Timing of cyclin D1 expression within G1 phase is controlled by Rho

Mechanisms of Rho GTPases in regulating tumor proliferation, migration and invasion

The occurrence of most cancers is due to the clonal proliferation of tumor cells, immune evasion, and the ability to spread to other body parts. Rho GTPases, a family of small GTPases, are key regulators of cytoskeleton reorganization and cell polarity. Additionally, Rho GTPases are key proteins that induce the proliferation and metastasis of tumor cells. This review focuses on the complex regulatory mechanisms of Rho GTPases, exploring their critical role in promoting tumor cell proliferation and dissemination. Regarding tumor cell proliferation, attention is given to the role of Rho GTPases in regulating the cell cycle and mitosis. In terms of tumor cell dissemination, the focus is on the role of Rho GTPases in regulating cell migration and invasion. Overall, this review elucidates the mechanisms of Rho GTPases members in the development of tumor cells, aiming to provide theoretical references for the treatment of mammalian tumor diseases and related applications.

Immobilizing c(RGDfc) on the surface of metal-phenolic networks by thiol-click reaction for accelerating osteointegration of implant

The limited osteointegration often leads to the failure of implant, which can be improved by fixing bioactive molecules onto the surface, such as arginyl-glycyl-aspartic acid (RGD): a cell adhesion motif. Metal-Phenolic Networks (MPNs) have garnered increasing attention from different disciplines in recent years due to their simple and rapid process for depositing on various substrates or particles with different shapes. However, the lack of cellular binding sites on MPNs greatly blocks its application in tissue engineering. In this study, we present a facile and efficient approach for producing PC/Fe@c(RGDfc) composite coatings through the conjugation of c(RGDfc) peptides onto the surface of PC/Fe-MPNs utilizing thiol-click reaction. By combined various techniques (ellipsometry, X-ray photoelectron spectroscopy, Liquid Chromatography-Mass Spectrometry, water contact angle, scanning electronic microscopy, atomic force microscopy) the physicochemical properties (composition, coating mechanism and process, modulus and hydrophilicity) of PC/Fe@c(RGDfc) surface were characterized in detail. In addition, the PC/Fe@c(RGDfc) coating exhibits the remarkable ability to positively modulate cellular attachment, proliferation, migration and promoted bone-implant integration in vivo, maintaining the inherent features of MPNs: anti-inflammatory, anti-oxidative properties, as well as multiple substrate deposition. This work contributes to engineering MPNs-based coatings with bioactive molecules by a facile and efficient thiol-click reaction, as an innovative perspective for future development of surface modification of implant materials.

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Insights into the aberrant CDK4/6 signaling pathway as a therapeutic target in tumorigenesis

The recent findings advance our knowledge for the prevention of the premature activation of the major oncogenic pathways including MYC and the cyclin D-cyclin-dependent kinases 4 and 6 (CDK4/6) axis. D-type cyclins are frequently deregulated in human cancer and promote cell division in part through activation of CDK4/6. Therefore, the activation of the cyclin D-CDK4/6 axis stimulates cell proliferation and cancer progression, which represents a unique therapeutic target. However, we have shown that inhibition of CDK4/6 upregulates protein levels of RB1 and CDK6 for acquisition of drug resistance to CDK4/6 inhibitors. Here, we review new progress in the control of cyclin D-dependent cancer cell cycle and proliferation, along with identification of novel E3 ligase for the stability of cyclin D. Cullin4-RING E3 ligase (CRL4)AMBRA1 complex plays a critical role in regulating D-type cyclins through their protein destabilization to control S phase entry and maintain genomic integrity. We also summarize the strategy for inhibition of the cyclin D-associated kinases CDK4/6 and other potential cell cycle regulators for targeting cancer with altered cyclin D expression. We also uncover the function of CK1ɛ as an effective target to potentiate therapeutic efficacy of CDK4/6 inhibitors. Moreover, as the level of PD-L1 is considered in the severe clinical problem in the patients treated with CDK4 inhibitors, we assume that a therapeutic combination using PD-L1 immunotherapy might lower the development of drug resistance and targeting cyclin D will likely inhibit tumor growth and overcome resistance to cyclin D-associated CDK4/6 inhibitors.

WNT5A-RHOA Signaling Is a Driver of Tumorigenesis and Represents a Therapeutically Actionable Vulnerability in Small Cell Lung Cancer

WNT signaling represents an attractive target for cancer therapy due to its widespread oncogenic role. However, the molecular players involved in WNT signaling and the impact of their perturbation remain unknown for numerous recalcitrant cancers. Here, we characterize WNT pathway activity in small cell lung cancer (SCLC) and determine the functional role of WNT signaling using genetically engineered mouse models. β-Catenin, a master mediator of canonical WNT signaling, was dispensable for SCLC development, and its transcriptional program was largely silenced during tumor development. Conversely, WNT5A, a ligand for β-catenin-independent noncanonical WNT pathways, promoted neoplastic transformation and SCLC cell proliferation, whereas WNT5A deficiency inhibited SCLC development. Loss of p130 in SCLC cells induced expression of WNT5A, which selectively increased Rhoa transcription and activated RHOA protein to drive SCLC. Rhoa knockout suppressed SCLC development in vivo, and chemical perturbation of RHOA selectively inhibited SCLC cell proliferation. These findings suggest a novel requirement for the WNT5A-RHOA axis in SCLC, providing critical insights for the development of novel therapeutic strategies for this recalcitrant cancer. This study also sheds light on the heterogeneity of WNT signaling in cancer and the molecular determinants of its cell-type specificity.

SIGNIFICANCE

The p130-WNT5A-RHOA pathway drives SCLC progression and is a potential target for the development of therapeutic interventions and biomarkers to improve patient treatment.

Wound Healing from an Actin Cytoskeletal Perspective

Wound healing requires a complex cascade of highly controlled and conserved cellular and molecular processes. These involve numerous cell types and extracellular matrix molecules regulated by the actin cytoskeleton. This microscopic network of filaments is present within the cytoplasm of all cells and provides the shape and mechanical support required for cell movement and proliferation. Here, an overview of the processes of wound healing are described from the perspective of the cell in relation to the actin cytoskeleton. Key points of discussion include the role of actin, its binding proteins, signaling pathways, and events that play significant roles in the phases of wound healing. The identification of cytoskeletal targets that can be used to manipulate and improve wound healing is included as an emerging area of focus that may inform future therapeutic approaches to improve healing of complex wounds.

Clostridioides difficile toxins: mechanisms of action and antitoxin therapeutics

Clostridioides difficile is a Gram-positive anaerobe that can cause a spectrum of disorders that range in severity from mild diarrhoea to fulminant colitis and/or death. The bacterium produces up to three toxins, which are considered the major virulence factors in C. difficile infection. These toxins promote inflammation, tissue damage and diarrhoea. In this Review, we highlight recent biochemical and structural advances in our understanding of the mechanisms that govern host-toxin interactions. Understanding how C. difficile toxins affect the host forms a foundation for developing novel strategies for treatment and prevention of C. difficile infection.

