Rho GTPases, their post-translational modifications, disease-associated mutations and pharmacological inhibitors

Effects of Different Combinations of Phytochemical-Rich Fruits and Vegetables on Chronic Disease Risk Markers and Gene Expression Changes: Insights from the MiBLEND Study, a Randomized Trial

Adequate fruit and vegetable (F and V) intake, as recommended by the World Health Organization (over 400 g/day), is linked to reduced chronic disease risk. However, human intervention trials, especially with whole F and V and in complex combinations, are lacking. The MiBlend Study explored the effects of various phytochemical-rich F and V combinations on chronic disease risk markers, phytochemical absorption, and gene expression in blood. This randomized cross-over study involved participants consuming two of seven different F and V blends for 2 weeks (450 g/day), following a 2-week low F and V intake period (50 g/day). Each blend represented major phytochemical classes (flavonoids, anthocyanins, carotenoids, and glucosinolates) or combinations thereof. Markers of chronic disease risk, including DNA damage, oxidative stress, and retinal microvasculature, were measured. Increasing F and V intake significantly improved plasma antioxidant capacity, DNA damage protection, and retinal arteriolar dilation. Flavonoid-rich, carotenoid-rich, and complex blends notably reduced DNA damage susceptibility. Anthocyanin-rich and carotenoid-rich interventions were most effective in boosting antioxidant capacity, while blends high in flavonoids, especially combined with anthocyanins, significantly improved retinal microvasculature. Gene expression analysis revealed changes in DNA repair, signal transduction, and transcription processes, indicating mechanisms for these health benefits. The study suggests specific F and V blends can provide targeted health improvements, emphasizing the importance of both overall F and V intake and the specific phytochemical composition for personalized preventive strategies.

The Roles of RAC1 and RAC1B in Colorectal Cancer and Their Potential Contribution to Cetuximab Resistance

Colorectal cancer (CRC) is one of the most diagnosed cancers and a leading contributor to cancer-related deaths in the United States. Clinically, standard treatment regimens include surgery, radiation, and chemotherapy; however, there has been increasing development and clinical use of targeted therapies for CRC. Unfortunately, many patients develop resistance to these treatments. Cetuximab, the first targeted therapy approved to treat advanced CRC, is a monoclonal antibody that targets the epidermal growth factor receptor and inhibits downstream pathway activation to restrict tumor cell growth and proliferation. CRC resistance to cetuximab has been well studied, and common resistance mechanisms include constitutive signal transduction through downstream protein mutations and promotion of the epithelial-to-mesenchymal transition. While the most common resistance mechanisms are known, a proportion of patients develop resistance through unknown mechanisms. One protein predicted to contribute to therapy resistance is RAC1, a small GTPase that is involved in cytoskeleton rearrangement, cell migration, motility, and proliferation. RAC1 has also been shown to be overexpressed in CRC. Despite evidence that RAC1 and its alternative splice isoform RAC1B play important roles in CRC and the pathways known to contribute to cetuximab resistance, there is a need to directly study the relationship between RAC1 and RAC1B and cetuximab resistance. This review highlights the recent studies investigating RAC1 and RAC1B in the context of CRC and suggests that these proteins could play a role in resistance to cetuximab.

ROP GTPases with a geranylgeranylation motif modulate alkaloid biosynthesis in Catharanthus roseus

Rho of Plant (ROP) GTPases function as molecular switches that control signaling processes essential for growth, development, and defense. However, their role in specialized metabolism is poorly understood. Previously, we demonstrated that inhibition of protein geranylgeranyl transferase (PGGT-I) negatively impacts the biosynthesis of monoterpene indole alkaloids (MIA) in Madagascar periwinkle (Catharanthus roseus), indicating the involvement of prenylated proteins in signaling. Here, we show through biochemical, molecular, and in planta approaches that specific geranylgeranylated ROPs modulate C. roseus MIA biosynthesis. Among the six C. roseus ROP GTPases (CrROPs), only CrROP3 and CrROP5, having a C-terminal CSIL motif, were specifically prenylated by PGGT-I. Additionally, their transcripts showed higher expression in most parts than other CrROPs. Protein-protein interaction studies revealed that CrROP3 and CrROP5, but not ΔCrROP3, ΔCrROP5, and CrROP2 lacking the CSIL motif, interacted with CrPGGT-I. Further, CrROP3 and CrROP5 exhibited nuclear localization, whereas CrROP2 was localized to the plasma membrane. In planta functional studies revealed that silencing of CrROP3 and CrROP5 negatively affected MIA biosynthesis, while their overexpression upregulated MIA formation. In contrast, silencing and overexpression of CrROP2 had no effect on MIA biosynthesis. Moreover, overexpression of ΔCrROP3 and ΔCrROP5 mutants devoid of sequence coding for the CSIL motif failed to enhance MIA biosynthesis. These results implicate that CrROP3 and CrROP5 have a positive regulatory role on MIA biosynthesis and thus shed light on how geranylgeranylated ROP GTPases mediate the modulation of specialized metabolism in C. roseus.

Functional Conservation of the Small GTPase Rho5/Rac1-A Tale of Yeast and Men

Small GTPases are molecular switches that participate in many essential cellular processes. Amongst them, human Rac1 was first described for its role in regulating actin cytoskeleton dynamics and cell migration, with a close relation to carcinogenesis. More recently, the role of Rac1 in regulating the production of reactive oxygen species (ROS), both as a subunit of NADPH oxidase complexes and through its association with mitochondrial functions, has drawn attention. Malfunctions in this context affect cellular plasticity and apoptosis, related to neurodegenerative diseases and diabetes. Some of these features of Rac1 are conserved in its yeast homologue Rho5. Here, we review the structural and functional similarities and differences between these two evolutionary distant proteins and propose yeast as a useful model and a device for high-throughput screens for specific drugs.

ECM1 promotes migration and invasion in endometriosis

Extracellular matrix protein 1 (ECM1) is a glycoprotein that may be a key player in tumorigenesis and tumor progression. However, knowledge regarding the role of ECM1 in endometriosis (EM) is still lacking. Microarray analyses were performed to compare the mRNA expression patterns between paired EU tissues and ectopic endometrial (EC) tissues (n = 4) from EM patients. ECM1 expression was significantly increased in the eutopic endometrial (EU) tissues than paired EC tissues of endometriotic patients and normal endometrial (NE) tissues of controls without EM. Blocking ECM1 with siRNA attenuated the migration and invasion of hEM15A cells and modified the distribution of the F-actin cytoskeleton. We conducted microarray analyses and bioinformatics analyses to investigate the differentially expressed genes (DEGs) and related pathways regulated by ECM1. A total of 161 DEGs between the siECM1 and the negative control (siNC) treatments were identified, consisting of 79 downregulated genes and 82 upregulated genes. Enriched DEGs were associated with 9 gene ontology (GO) terms. Moreover, a protein-protein interaction (PPI) network was constructed for the hub genes and modules. Radixin (RDX) was the second most downregulated gene in the siECM1 group compared with the siNC group. ECM1 knockdown significantly decreased the expression of RDX, RhoC, ROCK1, N-cadherin and β-catenin but not ROCK2. ECM1 showed high tissue-specific expression in EU tissues from EM patients, and may contribute to the migration, invasion and reorganization of the F-actin cytoskeleton in eutopic endometrial stromal cells via the RhoC/ROCK1 signaling pathway in EM.

DIRAS3 enhances RNF19B-mediated RAC1 ubiquitination and degradation in non-small-cell lung cancer cells

Distant metastasis remains the leading cause of high mortality in patients with non-small-cell lung cancer (NSCLC). DIRAS3 is a candidate tumor suppressor protein that is decreased in various tumors. However, the regulatory mechanism of DIRAS3 on metastasis of NSCLC remains unclear. Here, we found that DIRAS3 suppressed the migration of NSCLC cells. Besides, DIRAS3 stimulated the polyubiquitination of RAC1 and suppressed its protein expression. Furthermore, RNF19B, a member of the RBR E3 ubiquitin ligase family, was observed to be the E3 ligase involved in the DIRAS3-induced polyubiquitination of RAC1. DIRAS3 could promote the binding of RAC1 and RNF19B, thus enhancing the degradation of RAC1 by the ubiquitin-proteasome pathway. Finally, the DIRAS3-RNF19B-RAC1 axis was confirmed to be associated with the malignant progression of NSCLC. These findings may be beneficial for developing potential prognostic markers of NSCLC and may provide an effective treatment strategy.

