The cytoskeleton in cancer cells

The β-Carboline Harmine Induces Actin Dynamic Remodeling and Abrogates the Malignant Phenotype in Tumorigenic Cells

Numerous studies have shown that alteration of actin remodeling plays a pivotal role in the regulation of morphologic and phenotypic changes leading to malignancy. In the present study, we searched for drugs that can regulate actin polymerization and reverse the malignant phenotype in cancer cells. We developed a cell-free high-throughput screening assay for the identification of compounds that induce the actin polymerization in vitro, by fluorescence anisotropy. Then, the potential of the hit compound to restore the actin cytoskeleton and reverse the malignant phenotype was checked in EWS-Fli1-transformed fibroblasts and in B16-F10 melanoma cells. A β-carboline extracted from Peganum harmala (i.e., harmine) is identified as a stimulator of actin polymerization through a mechanism independent of actin binding and requiring intracellular factors involved in a process that regulates actin kinetics. Treatment of malignant cells with non-cytotoxic concentrations of harmine induces the recovery of a non-malignant cell morphology accompanied by reorganization of the actin cytoskeleton, rescued cell–cell adhesion, inhibition of cell motility and loss of anchorage-independent growth. In conclusion, harmine induces the reversion of the malignant phenotype by a process involving the modulation of actin dynamics and is a potential anti-tumor agent acting principally through a non-cytotoxic process.

Cytoskeletal Proteins in Cancer and Intracellular Stress: A Therapeutic Perspective

Cytoskeletal proteins, which consist of different sub-families of proteins including microtubules, actin and intermediate filaments, are essential for survival and cellular processes in both normal as well as cancer cells. However, in cancer cells, these mechanisms can be altered to promote tumour development and progression, whereby the functions of cytoskeletal proteins are co-opted to facilitate increased migrative and invasive capabilities, proliferation, as well as resistance to cellular and environmental stresses. Herein, we discuss the cytoskeletal responses to important intracellular stresses (such as mitochondrial, endoplasmic reticulum and oxidative stresses), and delineate the consequences of these responses, including effects on oncogenic signalling. In addition, we elaborate how the cytoskeleton and its associated molecules present themselves as therapeutic targets. The potential and limitations of targeting new classes of cytoskeletal proteins are also explored, in the context of developing novel strategies that impact cancer progression.

Elasticity and tumorigenic characteristics of cells in a monolayer after nanosecond pulsed electric field exposure

Nanosecond pulsed electric fields (nsPEFs) applied to cells can induce different biological effects depending on pulse duration and field strength. One known process is the induction of apoptosis whereby nsPEFs are currently investigated as a novel cancer therapy. Another and probably related change is the breakdown of the cytoskeleton. We investigated the elasticity of rat liver epithelial cells WB-F344 in a monolayer using atomic force microscopy (AFM) with respect to the potential of cells to undergo malignant transformation or to develop a potential to metastasize. We found that the elastic modulus of the cells decreased significantly within the first 8 min after treatment with 20 pulses of 100 ns and with a field strength of 20 kV/cm but was still higher than the elasticity of their tumorigenic counterpart WB-ras. AFM measurements and immunofluorescent staining showed that the cellular actin cytoskeleton became reorganized within 5 min. However, both a colony formation assay and a cell migration assay revealed no significant changes after nsPEF treatment, implying that cells seem not to adopt malignant characteristics associated with metastasis formation despite the induced transient changes to elasticity and cytoskeleton that can be observed for up to 1 h.

Effects of cytochalasin congeners, microtubule-directed agents, and doxorubicin alone or in combination against human ovarian carcinoma cell lines in vitro

BACKGROUND

Although the actin cytoskeleton is vital for carcinogenesis and subsequent pathology, no microfilament-directed agent has been approved for cancer chemotherapy. One of the most studied classes of microfilament-directed agents has been the cytochalasins, mycotoxins known to disrupt the formation of actin polymers. In the present study, we sought to determine the effects of cytochalasin congeners toward human drug sensitive and multidrug resistant cell lines.

