Cell shape, cytoskeletal mechanics, and cell cycle control in angiogenesis

Cellular Alterations in Cultured Endothelial Cells Exposed to Therapeutic Ultrasound Irradiation

Restoration of blood supply to tissue with impaired perfusion depends on spontaneous or mediated angiogenesis, which among other mechanisms includes stimulation, migration, and proliferation of endothelial cells (ECs). Therapeutic ultrasound (US) irradiation is known as an inducer of cellular modifications and is used to accelerate wound healing. An in vitro setup was developed in order to allow for a comprehensive investigation of cellular alterations induced in cultured ECs after exposure to different modes of therapeutic US irradiation. Viability assays revealed a higher rate of proliferation in the sonicated groups, although cell death was not observed. Visualization of actin stress fibers demonstrated partial disassembly of the fibers immediately after US sonication, with a maximum after about 2 h. However, 24 h following sonication the fibers regain normal appearance. A similar behavior was observed with the microtubules and focal adhesion complexes. Utilizing a wound healing assay revealed that migration rate of ECs is enhanced by US irradiation. These findings hint that therapeutic US sonication of ECs results in temporarily cellular alterations, which may induce tissue remodeling via stimulation of EC proliferation and migration.

Mechanics, malignancy, and metastasis: the force journey of a tumor cell

A cell undergoes many genetic and epigenetic changes as it transitions to malignancy. Malignant transformation is also accompanied by a progressive loss of tissue homeostasis and perturbations in tissue architecture that ultimately culminates in tumor cell invasion into the parenchyma and metastasis to distant organ sites. Increasingly, cancer biologists have begun to recognize that a critical component of this transformation journey involves marked alterations in the mechanical phenotype of the cell and its surrounding microenvironment. These mechanical differences include modifications in cell and tissue structure, adaptive force-induced changes in the environment, altered processing of micromechanical cues encoded in the extracellular matrix (ECM), and cell-directed remodeling of the extracellular stroma. Here, we review critical steps in this "force journey," including mechanical contributions to tissue dysplasia, invasion of the ECM, and metastasis. We discuss the biophysical basis of this force journey and present recent advances in the measurement of cellular mechanical properties in vitro and in vivo. We end by describing examples of molecular mechanisms through which tumor cells sense, process and respond to mechanical forces in their environment. While our understanding of the mechanical components of tumor growth, survival and motility remains in its infancy, considerable work has already yielded valuable insight into the molecular basis of force-dependent tumor pathophysiology, which offers new directions in cancer chemotherapeutics.

Design and fabrication of 3D porous scaffolds to facilitate cell-based gene therapy

Biomaterials capable of efficient gene delivery by embedded cells provide a fundamental tool for the treatment of acquired or hereditary diseases. A major obstacle is maintaining adequate nutrient and oxygen diffusion to cells within the biomaterial. In this study, we combined the solid free-form fabrication and porogen leaching techniques to fabricate three-dimensional scaffolds, with bimodal pore size distribution, for cell-based gene delivery. The objective of this study was to design micro-/macroporous scaffolds to improve cell viability and drug delivery. Murine bone marrow-derived mesenchymal stromal cells (MSCs) genetically engineered to secrete erythropoietin (EPO) were seeded onto poly-L-lactide (PLLA) scaffolds with different microporosities. Over a period of 2 weeks in culture, an increase in cell proliferation and metabolic activity was observed with increasing scaffold microporosity. The concentration of EPO detected in supernatants also increased with increasing microporosity level. Our study shows that these constructs can promote cell viability and release of therapeutic proteins, and clearly demonstrates their capacity for a dual role as scaffolds for tissue regeneration and as delivery systems for soluble gene products.

A comparative study on the anti-angiogenic effects of DNA-damaging and cytoskeletal-disrupting agents

The discovery of molecules with anti-angiogenic properties has led to promising new strategies for the treatment of diseases characterized by excessive new vessel growth, such as cancer and haemangioma. We have assessed the effects of DNA-damaging and cytoskeletal-disrupting agents in vitro on several endothelial cell functions. We report that bleomycin, mitomycin C and cytoskeletal-disrupting drugs (2-methoxyestradiol, taxol, vincristine, vinblastine, colchicine, nocodazole, and cytochalasin D) exhibit anti-angiogenic activities of varying potency. Bleomycin and the various cytoskeletal-disrupting drugs inhibited endothelial cell migration, while mitomycin C had a marginal effect. Both DNA-damaging and cytoskeletal-disrupting drugs decreased endothelial cell growth in a dose-dependent manner, and this was accompanied by the induction of apoptosis. The growth inhibitory and apoptotic effects of cytoskeletal-disrupting drugs were the most pronounced. We also show that both classes of drugs inhibited capillary-like tube formation in an assay of in vitro angiogenesis, with cytoskeletal-disrupting agents inhibiting in vitro angiogenesis with greater potency. A targeted approach incorporating several compounds with different mechanisms of action may be useful for the treatment of angiogenesis-dependent diseases such as hemangiomas of infancy.

Elongation, proliferation & migration differentiate endothelial cell phenotypes and determine capillary sprouting

BACKGROUND

Angiogenesis, the growth of capillaries from preexisting blood vessels, has been extensively studied experimentally over the past thirty years. Molecular insights from these studies have lead to therapies for cancer, macular degeneration and ischemia. In parallel, mathematical models of angiogenesis have helped characterize a broader view of capillary network formation and have suggested new directions for experimental pursuit. We developed a computational model that bridges the gap between these two perspectives, and addresses a remaining question in angiogenic sprouting: how do the processes of endothelial cell elongation, migration and proliferation contribute to vessel formation?

RESULTS

We present a multiscale systems model that closely simulates the mechanisms underlying sprouting at the onset of angiogenesis. Designed by agent-based programming, the model uses logical rules to guide the behavior of individual endothelial cells and segments of cells. The activation, proliferation, and movement of these cells lead to capillary growth in three dimensions. By this means, a novel capillary network emerges out of combinatorially complex interactions of single cells. Rules and parameter ranges are based on literature data on endothelial cell behavior in vitro. The model is designed generally, and will subsequently be applied to represent species-specific, tissue-specific in vitro and in vivo conditions. Initial results predict tip cell activation, stalk cell development and sprout formation as a function of local vascular endothelial growth factor concentrations and the Delta-like 4 Notch ligand, as it might occur in a three-dimensional in vitro setting. Results demonstrate the differential effects of ligand concentrations, cell movement and proliferation on sprouting and directional persistence.

CONCLUSION

This systems biology model offers a paradigm closely related to biological phenomena and highlights previously unexplored interactions of cell elongation, migration and proliferation as a function of ligand concentration, giving insight into key cellular mechanisms driving angiogenesis.

