Interplay of cyclic AMP and microtubules in modulating the initiation of DNA synthesis in 3T3 cells

A growth regulatory factor that can both inhibit and stimulate growth

A growth inhibitor that is produced by BSC-1 cells (African green monkey kidney epithelial cells) has been isolated from conditioned medium. It has been purified by gel chromatography and high performance liquid chromatography. It appears to be a protein with a relative molecular mass (Mr) of 24 000. It is extremely active as a growth inhibitor with some cells, but not with others. Approximately 50% inhibition of thymidine incorporation is observed with CCL64 cells at 0.05 ng/ml and with BSC-1 cells at 1 ng/ml. The growth inhibitor induces BSC-1 cells to synthesize and secrete a glycoprotein of approximately 48 000 Mr. It inhibits Na+ accumulation in BSC-1 cells. Recently, in collaboration with R. F. Tucker, G.D. Shipley and H. L. Moses (Mayo Foundation & Medical School), we have found that the growth inhibitor is very similar to and may be identical with transforming growth factor beta (TGF-beta). Our growth inhibitor stimulates colony formation in soft agar by AKR-2B cells, and it competes with TGF-beta in binding to cell surface receptors. TGF-beta, from human platelets, is extremely active as an inhibitor of thymidine incorporation by BSC-1 cells and CCL64 cells. The growth inhibitor/TGF-beta can, therefore, stimulate or inhibit growth, depending on the cells and the growth conditions.

Interplay between exchange protein directly activated by cAMP (Epac) and microtubule cytoskeleton

"Exchange protein directly activated by cAMP" (Epac) is a newly discovered cAMP receptor that mediates the intracellular cAMP actions in addition to the classic cAMP-dependent protein kinase system. In this study, we show that Epac interacts directly with tubulin, co-purifies with cellular microtubules, and co-localizes with the mitotic spindle assembly. Association with microtubules suppresses Epac-mediated Rap1 activation, while the binding of Epac promotes microtubule formation. These results demonstrate that Epac plays an important role in connecting the microtubule cytoskeleton network and intracellular cAMP-signalling.

Toxic effects of griseofulvin: disease models, mechanisms, and risk assessment

Griseofulvin (GF) has been in use for more than 30 years as a pharmaceutical drug in humans for the treatment of dermatomycoses. Animal studies give clear evidence that it causes a variety of acute and chronic toxic effects, including liver and thyroid cancer in rodents, abnormal germ cell maturation, teratogenicity, and embroyotoxicity in various species. No sufficient data from human studies are available at present to exclude a risk in humans: therefore, attempts were made to elucidate the mechanisms responsible for the toxic effects of GF and to address the question whether such effects might occur in humans undergoing GF therapy. It is well documented that GF acts as a spindle poison and its reproductive toxicity as well as the induction of numerical chromosome aberrations and of micronuclei in somatic cells possibly may result from disturbance of microtubuli formation. Likewise, a causal relationship between aneuploidy and cancer has been repeatedly postulated. However, a critical survey of the data available on aneuploidogenic chemicals revealed insufficient evidence for such an association. Conceivably, other mechanisms may be responsible for the carcinogenic effects of the drug. The induction of thyroid tumors in rats by GF is apparently a consequence of the decrease of thyroxin levels and it is unlikely that such effects occur in GF-exposed humans. The appearance of hepatocellular carcinomas (HCC) in mice on GF-supplemented diet is preceded by various biochemical and morphological changes in the liver. Among these, hepatic porphyria is prominent, it may result from inhibition of ferrochelatase and (compensatory) induction of ALA synthetase. GF-induced accumulation of porphyrins in mouse liver is followed by cell damage and necrotic and inflammatory processes. Similar changes are known from certain human porphyrias which are also associated with an increased risk for HCC. However, the porphyrogenic effect of GF therapy in humans is moderate compared with that in the mouse model, although more detailed studies should be performed in order to clarify this relationship on a quantitative basis. A further important effect of GF-feeding in mice is the formation of Mallory bodies (MBs) in hepatocytes. These cytoskeletal abnormalities occur also in humans, although under different conditions; their appearance is associated with the induction of liver disease and HCC. Chronic liver damage associated with porphyria and MB formation, enhanced cell proliferation, liver enlargement, and enzyme induction all may contribute to the hepatocarcinogenic effect of GF in mice. In conclusion, further investigation is required for adequate assessment of health risks to humans under GF therapy.

