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

MiR-200a Regulates Nasopharyngeal Carcinoma Cell Migration and Invasion by Targeting MYH10

Nasopharyngeal carcinoma (NPC), is one of the most common malignant tumor in southern China and southeast Asia. MYH10 is a coding gene of the NMMHC-IIB protein. Previous studies have shown that MYH10 expression was up-regulated in breast cancer, glioma and meningioma. Moreover, it was targeted by miR200 family. However, no relevant studies have been found in NPC. In present study, we found in 48 NPC specimens, MYH10 level was lower in most cancer areas than that in the adjacent normal tissue. Moreover, the depletion of MYH10 can promote the migration and invasion of NPC. In addition, we demonstrated that miR-200a has the strongest regulation to MYH10 among miR-200 family. miR-200a mimics could decrease MYH10 expression, while miR-200a inhibitor increase MYH10 expression. Next, we found that miR-200a bound directly to MYH10 using Dual-luciferase reporter. Finally, it was demonstrated that siMYH10 could reverse the effect of miR-200a inhibitor on NPC cell migration and invasion. Taken together, it can be concluded that MYH10 is lowly expressed in NPC compared with adjacent tissues, and the loss of MYH10 can promote the migration and invasion of NPC cells; Among the miR-200 family, miR-200a has the strongest regulatory effect on MYH10; MYH10 is a direct target gene of miR200a, and miR200a targets MYH10 to regulate the migration and invasion of NPC cells.

X-RAY IRRADIATION AFFECTS MORPHOLOGY, PROLIFERATION AND MIGRATION RATE OF HEALTHY AND CANCER CELLS

Cytoskeleton plays a central role in many cellular processes, such as migration, adhesion and proliferation. Alterations of its structural properties are commonly associated with different diseases (malignancy, cardiac hypertrophy, etc.). In this work, we studied the effects of X-radiations on cytoskeleton architecture of two cell lines: BALBc/3T3 and Simian virus 40-transformed BALBc/3T3 (SVT2) cells. In agreement with the current literature, we observed reduced adhesion and increased motility of SVT2 cells respect to non-transformed BALBc/3T3. In addition, we showed that two different doses of X-rays (1 and 2 Gy) increased cell-dish adhesiveness and reduced cell proliferation and cell motility of transformed cells, whereas minor effects were measured on the normal counterpart. These results suggested that low doses or fractioning of X-rays may have a normalization effect on the investigated parameters for the transformed cell phenotype.

The non-muscle functions of actinins: an update

α-Actinins are a major class of actin filament cross-linking proteins expressed in virtually all cells. In muscle, actinins cross-link thin filaments from adjacent sarcomeres. In non-muscle cells, different actinin isoforms play analogous roles in cross-linking actin filaments and anchoring them to structures such as cell-cell and cell-matrix junctions. Although actinins have long been known to play roles in cytokinesis, cell adhesion and cell migration, recent studies have provided further mechanistic insights into these functions. Roles for actinins in synaptic plasticity and membrane trafficking events have emerged more recently, as has a 'non-canonical' function for actinins in transcriptional regulation in the nucleus. In the present paper we review recent advances in our understanding of these diverse cell biological functions of actinins in non-muscle cells, as well as their roles in cancer and in genetic disorders affecting platelet and kidney physiology. We also make two proposals with regard to the actinin nomenclature. First, we argue that naming actinin isoforms according to their expression patterns is problematic and we suggest a more precise nomenclature system. Secondly, we suggest that the α in α-actinin is superfluous and can be omitted.

Actinin-4 in keratinocytes regulates motility via an effect on lamellipodia stability and matrix adhesions

During wound repair, epidermal cells at the edge of an injury establish front-rear polarity through orchestrated changes in their cytoskeleton and adhesion structures. The polarity and directed migration of such cells is determined by the assembly, extension, and stabilization of a lamellipodium. Actinin-4 associates with lamellipodia and has been implicated in regulating lamellipodial structure, function and assembly. To study the functions of actinin-4 in human keratinocytes, we used shRNA to generate knockdown cells and compared their motility behavior and matrix adhesion assembly to scrambled shRNA treated control keratinocytes. Actinin-4 knockdown keratinocytes lack polarity, assemble multiple lamellipodia with a 2× increased area over controls, display reduced activity of the actin remodeling protein cofilin, and fail to migrate in a directional manner. This motility defect is rescued by plating knockdown cells on preformed laminin-332 matrix. In actinin-4-knockdown keratinocytes, focal contact area is increased by 25%, and hemidesmosome proteins are mislocalized. Specifically, α6β4 integrin localizes to large lamellipodial extensions, displays reduced dynamics, and fails to recruit its bullous pemphigoid antigen binding partners. Together, our data indicate a role for actinin-4 in regulating the steering mechanism of keratinocytes via profound effects on their matrix adhesion sites.

Rac-dependent doubling of HeLa cell area and impairment of cell migration and cell cycle by compounds from Iris germanica

Plants are an infinite source of bioactive compounds. We screened the Israeli flora for compounds that interfere with the organization of the actin cytoskeleton. We found an activity in lipidic extract from Iris germanica that was able to increase HeLa cell area and adhesion and augment the formation of actin stress fibers. This effect was not observed when Ref52 fibroblasts were tested and was not the result of disruption of microtubules. Further, the increase in cell area was Rac1-dependent, and the iris extract led to slight Rac activation. Inhibitor of RhoA kinase did not interfere with the ability of the iris extract to increase HeLa cell area. The increase in HeLa cell area in the presence of iris extract was accompanied by impairment of cell migration and arrest of the cell cycle at G1 although the involvement of Rac1 in these processes is not clear. Biochemical verification of the extract based on activity-mediated fractionation and nuclear magnetic resonance analysis revealed that the active compounds belong to the group of iridals, a known group of triterpenoid. Purified iripallidal was able to increase cell area of both HeLa and SW480 cells.

Activity-based protein profiling of host-virus interactions

Virologists have benefited from large-scale profiling methods to discover new host–virus interactions and to learn about the mechanisms of pathogenesis. One such technique, referred to as activity-based protein profiling (ABPP), uses active site-directed probes to monitor the functional state of enzymes, taking into account post-translational interactions and modifications. ABPP gives insight into the catalytic activity of enzyme families that does not necessarily correlate with protein abundance. ABPP has been used to investigate several viruses and their interactions with their hosts. Differential enzymatic activity induced by viruses has been monitored using ABPP. In this review, we present recent advances and trends involving the use of ABPP methods in understanding host–virus interactions and in identifying novel targets for diagnostic and therapeutic applications.

