Chemoattractant-stimulated NF-kappaB activation is dependent on the low molecular weight GTPase RhoA

G12/13 signaling in asthma

Shortening of airway smooth muscle and bronchoconstriction are pathognomonic for asthma. Airway shortening occurs through calcium-dependent activation of myosin light chain kinase, and RhoA-dependent calcium sensitization, which inhibits myosin light chain phosphatase. The mechanism through which pro-contractile stimuli activate calcium sensitization is poorly understood. Our review of the literature suggests that pro-contractile G protein coupled receptors likely signal through G12/13 to activate RhoA and mediate calcium sensitization. This hypothesis is consistent with the effects of pro-contractile agonists on RhoA and Rho kinase activation, actin polymerization and myosin light chain phosphorylation. Recognizing the likely role of G12/13 signaling in the pathophysiology of asthma rationalizes the effects of pro-contractile stimuli on airway hyperresponsiveness, immune activation and airway remodeling, and suggests new approaches for asthma treatment.

Osteogenic growth peptide enhances osteogenic differentiation of human periodontal ligament stem cells

Bone tissue engineering consists of three major components namely cells, scaffolds, and signaling molecules to improve bone regeneration. These integrated principles can be applied in patients suffered from bone resorption diseases, such as osteoporosis and periodontitis. Osteogenic growth peptide (OGP) is a fourteen-amino acid sequence peptide that has the potential to regenerate bone tissues. This study aimed to disseminate the osteogenic differentiation of human periodontal ligament stem cells (hPDLSCs) with OGP treatment. OGP was elaborated for proliferation, cytotoxicity, osteogenic differentiation effects, and the involvement of osteogenic related signaling pathways in vitro. This study found that OGP at lower concentration shows better effects on cytotoxicity and proliferation. Moreover, OGP at concentration 0.01 nM had the most potential to differentiate hPDLSCs toward osteogenic lineage comparing with higher concentrations of OGP. The phenomenon was mainly involving transforming growth factor-beta (TGF-β), bone morphogenetic protein (BMP), Hedgehog, and Wingless-related (Wnt) pathways. Further, SB-431542 treatment demonstrated the partial involvement of OGP in regulating osteogenic differentiation of hPDLSCs. In conclusion, OGP at low concentration enhances osteogenic differentiation of hPDLSCs by governing TGF-β signaling pathway.

Annexin A1 as a Regulator of Immune Response in Cancer

Annexin A1 is a 37 kDa phospholipid-binding protein that is expressed in many tissues and cell types, including leukocytes, lymphocytes and epithelial cells. Although Annexin A1 has been extensively studied for its anti-inflammatory activity, it has been shown that, in the cancer context, its activity switches from anti-inflammatory to pro-inflammatory. Remarkably, Annexin A1 shows pro-invasive and pro-tumoral properties in several cancers either by eliciting autocrine signaling in cancer cells or by inducing a favorable tumor microenvironment. Indeed, the signaling of the N-terminal peptide of AnxA1 has been described to promote the switching of macrophages to the pro-tumoral M2 phenotype. Moreover, AnxA1 has been described to prevent the induction of antigen-specific cytotoxic T cell response and to play an essential role in the induction of regulatory T lymphocytes. In this way, Annexin A1 inhibits the anti-tumor immunity and supports the formation of an immunosuppressed tumor microenvironment that promotes tumor growth and metastasis. For these reasons, in this review we aim to describe the role of Annexin A1 in the establishment of the tumor microenvironment, focusing on the immunosuppressive and immunomodulatory activities of Annexin A1 and on its interaction with the epidermal growth factor receptor.

Novel CTRP8-RXFP1-JAK3-STAT3 axis promotes Cdc42-dependent actin remodeling for enhanced filopodia formation and motility in human glioblastoma cells

C1q tumor necrosis factor‐related peptide 8 (CTRP8) is the least studied member of the C1Q‐TNF‐related peptide family. We identified CTRP8 as a ligand of the G protein‐coupled receptor relaxin family peptide receptor 1 (RXFP1) in glioblastoma multiforme (GBM). The CTRP8‐RXFP1 ligand–receptor system protects human GBM cells against the DNA‐alkylating damage‐inducing temozolomide (TMZ), the drug of choice for the treatment of patients with GBM. The DNA protective role of CTRP8 was dependent on a functional RXFP1‐STAT3 signaling cascade and targeted the monofunctional glycosylase N‐methylpurine DNA glycosylase (MPG) for more efficient base excision repair of TMZ‐induced DNA‐damaged sites. CTRP8 also improved the survival of GBM cells by upregulating anti‐apoptotic BCl‐2 and BCL‐XL. Here, we have identified Janus‐activated kinase 3 (JAK3) as a novel member of a novel CTRP8‐RXFP1‐JAK3‐STAT3 signaling cascade that caused an increase in cellular protein content and activity of the small Rho GTPase Cdc42. This is associated with significant F‐actin remodeling and increased GBM motility. Cdc42 was critically important for the upregulation of the actin nucleation complex N‐Wiskott–Aldrich syndrome protein/Arp3/4 and actin elongation factor profilin‐1. The activation of the RXFP1‐JAK3‐STAT3‐Cdc42 axis by both RXFP1 agonists, CTRP8 and relaxin‐2, caused extensive filopodia formation. This coincided with enhanced activity of ezrin, a key factor in tethering F‐actin to the plasma membrane, and inhibition of the actin filament severing activity of cofilin. The F‐actin remodeling and pro‐migratory activities promoted by the novel RXFP1‐JAK3‐STAT3‐Cdc42 axis were blocked by JAK3 inhibitor tofacitinib and STAT3 inhibitor STAT3 inhibitor VI. This provides a new rationale for the design of JAK3 and STAT3 inhibitors with better brain permeability for clinical treatment of the pervasive brain invasiveness of GBM.

