Transient induction of vinculin gene expression in 3T3 fibroblasts stimulated by serum-growth factors

Comparable respiratory activity in attached and suspended human fibroblasts

Measurement of oxygen consumption of cultured cells is widely used for diagnosis of mitochondrial diseases, drug testing, biotechnology, and toxicology. Fibroblasts are cultured in monolayers, but physiological measurements are carried out in suspended or attached cells. We address the question whether respiration differs in attached versus suspended cells using multiwell respirometry (Agilent Seahorse XF24) and high-resolution respirometry (Oroboros O2k), respectively. Respiration of human dermal fibroblasts measured in culture medium was baseline-corrected for residual oxygen consumption and expressed as oxygen flow per cell. No differences were observed between attached and suspended cells in ROUTINE respiration of living cells and LEAK respiration obtained after inhibition of ATP synthase by oligomycin. The electron transfer capacity was higher in the O2k than in the XF24. This could be explained by a limitation to two uncoupler titrations in the XF24 which led to an underestimation compared to multiple titration steps in the O2k. A quantitative evaluation of respiration measured via different platforms revealed that short-term suspension of fibroblasts did not affect respiratory activity and coupling control. Evaluation of results obtained by different platforms provides a test for reproducibility beyond repeatability. Repeatability and reproducibility are required for building a validated respirometric database.

Influence of Topological Cues on Fibronectin Adsorption and Contact Guidance of Fibroblasts on Microgrooved Titanium

The choice of suitable nano- and microstructures of biomaterials is crucial for successful implant integration within the human body. In particular, surface characteristics affect the adsorption of various extra cellular matrix proteins. This work illustrates the interaction of protein adsorption and early cell adhesion on bulk microstructured titanium surfaces with parallel grooves of 27 to 35 μm widths and 15 to 19 μm depths, respectively. In contact with low concentrated fibronectin solutions, distinct adsorption patterns are observed on the edges of the ridges. Moreover, NIH/3T3 fibroblasts cultured in serum-free medium for 1 h, 3 h, and 1 day show enhanced early cell adhesion on fibronectin coated samples compared to uncoated ones. In fact, early adhesion and cell contacts occur mainly on the groove edges where fibronectin adsorption was preferentially detected. Such adsorption patterns support cellular contact guidance on short time scales since the adsorbed fibronectin proteins acted as a chemical boundary superimposing the topographical cues of the grooved microstructure. In fibronectin-free conditions, this chemical boundary is absent after cell seeding and initial cell-surface interaction. Here, cellular fibronectin released by the fibroblasts adsorbs along the grooves after 3 h and contact guidance occurs delayed. After 1 day, cell adhesion and cell morphology on uncoated and fibronectin coated titanium microgrooves were nearly equilibrated. Thus, surface structures can promote directed adsorption of low concentrated fibronectin, which, furthermore, facilitates early cell adhesion. These results give rise to new developments in surface engineering of biomedical implants for improved osseointegration.

Calreticulin and focal-contact-dependent adhesion

Cell adhesion is regulated by a variety of Ca2+-regulated pathways that depend on Ca2+-binding proteins. One such protein is calreticulin, an ER-resident protein. Calreticulin signalling from within the ER can affect processes outside the ER, such as expression of several adhesion-related genes, most notably vinculin and fibronectin. In addition, changes in the expression level of calreticulin strongly affect tyrosine phosphorylation of cellular proteins, which is known to affect many adhesion-related functions. While calreticulin has been localized to cellular compartments other than the ER, it appears that only the ER-resident calreticulin affects focal-contact-dependent adhesion. In contrast, calreticulin residing outside the ER may be involved in contact disassembly and other adhesion phenomena. Here, we review the role of calreticulin in focal contact initiation, stabilization, and turnover. We propose that calreticulin may regulate cell-substratum adhesion by participating in an "ER-to-nucleus" signalling and in parallel "ER-to-cell surface" signalling based on posttranslational events.

An en masse phenotype and function prediction system for Mus musculus

BACKGROUND

Individual researchers are struggling to keep up with the accelerating emergence of high-throughput biological data, and to extract information that relates to their specific questions. Integration of accumulated evidence should permit researchers to form fewer - and more accurate - hypotheses for further study through experimentation.

RESULTS

Here a method previously used to predict Gene Ontology (GO) terms for Saccharomyces cerevisiae (Tian et al.: Combining guilt-by-association and guilt-by-profiling to predict Saccharomyces cerevisiae gene function. Genome Biol 2008, 9(Suppl 1):S7) is applied to predict GO terms and phenotypes for 21,603 Mus musculus genes, using a diverse collection of integrated data sources (including expression, interaction, and sequence-based data). This combined 'guilt-by-profiling' and 'guilt-by-association' approach optimizes the combination of two inference methodologies. Predictions at all levels of confidence are evaluated by examining genes not used in training, and top predictions are examined manually using available literature and knowledge base resources.

CONCLUSION

We assigned a confidence score to each gene/term combination. The results provided high prediction performance, with nearly every GO term achieving greater than 40% precision at 1% recall. Among the 36 novel predictions for GO terms and 40 for phenotypes that were studied manually, >80% and >40%, respectively, were identified as accurate. We also illustrate that a combination of 'guilt-by-profiling' and 'guilt-by-association' outperforms either approach alone in their application to M. musculus.

