PDZ1 and PDZ3 domains in complex with the PDZ-binding motif (PBM) motifs of E6 from HPV strains 16, 18 and 66. Our findings reveal distinct interaction patterns for each E6 PBM motif from a given HPV strain, suggesting that a complex molecular interplay exists that underpins the overt Scribble-HPV E6 interaction and controls E6 carcinogenic potential.

Cell polarity signalling at the birth of multicellularity: What can we learn from the first animals

The innovation of multicellularity has driven the unparalleled evolution of animals (Metazoa). But how is a multicellular organism formed and how is its architecture maintained faithfully? The defining properties and rules required for the establishment of the architecture of multicellular organisms include the development of adhesive cell interactions, orientation of division axis, and the ability to reposition daughter cells over long distances. Central to all these properties is the ability to generate asymmetry (polarity), coordinated by a highly conserved set of proteins known as cell polarity regulators. The cell polarity complexes, Scribble, Par and Crumbs, are considered to be a metazoan innovation with apicobasal polarity and adherens junctions both believed to be present in all animals. A better understanding of the fundamental mechanisms regulating cell polarity and tissue architecture should provide key insights into the development and regeneration of all animals including humans. Here we review what is currently known about cell polarity and its control in the most basal metazoans, and how these first examples of multicellular life can inform us about the core mechanisms of tissue organisation and repair, and ultimately diseases of tissue organisation, such as cancer.

Stepwise progression of β-selection during T cell development involves histone deacetylation

During T cell development, the first step in creating a unique T cell receptor (TCR) is genetic recombination of the TCRβ chain. The quality of the new TCRβ is assessed at the β-selection checkpoint. Most cells fail this checkpoint and die, but the coordination of fate at the β-selection checkpoint is not yet understood. We shed new light on fate determination during β-selection using a selective inhibitor of histone deacetylase 6, ACY1215. ACY1215 disrupted the β-selection checkpoint. Characterising the basis for this disruption revealed a new, pivotal stage in β-selection, bookended by up-regulation of TCR co-receptors, CD28 and CD2, respectively. Within this "DN3b^Pre" stage, CD5 and Lef1 are up-regulated to reflect pre-TCR signalling, and their expression correlates with proliferation. These findings suggest a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD28, CD5 and Lef1 allows for modulating TCR signalling strength and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.

PTPN2 elicits cell autonomous and non-cell autonomous effects on antitumor immunity in triple-negative breast cancer

The tumor-suppressor PTPN2 is diminished in a subset of triple-negative breast cancers (TNBCs). Paradoxically, PTPN2-deficiency in tumors or T cells in mice can facilitate T cell recruitment and/or activation to promote antitumor immunity. Here, we explored the therapeutic potential of targeting PTPN2 in tumor cells and T cells. PTPN2-deficiency in TNBC associated with T cell infiltrates and PD-L1 expression, whereas low PTPN2 associated with improved survival. PTPN2 deletion in murine mammary epithelial cells TNBC models, did not promote tumorigenicity but increased STAT-1-dependent T cell recruitment and PD-L1 expression to repress tumor growth and enhance the efficacy of anti-PD-1. Furthermore, the combined deletion of PTPN2 in tumors and T cells facilitated T cell recruitment and activation and further repressed tumor growth or ablated tumors already predominated by exhausted T cells. Thus, PTPN2-targeting in tumors and/or T cells facilitates T cell recruitment and/or alleviates inhibitory constraints on T cells to combat TNBC.

The enigmatic Placozoa part 2: Exploring evolutionary controversies and promising questions on earth and in space

The placozoan Trichoplax adhaerens has been bridging gaps between research disciplines like no other animal. As outlined in part 1, placozoans have been subject of hot evolutionary debates and placozoans have challenged some fundamental evolutionary concepts. Here in part 2 we discuss the exceptional genetics of the phylum Placozoa and point out some challenging model system applications for the best known species, Trichoplax adhaerens.

The enigmatic Placozoa part 1: Exploring evolutionary controversies and poor ecological knowledge

The placozoan Trichoplax adhaerens is a tiny hairy plate and more simply organized than any other living metazoan. After its original description by F.E. Schulze in 1883, it attracted attention as a potential model for the ancestral state of metazoan organization, the "Urmetazoon". Trichoplax lacks any kind of symmetry, organs, nerve cells, muscle cells, basal lamina, and extracellular matrix. Furthermore, the placozoan genome is the smallest (not secondarily reduced) genome of all metazoan genomes. It harbors a remarkably rich diversity of genes and has been considered the best living surrogate for a metazoan ancestor genome. The phylum Placozoa presently harbors three formally described species, while several dozen "cryptic" species are yet awaiting their description. The phylogenetic position of placozoans has recently become a contested arena for modern phylogenetic analyses and view-driven claims. Trichoplax offers unique prospects for understanding the minimal requirements of metazoan animal organization and their corresponding malfunctions.

Crystallographic Studies of PDZ Domain-Peptide Interactions of the Scribble Polarity Module

The determination of high-resolution crystal structures of cell polarity regulatory proteins bound to their functional interactors has proven to be invaluable for deciphering the underlying molecular mechanisms. Here we describe methods to identify suitable complexes of cell polarity protein domains bound to interacting ligands with subsequent preparation of such complexes for X-ray crystallographic analysis.

Structural basis of coronavirus E protein interactions with human PALS1 PDZ domain

SARS-CoV-2 infection leads to coronavirus disease 2019 (COVID-19), which is associated with severe and life-threatening pneumonia and respiratory failure. However, the molecular basis of these symptoms remains unclear. SARS-CoV-1 E protein interferes with control of cell polarity and cell-cell junction integrity in human epithelial cells by binding to the PALS1 PDZ domain, a key component of the Crumbs polarity complex. We show that C-terminal PDZ binding motifs of SARS-CoV-1 and SARS-CoV-2 E proteins bind the PALS1 PDZ domain with 29.6 and 22.8 μM affinity, whereas the related sequence from MERS-CoV did not bind. We then determined crystal structures of PALS1 PDZ domain bound to both SARS-CoV-1 and SARS-CoV-2 E protein PDZ binding motifs. Our findings establish the structural basis for SARS-CoV-1/2 mediated subversion of Crumbs polarity signalling and serve as a platform for the development of small molecule inhibitors to suppress SARS-CoV-1/2 mediated disruption of polarity signalling in epithelial cells.