METHODS

SKOV3 human ovarian carcinoma and several multidrug resistant derivatives were tested for sensitivity against a panel of nine cytochalasin congeners, as well as three clinically approved chemotherapeutic agents (doxorubicin, paclitaxel, and vinblastine). In addition, verapamil, a calcium ion channel blocker known to reverse P-glycoprotein (P-gp) mediated drug resistance, was used in combination with multiple cytochalasin congeners to determine whether drug sensitivity could be increased.

RESULTS

While multidrug resistant SKVLB1 had increased drug tolerance (was more resistant) to most cytochalasin congeners in comparison to drug sensitive SKOV3, the level of resistance was 10 to 1000-fold less for the cytochalasins than for any of the clinically approved agents. While cytochalasins did not appear to alter the expression of ATP binding cassette (ABC) transporters, several cytochalasins appeared to inhibit the activity of ABC transporter-mediated efflux of rhodamine 123 (Rh123), suggesting that these congeners do have affinity for drug efflux pumps. Cytochalasins also appeared to significantly decrease the F/G-actin ratio in both drug sensitive and drug resistant cells, indicative of marked microfilament inhibition. The cytotoxicity of most cytochalasin congeners could be increased with the addition of verapamil, and the drug sensitivity of resistant SKVLB1 to the clinically approved antineoplastic agents could be increased with the addition of cytochalasins. As assessed by isobolographic analysis and Chou-Talalay statistics, cytochalasin B and 21,22-dihydrocytochalasin B (DiHCB) demonstrated notable synergy with doxorubicin and paclitaxel, warranting further investigation in a tumor-bearing mammalian model.

CONCLUSION

Cytochalasins appear to inhibit the activity of P-gp and potentially other ABC transporters, and may have novel activity against multidrug resistant neoplastic cells that overexpress drug efflux proteins.

Chemotherapy in vivo against M109 murine lung carcinoma with cytochalasin B by localized, systemic, and liposomal administration

Cytochalasin B is a potentially novel microfilament-directed chemotherapeutic agent that prevents actin polymerization, thereby inhibiting cytokinesis. Although cytochalasin B has been extensively studied in vitro, only limited data are available to assess its in vivo potential. Cytochalasin B was administered to Balb/c mice challenged i.d. with M109 murine lung carcinoma to determine whether the agent could affect an established i.d. tumor when the compound is administered s.c. in the region of the i.d. tumor, but not in direct contact with it. Cytochalasin B was also administered either i.p. or s.c. at a distant site or i.v. to determine whether it could affect the long-term development of an established i.d. tumor. Cytochalasin B was then liposome encapsulated to determine whether the maximum tolerated dose (MTD) of the compound could be increased, while reducing immunosuppression that we have previously characterized. Liposomal cytochalasin B was also administered to mice challenged i.d. with M109 lung carcinoma to assess its chemotherapeutic efficacy. The results can be summarized as follows: 1) cytochalasin B substantially delayed the growth of i.d. M109 tumor nodules, inhibited metastatic progression in surrounding tissues, and produced long-term cures in treated mice; 2) liposomal cytochalasin B increased the i.p. MTD by more than 3-fold, produced a different distribution in tissue concentrations, and displayed antitumor effects against M109 lung carcinoma similar to non-encapsulated cytochalasin B. These data show that cytochalasin B exploits unique chemotherapeutic mechanisms and is an effective antineoplastic agent in vivo in pre-clinical models, either in bolus form or after liposome encapsulation.

