Oncogene ect2 is related to regulators of small GTP-binding proteins

Small interfering RNA-induced inhibition of epithelial cell transforming sequence 2 suppresses the proliferation, migration and invasion of osteosarcoma cells

Osteosarcoma (OS) is the most common malignant tumor in bones. Although the five-year survival rate has improved to ∼60% for patients without metastasis, the prognosis remains poor for patients with metastatic OS. Epithelial cell transforming sequence 2 (ECT2) has been shown to act as an oncogene in human malignancies. More recently, ETC2 was shown to be involved in the development and progression of OS; however, the detailed role of ECT2 in the regulation of cellular biological processes in OS cells remains largely unknown. Therefore, it was investigated in the present study. It was found that the expression of ECT2 was notably increased in OS tissues when compared with that in matched normal adjacent tissues. Furthermore, it was established that the downregulation of ECT2 induced by transfection with ECT2-specific small interfering RNA effectively inhibited OS cell proliferation and induced cell apoptosis. Further investigation revealed that the inhibition of ECT2 expression suppressed OS cell migration and invasion, indicating that the overexpression of ECT2 promotes OS cell migration and invasion, while. In addition, western blotting results indicated that matrix metalloproteinases 2 and 9 may be involved in the ECT2-mediated OS cell invasion. In conclusion, the current study suggested that ECT2 acted as an oncogene in OS, and it may become a promising therapeutic target for the prevention and treatment of OS.

RhoGTPases - A novel link between cytoskeleton organization and cisplatin resistance

For more than three decades, platinum compounds have been the first line treatment for a wide spectrum of solid tumors. Yet, cisplatin resistance is a major impediment in cancer therapy, and deciphering the mechanisms underlying chemoresistance is crucial for the development of novel therapies with enhanced efficacy. The Rho subfamily of small GTPases plays a significant role in cancer progression, and a growing body of evidence points toward the involvement of these proteins in anticancer drug resistance, including cisplatin resistance. The cycling between active and inactive states, governed by the balance between their GEFs, GAPs and GDIs, RhoGTPases, acts as molecular switches with a pivotal role in actin cytoskeleton organization. The Rho subfamily of proteins is involved in many key cellular processes including adhesion, vesicular trafficking, proliferation, survival, cell morphology and cell-matrix interactions. Although RhoA, RhoB and RhoC are highly homologous and share some upstream regulators and downstream effectors, they each have different roles in cancer progression and chemoresistance. While RhoA and RhoC are upregulated in many tumors and can stimulate transformation, RhoB appears to exhibit tumor suppressor characteristics with proapoptotic effects. In the current review, we discuss the role of Rho subfamily of proteins in cancer, and focus on their involvement in intrinsic and acquired drug resistance.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

Crystal structure of triple-BRCT-domain of ECT2 and insights into the binding characteristics to CYK-4

Homo sapiens ECT2 is a cell cycle regulator that plays critical roles in cytokinesis. ECT2 activity is restrained during interphase via intra-molecular interactions that involve its N-terminal triple-BRCT-domain and its C-terminal DH-PH domain. At anaphase, this self-inhibitory mechanism is relieved by Plk1-phosphorylated CYK-4, which directly engages the ECT2 BRCT domain. To provide a structural perspective for this auto-inhibitory property, we solved the crystal structure of the ECT2 triple-BRCT-domain. In addition, we systematically analyzed the interaction between the ECT2 BRCT domains with phospho-peptides derived from its binding partner CYK-4, and have identified Ser164 as the major phospho-residue that links CYK-4 to the second ECT2 BRCT domain.

ECT2 amplification and overexpression as a new prognostic biomarker for early-stage lung adenocarcinoma

Genetic abnormality in early-stage lung adenocarcinoma was examined to search for new prognostic biomarkers. Six in situ lung adenocarcinomas and nine small but invasive adenocarcinomas were examined by array-comparative genomic hybridization, and candidate genes of interest were screened. To examine gene abnormalities, 83 cases of various types of lung carcinoma were examined by quantitative real-time genomic PCR and immunohistochemistry. The results were then verified using another set of early-stage adenocarcinomas. Array-comparative genomic hybridization indicated frequent amplification at chromosome 3q26. Of the seven genes located in this region, we focused on the epithelial cell transforming sequence 2 (ECT2) oncogene, as ECT2 amplification was detected only in invasive adenocarcinoma, and not in in situ carcinoma. Quantitative PCR and immunohistochemistry analyses also detected overexpression of ECT2 in invasive adenocarcinoma, and this was correlated with both the Ki-67 labeling index and mitotic index. In addition, it was associated with disease-free survival and overall survival of patients with lung adenocarcinoma. These results were verified using another set of early-stage adenocarcinomas resected at another hospital. Abnormality of the ECT2 gene occurs at a relatively early stage of lung adenocarcinogenesis and would be applicable as a new biomarker for prognostication of patients with lung adenocarcinoma.

Centralspindlin assembly and 2 phosphorylations on MgcRacGAP by Polo-like kinase 1 initiate Ect2 binding in early cytokinesis

Cytokinesis is the final step of cell division which partitions genetic and cytosolic content into daughter cells. Failed cytokinesis causes polyploidy, genetic instability, and cancer. Kinases use phosphorylation to regulate the timing and location of the cytokinetic furrow. Polo-like kinase 1 (Plk1) is an essential mitotic kinase that triggers cytokinesis by phosphorylating MgcRacGAP to create a docking site for Ect2 at the central spindle. Ect2 binds to MgcRacGAP via its N-terminal BRCT domain (BRCA1 C-terminal), which docks at specific phosphorylated residues. Here we investigate the minimal Plk1-dependent phosphorylation sites required for cytokinesis onset. We demonstrate that phosphorylation of the major MgcRacGAP site, S157, is necessary but not sufficient to bind the Ect2 BRCT domain. Phosphorylation of an additional residue on MgcRacGAP at S164 is also required to elicit efficient binding. Surprisingly, BRCT binding additionally requires MKLP1 and its cognate interacting N-terminal domain of MgcRacGAP. Our findings indicate that central spindle assembly and 2 Plk1-dependent phosphorylations are required to establish efficient binding of the Ect2 BRCT in early cytokinesis. We propose that these requirements establish a high threshold to restrain premature or ectopic cytokinesis.

Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway

Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. We have discovered a new mechanism for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. Our results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.

Rho guanine nucleotide exchange factors: regulators of Rho GTPase activity in development and disease

The aberrant activity of Ras homologous (Rho) family small GTPases (20 human members) has been implicated in cancer and other human diseases. However, in contrast to the direct mutational activation of Ras found in cancer and developmental disorders, Rho GTPases are activated most commonly in disease by indirect mechanisms. One prevalent mechanism involves aberrant Rho activation via the deregulated expression and/or activity of Rho family guanine nucleotide exchange factors (RhoGEFs). RhoGEFs promote formation of the active GTP-bound state of Rho GTPases. The largest family of RhoGEFs is comprised of the Dbl family RhoGEFs with 70 human members. The multitude of RhoGEFs that activate a single Rho GTPase reflects the very specific role of each RhoGEF in controlling distinct signaling mechanisms involved in Rho activation. In this review, we summarize the role of Dbl RhoGEFs in development and disease, with a focus on Ect2 (epithelial cell transforming squence 2), Tiam1 (T-cell lymphoma invasion and metastasis 1), Vav and P-Rex1/2 (PtdIns(3,4,5)P3 (phosphatidylinositol (3,4,5)-triphosphate)-dependent Rac exchanger).

