A machine learning pipeline revealing heterogeneous responses to drug perturbations on vascular smooth muscle cell spheroid morphology and formation

Machine learning approaches have shown great promise in biology and medicine discovering hidden information to further understand complex biological and pathological processes. In this study, we developed a deep learning-based machine learning algorithm to meaningfully process image data and facilitate studies in vascular biology and pathology. Vascular injury and atherosclerosis are characterized by neointima formation caused by the aberrant accumulation and proliferation of vascular smooth muscle cells (VSMCs) within the vessel wall. Understanding how to control VSMC behaviors would promote the development of therapeutic targets to treat vascular diseases. However, the response to drug treatments among VSMCs with the same diseased vascular condition is often heterogeneous. Here, to identify the heterogeneous responses of drug treatments, we created an in vitro experimental model system using VSMC spheroids and developed a machine learning-based computational method called HETEROID (heterogeneous spheroid). First, we established a VSMC spheroid model that mimics neointima-like formation and the structure of arteries. Then, to identify the morphological subpopulations of drug-treated VSMC spheroids, we used a machine learning framework that combines deep learning-based spheroid segmentation and morphological clustering analysis. Our machine learning approach successfully showed that FAK, Rac, Rho, and Cdc42 inhibitors differentially affect spheroid morphology, suggesting that multiple drug responses of VSMC spheroid formation exist. Overall, our HETEROID pipeline enables detailed quantitative drug characterization of morphological changes in neointima formation, that occurs in vivo, by single-spheroid analysis.

LINC00665 promotes the progression of acute myeloid leukemia by regulating the miR-4458/DOCK1 pathway

This study aimed to explore the role of LINC00665, miR-4458 and DOCK1 and their interactions in the development of acute myeloid leukemia (AML). The relative expression of LINC00665, miR-4458 and DOCK1 in AML samples was measured using qRT-PCR, and the protein level of DOCK1 in AML cell lines was examined using western blot. CCK8, BrdU, transwell, cell adhesion, and caspase-3 activity assays were carried out to evaluate the viability, proliferation, migration, adhesion, and apoptosis of AML cells, respectively. Luciferase reporter, RIP, and RNA pull-down assays were also performed to confirm the target relationship among LINC00665, miR-4458 and DOCK1. Findings revealed that LINC00665 and DOCK1 were aberrantly overexpressed in AML tissues and that the expression of miR-4458 was low in AML tissues. Silencing LINC00665 or DOCK1 presented significant restriction to the proliferation, migration and adhesion of AML cells. Apart from that, it was found that inhibiting miR-4458 could enhance the proliferation, migration and adhesion of AML cells but suppress the apoptosis of AML cells. Experimental results also indicated that LINC00665 exerted its positive function on AML cells by sponging miR-4458 and that miR-4458 influenced the progression of AML cells by targeting DOCK1 directly. Overall, this finding not only provided a novel molecular pathway for the diagnosis and treatment of AML but also showed that LINC00665 could enhance the progression of AML by regulating the miR-4458/DOCK1 pathway.

Identifying and treating ROBO1-ve /DOCK1+ve prostate cancer: An aggressive cancer subtype prevalent in African American patients

BACKGROUND

There is a need to develop novel therapies which could be beneficial to patients with prostate cancer (CaP) including those who are predisposed to poor outcome, such as African-Americans. This study investigates the role of ROBO1-pathway in predicting outcome and race-based disparity in patients with CaP.

METHODS AND RESULTS

Aided by RNA sequencing-based DECIPHER-testing and immunohistochemical (IHC) analysis of tumors we show that ROBO1 is lost during the progressive stages of CaP, a prevalent feature in African-Americans. We show that the loss of ROBO1 predicts high-risk of recurrence, metastasis and poor outcome of androgen-deprivation therapy in radical prostatectomy-treated patients. These data identified an aggressive ROBO1deficient /DOCK1+ve sub-class of CaP. Combined genetic and IHC data showed that ROBO1 loss is accompanied by DOCK1/Rac1 elevation in grade-III/IV primary-tumors and Mets. We observed that the hypermethylation of ROBO1-promoter contributes to loss of expression that is highly prevalent in African-Americans. Because of limitations in restoring ROBO1 function, we asked if targeting the DOCK1 could be an ideal strategy to inhibit progression or treat ROBO1deficient metastatic-CaP. We tested the pharmacological efficacy of CPYPP, a selective inhibitor of DOCK1 under in vitro and in vivo conditions. Using ROBO1-ve and ROBO1+ve CaP models, we determined the median effective concentration of CPYPP for growth. DOCK1-inhibitor treatment significantly decreased the (a) Rac1-GTP/β-catenin activity, (b) transmigration of ROBO1deficient cells across endothelial lining, and (c) metastatic spread of ROBO1deficient cells through the vasculature of transgenicfl Zebrafish model.

CONCLUSION

We suggest that ROBO1 status forms as predictive biomarker of outcome in high-risk populations such as African-Americans and DOCK1-targeting therapy has a clinical potential for treating metastatic-CaP.

The role of Rho GTPases' substrates Rac and Cdc42 in osteoclastogenesis and relevant natural medicinal products study

Recently, Rho GTPases substrates include Rac (Rac1 and Rac2) and Cdc42 that have been reported to exert multiple cellular functions in osteoclasts, the most prominent of which includes regulating the dynamic actin cytoskeleton rearrangements. In addition, natural products and their molecular frameworks have a long tradition as valuable starting points for medicinal chemistry and drug discovery. Although currently, there are reports about the natural product, which could play a therapeutic role in bone loss diseases (osteoporosis and osteolysis) through the regulation of Rac1/2 and Cdc42 during osteoclasts cytoskeletal structuring. There have been several excellent studies for exploring the therapeutic potentials of various natural products for their role in inhibiting cancer cells migration and function via regulating the Rac1/2 and Cdc42. Herein in this review, we try to focus on recent advancement studies for extensively understanding the role of Rho GTPases substrates Rac1, Rac2 and Cdc42 in osteoclastogenesis, as well as therapeutic potentials of natural medicinal products for their properties on the regulation of Rac1, and/or Rac2 and Cdc42, which is in order to inspire drug discovery in regulating osteoclastogenesis.

HACE1 Prevents Lung Carcinogenesis via Inhibition of RAC-Family GTPases

HACE1 is an E3 ubiquitin ligase with important roles in tumor biology and tissue homeostasis. Loss or mutation of HACE1 has been associated with the occurrence of a variety of neoplasms, but the underlying mechanisms have not been defined yet. Here, we report that HACE1 is frequently mutated in human lung cancer. In mice, loss of Hace1 led to enhanced progression of KRasG12D -driven lung tumors. Additional ablation of the oncogenic GTPase Rac1 partially reduced progression of Hace1-/- lung tumors. RAC2, a novel ubiquitylation target of HACE1, could compensate for the absence of its homolog RAC1 in Hace1-deficient, but not in HACE1-sufficient tumors. Accordingly, ablation of both Rac1 and Rac2 fully averted the increased progression of KRasG12D -driven lung tumors in Hace1-/- mice. In patients with lung cancer, increased expression of HACE1 correlated with reduced levels of RAC1 and RAC2 and prolonged survival, whereas elevated expression of RAC1 and RAC2 was associated with poor prognosis. This work defines HACE1 as a crucial regulator of the oncogenic activity of RAC-family GTPases in lung cancer development. SIGNIFICANCE: These findings reveal that mutation of the tumor suppressor HACE1 disrupts its role as a regulator of the oncogenic activity of RAC-family GTPases in human and murine lung cancer. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/14/3009/F1.large.jpg.

Distinct Interaction Sites of Rac GTPase with WAVE Regulatory Complex Have Non-redundant Functions in Vivo

Cell migration often involves the formation of sheet-like lamellipodia generated by branched actin filaments. The branches are initiated when Arp2/3 complex [1] is activated by WAVE regulatory complex (WRC) downstream of small GTPases of the Rac family [2]. Recent structural studies defined two independent Rac binding sites on WRC within the Sra-1/PIR121 subunit of the pentameric WRC [3, 4], but the functions of these sites in vivo have remained unknown. Here we dissect the mechanism of WRC activation and the in vivo relevance of distinct Rac binding sites on Sra-1, using CRISPR/Cas9-mediated gene disruption of Sra-1 and its paralog PIR121 in murine B16-F1 cells combined with Sra-1 mutant rescue. We show that the A site, positioned adjacent to the binding region of WAVE-WCA mediating actin and Arp2/3 complex binding, is the main site for allosteric activation of WRC. In contrast, the D site toward the C terminus is dispensable for WRC activation but required for optimal lamellipodium morphology and function. These results were confirmed in evolutionarily distant Dictyostelium cells. Moreover, the phenotype seen in D site mutants was recapitulated in Rac1 E31 and F37 mutants; we conclude these residues are important for Rac-D site interaction. Finally, constitutively activated WRC was able to induce lamellipodia even after both Rac interaction sites were lost, showing that Rac interaction is not essential for membrane recruitment. Our data establish that physical interaction with Rac is required for WRC activation, in particular through the A site, but is not mandatory for WRC accumulation in the lamellipodium.