Epithelial Mesenchymal Transition (EMT) and Associated Invasive Adhesions in Solid and Haematological Tumours

In solid tumours, cancer cells that undergo epithelial mesenchymal transition (EMT) express characteristic gene expression signatures that promote invasive migration as well as the development of stemness, immunosuppression and drug/radiotherapy resistance, contributing to the formation of currently untreatable metastatic tumours. The cancer traits associated with EMT can be controlled by the signalling nodes at characteristic adhesion sites (focal contacts, invadopodia and microtentacles) where the regulation of cell migration, cell cycle progression and pro-survival signalling converge. In haematological tumours, ample evidence accumulated during the last decade indicates that the development of an EMT-like phenotype is indicative of poor disease prognosis. However, this EMT phenotype has not been directly linked to the assembly of specific forms of adhesions. In the current review we discuss the role of EMT in haematological malignancies and examine its possible link with the progression towards more invasive and aggressive forms of these tumours. We also review the known types of adhesions formed by haematological malignancies and speculate on their possible connection with the EMT phenotype. We postulate that understanding the architecture and regulation of EMT-related adhesions will lead to the discovery of new therapeutic interventions to overcome disease progression and resistance to therapies.

Rho GTPases: Non-canonical regulation by cysteine oxidation

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

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.

RhoGTPase Signalling in Cancer Progression and Dissemination

Rho GTPases are a family of small G proteins that regulate a wide array of cellular processes related to their key roles controlling the cytoskeleton. On the other hand, cancer is a multi-step disease caused by the accumulation of genetic mutations and epigenetic alterations, from the initial stages of cancer development when cells in normal tissues undergo transformation, to the acquisition of invasive and metastatic traits, responsible for a large number of cancer related deaths. In this review, we discuss the role of Rho GTPase signalling in cancer in every step of disease progression. Rho GTPases contribute to tumour initiation and progression, by regulating proliferation and apoptosis, but also metabolism, senescence and cell stemness. Rho GTPases play a major role in cell migration, and in the metastatic process. They are also involved in interactions with the tumour microenvironment and regulate inflammation, contributing to cancer progression. After years of intensive research, we highlight the importance of relevant models in the Rho GTPase field, and we reflect on the therapeutic opportunities arising for cancer patients.

Targeting the actin/tropomyosin cytoskeleton in epithelial ovarian cancer reveals multiple mechanisms of synergy with anti-microtubule agents

BACKGROUND

Anti-microtubule agents are widely used to treat ovarian cancers, but the efficacy is often compromised by drug resistance. We investigated co-targeting the actin/tropomyosin cytoskeleton and microtubules to increase treatment efficacy in ovarian cancers and potentially overcome resistance.

METHODS

The presence of tropomyosin-3.1 (Tpm3.1) was examined in clinical specimens from ovarian cancer patients using immunohistochemistry. Combinatorial effects of an anti-Tpm3.1 compound, ATM-3507, with vinorelbine and paclitaxel were evaluated in ovarian cancer cells via MTS and apoptosis assays. The mechanisms of action were established using live- and fixed-cell imaging and protein analysis.

RESULTS

Tpm3.1 is overexpressed in 97% of tumour tissues (558 of 577) representing all histotypes of epithelial ovarian cancer. ATM-3507 displayed synergy with both anti-microtubule agents to reduce cell viability. Only vinorelbine synergised with ATM-3507 in causing apoptosis. ATM-3507 significantly prolonged vinorelbine-induced mitotic arrest with elevated activity of the spindle assembly checkpoint and mitotic cell death; however, ATM-3507 showed minor impact on paclitaxel-induced mitotic defects. Both combinations substantially increased post-mitotic G1 arrest with cyclin D1 and E1 downregulation and an increase of p21^Cip and p27^Kip.

CONCLUSION

Combined targeting of Tpm3.1/actin and microtubules is a promising treatment strategy for ovarian cancer that should be further tested in clinical settings.

Rho Family GTPases and Rho GEFs in Glucose Homeostasis

Dysregulation of glucose homeostasis leading to metabolic syndrome and type 2 diabetes is the cause of an increasing world health crisis. New intriguing roles have emerged for Rho family GTPases and their Rho guanine nucleotide exchange factor (GEF) activators in the regulation of glucose homeostasis. This review summates the current knowledge, focusing in particular on the roles of Rho GEFs in the processes of glucose-stimulated insulin secretion by pancreatic β cells and insulin-stimulated glucose uptake into skeletal muscle and adipose tissues. We discuss the ten Rho GEFs that are known so far to regulate glucose homeostasis, nine of which are in mammals, and one is in yeast. Among the mammalian Rho GEFs, P-Rex1, Vav2, Vav3, Tiam1, Kalirin and Plekhg4 were shown to mediate the insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane and/or insulin-stimulated glucose uptake in skeletal muscle or adipose tissue. The Rho GEFs P-Rex1, Vav2, Tiam1 and β-PIX were found to control the glucose-stimulated release of insulin by pancreatic β cells. In vivo studies demonstrated the involvement of the Rho GEFs P-Rex2, Vav2, Vav3 and PDZ-RhoGEF in glucose tolerance and/or insulin sensitivity, with deletion of these GEFs either contributing to the development of metabolic syndrome or protecting from it. This research is in its infancy. Considering that over 80 Rho GEFs exist, it is likely that future research will identify more roles for Rho GEFs in glucose homeostasis.

Clostridioides difficile Toxin A-Induced Wnt/β-Catenin Pathway Inhibition Is Mediated by Rac1 Glucosylation

Clostridioides difficile toxin A (TcdA) has been shown to inhibit cellular Wnt signaling, the major driving force behind the proliferation of epithelial cells in colonic crypts, likely through the inhibition of β-catenin nuclear translocation. Herein, we aimed to advance the understanding of this mechanism by replicating the findings in vivo and by investigating the specific role of Rac1, a member of the Rho GTPase family, on the inhibition of the Wnt-induced β-catenin nuclear translocation triggered by TcdA. To investigate the effects of TcdA on the Wnt/β-catenin pathway in vivo, we injected the ileal loops of C57BL/6 mice with TcdA [phosphate-buffered saline (PBS) as the control] to induce C. difficile disease-like ileitis. After 4 h post-injection, we obtained ileum tissue samples to assess Wnt signaling activation and cell proliferation through Western blotting, immunohistochemistry, and qPCR. To assess the role of Rac1 on Wnt signaling inhibition by TcdA, we transfected rat intestinal epithelial cells (IEC-6) with either a constitutively active Rac1 plasmid (pcDNA3-EGFP-Rac1-Q61L) or an empty vector, which served as the control. We incubated these cells with Wnt3a-conditioned medium (Wnt3a-CM) to induce Wnt/β-catenin pathway activation, and then challenged the cells with TcdA. We assessed Wnt signaling activation in vitro with TOP/FOPflash luciferase assays, determined nuclear β-catenin translocation by immunofluorescence, measured cyclin D1 protein expression by Western blotting, and quantified cell proliferation by Ki67 immunostaining. In vivo, TcdA decreased β-catenin, cyclin D1, and cMYC expression and inhibited the translocation of β-catenin into the nucleus in the ileum epithelial cells. In addition, TcdA suppressed cell proliferation and increased Wnt3a expression, but did not alter Rac1 gene expression in the ileum tissue. In vitro, constitutively active Rac1 prevented Wnt signaling inhibition by enabling the β-catenin nuclear translocation that had been blocked by TcdA. Our results show that TcdA inhibits Wnt/β-catenin pathway in vivo and demonstrate that this inhibition is likely caused by a Rac1-mediated mechanism.