RhoB as a tumor suppressor: It’s all about localization

The small GTPase RhoB is distinguished from other Rho proteins by its unique subcellular localization in endosomes, multivesicular bodies, and nucleus. Despite high sequence homology with RhoA and RhoC, RhoB is mainly associated with tumor suppressive function, while RhoA and RhoC support oncogenic transformation in most malignancies. RhoB regulates the endocytic trafficking of signaling molecules and cytoskeleton remodeling, thereby controlling growth, apoptosis, stress response, immune function, and cell motility in various contexts. Some of these functions may be ascribed to RhoB's unique subcellular localization to endocytic compartments. Here we describe the pleiotropic roles of RhoB in cancer suppression in the context of its subcellular localization, and we discuss possible therapeutic avenues to pursue and highlight priorities for future research.

Targeting small GTPases: emerging grasps on previously untamable targets, pioneered by KRAS

Small GTPases including Ras, Rho, Rab, Arf, and Ran are omnipresent molecular switches in regulating key cellular functions. Their dysregulation is a therapeutic target for tumors, neurodegeneration, cardiomyopathies, and infection. However, small GTPases have been historically recognized as "undruggable". Targeting KRAS, one of the most frequently mutated oncogenes, has only come into reality in the last decade due to the development of breakthrough strategies such as fragment-based screening, covalent ligands, macromolecule inhibitors, and PROTACs. Two KRAS^G12C covalent inhibitors have obtained accelerated approval for treating KRAS^G12C mutant lung cancer, and allele-specific hotspot mutations on G12D/S/R have been demonstrated as viable targets. New methods of targeting KRAS are quickly evolving, including transcription, immunogenic neoepitopes, and combinatory targeting with immunotherapy. Nevertheless, the vast majority of small GTPases and hotspot mutations remain elusive, and clinical resistance to G12C inhibitors poses new challenges. In this article, we summarize diversified biological functions, shared structural properties, and complex regulatory mechanisms of small GTPases and their relationships with human diseases. Furthermore, we review the status of drug discovery for targeting small GTPases and the most recent strategic progress focused on targeting KRAS. The discovery of new regulatory mechanisms and development of targeting approaches will together promote drug discovery for small GTPases.

Novel Loss of Function (G15D) Mutation on RAC2 in a Family with Combined Immunodeficiency and Increased Levels of Immunoglobulin G, A, and E

Ras-related C3 botulinum toxin substrate 2 (RAC2) is a small guanine nucleotide binding molecule that is exclusively expressed in hematopoietic cell lineages as a switcher. Based on in vivo and/or in vitro model experiments, RAC2 plays important roles in different cells through proliferation, secretion, and phagocytosis. It also performs a suppressing function in immunoglobulin (Ig) switching in Rac2-/- animals or cells. Several RAC2 natural mutations have been described in patients with primary immunodeficiency. RAC2 mutations can be classified into loss-of-function inactivating (LoF-I) and gain-of-function activating mutations according to their functional effects. Only two LoF-I mutations on RAC2 have been reported, including a dominant D57N mutation in several cases that exhibit granulocyte function defects and a recessive D56X mutation in cases with common variable immunodeficiency. Regardless of the type of mutation, most of the reported RAC2 mutant cases have shown reduced IgG, IgA, and IgM levels. Herein, we report on a family with three members that suffer from persistent HPV infection, recurrent respiratory infections, bronchiectasis, and autoimmune disease. The immunologic profile suggests that the family was affected by combined immunodeficiency (CID) with increased serum levels of IgG, IgA, and IgE. Exome sequencing identified a de novo RAC2 mutation (c.44G > A/p.G15D) that was co-segregated with the disease in the family. Gene functional experiments identified that such mutation results in reduced guanosine triphosphate binding activity and RAC2 protein expression. In patients' lymphocytes, impaired aggregation and proliferation effects, decreased mitochondrial membrane potential, and increased levels of cell apoptosis were observed, although no functional abnormalities were detected in neutrophils. To our knowledge, this study was the first to identify a LoF-I mutation of RAC2 affecting lymphocyte function that consequently led to CID and increased levels of serum IgG, IgE, and IgA. This study presents a novel subtype of RAC2-related immune disorder.

Scar/WAVE has Rac GTPase-independent functions during cell wound repair

Rho family GTPases regulate both linear and branched actin dynamics by activating downstream effectors to facilitate the assembly and function of complex cellular structures such as lamellipodia and contractile actomyosin rings. Wiskott-Aldrich Syndrome (WAS) family proteins are downstream effectors of Rho family GTPases that usually function in a one-to-one correspondence to regulate branched actin nucleation. In particular, the WAS protein Scar/WAVE has been shown to exhibit one-to-one correspondence with Rac GTPase. Here we show that Rac and SCAR are recruited to cell wounds in the Drosophila repair model and are required for the proper formation and maintenance of the dynamic actomyosin ring formed at the wound periphery. Interestingly, we find that SCAR is recruited to wounds earlier than Rac and is still recruited to the wound periphery in the presence of a potent Rac inhibitor. We also show that while Rac is important for actin recruitment to the actomyosin ring, SCAR serves to organize the actomyosin ring and facilitate its anchoring to the overlying plasma membrane. These differing spatiotemporal recruitment patterns and wound repair phenotypes highlight the Rac-independent functions of SCAR and provide an exciting new context in which to investigate these newly uncovered SCAR functions.

Identification of a small RhoA GTPase inhibitor effective in fission yeast and human cells

The Rho GTPase family proteins are key regulators of cytoskeletal dynamics. Deregulated activity of Rho GTPases is associated with cancers and neurodegenerative diseases, and their potential as drug targets has long been recognized. Using an economically effective drug screening workflow in fission yeast and human cells, we have identified a Rho GTPase inhibitor, O1. By a suppressor mutant screen in fission yeast, we find a point mutation in the rho1 gene that confers resistance to O1. Consistent with the idea that O1 is the direct inhibitor of Rho1, O1 reduced the cellular amount of activated, GTP-bound Rho1 in wild-type cells, but not in the O1-resistant mutant cells, in which the evolutionarily conserved Ala62 residue is mutated to Thr. Similarly, O1 inhibits activity of the human orthologue RhoA GTPase in tissue culture cells. Our studies illustrate the power of yeast phenotypic screens in the identification and characterization of drugs relevant to human cells and have identified a novel GTPase inhibitor for fission yeast and human cells.

Bacterial infection promotes tumorigenesis of colorectal cancer via regulating CDC42 acetylation

Increasing evidence highlights the role of bacteria in promoting tumorigenesis. The underlying mechanisms may be diverse and remain poorly understood. Here, we report that Salmonella infection leads to extensive de/acetylation changes in host cell proteins. The acetylation of mammalian cell division cycle 42 (CDC42), a member of the Rho family of GTPases involved in many crucial signaling pathways in cancer cells, is drastically reduced after bacterial infection. CDC42 is deacetylated by SIRT2 and acetylated by p300/CBP. Non-acetylated CDC42 at lysine 153 shows an impaired binding of its downstream effector PAK4 and an attenuated phosphorylation of p38 and JNK, consequently reduces cell apoptosis. The reduction in K153 acetylation also enhances the migration and invasion ability of colon cancer cells. The low level of K153 acetylation in patients with colorectal cancer (CRC) predicts a poor prognosis. Taken together, our findings suggest a new mechanism of bacterial infection-induced promotion of colorectal tumorigenesis by modulation of the CDC42-PAK axis through manipulation of CDC42 acetylation.