Effects of Dressing Type on 3D Tissue Microdeformations During Negative Pressure Wound Therapy: A Computational Study

Vacuum-assisted closure® (VAC®) therapy, also referred to as vacuum-assisted closure® negative pressure wound therapy (VAC® NPWT), delivered to various dermal wounds is believed to influence the formation of granulation tissue via the mechanism of microdeformational signals. In recent years, numerous experimental investigations have been initiated to study the cause-effect relationships between the mechanical signals and the transduction pathways that result in improved granulation response. To accurately quantify the tissue microdeformations during therapy, a new three-dimensional finite element model has been developed and is described in this paper. This model is used to study the effect of dressing type and subatmospheric pressure level on the variations in the microdeformational strain fields in a model dermal wound bed. Three-dimensional geometric models representing typical control volumes of NPWT dressings were generated using micro-CT scanning of VAC® GranuFoam®, a reticulated open-cell polyurethane foam (ROCF), and a gauze dressing (constructed from USP Class VII gauze). Using a nonlinear hyperfoam constitutive model for the wound bed, simulated tissue microdeformations were generated using the foam and gauze dressing models at equivalent negative pressures. The model results showed that foam produces significantly greater strain than gauze in the tissue model at all pressures and in all metrics (p<0.0001 for all but εvol at −50mmHg and −100mmHg where p<0.05). Specifically, it was demonstrated in this current work that the ROCF dressing produces higher levels of tissue microdeformation than gauze at all levels of subatmospheric pressure. This observation is consistent across all of the strain invariants assessed, i.e., εvol, εdist, the minimum and maximum principal strains, and the maximum shear strain. The distribution of the microdeformations and strain appears as a repeating mosaic beneath the foam dressing, whereas the gauze dressings appear to produce an irregular distribution of strains in the wound surface. Strain predictions from the developed computational model results agree well with those predicted from prior two-dimensional experimental and computational studies of foam-based NPWT (Saxena, V., et al., 2004, “Vacuum-assisted closure: Microdeformations of Wounds and Cell Proliferation,” Plast. Reconstr. Surg., 114(5), pp. 1086–1096). In conjunction with experimental in vitro and in vivo studies, the developed model can now be extended into more detailed investigations into the mechanobiological underpinnings of VAC® NPWT and can help to further develop and optimize this treatment modality for the treatment of challenging patient wounds.

Biomodulation with low-level laser radiation induces changes in endothelial cell actin filaments and cytoskeletal organization

The cytoskeleton is a central and vital structure of eukaryotic cells. It consists of a dynamic network of partially interconnected polymers. This extended network controls the mechanical properties of animal cells, serves as intracellular transport "pathways", and plays a prominent role in cell motility, proliferation, and adhesion. In order to evaluate the action of laser irradiation on the cytoskeleton and proliferation of endothelial cells, rabbit aortic endothelial cells (RAEC) were irradiated with 685-nm low-level laser (20 mW output power). Fluorescent dye rhodamine-phalloidin staining was used to visualize the effect of laser irradiation on actin filaments. Irradiation with 8 J/cm(2) was performed four times at 12-h intervals for 24 min. Cells cultured under low fetal bovine serum condition (5% FBS) for 7 days presented actin staining predominantly in the cortical membrane region and a few actin filament stress fibers. However, the formation of stress fibers similar to those of control cells increased significantly in irradiated cells. It was concluded that laser irradiation induces changes in the cytoskeleton of endothelial cells through the reorganization of actin filaments and neo-formation of stress fibers, allowing evident cellular proliferation.

Contractility-dependent modulation of cell proliferation and adhesion by microscale topographical cues

Engineering of cellular assembly on biomaterial scaffolds by utilizing microscale topographical cues has emerged as a powerful strategy in cardiovascular tissue engineering and regenerative medicine. However, the mechanisms through which these cues are processed to yield changes in canonical cell behaviors remain unclear. Previously, we showed that when mixtures of cardiomyocytes and fibroblasts were cultured on polydimethylsiloxane surfaces studded with microscale pillars (micropegs), fibroblast proliferation was dramatically suppressed, which suggests that the micropegs could be exploited to minimize fibrosis and scar formation. Here, we demonstrate that this effect relies on altered adhesive and micromechanical interactions between individual cells and micropegs. First, we show that the proliferation of a cell physically attached to a micropeg is significantly lower than that of a cell cultured on a featureless region of the substrate. Micropeg adhesion is accompanied by a marked elongation in cell and nuclear shape. When fibroblast contractility is pharmacologically attenuated through low-dose inhibition of either Rho-associated kinase or myosin light chain kinase, the potency with which micropeg adhesion suppresses cell proliferation is significantly reduced. Together, our results support a model in which cell fate decisions may be directly manipulated within tissue engineering scaffolds by the inclusion of microtopographical structures that alter cellular mechanics.

The alteration of a mechanical property of bone cells during the process of changing from osteoblasts to osteocytes

Osteocytes acquire their stellate shape during the process of changing from osteoblasts in bone. Throughout this process, dynamic cytoskeletal changes occur. In general, changes of the cytoskeleton affect cellular mechanical properties. Mechanical properties of living cells are connected with their biological functions and physiological processes. In this study, we for the first time analyzed elastic modulus, a mechanical property of bone cells. Bone cells in embryonic chick calvariae and in isolated culture were identified using fluorescently labeled phalloidin and OB7.3, a chick osteocyte-specific monoclonal antibody, and then observed by confocal laser scanning microscopy. The elastic modulus of living cells was analyzed with atomic force microscopy. To examine the consequences of focal adhesion formation on the elastic modulus, cells were pretreated with GRGDS and GRGES, and then the elastic modulus of the cells was analyzed. Focal adhesions in the cells were visualized by immunofluorescence of vinculin. From fluorescence images, we could distinguish osteoblasts, osteoid osteocytes and mature osteocytes both in vivo and in vitro. The elastic modulus of peripheral regions of cells in all three populations was significantly higher than in their nuclear regions. The elastic modulus of the peripheral region of osteoblasts was 12053+/-934 Pa, that of osteoid osteocytes was 7971+/-422 Pa and that of mature osteocytes was 4471+/-198 Pa. These results suggest that the level of elastic modulus of bone cells was proportional to the stage of changing from osteoblasts to osteocytes. The focal adhesion area of osteoblasts was significantly higher than that of osteocytes. The focal adhesion area of osteoblasts was decreased after treatment with GRGDS, however, that of osteocytes was not. The elastic modulus of osteoblasts and osteoid osteocytes were decreased after treatment with GRGDS. However, that of mature osteocytes was not changed. There were dynamic changes in the mechanical property of elastic modulus and in focal adhesions of bone cells.