Cell Cycle Deficits in Neurodegenerative Disorders: Uncovering Molecular Mechanisms to Drive Innovative Therapeutic Development

Cell cycle dysregulation has been implicated in the pathogenesis of neurodegenerative disorders. Specialised function obligates neuronal cells to subsist in a quiescent state of cell cycle once differentiated and therefore the circumstances and mechanisms underlying aberrant cell cycle activation in post-mitotic neurons in physiological and disease conditions remains an intriguing area of research. There is a strict requirement of concurrence to cell cycle regulation for neurons to ensure intracellular biochemical conformity as well as interrelationship with other cells within neural tissues. This review deliberates on various mechanisms underlying cell cycle regulation in neuronal cells and underscores potential implications of their non-compliance in neural pathology. Recent research suggests that successful duplication of genetic material without subsequent induction of mitosis induces inherent molecular flaws that eventually assert as apoptotic changes. The consequences of anomalous cell cycle activation and subsequent apoptosis are demonstrated by the increased presence of molecular stress response and apoptotic markers. This review delineates cell cycle events under normal physiological conditions and deficits amalgamated by alterations in protein levels and signalling pathways associated with cell-division are analysed. Cell cycle regulators essentially, cyclins, CDKs, cip/kip family of inhibitors, caspases, bax and p53 have been identified to be involved in impaired cell cycle regulation and associated with neural pathology. The pharmacological modulators of cell cycle that are shown to impart protection in various animal models of neurological deficits are summarised. Greater understanding of the molecular mechanisms that are indispensable to cell cycle regulation in neurons in health and disease conditions will facilitate targeted drug development for neuroprotection.

Feeling Things Out: Bidirectional Signaling of the Cell-ECM Interface, Implications in the Mechanobiology of Cell Spreading, Migration, Proliferation, and Differentiation

Biophysical cues stemming from the extracellular environment are rapidly transduced into discernible chemical messages (mechanotransduction) that direct cellular activities-placing the extracellular matrix (ECM) as a potent regulator of cell behavior. Dynamic reciprocity between the cell and its associated matrix is essential to the maintenance of tissue homeostasis and dysregulation of both ECM mechanical signaling, via pathological ECM turnover, and internal mechanotransduction pathways contribute to disease progression. This review covers the current understandings of the key modes of signaling used by both the cell and ECM to coregulate one another. By taking an outside-in approach, the inherent complexities and regulatory processes at each level of signaling (ECM, plasma membrane, focal adhesion, and cytoplasm) are captured to give a comprehensive picture of the internal and external mechanoregulatory environment. Specific emphasis is placed on the focal adhesion complex which acts as a central hub of mechanical signaling, regulating cell spreading, migration, proliferation, and differentiation. In addition, a wealth of available knowledge on mechanotransduction is curated to generate an integrated signaling network encompassing the central components of the focal adhesion, cytoplasm and nucleus that act in concert to promote durotaxis, proliferation, and differentiation in a stiffness-dependent manner.

Dermatopontin inhibits papillary thyroid cancer cell proliferation through MYC repression

Dermatopontin (DPT), a noncollagenous extracellular matrix component, has been illustrated to regulate cellular proliferation and invasiveness in several types of neoplasms. Nevertheless, the biological functions of DPT in cell proliferation, especially papillary thyroid cancer (PTC) cell proliferation, remain unknown, as do the mechanisms underlying its effects. In this study, we detected low DPT expression in PTC, which was related to higher T classifications. Ectopic DPT expression impeded cell proliferation both in vitro and in vivo. Furthermore, we illustrated that DPT down-regulated MYC, which in turn targeted CDK4, CDK6 and p21, through the ERK pathway. These results suggest that DPT regulates CDK4, CDK6 and p21, through MEK-ERK-MYC signaling to repress PTC proliferation.

Porf-2 = Arhgap39 = Vilse: A Pivotal Role in Neurodevelopment, Learning and Memory

Small GTP-converting enzymes, GTPases, are essential for the efficient completion of many physiological and developmental processes. They are regulated by GTPase activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). Arhgap39, also known as preoptic regulatory factor-2 (Porf-2) or Vilse, a member of the Rho GAP group, was first identified in 1990 in the rat CNS. It has since been shown to regulate apoptosis, cell migration, neurogenesis, and cerebral and hippocampal dendritic spine morphology. It plays a pivotal role in neurodevelopment and learning and memory. Homologous or orthologous genes are found in more than 280 vertebrate and invertebrate species, suggesting preservation through evolution. Not surprisingly, loss of the Arhgap39/Porf-2 gene in mice manifests as an embryonic lethal condition. Although Arhgap39/Porf-2 is highly expressed in the brain, it is also widely distributed throughout the body, with potential additional roles in oncogenesis and morphogenesis. This review summarizes, for the first time, the known information about this gene under its various names, in addition to considering its transcripts and proteins. The majority of findings described have been made in rats, mice, humans, and fruit flies. This work surveys the known functions, functional mediators, variables modifying expression and upstream regulators of expression, and potential physiological and pathological roles of Arhgap39/Porf-2 in health and disease.

Cell adhesion is regulated by CDK1 during the cell cycle

In most tissues, anchorage-dependent growth and cell cycle progression are dependent on cells engaging extracellular matrices (ECMs) via integrin-receptor adhesion complexes. In a highly conserved manner, cells disassemble adhesion complexes, round up, and retract from their surroundings before division, suggestive of a primordial link between the cell cycle machinery and the regulation of cell adhesion to the ECM. In this study, we demonstrate that cyclin-dependent kinase 1 (CDK1) mediates this link. CDK1, in complex with cyclin A2, promotes adhesion complex and actin cytoskeleton organization during interphase and mediates a large increase in adhesion complex area as cells transition from G1 into S. Adhesion complex area decreases in G2, and disassembly occurs several hours before mitosis. This loss requires elevated cyclin B1 levels and is caused by inhibitory phosphorylation of CDK1-cyclin complexes. The inactivation of CDK1 is therefore the trigger that initiates remodeling of adhesion complexes and the actin cytoskeleton in preparation for rapid entry into mitosis.

Mechanical and Systems Biology of Cancer

Mechanics and biochemical signaling are both often deregulated in cancer, leading toincreased cell invasiveness, proliferation, and survival. The dynamics and interactions of cytoskeletal components control basic mechanical properties, such as cell tension, stiffness, and engagement with the extracellular environment, which can lead to extracellular matrix remodeling. Intracellular mechanics can alter signaling and transcription factors, impacting cell decision making. Additionally, signaling from soluble and mechanical factors in the extracellular environment, such as substrate stiffness and ligand density, can modulate cytoskeletal dynamics. Computational models closely integrated with experimental support, incorporating cancer-specific parameters, can provide quantitative assessments and serve as predictive tools toward dissecting the feedback between signaling and mechanics and across multiple scales and domains in tumor progression.