Aptamer-derived peptide inhibitors of Rho guanine nucleotide exchange factors

Small G proteins of the Rho family and their activators the guanine nucleotide exchange factors (RhoGEFs) regulate essential cellular functions and their deregulation has been associated with an amazing variety of human disorders, including cancer, inflammation, vascular diseases, and mental retardation. Rho GTPases and RhoGEFs therefore represent important targets for inhibition, not only in basic research but also for therapeutic purposes, and strategies to inhibit their function are actively being sought. Our lab has been very active in this field and has used the peptide aptamer technology to develop the first RhoGEF inhibitor, using the RhoGEF Trio as a model. Trio function has been described mainly in cell motility and axon growth in the nervous system via Rac1 GTPase activation, but recent findings suggest it to play also a role in the aggressive phenotype of various cancers, making it an attractive target for drug discovery. The object of this chapter is to demonstrate that targeting a RhoGEF using the peptide aptamer technology represents a valid and efficient approach to inhibit cellular processes in which Rho GTPase activity is upregulated. This is illustrated here by the first description of a peptide inhibitor of the oncogenic RhoGEF Tgat, TRIP(E32G), which is functional in vivo. On a long-term perspective, these peptide inhibitors can also serve as therapeutic tools or as guides for the discovery of small-molecule drugs, using an aptamer displacement screen.

Regulation of Drosophila mesoderm migration by phosphoinositides and the PH domain of the Rho GTP exchange factor Pebble

The Drosophila RhoGEF Pebble (Pbl) is required for cytokinesis and migration of mesodermal cells. In a screen for genes that could suppress migration defects in pbl mutants we identified the phosphatidylinositol phosphate (PtdInsP) regulator pi5k59B. Genetic interaction tests with other PtdInsP regulators suggested that PtdIns(4,5)P2 levels are important for mesoderm migration when Pbl is depleted. Consistent with this, the leading front of migrating mesodermal cells was enriched for PtdIns(4,5)P2. Given that Pbl contains a Pleckstrin Homology (PH) domain, a known PtdInsP-binding motif, we examined PtdInsP-binding of Pbl and the importance of the PH domain for Pbl function. In vitro lipid blot assays showed that Pbl binds promiscuously to PtdInsPs, with binding strength associated with the degree of phosphorylation. Pbl was also able to bind lipid vesicles containing PtdIns(4,5)P2 but binding was strongly reduced upon deletion of the PH domain. Similarly, in vivo, loss of the PH domain prevented localisation of Pbl to the cell cortex and severely affected several aspects of early mesoderm development, including flattening of the invaginated tube onto the ectoderm, extension of protrusions, and dorsal migration to form a monolayer. Pbl lacking the PH domain could still localise to the cytokinetic furrow, however, and cytokinesis failure was reduced in pbl(ΔPH) mutants. Taken together, our results support a model in which interaction of the PH-domain of Pbl with PtdIns(4,5)P2 helps localise it to the plasma membrane which is important for mesoderm migration.

Changes in Ect2 localization couple actomyosin-dependent cell shape changes to mitotic progression

As they enter mitosis, animal cells undergo profound actin-dependent changes in shape to become round. Here we identify the Cdk1 substrate, Ect2, as a central regulator of mitotic rounding, thus uncovering a link between the cell-cycle machinery that drives mitotic entry and its accompanying actin remodeling. Ect2 is a RhoGEF that plays a well-established role in formation of the actomyosin contractile ring at mitotic exit, through the local activation of RhoA. We find that Ect2 first becomes active in prophase, when it is exported from the nucleus into the cytoplasm, activating RhoA to induce the formation of a mechanically stiff and rounded metaphase cortex. Then, at anaphase, binding to RacGAP1 at the spindle midzone repositions Ect2 to induce local actomyosin ring formation. Ect2 localization therefore defines the stage-specific changes in actin cortex organization critical for accurate cell division.

Regulation of the DH-PH tandem of guanine nucleotide exchange factor for Rho GTPases by phosphoinositides

Rho GTPases act as molecular switches central in cellular processes such as cytoskeleton dynamics, migration, cell proliferation, growth or survival. Their activation is tightly regulated downstream of cell surface receptors by Guanine nucleotide Exchange Factors (GEFs), that are responsible for the specificity, the accuracy, and the spatial restriction of Rho GTPases response to extracellular cues. Because there is about four time more RhoGEFs that Rho GTPases, and GEFs do not always show a strict specificity for GTPases, it is clear that their regulation depends on specific interactions with the subcellular environment. RhoGEFs bear a peculiar structure, highly conserved though evolution, consisting of a DH-PH tandem, the DH (Dbl homology) domain being responsible for the exchange activity. The function of the PH (Pleckstrin homology) domain known to bind phosphoinositides, however, remains elusive, and reports are in many cases rather confusing. This review summarizes data on the regulation of RhoGEFs activity through interaction of the PH-associated DH domain with phosphoinositides which are considered as critical players in the spatial organization of major signaling pathways.

Control of protein signaling using a computationally designed GTPase/GEF orthogonal pair

Signaling pathways depend on regulatory protein-protein interactions; controlling these interactions in cells has important applications for reengineering biological functions. As many regulatory proteins are modular, considerable progress in engineering signaling circuits has been made by recombining commonly occurring domains. Our ability to predictably engineer cellular functions, however, is constrained by complex crosstalk observed in naturally occurring domains. Here we demonstrate a strategy for improving and simplifying protein network engineering: using computational design to create orthogonal (non-crossreacting) protein-protein interfaces. We validated the design of the interface between a key signaling protein, the GTPase Cdc42, and its activator, Intersectin, biochemically and by solving the crystal structure of the engineered complex. The designed GTPase (orthoCdc42) is activated exclusively by its engineered cognate partner (orthoIntersectin), but maintains the ability to interface with other GTPase signaling circuit components in vitro. In mammalian cells, orthoCdc42 activity can be regulated by orthoIntersectin, but not wild-type Intersectin, showing that the designed interaction can trigger complex processes. Computational design of protein interfaces thus promises to provide specific components that facilitate the predictable engineering of cellular functions.

Rac signaling in breast cancer: a tale of GEFs and GAPs

Rac GTPases, small G-proteins widely implicated in tumorigenesis and metastasis, transduce signals from tyrosine-kinase, G-protein-coupled receptors (GPCRs), and integrins, and control a number of essential cellular functions including motility, adhesion, and proliferation. Deregulation of Rac signaling in cancer is generally a consequence of enhanced upstream inputs from tyrosine-kinase receptors, PI3K or Guanine nucleotide Exchange Factors (GEFs), or reduced Rac inactivation by GTPase Activating Proteins (GAPs). In breast cancer cells Rac1 is a downstream effector of ErbB receptors and mediates migratory responses by ErbB1/EGFR ligands such as EGF or TGFα and ErbB3 ligands such as heregulins. Recent advances in the field led to the identification of the Rac-GEF P-Rex1 as an essential mediator of Rac1 responses in breast cancer cells. P-Rex1 is activated by the PI3K product PIP3 and Gβγ subunits, and integrates signals from ErbB receptors and GPCRs. Most notably, P-Rex1 is highly overexpressed in human luminal breast tumors, particularly those expressing ErbB2 and estrogen receptor (ER). The P-Rex1/Rac signaling pathway may represent an attractive target for breast cancer therapy.

