Hormones Centrosomes and Genomic Instability in Mammary Carcinogenesis

William R. Brinkley, David L. Stenoien, and Thea Goepfert Introduction

The centrosome, a cytoplasmic organelle acquired at fertilization via paternal inheritance, plays a vital role in cell division and in the maintenance of cell polarity throughout development. The importance of the centrosome in achieving proper orientation and segregation of duplicated chromosomes and assuring stability of the eukaryotic genome is well established. The centrosome can be recognized in the cytoplasm of most eukaryotic cells as a discrete, microscopic domain that functions as the cell's principal microtubule organizing center (MTOC). Like the genome, it undergoes duplication during late S-phase, producing a pair of centrosomes that function to organize the bipolar spindle in mitosis.

The centrosome duplication cycle, like the cell cycle, is regulated by a vital system of checkpoint-signaling proteins, about which little is currently known. Errors in centrosome duplication and/or distribution can result in aberrant daughter cells that either lack centrosomes (acentric cells) or receive a single centrosome, producing mono-polar spindles, or they can receive more than two centrosomes, resulting in the assembly of multipolar spindles during mitosis. These aberrations can lead to catastrophic errors in chromosome distribution resulting either in cell death or transformation to become tumor cells. Using antibodies specific for centrosome associated proteins, we and others have noted that many tumors, both in-vitro and in-vivo, display cells with a variety of centrosome aberrations (1-7). The most commonly reported defect causes cells to develop supernumerary centrosomes (greater than the expected 1-2 centrosomes/cell), a process identified as "centrosome amplification." Retention and maintenance of centrosome duplicity is critical in the cell cycle to assure bipolar spindles and the maintenance of a diploid genome. Centrosome amplification could therefore account, in part, for the genomic instability and aneuploidy commonly found in cancer cells. Although bipolar spindles can form in the absence of centrosomes in some germ cells of eukaryotes this organelle plays a prominent role in spindle assembly and function in somatic cells (reviewed in 5). Thus, it becomes essential to identify and characterize the molecular components involved in maintaining the correct centrosome number in diploid cells and to understand the cause of aberrations.

Although little is known about the role of hormones in the biology of centrosomes, they serve an important role in normal cell growth and developments, and are implicated in tumorigenesis. In a later section of this report, we review recent studies in our laboratory of centrosomes and their involvement in hormone mediated aneuploidy and cancer in an experimental rat mammary model for carcinogenesis.

The Resurrection

The implication of centrosome anomalies in cancer were first identified at the dawn of the 20th Century. In a 1914 treatise, Zur Frage der Enstehung Maligner Tumoren, Theodore Boveri (8) proposed that sea urchin zygotes containing more than two centrosomes, segregated their chromosomes abnormally due to the presence of extra spindle poles induced by polyspermy (Figure 1). He noted that although multipolarity was generally lethal, occasional segregants survived to produce embryos with tumor-like outgrowths. As one of the early experts on the role of centrosomes in cell division, Boveri was the first to propose a direct link between oncogenesis and the presence of multipolar spindles, aneuploidy and loss oftissue architecture in these embryos. Although he never studied cancer cells per se, his astute observations ofthis simple invertebrate system allowed him to derive many important postulates that still apply to cancer, including the concept of oncogenes, cell cycle checkpoints, tumor-suppressor genes, genetic instability, the clonal origin of tumors, chromosome specific "weakness" (telomeres), loss of cell adhesion, and genetic mosaicism. Moreover, he achieved this monumental task with little more than a primitive light microscope, a keen sense ofobservation and a truly remarkable intuition.