Airah Javorsky et al. present the crystal structures of SARS-CoV-1 and SARS-CoV-2 E proteins in complex with the PALS1 PDZ domain. Their results suggest that the coronavirus E protein can interfere with normal PALS1 binding, potentially disrupting epithelial tissue integrity, and may provide future insight into the development of small molecule inhibitors against SARS-CoV-1/2.

Ancient and conserved functional interplay between Bcl-2 family proteins in the mitochondrial pathway of apoptosis

In metazoans, Bcl-2 family proteins are major regulators of mitochondrially mediated apoptosis; however, their evolution remains poorly understood. Here, we describe the molecular characterization of the four members of the Bcl-2 family in the most primitive metazoan, Trichoplax adhaerens All four trBcl-2 homologs are multimotif Bcl-2 group, with trBcl-2L1 and trBcl-2L2 being highly divergent antiapoptotic Bcl-2 members, whereas trBcl-2L3 and trBcl-2L4 are homologs of proapoptotic Bax and Bak, respectively. trBax expression permeabilizes the mitochondrial outer membrane, while trBak operates as a BH3-only sensitizer repressing antiapoptotic activities of trBcl-2L1 and trBcl-2L2. The crystal structure of a trBcl-2L2:trBak BH3 complex reveals that trBcl-2L2 uses the canonical Bcl-2 ligand binding groove to sequester trBak BH3, indicating that the structural basis for apoptosis control is conserved from T. adhaerens to mammals. Finally, we demonstrate that both trBax and trBak BH3 peptides bind selectively to human Bcl-2 homologs to sensitize cancer cells to chemotherapy treatment.

The polarity protein Scrib limits atherosclerosis development in mice

AIMS

The protein Scrib (Scribble 1) is known to control apico-basal polarity in epithelial cells. The role of polarity proteins in the vascular system remains poorly characterized; however, we previously reported that Scrib maintains the endothelial phenotype and directed migration. On this basis, we hypothesized that Scrib has anti-atherosclerotic functions.

METHODS AND RESULTS

Tamoxifen-induced Scrib-knockout mice were crossed with ApoE-/- knockout mice and spontaneous atherosclerosis under high-fat diet (HFD), as well as accelerated atherosclerosis in response to partial carotid artery ligation and HFD, was induced. Deletion of Scrib resulted in increased atherosclerosis development in both models. Mechanistically, flow- as well as acetylcholine-induced endothelium-dependent relaxation and AKT phosphorylation was reduced by deletion of Scrib, whereas vascular permeability and leucocyte extravasation were increased after Scrib knockout. Scrib immune pull down in primary carotid endothelial cells and mass spectrometry identified Arhgef7 (Rho Guanine Nucleotide Exchange Factor 7, βPix) as interaction partner. Scrib or Arhgef7 down-regulation by siRNA reduced the endothelial barrier function in human umbilical vein endothelial cells. Gene expression analysis from murine samples and from human biobank material of carotid endarterectomies indicated that loss of Scrib resulted in endothelial dedifferentiation with a decreased expression of endothelial signature genes.

CONCLUSIONS

By maintaining a quiescent endothelial phenotype, the polarity protein Scrib elicits anti-atherosclerotic functions.

A genome-wide Drosophila epithelial tumorigenesis screen identifies Tetraspanin 29Fb as an evolutionarily conserved suppressor of Ras-driven cancer

Oncogenic mutations in the small GTPase Ras contribute to ~30% of human cancers. However, Ras mutations alone are insufficient for tumorigenesis, therefore it is paramount to identify cooperating cancer-relevant signaling pathways. We devised an in vivo near genome-wide, functional screen in Drosophila and discovered multiple novel, evolutionarily-conserved pathways controlling Ras-driven epithelial tumorigenesis. Human gene orthologs of the fly hits were significantly downregulated in thousands of primary tumors, revealing novel prognostic markers for human epithelial tumors. Of the top 100 candidate tumor suppressor genes, 80 were validated in secondary Drosophila assays, identifying many known cancer genes and multiple novel candidate genes that cooperate with Ras-driven tumorigenesis. Low expression of the confirmed hits significantly correlated with the KRAS^G12 mutation status and poor prognosis in pancreatic cancer. Among the novel top 80 candidate cancer genes, we mechanistically characterized the function of the top hit, the Tetraspanin family member Tsp29Fb, revealing that Tsp29Fb regulates EGFR signaling, epithelial architecture and restrains tumor growth and invasion. Our functional Drosophila screen uncovers multiple novel and evolutionarily conserved epithelial cancer genes, and experimentally confirmed Tsp29Fb as a key regulator of EGFR/Ras induced epithelial tumor growth and invasion.

Cancer involves the cooperative interaction of many gene mutations. The Ras signaling pathway is upregulated in many human cancers, but upregulated Ras signaling alone is not sufficient to induce malignant tumors. We have undertaken a genome-wide genetic screen using a transgenic RNAi library in the vinegar fly, Drosophila melanogaster, to identify tumor suppressor genes that cooperate with the Ras oncogene (Ras^V12) in conferring overgrown invasive tumors. We stratified the hits by analyzing the expression of human orthologs of these genes in human epithelial cancers, revealing genes that were strongly downregulated in human cancer. By conducting secondary genetic interaction tests, we validated 80 of the top 100 genes. Pathway analysis of these genes revealed that 55 fell into known pathways involved in human cancer, whereas 25 were unique genes. We then confirmed the tumor suppressor properties of one of these genes, Tsp29Fb, encoding a Tetraspanin membrane protein, and showed that Tsp29Fb functions as a tumor suppressor by inhibiting Ras signaling and by maintaining epithelial cell polarity. Altogether, our study has revealed novel Ras-cooperating tumor suppressors in Drosophila and suggests that these genes may also be involved in human cancer.

