Mechanisms of Cell Fusion in Cancer

Cell-cell fusion is a normal physiological mechanism that requires a well-orchestrated regulation of intracellular and extracellular factors. Dysregulation of this process could lead to diseases such as osteoporosis, malformation of muscles, difficulties in pregnancy, and cancer. Extensive literature demonstrates that fusion occurs between cancer cells and other cell types to potentially promote cancer progression and metastasis. However, the mechanisms governing this process in cancer initiation, promotion, and progression are less well-studied. Fusogens involved in normal physiological processes such as syncytins and associated factors such as phosphatidylserine and annexins have been observed to be critical in cancer cell fusion as well. Some of the extracellular factors associated with cancer cell fusion include chronic inflammation and inflammatory cytokines, hypoxia, and viral infection. The interaction between these extracellular factors and cell's intrinsic factors potentially modulates actin dynamics to drive the fusion of cancer cells. In this review, we have discussed the different mechanisms that have been identified or postulated to drive cancer cell fusion.

Inhibition of insulin-like growth factor 2 mRNA-binding protein 1 sensitizes colorectal cancer cells to chemotherapeutics

Although colorectal cancer (CRC) treatment has seen a remarkable improvement in the recent years, many patients will develop metastasis due to the resistance of cancer cells to chemotherapeutics. Targeting mechanisms driving the resistance of CRC cells to treatment would significantly reduce cases of metastasis and death. Induction of insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1), a direct target of the Wnt/β-catenin signaling pathway, might promote resistance of CRC cells to treatment via activation of anti-apoptotic pathways and induction of the multidrug resistance (MDR1) membrane transporter that pumps drugs out of the cells. We hypothesized that inhibition of IGF2BP1 will sensitize CRC cells to chemotherapeutics. We used CRC cell lines with different status of activation of Wnt signaling to show that inhibition of IGF2BP1 potentiates the anti-growth and anti-proliferative effects of chemotherapeutics on CRC cells with activated Wnt/β-catenin signaling pathway. We observed that the inhibition of IGF2BP1 significantly increases apoptosis in the same cells. A remarkable reduction in the migratory capability of those cells was noted as well. We found that inhibition of IGF2BP1 is sufficient to decrease the resistance of chemotherapy-resistant cancer cells with activated Wnt/β-catenin signaling pathway. These findings portray IGF2BP1 as a good candidate for CRC therapy.

Detection and quantification of γ-H2AX using a dissociation enhanced lanthanide fluorescence immunoassay

Phosphorylation of the histone protein H2AX to form γ-H2AX foci directly represents DNA double-strand break formation. Traditional γ-H2AX detection involves counting individual foci within individual nuclei. The novelty of this work is the application of a time-resolved fluorescence assay using dissociation-enhanced lanthanide fluorescence immunoassay for quantitative measurements of γ-H2AX. For comparison, standard fluorescence detection was employed and analyzed either by bulk fluorescent measurements or by direct foci counting using BioTek Spot Count algorithm and Gen 5 software. Etoposide induced DNA damage in A549 carcinoma cells was compared across all test platforms. Time resolved fluorescence detection of europium as a chelated complex enabled quantitative measurement of γ-H2AX foci with nanomolar resolution. Comparative bulk fluorescent signals achieved only micromolar sensitivity. Lanthanide based immunodetection of γ-H2AX offers superior detection and a user-friendly workflow. These approaches have the potential to improve screening of compounds that either enhance DNA damage or protect against its deleterious effects.

