Evidence that microtubule depolymerization early in the cell cycle is sufficient to initiate DNA synthesis

Effects of colchicine on the induction of ornithine decarboxylase and its gene expression by the phorbol ester tumour promoter

The activity and gene expression of ornithine decarboxylase (ODC, an indicator of tumour promotion) were induced by the phorbol ester tumour promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), in mouse skin. In the present study, the effect of colchicine, a microtubule-disrupting agent, on ODC activity and its gene expression were investigated. On administration of colchicine (100 micrograms) intraperitoneally 1.5 h before TPA treatment, ODC activity and ODC mRNA levels stimulated by TPA were suppressed to about 52 and 64%, respectively. These results suggest the involvement of a microtubule or colchicine-sensitive substrate in the signal transduction system for gene expression.

Production of a unique antibody specific for membrane ruffles and its use to characterize the behavior of two distinct types of ruffles

I have produced a new monoclonal antibody, YF-169, against membrane ruffle specific 55-kD protein. YF-169 stained membrane ruffles of chick embryo fibroblasts so definitely that it enabled clear and reliable analyses of membrane ruffles. Fibroblasts organized two distinct types of membrane ruffles. One type of the ruffles were transiently formed in serum-starved cells (Type I) when stimulated by serum or platelet-derived growth factor. After spontaneous degradation of Type I ruffles, the other type of ruffles containing many microspikes were gradually organized at leading edges (Type II). The formation of Type I ruffles was not affected by either nocodazole, a microtubule-disrupting drug, or taxol, a microtubule-stabilizing reagent. However, Type II ruffles were entirely destroyed not only by nocodazole but also by taxol, suggesting that regulated organization of microtubule network is important to maintain Type II ruffles. H8, a protein kinase inhibitor prevented the spontaneous degradation of Type I ruffles and also reduced the destructive effect of nocodazole on Type II ruffles without affecting microtubule-disrupting activity. Protein kinases may be involved in the degradation processes of both types of ruffles. W7, a calmodulin antagonist, strongly inhibited Type I ruffle formation and completely destroyed Type II ruffles. W7 was also found to induce a remarkable change of 55-kD protein localization. After degradation of Type II ruffles, most of 55-kD protein was incorporated into newly formed unusual thick fibers. These results suggest that regulated organization of microtubule network is not necessary to form Type I ruffles but is important to maintain Type II ruffles, while calmodulin function is essential for both types of membrane ruffles.

Disruption of microtubules inhibits the stimulation of tissue plasminogen activator expression and promotes plasminogen activator inhibitor type 1 expression in human endothelial cells

The expression of certain proteolytic enzymes involved in cell migration (collagenase, urokinase) can be enhanced by the disruption of cellular cytoskeletal organization, suggesting an association between cell shape and gene expression. We have examined the effect of cytoskeleton-disrupting agents on the production and secretion of another proteolytic enzyme, tissue plasminogen activator (tPA), and its inhibitor, plasminogen activator inhibitor-1 (PAI-1), in human endothelial cells. Addition of 1 x 10(-6) M colchicine, 5 x 10(-6) M cytochalasin B, 10(-6) M nocodazole, or 10(-6) M tubulazole had no effect on the constitutive rate of release of tPA. However, the three microtubule-disrupting agents--colchicine, nocodazole, and tubulazole--depressed the stimulation of tPA secretion by phorbol myristate acetate (PMA) by 50- to 65%. Disruption of microfilament structure by cytochalasin B had no effect. In contrast, microtubule disruption in the absence or presence of PMA stimulated PAI-1 secretion by 2.5 and 2 times, respectively. The depression of tPA secretion was not due to inhibition of the secretory function since tPA did not accumulate intracellularly during colchicine treatment. Nor did colchicine affect the PMA activation of protein kinase C-alpha, upon which stimulation of tPA is dependent; neither translocation of the kinase nor phosphorylation of the protein kinase C substrate protein, P80, was inhibited. Measurement of tPA mRNA levels demonstrated that the increase which precedes PMA-enhanced tPA secretion was also inhibited by colchicine by 50%. However, tPA gene transcriptional activity was only reduced 13%, suggesting that a post-transcriptional event was affected by microtubule disruption. PAI-1 mRNA levels and transcription rates were elevated 3.5 times. This study suggests that the changes that occur in endothelial cells during PMA-induced signal transmission leading to enhanced tPA mRNA levels and tPA antigen production can be partly blocked by agents that disrupt microtubule organization.

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.