The ect2 rho Guanine nucleotide exchange factor is essential for early mouse development and normal cell cytokinesis and migration

Ect2 is a member of the human Dbl family of guanine nucleotide exchange factors (RhoGEFs) that serve as activators of Rho family small GTPases. Although Ect2 is one of at least 25 RhoGEFs that can activate the RhoA small GTPase, cell culture studies using established cell lines determined that Ect2 is essential for mammalian cell cytokinesis and proliferation. To address the function of Ect2 in normal mammalian development, we performed gene targeting to generate Ect2 knockout mice. The heterozygous Ect2(+/-) mice showed normal development and life span, indicating that Ect2 haplodeficiency was not deleterious for development or growth. In contrast, Ect2(-/-) embryos were not found at birth or postimplantation stages. Ect2(-/-) blastocysts were recovered at embryonic day 3.5 but did not give rise to viable outgrowths in culture, indicating that Ect2 is required for peri-implantation development. To further assess the importance of Ect2 in normal cell physiology, we isolated primary fibroblasts from Ect2(fl/fl) embryos (MEFs) and ablated Ect2 using adenoviral delivery of Cre recombinase. We observed a significant increase in multinucleated cells and accumulation of cells in G2/M phase, consistent with a role for Ect2 in cytokinesis. Ect2 deficiency also caused enlargement of the cytoplasm and impaired cell migration. Finally, although Ect2-dependent activation of RhoA has been implicated in cytokinesis, Ect2 can also activate Rac1 and Cdc42 to cause growth transformation. Surprisingly, ectopic expression of constitutively activated RhoA, Rac1, or Cdc42, known substrates of Ect2, failed to phenocopy Ect2 and did not rescue the defect in cytokinesis caused by loss of Ect2. In summary, our results establish the unique role of Ect2 in development and normal cell proliferation.

APC(cdh1) mediates degradation of the oncogenic Rho-GEF Ect2 after mitosis

Background

Besides regulation of actin cytoskeleton-dependent functions, Rho GTPase pathways are essential to cell cycle progression and cell division. Rho, Rac and Cdc42 regulate G1 to S phase progression and are involved in cytokinesis. RhoA GDP/GTP cycling is required for normal cytokinesis and recent reports have shown that the exchange factor Ect2 and the GTPase activating protein MgcRacGAP regulate RhoA activity during mitosis. We previously showed that the transcription factors E2F1 and CUX1 regulate expression of MgcRacGAP and Ect2 as cells enter S-phase.

Methodology/Principal Findings

We now report that Ect2 is subject to proteasomal degradation after mitosis, following ubiquitination by the APC/C complex and its co-activator Cdh1. A proper nuclear localization of Ect2 is necessary for its degradation. APC-Cdh1 assembles K11-linked poly-ubiquitin chains on Ect2, depending upon a stretch of ∼25 amino acid residues that contain a bi-partite NLS, a conventional D-box and two TEK-like boxes. Site-directed mutagenesis of target sequences generated stabilized Ect2 proteins. Furthermore, such degradation-resistant mutants of Ect2 were found to activate RhoA and subsequent signalling pathways and are able to transform NIH3T3 cells.

Conclusions/Significance

Our results identify Ect2 as a bona fide cell cycle-regulated protein and suggest that its ubiquitination-dependent degradation may play an important role in RhoA regulation at the time of mitosis. Our findings raise the possibility that the overexpression of Ect2 that has been reported in some human tumors might result not only from deregulated transcription, but also from impaired degradation.

Lip, a human gene detected by transfection of DNA from a human liposarcoma encodes a protein with homology to regulators of small g proteins

Purpose/Method. Transfection experiments have been used to identify activated oncogenes in a wide variety of tumour types. Here we describe the use of transfection experiments utilizing DNA from a human pleomorphic liposarcoma to identify a novel gene, designated lip which maps to chromosome 19.Results. lip was expressed in all sarcoma cell lines examined and a wide variety of normal tissues. Sequencing of cDNAs prepared from transcripts of the normal lip gene indicates that lip is predicted to encode a 966 amino acid protein with a region of homology to proteins such as vav, dbl, lbc and ect-2 which act as GDP-GTP exchange factors for the RAS superfamily of small GTP-binding proteins, and the N-terminal 830 amino acids are identical to the recently identified gene p115-RhoGEF, an exchange factor for RHOA. In transfectants, lip has undergone a rearrangement which results in C-terminal truncation of the predicted LIP protein. However, we failed to detect this alteration in the primary liposarcoma used in the original transfection experiments, or in other sarcoma specimens examined.Discussion. When considered together, these observations suggest that transforming lip sequences represent an alternatively spliced form of p115-RhoGEF that is activated for transformation by C-terminal truncation during transfection, and is not widely involved in sarcoma development.

Identification of novel Ras-cooperating oncogenes in Drosophila melanogaster: a RhoGEF/Rho-family/JNK pathway is a central driver of tumorigenesis

We have shown previously that mutations in the apico-basal cell polarity regulators cooperate with oncogenic Ras (Ras(ACT)) to promote tumorigenesis in Drosophila melanogaster and mammalian cells. To identify novel genes that cooperate with Ras(ACT) in tumorigenesis, we carried out a genome-wide screen for genes that when overexpressed throughout the developing Drosophila eye enhance Ras(ACT)-driven hyperplasia. Ras(ACT)-cooperating genes identified were Rac1 Rho1, RhoGEF2, pbl, rib, and east, which encode cell morphology regulators. In a clonal setting, which reveals genes conferring a competitive advantage over wild-type cells, only Rac1, an activated allele of Rho1 (Rho1(ACT)), RhoGEF2, and pbl cooperated with Ras(ACT), resulting in reduced differentiation and large invasive tumors. Expression of RhoGEF2 or Rac1 with Ras(ACT) upregulated Jun kinase (JNK) activity, and JNK upregulation was essential for cooperation. However, in the whole-tissue system, upregulation of JNK alone was not sufficient for cooperation with Ras(ACT), while in the clonal setting, JNK upregulation was sufficient for Ras(ACT)-mediated tumorigenesis. JNK upregulation was also sufficient to confer invasive growth of Ras(V12)-expressing mammalian MCF10A breast epithelial cells. Consistent with this, HER2(+) human breast cancers (where human epidermal growth factor 2 is overexpressed and Ras signaling upregulated) show a significant correlation with a signature representing JNK pathway activation. Moreover, our genetic analysis in Drosophila revealed that Rho1 and Rac are important for the cooperation of RhoGEF2 or Pbl overexpression and of mutants in polarity regulators, Dlg and aPKC, with Ras(ACT) in the whole-tissue context. Collectively our analysis reveals the importance of the RhoGEF/Rho-family/JNK pathway in cooperative tumorigenesis with Ras(ACT).