Role of a small GTPase Cdc42 in aging and age-related diseases

A small GTPase, Cdc42 is evolutionarily one of the most ancient members of the Rho family, which is ubiquitously expressed and involved in a wide range of fundamental cellular functions. The crucial role of Cdc42 includes regulation of the actin cytoskeleton, cell polarity, morphology and migration, endocytosis and exocytosis, cell cycle, and proliferation in many different cell types. Many studies have provided compelling yet contradicting evidence that Cdc42 dysregulation plays an important role in cellular and tissue aging. Furthermore, Cdc42 is a critical factor in the development and progression of aging-related pathologies, such as neurodegenerative and cardiovascular disorders, diabetes type 2, and aging-related disorders of the joints and bones, and the inhibition of the Cdc42 demonstrates potentially significant therapeutic and anti-aging effects in animal models of aging and disease. However, regulation of Cdc42 expression and activity is very complex and depends on many factors, such as the origin and complexity of the tissues, hormonal status, etc. Therefore, this review is focused on current advances in understanding the underlying cellular and molecular mechanisms associated with Cdc42 activity and regulation of senescence in different cell types since they may provide a foundation for novel therapeutic strategies and targeted drugs to reverse the aging process and treat aging-associated disorders.

Options of Therapeutics and Novel Delivery Systems of Drugs for the Treatment of Melanoma

Melanoma is one of the most severe cancerous diseases. The cells employ multiple signaling pathways, such as ERK, HGF/c-MET, WNT, and COX-2 to cause the cell proliferation, survival, and metastasis. Treatment of melanoma, including surgery, chemotherapy, immunotherapy, radiation, and targeted therapy, is based on 4 major or 11 substages of the disease. Fourteen drugs, including dacarbazine, interferon α-2b, interleukin-12, ipilimumab, peginterferon α-2b, vemurafenib, trametinib, talimogene laherparepvec, cobimetinib, pembrolizumab, dabrafenib, binimetinib, encorafenib, and nivolumab, have been approved by the FDA for the treatment of melanoma. All of them are in conventional dosage forms of injection solutions, suspensions, oral tablets, or capsules. Major drawbacks of the treatment are side effects of the drugs and patients' incompliance to them. These are consequences of high doses and long-term treatments for the diseases. Currently more than 350 NCI-registered clinical trials are being carried out to treat advanced and/or metastatic melanoma using novel treatment methods, such as immune cell therapy, cancer vaccines, and new therapeutic targets. In addition, novel delivery systems using biomaterials of the approved drugs have been developed attempting to increase the drug delivery, targeting, stability, bioavailability, thus potentially reducing the toxicity and increasing the treatment effectiveness. Nanoparticles and liposomes have been emerging as advanced delivery systems which can improve drug stability and systemic circulation time. In this review, the most recent findings in the options for treatment and development of novel drug delivery systems for the treatment of melanoma are comprehensively discussed.

Evaluation of autoantibody signatures in pituitary adenoma patients using human proteome arrays

PURPOSE

To identify the specific diagnostic biomarkers related to pituitary adenomas (PAs), we performed serological antibody profiles for three types of PAs, namely Acromegaly, Cushing's and Nonfunctional Pituitary Adenomas (NFPAs), using the human proteome (HuProt) microarray. This is the first study describing the serum autoantibody profile of PAs.

EXPERIMENTAL DESIGN

We performed serological autoantibody profiling of four healthy controls, four Acromegaly, three Cushing's and three NFPAs patient samples to obtain their autoantibody profiles, which were used for studying expression, interaction and altered biological pathways. Further, significant autoantibodies of PAs were compared with data available for glioma, meningioma and AAgAtlas for their specificity.

RESULTS

Autoantibody profile of PAs led to the identification of differentially expressed significant proteins such as AKNAD1 (AT-Hook Transcription Factor [AKNA] Domain Containing 1), NINJ1 (Nerve injury-induced protein 1), L3HYPDH (Trans-3-hydroxy-L-proline dehydratase), RHOG (Rho-related GTP-binding protein) and PTP4A1 (Protein Tyrosine Phosphatase Type IVA 1) in Acromegaly. Protein ABR (Active breakpoint cluster region-related protein), ST6GALNAC6 (ST6 N-acetylgalactosaminide alpha-2, 6-sialyltransferase 6), NOL3 (Nucleolar protein 3), ANXA8 (Annexin A8) and POLR2H (RNA polymerase II, I and III subunit H) showed an antigenic response in Cushing's patient's serum samples. Protein dipeptidyl peptidase 3 (DPP3) and reticulon-4 (RTN4) exhibited a very high antigenic response in NFPA patients. These proteins hold promise as potential autoantibody biomarkers in PAs.

Nascent RHOH acts as a molecular brake on actomyosin-mediated effector functions of inflammatory neutrophils

In contrast to molecular changes associated with increased inflammatory responses, little is known about intracellular counter-regulatory mechanisms that control signaling cascades associated with functional responses of neutrophils. Active RHO GTPases are typically considered as effector proteins that elicit cellular responses. Strikingly, we show here that RHOH, although being constitutively GTP-bound, limits neutrophil degranulation and the formation of neutrophil extracellular traps (NETs). Mechanistically, RHOH is induced under inflammatory conditions and binds to non-muscle myosin heavy chain IIA (NMHC IIA) in activated neutrophils in order to inhibit the transport of mitochondria and granules along actin filaments, which is partially reverted upon disruption of the interaction with NMHC IIA by introducing a mutation in RhoH at lysine 34 (RhoHK34A). In parallel, RHOH inhibits actin polymerization presumably by modulating RAC1 activity. In vivo studies using Rhoh-/- mice, demonstrate an increased antibacterial defense capability against Escherichia coli (E. coli). Collectively, our data reveal a previously undefined role of RHOH as a molecular brake for actomyosin-mediated neutrophil effector functions, which represents an intracellular regulatory axis involved in controlling the strength of an antibacterial inflammatory response.

Tripartite split-GFP assay to identify selective intracellular nanobody that suppresses GTPase RHOA subfamily downstream signaling

Strategies based on intracellular expression of artificial binding domains present several advantages over manipulating nucleic acid expression or the use of small molecule inhibitors. Intracellularly-functional nanobodies can be considered as promising macrodrugs to study key signaling pathways by interfering with protein-protein interactions. With the aim of studying the RAS-related small GTPase RHOA family, we previously isolated, from a synthetic phage display library, nanobodies selective towards the GTP-bound conformation of RHOA subfamily proteins that lack selectivity between the highly conserved RHOA-like and RAC subfamilies of GTPases. To identify RHOA/ROCK pathway inhibitory intracellular nanobodies, we implemented a stringent, subtractive phage display selection towards RHOA-GTP followed by a phenotypic screen based on F-actin fiber loss. Intracellular interaction and intracellular selectivity between RHOA and RAC1 proteins was demonstrated by adapting the sensitive intracellular protein-protein interaction reporter based on the tripartite split-GFP method. This strategy led us to identify a functional intracellular nanobody, hereafter named RH28, that does not cross-react with the close RAC subfamily and blocks/disrupts the RHOA/ROCK signaling pathway in several cell lines without further engineering or functionalization. We confirmed these results by showing, using SPR assays, the high specificity of the RH28 nanobody towards the GTP-bound conformation of RHOA subfamily GTPases. In the metastatic melanoma cell line WM266-4, RH28 expression triggered an elongated cellular phenotype associated with a loss of cellular contraction properties, demonstrating the efficient intracellular blocking of RHOA/B/C proteins downstream interactions without the need of manipulating endogenous gene expression. This work paves the way for future therapeutic strategies based on protein-protein interaction disruption with intracellular antibodies.

