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

Elevated expression of ECT2 predicts unfavorable prognosis in patients with colorectal cancer

Epithelial cell transforming sequence 2 (ECT2) is a well-studied guanine nucleotide exchange factor for the Rho family GTPase, which has been demonstrated as an oncogene in many types of human cancers. However, little is known about the prognostic value of ECT2 in colorectal cancer (CRC). In current study, we investigated the expression pattern and underlying clinical significance of ECT2 in CRC. ECT2 expression was detected in 345 CRC specimens by immunohistochemistry, and its correlation with clinicopathologic parameters and prognosis of CRC patients were analyzed. Data from Oncomine database and real-time PCR demonstrated that ECT2 expression was elevated in CRC compared with normal tissues. Among the clinical parameters analyzed, high expression level of ECT2 significantly associated with tumor size (P=0.020), serum CEA levels (P = 0.000) and TNM stage (P=0.027). Kaplan-Meier survival analysis showed that patients with high ECT2 expression had a remarkably shorter overall survival. Cox regression analysis revealed that ECT2 expression level was a significant and independent prognostic factor for overall survival rate of CRC patients. These data suggested that ECT2 is an unfavorable biomarker of prognosis in CRC and that ECT2 may be a potential therapeutic candidate for CRC treatment.

Copy number gains of FGFR1 and 3q chromosome in squamous cell carcinoma of the lung

Squamous cell carcinoma of the lung (SQCCL) remains a leading cause of cancer-related death. Unlike non-smoker adenocarcinoma of the lung, where highly efficient tyrosine kinase inhibitors are available for treating mutant EGFR or ALK-rearranged, no targetable biomarkers are available for SQCCL. The frequent and focal amplification of FGFR1 has generated great expectations in offering new therapeutical options in case of 16-22% of SQCCL patients. Broad 3q chromosome amplification is widely recognized as the most common chromosomal aberration found in SQCCL, where PIK3CA, SOX2, ACK1, PRKCI, TP63, PLD1, ECT2, and others genes are located. Although SOX2 has been postulated as a key regulator of basal stem cells transformation and tumor progression, it seems to confer a good prognosis in SQCCL. It is known that each patient might carry a different length of 3q chromosome amplicon. Thus, we suggest that the number and the biological importance of the genes spanned along each patient's 3q amplicon might help to explain inter-individual outcome variations of the disease and its potential predictive value, especially when relevant oncogenes such as those mentioned above are implicated. Currently, there is no clinical predictive data available from clinical trials. In this review, we have focused on the potential role of FGFR1 in SQCCL prognosis. Additionally, we have explored recently available public data on the comprehensive genomic characterization of SQCCL, in relation to the protein-coding genes that have a strong gene copy number - mRNA correlation in 3q chromosome, that were previously described as potential driver oncogenes or its modifiers in SQCCL.

The RNA binding protein FXR1 is a new driver in the 3q26-29 amplicon and predicts poor prognosis in human cancers

Aberrant expression of RNA-binding proteins has profound implications for cellular physiology and the pathogenesis of human diseases such as cancer. We previously identified the Fragile X-Related 1 gene (FXR1) as one amplified candidate driver gene at 3q26-29 in lung squamous cell carcinoma (SCC). FXR1 is an autosomal paralog of Fragile X mental retardation 1 and has not been directly linked to human cancers. Here we demonstrate that FXR1 is a key regulator of tumor progression and its overexpression is critical for nonsmall cell lung cancer (NSCLC) cell growth in vitro and in vivo. We identified the mechanisms by which FXR1 executes its regulatory function by forming a novel complex with two other oncogenes, protein kinase C, iota and epithelial cell transforming 2, located in the same amplicon via distinct binding mechanisms. FXR1 expression is a candidate biomarker predictive of poor survival in multiple solid tumors including NSCLCs. Because FXR1 is overexpressed and associated with poor clinical outcomes in multiple cancers, these results have implications for other solid malignancies.

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.

Up-regulation of ECT2 is associated with poor prognosis in gastric cancer patients

OBJECTIVES

The aim of this study was to investigate the expression of ECT2 in gastric cancer and its clinical significance.

