Table 3.
Interacting partners and functional consequences.
| Molecular Partners | Full Name | Function | Effect on Rac1 and Biological Impact | References Linked with CRC * |
|---|---|---|---|---|
| RAC1 Downstream Effectors; Nitric Oxide (NO), Reactive Oxygen Species (ROS) Production | ||||
| NOXA1 | NADPH oxidase activator 1, P67phox-Like Factor | Cytosolic subunit recruited to the plasma membrane to form the active NADPH oxidase complex (NOX1) with p22phox, NOXO1, Rac1 and NADPH oxidase1 that produces the superoxide anion | Activated Rac-1 interacts with NOXA1 in the NOX1 holoenzyme at the membrane leading to reactive oxygen species production involved in cell signaling and innate immune response [202]. | |
| NCF2, P67-Phox | Neutrophil cytosol factor 2 (NCF-2) (67 kDa neutrophil oxidase factor) (NADPH oxidase activator 2) (Neutrophil NADPH oxidase factor 2) (p67-phox) | Rac-GTP binds to the N-terminal tetratricopeptide repeat domain of p67phox. Cytosolic subunit is recruited to the membrane to form the active NADPH oxidase complex (NOX2) with p22phox, p40phox, p47phox, Rac1 (monocytes, Rac2 neutrophils) and gp91phox that produces the superoxide anion | Regulation of phagocytic NADPH oxidase activity. Activated Rac-1 interacts with p67-Phox in the NADPH complex at the membrane, causes a conformational change in the "activation domain" in p67-Phox leading to reactive oxygen species production and innate immune response [203,204,205,206,207,208]. | |
| APEX1/APE1 | Apurinic/Apyrimidinic Endodeoxyribonuclease 1 | Endonuclease. Involved in DNA repair and redox regulation of transcriptional factors | Interaction with RAC1 impairs NOX1 complex formation. Decreased ROS production [81]. | |
| BCL2 | BCL2 Apoptosis Regulator | Integral outer mitochondrial membrane protein. Inhibitor of apoptotic death | Increased ROS production. Mitochondrial oxidative stress. Although anti-apoptotic molecule, a meta-analysis suggests that Bcl-2 is for a favorable prognosis for patient with CRC [209]. | [210] |
| CYCS | Cytochrome c | Central component of the electron transport chain in inner membrane of mitochondria | Mitochondrial ROS production. Electron transfer from cytochrome c to Rac-1 modulates mitochondrial H2O2 production. Rac1b phosphorylation at Ser71 facilitates its interaction with cytochrome c. Rac1B may be involved in Hutchinson-Gilford progeria syndrome. [27,211]. | |
| NOS2 | Nitric oxide synthase-2 | Inducible by LPS and certain cytokines, generates nitric oxide mediator | Increased nitrite generation and NOS2 activity through subcellular redistribution [212]. | |
| Transcription (co)Factors | ||||
| CTNNB1 | Catenin Beta 1, Beta-catenin |
Component of the complex of proteins that constitute adherens junctions: interacts with E-cadherins. Component of the canonical Wnt signaling pathway: In the presence of Wnt ligand or consecutively to mutations of components of the complexes that trigger its phosphorylation and degradation by the proteasome (e.g., APC), β-catenin translocates in the nucleus, and acts as a coactivator for transcription factors of the TCF/LEF family, leading to transcription of Wnt target genes (e.g., Myc, Cyclin D1, MMP7) | Rac-1 interacts with β-catenin through its polybasic region. In the absence of APC in mouse intestinal epithelium, Rac1 is not required for β-catenin nuclear localization and/or for its functional activity. In LoVo colon cancer cells (mutant APC), Rac1 phosphorylation at Ser71 by PKCZ promotes nuclear β-catenin accumulation through PAK1 activation. In NIH3T3, SW480 