The Intracellular Distribution of the Small GTPase Rho5 and Its Dimeric Guanidine Nucleotide Exchange Factor Dck1/Lmo1 Determine Their Function in Oxidative Stress Response

Rho5, the yeast homolog of human Rac1, is a small GTPase which regulates the cell response to nutrient and oxidative stress by inducing mitophagy and apoptosis. It is activated by a dimeric GEF composed of the subunits Dck1 and Lmo1. Upon stress, all three proteins rapidly translocate from the cell surface (Rho5) and a diffuse cytosolic distribution (Dck1 and Lmo1) to mitochondria, with translocation of the GTPase depending on both GEF subunits. We here show that the latter associate with mitochondria independent from each other and from Rho5. The trapping of Dck1-GFP or GFP-Lmo1 to the mitochondrial surface by a specific nanobody fused to the transmembrane domain (TMD) of Fis1 results in a loss of function, mimicking the phenotypes of the respective gene deletions, dck1 or lmo1. Direct fusion of Rho5 to Fis1TMD, i.e., permanent attachment to the mitochondria, also mimics the phenotypes of an rho5 deletion. Together, these data suggest that the GTPase needs to be activated at the plasma membrane prior to its translocation in order to fulfill its function in the oxidative stress response. This notion is substantiated by the observation that strains carrying fusions of Rho5 to the cell wall integrity sensor Mid2, confining the GTPase to the plasma membrane, retained their function. We propose a model in which Rho5 activated at the plasma membrane represses the oxidative stress response under standard growth conditions. This repression is relieved upon its GEF-mediated translocation to mitochondria, thus triggering mitophagy and apoptosis.

Bending over backwards: BAR proteins and the actin cytoskeleton in mammalian receptor-mediated endocytosis

The role of the actin cytoskeleton during receptor-mediated endocytosis (RME) has been well characterized in yeast for many years. Only more recently has the interplay between the actin cytoskeleton and RME been extensively explored in mammalian cells. These studies have revealed the central roles of BAR proteins in RME, and have demonstrated significant roles of BAR proteins in linking the actin cytoskeleton to this cellular process. The actin cytoskeleton generates and transmits mechanical force to promote the extension of receptor-bound endocytic vesicles into the cell. Many adaptor proteins link and regulate the actin cytoskeleton at the sites of endocytosis. This review will cover key effectors, adaptors and signalling molecules that help to facilitate the invagination of the cell membrane during receptor-mediated endocytosis, including recent insights gained on the roles of BAR proteins. The final part of this review will explore associations of alterations to genes encoding BAR proteins with cancer.

Insights into the Mechanisms of Lactobacillus acidophilus Activity against Entamoeba histolytica by Using Thiol Redox Proteomics

Amebiasis is an intestinal disease transmitted by the protist parasite, Entamoeba histolytica. Lactobacillus acidophilus is a common inhabitant of healthy human gut and a probiotic that has antimicrobial properties against a number of pathogenic bacteria, fungi, and parasites. The aim of this study was to investigate the amebicide activity of L. acidophilus and its mechanisms. For this purpose, E. histolytica and L. acidophilus were co-incubated and the parasite's viability was determined by eosin dye exclusion. The level of ozidized proteins (OXs) in the parasite was determined by resin-assisted capture RAC (OX-RAC). Incubation with L. acidophilus for two hours reduced the viability of E. histolytica trophozoites by 50%. As a result of the interaction with catalase, an enzyme that degrades hydrogen peroxide (H2O2) to water and oxygen, this amebicide activity is lost, indicating that it is mediated by H2O2 produced by L. acidophilus. Redox proteomics shows that L. acidophilus triggers the oxidation of many essential amebic enzymes such as pyruvate: ferredoxin oxidoreductase, the lectin Gal/GalNAc, and cysteine proteases (CPs). Further, trophozoites of E. histolytica incubated with L. acidophilus show reduced binding to mammalian cells. These results support L. acidophilus as a prophylactic candidate against amebiasis.

Structure-based design of CDC42 effector interaction inhibitors for the treatment of cancer

CDC42 family GTPases (RHOJ, RHOQ, CDC42) are upregulated but rarely mutated in cancer and control both the ability of tumor cells to invade surrounding tissues and the ability of endothelial cells to vascularize tumors. Here, we use computer-aided drug design to discover a chemical entity (ARN22089) that has broad activity against a panel of cancer cell lines, inhibits S6 phosphorylation and MAPK activation, activates pro-inflammatory and apoptotic signaling, and blocks tumor growth and angiogenesis in 3D vascularized microtumor models (VMT) in vitro. Additionally, ARN22089 has a favorable pharmacokinetic profile and can inhibit the growth of BRAF mutant mouse melanomas and patient-derived xenografts in vivo. ARN22089 selectively blocks CDC42 effector interactions without affecting the binding between closely related GTPases and their downstream effectors. Taken together, we identify a class of therapeutic agents that influence tumor growth by modulating CDC42 signaling in both the tumor cell and its microenvironment.

Rho GTPases: Non-canonical regulation by cysteine oxidation

Rho GTPases are critically important and are centrally positioned regulators of the actomyosin cytoskeleton. By influencing the organization and architecture of the cytoskeleton, Rho proteins play prominent roles in many cellular processes including adhesion, migration, intra-cellular transportation, and proliferation. The most important method of Rho GTPase regulation is via the GTPase cycle; however, post-translational modifications (PTMs) also play critical roles in Rho protein regulation. Relative to other PTMs such as lipidation or phosphorylation that have been extensively characterized, protein oxidation is a regulatory PTM that has been poorly studied. Protein oxidation primarily occurs from the reaction of reactive oxygen species (ROS), such as hydrogen peroxide (H₂ O₂ ), with amino acid side chain thiols on cysteine (Cys) and methionine (Met) residues. The versatile redox modifications of cysteine residues exemplify their integral role in cell signalling processes. Here we review prominent members of the Rho GTPase family and discuss how lipidation, phosphorylation, and oxidation on conserved cysteine residues affects their regulation and function.

Electrostatic Forces Mediate the Specificity of RHO GTPase-GDI Interactions

Three decades of research have documented the spatiotemporal dynamics of RHO family GTPase membrane extraction regulated by guanine nucleotide dissociation inhibitors (GDIs), but the interplay of the kinetic mechanism and structural specificity of these interactions is as yet unresolved. To address this, we reconstituted the GDI-controlled spatial segregation of geranylgeranylated RHO protein RAC1 in vitro. Various biochemical and biophysical measurements provided unprecedented mechanistic details for GDI function with respect to RHO protein dynamics. We determined that membrane extraction of RHO GTPases by GDI occurs via a 3-step mechanism: (1) GDI non-specifically associates with the switch regions of the RHO GTPases; (2) an electrostatic switch determines the interaction specificity between the C-terminal polybasic region of RHO GTPases and two distinct negatively-charged clusters of GDI1; (3) a non-specific displacement of geranylgeranyl moiety from the membrane sequesters it into a hydrophobic cleft, effectively shielding it from the aqueous milieu. This study substantially extends the model for the mechanism of GDI-regulated RHO GTPase extraction from the membrane, and could have implications for clinical studies and drug development.

Targeting small GTPases and their downstream pathways with intracellular macromolecule binders to define alternative therapeutic strategies in cancer

The RAS superfamily of small GTPases regulates major physiological cellular processes. Mutation or deregulation of these small GTPases, their regulators and/or their effectors are associated with many diseases including cancer. Hence, targeting these classes of proteins is an important therapeutic strategy in cancer. This has been recently achieved with the approval of the first KRASG12C covalent inhibitors for the clinic. However, many other mutants and small GTPases are still considered as 'undruggable' with small molecule inhibitors because of a lack of well-defined pocket(s) at their surface. Therefore, alternative therapeutic strategies have been developed to target these proteins. In this review, we discuss the use of intracellular antibodies and derivatives - reagents that bind their antigen inside the cells - for the discovery of novel inhibitory mechanisms, targetable features and therapeutic strategies to inhibit small GTPases and their downstream pathways. These reagents are also versatile tools used to better understand the biological mechanisms regulated by small GTPases and to accelerate the drug discovery process.