Matrix Stiffness Enhances VEGFR-2 Internalization, Signaling, and Proliferation in Endothelial Cells

Vascular endothelial growth factor (VEGF) can mediate endothelial cell migration, proliferation, and angiogenesis. During cancer progression, VEGF production is often increased to stimulate the growth of new blood vessels to supply growing tumors with the additional oxygen and nutrients they require. Extracellular matrix stiffening also occurs during tumor progression, however, the crosstalk between tumor mechanics and VEGF signaling remains poorly understood. Here, we show that matrix stiffness heightens downstream endothelial cell response to VEGF by altering VEGF receptor-2 (VEGFR-2) internalization, and this effect is influenced by cell confluency. In sub-confluent endothelial monolayers, VEGFR-2 levels, but not VEGFR-2 phosphorylation, are influenced by matrix rigidity. Interestingly, more compliant matrices correlated with increased expression and clustering of VEGFR-2; however, stiffer matrices induced increased VEGFR-2 internalization. These effects are most likely due to actin-mediated contractility, as inhibiting ROCK on stiff substrates increased VEGFR-2 clustering and decreased internalization. Additionally, increasing matrix stiffness elevates ERK 1/2 phosphorylation, resulting in increased cell proliferation. Moreover, cells on stiff matrices generate more actin stress fibers than on compliant substrates, and the addition of VEGF stimulates an increase in fiber formation regardless of stiffness. In contrast, once endothelial cells reached confluency, stiffness-enhanced VEGF signaling was no longer observed. Together, these data show a complex effect of VEGF and matrix mechanics on VEGF-induced signaling, receptor dynamics, and cell proliferation that is mediated by cell confluency.

The role of toxins in Clostridium difficile infection

Clostridium difficile is a bacterial pathogen that is the leading cause of nosocomial antibiotic-associated diarrhea and pseudomembranous colitis worldwide. The incidence, severity, mortality and healthcare costs associated with C. difficile infection (CDI) are rising, making C. difficile a major threat to public health. Traditional treatments for CDI involve use of antibiotics such as metronidazole and vancomycin, but disease recurrence occurs in about 30% of patients, highlighting the need for new therapies. The pathogenesis of C. difficile is primarily mediated by the actions of two large clostridial glucosylating toxins, toxin A (TcdA) and toxin B (TcdB). Some strains produce a third toxin, the binary toxin C. difficile transferase, which can also contribute to C. difficile virulence and disease. These toxins act on the colonic epithelium and immune cells and induce a complex cascade of cellular events that result in fluid secretion, inflammation and tissue damage, which are the hallmark features of the disease. In this review, we summarize our current understanding of the structure and mechanism of action of the C. difficile toxins and their role in disease.

Tyr42 phosphorylation of RhoA GTPase promotes tumorigenesis through nuclear factor (NF)-κB

Dysregulation of reactive oxygen species (ROS) levels is implicated in the pathogenesis of several diseases, including cancer. However, the molecular mechanisms for ROS in tumorigenesis have not been well established. In this study, hydrogen peroxide activated nuclear factor-κB (NF-κB) and RhoA GTPase. In particular, we found that hydrogen peroxide lead to phosphorylation of RhoA at Tyr42 via tyrosine kinase Src. Phospho-Tyr42 (p-Tyr42) residue of RhoA is a binding site for Vav2, a guanine nucleotide exchange factor (GEF), which then activates p-Tyr42 form of RhoA. P-Tyr42 RhoA then binds to IκB kinase γ (IKKγ), leading to IKKβ activation. Furthermore, RhoA WT and phospho-mimic RhoA, RhoA Y42E, both promoted tumorigenesis, whereas the dephospho-mimic RhoA, RhoA Y42F suppressed it. In addition, hydrogen peroxide induced NF-κB activation and cell proliferation, along with expression of c-Myc and cyclin D1 in the presence of RhoA WT and RhoA Y42E, but not RhoA Y42F. Indeed, levels of p-Tyr42 Rho, p-Src, and p-65 are significantly increased in human breast cancer tissues and show correlations between each of the two components. Conclusively, the posttranslational modification of as RhoA p-Tyr42 may be essential for promoting tumorigenesis in response to generation of ROS.

CRMP4 Inhibits Bone Formation by Negatively Regulating BMP and RhoA Signaling

We identified the neuroprotein collapsing response mediator protein-4 (CRMP4) as a noncanonical osteogenic factor that regulates the differentiation of mouse bone marrow skeletal stem cells (bone marrow stromal stem cells [mBMSCs]) into osteoblastic cells. CRMP4 is the only member of the CRMP1-CRMP5 family to be expressed by mBMSCs and in osteoprogenitors of both adult mouse and human bones. In vitro gain-of-function and loss-of-function of CRMP4 in murine stromal cells revealed its inhibitory effect on osteoblast differentiation. In addition, Crmp4-deficient mice (Crmp4^-/- ) displayed a 40% increase in bone mass, increased mineral apposition rate, and bone formation rate, compared to wild-type controls. Increased bone mass in Crmp4^-/- mice was associated with enhanced BMP2 signaling and BMP2-induced osteoblast differentiation in Crmp4^-/- osteoblasts (OBs). Furthermore, Crmp4^-/- OBs exhibited enhanced activation of RhoA/focal adhesion kinase (FAK) signaling that led to cytoskeletal changes with increased cell spreading. In addition, Crmp4^-/- OBs exhibited increased cell proliferation that was mediated via inhibiting cyclin-dependent kinase inhibitor 1B, p27^Kip1 and upregulating cyclin D1 expression which are targets of RhoA signaling pathway. Our findings identify CRMP4 as a novel negative regulator of osteoblast differentiation. © 2016 American Society for Bone and Mineral Research.

Visualizing mechanical modulation of nanoscale organization of cell-matrix adhesions

The mechanical properties of the extracellular matrix influence cell signaling to regulate key cellular processes, including differentiation, apoptosis, and transformation. Understanding the molecular mechanisms underlying mechanotransduction is contingent upon our ability to visualize the effect of altered matrix properties on the nanoscale organization of proteins involved in this signalling. The development of super-resolution imaging techniques has afforded researchers unprecedented ability to probe the organization and localization of proteins within the cell. However, most of these methods require use of substrates like glass or silicon wafers, which are artificially rigid. In light of a growing body of literature demonstrating the importance of mechanical properties of the extracellular matrix in regulating many aspects of cellular behavior and signaling, we have developed a system that allows scanning angle interference microscopy on a mechanically tunable substrate. We describe its implementation in detail and provide examples of how it may be used to aide investigations into the effect of substrate rigidity on intracellular signaling.

Vascular CaMKII: heart and brain in your arteries

First characterized in neuronal tissues, the multifunctional calcium/calmodulin-dependent protein kinase II (CaMKII) is a key signaling component in several mammalian biological systems. Its unique capacity to integrate various Ca(2+) signals into different specific outcomes is a precious asset to excitable and nonexcitable cells. Numerous studies have reported roles and mechanisms involving CaMKII in brain and heart tissues. However, corresponding functions in vascular cell types (endothelium and vascular smooth muscle cells) remained largely unexplored until recently. Investigation of the intracellular Ca(2+) dynamics, their impact on vascular cell function, the regulatory processes involved and more recently the spatially restricted oscillatory Ca(2+) signals and microdomains triggered significant interest towards proteins like CaMKII. Heteromultimerization of CaMKII isoforms (four isoforms and several splice variants) expands this kinase's peculiar capacity to decipher Ca(2+) signals and initiate specific signaling processes, and thus controlling cellular functions. The physiological functions that rely on CaMKII are unsurprisingly diverse, ranging from regulating contractile state and cellular proliferation to Ca(2+) homeostasis and cellular permeability. This review will focus on emerging evidence of CaMKII as an essential component of the vascular system, with a focus on the kinase isoform/splice variants and cellular system studied.