and HCT116 cells, Rac-1 silencing or overexpression do not influence this nuclear accumulation. In contrast upon Wnt stimulation, active Rac1 induces the redistribution of Rac1/ β-catenin protein complex from the plasma membrane to the nucleus, favors the formation of β-catenin/LEF1 complex and potentiates the transactivation of Wnt responsive genes, via Jnk -induced β-catenin phosphorylation. These discrepancies might originate from the level of Rac1 accumulation and activity, and from the cellular context [26,56,58,213,214] | [3,4] |
| DVL3 | Dishevelled Segment Polarity Protein 3 | Cytoplasmic phosphoprotein involved in Wnt signaling pathway | RAC1b interacts with Dishevelled-3 to form a tetramer with β-catenin/TCF. Transcription of canonical Wnt target genes [112]. | |
| STAT3 | Signal Transducer and Activator of Transcription 3 | Transcription factor activated by receptor associated kinases | Activated Rac1 stimulated STAT3 phosphorylation on both tyrosine and serine residues. Epithelial-mesenchymal transition and invasion of CRC cells. Overexpressed in CRC [215]. | [89,216] |
| UNKL, Unkempt | Unk Like Zinc Finger | Contributes to E3 ligase activity. Nuclear localization | Activated Rac1 interacts with UNKL and promotes ubiquitination of BAF60b, a component of SWI/SNF chromatin remodeling complexes involved in regulation of transcription and chromatin remodeling [217]. | |
| Kinases | ||||
| PAK1 | P21 (RAC1) Activated Kinase 1 | Serine/threonine-protein kinase PAK1, belongs to the subgroup I of PAKs. Interacts with the p21-binding domains PBDs of Rac1 | PAK1 is activated upon binding to GTP-Rac1. Implicated in cytoskeleton dynamics, cell adhesion, migration, proliferation, apoptosis, mitosis, DNA Damage Repair, and vesicle-mediated transport processes. PAK1 phosphorylates Bad leading to uncoupling of Bad/Bcl-2 and enhanced cell survival; effectors of cytoskeletal reorganization. Links Rac1 to JNK/MAPK pathways: phosphorylates and activates MAP2K1, RAF1. Regulates transcription through association and/or phosphorylation of transcription factors, co-regulators and cell cycle-related proteins. PAK1 phosphorylation of Snail favors its nuclear accumulation and promotes transcriptional repression of E-cadherin. Phosphorylation of NF-kB triggers nuclear translocation and the transcriptional activity of the p65 subunit. Implicated in mediating signaling from Rac1 to JNK and to actin cytoskeleton. PAK1 expression is associated with CRC metastasis [123,218,219,220,221,222]. | [219,220] |
| PAK4 | p21 protein (Cdc42/Rac)-activated kinase 4 Serine/threonine-protein kinase PAK4 | Serine/threonine-protein kinase PAK4, Belongs to the subgroup II of PAKs. Interacts with the p21-binding domains PBDs of Rac1 | PAK4 nuclear accumulation enhances β-catenin nuclear import and increases TCF/LEF transcriptional activity. PAK4 expression is associated with CRC metastasis. | [219,220,223] |
| PAK5 | p21 protein (Cdc42/Rac)-activated kinase 5 Serine/threonine-protein kinase PAK5 | Serine/threonine-protein kinase PAK5, belongs to the subgroup II of PAKs, mitochondrial localization | Overexpressed in CRC, correlates with tumor stage and dedifferentiation. | [224] |
| MAP3K1, MEKK1 | Mitogen-Activated Protein Kinase Kinase Kinase 1 | Serine/threonine kinase. Involved in signal transduction cascades of ERK and JNK kinase and NF-kappa-B pathways. Nuclear and post-Golgi vesicle-like compartment | Interacts with active GTP-bound Rac1. Activation of the Erk and JNK kinases via MAP2K1 and MAP2K4 [225]. |
|