HAMLET binding to α-actinin facilitates tumor cell detachment

Cell adhesion is tightly regulated by specific molecular interactions and detachment from the extracellular matrix modifies proliferation and survival. HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) is a protein-lipid complex with tumoricidal activity that also triggers tumor cell detachment in vitro and in vivo, suggesting that molecular interactions defining detachment are perturbed in cancer cells. To identify such interactions, cell membrane extracts were used in Far-western blots and HAMLET was shown to bind α-actinins; major F-actin cross-linking proteins and focal adhesion constituents. Synthetic peptide mapping revealed that HAMLET binds to the N-terminal actin-binding domain as well as the integrin-binding domain of α-actinin-4. By co-immunoprecipitation of extracts from HAMLET-treated cancer cells, an interaction with α-actinin-1 and -4 was observed. Inhibition of α-actinin-1 and α-actinin-4 expression by siRNA transfection increased detachment, while α-actinin-4-GFP over-expression significantly delayed rounding up and detachment of tumor cells in response to HAMLET. In response to HAMLET, adherent tumor cells rounded up and detached, suggesting a loss of the actin cytoskeletal organization. These changes were accompanied by a reduction in β1 integrin staining and a decrease in FAK and ERK1/2 phosphorylation, consistent with a disruption of integrin-dependent cell adhesion signaling. Detachment per se did not increase cell death during the 22 hour experimental period, regardless of α-actinin-4 and α-actinin-1 expression levels but adherent cells with low α-actinin levels showed increased death in response to HAMLET. The results suggest that the interaction between HAMLET and α-actinins promotes tumor cell detachment. As α-actinins also associate with signaling molecules, cytoplasmic domains of transmembrane receptors and ion channels, additional α-actinin-dependent mechanisms are discussed.

Polymorphism of highly cross-linked F-actin networks: probing multiple length scales

The assembly properties of F-actin filaments in the presence of different biological cross-linker concentrations and types have been investigated using a combined approach of fluorescence confocal microscopy and coarse-grained molecular dynamics simulation. In particular for highly cross-linked regimes, new network morphologies are observed. Complex network formation and the details of the resulting structure are strongly dependent on the ratio of cross-linkers to actin monomers and cross-linker shape but only weakly dependent on overall actin concentration and filament length. The work presented here may help to provide some fundamental understanding of how excessive cross-linkers interact with the actin filament solution, creating different structures in the cell under high cross-linker concentrations. F-actin is not only of biological importance but also, as an example of a semiflexible polymer, has attracted significant interest in its physical behavior. In combination with different cross-linkers semiflexible filaments may provide new routes to bio-materials development and act as the inspiration for new hierarchical network-based materials.

Isoform-specific contributions of alpha-actinin to glioma cell mechanobiology

Glioblastoma Multiforme (GBM) is a malignant astrocytic tumor associated with low survival rates because of aggressive infiltration of tumor cells into the brain parenchyma. Expression of the actin binding protein alpha-actinin has been strongly correlated with the invasive phenotype of GBM in vivo. To probe the cellular basis of this correlation, we have suppressed expression of the nonmuscle isoforms alpha-actinin-1 and alpha-actinin-4 and examined the contribution of each isoform to the structure, mechanics, and motility of human glioma tumor cells in culture. While subcellular localization of each isoform is distinct, suppression of either isoform yields a phenotype that includes dramatically reduced motility, compensatory upregulation and redistribution of vinculin, reduced cortical elasticity, and reduced ability to adapt to changes in the elasticity of the extracellular matrix (ECM). Mechanistic studies reveal a relationship between alpha-actinin and non-muscle myosin II in which depletion of either alpha-actinin isoform reduces myosin expression and maximal cell-ECM tractional forces. Our results demonstrate that both alpha-actinin-1 and alpha-actinin-4 make critical and distinct contributions to cytoskeletal organization, rigidity-sensing, and motility of glioma cells, thereby yielding mechanistic insight into the observed correlation between alpha-actinin expression and GBM invasiveness in vivo.

Overexpression of Plk3 causes morphological change and cell growth suppression in Ras pathway-activated cells

To unravel the growth inhibition mechanism of Polo-like kinase 3 (Plk3), the effect of overexpression of Plk3 was examined in 293T cells. Cell rounding, changes in actin organization and cellular detachment were induced by Plk3 transfection in a kinase activity-dependent manner. Although apoptosis was not observed, Plk3 overexpression suppressed cellular growth in a long-term colony-forming assay. Because both Plk3 and Ras affect F-actin organization, the effect of co-transfection of Plk3 and Ras was evaluated. Adhesion was synergistically lost by co-transfection of these two genes, compared with transfection of Plk3 alone. Furthermore, overexpression of Plk3 caused long-term growth suppression in Ras-transformed NIH3T3. Collectively, Plk3 activation might cause cytoskeleton re-organization and result in growth suppression more pronouncedly in Ras pathway-activated cells.

Oncogenic Ras-induced morphologic change is through MEK/ERK signaling pathway to downregulate Stat3 at a posttranslational level in NIH3T3 cells

Ras is a key regulator of the MAP kinase-signaling cascade and may cause morphologic change of Ras-transformed cells. Signal transducer and activator of transcription 3 (Stat3) can be activated by cytokine stimulation. In this study, we unravel that Ha-ras(V12) overexpression can downregulate the expression of Stat3 protein at a posttranslational level in NIH3T3 cells. Furthermore, we demonstrate that Stat3 expression downregulated by Ha-ras(V12) overexpression is through proteosome degradation and not through a mTOR/p70S6K-related signaling pathway. The suppression of Stat3 accompanied by the morphologic change induced by Ha-ras(V12) was through mitogen extracellular kinase (MEK)/extracellular-regulated kinase (ERK) signaling pathway. Microtubule disruption is involved in Ha-ras(V12)-induced morphologic change, which could be reversed by overexpression of Stat3. Taken together, we are the first to demonstrate that Stat3 protein plays a critical role in Ha-ras(V12)-induced morphologic change. Oncogenic Ras-triggered morphologic change is through the activation of MEK/ERK to posttranslationally downregulate Stat3 expression. Our finding may shed light on developing novel therapeutic strategies against Ras-related tumorigenesis.

Profilin-1 is a negative regulator of mammary carcinoma aggressiveness

Expression of profilin-1 (Pfn1) is downregulated in breast cancer cells, the functional significance of which is yet to be understood. To address this question, in this study we evaluated how perturbing Pfn1 affects motility and invasion of breast cancer cells. We show that loss of Pfn1 expression leads to enhanced motility and matrigel invasiveness of MDA-MB-231 breast cancer cells. Interestingly, silencing Pfn1 expression is associated with downregulation of both cell–cell and cell–matrix adhesions with concomitant increase in motility and dramatic scattering of normal human mammary epithelial cells. Thus, these data for the first time suggest that loss of Pfn1 expression may have significance in breast cancer progression. Consistent with these findings, even a moderate overexpression of Pfn1 induces actin stress-fibres, upregulates focal adhesion, and dramatically inhibits motility and matrigel invasiveness of MDA-MB-231 cells. Using mutants of Pfn1 that are defective in binding to either actin or proline-rich ligands, we further show that overexpressed Pfn1 must have a functional actin-binding site to suppress cell motility. Finally, animal experiments reveal that overexpression of Pfn1 suppresses orthotopic tumorigenicity and micro-metastasis of MDA-MB-231 cells in nude mice. These data imply that perturbing Pfn1 could be a good molecular strategy to limit the aggressiveness of breast cancer cells.