Cell cycle, cytoskeleton dynamics and beyond: the many functions of cyclins and CDK inhibitors

While targeting experiments carried out on the genes encoding many cell cycle regulators have challenged our views of cell cycle control, they also suggest that redundancy might not be the only explanation for the observed perplexing phenotypes. Indeed, several observations hint at functions of cyclins and CDK inhibitors that cannot be accounted for by their sole role as kinase regulators. They are found involved in many cellular transactions, depending or not on CDKs that are not directly linked to cell cycle control, but participating to general mechanisms such as transcription, DNA repair or cytoskeleton dynamics. In this review we discuss the roles that these alternative functions might have in cancer cell proliferation and migration that sometime even challenge their definition as proliferation markers.

Mice Deficient in AKAP13 (BRX) Are Osteoporotic and Have Impaired Osteogenesis

Mechanical stimulation is crucial to bone growth and triggers osteogenic differentiation through a process involving Rho and protein kinase A. We previously cloned a gene (AKAP13, aka BRX) encoding a protein kinase A-anchoring protein in the N-terminus, a guanine nucleotide-exchange factor for RhoA in the mid-section, coupled to a carboxyl region that binds to estrogen and glucocorticoid nuclear receptors. Because of the critical role of Rho, estrogen, and glucocorticoids in bone remodeling, we examined the multifunctional role of Akap13. Akap13 was expressed in bone, and mice haploinsufficient for Akap13 (Akap13(+/-)) displayed reduced bone mineral density, reduced bone volume/total volume, and trabecular number, and increased trabecular spacing; resembling the changes observed in osteoporotic bone. Consistent with the osteoporotic phenotype, Colony forming unit-fibroblast numbers were diminished in Akap13(+/-) mice, as were osteoblast numbers and extracellular matrix production when compared to control littermates. Transcripts of Runx2, an essential transcription factor for the osteogenic lineage, and alkaline phosphatase (Alp), an indicator of osteogenic commitment, were both reduced in femora of Akap13(+/-) mice. Knockdown of Akap13 reduced levels of Runx2 and Alp transcripts in immortalized bone marrow stem cells. These findings suggest that Akap13 haploinsufficient mice have a deficiency in early osteogenesis with a corresponding reduction in osteoblast number, but no impairment of mature osteoblast activity.

Screening a novel FGF3 antagonist peptide with anti-tumor effects on breast cancer from a phage display library

Accumulating evidence has suggested that fibroblast growth factor 3 (FGF3) is expressed in breast cancer and correlates with the stage and grade of the disease. In the present study, a specific FGF3‑binding peptide (VLWLKNR, termed FP16) was isolated from a phage display heptapeptide library with FGF3. The peptide FP16 contained four identical (WLKN) amino acids and demonstrated high homology to the peptides of the 188‑194 (TMRWLKN) site of the high‑affinity FGF3 receptor fibroblast growth factor receptor 2. Functional analyses indicated that FP16 mediated significant inhibition of FGF3‑induced cell proliferation, arrested the cell cycle at the G0/G1 phase by increasing proliferation‑associated protein 2G4, suppressing cyclin D1 and proliferating cell nuclear antigen, and inhibited the FGF3‑induced activation of extracellular signal‑regulated kinase 1/2 and Akt kinase. Taken together, these results demonstrated that the peptide FP16, acting as an FGF3 antagonist, is a promising therapeutic agent for the treatment of breast cancer.

Regulation of cell proliferation by ERK and signal-dependent nuclear translocation of ERK is dependent on Tm5NM1-containing actin filaments

Tropomyosin Tm5NM1 regulates cell proliferation and organ size. It mediates this effect by regulating the interaction of pERK and Imp7, leading to the regulation of pERK nuclear translocation. This demonstrates a role for a specific population of actin filaments in regulating a critical step in the MAPK/ERK signaling pathway.

ERK-regulated cell proliferation requires multiple phosphorylation events catalyzed first by MEK and then by casein kinase 2 (CK2), followed by interaction with importin7 and subsequent nuclear translocation of pERK. We report that genetic manipulation of a core component of the actin filaments of cancer cells, the tropomyosin Tm5NM1, regulates the proliferation of normal cells both in vitro and in vivo. Mouse embryo fibroblasts (MEFs) lacking Tm5NM1, which have reduced proliferative capacity, are insensitive to inhibition of ERK by peptide and small-molecule inhibitors, indicating that ERK is unable to regulate proliferation of these knockout (KO) cells. Treatment of wild-type MEFs with a CK2 inhibitor to block phosphorylation of the nuclear translocation signal in pERK resulted in greatly decreased cell proliferation and a significant reduction in the nuclear translocation of pERK. In contrast, Tm5NM1 KO MEFs, which show reduced nuclear translocation of pERK, were unaffected by inhibition of CK2. This suggested that it is nuclear translocation of CK2-phosphorylated pERK that regulates cell proliferation and this capacity is absent in Tm5NM1 KO cells. Proximity ligation assays confirmed a growth factor–stimulated interaction of pERK with Tm5NM1 and that the interaction of pERK with importin7 is greatly reduced in the Tm5NM1 KO cells.

A novel role of actomyosin bundles in ERK signaling

Intracellular and extracellular mechanical environments have a significant impact on survival and proliferation of cells. While the extracellular signal-regulated kinase (ERK) subfamily of MAP kinases plays critical roles in regulations of these cellular behaviors, activation of ERK is affected by mechanical conditions of cells. We have recently found that ERK is activated on contractile actomyosin bundles. ERK activation on actomyosin bundles depends on tension in the bundles, which is generated by either myosin II activity of external forces. In this Addendum, we discuss a novel, potential role of actomyosin bundles in ERK signaling and mechanical regulation of cell survival and proliferation.

Hydrostatic pressure promotes the proliferation and osteogenic/chondrogenic differentiation of mesenchymal stem cells: The roles of RhoA and Rac1

Our previous studies have shown that hydrostatic pressure can serve as an active regulator for bone marrow mesenchymal stem cells (BMSCs). The current work further investigates the roles of cytoskeletal regulatory proteins Ras homolog gene family member A (RhoA) and Ras-related C3 botulinum toxin substrate 1 (Rac1) in hydrostatic pressure-related effects on BMSCs. Flow cytometry assays showed that the hydrostatic pressure promoted cell cycle initiation in a RhoA- and Rac1-dependent manner. Furthermore, fluorescence assays confirmed that RhoA played a positive and Rac1 displayed a negative role in the hydrostatic pressure-induced F-actin stress fiber assembly. Western blots suggested that RhoA and Rac1 play central roles in the pressure-inhibited ERK phosphorylation, and Rac1 but not RhoA was involved in the pressure-promoted JNK phosphorylation. Finally, real-time polymerase chain reaction (PCR) experiments showed that pressure promoted the expression of osteogenic marker genes in BMSCs at an early stage of osteogenic differentiation through the up-regulation of RhoA activity. Additionally, the PCR results showed that pressure enhanced the expression of chondrogenic marker genes in BMSCs during chondrogenic differentiation via the up-regulation of Rac1 activity. Collectively, our results suggested that RhoA and Rac1 are critical to the pressure-induced proliferation and differentiation, the stress fiber assembly, and MAPK activation in BMSCs.

