ON THE ACTION OF COLCHICINE, THE MELANOCYTE MODEL

Colchicine effects on the mouse liver

With histochemical methods an instantaneous and marked inhibitory effect of a single dose of colchicine on the activity of succinic-, lactic-, glucose-6-phosphate-, NADH- and NADPH-dehydrogenase in the mouse liver was observed. This effect was essentially the same as that previously described in the intestinal mucosa and not dependent on or a consequence of the inhibitory effect of colchicine on cell division. The enzymatic inhibitory effect of colchicine was qualitatively similar regardless of the dose of the drug given, although a single large dose (1.5 mg/kg body weight) produced alterations and depression of dehydrogenase activity more marked and not so rapidly repaired as those appearing after a small dose of colchicine (0.24 mg/kg body weight). Following repeated doses of colchicine the degree of histochemically demonstrable depression of dehydrogenase activity was not obviously different from that found after a single dose. After large repeated doses morphological alterations and prolonged depression of dehydrogenase activity of liver cells were found and many of the animals died. After repeated administration of the small dose (0.24 mg/kg body weight) no morphological alterations were found and the activity of dehydrogenases was normalized, although the injection of colchicine was continued.

Small molecules, big impact: a history of chemical inhibitors and the cytoskeleton

Chemical inhibitors, whether natural products or synthetic, have had an enormous impact on the study of the eukaryotic cytoskeleton. Here we review the history of some of the most widely used cytoskeletal poisons and their influence on our understanding of cytoskeletal functions. We then highlight several new inhibitors and the targeted screens used to identify them and discuss why this approach has been successful.

Photosensitivity in the skin of the lizard, Anolis carolinensis

Skin of the lizard, Anolis carolinensis, is remarkably sensitive to light. Illuminated in vitro with visible light from a tungsten source (110 W (m-2)), skin changes from brilliant green to dark brown (50% reduction in reflectance) within 2-4 min as a result of dispersion of melanin from a perinuclear position within dermal melanophores into superficial dendritic processes. Reversal of the process, reaggregation of pigment, will occur within 2.0 min upon return to darkness. This photic response can be initiated with light levels as low as 5.0 W m(-2) and is maximized by light levels only 5% that of midwinter sunshine. Pigment dispersion in response to both melanocyte simulating hormone and to light is inhibited by cytochalasin-B, indicating that microfilaments may be the motor element for pigment movement in that direction. Colchicine, but not cytochalasin-B, totally blocks pigment reaggregation following melanocyte stimulating hormone exposure and partially blocks it in the dark phase of the photic response. The results of this study are consistent with a model for pigment movement in A. carolinensis that provides microfilaments for pigment dispersion and microtubule involvement in both dispersion and aggregation. Finally, because it is readily visible, easily quantified, rapid and reversible, photic response in the skin of A. carolinensis is recommended as a valuable model system for the study of saltatory movement of organelles within cells.

In vitro activation of amphibian dermal melanocytes by nicotine

Epidemiologic investigations have shown that nicotine could be a factor causing oral melanin pigmentation in man. Nicotine was therefore tested on amphibian skin melanocytes. Cell reactions were registered by spectrophotometry, in vitro light microscopy, photomicrography and light and electron microscopy. It was found that dermal melanocytes in the toad Bufo bufo and the frog Rana arvalis were activated by nicotine and that this activation was not mediated via beta-adrenergic receptors in melanocytes.

The pineal and regulation of fibrosis: pinealectomy as a model of primary biliary cirrhosis: roles of melatonin and prostaglandins in fibrosis and regulation of T lymphocytes