METHODS AND RESULTS

We investigated the differentially expressed genes between gastric cancer tissues and normal gastric mucosa by cDNA microarray, and then we found ECT2 was up-regulated in gastric cancer. What is more, we verified ECT2 expression level by quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) and measured its protein level by immunohistochemistry (IHC). qRT-PCR analysis indicated ECT2 was significantly up-regulated in gastric cancer and Immunohistochemistry confirmed the percentage of ECT2-positive specimens was significantly higher in gastric carcinoma than in non-tumor tissues. Up-regulation of ECT2 is associated with the degree of histological differentiation (P = 0.007), invasion depth (P = 0.047), lymph node metastasis (P = 0.016), distant metastasis (P = 0.021) and TNM stage (P = 0.016), patients with up-regulated ECT2 had a lower overall survival rate (P = 0.000). Cox regression analysis revealed that up-regulation of ECT2 is an independent prognostic factor in gastric cancer patients (P = 0.012).

CONCLUSION

Up-regulation of ECT2 might contribute to the progression of gastric carcinogenesis and may be a useful prognostic indicator in gastric cancer.

Protein kinase C and cancer: what we know and what we do not

Since their discovery in the late 1970s, protein kinase C (PKC) isozymes represent one of the most extensively studied signaling kinases. PKCs signal through multiple pathways and control the expression of genes relevant for cell cycle progression, tumorigenesis and metastatic dissemination. Despite the vast amount of information concerning the mechanisms that control PKC activation and function in cellular models, the relevance of individual PKC isozymes in the progression of human cancer is still a matter of controversy. Although the expression of PKC isozymes is altered in multiple cancer types, the causal relationship between such changes and the initiation and progression of the disease remains poorly defined. Animal models developed in the last years helped to better understand the involvement of individual PKCs in various cancer types and in the context of specific oncogenic alterations. Unraveling the enormous complexity in the mechanisms by which PKC isozymes have an impact on tumorigenesis and metastasis is key for reassessing their potential as pharmacological targets for cancer treatment.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Tyrosine phosphorylation of Dbl regulates GTPase signaling

Rho GTPases are molecular "switches" that cycle between "on" (GTP-bound) and "off" (GDP-bound) states and regulate numerous cellular activities such as gene expression, protein synthesis, cytoskeletal rearrangements, and metabolic responses. Dysregulation of GTPases is a key feature of many diseases, especially cancers. Guanine nucleotide exchange factors (GEFs) of the Dbl family are activated by mitogenic cell surface receptors and activate the Rho family GTPases Cdc42, Rac1, and RhoA. The molecular mechanisms that regulate GEFs from the Dbl family are poorly understood. Our studies reveal that Dbl is phosphorylated on tyrosine residues upon stimulation by growth factors and that this event is critical for the regulated activation of the GEF. These findings uncover a novel layer of complexity in the physiological regulation of this protein.

Rewiring cell polarity signaling in cancer

Disrupted cell polarity is a feature of epithelial cancers. The Crumbs, Par and Scribble polarity complexes function to specify and maintain apical and basolateral membrane domains, which are essential to organize intracellular signaling pathways that maintain epithelial homeostasis. Disruption of apical-basal polarity proteins facilitates rewiring of oncogene and tumor suppressor signaling pathways to deregulate proliferation, apoptosis, invasion and metastasis. Moreover, apical-basal polarity integrates intracellular signaling with the microenvironment by regulating metabolic signaling, extracellular matrix remodeling and tissue level organization. In this review, we discuss recent advances in our understanding of how polarity proteins regulate diverse signaling pathways throughout cancer progression from initiation to metastasis.

Atypical protein kinase Cι as a human oncogene and therapeutic target

Protein kinase inhibitors represent a major class of targeted therapeutics that has made a positive impact on treatment of cancer and other disease indications. Among the promising kinase targets for further therapeutic development are members of the Protein Kinase C (PKC) family. The PKCs are central components of many signaling pathways that regulate diverse cellular functions including proliferation, cell cycle, differentiation, survival, cell migration, and polarity. Genetic manipulation of individual PKC isozymes has demonstrated that they often fulfill distinct, nonredundant cellular functions. Participation of PKC members in different intracellular signaling pathways reflects responses to varying extracellular stimuli, intracellular localization, tissue distribution, phosphorylation status, and intermolecular interactions. PKC activity, localization, phosphorylation, and/or expression are often altered in human tumors, and PKC isozymes have been implicated in various aspects of transformation, including uncontrolled proliferation, migration, invasion, metastasis, angiogenesis, and resistance to apoptosis. Despite the strong relationship between PKC isozymes and cancer, to date only atypical PKCiota has been shown to function as a bona fide oncogene, and as such is a particularly attractive therapeutic target for cancer treatment. In this review, we discuss the role of PKCiota in transformation and describe mechanism-based approaches to therapeutically target oncogenic PKCiota signaling in cancer.