Advances in Our Understanding of the Molecular Mechanisms of Action of Cisplatin in Cancer Therapy

Cisplatin and other platinum-based chemotherapeutic drugs have been used extensively for the treatment of human cancers such as bladder, blood, breast, cervical, esophageal, head and neck, lung, ovarian, testicular cancers, and sarcoma. Cisplatin is commonly administered intravenously as a first-line chemotherapy for patients suffering from various malignancies. Upon absorption into the cancer cell, cisplatin interacts with cellular macromolecules and exerts its cytotoxic effects through a series of biochemical mechanisms by binding to Deoxyribonucleic acid (DNA) and forming intra-strand DNA adducts leading to the inhibition of DNA synthesis and cell growth. Its primary molecular mechanism of action has been associated with the induction of both intrinsic and extrinsic pathways of apoptosis resulting from the production of reactive oxygen species through lipid peroxidation, activation of various signal transduction pathways, induction of p53 signaling and cell cycle arrest, upregulation of pro-apoptotic genes/proteins, and down-regulation of proto-oncogenes and anti-apoptotic genes/proteins. Despite great clinical outcomes, many studies have reported substantial side effects associated with cisplatin monotherapy, while others have shown substantial drug resistance in some cancer patients. Hence, new formulations and several combinational therapies with other drugs have been tested for the purpose of improving the clinical utility of cisplatin. Therefore, this review provides a comprehensive understanding of its molecular mechanisms of action in cancer therapy and discusses the therapeutic approaches to overcome cisplatin resistance and side effects.

Vernonia calvoana Shows Promise towards the Treatment of Ovarian Cancer

The treatment for ovarian cancers includes chemotherapies which use drugs such as cisplatin, paclitaxel, carboplatin, platinum, taxanes, or their combination, and other molecular target therapies. However, these current therapies are often accompanied with side effects. Vernonia calvoana (VC) is a valuable edible medicinal plant that is widespread in West Africa. In vitro data in our lab demonstrated that VC crude extract inhibits human ovarian cancer cells in a dose-dependent manner, suggesting its antitumor activity. From the VC crude extract, we have generated 10 fractions and VC fraction 7 (F7) appears to show the highest antitumor activity towards ovarian cancer cells. However, the mechanisms by which VC F7 exerts its antitumor activity in cancer cells remain largely unknown. We hypothesized that VC F7 inhibits cell proliferation and induces DNA damage and cell cycle arrest in ovarian cells through oxidative stress. To test our hypothesis, we extracted and fractionated VC leaves. The effects of VC F7 were tested in OVCAR-3 cells. Viability was assessed by the means of MTS assay. Cell morphology was analyzed by acridine orange and propidium iodide (AO/PI) dye using a fluorescent microscope. Oxidative stress biomarkers were evaluated by the means of lipid peroxidation, catalase, and glutathione peroxidase assays, respectively. The degree of DNA damage was assessed by comet assay. Cell cycle distribution was assessed by flow cytometry. Data generated from the MTS assay demonstrated that VC F7 inhibits the growth of OVCAR-3 cells in a dose-dependent manner, showing a gradual increase in the loss of viability in VC F7-treated cells. Data obtained from the AO/PI dye assessment revealed morphological alterations and exhibited characteristics such as loss of cellular membrane integrity, cell shrinkage, cell membrane damage, organelle breakdown, and detachment from the culture plate. We observed a significant increase (p < 0.05) in the levels of malondialdhyde (MDA) production in treated cells compared to the control. A gradual decrease in both catalase and glutathione peroxidase activities were observed in the treated cells compared to the control. Data obtained from the comet assay showed a significant increase (p < 0.05) in the percentages of DNA cleavage and comet tail length. The results of the flow cytometry analysis indicated VC F7 treatment caused cell cycle arrest at the S-phase checkpoint. Taken together, our results demonstrate that VC F7 exerts its anticancer activity by inhibiting cell proliferation, inducing DNA damage, and causing cell cycle arrest through oxidative stress in OVAR-3 cells. This finding suggests that VC F7 may be a potential alternative dietary agent for the prevention and/or treatment of ovarian cancer.