Autoinhibition of GEF activity in Intersectin 1 is mediated by the short SH3-DH domain linker

Intersectin 1L (ITSN1L) acts as a specific guanine nucleotide exchange factor (GEF) for the small guanine nucleotide binding protein Cdc42 via its C-terminal DH domain. Interestingly, constructs of ITSN1L that comprise additional domains, for instance the five SH3 domains amino-terminal of the DH domain, were shown to be inhibited in their exchange factor activity. Here, we investigate the inhibitory mechanism of ITSN1L in detail and identify a novel short amino acid motif which mediates autoinhibition. We found this motif to be located in the linker region between the SH3 domains and the DH domain, and we show that within this motif W1221 acts as key residue in establishing the inhibitory interaction. This assigns ITSN1L to a growing class of GEFs that are regulated by a short amino acid motif inhibiting GEF activity by an intramolecular interaction. Moreover, we quantify the interaction between the ITSN1L SH3 domains and the Cdc42 effector N-WASP using fluorescence anisotropy binding experiments. As the SH3 domains are not involved in autoinhibition, binding of N-WASP does not release inhibition of nucleotide exchange activity in kinetic experiments, in contrast to earlier observations.

The nuclear guanine nucleotide exchange factors Ect2 and Net1 regulate RhoB-mediated cell death after DNA damage

Commonly used antitumor treatments, including radiation and chemotherapy, function by damaging the DNA of rapidly proliferating cells. However, resistance to these agents is a predominant clinical problem. A member of the Rho family of small GTPases, RhoB has been shown to be integral in mediating cell death after ionizing radiation (IR) or other DNA damaging agents in Ras-transformed cell lines. In addition, RhoB protein expression increases after genotoxic stress, and loss of RhoB expression causes radio- and chemotherapeutic resistance. However, the signaling pathways that govern RhoB-induced cell death after DNA damage remain enigmatic. Here, we show that RhoB activity increases in human breast and cervical cancer cell lines after treatment with DNA damaging agents. Furthermore, RhoB activity is necessary for DNA damage-induced cell death, as the stable loss of RhoB protein expression using shRNA partially protects cells and prevents the phosphorylation of c-Jun N-terminal kinases (JNKs) and the induction of the pro-apoptotic protein Bim after IR. The increase in RhoB activity after genotoxic stress is associated with increased activity of the nuclear guanine nucleotide exchange factors (GEFs), Ect2 and Net1, but not the cytoplasmic GEFs p115 RhoGEF or Vav2. Importantly, loss of Ect2 and Net1 via siRNA-mediated protein knock-down inhibited IR-induced increases in RhoB activity, reduced apoptotic signaling events, and protected cells from IR-induced cell death. Collectively, these data suggest a mechanism involving the nuclear GEFs Ect2 and Net1 for activating RhoB after genotoxic stress, thereby facilitating cell death after treatment with DNA damaging agents.

Clinical validation of colorectal cancer biomarkers identified from bioinformatics analysis of public expression data

PURPOSE

Identification of novel biomarkers of cancer is important for improved diagnosis, prognosis, and therapeutic intervention. This study aimed to identify marker genes of colorectal cancer (CRC) by combining bioinformatics analysis of gene expression data and validation experiments using patient samples and to examine the potential connection between validated markers and the established oncogenes such as c-Myc and K-ras.

EXPERIMENTAL DESIGN

Publicly available data from GenBank and Oncomine were meta-analyzed leading to 34 candidate marker genes of CRC. Multiple case-matched normal and tumor tissues were examined by RT-PCR for differential expression, and 9 genes were validated as CRC biomarkers. Statistical analyses for correlation with major clinical parameters were carried out, and RNA interference was used to examine connection with major oncogenes.

RESULTS

We show with high confidence that 9 (ECT2, ETV4, DDX21, RAN, S100A11, RPS4X, HSPD1, CKS2, and C9orf140) of the 34 candidate genes are expressed at significantly elevated levels in CRC tissues compared to normal tissues. Furthermore, high-level expression of RPS4X was associated with nonmucinous cancer cell type and that of ECT2 with lack of lymphatic invasion while upregulation of CKS2 was correlated with early tumor stage and lack of family history of CRC. We also demonstrate that RPS4X and DDX21 are regulatory targets of c-Myc and ETV4 is downstream to K-ras signaling.

CONCLUSIONS

We have identified multiple novel biomarkers of CRC. Further analyses of their function and connection to signaling pathways may reveal potential value of these biomarkers in diagnosis, prognosis, and treatment of CRC.

Oncogenic activity of Ect2 is regulated through protein kinase C iota-mediated phosphorylation

The Rho GTPase guanine nucleotide exchange factor Ect2 is genetically and biochemically linked to the PKCι oncogene in non-small cell lung cancer (NSCLC). Ect2 is overexpressed and mislocalized to the cytoplasm of NSCLC cells where it binds the oncogenic PKCι-Par6 complex, leading to activation of the Rac1 small GTPase. Here, we identify a previously uncharacterized phosphorylation site on Ect2, threonine 328, that serves to regulate the oncogenic activity of Ect2 in NSCLC cells. PKCι directly phosphorylates Ect2 at Thr-328 in vitro, and RNAi-mediated knockdown of either PKCι or Par6 leads to a decrease in phospho-Thr-328 Ect2, indicating that PKCι regulates Thr-328 Ect2 phosphorylation in NSCLC cells. Both wild-type Ect2 and a phosphomimetic T328D Ect2 mutant bind the PKCι-Par6 complex, activate Rac1, and restore transformed growth and invasion when expressed in NSCLC cells made deficient in endogenous Ect2 by RNAi-mediated knockdown. In contrast, a phosphorylation-deficient T328A Ect2 mutant fails to bind the PKCι-Par6 complex, activate Rac1, or restore transformation. Our data support a model in which PKCι-mediated phosphorylation regulates Ect2 binding to the oncogenic PKCι-Par6 complex thereby activating Rac1 activity and driving transformed growth and invasion.

Ras superfamily GEFs and GAPs: validated and tractable targets for cancer therapy?

There is now considerable and increasing evidence for a causal role for aberrant activity of the Ras superfamily of small GTPases in human cancers. These GTPases function as GDP-GTP-regulated binary switches that control many fundamental cellular processes. A common mechanism of GTPase deregulation in cancer is the deregulated expression and/or activity of their regulatory proteins, guanine nucleotide exchange factors (GEFs) that promote formation of the active GTP-bound state and GTPase-activating proteins (GAPs) that return the GTPase to its GDP-bound inactive state. In this Review, we assess the association of GEFs and GAPs with cancer and their druggability for cancer therapeutics.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Cytokinesis and cancer: Polo loves ROCK'n' Rho(A)

Cytokinesis is the last step of the M (mitosis) phase, yet it is crucial for the faithful division of one cell into two. Cytokinesis failure is often associated with cancer. Cytokinesis can be morphologically divided into four steps: cleavage furrow initiation, cleavage furrow ingression, midbody formation and abscission. Molecular studies have revealed that RhoA as well as its regulators and effectors are important players to ensure a successful cytokinesis. At the same time, Polo-like kinase 1 (Plk1) is an important kinase that can target many substrates and carry out different functions during mitosis, including cytokinesis. Recent studies are beginning to unveil a closer tie between Plk1 and RhoA networks. More specifically, Plk1 phosphorylates the centralspindlin complex Cyk4 and MKLP1/CHO1, thus recruiting RhoA guanine nucleotide-exchange factor (GEF) Ect2 through its phosphopeptide-binding BRCT domains. Ect2 itself can be phosphorylated by Plk1 in vitro. Plk1 can also phosphorylate another GEF MyoGEF to regulate RhoA activity. Once activated, RhoA-GTP will activate downstream effectors, including ROCK1 and ROCK2. ROCK2 is among the proteins that associate with Plk1 Polo-binding domain (PBD) in a large proteomic screen, and Plk1 can phosphorylate ROCK2 in vitro. We review current understandings of the interplay between Plk1, RhoA proteins and other proteins (e.g., NudC, MKLP2, PRC1, CEP55) involved in cytokinesis, with particular emphasis of its clinical implications in cancer.

