Deciphering tumor-suppressor signaling in flies: Genetic link between Scribble/Dlg/Lgl and the Hippo pathways

Loss of apico-basal polarity is one of the crucial factors that drives epithelial tumor progression.scribble/discs large/lethal giant larvae (scrib/dlg/lgl),a group of apico-basal polarity genes,were initially identified as members of “neoplastic” tumor-suppressors in flies.The components of the Hippo signaling pathway,which is crucial for organ size control and cancer development,were also identified through Drosophila genetic screens as members of “hyperplastic” tumor-suppressors.Accumulating evidence in recent studies implies that these two tumor-suppressor signaling pathways are not mutually exclusive but rather cooperatively act to give rise to highly malignant tumors.The interaction of these tumor-suppressor pathways could include deregulations of actin cytoskeleton,cell-cell contact,and apical-domain size of the epithelial cell.     

The hippo pathway promotes Notch signaling in regulation of cell differentiation, proliferation, and oocyte polarity

Specification of the anterior-posterior axis in Drosophila oocytes requires proper communication between the germ-line cells and the somatically derived follicular epithelial cells. Multiple signaling pathways, including Notch, contribute to oocyte polarity formation by controlling the temporal and spatial pattern of follicle cell differentiation and proliferation. Here we show that the newly identified Hippo tumor-suppressor pathway plays a crucial role in the posterior follicle cells in the regulation of oocyte polarity. Disruption of the Hippo pathway, including major components Hippo, Salvador, and Warts, results in aberrant follicle-cell differentiation and proliferation and dramatic disruption of the oocyte anterior-posterior axis. These phenotypes are related to defective Notch signaling in follicle cells, because misexpression of a constitutively active form of Notch alleviates the oocyte polarity defects. We also find that follicle cells defective in Hippo signaling accumulate the Notch receptor and display defects in endocytosis markers. Our findings suggest that the interaction between Hippo and classic developmental pathways such as Notch is critical to spatial and temporal regulation of differentiation and proliferation and is essential for development of the body axes in Drosophila.     

Lgl/aPKC and Crb regulate the Salvador/Warts/Hippo pathway

A key goal of developmental biology is to understand the mechanisms that coordinate organ growth. It has long been recognized that the genes that control apico-basal cell polarity also regulate tissue growth. How loss of cell polarity contributes to tissue overgrowth has been the subject of much speculation. Do loss-of-function mutations in cell polarity regulators result in secondary effects that globally deregulate cell proliferation, or do these genes specifically control growth pathways? Three recent papers have shown that the apico-basal polarity determinants Lgl/aPKC and Crb regulate tissue growth independently of their roles in cell polarity and coordinately regulate cell proliferation and cell death via the Salvador/Warts/Hippo (SWH) pathway. Lgl/aPKC are required for the correct localization of Hippo (Hpo)/Ras associated factor (RASSF), whilst Crb regulates the levels and localization of Expanded (Ex), indicating that cell polarity determinants modify SWH pathway activity by distinct mechanisms. Here, we review the key data that support these conclusions, highlight remaining questions and speculate on the underlying mechanisms by which the cell polarity complexes interact with the SWH pathway. Understanding the interactions between cell polarity regulators and the SWH pathway will improve our knowledge of how epithelial organization and tissue growth are coordinated during development and perturbed in disease states such as cancer.     

Lgl/aPKC and Crb regulate the Salvador/Warts/Hippo pathway

A key goal of developmental biology is to understand the mechanisms that coordinate organ growth. It has long been recognized that the genes that control apico-basal cell polarity also regulate tissue growth. How loss of cell polarity contributes to tissue overgrowth has been the subject of much speculation. Do loss-of-function mutations in cell polarity regulators result in secondary effects that globally deregulate cell proliferation, or do these genes specifically control growth pathways? Three recent papers have shown that the apico-basal polarity determinants Lgl/aPKC and Crb regulate tissue growth independently of their roles in cell polarity and coordinately regulate cell proliferation and cell death via the Salvador/Warts/Hippo (SWH) pathway. Lgl/aPKC are required for the correct localization of Hippo (Hpo)/Ras associated factor (RASSF), while Crb regulates the levels and localization of Expanded (Ex), indicating that cell polarity determinants modify SWH pathway activity by distinct mechanisms. Here, we review the key data that support these conclusions, highlight remaining questions and speculate on the underlying mechanisms by which the cell polarity complexes interact with the SWH pathway. Understanding the interactions between cell polarity regulators and the SWH pathway will improve our knowledge of how epithelial organization and tissue growth are coordinated during development and perturbed in disease states such as cancer.Key words: Drosophila, tumor suppressor gene, cell polarity, Hippo pathway, Crb, Lgl, aPKC     