Differential usage of signal transduction pathways defines two types of serum response factor target gene

Activation of the transcription factor serum response factor (SRF) is dependent on Rho-controlled changes in actin dynamics. We used pathway-specific inhibitors to compare the roles of actin dynamics, extracellular signal-regulated kinase (ERK) signaling, and phosphatidylinositol 3-kinase in signaling either to SRF itself or to four cellular SRF target genes. Serum, lysophosphatidic acid, platelet-derived growth factor, and phorbol 12-myristate 13-acetate (PMA) each activated transcription of a stably integrated SRF reporter gene dependent on functional RhoA GTPase. Inhibition of mitogen-activated protein kinase-ERK kinase (MEK) signalling reduced activation of the SRF reporter by all stimuli by about 50%, except for PMA, which was effectively blocked. Inhibition of phosphatidylinositol 3-kinase slightly reduced reporter activation by serum and lysophosphatidic acid but substantially inhibited activation by platelet-derived growth factor and PMA. Reporter induction by all stimuli was absolutely dependent on actin dynamics. Regulation of the SRF (srf) and vinculin (vcl) genes was similar to that of the SRF reporter gene; activation by all stimuli was Rho-dependent and required actin dynamics but was largely independent of MEK activity. In contrast, activation of fos and egr1 occurred independently of RhoA and actin polymerization but was almost completely dependent on MEK activation. These results show that at least two classes of SRF target genes can be distinguished on the basis of their relative sensitivity to RhoA-actin and MEK-ERK signaling pathways.

The dual role of cytoskeletal anchor proteins in cell adhesion and signal transduction

beta-Catenin and plakoglobin are homologous proteins having a dual role in cell adhesion and in transactivation together with LEF/TCF transcription factors. Overexpression of plakoglobin suppresses tumorigenicity, whereas increased beta-catenin levels are considered oncogenic. We compared the nuclear translocation and transactivation by beta-catenin and plakoglobin. Overexpression of each protein resulted in nuclear translocation and formation of structures that also contained LEF-1 and vinculin with beta-catenin, but not with plakoglobin. Transfection of LEF-1 translocated endogenous beta-catenin, but not plakoglobin into the nucleus. Chimeras of the Gal4 DNA-binding domain and the transactivation domains of either plakoglobin or beta-catenin were equally potent in transactivation, but induction of LEF-1-responsive transcription was higher with beta-catenin. Overexpression of wt plakoglobin or mutant beta-catenin lacking the transactivation domain induced nuclear accumulation of the endogenous beta-catenin and LEF-1-responsive transactivation. The nuclear localization and constitutive beta-catenin-dependent transactivation in SW480 cancer cells were inhibited by overexpressing cadherin or alpha-catenin. Moreover, transfecting the cytoplasmic tail of cadherin inhibited transactivation, by competition with LEF-1 in the nucleus for beta-catenin binding. The results indicate that (1) plakoglobin and beta-catenin differ in nuclear translocation and complexing with LEF-1 and vinculin, (2) LEF-1-dependent transactivation is mainly driven by beta-catenin, (3) cadherin and alpha-catenin can sequester beta-catenin, inhibit its transcriptional activity, and antagonize its oncogenic action.

Autoregulation of actin synthesis requires the 3'-UTR of actin mRNA and protects cells from actin overproduction

Monomeric (G) actin was shown to be involved in inhibiting its own synthesis by an autoregulatory mechanism that includes enhanced degradation of the actin mRNA [Bershadsky et al., 1995; Lyubimova et al., 1997]. We show that the 3'-untranslated region (3'-UTR) of beta-actin mRNA, but not its 5'-untranslated region, is important for this regulation. The level of full-length beta-actin mRNA in cells was reduced when actin filaments were depolymerized by treatment with latrunculin A and elevated when actin polymerization was induced by jasplakinolide. By contrast, the level of actin mRNA lacking the 3'-UTR remained unchanged when these drugs modulated the dynamics of actin assembly in the cell. Moreover, the transfection of cells with a construct encoding the autoregulation-deficient form of beta-actin mRNA led to very high levels of actin expression compared with transfection with the control actin construct and was accompanied by characteristic changes in cell morphology and the structure of the actin cytoskeleton. These results suggest that the autoregulatory mechanism working via the 3'-UTR of actin mRNA is involved in controlling the maintenance of a defined pool of actin monomers that could be necessary for the proper organization of the microfilament system and the cytoskeleton-mediated signaling.

Signal-regulated activation of serum response factor is mediated by changes in actin dynamics

Serum response factor (SRF) regulates transcription of many serum-inducible and muscle-specific genes. Using a functional screen, we identified LIM kinase-1 as a potent activator of SRF. We show that SRF activation by LIM kinase-1 is dependent on its ability to regulate actin treadmilling. LIM kinase activity is not essential for SRF activation by serum, but signals depend on alterations in actin dynamics. Studies with actin-binding drugs, the actin-specific C2 toxin, and actin overexpression demonstrate that G-actin level controls SRF. Regulation of actin dynamics is necessary for serum induction of a subset of SRF target genes, including vinculin, cytoskeletal actin, and srf itself, and also suffices for their activation. Actin treadmilling provides a convergence point for both serum- and LIM kinase-1-induced signaling to SRF.