Focal adhesion interactions with topographical structures: a novel method for immuno-SEM labelling of focal adhesions in S-phase cells

Current understanding of the mechanisms involved in osseointegration following implantation of a biomaterial has led to adhesion quantification being implemented as an assay of cytocompatibility. Such measurement can be hindered by intra-sample variation owing to morphological changes associated with the cell cycle. Here we report on a new scanning electron microscopical method for the simultaneous immunogold labelling of cellular focal adhesions and S-phase nuclei identified by BrdU incorporation. Prior to labelling, cellular membranes are removed by tritonization and antigens of non-interest blocked by serum incubation. Adhesion plaque-associated vinculin and S-phase nuclei were both separately labelled with a 1.4 nm gold colloid and visualized by subsequent colloid enhancement via silver deposition. This study is specifically concerned with the effects microgroove topographies have on adhesion formation in S-phase osteoblasts. By combining backscattered electron (BSE) imaging with secondary electron (SE) imaging it was possible to visualize S-phase nuclei and the immunogold-labelled adhesion sites in one energy 'plane' and the underlying nanotopography in another. Osteoblast adhesion to these nanotopographies was ascertained by quantification of adhesion complex formation.

DNA nanoparticles encapsulated in 3D tissue-engineered scaffolds enhance osteogenic differentiation of mesenchymal stem cells

In this study, we enhanced the expression of a plasmid DNA in mesenchymal stem cells (MSC) by the combination of three-dimensional (3D) tissue-engineered scaffold and nonviral gene carrier. To function as an enhanced delivery of plasmid DNA, acetic anhydride was reacted with polyethylenimine (PEI) to acetylate 80% of the primary and 20% of the secondary amines (PEI-Ac(80)). This acetylated PEI has been demonstrated to show enhanced gene-delivery efficiency over unmodified PEI. Collagen sponges reinforced by incorporating of poly(glycolic acid) (PGA) fibers were used as the scaffold material. DNA nanoparticles formed through simple mixing of plasmid DNA encoding bone morphogenetic protein-2 (BMP-2) and PEI-Ac(80) solutions were encapsulated within these scaffolds. MSC were seeded into each scaffold and cultured for several weeks. Within these scaffolds, the level of BMP-2 expression by transfected MSC was significantly enhanced compared to MSC transfected by DNA nanoparticles in solution (in 2D tissue culture plates). Homogeneous bone formation was histologically observed throughout the sponges seeded with transfected MSC by using DNA nanoparticles after subcutaneous implantation into the back of rats. The level of alkaline phosphatase activity and osteocalcin content at the implanted sites of sponges seeded with transfected MSC by using DNA nanoparticles were significantly higher when compared with those seeded with other agents.

Circumferential vibration of microtubules with long axial wavelength

This paper uses an orthotropic shell model to investigate in detail the long axial wavelength circumferential vibration of microtubules (MTs). The deformation patterns in the vibrations were explored and their phonon dispersion relations were presented for MTs with increasing radius. It was shown that with the growth of the axial wavelength, the associated frequency of MTs would finally approach a nonzero asymptotic value, rising considerably with the increase of circumferential wave number but dropping linearly with the growing radius. This study corrects the previous misunderstanding drawn by an oversimplified model, and points out that a parabolic dispersion law does not apply to the circumferential modes when the MT bending stiffness is properly considered.

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

Methods for studying mechanical control of angiogenesis by the cytoskeleton and extracellular matrix

Mechanical forces that capillary endothelial cells generate in their cytoskeleton and exert on their extracellular matrix adhesions feed back to modulate cell sensitivity to soluble angiogenic factors, and thereby control vascular development. Here we describe various genetic, biochemical, and engineering methods that can be used to study, manipulate, and probe this physical mechanism of developmental control. These techniques are useful as in vitro angiogenesis models and for analyzing the molecular and biophysical basis of vascular control.

Cell-cycle progression without an intact microtuble cytoskeleton

For mammalian somatic cells, the importance of microtubule cytoskeleton integrity during interphase cell-cycle progression is uncertain. The loss, suppression, or stabilization of the microtubule cytoskeleton has been widely reported to cause a G1 arrest in a variable, and often high, proportion of cell populations, suggesting the existence of a "microtubule damage," "microtubule integrity," or "postmitotic" checkpoint in G1 or G2. We found that when normal human cells (hTERT RPE1 and primary fibroblasts) are continuously exposed to nocodazole, they remain in mitosis for 10-48 hr before they slip out of mitosis and arrest in G1; this finding is consistent with previous reports. To eliminate the persistent effects of prolonged mitosis, we isolated anaphase-telophase cells that were just finishing a mitosis of normal duration, then we rapidly and completely disassembled microtubules by chilling the preparations to 0 degrees C for 10 minutes in the continuous presence of nocodazole or colcemid treatment to ensure that the cells entered G1 without a microtubule cytoskeleton. Without microtubules, cells progressed from anaphase to a subsequent mitosis with essentially normal kinetics. Similar results were obtained for cells in which the microtubule cytoskeleton was partially diminished by lower nocodazole doses or augmented and stabilized with taxol. Thus, after a preceding mitosis of normal duration, the integrity of the microtubule cytoskeleton is not subject to checkpoint surveillance, nor is it required for the normal human cell to progress through G1 and the remainder of interphase.

Pseudolaric acid B induces apoptosis, senescence, and mitotic arrest in human breast cancer MCF-7

AIM

The aim of the present study was to investigate the inhibitory effect of pseudolaric acid B (PAB) on human breast cancer MCF-7 cells.

METHODS

3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis, morphological changes, acridine orange staining, and agarose gel electrophoresis were applied to detect apoptosis. The percentage of apoptotic and necrotic cells was calculated by the lactate dehydrogenase activity-based cytotoxicity assay; senescence associated (SA)-beta-galactosidase activity was detected to evaluate senescence; flow cytometric analysis of propidium iodide staining was carried out to investigate the distribution of cell cycle, and the protein expression was examined by Western blot analysis.

RESULTS

During apoptosis, the half maximal inhibitory concentration IC(50)was 3.4 and 1.35 micromol/L at 36 and 48 h after PAB treatment, respectively. The MCF-7 cells exposed to PAB showed typical characteristics of apoptosis, including the morphological changes and DNA fragmentation. The MCF-7 cells treated with 4 micromol/L PAB for 36 h underwent apoptosis, but not necrosis. The apoptosis induced by PAB was independent of the death receptor pathway. The senescent cells became larger and flatter, and the SA-beta-galactosidase staining was positive. PAB induced obvious mitotic arrest and it preceded apoptosis and senescence. The expressions of p21 and p53 was upregulated with PAB treatment, and cyclin B1 was upregulated and transported from the cytoplasm to nuclei, and sustained stable levels.

CONCLUSION

PAB induced mitotic arrest in the MCF-7 cells and inhibited proliferation through apoptosis and senescence. The apoptosis was independent of the death receptor pathway.