Abstract LB-183: Investigating the role of polarity protein, SCRIB, in hematopoiesis

Acting as both a polarity regulator and a scaffolding protein, Scribble (SCRIB), coordinates intracellular signalling pathways that regulate cell proliferation, migration and apoptosis. Deregulation of SCRIB has been linked to multiple epithelial cancers but the role of SCRIB in hematopoiesis and hematopoietic malignancies remains relatively unexplored. Using different approaches, we show that SCRIB is expressed in hematopoietic stem and progenitor cells and in their differentiated progenies, implying a role for SCRIB in blood production. We investigated the effect of SCRIB loss in hematopoiesis using two conditional knockout mouse models in conjunction with an extensive phenotypic labelling regime and multi-color flow cytometry. We reveal a role for SCRIB in T cell development although subtle differences between the two mouse models were noted. Compensatory mechanisms often mask the effects of single protein deletion during steady-state so stress hematopoiesis was induced through a transplant assay to minimise these effects. Lethally irradiated recipient mice receiving a transplant of SCRIB knockout bone marrow revealed a more striking phenotype with significantly altered populations of granulocyte, macrophage, erythroid, T and B cell progenitors, and mature macrophage fractions compared to controls. Collectively, our data shows SCRIB is required for the development of multiple hematopoietic fractions and studies are currently underway to identify the underlying mechanism/s and elucidate how deregulation of SCRIB impacts on hematopoietic malignancies.

Citation Format: Novita Novita, Alysha Dew, Chad Johnson, Carl R. Walkley, Louise E. Purton, Patrick O. Humbert, Phillip K. Darcy, Sarah L. Ellis. Investigating the role of polarity protein, SCRIB, in hematopoiesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-183.

Identification of Pik3ca Mutation as a Genetic Driver of Prostate Cancer That Cooperates with Pten Loss to Accelerate Progression and Castration-Resistant Growth

Genetic alterations that potentiate PI3K signaling are frequent in prostate cancer, yet how different genetic drivers of the PI3K cascade contribute to prostate cancer is unclear. Here, we report PIK3CA mutation/amplification correlates with poor survival of patients with prostate cancer. To interrogate the requirement of different PI3K genetic drivers in prostate cancer, we employed a genetic approach to mutate Pik3ca in mouse prostate epithelium. We show Pik3caH1047R mutation causes p110α-dependent invasive prostate carcinoma in vivo Furthermore, we report that PIK3CA mutation and PTEN loss coexist in patients with prostate cancer and can cooperate in vivo to accelerate disease progression via AKT-mTORC1/2 hyperactivation. Contrasting single mutants that slowly acquire castration-resistant prostate cancer (CRPC), concomitant Pik3ca mutation and Pten loss caused de novo CRPC. Thus, Pik3ca mutation and Pten deletion are not functionally redundant. Our findings indicate that PIK3CA mutation is an attractive prognostic indicator for prostate cancer that may cooperate with PTEN loss to facilitate CRPC in patients.Significance: We show PIK3CA mutation correlates with poor prostate cancer prognosis and causes prostate cancer in mice. Moreover, PIK3CA mutation and PTEN loss coexist in prostate cancer and can cooperate in vivo to accelerate tumorigenesis and facilitate CRPC. Delineating this synergistic relationship may present new therapeutic/prognostic approaches to overcome castration/PI3K-AKT-mTORC1/2 inhibitor resistance. Cancer Discov; 8(6); 764-79. ©2018 AACR.See related commentary by Triscott and Rubin, p. 682This article is highlighted in the In This Issue feature, p. 663.

The Scribble Cell Polarity Module in the Regulation of Cell Signaling in Tissue Development and Tumorigenesis

The Scribble cell polarity module, comprising Scribbled (Scrib), Discs-large (Dlg) and Lethal-2-giant larvae (Lgl), has a tumor suppressive role in mammalian epithelial cancers. The Scribble module proteins play key functions in the establishment and maintenance of different modes of cell polarity, as well as in the control of tissue growth, differentiation and directed cell migration, and therefore are major regulators of tissue development and homeostasis. Whilst molecular details are known regarding the roles of Scribble module proteins in cell polarity regulation, their precise mode of action in the regulation of other key cellular processes remains enigmatic. An accumulating body of evidence indicates that Scribble module proteins play scaffolding roles in the control of various signaling pathways, which are linked to the control of tissue growth, differentiation and cell migration. Multiple Scrib, Dlg and Lgl interacting proteins have been discovered, which are involved in diverse processes, however many function in the regulation of cellular signaling. Herein, we review the components of the Scrib, Dlg and Lgl protein interactomes, and focus on the mechanism by which they regulate cellular signaling pathways in metazoans, and how their disruption leads to cancer.

Drosophila melanogaster Guk-holder interacts with the Scribbled PDZ1 domain and regulates epithelial development with Scribbled and Discs Large

Epithelial cell polarity is controlled by components of the Scribble polarity module, and its regulation is critical for tissue architecture and cell proliferation and migration. In Drosophila melanogaster, the adaptor protein Guk-holder (Gukh) binds to the Scribbled (Scrib) and Discs Large (Dlg) components of the Scribble polarity module and plays an important role in the formation of neuromuscular junctions. However, Gukh's role in epithelial tissue formation and the molecular basis for the Scrib-Gukh interaction remain to be defined. We now show using isothermal titration calorimetry that the Scrib PDZ1 domain is the major site for an interaction with Gukh. Furthermore, we defined the structural basis of this interaction by determining the crystal structure of the Scrib PDZ1-Gukh complex. The C-terminal PDZ-binding motif of Gukh is located in the canonical ligand-binding groove of Scrib PDZ1 and utilizes an unusually extensive network of hydrogen bonds and ionic interactions to enable binding to PDZ1 with high affinity. We next examined the role of Gukh along with those of Scrib and Dlg in Drosophila epithelial tissues and found that Gukh is expressed in larval-wing and eye-epithelial tissues and co-localizes with Scrib and Dlg at the apical cell cortex. Importantly, we show that Gukh functions with Scrib and Dlg in the development of Drosophila epithelial tissues, with depletion of Gukh enhancing the eye- and wing-tissue defects caused by Scrib or Dlg depletion. Overall, our findings reveal that Scrib's PDZ1 domain functions in the interaction with Gukh and that the Scrib-Gukh interaction has a key role in epithelial tissue development in Drosophila.