Breast tumor cell hybrids form spontaneously in vivo and contribute to breast tumor metastases

Cancer cell fusion was suggested as a mechanism of metastasis about a century ago. Since then, many additional modes of material transfer (i.e., tunneling nanotubes, and exosomes) to generate cell hybrids have been identified. However, studies documenting spontaneous tumor hybrid formation in vivo as a mechanism that enables metastasis are still lacking. Here, we tested whether spontaneous hybrid formation in vivo contributes to bona fide metastatic tumors. We first used single cell RNASeq to analyze the gene expression profile of spontaneously formed cancer cell-stromal hybrids, and results revealed that hybrids exhibit a clustering pattern that is distinct from either parental cell and suggestive of substantial diversity of individual hybrids. Despite the newly gained diversity, hybrids can retain expression of critical genes of each parental cell. To assess the biological impact of cancer cell hybrids in vivo, we transfected murine mammary tumor cells, isolated from FVB/N-Tg(MMTV-PyVT)634Mul/J mice (PyVT) with Cre recombinase prior to injection to the murine fat pad of FVB.129S6(B6)-Gt(ROSA)26Sor^tm1(Luc)Kael/J mice such that luciferase expression is induced with hybrid formation; luciferase expression was tracked for up to four months. We observed that hybrid formation occurs spontaneously in vivo and that a significantly higher number of hybrids reside in metastases compared to the primary tumor, supporting the possibility that hybrids can emerge from the primary tumor and proliferate to help create a new tumor at a distant site. Additional studies are now warranted to delineate the mechanisms of cancer cell hybrid transit to metastases since drugs to inhibit hybrid formation might prevent metastatic spread.

Anticancer Properties of Graviola (Annona muricata): A Comprehensive Mechanistic Review

Graviola (Annona muricata) is a small deciduous tropical evergreen fruit tree, belonging to the Annonaceae family, and is widely grown and distributed in tropical and subtropical regions around the world. The aerial parts of graviola have several functions: the fruits have been widely used as food confectionaries, while several preparations, especially decoctions of the bark, fruits, leaves, pericarp, seeds, and roots, have been extensively used in traditional medicine to treat multiple ailments including cancers by local communities in tropical Africa and South America. The reported therapeutic benefits of graviola against various human tumors and disease agents in in vitro culture and preclinical animal model systems are typically tested for their ability to specifically target the disease, while exerting little or no effect on normal cell viability. Over 212 phytochemical ingredients have been reported in graviola extracts prepared from different plant parts. The specific bioactive constituents responsible for the major anticancer, antioxidant, anti-inflammatory, antimicrobial, and other health benefits of graviola include different classes of annonaceous acetogenins (metabolites and products of the polyketide pathway), alkaloids, flavonoids, sterols, and others. This review summarizes the current understanding of the anticancer effects of A. muricata and its constituents on diverse cancer types and disease states, as well as efficacy and safety concerns. It also includes discussion of our current understanding of possible mechanisms of action, with the hope of further stimulating the development of improved and affordable therapies for a variety of ailments.

Graviola (Annona muricata) Exerts Anti-Proliferative, Anti-Clonogenic and Pro-Apoptotic Effects in Human Non-Melanoma Skin Cancer UW-BCC1 and A431 Cells In Vitro: Involvement of Hedgehog Signaling

Non-melanoma skin cancers (NMSCs) are the leading cause of skin cancer-related morbidity and mortality. Effective strategies are needed to control NMSC occurrence and progression. Non-toxic, plant-derived extracts have been shown to exert multiple anti-cancer effects. Graviola (Annona muricata), a tropical fruit-bearing plant, has been used in traditional medicine against multiple human diseases including cancer. The current study investigated the effects of graviola leaf and stem extract (GLSE) and its solvent-extracted fractions on two human NMSC cell lines, UW-BCC1 and A431. GLSE was found to: (i) dose-dependently suppress UW-BCC1 and A431 cell growth, motility, wound closure, and clonogenicity; (ii) induce G₀/G₁ cell cycle arrest by downregulating cyclin/cdk factors while upregulating cdk inhibitors, and (iii) induce apoptosis as evidenced by cleavage of caspases-3, -8 and PARP. Further, GLSE suppressed levels of activated hedgehog (Hh) pathway components Smo, Gli 1/2, and Shh while inducing SuFu. GLSE also decreased the expression of pro-apoptotic protein Bax while decreasing the expression of the anti-apoptotic protein Bcl-2. We determined that these activities were concentrated in an acetogenin/alkaloid-rich dichloromethane subfraction of GLSE. Our data identify graviola extracts and their constituents as promising sources for new chemopreventive and therapeutic agent(s) to be further developed for the control of NMSCs.