Increased calmodulin affects cell morphology and mRNA levels of cytoskeletal protein genes

We have previously described stable mouse C127 cell lines in which a CaM mini-gene has been expressed in a bovine papilloma virus-based expression vector (Rasmussen and Means: EMBO J. 6:3961-3968, 1987). Elevation of CaM to levels five-fold higher than in control cells caused an acceleration in cell cycle progression by reducing the length of the G1 period. When these cell lines were originally isolated it was observed that cells in which CaM levels were increased had a flattened morphology. In this study we have examined the localization of actin, vimentin, and tubulin in these cells as compared to the BPV-transformed control cell line in order to determine if changes in shape were accompanied by differences in the cytoskeletal organization. Cell-cycle-dependent changes in the levels of mRNAs for histone H4, glyceraldehyde-3-phosphate dehydrogenase, beta-actin, vimentin, and beta-tubulin have also been examined. Our results indicate that increased CaM causes differences in the organization of microfilaments, intermediate filaments, and microtubules and that these changes are accompanied by selective differences in the cell-cycle-dependent expression of some mRNAs. Elevated CaM was also correlated with a reduced stability of beta-tubulin mRNA. These studies indicate that CaM has pleiotropic effects on cell function and suggest that stable cell lines with altered CaM levels may provide a useful model system for understanding the molecular basis of CaM-dependent regulation of cellular processes.

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.

Glycosylation of high-affinity thrombin receptors appears necessary for thrombin binding

Monosaccharide binding competition, lectin affinity chromatography, and glycosylation inhibitors have been used to determine if glycosylation plays a role in thrombin-receptor interactions. Mannose appeared to specifically inhibit thrombin binding to mouse embryo (ME) and hamster fibroblasts. Concanavalin A bound to antibody-purified receptor fractions, and was used as an affinity ligand to purify receptor fractions that retained thrombin binding activity. Cells treated with tunicamycin (6.25 ng/ml) for 24 h lost approximately 35% of their high-affinity thrombin binding sites, yet binding of receptor monoclonal antibody TR-9 was not affected, indicating that the receptor was present in the membrane, but unable to bind thrombin. Thus thrombin receptor glycosylation may be directly involved in thrombin binding.

The clinical pharmacology and use of antimicrotubule agents in cancer chemotherapeutics

Although there has been a rapid expansion of the number of classes of compounds with antineoplastic activity, few have played a more vital role in the curative and palliative treatment of cancers than the antimicrotubule agents. Although the vinca alkaloids have been the only subclass of antimicrotubule agents that have had broad experimental and clinical applications in oncologic therapeutics over the last several decades, the taxanes, led by the prototypic agent taxol, are emerging as another very active class of antimicrotubule agents. After briefly reviewing the mechanisms of antineoplastic action and resistance, this article comprehensively reviews the clinical pharmacology, therapeutic applications, and clinical toxicities of selected antimicrotubule agents.

Activation of microtubule-associated protein kinase by microtubule disruption in quiescent rat 3Y1 cells

Treatment of quiescent rat fibroblastic cells (3Y1) with colchicine, a microtubule-disrupting agent, which could induce the initiation of DNA synthesis [Y. Shinohara, E. Nishida, and H. Sakai (1989) Eur. J. Biochem. 183, 275-280], activated a serine/threonine-specific protein kinase activity in cell extracts that preferentially phosphorylated exogenous microtubule-associated protein 2 (MAP2). Vinblastine treatment also activated the kinase activity, and taxol pretreatment inhibited the colchicine-induced activation of this kinase activity. The detailed biochemical characterization indicated that this microtubule disruption-activated MAP2 kinase was very similar or identical to the mitogen-activated MAP kinase in the substrate specificity and chromatographic behaviors on phosphocellulose, DEAE-cellulose, gel filtration, and phenyl-Sepharose. Pretreatment of the cells with protein synthesis inhibitors did not prevent the MAP2 kinase activation by colchicine. Moreover, phosphatase treatment inactivated the colchicine-activated MAP2 kinase activity. These data suggest that microtubule disruption activates MAP kinase through phosphorylation.

Inhibition of cell proliferation by glycerol

The effect of glycerol on proliferation of BHK, CHO, HBL, MCF-7, and human glioma cells was studied. Cell proliferation was significantly decreased in all the cell lines at glycerol concentrations of 2-4% in the culture medium. The inhibition was dose-dependent, complete suppression of proliferation occurring at a glycerol concentration of 4% for the MCF-7 cell line and 6-8% for the BHK, CHO and human glioma cells. Studies on [3H]thymidine incorporation correlate with the effect on cell proliferation. The viability of the cells was not significantly affected until higher concentrations of glycerol (12% +) were present. Recovery studies with BHK cells indicated that replacement of the glycerol medium with glycerol-free medium resulted in full recovery following exposure to 4% glycerol and only partial recovery (65%) of proliferation rate following exposure to 10-12% glycerol. It is concluded that glycerol, a substance that is normally present in tissues, can serve as a potent inhibitor of cell proliferation.