Regulation of cellular and PCP signalling by the Scribble polarity module

Since the first identification of the Scribble polarity module proteins as a new class of tumour suppressors that regulate both cell polarity and proliferation, an increasing amount of evidence has uncovered a broader role for Scribble, Dlg and Lgl in the control of fundamental cellular functions and their signalling pathways. Here, we review these findings as well as discuss more specifically the role of the Scribble module in PCP signalling.

Structural basis for the differential interaction of Scribble PDZ domains with the guanine nucleotide exchange factor β-PIX

Scribble is a highly conserved protein regulator of cell polarity that has been demonstrated to function as a tumor suppressor or, conversely, as an oncogene in a context-dependent manner, and it also controls many physiological processes ranging from immunity to memory. Scribble consists of a leucine-rich repeat domain and four PDZ domains, with the latter being responsible for most of Scribble's complex formation with other proteins. Given the similarities of the Scribble PDZ domain sequences in their binding grooves, it is common for these domains to show overlapping preferences for the same ligand. Yet, Scribble PDZ domains can still exhibit unique binding profiles toward other ligands. This raises the fundamental question as to how these PDZ domains discriminate ligands and exert specificities in Scribble complex formation. To better understand how Scribble PDZ domains direct cell polarity signaling, we investigated here their interactions with the well-characterized Scribble binding partner β-PIX, a guanine nucleotide exchange factor. We report the interaction profiles of all isolated Scribble PDZ domains with a β-PIX peptide. We show that Scribble PDZ1 and PDZ3 are the major interactors with β-PIX and reveal a distinct binding hierarchy in the interactions between the individual Scribble PDZ domains and β-PIX. Furthermore, using crystal structures of PDZ1 and PDZ3 bound to β-PIX, we define the structural basis for Scribble's ability to specifically engage β-PIX via its PDZ domains and provide a mechanistic platform for understanding Scribble-β-PIX-coordinated cellular functions such as directional cell migration.

Autophagy suppresses Ras-driven epithelial tumourigenesis by limiting the accumulation of reactive oxygen species

This corrects the article DOI: 10.1038/onc.2017.175.

The Asymmetric Cell Division Regulators Par3, Scribble and Pins/Gpsm2 Are Not Essential for Erythroid Development or Enucleation

Erythroid enucleation is the process by which the future red blood cell disposes of its nucleus prior to entering the blood stream. This key event during red blood cell development has been likened to an asymmetric cell division (ACD), by which the enucleating erythroblast divides into two very different daughter cells of alternate molecular composition, a nucleated cell that will be removed by associated macrophages, and the reticulocyte that will mature to the definitive erythrocyte. Here we investigated gene expression of members of the Par, Scribble and Pins/Gpsm2 asymmetric cell division complexes in erythroid cells, and functionally tested their role in erythroid enucleation in vivo and ex vivo. Despite their roles in regulating ACD in other contexts, we found that these polarity regulators are not essential for erythroid enucleation, nor for erythroid development in vivo. Together our results put into question a role for cell polarity and asymmetric cell division in erythroid enucleation.

A Phenotypic High-Content Screening Assay to Identify Regulators of Membrane Protein Localization

Correct subcellular localization of proteins is a requirement for appropriate function. This is especially true in epithelial cells, which rely on the precise localization of a diverse array of epithelial polarity and cellular adhesion proteins. Loss of cell polarity and adhesion is a hallmark of cancer, and mislocalization of core polarity proteins, such as Scribble, is observed in a range of human epithelial tumors and is prognostic of poor survival. Despite this, little is known about how Scribble membrane localization is regulated. Here, we describe the development and application of a phenotypic high-content screening assay that is designed to specifically quantify membrane levels of Scribble to identify regulators of its membrane localization. A screening platform that is capable of resolving individual cells and quantifying membrane protein localization in confluent epithelial monolayers was developed by using the cytoplasm-to-cell-membrane bioapplication integrated with the Cellomics ArrayScan high-content imaging platform. Application of this method to a boutique human epithelial polarity and signaling small interfering RNA (siRNA) library resulted in highly robust coefficient-of-variance and Z' factor values. As proof of concept, we present two candidate genes whose depletion specifically reduces Scribble protein levels at the membrane. Data mining revealed that these proteins interact with components of the Scribble polarity complex, providing support for the utility of the screening approach. This method is broadly applicable to genome-wide and large-scale compound screening of membrane-bound proteins, and when coupled with pathway analysis the dataset becomes even more valuable and can provide predictive mechanistic insight.

Calcium Signaling Is Required for Erythroid Enucleation

Although erythroid enucleation, the property of erythroblasts to expel their nucleus, has been known for 7ore than a century, surprisingly little is known regarding the molecular mechanisms governing this unique developmental process. Here we show that similar to cytokinesis, nuclear extrusion requires intracellular calcium signaling and signal transduction through the calmodulin (CaM) pathway. However, in contrast to cytokinesis we found that orthochromatic erythroblasts require uptake of extracellular calcium to enucleate. Together these functional studies highlight a critical role for calcium signaling in the regulation of erythroid enucleation.

Asymmetric cell division during T cell development controls downstream fate

T cell precursors undergo asymmetric cell division after T cell receptor genomic recombination, with stromal cell cues controlling the differential inheritance of fate determinants Numb and α-Adaptin by the daughters of a dividing DN3a T cell precursor.