The PRKCI and SOX2 oncogenes are coamplified and cooperate to activate Hedgehog signaling in lung squamous cell carcinoma

We report that two oncogenes coamplified on chromosome 3q26, PRKCI and SOX2, cooperate to drive a stem-like phenotype in lung squamous cell carcinoma (LSCC). Protein kinase Cι (PKCι) phosphorylates SOX2, a master transcriptional regulator of stemness, and recruits it to the promoter of Hedgehog (Hh) acyltransferase (HHAT) that catalyzes the rate-limiting step in Hh ligand production. PKCι-mediated SOX2 phosphorylation is required for HHAT promoter occupancy, HHAT expression, and maintenance of a stem-like phenotype. Primary LSCC tumors coordinately overexpress PKCι, SOX2, and HHAT and require PKCι-SOX2-HHAT signaling to maintain a stem-like phenotype. Thus, PKCι and SOX2 are genetically, biochemically, and functionally linked in LSCC, and together they drive tumorigenesis by establishing a cell-autonomous Hh signaling axis.

The Role of Ect2 Nuclear RhoGEF Activity in Ovarian Cancer Cell Transformation

Ect2, a Rho guanine nucleotide exchange factor (RhoGEF), is atypical among RhoGEFs in its predominantly nuclear localization in interphase cells. One current model suggests that Ect2 mislocalization drives cellular transformation by promoting aberrant activation of cytoplasmic Rho family GTPase substrates. However, in ovarian cancers, where Ect2 is both amplified and overexpressed at the mRNA level, we observed that the protein is highly expressed and predominantly nuclear and that nuclear but not cytoplasmic Ect2 increases with advanced disease. Knockdown of Ect2 in ovarian cancer cell lines impaired their anchorage-independent growth without affecting their growth on plastic. Restoration of Ect2 expression rescued the anchorage-independent growth defect, but not if either the DH catalytic domain or the nuclear localization sequences of Ect2 were mutated. These results suggested a novel mechanism whereby Ect2 could drive transformation in ovarian cancer cells by acting as a RhoGEF specifically within the nucleus. Interestingly, Ect2 had an intrinsically distinct GTPase specificity profile in the nucleus versus the cytoplasm. Nuclear Ect2 bound preferentially to Rac1, while cytoplasmic Ect2 bound to RhoA but not Rac. Consistent with nuclear activation of endogenous Rac, Ect2 overexpression was sufficient to recruit Rac effectors to the nucleus, a process that required a functional Ect2 catalytic domain. Furthermore, expression of active nuclearly targeted Rac1 rescued the defect in transformed growth caused by Ect2 knockdown. Our work suggests a novel mechanism of Ect2-driven transformation, identifies subcellular localization as a regulator of GEF specificity, and implicates activation of nuclear Rac1 in cellular transformation.

PKCι maintains a tumor-initiating cell phenotype that is required for ovarian tumorigenesis

Protein kinase Cι (PKCι) has oncogenic potential and is an attractive therapeutic target for treatment of lung cancer, particularly those tumors that express elevated PKCι. However, whether PKCι is a viable target in ovarian cancer is unknown, and virtually nothing is known about the mechanism by which PKCι drives ovarian tumorigenesis. Here, it is demonstrated that PKCι maintains a tumor-initiating cell (TIC) phenotype that drives ovarian tumorigenesis. A highly tumorigenic population of cells from human ovarian cancer cell lines exhibit cancer stem-like TIC properties, including self-renewal, clonal expansion, expression of stem-related genes, enhanced transformed growth in vitro, and aggressive tumor-initiating activity in vivo. Genetic disruption of PKCι inhibits the proliferation, clonal expansion, anchorage-independent growth, and enhanced tumorigenic properties of ovarian TICs. Biochemical analysis demonstrates that PKCι acts through its oncogenic partner Ect2 to activate a MEK/ERK signaling axis that drives the ovarian TIC phenotype. Genomic analysis reveals that PKCι and Ect2 are coordinately amplified and overexpressed in the majority of primary ovarian serous tumors, and these tumors exhibit evidence of an active PKCι-Ect2 signaling axis in vivo. Finally, this study reveals that auranofin, a potent and selective inhibitor of oncogenic PKCι signaling, inhibits the tumorigenic properties of ovarian TIC cells in vitro and in vivo. These data demonstrate that PKCι is required for a TIC phenotype in ovarian cancer, and that auranofin is an attractive therapeutic option to target deadly ovarian TICs in ovarian cancer patients.