Neuropeptide Y/Y5 Receptor Pathway Stimulates Neuroblastoma Cell Motility Through RhoA Activation

Neuropeptide Y (NPY) has been implicated in the regulation of cellular motility under various physiological and pathological conditions, including cancer dissemination. Yet, the exact signaling pathways leading to these effects remain unknown. In a pediatric malignancy, neuroblastoma (NB), high NPY release from tumor tissue associates with metastatic disease. Here, we have shown that NPY stimulates NB cell motility and invasiveness and acts as a chemotactic factor for NB cells. We have also identified the Y5 receptor (Y5R) as the main NPY receptor mediating these actions. In NB tissues and cell cultures, Y5R is highly expressed in migratory cells and accumulates in regions of high RhoA activity and dynamic cytoskeleton remodeling. Y5R stimulation activates RhoA and results in Y5R/RhoA-GTP interactions, as shown by pull-down and proximity ligation assays, respectively. This is the first demonstration of the role for the NPY/Y5R axis in RhoA activation and the subsequent cytoskeleton remodeling facilitating cell movement. These findings implicate Y5R as a target in anti-metastatic therapies for NB and other cancers expressing this receptor.

FPR1 mediates the tumorigenicity of human cervical cancer cells

Purpose

The present study aimed to investigate the role of FPR1 and the downstream effectors such as NF-κB and IL-6/8 in the development of cervical cancer.

Patients and methods

FPR1 protein expression was detected via immunohistochemical staining in tissue microarrays containing cervical cancer tissues from 185 patients. Following FPR1 silencing in SiHa cells using lentiviral siRNA delivery, biological characteristics and tumor formation were evaluated in vitro and in vivo, respectively. Phosphorylated NF-κB levels were detected by Western blotting, while IL-6 and IL-8 secretion were detected by ELISA in both FPR1 knockdown and control SiHa cells. Human umbilical vein endothelial cell tube formation assays were performed to evaluate the angiogenesis-promoting ability of IL-6 and IL-8 secretion in FPR1 knockdown and control SiHa cells. Neovascularization, proliferation and apoptosis markers were detected by immunohistochemical staining to analyze the tumorigenic role of FPR1.

Results

Immunohistochemistry of cervical cancer tissues from 185 patients revealed high FPR1 expression levels in patients with advanced-stage disease and/or poor prognosis. Compared with control cells, cervical cancer cells in which FPR1 was silenced exhibited inhibition of cell invasion, migration and proliferation and higher levels of apoptosis. NF-κB was inhibited in FPR1 knockdown in SiHa cells. IL-6/8 upregulation by FPR1 activation stimulated angiogenesis. FPR1 deficiency inhibited the tumorigenicity of cervical cancer cells in nude mice. FPR1, IL-6, IL-8, CD31 and Ki67 levels were all reduced, whereas cleaved caspase-3 was upregulated, in the FPR1 knockdown group compared with the levels in the control group.

Conclusion

High FPR1 expression was associated with advanced stage and poor prognosis in cervical cancer patients. FPR1 activation induced NF-κB nuclear translocation to promote cervical cancer development through the upregulation of IL-6 and IL-8 expression. Inhibiting FPR1 activity may thus have potential therapeutic value in cervical cancer patients.

Inhibiting HDAC1 Enhances the Anti-Cancer Effects of Statins through Downregulation of GGTase-Iβ Expression

Hydroxy-methyl-glutaryl-coenzyme A (HMG-CoA) reductase inhibitors, namely statins, are potential anti-tumor agents. Previously, we showed that a pan-histone deacetylase (HDAC) inhibitor enhances the anti-tumor effects of the HMG-CoA inhibitor. However, the underlying mechanisms were not fully understood. Cancer cell lines (CAL-27 and SACC-83) were exposed to pan-HDAC inhibitor, or HDAC1 inhibitor, or geranylgeranyl transferase type I (GGTase-I) inhibitor alone or in combination with statin. Cell viability, apoptosis, migration, and invasion were assessed by Cell Count Kit-8, 4′,6-diamidino-2-phenylindole staining, and transwell assay, respectively. A xenograft model was used for assessing tumor growth in vivo. Western blot and real-time PCR were used to assess the expression of genes. We observed that inhibiting HDAC1 could enhance the anti-tumor effects of statins both in vitro and in vivo. Inhibiting HDAC1 blocked the statin-induced upregulation of geranylgeranyl transferase type Iβ subunit (GGTase-Iβ), resulting in an enhancement of the anti-cancer effects of statin. Overexpression of GGTase-Iβ or constitutively active RhoA abolished the enhancement by inhibiting HDAC1 on anti-tumor effects of statins. The HDAC1 inhibitor failed to enhance cytotoxicity in non-tumor primary cells treated with statin. Inhibiting HDAC1 enhanced the anti-cancer effects of statins through downregulation of GGTase-Iβ expression, and thus further inactivation of RhoA. A combination of statin with HDAC1 or GGTase-I inhibitor would be a new strategy for cancer chemotherapy.

The Role of Rho GTPases in Toxicity of Clostridium difficile Toxins

Clostridium difficile (C. difficile) is the main cause of antibiotic-associated diarrhea prevailing in hospital settings. In the past decade, the morbidity and mortality of C. difficile infection (CDI) has increased significantly due to the emergence of hypervirulent strains. Toxin A (TcdA) and toxin B (TcdB), the two exotoxins of C. difficile, are the major virulence factors of CDI. The common mode of action of TcdA and TcdB is elicited by specific glucosylation of Rho-GTPase proteins in the host cytosol using UDP-glucose as a co-substrate, resulting in the inactivation of Rho proteins. Rho proteins are the key members in many biological processes and signaling pathways, inactivation of which leads to cytopathic and cytotoxic effects and immune responses of the host cells. It is supposed that Rho GTPases play an important role in the toxicity of C. difficile toxins. This review focuses on recent progresses in the understanding of functional consequences of Rho GTPases glucosylation induced by C. difficile toxins and the role of Rho GTPases in the toxicity of TcdA and TcdB.

