Patterns of organization of actin and myosin in normal and transformed cultured cells

Cytoplasmic Beta and Gamma Actin Isoforms Reorganization and Regulation in Tumor Cells in Culture and Tissue

The cytoplasmic actin isoforms (β- and γ-actins) contribute greatly to cellular processes such as cel-cell and cell-matrix interactions, as well as cell polarization, motility and division. Distinct isoforms modulations are linked to serious pathologies, so investigations of underlying mechanisms would be of major relevance not only for fundamental research but also for clinical applications. Therefore, the study of the relevant mechanisms of change in the isoform’s balance is important for basic research and for clinical studies. The disruption of actin cytoskeleton and intercellular adhesions contribute to the neoplastic transformation, as it is important for the tumor growth, invasiveness and metastasis. Cytoplasmic actins display the functional diversity: β-actin is responsible for contractility, whereas γ-actin participates in the submembrane flexible cortex organization and direction cell motility. The involvement of β- and γ-actin in cell architecture, motility, division, and adhesion junctions in normal cells is not equivalent, and the major question was following: whether isoform ratio and the distribution in the cell corresponds to pathological function. Significant data were obtained in the study of tumor and normal cells in culture, as well as on clinical material of human tissues, and via selective regulation of β- and γ-actin’s expression. Investigation of the actins’ diversity and function in cancers may help to choose the benefit treatment strategies, and to design new therapies.

Universal glass-forming behavior of in vitro and living cytoplasm

Physiological processes in cells are performed efficiently without getting jammed although cytoplasm is highly crowded with various macromolecules. Elucidating the physical machinery is challenging because the interior of a cell is so complex and driven far from equilibrium by metabolic activities. Here, we studied the mechanics of in vitro and living cytoplasm using the particle-tracking and manipulation technique. The molecular crowding effect on cytoplasmic mechanics was selectively studied by preparing simple in vitro models of cytoplasm from which both the metabolism and cytoskeletons were removed. We obtained direct evidence of the cytoplasmic glass transition; a dramatic increase in viscosity upon crowding quantitatively conformed to the super-Arrhenius formula, which is typical for fragile colloidal suspensions close to jamming. Furthermore, the glass-forming behaviors were found to be universally conserved in all the cytoplasm samples that originated from different species and developmental stages; they showed the same tendency for diverging at the macromolecule concentrations relevant for living cells. Notably, such fragile behavior disappeared in metabolically active living cells whose viscosity showed a genuine Arrhenius increase as in typical strong glass formers. Being actively driven by metabolism, the living cytoplasm forms glass that is fundamentally different from that of its non-living counterpart.

A peptide functionalized poly(ethylene glycol) (PEG) hydrogel for investigating the influence of biochemical and biophysical matrix properties on tumor cell migration

To address the challenges associated with defined control over matrix properties in 3D cell culture systems, we employed a peptide functionalized poly(ethylene glycol) (PEG) hydrogel matrix in which mechanical modulus and adhesive properties were tuned. An HT-1080 human fibrosarcoma cell line was chosen as a model for probing matrix influences on tumor cell migration using the PEG hydrogel platform. HT-1080 speed varied with a complex dependence on both matrix modulus and Cys-Arg-Gly-Asp-Ser (CRGDS) adhesion ligand concentration, with regimes in which motility increased, decreased, or was minimally altered being observed. We further investigated cell motility by forming matrix interfaces that mimic aspects of tissue boundaries that might be encountered during invasion by taking advantage of the spatial control of the thiol-ene photochemistry to form patterned regions of low and high cross-linking densities. HT-1080s in 100 Pa regions of patterned PEG hydrogels tended to reverse direction or aggregate at the interface when they encountered a 360 Pa boundary. In contrast, HT-1080s were apparently unimpeded when migrating from the stiff to the soft regions of PEG peptide hydrogels, which may indicate that cells are capable of "reverse durotaxis" within at least some matrix regimes. Taken together, our results identified matrix regimes in which HT-1080 motility was both positively and negatively influenced by cell adhesion or matrix modulus.

A quantitative comparison of human HT-1080 fibrosarcoma cells and primary human dermal fibroblasts identifies a 3D migration mechanism with properties unique to the transformed phenotype

Here, we describe an engineering approach to quantitatively compare migration, morphologies, and adhesion for tumorigenic human fibrosarcoma cells (HT-1080s) and primary human dermal fibroblasts (hDFs) with the aim of identifying distinguishing properties of the transformed phenotype. Relative adhesiveness was quantified using self-assembled monolayer (SAM) arrays and proteolytic 3-dimensional (3D) migration was investigated using matrix metalloproteinase (MMP)-degradable poly(ethylene glycol) (PEG) hydrogels (“synthetic extracellular matrix” or “synthetic ECM”). In synthetic ECM, hDFs were characterized by vinculin-containing features on the tips of protrusions, multipolar morphologies, and organized actomyosin filaments. In contrast, HT-1080s were characterized by diffuse vinculin expression, pronounced β1-integrin on the tips of protrusions, a cortically-organized F-actin cytoskeleton, and quantitatively more rounded morphologies, decreased adhesiveness, and increased directional motility compared to hDFs. Further, HT-1080s were characterized by contractility-dependent motility, pronounced blebbing, and cortical contraction waves or constriction rings, while quantified 3D motility was similar in matrices with a wide range of biochemical and biophysical properties (including collagen) despite substantial morphological changes. While HT-1080s were distinct from hDFs for each of the 2D and 3D properties investigated, several features were similar to WM239a melanoma cells, including rounded, proteolytic migration modes, cortical F-actin organization, and prominent uropod-like structures enriched with β1-integrin, F-actin, and melanoma cell adhesion molecule (MCAM/CD146/MUC18). Importantly, many of the features observed for HT-1080s were analogous to cellular changes induced by transformation, including cell rounding, a disorganized F-actin cytoskeleton, altered organization of focal adhesion proteins, and a weakly adherent phenotype. Based on our results, we propose that HT-1080s migrate in synthetic ECM with functional properties that are a direct consequence of their transformed phenotype.