IMPLICATIONS

PKCι drives a tumor-initiating cell phenotype in ovarian cancer cells that can be therapeutically targeted with auranofin, a small molecule inhibitor of PKCι signaling.

Molecular Insights on Post-chemotherapy Retinoblastoma by Microarray Gene Expression Analysis

PURPOSE

Management of Retinoblastoma (RB), a pediatric ocular cancer is limited by drug-resistance and drug-dosage related side effects during chemotherapy. Molecular de-regulation in post-chemotherapy RB tumors was investigated.

MATERIALS AND METHODS

cDNA microarray analysis of two post-chemotherapy and one pre-chemotherapy RB tumor tissues was performed, followed by Principle Component Analysis, Gene ontology, Pathway Enrichment analysis and Biological Analysis Network (BAN) modeling. The drug modulation role of two significantly up-regulated genes (p≤0.05) - Ect2 (Epithelial-cell-transforming-sequence-2), and PRAME (preferentially-expressed-Antigen-in-Melanoma) was assessed by qRT-PCR, immunohistochemistry and cell viability assays.

RESULTS

Differential up-regulation of 1672 genes and down-regulation of 2538 genes was observed in RB tissues (relative to normal adult retina), while 1419 genes were commonly de-regulated between pre-chemotherapy and post- chemotherapy RB. Twenty one key gene ontology categories, pathways, biomarkers and phenotype groups harboring 250 differentially expressed genes were dys-regulated (EZH2, NCoR1, MYBL2, RB1, STAMN1, SYK, JAK1/2, STAT1/2, PLK2/4, BIRC5, LAMN1, Ect2, PRAME and ABCC4). Differential molecular expressions of PRAME and Ect2 in RB tumors with and without chemotherapy were analyzed. There was neither up- regulation of MRP1, nor any significant shift in chemotherapeutic IC50, in PRAME over-expressed versus non-transfected RB cells.

CONCLUSION

Cell cycle regulatory genes were dys-regulated post-chemotherapy. Ect2 gene was expressed in response to chemotherapy-induced stress. PRAME does not contribute to drug resistance in RB, yet its nuclear localization and BAN information, points to its possible regulatory role in RB.

Protein kinase C pharmacology: refining the toolbox

PKC (protein kinase C) has been in the limelight since the discovery three decades ago that it acts as a major receptor for the tumour-promoting phorbol esters. Phorbol esters, with their potent ability to activate two of the three classes of PKC isoenzymes, have remained the best pharmacological tool for directly modulating PKC activity. However, with the discovery of other phorbol ester-responsive proteins, the advent of various small-molecule and peptide modulators, and the need to distinguish isoenzyme-specific activity, the pharmacology of PKC has become increasingly complex. Not surprisingly, many of the compounds originally touted as direct modulators of PKC have subsequently been shown to hit many other cellular targets and, in some cases, not even directly modulate PKC. The complexities and reversals in PKC pharmacology have led to widespread confusion about the current status of the pharmacological tools available to control PKC activity. In the present review, we aim to clarify the cacophony in the literature regarding the current state of bona fide and discredited cellular PKC modulators, including activators, small-molecule inhibitors and peptides, and also address the use of genetically encoded reporters and of PKC mutants to measure the effects of these drugs on the spatiotemporal dynamics of signalling by specific isoenzymes.

Centralspindlin: at the heart of cytokinesis

The final step in the cell cycle is the formation of two genetically identical daughter cells by cytokinesis. At the heart of cytokinesis in animal cells is the centralspindlin complex which is composed of two proteins, a kinesin-like protein, Mitotic kinesin-like protein 1, and a Rho GTPase activating protein (RhoGAP), CYK-4. Through its targeted localization to a narrow region of antiparallel microtubule overlap immediately following chromosome segregation, centralspindlin initiates central spindle assembly. Centralspindlin has several critical functions during cell division including positioning of the division plane, regulation of Rho family GTPases, as well as midbody assembly and abscission. In this review, we will examine the biochemistry of centralspindlin and its multiple functions during cell division. Remarkably, several of its critical functions are somewhat unexpected. Although endowed with motor domains, centralspindlin has an important role in generating stable, antiparallel microtubule bundles. Although it contains a Rho family GAP domain, it has a central role in the activation of RhoA during cytokinesis. Finally, centralspindlin functions as a motor protein complex, as a scaffold protein for key regulators of abscission and as a conventional RhoGAP. Because of these diverse functions, centralspindlin lies at the heart of the cytokinetic mechanism.

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 APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

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.