Overexpression of full- or partial-length MAP4 stabilizes microtubules and alters cell growth

To investigate the in vivo functions of MAP4, a microtubule-associated protein expressed almost ubiquitously in vertebrate cells, we prepared stably transfected clonal mouse Ltk- cell lines expressing full-length MAP4 (L-MAP4 cells) or its MT-binding domain (L-MTB cells). Although transfectants showed no dramatic defect in morphology, organellar distribution, or level of MT polymer, as compared to naive Ltk- cells or L-MOCK cells (transfected with vector alone), MTs in L-MAP4 and L-MTB cells showed greater stability than those in control cells, as monitored by the level of post-translationally detyrosinated alpha-tubulin and by a quantitative nocodazole-resistance assay. In vivo, the MT-binding domain of MAP4 stabilized MTs less potently than full-length MAP4, in contrast to the equivalent efficacy demonstrated in studies of in vitro MT polymerization (Aizawa et al. (1991), J. Biol. Chem. 266, 9841–9846), L-MAP4 and L-MTB cells grew significantly more slowly than control cells; this growth inhibition was not due to mitotic arrest or cell death. L-MAP4 and L-MTB cells also exhibited greater tolerance to the MT-depolymerizing agent, nocodazole, but not to the MT-polymerizing agent, Taxol. Our results demonstrate that MAP4 and its MT-binding domain are capable of MT stabilization in vivo, and that increasing the intracellular level of MAP4 affects cell growth parameters.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

In vitro endothelial wound repair. Interaction of cell migration and proliferation

Most re-endothelialization requires both cell migration and cell proliferation. To study the association between cell migration and the initiation of DNA synthesis in an in vitro wound model, confluent cultures of porcine aortic endothelial cells grown on glass coverslips were scraped to remove half of the monolayer. Treatment with 1 ng/ml of taxol, a microtubule stabilizing drug, for 24 hours resulted in no visible change in F-actin or microtubule organization as assessed by fluorescence and immunofluorescence microscopy. There was, however, a reduction of wound re-endothelialization and an associated reduction in the proportion of cells with centrosomes redistributed toward the wound edge. No significant differences, however, were seen in the labeling indices for the first two rows of cells at the wound edge as revealed by 3H-thymidine autoradiography. Labeling of nuclei in Rows 3 to 8 and in a zone deeper within the monolayer was reduced in treated cultures. The data suggest that endothelial proliferation in cells within an area bordering a wound is dependent on both denudation, which is sufficient to promote maximal proliferation in the two rows adjacent to the wound, and cell migration, which is required for the propagation of proliferation in cells further away from the wound edge.

Cyclic AMP increasing agents rapidly stimulate vimentin phosphorylation in quiescent cultures of Swiss 3T3 cells

The results presented here demonstrate that an elevation in the cellular levels of cyclic AMP (cAMP) increases the phosphorylation of an Mr = 58,000 cellular protein in quiescent cultures of Swiss 3T3 cells. The enhancement of 32Pi incorporation into the Mr 58,000 cellular protein was detected as early as 1 min and reached a maximum after 20 min of treatment. The role of cAMP in the phosphorylation of Mr = 58,000 protein is substantiated by the following lines of evidence: a) a variety of agents that cause cAMP accumulation in 3T3 cells, including cholera toxin, 5'-N-ethylcarboxamideadenosine (NECA), PGE1, and 3-isobutyl-1-methyl-xanthine (IBMX) increased the phosphorylation of the same Mr 58,000 cellular protein as demonstrated by peptide mapping; b) inhibitors of cyclic nucleotide phosphodiesterase potentiated the ability of low concentrations of the adenylate cyclase activators NECA, PGE1, and forskolin to increase Mr 58,000 phosphorylation; and c) permeable derivatives of cAMP such as 8BrcAMP were also effective and specific in promoting Mr 58,000 phosphorylation. Detergent extraction, immunoblotting, and immunoprecipitation identified the Mr = 58,000 phosphoprotein as vimentin, the main protein subunit of the intermediate filaments of mesenchymal cells including Swiss 3T3 cells. Studies with intact 3T3 cells revealed that an increase in the intracellular level of cAMP induced a marked redistribution and collapse of the intermediate filaments. These results raise the possibility that an intact intermediate filament network may restrict the reinitiation of DNA synthesis.

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.

Cytoskeletal alterations as a parameter for assessment of toxicity

1. Some environmental substances, drugs and pollutants affect the growth of cultured cells, and produce cytoskeletal alterations. 2. These have been used as parameters for toxicity assessment of cholera toxin and pertussis toxin in Chinese Hamster Ovary cells. 3. Cholera toxin stabilized microtubules and had no effect on microfilaments and intermediate filaments. 4. Pertussis toxin affected microfilaments but appeared to have no effect on microtubules and intermediate filaments.