Boveri's novel hypothesis that centrosome anomalies could be responsible for aneuplody and the ontogeny of cancer, created a brief but spirited debate at the time, but was never widely accepted by the cancer establishment and it was essentially ignored until recently. The development and rise of Drosophila genetics by the Morgan school of genetics (9) and the discovery by Muller, et al. (10) that x-rays were potent mutagens and carcinogens, led to the widely accepted view that cancer is caused by a somatic gene mutation. The subsequent discovery of cellular oncogenes and tumor suppressor genes in the latter half ofthe 20th Century added considerable reinforcement to the somatic mutation idea (11-12). Despite the complex genetic basis of cancer, mutations in somatic genes remains the accepted hypothesis for oncogenesis by most cancer researchers today (for alternative view, see 13). However, recent reports of centrosome anomalies associated with many tumor cells (3-6), along with the well-established role of this organelle in mitotic spindle assembly and chromosome segregation (6, 7, 14), has revived Boveri's 96 year-old hypothesis that aberrant centrosomes and aneuploidy (genomic instability) are incipient events in oncogenesis.

Theodor Boveri Sea Urchins

Figure 1. Theodore Boveri, whose portrait is shown here in 1908, proposed that aneuploidy and alterations in cellular polarity, distinguishing characteristics of most cancer cells, resulted from defects in the function of the centrosome. His early drawings, shown here, depict possible arrangements of chromosome and patterns of segregation in dividing cells with three (tripolar) or four (tetrapolar) spindle poles resulting from double fertilization of sea urchin eggs. (Reproduced with permission from University ofCalifornia Press Berkeley and Los Angeles, CA. Cambridge University Press London, England. Copyright 1967, by Regents ofThe University of California).

The role of centrosomes in cancer resurfaced again in 1996 when Fukasawa, et al. (1) reported centrosome amplification in mouse embryonic fibroblast null for the tumor suppressor p53. This group used anti-y tubulin antibodies to detect and count centrosomes by immunofluorescence and reported that cells with the p53 null phenotype displayed more than the normal complement of centrosomes, whereas wild type and heterozygous cells displayed normal numbers of centrosomes. Three additional manuscripts published in 1998 from the laboratories of Roop and Brinkley (2), Salisbury (3), Pihan, Doxy, et al. (4) reported centrosome anomalies, especially centrosome amplification, in tumor cells in vivo. These reports were followed by additional findings that centrosomal abnormalities are common to many human cancers (reviewed in 6) and have sparked a lively resurrection of Boveri's original hypothesis (14).

The Enlightenment: The Aurora Family of Serine/Theronine Kinases

The remarkable re-discovery of centrosome amplification in many common cancers led to a renewed interest in aneuploidy in neoplasia and initiated a search for a cellular and/or biochemical mechanism responsible for this phenomenon. It was immediately obvious; however that progress would be hampered by a dearth of knowledge about the molecular basis of centrosome maturation and replication in eukaryotic cells (15, 16). Theoretically, centrosome amplification (more than the normal 1-2 centrosomes/cell) can occur by one of several pathways: (a) through a defect in a checkpoint pathway that controls centrosome duplication in late S-phase resulting in the over duplication, (b) via failure to partition duplicated centrosomes into daughter cells due to arrested or aberrant cytoplasmic cleavage of cytokinesis (6, 17), or (c) due, possibly, to fragmentation of pericentriolar material into small dispersed bodies that retain their capacity serve as MTOCs.

New light was cast onto the mechanism of centrosome amplification with the discovery of mitotic serine/threonine kinases representing the Aurora kinase family that included the prototypic yeast Ipll and the Drosophila Aurora kinases (reviewed in 18, 19). Two groups from the laboratories of Sen and Brinkley at Houston (20) and the Bischoff group at Los Angeles (21) reported elevated expression of Aurora A (AurA) in many human cancers. Moreover, antibodies made against AurA were found to localize to the centrosomes of both interphase and mitotic cells (Figure 2).

Figure 2.