Oral cancer diagnosis by mechanical phenotyping

Oral squamous cell carcinomas are among the 10 most common cancers and have a 50% lethality rate after 5 years. Despite easy access to the oral cavity for cancer screening, the main limitations to successful treatment are uncertain prognostic criteria for (pre-)malignant lesions. Identifying a functional cellular marker may represent a significant improvement for diagnosis and treatment. Toward this goal, mechanical phenotyping of individual cells is a novel approach to detect cytoskeletal changes, which are diagnostic for malignant change. The compliance of cells from cell lines and primary samples of healthy donors and cancer patients was measured using a microfluidic optical stretcher. Cancer cells showed significantly different mechanical behavior, with a higher mean deformability and increased variance. Cancer cells (n approximately 30 cells measured from each patient) were on average 3.5 times more compliant than those of healthy donors [D(normal) = (4.43 +/- 0.68) 10(-3) Pa(-1); D(cancer) = (15.8 +/- 1.5) 10(-3) Pa(-1); P < 0.01]. The diagnosis results of the patient samples were confirmed by standard histopathology. The generality of these findings was supported by measurements of two normal and four cancer oral epithelial cell lines. Our results indicate that mechanical phenotyping is a sensible, label-free approach for classifying cancer cells to enable broad screening of suspicious lesions in the oral cavity. It could in principle be applied to any cancer to aid conventional diagnostic procedures.

Optical deformability as an inherent cell marker for testing malignant transformation and metastatic competence

The relationship between the mechanical properties of cells and their molecular architecture has been the focus of extensive research for decades. The cytoskeleton, an internal polymer network, in particular determines a cell's mechanical strength and morphology. This cytoskeleton evolves during the normal differentiation of cells, is involved in many cellular functions, and is characteristically altered in many diseases, including cancer. Here we examine this hypothesized link between function and elasticity, enabling the distinction between different cells, by using a microfluidic optical stretcher, a two-beam laser trap optimized to serially deform single suspended cells by optically induced surface forces. In contrast to previous cell elasticity measurement techniques, statistically relevant numbers of single cells can be measured in rapid succession through microfluidic delivery, without any modification or contact. We find that optical deformability is sensitive enough to monitor the subtle changes during the progression of mouse fibroblasts and human breast epithelial cells from normal to cancerous and even metastatic state. The surprisingly low numbers of cells required for this distinction reflect the tight regulation of the cytoskeleton by the cell. This suggests using optical deformability as an inherent cell marker for basic cell biological investigation and diagnosis of disease.

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.

Reversible tumorigenesis induced by deficiency of vasodilator-stimulated phosphoprotein

Random homozygous knockout (RHKO) is an antisense RNA strategy capable of identifying genes whose homozygous functional inactivation yields a selectable phenotype in cells growing in culture. Using this approach, we isolated NIH 3T3 fibroblast clones that showed the ability to form colonies on 0.5% agar and tumors in nude mice. The gene inactivated in one of these clones was found to encode VASP (vasodilator-stimulated phosphoprotein), a previously identified protein that binds to components of the cadherin-catenin junctional complex and has been implicated in cell-cell interactions, the formation of actin filaments, and the transmission of signals at the cytoskeleton-membrane interface. Fibroblasts made deficient in VASP by RHKO showed loss of contact inhibition, and consequently, continued cell division past confluence. Restoration of VASP function by reversal of RHKO yielded cells that had lost the neoplastic capabilities acquired during RHKO. Overproduction of VASP mRNA in the sense or antisense orientation from expression constructs introduced by transfection into naive NIH 3T3 fibroblasts also resulted in neoplastic transformation, implying that normal cell growth may require the maintenance of VASP expression within a narrow range. Our results implicate VASP in tumorigenesis and/or cancer progression.

A zyxin-related protein whose synthesis is reduced in virally transformed fibroblasts

We have cloned the gene for a novel LIM-domain protein from human fibroblasts whose expression is substantially decreased in simian-virus-40-(SV40)-transformed cells. This protein has a calculated molecular mass of 61 kDa and comprises a proline-rich domain followed by three LIM motifs. It appears to be identical to the focal adhesion protein p83 that has recently been isolated and characterized from porcine and human platelets. Hybridization experiments demonstrate a very low degree of evolutionary conservation of its sequence between mammals and birds. It is therefore possible that the novel protein represents the human equivalent of the chicken protein zyxin as the two proteins display a very similar overall structure, although their amino acid sequences diverge markedly from each other. The repression of this zyxin-related protein in virally transformed fibroblasts may explain, at least in part, the dramatic morphological changes that occur at the cell surface and in the cytoskeleton of transformed cells.