Cell shape-dependent Control of Ca2+ influx and cell cycle progression in Swiss 3T3 fibroblasts

The ability of adherent cells such as fibroblasts to enter the cell cycle and progress to S phase is strictly dependent on the extent to which individual cells can attach to and spread on a substratum. Here we have used microengineered adhesive islands of 22 and 45 mum diameter surrounded by a nonadhesive substratum of polyhydroxyl methacrylate to accurately control the extent to which individual Swiss 3T3 fibroblasts may spread. The effect of cell shape on mitogen-evoked Ca2+ signaling events that accompany entry into the cell cycle was investigated. In unrestricted cells, the mitogens bombesin and fetal calf serum evoked a typical biphasic change in the cytoplasmic free Ca2+ concentration. However, when the spreading of individual cells was restricted, such that progression to S phase was substantially reduced, both bombesin and fetal calf serum caused a rapid transient rise in the cytoplasmic free Ca2+ concentration but failed to elicit the normal sustained influx of Ca2+ that follows Ca2+ release. As expected, restricting cell spreading led to the loss of actin stress fibers and the formation of a ring of cortical actin. Restricting cell shape did not appear to influence mitogen-receptor interactions, nor did it influence the presence of focal adhesions. Because Ca2+ signaling is an essential component of mitogen responses, these findings implicate Ca2+ influx as a necessary component of cell shape-dependent control of the cell cycle.

Using Angiogenesis in Chronic Wound Care with Becaplermin and Oxidized Regenerated Cellulose/Collagen

For most of the last century, chronic wound care was a practice of passive techniques, designed to prevent the progression of the wound. In the last decade, however, advanced techniques have focused on improving the wound at the molecular level to accelerate wound healing. Successful modalities include tissue-engineered products, hyperbaric oxygen, negative pressure therapy, electrical stimulation, and recombinant growth factors. This shift in the treatment of wound care saw the development of a recombinant human platelet-derived growth factor, becaplermin, which stimulates granulation and increases the incidence of complete wound closure. Another product is oxidized regenerated cellulose/collagen, which protects growth factors and granulation tissue by inhibiting wound proteases. Used together, an optimal environment for wound healing can be created.

Synergistic antiangiogenic effects of stathmin inhibition and taxol exposure

Stathmin is one of the key regulators of the microtubule cytoskeleton and the mitotic spindle in eukaryotic cells. It is expressed at high levels in a wide variety of human cancers and may provide an attractive target for cancer therapy. We had previously shown that stathmin inhibition results in the abrogation of the malignant phenotype. The microtubule-interfering drug, taxol, has both antitumorigenic and antiangiogenic properties. We had also shown that the antitumor activities of taxol and stathmin inhibition are synergistic. We hypothesized that taxol and stathmin inhibition may also have synergistic antiangiogenic activities. A replication-deficient bicistronic adenoviral vector that coexpresses green fluorescent protein and an anti-stathmin ribozyme was used to target stathmin mRNA. Exposure of endothelial cells to anti-stathmin adenovirus alone resulted in a dose-dependent inhibition of proliferation, migration, and differentiation into capillary-like structures. This inhibition was markedly enhanced by exposure of transduced endothelial cells to very low concentrations of taxol, which resulted in a virtually complete loss of proliferation, migration, and differentiation of endothelial cells. In contrast, exposure of nontransduced endothelial cells to taxol alone resulted in a modest inhibition of proliferation, migration, and differentiation. Our detailed analysis showed that the antiangiogenic effects of the combination of stathmin inhibition and taxol exposure are synergistic. Our studies also showed that the mechanism of this synergistic interaction is likely to be mediated through the stabilization of microtubules. Thus, this novel combination may provide an attractive therapeutic strategy that combines a synergistic antitumor activity with a synergistic antiangiogenic activity.

Real-time detection of stress in 3D tissue-engineered constructs using NF-kappaB activation in transiently transfected human dermal fibroblast cells

The main objective of this study was to develop a nondestructive reporter system for assessing the response of human cells contained within a three-dimensional (3D) tissue-engineered construct to exogenous stress. Dermal fibroblasts were transiently transfected with a reporter construct linked to nuclear factor kappaB (NF-kappaB) activation which led to expression of a nonstable form of enhanced green fluorescent protein (d2EGFP) after stimulation. This led to a temporary production of fluorescence, which could be readily detected but was not intrinsically toxic, as cells were able to metabolize the initial cycle of d2EGFP produced. This permitted the model to be used for restimulation post recovery. To investigate the performance and predictive ability of this method for assessing cellular response to stress in 3D, we used a range of compounds known to have pro-inflammatory or oxidative properties. Tumor necrosis factor-alpha (TNF-alpha) and interleukin-1-beta (IL-1beta) were selected for having a direct cytokine action; lipopolysaccharide (LPS) was selected for modeling bacterial-mediated inflammation; and hydrogen peroxide was selected as a crude method for delivering an oxidative stress. Transfected cells were stimulated with the above compounds in 3D and the synthesis of d2EGFP was detected as a measure of NF-kappaB activation. The resultant fluorescence was scored using a series of photomicrographs taken by epifluorescence microscopy. All agents activated NF-kappaB when cells were grown in 3D scaffolds but did not cause any significant reduction in cell viability as measured by a standard MTT-ESTA viability test. Parallel NF-kappaB activation and MTT measurements was also conducted in two-dimension (2D) and confirmed findings in 3D. The 3D model described using a fluorescent reporter gene is a highly sensitive and reliable method for detecting cellular stress and represents a key step in developing tissue engineering models with the potential for screening pharmaceutical and cosmetic compounds, as an alternative to existing in vitro and in vivo methods.

Hela l-CaD is implicated in the migration of endothelial cells/endothelial progenitor cells in human neoplasms

Caldesmon (CaD) is a major actin-binding protein distributed in a variety of cell types. No functional differences among the isoforms in in vitro studies were found so far. In a previous study we found that the low molecular caldesmon isoform (Hela l-CaD) is expressed in endothelial cells (ECs)/endothelial progenitor cells (EPCs) in tumor vasculature of various human tumors. Activation of cell motility is necessary for the navigation of the tip ECs during angiogenesis, and migration of EPCs from the bone marrow during vasculogenesis. In the present study we searched for features of motility and the intracellular expression sites of Hela l-CaD in ECs/EPCs of various human tumors under histologically preserved microenviroment. We discovered a variety of motility-related cell protrusions like filopodia, microspikes, lamellipodia, podosomes, membrane blebs and membrane ruffles in the activated ECs/EPCs. Hela l-CaD appeared to be invariably expressed in the subregions of these cell protrusions. The findings suggest that Hela l-CaD is implicated in the migration of ECs/EPC in human neoplasms where they contribute to tumor vasculogenesis and angiogenesis.

Cell cycle progression and de novo centriole assembly after centrosomal removal in untransformed human cells

How centrosome removal or perturbations of centrosomal proteins leads to G1 arrest in untransformed mammalian cells has been a mystery. We use microsurgery and laser ablation to remove the centrosome from two types of normal human cells. First, we find that the cells assemble centrioles de novo after centrosome removal; thus, this phenomenon is not restricted to transformed cells. Second, normal cells can progress through G1 in its entirety without centrioles. Therefore, the centrosome is not a necessary, integral part of the mechanisms that drive the cell cycle through G1 into S phase. Third, we provide evidence that centrosome loss is, functionally, a stress that can act additively with other stresses to arrest cells in G1 in a p38-dependent fashion.