RHO GTPase-Related Long Noncoding RNAs in Human Cancers

RHO GTPases are critical signal transducers that regulate cell adhesion, polarity, and migration through multiple signaling pathways. While all these cellular processes are crucial for the maintenance of normal cell homeostasis, disturbances in RHO GTPase-associated signaling pathways contribute to different human diseases, including many malignancies. Several members of the RHO GTPase family are frequently upregulated in human tumors. Abnormal gene regulation confirms the pivotal role of lncRNAs as critical gene regulators, and thus, they could potentially act as oncogenes or tumor suppressors. lncRNAs most likely act as sponges for miRNAs, which are known to be dysregulated in various cancers. In this regard, the significant role of miRNAs targeting RHO GTPases supports the view that the aberrant expression of lncRNAs may reciprocally change the intensity of RHO GTPase-associated signaling pathways. In this review article, we summarize recent advances in lncRNA research, with a specific focus on their sponge effects on RHO GTPase-targeting miRNAs to crucially mediate gene expression in different cancer cell types and tissues. We will focus in particular on five members of the RHO GTPase family, including RHOA, RHOB, RHOC, RAC1, and CDC42, to illustrate the role of lncRNAs in cancer progression. A deeper understanding of the widespread dysregulation of lncRNAs is of fundamental importance for confirmation of their contribution to RHO GTPase-dependent carcinogenesis.

RAC1 Activation as a Potential Therapeutic Option in Metastatic Cutaneous Melanoma

Metastasis is a complex process by which cancer cells escape from the primary tumor to colonize distant organs. RAC1 is a member of the RHO family of small guanosine triphosphatases that plays an important role in cancer migration, invasion, angiogenesis and metastasis. RAC1 activation has been related to most cancers, such as cutaneous melanoma, breast, lung, and pancreatic cancer. RAC1P29S driver mutation appears in a significant number of cutaneous melanoma cases. Likewise, RAC1 is overexpressed or hyperactivated via signaling through oncogenic cell surface receptors. Thus, targeting RAC1 represents a promising strategy for cutaneous melanoma therapy, as well as for inhibition of other signaling activation that promotes resistance to targeted therapies. In this review, we focus on the role of RAC1 in metastatic cutaneous melanoma emphasizing the anti-metastatic potential of RAC1- targeting drugs.

Role of PLEXIND1/TGFβ Signaling Axis in Pancreatic Ductal Adenocarcinoma Progression Correlates with the Mutational Status of KRAS

Simple Summary

Pancreatic cancer is among the most lethal cancers. The expression of PLEXIND1, a receptor, is upregulated in many cancers (including pancreatic cancer). Traditionally, PLEXIND1 is known to be involved in neuron development and mediate semaphorin signaling. However, its role and signaling in cancer is not fully understood. In our study, we present a new mechanism through which PLEXIND1 mediates its roles in cancer. For the first time, we demonstrate that it can function as a transforming growth factor beta coreceptor and modulate SMAD3 signaling. Around 90% of pancreatic cancer patients have mutant KRAS. Our work suggests that PLEXIND1 functions differently in pancreatic cancer cell lines, and the difference correlates with KRAS mutational status. Additionally, we demonstrate a novel peptide based therapeutic approach to target PLEXIND1 in cancer cells. Our work is valuable to both neuroscience and cancer fields, as it demonstrates an association between two previously unrelated signaling pathways.

Abstract

PLEXIND1 is upregulated in several cancers, including pancreatic ductal adenocarcinoma (PDAC). It is an established mediator of semaphorin signaling, and neuropilins are its known coreceptors. Herein, we report data to support the proposal that PLEXIND1 acts as a transforming growth factor beta (TGFβ) coreceptor, modulating cell growth through SMAD3 signaling. Our findings demonstrate that PLEXIND1 plays a pro-tumorigenic role in PDAC cells with oncogenic KRAS (KRAS^mut). We show in KRAS^mut PDAC cell lines (PANC-1, AsPC-1,4535) PLEXIND1 downregulation results in decreased cell viability (in vitro) and reduced tumor growth (in vivo). Conversely, PLEXIND1 acts as a tumor suppressor in the PDAC cell line (BxPC-3) with wild-type KRAS (KRAS^wt), as its reduced expression results in higher cell viability (in-vitro) and tumor growth (in vivo). Additionally, we demonstrate that PLEXIND1-mediated interactions can be selectively disrupted using a peptide based on its C-terminal sequence (a PDZ domain-binding motif), an outcome that may possess significant therapeutic implications. To our knowledge, this is the first report showing that (1) PLEXIND1 acts as a TGFβ coreceptor and mediates SMAD3 signaling, and (2) differential roles of PLEXIND1 in PDAC cell lines correlate with KRAS^mut and KRAS^wt status.

Post-Translational Modification and Subcellular Compartmentalization: Emerging Concepts on the Regulation and Physiopathological Relevance of RhoGTPases

Cells and tissues are continuously exposed to both chemical and physical stimuli and dynamically adapt and respond to this variety of external cues to ensure cellular homeostasis, regulated development and tissue-specific differentiation. Alterations of these pathways promote disease progression-a prominent example being cancer. Rho GTPases are key regulators of the remodeling of cytoskeleton and cell membranes and their coordination and integration with different biological processes, including cell polarization and motility, as well as other signaling networks such as growth signaling and proliferation. Apart from the control of GTP-GDP cycling, Rho GTPase activity is spatially and temporally regulated by post-translation modifications (PTMs) and their assembly onto specific protein complexes, which determine their controlled activity at distinct cellular compartments. Although Rho GTPases were traditionally conceived as targeted from the cytosol to the plasma membrane to exert their activity, recent research demonstrates that active pools of different Rho GTPases also localize to endomembranes and the nucleus. In this review, we discuss how PTM-driven modulation of Rho GTPases provides a versatile mechanism for their compartmentalization and functional regulation. Understanding how the subcellular sorting of active small GTPase pools occurs and what its functional significance is could reveal novel therapeutic opportunities.

Rac1/ROCK-driven membrane dynamics promote natural killer cell cytotoxicity via granzyme-induced necroptosis

BACKGROUND

Natural killer (NK) cells play an important role in cancer immunosurveillance and therapy. However, the target selectivity of NK cell activity is still poorly understood.

RESULTS

Here, we used live-cell reporters to unravel differential epithelial cancer target killing by primary human NK cells. We found highly variable fractions of killing by distinct NK cell cytotoxic modes that were not determined by NK ligand expression. Rather, epithelial plasma membrane dynamics driven by ROCK-mediated blebs and/or Rac1-mediated lamellipodia promoted necrotic mode in preference to the apoptotic mode of killing. Inhibition of granzyme B and key necroptosis regulators RIP1, RIP3, and MLKL significantly attenuated the necrotic killing, revealing a novel NK cell cytotoxic pathway by granzyme-induced necroptosis that conferred target selectivity.

CONCLUSIONS

Our results not only elucidate a new NK cell effector mechanism but also suggest that tissue microenvironment and oncogenic signaling pathways that promote membrane dynamics, e.g., Rac1 and Rho/ROCK, could be exploited to enhance proinflammatory NK cell killing.

The interplay of Rac1 activity, ubiquitination and GDI binding and its consequences for endothelial cell spreading

Signaling by the Rho GTPase Rac1 is key to the regulation of cytoskeletal dynamics, cell spreading and adhesion. It is widely accepted that the inactive form of Rac1 is bound by Rho GDI, which prevents Rac1 activation and Rac1-effector interactions. In addition, GDI-bound Rac1 is protected from proteasomal degradation, in line with data showing that Rac1 ubiquitination occurs exclusively when Rac1 is activated. We set out to investigate how Rac1 activity, GDI binding and ubiquitination are linked. We introduced single amino acid mutations in Rac1 which differentially altered Rac1 activity, and compared whether the level of Rac1 activity relates to Rac1 ubiquitination and GDI binding. Results show that Rac1 ubiquitination and the active Rac1 morphology is proportionally increased with Rac1 activity. Similarly, we introduced lysine-to-arginine mutations in constitutively active Rac1 to inhibit site-specific ubiquitination and analyze this effect on Rac1 signaling output and ubiquitination. These data show that the K16R mutation inhibits GTP binding, and consequently Rac1 activation, signaling and-ubiquitination, while the K147R mutation does not block Rac1 signaling, but does inhibits its ubiquitination. In both sets of mutants, no direct correlation was observed between GDI binding and Rac1 activity or -ubiquitination. Taken together, our data show that a strong, positive correlation exists between Rac1 activity and its level of ubiquitination, but also that GDI dissociation does not predispose Rac1 to ubiquitination.