Identification of candidate growth promoting genes in ovarian cancer through integrated copy number and expression analysis

Ovarian cancer is a disease characterised by complex genomic rearrangements but the majority of the genes that are the target of these alterations remain unidentified. Cataloguing these target genes will provide useful insights into the disease etiology and may provide an opportunity to develop novel diagnostic and therapeutic interventions. High resolution genome wide copy number and matching expression data from 68 primary epithelial ovarian carcinomas of various histotypes was integrated to identify genes in regions of most frequent amplification with the strongest correlation with expression and copy number. Regions on chromosomes 3, 7, 8, and 20 were most frequently increased in copy number (>40% of samples). Within these regions, 703/1370 (51%) unique gene expression probesets were differentially expressed when samples with gain were compared to samples without gain. 30% of these differentially expressed probesets also showed a strong positive correlation (r≥0.6) between expression and copy number. We also identified 21 regions of high amplitude copy number gain, in which 32 known protein coding genes showed a strong positive correlation between expression and copy number. Overall, our data validates previously known ovarian cancer genes, such as ERBB2, and also identified novel potential drivers such as MYNN, PUF60 and TPX2.

Understanding cytokinesis failure

Cytokinesis is the final step in cell division. The process begins during chromosome segregation, when the ingressing cleavage furrow begins to partition the cytoplasm between the nascent daughter cells. The process is not completed until much later, however, when the final cytoplasmic bridge connecting the two daughter cells is severed. Cytokinesis is a highly ordered process, requiring an intricate interplay between cytoskeletal, chromosomal and cell cycle regulatory pathways. A surprisingly broad range of additional cellular processes are also important for cytokinesis, including protein and membrane trafficking, lipid metabolism, protein synthesis and signaling pathways. As a highly regulated, complex process, it is not surprising that cytokinesis can sometimes fail. Cytokinesis failure leads to both centrosome amplification and production of tetraploid cells, which may set the stage for the development of tumor cells. However, tetraploid cells are abundant components of some normal tissues including liver and heart, indicating that cytokinesis is physiologically regulated. In this chapter, we summarize our current understanding of the mechanisms of cytokinesis, emphasizing steps in the pathway that may be regulated or prone to failure. Our discussion emphasizes findings in vertebrate cells although we have attempted to highlight important contributions from other model systems.

Promoter-wide analysis of Smad4 binding sites in human epithelial cells

Smad4, the common partner Smad, is a key molecule in transforming growth factor-beta (TGF-beta) family signaling. Loss of Smad4 expression is found in several types of cancer, including pancreatic cancer and colon cancer, and is related to carcinogenesis. Here we identified Smad4 binding sites in the promoter regions of over 25 500 known genes by chromatin immunoprecipitation on a microarray (ChIP-chip) in HaCaT human keratinocytes. We identified 925 significant Smad4 binding sites. Approximately half of the identified sites overlapped the binding regions of Smad2 and Smad3 (Smad2/3, receptor-regulated Smads in TGF-beta signaling), while the rest of the regions appeared dominantly occupied by Smad4 even when a different identification threshold for Smad2/3 binding regions was used. Distribution analysis showed that Smad4 was found in the regions relatively distant from the transcription start sites, while Smad2/3 binding regions were more often present near the transcription start sites. Motif analysis also revealed that activator protein 1 (AP-1) sites were especially enriched in the sites common to Smad2/3 and Smad4 binding regions. In contrast, GC-rich motifs were enriched in Smad4-dominant binding regions. We further determined putative target genes of Smad4 whose expression was regulated by TGF-beta. Our findings revealed some general characteristics of Smad4 binding regions, and provide resources for examining the role of Smad4 in epithelial cells and cancer pathogenesis.

Ect2 links the PKCiota-Par6alpha complex to Rac1 activation and cellular transformation

Protein kinase Ciota (PKCiota) promotes non-small cell lung cancer (NSCLC) by binding to Par6alpha and activating a Rac1-Pak-Mek1,2-Erk1,2 signaling cascade. The mechanism by which the PKCiota-Par6alpha complex regulates Rac1 is unknown. Here we show that epithelial cell transforming sequence 2 (Ect2), a guanine nucleotide exchange factor for Rho family GTPases, is coordinately amplified and overexpressed with PKCiota in NSCLC tumors. RNA interference-mediated knockdown of Ect2 inhibits Rac1 activity and blocks transformed growth, invasion and tumorigenicity of NSCLC cells. Expression of constitutively active Rac1 (RacV12) restores transformation to Ect2-deficient cells. Interestingly, the role of Ect2 in transformation is distinct from its well-established role in cytokinesis. In NSCLC cells, Ect2 is mislocalized to the cytoplasm where it binds the PKCiota-Par6alpha complex. RNA interference-mediated knockdown of either PKCiota or Par6alpha causes Ect2 to redistribute to the nucleus, indicating that the PKCiota-Par6alpha complex regulates the cytoplasmic localization of Ect2. Our data indicate that Ect2 and PKCiota are genetically and functionally linked in NSCLC, acting to coordinately drive tumor cell proliferation and invasion through formation of an oncogenic PKCiota-Par6alpha-Ect2 complex.

Aptamer-derived peptides as potent inhibitors of the oncogenic RhoGEF Tgat

Guanine nucleotide exchange factors (GEFs) activate the Rho GTPases by accelerating their GDP/GTP exchange rate. Some RhoGEFs have been isolated based on their oncogenic potency, and strategies to inhibit their activity are therefore actively being sought. In this study we devise a peptide inhibitor screening strategy to target the GEF activity of Tgat, an oncogenic isoform of the RhoGEF Trio, based on random mutations of the Trio inhibitor TRIP alpha, which we previously isolated using a peptide aptamer screen. This identifies one peptide, TRIP(E32G), which specifically inhibits Tgat GEF activity in vitro and significantly reduces Tgat-induced RhoA activation and foci formation. Furthermore, subcutaneous injection of cells expressing Tgat and TRIP(E32G) into nude mice reduces the formation of Tgat-induced tumors. Our approach thus demonstrates that peptide aptamers are potent inhibitors that can be used to interfere with RhoGEF functions in vivo.