Actomyosin bundles serve as a tension sensor and a platform for ERK activation

Tensile forces generated by stress fibers drive signal transduction events at focal adhesions. Here, we report that stress fibers per se act as a platform for tension-induced activation of biochemical signals. The MAP kinase, ERK is activated on stress fibers in a myosin II-dependent manner. In myosin II-inhibited cells, uniaxial stretching of cell adhesion substrates restores ERK activation on stress fibers. By quantifying myosin II- or mechanical stretch-mediated tensile forces in individual stress fibers, we show that ERK activation on stress fibers correlates positively with tensile forces acting on the fibers, indicating stress fibers as a tension sensor in ERK activation. Myosin II-dependent ERK activation is also observed on actomyosin bundles connecting E-cadherin clusters, thus suggesting that actomyosin bundles, in general, work as a platform for tension-dependent ERK activation.

From cell membrane to the nucleus: an emerging role of E-cadherin in gene transcriptional regulation

E-cadherin is a well-known mediator of cell–cell adherens junctions. However, many other functions of E-cadherin have been reported. Collectively, the available data suggest that E-cadherin may also act as a gene transcriptional regulator. Here, evidence supporting this claim is reviewed, and possible mechanisms of action are discussed. E-cadherin has been shown to modulate the activity of several notable cell signalling pathways, and given that most of these pathways in turn regulate gene expression, we proposed that E-cadherin may regulate gene transcription by affecting these pathways. Additionally, E-cadherin has been shown to accumulate in the nucleus where documentation of an E-cadherin fragment bound to DNA suggests that E-cadherin may directly regulate gene transcription. In summary, from the cell membrane to the nucleus, a role for E-cadherin in gene transcription may be emerging. Studies specifically focused on this potential role would allow for a more thorough understanding of this transmembrane glycoprotein in mediating intra- and intercellular activities.

Identification of a novel peptide that blocks basic fibroblast growth factor-mediated cell proliferation

Basic fibroblast growth factor (bFGF) has been implicated in tumor growth via interactions with its receptors (FGFRs) on the cell surface and therefore, bFGF/FGFRs are considered essential targets for cancer therapy. Herein, a consensus heptapeptide (LSPPRYP) was identified for the first time from a phage display heptapeptide library after three sequential rounds of biopanning against FGFR-expressing cells with competitive displacement of phage by bFGF, followed by subtraction of non-specific binding by FGFR-deficient cells. Phage bearing LSPPRYP showed high levels of binding to Balb/c 3T3 cells expressing high-affinity bFGF-binding FGFR (bFGFR), but not to the cells that do not express bFGFR (Cos-7), or express a very low affinity bFGFR (HaCat). The selected-phage-derived peptide synthesized by solid phase method using a rapid and practical Fmoc strategy was found to specifically compete with bFGF for binding to its receptors, inhibit bFGF-stimulated cell proliferation by inducing cell cycle arrest, and block bFGF-induced activation of Erk1 and Erk2 kinase in B16-F10 melanoma cells. Importantly, treatment of melanoma-bearing mice with the synthetic peptide significantly suppressed tumor growth. The results demonstrate a strong anticancer activity of the isolated bFGFR-binding peptide (and its future derivatives), which may have great potential for cancer therapy.

Function-blocking ERBB3 antibody inhibits the adaptive response to RAF inhibitor

ERBB3/HER3 expression and signaling are upregulated in mutant BRAF melanoma as an adaptive, prosurvival response to FDA-approved RAF inhibitors. Because compensatory ERBB3 signaling counteracts the effects of RAF inhibitors, cotargeting ERBB3 may increase the efficacy of RAF inhibitors in mutant BRAF models of melanoma. Here, we corroborate this concept by showing that the ERBB3 function-blocking monoclonal antibody huHER3-8 can inhibit neuregulin-1 activation of ERBB3 and downstream signaling in RAF-inhibited melanoma cells. Targeting mutant BRAF in combination with huHER3-8 decreased cell proliferation and increased cell death in vitro, and decreased tumor burden in vivo, compared with targeting either mutant BRAF or ERBB3 alone. Furthermore, the likelihood of a durable tumor response in vivo was increased when huHER3-8 was combined with RAF inhibitor PLX4720. Together, these results offer a preclinical proof of concept for the application of ERBB3-neutralizing antibodies to enhance the efficacy of RAF inhibitors in melanoma to delay or prevent tumor regrowth. As ERBB3 is often upregulated in response to other kinase-targeted therapeutics, these findings may have implications for other cancers as well.

RhoA/Rho kinase mediates TGF-β1-induced kidney myofibroblast activation through Poldip2/Nox4-derived reactive oxygen species

The small G proteins Rac1 and RhoA regulate actin cytoskeleton, cell shape, adhesion, migration, and proliferation. Recent studies in our laboratory have shown that NADPH oxidase Nox4-derived ROS are involved in transforming growth factor (TGF)-β1-induced rat kidney myofibroblast differentiation assessed by the acquisition of an α-smooth muscle actin (α-SMA) phenotype and expression of an alternatively spliced fibronectin variant (Fn-EIIIA). Rac1 and RhoA are essential in signaling by some Nox homologs, but their role as effectors of Nox4 in kidney myofibroblast differentiation is not known. In the present study, we explored a link among Rac1 and RhoA and Nox4-dependent ROS generation in TGF-β1-induced kidney myofibroblast activation. TGF-β1 stimulated an increase in Nox4 protein expression, NADPH oxidase activity, and abundant α-SMA and Fn-EIIIA expression. RhoA but not Rac1 was involved in TGF-β1 induction of Nox4 signaling of kidney myofibroblast activation. TGF-β1 stimulated active RhoA-GTP and increased Rho kinase (ROCK). Inhibition of RhoA with small interfering RNA and ROCK using Y-27632 significantly reduced TGF-β1-induced stimulation of Nox4 protein, NADPH oxidase activity, and α-SMA and Fn-EIIIA expression. Treatment with diphenyleneiodonium, an inhibitor of NADPH oxidase, did not decrease RhoA activation but inhibited TGF-β1-induced α-SMA and Fn-EIIIA expression, indicating that RhoA is upstream of ROS generation. RhoA/ROCK also regulated polymerase (DNA-directed) δ-interacting protein 2 (Poldip2), a newly discovered Nox4 enhancer protein. Collectively, these data indicate that RhoA/ROCK is upstream of Poldip2-dependent Nox4 regulation and ROS production and induces redox signaling of kidney myofibroblast activation and may broader implications in the pathophysiology of renal fibrosis.