During mammalian T cell development, the requirement for expansion of many individual T cell clones, rather than merely expansion of the entire T cell population, suggests a possible role for asymmetric cell division (ACD). We show that ACD of developing T cells controls cell fate through differential inheritance of cell fate determinants Numb and α-Adaptin. ACD occurs specifically during the β-selection stage of T cell development, and subsequent divisions are predominantly symmetric. ACD is controlled by interaction with stromal cells and chemokine receptor signaling and uses a conserved network of polarity regulators. The disruption of polarity by deletion of the polarity regulator, Scribble, or the altered inheritance of fate determinants impacts subsequent fate decisions to influence the numbers of DN4 cells arising after the β-selection checkpoint. These findings indicate that ACD enables the thymic microenvironment to orchestrate fate decisions related to differentiation and self-renewal.

Abstract C203: Cracking the (ultra)structural biology of Scribble to understand its role as a cell polarity regulator and tumor suppressor protein

Cell polarity or cell asymmetry is crucial for the normal biological function of all metazoan cells. Loss of cell polarity disrupts cellular organization that represents one of the hallmarks of cancer progression. Scribble, a member of the core polarity SCRIB module is deregulated in many cancers such as breast cancer, cervical cancer and prostate cancer. Scribble is a member of the LAP family that consists of 16 Leucine Rich Repeats and 4 PSD-95/Discs-large/ZO-1 (PDZ) domains. The PDZ domains (4PDZ) in particular are important in Scribble's interactions with molecules that are involved in many crucial signaling pathways. We hypothesized that Scribble acts as a scaffold protein to orchestrate these interactions. Our aim is to decipher Scribble's mechanism of action through characterization of Scribble interactions with βPIX (PAK-interacting exchange factor beta), which is well establish in exocytosis and neuronal transmission. Herein, we used pull down assay, isothermal titration calorimetry and small angle X-ray scattering to unravel the interaction profile of the region of Scribble encoding for all four PDZ domains. Our study proposes a working model to illustrate how PDZ domains modulate Scribble and βPIX interactions.

Citation Format: Krystle YB Lim, Sofia Caria, Colin M. House, Nathan J. Godde, Allison J. Ogden, Patrick O. Humbert, Marc Kvansakul. Cracking the (ultra)structural biology of Scribble to understand its role as a cell polarity regulator and tumor suppressor protein. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C203.

A Chemical Screening Approach to Identify Novel Key Mediators of Erythroid Enucleation

Erythroid enucleation is critical for terminal differentiation of red blood cells, and involves extrusion of the nucleus by orthochromatic erythroblasts to produce reticulocytes. Due to the difficulty of synchronizing erythroblasts, the molecular mechanisms underlying the enucleation process remain poorly understood. To elucidate the cellular program governing enucleation, we utilized a novel chemical screening approach whereby orthochromatic cells primed for enucleation were enriched ex vivo and subjected to a functional drug screen using a 324 compound library consisting of structurally diverse, medicinally active and cell permeable drugs. Using this approach, we have confirmed the role of HDACs, proteasomal regulators and MAPK in erythroid enucleation and introduce a new role for Cyclin-dependent kinases, in particular CDK9, in this process. Importantly, we demonstrate that when coupled with imaging analysis, this approach provides a powerful means to identify and characterize rate limiting steps involved in the erythroid enucleation process.

Lethal giant larvae-1 deficiency enhances the CD8(+) effector T-cell response to antigen challenge in vivo

Lethal giant larvae-1 (Lgl-1) is an evolutionary conserved protein that regulates cell polarity in diverse lineages; however, the role of Lgl-1 in the polarity and function of immune cells remains to be elucidated. To assess the role of Lgl-1 in T cells, we generated chimeric mice with a hematopoietic system deficient for Lgl-1. Lgl-1 deficiency did not impair the activation or function of peripheral CD8(+) T cells in response to antigen presentation in vitro, but did skew effector and memory T-cell differentiation. When challenged with antigen-expressing virus or tumor, Lgl-1-deficient mice displayed altered T-cell responses. This manifested in a stronger antiviral and antitumor effector CD8(+) T-cell response, the latter resulting in enhanced control of MC38-OVA tumors. These results reveal a novel role for Lgl-1 in the regulation of virus-specific T-cell responses and antitumor immunity.

Polarity Protein Scrib Facilitates Endothelial Inflammatory Signaling

OBJECTIVE

The polarity protein Scrib is highly expressed in endothelial cells and is required for planar cell polarity. Scrib also facilitates recycling of integrin α5 to the plasma membrane. Because integrin α5 signals the presence of the inflammatory matrix protein fibronectin, we hypothesized that Scrib contributes to endothelial inflammatory signaling.

APPROACH AND RESULTS

Cytokine treatment of human umbilical vein endothelial cells induced an inflammatory response as evident by the induction of vascular cell adhesion molecule-1 (VCAM-1). Downregulation of Scrib greatly attenuated this effect. In endothelial-specific conditional Scrib knockout mice, in vivo lipopolysaccharide treatment resulted in an impaired VCAM-1 induction. These effects were functionally relevant because Scrib small interfering RNAs in human umbilical vein endothelial cells attenuated the VCAM-1-mediated leukocyte adhesion in response to tumor necrosis factor-α. In vivo, tamoxifen-induced endothelial-specific deletion of Scrib resulted in a reduced VCAM-1-mediated leukocyte adhesion in response to tumor necrosis factor-α in the mouse cremaster model. This effect was specific for Scrib and not mediated by other polarity proteins. Moreover, it did not involve integrin α5 or classic pathways supporting inflammatory signaling, such as nuclear factor κ light chain enhancer of activated B-cells or MAP kinases. Co-immunoprecipitation/mass spectrometry identified the zinc finger transcription factor GATA-like protein-1 as a novel Scrib interacting protein. Small interfering RNA depletion of GATA-like protein-1 decreased the tumor necrosis factor-α-stimulated VCAM-1 induction to a similar extent as loss of Scrib did. Silencing of Scrib reduced GATA-like protein-1 protein, but not mRNA abundance.