From skeletal muscle to cancer: insights learned elucidating the function of tropomyosin

The tropomyosins (Tms) are a family of actin filament binding proteins that possess a simple dimeric α-helical coiled-coil structure along their entire length. Our knowledge of Tm structure and function has greatly expanded since they were first discovered in skeletal muscle almost 65 years ago. In multicellular organisms they exhibit extensive cell type specific isoform diversity. In this essay we discuss the genetic mechanisms by which this diversity is generated and its significance to actin-based cellular functions.

Transcriptomic effects of di-(2-ethylhexyl)-phthalate in Syrian hamster embryo cells: an important role of early cytoskeleton disturbances in carcinogenesis?

Background

Di-(2-ethylhexyl)-phthalate (DEHP) is a commonly used plasticizer in polyvinylchloride (PVC) formulations and a potentially non-genotoxic carcinogen. The aim of this study was to identify genes whose level of expression is altered by DEHP by using a global wide-genome approach in Syrian hamster embryo (SHE) cells, a model similar to human cells regarding their responses to this type of carcinogen. With mRNA Differential Display (DD), we analysed the transcriptional regulation of SHE cells exposed to 0, 12.5, 25 and 50 μM of DEHP for 24 hrs, conditions which induced neoplastic transformation of these cells. A real-time quantitative polymerase chain reaction (qPCR) was used to confirm differential expression of genes identified by DD.

Results

Gene expression profiling showed 178 differentially-expressed fragments corresponding to 122 genes after tblastx comparisons, 79 up-regulated and 43 down-regulated. The genes of interest were involved in many biological pathways, including signal transduction, regulation of the cytoskeleton, xenobiotic metabolism, apoptosis, lipidogenesis, protein conformation, transport and cell cycle. We then focused particularly on genes involved in the regulation of the cytoskeleton, one of the processes occurring during carcinogenesis and in the early steps of neoplastic transformation. Twenty one cytoskeleton-related genes were studied by qPCR. The down-regulated genes were involved in focal adhesion or cell junction. The up-regulated genes were involved in the regulation of the actin cytoskeleton and this would suggest a role of cellular plasticity in the mechanism of chemical carcinogenesis. The gene expression changes identified in the present study were PPAR-independent.

Conclusion

This study identified a set of genes whose expression is altered by DEHP exposure in mammalian embryo cells. This is the first study that elucidates the genomic changes of DEHP involved in the organization of the cytoskeleton. The latter genes may be candidates as biomarkers predictive of early events in the multistep carcinogenic process.

Interior decoration: tropomyosin in actin dynamics and cell migration

Cell migration and invasion requires the precise temporal and spatial orchestration of a variety of biological processes. Filaments of polymerized actin are critical players in these diverse processes, including the regulation of cell anchorage points (both cell-cell and cell-extracellular matrix), the uptake and delivery of molecules via endocytic pathways and the generation of force for both membrane protrusion and retraction. How the actin filaments are specialized for each of these discrete functions is yet to be comprehensively elucidated. The cytoskeletal tropomyosins are a family of actin associating proteins that form head-to-tail polymers which lay in the major groove of polymerized actin filaments. In the present review we summarize the emerging isoform-specific functions of tropomyosins in cell migration and invasion and discuss their potential roles in the specialization of actin filaments for the diverse cellular processes that together regulate cell migration and invasion.

Beta and gamma-cytoplasmic actins display distinct distribution and functional diversity

Using newly generated monoclonal antibodies, we have compared the distribution of beta- and gamma-cytoplasmic actin in fibroblastic and epithelial cells, in which they play crucial roles during various key cellular processes. Whereas beta-actin is preferentially localized in stress fibers, circular bundles and at cell-cell contacts, suggesting a role in cell attachment and contraction, gamma-actin displays a more versatile organization, according to cell activities. In moving cells, gamma-actin is mainly organized as a meshwork in cortical and lamellipodial structures, suggesting a role in cell motility; in stationary cells, gamma-actin is also recruited into stress fibers. beta-actin-depleted cells become highly spread, display broad protrusions and reduce their stress-fiber content; by contrast, gamma-actin-depleted cells acquire a contractile phenotype with thick actin bundles and shrinked lamellar and lamellipodial structures. Moreover, beta- and gamma-actin depleted fibroblasts exhibit distinct changes in motility compared with their controls, suggesting a specific role for each isoform in cell locomotion. Our results reveal new aspects of beta- and gamma-actin organization that support their functional diversity.

Proteomics Reveals Protein Profile Changes in Cyclooxygenase-2 Inhibitor-Treated Endometrial Cancer Cells

Objective:

To examine effects of an inhibitor of cyclooxygenase (COX)-2, NS-398, on the proliferation, apoptosis and invasion characteristics of endometrial cancer cell RL95-2.

Methods:

(1) Western blotting was carried out to determine COX-2 protein expression in RL95-2 cells and normal endometrium specimens. (2) The effect of NS-398 treatment on the cell proliferation, apoptosis, and invasion was assessed by methyl thiazolyl tetrazolium assay, flow cytometry, and matrigel invasion assay, respectively. (3) Finally, the proteomic analysis was used to find out proteins that are differentially expressed because of NS-398 treatment.

Results:

(1) COX-2 protein in RL95-2 cell line was significantly higher than that in normal endometrium. (2) NS-398 had significant growth inhibition effects on RL95-2 cells in a dose- and time-dependent manner. (3) NS-398 increased the proportion of cells in G1 and decreased the proportion of cells in the G2 phase in RL95-2 cells. (4) NS-398 could restrain endometrial cancer cells invasion. (5) The proteomic analysis revealed several proteins that are differentially expressed because of NS-398 treatment; the down-regulated proteins identified are hnRNP K, α enolase, Hsp70, tropomyosin, and protein disulfide isomerase, the up-regulated protein is phosphatidylethanolamine binding protein.

Conclusions:

The expression of COX-2 plays an important role in tumorigenesis of endometrial cancer. NS-398 can inhibit the ability of RL95-2 cell proliferation, viability, and invasion. In this study, the well-resolved reproducible 2-DE maps of NS-398 treated and control RL95-2 cells were established, and the significantly different expressed proteins are preliminary identified.