ATG4B promotes colorectal cancer growth independent of autophagic flux

Autophagy is reported to suppress tumor proliferation, whereas deficiency of autophagy is associated with tumorigenesis. ATG4B is a deubiquitin-like protease that plays dual roles in the core machinery of autophagy; however, little is known about the role of ATG4B on autophagy and proliferation in tumor cells. In this study, we found that ATG4B knockdown induced autophagic flux and reduced CCND1 expression to inhibit G 1/S phase transition of cell cycle in colorectal cancer cell lines, indicating functional dominance of ATG4B on autophagy inhibition and tumor proliferation in cancer cells. Interestingly, based on the genetic and pharmacological ablation of autophagy, the growth arrest induced by silencing ATG4B was independent of autophagic flux. Moreover, dephosphorylation of MTOR was involved in reduced CCND1 expression and G 1/S phase transition in both cells and xenograft tumors with depletion of ATG4B. Furthermore, ATG4B expression was significantly increased in tumor cells of colorectal cancer patients compared with adjacent normal cells. The elevated expression of ATG4B was highly correlated with CCND1 expression, consistently supporting the notion that ATG4B might contribute to MTOR-CCND1 signaling for G 1/S phase transition in colorectal cancer cells. Thus, we report that ATG4B independently plays a role as a positive regulator on tumor proliferation and a negative regulator on autophagy in colorectal cancer cells. These results suggest that ATG4B is a potential biomarker and drug target for cancer therapy.

Serum-dependence of affinity-mediated VEGF release from biomimetic microspheres

Vascular endothelial growth factor (VEGF) activity is highly regulated via sequestering within the ECM and cell-demanded proteolysis to release the sequestered VEGF. Numerous studies have demonstrated that VEGF activity mediates cellular events leading to angiogenesis and capillary formation in vivo. This has motivated the study of biomaterials to sustain VEGF release, and in many cases, the materials are inspired by the structure and function of the native ECM. However, there remains a need for materials that can bind to VEGF with high specificity, as the in vivo environment is rich in a variety of growth factors (GFs) and GF-binding moieties. Here we describe a strategy to control VEGF release using hydrogel microspheres with tethered peptides derived from VEGF receptor 2 (VEGFR2). Using biomaterials covalently modified with varying concentrations of two distinct VEGFR2-derived peptides with varying serum stability, we analyzed both biomaterial and environmental variables that influence VEGF release and activity. The presence of tethered VEGF-binding peptides (VBPs) resulted in significantly extended VEGF release relative to control conditions, and the resulting released VEGF significantly increased the expansion of human umbilical vein endothelial cells in culture. VEGF release rates were also strongly influenced by the concentration of serum. The presence of Feline McDonough Sarcoma-like tyrosine kinase 1 (sFlt-1), a serum-borne receptor fragment derived from VEGF receptor 1, increased VEGF release rates, although sFlt-1 was not sufficient to recapitulate the release profile of VEGF in serum. Further, the influence of serum on VEGF release was not due to protease activity or nonspecific VEGF interactions in the presence of serum-borne heparin. VEGF release kinetics correlated well with a generalizable mathematical model describing affinity-mediated release of VEGF from hydrogel microspheres in defined conditions. Modeling results suggest a potential mechanism whereby competition between VEGF and multiple VEGF-binding serum proteins including sFlt-1, soluble kinase insert domain receptor (sKDR), and α2-macroglobulin (α2-M) likely influenced VEGF release from microspheres. The materials and mathematical model described in this approach may be useful in a range of applications in which sustained, biologically active GF release of a specific GF is desirable.

Rho GTPases control specific cytoskeleton-dependent functions of hematopoietic stem cells

The Rho family of guanosine triphosphatases (GTPases) is composed of members of the Ras superfamily of proteins. They are GTP-bound molecules with a modest intrinsic GTPase activity that can be accelerated upon activation/localization of specialized guanine nucleotide exchange factors. Members of this family act as molecular switches and are required for coordinated cytoskeletal rearrangements that are crucial in a set of specialized functions of mammalian stem cells. These functions include self-renewal, adhesion, and migration. Mouse gene-targeting studies have provided convincing evidence of the indispensable and dispensable roles of individual members of the Rho GTPase family and the putative upstream and downstream mediators in stem cell-specific functions. The role of Rho GTPases and related signaling pathways previously seen in other cell types and organisms have been confirmed in mammalian hematopoietic stem cells (HSCs), and new signaling pathways and unexpected functions unique to HSCs have been identified and dissected. This review summarizes our current understanding of the role of Rho family of GTPases on HSC and progenitor activity through cytoskeleton-mediated signaling pathways, providing insight about relevant signaling pathways that regulate mammalian stem cell self-renewal, adhesion, and migration.

Rho GTPases modulate malignant transformation of tumor cells

Rho GTPases are involved in the acquisition of all the hallmarks of cancer, which comprise 6 biological capabilities acquired during the development of human tumors. The hallmarks include proliferative signaling, evading growth suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating invasion and metastasis programs, as defined by Hanahan and Weinberg. (1) Controlling these hallmarks are genome instability and inflammation. Emerging hallmarks are reprogramming of energy metabolism and evading immune destruction. To give a different view to the readers, we will not be focusing on invasion, metastasis, or cytoskeletal remodeling, but we will review here how Rho GTPases contribute to other hallmarks of cancer with a special emphasis on malignant transformation.

Clostridium difficile toxin A attenuates Wnt/β-catenin signaling in intestinal epithelial cells

Clostridium difficile toxins A and B (TcdA and TcdB) are homologous glycosyltransferases that inhibit a group of small GTPases within host cells, but several mechanisms underlying their pathogenic activity remain unclear. In this study, we evaluated the effects of TcdA on the Wnt/β-catenin pathway, the major driving force behind the proliferation of epithelial cells in colonic crypts. IEC-6 and RKO cells stimulated with Wnt3a-conditioned medium were incubated with 10, 50, and 100 ng/ml of TcdA for 24 h, resulting in a dose-dependent inhibition of the Wnt signaling, as demonstrated by a T-cell factor (TCF) reporter assay. This was further confirmed by immunofluorescence staining for nuclear localization of β-catenin and Western blotting for β-catenin and c-Myc (encoded by a Wnt target gene). Moreover, our Western blot analysis showed a decrease in the β-catenin protein levels, which was reversed by z-VAD-fmk, a pan-caspase inhibitor. Nonetheless, TcdA was still able to inhibit the Wnt/β-catenin pathway even in the presence of z-VAD-fmk, lithium chloride (a GSK3β inhibitor), or constitutively active β-catenin, as determined by a TCF reporter assay. Furthermore, preincubation of RKO cells with TcdA for 12 h also attenuated Wnt3a-mediated activation of Wnt signaling, suggesting that inactivation of Rho GTPases plays a significant role in that inhibition. Taken together, these findings suggest that attenuation of the Wnt signaling by TcdA is important for TcdA antiproliferative effects.