HGF-induced formation of the MET-AXL-ELMO2-DOCK180 complex promotes RAC1 activation, receptor clustering, and cancer cell migration and invasion

The MET proto-oncogene-encoded receptor tyrosine kinase (MET) and AXL receptor tyrosine kinase (AXL) are independently operating receptor tyrosine kinases (RTKs) that are functionally associated with aggressive and invasive cancer cell growth. However, how MET and AXL regulate the migratory properties of cancer cells remains largely unclear. We report here that the addition of hepatocyte growth factor (HGF), the natural ligand of MET, to serum-starved human glioblastoma cells induces the rapid activation of both MET and AXL and formation of highly polarized MET-AXL clusters on the plasma membrane. HGF also promoted the formation of the MET and AXL protein complexes and phosphorylation of AXL, independent of AXL's ligand, growth arrest-specific 6 (GAS6). The HGF-induced MET-AXL complex stimulated rapid and dynamic cytoskeleton reorganization by activating the small GTPase RAC1, a process requiring both MET and AXL kinase activities. We further found that HGF also promotes the recruitment of ELMO2 and DOCK180, a bipartite guanine nucleotide exchange factor for RAC1, to the MET-AXL complex and thereby stimulates the RAC1-dependent cytoskeleton reorganization. We also demonstrated that the MET-AXL-ELMO2-DOCK180 complex is critical for HGF-induced cell migration and invasion in glioblastoma or other cancer cells. Our findings uncover a critical HGF-dependent signaling pathway that involves the assembly of a large protein complex consisting of MET, AXL, ELMO2, and DOCK180 on the plasma membrane, leading to RAC1-dependent cell migration and invasion in various cancer cells.

Targeting Rac and Cdc42 GTPases in Cancer

Rac and Cdc42 are small GTPases that have been linked to multiple human cancers and are implicated in epithelial to mesenchymal transition, cell-cycle progression, migration/invasion, tumor growth, angiogenesis, and oncogenic transformation. With the exception of the P29S driver mutation in melanoma, Rac and Cdc42 are not generally mutated in cancer, but are overexpressed (gene amplification and mRNA upregulation) or hyperactivated. Rac and Cdc42 are hyperactivated via signaling through oncogenic cell surface receptors, such as growth factor receptors, which converge on the guanine nucleotide exchange factors that regulate their GDP/GTP exchange. Hence, targeting Rac and Cdc42 represents a promising strategy for precise cancer therapy, as well as for inhibition of bypass signaling that promotes resistance to cell surface receptor-targeted therapies. Therefore, an understanding of the regulatory mechanisms of these pivotal signaling intermediates is key for the development of effective inhibitors. In this review, we focus on the role of Rac and Cdc42 in cancer and summarize the regulatory mechanisms, inhibitory efficacy, and the anticancer potential of Rac- and Cdc42-targeting agents. Cancer Res; 78(12); 3101-11. ©2018 AACR.

Directional migration of mesenchymal stem cells under an SDF-1α gradient on a microfluidic device

Homing of peripheral stem cells is regulated by one of the most representative homing factors, stromal cell-derived factor 1 alpha (SDF-1α), which specifically binds to the plasma membrane receptor CXCR4 of mesenchymal stem cells (MSCs) in order to initiate the signaling pathways that lead to directional migration and homing of stem cells. This complex homing process and directional migration of stem cells have been mimicked on a microfluidic device that is capable of generating a chemokine gradient within the collagen matrix and embedding endothelial cell (EC) monolayers to mimic blood vessels. On the microfluidic device, stem cells showed directional migration toward the higher concentration of SDF-1α, whereas treatment with the CXCR4 antagonist AMD3100 caused loss of directionality of stem cells. Furthermore, inhibition of stem cell's main migratory signaling pathways, Rho-ROCK and Rac pathways, caused blockage of actomyosin and lamellipodia formation, decreasing the migration distance but maintaining directionality. Stem cell homing regulated by SDF-1α caused directional migration of stem cells, while the migratory ability was affected by the activation of migration-related signaling pathways.

Ras-related C3 Botulinum Toxin Substrate (Rac) and Src Family Kinases (SFK) Are Proximal and Essential for Phosphatidylinositol 3-Kinase (PI3K) Activation in Natural Killer (NK) Cell-mediated Direct Cytotoxicity against Cryptococcus neoformans

The activity of Rac in leukocytes is essential for immunity. However, its role in NK cell-mediated anti-microbial signaling remains unclear. In this study, we investigated the role of Rac in NK cell mediated anti-cryptococcal killing. We found thatCryptococcus neoformansindependently activates both Rac and SFK pathways in NK cells, and unlike in tumor killing,Cryptococcusinitiated a novel Rac → PI3K → Erk cytotoxicity cascade. Remarkably, Rac was not required for conjugate formation, despite its essential role in NK cytotoxicity againstC. neoformans Taken together, our data show that, unlike observations with tumor cells, NK cells use a novel Rac cytotoxicity pathway in conjunction with SFK, to killC. neoformans.

Cdc42 and k-Ras Control Endothelial Tubulogenesis through Apical Membrane and Cytoskeletal Polarization: Novel Stimulatory Roles for GTPase Effectors, the Small GTPases, Rac2 and Rap1b, and Inhibitory Influence of Arhgap31 and Rasa1

A critical and understudied property of endothelial cells is their ability to form lumens and tube networks. Although considerable information has been obtained concerning these issues, including the role of Cdc42 and Rac1 and their effectors such as Pak2, Pak4, Par6b, and co-regulators such as integrins, MT1-MMP and Par3; many key questions remain that are necessary to elucidate molecular and signaling requirements for this fundamental process. In this work, we identify new small GTPase regulators of EC tubulogenesis including k-Ras, Rac2 and Rap1b that act in conjunction with Cdc42 as well as the key downstream effectors, IQGAP1, MRCKβ, beta-Pix, GIT1, and Rasip1 (which can assemble into multiprotein complexes with key regulators including α2β1 integrin and MT1-MMP). In addition, we identify the negative regulators, Arhgap31 (by inactivating Cdc42 and Rac) and Rasa1 (by inactivating k-Ras) and the positive regulator, Arhgap29 (by inactivating RhoA) which play a major functional role during the EC tubulogenic process. Human EC siRNA suppression or mouse knockout of Rasip1 leads to identical phenotypes where ECs form extensive cord networks, but cannot generate lumens or tubes. Essential roles for these molecules during EC tubulogenesis include; i) establishment of asymmetric EC cytoskeletal polarization (subapical distribution of acetylated tubulin and basal membrane distribution of F-actin); and ii) directed membrane trafficking of pinocytic vacuoles or other intracellular vesicles along acetylated tubulin tracks to the developing apical membrane surface. Cdc42 co-localizes subapically with acetylated tubulin, while Rac1 and k-Ras strongly label vacuole/ vesicle membranes which accumulate and fuse together in a polarized, perinuclear manner. We observe polarized apical membrane and subapical accumulation of key GTPases and effectors regulating EC lumen formation including Cdc42, Rac1, Rac2, k-Ras, Rap1b, activated c-Raf and Rasip1 to control EC tube network assembly. Overall, this work defines novel key regulators and their functional roles during human EC tubulogenesis.

Inhibition of prostate smooth muscle contraction and prostate stromal cell growth by the inhibitors of Rac, NSC23766 and EHT1864

BACKGROUND AND PURPOSE

Medical therapy of lower urinary tract symptoms (LUTS) suggestive of benign prostatic hyperplasia (BPH) targets smooth muscle contraction in the prostate, or prostate growth. However, current therapeutic options are insufficient. Here, we investigated the role of Rac in the control of smooth muscle tone in human prostates and growth of prostate stromal cells.

EXPERIMENTAL APPROACH

Experiments were performed using human prostate tissues from radical prostatectomy and cultured stromal cells (WPMY-1). Expression of Rac was examined by Western blot and fluorescence staining. Effects of Rac inhibitors (NSC23766 and EHT1864) on contractility were assessed in the organ bath. The effects of Rac inhibitors were assessed by pull-down, cytotoxicity using a cell counting kit, cytoskeletal organization by phalloidin staining and cell growth using an 5-ethynyl-2'-deoxyuridine assay.

KEY RESULTS

Expression of Rac1-3 was observed in prostate samples from each patient. Immunoreactivity for Rac1-3 was observed in the stroma, where it colocalized with the smooth muscle marker, calponin. NSC23766 and EHT1864 significantly reduced contractions of prostate strips induced by noradrenaline, phenylephrine or electrical field stimulation. NSC23766 and EHT1864 inhibited Rac activity in WPMY-1 cells. Survival of WPMY-1 cells ranged between 64 and 81% after incubation with NSC23766 (50 or 100 μM) or EHT1864 (25 μM) for 24 h. NSC23766 and EHT1864 induced cytoskeletal disorganization in WPMY-1 cells. Both inhibitors impaired the growth of WPMY-1 cells.