Tropomyosin as a regulator of cancer cell transformation

Tropomyosins (Tms) are among the most studied structural proteins of the actin cytoskeleton that are implicated in neoplastic-specific alterations in actin filament organization. Decreased expression of specific nonmuscle Tm isoforms is commonly associated with the transformed phenotype. These changes in Tm expression appear to contribute to the rearrangement of microfilament bundles and morphological alterations, increased cell motility and oncogenic signaling properties of transformed cells. Below we review aspects of Tm biology as it specifically relates to transformation and cancer including its expression in culture models of transformed cells and human tumors, mechanisms that regulate Tm expression and the role of Tm in oncogenic signaling.

Tropomyosins as interpreters of the signalling environment to regulate the local cytoskeleton

A key regulator of cell morphology is the actin cytoskeleton and it has long been appreciated that the cytoskeleton is characteristically altered in cancer. Actin is organized into polymeric structures with distinct dynamics which in turn participate in a wide variety of cell processes including adhesion, migration, cell division and apoptosis. Despite displaying an altered actin cytoskeleton, transformed cells retain--and in many cases increase--their ability to adhere, move, divide and respond to apoptotic stimuli. Thus cancer cells maintain responsive actin cytoskeletons. Actin dynamics are regulated by numerous actin-binding proteins and chief among these are the tropomyosins which are core components of the microfilament. Recent advances in genomic and proteomic profiling confirm that Tm expression profiles are profoundly changed in transformed cells. It is therefore timely to review the role of Tms in the regulation of actin dynamics that pertain to crucial phenotypic changes in cancer. In this review we discuss how actin filaments containing different Tm isoforms respond to the activation of cell signalling pathways and consider the implications of this for cancer progression and therapy.

Binding of Par-4 to the actin cytoskeleton is essential for Par-4/Dlk-mediated apoptosis

Prostate apoptosis response-4 (Par-4) is a 38-kDa protein originally identified as a gene product upregulated in prostate cancer cells undergoing apoptosis. Cell death mediated by Par-4 and its interaction partner DAP like kinase (Dlk) is characterized by dramatic changes of the cytoskeleton. To uncover the role of the cytoskeleton in Par-4/Dlk-mediated apoptosis, we analyzed Par-4 for a direct association with cytoskeletal structures. Confocal fluorescence microscopy revealed that endogenous Par-4 is specifically associated with stress fibers in rat fibroblasts. In vitro cosedimentation analyses and in vivo FRET analyses showed that Par-4 directly binds to F-actin. Actin binding is mediated by the N-terminal 266 amino acids, but does not require the C-terminal region of Par-4 containing the leucine zipper and the death domain. Furthermore, the interaction of Par-4 with actin filaments leads to the formation of actin bundles in vitro and in vivo. In rat fibroblasts, this microfilament association is essential for the pro-apoptotic function of Par-4, since both disruption of the actin cytoskeleton by cytochalasin D treatment and overexpression of Par-4 constructs impaired in actin binding result in a significant decrease of apoptosis induction by Par-4 and Dlk. We propose a model, in which Par-4 recruits Dlk to stress fibers, leading to enhanced phosphorylation of the regulatory light chain of myosin II (MLC) and to the induction of apoptosis.

Activation of either ERK1/2 or ERK5 MAP kinase pathways can lead to disruption of the actin cytoskeleton

Oncogenic transformation often leads to the disruption of the actin cytoskeleton. Activation of the classical Ras-Raf-MEK1/2-ERK1/2 signalling cascade has been implicated in the effects of oncogenes such as Ras and Src on the cytoskeleton. Many of the studies of the effects of oncogenes on the cytoskeleton have made use of chemical inhibitors of MEK1/2 but it is now clear that these inhibitors also inactivate MEK5 in the MEK5-ERK5 MAP kinase pathway raising the possibility that this pathway may also be involved in oncogenic transformation. We therefore investigated whether activation of ERK5 can lead to disruption of the actin cytoskeleton. We show that activation of ERK5 can lead to loss of actin stress fibres, but by a distinct mechanism to ERK1/2. We demonstrate that ERK5 is activated by oncogenic Src as demonstrated by translocation of endogenous ERK5 from the cytoplasm to nucleus and activation of an ERK5-dependent transcriptional reporter and that ERK5 activation is required for Src-mediated transformation. We also show that in Src-transformed cells inhibition of ERK1/2 signalling is not sufficient for reappearance of the actin cytoskeleton and that ERK5 activation contributes to cytoskeletal disruption by Src. Our results suggest that multiple MAP kinase pathways downstream of oncogenes participate in cytoskeletal alterations.

The rat homologue of Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP) associates with actin filaments, recruits N-WASP from the nucleus, and mediates mobilization of actin from stress fibers in favor of filopodia formation

We cloned and characterized the rat homologue of the Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP). Rat WIP shows 86% amino acid sequence identity to human WIP. Northern analyses revealed two major mRNA species of 5.0 and 3.8 kb, which were ubiquitously expressed, though predominantly in spleen and lung. Minor species of 2.4, 1.8, 1.4, and 1.1 kb were also detected in some tissues and cell lines. Thus, WIP is subject to tissue-specific alternative splicing. WIP bound to N-WASP in vivo, as revealed by co-immunoprecipitation. Expression of WIP in rat fibroblasts revealed a clear co-localization with actin stress fibers. However, expression in tumor cells lacking actin cables did not restore these structures. Interestingly, co-expression of WIP and N-WASP resulted in redistribution of N-WASP, abrogating its dominant nuclear expression and leading to co-localization with WIP in the perinuclear area and with actin in membrane protrusions. Moreover, stress fibers and, concomitantly, the associated WIP were largely dissolved. Very similar effects were seen upon epidermal growth factor stimulation of serum-starved cells. Our results suggest that WIP might be involved in transmitting mitogenic signals to cytoskeletal functions, perhaps by modulating the subcellular localization of N-WASP. Interaction of N-WASP with WIP may in turn lead to mobilization of actin from stress fibers and nucleation of new actin filaments in filopodia.

