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Advances in Brief |
Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461
| ABSTRACT |
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| Introduction |
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ß-tubulin dimers and prevent assembly of microtubules. However, low concentrations of both Taxol and vinblastine suppress growing and shortening at the ends of microtubules and appear to block mitosis by dynamically stabilizing spindle microtubules (4
, 5)
. Cell death occurs in cancer cells treated with Taxol (6) , and the question arises as to the relationship between mitotic block and cell death. Although Taxol-induced apoptosis may occur after a prior mitotic arrest (7) , it could also be induced independently of G2-M arrest (8) . Previous studies with lung carcinoma A549 cells found that low concentrations of Taxol inhibited cell proliferation without blocking cells at mitosis (9) and induced a large population of hypodiploid cells seen close to the G1 peak (10) . It was speculated that the aneuploid cells might originate from aberrant mitosis (10) , although the mechanism remained to be determined.
Because suppression of spindle dynamics by low concentrations of drugs is a common mechanism responsible for mitotic block, induced by both microtubule-stabilizing and -destabilizing agents (4 , 5) , we questioned whether other microtubule-interacting drugs would, like Taxol, induce aneuploid cells without blocking cells at mitosis. The ability of six antimitotic drugs to initiate aberrant mitosis and to induce an aneuploid population of A549 cells has been analyzed. Our results indicate that the stabilizing drugs, but not the destabilizing agents, induce aberrant mitosis through multipolar spindle formation that results in aneuploid populations of cells.
| Materials and Methods |
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Flow Cytometry.
Distribution of DNA content in A549 cells treated with microtubule interacting drugs was determined by flow cytometry (10)
. Briefly, cells treated with drugs were fixed in 70% ethanol and resuspended in PBS containing PI and DNase-free RNase A. Forward and orthogonal scatter lights, as well as red fluorescence were analyzed by fluorescence-activated cell sorting. A dual parameter dot plot of the integrated area verses the width of the fluorescence pulse was displayed to exclude cell aggregates from cell counting and analysis. The histogram of DNA distribution was modeled as a sum of G1 (Fig. 1B
2N), G2-M (Fig. 1B
, 4N), S phase, and an aneuploid population close to the G1 peak, by using ModFitLT software (Fig. 1B)
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-tubulin antibody for 1 h at 37°C. The secondary antibody, Alex 488-conjugated goat antimouse IgG, was added to the cells for 1 h, in the presence of rhodamine-phalloidins for actin staining. Chromosomes were stained with 1 µg/ml DAPI in PBS. After washing with PBS, the slides were mounted and sealed. Fluorescent images were acquired with an Olympus high-resolution microscope, connected to a CCD camera. Tricolor images were merged using Photoshop 5.5. Approximately 100 mitotic cells were counted to calculate the number of spindles per cell. | Results and Discussion |
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Aberrant Spindles and Cell Division.
Mitoses were examined by fluorescent microscopy, and four kinds of spindle structures were observed in different cells before division (Fig. 3A)
. Some cells (Fig. 3, A.1)
contained a ring of condensed chromosomes with a monopolar spindle pole in the center. Bipolar spindles (Fig. 3, A.2)
were present not only in normal cells, but also in drug-treated cells. Lagging chromosomes close to spindle poles were occasionally found in drug-treated metaphase cells. In the absence of proper spindle attachment, lagging chromosomes may activate a spindle checkpoint and block anaphase transition (13)
. Tripolar (Fig. 3, A.3)
and quadripolar (Fig. 3, A.4)
spindles, with abnormal chromosome alignments, also were seen in cells treated with the stabilizing drugs. Multipolar spindles have been observed previously in cells treated with taxanes (14
, 15)
. For a detailed view of bipolar, tripolar, and tetrapolar spindles and their associated chromosomes in EpoB-treated cells, three-dimensional movies have been generated from confocal fluorescent microscopy and are available from the authors or on the Internet.4
(QuickTime software is required to view the files.)
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As long as chromosomes can congress and align, although they are distorted (Fig. 3, A.3 and A.4)
, the presence of multipolar spindles does not alert the checkpoint controlling the exit of mitosis (17)
. Mitotic cells with multipolar spindles resulted in aberrant cell divisions. A few cells gave rise to unequal division (Fig. 3, C.1)
, which would produce not only hypodiploid cells but also G1 cells with more than 2N DNA. Tripolar cell divisions were significantly increased in cells treated with microtubule-stabilizing drugs compared with control cells (Fig. 3, C.2 and C.3)
. At the drug concentrations used in Fig. 3B
,
15% of the telophase cells, after incubation with Taxol or EpoB, produced three daughter cells connected by multiple midbodies, whereas the remaining 85% of the telophase cells produced two daughter cells. With discodermolide, 25% of the telophase cells produced tripolar cell division. In contrast, no tripolar cell division was observed in cells treated with the destabilizing drugs at the concentrations indicated in Fig. 3B
, nor in cells treated with 9 and 11 nM of vinblastine (data not shown). Both tripolar cell division and multipolar spindles occurred only in cells treated with the stabilizing drugs. A higher percentage of tripolar cell division might have been expected if all mitotic cells with tripolar spindles had progressed to tripolar cell division. However, the number of tripolar cells may be underestimated if one cell was separated early from the two other connected cells. Our results suggest that a significant percentage of mitotic cells contained multipolar spindles that led to aberrant cell division and produced aneuploid G1 cells. Multipolar spindles have been observed in many tumor cells and are responsible for the induction of aneuploidy (18)
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Aneuploidy and Drug Sensitivity.