Pinealectomy leads to increased formation of fibrous tissue in the abdominal cavity, increased skin pigmentation and elevated cholesterol and alkaline phosphatase levels. It also leads to reduced formation and/or action of prostaglandin (PG) E1 and thromboxane (TX) A2. PGE1 plays an important role in enhancing function of T suppressor lymphocytes which control overactive antibody-producing B lymphocytes. In primary biliary cirrhosis there are increased skin pigmentation, hepatic fibrosis, elevated cholesterol and alkaline phosphatase levels, defective T lymphocytes and hyperactive B lymphocytes. Primary biliary cirrhosis may be a pineal deficiency disease. Serotonin is important in the pineal and the serotonin antagonist methysergide may cause retroperitoneal fibrosis by interfering with pineal function. There is a good deal of other evidence which suggests that melatonin PGE1 and TXA2 are important in the regulation of fibrosis in other situations such as "collagen" diseases, lithium-induced fibrosis and cardiomyopathies. This suggests that enhancement of formation of PGE1 and TXA2 may be of value in diseases associated with excess fibrosis and defective T suppressor cell function. PGE1 levels may be raised by zinc, penicillin, penicillamine and essential fatty acids. TXA2 levels may be raised by low dose colchicine. These new approaches to treatment may prove safer and more effective than existing ones. They may be of value in disorders such as cardiomyopathy, Hodgkin's disease and other lymphomas, multiple sclerosis, Crohn's disease, atopy and other diseases in which defective T cell function is suspected.

The effects of lumicolchicine, colchicine and vinblastine on pigment migration in fish chromatophores

The effects of lumicolchicine, colchicine, vinblastine and cold temperature on the pigment migration in melanophores and xanthophores of Fundulus heteroclitus and Oryzias latipes were examined by light and electron microscopy. Xanthophores of both species which contain numerous microfilaments and a poorly developed microtubule system were extremely sensitive to the alkaloids. Lumicolchicine and colchicine induced irreversible dispersion while vinblastine caused permanent aggregation of the pigments. Treatment in lumicolchicine or colchicine at 5 mM for 60 minutes did not disrupt microtubules of melanophores to an appreciable degree, however, melanosome aggregation was partially inhibited by these drug in Oryzias. When melanophores were kept in the cold in the presence of colchicine at 1 mM, almost all microtubules were disrupted and their repolymerization at room temperature was nearly completely inhibited by colchicine. These melanophores lacking in microtubules responded to epinephrine with slow aggregation. Vinblastine at 0.1 mM induced partial aggregation of melanosomes and disruption of microtubules but most melanophores were still able to respond with pigment migration. Vinblastine at 1 mM made all melanophores punctate and immobile. Large vinblastine-induced crystals were frequently seen in the dendritic processes. The results of the present investigation suggest that cytoplasmic microtubules in fish melanophores facilitate melanosome migration only in directional orientation and appear not be responsible for the motive force.

Effects of colchicine on cyclic AMP levels in human leukocytes

The increase in human leukocyte adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels seen in response to various substances was markedly potentiated by colchicine and other agents that affect microtubule assembly. Addition of dl-isoproterenol (2 muM) or prostaglandin E(1) (10 muM), together with the phosphodiesterase inhibitor isobutylmethylxanthine (1 mM), caused a much greater increase in cyclic AMP in colchicine-pretreated cells than in control cells. With isoproterenol (2 muM) plus isobutylmethylaxanthine (1 mM), cyclic AMP levels rose about 3-fold but, in combination with colchicine, these drugs caused a more than 15-fold increase in cyclic AMP. The effects of colchicine were both time- and dose-dependent; they could be seen within 1 min after addition of colchicine or at concentrations as low as 10 nM. In addition to its potentiation of hormonally induced increases in cyclic AMP levels, colchicine also potentiated the effect of isobutylmethylxanthine alone on leukocyte cyclic AMP levels. Vinblastine, vincristine, podophyllotoxin, and oncodazole all had effects similar to those of colchicine but lumicolchicine did not. The data suggest that cytoplasmic microtubules interact with the leukocyte plasma membrane to impose constraints on the expression of hormone-sensitive adenylate cyclase; the therapeutic effects of colchicine may depend in part upon the relaxation of such constraints. Moreover, the synergism described here between colchicine-like agents and hormones is of potential therapeutic importance in clinical conditions in which either alkaloid or hormone has been useful separately.

