Role of surface modulating assemblies in growth control of normal and transformed fibroblasts

EGF receptor-mediated signals are differentially modulated by concanavalin A

NIH 3T3 cells expressing high levels of the human epidermal growth factor (EGF) receptor were used to examine the effects of the lectin concanavalin A (Con A) on EGF-mediated signaling events. Proliferation of NIH 3T3 cells expressing high levels of the human EGF receptor was inhibited in a dose-dependent manner by Con A. At the same time, Con A also inhibited both dimerization and tyrosine phosphorylation of the EGF receptor. Tyrosine phosphorylation of the enzyme phospholipase C-gamma, a substrate of the phosphorylated EGF receptor kinase, was also inhibited. In contrast, EGF-stimulated changes in pH, calcium, and levels of inositol phosphates were unaffected by the presence of Con A. These results indicate that certain signals (changes in the levels of intracellular calcium, pH, and inositol phosphates) mediated by EGF binding to its receptor still occur when receptor dimerization and phosphorylation are dramatically decreased, suggesting that multiple independent signals are transmitted by the binding of EGF to its receptor.

Thrombin, epidermal growth factor, and phorbol myristate acetate stimulate tubulin polymerization in quiescent cells: a potential link to mitogenesis

Previous studies suggest that alterations in the microtubule (MT)-tubulin equilibrium during G0/G1 affect mitogenesis. To determine the effect of growth factors on the MT-tubulin equilibrium, we developed a radioactive monoclonal antibody binding assay (Ball et al.: J. Cell. Biol. 103:1033-1041, 1986). With this assay, 3H-Ab 1-1.1 binding to cytoskeletons in confluent populations of cultured cells is proportional to the number of tubulin subunits polymerized into MTs. We now show that purified alpha-thrombin increases 3H-Ab 1-1.1 binding to cytoskeletons of serum-arrested mouse embryo (ME) fibroblasts from 1.5- to 3-fold. This stimulation is dose-dependent and correlates with concentrations of thrombin required for initiation of DNA synthesis. Other mitogenic factors, epidermal growth factor (EGF) and phorbol 12-myristate 13-acetate (PMA), also stimulate MT polymerization. Addition of colchicine (0.3 microM) eight hours after growth factor addition, blocks stimulation of 3H-thymidine incorporation by thrombin, EGF, or PMA, suggesting that tubulin polymerization or subsequent events triggered by MT polymerization are required for cells to enter a proliferative cycle. Consistent with models for autoregulation of tubulin synthesis, thrombin, EGF, and PMA all increase tubulin synthesis 9 to 15 hr after growth factor addition, raising the possibility that the decrease in free tubulin and subsequent stimulation of tubulin synthesis is linked to progression of cells into a proliferative cycle. Colchicine addition to these cells also stimulates DNA synthesis, but colchicine-stimulated cells enter S phase 6 to 8 hr later than those stimulated by growth factors. This delayed stimulation may be related to the time required for degradation of tubulin-colchicine complexes below a critical level. These data suggest that regulation of cell proliferation may be linked to increased MT polymerization and the resulting decrease in free tubulin pools.

Cytotactin binding: inhibition of stimulated proliferation and intracellular alkalinization in fibroblasts

Cytotactin is an extracellular matrix protein that is dynamically and transiently expressed in a place-dependent fashion during development by glial cells, fibroblasts, and several other cell types. In the present study, the effects of cytotactin on cell proliferation were examined in fibroblastic cells in culture. NIH 3T3 mouse cells plated on tissue culture substrata in the presence of soluble cytotactin remained rounded for longer periods than untreated control cells, similar to their response to cytotactin-coated substrates. These rounding effects could be prevented by pretreatment of the cells with nocodazole, a microtubule-disrupting agent. Cytotactin inhibited the proliferation of fibroblasts in culture in a dose- and time-dependent manner, and this inhibition occurred even after nocodazole treatment. In addition, the presence of cytotactin inhibited proliferation stimulated by growth factors or tumor promoter. These effects on cell growth were accompanied by an early inhibition of the intracellular alkalinization that normally occurs upon mitogenic stimulation by a number of growth-promoting agents. Together these observations suggest that cytotactin is an endogenous cell surface modulatory protein and provide a possible mechanism whereby cytotactin may contribute to pattern formation during development, regeneration, tumorigenesis, and wound healing.