CONCLUSIONS AND IMPLICATIONS

Rac may be a link connecting the control of prostate smooth muscle tone with proliferation of smooth muscle cells. Improvements in LUTS suggestive of BPH by Rac inhibitors appears possible.

Pharmacokinetics of Rac inhibitor EHop-016 in mice by ultra-performance liquid chromatography tandem mass spectrometry

The Rho GTPase Rac is an important regulator of cancer cell migration and invasion; processes required for metastatic progression. We previously characterized the small molecule EHop-016 as a novel Rac inhibitor in metastatic breast cancer cells and recently found that EHop-016 was effective at reducing tumor growth in nude mice at 25 mg/kg bodyweight (BW). The purpose of this study was to compare the pharmacokinetics and bioavailability of EHop-016 at different dosages in a single dose input scheme (10, 20 and 40 mg/kg BW) following intraperitoneal (IP) and oral gavage (PO) administration to nude mice. We developed and validated a rapid and sensitive method for the quantitation of EHop-016 in mouse plasma by ultra high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (UPLC/MS/MS). Separation was carried out on an Agilent Poroshell 120 EC-C18 column (3.0 mm × 50 mm) using organic and aqueous mobile phases. EHop-016 was identified from its accurate mass and retention time from the acquired full-scan chromatogram and quantified by its peak area. The validated method was linear (R(2)>0.995) over the range of 5-1000 ng/mL (1/x(2) weighting). Pharmacokinetic parameters were obtained by non-compartmental analysis using WinNonlin. The area under the curve (AUC₀-∞) ranged from 328 to 1869 ng h/mL and 133-487 ng h/mL for IP and PO dosing, respectively. The elimination half-life (t₁/₂) ranged from 3.8-5.7 h to 3.4-26.8 h for IP and PO dosing, respectively. For both IP and PO administration, the AUC₀-∞values were proportional to the tested doses demonstrating linear PK profiles. The relative bioavailability of EHop-016 after oral gavage administration ranged from 26% to 40%. These results support further preclinical evaluation of EHop-016 as a new anti-cancer therapy.

Vav1 increases Bcl-2 expression by selective activation of Rac2-Akt in leukemia T cells

Vav proteins are guanine nucleotide exchange factors (GEFs) that activate a group of small G proteins (GTPases). Vav1 is predominantly expressed in hematopoietic cells, whereas Vav2 and Vav3 are ubiquitously distributed in almost all human tissues. All three Vav proteins contain conserved structural motifs and associate with a variety of cellular activities including proliferation, migration, and survival. Previous observation with Jurkat leukemia T cells showed that Vav1 possessed anti-apoptotic activity by enhancing Bcl-2 transcription. However the mechanism has not been unveiled. Here, we explored the effectors of Vav1 in promoting Bcl-2 expression in Jurkat cells and revealed that Rac2-Akt was specifically evoked by the expression of Vav1, but not Vav2 or Vav3. Although all three Vav isoforms existed in Jurkat cells, Rac2 was distinguishably activated by Vav1 and that led to enhanced Bcl-2 expression and cell survival. Akt was modulated downstream of Vav1-Rac2, and the activation of Akt was indispensable in the enhanced transcription of Bcl-2. Intriguingly, neither Vav2 nor Vav3 was able to activate Rac2-Akt pathway as determined by gene silencing approach. Our data illustrated a unique role of Vav1 in T leukemia survival by selectively triggering Rac2-Akt axis and elevating the expression of anti-apoptotic Bcl-2.

Pak and Rac GTPases promote oncogenic KIT-induced neoplasms

An acquired somatic mutation at codon 816 in the KIT receptor tyrosine kinase is associated with poor prognosis in patients with systemic mastocytosis and acute myeloid leukemia (AML). Treatment of leukemic cells bearing this mutation with an allosteric inhibitor of p21-activated kinase (Pak) or its genetic inactivation results in growth repression due to enhanced apoptosis. Inhibition of the upstream effector Rac abrogates the oncogene-induced growth and activity of Pak. Although both Rac1 and Rac2 are constitutively activated via the guanine nucleotide exchange factor (GEF) Vav1, loss of Rac1 or Rac2 alone moderately corrected the growth of KIT-bearing leukemic cells, whereas the combined loss resulted in 75% growth repression. In vivo, the inhibition of Vav or Rac or Pak delayed the onset of myeloproliferative neoplasms (MPNs) and corrected the associated pathology in mice. To assess the role of Rac GEFs in oncogene-induced transformation, we used an inhibitor of Rac, EHop-016, which specifically targets Vav1 and found that EHop-016 was a potent inhibitor of human and murine leukemic cell growth. These studies identify Pak and Rac GTPases, including Vav1, as potential therapeutic targets in MPN and AML involving an oncogenic form of KIT.

Rac1 is a novel regulator of contraction-stimulated glucose uptake in skeletal muscle

In skeletal muscle, the actin cytoskeleton-regulating GTPase, Rac1, is necessary for insulin-dependent GLUT4 translocation. Muscle contraction increases glucose transport and represents an alternative signaling pathway to insulin. Whether Rac1 is activated by muscle contraction and regulates contraction-induced glucose uptake is unknown. Therefore, we studied the effects of in vivo exercise and ex vivo muscle contractions on Rac1 signaling and its regulatory role in glucose uptake in mice and humans. Muscle Rac1-GTP binding was increased after exercise in mice (~60-100%) and humans (~40%), and this activation was AMP-activated protein kinase independent. Rac1 inhibition reduced contraction-stimulated glucose uptake in mouse muscle by 55% in soleus and by 20-58% in extensor digitorum longus (EDL; P < 0.01). In agreement, the contraction-stimulated increment in glucose uptake was decreased by 27% (P = 0.1) and 40% (P < 0.05) in soleus and EDL muscles, respectively, of muscle-specific inducible Rac1 knockout mice. Furthermore, depolymerization of the actin cytoskeleton decreased contraction-stimulated glucose uptake by 100% and 62% (P < 0.01) in soleus and EDL muscles, respectively. These are the first data to show that Rac1 is activated during muscle contraction in murine and human skeletal muscle and suggest that Rac1 and possibly the actin cytoskeleton are novel regulators of contraction-stimulated glucose uptake.

Inhibition of Rac and ROCK signalling influence osteoblast adhesion, differentiation and mineralization on titanium topographies

Reducing the time required for initial integration of bone-contacting implants with host tissues would be of great clinical significance. Changes in osteoblast adhesion formation and reorganization of the F-actin cytoskeleton in response to altered topography are known to be upstream of osteoblast differentiation, and these processes are regulated by the Rho GTPases. Rac and RhoA (through Rho Kinase (ROCK)). Using pharmacological inhibitors, we tested how inhibition of Rac and ROCK influenced osteoblast adhesion, differentiation and mineralization on PT (Pre-treated) and SLA (sandblasted large grit, acid etched) topographies. Inhibition of ROCK, but not Rac, significantly reduced adhesion number and size on PT, with adhesion size consistent with focal complexes. After 1 day, ROCK, but not Rac inhibition increased osteocalcin mRNA levels on SLA and PT, with levels further increasing at 7 days post seeding. ROCK inhibition also significantly increased bone sialoprotein expression at 7 days, but not BMP-2 levels. Rac inhibition significantly reduced BMP-2 mRNA levels. ROCK inhibition increased nuclear translocation of Runx2 independent of surface roughness. Mineralization of osteoblast cultures was greater on SLA than on PT, but was increased by ROCK inhibition and attenuated by Rac inhibition on both topographies. In conclusion, inhibition of ROCK signalling significantly increases osteoblast differentiation and biomineralization in a topographic dependent manner, and its pharmacological inhibition could represent a new therapeutic to speed bone formation around implanted metals and in regenerative medicine applications.

Oxidized LDL/CD36 interaction induces loss of cell polarity and inhibits macrophage locomotion

Cell polarization is essential for migration and the exploratory function of leukocytes. However, the mechanism by which cells maintain polarity or how cells revert to the immobilized state by gaining cellular symmetry is not clear. Previously we showed that interaction between oxidized low-density lipoprotein (oxLDL) and CD36 inhibits macrophage migration; in the current study we tested the hypothesis that oxLDL/CD36-induced inhibition of migration is the result of intracellular signals that regulate cell polarity. Live cell imaging of macrophages showed that oxLDL actuated retraction of macrophage front end lamellipodia and induced loss of cell polarity. Cd36 null and macrophages null for Vav, a guanine nucleotide exchange factor (GEF), did not show this effect. These findings were caused by Rac-mediated inhibition of nonmuscle myosin II, a cell polarity determinant. OxLDL induced dephosphorylation of myosin regulatory light chain (MRLC) by increasing the activity of Rac. Six-thioguanine triphosphate (6-thio-GTP), which inhibits Vav-mediated activation of Rac, abrogated the effect of oxLDL. Activation of the Vav-Rac-myosin II pathway by oxidant stress may induce trapping of macrophages at sites of chronic inflammation such as atherosclerotic plaque.