Moving Upwards: A Simple and Flexible In Vitro Three-dimensional Invasion Assay Protocol

Although 3D invasion assays have been developed, the challenge remains to study cells without affecting the integrity of their microenvironment. Traditional 3D assays such as the Boyden Chamber require that cells are displaced from the original culture location and moved to a new environment. Not only does this disrupt the cellular processes that are intrinsic to the microenvironment, but it often results in a loss of cells. These problems are especially challenging when dealing with cells that are either rare, or extremely sensitive to their microenvironment. Here, we describe the development of a 3D invasion assay that avoids both concerns. In this assay, cells are plated within a small well and an ECM matrix containing a chemoattractant is laid atop the cells. This requires no cell displacement, and allows the cells to invade upwards into the matrix. In this assay, cell invasion as well as cell morphology can be assessed within the collagen gel. Using this assay, we characterize the invasive capacity of rare and sensitive cells; the hybrid cells resulting from fusion between breast cancer cells MCF7 and mesenchymal/multipotent stem/stroma cells (MSCs).

An In Vitro Inverted Vertical Invasion Assay to Avoid Manipulation of Rare or Sensitive Cell Types

The ability to quantify cell migration and invasion is critical in the study of cancer metastasis. Current invasion assays, such as the Boyden Chamber, present difficulties in the measurement of the invasion of cells that are few in number and are intrinsically tied to the cell microenvironment. There exists a need for a three-dimensional invasion assay that is easily reproduced, accessible for most laboratories, and requires no displacement of cells from their original microenvironment. Here we present a simple design for an inverted vertical invasion assay able to assess the invasion capabilities of cells in a three dimensional, extracellular matrix-based environment without displacement from the original culture location. We used the assay to determine the migratory capacity of hybrids between mesenchymal/multipotent stem/stroma cells (MSCs) and breast cancer cells MCF7. These hybrids are formed reliably but rarely (1 in 1,000 cells) and for this reason require an invasion assay that does not involve extensive cell manipulation. Using this assay, we found that MSCs, breast cancer cells, and corresponding fusion products are able to migrate and invade through the extracellular matrix and that hybrids invade in a manner more similar to stromal cells than cancer cells. Thus, this assay can aid the study of the invasive capacity of both cancerous cells and associated fusion hybrids and could augment testing of therapeutic strategies to inhibit metastatic spread.

Cancer Cell Fusion: Mechanisms Slowly Unravel

Although molecular mechanisms and signaling pathways driving invasion and metastasis have been studied for many years, the origin of the population of metastatic cells within the primary tumor is still not well understood. About a century ago, Aichel proposed that cancer cell fusion was a mechanism of cancer metastasis. This hypothesis gained some support over the years, and recently became the focus of many studies that revealed increasing evidence pointing to the possibility that cancer cell fusion probably gives rise to the metastatic phenotype by generating widespread genetic and epigenetic diversity, leading to the emergence of critical populations needed to evolve resistance to the treatment and development of metastasis. In this review, we will discuss the clinical relevance of cancer cell fusion, describe emerging mechanisms of cancer cell fusion, address why inhibiting cancer cell fusion could represent a critical line of attack to limit drug resistance and to prevent metastasis, and suggest one new modality for doing so.

