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Advances in Brief |
Department of Laboratories, North Shore University Hospital, Manhasset, New York 11030 and Department of Pathology, New York University School of Medicine, New York, New York 10016
| ABSTRACT |
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| Introduction |
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The molecular mechanism of arsenic therapy is not well understood. It has been proposed that arsenic targets PML fusion proteins in APL with a t(15;17) translocation (3 , 7) , but recent studies have shown that PML fusion protein is not required for the arsenic effect (8) . The consensus of several reports is that arsenic can induce apoptosis in leukemia cells by activating apoptotic genes (3 , 5) . However, the primary cellular target of arsenic in inducing apoptosis has remained unknown.
Trivalent arsenic causes toxicity by binding and inactivating a number of sulfhydryl-containing proteins and enzyme systems. Arsenic affects mitochondrial enzymes and impairs tissue respiration, inhibits succinic dehydrogenase activity, and uncouples oxidative phosphorylation (9)
. The cytoskeleton is another cellular target for arsenic. Several lines of evidence suggest that arsenic has a particular affinity for the cytoskeleton, which contains proteins with a higher sulfhydryl content than the soluble fraction (10)
. Subunits of the microtubule, tubulin
and ß, have been isolated by arsenic affinity chromatography (11)
. The functional ability of centrosomes to nucleate microtubule assembly is inhibited by arsenite (12)
. Disruption of microtubule assembly and spindle formation by sodium arsenite can induce aneuploidy (13)
. Because the microtubule is a primary target for many antileukemia drugs, we have investigated cell cycle changes in arsenic trioxide (As2O3)-treated leukemia cells and provide evidence for a mechanism of arsenic-induced apoptosis through microtubule inhibition.
| Materials and Methods |
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Treatment of Cultured Cells.
K562 cells were incubated in presence of As2O3, arsanilic acid, PMA, BAL, GTP, Taxol, actinomycin D (Sigma), and Colcemid (Life Technologies, Inc.), alone or in various combinations, at indicated concentrations. The viable cell counts were determined by trypan blue staining and expressed as a percentage of untreated control cells. Mitotic cells and cells in telophase were analyzed after staining with Wright-Giemsa. At least 1000 cells were counted for each sample. Data represent three or more independent experiments.
Flow Cytometry and Immunostaining.
Flow cytometry analysis was performed using FACScan (Becton Dickinson). Cells were stained with directly labeled mouse antihuman glycophorin A, HLA-DR, CD33, CD34 (Becton Dickinson), or annexin V protein (Clontech) using the manufacturers protocol. Individual histograms were overlaid with isotype controls. Immunostaining was performed with the annexin V apoptotic stain kit (Clontech).
Tubulin Polymerization.
Bovine tubulin (Sigma; 5 mg/ml) was kept on ice to maintain monomeric conditions. The tubulin assembly assay was performed following the manufacturers protocol (Sigma). For pretreatment, tubulin was incubated with the indicated reagents or control buffer for 30 min at room temperature and then depolymerized by freezing and thawing three times. Monomeric tubulin was purified by centrifugation at 14,000 x g for 20 min at 4°C. Tubulin polymerization was determined by measuring A350 nm with the spectrophotometer.
Electrophoresis of Tubulin.
The degree of polymerization of tubulin was determined by SDS-PAGE. Tubulin (5 mg/ml) was incubated with the indicated reagents at 37°C for 1 h. Monomeric and polymerized tubulin were separated by centrifugation. Both supernatants (monomers) and pellets (polymers) were subjected to electrophoresis under denaturing conditions. The amount of tubulin in different states was estimated after staining the gel with Brilliant Blue stain (Sigma).
| Results and Discussion |
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Effects of Arsenic on Mitosis.