Serum- and polypeptide growth factor-inducible gene expression in mouse fibroblasts

Complex cellular processes such as proliferation, differentiation, and apoptosis are regulated in part by extracellular signaling molecules: for example, polypeptide growth factors, cytokines, and peptide hormones. Many polypeptide growth factors exert their mitogenic effects by binding to specific cell surface receptor protein tyrosine kinases. This interaction triggers numerous biochemical responses, including changes in phospholipid metabolism, the activation of a protein phosphorylation cascade, and the enhanced expression of specific immediate-early, delayed-early, or late response genes. In this review, I summarize the major findings obtained from studies investigating the effects of serum or individual polypeptide growth factors on gene expression in murine fibroblasts. Several experimental approaches, including differential hybridization screening of cDNA libraries and differential display, have been employed to identify mRNA species that are expressed at elevated levels in serum- or polypeptide growth factor-stimulated cells. These studies have demonstrated that serum- and growth factor-inducible genes encode a diverse family of proteins, including DNA-binding transcription factors, cytoskeletal and extracellular matrix proteins, metabolic enzymes, secreted chemokines, and serine-threonine kinases. Some of these gene products act as effectors of specific cell cycle functions (e.g., enzymes involved in nucleotide and DNA synthesis), others are required to successfully convert a metabolically inactive cell to a metabolically active cell that will eventually increase in size and then divide (e.g., glucose-metabolizing enzymes), and some actually function as positive or negative regulators of cell cycle progression. In conclusion, research conducted during the past 15 years on serum- and growth factor-regulated gene expression in murine fibroblasts has provided significant insight into mitogenic signal transduction and cell growth control.

Cell-cycle regulated expression and serine phosphorylation of the myristylated protein kinase C substrate, SSeCKS: correlation with culture confluency, cell cycle phase and serum response

We recently identified a novel myristylated protein kinase C (PKC) substrate, named SSeCKS (pronounced essex), whose transcription is suppressed > 15 fold in src- or ras-transformed rodent fibroblasts, but not in raf-transformed cells [1, 2]. SSeCKS associates with and controls the elaboration of a cortical cytoskeletal matrix in response to phorbol esters [2], and overexpression of SSeCKS causes growth arrest of untransformed NIH3T3 cells [3]. Our preliminary data suggested that SSeCKS functions as a negative mitogenic regulator by controlling cytoskeletal architecture and that serine phosphorylation of SSeCKS by kinases such as PKC alters its interaction with cytoskeletal matrices and its ability to control mitogenesis. Here, we determine the effects of culture confluency, growth arrest and serum response on the steady-state abundance of SSeCKS RNA and protein and on the relative level of phosphoserine-free SSeCKS. SSeCKS transcription is initially induced by serum factors and by contact-inhibited growth rather than by cell-cycle arrest induced by serum starvation, hydroxyurea or nocodazole, and following serum-induced G1/S progression, SSeCKS transcription is suppressed. SSeCKS protein is hyperphosphorylated on serine residues during G1/S progression but not during the G2/M phase. Finally, we show that the induction of SSeCKS protein expression by contact inhibition is independent of SSeCKS' serum responsiveness. These data suggest that SSeCKS expression and function can be controlled at either the transcriptional or post-translational level in response to serum factors and culture confluency. The data strengthen the notion that SSeCKS plays an important, yet transient, role in cell cycle progression from G0 to G1 that differs from its role in controlling contact-inhibited growth.

Modulation of bacterial entry into epithelial cells by association between vinculin and the Shigella IpaA invasin

Shigella flexneri is the causative agent of bacillary dysentery in humans. Shigella invasion of epithelial cells is characterized by cytoskeletal rearrangements and formation of cellular projections engulfing the bacterium in a macropinocytic process. We show here that vinculin, a protein involved in linking actin filaments to the plasma membrane, is a direct target of Shigella during cell invasion. IpaA, a Shigella protein secreted upon cell contact, rapidly associates with vinculin during bacterial invasion. Although defective for cell entry, an ipaA mutant is still able to induce foci of actin polymerization, but differs from wild-type Shigella in its ability to recruit vinculin and alpha-actinin. Presumably, IpaA-vinculin interaction initiates the formation of focal adhesion-like structures required for efficient invasion.