Loss of Stromal IMP1 Promotes a Tumorigenic Microenvironment in the Colon

The colon tumor microenvironment is becoming increasingly recognized as a complex but central player in the development of many cancers. Previously, we identified an oncogenic role for the mRNA-binding protein IMP1 (IGF2BP1) in the epithelium during colon tumorigenesis. In the current study, we reveal the contribution of stromal IMP1 in the context of colitis-associated colon tumorigenesis. Interestingly, stromal deletion of Imp1 (Dermo1Cre;Imp1(LoxP/LoxP), or Imp1(ΔMes)) in the azoxymethane/dextran sodium sulfate (AOM/DSS) model of colitis-associated cancer resulted in increased tumor numbers of larger size and more advanced histologic grade than controls. In addition, Imp1(ΔMes) mice exhibited a global increase in protumorigenic microenvironment factors, including enhanced inflammation and stromal components. Evaluation of purified mesenchyme from AOM/DSS-treated Imp1(ΔMes) mice demonstrated an increase in hepatocyte growth factor (HGF), which has not been associated with regulation via IMP1. Genetic knockdown of Imp1 in human primary fibroblasts confirmed an increase in HGF with Imp1 loss, demonstrating a specific, cell-autonomous role for Imp1 loss to increase HGF expression. Taken together, these data demonstrate a novel tumor-suppressive role for IMP1 in colon stromal cells and underscore an exquisite, context-specific function for mRNA-binding proteins, such as IMP1, in disease states.

IMPLICATIONS

The tumor-suppressive role of stromal IMP1 and its ability to modulate protumorigenic factors suggest that IMP1 status is important for the initiation and growth of epithelial tumors.

Apoptosis-induced cancer cell fusion: a mechanism of breast cancer metastasis

Although cancer cell fusion has been suggested as a mechanism of cancer metastasis, the underlying mechanisms defining this process are poorly understood. In a recent study, apoptotic cells were newly identified as a type of cue that induces signaling via phosphatidylserine receptors to promote fusion of myoblasts. The microenvironment of breast tumors is often hypoxic, and because apoptosis is greatly increased in hypoxic conditions, we decided to investigate whether the mechanism of breast cancer cell fusion with mesenchymal stem/multipotent stromal cells (MSCs) involves apoptosis. We used a powerful tool for identification and tracking of hybrids based on bimolecular fluorescence complementation (BiFC) and found that breast cancer cells fused spontaneously with MSCs. This fusion was significantly enhanced with hypoxia and signaling associated with apoptotic cells, especially between nonmetastatic breast cancer cells and MSCs. In addition, the hybrids showed a significantly higher migratory capacity than did the parent cells. Taken together, these findings describe a mechanism by which hypoxia-induced apoptosis stimulates fusion between MSCs and breast tumor cells resulting in hybrids with an enhanced migratory capacity that may enable their dissemination to distant sites or metastases. In the long run, this study may provide new strategies for developing novel drugs for preventing cancer metastasis.

Prodifferentiation, anti-inflammatory and antiproliferative effects of delphinidin, a dietary anthocyanidin, in a full-thickness three-dimensional reconstituted human skin model of psoriasis

BACKGROUND

Psoriasis is a chronic inflammatory disorder of skin and joints for which conventional treatments that are effective in clearing the moderate-to-severe disease are limited due to long-term safety issues. This necessitates exploring the usefulness of botanical agents for treating psoriasis. We previously showed that delphinidin, a diet-derived anthocyanidin endowed with antioxidant and anti-inflammatory properties, induces normal epidermal keratinocyte differentiation and suggested its possible usefulness for the treatment of psoriasis [1].

OBJECTIVES

To investigate the effect of delphinidin (0-20 μM; 2-5 days) on psoriatic epidermal keratinocyte differentiation, proliferation and inflammation using a three-dimensional reconstructed human psoriatic skin equivalent (PSE) model.