Putative role of heparan sulfate proteoglycan expression and shedding on the proliferation and survival of cells after photodynamic therapy

INTRODUCTION

Photodynamic therapy is based on the selective retention of a photosensitizer by highly proliferating cells and its activation with light at the appropriate wavelength. This combination generates reactive oxygen species that ultimately kill the cells. Some cells, however, may survive photodynamic therapy and the interaction of these cells with the extracellular matrix has profound effect in tumor biology. The knowledge of photodynamic therapy action on the extracellular matrix has not been fully explored. It has been focused mainly on integrins, matrix metalloproteinases and on growth factors and immunological mediators. Other important molecules involved in the regulation of many cell processes are the glycosaminoglycans, polymers of disaccharide units, present on the cell surface and in the extracellular matrix. In most cases, the glycosaminoglycans occur as proteoglycans.

AIMS

The purpose of the present investigation is to evaluate heparan sulfate proteoglycan expression and shedding, and its relation to the survival of the remaining cells, after a liposomal-AlClPc based photodynamic treatment.

MATERIALS

A wild-type endothelial cell derived from rabbit aorta and its counterpart transfected with EJ-ras oncogene were used.

RESULTS

Both cell lines presented augmented heparan sulfate proteoglycan syndecan-4 mRNA expression, augmented synthesis of heparan sulfate chains and increased shedding. Also, the formation of stress fibers on the border of the cells and the arrest in G(1) phase of the cell cycle was observed.

CONCLUSIONS

These results show that surviving cells after photodynamic therapy exhibit changes in their morphology and cell processes that differ from that of non-treated cells, and these changes are probably hindering the cells from resuming normal proliferation.

Effect of the cell type and cell density on the binding of living cells to a silica particle: An atomic force microscope study

We used the atomic force microscope to study how the cell type and the density of cells adsorbed at a substrate can affect the adhesion between a living cell and a model drug delivery system (DDS) carrier nano-particle. We used three different anchorage-dependent cells, i.e., a living mouse fibroblast cell (L929), a living human colon cancer cell (Caco2), and a living mouse malignant melanoma cell (B16F10). For the DDS model nano-particle, we used a silica colloid. In order to correlate the adhesion force with the cell types, the growth curve of the cells were determined with a haemocytometer. The shapes of the cells at the different stages were monitored by light microscopy, and the morphology of their surfaces obtained by tapping mode atomic force microscopy. Force measurements showed that the Caco2 cell bound little to a silica particle, regardless of the cell density. The L929 cell bound well to a silica particle for low and high cell densities. The B16F10 cell bound little to a silica particle for low cell densities, but bound well for high cell densities. AFM images showed that the L929 cell did not contain folds. The B16F10 cells, however, displayed folds in the cell surface for low cell densities, but no folds in the cell for high cell densities. As literature also reported that the Caco2 cell contains folds, these results suggested that cells with folds showed less adhesion to a silica particle than cells without folds. The presence of folds in the cell presumably decreased the number of sites on the cell that could hydrogen bond or undergo van der Waals binding with the silanol groups of the silica particle.

Reduction of anabolic signals and alteration of osteoblast nuclear morphology in microgravity

Bone loss has been repeatedly documented in astronauts after flight, yet little is known about the mechanism of bone loss in space flight. Osteoblasts were activated during space flight in microgravity (microg) with and without a 1 gravity (1 g) field and 24 genes were analyzed for early induction. Induction of proliferating cell nuclear antigen (PCNA), transforming growth factor beta (TGFbeta), cyclo-oxygenase-2 (cox-2), cpla2, osteocalcin (OC), c-myc, fibroblast growth factor-2 (fgf-2), bcl2, bax, and fgf-2 message as well as FGF-2 protein were significantly depressed in microg when compared to ground (gr). Artificial onboard gravity normalized the induction of c-myc, cox-2, TGFbeta, bax, bcl2, and fgf-2 message as well as FGF-2 protein synthesis in spaceflight samples. In normal gravity, FGF-2 induces bcl2 expression; we found that bcl2 expression was significantly reduced in microgravity conditions. Since nuclear shape is known to elongate in the absence of mitogens like FGF-2, we used high-resolution image-based morphometry to characterize changes in osteoblast nuclear architecture under microgravity, 1 g flight, and ground conditions. Besides changes in cell shape (roundish/elliptic), other high-resolution analyses show clear influences of gravity on the inner nuclear structure. These changes occur in the texture, arrangement, and contrast of nuclear particles and mathematical modeling defines the single cell classification of the osteoblasts. Changes in nuclear structure were evident as early as 24 h after exposure to microgravity. This documented alteration in nuclear architecture may be a direct result of decreased expression of autocrine and cell cycle genes, suggesting an inhibition of anabolic response in microg. Life on this planet has evolved in a normal gravity field and these data suggest that gravity plays a significant role in regulation of osteoblast transcription.

NPI-2358 is a tubulin-depolymerizing agent: in-vitro evidence for activity as a tumor vascular-disrupting agent

The diketopiperazine NPI-2358 is a synthetic analog of NPI-2350, a natural product isolated from Aspergillus sp., which depolymerizes microtubules in A549 human lung carcinoma cells. Although structurally different from the colchicine-binding site agents reported to date, NPI-2358 binds to the colchicine-binding site of tubulin. NPI-2358 has potent in-vitro anti-tumor activity against various human tumor cell lines and maintains activity against tumor cell lines with various multidrug-resistant (MDR) profiles. In addition, when evaluated in proliferating human umbilical vein endothelial cells (HUVECs), concentrations as low as 10 nmol/l NPI-2358 induced tubulin depolymerization within 30 min. Furthermore, NPI-2358 dose dependently increases HUVEC monolayer permeability--an in-vitro model of tumor vascular collapse. NPI-2358 was compared with three tubulin-depolymerizing agents with vascular-disrupting activity: colchicine, vincristine and combretastatin A-4 (CA4). Results showed that the activity of NPI-2358 in HUVECs was more potent than either colchicine or vincristine; the profile of CA4 approached that of NPI-2358. Altogether, our data show that NPI-2358 is a potent anti-tumor agent which is active in MDR tumor cell lines, and is able to rapidly induce tubulin depolymerization and monolayer permeability in HUVECs. These data warrant further evaluation of NPI-2358 as a vascular-disrupting agent in vivo. Currently, NPI-2358 is in preclinical development for the treatment of cancer.