Effect of lectin from the ascidian on the growth and the adhesion of HeLa cells

Cell adhesion molecules, some of which are lectins, play a key role in the control of normal and pathological processes of various living organisms. We found herein that N-acetyl-D-glucosamine-specific lectin, isolated from the ascidian Didemnum ternatanum (DTL), alters the growth properties of HeLa tumor cells depending on the anchorage. DTL was shown to increase the proliferation of HeLa cells grown in soft agar greatly (in anchorage-independent fashion). In contrast, DTL inhibits the proliferative activity of HeLa cells grown on solid substrate and acts as inductor of differentiation, slowing cell growth, increasing the cell attachment and spreading. Scanning electron microscopic data have demonstrated that DTL treatment resulted in pronounced changes of the shape and surface of HeLa cells. Changes of cellular morphology correlated with essential redistribution of actin microfilaments.

Regulation of the formation of tumor cell pseudopodia by the Na(+)/H(+) exchanger NHE1

The Na(+)/H(+) exchanger NHE1 is involved in intracellular pH homeostasis and cell volume regulation and accumulates with actin in the lamellipodia of fibroblasts. In order to determine the role of NHE1 following epithelial transformation and the acquisition of motile and invasive properties, we studied NHE1 expression in polarized MDCK cells, Moloney Sarcoma virus (MSV) transformed MDCK cells and an invasive MSV-MDCK cell variant (MSV-MDCK-INV). Expression of NHE1 was significantly increased in MSV-MDCK-INV cells relative to MSV-MDCK and MDCK cells. NHE1 was localized with b-actin to the tips of MSV-MDCK-INV cell pseudopodia by immunofluorescence. Sensitivity of NHE1-mediated (22)Na uptake to ethylisopropylamiloride, a specific inhibitor of NHE1, was increased in MSV-MDCK cells relative to MDCK cells. Changes in intracellular pH induced upon EIPA treatment were also of higher magnitude in MSV-MDCK and MSV-MDCK-INV cells compared to wild-type MDCK cells, especially in Hepes-buffered DMEM medium. Inhibition of NHE1 by 50 microM ethylisopropylamiloride induced the disassembly of actin stress fibers and redistribution of the actin cytoskeleton in all cell types. However, in MSV-MDCK-INV cells, the effect of ethylisopropylamiloride treatment was more pronounced and associated with the increased reversible detachment of the cells from the substrate. Videomicroscopy of MSV-MDCK-INV cells revealed that within 20 minutes of addition, ethylisopropylamiloride induced pseudopodial retraction and inhibited cell motility. The ability of ethylisopropylamiloride to prevent nocodazole-induced formation of actin stress fibers in MSV-MDCK cells was more pronounced in Hepes medium relative to NaHCO(3) medium, showing that NHE1 can regulate actin stress fiber assembly in transformed MSV-MDCK cells via its intracellular pH regulatory effect. These results implicate NHE1 in the regulation of the actin cytoskeleton dynamics necessary for the adhesion and pseudopodial protrusion of motile, invasive tumor cells.

Disruption of the actin cytoskeleton leads to inhibition of mitogen-induced cyclin E expression, Cdk2 phosphorylation, and nuclear accumulation of the retinoblastoma protein-related p107 protein

The actin cytoskeleton has been found to be required for mitogen-stimulated cells to passage through the cell cycle checkpoint. Here we show that selective disruption of the actin cytoskeleton by dihydrocytochalasin B (H(2)CB) blocked the mitogenic effect in normal Swiss 3T3 cells, leading to cell cycle arrest at mid to late G(1) phase. Cells treated with H(2)CB remain tightly attached to the substratum and respond to mitogen-induced MAP kinase activation. Upon cytoskeleton disruption, however, growth factors fail to induce hyperphosphorylation of the retinoblastoma protein (pRb) and the pRb-related p107. While cyclin D1 induction and cdk4-associated kinase activity are not affected, induction of cyclin E expression and activation of cyclin E-cdk2 complexes are greatly inhibited in growth-stimulated cells treated with H(2)CB. The inhibition of cyclin E expression appears to be mediated at least in part at the RNA level and the inhibition of cdk2 kinase activity is also attributed to the decrease in cdk2 phosphorylation and proper subcellular localization. The expression patterns of cdk inhibitors p21 and p27 are similar in both untreated and H(2)CB-treated cells upon serum stimulation. In addition, the changes in subcellular localization of pRb and p107 appear to be linked to their phosphorylation states and disruption of normal actin structure affects nuclear migration of p107 during G(1)-to-S progression. Taken together, our results suggest that the actin cytoskeleton-dependent G(1) arrest is linked to the cyclin-cdk pathway. We hypothesize that normal actin structure may be important for proper localization of certain G(1) regulators, consequently modulating specific cyclin and kinase expression.

Fibroblast transgelin and smooth muscle SM22alpha are the same protein, the expression of which is down-regulated in many cell lines

In this report we investigate the expression and relationship of transgelin (Tg), a transformation and shape-change sensitive actin gelling protein found in fibroblasts and smooth muscle [Shapland et al., 1988: J. Cell. Biol. 107:153-161; Shapland et al., 1993: J. Cell. Biol. 121:1065-1073], to SM22alpha, a smooth muscle protein of unknown function [Lees-Millar et al., 1987: J. Biol. Chem. 262:2988-2993; Solway et al., 1995: J. Biol. Chem. 270:13460-13469]. To clarify their relationship we have cloned and sequenced the cDNA encoding Tg from cultures of rat embryo fibroblasts. The sequences of fibroblast Tg and the smooth muscle isoform SM22 are identical [Prinjha et al., 1994: Cell Motil. Cytoskeleton 28:243-255; Shanahan et al., 1993: Circ. Res. 73:193-204; Solway et al., 1995]. These data, coupled with our immunoblot analysis and previous observations [Shapland et al., 1988; Shapland et al., 1993], demonstrate that Tg expression is not restricted to smooth muscle since this protein is also present in normal mesenchymal cells. However, we also show that Tg, although present in secondary cultures of mouse and rat embryo fibroblasts, is absent in many apparently normal fibroblast cell lines. Tg down-regulation may therefore be an early and sensitive marker for the onset of transformation. A functional role for Tg is unlikely to directly involve Ca2+ since it neither contains a functional EF hand nor binds 45Ca2+.