Oncogenic signaling pathways and deregulated target genes

A limited number of somatic mutations are known to trigger malignancy via chronic activation of cellular signaling pathways. High-throughput analysis of gene expression in cancer cells has revealed a plethora of deregulated genes by far exceeding the number of known genetic alterations. Targeted tumor therapy takes advantage of deregulated signaling in cancer. However, cancer cells may evade successful therapy, e.g., targeting oncogenic kinases, due to mutation of the target protein or to resistance mechanisms acting downstream of or parallel to the therapeutic block. To improve therapy and molecular diagnostics, we need detailed information on the wiring of pathway components and targets that ultimately execute the malignant properties of advanced tumors. Here we review work on Ras-mediated signal transduction and Ras pathway-responsive targets. We introduce the concept of signal-regulated transcriptional modules comprising groups of target genes responding to individual branches of the pathway network. Furthermore, we discuss functional approaches based on RNA interference for elucidating critical nodes in oncogenic signaling and the targets essential for malignancy.

Interaction of zonula occludens-1 (ZO-1) with alpha-actinin-4: application of functional proteomics for identification of PDZ domain-associated proteins

The use of recombinant "bait" proteins to capture protein-binding partners, followed by identification of protein interaction networks by mass spectrometry (MS), has gained popularity and widespread acceptance. We have developed an approach using recombinant PDZ protein interaction modules of the membrane-associated guanylate kinase (MAGUK) protein zonula occludens-1 (ZO-1) to pull-down and screen for proteins that interact with these modules via their PDZ domain binding motifs. Identification of proteins by MS of pull-down material was achieved using a vacuum-based chromatography sample preparation device designed for matrix-assisted laser desorption/ionization (MALDI) MS. MS analysis of tryptic fragments in pull-down material revealed a number of potential ZO-1 interacting candidates, including the presence of peptides corresponding to the cortical membrane scaffolding protein alpha-actinin-4. Interaction of alpha-actinin-4 with ZO-1 was confirmed by coimmunoprecipitation of these two proteins from cultured cells, as well as from brain, liver, and heart, and by immunoblot detection of alpha-actinin-4 after pull-down with the first PDZ domain of ZO-1. In contrast, the highly homologous alpha-actinin family member, alpha-actinin-1, displayed no association with ZO-1. Immunofluorescence showed colocalization of alpha-actinin-4 with ZO-1 in cultured HeLa and C6 glioma cells, as well as in a variety of tissues in vivo, including brain, heart, liver, and lung. This study demonstrates the utility of MS-based functional proteomics for identifying cellular components of the ZO-1 scaffolding network. Our finding of the interaction of ZO-1 with alpha-actinin-4 provides a mechanism for linking the known protein recruitment and signaling activities of ZO-1 with alpha-actinin-4-associated plasma membrane proteins that have regulatory activities at cell-cell and cell-extracellular matrix contacts.

Drosophila alpha-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

Roles for Rho/ROCK and vinculin in parietal endoderm migration

The first cell migration event in the mouse embryo is the movement of parietal endoderm cells from the surface of the inner cell mass facing the blastocoel cavity to line the inner surface of the trophectoderm. F9 embryoid bodies provide an in vitro model for this event. They have an inner core of undifferentiated stem cells surrounded by an outer visceral endoderm layer. When plated on a laminin coated substrate, visceral endoderm transitions to parietal endoderm and migrates onto the dish, away from the attached embryoid body. We now show that this outgrowth contains abundant focal complexes and focal adhesions, as well as lamellipodia and filopodia. Treatment with the ROCK inhibitor Y-27632 promotes a 2-fold increase in outgrowth, and a transition from focal adhesions and associated stress fibers, to focal complexes and a decrease in stress fibers. ROCK inhibition also leads to an increase in lamellipodia. Inhibition of RhoA by transfection of a vector encoding C3 transferase, direct administration of the C3 enzyme, or transfection of a vector encoding p190 Rho GTPase Activating Protein also promotes outgrowth and an apparent transition from focal adhesions to focal complexes. Parietal endoderm outgrowth generated using vinculin-deficient F9 stem cells migrates 2-fold further than wild type cultures, but this outgrowth retains the morphology of wild type parietal endoderm, including focal adhesions and stress fibers. Addition of Y-27632 to vinculin-null outgrowth cultures further stimulates migration an additional 2-fold, supporting the conclusion that Rho/ROCK and vinculin regulate parietal endoderm outgrowth by distinct pathways.

Amino terminal, but not the carboxy terminal, sequences of tropomyosin-1 are essential for the induction of stress fiber assembly in neoplastic cells

The presence of aberrant cytoskeleton, arising from the downregulation of key cytoskeletal proteins such as tropomyosins (TMs), is a prominent feature of many malignant cells and is suggested to promote neoplastic growth. While our previous work demonstrated that tropomyosin-1 (TM1) promotes stress fiber assembly and suppresses malignant growth, the molecular basis of the anti-oncogenic effects of TM1 has not been determined. By employing chimeric TMs, here we demonstrate that the amino terminal portion of TM1, but not the carboxy terminal portion which contains the alternatively spliced exon-coded sequences, is essential for stress fiber assembly and suppression of malignant growth. These studies also indicate that the amino and carboxy termini of TM1 coordinately function to regulate microfilament organization during cytokinesis.

Cell motility and local viscoelasticity of fibroblasts

Viscoelastic changes of the lamellipodial actin cytoskeleton are a fundamental element of cell motility. Thus, the correlation between the local viscoelastic properties of the lamellipodium (including the transitional region to the cell body) and the speed of lamellipodial extension is studied for normal and malignantly transformed fibroblasts. Using our atomic force microscopy-based microrheology technique, we found different mechanical properties between the lamellipodia of malignantly transformed fibroblasts (H-ras transformed and SV-T2 fibroblasts) and normal fibroblasts (BALB 3T3 fibroblasts). The average elastic constants, K, in the leading edge of SV-T2 fibroblasts (0.48 +/- 0.51 kPa) and of H-ras transformed fibroblasts (0.42 +/- 0.35 kPa) are significantly lower than that of BALB 3T3 fibroblasts (1.01 +/- 0.40 kPa). The analysis of time-lapse phase contrast images shows that the decrease in the elastic constant, K, for malignantly transformed fibroblasts is correlated with the enhanced motility of the lamellipodium. The measured mean speeds are 6.1 +/- 4.5 microm/h for BALB 3T3 fibroblasts, 13.1 +/- 5.2 microm/h for SV-T2 fibroblasts, and 26.2 +/- 11.5 microm/h for H-ras fibroblasts. Furthermore, the elastic constant, K, increases toward the cell body in many instances which coincide with an increase in actin filament density toward the cell body. The correlation between the enhanced motility and the decrease in viscoelastic moduli supports the Elastic Brownian Ratchet model for driving lamellipodia extension.