G Protein-Coupled Receptor and RhoA-Stimulated Transcriptional Responses: Links to Inflammation, Differentiation, and Cell Proliferation

The low molecular weight G protein RhoA (rat sarcoma virus homolog family member A) serves as a node for transducing signals through G protein-coupled receptors (GPCRs). Activation of RhoA occurs through coupling of G proteins, most prominently, G12/13, to Rho guanine nucleotide exchange factors. The GPCR ligands that are most efficacious for RhoA activation include thrombin, lysophosphatidic acid, sphingosine-1-phosphate, and thromboxane A2. These ligands also stimulate proliferation, differentiation, and inflammation in a variety of cell and tissues types. The molecular events underlying these responses are the activation of transcription factors, transcriptional coactivators, and downstream gene programs. This review describes the pathways leading from GPCRs and RhoA to the regulation of activator protein-1, NFκB (nuclear factor κ-light-chain-enhancer of activated B cells), myocardin-related transcription factor A, and Yes-associated protein. We also focus on the importance of two prominent downstream transcriptional gene targets, the inflammatory mediator cyclooxygenase 2, and the matricellular protein cysteine-rich angiogenic inducer 61 (CCN1). Finally, we describe the importance of GPCR-induced activation of these pathways in the pathophysiology of cancer, fibrosis, and cardiovascular disease.

Unprenylated RhoA contributes to IL-1β hypersecretion in mevalonate kinase deficiency model through stimulation of Rac1 activity

Protein prenylation is a post-translational modification whereby non-sterol isoprenoid lipid chains are added, thereby modifying the molecular partners with which proteins interact. The autoinflammatory disease mevalonate kinase deficiency (MKD) is characterized by a severe reduction in protein prenylation. A major class of proteins that are affected are small GTPases, including Rac1 and RhoA. It is not clear how protein prenylation of small GTPases relates to GTP hydrolysis activity and downstream signaling. Here, we investigated the contribution of RhoA prenylation to the biochemical pathways that underlie MKD-associated IL-1β hypersecretion using human cell cultures, Rac1 and RhoA protein variants, and pharmacological inhibitors. We found that when unprenylated, the GTP-bound levels of RhoA decrease, causing a reduction in GTPase activity and increased protein kinase B (PKB) phosphorylation. Cells expressing unprenylated RhoA produce increased levels of interleukin 1β mRNA. Of other phenotypic cellular changes seen in MKD, increased mitochondrial potential and mitochondrial elongation, only mitochondrial elongation was observed. Finally, we show that pharmacological inactivation of RhoA boosts Rac1 activity, a small GTPase whose activity was earlier implied in MKD pathogenesis. Together, our data show that RhoA plays a pivotal role in MKD pathogenesis through Rac1/PKB signaling toward interleukin 1β production and elucidate the effects of protein prenylation in monocytes.

G protein-coupled receptor FPR1 as a pharmacologic target in inflammation and human glioblastoma

Formylpeptide receptor1 (FPR1) is a G protein-coupled receptor (GPCR) originally identified in phagocytic leucocytes and mediates cell chemotaxis and activation in response to bacterial formylated chemotactic peptides. However, FPR1 also participates in a signal relay which regulates the infiltration of phagocytes, in particular neutrophils, to inflammatory sites in response to tissue-derived chemoattractant ligands. In addition to participating in innate immune responses, recently, FPR1 has been shown to be expressed by highly malignant glioblastoma (GBM) cells. Upon activation by an endogenous agonist Annexin 1 (Anx A1) released by necrotic glioma cells, FPR1 transactivates the receptor for epithelial growth factor (EGFR) and consequently to promote glioma cell chemotaxis, invasion, growth and production of angiogenic factors. The observations demonstrate that FPR1, as a multifunctional GPCR with pattern recognition properties, is not only involved in innate immune responses but also in the progression of GBM. Thus, FPR1 is an immunopharmacologic target for development of novel therapies.

Synthetic immunomodulatory peptide IDR-1002 enhances monocyte migration and adhesion on fibronectin

Regulation of the immune system by immunomodulatory agents, such as the synthetic innate defense regulator (IDR) peptides, has been proposed as a potential strategy to strengthen host immune responses against infection. IDR peptides confer protection in vivo against a range of bacterial infections and have been developed as components of single-dose vaccine adjuvants due to their ability to modulate innate immunity, correlating with an increased recruitment of monocytes to sites of infection or immunization. However, the mechanisms by which IDR peptides augment monocyte recruitment remain poorly defined. Anti-infective peptide IDR-1002 was demonstrated here to lack direct monocyte chemoattractive activity yet enhance, by up to 5-fold, the ability of human monocytes to migrate on fibronectin towards chemokines. This effect correlated with an increased adhesion of monocytes and THP-1 cells to fibronectin by IDR-1002 and other IDR peptides and the adhesion of THP-1 cells to fibronectin occurred in a β(1)-integrin-dependent manner, corresponding with an increased activation of β(1)-integrins and the phosphoinositide 3-kinase (PI3K)-Akt pathway. PI3K- and Akt-specific inhibitors abrogated IDR-1002-induced adhesion and activation of β(1)-integrins, whereas p38 and MEK1 inhibitors did not affect, or moderately inhibited, adhesion, respectively. Furthermore, IDR-1002 enhancement of monocyte migration towards chemokines and activation of β(1)-integrins was abrogated in the presence of PI3K- and Akt-specific inhibitors. In summary, IDR-1002 enhanced monocyte migration on fibronectin through promotion of β(1)-integrin-mediated interactions regulated by the PI3K-Akt pathway, revealing a mechanism by which IDR-1002 promotes monocyte recruitment.