Physiological regulation of total tubulin and polymerized tubulin in tissues

Polymerized and depolymerized forms of tubulin were measured in rat and mouse liver, rat islets, human lymphocytes, and platelets. The percent of the total tubulin present in the polymerized form varied from 30.3 +/- 1.5% in the liver of the fed rat to 89.2 +/- 0.2% in human platelets. Fasting decreased the total tubulin and to a greater extent the polymerized form of tubulin in both rat and mouse liver. Glucose feeding increased the polymerized tubulin without affecting the total tubulin content in rat liver. Phytohemagglutinin-stimulated lymphocytes exhibited at least a three-fold increase in total tubulin (expressed in terms of DNA content), which during the initial 48 h of incubation was accounted for in toto by an increase in polymerized tubulin. It is suggested that the lectin not only accelerates tubulin synthesis but also stimulated the polymerization process. Storage of platelets at 4 degrees C for 6 days resulted in a marked decrease in total tubulin and an even greater reduction in the polymerized form. It is concluded that both the total tubulin content and its degree of polymerization can be modulated independently by a wide variety of physiological factors.

Effect of colchicine on the secretion of matrices of dentine and enamel in the rat incisor: an autoradiographic study using [3H]-proline

Effects of colchicine on the incorporation of [3H]-proline into odontoblasts and ameloblasts at various development stages and on the secretion of the radioactive materials from these cells into dentinal and enamel matrices were studied autoradiographically, at 2,4,8 and 24 h after the injection of colchicine. In young functional and functional odontoblasts and ameloblasts, the number of autoradiographic silver grains over the cells increased while those over the matrices decreased 2,4, and 8 h after the injection of colchicine. However, the total number of grains over the cells and matrices were fairly uniform in all stages of cell development at all times after the injection of the drug. It is suggested that the drug does not appreciably affect the incorporation of [3H]-proline or the successive synthesis of the dentinal and enamel matrices but affects the secretion of the matrices in both functional odontoblasts and ameloblasts by interfering with the structure and function of microtubules in these cells.

The mechanism of microtubule-dependent movement of pigment granules in teleost chromatophores

The relationship of cytoplasmic microtubules to intracellular transport has been investigated in teleost chromatophores. Antimitotic agents, including colchicine, vinblastine, hydrostatic pressure, and low temperature remove microtubules in these cells and simultaneously disrupt the alignment and arrest the movement of pigment granules. During recovery, the return of alignment and movement corresponds in both time and space with the repolymerization of microtubules. These results demonstrate that microtubules are essential for the intracellular transport of pigment. Investigations of the mechanism of movement show that microtubules do not change in number or location during the redistribution of pigment in Fundulus melanophores. Microtubules in melanophores also behave as semi-stable organelles as determined by investigation with colchicine and hydrostatic pressure. These observations and others rule out a push-pull mechanism based on the polymerization and depolymerization of microtubules or one that distinguishes two operationally different sets of microtubules. It is proposed instead that particles move by sliding along a fixed array of microtubules.

Colchicine binding to bovine anterior pituitary slices and inhibition of growth-hormone release

The uptake of [ring C-methoxyl-3H]colchicine into bovine anterior pituitary slices was studied. The data suggest that more than one site exists for the binding of colchicine. At low concentrations colchicine binds to saturable trypsin-sensitive site(s), with a dissociation constant of 3.1 +/- 0.69 mug. The binding capacity of these sites is 8.58 +/- 0.60 pmol of colchicine/mg of wet pituitary. At higher colchicine concentrations binding occurs predominantly to sites which exhibit non-saturation kinetics. Subcellular fractionation of colchicine-labelled slices shows that 90% of the saturable sites are present in the fraction containing cytosol, where the binding protein has a molecular weight of about 11.9 x 10(4) and constitutes 0.7% of the protein present. The nuclear fraction contains 10% of the saturable sites, and the mitochondria and granule fraction contain only non-saturable sites. The rate of colchicine uptake was studied at 0.84 mm- and 2mum-colchicine. At both concentrations the colchicine space exceeded the total tissue water within 10 min. Equilibration with the saturable binding sites was complete in 120 min at 2mum-colchicine. A concentration of colchicine (13.4 mum) which would give 81% maximum binding was found to decrease the length of observable microtubules in tissue fixed at 37 degrees C in glutaraldehyde by 83 +/- 4%. The colchicine-binding protein could be partially purified by using a standard procedure for isolation of brain tubulin. Colchicine inhibits the release of growth hormone in the presence of 3-isobutyl-1-methylxanthine (0.1 mm), but does not alter basal release. The concentration-dependence of colchicine inhibition is similar to that of colchicine binding, but maximum inhibition is only 35%.