Apico-basal polarity complex and cancer

Apico-basal polarity is a cardinal molecular feature of adult eukaryotic epithelial cells and appears to be involved in several key cellular processes including polarized cell migration and maintenance of tissue architecture. Epithelial cell polarity is maintained by three well-conserved polarity complexes, namely, PAR, Crumbs and SCRIB. The location and interaction between the components of these complexes defines distinct structural domains of epithelial cells. Establishment and maintenance of apico-basal polarity is regulated through various conserved cell signalling pathways including TGFβ, Integrin and WNT signalling. Loss of cell polarity is a hallmark for carcinoma, and its underlying molecular mechanism is beginning to emerge from studies on model organisms and cancer cell lines. Moreover, deregulated expression of apico-basal polarity complex components has been reported in human tumours. In this review, we provide an overview of the apico-basal polarity complexes and their regulation, their role in cell migration, and finally their involvement in carcinogenesis.     

顶- 底极性复合物与癌症的关系研究进展

顶-底极性是上皮细胞的一项主要特征,参与细胞形态、迁移、功能维持等多个生物学事件。上皮细胞顶-底极性复合物包括PAR复合物、SCRIB复合物和CRB复合物。丧失极性是细胞癌化的标志之一,并且在人类癌症中也发现了顶-底极性复合物的异常表达。本文将就目前有关顶-底极性复合物在癌症方面的研究进行综述,重点阐述顶-底极性复合物在肿瘤发生、发展过程中的作用及调控机制。     

CREB3 subfamily transcription factors are not created equal: Recent insights from global analyses and animal models

The epithelial to mesenchymal transition (EMT) plays crucial roles in the formation of the body plan and also in the tumor invasion process. In addition, EMT also causes disruption of cell-cell adherence, loss of apico-basal polarity, matrix remodeling, increased motility and invasiveness in promoting tumor metastasis. The tumor microenvironment plays an important role in facilitating cancer metastasis and may induce the occurrence of EMT in tumor cells. A large number of inflammatory cells infiltrating the tumor site, as well as hypoxia existing in a large area of tumor, in addition many stem cells present in tumor microenvironment, such as cancer stem cells (CSCs), mesenchymal stem cells (MSCs), all of these may be the inducers of EMT in tumor cells. The signaling pathways involved in EMT are various, including TGF-β, NF-κB, Wnt, Notch, and others. In this review, we discuss the current knowledge about the role of the tumor microenvironment in EMT and the related signaling pathways as well as the interaction between them.     

Polarity complex proteins

The formation of functional epithelial tissues involves the coordinated action of several protein complexes, which together produce a cell polarity axis and develop cell-cell junctions. During the last decade, the notion of polarity complexes emerged as the result of genetic studies in which a set of genes was discovered first in Caenorhabditis elegans and then in Drosophila melanogaster. In epithelial cells, these complexes are responsible for the development of the apico-basal axis and for the construction and maintenance of apical junctions. In this review, we focus on apical polarity complexes, namely the PAR3/PAR6/aPKC complex and the CRUMBS/PALS1/PATJ complex, which are conserved between species and along with a lateral complex, the SCRIBBLE/DLG/LGL complex, are crucial to the formation of apical junctions such as tight junctions in mammalian epithelial cells. The exact mechanisms underlying their tight junction construction and maintenance activities are poorly understood, and it is proposed to focus in this review on establishing how these apical polarity complexes might regulate epithelial cell morphogenesis and functions. In particular, we will present the latest findings on how these complexes regulate epithelial homeostasis.     