Epidermal ROCK2 induces AKT1/GSK3β/β-catenin, NFκB and dermal tenascin C; but enhanced differentiation and p53/p21 inhibit papilloma

ROCK2 roles in epidermal differentiation and carcinogenesis have been investigated in mice expressing an RU486-inducible, 4HT-activated ROCK2 transgene (K14.creP/lslROCKer). RU486/4HT-mediated ROCKer activation induced epidermal hyperplasia similar to cutaneous oncogenic rasHa (HK1.ras); however ROCKer did not elicit papillomas. Instead, anomalous basal-layer ROCKer expression corrupted normal ROCK2 roles underlying epidermal rigidity/stiffness and barrier maintanance, resulting in premature keratin K1, loricrin and filaggrin expression. Also, hyperproliferative/stress-associated keratin K6 was reduced; possibly reflecting altered ROCK2 roles in epidermal rigidity and keratinocyte flexibility/migration during wound healing. Consistent with increased proliferation, K14.creP/lslROCKer hyperplasia displayed supra-basal-to-basal increases in activated p-AKT1, inactivated p-GSK3β ser9 and membranous/nuclear β-catenin expression together with weak NFκB, which were absent in equivalent HK1.ras hyperplasia. Furthermore, ROCKer-mediated increases in epidermal rigidity via p-MypT1 inactivation/elevated MLC, coupled to anomalous β-catenin expression, induced tenascin C-positive dermal fibroblasts. Alongside an altered ECM, these latent tenascin C-positive dermal fibroblasts may become putative pre-cancer-associated fibroblasts (pre-CAFs) and establish a susceptibility that subsequently contributes to tumour progression. However, anomalous differentiation was also accompanied by an immediate increase in basal-layer p53/p21 expression; suggesting that while ROCK2/AKT1/β-catenin activation increased keratinocyte proliferation resulting in hyperplasia, compensatory p53/p21 and accelerated differentiation helped inhibit papillomatogenesis.

Acquisition of New Migratory Properties by Highly Differentiated CD4+CD28null T Lymphocytes in Rheumatoid Arthritis Disease

Expanded CD4+CD28^null T lymphocytes are found in the tissues and peripheral blood of patients with many autoimmune diseases, such as rheumatoid arthritis (RA). These highly differentiated cells present potent inflammatory activity and capability to induce tissue destruction, which has been suggested to predispose to the development of more aggressive disease. In fact, preferential migration to inflammatory sites has been proposed to be a contributing factor in the progression of autoimmune and cardiovascular diseases frequently found in these patients. The functional activity of CD4+CD28^null T lymphocytes is largely dependent on interleukin 15 (IL-15), and this cytokine may also act as a selective attractor of these cells to local inflammatory infiltrates in damaged tissues. We have analysed, in RA patients, the migratory properties and transcriptional motility profile of CD4+CD28^null T lymphocytes compared to their counterparts CD28+ T lymphocytes and the enhancing role of IL-15. Identification of the pathways involved in this process will allow us to design strategies directed to block effector functions that CD4+CD28^null T lymphocytes have in the target tissue, which may represent therapeutic approaches in this immune disorder.

Rac1, A Potential Target for Tumor Therapy

RAS-related C3 botulinum toxin substrate 1 (Rac.1) is one of the important members of Rho GTPases. It is well known that Rac1 is a cytoskeleton regulation protein that regulates cell adhesion, morphology, and movement. Rac1 is highly expressed in different types of tumors, which is related to poor prognosis. Studies have shown that Rac1 not only participates in the tumor cell cycle, apoptosis, proliferation, invasion, migration and angiogenesis, but also participates in the regulation of tumor stem cell, thus promoting the occurrence of tumors. Rac1 also plays a key role in anti-tumor therapy and participates in immune escape mediated by the tumor microenvironment. In addition, the good prospects of Rac1 inhibitors in cancer prevention and treatment are exciting. Therefore, Rac1 is considered as a potential target for the prevention and treatment of cancer. The necessity and importance of Rac1 are obvious, but it still needs further study.

Rac1-PAK1 regulation of Rab11 cycling promotes junction destabilization

Rac1 GTPase is hyperactivated in tumors and contributes to malignancy. Rac1 disruption of junctions requires its effector PAK1, but the precise mechanisms are unknown. Here, we show that E-cadherin is internalized via micropinocytosis in a PAK1–dependent manner without catenin dissociation and degradation. In addition to internalization, PAK1 regulates E-cadherin transport by fine-tuning Rab small GTPase function. PAK1 phosphorylates a core Rab regulator, RabGDIβ, but not RabGDIα. Phosphorylated RabGDIβ preferentially associates with Rab5 and Rab11, which is predicted to promote Rab retrieval from membranes. Consistent with this hypothesis, Rab11 is activated by Rac1, and inhibition of Rab11 function partially rescues E-cadherin destabilization. Thus, Rac1 activation reduces surface cadherin levels as a net result of higher bulk flow of membrane uptake that counteracts Rab11-dependent E-cadherin delivery to junctions (recycling and/or exocytosis). This unique small GTPase crosstalk has an impact on Rac1 and PAK1 regulation of membrane remodeling during epithelial dedifferentiation, adhesion, and motility.

Nanobody-Based Quantification of GTP-Bound RHO Conformation Reveals RHOA and RHOC Activation Independent from Their Total Expression in Breast Cancer

As key regulators of the actin cytoskeleton, RHO GTPase expression and/or activity are deregulated in tumorigenesis and metastatic progression. Nevertheless, the vast majority of experiments supporting this conclusion was conducted on cell lines but not on human tumor samples that were mostly studied at the expression level only. Up to now, the activity of RHO proteins remains poorly investigated in human tumors. In this article, we present the development of a robust nanobody-based ELISA assay, with a high selectivity that allows an accurate quantification of RHO protein GTP-bound state in the nanomolar range (1 nM; 20 μg/L), not only in cell lines after treatment but also in tumor samples. Of note, we present here a fine analysis of RHOA-like and RAC1 active state in tumor samples with the most comprehensive study of RHOA-GTP and RHOC-GTP levels performed on human breast tumor samples. We revealed increased GTP-bound RHOA and RHOC protein activities in tumors compared to normal tissue counterparts, and demonstrated that the RHO active state and RHO expression are two independent parameters among different breast cancer subtypes. Our results further highlight the regulation of RHO protein activation in tumor samples and the relevance of directly studying RHO GTPase activities involvement in molecular pathways.

GTPases, genome, actin: A hidden story in DNA damage response and repair mechanisms

The classical small Rho GTPase (Rho, Rac, and Cdc42) protein family is mainly responsible for regulating cell motility and polarity, membrane trafficking, cell cycle control, and gene transcription. Cumulative recent evidence supports important roles for these proteins in the maintenance of genomic stability. Indeed, DNA damage response (DDR) and repair mechanisms are some of the prime biological processes that underlie several disease phenotypes, including genetic disorders, cancer, senescence, and premature aging. Many reports guided by different experimental approaches and molecular hypotheses have demonstrated that, to some extent, direct modulation of Rho GTPase activity, their downstream effectors, or actin cytoskeleton regulation contribute to these cellular events. Although much attention has been paid to this family in the context of canonical actin cytoskeleton remodeling, here we provide a contextualized review of the interplay between Rho GTPase signaling pathways and the DDR and DNA repair signaling components. Interesting questions yet to be addressed relate to the spatiotemporal dynamics of this collective response and whether it correlates with different subcellular pools of Rho GTPases. We highlight the direct and indirect targets, some of which still lack experimental validation data, likely associated with Rho GTPase activation that provides compelling evidence for further investigation in DNA damage-associated events and with potential therapeutic applications in translational medicine.

A gain-of-function RAC2 mutation is associated with bone-marrow hypoplasia and an autosomal dominant form of severe combined immunodeficiency

Severe combined immunodeficiencies (SCIDs) constitute a heterogeneous group of life-threatening genetic disorders that typically present in the first year of life. They are defined by the absence of autologous T cells and the presence of an intrinsic or extrinsic defect in the B-cell compartment. In three newborns presenting with frequent infections and profound leukopenia, we identified a private, heterozygous mutation in the RAC2 gene (p.G12R). This mutation was de novo in the index case, who had been cured by hematopoietic stem cell transplantation but had transmitted the mutation to her sick daughter. Biochemical assays showed that the mutation was associated with a gain of function. The results of in vitro differentiation assays showed that RAC2 is essential for the survival and differentiation of hematopoietic stem/progenitor cells. Therefore, screening for RAC2 gain-of-function mutations should be considered in patients with a SCID phenotype and who lack a molecular diagnosis.