Protein phosphorylation in response to PDGF stimulation in cultured neurons and astrocytes

Platelet-derived growth factor (PDGF) is an important growth factor for a variety of cells, including neurons and glial cells. PDGF signal transduction pathways have been studied primarily in mesenchyme-derived cells (such as fibroblasts and smooth muscle cells). However, little is known about these pathways in the central nervous system (CNS). It is believed that phosphorylation is a critical aspect of several steps in the signal transduction pathway. In this study, neurons and type 1 astrocytes in vitro were radiolabeled with 32P-orthophosphate (32P-Pi). The cells were lysed, and labeled proteins were separated by two-dimensional gel electrophoresis. Autoradiograms of PDGF-stimulated and control samples were compared. We found that in neurons and type 1 astrocytes in vitro, PDGF-BB greatly enhances protein phosphorylation while PDGF-AA has less of an effect on protein phosphorylation. Furthermore, because PDGF signal transduction pathways are likely to affect the cytoskeleton, we studied changes in actin-binding proteins induced by PDGF-BB. We found that PDGF-BB alters the expression, migration pattern and/or avidity of some actin-binding proteins in neurons. In conclusion, protein phosphorylation is up-regulated by PDGF in mouse cortical neurons and type 1 astrocytes in vitro. PDGF's effects on phosphorylation of cytoskeletal proteins might be a important mechanism by which PDGF affects the development and normal functions of central nervous system cells.

Contrasting effects of K8 and K18 on stabilizing K19 expression, cell motility and tumorigenicity in the BSp73 adenocarcinoma

The co-expression of vimentin and keratin-type intermediate filaments in the same cell was often reported to correlate with increased invasiveness and a more aggressive tumorigenic phenotype. To address the possible physiological relevance of these observations, we transfected simple keratins (K8 and 18) either individually, or in combination, into a tumorigenic but non-metastatic pancreatic adenocarcinoma that expresses vimentin but no keratins. Expression of K8 resulted in the stabilization of endogenous K19 in these cells, and formation of keratin filaments containing K8 and K19. Transfection of K18 yielded unstable K18 protein, but K18 could be stabilized when K8 was co-expressed in the same cells. Clones expressing K18 alone, or together with K8, displayed a reduced ability to grow in soft agar and decreased motility when compared to control, or K8/19 expressing cells. Moreover, K18 expressing cells were dramatically inhibited in their ability to form tumors when injected into syngeneic animals. The extent of suppression in the tumorigenicity of these cells correlated with the level of K18 expressed by these cells. The results show that K18 expression in cells may result in the suppression of the motile and tumorigenic abilities of this adenocarcinoma.

The use of two-dimensional gel electrophoresis in studies on the role of cytoskeletal plaque proteins as tumor suppressors

The morphology and functions of cells and tissues are determined, in a large part, by mechanical forces generated at cell-cell and cell-extracellular matrix (ECM) contacts. At these sites, transmembrane adhesion receptors of the integrin and cadherin families are linked, via their cytoplasmic domain, to the cytoskeleton by submembranal plaque proteins such as vinculin, alpha-actinin and the cell-cell junctional plaque proteins alpha- and beta-catenin and plakoglobin (or gamma-catenin). Recent studies have implicated this link of structural molecules between the outside and inside of the cell in signal transduction. We have shown that the expression of junctional plaque proteins is modulated during growth stimulation and differentiation, and is dramatically reduced in certain tumor cells. To study the functional significance of these changes in expression, we have used recombinant DNA technologies to overexpress or suppress the levels of junctional plaque proteins. In addition, we eliminated the expression of vinculin in embryonal stem (ES) cells and in the embryonal carcinoma F9 line by gene disruption employing homologous recombination. The results have indicated that moderate overexpression of cell-ECM plaque proteins results in reduced cell motility. In contrast, suppression of their expression, by antisense transfection, led to enhanced motility and conferred anchorage independent growth and tumorigenicity, upon injection into nude mice. These findings suggest that submembranal plaque proteins can act as effective tumor suppressors. In agreement with this notion, we found in several tumor cell lines diminished levels of junctional plaque proteins. Restoration of their level to that found in normal cells resulted in tumor suppression after their injection into experimental animals. Here we demonstrate the usefulness of the application of two dimensional (2-D) gel electrophoresis in these studies.

Fibroblast growth factor-1-inducible gene FR-17 encodes a nonmuscle alpha-actinin isoform

Polypeptide growth factor binding to cell surface receptors activates a cytoplasmic signaling cascade that ultimately promotes the expression of specific nuclear genes. As an approach to investigate the molecular mechanism of fibroblast growth factor (FGF)-1 mitogenic signaling, we have begun to identify and characterize FGF-1-inducible genes in murine NIH 3T3 cells. Here we report that one of these genes, termed FGF-regulated (FR)-17, is predicted to encode a nonmuscle isoform of alpha-actinin, an actin cross-linking protein found along microfilaments and in focal adhesion plaques. FGF-1 induction of alpha-actinin mRNA expression is first detectable at 2 h after mitogen addition and is dependent on the novo RNA and protein synthesis. Maximal alpha-actinin mRNA expression, corresponding to an approximately nineteenfold level of induction, is present after 12 h of FGF-1 stimulation. Western blot analysis indicated that FGF-1-stimulated cells also produce an increased amount of alpha-actinin protein. The FGF-1-related mitogen FGF-2, calf serum, several of the polypeptide growth factors present in serum, and the tumor promoter phorbol myristate acetate can also induce alpha-actinin mRNA expression. Finally, nonmuscle alpha-actinin mRNA is expressed in vivo in a tissue-specific manner, with relatively high levels detected in adult mouse intestine and kidney. These results indicate that nonmuscle alpha-actinin is a serum-, polypeptide growth factor-, and tumor promoter-inducible gene in mouse fibroblasts.