Initiation of DNA synthesis by microtubule disruption in quiescent rat 3Y1 cells

Disruption of cytoplasmic microtubules by colchicine, colcemid or vinblastine induced the initiation of DNA synthesis and cell division in quiescent cultures of rat embryo fibroblast cell line, 3Y1, in the absence of growth factors. The microtubule-disruption-induced DNA synthesis was so marked that we could initiate analysis of the mechanism. Incubation of quiescent 3Y1 cells with high concentrations of vinblastine or those of vinblastine plus colcemid, which formed large tubulin paracrystals concomitant with the depolymerization of cytoplasmic microtubules, initiated DNA synthesis in the cells. Because these treatments did not increase the free tubulin level in the cells, but rather decreased it, an increase in the free tubulin level may not be required for initiation of DNA synthesis induced by microtubule-disrupting agents. Culture fluid of the 3Y1 cells incubated with colchicine did not stimulate the initiation of DNA synthesis when added to other quiescent 3Y1 cells after conversion of the colchicine in the culture fluid to lumicolchicine by ultraviolet irradiation. This suggested that colchicine treatment did not induce secretion of any mitogenic factors from 3Y1 cells. These results suggest that disruption of normal microtubules may elicit intracellular signals leading to cell proliferation.

Low-dose gold compounds inhibit fibroblast proliferation and do not affect interleukin-1 secretion by macrophages

We examined the effect of low concentrations of gold compounds on the proliferation of human fibroblasts. Gold sodium thiomalate (GST) inhibited both basal and interleukin-1-induced tritiated thymidine incorporation into fibroblasts in a dose- and time-dependent manner. Significant inhibition was observed at the level of 5 micrograms/ml GST, and greater than 50% inhibition was attained at 10 micrograms/ml. These concentrations are attainable in the serum of treated patients. Similar inhibition was observed when less than 1 micrograms/ml auranofin, which is also within a serum-attainable range, was added. Low concentrations of GST (0-10 micrograms/ml) did not affect interleukin-1 secretion from lipopolysaccharide-stimulated human mononuclear phagocytes (M phi) when assessed by both human fibroblast and C3H/HeJ mouse thymocyte proliferation assays. When M phi precultured for 48 hours with GST (0-10 micrograms/ml) were added to the fibroblast culture in the presence or absence of lipopolysaccharide, there was no significant inhibition of M phi-induced DNA synthesis of fibroblasts. In contrast, when fibroblasts were precultured with GST (0-10 micrograms/ml) for 48 hours and freshly separated M phi were added, significant inhibition was observed in M phi-induced fibroblast proliferation at 5 micrograms/ml. These results suggest that low concentrations of GST directly cause a reduction of fibroblast proliferation, but do not affect the capability of M phi for induction of fibroblast proliferation. Therefore, gold compounds may play a role in the inhibition of the growth of rheumatoid pannus by direct inhibition of fibroblast proliferation.

Colchicine acts as a progression factor to initiate DNA synthesis in quiescent Balb/c 3T3 cells

In quiescent Balb/c 3T3 cells, competence factors such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and platelet-derived growth factor (PDGF) synergize with progression factors such as insulin to initiate DNA synthesis. In this study, we found that colchicine, a microtubule-disrupting agent, acted synergistically with TPA, but not with insulin, to induce the maximal stimulation of DNA synthesis. Colchicine also synergized with PDGF in the presence of epidermal growth factor to elicit nearly the optimal induction of DNA synthesis. Moreover, it acted synergistically with fibroblast growth factor, another competence factor. These results suggest that colchicine acts as a progression factor like insulin in quiescent Balb/c 3T3 cells.

Scanning acoustic microscopy visualizes cytomechanical responses to cytochalasin D

The reactions of XTH-2 cells (line derived from Xenopus laevis tadpole hearts) to cytochalasin D (CD) were followed using scanning acoustic microscopy (SAM) at 0.9 GHz, fluorescence and electron microscopy. The first reaction to CD which can be detected by SAM is a loss of image contrast, indicating a decrease in acoustic impedance of about 30%. Based on structural changes revealed by staining of actin with TRITC-phalloidin, and taking theoretical considerations into account, a relationship between impedance decrease and tension in the actin fibrillar system is deduced.

Inhibition by cytochalasin B of DNA synthesis in a thermosensitive anchorage-independent growth mutant

After a shift from a nonpermissive to a permissive temperature, synchronized DNA synthesis and cell division were observed in a cold-sensitive anchorage-independent growth mutant (cs-17-25) of Chinese hamster lung cells in Methocel culture. Only 15 min exposure to the permissive temperature was sufficient for induction of DNA synthesis in the cells. A low dose of actinomycin D (0.02 micrograms/ml) or cytochalasin B (5 micrograms/ml) was able to inhibit the DNA synthesis when added at an early period after the temperature shift. The inhibitory effects of actinomycin D and cytochalasin B on RNA and protein synthesis were very similar at both temperatures. The degree of multinucleation caused by cytochalasin B was altered quickly depending on the incubation temperature. These results suggest that stimulation of this mutant involves an obligatory actin-mediated step closely correlated to transcription of early mRNAs.