Taxol inhibits stimulation of cell DNA synthesis by human cytomegalovirus

The microtubule (MT)-stabilizing drug, taxol, inhibited human cytomegalovirus (CMV)-initiated cell DNA synthesis by up to 100% in serum-arrested mouse embryo (ME) fibroblasts that were abortively infected by CMV. Taxol concentrations known to increase MT polymerization and to stabilize existing MTs (10 to 20 micrograms/ml) blocked CMV-stimulated cell DNA synthesis, while taxol concentrations of 2.5 micrograms/ml, or less, did not. Taxol maximally inhibited CMV initiation of cell DNA synthesis when added 3 h after virus infection and inhibited this initiation by greater than 50% when added up to 12 h after CMV infection. Control experiments suggest that taxol specifically inhibited CMV-stimulated cell DNA synthesis. Pretreatment of CMV stock with taxol did not reduce the stimulatory effect of CMV on cell DNA synthesis and taxol had no detectable effect on CMV-specific early protein synthesis. Moreover, taxol did not appear to alter thymidine pool sizes, affect cell viability, or compromise the DNA synthetic machinery in CMV-infected cells. Since taxol increases tubulin polymerization and inhibits MT disassembly, these results suggest that dynamic changes in MTs or in the pool of free tubulin subunits are necessary for CMV to stimulate cell entry into a proliferative cycle.

The growth-regulated gene 1B6 is identified as the heavy chain of calpactin I

The expression of 1B6, a growth-regulated sequence isolated from a Syrian hamster fibroblast cDNA library, was studied in BALB/c 3T3 cells. The level of cytoplasmic 1B6 mRNA (1600 bases) was low in quiescent cells and plateaued in mid/late G1 after the cells were stimulated with 15% fetal calf serum (FCS). Protein synthesis was not required for the induction of 1B6 mRNA; therefore, the expression of 1B6 is a primary response to serum stimulation. The induction of 1B6 mRNA was also observed after stimulation with insulin, epidermal growth factor, and fibroblast growth factor but not with platelet-derived growth factor. When quiescent cells were serum-stimulated, the percentage of cells that became committed to enter DNA synthesis was proportional to the length of their incubation with serum. To determine if 1B6 expression was also correlated with the time of exposure to serum, quiescent cells were stimulated with a pulse of 15% FCS and the abundance level of 1B6 induced by that pulse was determined. The amount of 1B6 mRNA increased with increasing time of exposure to serum and paralleled the increase in the percentage of nuclei that were induced into DNA synthesis by the serum pulse. Comparison of the nucleotide sequence of the p1B6 cDNA to the GenBank database revealed a striking identity of 1B6 to the 3' end of p36, the heavy chain of calpactin I. The previous characterization of p36 as a substrate for tyrosine kinases suggests a possible role for 1B6/p36 in cell proliferation.

Autographa californica M nuclear polyhedrosis virus: microtubules and replication

Progressive reorganization and depolymerization of microtubules corresponded with virus-induced rounding of Autographa californica M nuclear polyhedrosis virus (AcMNPV)-infected Spodoptera frugiperda IPLB-Sf-21 cells, suggesting that microtubules were instrumental in maintaining the normal shape of these cells. Depolymerization of all cortical and most of the paranuclear microtubules with colchicine also resulted in cell rounding, confirming this hypothesis. Studies with aphidicolin and cycloheximide indicated the virus-induced effects on the microtubules were mediated by both early and late viral gene products. Microtubules in cells infected with a p10 deletion mutant depolymerized microtubules in a manner similar to those in wild-type virus-infected cells, indicating p10 was not responsible for virus-induced changes in the microtubules. Nevertheless, evidence for the association of p10 and microtubules was obtained by fluorescence microscopy and immunoelectron microscopy. Colchicine depolymerization of microtubules before and throughout infection did not interfere with virus replication, but treatment of cells with taxol, a microtubule-stabilizing agent, both delayed and depressed virus replication. The taxol-induced effect was relieved by the addition of colchicine. These results suggested that AcMNPV-induced depolymerization of microtubules may be a necessary event in, rather than a tangential effect of, virus replication. Attempts to monitor the effects of virus infection on intermediate filaments were unsuccessful due to the lack of cross-reactivity between antibodies to intermediate filament proteins and IPLB-Sf-21 cells, indicating these proteins are not highly conserved in lepidopteran insect cells.