Complex regulation of plasminogen activator inhibitor type-1 (PAI-1) gene expression by serum and substrate adhesion

Expression of plasminogen activator inhibitor type-1 (PAI-1), a member of the serine protease inhibitor (SERPIN) superfamily that functions to negatively regulate the plasmin-based pericellular proteolytic cascade, was induced early after exposure of growth-arrested normal rat kidney (NRK) cells to serum-containing medium. Increased PAI-1 transcription was rapid (evident within 10 min of serum addition) and involved immediate-early response kinetics. [3H]Thymidine autoradiography was used to map the time frame of PAI-1 expression during a synchronous growth cycle. PAI-1 transcript accumulation peaked in mid-G1 phase (approx. 4-6 h post-stimulation) and declined prior to, or concomitant with, the onset of DNA synthetic phase. Serum increased PAI-1 expression in NRK cells in agarose suspension, as well as monolayer, culture; induction in suspended cells (similar to monolayer culture conditions) also occurred in the presence of cyclohexamide or puromycin. The serum-inductive pathway leading to PAI-1 gene activation is thus functional regardless of adhesive conditions or capacity for de novo protein synthesis. The amplitude of induction and maintenance of expression in later stages of G1, however, were subject to adhesive influences. PAI-1 transcript accumulation at 4 and 8 h post-stimulation in newly adherent cells, moreover, was blocked by puromycin, indicating that both immediate-early and secondary mechanisms regulate PAI-1 mRNA levels during progression of NRK cells through an 'activated' G1 growth phase.

Targeted disruption of vinculin genes in F9 and embryonic stem cells changes cell morphology, adhesion, and locomotion

Vinculin, a major constituent of focal adhesions and zonula adherens junctions, is thought to be involved in linking the microfilaments to areas of cell-substrate and cell-cell contacts. To test the role of vinculin in cell adhesion and motility, we used homologous recombination to generate F9 embryonal carcinoma and embryonic stem cell clones homozygous for a disrupted vinculin gene. When compared to wild-type cells, vinculin-mutant cells displayed a rounder morphology and a reduced ability to adhere and spread on plastic or fibronectin. Decreased adhesion of the mutant cells was associated with a reduction in lamellipodial extensions, as observed by time-lapse video microscopy. The locomotive activities of control F9 and the vinculin-null cells were compared in two assays. Loss of vinculin resulted in a 2.4-fold increase in cell motility. These results demonstrate an important role for vinculin in determining cell shape, adhesion, surface protrusive activity, and cell locomotion.

Release from quiescence stimulates the expression of integrin alpha 5 beta 1 which regulates DNA synthesis in human fibrosarcoma HT1080 cells

We show that integrin alpha 5 subunit expression is stimulated when human fibrosarcoma HT1080 cells are released from quiescence. The alpha 5 subunit mRNA level in quiescent HT1080 cells was increased 24 hr after their release by 10% fetal bovine serum-containing medium reaching a maximum of 2.5 fold on day 2. Similar levels of induction of cell-surface alpha 5 subunit protein as well as beta 1 subunit protein were also observed. This resulted in a significant increase of cell attachment to fibronectin. The serum stimulation also increased alpha 5 subunit promoter activity by twofold which was protein synthesis independent. Subsequent deletion of alpha 5 subunit promoter DNA showed that the cis-element responsible for the activation is located between -92 bp and the transcription start site. The promoter activity was not induced until 12 hr after the release. Comparison of the effect of a serum-free medium and a 10% fetal bovine serum-supplemented medium revealed that both the DNA synthesis and alpha 5 subunit induction were independent of exogenous growth factors. The increased integrin alpha 5 beta 1 appears to function by reducing mitogenic activity since blockade of fibronectin binding to its receptor with a RGD peptide, a monoclonal anti-fibronectin antibody, or a monoclonal anti-alpha 5 subunit antibody during the release from quiescence significantly stimulated DNA synthesis. On the other hand, stable overexpression of the alpha 5 subunit resulted in decreased DNA synthesis.

Impact of altered actin gene expression on vinculin, talin, cell spreading, and motility

Previous studies have demonstrated a strong correlation between the expression of vinculin and the shape and motility of a cell (Rodriguez Fernandez et al., 1992a, b, 1993). This hypothesis was tested by comparing the expression of vinculin and talin with the motility of morphologically altered myoblasts. These mouse C2 myoblasts were previously generated by directly perturbing the cell cytoskeleton via the stable transfection of a mutant-form of the beta-actin gene (beta sm) and three different forms of the gamma-actin gene; gamma, gamma minus 3'UTR (gamma delta'UTR), and gamma minus intron III (gamma delta IVSIII) (Schevzov et al., 1992; Lloyd and Gunning, 1993). In the case of the beta sm and gamma-actin transfectants, a two-fold decrease in the cell surface area was coupled, as predicted, with a decrease in vinculin and talin expression. In contrast, the gamma delta IVSIII transfectants with a seven-fold decrease in the cell surface area showed an unpredicted slight increase in vinculin and talin expression and the gamma delta 3'-UTR transfectants with a slight increase in the cell surface area showed no changes in talin expression and a decrease in vinculin expression. We conclude that changes in actin gene expression alone can impact on the expression of vinculin and talin. Furthermore, we observed that these actin transfectants failed to show a consistent relationship between cell shape, motility, and the expression of vinculin. However, a relationship between talin and cell motility was found to exist, suggesting a role for talin in the establishment of focal contacts necessary for motility.