Combination of 3D tissue engineered scaffold and non-viral gene carrier enhance in vitro DNA expression of mesenchymal stem cells

The objective of this study is to enhance the expression of a plasmid DNA for mesenchymal stem cells (MSC) by combination of 3-dimensional (3D) tissue engineered scaffolds and non-viral gene carrier. As a carrier of plasmid DNA, dextran-spermine cationic polysaccharide was prepared by means of reductive-amination between oxidized dextran and the natural oligoamine, spermine. As the MSC scaffold, collagen sponges reinforced by incorporation of poly(glycolic acid) (PGA) fibers were used. A complex of the cationized dextran and plasmid DNA of BMP-2 was impregnated into the scaffolds. MCS were seeded into each scaffold and cultured by a 3D culture method. When MSC were cultured in the PGA-reinforced sponge, the level of BMP-2 expression was significantly enhanced by the cationized dextran-plasmid DNA complex impregnated into the scaffold than by the cationized dextran-plasmid DNA complex in 2-dimensional (2D) (tissue culture plate) culture method. The alkaline phosphatase activity and osteocalcin content of transfected MSC cultured in the PGA-reinforced sponge were significantly higher compared with 2D culture method. We conclude that combination of cationized dextran plasmid DNA complex and 3D tissue engineered scaffold was promising to promote the in vitro gene expression for MSC.

Osteogenic differentiation of mesenchymal stem cells in self-assembled peptide-amphiphile nanofibers

The proliferation and differentiation of mesenchymal stem cells (MSC) was investigated in a three dimensional (3-D) network of nanofibers formed by self-assembly of peptide-amphiphile (PA) molecules. PA was synthesized by standard solid phase chemistry that ends with the alkylation of the NH(2) terminus of the peptide. The sequence of arginine-glycine-aspartic acid (RGD) was included in peptide design as well. A 3-D network of nanofibers was formed by mixing cell suspensions in media with dilute aqueous solution of PA. Scanning electron microscopy (SEM) observation revealed the formation of fibrous assemblies with an extremely high aspect ratio and high surface areas. When rat MSC were seeded into the PA nanofibers with or without RGD, larger number of cells attached was observed in the PA nanofibers including RGD. When measured to evaluate the osteogenic differentiation of MSC, the alkaline phosphatase (ALP) activity and osteocalcin content became maximum for the PA nanofibers including RGD compared with those without RGD, although both the values were significantly higher compared with those in the static tissue culture plate (2-D culture). We concluded that the attachment, proliferation, and osteogenic differentiation of MSC were influenced by PA nanofibers as the cell scaffold.

Pseudolarix acid B, a new tubulin-binding agent, inhibits angiogenesis by interacting with a novel binding site on tubulin

Tubulin-binding agents have received considerable interest as potential tumor-selective angiogenesis-targeting drugs. Herein, we report that pseudolarix acid B (PAB), isolated from the traditional Chinese medicinal plant Pseudolarix kaempferi Gordon, is a tubulin-binding agent. We further demonstrate that PAB significantly and dose-dependently inhibits proliferation, migration, and tube formation by human microvessel enthothelial cells. It is noteworthy that PAB eliminated newly formed endothelial tubes and microvessels both in vitro and in vivo. In addition, PAB dramatically arrested the cell cycle at G2/M phase. PAB also induced endothelial cell retraction, intercellular gap formation, and promoted actin stress fiber formation in conjunction with disruption of the tubulin and actin cytoskeletons. All of these effects occurred at noncytotoxic concentrations of PAB. We found that these effects of PAB are attributable to depolymerization of tubulin by direct interaction with a distinct binding site on tubulin compared with those of colchicine and vinblastine. Taken together, these findings show that PAB is a candidate antiangiogenic agent for use in cancer therapy, and they provide proof of principle for targeting this novel binding site on tubulin as a new strategy for treating cancer.

Human fibroblast reactions to standard and electropolished titanium and Ti-6Al-7Nb, and electropolished stainless steel

Stainless steel (SS), titanium (cpTi), and Ti-6Al-7Nb (TAN) are frequently used metals in orthopedic internal fracture fixation. Although reactivity to SS and cpTi are noted in reference, the soft tissue compatibility of TAN has not been comprehensively studied. This study focuses on the in vitro soft tissue compatibility of TAN in comparison to SS and cpTi using a human fibroblast model. The industrial standard surface finishes of these three materials vary considerably in view of their use in similar applications. To distinguish between material parameters of topography and chemistry, we have included electropolished (e.p) counterparts of the standard preparations of cpTi and TAN in the study (standard SS is e.p). All materials were characterized using atomic force microscopy, profilometry, and scanning electron microscopy. Our findings demonstrate that cell morphology and growth rate was similar for SS, and e.p. cpTi and TAN, with cells well spread and forming a confluent monolayer by 10 days. Cell growth on standard cpTi was similar to the electropolished samples; however, they showed a less spread morphology with more filopodia and surface ruffling present. Cell morphology on standard TAN was rounded or elongated and proliferation was inhibited at all time points, with possible cell necrosis by day 10. We found evidence of endocytosis of beta-phase particles originating from the standard TAN surface. We believe that the particle uptake coupled with the characteristic surface topography contribute to the noncytocompatibility of fibroblasts on standard TAN.

Culture of skin cells in 3D rather than 2D improves their ability to survive exposure to cytotoxic agents

In this study, we asked the question of whether cells in 3D culture cope more effectively with cytotoxic agents than cells in 2D. The sensitivities of human skin cells (keratinocytes, dermal fibroblasts and endothelial cells) to oxidative stress (hydrogen peroxide) and to a potentially toxic heavy metal (silver) when cultured under 2D and 3D conditions were investigated. The results show a marked resistance of cells to a given dose of hydrogen peroxide or silver nitrate causing a 50% loss of viability in 3D cultures, when compared to the same cells grown in 2D. There was also an improvement in the ability of cells to withstand both stresses when cells were in co-culture rather than in mono-culture. Foetal calf serum was found to have a mild protective effect in 2D culture but this was not extended to findings in 3D culture. This study suggests that dermatotoxicity testing using 3D co-cultures might be more likely to reflect true physiological responses to xenobiotic materials than existing models that rely on 2D mono-cultures.

Endothelial-cell apoptosis induced by cleaved high-molecular-weight kininogen (HKa) is matrix dependent and requires the generation of reactive oxygen species

High-molecular-weight kininogen (HK) is an abundant plasma protein that plays a central role in activation of the kallikrein-kinin system. Cleavage of HK by plasma kallikrein results in release of the nonapeptide bradykinin (BK), leaving behind cleaved high-molecular-weight kininogen (HKa). Previous studies have demonstrated that HKa induces apoptosis of proliferating endothelial cells and inhibits angiogenesis in vivo, activities mediated primarily through its domain 5. However, the mechanisms by which these effects occur are not well understood. Here, we demonstrate that HKa induces apoptosis of endothelial cells cultured on gelatin, vitronectin, fibronectin, or laminin but not collagen type I or IV. The ability of HKa to induce endothelial-cell apoptosis is dependent on the generation of intracellular reactive oxygen species and associated with depletion of glutathione and peroxidation of endothelial-cell lipids, effects that occur only in cells cultured on matrix proteins permissive for HKa-induced apoptosis. Finally, the ability of HKa to induce endothelial-cell apoptosis is blocked by the addition of reduced glutathione or N-acetylcysteine. These studies demonstrate a unique role for oxidant stress in mediating the activity of an antiangiogenic polypeptide and highlight the importance of the extracellular matrix in regulating endothelial-cell survival.