Rac2-MRC-cIII-generated ROS cause genomic instability in chronic myeloid leukemia stem cells and primitive progenitors

Chronic myeloid leukemia in chronic phase (CML-CP) is induced by BCR-ABL1 oncogenic tyrosine kinase. Tyrosine kinase inhibitors eliminate the bulk of CML-CP cells, but fail to eradicate leukemia stem cells (LSCs) and leukemia progenitor cells (LPCs) displaying innate and acquired resistance, respectively. These cells may accumulate genomic instability, leading to disease relapse and/or malignant progression to a fatal blast phase. In the present study, we show that Rac2 GTPase alters mitochondrial membrane potential and electron flow through the mitochondrial respiratory chain complex III (MRC-cIII), thereby generating high levels of reactive oxygen species (ROS) in CML-CP LSCs and primitive LPCs. MRC-cIII-generated ROS promote oxidative DNA damage to trigger genomic instability, resulting in an accumulation of chromosomal aberrations and tyrosine kinase inhibitor-resistant BCR-ABL1 mutants. JAK2(V617F) and FLT3(ITD)-positive polycythemia vera cells and acute myeloid leukemia cells also produce ROS via MRC-cIII. In the present study, inhibition of Rac2 by genetic deletion or a small-molecule inhibitor and down-regulation of mitochondrial ROS by disruption of MRC-cIII, expression of mitochondria-targeted catalase, or addition of ROS-scavenging mitochondria-targeted peptide aptamer reduced genomic instability. We postulate that the Rac2-MRC-cIII pathway triggers ROS-mediated genomic instability in LSCs and primitive LPCs, which could be targeted to prevent the relapse and malignant progression of CML.

Rac signaling in osteoblastic cells is required for normal bone development but is dispensable for hematopoietic development

Hematopoietic stem cells (HSCs) interact with osteoblastic, stromal, and vascular components of the BM hematopoietic microenvironment (HM) that are required for the maintenance of long-term self-renewal in vivo. Osteoblasts have been reported to be a critical cell type making up the HSC niche in vivo. Rac1 GTPase has been implicated in adhesion, spreading, and differentiation of osteoblast cell lines and is critical for HSC engraftment and retention. Recent data suggest a differential role of GTPases in endosteal/osteoblastic versus perivascular niche function. However, whether Rac signaling pathways are also necessary in the cell-extrinsic control of HSC function within the HM has not been examined. In the present study, genetic and inducible models of Rac deletion were used to demonstrate that Rac depletion causes impaired proliferation and induction of apoptosis in the OP9 cell line and in primary BM stromal cells. Deletion of Rac proteins caused reduced trabecular and cortical long bone growth in vivo. Surprisingly, HSC function and maintenance of hematopoiesis in vivo was preserved despite these substantial cell-extrinsic changes. These data have implications for therapeutic strategies to target Rac signaling in HSC mobilization and in the treatment of leukemia and provide clarification to our evolving concepts of HSC-HM interactions.

Inhibition of Rac GTPase signaling and downstream prosurvival Bcl-2 proteins as combination targeted therapy in MLL-AF9 leukemia

The Rac family of small Rho GTPases coordinates diverse cellular functions in hematopoietic cells including adhesion, migration, cytoskeleton rearrangements, gene transcription, proliferation, and survival. The integrity of Rac signaling has also been found to critically regulate cellular functions in the initiation and maintenance of hematopoietic malignancies. Using an in vivo gene targeting approach, we demonstrate that Rac2, but not Rac1, is critical to the initiation of acute myeloid leukemia in a retroviral expression model of MLL-AF9 leukemogenesis. However, loss of either Rac1 or Rac2 is sufficient to impair survival and growth of the transformed MLL-AF9 leukemia. Rac2 is known to positively regulate expression of Bcl-2 family proteins toward a prosurvival balance. We demonstrate that disruption of downstream survival signaling through antiapoptotic Bcl-2 proteins is implicated in mediating the effects of Rac2 deficiency in MLL-AF9 leukemia. Indeed, overexpression of Bcl-xL is able to rescue the effects of Rac2 deficiency and MLL-AF9 cells are exquisitely sensitive to direct inhibition of Bcl-2 family proteins by the BH3-mimetic, ABT-737. Furthermore, concurrent exposure to NSC23766, a small-molecule inhibitor of Rac activation, increases the apoptotic effect of ABT-737, indicating the Rac/Bcl-2 survival pathway may be targeted synergistically.

Inhibition of Rac activity alleviates lipopolysaccharide-induced acute pulmonary injury in mice

BACKGROUND

Rac small GTPases play important roles in cytoskeleton and many cell functions including cell cycle, cell growth, cell adhesion and gene transcription. Here, we investigated the roles of Rac including Rac1 and Rac2 in lipopolysaccharide (LPS)-induced pulmonary injury.

METHODS

After LPS was intratracheally instilled to lungs in mice, Rac, CDC42 and RhoA activation assay by pull-down and West blot, inflammatory cell infiltration assay by counting cell numbers and lung histological examination, pro-inflammatory mediator mRNA expression assay by quantitative RT-PCR, measurement of myeloperoxidase (MPO) activity, Evans Blue and albumin accumulation by spectrophotometry were performed to evaluate the roles of Rac in pulmonary injury by using its specific inhibitor, NSC23766.

RESULTS

LPS challenge led to increases of both Rac1 and Rac2, but not CDC42 or RhoA activities in lungs, and intraperitoneal administration with NSC23766 inhibited both Rac1 and Rac2, but not CDC42 or RhoA activities. Treatment with NSC23766 at 1 or 3mg/kg not only reduced the inflammatory cells infiltration and MPO activities, but also inhibited pro-inflammatory mediators, tumor necrosis factor-α and interleukin-1β, mRNA expression. Moreover, in vitro neutrophil migration assay and in vivo microvascular permeability assay indicated that NSC23766 not only inhibited neutrophil transwell migration toward a chemoattractant, fMLP, but also reduced Evans Blue and albumin accumulation in LPS-challenged lungs. LPS activated both Rac1 and Rac2, but not CDC42 or RhoA activities in lungs, and specific inhibition of Rac activities by NSC23766 effectively alleviated LPS-induced injury.

GENERAL SIGNIFICANCE

Rac could be a potential target for therapeutic intervention of pulmonary inflammation.

Rac1 regulates the activity of mTORC1 and mTORC2 and controls cellular size

Mammalian target of rapamycin (mTOR) is a serine/threonine kinase that exists in two separate complexes, mTORC1 and mTORC2, that function to control cell size and growth in response to growth factors, nutrients, and cellular energy levels. Low molecular weight GTP-binding proteins of the Rheb and Rag families are key regulators of the mTORC1 complex, but regulation of mTORC2 is poorly understood. Here, we report that Rac1, a member of the Rho family of GTPases, is a critical regulator of both mTORC1 and mTORC2 in response to growth-factor stimulation. Deletion of Rac1 in primary cells using an inducible-Cre/Lox approach inhibits basal and growth-factor activation of both mTORC1 and mTORC2. Rac1 appears to bind directly to mTOR and to mediate mTORC1 and mTORC2 localization at specific membranes. Binding of Rac1 to mTOR does not depend on the GTP-bound state of Rac1, but on the integrity of its C-terminal domain. This function of Rac1 provides a means to regulate mTORC1 and mTORC2 simultaneously.

The diverse roles of Rac signaling in tumorigenesis

Rac is a member of the Rho family of small GTPases, which act as molecular switches to control a wide array of cellular functions. In particular, Rac signaling has been implicated in the control of cell-cell adhesions, cell-matrix adhesions, cell migration, cell cycle progression and cellular transformation. As a result of its functional diversity, Rac signaling can influence several aspects of tumorigenesis. Consistent with this, in vivo evidence that Rac signaling contributes to tumorigenesis is continuously emerging. Additionally, our understanding of the mechanisms by which Rac signaling is regulated is rapidly expanding and consequently adds to the complexity of how Rac signaling could be modulated during tumorigenesis. Here we review the numerous biological functions and regulatory mechanisms of Rac signaling and discuss how they could influence the different stages of tumorigenesis.