Localization and expression of Aurora kinase in mammalian cells. (A) Antibodies against AurA are localized in the centrosomes of HeLa cells and when the gene, STK15 is overexpressed by transfection, multiple centrosomes appear. Two centrosomes appear in cells transfected with the vector, while 20% of the STK15 transfected cells displayed > 3 centrosomes/cell. (B) Centrosomes counts in 200 vector and STK15-transfected cells are shown in (C) Figures 1D-E show growth of cells in agar of stable transfected cells with vector (D) and NIH 3T3 cell transfected with STK 15 and grown in 0.5% bovine calf serum. Micrographs were taken at a total magnification of X100. F, Western blot analysis of STK 15-transfected 3T3 clones showing expression of STK15. [From Figure 5 in Zhou, et al., 1998 (20)].

Breast Cancer Carcinogenesis

Although mRNA levels and AurA expression remain low during Gi and S phase ofthe cell cycle, message levels peak during G2/M phase and fall as cells exit mitosis (18, 21, 22). Even though AurA kinase was found to be expressed in normal cells, especially those progressing through the cell cycle, experimental overexpression of AurA kinase was shown to cause transformation of human and rodent cells in vitro and in vivo (20,21). Thus, this kinase appears to be oncogenic and centrosomes appear to be its primary target organelle. Much less is known, however, about the mutational status ofthe STK15 allele and its natural substrates.

The Aurora kinase family is highly conserved and has been isolated from a variety of species including Saccharomyces cerevisiae, Drosophila melanogaster, C. elegans, Xenopus laevis, Mus musculus, Rattus Norvegicus, and Homo sapiens (reviewed in 18). Three members ofthe family are found in human cells including AurA, B, and C. For comparison, the structure and functional domains of these three kinases are shown in Figure 3. Recent studies indicated that an orthologue of human AurA is also overexpressed in rat mammary tissues in virgin females exposed to a carcinogen, NMU, and the expression levels, along with centrosome amplification, appear to be early markers for tumorigenisis in this animal model (19).

Regulatory domain



Catalytic domain

Activation motif D»struction bos


Regulatory domain





A box 11







A box 11





i phofphciyLabon of rhit ferine p revents '.ry r: by Cdhl/AÎC "

Activation motif

Aur-A dfgwsvhapsserttlcgtldylppe: Aur-fi DFGWEVHAPSLRRKTMCGTLDYI.P]>£ Aur-C dfgwsvhtpslrrktmcgtldylppe

PK.A rkkMyfcdlM

Dcitmctl«) box


RizLixViE ([Dnnmnf)

Figure 3. The aurora kinase family. Structure of Aur-A, B, and C kinases with alignment of the variable amino terminal regulatory domains and conserved carboxyl terminal catalytic domains. Three putative Aur boxes (A-box I, II, and III) with sequence motifs are shown. Sequence motifs are also for the activation motif and destruction box. [From Katayama, et al., 2003 (18)].

Although the search continues for natural substrates and the nature of binding sites within cells, we have investigated AurA binding properties at centrosomes and spindles of human cells and determined it to be highly dynamic in comparison to other centrosome-associated proteins (23). This was accomplished by expressing a GFP-AurA in living cells to mark the centrosomes and by analyzing the cells in various stages of division by fluorescence microscopy. GFP-AurA expression was induced by doxycycline treatment in stably transfected HeLa Tet-on cells and detectable levels appeared as early as 2-4 hours following induction (Figure 4). As shown in Figures 5A-O, it was possible to detect the kinase at centrosomes throughout the cell cycle and to observe a rapid migration from spindle poles to the mitotic spindle at prometaphase (Figure 5C, D). The dynamics of AurA kinase were apparent at two levels: first within the duplicated centrosomes themselves, where dynamic oscillations and the unique tumbling motions of the entire organelle were detected (Figure 6), and secondly, at the molecular level as detected by fluorescence recovery after photobleach (FRAP). The latter procedure demonstrated a rapid exchange into an out of the centrosome and spindle (ti/2~3sec), indicating a dynamic state of equilibrium between bound kinase and unbound component in the microenvironment (Figure 7). In contrast, the tubulins and were found to be relatively stable and immobile (23). These novel observations provide considerable evidence for a kinetic vs structural role for AurA kinase in regulating spindle activity.