METHODS

PSEs and normal skin equivalents (NSEs) established on fibroblast-contracted collagen gels with respective psoriatic and normal keratinocytes and treated with/without delphinidin were analyzed for histology, expression of markers of differentiation, proliferation and inflammation using histomorphometry, immunoblotting, immunochemistry, qPCR and cultured supernatants for cytokine with a Multi-Analyte ELISArray Kit.

RESULTS

Our data show that treatment of PSE with delphinidin induced (1) cornification without affecting apoptosis and (2) the mRNA and protein expression of markers of differentiation (caspase-14, filaggrin, loricrin, involucrin). It also decreased the expression of markers of proliferation (Ki67 and proliferating cell nuclear antigen) and inflammation (inducible nitric oxide synthase and antimicrobial peptides S100A7-psoriasin and S100A15-koebnerisin, which are often induced in psoriatic skin). ELISArray showed increased release of psoriasis-associated keratinocyte-derived proinflammatory cytokines in supernatants of the PSE cultures, and this increase was significantly suppressed by delphinidin.

CONCLUSIONS

These observations provide a rationale for developing delphinidin for the management of psoriasis.

IMP1 promotes tumor growth, dissemination and a tumor-initiating cell phenotype in colorectal cancer cell xenografts

Igf2 mRNA binding protein 1 (IMP1, CRD-BP, ZBP-1) is a messenger RNA binding protein that we have shown previously to regulate colorectal cancer (CRC) cell growth in vitro. Furthermore, increased IMP1 expression correlates with enhanced metastasis and poor prognosis in CRC patients. In the current study, we sought to elucidate IMP1-mediated functions in CRC pathogenesis in vivo. Using CRC cell xenografts, we demonstrate that IMP1 overexpression promotes xenograft tumor growth and dissemination into the blood. Furthermore, intestine-specific knockdown of Imp1 dramatically reduces tumor number in the Apc (Min/+) mouse model of intestinal tumorigenesis. In addition, IMP1 knockdown xenografts exhibit a reduced number of tumor cells entering the circulation, suggesting that IMP1 may directly modulate this early metastatic event. We further demonstrate that IMP1 overexpression decreases E-cadherin expression, promotes survival of single tumor cell-derived colonospheres and promotes enrichment and maintenance of a population of CD24+CD44+ cells, signifying that IMP1 overexpressing cells display evidence of loss of epithelial identity and enhancement of a tumor-initiating cell phenotype. Taken together, these findings implicate IMP1 as a modulator of tumor growth and provide evidence for a novel role of IMP1 in early events in CRC metastasis.

Transcriptional Regulation of CRD-BP by c-myc: Implications for c-myc Functions

The coding region determinant binding protein, CRD-BP, is a multifunctional RNA binding protein involved in different processes such as mRNA turnover, translation control, and localization. It is mostly expressed in fetal and neonatal tissues, where it regulates many transcripts essential for normal embryonic development. CRD-BP is scarce or absent in normal adult tissues but reactivated and/or overexpressed in various neoplastic and preneoplastic tumors and in most cell lines. Its expression has been associated with the most aggressive form of some cancers. CRD-BP is an important regulator of different genes including a variety of oncogenes or proto-oncogenes (c-myc, β-TrCP1, GLI1, etc.). Regulation of CRD-BP expression is critical for proper control of its targets as its overexpression may play an important role in abnormal cell proliferation, suppression of apoptosis, invasion, and metastasis. Molecular bases of the regulatory mechanisms governing CRD-BP expression are still not completely elucidated. In this article, we have identified c-myc as a novel transcriptional regulator of CRD-BP. We show that c-myc binds to CRD-BP promoter and induces its transcription. This induction of CRD-BP expression contributes to the role of c-myc in the regulation of translation, increase in cell size, and acceleration of cell cycle progression via a mechanism involving upregulation of β-TrCP1 levels and activities and accelerated degradation of PDCD4.