Calmodulin, cell growth and gene expression

Calmodulin is thought to regulate a number of intracellular processes, including cell proliferation. Previous studies using drugs that antagonize calmodulin function have indicated that calmodulin is required for progression at specific points in the eukaryotic cell cycle. However, interpretation of these results has previously been difficult due to the lack of specificity of these inhibitors in living cells. Recent studies have used a combination of molecular biological and genetic analysis techniques to approach the study of calmodulin-dependent cell cycle control with greater precision and specificity. These studies have confirmed that calmodulin is an important regulator of the cell cycle, and provide new ways in which to examine the cellular mechanisms involved in calmodulin-dependent cell cycle control.

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.

S-100 antigen and calmodulin in human leukemic cells

Both S-100 antigen and calmodulin were shown in normal lymphocytes with S-100 being decreased in lymphocytic leukemia cells. Although small amounts of S-100 antigen and calmodulin were shown in acute myeloblastic leukemia cells, they could not be detected in normal granulocytes. In lymphoblastic leukemia, S-100 antigen levels in T-cell leukemia cells were higher than in B-cell leukemia cells, while calmodulin was decreased in chronic leukemia cells. In mitogen-stimulated lymphocytes, the levels of S-100 antigen were decreased, while those of calmodulin were either increased or unchanged. Calcium-dependent cyclic nucleotide phosphodiesterase was highest in acute lymphoblastic leukemia. These data suggest, therefore, that calcium ions may play a role in the proliferation, differentiation or leukemic change in lymphocytes and, hence, that measurement of calcium binding proteins may be useful in the investigation of leukemia cells or lymphocytes.

A single calcium flux triggers chromosome replication, segregation and septation in bacteria: a model

Abrupt changes in the concentration of intracellular calcium, through the mediation of calmodulin, is presumed to play an essential role in many molecular processes in eukaryotes including triggering cell cycle events. Although early studies failed to establish any role for calcium in the growth of bacteria, recent studies have demonstrated that bacteria have several calcium transport systems, and an intracellular concentration of free calcium identical to that of higher organisms, which appears to fluctuate during the cell cycle. Moreover, calmodulin-like proteins have been reported in bacteria, and the growth of E. coli is sensitive to calmodulin inhibitors. In this article we propose that a single flux of calcium, abruptly raising the intracellular concentration of free calcium, is responsible for the triggering in bacteria of the major cell cycle events, initiation of DNA replication, chromosome partition and cell division. We predict that major roles in this process will involve a bacterial calmodulin-like protein and a primitive cytoskeleton. The mechanism of triggering different cell cycle events by a single calcium flux is discussed.

Antimitogenic actions of lectins in cultured human fibroblasts

It was previously reported that the lectins wheat germ agglutinin (WGA) and concanavalin A (Con A) inhibit the mitogenic actions of multiple peptide growth factors in human fibroblasts without having a significant effect on mitogen binding. The current studies were designed to further examine the mechanisms of this antimitogenic action of lectins. Addition of WGA at progressively later times after stimulation of fibroblasts with peptide mitogens revealed significant inhibition of DNA synthesis even when the lectin was added 16-20 h after growth factors. This suggests an inhibitory effect on a pathway occurring late in G1, or close to the G1/S boundary. WGA also inhibited stimulation of DNA synthesis by non-peptide agents such as colchicine and vanadate ion, indicating that the lectin inhibits a common distal step in the mitogenic response, rather than acting primarily on events occurring at the level of the growth factor-receptor interaction. WGA had a rapid (within 30 min) inhibitory effect on insulin-stimulated amino acid uptake, but Con A, which like WGA blocked mitogen-stimulated 3H-dT incorporation, had little effect on stimulation of amino acid uptake. Thus the inhibition of DNA synthesis and amino acid uptake by lectins appear to be mediated by distinct mechanisms. WGA binding to fibroblasts persisted even when the lectin was removed from the incubation medium, but unlike 125I-EGF, which was rapidly internalized at 24 degrees C, little 125I-WGA was internalized. Incubation of fibroblasts for 20 h with WGA or Con A was not toxic to cells, since reversal of lectin binding by the appropriate saccharide allowed normal subsequent stimulation of DNA synthesis by EGF and insulin. However, the observation that cells exposed to antimitogenic lectins undergo a marked decrease in cell spreading suggests that changes in cell shape may be relevant to the mechanism by which lectin-treated fibroblasts become unresponsive to mitogenic stimulation.