Regulation of microfilament organization and anchorage-independent growth by tropomyosin 1

Variants of chemically immortalized Syrian hamster embryo cells that had either retained (supB+) or lost (supB-) the ability to suppress tumorigenicity when hybridized with a fibrosarcoma cell line were subcloned. Both supB cell types are nontumorigenic; however, the supB- but not supB+ cells exhibit conditional anchorage-independent growth. Alterations of actin microfilament organization were observed in supB- but not supB+ cells that corresponded to a significant reduction of the actin-binding protein tropomyosin 1 (TM-1) in subB- cells. To examine the possibility of a direct relationship between TM-1 expression and the subB- phenotype, subB+ cells were transfected with an expression vector containing the TM-1 cDNA in an antisense orientation. The antisense-induced reduction of TM-1 levels in supB+ clones caused a microfilament reorganization and conferred anchorage-independent growth potential that were indistinguishable from those characteristic of supB- cells. These data provide direct evidence that TM-1 regulates both microfilament organization and anchorage-independent growth and suggest that microfilament alterations are sufficient for anchorage-independent growth.

Phosphorylation of calmodulin by plasma-membrane-associated protein kinase(s)

Plasma-membrane-associated protein kinase(s) from normal rat liver phosphorylates exogenous bovine brain calmodulin in the absence of Ca2+ and in the presence of histone or poly(L-lysine). Maximum levels of calmodulin phosphorylation are obtained at a poly(L-lysine)/calmodulin molar ratio of 0.4. Phosphoamino acid analysis revealed that calmodulin is phosphorylated on serine, threonine and tyrosine residues. Endogenous plasma-membrane-associated calmodulin was also phosphorylated by plasma-membrane-associated protein kinase(s) in the absence of added cationic protein or polypeptide. The identity of endogenous phosphocalmodulin was confirmed by immunoprecipitation with a specific anti-calmodulin monoclonal antibody. Ehrlich ascites tumor cell plasma membranes do not contain endogenous calmodulin. However, membrane-associated protein kinase(s) from these tumor cells phosphorylates bovine brain calmodulin in the presence of poly(L-lysine). These data demonstrate that phosphocalmodulin is present in liver plasma membranes and suggest that this post-translational modification could have a physiological role in this location.

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.

Growth, morphologic, and invasive characteristics of early and late passages of a human endometrial carcinoma cell line (RL95-2)

Two in vitro passages of a human endometrial adenocarcinoma continuous cell line (RL95-2), an early (subcultured less than 30 times) and a late passage (subcultured greater than 200 times) have provided an interesting model to study the growth, morphologic, and invasive properties of endometrial tumors. The early passage, which has been shown to be estrogen-receptor positive, has characteristics closely resembling a primary tumor, whereas the estrogen receptor negative late passage exhibits several features of the metastatic phenotype. Compared to the early passage cells, the late passage cells were less serum dependent, formed foci, demonstrated a faster rate of growth (due to their shorter doubling times), and attained higher saturation densities. The late passage cells also displayed an altered morphology which was accompanied by alterations in the distribution of F-actin. Even though early and late passages showed similar invasive potential in an in vitro invasion assay, the late passage cells, by virtue of their several transformed characteristics, maintain distinctive properties compared with their early passage counterparts.

Antioncogenic effect of adenovirus E1A in human tumor cells

Stable expression of the adenovirus 5 E1A gene reduced anchorage-independent growth and tumorigenic potential, caused cytoskeletal reorganization, induced flat morphology, and restored contact inhibition in three human tumor cell lines. By these criteria, E1A appears to be functionally indistinguishable from a tumor suppressor gene in this context. The apparent paradox accorded by the observations of the ability of E1A to transform rodent cells in cooperation with other oncogenes suggests that E1A may be the prototype of a class of growth-regulatory proteins having context-specific transforming and antioncogenic activities.