Synergistic activation of NF-{kappa}B by bacterial chemoattractant and TNF{alpha} is mediated by p38 MAPK-dependent RelA acetylation

In the host immune system, leukocytes are often exposed to multiple inflammation inducers. NF-κB is of considerable importance in leukocyte function because of its ability to activate the transcription of many proinflammatory immediate-early genes. Tremendous efforts have been made toward understanding how NF-κB is activated by various inducers. However, most research on NF-κB regulation has been focused on understanding how NF-κB is activated by a single inducer. This is unlike the situation in the human immune system where multiple inflammation inducers, including both exogenous and endogenous mediators, are present concurrently. We now present evidence that the formylated peptide f-Met-Leu-Phe (fMLP), a bacterial chemoattractant, synergizes with TNFα to induce NF-κB activation and the resultant inflammatory response in vitro and in vivo. The mechanism of synergistic activation of NF-κB by bacterial fMLP and TNFα may be involved in the induction of RelA acetylation, which is regulated by p38 MAPK. Thus, this study provides direct evidence for the synergistic induction of NF-κB-dependent inflammatory responses by both exogenous and endogenous inducers. The ability of fMLP to synergize with TNFα and activate NF-κB represents a novel and potentially important mechanism through which bacterial fMLP not only attracts leukocytes but also directly contributes to inflammation by synergizing with the endogenous mediator TNFα.

Synergistic induction of inflammation by bacterial products lipopolysaccharide and fMLP: an important microbial pathogenic mechanism

A wide variety of stimuli have been shown to induce inflammation, but bacteria products/components are considered the major inducers during bacterial infections. We previously demonstrated that bacterial products/components such as LPS, a glycolipid component of the bacterial outer membrane, and formylated peptides (fMLP), a bacterial-derived peptide, induced proinflammatory cytokine gene expression in human peripheral blood monocytes. We now present evidence that mixtures of bacterial products/components LPS and fMLP behave synergistically in the induction of inflammation in vitro and in vivo. Furthermore, our results indicate that the TLR4 and the IKKbeta-IkappaBalpha signaling pathways are involved in the synergistic induction of inflammatory cytokines. The mechanism of synergistic activation of NF-kappaB is depended on nuclear translocation of p65 and phosphorylation of p65 at both Ser536 and Ser276 sites. These results demonstrate an important role for bacterial products/components from lysed bacteria in the pathogenesis of infectious diseases. We believe that this synergistic induction of inflammation by bacterial products LPS and fMLP represents an important pathogenic mechanism during bacterial infection, which may suggest novel therapeutic strategies or targets to minimize host injury following bacterial infection.

Receptor "hijacking" by malignant glioma cells: a tactic for tumor progression

Gliomas are the most common and deadly tumors in the central nervous system (CNS). In the course of studying the role of chemoattractant receptors in tumor growth and metastasis, we discovered that highly malignant human glioblastoma and anaplastic astrocytoma specimens were stained positively for the formylpeptide receptor (FPR), which is normally expressed in myeloid cells and accounts for their chemotaxis and activation induced by bacterial peptides. Screening of human glioma cell lines revealed that FPR was expressed selectively in glioma cell lines with a more highly malignant phenotype. FPR expressed in glioblastoma cell lines mediates cell chemotaxis, proliferation and production of an angiogenic factor, vascular endothelial growth factor (VEGF), in response to agonists released by necrotic tumor cells. Furthermore, FPR in glioblastoma cells activates the receptor for epidermal growth factor (EGFR) by increasing the phosphorylation of a selected tyrosine residue in the intracellular tail of EGFR. Thus, FPR hijacked by human glioblastoma cells exploits the function of EGFR to promote rapid tumor progression.

Emodin-induced generation of reactive oxygen species inhibits RhoA activation to sensitize gastric carcinoma cells to anoikis

RhoA is a critical signaling molecule regulating a variety of cellular processes, such as cytoskeletal organization, adhesion, and apoptosis. It is recently considered responsive to reactive oxygen species (ROS). Nevertheless, how RhoA regulates anoikis, a detachment-initiated apoptosis, and how this regulation is affected by ROS are not clear. The present study investigated the role of RhoA in apoptosis/anoikis in gastric cancer cells and the changes of RhoA and anoikis under oxidative stress. Immunohistochemistry showed that RhoA expression was upregulated in the primary gastric carcinoma compared with normal gastric mucosa. Overactivation of RhoA by transfection with the V14RhoA mutant prevented gastric cancer line SGC-7901 cells from arsenic-induced apoptosis and conferred anoikis resistance through, at least in part, promoting formations of F-actin fibers and focal adhesion. Oxidative stress caused by emodin, an ROS producer, in combination with arsenic trioxide (ATO) led to RhoA inactivation that triggered structural disruption of focal adhesion complex and eventually resulted in anoikis, and these effects could be partially reversed by antioxidant N-acetylcysteine (NAC). In conclusion, activation of RhoA is required for the maintenance of anoikis resistance phenotype of gastric cancer cells, and oxidative stress might be a therapeutic strategy for the inhibition of RhoA in cancer cells.

Regulator of G-protein signalling 3 redirects prototypical Gi-coupled receptors from Rac1 to RhoA activation

The small GTPases, Rac1 and RhoA, are pivotal regulators of several essential, but distinct cellular processes. Numerous G-protein-coupled receptors signal to these GTPases, but with different specificities. Specifically, Gi-coupled receptors (GiPCRs) are generally believed to activate Rac1, but not RhoA, a process involving Gbetagamma-dimers and phosphatidylinositol 3-kinase (PI3K). Here we show that, depending on the expression level of the 519 amino acid isoform of regulator of G-protein signalling 3 (RGS3L), prototypical GiPCRs, like M2 muscarinic, A1 adenosine, and alpha2-adrenergic receptors, activate either Rac1 or RhoA in human embryonic kidney cells and neonatal rat cardiomyocyte-derived H10 cells. The switch from Rac1 to RhoA activation in H10 cells was controlled by fibroblast growth factor-2 (FGF-2), lowering the expression of RGS3L. Activation of both, Rac1 and RhoA, seen at low and high expression levels of RGS3L, respectively, was sensitive to pertussis toxin and the PI3K inhibitor LY294002 and mediated by Gbetagamma-dimers. We conclude that RGS3L functions as a molecular switch, redirecting GiPCRs via Gbetagamma-dimers and PI3K from Rac1 to RhoA activation. Considering the essential roles of Rac1 and RhoA in many signalling pathways, this additional function of RGS3L indicates a specific role of this protein in cellular signalling networks.

