Table of Contents

Volume 3, Issue 3

  July/August/September 2012

  Pages 137 - 197

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 Issue Catalogue (MARC XML)

EDITOR'S CORNER

Open Access Article

In this issue of Small GTPases

Elaine L. Ellerton
Pages 137 - 138
http://dx.doi.org/10.4161/sgtp.21536
Full Text | PDF




RESEARCH PAPER

Open Access Article

Palmitoylation regulates vesicular trafficking of R-Ras to membrane ruffles and effects on ruffling and cell spreading

Jeremy G.T. Wurtzel, Puneet Kumar and Lawrence E. Goldfinger
Pages 139 - 153
http://dx.doi.org/10.4161/sgtp.21084
Abstract | Full Text | PDF

In this study we investigated the dynamics of R-Ras intracellular trafficking and its contributions to the unique roles of R-Ras in membrane ruffling and cell spreading. Wild type and constitutively active R-Ras localized to membranes of both Rab11- and transferrin-positive and -negative vesicles, which trafficked anterograde to the leading edge in migrating cells. H-Ras also co-localized with R-Ras in many of these vesicles in the vicinity of the Golgi, but R-Ras and H-Ras vesicles segregated proximal to the leading edge, in a manner dictated by the C-terminal membrane-targeting sequences. These segregated vesicle trafficking patterns corresponded to distinct modes of targeting to membrane ruffles at the leading edge. Geranylgeranylation was required for membrane anchorage of R-Ras, whereas palmitoylation was required for exit from the Golgi in post-Golgi vesicle membranes and trafficking to the plasma membrane. R-Ras vesicle membranes did not contain phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃), whereas R-Ras co-localized with PtdIns(3,4,5)P₃ in membrane ruffles. Finally, palmitoylation-deficient R-Ras blocked membrane ruffling, R-Ras/PI3-kinase interaction, enrichment of PtdIns(3,4,5)P₃ at the plasma membrane, and R-Ras-dependent cell spreading. Thus, lipid modification of R-Ras dictates its vesicle trafficking, targeting to membrane ruffles, and its unique roles in localizing PtdIns(3,4,5)P₃ to ruffles and promoting cell spreading.




COMMENTARIES

Open Access Article

RAS/ERK pathway transcriptional regulation through ETS/AP-1 binding sites

Peter C. Hollenhorst
Pages 154 - 158
http://dx.doi.org/10.4161/sgtp.19630
Abstract | Full Text | PDF

The RAS/RAF/MEK/ERK signaling pathway is activated by mutation in many cancers. Neighboring ETS and AP-1 DNA binding sequences can act as response elements for transcriptional activation by this pathway. ERK phosphorylation of an ETS transcription factor is one mechanism of activating the RAS/ERK gene expression program that can promote cancer cell phenotypes such as proliferation, invasion, and metastasis. Recent genome-wide mapping of ETS proteins overexpressed by chromosomal rearrangement in prostate cancer reveals a second mechanism for activation of this gene expression program. An oncogenic subset of ETS transcription factors can activate RAS/ERK target genes even in the absence of RAS/ERK pathway activation by binding ETS/AP-1 sequences. Thus, regulation of cancer cell invasion and metastasis via ETS/AP-1 sequence elements depends on which ETS protein is bound, and the status of the RAS/ERK pathway. This commentary will focus on what is known about the selectivity of ETS/AP-1 sequences for different ETS transcription factors and the transcriptional consequences of ETS protein selection.

Open Access Article

A big new job for small GTPases

Ana Carmena
Pages 159 - 162
http://dx.doi.org/10.4161/sgtp.19631
Abstract | Full Text | PDF

Members of the Ras superfamily of small guanosine triphosphatases (GTPases) function as key nodes within signaling networks in a remarkable range of cellular processes, including cell proliferation, differentiation, growth, cell-cell adhesion and apoptosis. We recently described a novel role for the Ras-like small GTPases Rap1 and Ral in regulating cortical polarity and spindle orientation during asymmetric neuroblast division in Drosophila. The participation of these proteins in promoting cell polarization seems to be a common theme throughout evolution.