Phosphatidylinositol-4,5-bisphosphate influences Nt-Rac5-mediated cell expansion in pollen tubes of Nicotiana tabacum

The regulation of pollen tube growth by the phospholipid phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2) ) is not well understood. The Arabidopsis genome encodes two type A phosphatidylinositol-4-phosphate (PI4P) 5-kinases, PIP5K10 and PIP5K11, which are exclusively expressed in pollen and produce PtdIns(4,5)P(2) in vitro. Fluorescence-tagged PIP5K10 and PIP5K11 localized to lateral subapical plasma membrane microdomains in tobacco pollen tubes in a pattern closely resembling the distribution of PtdIns(4,5)P(2,) with the exception of notably weaker association at the extreme apex. Overexpression of PIP5K10 or PIP5K11 in tobacco pollen tubes resulted in severe tip swelling and altered actin fine structure similar to that reported for overexpression of tobacco Nt-Rac5, a monomeric GTPase known to regulate the actin cytoskeleton. Increased sensitivity of Arabidopsis pip5k10 pip5k11 double mutant pollen tubes to Latrunculin B (LatB) further supports a role for type A PI4P 5-kinases in controlling the actin cytoskeleton. Despite the disruption of both its type A PI4P 5-kinases, the pip5k10 pip5k11 double mutant was fertile, indicating that one of the remaining type B PI4P 5-kinase isoforms might be functionally redundant with PIP5K10 and PIP5K11. Antagonistic effects of PIP5K11 and the Nt-Rac5-specific guanine nucleotide dissociation inhibitor, Nt-RhoGDI2, on tip swelling observed in coexpression-titration experiments indicate a link between PtdIns(4,5)P(2) and Rac-signaling in pollen tubes. The data suggest that type A PI4P 5-kinases influence the actin cytoskeleton in pollen tubes in part by counteracting Nt-RhoGDI2, possibly contributing to the control of the pool of plasma membrane-associated Nt-Rac5.

Rac inhibition reverses the phenotype of fibrotic fibroblasts

Background

Fibrosis, the excessive deposition of scar tissue by fibroblasts, is one of the largest groups of diseases for which there is no therapy. Fibroblasts from lesional areas of scleroderma patients possess elevated abilities to contract matrix and produce α−smooth muscle actin (α-SMA), type I collagen and CCN2 (connective tissue growth factor, CTGF). The basis for this phenomenon is poorly understood, and is a necessary prerequisite for developing novel, rational anti-fibrotic strategies.

Methods and Findings

Compared to healthy skin fibroblasts, dermal fibroblasts cultured from lesional areas of scleroderma (SSc) patients possess elevated Rac activity. NSC23766, a Rac inhibitor, suppressed the persistent fibrotic phenotype of lesional SSc fibroblasts. NSC23766 caused a decrease in migration on and contraction of matrix, and α−SMA, type I collagen and CCN2 mRNA and protein expression. SSc fibroblasts possessed elevated Akt phosphorylation, which was also blocked by NSC23766. Overexpression of rac1 in normal fibroblasts induced matrix contraction and α−SMA, type I collagen and CCN2 mRNA and protein expression. Rac1 activity was blocked by PI3kinase/Akt inhibition. Basal fibroblast activity was not affected by NSC23766.

Conclusion

Rac inhibition may be considered as a novel treatment for the fibrosis observed in SSc.

Human neutrophils coordinate chemotaxis by differential activation of Rac1 and Rac2

Rac1 and Rac2, members of the small Rho GTPase family, play essential roles in coordinating directional migration and superoxide production during neutrophil responses to chemoattractants. Although earlier studies in Rac1 and Rac2 knockout mice have demonstrated unique roles for each Rac isoform in chemotaxis and NADPH oxidase activation, it is still unclear how human neutrophils use Rac1 and Rac2 to achieve their immunological responses to foreign agent stimulation. In the current study, we used TAT dominant-negative Rac1-T17N and Rac2-T17N fusion proteins to acutely alter the activity of Rac1 and Rac2 individually in human neutrophils. We demonstrate distinct activation kinetics and different roles for Rac1 and Rac2 in response to low vs high concentrations of fMLP. These observations were verified using neutrophils from mice in which Rac1 or Rac2 was genetically absent. Based on these results, we propose a model to explain how human neutrophils kill invading microbes while limiting oxidative damage to the adjacent surrounding healthy tissue through the differential activation of Rac1 and Rac2 in response to different concentrations of chemoattractant.

Inhibition of Rac by the GAP activity of centralspindlin is essential for cytokinesis

During cytokinesis, the guanosine triphosphatase (GTPase) RhoA orchestrates contractile ring assembly and constriction. RhoA signaling is controlled by the central spindle, a set of microtubule bundles that forms between the separating chromosomes. Centralspindlin, a protein complex consisting of the kinesin-6 ZEN-4 and the Rho family GTPase activating protein (GAP) CYK-4, is required for central spindle assembly and cytokinesis in Caenorhabditis elegans. However, the importance of the CYK-4 GAP activity and whether it regulates RhoA remain unclear. We found that two separation-of-function mutations in the GAP domain of CYK-4 lead to cytokinesis defects that mimic centralspindlin loss of function. These defects could be rescued by depletion of the GTPase Rac or its effectors, but not by depletion of RhoA. Thus, inactivation of Rac by centralspindlin functions in parallel with RhoA activation to drive contractile ring constriction during cytokinesis.

Primary granule exocytosis in human neutrophils is regulated by Rac-dependent actin remodeling

The actin cytoskeleton regulates exocytosis in all secretory cells. In neutrophils, Rac2 GTPase has been shown to control primary (azurophilic) granule exocytosis. In this report, we propose that Rac2 is required for actin cytoskeletal remodeling to promote primary granule exocytosis. Treatment of neutrophils with low doses (< or = 10 microM) of the actin-depolymerizing drugs latrunculin B (Lat B) or cytochalasin B (CB) enhanced both formyl peptide receptor- and Ca(2+) ionophore-stimulated exocytosis. Higher concentrations of CB or Lat B, or stabilization of F-actin with jasplakinolide (JP), inhibited primary granule exocytosis measured as myeloperoxidase release but did not affect secondary granule exocytosis determined by lactoferrin release. These results suggest an obligatory role for F-actin disassembly before primary granule exocytosis. However, lysates from secretagogue-stimulated neutrophils showed enhanced actin polymerization activity in vitro. Microscopic analysis showed that resting neutrophils contain significant cortical F-actin, which was redistributed to sites of primary granule translocation when stimulated. Exocytosis and actin remodeling was highly polarized when cells were primed with CB; however, polarization was reduced by Lat B preincubation, and both polarization and exocytosis were blocked when F-actin was stabilized with JP. Treatment of cells with the small molecule Rac inhibitor NSC23766 also inhibited actin remodeling and primary granule exocytosis induced by Lat B/fMLF or CB/fMLF, but not by Ca(2+) ionophore. Therefore, we propose a role for F-actin depolymerization at the cell cortex coupled with Rac-dependent F-actin polymerization in the cell cytoplasm to promote primary granule exocytosis.

Rac GTPases as key regulators of p210-BCR-ABL-dependent leukemogenesis

Chronic myelogenous leukemia (CML) is a malignant disease characterized by expression of p210-BCR-ABL, the product of the Philadelphia chromosome. Survival of CML patients has been significantly improved with the introduction of tyrosine kinase inhibitors that induce long-term hematologic remissions. However, mounting evidence indicates that the use of a single tyrosine kinase inhibitor does not cure this disease due to the persistence of p210-BCR-ABL at the molecular level or the acquired resistance in the stem cell compartment to individual inhibitors. We have recently shown in a murine model that deficiency of the Rho GTPases Rac1 and Rac2 significantly reduces p210-BCR-ABL-mediated proliferation in vitro and myeloproliferative disease in vivo, suggesting Rac as a potential therapeutic target in p210-BCR-ABL-induced disease. This target has been further validated using a first-generation Rac-specific small molecule inhibitor. In this review we describe the role of Rac GTPases in p210-BCR-ABL-induced leukemogenesis and explore the possibility of combinatorial therapies that include tyrosine kinase inhibitor(s) and Rac GTPase inhibitors in the treatment of CML.

Anti-inflammatory effects of 8-hydroxy-2'-deoxyguanosine on lipopolysaccharide-induced inflammation via Rac suppression in Balb/c mice

Recently, we observed that 8-hydroxyguanosine triphosphate and 8-hydroxy-2'-deoxyguanosine (oh(8)dG) inactivate Rac and consequently down-regulate the Rac-linked NADPH oxidase, iNOS, and Cox2. Based on these observations, we tested whether oh(8)dG has anti-inflammatory activity in vivo in lipopolysaccharide (LPS)-treated mice. LPS (1 mg/kg, ip)-treated mice exhibit marked inflammatory responses, including increases in proinflammatory cytokines (TNF-alpha, IL-6, IL-18, and IL-12p70) in serum and infiltration of neutrophils, increased translocation of NF-kappaB p50 from the cytosol to the nucleus, and phosphorylation of c-Jun in lung tissues. Mice were pretreated with oh(8)dG (up to 60 mg/kg, ip) 4 h before LPS injection, and this pretreatment dose-dependently inhibited the inflammatory responses; the inhibitions observed with 60 mg/kg oh(8)dG were statistically significant. At the same time, oh(8)dG pretreatment inactivated Rac in lung tissues. Oh(8)dG pretreatment (50 mg/kg, ip) also significantly protected against LPS-induced septic death. Furthermore, oh(8)dG was more effective than acetyl salicylic acid in inhibiting these inflammatory responses. 8-Hydroxyguanosine also had some effect but was much weaker than oh(8)dG. The effects of normal nucleosides (dG, G, and A) were negligible or not significant. These results support an anti-inflammatory activity for oh(8)dG, which could be ascribed to its Rac-inactivating action.