Azatyrosine. Mechanism of action for conversion of transformed phenotype to normal

Azatyrosine [L-beta-(5-hydroxy-2-pyridyl)-alanine] has the unique property of converting ras- or c-erbB-2 transformed phenotype to normal. The administration of azatyrosine also inhibits tumor formation in transgenic mice harboring the normal human c-Ha-ras which is mutated during treatment with various chemical carcinogens. To elucidate the molecular mechanism, we investigated how azatyrosine functions and what are its major targets. Azatyrosine functions downstream of ras; azatyrosine does not alter either the level of GTP-bound Ras or the total amount of Ras. Instead, azatyrosine inhibits the activation of c-Raf-1 kinase by oncogenic c-ErbB-2, resulting in inactivation of AP1. It is interesting that azatyrosine also restores the expression of the rhoB gene, the product of which regulates the formation of actin stress fibers. Azatyrosine is incorporated into cellular proteins to replace tyrosine. Several experiments indicate that replacement of tyrosine is likely to be a cause for its conversion of transformed phenotype to normal. To prove this hypothesis, we are attempting to develop a mutant of tyrosyl-tRNA synthetase that, unlike wild type, can aminoacylate azatyrosine more efficiently than can tyrosine.

Cloning and characterization of rat BAT3 cDNA

HLA-B-associated transcript 3 (BAT3) was originally identified as one of the genes located within human major histocompatibility complex. It encodes a large proline-rich protein with unknown function. In this study, we found that a fragment of the BAT3 gene product interacts with a candidate tumor suppressor, DAN, in the yeast-based two-hybrid system. We cloned the full-length rat BAT3 cDNA from a fibroblast 3Y1 cDNA library. Our sequence analysis has demonstrated that rat BAT3 cDNA is 3617 nucleotides in length and encodes a full-length BAT3 (1098 amino acids) with an estimated molecular mass of 114,801 daltons, which displays an 87.4% identity with human BAT3. The deletion experiment revealed that the N-terminal region (amino acid residues 1-80) of DAN was required for the interaction with BAT3. Green fluorescent protein-tagged BAT3 was largely localized in the cytoplasm of COS cells. Northern hybridization showed that BAT3 mRNA was expressed in all the adult rat tissues examined but predominantly in testis. In addition, the level of BAT3 mRNA expression was more downregulated in some of the transformed cells, including v-mos- and v-Ha-ras-transformed 3Y1 cells, than in the parental cells.

Decrease in protein tyrosine phosphorylation is associated with F-actin reorganization by retinoic acid in human endometrial adenocarcinoma (RL95-2) cells

Transformed cells often express elevated levels of tyrosine-phosphorylated proteins. Inhibition of protein tyrosine kinases causes reversion of malignant cells to the normal phenotype. In the present study, we evaluated the possibility that the reversion of human endometrial adenocarcinoma RL95-2 cells to a stationary phenotype induced by retinoic acid was associated with inhibition of tyrosine phosphorylation of cellular proteins. We found that retinoic acid decreased the levels of tyrosine-phosphorylated proteins, as assessed by immunostaining and immunoprecipitations using specific anti-phosphotyrosine antibodies. In addition, the inhibitors of tyrosine kinases herbimycin A and tyrphostin mimicked retinoic acid, inducing F-actin reorganization and increasing the size of RL95-2 cells, as determined by measurement of cell perimeters. Because focal adhesions that connect actin filaments with the plasma membrane are major sites of tyrosine phosphorylation, we further investigated whether selected focal adhesion proteins were affected by retinoic acid. We found that retinoic acid altered the localization of focal adhesion kinase. All-trans retinoic acid was effective in reducing the levels of focal adhesion kinase and paxillin protein. Thirteen-cis retinoic acid increased the levels of vinculin protein in the cytosolic fraction of cells. These changes are consistent with actin reorganization and reversion toward a stationary phenotype induced by retinoic acid in endometrial adenocarcinoma RL95-2 cells. Our results indicate that the differentiating effects of retinoids on endometrial cells are associated with decreases in tyrosine phosphorylation and changes in the levels and distribution of focal adhesion proteins. These findings suggest that signaling pathways that involve tyrosine kinases are potential targets for drug design against endometrial cancer.

Suppression of glial tumor growth by expression of glial fibrillary acidic protein

We examined the effect of expression of glial fibrillary acidic protein (GFAP) on the tumor growth of astrocytoma in vivo. When rat astrocytoma C6 cells were injected subcutaneously in athymic mice, the cells produced tumors that grew rapidly. The tumor growth of C6 cells transfected with GFAP cDNA was significantly reduced compared to that of control NeoC6 cells transfected only with the neomycin resistant gene. After implantation of C6 cells transfected with mutated GFAP cDNA at the phosphorylation sites, the tumor growth was suppressed similar to that of the wild GFAP transfectants. To determine whether the cell growth suppression by GFAP is specific for astroglial cells, we assessed the effect of GFAP on the cell growth of an L cell of fibroblast origin in vitro. By GFAP cDNA transfection, L cells showed morphological changes, but the cell growth was not reduced. These results suggest that GFAP is a critical regulator of the tumor growth of astrocytoma.

Molecular basis for tissue expansion: clinical implications for the surgeon

A wide variety of tissue expansion techniques have been used for breast reconstruction, craniofacial surgery, and burn care in plastic reconstructive surgery. However, the basic mechanism by which skin and surrounding tissue respond to mechanical expansion remains unclear. Recent studies have revealed the biomechanical aspects of cells subjected to strain and various factors involved in the stretch-induced signal transduction pathway. In this regard, we have reported previously that mechanical force increases keratinocyte growth and protein synthesis and alters cell morphology. The mechanism by which strain causes an enhancement of cellular growth appears to be a network of several integrated cascades, implicating growth factors, cytoskeleton, and the protein kinase family. Recently, additional evidence has accumulated that mechanical strain stimulates signal transduction pathways that could trigger a series of cascades eventually leading to a new skin production. For example, we have evidence suggesting a key role for protein kinase C (PKC) in mechanosignaling as PKC is activated and translocated in keratinocytes subjected to strain in an isoform-specific manner. In this report, molecular mechanisms leading to enhancement of skin surface area are reviewed, and possible future applications are discussed.