Polarity complex proteins

The formation of functional epithelial tissues involves the coordinated action of several protein complexes, which together produce a cell polarity axis and develop cell-cell junctions. During the last decade, the notion of polarity complexes emerged as the result of genetic studies in which a set of genes was discovered first in Caenorhabditis elegans and then in Drosophila melanogaster. In epithelial cells, these complexes are responsible for the development of the apico-basal axis and for the construction and maintenance of apical junctions. In this review, we focus on apical polarity complexes, namely the PAR3/PAR6/aPKC complex and the CRUMBS/PALS1/PATJ complex, which are conserved between species and along with a lateral complex, the SCRIBBLE/DLG/LGL complex, are crucial to the formation of apical junctions such as tight junctions in mammalian epithelial cells. The exact mechanisms underlying their tight junction construction and maintenance activities are poorly understood, and it is proposed to focus in this review on establishing how these apical polarity complexes might regulate epithelial cell morphogenesis and functions. In particular, we will present the latest findings on how these complexes regulate epithelial homeostasis.     

Role of cell polarity and planar cell polarity (PCP) proteins in spermatogenesis

Abstract

Studies on cell polarity proteins and planar cell polarity (PCP) proteins date back to almost 40?years ago in Drosophila and C. elegans when these proteins were shown to be crucial to support apico-basal polarity and also directional alignment of polarity cells across the plane of an epithelium during morphogenesis. In adult mammals, cell polarity and PCP are most notable in cochlear hair cells. However, the role of these two groups of proteins to support spermatogenesis was not explored until a decade earlier when several proteins that confer cell polarity and PCP proteins were identified in the rat testis. Since then, there are several reports appearing in the literature to examine the role of both cell polarity and PCP in supporting spermatogenesis. Herein, we provide an overview regarding the role of cell polarity and PCP proteins in the testis, evaluating these findings in light of studies in other mammalian epithelial cells/tissues. Our goal is to provide a timely evaluation of these findings, and provide some thought provoking remarks to guide future studies based on an evolving concept in the field.     

Planar cell polarity signaling in craniofacial development

Out of the several signaling pathways controlling craniofacial development, the role of planar cell polarity (PCP) signaling is relatively poorly understood. This pathway, originally identified as a mechanism to maintain cell polarity within the epithelial cells of the Drosophila wing, has been linked to the proper development of a wide variety of tissues in vertebrates and invertebrates. While many of the pathway members are conserved, it appears that some of the members of the pathway act in a tissue-specific manner. Here, we discuss the role of this pathway in vertebrate craniofacial development, highlighting cranial neural crest migration, skull and palate formation and the role of non-traditional modulators of PCP signaling within this developmental process.     

Planar cell polarity signaling in craniofacial development

Out of the several signaling pathways controlling craniofacial development, the role of planar cell polarity (PCP) signaling is relatively poorly understood. This pathway, originally identified as a mechanism to maintain cell polarity within the epithelial cells of the Drosophila wing, has been linked to the proper development of a wide variety of tissues in vertebrates and invertebrates. While many of the pathway members are conserved, it appears that some of the members of the pathway act in a tissue-specific manner. Here, we discuss the role of this pathway in vertebrate craniofacial development, highlighting cranial neural crest migration, skull and palate formation and the role of non-traditional modulators of PCP signaling within this developmental process.     

CKIepsilon/discs overgrown promotes both Wnt-Fz/beta-catenin and Fz/PCP signaling in Drosophila

The related Wnt-Frizzled(Fz)/beta-catenin and Fz/planar cell polarity (PCP) pathways are essential for the regulation of numerous developmental processes and are deregulated in many human diseases. Both pathways require members of the Dishevelled (Dsh or Dvl) family of cytoplasmic factors for signal transduction downstream of the Fz receptors. Dsh family members have been studied extensively, but their activation and regulation remains largely unknown. In particular, very little is known about how Dsh differentially signals to the two pathways. Recent work in cell culture has suggested that phosphorylation of Dsh by Casein Kinase I epsilon (CKIepsilon) may act as a molecular "switch," promoting Wnt/beta-catenin while inhibiting Fz/PCP signaling. Here, we demonstrate in vivo in Drosophila through a series of loss-of-function and coexpression assays that CKIepsilon acts positively for signaling in both pathways, rather than as a switch. Our data suggest that the kinase activity of CKIepsilon is required for peak levels of Wnt/beta-catenin signaling. In contrast, CKIepsilon is a mandatory signaling factor in the Fz/PCP pathway, possibly through a kinase-independent mechanism. Furthermore, we have identified the primary kinase target residue of CKIepsilon on Dsh. Thus, our data suggest that CKIepsilon modulates Wnt/beta-catenin and Fz/PCP signaling pathways via kinase-dependent and -independent mechanisms.     