ELMO1 and Dock180, a bipartite Rac1 guanine nucleotide exchange factor, promote human glioma cell invasion

A distinct feature of malignant gliomas is the intrinsic ability of single tumor cells to disperse throughout the brain, contributing to the failure of existing therapies to alter the progression and recurrence of these deadly brain tumors. Regrettably, the mechanisms underlying the inherent invasiveness of glioma cells are poorly understood. Here, we report for the first time that engulfment and cell motility 1 (ELMO1) and dedicator of cytokinesis 1 (Dock180), a bipartite Rac1 guanine nucleotide exchange factor (GEF), are evidently linked to the invasive phenotype of glioma cells. Immunohistochemical analysis of primary human glioma specimens showed high expression levels of ELMO1 and Dock180 in actively invading tumor cells in the invasive areas, but not in the central regions of these tumors. Elevated expression of ELMO1 and Dock180 was also found in various human glioma cell lines compared with normal human astrocytes. Inhibition of endogenous ELMO1 and Dock180 expression significantly impeded glioma cell invasion in vitro and in brain tissue slices with a concomitant reduction in Rac1 activation. Conversely, exogenous expression of ELMO1 and Dock180 in glioma cells with low level endogenous expression increased their migratory and invasive capacity in vitro and in brain tissue. These data suggest that the bipartite GEF, ELMO1 and Dock180, play an important role in promoting cancer cell invasion and could be potential therapeutic targets for the treatment of diffuse malignant gliomas.

Ultrastructural analysis of the interaction between F-actin and respiratory syncytial virus during virus assembly

During respiratory syncytial virus (RSV) infection there is a close physical interaction between the filamentous actin (F-actin) and the virus, involving both inclusion bodies and the virus filaments. This interaction appears to occur relatively early in the replication cycle, and can be detected from 8 h post-infection. Furthermore, during virus assembly we obtained evidence for the participation of an F-actin-associated signalling pathway involving phosphatidyl-3-kinase (PI3K). Treatment with the PI3K inhibitor LY294002 prevented the formation of virus filaments, although no effect was observed either on virus protein expression, or on trafficking of the virus glycoproteins to the cell surface. Inhibition of the activity of Rac GTPase, a down-stream effector of PI3K, by treatment with the Rac-specific inhibitor NSC23766 gave similar results. These data suggest that an intimate interaction occurs between actin and RSV, and that actin-associated signalling pathway, involving PI3K and Rac GTPase, may play an important role during virus assembly.

Genetic and pharmacologic evidence implicating the p85 alpha, but not p85 beta, regulatory subunit of PI3K and Rac2 GTPase in regulating oncogenic KIT-induced transformation in acute myeloid leukemia and systemic mastocytosis

Oncogenic activation loop KIT mutations are observed in acute myeloid leukemia (AML) and systemic mastocytosis (SM); however, unlike the KIT juxtamembrane mutants, the activation loop mutants are insensitive to imatinib mesylate. Furthermore, as prior studies primarily used heterologous cell lines, the molecular mechanism(s) underlying oncogenic KIT-induced transformation in primary cells is poorly understood. We demonstrate that expression of KITD814V in primary hematopoietic stem/progenitor cells (HSC/Ps) and mast cell progenitors (MCps) induces constitutive KIT autophosphorylation, supports ligand-independent hyperproliferation, and promotes promiscuous cooperation with multiple cytokines. Genetic disruption of p85 alpha, the regulatory subunit of class IA lipid kinase phosphoinositol-3-kinase (PI3K), but not of p85 beta, or genetic disruption of the hematopoietic cell-specific Rho GTPase, Rac2, normalizes KITD814V-induced ligand-independent hyperproliferation. Additionally, deficiency of p85 alpha or Rac2 corrects the promiscuous hyperproliferation observed in response to multiple cytokines in both KITD814V-expressing HSC/Ps and MCps. Treatment of KITD814V-expressing HSC/Ps with a Rac inhibitor (NC23766) or with rapamycin showed a dose-dependent suppression in ligand-independent growth. Taken together, our results identify p85 alpha and Rac2 as potential novel therapeutic targets for the treatment of KITD814V-bearing AML and SM.

Antiviral activity of a Rac GEF inhibitor characterized with a sensitive HIV/SIV fusion assay

A virus-dependent fusion assay was utilized to examine the activity of a panel of HIV-1, -2, and SIV isolates of distinct coreceptor phenotypes. This assay allowed identification of entry inhibitors, and characterization of an antagonist of a Rac guanine nucleotide exchange factor, as an inhibitor of HIV-mediated fusion.

The GTPase Rac regulates the proliferation and invasion of fibroblast-like synoviocytes from rheumatoid arthritis patients

Fibroblast-like synoviocytes (FLS) isolated from joints of rheumatoid arthritis (RA) patients display proliferative and invasive properties reminiscent of those of malignant tumor cells. Rac small GTPases play an important role in tumor cell proliferation and invasion. We therefore investigated the potential role of Rac proteins in the proliferative and invasive behavior of RA-FLS. We showed that inhibiting Rac activity with the Rac-specific small molecule inhibitor NSC23766 causes a strong inhibition of RA-FLS proliferation, without affecting cell survival. Rac inhibition also results in a strong reduction in RA-FLS invasion through reconstituted extracellular matrix and a less marked inhibition of two-dimensional migration as measured by monolayer wound healing. We also showed that small interfering RNA-mediated depletion of Rac1 inhibits RA-FLS proliferation and invasion to a similar extent as NSC23766. These results demonstrate for the first time that Rac proteins play an important role in the aggressive behavior of FLS isolated from RA patients. In addition, we observed that inhibiting Rac proteins prevents JNK activation and that the JNK inhibitor SP600125 strongly inhibits RA-FLS invasion, suggesting that Rac-mediated JNK activation contributes to the role of Rac proteins in the invasive behavior of RA-FLS. In conclusion, Rac-controlled signaling pathways may present a new source of drug targets for therapeutic intervention in RA.

Paxillin-β-catenin interactions are involved in Rac/Cdc42-mediated endothelial barrier-protective response to oxidized phospholipids

Oxidized phospholipids may appear in the pulmonary circulation as a result of acute lung injury or inflammation. We have previously described barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine (OxPAPC) on human pulmonary endothelial cells (EC) mediated by small GTPases Rac and Cdc42. This work examined OxPAPC-induced focal adhesion (FA) and adherens junction (AJ) remodeling and potential interactions between FA and AJ protein complexes involved in OxPAPC-induced EC barrier enhancement. Immunofluorescence analysis, subcellular fractionation, and coimmunoprecipitation assays have shown that OxPAPC induced translocation and peripheral accumulation of FA complexes containing paxillin, focal adhesion kinase, vinculin, GIT1, and GIT2, increased association of AJ proteins vascular endothelial-cadherin, p120-catenin, α-, β-, and γ-catenins, and dramatically enhanced cell junction areas covered by AJ. Coimmunoprecipitation, pulldown assays, and confocal microscopy studies have demonstrated that OxPAPC promoted novel interactions between FA and AJ complexes via paxillin and β-catenin association, which was critically dependent on Rac and Cdc42 activities and was abolished by pharmacological or small interfering RNA (siRNA)-mediated inhibition of Rac and Cdc42. Depletion of β-catenin using the siRNA approach attenuated OxPAPC-induced paxillin translocation to the cell periphery, but also significantly decreased interaction of paxillin with AJ protein complex. In turn, paxillin knockdown by specific siRNA attenuated AJ enhancement in response to OxPAPC. These results show for the first time the novel interactions between FA and AJ protein complexes critical for EC barrier regulation by OxPAPC.

Tumor cytotoxicity and endothelial Rac inhibition induced by TNP-470 in anaplastic thyroid cancer

Anaplastic thyroid carcinoma is an aggressive form of cancer with no treatment. Angiogenesis inhibitors, such as TNP-470, a synthetic derivative of fumagillin, have been shown to reduce tumor size and increase survival in heterotopic animal models of thyroid cancer. Our goals were to determine the effect of TNP-470 on anaplastic thyroid cancer using an orthotopic murine model, to identify the molecular pathways of TNP-470 actions on endothelial cells, and to determine the non-endothelial tumor effects of TNP-470. We injected human anaplastic thyroid carcinoma cells (DRO'90) into the thyroid glands of nude mice. Mice received TNP-470 (30 mg/kg) s.c. for 6 weeks. TNP-470 prolonged survival and reduced liver metastases. TNP-470 had direct cytotoxic effects on anaplastic thyroid carcinoma cells in vitro and in vivo. Paradoxically, TNP-470 increased vascular endothelial growth factor secretion from tumor cells in vitro and in vivo. However, there was no associated increase in tumor microvessel density. In endothelial cells, TNP-470 prevented vascular endothelial growth factor-induced endothelial permeability, intercellular gap formation, and ruffle formation by preventing Rac1 activation.