Apparent gene conversion between beta-tubulin genes yields multiple regulatory pathways for a single beta-tubulin polypeptide isotype

We have determined the complete nucleotide sequences of two chicken beta-tubulin genes, beta 1 and beta 2. These genes display an unusual pattern of segmental homology which indicates that they originally arose by gene duplication and have subsequently coevolved by a process that included localized gene conversion or intergenic recombination. Since the beta-tubulin polypeptides encoded by the two genes are virtually identical (99.5%), particularly in the major beta-tubulin isotype defining regions, they almost certainly constitute a single isotypic class of beta tubulin. However, the regulatory properties of the two genes are highly divergent as indicated by analysis of their patterns of expression in different chicken cell types. beta 1 is the major transcript detected in skeletal muscle myoblasts, whereas beta 2 is the major beta-tubulin transcript in cultured sympathetic neurons. The existence of these two genes appears to derive from a regulatory requirement whereby the expression of a single tubulin isotype is mediated through different regulatory programs in development and differentiation. These results thus provide direct experimental support for the hypothesis that gene conversion and intergenic recombination play an important role in evolution by uncoupling the evolution of structural genes from the regulatory sequences which control them.

Effects of cholera toxin and isobutylmethylxanthine on growth of human fibroblasts

Cholera toxin produced a dose-dependent decrease in the restimulation of G0/G1 traverse in density-arrested human fibroblasts but did not inhibit the stimulation of cells arrested in G0 after serum starvation at low density. In addition, cholera toxin did not inhibit the proliferation of sparse logarithmically growing human fibroblasts, even when low concentrations of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) were also present. However, the final density to which sparse cells grew was limited by cholera toxin, when added either alone or together with low concentrations of IBMX. In contrast, high concentrations of the phosphodiesterase inhibitor alone produced a profound inhibition in the growth of sparse human fibroblasts. IBMX produced an inhibition both in the G1 and in the G2 phases of the cell cycle by a mechanism(s) that was not related to the magnitude of the increases in adenosine 3',5'-cyclic monophosphate concentrations.

Apparent gene conversion between beta-tubulin genes yields multiple regulatory pathways for a single beta-tubulin polypeptide isotype

We have determined the complete nucleotide sequences of two chicken beta-tubulin genes, beta 1 and beta 2. These genes display an unusual pattern of segmental homology which indicates that they originally arose by gene duplication and have subsequently coevolved by a process that included localized gene conversion or intergenic recombination. Since the beta-tubulin polypeptides encoded by the two genes are virtually identical (99.5%), particularly in the major beta-tubulin isotype defining regions, they almost certainly constitute a single isotypic class of beta tubulin. However, the regulatory properties of the two genes are highly divergent as indicated by analysis of their patterns of expression in different chicken cell types. beta 1 is the major transcript detected in skeletal muscle myoblasts, whereas beta 2 is the major beta-tubulin transcript in cultured sympathetic neurons. The existence of these two genes appears to derive from a regulatory requirement whereby the expression of a single tubulin isotype is mediated through different regulatory programs in development and differentiation. These results thus provide direct experimental support for the hypothesis that gene conversion and intergenic recombination play an important role in evolution by uncoupling the evolution of structural genes from the regulatory sequences which control them.

Interfacial properties of hydrophilic surfaces of phospholipid films as determined by the method of contact angles. Comparison with cell surfaces

Hydrophilic films of phospholipids were deposited onto plastic substrates (surface-treated for cell cultures) and shown to adhere sufficiently for measuring their interfacial properties by the method of contact angles. Both by absolute magnitude and by their dependence on temperature, the interfacial properties of these phospholipid films were indistinguishable from those determined for black lipid bilayer membranes with a different method by other authors. According to both their vesicular micromorphology and water permeability, the surface films can be interpreted to consist essentially of multibilayer vesicles with the hydrophilic groups facing outward. Treatment of these films with cell-culture medium containing calf serum results in changes of interfacial properties that are very similar to those effected on virus-transformed 3T3 cells (earlier work). These interfacial effects may be attributed essentially to serum proteins (such as albumin) adsorbing to phospholipid or cellular surfaces. The interfacial properties of nontransformed 3T3 cells are much less affected by serum treatment (earlier work), which correlates closely with their higher serum requirement for proliferation. Comparison of these results with those on the interfacial effects of serum on phospholipid films suggests that at least part of the proliferation-stimulating effect of serum is mediated by changes of interfacial properties of cell membranes upon adsorption of serum proteins such as albumin. Treatment of phospholipid films with concanavalin A, an inhibitor of cell proliferation, does not result in effects on their interfacial properties correlating with those on cellular membranes. This confirms previous suggestions that the latter depends on specific binding of concanavalin A to specific carbohydrates on the cell membrane.