Disruption of keratin intermediate filaments by ultraviolet radiation in cultured human keratinocytes

Fluorescence microscopy has been utilized to investigate the effects of UV irradiation on the organization of keratin intermediate filaments in normal human epidermal keratinocytes. Sun lamp irradiation induced the condensation of keratin intermediate filaments into the perinuclear region and inhibited the reorganization of keratin filaments normally induced by Ca2+. Exposure to UVC appeared to disrupt keratin filaments similarly, whereas UVA had no discernible effect.

Changes in adhesive properties of tumor cells do not necessarily influence metastasizing capacity

BSp6S and BSp73AS are two rat tumors which grow locally after intra-footpad (ifp) application. BSp73ASML and BSp6AS are variants, which metastasize via the lymphatics. Both variants have lost adherence properties, as shown by in vitro culture on plastic surfaces, suggesting that loss of adherence may be accompanied by increased metastasizing capacity. However, after growth of BSp6S and BSp73AS in vitro on poly(2-hydroxyethylmethacrylate) (polyHEMA)-coated plates, which resulted in loss of adherence and spreading, and subsequent intravenous (iv) or ifp injection of non-adherent tumor cells into syngeneic rats, metastasizing capacity was not increased. It is concluded that loss of adherence may facilitate metastatic spread, but certainly is not sufficient for initiation.

Biochemical localization of the transformation-sensitive 52 kDa (p52) protein to the substratum contact regions of cultured rat fibroblasts. Butyrate induction, characterization, and quantification of p52 in v-ras transformed cells

A 52 kDa protein (p52) was identified, using differential extraction and electrophoretic criteria, as a major extracellular and substrate-associated component of normal rat kidney (NRK) fibroblasts. Cells transformed with Kirsten murine sarcoma virus (KNRK cells) did not express p52 constitutively, but were inducible for both p52 production and its substrate association during culture in sodium butyrate (NaB)-supplemented growth medium. Comparative analysis of the relative molecular mass, subcellular distribution, and isoelectric complexity (five variants ranging in pI from 5.4 to 6.2) of the 52 kDa species constitutively and inducibly expressed by NRK and KNRK/NaB cells respectively, indicated that they were, indeed, the same protein. p52 selectively localized to cellular fractions enriched in substrate focal contact sites and associated ventral undersurface components. NaB induction of p52 in KNRK cells occurred before cell spreading; other polar compounds, such as dimethyl sulphoxide, which did not induce KNRK cell spreading, similarly failed to elicit p52 production. p52 accumulated more rapidly in (and was quickly released from) the focal-contact-enriched protein fraction of NRK cells compared with its time course of appearance in the medium. These data collectively suggest that p52 is one of a relatively small number of proteins the synthesis of which is either involved in determination of cell shape or regulated as a consequence of cell-shape changes.

Isolation of a cDNA corresponding to a developmentally regulated transcript in rat intestine

We report the isolation of a cDNA clone corresponding to a transcript that is accumulated differentially in rat intestine during development. Clone OCI-5 was selected from the rat intestinal cell line IEC-18, which represents primitive intestinal epithelial crypt cells. Expression was high in rat fetal intestine between 15 and 19 days of development and thereafter was progressively down regulated, becoming undetectable after weaning. Clone OCI-5 detected homologous sequences in human and murine cells. In particular, a high level of expression was detected in CaCo-2, a human colon carcinoma cell line, which is known to express molecules characteristic of fetal small intestinal cells. Expression of a homologous gene was also detected in F9 murine teratocarcinoma cells when they were induced to differentiate into parietal or visceral endodermlike cells. When IEC-18 cells were transformed by activated H-ras or v-src genes, expression of clone OCI-5 was suppressed; the degree of down-regulation correlated with the extent of morphological change induced in the transformed IEC-18 cells. The sequence of clone OCI-5 showed an open reading frame that was capable of encoding a protein of 597 amino acids, but no strong homology was found with any of the proteins registered in the protein sequence data base.