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

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

Human diploid fibroblasts are resistant to MEK/ERK-mediated disruption of the actin cytoskeleton and invasiveness stimulated by Ras

Ras-induced transformation is characterized not only by uncontrolled proliferation but also by drastic morphological changes accompanied by the disruption of the actin cytoskeleton. Previously, we reported that human fibroblasts are more resistant than rodent fibroblasts to Ras-induced transformation. To explore the molecular basis for the difference in susceptibility to Ras-induced transformation, we investigated the effect of activated H-Ras on the actin cytoskeleton in human diploid fibroblasts and in rat embryo fibroblasts, both of which are immortalized by SV40 early region. We demonstrate here that Ras-induced morphological changes, decreased expression of tropomyosin isoforms, and suppression of the ROCK/LIMK/Cofilin pathway observed in the rat fibroblasts were not detected in the human fibroblasts even with high expression levels of Ras. We also show that activation of the MEK/ERK pathway sufficed to induce all of these alterations in the rat fibroblasts, whereas the human fibroblasts were refractory to these MEK/ERK-mediated changes. In addition to morphological changes, we demonstrated that the expression of activated Ras induced an invasive phenotype in the rat, but not in the human fibroblasts. These studies provide evidence for the existence of human-specific mechanisms that resist Ras/MEK/ERK-mediated transformation.

Transcriptional basis of KRAS oncogene-mediated cellular transformation in ovarian epithelial cells

To understand the relationship between oncogenic signaling and the reprogramming of gene expression, we performed transcriptional profiling in rat ovarian surface epithelial cells (ROSE), in which neoplastic transformation is driven by a mutated KRAS oncogene. We identified >200 genes whose expression was elevated or reduced following permanent KRAS expression. Deregulated KRAS-responsive genes encode transcriptional regulators, signaling effectors, proteases, extracellular matrix and adhesion proteins, transformation-suppressing proteins and negative growth regulators. Many of them have not been previously identified in cells expressing oncogenic RAS genes or in other well-studied models of oncogenic signaling. The number of critical genes related to the execution of anchorage-independent proliferation and epithelial-mesenchymal transition was narrowed down to 79 by selectively inhibiting the mitogen-activated protein kinase (MAPK/ERK) and phosphatidylinositol 3-kinase (PI3K) pathways. Blocking MAPK/ERK-signaling caused reversion to the normal epithelial phenotype in conjunction with the reversal of deregulated target transcription to pretransformation levels. In addition, silencing of the overexpressed transcriptional regulator Fra-1 by RNA interference resulted in growth reduction, suggesting that this factor partially contributes to, but is not sufficient for the proliferative capacity of KRAS-transformed epithelial cells.

M34 Actin Regulatory Protein Is a Sensitive Diagnostic Marker for Early- and Late-Stage Mammary Carcinomas

PURPOSE

At present, there is no available molecular marker that reliably detects the earliest stages of epithelial transformation in the majority of patients affected with incipient breast carcinoma. Here we introduce M34 protein, a mammalian actin filament regulatory protein, as a highly sensitive and easily detected positive cellular marker for both early and late stages of breast carcinoma.

EXPERIMENTAL DESIGN

In this study, 24 human lactation duct neoplasms from postmenopausal women, including fibroadenoma, ductal carcinoma in situ, intraductal lobular papilloma, and metastatic adenocarcinoma, were analyzed for the presence of M34 protein by histochemical staining of paraffin and fresh-frozen sections.

RESULTS

All 24 neoplasias tested positive for M34, whereas none of the 4 normal breast tissues stained for the protein. M34 identification was strongly positive for transformed epithelium in all tumor types tested. Twelve precancerous lesions of fibroadenoma (n = 4), intraductal papilloma (n = 4), and incipient ductal carcinoma in situ (n = 4) all showed high levels of M34 staining, suggesting that precancerous tumors, as well as the earliest stages of mammary carcinoma, can be sensitively detected. Furthermore, anti-M34 antibody selectively stained all 12 advanced-stage metastatic adenocarcinoma cell masses and micrometastases in axillary lymph nodes tested. Single-cell micrometastases embedded in connective tissue or lymph node parenchyma could be clearly resolved by M34 with horseradish peroxidase staining. Lymphocytes, normal ductal endothelium, and vascular endothelial cells were M34-negative, as were muscle, nerve, and adipose tissues. Low-level M34 staining was detected in connective tissue fibroblasts, macrophages, and neutrophils.

CONCLUSIONS

To our knowledge, no previously reported markers have shown high sensitivity of detection for both the earliest and most advanced stages of breast carcinoma. Consequently, M34 appears uniquely suited for diagnosis of the earliest stages of lactation duct transformation as well as for advanced-stage mammary carcinoma metastases in surgical margins and axillary lymph nodes.

Mutant α-actinin-4 promotes tumorigenicity and regulates cell motility of a human lung carcinoma

The precise role of alpha-actinin-4 encoding gene (ACTN4) is not very well understood. It has been reported to elicit tumor suppressor activity and to regulate cellular motility. To further assess the function of human ACTN4, we studied a lung carcinoma cell line expressing a mutated alpha-actinin-4, which is recognized as a tumor antigen by autologous CD8(+) cytotoxic T lymphocytes (CTL). Confocal immunofluorescence microscopy indicated that, while wild-type (WT) alpha-actinin-4 stains into actin cytoskeleton and cell surface ruffles, the mutated protein is only dispersed in the cytoplasm of the lung carcinoma cells. This loss of association with the cell surface did not appear to correlate with a decrease in in vitro alpha-actinin-4 crosslinking to filamentous (F)-actin. Interestingly, experiments using cell lines stably expressing ACTN4 demonstrated that as opposed to WT gene, mutant ACTN4 was unable to inhibit tumor cell growth in vitro and in vivo. Moreover, the expression of mutant alpha-actinin-4 resulted in the loss of tumor cell capacity to migrate. The identification of an inactivating mutation in ACTN4 emphasizes its role as a tumor suppressor gene and underlines the involvement of cytoskeleton alteration in tumor development and metastasis.