Action of temperature-sensitive mutants of myeloproliferative sarcoma virus suggests that fibroblast-transforming and hematopoietic transforming viral properties are related

The myeloproliferative sarcoma virus is molecularly related to the Moloney sarcoma virus (Pragnell et al., J. Virol. 38:952-957, 1981), but causes both fibroblast transformation in vitro and leukemic changes--including spleen focus formation--in adult mice. The fibroblast transforming properties of myeloproliferative sarcoma virus were used to select viral temperature-sensitive mutants at 39.5 degrees C, the nonpermissive temperature. These mutants are temperature sensitive in the maintenance of the transformed state. This was also shown by cytoskeletal changes of the infected cells at permissive and nonpermissive temperatures. Viruses released from cells maintained at both the permissive and nonpermissive temperature are temperature sensitive in fibroblast transformation functions. All temperature-sensitive mutants show only a low reversion rate to wild-type transforming function. The myeloproliferative sarcoma virus temperature-sensitive mutants are inefficient in causing leukemic transformation (spleen enlargement, focus formation) in mice at the normal temperature. A method to maintain a low body temperature (33 to 34 degrees C) in mice is described. One temperature-sensitive mutant was checked at low body temperature and did not induce leukemia. These data thus indicate that the same or related viral functions are responsible for hematopoietic and fibroblast transformation.

Glucocorticoid hormone modulation of both cell surface and cytoskeleton related to growth control of rat glioma cells

We have shown that glucocorticoids reversibly change the growth control of rat C6 glioma cells from a transformed to a normal pattern. Here we report that the glucocorticoid hormone hydrocortisone (Hy) modulates structure and function of cell surface and cytoskeleton. The hormone is shown to cause: (a) increased flattening and adhesion to solid substrates and to fibrin layers, (b) inhibition of the cell shape change triggered by catecholamines and cAMP, (c) extensive fibronectin deposition on normally fibronectinless cells' surface, and (d) microtubule rearrangement. Comparison of Hy-hypersensitive and Hy-resistant variants showed that microtubule rearrangements correlate with the growth control change induced by Hy, whereas fibronectin deposition does not.

Cell surface and cytoskeletal interactions in a temperature-sensitive strain of SV40-transformed mouse fibroblasts

In order to assess the role of cytoskeletal structure in modulating cell surface topography during cell transformation, cytoskeletal organization of 3T3 mouse cells transformed with a tsA mutant of simian virus 40 (SV40) was studied in detail by correlative light and electron microscopy. Detergent-extracted, critical-point dried whole cells observed in the electron microscope were seen to contain well-organized microfilament bundles (stress fibers) traversing the longitudinal axis of cells grown at the restrictive temperature (39 degrees C). When grown at the permissive temperature (32 degrees C), cells prepared in this manner were not observed to contain such structures. However, when semithin sections (0.5 micron) were viewed by transmission electron microscopy at 120 kV, short microfilament bundles were seen in 32 degrees C-grown cells. There was an alteration in the morphology of these structures at sites of attachment to the substratum (focal contacts), and they were shorter in length than microfilament bundles of 39 degrees C-grown cells. A difference was also observed between the two phenotypes in the layer of microfilaments associated with the dorsal cell surface. Since it is this layer that directly determines cell surface architecture, it is proposed that changes in microfilament bundle-generated surface tension are responsible for alterations of this layer, leading to an altered cell surface morphology. Tension may be modified by disturbances in focal contacts (or adjacent regions) or altered actin-associated protein(s).

Commitment and proliferation kinetics of human lymphocytes stimulated in vitro: effects of colchicine on mitogen response

We have examined the effects of colchicine on concanavalin A (Con A)-and phytohemagglutinin (PHA)-stimulated human peripheral blood lymphocytes and from the time course of proliferation have extracted the relative size of the responding cell population, the rate of entry of this population into S-phase, and the length of the lag period. Additions of colchicine at any time did not appear to influence the size of the responding population nor did it greatly affect the duration of the lag period. Only the rate at which the cell population enters initial S-phase is a function of the time of previous exposure to colchicine. Colchicine does not appear to inhibit the commitment of stimulated lymphocytes to enter the cell cycle. Rather, it merely serves to decrease the biochemical processes responsible for fixing a maximal rate of entry into S-phase.

Density-dependent regulation of cell growth: an example of a cell-cell recognition phenomenon

Cell-to-cell contact can result in a variety of changes in the cell's physiology. For different cell types, this may include both the initiation as well as the cessation of cell growth and changes in the state of differentiation. This review examines in detail one such phenomenon, density-dependent inhibition of growth, which is observed with many fibroblasts in culture. Data are summarized which demonstrate that the cessation of growth at high cell density is in part a consequence of cell-to-cell contact. An approach to the study of the molecular basis of this phenomenon is presented based on the demonstration that plasma membranes, when bound to sparse growing cells, mimic contact inhibition of growth. The present status of attempts to purify plasma membrane proteins responsible for this effect are summarized, and the properties of these membrane proteins are compared to those of previously described "soluble" proteins that inhibit cellular growth.