Perfusion culture enhances osteogenic differentiation of rat mesenchymal stem cells in collagen sponge reinforced with poly(glycolic Acid) fiber

The objective of this study was to obtain fundamental knowledge about in vitro culture systems to enhance the proliferation and differentiation of mesenchymal stem cells (MSCs) in collagen sponge reinforced by the incorporation of poly(glycolic acid) (PGA) fiber. A collagen solution with PGA fiber homogeneously localized at PGA:collagen weight ratios of 0.67, 1.25, 2.5, and 5 was freezedried, followed by cross-linking of combined dehydrothermal, glutaraldehyde, and ultraviolet treatment. Scanning electron microscopy revealed that collagen sponges exhibited homogeneous and interconnected pore structures with an average size of 180 microm, irrespective of PGA fiber incorporation. When rat MSCs were seeded into collagen sponge with or without PGA fiber incorporation, more attached cells were observed in collagen sponge incorporating PGA fiber than in collagen sponge without PGA fiber incorporation, irrespective of the PGA:collagen ratio. The proliferation and osteogenic differentiation of MSCs in PGA-reinforced sponge at a weight ratio of 5 were greatly influenced by the culture method and growth conditions. Alkaline phosphatase (ALP) activity and osteocalcin content of MSCs cultured in PGA-reinforced sponge by the perfusion method became maximum at a flow rate of 0.2 mL/min, although they increased with culture time period. It may be concluded that appropriate perfusion conditions enable MSCs to positively improve the extent of proliferation and differentiation.

Critical role of actin in modulating BBB permeability

A major obstacle in the treatment of degenerative manifestations and debilitating diseases in the central nervous system (CNS) lies in the impediment of drug delivery into these tissues. The impediment is due to a membrane barrier referred to as the blood-brain barrier (BBB). It is known that the BBB is a unique membranous structure in brain capillaries that tightly segregates the brain from systemic blood circulation. It is imperative to have a thorough understanding of the molecular components and their integrated function of this barrier to develop effective therapeutics for CNS disorders and diseases. Although there are other cell and biochemical properties that underlie this barrier function, it is well established that the barrier is mainly made up of the physical elements of tight junction (TJ) complex. The major constituents of TJ, such as occludin, claudins, zonula occludens (ZOs) and junctional adhesion molecule (JAM) have been subjects of intensive studies and reviews. However, after examining currently proposed models, we have come to believe that a cytoskeletal component-actin may play a critical role in interacting TJ molecular constituents and modulating functional TJ complex. In this review, we will discuss the correlation of temporal and spatial distribution and remodeling of actin filaments with altering integrity of TJ complexes in various systems and present a hypothesis to depict its potential role in modulating BBB permeability.

Plasmid delivery in vivo from porous tissue-engineering scaffolds: transgene expression and cellular transfection

Tissue engineering scaffolds capable of sustained plasmid release can promote gene transfer locally and stimulate new tissue formation. We have investigated the scaffold design parameters that influence the extent and duration of transgene expression and have characterized the distribution of transfected cells. Porous scaffolds with encapsulated plasmid were fabricated from poly(lactide-co-glycolide) with a gas foaming procedure, with wet granulation employed to mix the components homogeneously prior to foaming. Wet granulation enhanced plasmid incorporation relative to standard procedures and also enhanced in vivo transgene expression, possibly through the increased loading and maintenance of the scaffold pore structure. The plasmid loading regulated the quantity and duration of transgene expression, with expression for 105 days achieved at the highest dosage. Expression was localized to the implantation site, though the distribution of transfected cells varied with time. Transfected cells were initially observed at the scaffold periphery (day 3), then within the pores and adjacent to the polymer (day 17), and finally throughout the scaffold interior (day 126). Delivery of a plasmid encoding VEGF increased the blood vessel density relative to control. Correlating scaffold design with gene transfer efficiency and tissue formation will facilitate application of plasmid-releasing scaffolds to multiple tissues.

How actin crosslinking and bundling proteins cooperate to generate an enhanced cell mechanical response

Actin-crosslinking proteins organize actin filaments into dynamic and complex subcellular scaffolds that orchestrate important mechanical functions, including cell motility and adhesion. Recent mutation studies have shown that individual crosslinking proteins often play seemingly non-essential roles, leading to the hypothesis that they have considerable redundancy in function. We report live-cell, in vitro, and theoretical studies testing the mechanical role of the two ubiquitous actin-crosslinking proteins, alpha-actinin and fascin, which co-localize to stress fibers and the basis of filopodia. Using live-cell particle tracking microrheology, we show that the addition of alpha-actinin and fascin elicits a cell mechanical response that is significantly greater than that originated by alpha-actinin or fascin alone. These live-cell measurements are supported by quantitative rheological measurements with reconstituted actin filament networks containing pure proteins that show that alpha-actinin and fascin can work in concert to generate enhanced cell stiffness. Computational simulations using finite element modeling qualitatively reproduce and explain the functional synergy of alpha-actinin and fascin. These findings highlight the cooperative activity of fascin and alpha-actinin and provide a strong rationale that an evolutionary advantage might be conferred by the cooperative action of multiple actin-crosslinking proteins with overlapping but non-identical biochemical properties. Thus the combination of structural proteins with similar function can provide the cell with unique properties that are required for biologically optimal responses.

Impregnation of Plasmid DNA into Three-Dimensional Scaffolds and Medium Perfusion Enhancein VitroDNA Expression of Mesenchymal Stem Cells

This article describes the development of an in vitro culture system to enhance the expression of a plasmid DNA for mesenchymal stem cells (MSCs) by a combination of plasmid DNA impregnation into three-dimensional cell scaffolds and culture methods. Gelatin was cationized by introducing spermine to the carboxyl groups for complexation with the plasmid DNA. As the MSC scaffold, poly(glycolic acid) (PGA) fiber fabrics, collagen sponges, and collagen sponges reinforced by incorporation of PGA fibers were used. A complex of cationized gelatin and plasmid DNA encoding bone morphogenetic protein 2 (BMP-2) was impregnated into the scaffolds. Plasmid DNA was released from PGA-reinforced collagen sponge for longer than from the other scaffolds. MCS were seeded into each type of scaffold and cultured by static, stirring, and perfusion methods. When MSCs were cultured in PGA-reinforced sponge, the level of BMP-2 expression was significantly enhanced by perfusion culture compared with the other culture methods, and the time of expression was prolonged. Irrespective of the culture method, the expression level was significantly higher from plasmid DNA impregnated in scaffold than by plasmid DNA in medium. The alkaline phosphatase activity and osteocalcin content of MSCs cultured in PGA-reinforced sponge by the perfusion method were significantly higher compared with those of other methods, and a significantly higher amount of plasmid DNA internalized into MSCs was observed. We conclude that a combination of plasmid DNA-impregnated PGA-reinforced sponge and the perfusion method was promising to promote in vitro gene expression for MSCs.