Isolation of a cDNA corresponding to a developmentally regulated transcript in rat intestine

We report the isolation of a cDNA clone corresponding to a transcript that is accumulated differentially in rat intestine during development. Clone OCI-5 was selected from the rat intestinal cell line IEC-18, which represents primitive intestinal epithelial crypt cells. Expression was high in rat fetal intestine between 15 and 19 days of development and thereafter was progressively down regulated, becoming undetectable after weaning. Clone OCI-5 detected homologous sequences in human and murine cells. In particular, a high level of expression was detected in CaCo-2, a human colon carcinoma cell line, which is known to express molecules characteristic of fetal small intestinal cells. Expression of a homologous gene was also detected in F9 murine teratocarcinoma cells when they were induced to differentiate into parietal or visceral endodermlike cells. When IEC-18 cells were transformed by activated H-ras or v-src genes, expression of clone OCI-5 was suppressed; the degree of down-regulation correlated with the extent of morphological change induced in the transformed IEC-18 cells. The sequence of clone OCI-5 showed an open reading frame that was capable of encoding a protein of 597 amino acids, but no strong homology was found with any of the proteins registered in the protein sequence data base.

Cytoskeleton organization of normal and neoplastic lymphocytes and lymphoid cell lines of T and B origin

An anomalous organization of the cytoskeleton has been described in lymphocytes from chronic lymphatic leukaemia and in only few cell lines. We have now studied normal and neoplastic lymphocytes and lymphoid cell lines of both T and B lineage in order to detect morphological differences in the expression of microfilaments and intermediate filaments. Microfilaments appear to be well expressed by all the B cells, whereas a rich network of intermediate filaments is present in T cells and plasma cells. Most prominent changes occur in the latter system, which is almost lacking in cells of B chronic lymphatic and hairy cell leukaemia. Although the significance of the present findings is not yet clear, one might speculate that such alterations account for some of the aberrant functions and peculiar biologic properties of neoplastic lymphocytes.

Isolation of the oncogene and epidermal growth factor-induced transin gene: complex control in rat fibroblasts

Various oncogenes or epidermal growth factor (EGF) induce transcription of a 1.9-kilobase RNA (transin RNA) in rat fibroblasts. The induction by EGF can be blocked by cycloheximide. Thus the response of the transin gene to EGF appears to require de novo protein synthesis. Transin RNA induction is specific to EGF, as neither insulin, platelet-derived growth factor, fibroblast growth factor, nor transforming growth factor beta could elicit the same response. However, transforming growth factor beta could block the EGF induction of transin RNA. Whereas the calcium ionophore A23187 and the tumor promoter TPA, either alone or administered together, did not increase transin RNA levels, TPA could synergise with a serum factor to effect such an increase. Dibutyryl cyclic AMP also induced transin RNA. Treatment of cells with the microfilament-disrupting agent cytochalasin B, but not the microtubule-disrupting agent colcemid, resulted in an increase in transin RNA levels, suggesting a role for the cytoskeleton in control of transin gene expression. The transin RNA does not contain repeated sequences and appears to be encoded by a single-copy gene. The protein sequence encoded by the last four exons of the transin gene shows some homology to two regions of the heme-binding protein hemopexin.

Concanavalin A-agglutinability of membrane-skeleton-free vesicles and aged cellular remnants derived from human erythrocytes. Is the membrane skeleton required for agglutination?