Figure 4. Regulated expression of GFP-AurA. (A) GFP-AurA subcloned into the pTRE vector (Clontech). A stable cell line was generated in HeLa Tet-on cells. GRP-AurA expression was induced by doxycycline addition and detectable levels of AurA were evident at 2-4 h. Following induction, GFP-AurA became localized to centrosome during interphase and prometaphase (B) before moving into the spindle (C) during prometaphase. [From Stenoien, et al., 2003 (23)].

Specific domains of AurA are needed to target the kinase to the centrosome and other sites in the mitotic spindle as indicated by mutational analysis (23). As shown in Figure 8, AurA targeting in human cells requires a minimum of amino acids 1-193 for targeting, but amino acids 1-310 assure a more pronounced association. Deletion ofthe amino terminal 129 amino acids results in a protein that shows a more stabilized centrosomal localization as indicated by reduced recovery after photobleaching (FRAP). These experiments are still ongoing, but it is clear from our initial FRAP analysis that AurA kinase is highly dynamic with respect to its association with the centrosome, indicating a tendency to reside at the functional

tet-an GFP-BTAK (hours in tet)

tet-an GFP-BTAK (hours in tet)

sites for very brief periods, in contrast to other proteins such as tubulins, NuMA, and other spindle proteins. The ability to analyze fusion protein bioluminescence and protein dynamics in live cells, during real time, has provided remarkable new insight into the micro-environment, both in the nucleus and in the cytoplasm (23, 24).

Figure 5. GFP-AurA localization during the cell cycle. Note the abrupt progression of GFP-AurA from the centrosomes to the spindle in frames C and D. Images were recorded live over a 2 h time-frame [From Stenoien, et al., 2003 (23)].

Figure 6. Analysis of GFP-AurA-marked centrosomes during the G2 phase of a HeLa cell cycle. Note that the two centrosomes undergo oscillatory movements and appear to rotate with respect to the plane parallel to the monolayer axis (compare frames at 45 min with frames at 55 and 65 min). The distance between centrosomes changes over time as shown by the graph (B). Depolymerization of microtubules by nocodazole (C) or depletion of ATP levels (D) markedly reduces the oscillatory motion [From Stenoien, et al., 2003, (23)].

Figure 5. GFP-AurA localization during the cell cycle. Note the abrupt progression of GFP-AurA from the centrosomes to the spindle in frames C and D. Images were recorded live over a 2 h time-frame [From Stenoien, et al., 2003 (23)].

Time Frame Interphase

^ Interphase





U Mitosis




Figure 7. GF-AurA is a dynamic component of the centrosome and mitotic spindle. FRAP analysis was performed on HeLa cells stably transfected with GFP-AurA. Following a short photobleach in the region denoted by the box, GFP-AurA recovers very rapidly in both the centrosome (A) and spindle (B). The recovery curves from 10 cells are shown in (C). (D) The calculated t|/2s of GFP-AurA in the interphase centrosome and mitotic spindle are 2.1 ± 0.2 sec, respectively [From Stenoien, et al., 2003, (23)].

interphase Mitosis

Figure 8. AurA targeting domains. Deletion analysis of centrosome targeting comparing the full-length AurA to deleted domains. AurA deletions were generated as GFP fusions to analyze the effects on AurA domains on centrosomeal targeting. DL1 (1-133) did not target to the centrosome whereas DL2 and DL3 showed moderate to strong association (inset photos). [From Stenoien, et al., 2003 (23)].

Figure 8. AurA targeting domains. Deletion analysis of centrosome targeting comparing the full-length AurA to deleted domains. AurA deletions were generated as GFP fusions to analyze the effects on AurA domains on centrosomeal targeting. DL1 (1-133) did not target to the centrosome whereas DL2 and DL3 showed moderate to strong association (inset photos). [From Stenoien, et al., 2003 (23)].