Adhesion systems in normal breast and in invasive breast carcinoma

To analyze the role of various elements of the adhesion system in the organization of the normal mammary gland and in breast carcinoma, we have studied simultaneously the expression of integrins, E- and P-cadherins, and cytoplasmic constituents of adherens junctions. In the normal gland, E-cadherin and alpha-catenin are present in luminal epithelial and myoepithelial cells, whereas integrins are more abundant in acinar epithelial and in myoepithelial cells. We demonstrate here that, in addition, myoepithelial cells express much more vinculin and alpha-actinin than luminal epithelial cells, whereas talin and focal adhesion kinase (pp125FAK) are restricted to the basal cell layer. In invasive carcinoma, E-cadherin is usually present although often in reduced amount; different integrin subunits are expressed either by a fraction or by all of the cells or are absent. However, the cytoplasmic components of adherens junctions, such as alpha-catenin, vinculin, alpha-actinin, talin, and pp125FAK, are expressed at low levels or cannot be detected in the carcinoma cells. Our data suggest that 1), in the normal mammary gland, the myoepithelial cells, being particularly rich in integrins and cytoplasmic components of the adherens junctions, play an important role in the maintenance of tissue integrity; 2), in invasive carcinoma, cell aggregates may be maintained due to varying levels of expression of E-cadherin and/or integrins; and 3), interaction of the transmembrane adhesion molecules with the cytoskeleton in carcinoma may be impaired as revealed by reduced levels of expression of alpha-catenin, vinculin, alpha-actinin, talin, and pp125FAK. Importantly, carcinoma cells, when exposed to stroma during invasion, do not acquire the adhesion apparatus characteristic of normal cells in contact with the extracellular matrix.

The state of actin assembly regulates actin and vinculin expression by a feedback loop

Actin filaments are major determinants of cell shape, motility and adhesion, which control important biological processes including embryonic development and wound healing. These processes are associated with changes in actin assembly, which is regulated by controlling the balance between polymerized and non-polymerized actin. To maintain a significant pool of non-polymerized actin, mechanism(s) linking actin synthesis to its state of polymerization were proposed. We have studied this relationship between actin synthesis and organization by modulating actin assembly using different drugs. Unassembled actin was increased in 3T3 cells using either the Clostridium botulinum C2 toxin, which ADP-ribosylates actin, or by latrunculin A, a Red Sea sponge product, which binds monomeric actin. The synthesis of actin was dramatically reduced in these cells owing to a concomitant decrease in actin RNA level. Similar results were obtained with HeLa cells grown in both monolayer and in suspension, suggesting that cell shape changes associated with drug treatment are not the primary cause for the effect on actin synthesis. In contrast, the scrape-loading of 3T3 cells with phalloidin, a stabilizer of polymerized actin that increased the level of assembled actin, resulted in elevated actin synthesis and RNA content. The expression of vinculin, a major component of adhesion plaques and cell-cell junctions, which is involved in actin-membrane associations, was altered in parallel with that of actin in cells treated with these drugs. The decrease in actin RNA resulted from destabilization of actin mRNA in cells where unassembled actin level was elevated. This is suggested by the unchanged transcription of actin in isolated nuclei from drug-treated cells, and by demonstrating that actin mRNA was degraded faster in cells after C2 toxin treatment than in control cells. This feedback control mechanism is mainly confined to the cytoplasm, as it remained active in enucleated cells. The results suggest the existence of an autoregulatory pathway for the expression of actin and other microfilament-associated proteins which is linked to the state of actin polymerization in the cell.

Contractile activity and cell-cell contact regulate myofibrillar organization in cultured cardiac myocytes

Adult feline ventricular myocytes cultured on a laminin-coated substratum reestablish intercellular junctions, yet disassemble their myofibrils. Immunofluorescence microscopy reveals that these non-beating heart cells lack vinculin-positive focal adhesions; moreover, intercellular junctions are also devoid of vinculin. When these quiescent myocytes are stimulated to contract with the beta-adrenergic agonist, isoproterenol, extensive vinculin-positive focal adhesions and intercellular junctions emerge. If solitary myocytes are stimulated to beat, an elaborate series of vinculin-positive focal adhesions develop which appear to parallel the reassembly of myofibrils. In cultures where neighboring myocytes reestablish cell-cell contact, myofibrils appear to reassemble from the fascia adherens rather than focal contacts. Activation of beating is accompanied by a significant reduction in the rate of total and cytoskeletal protein synthesis; in fact, myofibrillar reassembly, redevelopment of focal adhesions and fascia adherens junctions require no protein synthesis for at least 24 h, implying the existence of an assembly competent pool of cytoskeletal proteins. Maturation of the fasciae adherens and the appearance of vinculin within Z-line/costameres, does require de novo synthesis of new cytoskeletal proteins. Changes in cytoskeletal protein turnover appear dependent on beta agonist-induced cAMP production, but myofibrillar reassembly is a cAMP-independent event. Such observations suggest that mechanical forces, in the guise of contractile activity, regulate vinculin distribution and myofibrillar order in cultured adult feline heart cells.

