ReviewRac signaling in breast cancer: A tale of GEFs and GAPs
Highlights
► Rac GTPases are important players in breast tumorigenesis and metastasis. ► Receptors and oncogenes signal via Rac for proliferation and migration. ► Rac signaling is hyperactive in breast cancer. ► The Rac-GEF P-Rex1 is overexpressed in breast cancer. ► The P-Rex1/Rac signaling pathway is an attractive target for breast cancer therapy.
Introduction
The Rho/Rac GTPases are a family of small G-proteins widely implicated in normal physiology and disease. They play an important role in cytoskeleton rearrangements and are key regulators of cellular adhesion, migration, proliferation, survival, differentiation and malignant transformation [1]. Members of this family in humans are divided into 6 classes: Rho (RhoA, RhoB and RhoC), Rac (Rac1, Rac2, Rac3 and RhoG), Cdc42 (Cdc42, Tc10, TCL, Chp/Wrch-2 and Wrch-1), RhoBTB, Rnd and RhoT [2]. The most studied members are RhoA, Rac1 and Cdc42. Like most GTPases, Rho, Rac and Cdc42 function as molecular switches that cycle between an inactive state that binds GDP and an active state that binds GTP. GTP is hydrolyzed to GDP through their intrinsic GTPase activity to render the G-protein inactive [3]. The switch between GDP and GTP is primarily regulated by two types of proteins: GEFs (Guanine nucleotide Exchange Factors) that facilitate GTP loading and thereby activate the small G-protein, and GAPs (GTPase Activating Proteins) that stimulate the hydrolysis of GTP by enhancing intrinsic GTPase activity, thus leading to G-protein inactivation. A third class of proteins known as Rho GDIs (GDP Dissociation Inhibitors) sequesters the inactive GTPases in the cytosol, preventing their translocation and subsequent activation. Dissociation from Rho GDI becomes essential for proper activation of the G proteins [4], [5], [6].
Rac isoforms have a very high degree of homology. The greatest divergence is in the C-terminal end, which is also the hypervariable region in Ras [7]. This domain is important for driving subcellular localization and binding to specific cellular regulators [8]. Rac2 shares significant nucleotide sequence identity (~ 88%) with the other Rac isoforms. At the nucleotide level Rac3 has 77% identity with Rac1, 83% identity with Rac2 and 69% identity with RhoG. At the amino acid level, Rac3 has 92% identity with Rac1 and 89% identity with Rac2.
The Rac1 gene localizes to chromosome 7 (7p22) and comprises 7 exons over a length of 29 kB [9]. Rac1, but not Rac2 or Rac3 genes, contains an additional exon 3b that is included by alternative splicing in the variant Rac1b, a constitutively active mutant that is expressed mainly in colon and breast cancer [10], [11]. Rac1 is ubiquitously expressed, and it is involved in signal transduction pathways that control proliferation, adhesion, and migration. Its inactivation by gene targeting in mice leads to embryonic lethality caused by both gastrulation defects and apoptosis of mesodermal cells [12].
The Rac2 gene contains 7 exons in chromosome 22 (22q13.1) [13] and its expression is silenced in non-hematopoietic cells by DNA methylation [14]. Rac2-deficient mice show defects in neutrophil, macrophage, mast cell, lymphocyte B and lymphocyte T function [15], [16]. Rac2 plays an important role in integrin-mediated hematopoietic stem-cell adhesion [17]. Patients with impaired Rac2 function display major alterations in hematopoiesis and an immunodeficiency syndrome [18], [19].
The Rac3 gene encompasses 6 exons in chromosome 17 (17q25.3). Rac3 is primarily expressed in brain, although its expression has been reported in some human cell lines including GM04155 (lymphoblastic leukemia), K562 (chronic myelogenous leukemia), 5838 (Ewing sarcoma), HL60 (promyelocytic leukemia) and DU4475 (breast cancer) [8]. Rac3−/− mice display slight motor coordination problems and hyperactive behavior [20].
Rac proteins associate with membranes in order to carry out their biological functions. However, unlike other Ras superfamily proteins, this anchoring step is not achieved during biosynthesis but rather requires a combination of intrinsic and cooperative signaling. The first and most crucial signal is the post-translational modification of the “CAAX box” by incorporation of a geranyl–geranyl group or less frequently a farnesyl group. In cooperation with the CAAX box a closely located proline-rich domain contributes to the association of Rac with specific proteins in focal adhesion complexes [21], [22].