Griseofulvin

Griseofulvin (GF) is a mycotoxin produced by various species of Penicillium including P. griseofulvum Dierckx, P. janczewski (P. nigricans) and P. patulum. It is active against dermatophytic fungi of different species in the genera Microsporum, Trychophyton and Epidermophyton. Because of its capacity to concentrate in the keratinous layer of the epidermis and its relatively low toxicity in man, it has been extensively used in the therapy of dermatophytoses by oral administration. The biological activity of GF towards fungi is manifested as nuclear and mitotic abnormalities followed by distortions in the hyphal morphology. Mitotic segregation is also induced in fungi by GF treatment. In higher eukaryotes the cytostatic action of GF is essentially due to a mitotic arrest at late metaphase/early anaphase. The cytological effects observable both in vivo and in vitro on different plant and animal cell systems, include C-mitoses, multipolar mitoses and multinuclearity. Prolonged GF treatment in experimental animals provokes biochemical changes consisting mainly of disturbances of porphyrin metabolism, variation in the microsomal cytochrome levels and formation of Mallory bodies. In mice these alterations are followed by the development of multiple hepatomas. Evidence of tumor induction by GF has been obtained in mice and rats, but not in hamsters. GF may also act either as a promoting or a co-carcinogenic agent, depending on the circumstances of its administration. It has been found to increase the frequency of cell transformation induced by polyoma virus, but not to induce cell transformation per se. Induction of sperm abnormalities has been observed in GF-treated mice. The embryotoxic and teratogenic action of GF has been demonstrated in pregnant rats exposed during organogenesis. Genetic effects of GF have been investigated by the following tests: Salmonella/microsome mutagenicity assay, point mutations in mammalian and plant cells, DNA damage and repair, SCE, chromosome aberrations, micronuclei, dominant lethals, aneuploidy in lower and higher eukaryotes. A positive response has been obtained in the assays on numerical chromosome changes in all the systems analyzed; limited or inconclusive evidence has been obtained for SCE and structural chromosome changes. Doubled or highly polyploid sets can be detected in all types of cells during or immediately after GF treatment. A marked increase in chromosome number variation is observed at various times after withdrawal of the drug, with prevailing hyperdiploid and reduced sets in animal cells and plant cells respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.

The induction of class I HLA by interferon-alpha is independent of the cell cycle, but the expression is enhanced by a G1/S block

The induction of class I HLA expression by interferon-alpha (IFN-alpha) was studied in lymphoid cells arrested or traversing different stages of the cell cycle. Exponential cultures of MOLT-4 cells and the MOLT-4 cell variant YHHH were treated with the cell cycle inhibitors aphidicolin and colcemid to obtain cell populations arrested in G1/S and G2/M, respectively, and also cells traversing from S to M and vice versa. Cytofluorimetry with the monoclonal antibody YTH/76.3 (which specifically detects those class I molecules which are most susceptible to IFN-alpha induction) was used to quantitate the class I HLA response to IFN-alpha. The results showed that the response to IFN-alpha is not restricted to a given stage of the cell cycle. These studies also revealed that when the cells were arrested at G1/S, the absolute level of class I HLA expression was enhanced 2-3-fold, both in the presence or absence of either IFN-alpha or IFN-gamma. Therefore, even when absolute levels changed, the ratio of IFN-induced expression to basal expression remained constant at all cell cycle stages. The level of expression of another surface antigen (the CD1 antigen HTA-1) was not affected by the G1/S block. The results were confirmed by dot blot hybridization of poly(A)+ RNA using cDNA-specific probes. These findings suggest that the effect of IFN-alpha is continuous throughout the cell cycle but that a G1-dependent event determines the extent of class I HLA expression, and leads to a synergistic superinduction by IFN in G1/S-arrested cells.

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.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.

Microtubule-disrupting agents reverse the inhibitory effect of interferon on mitogenesis in 3T3 cells

Interferon can inhibit the stimulation of DNA synthesis in quiescent 3T3 cells exposed to combinations of purified growth factors, but the extent of inhibition varies with the number and combination of mitogens used. As the number of growth factors used to stimulate the cells is increased from two to three, the inhibitory effect of IFN is reduced, and if the third mitogen is a microtubule-disrupting agent such as colchicine or nocodazole, it is abolished altogether. The antagonistic effect of microtubule-disrupting agents on interferon-induced inhibition of DNA synthesis suggests that an intact tubulin network is required for this action of interferon. Interferon and tubulin disrupting agents also show similar kinetics in establishing an effect on DNA synthesis which could imply that they have opposite effects on tubulin assembly.

Antiviral and antiproliferative effects of interferons in quiescent fibroblasts are dissociable

Quiescent mouse Swiss 3T3 cells or human fibroblasts treated with interferon for only 2 hr before the addition of serum are effectively protected against virus infection for up to 24 hr after the addition of serum and the removal of interferon. In sharp contrast, exposure of quiescent fibroblasts to interferon only before addition of serum or the peptides epidermal growth factor and insulin, has no inhibitory effect on cell proliferation. Further, even when interferon is added after the addition of growth factors, an exposure of several hours is required to inhibit stimulation of DNA synthesis. These findings strongly suggest that the antiviral action of interferons is dissociable from their ability to inhibit cell proliferation. The implications of these findings for elucidating the molecular events leading to the biological responses elicited by interferon are discussed.