Insulin release and the microtubular system of the islets of Langerhans. Identification and characterization of tubulin-like protein

1. Incubation of islets of Langerhans in vitro in the presence of colchicine produced a progressive inhibition of the insulin-secretory response to glucose, which was dependent on the time of incubation. 2. The uptake of [3-H]colchicine by islet cells was a rapid process, equilibrium being reached in less than 30 min. Part of the colchicine taken up was bound to protein material, which was recovered largely in a post-microsomal supernatant fraction prepared from the islets. In contrast with this rapid uptake, the binding of colchicine by islet-cell proteins in intact islets or in islet homogenates was a slow process, and equilibrium was not reached for 60-90 min. After an initial 30 min delay, the time-course of the binding of [3-H]colchicine to islet-cell proteins paralleled that for the inhibitory effect of colchicine on insulin release. 3. Some purification of the colchicine-binding material present in islet homogenates could be achieved by precipitation of the protein with 2mM-CaCl2 (2.8-fold). However, ion-exchange chromatography on DEAE-Sephadex produced a further 27-fold purification on elution with 0.6M-NaCl. 4. Colchicine-binding protein prepared from islets by ion-exchange chromatography showed an intrinsic association constant for colchicine of 1.4muM and an apparent molecular weight on gel filtration of 110000. 5. These results suggest that colchicine-binding protein in islet cells closely resembles tubulin extracted from the other tissues. The delayed effectiveness of colchicine in inhibiting insulin secretion is not due to poor penetration of colchicine into the cells but rather to slow binding of the alkaloid to islet-cell tubulin. It seems likely that, as in other tissues, this binding prevents polymerization of the tubulin into microtubules, and thus interferes with the release process.

Alkylthiocolchicines and N-deacetyl-alkylthiocolchicines and their antileukemic activity

A series of alkylthiocolchcines (methyl, ethyl, n-butyl, n-hexy, n-octyl, and pinanyl) was prepared from colchicine by treatment with the appropriate alkyl mercaptan and p-toluenesulfonic acid. Some of these compounds (methyl-, ethyl-, and n-butylthiocolchicines) were deacetylated in good yields with 2 N hydrochloric acid in methanol. These compounds were tested for their antileukemic activity in an in vitro assay against L-1210 (mouse leukemia). Preliminary results showed that methylthiocolchicine is more active and the other alkylthiocolchicines are much less active than colchicine. N-Deacetyl-methylthiocolchicine is as active as colchicine.

The changes in cell shape during pigment migration in melanophores of a teleost, Oryzias latipes

The changes in cell shape of fish melanophores during pigment displacement, and the effects of colchicine and cytochalasin B on the surface morphology were studied by scanning electron microscopy. Dispersed melanophores are generally flat, with thick radiating dendritic processes. Aggregated melanophores are characterized by their swollen, hemispherical centrospheres and thin, collapsed dendrites. Colchicine induces a flattening of the entire surface of the cell while cytochalasin B elicits the swelling of the centrosphere accompanied by a partial migration (aggregation) of melanosomes.

The role of microtubules in the movement of pigment granules in teleost melanophores

When microtubules in teleost melanophores are disrupted with antimitotic agents, colchicine, high hydrostatic pressure, low temperature, and vinblastine, the alignment and movement of the pigment granules in these cells disappear; during recovery, the return of alignment and movement corresponds in both time and space with the repolymerization of microtubules. Furthermore, analysis of nearest neighbor distances in untreated melanophores reveals that pigment granules are closely associated with microtubules. Other structures such as microfilaments, the endoplasmic reticulum, and the cytoplasmic matrix do not appear to be involved. Thus we conclude that microtubules determine the alignment and are essential for the selective movements of the pigment granules in these cells. Investigations of the mechanism of movement show that microtubules are required for both centrifugal and centripetal migrations and that they do not change in number or location during redistribution of pigment. Our results further indicate that microtubules in melanophores behave as semistable organelles as determined by investigation with colchicine and hydrostatic pressure. These observations and others rule out a push-pull mechanism based on the polymerization and depolymerization of microtubules or one which distinguishes two operationally different sets of microtubules. We propose instead that particles move by sliding along a fixed array of microtubules.