Section snippets
Regulation of Rac activity by GEFs and GAPs
As mentioned above, Rac cycles between inactive and active states, two conformations that depend on the binding of GDP and GTP, respectively. Guanine nucleotides have picomolar affinities for Rac, and as a consequence their dissociation rate from the G-protein is slow. In order to lead to fast responses such as actin cytoskeleton reorganization, GEFs accelerate GDP/GTP exchange by several orders of magnitude [23]. GEFs catalyze the dissociation of the nucleotide from the G-protein by modifying
Rac function and effectors
A main role for Rac is the regulation of cytoskeleton reorganization, as it promotes actin assembly required for the formation of lamellipodia and membrane ruffles [38]. Regulation of cytoskeleton dynamics is essential for the maintenance of cellular morphology, polarity, adhesion and migration [2]. The control of cytoskeleton reorganization via Rac involves at least two different mechanisms (Fig. 2). One is through the activation of Arp2/3, which has a prominent role in actin polymerization
Rac and cancer
Malignant cancer cells display abnormal migratory properties and have the ability to invade and metastasize. Since Rac modulates actin cytoskeleton reorganization and cell motility, impairment of Rac function by expressing a dominant-negative Rac mutants or Rac-GAPs markedly affects migration and invasiveness of cancer cells [32]. Of great interest, a recent study by Marshall and co-workers in melanoma models found that the Rac-specific GEF DOCK3 and the Rac effector WAVE2 are essential for
Rac and breast cancer
Rac1 was reported to be overexpressed or hyperactive in breast cancer tissues. Moreover, the active variant Rac1b is also expressed in human breast tumors [11]. Accumulating evidence indicates that the Rac effector Pak1 is implicated in breast cancer progression. Indeed, more than 50% of human breast tumors show overexpression and/or hyperactivation of Pak1 [87]. Early studies by Kumar and coworkers showed that expression of a kinase-dead Pak1 mutant reduced invasiveness in MDA-MB-231 breast
P-Rex Rac-GEFs: novel players in breast cancer
Recent studies identified the Rac-GEFs P-Rex1 and P-Rex2 as important players in cancer, particularly in breast and prostate cancer [76], [77], [78]. P-Rex1 mediates Rac activation and cell motility in breast cancer cells in response to stimulation of tyrosine-kinase receptors and GPCRs. Moreover, P-Rex1 is highly expressed in human breast tumors relative to normal mammary tissue, arguing for a potential role for this Rac-GEF in breast cancer progression [77]. In addition, P-Rex2 is a crucial
Concluding remarks: targeting the Rac pathway as an approach for breast cancer treatment?
Rac GTPases function as tightly regulated signaling nodes that mediate inputs from receptors and oncogenes. The aberrant expression and/or activity of Rac regulators, particularly Rac-GEFs, offer a number of possibilities for targeting the Rac pathway. Based on current structural-function information on the interaction of Rac with GEFs it has been possible to identify small molecules that fit in the surface groove of Rac1 which determines GEF specification. One good example is the compound
Acknowledgments
This work was supported by grants CA74197, CA129133, and CA139120 from NIH, and KG090522 from Susan G. Komen for the Cure (M.G.K.).
References (152)
Dev. Cell
(2002)- et al.
J. Biol. Chem.
(2003) - et al.
J. Biol. Chem.
(1989) - et al.
J. Biol. Chem.
(1997) - et al.
Biochem. Biophys. Res. Commun.
(2000) - et al.
Genomics
(1997) - et al.
Gene
(2004) - et al.
Immunity
(1999) - et al.
Blood
(2000) - et al.
Cell
(1999)
Cell
Cell
FEBS Lett.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
Cell
Curr. Biol.
Cell
Cell
Biochem. Biophys. Res. Commun.
Biochem. Biophys. Res. Commun.
Mol. Cells
J. Biol. Chem.
Mol. Cell
FEBS Lett.
Cell
Gastroenterology
Gastroenterology
Gastroenterology
Biochem. Biophys. Res. Commun.
J. Clin. Neurosci.
Cell
Mol. Cell
FEBS Lett.
Methods Enzymol.
Am. J. Pathol.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
J. Biol. Chem.
Bioessays
J. Cell Sci.
Nat. Cell Biol.
Nature
Oncogene
Oncogene
Cited by (155)
Phosphoinositide 3-kinase as a therapeutic target in angiogenic disease
2023, Experimental Eye ResearchCharacterisation and outcome of RAC1 mutated melanoma
2023, European Journal of CancerRepurposing of Guanabenz acetate by encapsulation into long-circulating nanopolymersomes for treatment of triple-negative breast cancer
2021, International Journal of PharmaceuticsThe Rac inhibitor HV-107 as a potential therapeutic for metastatic breast cancer
2023, Molecular Medicine
- 1
Current address: Division of Hematology and Oncology, Department of Medicine, Mount Sinai School of Medicine, New York, NY 10029, USA.