Biological role of the N-formyl peptide receptors

Ligation of N-formyl-methionyl-leucyl-phenylalanine (fMLP) to its specific cell surface receptors triggers different cascades of biochemical events, eventually leading to cellular activation. The formyl peptide receptors (FPRs) are members of the seven-transmembrane, G-protein coupled receptors superfamily, expressed at high levels on polymorphonuclear and mononuclear phagocytes. The main responses elicited upon ligation of formylated peptides, referred to as cellular activation, are those of morphological polarization, locomotion, production of reactive-oxygen species and release of proteolytic enzymes. FPRs have in recent years been shown to be expressed also in several non myelocytic populations, suggesting other unidentified functions for this receptor family, independent of the inflammatory response. Finally, a number of ligands acting as exogenous or host-derived agonists for FPRs, as well as ligands acting as FPRs antagonists, have been described, indicating that these receptors may be differentially modulated by distinct molecules.

Statins revert doxorubicin resistance via nitric oxide in malignant mesothelioma

Human malignant mesothelioma (HMM) is resistant to many anticancer drugs, including doxorubicin. Mevastatin and simvastatin, 2 inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase, potentiated the intracellular accumulation and the cytotoxicity of doxorubicin in HMM cells constitutively expressing P-glycoprotein and multidrug resistance-associated protein 3. This effect of statins was nitric oxide (NO)-dependent, since it was reverted by either an NO synthase inhibitor or an NO scavenging system. The NO synthase up-regulation in HMM and other cells is known to be associated with the activation of the transcription factor NF-kappaB: in HMM cells statins increased the NF-kappaB translocation into the nucleus, decreased the level of the NF-kappaB inhibitor IkBalpha and increased the phosphorylation/activation of IkB kinase alpha (IKKalpha). IKKalpha is under the negative control exerted by RhoA in its prenylated (active) form: incubation of HMM cells with statins lowered the amount of active RhoA and the level of Rho-associated kinase activity. All statins' effects were reverted by mevalonic acid, thus suggesting that they were mediated by the inhibition of HMGCoA reductase and were likely to be subsequent to the reduced availability of precursor molecules for RhoA prenylation. Both the Rho kinase inhibitor Y27632 and the RhoA inhibitor toxin B (from Clostridium difficile) mimicked the statins' effects, enhancing doxorubicin accumulation, NO synthesis and IKKalpha phosphorylation and decreasing the amount of IkBalpha in HMM cells. Simvastatin, Y27632 and toxin B elicited tyrosine nitration in the P-glycoprotein, thus providing a likely mechanism by which NO reverts the doxorubicin resistance in HMM cells.

Perturbation of actin dynamics induces NF-kappaB activation in myelomonocytic cells through an NADPH oxidase-dependent pathway

Although several reports showed the effect of compounds disrupting microtubules on NF-kappaB (nuclear factor kappaB) activation, nothing is known about agents perturbing actin dynamics. In the present study, we have shown that actin cytoskeleton disruption induced by actin-depolymerizing agents such as cytochalasin D and latrunculin B and actin-polymerizing compounds such as jasplakinolide induced NF-kappaB activation in myelomonocytic cells. The transduction pathway involved the IkappaB (inhibitory kappaB) kinase complex and a degradation of IkappaBalpha. We have shown that NF-kappaB activation in response to the perturbation of actin dynamics required reactive oxygen species, as demonstrated by the effect of antioxidants. Actin cytoskeleton disruption by cytochalasin D induced O2- release from human monocytes, through the activation of the NADPH oxidase, as confirmed by the phosphorylation and by the membrane translocation of p47phox. NF-kappaB activation after actin cytoskeleton disruption could be physiologically relevant during monocyte activation and/or recruitment into injured tissues, where cellular attachment, migration and phagocytosis result in cyclic shifts in cytoskeletal organization and disorganization.

Bacterial cytotoxins: targeting eukaryotic switches

Many bacterial cytotoxins act on eukaryotic cells by targeting the regulators that are involved in controlling the cytoskeleton or by directly modifying actin, with members of the Rho GTPase family being particularly important targets. The actin cytoskeleton, and especially the GTPase 'molecular switches' that are involved in its control, have crucial functions in innate and adaptive immunity, and have pivotal roles in the biology of infection. In this review, we briefly discuss the role of the actin cytoskeleton and the Rho GTPases in host-pathogen interactions, and review the mode of actions of bacterial protein toxins that target these components.

A novel protein kinase C (PKCepsilon) is required for fMet-Leu-Phe-induced activation of NF-kappaB in human peripheral blood monocytes

We have reported that the chemoattractant, fMet-Leu-Phe (fMLP), induces the activation of NF-kappaB in human peripheral blood monocytes and that this requires the activity of small GTPase, RhoA (Huang, S., Chen, L.-Y., Zuraw, B. L., Ye, R. D., and Pan, Z. K. (2001) J. Biol. Chem. 276, 40977-40981). Here we showed that the novel protein kinase C isozyme, PKCepsilon, associates functionally with RhoA in fMLP-stimulated monocytes and that PKCepsilon acted as a signaling component downstream of the GTPase RhoA during fMLP-induced activation of NF-kappaB. Stimulation of monocytes with fMLP resulted in activation of both PKCepsilon and NF-kappaB. This latter activation was largely blocked by specific inhibitors of PKCepsilon by transient expression of a dominant-negative form of PKCepsilon and by PKCepsilon-specific short interfering RNA. These findings demonstrate, for the first time, that fMLP-induced activation of NF-kappaB utilizes a signaling pathway, which requires activity of PKCepsilon, and that PKCepsilon acts as a signaling component downstream of RhoA in cytokine gene transcription stimulated by a chemoattractant. The specificity of this response suggests an important role for the Rho GTPase-PKCepsilon-NF-kappaB pathway in host defense and represents a novel and potentially important mechanism through which fMLP not only attracts leukocytes but may also contribute directly to inflammation.