Centrosome Amplification and AurA Expression in Rat Mammary Carcinogenesis: Hormone Mediation

Although AurA kinase expression and centrosome amplification have been investigated in cells in vitro and has been reported in a number ofhuman tumors in vivo, few studies have utilized animal models where experimental manipulation is possible. For this reason, we adapted a novel rat mammary model, developed in the Medina laboratory (25), to explore the effects of carcinogens and hormones on centrosome amplification and AurA expression in vivo.

The rat mammary model used in our studies was initially established to investigate the paradigm of the hormone-induced refractory state imposed by the exposure of virgin female rats to elevated levels of estrogen and progesterone (E + P) at a critical period in development (26). It is our working hypothesis that in the mammary gland of the immediate post-pubescent female, hormones induce a molecular switch in a population of stem cells that renders the descendent mammary cells refractory to carcinogens such as methylnitrosourea (MNU) and others (25). A schema illustrating this hypothesis is diagrammed in Figure 9. Cells progressing through pathway 1 are susceptible to the carcinogen and become neoplastic, but mammary cells from females exposed to elevated E + P, via pregnancy or experimental manipulation (pathway 2), at the appropriate stage of development, become resistant to MNU exposure. Using this model, we analyzed centrosome amplification and AurA expression in cells progressing through each of the pathways. The details of these experiments and the results have been published elsewhere (27), but they are reiterated here to establish that centrosome amplification and overexpression of AurA are early events in Carcinogenesis and sensitive to hormone action. Thus, E + P treatment under these experimental conditions, suppresses Carcinogenesis via pathways that are, as yet, poorly understood.

Figure 9. The rat mammary model. Scheme illustrates the proposed E + P switch (black box) showing pathway 1 of MNU-

susceptible rats and pathway 2 animals that are resistant.

Centrosome Animal Cell

Centrosome counts were obtained from both epithelial and stromal cells exposed to MNU according to schemes 1 and 2 shown in Figures 10 A-D. Mammary epithelia and associated stromal cells provide an excellent built-in control for centrosome counts. Thus, proliferating cell populations (mammary epithelium) generally display cells with 1-2 centrosomes/cell, where as the non-dividing stromal cells in G0, contain mostly single centrosomes (Figure 10B). The same populations of mammary cells from animals exposed to MNU at 97-104 generally display elevated numbers of epithelial cells with 3 or more centrosomes/cell (Figure 10D). Centrosome amplification is considered significant when 10% or more ofthe cells counted contained more than 2 centrosomes per cell. As shown in Figure 10D, the incidence of centrosome amplification in cells from MNU-treated rats varied from a low of 10% to a high of 46% (300 cells were scored in each mammary gland). The nonproliferating populations of stromal cells from both normal glands as well as tumors displayed mostly single centrosomes, as anticipated.

Centrosome Microscope

Figure 10. (A) Confocal microscope optical sections of mammary epithelium showing centrosome at the basal region below nuclei of normal mammary gland. The centrosome plot in (B) shows centrosome distribution in both stroma and normal epithelium from age matched virgin rats (AMV). Confocal image for mammary tumor (C) and centrosome plot (D) show centrosome plots of eight different mammary tumors (A-H). Details on the centrosome quantitation age given in Goepfert, et al., 2002 (27).

Figure 10. (A) Confocal microscope optical sections of mammary epithelium showing centrosome at the basal region below nuclei of normal mammary gland. The centrosome plot in (B) shows centrosome distribution in both stroma and normal epithelium from age matched virgin rats (AMV). Confocal image for mammary tumor (C) and centrosome plot (D) show centrosome plots of eight different mammary tumors (A-H). Details on the centrosome quantitation age given in Goepfert, et al., 2002 (27).