Constitutively active RhoA inhibits proliferation by retarding G(1) to S phase cell cycle progression and impairing cytokinesis

The actions of RhoA in cytoskeletal regulation have been extensively studied. RhoA also contributes to proliferation and oncogenic transformation by less well-characterized means. Elevated RhoA signalling has been associated with human cancer; through increased RhoA expression, mutation or elevated expression of activating Rho guanine-nucleotide exchange factors (GEFs), or from deletion or decreased expression of inhibitory Rho GTPase-activating proteins (GAPs). Unlike the Ras oncogene, constitutively-activated GTPase-deficient RhoA mutants have not been identified in tumours. To investigate the effects of active RhoA on proliferation, we generated Swiss3T3 cells that inducibly express wild-type RhoA or GTPase-deficient active V14RhoA. We found that V14RhoA inhibited cell proliferation by retarding entry into the DNA synthetic cell cycle phase and blocking successful completion of cytokinesis, resulting in an increased incidence of binucleate cells. These effects were associated with inhibition of mitogen-induced activation of the MAPK pathway, and suppression of several proteins involved in mitosis, including anillin, ECT2 and cyclin B1 which would be expected to result in reduced activation of endogenous RhoA at the cell equator. Accumulation of active RhoA protein in the midbody of cells in telophase was inhibited in V14RhoA-expressing cells, suggesting that RhoA inactivation must occur prior to re-activation. Defective cytokinesis was also associated with prominent actin structures in V14RhoA-expressing cells, which might be incompatible with equatorial furrowing. Using super-resolution imaging based on single-molecule switching, we have significantly improved the resolution of active RhoA in midbodies. These results indicate that constitutively-active RhoA antagonizes several cellular activities that contribute to proliferation, highlighting the importance for cycling between GTP/GDP-bound states.

CUX1 and E2F1 regulate coordinated expression of the mitotic complex genes Ect2, MgcRacGAP, and MKLP1 in S phase

Rho GTPases are critical for mitosis progression and completion of cytokinesis. During mitosis, the GDP/GTP cycle of Rho GTPases is regulated by the exchange factor Ect2 and the GTPase activating protein MgcRacGAP which associates with the kinesin MKLP1 in the centralspindlin complex. We report here that expression of Ect2, MgcRacGAP, and MKLP1 is tightly regulated during cell cycle progression. These three genes share similar cell cycle-related signatures within their promoter regions: (i) cell cycle gene homology region (CHR) sites located at -20 to +40 nucleotides of their transcription start sites that are required for repression in G(1), (ii) E2F binding elements, and (iii) tandem repeats of target sequences for the CUX1 transcription factor. CUX1 and E2F1 bind these three promoters upon S-phase entry, as demonstrated by chromatin immunoprecipitation, and regulate transcription of these genes, as established using promoter-luciferase reporter constructs and expression of activated or dominant negative transcription factors. Overexpression of either E2F1 or CUX1 increased the levels of the endogenous proteins whereas small interfering RNA knockdown of E2F1 or use of a dominant negative E2F1 reduced their expression levels. Thus, CUX1, E2F, and CHR elements provide the transcriptional controls that coordinate induction of Ect2, MgcRacGAP, and MKLP1 in S phase, leading to peak expression of these interacting proteins in G(2)/M, at the time they are required to regulate cytokinesis.

Cdc42 protein acts upstream of IQGAP1 and regulates cytokinesis in mouse oocytes and embryos

Cdc42 and Rac1 Rho family GTPases, and their interacting protein IQGAP1 are the key regulators of cell polarity. We examined the role of Cdc42 and IQGAP1 in establishing the polarity of mouse oocyte and regulation of meiotic and mitotic divisions. We showed that Cdc42 was localized on the microtubules of meiotic and mitotic spindle and in the cortex of mouse oocytes and cleaving embryos. IQGAP1 was present in the cytoplasm and cortex of growing and fully-grown oocytes. During maturation it disappeared from the cortex and during meiotic and mitotic cytokinesis it concentrated in the contractile ring. Toxin B inhibition of the binding activity of Cdc42 changed the localization of IQGAP1, inhibited emission of the first polar body, and caused disappearance of the cortical actin without affecting the migration of meiotic spindle. This indicates, that in maturing oocytes accumulation of cortical actin is not indispensable for spindle migration. In zygotes treated with toxin B actin cytoskeleton was rearranged and the first and/or subsequent cytokinesis were inhibited. Our results indicate that Cdc42 acts upstream of IQGAP1 and is involved in regulation of cytokinesis in mouse oocytes and cleaving embryos, rather than in establishing the polarity of the oocyte.

Nm23-H1 modulates the activity of the guanine exchange factor Dbl-1

Cytoskeleton rearrangement is necessary for tumor invasion and metastasis. Cellular molecules whose role is to regulate components of the cytoskeletal structure can dictate changes in cellular morphology. One of these molecules is the suppressor of tumor metastasis Nm23-H1. The level of Nm23-H1 expression has been linked to the invasiveness and metastatic potential of human cancers including melanoma and breast cancer. In this report, we demonstrate an interaction between the suppressor of tumor metastasis Nm23-H1, and Dbl-1, an oncoprotein which is associated with guanine exchange and belongs to a family of Guanine Exchange Factors (GEF). Nm23-H1 also was shown to bind pDbl which is the proto-oncoprotein of Dbl. Interestingly, the interaction between Nm23-H1 and Dbl-1 rescues the suppression of the cell motility activity Nm23-H1. Moreover, this interaction results in loss of the ability of the Dbl-1 oncoprotein to function as a GEF for the critical Rho-GTPase family member Cdc42. The loss of GTP loading onto Cdc42 resulted in a dramatic reduction in adhesion stimulated ruffles and suggests that Nm23-H1 can negatively regulate cell migration and tumor metastasis by modulating the activity of Cdc42 through direct interaction with Dbl-1.

Novel insights into FGD3, a putative GEF for Cdc42, that undergoes SCF(FWD1/beta-TrCP)-mediated proteasomal degradation analogous to that of its homologue FGD1 but regulates cell morphology and motility differently from FGD1

We previously demonstrated that FGD1, the Cdc42 guanine nucleotide exchange factor (GEF) responsible for faciogenital dysplasia, is targeted by the ubiquitin ligase SCF(FWD1/beta-TrCP) upon phosphorylation of two serine residues in its DSGIDS motif and subsequently degraded by the proteasome. Here we show that FGD3, which was identified as a homologue of FGD1 but has been poorly characterized, has conserved the same motif and is down-regulated similarly by SCF(FWD1/beta-TrCP). Although FGD3 and FGD1 share strikingly similar Dbl homology (DH) domains and adjacent pleckstrin homology (PH) domains, both of which are responsible for guanine nucleotide exchange, there also exist remarkable differences in their structures. Indeed, FGD1 and FGD3 induced significantly different morphological changes in HeLa Tet-Off cells: whereas FGD1 induced long finger-like protrusions, FGD3 induced broad sheet-like protrusions when the level of GTP-bound Cdc42 was significantly increased by the inducible expression of FGD3. Furthermore, FGD1 and FGD3 reciprocally regulated cell motility: when inducibly expressed in HeLa Tet-Off cells, FGD1 stimulated cell migration whereas FGD3 inhibited it. Thus we demonstrate that the highly homologous GEFs, FGD1 and FGD3 play different roles to regulate cellular functions but that their intracellular levels are tightly controlled by the same destruction pathway through SCF(FWD1/beta-TrCP).