Two different pathways link G-protein-coupled receptors with tyrosine kinases for the modulation of growth and survival in human hematopoietic progenitor cells

The G-protein-coupled receptor agonists CXCL12 (SDF-1, a chemokine) and thrombin showed opposite effects on growth and survival of multipotent and erythroid human hematopoietic progenitor cells. CXCL12 promoted growth in multipotent cells by activating the RhoA-Rho kinase pathway. Its effect was largely blocked by Y-27632, a specific inhibitor of Rho kinase, and by clostridial toxin B, a specific inhibitor of Rho family proteins. Rho activation required a G(i)-mediated stimulation of tyrosine kinases, which was blocked by PP2 and tyrphostin AG 490, inhibitors of Src and Jak type kinases, respectively. By contrast, in erythroid cells, inhibitors of Src family and c-Abl tyrosine kinases (tyrphostin AG 82, PP2, imatinib) enhanced protein kinase C (PKC)-dependent cell growth and antagonized thrombin-promoted apoptosis by specifically stimulating PKCbeta activity. The PKC activating phorbol ester PMA (a growth factor in erythroid cells) induced the activation of Lyn and c-Abl tyrosine kinases, thus establishing a feedback inhibition of PKCbeta. Hence, developmental stage-specific crosstalk between PKC subtypes and tyrosine kinases appear to determine whether growth and survival of hematopoietic cells are promoted or inhibited by G-protein-coupled receptor agonists.

Interleukin-8 induces nuclear transcription factor-kappaB through a TRAF6-dependent pathway

Considering the potential role of interleukin-8 (IL-8) in inflammation, angiogenesis, tumorigenesis, and metastasis, we investigated the molecular mechanism involved in IL-8-mediated signaling. In this report we provide evidence that like TNF, an inducer of NF-kappaB and also a NF-kappaB-dependent gene product, IL-8 induces NF-kappaB in a unique pathway. IL-8 induces NF-kappaB activation in a dose-dependent manner in different cell types as detected by a DNA-protein binding assay. IL-8 induces NF-kappaB-dependent reporter gene expression as well as ICAM-1, VCAM-1, and Cox-2 expression. IL-8 also induces IkappaBalpha phosphorylation followed by degradation and p65 translocation. IL-8 induces c-Jun N-terminal kinase (JNK) and mitogen-activated protein kinase (MAPK) in a dose- and time-dependent manner. IL-8-induced NF-kappaB activation is for the most part unaltered when cells are transfected with dominant-negative TRADD, FADD, or TRAF2, but is inhibited with dominant-negative TRAF6-, NIK-, IKK-, or IkappaBalpha-transfected cells. The data suggest that IL-8-induced NF-kappaB activation proceeds through a TRAF2-independent but TRAF6-dependent pathway, followed by recruitment of IRAK and activation of IKK. IL-8-induced NF-kappaB activation is not observed in a cell-permeable peptide that has TRAF6 binding motif-treated cells or IRAK-deficient cells. IL-8-induced NF-kappaB activation proceeds mostly through interaction with TRAF6 and partially through the Rho-GTPase pathways. This is the first report that IL-8 induces NF-kappaB in a distinct pathway, and activation of NF-kappaB and its dependent genes may be one of the pathways of IL-8-induced inflammation and angiogenesis.

Signaling mechanisms responsible for lysophosphatidic acid-induced urokinase plasminogen activator expression in ovarian cancer cells

Lysophosphatidic acid (LPA) enhances urokinase plasminogen activator (uPA) expression in ovarian cancer cells; however, the molecular mechanisms responsible for this event have not been investigated. In this study, we used the invasive ovarian cancer SK-OV-3 cell line to explore the signaling molecules and pathways essential for LPA-induced uPA up-regulation. With the aid of specific inhibitors and dominant negative forms of signaling molecules, we determined that the G(i)-associated pathway mediates this LPA-induced event. Moreover, constitutively active H-Ras and Raf-1-activating H-Ras mutant enhance uPA expression, whereas dominant negative H-Ras and Raf-1 block LPA-induced uPA up-regulation, suggesting that the Ras-Raf pathway works downstream of G(i) to mediate this LPA-induced process. Surprisingly, dominant negative MEK1 or Erk2 displays only marginal inhibitory effect on LPA-induced uPA up-regulation, suggesting that a signaling pathway distinct from Raf-MEK1/2-Erk is the prominent pathway responsible for this process. In this report, we demonstrate that LPA activates NF-kappaB in a Ras-Raf-dependent manner and that blocking NF-kappaB activation with either non-phosphorylable IkappaB or dominant negative IkappaB kinase abolished LPA-induced uPA up-regulation and uPA promoter activation. Furthermore, introducing mutations to knock out the NF-kappaB binding site of the uPA promoter results in over 80% reduction in LPA-induced uPA promoter activation, whereas this activity is largely intact with the promoter containing mutations in the AP1 binding sites. Thus these results suggest that the G(i)-Ras-Raf-NF-kappaB signaling cascade is responsible for LPA-induced uPA up-regulation in ovarian cancer cells.

Cardioprotective mechanisms of Rho-kinase inhibition associated with eNOS and oxidative stress-LOX-1 pathway in Dahl salt-sensitive hypertensive rats

OBJECTIVES

Rho-kinase plays a crucial role in various cellular functions. To elucidate molecular mechanisms of Rho-kinase-mediated cardiovascular remodeling in vivo, we evaluated whether a signaling pathway through Rho is involved, and whether Y-27632, a specific Rho-kinase inhibitor, stimulates endothelial nitric oxide synthase (eNOS) and suppresses the oxidative stress and lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) pathway in the left ventricle of Dahl salt-sensitive hypertensive (DS) rats.

METHODS

Y-27632 (3 mg/kg per day) or vehicle were given for 5 weeks, from age 6 weeks to a stage of left ventricular hypertrophy (11 weeks). Age-matched Dahl salt-resistant (DR) rats fed the same diet served as a control group.