Regulation of the nuclear speckle localization and function of Rac1

Despite significant evidence that Rac1 is localized to the nucleus, little is known regarding the function and biological significance of nuclear Rac1. Here, we showed that in response to EGF Rac1 was translocated to nuclear speckles and co-localized with the nuclear speckle marker Serine/arginine-rich splicing factor 2 (SRSF2) in Cos-7 cells. We also showed that the nuclear speckle localization of Rac1 was dependent on its T108 phosphorylation and facilitated by Rac1 polybasic region (PBR) that contains a nuclear localization signal and Rac1 GTPase activity. To gain insight into the function of Rac1 in nuclear speckles, we searched for Rac1 binding proteins in the nucleus. We isolated nuclear fraction of HEK 293 cells and incubated with GST-Rac1 and the phosphomimetic GST-Rac1T108E. We identified 463 proteins that were associated with GST-Rac1T108E, but not with GST-Rac1 by LC-MS/MS. Three notable groups of these proteins are: the heterogeneous nuclear ribonucleoproteins (hnRNPs), small nuclear ribonucleoproteins (snRNPs), and SRSFs, all of which are involved in pre-mRNA splicing and associated with nuclear speckles. We further showed by co-immunoprecipitation that Rac1 interacts with SRSF2, hnRNPA1, and U2A' in response to EGF. The interaction is dependent on T108 phosphorylation and facilitated by Rac1 PBR and GTPase activity. We showed that hnRNPA1 translocated in and out of nucleus in response to EGF in a similar pattern to Rac1. Rac1 only partially colocalized with U2A' that localizes to the actual splicing sites adjacent to nuclear speckle. Finally, we showed that Rac1 regulated EGF-induced pre-mRNA splicing and this is mediated by T108 phosphorylation. We conclude that in response to EGF, T108 phosphorylated Rac1 is targeted to nuclear speckles, interacts with multiple groups of proteins involved in pre-mRNA splicing, and regulates EGF-induced pre-mRNA splicing.

Rho GTPases as Key Molecular Players within Intestinal Mucosa and GI Diseases

Rho proteins operate as key regulators of the cytoskeleton, cell morphology and trafficking. Acting as molecular switches, the function of Rho GTPases is determined by guanosine triphosphate (GTP)/guanosine diphosphate (GDP) exchange and their lipidation via prenylation, allowing their binding to cellular membranes and the interaction with downstream effector proteins in close proximity to the membrane. A plethora of in vitro studies demonstrate the indispensable function of Rho proteins for cytoskeleton dynamics within different cell types. However, only in the last decades we have got access to genetically modified mouse models to decipher the intricate regulation between members of the Rho family within specific cell types in the complex in vivo situation. Translationally, alterations of the expression and/or function of Rho GTPases have been associated with several pathological conditions, such as inflammation and cancer. In the context of the GI tract, the continuous crosstalk between the host and the intestinal microbiota requires a tight regulation of the complex interaction between cellular components within the intestinal tissue. Recent studies demonstrate that Rho GTPases play important roles for the maintenance of tissue homeostasis in the gut. We will summarize the current knowledge on Rho protein function within individual cell types in the intestinal mucosa in vivo, with special focus on intestinal epithelial cells and T cells.

RAC1 nitration at Y32 IS involved in the endothelial barrier disruption associated with lipopolysaccharide-mediated acute lung injury

Acute lung injury (ALI), a devastating illness induced by systemic inflammation e.g., sepsis or local lung inflammation e.g., COVID-19 mediated severe pneumonia, has an unacceptably high mortality and has no effective therapy. ALI is associated with increased pulmonary microvascular hyperpermeability and alveolar flooding. The small Rho GTPases, RhoA and Rac1 are central regulators of vascular permeability through cytoskeleton rearrangements. RhoA and Rac1 have opposing functional outcome: RhoA induces an endothelial contractile phenotype and barrier disruption, while Rac1 stabilizes endothelial junctions and increases barrier integrity. In ALI, RhoA activity is increased while Rac1 activity is reduced. We have shown that the activation of RhoA in lipopolysaccharide (LPS)-mediated ALI, is dependent, at least in part, on a single nitration event at tyrosine (Y)³⁴. Thus, the purpose of this study was to determine if the inhibition of Rac1 is also dependent on its nitration. Our data show that Rac1 inhibition by LPS is associated with its nitration that mass spectrometry identified as Y³², within the switch I region adjacent to the nucleotide-binding site. Using a molecular modeling approach, we designed a nitration shielding peptide for Rac1, designated NipR2 (nitration inhibitor peptide for the Rho GTPases 2), which attenuated the LPS-induced nitration of Rac1 at Y³², preserves Rac1 activity and attenuates the LPS-mediated disruption of the endothelial barrier in human lung microvascular endothelial cells (HLMVEC). Using a murine model of ALI induced by intratracheal installation of LPS we found that NipR2 successfully prevented Rac1 nitration and Rac1 inhibition, and more importantly attenuated pulmonary inflammation, reduced lung injury and prevented the loss of lung function. Together, our data identify a new post-translational mechanism of Rac1 inhibition through its nitration at Y³². As NipR2 also reduces sepsis induced ALI in the mouse lung, we conclude that Rac1 nitration is a therapeutic target in ALI.

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Endotoxin exposure induces site specific nitration of Rac1 at Y³² via peroxynitrite stress.

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Rac1 nitration at Y³² leads to persistent Rac GTPase inhibition and endothelial barrier disruption.

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Novel Rac1 nitration shielding peptide, NipR2 blocks Rac1 nitration and rescues endotoxin induced lung inflammation.

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NipR2 is potentially an effective therapy for sepsis induced lung injury by targeting Rac1 Y³² nitration.

Regulation and functions of the RhoA regulatory guanine nucleotide exchange factor GEF-H1

Since the discovery by Madaule and Axel in 1985 of the first Ras homologue (Rho) protein in Aplysia and its human orthologue RhoB, membership in the Rho GTPase family has grown to 20 proteins, with representatives in all eukaryotic species. These GTPases are molecular switches that cycle between active (GTP bound) and inactivate (GDP bound) states. The exchange of GDP for GTP on Rho GTPases is facilitated by guanine exchange factors (GEFs). Approximately 80 Rho GEFs have been identified to date, and only a few GEFs associate with microtubules. The guanine nucleotide exchange factor H1, GEF-H1, is a unique GEF that associates with microtubules and is regulated by the polymerization state of microtubule networks. This review summarizes the regulation and functions of GEF-H1 and discusses the roles of GEF-H1 in human diseases.

Cullin 3/KCTD5 Promotes the Ubiqutination of Rho Guanine Nucleotide Dissociation Inhibitor 1 and Regulates Its Stability

Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1) plays important roles in numerous cellular processes, including cell motility, adhesion, and proliferation, by regulating the activity of Rho GTPases. Its expression is altered in various human cancers and is associated with malignant progression. Here, we show that RhoGDI1 interacts with Cullin 3 (CUL3), a scaffold protein for E3 ubiquitin ligase complexes. Ectopic expression of CUL3 increases the ubiquitination of RhoGDI1. Furthermore, potassium channel tetramerization domain containing 5 (KCTD5) also binds to RhoGDI1 and increases its interaction with CUL3. Ectopic expression of KCTD5 increases the ubiquitination of RhoGDI1, whereas its knockdown by RNA interference has the opposite effect. Depletion of KCTD5 or expression of dominant-negative CUL3 (DN-CUL3) enhances the stability of RhoGDI1. Our findings reveal a previously unknown mechanism for controlling RhoGDI1 degradation that involves a CUL3/KCTD5 ubiquitin ligase complex.