A significant aneuploid population was induced by treatment with 8 nM Taxol for 18 h (Fig. 2)
. In response to an additional treatment with 36 nM Taxol that blocked cells in mitosis, the accumulation of cells at the G2-M phase was significantly slower compared with cells without a preincubation (data not shown). This suggests that part of the cells were not cycling after preincubation with Taxol. Taxol may have activated the G1 checkpoint genes p53 and p21 (9)
, and induced a G1 block in cells after division (19)
. Consistent with previous observations in Taxol-treated cells (20
, 21)
, drug sensitivity for microtubule-stabilizing agents corresponds to the induction of aneuploid G1 cells. The drug concentrations required for 50% inhibition of cell growth after a 72-h incubation (IC50) with Taxol and discodermolide were 3.4 and 9.5 nM, respectively. These concentrations were closer to the concentrations of drug required for the initiation of aneuploidy than for the induction of mitotic arrest (Fig. 2)
. Interestingly, EpoB had an IC50 value of 0.57 nM, which was even lower than the drug concentration (3.3 nM) needed for one-half maximal induction of aneuploidy (Fig. 2)
. This suggests that induction of an aneuploid population may not fully account for the EpoB cytotoxicity, and that other mechanisms may be involved. Alternatively, more accurate methods such as fluorescence in situ hybridization may be required to measure aneuploidy induction by EpoB at concentrations close to its IC50 value.
To further examine the significance of aneuploidy induction on EpoB sensitivity, an EpoB-resistant cell line, A549.EpoB40, was examined. This cell line has a ß-tubulin mutation at amino acid 292 and is 95-fold resistant to EpoB compared with the parental cells (11)
. Although a mitotic block did occur in response to increasing concentrations of EpoB, there was a large decrease in the aneuploid population compared with that in the drug-sensitive cells (compare Fig. 4A
with EpoB in Fig. 2
). In contrast, the A549.EpoB40 resistant cell line that does not demonstrate cross-resistance to discodermolide (11)
, exhibited a large increase in the aneuploid population in response to discodermolide (Fig. 4B)
. These results indicate that EpoB drug resistance is partly determined by the inhibition of aneuploid induction.
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In contrast to mitotic arrest, aberrant mitosis was induced only by microtubule-stabilizing drugs and not by destabilizing agents. This suggests that suppressing spindle dynamics cannot fully account for aberrant mitosis. In addition, a complete mitotic block, but not aneuploidy, was induced by higher concentrations of EpoB in A549.EpoB40-resistant cells (Fig. 4)
. This further supports the concept that the mechanisms responsible for mitotic block and aberrant mitosis may not be the same. Interference with the function of centrosomes may contribute to aberrant mitosis (15)
, because Taxol has been reported to preferentially bind to centrosomes (24)
. Preliminary studies using antibodies against
-tubulin and pericentrin found two major centrosomes and additional minor ones in some aberrant mitotic cells after treatment with 4.8 nM EpoB (data not shown). Although centrosomes may not be required for the function of spindle poles, the frequency of midbody formation and successful division is higher when centrosomes are present (25)
. Centrosome amplification has been associated with induction of multipolar spindles and aneuploidy in cancer cells (18)
. Further research is needed to elucidate the molecular mechanisms of multipolar spindle induction by the microtubule-stabilizing drugs.
In summary, we have demonstrated that the response to different microtubule-interacting agents can be distinct. The occurrence of aberrant mitosis and aneuploidy is dependent on both the mechanism of drug action and the concentration of drug. The sensitivity of cells to microtubule-stabilizing drugs is mainly the result of aberrant mitosis and not mitotic block.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by Department of Defense Breast Cancer Research Program DAMD17-00-1-0674 (to J-G. C.) and USPHS Grants CA 39821 and CA 77263 (to S. B. H.). ![]()
2 To whom requests for reprints should be addressed, at Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461. Phone: (718) 430-2163; Fax: (718) 430-8959; E-mail: shorwitz{at}aecom.yu.edu ![]()
3 The abbreviations used are: EpoB, epothilone B; PI, propidium iodide; DAPI, 4',6-diamidino-2-phenylindole. ![]()
4 Internet address: www.aecom.yu.edu/aif/users/s_horwitz/mitosis.htm. ![]()
Received 12/21/01. Accepted 2/14/02.
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