| MAP3K10, MLK2 | Mitogen-Activated Protein Kinase Kinase Kinase 10 | Serine/threonine kinase. Activates MAPK8/JNK and MKK4 pathway | CRIB domain interacts with GTP-bound form of Rac1. Activates MAPK8/JNK and MKK4/SEK1 [226]. | |
| MAP3K4, MEKK4 | Mitogen-Activated Protein Kinase Kinase Kinase 4 | Serine threonine kinase. Activates MAPK14 (P38alpha) and JNK pathways, but not ERK | Interacts with active GTP-bound Rac1. JNK activation [225,227]. | |
| MAP3K11, MLK3 | Mitogen-Activated Protein Kinase Kinase Kinase 11 | Activates MAPK8/JNK and NF-kappaB transcriptional activity | CRIB domain interacts with GTP-bound form of Rac1. JNK activation [226]. | |
| Scaffolding Molecules/ Rac1 Subcellular Targeting/Cytoskeletton Remodeling, Membrane Ruffling | ||||
| IQGAP1, P195 | IQ Motif Containing GTPase Activating Protein 1; Ras GTPase-activating-like protein IQGAP1 | Scaffolding molecule. Regulates dynamics and assembly of actin cytoskeleton | Activated Rac1/Cdc42, IQGAP1, and CLIP-170 form a tripartite complex; activated Rac1 recruits MTs through IQGAP1. Role in cell polarization, actin crosslinking protein, accumulates at the polarized leading edge and areas of membrane ruffling. Overexpressed in tumor tissues as compared with control mucosa; localized at invasion front [228,229,230]. | [231] |
| IQGAP2 | Ras GTPase-activating-like protein IQGAP2 | Scaffolding molecule. Dynamics and assembly of actin cytoskeleton | Role in generation of specific actin structures; subcellular Rac1 localization. Expression in CRC correlates positively with patient survival [232]. | [233] |
| IQGAP3 | Ras GTPase-activating-like protein IQGAP3 | Scaffolding molecule. Dynamics and assembly of actin cytoskeleton | Overexpressed in MSS TP53 mutant CRC, expression levels correlated inversely with survival. | [233,234] |
| SH3RF1, POSH | SH3 Domain Containing Ring Finger 1, Plenty of SH3 Domains | Scaffolding molecules for components of the JNK signaling pathway. E3 ubiquitin-protein ligase activity | Links activated Rac1 and downstream JNK kinase cascade (MLKs, MLK4/7, JNK1/2). Induction apoptotic cascade [235,236]. | |
| SH3RF3, POSH2 | SH3 Domain Containing Ring Finger 3 | Scaffolding molecules for components of the JNK signaling pathway. E3 ubiquitin-protein ligase activity | Scaffold for a multiprotein complex that transduces signals from GTP-loaded Rac1 to JNK activation [237]. | |
| CCM2 | CCM2 Scaffold Protein, osmosensing scaffold for MEKK3 | Involved in stress-activated p38 Mitogen-activated protein kinase signaling cascade | Binds to actin, Rac1, MEKK3 and MKK3. Role in osmoregulation [238]. | |
| KPNA2 | Karyopherin alpha2, importin α-1 | Nuclear transport of proteins (binds NLS) | Interacts with Rac1 nuclear localization signal (NLS, C-terminal polybasic region). Nuclear import, (independent GDP/GTP loading), requires Rac1 activation. Nuclear Rac1 coimmunoprecipitates with numerous proteins. Overexpressed in CRC [239]. | [240] |
| YWHA, 14-3-3 | Tyrosine 3-Monooxygenase/Tryptophan 5-Monooxygenase Activation Protein | Binds to phosphoserine-containing proteins | Rac1 Ser71 phosphorylation increases affinity for 14-3-3 proteins. Interaction increases EGF -induced Rac1 activation. Cytoplasmic localisation of the complexe. Overexpressed in CRC [93]. | [241] |