Stimulating the proliferation of quiescent 3T3 fibroblasts by peptide growth factors or by agents which elevate cellular cyclic AMP level has opposite effects on motility

The effects of some chemically defined growth factors on the locomotion of quiescent Swiss 3T3 fibroblasts have been studied. A computer digitiser has been used to facilitate recording the paths followed by cells in time-lapse films; this method allows 500 cell-hours to be recorded in 1 h of real time. Individual cells in the same culture vary widely in speed. This variation is not associated with the positions of the cells in the cell cycle; a small deceleration which seems to occur in G2 cannot account for any significant part of the variation seen. Nor is it related to the time elapsing before the cell divides, although this is equally variable; the speed and age at division of particular cells appear to be entirely independent of one another. Nevertheless, good reproducibility is seen between the mean speeds of large numbers of cells from the same type of culture. The mean speed of quiescent cells is less than 2 microns/h. A mixture of epidermal growth factor (EGF) and vasopressin, in the presence of insulin, is known to be a potent promoter of proliferation in this system. We have found it to increase speed to 30 microns/h. Agents which stimulate the cellular level of cAMP are also known to be potent promoters of proliferation in the presence of insulin. We have found these agents to be inhibitors of locomotion; several cycles of cell division take place while the cells move at a speed no greater than that seen in the presence of cytochalasin B (CB) or colchicine. These findings therefore give further support to the idea that there may be two different classes of growth-promoting factors, with major differences in their mode of action. They show that some members of these two different classes have opposite effects on motility.

Mammalian brain microtubules are sensitive to cyclic AMP in vitro

Microtubules assembled in vitro with ATP were depolymerized by the addition of cyclic AMP, which correlates with a stimulation of the endogeneous phosphorylation reaction. When assembled with GTP, however, microtubules were only sensitive to cyclic AMP when ATP was present. This nucleoside triphosphate induced the disassembly of microtubules in a concentration-dependent, cyclic nucleotide-stimulated manner. Since UTP, CTP and the nonhydrolyzable ATP analog adenosine-5'-(beta, gamma-methylene)triphosphate were without comparable effect, it was assumed that phosphorylation of the microtubule-associated proteins may represent a physiological mechanism by which microtubules in the living cell respond to external stimuli.

Beta-adrenergic stimulation inhibits calcium efflux and alters the morphology of cultured arterial muscle cells

Beta-adrenoreceptor stimulation with isoproterenol (IP) rapidly and reversibly rounded and arborized smooth muscle cells (SMC) cultured from rat aorta. The arborized SMC remained firmly attached to the substratum via a network of long, dendritelike processes. Arborization of the SMC correlated closely with increases in cellular cAMP produced by IP and a variety of other compounds. The intracellular calcium antagonist 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) also rounded and arborized the SMC. Antitubulin compounds potently blocked arborized by IP, dibutyryl cAMP, and TMB-8. The release of cell-bound 45Ca2+ was followed in the presence and absence of IP. IP strikingly increased the amount of 45Ca2+ that remained cell bound between 20 and 120 min. Propranolol and colchicine prevented IP from inhibiting the release of cell-bound 45Ca2+. These results suggest that the modulation of Ca2+ transport is involved in the arborization of cultured SMC by cAMP.

Initiation of proliferative events by human alpha-thrombin requires both receptor binding and enzymic activity

To determine the role of thrombin high-affinity receptor occupancy and enzymic activity in thrombin initiation of cell proliferation, we have utilized thrombin derivatives which separate these functions. We previously showed that enzymically active gamma-thrombin stimulates ion fluxes without binding to high-affinity sites, whereas proteolytically inhibited DIP-alpha-thrombin which binds to high-affinity receptors does not. Since neither derivative initiates DNA synthesis by itself, this suggested that two separate sequences of events might be necessary for a complete initiation signal. We now report that the combination of DIP-alpha-thrombin and gamma-thrombin initiate DNA synthesis and cell proliferation to levels approaching the maximal initiation by native alpha-thrombin. This combinatory effect is dose-dependent for both gamma-thrombin and DIP-alpha-thrombin in the same concentration range as alpha-thrombin alone. Thus, these same concentrations of alpha-thrombin alone may be required to initiate each sequence of events. The combinatory stimulation could be achieved even if the derivatives were added individually up to 8 hr apart. Moreover, preincubation with either derivative shortened the lag period for initiation of DNA synthesis by native alpha-thrombin. These results indicate that both receptor occupancy and enzymic activity are necessary for thrombin initiation of cell proliferation and that each action initiates a sequence of early events which moves the cell forward toward entry into a proliferative cycle.