Actin organization associated with the expression of multidrug resistant phenotype in osteosarcoma cells and the effect of actin depolymerization on drug resistance

We have previously reported that P-glycoprotein (Pgp)-overexpressing multidrug resistant (MDR) osteosarcoma cells were functionally more differentiated than their parent cells. The present study showed that in the parent cells, the actin filaments were sparsely distributed or were diffusely spread throughout the cytoplasm, whereas the MDR osteosarcoma cells exhibited a remarkable increase in well-organized actin stress fibers. Furthermore, dihydrocytochalasin B, a specific inhibitor of actin polymerization, dramatically disrupted this network of stress fibers, increased the intracellular accumulation of doxorubicin (DOX) and modified the resistance against DOX. These results indicate that the organization of actin filaments associated with cellular differentiation may be involved in the expression of Pgp function in the MDR osteosarcoma cells.

Expression of the pRb-binding regions of E1A enables efficient transformation of primary epithelial cells by v-src

Primary cultures of rat embryo fibroblasts have been shown to be resistant to transformation by dominant oncogenes such as v-src. We sought to determine if similar resistance is displayed by primary epithelial cells, and, if so, whether an immortalizing oncogene such as E1A could enhance transformation of primary epithelial cells by v-src. Transformation of primary rat epithelial cells by v-src was synergistically enhanced when E1A expression plasmids were cotransfected with a v-src expression plasmid. Foci were more numerous and observed earlier (9 to 14 days) with E1A plus v-src than with v-src alone (18 to 28 days). This cotransformation ability was abrogated by deletions in CR1 or CR2 of E1A, which encode the binding regions for the pRb family and are responsible for E1A-mediated cell cycle activation. Mutations in the p300 binding site or the second exon, which abolish immortalization, did not affect v-src cooperation, in contrast to ras and adenovirus E1B. While kinase activation was required for growth in soft agar, differential activation of Src kinase did not correlate with transformation efficiency. Cell morphology and actin structures were not dramatically impacted by E1A expression; thus, hypertransformation, as previously described for ras cotransformation, was not observed with v-src and second-exon mutants of E1A. However, growth rates for cells expressing both E1A and v-Src were higher than those for cells expressing only v-Src. These results suggest that functions involved in cell cycle activation encoded by E1A first exon can enhance v-src transformation of primary epithelial cells.

Interaction of vaccinia virus with the actin cytoskeleton

Vaccinia virus infection results in large rearrangements of the host actin cytoskeleton including the formation of actin tails that are strikingly similar to those seen in Listeria, Shigella and Rickettsia infections. Using actin polymerization as the driving force the intracellular enveloped form of the vaccinia virus (IEV) is propelled on the tip of actin tails at a speed of 2.8 microns/min, both intra- and intercellularly. The similarities between the actin-based motility of the vaccinia virus, Listeria, Shigella and Rickettsia suggest that intracellular pathogens have developed a common strategy to exploit the actin cytoskeleton of the host to facilitate their intercellular spread. This review focuses on our current understanding of the interactions between the vaccinia virus and the actin cytoskeleton.

Cytoplasmic microtubules in two different mouse melanoma cell lines: a qualitative and quantitative analysis using confocal laser scanning microscopy and computer-assisted image analysis

The microtubular system as one part of the cellular cytoskeleton is not only necessary for mitotic activity of malignant cells but also for invading neighboring tissues and for the formation of distant metastases. In the present study, the amount and distribution of tubulin in two murine melanoma cell lines (K1735-M2: high metastatic clone; K1735-c116: low metastatic clone) were determined quantitatively using an indirect immunofluorescence technique, confocal laser scanning microscopy (CLSM) and computer-assisted image analysis. Additionally, qualitative and quantitative changes after application of the microtubule-inhibitor nocodazole were investigated. Quantitative analysis showed a significant difference between the high and low metastatic cell line for the parameter TEXTURE, indicating a finer structured network within the high metastatic cells. After treatment with nocodazole the parameters TEXTURE and DENSITY were reduced, suggesting a decrease of assembled tubulin and a less delicate structure of the remaining microtubules. Our study shows that CLSM combined with computer-assisted image analysis provides a new method to examine quantitative variations of the cytoskeleton possibly related to cell function.

Cytoskeletal and morphological changes associated with the specific suppression of the epidermal growth factor receptor tyrosine kinase activity in A431 human epidermoid carcinoma

The epidermal growth factor (EGF) receptor is well known as a mediator of mitogenic signaling and its tyrosine kinase activity has been suggested as a viable target in cancer chemotherapy. To explore the consequences of abolishing the kinase activity of this receptor, we have utilized a potent and specific inhibitor of the enzyme, PD 153035, to sustain a long-term suppression of its activity. This compound inhibits EGF receptor autophosphorylation in cells with an IC50 in the low nanomolar range and does not block PDGF or FGF receptor kinase until concentrations are greater than 10 microM. [1] Human epidermoid carcinoma A431 cells were grown in the presence of PD 153035 and were passed weekly until cells grew in the presence of 1 microM inhibitor. These cells, referred to as A431R, showed a remarkable change in morphology, becoming flattened and spread out. A comparison of the sensitivity of EGF receptor autophosphorylation to PD 153035 between A431 and A431R showed a similar dose response, indicating that the cells had not developed any defect in the kinase which might make it resistant to the inhibitor. Likewise, EGF receptor autophosphorylation in response to exogenously added EGF, as well as receptor internalization, was similar between the two cell lines. Furthermore, analysis of A431R cells by flow cytometry showed no significant change in DNA content or percentage of cells in any one phase of the cell cycle compared to the parent line. 125I-labeled EGF/receptor binding studies showed that receptor number in the A431R cells was equivalent to that of the parent line; however, the Scatchard plot was linear, in contrast to the typical biphasic plot obtained with the parent cells, implying a loss of high-affinity receptors. Cytoskeletal preparations from both cell lines indicated that the A431R had fourfold less EGF receptor associated with the cytoskeleton than A431. This was accompanied by a remarkable increase in polymerized actin stress fibers throughout the A431R cells, which most likely accounts for their flattened morphology. The A431R cells also exhibited a twofold increase in the expression of focal adhesion kinase, which is consistent with a greater contact area for their cell surface and increase in focal adhesions. Finally, although the A431R cells have a doubling time of 24 h, similar to that of the parent line, these cells stop growing as the monolayer approaches confluence, reminiscent of the contact inhibition seen in nontransformed cells. These data indicate that long-term suppression of the EGF receptor tyrosine kinase activity in A431 human epidermoid carcinoma results in certain cellular properties which are more consistent with a differentiated and nontransformed phenotype.