Fluvastatin inhibits hypoxic proliferation and p38 MAPK activity in pulmonary artery fibroblasts

The earliest structural change in hypoxia-induced pulmonary hypertension is increased proliferation of adventitial fibroblasts. This fibroproliferative response occurs in acute and chronic hypoxic models, is dependent on p38 mitogen-activated protein (MAP) kinase activation, is selective for the pulmonary circulation, and would seem an important therapeutic target. Simvastatin attenuates pulmonary vascular remodeling in animal models, but additional information regarding mechanisms of action, differential antiproliferative effects and dose responses of available statins is required for appropriate clinical trial design. Our objectives were to determine the effects of statins on acute hypoxia-induced proliferation and p38 MAP kinase activation in pulmonary and systemic artery fibroblasts, to assess the effects of cholesterol intermediates, prenyltransferase and related inhibitors, and to determine the statin's mechanism of action. Atorvastatin, fluvastatin, and simvastatin inhibited adventitial fibroblast proliferation. At low doses (1 microM), this effect was selective for hypoxic (versus serum-induced) proliferation and was also selective for pulmonary (versus systemic) fibroblasts. Complete inhibition of hypoxia-induced p38 MAP kinase activity was achieved at this 1-microM dose. The lipophilic statins exhibited similar potency. The statin effect was reversed by geranylgeranyl pyrophosphate and mimicked by geranylgeranyl transferase and Rac1 inhibitors. Hypoxia-induced p38 MAP kinase activation and proliferation in pulmonary adventitial fibroblasts is dependent on a geranylgeranylated signaling protein, probably Rac1. One micromolar of fluvastatin exhibits a circulation- and stimulus-selective antiproliferative effect on pulmonary artery fibroblasts. The pharmacokinetics of fluvastatin would suggest that its antiproliferative effects may be useful in pulmonary hypertension associated with hypoxia.

Expansion of normal and leukemic human hematopoietic stem/progenitor cells requires Rac-mediated interaction with stromal cells

OBJECTIVE

To determine the involvement of Rac signaling in self-renewal and expansion on bone marrow stroma of normal CD34+ cells vs leukemic CD34+ cells from acute myeloid leukemia (AML) patients.

MATERIALS AND METHODS

Rac signaling was modulated by retroviral introduction of Racl-N17, Racl-V12, or by using the Rac inhibitor NSC23766. In long-term MS5 cocultures (leukemic) expansion, migration, adhesion, and presence of stem/progenitor cells were monitored in both normal as well as leukemic CD34+ cells.

RESULTS

Inhibition of Rac signaling impaired migration and adhesion of cord blood (CB) CD34+ cells on MS5 stroma. Long-term inhibition of Rac during a 5-week coculture period on stroma prevented association of hematopoietic progenitors with the bone marrow stromal cells and resulted in a dramatic decrease in the primitive stem cell frequency (long-term culture-initiating cell) in a dose-dependent manner. Many of these phenotypes were reversed in the presence of activated Racl-V12, including improved migration toward, and association with, MS5 cells. CD34+ AML cells were characterized by elevated levels of Rac activity (five of seven patients) and enhanced migration and adhesion to MS5 bone marrow stroma as compared to CB CD34+ cells. A dramatic decrease was observed in the formation of leukemic cobblestone area-forming cells as well as strongly diminished clonal expansion in the presence of the Rac inhibitor NSC23766.

CONCLUSION

Our data indicate that Rac signal transduction is required for the maintenance and expansion of both normal as well as leukemic stem/progenitor cells by mediating their interaction with stromal cells.

Effects of pharmacological inhibition of small GTPases on axon extension and migration of enteric neural crest-derived cells

In the developing enteric nervous system, there is a close association between migrating neural crest-derived cells and the axons of early differentiating neurons. We used pharmacological inhibitors of small GTPases to determine if crest cell migration and axon growth could be uncoupled in cultured intact explants of embryonic mouse gut and slices of embryonic gut grown on collagen gels containing GDNF. Inhibition of the Rho effectors, ROCKI/II, or Rac/Cdc42 inhibited both cell migration and neurite growth in intact explants of embryonic gut. The effects of both ROCKI/II and Rac/Cdc42 inhibitors were more severe on cell migration and axon extension in gut explants from Ret(+/-) mice than in explants from wildtype mice, indicating that Rho GTPases probably act downstream of the receptor tyrosine kinase, Ret. Inhibition of ROCKI/II had different effects on migration and axon extension in gut slices grown on collagen gels containing GDNF from that seen in intact explants of gut. We conclude that ROCKI/II and Rac/Cdc42 are required for both neural crest-derived cell migration and axon growth in the developing gut.

Enhanced cardiac allograft survival by Vav1-Rac signaling blockade in a mouse model

BACKGROUND

Vav1-Rac signaling plays a pivotal role in TCR/antigen and CD28 signals for T cell activation. However, pharmacological interference of this signaling has not been tested in the prevention of alloimmune-mediated allograft rejection. It has been demonstrated that 6-thio-GTP, a metabolite of azathioprine, specifically inhibits Vav1-Rac activity in T lymphocytes. Here we show the immunosuppressive efficacy of 6-thio-GTP in the prevention of cardiac allograft rejection.

METHODS

T cell proliferations were measured by (3)H-thymidine uptake. The immunosuppressive activities of 6-thio-GTP were tested in the cardiac allograft model of C57BL/6 (H-2(b)) to Balb/c (H-2(d)) mice.

RESULTS

6-Thio-GTP inhibited TCR/alloantigen stimulated T cell proliferation and CD28-dependent T cell survival. Administration of 6-thio-GTP (0.5 mg/kg) prolonged graft survival to 13.8+/-2.39 days compared to 8.3+/-0.48 days in PBS controls (p<0.0001). Combination of 6-thio-GTP (0.5 mg/kg) with CsA (15 mg/kg) enhanced graft survival from 15.0+/-1.61 days in CsA treated recipients to 36.8+/-2.17 days in those received 20 days of combination therapy of CsA and 6-thio-GTP (p<0.0001), or to 42.7+/-16.63 days in the group treated with 20 days of CsA and 60 days of 6-thio-GTP (p<0.0001). Lymphocytes from 6-thio-GTP treated recipients with long-term surviving grafts (>60 days) displayed reduced proliferative response to alloantigen and higher frequencies of regulatory T cells (Treg).

CONCLUSION

Vav1-Rac inhibitor 6-thio-GTP prolongs allograft survival alone or in combination with CsA by suppression of alloreactive T cell activation. Our findings suggest the therapeutic potential of pharmacological interference of Vav1-Rac signaling for transplantation.

HGF attenuates thrombin-induced endothelial permeability by Tiam1-mediated activation of the Rac pathway and by Tiam1/Rac-dependent inhibition of the Rho pathway

Reorganization of the endothelial cell (EC) cytoskeleton and cell adhesive complexes provides a structural basis for increased vascular permeability implicated in the pathogenesis of many diseases, including asthma, sepsis, and acute respiratory distress syndrome (ARDS). We have recently described the barrier-protective effects of hepatocyte growth factor (HGF) on the human pulmonary EC. In the present study, we explored the involvement of Rac-GTPase and Rac-specific nucleotide exchange factor Tiam1 in the mechanisms of EC barrier protection by HGF. HGF protected EC monolayers from thrombin-induced hyperpermeability, disruption of intercellular junctions, and formation of stress fibers and paracellular gaps by inhibiting thrombin-induced activation of Rho GTPase, Rho association with nucleotide exchange factor p115-RhoGEF, and myosin light chain phosphorylation, which was opposed by stimulation of Rac-dependent signaling. The pharmacological Rac inhibitor or silencing RNA (siRNA) based depletion of either Rac or Tiam1 significantly attenuated HGF-induced peripheral translocation of Rac effector cortactin, cortical actin ring formation, and EC barrier enhancement. Moreover, Tiam1 knockdown using the siRNA approach, attenuated the protective effect of HGF against thrombin-induced activation of Rho signaling, monolayer disruption, and EC hyperpermeability. This study demonstrates the Tiam1/Rac-dependent mechanism of HGF-induced EC barrier protection and provides novel mechanistic insights into regulation of EC permeability via dynamic interactions between Rho- and Tiam1/Rac-mediated pathways.