Dependence of interfacial properties of normal and transformed 3T3 cell membranes on treatment with factors modifying proliferation

Interfacial properties of the outer cell membrane of normal and transformed in vitro cultures of mouse 3T3 cells have been investigated. The contact angles of sessile drops on dried cell preparations were measured and the interfacial tensions derived using the thermodynamic approach introduced by Neumann. Interfacial tensions were found to be within an order of magnitude of those determined for other cell and model membranes. Treatment of cells with calf serum, a stimulant to proliferation, resulted in a decrease in the interfacial tension of normal and transformed cells, whereas use of concanavalin A and its succinylated derivative lead to an increase of interfacial tensions of both cell types. These and further results show a detailed correlation between the growth-regulating effects and the effects on interfacial properties of these proliferation-modifying factors. An interpretation of the results of serum depression of the interfacial tension in terms of a binding equilibrium dependent on the concentration of humoral growth factors in the medium is attempted.

Effects of cytoskeletal disrupting agents on replication of bovine endothelium

Colchicine and vinblastine inhibited endothelial cell migration but had no effect on the stimulation of replication seen at wound edges in cultures of endothelium at stationary density. This is in contrast to the effects of cytochalasins which inhibit both migration and replication at wound edges. Moreover, colchicine and vinblastine stimulated cell replication in the unwounded, confluent monolayer. This effect has kinetics similar to the stimulation of replication at a wound edge and is associated with an initial retraction of cell borders, leaving gaps between cells. Cytochalasin D inhibited the growth response to microtubule disrupting agents but did not prevent cell retraction. Stimulation of replication by microtubule disrupting agents was not dependent on serum but was synergistic with serum in cultures rinsed repeatedly with serum-free medium. The replication occurred prior to any cell loss. When, however, cells were allowed to complete mitosis, about one-half of the daughter cells detached from the monolayer so that there was no increase in cell density. We conclude that microtubule disrupting agents are the first agents found to be effective in stimulating growth of vascular endothelium at saturation density. These data further suggest that colchicine and vinblastine stimulate cell growth in a manner similar to wounding, where cell movement is a prerequisite to cell replication.

Microtubule-disrupting agents affect two different events regulating the initiation of DNA synthesis in Swiss 3T3 cells

The stimulatory effect of epidermal growth factor, alone or with insulin, on the rate of initiation of DNA synthesis in Swiss 3T3 cells can be synergistically enhanced by the addition of either Colcemid or colchicine at 1 microM. However, both Colcemid and colchicine can exert the synergistic effect only when added earlier than 8 hr of the prereplicative period (lag phase). Removal of Colcemid (which allows for rapid reassembly of microtubules) earlier than 10 hr of the lag phase results in a loss of the synergistic effect. This suggests that microtubules must remain disrupted for longer times to accomplish some putative event(s) necessary for increasing the rate of initiation of DNA synthesis. Preincubation of quiescent cells with either Colcemid or colchicine for 8 hr prior to adding epidermal growth factor, alone or with insulin, shortens the lag phase by about 4 hr, irrespective of the resulting rate of initiation of DNA synthesis. These results suggest that the state of microtubules is affecting independently at least two different events involved in regulating time initiation of DNA synthesis.

Studies on cell adhesion and recognition. II. The kinetics of cell adhesion and cell spreading on surfaces coated with carbohydrate-reactive proteins (glycosidases and lectins) and fibronectin

The kinetics of cell attachment and cell spreading on the coated surfaces of two classes of carbohydrate-reactive proteins, enzymes and lectins, have been compared with those on fibronectin-coated surfaces with the following results: (a) A remarkable similarity between the kinetics of cell attachment to fibronectin-coated and glycosidase-coated surfaces was found. In contrast, cell attachment kinetics induced by lectin- and galactose oxidase-coated surfaces, in general, were strikingly different from those on fibronectin and glycosidase surfaces. The distinction between fibronectin- or glycosidase- and lectin- or galactose oxidase (an enzyme with lectin-type characteristics)-coated surfaces was further supported by the finding that cytochalasin B and EDTA inhibited cell attachment to fibronectin- and glycosidase-coated surfaces but not lectin-coated surfaces. (b) Fibronectin, if labeled and added to a cell suspension, showed only low or negligible interaction with the cell surface. However, fibronectin absorbed on plastic surfaces showed a high cell-attaching activity. It is assumed that fibronectin coated on plastic surfaces may form polyvalent attachment sites in contrast to its lower valency in aqueous solution. (c) Various inhibitors of cell attachment to both fibronectin-, galactose oxidase-, and lectin-coated surfaces were effective only during the first few minutes of the adhesion assay, after which time the attached cells became insensitive to the inhibitors. It is suggested that the initial specific recognition on either lectin-type or fibronectin-type surfaces is followed by an active cell-dependent attachment process. The primary role of the adhesion surface is to stimulate the cell-dependent attachment response. (d) Cells attached on tetravalent concanavalin A (Con A) spread very rapidly and quantitatively, whereas divalent succinyl Con A and monovalent Con A were effective stimulators of cell attachment but not cell spreading. Cross-linking of succinyl Con A restored the cell spreading activity. Tetravalent Con A surfaces specifically bind soluble glycoproteins, whereas succinyl Con A has a greatly reduced ability to bind the same glycoproteins. These results suggest that cross-linking of cell surface glycoproteins by the multivalent adhesive surface may trigger the cellular reaction leading to cell spreading.