| RAP1GDS1, SmgGDS | Rap1 GTPase-GDP dissociation stimulator 1 (Exchange factor smgGDS) (SMG GDS protein) (SMG P21 stimulatory GDP/GTP exchange protein) | C-terminal poly-basic region of Rac1 | SmgGDS is a GEF for RhoA and RhoC, but not for Rac1. SmgGDS interacts with RAC1 C-terminal polybasic region and triggers Rac1 nuclear translocation and degradation by proteasome. Rac1b interacts more efficiently to SmgGDS than Rac1. SmgGDS splice variants control Rac1 prenylation and membrane localization: SmgGDS-558 associates with prenylated Rac1, SmgGDS-607 associates with nonprenylated GTPases and regulates its entry into the prenylation pathway [111,242,243,244]. | |
| ARFIP2/POR1 | Arfaptin-2 (ADP-ribosylation factor-interacting protein 2) (Partner of RAC1) (Protein POR1) | Downstream effector, plasma membrane | POR1 binds preferentially to GTP-bound form of Rac1 (effector binding domain, aminoacid residues 26-48). Mediates Rac1-induced signals; membrane ruffling; regulates the organization of the actin cytoskeleton [245,246,247]. | |
| BAIAP2, IRSp53 | BAI1 Associated Protein 2 | Adaptor protein; links membrane bound G-proteins to cytoplasmic effector proteins | RAC1-mediated membrane ruffling. Involved in the regulation of the actin cytoskeleton by WASF family members and the Arp2/3 complex [248]. | |
| CTNND1/P120ctn | Catenin Delta 1/P120 catenin | Member of the Armadillo protein family. Involved in cell adhesion (binds E-cadherin) and transcription | P120 catenin interacts with the extra 19-amino acid sequence of RAC1b. Directed cell movement [111]. | |
| CAV1 | Caveolin-1 | Scaffolding protein, caveolar membranes | Coronin-1C and caveolin retrieve Rac1 from similar locations at the rear and sides of the cell. In absence of fibronectin, Coronin-1C-mediated Rac1-GDP extraction and recycling to the leading edge, and maintains Rac1 cellular levels. In absence of coronin-1C, caveolin-mediated endocytosis targets Rac1 for proteasomal degradation, consecutively to engagement of the fibronectin receptor syndecan-4 [249]. | |
| CORO1C | Coronin 1C | Member of the WD repeat protein family. Actin-binding proteins that regulate actin branching by inhibition of the Arp2/3 complex and stimulation of actin depolymerization by cofilin | Coro1C Binds Rac1-GDP. Release inactive Rac1 from non-protrusive membrane. Required for Rac1 redistribution to a protrusive tip and fibronectin-dependent Rac1 activation. Increases accumulation of activated RAC1 at the leading edge of migrating cells. Directional fibroblast migration [184,249]. | |
| CYFIP1, SRA-1 | Cytoplasmic FMR1 Interacting Protein 1 | Regulates cytoskeletal dynamics and protein translation. Component of the WAVE regulatory complex (WRC), through actin polymerization | RAC1 binds to CYFIP1, initiating WASF3 complex formation involved in cytoskeleton reorganization and polymerization. Promotes invasiveness of breast prostate and colon cancer cell lines [250,251]. | |
| FLNA | Filamin A | Actin binding protein, connects cell membrane constituents to actin cytoskeleton | Mechanical force through β1-integrins triggers apoptosis through Rac1/Pak1/p38 signaling pathway. FLNa recruits ARHGAP24 to sites of force application suppressing Rac1 activation, lamallae formation and Rac1/p38-mediated apoptosis. Down-regulated in CRC. | [252,253] |
| FLNB | Filamin B | Actin binding protein, connects cell membrane constituents to actin cytoskeleton | JNK activation and induction of apoptosis in response to type I Interferon [254,255]. | |
| FMNL1, FRL | Formin Like 1 | Actin polymerization, morphogenesis, cytokinesis, and cell polarity | Interacts preferentially with GTP-bound Rac1. Regulation of motility and survival of macrophages [256]. | |