Dickkopf 1 (DKK1) regulates skin pigmentation and thickness by affecting Wnt/beta-catenin signaling in keratinocytes

The epidermis (containing primarily keratinocytes and melanocytes) overlies the dermis (containing primarily fibroblasts) of human skin. We previously reported that dickkopf 1 (DKK1) secreted by fibroblasts in the dermis elicits the hypopigmented phenotype of palmoplantar skin due to suppression of melanocyte function and growth via the regulation of two important signaling factors, microphthalmia-associated transcription factor (MITF) and beta-catenin. We now report that treatment of keratinocytes with DKK1 increases their proliferation and decreases their uptake of melanin and that treatment of reconstructed skin with DKK1 induces a thicker and less pigmented epidermis. DNA microarray analysis revealed many genes regulated by DKK1, and several with critical expression patterns were validated by reverse transcriptase-polymerase chain reaction and Western blotting. DKK1 induced the expression of keratin 9 and alpha-Kelch-like ECT2 interacting protein (alphaKLEIP) but down-regulated the expression of beta-catenin, glycogen synthase kinase 3beta, protein kinase C, and proteinase-activated receptor-2 (PAR-2), which is consistent with the expression patterns of those proteins in human palmoplantar skin. Treatment of reconstructed skin with DKK1 reproduced the expression patterns of those key proteins observed in palmoplantar skin. These findings further elucidate why human skin is thicker and paler on the palms and soles than on the trunk through topographical and site-specific differences in the secretion of DKK1 by dermal fibroblasts that affects the overlying epidermis.

An ECT2-centralspindlin complex regulates the localization and function of RhoA

In anaphase, the spindle dictates the site of contractile ring assembly. Assembly and ingression of the contractile ring involves activation of myosin-II and actin polymerization, which are triggered by the GTPase RhoA. In many cells, the central spindle affects division plane positioning via unknown molecular mechanisms. Here, we dissect furrow formation in human cells and show that the RhoGEF ECT2 is required for cortical localization of RhoA and contractile ring assembly. ECT2 concentrates on the central spindle by binding to centralspindlin. Depletion of the centralspindlin component MKLP1 prevents central spindle localization of ECT2; however, RhoA, F-actin, and myosin still accumulate on the equatorial cell cortex. Depletion of the other centralspindlin component, CYK-4/MgcRacGAP, prevents cortical accumulation of RhoA, F-actin, and myosin. CYK-4 and ECT2 interact, and this interaction is cell cycle regulated via ECT2 phosphorylation. Thus, central spindle localization of ECT2 assists division plane positioning and the CYK-4 subunit of centralspindlin acts upstream of RhoA to promote furrow assembly.

Novel functions of Ect2 in polar lamellipodia formation and polarity maintenance during "contractile ring-independent" cytokinesis in adherent cells

Some mammalian cells are able to divide via both the classic contractile ring-dependent method (cytokinesis A) and a contractile ring-independent, adhesion-dependent method (cytokinesis B). Cytokinesis A is triggered by RhoA, which, in HeLa cells, is activated by the guanine nucleotide-exchange factor Ect2 localized at the central spindle and equatorial cortex. Here, we show that in HT1080 cells undergoing cytokinesis A, Ect2 does not localize in the equatorial cortex, though RhoA accumulates there. Moreover, Ect2 depletion resulted in only modest multinucleation of HT1080 cells, enabling us to establish cell lines in which Ect2 was constitutively depleted. Thus, RhoA is activated via an Ect2-independent pathway during cytokinesis A in HT1080 cells. During cytokinesis B, Ect2-depleted cells showed narrower accumulation of RhoA at the equatorial cortex, accompanied by compromised pole-to-equator polarity, formation of ectopic lamellipodia in regions where RhoA normally would be distributed, and delayed formation of polar lamellipodia. Furthermore, C3 exoenzyme inhibited equatorial RhoA activation and polar lamellipodia formation. Conversely, expression of dominant active Ect2 in interphase HT1080 cells enhanced RhoA activity and suppressed lamellipodia formation. These results suggest that equatorial Ect2 locally suppresses lamellipodia formation via RhoA activation, which indirectly contributes to restricting lamellipodia formation to polar regions during cytokinesis B.

Overexpression of the Rho-guanine nucleotide exchange factor ECT2 inhibits nuclear translocation of nuclear receptor CAR in the mouse liver

Various drugs such as phenobarbital (PB) trigger translocation of constitutive active/adrostane receptor (CAR) from the cytoplasm into the nucleus of mouse liver cells without directly binding to the receptor. We have now characterized the guanine nucleotide exchange factor epithelial cell-transforming gene 2 (ECT2) as a PB-inducible factor as well as a cellular signal that represses PB-triggered nuclear translocation of CAR. When CFP-tagged ECT2 was co-expressed with YFP-tagged CAR in the liver of Car(-/-) mice, ECT2 repressed CAR nuclear translocation. Coexpression of various deletion mutants delineated this repressive activity to the tandem Dbl homology/pleckstrin homology domains of ECT2 and to their cytosolic expression. CAR directly bound to the PH domain. Thus, ECT2 may comprise a part of the PB response signal regulating the intracellular trafficking of CAR.

A novel shRNA vector that enables rapid selection and identification of knockdown cells

Small interference RNA (siRNA) is a powerful tool for disrupting expression of specific genes in a variety of cells. We have developed a vector, piMARK, which mediates expression of both small hairpin RNA (shRNA) and the blasticidin resistance (Bsr) protein fused with enhanced green fluorescent protein (EGFP), enabling rapid selection and identification of knockdown cells. Using this vector, we targeted Ect2, a gene encoding a guanine nucleotide exchange factor for several small GTPases, in human cell lines. Incubation in the presence of 10 microg/ml blasticidin S rapidly killed untransfected cells, so that after 24 h >90% of surviving HeLa S3 cells emitted green fluorescence and >70% were binucleate as a result of the frequent failure of cell division. The GFP-Bsr fluorescence enabled easy identification of individual knockdown cells under a fluorescence microscope, which in turn enabled unambiguous assessment of the morphological consequences of silencing Ect2. Moreover, because untransfected cells rapidly died and detached from the substrate, they were easily removed by simply rinsing the culture dishes. It thus should be possible to analyze the biochemical consequences of gene silencing en masse in the absence of a background of untransfected cells.

The BTB-Kelch Protein KLEIP Controls Endothelial Migration and Sprouting Angiogenesis

Sprouting and invasive migration of endothelial cells are important steps of the angiogenic cascade. Vascular endothelial growth factor (VEGF) induces angiogenesis by activating intracellular signal transduction cascades, which regulate endothelial cell morphology and function. BTB-kelch proteins are intracellular proteins that control cellular architecture and cellular functions. The BTB-kelch protein KLEIP has been characterized as an actin-binding protein that interacts with the nucleotide exchange factor ECT2. We report that KLEIP is preferentially expressed in endothelial cells, suggesting that it may play a critical role in controlling the functions of migrating, proliferating, and invading endothelial cells during angiogenesis. KLEIP mRNA level in endothelial cells is strongly regulated by hypoxia which is controlled by hypoxia-inducible factor-1α. Functional analysis of KLEIP in endothelial cells revealed that it acts as an essential downstream regulator of VEGF- and basic fibroblast growth factor-induced migration and in-gel sprouting angiogenesis. Yet, it is not involved in controlling VEGF- or basic fibroblast growth factor-mediated proliferative responses. The depletion of KLEIP in endothelial cells blunted the VEGF-induced activation of the monomeric GTPase RhoA but did not alter the VEGF-stimulated activation of extracellular signal-regulated kinase 1/2. Moreover, VEGF induced a physical association of KLEIP with the guanine nucleotide-exchange factor ECT2, the depletion of which also blunted VEGF-induced sprouting. We conclude that the BTB-kelch protein KLEIP is a novel regulator of endothelial function during angiogenesis that controls the VEGF-induced activation of Rho GTPases.