Apical Accumulation of the Sevenless Receptor Tyrosine Kinase During Drosophila Eye Development Is Promoted by the Small GTPase Rap1

The Ras/MAPK-signaling pathway plays pivotal roles during development of metazoans by controlling cell proliferation and cell differentiation elicited, in several instances, by receptor tyrosine kinases (RTKs). While the internal mechanism of RTK-driven Ras/MAPK signaling is well understood, far less is known regarding its interplay with other corequired signaling events involved in developmental decisions. In a genetic screen designed to identify new regulators of RTK/Ras/MAPK signaling during Drosophila eye development, we identified the small GTPase Rap1, PDZ-GEF, and Canoe as components contributing to Ras/MAPK-mediated R7 cell differentiation. Rap1 signaling has recently been found to participate in assembling cadherin-based adherens junctions in various fly epithelial tissues. Here, we show that Rap1 activity is required for the integrity of the apical domains of developing photoreceptor cells and that reduced Rap1 signaling hampers the apical accumulation of the Sevenless RTK in presumptive R7 cells. It thus appears that, in addition to its role in cell–cell adhesion, Rap1 signaling controls the partitioning of the epithelial cell membrane, which in turn influences signaling events that rely on apico-basal cell polarity.     

Roles of the JNK signaling pathway in Drosophila morphogenesis.

Epithelial cell differentiation and morphogenesis are crucial in many aspects of metazoan development. Recent genetic studies in Drosophila have revealed that the conserved Jun amino-terminal kinase (JNK) signaling pathway regulates epithelial morphogenesis during the process of embryonic dorsal closure and participates in the control of planar polarity in several tissues. Importantly, these studies have linked the JNK pathway to the decapentaplegic and Frizzled pathways in these processes, suggesting a high degree of integrative signaling during epithelial morphogenesis.     

Proteome Dynamics in Biobanked Horse Peripheral Blood Derived Lymphocytes (PBL) with Induced Autoimmune Uveitis

Equine recurrent uveitis is the only spontaneous model for recurrent autoimmune uveitis in humans, where T cells target retinal proteins. Differences between normal and autoaggressive lymphocytes were identified in this study by analyzing peripheral blood derived lymphocytes (PBL) proteomes from the same case with interphotoreceptor retinoid binding protein induced uveitis sampled before (Day 0), during (Day 15), and after uveitic attack (Day 23). Relative protein abundances of PBL were investigated in a quantitative, label‐free differential proteome analysis in cells that were kept frozen for 14 years since the initial experiment. Quantitative data could be acquired for 2632 proteins at all three time points. Profound changes (≥2‐fold change) in PBL protein abundance were observed when comparing Day 0 with 15, representing acute inflammation (1070 regulated proteins) and Day 0 with 23 (cessation; 1571 regulated). Significant differences applied to proteins with functions in integrin signaling during active uveitis, involving “Erk and pi‐3 kinase are necessary for collagen binding in corneal epithelia,” “integrins in angiogenesis,” and “integrin‐linked kinase signaling” pathways. In contrast, at cessation of uveitic attack, significantly changed proteins belonged to pathways of “nongenotropic androgen signaling,” “classical complement pathway,” and “Amb2 integrin signaling.” Several members of respective pathways were earlier shown to be changed in naturally occurring uveitis, underscoring the significance of these findings here and proofing the value of the induced model in mimicking spontaneous autoimmune uveitis. All MS data have been deposited to the ProteomeXchange consortium via the PRIDE partner repository (dataset identifier PXD005580).     

Wnt/Ca2+ signaling pathway: a brief overview

The non-canonical Wnt/Ca(2+) signaling cascade is less characterized than their canonical counterpart, the Wnt/β-catenin pathway. The non-canonical Wnt signaling pathways are diverse, defined as planer cell polarity pathway, Wnt-RAP1 signaling pathway, Wnt-Ror2 signaling pathway, Wnt-PKA pathway, Wnt-GSK3MT pathway, Wnt-aPKC pathway, Wnt-RYK pathway, Wnt-mTOR pathway, and Wnt/calcium signaling pathway. All these pathways exhibit a considerable degree of overlap between them. The Wnt/Ca(2+) signaling pathway was deciphered as a crucial mediator in development. However, now there is substantial evidence that the signaling cascade is involved in many other molecular phenomena. Many aspects of Wnt/Ca(2+) pathway are yet enigmatic. This review will give a brief overview of the fundamental and evolving concepts of the Wnt/Ca(2+) signaling pathway.