Microtubule and microfilament distribution and tubulin content in the cell cycle of Indian muntjac cells

Using DAPI, rabbit antitubulin antibody, FITC-labeled goat anti-rabbit IgG, and TRITC-phalloidin to stain individual cells, the microspectrophotometric analysis showed that three markers that represent the nucleus, microtubules (MT), and microfilaments (MF), respectively, could be recognized in individual cells without interference. The phase of the cell cycle was determined by DNA content. We found that in Indian muntjac (IM) cells, the amount of tubulin in G2 and M phases was about twice as much as that in G1 phase. In G2 cells, the cytoplasmic microtubule complex (CMTC) became denser than in G1 cells. The cytoplasmic MT extent in basically the same orientation as MF bundles in interphase. The regions where the MT is denser also have a denser MF distribution.

New synthesis of cyclic AMP-dependent protein kinases during liver regeneration

As has been previously reported one surge in cytosolic calmodulin is produced between 4 and 12 h after a partial hepatectomy. Moreover, a surge in cytosolic cyclic AMP and another in cyclic AMP-dependent protein kinase activity can be detected during the late period of the prereplicative phase of liver regeneration after a partial hepatectomy. It is known that these three surges are involved in triggering DNA synthesis. By kinetic studies and by injecting transcription (actinomycin D) and translation (cycloheximide) inhibitors into hepatectomized rats we have demonstrated that the cyclic AMP-dependent protein kinase surge is produced by new synthesis of the enzyme. In thyroparathyroidectomized rats subjected to a partial hepatectomy the calmodulin surge was similar to that observed in normal hepatectomized rats whereas the surge in cyclic AMP-dependent protein kinase activity was strongly decreased suggesting that the cyclic AMP-dependent protein kinase surge is not generated by the previous surge in calmodulin.

Inhibitory effect of dimethyl sulfoxide on the proliferation of cultured arterial smooth muscle cells: relationship to the cytoplasmic microtubules

Our preliminary study has shown that dimethyl sulfoxide (DMSO) has an inhibitory effect on the proliferation of cultured arterial smooth muscle cells and promotes phenotypic modulation from the synthetic state to the contractile state. In the present study we have examined the effect of DMSO with special attention to relationship between cell growth and cytoplasmic microtubules. DMSO inhibited DNA synthesis and cell division, and concomitantly promoted microtubule assembly. Initiation of DNA synthesis and cell division of nonproliferating cells by platelet-derived growth factor (PDGF) was also inhibited in the presence of 1% DMSO. A 12-hr incubation with 10(-6) M colchicine caused disruption of microtubules; however, pretreatment with 1% DMSO for 24 hr prevented the disruption, indicating that DMSO has a stabilizing action on microtubules. A 24-hr exposure to PDGF resulted in microtubule depolymerization, while the addition of 1% DMSO prevented the depolymerization. These results suggest that DMSO inhibits DNA synthesis of cultured smooth muscle cells by stabilizing cytoplasmic microtubules.

Tobacco components induce alterations in keratinocyte lipid membrane fluidity and inhibit cell proliferation in culture

Guinea pig epidermal keratinocytes were harvested and grown in vitro (20). The cells were treated with polyphenolic compounds, acetate/shikimate aromatics, that are ubiquitous in plants. In addition, the cells were exposed to tobacco glycoprotein (TGP), a plant-derived substance prepared from cured tobacco (19). All of the polyphenolic compounds enhanced the attachment of cells to a defined substratum and inhibited cell proliferation. Tobacco glycoprotein (TGP), rutin conjugates and ellagic acid inhibited cell spreading and enhanced the apparent microviscosity of keratinocyte membrane lipids. These data suggest that the use of these compounds may be associated with alterations in cytoskeletal architecture and membrane fluid dynamics that might result in clinical alterations characterized by whitening of mucosae.

Monoclonal antibody to the thrombin receptor stimulates DNA synthesis in combination with gamma-thrombin or phorbol myristate acetate

Studies with various thrombin derivatives have shown that initiation of cell proliferation by thrombin requires two separate types of signals: one, generated by high affinity interaction of thrombin or DIP-thrombin (alpha-thrombin inactivated at ser 205 of the B chain by diisopropylphosphofluoridate) with receptors and the other, by thrombin's enzymic activity. To further study the role of high affinity thrombin receptors in initiation, we immunized mice with whole human fibroblasts and selected antibodies that blocked the binding of 125I-thrombin to high affinity receptors on hamster fibroblasts. One of these antibodies, TR-9, inhibits from 80 to 100% of 125I-thrombin binding, exhibits an immunofluorescent pattern indistinguishable from that of thrombin bound to receptors on these cells, and selectively binds solubilized thrombin receptors. By itself, TR-9 did not initiate DNA synthesis nor did it block thrombin initiation, but TR-9 addition to cells in the presence of alpha-thrombin, gamma-thrombin (0.5 microgram/ml), or PMA stimulated thymidine incorporation up to threefold over controls. In all cases, maximal stimulation was observed at concentrations of TR-9, ranging from 1 to 4 nM corresponding to concentrations required to inhibit from 30 to 100% of 125I-thrombin binding. These results demonstrate that the binding of the monoclonal antibody to the alpha-thrombin receptor can mimic the effects of thrombin's high affinity interaction with this receptor in stimulating cell proliferation.