Polyoma virus middle t antigen: a tumor progression factor

Polyoma virus (PyV) deletion mutant dl23 (affecting both large T and middle t but not small t antigens) was used to study transformation of 3T3 rat cells. This mutant generated stable transformants in the agar assay at a frequency similar to that of wild-type virus (WT). However, WT-induced transformants were detected 3 weeks after infection, whereas those induced by the mutant could not be detected until 6 to 8 weeks after infection. In this respect, dl23 PyV behaved like WT simian virus 40 (SV40). Cells transformed by WT SV40 or by dl23 PyV were similar in all their transformed properties. Those transformed by WT PyV were different from the others on the basis of morphology, cell adhesion to the substrate, release of protease activity, efficiency of doubling in agar, growth rate, and time required for tumor formation. Saturation density, the ability to grow in agar, the serum requirement for cloning, and the ability to grow on a cell monolayer were similar for all transformants. Middle t antigen enhanced membrane alterations and growth rate of the transformed cells, shortening the time required for tumor formation in rats.

The protein encoded by the transforming gene of avian sarcoma virus (pp60src) and a homologous protein in normal cells (pp60proto-src) are associated with the plasma membrane

Oncogenesis by avian sarcoma virus is attributable to a single viral gene (src) which encodes a phosphoprotein (pp60src) with the enzymatic activity of a protein kinase. A closely related protein, pp60proto-src, occurs in uninfected cells from a wide variety of vertebrate species and is presumed to be the product of a cellular gene that served as progenitor for src. We explored the location of these proteins within the cell by using immunoprecipitation to analyze subcellular fractions prepared from avian sarcoma virus-transformed rat and chicken cells and from uninfected rat cells. We found that both pp60src and pp60proto-src were associated with the plasma membrane as active protein kinases and could be recovered efficiently only by disrupting the membranes with nonionic detergent. Our findings, in conjunction with those of other investigators, indicate that both proteins are embedded in the membrane by means of a hydrophobic domain(s); available evidence indicates that pp60src is not exposed on the surface of the cell but is accessible at the cytoplasmic aspect of the plasma membrane. These conclusions lend credence to two current speculations. First, pp60src and pp60proto-src may have similar or even identical functions. Second, neoplastic transformation may originate from derangements in the plasma membrane or its affiliated structures.

Density-dependent stimulation and inhibition of cell growth by agents that disrupt microtubules

We have previously reported that agents that disrupt microtubules, such as colchicine, inhibit the growth stimulation of lymphocytes and arrested fibroblasts; other workers have recently reported enhanced stimulation of fibroblasts in the presence of the same drugs. In the present studies, we resolve this conflict by demonstrating that the stimulatory and inhibitory effects of microtubule disruption depend upon the density and the cell type of the treated cultures. Our analysis included an examination of three variables: (i) cell density (sparse or confluent), (ii) cell type (resting fibroblasts from mouse or chicken embryos or from the permanent 3T3 mouse fibroblast line), and (iii) treatment with colchicine and related drugs in the presence or absence of various mitogens such as serum, insulin, and epidermal growth factor. We found that colchicine augmented mitogenesis in confluent cultures of all cell types. Colchicine by itself appeared to be mitogenic only for confluent chicken embryo fibroblasts. In sparse cultures with minimal cell-cell contacts, however, there were differences between embryonic cells and the 3T3 cell line. In confirmation of our previous reports, disruption of microtubules by colchicine inhibited the mitogenic stimulation of sparse cultures of embryonic chicken or mouse fibroblasts. In contrast, fibroblasts of the permanent 3T3 line in sparse cultures were stimulated by some mitogens despite the presence of colchicine. The augmentative effects of colchicine on the stimulation of confluent cultures were synergistic with the mitogens, and colchicine allowed response to otherwise submitogenic doses of growth factors. Kinetic studies indicated that the stimulatory and inhibitory effects of colchicine are separable and that both can operate simultaneously. The experiments suggest that the regulation of growth by nutrient deprivation and the regulation by density dependence proceed at least in part by different mechanisms. All of the results suggest that microtubular integrity can be associated with the expression of either negative or positive controls on cell growth, depending upon the confluence or lineage of the cells in culture.