Expression of the Arp11 gene suppresses the tumorigenicity of PC-14 human lung adenocarcinoma cells

We recently reported the isolation of a novel actin-related gene, Arp11, from a PC-14 human lung adenocarcinoma clone with low metastatic and tumorigenic ability. In this paper, we report the effect of Arp11 expression on PC-14 tumor growth. We transfected a highly metastatic and tumorigenic PC-14 line, Lu-2, with an Arp11 expression vector, and obtained five stable transfectants that each express Arp11 at different levels. The tumor growth rate of these transformants in nude mice and their colony formation in soft agar were reduced in proportion to their levels of Arp11 expression. Thus, overexpression of Arp11 may suppress tumorigenicity in nude mice. The morphology of the transfectants expressing high levels of Arp11 differed clearly from that of Lu-2 and the mock-transfectants as the Arp11-transfectants were flat and fibrous, while Lu-2 and the mock-transfectants were round and were released into the culture medium. An increase in the number of actin stress fibers was observed in the Arp11-transfectants 1-12-3-x and 2-9-2 but not in the mock-transfected lines. Our results suggest that the overexpression of Arp11 regulates the transcription of multiple genes that are involved in suppressing tumorigenicity.

Tumor suppressor activity of profilin requires a functional actin binding site

Profilin 1 (PFN1) is a regulator of the microfilament system and is involved in various signaling pathways. It interacts with many cytoplasmic and nuclear ligands. The importance of PFN1 for human tissue differentiation has been demonstrated by the findings that human cancer cells, expressing conspicuously low PFN1 levels, adopt a nontumorigenic phenotype upon raising their PFN1 level. In the present study, we characterize the ligand binding site crucial for profilin's tumor suppressor activity. Starting with CAL51, a human breast cancer cell line highly tumorigenic in nude mice, we established stable clones that express PFN1 mutants differentially defective in ligand binding. Clones expressing PFN1 mutants with reduced binding to either poly-proline-stretch ligands or phosphatidyl-inositol-4,5-bisphosphate, but with a functional actin binding site, were normal in growth, adhesion, and anchorage dependence, with only a weak tendency to elicit tumors in nude mice, similar to controls expressing wild-type PFN1. In contrast, clones expressing a mutant with severely reduced capacity to bind actin still behaved like the parental CAL51 and were highly tumorigenic. We conclude that the actin binding site on profilin is instrumental for normal differentiation of human epithelia and the tumor suppressor function of PFN1.

Phosphoinositide binding inhibits alpha-actinin bundling activity

alpha-Actinin is an abundant actin-bundling and adhesion protein that directly links actin filaments to integrin receptors. Previously, in platelet-derived growth factor-treated fibroblasts, we demonstrated that phosphoinositides bind to alpha-actinin, regulating its localization (Greenwood, J. A., Theibert, A. B., Prestwich, G. D., and Murphy-Ullrich, J. E. (2000) J. Cell Biol. 150, 627- 642). In this study, phosphoinositide binding and regulation of alpha-actinin function is further characterized. Phosphoinositide binding specificity, determined using a protein-lipid overlay procedure, suggests that alpha-actinin interacts with phosphates on the 4th and 5th position of the inositol head group. Binding assays and mutational analyses demonstrate that phosphoinositides bind to the calponin homology domain 2 of alpha-actinin. Phosphoinositide binding inhibited the bundling activity of alpha-actinin by blocking the interaction of the actin-binding domain with actin filaments. Consistent with these results, excessive bundling of actin filaments was observed in fibroblasts expressing an alpha-actinin mutant with decreased phosphoinositide affinity. We conclude that the interaction of alpha-actinin with phosphoinositides regulates actin stress fibers in the cell by controlling the extent to which microfilaments are bundled.

FHL3 is an actin-binding protein that regulates alpha-actinin-mediated actin bundling: FHL3 localizes to actin stress fibers and enhances cell spreading and stress fiber disassembly

Four and a half LIM domain (FHL) proteins are members of the LIM protein superfamily. Several FHL proteins function as co-activators of CREM/CREB transcription factors and the androgen receptor. FHL3 is highly expressed in skeletal muscle, but its function is unknown. FHL3 localized to the nucleus in C2C12 myoblasts and, following integrin engagement, exited the nucleus and localized to actin stress fibers and focal adhesions. In mature skeletal muscle FHL3 was found at the Z-line. Actin was identified as a potential FHL3 binding partner in yeast two-hybrid screening of a skeletal muscle library. FHL3 complexed with actin both in vitro and in vivo as shown by glutathione S-transferase pull-down assays and co-immunoprecipitation of recombinant and endogenous proteins. FHL3 promoted cell spreading and when overexpressed in spread C2C12 cells disrupted actin stress fibers. Increased FHL3 expression was detected in highly motile cells migrating into an artificial wound, compared with non-motile cells. The molecular mechanism by which FHL3 induced actin stress fiber disassembly was demonstrated by low speed actin co-sedimentation assays and electron microscopy. FHL3 inhibited alpha-actinin-mediated actin bundling. These studies reveal FHL3 as a significant regulator of actin cytoskeletal dynamics in skeletal myoblasts.

EPLIN regulates actin dynamics by cross-linking and stabilizing filaments

Epithelial protein lost in neoplasm (EPLIN) is a cytoskeleton-associated protein encoded by a gene that is down-regulated in transformed cells. EPLIN increases the number and size of actin stress fibers and inhibits membrane ruffling induced by Rac. EPLIN has at least two actin binding sites. Purified recombinant EPLIN inhibits actin filament depolymerization and cross-links filaments in bundles. EPLIN does not affect the kinetics of spontaneous actin polymerization or elongation at the barbed end, but inhibits branching nucleation of actin filaments by Arp2/3 complex. Side binding activity may stabilize filaments and account for the inhibition of nucleation mediated by Arp2/3 complex. We propose that EPLIN promotes the formation of stable actin filament structures such as stress fibers at the expense of more dynamic actin filament structures such as membrane ruffles. Reduced expression of EPLIN may contribute to the motility of invasive tumor cells.

Identification of Nd1, a novel murine kelch family protein, involved in stabilization of actin filaments

We isolated Nd1, a novel kelch family gene that encodes two forms of proteins, Nd1-L and Nd1-S. Nd1-L contains a BTB/POZ domain in its N terminus and six kelch repeats in the C terminus. Nd1-S has the BTB/POZ domain but lacks the six kelch repeats. Nd1-L but not Nd1-S mRNA is detected ubiquitously in normal mouse tissues. Nd1-L and Nd1-S proteins can form a dimer through the BTB/POZ domain. Nd1-L colocalizes with actin filaments detected using a confocal microscope, and its kelch repeats bind to them in vitro. Overexpression of Nd1-L in NIH3T3 cells delayed cell growth by affecting the transition of cytokinesis. Furthermore, the overexpression prevented NIH3T3 cells from cell death induced by actin destabilization but not by microtubule dysfunction. These data suggest that Nd1-L functions as a stabilizer of actin filaments as an actin-binding protein and may play a role in the dynamic organization of the actin cytoskeleton.