The effects of parathyroid hormone, colchicine, and calcitonin on the ultrastructure and the activity of osteoclasts in organ culture

The ultrastructure of osteoclasts was examined in fetal rat bones after stimulation or inhibition of resorption in culture. A central ruffled border area completely encircled by a clear zone was considered to represent the resorbing system of the cell. The proportion of ruffled border and clear zone in osteoclast cross sections was compared with changes in bone resorption as measured by the release of previously incorporated radioactive calcium ((45)Ca). In control cultures 55% of the osteoclast cross sections showed an area closely apposed to bone and this consisted mainly of clear zone; only 11% showed ruffled borders. Treatment with parathyroid hormone (PTH) increased (45)Ca release, increased the frequency of finding areas closely apposed to bone (79%), and markedly increased the frequency of the ruffled border area (64%). Colchicine given concurrently with PTH decreased the number of osteoclasts. Colchicine or calcitonin treatment after PTH stimulation decreased the proportion of ruffled border area significantly by 1 h; this was followed by a decrease in (45)Ca release. These inhibited osteoclasts resembled osteoclasts from control, unstimulated cultures, suggesting that the cells had returned to their inactive state. Colchicine-treated osteoclasts also showed a loss of microtubules and a massive accumulation of 100 A filaments, suggesting that synthesis of microtubular subunits had increased.

Ultrastructure and cell biology of pigment cells. Intracellular dynamics and the fine structure of melanocytes with special reference to the effects of MSH and cyclic AMP on microtubules and 10-nm filaments

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Two actions of cyclic AMP on melanosome movement in frog skin. Dissection by cytochalasin B

Photomicrography and reflectance microphotometry were used to monitor melanosome movement in frog skin melanocytes in vitro in response to hormonal stimulation and cytochalasin B (CB). Melanocyte-stimulating hormone (MSH), theophylline, and dibutyryl cyclic AMP (DiBcAMP) induced melanosome dispersion (darkening) which was promptly arrested by cytochalasin B in concentrations of 5-20 microg/ml. Melanosome aggregation (skin lightening) occurred only after removal of the darkening agent (MSH, theophylline, or DiBcAMP) and proceeded in the presence or absence of CB. When CB was added to darkened skins, they did not lighten and melanosomes remained in the dispersed state. Use of CB has permitted the dissection of cyclic AMP-mediated melanosome dispersion into two distinct events. The first, induction of melanosome dispersion, is CB sensitive. The second action of intracellular cyclic AMP involves an uncoupling of the centripetal motive force, and is CB insensitive. In the latter process, production of cyclic AMP appears to produce the same result as application of microtubule-disrupting agents.

Cytochalasin B: effect on lysosomal enzyme release from human leukocytes

The morphological and biochemical consequences of treatment of human peripheral blood leukocytes with cytochalasin B were studied. Incubation of human polymorphs with cytochalasin B resulted in nuclear and cytoplasmic spreading, but not in spontaneous release of lysosomal enzymes. Cytochalasin B inhibited particle uptake. Consequently, phagocytic vacuoles were not observed; instead, granule contents were discharged directly into the surrounding medium when cytochalasin B-treated cells were challenged with zymosan particles. Cytochalasin B enhanced the release of lysosomal enzymes from human polymorphonuclear leukocytes whether these encountered zymosan particles or immune complexes on a nonphagocytosable Millipore filter. Cytochalasin B-treated leukocytes thus constitute a model system for quantitative study of lysosome fusion. Augmented enzyme release was blocked by prior treatment of cells with pharmacological doses of agents that influence the accumulation of cyclic nucleotides (cyclic nucleotides themselves, prostaglandin E(1)) or by compounds that interfere with microtubule function (e.g., colchicine, vinblastine). These observations suggest that one action of cytochalasin B on phagocytic cells is to remove the normal constraints to merger of granules, either with each other or with the plasma membrane, and that intact microtubule function is required for translocation of lysosomes.