CONCLUSIONS

Scrib is a novel proinflammatory regulator in endothelial cells, which maintains the protein expression of GATA-like protein-1.

The polarity protein Scribble regulates myelination and remyelination in the central nervous system

The development and regeneration of myelin by oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), requires profound changes in cell shape that lead to myelin sheath initiation and formation. Here, we demonstrate a requirement for the basal polarity complex protein Scribble in CNS myelination and remyelination. Scribble is expressed throughout oligodendroglial development and is up-regulated in mature oligodendrocytes where it is localised to both developing and mature CNS myelin sheaths. Knockdown of Scribble expression in cultured oligodendroglia results in disrupted morphology and myelination initiation. When Scribble expression is conditionally eliminated in the myelinating glia of transgenic mice, myelin initiation in CNS is disrupted, both during development and following focal demyelination, and longitudinal extension of the myelin sheath is disrupted. At later stages of myelination, Scribble acts to negatively regulate myelin thickness whilst suppressing the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAP) kinase pathway, and localises to non-compact myelin flanking the node of Ranvier where it is required for paranodal axo-glial adhesion. These findings demonstrate an essential role for the evolutionarily-conserved regulators of intracellular polarity in myelination and remyelination.

The polarity protein Scribble regulates the formation and properties of myelin sheaths in the central nervous system during development and after demyelinating injury.

The formation of myelin, a fatty, multilayered structure that surrounds certain neuronal axons in the nervous system, is essential for the proper communication of electrical signals by neurons, acting both as an insulator and to promote metabolic support to the axon. Loss of myelin can have severe functional consequences and trigger serious diseases, such as multiple sclerosis. Bidirectional communication between the oligodendrocytes, the myelinating cells of the central nervous system, and the axon is essential for the proper formation and function of myelin membranes; however, the signals that control myelination by oligodendrocytes in the central nervous system are poorly understood. In this paper, we use a combination of cell culture and animal studies to demonstrate that the protein Scribble, which is known to be a highly evolutionarily conserved regulator of cell polarity, plays a role in controlling whether oligodendrocytes myelinate axons. We show that Scribble regulates the length and thickness of myelin sheaths formed, as well as the tight adhesion of oligodendroglial membranes to the axonal surface, which is required for the organization of the axon into specialized domains at the nodes of Ranvier (gaps formed between the myelin sheaths generated by different cells). In addition, we show that Scribble plays a key role in the repair of myelin sheaths in a mouse model of demyelinating disease. The discovery of novel regulators of myelination in the central nervous system may allow for the identification of novel therapeutic targets for the promotion of myelin repair in patients suffering from demyelinating diseases.

Muscle stem cell fate is controlled by the cell-polarity protein Scrib

Satellite cells are resident skeletal muscle stem cells that supply myonuclei for homeostasis, hypertrophy, and repair in adult muscle. Scrib is one of the major cell-polarity proteins, acting as a potent tumor suppressor in epithelial cells. Here, we show that Scrib also controls satellite-cell-fate decisions in adult mice. Scrib is undetectable in quiescent cells but becomes expressed during activation. Scrib is asymmetrically distributed in dividing daughter cells, with robust accumulation in cells committed to myogenic differentiation. Low Scrib expression is associated with the proliferative state and preventing self-renewal, whereas high Scrib levels reduce satellite cell proliferation. Satellite-cell-specific knockout of Scrib in mice causes a drastic and insurmountable defect in muscle regeneration. Thus, Scrib is a regulator of tissue stem cells, controlling population expansion and self-renewal with Scrib expression dynamics directing satellite cell fate.

Keratin 76 is required for tight junction function and maintenance of the skin barrier

Keratins are cytoskeletal intermediate filament proteins that are increasingly being recognised for their diverse cellular functions. Here we report the consequences of germ line inactivation of Keratin 76 (Krt76) in mice. Homozygous disruption of this epidermally expressed gene causes neonatal skin flaking, hyperpigmentation, inflammation, impaired wound healing, and death prior to 12 weeks of age. We show that this phenotype is associated with functionally defective tight junctions that are characterised by mislocalization of the integral protein CLDN1. We further demonstrate that KRT76 interacts with CLDN1 and propose that this interaction is necessary to correctly position CLDN1 in tight junctions. The mislocalization of CLDN1 has been associated in various dermopathies, including the inflammatory disease, psoriasis. These observations establish a previously unknown connection between the intermediate filament cytoskeleton network and tight junctions and showcase Krt76 null mice as a possible model to study aberrant tight junction driven skin diseases.

Dissecting the role of polarity regulators in cancer through the use of mouse models

Loss of cell polarity and tissue architecture is a hallmark of aggressive epithelial cancers. In addition to serving as an initial barrier to tumorigenesis, evidence in the literature has pointed towards a highly conserved role for many polarity regulators during tumor formation and progression. Here, we review recent developments in the field that have been driven by genetically engineered mouse models that establish the tumor suppressive and context dependent oncogenic function of cell polarity regulators in vivo. These studies emphasize the complexity of the polarity network during cancer formation and progression, and reveal the need to interpret polarity protein function in a cell-type and tissue specific manner. They also highlight how aberrant polarity signaling could provide a novel route for therapeutic intervention to improve our management of malignancies in the clinic.