The vaccinia virus F17R protein interacts with actin

We have examined the possible role of the F17R protein in vaccinia virus-induced rearrangements of the host actin cytoskeleton. F17R is localized to vaccinia-induced actin tails late during infection. The recombinant vaccinia strain vRO11k is able to induce actin tails that are indistinguishable from controls in the absence of F17R expression. The association of vaccinia and myxoma virus F17R with the actin cytoskeleton in the absence of additional viral factors suggests a basic region in the N-terminal half of the protein is important for this interaction. A peptide corresponding to this region efficiently bundles actin filaments in vitro, confirming that the protein interacts directly with actin. Our results show F17R is not required for actin tail formation and highlight the difficulty in discriminating functional actin-binding proteins from those that associate by virtue of their basic nature.

Transcriptional repression of smooth-muscle alpha-actin gene associated with human papillomavirus type 16 E7 expression

To explore the role of the E7 viral oncogene from human papillomavirus type 16 (HPV 16) in the regulation of cytoskeletal organization, we investigated alterations in particular cytoskeletal components in rat embryonal fibroblasts and three transformants of rat embryonal fibroblast cells produced by transfections with HPV16 E7 alone (TF1), HPV16 E7 plus adenovirus type 5 E1B (TF3), and HPV16 E7 plus activated Ha-ras (TF4). Marked reductions in smooth-muscle (SM) alpha-actin content and disrupted organization of stress fibers detected by anti-SM alpha-actin antibody were evident in all the transformants. These cytoskeletal manifestations were associated with a significant reduction in the mRNAs in these cells. Transcriptional repression by the E7 gene was observed after transient transfection of a chloramphenicol acetyltransferase reporter gene with SM alpha-actin gene promoter. Nucleotides -123 to -39 of the SM alpha-actin gene promoter were required for the HPV16 E7 transcriptional repression as shown by the chloramphenicol acetyltransferase assay. The downregulation of this actin isoform mediated by the E7 oncoprotein may play an important role in cell transformation by HPV16.

Direct transforming activity of TGF-beta on rat fibroblasts

Treatment of NRK 536 fibroblasts with EGF and transforming growth factor type beta (TGF-beta) is known to lead to the reversible induction of the transformed phenotype in a large percentage of cells. Maintenance of the transformed state is dependent on the continuous presence of the cytokines. Here we show that treatment of these cells with TGF-beta alone leads to the formation of a small percentage of colonies, the majority of which stably retain the transformed phenotype after removal of the cytokine. Colony induction is dependent on the concentration of TGF-beta originally present. Stably transformed cells grow in soft agar without further addition of exogenous TGF-beta, and exhibit criss-cross morphology in monolayer culture and a diffuse actin pattern. Transformation of NRK 536 cells can be demonstrated only for individualized cells treated with TGF-beta in soft agar, as treatment in monolayer leads to the induction of apoptosis in newly transformed cells by neighboring normal cells. Transformation of NRK 536 fibroblasts by TGF-beta is not due to the induction of point mutations by TGF-beta.

Actin, its associated proteins and metastasis

In this short article we have overviewed the effect of transformation on AAPs. We have not dealt with molecules indirectly associated with actin which are modified by transformation, such as the catenins, cadherins, vinculin, and integrins [Tsukita et al., 1993], although their relationship with and importance to the transformed phenotype cannot be overemphasised. Similarly, alterations in polyphosphoinositide metabolism that occur in transformed cells may also promote or induce alterations in the microfilament cytoskeleton via interactions with proteins such as gelsolin, alpha-actinin, and MARCKS [Bretscher, 1993]. At present, there are no clear-cut rules regarding the effect that oncogenesis has on AAPs, since the expression level of one oncogenically modified AAP can vary from one tumour to another, even within the same type of tumour [Yamamoto et al., 1993]. This apparent inconsistency may well be because we are "playing with an incomplete pack of cards" [Pollard, 1993], or it may reflect this highly complex control mechanism in which any perturbation of the normal expression levels of actin/AAPs unbalances the delicate equilibrium which controls normal cell behaviour [Bray and Vasiliev, 1989]. For example, it is possible that oncogenically induced alterations to actin isoform expression [Lin et al., 1985] exert a downstream effect on actin-associated protein expression. Oncogenically induced actin based cell movements may be activated by alterations to either a single AAP or a synergistic functional unit of these molecules [Shapland et al., 1993; Vandekerckhove et al., 1990].(ABSTRACT TRUNCATED AT 250 WORDS)

Electric field-directed fibroblast locomotion involves cell surface molecular reorganization and is calcium independent