Antitumor cytotoxic T-lymphocyte response in human lung carcinoma: identification of a tumor-associated antigen

We have isolated several cytotoxic T lymphocyte (CTL) clones from lymphocytes infiltrating a lung carcinoma of a patient with long survival. These clones showed a CD3+, CD8+, CD4-, CD28- phenotype and expressed a T-cell receptor (TCR) encoded either by Vbeta8-Jbeta1.5 or Vbeta22-Jbeta1.4 rearrangements. Functional studies indicated that these clones mediated a high human leukocyte antigen (HLA)-A2.1-restricted cytotoxic activity against the autologous tumor cell line. Interestingly, TCRbeta chain gene usage indicated that CTL clones identified in vitro were selectively expanded in vivo at the tumor site as compared to autologous peripheral blood lymphocytes (PBL). These findings provide evidence that an immune response may take place in non-small cell lung carcinoma and that effector T cells may contribute to tumor regression. Further study indicated that the CTL clones recognized the same decamer peptide encoded by a mutated alpha-actinin-4 gene. Using tetramers of soluble HLA-A2 molecules loaded with the mutated antigenic peptide, we have derived several anti-alpha-actinin-4 T-cell clones from patient PBL. These CTL, recognizing a truly tumor-specific antigen, may play a role in the clinical evolution of this lung cancer patient. Adoptive transfer of CTL clones in a SCID/NOD mice model transplanted with autologous tumor supported their antitumor effect in vivo.

Post-transcriptional down-regulation of ROCKI/Rho-kinase through an MEK-dependent pathway leads to cytoskeleton disruption in Ras-transformed fibroblasts

Transformation by oncogenic Ras profoundly alters actin cytoskeleton organization. We investigated Ras-dependent signaling pathways involved in cytoskeleton disruption by transfecting normal rat kidney (NRK) cells with different Ras mutants. RasV12S35, a mutant known to activate specifically the Raf/MAPK pathway, led to stress fiber and focal contact disruption, whereas the adherens junctions remained intact. Next, we found that pharmacological inhibition of MEK was sufficient to restore the cytoskeletal defects of ras-transformed NRK cells, including assembly of stress fibers and focal contacts, but it did not induce reorganization of the cell-cell junctions. Investigating the mechanism underlying this phenotypic reversion, we found that the sustained MAPK signaling resulting from Ras-transformation down-regulated the expression of ROCKI and Rho-kinase, two-Rho effectors required for stress fiber formation, at the post-transcriptional level. On MEK inhibition, ROCKI/Rho-kinase expression and cofilin phosphorylation were increased, demonstrating that the Rho-kinase/LIM-kinase/cofilin pathway was functionally restored. Finally, using dominant negative or constitutively active mutants, we demonstrated that expression of ROCKI/Rho-kinase was both necessary and sufficient to promote cytoskeleton reorganization in NRK/ras cells. These findings further establish the Ras/MAPK pathway as the critical pathway involved in cytoskeleton disruption during Ras-transformation, and they suggest a new mechanism, involving alteration in ROCKI/Rho-kinase expression, by which oncogenic Ras can specifically target the actin-based cytoskeleton and achieve morphological transformation of the cells.

Inhibition of anchorage-independent growth of transformed NIH3T3 cells by epithelial protein lost in neoplasm (EPLIN) requires localization of EPLIN to actin cytoskeleton

Epithelial protein lost in neoplasm (EPLIN) is a cytoskeleton-associated protein characterized by the presence of a single centrally located lin-11, isl-1, and mec-3 (LIM) domain. We have reported previously that EPLIN is down-regulated in transformed cells. In this study, we have investigated whether ectopic expression of EPLIN affects transformation. In untransformed NIH3T3 cells, retroviral-mediated transduction of EPLIN did not alter the cell morphology or growth. NIH3T3 cells expressing EPLIN, however, failed to form colonies when transformed by the activated Cdc42 or the chimeric nuclear oncogene EWS/Fli-1. This suppression of anchorage-independent growth was not universal because EPLIN failed to inhibit the colony formation of Ras-transformed cells. Interestingly, the localization of EPLIN to the actin cytoskeleton was maintained in the EWS/Fli-1- or Cdc42-transformed cells, but not in Ras-transformed cells where it was distributed heterogeneously in the cytoplasm. Using truncated EPLIN constructs, we demonstrated that the NH(2)-terminal region of EPLIN is necessary for both the localization of EPLIN to the actin cytoskeleton and suppression of anchorage-independent growth of EWS/Fli-1-transformed cells. The LIM domain or the COOH-terminal region of EPLIN could be deleted without affecting its cytoskeletal localization or ability to suppress anchorage-dependent growth. Our study indicates EPLIN may function in growth control by associating with and regulating the actin cytoskeleton.

Post-transcriptional down-regulation of ROCKI/Rho-kinase through an MEK-dependent pathway leads to cytoskeleton disruption in Ras-transformed fibroblasts

Transformation by oncogenic Ras profoundly alters actin cytoskeleton organization. We investigated Ras-dependent signaling pathways involved in cytoskeleton disruption by transfecting normal rat kidney (NRK) cells with different Ras mutants. RasV12S35, a mutant known to activate specifically the Raf/MAPK pathway, led to stress fiber and focal contact disruption, whereas the adherens junctions remained intact. Next, we found that pharmacological inhibition of MEK was sufficient to restore the cytoskeletal defects of ras-transformed NRK cells, including assembly of stress fibers and focal contacts, but it did not induce reorganization of the cell-cell junctions. Investigating the mechanism underlying this phenotypic reversion, we found that the sustained MAPK signaling resulting from Ras-transformation down-regulated the expression of ROCKI and Rho-kinase, two-Rho effectors required for stress fiber formation, at the post-transcriptional level. On MEK inhibition, ROCKI/Rho-kinase expression and cofilin phosphorylation were increased, demonstrating that the Rho-kinase/LIM-kinase/cofilin pathway was functionally restored. Finally, using dominant negative or constitutively active mutants, we demonstrated that expression of ROCKI/Rho-kinase was both necessary and sufficient to promote cytoskeleton reorganization in NRK/ras cells. These findings further establish the Ras/MAPK pathway as the critical pathway involved in cytoskeleton disruption during Ras-transformation, and they suggest a new mechanism, involving alteration in ROCKI/Rho-kinase expression, by which oncogenic Ras can specifically target the actin-based cytoskeleton and achieve morphological transformation of the cells.

Differentiation genes: are they primary targets for human carcinogenesis?