At concentrations between 0.5 and 5 µM, As2O3 blocks the proliferation of leukemia cell lines representing various stages and lineages, including K562 (erythroid myeloid cells), U937 (macrophage-like cells), HL60 (promyelocytic cells), and NB4 (APL cells). A striking similarity of the effect of arsenic on these cells is mitotic arrest, resembling the effect of antitubulin drugs. As demonstrated in K562 cells, most untreated cells are normally in interphase (Fig. 1A)
, and only 12% of cells are in different stages of mitosis (Fig. 1, BE)
. When treated by microtubule-disrupting drugs such as Taxol and Colcemid (a colchicine derivative), cells exhibit distinct types of mitotic arrest. The chromosome pattern of Taxol-treated cells resembles prophase (Fig. 1F)
, whereas the colchicine-treated cells show condensed nuclei due to the loss of the microtubule (Fig. 1G)
. At 2.5 µM, As2O3 induces chromosomal morphological changes with features of both Taxol (Fig. 1H)
and colchicine (Fig. 1I)
. These morphological changes were also observed in other leukemia cell lines such as APL NB4 cells. However, PMA-differentiated K562 cells remain unchanged with As2O3 treatment at this level (Fig. 1J)
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Effects of Arsenic on Tubulin.
Consistent with the cellular effect, the in vitro microtubule assembly assay showed that prior treatment of monomeric tubulin with As2O3 markedly inhibits GTP-induced polymerization and microtubule formation (Fig. 4A)
. The addition of Taxol did not enhance GTP-dependent polymerization in the presence of arsenic. However, when tubulin is preincubated with GTP, the addition of arsenic did not change the state of tubulin polymerization (Fig. 4B)
. Arsenic slightly decreased background tubulin polymerization in the absence of added GTP. These cross-inhibition experiments suggest that As2O3 is a noncompetitive inhibitor of GTP binding to tubulin. Analysis of monomeric tubulin by electrophoresis showed that arsenic, like Colcemid, could prevent tubulin polymerization, whereas Taxol enhanced tubulin polymerization (Fig. 4C)
. These results indicate that arsenic targets free tubulins in a manner similar to colchicine. This does not rule out the possibility that arsenic may bind to polymerized tubulins and stabilize microtubules.
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A unique target site of arsenic action on tubulin may account for the observation that the mitotic arrest of arsenic-treated cells resembles the effects of both the tubulin polymerization inhibitor and promoter. Because arsenic prevents tubulin polymerization but does not depolymerize the GTP-stabilized microtubule (Fig. 4B)
, it is possible that arsenic binds both tubulin monomer and polymer and stabilizes them. The relatively narrow range of the effective concentration of arsenic could be determined by the availability of arsenic binding sites, which would depend on the intracellular GTP level. If sensitive myeloid cells have limited GTP production during mitosis, arsenic might bind to tubulin more readily. Leukemic cells apparently have an increased level of tubulin and a higher ratio of monomeric tubulin:polymerized tubulin, which will provide a large number of targets for arsenic (20)
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In summary, data presented here suggest that in myeloid cells, trivalent arsenic binds two cysteine residues in tubulin, blocks the binding site for GTP, and disrupts the normal dynamic of microtubules during mitosis. As seen with other microtubule inhibitors, this activates a cascade of genes for programmed cell death and leads to apoptosis. These results suggest that tubulin is the molecular target for arsenic in leukemia therapy. Consistent with clinical observations, these results also suggest that arsenic can be used at a low concentration that selectively targets the microtubules of rapidly dividing tumor cells, thus minimizing general toxicity. Evidence that the cross-linking of vicinal dithiols in tubulin has an important role in leukemia therapy raises the possibility of developing rationally designed arsenic-based antimitotic agents.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at Department of Laboratories, North Shore University Hospital, 300 Community Drive, Manhasset, NY 11030. E-mail:YMLI{at}AOL.COM ![]()
2 The abbreviations used are: APL, acute promyelocytic leukemia; PMA, phorbol 12-myristate 13-acetate; PI, propidium iodide; PML, promyelocytic leukemia gene; BAL, British Anti-Lewisite, 2,3-dimercapro-1-propanol. ![]()
Received 11/24/98. Accepted 12/31/98.
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