Scribble modulates the MAPK/Fra1 pathway to disrupt luminal and ductal integrity and suppress tumour formation in the mammary gland

Polarity coordinates cell movement, differentiation, proliferation and apoptosis to build and maintain complex epithelial tissues such as the mammary gland. Loss of polarity and the deregulation of these processes are critical events in malignant progression but precisely how and at which stage polarity loss impacts on mammary development and tumourigenesis is unclear. Scrib is a core polarity regulator and tumour suppressor gene however to date our understanding of Scrib function in the mammary gland has been limited to cell culture and transplantation studies of cell lines. Utilizing a conditional mouse model of Scrib loss we report for the first time that Scrib is essential for mammary duct morphogenesis, mammary progenitor cell fate and maintenance, and we demonstrate a critical and specific role for Scribble in the control of the early steps of breast cancer progression. In particular, Scrib-deficiency significantly induced Fra1 expression and basal progenitor clonogenicity, which resulted in fully penetrant ductal hyperplasia characterized by high cell turnover, MAPK hyperactivity, frank polarity loss with mixing of apical and basolateral membrane constituents and expansion of atypical luminal cells. We also show for the first time a role for Scribble in mammalian spindle orientation with the onset of mammary hyperplasia being associated with aberrant luminal cell spindle orientation and a failure to apoptose during the final stage of duct tubulogenesis. Restoring MAPK/Fra1 to baseline levels prevented Scrib-hyperplasia, whereas persistent Scrib deficiency induced alveolar hyperplasia and increased the incidence, onset and grade of mammary tumours. These findings, based on a definitive genetic mouse model provide fundamental insights into mammary duct maturation and homeostasis and reveal that Scrib loss activates a MAPK/Fra1 pathway that alters mammary progenitor activity to drive premalignancy and accelerate tumour progression.

Lethal giant larvae 1 tumour suppressor activity is not conserved in models of mammalian T and B cell leukaemia

In epithelial and stem cells, lethal giant larvae (Lgl) is a potent tumour suppressor, a regulator of Notch signalling, and a mediator of cell fate via asymmetric cell division. Recent evidence suggests that the function of Lgl is conserved in mammalian haematopoietic stem cells and implies a contribution to haematological malignancies. To date, direct measurement of the effect of Lgl expression on malignancies of the haematopoietic lineage has not been tested. In Lgl1^−/− mice, we analysed the development of haematopoietic malignancies either alone, or in the presence of common oncogenic lesions. We show that in the absence of Lgl1, production of mature white blood cell lineages and long-term survival of mice are not affected. Additionally, loss of Lgl1 does not alter leukaemia driven by constitutive Notch, c-Myc or Jak2 signalling. These results suggest that the role of Lgl1 in the haematopoietic lineage might be restricted to specific co-operating mutations and a limited number of cellular contexts.

Increasing intracellular bioavailable copper selectively targets prostate cancer cells

The therapeutic efficacy of two bis(thiosemicarbazonato) copper complexes, glyoxalbis[N4-methylthiosemicarbazonato]Cu(II) [Cu(II)(gtsm)] and diacetylbis[N4-methylthiosemicarbazonato]Cu(II) [Cu(II)(atsm)], for the treatment of prostate cancer was assessed in cell culture and animal models. Distinctively, copper dissociates intracellularly from Cu(II)(gtsm) but is retained by Cu(II)(atsm). We further demonstrated that intracellular H2gtsm [reduced Cu(II)(gtsm)] continues to redistribute copper into a bioavailable (exchangeable) pool. Both Cu(II)(gtsm) and Cu(II)(atsm) selectively kill transformed (hyperplastic and carcinoma) prostate cell lines but, importantly, do not affect the viability of primary prostate epithelial cells. Increasing extracellular copper concentrations enhanced the therapeutic capacity of both Cu(II)(gtsm) and Cu(II)(atsm), and their ligands (H2gtsm and H2atsm) were toxic only toward cancerous prostate cells when combined with copper. Treatment of the Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) model with Cu(II)(gtsm) (2.5 mg/kg) significantly reduced prostate cancer burden (∼70%) and severity (grade), while treatment with Cu(II)(atsm) (30 mg/kg) was ineffective at the given dose. However, Cu(II)(gtsm) caused mild kidney toxicity in the mice, associated primarily with interstitial nephritis and luminal distention. Mechanistically, we demonstrated that Cu(II)(gtsm) inhibits proteasomal chymotrypsin-like activity, a feature further established as being common to copper-ionophores that increase intracellular bioavailable copper. We have demonstrated that increasing intracellular bioavailable copper can selectively kill cancerous prostate cells in vitro and in vivo and have revealed the potential for bis(thiosemicarbazone) copper complexes to be developed as therapeutics for prostate cancer.

Scribble regulates an EMT polarity pathway through modulation of MAPK-ERK signaling to mediate junction formation

The crucial role the Crumbs and Par polarity complexes play in tight junction integrity has long been established, however very few studies have investigated the role of the Scribble polarity module. Here, we use MCF10A cells, which fail to form tight junctions and express very little endogenous Crumbs3, to show that inducing expression of the polarity protein Scribble is sufficient to promote tight junction formation. We show this occurs through an epithelial-to-mesenchymal (EMT) pathway that involves Scribble suppressing ERK phosphorylation, leading to downregulation of the EMT inducer ZEB. Inhibition of ZEB relieves the repression on Crumbs3, resulting in increased expression of this crucial tight junction regulator. The combined effect of this Scribble-mediated pathway is the upregulation of a number of junctional proteins and the formation of functional tight junctions. These data suggests a novel role for Scribble in positively regulating tight junction assembly through transcriptional regulation of an EMT signaling program.

Localization, not important in all tumor-suppressing properties: a lesson learnt from scribble

Aberrant localization of proteins is increasingly being suggested as a causal player in epithelial cancers. Despite this, few studies have investigated how mislocalization of a protein can alter individual biological processes that contribute to cancer progression. Using Ras as a model of transformation, we investigate how localization of the polarity protein Scribble contributes to its tumor-suppressive properties. Wild-type Scribble has been shown to modulate Ras-mitogen-activated protein kinase (MAPK) transformation both in vitro and in vivo. By utilizing a construct that carries a mutation in the LRR domain of Scribble (Scribble P305L) resulting in a cytosolic rather than the usual membrane-bound localization, we report that discrete tumor suppressive properties of Scribble are differentially sensitive to the localization of Scribble. We find that although the Scribble P305L mislocalization mutant can no longer suppress Ras-MAPK-induced invasion or epithelial to mesenchymal transition phenotypes, mislocalized Scribble can still suppress anchorage-independent cell growth. This study illustrates that the manner in which protein mislocalization contributes to cancer is likely complex and highlights the need for careful interrogation as to how cell polarity protein mislocalization, its secondary consequences, and the mutations that give rise to their mislocalization may contribute to specific aspects of cancer progression.