Open Access Article

RAS, cellular senescence and transformation: The BRCA1 DNA repair pathway at the crossroads

Zhigang Tu, Katherine M. Aird and Rugang Zhang
Pages 163 - 167
http://dx.doi.org/10.4161/sgtp.19884
Abstract | Full Text | PDF

The definition of an oncogene is a gene that actively promotes tumorigenesis. For example, activation of RAS oncogene promotes cell transformation and cancer. Paradoxically, in primary mammalian cells, oncogenic RAS typically triggers cellular senescence, a state of irreversible cell growth arrest. Oncogene-induced senescence is an important tumor suppression mechanism in vivo. Here, we discuss our recent evidence that RAS-induced suppression of DNA repair response via dissociation of BRCA1 from chromatin promotes senescence while predisposing cells to senescence bypass and transformation by allowing for secondary hits. The molecular mechanism we uncovered helps reconcile the tumor-promoting nature of oncogenic RAS with the tumor-suppressing role of oncogene-induced senescence.

Open Access Article

A novel function of Rab5 in mitosis

Letizia Lanzetti
Pages 168 - 172
http://dx.doi.org/10.4161/sgtp.19987
Abstract | Full Text | PDF

Several membrane trafficking proteins have been shown to participate in spindle assembly and stability during mitosis. Despite the fact that the role of some of them has been clarified, the requirement for these molecules in mitosis is still poorly understood.

Recently, we and others^1,2 found that a key player in endocytosis, the small GTPase Rab5, controls the alignment of chromosomes on the metaphase plate both in mammalian cells and in the Drosophila model organism. Although the underlying mechanisms appear to be distinct, depletion of Rab5 affects progression through mitosis and the correct execution of chromosome segregation in the daughter cells in both systems, indicating that this function of Rab5 is conserved through evolution.

After outlining the common requirements for Rab5 in metazoans mitosis, I will comment on the involvement of Rab5 in spindle stability and in the localization of the centromere-associated protein CENP-F to kinetochores of mammalian cells.

Open Access Article

Phosphorylation is the switch that turns PEA-15 from tumor suppressor to tumor promoter

Florian J. Sulzmaier, John Opoku-Ansah and Joe W. Ramos
Pages 173 - 177
http://dx.doi.org/10.4161/sgtp.20021
Abstract | Full Text | PDF

Abnormal ERK signaling is implicated in many human diseases including cancer. This signaling cascade is a good target for the therapy of certain malignancies because of its important role in regulating cell proliferation and survival. The small phosphoprotein PEA-15 is a potent regulator of the ERK signaling cascade, and, by acting on this pathway, has been described to have both tumor-suppressor and tumor-promoter functions. However, the exact mechanism by which PEA-15 drives the outcome one way or the other remains unclear. We propose that the cellular environment is crucial in determining PEA-15 protein function by affecting the protein’s phosphorylation state. We hypothesize that only unphosphorylated PEA-15 can act as a tumor-suppressor and that phosphorylation alters the interaction with binding partners to promote tumor development. In order to use PEA-15 as a prognostic marker or therapeutic target it is therefore important to evaluate its phosphorylation status.

Open Access Article

Rho/RacGAPs: Embarras de richesse?

Roland Csépányi-Kömi, Magdolna Lévay and Erzsébet Ligeti
Pages 178 - 182
http://dx.doi.org/10.4161/sgtp.20040
Abstract | Full Text | PDF

Regulatory proteins such as guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) determine the activity of small GTPases. In the Rho/Rac family, the number of GEFs and GAPs largely exceeds the number of small GTPases, raising the question of specific or overlapping functions. In our recent study we investigated the first time ARHGAP25 at the protein level, determined its activity as RacGAP and showed its involvement in phagocytosis. With the discovery of ARHGAP25, the number of RacGAPs described in phagocytes is increased to six. We provide data that indicate the specific functions of selected Rho/RacGAPs and we show an example of differential regulation of a Rho/Rac family GAP by different kinases. We propose that the abundance of Rho/Rac family GAPs is an important element of the fine spatiotemporal regulation of diverse cellular functions.