Interferon effects on microfilament organization, cellular fibronectin distribution, and cell motility in human fibroblasts

We have shown previously (Pfeffer et al., 1979, Exp. Cell Res. 121:111-120) that treatment of human fibroblasts, planted at a density of 2x10(3) cells/cm(2), with purified human fibroblasts interferon (640 U/ml) for 3 d at 37 degrees C decreases the overall rate of cell proliferation to 35-40 percent of the control value. In the present experiments we have characterized the phenotype of interferon-inhibited fibroblasts. The mean volume of trypsinized, interferon-treated cells was increased 31 percent abover that of control cells. The interferon-treated population was much more heterogeneous than the control population with respect to volume, and there was a considerable overlap in the volume distributions of the two populations. The cell surface area was, on the average, increased 65 percent after interferon treatment. More than 80 percent of the treated cells had enlarged nuclei, many of which were lobed, and the fraction of binucleated cells was increased fivefold. After interferon treatment, over 40 percent of the cells showed large actin-containing fibers in the form of multiple parallel arrays. Fewer than 5 percent of the control cells contained such large actin fibers. The number of actin fibers of all sizes was tripled in the treated fibroblasts on a per cell basis and, calculated per unit surface area of the cells, the number was increased 82 percent. In contrast, 10-nm filaments and microtubules did not appear to be increased in number per unit surface area of the cells. The increases per cell in the abundance of these structures were directly related to increased cell size. After interferon treatment, fibronection was distributed in arrays of long filaments covering most portions of the cell surface. Interferon treatment markedly decreased the rate of cell locomotion as well as membrane ruffling and saltatory movements of intracellular granules.

Cytoskeleton-disrupting drugs enhance effect of growth factors and hormones on initiation of DNA synthesis

Addition of growth factors, such as prostaglandin F2 alpha or fibroblastic growth factor, to quiescent Swiss mouse 3T3 cells resulted in an abrupt increase in the rate of initiation of DNA synthesis after a lag phase of 13-15 hr. This increase could be quantified by a rate constant k. Addition of colchicine, Colcemid, or vinblastine had a synergistic effect on the initiation of DNA synthesis triggered by PGF2 alpha or FGF by increasing the value of k. These drugs alone had no effect. Colchicine had a synergistic effect only if added within 8 hr of the PGF2 alpha or FGF addition. Also, colchicine exerted its full effect when it was present only for the first 5 hr with either growth factor. These results suggest that an intact cytoskeleton is not required for the initiation of DNA synthesis. Furthermore, cytoskeleton-disrupting drugs enhance the stimulatory effect of the growth factors.

Quantitative biochemical analysis of microtubule content in normal and transformed 3T3 cells

Microtubules in normal and transformed BALB 3T3 cells were preserved in a stabilizing medium and measured by a [3H]colchicine-binding tubulin assay, and compared to total cellular tubulin measured under nonstabilizing conditions. Essentially no change in tubulin or microtubule content was seen with changes in cell density or with changes in cellular morphology at various stages of growth of normal or transformed cells or induced by dibutyryl cAMP treatment of transformed cells. Of five cell lines transformed by a variety of agents, four had a significantly higher total tubulin content than untransformed 3T3 cells and all of them had an increased microtubule content. None of the transformed lines had a lower fraction of tubulin recoverable as sedimentable microtubules compared to untransformed cells, and in three of them this fraction was significantly higher. These results establish that microtubules are present in transformed cells to at least the extent (if not greater) than in normal cells but that there are variations in the total amount of tubulin and microtubules as well as the fraction of the total tubulin present as microtubules which are not strictly correlated with transformation or cell morphology.

Antitubulin agents enhance the stimulation of DNA synthesis by polypeptide growth factors in 3T3 mouse fibroblasts

Colchicine and other antitubulin agents markedly enhanced the stimulation of DNA synthesis by combinations of various growth factors such as epidermal growth factor, insulin, fibroblast-derived growth factor, and vasopressin in serum-free cultures of several quiescent 3T3 mouse fibroblast cell lines. Enhancing effects were observed based on continuous incorporation of [3H]thymidine into DNA as well as by autoradiographic labeling of cell nuclei. The concentration of colchicine and podophyllotoxin required to produce half-maximal enhancement of DNA synthesis stimulated by epidermal growth factor and insulin was 25-50 nM. Lumicolchicine did not produce enhancing effects. The disassembly of microtubules resulting from the action of colchicine, Colcemid, and vinblastine did not inhibit the stimulation of DNA synthesis in quiescent Swiss 3T3 fibroblasts by fetal bovine serum. We conclude that the cytoplasmic microtubule network in 3T3 mouse fibroblasts does not exert a positive regulatory function in the initiation of DNA synthesis but rather can produce a constraint on the initial action of the peptide growth factors in serum-free media.

Thermolabile protein kinase molecules in a temperature-sensitive murine sarcoma virus pseudotype

Murine sarcoma virus-associated protein kinases that bind to actin have been purified by affinity chromatography on actin coupled to Sepharose. Heat inactivation studies showed the presence of thermolabile enzyme activity in pseudotypes containing a temperature-sensitivity mutant of murine sarcoma virus (MSV) but not in two independent wild-type MSV pseudotypes. Studies with Sephadex G-75 column fractions showed that a low molecular weight form, approximately 15,000, is the major thermolabile kinase in the temperature-sensitive MSV virions. Antibodies raised against the MSV-coded p60 protein, when added to the in vitro reaction mixtures, showed specific phosphorylation of the IgG heavy chain and a simultaneous reduction in the extent of phosvitin phosphorylation catalyzed by the various MSV pseudotype kinases. Thus a transforming retrovirus-coded enzyme activity that interacts directly with a major cytoskeletal protein and whose activity parallels the transforming ability of a conditional MSV mutant has now been identified.