Vesicles and cell remnants have been obtained by aging of erythrocytes in vitro. The vesicles lacking the membrane skeletal proteins and the remnants known to possess a rigid skeleton have been used to assess the role of membrane skeletal proteins in the process of Con A (concanavalin A)-mediated agglutination of erythrocytes. Both the vesicles and the remnants were found to bind Con A at the same density as did intact cells. The vesicles, isolated from normal as well as from the Con A-agglutinable trypsin- and Pronase-treated cells, failed to agglutinate with Con A. They were, however, well agglutinated by WGA (wheat-germ agglutinin) and RCA [Ricinus communis (castor bean) agglutinin], indicating that the vesicles are not defective in agglutination. Large, cytoskeleton-free, vesicles prepared by another procedure also gave the same results. The aged remnants from trypsin- and Pronase-treated erythrocytes showed significantly decreased agglutination with Con A, but were agglutinated as well as the fresh cells by WGA and RCA. The agglutination with Con A is thus abolished when the membrane skeleton is absent, and reduced when it is rigid, suggesting that the skeleton may play an important role in the agglutination of erythrocytes by Con A.

Cell-contact and -architecture of malignant cells and their relationship to metastasis

The interaction of metastatic cells with the host environment occurs, to a large extent, through the cell surface, and the cell cytoskeletal system controls the distribution and motility of cell surface receptors. During metastasis, tumor cells migrate from one organ to another, and the dynamic properties and mechanochemical deformability of disseminated cells play a central role in the process. The studies described here under suggest an interrelationship between the cytoskeleton and cell adhesion, which can control and augment the expression of the metastatic phenotype of neoplastic cells.

Epidermal growth factor receptors associated to cytoskeletal elements of epidermoid carcinoma (A431) cells

The structural interaction of the epidermal growth factor (EGF) receptor and the cytoskeleton of A431 cells has been studied using a monoclonal anti-EGF receptor antibody. This has been done with immunogold labeling using a variety of electron microscopical preparation procedures and EGF binding studies. By providing an image of the membrane-associated cytoskeleton, the dry cleavage method reveals a preferential localization of EGF receptors superimposed upon cytoskeletal filaments. The colocalization of gold particles with cytoskeletal filaments is not affected when pre-labeled cells are extracted with the non-ionic detergent Triton X-100, as visualized by dry cleavage. Using surface replication, this treatment results in visualization of the cytoskeleton. In these latter preparations, it is also observed that EGF receptor-coupled gold particles remain associated with cytoskeletal elements. Moreover, Triton extraction performed before immunogold labeling of EGF receptors demonstrates that isolated cytoskeletons contained binding sites for anti-EGF receptor antibodies. Using stereo micrographs of replica's obtained from these isolated cytoskeletons, it is shown that gold-labeled EGF receptors are exclusively present on the cortical membrane-associated region of the cytoskeleton and not on more intracellular-located filaments. Scatchard analysis of EGF binding to cells fixed with glutaraldehyde and treated with Triton X-100 before and after EGF binding indicates that a high affinity EGF binding site is associated with the Triton X-100 insoluble cytoskeleton.

Isolation of the oncogene and epidermal growth factor-induced transin gene: complex control in rat fibroblasts

Various oncogenes or epidermal growth factor (EGF) induce transcription of a 1.9-kilobase RNA (transin RNA) in rat fibroblasts. The induction by EGF can be blocked by cycloheximide. Thus the response of the transin gene to EGF appears to require de novo protein synthesis. Transin RNA induction is specific to EGF, as neither insulin, platelet-derived growth factor, fibroblast growth factor, nor transforming growth factor beta could elicit the same response. However, transforming growth factor beta could block the EGF induction of transin RNA. Whereas the calcium ionophore A23187 and the tumor promoter TPA, either alone or administered together, did not increase transin RNA levels, TPA could synergise with a serum factor to effect such an increase. Dibutyryl cyclic AMP also induced transin RNA. Treatment of cells with the microfilament-disrupting agent cytochalasin B, but not the microtubule-disrupting agent colcemid, resulted in an increase in transin RNA levels, suggesting a role for the cytoskeleton in control of transin gene expression. The transin RNA does not contain repeated sequences and appears to be encoded by a single-copy gene. The protein sequence encoded by the last four exons of the transin gene shows some homology to two regions of the heme-binding protein hemopexin.