Suppression of vinculin expression by antisense transfection confers changes in cell morphology, motility, and anchorage-dependent growth of 3T3 cells

The expression of vinculin, a major component of adhesion plaques and cell-cell junctions, is markedly modulated in cells during growth activation, differentiation, motility and cell transformation. The stimulation of quiescent cells by serum factors and the culturing of cells on highly adhesive matrices induce vinculin gene expression, whereas the transformation of fibroblast and epithelial cells often results in decreased vinculin expression (reviewed in Rodríguez Fernández, J. L., B. Geiger, D. Salomon, I. Sabanay, M. Zöller, and A. Ben-Ze'ev. 1992. J. Cell Biol. 119:427). To study the effect of reduced vinculin expression on cell behavior, 3T3 cells were transfected with an antisense vinculin cDNA construct, and clones displaying decreased vinculin levels down to 10-30% of control levels were isolated. These cells showed a round phenotype with smaller and fewer vinculin-positive plaques localized mostly at the cell periphery. In addition, they displayed an increased motility compared to controls, manifested by a faster closure of "wounds" introduced into the monolayer, and by the formation of longer phagokinetic tracks. Moreover, the antisense transfectants acquired a higher cloning efficiency and produced larger colonies in soft agar than the parental counterparts. The results demonstrate that the regulation of vinculin expression in cells can affect, in a major way, cell shape and motility, and that decreased vinculin expression can induce cellular changes reminiscent of those found in transformed cells.

Expression of chicken vinculin complements the adhesion-defective phenotype of a mutant mouse F9 embryonal carcinoma cell

A mutant cell line, derived from the mouse embryonal carcinoma cell line F9, is defective in cell-cell adhesion (compaction) and in cell-substrate adhesion. We have previously shown that neither uvomorulin (E-cadherin) nor integrins are responsible for the mutant phenotype (Calogero, A., M. Samuels, T. Darland, S. A. Edwards, R. Kemler, and E. D. Adamson. 1991. Dev. Biol. 146:499-508). Several cytoskeleton proteins were assayed and only vinculin was found to be absent in mutant (5.51) cells. A chicken vinculin expression vector was transfected into the 5.51 cells together with a neomycin-resistance vector. Clones that were adherent to the substrate were selected in medium containing G418. Two clones, 5.51Vin3 and Vin4, were analyzed by Nomarski differential interference contrast and laser confocal microscopy as well as by biochemical and molecular biological techniques. Both clones adhered well to substrates and both exhibited F-actin stress fibers with vinculin localized at stress fiber tips in focal contacts. This was in marked contrast to 5.51 parental cells, which had no stress fibers and no vinculin. The mutant and complemented F9 cell lines will be useful models for examining the complex interactions between cytoskeletal and cell adhesion proteins.

Suppression of tumorigenicity in simian virus 40-transformed 3T3 cells transfected with alpha-actinin cDNA

Human cytoskeletal alpha-actinin cDNA was transfected into highly malignant simian virus 40-transformed BALB/c 3T3 (SVT2) cells that express 6-fold lower levels of alpha-actinin than nontransformed BALB/c 3T3 cells. SVT2 clones expressing various levels of alpha-actinin were isolated and their structure and tumorigenic properties were determined. Transfected SVT2 clones expressing alpha-actinin at levels found in nontumorigenic 3T3 cells displayed a flatter phenotype, a decreased ability to grow in suspension culture in soft agar, and a marked reduction in their ability to form tumors in syngeneic BALB/c mice and in athymic nude mice. Clones overexpressing alpha-actinin at the highest level (about 2-fold higher than 3T3 cells) were completely suppressed in their ability to form tumors in syngeneic BALB/c mice. The results suggest that alpha-actinin, an actin-crosslinking protein that is also localized in cell junctions, may have an effective suppressive ability on the transformed phenotype.

Suppression of tumorigenicity in transformed cells after transfection with vinculin cDNA

Transfection of chicken vinculin cDNA into two tumor cell lines expressing diminished levels of the endogenous protein, brought about a drastic suppression of their tumorigenic ability. The SV-40-transformed Balb/c 3T3 line (SVT2) contains four times less vinculin than the parental 3T3 cells, and the rat adenocarcinoma BSp73ASML has no detectable vinculin. Restoration of vinculin in these cells, up to the levels found in 3T3 cells, resulted in an apparent increase in substrate adhesiveness, a decrease in the ability to grow in soft agar, and suppression of their capacity to develop tumors after injection into syngeneic hosts or nude mice. These results suggest that vinculin, a cytoplasmic component of cell-matrix and cell-cell adhesions, may have a major suppressive effect on the transformed phenotype.