Colchicine activates cell cycle-dependent genes in growth-arrested rat 3Y1 cells

When growth-arrested 3Y1 cells (Fischer rat fibroblasts) were exposed to 3 X 10(-5) M colchicine, they entered S phase after a 12-h lag period which is the same as that in serum-stimulated cells. The expression of genes such as c-fos, c-myc, JE, KC, ornithine decarboxylase, and histone H3, analyzed by Northern blotting, increased in a cell-cycle dependent manner after colchicine treatment. The increased level of mRNAs was much smaller in colchicine-stimulated cells than in serum-stimulated cells, corresponding to the lower frequency of the former cells entering S phase. The course of the prereplicative phase seems to be similar in terms of the expression of cell cycle-dependent genes in cells stimulated with colchicine and in those stimulated with serum.

Taxol counteracts colchicine blockade of axonal transport in neurites of cultured dorsal root ganglion cells

Taxol does not affect axonal transport in cultured neurites. Preincubation of this drug inhibits the effects of colchicine on axonal transport and morphology in cultured neurites. A reduction of the free tubulin density by preincubated taxol might prevent the disruption of microtubules by colchicine.

Stimulation by serum of the Na+/H+ antiporter in quiescent pig kidney epithelial (LLC-PK1) cells and role of the antiporter in the reinitiation of DNA synthesis

LLC-PK1 cells can be brought into a classical quiescent state by depriving them of serum for 6 days. At this time, pulse-labeling with [3H]-thymidine shows that only 3% of the cells are synthesizing DNA, but the quiescent cells can be stimulated with serum to re-enter the cell cycle at a point early in G1. The rate of amiloride-sensitive 22Na+ uptake (as a measure of the Na+/H+ antiporter) is relatively low during quiescence; it rises 2- to 3-fold within 4 h after serum addition. This increase in antiporter activity appears to be required for the resumption of DNA synthesis in the absence of bicarbonate, because ethylisopropylamiloride (EIPA) blocks [3H]-thymidine incorporation when serum is added to cells in bicarbonate-free medium. In the presence of bicarbonate, however, EIPA has no effect on [3H]-thymidine incorporation, indicating that another (bicarbonate-dependent) transport system can substitute for the antiporter under these conditions.

Effects of ATP and cyclic AMP on the in vitro assembly and stability of mammalian brain microtubules

The relevance of protein phosphorylation, transphosphorylation and binding phenomena in the kinetics of the ATP-induced assembly of cycle-purified microtubule protein from mammalian brain were studied. ATP was able to induce the polymerization of microtubules of normal appearance. However, the assembled structures, were unstable and microtubules depolymerized after achievement of a transitory maximum. Cyclic AMP reduced the amplitude of the polymerization maximum in a concentration-dependent manner, correlating with the stimulation of the endogenous phosphorylation reaction. When microtubule assembly was induced by GTP, in the presence of various concentrations of ATP, the slope of the depolymerization phase was found to depend on the concentration of ATP. Fluoride ion inhibited the endogenous phosphorylation reaction and reduced the disassembly rate, in a concentration-dependent manner. Evidence is also presented indicating that ATP did not bind to phosphocellulose-purified tubulin. These results further contribute to indicate that ATP and cyclic AMP, acting coordinately to control the phosphorylation extent of microtubule proteins are important factors to determine microtubule stability within the cell. Some implications of this mechanism for the regulation by cAMP of the initiation of DNA synthesis and mitosis are considered.

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.

Scanning microfluorometric measurement of immunofluorescently labelled microtubules in cultured cells. Dependence of microtubule content on cell density

A method for evaluation of microtubule content in cultured cells has been developed. The method is based on scanning microfluorometric measurement of immunofluorescently labelled microtubules. The method has been applied to the comparison of microtubule content in epithelial XTH-2 cells grown in culture at various cell densities. The results have shown that the microtubule content in the cells is not dependent on their proliferative state rather than it depends on cellular contacts.