IMP-1 displays cross-talk with K-Ras and modulates colon cancer cell survival through the novel proapoptotic protein CYFIP2

Insulin-like growth factor 2 mRNA-binding protein-1 (IMP-1) is an oncofetal protein that binds directly to and stabilizes oncogenic c-Myc and regulates, in turn, its posttranscriptional expression and translation. In contrast to normal adult tissue, IMP-1 is reexpressed and/or overexpressed in human cancers. We show that knockdown of c-Myc in human colon cancer cell lines increases the expression of mature let-7 miRNA family members and downregulates several of its mRNA targets: IMP-1, Cdc34, and K-Ras. We further show that loss of IMP-1 inhibits Cdc34, Lin-28B, and K-Ras, suppresses SW-480 cell proliferation and anchorage-independent growth, and promotes caspase- and lamin-mediated cell death. We also found that IMP-1 binds to the coding region and 3'UTR of K-Ras mRNA. RNA microarray profiling and validation by reverse transcription PCR reveals that the p53-inducible proapoptotic protein CYFIP2 is upregulated in IMP-1 knockdown SW480 cells, a novel finding. We also show that overexpression of IMP-1 increases c-Myc and K-Ras expression and LIM2405 cell proliferation. Furthermore, we show that loss of IMP-1 induces Caspase-3- and PARP-mediated apoptosis, and inhibits K-Ras expression in SW480 cells, which is rescued by CYFIP2 knockdown. Importantly, analysis of 228 patients with colon cancers reveals that IMP-1 is significantly upregulated in differentiated colon tumors (P ≤ 0.0001) and correlates with K-Ras expression (r = 0.35, P ≤ 0.0001) relative to adjacent normal mucosa. These findings indicate that IMP-1, interrelated with c-Myc, acts upstream of K-Ras to promote survival through a novel mechanism that may be important in colon cancer pathogenesis.

Wnt signaling stimulates transcriptional outcome of the Hedgehog pathway by stabilizing GLI1 mRNA

Wnt and Hedgehog signaling pathways play central roles in embryogenesis, stem cell maintenance, and tumorigenesis. However, the mechanisms by which these two pathways interact are not well understood. Here, we identified a novel mechanism by which Wnt signaling pathway stimulates the transcriptional output of Hedgehog signaling. Wnt/beta-catenin signaling induces expression of an RNA-binding protein, CRD-BP, which in turn binds and stabilizes GLI1 mRNA, causing an elevation of GLI1 expression and transcriptional activity. The newly described mode of regulation of GLI1 seems to be important to several functions of Wnt, including survival and proliferation of colorectal cancer cells.

CRD-BP protects the coding region of betaTrCP1 mRNA from miR-183-mediated degradation

miRNAs are largely known to base pair with the 3'UTR of target mRNAs, downregulating their stability and translation. mRNA of betaTrCP1 ubiquitin ligase is very unstable, but unlike the majority of mRNAs where 3'UTR determines the rate of mRNA turnover, betaTrCP1 mRNA contains cis-acting destabilizing elements within its coding region. Here we show that degradation of mRNA of betaTrCP1 is miRNA dependent and identify miR-183 as a microRNA that interacts with the coding region of betaTrCP1 mRNA. Argonaute2 interacts with the same region of betaTrCP1 mRNA in an miR-183-dependent manner. Inhibition of miR-183 function or disruption of the miR-183-binding site stabilizes betaTrCP1 mRNA and elevates betaTrCP1 levels, resulting in activation of the SCF(betaTrCP) E3 ubiquitin ligase. We previously showed that the RNA-binding protein CRD-BP binds to the coding region of betaTrCP1 mRNA and stabilizes it. Here we demonstrate that CRD-BP prevents degradation of betaTrCP1 mRNA by attenuating its miR-183-dependent interaction with Ago2.