In spite of extensive research in molecular carcinogenesis, genes that can be considered primary targets in human carcinogenesis remain to be identified. Mutated oncogenes or cellular growth regulatory genes, when incorporated into normal human epithelial cells, failed to immortalize or transform these cells. Therefore, they may be secondary events in human carcinogenesis. Based on some experimental studies we have proposed that downregulation of a differentiation gene may be the primary event in human carcinogenesis. Such a gene could be referred to as a tumor-initiating gene. Downregulation of a differentiation gene can be accomplished by a mutation in the differentiation gene, by activation of differentiation suppressor genes, and by inactivation of tumor suppressor genes. Downregulation of a differentiation gene can lead to immortalization of normal cells. Mutations in cellular proto-oncogenes, growth regulatory genes, and tumor suppressor genes in immortalized cells can lead to transformation. Such genes could be called tumor-promoting genes. This hypothesis can be documented by experiments published on differentiation of neuroblastoma (NB) cells in culture. The fact that terminal differentiation can be induced in NB cells by adenosine 3',5'-cyclic monophosphate (cAMP) suggests that the differentiation gene in these cells is not mutated, and thus can be activated by an appropriate agent. The fact that cAMP-resistant cells exist in NB cell populations suggests that a differentiation gene is mutated in these cancer cells, or that differentiation regulatory genes have become unresponsive to cAMP. In addition to cAMP, several other differentiating agents have been identified. Our proposed hypothesis of carcinogenesis can also be applied to other human tumors such as melanoma, pheochromocytoma, medulloblastoma, glioma, sarcoma, and colon cancer.

Cytoskeletal organization in tropomyosin-mediated reversion of ras-transformation: Evidence for Rho kinase pathway

Tropomyosin (TM) family of cytoskeletal proteins is implicated in stabilizing actin microfilaments. Many TM isoforms, including tropomyosin-1 (TM1), are down-regulated in transformed cells. Previously we demonstrated that TM1 is a suppressor of the malignant transformation, and that TM1 reorganizes microfilaments in the transformed cells. To investigate how TM1 induces microfilament organization in transformed cells, we utilized ras-transformed NIH3T3 (DT) cells, and those transduced to express TM1, and/or TM2. Enhanced expression of TM1 alone, but not TM2, results in re-emergence of microfilaments; TM1, together with TM2 remarkably improves microfilament architecture. TM1 induced cytoskeletal reorganization involves an enhanced expression of caldesmon, but not vinculin, alpha-actinin, or gelsolin. In addition, TM1-induced cytoskeletal reorganization and the revertant phenotype appears to involve re-activation of RhoA controlled pathways in DT cells. RhoA expression, which is suppressed in DT cells, is significantly increased in TM1-expressing cells, without detectable changes in the expression of Rac or Cdc42. Furthermore, expression of a dominant negative Rho kinase, or treatment with Y-27632 disassembled microfilaments in normal NIH3T3 and in TM1 expressing cells. These data suggest that reactivation of Rho kinase directed pathways are critical for TM1-mediated microfilament assemblies.

Dynamics of alpha-actinin in focal adhesions and stress fibers visualized with alpha-actinin-green fluorescent protein

Motile cells undergo changes in cell adhesion, behavior, and shape that are mediated by small-scale cytoskeletal rearrangements. These rearrangements have proven difficult to follow quantitatively in living cells, without disrupting the very structures and delicate protein balances under study. We have expressed a prominent cytoskeletal protein, alpha-actinin, as a fusion with green fluorescent protein (alpha AGFP), and have followed this construct's movements within transfected mouse Swiss 3T3 and BALB/c fibroblasts. alpha AGFP was expressed at low levels to avoid overexpression artifacts. alpha AGFP localized to cellular structures, including stress fibers, focal adhesions, microspikes, and lamellipodia. High-resolution video-microscopy revealed that the alpha AGFP construct could be seen relocating to focal adhesions early in their formation and shortly thereafter to stress-fiber dense bodies. By Fluorescent Recovery After Photo-bleaching (FRAP) techniques, alpha AGFP was found to have similar exchange rates and protein stability in focal adhesions and stress fibers (despite the known differences in protein composition in these two structures). This raises the possibility that the two structures share common key regulatory factors and may not be as affected by protein-protein binding interactions as previously suggested. Additionally, the exchange rates revealed by video-microscopy and FRAP analysis of alpha AGFP are more rapid than those reported previously, which were obtained using microinjection of large excesses of fluorescently-tagged protein.

Cytoskeletal changes in cell transformation and tumorigenesis

Research during the past couple of years has provided important new information as to how the actin cytoskeleton contributes to growth control in both normal and transformed cells. The cytoskeleton can no longer be viewed as simply a structural framework playing a role in cell shape and motile events such as cell movement, intracellular transport, contractile-ring formation and chromosome movement. More recent experiments show that the cytoskeleton plays a critical role in the regulation of various cellular processes linked to transformation including proliferation, contact inhibition, anchorage-independent cell growth, and apoptosis.

Suppression of the tumorigenicity of mutant p53-transformed rat embryo fibroblasts through expression of a newly cloned rat nonmuscle myosin heavy chain-B

In our previous study, a rat homolog of human nonmuscle myosin heavy chain-B (nmMHC-B) was identified by mRNA differential display comparing of transformed against nontransformed Rat 6 cells overexpressing mutant p53val135 gene. The nmMHC-B was found to be expressed in normal Rat 6 embryo fibroblast cell line, but markedly suppressed in the mutant p53val135-transformed Rat 6 cells. To examine the possible involvement of nmMHC-B in cell transformation, we first cloned and sequenced the full length cDNA of rat nmMHC-B, which was then cloned into an ecdysone-expression vector. The resulting construct was introduced into the T2 cell line, a mutant p53val135-transformed Rat 6 cells lacking the expression of the endogenous nmMHC-B. The clonal transfectants, expressing muristerone A-induced nmMHC-B, displayed a slightly flatter morphology and reached to a lower saturation density compared to the parental transformed cells. Reconstitution of actin filamental bundles was also clearly seen in cells overexpressing the nmMHC-B. In soft agar assays, nmMHC-B transfectants formed fewer and substantially smaller colonies than the parental cells in response to muristerone A induction. Moreover, it was strikingly effective in suppressing the tumorigenicity of the T2 cells when tested in nude mice. Thus, the nmMHC-B, known as a component of the cytoskeletal network, may act as a tumor suppressor gene. Our current finding may reveal a novel role of nmMHC-B in regulating cell growth and cell signaling in nonmuscle cells. Oncogene (2001) 20, 58 - 68.