Hormones Secretion and Rho GTPases in Neuroendocrine Tumors

Neuroendocrine tumors (NETs) belong to a heterogeneous group of neoplasms arising from hormone secreting cells. These tumors are often associated with a dysfunction of their secretory activity. Neuroendocrine secretion occurs through calcium-regulated exocytosis, a process that is tightly controlled by Rho GTPases family members. In this review, we compiled the numerous mutations and modification of expression levels of Rho GTPases or their regulators (Rho guanine nucleotide-exchange factors and Rho GTPase-activating proteins) that have been identified in NETs. We discussed how they might regulate neuroendocrine secretion.

Covalent Inhibitors Allosterically Block the Activation of Rho Family Proteins and Suppress Cancer Cell Invasion

The Rho family GTPases are crucial drivers of tumor growth and metastasis. However, it is difficult to develop GTPases inhibitors due to a lack of well-characterized binding pockets for compounds. Here, through molecular dynamics simulation of the RhoA protein, a groove around cysteine 107 (Cys107) that is relatively well-conserved within the Rho family is discovered. Using a combined strategy, the novel inhibitor DC-Rhoin is discovered, which disrupts interaction of Rho proteins with guanine nucleotide exchange factors (GEFs) and guanine nucleotide dissociation inhibitors (GDIs). Crystallographic studies reveal that the covalent binding of DC-Rhoin to the Cys107 residue stabilizes and captures a novel allosteric pocket. Moreover, the derivative compound DC-Rhoin04 inhibits the migration and invasion of cancer cells, through targeting this allosteric pocket of RhoA. The study reveals a novel allosteric regulatory site within the Rho family, which can be exploited for anti-metastasis drug development, and also provides a novel strategy for inhibitor discovery toward "undruggable" protein targets.

Dysregulation of Rho GTPases in Human Cancers

Rho GTPases play central roles in numerous cellular processes, including cell motility, cell polarity, and cell cycle progression, by regulating actin cytoskeletal dynamics and cell adhesion. Dysregulation of Rho GTPase signaling is observed in a broad range of human cancers, and is associated with cancer development and malignant phenotypes, including metastasis and chemoresistance. Rho GTPase activity is precisely controlled by guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors. Recent evidence demonstrates that it is also regulated by post-translational modifications, such as phosphorylation, ubiquitination, and sumoylation. Here, we review the current knowledge on the role of Rho GTPases, and the precise mechanisms controlling their activity in the regulation of cancer progression. In addition, we discuss targeting strategies for the development of new drugs to improve cancer therapy.

Intratumor δ-catenin heterogeneity driven by genomic rearrangement dictates growth factor dependent prostate cancer progression

Only a small number of genes are bona fide oncogenes and tumor suppressors such as Ras, Myc, β-catenin, p53, and APC. However, targeting these cancer drivers frequently fail to demonstrate sustained cancer remission. Tumor heterogeneity and evolution contribute to cancer resistance and pose challenges for cancer therapy due to differential genomic rearrangement and expression driving distinct tumor responses to treatments. Here we report that intratumor heterogeneity of Wnt/β-catenin modulator δ-catenin controls individual cell behavior to promote cancer. The differential intratumor subcellular localization of δ-catenin mirrors its compartmentalization in prostate cancer xenograft cultures as result of mutation-rendered δ-catenin truncations. Wild-type and δ-catenin mutants displayed distinct protein interactomes that highlight rewiring of signal networks. Localization specific δ-catenin mutants influenced p120ctn-dependent Rho GTPase phosphorylation and shifted cells towards differential bFGF-responsive growth and motility, a known signal to bypass androgen receptor dependence. Mutant δ-catenin promoted Myc-induced prostate tumorigenesis while increasing bFGF-p38 MAP kinase signaling, β-catenin-HIF-1α expression, and the nuclear size. Therefore, intratumor δ-catenin heterogeneity originated from genetic remodeling promotes prostate cancer expansion towards androgen independent signaling, supporting a neomorphism model paradigm for targeting tumor progression.

Rho GTPases: Promising candidates for overcoming chemotherapeutic resistance

Despite therapeutic advances, resistance to chemotherapy remains a major challenge to patients with malignancies. Rho GTPases are essential for the development and progression of various diseases including cancer, and a vast number of studies have linked Rho GTPases to chemoresistance. Therefore, understanding the underlying mechanisms can expound the effects of Rho GTPases towards chemotherapeutic agents, and targeting Rho GTPases is a promising strategy to downregulate the chemo-protective pathways and overcome chemoresistance. Importantly, exceptions in certain biological conditions and interactions among the members of Rho GTPases should be noted. In this review, we focus on the role of Rho GTPases, particularly Rac1, in regulating chemoresistance and provide an overview of their related mechanisms and available inhibitors, which may offer novel options for future targeted cancer therapy.

Rac1 activates non-oxidative pentose phosphate pathway to induce chemoresistance of breast cancer

Resistance development to one chemotherapeutic reagent leads frequently to acquired tolerance to other compounds, limiting the therapeutic options for cancer treatment. Herein, we find that overexpression of Rac1 is associated with multi-drug resistance to the neoadjuvant chemotherapy (NAC). Mechanistically, Rac1 activates aldolase A and ERK signaling which up-regulates glycolysis and especially the non-oxidative pentose phosphate pathway (PPP). This leads to increased nucleotides metabolism which protects breast cancer cells from chemotherapeutic-induced DNA damage. To translate this finding, we develop endosomal pH-responsive nanoparticles (NPs) which deliver Rac1-targeting siRNA together with cisplatin and effectively reverses NAC-chemoresistance in PDXs from NAC-resistant breast cancer patients. Altogether, our findings demonstrate that targeting Rac1 is a potential strategy to overcome acquired chemoresistance in breast cancer.

Acquired resistance to chemotherapy can lead to multi-drug resistance and poor prognosis in cancer. Here, the authors show that Rac1 increases glycolysis and non-oxidative pentose phosphate pathway activity leading to neoadjuvant chemotherapy (NAC) resistance, thus its inhibition sensitizes resistant breast cancer PDXs to NAC.

Rho-ROCK Signaling in Normal Physiology and as a Key Player in Shaping the Tumor Microenvironment

The Rho-ROCK signaling network has a range of specialized functions of key biological importance, including control of essential developmental processes such as morphogenesis and physiological processes including homeostasis, immunity, and wound healing. Deregulation of Rho-ROCK signaling actively contributes to multiple pathological conditions, and plays a major role in cancer development and progression. This dynamic network is critical in modulating the intricate communication between tumor cells, surrounding diverse stromal cells and the matrix, shaping the ever-changing microenvironment of aggressive tumors. In this chapter, we overview the complex regulation of the Rho-ROCK signaling axis, its role in health and disease, and analyze progress made with key approaches targeting the Rho-ROCK pathway for therapeutic benefit. Finally, we conclude by outlining likely future trends and key questions in the field of Rho-ROCK research, in particular surrounding Rho-ROCK signaling within the tumor microenvironment.

Enhanced Dendritic Actin Network Formation in Extended Lamellipodia Drives Proliferation in Growth-Challenged Rac1P29S Melanoma Cells

Actin assembly supplies the structural framework for cell morphology and migration. Beyond structure, this actin framework can also be engaged to drive biochemical signaling programs. Here, we describe how the hyperactivation of Rac1 via the P29S mutation (Rac1^P29S) in melanoma hijacks branched actin network assembly to coordinate proliferative cues that facilitate metastasis and drug resistance. Upon growth challenge, Rac1^P29S-harboring melanoma cells massively upregulate lamellipodia formation by dendritic actin polymerization. These extended lamellipodia form a signaling microdomain that sequesters and phospho-inactivates the tumor suppressor NF2/Merlin, driving Rac1^P29S cell proliferation in growth suppressive conditions. These biochemically active lamellipodia require cell-substrate attachment but not focal adhesion assembly and drive proliferation independently of the ERK/MAPK pathway. These data suggest a critical link between cell morphology and cell signaling and reconcile the dichotomy of Rac1's regulation of both proliferation and actin assembly by revealing a mutual signaling axis wherein actin assembly drives proliferation in melanoma.