RESULTS

Increased left ventricular weight in the hypertrophy stage was significantly ameliorated by Y-27632. Upregulated RhoA protein, Rho-kinase gene expression and myosin light-chain phosphorylation in the hypertrophy stage were suppressed by Y-27632. Increased expression of NAD(P)H oxidase p22phox, p47phox, gp91phox and LOX-1 in DS rats were inhibited by Y-27632. Upregulated protein kinase Cepsilon and p65 nuclear factor-kappaB phosphorylation in DS rats was reduced by Y-27632. In contrast, downregulated eNOS expression in hypertrophy stage was upregulated by Y-27632. Y-27632 effectively inhibited vascular lesion formation, such as medial thickness and perivascular fibrosis, and suppressed transforming growth factor-beta1, type I and III collagen, and fibronectin gene expression.

CONCLUSIONS

Inhibiting the Rho-kinase pathway may play a key role in the cardioprotective effect on cardiovascular remodeling associated with eNOS and the oxidative stress-LOX-1 pathway in DS rats, and may be at least a potential therapeutic strategy for hypertension with cardiac hypertrophy.

Lysophosphatidic acid promotes survival of androgen-insensitive prostate cancer PC3 cells via activation of NF-kappaB

BACKGROUND

Dysregulated cell survival contributes to the poor efficacy of many chemotherapeutic regimens for patients with advanced prostate cancer. In this study we examined ability of the lipid growth factor lysophosphatidic acid (LPA), a G protein-coupled receptor (GPCR) ligand, to promote prostate cell survival.

METHODS

PC3 cells were used as a model to study mechanisms involved in survival of androgen-insensitive prostate cancer cells. Cell survival was measured by FACS analysis of cell cycle parameters after propidium iodide or annexin V and 7-AAD immunostaining. Activation state of nuclear facor-kappaB (NF-kappaB) was determined biochemically by nuclear translocation and transcriptional activation. Human tissue was analyzed for nuclear expression of NF-kappaB by immunohistochemistry.

RESULTS

Molecular dissection of the LPA-regulated PC3 cell survival revealed the sequential phosphorylation of Akt, IkappaB, and transcriptional activation of NF-kappaB. Both Akt and NF-kappaB were required to escape serum deprivation-induced cell death since their inhibition abrogated the LPA-mediated PC3 cell survival. Data from archival human tissue show that NF-kappaB is constitutively activated in prostate cancers, but not in benign prostate tissues.

CONCLUSIONS

Targeted disruption of the LPA receptor-Akt-NF-kappaB signaling axis may be effective for the treatment of androgen-insensitive prostate cancer.

RhoA and Rac1 signals in fMLP-induced NF-kappaB activation in human blood monocytes

GTPase RhoA is required for fMet-Leu-Phe (fMLP)-stimulated NF-kappaB activation in human peripheral blood monocytes. Here we have investigated different members of the Rho family of GTPases Rac1, Cdc42, and RhoA in regulating the transcription factor nuclear factor-kappaB (NF-kappaB) in human peripheral blood monocytes. Stimulation of monocytes with fMLP rapidly activated Rac1, Cdc42, and RhoA and cotransfection of the monocytic THP1 cells with dominant negative forms of Rho GTPases, we found that Rac1 and RhoA, but not Cdc42, involved fMLP-stimulated kappaB reporter gene expression. These results indicate that fMLP stimulates three members of the Rho family of GTPases Rac1, Cdc42, and RhoA activity in monocytes, and that Rac1 and RhoA, but not Cdc42, is required for fMLP-induced NF-kappaB activation. Furthermore, our data also suggest that RhoA is mediated by signals independent of Rac1 in NF-kappaB activation in human peripheral blood monocytes stimulated with bacterial products.

A Rho exchange factor mediates fMet-Leu-Phe-induced NF-kappaB activation in human peripheral blood monocytes

We reported previously that fMLP stimulates NF-kappaB activation, and this function of fMLP requires small GTPase RhoA in human peripheral blood monocytes (Huang, S., Chen, L.-Y., Zuraw, B. L., Ye, R. D., and Pan, Z. K. (2001) J. Biol. Chem. 276, 40977-40981). Here we present evidence that RhoA associates specifically with the guanine nucleotide exchange factor Lbc in human peripheral blood monocytes stimulated with fMLP and that Lbc specifically catalyzes the guanine nucleotide exchange activity of RhoA in human peripheral blood monocytes. Cotransfection of the monocytic THP1 cells with lbc with a kappaB promoter reporter plasmid results in a marked increase in NF-kappaB-mediated reporter gene expression. Finally, Lbc-enhanced NF-kappaB activation is inhibited by a RhoA inhibitor, C3 transferase from Clostridium botulinum. A dominant-negative form of RhoA (T19N) also inhibited Lbc-enhanced reporter gene expression in a kappaB-dependent manner. These results indicate that guanine nucleotide exchange factor Lbc is a novel signal transducer for RhoA-mediated NF-kappaB activation in human peripheral blood monocytes stimulated with bacterial products.

Statin-mediated correction of STAT1 signaling and inducible nitric oxide synthase expression in cystic fibrosis epithelial cells

The expression of the inducible form of nitric oxide synthase (NOS2) is reduced in cystic fibrosis (CF) epithelium despite the presence of aggressive inflammation. A potential mechanism for reduced NOS2 expression in CF is diminished signal transducer and activator of transcription-1 (STAT1) activity, possibly due to an increase in expression of protein inhibitor of activated STAT1 (PIAS1). Previous evidence also suggests that NOS2 expression can be negatively regulated by increased activation of the GTPase RhoA, leading to the hypothesis that CF-related increases in PIAS1 expression and altered STAT1 signaling may be mediated by Rho GTPase function. Consistent with this hypothesis, data demonstrate increased expression of RhoA in two models of CF epithelium with a proportional increase in the active GTP-bound RhoA. Mouse embryonic fibroblasts null for p190B Rho GTPase-activating protein exhibit increased RhoA protein content and activation, similar to what is observed in CF models, and also exhibit CF-like alterations in STAT1 regulation, including decreased STAT1 activation, increased PIAS1 protein expression, and reduced NOS2 induction, implicating RhoA-mediated signaling in CF-related STAT1 alterations. Inhibition of the Rho GTPase pathway at the level of isoprenoid/cholesterol synthesis with mevastatin reduces PIAS1 expression, increases STAT1 activation, and restores NOS2 expression in models of CF epithelium, suggesting that pharmacological inhibition of the isoprenoid synthesis/Rho GTPase pathway may represent a potential avenue for therapeutic intervention for CF.