Actinin-4 is preferentially involved in circular ruffling and macropinocytosis in mouse macrophages: analysis by fluorescence ratio imaging

We have applied fluorescence ratio imaging to the analysis of an actin-binding protein concentration relative to F-actin in macrophages, in order to explore the role of a novel (alpha)-actinin isoform, actinin-4, relative to that of the classical isoform, actinin-1. Conventional immunofluorescence images showed that both isoforms were enriched in F-actin-rich regions such as cell surface ruffles. However, ratio images further demonstrated that actinin-4 concentrations relative to F-actin were higher in peripheral inward curved ruffles and dorsal circular ruffles, presumed precursor forms of macropinosomes, than in straight linear ruffles, while actinin-1 concentrations were uniform among the different types of ruffles. Macropinosome pulse-labeling and chase experiments indicated that actinin-4 was also closely associated with newly formed macropinosomes and gradually dissociated with their maturation. Consistent with ratio imaging data, macrophages scrape-loaded with anti-actinin-4 showed a more reduced rate of macropinocytosis than those loaded with anti-actinin-1. Altogether, these results indicate that actinin-4 and actinin-1 contribute differently to F-actin dynamics, that actinin-4 is more preferentially involved in early stages of macropinocytosis than actinin-1. A similar redistribution of actinin-4 was also observed during phagocytosis, suggesting that actinin-4 may play the same role in the two mechanistically analogous types of endocytosis, i.e. macropinocytosis and phagocytosis.

Suppression of tumorigenicity in breast cancer cells by the microfilament protein profilin 1

Differential display screening was used to reveal differential gene expression between the tumorigenic breast cancer cell line CAL51 and nontumorigenic microcell hybrids obtained after transfer of human chromosome 17 into CAL51. The human profilin 1 (PFN1) gene was found overexpressed in the microcell hybrid clones compared with the parental line, which displayed a low profilin 1 level. A comparison between several different tumorigenic breast cancer cell lines with nontumorigenic lines showed consistently lower profilin 1 levels in the tumor cells. Transfection of PFN1 cDNA into CAL51 cells raised the profilin 1 level, had a prominent effect on cell growth, cytoskeletal organization and spreading, and suppressed tumorigenicity of the stable, PFN1-overexpressing cell clones in nude mice. Immunohistochemical analysis revealed intermediate and low levels of profilin 1 in different human breast cancers. These results suggest profilin 1 as a suppressor of the tumorigenic phenotype of breast cancer cells.

v-Src suppresses SHPS-1 expression via the Ras-MAP kinase pathway to promote the oncogenic growth of cells

We investigated the effect of cell transformation by v-src on the expression and tyrosine phosphorylation of SHPS-1, a putative docking protein for SHP-1 and SHP-2. We found that transformation by v-src virtually inhibited the SHPS-1 expression at mRNA level. While nontransforming Src kinases including c-Src, nonmyristoylated forms of v-Src had no inhibitory effect on SHPS-1 expression, transforming Src kinases including wild-type v-Src and chimeric mutant of c-Src bearing v-Src SH3 substantially suppressed the SHPS-1 expression. In cells expressing temperature sensitive mutant of v-Src, suppression of the SHPS-1 expression was temperature-dependent. In contrast, tyrosine phosphorylation of SHPS-1 was rather activated in cells expressing c-Src or nonmyristoylated forms of v-Src. SHPS-1 expression in SR3Y1 was restored by treatment with herbimycin A, a potent inhibitor of tyrosine kinase, or by the expression of dominant negative form of Ras. Contrary, active form of Mekl markedly suppressed SHPS-1 expression. Finally, overexpression of SHPS-1 in SR3Y1 led to the drastic reduction of anchorage independent growth of the cells. Taken together, our results suggest that the suppression of SHPS-1 expression is a pivotal event for cell transformation by v-src, and the Ras-MAP kinase cascade plays a critical role in the suppression.

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.

The human non-muscle alpha-actinin protein encoded by the ACTN4 gene suppresses tumorigenicity of human neuroblastoma cells

alpha-Actinins are actin-binding proteins important in organization of the cytoskeleton and in cell adhesion. We have cloned and characterized a cDNA from human neuroblastoma cell variants which encodes the second non-muscle alpha-actinin isoform designated ACTN4 (actinin-4). mRNA encoded by the ACTN4 gene, mapped to chromosome 4, is abundant in non-tumorigenic, substrate-adherent human neuroblastoma cell variants but absent or only weakly expressed in malignant, poorly substrate-adherent neuroblasts. It is also present in many adherent tumor cell lines of diverse tissue origins. Cell lines typically co-express ACTN4 and ACTN1, a second non-muscle alpha-actinin gene. Expression is correlated with substrate adhesivity. Analysis of deduced amino acid sequences suggests that the two isoforms may differ in function and in regulation by calcium. Moreover, ACTN4 exhibits tumor suppressor activity. Stable clones containing increased levels of alpha-actinin, isolated from highly malignant neuroblastoma stem cells [BE(2)-C] after transfection with a full-length ACTN4 cDNA, show decreased anchorage-independent growth ability, loss of tumorigenicity in nude mice, and decreased expression of the N-myc proto-oncogene.

Tyrosine phosphorylation of alpha-actinin in activated platelets

The integrin alpha(IIb)beta(3) mediates tyrosine phosphorylation of a 105-kDa protein (pp105) in activated platelets. We have partially purified a 105-kDa tyrosine-phosphorylated protein from platelets stimulated with phorbol 12-myristate 13-acetate and obtained the sequence of an internal 12-mer peptide derived from this protein. The sequence was identical to human alpha-actinin sequences deposited in the Swiss Protein Database. alpha-Actinin, a 105-kDa protein in platelets, was subsequently purified from activated platelets by four sequential chromatographic steps. Fractions were analyzed by Western blotting and probed with alpha-actinin and anti-phosphotyrosine antibodies. The distribution of alpha-actinin and pp105 overlapped throughout the purification. Furthermore, in the course of this purification, a 105-kDa tyrosine-phosphorylated protein was only detected in fractions that contained alpha-actinin. The purified alpha-actinin protein was immunoprecipitated with antibodies to phosphotyrosine in the absence but not in the presence of phenyl phosphate. alpha-Actinin resolved by two-dimensional gel electrophoresis of activated platelet lysates was recognized by the antibodies to phosphotyrosine, whereas pretreatment of the platelets with bisindolylmaleimide, a protein kinase C inhibitor that prevents tyrosine phosphorylation of pp105, inhibited the reactivity of the antibodies to phosphotyrosine with alpha-actinin. Taken together, these data demonstrate that a fraction of alpha-actinin is tyrosine-phosphorylated in activated platelets.

G proteins, phosphoinositides, and actin-cytoskeleton in the control of cancer growth

Almost three decades have passed since actin-cytoskeleton (acto-myosin complex) was first discovered in non-muscle cells. A combination of cell biology, biochemistry, and molecular biology has revealed the structure and function of many actin-binding proteins and their physiological role in the regulation of cell motility, shape, growth, and malignant transformation. As molecular oncologists, we would like to review how the function of actin-cytoskeleton is regulated through Ras/Rho family GTPases- or phosphoinosites-mediated signaling pathways, and how malignant transformation is controlled by actin/phosphoinositides-binding proteins or drugs that block Rho/Rac/CDC42 GTPases-mediated signaling pathways.