Identification of a novel aFGF-binding peptide with anti-tumor effect on breast cancer from phage display library

It has been reported that acidic fibroblast growth factor (aFGF) is expressed in breast cancer and via interactions with fibroblast growth factor receptors (FGFRs) to promote the stage and grade of the disease. Thus, aFGF/FGFRs have been considered essential targets in breast cancer therapy. We identified a specific aFGF-binding peptide (AGNWTPI, named AP8) from a phage display heptapeptide library with aFGF after four rounds of biopanning. The peptide AP8 contained two (TP) amino acids identical and showed high homology to the peptides of the 182-188 (GTPNPTL) site of high-affinity aFGF receptor FGFR1. Functional analyses indicated that AP8 specifically competed with the corresponding phage clone A8 for binding to aFGF. In addition, AP8 could inhibit aFGF-stimulated cell proliferation, arrested the cell cycle at the G0/G1 phase by increasing PA2G4 and suppressing Cyclin D1 and PCNA, and blocked the aFGF-induced activation of Erk1/2 and Akt kinase in both breast cancer cells and vascular endothelial cells. Therefore, these results indicate that peptide AP8, acting as an aFGF antagonist, is a promising therapeutic agent for the treatment of breast cancer.

SmgGDS-558 regulates the cell cycle in pancreatic, non-small cell lung, and breast cancers

Oncogenic mutation or misregulation of small GTPases in the Ras and Rho families can promote unregulated cell cycle progression in cancer. Post-translational modification by prenylation of these GTPases allows them to signal at the cell membrane. Splice variants of SmgGDS, named SmgGDS-607 and SmgGDS-558, promote the prenylation and membrane trafficking of multiple Ras and Rho family members, which makes SmgGDS a potentially important regulator of the cell cycle. Surprisingly little is known about how SmgGDS-607 and SmgGDS-558 affect cell cycle-regulatory proteins in cancer, even though SmgGDS is overexpressed in multiple types of cancer. To examine the roles of SmgGDS splice variants in the cell cycle, we compared the effects of the RNAi-mediated depletion of SmgGDS-558 vs. SmgGDS-607 on cell cycle progression and the expression of cyclin D1, p27, and p21 in pancreatic, lung, and breast cancer cell lines. We show for the first time that SmgGDS promotes proliferation of pancreatic cancer cells, and we demonstrate that SmgGDS-558 plays a greater role than SmgGDS-607 in cell cycle progression as well as promoting cyclin D1 and suppressing p27 expression in multiple types of cancer. Silencing both splice variants of SmgGDS in the cancer cell lines produces an alternative signaling profile compared with silencing SmgGDS-558 alone. We also show that loss of both SmgGDS-607 and SmgGDS-558 simultaneously decreases tumorigenesis of NCI-H1703 non-small cell lung carcinoma (NSCLC) xenografts in mice. These findings indicate that SmgGDS promotes cell cycle progression in multiple types of cancer, making SmgGDS a valuable target for cancer therapeutics.

SUMOylation of RhoGDIα is required for its repression of cyclin D1 expression and anchorage-independent growth of cancer cells

Selective activation of Rho GTPase cascade requires the release of Rho from RhoGDI (GDP-dissociation inhibitors) complexes. Our previous studies identified RhoGDIα SUMOylation at Lys-138 and its function in the regulation of cancer cell invasion. In the current study, we demonstrate that RhoGDIα SUMOylation has a crucial role in the suppression of cancer cell anchorage-independent growth as well as the molecular mechanisms underlying this suppression. We found that ectopic expression of RhoGDIα resulted in marked inhibition of an anchorage-independent growth with induction of G0/G1 cell cycle arrest, while point mutation of RhoGDIα SUMOylation at residue Lys-138 (K138R) abrogated this growth suppression and G0/G1 cell cycle arrest in cancer cells. Further studies showed that SUMOylation at Lys-138 was critical for RhoGDIα down-regulation of cyclin D1 protein expression and that MEK1/2-Erk was a specific downstream target of SUMOylated RhoGDIα for its inhibition of C-Jun/AP-1 cascade, cyclin d1 transcription, and cell cycle progression. These results strongly demonstrate that SUMOylated RhoGDIα suppressed C-Jun/AP-1-dependent transactivation specifically via targeting MEK1/2-Erk, subsequently leading to the down-regulation of cyclin D1 expression and anti-cancer activity. Our results provide new mechanistic insights into the understanding of essential role of SUMOylation at Lys-138 in RhoGDIα's biological function.

Specific VEGF sequestering to biomaterials: influence of serum stability

Vascular endothelial growth factor (VEGF) was originally discovered as a tumor-derived factor that is able to induce endothelial cell behavior associated with angiogenesis. It has been implicated during wound healing for the induction of endothelial cell proliferation, tube formation and blood vessel remodeling. However, previous investigations into the biological effect of VEGF concluded that a particular range of growth factor concentrations are required for healthy vasculature to form, motivating recent studies to regulate VEGF activity via molecular sequestering to biomaterials. Numerous VEGF sequestering strategies have been developed, and they have typically relied on extracellular matrix mimicking moieties that are not specific for VEGF and can affect many growth factors simultaneously. We describe here a strategy for efficient, specific VEGF sequestering with poly(ethylene glycol) (PEG) microspheres, using peptides designed to mimic VEGF receptor type 2 (VEGFR2). By immobilizing two distinct peptides with different serum stabilities, we examined the effect of serum on the specific interaction between peptide-containing PEG microspheres and VEGF. We addressed the hypothesis that VEGF sequestering in serum-containing solutions would be influenced by the serum stability of the VEGF-binding peptide. We further hypothesized that soluble VEGF could be sequestered in serum-containing cell culture media, resulting in decreased VEGF-dependent proliferation of human umbilical vein endothelial cells. We show that soluble VEGF concentration can be effectively regulated in serum-containing environments via specific molecular sequestering, which suggests potential clinical applications.

β-Escin inhibits NNK-induced lung adenocarcinoma and ALDH1A1 and RhoA/Rock expression in A/J mice and growth of H460 human lung cancer cells

Lung cancer is the leading cause of cancer-related deaths. β-Escin, a triterpene saponin isolated from horse chestnut seeds, was tested for inhibition of lung adenoma and adenocarcinoma induced by the tobacco carcinogen 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in female A/J mice; and its possible mode of action was evaluated using the H460 human lung cancer cell line. At 6 weeks of age, 35 mice were fed AIN-76A-modified diet, and one week later, lung tumors were induced with a single intraperitoneal (i.p.) injection of 10 μmol NNK/mouse. Three weeks after the NNK treatment, groups of mice were fed either control or experimental diets containing 500 ppm for 20 weeks (10 control, 5 β-escin) or 36 weeks (15 control, 5 β-escin) and evaluated for lung tumor via histopathologic methods. Administration of 500 ppm β-escin significantly suppressed lung tumor (adenoma + adenocarcinoma) formation by more than 40% (P < 0.0015) at 20 weeks and by 53.3% (P < 0.0001) at 37 weeks. β-Escin inhibited NNK-induced lung adenocarcinoma formation by 65% (P < 0.001) at 20 weeks and by 53% (P < 0.0001) at 37 weeks. Immunohistochemical analysis revealed that lung tumors from mice exposed to β-escin showed significantly reduced aldehyde dehydrogenase (ALDH)1A1 and phospho-Akt (p-Akt) expression when compared with those in mice fed control diet. Aldefluor assay for ALDH revealed that among H460 lung cancer cells treated with different concentrations of β-escin (0-40 μmol/L), the subpopulation of cells with elevated ALDH activity was inhibited significantly. Our findings suggest that β-escin inhibits tobacco carcinogen-induced lung tumor formation by modulating ALDH1A1-positive cells and RhoA/Rock signaling.

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.