Constitutively active Gq/11-coupled receptors enable signaling by co-expressed G(i/o)-coupled receptors

Co-expression of guanine nucleotide-binding regulatory (G) protein-coupled receptors (GPCRs), such as the G(i/o)-coupled human 5-hydroxytryptamine receptor 1B (5-HT(1B)R), with the G(q/11)-coupled human histamine 1 receptor (H1R) results in an overall increase in agonist-independent signaling, which can be augmented by 5-HT(1B)R agonists and inhibited by a selective inverse 5-HT(1B)R agonist. Interestingly, inverse H1R agonists inhibit constitutively H1R-mediated as well as 5-HT(1B)R agonist-induced signaling in cells co-expressing both receptors. This phenomenon is not solely characteristic of 5-HT(1B)R; it is also evident with muscarinic M2 and adenosine A1 receptors and is mimicked by mastoparan-7, an activator of G(i/o) proteins, or by over-expression of Gbetagamma subunits. Likewise, expression of the G(q/11)-coupled human cytomegalovirus (HCMV)-encoded chemokine receptor US28 unmasks a functional coupling of G(i/o)-coupled CCR1 receptors that is mediated via the constitutive activity of receptor US28. Consequently, constitutively active G(q/11)-coupled receptors, such as the H1R and HCMV-encoded chemokine receptor US28, constitute a regulatory switch for signal transduction by G(i/o)-coupled receptors, which may have profound implications in understanding the role of both constitutive GPCR activity and GPCR cross-talk in physiology as well as in the observed pathophysiology upon HCMV infection.

Tie-2-dependent activation of RhoA and Rac1 participates in endothelial cell motility triggered by angiopoietin-1

Angiopoietin-1 is implicated in the maturation and remodeling of the vascular network during embryo development and in adult life. Through its tyrosine kinase receptor Tie-2 it stimulates endothelial cells to migrate and change shape. Here we show that angiopoietin-1 elicits chemokinesis of endothelial cells by a phosphoinositide 3-OH kinase/son of sevenless-dependent modulation of Rac1 and RhoA. The resulting temporal events are associated with cytoskeletal rearrangements and occur in discrete zones of the cell. Endothelial cells carrying dominant-negative mutants of RhoA and Rac1 or treated with LY294002, an inhibitor of phosphoinositide 3-OH kinase, dramatically decrease their chemokinetic velocity. Taken together, these results further expand our understanding of angiopoietin-1-mediated endothelial cell motility during vascular network assembly and angiogenesis.

Neurotensin stimulates IL-8 expression in human colonic epithelial cells through Rho GTPase-mediated NF-kappa B pathways

Neurotensin (NT), a neuropeptide highly expressed in the gastrointestinal tract, participates in the pathophysiology of intestinal inflammation. We recently showed that NT stimulates interleukin-8 (IL-8) expression in NCM460 nontransformed human colonic epithelial cells via both mitogen-activating protein kinase (MAPK)- and NF-kappaB-dependent pathways. However, the molecular mechanism by which NT induces expression of proinflammatory cytokines such as IL-8 has not been investigated. In this study we show that inhibition of endogenous Rho family proteins (RhoA, Rac1, and Cdc42) by their respective dominant negative mutants inhibits NT-induced IL-8 protein production and promoter activity. Western blot experiments demonstrated that NT strongly activated RhoA, Rac1, and Cdc42. Overexpression of the dominant negative mutants of RhoA, Rac1, and Cdc42 significantly inhibited NT-induced NF-kappaB-dependent reporter gene expression and NF-kappaB DNA binding activity. NT also stimulated p38 MAPK phosphorylation, and overexpression of dominant negative mutants of RhoA, Rac1, and Cdc42 did not significantly alter p38 and ERK1/2 phosphorylation in response to NT. Together, our findings indicate that NT-stimulated IL-8 expression is mediated via a Rho-dependent NF-kappaB-mediated pathway.

Distinct roles of receptor phosphorylation, G protein usage, and mitogen-activated protein kinase activation on platelet activating factor-induced leukotriene C(4) generation and chemokine production

Platelet activating factor (PAF) interacts with cell surface G protein-coupled receptors on leukocytes to induce degranulation, leukotriene C(4) (LTC(4)) generation, and chemokine CCL2 production. Using a basophilic leukemia RBL-2H3 cell line expressing wild-type PAF receptor (PAFR) and a phosphorylation-deficient mutant (mPAFR), we have previously demonstrated that receptor phosphorylation mediates desensitization of PAF-induced degranulation. Here, we sought to determine the role of receptor phosphorylation on PAF-induced LTC(4) generation and CCL2 production. We found that PAF caused a significantly enhanced LTC(4) generation in cells expressing mPAFR when compared with PAFR cells. In contrast, PAF-induced CCL2 production was greatly reduced in mPAFR cells. Pertussis toxin and U0126, which inhibit G(i) and p44/42 mitogen-activated protein kinase (ERK) activation, respectively, caused very little inhibition of PAF-induced CCL2 production (approximately 20% inhibition). In contrast, these inhibitors almost completely blocked both PAF-induced ERK phosphorylation and LTC(4) generation in PAFR cells. However, in mPAFR cells pertussis toxin only partially inhibited PAF-induced ERK phosphorylation. A Ca(2+)/calmodulin inhibitor had no effect on PAF-induced ERK phosphorylation in PAFR cells but completely blocked the response in mPAFR cells. These data demonstrate that receptor phosphorylation, which serves to desensitize PAF-induced LTC(4) generation, is required for chemokine CCL2 production. They also indicate a previously unrecognized selectivity in G protein usage and ERK activation for PAF-induced responses. Whereas PAF-induced CCL2 production is, in large part, mediated independently of G(i) activation or ERK phosphorylation, LTC(4) generation requires ERK phosphorylation, which is mediated by different G proteins depending on the phosphorylation status of the receptor.