Metabolic stimulation of polymorphonuclear leucocytes: effects of tetravalent and divalent concanavalin A

Polymorphonuclear leucocytes (PMNL) undergo a marked activation of their oxidative metabolism upon interaction with surface-reactive soluble stimuli as well as with phagocytosable objects. To get some insight into the mechanism of this stimulation, we have compared the stimulatory activity of the tetravalent lectin concanavalin A (Con A) with that of the divalent derivative succinyl-Con A (S-Con A). Both lectins bind to the PMNL surface to the same extent. S-Con A, however, is much less efficient in stimulating the PMNL metabolism. When S-Con A-treated PMNL are further reacted with antiserum to Con A, a potentiation of the metabolic stimulation is observed. Normal serum or addition to PMNL of antiserum to Con A in the absence of lectin has no effect. Furthermore, if S-Con A is displaced from its receptors on the cell membrane with alpha-methyl mannopyranoside, the addition of antiserum fails to cause a respiratory stimulation. These results suggest that the initial triggering of the metabolic stimulation of PMNL is in part accomplished through cross-linkage of membrane constituents.

Assay for early cytoplasmic effects of the src gene product of Rous sarcoma virus

When microinjected into normal fibroblasts, cytoplasmic extracts of cells transformed by Rous sarcoma virus caused dissolution of microfilament bundles. This activity was not found in extracts of normal cells. The maximum effect was seen within 30 min of injection, and the activity could still be measured after a 10-fold dilution of the cytoplasmic extracts (14 mg/ml original protein concentration). The activity was trypsin sensitive and was destroyed by boiling, but was not RNase sensitive. Protein synthesis was not required for the disruption of actin-containing stress fibers by the injected activity. Microinjected cytoplasts prepared from normal 3T3 cells also showed dissolution of microfilament bundles, indicating that the cell nucleus was not required for expression of activity. Extracts made from fibroblasts transformed by Rous sarcoma virus having a temperature-sensitive mutation in the src gene were also temperature sensitive in the microinjection assay. Thus, the activity of extracts from cells infected with src mutant virus, but not from cells infected with wild-type virus, was destroyed either by in vitro incubation of the extract at the nonpermissive temperature before injection or by incubation of recipient cells at the nonpermissive temperature after injection. We conclude that the microinjection assay can detect a cytoplasmic activity coded for by the src gene of Rous sarcoma virus and that an early direct or indirect target of the src gene product is the cytoskeleton and cell motility system. This result is discussed in relation to the hypothesis that submembranous arrays of microfilaments, microtubules, and their associated proteins interact with cell surface receptors to form a surface modulating assembly that functions as a key regulator of cell growth.

Evidence for unaltered structure and in vivo assembly of microtubules in transformed cells

By using immunoperoxidase cytochemistry at the light and electron microscopic level, microtubles were visualized in a number of "normal" nontumorigenic and transformed tumorigenic cell lines. A well-defined cytoplasmic microtubule complex exists in both normal and transformed interphase cells. The distribution of this complex closely correlates with the cell shape and the degree of cell spreading. Our data support the idea that these properties determine the pattern of the cytoplasmic microtubule complex, rather than the reverse. Ultrastructural observations of immunoperoxidase-stained tumor cells showed characteristic microtubules in cells in which the microtubules were poorly resolved at the light microscopic level. The results suggest that microtubule assembly and structure are unaltered in transformed cells. However, this conclusion does not exclude the possibility that some of the microtubules' functions might be impaired in a yet-unknown way.

Studies on secretion and endocytosis of macromolecules by cultivated skin fibroblasts. Effects of anti-microtubular agents on secretion and endocytosis of lysosomal hydrolases and of sulphated glycosaminoglycans

Fibroblasts were incubated in the presence of the anti-microtubular drugs colchicine, vinblastine and vincristine. In concentrations between 10nm and 1 mM these drugs stimulated the secretion of beta-N-acetylglucosaminidase, alpha-N-acetylglucosaminidase and beta-glucuronidase, but not of beta-galactosidase. The endocytosis of beta-N-acetylhexosaminidase and alpha-N-acetylglucosaminidase, but not of beta-glucuronidase, was inhibited at drug concentrations higher than 0.1 micrometer. Formation, secretion and association with the cell membrane of sulphated proteoglycans were not affected by anti-microtubular drugs. Endocytosis of sulphated proteoglycans and their subsequent degradation was inhibited by drug concentrations above 0.1 micrometer. The inhibition of intracellular glycosaminoglycan degradation led to a moderate storage of these compounds. These results suggest that microtubules participate in the control of secretion and endocytosis of lysosomal enzymes, and in the endocytosis and degradation of lysosomal substrates such as sulphated proteoglycans.