In order to determine when the first indications of centrosome amplification occurred after exposure to MNU, we examined mammary glands at various intervals after exposure. We found mostly normal centrosome profiles, but also detected small foci of cells where centrosome amplification was occurring as early as 40 days post treatment. Histological examination ofthese samples revealed no indication of pre-malignant or malignant pathology. Mammary tumors arise within 15 weeks after exposure to MNU, and AurA mRNA levels and centrosome amplification correlate with tumorigenesis (Figure 11).

Mammary Tumors


Figure 11. AurA mRNA expression and centrosome amplification correlated with tumorigenesis in rat mammary gland. Northern blot containing PolyA+ RNA (5 ug) from rat mammary gland tissue of different regimens. Tissue of AMVs of animals subjected to hormonal treatment prior to MNU (E + P/MNU), of animals subjected to MNU treatment. The confocal images show sections of immunostained tissues from the normal mammary gland (NMG) and from ADH, CA, and DCIS. The corresponding centrosome plots are shown below each image. [From Goepfert, et al., 2002 (27)].


Figure 11. AurA mRNA expression and centrosome amplification correlated with tumorigenesis in rat mammary gland. Northern blot containing PolyA+ RNA (5 ug) from rat mammary gland tissue of different regimens. Tissue of AMVs of animals subjected to hormonal treatment prior to MNU (E + P/MNU), of animals subjected to MNU treatment. The confocal images show sections of immunostained tissues from the normal mammary gland (NMG) and from ADH, CA, and DCIS. The corresponding centrosome plots are shown below each image. [From Goepfert, et al., 2002 (27)].

To correlate centrosome amplification with overexpression of AurA, we analyzed the same rat mammary tissue samples using our human probe. However, we soon found it necessary to clone a rat orthologue to human AurA/STK15. The rat AurA (rAurA) displayed 85% identity to human kinase and 96% to the mouse orthologue (27). Northern analysis was applied to examine AurA gene expression following various treatments. As shown in Figure 11, AurA mRNA expression levels were significantly elevated [compared to control aged-matched virgin (AMV

glands)] in all rat mammary tumors examined, as well as in mammary gland examined as early as 40 days after treatment. Tissues from pregnant rats, and from rats pretreated with E + P, prior to exposure, displayed control levels of AurA expression and mammary epithelial cells displayed normal centrosomes.

Rat AurA gene expression appeared to be specifically influenced by hormone treatment as indicated by mRNA levels throughout pregnancy. Early in pregnancy (6 days), message levels were elevated (compared to tissues from the same time points from E+P treatment), but later (18 days) and during lactation (10 days) message levels decreased significantly to levels comparable to AMVs (Figure 12). Such fluctuation in the expression ofthis kinase gene is likely due to a natural elevation in mitotic index known to occur in proliferating mammary epithelia at early pregnancy. Proliferation levels remained high initially, but decreased at the end of pregnancy and the onset of lactation. Such mitotic activity clearly influences the normal ebb and flow of AurA expression in populations of tissue cells during developmental stages of growth and differentiation and must be taken into account when analyzing mRNA expression in normal or neoplastic cells in vivo.

Figure 12. Northern blot containing poly A+ RNA (5 ng) from mammary gland tissue of animals subjected to E; 20 yg), P; 20 mg, and prolactin (PRL;10 Ug/g body weight) stimulation for 6 days. RNA samples were probed with radioactive labeled ratAurA cDNA as indicated in the figure. The blot was exposed to a Phosphorlmager, and the results were quantitated using the software Image-Quant (Molecular Dynamics). S12 ribosomal protein was used as the internal control. The data for rat AurA were plotted in arbitrary relative volume units/ug RNA normalized for S12 ribosomal protein expression. The columns show the range of variance of two treatments (n = 2); bars, SD. [From Goepfert, et al., 2002 (27)].