Open Access Article

A shortcut from GPCR signaling to Rac-mediated actin cytoskeleton through an ELMO/DOCK complex

Xuehua Xu and Tian Jin
Pages 183 - 185
http://dx.doi.org/10.4161/sgtp.20271
Abstract | Full Text | PDF

Chemotaxis, chemoattractant-guided directional cell migration, plays major roles in human innate immunity and in development of a model organism Dictyostelium discoideum. Human leukocytes and D. disscoideum share remarkable similarities in the molecular mechanisms that control chemotaxis. These cells use G-Protein-Coupled Receptors (GPCRs), such as chemokine receptors, to control a signaling network that carries out chemotactic gradient sensing and directs cell migration. Diverse chemokines bind to their receptors to activate small G protein Rac through an evolutionarily conserved mechanism. Elmo and Dock180 proteins form ELMO/Dock180 complexes functioning as guanine nucleotide exchange factors (GEFs) for Rac activation. However, the linkage between GPCR to Elmo/Dock180 for Rac activation that controls F-actin dynamics remained unclear. Recently, we discovered a novel function of an ELMO protein in Dictyostelium discoideum linking GPCR signaling from Gβ to actin dynamics through regulating Rac activation during chemotaxis.

Open Access Article

Ral inhibits ligand-independent Notch signaling in Drosophila

Bomsoo Cho and Janice A. Fischer
Pages 186 - 191
http://dx.doi.org/10.4161/sgtp.19802
Abstract | Full Text | PDF

We discovered recently that the Drosophila Ral GTPase regulates Notch signaling and thereby affects cell patterning in the eye. Although Ral functions in the ligand signaling cells, Ral does not stimulate ligand signaling directly. Rather, in cells that express both Notch receptor and ligand, Ral activity promotes a cell to become the signaler by inhibiting Notch receptor activation in that cell. Moreover, Ral inhibits a particular pathway of Notch activation—receptor activation that occurs independent of ligand binding. In this Commentary, we discuss the phenomenon of ligand-independent Notch receptor activation and how this event might be regulated by Ral.

Rap1 and its regulatory proteins: The tumor suppressor, oncogene, tumor suppressor gene axis in head and neck cancer

Rajat Banerjee, Nickole Russo, Min Liu, Elizabeth Van Tubergen and Nisha J. D'Silva
Pages 192 - 197
http://dx.doi.org/10.4161/sgtp.20413
Abstract | Full Text | PDF

Squamous cell carcinoma of the head and neck (SCCHN) is the sixth most common cancer, globally. Previously, we showed that Rap1GAP is a tumor suppressor gene that inhibits tumor growth, but promotes invasion in SCCHN. In this work, we discuss the role of Rap1 and Rap1GAP in SCCHN progression in the context of a microRNA-oncogene-tumor suppressor gene axis, and investigate the role of Rap1GAP in EZH2-mediated invasion. Loss of expression of microRNA-101 in SCCHN leads to upregulation of EZH2, a histone methyltransferase. Overexpression of EZH2 silences Rap1GAP via methylation, thereby promoting activation of its target, Rap1. This microRNA-controlled activation of Rap1, via EZH2-mediated silencing of Rap1GAP, is a novel mechanism of Rap1 regulation. In two independent SCCHN cell lines, downregulation of EZH2 inhibits proliferation and invasion. In both cell lines, stable knockdown of EZH2 (shEZH2) recovers Rap1GAP expression and inhibits proliferation. However, siRNA-mediated knockdown of Rap1GAP in these cells rescues proliferation but not invasion. Thus, EZH2 promotes proliferation and invasion via Rap1GAP-dependent and –independent mechanisms, respectively. Although the studies presented here are in the context of SCCHN, our results may have broader implications, given that Rap1GAP acts as a tumor suppressor in pancreatic cancer, thyroid cancer, and melanoma.