Regulation of proto-oncogenic dbl by chaperone-controlled, ubiquitin-mediated degradation

The dbl proto-oncogene product is a prototype of a growing family of guanine nucleotide exchange factors (GEFs) that stimulate the activation of small GTP-binding proteins from the Rho family. Mutations that result in the loss of proto-Dbl's amino terminus produce a variant with constitutive GEF activity and high oncogenic potential. Here, we show that proto-Dbl is a short-lived protein that is kept at low levels in cells by efficient ubiquitination and degradation. The cellular fate of proto-Dbl is regulated by interactions with the chaperones Hsc70 and Hsp90 and the protein-ubiquitin ligase CHIP, and these interactions are mediated by the spectrin domain of proto-Dbl. We show that CHIP is the E3 ligase responsible for ubiquitination and proteasomal degradation of proto-Dbl, while Hsp90 functions to stabilize the protein. Onco-Dbl, lacking the spectrin homology domain, cannot bind these regulators and therefore accumulates in cells at high levels, leading to persistent stimulation of its downstream signaling pathways.

Nucleotide exchange factor ECT2 regulates epithelial cell polarity

Cell polarity regulates diverse biological events such as localization of embryonic determinants and establishment of tissue and organ architecture. Epithelial cell polarity is regulated by the polarity complex Par6/Par3/atypical protein kinase C (aPKC). We previously found that the nucleotide exchange factor ECT2 associates with this polarity complex and regulates aPKC activity, but the role of ECT2 in cell polarity is still unclear. Here we show that expression of a dominant negative (ECT2-N2) or constitutively active (ECT2-DeltaN5) form of ECT2 inhibits normal cyst formation of MDCK cells in 3-dimensional collagen gels. Central lumens were not observed in cysts formed by cells expressing either ECT2-DeltaN5 or ECT2-N2. Apical localization of ZO-1 and basolateral localization of beta-catenin were no longer observed in these cells. Interestingly, cells expressing ECT2-N2 did form normal cysts when cultured in the basement membrane matrix Matrigel instead of collagen gels. Addition of a major Matrigel component, laminin, partially rescued the normal cyst formation inhibited by ECT2-N2 in 3-dimensional collagen gels. Thus, signaling through laminin might override the defects of signaling through collagen and ECT2. Whereas ECT2-N2 inhibited the lumen formation of MDCK cysts, caspase-3, which is reportedly involved in lumen formation through apoptosis, was activated at various locations of cells in the cysts. It is likely that perturbation of ECT2 signaling inhibits the establishment of epithelial cell polarity leading to the inhibition of selected elimination of cells at the center of cysts. Thus, ECT2 appears to play a critical role in epithelial cell polarity.

Expression of the Rho-GEF Pbl/ECT2 is regulated by the UBE3A E3 ubiquitin ligase

We applied genetic tools available in Drosophila to identify candidate substrates of the UBE3A ubiquitin ligase, the gene responsible for Angelman syndrome (AS). Human UBE3A was expressed in Drosophila heads to identify proteins differentially regulated in UBE3A-expressing versus wild-type extracts. Using two-dimensional gel and MALDI-TOF analysis, we detected 20 proteins that were differentially regulated by over-expression of human UBE3A in Drosophila heads. One protein responsive to UBE3A was the Rho-GEF pebble (pbl). Here, we present three lines of evidence suggesting that UBE3A regulates Pbl. First, we show genetic evidence that UBE3A and the Drosophila de-ubiquitinase fat facets (faf) exert opposing effects on Pbl function. Secondly, we find that both Pbl and ECT2, the mammalian orthologue of Pbl called epithelial cell transforming sequence 2 oncogene, physically interact with their respective ubiquitin E3 ligases. Finally, we show that Ect2 expression is regulated by Ube3a in mouse neurons as the pattern of Ect2 expression is dramatically altered in the hippocampus and cerebellum of Ube3a null mice. These results suggest that an orthologous UBE3A post-translational regulatory pathway regulates neuronal outgrowth in the mammalian brain and that dysregulation of this pathway may result in neurological phenotypes including AS and possibly other autism spectrum disorders.

The epithelial cell transforming sequence 2, a guanine nucleotide exchange factor for Rho GTPases, is repressed by p53 via protein methyltransferases and is required for G1-S transition

The epithelial cell transforming sequence 2 (ECT2), a member of the Dbl family of guanine nucleotide exchange factor for Rho GTPases, is required for cytokinesis. The tumor suppressor p53 plays a crucial role in coordinating cellular processes, such as cell cycle arrest and apoptosis, in response to stress signals. Here, we showed that ECT2 is negatively regulated by wild-type p53 but not tumor-derived mutant p53 or other p53 family members. In addition, ECT2 is down-regulated in multiple cell lines by DNA damage agents and Nutlin-3, an MDM2 antagonist, in a p53-dependent manner. We also showed that the activity of the ECT2 promoter is repressed by wild-type p53, and to a lesser extent, by p21. In addition, the second activation domain in p53 is necessary for the efficient repression of ECT2. Importantly, we found that the ECT2 gene is bound by p53 in vivo in response to DNA damage and Nutlin-3 treatment. Furthermore, we provided evidence that inhibition of protein methyltransferases, especially arginine methyltransferases, relieve the repression of ECT2 induced by DNA damage or Nutlin-3 in a p53-dependent manner. Finally, we generated multiple cell lines in which ECT2 is inducibly knocked down and found that ECT2 knockdown triggers cell cycle arrest in G1. Taken together, we uncovered a novel function for ECT2 and provided a novel mechanism by which p53 represses gene expression via protein methyltransferases.

RB silencing compromises the DNA damage-induced G2/M checkpoint and causes deregulated expression of the ECT2 oncogene

As alterations in retinoblastoma (RB)/E2F pathway are commonly found in human cancers, the molecular mechanism underlying cell cycle deregulation caused by the mutations in the RB/E2F pathway needs to be investigated extensively. Compared with good understanding of RB/E2F functions in G1-S cell cycle progression, it is not fully understood how an abrogated RB pathway affects the G2-M phase of the cell cycle. Here, we report that disruption of RB accelerated G2-M progression in the presence of DNA damage by elevating the expression of a set of mitotic regulatory genes. We generated RB(+)- and (-)-matched cells using short hairpin RNA. In the RB(-) cells, the G2/M checkpoint mediated by a DNA-damaging agent was over-ridden. With microarray analysis, we found that the expression of key G2-M regulatory genes was upregulated in RB(-) cells. In particular, we demonstrated that the proto-oncogene ECT2 was directly regulated by E2Fs. Furthermore, suppression of ECT2 expression by small interfering RNA in RB(-) cells resulted in cytokinesis arrest, suggesting that RB(-) cells lack the regulation of E2F-mediated cytokinesis. These results indicate that aberrant ECT2 expression, observed in various human tumors, could be the direct result of RB/E2F pathway deficiency, thereby contributing to cell division in cancers.