Regulation of adherens junction protein expression in growth-activated 3T3 cells and in regenerating liver

The expression of the adherens junction proteins vinculin, alpha-actinin, and talin was compared in serum-stimulated 3T3 cells and in regenerating rat liver following partial hepatectomy. The levels of vinculin RNA and protein synthesis were rapidly and transiently elevated in growth-activated fibroblasts (peaking at 2-3 h) and in regenerating liver (at 4-8 h), preceding the replicative stage. alpha-Actinin expression was also induced, but more slowly (peaking at 6-8 h in 3T3 cells and at 28 h in regenerating liver), and remained elevated when DNA synthesis was proceeding in both systems. The expression of talin RNA was only slightly elevated in 3T3 cells following serum stimulation, and it remained largely unchanged in regenerating liver. The levels of RNA coding for fibronectin and for the beta 1-integrin subunit were transiently and extensively induced during liver regeneration (fibronectin with a peak at 8 h and beta 1-integrin at 12 h). The uvomorulin RNA level, and the expression of the liver-specific genes albumin and transthyretin, decreased in regenerating liver. The results suggest a physiologically significant regulation in the expression of structural components which link the extracellular matrix to the microfilament system in growth-activated fibroblasts and in regenerating liver.

Differential expression of vinculin between weakly and highly metastatic B16-melanoma cell lines

We previously reported on the altered expression of a third actin in mouse-B16 melanoma associated with malignant progression. While further investigating the relationship of cytoskeletal proteins to malignancy, we found that the expression of vinculin was higher in weakly metastatic B16-F1 cells than in highly metastatic B16-F10 cells. By Northern blot analysis, the mRNA expression of vinculin in B16-F1 was also shown to be higher than in B16-F10. Immunofluorescence staining showed a clear dotted distribution of vinculin in B16-F1, but only a weak and diffuse distribution in B16-F10. The dotted distribution tended to be larger in B16-F1 and when cultured on Matrigel and fibronectin than on laminin and type IV collagen. An alteration in the expression of vinculin was also observed in other cell systems. Vinculin was detected in both normal 3Y1 and in relatively weakly malignant transformed 3Y1 cell lines, while vinculin was either scarcely detected or not detected at all in more malignant cell lines. These results suggest that the suppression of vinculin is closely related to malignant progression in both the B16-melanoma and 3Y1 cell systems.

Transfection of nonmuscle beta- and gamma-actin genes into myoblasts elicits different feedback regulatory responses from endogenous actin genes

We have examined the role of feedback-regulation in the expression of the nonmuscle actin genes. C2 mouse myoblasts were transfected with the human beta- and gamma-actin genes. In gamma-actin transfectants we found that the total actin mRNA and protein pools remained unchanged. Increasing levels of human gamma-actin expression resulted in a progressive down-regulation of mouse beta- and gamma-actin mRNAs. Transfection of the beta-actin gene resulted in an increase in the total actin mRNA and protein pools and induced an increase in the levels of mouse beta-actin mRNA. In contrast, transfection of a beta-actin gene carrying a single-point mutation (beta sm) produced a feedback-regulatory response similar to that of the gamma-actin gene. Expression of a beta-actin gene encoding an unstable actin protein had no impact on the endogenous mouse actin genes. This suggests that the nature of the encoded actin protein determines the feedback-regulatory response of the mouse genes. The role of the actin cytoskeleton in mediating this feedback-regulation was evaluated by disruption of the actin network with Cytochalasin D. We found that treatment with Cytochalasin D abolished the down-regulation of mouse gamma-actin in both the gamma- and beta sm-actin transfectants. In contrast, a similar level of increase was observed for the mouse beta-actin mRNA in both control and transfected cells. These experiments suggest that the down-regulation of mouse gamma-actin mRNA is dependent on the organization of the actin cytoskeleton. In addition, the mechanism responsible for the down-regulation of beta-actin may be distinct from that governing gamma-actin. We conclude that actin feedback-regulation provides a biochemical assay for differences between the two nonmuscle actin genes.

Overexpression of vinculin suppresses cell motility in BALB/c 3T3 cells

The content of vinculin, a cytoplasmic protein found in focal contacts and cell-cell junctions, was increased in BALB/c 3T3 cells by gene transfection. The vinculin expressed from the full length chicken cDNA, incorporated into focal contacts and its pattern was identical to that of the endogenous protein. Cells stably expressing vinculin by 20% over the endogenous level had altered locomotory properties. In these cells, the ability to migrate into a wound formed in a confluent monolayer and the locomotion of individual cells were drastically reduced. The results provide direct evidence that cell locomotion can be regulated by modulating vinculin expression.

Animal cell shape changes and gene expression

Cell shape and cell contacts are determined by transmembrane receptor-mediated associations of the cytoskeleton with specific extracellular matrix proteins and with ligands on the surface of adjacent cells. The cytoplasmic domains of these microfilament-membrane associations at the adherens junction sites, also localize a variety of regulatory molecules involved in signal transduction and gene regulation. The stimulation of cells with soluble polypeptide factors leads to rapid changes in cell shape and microfilament component organization. In addition, this stimulation also activates the phosphoinositide signaling pathway. Recently, a linkage between actin-binding proteins and the phosphoinositide signaling pathway, was discovered. It is suggested that by the association with the second messenger system, and/or by controlling the localization of regulatory molecules, the cytoskeleton may regulate gene expression.