An in vivo large-scale chemical screening platform using Drosophila for anti-cancer drug discovery

Anti-cancer drug development involves enormous expenditure and risk. For rapid and economical identification of novel, bioavailable anti-tumour chemicals, the use of appropriate in vivo tumour models suitable for large-scale screening is key. Using a Drosophila Ras-driven tumour model, we demonstrate that tumour overgrowth can be curtailed by feeding larvae with chemicals that have the in vivo pharmacokinetics essential for drug development and known efficacy against human tumour cells. We then develop an in vivo 96-well plate chemical screening platform to carry out large-scale chemical screening with the tumour model. In a proof-of-principle pilot screen of 2000 compounds, we identify the glutamine analogue, acivicin, a chemical with known activity against human tumour cells, as a potent and specific inhibitor of Drosophila tumour formation. RNAi-mediated knockdown of candidate acivicin target genes implicates an enzyme involved in pyrimidine biosynthesis, CTP synthase, as a possible crucial target of acivicin-mediated inhibition. Thus, the pilot screen has revealed that Drosophila tumours are glutamine-dependent, which is an emerging feature of many human cancers, and has validated the platform as a powerful and economical tool for in vivo chemical screening. The platform can also be adapted for use with other disease models, thus offering widespread applications in drug development.

Characterization of in vivo Dlg1 deletion on T cell development and function

Background

The polarized reorganization of the T cell membrane and intracellular signaling molecules in response to T cell receptor (TCR) engagement has been implicated in the modulation of T cell development and effector responses. In siRNA-based studies Dlg1, a MAGUK scaffold protein and member of the Scribble polarity complex, has been shown to play a role in T cell polarity and TCR signal specificity, however the role of Dlg1 in T cell development and function in vivo remains unclear.

Methodology/Principal Findings

Here we present the combined data from three independently-derived dlg1-knockout mouse models; two germline deficient knockouts and one conditional knockout. While defects were not observed in T cell development, TCR-induced early phospho-signaling, actin-mediated events, or proliferation in any of the models, the acute knockdown of Dlg1 in Jurkat T cells diminished accumulation of actin at the IS. Further, while Th1-type cytokine production appeared unaffected in T cells derived from mice with a dlg1germline-deficiency, altered production of TCR-dependent Th1 and Th2-type cytokines was observed in T cells derived from mice with a conditional loss of dlg1 expression and T cells with acute Dlg1 suppression, suggesting a differential requirement for Dlg1 activity in signaling events leading to Th1 versus Th2 cytokine induction. The observed inconsistencies between these and other knockout models and siRNA strategies suggest that 1) compensatory upregulation of alternate gene(s) may be masking a role for dlg1 in controlling TCR-mediated events in dlg1 deficient mice and 2) the developmental stage during which dlg1 ablation begins may control the degree to which compensatory events occur.

Conclusions/Significance

These findings provide a potential explanation for the discrepancies observed in various studies using different dlg1-deficient T cell models and underscore the importance of acute dlg1 ablation to avoid the upregulation of compensatory mechanisms for future functional studies of the Dlg1 protein.

Abstract 3291: Defining the role of the cellular polarity regulator Scrib in epithelial tumorigenesis

In recent years, loss of cell polarity and tissue architecture has been defined as a hallmark of epithelial cancer yet the molecular mechanisms underlying this predisposition are not fully understood. Two distinct polarity programs are required for the correct assembly and function of all mammalian epithelial tissues; apical-basal polarity and non-canonical Wnt/planar cell polarity (1). The Scribble complex functions to establish and maintain both aspects of epithelial polarity and has been shown to mediate cell adhesion, migration and asymmetric cell division (2). In human colorectal, breast, and cervical cancers, expression of the Scribble complex member SCRIB is often mislocalised and deregulated, suggesting that polarity regulators plays a role in maintaining homeostasis of epithelial tissues (1,3). Scrib null mice are neonatal lethal owing to a neural tube closure defect. To determine the tumor suppressive role of Scrib within epithelial tissues, we have employed tissue specific Cre-LoxP technology to deplete Scrib specifically within the prostate, mammary or lung. Scrib depletion in these epithelial tissues results in tumor initiation, supporting previous work in Drosophila (1). We and others have previously shown that SCRIB loss stimulates Ras/MAPK signaling in vitro, suggesting that SCRIB negatively regulates the Ras/MAPK cascade to suppress tumorigenesis (4,5). We now show elevated p-ERK1/2 expression is a common feature of Scrib deficient tumors in vivo. Accordingly, administration of the MEK inhibitor PD0325901 in Scrib+/− mice partially rescued the hyperplastic prostate phenotype. Together, these studies establish that depletion of the polarity regulator Scrib can initiate murine tumorigenesis in several epithelial tissues in a process involving deregulation of the Ras/MAPK cascade. Interestingly, tissue specific activation of oncogenic K-ras in combination with Scrib depletion accelerated tumor progression in both the prostate and lung. Thus, K-ras activation and Scrib deficiency synergise to facilitate tumor growth, demonstrating that Scrib depletion can contribute to tumor progression in the presence of an additional oncogenic mutation. The clinical relevance of this research is highlighted by our observation that SCRIB deregulation strongly correlated with PSA recurrence free survival in human prostate cancer (3). Together, our findings suggest that the polarity network may hold prognostic value or provide a novel route for therapeutic intervention. (1) Humbert PO et al (2008) Oncogene 27(55):6888-6907 (2) Kallay LM et al (2006) J Cell Biochem 99: 647-64 (3) Pearson HB et al (2011) J Clin Invest 121(11):4257-4267 (4) Dow LE et al (2008) Oncogene 27(46):5988-6001 (5) Nagasaka K et al (2010) Oncogene 29(38):5311-5321

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3291. doi:1538-7445.AM2012-3291