Inhibiting myosin light chain kinase induces apoptosis in vitro and in vivo

Previous short-term studies have correlated an increase in the phosphorylation of the 20-kDa light chain of myosin II (MLC20) with blebbing in apoptotic cells. We have found that this increase in MLC20 phosphorylation is rapidly followed by MLC20 dephosphorylation when cells are stimulated with various apoptotic agents. MLC20 dephosphorylation is not a consequence of apoptosis because MLC20 dephosphorylation precedes caspase activation when cells are stimulated with a proapoptotic agent or when myosin light chain kinase (MLCK) is inhibited pharmacologically or by microinjecting an inhibitory antibody to MLCK. Moreover, blocking caspase activation increased cell survival when MLCK is inhibited or when cells are treated with tumor necrosis factor alpha. Depolymerizing actin filaments or detaching cells, processes that destabilize the cytoskeleton, or inhibiting myosin ATPase activity also resulted in MLC20 dephosphorylation and cell death. In vivo experiments showed that inhibiting MLCK increased the number of apoptotic cells and retarded the growth of mammary cancer cells in mice. Thus, MLC20 dephosphorylation occurs during physiological cell death and prolonged MLC20 dephosphorylation can trigger apoptosis.

Osteoblast Elastic Modulus Measured by Atomic Force Microscopy Is Substrate Dependent

The actin and microtubule cytoskeleton have been found to contribute to the elastic modulus of cells, which may be modulated by adhesion to extracellular matrix (ECM) proteins and subsequent alterations in the cytoskeleton. In this study, the apparent elastic modulus (Eapp) of osteoblast-like MC3T3-E1 cells adhered to fibronectin (FN), vitronectin (VN), type I collagen (COLI), fetal bovine serum (FBS), or poly-l-lysine (PLL), and bare glass were determined using an atomic force microscope (AFM). The E(app) of osteoblasts adhered to ECM proteins (FN, VN, COLI, and FBS) that bind cells via integrins were higher compared to cells on glass and PLL, which adhere cells through nonspecific binding. Also, osteoblasts adhered to FN, VN, COLI, and FBS had F-actin stress fiber formation, while osteoblasts on glass and PLL showed few F-actin fibers. Disruption of the actin cytoskeleton decreased E(app) of osteoblasts plated on FN to the level of osteoblasts plated on glass, while microtubule disruption had no significant effect. This suggests that the elevated modulus of osteoblasts adhered to FN was due to remodeling of the actin cytoskeleton upon adhesion to ECM proteins. Modulation of cell stiffness upon adhesion to various substrates may influence mechanosignal transduction in osteoblasts.

SRC-mediated phosphorylation of focal adhesion kinase couples actin and adhesion dynamics to survival signaling

Integrin-associated focal adhesions not only provide adhesive links between cellular actin and extracellular matrix but also are sites of signal transmission into the cell interior. Many cell responses signal through focal adhesion kinase (FAK), often by integrin-induced autophosphorylation of FAK or phosphorylation by Src family kinases. Here, we used an interfering FAK mutant (4-9F-FAK) to show that Src-dependent FAK phosphorylation is required for focal adhesion turnover and cell migration, by controlling assembly of a calpain 2/FAK/Src/p42ERK complex, calpain activation, and proteolysis of FAK. Expression of 4-9F-FAK in FAK-deficient fibroblasts also disrupts F-actin assembly associated with normal adhesion and spreading. In addition, we found that FAK's ability to regulate both assembly and disassembly of the actin and adhesion networks may be linked to regulation of the protease calpain. Surprisingly, we also found that the same interfering 4-9F-FAK mutant protein causes apoptosis of serum-deprived, transformed cells and suppresses anchorage-independent growth. These data show that Src-mediated phosphorylation of FAK acts as a pivotal regulator of both actin and adhesion dynamics and survival signaling, which, in turn, control apparently distinct processes such as cell migration and anchorage-independent growth. This also highlights that dynamic regulation of actin and adhesions (which include the integrin matrix receptors) is critical to signaling output and biological responses.

Cell stiffness and receptors: evidence for cytoskeletal subnetworks

Viscoelastic models of cells often treat cells as homogeneous objects. However, studies have demonstrated that cellular properties are local and can change dramatically on the basis of the location probed. Because membrane receptors are linked in various ways to the intracellular space, with some receptors linking to the cytoskeleton and others diffusing freely without apparent linkages, the cellular physical response to mechanical stresses is expected to depend on the receptor engaged. In this study, we tested the hypothesis that cellular mechanical stiffness as measured via cytoskeletally linked receptors is greater than stiffness measured via receptors that are not cytoskeletally linked. We used a magnetic micromanipulator to apply linear stresses to magnetic beads attached to living cells via selected receptors. One of the receptor classes probed, the dystroglycan receptors, is linked to the cytoskeleton, while the other, the transferrin receptors, is not. Fibronectin-coated beads were used to test cellular mechanical properties of the cytoskeleton without membrane dependence by allowing the beads to endocytose. For epithelial cells, transferrin-dependent stiffness and endocytosed bead-dependent stiffness were similar, while dystroglycan-dependent stiffness was significantly lower. For smooth muscle cells, dystroglycan-dependent stiffness was similar to the endocytosed bead-dependent stiffness, while the transferrin-dependent stiffness was lower. The conclusion of this study is that the measured cellular stiffness is critically influenced by specific receptor linkage and by cell type and raises the intriguing possibility of the existence of separate cytoskeletal networks with distinct mechanical properties that link different classes of receptors.

Interaction of cortactin and Arp2/3 complex is required for sphingosine-1-phosphate-induced endothelial cell remodeling

Sphingosine-1-phosphate (S1P) induces capillary formation of endothelial cells on Matrigel in accompany with actin assembly and accumulation of cortactin and Arp2/3 complex at the cell-leading edge. Suppression of cortactin expression with a cortactin antisense oligo significantly impaired S1P-induced capillary formation, migration of endothelial cells, and actin assembly at the cell periphery. Overexpression of wild-type cortactin tagged by green fluorescent protein (GFP) increased the S1P-induced tube formation and cell motility, whereas the cells overexpressing the mutant formed poorly capillary network and became less motile in response to S1P. Analysis of distribution in Triton X-100 insoluble fractions demonstrated that the cortactin mutant inhibited the association of wild-type cortactin and Arp2/3 complex with the actin-enriched complex. Furthermore, actin polymerization at and distribution of Arp2/3 complex as well as endogenous cortactin into the cell-leading edge mediated by S1P was disturbed. These data suggest that the interaction between cortactin and Arp2/3 complex plays an important role in S1P-mediated remodeling of endothelial cells.