Colcemid inhibits growth during early G1 in normal but not in tumorigenic lymphocytes

Mitogenically stimulated human and mouse lymphocytes enter the cell cycle (G0, G1A, G1B, S, G2+M) via a newly recognized subphase, G1'. This subphase precedes G1A and is distinct from G0. The G1' subphase is absent in immortalized and tumorigenic lymphoblastoid cell lines (LCLs) by cytofluorimetric criteria. Furthermore, colcemid inhibits transition through the G0/G1' as well as G2 phases in mitogen-stimulated lymphocytes and in LCLs. Tumorigenic LCLs are not sensitive to growth inhibition by colcemid during early G1. These observations suggest that a progressive series of changes have occurred during G0/G1' which lead to deregulation of growth control.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Occurrence of two beta-tubulin isoforms with different polymerizing abilities in L5178Y cells

Mouse lymphoma L5178Y cells express at least two isoforms of beta-tubulin, designated M beta I and M beta II, as revealed by isoelectrofocusing, whereas two independently isolated normal T-cell clones, 3D10 and K23, express only M beta I. M beta II-tubulin is more acidic (pI, 5.10) than M beta I-tubulin (pI, 5.15). L5178Y cells were disrupted under the microtubule-stabilizing conditions, followed by centrifugation to separate fractions containing polymerized and unpolymerized tubulin. We found that a proportion of M beta II to total beta-tubulins is larger in the fraction containing unpolymerized tubulin than in that containing polymerized tubulin. In addition, when tubulin was purified from extracts of L5178Y cells by repeated cycles of polymerization-depolymerization, the M beta II-tubulin isoform was gradually lost during the successive purification steps. The low recovery of M beta II-tubulin was observed, irrespective of the presence or absence of MAPs, and even in the presence of an excess amount of essentially polymerizable porcine brain tubulin. These results indicate that M beta II-tubulin is less able to polymerize than is M beta I-tubulin, both in vivo and in vitro.

Distribution of interdivisional times in proliferating and differentiating Friend murine erythroleukaemia cells

The interdivisional times of Friend murine erythroleukaemia cells which are growing continuously, or during terminal erythroid differentiation after exposure to dimethyl sulphoxide (DMSO), were determined by time lapse video photography. The median interdivisional times were found to increase from 11.75 hr before exposure to DMSO, to 24.0 hr at 72 hr after exposure. This increase in median interdivisional time was accompanied by an increase in heterogeneity of interdivisional times (%CV = 8.5----40.8), by an increase in the similarity of sister interdivisional times (ryy = 0.622----0.925), and by a decrease in the fraction of cells observed to divide (F = 1.0----0.807). Cells exposed to DMSO for 72 hr can be induced to divide at least once with nearly normal interdivisional times, if they are resuspended at a tenfold higher cell concentration. Computer simulations of cell cycle regulation, based on the opposing reactions model of Murphy, generate interdivisional time distributions which resemble the experimental data better than the single transition probability model of Smith and Martin.

A radiolabeled monoclonal antibody binding assay for cytoskeletal tubulin in cultured cells

To detect changes in the extent of tubulin polymerization in cultured cells, we have developed a radioactive antibody binding assay that can be used to quantitate total cytoskeletal tubulin or specific antigenic subsets of polymerized tubulin. Fibroblastic cells, grown to confluence in multiwell plates, were permeabilized and extracted with 0.5% Triton X-100 in a microtubule-stabilizing buffer. These extracted cytoskeletons were then fixed and incubated with translationally radiolabeled monoclonal antitubulin antibody (Ab 1-1.1), an IgM antibody specific for the beta subunit of tubulin. Specific binding of Ab 1-1.1 to the cytoskeletons was saturable and of a single apparent affinity. All specific binding was blocked by preincubation of the radiolabeled antibody with excess purified brain tubulin. Specific Ab 1-1.1 binding appeared to represent binding to cytoskeletal tubulin inasmuch as: pretreatment of cells with colchicine decreased Ab 1-1.1 binding in a dose-dependent manner which correlated with the amount of polymerized tubulin visualized in parallel cultures by indirect immunofluorescence, taxol pretreatment alone caused an increase in Ab 1-1.1 binding and prevented in a dose-dependent manner the colchicine-induced decrease in antibody binding, in cells pretreated with colcemid and returned to fresh medium, Ab 1-1.1 binding decreased and recovered in parallel with the depolymerization and regrowth of microtubules in these cells, and comparison of maximal antibody binding per cell between primary mouse embryo, 3T3, and human foreskin fibroblasts correlated with immunofluorescence visualization of microtubules in these cells. Thus, this assay can be used to measure relative changes in the level of polymerized cytoskeletal tubulin. Moreover, by Scatchard-type analysis of the binding data it is possible to estimate the total number of antibody binding sites per cell. Therefore, depending on the stoichiometry of antibody binding, this type of assay may be used for quantitating total cytoskeletal tubulin, specific antigenic subsets of cytoskeletal tubulin, or other cytoskeletal proteins.