Directional cellular locomotion is thought to involve localized intracellular calcium changes and the lateral transport of cell surface molecules. We have examined the roles of both calcium and cell surface glycoprotein redistribution in the directional migration of two murine fibroblastic cell lines, NIH 3T3 and SV101. These cell types exhibit persistent, cathode directed motility when exposed to direct current electric fields. Using time lapse phase contrast microscopy and image analysis, we have determined that electric field-directed locomotion in each cell type is a calcium independent process. Both exhibit cathode directed motility in the absence of extracellular calcium, and electric fields cause no detectable elevations or gradients of cytosolic free calcium. We find evidence suggesting that galvanotaxis in these cells involves the lateral redistribution of plasma membrane glycoproteins. Electric fields cause the lateral migration of plasma membrane concanavalin A receptors toward the cathode in both NIH 3T3 and SV101 fibroblasts. Exposure of directionally migrating cells to Con A inhibits the normal change of cell direction following a reversal of electric field polarity. Additionally, when cells are plated on Con A-coated substrata so that Con A receptors mediate cell-substratum adhesion, cathode-directed locomotion and a cathodal accumulation of Con A receptors are observed. Immunofluorescent labeling of the fibronectin receptor in NIH 3T3 fibroblasts suggests the recruitment of integrins from large clusters to form a more diffuse distribution toward the cathode in field-treated cells. Our results indicate that the mechanism of electric field directed locomotion in NIH 3T3 and SV101 fibroblasts involves the lateral redistribution of plasma membrane glycoproteins involved in cell-substratum adhesion.

Expression, cytoskeletal utilization and dimer formation of tropomyosin derived from retroviral-mediated cDNA transfer. Metabolism of tropomyosin from transduced cDNA

Expression of the tropomyosin-1 isoform was enhanced by cDNA transfer in non-transformed murine 3T3 fibroblasts and also in v-Ki-ras transformed fibroblasts in which native tropomyosin-1 expression had been reduced and tropomyosin-2 synthesis virtually eliminated by action of the oncogene. The level of synthesis of insert-derived tropomyosin-1 was similar in normal and transformed transductants (3-5 times normal levels). The high level of insert-derived tropomyosin-1 expression resulted in a considerable increase in tropomyosin-1 utilization in the cytoskeleton of transformed cells, but this expression still did not reach normal levels, suggesting an oncogene-related inhibition of tropomyosin utilization. A large proportion of newly synthesized native tropomyosin-1 in normal, unmodified fibroblasts appeared in homodimers which, upon prolonged incubation, were largely converted to the heterodimers. Excess tropomyosin-1 derived from the inserted cDNA also appeared largely as the homodimer in both normal and transformed cells. This homodimer was utilized effectively in the formation of cytoskeletal structures but was partially converted to heterodimer by chain exchange. Under steady-state conditions, approximately 33% of the cytoskeletal tropomyosin-1-containing dimers were homodimers, compared to approximately 10% in normal fibroblasts. The results show that the increased amount of tropomyosin-1 homodimer entering the cytoskeleton under conditions of tropomyosin-1 excess, results in an atypical microfilament composition. The effect of this excess of tropomyosin-1 homodimers on stability or function of microfilament fibers remains to be determined. The results also confirm that the mechanisms of rapid homodimer formation with conversion to heterodimers by chain exchange, known from in vitro studies, also occur in vivo.

Increased actin cable organization after single chromosome introduction: association with suppression of in vitro cell growth rather than tumorigenic suppression

We previously showed that introduction of a single human chromosome 1, 6, or 9 derived from normal fibroblasts into HHUA endometrial carcinoma cells resulted in suppression of tumorigenicity. The tumorigenic suppression was accompanied by remarkable morphological changes in the microcell hybrids containing an extra copy of chromosome 1. The study presented here was undertaken to search for target cytoskeletal components affected by chromosome 1 transfer into endometrial carcinoma cells. We found that the microcell hybrids containing an extra copy of chromosome 1 were characterized by intracellular actin bundle formation and an excessive accumulation of actin and vinculin. The latter was a result of increased stabilization of the proteins. Additionally, chromosome 3 introduction into RCC23 human renal carcinoma cells resulted in prolongation of cell division and in senescence of a significant proportion of the microcell hybrids. In these microcell hybrids, the intracellular actin network was also reorganized, but the amounts of actin and vinculin protein were not increased. These findings suggest that the increased actin organization, which appeared not to cause tumorigenic suppression in the microcell hybrids, is associated with complementation of tumor suppressor genes and senescence by multiple mechanisms.

A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype

A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.

Discovery and characterization of two novel human cancer-related proteins using two-dimensional gel electrophoresis

Comparative examination of protein synthesis in normal and neoplastic human fibroblasts led to the discovery of two novel microfilament proteins with roles in human neoplasia. One protein, a mutant beta-actin was found to convert nontumorigenic human fibroblasts to tumorigenicity. Recently, the oncogenic potential of this mutant beta-actin was verified independently and shown to alter the metastatic phenotype of human cells in conjunction with the myc and ras oncogenes. A second protein, leukocyte plastin, was discovered to be a marker of a majority of human cancer cells of nonhemopoietic origin. A survey of SV40-transformed human fibroblasts and human sarcoma and carcinoma cell types demonstrated that the L-plastin gene was activated at widely varying degrees in nearly all human cancer cells. Activation of the L-plastin gene was not detected in normal nonhemopoietic cells using sensitive reverse transcript-polymerase chain reaction, excepting those cells that expressed estrogen and progesterone receptors which mediate activation of L-plastin synthesis in reproductive tissues. Our most recent findings have revealed that activation of L-plastin synthesis in neoplastic cells that cannot phosphorylate L-plastin (e.g. those neoplastic cell types that express only trace amounts of L-plastin) results in the coinduction of two alternative inflammatory programs of gene expression which mediate cytolytic effects on surrounding cells. This inflammatory response appears to be mediated by "inappropriate" constitutive synthesis of L-plastin and failure of the induced cell to phosphorylate L-plastin. Our findings suggest explanations for the novel resistance of human cells to in vitro transformation and one role of oncogene activation in cancer. As a consequence of the interplay of two-dimensional (2-D) gel electrophoretic analyses with other sophisticated techniques of molecular biology, the formal characterization of two fundamentally important multigene families was completed with determination of many aspects of the structure and function of these proteins and their genes. The discovery and characterization of the mutant beta-actin and L-plastin and their relationship to the human neoplastic phenotype serve as useful models for the discovery of other important disease-related proteins/genes using 2-D gel electrophoresis.