Future Directions

Much more can be learned about the Aurora kinase family of kinases and their involvement in cell transformation when investigators more clearly identify and define molecular substrates for these catalytic enzymes, especially AurA. Until then, the role of centrosomes in cell transformation and tumorigenesis remains a "chicken vs egg" paradox. Which comes first, centrosome amplification or some prior condition that leads to genomic instability, with subsequent, downstream disregulation of centrosomes? Our results with the rat mammary model strongly suggest that both overexpression of AurA and centrosome amplification represent early, perhaps incipient events in cell transformation. Since centrosomes are responsible for achieving spindle bipolarity, a condition absolutely required for proper segregation of chromosomes, disregulation of centrosome number may be the "Rosetta stone" for genomic instability. Thus, an aberration in the process of centrosome duplication, as illustrated in Figure 13, may switch cells to an errant pathway causing aneuploidy, and initiating a selection process that results in clonal outgrowth of tumor cells. Alternatively, a similar fate could arise from a defect in cytokinesis causing misdivision of the cytoplasm and nucleus, resulting in supernumerary centrosomes and tetraploidization, as reported by Meraldi, et al. (17). The later pathway (Figure 13) argues that AurA overexpression does not necessarily disregulate by over-duplicating centrosomes, but produces cells with an extra burden of centrosomes through misdivision that will initially double the centrosome number. Further variations in centrosome numbers could arise in subsequent divisions causing multipolar spindles to form leading to aneuploidy and apoptosis, in most cells, or perhaps a favorable partitioning ofthe genome in a rare segrant, favoring survival, clonal expansion from daughter cells and tumor production.

If the initial lesion that gives rise to genomic instability involves targets within the fundamental machinery of cell division, such as those that maintain correct centrosome number and distribution, the surviving cancer cells must ultimately compensate for this harmful defect. Otherwise mitotic chaos would persist. Mature cancer cells divide as virtual "mitotic machines," however, ultimately overgrowing and out-competing normal diploid cells for space and nutrients, leading to the death ofthe host. Thus, it seems highly likely that a natural selection process exists to set the mitotic process back on track, rescuing tumor cells from an initial chaotic state of division, to one where mitotic efficiency reigns supreme (7).

Cancer, like all living forms, is thus, a product of Darwinian evolution that capitalizes on the time-honored process ofnatural selection that leads to diversity in all life forms. The successful development offuture anti-cancer drugs and treatment regimens not only requires additional knowledge of the molecular basis of cell division, but also must take into account the basic concepts of evolution and natural selection within tissue cells. Accordingly, a cancer cell is a "survivor" of the harsh natural ecosystem of the host, and, in some cases, the pharmaceutical armamentarium of modern medicine, --and becomes a species all its own (13).

Figure 13. Centrosome pathways. Centrosomes are duplicated once, and only once during a normal cell cycle, producing a pair of centrosomes that separate and migrate to opposite poles of the mitotic spindle during mitosis (M). The replicated chromosomes are aligned on the metaphase plate of the bipolar spindle and partitioned equally to each daughter cell producing diploid progeny. At telophase, each daughter receives a single centrosome that becomes the major MTOC at Gr Centrosome amplification occurs when errors occur, either in the process of duplication (1), or partitioning of centrosomes (2) such that the cells receive extra copies of centrosomes. In pathway 1, centrosomes undergo multiple rounds of replication during the cell cycle, possibly due to failure of a centrosome-associated checkpoint, producing supernumerary centrosomes and a multipolar spindle at the subsequent M. In pathway 2, centrosomes accumulate in a cell that has undergone endoreduplication and failure to complete cytokinesis. Either pathway produces dis-regulation ofchromosome segregation, resulting in aneuploidy followed by cell death (apoptosis). According to this scheme, a rare daughter cell receives a favorable complement of genes, growth factors, etc., (the imitator phenotype) producing progeny that proliferate and survive to produce a cancer.


The authors are grateful to Janet Hom for administrative support, to Kevin Brinkley for editorial assistance and to Rebecca Moore and Joshua Newton for technical assistance. This research was supported in part by grants from NIH CA 64255 and CA 41434. We dedicate this work to the memory of Professor T.C. Hsu.


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