Changes in the extent of microtubule assembly can regulate initiation of DNA synthesis

We have shown that MT depolymerization by colchicine and other drugs is sufficient to initiate DNA synthesis in serum-free cultures of embryonic fibroblasts and that stabilization of MTs with taxol inhibits this initiation. Growth factors and oncogenic DNA viruses also initiate DNA synthesis by a taxol-sensitive mechanism that appears to require MT depolymerization or rearrangements. Because we have shown that microtubule heterogeneity exists within single fibroblastic cells, we have carried out a series of experiments to determine the extent of microtubule disruption necessary to initiate DNA synthesis. We have compared the effects of various concentrations of colchicine and taxol on initiation of DNA synthesis with their effects on cytoplasmic MT complexes as visualized by indirect immunofluorescence microscopy and quantitated by direct binding of radiolabeled monoclonal antibody to cytoskeletons. The opposing effects of these drugs on MTs shows that there is a correlation between the extent of MT depolymerization and initiation of DNA synthesis. Initiation of DNA synthesis by colchicine in the presence of taxol is half-maximal when taxol and colchicine are added to cultures at a ratio of about 13 to 1. At this drug ratio, taxol stabilizes MTs near the nucleus, but MTs near the cell periphery are depolymerized. Maximal inhibition of DNA synthesis by taxol occurs only at taxol to colchicine ratios where MTs extend throughout the cytoplasm to the cell periphery. Thus, depolymerization of a small fraction of total MTs, particularly those near the periphery, may be sufficient to initiate proliferative events.

Independence of centriole formation and initiation of DNA synthesis in Chinese hamster ovary cells

The relationship between centriole formation and DNA synthesis was investigated by examining the effect of taxol on the centriole cycle and the initiation of DNA synthesis in synchronized cells. The centriole cycle was monitored by electron microscopy of whole-mount preparations [Kuriyama and Borisy, J. Cell Biol., 1981, 91:814-821]. A short daughter centriole appeared in perpendicular orientation to each parent during late G1 or early S and elongated slowly during S to G2. Addition of 5-20 micrograms/ml taxol to a synchronous population of cells in S phase did not inhibit centriole elongation; rather, elongation was accelerated. In contrast, when taxol was added to M phase or early G1 cells, centriole duplication was completely inhibited. The taxol block was reversible since nucleation and elongation of centrioles resumed as soon as the drug was removed. Cells exposed to taxol progressed through the cell cycle and became blocked in mitosis, as indicated by an increase in the mitotic index, but eventually the mitotic arrest was overcome, resulting in formation of multinucleated cells. A peak in mitotic index was seen in the following generation, indicating that chromosomes duplicated in the presence of taxol. Incorporation of 3H-thymidine followed by autoradiography confirmed that DNA synthesis was initiated in the presence of taxol even though formation of daughter centrioles was inhibited. It seems, therefore, that centriole duplication is not a prerequisite for entry into S phase. Since DNA synthesis has already been demonstrated not to be necessary for centriole duplication, these two events, normally coordinated in time, appear to be independent of each other.

Interaction between epidermal growth factor and triamcinolone acetonide in mouse palatal mesenchymal cells in vitro

Palatal mesenchymal cells from mouse embryos (MEPM cells) had a high number of specific receptors for epidermal growth factor (EGF). At 37 degrees C, the number of high-affinity receptors (dissociation constant, Kd of 6.0 X 10(-10) M) was 2.0 X 10(5) per cell and of low-affinity receptors (Kd of 2.9 X 10(-9) M), 3.8 X 10(5) per cell. At 2 degrees C, a single class of receptors (Kd of 4.8 X 10(-9) M) was detected at 1.8 X 10(5) per cell. Triamcinolone acetonide, a potent cleft-palate-inducing agent, slightly inhibited the recovery of 125I-labelled EGF binding capacity in MEPM cells after down regulation of EGF receptors; it did not affect the binding properties of 125I-labelled EGF in these cells.

Distinction of viral and host-derived glycopolypeptides induced by "early" functions of human cytomegalovirus

Human cytomegalovirus is shown to induce in phosphonoacetic acid-treated human fibroblasts glycosylation of five polypeptides with approximate molecular weights of 200-250, 150, 135, 130 and 100 kilodaltons (kd). Except for the 130 kd product, these glycopolypeptides (gp) separate with the cytoplasmic fraction, only one (200-250 kd) with the chromatin fraction as well. The gp of 135 and 100 kd were found to be virus-specified as determined by immunoblotting and immunoprecipitation. The gp of 200-250 kd exhibited an immunological relatedness to fibronectin and are therefore considered host-specific products. Both subsets of gp participate in virus-induced surface membrane alterations as documented by living cell immunofluorescence.