Prostaglandins PGE(2) and PGI(2) promote endothelial barrier enhancement via PKA- and Epac1/Rap1-dependent Rac activation

Prostaglandin E(2) (PGE(2)) and prostacyclin are lipid mediators produced by cyclooxygenase and implicated in the regulation of vascular function, wound repair, inflammatory processes, and acute lung injury. Although protective effects of these prostaglandins (PGs) are associated with stimulation of intracellular cAMP production, the crosstalk between cAMP-activated signal pathways in the regulation of endothelial cell (EC) permeability is not well understood. We studied involvement of cAMP-dependent kinase (PKA), cAMP-Epac-Rap1 pathway, and small GTPase Rac in the PGs-induced EC barrier protective effects and cytoskeletal remodeling. PGE(2) and PGI(2) synthetic analog beraprost increased transendothelial electrical resistance and decreased dextran permeability, enhanced peripheral F-actin rim and increased intercellular adherens junction areas reflecting EC barrier-protective response. Furthermore, beraprost dramatically attenuated thrombin-induced Rho activation, MLC phosphorylation and EC barrier dysfunction. In vivo, beraprost attenuated lung barrier dysfunction induced by high tidal volume mechanical ventilation. Both PGs caused cAMP-mediated activation of PKA-, Epac/Rap1- and Tiam1/Vav2-dependent pathways of Rac1 activation and EC barrier regulation. Knockdown of Epac, Rap1, Rac-specific exchange factors Tiam1 and Vav2 using siRNA approach, or inhibition of PKA activity decreased Rac1 activation and PG-induced EC barrier enhancement. Thus, our results show that barrier-protective effects of PGE(2) and prostacyclin on pulmonary EC are mediated by PKA and Epac/Rap pathways, which converge on Rac activation and lead to enhancement of peripheral actin cytoskeleton and adherens junctions. These mechanisms may mediate protective effects of PGs against agonist-induced lung vascular barrier dysfunction in vitro and against mechanical stress-induced lung injury in vivo.

Requirement of Rac1 distinguishes follicular from interfollicular epithelial stem cells

Epithelial stem cells in the bulge region within the hair follicle maintain the cyclic hair growth, but whether these stem cells also contribute to the epidermal renewal remains unclear. Here, we observed that the conditional deletion of the Rac1 gene in the mouse skin, including the potential follicular and epidermal stem cell compartments, results in alopecia owing to defective hair development. Surprisingly, mice lacking the expression of this Rho GTPase do not display major alterations in the interfollicular skin. Furthermore, Rac1 excision from primary epithelial keratinocytes results in the inability to reconstitute hair follicles and sebaceous glands when grafted onto mice, but epithelial cells lacking Rac1 can nonetheless form a healthy epidermis. Together, these findings support the emerging view that the epidermis and the hair follicles are maintained by different epithelial stem cells, and provide evidence that the requirement for Rac1 function can distinguish these distinct stem cells populations.

Phospholipase C gamma negatively regulates Rac/Cdc42 activation in antigen-stimulated mast cells

The Rho GTPases Rac and Cdc42 play a central role in the regulation of secretory and cytoskeletal responses in antigen-stimulated mast cells. In this study, we examine the kinetics and mechanism of Rac and Cdc42 activation in the rat basophilic leukemia RBL-2H3 cells. The activation kinetics of both Rac and Cdc42 show a biphasic profile, consisting of an early transient peak at 1 min and a late sustained activation phase at 20-40 min. The inhibition of phospholipase C (PLC)gamma causes a twofold increase in Rac and Cdc42 activation that coincides with a dramatic production of atypical filopodia-like structures. Inhibition of protein kinase C using bisindolylmaleimide mimics the effect of PLCgamma inhibition on Rac activation, but not on Cdc42 activation. In contrast, depletion of intracellular calcium leads to a complete inhibition of the early activation peak of both Rac and Cdc42, without significant effects on the late sustained activation. These data suggest that PLCgamma is involved in a negative feedback loop that leads to the inhibition of Rac and Cdc42. They also suggest that the presence of intracellular calcium is a prerequisite for both Rac and Cdc42 activation.

Merlin/Neurofibromatosis Type 2 Suppresses Growth by Inhibiting the Activation of Ras and Rac

The small G-protein Ras is a tightly controlled regulator of cell fate. Prolonged or persistent arrest in the activated GTP-loaded state by mutation of Ras as in lung cancer or in a Ras-GTPase-activating protein as in neurofibromatosis type 1 promotes tumorigenesis. We now show that the tumor-suppressor protein merlin (mutated in neurofibromatosis type 2) also controls Ras activity. Systematic analysis of growth factor signaling located the step of merlin interference to the activation of Ras and Rac. Merlin independently uncouples both Ras and Rac from growth factor signals. In the case of Ras, merlin acts downstream of the receptor tyrosine kinase-growth factor receptor binding protein 2 (Grb2)-SOS complex. However, merlin does not bind either SOS or Ras, but it counteracts the ERM (ezrin, radixin, moesin)-dependent activation of Ras, which correlates with the formation of a complex comprising ERM proteins, Grb2, SOS, Ras, and filamentous actin. Because efficient signaling from Ras requires Rac-p21-activated kinase-dependent phosphorylations of Raf and mitogen-activated protein/extracellular signal-regulated kinase kinase, merlin can also inhibit signal transfer from dominantly active Ras mutants. We propose that the interference of merlin with Ras- and Rac-dependent signal transfer represents part of the tumor-suppressive action of merlin.

p120-catenin and p190RhoGAP regulate cell-cell adhesion by coordinating antagonism between Rac and Rho

Integration of receptor tyrosine kinase, integrin, and cadherin activities is crucial for normal cell growth, motility, and adhesion. Here, we describe roles for p120-catenin (p120) and p190RhoGAP that coordinate crosstalk between these systems and regulate cadherin function. Surprisingly, PDGFR-induced actin remodeling in NIH3T3 cells is blocked in the absence of p120, and the cells are partially transformed via constitutive activation of Rho. We have traced the mechanism to unexpected codependent roles for p120 and p190RhoGAP in regulating Rac-dependent antagonism of Rho. Receptor-induced Rac activity causes translocation of p190RhoGAP to adherens junctions (AJs), where it couples to the cadherin complex via interaction with p120. AJ formation is dependent on this p120-p190RhoGAP interaction and fails altogether if either of these proteins are compromised. We propose that Rac activation links diverse signaling systems to AJ assembly by controlling transient p190RhoGAP interactions with p120 and localized inhibition of Rho.

Signaling pathways involved in OxPAPC-induced pulmonary endothelial barrier protection

Increased tissue or serum levels of oxidized phospholipids have been detected in a variety of chronic and acute pathological conditions such as hyperlipidemia, atherosclerosis, heart attack, cell apoptosis, acute inflammation and injury. We have recently described signaling cascades activated by oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (OxPAPC)in the human pulmonary artery endothelial cells (EC) and reported potent barrier-protective effects of OxPAPC, which were mediated by small GTPases Rac and Cdc42. In this study we have further characterized signal transduction pathways involved in the OxPAPC-mediated endothelial barrier protection. Inhibitors of small GTPases, protein kinase A (PKA), protein kinase C (PKC), Src family kinases and general inhibitors of tyrosine kinases attenuated OxPAPC-induced barrier-protective response and EC cytoskeletal remodeling. In contrast, small GTPase Rho, Rho kinase, Erk-1,2 MAP kinase and p38 MAP kinase and PI3-kinase were not involved in the barrier-protective effects of OxPAPC. Inhibitors of PKA, PKC, tyrosine kinases and small GTPase inhibitor toxin B suppressed OxPAPC-induced Rac activation and decreased phosphorylation of focal adhesion kinase (FAK) and paxillin. Barrier-protective effects of OxPAPC were not reproduced by platelet activating factor (PAF), which at high concentrations induced barrier dysfunction, but were partially attenuated by PAF receptor antagonist A85783. These results demonstrate for the first time upstream signaling cascades involved in the OxPAPC-induced Rac activation, cytoskeletal remodeling and barrier regulation and suggest PAF receptor-independent mechanisms of OxPAPC-mediated endothelial barrier protection.

Cross-linking of MHC class II molecules interferes with phorbol 12,13-dibutyrate-induced differentiation of resting B cells by inhibiting Rac-associated ROS-dependent ERK/p38 MAP kinase pathways leading to NF-kappaB activation

In addition to their essential role in antigen presentation, major histocompatibility complex (MHC) class II molecules have been described as the receptor associated with signal transduction regulating B-cell function. In previous experiments, we found that cross-linking of MHC class II molecules with corresponding anti-MHC class II antibodies inhibited NF-kappaB-activated signaling pathways associated with the proliferation and differentiation of the LPS-stimulated primary and resting B-cell line, 38B9. We also found that exposure to the anti-MHC class II antibody reduced the production of ROS, which function as secondary signal transducers, in the phorbol 12,13-dibutyrate (PDBU)-treated (but not in the LPS-treated) resting B-cell line. In this study, we investigated the molecular mechanisms in the ROS-associated signaling pathway leading to PDBU-induced NF-kappaB activation that results in B-cell differentiation and speculated that the signaling pathway was inhibited by exposure to the anti-MHC class II antibody. We also found that this inhibition was mediated through down-regulation of the activated Rac/ROS-associated ERK/p38 MAPK signaling pathway in PDBU-treated 38B9 cells. Collectively, these findings suggest that ROS-associated molecules are involved in MHC class II-associated negative signal transduction in resting B cells.