The guanine nucleotide exchange factor (GEF) Ect2 is an oncogene in human cancer
Introduction
Small guanosine triphosphatases (GTPases; EC 3.6.5.2) act as molecular switches in diverse signaling pathways that regulate actin cytoskeleton remodeling, cell motility, cell adhesion, cell cycle progression and gene transcription. The discovery of mutations in the RAS gene, the prototypical member of the GTPase family, in human tumors prompted a search for mutations in other GTPases, including the well characterized members of the Rho family GTPases, RhoA, Rac1 and Cdc42. To date, mutations in these Rho GTPases have not been shown to occur naturally in human cancers. The Rho GTPases, however, have been implicated in the malignant phenotype of many human cancers as a result of their participation in aberrant signaling in tumor cells and overexpression in human tumors (reviewed in Gomez del Pulgar et al., 2005). Similar to other GTPases, RhoA, Rac1 and Cdc42 cycle between a GTP bound active state and a GDP bound inactive state. The weak intrinsic GTPase activity of the Rho GTPases is enhanced by association with GTPase activating proteins (GAPs) that promote hydrolysis of GTP and the return of the GTPase to the inactive state. The activity of Rho GTPases is negatively regulated by GTP dissociation inhibitors (GDIs) which bind GTPases and maintain them in the GDP bound state. Guanine nucleotide exchange factors (GEFs) catalyze the exchange of GDP for GTP, thereby activating the Rho GTPases in signal transduction. GEFs for Rho GTPases have also been implicated in cancer, some of which were initially identified as potential oncogenes. One of the Rho-GEFs reported to promote transformation is Epithelial Cell Transforming Sequence 2 (Ect2), a member of the Dbl family of GEFs. This report summarizes the physiologic functions of Ect2, mechanisms of its regulation and its role in cellular transformation. Particular focus will be placed on recent studies demonstrating that Ect2 is an oncogene in human tumors.
Section snippets
Ect2 structure and expression
Pebble (Pbl), the fly ortholog of the human ECT2 gene was the first member of the Ect2 protein family to be identified during a screen for recessive embryonic lethal mutations that result in cuticular pattern defects in Drosophila melanogaster (Jügens et al., 1984). Pbl was subsequently found to function as a Rho-GEF required for cytokinesis through activation of Rho1 (a Drosophila ortholog of mammalian RhoA) (Lehner, 1992, Prokopenko et al., 1999). Mammalian ECT2 was later isolated as a
Physiological functions of Ect2
Functional studies in Drosophila established a critical role for Pbl, the Drosophila ortholog of Ect2, in cytokinesis. At the onset of cytokinesis, Pbl is associated with the cleavage furrow where it activates Rho1 to initiate assembly of the contractile ring and promote cytokinesis (Prokopenko et al., 1999). Mutations in Pbl result in a lack of contractile ring formation and failure of Drosophila cells to undergo cytokinesis during embryogenesis. Likewise, Let-21 the C. elegans ortholog of
Regulation of Ect2 function
Ect2 function is regulated through a number of mechanisms including phosphorylation, intracellular localization and intra- and inter-molecular interactions. Ect2 becomes phosphorylated during the G2 phase of the cell cycle, remains phosphorylated during mitosis and becomes dephosphorylated at the time of cytokinesis. Phosphatase treatment of immunoprecipitated Ect2 significantly decreases its GEF activity, suggesting that Ect2 exchange activity is regulated by phosphorylation. At least 2 sites
Ect2 and cancer
The ECT2 gene was initially identified as a proto-oncogene capable of transforming NIH/3T3 fibroblasts (Miki et al., 1993). Subsequent analysis revealed that the originally characterized oncogenic Ect2 clone actually consisted of a carboxyl-terminal truncation of the full-length ECT2 gene. This truncated clone encoded a protein consisting of the DH-PH-C domains of Ect2 similar to that described above. This mutant localized to the cytoplasm, possessed constitutive GEF activity and could
Summary
Ect2 is an oncogene in multiple human cancers. Ect2 is aberrantly overexpressed in multiple human tumor types, often as a result of targeted amplification of the ECT2 gene as part of the 3q26 amplicon. Ect2 is important for proliferation, migration and invasion of various types of cancer cells in vitro, and for NSCLC tumorigenicity in vivo. The role of Ect2 in cellular transformation is distinct from its physiologic role in cytokinesis, and many tumor cells appear to have evolved
Acknowledgments
The authors wish to thank their colleagues in the Fields laboratory for helpful suggestions and critical review of the manuscript. The authors also wish to apologize to colleagues who have made important contributions to this area, but whose work could not be cited due to space limitations. The work from the Fields laboratory discussed in this article was supported by grants from the National Institutes of Health (CA081436-13), the American Lung Association/LUNGevity, the V Foundation, and the
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