CRD-BP mediates stabilization of betaTrCP1 and c-myc mRNA in response to beta-catenin signalling

Although constitutive activation of beta-catenin/Tcf signalling is implicated in the development of human cancers, the mechanisms by which the beta-catenin/Tcf pathway promotes tumorigenesis are incompletely understood. Messenger RNA turnover has a major function in regulating gene expression and is responsive to developmental and environmental signals. mRNA decay rates are dictated by cis-acting elements within the mRNA and by trans-acting factors, such as RNA-binding proteins (reviewed in refs 2, 3). Here we show that beta-catenin stabilizes the mRNA encoding the F-box protein betaTrCP1, and identify the RNA-binding protein CRD-BP (coding region determinant-binding protein) as a previously unknown target of beta-catenin/Tcf transcription factor. CRD-BP binds to the coding region of betaTrCP1 mRNA. Overexpression of CRD-BP stabilizes betaTrCP1 mRNA and elevates betaTrCP1 levels (both in cells and in vivo), resulting in the activation of the Skp1-Cullin1-F-box protein (SCF)(betaTrCP) E3 ubiquitin ligase and in accelerated turnover of its substrates including IkappaB and beta-catenin. CRD-BP is essential for the induction of both betaTrCP1 and c-Myc by beta-catenin signalling in colorectal cancer cells. High levels of CRD-BP that are found in primary human colorectal tumours exhibiting active beta-catenin/Tcf signalling implicates CRD-BP induction in the upregulation of betaTrCP1, in the activation of dimeric transcription factor NF-kappaB and in the suppression of apoptosis in these cancers.

Genetic analysis of wild-isolated Neurospora crassa strains identified as dominant suppressors of repeat-induced point mutation

Repeat-induced point mutation (RIP) in Neurospora results in inactivation of duplicated DNA sequences. RIP is thought to provide protection against foreign elements such as retrotransposons, only one of which has been found in N. crassa. To examine the role of RIP in nature, we have examined seven N. crassa strains, identified among 446 wild isolates scored for dominant suppression of RIP. The test system involved a small duplication that targets RIP to the easily scorable gene erg-3. We previously showed that RIP in a small duplication is suppressed if another, larger duplication is present in the cross, as expected if the large duplication competes for the RIP machinery. In two of the strains, RIP suppression was associated with a barren phenotype--a characteristic of Neurospora duplications that is thought to result in part from a gene-silencing process called meiotic silencing by unpaired DNA (MSUD). A suppressor of MSUD (Sad-1) was shown not to prevent known large duplications from impairing RIP. Single-gene duplications also can be barren but are too short to suppress RIP. RIP suppression in strains that were not barren showed inheritance that was either simple Mendelian or complex. Adding copies of the LINE-like retrotransposon Tad did not affect RIP efficiency.

Evidence for dominant suppression of repeat-induced point mutation (RIP) in crosses with the wild-isolated Neurospora crassa strains Sugartown and Adiopodoume-7

A convenient assay to score repeat-induced point mutation (RIP) in Neurospora employs the erg-3 locus as a mutagenesis target. Using this assay we screened 132 wild-isolated Neurospora crassa strains for ability to dominantly suppress RIP. RIP was exceptionally inefficient in crosses with the wild isolates Sugartown (P0854) and Adiopodoume-7 (P4305), thereby suggesting the presence of dominant RIP suppressors in these strains. In other experiments, we found no evidence for dominant RIP suppression by the Spore killer haplotypes Sk-2 and Sk-3.

Repeat-induced point mutation (RIP) in crosses with wild-isolated strains of Neurospora crassa: evidence for dominant reduction of RIP

Seventy-one wild-isolated strains of Neurospora crassa were examined for their ability to support repeat-induced point mutation (RIP) in the erg-3 locus. RIP was exceptionally inefficient but detectable in crosses with the strain FGSC 430 from Adiopodoume, Ivory Coast. We could find no consistent differences in ascospore yields when wild isolates identified as "low-RIP" or "high-RIP" strains were crossed with strains bearing the segmental duplication Dp(IIIR > [I; II])AR17. This suggested that RIP may not be responsible for the barren phenotype of crosses involving segmental duplication strains.