| FMNL2 | Formin Like 2 | Actin polymerization, morphogenesis, cytokinesis, and cell polarity | Activation of Rac1 steers FMNL2 to de novo junctional actin formation at newly formed cell-cell contacts and adherens junction formation. FMNL2 enhances proliferation, motility and invasiveness of colon cancer cell lines. Overexpressed in CRC and in liver metastases [257]. | [258,259] |
| LRRK2 | Leucine Rich Repeat Kinase 2 | Member leucine-rich repeat kinase family (protein with ankryin repeat region, leucine-rich repeat domain, kinase domain, DFG-like motif, RAS domain, GTPase domain, MLK-like domain, and WD40 domain). Mutation causes dominant-inherited Parkinson’s disease | Overexpression and knockdown of LKRR2 simulates Rac1 activity. Role in maintenance of neurite morphology. Role in synaptic vesicle trafficking [260,261]. | |
| NEDD4-1 | Neural Precursor Cell Expressed, Developmentally Down-Regulated 4, E3 Ubiquitin Protein Ligase | E3 Ubiquitin Protein Ligase | Rac1 stimulates Nedd4 activity and increases ubiquitylation and degradation of the adapter protein dishevelled-1 that transduces Wnt signal downstream frizzled receptor. Maturation of epithelial cell-cell contacts. Overexpressed in CRC. [262]. | [263] |
| PARD6A, PARD6B, PARD6G | Par-6 Family Cell Polarity Regulator Alpha/Beta/Gamma | Protein with a PDZ domain and a semi-Cdc42/Rac interactive binding (CRIB) domain. Involved in asymmetrical cell division and cell polarization | Interacts with GTP-bound Rac1. PAR6, Rac1 and atypical PKC colocalize as a ternary complex in membrane ruffles (leading edge of polarized cells during movement) [264]. | |
| PIK3R1, P85A | Phosphoinositide-3-Kinase Regulatory Subunit 1 | Regulatory subunit of PI3K | P85 binds GTP bound Rac1. Rac1 triggers P85a nuclear translocation. Activation of ERK and JNK Signaling Cascades. Downregulated in CRC [265]. | [4,17,266] |
| PLCB2 | Phospholipase C Beta 2 | Hydrolysis of phosphatidylinositol 4,5-bisphosphate to 1,4,5-trisphosphate (IP3) and diacylglycerol | Rac1 engages the PH domain of PLC-beta2 and optimizes its orientation for substrate membranes. Increased PLC activity [267]. | |
| SET | SET nuclear proto-oncogene; protein phosphatase type 2A inhibitor | Inhibits acetylation of nucleosomes (especially histone H4) | SET potentiates Rac1-mediated cell migration. Phosphorylation at Ser9 dissociates SET dimers and allows SET redistribution from nucleus to cytoplasm. The SET/activated Rac1 complex is recruited to the plasma membrane, and stimulates kinase-mediated signaling. SET enhances cell migration, EMT, and induces MYC expression in CRC cells. Overexpressed is in early-stage CRC, associated with progression and aggressiveness, and a poor outcome. Lower levels of SET in MSI CRCs compared to MSS CRC [268]. | [269,270] |
| TOP2A | DNA Topoisomerase II Alpha | Catalyzes transient breaking and rejoining of two strands of duplex DNA to relieve torsional stress that occurs during DNA transcription and replication | Rac1 is required for DNA damage induction and subsequent activation of DNA Damage Repair following treatment with topo II inhibitors. Overexpressed in CRC. Knowdown in CRC cells decreases Akt and Erk activity, and suppresses cell proliferation and invasion [269,271]. | [272] |
* Highlighted references correspond to the involvement of the corresponding proteins (positively Red, negatively Green) to colorectal carcinogenesis, CRC aggressiveness, response to chemotherapy, or patient overall survival. Some discrepancies between their biological impact on Rac1 and their link with CRC might be related to the mobilization of distinct signaling pathways.